Targeted proteomic analyses of histone H4 acetylation changes associated with homologous-recombination-deficient high-grade serous ovarian carcinomas

Approximately 20% of high-grade serous ovarian cancers are homologousrecombination (HR)-deficient due to genetic and epigenetic mutations of HR pathway genes including the tumor suppressor genes BRCA1 and 2. HR deficiency (HRD) compromises cells' ability to efficiently repair DNA damage, but it also increases sensitivity to chemotherapeutic treatment strategies; however, not all ovarian cancer patients with HRD tumors exhibit positive responses to chemotherapy. Our previous iTRAQ-based comprehensive proteomic characterization of high-grade serous ovarian carcinomas found that lower levels of histone H4 acetylation at Lys12 and Lys16 (H4-K12acK16ac) were associated with HRD tumors compared with non-HRD tumors. In the current study, we developed and validated an H4-K12acK16ac parallel-reaction-monitoring (PRM)-Targeted mass-spectrometry-based assay to analyze acetylation changes of histone H4 and to determine the association of these changes with total H4, histone acetyltransferase, and histone deacetylase (HDAC) levels. Whereas the levels of H4 and histone acetyltransferases were stable irrespective of HRD status, the levels of histone H4 acetylation and one HDAC, HDAC6, were elevated in the HRD tumors. Relative H4 acetylation levels were also analyzed by an antibody-based approach in additional ovarian tumors. It is possible that specific H4 acetylation at Lys12 and Lys16 associated with HRD could inform chemotherapeutic treatment modalities to improve ovarian cancer patients' treatment response.