Functional study of a novel SCN4A variant, c.611C>T, identified in a Japanese patient with myasthenia

Tomoya Kubota^1*Natsuki Kira²Kosuke Yoshida³Satoe Takahashi⁴Ayami Yamanaka²Takashi Kimura³Kazuaki Homma⁴Masanori P Takahashi²

• ¹Osaka University, Suita, Japan
• ²Osaka Daigaku, Suita, Japan
• ³National Hospital Organization Asahikawa Medical Center, Asahikawa, Japan
• ⁴Northwestern University, Chicago, United States

Recent advances in sequencing technology have significantly contributed to the identification of disease-associated gene variants. However, a substantial number of patients, particularly those presenting with atypical neuromuscular phenotypes, still remain genetically undiagnosed. Here, we report a Japanese case with a myasthenic symptom in eyelids and limbs rather than periodic paralysis with a novel heterozygous variant, c.611C>T, located at the 3' end of exon 4 in SCN4A. The analysis of the proband's SCN4A mRNA showed that this variant causes an alanine-to-valine missense change at the amino acid position of 204 (p.A204V, 39%) and a disruption of the splicing of exons 4 and 5 that leads to the production of truncated Nav1.4 variant protein (p.A204Vfs*94, 4%). We anticipated that the p.A204V missense change would impair Nav1.4 function; however, the ion channel activity and membrane targeting of p.A204V Nav1.4 were found to be wild-type (WT)-like. We also examined cytotoxicity of p.A204V and p.A204Vfs*94 variants; however, cell lines heterologously overexpressing these Nav1.4 variant proteins induced cell death no more than the WT control. Although a loss or gain of anomalous ion channel function that is commonly suspected in channelopathies has been ruled out, the precise mechanism for the pathogenic role of c.611C>T SCN4A remains to be elucidated.