PPP2R1A mutations cause ATR inhibitor synthetic lethality in ARID1A deficient ovarian clear cell carcinoma

Ovarian clear cell carcinoma (OCCC) is a cancer of unmet need characterized by ARID1A mutation. Prior work identified an ARID1A/ATR synthetic lethality, information that led to phase II clinical trials. Using genome-wide CRISPR-Cas9 mutagenesis and interference screens, we identified protein phosphatase 2A (PP2A) subunits, including PPP2R1A, as determinants of ATRi sensitivity in ARID1A mutant OCCC. Analysis of an OCCCs cohort indicated that >1/3 possessed both PPP2R1A and ARID1A loss-of-function mutations. CRISPR-prime editing demonstrated that oncogenic PPP2R1A p.R183 missense mutations enhance in vitro and in vivo ATRi sensitivity in ARID1A mutant OCCC. OCCC patients with both ARID1A and PPP2R1A mutations also showed clinical responses to ATRi in a phase II trial. Mechanistically, this synthetic lethal effect is dependent upon WNK1 kinase, which opposes PP2A function. This data suggests that co-occurrence of PPP2R1A and ARID1A mutations in OCCC should be assessed as a biomarker of ATRi response in on-going clinical trials.