Rapsyn Mutations in Humans Cause Endplate Acetylcholine-Receptor Deficiency and Myasthenic Syndrome

Abstract

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.

Acetylcholine Receptors Direct Rapsyn Clusters to the Neuromuscular Synapse in Zebrafish

“…Thus the mutated ␦ subunit might be unable to associate properly with the other subunits of the AChR. Given that other zebrafish neuromuscular mutants have increased our understanding of human neuromuscular diseases such as myasthenic syndrome (Ohno et al, 2002;Ono et al, 2002) and slow channel syndrome (Engel et al, 2003;Lefebvre et al, 2004), it seems important to consider the potential implications of a ␦ subunit null in the human population. In frog the ␦ subunit is absolutely required for functional expression of both embryonic and adult-type AChRs (Paradiso and Brehm, 1998), and our findings from sop indicate that a similar dependence exists for fish.…”

“…The mutant twitch once shows weak and abbreviated swimming in response to touch because of a mutation in muscle rapsyn (Ono et al, 2002). Mutations in the same domain of human rapsyn recently were shown to underlie congenital forms of myasthenic syndrome (Ohno et al, 2002). Another prevalent defect in swimming is discoordinated and synchronous contractions of opposing muscles with touch.…”

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…”

Muscle-like nicotinic receptor accessory molecules in sensory hair cells of the inner ear

“…That rapsyn-AChR aggregates are necessary for AChR clustering in muscle as is also clearly demonstrated by mutant mice lacking rapsyn, which show severe neuromuscular dysfunction with no detectable nAChR clusters on the muscle fiber . Furthermore, rapsyn deficiencies affect the formation of nAChR aggregates, can lead to a reduction in the density of synaptic nAChRs (Eckler et al, 2005;Fuhrer et al, 1999) and may be associated with human diseases such as congenital myasthenic syndrome (Ohno et al, 2002;Ohno et al, 2003;Ono et al, 2001). Although it is not known whether rapsyn interacts with the nAChRs in hair cells or induces clustering of these receptors at the cell surface, it is worthwhile to note that myasthenic gravis patients have reduced auditory function, which is reversed after administration of a cholinesterase inhibitor (Paludetti et al, 2001).…”