Rare breast cancer subtype shows poor outcomes and distinct genomic profile

Researchers at a tertiary cancer center analyzed 77 patients diagnosed between 2000 and 2025, all cases undergoing centralized pathology review, and compared them with a comparator cohort of 37 metaplastic carcinomas lacking matrix production. The study, published in Breast Cancer Research, combined clinicopathologic assessment, germline genetic testing review, and retrospective analysis of targeted tumor-normal sequencing results.

The clinical picture that emerged was sobering. Ninety-four percent of the MPMC tumors were Nottingham grade 3, and 91 percent were triple-negative. Among the 21 patients who received neoadjuvant chemotherapy, not a single pathologic complete response was observed. With a median follow-up of 56 months, 17 percent of patients experienced locoregional recurrence, 33 percent developed distant metastases, and 29 percent died of the disease.

Genetic susceptibility findings were a central focus of the analysis. Germline pathogenic or likely pathogenic variants were identified in 32 percent of tested patients (15 of 47), predominantly in genes associated with homologous recombination deficiency, including BRCA1 and BRCA2, which accounted for 23 percent. When compared with the non-MPMC cohort, MPMC showed significantly higher rates of germline pathogenic variants overall (32 percent versus 8 percent; P = 0.014) and of HRD-associated germline variants specifically (28 percent versus 3 percent; P = 0.002). The subtype was also far more likely to arise in association with atypical microglandular adenosis, a putative precursor lesion, present in 22 percent of MPMC cases versus none of the comparator tumors (P = 0.0012).

Tumor sequencing revealed a recurrent genomic signature. TP53 alterations were present in 90 percent of MPMC tumors, and chromosome 8q copy-number alterations were frequent, including MYC amplification in 48 percent of cases. Relative to metaplastic carcinomas without matrix production, MPMC showed enrichment of multiple 8q-associated alterations including MYC, along with higher large-scale state transition scores and HRD-associated loss-of-heterozygosity scores, both genomic scar metrics linked to homologous recombination deficiency. In contrast, alterations in PIK3CA, TERT, and CDKN2A/B were less frequent in MPMC than in the comparator group, further distinguishing the subtype at the genomic level.

The authors note an important nuance: within the MPMC cohort itself, HRD-associated germline variants and atypical microglandular adenosis were not significantly associated with each other. However, the fact that both features are enriched in MPMC relative to other metaplastic carcinomas supports further investigation of hereditary susceptibility and precursor lesion biology in this rare subtype. The complete absence of pathologic complete responses after neoadjuvant therapy, the authors conclude, highlights the need for improved treatment strategies and supports the study of biomarker-driven therapeutic approaches for patients with this aggressive and genomically distinct form of breast cancer.

Subject of Research: Clinicopathologic, germline, and genomic characterization of matrix-producing metaplastic carcinoma of the breast

Article Title: Matrix-producing metaplastic carcinoma of the breast: clinicopathologic, germline, and genomic characterization of a distinct metaplastic carcinoma subtype

Article References: Mack, T., Momeni-Boroujeni, A., Kiernan, R., Terraf, P., Grabenstetter, A., Ross, D. S., Abuhadra, N., Montagna, G., Brogi, E., Wen, H. Y., & Schwartz, C. J. (2026). Matrix-producing metaplastic carcinoma of the breast: clinicopathologic, germline, and genomic characterization of a distinct metaplastic carcinoma subtype. Breast Cancer Research. https://doi.org/10.1186/s13058-026-02396-4

Image Credits: AI Generated

DOI: 10.1186/s13058-026-02396-4

Keywords: matrix-producing metaplastic carcinoma, triple-negative breast cancer, BRCA1, BRCA2, homologous recombination deficiency, MYC amplification, TP53, atypical microglandular adenosis, neoadjuvant chemotherapy, genomic profiling, germline variants, metaplastic carcinoma