Abstract A061: Identification and validation of integrin β1 as an <i>in vivo</i> -specific combination partner with MAPK pathway inhibitors in BRAF mutant colorectal cancer

Abstract Encorafenib (BRAFi) + cetuximab (EGFRi) + chemotherapy is the standard of care for patients with BRAF V600 mutant Colorectal Cancer (CRC). However, the median progression-free survival is 12.8 months. Herein, we performed in vivo shRNA screens with a targeted library in multiple BRAF mutant CRC models, including two PDX-derived cell lines, in the presence or absence of the combination of MAPK pathway inhibitors, dabrafenib (BRAFi) and trametinib (MEK1/2i). We implemented doxycycline inducible genetic depletion approaches to validate the in-vivo-specific top dependency in the presence of dabrafenib + trametinib. We also explored the mechanisms of action of the combination approach through unbiased transcriptomic analysis and immunohistochemistry. The in vivo shRNA screens identified Integrin β1 (ITGB1) as a top-ranked in-vivo-specific dependency in tumors from two PDX-derived cell lines. Depletion of ITGB1, combined with dabrafenib + trametinib, significantly delayed tumor progression and extended the survival of tumor-bearing mice, compared to either ITGB1-depletion itself or dabrafenib + trametinib treatment. With the help of AI-based tools, HALO and QuPath, tumor sections stained with H&amp;E and Ki67 were classified and quantified. The therapeutic benefits of the combination in vivo were associated with a reduction in tumoral Ki67 positive percentage. Interestingly, the combination also reduced the tumor’s mucinous phenotype. Unbiased analysis of RNA sequencing identified Mucin 2 (MUC2) as one of the up-regulated differential expression genes (DEGs) in the presence of dabrafenib + trametinib, compared to untreated tumors. Depletion of ITGB1 inhibited the dabrafenib+trametinib-induced increase in MUC2 gene expression. Depletion of ITGB1 also enhanced Bcl2 Modifying Factor (BMF) expression. Moreover, depletion of ITGB1 inhibited phosphorylation of Focal Adhesion Kinase (FAK) and reduced the expression of its binding partner, Integrin α5 (ITGA5), which are key regulators in extracellular matrix (ECM) signaling. Our findings suggest that ITGB1 is a key survival mechanism for BRAF mutant CRC tumors under treatment with MAPK pathway inhibitors, potentially via regulating MUC2, mucinous phenotype, pro-apoptotic protein expression and key regulators in ECM signaling. Targeting ITGB1 provides a promising combinational therapeutic opportunity with MAPK pathway inhibition to treat BRAF mutant CRC. Citation Format: Hao Chen, Lujian Tan, Jing Yuan, Varunika Vivekanandan, Khushboo Jindal, Steve Woolfenden, Ned Kirkpatrick, Lori Iaconis, Vivek Sagar, Vesselina Cooke, David Kodack. Identification and validation of integrin β1 as an in vivo-specific combination partner with MAPK pathway inhibitors in BRAF mutant colorectal cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A061.