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Mismatch repair-deficient (dMMR) colon cancer, also known as microsatellite instability-high (MSI-H),is a biologically distinct form of colorectal cancercaused by mutations in some of the genes that are involved in correcting mistakes made when DNA is copied in a cell. This type of colon cancer is highly responsive to treatment with immunotherapy, which hasreshaped howԾԲ’Dz,leading to major advances across both early-stage and metastatic disease.
Here, we take a closer look at what makesdMMRcolon cancer different, why these tumors respond so well to therapiesthat activate the貹پԳ’sown immune system, known as immunotherapy, and how recent research is changinghow we approach patients with these tumors. From new first-line treatments for metastatic disease to promising approaches before surgery, these advances arehelpingmanypatients withcolon cancerlivelongerand feelhealthier, oftenwith fewer side effects than older chemotherapy-only strategies. Understanding the science behindthisdMMRcolon cancer subtypeis helpingdoctorsmatch the right treatment to the right patient at the right time.
When cells divide, they must copy DNA accurately. The mismatch repair system acts as a molecular proofreader, correcting small errors that occur duringcellreplication. If this system fails, mutations accumulate throughout the genome.These mutations are thentranscriptedto proteins that have mutations or alterations. When tumorsdevelop under these conditions, theseabnormal proteins, known as neoantigens,are readily recognized bythe immune system as foreign. This heightened immune visibility explains whydMMRtumors often showaheavy immune cell infiltration and why they respond so well toa type of immunotherapy known asimmune checkpoint inhibitors.
Mismatch repair deficiency can occurspontaneously in the bodyordue to other genetic conditions, such asLynch syndrome. Lynch syndrome isan inherited conditionthat is passed down in families andresults inmutationsin thesemismatch repair genes, resultinginafaulty proofreading system. Lynch syndrome affects roughlyonein 280 Americans andhavinga mutation in one of these repair genesincreases theoverallrisk of developingcancerand at a younger age. People with Lynch syndromeareespeciallyat risk for developing colorectal and endometrial cancers.
Research has shown that approximately12–15 percent ofallcolorectal cancers aremismatch repairdeficient.Whether the deficiency arises from Lynch syndrome or sporadic causes, immunotherapy responses are similarly strong because the underlying featureofthe high mutation burdenremains the key driverfor these cancers.
The prevalence ofdMMRvaries by stage. It is more common in early-stage colon cancer and less frequent in metastatic disease, suggesting that tumors capable of spreading may have developed mechanisms to evade immune control.
In stage II and some stage IIIcoloncancers,dMMRstatus has historically been associated with a better prognosis.Thesegrow more slowly and are recognized by the immune system more effectively.Patientswith stage IIdMMRcolon cancer generally have lower rates of recurrence and improved overall survival compared withstage II colon cancer withoutdMMR.
That advantage appears to diminish with extensive lymph node involvement and is not seen once the disease becomes metastatic.In metastatic colon cancer,dMMRtumors behave more aggressively. Historically,outcomesfor this subset of patientshave beensimilar to, or slightly worse than, mismatch repair–proficient tumors when treated with chemotherapy alone.Thisresistance to standard cytotoxic regimenshaslikely contributed to poorer historical outcomesfor patients with metastatic mismatch repair deficient colon cancer. Advances in immunotherapy anditsincorporation into treatment regimens is changing that.
While initial surgical management is similar for all colon cancers,dMMRtumors have historically not responded as well to chemotherapy, such as FOLFOX or FOLFIRI,compared with other types of colon cancer.This limitedthelong-termcancercontrol options for these tumors until the introduction of immune checkpoint inhibitors. Checkpoint inhibitors are a form of immunotherapy that work by helping to lift the brakes off the immune cells (T cells) that are responsible for fighting cancer, allowing them to better recognize the cancercellsin the body.
The most dramatictreatmentadvancewith immune checkpoint inhibitorshas occurred in metastaticdMMRcolorectal cancer.Multiple large clinical trials demonstrated that PD-1 inhibitors such as pembrolizumabor nivolumabproduce higher response rates and more durable disease control than standard chemotherapy.Patients receiving immunotherapy as their first treatment often experience fewer symptoms, delayed disease progression, and in many cases, animprovement in overall quality of life.Theseclinical trialresultshaveestablished immunotherapy as the preferred first-line treatment for metastaticdMMRdisease, largely replacing chemotherapyfor most patients with this subtypeof colon cancer.
Combination immunotherapy has further improved outcomesfor some patients. Regimens pairing PD-1 inhibitors with CTLA-4 inhibitors, such as nivolumab plus ipilimumab, have shown superior progression-free survival compared with single-agent immunotherapy or chemotherapy.This approach aims to activate the immune system from multiple angles to generate a more powerful response.
While dual therapy can increase immune-related side effects,such as skin, liver, or gastrointestinal tract inflammation,most are manageable with careful monitoring.Because of this,immunotherapytreatmentselection is often individualized, with combination therapy favored for younger, fitter patients or those with bulky disease, and single-agent therapy chosen for older or more frail individuals.
As ongoingresearchstudieshelp to furtherrefine these strategies, immunotherapy continues to reshape the outlook forpatients withmetastaticdMMRcolorectal cancer. What was once averydifficulttotreatform of colon cancernow has moretreatmentoptions available.
The success of immunotherapy for metastaticdMMRcolon cancerhas led to researchevaluatingthese treatments in earlier stagesof the disease.For years, stage III colon cancer was treated with six months of adjuvant chemotherapyfollowing surgery, which wasdetermined tonotbeas effective indMMRtumors.“Building on the advances of using immunotherapy to treat stagefourdMMRcolon cancer, investigators contemplated the use of immunotherapyearlier on, in patients withstage IIIdMMRcolon cancer,” explains Dr. Shah.
A recent studyshowedthat adding immunotherapy to chemotherapy for stage IIIdMMRcolorectaltumors significantly reducesrisk ofrecurrence, leading tosignificantupdates in treatment guidelines and integrationofimmune-based approaches earlier in the disease course.These findings havechanged the wayphysiciansapproachcaring for patients withstage IIIdMMRcolon cancer, andthese research resultshaveopened the door tousingimmunotherapy inthose withhigh-risk stage IIcolon canceras well.
The role of neoadjuvant immunotherapy,giving the immunotherapytreatmentbefore surgery,is another promising strategy.Treating earlier-stagedMMRtumors before surgery has the advantage ofleveraging theintact tumor tissue totry tohelp stimulate a stronger immune response.Early trials have reported remarkably high rates of tumor regression after just a few doses of immune checkpoint inhibitors, with many patients achieving complete pathologic response. These results raise the possibility that some individuals may eventually require less extensive surgeryor shorter courses of postoperative therapy.There may even be potential for some patients to avoid surgery entirely.
Despitethesuccessand promise, not alldMMRtumors respond to immunotherapy, and somepatientseventually developtreatmentresistance. Research suggests thatthis “immune escape”can occur through disruptions in antigen presentation, alterations in T-cell receptor signaling and changes in interferon pathways.
BecausedMMRtumors share certain recurrent mutations, investigators are exploring vaccine-based approachesfor early-stage disease and those at increased risk for developing these cancers due to Lynch syndrome. This vaccine approach may helpstimulatethe immune system torecognize andkillthe cancerearly, as well asprevent cancers from forming in the first place. These strategiesaim tobalance potential benefit with the risk of immune-related side effects.
Since Lynch syndrome increases a person’s risk of multiple cancers over a lifetime, immunotherapy is now being explored not just as a treatment strategy,but also as a potential preventive tool in high‑risk individuals. For families affected by Lynch syndrome, genetic counseling, coordinated surveillance, and emerging prevention strategies form a comprehensive approach that supports both patients and at‑risk relatives.
Dr. Shah highlights that “the future ofdMMRcolon cancer care is moving towardsmore precise and less toxic treatment, which is a real benefit that patients truly appreciate.”Researchers are studying novel drug combinations, targeted therapies such as synthetic lethal approaches and cellular therapies. Circulating tumor DNA and immune monitoring may help determine which patients can safely stop treatment and which need longer therapy to preventdiseaserelapse.
What is clear is that understanding the biology of mismatch repair deficiency hastransformedcare. Immunotherapy has shifted the paradigm from conventional chemotherapy to immunotherapy-based strategies that offer longer survival and, in some cases, the potential for cure. As research continues,treatment is expected to become even more personalized, aiming to maximize effectiveness while minimizing the burden of therapy.
To hear a more in-depth discussion about this topic, tune in to theCancerCastpodcast where hostDr. Manish Shahspeaks with two experts to discuss mismatch repair deficient colon cancer and how this subset of colon cancer is managed and treated differently. Listen on,,, or the.
