Genotype-phenotype correlation of novel SCN1A variants selected from the Italian Registry of Dravet Syndrome Residras

Purpose: Variants in the SCN1A gene, encoding the Nav1.1 sodium channel, are linked to a range of neurological disorders from mild genetic epilepsy with febrile seizures plus (GEFS+) to severe Dravet syndrome (DS). These variants may lead to loss-of-function (LoF), gain-of-function (GoF), or mixed LoF/GoF effects, resulting in significant phenotypic variability and diverse treatment responses (Guerrini et al., Physiol. Rev. , 103, 433–513, 2023). Therefore, functional studies are critical to establish genotype-phenotype correlations and support therapeutic choices, especially to avoid contraindicated drugs. From the Italian Registry of Dravet Syndrome (Residras), novel SCN1A variants associated with DS (R1634Q, F984L) and GEFS+ (V1339A, A1940S, F773L) were selected (Balestrini et al., Epilepsia Open , 8(2):517-534, 2023). Except for A1940S, variants were predicted to cause pathogenic LoF or GoF effects using a machine learning model (Heyne et al., Sci Transl Med , 12(556):eaay6848, 2020). This study aimed to characterize Nav1.1 mutant channels and guide treatment strategies. Method: Nav1.1 wild-type (WT) and mutant channels were expressed in HEK293 cells, and sodium currents were recorded via whole-cell patch-clamp. Results: Mutant channels displayed normal current amplitude and voltage-dependent activation. R1634Q, F984L, and V1339A negatively shifted voltagedependent fast inactivation by +16, +10, and +7 mV, respectively, compared with WT. R1634Q also slowed recovery from fast inactivation. F773L accelerated both inactivation kinetics and recovery, while A1940S had no impact on Nav1.1 activity. Conclusion: R1634Q and F984L cause LoF defects consistent with DS, emphasizing the need to avoid sodium channel blockers. V1339A correlates with mild GEFS+, while F773L exhibits mixed LoF/GoF behavior atypical for GEFS+. These results underline the importance of functional studies to personalize treatment for SCN1A -related epilepsies. (PNRR: M6/C2_CALL 2022, Project Code: PNRR-MR1-2022-12376642)