Biallelic Truncating Variants in SCN3B Encoding Nav Channel Subunit β3 Lead to Neurodevelopmental Phenotype with and without Epilepsy and Ataxia.

SCN3B encodes the β3 auxiliary subunit, essential for voltage-gated Na+ (Nav) channel trafficking and gating. Although SCN3B has been associated with cardiac disorders, a link with neurodevelopmental disorders (NDD) has not been established. Using a genotype-first approach, we identified homozygous truncating variants (c.281G>A-β3W94*, c.584 + 1G>A-β3S196*) in 2 consanguineous Pakistani families, leading to global developmental delay, intellectual disability and autism, with severe cognitive impairment, ataxia, and seizures in the case of β3W94*. Electrophysiological analysis revealed subtype-specific gating alterations on multiple brain Nav channel subtypes. This is the first report linking SCN3B mutations to NDD, expanding our understanding of Nav channelopathies. ANN NEUROL 2025;98:864-870.