MED13L pathogenic missense variants impair protein stability and interaction, underlying diverse clinical outcomes.

Heterozygous pathogenic variants in the Mediator complex subunit 13-like gene located in the locus 12q21.21 (MED13L) are associated with intellectual disability, developmental delay, and distinctive facial features. While nonsense and frameshift variants typically cause haploinsufficiency, resulting in a well-characterized clinical presentation, missense variants have been associated with a broader range of phenotypes, including epilepsy and severe motor delay. In this study, we investigated five pathogenic missense variants in MED13L-c.2597C>T p.Pro866Leu, c.2605C>T p.Pro869Ser, c.3392G>A p.Cys1131Tyr, c.5695G>A p.Gly1899Arg, and c.6485C>T p.Thr2162Met-associated with different clinical severities. We identified significant reductions in protein stability across these variants, with some exhibiting aberrant cytoplasmic localization, suggesting disruptions in structural integrity and function. In particular, exon 15 variants (p.Pro866Leu and p.Pro869Ser) correlated with severe phenotypes, including epilepsy and severe motor impairment, whereas p.Gly1899Arg and p.Thr2162Met were associated with milder manifestations. 3D protein modeling suggested that these missense variants may disrupt MED13L's interaction with the CDK8 kinase module, leading to functional deficits. Our findings highlight different pathogenic mechanisms, ranging from protein instability to altered molecular interactions, that contribute to the clinical variability observed in MED13L-related disorders.