Congenital myasthenic syndrome due to mutations in MUSK suggests that the level of MuSK phosphorylation is crucial for governing synaptic structure.

Pedro M Rodríguez Cruz,Sandeep Jayawant,Ruth Herbst,David Beeson,Jonathan Cheung,Alexandr P Kornev,Dominic Waithe,Susan Maxwell,Judith Cossins,Jacqueline Palace

doi:10.1002/humu.23949

Abstract

MUSK encodes the muscle‐specific receptor tyrosine kinase (MuSK), a key component of the agrin‐LRP4‐MuSK‐DOK7 signaling pathway, which is essential for the formation and maintenance of highly specialized synapses between motor neurons and muscle fibers. We report a patient with severe early‐onset congenital myasthenic syndrome and two novel missense mutations in MUSK (p.C317R and p.A617V). Functional studies show that MUSK p.C317R, located at the frizzled‐like cysteine‐rich domain of MuSK, disrupts an integral part of MuSK architecture resulting in ablated MuSK phosphorylation and acetylcholine receptor (AChR) cluster formation. MUSK p.A617V, located at the kinase domain of MuSK, enhances MuSK phosphorylation resulting in anomalous AChR cluster formation. The identification and evidence for pathogenicity of MUSK mutations supported the initiation of treatment with β2‐adrenergic agonists with a dramatic improvement of muscle strength in the patient. This work suggests uncharacterized mechanisms in which control of the precise level of MuSK phosphorylation is crucial in governing synaptic structure.

Highlights

The neuromuscular junction (NMJ) is a complex and highly specialized synapse between the motor neuron and the skeletal muscle fiber
These results show that the abnormalities seen in tyrosine phosphorylation derived from the p.A617V substitution are not caused by differences in the S751‐related modulation of muscle‐specific receptor tyrosine kinase (MuSK) activity
MuSK is considered the key organizer of NMJ formation and maintenance and is an essential element of the agrin‐lipoprotein receptor‐related protein 4 (LRP4)‐MuSK‐ DOK7 signaling pathway (Burden et al, 2013; DeChiara et al, 1996)

Summary

| INTRODUCTION

The neuromuscular junction (NMJ) is a complex and highly specialized synapse between the motor neuron and the skeletal muscle fiber. After its release by the nerve terminal, agrin binds to the low‐ density lipoprotein receptor‐related protein 4 (LRP4) on the postsynaptic membrane forming a tetrameric complex (Zong et al, 2012) This enhances the binding of LRP4 to the first IgG‐like domain of MuSK (Zhang et al, 2012), resulting in MuSK activation via dimerization and trans‐autophosphorylation of specific tyrosine residues within the cytoplasmic region (Schlessinger, 2000; Figure 1b). The activation loop occupies the active site cleft of the MuSK kinase domain by adopting a pseudo‐substrate conformation that impedes ATP and other substrates binding blocking kinase activation (Till et al, 2002) These regulatory mechanisms seem important to control postsynaptic differentiation of the NMJ (Madhavan & Peng, 2005). MUSK mutations have been shown to diminish the expression and stability of MuSK (Chevessier et al, 2004), impair MuSK‐DOK7 interaction (Maselli et al, 2010), and reduce

| MATERIALS AND METHODS

| RESULTS

Findings

| DISCUSSION