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

Induction of a p95/Nbs1-mediated S phase checkpoint by telomere 3' overhang specific DNA

Telomere shortening induces a nonproliferative senescent phenotype, believed to reduce cancer risk, and telomeres are involved in a poorly understood manner in responses to DNA damage. Although telomere disruption induces p53 and triggers apoptosis or cell cycle arrest, the features of the disrupted telomere that trigger this response and the precise mechanism involved are poorly understood. Using human cells, we show that DNA oligonucleotides homologous to the telomere 3' overhang sequence specifically induce and activate p53 and activate an S phase checkpoint by modifying the Nijmegen breakage syndrome protein, known to mediate the S phase checkpoint after DNA damage. These responses are mediated, at least in part, by the ATM kinase and are not attributable to disruption of cellular telomeres. Based on these and earlier data, we propose that these oligonucleotides mimic a physiological signal, exposure of the telomere 3' overhang due to opening of the normal telomere loop structure, and hence evoke these protective antiproliferative responses in the absence of DNA damage or telomere disruption.

Partial T and B lymphocyte immunodeficiency and predisposition to lymphoma in patients with hypomorphic mutations in Artemis

We have previously described the identification of Artemis, a factor involved in the nonhomologous end joining (NHEJ) phase of V(D)J recombination of T and B cell receptor genes. Null mutations of the Artemis gene result in a complete absence of T and B lymphocytes that is associated with increased cell radiosensitivity, causing the radiosensitive T(-)B(-) SCID (RS-SCID) condition. We presently report the occurrence of hypomorphic mutations of the Artemis gene in four patients from two kindreds. Partially preserved in vivo activity of Artemis is associated with the presence of polyclonal T and B lymphocyte populations, albeit in reduced numbers, along with chromosomal instability and development of EBV-associated lymphoma in two of four patients. This syndrome emphasizes the role of Artemis in the NHEJ pathway of DNA repair and suggests that other, yet ill-defined, conditions associating immunodeficiency and lymphoma could be caused by mutations in genes encoding NHEJ factors.

DNA damage activates ATM through intermolecular autophosphorylation and dimer dissociation

The ATM protein kinase, mutations of which are associated with the human disease ataxia-telangiectasia, mediates responses to ionizing radiation in mammalian cells. Here we show that ATM is held inactive in unirradiated cells as a dimer or higher-order multimer, with the kinase domain bound to a region surrounding serine 1981 that is contained within the previously described 'FAT' domain. Cellular irradiation induces rapid intermolecular autophosphorylation of serine 1981 that causes dimer dissociation and initiates cellular ATM kinase activity. Most ATM molecules in the cell are rapidly phosphorylated on this site after doses of radiation as low as 0.5 Gy, and binding of a phosphospecific antibody is detectable after the introduction of only a few DNA double-strand breaks in the cell. Activation of the ATM kinase seems to be an initiating event in cellular responses to irradiation, and our data indicate that ATM activation is not dependent on direct binding to DNA strand breaks, but may result from changes in the structure of chromatin.

The mismatch repair system is required for S-phase checkpoint activation

Defective S-phase checkpoint activation results in an inability to downregulate DNA replication following genotoxic insult such as exposure to ionizing radiation. This 'radioresistant DNA synthesis' (RDS) is a phenotypic hallmark of ataxia-telangiectasia, a cancer-prone disorder caused by mutations in ATM. The mismatch repair system principally corrects nucleotide mismatches that arise during replication. Here we show that the mismatch repair system is required for activation of the S-phase checkpoint in response to ionizing radiation. Cells deficient in mismatch repair proteins showed RDS, and restoration of mismatch repair function restored normal S-phase checkpoint function. Catalytic activation of ATM and ATM-mediated phosphorylation of the protein NBS1 (also called nibrin) occurred independently of mismatch repair. However, ATM-dependent phosphorylation and activation of the checkpoint kinase CHK2 and subsequent degradation of its downstream target, CDC25A, was abrogated in cells lacking mismatch repair. In vitro and in vivo approaches both show that MSH2 binds to CHK2 and that MLH1 associates with ATM. These findings indicate that the mismatch repair complex formed at the sites of DNA damage facilitates the phosphorylation of CHK2 by ATM, and that defects in this mechanism form the molecular basis for the RDS observed in cells deficient in mismatch repair.

Nijmegen breakage syndrome gene, NBS1, and molecular links to factors for genome stability

DNA double-strand breaks represent the most potentially serious damage to a genome and hence, at least two pathways of DNA repair have evolved; namely, homologous recombination repair and non-homologous end joining. Defects in both rejoining processes result in genomic instability including chromosome rearrangements, LOH and gene mutations, which may lead to development of malignancies. Nijmegen breakage syndrome is a recessive genetic disorder, characterized by elevated sensitivity to ionizing radiation that induces double-strand breaks, and high frequency of malignancies. NBS1, the product of the gene underlying the disease, forms a multimeric complex with hMRE11/hRAD50 nuclease and recruits them to the vicinity of sites of DNA damage by direct binding to phosphorylated histone H2AX. The combination of the highly-conserved NBS1 forkhead associated domain and BRCA1 C-terminus domain has a crucial role for recognition of damaged sites. Thereafter, the NBS1-complex proceeds to rejoin double-strand breaks predominantly by homologous recombination repair in vertebrates. This process collaborates with cell-cycle checkpoints at S and G2 phase to facilitate DNA repair. NBS1 is also associated with telomere maintenance and DNA replication. Based on recent knowledge regarding NBS1, we propose here a two-step binding mechanism for damage recognition by repair proteins, and describe the molecular links to factors for genome stability.

Regulation of ionizing radiation-induced Rad52 nuclear foci formation by c-Abl-mediated phosphorylation

The RAD52 epistasis group of proteins, including Rad51, Rad52, and Rad54, plays an important role in the homologous recombination repair of double strand breaks. A well characterized feature associated with the ability of these proteins to repair double strand breaks is inducible nuclear foci formation at the sites of damage. How the process is functionally regulated in response to DNA damage, however, remains elusive. We show here that c-Abl tyrosine kinase associates with and phosphorylates Rad52 on tyrosine 104. Importantly, the very same site of Rad52 is phosphorylated on exposure of cells to ionizing radiation (IR). The functional significance of c-Abl-dependent phosphorylation of Rad52 is underscored by our findings that cells that express the phosphorylation-resistant Rad52 mutant, in which tyrosine 104 is replaced by phenylalanine, exhibit compromised nuclear foci formation in response to IR. Furthermore, IR-induced Rad52 nuclear foci formation is markedly suppressed by the expression of dominant-negative c-Abl. Together our data support a mode of post-translational regulation of Rad52 mediated by the c-Abl tyrosine kinase.

Role of RAD52 epistasis group genes in homologous recombination and double-strand break repair

The process of homologous recombination is a major DNA repair pathway that operates on DNA double-strand breaks, and possibly other kinds of DNA lesions, to promote error-free repair. Central to the process of homologous recombination are the RAD52 group genes (RAD50, RAD51, RAD52, RAD54, RDH54/TID1, RAD55, RAD57, RAD59, MRE11, and XRS2), most of which were identified by their requirement for the repair of ionizing-radiation-induced DNA damage in Saccharomyces cerevisiae. The Rad52 group proteins are highly conserved among eukaryotes, and Rad51, Mre11, and Rad50 are also conserved in prokaryotes and archaea. Recent studies showing defects in homologous recombination and double-strand break repair in several human cancer-prone syndromes have emphasized the importance of this repair pathway in maintaining genome integrity. Although sensitivity to ionizing radiation is a universal feature of rad52 group mutants, the mutants show considerable heterogeneity in different assays for recombinational repair of double-strand breaks and spontaneous mitotic recombination. Herein, I provide an overview of recent biochemical and structural analyses of the Rad52 group proteins and discuss how this information can be incorporated into genetic studies of recombination.

Interaction of FANCD2 and NBS1 in the DNA damage response

Fanconi anaemia (FA) and Nijmegen breakage syndrome (NBS) are autosomal recessive chromosome instability syndromes with distinct clinical phenotypes. Cells from individuals affected with FA are hypersensitive to mitomycin C (MMC), and cells from those with NBS are hypersensitive to ionizing radiation. Here we report that both NBS cell lines and individuals with NBS are hypersensitive to MMC, indicating that there may be functional linkage between FA and NBS. In wild-type cells, MMC activates the colocalization of the FA subtype D2 protein (FANCD2) and NBS1 protein in subnuclear foci. Ionizing radiation activates the ataxia telangiectasia kinase (ATM)-dependent and NBS1-dependent phosphorylation of FANCD2, resulting in an S-phase checkpoint. NBS1 and FANCD2 therefore cooperate in two distinct cellular functions, one involved in the DNA crosslink response and one involved in the S-phase checkpoint response.

DNA double-strand break repair signalling: the case of RAD51 post-translational regulation

DNA double-strand breaks (DSBs) are the major lethal lesion induced by ionizing radiation or by replication block. However, cells can take advantage of DSB-induced recombination in order to generate genetic diversity in physiological processes such as meiosis and V(D)J recombination. Two main alternative pathways compete for DSB repair: homologous recombination (HR) and non-homologous end-joining (NHEJ). This review will briefly present the mechanisms and the enzymatic complex for HR and NHEJ. The signalling of the DSB through the ATM pathway will be presented. Then, we will focus on the case of the RAD51 protein, which plays a pivotal role in HR and is conserved from bacteria to humans. Post-translational regulation of RAD51 is presented. Two contrasting situations are discussed: one with up-regulation (expression of the oncogene BCR/ABL) and one with a down-regulation (expression of the oncogene BCL-2) of RAD51, associated with apoptosis inhibition and tumour predisposition.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

Immunological disorders and DNA repair

Cellular DNA continuously incurs damage and a range of damage response mechanisms function to maintain genomic integrity in the face of this onslaught. During the development of the immune response, the cell utilises three defined processes, V(D)J recombination, class switch recombination and somatic hypermutation, to create genetic diversity in developing T and B cells. Curiously, the damage response mechanisms employed to maintain genomic stability in somatic cells have been exploited and adapted to help generate diversity during immune development. As a consequence of this overlap, there is mounting evidence that disorders attributable to impaired damage response mechanisms display associated immunodeficiency. Since double strand breaks (DSB) are created during at least two of the mechanisms used to create immunoglobulin diversity, namely V(D)J recombination and class switch recombination, it is not surprising that disorders associated with defects in the response to double strand breaks are those most associated with immunodeficiency. Here, we review the steps involved in the generation of genetic diversity during immune development with a focus on the damage response mechanisms employed and then consider human immunodeficiency disorders associated with impaired damage response mechanisms.

Mutations and molecular variants of the NBS1 gene in non-Hodgkin lymphoma

Non-Hodgkin lymphomas (NHLs) are characterized by chromosomal translocations that juxtapose loci encoding lymphoid antigen receptors with cellular proto-oncogenes. These translocations are thought to arise from inaccurate processing of DNA breaks created during physiologic recombination of the antigen receptor genes in lymphocytes. The inherited disorders ataxia-telangiectasia and Nijmegen breakage syndrome are caused by mutations in the ATM and NBS1 genes, respectively, and are characterized by generalized genomic instability and a high incidence of lymphoid cancers. Lymphoid cells from patients with either disorder frequently have chromosomal translocations involving T-cell-receptor or immunoglobulin loci. To investigate the potential role of the NBS1 gene in the pathogenesis of NHL, we screened tumor DNA samples from 91 sporadic cases of NHL and genomic DNA from 154 control individuals for mutations in all 16 exons of the NBS1 gene and in flanking intronic sequences. One NHL case with a truncating mutation in NBS1 and a second NHL case with a putative missense mutation were detected. Neither mutation was observed among controls. Three additional putative missense mutations were observed only in the normal control samples. A panel of six common polymorphisms spanning the NBS1 gene was genotyped and provided no evidence for loss of heterozygosity in the NHL cases with mutations or in the NHL population overall. These results suggest that mutations in NBS1 do not play a major role in the development of NHL in the United States.

Characteristics of the end-joining of DNA double-strand breaks by the ataxia-telangiectasia nuclear extract

A double-strand break was introduced in plasmid pZErO-2 at a specific site within the ccdB gene that is lethal to Escherichia coli cells and treated with nuclear extracts from human cells. The efficiency of rejoining was monitored by Southern blot analysis and the fidelity of rejoining was measured by expressing the ccdB gene after bacterial transformation. The efficiency of rejoining in the nuclear extract from an ataxia-telangiectasia (A-T) cell line was comparable to that from a control cell line. However, the accuracy of rejoining was much lower for the A-T cell extract than for the control cell extract. All mutations were deletions, most of which contained short direct repeats at the breakpoint junctions. The deletion spectrum caused by the A-T nuclear extract was distinct from that of the control extract. These results indicate that the ccdB gene is useful for analysis of mis-rejoining and that A-T cells have certain deficiencies in end-joining of double-strand breaks in DNA.

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.

Neocarzinostatin induces Mre11 phosphorylation and focus formation through an ATM- and NBS1-dependent mechanism

DNA double-strand breaks, if unrepaired, may lead to the accumulation of chromosomal aberrations and eventually cancer cell formation. Components of the Rad50/NBS/Mre11 nuclease complex are essential for the effective repair of DNA double-stranded breaks. Here, we show that neocarzinostatin, a radiomimetic enediyne antibiotic, induces phosphorylation and nuclear focus formation of Mre11 and NBS1 through a cell cycle-independent mechanism. Furthermore, neocarzinostatin-induced Mre11 phosphorylation and nuclear focus formation are defective in AT and NBS cells, but not wild type cells. Our results suggest that ATM and NBS1 are required for the effective repair of neocarzinostatin-induced DNA double-strand breaks by both non-homologous end joining and homologous recombinational repair pathways.

Insulin-like growth factor I receptor is expressed at normal levels in Nijmegen breakage syndrome cells

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder sharing a pleiotropic phenotype with ataxia-telangiectasia (A-T), including increased radiosensitivity and cancer disposition. Insulin-like growth factor I receptor (IGF-IR) expression is reportedly decreased in A-T cells, which is thought to contribute to its increased radiosensitivity. In this study, we investigated whether the same mechanism underlies the radiosensitivity of NBS cells. GM7166VA7 cells lacking NBS1 protein displayed a phenotype of increased radiosensitivity, while the introduction of NBS1 cDNA conferred radioresistance comparable to normal cells. IGF-IR expression levels were essentially the same among normal, NBS, and NBS1-complemented NBS cells. There was no significant difference between NBS and NBS1-complemented cells in activation of major downstream pathways of IGF-IR upon IGF-I stimulation, including phosphatidylinositol-3(') kinase (PI3-K) and mitogen-activated protein kinase (MAPK). Collectively, IGF-IR-related events are unlikely to be disrupted in NBS cells, and therefore, defects in IGF-IR signaling do not explain the increased radiosensitivity of NBS cells.

ATM mutations in sporadic lymphoid tumours

Patients with the autosomal recessive disorder ataxia telangiectasia (A-T) show the biallelic inactivation of the ataxia telangiectasia mutated (ATM) gene. A-T patients exhibit a predisposition to the development of a wide range of lymphoid tumours, suggesting that the ATM protein normally plays an important role in the prevention of both T and B cell malignancies. The ATM protein is a 370 kDa protein kinase implicated in the integration of different cellular responses to particular forms of DNA damage. Several recent studies have reported the possibility that the ATM gene can act as a tumour suppressor gene in non A-T individuals. Frequent ATM inactivation was confirmed in three sporadic lymphoid tumours of mature phenotype: T cell prolymphocytic leukaemia (T-PLL), B-cell chronic lymphocytic leukaemia (B-CLL) and mantle cell lymphoma (MCL). Here, we provide a summary of the published ATM mutations in sporadic lymphoid tumours, including our own study on the role of ATM mutations in the pathogenesis of sporadic B-CLL. The published results suggest possible differences in the origin, the nature and distribution of ATM mutations between sporadic B-CLL, MCL and T-PLL. While ATM mutations in mature B cell tumours (B-CLL and MCL) represent a mixture of missense and truncating errors distributed across the whole of the ATM coding sequence, mutations in sporadic T-PLL appear to be predominantly missense, clustering in the region encoding the PI-3 kinase catalytic domain of the protein. The reason for this difference is unclear, but the difference itself supports the notion that the pathogenesis of B and T cell tumours on an ATM deficient background might be different. In addition, in both B-CLL and MCL ATM mutation carriers have been reported, raising the possibility that ATM mutation carriers may have an increased risk of developing these tumours. The existence as well as magnitude of the risk, however, remains to be established. Furthermore, our own studies indicate that the presence of ATM mutations in sporadic B-CLL causes a distinctive defect in response to DNA damaging agents, offering a possible explanation for the poor response of ATM mutant tumours to standard treatment. Therefore, one of the future challenges will be to devise strategies to bypass the existing defect in response to DNA damage and activate apoptosis in ATM mutant sporadic lymphoid tumours.

Radiation-induced oral mucositis in mice: strain differences

The present study was initiated to investigate strain differences in oral mucosal radiosensitivity in mice with regard to induction of clinically manifest ulceration. Mouse ventral tongue epithelium was used as an established animal model for radiobiological studies of radiation-induced mucositis. Mice from two different strains, C3H/Neu (n = 40) from the Dresden colony, and B6D2F1 (n = 50) from the Harlan/Winkelmann UK colony were subjected to irradiation of tongue mucosa. Graded single doses were applied to a 3 x 3 mm2 test field in the centre of the lower tongue with 25 kV X-rays in order to generate full dose-effect curves for acute mucosal ulceration, as a clinically relevant reaction. For both groups, dose-effect curves were computed by logit analysis; comparison of the curves was by maximum-likelihood chi2 test. In addition, the time course of ulceration, i.e. latent time and individual ulcer duration, was analysed. In both mouse strains, a well-defined dose effect was observed. The ED50 values, i.e. the doses at which ulceration is expected in 50% of the animals irradiated, and their standard deviation sigma, calculated by logit analysis, can be used to describe radiosensitivity. The ED50 was 11.0 +/- 3.4 Gy (95% confidence interval (7.2; 15.4), P for dose dependence: 0.014) and 13.4 +/- 3.6 Gy (95% confidence interval (10.6; 16.1), P for dose dependence: 0.0002) in C3H and BDF1 mice, respectively. Hence, oral mucosa in BDF1 mice was found to be marginally more radioresistant (P = 0.1). The latent time to ulceration, i.e. the time between irradiation and first diagnosis of ulcer, was 11.6 +/- 0.2 days (mean +/- SEM, n = 18) in C3H mice and 5.6 +/- 0.1 days (n = 27) in BDF1 mice (P = 0.0001). Both were independent of dose (PC3H = 0.94, PBDF1 = 0.33) and hence were calculated for all responding animals of the respective strain. Ulcer duration was 2.8 +/- 0.2 days and 2.4 +/- 0.1 days in C3H and B6 mice, respectively, and was also independent of dose (PC3H = 0.25, PBDF1 = 0.99), but was dependent on the mouse strain (P = 0.036). In conclusion, no statistically significant difference in oral mucosal radiosensitivity was observed between the mouse strains. This, however, may be attributed to the small number of animals used per dose group. The time course data, with a shorter latency and ulcer duration in BDF1 mice, are in good accordance with the higher proliferation rates reported for oral mucosa in this mouse strain.

The two faces of BRCA2, a FANCtastic discovery

The molecular defects responsible for the cancer predisposition syndrome Fanconi's Anemia (FA) have been elusive. A recent study reports that the FANC-B and -D1 subgroups result from hypomorphic mutations in BRCA2. Given that BRCA2 protein participates in homologous recombination, this finding connects at least a subset of the FA phenotypes to defective DNA repair.

Nbs1 promotes ATM dependent phosphorylation events including those required for G1/S arrest

Cell lines from Nijmegen Breakage Syndrome (NBS) and ataxia telangiectasia (A-T) patients show defective S phase checkpoint arrest. In contrast, only A-T but not NBS cells are significantly defective in radiation-induced G1/S arrest. Phosphorylation of some ATM substrates has been shown to occur in NBS cells. It has, therefore, been concluded that Nbs1 checkpoint function is S phase specific. Here, we have compared NBS with A-T cell lines (AT-5762ins137) that express a low level of normal ATM protein to evaluate the impact of residual Nbs1 function in NBS cells. The radiation-induced cell cycle response of these NBS and 'leaky' A-T cells is almost identical; normal G2/M arrest after 2 Gy, intermediate G1/S arrest depending on the dose and an A-T-like S phase checkpoint defect. Thus, the checkpoint assays differ in their sensitivity to low ATM activity. Radiation-induced phosphorylation of the ATM-dependent substrates Chk2, RPAp34 and p53-Ser15 are similarly impaired in AT-5762ins137 and NBS cells in a dose dependent manner. In contrast, NBS cells show normal ability to activate ATM kinase following irradiation in vitro and in vivo. We propose that Nbs1 facilitates ATM-dependent phosphorylation of multiple downstream substrates, including those required for G1/S arrest.

Regulation of genome stability by TEL1 and MEC1, yeast homologs of the mammalian ATM and ATR genes

In eukaryotes, a family of related protein kinases (the ATM family) is involved in regulating cellular responses to DNA damage and telomere length. In the yeast Saccharomyces cerevisiae, two members of this family, TEL1 and MEC1, have functionally redundant roles in both DNA damage repair and telomere length regulation. Strains with mutations in both genes are very sensitive to DNA damaging agents, have very short telomeres, and undergo cellular senescence. We find that strains with the double mutant genotype also have approximately 80-fold increased rates of mitotic recombination and chromosome loss. In addition, the tel1 mec1 strains have high rates of telomeric fusions, resulting in translocations, dicentrics, and circular chromosomes. Similar chromosome rearrangements have been detected in mammalian cells with mutations in ATM (related to TEL1) and ATR (related to MEC1) and in mammalian cells that approach cell crisis.

A single low dose of X-rays induces high frequencies of genetic instability (aneuploidy) and heritable damage (apoptosis), dependent on cell type and p53 status

We harvested and analyzed cells from four different non-transformed cell lines surviving a single X-ray exposure. Evidence of radiation-induced karyotype instability was observed in 100% of C3H 10T1/2 fibroblast clones and 11.3% of V79 fibroblast clones. Heritable damage: predisposition to apoptosis, but not karyotype instability, was induced in TK6 (p53(wt/wt)) and WTK1 (p53(mut/mut)) human B-lymphoblastoid cell clones. The studies indicate: (1) genetic instability and/or heritable damage are induced in cells exposed to radiation at a high frequency, and induction of genetic instability is not limited to morphologically transformed cells [Radiat. Res. 138 (1994) S105; Radiat. Environ. Biophys. 36 (1998) 255]; (2) sensitivity to genetic instability and heritable damage depend on cell type; (3) checkpoint stringency and p53 status significantly influence the frequency of radiation-induced genetic instability and heritable damage; (4) in some cell lines, damage induced by low doses of radiation (below 2 Gy) leads to heritable cytotoxic and genotoxic effects in 100% of cells exposed. The data suggest that mammalian cells misinterpret damage induced by ionizing radiation as if it were a physiological cell signal. This contrasts strongly with the response of mammalian cells to damage induced by other types of DNA-toxic agents where damage-specific repair mechanisms are activated.

The Mre11 complex: at the crossroads of dna repair and checkpoint signalling

The Mre11 complex is a multisubunit nuclease that is composed of Mre11, Rad50 and Nbs1/Xrs2. Mutations in the genes that encode components of this complex result in DNA- damage sensitivity, genomic instability, telomere shortening and aberrant meiosis. The molecular defect that underlies these phenotypes has long been thought to be related to a DNA repair deficiency. However, recent studies have uncovered functions for the Mre11 complex in checkpoint signalling and DNA replication.

G2-phase radiation response in lymphoblastoid cell lines from Nijmegen breakage syndrome

The relationship between G2-phase checkpoint activation, cytoplasmic cyclin-B1 accumulation and nuclear phosphorylation of p34CDC2 was studied in Nijmegen breakage syndrome cells treated with DNA damaging agents. Experiments were performed on lymphoblastoid cell lines from four Nijmegen breakage syndrome patients with different mutations, as well as on cells from an ataxia telangiectasia patient. Lymphoblastoid cell lines were irradiated with 0.50-2 Gy X-rays and the percentage of G2-phase accumulated cells was evaluated by means of flow cytometry in samples that were harvested 24 h later. The G2-checkpoint activation was analysed by scoring the mitotic index at 2 and 4 h after treatment with 0.5 and 1 Gy X-rays and treatment with the DNA double-strand break inducer calicheamicin-gamma1. Cytoplasmic accumulation of cyclin-B1 was evaluated by means of fluorescence immunostaining or Western blotting, in cells harvested shortly after irradiation with 1 and 2 Gy. The extent of tyrosine 15-phosphorylated p34CDC2 was assessed in the nuclear fractions. Nijmegen breakage syndrome cells showed suboptimal G2-phase checkpoint activation respect to normal cells and were greatly different from ataxia telangiectasia cells. Increased cytoplasmic cyclin-B1 accumulation was detected by both immunofluorescence and immunoblot in normal as well as in Nijmegen breakage syndrome cells. Furthermore, nuclear p34CDC2. phosphorylation was detected at a higher level in Nijmegen breakage syndrome than in ataxia telangiectasia cells. In conclusion, our data do not suggest that failure to activate checkpoints plays a major role in the radiosensitivity of Nijmegen breakage syndrome cells.

A murine model of Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder characterized by microcephaly, immunodeficiency, and predisposition to hematopoietic malignancy. The clinical and cellular phenotypes of NBS substantially overlap those of ataxia-telangiectasia (A-T). NBS is caused by mutation of the NBS1 gene, which encodes a member of the Mre11 complex, a trimeric protein complex also containing Mre11 and Rad50. Several lines of evidence indicate that the ataxia-telangiectasia mutated (ATM) kinase and the Mre11 complex functionally interact. Both NBS and A-T cells exhibit ionizing radiation (IR) sensitivity and defects in the intra S phase checkpoint, resulting in radioresistant DNA synthesis (RDS)-the failure to suppress DNA replication origin firing after IR exposure. NBS1 is phosphorylated by ATM in response to IR, and this event is required for activation of the intra S phase checkpoint (the RDS checkpoint). We derived a murine model of NBS, the Nbs1(DeltaB/DeltaB) mouse. Nbs1(DeltaB/DeltaB) cells are phenotypically identical to those established from NBS patients. The Nbs1(DeltaB) allele was synthetically lethal with ATM deficiency. We propose that the ATM-Mre11 complex DNA damage response pathway is essential and that ATM or the Mre11 complex serves as a nexus to additional components of the pathway.

Suppression of genome instability by redundant S-phase checkpoint pathways in Saccharomyces cerevisiae

Cancer cells show increased genome rearrangements, although it is unclear what defects cause these rearrangements. Previous studies have implicated the Saccharomyces cerevisiae replication checkpoint in the suppression of spontaneous genome rearrangements. In the present study, low doses of methyl methane sulfonate that activate the intra-S checkpoint but not the G1 or G2 DNA damage checkpoints were found to cause increased accumulation of genome rearrangements in both wild-type strains and to an even greater extent in strains containing mutations causing defects in the intra-S checkpoint. The rearrangements were primarily translocations or events resulting in deletion of a portion of a chromosome arm along with the addition of a new telomere. Combinations of mutations causing individual defects in the RAD24 or SGS1 branches of the intra-S checkpoint or the replication checkpoint showed synergistic interactions with regard to the spontaneous genome instability rate. PDS1 and the RAD50-MRE11-XRS2 complex were found to be important members of all the S-phase checkpoints in suppressing genome instability, whereas RAD53 only seemed to play a role in the intra-S checkpoints. Combinations of mutations that seem to result in inactivation of the S-phase checkpoints and critical effectors resulted in as much as 12,000-14,000-fold increases in the genome instability rate. These data support the view that spontaneous genome rearrangements result from DNA replication errors and indicate that there is a high degree of redundancy among the checkpoints that act in S phase to suppress such genome instability.

ATM: genome stability, neuronal development, and cancer cross paths

One of the cornerstones of the web of signaling pathways governing cellular life and differentiation is the DNA damage response. It spans a complex network of pathways, ranging from DNA repair to modulation of numerous processes in the cell. DNA double-strand breaks (DSBs), which are formed as a result of genotoxic stress or normal recombinational processes, are extremely lethal lesions that rapidly mobilize this intricate defense system. The master controller that pilots cellular responses to DSBs is the ATM protein kinase, which turns on this network by phosphorylating key players in its various branches. ATM is the protein product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, sterility, genomic instability, cancer predisposition, and radiation sensitivity. The clinical and cellular phenotype of A-T attests to the numerous roles of ATM, on the one hand, and to the link between the DNA damage response and developmental processes on the other hand. Recent studies of this protein and its effectors, combined with a thorough investigation of animal models of A-T, have led to new insights into the mode of action of this master controller of the DNA damage response. The evidence that ATM is involved in signaling pathways other than those related to damage response, particularly ones relating to cellular growth and differentiation, reinforces the multifaceted nature of this protein, in which genome stability, developmental processes, and cancer cross paths.

Human MRE11 is inactivated in mismatch repair-deficient cancers

Mutations of the ATM and NBS1 genes are responsible for the inherited Ataxia-Telangiectasia and Nijmegen Breakage Syndrome, both of which are associated with a predisposition to cancer. A related syndrome, the Ataxia-Telangiectasia-like disorder, is due to mutations of the MRE11 gene. However, the role of this gene in cancer development has not been established. Here we describe an often homozygous mutation of the poly(T)11 repeat within human MRE11 intron 4 that leads to aberrant splicing, impairment of wild-type MRE11 expression and generation of a truncated protein. This mutation is present in mismatch repair-deficient, but not proficient, colorectal cancer cell lines and primary tumours and is associated with reduced expression of the MRE11--NBS1--RAD50 complex, an impaired S-phase checkpoint and abrogation of MRE11 and NBS1 ionizing radiation-induced nuclear foci. Our findings identify MRE11 as a novel and major target for inactivation in mismatch repair-defective cells and suggest its impairment may contribute to the development of colorectal cancer.

Arsenic-induced Mre11 phosphorylation is cell cycle-dependent and defective in NBS cells

Cancer-prone diseases ataxia-telangiectasia (AT), Nijmegen breakage syndrome (NBS) and ataxia-telangiectasia-like disorder (ATLD) are defective in the repair of DNA double-stranded break (DSB). On the other hand, arsenic (As) has been reported to cause DSB and to be involved in the occurrence of skin, lung and bladder cancers. To dissect the repair mechanism of As-induced DSB, wild type, AT and NBS cells were treated with sodium arsenite to study the complex formation and post-translational modification of Rad50/NBS1/Mre11 repair proteins. Our results showed that Mre11 went through cell cycle-dependent phosphorylation upon sodium arsenite treatment and this post-translational modification required NBS1 but not ATM. Defective As-induced Mre11 phosphorylation was rescued by reconstitution with full length NBS1 in NBS cells. Although As-induced Mre11 phosphorylation was not required for Rad50/NBS1/Mre11 complex formation, it might be required for the formation of Rad50/NBS1/Mre11 nuclear foci upon DNA damage.

Fission yeast Rad26 is a regulatory subunit of the Rad3 checkpoint kinase

Fission yeast Rad3 is a member of a family of phosphoinositide 3-kinase -related kinases required for the maintenance of genomic stability in all eukaryotic cells. In fission yeast, Rad3 regulates the cell cycle arrest and recovery activities associated with the G2/M checkpoint. We have developed an assay that directly measures Rad3 kinase activity in cells expressing physiological levels of the protein. Using the assay, we demonstrate directly that Rad3 kinase activity is stimulated by checkpoint signals. Of the five other G2/M checkpoint proteins (Hus1, Rad1, Rad9, Rad17, and Rad26), only Rad26 was required for Rad3 kinase activity. Because Rad26 has previously been shown to interact constitutively with Rad3, our results demonstrate that Rad26 is a regulatory subunit, and Rad3 is the catalytic subunit, of the Rad3/Rad26 kinase complex. Analysis of Rad26/Rad3 kinase activation in rad26.T12, a mutant that is proficient for cell cycle arrest, but defective in recovery, suggests that these two responses to checkpoint signals require quantitatively different levels of kinase activity from the Rad3/Rad26 complex.

Two molecularly distinct G(2)/M checkpoints are induced by ionizing irradiation

Cell cycle checkpoints are among the multiple mechanisms that eukaryotic cells possess to maintain genomic integrity and minimize tumorigenesis. Ionizing irradiation (IR) induces measurable arrests in the G(1), S, and G(2) phases of the mammalian cell cycle, and the ATM (ataxia telangiectasia mutated) protein plays a role in initiating checkpoint pathways in all three of these cell cycle phases. However, cells lacking ATM function exhibit both a defective G(2) checkpoint and a prolonged G(2) arrest after IR, suggesting the existence of different types of G(2) arrest. Two molecularly distinct G(2)/M checkpoints were identified, and the critical importance of the choice of G(2)/M checkpoint assay was demonstrated. The first of these G(2)/M checkpoints occurs early after IR, is very transient, is ATM dependent and dose independent (between 1 and 10 Gy), and represents the failure of cells which had been in G(2) at the time of irradiation to progress into mitosis. Cell cycle assays that can distinguish mitotic cells from G(2) cells must be used to assess this arrest. In contrast, G(2)/M accumulation, typically assessed by propidium iodide staining, begins to be measurable only several hours after IR, is ATM independent, is dose dependent, and represents the accumulation of cells that had been in earlier phases of the cell cycle at the time of exposure to radiation. G(2)/M accumulation after IR is not affected by the early G(2)/M checkpoint and is enhanced in cells lacking the IR-induced S-phase checkpoint, such as those lacking Nbs1 or Brca1 function, because of a prolonged G(2) arrest of cells that had been in S phase at the time of irradiation. Finally, neither the S-phase checkpoint nor the G(2) checkpoints appear to affect survival following irradiation. Thus, two different G(2) arrest mechanisms are present in mammalian cells, and the type of cell cycle checkpoint assay to be used in experimental investigation must be thoughtfully selected.

DNA damage processing defects and disease

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

Ataxia telangiectasia mutated-deficient B-cell chronic lymphocytic leukemia occurs in pregerminal center cells and results in defective damage response and unrepaired chromosome damage

B-cell chronic lymphocytic leukemia (B-CLL) is a heterogeneous disease involving more than one molecular mechanism that leads to the transformation of CD5(+) B cells at either the pregerminal or postgerminal center stage of differentiation. It was previously demonstrated that ataxia telangiectasia mutated (ATM) gene mutations can occur in B-CLL and cause a defect in the p53 pathway. Here the role of ATM mutations in the pathogenesis of B-CLL is addressed. Of 50 B-CLL tumors with fully analyzed ATM and TP53, 16 had ATM mutations. Six of 50 B-CLLs showed mutations in TP53 and the remaining 28 tumors had wild-type ATM or TP53. No tumor had both ATM and TP53 mutations. Remarkably, all 16 ATM mutant B-CLLs showed the absence of somatic variable region heavy chain hypermutation indicating a pregerminal center cell origin and a common pathogenesis for these tumors. Furthermore, in 5 of the 16 B-CLLs, ATM mutation preceded the transformation stage of differentiation. At the cellular level, ATM mutant tumors exhibited a deficient ATM-dependent p53 response to gamma irradiation, failure to up-regulate TRAIL-R2, a downstream target that links irradiation-induced p53 response with apoptosis, and an inability to repair induced chromosome breaks. Mantle cell lymphoma (MCL) is also of pregerminal center origin and ATM mutations are frequent in this malignancy. It is concluded that ATM is likely to play an important role at the pregerminal center stage and a model is proposed where loss of ATM function during B-cell ontogeny drives B-CLL tumorigenesis in pregerminal B cells by a dual defect in p53 damage response and repair of chromosome breaks.

An antisense construct of full-length human RAD50 cDNA confers sensitivity to ionizing radiation and alkylating agents on human cell lines

In Saccharomyces cerevisiae, Rad50 is reported to participate in the repair of double-stranded DNA breaks, and most rad50 mutants are unable to repair gamma-ray-induced DNA damage. In this study, we examined whether human RAD50 is involved in the repair of DNA damage induced by gamma radiation, radiomimetic alkylating agents, or UVB radiation in cultured human cells. Because homozygous null RAD50 mutant cells could not be isolated, human 293 embryonic kidney cells and A431 epithelial tumor cells were transfected with antisense RAD50 cDNA to obtain viable cell lines which expressed reduced RAD50. Selected individual clones were subjected to PCR-Southern and Western blot analyses to confirm the integrity of the antisense RAD50 construct and the reduced RAD50 expression levels. The cells engineered to express reduced RAD50 levels showed significantly increased sensitivity to gamma radiation, mitomycin C and methylmethane sulfonate compared with control cells that were transfected with the vector alone. However, there were no differences in viability of cells with reduced RAD50 levels and control cells treated with UVB radiation. These results indicate that human RAD50 is involved in the repair of DNA damage induced by gamma radiation and alkylating agents in mammalian cells and suggest the possible application of antisense RAD50 cDNA transfection as a radiation sensitizer in radiation oncology.

Immune diversity and genomic stability: opposite goals but similar paths

DNA damage response mechanisms serve to protect cells from exogenous and endogenous DNA damaging agents with the aim of maintaining genomic stability. In contrast, the generation of an efficient immune response requires the creation of a repertoire of distinct immunoglobulin and T cell receptor genes able to recognise the huge array of antigens that may be encountered in a lifetime. Surprisingly, cells have exploited the same mechanisms used to maintain genomic integrity to create genetic diversity during immune development. Here, we review the damage response mechanisms operating on DNA double strand breaks and their function during development of the immune response. We discuss disorders that are associated with immunodeficiency and defective responses to the presence of DNA double strand breaks.

Decreased immunoglobulin class switching in Nijmegen Breakage syndrome due to the DNA repair defect

Nijmegen breakage syndrome (NBS) is a rare chromosomal-instability syndrome associated with defective DNA repair. Approximately 90% of NBS patients are homozygous for a truncating mutation of the NBS1 gene. As development of the immune system relies on recombination, which involves repair of DNA breaks, one might predict that mutations in the NBS1 gene could cause immunodeficiency. We immunologically investigated the world's largest series of NBS patients (n = 74), confirmed immunodeficiency, and found a discrepancy between relatively normal IgM concentrations, and decreased IgG and IgA concentrations. In addition, a significant relation between low IgA and low IgG levels was found. These data are compatible with a defective class switching in NBS and can be explained by a role of the NBS1 protein in DNA repair, signal transduction, cell cycle regulation or apoptosis.

High frequency of deletions at the hypoxanthine-guanine phosphoribosyltransferase locus in an ataxia-telangiectasia lymphoblastoid cell line irradiated with gamma-rays

The molecular nature of gamma-ray-induced mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus in an ataxia-telangiectasia (A-T) lymphoblastoid cell line was investigated. Twelve of 15 gamma-ray-induced HPRT-deficient mutants showed deletions. Eight of them had lost the entire HPRT gene, one showed a 1.9-kb deletion, and three had deletions of about 40-150 base pairs. Of the eight mutants that lost the entire gene, five had also lost both DXS79 and DXS86, flanking markers of the HPRT locus. The spectrum of mutations induced by gamma-irradiation in the A-T cells showed a high frequency of deletions in comparison with that in a control cell line, WIL2-NS. Sequence analysis of breakpoint junctions in four mutants revealed that three of them had junctions between short identical sequences at each breakpoint, leaving one copy at the junction. These results suggest that non-homologous end-joining is the major mechanism for deletion formation in A-T cells.

Promotion of Dnl4-catalyzed DNA end-joining by the Rad50/Mre11/Xrs2 and Hdf1/Hdf2 complexes

S. cerevisiae RAD50, MRE11, and XRS2 genes are required for telomere maintenance, cell cycle checkpoint signaling, meiotic recombination, and the efficient repair of DNA double-strand breaks (DSB)s by homologous recombination and nonhomologous end-joining (NHEJ). Here, we demonstrate that the complex formed by Rad50, Mre11, and Xrs2 proteins promotes intermolecular DNA joining by DNA ligase IV (Dnl4) and its associated protein Lif1. Our results show that the Rad50/Mre11/Xrs2 complex juxtaposes linear DNA molecules via their ends to form oligomers and interacts directly with Dnl4/Lif1. We also demonstrate that Rad50/Mre11/Xrs2-mediated intermolecular DNA joining is further stimulated by Hdf1/Hdf2, the yeast homolog of the mammalian Ku70/Ku80 heterodimer. These studies reveal specific functional interplay among the Hdf1/Hdf2, Rad50/Mre11/Xrs2, and Dnl4/Lif1 complexes in NHEJ.

Abnormal regulation of DDB2 gene expression in xeroderma pigmentosum group E strains

A damage-specific DNA binding protein (DDB) activity is absent from a subset (DDB(-)) of cells from individuals initially classified as group E of xeroderma pigmentosum (XP), a hereditary, photosensitive disease with a high incidence of skin malignancies. In these cases, mutations have been identified in the DDB2 gene (DDB2(-)) that codes for the small subunit, p48, of the DDB heterodimer. In four DDB2(- )strains, neither p48 nor DDB activity were observed before or after UV-irradiation, despite an unusually strong up-regulation of DDB2 mRNA levels after UV-irradiation. In a fifth strain, XP82TO, p48 was detectable and both DDB2 mRNA and p48 levels were more up-regulated after UV-irradiation than in normal primary cells. Moreover, DDB activity also became apparent after irradiation. XP82TO showed very mild clinical manifestations compared with the other DDB(-) patients. These results, coupled with our findings that most, if not all DDB(+) cells classified as XP-E were misclassified, suggests a direct correlation between DDB2 levels and the XP-E phenotype.

ATM as a target for novel radiosensitizers

DNA damage checkpoints are complex signal transduction pathways that are critical for normal cellular recovery following potentially lethal genotoxic insults. The ataxia-telangiectasia mutated (ATM) protein kinase is a critical component in these pathways and integrates the cellular response to damage by phosphorylating key proteins involved in cell cycle regulation and DNA repair. Lack of normal ATM function in the inherited ataxia-telangiectasia (A-T) syndrome results in a pleiotropic clinical syndrome characterized by a marked increased risk of cancer and profound hypersensitivity to ionizing radiation. Cells derived from patients with A-T share some of these attributes with genomic instability, loss of normal cell cycle arrest pathways, defects in DNA repair and increased radiation sensitivity. The radiosensitivity of A-T cells suggests that pharmacological inhibitors of the ATM kinase should be effective radiosensitizing agents. In fact, caffeine inhibits ATM kinase activity at concentrations that result in an A-T-like phenotype with loss of cell cycle checkpoints and hypersensitivity to ionizing radiation. Although the clinical use of caffeine as a radiosensitizer is limited by potentially lethal systemic toxicities, more potent methyl xanthines may selectively inhibit the ATM pathway at clinically achievable levels. Interestingly, caffeine and other methyl xanthines preferentially radiosensitize cells that lack normal p53 function. Because p53 is commonly inactivated in epithelial malignancies, this suggests that small molecule inhibitors of ATM might selectively sensitize the majority of tumors to the lethal effects of ionizing radiation while sparing normal tissues.

Environmental causes of human cancers

Epidemiological studies have clearly shown a causal association between tobacco exposure and various human cancers, hepatitis B and C infection and hepatocellular carcinoma, human papilloma viruses and cervical cancer, and the occupational origin of certain human cancers is well established. The identification of the environmental causes of human cancers has been a long and difficult process. Much remains to be understood about the role of specific components of the diet and the interaction of different risk factors in the aetiology of human cancers. Withstanding the progress made on the understanding of the cancer process and their potential impact in the therapy of cancer, primary prevention remains, in developed and developing countries, the most effective measure to reduce cancer mortality.

A case of PDH-E1 alpha mosaicism in a male patient with severe metabolic lactic acidosis

We have characterized a novel mutation in a male patient that affects the coding sequence of PDH-E1 alpha gene and changes arginine-141 to a leucine. This nucleotide substitution was found in about 75% of the studied DNA (fibroblasts, liver and muscle), a scenario that would indicate a case of E1 alpha mosaicism in a male patient. When the mutant E1 alpha protein was expressed in human skin fibroblasts with zero endogenous pyruvate dehydrogenase complex activity and E1 alpha protein expression, no significant restoration of activity was recorded, in contrast to the wild-type cDNA. even though both wild-type and mutant protein levels were comparable. We concluded that the R141L mutation is a severe one and that it must have occurred in one of the E1 alpha alleles during early embryogenesis.

Futile caspase-8 activation during the apoptotic cell death induced by DNA damaging agents in human B-lymphoblasts

Caspase-8 plays an essential role in apoptosis induced by Fas activation. Moreover, caspase-8 can be processed also in response to exposure to genotoxic agents. To decipher the role of caspase-8 in DNA damaging agent (DDA)-induced apoptosis as well as the pathway(s) leading to its activation in response to genotoxic stress, we investigated caspase-8 processing induced by ionizing radiation (IR) or mitomycin C (MMC) treatment in human B-lymphoblasts. Altogether, our observations establish that caspase-8 is actively processed in both receptor-mediated and DDA-induced cell death. However, while Fas-dependent apoptosis absolutely required caspase-8 activity, it is not necessary for completion of the apoptotic program induced by IR and MMC. Experiments performed to understand the molecular pathway(s) of the caspase-8 activation after DDA demonstrated that for both IR and MMC, the Fas/Fas-L interaction is dispensable. Data obtained from caspase inhibitors and from lymphoblasts carrying mutations in ATM and FANCC proteins, involved in DDA response, clearly showed that distinct mechanisms are responsible for caspase-8 activation by IR and MMC in B-lymphoblasts. IR-dependent processing of caspase-8 involves ATM, mitochondrial collapse, FANCC, and caspase-3 activation. Caspase-8 activation by MMC evokes the mitochondrial pathways involving FANCC but not ATM. Collectively, our data indicate that caspase-8 activation is essentially a bystander effect and not a major determinant of the behavior of DDA-exposed cells.

Residual ataxia telangiectasia mutated protein function in cells from ataxia telangiectasia patients, with 5762ins137 and 7271T-->G mutations, showing a less severe phenotype

We have assessed several ataxia Telangiectasia mutated (ATM)-dependent functions in cells derived from ataxia telangiectasia patients, carrying either an ATM 5762ins137 splice site or a 7271T-->G missense mutation, with a less severe phenotype compared with the classical disorder. ATM kinase in vitro, from 5762ins137 cells, showed the same specific activity as ATM in normal cells, but the protein was present at low levels. In contrast, mutant ATM kinase activity in the 7271T-->G cells was only about 6% that of the activity in normal cells, although the level of mutant protein expressed was similar to normal cells. Phosphorylation of the DNA double strand break repair proteins Nbs1 and hMre11, following DNA damage, was observed in normal and 7271T-->G cells but was almost absent in both 5762ins137 and classical ataxia telangiectasia cells. The kinetics of p53 response was intermediate between normal and classical ataxia telangiectasia cells in both the 7271T-->G and 5762ins137 cells, but interestingly, c-Jun kinase activation following DNA damage was equally deficient in cell lines derived from all the ataxia telangiectasia patients. Our results indicate that levels of ATM kinase activity, but not induction of p53 or c-Jun kinase activity, in these cells correlate with the degree of neurological disorder in the patients.

Mre11 protein complex prevents double-strand break accumulation during chromosomal DNA replication

Mre11 complex promotes repair of DNA double-strand breaks (DSBs). Xenopus Mre11 (X-Mre11) has been cloned, and its role in DNA replication and DNA damage checkpoint studied in cell-free extracts. DSBs stimulate the phosphorylation and 3'-5' exonuclease activity of X-Mre11 complex. This induced phosphorylation is ATM independent. Phosphorylated X-Mre11 is found associated with replicating nuclei. X-Mre11 complex is required to yield normal DNA replication products. Genomic DNA replicated in extracts immunodepleted of X-Mre11 complex accumulates DSBs as demonstrated by TUNEL assay and reactivity to phosphorylated histone H2AX antibodies. In contrast, the ATM-dependent DNA damage checkpoint that blocks DNA replication initiation is X-Mre11 independent. These results strongly suggest that the function of X-Mre11 complex is to repair DSBs that arise during normal DNA replication, thus unraveling a critical link between recombination-dependent repair and DNA replication.

Role of Nijmegen breakage syndrome protein in specific T-lymphocyte activation pathways

Nijmegen breakage syndrome (NBS) is a genetic disorder characterized by immunodeficiency, microcephaly, and "bird-like" facies. NBS shares some clinical features with ataxia telangiectasia (AT), including increased sensitivity to ionizing radiation, increased spontaneous and induced chromosome fragility, and strong predisposition to lymphoid cancers. The mutated gene that results in NBS codes for a novel double-stranded DNA break repair protein, named nibrin. In the present work, a Spanish NBS patient was extensively characterized at the immunological and the molecular DNA levels. He showed low CD3(+)-cell numbers and an abnormal low CD4(+) naive cell/CD4(+) memory cell ratio, previously described in AT patients and also described in the present report in the NBS patient. The proliferative response of peripheral blood lymphocytes in vitro to mitogens is deficient in NBS patients, but the possible link among NBS mutations and the abnormal immune response is still unknown.