# Mismatch repair deficiency and microsatellite instability in triple-negative breast cancer: a retrospective study of 440 patients

Source: https://onco.cc/key-papers/paper-ren-tnbc-mmr-msi-440-front-oncol-2021/  
OnCo record `paper-ren-tnbc-mmr-msi-440-front-oncol-2021` (Key paper). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Among 440 Chinese women with triple-negative breast cancer, none had high microsatellite instability by PCR and only one had lost a mismatch repair protein, so the tumour-agnostic immunotherapy route through microsatellite instability almost never applies.

## Summary

Tissue microarrays from 440 TNBC patients (median age 49, median follow-up 68 months) were stained for MLH1, MSH2, MSH6 and PMS2; 195 were also tested by MSI PCR. All samples were proficient except one with MSH2 loss and intact MSH6; PCR found no MSI-high case, 14 (7.2%) MSI-low and 181 (92.8%) stable. The dMMR sample had low-frequency instability and a possible EPCAM deletion. MMR and MSI status did not correlate with clinicopathological features, PD-1/PD-L1 expression or survival.

## Fields

- Kind: Key paper
- Last checked: 2026-09-24
- Journal: Frontiers in Oncology
- Year: 2021
- DOI: 10.3389/fonc.2021.570623
- Authors: Ren XY, Song Y, Wang J, et al.
- Findings: dMMR by IHC in 1 of 440 (0.2%); MSI-high by PCR in 0 of 195.; MMR/MSI status unrelated to PD-L1 or survival.
- What it means: MSI testing has a very low yield in TNBC; TMB and PD-L1 are the immunotherapy biomarkers worth pursuing.
- Caveats: Single Chinese centre; tissue microarray cores may miss heterogeneous loss.; PCR performed in fewer than half the cohort.

## Sources

- Ren et al., Front Oncol 2021: mismatch repair and MSI in 440 Chinese TNBC patients: https://doi.org/10.3389/fonc.2021.570623
- PubMed: https://pubmed.ncbi.nlm.nih.gov/33747906/

## Connected records

- biomarkers: [dMMR (mismatch repair deficiency by IHC)](https://onco.cc/biomarkers/dmmr-ihc/), [MSI-high (microsatellite instability by PCR or sequencing)](https://onco.cc/biomarkers/msi-high/)
- cancers: [Triple-negative breast cancer (TNBC)](https://onco.cc/cancers/tnbc/)
- technologies: [MSI and mismatch-repair testing](https://onco.cc/technologies/msi-mmr-testing/)
- targets: [Mismatch repair proteins (MLH1, MSH2, MSH6, PMS2)](https://onco.cc/targets/mmr/)
- terms: [Microsatellite instability (MSI-H) / mismatch repair deficiency (dMMR)](https://onco.cc/terms/msi/)

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