microsatellite instability
Also written as: MSI — Microsatellite Instability
Genetic condition where DNA mismatch repair system failures lead to tumor hypermutability and distinct therapeutic implications.
Full Definition
Microsatellite instability (MSI) occurs when cellular DNA mismatch repair mechanisms fail, resulting in accumulation of mutations in repetitive DNA sequences called microsatellites. MSI-high tumors, found in approximately 15% of colorectal cancers and various other malignancies, demonstrate unique characteristics including better response to immunotherapy but relative resistance to certain chemotherapeutic agents. Testing for MSI status has become standard practice in colorectal cancer management as it guides treatment decisions and identifies patients who may benefit from immune checkpoint inhibitors.
Usage
Usage note: Often reported as MSI-high, MSI-low, or microsatellite stable (MSS).
In Context
- "Molecular testing revealed microsatellite instability-high status, indicating potential benefit from immunotherapy." — molecular pathology report
- "The tumor demonstrated microsatellite instability with loss of MLH1 expression on immunohistochemistry." — diagnostic workup summary