Microsatellite Instability (MSI-H)

Microsatellite instability is a pattern of DNA errors in which short, repeating sequences (microsatellites) become abnormally long or short because the cell's mismatch repair system is broken. When many of these sequences are affected, the tumor is called MSI-High (MSI-H). MSI-H tumors often respond well to immunotherapy drugs called checkpoint inhibitors.

Origin

The term "microsatellite" was coined in the 1980s to describe short tandem DNA repeats. MSI was first characterized in colorectal cancers in 1993 and tied to defective mismatch repair (MMR) genes; the MSI-H designation comes from the 1997 Bethesda Guidelines for Lynch syndrome testing.

Significance in Asbestos Context

MSI-H isn't a hallmark of asbestos-driven cancers. Pleural and peritoneal mesotheliomas are almost always microsatellite stable, and asbestos-related lung cancers usually are as well. Its relevance to asbestos patients is mostly about treatment options: if your tumor is tested and comes back MSI-H, you may qualify for immune checkpoint inhibitors such as pembrolizumab, which are FDA-approved across many MSI-H solid tumors. MSI-H can also point to an inherited cause (Lynch syndrome) that is separate from asbestos and worth discussing with a genetic counselor, since it affects family members and future cancer screening.

Example Sentence

Her oncologist ordered MSI testing on the tumor sample, explaining that an MSI-H result would open the door to immunotherapy in addition to standard treatment.

Related Terms

DNA Mismatch Repair (MMR), Tumor Mutational Burden (TMB), Lynch Syndrome, Immunohistochemistry, Pembrolizumab, BAP1

Notes

• Ask your oncologist whether your tumor has been tested for MSI status and mismatch repair (MMR) proteins. These are standard biomarkers that can change treatment options.
• If your result is MSI-H, ask specifically about eligibility for checkpoint inhibitor immunotherapy (such as pembrolizumab), which has tumor-agnostic FDA approval for MSI-H cancers.
• An MSI-H result should prompt a referral to a genetic counselor to evaluate for Lynch syndrome; this matters for your blood relatives' cancer screening.
• Keep copies of all molecular pathology reports (MSI, MMR, NGS panels) with your other medical records. They document treatment decisions and may be relevant to your legal claim.