Concurrent sodium channel myotonia and amyotrophic lateral sclerosis supports shared pathogenesis

Abstract

Amyotrophic lateral sclerosis (ALS) is an invariably fatal adult-onset neurodegenerative disorder; 10% of ALS is autosomal dominant but even sporadic ALS exhibits significant heritability. Dysfunctional voltage- gated sodium channels (NaVs) have been implicated in both sporadic and familial forms of the disease, but it is unclear whether these are an upstream cause or a downstream consequence. Rare missense mutations in the SCN4A gene, encoding NaV1.4, disrupt membrane excitability and cause autosomal dominant muscle channelopathies such as myotonia and hypo/hyperkalaemic periodic paralysis. We report two patients with congenital SCN4A channelopathy due to p.S1159P and p.R672H mutations who developed ALS in later life. Based on our finding we hypothesised that ALS may be linked to genetic mutations in other voltage gated ion channels. Rare-variant burden testing using whole genome sequenc- ing data from 4,495 ALS patients and 1,925 controls within the superfamily of voltage-gated ion channels (Accession: ssf81324) identified one gene, SCN7A, which passed multiple testing correction (p=0.0002, Firth logistic regression) consistent with an enrichment of ALS-associated mutations. We propose that genetic mutations within NaVs leading to neuromuscular excitotoxicity are an upstream cause of ALS. Early identification of ALS patients carrying NaV mutations might lead to personalised therapy with a sodium channel-blocking agent such as riluzole.j.franklin@doctors.org.uk