Abstract

Secretory breast carcinomas (<0.15% of breast tumors) are associated with a characteristic morphology and a favorable prognosis. Remarkably, this entity is the only epithelial tumor of the breast with a balanced translocation, t(12;15), that creates an ETV6-NTRK3 gene fusion encoding chimeric tyrosine kinase also encountered in cellular mesoblastic nephroma and infantile fibrosarcoma. The aim of this study was to determine the phenotypic class (ie luminal A/B, ERBB2, basal-like) of secretory breast carcinoma. A series of six secretory breast carcinomas were identified in our files. The ETV6 rearrangement was confirmed in all cases by fluorescence in situ hybridization. Immunophenotype was assessed with anti-ER, PR, ERBB2, KIT, EGFR, E-cadherin, vimentin, PS100, smooth muscle actin, basal (CK5/6 and 14), luminal cytokeratins (CK8/18) and p63 antibodies. In situ and invasive components shared the same immunoprofile and were ER, PR, ERBB2 negative with expression of basal cytokeratins. ETV6 gene alterations were present in both in situ and invasive components, highlighting their genetic similarities. The immunoprofile data (triple-negative with expression of basal markers) showed that secretory breast carcinomas with ETV6-NTRK3 fusion gene belong to the phenotypic basal-like spectrum of breast carcinomas. These results support the hypothesis that secretory breast carcinomas have immunohistochemical and genetic features that distinguish them from other basal-like tumors of the breast.

Secretory breast carcinoma is one of the rarest types of breast cancer, accounting for less than 0.15% of all breast cancers.¹ This entity was initially termed ‘Juvenile breast carcinoma’ by McDivitt and Stewart,² as the average age of the seven patients in their series was 9 years, with a range of 3–15 years. More cases in children and adults have subsequently been described.^{3, 4} The age at presentation ranges from 3 to 87 years with a median of 25 years.¹ It was therefore recommended to replace the term ‘juvenile breast carcinoma’ by the descriptive term ‘secretory breast carcinoma’.

Secretory breast carcinomas are distinguished from invasive ductal carcinomas of the breast by their characteristic histomorphology. Secretory breast carcinomas can demonstrate several histologic patterns including, solid, microcystic and tubular, and many tumors contain all three patterns. Tumor cells are polygonal with granular eosinophilic cytoplasm. Atypia is minimal or absent and mitotic activity is low. A typical finding is the presence of intracellular and extracellular secretory material (periodic acid-Schiff and Alcian blue positive).

Tognon et al⁵ recently showed that secretory breast carcinomas are associated with a characteristic balanced translocation, t(12;15), that creates a ETV6-NTRK3 gene fusion. The fusion between ETV6 (TEL) transcription factor and the NTRK3—neurotrophin-3 tyrosine kinase protein occurs in various cell types and lineages.⁵ This specific translocation was first cloned in pediatric mesenchymal cancers: congenital fibrosarcoma and congenital cellular mesoblastic nephroma, two morphologically similar pediatric mesenchymal tumors with no epithelial features.^{6, 7} The biological consequence of this translocation is the expression of a chimeric tyrosine kinase protein with a potent transforming activity in very different cell lineages including fibroblasts, and breast epithelial cells.⁸ The finding of a fusion transcript in secretory breast carcinomas and the demonstration that ETV6-NTRK3 can transform normal murine mammary epithelial cells strongly suggest that this chimeric protein constitutes a primary genetic lesion in this subtype of breast carcinoma.⁸ The ETV6-NTRK3 fusion activates RAS-MAP kinases and PI3K–Akt pathways which are important for breast cell proliferation and survival.⁸

Gene expression profile analysis has recently identified at least six different breast carcinoma types: luminal A, B, C, ERBB2, basal-like and normal-like types.^{9, 10} Subsequent immunophenotype analyses provided surrogate markers to identify these molecular types in clinical practice.^{11, 12}

Secretory breast carcinoma is a paradoxical entity harboring a translocation known to be oncogenic in two other tumor types, including a mesenchymal tumors, while also demonstrating true epithelial differentiation with secretory activity. In this context, the aim of this study was to identify the secretory breast carcinoma phenotypic class. Six histologically and molecularly confirmed secretory breast carcinomas were evaluated. Secretory breast carcinomas were found to be low-grade triple-negative (ER, PR and ERBB2 negative) carcinomas that express basal-cell markers. This observation suggests that secretory breast carcinomas with ETV6-NTRK3 fusion gene are part of the basal-like spectrum. ETV6 gene alterations were present in both the in situ and invasive components, highlighting their genetic similarities. These results also show that secretory breast carcinomas have genetic features that distinguish them from other basal-like tumors of the breast.

Pathologic Findings

Tumor size ranged from 5 to 40 mm, with a mean of 17 mm. Microscopically, all tumors presented both invasive and in situ components. Tumors demonstrated several histologic patterns including, solid, microcystic, and tubular, and many tumors contained all three patterns. Tumor cells were polygonal with granular eosinophilic cytoplasm. Atypia was moderate to marked. Nuclei were ovoid with small nucleoli. Mitotic activity was zero or low (mitotic activity ranged from 0 to 8/10 HPF). A typical finding was the presence of intracellular (intracytoplasmic lumina) and extracellular secretory material (PAS and Alcian blue positive). Four tumors were Elston and Ellis grade 2 (66%) and two were grade 1 (33%). Vascular invasion was present in one case. Perineural infiltration was absent in all cases. Necrosis was absent in the invasive component. All tumors presented an in situ secretory component with cribriform, microcystic or massive architecture at the margins or within the tumor. In situ nuclear grade was intermediate without necrosis except for one tumor which presented a high-grade in situ component with necrosis. Pathologic findings are reported in Table 2 and illustrated in Figure 1a–c.

Immunohistochemical Findings

Immunohistochemical findings are reported in Table 3 and illustrated in Figure 1d–i. All tumors were negative for ER, PR and ERBB2. All tumors expressed CK8/18 at least focally (1–80% of cells). E-cadherin expression (membranous immunoreactivity of 5–90% of cells) was present in all tumors. Five tumors (83%) expressed CK5/6 (1–80% of cells), vimentin (5–60% of cells). Four tumors focally expressed KIT (1–5% of cells). CK14 expression was present in two tumors and EGFR expression was present in three tumors. Five tumors (83%) expressed S-100 (strong and diffuse expression in 100% of cells) with focal smooth muscle actin expression (2–10% of cells). None of the tumors cells expressed p63. In situ and invasive components presented the same immunoprofile.