Chromosome constitution and its bearing on the chromosomal radiosensitivity in man

Radiotherapy and genetic predisposition to breast cancer

Cancer genetics is becoming increasingly integrated into oncological care; particularly in breast cancer management. The recognition of monogenic breast cancer predisposition syndromes, such as BRCA, is critical as there is also a risk of other cancers in addition to a markedly elevated risk of contralateral breast cancer. In individuals with breast cancer due to some predisposition genes, radiation should be avoided, e.g. the ATM and TP53 genes, but there is still controversy as to whether radiation should be used in BRCA1/2 mutation carriers and more follow-up is needed. There are some radiation-sensitive genetic conditions where, if breast cancer occurs (as it may occur in any individual, not necessarily due to an increased risk associated with the condition), radiation should be avoided. These conditions are often associated with dysmorphic features. If such features are noticed then the advice of a geneticist should be sought urgently prior to giving radiation.

Cytogenetic radiosensitivity of g0-lymphocytes of breast and esophageal cancer patients as determined by micronucleus assay

Enhanced chromosomal radiosensitivity is a feature of many cancer predisposition conditions, indicative of the important role of chromosomal alterations in carcinogenesis. In this study the cytokinesis-blocked micronucleous assay was used to compare the radiosensitivity of blood lymphocytes obtained from Iranian breast or esophageal cancer patients (n = 50, n = 16; respectively) with that of control individuals (n = 40). For each sample, one thousand binucleate lymphocytes were analyzed before and after in vitro exposure to 3 Gy of gamma rays. The radiation-induced frequency of micronucleus was significantly higher in the breast cancer group (261/1,000 binucleated cells) than in esophageal cancer group (241/1,000 binucleated cells, P < 0.01) or in the control group (240/1,000 binucleated cells, P < 0.01). The results indicate that breast cancer patients are more radiosensitive compared to normal healthy individuals or esophageal cancer patients. Increased radiosensitivity could be due to defects in DNA repair genes involved in breast cancer formation. Since patients with esophageal cancer did not show elevated radiosensitivity, it is assumed that the contribution of radiosensitivity-related genes to the development of esophageal cancer may be smaller than the contribution of those genes to breast cancer.

Effect of estradiol on radiation-induced chromosome aberrations in human lymphocytes

As a part of studies on physiological factors that affect radiosensitivity, we examined the in vitro effect of estradiol (E2) on the yield of radiation-induced chromosome aberrations in human peripheral lymphocytes. Lymphocytes were cultured for 3 days in the medium containing E2 at 0-100,000 ng/ml. On the second day, they were irradiated by X-rays at 3 Gy, and then 2% phytohemagglutinin and 0.05 microgram/ml colcemid were added to the medium. After further 48 h, mitotic indices and the yields of chromosome aberrations were examined at various E2 concentrations. E2 treatment at concentrations above 1000 ng/ml resulted in dose-related inhibition of mitosis. Repeated experiments showed that the yield of dicentrics plus centric rings in the culture containing E2 at 100 ng/ml was significantly higher than the yields at 0 ng/ml. Similarly, the yield of total chromosome breaks in the culture containing E2 at 100 ng/ml was significantly higher than that at 1 ng/ml. This study provides the direct evidence in human that radiosensitivity may vary in relation to hormonal conditions.

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.

Atypical background somatic mutant frequencies at the HPRT locus in children and adults with Down syndrome

People with Down syndrome are 10-30 fold more likely to develop leukemia than the normal population. To date, little is known regarding the molecular mechanisms underlying this phenomenon. We have previously demonstrated that the spontaneous somatic mutant frequency (Mf) at a reporter gene, hypoxanthine-guanine phosphoribosyl transferase (HPRT), from a normal population showed a strict age dependency with an exponential increase in Mf from birth to late adolescents with a subsequent linear 2-5% increase per year in adults. In this study, we compared HPRT Mf in children and adults with Down syndrome using the HPRT T-cell cloning assay. We determined the Mf at the HPRT locus in 27 subjects with Down syndrome from ages 6 months to 53.4 years. Results demonstrated that background somatic Mf at the HPRT locus in children and adults with Down syndrome are not dependent on age as seen in a normal control population. Results also show that adults with Down syndrome have a significantly lower Mf than normal adults, and that children with Down syndrome have a significantly higher Mf than normal children, although the latter appears to be due to a decreased cloning efficiency (CE). These observations demonstrate that the frequency of spontaneous somatic mutations in children and adults with Down syndrome are atypical compared to normal controls, and suggest that the genetic mechanisms associated with background somatic mutational events in children and adults with Down syndrome may be different.

Radiation-induced micronucleus induction in lymphocytes identifies a high frequency of radiosensitive cases among breast cancer patients: a test for predisposition?

Enhanced sensitivity to the chromosome-damaging effects of ionizing radiation is a feature of many cancer-predisposing conditions. We previously showed that 42% of an unselected series of breast cancer patients and 9% of healthy control subjects showed elevated chromosomal radiosensitivity of lymphocytes irradiated in the G2 phase of the cell cycle. We suggested that, in addition to the highly penetrant genes BRCA1 and BRCA2, which confer a very high risk of breast cancer and are carried by about 5% of all breast cancer patients, there are also low-penetrance predisposing genes carried by a much higher proportion of breast cancer patients, a view supported by recent epidemiological studies. Ideally, testing for the presence of these putative genes should involve the use of simpler methods than the G2 assay, which requires metaphase analysis of chromosome damage. Here we report on the use of a simple, rapid micronucleus assay in G0 lymphocytes exposed to high dose rate (HDR) or low dose rate gamma-irradiation, with delayed mitogenic stimulation. Good assay reproducibility was obtained, particularly with the HDR protocol, which identified 31% (12 out of 39) of breast cancer patients compared with 5% (2 out of 42) of healthy controls as having elevated radiation sensitivity. In the long term, such cytogenetic assays may have the potential for selecting women for intensive screening for breast cancer.

Elevated chromosome aberration frequency after X-ray exposure of cultured fibroblasts derived from patients with porokeratosis

Porokeratosis (PK) is a rare genetic skin disorder inherited as an autosomal dominant trait and regarded as a disease predisposing to cancer. To evaluate chromosomal radiosensitivity of PK cells, we examined chromosome aberration frequency after X-irradiation of cultured skin fibroblasts derived from PK patients and controls. Without X-ray exposure, frequencies of chromosome-type aberrations (exchanges or deletions) were not different between the patients and controls. Following X-ray irradiation, frequencies of deletions in the patient group were significantly increased, whereas those of exchanges were not elevated. No differences in chromatid-type aberration frequency were found between the patients and controls with or without exposure to X-ray. The observed radiosensitivity, though not as high as in ataxia telangiectasia (AT) cells, agrees well with the previously reported higher radiosensitivity of PK fibroblasts in survival analysis.

Defective G2 repair in Down syndrome: effect of caffeine, adenosine and niacinamide in control and X-ray irradiated lymphocytes

Lymphocytes from both Down syndrome (DS) patients and age-matched control donors have been investigated to identify a possible disturbance in chromosomal G2 repair. Analyses of caffeine treatments during G2 have shown that the frequency of chromosomal aberrations is higher in DS lymphocytes than in normal lymphocytes. Likewise, G2 duration is longer in DS cells than in normal cells. In both control and DS lymphocytes, caffeine treatments increase the frequencies of chromatid breakages and decrease the average of G2 duration. The reversal of the caffeine potentiation effect by adenosine and niacinamide is higher in DS cells than in normal cells. Furthermore, ATP content per cell in DS lymphocytes is one third of that estimated in normal lymphocytes. The increase of ATP level produced by adenosine or niacinamide generally correlates with the reversal of the caffeine effect on chromosome aberrations. Under the experimental conditions tested, a good negative exponential correlation between ATP level and chromosome aberrations has been detected in both normal and DS lymphocytes which were or were not X-irradiated. Finally, we postulate a decrease in G2 repair capability of DS lymphocytes caused by a low availability of ATP and/or some other factor correlating with it.

Evidence of different complementation groups amongst human genetic disorders characterized by radiosensitivity

The genetic diversity of a clinically heterogeneous group of ionizing radiation-sensitive human mutants has been examined. In this group, the relationship between ataxia telangiectasia (A-T), Alzheimer's disease (AD) and Down's syndrome (DS) was studied, on the basis of their cellular radiosensitivity. Cell-fusion analysis was used to determine the presence of different complementation groups. In a series of 4A-T, 5AD and 4DS cell lines, 8 complementation groups were documented. These findings suggest that this group of primary neuronal degenerative disorders might have some overlap in their genetic defects.

Increased radiosensitivity of cell lines derived from a Down's syndrome patient with ocular telangiectasia

Studies on radiosensitivity of cells from Down's syndrome (DS) patients were stimulated by the observation of their increased susceptibility to leukemia. While lymphocytes from DS patients were found to consistently show increased chromosomal aberrations after exposure to ionizing radiation, conflicting reports have been published on the radiosensitivity of fibroblasts and lymphoblastoid cell lines (LCL) derived from these patients. In the present study, cultured skin fibroblast lines developed from a DS patient with ocular telangiectasia and five normal subjects were compared for both cell killing and chromosomal aberrations (breaks, translocations, inversions, dicentrics, and rings) after low dose-rate gamma-irradiation. The LCLs developed from the patient and two normal persons were also compared for chromosomal radiosensitivity using the same irradiation protocol. A comparison of the D10 (radiation dose resulting in 10% survival) values estimated from the survival curves and the frequencies of induced chromosome aberrations in different cell lines showed that the DS cells were more radiosensitive than the respective controls. The increased cellular radiosensitivity of the DS patient reported here could be due to a combination of genetic factors (DS plus a gene for hypersensitivity to radiation) and, thus, may not be representative of all DS patients. Alternatively, the use of low dose-rate irradiation could be a factor in revealing the radiosensitivity of DS fibroblasts in general.

Gender differences and the interpretation of genetic instability in Alzheimer's disease

The neuronal degeneration and death which characterize Alzheimer's disease (AD) may stem from a constitutive genetic instability related to DNA repair deficits. To test this hypothesis, we treated peripheral blood lymphocytes from persons with AD, age-matched controls, and young controls with two drugs that induce chromosome breakage. Bleomycin, a radiomimetic antineoplastic drug, causes single- and double-stranded DNA breaks through the generation of activated oxygen radicals. Methyl methane-sulfonate (MMS) is a monofunctional alkylating agent that binds covalently to DNA. Cells were grown in culture for 72 h, with drug treatments for 4 h (bleomycin) or 24 h (MMS) prior to harvest. Fifty cells per subject per drug were scored for chromosome breakage. Breakage rates for both drugs in AD women were significantly higher than those in age-matched control women. This was not the case in men, due to the very high induced breakage rates seen in the age-matched normal control men. Because the induced breakage rates in AD women and AD men are equivalent, it seems likely that an independent factor may be contributing to genetic instability in the normal control men. Our findings indicate that the interpretation of the response of AD lymphocyte chromosomes to DNA-damaging chemicals can be strongly confounded by the effects of gender ratio in the control population sampled. These findings have important implications for the design of future studies of Alzheimer's disease, as well as for the assessment of health risks in unaffected elderly populations.

The effect of aging on cell-cycle kinetics and X-ray-induced chromosome aberrations in cultured lymphocytes from patients with Down syndrome

To evaluate the effects of aging on cytogenetic characteristics of lymphocytes from Down syndrome (DS), cell-cycle kinetics after PHA stimulation and chromosome-type aberration frequencies after X-ray exposure were investigated in vitro in the lymphocytes derived from 4 (or 3 for X-ray treatment) age groups of DS patients and age-matched controls. The results clearly showed higher mitotic and proliferation index levels in younger groups compared to older groups at the various culture intervals, whether the lymphocytes were from the DS patients or controls. The age-related changes of the proliferation index were mainly attributed to a delayed response to PHA as age increased. The changes of PHA responses seemed to be particularly marked during adolescence. Nonetheless, no significant differences were observed between the DS patients and age-matched controls for each age group. In all age groups, frequencies of both chromosome-type exchanges and deletions were elevated in the DS patients by about 1.3 times in comparison with the controls. The magnitude of radiosensitivity, however, seemed to decrease slightly in the 40-49-year group. To our knowledge, the present study is the first report in the literature to deal with the effect of aging on the greater radiosensitivity of DS lymphocytes.

Heterogeneity in Alzheimer's disease: evidence from cellular radiosensitivity and complementation of this phenotype

Radiosensitivity was studied in a series of Alzheimer's disease (AD) patients and normal controls by examining clonogenic survival and radiation-induced chromosome aberrations in lymphoblastoid cell lines. D0 values based on colony survival for AD and normals following exposure to gamma-rays were 0.86 +/- 0.04 and 1.14 +/- 0.03 Gy respectively. However, 2 of the AD cell lines had D0 values in the normal range. This increased radiosensitivity in AD cells was confirmed by an increased number of gamma-ray-induced chromosome aberrations in these cells. Cell fusion was employed to investigate the presence of different complementation groups for the radiosensitive phenotype in AD using frequency of radiation-induced chromosome aberrations as a means of distinguishing different groups. Four complementation groups were found among 5 AD cell lines. These findings provide additional experimental evidence in support of heterogeneity in AD.

Chromosomal radiosensitivity of lymphocytes from Alzheimer's disease patients

We have examined chromosome aberrations in gamma irradiated (3 Gy) lymphocytes from five patients with Alzheimer's disease (AD). In each case, the number of dicentrics was significantly higher than the number in irradiated lymphocytes from five age matched normal subjects, the mean value for AD cells being about 25% higher. There was no significant difference in number of acentrics between AD and normal cells. Examination of the number of first, second, and third division metaphases, using fluorescence plus Giemsa staining, indicated that there was no difference in cycling time between AD and normal cells, and that after irradiation both groups showed the same mitotic delay. The similarity of our findings to those of others with irradiated Down's syndrome cells (from adult patients) is discussed.

Normal inhibition of DNA synthesis following gamma-irradiation of radiosensitive cell lines from patients with Down's syndrome and Alzheimer's disease

Inhibition of DNA synthesis was studied in gamma-irradiated lymphoblastoid cells from patients with Alzheimer's disease and Down's syndrome. A normal biphasic pattern of inhibition was observed over a dose range of 0-4 krad of gamma-rays in all of the cell lines. 3 out of 4 Down's and all the Alzheimer's cell lines were shown to be hypersensitive to ionizing radiation based on induced chromosomal aberrations. Increased G2 phase delay, comparable to that occurring in ataxia-telangiectasia cells, was observed for some of the cell lines, after exposure to gamma-rays. Contrary to other data in the literature these results demonstrate that radioresistant DNA synthesis is not an intrinsic feature of all disorders characterized by radiosensitivity.

Mitomycin-C-induced sister-chromatid exchanges and cell-cycle kinetics in lymphocytes from patients with Klinefelter syndrome

The chromosomal sensitivity to mitomycin-C (MMC) and cell-cycle kinetics in cells from patients with Klinefelter syndrome, a sex chromosomal disorder giving a high risk of malignant tumor, were studied by techniques of sister-chromatid exchanges (SCEs). The frequencies of MMC-induced SCEs increased in proportion to the increase in MMC concentration in both patient and normal control cells. At low levels of MMC there were no significant differences in SCE frequencies between the patient and normal control cells, but at MMC concentrations of 3 X 10(-8) M (p less than 0.05) and 1 X 10(-7) M (p less than 0.01), significant increases in the frequency of MMC-induced SCEs were observed in cells from patients compared to cells from normal controls. Although the analysis of cell-cycle kinetics both after various culture times and after treatment with MMC revealed that there were no significant differences between the patient and normal control cells, patients with Klinefelter syndrome showed a tendency to cell-cycle delays after treatment with MMC in comparison with normal controls.

Increased frequency of spontaneous and X-ray-induced chromosomal aberrations in lymphocytes from neonates and the influence of caffeine--an in vitro study

We have examined lymphocytes from human preterm (PT) and fullterm (FT) babies for an effect of gestational age (GA) on chromosomal aberrations either occurring spontaneously or induced by treatment with X-rays (1 Gy) alone; or with caffeine (1,3,7-trimethylxanthine) supplementation (5 X 10(-4) M), in comparison to the lymphocytes of healthy adults (AD). Percent of abnormal cells (%Abn) was used as an indicator of chromosome sensitivity to the different treatments. PT babies had significantly higher spontaneous and X-ray-induced %Abn values than AD, but were comparable to FT. After X-irradiation + caffeine the yield of aberrations in any 2 groups was not significantly different. Chromosomal sensitivity may result from factors other than GA. This in vitro model may permit study of the mechanisms of chromosomal damage repair and prevention of free radical damage of DNA during the perinatal period.

The effect of 3-aminobenzamide on X-ray induction of chromosome aberrations in Down syndrome lymphocytes

Human lymphocytes from normal and Down syndrome (DS) subjects were examined to determine the effect of 3-aminobenzamide (3AB) on X-ray-induced chromosome aberrations. Lymphocytes were treated with 150 or 300 rad of X-rays in the presence of 3 mM 3AB for various times after irradiation, and then the cells were analyzed for the presence of chromosome aberrations in mitotic cells. 3-Aminobenzamide had no effect on the frequency of chromosome aberrations produced by X-rays in G0 lymphocytes from normal subjects. In contrast, lymphocytes from DS patients displayed an increase in the frequency of chromosome aberrations as a result of treatment with X-rays in the presence of 3AB. These observations indicate that DS lymphocytes are more sensitive to the inhibition of poly(ADP)ribose synthetase than normal lymphocytes.

Increased methotrexate-induced chromosome breakage in patients with free trisomy 21 and their parents

Increased susceptibility of chromosomes from peripheral blood lymphocytes to the antimetabolite methotrexate (2 X 10(-6) M) has been found in patients with free trisomy 21 and their parents (N = 14). The level of induced chromatid and chromosome breaks is lowest in normal controls intermediate in patients' mothers and fathers, and highest in trisomy 21 patients. The findings are viewed as a special type of cytogenetic polymorphism or as a defective chromosomal infrastructure, also in the parents of trisomic children.

Radiosensitive Down syndrome lymphoblastoid lines have normal ionizing-radiation-induced inhibition of DNA synthesis

The extent of X-ray-induced inhibition of DNA synthesis was determined in radiosensitive lymphoblastoid lines from 3 patients with Down syndrome and 3 patients with ataxia telangiectasia (AT). Compared to 6 normal control lines, the 3 AT lines were abnormally resistant to X-ray-induced inhibition of DNA synthesis, while the 3 Down syndrome lines had normal inhibition. These results demonstrate that radiosensitive human cells can have normal X-ray-induced inhibition of DNA synthesis and provide new evidence for the dissociation of radiosensitivity from radioresistant DNA synthesis.

Effects of bleomycin on Down's syndrome lymphocytes in culture

Blood lymphocytes from 3 Down's syndrome (DS) and 3 age- and sex-matched normal probands were studied for the induction of chromosomal aberrations and sister-chromatid exchange (SCEs). Treatment with bleomycin (30 and 60 ng) at the initiation of culture showed a dose-dependent increase in the incidence of dicentric and ring chromosome aberrations. In contrast, the cells which were treated for the last 24 h in culture with bleomycin did not show an increase in chromosome-type aberrations. The proportion of metaphases in M1, M2, and M3 in cultures was not different between DS and normal cells. Sister-chromatid exchange frequency did not show significant changes between DS and normal individuals.

Chromosomal radiosensitivity of Down syndrome lymphocytes at different stages of the cell cycle

Down syndrome (DS) is one of the most common types of congenital anomalies. In addition to a wide spectrum of developmental abnormalities, DS patients are also highly sensitive to the induction of chromosomal aberrations when their GO lymphocytes are exposed to ionizing radiation. We conducted the present study to evaluate the effect of X-rays on proliferating lymphocytes from DS and normal individuals. We found that DS lymphocytes were significantly more sensitive to X-ray induction of chromosome aberrations than normal cells, when they were irradiated at the G0, G1 and S phases of the cell cycle. The S phase was the most radiosensitive phase and would lead to extensive cell killing, whereas the G1 phase seemed to be more prone to the induction of chromosome rearrangements that would potentially lead to serious long-term consequences.

Sister chromatid exchange induction by benzo(a)pryene in cultured peripheral blood lymphocytes of lung cancer patients and healthy individuals with or without familial history of neoplasms

The inducibility of sister chromatid exchanges (SCEs) by benzo(a)pyrene (BP) was studied in cultured peripheral blood lymphocytes of 15 untreated lung cancer patients and 25 healthy persons including 11 high- and 14 low-cancer-risk individuals tentatively classified by the familial history of lung cancer and other neoplasms. The baseline SCE frequency in cultured lymphocytes was significantly high in lung cancer patients, as compared with all healthy persons or low-cancer-risk individuals. Following exposure to BP, the lymphocytes of lung-cancer patients and high-cancer-risk individuals exhibited significantly greater SCE yields than those of persons at low risk, although no significant difference was observed in the lymphocyte SCE yields when the levels of lung cancer patients were compared with those of all healthy persons. A comparison of the net SCE increase (delta SCE) in BP-exposed lymphocytes among the study groups, however, revealed a significant difference in delta SCE values only between high- and low-cancer-risk individuals. The present findings on both the observed SCE yields and delta SCE values suggest that lymphocytes of high-risk individuals may be more susceptible to BP-induced DNA damage than those of persons at low risk, and that such a chromosomal hypersensitivity to genotoxins may be associated with a high risk of neoplasms.

An early, nonrandom karyotypic change in immortal Syrian hamster cell lines transformed by asbestos: trisomy of chromosome 11

Cytogenetic studies were performed on eight early passage Syrian hamster embryo cell lines independently derived following asbestos exposure. The modal chromosome number of all the immortal cell lines was near-diploid. At the earliest passage examined, six of eight cell lines had only numerical chromosome changes. Cells in each of these six cell lines had an extra chromosome #11, either as a sole karyotypic change or with other numerical changes. The remaining two cell lines displayed both numerical and structural chromosome changes, but without involvement of chromosome #11. Common abnormalities were -X or -Y, +3, and 8p- in one cell line, and -13 and t(13;21) in the other cell line. A nonrandom gain of chromosome #8 was also found in four cell lines. In three of the four cell lines, trisomy of chromosome #8 seems to have occurred during karyotypic progression. The observation that nonrandom changes in chromosome number are an early karyotypic change after carcinogen treatment supports our hypothesis that induction of aneuploidy by asbestos is mechanistically important in the transformation of Syrian hamster embryo cells in culture and, further, suggests that trisomy 11 plays a major role in the early steps of immortalization and neoplastic progression.

Higher incidence of spontaneous sister-chromatid exchanges (SCEs) and X-ray-induced chromosome aberrations in peripheral blood lymphocytes during pregnancy

In vitro cultures of peripheral blood lymphocytes from human and muntjac (barking deer) females who were at an advanced stage of pregnancy (32-37 weeks pregnant women and 20-24 weeks pregnant muntjacs) showed an enhanced frequency of SCEs and X-ray-induced chromosome aberrations when compared with those of nonpregnant females. Lymphocyte cultures of nonpregnant females to which sex hormones progesterone, oestrogen and human chorionic gonadotropin (HCG) were added together exogenously also showed higher frequency of SCEs. The plausible reason(s) for such high incidence of SCEs during pregnancy is discussed.

Cellular studies on retinoblastoma

Retinoblastoma may be hereditary or non-hereditary. The hereditary form involves either a predisposing gene transmissible as an autosomal dominant or a deletion at chromosome 13q14. An abnormal cellular response to ionizing radiation was suggested by the occurrence of secondary neoplasms within the field of therapeutic radiation in hereditary retinoblastoma patients. Hereditary retinoblastoma patients also show a predisposition to second neoplasms not related to therapy. In vitro studies on the radiation response of cells from retinoblastoma patients have generated conflicting results. Some laboratories, including our own, find that survival following ionizing irradiation of fibroblasts is within the normal range, other laboratories find an abnormal decrease in cell survival. X-ray-induced chromosome damage in G0-irradiated lymphocytes was slightly elevated compared to control subjects. Recent studies using chromosome 13 genetic markers suggest that retinoblastoma tumour cells are homo- or hemi-zygous for the mutant retinoblastoma gene. It seems unlikely that the mutant gene causes sensitivity to ionizing radiation but any tendency to chromosomal rearrangement in a gene carrier would increase the probability of tumour development.

An imbalance in sex chromosomes alters cell survival of human skin fibroblasts exposed to ionizing radiation in vitro

We report that a cell clone with a 45,X(-Y) karyotype, obtained from a male individual who is a gene carrier for hereditary adenomatosis of the colon and rectum (ACR), was considerably more resistent to gamma-ray irradiation (D0 range 184-240) than were diploid ACR skin fibroblasts (SF) obtained from sister biopsies of the same individual or of normal controls (D0 range 90-125). In addition, cell clones obtained from a Turner's syndrome female patient and from a female patient with focal endometrial hyperplasia, each with a 45,X(-X) karyotype, were abnormally resistent to gamma-ray irradiation (D0 range 168-195). These results may suggest an association between the status of the sex chromosomes and an altered response to gamma-ray-induced cytotoxicity of human SF.

Mitomycin C and ethyl methanesulphonate-induced sister-chromatid exchanges in lymphocytes from individuals with Alzheimer's pre-senile dementia

Baseline and mutagen-induced sister-chromatid exchange frequencies were compared in peripheral blood lymphocytes from patients with Alzheimer's pre-senile dementia and control individuals. No significant differences were found between the two groups.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

High radiosensitivity of the MOLT-4 leukaemic cell line

Radiation survival of MOLT-4, a leukaemic T-lymphocyte cell line, was measured by counting colonies formed in 0.8 per cent methyl cellulose. The survival curve was a simple exponential and showed the cells to be radiation sensitive, with D0 = 0.49 +/- 0.02 Gy and extrapolation number n = 0.92 +/- 0.09. No increase in survival as measured by colony-forming ability or trypan blue dye exclusion was seen when the dose was split into two fractions, separated by a 5 h incubation period. Electron microscopy and trypan blue dye exclusion showed that 5 h after exposure to high doses, MOLT-4 cells began to die and displayed condensed, marginated chromatin and cellular vesiculation.

Hypersensitivity to ionizing radiation in cultured cells from Down syndrome patients

Down syndrome is caused by trisomy of chromosome 21 and is comprised of a constellation of abnormalities including neuropathological features that closely resemble those characterizing the neurodegeneration of Alzheimer disease. Because cultured cell lines from patients with Alzheimer disease and other neurodegenerations have a hypersensitivity to the lethal effects of DNA-damaging agents, we studied the response of Down syndrome lymphoblastoid lines to the lethal effects of ionizing and ultraviolet radiation. Lines from the four Down syndrome patients were more sensitive to X-rays than lines from 28 normal donors (P = 10(-4)), while survival of the Down syndrome lines after ultraviolet irradiation was not significantly different from normal. This hypersensitivity to X-rays, which may reflect defective repair of X-ray-induced DNA damage, represents the first abnormality common to cultured cells from both Down syndrome and Alzheimer disease patients.

Bleomycin-induced chromosomal aberrations and sister chromatid exchanges in Down lymphocyte cultures

Peripheral blood lymphocytes from three patients with Down syndrome (DS; trisomy 21; aged 5-6 years) and three age-matched control children were studied for the induction of chromosomal aberrations and sister chromatid exchanges (SCEs). Cells in G0 were exposed to bleomycin (20-100 micrograms/ml) for 3 h, and then cultured in medium containing 5-bromodeoxyuridine and phytohemagglutinin for 66 h. By the sister chromatid differential staining method, chromosome analyses were performed on metaphase cells that had divided one, two, or three or more times after treatment. The results indicate that DS cells exposed to bleomycin are hypersensitive to the production of dicentric and ring chromosomes compared to normal cells. Bleomycin also led to a dose-related increase in the frequency of SCEs, but no difference was found between the SCE frequencies in DS or normal lymphocytes exposed to bleomycin.

Influence of age on the frequency of sister-chromatid exchanges and X-ray-induced chromosome aberrations in muntjac

The BrdU-differential staining technique was used in a study of the frequency of sister-chromatid exchanges (SCEs) and X-ray-induced chromosome aberrations in peripheral blood lymphocytes of the same individual muntjacs. Blood was collected periodically from immediately after birth (1 day old) to the adult stage (1 year). The results showed that both the frequency of base-line SCEs and induced chromosome aberrations changed as a function of age. At a young age, the frequency of SCEs was significantly low, whereas a high frequency of chromosome aberrations was observed. But with increase in age of the individuals, an enhanced frequency of SCEs and a decreased frequency of induced chromosome aberrations were observed; and as the age advanced further, the frequencies of both SCEs and chromosomal aberrations came to a steady level.

The influence of cell cycle kinetics on the radiosensitivity of Down's syndrome lymphocytes

In agreement with previous work, [60Co]gamma-irradiation shortly after phytohemagglutinin (PHA) stimulation, induces higher frequencies of chromosomal aberrations in trisomy 21 lymphocytes compared to normal controls. However, equal frequencies of chromatid aberrations are induced in fully-stimulated trisomy 21 and normal lymphocytes by irradiation during G2. We have observed that trisomic lymphocytes respond more rapidly to PHA stimulation than normal lymphocytes. Furthermore, we have observed that chromosomal radiosensitivity increases as a function of time after PHA stimulation in normal lymphocytes. When normal lymphocytes are irradiated 8 h after PHA stimulation, the frequencies of chromosomal aberrations induced are comparable to those induced in trisomy 21 lymphocytes irradiated 30 min after PHA stimulation.

Down syndrome--a disruption of homeostasis

A major question in human genetics concerns the relationship between the extra chromosome material in the Down syndrome (DS) and its effects. It is suggested here that a generalized disruption of evolved genetic balance in cells of affected individuals leads to decreased developmental and physiological buffering against genetic and environmental forces. Examples of consequences in DS of this model of disruption of homeostasis are presented: i) increased variance for metric traits, ii) amplified instability of developmental pathways, iii) reduced precision of physiological homeostatic controls, and iv) generalized increased morbidity. Evolution has selected for interacting systems. When this evolved balance is disrupted, as in autosomal aneuploidy, the organism is generally disrupted. The model emphasizes the role of environment in producing much of the DS phenotype. Traits less buffered than others in the general population are the ones most disturbed in DS and account for much of the DS phenotype.

Bleomycin-induced chromosomal aberrations in Down's syndrome lymphocytes

Blood samples from 4 Down's syndrome (DS) patients with a 47,XY,21 + karyotype and from 4 normal male probands were cultured for 72 h in the presence of BrdU and lymphocytes analysed at their first mitosis for chromosomal aberrations. The frequencies of spontaneous aberrations and the proportions of cells in the first or later mitoses in culture were not different between the groups. Treatment with various doses of bleomycin in vitro resulted in similar delays in cell development for both DS and normal lymphocytes and dose-dependent increases in the incidence of chromosome-type aberrations. However, the induction of both dicentric aberrations and acentric fragments was significantly enhanced in DS cells relative to cells of normal karyotype.

Absence of correlation between the chromosomal radiosensitivity of peripheral blood lymphocytes and stem-cell spermatogonia in mammals

To evaluate the reliability of quantitative extrapolation of radiation-induced chromosomal damage from somatic cells to germ cells, data on the effects of several biological and physical factors on the chromosomal radiosensitivity of blood lymphocytes and stem-cell spermatogonia have been collected from the literature. The results show that most of the factors considered, such as chromosomal constitution, age, genetic constitution, species, sampling time and dose fractionation, had differential effects on the induction of chromosomal aberrations in both systems. These differential effects can easily be explained in terms of the biological differences between in-vitro-stimulated peripheral blood lymphocytes and stem-cell spermatogonia. It is concluded that only direct experiments on germ cells of higher primates and man can be used for a quantitative estimation of human genetic radiation risks arising from structural chromosomal aberrations.

The implications of S-phase exchanges for the mechanisms of radiosensitivity in trisomy 21

Human lymphocytes obtained from four patients with Down syndrome and from two normal individuals were irradiated with X-rays during their S phase and examined for chromatid type aberrations. It is suggested that the significantly increased frequency of asymmetrical chromatid interchanges found in trisomic cells is related to an altered DNA repair system. This altered repair system is probably responsible for the increased frequency of chromosome aberrations that can be induced in these cells by x-rays and the increased tendency for leukemia observed in Down syndrome as well.

Radiosensitivity of fibroblasts from patients with retinoblastoma and chromosome-13 anomalies

Diploid fibroblast cell strains derived from 14 patients with various forms of retinoblastoma (RB) and 5 non-RB patients with constitutional chromosome anomalies involving chromosome 13 were assayed for their clonogenic survival after X-irradiation. Cells from a patient with ataxia telangiectasia (AT) was used as a radiosensitive reference strain. When compared with cell strains from 7 healthy persons as normal controls, a marked radiosensitivity was observed in strain from an AT patient. However, none of the cell strains derived from RB patients or patients with inborn anomalies in chromosome 13 showed pronounced deviation from the normal range of radiosensitivity. The findings thus did not warrant either the RB as radiosensitive genetic disease or the presence of repair locus on chromosome 13, deletion or triplication of which was previously suggested to link to radiosensitivity.

Sister chromatid exchanges and structural chromosome aberrations in relation to age and sex

Sister chromatid exchanges (SCE) and structural chromosome aberrations were analyzed in peripheral blood lymphocytes of 100 individuals, and correlated to age and sex. No correlation was found between the frequency of SCE and age, but older individuals had significantly more structural aberrations than younger. Females had significantly more SCE as well as structural chromosome aberrations than males. The positive correlations of SCE and structural aberrations to age and sex were also significant when these factors, as well as smoking habits, were taken into consideration in an analysis of covariance.

Abnormalities of human chromosome 13 and in vitro radiosensitivity; a study of 19 fibroblast strains

The in vitro X-ray sensitivity of 19 fibroblast strains derived from patients bearing a deletion, trisomy, inversion, or translocation of all or part of chromosome 13 were determined with a clonogenic survival assay. The results were compared with data from similar experiments involving strains from normal controls and from individuals trisomic for 3 other autosomes. The results suggest the involvement of this chromosome with hypersensitivity to the cytotoxic effects of X-irradiation. Experiments involving the partial monosomies and trisomies seem to implicate a locus on 13q14.