DNA excision repair mediates adaptive response to low doses of ionizing radiation in C57BL/6J mice
1Canadian Nuclear Laboratories, Chalk River, Ontario, Canada
Our previous studies showed that exposure to low dose gamma-radiation can increase life span and latency times for tumor formation in C57BL/6J mice. In this study, we examined whether this systemic radioadaptive response can be mediated by activated DNA repair mechanisms.
For this study, we conducted in-vivo mice experiments of damage stemming from external gamma radiation exposure. Repair of DNA double strand breaks was determined by measuring gamma-H2AX levels in splenic lymphocytes by flow cytometry and western blot. DNA excision repair including base (BER) and nucleotide (NER) excision repair were determined in spleen by: (1) DNA excision and synthesis repair functional assay, (2) gene expression quantitative PCR analysis, and (3) western blot analyses
The results of our study indicate that exposure of mice to 20 or 100 mGy of gamma-radiation 24 h prior to a 2 Gy challenging dose did not result in enhanced repair of DNA doubles-strand breaks in splenic lymphocytes, measured as gamma-H2AX formation and loss by flow cytometry and western blot. In contrast, base (BER) and nucleotide excision repair (NER) rates, measured by the DNA excision and synthesis repair functional assay, were significantly higher in cells from low dose irradiated compared to non-irradiated mice. In quantitative RT-PCR experiments we measured expression levels of 84 key genes involved in various DNA repair pathways. We showed that only DNA excision repair genes were modulated by low dose radiation. Using western blot, we validated these results for Ddb1, Xpd, Rad51, Apex2 and Brca2 genes. Altogether, our data provide strong evidence that DNA excision repair is activated in response to low dose radiation in vivo.
Given our previous results showing increased tumor latency times following exposure to low dose radiation in the same experimental mouse model, our results suggest the role of BER and NER in increased genome stability and anti-tumor effects triggered by low dose radiation.