Uncovering functional insights into human pathogenic variants in CDK19 using Drosophila models

Heterozygous missense variants in CDK19 have been found in patients diagnosed with Developmental and Epileptic Encephalopathy-87 (DEE87) who present with global developmental delay, intellectual disability and other neuromuscular deficiencies. Two missense variants in CDK19, Y32H and T196A, were first proposed to be loss of function based on experiments in Drosophila models of DEE87. Subsequently, it was proposed that Y32H is a gain of function with elevated kinase activity. We present a detailed functional evaluation of these dominant missense variants in several contexts. We use fly models of DEE87 in which endogenous cdk8, the fly ortholog to human CDK8 and CDK19, is depleted through RNA interference (RNAi) while expressing the human genes. Depletion of Drosophila cdk8 causes thicker muscle myofibrils, fused mitochondria, and climbing defects. The expression of wild-type human CDK19 in a fly cdk8 knockdown background rescues these defects, highlighting functional conservation. In our assays, we used a cdk8 depleted background and individually expressed either the variant or wildtype CDK19 to compare the function of the variants relative to wild-type. We demonstrate that Y32H can rescue defects caused by cdk8 depletion, while T196A is unable to due to possible loss of function. Further, we find that supplementation of the fly diet with an antioxidant improves T196A phenotypes. Our Drosophila studies allowed us to assay these variants for further insight into their functional nature and to obtain translational knowledge that may be applied back to human health.