Predominant causes of head and neck cancer recurrence after radiotherapy are rapid repopulation, hypoxia, fraction of cancer stem cells, and intrinsic radioresistance. Currently, intrinsic radioresistance can only be assessed by ex vivo colony assays. Besides being time-consuming, colony assays do not identify causes of intrinsic resistance. We aimed to identify a biomarker for intrinsic radioresistance to be used before start of treatment and to reveal biologic processes that could be targeted to overcome intrinsic resistance.

We analyzed both microRNA and mRNA expression in a large panel of head and neck squamous cell carcinoma (HNSCC) cell lines. Expression was measured on both irradiated and unirradiated samples. Results were validated using modified cell lines and a series of patients with laryngeal cancer.

miRs, mRNAs, and gene sets that correlated with resistance could be identified from expression data of unirradiated cells. The presence of epithelial-to-mesenchymal transition (EMT) and low expression of miRs involved in the inhibition of EMT were important radioresistance determinants. This finding was validated in two independent cell line pairs, in which the induction of EMT reduced radiosensitivity. Moreover, low expression of the most important miR (miR-203) was shown to correlate with local disease recurrence after radiotherapy in a series of patients with laryngeal cancer.

These findings indicate that EMT and low expression of EMT-inhibiting miRs, especially miR-203, measured in pretreatment material, causes intrinsic radioresistance of HNSCC, which could enable identification and treatment modification of radioresistant tumors.

In head and neck squamous cell carcinomas (HNSCC), radiation is a major treatment modality. Intrinsic radioresistance of tumor cells is one of the predominant causes of head and neck cancer recurrence. This phenomenon can only be examined by ex vivo colony assays, but these take too much time to be clinically useful and do not reveal the biologic mechanisms of intrinsic radioresistance. Using microRNA and mRNA expression profiles of HNSCC cell lines and tumors, we found that low expression of certain microRNAs that suppress epithelial-to-mesenchymal transition, measured prior to treatment, is causally related to intrinsic resistance to radiation. This finding provides an important step toward modification and thereby improvement of the treatment of radioresistant tumors.