Hypersensitivity to ionizing radiation, in vitro, in a new chromosomal breakage disorder, the Nijmegen Breakage Syndrome

Investigating Molecular Determinants of Cancer Cell Resistance to Ionizing Radiation Through an Integrative Bioinformatics Approach

Eradication of cancer cells through exposure to high doses of ionizing radiation (IR) is a widely used therapeutic strategy in the clinical setting. However, in many cases, cancer cells can develop remarkable resistance to radiation. Radioresistance represents a prominent obstacle in the effective treatment of cancer. Therefore, elucidation of the molecular mechanisms and pathways related to radioresistance in cancer cells is of paramount importance. In the present study, an integrative bioinformatics approach was applied to three publicly available RNA sequencing and microarray transcriptome datasets of human cancer cells of different tissue origins treated with ionizing radiation. These data were investigated in order to identify genes with a significantly altered expression between radioresistant and corresponding radiosensitive cancer cells. Through rigorous statistical and biological analyses, 36 genes were identified as potential biomarkers of radioresistance. These genes, which are primarily implicated in DNA damage repair, oxidative stress, cell pro-survival, and apoptotic pathways, could serve as potential diagnostic/prognostic markers cancer cell resistance to radiation treatment, as well as for therapy outcome and cancer patient survival. In addition, our findings could be potentially utilized in the laboratory and clinical setting for enhancing cancer cell susceptibility to radiation therapy protocols.

The Nucleoshuttling of the ATM Protein: A Unified Model to Describe the Individual Response to High- and Low-Dose of Radiation?

The evaluation of radiation-induced (RI) risks is of medical, scientific, and societal interest. However, despite considerable efforts, there is neither consensual mechanistic models nor predictive assays for describing the three major RI effects, namely radiosensitivity, radiosusceptibility, and radiodegeneration. Interestingly, the ataxia telangiectasia mutated (ATM) protein is a major stress response factor involved in the DNA repair and signaling that appears upstream most of pathways involved in the three precited RI effects. The rate of the RI ATM nucleoshuttling (RIANS) was shown to be a good predictor of radiosensitivity. In the frame of the RIANS model, irradiation triggers the monomerization of cytoplasmic ATM dimers, which allows ATM monomers to diffuse in nucleus. The nuclear ATM monomers phosphorylate the H2AX histones, which triggers the recognition of DNA double-strand breaks and their repair. The RIANS model has made it possible to define three subgroups of radiosensitivity and provided a relevant explanation for the radiosensitivity observed in syndromes caused by mutated cytoplasmic proteins. Interestingly, hyper-radiosensitivity to a low dose and adaptive response phenomena may be also explained by the RIANS model. In this review, the relevance of the RIANS model to describe several features of the individual response to radiation was discussed.

The Lowest Radiation Dose Having Molecular Changes in the Living Body

We herein attempted to identify the lowest radiation dose causing molecular changes in the living body. We investigated the effects of radiation in human cells, animals, and humans. DNA double-strand breaks (DSBs) formed in cells at γ- or X-ray irradiation doses between 1 mGy and 0.5 Gy; however, the extent of DSB formation differed depending on the cell species. The formation of micronuclei (MNs) and nucleoplasmic bridges (NPBs) was noted at radiation doses between 0.1 and 0.2 Gy. Stress-responsive genes were upregulated by lower radiation doses than those that induced DNA DSBs or MN and NPBs. These γ- or X-ray radiation doses ranged between approximately 10 and 50 mGy. In animals, chromosomal aberrations were detected between 50 mGy and 0.1 Gy of low linear energy transfer radiation, 0.1 Gy of metal ion beams, and 9 mGy of fast neutrons. In humans, DNA damage has been observed in children who underwent computed tomography scans with an estimated blood radiation dose as low as 0.15 mGy shortly after examination. The frequencies of chromosomal translocations were lower in residents of high background areas than in those of control areas. In humans, systemic adaptive responses may have been prominently expressed at these radiation doses.

A Multiplexed Mass Spectrometry-Based Assay for Robust Quantification of Phosphosignaling in Response to DNA Damage

A lack of analytically robust and multiplexed assays has hampered studies of the large, branched phosphosignaling network responsive to DNA damage. To address this need, we developed and fully analytically characterized a 62-plex assay quantifying protein expression and post-translational modification (phosphorylation and ubiquitination) after induction of DNA damage. The linear range was over 3 orders of magnitude, the median inter-assay variability was 10% CV and the vast majority (∼85%) of assays were stable after extended storage. The multiplexed assay was applied in proof-of-principle studies to quantify signaling after exposure to genotoxic stress (ionizing radiation and 4-nitroquinoline 1-oxide) in immortalized cell lines and primary human cells. The effects of genomic variants and pharmacologic kinase inhibition (ATM/ATR) were profiled using the assay. This study demonstrates the utility of a quantitative multiplexed assay for studying cellular signaling dynamics, and the potential application to studies on inter-individual variation in the radiation response.

A New Era of Low-Dose Radiation Epidemiology

The last decade has introduced a new era of epidemiologic studies of low-dose radiation facilitated by electronic record linkage and pooling of cohorts that allow for more direct and powerful assessments of cancer and other stochastic effects at doses below 100 mGy. Such studies have provided additional evidence regarding the risks of cancer, particularly leukemia, associated with lower-dose radiation exposures from medical, environmental, and occupational radiation sources, and have questioned the previous findings with regard to possible thresholds for cardiovascular disease and cataracts. Integrated analysis of next generation genomic and epigenetic sequencing of germline and somatic tissues could soon propel our understanding further regarding disease risk thresholds, radiosensitivity of population subgroups and individuals, and the mechanisms of radiation carcinogenesis. These advances in low-dose radiation epidemiology are critical to our understanding of chronic disease risks from the burgeoning use of newer and emerging medical imaging technologies, and the continued potential threat of nuclear power plant accidents or other radiological emergencies.

NBN phosphorylation regulates the accumulation of MRN and ATM at sites of DNA double-strand breaks

In response to ionizing radiation, the MRE11/RAD50/NBN (MRN) complex re-distributes to the sites of DNA double strand breaks (DSBs) where each of its individual components is phosphorylated by the serine-threonine kinase, ATM. ATM phosphorylation of NBN is required for activation of the S-phase checkpoint, but the mechanism whereby these phosphorylation events signal the checkpoint machinery remains unexplained. Here, we describe the use of direct protein transduction of the homing endonuclease, I-PpoI, into human cells to generate site-specific DSBs. Direct transduction of I-PpoI protein results in rapid accumulation and turnover of the endonuclease in live cells, facilitating comparisons across multiple cell lines. We demonstrate the utility of this system by introducing I-PpoI into isogenic cell lines carrying mutations at the ATM phosphorylation sites in NBN and assaying the effects of these mutations on the spatial distribution and temporal accumulation of NBN and ATM at DSBs by chromatin immunoprecipitation, as well as timing and extent of DSB repair. Although the spatial distribution of NBN and ATM recruited to the sites of DSBs was comparable between control cells and those expressing phosphorylation mutants of NBN, the timing of accumulation of NBN and ATM was altered. Serine to alanine mutations that blocked phosphorylation resulted in delayed recruitment of both NBN and ATM to DSBs. Serine to glutamic acid substitutions that mimicked the phosphorylation event resulted in both increased and prolonged accumulation of both NBN and ATM at DSBs. The repair of DSBs in cells lacking full-length NBN was significantly delayed compared to control cells while blocking phosphorylation of NBN resulted in a more modest delay in repair. These data indicate that following the induction of DSBs, phosphorylation of NBN regulates its accumulation, and that of ATM, at sites of DNA DSB as well as the timing of the repair of these sites.

Sarcoma risk after radiation exposure

Sarcomas were one of the first solid cancers to be linked to ionizing radiation exposure. We reviewed the current evidence on this relationship, focusing particularly on the studies that had individual estimates of radiation doses. There is clear evidence of an increased risk of both bone and soft tissue sarcomas after high-dose fractionated radiation exposure (10 + Gy) in childhood, and the risk increases approximately linearly in dose, at least up to 40 Gy. There are few studies available of sarcoma after radiotherapy in adulthood for cancer, but data from cancer registries and studies of treatment for benign conditions confirm that the risk of sarcoma is also increased in this age-group after fractionated high-dose exposure. New findings from the long-term follow-up of the Japanese atomic bomb survivors suggest, for the first time, that sarcomas can be induced by acute lower-doses of radiation (<5 Gy) at any age, and the magnitude of the risk is similar to that observed for other solid cancers. While there is evidence that individuals with certain rare familial genetic syndromes predisposing to sarcoma, particularly Nijmegen Breakage Syndrome, are particularly sensitive to the effects of high dose radiation, it is unclear whether this is also true in very low-dose settings (<0.1 Gy). The effects of common low-penetrance alleles on radiosensitivity in the general population have not been well-characterized. Some evidence suggests that it may be possible to identify radiation-induced sarcomas by a distinct molecular signature, but this work needs to be replicated in several dose settings, and the potential role of chemotherapy and tumor heterogeneity needs to be examined in more detail. In summary, radiation exposure remains one of the few established risk factors for both bone and soft tissue sarcomas. Similar to many other cancers children have the highest risks of developing a radiation-related sarcoma. Efforts to limit unnecessary high-dose radiation exposure, particularly in children, therefore remain important given the high fatality rates associated with this disease.

Nijmegen breakage syndrome (NBS)

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive syndrome of chromosomal instability mainly characterized by microcephaly at birth, combined immunodeficiency and predisposition to malignancies. Due to a founder mutation in the underlying NBN gene (c.657_661del5) the disease is encountered most frequently among Slavic populations. The principal clinical manifestations of the syndrome are: microcephaly, present at birth and progressive with age, dysmorphic facial features, mild growth retardation, mild-to-moderate intellectual disability, and, in females, hypergonadotropic hypogonadism. Combined cellular and humoral immunodeficiency with recurrent sinopulmonary infections, a strong predisposition to develop malignancies (predominantly of lymphoid origin) and radiosensitivity are other integral manifestations of the syndrome. The NBN gene codes for nibrin which, as part of a DNA repair complex, plays a critical nuclear role wherever double-stranded DNA ends occur, either physiologically or as a result of mutagenic exposure. Laboratory findings include: (1) spontaneous chromosomal breakage in peripheral T lymphocytes with rearrangements preferentially involving chromosomes 7 and 14, (2) sensitivity to ionizing radiation or radiomimetics as demonstrated in vitro by cytogenetic methods or by colony survival assay, (3) radioresistant DNA synthesis, (4) biallelic hypomorphic mutations in the NBN gene, and (5) absence of full-length nibrin protein. Microcephaly and immunodeficiency are common to DNA ligase IV deficiency (LIG4 syndrome) and severe combined immunodeficiency with microcephaly, growth retardation, and sensitivity to ionizing radiation due to NHEJ1 deficiency (NHEJ1 syndrome). In fact, NBS was most commonly confused with Fanconi anaemia and LIG4 syndrome. Genetic counselling should inform parents of an affected child of the 25% risk for further children to be affected. Prenatal molecular genetic diagnosis is possible if disease-causing mutations in both alleles of the NBN gene are known. No specific therapy is available for NBS, however, hematopoietic stem cell transplantation may be one option for some patients. Prognosis is generally poor due to the extremely high rate of malignancies.

Zespół Nijmegen (Nijmegen breakage syndrome; NBS) jest rzadkim schorzeniem z wrodzoną niestabilnością chromosomową dziedziczącym się w sposób autosomalny recesywny, charakteryzującym się przede wszystkim wrodzonym małogłowiem, złożonymi niedoborami odporności i predyspozycją do rozwoju nowotworów.

Choroba występuje najczęściej w populacjach słowiańskich, w których uwarunkowana jest mutacją założycielską w genie NBN (c.657_661del5). Do najważniejszych objawów zespołu zalicza się: małogłowie obecne od urodzenia i postępujące z wiekiem, charakterystyczne cechy dysmorfii twarzy, opóźnienie wzrastania, niepełnosprawność intelektualną w stopniu lekkim do umiarkowanego oraz hipogonadyzm hipogonadotropowy u dziewcząt. Na obraz choroby składają się także: niedobór odporności komórkowej i humoralnej, który jest przyczyną nawracających infekcji, znaczna predyspozycja do rozwoju nowotworów złośliwych (zwłaszcza układu chłonnego), a także zwiększona wrażliwość na promieniowanie jonizujące. Wyniki badań laboratoryjnych wykazują: (1) spontaniczną łamliwość chromosomów w limfocytach T krwi obwodowej, z preferencją do rearanżacji chromosomów 7 i 14, (2) nadwrażliwość na promieniowanie jonizujące lub radiomimetyki, co można wykazać metodami in vitro, (3) radiooporność syntezy DNA, (4) hipomorficzne mutacje na obu allelach genu NBN, oraz (5) brak w komórkach pełnej cząsteczki białka, nibryny. Małogłowie i niedobór odporności występują także w zespole niedoboru ligazy IV (LIG4) oraz w zespole niedoboru NHEJ1. Rodzice powinni otrzymać poradę genetyczną ze względu na wysokie ryzyko (25%) powtórzenia się choroby u kolejnego potomstwa. Możliwe jest zaproponowanie molekularnej diagnostyki prenatalnej jeżeli znane są obie mutacje będące przyczyną choroby. Nie ma możliwości zaproponowania specyficznej terapii, ale przeszczep szpiku może być alternatywą dla niektórych pacjentów. Generalnie prognoza nie jest pomyślna z uwagi na wysokie ryzyko rozwoju nowotworu.

Clinical radiation sensitivity with DNA repair disorders: an overview

Adverse reactions to radiotherapy represent a confounding phenomenon in radiation oncology. These reactions are rare, and many have been associated with individuals with DNA repair disorders such as ataxia-telangiectasia and Nijmegen Breakage syndrome. A paucity of published data is available detailing such circumstances. This overview describes four exemplary situations, a comprehensive list of 32 additional cases, and some insights gleaned from this overall experience. Fanconi anemia was associated with more than one-half of the reports. The lowest dose given to a patient that resulted in a reaction was 3 Gy, given to an ataxia-telangiectasia patient. Most patients died within months of exposure. It is clear that the patients discussed in this report had complicated illnesses, in addition to cancer, and the radiotherapy administered was most likely their best option. However, the underlying DNA repair defects make conventional radiation doses dangerous. Our findings support previous wisdom that radiotherapy should either be avoided or the doses should be selected with great care in the case of these radiosensitive genotypes, which must be recognized by their characteristic phenotypes, until more rapid, reliable, and functional assays of DNA repair become available.

Division of labor: DNA repair and the cell cycle specific functions of the Mre11 complex

Genomic integrity is maintained via the concerted action of proteins that coordinate and control DNA replication and those that respond to DNA damage. The Mre11 complex is a mediator of the DNA damage response through its functions in DNA double strand break (DSB) sensing, checkpoint activation and recombinational DNA repair. The complex responds to mitotic and meiotic DSBs, and is also activated in cells experiencing DNA replication stress. The Mre11 complex's role in recombinational repair primarily concerns the promotion of homologous recombination (HR), but it is also implicated in non-homologous end joining (NHEJ)--a DSB repair mechanism prevalent in non-dividing cells. We recently characterized deletion of the Mre11 complex member, Rad50, in a number of postmitotic and proliferative tissues of the mouse. These studies indicated that the complex is dispensable in postmitotic tissues, but loss of Rad50 in proliferating cells resulted in accumulation of unrepaired, DNA replication-dependent lesions. The data suggest that the Mre11 complex is not a major contributor to NHEJ and support the interpretation that its role in recombinational DNA repair is largely restricted to dividing cells, in which repair involving sister chromatids predominates. An exception to this concept is manifest in previous work from our laboratory revealing that the mammalian Mre11 complex promotes meiotic DSB repair, an event involving recombination between sister chromatids of homologous chromosomes and taking place in cells not undergoing replication. Together these studies highlight the importance of cell cycle and cell type specific modulation of the Mre11 complex's repair activities in vivo.

NBS1 prevents chromatid-type aberrations through ATM-dependent interactions with SMC1

Nijmegen breakage syndrome shares several common cellular features with ataxia telangiectasia, including chromosomal instability and aberrant S- and G2-phase checkpoint regulation. We show here that after irradiation, NBS1 interacts physically with both BRCA1 and SMC1, a component of the cohesin complex, and that their interactions are completely abolished in AT cells. It is noted that BRCA1 is required for the interaction of NBS1 with SMC1, whereas the reverse is not the case, since BRCA1 is able to bind to NBS1 in the absence of an NBS1/SMC1 interaction as observed in MRE11- or RAD50-deficient cells. This indicates that ATM and BRCA1 are upstream of the NBS1/SMC1 interaction. Furthermore, the interaction of NBS1 with SMC1 requires both conserved domains of NBS in the N-terminus and the C-terminus, since they are indispensable for binding of NBS1 to BRCA1 and to MRE11/ATM, respectively. The interaction of NBS1 with SMC1 and the resulting phosphorylation are compromised in the clones lacking either the N- or C-terminus of NBS1, and as a consequence, chromatid-type aberrations are enhanced after irradiation. Our results reveal that ATM plays a fundamental role in promoting the radiation-induced interaction of NBS1 with SMC1 in the presence of BRCA1, leading to the maintenance of chromosomal integrity.

ATM and the Mre11-Rad50-Nbs1 complex respond to nucleoside analogue-induced stalled replication forks and contribute to drug resistance

The Mre11-Rad50-Nbs1 complex and autophosphorylated Ser(1981)-ATM are involved in recognizing and repairing DNA damage, such as double-strand breaks (DSB). However, the role of these factors in response to stalled replication forks is not clear. Nucleoside analogues are agents that are incorporated into DNA during replication, which cause stalling of replication forks. The molecular mechanisms that sense these events may signal for DNA repair and contribute to survival but are poorly understood. Cellular responses to both DSBs and stalled replication forks are marked by H2AX phosphorylation on Ser(139) (gamma-H2AX), which forms nuclear foci at sites of DNA damage. Here, concentrations of the nucleoside analogues 1-beta-d-arabinofuranosylcytosine (cytarabine; ara-C), gemcitabine, and troxacitabine, which inhibited DNA synthesis by 90% within 2 hours, were determined for each agent. Using gamma-H2AX as a marker for changes in chromatin structure, we show that Mre11, Rad50, Nbs1, and phosphorylated ATM respond to nucleoside analogue-induced stalled replication forks by forming nuclear foci that colocalize with gamma-H2AX within 2 hours. Because neither DSBs nor single-strand breaks were detectable after nucleoside analogue exposure, we conclude that this molecular response is not due to the presence of DNA breaks. Deficiencies in ATM, Mre11, or Rad50 led to a 2- to 5-fold increase in clonogenic sensitization to gemcitabine, whereas Nbs1 and H2AX deficiency did not affect reproductive growth. Taken together, these results suggest that ATM, Mre11, and Rad50 are required for survival after replication fork stalling, whereas Nbs1 and H2AX are inconsequential.

E2F1 induces MRN foci formation and a cell cycle checkpoint response in human fibroblasts

Deregulation of the Rb/E2F pathway in human fibroblasts results in an E2F1-mediated apoptosis dependent on Atm, Nbs1, Chk2 and p53. Here, we show that E2F1 expression results in MRN foci formation, which is independent of the Nbs1 interacting region and the DNA-binding domain of E2F1. E2F1-induced MRN foci are similar to irradiation-induced foci (IRIF) that result from double-strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci, do not form in NBS cells, do form in AT cells and do not correlate with cell cycle entry. In fact, we find that in human fibroblasts deregulated E2F1 causes a G1 arrest, blocking serum-induced cell cycle progression, in part through an Nbs1/53BP1/p53/p21(WAF1/CIP1) checkpoint pathway. This checkpoint protects against apoptosis because depletion of 53BP1 or p21(WAF1/CIP1) increases both the rate and extent of apoptosis. Nbs1 and p53 contribute to both checkpoint and apoptosis pathways. These results suggest that E2F1-induced foci generate a cell cycle checkpoint that, with sustained E2F1 activity, eventually yields to apoptosis. Uncontrolled proliferation due to Rb/E2F deregulation as well as inactivation of both checkpoint and apoptosis programs would then be required for transformation of normal cells to tumor cells.

Telomeres, chromosome instability and cancer

Telomeres are composed of repetitive G-rich sequence and an abundance of associated proteins that together form a dynamic cap that protects chromosome ends and allows them to be distinguished from deleterious DSBs. Telomere-associated proteins also function to regulate telomerase, the ribonucleoprtotein responsible for addition of the species-specific terminal repeat sequence. Loss of telomere function is an important mechanism for the chromosome instability commonly found in cancer. Dysfunctional telomeres can result either from alterations in the telomere-associated proteins required for end-capping function, or from alterations that promote the gradual or sudden loss of sufficient repeat sequence necessary to maintain proper telomere structure. Regardless of the mechanism, loss of telomere function can result in sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles, leading to extensive DNA amplification and large terminal deletions. B/F/B cycles terminate primarily when the unstable chromosome acquires a new telomere, most often by translocation of the ends of other chromosomes, thereby providing a mechanism for transfer of instability from one chromosome to another. Thus, the loss of a single telomere can result in on-going instability, affect multiple chromosomes, and generate many of the types of rearrangements commonly associated with human cancer.

Radiation-induced DNA damage and repair in peripheral blood mononuclear cells from Nijmegen breakage syndrome patients and carriers assessed by the Comet assay

Nijmegen breakage syndrome (NBS) patients and carriers are predisposed to malignancy and are often treated with X-irradiation. In the present study, the single-cell gel electrophoresis (Comet) assay was used to examine radiation-induced DNA damage and repair in peripheral blood mononuclear cells from NBS patients (n=13) and carriers (n=36) of six unrelated families. Cells from apparently healthy donors (n=10) and from breast cancer patients with normal clinical radiosensitivity (n=10) served as controls. Cells were irradiated with 5 Gy of X-rays and assayed for initial DNA damage and for residual DNA damage after 40 min of repair; the kinetics of DNA repair also was estimated. In addition, the nuclear area of unirradiated cells was extracted from the Comet data. The initial radiation-induced DNA fragmentation indicated that cells from members of two out of six NBS families were significantly more sensitive to X-irradiation than cells from the controls. Cells from four NBS families had longer DNA repair half-time values, while cells from five NBS families had significantly increased residual DNA damage following repair. The mean nuclear area of unirradiated cells processed in the Comet assay was 1.3-fold higher in cells from all NBS families than in the controls (P<0.05). Notably, the Comet assay parameters (initial and residual DNA damage and the repair kinetics) of irradiated NBS cells predicted the carrier status of the majority (86%) of blindly tested individuals. The prediction of NBS status was higher if the nuclear area of unirradiated cells was used as the endpoint. The results of this study suggest that the impaired radiation response of NBS cells should be taken into account if radiotherapy of NBS patients and carriers is required.

Molecular pathology of ataxia telangiectasia

Ataxia telangiectasia (A-T) is one of a group of autosomal recessive cerebellar ataxias. Presentation is usually by the age of 2 years and ataxia of both upper and lower limbs develops, such that by early teenage most patients require a wheelchair for mobility. Speech and eye movement are also affected. Other important features are t(7;14) translocations, immunodeficiency, a high serum alpha fetoprotein concentration, growth retardation, telangiectasia-most noticeably on the bulbar conjunctiva-and a very high risk of developing a lymphoid tumour. Patients also show an increased sensitivity to ionising radiation. The classic form of A-T results from the presence of two truncating ATM mutations, leading to total loss of the ATM protein, a protein kinase. Importantly, A-T shows clinical heterogeneity, including milder forms where neurological progression may be slower or of later onset. In these cases there is a correlation between the preservation of neurological function, decreased radiosensitivity, and the degree of retained ATM protein kinase activity. Considerable scope remains for understanding the progress of the disorder in relation to the types of ATM mutation present.

Nbs1 is required for ATR-dependent phosphorylation events

Nijmegen breakage syndrome (NBS) is characterised by microcephaly, developmental delay, characteristic facial features, immunodeficiency and radiosensitivity. Nbs1, the protein defective in NBS, functions in ataxia telangiectasia mutated protein (ATM)-dependent signalling likely facilitating ATM phosphorylation events. While NBS shares overlapping characteristics with ataxia telangiectasia, it also has features overlapping with ATR-Seckel (ATR: ataxia-telangiectasia and Rad3-related protein) syndrome, a subclass of Seckel syndrome mutated in ATR. We show that Nbs1 also facilitates ATR-dependent phosphorylation. NBS cell lines show a similar defect in ATR phosphorylation of Chk1, c-jun and p-53 in response to UV irradiation- and hydroxyurea (HU)-induced replication stalling. They are also impaired in ubiquitination of FANCD2 after HU treatment, which is ATR dependent. Following HU-induced replication arrest, NBS and ATR-Seckel cells show similarly impaired G2/M checkpoint arrest and an impaired ability to restart DNA synthesis at stalled replication forks. Moreover, NBS cells fail to retain ATR in the nucleus following HU treatment and extraction. Our findings suggest that Nbs1 functions in both ATR- and ATM-dependent signalling. We propose that the NBS clinical features represent the result of these combined defects.

The genetics of familial lymphomas

Whereas familial clustering of malignant lymphoma is well documented, the molecular changes underlying familial lymphoma syndromes remain unclear. An understanding of the hereditary basis of lymphoma may lead to the identification of new molecular markers for disease or novel therapeutic targets. This paper reviews the genetics of familial lymphoma, focusing on germline susceptibilities to lymphoma as well as germline susceptibilities to environmental exposures that have been linked to lymphoma.

Reduced DNA-dependent protein kinase activity in two cell lines derived from adult cancer patients with late radionecrosis

Epstein-Barr virus-immortalized lymphoblastoid cell lines were derived from five patients with late radionecrosis. Two of these cell lines exhibited postradiation viability levels intermediate between normal cell lines and that from an individual with ataxia telangiectasia. Compared with controls, these two cell lines exhibited impaired ability to rejoin DNA double-strand breaks on pulsed-field gel electrophoresis and 6-10-fold reduced DNA-dependent protein kinase (DNA-PK) activity in vitro in cell-free extracts. Immunoblotting showed normal levels of Ku70, Ku80 and XRCC4 and the presence of DNA-PKcs in both cell lines. These findings suggest that DNA-PK might be an important factor affecting the predisposition of radiotherapy patients to late radionecrosis.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Differing responses of Nijmegen breakage syndrome and ataxia telangiectasia cells to ionizing radiation

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder. Originally thought to be a variant of ataxia telangiectasia (AT), the cellular phenotype of NBS has been described as almost indistinguishable from that of AT. Since the gene involved in NBS has been cloned and its functions studied, we sought to further characterize its cellular phenotype by examining the response of density-inhibited, confluent cultures of human diploid fibroblasts to irradiation in the G(0)/G(1) phase of the cell cycle. Both NBS and AT cells were markedly sensitive to the cytotoxic effects of radiation. NBS cells, however, were proficient in recovery from potentially lethal damage and exhibited a pronounced radiation-induced G(1)-phase arrest. Irradiated AT cells showed no potentially lethal damage and no G(1)-phase arrest. Both cell types were hypersensitive to the induction of chromosomal aberrations, whereas the distribution of aberrations in irradiated NBS cells was similar to that of normal controls, AT cells showed a high frequency of chromatid-type aberrations. TP53 and CDKN1A (also known as p21(Waf1)) expression was attenuated in irradiated NBS cells, but maximal induction occurred 2 h postirradiation, as was observed in normal controls. The similarities and differences in cellular phenotype between irradiated NBS and AT cells are discussed in terms of the functional properties of the signaling pathways downstream of AT involving the NBS1 and TP53 proteins.

Mre11 complex and DNA replication: linkage to E2F and sites of DNA synthesis

We show that the Mre11 complex associates with E2F family members via the Nbs1 N terminus. This association and Nbs1 phosphorylation are correlated with S-phase checkpoint proficiency, whereas neither is sufficient individually for checkpoint activation. The Nbs1 E2F interaction occurred near the Epstein-Barr virus origin of replication as well as near a chromosomal replication origin in the c-myc promoter region and was restricted to S-phase cells. The Mre11 complex colocalized with PCNA at replication forks throughout S phase, both prior to and coincident with the appearance of nascent DNA. These data suggest that the Mre11 complex suppresses genomic instability through its influence on both the regulation and progression of DNA replication.

The yeast Xrs2 complex functions in S phase checkpoint regulation

The Nbs1 complex is an evolutionarily conserved multisubunit nuclease composed of the Mre11, Rad50, and Nbs1 proteins. Hypomorphic mutations in the NBS1 or MRE11 genes in humans result in conditions characterized by DNA damage sensitivity, cell cycle checkpoint deficiency, and high cancer incidence. The equivalent complex in the yeast Saccharomyces cerevisiae (Xrs2p complex) has been implicated in DNA double-strand break repair and in telomere length regulation. Here, we find that xrs2Delta, mre11Delta, and rad50Delta mutants are markedly defective in the initiation of the intra-S phase checkpoint in response to DNA damage. Furthermore, the absence of a functional Xrs2p complex leads to sensitivity to deoxynucleotide depletion and to an inability to efficiently slow down cell cycle progression in response to hydroxyurea. The checkpoint appears to require the nuclease activity of Mre11p and its defect is associated with the abrogation of the Tel1p/Mec1p signaling pathway. Notably, DNA damage induces phosphorylation of both Xrs2p and Mre11p in a Tel1p-dependent manner. These results indicate that the Tel1p/ATM signaling pathway is conserved from yeast to humans and suggest that the Xrs2p/Nbs1 complexes act as signal modifiers.

Genetic disorders associated with cancer predisposition and genomic instability

Genomic instability in its broadest sense is a feature of virtually all neoplastic cells. In addition to the mutations and/or gene amplifications that appear to be a prerequisite for the acquisition of a neoplastic phenotype, human cancers exhibit other "markers" of genomic instability--in particular, a high degree of aneuploidy. Indeed, many studies have shown that aneuploidy is an almost invariant feature of cancer cells, and it has been argued by some that the emergence of aneuploid cells is a necessary step during tumorigenesis. The functional link between genomic instability and cancer is strengthened by the existence of several "genetic instability" disorders of humans that are associated with a moderate to severe increase in the incidence of cancers. These disorders include ataxia telangiectasia, Bloom's syndrome, Fanconi anemia, xeroderma pigmentosum, and Nijmegen breakage syndrome, all of which are very rare and are inherited in a recessive manner. Analysis of the cells from such cancer-prone individuals is clearly a potentially fruitful approach for delineating the genetic basis for instability in the genome. It is assumed that by identifying the underlying cause of genetic instability in these disorders, one can derive valuable information not only about the basis of particular genetic diseases, but also about the underlying causes of genomic instability in sporadic cancers in the general population. In this article, we review the clinical and cellular properties of genetic instability disorders associated with cancer predisposition. In particular, we focus on the rapid advances made in our understanding of these disorders that have derived from the cloning of the genes mutated in each case. Because in many instances the affected genes have analogs in lower eukaryotic species, we shall discuss how studies in yeasts in particular have proved valuable in our understanding of human diseases and predisposition to cancer.

Determination of the frequency of the common 657Del5 Nijmegen breakage syndrome mutation in the German population: no association with risk of breast cancer

Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability syndrome characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. It shares a number of features with the Ataxia telangiectasia (AT) syndrome: the most notable are high sensitivity to ionizing radiation and predisposition to cancer. Recently, the gene responsible for NBS has been identified on chromosome band 8q21. It encodes a DNA double-strand break repair protein, named Nibrin. A truncating 5-bp deletion (657Del5) has been identified in 90% of NBS patients and this is presumed to be of Slavic origin. There is evidence that heterozygous AT mutation carriers are predisposed to breast cancer. Since the NBS phenotype at the cellular level is very similar to AT, we have screened 477 German breast cancer patients, aged under 51 years, and 866 matched controls for the common NBS mutation. We have identified one carrier among the cases and one among the controls, indicating that the population frequency of this NBS mutation is 1 in 866 people (95% CI = 1 in 34,376 to 1 in 156) and the estimated prevalence of NBS is thus 1 in 3 million people. The proportion of breast cancer attributable to this mutation is less than 1%. Genes Chromosomes Cancer 25:393-395, 1999.

Fine localization of the Nijmegen breakage syndrome gene to 8q21: evidence for a common founder haplotype

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder characterized by microcephaly, a birdlike face, growth retardation, immunodeficiency, lack of secondary sex characteristics in females, and increased incidence of lymphoid cancers. NBS cells display a phenotype similar to that of cells from ataxia-telangiectasia patients, including chromosomal instability, radiation sensitivity, and aberrant cell-cycle-checkpoint control following exposure to ionizing radiation. A recent study reported genetic linkage of NBS to human chromosome 8q21, with strong linkage disequilibrium detected at marker D8S1811 in eastern European NBS families. We collected a geographically diverse group of NBS families and tested them for linkage, using an expanded panel of markers at 8q21. In this article, we report linkage of NBS to 8q21 in 6/7 of these families, with a maximum LOD score of 3.58. Significant linkage disequilibrium was detected for 8/13 markers tested in the 8q21 region, including D8S1811. In order to further localize the gene for NBS, we generated a radiation-hybrid map of markers at 8q21 and constructed haplotypes based on this map. Examination of disease haplotypes segregating in 11 NBS pedigrees revealed recombination events that place the NBS gene between D8S1757 and D8S270. A common founder haplotype was present on 15/18 disease chromosomes from 9/11 NBS families. Inferred (ancestral) recombination events involving this common haplotype suggest that NBS can be localized further, to an interval flanked by markers D8S273 and D8S88.

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

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

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Chromosome instability with bleomycin and X-ray hypersensitivity in a boy with Nijmegen breakage syndrome

We report on a Mexican boy with microcephaly, short stature, and a high frequency of chromosome aberrations with rearrangements involving chromosomes 7 and 14, typical of ataxia telangiectasia (AT) patients. He had neither ataxia nor telangiectasia, and his immunological status and serum alpha feto protein (AFP) level were normal. Bleomycin hypersensitivity, which has been demon-strated in AT patients, was tested in the patient using AT and normal subjects for comparison. The frequency of spontaneously occurring chromosome aberrations in lymphocyte cultures was significantly higher in the patient and the AT patient than in the normal subject. Four cells from the patient showed structural rearrangements involving chromosomes 7 or 14, with breakpoints typical for AT. When exposed to 5.0 micrograms bleomycin, the lymphocytes from the AT patient showed the highest sensitivity to this agent; our patient had an intermediate sensitivity. In both patients several rearrangements involving chromosomes 7 and 14 were scored, while none were observed in the normal subject. A colony survival assay (CSA) [Huo et al., 1994: Cancer Res 54:2544-2547], using a lymphoblastoid cell line (LCL) derived from our patient, showed a survival fraction (SF) of 7%, which is in the same range as in AT patients. The clinical picture, together with the cytogenetic and radiosensitivity results, suggests that our patient fits the variable spectrum of Nijmegen breakage syndrome.

A variant of the Nijmegen breakage syndrome with unusual cytogenetic features and intermediate cellular radiosensitivity

We report the first Italian case of Nijmegen breakage syndrome (NBS). The proband is an immunodeficient, microcephalic, 11 year old boy with a "bird-like" face. He developed a T cell rich B cell lymphoma. Spontaneous chromosomal instability was detected in T and B lymphocytes and fibroblasts; chromosomes 7 and 14 were only sporadically involved in the rearrangements and no clonal abnormality was present. The patient appeared to be sensitive both to ionising radiation and to bleomycin, although his sensitivity did not reach the level of AT reference cells. After bleomycin treatment, inhibition of DNA synthesis was low when compared with normal cells, but higher than observed in an AT reference strain. Moreover, cell cycle analysis, after drug exposure, showed a progressive reduction in the percentage of S phase cells, but the G1 arrest, found in normal cells, was not observed. On clinical evaluation our patient shares features with NBS subjects, but cytogenetic and cell biological data do not completely overlap with those reported in Nijmegen breakage syndrome. The ethnic origin of our patient might account for these differences, as expression of different allelic forms at the NBS locus.

Possible new variant of Nijmegen breakage syndrome

We report on a child with microcephaly, small facial and body size, and immune deficiency. The phenotype is consistent with Nijmegen breakage syndrome (NBS), with additional clinical manifestations and laboratory findings not reported heretofore. Most investigations, including the results of radiation-resistant DNA synthesis, concurred with the diagnosis of NBS. Cytogenetic analysis documented abnormalities in virtually all cells examined. Along with the high frequency of breaks and rearrangements of chromosomes 7 and 14, we found breakage and monosomies involving numerous other chromosomes. Because of some variation in the clinical presentation and some unusual cytogenetic findings, we suggest that our patient may represent a new variant of Nijmegen breakage syndrome.

Inactivation of the HR6B ubiquitin-conjugating DNA repair enzyme in mice causes male sterility associated with chromatin modification

The ubiquitin-conjugating yeast enzyme RAD6 and its human homologs hHR6A and hHR6B are implicated in postreplication repair and damage-induced mutagenesis. The yeast protein is also required for sporulation and may modulate chromatin structure via histone ubiquitination. We report the phenotype of the first animal mutant in the ubiquitin pathway: inactivation of the hHR6B-homologous gene in mice causes male infertility. Derailment of spermatogenesis becomes overt during the postmeiotic condensation of chromatin in spermatids. These findings provide a parallel between yeast sporulation and mammalian spermatogenesis and strongly implicate hHR6-dependent ubiquitination in chromatin remodeling. Since heterozygous male mice and even knockout female mice are completely normal and fertile and thus able to transmit the defect, similar hHR6B mutations may cause male infertility in man.

Human chromosome 11 complements ataxia-telangiectasia cells but does not complement the defect in AT-like Chinese hamster cell mutants

It has been shown that the X-ray-sensitive Chinese hamster V79 mutants (V-E5, V-C4 and V-G8) are similar to ataxia-telangiectasia (A-T) cells. To determine whether the AT-like rodent cell mutants are defective in the gene homologous to A-T (group A, C or D), human chromosome 11 was introduced to the V-E5 and V-G8 mutant cells by microcell-mediated chromosome transfer. Forty independent hybrid clones were obtained in which the presence of chromosome 11 was determined by in situ hybridization. The presence of the region of chromosome 11q22-23 was shown by molecular analysis using polymorphic DNA markers specific for the ATA, ATC and ATD loci. Seventeen of the obtained monochromosomal Chinese hamster hybrids contained a cytogenetically normal human chromosome 11, but only twelve hybrid cell lines were shown to contain an intact 11q22-23 region. Despite the complementation of the X-ray sensitivity by a normal chromosome 11 introduced to A-T cells (complementation group D), these twelve Chinese hamster hybrid clones showed lack of complementation of X-ray and streptonigrin hypersensitivity. The observed lack of complementation does not seem to be attributable to hypermethylation of the human chromosome 11 in the rodent cell background, since 5-azacytidine treatment had no effect on the streptonigrin hypersensitivity of the hybrid cell lines. These results indicate that the gene defective in the AT-like rodent cell mutants is not homologous to the ATA, ATC or ATD genes and that the human gene complementing the defect in the AT-like mutants seems not to be located on human chromosome 11.

Lack of correlation of human fibroblast radiosensitivity in vitro with early skin reactions in patients undergoing radiotherapy

Fibroblasts from breast cancer patients were obtained as outgrowths in vitro from punch biopsies and their radiosensitivity tested in early passages. Skin erythema reactions in the same patients were also measured, as degree of redness using reflectance spectrophotometry. Measurements were taken before and during a 4-week radiotherapy treatment with electrons to the thoracic wall. Of 59 biopsies studied, radiosensitivity and erythema were concurrently studied in 32. In 24, evaluable data from both clinic and laboratory were obtained. A population growth assay in 96-well plates, using absorption of sulphur rhodamine B as the stain for cell numbers, showed good agreement with the colony-formation assay. Plating efficiencies and growth rates in the colony assay were higher using human serum in place of foetal calf serum. Cell survival curves with human serum were mostly exponential with little shoulder. The parameters of survival at 2 Gy (SF2) and the dose required to give 10% survival (D10) were used in the correlations with clinical data; these were 0.25 +/- 0.09 and 3.03 +/- 0.50 Gy, respectively. There was a strong correlation between these two survival curve parameters (r = 0.98). Skin redness was found to linearly increase with time during radiotherapy. The slope of the increase differed markedly from patient to patient, with a range of a factor approx. 10. No correlation was found between SF2 and erythema response in the 24 evaluable patients (r = 0.13, p > 0.5). A similar lack of correlation was found using D10 as the radiosensitivity parameter (r = 0.12, p > 0.5). These data indicate that fibroblast radiosensitivity measured in vitro cannot be used to predict erythema reactions to radiotherapy in breast cancer patients.

Use of low-dose rate irradiation to measure the intrinsic radiosensitivity of human T-lymphocytes

A comparison has been made of high (1.55 Gy min-1) and low dose-rate (0.0098 Gy min-1) irradiation in determining the intrinsic radiosensitivity of peripheral blood lymphocytes from normal individuals. Samples from 19 people were assessed using a limiting dilution assay and used to investigate the variability associated with evaluation of lymphocyte radiosensitivity after both high and low dose-rate irradiation. Repeat experiments on a single sample from one donor stored over a period of 6 months have been used to compare assay variability using the different dose-rates. Multiple samples taken from a single person over a period of 5 months were assayed at low dose-rate to assess intraindividual variation in measured in vitro radiosensitivity. At high dose-rate significant interexperimental variation in the measured parameters was demonstrated and, after allowing for this variability, no significant interindividual differences were found. At low dose-rate, sparing of cell kill was seen for all lymphocytes and led to an increase in the spread of data between individuals such that interindividual differences reached statistical significance for surviving fraction at 4 Gy, alpha (linear fit) and D with p < 0.004 for all parameters.

Atypical clinical presentation of ataxia telangiectasia

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by progressive cerebellar ataxia, oculocutaneous telangiectasias, immunodeficiency with recurrent infections, IgA and IgE deficiency, and increased incidence of malignancies. The pathognomonic biological abnormalities consist of spontaneous chromosomal instability resulting in a high in vivo occurrence of cells with translocations, especially involving chromosomes 7 and 14, and a relative insensitivity of DNA replication in vitro to radiation exposure. We report on a patient with the biological hallmarks of AT but with atypical clinical manifestations. Although progressive cerebellar ataxia was present, the neurological picture was broader than that usually seen in AT and included peripheral polyneuropathy and spinal atrophy. On the other hand, telangiectasias, recurrent infections, malignancies, IgA deficiency, or other immunological abnormalities were not present. This illustrates that the clinical picture of AT is broad and nonspecific, and highlights the diagnostic value of cytogenetic analysis and studies of radioresistance of DNA synthesis.

Chromosome instability and X-ray hypersensitivity in a microcephalic and growth-retarded child

We report on a microcephalic, growth-retarded newborn girl without major anomalies who has chromosome instability in lymphocytes and fibroblasts. Frequent involvement of bands 7p13, 7q34, 14q11, and 14q32 suggested the diagnosis of ataxia telangiectasia (AT) or a related disorder. Supportive evidence was radioresistant DNA synthesis in fibroblasts and radiation hypersensitivity of short-term lymphocyte cultures. Follow-up for nearly 4 years showed largely normal development, and no signs of telangiectasia, ataxia, or immunodeficiency. Serum AFP levels turned from elevated at age 5 months to normal at age 2 years. We propose that our patient belongs to the expanding category of "AT-related" genetic disorders, probably to the Nijmegen breakage syndrome.

First-trimester prenatal diagnosis of the Nijmegen breakage syndrome and ataxia telangiectasia using an assay of radioresistant DNA synthesis

Prenatal diagnosis was performed in two pregnancies at risk of the Nijmegen breakage syndrome. In one pregnancy, an affected fetus was diagnosed by demonstration of radioresistant DNA synthesis, using autoradiographic detection of incorporated tritiated thymidine in cultured chorionic villus cells. The diagnosis was confirmed in fetal skin fibroblasts. In the other case, the fetus appeared unaffected. Using the same procedure, unaffected fetuses were predicted from chorionic villus cells in two pregnancies at risk of ataxia telangiectasia, which is another genetic disorder showing the feature of radioresistant DNA synthesis. The present biochemical method for prenatal detection of Nijmegen breakage syndrome and ataxia telangiectasia can be used as a simplified alternative to the cytogenetic procedures reported earlier for ataxia telangiectasia.

An instance of clinical radiation morbidity and cellular radiosensitivity, not associated with ataxia-telangiectasia

A 14-year-old boy received standard induction chemotherapy for acute lymphoblastic leukaemia followed by standard dose cranial radiation prophylaxis (18 Gy). Severe chemosensitivity and acute radiation reactions occurred and he died at 8 months from late radiation damage. In vitro radiobiological studies of the boy's fibroblasts in culture demonstrated an enhanced radiosensitivity indistinguishable from ataxia-telangiectasia (A-T) cells. However, unlike A-T cells, DNA synthesis following irradiation was inhibited in a normal manner. This patient represents yet another example of extreme radiosensitivity, and the possibility of clinical prediction in the future is discussed.

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

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

Further delineation of the Nijmegen breakage syndrome

We report on five independent families with a chromosome instability disorder that earlier had been called the Nijmegen breakage syndrome (NBS). These families, two from the Netherlands and three from Czechoslovakia, had a total of eight patients, five of whom are still alive. The main clinical manifestations were microcephaly, short stature, a "bird-like" face, immunological defects involving both the humoral and cellular system. In four of the five living patients it has been possible to study the chromosomes of cultured lymphocytes. The basic karyotype in these patients were normal, but in 17% to 35% of the metaphases rearrangements were found, preferentially involving chromosomes 7 and/or 14 at the sites 7p13, 7q34, and 14q11. The chromosomes of all five living patients were very sensitive to ionizing radiation. In addition, the DNA synthesis in their cultured lymphocytes and fibroblasts was more resistant to X-rays than in cells from controls. The NBS shares a number of important features with ataxia telangiectasia (AT). Both syndromes are characterized by the occurrence of typical rearrangements of chromosomes 7 and/or 14, cellular and chromosomal hypersensitivity to X-irradiation, radioresistance of DNA replication and immunodeficiency. However, there are also obvious differences: NBS patients have microcephaly but neither ataxia nor telangiectasia, and in contrast to the situation in AT the alpha-fetoprotein level in their serum is normal.

Ataxia-telangiectasia: a variant with altered in vitro phenotype of fibroblast cells

The clinical and cellular phenotype of ataxia telangiectasia (AT) has been extensively documented in numerous patients of different ethnic groups and is characterized by several specific laboratory hallmarks, such as chromosomal instability, profound radiosensitivity and radioresistant DNA synthesis. Several recent reports have, however, shown variations on this theme. This article describes 2 Turkish siblings with AT, who showed a typical but somewhat more prolonged clinical course of the disease and altered characteristics of fibroblast cells, compared to the 'classical' AT cellular phenotype. Fibroblast strains derived from these patients showed a normal cellular life span, moderate degrees of chromosomal instability and sensitivity to the lethal effects of X-rays and neocarzinostatin, and lack of radioresistant DNA synthesis. A compilation of the literature on 'AT variants' and 'AT-like' syndromes shows that in addition to the internal variability of AT, this disease occupies a limited segment within a large spectrum of clinical and cellular features, which are common to a variety of syndromes. Each of these syndromes covers a different segment in this spectrum. The genetic basis of this family of disorders might be complex.

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

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

Variant forms of ataxia telangiectasia

Two ataxia telangiectasia patients with unusual clinical and cellular features are described. Cultured fibroblasts and PHA stimulated lymphocytes from these two patients showed a smaller increase of radiosensitivity than cells from other A-T patients, as measured by colony forming ability or induced chromosome damage respectively, after exposure to ionising radiation. The response of DNA synthesis to irradiation of these cells was, however, the same as for other A-T patients. Cells from a third patient with some clinical features of A-T but with a very protracted course also showed low levels of radiation induced chromosome damage, but colony forming ability and the response of DNA synthesis after irradiation were no different from cells of normal subjects. There was, however, an increased level of translocations and unstable chromosomal rearrangements in this patient's lymphocytes.

Ataxia-telangiectasia: an inherited disorder of ionizing-radiation sensitivity in man. Progress in the elucidation of the underlying biochemical defect

This review summarizes the current research on the biochemical defect leading to ataxia-telangiectasia (AT). A DNA repair defect has been linked to AT, although the precise defect has not been found. A critical examination of the evidence for and against a DNA repair defect in AT is presented. Consideration of other recent data on AT raises the possibility that AT may not primarily be the result of a DNA repair defect. Therefore, in this review AT is approached as a syndrome which is defective in the ability to respond to ionizing-radiation-type damage, rather than defective in the ability to repair this damage. However, this does not necessarily exclude the potential involvement of a DNA repair defect in some of the genetically distinct subsets present in AT. Other recent anomalies found in AT, including an altered cell cycle and DNA synthesis profile following ionizing-radiation damage, are also assessed. A suggestion to account for the underlying defect in AT, based on the various research reports, is presented.

Re-evaluation of in vitro radiosensitivity of human fibroblasts of different genetic origins

A statistical analysis of the radiosensitivity of 204 different survival curves of nontransformed human fibroblast cell strains of different genetic origins was made using three criteria: the multi-target one-hit model (characterized by parameters n and D0), the surviving fraction for a 2 Gy dose (S2) and the mean inactivation dose (D). D is found to be the best parameter for characterization of anomalous radiosensitivity linked to a genetic disorder and for discrimination between groups of cell strains of differing radiosensitivity. Its use allows the description of a range of 'normal' radiosensitivity for control fibroblasts and the classification of the various genetic disorders as a function of their mean radiosensitivity expressed in terms of D. Nine groups of cell strains appear to exhibit radiosensitivity which differs significantly from that of the controls: seven groups are hypersensitive (ataxia-telangiectasia homozygotes and heterozygotes, Cockayne's syndrome, Gardner's syndrome, 5-oxoprolinuria homozygotes and heterozygotes, Fanconi's anaemia) and two groups are more radioresistant (fibroblasts from retinoblastoma patients and from individuals with chromosome 13 anomalies). Since the coupled parameter n and D0 failed to discriminate between the radiosensitivity of the different genetic groups, we recommend the use of D to make an intercomparison of intrinsic radiosensitivity of nontransformed human fibroblasts.

Radioresistant DNA synthesis in cells of patients showing increased chromosomal sensitivity to ionizing radiation

The rate of DNA synthesis after gamma-irradiation was studied either by analysis of the steady-state distribution of daughter [3H]DNA in alkaline sucrose gradients or by direct assay of the amount of [3H]thymidine incorporated into DNA of fibroblasts derived from a normal donor (LCH882) and from Down's syndrome (LCH944), Werner's syndrome (WS1LE) and xeroderma pigmentosum (XP2LE) patients with chromosomal sensitivity to ionizing radiation. Doses of gamma-irradiation that markedly inhibited the rate of DNA synthesis in normal human cells caused almost no inhibition of DNA synthesis in the cells from the affected individuals. The radioresistant DNA synthesis in Down's syndrome cells was mainly due to a much lower inhibition of replicon initiation than that in normal cells; these cells were also more resistant to damage that inhibited replicon elongation. Our data suggest that radioresistant DNA synthesis may be an intrinsic feature of all genetic disorders showing increased radiosensitivity in terms of chromosome aberrations.