Wnt/β-catenin signal alteration and its diagnostic utility in basal cell adenoma and histologically similar tumors of the salivary gland

Differential diagnosis among basal cell adenoma (BCA), basal cell adenocarcinoma (BCAC), adenoid cystic carcinoma (ACC) and pleomorphic adenoma (PA) of the salivary gland can be challenging due to their similar histological appearance. Although frequent nuclear β-catenin expression and CTNNB1 mutations have been reported in BCA, further details of the Wnt/β-catenin signal alterations are unclear. The aim of this study was to assess the diagnostic utility of Wnt/β-catenin signal alteration in BCA and morphological mimics. We performed immunohistochemical staining for β-catenin and mutation analysis for Wnt/β-catenin-related genes (CTNNB1, APC, AXIN1 and AXIN2) in BCA (n = 34), BCAC (n = 3), ACC (n = 67) and PA (n = 31). We also analyzed ACC-specific MYB and MYBL1 gene rearrangements by fluorescence in situ hybridization (FISH). Nuclear β-catenin expression (≥3%) was present in 32/34 cases (94.1%) of BCA, and the nuclear β-catenin labeling index was significantly higher than in other tumor types (p = 0.0001). In BCA, we found mutations in CTNNB1, APC and AXIN1 genes (41.1%, 2.9% and 8.8%, respectively). In BCAC, nuclear β-catenin expression with CTNNB1 mutation was present in 1/3 cases (33.3%). As for ACC, nuclear β-catenin expression was observed in 3/67 cases (4.4%), but all 3 cases harbored either MYB or MYBL1 gene rearrangement. The results suggest that nuclear β-catenin immunoreactivity with appropriate criteria may be helpful to distinguish BCA from histologically similar tumors. However, a minor subset of ACCs with nuclear β-catenin expression require careful diagnosis. In addition, Wnt/β-catenin signal alteration may play a role in the pathogenesis of BCA and BCAC.