microsatellite instability
Also written as: MSI — Microsatellite Instability
A molecular phenotype caused by defective DNA mismatch repair, resulting in hypermutation and immunotherapy sensitivity.
Full Definition
Microsatellite instability (MSI) is a molecular phenotype characterized by the accumulation of mutations in repetitive DNA sequences called microsatellites due to deficient mismatch repair (MMR) mechanisms. MSI occurs in approximately 15% of colorectal cancers and smaller percentages of other tumor types, either through germline mutations (Lynch syndrome) or somatic inactivation of MMR genes. Tumors with MSI-high status typically exhibit increased tumor-infiltrating lymphocytes and respond favorably to immune checkpoint inhibitors, making MSI testing an important predictive biomarker.
Usage
Usage note: Specify MSI-high, MSI-low, or microsatellite stable status when reporting results.
In Context
- "Molecular testing revealed microsatellite instability-high status, indicating potential benefit from pembrolizumab therapy." — Molecular pathology report
- "Universal screening for microsatellite instability is now recommended for all newly diagnosed colorectal cancers." — Clinical practice guideline