MSI genetic testing, or Microsatellite Instability testing, identifies tumors caused by defective mismatch repair (MMR) genes. This test compares the number of nucleotide repeats in microsatellite markers between normal tissue and tumor tissue from the same individual.
Understanding Microsatellite Instability (MSI)
Microsatellites are short, repetitive DNA sequences found throughout the genome. These sequences are prone to errors during DNA replication. Normally, the mismatch repair (MMR) system corrects these errors. However, when the MMR system is defective, these errors accumulate, leading to microsatellite instability.
How MSI Testing Works
MSI testing analyzes a panel of microsatellite markers in both normal and tumor tissue samples from the same patient. The number of repeats in these markers are compared.
- Stable Microsatellites: If the microsatellites have the same number of repeats in both tissues, they are considered stable.
- Unstable Microsatellites: If the microsatellites show a different number of repeats in the tumor tissue compared to the normal tissue, it indicates microsatellite instability (MSI).
Interpretation of MSI Results
MSI results are generally categorized as:
- MSI-High (MSI-H): High levels of microsatellite instability, indicating a high probability of defective MMR.
- MSI-Low (MSI-L): Low levels of microsatellite instability.
- Microsatellite Stable (MSS): No microsatellite instability detected.
Clinical Significance of MSI Testing
MSI status can influence treatment decisions, particularly in certain cancers like colorectal cancer and endometrial cancer.
- Immunotherapy: MSI-H tumors are often more responsive to immunotherapy.
- Prognosis: MSI status can provide prognostic information, helping to predict the likely course of the disease.
Example of MSI Testing Application
In colorectal cancer, for example, identifying MSI-H tumors helps determine which patients might benefit from immunotherapy and can also provide information about the likelihood of response to certain chemotherapies.
