Identification of novel CDK19 variants and Drosophila-based in vivo functional evidence supporting pathogenicity in neurodevelopmental disorders.

Purpose

CDK19 (HGNC:19338) is a Mediator-associated kinase that regulates transcription during neurodevelopment. Although CDK19 variants have been linked to neurodevelopmental disorders, the full phenotypic spectrum and underlying pathogenic mechanisms remain incompletely defined. We aimed to further characterize clinical-genetic correlations and elucidate disease mechanisms through integrative functional and transcriptomic approaches.

Methods

Through an international collaboration, 28 individuals with CDK19 variants (12 newly identified and 16 previously reported) were evaluated using systematic clinical phenotyping. To assess the functional consequences of variants identified in affected individuals, Drosophila models expressing CDK19 alleles were generated and examined for viability, lifespan, motor behavior, and seizure susceptibility. Protein stability, subcellular localization, tissue morphology, and transcriptomic alterations were assessed by western blot, immunofluorescence, and RNA sequencing of larval brains.

Results

All individuals demonstrated global developmental delay, with speech delay as a consistently prominent feature; 67% had epilepsy, most commonly infantile epileptic spasms syndrome. Hypotonia (50%), craniofacial dysmorphisms, and abnormal brain MRI findings were also frequent. Several pathogenic alleles exhibited reduced protein stability, aberrant nuclear localization, and tissue toxicity, with associated structural abnormalities including elongated mitochondria, thickened myofibrils, and shortened sarcomeres. Behavioral assays revealed motor incoordination and allele-specific seizure susceptibility. RNA-seq identified consistent downregulation of synaptic and neuromuscular genes, including GluRIIA, VGAT, nAChRβ1, and Syn, implicating disrupted neurotransmission and impaired neuromuscular junction integrity. These findings, obtained in an overexpression-based allele-comparison system, are interpreted as supportive functional evidence of variant pathogenicity.

Conclusion

These findings corroborate and further refine the clinical spectrum of CDK19-related NDDs, and provide in vivo functional evidence that pathogenic CDK19 variants are associated with disruption of transcriptional programs relevant to synaptic and neuromuscular function, supporting CDK19 dysfunction as part of the emerging spectrum of Mediator complexopathies.