Effect of x-rays on chromosome 21 nondisjunction

In a series of 156 females and 149 males with a Down syndrome (DS) child, a case-control study was performed to evaluate the effect of abdominal-pelvic exposure to diagnostic x-rays prior to conception on nondisjunction (ND). Cytogenetic analysis using QFQ banding allowed unequivocal identification of ND parents as cases. Partners of ND parents were treated as control group. Odds ratio for the association of x-rays exposure and ND occurrence (stratified for sex and age) was 1.85 (borderline to significance: with a 95% confidence interval 1-3.44). Such an association appeared highly significant in older fathers and borderline to significant in younger mothers, when age groups were analyzed separately. By comparing mean parental ages at birth of the propositus, the prevalence of exposure to x-rays appeared moderately associated with aging in control parents of both sexes. Furthermore, the mean age of unexposed ND parents of paternally derived SD cases was the same as the referent population's, suggesting that age is not a risk factor for ND in the male, except for being associated with increasing exposure risk. Conversely, risk attributable to x-rays exposure in the female appears to be progressively diluted with increasing age, by strongly age-dependent high risk, presumably due to biologic factors that are not affected by environmental exposure.

[Down syndrome: pathogenesis, radioresistant DNA synthesis and chromosomal instability]

Down syndrome (DS) is a frequent chromosomal aberration. Triplication of the fragment 21q22 of chromosome 21 is sufficient to cause the DS phenotype including immunodeficiency, premature aging, mental retardation, and an increased risk of leukemia. Chromosomal aberrations caused by X-ray irradiation were observed in DS lymphocytes and DS fibroblasts, but the correlation between chromosomal sensitivity, repair deficiency, and radioresistant DNA synthesis was not clear. Here some insight into the nature of this problem has been made. Besides, new arguments have been provided in favour of genetic heterogeneity of this genetic disorder.

Joining of correct and incorrect DNA ends at double-strand breaks produced by high-linear energy transfer radiation in human fibroblasts

DNA double-strand breaks (DSBs) were measured within a 3.2-Mbp NotI fragment on chromosome 21 of cells of a normal human fibroblast cell line. Correct rejoining of DSBs was followed by measuring reconstitution of the original-size NotI fragment, and this was compared to total rejoining as measured by a conventional pulsed-field gel electrophoresis technique (FAR assay). After 80 Gy of particle irradiations with LETs in the range of 7-150 keV/microm, it was found that the repair kinetics was generally slower after irradiation with high-LET particles compared to X irradiation and that a larger proportion of the breaks remained unrepaired after 24 h. On the other hand, the misrejoining frequency as measured by the difference between correct and total rejoining after 24 h did not change with LET, but was approximately the same for all radiations at this dose, equal to 25-30% of the initial breaks. This result is discussed in relation to formation of chromosomal aberrations, deletion mutations and other biological end points.

Relative involvement of chromosome #21 in radiation induced exchange aberrations in lymphocytes of Down syndrome patients

It is not yet resolved as to what type of DNA double strand break repair operates in G0 lymphocytes. We have employed Down syndrome (DS) lymphocytes with three copies of chromosome #21 to answer the question whether the presence of three copies reduces the frequency of exchange aberrations involving this chromosome in comparison to normal cells with two copies of #21. Peripheral blood lymphocytes from three DS patients and two normal individuals were X-irradiated with 1 and 3 Gy. The frequencies of unstable aberrations were found to be higher in DS lymphocytes than normal lymphocytes after 3 Gy of X-rays. FISH studies employing chromosome specific DNA libraries for chromosomes #21 and #22 indicated that the frequencies of exchange aberrations per chromosome are similar in both disomic and trisomic condition. This indicates that the presence of an extra copy of chromosome #21 does not alter the yield, suggesting that homologous recombination does not play a major role in the repair of DNA strand breaks in human G0 lymphocytes.

Aneuploidy in somatic cells of in utero exposed A-bomb survivors in Hiroshima

Cytogenetic data on cultured lymphocytes of the in utero exposed A-bomb survivors in the RERF Adult Health Study cohort have been analyzed using the G-banding technique to determine the frequency of aneuploid cells. The data consist of blood samples collected between 1985 and 1987 from 264 Hiroshima individuals for whom DS86 maternal uterine dose estimates are available: 124 proximally exposed (74 males and 50 females) with an estimated dose of 0.005 Sv or more, and 140 distally exposed (76 males and 64 females) with a dose estimate of 0 Sv, assuming the neutron relative biological effectiveness (RBE) of 10. A main feature of aneuploidy was that aneuploid frequency in autosomes depended generally on chromosome length; aneuploidies were significantly more frequent in shorter chromosomes than in longer chromosomes. The frequency of aneuploidies also depended on type, with chromosome loss approximately five times more frequent than chromosome gain. However, chromosome 21, as well as the sex chromosomes, were notable in that aneuploidy was much more frequent for these chromosomes than would be predicted from a simple relationship with length. X chromosome aneuploidies were significantly more frequent in females than in males. There was no dependence of aneuploid frequencies on dose when measured 40 years after the exposure.

Genetic mapping based on radiation hybrid data

Radiation hybrid mapping is a recently developed technique for constructing high-resolution maps of mammalian chromosomes. The chromosome of interest is exposed to X rays, which cause it to break into numerous fragments. The fragments are randomly recovered into hamster cells. The resulting hybrids are then analyzed for the presence or absence of DNA markers of interest. A simple method for using this information to order the markers on the chromosome is developed here. For any particular ordering of the loci, the least number of breaks consistent with the data can be determined. The best order is taken to be that which minimizes this number of obligatory breaks. Some statistical properties of the method are examined, both theoretically and by use of a simulation study. Except for some extreme cases, the method gives good results, which compare well with results obtained by a full maximum likelihood analysis. The method is used to order a set of 14 loci on chromosome 21 with encouraging results.

The accident at Chernobyl and trisomy 21 in Finland

Our objective was to explore whether the radiation fallout in Finland after the accident at the Chernobyl nuclear power plant in April 1986 led to an increased incidence of trisomy 21. In this geographic and temporal cohort study, the country was divided into three zones according to the amounts of radioactive fallout and internal radiation caused by two cesium isotopes. The 518 cytologically verified cases of trisomy 21 were divided into a control group (conceived before the accident), and a study group of children whose expected dates of birth were in the post-accident years 1987-1988, i.e., pregnancies commenced after May 1986. The cases were also divided into three subgroups according to the zones of radiation. There were no significant differences in prevalence of trisomy 21 between the control and study groups nor between the three zones in spite of the significant differences in the levels of radiation and in the body burden that prevailed throughout the study period. Power estimates showed that in the two zones of lower radiation, an increase of 0.5% in the prevalence would have been detected with a power of 0.85, and in the somewhat smaller zone of the highest radiation, with a power of 0.70. The study lends no further support to the view that the low radiation fallout in western Europe would have been causally associated with trisomy 21.

Statistical methods for multipoint radiation hybrid mapping

On the basis of the earlier work of Goss and Harris, Cox et al. introduced radiation hybrid (RH) mapping, a somatic cell genetic technique for constructing fine-structure maps of human chromosomes. Radiation hybrid mapping uses X-ray breakage of chromosomes to order a set of genetic loci and to estimate distances between them. To analyze RH mapping data Cox et al. derived statistical methods that employ information on sets of two and four loci, to build an overall locus order. Here we describe alternative nonparametric and maximum-likelihood methods for the analysis of RHs that use information on many loci simultaneously, including information on partially typed hybrids. Combination of these multipoint methods provides a statistically more efficient solution to the locus-ordering problem. We illustrate our approach by applying it to RH mapping data on 14 markers in 99 radiation hybrids for the proximal long arm of human chromosome 21.