Myotonic Mutations of Nav1.4 Located At EF Hand-Like Motif in C-Terminus Impair Fast Inactivation

Abstract

Mutations of the voltage-gated sodium channel gene (SCN4A), which encodes Nav1.4, cause non-dystrophic myotonia that occasionally contributes to sudden infant death syndrome (SIDS). Nav1.4 mainly comprises four homologous domains (DI-DIV) and has a C-terminal tail (CTerm) downstream of DIV. The CTerm is known to have several functional motifs including the EF-hand like motif, the IQ motif, and the calmodulin-binding site. Recently, case reports of non-dystrophic myotonia due to mutations located within the EF hand-like motif of Nav1.4 Cterm have accumulated, but functional analysis of such mutations is scarce. We identified SCN4A mutations in two families with myotonia, one with a novel mutation (E1702del) and the other with a known mutation (E1702K). The proband with E1702K experienced repeated rhabdomyolysis, and the daughter showed laryngospasm and cyanosis, a risk for SIDS. The T1700_E1703del mutation has previously been reported as causative for myotonia. In this study, functional analysis of these three mutations, all located on the EF hand-like motif in CTerm, was conducted with whole-cell patch-clamp recording of heterologously expressed channel. As a result, all mutations displayed impaired fast inactivation, suggesting that the CTerm of Nav1.4 is vital for regulating fast inactivation. In addition, our data indicate that the negative charge at amino acid position E1702 in Nav1.4 would be functionally important. This study may contribute to the elucidation of molecular mechanisms of CTerm of Nav1.4. This study is supported by Research Grants from MHLW (H29-Nanchitou (Nan)-Ippan-030) and AMED (JP19ek0109230) to MPT, and by JSPS KAKENHI Grant Number JP18K07524 and by Takeda Science Foundation Grant for medical science research to T.K.