For the hundreds of thousands of women diagnosed annually with epithelial ovarian cancer, PARP inhibitors represent a frontline defense — yet most patients eventually see their tumors escape treatment. A newly identified molecular circuit explains one major escape route: the acidic conditions tumors engineer around themselves appear to chemically disable the very mechanism that makes PARP inhibitors lethal to cancer cells.
Working in ovarian cancer models, researchers used a CRISPR-Cas9 genetic screen to pinpoint p300 — an acetyltransferase enzyme — as a central driver of treatment resistance under acidic tumor conditions. Pathologically low pH activates an ERK–p300–PARP1 signaling cascade that places an acetyl group on PARP1 at lysine residue 505. This single modification, designated PARP1 K505Ac, is sufficient to prevent PARP inhibitors from trapping PARP1 on damaged DNA — the core mechanism through which drugs like olaparib or niraparib kill cancer cells. Critically, elevated PARP1 K505Ac levels were detected in clinical tumor samples and correlated with both PARPi resistance and reduced overall survival. Combining a p300 inhibitor with a PARP inhibitor in patient-derived and syngeneic mouse models suppressed tumor growth synergistically.
This finding matters beyond ovarian cancer biology. Tumor acidosis is a near-universal feature of solid tumors, meaning the ERK–p300–PARP1 resistance axis could theoretically apply wherever PARP inhibitors are used, including breast and prostate cancers with BRCA mutations. The mechanistic precision here — a specific acetylation site on a well-characterized protein — is a meaningful strength, as it creates a biomarker hypothesis (PARP1 K505Ac as a resistance predictor) alongside a therapeutic one. That said, the data remain largely preclinical; no p300 inhibitor is yet approved, and translating synergistic effects from mouse models to humans carries substantial uncertainty. Still, this is a genuinely clarifying piece of work that advances the resistance-mechanism field from correlation to mechanism, positioning p300 inhibition as a rational clinical combination strategy worth pursuing in early-phase trials.