The molecular landscape of breast mucoepidermoid carcinoma

Abstract

Mucoepidermoid carcinoma (MEC) of the breast is an extremely rare salivary gland-type tumor characterized by epidermoid, basaloid, intermediate, and/or mucinous cells arranged in solid and cystic patterns. Despite their triple-negative phenotype, breast MECs are generally considered low-risk malignancies but their biology is largely unexplored; therefore, guidelines for clinical management are lacking. Here, we sought to characterize the molecular landscape of breast MECs. Thirteen cases were histologically reviewed, characterized for tumor-infiltrating lymphocytes (TILs), and were subjected to immunohistochemistry for programmed death-ligand 1 (PD-L1, clone 22C3), EGFR, and amphiregulin (AREG). Rearrangements in MAML2 and EWSR1 were investigated by fluorescent in situ hybridization. Targeted next-generation sequencing of 161 genes was performed on eight cases. Most MECs had low histological grade (n = 10, 77%), with the presence of TILs (n = 9/12; 75%) and PD-L1 combined positive score ranging from 10 to 20 (n = 4/6; 67%). All cases showed EGFR and AREG overexpression and were fusion negative. Enrichment of genetic alterations was observed in PI3K/AKT/mTOR and cell cycle regulation pathways, while only one case harbored TP53 mutations. This is the first study providing extensive molecular data on breast MECs and the largest collection of cases available to date in the literature. Breast MECs lack TP53 mutations found in high-grade forms of triple-negative breast cancers and MAML2 or EWSR1 rearrangements pathognomonic of salivary MECs. Triple-negativity and PD-L1 positivity suggest a window of opportunity for immunotherapy in these patients. The EGFR/AREG axis activation, coupled with the mutational patterns in PI3K/AKT/mTOR and cell cycle pathways warrants caution in considering MECs as low-risk neoplasms.

Keywords: biomarkers; breast cancer; diagnosis; molecular profiling; mucoepidermoid carcinoma; rare tumors; triple-negative breast cancer.

FIGURE 1

Representative micrograph showing the histopathological features of two primary breast mucoepidermoid carcinomas. Case #05 was a low‐grade carcinoma showing cystic ductal spaces lined by mucinous epithelial cells showing an unremarkable degree of nuclear pleomorphism and no mitotic count (A, H&E original magnification ×100; inset original magnification ×400), surrounded by a paucicellular myxoid stroma, as highlighted by Alcian blue stain (B, original magnification ×200). No PD‐L1 positivity was restricted to the neoplastic cells, with a CPS score of 10 (C, original magnification ×200). This neoplasm showed moderate cytoplasmic staining for EGFR in the majority of tumor cells and was scored as 2+ (D, original magnification ×200), while AREG expression was low (E, original magnification ×200). Case #3 was a high‐grade carcinoma showing nests of tumor cells with mucinous and squamoid features with no keratinization, minimal/null cystic formation, variable degree of nuclear atypia, occasional mitoses, karyopyknosis (F, H&E original magnification ×100; inset original magnification ×400), and diminished stromal mucin production in the presence of sparse mucin pools between the neoplastic clusters (G, original magnification ×200). The presence of TILs was confirmed by the expression of PD‐L1, with a CPS scored as 25 (H, original magnification ×200). This neoplasm was EGFR‐positive (I, original magnification ×200) and AREG low (J, original magnification ×200).

FIGURE 2

Heatmap illustrating selected clinicopathologic features, histochemical, and mutational status, including single‐nucleotide variants and copy‐number alterations, of MECs. Each column represents a patient, according to their ID, each row represents a clinicopathologic and genetic parameter, color‐coded according to the legend on the bottom right. The number of genomic alterations detected in each case is represented in the bar chart at the bottom of the figure, while the frequency of recurrent actionable/pathogenic mutations is reported on the right as a percentage. AREG, amphiregulin; CK, cytokeratin; CNV, copy‐number variations; CPS, combined positive score; ER, estrogen receptor; MMR, mismatch repair; PD‐L1, programmed death‐ligand 1; SNV, single‐nucleotide variants; TILs, tumor‐infiltrating lymphocytes; WT, wild‐type.

FIGURE 3

Recurrently altered pathways in breast MECs. Each column represents a patient, each row a pathway; the number of molecular aberrations is annotated as reported on the bottom.