1103 - SETD2 Loss Predicts Response to Chemoradiotherapy in Rectal Cancer: Clinical Outcomes and Preclinical Validation in Orthotopic Models

Purpose/Objective(s): SETD2, a key regulator of histone H3K36 trimethylation (H3K36me3), plays a critical role in maintaining genomic stability and mediating DNA damage repair. SETD2 has been implicated in therapy resistance in several cancers, but its impact in rectal cancer is not well understood. This study investigates the clinical relevance of SETD2 loss in rectal cancer patients undergoing neoadjuvant chemoradiotherapy (nCRT) and uses orthotopic models to identify potential strategies to overcome radiation resistance.

Materials/Methods: A retrospective analysis of 52 rectal cancer patients treated with nCRT followed by surgery were performed. SETD2 expression and H3K36me3 status were evaluated by immunohistochemistry on pre-treatment biopsy specimens and correlated with tumor regression grade (TRG), recurrence-free survival (RFS), and overall survival (OS). Orthotopic rectal cancer models were established by injecting SETD2-deficient or SETD2-proficient HCT116 colorectal cells into the rectal wall of immunodeficient mice. Mice were treated with radiation (5 Gy x 5 fractions) utilizing the small animal radiation therapy platform (SARRP). Tumor growth, DNA damage (?H2AX), and apoptosis (cleaved caspase-3) were assessed.

Results: SETD2 loss was observed in 29% of the patients and was significantly associated with improved response to nCRT, as indicated by a higher rate of TRG 0-1 (90% vs 50% p < 0.01). Clinically, SETD2 loss correlated with improved RFS (HR 0.47; 95% CI, 0.31–0.71) and OS (OS; HR 0.4; 95% CI, 0.27-0.59). In orthotopic rectal cancer models, SETD2-deficient tumors exhibited enhanced radiosensitivity compared to SETD2-proficient tumors. Tumors with SETD2 loss exhibited elevated ?H2AX levels, suggesting sustained DNA damage after radiation and increased cleaved caspase-3, indicative of enhanced apoptosis. These molecular changes translated into a significant therapeutic benefit, with a 60% reduction in tumor volume at 4 weeks post-radiation in SETD2-deficient tumors compared to SETD2-proficient tumors (p < 0.01).

Conclusion: SETD2 loss serves as a predictive biomarker of improved response to chemoradiotherapy and favorable clinical outcomes in rectal cancer. Preclinical orthotopic models show that SETD2-deficient tumors are more radiosensitive. These findings provide a strong rationale for tailoring treatment strategies to enhance outcomes by targeting SETD2 in rectal cancer patients undergoing radiation therapy.