Choline acetyltransferase mutations cause myasthenic syndrome associated with episodic apnea in humans

Abstract

Choline acetyltransferase (ChAT; EC 2.3.1.6 ) catalyzes the reversible synthesis of acetylcholine (ACh) from acetyl CoA and choline at cholinergic synapses. Mutations in genes encoding ChAT affecting motility exist in Caenorhabditis elegans and Drosophila , but no CHAT mutations have been observed in humans to date. Here we report that mutations in CHAT cause a congenital myasthenic syndrome associated with frequently fatal episodes of apnea (CMS-EA). 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. These findings point to a defect in ACh resynthesis or vesicular filling and to CHAT as one of the candidate genes. 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. 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.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

“…22 However, the most striking association of ChAT deficiency causing disease is seen in CMS with episodic apnoea (CMS-EA) 6 .…”

“…6 The course of disease seems to follow two distinct trends with either neonatal onset respiratory distress with progressive improvement over time but then further respiratory relapses in adulthood or late onset (during infancy or childhood) respiratory crises with a more unpredictable course of disease. [25][26] Treatment is with AChE inhibitor therapy in variable doses with or without addition of 3, 4-diaminopyridine (3, 4 DAP) 27 .…”

“…The CHAT gene encodes the choline acetyltransferase protein, which is located at the pre-synaptic nerve terminal and is responsible for acetylcholine synthesis from acetyl coenzyme A and choline 6 . Mutations in this gene have been shown to be associated with a reduction in ChAT synthesis or impairment in ChAT catalytic function.…”

“…The kinetic mutations fall into two categories according to whether they induce slow or fast channel syndromes [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] .…”

“…[4][5] To date 13 genes have been found to cause CMS when mutated. These are the pre-synaptic acetyltransferase CHAT 6 , the gene COLQ 7-8-9 encoding the triple stranded collagenic tail of the synaptic acetylcholinesterase (AChE), the genes encoding the different subunits of the postsynaptic acetylcholine receptors (AChR) CHRNA1, CHRNB1, CHRND, CHRNE and CHRNG, the RAPSN gene encoding the postsynaptic protein rapsyn 10 , the postsynaptic muscle specific kinase (MUSK) gene 11 , the postsynaptic sodium channel (SCN4) 12 , the DOK7 gene encoding the postsynaptic Dok-7 protein [13][14] , the LAMB2 encoding the synaptic Laminin B2 protein 15 , the AGRN gene encoding the postsynaptic agrin protein 16 and the PLEC1 gene encoding the postsynaptic protein plectin. 17-18-19 Genetic classification of the different CMS cohorts reveals that the majority of CMS patients have mutations in genes expressing postsynaptic endplate proteins [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] (table 1).…”