SCN1A-related epilepsy with recessive inheritance: Two further families

Variants in SCN1A gene, encoding the voltage-gated sodium channel Na_v1.1, are associated with distinct epilepsy syndromes ranging from the relatively benign genetic epilepsy with febrile seizures plus (GEFS+) to Dravet syndrome, a severe developmental and epileptic encephalopathy (DEE). Most SCN1A pathogenic variants are heterozygous changes inherited in a dominant or de novo inheritance and many cause a loss-of-function of one allele. To date, recessive inheritance has been suggested in only two families with affected children harboring homozygous SCN1A missense variants while their heterozygous parents were asymptomatic. The aim of this report is to describe two additional families in which affected individuals have biallelic SCN1A variants possibly explaining their phenotype.

We report two novel homozygous SCN1A missense variants in two patients from related parents. Both patients had fever-sensitive epilepsy beginning in the first months of life, followed by afebrile seizures, without severe cognitive impairment. Parents were asymptomatic. Next generation sequencing excluded a pathogenic variant in other genes involved in DEE. Estimation of pathogenicity scores by in-silico tools suggests that the impact of these SCN1A variants is less damaging than that of dominant pathogenic variants.

This study provides additional evidence that homozygous variants in SCN1A can cause GEFS+. This recessive inheritance would imply that hypomorphic variants may not necessarily cause epilepsy at the heterozygous state but may decrease the seizure threshold when combined.