MSI-H Colorectal Cancer: Meaning

Microsatellites are short pieces of DNA where a few letters repeat over and over. Cells copy them clumsily. A repair crew normally fixes the slips.

That crew is the mismatch repair system, and four proteins do most of the work: MLH1, MSH2, MSH6, and PMS2. A fifth gene, EPCAM, can switch off MSH2.

When the crew fails, those repeats end up with the wrong number of copies. That is microsatellite instability. When many sites are affected, the tumor is called MSI-H, meaning microsatellite instability-high.

You will also see dMMR, short for mismatch repair deficient. In practice the two labels point at the same broken system. One names the cause, the other names the fingerprint it leaves.

Labs get there by two routes, and your report will name one or both.

Immunohistochemistry (IHC) stains the tumor for the four proteins. All four present is normal. One or more missing means dMMR, and which protein is missing is a clue to why.

MSI testing reads the repeat sequences themselves, by PCR or by gene sequencing. It counts how many are unstable.

Both are accepted. In the trial that changed first-line treatment, MSI-H or dMMR status was determined locally by either PCR or IHC.

If your pathology report mentions only one method, that is normal. If it mentions neither, ask whether the test was done at all.

Finding dMMR is not the end. The next question is whether the cause is inherited.

Lynch syndrome is the inherited version. It comes from a germline change in MLH1, MSH2, MSH6, PMS2, or EPCAM. It is the most common inherited colorectal cancer syndrome. It accounts for roughly 2% to 3% of all colorectal cancers. All high-penetrance inherited genes together explain only 5% to 6% of cases.

Most MSI-H tumors are not inherited. In sporadic cases, the MLH1 gene is silenced by hypermethylation of its promoter. That is a chemical switch. It shuts the gene off in the tumor alone.

Two follow-on tests sort this out. MLH1 promoter methylation analysis points toward sporadic disease. So does a BRAF pathogenic variant. It is absent in colorectal cancers from people with Lynch syndrome. It is rare even in sporadic adenomatous polyps.

The current advice is universal tumor screening. Test every new colorectal cancer, whatever the person's age, whatever the family history. Older criteria built on relatives missed too many people.

For advanced disease, MSI-H moves you off standard chemotherapy. It moves you onto immune checkpoint inhibitors, drugs that release a brake on the immune system.

The pivotal trial was KEYNOTE-177. It enrolled 307 people with untreated MSI-H or dMMR colorectal cancer that could not be removed or had spread. Half got pembrolizumab by vein every 3 weeks. Half got chemotherapy, either mFOLFOX6 or FOLFIRI, with or without bevacizumab or cetuximab.

Median progression-free survival was 16.5 months with pembrolizumab. It was 8.2 months with chemotherapy. The hazard ratio was 0.60, with a 95% confidence interval of 0.45 to 0.80. The two-sided p value was 0.0004. The FDA approved pembrolizumab as first-line treatment for this group on June 29, 2020. It is given by drip either every 3 weeks or, at a larger amount, every 6 weeks, whichever suits the clinic and you.

On April 8, 2025, the FDA approved nivolumab with ipilimumab for the same setting. It covers adults and children 12 and older whose MSI-H or dMMR colorectal cancer cannot be removed or has spread.

The trial was CHECKMATE-8HW, a three-arm open-label study. Nobody in it had received immunotherapy before. In the first-line comparison against chemotherapy, median progression-free survival was not reached with the combination. It was 5.8 months with chemotherapy. The hazard ratio was 0.21, with a 95% confidence interval of 0.14 to 0.32.

The schedule pairs nivolumab with ipilimumab every 3 weeks for four rounds, then continues nivolumab alone every 4 weeks. The amounts are fixed by the protocol and prepared for you.

Two active options with very different side effect profiles now exist for the same biomarker. Adding ipilimumab raises the rate of immune-related side effects. That trade-off is a real conversation, not a formality.

On May 23, 2017, pembrolizumab became the first drug the FDA approved based on a biomarker rather than the organ the cancer started in. The indication covered adults and children with MSI-H or dMMR solid tumors that had grown after prior treatment and could not be removed or had spread.

The evidence came from 149 patients across five uncontrolled single-arm trials. Ninety had colorectal cancer and 59 had one of 14 other cancer types. The overall response rate was 39.6%. Among people who responded, 78% had responses lasting six months or longer. Response rates were similar in colorectal cancer at 36% and other cancers at 46%.

That is why an MSI-H result on a colorectal specimen is treated as important information even outside colorectal treatment plans.

• Whether MSI or MMR testing was done, and by which method
• If IHC was used, which of the four proteins were present and which were absent
• If MLH1 was absent, whether MLH1 promoter methylation testing was done
• Whether BRAF testing was done
• Whether germline genetic testing for Lynch syndrome has been ordered or completed
• Whether a genetic counselor has been involved

If the tumor result points toward Lynch syndrome and germline testing confirms it, the finding is no longer only about this cancer.

It changes your own follow-up for other cancers. It also gives first-degree relatives a specific gene to test for, rather than a vague family history. First-degree relatives are parents, siblings, and children. Testing them is the point of finding it.

If testing points to sporadic disease instead, that is genuine information too. It closes the question for your family and keeps the focus on treatment.

Either way, this is one of the few pathology findings in colorectal cancer that changes three things at once: drug choice, follow-up, and family screening. Read the actual report, not a summary line.

The result itself is a lab finding, not an emergency. What it leads to is not. Checkpoint inhibitors work by releasing a brake on the immune system, so the immune system can inflame healthy organs, and the ipilimumab combination does this more often.

• Call 911 or go to an emergency department if you have chest pain, a new or worsening cough, or trouble breathing while on pembrolizumab or nivolumab with ipilimumab. Lung inflammation is one of the reactions these drugs cause.
• Call your care team the same day if you have diarrhea that is more frequent than usual, blood or mucus in your stools, or severe stomach pain. Inflammation of the bowel is the reaction most often seen on these drugs, and it is treated with steroids rather than anti-diarrhoea medicine.
• Call your care team the same day if your skin or the whites of your eyes turn yellow, or your urine goes dark. That points to the liver.
• Call your care team the same day if you have confusion, memory problems, a stiff neck, new weakness, or blistering and peeling skin.
• Call your care team the same day if you cannot pass stool or gas, are vomiting repeatedly, and your abdomen is swollen. A colorectal tumor can block the bowel.
• Call your care team within a day or two if you feel unusually tired, thirsty, or cold, or you have a new rash, joint pain, or blurred vision. Checkpoint inhibitors can affect hormone glands and other organs, and these signs build slowly.

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