Predictive and prognostic implications of microsatellite instability and mismatch repair deficiency in carcinomas of the gastrointestinal tract, liver, and pancreas

Defects in DNA mismatch repair and the resultant microsatellite instability (MSI) phenotype in various organ systems have become a topic of great interest in recent years due to the demonstrated safety and efficacy of various immune checkpoint inhibitors in tumors with high neoantigen load. In the gastrointestinal tract, evaluating for Lynch syndrome was the initial indication for assessing MSI; this has since evolved to appraising eligibility for immune checkpoint inhibition. The purpose of this review is to outline the clinicopathologic spectrum of MSI and mismatch repair deficiency (MMRD) in the gastrointestinal tract, liver, and pancreas.