Evaluation of Predictive Markers for Immunotherapy in Colorectal Cancer: Concordance Between MMR Protein Expression and Microsatellite Instability in a Retrospective Series

Background/Objectives: In metastatic colorectal cancer (mCRC) the evaluation of mismatch repair (MMR) and microsatellite instability (MSI) status is essential to identify patients eligible for treatment with immune-checkpoint inhibitors (ICI). This study aims to evaluate the potential utility of Comprehensive Genomic Profiling (CGP) in assessing MSI status, in addition to other immunotherapy-predictive biomarkers such as high tumor molecular burden (TMB) and the POLE and POLD1 mutations. Methods: A total of 138 mCRC tumor samples underwent a first-level molecular test (MMR status by immunohistochemistry, MSI by a melting-based PCR approach and RAS/BRAF mutational status by a small next-generation sequencing (NGS) panel) and second-level CGP analysis by the FoundationOne CDx assay. The prevalence of dMMR and MSI tumors was reported. Moreover, the concordance between the MMR and MSI status was determined, and discordant cases were discussed. Results: Twelve cases (8.7%) were MMR-deficient (dMMR); 10 showed high MSI and TMB (>10 mut/Mb). MSI status assessed by CGP and PCR was concordant in all cases except one MSH6-deficient tumor. Two dMMR cases were stable with low TMB. Moreover, in two MLH1/PMS2-deficient cases CGP revealed pathogenic alterations in the MSH2 and MSH6 genes; in both cases, the MLH1 promoter was hypermethylated. A high TMB was the only positive biomarker in 11 cases with a proficient MMR system and no MSI. Conclusions: MSI assessment by CGP analysis showed high concordance (98%) with MMR and was helpful in evaluating ICI eligibility in three out of twelve dMMR cases. Overall, compared to standard methods, analyzing a broader range of microsatellite loci and the simultaneous assessment of multiple predictive biomarkers by CGP may increase diagnostic accuracy and improve therapeutic assessment.