Cyclin C nuclear release and mitochondrial dysfunction define molecular signatures of MED13L syndrome.

The mediator kinase module (MKM) coordinates transcriptional programs regulating cellular metabolism, stress responses, and differentiation. Heterozygous variants of MED13L, a core MKM component, cause a neurodevelopmental disorder characterized by variable intellectual disability, developmental delay, hypotonia and motor impairment, and congenital anomalies. However, the molecular basis underlying this clinical heterogeneity is poorly defined. Previously, we identified mitochondrial dysfunction and aberrant nuclear release of another MKM component, cyclin C (CCNC), in a single fibroblast line derived from an individual with MED13L syndrome. Here, we expand these studies across 12 fibroblast lines derived from individuals with 11 distinct MED13L variants. We identify mitochondrial dysfunction as a consistent feature of MED13L variation, characterized by reduced mitochondrial ATP production, decreased mitochondrial DNA abundance, elevated reactive oxygen species, and impaired transcription of genes involved in mitochondrial biogenesis. In parallel, all variant lines exhibit aberrant cytoplasmic CCNC localization, consistent with its established role in mitochondrial fission. Longitudinal analyses further reveal progressive declines in mitochondrial function along with markers associated with premature cellular aging. Importantly, the severity of mitochondrial dysfunction shows an association with variant position within MED13L and with clinical functional measures, suggesting that mutation location may partially predict disease severity. Together, these findings establish mitochondrial dysfunction as a consistent cellular feature of MED13L heterozygosity and identify CCNC mislocalization as a candidate biomarker of MKM disruption. More broadly, this work reveals an intersection between transcriptional control and mitochondrial homeostasis in MED13L syndrome, forming the framework for biomarker-driven therapeutic development in MED13L-associated and related neurodevelopmental disorders.