Cancer Stem Cells in Squamous Cell Carcinoma of the Oral Tongue

Abstract

Background: Given the discovery of cancer stem cells (CSCs) within haematological and solid tumours and the expression of primitive markers by infantile haemangioma, it was hypothesised that a CSC population would also be present in oral tongue squamous cell carcinoma (OTSCC). A deeper understanding of the cells that drive tumourigenesis is required for the development of mechanism-based therapies for OTSCC. Aims: This study aimed to identify and characterise the CSC population within OTSCC based on their protein and gene expression profiles. Methods: Immunohistochemical staining, Western Blotting, mass spectrometry and Nanostring analysis were employed to investigate the expression of a panel of proteins and genes in formalin-fixed paraffin-embedded tissues, and snap frozen tissues of OTSCC from 21 patients. Markers used include epithelial cancer markers (p63 and EMA), CSC markers (CD44, CD133 and SOX2) and embryonic stem cell (ESC) markers (Oct-4, Nanog and pSTAT3). Results: Widespread and overlapping expression of p63, EMA, CD44, SOX2, pSTAT3, Nanog and Oct-4 was identified within the OTSCC. Co-expression of CD44, SOX2, Oct-4, Nanog and pSTAT3, was found within the OTSCC cells, while cells scattered within the peri-tumoural stroma expressed CD44, CD133, pSTAT3, Nanog and Oct-4. A CD44+/SOX2+/Nanog+/Oct-4+/pSTAT3+ but CD133-/EMA-/p63- CSC population was identified in OTSCC. Expression of CSC and ESC markers by differentiated phenotype structures, and the presence of polyploid giant cancer cells that are CD44+/Nanog+ were novel findings. Conclusions: The unique widespread distribution of the CSC population with co-expression of epithelial cancer cell, CSC and ESC markers within OTSCC suggests that these cells are comprised of several overlapping sub-populations that are organised hierarchically within the tumour. The lack of CD133 expression in OTSCC tumour cells brings into question the utility of this protein as a CSC marker. The findings demonstrate a unique expression signature for the CSC population within the OTSCC samples investigated and support a hierarchical CSC model of carcinogenesis, while the novel findings provide interesting avenues for further research.