PRIMPOL shields replication forks after multiple rounds of cisplatin in BRCA-deficient cancer cells.

  • Major Finding: PRIMPOL shields replication forks after multiple rounds of cisplatin in BRCA-deficient cancer cells.

  • Mechanism: The protective effect relies on PRIMPOL's primase activity and is dependent on the ATR pathway.

  • Impact: This study elucidates the role of PRIMPOL in the DNA-damage response in cells treated with cisplatin.

BRCA proteins shield reversed replication forks, protecting them from damage that could otherwise be incurred during DNA-replication stress, such as that caused by some chemotherapeutic agents. However, cells of many cancer types (particularly breast and ovarian cancers) commonly have inactivating mutations in BRCA proteins. Quinet and colleagues studied the ways DNA replication is altered in BRCA1-deficient cancer cells repeatedly treated with cisplatin, a DNA–cross-linking chemotherapy drug often used to treat ovarian cancers. They found that in a BRCA1-null human ovarian cancer cell line and a human osteosarcoma cell line depleted for BRCA1 using siRNA, repeated dosing with cisplatin prevented the degradation of nascent DNA strands typically seen with replication stress in BRCA-deficient cells, indicating a compensatory mechanism must have been activated. Further experiments revealed that this compensatory mechanism was based on PRIMPOL, a DNA polymerase and primase involved in translesion DNA synthesis; specifically, the mechanism was dependent on PRIMPOL's primase activity, not its polymerase activity. The PRIMPOL-mediated adaptive response to multiple rounds of cisplatin treatment required the activity of the ATR pathway, named for the serine/threonine kinase ATR, which is involved in the DNA-damage response. Overexpression of PRIMPOL alone appeared to be sufficient to prevent deleterious levels of replication-fork reversal following cisplatin dosing in BRCA1-deficient cells, and suppressing fork reversal in both BRCA1-deficient and BRCA1-proficient cells caused a shift toward PRIMPOL-mediated DNA repriming after cisplatin treatment. Hinting at the potential clinical relevance of these findings, PRIMPOL overexpression in BRCA1-deficient cells reduced their sensitivity to treatment with the combination of an ATR inhibitor and cisplatin, which is currently being tested in clinical trials. Further, PRIMPOL appeared to be indispensable for cell survival in the absence of BRCA1, implying that targeting PRIMPOL may be a useful strategy. Collectively, these findings establish a new role for PRIMPOL in the response to replication stress and provide mechanistic insight into how cells cope with multiple doses of DNA-damaging drugs such as cisplatin.

Quinet A, Tirman S, Jackson J, Šviković S, Lemaçon D, Carvajal-Maldonado D, et al. PRIMPOL-mediated adaptive response suppresses replication fork reversal in BRCA-deficient cells. Mol Cell 2019 Oct 29 [Epub ahead of print].

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