CDC7 and CDK8 kinases cooperate to support DNA replication origin firing in human cells

The coordinated activation of DNA replication origins is important for efficient DNA synthesis and genome stability. S-phase cyclin-dependent kinases (CDKs), together with CDC7 kinase, are essential for origin activation by converting the pre-replicative complex into a fully active helicase. Here, in order to identify genes that tune DNA replication, we performed a chemogenetic genome-wide CRISPR-KO screen in cells challenged with the CDC7 inhibitor XL413. By developing a methodology based on genetic co-essentiality to functionally cluster the hits, we uncover the transcriptional CDK8/CCNC kinase in a cluster with replication initiation factors. Depletion of CDK8 further reduces the rate of DNA synthesis imposed by CDC7 inhibitors. DNA fibre experiments further support that CDK8 and CDC7 cooperate in origin activation. Suppression of DNA synthesis by CDK8 inhibition requires the binding of CDK8/CCNC to the MDM2 Binding Protein (MTBP), and we show that CDC7 and CDK8 associate with the MCM complex and individually contribute to the phosphorylation of the replicative helicase. In conclusion, this work adds CDK8 to the list of kinases directly involved in origin activation in human cells.