Abstract
Microsatellites are composed of many tandemly repeated short-sequence units. They are known to have high spontaneous mutation rates in the number of repeat units in germ cells. Therefore, we chose microsatellite loci as targets for monitoring A-bomb radiation-induced mutation rates in germ cells. We measured mutation rates at 40 microsatellite loci among 129 offspring (62 and 4 are from radiation-exposed families having one and both exposed parents, respectively, and 63 from the unexposed controls). In the study we screened for mutations at a total of 2789 microsatellite alleles in 70 exposed germ cells (average dose 1.56 Gy) and at 7465 alleles in 188 unexposed germ cells.
For detecting length-modified mutant alleles via gain or loss of the repeat units, we amplified each microsatellite by PCR and analyzed products by capillary electrophoresis. We compared allele sizes in children with those of their parents. When allele sizes of the children were different from those of either of their parents, we scored the alleles as mutated. For the first screening, we analyzed DNA extracted from established lymphoblastoid cells. Because genetic instability at microsatellites in EBV-transformed cell lines was reported, we confirmed putative mutations detected in the first screening using DNA from uncultured cells.
We found 20 and 17 mutations in the children from the exposed and the control families, respectively. Although it is not yet possible to determine parental origins of four mutations, either from the exposed parents or the unexposed parents, the mean mutation rates of exposed and unexposed alleles are 0.39% (7+4/2789) and 0.35% (26/7465), respectively, even if we assume that all of the 4 mutations were derived from the exposed parents. In conclusion, the present results do not provide evidence for an increased mutation rate at microsatellite loci attributable to A-bomb radiation.
