Background: DNA repair genes play a critical role in maintaining genome stability and have been implicated in tumorigenesis. Head and neck squamous cell carcinoma (HNSCC) often shows chromosomal instability. We examined the expression of human RAD17, a DNA damage cell cycle checkpoint gene, in primary head and neck cancer tissue.
Methods: Significance analysis of microarrays was applied to expression array results examining more than 12,000 genes in 7 samples of primary HNSCC and 6 samples of normal control oral epithelial tissue. Additional confirmation was performed by quantitative reverse transcription-polymerase chain reaction (RT-PCR) in these samples and western blot with an additional 12 primary HNSCC and 7 normal samples, followed by loss of heterozygosity (LOH) analysis and quantitative PCR at the RAD17 locus.
Results: Multiple checkpoint and DNA repair genes were downregulated in primary head and neck tumor tissue compared with normal control epithelial tissue, including hRAD17. Its Z-score and fold change were -2.5 and 0.39, respectively. The results of normalized, quantitative RT-PCR showed decreased expression of hRAD17 mRNA in tumor tissue (mean value 0.2166) when compared with normal tissue (mean value 0.3957, p < .05). Western blot demonstrated undetectable expression of hRAD17 protein in primary tumor tissue (0/12), while there was strong expression of hRAD17 protein in normal oral mucosal tissue (6/7). To determine possible mechanisms of inactivation, the hRAD17 locus at 5q13 was analyzed using microsatellite markers, showing 70% LOH in 30 primary HNSCCs. Quantitative PCR showed that RAD17 DNA copy number was decreased in the majority of head and neck tumor tissue samples.
Conclusion: Loss of hRAD17 expression occurs frequently in HNSCC, is often due to genomic deletion, and may facilitate genomic instability in HNSCC.