NIEUWE GAIN-OF-FUNCTION VARIANTEN IN HET IONENKANAAL TRPM3 GEASSOCIEERD MET GLOBALE ONTWIKKELINGSSTOORNISSEN

Developmental and epileptic encephalopathies (DEE) are a heterogenous group of severe neurodevelopmental disorders characterized by global developmental delay, moderate to severe intellectual disability, and epilepsy, variably associated with other clinical features such as continuous spike-and-wave activation during sleep (CSWS), hypotonia and facial dysmorphisms. Recent genetic studies have identified de novo heterozygous mutations in the Trpm3 gene, which encodes a Ca2+-permeable ion channel, in patients with DEE. However, the functional consequences of the mutations remained poorly understood. To investigate TRPM3 variants, we used electrophysiological and microfluorimetric techniques in a heterologous overexpression system. First, functional analyses of two disease-associated mutations (p.Val1002Met and p.Pro1102Gln) revealed a robust gain-of-function (GoF) phenotype, characterized by increased basal activity, elevated sensitivity to the endogenous neurosteroid pregnenolone sulfate (PS) and heat, and altered ligand modulation. The most recurrent mutation, p.Val1002Met, disrupted non-canonical pore gating, leading to aberrant inward cation currents and Ca2+ overload. Next, we identified seven additional heterozygous missense variants, all exhibiting a robust GoF phenotype, with increased basal activity, elevated Ca2+ influx, and enhanced responses to PS. Given the pathogenic GoF effects, we explored the therapeutic potential of the FDA-approved antiseizure medication primidone, a known TRPM3 antagonist. Primidone effectively reduced the basal activity of all mutant channels, suggesting its potential as a targeted treatment. Furthermore, we identified two children with pharmacoresistant DEE-SWAS and developmental regression, each carrying a de novo TRPM3 mutation (p.Val1002Met or p.Ser1133Pro). Both mutations resulted in a GoF phenotype. Remarkably, off-label primidone treatment halted developmental regression, improved psychomotor development, and eliminated CSWS. These findings establish TRPM3 GoF mutations as a driver of a spectrum of neurodevelopmental disorders, frequently involving cerebellar dysfunction. We propose that TRPM3 GoF mutations disrupt cerebellar and cortical development via Ca2+-mediated excitotoxicity, dysregulated neuronal proliferation, and altered network excitability. However, further studies using neuronal cultures and in vivo models are essential to validate this hypothesis and refine therapeutic strategies targeting TRPM3 dysfunction.