High Tumor Mutational Burden in Microsatellite Stable Metastatic Colorectal Cancer: Enhancing POLE Mutation Selection Through SBS Mutational Signatures.

Tumor Mutational Burden (TMB) has emerged as a crucial surrogate biomarker for mapping the mutational landscape of colorectal cancer (CRC) and guiding treatment decisions, particularly regarding immune checkpoint inhibitors. In the case of mismatch repair deficient/microsatellite unstable (MMRd/MSI) CRC, staining evaluations are considered the gold standard for identifying this hypermutated subgroup. However, the Polymerase Epsilon (POLE) gene presents a unique challenge, as only a subset of variants disrupt proofreading ability while maintaining polymerase functionality. Although TMB may indirectly indicate which tumors exhibit a hypermutated phenotype, technical challenges and resource limitations can hinder its assessment. To address these complexities, mutational signature analysis can provide valuable insights into the etiology of carcinogenesis, potentially illuminating the pathogenicity of various genetic variants. We conducted a comprehensive analysis using a highly selective mutational signature approach on a large cohort of tumors profiled through whole exome sequencing (WES). This analysis aimed to identify specific POLE mutations associated with loss of proofreading function, classifying tumors into POLE loss of proofreading (LOP) and non-LOP categories. Subsequently, we evaluated the response to immunotherapy in patients with both POLE LOP and non-LOP colorectal tumors, with a focus on treatment efficacy and clinical outcomes. From a clinical perspective, these findings are instrumental in refining patient selection for immunotherapy, ensuring that those who stand to benefit most are prioritized. Moreover, our data supports the hypothesis that immunotherapy should be explored as a frontline treatment for patients with LOP POLE mutant metastatic CRC.