Metaplastic breast cancers: Genomic profiling, mutational burden and tumor-infiltrating lymphocytes.

Metaplastic breast cancer (MPBC) is a rare subtype that accounts for <1% of all breast cancers. Although these are typically "triple negative," they are relatively chemotherapy-refractory compared to conventional triple negative invasive breast cancers with more aggressive features and an overall poor prognosis. MPBC is a heterogeneous group of tumors that are enriched for TP53 and PIK3CA mutations, and have been found to have high PD-L1 expression though the mechanisms underlying its immunogenicity remain unclear. We perform comprehensive genomic profiling in the largest MPBC dataset (n = 192) to date and assess for other potential biomarkers of immune response.