NKX3.1 Helps Distinguish Hyalinizing Clear Cell Carcinoma From Other Clear Cell Salivary Gland Neoplasms

Hyalinized clear cell carcinoma (HCCC) is a rare tumor of the minor salivary gland, characterized by pale cytoplasm and EWSR1::ATF1 fusion. Recently, new fusions, such as EWSR1::LARP4 and SMARCA2::CREM, have also been identified. Histologically, HCCC closely resembles other salivary gland tumors like mucoepidermoid carcinoma and myoepithelial carcinoma, and there are no specific immunohistological markers for its identification. In this study, we investigated potential markers for HCCC based on the characteristics of minor salivary gland acini, from which these tumors may originate. SOX10 is a known marker for serous gland clusters and NKX3.1 for mucus gland clusters. Fluorescence intensity analysis of double staining, objectively evaluated by artificial intelligence, revealed variations in the positive intensity of cells single positive for NKX3.1 and SOX10, as well as cells positive for both markers, which are commonly observed in normal minor salivary glands. We evaluated NKX3.1 expression by immunohistochemistry in 12 HCCC cases (including 9 EWSR1::ATF1, 1 EWSR1::LARP4, and 1 SMARCA2::CREM), 12 myoepithelial carcinoma cases, and tissue microarray containing 88 cases of multiple salivary gland tumors using immunohistochemistry. NKX3.1 was expressed in all 12 HCCC cases (100%), with NKX3.1-positive cells ≧90% in 3 cases, ≧60% 1 case, ≧30% 4 cases, and <30% 4 cases, respectively. SOX10 was negative in 10 cases and weakly positive in 2 cases. This finding mimics the pattern of expression in minor salivary glands and may explain the occurrence of weak NKX3.1 staining and SOX10-positive cases in HCCC. Additionally, in the tissue microarray analysis, NKX3.1 staining was observed in only 1 HCCC case. These findings indicate that NKX3.1 is a useful marker for distinguishing HCCC from other clear cell salivary gland neoplasms. This study suggests that NKX3.1, along with SOX10 and CK7, can be utilized to improve the accuracy of HCCC diagnosis.