GRN-300 restores PARP inhibitor sensitivity through inhibition of the HDAC-MEF2A-RAD51 pathway in BRCA2-revertant ovarian cancer

Genomic instability is a hallmark of cancer. Germline mutations in DNA repair genes such as BRCA1 and BRCA2 predispose to tumor development but also confer vulnerabilities exploitable by therapy. Poly ADP-ribose polymerase (PARP) inhibitors selectively target tumors with homologous recombination deficiency (HRD) caused by BRCA1/2 mutations and elicit strong responses in ovarian cancer. However, resistance frequently emerges through secondary BRCA1/2 reversion mutations that partially restore DNA repair function, underscoring the need for strategies to overcome PARP inhibitor resistance. Salt-inducible kinase 2 (SIK2), an AMPK-related kinase, is required for ovarian cancer cell proliferation and metastasis. The selective SIK2 inhibitor GRN-300 induces DNA double-strand breaks (DSBs) in HR-proficient cells and synergizes with PARP inhibitors to produce synthetic lethality. Here, we investigated whether GRN-300 can restore PARP inhibitor sensitivity in BRCA2-revertant ovarian cancer. GRN-300 markedly reduced the IC₅₀ of olaparib in both PARP-sensitive PEO1 cells (1.98 to 0.64 µM) and resistant BRCA2-revertant PEO1-C4-2 (6.87 to 0.92 µM) and PEO4 (3.47 to 1.17 µM) ovarian cancer cells, restoring responsiveness. GRN-300 increased ROS production, enhanced caspase-3/7 activation, and promoted olaparib-induced apoptosis while suppressing colony formation. In PEO1-C4-2 xenografts, GRN-300 plus olaparib significantly inhibited tumor growth without affecting body weight. Combination treatment increased gamma-H2AX and comet-tail formation, indicating enhanced DNA damage accumulation. Mechanistically, GRN-300 reduced RAD51 expression and RAD51 foci formation. SIK2 inhibition decreased phosphorylation of class IIa HDAC4/5/7, abrogating HDAC class II-MEF2A transcriptional activity. ChIP-qPCR revealed reduced MEF2A binding to regulatory regions of DNA repair genes, including RAD51 and RAD50, thereby suppressing DSB end resection and strand invasion mediated by the MRN complex. In summary, GRN-300 restores PARP inhibitor sensitivity in BRCA2-revertant ovarian cancer through inhibition of the HDAC-MEF2-RAD51 pathway, providing a promising therapeutic strategy to overcome PARP resistance.