Congenital myasthenic syndrome 6 (CMS6, CMS with episodic apnea/CMS-EA, CHAT-CMS, historically "familial infantile myasthenia") is an autosomal recessive presynaptic congenital myasthenic syndrome caused by biallelic pathogenic variants in CHAT, encoding choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine (ACh) from acetyl-CoA and choline at the motor nerve terminal. It is the prototype and most common presynaptic CMS. Unlike postsynaptic or synaptic CMS, endplate acetylcholine receptor density and acetylcholinesterase activity are normal; the defect is a use-dependent failure to resynthesize/repackage ACh under sustained functional demand. Patients classically present at birth or in early infancy with hypotonia, ptosis, and bulbar weakness, and experience sudden, recurrent, potentially fatal apneic crises precipitated by fever, infection, excitement, or exertion, with only mild or no myasthenic symptoms between crises. A phenotypic spectrum is increasingly recognized: alongside this classic severe neonatal/infantile presentation, a milder, later-onset (childhood) phenotype has been described in which exercise-induced leg fatigability is the sole or presenting manifestation, without apnea, ptosis, or ophthalmoparesis, and without progression over years of follow-up.
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name: Congenital Myasthenic Syndrome 6
creation_date: "2026-07-30T00:00:00Z"
category: Mendelian
disease_term:
preferred_term: Congenital Myasthenic Syndrome 6
term:
id: MONDO:0009689
label: congenital myasthenic syndrome 6
parents:
- Congenital Myasthenic Syndrome
description: >-
Congenital myasthenic syndrome 6 (CMS6, CMS with episodic apnea/CMS-EA,
CHAT-CMS, historically "familial infantile myasthenia") is an autosomal
recessive presynaptic congenital myasthenic syndrome caused by biallelic
pathogenic variants in CHAT, encoding choline acetyltransferase (ChAT), the
enzyme that synthesizes acetylcholine (ACh) from acetyl-CoA and choline at
the motor nerve terminal. It is the prototype and most common presynaptic
CMS. Unlike postsynaptic or synaptic CMS, endplate acetylcholine receptor
density and acetylcholinesterase activity are normal; the defect is a
use-dependent failure to resynthesize/repackage ACh under sustained
functional demand. Patients classically present at birth or in early
infancy with hypotonia, ptosis, and bulbar weakness, and experience sudden,
recurrent, potentially fatal apneic crises precipitated by fever, infection,
excitement, or exertion, with only mild or no myasthenic symptoms between
crises. A phenotypic spectrum is increasingly recognized: alongside this
classic severe neonatal/infantile presentation, a milder, later-onset
(childhood) phenotype has been described in which exercise-induced leg
fatigability is the sole or presenting manifestation, without apnea, ptosis,
or ophthalmoparesis, and without progression over years of follow-up.
inheritance:
- name: Autosomal recessive
inheritance_term:
preferred_term: Autosomal recessive inheritance
term:
id: HP:0000007
label: Autosomal recessive inheritance
description: >-
All reported CMS6 patients carry biallelic (homozygous or compound
heterozygous) CHAT variants inherited from unaffected heterozygous
carrier parents; heterozygous CHAT null-allele carriers are asymptomatic.
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Restriction analysis of DNA from relatives revealed that a living affected sibling harbors both mutations, the asymptomatic parents carry a single mutation, and other unaffected relatives carry either no or a single mutation (data not shown). Therefore, each mutation is heteroallelic and recessive."
explanation: Establishes the autosomal recessive, biallelic segregation of CHAT mutations in CMS6 families.
pathophysiology:
- name: Choline Acetyltransferase Catalytic Deficiency
biological_scale: MOLECULAR
description: >-
Biallelic CHAT missense, nonsense, or frameshift variants reduce ChAT
protein expression and/or catalytic efficiency for its two substrates,
acetyl-CoA and choline. Mutant severity spans a spectrum: null/frameshift
alleles abolish expression entirely, while missense substitutions near the
active-site tunnel or substrate-binding domains variably impair
acetyl-CoA/choline affinity and catalytic turnover, reducing acetylcholine
(ACh) biosynthesis at the presynaptic nerve terminal. Endplate AChR
density and acetylcholinesterase activity are unaffected.
cell_types:
- preferred_term: Motor neuron presynaptic terminal
term:
id: CL:0000100
label: motor neuron
biological_processes:
- preferred_term: Acetylcholine biosynthetic process
term:
id: GO:0008292
label: acetylcholine biosynthetic process
modifier: DECREASED
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Direct sequencing of CHAT reveals 10 recessive mutations in five patients with CMS-EA. One mutation (523insCC) is a frameshifting null mutation. Three mutations (I305T, R420C, and E441K) markedly reduce ChAT expression in COS cells."
explanation: Establishes recessive CHAT loss-of-function mutations, including a null frameshift allele and missense alleles with reduced expression, as the molecular lesion in CMS6.
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Kinetic studies of nine bacterially expressed ChAT mutants demonstrate that one mutant (E441K) lacks catalytic activity, and eight mutants (L210P, P211A, I305T, R420C, R482G, S498L, V506L, and R560H) have significantly impaired catalytic efficiencies."
explanation: Direct biochemical demonstration that CMS6 CHAT mutants are catalytically deficient for ACh synthesis.
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "The characterization of mutants showed a decrease in the overall catalytic efficiency of ChAT; in particular, those located near the active-site tunnel produced the most seriously disruptive phenotypic effects."
explanation: Confirms reduced ChAT catalytic efficiency as the shared biochemical consequence of CMS6-causing mutations and links mutation location/severity to disease severity.
downstream:
- target: Use-Dependent Failure of Acetylcholine Resynthesis and Vesicular Repackaging
causal_link_type: DIRECT
description: >-
Reduced ChAT catalytic activity limits the rate at which cytoplasmic ACh
can be resynthesized to keep pace with vesicular release, producing a
use-dependent (rather than resting) neurotransmission defect.
- name: Use-Dependent Failure of Acetylcholine Resynthesis and Vesicular Repackaging
biological_scale: CELLULAR
description: >-
In contrast to postsynaptic or synaptic CMS, resting (unstimulated)
neuromuscular transmission in CMS6 is essentially normal: endplate AChR
number, quantal content of the endplate potential, and miniature endplate
potential (MEPP) amplitude are all normal at rest. The defect only
manifests under sustained functional demand — after a conditioning train
of high-frequency (10 Hz) nerve stimulation, MEPP amplitude falls
(consistent with fewer ACh molecules per synaptic vesicle) because
depleted synaptic vesicles cannot be adequately refilled with newly
synthesized ACh. Neuromuscular junction ultrastructure is normal on
electron microscopy.
cell_types:
- preferred_term: Motor neuron presynaptic terminal
term:
id: CL:0000100
label: motor neuron
- preferred_term: Skeletal muscle fiber endplate
term:
id: CL:0008002
label: skeletal muscle fiber
biological_processes:
- preferred_term: Neurotransmitter loading into synaptic vesicle
term:
id: GO:0098700
label: neurotransmitter loading into synaptic vesicle
modifier: DECREASED
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Studies of the neuromuscular junction in this disease show a stimulation-dependent decrease of the amplitude of the miniature endplate potential and no deficiency of the ACh receptor."
explanation: Documents the defining electrophysiological signature of CMS6 - a stimulation-dependent, not resting, defect - with normal AChR number.
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Instead, there is a use-dependent reduction of the quantal size due to a decrease in the number of ACh molecules per synaptic vesicle (Engel and Lambert, 1987; Mora et al., 1987) with normal ultrastructure of the neuromuscular junction (NMJ) (Maselli et al., 2003)."
explanation: Confirms the mechanism as a use-dependent fall in vesicular ACh quantal content rather than a structural NMJ defect.
downstream:
- target: Impaired Neuromuscular Junction Transmission
causal_link_type: DIRECT
description: >-
Insufficient ACh resynthesis during sustained nerve activity lowers
endplate potential amplitude below the threshold needed to reliably
trigger a muscle fiber action potential, especially during repetitive or
prolonged demand.
- name: Impaired Neuromuscular Junction Transmission
biological_scale: CELLULAR
description: >-
The terminal shared mechanism of CMS6: a use-dependent reduction of the
safety factor of cholinergic neuromuscular transmission. Because the
defect only becomes electrophysiologically and clinically apparent under
sustained or repetitive demand, basal (2 Hz) repetitive nerve stimulation
and single-fiber EMG can be falsely normal, particularly in neonates; a
decremental compound muscle action potential (CMAP) response classically
appears only after a prolonged conditioning train of high-frequency (10
Hz, 5 minute) stimulation.
cell_types:
- preferred_term: Skeletal muscle fiber (postsynaptic motor endplate)
term:
id: CL:0008002
label: skeletal muscle fiber
biological_processes:
- preferred_term: Neuromuscular synaptic transmission
term:
id: GO:0007274
label: neuromuscular synaptic transmission
modifier: DECREASED
evidence:
- reference: PMID:23292760
reference_title: "Congenital myasthenic syndrome due to choline acetyltransferase mutations in infants: clinical suspicion and comprehensive electrophysiological assessment are important for early diagnosis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Electromyography showed myogenic signs, and basal repetitive nerve stimulation yielded negative findings. ... Prolonged subtetanic repetitive nerve stimulation disclosed a marked decremental response compatible with suspected congenital myasthenic syndrome with episodic apnea."
explanation: Demonstrates that basal RNS can be falsely negative in CMS6 and that the use-dependent transmission defect requires prolonged high-frequency stimulation to unmask.
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Electromyographic studies in the five patients between crises revealed no decremental response on 2-Hz stimulation of motor nerves, but such a response appeared after a conditioning train of 10-Hz stimuli for 5 min."
explanation: Establishes the classic CMS6 electrodiagnostic pattern of a normal 2 Hz response with a decrement unmasked only by prolonged 10 Hz conditioning stimulation.
downstream:
- target: Sudden Episodic Apnea
causal_link_type: DIRECT
description: >-
Acute, severe demand-driven failure of ACh resynthesis at the
diaphragm and respiratory accessory muscle endplates during fever,
infection, excitement, or exertion produces abrupt, sometimes fatal,
respiratory failure.
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "If the infant survives, the symptoms improve, but the crises recur abruptly with infections, fever, excitement, vomiting, or overexertion. During crises, episodes of apnea can cause sudden death or anoxic brain injury."
explanation: Establishes fever/infection/excitement/exertion as the precipitants of the life-threatening apneic crises characteristic of CMS6.
- target: Fatigable Muscle Weakness
causal_link_type: DIRECT
description: >-
Between crises, the reduced safety factor of transmission produces
exertion-induced fatigable weakness of ocular, bulbar, and limb
muscles.
phenotypes:
- category: Respiratory
name: Sudden Episodic Apnea
diagnostic: true
description: >-
The hallmark and namesake manifestation of CMS6: sudden, recurrent
episodes of apnea and respiratory failure precipitated by fever,
infection, excitement, vomiting, or overexertion, which can be fatal or
cause anoxic brain injury; between crises, weakness may be mild or
absent.
phenotype_term:
preferred_term: Sudden episodic apnea
term:
id: HP:0002882
label: Sudden episodic apnea
temporality: RECURRENT
frequency: VERY_FREQUENT
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "we report that mutations in CHAT cause a congenital myasthenic syndrome associated with frequently fatal episodes of apnea (CMS-EA)"
explanation: Establishes recurrent, frequently fatal apneic episodes as the defining phenotype of CHAT-related CMS6.
- reference: PMID:19520274
reference_title: "Congenital myasthenic syndrome with episodic apnea."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "We present an infant with nine severe episodes of apnea in her first 6 months who underwent a prolonged evaluation before ptosis was evident, leading to a diagnosis of choline acetyltransferase deficiency"
explanation: Documents the recurrent apneic crises (nine episodes in six months) that can precede recognition of other myasthenic signs.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Apneic crises 86% (36/42) 0/5"
explanation: Quantifies apneic crises in a literature-aggregated cohort of 42 previously published CHAT-CMS cases (contrasted with 0/5 in the authors' own mild non-apneic cohort).
- category: Neurologic
name: Fatigable Muscle Weakness
diagnostic: true
description: >-
Fluctuating, exertion-induced fatigable weakness, frequently precipitated
by exercise, and in some patients by exposure to cold water.
phenotype_term:
preferred_term: Fatigable weakness
term:
id: HP:0003473
label: Fatigable weakness
frequency: VERY_FREQUENT
evidence:
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "had a history of mild to moderate symptoms consisting of fatigable muscle weakness and repeated falls precipitated by exercise"
explanation: Documents exercise-induced fatigable weakness in genetically confirmed CMS6 patients.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Fatigable leg muscle weakness 96% (27/28) 5/5"
explanation: >-
Quantifies fatigable muscle weakness specifically, from the
course-of-disease block of Table 1 in a literature-aggregated cohort of
previously published CHAT-CMS cases (27/28) plus all five current cases.
96% maps to VERY_FREQUENT.
- category: Neurologic
name: Hypotonia
description: >-
Severe hypotonia from birth is common in the more severely affected
infantile presentations of CMS6.
phenotype_term:
preferred_term: Hypotonia
term:
id: HP:0001252
label: Hypotonia
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "One form of presynaptic CMS presents at birth or in the neonatal period with hypotonia, variable eyelid ptosis, severe bulbar weakness causing dysphagia, and respiratory insufficiency with cyanosis and apnea."
explanation: Establishes hypotonia as part of the severe neonatal presentation of CMS6.
- category: HEENT
name: Ptosis
frequency: VERY_FREQUENT
phenotype_term:
preferred_term: Ptosis
term:
id: HP:0000508
label: Ptosis
evidence:
- reference: PMID:19520274
reference_title: "Congenital myasthenic syndrome with episodic apnea."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "At six months, bilateral ptosis was noted. Her mother reported that this had been present for three weeks, worse at the end of the day or with fatigue."
explanation: Documents fatigable ptosis, which in this case emerged only after recurrent apneic episodes and prompted the diagnosis.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Ptosis 88% (36/41) 0/5"
explanation: Quantifies ptosis frequency in a literature-aggregated cohort of previously published CHAT-CMS cases (absent in the authors' own mild non-apneic cohort).
- category: HEENT
name: Ophthalmoplegia
description: >-
Impaired or absent extraocular movements, ranging from mild fatigable
diplopia to complete external ophthalmoplegia in severely affected
patients.
phenotype_term:
preferred_term: Ophthalmoplegia
term:
id: HP:0000602
label: Ophthalmoplegia
frequency: OCCASIONAL
evidence:
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The examination showed ptosis, absent external ocular movements, severe facial and limb weakness and intact reflexes."
explanation: Documents complete external ophthalmoplegia in a severely affected, homozygous CMS6 patient.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Strabismus or ophtalmoparesis 23% (9/39) 0/5"
explanation: Quantifies strabismus/ophthalmoparesis frequency in a literature-aggregated cohort of previously published CHAT-CMS cases.
- category: Neurologic
name: Bulbar Weakness
frequency: OCCASIONAL
phenotype_term:
preferred_term: Bulbar palsy
term:
id: HP:0001283
label: Bulbar palsy
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "severe bulbar weakness causing dysphagia, and respiratory insufficiency with cyanosis and apnea"
explanation: Documents severe bulbar weakness with dysphagia as part of the neonatal CMS6 presentation.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Bulbar weakness 28% (12/42) 0/5"
explanation: Quantifies bulbar weakness frequency in a literature-aggregated cohort of previously published CHAT-CMS cases.
- category: Respiratory
name: Respiratory Insufficiency
description: >-
Severe cases require prolonged mechanical ventilation and tracheotomy,
not only during discrete apneic crises but as chronic respiratory
support.
phenotype_term:
preferred_term: Respiratory insufficiency due to muscle weakness
term:
id: HP:0002747
label: Respiratory insufficiency due to muscle weakness
evidence:
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The patient required a gastric tube, tracheotomy and mechanical ventilation. She was nonverbal and had profound developmental delay."
explanation: Documents chronic ventilatory dependence in a severely affected, homozygous CMS6 patient (p.Arg207His).
- category: Neurologic
name: Global Developmental Delay
description: >-
Developmental delay is reported in severely affected patients, and has
been specifically linked to hypoxic-ischemic brain injury sustained
during recurrent apneic crises.
phenotype_term:
preferred_term: Global developmental delay
term:
id: HP:0001263
label: Global developmental delay
frequency: FREQUENT
evidence:
- reference: PMID:15701560
reference_title: "Clinical variability of CMS-EA (congenital myasthenic syndrome with episodic apnea) due to identical CHAT mutations in two infants."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "the male patient (patient 1) experienced recurrent and severe episodes of apnea despite adequate treatment with AChE-inhibitors and shows a global developmental delay with delayed myelination and signs of hypoxic-ischemic injury in brain imaging"
explanation: Directly links recurrent apneic crises to secondary hypoxic-ischemic brain injury and global developmental delay, illustrating that the same identical CHAT genotype can produce markedly different severity between patients.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Psychomotor delay 57% (21/37) 0/5"
explanation: Quantifies psychomotor/developmental delay frequency in a literature-aggregated cohort of previously published CHAT-CMS cases (absent in the authors' own mild non-apneic cohort).
- category: Diagnostic
name: Decremental RNS Response After Prolonged Stimulation
diagnostic: true
description: >-
Repetitive nerve stimulation (RNS) at rest/low frequency (2-3 Hz) is
often normal in CMS6; the characteristic decremental compound muscle
action potential (CMAP) response only appears after a conditioning train
of high-frequency (10 Hz) stimulation for several minutes, reflecting the
use-dependent nature of the ACh resynthesis defect. Basal electrodiagnostic
testing can therefore be falsely reassuring, especially in neonates.
phenotype_term:
preferred_term: Decremental CMAP response to repetitive nerve stimulation
term:
id: HP:0003403
label: "EMG: decremental response of compound muscle action potential to repetitive nerve stimulation"
evidence:
- reference: PMID:23292760
reference_title: "Congenital myasthenic syndrome due to choline acetyltransferase mutations in infants: clinical suspicion and comprehensive electrophysiological assessment are important for early diagnosis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "basal repetitive nerve stimulation yielded negative findings. ... Prolonged subtetanic repetitive nerve stimulation disclosed a marked decremental response compatible with suspected congenital myasthenic syndrome with episodic apnea."
explanation: Documents the diagnostically critical pattern of a normal basal RNS with a decrement unmasked only by prolonged high-frequency conditioning stimulation.
- category: Musculoskeletal
name: Proximal Muscle Weakness
frequency: FREQUENT
phenotype_term:
preferred_term: Proximal muscle weakness
term:
id: HP:0003701
label: Proximal muscle weakness
evidence:
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Proximal muscle weakness 67% (26/39) 0/5"
explanation: Quantifies proximal muscle weakness frequency in a literature-aggregated cohort of previously published CHAT-CMS cases.
- category: Neurologic
name: Exercise Intolerance
description: >-
In the recently recognized mild, childhood-onset CMS6 phenotype,
exercise-induced leg fatigability can be the sole presenting and only
manifestation for years, without apnea, ptosis, or ophthalmoparesis, and
without progression on long-term follow-up. A modified repetitive nerve
stimulation protocol (recording from tibialis anterior after 15-20 minutes
of exercise) can unmask the decrement in these patients when standard
testing is uninformative.
phenotype_term:
preferred_term: Exercise intolerance
term:
id: HP:0003546
label: Exercise intolerance
evidence:
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "we describe five individuals with exercise intolerance caused by single nucleotide variants in the CHAT gene"
explanation: Directly documents exercise intolerance as the presenting manifestation in five genetically confirmed CHAT-CMS patients.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The age of onset ranged from 1 to 2.5 years, and all patients exhibited a fluctuating course of congenital myasthenic syndrome without disease progression over several years."
explanation: Establishes the later childhood onset and non-progressive, fluctuating course of the mild CMS6 phenotype.
- category: Neurologic
name: Cold-Induced Muscle Weakness
description: >-
Symptom worsening on exposure to cold is reported in a subset of CMS6
patients and, among CMS subtypes, appears relatively distinctive to
CHAT-CMS, making it a useful clinical diagnostic clue.
phenotype_term:
preferred_term: Cold paresis
term:
id: HP:0031372
label: Cold paresis
evidence:
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The presence of this symptom is unique to CHAT-associated CMS when compared to other forms of CMS and can serve as a valuable clinical clue for diagnosis"
explanation: Establishes cold-induced symptom worsening as a diagnostically useful feature relatively distinctive to CHAT-CMS among congenital myasthenic syndromes.
genetic:
- name: CHAT
gene_term:
preferred_term: CHAT
term:
id: hgnc:1912
label: CHAT
association: Causal
relationship_type: CAUSATIVE
notes: >-
CHAT (10q11.2, in the cholinergic gene locus that also harbors SLC18A3
within its first intron) encodes choline acetyltransferase (ChAT), the
sole enzyme identified to date whose deficiency causes congenital ACh
synthesis defects. Reported CMS6 alleles include frameshift/nonsense
null variants and missense variants scattered throughout the coding
region with no apparent mutational hotspot; missense substitutions
predominantly reduce affinity for the acetyl-CoA and/or choline
substrates and/or reduce protein expression/stability. Phenotypic
severity correlates with the degree of residual catalytic
efficiency/expression, but can vary even between patients or siblings
sharing an identical genotype.
evidence:
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Choline acetyltransferase (ChAT; EC ) catalyzes the reversible synthesis of acetylcholine (ACh) from acetyl CoA and choline at cholinergic synapses. ... Here we report that mutations in CHAT cause a congenital myasthenic syndrome associated with frequently fatal episodes of apnea (CMS-EA)."
explanation: Founding report identifying CHAT as the CMS6-causing gene.
- reference: PMID:19520274
reference_title: "Congenital myasthenic syndrome with episodic apnea."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The mutations identified to date, including the current ones, have mostly been scattered throughout the gene, with no apparent \"hotspots\"."
explanation: Documents the lack of a CHAT mutational hotspot across reported CMS6 alleles.
- reference: PMID:15701560
reference_title: "Clinical variability of CMS-EA (congenital myasthenic syndrome with episodic apnea) due to identical CHAT mutations in two infants."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "sequencing of the CHAT gene revealed identical, compound heterozygous mutations S694C and T354M in both children. These findings are in line with a remarkable clinical heterogeneity observed in patients with CHAT mutations"
explanation: Documents marked clinical variability despite an identical CHAT genotype, indicating additional modifying factors beyond the primary CHAT lesion.
treatments:
- name: Acetylcholinesterase Inhibitor Therapy
description: >-
Pyridostigmine is the primary and first-line therapy for CMS6: by
prolonging ACh action at the endplate, it compensates for reduced ACh
availability and can prevent or mitigate apneic crises and stabilize
respiratory status, though response can be partial in severely affected
patients.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: acetylcholinesterase inhibitor therapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: pyridostigmine
term:
id: CHEBI:8665
label: Pyridostigmine
evidence:
- reference: PMID:19520274
reference_title: "Congenital myasthenic syndrome with episodic apnea."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Pyridostigmine is the primary therapy for choline acetyltransferase deficiency, but the efficacy of midazolam during this patient's episodes of apnea is interesting, and warrants further study."
explanation: Establishes pyridostigmine as the primary/first-line therapy for CMS6.
- reference: PMID:11172068
reference_title: "Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The crises can be prevented or mitigated by anticholinesterase drugs."
explanation: Documents that anticholinesterase (acetylcholinesterase inhibitor) therapy can prevent or mitigate the apneic crises characteristic of CMS6.
- name: Amifampridine (3,4-Diaminopyridine) Adjunct Therapy
description: >-
In some severely affected patients with partial response to
cholinesterase inhibitors alone, amifampridine (3,4-diaminopyridine), a
potassium-channel blocker that enhances presynaptic ACh release, is added
as adjunct therapy.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: amifampridine
term:
id: CHEBI:135948
label: amifampridine
evidence:
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Patient partially responded to combined treatment with an anticholinesterase drug and DAP."
explanation: Documents amifampridine (3,4-DAP) as adjunct therapy alongside an acetylcholinesterase inhibitor in a severely affected CMS6 patient.
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Treatment with 3,4-diaminopyridine showed clear improvement in two children, while one patient experienced severe adverse effects and is currently receiving a combination of Salbutamol Syrup and pyridostigmine with slight positive effects."
explanation: Documents both clear benefit in some patients and a severe adverse reaction in another, indicating a mixed and individually variable response to amifampridine in CMS6.
- name: Beta-2 Adrenergic Agonist Adjunct Therapy (Salbutamol)
description: >-
Salbutamol (albuterol), typically combined with an acetylcholinesterase
inhibitor, has been used as an adjunct in CMS6 patients with an
inadequate response to acetylcholinesterase inhibitors or amifampridine
alone, with modest benefit reported.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: salbutamol
term:
id: CHEBI:2549
label: albuterol
evidence:
- reference: PMID:38304750
reference_title: "Mild phenotype of CHAT-associated congenital myasthenic syndrome: case series."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Treatment with 3,4-diaminopyridine showed clear improvement in two children, while one patient experienced severe adverse effects and is currently receiving a combination of Salbutamol Syrup and pyridostigmine with slight positive effects."
explanation: Documents salbutamol combined with pyridostigmine as an adjunct option after a severe amifampridine adverse reaction, with only slight benefit in this patient.
- name: Mechanical Ventilatory Support
description: >-
Acute apneic crises frequently require emergency intubation and
mechanical ventilation; the most severely affected patients require
tracheotomy and chronic ventilatory support.
therapeutic_modality: DEVICE
treatment_term:
preferred_term: invasive ventilation
term:
id: NCIT:C191573
label: Invasive Mechanical Ventilation
evidence:
- reference: PMID:26080897
reference_title: "Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The patient had developmental delay and required a gastric tube, tracheotomy and mechanical ventilation."
explanation: Documents tracheotomy and chronic mechanical ventilation as management for the most severely affected CMS6 patients.
- name: Genetic Counseling
description: >-
Counseling for affected individuals and families, given autosomal
recessive inheritance with a 25% recurrence risk for future pregnancies
of carrier parents.
treatment_term:
preferred_term: Genetic Counseling
term:
id: NCIT:C15240
label: Genetic Counseling
references:
- reference: PMID:20301347
title: "Congenital Myasthenic Syndromes Overview."
tags:
- GeneReviews
datasets: []
Scope and nomenclature note. In current disease-specific literature, congenital myasthenic syndrome 6 (CMS6) means CHAT-associated congenital myasthenic syndrome with episodic apnea (CMS-EA). It should not be confused with COLQ-related endplate acetylcholinesterase deficiency, which has a different mechanism and treatment profile. The evidence base is exceptionally small—approximately 50 patients had been reported by 2024—and consists predominantly of families, case reports, and small case series rather than controlled trials. The findings below therefore describe reported cases, not population-level certainties. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2)
The following table provides a knowledge-base-ready summary.
| domain | curated finding | quantitative evidence | suggested ontology terms |
|---|---|---|---|
| Identity / identifiers | Congenital myasthenic syndrome 6 (CMS6) corresponds to CHAT-associated congenital myasthenic syndrome with episodic apnea (CMS-EA); recent disease-specific literature explicitly states “CMS-EA, also known as CMS type 6.” MONDO for the exact subtype was not confirmed from available evidence; broader congenital myasthenic syndrome is MONDO_0018940. MeSH for the broader class: Myasthenic Syndromes, Congenital (D020294). (murtazina2024mildphenotypeof pages 1-2, NCT01203592 chunk 1, OpenTargets Search: congenital myasthenic syndrome-CHAT) | ~50 patients reported by 2024 (murtazina2024mildphenotypeof pages 1-2) | MONDO: congenital myasthenic syndrome (MONDO_0018940, broader); MeSH: D020294; disease synonym candidates: “CMS-EA”, “CHAT-associated CMS” |
| Gene / inheritance | Cause is biallelic pathogenic variants in CHAT encoding choline acetyltransferase; disorder is a presynaptic CMS and is autosomal recessive. Most known variants are missense. (murtazina2024mildphenotypeof pages 1-2, pugliese2023presynapticcongenitalmyasthenic pages 7-9, OpenTargets Search: congenital myasthenic syndrome-CHAT) | CHAT accounts for ~4–5% of all CMS cases (pugliese2023presynapticcongenitalmyasthenic pages 7-9) | HGNC gene: CHAT; GO: acetylcholine biosynthetic process (suggested), chemical synaptic transmission (suggested) |
| Molecular mechanism | ChAT catalyzes resynthesis of acetylcholine from choline and acetyl-CoA in the nerve terminal; impaired ChAT reduces presynaptic ACh resynthesis, causing failure of sustained neuromuscular transmission, especially under repetitive activity or stress. (ohno2023clinicalandpathologic pages 4-6, pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2) | Qualitative; no disease-wide effect size available | GO: acetylcholine biosynthetic process; GO: synaptic vesicle cycle; GO: neuromuscular synaptic transmission; CL: motor neuron; UBERON: neuromuscular junction |
| Major phenotype: apneic crises / respiratory involvement | Classical CMS6 commonly presents in infancy with apnea, respiratory insufficiency, and episodic respiratory crises; crises may be severe or fatal. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2) | Literature summary in CHAT cohort: apneic crises 86% (36/42), ventilation 63% (26/41), tracheostomy 25% (9/36), respiratory insufficiency 48% (20/42), apnea 28% (12/42) (murtazina2024mildphenotypeof pages 5-6) | HPO: Apnea; Respiratory insufficiency; Recurrent respiratory infections/crises (suggested) |
| Major phenotype: ocular / bulbar | Ocular findings and bulbar weakness are common in typical disease, though absent in the 2024 mild series. (murtazina2024mildphenotypeof pages 5-6, murtazina2024mildphenotypeof pages 1-2) | Ptosis 88% (36/41), strabismus/ophthalmoparesis 23% (9/39), bulbar weakness 28% (12/42) (murtazina2024mildphenotypeof pages 5-6) | HPO: Ptosis; Ophthalmoparesis; Strabismus; Dysphagia; Dysarthria |
| Major phenotype: limb weakness / fatigability | Generalized, proximal, or exercise-induced fatigable weakness is typical; a mild phenotype may present mainly with exercise intolerance and leg fatigability without apnea or ocular signs. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2, murtazina2024mildphenotypeof pages 5-6) | General muscle weakness 45% (17/38), proximal weakness 67% (26/39), fatigable leg weakness 96% (27/28); in 2024 mild series 5/5 had leg fatigability and 0/5 had apnea or ptosis (murtazina2024mildphenotypeof pages 5-6, murtazina2024mildphenotypeof pages 1-2) | HPO: Muscle weakness; Proximal muscle weakness; Exercise intolerance; Fatigability |
| Neurodevelopment / CNS features | Some patients, especially severe early-onset cases, may have delayed motor milestones, intellectual disability, loss of consciousness, or seizures; recent mild cases had normal neurologic status aside from fatigability. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2) | Psychomotor delay 57% (21/37) in summarized literature; 0/5 in 2024 mild cohort (murtazina2024mildphenotypeof pages 5-6) | HPO: Global developmental delay; Seizure; Loss of consciousness |
| Temporal development / course | Typical onset is birth or early infancy for severe disease; mild phenotype can begin at 1–2.5 years. Course is often fluctuating; the 2024 mild series showed no progression over several years. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2, murtazina2024mildphenotypeof pages 5-6) | Mild cohort onset 1–2.5 years, 5/5 fluctuating without progression (murtazina2024mildphenotypeof pages 1-2, murtazina2024mildphenotypeof pages 5-6) | HPO: Infantile onset; Childhood onset; Fluctuating weakness |
| Triggers / gene-environment interaction | Infection, fever, and other stressful conditions can provoke sudden respiratory crises in infancy; cold may aggravate weakness; prolonged exertion unmasks fatigability. These are triggers/modifiers, not primary causes. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6) | Qualitative; no pooled effect estimates available | HPO: Cold-induced myasthenic symptoms (suggested); Exercise-induced weakness |
| Pathogenic variants | 2024 mild series identified four novel missense SNVs and one recurrent severe-associated missense variant; example variants include c.404C>G p.(Pro135Arg) (likely pathogenic) and c.1061C>T p.(Thr354Met). Three novel SNVs were VUS at publication. (murtazina2024mildphenotypeof pages 5-6) | 5 patients from 4 families; 4 novel missense SNVs; 1 recurrent severe-associated missense variant (murtazina2024mildphenotypeof pages 5-6) | Sequence Ontology: missense_variant; nonsense_variant (reported in one family context) |
| Diagnostics: electrophysiology | Standard low-frequency RNS may be normal; decrement often appears after prolonged high-frequency stimulation or exercise. In the pediatric mild series, modified 3 Hz peroneal RNS after 15–20 min exercise was informative. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6, NCT01203592 chunk 1) | Standard 3 Hz decrement >10% in only a small percentage; prolonged 10 Hz for 5 min positive in 9/10; modified post-exercise 3 Hz showed 22–37% decrement improving after 3–4 min rest; in literature 44% (14/32) had increased decrement at 3 Hz after 20 s exercise (murtazina2024mildphenotypeof pages 5-6) | LOINC/SNOMED not confirmed; HPO: Abnormal repetitive nerve stimulation; MAXO: Electromyography |
| Diagnostics: genetic testing | Accurate diagnosis requires genetic testing because clinical/electrophysiologic features alone do not identify the defective molecule. Exome/genome sequencing captured CHAT variants in recent series; trial inclusion criteria for CMS required seronegativity to AChR and MuSK plus decremental EMG. (OpenTargets Search: congenital myasthenic syndrome-CHAT, murtazina2024mildphenotypeof pages 1-2, NCT01203592 chunk 1) | WES mean coverage ×76.1 in 2024 series; WGS average on-target coverage 30× (murtazina2024mildphenotypeof pages 1-2) | MAXO: Sequence analysis of gene panel / exome / genome (suggested); MeSH: Myasthenic Syndromes, Congenital |
| Differential diagnosis / distinction | Important distinction: CMS6 is CHAT-related, not COLQ-related endplate AChE deficiency. COLQ-CMS is the subtype where AChE inhibitors are usually contraindicated and β-adrenergic agonists often first-line; that treatment rule should not be misapplied to CMS6. (murtazina2024mildphenotypeof pages 1-2, ohno2023clinicalandpathologic pages 4-6) | Not quantitative | Differential terms: COLQ-related congenital myasthenic syndrome; autoimmune myasthenia gravis |
| Treatment | First-line therapy is usually acetylcholinesterase inhibitors; if response is limited/absent, 3,4-diaminopyridine may be added, and some patients may receive salbutamol/albuterol combinations. Evidence is mainly case based. (murtazina2024mildphenotypeof pages 5-6, pugliese2023presynapticcongenitalmyasthenic pages 7-9) | Response to AChE inhibitors ~73% (27/37); response to 3,4-DAP 71% (5/7); in 2024 mild series, 2 children improved clearly on 3,4-DAP, 1 had only slight benefit on salbutamol+pyridostigmine, and 1 had severe adverse effects to 3,4-DAP (murtazina2024mildphenotypeof pages 5-6, murtazina2024mildphenotypeof pages 1-2) | CHEBI/drugs: pyridostigmine, amifampridine/3,4-diaminopyridine, salbutamol/albuterol; MAXO: Acetylcholinesterase inhibitor therapy; Adrenergic agonist therapy; Ventilatory support |
| Clinical trials / real-world implementation | Broad CMS interventional studies exist for albuterol and ephedrine, but they were not specific to CHAT/CMS6; ephedrine trial targeted COLQ-deficient kindred. Real-world use in CMS is genotype-guided. (NCT01203592 chunk 1, NCT00541216 chunk 1, ohno2023clinicalandpathologic pages 4-6) | Albuterol study NCT01203592 enrolled 21; ephedrine study NCT00541216 planned 15 and was COLQ-specific (NCT01203592 chunk 1, NCT00541216 chunk 1) | ClinicalTrials.gov: NCT01203592; NCT00541216 |
| Prognosis / outcomes | Prognosis is variable. Severe neonatal/infantile disease can be life-threatening, with fatal infancy cases reported; however, a mild non-apneic phenotype appears to have favorable medium-term outcome without progression. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6) | Wheelchair dependency 25% (7/28) in summarized literature; 0/5 in 2024 mild cohort (murtazina2024mildphenotypeof pages 5-6) | HPO: Respiratory failure; Wheelchair dependence; Favorable prognosis (not an HPO term; narrative only) |
| Epidemiology / population | CMS overall is rare and likely underdiagnosed; pediatric prevalence estimates for all CMS vary by region. Subtype-specific prevalence for CMS6 was not available from retrieved sources. (ohno2023clinicalandpathologic pages 4-6, murtazina2024mildphenotypeof pages 1-2) | CMS prevalence under age 18: UK average 9.2/million (range 2.8–14.8), Brazil 1.8/million, Slovenia 22.2/million, Spain 1.8/million (ohno2023clinicalandpathologic pages 4-6) | MONDO: congenital myasthenic syndrome (broader); note: CMS6-specific prevalence unavailable |
| Anatomy / cell types | Primary site is the presynaptic motor nerve terminal at the neuromuscular junction of skeletal muscle; clinically affects ocular, bulbar, respiratory, axial, and limb muscles. Central nervous system expression of CHAT may help explain occasional CNS manifestations. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2, ohno2023clinicalandpathologic pages 4-6) | Qualitative | UBERON: neuromuscular junction; skeletal muscle; diaphragm; extraocular muscle; CL: motor neuron; skeletal muscle fiber |
| Model organisms / natural disease | Models include Chat-targeted mouse knockout (heterozygotes normal; homozygous pups die at birth with flaccid paralysis and absent spontaneous/nerve-evoked postsynaptic potentials), zebrafish bajan and chatatk64 mutants (reduced movement and synaptic responses), and a naturally occurring Old Danish Pointing Dog CHAT mutation causing exercise-induced fatigability. A 2024 AAV9 CHAT mouse gene therapy study was identified as emerging preclinical work, but detailed results were not available in retrieved text. (pugliese2023presynapticcongenitalmyasthenic pages 7-9) | Mouse homozygotes die at birth; zebrafish homozygotes show absent coiling and reduced touch response; affected dogs tolerate only ~5–30 min walking/running before weakness (pugliese2023presynapticcongenitalmyasthenic pages 7-9) | NCBI Taxon suggestions: Mus musculus, Danio rerio, Canis lupus familiaris; GO: neuromuscular synaptic transmission |
| Data provenance / evidence level | Evidence is derived from aggregated disease-level resources, recent reviews, case series/case reports, and trial registry entries, not EHR-only datasets. Many subtype-specific claims remain based on small case numbers. (OpenTargets Search: congenital myasthenic syndrome-CHAT, murtazina2024mildphenotypeof pages 1-2, NCT01203592 chunk 1, NCT00541216 chunk 1) | Highest disease-specific 2024 cohort size in retrieved evidence: n=5; historical total ~50 cases reported (murtazina2024mildphenotypeof pages 1-2) | Evidence type labels: human clinical, review, model organism, trial registry |
Table: This table summarizes high-value, citable knowledge-base facts for Congenital Myasthenic Syndrome 6, resolved as CHAT-associated CMS with episodic apnea. It emphasizes disease identity, genotype-mechanism links, quantitative phenotypes, diagnostics, treatment, and model systems while clearly marking broader-class versus subtype-specific evidence.
CMS6 is an inherited presynaptic neuromuscular-junction disorder caused by biallelic pathogenic variants in CHAT, the gene encoding choline O-acetyltransferase (ChAT). Deficient acetylcholine resynthesis impairs sustained neuromuscular transmission. The classical phenotype comprises neonatal or infantile fatigable weakness with recurrent, potentially fatal apneic crises, although a non-apneic, childhood-onset phenotype is now well established. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2)
Names and identifiers
The evidence is principally aggregated disease-level literature plus individual-patient case series, not an EHR-derived cohort. The 2024 study used WES/WGS and detailed phenotyping of five individuals from four families. (murtazina2024mildphenotypeof pages 1-2)
CMS6 is a monogenic, autosomal-recessive disorder caused by germline biallelic CHAT variants. Most reported variants are missense, although truncating and splice-altering alleles may occur. It is not caused by autoantibodies, infection, toxin exposure, diet, or lifestyle. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2)
The principal risk is inheritance of two pathogenic or likely pathogenic CHAT alleles. Siblings of an affected person have, under standard autosomal-recessive assumptions, a 25% recurrence probability when both parents are heterozygous carriers. Consanguinity can increase the probability of homozygosity, but no CMS6-specific estimate was retrieved. Sex does not determine inheritance.
No validated modifier gene, protective allele, polygenic risk score, penetrance estimate, or CMS6-specific carrier frequency was found. Clinical variability—including mild and severe disease associated with the recurrent c.1061C>T, p.(Thr354Met) allele—indicates that genotype alone does not fully predict severity. (murtazina2024mildphenotypeof pages 5-6)
Infection, fever, and other physiologic stress can precipitate acute respiratory or bulbar crises; cold may exacerbate weakness, and prolonged exercise can unmask a neuromuscular decrement. These are phenotypic triggers, not causes. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6)
No established diet, exercise regimen, toxin avoidance measure, medication, or genetic factor prevents disease occurrence. Practical protection is instead directed toward preventing crises: prompt infection treatment, avoidance of unmonitored respiratory depressants or neuromuscular blockers, an emergency respiratory plan, and adherence to effective symptomatic therapy.
Reported frequencies below derive from a literature aggregation of approximately 42 patients and have variable denominators; ascertainment was biased toward severe CMS-EA. (murtazina2024mildphenotypeof pages 5-6)
The 2024 case series described five patients whose onset was at 1–2.5 years and whose sole or initial manifestation was exercise-induced leg fatigability. All five lacked apnea, respiratory insufficiency, ptosis, and ophthalmoparesis and showed a fluctuating but non-progressive course over several years. The authors’ abstract states: “we propose the existence of a mild phenotype characterized by the absence of apneic episodes.” (murtazina2024mildphenotypeof pages 1-2)
No CMS6-specific EQ-5D, SF-36, PROMIS, or utility-weight study was found. Quality-of-life impairment should therefore be represented through respiratory risk, activity limitation, schooling/work effects, treatment burden, and possible ventilatory or wheelchair dependence rather than an unsupported numerical score.
Causal gene: CHAT, chromosome 10q11.23; protein: choline O-acetyltransferase. Open Targets identifies CHAT as ENSG00000070748 and associates it with the broader congenital myasthenic syndrome category. (OpenTargets Search: congenital myasthenic syndrome-CHAT)
The 2024 series identified four novel missense SNVs. c.404C>G, p.(Pro135Arg) was classified as likely pathogenic because another missense substitution at the same residue had been reported; three other novel missense variants remained VUS pending stronger functional evidence. The recurrent c.1061C>T, p.(Thr354Met) variant had previously been associated with infantile onset and apneic crises, yet also occurred in milder families, underscoring incomplete genotype–phenotype predictability. (murtazina2024mildphenotypeof pages 5-6)
Variants are constitutional/germline, not somatic cancer mutations. The expected functional category is loss or reduction of ChAT expression, stability, catalytic activity, or substrate kinetics, producing inadequate acetylcholine resynthesis under sustained demand. Population allele frequencies were not available in the retrieved text and must be obtained variant-by-variant from the current gnomAD release. Likewise, ClinVar classifications should be recorded per accession and review status rather than generalized from publication assertions.
No reproducible modifier genes, disease-specific methylation signature, chromatin abnormality, aneuploidy, translocation, or recurrent pathogenic copy-number alteration has been established. A chromosomal microarray cannot exclude CMS6, although exon-level CNV analysis remains appropriate when sequencing finds only one pathogenic allele.
There is no environmental, occupational, lifestyle, toxic, or infectious etiology. Infection and fever are clinically important crisis triggers, cold may aggravate fatigability, and exertion reveals limited presynaptic acetylcholine reserve. Smoking, alcohol, diet, pollution, and radiation have no demonstrated causal role. Pathogens are therefore not disease agents, and the disorder is neither communicable nor zoonotic. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6)
The 2023 comprehensive review describes the normal step directly: “Choline acetyltransferase (ChAT, CHAT) in the nerve terminal generates ACh from up taken choline and acetyl-CoA.” (ohno2023clinicalandpathologic pages 4-6)
This is primarily a neurotransmitter-biosynthesis and synaptic-transmission disorder, not an inflammatory myopathy, autoimmune disease, primary mitochondrial disorder, degenerative motor-neuron disease, or structural ACh-receptor channelopathy. Secondary muscle atrophy, developmental delay, or hypoxic injury may occur downstream.
Suggested annotations include GO acetylcholine biosynthetic process, neuromuscular synaptic transmission, chemical synaptic transmission, synaptic vesicle cycle, and regulation of neurotransmitter levels; GO cellular components presynaptic active zone, axon terminal, synaptic vesicle, and neuromuscular junction; CL motor neuron and skeletal muscle fiber.
No validated CMS6-specific transcriptomic, proteomic, metabolomic, lipidomic, epigenomic, single-cell, spatial-transcriptomic, or integrated multi-omics signature is available. ChAT activity and electrophysiologic transmission are mechanistically relevant but are not validated circulating biomarkers.
The primary lesion is at cholinergic motor-neuron presynaptic terminals of the neuromuscular junction. Functionally affected tissues include skeletal muscle innervated by somatic motor neurons—especially the diaphragm and other respiratory muscles, extraocular and levator muscles, bulbar/pharyngeal musculature, proximal limb muscles, and distal leg muscles. Laterality is generally bilateral rather than focal. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6)
Suggested UBERON concepts: neuromuscular junction, skeletal muscle organ, diaphragm, extraocular muscle, pharyngeal muscle, and limb muscle. Suggested GO cellular components are presynaptic membrane, axon terminus, synaptic vesicle, and cytosol, where ChAT catalysis occurs.
The classical severe form begins at birth or in early infancy, sometimes abruptly with apnea. The milder form may become evident in early childhood after sustained walking or exercise. Disease is chronic and lifelong, but manifestations fluctuate. Severe neonatal disease may cause fatal crises, whereas the five 2024 mild cases remained non-progressive for several years. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6, murtazina2024mildphenotypeof pages 1-2)
There is no accepted staging system. Clinically useful states are: baseline compensated weakness; exertional deterioration; bulbar/respiratory exacerbation; and acute apneic crisis requiring ventilation. Early diagnosis is a critical intervention window because respiratory support and genotype-appropriate medication may prevent hypoxic injury or death. True spontaneous molecular remission is not expected, although symptom-free intervals and treatment-induced functional improvement occur.
Inheritance is autosomal recessive, with variable expressivity. Penetrance for individuals with two definitively pathogenic alleles is presumed high but has not been quantified. Anticipation is not expected. No CHAT-specific germline-mosaicism rate, founder allele, sex ratio, ethnic enrichment, or carrier-frequency estimate was established by the retrieved evidence. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2)
CHAT-CMS represents approximately 4–5% of all CMS cases, but only about 50 CMS-EA patients had been reported by 2024. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 1-2)
Subtype-specific incidence and prevalence are unknown. For context—not as a CMS6 estimate—the pediatric prevalence of all CMS was 9.2 per million in the UK, with regional estimates of 2.8–14.8 per million; published estimates were 1.8 per million in Brazil, 22.2 per million in Slovenia, and 1.8 per million in Spain. Investigators considered these underestimates because patients remain undiagnosed. (ohno2023clinicalandpathologic pages 4-6)
Suspect CMS6 in neonatal/infantile episodic apnea, especially with ptosis, ophthalmoparesis, bulbar weakness, hypotonia, or fatigability; also consider it in children with isolated exertional leg weakness. Standard 3-Hz repetitive nerve stimulation may be falsely normal. Prolonged 10-Hz stimulation for five minutes was positive in 9/10 reported patients. In the 2024 mild cohort, 3-Hz peroneal stimulation after 15 minutes of exercise produced a 22–37% CMAP decrement, which improved after 3–4 minutes’ rest. (murtazina2024mildphenotypeof pages 5-6)
Single-fiber EMG can demonstrate increased jitter/blocking but may be difficult in children. Routine CK, imaging, muscle biopsy, and histopathology are not diagnostic and may be normal or nonspecific. Pulmonary-function testing, pulse oximetry, sleep/ventilation assessment, swallowing assessment, and ECG/cardiovascular review before sympathomimetic therapy are clinically useful.
A CMS/NMJ panel containing CHAT is efficient when the phenotype is recognizable. WES or WGS is appropriate when panel testing is negative, the presentation is atypical, or CNV/noncoding analysis is needed. The 2024 series used WES at mean ×76.1 coverage and WGS at approximately ×30 coverage. Pathogenicity assessment should incorporate phase, segregation, population frequency, computational evidence, RNA studies for splice variants, and functional ChAT assays where feasible. (murtazina2024mildphenotypeof pages 1-2)
CMA, karyotyping, FISH, mitochondrial testing, and repeat-expansion testing are not first-line for isolated CMS6. RNA sequencing may resolve splice variants but is not an established routine assay. Prenatal and preimplantation testing become possible after familial variants are established.
Major alternatives include other presynaptic CMS—especially SLC5A7 and SLC18A3 disorders—RAPSN-associated CMS, congenital AChR deficiency, DOK7-CMS, COLQ-CMS, congenital myopathies, spinal muscular atrophy, mitochondrial disease, infantile epilepsy or central apnea, botulism, and autoimmune myasthenia gravis. AChR/MuSK seronegativity supports congenital rather than autoimmune myasthenia but is not by itself diagnostic. The albuterol study required a typical history, AChR/MuSK seronegativity, and decremental EMG. (NCT01203592 chunk 1)
No reliable 5- or 10-year survival rate, life-expectancy estimate, or CMS6-specific mortality rate exists. Historical severe cases include death during infantile apneic crises; ventilation, tracheostomy, developmental disability, impaired mobility, and hypoxic complications drive morbidity. Conversely, mild non-apneic disease can remain stable for years with preserved routine neurologic function. (pugliese2023presynapticcongenitalmyasthenic pages 7-9, murtazina2024mildphenotypeof pages 5-6)
Likely adverse prognostic features are neonatal onset, recurrent apnea, need for early ventilation, severe bulbar weakness, delayed diagnosis, and poor drug response. Favorable features include isolated exertional fatigability, no respiratory events, and treatment responsiveness. These are clinical observations, not validated prediction-model variables or biomarkers.
Pyridostigmine, an acetylcholinesterase inhibitor, is generally first-line for CHAT-CMS because prolonging acetylcholine action can partly compensate for reduced synthesis. Historical response was reported in 27/37 patients (73%). If response is incomplete, amifampridine/3,4-diaminopyridine, which prolongs the presynaptic action potential and increases calcium-dependent acetylcholine release, may be added; 5/7 reported patients (71%) responded. (ohno2023clinicalandpathologic pages 4-6, murtazina2024mildphenotypeof pages 5-6)
In the 2024 mild series, two children improved clearly on 3,4-diaminopyridine, while one experienced severe adverse effects and later had slight benefit from salbutamol plus pyridostigmine. Treatment must therefore be initiated and titrated by a neuromuscular specialist, with cardiac and seizure-risk consideration for amifampridine and cardiovascular monitoring for β-agonists. (murtazina2024mildphenotypeof pages 1-2)
Important subtype distinction: acetylcholinesterase inhibitors are contraindicated or harmful in many COLQ-CMS patients, but this rule does not apply automatically to CHAT/CMS6. The recent CMS review explicitly reserves that contraindication for COLQ- and LAMB2-related disease. (ohno2023clinicalandpathologic pages 4-6)
Apneic crisis requires immediate airway support, bag-mask ventilation or mechanical ventilation as indicated, oxygenation and CO₂ monitoring, treatment of infection/fever, and avoidance of diagnostic delay. Families should have a written emergency plan and resuscitation training when recurrent apnea is a risk. Supportive care may include noninvasive ventilation, tracheostomy in refractory disease, feeding/swallowing support, physical and occupational therapy, and school/activity accommodations.
Suggested MAXO annotations include genetic testing, electromyography, acetylcholinesterase inhibitor therapy, potassium-channel blocker therapy, adrenergic agonist therapy, noninvasive positive-pressure ventilation, mechanical ventilation, tracheostomy, physical therapy, occupational therapy, and genetic counseling.
No controlled trial specific to CHAT-CMS6 was identified. NCT01203592, an open-label Phase 1 albuterol study, enrolled 21 heterogeneous CMS patients; it was not CHAT-specific. NCT00541216 tested ephedrine in a COLQ-deficient kindred and should not be represented as CMS6 evidence. (NCT01203592 chunk 1, NCT00541216 chunk 1)
A 2024 report of AAV9-mediated CHAT gene therapy in ChAT-deficient mice represents an important preclinical development, not an approved or clinically validated treatment. No human gene-, RNA-, or cell-therapy implementation for CMS6 was identified.
Primary prevention by lifestyle or vaccination is impossible because CMS6 is inherited. Reproductive prevention options after molecular diagnosis include carrier testing of relatives, partner testing, prenatal diagnosis, and preimplantation genetic testing. Population newborn screening is unavailable, but targeted neonatal testing is justified in an at-risk pregnancy or symptomatic neonate.
Secondary prevention comprises early molecular diagnosis, family cascade testing, respiratory monitoring, and early effective therapy. Tertiary prevention includes infection and fever management, crisis planning, vaccination according to routine schedules to reduce preventable respiratory illness, ventilatory/feeding support, rehabilitation, and avoidance of medications or anesthesia practices that may worsen neuromuscular transmission. Vaccines do not prevent the genetic disease itself.
A naturally occurring CHAT-associated myasthenic disorder occurs in the Old Danish Pointing Dog (Canis lupus familiaris, NCBI Taxonomy 9615). Affected dogs carry a CHAT valine-to-methionine substitution and develop fore- and hind-limb fatigability after only 5–30 minutes of walking or running; prolonged 3-Hz stimulation induces a decrement. This provides strong comparative evidence for conserved activity-dependent failure of cholinergic transmission. (pugliese2023presynapticcongenitalmyasthenic pages 7-9)
There is no zoonotic transmission. Veterinary breed and VBO identifiers should be obtained from live VBO/OMIA records before ingestion.
The most important 2023–2024 advance is recognition that “episodic apnea” is not obligatory: CHAT disease spans lethal neonatal apnea through isolated childhood exercise intolerance. The 2024 series also showed that exercise-conditioned RNS can reveal abnormalities missed by routine testing. These findings favor phenotype expansion, exertion-provoked electrophysiology, and broad genetic testing rather than exclusion of CHAT because apnea is absent. (murtazina2024mildphenotypeof pages 5-6, murtazina2024mildphenotypeof pages 1-2)
Major gaps are CMS6-specific prevalence, prospective natural history, standardized patient-reported outcomes, variant-level functional assays, validated prognostic markers, controlled comparative treatment trials, and human gene-therapy safety data. Expert reviews consequently emphasize genotype-guided treatment and molecular confirmation because clinical and electrophysiologic findings alone cannot reliably identify the affected CMS molecule. (ohno2023clinicalandpathologic pages 4-6)
Evidence-type labels: Murtazina et al. is human clinical case-series evidence; Pugliese et al. and Ohno et al. are authoritative reviews integrating human, in vitro, and model-organism studies; the NCT records are trial-registry evidence. Exact PMID values were not exposed in the retrieved records and are therefore not supplied rather than risked as unverified identifiers.
References
(pugliese2023presynapticcongenitalmyasthenic pages 7-9): Alessia Pugliese, Stephen H. Holland, Carmelo Rodolico, Hanns Lochmüller, and Sally Spendiff. Presynaptic congenital myasthenic syndromes: understanding clinical phenotypes through in vivo models. Journal of Neuromuscular Diseases, 10:731-759, Sep 2023. URL: https://doi.org/10.3233/jnd-221646, doi:10.3233/jnd-221646. This article has 22 citations and is from a peer-reviewed journal.
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(NCT01203592 chunk 1): Andrew Engel. Efficacy of Albuterol in the Treatment of Congenital Myasthenic Syndromes. Mayo Clinic. 2010. ClinicalTrials.gov Identifier: NCT01203592
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(NCT00541216 chunk 1): Ephedrine for the Treatment of Congenital Myasthenia. Hadassah Medical Organization. 2007. ClinicalTrials.gov Identifier: NCT00541216