Mismatch repair (MMR) assay uptake and use in early colorectal cancer patients with MMR-deficient and MMR-proficient tumors

Three biomarkers for immune checkpoint inhibitors—mismatch repair (MMR)/microsatellite instability, tumor mutation burden, and PD-L1 ligand expression—are currently approved for utilization in therapeutic decisions. The adoption of these biomarkers has gained ground gradually following demonstrated clinical utility. The colorectal cancer database of our cancer program was reviewed, and data on the availability and on the results of MMR molecular evaluation of patients diagnosed with stage 1–3 colorectal cancer over 8 years were extracted from medical records. MMR evaluation was available in a higher percentage of patients diagnosed with non-metastatic colorectal cancer in most recent years. MMR was performed more often in patients with more advanced stage (stage 2 and 3) and in patients with right colon locations. MMR deficiency was associated with right colon cancers, high grade, and thrombocytosis. The prevalence of MMR deficiency was 23.1%, and the main defects leading to MMR deficiency were losses of MLH1 and PMS2 nuclear staining, which were present in 86.7% of patients with MMR deficiency. MMR testing showed an increasing up-take over the years, with a significant increase of performance since the approval of immunotherapy in metastatic patients with microsatellite instability/MMR deficiency.