Abstract 4044: Hypomethylation and overexpression of transcription factor AP2B promote growth in ILC via cell cycle modulation

Abstract Invasive lobular carcinoma (ILC) is responsible for 10%-15% of all invasive breast cancers and is associated with worse long-term outcomes compared to the most common no special type (NST). Despite known differences between ILC and NST, the treatment for ILC is the same as stage matched NST. Therefore, identifying the biological molecules that are specific for ILC and can be therapeutically targeted is an urgent need. In this context, we recently identified the transcription factor AP2b (TFAP2B) as one of the most hypomethylated genes in ILC compared with NST and normal mammary tissues. Consistently, differential gene expression analysis in TCGA, METABRIC and SCAN-B datasets confirmed the upregulation of TFAP2B expression in ILC. Methylation of CG island in the first exon possesses the highest correlation with mRNA expression levels. Here, we examined the role of this transcription factor in ILC using various models including cell lines, patient derived organoids (PDOs) and tumor samples. Immunoblotting and RT-qPCR confirmed the specific upregulation of TFAP2B in ILC compared with NST cell lines and PDOs. Immunofluorescence showed a clear nuclear localization of TFAP2B and immunohistochemistry revealed a strong and uniform expression in tissues isolated from ILC patients. TFAP2B knockdown (using siRNA and inducible shRNA) significantly reduces the growth of ILC cell lines and PDOs in anchorage dependent and independent conditions as well as colony formation assay. The actions of TFAP2B seem to be mediated, at least in part, through cell cycle as knockdown of this factor decreases and increases the percentage of cells in S and G0/G1 phase, respectively. CDH1 knockout or inducible expression in NST or ILC cell lines failed to affect TFAP2B levels indicating that changes in E-cadherin levels, in the transformed cells, are not sufficient to alter TFAP2B expression. Preliminary data from CUT and RUN sequencing indicate possible binding sites for TFAP2B in growth factors and cell cycle related genes. Collectively, the data support the specific enhancement of TFAP2B in ILC and its importance for cell proliferation. Future studies will be conducted to uncover the interaction partners and pathways regulated by TFAP2B. This could serve as a basis for the development of novel therapeutic approaches that can be tailored specifically to patients with ILC tumors. Citation Format: Abdalla M. Wedn, Hunter Waltermire, Haley Arbore, Daniel Brown, Osama Shah, Fangyuan Chen, Sanghoon Lee, Jagmohan Hooda, Adrian Lee, Steffi Oesterreich. Hypomethylation and overexpression of transcription factor AP2B promote growth in ILC via cell cycle modulation [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 4044.