| Summary: | 碩士 === 國立陽明大學 === 放射醫學科學研究所 === 92 === Radiobiological effect under low doses of most studies have been obtained by back extrapolating the cell survival from high doses. It does not demonstrate an accurate response of the effects of most environmental exposures, which tend to be of low dose and protracted over time.
For this reason, the effect of high and low dose-rate (HDR, LDR), low doses ionizing radiation (IR) on the mutant frequency (MF) and the mutation spectrum at the hypoxanthine phosphoribosyl transferase (hprt) locus was studied in Chinese hamster ovary (CHO-10B2) cell line. Cells were cultured in the medium containing 6-thioguanine (6-TG) for 14 days to select for hprt mutants following 0, 0.2, 0.5, 0.75 and 1.0 Gy Co-60 γ- ray exposure. The mutation spectrum was determined for exon deletion analysis by multiplex polymerase chain reaction (M-PCR). Intact mutants were further analyzed by sequencing. In addition, cell cycle distribution analysis and mitotic cells were confirmed by flow cytometry and mitotic index (MI), respectively.
The results showed that hprt MF was increased with radiation doses under both HDR and LDR irradiation. However, the higher MF induced by 0.2 Gy suggests that hyper-radiosensitivity (HRS) and inverse dose-rate effect likely exist around this dose range. The dose response for the induction of hprt large deletions were significantly increased with radiation dose (r2=0.99, p=0.0064) compared to the point mutation (r2=0.29, p=0.35) induced by LDR IR which is the major of spontaneous mutations. In addition, exons deletions were found mostly at both ends of hprt gene. Terminal deletions were the predominant mutation type (r2=0.95, p=0.02). The higher deletion mutation at 0.2 Gy under LDR irradiation was also found. Furthermore, base sequencing of intact mutations seemed to indicate that hprt exons 3-9, and bases A or T were mostly substituted and about 22% were silent mutations.
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