Pathogenic Germline Variants in Patients With Metaplastic Breast Cancer.

Importance: Metaplastic breast cancer (MpBC) is a rare, heterogeneous disease often associated with inferior outcomes. A growing body of literature describes the clinical and molecular features of MpBC, yet limited data describe the pathogenic germline variants (PGVs) in breast cancer susceptibility genes among affected individuals. Objective: To examine the frequency and types of PGVs in breast cancer genes among patients with MpBC. Design, Setting, and Participants: This is a descriptive retrospective cohort study of patients who received a diagnosis of MpBC at the University of Pennsylvania between January 2010 and May 2023. Electronic medical records were reviewed for demographic, clinicopathologic, and germline genetic testing information. Germline variant status was independently confirmed by a licensed genetic counselor and a physician with expertise in genetics. MpBC diagnosis and subtype were confirmed by a breast pathologist. Participants were identified via query of an institutional pathology database for reports signed between January 2010 and May 2023 including the term metaplastic. Among 320 initially obtained reports, 272 individuals had confirmed MpBC and were included in the study. Exposure: Germline genetic testing to investigate the presence of PGVs in breast cancer susceptibility genes. Main Outcomes and Measures: The primary outcome measurement was the prevalence of PGVs in breast cancer susceptibility genes among participants. The hypothesis that individuals with MpBC have an enrichment of PGVs in genes associated with inherited breast cancer risk was formulated before data collection. Results: The total sample size was 272 women, and the median age at diagnosis was 58 years (range, 20-102 years); all were biological female patients; 143 of 272 (52.6%) had documentation of germline genetic testing; and participants with testing were significantly younger than those without (median age, 53 years [range, 20-79 years] vs 63 years [range, 29-102 years]; P < .001). Of the 143 patients, 24 (16.8%) had a PGV in a breast cancer susceptibility gene (BRCA1, n = 17; BRCA2, n = 5; PALB2, n = 1; CHEK2, n = 1). Patients with PGV-associated MpBC received a diagnosis at a younger age than those with sporadic disease, but there were no significant differences in hormone receptor positivity, ERBB2 status, or metaplastic subtype. Conclusions and Relevance: In this cohort study of patients with MpBC, a substantial proportion of clinically tested patients had a PGV in a breast cancer susceptibility gene, most commonly BRCA1. Germline testing was high yield in patients with MpBC, many of whom would be included in current germline testing eligibility criteria.