How MSI-High and Mismatch Repair Testing Guides Immunotherapy

MSI-High and mismatch repair deficiency describe tumors that have lost the cellular machinery for correcting DNA copying errors. When that repair system fails, mutations pile up across the genome, the tumor starts to look foreign to the immune system, and it becomes unusually likely to respond to immunotherapy that releases the brakes on that immune response. That single piece of biology is why, in 2017, the U.S. Food and Drug Administration approved a drug for the first time based on a shared molecular feature rather than the organ where the cancer began.

I write this as a physician-scientist who evaluates the evidence behind biomarkers, not as an oncologist directing anyone's treatment. The story here is a good case study in what it takes for a laboratory finding to earn the weight of a regulatory decision. This is general education, not medical advice, and questions about a specific cancer belong with the treating team.

What mismatch repair does, and what happens when it fails

Every time a cell divides, it copies about three billion base pairs of DNA, and it makes mistakes. A dedicated proofreading system, called DNA mismatch repair, catches and fixes the small insertion, deletion, and base-pairing errors that slip past the main copying enzymes. Four proteins do most of this work, and they operate in pairs: MLH1 with PMS2, and MSH2 with MSH6. As the ESMO consensus recommendations on MSI testing describe, these are the proteins a laboratory checks first when it wants to know whether the repair system is intact.

When one of these proteins is lost, the proofreading breaks down. Errors accumulate fastest in short, repetitive stretches of DNA called microsatellites, where the copying machinery is most prone to slipping. A tumor with many of these unrepaired errors is called microsatellite instability-high, or MSI-High. Because the underlying cause is the failed repair system, the terms travel together: a mismatch repair-deficient (dMMR) tumor is generally MSI-High, and the two labels point at the same broken pathway from two different testing angles.

The loss can be inherited or acquired. In Lynch syndrome, a person inherits one faulty copy of a mismatch repair gene, and cancers arise when the second copy is lost in a tissue. Far more often, the deficiency is sporadic, commonly from the tumor silencing the MLH1 gene. Either way, the downstream consequence is the same: a genome that can no longer keep itself tidy.

Why a broken repair system makes a tumor visible to the immune system

Here is the link that makes this biology matter for treatment. A tumor that cannot repair its own errors generates a very large number of mutations, and many of those mutations produce abnormal proteins the body has never seen. Fragments of those proteins, called neoantigens, appear on the tumor's surface. The immune system reads them as foreign.

Many tumors survive this scrutiny by exploiting a natural off-switch, the PD-1 pathway, that tells T cells to stand down. Drugs called PD-1 inhibitors block that off-switch. In a tumor already carrying a heavy load of neoantigens, releasing the brake can unleash a strong and durable immune attack. In a tumor with few mutations and little for the immune system to recognize, the same drug often has little to work with. That difference in mutational burden is the mechanistic reason MSI-High and dMMR predict response, and it is why the marker is described as predictive rather than merely prognostic.

How the deficiency is actually detected

Two established methods answer the question, and they approach it from different directions.

Immunohistochemistry (IHC) stains a tissue sample for the four repair proteins. If one is missing, the stain shows its absence directly. A normal result shows all four present; loss of MLH1 and PMS2 together, or of MSH2 and MSH6 together, signals deficiency. The ESMO recommendations describe IHC for these four proteins as the first action used to assess for deficiency, because it is widely available and inexpensive.

MSI testing by PCR or next-generation sequencing looks at the microsatellite regions themselves and measures how unstable they have become compared with the patient's normal tissue. Rather than checking whether the repair proteins are present, it checks whether the damage they were supposed to prevent has occurred.

The two approaches agree in the large majority of cases, which is why either can support the same clinical conclusion. When results are discordant or ambiguous, the tests are used together. An abnormal result also carries a second signal: certain patterns of protein loss raise the possibility of Lynch syndrome and can prompt germline testing and genetic counseling, which has implications for the patient's relatives as well.

From biomarker to a tissue-agnostic approval

The regulatory milestone rests on this biology. In May 2017, the FDA granted accelerated approval to the PD-1 inhibitor pembrolizumab for unresectable or metastatic MSI-High or dMMR solid tumors that had progressed after prior treatment and had no satisfactory alternatives. The FDA itself termed it the first tissue- and site-agnostic approval, as reported in The ASCO Post. The supporting data came from patients with MSI-High or dMMR cancers across multiple tumor types, all sharing the marker rather than an anatomic home.

Accelerated approval is a promise pending confirmation, and the confirmation came. On March 29, 2023, the FDA converted the indication to full approval. Per Merck's announcement, the decision drew on a pooled analysis of 504 patients across the KEYNOTE-158, KEYNOTE-164, and KEYNOTE-051 trials, spanning more than 30 cancer types. The overall response rate was 33.3 percent, and the responses proved durable: among patients who responded, 77 percent had responses lasting at least a year and 39 percent lasted three years or longer. The FDA approval summary published in the peer-reviewed literature laid out the same evidentiary basis for the accelerated approval that preceded it.

Two cautions keep this honest. The indication is specific: it applies to advanced disease that has progressed after prior treatment and lacks satisfactory alternatives, and it requires detection by an FDA-approved test. And a one-third response rate, however historic, means most patients with the marker did not have a measured response, so the biomarker sorts a population toward better odds rather than guaranteeing an outcome for any individual.

Why this matters as a way of thinking

The lasting lesson is about appraisal. A biomarker earns a tissue-agnostic indication only when a plausible mechanism, a reproducible test, and durable clinical benefit line up across many tumor types. MSI-High and dMMR cleared that bar because the biology, the diagnostics, and the trial data told one coherent story. That is the standard any future claim of a universal biomarker should be measured against.