When a Breast Tumor Behaves Like a Colon Cancer: dMMR/MSI-H and TMB Testing

A triple-negative breast carcinoma comes back on a broad panel with a surprise: microsatellite instability–high, tumor mutational burden 14 mutations per megabase. It's an uncommon result in this disease, and it stops the sign-out cold. That single flag can move a rare breast cancer out of the conventional treatment lane and into a class of therapy defined not by where the tumor started but by how it mutates. Understanding why requires walking through the biology, the assays, and—critically—the limits of what these tests can and cannot tell you.

What the Tests Measure

Start with the DNA repair machinery. The mismatch repair (MMR) system corrects the small base-pair errors that DNA polymerase leaves behind during replication. Four proteins do the heavy lifting: MLH1, PMS2, MSH2, and MSH6. They work as pairs. When one is lost, replication errors accumulate—especially in microsatellites, the short repetitive DNA sequences scattered across the genome. Those regions slip and stutter without correction. That instability is what "MSI-high" describes.

Deficient MMR (dMMR) and MSI-H are two windows onto the same underlying defect. One is measured at the protein level, the other at the DNA level. They usually agree.

Tumor mutational burden is a related but distinct concept. TMB counts the total number of somatic mutations per megabase of sequenced tumor DNA. A tumor with broken repair machinery tends to accumulate mutations, so dMMR tumors are often TMB-high. But the reverse isn't guaranteed. A tumor can carry a high mutational load from other causes—UV exposure, tobacco, or defects in different repair pathways—while its microsatellites remain stable. That's why these markers overlap without being interchangeable.

The therapeutic logic ties it together. Tumors that accumulate many mutations produce more abnormal proteins, more neoantigens, and thus become more visible to the immune system. That visibility is what immune checkpoint blockade exploits.

How It's Tested

Three assay families are in play, all run on formalin-fixed, paraffin-embedded (FFPE) tissue.

Immunohistochemistry for the four MMR proteins is the fastest, cheapest entry point. The pathologist scores nuclear staining. Intact expression means the protein is present; complete loss of nuclear staining in tumor cells—with retained staining in adjacent normal cells serving as the internal control—signals deficiency. That internal control is not optional. Without a positive internal control, a "loss" result is uninterpretable, because you cannot distinguish true protein absence from a failed stain.

PCR or NGS-based MSI testing interrogates a defined panel of microsatellite loci and compares tumor to normal. A tumor scored unstable at a sufficient fraction of markers is called MSI-high.

NGS for TMB requires sequencing enough of the genome to generate a stable mutation count, then applying a threshold. The tumor-agnostic cutoff is ≥10 mutations per megabase [1].

Preanalytics matter more here than clinicians often appreciate. Fixation time, tissue age, and tumor cellularity all shape results. Low tumor purity dilutes the mutant signal and can pull a genuinely high TMB below threshold. Degraded DNA from over-fixed or old blocks compromises sequencing quality. In practice, the most common interpretive pitfall isn't reading the stain—it's accepting a TMB value from a specimen with marginal tumor content without questioning it.

What Each Result Means

dMMR/MSI-H: repair machinery is broken and the genome is unstable. This is the result with the strongest predictive weight for checkpoint blockade.

pMMR/MSS: proficient repair, stable microsatellites. The immunotherapy rationale grounded in repair deficiency does not apply. The tumor may still carry other actionable features.

TMB-high (≥10 mut/Mb): a heavy mutational load, whatever the cause. It may coincide with dMMR or stand alone.

What It Determines for Treatment Eligibility

These are predictive biomarkers, and their function is narrow and specific: they inform eligibility for a class of drug—immune checkpoint inhibitors targeting PD-1.

Regulatory approval for this class in MSI-H/dMMR and TMB-high solid tumors is tumor-agnostic, meaning eligibility follows the biomarker rather than the anatomic site. The evidence base comes from KEYNOTE-158, in which pembrolizumab produced durable responses across a range of previously treated solid tumors selected by these markers [1]. The TMB-high analysis from that trial supported the ≥10 mut/Mb threshold as a pan-cancer predictor of response [1], and this evidence underpinned the corresponding FDA approvals [2,3].

For breast cancer specifically, this is where honesty is required. MSI-H and TMB-high are rare in breast carcinoma. The tumor-agnostic pathway means a qualifying breast tumor may fall under this indication, but the breast-specific evidence is thin, drawn from small numbers within larger pan-tumor cohorts. The framework tells you a patient's tumor is biomarker-eligible for consideration of this drug class. It does not, by itself, tell you the tumor will respond—and it is never a directive that any individual should receive the drug.

Caveats and What's Evolving

Discordant results are uncommon but genuinely difficult, and they land on the pathologist's desk more often than the literature admits. IHC shows intact protein while MSI testing calls instability; or IHC shows loss while the microsatellite panel reads stable. Neither assay is infallible. IHC can mislead when a mutation produces a nonfunctional but antigenically intact protein—the stain looks retained, but the protein doesn't work. MSI panels can miss instability that IHC catches, particularly with certain protein losses. When the two disagree, the practical response is to treat neither as automatically correct: confirm the internal control on IHC, review tumor cellularity for the molecular assay, and consider orthogonal or repeat testing before committing to a result that will steer therapy. That's not overcaution. That's diligence.

MMR protein loss also carries a germline dimension. Loss of MSH2, MSH6, or PMS2—and MLH1 loss not attributable to promoter hypermethylation—can signal an underlying hereditary MMR defect, historically linked to Lynch syndrome. In breast cancer the association is less established than in colorectal or endometrial disease, but a somatic finding that raises germline suspicion warrants a conversation about genetic evaluation. The tumor block, in other words, occasionally speaks for the whole family.

The TMB threshold itself is contested. A single ≥10 mut/Mb cutoff, validated largely in one assay context, is applied across tumor types with very different background mutation rates. Whether that pan-cancer line is the right one for breast cancer—a disease with generally modest mutational burden—remains an open question, and assay-to-assay concordance in TMB measurement is imperfect [1]. A value near the cutoff on one platform may not reproduce on another.

None of this diminishes the value of the testing. It sharpens how the results should be read. The unresolved problem for breast pathology is precisely the one that scenario in the introduction poses: when a rare breast tumor crosses a threshold built from other cancers, how much confidence does that number actually earn?

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 Oncol. 2020;21(10):1353–1365.
2. Marabelle A, Le DT, Ascierto PA, et al. Efficacy of pembrolizumab in patients with noncolorectal high microsatellite instability/mismatch repair–deficient cancer: results from the phase II KEYNOTE-158 study. J Clin Oncol. 2020;38(1):1–10.
3. U.S. Food and Drug Administration. Pembrolizumab tumor-agnostic approvals for MSI-H/dMMR and TMB-high solid tumors. FDA prescribing information and approval records.