Human endplate acetylcholinesterase deficiency caused by mutations in the collagen-like tail subunit (ColQ) of the asymmetric enzyme

Abstract

In skeletal muscle, acetylcholinesterase (AChE) exists in homomeric globular forms of type T catalytic subunits (ACHE T ) and heteromeric asymmetric forms composed of 1, 2, or 3 tetrameric ACHE T attached to a collagenic tail (ColQ). Asymmetric AChE is concentrated at the endplate (EP), where its collagenic tail anchors it into the basal lamina. The ACHE T gene has been cloned in humans; COLQ cDNA has been cloned in Torpedo and rodents but not in humans. In a disabling congenital myasthenic syndrome, EP AChE deficiency (EAD), the normal asymmetric species of AChE are absent from muscle. EAD could stem from a defect that prevents binding of ColQ to ACHE T or the insertion of ColQ into the basal lamina. In six EAD patients, we found no mutations in ACHE T . We therefore cloned human COLQ cDNA, determined the genomic structure and chromosomal localization of COLQ, and then searched for mutations in this gene. We identified six recessive truncation mutations of COLQ in six patients. Coexpression of each COLQ mutant with wild-type ACHE T in SV40-transformed monkey kidney fibroblast (COS) cells reveals that a mutation proximal to the ColQ attachment domain for ACHE T prevents association of ColQ with ACHE T ; mutations distal to the attachment domain generate a mutant Ϸ10.5S species of AChE composed of one ACHE T tetramer and a truncated ColQ strand. The Ϸ10.5S species lack part of the collagen domain and the entire C-terminal domain of ColQ, or they lack only the C-terminal domain, which is required for formation of the triple collagen helix, and this likely prevents their insertion into the basal lamina.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

“…[7][8] One, two or three globular subunits form the main structure of this synaptic enzyme, which needs to be anchored to the basal lamina for its proper function. The…”

“…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] .…”

“…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). Despite tremendous progress in identifying the molecular basis of the different CMS subtypes, almost half of the CMS population remains genetically undiagnosed.…”

Therapeutic Strategies in Congenital Myasthenic Syndromes

“…These are inherited disorders, caused by various genetic defects, and all but the slow-channel CMS by recessive inheritance. [1][2][3][4][5] To date, mutations in 10 different genes have been found to cause a CMS: CHAT, coding for the presynaptic choline acetyltransferase 6 ; COLQ, coding for the endplate acetylcholine esterase 7,8 ; CHRNA1, CHRNB1, CHRND, and CHRNE coding for four different AChR subunits 9,10 ; RAPSN, coding for the postsynaptic protein rapsyn 11 ; MUSK, coding for the muscle-specific kinase 12 ; DOK7, coding for the downstream of kinase 7 protein 13 ; and SCN4A, coding for the postsynaptic voltage-gated sodium channel Na v 1.4. 14…”

Assembly and regulation of acetylcholinesterase at the vertebrate neuromuscular junction

“…Perlecan is a multifunctional heparin sulfate proteoglycan that is also concentrated on the synaptic basal lamina, and the carboxyl terminal domain of ColQ is necessary and sufficient for its attachment. Several mutations in the C-terminal domain of COLQ indicate that this region of the collagenic tail is essential for AChE localization at the synapse [14][15][16]. To determine whether the entire collagenic tail was necessary for attachment, or only the Cterminal domain, we generated several fusion proteins consisting of GFP fused to varying lengths of COLQ.…”