Abstract 2034: The NAE1 inhibitor MLN4924 sensitizes both BRCA1 wild type and mutant triple negative breast cancers to cisplatin by reducing the nucleotide excision repair protein XPC level

Abstract Background: Triple-negative breast cancer (TNBC) shows poor survival and prognosis, high recurrence, and poor response to conventional therapeutics. To develop novel therapeutics for TNBC, we explored the efficacy of the neddylation inhibitor, MLN4924. We showed that MLN4924 displayed increased sensitivity against TNBC compared to non-TNBC subtypes. Mechanistically, MLN4924 induced extensive re-replication and DNA damage leading to cell death. Furthermore, MLN4924 improved cisplatin efficacy by elevating the DNA damage levels. To demonstrate the mechanisms of MLN4924+cisplatin-mediated enhanced cell death, we examined the XPC level, a key factor in the nucleotide excision repair (NER) pathway, playing a central role in cisplatin damage repair. Neddylation of CUL4A promotes XPC ubiquitination which influences XPC binding to the DNA damage sites for NER. Methods: We examined the XPC protein level by Western blot assay and RNA level by Real Time PCR using both BRCA1-wild type (MDA-MB-231) and BRCA1-mutated (MDA-MB-436) cell lines and mouse xenograft tumor samples. We evaluated if the proteosome inhibitor MG132 stabilizes XPC ubiquitination and protein levels by treating these cells with MG132 for various times. We also overexpressed XPC in these cell lines and examined the effect on survival upon MLN4924 and MLN4924+cisplatin treatments. Results: Our results demonstrate that MLN4924 reduces XPC ubiquitination upon shorter drug treatment but reduces the protein level upon longer drug treatment in both BRCA1-wild type and BRCA1-mutated cell lines. MG132 treatment did not stabilize the XPC ubiquitination level upon shorter drug treatment but stabilized the protein level upon longer drug treatment. We also observed a reduction in the XPC protein level in tumor samples which correlated with the increase in the DNA damage level. Surprisingly, XPC overexpression improved the cell survival in both MLN4924- and MLN4924+cisplatin-treated BRCA1-wild type and mutated TNBC cells. Conclusion: We hypothesize that a reduction in the XPC ubiquitination and protein levels plays a role in cisplatin-induced DNA damage repair reducing NER efficiency and consequently increasing the DNA damage upon MLN4924+cisplatin treatment. Since overexpression of XPC results in improved survival of both MLN4924-and MLN4924+cisplatin-treated cells, we anticipate that XPC not only regulates cisplatin damage repair through NER but may have a broader role in MLN4924 sensitization beyond the NER pathway. Importantly, MLN4924 treatment does not affect the XPC mRNA level emphasizing that neddylation influences the XPC protein level. In summary, our research uncovers novel insights into the role of XPC in both MLN4924 and MLN4924+cisplatin sensitization, shedding light on potential avenues for innovative therapeutic strategies for TNBC patients. Citation Format: Shrilekha Misra, Mateusz Opyrchal, Alo Ray. The NAE1 inhibitor MLN4924 sensitizes both BRCA1 wild type and mutant triple negative breast cancers to cisplatin by reducing the nucleotide excision repair protein XPC level [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2034.