Repair of DNA double-strand breaks in BRCA2-deficient cells by POLθ and CIP2A

• Applicant: Professor Dr. Markus Löbrich
• Subject Area: Cell Biology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 553222300

BRCA2 is an important tumour suppressor whose loss is associated with the onset of various cancer types. BRCA2-deficient tumor cells fail to repair DNA double-strand breaks (DSBs) by homologous recombination (HR) and are exquisitely sensitive to therapeutic treatments which induce DSBs. The best-known examples are treatments with PARP inhibitors (PARPi) that cause DSBs during the replicative cell cycle. However, the success of PARPi in the clinic is limited by resistance mechanisms which includes DSB repair in BRCA2-deficient cells by pathways other than HR. POLtheta-mediated end-joining (TMEJ) is one such alternative DSB repair pathway which was recently shown to repair replication-associated DSBs during the mitotic phase of the cell cycle. Another example is CIP2A-dependent DSB repair which also operates during mitosis and contributes to the survival of BRCA2-deficient cells after treatment with PARPi or other DNA damaging agents. Although PARP inhibitors are already frequently applied in the clinic and POLtheta as well as CIP2A inhibitors are currently being developed, little is known about how these two mitotic repair pathways are regulated in BRCA2-deficient cells and how they individually and together contribute to the survival of BRCA2-deficient cells after PARPi treatment. Moreover, although BRCA2 can suppress the activity of POLtheta, it is unknown which of the many known mutations of BRCA2 negatively affect POLtheta- and/or CIP2A-dependent mitotic DSB repair. Here, we strive to establish this knowledge by applying cell biology, molecular biology and cytogenetic technologies to study DSB repair in BRCA2-deficient cells treated with PARPi. We will investigate which domains of BRCA2 allow timely DSB repair by restricting TMEJ until the onset of mitosis and which domains and mutations of BRCA2 prevent TMEJ usage during mitosis. We will also study the interplay between POLtheta- and CIP2A-dependent mitotic DSB repair and elucidate if and how these pathways synergize to repair DSBs in BRCA2-deficient cells. Finally, we will investigate which domains and mutations of BRCA2 differentially affect mitotic DSB repair by POLtheta and CIP2A and which BRCA2 mutations render cells susceptible to the individual or combined inhibition of POLtheta or CIP2A. Collectively, our studies will provide a rationale for using inhibitors of POLtheta and CIP2A in the clinic and will help to build a conceptual framework which is needed for the best possible treatment of a specific BRCA2-mutated tumor with combination therapies of PARPi and inhibitors of POLtheta and/or CIP2A.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Wolf-Dietrich Heyer