PTEN deficiency confers sensitivity to ATR inhibitor-based treatment in high-grade serous ovarian cancer.

Ataxia telangiectasia and Rad3-related (ATR) inhibition is under evaluation for the treatment of high-grade serous ovarian cancer (HGSOC) to reverse acquired resistance to poly (ADP-ribose) polymerase (PARP) inhibition and to exacerbate chemotherapy-induced replicative stress. Here, we define PTEN deficiency as a predictive biomarker for the response to ATR inhibition, as monotherapy and in combination with PARP inhibition or gemcitabine. In response to ATR inhibition and compared with PTEN-proficient cells, PTEN-deficient cells are prone to (a) uncoupling of DNA polymerase and helicase activities, leading to excessive ssDNA and replication stress; (b) cytoplasmic sequestration of checkpoint kinase 1 (CHK1), compromising cell-cycle checkpoint control with reduced compensatory effects by ataxia-telangiectasia mutated (ATM) and DNA-dependent proteinase K (DNA-PK), leading to mitotic catastrophe; and (c) reduced DNA repair protein RAD51 homolog 1 (RAD51) recruitment, exacerbating replication fork instability, also leading to lethality. Retrospective analyses revealed that patients with HGSOC who expressed low PTEN levels experienced greater clinical benefit on ATR inhibitor-based trials than did those with high PTEN levels. These results justify prospective trials evaluating ATR inhibition as a therapeutic strategy for PTEN-deficient tumors.