Abstract
Ionizing irradiation directly causes DNA damages as well as indirectly influences genomic integrity via modifying some cellular functions. The mechanism of the indirect effects has not been well studied, however, because it is difficult to separately investigate the direct and indirect effects. We recently developed human cell lines to generate DNA double strand breaks (DSB) in the genome using meganuclease I-SceI (Honma et al., EMM 42, 288, 2003). They are suitable models to study the fate of DSB directly cause by irradiation. We now investigated the effect of low-dose irradiation on the repair of DSB using these cell models. Human ymphoblastoid cell lines TSCE5 and TSCER2 are heterozygous (+/-) or compound heterozygous (-/-), respectively, for the thymidine kinase gene (TK), and were introduced an I-SceI endnuclease site into the gene. NHEJ for a DSB at the I-SceI site results in TK-deficient mutants in TSCE5 cells, while HR between the alleles produces TK-proficient revertants in TSCER2 cells. Because the expression of I-SceI enzyme produced a DSB at the TK gene 10,000-times more efficiently than ionizing irradiation, we could ignore the irradiation-inducing direct DNA damages. We examined the pretreatment effect of 30 mGy gamma ray for 24h on the repair of I-SceI-inducing DSB. While NHEJ efficiency was never changed, the frequency of HR increased approximately 2-times comparing non-irradiated cells. This indicates that low-dose irradiation influences something on the HR mechanism. We now investigated the manner of HR modified by irradiation.
