Abstract C031: Disarming colorectal plasticity: Dual HDAC1/2 inhibition reprograms tumor cell fate via H3K27ac-dependent differentiation

Abstract Colorectal cancer (CRC) remains one of the most difficult solid tumors to treat, owing in part to its remarkable cell state plasticity - a property that enables tumor cells to resist therapy, evade differentiation, and adapt to selective pressures. While targeted therapies have succeeded in subsets of patients, most tumors persist and eventually progress. To pursue a fundamentally new therapeutic strategy, we conducted an unbiased chemical screen using a custom-designed dual reporter system designed to broadcast cell state dynamics in real time by fluorescently tagging the endogenous SOX9 (aberrant stem-like) and KRT20 (differentiated) loci. This screen targeted epigenetic regulators capable of supporting CRC plasticity and preventing differentiation. We discovered that selective inhibition of histone deacetylases HDAC1/2 promotes differentiation and reduces tumor growth across human and mouse CRC models. This effect was not a consequence of cytotoxicity but rather epigenetic reprogramming. Through mass spectrometry-based histone modification profiling, we identified H3K27ac and H3K9ac as key epigenetic marks enriched at HDAC1/2-bound loci. These marks correlated with enhanced chromatin accessibility and the upregulation of differentiation-associated genes. Degradation of EP300, a major acetyltransferase, reversed the phenotype, confirming the functional relevance of histone acetylation in mediating this differentiation response. Furthermore, a focused CRISPR screen uncovered DAPK3 as a previously unrecognized downstream effector, providing a mechanistic foothold for future biomarker development. Our findings introduce a paradigm shift in CRC therapy: rather than targeting plastic tumor cells for elimination, we aim to force them into a differentiated, less adaptive state, making them more vulnerable to conventional and targeted treatments. This reprogramming effect is trackable via specific histone acetylation patterns, which we propose as candidate predictive biomarkers of response. Based on these insights, we hypothesize that HDAC1/2 inhibitors, when combined with KRAS or EZH2 inhibitors and/or chemotherapy, could yield synergistic therapeutic benefit. We are actively evaluating these combinations in preclinical models. In sum, this work positions HDAC1/2 inhibition as a precision epigenetic strategy to suppress CRC plasticity through enforced differentiation. By integrating multiomic profiling, functional genomics, and novel reporter technologies, we offer a blueprint for targeting the epigenetic underpinnings of therapy resistance in gastrointestinal malignancies, one that prioritizes reprogramming over destruction. Citation Format: Pornlada Likasitwatanakul, Zhixin Li, Paul Doan, Sandor Spisak, Kishore Raghawan. Akhouri, Qi Liu, Priscilla Liow, Sunwoo Lee, David Chen, Pratyusha Bala, Pranshu Sahgal. Sethi, Daulet Aitymbayev, Jennifer Susan . Thalappillil, Malvina Papanastasiou, William Hawkins, Steven Carr, Haeseong Park, James Cleary, Jun Qi, Nilay Sethi. Disarming colorectal plasticity: Dual HDAC1/2 inhibition reprograms tumor cell fate via H3K27ac-dependent differentiation [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 C031.