Mismatch Repair Status and Microsatellite Instability in Ovarian Cancer: What the Test Tells Us

Why This Test Exists

Most prostate cancers won't respond to immune checkpoint blockade. But a small subset carry a defect that changes the calculus entirely, and the only way to find them is to look. Mismatch repair (MMR) status and microsatellite instability (MSI) testing exist precisely because the tumors that benefit are rare, and rare things get missed unless you test systematically.

This article explains what the assay measures, how it's performed, what the results mean, and how a dMMR/MSI-H result informs eligibility for a class of therapy, never a recommendation for any individual.

What the Test Measures

The DNA mismatch repair system is a proofreading apparatus. As cells divide, DNA polymerase occasionally inserts the wrong base or slips at repetitive sequences. Four core proteins, MLH1, PMS2, MSH2, and MSH6, work in pairs to recognize and excise these errors [3][4]. When one of these proteins is lost or non-functional, repair fails.

The consequence is genomic. Errors accumulate, and they accumulate most visibly at microsatellites, short, repeated DNA motifs scattered throughout the genome. When these repeats expand or contract from their germline length, the tumor is called microsatellite-instable-high (MSI-H). The functional protein deficiency is termed mismatch repair-deficient (dMMR). In practice, dMMR and MSI-H are two windows onto the same underlying biology, which is why we measure them by complementary methods.

The downstream effect that matters therapeutically is a high mutational burden. A tumor that can't repair its own DNA generates abnormal proteins, neoantigens, that the immune system can recognize. That's the biological rationale linking dMMR/MSI-H to checkpoint blockade [1][3].

How It's Tested

Two assay families are in routine use, and they're often complementary rather than interchangeable.

Immunohistochemistry (IHC) for the four MMR proteins is performed on formalin-fixed, paraffin-embedded (FFPE) tissue. The pathologist evaluates nuclear staining in tumor cells against internal controls, normal stroma, lymphocytes, and benign glands, which should retain expression. Intact nuclear staining for all four proteins indicates proficient MMR (pMMR). Loss of staining in one or more proteins, most commonly in paired combinations reflecting the protein-heterodimer biology, indicates dMMR [6]. IHC is fast, inexpensive, and widely available.

PCR- or NGS-based MSI testing interrogates a defined panel of microsatellite loci and compares tumor to normal, or uses tumor-only panel-based algorithms. A tumor is scored MSI-H when a threshold fraction of the interrogated loci show instability [7].

Preanalytic quality governs everything here. FFPE fixation must be adequate, under-fixation or prolonged ischemia degrades both antigen and nucleic acid. Weak or absent internal controls make an IHC slide uninterpretable rather than negative. For MSI assays, low tumor cellularity can dilute the instability signal below detection. When IHC and molecular results disagree, and they occasionally do, the discordance itself is informative and usually prompts reflex confirmatory testing.

What Each Result State Means

dMMR / MSI-H. The tumor has lost functional mismatch repair. This is the actionable, and the less common, result in prostate cancer. It flags a tumor with a hypermutated phenotype and raises the possibility of an underlying germline mismatch repair defect (discussed below).

pMMR / MSS. Mismatch repair is intact and microsatellites are stable. This is the large majority of prostate cancers. A pMMR/MSS result does not, by itself, exclude a patient from other biomarker-driven pathways, it means this particular door is closed.

One practical caution: a pMMR/MSS result reflects the tissue tested. In a disease as heterogeneous as prostate cancer, sampling a single metastatic site or an older archival block may not capture a subclone. That's a limitation of the specimen, not a verdict on the biology.

What It Determines for Treatment Eligibility

The dMMR/MSI-H result informs eligibility for tumor-agnostic checkpoint inhibitor therapy, a drug class approved on the basis of biomarker status across tumor types rather than by organ of origin. The regulatory foundation includes basket-trial data such as KEYNOTE-158, in which durable responses to pembrolizumab were observed in dMMR/MSI-H tumors across a range of cancers [1].

In prostate cancer specifically, the evidence continues to mature. Panel-based mutational signatures of mismatch repair deficiency have been associated with durable responses to pembrolizumab in metastatic castration-resistant prostate cancer (mCRPC), supporting the use of these assays to identify candidates [2].

The central number to hold in mind is one of scarcity: dMMR/MSI-H is uncommon in prostate cancer, and only a minority of tumors, a small single-digit percentage of mCRPC, carry the alteration [2][5]. That rarity is exactly why systematic testing matters. When a predictive biomarker appears in only a few percent of cases, you can't rely on clinical suspicion to find it. You test broadly, so the few aren't overlooked.

To be explicit about framing: a positive result establishes that a patient belongs to a group for whom a therapeutic class is an option. It does not tell any individual what they should do. That decision belongs to a treating clinician weighing the whole clinical picture.

Caveats and What's Evolving

Assay concordance and thresholds. IHC and MSI testing usually agree, but not always. Rare dMMR tumors retain apparently intact staining, and rare MSI-H calls arise where protein loss isn't evident. Panel-based tumor-only MSI algorithms use analytic thresholds that continue to be refined, and cutoffs optimized in colorectal cancer may not transfer perfectly to prostate tissue [7][8]. This is an area where the evidence is still settling.

Germline versus somatic MMR loss. A dMMR/MSI-H result raises a question that reaches beyond the tumor. Mismatch repair deficiency can be somatic, acquired within the tumor, or germline, as in Lynch syndrome. The distinction has real consequences. A germline defect carries implications for the patient's future cancer risk and for cascade testing of blood relatives [4]. In practice, a dMMR result in prostate cancer isn't only a therapeutic flag; it can be the first thread that leads to a hereditary cancer evaluation. Confirming germline status requires dedicated genetic testing and counseling, the tumor assay alone can't make that determination.

Tissue heterogeneity and reflex logic. Because loss can be focal or subclonal, some laboratories build reflex algorithms, IHC first, molecular confirmation for equivocal or discordant cases, to reduce false-negatives. Testing strategy still varies between institutions.

The Bottom Line

In a cancer where mismatch repair deficiency shows up in only a handful of cases out of a hundred, the value of this biomarker lies in not missing them. A dMMR/MSI-H result identifies the small group of patients whose tumors carry a hypermutated, immune-visible phenotype and who are therefore eligible for consideration of checkpoint blockade, and it can simultaneously open the door to germline evaluation for the patient and their family. Systematic, quality-controlled testing is what ensures that rarity doesn't become invisibility.

References

1. Marabelle A, Fakih M, Lopez J, et al. Association of tumour mutational burden with outcomes in patients with advanced solid tumours treated with pembrolizumab: prospective biomarker analysis of the multicohort, open-label, phase 2 KEYNOTE-158 study. Lancet Oncology. 2020;21(10):1353, 1365. PMID 32919526. doi:10.1016/S1470-2045(20)30445-9.

2. Abida W, Cheng ML, Armenia J, et al. Analysis of the prevalence of microsatellite instability in prostate cancer and response to immune checkpoint blockade. JAMA Oncology. 2019;5(4):471, 478. PMID 30589920. doi:10.1001/jamaoncol.2018.5801.

3. Le DT, Durham JN, Smith KN, et al. Mismatch repair deficiency predicts response of solid tumors to PD-1 blockade. Science. 2017;357(6349):409, 413. PMID 28596308. doi:10.1126/science.aan6733.

4. Latham A, Srinivasan P, Kemel Y, et al. Microsatellite instability is associated with the presence of Lynch syndrome pan-cancer. Journal of Clinical Oncology. 2019;37(4):286, 295. PMID 30376427. doi:10.1200/JCO.18.00283.

5. Middha S, Zhang L, Nafa K, et al. Reliable pan-cancer microsatellite instability assessment by using targeted next-generation sequencing data. JCO Precision Oncology. 2017;1:PO.17.00084. PMID 30211344. doi:10.1200/PO.17.00084.

6. Guedes LB, Antonarakis ES, Schweizer MT, et al. MSH2 loss in primary prostate cancer. Clinical Cancer Research. 2017;23(22):6863, 6874. PMID 28790115. doi:10.1158/1078-0432.CCR-17-0955.

7. Hempelmann JA, Lockwood CM, Konnick EQ, et al. Microsatellite instability in prostate cancer by PCR or next-generation sequencing. Journal for ImmunoTherapy of Cancer. 2018;6(1):63. PMID 29665853. doi:10.1186/s40425-018-0341-y.

8. Bartley AN, Mills AM, Konnick E, et al. Mismatch repair and microsatellite instability testing for immune checkpoint inhibitor therapy: guideline from the College of American Pathologists in collaboration with the Association for Molecular Pathology and Fight Colorectal Cancer. Archives of Pathology & Laboratory Medicine. 2022;146(10):1194, 1210. PMID 35920830. doi:10.5858/arpa.2021-0632-CP.