Invasive apocrine carcinoma of the breast: clinicopathologic features and comprehensive genomic profiling of 18 pure triple-negative apocrine carcinomas

Pure invasive apocrine carcinoma is a rare type of primary breast cancer, constituting similar to 1% of all breast cancers. Since most pure invasive apocrine carcinomas are triple negative, the lack of targeted therapies for triple-negative breast cancer has fostered efforts to discover actionable molecular targets in these tumors. In this study, we analyzed the clinicopathologic characteristics and comprehensive genomic profiling of 18 patients with pure triple-negative apocrine carcinomas (TNACs) using a 324-gene panel assay (FoundationOne CDx). The median age of these patients was 55.5 years, and the postmenopausal status rate was 77.8%. In total, 83.3% of patients were diagnosed with histological grade II, and 16.7% were diagnosed with grade III. The majority of patients presented at an early tumor-node-metastasis (TNM) stage (I: 38.9%; II: 50.0%; and III: 11.1%). The mean Ki-67 index was 9.7%, and the percent of PD-L1 positivity was 11.7%. With a median follow-up period of 76.5 months, one patient died, and two experienced distant metastases. There were 61 clinically relevant genomic alterations among all 18 pure TNACs, and the mean tumor mutation burden (TMB) was 3 Mut/Mb. The top ranked altered genes were PIK3CA (72.2%), PTEN (33.3%) and TP53 (27.8%). There were four novel mutations found in PTEN and an actionable rearrangement involving FGFR2-TACC2 that has not been reported in breast cancer before. In total, 88.9%, 50%, 44.4%, and 16.7% of TNACs had at least one clinically relevant genomic alteration in genes involved in the PI3K/mTOR, cell cycle, RAS/RAF/MEK and growth factor receptor-related pathways, respectively. All patients had at least one clinically relevant genomic alteration, and 94.4% had at least one actionable alteration. To the best of our knowledge, this study is the largest genomic sequencing cohort of pure TNACs. Incorporation of comprehensive genomic profiling into TNACs might shed light on potential therapeutic opportunities for both targeted drugs and immune checkpoint inhibitors.