Clear cell squamous cell carcinoma of the tongue exhibits characteristics as an undifferentiated squamous cell carcinoma

Clear cell squamous cell carcinoma (CCSCC), where cells show abundant clear cytoplasm, -is a variant of squamous cell carcinoma (SCC) and a rare entity in the oral cavity. The characteristics of CCSCC, especially in immunohistochemical features, remain unclear. We characterized a case of CCSCC arising from the oral mucosal epithelium of tongue, where the clear cell lesion accounted for a predominant portion of the tumor. This CCSCC, which was partially surrounded by conventional SCC, exhibited cellular atypia immunohistopathologically and histopathologically with a high Ki-67 index, increased number of mitotic figures and enlarged nuclei. Intravascular invasion of the carcinoma cells was also observed. Furthermore, the CCSCC recurred and metastasized to the cervical lymph nodes and both lungs three months after resection. Immunohistochemical analyses demonstrated decreased expression of p40 (an isoform of SCC marker p63), ADP-ribosylation factor (ARF)-like 4c (ARL4C), yes-associated protein (YAP) and 5-methylcytosine (5mC) in the CCSCC lesion compared with the surrounding SCC lesion, where the expression of ARL4C was upregulated compared with non-tumor region and YAP showed nuclear translocation. In addition, siRNA loss-of-function experiments revealed that p63 expression was required for ARL4C expression and DNA methylation was induced by p63 and YAP/transcriptional co-activator with PDZ-binding motif (TAZ) signaling in oral SCC cell lines. These results suggest that CCSCC, in which several markers of SCC-associated intracellular signaling pathways are downregulated, together with evidence of altered epigenetic regulation, is characterized as an undifferentiated SCC variant.