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J D Tucker

Publications and source records attributed to J D Tucker.

At least 37 records · Page 2Linked to original sources
PubMed1999

Radiation-induced breakpoint misrejoining in human chromosomes: random or non-random?

PURPOSE: To investigate whether radiation-induced misrejoining of chromosome breakpoints is randomly or non-randomly distributed throughout the human genome. MATERIALS AND METHODS: Data were combined from as many published cytogenetic studies as possible. The percentage of radiation-induced breaks per megabase (Mb) of DNA between all human chromosomes was calculated, and the observed and expected numbers of breakpoints based on DNA content between and within chromosomes were compared. RESULTS: A DNA-proportional distribution of breakpoints in 14 autosomes and a statistically significant deviation from proportionality in the other eight autosomes and the sex chromosomes was found. Regression analysis showed no significant change in breakpoint frequency per Mb of DNA relative to autosome size. Analysis between chromosome arms showed a non-random distribution of induced breakpoints within certain autosomes, particularly the acrocentrics. In cases of non-random distributions, a prevalence of events was found at heterochromatic regions and/or telomeres, and a clustering of breakpoints was found near the centromeres of many chromosomes. CONCLUSIONS: There is an approximately linear proportionality between autosomal DNA content and observed breakpoint number, suggesting that subsets of autosomes can be used to estimate accurately the overall genomic frequency of misrejoined breakpoints contingent upon a carefully selected subset. However, this conclusion may not apply to the sex chromosomes. The results also support the influence of chromatin organization and/or preferential DNA repair/misrejoining on the distribution of induced breakpoints. However, these effects are not sufficient at a global level to dismiss the value of cytogenetic analysis using a genome subset for biodosimetry.

Chromosome Breakage↗
PubMed1999

Chromosome aberrations of clonal origin in irradiated and unexposed individuals: assessment and implications.

Chromosome painting has proven useful for the detection of chromosomal rearrangements, although the presence of cells containing clonal aberrations can have an effect on the outcome of cytogenetic analyses (e.g. aberration frequency and chromosomal distribution studies). Cells with clonal chromosomal changes have been found in studies of both radiation-exposed Chernobyl cleanup workers ("liquidators") and healthy unexposed human subjects. We have used a simple statistical method to aid in the identification of individuals from distinct Chernobyl radiation-exposed and unexposed control populations who may possess cells containing clonal rearrangements. A chi2 value determined from the observed number of aberrations and the expected number based on chromosome length that corresponds to a probability less than 0.005 appears to be an indicator of clonality. These selected individuals can be analyzed further for clonality, thereby sparing detailed examination of the entire population. Here we present an analysis of individuals possessing clonal aberrations to assess the influence of clonality on the results of cytogenetic studies. Our results show that the subtraction of clonal events from the chi2 calculation for the "outliers" results in nearly all of these values losing their statistically significant deviation from proportionality. These adjustments can also be made to prevent the overestimation of frequencies of chromosome aberrations for biodosimetry. The frequency of clonal aberrations appears to increase as a function of age in control subjects, whereas an age effect was not evident in Chernobyl liquidators. This suggests that spontaneous and radiation-induced clonal expansion are occurring in control subjects and liquidators, respectively.

Adult↗
PubMed1999

Biological dosimetry of chernobyl cleanup workers: inclusion of data on age and smoking provides improved radiation dose estimates.

We report the results of a study of chromosome translocations in 126 Russian subjects who participated in the cleanup activities at Chernobyl and another 53 subjects, from other places in Russia, who were not exposed at Chernobyl. In agreement with our earlier study, we find increased translocation frequencies among the exposed compared to Russian controls. We describe statistical methods for estimating the dose of ionizing radiation determined by scoring chromosome translocations found in circulating lymphocytes sampled several years after exposure. Two statistical models were fitted to the data. One model assumed that translocation frequencies followed an overdispersed Poisson distribution. The second model assumed that translocation frequencies followed a negative binomial distribution. In addition, the effects of radiation exposure were modeled as additive or as multiplicative to the effects of age and smoking history. We found that the negative binomial model fit the data better than the overdispersed Poisson model. We could not distinguish between the additive and the multiplicative model with our data. Individual dose estimates ranged from 0 (for 43 subjects) to 0.56 Gy (mean 0.14 Gy) under the multiplicative model and from 0 to 0.95 Gy (mean 0.15 Gy) under the additive model. Dose estimates were similar under the two models when the number of translocations was less than 4 per 100 cells. The additive model tended to estimate larger doses when the number of translocations was greater than 4 per 100 cells. We also describe a method for estimating upper 95% tolerance bounds for numbers of translocations in unexposed individuals. We found that inclusion of data on age and smoking history was important for dose estimation. Ignoring these factors could result in gross overestimation of exposures, particularly in older subjects who smoke.

Adult↗
PubMed1998

Detection of structural chromosome damage in rat interphase cells using region-specific fluorescence in situ hybridization probes developed by microdissection.

Cytogenetic analysis using fluorescence in situ hybridization (FISH) was employed to detect structural chromosome aberrations in interphase. We generated DNA probes specific for rat chromosome regions 1q11-12, 1q31-35 and 1q51-53 by microdissection and degenerate oligonucleotide-primed PCR. Targeted regions were labeled in unique colors by FISH. Abnormal cells were identified on the basis of alterations in the physical distance between the hybridization signals. To evaluate the ability of these probes to quantify chromosome aberrations in interphase, rats were acutely exposed whole-body to 0, 1, 2, 3 or 4 Gy of 137Cs gamma rays. Eight days later, peripheral blood, bone marrow, lung and pancreas were removed and hybridized with the probes. Multi-color FISH analysis showed dose-responsive frequencies of abnormal interphase nuclei in peripheral blood and bone marrow cells. In lung and pancreas, on the contrary, no increase in the frequency of the abnormal interphases was observed. However, chromosome damage was observed when primary lung cells, obtained from rats irradiated 8 days previously, were cultured for three days. Detection of rearranged signals after in vitro tissue culture was attributed to the movement of chromosome domains that accompanies mitosis. The use of region-specific painting probes appears useful for detecting structural chromosome damage in interphase cells of rat tissues, although further optimization is still needed to improve the method.

Animals↗
PubMed1998

The accumulation of chromosome aberrations and Dlb-1 mutations in mice with highly fractionated exposure to gamma radiation.

The dichotomy between the doses at which experimental measurements of genetic effects can be made and the doses to which people are exposed is often different by two or more orders of magnitude. This presents a significant problem when determining the effects of low doses of radiation or chemicals. The solution has usually involved extrapolating the data by curve-fitting or by applying theoretical considerations. Both approaches are unsatisfactory due to uncertainties of the assumptions used in each process. The alternative solution has been to increase the sample size enormously at the lower doses. This is impractical beyond a certain point due to the variation in the spontaneous frequency and the need to quadruple the sample size for a doubling of precision. The development of new methods for measuring stable genetic effects, however, permits a simple and effective approach to this problem: if the genetic events being detected have no effect on survival, i.e., are selectively neutral, then the effects of multiple independent treatments will be additive. If the independent treatments are identical, then the effect of each is easily calculated by dividing the total effect by the number of treatments. Here we report a limited test of this approach using mice. Chromosome aberrations induced in lymphocytes and Dlb-1 mutations induced in the small intestine were measured after daily doses of 0.64, 1.85 or 5.5 cGy 137Cs gamma rays administered for 21, 42 or 63 days. The dose response curve for chromosome translocations obtained in this way, combined with the data from single larger acute doses, shows no evidence for a threshold over a 500-fold dose range. Dlb-1 mutations were increased at each dose and time but the results do not permit reliable extrapolations. The results suggest that translocations might be useful for quantifying the effect of doses below 0.05 cGy and that the effect of dose rate and dose fractionation at much lower doses than reported here could be investigated.

Animals↗
PubMed1998

Chromosome aberrations induced in mice by chronic feeding of 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ).

Dietary intake of mutagenic compounds is considered to be an important factor for the induction of some human cancers. Highly mutagenic compounds are known to be formed in meat during the cooking process. Since the discovery of such compounds, many studies have been conducted to evaluate their carcinogenic potential. One of the most mutagenic compounds formed in the cooking of meat is 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ). The recent development of mouse chromosome painting probes expand the capability of evaluating these food mutagens as potential clastogens in vivo. In this paper, we demonstrate the induction of chromosome aberrations in mice chronically exposed to MeIQ in their diet. CDF1 female mice were fed 400 ppm MeIQ beginning at 7 wk of age. At 76 wk of age, five control and eight exposed mice were euthanized. Blood and bone marrow cells were obtained and arrested in metaphase. Whole chromosome painting probes were used for fluorescence in situ hybridization of metaphase cells from blood and bone marrow. MeIQ-exposed mice were found to have a twofold increase in translocations and a 16-fold increase in fragments in their peripheral blood compared with controls. No aberrations were observed in the bone marrow. All organs were examined for the presence of tumours and routine histopathological analysis was performed on all organs as well as any tissue with macroscopic abnormalities. Forestomach and/or liver tumours developed in all but one of the mice fed MeIQ, but no such tumours were observed in the control mice. These data indicate that MeIQ is clastogenic and carcinogenic in vivo.

Animal Feed↗
PubMed1998

XRCC2 and XRCC3, new human Rad51-family members, promote chromosome stability and protect against DNA cross-links and other damages.

The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

Animals↗
PubMed1998

Frequency, distribution and clonality of chromosome damage in human lymphocytes by multi-color FISH.

Whole chromosome painting was used to determine whether the use of different sets of paints would influence results obtained from the analysis of human peripheral blood lymphocytes from 27 healthy unexposed subjects. Painting was also used to determine if aberration frequencies are proportional to the size of selected chromosomes in human lymphocytes irradiated in vitro. The in vitro results showed that the frequencies of radiation-induced stable aberrations (i.e. translocations and insertions) in chromosomes 3, 5 and 6 painted in unique colors are proportional to chromosome size. Aberration frequencies in the normal subjects were measured using two different sets of paints, one set for chromosomes 3, 5 and 6 where each chromosome was labeled in a unique color and one set where chromosomes 1, 2 and 4 were painted in a single color. The frequency of aberrations among chromosomes 1-6 in the population as a whole was also found to be proportional to chromosome size. However, some individual subjects had a distribution of damage that was not proportional to chromosome size due to the presence of clones of abnormal cells. Aberration frequencies measured in chromosomes 3, 5 and 6 as a set were highly correlated with those observed in chromosomes 1, 2 and 4 as a set, after adjusting for the different amounts of the genome that were painted. We also determined whether differences exist in the aberration frequencies measured by two scoring systems: the classical method, where reciprocal exchanges are scored as single events, and PAINT, where each break junction is scored as a single event. The two scoring systems gave highly correlated results for translocations and differed by a constant value (PAINT x 0.58 = classical method). Approximately 27% of translocations were observed to be non-reciprocal due to a failure to detect exchanges involving small amounts of material or to a non-reciprocal exchange mechanism. Our results support the hypothesis that cytogenetic evaluations for biodosimetry can be performed with any one or more of the chromosomes studied here and indicate that the aberration frequency measurements are independent of the scoring system selected for the evaluation. We also present a simple statistical method for identifying subjects that may possess clonal aberrations.

Adolescent↗
PubMed1998

Persistence of radiation-induced translocations in human peripheral blood determined by chromosome painting.

We have investigated the persistence of translocations and other types of chromosome damage with time using human peripheral blood acutely exposed in vitro to 137Cs gamma rays at doses ranging from 0.5 to 4 Gy. Freshly drawn blood from one donor was irradiated and metaphase chromosomes were prepared 2 to 7 days after exposure. Chromosomes 1, 2 and 4 were painted red-orange and chromosomes 3, 5 and 6 were painted green by fluorescence in situ hybridization (FISH) using "semi-directly" labeled whole-chromosome painting probes. This type of labeling combines direct and indirect labeling and showed significant advantages over both these other methods. All types of structural chromosome aberrations were classified by the Protocol for Aberration Identification and Nomenclature Terminology (PAINT) system. The yields of dicentric chromosomes, acentric fragments and ring chromosomes diminished with time as expected. Translocations exhibited greater persistence but showed a clear and statistically significant reduction in frequency at all doses. The mathematical model suggested that the translocation frequencies would reach a plateau of approximately 4, 15, 51, 106 and 179 translocations per 100 cell equivalents after irradiation with 0.5, 1, 2, 3 and 4 Gy, respectively. When translocations were classified by the conventional system, an analysis of the distribution of translocations and dicentrics per cell indicated that both types of exchanges were Poisson-distributed 48 h postirradiation. However, cells bearing translocations have a higher possibility of having dicentrics than cells without translocations. These findings suggest that dicentrics may contribute to a decline of translocation frequencies with time, and that some translocations are not completely persistent. The results obtained here using human blood exposed in vitro may influence the use of translocations as a retrospective biodosimeter of exposure to ionizing radiation in humans.

Cells, Cultured↗
PubMed1998

Do recorded doses overestimate true doses received by Chernobyl cleanup workers? Results of cytogenetic analyses of Estonian workers by fluorescence in situ hybridization.

Studies of workers who were sent to Chernobyl after the 1986 reactor accident are being conducted to provide a better understanding of the effects of chronic low-dose radiation exposures. A crucial component to these investigations is an accurate assessment of the radiation doses received during the cleanup activities. To provide information on biological measurements of dose, fluorescence in situ hybridization (FISH) with whole-chromosome painting probes has been applied to quantify stable chromosome aberrations (translocations and insertions) among a defined cohort of 4,833 cleanup workers from Estonia. Cytogenetic analysis of 48-h lymphocyte cultures from 118 Estonian cleanup workers (10.3 cGy mean recorded dose; 25 cGy maximum), 29 Estonian population controls and 21 American controls was conducted by three laboratories. More than 258,000 painted metaphases were evaluated. Overall, we observed lower translocation frequencies than has been reported in previous studies using FISH among Chernobyl cleanup workers. In our data, a clear association with increased levels of translocations was seen with increasing age at blood drawing. There was no correlation, however, between aberration frequency and recorded measurements of physical dose or any category of potential high-dose and high-dose-rate exposure such as being sent to Chernobyl in 1986, working on the roof near the damaged nuclear reactor, working in special zones or having multiple tours. In fact, the translocation frequency was lower among the exposed workers than the controls, though not significantly so. To estimate the level of effect that would have been expected in a population of men having an average dose of approximately 10 cGy, blood from six donors was exposed to low-LET radiation, and more than 32,000 metaphases were scored to estimate dose-response coefficients for radiation-induced translocations in chromosome pairs 1, 2 and 4. Based on these results, we estimate that had this group of 118 men received an average whole-body dose of 10-11 cGy, as chronic or acute exposures, an increase in the mean frequency of chromosome translocations of more than 40-65% would have been observed in their lymphocytes compared to findings in nonirradiated controls. In spite of evaluating more than a quarter of a million metaphases, we were unable to detect any increase in the mean, median or range in chromosome aberrations in lymphocyte cultures from a group of Estonian men who took part in the cleanup of the Chernobyl nuclear power site and those who did not. We conclude that it is likely that recorded doses for these cleanup workers overestimate their average bone marrow doses, perhaps substantially. These results are consistent with several negative studies of cancer incidence in Chernobyl cleanup workers and, if borne out, suggest that future studies may not be sufficiently powerful to detect increases in leukemia or cancer, much less distinguish differences between the effects of chronic compared to brief radiation exposures.

Adult↗
PubMed1998

Chronic ingestion of clastogens by mice and the frequency of chromosome aberrations.

Environmental exposure to mutagens is believed to play a significant role in human carcinogenesis. Determination of the in vivo effects of a single mutagen is best done in laboratory animals because humans are exposed to a variety of mutagens both in their diet and in the rest of their environment. In this study, C57BL/6N female mice were used to analyze the effect on chromosomes of chronic ingestion of a mutagen dissolved in drinking water. Cyclophosphamide (CP) or urethane (ethyl carbamate, EC) were dissolved in sterile drinking water at concentrations of 0, 32, 64, and 96 ppm or 0, 5,000, 10,000, and 15,000 ppm, respectively. All exposures began at 8 weeks of age and continued through the 20th week unless terminated earlier due to toxicity. Body weights and water consumption were measured weekly. Blood and bone marrow were taken from approximately five mice per exposure group at 4, 8, and 12 weeks from the start of exposure. All mice remaining after 12 weeks received drinking water without any carcinogen for an additional 6 weeks to determine if induced aberrations persisted. Chromosome translocations, measured by painting, were not induced in blood or bone marrow cells at any time point for either chemical. However, both carcinogens induced significant increases in micronucleated normochromatic erythrocytes, indicating that the carcinogens reached the tissues examined in these experiments. These results indicate that chronic exposure of mice to chemical carcinogens induces chromosome breakage measurable by micronuclei. However, the breakage and reunion necessary to see chromosome exchanges such as translocations were not observed in this study.

Administration, Oral↗
PubMed1997

The human XRCC9 gene corrects chromosomal instability and mutagen sensitivities in CHO UV40 cells.

The Chinese hamster ovary (CHO) mutant UV40 cell line is hypersensitive to UV and ionizing radiation, simple alkylating agents, and DNA cross-linking agents. The mutant cells also have a high level of spontaneous chromosomal aberrations and 3-fold elevated sister chromatid exchange. We cloned and sequenced a human cDNA, designated XRCC9, that partially corrected the hypersensitivity of UV40 to mitomycin C, cisplatin, ethyl methanesulfonate, UV, and gamma-radiation. The spontaneous chromosomal aberrations in XRCC9 cDNA transformants were almost fully corrected whereas sister chromatid exchanges were unchanged. The XRCC9 genomic sequence was cloned and mapped to chromosome 9p13. The translated XRCC9 sequence of 622 amino acids has no similarity with known proteins. The 2.5-kb XRCC9 mRNA seen in the parental cells was undetectable in UV40 cells. The mRNA levels in testis were up to 10-fold higher compared with other human tissues and up to 100-fold higher compared with other baboon tissues. XRCC9 is a candidate tumor suppressor gene that might operate in a postreplication repair or a cell cycle checkpoint function.

Amino Acid Sequence↗
PubMed1997

'Spontaneous' genetic damage in man: evaluation of interindividual variability, relationship among markers of damage, and influence of nutritional status.

The 'spontaneous' frequency of genetic damage (normal background) and the possible relationship of this damage to nutritional variables in humans were investigated in 22 subjects using several indices of genetic damage. The subjects were chosen, out of 122 initially analyzed, for being at the extremes of the highest and lowest values of one index of genetic damage, the frequency of micronucleated erythrocytes in peripheral blood. This index reflects chromosomal damage and loss in bone marrow erythropoietic cells. The assay for micronuclei is convenient but is restricted to splenectomized individuals because the human spleen removes micronucleated cells. The initial 122 subjects were splenectomized, but all were normal and healthy at the time of this study and none had a previous history of neoplastic disease. Factors investigated were stability of micronucleus frequency as a function of time, correlations among multiple markers of genetic damage, and influence on damage indices of nutritional variables, including blood levels of folate, B12 and antioxidant vitamins. Among different individuals, the range of values was 10-fold or more in the erythrocyte micronucleus, glycophorin A, plasma ascorbate and urinary 8-hydroxydeoxyguanosine (oxo8dG) assays, was approximately 6-fold in the lymphocyte micronucleus assay, and was 2-fold in the lymphocyte sister chromatid exchange (SCE) assay. Red blood cell folate and plasma folate, B12 and alpha-tocopherol values varied by up to 10-fold among individuals. Micronucleus frequencies in erythrocytes and peripheral blood lymphocytes ranged from < 0.3 to 16.9/1000 in mature red blood cells, < 1 to 33/1000 in reticulocytes, and 2.5 to 15/1000 in binucleate lymphocytes. Frequencies of glycophorin A variant erythrocytes ranged from 5.6 to 77.3 x 10(6) N/0 cells and 3.2 to 16.2 x 10(6) N/N cells, and oxo8dG excretion varied from 32 to 397 pmol/kg/day. Although a wide range of values was observed in each genetic endpoint, the extreme values for various endpoints of genetic damage were not observed in the same individuals. The frequency of micronucleated erythrocytes varied over time within individuals and indicated that individuals with the highest levels of damage exhibit greater variability than those with lower levels. In some subjects, frequencies of micronucleated erythrocytes changed dramatically over an interval of 2-3 years: four subjects with initial micronucleated reticulocyte frequencies of 20.4, 5.9, 6.4 and 33/1000 changed to 2.5, 20.5, 18.5 and 12/1000, respectively. Among more than 150 individuals we have studied, including the 64 individuals studied by Everson et al. [(1988) J. Natl. Cancer Inst., 80, 525-529] and Smith et al. [(1990) Cancer Res., 50, 5049-5054], the seven individuals with the highest observed frequencies of micronucleated erythrocytes all had exceptionally low values of plasma folate, red cell folate, or plasma B12, suggesting that folate and B12 status are the major determinants of the types of damage that lead to spontaneous micronucleus formation in erythrocytic cells.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1997

Differential effect of acetyltransferase expression on the genotoxicity of heterocyclic amines in CHO cells.

We earlier developed the Chinese hamster ovary UV5P3 cell line that expresses cytochrome P4501A2 and lacks nucleotide excision repair for studying metabolism and mutagenicity of heterocyclic amines. The Chinese hamster ovary UV5P3 cells are approximately 50-fold more sensitive to the cooked food mutagen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) than 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), another genotoxic compound found in cooked food, with respect to cytotoxicity and mutation induction at the adenine phosphoribosyltransferase (aprt) locus. To test the hypothesis that the important missing activity in our CHO system for IQ genotoxicity was acetyltransferase, we transfected the UV5P3 cells with cDNA plasmids of either the human NAT2 N-acetyltransferase gene or a bacterial O-acetyltransferase gene. Functionally transformed clones were determined by the differential cytotoxicity assay using IQ, and confirmed by measuring the enzyme activity with isoniazid as substrate. Two clones designated 5P3NAT2 and 5P3YG (expressing human and bacterial transferases, respectively) were characterized. Both cell lines were sensitive to killing by IQ at concentrations as low as 4 ng/ml. Based on the D37 value, the dose that reduced the survival to 37% relative to untreated controls, the acetyltransferase expressing lines showed approximately 1000-fold increase in sensitivity to the killing effect of IQ over the parental UV5P3 cell line. The same dramatic change in sensitivity was also seen in mutation response at the aprt locus and with chromosomal aberrations and sister chromatid exchanges. In contrast, these cell lines showed cytotoxicity to PhIP similar to that of the parental line UV5P3. These results suggest that PhIP does not require acetyltransferase for metabolic activation leading to genotoxicity in these cells. These new cell lines constitute a sensitive cell system for assessing genotoxicity of compounds requiring metabolic activation by both P450IA2 and acetyltransferase, as well as for studying the molecular processes by which DNA damage can lead to mutation and cancer.

Acetyltransferases↗
PubMed1997

Persistence of radiation-induced translocations in rat peripheral blood determined by chromosome painting.

In this article, we address the issue of persistence of chromosome exchanges following acute in vitro exposure of rat peripheral blood to 137Cs. Irradiation occurred 24 hr after culture initiation, and metaphase chromosomes were prepared 2, 3, 4, and 5 days later. Chromosomes 1, 2, and 4 were painted in unique colors and scored for structural aberrations. Dicentric chromosomes and acentric fragments diminished rapidly with time, as expected. Translocations exhibited greater persistence, but still showed a reduction in frequency, reaching a plateau of approximately 65 and 55% of their initial values, 4 days after exposure to 1 and 2 Gy, respectively. An exponentially declining model was fit to the combined dicentric, acentric fragment, and translocation frequencies, which showed that all three aberration types declined at equivalent rates. The frequencies of dicentrics and fragments declined to a plateau of zero, while translocations reached a plateau at frequencies significantly greater than zero. The decline in translocations with time is inconsistent with prevailing theoretical expectations, but is consistent with a model where some translocations are fully stable (persistent) and some are unstable (not persistent) through cell division. These results may have implications for radiation biodosimetry in humans.

Animals↗
PubMed1997

X chromosome inactivation and micronuclei in normal and Turner individuals.

Studies on aneuploidy have shown that the X is the most frequently lost chromosome in females, and that the number of X chromosome-positive micronuclei increases with age in women. Recently, we showed that the inactive X chromosome is incorporated preferentially in micronuclei. The objectives of the current study were, firstly, to determine the incidence of X chromosome incorporation into micronuclei in males and, secondly, to determine the incidence of X chromosome incorporation into micronuclei of females with Turner syndrome. Blood samples were obtained from 18 male newborns and 35 normal adult males ranging in age from 22 to 79 years and from seven women with non-mosaic Turner syndrome aged 11-39 years. Isolated lymphocytes were cultured in the presence of cytochalasin B and 2000 binucleated cells per subject were scored for micronuclei. Cells were then hybridized with the biotinylated X centromere-specific probe, pBamX7, and visualized with fluorescein-conjugated avidin. All micronucleated cells were relocated and evaluated for the presence or absence of the X chromosome. Of the 335 micronuclei observed, 6.6% (22/335) contained an X chromosome. Analysis of variance shows a statistically significant increase, for both males and Turner females, in the number of X chromosome-positive micronuclei with age (P < 0.001). These data also show that the X chromosome is included in micronuclei from males more often than would be expected by chance (P < 0.005; chi 2 analysis, 15 df). Here we show that there is a tenfold difference in the frequency of X chromosome-positive micronuclei in 46,XX females compared to 46,XY males and 45,X females, providing further support to our previous finding that the X chromosome in micronuclei is the inactive chromosome.

Adolescent↗