Abstract 1704: Interplay of MITF and NCOA3 in orchestrating oncogenic transcriptional network in uveal melanoma

Abstract Uveal melanoma (UM) originates from melanocytes residing in the uveal tract of an eye. Despite effective local control, 50% of patients develop metastatic disease responding poorly to chemotherapy, targeted therapy, and immunotherapy. The molecular basis of events leading to initiation of UM is related to mutations in G alpha proteins GNAQ or GNA11, rendering them constitutively active and activating downstream PKC, ERK and AKT pathways sustaining cell proliferation. In our previous work, we showed that NCOA3 regulates proliferation of UM cells in vitro and the growth of UM xenografts in vivo downstream of GNAQ or GNA11. The critical role of NCOA3 in UM is manifested by transcriptional regulation of proliferation-related genes that are massively suppressed by NCOA3 knockout or knockdown. Here, we interrogated transcription factors that NCOA3 interacts with in UM to execute oncogenic programs and identified Melanocyte inducing transcription factor (MITF) as its critical partner. Chromatin immunoprecipitation (ChIP) using an NCOA3 recognizing antibody followed by sequencing was performed to determine NCOA3 binding sites in the human genome. The known transcription binding motifs within NCOA3 peaks were discovered using HOMER algorithm. Co-immunoprecipitation was performed to examine direct NCOA3 and MITF interaction. ChIP sequencing for MITF was performed to determine co-localization of NCOA3 and MITF at chromatin. Proliferation of UM cells after inhibition of MITF by either siRNA mediated knockdown or MITF inhibitor ML329 was evaluated using MTT assay. Functional enrichment analysis of transcriptome was used to identify classes of genes regulated by MITF, with specific attention to the subset regulated by MITF partnered with NCOA3. NCOA3 chromatin peaks are highly enriched in MITF binding motifs, and 77% of the NCOA3 peaks co-localizes with the MITF peaks. NCOA3 co-immunoprecipitates with MITF. Among transcriptional programs regulated by MITF in UM are those related to melanocyte biology, differentiation, and melanin production, but also those related to melanoma growth, especially cell division, DNA replication, myc response, purine and pyrimidine metabolism, and mTOR signaling. Strong suppression of these transcriptional signatures by inhibition of MITF using siRNA-mediated knockdown or pharmacological inhibition correlates with dramatic loss of cell viability. Clinical data indicate that higher expression of MITF-positively regulated genes correlates with lower survival rates of UM patients. Importantly, the transcriptional programs regulated by MITF cooperating with NCOA3 were related to TGF beta signaling, cell adhesion, ECM interaction and actin cytoskeleton. Importance of MITF as an oncogene is manifested in uveal melanoma cell lines and clinical data. Oncogenic function of MITF is partially dependent on its interaction with a transcription co-activator NCOA3. Citation Format: Aleksandra Rusin, Maria E. Echartea, Sai P. Manikonda, Darlene G. Skapura, Christel M. Davis, Erik A. Ehli, Sandra L. Grimm, Cristian Coarfa, Salma Kaochar. Interplay of MITF and NCOA3 in orchestrating oncogenic transcriptional network in uveal melanoma [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 1704.