PRMT5 inhibition impairs Fanconi Anemia pathway-mediated homologous recombination and enhances the antitumor efficacy of Temozolomide in glioblastoma

Despite multimodal therapy of surgical resection, radiation, and chemotherapy, glioblastoma patients show a dismal prognosis. Protein Arginine Methyltransferase 5 (PRMT5) is overexpressed in glioblastoma, and its inhibition imparts an anti-tumor effect. Tumor cells invariably develop resistance to Temozolomide (TMZ), the standard chemotherapeutic agent for glioblastoma. However, the mechanistic role of PRMT5 in treatment-resistant glioblastoma is unknown. Patient-derived glioma stem-like cells (GSCs), treated with PRMT5 inhibitor (LLY-283) or transfected with PRMT5-target-specific siRNA, were treated with TMZ and subjected to in vitro functional and mechanistic studies. The intracranial mouse xenograft model was used to test the in vivo antitumor efficacy of combination treatment. We found that PRMT5 inhibition increased the cytotoxic effect of TMZ in GSCs. Unbiased transcriptomic profiling revealed negative enrichment of DNA damage repair pathways, with prominent suppression of the Fanconi anemia (FA) pathway. PRMT5 inhibition abrogated the TMZ-induced G2/M cell cycle arrest. Importantly, combination treatment increased the DNA double-strand breaks (γH2AX foci) and enhanced the DNA damage (comet assay). Specifically, the LLY-283 treatment blocked the FA pathway-mediated homologous recombination repair in GSCs. In vivo, the LLY-283 and TMZ combination significantly curbs tumor growth and prolongs the survival of tumor-bearing mice. Furthermore, compared to monotherapy, there was a significant reduction in the proliferation marker Ki-67, while the apoptosis marker cleaved caspase 3 and the DNA damage response marker γH2AX were upregulated. Collectively, these findings identify PRMT5 as a critical regulator of the FA pathway in glioblastoma and demonstrate that PRMT5 inhibition potentiates TMZ efficacy by disrupting FA-dependent homologous recombination repair, indicating that the combination of PRMT5 inhibition and TMZ could be a novel therapeutic strategy for glioblastoma.