Synthetic lethality from the combination of a histone methyltransferase SUV39H2 inhibitor and a poly (ADP-ribose) polymerase inhibitor for uterine leiomyosarcoma.

BACKGROUND: Uterine leiomyosarcoma (uLMS) has a poor prognosis owing to its resistance to chemotherapy. Therefore, novel therapeutic targets for uLMS should be identified. Suppressor license of variegation 3-9 homolog 2 (SUV39H2) is a histone methyltransferase that promotes the repair of double-strand DNA breaks by recruiting phosphorylated H2AX (γH2AX). In this study, we investigated the potential therapeutic targets of SUV39H2 in uLMS and the mechanism of synthetic lethality between PARP inhibitors and the SUV39H2 inhibitor OTS186935. METHODS: First, we analyzed the mRNA and protein expression of SUV39H2 in the clinical tissues of uLMS, normal myometrium, and leiomyomas using real-time polymerase chain reaction and immunohistochemistry, respectively. Next, we conducted drug sensitivity assays for OTS186935 alone and in combination with olaparib, a poly (ADP-ribose) polymerase inhibitor, using the uLMS cell lines SK-LMS-1 and SK-UT-1. We performed western blotting, immunofluorescence, and chromatin immunoprecipitation sequencing (ChIP-seq) to investigate γH2AX following OTS186935 treatment in addition to in vivo experiments using nude mice with subcutaneously implanted uLMS. RESULTS: SUV39H2 expression in uLMS was significantly higher than that in the normal myometrium and leiomyomas. OTS186935 decreased the viability of both cell lines, and its combination with olaparib resulted in synthetic lethality in SK-UT-1 cells (combination index = 0.88). After treatment with OTS186935, γH2AX accumulation decreased. ChIP-seq also showed downregulation of γH2AX following OTS186935 treatment. Notably, the combination of OTS186935 and a PARP inhibitor was significantly more effective in vivo. CONCLUSION: OTS186935 inhibited double-strand DNA break repair, as evidenced by γH2AX downregulation by ChIP-seq and other assays. Combining OTS186935 with olaparib demonstrated activity resembling synthetic lethality; however, further validation is required before clinical translation.