Objective:
A screen for differentially expressed DNA repair proteins in HPV+ and HPV- HNSCC cell lines revealed very low expression levels of the central DNA damage response kinase ATM in the most radiosensitive HPV+ strains. Since ATM also shows an enhanced rate of copy number loss in HPV+ HNSCC we tested a possible functional role of the reduced ATM expression for cellular radiosensitivity and therefore also a potential role as a molecular marker for deintensification of therapy.
Methods:
Western Blot analyses, colony formation assay, immunofluorescence microscopy, GFP-based DNA repair reporter gene assay, ATM inhibition using KU55933.
Results:
Irrespective of their ATM-expression level, radiosensitive HPV+ HNSCC cells displayed a DNA double-strand break repair kinetic similar to ATM-deficient cells. ATM inhibition sensitized both, HPV+ and HPV- HNSCC cell lines towards radiation but the extent of sensitization was greater for HPV- cells. Investigating possible mechanisms of the differential response we tested the influence of ATM-inhibition on homologous recombination repair (HRR). Unexpectedly a GFP-based reporter gene assay for HRR was not influenced by ATM inhibition in both groups and we could not detect an obvious difference in radiation induced ATM autophosphorylation or the phosphorylation of ATM target proteins.
Conclusion:
Our results point towards a possible role of deficits in the ATM-orchestration of DNA-repair processes in HPV+ cells but remain partly controversial. Further analyses, such as the ATM-dependency of the non-homologous endjoining pathway or a possible association of tumor ATM expression and patient outcome, are planned and required.
