Development of (a human X-chromosome * hamster) hybrid cell assay system, which is sensitive to tritium exposure.

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HPRT mutation was used to assay the radiation effect, since it is possible to isolate by positeve-negative selection with 6-thioguanin and HAT (hypoxunthin-aminopterin-thymidine). Howeverr, radiation is known frequantly to induce DNA-deletion type mutation, which possibly decrease the HPRT mutation rate. To detect the deletion mutation, we developped HPRT deficient hamster-based hybrid cells, where an intanct human X-chromosome. was transfered our experiments showed that appropriate expression time of these cells is 8-10 days after irradiation. The mutation incidence of theses cells was 450 mutants/ 10^5 survived cells after 4-Gy irradiation and it was contrast with mutantion rate in conventional rodent assay system : 5-50 mutants/10^5 survived cells. Thus, newly developped assay system indicated 10-100 fold highre sesnsitivity than that used to assay for radiation effect. Existance of a transferred human chromosome in their mutant cells were confirmed by FISH ana1ysis, implying mutation induction with the of HPRT gene. Mutation incidences were linearly increased with radiation dose of ^<60>Co-gamma-rays and this dose-relationship showed the approxinately 100-fold high sensitivity, compared with that of conventional HPRT mutation assay. When this incidence was compared with that of tritium-beta rays, the biological effectiveness RBE indicated the consistent value, 1.24, with previous observation. As a results, our studies demonstrated the high sensitivity and usefulness of hamster hybrid cells cantaining an intact human X-chromosome for evaluation of tritium biological effects.