MSI, or microsatellite instability, is a characteristic of some cancers that can help predict how well a patient will respond to immunotherapy. MSI testing, performed on a tumor sample, assesses the stability of microsatellites.
Understanding Microsatellites and MSI
Microsatellites are short, repetitive DNA sequences scattered throughout the genome. Because of their repetitive nature, they are prone to errors during DNA replication. Normally, these errors are corrected by the mismatch repair (MMR) system. However, when the MMR system is deficient, these errors accumulate, leading to changes in the length of microsatellites, resulting in MSI.
MSI Testing and Immunotherapy Response
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Predictive Biomarker: MSI status can serve as a biomarker to identify cancer patients who are more likely to benefit from immunotherapy.
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Mechanism: Cancers with high MSI (MSI-H) tend to have a higher number of mutations. These mutations can lead to the production of abnormal proteins (neoantigens) that the immune system can recognize and target. Immunotherapy drugs, particularly checkpoint inhibitors, can then enhance the immune response against these cancer cells.
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Testing Method: MSI testing is typically performed on a sample from a patient's tumor. According to the reference, MSI testing is generally based on five microsatellite markers.
Clinical Significance
Identifying MSI-H cancers is crucial because these cancers often show a better response to immune checkpoint inhibitors, a type of immunotherapy drug. This knowledge helps oncologists make informed treatment decisions and potentially improve patient outcomes.
