A double-strand break repair defect in ATM-deficient cells contributes to radiosensitivity

[Ataxia-telangiectasia: a review]

Ataxia-telangiectasia (AT) is an autosomal recessive inherited disease caused by mutational inactivation of the ATM gene. It is a multisystemic disease, characterized by progressive neurological dysfunction, especially in the cerebellum, oculo-cutaneous telangiectasia, immunodeficiency, recurrent sino-pulmonary infections and high incidence of neoplasms. The responsible gene, ATM, encodes a large protein that belongs to a family of protein kinases with a phosphatidylinositol 3-kinase (Pi3K) domain. ATM is a key regulator of cell cycle checkpoints that causes DNA repair or apoptosis. Several studies report ATM function in target cells (such as neurons, fibroblast, endothelium, germ cells, lymphocytes). The pleiotropic phenotypes of AT reflect the multifaceted activities of ATM protein. In nucleus (lymphocytes, fibroblasts, germ cells) ATM is involved in regulation of cell-cycle checkpoints; in cytoplasm ATM regulates redox state (neurons).

Atm-haploinsufficiency enhances susceptibility to carcinogen-induced mammary tumors

Ataxia-telangiectasia (A-T), which is due to mutations in the ATM gene, is a rare autosomal recessive genomic instability syndrome characterized by radiosensitivity and predisposition to cancer. Epidemiological studies have suggested that relatives of A-T patients (A-T carriers) have increased risks of developing breast cancer. We propose that increased breast cancer risks in A-T carriers may be due to exposure to various environmental carcinogens and/or dietary consumption. To test this hypothesis, we treated a congenic strain of Atm+/- mice with DMBA (7,12-dimethylbenz(alpha)anthracene), a mammary carcinogen, and observed mammary tumor incidence. It was found that Atm+/- mice have a 2-fold increase, as well as early onset, in mammary tumor incidence relative to wild-type mice (P<0.005). The increased mammary tumor development is correlated with a 3-fold increase in the development of mammary dysplasia in Atm+/- compared with wild-type mice (P<0.05). We also found that Ras signaling pathway was not activated in DMBA-induced mammary tumors irrespective of the Atm status. At the cellular level, Atm-haploinsufficiency confers increased cellular stress manifested by an increased p53 expression and a slightly enhanced survival of mammary epithelial cells in response to radiation. Our results demonstrate that Atm heterozygotes are predisposed to mammary tumor development and support the hypothesis that exposure to environmental carcinogens contributes to the increased rate of breast cancer development in A-T carriers. Given that 1% of the general population are ATM heterozygotes (A-T carriers), this study has great implications in breast cancer development in this population.

Assessing the likelihood of severe side effects in radiotherapy

Ionizing radiation is a powerful tool to treat cancer. The curing effect is mainly based on the efficiency of ionizing radiation to kill the cancer cells and it is believed that DNA double-strand breaks (DSBs) represent the most significant genetic lesion introduced by radiation that causes cell killing. One limitation in radiotherapy is the unavoidable damage delivered to the normal, noncancer cells that can give rise to side effects. The ultimate goal in treatment planning is to maximize cell killing in the tumor by minimizing damage induction in the normal tissue surrounding the tumor. The biological response to the induction of DSBs is largely affected by DSB repair processes and it has, therefore, been a long-standing goal to determine a patient's DSB repair capacity to "individualize" treatment planning. A recently developed DSB repair assay that allows the assessment of patients' repair capacity under in vivo conditions may provide a new approach to predict individuals' responses to radiotherapy and may be able to contribute to improvements in treatment planning.

Less efficient g2-m checkpoint is associated with an increased risk of lung cancer in African Americans

Cell cycle checkpoints play critical roles in the maintenance of genomic integrity. The inactivation of checkpoint genes by genetic and epigenetic mechanisms is frequent in all cancer types, as a less-efficient cell cycle control can lead to genetic instability and tumorigenesis. In an on-going case-control study consisting of 216 patients with non-small cell lung cancer, 226 population-based controls, and 114 hospital-based controls, we investigated the relationship of gamma-radiation-induced G2-M arrest and lung cancer risk. Peripheral blood lymphocytes were cultured for 90 hours, exposed to 1.0 Gy gamma-radiation, and harvested at 3 hours after gamma-radiation treatment. gamma-Radiation-induced G2-M arrest was measured as the percentage of mitotic cells in untreated cultures minus the percentage of mitotic cells in gamma-radiation-treated cultures from the same subject. The mean percentage of gamma-radiation-induced G2-M arrest was significantly lower in cases than in population controls (1.18 versus 1.44, P < 0.01) and hospital controls (1.18 versus 1.40, P = 0.01). When dichotomized at the 50th percentile value in combined controls (population and hospital controls), a lower level of gamma-radiation-induced G2-M arrest was associated with an increased risk of lung cancer among African Americans after adjusting for baseline mitotic index, age, gender, and pack-years of smoking [adjusted odd ratio (OR), 2.25; 95% confidence interval (95% CI), 0.97-5.20]. A significant trend of an increased risk of lung cancer with a decreased level of G2-M arrest was observed (P(trend) = 0.02) among African Americans, with a lowest-versus-highest quartile adjusted OR of 3.74 (95% CI, 0.98-14.3). This trend was most apparent among African American females (P(trend) < 0.01), with a lowest-versus-highest quartile adjusted OR of 11.75 (95% CI, 1.47-94.04). The results suggest that a less-efficient DNA damage-induced G2-M checkpoint is associated with an increased risk of lung cancer among African Americans. Interestingly, we observed a stronger association of DNA damage-induced G2-M arrest and lung cancer among African Americans when compared with Caucasians. If replicated, these results may provide clues to the exceedingly high lung cancer incidence experienced by African Americans.

Interstitial chromatin alteration causes persistent p53 activation involved in the radiation-induced senescence-like growth arrest

Various stresses including ionizing radiation give normal human fibroblasts a phenotype of senescence-like growth arrest (SLGA), manifested by p53-dependent irreversible G1 arrest. To determine the mechanism of persistent activation of p53, we examined phosphorylated Ataxia telangiectasia mutated (ATM) and phosphorylated histone H2AX foci formation after X-irradiation. Although the multiple tiny foci, detected soon after (<30 min) irradiation, gradually disappeared, some of these foci changed to large foci and persisted for 5 days. Large foci containing phosphorylated ATM and gamma-H2AX co-localized and foci with p53 phosphorylated at serine 15 also showed the same distribution. Interestingly, the signals obtained by telomere fluorescence in situ hybridization (FISH) assay did not co-localize with 90% of the large foci. Our results indicate that chromatin alteration in interstitial chromosomal regions is the most likely cause of continuous activation of p53, which results in the induction of SLGA by ionizing radiation.

Sensitization of breast carcinoma cells to ionizing radiation by small molecule inhibitors of DNA-dependent protein kinase and ataxia telangiectsia mutated

DNA-PK and ATM are members of the phosphatidylinositol 3'-kinase like kinase (PIKK) family of serine/threonine protein kinases and have critical roles in the cellular response to DNA double-strand breaks. Genetic loss of either activity leads to pronounced sensitivity to ionizing radiation (IR). Hence, these enzymes are potential targets to confer enhanced radiosensitivity on tumour cells. We show that novel inhibitors of either DNA-PK or ATM sensitize breast carcinoma cells to IR. Radiosensitization was accompanied by an apparent DNA repair deficit as measured by the persistence of IR-induced foci of phosphorylated histone H2AX (gammaH2AX foci). These specific inhibitors also allowed us to probe the biochemistry and kinetics of histone H2AX phosphorylation following gamma-irradiation in breast cancer cells with the aim of validating H2AX as a biomarker for DNA-PK or ATM inhibition in vivo. ATM inhibition reduced the initial average intensity of gammaH2AX foci while inhibition of DNA-PK had only a small effect on the initial phosphorylation of H2AX. However, simultaneous treatment with both compounds dramatically reduced gammaH2AX focus intensity, consistent with the reported role of ATM and DNA-PK in IR induced phosphorylation of H2AX.

Pre-exposure to Low Doses: Modulation of X-Ray-Induced DNA Damage and Repair?

The adaptive response to ionizing radiation may be mediated by the induction of antioxidant defense mechanisms, accelerated repair or altered cell cycle progression after the conditioning dose. To gain new insight into the mechanism of the adaptive response, nondividing lymphocytes and fibroblasts were used to eliminate possible contributions of cell cycle effects. The effect of conditioning doses of 0.05 or 0.1 Gy followed by challenging doses up to 8 Gy (with a 4-h interval between exposures) on induction and repair of DNA damage was determined by single-cell gel electrophoresis (comet assay), premature chromosome condensation, and immunofluorescence labeling for gamma-H2AX. The conditioning dose reduced the induction of DNA strand breaks, but the kinetics of strand break rejoining was not influenced by the conditioning dose in nondividing cells of either cell type. We conclude that adaptation in nondividing cells is not mediated by enhanced strand break rejoining and that protection against the induction of DNA damage is rather small. Therefore, the adaptive response is most likely a reflection of perturbation of cell cycle progression.

Caffeine Sensitizes Nondividing Human Fibroblasts to X Rays by Inducing a High Frequency of Misrepair

Caffeine sensitizes cells to ionizing radiation, and this effect is believed to be associated with the disruption of DNA damage-responsive cell cycle checkpoints, which is controlled by ATM. Recent studies suggest that misrejoining of DSBs is one of the underlying mechanisms of AT cell hyper-radiosensitivity. In this study, we investigated the effects of caffeine and radiation on nongrowing G0 normal human fibroblast cells by determining cell survival and scoring aberrations in calyculin A-induced G2 chromosomes. Results from the cell survival study indicate that after X-ray exposure G0 cells were sensitized by 24 h treatment with caffeine. Analysis of chromosome aberrations using FISH (fluorescence in situ hybridization) revealed a high frequency of aberrant cells and color junctions in the caffeine-treated cells. Since most DNA repair in nongrowing G0 cells is believed to result from nonhomologous end joining (NHEJ), caffeine may influence the fidelity of the NHEJ pathway in irradiated G0 cells.

Harmonising the response to DSBs: a new string in the ATM bow

Ataxia telangiestasia mutated protein (ATM) is the major kinase that initiates the DNA damage signal transduction response following exposure to ionising radiation (IR) in mammalian cells. DNA non-homologous end-joining (NHEJ) is the most significant double strand break (DSB) repair pathway in mammalian cells. ATM-defective cell lines display cell cycle checkpoint defects and show pronounced radiosensitivity. ATM signalling was previously thought to be dispensable for NHEJ. This review discusses recent findings that ATM activates an end-processing mechanism dependent upon Artemis, a nuclease that also functions to cleave the hairpin intermediate generated during V(D)J recombination. ATM/Artemis-dependent end-processing is required for the repair of a sub-fraction (approximately 10%) of DSBs induced by IR and makes a significant contribution to survival following exposure to ionising radiation. This result represents a new role for ATM and demonstrates a novel cross communication between the DNA repair and signal transduction machinery.

In vivo formation and repair of DNA double-strand breaks after computed tomography examinations

Ionizing radiation can lead to a variety of deleterious effects in humans, most importantly to the induction of cancer. DNA double-strand breaks (DSBs) are among the most significant genetic lesions introduced by ionizing radiation that can initiate carcinogenesis. We have enumerated gamma-H2AX foci as a measure for DSBs in lymphocytes from individuals undergoing computed tomography examination of the thorax and/or the abdomen. The number of DSBs induced by computed tomography examination was found to depend linearly on the dose-length product, a radiodiagnostic unit that is proportional to both the local dose delivered and the length of the body exposed. Analysis of lymphocytes sampled up to 1 day postirradiation provided kinetics for the in vivo loss of gamma-H2AX foci that correlated with DSB repair. Interestingly, in contrast to results obtained in vitro, normal individuals repair DSBs to background levels. A patient who had previously shown severe side effects after radiotherapy displayed levels of gamma-H2AX foci at various sampling times postirradiation that were several times higher than those of normal individuals. Gamma-H2AX and pulsed-field gel electrophoresis analysis of fibroblasts obtained from this patient confirmed a substantial DSB repair defect. Additionally, these fibroblasts showed significant in vitro radiosensitivity. These data show that the in vivo induction and repair of DSBs can be assessed in individuals exposed to low radiation doses, adding a further dimension to DSB repair studies and providing the opportunity to identify repair-compromised individuals after diagnostic irradiation procedures.

Deficiency in the catalytic subunit of DNA-dependent protein kinase causes down-regulation of ATM

Previous reports have suggested a connection between reduced levels of the catalytic subunit of DNA-dependent protein kinases (DNA-PKcs), a component of the nonhomologous DNA double-strand breaks end-joining system, and a reduction in ATM. We studied this possible connection in other DNA-PKcs-deficient cell types, and following knockdown of DNA-PKcs with small interfering RNA, Chinese hamster ovary V3 cells, lacking DNA-PKcs, had reduced levels of ATM and hSMG-1, but both were restored after transfection with PRKDC. Atm levels were also reduced in murine scid cells. Reduction of ATM in a human glioma cell line lacking DNA-PKcs was accompanied by defective signaling through downstream substrates, post-irradiation. A large reduction of DNA-PKcs was achieved in normal human fibroblasts after transfection with two DNA-PKcs small interfering RNA sequences. This was accompanied by a reduction in ATM. These data were confirmed using immunocytochemical detection of the proteins. Within hours after transfection, a decline in PRKDC mRNA was seen, followed by a more gradual decline in DNA-PKcs protein beginning 1 day after transfection. No change in ATM mRNA was observed for 2 days post-transfection. Only after the DNA-PKcs reduction occurred was a reduction in ATM mRNA observed, beginning 2 days post-transfection. The amount of ATM began to decline, starting about 3 days post-treatment, then it declined to levels comparable to DNA-PKcs. Both proteins returned to normal levels at later times. These data illustrate a potentially important cross-regulation between the nonhomologous end-joining system for rejoining of DNA double-strand breaks and the ATM-dependent damage response network of pathways, both of which operate to maintain the integrity of the genome.

Ionizing radiation-dependent gamma-H2AX focus formation requires ataxia telangiectasia mutated and ataxia telangiectasia mutated and Rad3-related

The histone variant H2AX is rapidly phosphorylated at the sites of DNA double-strand breaks (DSBs). This phosphorylated H2AX (gamma-H2AX) is involved in the retention of repair and signaling factor complexes at sites of DNA damage. The dependency of this phosphorylation on the various PI3K-related protein kinases (in mammals, ataxia telangiectasia mutated and Rad3-related [ATR], ataxia telangiectasia mutated [ATM], and DNA-PKCs) has been a subject of debate; it has been suggested that ATM is required for the induction of foci at DSBs, whereas ATR is involved in the recognition of stalled replication forks. In this study, using Arabidopsis as a model system, we investigated the ATR and ATM dependency of the formation of gamma-H2AX foci in M-phase cells exposed to ionizing radiation (IR). We find that although the majority of these foci are ATM-dependent, approximately 10% of IR-induced gamma-H2AX foci require, instead, functional ATR. This indicates that even in the absence of DNA replication, a distinct subset of IR-induced damage is recognized by ATR. In addition, we find that in plants, gamma-H2AX foci are induced at only one-third the rate observed in yeasts and mammals. This result may partly account for the relatively high radioresistance of plants versus yeast and mammals.

ATM signaling and genomic stability in response to DNA damage

DNA double strand breaks represent the most threatening lesion to the integrity of the genome in cells exposed to ionizing radiation and radiomimetic chemicals. Those breaks are recognized, signaled to cell cycle checkpoints and repaired by protein complexes. The product of the gene (ATM) mutated in the human genetic disorder ataxia-telangiectasia (A-T) plays a central role in the recognition and signaling of DNA damage. ATM is one of an ever growing number of proteins which when mutated compromise the stability of the genome and predispose to tumour development. Mechanisms for recognising double strand breaks in DNA, maintaining genome stability and minimizing risk of cancer are discussed.

Combined Haploinsufficiency for ATM and RAD9 as a Factor in Cell Transformation, Apoptosis, and DNA Lesion Repair Dynamics

Loss of function of oncogenes, tumor suppressor genes and DNA damage processing genes has been implicated in the development of many types of cancer, but for the vast majority of cases, there is no link to specific germ line mutations. In the last several years, heterozygosity leading to haploinsufficiency for proteins involved in DNA repair pathways was shown to play a role in genomic instability and carcinogenesis after DNA damage is induced. Because the effect of haploinsufficiency for one protein is relatively small, we hypothesize that predisposition to cancer could be a result of the additive effect of heterozygosity for two or more genes, critical for pathways that control DNA damage signaling, repair or apoptosis. To address this issue, primary mouse cells, haploinsufficient for one or two proteins, ATM and RAD9, related to the cellular response to DNA damage were examined. The results show that cells having low levels of both ATM and RAD9 proteins are more sensitive to transformation by radiation, have different DNA double-strand break repair dynamics and are less apoptotic when compared with wild-type controls or those cells haploinsufficient for only one of these proteins. Our conclusions are that under stress conditions, the efficiency and capacity for DNA repair mediated by the ATM/RAD9 cell signaling network depend on the abundance of both proteins and that, in general, DNA repair network efficiencies are genotype-dependent and can vary within a specific range.

Activation of Ataxia Telangiectasia Mutated by DNA Strand Break-inducing Agents Correlates Closely with the Number of DNA Double Strand Breaks

The protein kinase ataxia telangiectasia mutated (ATM) is activated when cells are exposed to ionizing radiation (IR). It has been assumed that ATM is specifically activated by the few induced DNA double strand breaks (DSBs), although little direct evidence for this assumption has been presented. DSBs constitute only a few percent of the IR-induced DNA damage, whereas the more frequent single strand DNA breaks (SSBs) and base damage account for over 98% of the overall DNA damage. It is therefore unclear whether DSBs are the only IR-induced DNA lesions that activate ATM. To test directly whether or not DSBs are responsible for ATM activation, we exposed cells to drugs and radiation that produce different numbers of DSBs and SSBs. We determined the resulting ATM activation by measuring the amount of phosphorylated Chk2 and the numbers of SSBs and DSBs in the same cells after short incubation periods. We found a strong correlation between the number of DSBs and ATM activation but no correlation with the number of SSBs. In fact, hydrogen peroxide, which, similar to IR, induces DNA damage through hydroxyl radicals but fails to induce DSBs, did not activate ATM. In contrast, we found that calicheamicin-induced strand breaks activated ATM more efficiently than IR and that ATM activation correlated with the relative DSB induction by these agents. Our data indicate that ATM is specifically activated by IR-induced DSBs, with little or no contribution from SSBs and other types of DNA damage. These findings have implications for how ATM might recognize DSBs in cells.

The accumulation of MMS-induced single strand breaks in G1 phase is recombinogenic in DNA polymerase beta defective mammalian cells

DNA polymerase (Pol) beta null mouse embryonic fibroblasts provide a useful cell system to investigate the effects of alterations in base excision repair (BER) on genome stability. These cells are characterized by hypersensitivity to the cytotoxic effects of methyl methanesulfonate (MMS) and by decreased repair of the MMS-induced DNA single strand breaks (SSB). Here, we show that, in the absence of Pol beta, SSB accumulate in G1 phase cells, accompanied by the formation of proliferating cell nuclear antigen foci in the nuclei. When replicating Pol beta null cells are treated with MMS, a rapid phosphorylation of histone H2AX is detected in the nuclei of S phase cells, indicating that double strand breaks (DSB) are formed in response to unrepaired SSB. This is followed by relocalization within the nuclei of Rad51 protein, which is essential for homologous recombination (HR). These findings are compatible with a model where, in mammalian cells, unrepaired SSB produced during BER are substrates for the HR pathway via DSB formation. This is an example of a coordinated effort of two different repair pathways, BER and HR, to protect mammalian cells from alkylation-induced cytotoxicity.

A Pathway of Double-Strand Break Rejoining Dependent upon ATM, Artemis, and Proteins Locating to γ-H2AX Foci

The hereditary disorder ataxia telangiectasia (A-T) is associated with striking cellular radiosensitivity that cannot be attributed to the characterized cell cycle checkpoint defects. By epistasis analysis, we show that ataxia telangiectasia mutated protein (ATM) and Artemis, the protein defective in patients with RS-SCID, function in a common double-strand break (DSB) repair pathway that also requires H2AX, 53BP1, Nbs1, Mre11, and DNA-PK. We show that radiation-induced Artemis hyperphosphorylation is ATM dependent. The DSB repair process requires Artemis nuclease activity and rejoins approximately 10% of radiation-induced DSBs. Our findings are consistent with a model in which ATM is required for Artemis-dependent processing of double-stranded ends with damaged termini. We demonstrate that Artemis is a downstream component of the ATM signaling pathway required uniquely for the DSB repair function but dispensable for ATM-dependent cell cycle checkpoint arrest. The significant radiosensitivity of Artemis-deficient cells demonstrates the importance of this component of DSB repair to survival.

Autophosphorylation of ataxia-telangiectasia mutated is regulated by protein phosphatase 2A

Ionizing radiation induces autophosphorylation of the ataxia-telangiectasia mutated (ATM) protein kinase on serine 1981; however, the precise mechanisms that regulate ATM activation are not fully understood. Here, we show that the protein phosphatase inhibitor okadaic acid (OA) induces autophosphorylation of ATM on serine 1981 in unirradiated cells at concentrations that inhibit protein phosphatase 2A-like activity in vitro. OA did not induce gamma-H2AX foci, suggesting that it induces ATM autophosphorylation by inactivation of a protein phosphatase rather than by inducing DNA double-strand breaks. In support of this, we show that ATM interacts with the scaffolding (A) subunit of protein phosphatase 2A (PP2A), that the scaffolding and catalytic (C) subunits of PP2A interact with ATM in undamaged cells and that immunoprecipitates of ATM from undamaged cells contain PP2A-like protein phosphatase activity. Moreover, we show that IR induces phosphorylation-dependent dissociation of PP2A from ATM and loss of the associated protein phosphatase activity. We propose that PP2A plays an important role in the regulation of ATM autophosphorylation and activity in vivo.

Activation of ATM and Chk2 kinases in relation to the amount of DNA strand breaks

The diverse checkpoint responses to DNA damage may reflect differential sensitivities by molecular components of the damage-signalling network to the type and amount of lesions. Here, we determined the kinetics of activation of the checkpoint kinases ATM and Chk2 (the latter substrate of ATM) in relation to the initial yield of genomic DNA single-strand (SSBs) and double-strand breaks (DSBs). We show that doses of gamma-radiation (IR) as low as 0.25 Gy, which generate vast numbers of SSBs but only a few DSBs per cell (<8), promptly activate ATM kinase and induce the phosphorylation of the ATM substrates p53-Ser15, Nbs1-Ser343 and Chk2-Thr68. The full activation of Chk2 kinase, however, is triggered by treatments inflicting >19 DSBs per cell (e.g. 1 Gy), which cause Chk2 autophosphorylation on Thr387, Chk2-dependent accumulation of p21waf1 and checkpoint arrest in the S phase. Our results indicate that, in contrast to ATM, Chk2 activity is triggered by a greater number of DSBs, implying that, below a certain threshold level of lesions (<19 DSBs), DNA repair can occur through ATM, without enforcing Chk2-dependent checkpoints.

DNA double strand break quantification in skin biopsies

BACKGROUND AND PURPOSE

Following induction of double strand breaks the histone H2AX is rapidly phosphorylated at regions flanking the breaks resulting in nuclear gamma H2AX foci. The purpose of this study was to use this endogenous signalling system to quantify the in vivo response to radiation in normal tissue.

PATIENTS AND METHODS

Skin biopsies were taken from prostate cancer patients undergoing radiotherapy with a curative intent. Biopsies were taken at locations corresponding to 5 different doses in the range below 1.1 Gy per fraction. Biopsies were taken from patients 30 min following the first fraction and then once again following the fraction given after the first weekend break in the treatment course. The DNA double strand breaks were visualised as gamma H2AX foci using immunohistochemistry. Images were acquired using a CCD-camera and a fluorescence microscope and the gamma H2AX foci were quantified using digital image analysis including the basic procedures of top-hat transformation, threshold setting and labelling.

RESULTS

Repeated assessments of the biopsies showed a high reproducibility in quantifying the number of foci per DNA area of the nucleated cells in epidermis. The reproducibility was equally good for the two biopsy occasions. A linear dose response was observed for the epidermis in the dose region 0-1 Gy.

CONCLUSIONS

We have established a method to measure the relative amount of DNA double strand breaks by detecting gamma H2AX foci in patients exposed to radiotherapy. The method provides a tool to study induction and repair of DNA double strand breaks and has the potential to predict individual radiosensitivity.

The DNA damage checkpoint pathways exert multiple controls on the efficiency and outcome of the repair of a double-stranded DNA gap

A DNA gap repair assay was used to determine the effect of mutations in the DNA damage checkpoint system on the efficiency and outcome (crossover/non-crossover) of recombinational DNA repair. In Saccharomyces cerevisiae gap repair is largely achieved by homologous recombination. As a result the plasmid either integrates into the chromosome (indicative of a crossover outcome) or remains extrachromosomal (indicative of a non-crossover outcome). Deletion mutants of the MEC1 and RAD53 checkpoint kinase genes exhibited a 5-fold decrease in gap repair efficiency, showing that 80% of the gap repair events depended on functional DNA damage checkpoints. Epistasis analysis suggests that the DNA damage checkpoints affect gap repair by modulating Rad51 protein-mediated homologous recombination. While in wild-type cells only approximately 25% of the gap repair events were associated with a crossover outcome, Mec1-deficient cells exhibited a >80% crossover association. Also mutations in the effector kinases Rad53, Chk1 and Dun1 were found to affect crossover association of DNA gap repair to various degrees. The data suggest that the DNA damage checkpoints are important for the optimal functioning of recombinational DNA repair with multiple terminal targets to modulate the efficiency and outcome of homologous recombination.

Enhanced fidelity for rejoining radiation-induced DNA double-strand breaks in the G2 phase of Chinese hamster ovary cells

The influence of cell cycle phase on the fidelity of DNA double-strand break (DSB) repair is largely unknown. We investigated the rejoining of correct and incorrect DSB ends in synchronized populations of Chinese hamster ovary cells irradiated with 80 Gy X-rays. A specialized pulsed-field gel electrophoresis assay based on quantitative Southern hybridization of individual large restriction fragments was employed to measure correct DSB rejoining by monitoring restriction fragment reconstitution. Total DSB repair, representing both correct and incorrect rejoining, was analyzed using conventional pulsed-field gel electrophoresis. We present evidence that restriction fragment reconstitution is more efficient in G2 than in G1, suggesting that DSB rejoining in G2 proceeds with higher fidelity. DNA-dependent protein kinase-deficient V3 and xrs-6 cells show impaired restriction fragment reconstitution in G1 and G2 compared with wild-type AA8 and K1 cells, demonstrating that the enhanced fidelity of DSB rejoining in G2 occurs by non- homologous end joining. Additionally, homologous recombination-deficient irs1SF and wild-type cells show identical DSB rejoining in G1 and G2. We propose that structural characteristics of G2 phase chromatin, such as the cohesion of sister chromatids in replicated chromatin, limit the mobility of radiation-induced break ends and enhance the fidelity of DSB rejoining.

The rare phakomatoses

This article outlines the clinical, central nervous system, and neuropathologic features,pathogenesis, genetics, molecular biology, and neuroimaging characteristics of the rare vascular phakomatoses, melanophakomatoses, and organoid phakomatoses.