Abstract
To evaluate the risk of diagnostic irradiation for infant and children, we studied the mutation in Aprt locus induced by X-ray exposure using mouse kidney cells. Aprt is an enzyme that catalyzes phosphoribosylation of adenine molecule in a salvage pathway. A lack of Aprt activity causes a disease known as APRT deficiency in human associated with marked interstitial fibrosis and/or urolithiasis, however, not essential for survival, because of de novo synthetic pathway. Therefore, the non-essentiality of Aprt activity enables us to use it for mutation detection. We used the Aprt heterozygous mouse for X-ray irradiation and subsequent mutation detection experiment to evaluate the age-dependent risk of X-ray.
Aprt+/- heterozygous B6C3F1 mice were were exposed to 0Gy, 1Gy, or 4Gy of X-rays at 1 or 7 week old. F1 mice were sacrificed after 8 weeks of X-ray exposure and the cells derived from each kidney were cultured in the presence of 1mM 2,6-diaminopurine (DAP) to select Aprt-/- cells. The frequency of DAP-resistant colonies was 2.3 x 10-5 in average in the un-irradiated control. When mice were exposed with 1Gy of X-rays at 7 weeks old, the frequency of DAP-resistant clone was 61% of unexposed control. This result was unexpectedly lower than the one of the un-irradiated control. When the same dose was applied to the mice at 1 week old, the frequency increased to 200% of unexposed control. This result suggested that X-ray has more suppressive effect on Aprt activity in younger mouse kidney cells. Also mice were exposed with 4Gy of X-rays at 7 weeks old, and the result was 435% increase in mutant frequency compared to the unexposed group. When 4Gy of X-ray was exposed to the mice at 1 week old, the frequency of DAP-resistant colony was further increased to 604% of unexposed control. These data suggested that the higher the dose of X-rays and the younger the age of irradiation, the more Aprt-deficient colonies were induced.
