A Genomic Study of Radiation Damage and Heat-Acclimation Mediated Radioprotection

Toxicity to non-tumor tissues is a significant obstacle in cancer radiotherapy. Treatment of head and neck malignancies often includes salivary glands in the radiation field. Radiation irreversibly damages these glands, leading to xerostomia and oral malfunctions. Heat- acclimation (a conserved phenotypic adaptation to elevated ambient temperatures) mediated cross-tolerance confers protection to radiation damage to the salivary glands. Gene transfer to over-express HSP70 a specific protective gene product, constitutively increasing upon heat-acclimation did not protect the salivary glands.

Objectives: To construct and use a customized cDNA array to shed light on the mechanisms of radiation damage and radioprotection.

Methods: Following an extensive literature search, 54 genes were selected and printed onto hybridization membranes. Samples were labeled with a radioactive probe (P32) and hybridized to the membranes, which were then exposed to a phosphor screen. The image was detected using a bioimage analyzer. Initial image analysis was performed using VisualGrid®. Results were normalized to controls and confirmed using PCR.

Results: The array highlighted marked genomic differences between acclimated and viral treated groups. Many of genes expressed in the acclimated groups behaved differently in the gene transfer groups e.g. up/down-regulation instead of silenced. These differences allowed us to formulate a list of potentially protective genes.

Conclusions: Based on our findings, most of the potentially protective genes are involved in the stress response in general and the Heat Shock Response in particular, some play a role in apoptosis and respond to DNA damage. Radiation altered expression of genes involved in the stress response, oxidative stress and apoptosis. Radiation induced long term genotypic modifications. In acclimated animals the genenotype and phenotype showed parallelism following radiation. Long term genotypic changes remained in the acclimated animals despite the phenotypic return to control behavior. Silencing of these protective genes, despite HSP70 up-regulation by gene transfer abolished radioprotection.