What Is Pathogenic Mismatch Repair (PMMR) Status?

The “p” in pMMR stands for “proficient,” not “pathogenic.” The result means the tumor’s DNA repair system is still working.1UT MD Anderson. DNA mismatch repair status: What colorectal cancer patients should know It is the most common result in colorectal cancer, and many endometrial and other tumors also come back pMMR.2Colontown University. Is my tumor MSS (pMMR) or MSI-H (dMMR)? A working repair system says nothing about how mild or aggressive the cancer is. It is a sorting label that helps doctors choose treatment, and because the tumor is what gets tested, it describes the cancer cells, not your whole body.

What Mismatch Repair Does and What Proficient Means

Each time a cell divides, it copies its DNA, and the copying machinery itself avoids most errors.3PubMed Central. Eukaryotic Mismatch Repair in Relation to DNA Replication Errors that slip through are caught and fixed during copying.4PubMed Central. Eukaryotic Mismatch Repair in Relation to DNA Replication The rare ones left after that go to mismatch repair, a final proofreading step.5PubMed Central. Eukaryotic Mismatch Repair in Relation to DNA Replication It mainly fixes single wrong DNA letters and small extra or missing pieces.6PubMed. Mechanisms and functions of DNA mismatch repair

Tumors whose repair system still works are called proficient (pMMR). Tumors whose system has failed are called deficient (dMMR).7UT MD Anderson. DNA mismatch repair status: What colorectal cancer patients should know The system depends on four proteins, named MLH1, MSH2, MSH6 and PMS2, that work in pairs.8Caris Life Sciences. Gene-specific features (MLH1, MSH2, MSH6, PMS2) of mismatch repair (MMR) protein expression and somatic mutations, microsatellite instability (MSI), and tumor mutational burden (TMB) in MSI-H and MMR-mutated tumor genomic profiles MSH2 teams with MSH6 and MLH1 with PMS2. The MSH2-MSH6 pair works first, scanning newly copied DNA for a mismatched letter or a small loop. The MLH1-PMS2 pair then arrives to help cut out the faulty stretch so the cell can recopy it correctly. That pairing is why a report often lists the proteins in pairs, and why losing one partner can make its partner vanish from the stain too. Your report may name all four, and pMMR means all four were present.9Caris Life Sciences. Gene-specific features (MLH1, MSH2, MSH6, PMS2) of mismatch repair (MMR) protein expression and somatic mutations, microsatellite instability (MSI), and tumor mutational burden (TMB) in MSI-H and MMR-mutated tumor genomic profiles

How the Test Is Done and How It Reads on Your Report

The most common method stains thin slices of tumor to see whether the four repair proteins are present.10Caris Life Sciences. Gene-specific features (MLH1, MSH2, MSH6, PMS2) of mismatch repair (MMR) protein expression and somatic mutations, microsatellite instability (MSI), and tumor mutational burden (TMB) in MSI-H and MMR-mutated tumor genomic profiles Your report may say expression is “retained” or “intact” for all four, which means pMMR. The pathologist checks the stain in the tumor against normal tissue nearby that should always show the proteins.11PubMed. Immunohistochemical expression pattern of MMR protein can specifically identify patients with colorectal cancer microsatellite instability The repair proteins sit inside the cell nucleus, so the pathologist looks for stained nuclei in the tumor cells. If a protein is missing from the tumor but present in those neighbors, that counts as true loss and points to dMMR. If the normal tissue doesn’t stain either, the test is considered unreliable and is usually repeated. Pathologists also increasingly treat loss in only part of the tumor as meaningful, not just complete loss.12PubMed. Detecting mismatch repair deficiency in solid neoplasms: immunohistochemistry, microsatellite instability, or both?

Another test looks at short repeated stretches of DNA that grow or shrink when repair fails.13PubMed Central. Microsatellite instability and mismatch repair protein deficiency: equal predictive markers? A tumor whose repeats are unchanged is called microsatellite stable (MSS), which matches pMMR.14Colontown University. Is my tumor MSS (pMMR) or MSI-H (dMMR)? When repair fails, the result is called MSI-H, microsatellite instability high, which matches dMMR.15Colontown University. Is my tumor MSS (pMMR) or MSI-H (dMMR)? With a standard five-marker panel, the lab calls a tumor MSI-H when 2 or more of the 5 marker stretches have shifted in size compared with your normal tissue.16PubMed Central. Microsatellite Status Detection of Colorectal Cancer: Evaluation of Inconsistency between PCR and IHC Some labs use broader tumor DNA sequencing, which can check repair status and other genes from one small piece of tissue.17PubMed Central. Comparative Analysis of Next-Generation Sequencing and Immunohistochemistry in MSI/MMR Testing Your report may show one label or both, depending on which test the lab ran. You may also see MSI-L, meaning one marker shifted, which is generally handled like stable.

You don’t need to do anything extra, because tissue already removed at biopsy or surgery is used. Results usually come with or shortly after the pathology report, and you can ask for a copy showing the result for each protein.

What pMMR Means for Treatment

In colon cancer, fluorouracil chemotherapy after surgery lowered the chance of the cancer returning by about a third for people with pMMR tumors.18PubMed Central. Defective Mismatch Repair As a Predictive Marker for Lack of Efficacy of Fluorouracil-Based Adjuvant Therapy in Colon Cancer People with dMMR tumors gained no benefit from the same drug compared with surgery alone.19PubMed Central. Defective Mismatch Repair As a Predictive Marker for Lack of Efficacy of Fluorouracil-Based Adjuvant Therapy in Colon Cancer Combined studies of stage II and III colorectal cancer agree that this chemotherapy improves survival in pMMR patients and does less for dMMR ones.20PubMed. Prognostic and predictive role of DNA mismatch repair status in stage II-III colorectal cancer: A systematic review and meta-analysis A pMMR result therefore supports the standard chemotherapy plans your oncologist may offer.

For colon cancer, the decision rests mainly on stage and on features such as lymph node involvement, with pMMR status as one input. In stage II, a pMMR result without other high-risk features may lead to a discussion of whether chemotherapy is needed at all, since surgery alone cures many people. In stage III, chemotherapy after surgery is standard, and pMMR supports that plan. Rectal and endometrial cancers follow their own pathways, so what applies to colon cancer doesn’t carry over automatically.

Immunotherapy is treatment that helps your immune system attack cancer. In advanced MSI-H colorectal cancer, the immunotherapy drug pembrolizumab kept the cancer from growing for a median of 16.5 months versus 8.2 with chemotherapy.21New England Journal of Medicine. Pembrolizumab in Microsatellite-Instability-High Advanced Colorectal Cancer Tumors with broken repair carry many more DNA errors, which makes them easier for the immune system to spot.22PubMed. Combined hereditary and somatic mutations of replication error repair genes result in rapid onset of ultra-hypermutated cancers For that reason, pMMR tumors are generally not offered immunotherapy alone as the standard first treatment. Even among dMMR cancers a fair share resist immunotherapy, so it is no guaranteed cure for that group either.23PubMed Central. Mismatch Repair-Deficient Colorectal Cancer: Building on Checkpoint Blockade

Chemotherapy has a strong track record in pMMR colon cancer, and doctors also test other genes such as KRAS and BRAF to guide treatment.

Immunotherapy can still come into play. Some pMMR endometrial cancers in the advanced setting are treated with immunotherapy combined with chemotherapy, so the answer depends on the cancer type. Trials combine immunotherapy with other drugs to see whether pMMR tumors can be made more responsive, and they are worth asking about.

At your appointment, you might ask how the pMMR result fits with your cancer’s stage, which other tumor gene results are still pending, and whether chemotherapy after surgery is recommended in your case and why.

Is pMMR Good or Bad for Outlook

In early-stage colorectal cancer, dMMR tumors on average carry a lower risk of coming back than pMMR tumors.24PubMed Central. Prognostic and Predictive Values of Mismatch Repair Deficiency in Non-Metastatic Colorectal Cancer The difference is larger in stage II than in stage III.25PubMed Central. Implications of mismatch repair-deficient status on management of early stage colorectal cancer One reason is that dMMR tumors are often caught before spreading to lymph nodes.26PubMed Central. Different surgical outcome and follow-up status between dMMR and pMMR colorectal cancer patients who fulfilled with Amsterdam-II criteria Still, this is an average across groups, and stage and treatment matter much more to any one person.

Stage is the strongest single guide to outlook in colorectal cancer, and cancer found in lymph nodes raises the chance of return regardless of repair status. A person with stage I pMMR colon cancer generally has a better outlook than a person with stage III dMMR disease. Repair status refines the estimate within a stage rather than replacing it.

Most colorectal cancers are pMMR, so most survival figures you read already describe people with this result.27Colontown University. Is my tumor MSS (pMMR) or MSI-H (dMMR)? In pMMR colon cancer, chemotherapy after surgery helps, which partly closes the gap.28PubMed Central. Defective Mismatch Repair As a Predictive Marker for Lack of Efficacy of Fluorouracil-Based Adjuvant Therapy in Colon Cancer A headline about a “better prognosis for dMMR” compares two groups. It doesn’t mean pMMR patients do poorly, and many early-stage pMMR cancers are cured with surgery, with or without chemotherapy.

Within the pMMR group, outcomes range widely, and many people remain cancer-free for years. Tumor grade, whether the cancer grew through the bowel wall, and how completely it was removed also feed into your risk. Ask your doctor for figures that match your own stage.

What pMMR Means for Inherited Cancer Risk and Your Family

Lynch syndrome is an inherited condition, passed down from either parent, that raises the risk of several cancers.29PubMed Central. Mismatch repair genes in Lynch syndrome: a review It comes from a faulty copy of one of the four repair genes.30PubMed. Screening for germline mutations in mismatch repair genes in patients with Lynch syndrome by next generation sequencing It accounts for about 1 to 7 in 100 colorectal cancers.31PubMed Central. Mismatch repair genes in Lynch syndrome: a review Because of this, guidelines recommend testing repair status in every colorectal and endometrial cancer, along with several others.32PubMed Central. Canadian consensus for the assessment and testing of Lynch syndrome Finding Lynch syndrome lets relatives be tested, which can be cost-effective for health systems.33PubMed Central. Swiss cost-effectiveness analysis of universal screening for Lynch syndrome of patients with colorectal cancer followed by cascade genetic testing of relatives

Lynch cancers usually show dMMR, so a pMMR result makes Lynch syndrome much less likely. It does not erase the chance. The test looks at the tumor, whereas Lynch syndrome is a change in the genes you were born with, so the two can occasionally disagree. Other inherited conditions raise cancer risk through genes unrelated to mismatch repair, and pMMR says nothing about those.

Most dMMR colorectal cancers are not inherited: about 12% of cases come from a gene switched off within the tumor, versus 3% from Lynch.34PubMed Central. Patients with colorectal cancer associated with Lynch syndrome and MLH1 promoter hypermethylation have similar prognoses And about 5% of colorectal cancers that keep all four proteins still turn out unstable on DNA testing.35PubMed Central. Heterogenous loss of mismatch repair (MMR) protein expression: a challenge for immunohistochemical interpretation and microsatellite instability (MSI) evaluation So if several relatives had colorectal, endometrial or related cancers, ask about genetic counselling even with pMMR.

A genetic counselling visit starts with your family tree, noting who had which cancers and at what age. Cancers at younger ages, and several cases on the same side of the family, make the pattern more worrying. If testing is appropriate, it is usually a blood or saliva sample that checks the inherited copies of the repair genes and sometimes others. The counsellor then explains what a positive, negative or unclear result means, and the result can guide your screening and your relatives’.

For relatives, a pMMR result alone does not call for urgent testing, and without a worrying family history they can usually keep to normal screening, such as colonoscopy at the recommended age. A strong family history is a reason to see a genetic counsellor whatever your result.

Can a pMMR Result Be Wrong

A tumor can contain both working-repair and broken-repair areas, so the sampled piece may not show both.36PubMed Central. Discordance between immunochemistry of mismatch repair proteins and molecular testing of microsatellite instability in colorectal cancer Poor tissue quality, few tumor cells, or inexperienced reading can also cause mismatched results.37PubMed Central. Discordance between immunochemistry of mismatch repair proteins and molecular testing of microsatellite instability in colorectal cancer Some faulty proteins still show up on the stain even though they no longer work, which can make a tumor look pMMR when repair has actually failed.38PubMed Central. Heterogenous loss of mismatch repair (MMR) protein expression: a challenge for immunohistochemical interpretation and microsatellite instability (MSI) evaluation Certain DNA markers vary naturally in some people of African ancestry, which can confuse the DNA test.39PubMed Central. Discordance between immunochemistry of mismatch repair proteins and molecular testing of microsatellite instability in colorectal cancer Even so, in one comparison a DNA sequencing method agreed with the stain in about 84% of cases.40PubMed Central. A novel NGS-based microsatellite instability (MSI) status classifier with 9 loci for colorectal cancer patients

The two tests ask slightly different questions. The stain asks whether the repair proteins are present, while the DNA test asks whether failed repair has left its mark on the DNA. A small number of tumors give different answers on each without either test being careless. The cases that disagree are the ones worth a second look.

Radiation or chemotherapy before surgery can alter how tissue looks and complicate reading the stain. Pathologists comparing a biopsy with the larger surgical specimen sometimes find the larger sample more informative.

Consider asking for a second test if only one was done and your family history is strong, if the biopsy was very small or had few tumor cells, or if your report mentions patchy or partial staining. A DNA instability test or sequencing can confirm a staining result.

How Common pMMR Is in Different Cancers

By stage, about 23% of stage II colon cancers were dMMR in a large group of trial patients.46PubMed Central. Prognostic and Predictive Values of Mismatch Repair Deficiency in Non-Metastatic Colorectal Cancer dMMR is rarer in advanced colorectal cancer, which makes pMMR even more usual there. Endometrial cancer stands out because repair status is now part of how these cancers are classified. It also guides Lynch screening, since these cancers are a common first sign of the condition. Your gynecologic oncologist may combine it with other tumor markers.