SCN2A total or partial loss-of-function variants occur across diverse epilepsy and neurodevelopmental phenotypes, with 70% recurrent variants seen in multiple phenotypes.
Do specific biophysical mechanisms of SCN2A loss-of-function variants predict clinical phenotypes such as epilepsy, intellectual disability, or autism spectrum disorder?
Distinct SCN2A loss-of-function phenotypes cannot be reliably linked to specific biophysical mechanisms, highlighting the need to not rely solely on clinical phenotype to predict the underlying mechanism.
Abstract Objective SCN2A loss‐of‐function (LoF) variants are associated with epilepsy (onset age ≥ 3 months), intellectual disability (ID), and autism spectrum disorder (ASD). Despite numerous identified variants and the description of phenotypic subgroups, relationships between Na v 1.2 channel dysfunction and clinical phenotypes remain unclear. This study examined how distinct LoF mechanisms relate to phenotypic outcomes. Methods Whole‐cell patch‐clamp electrophysiology was used to characterize 15 presumed LoF SCN2A variants. Mechanism–phenotype correlations were assessed in 33 patients with these variants (six recurrent) and 41 patients with 15 previously characterized LoF variants (four recurrent). Phenotypic subgroups were categorized as later onset epilepsy–midinfancy (onset between 3 and 18 months), later onset epilepsy–childhood (onset after 18 months), ID/ASD without epilepsy, and “other” for unclassified cases. Results Of the 15 electrophysiologically characterized SCN2A variants, 11 caused total Na v 1.2 LoF, three caused partial LoF, and one showed mixed LoF and gain‐of‐function (GoF) effects. Among previously published variants, seven showed total LoF, five partial LoF, and two mixed LoF/GoF, and one was undetermined. Across both cohorts, seven of 10 recurrent variants (70%) were associated with multiple phenotypic subgroups. Partial or total Na v 1.2 LoF variants were identified in all subgroups. Notably, a midinfancy epilepsy phenotype was observed in 22 of 24 individuals (92%) carrying a mixed LoF variant, with phenotype data unavailable for seven additional individuals. A novel LoF‐associated phenotype—episodic ataxia with or without developmental delay or ID—was identified in five of six individuals with the L1650P variant. Although episodic ataxia has been previously associated with GoF variants in SCN2A , this is the first reported instance in individuals with a confirmed LoF variant. Significance Distinct SCN2A LoF phenotypes cannot be reliably linked to specific biophysical mechanisms, as both total and partial Na v 1.2 LoF occurs across diverse phenotypes. For efficient personalized treatment, it is crucial not to rely solely on clinical phenotype to predict the underlying LoF mechanism.
Tan et al. (2026) studied this question. SCN2A total or partial loss-of-function variants occur across diverse epilepsy and neurodevelopmental phenotypes, with 70% recurrent variants seen in multiple phenotypes.