MUSK, a new target for mutations causing congenital myasthenic syndrome

Abstract

We report the first case of a human neuromuscular transmission dysfunction due to mutations in the gene encoding the muscle-specific receptor tyrosine kinase (MuSK). Gene analysis identified two heteroallelic mutations, a frameshift mutation (c.220insC) and a missense mutation (V790M). The muscle biopsy showed dramatic pre- and postsynaptic structural abnormalities of the neuromuscular junction and severe decrease in acetylcholine receptor (AChR) epsilon-subunit and MuSK expression. In vitro and in vivo expression experiments were performed using mutant MuSK reproducing the human mutations. The frameshift mutation led to the absence of MuSK expression. The missense mutation did not affect MuSK catalytic kinase activity but diminished expression and stability of MuSK leading to decreased agrin-dependent AChR aggregation, a critical step in the formation of the neuromuscular junction. In electroporated mouse muscle, overexpression of the missense mutation induced, within a week, a phenotype similar to the patient muscle biopsy: a severe decrease in synaptic AChR and an aberrant axonal outgrowth. These results strongly suggest that the missense mutation, in the presence of a null mutation on the other allele, is responsible for the dramatic synaptic changes observed in the patient.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

“…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).…”

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…”

Congenital Myasthenic Syndromes - Molecular Bases of Congenital Defects of Proteins at the Neuromuscular Junction

“…No fewer than 11 defective molecules at the NMJ have been identified to date. The mutant molecules include (i) acetylcholine receptor (AChR) subunits that forms nicotinic AChR and generate endplate potentials Sine et al, 1995), (ii) rapsyn that anchors and clusters AChRs at the endplate Milone et al, 2009), (iii) agrin that is released from nerve terminal and induces AChR clustering by stimulating the downstream LRP4/MuSK/Dok-7/rapsyn/AChR pathway (Huze et al, 2009), (iv) muscle-specific receptor tyrosine kinase (MuSK) that transmits the AChR-clustering signal from agrin/LRP4 to Dok-7/rapsyn/AChR (Chevessier et al, 2004;Chevessier et al, 2008), (v) Dok-7 that interacts with MuSK and exerts the AChR-clustering activity Hamuro et al, 2008), (vi) plectin that is an intermediate filament-associate protein concentrated at sites of mechanical stress (Banwell et al, 1999;Selcen et al, 2011), (vii) glutamine-fructose-6-phosphate aminotransferase 1 encoded by GFPT1, the function of which at the NMJ has not been elucidated (Senderek et al, 2011), (viii) skeletal muscle sodium channel type 1.4 (Na V 1.4) that spreads depolarization potential from endplate throughout muscle fibers , (ix) collagen Q that anchors acetylcholinesterase (AChE) to the synaptic basal lamina Kimbell et al, 2004), (x) 2-laminin that forms a cruciform heterotrimeric lamins-221, -421, and -521 and links extracellular matrix molecules to the -dystroglycan at the NMJ (Maselli et al, 2009), (xi) choline acetyltransferase (ChAT) that resynthesizes acetylcholine from recycled choline at the nerve terminal (Ohno et al, 2001). AChR (Lang & Vincent, 2009), MuSK (Hoch et al, 2001;Cole et al, 2008), and LRP4 (Higuchi et al, 2011) are also targets of myasthenia gravis, in which autoantibody against each molecule impairs the neuromuscular transmission.…”