Myasthenia Gravis & Neuromuscular Junction (NMJ) Disorders

BASIC CONCEPTS   Acetylcholine receptors (AChRs)     Disorders     Structure     Subunit mutations: α; β; ε; δ   Neuromuscular junction (NMJ)     Illustrations: A; B     Presynaptic     Postsynaptic   Diagnostic tests ACQUIRED NMJ DISORDERS   Botulism   Myasthenia gravis     Autoimmune myasthenia gravis     Childhood MG     Drug-induced MG     Neonatal MG       Transient       AChR inactivation       Arthrogryposis     Ocular     Anti-MuSK antibody positive     Anti-AChR-antibody-negative     Thymoma     Domestic animals   Myasthenic syndrome (Lambert-Eaton)   Snake venom toxins Synaptic and Post-synaptic molecules at the NMJ

CONGENITAL & FAMILIAL NMJ DISORDERS 2 Congenital MG syndromes (CMS)   General   Differential Diagnoses     Clinical     Molecular & Localization   AChRs: Kinetic abnormalities   Inheritance     Recessive: Most Dominant: Slow AChR Channels   Muscle pathology Postsynaptic NMJ defects: AChR disorders   AChR subunits: α, β, δ, ε, γ   Kinetic changes in AChR function     Numbers of AChRs at NMJs: Varied       Reduced AChR #         Slow AChR channels: ↑ Response to ACh               Delayed channel closure: AChR mutations               Repeated channel reopening: AChR mutations         Fast AChR channels: ↓ Response to ACh               Mode-switching kinetics: AChR ε subunit               Gating abnormality: AChR α or ε subunit               Low ACh affinity: AChR α, δ or ε subunit               Reduced # & Fast channel: AChR β;δ               Arthrogryposis: AChR δ subunit         Cl- channel conversion: AChR α         Also see: ε subunit disorders       Normal AChR #             Fast-channel syndromes               Low ACh-affinity of AChR; AChR ε subunit               Reduced channel openings: AChR α subunit             High conductance & Fast closure of AChRs       Increased AChR #             Slow AChR channels: AChR subunit β L262M   No kinetic abnormalities in AChR function       Reduced Numbers of AChRs at NMJs         AChR mutations           Usually: ε subunit: 17p13           Rarely other subunits: α; 2q24, β; 17p12, δ; 2q33   Arthrogryposis syndromes       Multiple pterygium (Escobar): AChR; RAPSN; TPM2       Recurrent congenital MG: Immune         Maternal antibodies vs fetal AChRs Synaptic (NMJ) basal lamina defects   Acetylcholinesterase deficiency: ColQ; 3p25   Laminin β2 (LAMB2) deficiency: 3p21   Laminin α5 (LAMA5) deficiency: 20q13 Postsynaptic NMJ defects: Other   LRP4: 11p11   MACF1: 1p34   MuSK: 9q31   Plectin deficiency: Plectin; 8q24   Rapsyn: 11p11     Digenic with AK9   Weakness, Apnea & Bulbar Δ: SCN4A; 17q35   Lethal congenital myopathy: Contactin-1; 12q11   Arthrogryposis     Arthrogryposis: UNC50; 2q11     Intellectual Disability: ZC4H2; Xq11     LCCS6: ZBTB42; 14q32 Presynaptic defects   Arthrogryposis, Distal 5D: ECEL1; 2q37   Congenital Myasthenic Gravis (MG)     Episodic apnea (Infantile)       ChAT; 10q11       SLC5A7 (CHT); 2q12       SLC18A3; 10q11     Lambert-Eaton       LEMS-like       SYT2: 1q32     MG + CNS       Munc13-1: 19p13       RPH3A: 12q24       SNAP25: 20p12     SLC25A1: 22q11     VAMP1: 12p13     Reduced synaptic vesicles & quantal release     Reduced quantal release   Episodic ataxia 2: CACNA1A; 19p13 Presynaptic + Postsynaptic defects   Agrin: 1p36   Centronuclear myopathies     MTM1     DNM2   Limb-Girdle MG     Familial: Dok-7; 4p16     Glycosylation disorders       ALG14: 1p21       GMPPB: 3p21       Limb-Girdle MG + Tubular Aggregates         CMSTA1: GFPT1; 2p13         CMSTA2: DPAGT1; 11q23         CMSTA3: ALG2; 9q22   Col13A1: 10q22   MYO9A: 15q23   PREPL: 2p21   CHD8: 14q11 CMS #   AChRs     1A; 1B: CHRNA1; 2q31     2A; 2C: CHRNB1; 17p13     3A, 3B; 3C: CHRND; 2q33     4A, 4B; 4C: CHRNE; 17p13       A = Slow AChRs; Dom       B = Fast AChRs; Rec       C = Reduced AChR #s; Rec   5: COLQ; 3p25   6: ChAT; 10q   7A; 7B: SYT2; 1q32   8: AGRN; 1p   9: MUSK; 9q31   10: DOK7; 4p   11: RAPSN; 11p11   12: GFPT1; 2p14   13: DPAGT1; 11q23   14: ALG2; 9q22   15: ALG14; 1p21   16: SCN4A; 17q35   17: LRP4; 11p12   18: SNAP25; 20p12   19: COL13A1; 10q22   20: SLC5A7 (CHT); 2q12   21: SLC18A3; 10q11   22: PREPL; 2p21   23: SLC25A1; 22q11   24: MYO9A; 15q23   25: VAMP1; 12p13 MG, RNS Decrement or Fatigue   Congenital MG     Face malform: Rapsyn; 11p11     Congenital + Acquired MG     SLC25A1   Centronuclear     CNM1: DNM2; 12q21     CNM2: BIN1; 2q14     CNMX: MTM1; Xq28   Myopathy     DES: 2q35     LGMD D1: DNAJB6; 7q36     PURA: 5q31     RyR1     TEFM: 17q11     TPM3   Familial Immune   Ophthalmoplegia + Curare sens   TOR1AP1: 1q25

Myasthenia Gravis: Autoimmune History Early descriptions External link: 1935 Anti-AChE Rx (YouTube) Clinical Features Weakness Extraocular Bulbar Generalized Distal Fatigue Onset age Childhood Adult: Early & Late peaks Differential Diagnosis Pathophysiology: Post-Synaptic changes Acetylcholine Receptors (AChRs): ↓ Number Post-synaptic Membrane: Damage Pathology Abnormal NMJs Lymphorrhages Diagnostic Testing Tensilon Serum Antibodies IgG vs AChR IgG vs MuSK IgG vs LRP4 IgG vs Agrin IgG vs Collagen XIII Anti-Striational Thymoma associations Titin Ryanodine receptor IgG vs Cortactin Other Repetitive Nerve Stimulation (RNS) Decrement Test: Proximal & Facial muscles Single fiber EMG: Increased jitter Treatment AChE inhibitors Immunosuppression Immune disorders: Treatment strategies

Variants Adult onset Early Late Neonatal & Childhood onset Arthrogryposis: Recurrent Congenital Neonatal: Transient AChR inactivation Ocular Drug-induced Anti-AChR-Ab-Negative No anti-AChR antibodies May become + with repeat testing Focal weakness Ocular Distal: Hands > Feet Respiratory failure Bulbar Slow channel syndrome: Acquired Brachio-cervical inflammatory myopathies Camptocormic myopathy + MG 36 Hereditary component to immune MG MG + Thymic hyperplasia Familial immune MG Animals Associations Genetic & HLA types Medications Pregnancy Systemic disorders Thymic pathology Thymoma: Patients older than 30 Hyperplasia Thyroid: Hyper- & Hypofunction GvHD: Rare Serum CK High Polymyositis Rippling muscle disease

Fatigue (Ptosis) in a patient with MG   Posey & Spiller: The Eye and Nervous System, 1906 Repetitive nerve stimulation: Decrement

Myasthenic Syndrome (Lambert-Eaton; LEMS): Autoimmune

Antibodies Clinical features Electrophysiology Epidemiology Neoplasms Subgroups Treatment Also see   Congenital LEMS syndrome

LEMS: Increment after exercise From M Al-Lozi

• Epidemiology ¹⁷
  • Prevalence: 1 in 100,000
  • Sex ratio: Males slightly more common than Female
• Clinical features
  • Onset
    • Age
      • Most > 40 years
      • Range = 7 to 80 years
      • Onset younger: No associated neoplasm
      • Childhood: 5%
    • Weakness (82%), especially legs
    • Usually precedes cancer: > 80%
  • Weakness
    • Proximal > Distal
    • Extremities
      • Legs (98%)
      • Arms (82%)
      • Legs may be more severe than Arms
    • Neck (30%)
    • Respiratory (15%): Rarely severe; Later in disease course
    • Bulbar: Dysphagia (22% to 56%); Dysarthria (Up to 80%)
    • Improves with: Brief sustained exercise
    • May worsen with: Sustained exercise; Heat or Fever
    • Fatigability (33%)
  • Extraocular muscle involvement
    • Ptosis: Occasional (30% to 50%)
    • Diplopia, Symptomatic (~ 40%): Transient
    • Eye movement limitation: Rarely present at presentation or on examination
    • Rule out: Concurrent myasthenia gravis
  • Muscle pain: Occasional
  • Sensory neuropathy: Distal; Symmetric
  • Autonomic neuropathy (75%)
    • Association stronger with cancer than with LEMS
    • Dryness: Mouth > Eyes
    • Impotence: Males
    • Blurred vision
    • Other (10% to 50%): Bladder; Obstipation; Hypohidrosis
  • Tendon reflexes
    • At rest: Diminished or absent (90%)
    • Reappear after: Brief maximal voluntary contraction; Repeated tendon percussion
  • Associated neurologic syndromes
    • CNS
      • Ataxia (5% to 56%)
      • Encephalopathy
      • More common in paraneoplastic LEMS
      • Anti-Hu syndromes
    • Weight loss (24%)
    • "Viral" prodrome (34%)
    • Cranial nerves: Usually normal
  • Course: Generally stable defect
  • Exacerbation of LEMS by drugs: Occasional
    • Neuromuscular junction blocking agents
      • For surgery
      • Botulinum toxin
    • Antibiotics: Aminoglycoside; Fluoroquinolone, Ofloxacin
    • Magnesium
    • Ca⁺⁺-channel blockers: Diltiazem, Verapamil
    • IV contrast agents: Iodinated
    • Varenicline (Chantix).
  • Differences from Myasthenia Gravis ¹²
    • LEMS never begins with ocular weakness
    • LEMS usually has weakness in Legs > Arms
  • Treatment: Often Beneficial ⁵⁵
    • Pharmacologic
      • 3,4 Diaminopyridine
        • 2 formulations
          • 3,4-diaminopyridine phosphate (3,4-DAPP; Amifampridine phosphate)
          • 3,4-diaminopyridine (3,4-DAP)
        • Usual doses: 5 mg to 25 mg 3x or 4x per day
        • Side effect: Seizures, especially doses > 100 mg/day
      • ± Pyridostigmine
    • Immunosuppression
      • Anti-T-cell drugs
        • Prednisone
        • Cyclosporine A
      • Plasma Exchange
      • IV Ig
    • Treatment of neoplasm
• LEMS Subgroups ⁵
  • LEMS without neoplasm (SCLC)
    • Epidemiology
      • Male (50%) = Female (50%)
      • Median age: 49.5 years
      • 40% of LEMS
    • Weakness
      • More confined to proximal legs
      • Many with some in arms
    • Autonomic: Dry mouth (84%); Erectile dysfunction; Constipation
    • Course: Slow progression with normal life expectancy
    • Immunosuppression (Corticosteroids + ..)
      • Improvement in most
      • Continued immunosuppression needed in most
    • Better prognosis with better clinical score at onset
    • No correlation with: Anti-P/Q-VGCC antibody titers; Size of increment on RNS
    • Lymphoma may be detected on follow-up (6%)
    • Associated immunologic disorders
      • Frequency: 27%
      • Common types: Myasthenia gravis; Pernicious anemia; Thyroid;
          Systemic lupus erythematosus; Celiac disease; Vitiligo; Diabetes
    • HLA association: DR3-B8
  • LEMS with neoplasm
    • Epidemiology
      • Male (70%) > Female (30%)
      • Median age: 58 years
      • Positive smoking history
      • Frequency in patients with small cell lung cancer
        • Clinical LEMS: 0.4% to 4%
        • VGCC antibody: 8%
    • Clinical
      • Progression: More rapid ²²
      • Weakness
        • Onset: Proximal legs
        • Common progression: To arms & distal legs
        • Respiratory failure more common
        • Dysarthria
      • Weight loss: More common
      • Autonomic: Dry mouth; Impotence
      • Shortened life expectancy: Probably related to neoplasm
      • CNS: Ataxia more common (9%)
    • Laboratory
      • Associated immunologic disorders: 6%
      • ESR: High
    • Associated neoplasm
      • Frequency: 25%
      • Small cell lung cancer
        • Common
        • May be less malignant ²³
          • Long survival more common
          • Average suvival with LEMS 2x longer: 18 months to 4 years vs 9 months
          • No clear increased survival with other paraneoplastic syndromes
      • ? Merkel cell carcinoma
  • LEMS without anti-(VGCC) antibodies ¹³
    • Frequency: 15% of LEMS patients
    • Clinical features: Similar to antibody positive group
    • Less associated small cell lung cancer: 12% vs 60% with antibodies
    • Survival when SCLC present: Shorter that antibody-positive patients
    • Serum IgG reduces quantal content of end plate potentials
• Pathophysiology: Presynaptic disorder
  • Reduced numbers of P/Q Ca⁺⁺ channels on presynaptic terminals
  • Ultrastructure: Active zone particles on presynaptic terminals reduced
  • Physiology
    • Reduced K⁺ stimulated Ca⁺⁺ flux into presynaptic terminals ®
    • Reduced Ca⁺⁺ dependent quantal ACh release
• Serum Antibodies
  • IgG vs Voltage Gated Calcium Channels (VGCC)
    • Location: Serum ± CSF
    • Frequency in LEMS
      • Overall: 85%
      • LEMS + Cancer: 98% Ab+
      • LEMS without cancer: 80% Ab+
      • More common: With decrement at slow stimulation rates
    • Other positive testing
      • CNS disorders: Cerebellar disorders with neoplasm
        • VGCC antibody frequency: 40%; More common than Hu
        • Neoplasm type: Lung most common, especially small cell
        • Intrathecal synthesis: Some patients
        • Response of cerebellar syndrome to treatment: Poor
        • LEMS clinical & electrodiagnostic features: Often not present
      • Small cell lung cancer
        • P/Q-type VGCC (50%)
        • N-type VGCC (21%)
      • False positives
        • Hypergammaglobulinemia
        • Chronic liver disorders
        • Infections
        • Normal population: < 3%
    • Target antigens
      • P/Q-type Ca⁺⁺ channel (85%): α_1A subunit; Especially domains II & IV
      • N-type Ca⁺⁺ channel (35%): ? α_1B subunit
  • Other associated serum antibodies
    • Glutamic acid decarboxylase (24%)
    • Thyroid (21%)
    • Parietal cell (14%)
      
    • Hu (3%)
    • Muscle nicotinic AChR (10%)
    • Synaptotagmin (20%)
    • M1 muscarinic AChRs
    • SOX1 antibodies ²⁷
      • Present
        • Small cell lung neoplasm (63%)
        • LEMS patients with associated Small cell lung neoplasm (64%)
        • IgG vs Voltage Gated Calcium Channels (50%)
      • Not present: LEMS patients without SCLC (0%)
      • Cerebellar staining pattern: Nuclei of Bergmann glia (AGNA)
      • Other associations
        • Hu Ab (32%)
        • Small cell lung cancer only (22%);
        • Voltage-gated K⁺ channel Ab
• Electrophysiology
  • Repetitive nerve stimulation
    • Increment
      • After rapid (50 Hz) RNS, or
      • Sustained muscle contraction
      • Prolonged by cooling
      • Muscles with most increment after 10 sec maximal voluntary contracture
      • Most specific for LEMS
        • > 60% to 100% in several muscles
        • > 400% in one muscle
        • Muscles: Abductor digiti minimi; Abductor policis brevis; Anconeus
    • Decrement on slow (5 Hz) RNS: Especially in small hand muscles
  • Small CMAP amplitude
  • Microphysiology
    • Ca⁺⁺ channel blockade: P most common; Q often; N minority of cases
    • Reduced quantal acetylcholine release from presynaptic nerve terminal
• Tumor
  • Clinical: See LEMS subgroups
  • Frequency of association with neoplasm
    • Overall: 50% to 60%
    • With risk factors: Up to 100%
  • Risk factors: Smoking; Increasing Age (> 50 years)
  • Associated neoplasms
    • Small Cell Lung Cancer
      • Most frequent associated neoplasm
      • Clincial LEMS: 3% to 5% of patients with SCLC
      • Serum IgG binding to VGCC: 10%; 8% to P-type calcium channel
      • Almost all SCLC with LEMS have smoking history
    • Lymphoproliferative: Reticulum-cell sarcoma; T-cell leukemia; Lymphoma; Castleman's disease
    • Other possible associations: Non-small cell lung; Prostate; Thymoma; Merkel cell carcinoma
  • Timing of neoplasm association
    • Usual (80% to 90%): LEMS onset 0.5 to 5 years before neoplasm detected
    • Neoplasm only found in follow-up
      • Frequency: 10%
      • Most commonly: < 1 year; Range 1 to 41 months
  • Work-up: Chest CT or MRI
    • If no neoplasm found at initial evaluation: 3 months, then q 1 year x 5 years later
    • Chest X-ray: Not sensitive
    • FDG-PET: May identify additional cases
• Also see: Congenital MG syndrome resembling LEMS
• Experimental model
  • Immunization with extracellular region (S5-S6 linker) of domain III of P/Q Ca⁺⁺ channel ¹

CONGENITAL MYASTHENIC SYNDROMES (CMS) ²

• Genetics: Inheritance
  • Recessive: Most congenital myasthenic disorders
  • Dominant: Slow AChR Channel syndromes
• Clinical
  • Onset
  • Early: Most congenital MG; Often infancy or childhood
  • Later
    • Slow-channel syndrome: Some
    • Familial limb-Girdle myasthenia (Dok-7)
    • Rapsyn: Some
    • AChR (Non-epsilon): Low expressor
  • Weakness
    • Ocular; Bulbar; Respiratory; Generalized
    • Wrist & finger extensors: Older patients with
      • Endplate AChE deficiency (ColQ)
      • Slow AChR channel syndrome
    • Limb-girdle & Axial: Dok7, GFPT1, DPAGT1, ALG2, ALG14, Rapsyn, ColQ
    • Stridor & vocal cord paralysis, Infancy: Dok-7 > ColQ, MuSK
    • Milder ocular involvement
      • Endplate AChE deficiency (ColQ)
      • Slow AChR channel syndrome
    • Pupil
      • Delayed light reflexes: Endplate AChE deficiency (ColQ)
      • Persistently constricted: Laminin-β2
  • Variable course
    • Fatigue & Fluctuation of signs: Especially with MG + episodic apnea (ChAT)
    • Episodic apnea in childhood
      • MG + episodic apnea (ChAT)
      • AChR syndromes
        • Fast-channel
        • AChR δ subunit mutation (E381K)
      • Rapsyn mutations
      • Sudden apneic episodes after fever or stress: ChAT, Rapsyn, Na⁺ channel
  • Tendon reflexes reduced
    • Endplate AChE deficiency;
    • Slow-channel syndrome (Severe)
    • Lambert-Eaton CMG
  • Arthrogryposis (Contractures)
    • Recurrent congenital MG: Maternal antibodies vs fetal AChRs
    • Multiple pterygium syndrome (Escobar): AChR δ or γ subunit mutations
    • Rapsyn
    • ChAT
    • ECEL1
    • ZC4H2
    • ZBTB42
  • Clinical: Other
    • Facial malformations: Rapsyn
    • Nephrotic syndrome & Ocular malformations: Laminin β2
    • Seizures or Intellectual disability: DPAGT1, ZC4H2
    • Epidermolysis bullosa simplex & Muscular dystrophy: Plectin
    • Episodic ataxia 2: CACNA1A
  • Progression
    • Usual: Slow or none in adolescence or adulthood
    • More progression
      • Slow AChR channel syndrome
      • Acetylcholinesterase deficiency (ColQ)
  • CMG: Treatments
    • Anti-AChE medications
      • Mechanism: Blocks ACh degradation by AChE
      • AChR deficiency syndromes
      • ChAT deficiency
      • Not effective: AChE deficiency
      • Refractory to, or worsened by, AChE inhibitors: ColQ, Dok-7, MuSK, Agrin, LRP4, Plectin, Laminin β2
    • 3,4-Diaminopyridine
      • Mechanism: Increases ACh release from motor nerve terminals
      • Disorders
        • Presynaptic: SNAP25; SYT2
        • Postsynaptic: MuSK, Plectin, Fast-channel syndromes
    • Salbutamol (Albuterol)
      • Mechanisms: Stimulates β2-adrenergic receptors; May activate MuSK; Effects on AChR clustering
      • Disorders: Dok-7, AChE deficiency, Laminin β2, LRP4, COL13A1
    • Ephedrine
      • Mechanism: Stimulates β2-adrenergic receptors; May activate MuSK
      • Disorders: Dok-7, AChE deficiency, Agrin, Laminin β2
    • Fluoxitine
      • Mechanism of action: AChR channels
        • Open time reduced
        • Reduced opening frequency: Stabilizes AChR desensitized state
      • Disorder: Slow-channel syndromes
    • Quinidine
      • Mechanism of action: AChR channel open time reduced
      • Disorder: Slow-channel syndromes
• Laboratory
  • Antibodies vs AChRs: Absent in hereditary myasthenic syndromes
  • Tensilon test: Often positive
    • Negative: Endplate AChE deficiency
• Electrophysiology
  • Decrement on Repetitive nerve stimulation
    • Common: At 2Hz stimulation
    • Absent: CMG with episodic apnea when asymptomatic
    • Absent at rest, but elicted by 10 Hz stimulation: ChAT deficiency
    • > 50% decrease of CMAP amplitude after 10 Hz stimulation for 5 min & slow recovery over 5–10 min: ChAT
    • Not corrected by edrophonium: Endplate AChE deficiency
  • Repetitive CMAP to single stimulus
    • Endplate AChE deficiency (ColQ)
    • Slow-channel syndrome: Dominant AChR subunit mutations
    • PURA-CMS
    • Patients taking high doses of AChE inhibitors
• Morphology
  • NMJ
    • No IgG or complement at NMJs
    • Small endplate regions, Multiple: Slow-channel syndrome; AChR deficiency syndromes
    • Absent AChE staining: Endplate AChE deficiency
    • Reduced numbers of junctional folds in post-synaptic membrane
    • Molecular evaluation of NMJs: AChE, AChR, Agrin, B₂-laminin, Utrophin, Rapsyn
  • Muscle
    • Type I muscle fiber predominance
    • Type II muscle fiber atrophy
    • Tubular aggregates: GFPT1, DPAGT1, ALG2
    • Autophagic myopathy: GFPT1, DPAGT1

• Presynaptic
  • Defects in evoked release of acetylcholine (ACh) quanta or ACh resynthesis (ChAT mutations)
  • Frequency: 8%
• Synaptic basal lamina
  • Defects caused by mutations in
    • Collagen tail of AChE: Frequency 16%
    • Laminin β2
• Postsynaptic
  • Many caused by mutations in subunits of AChRs
  • Other: Syndromes caused by other post-synaptic molecules
  • Frequency: 76%
• Glycosylation disorders
  • CMSTA1: GFPT1; 2p13
  • CMSTA2: DPAGT1; 11q23
  • CMSTA3: ALG2; 9q22
  • Familial MG: ALG14; 1p21

• Slow channel syndromes
  • Inheritance: Dominant; Occasionally recessive
  • Mutations: AChR subunits
  • Channel physiology: Opened for prolonged time
    • Endplate currents: Slow decay
    • Channel opening events: Prolonged
    • Open states: Stabilized
    • Closed states: Destabilized
    • Mechanisms of abnormality
      • AChR Affinity for ACh: Increased
      • AChR channel opening rate (β): Increased
      • AChR channel closing rate (α): Decreased
  • Clinical & Pathology: Slow channel syndromes
• Congenital myasthenic syndromes, Fast channel ³³
    ● CMS1B: Acetylcholine receptor α subunit (CHRNA1) ; Chromosome 2q31.1; Recessive
    ● CMS2B: Acetylcholine receptor β subunit (CHRNB) ; Chromosome 17p13.1; Recessive
    ● CMS3B: Acetylcholine receptor δ subunit (CHRND) ; Chromosome 2q37.1; Recessive
    ● CMS4B: Acetylcholine receptor ε subunit (CHRNE) ; Chromosome 17p13.2; Recessive
  
  
  • Genetics
    • Inheritance: Recessive
    • Mutation effects: Loss of function
    • Consequences of fast channel mutation unmasked by null mutation in other allele
    • CHRNA1: Allelic disorders
  • AChR channel physiology
    • Endplate currents: Rapid decay
    • Channel opening events: Brief
    • Open states: Destabilized
    • Closed states: Stabilized
    • Mechanisms of abnormality
      • Decreased affinity of AChR for ACh
      • Increased affinity of AChR for ACh with short open times (δ)
      • Decreased AChR channel opening rate (β)
      • Increased AChR channel closing rate (α)
      • Mode switching kinetics of AChR abnormal
  • Specific channel abnormalities: Brief, or reduced, channel activation due to several mechanisms
    • Decreased affinity for ACh
      • εP121L mutation: Common
        • Mutation effect
          • Reduced affinity for ACh in AChR open state
          • Number of AChRs at NMJs: Often normal
        • Location
          • Extracellular domain of ε subunit
          • Near ACh ligand binding site formed by α & ε subunits
        • Physiology: Slowed channel opening
        • Clinical phenotype: Moderately severe MG
      • εD175N mutation
        • Mutation effect: Reduced affinity for ACh in AChR closed state; Impaired gating efficiency
        • Location: Near ACh ligand binding site formed by α & ε subunits
      • δE59K mutation
        • Mutation effect: Reduced affinity of AChR for ACh
        • Location: Near ACh ligand binding site formed by α & δ subunits
        • Clinical phenotype: Multiple congenital joint contractures; Neonatal respiratory distress
    • Abnormal channel gating mechanism
      • εTrp55Arg mutation
        • Mutation effects: Gating abnormal & Reduced ACh affinity
          • Abnormal isomerization of AChR from closed to open state
          • Slow opening rate
          • Fast closing rat
          • Reduced open channel probability
        • Clinical phenotype: Severe; Congenital; Early death
      • εN182Y mutation
        • Mutation effect: Increases ACh affinity for the resting closed state
        • Clinical phenotype: Moderately severe MG
      • αV285I mutation
        • Location: M3 transmembrane domain of α subunit
        • Mutation effect: Impaired gating efficiency
        • Clinical phenotype: Mild MG
      • εR311W mutation
      • εAla411Pro mutation
        • Heterogeneous gating kinetics
        • Location: Amphipathic helix of the long cytoplasmic loop
        • Clinical phenotype: Mild to moderate MG
    • Abnormal mode-switching kinetics: ε1254ins18 mutation in cytoplasmic loop of ε subunit
    • Reduced probability of channel opening after ACh binding
      • αV132L mutation ¹⁵
        • Impaired ACh binding to AChRs in the resting closed state
        • Decreasing binding affinity for the second binding step 30-fold
        • Gating efficiency impaired only 2-fold: Similar mutation in δAChR subunit impairs gating
        • Location: Cys-loop domain of α subunit
        • Clinical phenotype: Most severe fast channel MG syndrome
          • Generalized weakness
          • Respiratory failure
    • Fast channel syndrome with arthrogryposis: δ subunit
    • Reduced expression of AChRs with fast channel properties
  • Clinical: General
    • Onset: Birth; Respiratory or Feeding problems; Fetal movements reduced
    • Episodic: Commonly develop life threatening episodes of apnea in childhood
    • Weakness: Proximal > Distal; Face & Neck
    • Ophthalmoplegia: Common
    • Treatments: Partial response to cholinesterase inhibitors & 3,4-diaminopyridine
  • Clinical correlations
    • Mild disease: Abnormal gating efficiency
    • Moderate severity: Unstable channel kinetics
    • Severe disease: Abnormal ACh affinity ± Impaired gating efficiency
    • Treatment
      • 3,4-diaminopyridine
      • Best response when normal numbers of AChRs at NMJs
  • Pathology
    • May be Normal
    • Reduced numbers of AChRs at NMJs
    • Multiple small endplate regions

Episodic Apnea

• Nosology: Other nammes
  • Defect in ACh resynthesis or packaging
  • Familial Infantile Myasthenia
• Gene mutations
  • Often missense
  • Occasionally 1 mutation is frame shifting
  • Most patients compound heterozygotes
  • Turkish families with homozygous I336T ¹⁸
• ChAT protein
  • ChAT protein : Normal features
    • Location: Presynaptic nerve terminals
    • Function: Facilitates transfer of acetate to choline; Produces acetylcholine
    • External link: Williams
  • Mutation effects on ChAT protein
    • Kinetic disorders
      • Cause loss of function of ChAT
      • Often produce altered affinity of ChAT for AcCoA
    • Protein expression: Often reduced, but not absent
• Clinical features: Variable
  • Onset
    • Birth, or Early childhood
    • Hypotonia
    • Weakness: Bulbar & Respiratory
    • Gradual improvement
  • Weakness: Episodic rapid exacerbations
    • Onset: Infancy or Childhood
    • Ptosis: Fluctuating
    • Crises
      • Signs: Respiratory failure; Episodic sudden apnea; Weakness; Bulbar dysfunction
      • Precipitated by: Fever; Exertion; Excitement
      • Gradual improvement: Respiratory impairment may persist for weeks
    • Partial or complete recovery after each episode
    • Between episodes
      • Normal, or
      • Moderate myasthenic symptoms: Ptosis; Fatigability
  • Course
    • Improvement with age: Fewer exacerbations
    • Occasional sudden death
  • Tendon reflexes: Normal
  • Other
    • Muscle: No myopathy; Normal muscle bulk
    • Autonomic: Normal
    • CNS: Normal
  • Treatment
    • AChE inhibitors: Prophylactic pyridostigmine
    • ? 3,4 diaminopyridine
    • Availability of emergency ventilatory apparatus (Bag)
    • Apnea monitor
    • Avoid: Aminoglycoside antibiotics; Quinidine
• Electrophysiology ¹⁰
  • Decrement with repetitive nerve stimulation
    • Variably present
      • Often not present in normal strength, rested muscles
      • More common during attacks
      • On 2 to 3 Hz RNS: Only in weak muscles
      • Decrement increased after
        • Exercise
        • 10 Hz stimulation of several minutes
      • Slow recovery of CMAP to normal amplitude after RNS finished
    • Corrected by
      • Rest: Improves slowly over > 2 minutes
      • Edrophonium
  • Single stimulus evokes single CMAP
  • CMAP amplitude: Normal at rest; Reduced with 10 Hz RNS for 5 minutes
  • MEPPs
    • Normal in resting muscle
    • Decreased Amplitude after 10 Hz RNS for 5 min: Persists for 10 to 15 minutes
  • End-plate potential (EPP): Quantal size Reduced; Quantal content normal
• Pathology: Presynaptic disorder
  • Probable mechanism: Reduced acetylcholine resynthesis or repackaging in presynaptic terminal
  • Synaptic vesicles
    • Rested muscle: Small synaptic vesicles
    • Stimulation: Increase or no change in size
  • Post-synaptic: Normal number of AChRs; Normal post-synaptic morphology

• Epidemiology: 4 patients; Yemeni Jewish; Philipino & Turkish
• Genetics
  • Mutations: c.557G>C, p.(Gly186Ala); Deletion; c.1192G.C, p.(Asp398His); c.1078G>C, p.Gly360Arg
  • Gene is exon in 1st intron of ChAT gene
  • More severe disease: Gly360Arg
• SLC18A3 protein
  • Presynaptic
  • Uptake of acetylcholine from cytoplasm into presynaptic vesicles
• Clinical
  • Onset age: Congenital
  • Eye
    • Ptosis
    • Ophthalmoplegia
  • Weakness
    • Proximal
    • Arms & Legs
    • Face, mild
    • Dysphagia
    • Respiratory
      • Fixed: More severe syndromes
      • Episodic: Cyanosis or Apnea
  • Fatigue: Worse with cold water
  • Tendon reflexes: Present
  • Skeletal
    • Contractures
    • Arthrogryposis: More severe patients
  • Cognitive defects: Mild, 1 patient
  • Treatment: Pyridostigmine improves fatigue & strength
• Laboratory
  • Repetitive nerve stimulation
    • 3 to 5 Hz Decrement
    • Postactivation exhaustion
    • Unmasked by isometric contraction
  • Cardiac: Abnormal LV function

• Epidemiology: 1 patient reported
• Clinical features
  • Onset: Birth
  • Weakness
    • Bulbar & Limb
    • Ptosis & Face
    • Extraocular movements: Limited in some patients
  • Fatigability
  • Treatment: AChE inhibitors
• Electrophysiology
  • Repetitive nerve stimulation: Decrement
  • Endplate potential (EPP): Quantal content reduced to 20% of normal
  • CMAP amplitude: Normal
• Synaptic morphology
  • Presynaptic nerve terminal
    • Synaptic Vesicles: Reduced in number by 80%
    • Terminal size: Normal
  • Post-synaptic: Normal folds; Normal number of AChRs
• Possible defect: Reduced synthesis or axonal transport of vesicle precursors

• Epidemiology
  • 10–15% of CMS
  • 3rd most common mutated molecule in congenital MG
  • Other common congenital MG: CHRNE; RAPSN
• Genetics

  >20 Different mutations described Mutation types Missense mutations (P59Q; D342E): Milder disease Point mutations → Stop codons: Severe disease Mutations G240X mutation: Common in Palestinian Arabs Homozygous missense (P59Q; R410W; Tyr430Ser): May have Teen onset Functional effects of mutations differ N-terminal (P59Q) or PRAD region (107del215) Reduced Binding of ColQ to catalytic subunit of AChE More distal mutations Insertion of ColQ into basal lamina: Reduced Formation of asymmetric forms of AChE: Reduced Protein: ColQ Collagen Collagen tail of asymmetric acetylcholinesterase Proline-rich attachment domain (PRAD) of ColQ binds tetramer of AChE subunits Produces asymmetric AChE: Concentrated at NMJ ColQ tail anchors catalytic AChE subunits to basal lamina AChE stabilization depends on binding to Perlecan-Dystroglycan MuSK: Responsible for AChE accumulation at synaptic cleft2 Tail formed by triple helix association of 3 collagen-like strands (ColQ) AChE Several isoforms Vary in C-terminal domain AChET variant at NMJ & Brain Located in synaptic cleft: Hydrolyzes ACh See: Anti-AChE antibodies

  Acetylcholinesterase A12, Asymmetric

• Clinical features
  • Onset
    • Birth to 2 years: Variable even with same mutations
    • Weakness: Generalized; Respiratory in some patients
  • Weakness
    • Slow motor milestones
    • Diffuse: Respiratory, Extraocular, Facial, Proximal trunk & Limb
    • Symmetric
    • Severely disabling: Most patients
  • Tendon reflexes: Reduced or Normal
  • Slow pupillary response to light: Some patients
  • Scoliosis: Older patients
  • Tensilon test: May make patient worse
  • Treatment
    • Ephedrine: 50 mg qid ²⁴
    • Salbutamol: 2mg tid
    • No response to AChE inhibitors
  • Course: May be progressive, stable or improve
• Electrophysiology
  • Decrement at 2 Hz RNS
    • In all muscles
    • Not corrected by edrophonium
    • Present in most patients: May develop during childhood
  • Repetitive CMAP response to single stimulus (80%)
    • Especially in well rested muscle
    • Present in all patients: May be absent during infancy
    • Repetitive CMAP is smaller amplitude & decrements faster than first CMAP
  • Microphysiology: Small EPP due to reduced number of immediately releasable quanta
  • No effect of AChE inhibitors on electrophysiological abnormalities
• Muscle Pathology: Endplate myopathy
  • AChE deficiency at neuromuscular junctions: Absent asymmetric forms
  • Small presynaptic motor nerve terminals
  • Simplified post-synaptic folds at some NMJs
  • Number of AChRs reduced
  • Schwann cell processes extend into the synaptic cleft
• AChE: Partial AChE deficiency
  • Onset: ~ 6 years
  • Milder weakness
  • Disabling during 2nd decade
• AChE Variant
  • Mild symptoms in childhood
  • Worsening in 5th decade with severe respiratory insufficiency
  • Mutations
    • Splicing (IVS1-1GtoA): Affects exon encoding proline-rich attachment domain (PRAD)
    • R236X
• Sphynx & Devon Rex Cats ⁴²
  • ColQ mutations: Homozygous; c.1190G>A (C397Y); Missense
  • Clinical: Weakness; Fatigability

• Epidemiology: Single patient
• Genetics
  • Heterozygous mutations: 1478delG, 4804delC
  • Allelic with
    • Pierson syndrome (Congenital nephrosis & Ocular defects)
    • Nephrotic syndrome, type 5
• LAMB2 protein
  • Laminins: Heterotrimeric glycoproteins containing &alpha, &beta & γ chains
  • Location: Basal lamina
  • NMJ laminins: Laminin-4, -9 & -11; All contain β2 chain
  • LAMB2 deficient mice: Defective neuromuscular synapses; Schwann cells intrude in synaptic cleft
• Clinical
  • Neonatal: Episodes of respiratory distress
  • Eyes
    • Pupils: Persistently constricted
    • Ptosis
    • EOM: Limited
    • Myopia: Impaired visual acuity
    • Fundus: Hypoplastic macula
  • Motor
    • Developoment: Delayed
    • Weakness: Proximal
  • Scoliosis
  • Treatments
    • Ephedrine
    • AChE inhibitor: Increased weakness
    • Renal transplant
• Laboratory
  • Nephrotic syndrome: Proteinuria
  • Repetitive nerve stimulation (3 Hz): Decrement
  • Endplate physiology
    • Reduced quantal content of end plate potentials (EPPs) evoked by nerve stimulation at 1 Hz
    • MEPP frequency & amplitudes: Reduced
  • Muscle biopsy
    • General morphology: Non-specific changes
      • Angular muscle fibers: Scattered
      • Type I muscle fiber predominance
    • LAMB2 staining: Absent
    • Neuromuscular junctions (NMJs)
      • Presynaptic
        • Axon terminals: Small size
        • Encasement of nerve endings by Schwann cells
        • Synaptic vesicles: Normal to reduced numbers
      • Primary synaptic cleft
        • Severe widening; Invasion by Schwann cell processes
        • Endplate acetylcholinesterase: Normal
      • Postsynaptic folds: Simplified
      • Endplate area: Reduced

• Epidemiology: 1 or 2 patients
• Genetics
  • LAMA5 mutation: Homozygous; Arg2659Trp
  • Other mutation: LAMA1, Homozygous, Asp210Asn
• LAMA5 protein
  • Laminin
  • Present at neuromuscular junction
  • Role in presynaptic differentiation
• Clinical
  • Onset age: Birth
  • Hypotonia
  • Weakness
    • Respiratory: Mechanical ventilation
    • Feeding: G-tube
    • Eyes: EOM limitation; Ptosis
    • Cranial: Face; Tongue; Soft palate
    • Neck flexion
    • Limbs: Proximal
  • Tendon reflexes: Reduced
  • Dysmorphism: Elongated face; Prosis; High arched palate
  • Systemic
    • GI: Inflammatory bowel disease
    • Myopia
    • Knee contractures
  • Treatment: Pyridostigmine sulfate + 3,4-DAP
• Laboratory
  • Repetitive nerve stimulation
    • 2Hz: 55% decrement
  • 30 s of maximal muscle activation: LEMS-like
    • 250% increment
  • Microphysiology
    • EPP quantal content: Reduced
    • MEPPs: Normal
  • Anconeus muscle biopsy
    • Type 1 muscle fiber predominance
    • Normal: AChE, Laminin α-5, AChRs
    • NMJ Ultrastructure
      • Post-synaptic folding folding: Normal or increased
      • Synaptic vesicile density: Moderately reduced

• Epidemiology
  • SC-CMS: > 60 families
  • CHRNA1 (CMS1A): ~20 patients
  • CHRNB (CMS2A): 9 patients
  • CHRND (CMS3A): 3 patients
  • CHRNE (CMS4A): > 20 patients
• Genetics: Multiple mutations identified
  • Mutations: General
    • Types: All missense
    • > 25 identified
  • Mutation features
    • Locations
      • Common: M2 (2nd) ion pore transmembrane domain of AChR subunits (60%)
      • Other: 1st & 4th transmembrane domains; Extracellular (M2-M3) linker; Near AChR binding site
    • Different functional effects
      • Slow AChR ion channel closure
      • Abnormal reopening of AChR
      • Enhanced agonist binding affinity of AChR
      • Increased AChR channel openings during ACh occupancy
      • Decreased desensitization of mutant AChR by ACh
    • Reduced number & density of AChRs at NMJs
    • Clinical correlations
      • Most severe with missense introduction of phenylalanine: εL269F ; αV249F
      • Generalized weakness may be present
    • Other SCS mutations: αT254I; βL263M ; εT264P; βV266M ; δS268F
  • α subunit (CHRNA1) mutations
    • CMS1A: Myasthenic syndrome, congenital, slow-channel
    • M1 transmembrane domain: αN217K
      • Clinical
        • Moderate weakness: Forearm & Face
        • Fatigability
      • Mutation effects
        • Slows rate of channel closing
        • Allows multiple reopenings per activation episode
      • Other M1 mutation: βV229F
    • Extracellular, near ACh binding site: αG153S
      • Decreased rate of dissociation of ACh from AChR
      • Repeated openings with normal open time
      • Mildest clinical syndrome
    • Extracellular: αV156M
      • ? Stabilizes open state
    • M2 domain: αV249F: Not facing channel lumen
      • Increased channel openings in absence of ACh
      • Increased ACh affinity for AChR in closed state
      • Prolonged or repeated opening in presence of ACh
      • Enhanced steady-state desensitization
      • Defects produce cationic overload & degeneration of junctional folds
      • Mutation in corresponding valine to phenylalanine in ε subunit (V259F)
        • Causes slow-channel syndrome
    • Extracellular between M2 & M3: αS269I
      • ? Alters coupling between ACh binding & channel gating
    • CHRNA1: Allelic disorders
  • β subunit (CHRNB) mutations
    • CMS2A: Myasthenic syndrome, congenital, 2A, slow-channel
    • Allelic disorders
      • βL262M mutation: More severe weakness
        • Near channel gate
        • Increased folding of postsynaptic membrane
        • Increased number of AChRs
      • CMS2C: AChR deficiency
        • Epidemiology: 1 family
        • Mutations: 9-BP DEL, NT1276; EX8DEL
  • δ subunit (CHRND) mutations
    • CMS3A: Myasthenic syndrome, congenital, slow-channel 3A (Ser268Phe)
    • Allelic disorders
      • CMS3B: Myasthenic syndrome, fast-channel congenital
      • CMS3C: AChR deficiency
        • δS268F mutation ⁴
        • Widening of synaptic cleft
        • Accumulation of debris in synaptic cleft
        • Normal post-synaptic morphology
        • AChR #: Mildly reduced
      • Multiple pterygium syndrome, lethal type
  • ε subunit (CHRNE) mutations
    • CMS4A: Myasthenic syndrome, congenital, slow-channel
    • Allelic disorders
      • CMS4B: Myasthenic syndrome, fast-channel congenital
      • CMS4C: AChR deficiency
    • εLeu221Phe mutation
      • Inheritance: Dominant: Variable penetrance
      • Physiology: Prolonged AChR ion channel activations
      • Mutation location: M1 domain of AChR molecule
      • Disease severity: Mild; Congenital
    • εV259F mutation
      • Clinical: Late walking; Nasal speech; Ptosis; Proximal & face weakness
      • Electrophysiology
        • RNS decrement (on 2nd study)
        • Jitter: Abnormal
        • No repetitive CMAP
      • Muscle pathology
        • Endplate myopathy
        • Tubulofilamentous inclusion bodies
      • Mutation in corresponding valine to phenylalanine in α subunit (V249F)
        • Causes severe slow-channel syndrome
    • εSer278Del + εArg217Leu
      • Inheritance: Recessive
      • Physiology: Prolonged AChR ion channel activations
      • Mutation location: Extracellular region of AChR molecule
      • Disease severity: Mild
• Pathophysiology: General
  • AChR Channel changes
    • Prolonged opening episodes
      • Slow decay of miniature endplate potential
      • Caused by: Increased affinity of AChR for acetylcholine; Increased gating efficiency of channel
    • Spontaneous channel openings of unliganded AChR
  • Activated by exposure to synaptic ACh & Serum choline
  • Cationic (Ca⁺⁺) overload of junctional sarcoplasm
    • Causes endplate myopathy with loss of junctional AChR
  • Depolarization block: Due to summation of prolonged endplate potentials
  • Endplate myopathy
    • Focal activation of caspases 3 & 9 at NMJs ²⁰
• Clinical features
  • General: Variable phenotypes
    • Mild: Later onset
    • Severe: Infantile onset
  • Onset: Infant to 7th decade; Typical 0 to 3 years
  • Cranial nerves
    • Often spared in milder cases
    • Ptosis
    • Ophthalmoplegia: More severe cases
    • Facial weakness: Especially eye closure
  • Weakness & Wasting
    • Cervical: Neck flexors
    • Scapular
    • Hand: Thumb abduction; Muscle wasting (Thenar eminence)
    • Extensors: Forearm & Fingers
    • Limbs: Upper > Lower
    • Respiratory
      • With disease progression
      • Failure to breathe after anesthetic
  • Fatigability
  • Spinal deformities: With disease progression
  • Progression: Slow; More with earlier onset
  • Treatments
    • Quinidine Sulfate (60%)
      • Dose: 200 mg 3 or 4 times per day; po
      • Serum concentrations 0.7-2.4 μg/ml
      • Mechanism of action: Blocker of open channel AChRs
      • Clinical response: Gradual improvement
    • Fluoxetine (100%) (Open AChR channel blocker): 80 mg to 160 mg/day
    • Salbutamol (β2-adrenergic receptor agonist): 2 mg to 4 mg tid
    • Ephedrine
    • Chloroquine
    • AChE inhibitors: No long term response; 50% with some initial response
• Electrophysiology
  • Repetitive nerve stimulation (75%): Decrement in weak muscles
  • Repetitive CMAP response to single stimulus
    • More with: Longer AChR open times (> 4x normal)
    • Exacerbated by: Edrophonium
  • Jitter: abnormal
  • Postsynaptic recordings
    • Disorders due to: Slow closure of AChR ion channel
    • Miniature endplate currents: Prolonged & Biexponentially decay
      • Staircase summation
      • Depolarization block during subtetanic stimulation
    • Single channel recordings
      • Dual population of AChR channels
        • Normal channel: Normal opening episodes
        • Mutant channel: Prolonged opening episodes
      • Mutant chanels open even in absence of ACh
• Pathology
  • Type I muscle fiber predominance
  • Endplate myopathy
    • Ca⁺⁺ increased in muscle cytoplasm near NMJ
    • Autophagic vacuoles: Regions neighboring NMJ
    • Junctional folds: Severe damage; Similar to Dok-7 mutations
    • Apoptosis: Nuclei at NMJs
    • Reduced nerve terminal area
  • Some patients with
    • Tubular aggregates
    • Tubulofilamentous inclusions
• Also see: Acquired slow channel syndrome

• Mutations: Heterozygous
  • εC128S
    • Prevents formation of conserved disulfide group in extracellular domain
    • Arrests subunit assembly before formation of AChR pentamers
  • ε1254ins18
    • Reduces AChR expression
    • Mutation adds new modes of gating to normal high efficiency
    • Effects of mutation on channel function
      • Slow opening of channels
      • Rapid closing of channels
• Clinical features
  • Onset < 2 years
  • May be More common in males
  • Weakness: Extraocular, Facial & Generalized
  • Course: Benign; Weakness persists into adulthood
  • Treatment: AChE Inhibitors; 3,4 Diaminopyridine
• Electrophysiology
  • Decrement in facial muscles
  • No exhaustion phenomenon
  • Short open time of AChR channel
• Muscle
  • Postsynaptic reduction in number of of AChRs
  • Elongated neuromuscular junctions
  • Simplified post synaptic folds
  • Compensatory increase in expression of γ AChR subunit
• AChR function: Heterozygous εC128S & ε1254ins18
  • Some channels with short open times
  • Other channels with long open times: Probably AChRs containing γ, instead of ε subunit
  • Mode switching kinetics
    • 2 new AChR modes, both with inefficient gate
    • In both modes channel opens more slowly and closes more rapidly than normal
    • Electrically silent during synaptic response to ACh

• Epidemiology: 1 patient
• Genetics
  • Mutation: Heterozygous; α1Leu251Arg
• Clinical
  • Onset age: Infancy
  • Feeding difficulties, Dribbling & Reduced facial expression,
  • Motor milestones: Delay
  • Eyes: Ptosis, bilateral & Fatigable; EOM limited
  • Weakness: Face; Neck flexor; Limbs normal
  • Fatigue: With exercise
• Laboratory
  • RNS: Decrement
  • SF-EMG: Increased jitter
  • AChR_α1L251R protein
    • Expression: Reduced on muscle surface
    • Channel properties: Conversion of ion selectivity from cationic to anionic
    • AChR changes from excitatory to inhibitory channel
  • Muscle biopsy: Type 2 fiber smallness

• Genetics: AChR mutations
  • ε AChR subunit
    • P121L; G-8R (Signal peptide); S143L (Glycosylation site); D175N
    • εP121L mutation: Normal number of AChRs at NMJs
  • α AChR subunit: Val132Leu; 381delC
• Physiology
  • Reduced affinity of AChR for ACh: In open channel and desensitized states
  • Affinity is mainly reduced in open & desensitized states of AChR
  • Decreased rate, number & time of AChR channel openings
  • Infrequent (Reduced) AChR openings during ACh occupancy
  • Effects of mutation on channel function
    • Brief AChR activation
    • Resistance to desensitization
• Clinical features
  • General: More severe than when main effect of mutation is on AChR gating
  • Onset: Congenital; Fetal movements reduced
  • Weakness: Generalized; Proximal > Distal; Face & Neck
  • Episodic: Commonly develop life threatening episodes of apnea in childhood
  • Ophthalmoplegia: Common
  • Treatment
    • 3,4-Diaminopyridine
    • ? AChE inhibitors (Mestinon)
• Laboratory
  • RNS:Decrement
  • Postsynaptic defect in AChR: Numbers normal or Reduced
  • NMJ morphology: Normal

• Mutations: Compound Heterozygous
  • α subunit
    • αV285I (M3 domain): Abnormal gating properties
    • αF233V: Low expressor; Null mutation
  • ε-subunit
    • εR311W: Abnormal gating properties; Slow channel opening speeds agonist dissociation
    • ε553del7: Low expressor; Null mutation
• Neuromuscular junction
  • Reduced Number of AChRs at NMJ
  • Some endplate regions have sparse or shallow junctional folds
  • Miniature endplate currents: Brief channel activation episodes
  • Channel properties
    • Slow opening
    • Fast closure
    • Reduced affinity for ACh: 2.5x
  • M3 domain of AChR subunits: Regulates rates of channel opening & closing in volume-dependent manner
• Clinical
  • General: Milder than when AChR affinity for ACh is low
  • Onset: Birth; Extraocular muscle paresis
  • Fatigability: Since early childhood
  • Weakness: Face; Neck flexor; Trunk; Limbs
  • Treatment: 3,4 Diaminopyridine + Mestinon

• Mutations: Heterozygous α subunit
  • αV132L (Cys-loop (β-hairpin) domain)
  • α381delC: Low expressor; Null mutation
• Neuromuscular junction
  • Normal number of AChRs at NMJ
  • Endplate ultrastructure normal
  • Miniature endplate currents
    • Brief channel activation episodes
    • Small amplitude
  • AChR properties
    • Opening probability after interaction with ACh reduced
    • Low affinity for ACh
    • Increased resistance to desensitization
  • Cys-loop of AChR subunits: Highly conserved domain
• Clinical
  • Onset: Birth
  • Fatigability: Since early childhood
  • Weakness: Diffuse; Respirator dependent
  • Repetitive nerve stimulation: 25% to 50% decrement
  • Treatment: Partial response to Mestinon

• Weakness: Ocular, Neck and patchy Limb
• Postsynaptic defect in AChR channel kinetics
• Decrement in hand muscles post-exercise
• Little response to AChE inhibitors

• Epidemiology: 1 patient
• Genetics
  • Inheritance: Recessive
  • Mutations
    • c.812C>T (p.P248L ((p.P271L)): In M1-M2 linker
    • 430 bp frame shift deletion (Loss of exon 8)
• CHRNB1
  • Functions: Channel gating
  • βP248L mutation
    • AChR channel opening time: Reduced to 36% of normal
    • MEPP decay time: 22% of normal
  • M1-M2 linker: Contributes to elementary state transitions that open & close AChR channel
• Clinical
  • Onset age: Congenital
  • Weakness
    • Severe
    • Respiratory
    • Bulbar: Dysphagia; Speech hypophonic
    • Gait: Onset 5 years; Ambulatory
    • Fatigability
  • Muscle size: Small
  • Joint contracture: Ankles
• Laboratory
  • RNS (2 Hz): 10% to 37% decrement in different muscles

• Epidemiology: 1 family, 3 patients
• Genetics
  • AChR Mutations: Compound heterozygosity
    • β1276DelEQE
      • Located in long cytoplasmic loop between transmembrane domains M3 & M4
      • Disrupts interaction between β &δ subunits
      • Impairs AChR assembly
      • Reduces incorporation of AChRs into surface membrane: More AChRs remain in cytoplasm
    • Exon 8 deletion: Abolishes expression of pentameric AChR
  • Allelic disorders
• Clinical
  • Onset: Birth
  • Weakness: Severe; Respiratory failure; Poor feeding
  • Repetitive nerve stimulation: Decrement
  • Treatment: Partial response to acetylcholinesterase (AChE) inhibitors
  • Course
    • Static weakness
    • Survival at least through childhood with support of vital functions
• Neuromuscular junction pathology
  • More small endplate regions
  • Endplate regions distributed over 3x increase in muscle fiber surface
  • NMJ ultrastructure
    • Structure: Normal presynaptic & post synaptic
    • Size: Small presynaptic terminals; Small postsynaptic area
  • AChR #: Reduced to 7% of normal
• NMJ physiology
  • AChR channels: 89% normal; 11% with features of fetal AChRs (containing γ subunits)
  • Quantal release by impulse: Normal
  • Miniature endplate potentials (MEPPs): Reduced amplitude
  • AChR opening time: Normal MEPCs

• Mutations: Heterozygous δ subunit
  • δ756ins2: Null mutation
  • δE59K
    • Short channel activations: Predict fast decay of endplate currents
    • Dysfunction of fetal & adult AChRs
• Clinical features
  • Reduced fetal movements
  • Birth: Joint contractures, Hand IP joints
  • Weakness
    • Onset: Congenital
    • Cranial nerve: Poor feeding; Ophthalmoplegia; Dysphagia
    • Respiratory failure
    • Distribution: Arms & Legs; Proximal & Distal
  • Fatigue
• Laboratory
  • Tensilon test: Positive
  • SFEMG: Abnormal, increased jitter
• Treatment: Anti-AChE medications
• Other Arthrogryposis syndromes with familial MG
  • Recurrent congenital MG: Maternal antibodies vs fetal AChRs
  • Multiple pterygium syndrome (Escobar): AChR γ-subunit mutations
  • Rapsyn mutations

• Epidemiology: Saudi Arabian patients
• Genetics
  • Mutation: P250Q missense in transmembrane domain 1; Homozygous
• Clinical
  • Onset
    • Severe weakness at birth
    • Respiratory insufficiency or failure
    • Hypotonia
  • General: Small habitus; Large ears; Micrognathia; High-arched palate
  • Weakness
    • Ophthalmoplegia: Nearly complete
    • Ptosis: Moderate
    • Facial diplegia
    • Bulbar: Markedly weak lingual & masticatory muscles
    • Neck flexion: Very weak
    • Limb muscles: Moderate weakness
  • Improvement with age
• Laboratory
  • Channel disorders
    • Reduced numbers of AChRs at NMJs
      • 16% to 34% of normal
      • Impaired δ/α subunit association during early AChR assembly
    • Fast channel effects: Changes in gating & ACh binding
      • Hindered opening of the doubly occupied closed receptor (A2R)
      • Rapid dissociation of acetylcholine from A2R
      • Fast decaying, low-amplitude endplate currents
      • Abnormally brief channel opening events
      • NOTE: Similar mutations in ε-AChR subunit have slow channel effects
    • Presynaptic: Number of quanta released by nerve impulse
      • Low-normal range
      • Differs from high normal range in α, β & ε mutations
  • Pathology
    • Multiple large endplate regions on each muscle fiber
    • Some reduction of 2° postsynaptic folds
    • Type II muscle fiber atrophy: Mild
  • Repetitive nerve stimulation: Decrement
  • Treatment: Partial improvement with combined pyridostigmine & 3,4-diaminopyridine
• Other: AChR Delta subunit mutation syndromes

• Nosology: Myasthenic syndrome, congenital, with pre- & postsynaptic defects (CMSPPD)
• Epidemiology: 6 families; 0.8% of Congenital Myasthenic syndromes
• Genetics
  • Mutations: Glu42Ala, Ala1506Thr, Gly1709Arg, G76S, Val1727Phe & Nonsense; Often homozygous
  • Allelic disorder
    • Fetal akinesia: Null mutations
• Agrin protein
  • Heparin sulfate proteoglycan
  • Mutant protein: Destabilized NMJs; Normal binding to α-dystroglycan
• Clinical
  • Onset
    • Age: Early childhood
    • Difficulty running
  • Weakness
    • Severity: Mild
    • Distribution
      • Limbs
        • Proximal & Distal
        • Some patients predominantly distal: Arms or Legs
        • Symmetric
      • Face (50%)
    • Muscle atrophy: Distal predominant
    • Eyes: Ptosis; Normal EOM & Pupils
    • Exacerbations: Menstrual periods; Pregnancy
    • Overall course: Non-progressive
  • Milder patients: No scoliosis or contractures
  • Intellectual disability: Few patients
  • Treatment
    • Ephedrine (50 md/day x 3 days, then 2 mg/kg qAM)
    • Anti AChE drugs: No benefit
    • 3,4-diaminopyridine: Small benefit
• Laboratory
  • RNS
    • 3 Hz: Decrement (10% to 72%)
    • 30Hz: No potentiation or Increment in 1 patient
    • Post-exercise increment: Up to 285%
  • EMG: Early recruitment
  • NCV: Normal; Single stimuli produce single, normal amplitude CMAPs
  • Muscle pathology: Disorganized NMJs
    • Nerve-terminals
      • Diameter decrease
      • Neurofilament destructuring
      • Sprouting
    • Postsynaptic
      • Terminal Schwann cells: Cytoplasmic extensions or Absent
      • Synaptic gutters: Fragmented
      • Synaptic remodeling
    • Muscle fibers
      • Varied fiber size
      • Myotendinous junctions: Large, Irregular
      • Type 1 fiber predomiance
  • MRI: Gastrocnemius & Semitendinosis atrophy
• Agrin variant: Fetal akinesia ⁵⁴
  • Epidemiology: 1 patient
  • Genetics
    • Inheritance: Recessive
    • Mutations: Nonsense
  • Clinical
    • Still born fetus
  • Laboratory
    • Prenatal ultrasound: Hypoechoic muscle; Joint contractures

• Epidemiology: Two families
• Genetics
  • MuSK mutations
    • Frameshift mutation: c.220insC
    • Missense: V790M; P344R
  • Allelic disorder: Fetal Akinesia Deformation Sequence 1 (FADS1) (Arthrogryposis)
• MuSK protein functions
  • Tyrosine kinase receptor
  • Role in aggregation of molecules in postsynaptic NMJ membrane
    • AChRs
    • ErbB family receptor tyrosine kinases
  • Presynaptic differentiation: Absence leads to aberrant innervation
  • Mutated MuSK protein: Increased degradation rate
• Clinical
  • Onset: Neonatal
  • Motor
    • Respiratory involvement: Episodes during 1st year
    • Bulbar involvement: During childhood; Episodic
    • Ocular
      • Ptosis
      • Eye movement limitation (100%): Variable Up & Lateral gaze
    • Weakness
      • Face: Mild
      • Arms > Legs
      • Proximal > Distal
      • Neck extensors
      • Respiratory: Neonatal & Later in course
    • Fatigue
  • Course
    • Improvement during childhood
    • Exacerbation during pregnancy
    • Progressive in adulthood
  • Treatment
    • 3,4-diaminopyridine: 50 mg per day
    • No benefit: Acetylcholinesterase inhibitors
• Laboratory
• Pathology
  • Nerve terminals
    • Aberrant branching
    • No 200 kD neurofilament subunits
  • Several AChR patches on surface of single fibers
• Molecular
  • Acetylcholine receptors
    • ε-subunit: Reduced expression at NMJs
    • γ-subunit: Upregulated
  • MuSK: Reduced at NMJs; Increased in perijunctional regions
• Family history: Sibling with congenital MG
  • Neonatal hypotonia
  • Respiratory failure
  • Vocal cord paralysis
  • Death 1.5 years
• Other: Myasthenia gravis with serum IgG binding to MuSK
  • MuSK DNA polymorphisms: Patient with MuSK-seropositive autoimmune myasthenia gravis

• Rapsyn genetics
  • ~45 mutations identified
  • Mutation type: Missense & Insertion
  • Mutation actions
    • Diminished co-clustering of AChR with rapsyn
    • Normal rapsyn self-association
  • Tetratricopeptide repeats (TPRs) of rapsyn: Subserve self association; L14P, N88K, 553ins5
  • N88K: Present in many patients of Western European ethnic origin
  • E-box of RAPSN promoter
    • Mechanism: Regulate transcriptional activities
    • -38A to G mutation: Homozygosity in Near-Eastern Jews
      • Marked facial malformations
      • Mild course
    • -27C to G mutation
  • Other: R164C; L283P
  • Allelic disorders
    • FADS2 (Arthrogryposis; Fetal Akinesia Deformation Sequence)
    • Congenital MG: Reduced AChRs at NMJs
    • Congenital MG with Facial Malformations
    • Congenital Limb-Girdle MG with Digenic RAPSN & AK9 mutations
• Rapsyn protein
  • 43-kDa postsynaptic protein
  • Function: Clustering of AChRs at the NMJ
    • Rapsyn-dependent clustering may be independent of nerve
    • Effector of neural agrin
    • Nerve-derived agrin acts through MuSK to promote apposition of nerve terminals to AChR clusters
  • Structure
    • Tetratricopeptide repeats (TPRs): Subserve self association; 7 repeats
    • Coiled-coil domain: Binds to AChR
    • RING-H2 domain: Associates with β-dystroglycan; Links rapsyn to subsynaptic cytoskeleton
• Clinical
  • Onset
    • Age: Birth or Infancy
    • Weakness: Ptosis; Respiratory; Bulbar
  • Severity
    • Mild to Severe
    • May vary with same mutation
  • MG at birth: Early onset group
    • May have same mutations as later onset group
    • Prenatal: Reduced fetal movements
    • Motor
      • Hypotonia
      • Weak suck & Cry
      • Delayed motor milestones
    • Ptosis
    • Joint & Skeletal
      • Arthrogryposis: Most patients
      • Palate: High arched
      • Spine: Scoliosis or Lordosis
    • Episodic crises
      • Often with respiratory failure
      • Precipitated by: Infection, fever or no obvious event
    • Less ophthalmoplegia than AChR ε-subunit mutations
  • Childhood:
    • Early: Respiratory failure
    • Improvement with age
    • Walk in 2nd year
    • Reduced contractures
    • Less severe long term disability than AChR ε-subunit mutations with early onset
  • Late childhood & Adult: Late onset group
    • Onset: 13 to 48 years
    • Eyes: Ptosis without ophthalmoparesis
    • Weakness
      • Mild to moderate
      • Diffuse or Proximal
      • Masticatory muscles
      • Face strength may be normal
      • Normal early motor milestones
    • Fatigue
    • Tendon reflexes: Present
    • Mouth (-38A to G mutations): Mandibular prognathism; Malocclusion; High-arched palate; Crowded teeth
    • Differential diagnosis: Sero-negative immune myasthenia gravis
  • Course
    • Often benign with anti-AChE treatment
    • Occasional exacerbations: Temporary
    • Rare patient: Death in Childhood
  • Treatment: Combined
    • Anti-AChE medications: Incomplete benefit
    • 3,4 Diaminopyridine: May add to benefit from anti-AChE treatment
• Laboratory
  • Pathology
    • Type 1 muscle fiber preponderance
    • Decreased staining at NMJs for
      • Rapsyn
      • AChRs: 8% to 48% of control; Less severe reduction than with AChR mutations
    • Normal AChE
    • Postsynaptic morphology
      • NMJ extended over increased length of muscle fiber
      • Increased number of end plate regions
      • Simplified post synaptic membrane: Reduced postsynaptic area & secondary clefts
  • Repetitive nerve stimulation
    • Decrement
      • 50% to 80% of patients
      • Normal: More frequent in
        • Congenital onset patients
        • Muscles with normal strength
      • Abnormal: Most later onset patients
    • Other
      • Decrement after exercise or subtetanic potentiation (10 Hz)
      • SFEMG abnormality: Most patients
  • Post synaptic physiology
    

    From M. Sadeh Congenital MG with Facial Malformations

    • MEPPs: Small
    • AChR function: Normal
    • ACh release: Normal
• Animal models
  • Zebrafish, Danio rerio
  • Mouse rapsyn knockout
    • Absence of initiation of postsynaptic differentiation

• Nosology: See Congenital MG with rapsyn mutations
• Genetics: -38A to G mutations
• Epidemiology: Inbred Iraqi & Iranian Jews
• Clinical
  • Extraocular
    • Ptosis
    • No ophthalmoplegia
  • Cranial nerves
    • Facial weakness
    • Dysarthria
    • Weak mastication
  • Face
    • Mandibular prognathism
    • Palate: High arched
    • Face: Elongated
  • Treatment: AChE inhibitors
  • Course: Stable & Benign
• Electrophysiology
  • Repetitive nerve stimulation
    • Decrement: Some patients
  • Single fiber EMG: Abnormal jitter

• Epidemiology: 1 family, 2 patients
• Genetics: Mutations
  • RAPSN: Arg164His, Homozygous
  • AK9: c.332-14A>G, Homozygous, Start-gain mutation; Introduces cryptic 5'-UTR signal in intron 5
• AK9 protein
  • Cytosolic
  • Catalyzes phosphorylation of AMP, dAMP, CMP & dCMP with ATP as phosphate donor
  • Mutation: May reduce nucleotide sugars for N-glycosylation pathway
• Clinical
  • Onset age: 2nd decade
  • Motor
    • Weakness: Proximal; Arms > Legs; Symmetric
    • Walking difficulty
    • Fatigability
    • Falls: Occasional
  • Atrophy: Proximal muscles
  • Eyes: Normal
  • Treatment: Pyridostigmine; Salbutamol
  • Course: Slow progression or Stable
• Laboratory
  • Tensilon test: Positive
  • Muscle biopsy: "Unremarkable"
  • Chest CT: Thymus hyperplasia

• Epidemiology: 4 families
• Genetics
  • Mutations: Pro65Leu & Arg104X; Asp74Asn, Val141Gly, Arg109Gln
  • Allelic disorders
    • CMS15
    • Intellectual developmental disorder + Epilepsy, Behavioral abnormalities & Coarse facies (IDDEBF)
    • Myopathy, Epilepsy & Progressive Cerebral Atrophy (MEPCA)
• ALG14 protein
  • Interacts with DPAGT1
  • ALG7/13/14 complex: Multiglycosyltransferase
  • Catalyses 1st two steps in biosynthesis of LLO precursor for N-glycan assembly
  • Asparagine (N)-glycosylation
  • Enriched at NMJs: Localized with α-Bungarotoxin
  • Mutation: Causes ~50% reduced numbers of AChRs at NMJs
• Clinical
  • Onset age
    • CMG syndrome: 1st decade
    • CMG + CNS: Antenatal or Infancy
  • Muscle & Strength
    • Early: Hypotonia
    • Limbs: Moderate weakness, Proximal & Distal
    • Difficulty climbing stairs & running
    • Face & Bulbar: Strong
    • Eye: Divergent exophoria; No ophthalmoplegia; ? Ptosis
    • Severe involvement: Respiratory failure
  • Contractures: Congenital; Knees & Elbows
  • CNS: Some patients
    • Epilepsy: Therapy resistant
  • Course
    • CMG: Stable or Mildly progressive
    • CMG + CNS: Death < 1 year
  • Treatment
    • Anti-AChE medication (Pyridostigmine): Benefit may be transient
• Laboratory
  • Serum CK: Normal
  • RNS: 19% Decrement
  • SFEMG: Jitter & Blocking
  • Muscle biopsy
    • Type 2 fiber predominance
    • Atrophy
    • No tubular aggregates
  • CNS
    • Cerebral atrophy: Progressive
    • Delayed myelination
    • Astrocytosis
    • Neuron loss: Multifocal
  • EEG
    • Burst-suppression
    • Hypsarrhythmia
• ALG14 variant: Myopathy, Epilepsy & Progressive Cerebral Atrophy (MEPCA)
  • Epidemiology: 6 infants, 4 families
  • Genetics
    • Inheritance: Recessive
    • Mutations: Missense; Asp74Asn (Common), Val141Gly, Arg109Gln, Ile175Asn
  • Clinical
    • Onset: Birth, in utero, or Postnatal
    • Pregnancy: Polyhydramnios; Reduced fetal movements
    • Hypotonia
    • Respiratory insufficiency
    • Feeding problems
    • Ptosis (50%)
    • Skeletal
      • Joint contractures; Foot deformities
      • High-arched palate
      • Dysmorphism: Turricephalus, Scaphocephalus, Micrognathia
    • Seizures: Refractory
    • Pyridostigmine treatment: Transient improvement
    • Death: < 1 year in most
  • Laboratory
    • EEG: Burst suppression; Hypsarrhythmia
    • Brain imaging: Supratentorial atrophy, progressive; Myelination delay
    • EMG: Denervation
    • RNS: Decrement
    • Muscle pathology
      • Type II fibers: Predominance
      • Fiber sizes: Atrophy & Varied
      • Internal architecture: NADH punctate
      • Mitochondria: Normal
    • Brain pathology: Reactive astrocytosis, Microglia, Decreased myelination

• Epidemiology
  • 3 families
• Genetics
  • SCN4A gene mutations
    • Missense: V1442E; R1454W; R1457H; R1460Q; R1460W
    • Loss of function
    • Associated mutation: Probable polymorphism: S246L
  • SCN4A, α-subunit: Allelic disorders
• Clinical
  • Onset
    • Age: Infancy
    • Hypotonia
    • Episodic respiratory failure
  • Episodic attacks
    • Sudden onset
    • Length: 3 to 30 mintues
    • Weakness
      • Respiratory failure
      • Bulbar dysfunction
  • Weakness
    • General
      • Feeding
      • Face; Trunk; Limb
      • Proximal & Distal
    • Ocular: Ptosis: Limited eye movements
  • Fatigue: Rapid over minutes
  • Myalgia
  • Skeletal
    • High-arched palate
    • Adduction deformity: Knees; ankles
    • Lumbar lordosis: Increased
  • CNS
    • Mental retardation: ? Related to apneic episodes
• SCNA4: Micro-electrophysiology
  • AChRs at NMJs: Normal number & function
  • Muscle fiber membrane potential: Normal
  • MEPP amplitude and quantal content of the EPP: Normal
  • Failure of action potential stimulation by normal EPP
    • Depolarization of membrane to -40mV: Fails to trigger action potentials
  • Na channels
    • Normal density
    • Trapping of mutant channels in fast-inactivated state
• Laboratory
  • Normokalemia
  • CMAP decrement: Normal to 85%
    • 10-Hz stimulation for 5 min
    • 50-Hz stimulation for 2 sec
    • 2-Hz stimulation
      • No decrement in rested muscle
      • 50% decrement after 10-Hz conditioning stimuli for 1 min
  • SF-EMG: Abnormal jitter
  • Muscle pathology
    • Type 1 muscle fiber smallness
    • NMJs: Normal presynaptic terminals & Postsynaptic folds
    • Vacuoles: Non-rimmed
  • MRI: Mild cerebral atrophy
• Carriers of recessive mutation: Myotonia on EMG

• Epidemiology: 2 families; 3 patients
• Genetics
  • Mutations
    • Missense: Glu1233Lys, Arg1277His
    • Location: 3rd LRP4 β-propeller domain
  • Allelic with
    • Cenani-Lenz syndactyly syndrome
    • Sclerosteosis 2
• LRP4 protein
  • Mutation effect: Impairs binding of LRP4 to agrin & MuSK
  • Antibody-related disorder: AChR Antibody-negative MG
• Clinical
  • Onset
    • Age: Congenital
    • Respiratory & feeding insufficiency
  • Fatigue: Walking distance limited
  • Weakness: Moderate; Proximal > Distal; Respiratory
  • Eyes
    • Ptosis
    • EOM: Limitation of lateral movements, mild
  • Treatment
    • Albuterol: Improved strength
    • Pyridostigmine: Causes increased weakness
  • Course: Intermittent use of wheelchair as young adult
• Laboratory
  • Repetitive nerve stimulation: Decrement in proximal muscles
  • Muscle
    • Histology: Type I muscle fiber predominance
    • AChRs at NMJs: Normal distribution; Numbers slightly reduced; Channel function normal
    • NMJ ultrastructure: Reduced size of presynaptic nerve terminal & postsynaptic region

• Epidemiology: 8 patients
• Genetics
  • Mutations: Nonsense (Met270X) & Large deletion
  • Allelic wtih: Recessive contiguous gene deletion syndromes
    • Hypotonia-Cystinuria syndrome (HCS) : with SLC3A1 disorder
    • Atypical hypotonia–cystinuria syndrome: CAMKMT
    • 2p21 deletion syndrome: PPM1B ; Most severe phenotype
• PREPL protein
  • Oligopeptidase
  • Interacts with: Clathrin-associated adaptor protein 1 (AP-1)
    • Effector
    • Binds to m1A subunit to release AP-1 from membrane target
  • Mutations: PREPL protein absent from NMJs & Muscle fibers
• Clinical
  • Onset age: Birth
  • Hypotonia
  • Feeding difficulties. At age 11 weeks, she had
  • Face: Eyelid ptosis; Tented upper lip
  • Weakness
    • Proximal & Neck predominant
    • Variable during day
    • Gait: Waddling
    • Dysarthria
  • Tendon reflexes: Normal
  • AChE inhibitor (edrophonium & pyridostigmine) response: Positive in younger patients
• Laboratory
  • Repetitive nerve stimulation (2 Hz): No decrement
  • End plate physiology: Pre- & Post-synaptic features
    • Quantal content of endplate potential: Decreased
    • Miniature endplate potentials: Reduced amplitude
  • NMJ AChRs: Normal number
  • Endplate geometry: Normal
  • Serum insulin-like growth factor (IGF)-1 level: Low
  • Growth hormone deficiency

• Epidemiology: 1 monozygous twin pair
• Genetics
  • Mutation: Missense; c.1732C>T (p.R578C); Heterozygous; Duplin domain
  • Inheritance: de novo
  • Allelic disorders
    • Intellectual dysability, Autism susceptibility, Overgrowth syndromes ± Macrocephaly : Microdeletions
    • Gastrointestinal disorders
• CHD8 protein
  • ATP-dependent chromatin remodelling enzyme
  • Regulation of DNA accessibility & gene expression
  • Transcriptional regulation of canonical WNT-pathway
  • Inhibits binding of β-catenin to TCF4
  • Cell distribution: Cytoplasmic, especially Golgi, NMJ & Subsarcolemmal
  • Muscle
    • Canonical WNT-signalling reduces rapsyn expression
    • NMJ development
• Clinical
  • Pregnancy: Pre-eclampsia
  • Birth: Respiratory distress; Ptosis; Weakness
  • Skeletal
    • Head: Macrocephaly; Hypertelorism;
    • Scoliosis
  • GI: Constipation
  • Motor
    • Leg weakness: Moderate
    • Walking: Onset 2.5 years; Waddling gait
    • Deterioration: Onset 6 years
    • Voice: Weak
    • Fatigability: Improvement with Tensilon; Worse with cold
    • EOM: Full
  • Intelligence: Normal
  • Drug treatment: 3,4-diaminopyridine (3,4-DAP; 0.5 mg/kg BW/d)
• Laboratory
  • NCV: Normal
  • RNS: No decrement
  • Brain MRI: Normal

CMYP12: Lethal Congenital Myopathy (MYPCN)

• Nosology: Compton-North congenital myopathy
• Epidemiology: Egyptian family
• Genetics: Mutation
  • Frameshift
  • Homozygous
  • c.871dupT
• Contactin-1 protein
  • Glycosyl phosphatidylinositol (GPI)-anchored
  • Neural adhesion molecule
    • Immunoglobulin superfamily
  • Ligand: NOTCH1
  • Muscle: Localized to NMJs in normals; Sarcolemmal membrane in dystroglycanopathies
  • Nerve: Paranode
  • Function: Role in neurite growth & establishment and stabilization of synaptic connections
  • Contactin-1 autoantibody (IgG): Immune demyelinating polyneuropathy
• Clinical
  • Pregnancy: Polyhydramnios; Growth retardation
  • Birth age: 28 to 35 weeks
  • Motor: No spontaneous movements or Intubation needed
  • Muscle mass: Reduced
  • Skeletal
    • Multiple contractures (Arthrogryposis)
    • Simian creases
    • Arachnodactyly with overlapping fingers
    • Camptodactyly
  • Death: Days to Months
• Laboratory
  • CK: Normal
  • Muscle pathology
    • Fiber size: Mild variation
    • Fiber architecture: Internal clear regions
    • Endomysial connective tissue: Normal or Mildly increased
    • Staining: Absent Integrin α7, β2-syntrophin, α-dystrobrevin

• CMS10: Familial Limb-Girdle Myasthenia ¹¹
    ● Dok-7 (C4ORF25) ; Chromosome 4p16.3; Recessive ²⁵
  
  • Epidemiology: Dok-7 MG
    • Common in different ethnic origins
    • > 70 patients described
  • Genetics: Dok-7 mutations
    • Type: Usually frameshift; Occasional missense
    • Location: Common in exon 7 (Frameshift c.1124_1127dupTGCC); Also exons 2, 3, 4, 5
    • Some mutations are complex: Identifiable only in cloned complementary DNA
    • Allelic disorder: Fetal Akinesia Deformation Sequence 3 (FADDS3) (Arthrogryposis; Multiple Pterygium syndromes)
  • Dok-7 protein
    • Location: Post-synaptic, cytoplasmic
    • Structure: Functional domains
      • N-terminal pleckstrin homology (PH): Membrane association
      • Central phosphotyrosine-binding (PTB): Dok-7 induced activation of MuSK
      • Large C-terminal domain: Contains multiple tyrosine residues
    • Functions
      • Required for neuromuscular synaptogenesis & maintenance of NMJs
      • Induces: Activation of MuSK & Clustering of AChRs
      • AChR β-subunit phosphorylation
  • Clinical features
    • Onset
      • Age
        • Common: Birth to 2nd year
        • Range: Birth to 3rd decade
        • Occasional: Decreased fetal movements
      • Hypotonia; Falling; Gait disorder; Weakness
      • May be acute or chronic
      • Associated with: Pregnancy
    • Weakness
      • Proximal
        • Arms & Legs
        • Trunk & Neck
        • Symmetric
        • Difficulty walking: Waddling gait
      • Respiratory: Common with Dok-7 mutations
      • Face: Weak; Ptosis, may be asymmetric
      • Extra-ocular muscles: Normal
      • Fatiguable: Worse after exercise & emotional events
      • Severity: Variable
        • Mild: Static weakness
        • Severe: Progressive generalized
      • Course
        • Progressive: Especially in severe patients
        • Intermittent exacerbations: Duration days to weeks
    • Other muscle features
      • Muscle wasting
      • Discomfort: Cramps; Pain
    • Skeletal
      • Scoliosis
      • No joint contractures
    • Occasional systemic features
      • Contractures
      • Cardiac repolarization defect
    • Treatments
      • Ephedrine
      • Salbutamol (Albuterol)
        • Adults: 2mg tid or 4 mg bid (6-18 mg/d)
        • Children (up to 6 years): 0.1-0.3 mg/kg/d
      • Fluoxetine: 40 mg/d; Partial responses reported
      • ? AChE inhibitors
        • May benefit or worsen some patients with limb-girdle MG syndromes
        • Dok-7 mutations
          • No long-term benefit
          • May have positive tensilon test
  • Laboratory
    • Serum CK: Normal to Mildly high; 160 to 320
    • Electrophysiology
      • RNS: Decrement
      • SF-EMG: Often increased jitter
      • Muscle fiber irritability: Some patients
    • Muscle pathology
      • Internal nuclei
      • Neuromuscular junctions
        • Small size: Some
        • Post-synaptic folds: Reduced
        • Single or Multiple
        • EM
          • Destruction & remodeling of EPs
          • Degeneration of post-synaptic folds
          • Similar to: Slow channel syndromes
        • Dok-7 protein
          • Dok-7 mutations: Normal or reduced amounts at NMJs
          • May be reduced with AChR mutations
        • AChR function: Normal
        • Presynaptic function: Normal
      • Tubular aggregates: Not present with Dok-7 mutations
  
  
• CMS12: Familial Limb-Girdle Myasthenia with Tubular Aggregates (CMSTA1) ³²
    ● Glutamine:Fructose-6-phosphate Amidotransferase 1 (GFPT1) ; Chromosome 2p13.3; Recessive
  
  • Epidemiology: > 75 patients, 40 families
  • Genetics
    • Mutations: Missense, Splice site or Stop
    • Location: Most regions of gene
    • Loss of function mutations: More severe phenotype
  • GFPT1 protein
    • Hexosamine pathway: Rate-limiting enzyme
    • Yields essential amino sugars for carbohydrate modifications
    • End product of pathway: UDP-N-acetlyglucosamine (Glc-NAc)
      • Substrate for N- & O-linked glycosylation
    • Required for neuromuscular transmission
  • Clinical
    • Onset age
      • Range: Infancy to 24 years
      • 1st decade in 80%; Mean 8 years
      • Normal at birth: Most patients
      • Congenital onset: Some patients with severe weakness
    • Weakness
      • Proximal > Distal
      • Shoulder & Pelvic girdle: 100%
      • Distal weakness: 30% to 50%
        • Forearm extensors, Intrinsic hand, Anterior leg
      • Cranial
        • Face: 30%
        • Ptosis: 5%
        • Ocular muscles: 0%
        • Bulbar: 0%
      • Neck: 25%
      • Respiratory: 5% to 12%
      • Permanent: > 30%
      • Course
        • Slowly progressive
        • Prognosis
          • Most (80%) remain ambulant
          • Myopathic weakness with less response to CMS drugs
    • Tendon reflexes: May be reduced
    • Contractures: Occasional; Mild
    • Sensation: Normal
    • Treatments: Partial benefit
      • Anti-AChE medications: More response than with Dok-7 mutations
      • 3,4 Diaminopyridine
      • Albuterol (6 mg/day)
  • Laboratory
    • EMG: Myopathic
      • Motor units: Small amplitude; Short (Myopathic); Unstable
      • Some patients: Spontaneous activity; Fibrillations & Positive sharp waves
    • Repetitive nerve stimulation: Decrement
    • SFEMG: Abnormal jitter & blocking
    • Serum CK: Normal in most; Occasionally High
    • Muscle MRI
      • Fatty infiltration (T1w imaging)
      • Nuscle involvement
        • Non-selective
        • Sparing: Adductor magnus; Semimembranosus; Gastrocnemius, medial
      • May be near normal with severe weakness
    • Muscle pathology
      • Tubular aggregates (80%): Especially in Type 2 fibers, May be small
      • Some patients
        • Myopathic: Varied fiber sizes & Endomysial fibrosis
        • Desmin or Dystrophin+ aggregates
        • Rimmed vacuoles (30%)
      • Ultrastructure: Autophagic & other vacuoles; Tubular aggregates; Granulofilamentous deposits
      • Glycosylation & Glycoprotein expression
        • Often reduced (70 kD band)
        • Reduced O-glycosylation
        • May be normal
        • α-dystroglycan staining: Normal
      • Sialylation of transmembrane proteins: Reduced in extra-junctional regions
        • Increased sarcolemma staining for peanut agglutinin (PNA)
      • NMJs
        • AChRs: Reduced or Normal numbers
        • Morphology
          • Normal or Post-synaptic simplification (Loss of folds)
          • Low nerve terminal area
          • Post synaptic: Small; Single or grape-like regions
          • Multiple AChE regions
      • Neuropathic features: Fiber type grouping in 30%
  
  
• CMS13: Familial Limb-Girdle Myasthenia with Tubular Aggregates 2 (CMSTA2) ³⁴
    ● Dolichyl-phosphate N-acetylglucosamine phosphotransferase (DPAGT1) ; Chromosome 11q23.3; Recessive
  
  • Epidemiology: 16 patients
  • Genetics
    • Mutations: Missense & Duplication
    • Allelic with: Congenital disorder of glycosylation, type IJ
  • DPAGT1 protein
    • Expression: Ubiquitous
    • Catalyzes first committed step of N-linked protein & lipid (LLO) glycosylation
      • Phosphorylated UDP-GlcNAc (from cytoplasmic UDP-GlcNAc) is added to dolichol phosphate (a transmembrane lipid carrier)
      • Dolichol-PP-glycan assembly
    • Required for efficient
      • Glycosylation of acetylcholine-receptor subunits
      • Export of AChRs to cell surface
  • Clinical
    • Onset age: 0.5 to 17 years; Median 2 years; Normal at birth
    • Early: Hypotonia
    • Weakness
    • Proximal: Most patients
    • Distal: Many patients
    • Arms ≥ Legs
    • Gait disorder
    • Neck flexion
    • Respiratory: Moderate
    • Face or Bulbar: Mild or None
    • Course: Slowly progressive
  • No: Ptosis or ophthalmoplegia;
  • Skeletal
    • Scoliosis: 50%
    • Joint contractures: 40%
  • Treatment: Cholinesterase inhibitors; 3,4-Diaminopyridine
  • Course: Slow progression or Improvement
• Laboratory
  • AChRs at NMJs: Reduced
  • Repetitive nerve stimulation: Decrement (30% to 40%)
  • SFEMG: Jitter & Blocking
  • Repetitive discharges after single stimulation: 1 patient
  • EMG: Myopathic
  • Serum CK: Normal
  • Transferrin IEF: Normal or Mildy reduced glycosylation
  • Muscle pathology
    • Tubular aggregates: More common in older patients
    • NMJs
      • Postsynaptic folding: Reduced
      • AChRs: Reduced number
      • AChE: Normal
    • Glycogen: Increased
    • Autophagic vacuoles
  
  
• CMS14: Familial Limb-Girdle Myasthenia with Tubular Aggregates 3 (CMSTA3) ³⁷
    ● ALG2 ; Chromosome 9q22.33; Recessive
  
  • Epidemiology: 5 patients in 2 families
  • Genetics
    • Mutations: Val68Gly; c.214_226delinsAGTCCCCGGC, p.72_75delinsSPR; Homozygous
    • Allelic with: Congenital disorder of glycosylation 1i (GDG 1i)
  • ALG2 protein
    • α-1,3-mannosyltransferase
    • Catalyses early steps in asparagine-linked glycosylation
    • Location: Enriched at NMJs in muscle
    • Mutant: Reduced ALG2 protein in muscle
  • Clinical
    • Onset age: Infancy to 4 years
    • Weakness
      • Early: Delayed motor milestones; Hypotonia
      • Pattern: Proximal > Distal
      • Face: Mild
      • Respiratory: Mild involvement
      • Eye: Normal; Full EOM; No ptosis
    • Tendon reflexes: May be reduced
    • Skeletal
      • Contractures: Knees (60%)
      • Spine: Generally normal
      • Joint Laxity: Distal (60%)
      • Palate: High arched
    • Symptoms exacerbated by: Stress; Infection
    • Progression: Slow; 80% never ambulate
    • Treatment: AChE inhibitors
  • Laboratory
    • Muscle
      • Fiber size: Varied
      • Myopathic
      • Tubular aggregates (50%)
      • Internal nuclei (50%)
      • Type 1 predominance (50%)
    • Repetitive nerve stimulation: Decrement up to 67%
    • SFEMG: Jitter & Blocking
    • Serum CK: Normal to Slightly high

• Familial immune MG
    ● Autosomal recessive
  • Epidemiology
    • + Family history in 3% to 5% of MG
    • Most commonly same generation: Sibs or cousins
  • Onset
    • Earlier than nonfamilial autoimmune MG
    • Age 2 to 20 years: Similar clinical features to non-familial adult patients
  • ? More commonly have no anti-AChR antibodies
  • Rule out
    • Transient neonatal MG
    • Recurrent congenital arthrogryposis
  • Variant syndrome: Familial autoimmune myasthenia gravis ³⁵
      ● Ecto-NOX Disulfide-Thiol exchanger 1 (ENOX1) ; Chromosome 13q14.11; Recessive
    • Epidemiology: 1 Sicilian family
    • Mutation
      • c.*35A>G: Located in 3' UTR of ENOX1 gene
      • Also present in 1 unaffected sibling
    • Clinical
      • Onset ages: 51 to 70 years
      • Ptosis
      • Eye movements: Diplopia; Fatigability of upward & lateral gaze
      • Weakness: Neck; Extremities
      • Fatigability
      • Treatments: Pyridostigmine; Corticosteroids
    • Laboratory
      • AChR antibodies: 75% positive
      • Repetitive nerve stimulation: Decrement
  
    ● Also see
  • Myasthenia gravis with thymus hyperplasia
  • Acquired MG: Genetic Associations
  
  
• Reduction in number of AChRs at NMJs and Paucity of Secondary Synaptic Clefts
  
  
• Presynaptic defects in ACh synthesis, mobilization or storage

• Most common type of congenital MG syndrome
• AChR mutations
  • Often recessive
  • Subunits: Usually ε mutations; Occasional α, β, or rarely δ, mutations
  • May be compound heterozygous or homozygous
  • Types
    • Premature termination of translation: Frame shifting; Splice site; Stop codon
    • Missense in signal peptide region (εG-8R)
    • Missense in amino acids needed for assembly of pentameric AChR: εS143L; εC128S; εR147L
    • Missense affecting both AchR expression & kinetics
    • Point mutations in promoter region of subunit gene
  • ε subunit nonsense mutations
    • ε1101insT
      • Reduced AChR expression
      • Normal channel function
    • ε1293insG
      • No channel function
    • Other frameshifting mutations
    • Many patients are heteroallelic
    • Expression of γ AChR (immature) at NMJs: ? Compensatory
• Rapsyn mutations
• Morphology
  • Endplate regions: Increased Number; Distributed over larger surface area of muscle fiber
  • Most junctions: Integrity of post-synaptic membrane folds is preserved
  • Occasional junctions: Endplate regions are simplified & small
  • AChR on junctional folds: Patchy distribution; Reduced Density
• Physiology
  • Reduced amplitiude of miniature endplate potentials & currents
  • High quantal release by nerve impulse: frequent
  • Fetal AChRs at NMJ containing γ instead of adult ε AChR subunits
• Clinical features: Variable
  • Severe form
  • Onset: Birth
  • Weakness
    • Diffuse: Ocular; Bulbar; Respiratory; Limbs; Trunk
    • Delayed motor milestones
    • Respiratory insufficiency: Frequent
  • Muscle bulk: Reduced
  • Tendon reflexes: Normal or Reduced
  • Skeletal
    • Facial deformities: Prognathism; Malocclusion
    • Scoliosis
  • Intermediate form
    • Onset: Early childhood
    • Ocular paresis & Ptosis: Onset 1st year
    • Weakness: Diffuse; Moderate disability
    • Fatigue
  • Mild form
  • Normal motor milestones
  • Mild ptosis common
  • Ophthalmoplegia: Partial
  • Motor: Clumsy; Poor running
  • Sensitivity to neuromuscular blocking agents
• Treatment
  • AChE inhibitors
  • 3,4-Diaminopyridine: 1 mg/kg/day in doses q 3 to 5 hours

• Epidemiology: 2 patients reported
• Genetics: Normal CACNA1A gene
• Clinical
  • Onset: Birth
  • Motor development: Delayed; Hypotonia
  • Weakness: Bulbar & limb
  • Mental retardation
• Electrophysiology: Repetitive nerve stimulation
  • Small initial CMAP
  • Rapid stimulation: Increment up to 5-fold
  • Slow stimulation: Decrement
• Neuromuscular junctions
  • AChRs: Normal number
  • MEPPS: Normal amplitude
  • EPP: Small quantal content; Increased after 40 Hz RNS
• 3,4 Diaminopyridine treatment
  • Improves electrophysiologic defects
  • Little clinical benefit

• Epidemiology: 7 families
• Genetics
  • Mutations: Ser306Leu; Asp307Ala; Pro308Leu; Ile371Lys; Inframe c.1082_1096del; Asp361_Leu365del
  • Allelic disorder: Congenital onset presynaptic myasthenic syndrome, Recessive (CMS7B)
• SYT2 protein
  • Calcium
    • Binding
    • Sensors in vesicular trafficking & exocytosis
  • Secretory granules & Synaptic vesicles
  • Binds: Botulinum toxins B & G
  • Role in: Synaptic vesicle exocytosis
    • Synchronous vesicle release at NMJs
  • Mutant SYT2 protein
    • Dominant-negative effect on synaptic vesicle exocytosis
    • Disease expression may relate to disorder of presynaptic NMJ or Motor axon
  • SYT1 mutations: Neurodevelopmental disorder (Baker-Gordon syndrome)
• Clinical
  • Onset age: Childhood, early
  • Foot deformities: Pes cavus; Hammer toes
  • Weakness
    • Proximal & Distal, or Distal only
    • Legs > Arms
    • May be asymmetric
    • Gait disorder
    • Ptosis: Few patients
  • Motor: Other
    • Fatigue: Improves with rest
    • Atrophy: Leg
  • Tendon reflexes: Decreased at ankles; Increase with exercise
  • Sensory loss: 25%
  • Course: Slow progression
  • Treatment: 3,4-diaminopyridine variably effective
• Laboratory
  • Electrodiagnostic
    • CMAP amplitudes: Small; Increment after exercise
    • SNAPs: Normal
    • Repetitive Nerve stimulation
      • Decrement with 0.5 Hz
      • Increment (Mean 90%) with maximal voluntary contraction or exercise
      • May be normal
        • Decay to normal over 60 minutes
          • Longer than LEMS (30 sec) & Botulism (2 to 20 minutes)
    • SFEMG: Jitter increased
    • EMG: Normal or Mild reinnervation
  • Muscle
    • Fiber sizes varied
    • NMJs: Nerve terminals small & inclusions; Postsynaptic folding overdeveloped
• SYT2 variant disorder: CMS7B, Congenital onset presynaptic myasthenic syndrome ⁵³
  • Epidemiology: 9 patients in 7 families
  • Genetics
    • Inheritance: Recessive
    • Mutations: Loss of function; Biallelic
  • Clinical
    • General: May be more severe than Dominant syndromes
    • Onset: Congenital
    • Pregnancy: Reduced fetal movements
    • Hypotonia
    • Weakness: Severe
      • Motor milestones: Delayed
      • Face
      • Axial
      • Limbs: Proximal > Distal
      • EOM: Slow; Limited
      • General function: Minimal head control to Sitting
    • Respiratory involvement: Varied
    • Tendon reflexes: Absent
    • Sensation: Normal
    • Scoliosis
    • Treatment: AChE inhibitor (Pyridostigmine or Neostigmine)
    • Cognition: Normal
  • Laboratory
    • RNS: Decrement at 3 to 5 Hz
    • NCV: CMAP amplitudes reduced
    • EMG: Denervation or Myopathy
    • Muscle: Fiber size varied; Tubular aggregates; NMJs segmented
    • Brain MRI: Corpus callosum thin

• Epidemiology: 1 patient
• Genetics
  • Mutations
    • Ile67Asn: Produces SNAP25b splice variant
    • Q117X: Likely produces truncated protein
    • Probable mechanism: Dominant negative
  • Allelic disorder
    • Epilepsy & Encephalopathy: Val48Phe
• SNAP25 protein
  • Location: Nerve terminal; Presynaptic
  • t-SNARE
  • Attached to vesicle target plasma membrane
  • Component of synaptic vesicle fusion machinery
  • Loss of SNAP25
    • Pool of vesicles primed for release: Empty
    • Fast calcium-triggered exocytosis: Abolished
  • Binds to: Munc13-1
  • Splice variants
    • SNAP25b: Plasma membrane of nerve terminals at motor endplates &central synapses
    • SNAP25a: Diffusely distributed in neuronal cytoplasm & axoplasm
  • Mutation: MEPP frequency reduced
  • Altered in: Schizophrenic brains
  • Toxic cleavage: Botulinum toxins A & E
• Clinical
  • Onset: in utero
    • Hypomobility
    • Polyhydramnios
  • Birth: Cyanosis; Contractures; Arthrogryposis
  • Motor
    • Walking: Delayed; Limited
    • Weakness: Fatigable
  • Episodes: Ptosis, Blank stare, Unresponsive
  • CNS
    • Intellectual disability
    • Ataxia: Dysarthria; Gait
  • Treatment
    • Strength: Improved by 3,4-diaminopyridine, not pyridostigmine
    • Seizures: Levetiracetam
  • Course: Early death in patient with stop mutation
• Laboratory
  • EEG: Generalized atypical polyspike & wave discharges; Diffuse slowing
  • Brain MRI: Normal
  • Repetitive nerve stimulation: 18% to 50% decrement
  • Muscle
    • Histochemistry: Normal
    • Synaptic areas: Pretzel-shaped or Ovoid; AChR & AChE normal
    • NMJ ultrastructure: Normal
    • Quantal release: Reduced probability
• Animals
  • Blind-drunk (+/Bdr) mouse: Ile67Thr mutation

• Epidemiology: 15 families
• Genetics
  • Mutations: Homozygous; Loss-of-function; c.1171delG, c.523-1delG
• COL13A1 protein
  • Collagen
  • alpha chain of atypical non-fibrillar collagen
  • Expressed at low levels in many tissues
  • Interacts with: Fibronectin, Heparin, Nidogen-2, Perlecan,15
  • Interactions with extracellular matrix
  • Anchored to plasma membrane by a hydrophobic transmembrane segment
  • Localized to NMJ: Postsynaptic membrane; Synaptic basement membrane
  • Target of autoantibodies in some patients with autoimmune MG
• Clinical
  • Onset age: Birth
  • Weakness
    • Respiratory:
      • Crises: Fewer with increased age
      • Vital cpacity: 14% to 62%
    • Bulbar: Feeding difficulty; Face; Poor suck
    • Truncal & Axial > Limbs
    • Limbs: Arms & Legs; Proximal & Distal
    • Milder patient: Distal arms
    • Muscle bulk: Often reduced
    • NO fluctuation or fatigability
    • Course: Slow improvement ove years
  • Ocular
    • Ptosis
    • Ophthalmoplegia: Often mild; May be asymmetric
  • Skeletal
    • Dysmorphic: Micrognathia, Ears low-set, Palate high-arched; Face elongated & narrow
    • Pectus carinatum & Pes cavus
    • Spinal rigidity
    • Scoliosis: Few patients
    • Distal joint laxity: Few patients
    • No contractures
  • CNS: some patients
    • Learning difficulties (50%)
    • Motor milestones delayed (70%)
  • Treatments
    • 3,4-diaminopyridine
    • Salbutamol
    • Ventilatory support
    • NOT: AChE inhibitors
• Laboratory
  • Repetitive nerve stimulation: Decrement (70%)
  • Jitter: Abnormal
  • Serum CK: Normal
  • Muscle biopsy
    • Fiber size: Varied
    • Immature muscle fibers
    • NADH: Halo-like fibers
    • Mitochondria: ? Large
    • May be normal
    • NMJ
      • Presynaptic: Incomplete clustering of synaptic vesicles
      • Postsynaptic: Defective maturation; AChE more extensive than AChRs
      • Nerve terminals: Abnormal localization
  • Brain & Muscle MRI: Normal

• Epidemiology: 3 patients, Kurdish & German families
• Genetics
  • Mutations: Missense; .R1517H, R2283H, D1698G
  • Allelic disorder: Arthrogryposis
• MYO9A protein
  • Unconventional myosin
  • Roles: NMJ development; Neurite extension & branching
  • Location: NMJ, Pre- & Post-synaptic; Growth cones; Brain
  • Single headed processive motor
  • Tail region: Contains Rho-GTPase domain
  • Postsynaptic: Binds to α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) GluA2 subunit
• Clinical
  • Onset age: Neonatal
  • Hypotonia
  • Apnea: Episodic
  • Dysphagia
  • Eyes: Variable
    • Ptosis: Bilateral
    • Ophthalmoplegia: Symmetric
    • Nystagmus
    • Oculomotor apraxia
  • Weakness: Distal & Proximal
  • Treatment: Partial responses to Pyridostigmine or 3.4-DAP
• Laboratory
  • Repetitive nerve stimulation: Decrement in face
  • SFEMG: Abnormal jitter
  • EMG: Normal
  • Brain MRI: Normal
  • Serum CK: Normal
• MYO9A variant: Arthrogryposis
  • Epidemiology: 1 family
  • Mutations: c.6845G>A (p.Gly2282Glu); c.608A>G (p.Tyr203C)
  • Clinical: Multiple limb contractures

• Epidemiology: 1 patient; Native American female
• Genetics
  • Mutation: Nonsense; Gln102*
  • Allelic disorder: ALS survival influence
• Munc13-1 protein
  • Location: Presynaptic
    • Active zones of NMJ nerve terminal & Glutamatergic synapses in brain
  • Reqired for
    • Synaptic vesicle release: Docking & Priming
    • Brain development
  • Binds to: Syntaxin 1B; SNAP25B
• Clinical
  • Onset
    • Age: Birth; Premature
    • Respiratory weakness
  • Weakness
    • Hypotonia
    • Respiratory failure: Cyanotic episodes
    • Feeding difficulties
    • Ptosis
    • Ophthalmoplegia
  • Head
    • Microcephaly: Circumference: < 5th percentile
    • Forehead: High, Frontotemporal narrowing
    • Nasal bridge: Wide
    • Palate: High arched
    • Jaw: Small
  • Skeletal
    • Thoracic kyphoscoliosis
    • Clinodactyly: Fifth digits
    • Contractures: Flexion; Proximal joints & knees
  • CNS: No speech
  • Death: 50 months
• Laboratory
  • MRI: Corpus callosum thin
  • EEG: Sharp waves
  • Nerve conduction
    • CMAP ampltudes: Small at rest
    • RNS 2 Hz: 20% to 40% decrement
    • RNS 50 Hz: 500% increment (0.8 to 4 mV)
  • Microphysiology: NMJ
    • MEPP frequency: 2% of normal
    • Quantal content of EPP (m): 6% of normal
    • α-bungarotoxin binding sites per EP: Normal
  • Muscle
    • Type 2 fibers: Small
    • Type 1 fibers: Large
    • Pre- & Post-synaptic morphology: Normal

• Epidemiology: 1 patient
• Genetics
  • Mutations: Missense; Arg269Gln, Val464Leu
• RPH3A protein
  • Interacts with small GTP-binding proteins Rab3A & Rab27a
  • Participates in trafficking & release of synaptic & secretory vesicles
• Clinical
  • Onset age: Earl;y childhood
  • Learning disabilities
  • Cerebellar: Tremor, Gait ataxia, Dysmetria
  • Bulbar
    • Speech: Nasal
    • Velopharyngeal insufficiency
    • Obstructive sleep apnea
  • Weakness; Mild; Neck, Proximal arms
  • Fatigability: Responsive to albuterol sulfate
  • Tendon reflexes: Normal
  • EOM: Normal
  • Abdominal pain
• Laboratory
  • Hyperglycemia: Transient
  • Repetitive nerve stimulation
    • 2 Hz: Normal
    • 30 Hz: 30% increase in CMAP area
  • SFEMG: Jitter increased
  • Muscle pathology
    • Type 1 predominance
    • Nonspecific filament bodies & mild Z-disk disruption in few fibers
  • NMJ pathology
    • Presynaptic terminals
      • Synaptic vesicle density: Marked reduction in SV density,
      • Double-membrane-bound sacs containing synaptic vesicles
      • Abundant endosomes
      • Degenerative lamellar bodies
    • AChR & AChE: Normal

• Onset: Infancy to 5 years
• Clinical
  • Myasthenic symptoms
    • Weakness
    • Extraocular muscles: Normal
  • Cerebellar: Trunk & limb ataxia; Nystagmus
• Electrophysiology
  • Decremental response on 2-Hz stimulation: Not improved by stimulation at higher frequencies
  • Normal first-evoked CMAP
  • Marked decrease in number of quanta released by 1-Hz stimulation
• Treatment: Some patients improve with combined treatment with pyridostigmine & 3,4-DAP

• Epidemiology +
  • Single patient: HLA type DR3-B8-A1
  • Sibling with congenital MG but no acquired disease: Thymectomy performed at 3 years
• Congential MG
  • AChR mutations
    • ε-subunit
    • Heteroallelic
    • Truncating: Y15X; 70insG
  • Onset: Birth; Ptosis
  • Slow motor development
  • Weakness
    • Severity: Moderate
    • Distribution: Limb, Ocular & Bulbar
    • Progression: None
  • Laboratory
    • Anti-AChR antibodies: None
    • SF-EMG: Mild jitter
  • Treatment: Anti-AChE agents (Pyridostigmine)
• Acquired MG
  • Onset age: 34 years; After pregnancy
  • Weakness
    • Severe
    • Increasing over 1 year
    • Respiratory failure
  • Laboratory
    • Anti-AChR antibodies: Present; Bind to adult & fetal AChRs
    • SF-EMG: Increased jitter
    • NMJ pathology
      • Reduced number of AChRs
      • Multiple endplate regions on individual muscle fibers
  • Treatment: Immunosuppression

Ocular Myopathy with Curare Sensitivity

• Epidemiology: 1 South Indian family, 9 patients
• Genetics: Recessive pattern of inheritance
• Clinical
  • Onset age: Infancy or Early childhood
  • Ophthalmoparesis
    • Bilateral
    • Often complete
    • Ptosis
    • Pupils: Normal
  • Strength: Limbs
    • Generally normal
    • Weakness: Provoked by low doses of curare
      • Proximal
      • Dysphagia
      • Ptosis: Increased
    • 1 patient with fixed proximal weakness
  • Tendon reflexes: Normal
  • Some curare sensitive family members asymptomatic
• Laboratory
  • EMG: Normal
  • Repetitive nerve stimulation (Ulnar): No decrement
  • Edrophonium test: Negative
  • Muscle biopsy: Normal

MUTATIONS OF AChR SUBUNITS

• Allelic disorders
  • Slow AChR channel syndromes: Dominant
  • AChR conversion into chloride conductance at positive potentials
  • Low-affinity, fast AChR channel syndrome: Recessive
  • Fast channel, gating MG syndrome: Recessive
  • Multiple pterygium syndrome: Recessive

• Genetics
  • Mutations
    • Often in exon 8
  • Allelic disorders
    • Slow channel syndromes: Dominant
    • Fast channel syndrome: Recessive
    • Congenital myasthenia gravis: Severe; Recessive
    • Fetal akinesia deformation sequence (FADS): Recessive
  • Myasthenia gravis, Acquired: Transcriptome association, rs4151121 ⁵⁶

• Epidemiology
  • ε subunit mutations: Most common cause of congenital MG syndromes
• Genetics
  • Mutations
    • Most patients compound heterozygotes
    • Each patient with one null & one missense mutation
    • North Africa: 1293insG founder mutation
    • ε subunit: Only AChR subunit with null mutations in congenital MG
  • Clinical: AChR dysfunction & phenotype correlates with missense mutation
  • Allelic disorders
    • Recessive: Congenital MG syndromes with
      • AChR Deficiency (CMS4C)
      • Fast channel syndromes (CMS4B)
        • Low AChR affinity for ACh
        • Kinetic Δ & AChR deficiency
      • North Africa founder mutation
      • Slow channel syndrome (CMS4A)
    • Dominant: Slow channel syndrome
    • Congenital MG with acquired MG developing in 4th decade
• AChR function in disease syndromes: Changes with mutations
  • Abnormal channel open times: Also see Slow channel syndromes
  • Abnormal ACh-AChR interaction: Low-affinity, fast-channel syndrome
  • Reduced AChR expression
    • Postsynaptic reduction in number of AChRs at NMJs
  • Short AChR open time
  • Compensation for ε subunit loss: Allows some NMJ function & survival of patient
    • γ subunit may be upregulated
    • Mutations between M3 & M4 (ε1267delG): Retention of intron 11 in some mRNA transcripts
      • Restores reading frame
      • Production of ε subunit at reduced level
      • Partially rescues adult AChR function
• Clinical features
  • Onset: Congenital
  • Weakness: Variable severity
    • Generalized: Proximal & Distal
    • Face
    • Ptosis
    • Ophthalmoplegia
  • Fatigue
• Laboratory
  • Electrophysiology
    • Decrement on RNS
    • Slow channel syndrome also has repetitive CMAP response to single stimulus
  • Pathology
    • Post-synaptic folding: Normal
    • Endplate morphology: Small, pale regions (Single or Multiple) distributed over larger region of muscle fiber
    • Type 2 fiber atrophy
• CHRNE variant syndrome: North Africa 1293insG founder mutation ²⁹
  • No fetal involvement
  • Moderate hypotonia & oculobulbar involvement
  • Disease course: Mild & Stable
  • Treatment: Good response to cholinesterase inhibitors

• Genetics
  • Mutation types: Point; Missense & Truncating
  • Allelic disorders
    • Fast channel syndromes
      • Reduced expression of AChRs with fast channel properties
        • δ subunit mutation (P250Q): Recessive
        • δ subunit mutation (L42P): Fast channel (Short open times); Increased affinity for ACh
        • General features: Reduced AChR expression; Fast channel syndromes
      • Fast channel syndrome with arthrogryposis: Recessive
    • Slow channel syndrome: δS268F; Dominant
    • Disruption of AChR clustering
    • Nonsense mutations
      • Multiple pterygium syndrome: Lethal; Recessive
• CHRND variant: Disruption of AChR clustering ²⁶
  • Genetics
    • Inheritance: Recessive
    • Mutations: E381K + null mutation
  • CHRND mutant protein: Reduced binding to rapsyn
  • Clinical
    • Early onset: Feeding difficulties
    • Ptosis
    • Weakness: Moderate, General, Proximal > Distal
    • Recurrent episodes of respiratory insufficiency: Provoked by infections
    • Treatment: Moderate response to anticholinesterase treatment
  • Electrodiagnostic
    • Decrement on 3 Hz stimulation

• CHRNG protein: Expressed in fetal, but not adult, muscle
• Allelic disorders
  • Multiple pterygium syndrome (Escobar variant; EVMPS)
    • Mutations: Missense & Stop
  • Multiple pterygium syndrome, Lethal form (LMPS)
    • Mutations: Stop

• Drug- induced myasthenic syndromes
• Patient information