Microsatellite instability (MSI) is when a tumor has a lot of mutations because the mismatch repair genes do not function.
MSI high tumors are instable tumors, and this can be used to screen for Lynch syndrome in colorectal cancer. MSI low tumors are also called MSS or microsatellite stable and are not indicative of Lynch syndrome.
Lynch syndrome is a genetic disorder that predisposes to the development of colorectal cancer. With this syndrome the individual has a mutation in the mismatch repair genes, which leads to the development of several cancers: colorectal, endometrial, uterine, stomach, and bladder cancer.
If the cancer is caught in the early stages this mutation confers good prognosis, and the tumors appear to have a less aggressive biology. This has prognostic as well as therapeutic implications. Early stage MSI high tumors have no benefit from adjuvant chemotherapy because of the good prognosis.
Patients with metastatic disease that are MSI high were found to have a very high response to single agent pembrolizumab which is immunotherapy, while the MSS patients had no response at all to the monoclonal antibody.
There are several ways to test for MSI high: immunochemistry, PCR (polymerase chain reaction), and NGS (next generation sequencing). In view of prognostic and therapeutic implications MSI testing should be included with EGFR, RAS, BRAF, and HER 2 status.