Microsatellite Instability as a Predictor of Outcomes in Colorectal Cancer in the Era of Immune-Checkpoint Inhibitors

The microsatellite instable phenotype resulting from errors in DNA mismatch repair proteins accounts for as far as 15 to 20% of non-hereditary colon cancers but is scarce in rectal cancer. It has been shown that the increased existence of tumor-specific neoantigens in hypermutated tumors is correlated with higher tumor-infiltrating lymphocytes (TILs) and overexpression of immune checkpoint receptors and ligands, mainly PD-1 and PD-L1. In particular, the data gained up to now gives evidence that neoantigen recognition constitutes a dominant component in the course of immunotherapies. This review's primary objective is to describe current approvals and summarize present knowledge about the outcomes of immuno-oncology treatment of microsatellite instable colorectal cancer (CRC). The secondary objective is to give a narrative report about testing methodologies, prognostics, and the predictive value of microsatellite instability. For this purpose, a literature review was performed, focusing on published clinical trial results, ongoing clinical trials and timelines, testing methods, and prognostic and predictive value of MSI. Following four recent FDA approvals of immunotherapy of MSI-high CRC, further work should be warranted by pathology societies towards standardization and rising concordance and reproducibility across the IHC/MSI testing landscape in order to facilitate professionals to offer better survival options for patients with CRC.