<SESSION>IN VIVO SOMATIC MUTATION IN MICE INDUCED BY TRITIATED WATER

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To estimate genetic risk of tritiated water to humans, we examined dose-response relationship of mutation in the whole body with the use of very sensitive in vivo method to detect somatic mutation in mince. PT females (a/a, b/b, pc^<ch>/pc^<ch> d/d) were mated with HT males (a/a, pa/pa ln/ln, pe/pe), and the F_1 embryos from PT x HT cross, heterozygous at 7 recessive coat color genes, were treated with a single i.p. injection of 4.4,2.2,and 0.7 MBq of tritiated water per g of body weight at 14 : 00 on the 10th day of gestation. Somatic mutation was detected as a colored spot on the non-agouti (black) background 4 weeks after birth. Frequency of in vivo somatic mutations induced by tritiated water increased almost linearly with doses of tritiated water; 22/750 (0.03), 13/133 (0.10), 12/92 (0.13), and 22/85 (0.26) at 0,0.7,2.2,and 4.4 MBq/g, respectively. Cummulative doses of tritium for 4 days from the 10th to 14th day of gestation, were approximately 70% of the total (30 rad as absorbed dose). Mutation rate per locus per μCi/g was calculated to be 1.8 x 10^<-7>. On the assumption that dose-response curve by the reference radiation (chronic X-ray exposure) is linear at 30 rad, RBE of tritiated water was estimated to be about 2.7 at present. These values are similar to those for spermatogonial mutation.
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