Monitoring of radiation-induced germline mutation in humans.

Estimating the genetic hazards of radiation and other mutagens in humans depends on extrapolation from experimental systems. Recent data have shown that minisatellite loci provide a useful and sensitive experimental approach for monitoring radiation-induced mutation in humans. This review describes the progress made in validating this approach and presents the results of recent publications on the analysis of minisatellite mutation rates in the irradiated families

Age-related accumulation of mutations reveals mechanisms of spontaneous mutation at tandem repeat DNA loci in mice

Expanded simple tandem repeat (ESTR) loci belong to the class of highly unstable loci in the mouse genome. The mechanisms underlying the very high spontaneous instability at these loci still remain poorly understood. Using single-molecule polymerase chain reaction, here we have compared the pattern of mutation accumulation in tissues with different proliferation capacities in male mice of age 12, 26, 48, and 96 weeks. In the nonproliferating brain, we did not observe any measurable age-related accumulation of ESTR mutations. In contrast, a highly elevated frequency of ESTR mutation was detected in the sperm samples taken from old mice; similar changes were also observed in the bone marrow tissue. The spectra of ESTR mutations accumulated in all tissues of young and old mice did not significantly differ. Taken together, these data clearly imply that spontaneous ESTR mutations occur almost exclusively in replication-proficient cells. To gain further insights into the mechanisms of ESTR mutation, we developed a stochastic model of age-related mutation accumulation. The observed spectra of ESTR mutants accumulated in the brain and sperm were fairly accurately approximated assuming the values of ESTR mutation rate, ranging from 0.01 to 0.04 per cell division. As these estimates dramatically exceed those for protein-coding genes and microsatellite loci, our data therefore suggest that ESTRs represent one of the most unstable loci in the mammalian genome. The results of our study also imply that ESTR loci can be regarded as a class of expanded microsatellites, with the mechanism of spontaneous mutation most probably attributed to replication slippage

Stage-specificity of spontaneous mutation at a tandem repeat DNA locus in the mouse germline

Stage-specificity of spontaneous mutation at a tandem repeat DNA locus in the mouse germlin

The genome-wide effects of ionizing radiation on mutation induction in the mammalian germline.

The ability to predict the genetic consequences of human exposure to ionizing radiation has been a long-standing goal of human genetics in the past 50 years. Here we present the results of an unbiased, comprehensive genome-wide survey of the range of germline mutations induced in laboratory mice after parental exposure to ionizing radiation and show irradiation markedly alters the frequency and spectrum of de novo mutations. Here we show that the frequency of de novo copy number variants (CNVs) and insertion/deletion events (indels) is significantly elevated in offspring of exposed fathers. We also show that the spectrum of induced de novo single-nucleotide variants (SNVs) is strikingly different; with clustered mutations being significantly over-represented in the offspring of irradiated males. Our study highlights the specific classes of radiation-induced DNA lesions that evade repair and result in germline mutation and paves the way for similarly comprehensive characterizations of other germline mutagens

Germline mutation rate in the hypervariable minisatellite CEB1 in the parents of children with leukaemia

Gardner and colleagues advanced the hypothesis that the Seascale leukaemia cluster may have been caused by germ cell mutagenesis due parental preconceptional irradiation (PPI) of fathers working at the Sellafield nuclear installation. Recent evidence has shown that PPI can lead to an increased germline mutation rate in certain minisatellite loci, allowing previously undetectable levels of germline sensitivity to IR to be measured. In this study, we have investigated the hypothesis that childhood leukaemia may be associated with parental germ cell sensitivity detected by an increased parental germline minisatellite mutation rate. To test this we compared the germline mutation rate of the hypervariable minisatellite locus, CEB1 in family trios (both parents and their child) of children with leukaemia (n = 109) compared with normal families (n = 64). We found no significant difference in mean mutation rate of parents of children with leukaemia and control children (0.0414 vs 0.0887; P > 0.05). The majority of germline mutations (95%) were paternal, and unaffected by the presence of leukaemia cells in the sample. We found no significant differences in mean minisatellite allele size (72 vs 68 repeats; p > 0.05), or mutational spectrum (gains or losses of repeats) in the parents of case and control children. Although preliminary, our results suggest that childhood leukaemia is unlikely to be associated with increased germline minisatellite instability resulting from exposure of parental germ cells to mutagens such as IR

The effects of MSH2 deficiency on spontaneous and radiation-induced mutation rates in the mouse germline

Mutation rates at two expanded simple tandem repeat (ESTR) loci were studied in the germline of mismatch repair deficient Msh2 knock-out mice. Spontaneous mutation rates in homozygous Msh2-/- males were significantly higher than those in isogenic wild-type (Msh2+/+) and heterozygous (Msh2+/-) mice. In contrast, the irradiated Msh2-/- mice did not show any detectable increases in their mutation rate, whereas significant ESTR mutation induction was observed in the irradiated Msh2+/+ and Msh2+/- animals. Considering these data and the results of other publications, we propose that the Msh2-deficient mice possess a mutator phenotype in their germline and somatic tissues while the loss of a single Msh2 allele does not affect the stability of heterozygotes

Maternal effects of the scid mutation on radiation-induced transgenerational instability in mice

The results of a number of recent studies show that mutation rates in the offspring of irradiated parents are substantially elevated, however the effect of parental genotype on transgenerational instability remains poorly understood. Here we have analysed the mutation frequency at an expanded simple tandem repeat (ESTR) locus in the germline and bone marrow of the first generation male offspring of control and irradiated male mice. The frequency of ESTR mutation was studied in the offspring of two reciprocal matings ♂scid x ♀BALB/c and ♂BALB/c x ♀scid, which were compared with that in BALB/c mice. In the offspring of the BALB/c x BALB/c and ♂scid x ♀BALB/c matings, which were conceived after paternal sperm irradiation, the frequency of ESTR mutation was significantly elevated in both tissues. In contrast, ESTR mutation frequency was only slightly elevated in the offspring of ♂BALB/c x ♀scid mating conceived after paternal irradiation. The results of this study suggest that the oocytes of scid females are unable to fully support the repair of double-strand breaks induced in paternal sperm which may in turn result in the elimination of cells/embryos containing high levels of DNA damage, thus partially preventing the manifestation of genomic instability

Paternal Exposure to Ethylnitrosourea Results in Transgenerational Genomic Instability in Mice

Paternal Exposure to Ethylnitrosourea Results in Transgenerational Genomic Instability in Mic

Maternal effects of the scid mutation on radiation-induced transgenerational instability in mice

The results of a number of recent studies show that mutation rates in the offspring of irradiated parents are substantially elevated, however the effect of parental genotype on transgenerational instability remains poorly understood. Here we have analysed the mutation frequency at an expanded simple tandem repeat (ESTR) locus in the germline and bone marrow of the first generation male offspring of control and irradiated male mice. The frequency of ESTR mutation was studied in the offspring of two reciprocal matings ♂scid x ♀BALB/c and ♂BALB/c x ♀scid, which were compared with that in BALB/c mice. In the offspring of the BALB/c x BALB/c and ♂scid x ♀BALB/c matings, which were conceived after paternal sperm irradiation, the frequency of ESTR mutation was significantly elevated in both tissues. In contrast, ESTR mutation frequency was only slightly elevated in the offspring of ♂BALB/c x ♀scid mating conceived after paternal irradiation. The results of this study suggest that the oocytes of scid females are unable to fully support the repair of double-strand breaks induced in paternal sperm which may in turn result in the elimination of cells/embryos containing high levels of DNA damage, thus partially preventing the manifestation of genomic instability

The effects of in utero irradiation on mutation induction and transgenerational instability in mice

Epidemiological evidence suggests that the deleterious effects of prenatal irradiation can manifest during childhood, resulting in an increased risk of leukaemia and solid cancers after birth. However, the mechanisms underlying the long-term effects of foetal irradiation remain poorly understood. This study was designed to analyse the impact of in utero irradiation on mutation rates at expanded simple tandem repeat (ESTR) DNA loci in directly exposed mice and their first-generation (F1) offspring. ESTR mutation frequencies in the germline and somatic tissues of male and female mice irradiated at 12 days of gestation remained highly elevated during adulthood, which was mainly attributed to a significant increase in the frequency of singleton mutations. The prevalence of singleton mutations in directly exposed mice suggests that foetal irradiation results in genomic instability manifested both in utero and during adulthood. The frequency of ESTR mutation in the F1 offspring of prenatally irradiated male mice was equally elevated across all tissues, which suggests that foetal exposure results in transgenerational genomic instability. In contrast, maternal in utero exposure did not affect the F1 stability. Our data imply that the passive erasure of epigenetic marks in the maternal genome can diminish the transgenerational effects of foetal irradiation and therefore provide important clues to the still unknown mechanisms of radiation-induced genomic instability. The results of this study offer a plausible explanation for the effects of in utero irradiation on the risk of leukaemia and solid cancers after birth