Defective excision repair of gamma-ray-damaged DNA in human (ataxia telangiectasia) fibroblasts

Ataxia-telangiectasia (A-T): An emerging dimension of premature ageing

A-T is a prototype genome instability syndrome and a multifaceted disease. A-T leads to neurodegeneration - primarily cerebellar atrophy, immunodeficiency, oculocutaneous telangiectasia (dilated blood vessels), vestigial thymus and gonads, endocrine abnormalities, cancer predisposition and varying sensitivity to DNA damaging agents, particularly those that induce DNA double-strand breaks. With the recent increase in life expectancy of A-T patients, the premature ageing component of this disease is gaining greater awareness. The complex A-T phenotype reflects the ever growing number of functions assigned to the protein encoded by the responsible gene - the homeostatic protein kinase, ATM. The quest to thoroughly understand the complex A-T phenotype may reveal yet elusive ATM functions.

A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

Ataxia-telangiectasia (A-T) is a rare multi-system disorder caused by mutations in the ATM gene. Significant heterogeneity exists in the underlying genetic mutations and clinical phenotypes. A number of mouse models have been generated that harbor mutations in the distal region of the gene, and a recent study suggests the presence of residual ATM protein in the brain of one such model. These mice recapitulate many of the characteristics of A-T seen in humans, with the notable exception of neurodegeneration. In order to study how an N-terminal mutation affects the disease phenotype, we generated an inducible Atm mutant mouse model (Atm(tm1Mmpl/tm1Mmpl), referred to as A-T [M]) predicted to express only the first 62 amino acids of Atm. Cells derived from A-T [M] mutant mice exhibited reduced cellular proliferation and an altered DNA damage response, but surprisingly, showed no evidence of an oxidative imbalance. Examination of the A-T [M] animals revealed an altered immunophenotype consistent with A-T. In contrast to mice harboring C-terminal Atm mutations that disproportionately develop thymic lymphomas, A-T [M] mice developed lymphoma at a similar rate as human A-T patients. Morphological analyses of A-T [M] cerebella revealed no substantial cellular defects, similar to other models of A-T, although mice display behavioral defects consistent with cerebellar dysfunction. Overall, these results suggest that loss of Atm is not necessarily associated with an oxidized phenotype as has been previously proposed and that loss of ATM protein is not sufficient to induce cerebellar degeneration in mice.

Intramolecular Binding of the Rad9 C Terminus in the Checkpoint Clamp Rad9-Hus1-Rad1 Is Closely Linked with Its DNA Binding

The human checkpoint clamp Rad9-Hus1-Rad1 (9-1-1) is loaded onto chromatin by its loader complex, Rad17-RFC, following DNA damage. The 120-amino acid (aa) stretch of the Rad9 C terminus (C-tail) is unstructured and projects from the core ring structure (CRS). Recent studies showed that 9-1-1 and CRS bind DNA independently of Rad17-RFC. The DNA-binding affinity of mutant 9(ΔC)-1-1, which lacked the Rad9 C-tail, was much higher than that of wild-type 9-1-1, suggesting that 9-1-1 has intrinsic DNA binding activity that manifests in the absence of the C-tail. C-tail added in trans interacted with CRS and prevented it from binding to DNA. We narrowed down the amino acid sequence in the C-tail necessary for CRS binding to a 15-aa stretch harboring two conserved consecutive phenylalanine residues. We prepared 9-1-1 mutants containing the variant C-tail deficient for CRS binding, and we demonstrated that the mutant form restored DNA binding as efficiently as 9(ΔC)-1-1. Furthermore, we mapped the sequence necessary for TopBP1 binding within the same 15-aa stretch, demonstrating that TopBP1 and CRS share the same binding region in the C-tail. Indeed, we observed their competitive binding to the C-tail with purified proteins. The importance of interaction between 9-1-1 and TopBP1 for DNA damage signaling suggests that the competitive interactions of TopBP1 and CRS with the C-tail will be crucial for the activation mechanism.

Radiological imaging in ataxia telangiectasia: a review

The human genetic disorder ataxia telangiectasia (A-T) is characterised by neurodegeneration, immunodeficiency, radiosensitivity, cell cycle checkpoint defects, genomic instability and cancer predisposition. Progressive cerebellar ataxia represents the most debilitating aspect of this disorder. At present, there is no therapy available to cure or prevent the progressive symptoms of A-T. While it is possible to alleviate some of the symptoms associated with immunodeficiency and deficient lung function, neither the predisposition to cancer nor the progressive neurodegeneration can be prevented. Significant effort has focused on improving our understanding of various clinical, genetic and immunological aspects of A-T; however, little attention has been directed towards identifying altered brain structure and function using MRI. To date, most imaging studies have reported radiological anomalies in A-T. This review outlines the clinical and biological features of A-T along with known radiological imaging anomalies. In addition, we briefly discuss the advent of high-resolution MRI in conjunction with diffusion-weighted imaging, which enables improved investigation of the microstructural tissue environment, giving insight into the loss in integrity of motor networks due to abnormal neurodevelopmental or progressive neurodegenerative processes. Such imaging approaches have yet to be applied in the study of A-T and could provide important new information regarding the relationship between mutation of the ataxia telangiectasia mutated (ATM) gene and the integrity of motor circuitry.

Lupus-like histopathology in bloom syndrome: reexamining the clinical and histologic implications of photosensitivity

Bloom syndrome is a rare genodermatosis of autosomal recessive inheritance. Although lupus-like skin lesions characterize this disorder, mechanisms of photosensitivity are poorly understood. In this case presentation, the authors report a patient with Bloom syndrome whose lupus-like facial rash revealed striking histopathologic similarities to cutaneous lupus erythematosus.

Enhanced Expression of Radiation-induced Leukocyte CDKN1A mRNA in Multiple Primary Breast Cancer Patients: Potential New Marker of Cancer Susceptibility

This study was designed to discover blood biomarkers of cancer susceptibility using invasive multiple (n = 21), single primary breast cancer (n = 21), and control subjects (n = 20). Heparinized whole blood was incubated at 37 °C for 2 hours after 0–10 Gy of radiation, then cell cycle arrest marker CDKN1A and apoptosis marker BBC3 mRNA were quantified. This epidemiological study was practically feasible because radiation-induced mRNA was preserved for at least 1 day whenever blood was stored at 4 °C (r² = 0.901). Moreover, blood could be stored frozen after radiation treatment (r² = 0.797). Radiation-induced CDKN1A and BBC3 mRNA were dose dependent, and the degree of induction of CDKN1A was correlated with that of BBC3 (r² = 0.679). Interestingly, multiple primary cases showed higher induction of CDKN1A mRNA than single primary and control groups, whereas BBC3 did not show such differences. The results suggested that cancer susceptibility represented by the multiple primary breast cancer cases was related to over-reaction of CDKN1A mRNA, not BBC3. The study also suggests that ex vivo gene expression analysis could potentially be used as a new tool in epidemiological studies for cancer and radiation sensitivity research.

Oxygen Effect for DNA Double-strand Break Induction Determined by Pulsed-field Gel Electrophoresis

The induction of DNA double-strand breaks (dsb) following irradiation under oxygenated and hypoxic conditions with and without misonidazole was measured by pulsed-field gel electrophoresis (PFGE) in a human bladder carcinoma cell line. The dose-response curve for DNA dsb detection by PFGE was biphasic with an apparent reduction in rate of dsb induced with dose. Oxygen enhancement ratios (OER) for cell survival (at a surviving fraction of 0.1) and for DNA damage assessed by PFGE (at 80% retained) were 2.0 and 3.0 respectively. Dose-modifying factors for misonidazole (15 mM), of 1.9 (survival) and 2.4 (DNA damage) were found. Although the magnitude of the inter-experiment variations limit the precision with which cell survival and DNA electrophoresis can be compared, the data do support a simple correlation between these two measures of response. When DNA dsb induction frequency was assessed from the number average molecular weight, values of 2.7 (+/- 0.3), 0.7 (+/- 0.1) and 2.6 (+/- 0.5) x 10(-9) dsb/bp/Gy were found for irradiation under oxic, hypoxic alone and hypoxic + misonidazole conditions respectively. This gives an OER of 3.9 and a DMF of 3.7.

The inhibition of poly(ADP-ribose) polymerase enhances growth rates of ataxia telangiectasia cells

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme which is activated in response to genotoxic insults by binding damaged DNA and attaching polymers of ADP-ribose to nuclear proteins at the expense of its substrate NAD+. In persons affected with ataxia telangiectasia (A-T), associated mutations in the ataxia telangiectasia mutated gene render cells unable to cope with the genotoxic stresses from ionizing radiation and oxidative damage, thus resulting in a higher concentration of unrepaired DNA damage and the activation of PARP in an uncontrolled manner. In primary A-T fibroblasts, we observed a 58-96% increase in PARP activity and a concomitant loss of cellular NAD+ and ATP content. PARP protein by Western blot analysis increased only slightly in these cells, supporting the observation that the steady state levels of DNA damage is higher in A-T cells than in normals. When treated with PARP inhibitors 3-aminobenzamide or 1,5-dihydroisoquinoline, cellular growth rates reached those observed in normal fibroblast cultures. The improvement of cellular growth and NAD+ levels in A-T cells with PARP inhibition suggests that the cellular metabolic status of A-T cells is compromised and the inhibition of PARP may relieve some of the drain on cellular pyridine nucleotides and ATP. Thus, therapy utilizing PARP inhibitors may provide a benefit for individuals affected with A-T.

Early diagnosis of ataxia-telangiectasia using radiosensitivity testing

OBJECTIVES

To utilize radiosensitivity testing to improve early diagnosis of patients with ataxia-telangiectasia (A-T).

STUDY DESIGN

We established normal ranges for the colony survival assay (CSA) by testing cells from 104 patients with typical A-T, 29 phenotypic normal patients, and 19 A-T heterozygotes. We also analyzed 61 samples from patients suspected of having A-T and 25 patients with related disorders to compare the CSA with other criteria in the diagnosis of A-T.

RESULTS

When cells were irradiated with 1.0 Gy, the mean survival fraction (microSF +/- 1 SD) for patients with A-T was 13.1% +/- 7.2% compared with 50.1% +/- 13.5% for healthy control patients. These data served to define a diagnostic range for the CSA (ie, <21%), a normal range (>36%), and a nondiagnostic intermediate range of 21% to 36%. The mutations of patients with A-T with intermediate radiosensitivity tended to cluster around the functional domains of the ATM gene.

CONCLUSIONS

The CSA is a useful adjunctive test for confirming an early clinical diagnosis of A-T. However, CSA is also abnormal in other chromosomal instability and immunodeficiency disorders.

DNA damage processing defects and disease

Inherited defects in DNA repair or the processing of DNA damage can lead to disease. Both autosomal recessive and autosomal dominant modes of inheritance are represented. The diseases as a group are characterized by genomic instability, with eventual appearance of cancer. The inherited defects frequently have a specific DNA damage sensitivity, with cells from affected individuals showing normal resistance to other genotoxic agents. The known defects are subtle alterations in transcription, replication, or recombination, with alternate pathways of processing permitting cellular viability. Distinct diseases may arise from different mutations in one gene; thus, clinical phenotypes may reflect the loss of different partial functions of a gene. The findings indicate that partial defects in transcription or recombination lead to genomic instability, cancer, and characteristic disease phenotypes.

Correction of radioresistant DNA synthesis in ataxia telangiectasia fibroblasts by prostaglandin E2 treatment

Cultured cells from patients inheriting the rare cancer-prone and radiotherapy-sensitive disorder ataxia telangiectasia (AT) exhibit defects in the activation of cell-cycle checkpoints after exposure to ionizing radiation. In particular, the failure of AT cells to arrest transiently the DNA de novo replication machinery immediately after irradiation--so-called radioresistant DNA synthesis (RDS)--is often taken as a molecular hallmark of the disease. Recently we reported that: (i) the radiation-responsive S-phase checkpoint operating in normal human cells is mediated by a signal transduction pathway involving Ca2+/calmodulin-dependent protein kinase II (CaMKII); and (ii) the RDS phenotype of AT cells is associated with failure to mobilize Ca2+ from intracellular stores, which is required for activation of the CaMKII-dependent S-phase arrest. In the present study, we demonstrate that the RDS phenotype of AT dermal fibroblasts can be rectified in the absence of ectopic expression of functional ATM, the 350-kDa protein kinase encoded by the gene mutated in AT. Correction of RDS was observed when AT fibroblasts were coincubated with normal fibroblasts under conditions in which the 2 different cell cultures shared the same medium but were completely separated physically. The RDS trait was also rectified when AT fibroblasts were briefly incubated with prostaglandin E2 in the absence of normal feeder cells, signifying that this ubiquitous eicosanoid can serve as the diffusible "RDS-correction factor" for AT cells in the aforementioned cocultivation studies. It would therefore appear that prostaglandin E2 can assume the role of an extracellular signaling modulator of the S-phase checkpoint in AT cells exposed to ionizing radiation, inducing DNA synthesis shutdown via an alternative, ATM-independent signal transduction pathway.

Genetic and biochemical studies with ataxia telangiectasia. A review

This article summarizes the genetics and clinical features of ataxia telangiectasia (AT) and then reviews recent cytogenetic, cellular, and biochemical studies which support the hypothesis that a defect in DNA repair is responsible for the various manifestations of the disease. The biochemical evidence further indicates that the defect specifically reduces the cellular capacity to remove bases and nucleotides damaged by ionizing radiation, without affecting the cells' ability to scavenge free radicals or to rejoin breaks in the sugar-phosphate backbone of DNA. Suggestions for additional research to more precisely identify the repair defect will also be presented.

Oligo-/monoclonal gammopathy and hypergammaglobulinemia in ataxia-telangiectasia. A study of 90 patients

We investigated the presence of hypergammaglobulinemia and oligo-/monoclonal gammopathy in 90 patients (from 80 families) with ataxia-telangiectasia ranging in age from 2 to 29 years. Of the 90 patients, 38.8% displayed hypergammaglobulinemia. An isolated increase in IgM was the most common finding (23.3%) followed by a simultaneous increase in IgM and IgG (8.8%), an isolated increase in IgA (3.3%), an elevated level of IgG (2.2%) and a concomitant increase in IgM and IgA (1.1%), respectively. Seven of the patients (8.1%) had oligo-/monoclonal gammopathy. The gammopathies included all major immunoglobulin isotypes. Chemotherapeutic intervention in 2 cases precipitated the emergence of new clones within a matter of weeks. Further investigation of oligo-/monoclonal gammopathies in these patients may lead to a clearer understanding of the clinical course and provide further insight into the underlying mechanisms of B-cell abnormalities in ataxia-telangiectasia.

Radiosensitivity and oxidative signalling in ataxia telangiectasia: an update

Radiosensitivity is a major hallmark of the human genetic disorder ataxia telangiectasia. This hypersensitivity to ionizing radiation has been demonstrated in vivo after exposure of patients to therapeutic doses of radiation and in cells in culture. Clearly an understanding of the nature of the molecular defect in ataxia telangiectasia will be of considerable assistance in delineating additional pathways that determine cellular radiosensitivity/radioresistance. Furthermore, since patients with this syndrome are also predisposed to developing a number of leukaemias and lymphomas, the possible connection between radiosensitivity and cancer predisposition is of interest. Now that the gene (ATM) responsible for this genetic disease has been cloned and identified, progress is being made in determining the role of the ATM protein in mediating the effects of cellular exposure to ionizing radiation and other forms of redox stress. Proteins such as the product of the tumour suppressor gene p53 and the proto-oncogene c-Abl (a protein tyrosine kinase) have been shown to interact with ATM. Since several intermediate steps in both the p53 and c-Abl pathways, activated by ionizing radiation, are known it will be possible to map the position of ATM in these pathways and describe its mechanism of action. What are the clinical implications of understanding the molecular basis of the defect in ataxia telangiectasia (A-T)? As outlined above, since radiosensitivity is a universal characteristic of A-T, understanding the mechanism of action of ATM will provide additional information on radiation signalling in human cells. With this information it may be possible to sensitize tumour cells to radiation and thus increase the therapeutic benefit of radiotherapy. This might involve the use of small molecules that would interfere with the normal ATM-controlled pathways and thus sensitize cells to radiation or alternatively it might involve the efficient introduction of ATM anti-sense cDNA constructs into tumours to achieve the same end-point.

Aneuploid nucleomegaly of bronchial cells in ataxia-telangiectasia: cytologic recognition in bronchial brushings

Ataxia-telangiectasia (AT) is an autosomal recessive disorder of childhood onset characterized by cerebellar ataxia and cutaneous and conjunctival telangiectasias, which affects many systems and organs. One histologic feature of AT is the presence of enlarged dystrophic nuclei, predominantly in satellite cells of sympathetic ganglia and dorsal roots. This paper describes the recognition of nucleomegaly of respiratory cells in bronchial brushings of a 9-year-old patient with AT. The enlarged nuclei displayed smooth nuclear contour, coarse and clumped chromatin granules, and one or two conspicuous nucleoli. The average size was 0.1015 mm in the AT case and 0.0573 mm in control cells. Ploidy analysis demonstrated an aneuploid population of cells with a DNA index of 1.31 and a S-G2M fase of 4.48% in the AT, while the control nuclei showed normal diploid values. To our knowledge, this is the first report of a description of aneuploid nucleomegaly of bronchial cells detected in bronchial smears from a patient with AT. Given that malignant transformations are usually preceded by ploidy alterations, it seems likely that the presence of an aneuploid cell population probably correlates with the increased cancer risk observed in AT patients. Cytopathologists must bear in mind these morphologic features of aneuploid nucleomegaly exhibited by certain cell populations when examining a smear from AT patients. Moreover, this finding may even represent a clue for diagnosis of AT in cases in which the disease has gone unrecognized.

p53 and ATM: cell cycle, cell death, and cancer

The development of a normal cell into a tumor cell appears to depend in part on mutations in genes that normally control cell cycle and cell death, thereby resulting in inappropriate cellular survival and tumorigenesis. ATM ("mutated in ataxia-telangiectasia") and p53 are two gene products that are believed to play a major role in maintaining the integrity of the genome such that alterations in these gene products may contribute to increased incidence of genomic changes such as deletions, translocations, and amplifications, which are common during oncogenesis. p53 is a critical participant in a signal transduction pathway that mediates either a G1 arrest or apoptosis in response to DNA damage. In addition, p53 is believed to be involved in the mitotic spindle checkpoint and in the regulation of centrosome function. Following certain cytotoxic stresses, normal ATM function is required for p53-mediated G1 arrest. ATM is also involved in other cellular processes such as S phase and G2-M phase arrest and in radiosensitivity. The understanding of the roles that both p53 and ATM play in cell cycle progression and cell death in response to DNA damage may provide new insights into the molecular mechanisms of cellular transformation and may help identify potential targets for improved cancer therapies.

The genetic defect in ataxia-telangiectasia

The autosomal recessive human disorder ataxia-telangiectasia (A-T) was first described as a separate disease entity 40 years ago. It is a multisystem disease characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, radiosensitivity, predisposition to lymphoid malignancies and immunodeficiency, with defects in both cellular and humoral immunity. The pleiotropic nature of the clinical and cellular phenotype suggests that the gene product involved is important in maintaining stability of the genome but also plays a more general role in signal transduction. The chromosomal instability and radiosensitivity so characteristic of this disease appear to be related to defective activation of cell cycle checkpoints. Greater insight into the nature of the defect in A-T has been provided by the recent identification, by positional cloning, of the responsible gene, ATM. The ATM gene is related to a family of genes involved in cellular responses to DNA damage and/or cell cycle control. These genes encode large proteins containing a phosphatidylinositol 3-kinase domain, some of which have protein kinase activity. The mutations causing A-T completely inactivate or eliminate the ATM protein. This protein has been detected and localized to different subcellular compartments.

Abnormal pattern of post-gamma-ray DNA replication in radioresistant fibroblast strains from affected members of a cancer-prone family with Li-Fraumeni syndrome

Non-malignant dermal fibroblast strains, cultured from affected members of a Li-Fraumeni syndrome (LFS) family with diverse neoplasms associated with radiation exposure, display a unique increased resistance to the lethal effects of gamma-radiation. In the studies reported here, this radioresistance (RR) trait has been found to correlate strongly with an abnormal pattern of post-gamma-ray DNA replicative synthesis, as monitored by radiolabelled thymidine incorporation and S-phase cell autoradiography. In particular, the time interval between the gamma-ray-induced shutdown of DNA synthesis and its subsequent recovery was greater in all four RR strains examined and the post-recovery replication rate was much higher and was maintained longer than in normal and spousal controls. Alkaline sucrose sedimentation profiles of pulse-labelled cellular DNA indicated that the unusual pattern of DNA replication in irradiated RR strains may be ascribed to anomalies in both replicon initiation and DNA chain elongation processes. Moreover, the RR strain which had previously displayed the highest post-gamma-ray clonogenic survival was found to harbour a somatic (codon 234) mutation (presumably acquired during culture in vitro) in the same conserved region of the p53 tumour-suppressor gene as the germline (codon 245) mutation in the remaining three RR strains from other family members, thus coupling the RR phenotype and abnormal post-gamma-ray DNA synthesis pattern with faulty p53 expression. Significantly, these two aberrant radioresponse end points, along with documented anomalies in c-myc and c-raf-1 proto-oncogenes, are unprecedented among other LFS families carrying p53 germline mutations. We thus speculate that this peculiar cancer-prone family may possess in its germ line a second, as yet unidentified, genetic defect in addition to the p53 mutation.

An ionizing radiation-sensitive mutant of CHO cells: irs-20. III. Chromosome aberrations, DNA breaks and mitotic delay

The ionizing radiation-sensitive mutant of CHO cells, irs-20, showed a defect in the rate and extent of rejoining of gamma-ray-induced DNA double-strand breaks (dsbs) compared with the parental CHO cells as measured using pulsed-field gel electrophoresis. Virtually all rejoined in wild-type cells but some 20-30% failed to rejoin in the mutant cells during a 5-h period after irradiation. An increased level of chromosome-type aberrations per unit dose was seen in irs-20 cells compared with wild-type cells irradiated during the G1 phase. For the irs-20 cells, about half the dose was required to produce the same chromosome-type aberration frequency. Chromatid-type aberrations were induced in G1-irradiated irs-20 cells at frequencies nearly the same as for chromosome types. For the parental wild-type CHO 10B2 cells, only chromosome types were seen. The distribution of aberrations among cells was not significantly different from Poisson for wild-type cells, but this was not the case for irs-20 cells where the overdispersion was highly significant. The mutant irs-20 cells displayed a much greater cell cycle delay per unit dose (about five-fold) in reaching mitosis after irradiation in G1 than the more radioresistant wild-type parental cells.

Radiosensitivity in ataxia-telangiectasia: anomalies in radiation-induced cell cycle delay

A number of anomalies have been described in the progression of ataxia-telangiectasia (AT) cells through the cell cycle post-irradiation. Some uncertainty still exists as to whether AT cells show increased or reduced division delay after exposure to ionizing radiation. We have attempted to resolve the apparent inconsistencies that exist by investigating the effects of radiation on AT cells at various stages of the cell cycle. Specific labelling of S phase cells with 5-bromodeoxyuridine (BrdU) followed by irradiation caused a prolonged accumulation of these cells in G2/M phase with only 2-7% of AT cells progressing through to G1 24h post-irradiation. In contrast, 23-28% of control cells irradiated in S phase reached G1 by 24 h after irradiation. As observed previously with AT fibroblasts, AT lymphoblastoid cells irradiated in G1 phase did not experience a delay in entering S phase. After progressing through S phase these cells also were delayed in G2/M, but not to the same extent as irradiated S phase cells. On the other hand, when AT cells were irradiated in G2 phase they showed less delay initially in entry to mitosis and the subsequent G1 phase than did irradiated control cells. The overall results demonstrate that AT cells fail to show an initial delay in transitions between the G1/S and G2/M phases of the cell cycle and in progression through these phases post-irradiation, but in the long-term, after passage through S phase, they experience a prolonged delay in G2/M. Since several AT complementation groups are represented in this study, the cell cycle anomalies appear to be universal in AT. These results implicate deficiencies in control of cell cycle progression in the increased radiosensitivity and cancer predisposition in AT.

Postirradiation exposure to hypotonic saline shows normal damage processing in radiation-sensitive cell lines

Three pairs of cell lines (one human and two Chinese hamster ovary (CHO) cell lines) each comprising a cell line with a normal radiation response and a radiation-sensitive mutant, were evaluated for recovery of potentially lethal damage (PLDR) and recovery of sublethal damage (SLDR). In all cases, the normal cell lines (GM1522, human; AA8-4 and K1, CHO) exhibited capacity for PLDR and SLDR was also normal in the two CHO lines. For the mutants (GM3395, human AT; V3 and 5-11, CHO) there was no ability for PLDR and SLDR was also absent in the two CHO cell lines. Postirradiation exposure to hypotonic NaCl solutions resulted in fixation of radiation damage in all the cell lines. This form of damage is repaired if left unperturbed after irradiation. This shows that cells have a large capacity for repair of this form of damage which accounts for much greater changes in survival than those observed in conventional PLDR experiments. These data show that the mutant cell lines retained their capacity to repair the damage which was susceptible to postirradiation fixation by anisotonic salt solutions. In addition, initial (i.e. preirradiation) DNA polymerase activities were measured in the four CHO cell lines; they were not correlated to radiation sensitivity.

Cutaneous granuloma with ataxia telangiectasia--a case report and review of literature

A patient is reported who had ataxia telangiectasia with multiple cutaneous lesions mainly on the limbs, which showed atrophy and scarring. Histopathology of these skin lesions showed tuberculoid granulomas without frank collagen necrobiosis in the dermis. Chromosomal analysis revealed a translocation between 7 and 14 as well as deletion of chromosomes 4 and 6.

Sensitivity of neutral filter elution but not PFGE can be modified by non-dsb chromatin damage

Hamster V79 fibroblast cells and human squamous carcinoma cells (Caski) were exposed to 60Co radiation and DNA double-strand break (dsb) induction was analysed by DNA elution at neutral pH from polycarbonate filter or out of an agarose matrix in pulsed-field electrophoresis (PFGE). While dsb yields were equal for the two cell lines (using 125-iodine calibration) a reduced responsiveness of filter elution was found for V79 versus Caski cells. This difference could be abolished when additional single-strand breaks (ssb) were introduced by an incubation at 10(-4) M H2O2 for up to 40 min that itself did not give a response in neutral elution. No such lack of specificity for the detection of dsb was seen in electrophoretic elution where also the influence of peroxide incubation was absent. The presumed potential of ssb to modify dsb detection was paralleled by the kinetics of dsb rejoining: a pronounced transient increase of DNA elution from filters was observed for V79 cells (less prominent with Caski cells) at 15-40 which is thought to reflect the occurrence of secondary ssb from incisions during base damage repair. Rejoining measured by PFGE did not show this behaviour. The results suggest that ssb may aid decondensation of the chromatin during lysis of cells required for an efficient release of dsb fragments when supported on filters, but which depends on cell type and is less critical in electrophoretic elution out of an agarose matrix. This involvement of ssb in the neutral filter elution assay appears to be contrary to published data obtained with different experimental systems. The finding of an increase of DNA elution from filters due to hyperthermia at 45 degrees C is also taken to indicate an involvement of non-dsb chromatin damage in the response of filter elution at neutral pH with V79 but not with Caski cells.

Studies on mutagen sensitive strains of Drosophila melanogaster. XI. Survival (dominant lethality) after X-irradiation and relation to recessive lethals and translocations

Muller-5 males of Drosophila melanogaster were irradiated in N2 or O2 and mated to excision repair deficient, post-replication repair deficient (mei-9a, mei-41D5, mus101D1, mus201D1, mus302D1, mus306D1 and mus308D2) or repair proficient females. The surviving fraction (dominant lethality) was estimated in the F1 and used to reassess existing recessive lethal and translocation data. The surviving fraction was found to decrease if repair deficient females were used (maternal effect). The dose-effect curves are often biphasic with a steeper slope at low doses than at high (> or = 5 Gy) doses of X-rays. The high dose part of the curve is sensitive to oxygenation during irradiation and is affected significantly by the mutants with low fertility (mei-9, mus101 and mus302). The low dose component is not sensitive to oxygenation during irradiation and seems influenced by all seven repair deficient mutants. The sensitivity of the high dose part to oxygenation suggests that this part is related mainly to DNA break damage, while in the low dose part base damage seems more important. Existing recessive lethal and translocation data were plotted against the surviving fraction for a reassessment. In excision repair deficient mutants translocation induction is lower compared to repair proficient flies at the same level of survival (i.e., dominant lethality). Likewise in post-replication repair deficient mutants induction of recessive lethals is decreased. However the frequency of respectively induced recessive lethals and translocations obtained at the same level of X-rays was the same in repair deficient and proficient backgrounds. It is concluded that genetic damage recovered in a repair deficient background is likely to be qualitatively different even if the frequency of the damage induced by a given dose is not altered.

Enhanced levels of radiation-induced G2 phase delay in ataxia telangiectasia heterozygotes

Ataxia telangiectasia (AT) is a multiform genetic disease characterized by immunodeficiency, cerebellar abnormalities, and cancer predisposition. Heterozygotes also have an increased risk of developing several different cancers. It has been estimated that as many as 18% of all patients with breast cancer, the cancer most clearly associated with AT heterozygotes, may be carriers of the AT gene. We describe an assay for AT heterozygotes that relies on the previous observation that cells from AT homozygotes show a greater and more prolonged radiation-induced accumulation in the G2 phase of the cell cycle than do normal controls. We showed that all 6 A-T heterozygotes show a greater extent of G2 phase delay at different times postirradiation than do controls. The degree of accumulation was less than that observed in AT homozygotes. Only two of 22 controls showed overlap with heterozygotes at 18 hours postirradiation, and that number was reduced to one at the 24-hour point. As a group, AT heterozygotes were intermediate between controls and AT homozygotes at both time points after irradiation. This assay is relatively simple and reliable and can be performed in any laboratory with access to both Epstein-Barr virus (EBV) for transformation of lymphocytes and a fluorescence-activated cell analyzer.

Mutagenesis by apurinic sites in normal and ataxia telangiectasia human lymphoblastoid cells

We used a shuttle vector based on the Epstein-Barr virus origin of plasmid replication (oriP) to determine the types of mutations induced by depurination in human cells. Plasmid DNA was incubated at pH 2 at 40 degrees C for various times to induce up to 20 apurinic (AP) sites per 9.7-kb plasmid and electroporated into lymphoblastoid cells derived from either a normal individual or an ataxia telangiectasia patient. After replication of the vector in the human cells, plasmid DNA was isolated and analyzed for mutations induced in the plasmid-encoded herpes simplex virus type 1-thymidine kinase gene. Both the frequencies and types of mutations induced by depurination were essentially identical for normal and ataxia telangiectasia cells. The mutant frequency at 20 AP sites/plasmid was 10-fold to 13-fold greater than that observed for untreated DNA. Deletion and frameshift events accounted for 46-55% of the mutants induced by depurination. The induced deletions were relatively small (median size, 100-150 bp) and characterized by short (1-5 bp) regions of sequence homology at the endpoints. These mutations and the frameshifts, a majority of which occurred in runs of identical nucleotides, are consistent with a model involving AP-site-induced template dislocation during DNA synthesis. A broad spectrum of base-substitution mutations, which accounted for 19-36% of the induced mutants, was observed. The apparent preference for insertion opposite AP sites in human cells was G (43-55%) greater than A approximately C (18-21%) greater than T (9-14%). Our results in human cells contrast markedly with those published previously for the mutational specificity of AP sites in Escherichia coli, in which a large majority of the mutants resulted from insertion of an A opposite the abasic site.

The repair of double-strand breaks and S1 nuclease-sensitive sites can be monitored chromosome-specifically in Saccharomyces cerevisiae using pulse-field gel electrophoresis

Repair under non-growth conditions of DNA double-stranded breaks (DSBs) and S1 nuclease-sensitive sites (SSSs; e.g. DNA damage which is processed by in vitro treatment with S1 nuclease to DSBs) induced by [60Co]-gamma-rays (200 Gy; anoxic conditions) was monitored in a diploid repair-competent strain of Saccharomyces cerevisiae. We used pulsed-field gel electrophoresis (PFGE), which allows the separation of chromosome-sized yeast DNA molecules, to determine the number of DSBs and SSSs in individual chromosome species of yeast. Our results indicate that SSSs which have been regarded as clusters of base damage in opposite DNA strands are repaired efficiently in a repair-proficient diploid strain of yeast. The time course of SSS repair is comparable to the one of DSB repair, indicating similarities in the molecular mechanism. Both types of repair kinetics are different for different chromosome species.

Repair of gamma ray-induced S1 nuclease hypersensitive sites in yeast depends on homologous mitotic recombination and a RAD18-dependent function

Repair under non-growth conditions of DNA double-strand breaks (DSB) and chromatin sites sensitive to S1 endonuclease (SSS) induced by 60Cobalt-gamma rays were monitored in repair-competent and deficient strains of Saccharomyces cerevisiae by pulsed field gel-electrophoresis. In stationary-phase cells of a repair-competent RAD diploid, and an excision-deficient rad3-2 diploid, SSS are repaired as efficiently as DSB, whereas in a repair-competent RAD haploid, and a rad 50-1 diploid, neither SSS nor DSB are repaired. The rad18-2 diploid repairs DSB well but is defective in SSS repair. Obviously, SSS repair in yeast chromatin, like DSB repair, depends on recombination, but unlike DSB repair depends additionally on RAD18 function.

Detection of base damage in DNA in human blood exposed to ionizing radiation at biologically relevant doses

The alkaline elution technique for the detection of DNA damage has been adapted to allow application on unlabelled blood cells. Both the induction and subsequent repair have been studied of two classes of DNA damage, viz, single-strand breaks and base damage recognized by the gamma-endonuclease activity in a cell-free extract of Micrococcus luteus bacteria. The high sensitivity of the assay permitted the measurement of induction and repair of base damage after in vitro exposure of full blood under aerobic conditions to biologically relevant doses of gamma-rays (1.5-4.5 Gy). After a radiation dose of 3 Gy about 50% of the base damage was removed within 1.5 h of repair. Base damage could still be detected at 24 h after exposure to 15 Gy.

Time-lapse microscopy and DNA double-strand breakage of Chinese hamster cells under conditions promoting or preventing PLD repair after irradiation with 60Co gamma rays

We have confirmed previous time-lapse microscopic observations (Suzuki 1985) using Chinese hamster hai and V79 cells. The proportion of non-dividing to dividing cells was the same under conditions of potentially lethal damage (PLD) repair and non-PLD repair after irradiation with 60Co gamma-rays. This finding suggested that the radiation-induced damage to cellular DNA was similarly repaired so that cells undergo a first division to the same extent under both sets of conditions. In fact, direct measurement of double-strand breaks (dsb) in DNA from the two cell lines by the neutral elution technique showed no differences either in the initial amount of damage or in the time-course under conditions promoting or preventing PLD repair. These results indicate that PLD repair (i.e. an increase in cell survival) cannot be simply explained by a difference in the repair of dsb, but it can perhaps be explained by assuming that DNA damage is repaired with either fewer or more errors in the presence or absence of PLD repair respectively.

Response of fibroblast cultures from ataxia-telangiectasia patients to oxidative stress

The object of this study was to determine whether ataxia-telangiectasia (AT) cells are more sensitive than normal cells to reduced oxygen species generated either during normal cell processes or resulting from metabolism of xenoblotics. To test this hypothesis four AT and four normal fibroblast cultures were exposed to hydrogen peroxide (H2O2) and the induction of micronucleated cells was assayed. AT cultures responded to the H2O2 treatment with a greater increase in micronucleus frequencies than that observed in normal cultures (P less than 0.01). At time course study showed that an elevation in micronucleus frequencies occurred earlier in AT cultures (significant increase by 1.5 h after treatment) than in normal cultures, possibly indicating a G2-phase sensitivity of AT cells to H2O2. The addition of an aqueous extract of areca nut to the cultures, as an example of exogenous stress, induced a greater frequency of micronucleated cells in AT cultures than in the normal cultures. These results suggest that the AT syndrome may serve as a model for investigating the role of reduced oxygen species in cancer.

Cancer in ataxia-telangiectasia patients

A gene locus for ataxia-telangiectasia (A-T) is in chromosome region 11q22 to 11q23 and predisposes to cancer. Ataxia-telangiectasia patients appear to have two separate clinical patterns of malignancy. One pattern involves solid tumors, which have not been stressed and which include malignancies in the oral cavity, breast, stomach, pancreas, ovary, and bladder. Detection of a solid tumor in an A-T patient should serve as a warning. It heralds a markedly elevated risk of another malignancy in that patient. The second pattern of neoplasia in A-T is well recognized and consists of lymphocytic leukemia and non-Hodgkin's lymphoma. These malignancies may relate to immunodeficiency in A-T and to chromosome breakage and rearrangement, which are a feature of A-T. These two patterns of malignancy may be truly separate and reflect different mechanisms of malignancy in A-T, or they may not really be separate but instead reflect a single mechanism of malignancy. The situation in A-T is reminiscent of that in the acquired immunodeficiency syndrome (AIDS), in which Kaposi's sarcoma occurs with mild immunodeficiency and pneumocystis carinii pneumonia occurs with more profound immunodeficiency owing to the human immunodeficiency virus. Next to pulmonary disease, cancer is the leading cause of death in A-T.

Chromosome-specific identification and quantification of S1 nuclease-sensitive sites in yeast chromatin by pulsed-field gel electrophoresis

Sites that are sensitive to the single-strand-specific endonuclease S1 ('S1-sensitive sites', SSS) occur in native chromatin and, like DNA double-stranded breaks (DSB), they are induced by DNA-damaging agents, such as ionizing radiation. We have developed a method to quantify SSS and DSB in yeast chromatin by using pulsed-field gel electrophoresis (PFGE) to separate the intact chromosomal-length DNA molecules from the lower molecular-weight broken ones. Direct evaluation of the photonegatives of the ethidium bromide-stained gels by laser densitometry enabled us to calculate the numbers of DSB and SSS per DNA molecule. These numbers were determined from the bulk of the non-separated genomic DNA of yeast, corresponding to a single band in the PFGE (pulse time 10 seconds), and in each of the eight largest yeast chromosomes, corresponding to distinct bands in the PFGE gels (pulse time 50 seconds), which were not superimposed by the smear of the broken, low molecular-weight DNA. Furthermore, the induction of DSB and SSS in a specific chromosome (circular chromosome III) was determined by Southern hybridization of the PFGE gels with a suitable centromere probe, followed by densitometry of the autoradiographs. Our method allows the chromosome-specific monitoring of DSB and all those DNA structures that are processed either in vivo or in vitro into DSB and which may not be distributed randomly within the genome.

Sister chromatid exchange and chromosome fragility in the nevoid basal cell carcinoma syndrome

Previous studies of chromosome stability in the nevoid basal cell carcinoma syndrome have yielded inconsistent results and suffered from small sample sizes and less than optimal controls. We investigated chromosome fragility and sister chromatid exchange in 20 affected individuals from five multiplex pedigrees, and 15 first- or second-degree unaffected relatives. The percentage of case and control cells showing breaks or rearrangements was compared using a test of proportions. A similar procedure was used to compare site-specific sister chromatid exchanges in baseline cultures from affected persons and controls. No significant differences were noted for either chromosome fragility or sister chromatid exchange between the two groups. These results suggest that cancer susceptibility in the nevoid basal cell carcinoma syndrome is not caused by or manifested as chromosome instability.

Cytogenetic investigations in a family with ataxia telangiectasia

Cytogenetic findings on a family with ataxia telangiectasia (A-T) in which three of four sibs were affected are described. The affected individuals had approximately twice the level of spontaneous chromosome breakage of a normal control, while the parents and the normal sib had no significant increase. Lymphocytes from all three A-T homozygotes showed specific stable chromosomal rearrangements involving chromosomes 7 and 14. All of these abnormalities involved breakage at the usual four sites associated with A-T (7p14, 7q35, 14q12, and 14q32). Two rearrangements detected in the eldest and most severely affected patient were clones, one of which [t(14;14)(p11;q12)] is not commonly found in A-T cells. No chromosomal rearrangements were encountered in lymphocytes from the control, the parents, or the normal sib. Lymphocytes from the A-T patients also were found to be 7-11 times more sensitive to the induction of chromatid aberrations by X-irradiation than control cells. Lymphocytes from the parents and normal sib showed a moderately increased frequency of X-ray induced aberrations compared with that of the control.

Identification of ataxia telangiectasia heterozygotes by flow cytometric analysis of X-ray damage

Flow cytometry was used to identify heterozygotes for the autosomal recessive DNA-repair deficiency disease ataxia telangiectasia (AT). Confluent G0/G1 fibroblasts from 4 homozygotes (at/at), 5 obligate heterozygotes (at/+) and 7 presumed normal controls (+/+) were X-irradiated with 200 Rad and subcultured immediately in medium containing 5-bromodeoxyuridine (BrdU). Cells were harvested 72 h later and stained with fluoresceinated anti-BrdU antibody to identify cells that had entered S phase. They were counterstained with propidium iodide to measure total DNA content. On the basis of relative release from G0/G1, the at/+ strains as a group (33 +/- 3% release) were distinguished from both the presumed +/+ strains (60 +/- 3%) and at/at strains (85 +/- 3%), although the individual values for some strains did show overlap between genotypes. When 10 cell strains were coded and analyzed in 'blind' experiments, all 4 heterozygotes were correctly assigned, although one poorly growing presumed normal line was incorrectly assigned as a heterozygote. By a similar assay in which exponentially growing cultures were pulsed briefly with BrdU 8 h after irradiation with 400 Rad and then harvested immediately, presumed +/+ cells as a group could be distinguished from at/at cells but not from at/- cells. This combination of assays assists in the identification of all 3 AT genotypes. This should be of both basic and diagnostic use, particularly in families known to segregate AT.

Abnormality in DNA-protein binding in ataxia-telangiectasia nuclear extracts

Anomalies in DNA replication, repair and recombination in ataxia-telangiectasia (A-T) point to a defect in structure or function of chromatin. In this study we have compared DNA-protein binding in nuclear extracts from control and A-T cells using two assay systems, filter-binding and DNA-accessibility. Interestingly, the extent of DNA protein binding over a range of protein concentration was significantly lower in A-T extracts. In addition the accessibility of the restriction enzyme Eco R1 to protein-bound plasmid was greater when A-T extracts were used. This is in keeping with the reduced binding observed in the filter-binding assay.