Chromosome instability syndromes

ATM-dependent phosphorylation of the checkpoint clamp regulates repair pathways and maintains genomic stability

Upon genotoxic stress and during normal S phase, ATM phosphorylates the checkpoint clamp protein Rad9 in a manner that depends on Ser272. Ser272 is the only known ATM-dependent phosphorylation site in human Rad9. However, Ser272 phosphorylation is not required for survival or checkpoint activation after DNA damage. The physiological function of Ser272 remains elusive. Here, we show that ATM-dependent Rad9(Ser272) phosphorylation requires the MRN complex and controls repair pathways. Furthermore, the mutant cells accumulate large numbers of chromosome breaks and induce gross chromosomal rearrangements. Our findings establish a new and unexpected role for ATM: it phosphorylates the checkpoint clamp in order to control repair pathways, thereby maintaining genomic integrity during unperturbed cell cycle and upon DNA damage.

Ataxia-telangiectasia in Iran: clinical and laboratory features of 104 patients

Ataxia-telangiectasia is a multisystem disorder characterized by progressive neurologic impairment, variable immunodeficiency, impaired organ maturation, x-ray hypersensitivity, oculocutaneous telangiectasia, and a predisposition to malignancy. To evaluate clinical and immunologic features of Iranian patients with ataxia-telangiectasia, the records of 104 patients with ataxia-telangiectasia (54 male, 50 female) with the age range of 1.6-23.5 years were reviewed. The Iranian Primary Immunodeficiency Registry was used as the data source. Progressive ataxia was seen in all the patients. Other symptoms were eye movement disorders (n = 84), slurred speech (n = 70), mental retardation (n = 10), and ocular (n = 87) and cutaneous (n = 73) telangiectasia. Three patients developed leukemia and lymphoma, and 17 patients had family history of malignancy. Positive correlation was seen between clinical immunologic symptoms and immunoglobulin deficiencies (P = 0.004). The predominant infections were sinopulmonary and acute and recurrent infections (78 cases). Infections included pneumonia (56 patients), otitis media (34 patients), and sinusitis (50 patients). Average serum alpha-fetoprotein level was 149 +/- 137 ng/dL. The incidence of ataxia-telangiectasia in Iran is high, possibly due to familial marriages. Treatment should be focused on supportive management to prolong survival.

Involvement of homologous recombination in carcinogenesis

DNA alterations of every type are associated with the incidence of carcinogenesis, often on the genomic scale. Although homologous recombination (HR) is an important pathway of DNA repair, evidence is accumulating that deleterious genomic rearrangements can result from HR. It therefore follows that HR events may play a causative role in carcinogenesis. HR is elevated in response to carcinogens. HR may also be increased or decreased when its upstream regulation is perturbed or components of the HR machinery itself are not fully functional. This chapter summarizes research findings that demonstrate an association between HR and carcinogenesis. Increased or decreased frequencies of HR have been found in cancer cells and cancer-prone hereditary human disorders characterized by mutations in genes playing a role in HR, such as ATM, Tp53, BRCA, BLM, and WRN genes. Another evidence linking perturbations in HR and carcinogenesis is provided by studies showing that exposure to carcinogens results in an increased frequency of HR resulting in DNA deletions in yeast, human cells, or mice.

Immunodeficiency, centromeric region instability, facial anomalies syndrome (ICF)

The Immunodeficiency, Centromeric region instability, Facial anomalies syndrome (ICF) is a rare autosomal recessive disease described in about 50 patients worldwide and characterized by immunodeficiency, although B cells are present, and by characteristic rearrangements in the vicinity of the centromeres (the juxtacentromeric heterochromatin) of chromosomes 1 and 16 and sometimes 9. Other variable symptoms of this probably under-diagnosed syndrome include mild facial dysmorphism, growth retardation, failure to thrive, and psychomotor retardation. Serum levels of IgG, IgM, IgE, and/or IgA are low, although the type of immunoglobulin deficiency is variable. Recurrent infections are the presenting symptom, usually in early childhood. ICF always involves limited hypomethylation of DNA and often arises from mutations in one of the DNA methyltransferase genes (DNMT3B). Much of this DNA hypomethylation is in 1qh, 9qh, and 16qh, regions that are the site of whole-arm deletions, chromatid and chromosome breaks, stretching (decondensation), and multiradial chromosome junctions in mitogen-stimulated lymphocytes. By an unknown mechanism, the DNMT3B deficiency that causes ICF interferes with lymphogenesis (at a step after class switching) or lymphocyte activation. With the identification of DNMT3B as the affected gene in a majority of ICF patients, prenatal diagnosis of ICF is possible. However, given the variety of DNMT3B mutations, a first-degree affected relative should first have both alleles of this gene sequenced. Treatment almost always includes regular infusions of immunoglobulins, mostly intravenously. Recently, bone marrow transplantation has been tried.

Molecular basis of ataxia telangiectasia and related diseases

Ataxia telangiectasia (AT) is a rare human disease characterized by extreme cellular sensitivity to radiation and a predisposition to cancer, with a hallmark of onset in early childhood. Several human diseases also share similar symptoms with AT albeit with different degrees of severity and different associated disorders. While all AT patients contain mutations in the AT-mutated gene (ATM), most other AT-like disorders are defective in genes encoding an MRN protein complex consisting of Mre11, Rad50 and Nbs1. Both ATM and MRN function as cellular sensors to DNA double-strand breaks, which lead to the recruitment and phosphorylation of an array of substrate proteins involved in DNA repair, apoptosis and cell-cycle checkpoints, as well as gene regulation, translation initiation and telomere maintenance. ATM is a member of the family of phosphatidylinositol 3-kinase-like protein kinases (PIKK), and the discovery of many ATM substrates provides the underlying mechanisms of heterologous symptoms among AT patients. This review article focuses on recent findings related to the initial recognition of double-strand breaks by ATM and MRN, as well as a DNA-dependent protein kinase complex consisting of the heterodimer Ku70/Ku80 and its catalytic subunit DNA-PKcs, another member of PIKK. This possible interaction suggests that a much greater complex is involved in sensing, transducing and co-ordinating cellular events in response to genome instability.

Controversial Cytogenetic Observations in Mammalian Somatic Cells Exposed to Radiofrequency Radiation

During the years 1990-2003 a large number of investigations were conducted using rodents, cultured rodent and human cells, and freshly collected human blood lymphocytes to determine the genotoxic potential of exposure to radiofrequency (RF) radiation. The results of most of these studies (58%) did not indicate increased damage to the genetic material (assessed from DNA strand breaks, incidence of chromosomal aberrations, micronuclei and sister chromatid exchanges) in cells exposed to RF radiation compared to sham-exposed and/or unexposed cells. Some investigations (23%) reported an increase in such damage in cells exposed to RF radiation. The observations from other studies (19%) were inconclusive. This paper reviews the investigations published in scientific journals during 1990-2003 and attempts to identify probable reason(s) for the conflicting results. Recommendations are made for future research to address some of the controversial observations.

Evidence that Unrejoined DNA Double-Strand Breaks are not Predominantly Responsible for Chromosomal Radiosensitivity of AT Fibroblasts

To examine more fully the nature of chromosomal radiosensitivity in ataxia telangiectasia (AT) cells, we employed 24-color combinatorial painting to visualize 137Cs gamma-ray-induced chromosome-type aberrations in cells of two AT and one normal primary human fibroblast strains irradiated in log-phase growth. As a measure of misrejoined radiation-induced DSBs, we quantified exchange breakpoints associated with both simple and complex exchanges. As a measure of unrejoined DSBs, we quantified breakpoints from terminal deletions as well as deletions associated with incomplete exchange. For each of these end points, the frequency of damage per unit dose was markedly higher in AT cells compared to normal cells, although the proportion of total breaks that remained unrejoined was rather similar. The majority of breakpoints in both cell types were involved in exchanges. AT cells had a much higher frequency of complex exchanges compared to normal cells given the same dose, but for doses that resulted in approximately the same level of total breakpoints, the relative contribution from complex damage was also similar. We conclude that although terminal deletions and incomplete exchanges contribute to AT cell radiosensitivity, their relative abundance does not-in apparent contrast to the situation in lymphoblastoid cells-overwhelmingly account for the increased damage we observed in cycling AT fibroblasts. Thus, from a cytogenetic perspective, a higher level of unrepaired DSBs does not provide a universal explanation for the radiation-sensitive AT phenotype.

Molecular cytogenetic parameters in fibroblasts of ataxia telangiectasia carrier

Ataxia telangiectasia (AT) is a pleiotropic and rare (1:40,000 to 1:100,000) recessive disease. Laboratory investigations have failed to detect any consistent anomaly in cells from AT heterozygotes. To estimate random aneuploidy, we applied a fluorescence in situ hybridization technique with alpha-satellite probes for chromosomes 8 and 9 and replication pattern for RB-1, HER-2/neu, and the imprinted SNRPN loci on primary AT carrier fibroblasts. Higher random aneuploidy was not found in the carrier fibroblasts compared to control amniocytic cells. The asynchrony pattern was higher in the AT carrier cells with the RB-1 locus (P=0.057) and significantly higher with the HER-2/neu locus (P < 0.001) compared to control cells. As for the imprinted locus SNRPN, there was a significantly lower asynchrony rate in the AT carriers (P < 10(-5)) compared to the control group. Molecular cytogenetic parameters of random aneuploidy and replication pattern may reflect predisposition for the development of cancer. It is possible that in some AT carriers the genetic instability phenomena associated with the abnormal replication pattern may represent their potential for developing malignancies.

Non-random distribution of spontaneous chromosome aberrations in two Bloom Syndrome patients

The distribution of breakpoints involved in spontaneous chromosome aberrations (CA) was analyzed in lymphocytes from a family with Bloom's Syndrome (BS) and 9 healthy individuals. Standard and G-banded metaphases from each individual were analyzed to allow the identification of the breakpoints involved in spontaneously occurring chromosome aberrations. A total of 85 breakpoints in BS patients, 17 in their parents and 35 in controls, could be exactly localized to specific chromosome bands. Breakpoint distribution was statistically analyzed considering the formula proposed by Brøgger (1977), showing a non-random pattern in BS patients. Thirteen bands non-randomly involved in spontaneous CA (p < 0.005) were recognized in BS, located at 1p36, 1q21, 1q32, 2q33, 3p24, 3p14, 3q27, 5q31, 6p21, 7q22, 9q13, 11q13, and 17q23. Only 1 band (1q21) was significantly implicated in both parents (p < 0.005), while controls showed a random distribution. BS non-random bands were correlated with the chromosomal location of fragile sites, oncogenes, and breakpoints involved in cancer rearrangements. A significant correlation with the location of fragile sites and cancer-breakpoints (p < 0.005), particularly with acute myeloid leukemia and malignant lymphomas rearrangements was found. These findings demonstrate that constitutional chromosome instability in BS might involve specific points, such as fragile sites and cancer breakpoints, suggesting an association with the increased incidence of cancer.

Spontaneous chromosome aberrations in Fanconi's anemia patients are located at fragile sites and acute myeloid leukemia breakpoints

Spontaneous chromosome aberrations (CA) were analyzed in 3 Fanconi's anemia (FA) patients, 8 family members, and 9 healthy individuals. Peripheral blood lymphocytes obtained from each individual were cultured and cytogenetic analysis was performed on standard and sequential G-banded metaphases. The numbers of abnormal cells and breaks were found to be higher in AF patients compared to the other groups (p < 0.0001). Breakpoint distribution was statistically analyzed considering the formula proposed by Brøgger (1977), showing a non-random pattern among FA patients but not among controls or relatives (p < 0.001). Five chromosomal bands located at 1p36, 1p22, 1q21, 3p14, and 3q21 were non-randomly involved in spontaneous CA in FA patients. These bands were correlated with the chromosomal location of fragile sites, oncogenes, and breakpoints involved in cancer-rearrangements. A significant correlation with the location of fragile sites (p < 0.03) and breakpoints involved in cancer-rearrangements (p < 0.001), particularly with AML chromosome anomalies (p < 0.03) was found, suggesting a possible relationship with the high predisposition to cancer observed in this disease.

Role of homologous recombination in carcinogenesis

Cancer develops when cells no longer follow their normal pattern of controlled growth. In the absence or disregard of such regulation, resulting from changes in their genetic makeup, these errant cells acquire a growth advantage, expanding into precancerous clones. Over the past decade many studies have revealed the relevance of genomic mutation in this process, be it by misreplication, environmental damage, or a deficiency in repairing endogenous and exogenous damage. Here we discuss the possibility of homologous recombination as an errant DNA repair mechanism that can result in loss of heterozygosity or genetic rearrangements. Some of these genetic alterations may play a primary role in carcinogenesis, but they are more likely to be involved in secondary and subsequent steps of carcinogenesis by which recessive oncogenic mutations are revealed. Patients, whose cells display an increased frequency of recombination, also have an elevated frequency of cancer, further supporting the link between recombination and carcinogenesis.

Genetic instability of specific chromosomes associated with a family history of cancer

This study evaluated the association between family history of cancer and bleomycin-induced mutagen breaks at specific chromosomes. The authors' hypothesis was that individuals exhibiting mutagen-induced specific chromatid breaks might have genetic instability and thus be more likely to report a family history of cancer. The study included 78 healthy individuals. All subjects completed a personal interview to collect epidemiologic information, including a detailed family history of cancer, and donated a 10-mL blood sample. Bleomycin-induced mutagen sensitivity on specific chromosomes was quantified by counting the bleomycin-induced specific chromosomal breaks with Q-banding techniques. We found that chromosome 4 breaks were significantly associated with a positive family history of cancer in first-degree relatives with an odds ratio of 3.18 and 95% confidence interval of 1.05-9.61. However, none of the other chromosomes showed significantly increased risk with family cancer history. In addition, the mutagen-induced chromosome 4 breaks were not associated with age, sex, ethnicity, or smoking status. These findings suggested that chromosome 4 mutagen sensitivity might be a predictor of familial susceptibility to cancer.

Homologous Recombination and Its Role in Carcinogenesis

Cancer develops when cells no longer follow their normal pattern of controlled growth. In the absence or disregard of such regulation, resulting from changes in their genetic makeup, these errant cells acquire a growth advantage, expanding into precancerous clones. Over the last decade, many studies have revealed the relevance of genomic mutation in this process, be it by misreplication, environmental damage, or a deficiency in repairing endogenous and exogenous damage. Here, we discuss homologous recombination as another mechanism that can result in a loss of heterozygosity or genetic rearrangements. Some of these genetic alterations may play a primary role in carcinogenesis, but they are more likely to be involved in secondary and subsequent steps of carcinogenesis by which recessive oncogenic mutations are revealed. Patients, whose cells display an increased frequency of recombination, also have an elevated frequency of cancer, further supporting the link between recombination and carcinogenesis.

Homologous recombination as a mechanism of carcinogenesis

Cancer develops when cells no longer follow their normal pattern of controlled growth. In the absence or disregard of such regulation, resulting from changes in their genetic makeup, these errant cells acquire a growth advantage, expanding into pre-cancerous clones. Over the last decade many studies have revealed the relevance of genomic mutation in this process, be it by misreplication, environmental damage or a deficiency in repairing endogenous and exogenous damage. Here we discuss homologous recombination as another mechanism that can result in loss of heterozygosity or genetic rearrangements. Some of these genetic alterations may play a primary role in carcinogenesis, but they are more likely to be involved in secondary and subsequent steps of carcinogenesis by which recessive oncogenic mutations are revealed. Patients whose cells display an increased frequency of recombination also have an elevated frequency of cancer, further supporting the link between recombination and carcinogenesis. In addition, homologous recombination is induced by a wide variety of carcinogens, many of which are classically considered to be efficiently repaired by other repair pathways. Overall, homologous recombination is a process that has been widely overlooked but may be more central to the process of carcinogenesis than previously described.

Two primary osteosarcomas in a patient with Rothmund-Thomson syndrome

Rothmund-Thomson syndrome is an autosomal recessive disorder characterized by poikilodermatous skin changes that develop in infancy. Associated manifestations include juvenile cataracts, sparse hair, short stature, skeletal defects, dystrophic nails and teeth, and hypogonadism. An increased incidence of malignancy, including osteosarcoma, has been reported in patients with Rothmund-Thomson syndrome. The molecular basis of the disorder is not known. This report describes a patient with Rothmund-Thomson syndrome in whom two primary osteosarcomas developed 12 years apart. The presentation, diagnosis, and treatment of osteosarcoma in this patient with Rothmund-Thomson syndrome are described. Cytogenetic and molecular analysis of peripheral blood and skin fibroblasts had low level mosaicism for trisomy of chromosomes 2 and 8. Although several patients have been described with mosaic trisomy 8 and i(2q) (mosaic isochromosome for the long arm of chromosome 2), the patient described here is the first to have mosaic trisomy for the entire chromosomes 2 and 8. The cytogenetic findings in this patient are consistent with an underlying defect in chromosomal stability.

Ataxia-telangiectasia, cancer and the pathobiology of the ATM gene

Ataxia-telangiectasia (A-T) is a pleiotropic inherited disease characterized by neurodegeneration, cancer, immunodeficiencies, radiation sensitivity, and genetic instability. Although A-T homozygotes are rare, the A-T gene may play a role in sporadic breast cancer and leukemia. ATM, the gene responsible for A-T, is homologous to several cell cycle checkpoint genes from other organisms. ATM is thought to play a crucial role in a signal transduction network that modulates cell cycle checkpoints, genetic recombination, apoptosis, and other cellular responses to DNA damage. New insights into the pathobiology of A-T have been provided by the creation of Atm-/- mice and by in vitro studies of ATM function. Analyses of ATM mutations in A-T patients and in sporadic tumors suggest the existence of two classes of ATM mutation: null mutations that lead to A-T and dominant negative missense mutations that may predispose to cancer in the heterozygous state.

Rothmund-Thomson syndrome: two case reports show heterogeneous cutaneous abnormalities, an association with genetically programmed ageing changes, and increased chromosomal radiosensitivity

Rothmund-Thomson syndrome is a rare, autosomal recessive disorder associated with characteristic cutaneous changes, sparse hair, juvenile cataracts, short stature, skeletal defects, dystrophic teeth and nails, and hypogonadism. Mental retardation is unusual. An increased incidence of certain malignancies has been reported. Clonal or mosaic chromosome abnormalities and abnormalities in DNA repair mechanisms have been reported in some cases. We report two cases of Rothmund-Thomson syndrome, both with intellectual handicap, associated in one with a previously undescribed histological appearance of involved skin, suggesting that the spectrum of abnormalities is even more heterogeneous than previously presumed. Both cases exhibited chromosomal radiosensitivity of lymphocytes which may be an indication of a DNA repair defect. This is the first report of an association between Rothmund-Thomson syndrome and unique, intrinsic, age related skin changes.

Increased micronucleus frequency in lymphocytes from smokers with lung cancer

We investigated whether lung cancer was associated with an increased micronucleus (MN) frequency in lymphocytes in a case-control study. Epidemiological data were obtained by an interviewer-administered questionnaire and included information on smoking history, intake of dietary micronutrients, general medical history, environmental and occupational exposures to mutagens and carcinogens, and family history of cancer. A modified cytokinesis-block method was used to determine individual MN frequency. Polymorphisms in glutathione S-transferase class mu were determined by PCR analysis. Overall, 55 controls and 42 cases were studied. MN frequency in cases and controls was not associated with age, smoking, metabolic genetic polymorphisms, environmental and occupational exposures, or medical history. Female controls had a significantly higher MN frequency than male controls (p = 0.05). Overall, MN frequency was significantly higher in cases than in controls (p < 0.01). Twenty-four cases (57%) had an MN frequency higher than the upper 95% confidence interval of the mean value for controls (11.5 MNs/1000 binucleated cells). Further analysis showed that, cases who were current and former smokers had significantly higher MN frequencies than controls (p = 0.04); this difference was not seen in the group that had never smoked. The significantly higher MN frequency among cases with a history of smoking may be attributable to the presence of lung neoplasm per se or to the interaction of smoking with endogenous factors associated with the development of lung cancer.

Cell cycle checkpoints and DNA repair preserve the stability of the human genome

Chemical carcinogenesis in the regenerating rat liver is cell-cycle-dependent. Proliferating hepatocytes were maximally susceptible to initiation by a single dose of benzo[a]pyrene diolepoxide I when at the G1/S border. Hepatocytes in early G1 or late S/G2/M were less susceptible and non-proliferating G0 hepatocytes were resistant to initiation. Radiation clastogenesis in proliferating human fibroblasts also is cell-cycle-dependent. Ultraviolet radiation (UV) induced maximal frequencies of chromosomal aberrations in synchronized cells that were at the G1/S border. Cells in early G1 or G2 were significantly less sensitive. For both initiation of chemical carcinogenesis and UV-clastogenesis, it appears that replication of damaged DNA is required and DNA repair before replication reduces cellular risk. If DNA repair is protective, cell cycle checkpoints which delay DNA replication and mitosis should augment this protective influence by providing more time for repair. The contribution of cell cycle checkpoint function to DNA repair during cell cycle-dependent clastogenesis was studied using ataxia telangiectasia (AT) fibroblasts. The AT cells displayed a defect in the coupling of DNA damage to checkpoints which control the G1/S and G2/M transitions and the rate of replicon initiation in S phase cells. UV-clastogenesis in AT cells was cell-cycle-dependent with irradiation at the G1/S boundary inducing 3-times more aberrations than treatment in G0 at the time of release into the cell cycle. Thus, DNA excision repair during the pre-replicative G1 phase was protective even in cells with defective checkpoint function. However, following irradiation at the G1/S border, AT cells displayed about 6-fold increased levels of UV-induced chromosome aberrations in comparison to normal human fibroblasts that were treated at this time. These observations indicate that secondary and tertiary DNA lesions that are produced during replication of UV-damaged DNA (replicative gaps and double-strand breaks) also depend on checkpoint function for repair. The replicon initiation and G2-delay checkpoints that operate after initiation of S phase appear to play a major role in protection against UV-clastogenesis.

Structural instability of a transmissible end-to-end dicentric chromosome in a xeroderma pigmentosum fibroblast clone

Cytogenetic analysis was performed in a fibroblast clone (XP9UV25) selected for anchorage-independent growth from an XP strain of normal origin and characterized by the presence of clonal chromosome rearrangements. A dicentric chromosome involving the 5p and 16q telomeric regions was observed in XP9UV25 cells at the fifth passage from colony isolation and at successive passages. The specific anomaly was present with increasing frequency (from 22 to 60% of mitoses) during culture propagation, undergoing rearrangements that gave rise to: 1) (5;16) dicentrics with deletions or duplications of the intercentromeric region; 2) homodicentrics for chromosomes 5 or 16, either end-to-end associations or rearranged; and 3) derivative 5p+ and 16q+ monocentric chromosomes. The frequency of other anomalies involving other chromosomes was negligible. These findings represent the first demonstration that a telomeric association leads to a variety of balanced and unbalanced chromosome rearrangements. These rearrangements may result from asymmetric interchanges between sister chromatids, "bridge-breakage-fusion" events during cell division, breakage and reunion of isochromatids, and breakage followed by healing of the ends. The type of anomaly and the sequence of karyotypic changes we observed in the XP9UV25 clone and their mechanisms of origin may be the same as those occurring during transformation from diploidy to aneuploidy in neoplastic cells.

Spontaneous chromosomal aberrations in cell cultures from patients with neurofibromatosis 1

Neurofibroma-derived cell cultures, skin fibroblast strains from NF1 patients, melanocyte cultures from café-au-lait spots and melanocytes from skin overlying neurofibroma were investigated with regard to the frequencies of spontaneous chromosomal aberrations. A 3.4-fold increased rate of stable and unstable chromosome aberrations in 34 neurofibroma cultures of 18 NF1 patients was noticed in comparison to 17 skin biopsy-derived cultures of healthy probands. Fibroblast strains from the unaffected skin of nine NF1 patients revealed a 2.6-fold higher rate of chromosome breakage compared with the control cultures. Likewise, an increase of spontaneous chromosomal instability by a factor of 13.5 was found in cultured melanocytes from café-au-lait spots and by a factor of 11.9 in the skin-melanocyte cultures of six NF1 patients in comparison to foreskin-derived melanocyte strains of five unaffected persons. Analyses of the distribution of chromatid and chromosome breaks along the chromosomes revealed a significant clustering of these events in the centromeric regions specifically in the four kinds of NF1-derived cultures.

Chromosome instability in lymphocytes from patients with celiac disease

Cytogenetic studies were performed in celiac disease (CD) patients to determine if the presence of chromosome instability is related to the predisposition to cancer. Chromosome aberrations (CA) and sister chromatid exchange (SCE) frequencies in peripheral blood lymphocyte cultures from untreated CD patients and healthy controls were analyzed. Patients showed aberrations in 23% of cells, while only 3% were detected in the control group (p < 0.0001). The mean frequencies of gaps, breaks and total CA were found to be higher in CD patients compared to controls (p < 0.0001). Breakpoint distribution was nonrandom among chromosomes from celiac patients (p = 0.01), but not among controls (p = 0.04). The frequency of SCE/cell showed a mean value of 6.9 +/- 0.6 in CD patients and 7.3 +/- 0.2 in controls. No statistical differences were found. Breakpoints involved in CD patients presented a strong coincidence with the location of fragile sites (78.6%) and sites of cancer chromosome rearrangements (57.1%), most of them (75%) associated with malignant non-Hodgkin lymphomas. These results suggest that CD is a condition with increased chromosome instability characterized by a high level of CA and normal SCE frequencies, probably related to the increased incidence of cancer.

G1 arrest and cell-cycle-dependent clastogenesis in UV-irradiated human fibroblasts

The demonstrations of frequent allelic deletions in lung and colon cancers have reemphasized the importance of clastogenesis in carcinogenesis. We have investigated the mechanisms of induction of chromosome aberrations in ultraviolet-irradiated diploid human fibroblasts. Cells were irradiated with UV at various times during a parasynchronous wave of cell proliferation and then harvested during the first mitosis that followed irradiation. Metaphase spreads were stained with Geimsa and the yields of chromosome aberrations were quantified. Ultraviolet irradiation induced primarily chromatid-type chromosome aberrations which included chromatid breaks and exchanges. Frequencies of aberrations displayed significant differences according to the phase of the cell cycle in which irradiation occurred and the time after irradiation when metaphases were harvested. Fibroblasts that were irradiated when in G0 and then immediately replated to stimulate cell division and cells that were at the S/G2 border when irradiated displayed the fewest numbers of aberrations. For G0-irradiated cells, the first entering mitosis carried a higher frequency of aberrations than those collected 2-4 h later. In contrast, for S/G2-irradiated cells the first into mitosis displayed fewer aberrations than subsequent fractions. Cells that were irradiated when at the G1/S border displayed the greatest numbers of aberrations with the frequencies of chromatic exchanges being significantly increased over all other times of irradiation. These studies confirm that UV is an S-phase-dependent clastogen and point to the G1/S border as a time of maximal sensitivity to clastogenesis. Irradiation of G1 cells was shown to produce a fluence-dependent reduction in the rate of entry of cells into the S-phase. There appeared to be a point late in G1 beyond which cells were resistant to irradiation and experienced less delay in S phase entry. Ataxia telangiectasia fibroblasts failed to delay entry to S phase following UV-irradiation in G1 and displayed hypersensitivity to UV-induced chromosomal aberrations. The delay in entry of damaged cells into the S phase may have the beneficial effect of providing more time for repair of potentially clastogenic DNA damage before the onset of DNA replication.

Ataxia-telangiectasia. Review of the literature and a case report

Cutaneous vascular abnormalities are frequently detected on initial examination of newborns. Many of these lesions are common variations of normal lesions such as nevus simplex and strawberry hemangiomas. Some of the vascular abnormalities, however, are a feature of a number of syndromes with multisystemic involvement. These syndromes have been described under the heading of neurocutaneous diseases. Ataxia-telangiectasia is a neurocutaneous syndrome that appears with progressive cerebellar ataxia, oculocutaneous telangiectasias, and abnormalities of many other organs. Oral mucosa is also affected. Current concepts on the pathogenesis of ataxia-telangiectasia and one case of the disease are presented in this article.

Creative blocks: cell-cycle checkpoints and feedback controls

Before division, cells must ensure that they finish DNA replication, DNA repair and chromosome segregation. They do so by using feedback controls which can detect the failure to complete replication, repair or spindle assembly to arrest the progress of the cell cycle at one of three checkpoints. Failures in feedback controls can contribute to the generation of cancer.

Ataxia-telangiectasia: linkage analysis in highly inbred Arab and Druze families and differentiation from an ataxia-microcephaly-cataract syndrome

Ataxia-telangiectasia (A-T) is a progressive autosomal recessive disease featuring neurodegeneration, immunodeficiency, chromosomal instability, radiation sensitivity and a highly increased proneness to cancer. A-T is ethnically widespread and genetically heterogeneous, as indicated by the existence of four complementation groups in this disease. Several "A-T-like" genetic diseases share various clinical and cellular characteristics with A-T. By using linkage analysis to study North American and Turkish A-T families, the ATA (A-T, complementation group A) gene has been mapped to chromosome 11q23. A number of Israeli Arab A-T patients coming from large, highly inbred families were assigned to group A. In one of these families, an additional autosomal recessive disease was identified, characterized by ataxia, hypotonia, microcephaly and bilateral congenital cataracts. In two patients with this syndrome, normal levels of serum immunoglobulins and alpha-fetoprotein, chromosomal stability in peripheral blood lymphocytes and skin fibroblasts, and normal cellular response to treatments with X-rays and the radiomimetic drug neocarzinostatin indicated that this disease does not share, with A-T, any additional features other than ataxia. These tests also showed that another patient in this family, who is also mentally retarded, is affected with both disorders. This conclusion was further supported by linkage analysis with 11q23 markers. Lod scores between A-T and these markers, cumulated over three large Arab families, were significant and confirmed the localization of the ATA gene to 11q23. However, another Druze family unassigned to a specific complementation group, showed several recombinants between A-T and the same markers, leaving the localization of the A-T gene in this family open.

Genetic and environmental modulations of chromosomal stability: their roles in aging and oncogenesis

The primary objective of this review is to suggest a major role of cytogenetic pathology in the genesis of both the neoplastic phenotype and the senescent phenotype of aging mammalian organisms. It is hypothesized that allelic variation at a number of genetic loci that have the potential to modulate various types of chromosomal mutation could account, in part, for species-specific variations in maximum life-span potentials and the times of onset of characteristic age-related neoplasms. As a corollary to this proposition, attention is directed to the potential importance of environmental clastogens and aneugens as both oncogenic and "gerontogenic" agents. By way of introduction, a series of definitions of our subject material is given (aging, oncogenesis, proliferative homeostasis, point mutation, chromosomal mutation, epimutation, mutagens, clastogens, aneugens, gerontogens). There follows a defense of the view that there is a tight coupling of abnormalities in proliferative homeostasis (including atrophies, hyperplasias, benign neoplasias and malignant neoplasias) with intrinsic biological aging. This view differs from those of other authors who either: 1) believe that, to qualify as a bona fide component of aging, the phenotype in question (in this case, neoplasia) must be "genetically programmed" or 2) insist that the rate of development of the neoplastic phenotype should parallel species-specific Gompertz kinetics for the survival of the population of organisms. After briefly reviewing the classification of chromosomal lesions, we consider evidence for constitutional genetic determinants of chromosomal stability, oncogenesis, and some other aspects of the senescent phenotype, both with respect to interspecific and intraspecific differences. The discussion will include the recent evidence characterizing a human segmental progeroid syndrome (Werner's syndrome) as a deletor mutator strain. A summary of research on environmental chemical clastogens and aneugens is given, showing both discordances and concordances of such assays with assays for point mutagens. An analysis of the literature indicates that there is no statistical evidence for the positive correlations when substances showing effects only at comparatively high concentrations (greater than 1 mM) are excluded from the analysis. Brief mention is also made of the roles of viral agents and ionizing radiation in the genesis of chromosomal mutations. Finally, some possible pathogenetic mechanisms common to chromosomal mutagenicity, oncogenicity and gerontogenicity are considered, including chemical free radicals (active oxygen species), DNA transposition, DNA amplification, DNA glycation, virally induced cell fusion, posttranslational modifications of centromeric and mitotic spindle proteins, and alterations in the metabolism of telomeric DNA.