Frequency Of Chromatid Breaks Research Articles

Suppression of X-Ray-Induced Chromatid Breaks in Human Tumor Cells by Introduction of Normal Chromosome 4

Human tumor cells in culture frequently show an infinite lifespan and are characterized by an abnormally high frequency of chromatid breaks after x-irradiation during G 2 phase of the cell cycle. This abnormally high frequency of persistent chromatid breaks results from deficient repair of the radiation-induced DNA damage. In three of four tumor cell lines, addition of a single human chromosome 4 from normal cells by microcell fusion resulted in efficient repair as evidenced by suppression of radiation-induced chromatid breaks to the level in normal cells. With respect to senescence-related gene (s), two of these four tumor cell lines belonged to complementation group A and one each to groups C and D. Chromosome 4 restored DNA repair efficiency in only one of the two tumor cell lines of complementation group A. These results suggest that chromosome 4 carries a DNA repair gene or gene(s) that complement the repair deficiency in three of these four tumor lines. Furthermore, the gene(s) involved in cellular senescence on chromosome 4 appear to differ from the gene(s) for repair of radiation-induced DNA damage during G 2.

The Contribution of Deficient DNA Repair to Chromosomal Radiosensitivity of CHO Cells After G 2 Irradiation

We compared cytogenetic responses of the parental Chinese hamster ovary (CHO) cell line and its DNA repair-deficient strains to irradiation during the G 2 phase. Chromatid breaks were quantified in cells entering metaphase in the presence or absence of cytosine arabinoside (ara-C) 0.5–1.5 hours after exposure to x-rays or UV-C. Addition of ara-C, an inhibitor of DNA repair replication, significantly increased chromatid break frequency (CBF) in the parental line, but not in the strains deficient in nucleotide excision repair (NER). This increase (ara-C effect) was comparable to that in repair-proficient normal human lymphocytes. We conclude that CBF in cells entering metaphase in the presence of ara-C 0.5–1.5 hours after DNA damage represents a functional in vitro assay for evaluating the DNA repair capacity of mammalian cells in culture. Published by Elsevier Science Inc.

Follow-up biological and genotoxicological monitoring of acrylonitrile- and dimethylformamide-exposed viscose rayon plant workers.

In order to investigate the genotoxic effects of occupational acrylonitrile (ACN) and dimethylformamide (DMF) exposures, clinical serum and urine parameters and genotoxicological endpoints such as chromosome aberration (CA), sister chromatid exchange (SCE), high frequency SCE (HFC), cell cycle kinetics, and UV-induced unscheduled DNA synthesis (UDS) were followed up three times during a 20-month period in peripheral blood lymphocytes (PBL) of 26 workers (13 maintainers and 13 fiber producers) occupationally exposed to ANC and/or DMF in a viscose rayon plant, 26 matched control subjects, and six industrial controls (all males). Six of the 26 exposed subjects were hospitalized because of liver dysfunction that had developed due to inhalative DMF exposure. The rate of smoking was estimated on the basis of serum thiocyanate (SCN) levels. Average peak air ACN and DMF concentrations were over the maximum concentration limits at the time of both investigations. Urine ACN and monomethyl-formamide (MMF) excretions of the exposed subjects were almost doubled after work shifts. An increase in lymphocyte count (in months 0 and 7), and severe alterations in the liver function were observed in the exposed subjects. In PBLs the proliferative rate index (PRI) was already increased in month 0 compared with the controls. In each study, significant increases in CA and SCE frequencies, as well as increases in UDS were found in PBLs of the exposed subjects. The frequencies of chromatid breaks and acentric fragments further increased in month 7 and remained constantly elevated in month 20. Increased yields of both chromatid and chromosome-type exchange aberrations first appeared in month 20, when HFCs were 2.72 times more frequent in fiber producers than in maintainers. The role of some important biological confounding factors (age, white blood cell count, and hematocrit) and lifestyle confounding factors (smoking and drinking habits) were subjected to an analysis of variance during the second study. Increased CA, SCE, and UDS were found both in control and exposed smokers when current smoking was established on the basis of the serum SCN levels. The cytogenetic data suggest that occupational exposures to ACN and DMF induce considerable genotoxic consequences and may increase the cancer risk in the exposed human populations.

The signal model: a possible explanation for the conversion of DNA double-strand breaks into chromatid breaks

. Purpose : To present and evaluate the 'signal' model for the formation of radiation-induced chromatid breaks. Conclusions : Chromatid breaks in human cells represent the apparent interstitial loss of up to about 40 Mbp of DNA, difficult to account for as single lesions under the classical 'breakageand-reunion' hypothesis. If breakage-first resulted from two interacting DNA double-strand breaks (dsb) with the loss or displacement of the intervening fragment, a dose-squared relationship would be predicted for chromatid breaks. However, the relationship between chromatid break frequency and dose for human cells is linear. The alternative 'exchange' model of Revell is based on the principle of the interaction of two initiating lesions, thus also predicting a dose-squared relationship for chromatid 'breaks'. The signal model explains the conversion of dsb into chromatid breaks on the assumption that a single dsb generates a signal which triggers the cell to initiate a recombinational exchange involving a large loop of chromatin. Incomplete exchanges would be observed as chromatid breaks. Possible candidates for the signalling molecule(s) are DNA protein kinase (DNA PK) and the ATM protein.

The evolution of chromosomal instability in Chinese hamster cells: a changing picture?

Purpose: To investigate the kinetics of chromosomal instability induced in clones of Chinese hamster cells following X-irradiation. Materials and methods: X-irradiated clones of GM10115, human hamster hybrid cells containing a single human chromosome 4 (HC4), have been previously established. These clones were defined as unstable if they contained three sub-populations of cells with unique rearrangements of HC4 as detected by FISH. Stable and unstable clones were analysed by FISH and Giemsa staining at various times post-irradiation. Results: While most of the stable clones continued to show chromosomal stability of HC4 over time, one became marginally unstable at approximately 45 population doublings postirradiation. Clones exhibiting chromosomal instability had one of several fates. Many of the unstable clones were showed similar levels of instability over time. However, one unstable clone became stable with time in culture, while another became even more unstable over time. Cytogenetic analyses of all clones after Giemsa staining indicated that in some clones the hamster chromosomes were rearranged independent of HC4, demonstrating increased frequencies of chromatid breaks and dicentric chromosomes. The majority of the unstable clones also had higher yields of chromatid gaps. Conclusions: These data demonstrate the dynamic nature of chromosomal instability as measured by two different cytogenetic assays.

Radiation–Induced Chromatid Breaks and DNA Repair in Blood Lymphocytes of Patients with Dysplastic Nevi and/or Cutaneous Melanoma

Each chromatid contains a single continuous molecule of double-stranded DNA, so chromatid breaks represent unrepaired DNA double-strand breaks. Frequencies of chromatid breaks after G2 phase x-irradiation were determined in phytohemagglutinin-stimulated blood lymphocytes from normal subjects and from four categories of patients with dysplastic nevi with or without cutaneous melanoma or with melanoma alone. Some cells were treated with an inhibitor of DNA repair replication to determine enzymatic incision activity at damaged sites after exposure to x-rays, UVC, or fluorescent light. Whereas one of 16 normal controls had deficient DNA repair, all 17 patients from families with hereditary dysplastic nevi with or without melanoma (category I) had a deficiency in repair of radiation-induced DNA damage, manifested as an abnormally high frequency of chromatid breaks after x-irradiation or a reduced capacity to incise the damaged sites after UV exposure. Four of 11 patients with sporadic dysplastic nevi alone (category II) and eight of 12 with sporadic dysplastic nevi and melanoma (category III) showed deficient DNA repair after x-irradiation. One of two patients with sporadic melanoma and no dysplastic nevi (category IV) was also deficient in repair of x-ray-induced damage. Deficient DNA repair thus appears to be associated with hereditary dysplastic nevi with or without melanoma. It also characterizes some patients with sporadic dysplastic nevi or melanoma.

Chromosome sensitivity to bleomycin in G2 lymphocytes from Down syndrome patients

Several studies have demonstrated that lymphocytes from patients with Down syndrome (DS) exhibit an increased frequency of chromosome aberrations when they are exposed to ionizing radiation or to chemicals at the G0 or G1 phases of the cell cycle, but not at G2, when compared to normal subjects. To determine the susceptibility of DS lymphocytes at G2 phase, bleomycin, a radiomimetic agent, was used to induce DNA breaks in blood cultures from 24 Down syndrome patients. All the patients with DS showed free trisomy 21 (47,XX + 21 or 47,XY + 21). Individuals that showed an average number of chromatid breaks per cell higher than 0.8 were considered sensitive to the drug. No control child showed susceptibility to bleomycin, and among the 24 patients with DS, only one was sensitive to the drug. No significant difference was observed between the two groups, regarding chromatid break frequencies in treated G2 lymphocytes. The distribution of bleomycin-induced breaks in each group of chromosomes was similar for DS and controls. No significant difference was found in the response to bleomycin between male and female subjects. Probably, the main factor involved in chromosome sensitivity of lymphocytes from patients with DS is the phase of the cell cycle in which the cell is treated.

Enhancement of frequencies of restriction endonuclease-induced chromatid breaks by arabinoside adenine in normal human and ataxia telangiectasia cells

. The effect of ara A (9- beta -D-arabinofuranosyladenine) a potent inhibitor of DNA synthesis on the frequencies of chromatid breaks induced by restriction endonucleases (RE) has been investigated in normal human and ataxia telangiectasia (AT) lymphoblastoid cells. PvuII, PstI and BamHI, which cause blunt-ended, 3-overhang and 5-overhang cohesive-ended DNA double-strand breaks (dsb) respectively, were introduced into two AT cell lines (AT-KM and AT-PA) and a normal human (N-SW) cell line by the use of streptolysin-O poration. Controls were exposed to gamma -irradiation and similarly treated with or without ara A. Both AT cell lines were found to exhibit higher frequencies of chromatid breaks when treated with RE alone as compared with the normal cell line. The pattern of chromatid response to the three RE was shown to be similar in all three cell lines i.e. PvuII was most clastogenic while PstI and BamHI were both less effective at inducing chromosomal aberrations. Incubation of cells with ara A resulted in an increase in frequencies of chromatid breaks in PvuII and PstI treated cells but no increase was observed in BamHI treated cells. Normal cells showed most response to ara A following treatment with PvuII and PstI (enhancement ratios 4.63 and 3.75 respectively) while AT cells were affected by ara A to a lesser extent indicating a reduced expression of damage by ara A in these lines. Since ara A is a potent inhibitor of DNA synthesis, it was concluded from the elevated frequency of chromosomal aberrations in the presence of ara A that rejoining of RE-induced dsb in genomic DNA of human cells involves nucleotide insertion at dsb termini prior to ligation.

Modulation of the clastogenic activity of bleomycin by reduced-glutathione, glutathione-ester and buthionine sulphoximine.

In this study an attempt has been made to establish a relationship between bleomycin (BLM)-induced DNA damage and buthionine sulphoximine (BSO)-mediated modified endogenous glutathione (GSH) status in normal human lymphocytes. Present results demonstrate that depletion of endogenous GSH by BSO reduced the clastogenic action of BLM, whereas elevation of endogenous GSH by treating the cells with GSH and GSH-ester, potentiates the cytotoxicity of BLM. A significant reduction in the frequency of deletions and chromatid breaks was observed when BSO-treated cells were treated with BLM. Again the frequency of these two types of aberrations was increased significantly when GSH- and GSH-ester-treated cells were treated with BLM. The observed reduction in the effect of BLM in GSH-depleted cells could be explained on the basis of the failure of reactivation of the oxidized BLM by reducing agent GSH which is present endogenously. Similarly, it appears that radicals which are generated due to reduction of oxidized BLM by the increased level of cellular GSH, after treating the cells with GSH or GSH-ester, could be responsible for the increasing frequency of deletion and chromatid breaks.

In Vitro Culture Studies of Childhood Myelodysplastic Syndrome: Establishment of the Cell Line MUTZ-1

Myelodysplastic syndrome (MDS) in childhood is considered to be very rare and detailed pathobiological data are scarce. More biological information regarding MDS in children is clearly needed and in vitro culture studies provide one possibility for gaining further pathophysiological insights into this malignancy. Here, we incubated bone marrow samples from 30 children with MDS in liquid suspension culture in order to grow the transformed cells in vitro. In most cultures, the hematopoietic cells died quickly and only fibroblastic (stromal) back-ground layers proliferated temporarily; several normal Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines (B-LCL) were established. Only in one instance, albeit from the peripheral blood and not from the bone marrow, could we establish a cell line, termed MUTZ-1, from the malignant cells of a 5-year-old girl with MDS (FAB subtype refractory anemia with excess of blasts). The MDS arose from a pre-existing Fanconi anemia and progressed quickly to an acute myeloid leukemia (FAB M2). Despite positivity for EBV, MUTZ-1 is not an EBV + B-LCL and further characterization of MUTZ-1 confirmed the derivation from the transformed clonal cells. Immunophenotyping showed a pre B-cell surface marker profile (CD10+ CD19+ cytoplasmic IgM+); receptor gene rearrangement analyses underlined the clonal B-cell nature of MUTZ-I cells. MUTZ-1 cells exhibit a highly rearranged, unstable karyotype with a high frequency of spontaneous chromatid breaks and exchanges; del(5q) and additional rearrangements involving chromosome 5 [der(15)t(5;15)] were detected. The present data and results from a few other MDS-derived cell lines suggest that the transforming event in MDS seems to occur in an immature pluripotent progenitor cell. The new MDS-derived continuous cell line MUTZ-1 provides a useful in vitro model system for studies on the pathogenetic events leading to MDS.

Decreased sensitivity to the cytogenetic effects of bleomycin in individuals occupationally exposed to ionizing radiation

In the present work, 12 individuals occupationally exposed to ionizing radiation and 11 unexposed ones were studied to determine the cytogenetic effect of a challenge dose of bleomycin on their phytohemagglutinin stimulated lymphocytes. After bleomycin treatment, the frequencies of chromatid breaks and gaps were significantly lower in the exposed population ( p < 0.025 for both types of chromatid alterations). These results could indicate that occupational exposure to ionizing radiation can induce an adaptive response that can be detected by a subsequent treatment with bleomycin.

Cytogenetic Monitoring of Farmers exposed to pesticides in Colombia.

We have monitored 30 pesticide-exposed workers and 30 matched controls for expression of chromosome aberrations (CA) and sister chromatid exchanges (SCE) in their lymphocytes. Peripheral blood cultures were set up within 3 hr after the collection of samples, and four cultures were set up from each donor. For CA analysis, 100 complete metaphase cells from each donor were evaluated. For the SCE assay, 50 complete metaphase cells from each donor were analyzed. The CA and SCE data were analyzed for differences between the two groups using the chi 2 and the Student's t-test, respectively. From the CA analysis it was obvious that the overwhelming majority of aberrations were chromatid breaks and isochromatid breaks; therefore, only these data are presented and used for statistical analysis. Isochromatid breaks were counted as two breaks each and chromatid breaks as one in calculating the total chromatid break frequencies. Statistical evaluation of the data indicates that there is no significant difference (p > 0.05; chi 2 test) between the exposed and the nonexposed groups based on chromatid breaks per 100 cells (1.2 +/- 0.3 and 1.5 +/- 0.2, respectively) and total chromatid breaks per 100 cells (1.7 +/- 0.3 and 2.1 +/- 0.2, respectively). No significantly difference between the two groups (p > 0.05, Student's t-test) was observed with SCE frequencies (5.0 +/- 1.1 and 4.8 +/- 0.9, respectively). Linear regression analysis indicates that the data were not influenced by age, cigarette smoking, or alcohol consumption. It is assuring that the exposure conditions among these Indian farmers have not caused detectable increases of chromosome damage using standard assays; this suggests the lack of serious long-term health problems. However, periodic monitoring of such exposed populations should be conducted using the same or other more sensitive assays. In addition, other populations with exposure to different types of pesticides in Colombia should also be investigated.

Chromosome aberrations in peripheral blood lymphocytes from control individuals

This study emphasizes the problems encountered in obtaining suitable contol levels for comparison with occupational studies of exposure to clastogens. Blood samples taken from 135 individuals (69 females and 66 males) were examined for chromosome aberrations. The data include 15 368 cells scored for chromosome aberrations. The frequencies of chromatid and chromosome breaks, acentric fragments and dicentrics were determined. The frequency of the most common chromosome aberration, chromosome breaks, was 1.1 x 10 -2 and for dicentries 0.26 x 10 3. The values obtained were investigatd in relation to sex, cigarette smoking habits, diagnostic X-ray exposure and use of antibiotics. In all the parameters, no significant differences were found.

Chromosome aberrations and response to γ-ray challenge in lymphocytes of workers exposed to 1,3-butadiene

An integrated population monitoring study was initiated to investigate whether occupational exposure to current low levels of butadiene is mutagenic to workers. Ten exposed workers (mean production area concentration of 3.5 ppm) and 10 matched plant controls(mean exposure to 0.03 ppm) were selected and blood samples were collected for our study. The standard cytogenetic assay was used to determine chromosome aberration frequencies. In addition, a challenge assay was used to determine response to γ-rays as an indication of DNA repair deficiencies. In the latter assay, cells were exposed to γ-rays at the G1 phase of the cell cycle in vitro and the frequencies of chromosome aberrations in the first post-irradiation metaphase cells were quantitated. Based on results of the cytogenetic assay, the exposed group had a higher frequency of cells with chromosome aberrations and higher chromatid breaks per 100 cells compared with the control. However, the difference was not significant ( p > 0.1). With the challenge assay, the exposed group had a higher frequency of aberrant cells ( p < 0.04), chromatid breaks ( p < 0.05), deletions ( p < 0.07), and dicentrics ( p < 0.02) than the controls. In addition, the dicentric frequencies from workers were significantly correlated with the presence of a butadiene metabolite [1,2-dihydroxy-4-( N-acetyl-cysteinyl- S)butane] in urine with a correlation of coefficient of 0.6 ( p < 0.01). Two outliers were identified and our interpretation of their responses will be discussed. This study indicates that the workers had exposure-induced mutagenic effects. Together with the observation of gene mutation in a subset of the present population, this study indicates that the current occupational exposure to butadiene may not be safe to workers.

G 2 phase repair of X-ray-induced chromosomal DNA damage in trichothiodystrophy cells

The repair of X-ray-induced DNA damage during G 2 cell-cycle phase has been examined in lines of skin fibroblasts from three patients with trichothiodystrophy (TTD), one with apparently normal and two with defective nucleotide excision repair (NER). These responses are compared with those of five lines from clinically normal controls, lines from xeroderma pigmentosum (XP), Cockayne syndrome (CS), Down syndrome (DS), and ataxia telangiectasia (AT) patients. Chromosomal DNA repair was measured as the chromatid aberration frequency (CAF) or total number of chromatid breaks and long gaps per 100 metaphase cells, determined 0.5–1.5 h after X-irradiation (53 rad). Chromatid breaks and gaps (as defined herein) represent unrepaired DNA strand breaks. Only one of the TTD lines, TTD 1BR, showed an abnormally high CAF. This line was shown subsequently to be of a different complementation group, representing a new nucleotide excision repair gene. An abnormally high CAF was also observed, as reported previously, in XP-C, AT and DS but not in CS skin fibroblasts. In addition, cell lines were examined for DNA incision activity by an indirect method in which chromatid aberrations were enumerated with or without ara-C, an inhibitor of repair synthesis, added after X-irradiation. All TTD lines had abnormally low incision activity.

DNA FRAGILITY AND REPAIR CAPABILITY ARE SEPARATE GENETIC PHENOTYPES - STUDIES ON IN-SITU NICK TRANSLATION AND CHROMOSOME BREAKAGE

Three separate experiments, designed to minimize the DNA repair process following exposures to mutagens, were performed to determine whether DNA repair capability is the sole factor responsible for differential mutagen sensitivity. These experiments included (i) very short bleomycin pulse treatments (2, 5 and 10 min), (ii) ionizing irradiation and (iii) bleomycin treatment with DNA repair inhibited by ethanol. Three lymphoblastoid cell lines, derived (i) from a patient with ataxia telangiectasia (AT), (ii) a patient with lupus erythematosus and (iii) a normal blood donor, were used as test cells. Mutagen sensitivity was measured either by the frequency of chromatid breaks or by grain counts in in situ nick translation preparations. In all cases, sensitivity rating followed the same pattern: AT>lupus>normal. The results support the notion that mutagen sensitivity may depend on two separate phenotype: intrinsic DNA fragility and repair capability.

Distribution of X-ray-induced G2 chromatid damage among Chinese hamster chromosomes: influence of chromatin conformation

Exponentially growing primary embryonic Chinese hamster cells, in which the pattern of distribution of heterochromatin is well characterized, were X-irradiated and fixed at 1,2,3, and 4 h following irradiation. In one set of cells repair of damage was completely blocked by ara A. The frequencies of chromatid breaks and exchanges were evaluated for individual chromosomes. An analysis of observed and estimated expected frequencies of chromosomal aberrations indicated that in general, (a) the initial damage was higher in euchromatic regions than the heterochromatic regions and (b) the repair of DNA lesions (as evaluated by the frequencies of chromatid gaps and breaks) was more efficient in euchromatic regions than heterochromatic regions.

Enhanced chromosomal response of ataxia-telangiectasia cells to specific types of DNA double-strand breaks

The chromosomal response of two ataxia-telangiectasia (A-T) lymphoblastoid cell lines (A-T-PA and A-T-KM) to restriction endonucleases (RE) is compared with that of a normal (N-SW) lymphoblastoid cell line. The RE used were PvuII (generating DNA double-strand breaks with blunt termini), BamHI (cohesive termini with 4 base, 5' overhangs) and PstI (cohesive termini with 4 base 3' overhangs). Chromatid aberrations were analysed in cells 5 h after treatment. Cells were porated using streptolysin O to allow entry of RE. Both A-T lines showed an enhanced frequency of chromatid breaks in G2 phase compared with normal cells in response to RE. The enhanced response of A-T cells was most marked in the case of PvuII treatment when the enhancement ratios were 2.5 and 4.2 for A-T-PA and A-T-KM respectively. However, the frequency of DNA double-strand breaks (dsb), measured by neutral filter elution, were considerably lower in A-T-PA cells than N-SW, due to a lower efficiency of poration. When A-T-PA cells were treated with streptolysin O at a higher concentration (0.3 Units/ml), a condition that apparently led to a similar level of poration in A-T-PA as in N-SW cells treated with 0.06 Units/ml as judged by the similar number of dsb induced in the two lines for a given PvuII concentration, the enhancement ratio for A-T-PA cells treated with PvuII increased from 2.5 to 5.8. BamHI and PstI were found to be less clastogenic in all three cell lines as found previously for Chinese hamster cells, although part of this effect may be due to a lower activity, particularly in the case of PstI. However, even at a 4-6-fold higher concentration, BamHI was still less clastogenic than PvuII. It is concluded that dsb with blunt termini are more clastogenic than those with cohesive termini. The results suggest that the chromosomal sensitivity of A-T cells may result from a defect causing a higher rate of conversion of dsb into chromatid aberrations.

Enhanced Chromosomal Response of Ataxia-telangiectasia Cells to Specific Types of DNA Double-strand Breaks

The chromosomal response of two ataxia-telangiectasia (A-T) lymphoblastoid cell lines (A-T-PA and A-T-KM) to restriction endonucleases (RE) is compared with that of a normal (N-SW) lymphoblastoid cell line. The RE used were PvuII (generating DNA double-strand breaks with blunt termini), BamHI (cohesive termini with 4 base, 5' overhangs) and PstI (cohesive termini with 4 base 3' overhangs). Chromatid aberrations were analysed in cells 5 h after treatment. Cells were porated using streptolysin O to allow entry of RE. Both A-T lines showed an enhanced frequency of chromatid breaks in G2 phase compared with normal cells in response to RE. The enhanced response of A-T cells was most marked in the case of PvuII treatment when the enhancement ratios were 2.5 and 4.2 for A-T-PA and A-T-KM respectively. However, the frequency of DNA double-strand breaks (dsb), measured by neutral filter elution, were considerably lower in A-T-PA cells than N-SW, due to a lower efficiency of poration. When A-T-PA cells were treated with streptolysin O at a higher concentration (0.3 Units/ml), a condition that apparently led to a similar level of poration in A-T-PA as in N-SW cells treated with 0.06 Units/ml as judged by the similar number of dsb induced in the two lines for a given PvuII concentration, the enhancement ratio for A-T-PA cells treated with PvuII increased from 2.5 to 5.8. BamHI and PstI were found to be less clastogenic in all three cell lines as found previously for Chinese hamster cells, although part of this effect may be due to a lower activity, particularly in the case of PstI. However, even at a 4-6-fold higher concentration, BamHI was still less clastogenic than PvuII. It is concluded that dsb with blunt termini are more clastogenic than those with cohesive termini. The results suggest that the chromosomal sensitivity of A-T cells may result from a defect causing a higher rate of conversion of dsb into chromatid aberrations.

Cytogenetic Response to G 2 -Phase X Irradiation in Relation to DNA Repair and Radiosensitivity in a Cancer-Prone Family with Li-Fraumeni Syndrome

Noncancerous skin fibroblasts from six family members with Li-Fraumeni syndrome, five with cancer of diverse tissue origin and one with a premalignant neoplasm, showed a high frequency of chromatid aberrations, 94 to 119 breaks and 58 to 95 gaps per 100 metaphase cells arrested with colcemid 0.5 to 1.5 h after X irradiation (1.75 x 10(-2) C/kg). This response results from deficient repair of the radiation-induced DNA damage. In contrast, skin fibroblasts from two unrelated normal controls and a spouse showed 19 breaks and 17 to 19 gaps per 100 cells. Whereas all six members of the cancer-prone family had a radioresistant phenotype, only four had an inherited p53 mutation. Fibroblasts from a radioresistant family member showed the same extent of chromatid damage directly (0 to 0.5 h) after G2-phase X irradiation as those from the radiosensitive control spouse. We conclude, therefore, that radiosensitivity, as determined by cell killing in asynchronous populations of skin fibroblasts, is unrelated to chromosomal sensitivity to G2-phase X irradiation. However, the persistence of a high frequency of chromatid breaks and gaps at 0.5 to 1.5 h after G2-phase X irradiation, a manifestation of deficient DNA repair, is associated with proneness to cancer in this family.