Abstract 7187: Epigenetic drugs inhibit the oncogenic activity of methyl CpG binding protein 2 (MECP2)

Abstract Purpose of the study: MECP2 amplification plays a role similar to activated oncogenic RAS in tumorigenesis, activating the MAPK and PI3K signaling pathways. MECP2 amplification and RAS mutations are mutually exclusive across human cancers; solid tumors such as high-grade serous ovarian cancer (HGSOC), triple-negative breast cancer (TNBC) and lung adenocarcinoma have 38%, 33% and 29% rates of MECP2 amplification respectively. The MECP2 protein has DNA- binding activity which is required for oncogenic transformation and binding is dependent on cytosine methylation. Therefore, MECP2 DNA binding may be targeted by inhibiting cytosine methylation with DNMTi. Additionally, since MECP2 protein requires HDAC activity for its stability, treatment with either HDACi and/orDNMTi could be therapeutic strategies in MECP2- dependent tumors. Experimental Procedures and Results: The MDAMB468 (TNBC) cell line was modified stably with a doxycycline-inducible shRNA construct targeting the 3’UTR of MECP2 (shMECP2-3’UTR) and growth was dependent on MECP2 in culture. The cells were then implanted in nude mice subcutaneously and tumors allowed to reach approximately 150 mm3 before shRNA was induced. Tumor growth in doxycycline-treated mice immediately ceased whereas tumors in vehicle-treated mice continued to grow, showing dependence on MECP2 activity in vivo. To verify that this was an on-target effect, the modified cell line was further infected with lentiviruses carrying both splicing isoforms of MECP2 without the 3’UTR shRNA target. This reconstitution allowed growth as a xenograft at a similar rate regardless of shRNA induction, confirming inhibition was on-target. We then assessed the therapeutic efficacy of HDACi and DNMTi alone and in combination in xenograft (for TNBC and lung) or PDX (HGSOC) models of MECP2-dependent and independent tumors. To determine if growth inhibition was an on-target effect of these drugs, MDAMB468 was stably transduced with activated KRAS or GFP as a control and was injected in nude mice. Activated KRAS rescued the epigenetic drug sensitivity of xenografts, demonstrating that these drugs inhibit tumor growth by preventing the ability of MECP2 to induce RAS-activated growth factor pathways. Summary: The antitumor effects of the commonly used epigenetic drugs are thought to result from cell reprogramming through re-expression of tumor suppressors that had been silenced by hypermethylation or by repressive chromatin marks. However, in this context, we show that these epigenetic drugs can act directly by interfering with the oncogenic activities of MECP2, culminating in inhibition of key growth factor pathways for transformation. Conclusion: Targeting MECP2 function with epigenetic drugs provides an unanticipated therapeutic opportunity for treating the subset of tumors that overexpress MECP2. Citation Format: Manish Neupane, Mukesh Kumar, Malela M. Werner, Colleen M. Donnelly, Alsu Ibragimova, Brian Curcio, Inna Chervoneva, Irina Vasilevskaya, Daniel P. Silver. Epigenetic drugs inhibit the oncogenic activity of methyl CpG binding protein 2 (MECP2) [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 7187.