A direct role for HIPK2 in homology-directed DNA repair and the regulation of PARP inhibitor sensitivity

Abstract

Our cells are under continuous attack by exogenously and endogenously evoked DNA damage resulting in different types of DNA lesions. DNA double-strand breaks (DSBs) are the most deleterious type of genome damage and are in principle repaired by two pathways, non-homologous end-joining (NHEJ) and homologous recombination (HR). Homology-directed repair of DSBs efficiently counteracts genome instability and cancer formation. Moreover, cancer cells frequently show defects in DNA repair and particularly HR-deficiency of cancer cells can be therapeutically exploited since such cells are sensitized to poly(ADP-ribose) polymerase inhibitor (PARPi) treatment.

Work from us and others has established the tumour suppressor Homeodomain Interacting Protein Kinase 2 (HIPK2) as a central player in DNA damage-induced cell fate control. Upon DSB induction by ionizing radiation (IR), HIPK2 is activated by a mechanism involving checkpoint kinase ATM and site-specific HIPK2 autophosphorylation, and triggers cell death through the p53 pathway. Although the cell death regulatory function of HIPK2 in response to irreparable DNA damage is well-established, its function upon repairable DNA damage remains largely obscure.

Intriguingly, our unpublished data identified a direct role for HIPK2 in homology-directed DSB repair. We found that HIPK2 forms a protein complex with DSB repair factors BRCA1 and 53BP1, and physically accumulates at IR-induced DSBs in an ATM-regulated manner. DSB reporter assays and Sister chromatid exchange analysis revealed an essential role for HIPK2 in DSB repair by HR. Mechanistically, we found that HIPK2 binds and site-specifically phosphorylates BRCA1, thereby regulating BRCA1 protein stability. Accordingly, HIPK2 depletion or pharmacological inhibition of HIPK2 results in declined BRCA1 levels and potentiates IR sensitivity and PARPi toxicity in BRCA1-proficient cells. Together, our results identify a direct role of HIPK2 in HR repair, and suggest HIPK2 inhibition as novel strategy to sensitize BRCA1-proficient cancer cells to PARPi.