Abstract MIP-043: OVARIAN CARCINOMA CDK12 MUTATIONS IMPAIR GENE-SPECIFIC POL II ELONGATION AND FAITHFUL REPAIR OF DNA DOUBLE STRAND BREAKS

The Cdk12/CycK complex is a novel transcription elongation-associated kinase that promotes expression of a subset of RNA polymerase II (Pol II) genes, including those of the DNA damage response (DDR) network. CDK12 is mutated in many cancers, particularly in high-grade serous ovarian carcinoma (HGS-OvCa), where it is one of only nine genes with statistically recurrent somatic mutations. However, the influence of these mutations on the kinase and their link to cancerogenesis remain ill-defined. Here, we show that most CDK12 mutations interfere with formation of the complex, rendering the kinase inactive. By examining the mutations within the Cdk12/CycK structure, we find that they likely provoke structural rearrangements detrimental to Cdk12 activation. Our mRNA expression analysis of the patient samples containing the CDK12 mutations identifies coordinated down-regulation of key DDR genes, particularly those of the homologous recombination (HR) repair pathway. Moreover, we note that production of nascent HR gene transcripts is compromised significantly upon CDK12 and CYCK knockdown. Indeed, Cdk12 occupies these genes and promotes phosphorylation of the Pol II9s C-terminal domain (CTD) at Serine 2 (Ser2-P), the hallmark of effective transcription elongation, indicating that the down-regulated genes are directly controlled by Cdk12/CycK. Functionally, we demonstrate that the mutant Cdk12 proteins fail to stimulate the faithful repair of DNA double strand breaks via HR. In summary, we provide molecular basis of how mutated CDK12 ceases to function in ovarian carcinoma. We propose that CDK12 is a tumor suppressor of which the loss-of-function mutations could elicit defects in HR-mediated and possibly other DNA repair pathways, leading to genomic instability underlying the genesis of HGS-OvCa. Citation Format: Kingsley M. Ekumi, Hana Paculova, Tina Lenasi, Vendula Pospichalova, Christian A. Bosken, Jana Rybarikova, Vitezslav Bryja, Matthias Geyer, Dalibor Blazek and Matjaz Barboric. OVARIAN CARCINOMA CDK12 MUTATIONS IMPAIR GENE-SPECIFIC POL II ELONGATION AND FAITHFUL REPAIR OF DNA DOUBLE STRAND BREAKS [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-043.