Abstract 4323: CARM1-mediated methylation of ZNF703 regulates its oncogenic functions in estrogen receptor positive breast cancer

Abstract ER-positive (ER+) breast cancer, characterized by estrogen receptor expression on tumor cells, constitutes approximately 70% of all breast cancer cases. Identifying specific oncogenes and elucidating relevant oncogenic mechanisms are critical for developing effective therapies. ZNF703, a NET/N1Z family transcription factor, drives 8p12 locus amplification found in 10-20% of ER+ breast tumors. ZNF703 has been reported as an oncogene that promotes ER+ breast tumorigenesis in vitro. However, its regulation and in vivo oncogenic function remain unclear. We showed that ZNF703 expression is induced by estradiol in ER+ cell lines. ShRNA-mediated ZNF703 knockdown significantly reduced cell proliferation in MCF7 and BT474 cells. ZNF703 is post-translationally modified, which may affect its nuclear localization as well as its functions. Using mass spectrometry, we identified two asymmetrically di-methylated arginine residues (R544 and R580) on ZNF703 in human breast tumors. Protein arginine (R) methylation is catalyzed by protein arginine methyltransferases (PRMTs), which are classified into Type I, II, and III based on the type of methyl-R they produced. Among these, CARM1, a Type I PRMT, catalyzes asymmetric di-methylation of arginine residues. We showed that ZNF703 is methylated by CARM1 in in vitro methyltransferase assay. Furthermore, ZNF703 protein levels were reduced in CARM1 knockout MCF7 cells, accompanied by an increased degradation rate, suggesting that ZNF703 may be stabilized by CARM1. Given the lack of an ER+ breast cancer mouse model, we derived an ER+ MG1 rat cell line from estrogen-induced rat breast tumors. Consistent with our findings in human ER+ cell lines, ZNF703 was validated as an estrogen-induced gene in MG1 cells. ZNF703 knockout in MG1 cells impaired cell proliferation and colony formation. Functional rescue experiments demonstrated that re-expression of wild-type ZNF703 fully restored cell growth, whereas the methylation-deficient mutants ZNF703 R580K and R544K only partially rescued the proliferation defect. Our results implicate that CARM1-mediated ZNF703 methylation may regulate its stability and oncogenic functions. Given that patients diagnosed with 8p12 locus amplification caused by ZNF703 typically have poor prognosis, our MG-1/ACI rat syngeneic model would enable the investigation of ZNF703’s oncogenic functions in vivo. Citation Format: Jingjing Zhou, Eui-Jun Kim, Gui Ma, Ang Gao, Wei Xu. CARM1-mediated methylation of ZNF703 regulates its oncogenic functions in estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4323.