Karbassi, S. | Search bioRxiv

bioRxiv2026

HIPK2 regulates homology-directed DNA repair and PARP inhibitor sensitivity

Repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) counteracts genome instability and carcinogenesis. Cancer cells frequently show defects in HR which can be therapeutically exploited by hypersensitivity to poly(ADP-ribose) polymerase inhibitor (PARPi) treatment. Here we identify an unforeseen function of HIPK2 in HR repair and PARPi sensitivity. HIPK2 accumulates at DSBs and associates with DSB repair factors at DNA damage foci. DSB recruitment of HIPK2 requires checkpoint kinase ATM activity. DNA repair pathway analysis revealed that HIPK2 depletion specifically impairs HR. Mechanistically, we found that HIPK2 binds BRCA1 and phosphorylates BRCA1 at Ser1191, a site that regulates damage-induced BRCA1 protein stability. Consistently, HIPK2 depletion or pharmacological inhibition of HIPK2 results in reduced BRCA1 protein levels, and sensitizes BRCA1-proficient cancer cells to IR damage and PARPi treatment. In sum, our results identify a role for HIPK2 in HR through regulating BRCA1 protein levels, and propose HIPK2 inhibition as a novel strategy to sensitize BRCA1-proficient cancer cells to PARPi treatment.

cell biology↗

bioRxiv2026

Role of a childhood cancer-linked BRIP1/FANCJ germline variant in genomic instability and cancer cell vulnerability

Childhood cancer is frequently associated with inherited pathogenic variants of cancer predisposition genes. Using Whole-Exome Sequencing, we identified an inherited, monoallelic pediatric cancer-linked germline variant of DNA helicase FANCJ/BRIP1, BRIP1R162Q of unclear clinical significance. Intriguingly, in vitro helicase assays demonstrated that BRIP1R162Q encodes a hyperactive DNA helicase. In cells, stable expression of BRIP1R162Q in BRIP1-proficient cells selectively sensitized to ectopic DNA replication stress. Moreover, BRIP1R162Q expressing cells showed chronic, steady-state activation of DNA replication stress indicated by decreased fork speed, fork stalling and impaired fork symmetry. Moreover, BRIP1R162Q expression triggered genomic instability indicated by elevated {gamma}H2AX foci numbers and chromosomal aberrations, suggesting BRIP1R162Q as a variant driving genomic instability. Mechanistically, BRIP1R162Q mislocalizes upon replication stress, and leads to accumulation of DNA secondary G-quadruplex (G4) structures and R-loops. RNaseH1 expression, which resolves R-loops, also reduced G4 levels and relieved BRIP1R162Q-induced replication stress. Finally, BRIP1R162Q introduces exploitable vulnerabilities for targeted therapies using G4-ligand Pyridostatin and DNA damage kinase ATR and DNA-PK inhibition. Our findings propose a mechanistic framework of how a childhood cancer-linked monoallic, hypermorphic BRIP1 germline variant participates in cancer development, and suggest potential therapeutic treatment strategies.

cancer biology↗