A new look at the clinical and molecular characteristics of SCN1A-related developmental and epileptic encephalopathies

SCN1A-related diseases are a heterogeneous group of disorders with an expanding spectrum of phenotypes. Until recently, mutations in this gene were associated with epileptic syndromes and epileptic and developmental encephalopathy – Dravet syndrome, which was contrasted with a new group of early-onset syndromes, non-Dravet developmental and epileptic encephalopathies (DEEs; OMIM: PS308350). The aim of this paper is to review published data on the phenotypic variability of SCN1A-related developmental and epileptic encephalopathies, particularly non-Dravet syndromes. These are disorders with very early onset, polymorphic, drug-resistant epileptic seizures, impaired psychomotor development and intellectual disability, as well as the presence of additional symptoms such as arthrogryposis, osteopenia, and hyperkinetic movement disorders. Unlike Dravet syndrome, epileptic seizures begin in the first few months of life and may have an epileptic spasm or tonic morphology. The ability to quickly recognise the non-Dravet developmental and epileptic encephalopathy is of significant clinical value, because the identification of pathogenic SCN1A variants and their functional evaluation have an impact on both treatment and prognosis. Studies on the aetiology of non-Dravet developmental and epileptic encephalopathies have shown that pathogenic variants of the gain of function (GOF) type are identified in these patients. Therefore, it is possible to treat such patients with medicaments from the group of sodium channel blockers, which were contraindicated in cases of loss of function (LOF) variants, occurring in Dravet syndrome.