DNA Methylation in Ovarian and Endometrial Cancers: Predictive and Mechanistic Roles in PARP Inhibitor and ICI Response.

Cancer treatment is shifting from an organ-based approach to one driven by biological phenotypes, emphasizing the need to understand molecular mechanisms. DNA methylation plays a pivotal role in tumor biology, not only through gene silencing but also by inducing distinct behaviors beyond genetic mutations. In gynecologic cancers, molecular diagnostics, such as homologous recombination deficiency status guiding poly(ADP-ribose) polymerase (PARP) inhibitor therapy in ovarian cancer and deficient mismatch repair/microsatellite instability-high status informing immune checkpoint inhibitor (ICI) therapy in endometrial cancer have already been used in clinical practice. However, tumors with epigenetically driven functional deficiencies, such as BRCA1 promoter methylation in homologous recombination-deficient ovarian cancers or MLH1 promoter methylation in deficient mismatch repair/microsatellite instability-high endometrial cancers, often exhibit poorer prognoses and reduced therapeutic responses compared to their genetically mutated counterparts. Given the unique impact of DNA methylation, precise detection is crucial. Integrating methylation analysis into molecular classification could refine diagnostics-both by identifying mechanistic contributors to treatment response and by serving as predictive biomarkers for therapy selection-thereby optimizing patient management. This review explores the role of DNA methylation in modulating responses to PARP inhibitors and ICIs, highlights its promise as a biomarker in precision oncology, and outlines current developments and clinical challenges in BRCA1 and MLH1 methylation assays.