Production of chromatid breaks by single dsb: evidence supporting the signal model

Blunt-ended DNA double-strand breaks induced by endonucleases PvuII and EcoRV are poor substrates for repair in Saccharomyces cerevisiae

Most mechanistic studies of repair of DNA double-strand breaks (DSBs) produced by in vivo expression of endonucleases have utilized enzymes that produce cohesive-ended DSBs such as HO, I-SceI and EcoRI. We have developed systems for expression of PvuII and EcoRV, nucleases that produce DSBs containing blunt ends, using a modified GAL1 promoter that has reduced basal activity. Expression of PvuII and EcoRV caused growth inhibition and strong cell killing in both haploid and diploid yeast cells. Surprisingly, there was little difference in sensitivities of wildtype cells and mutants defective in homologous recombination, nonhomologous end-joining (NHEJ), or both pathways. Physical analysis using standard and pulsed field gel electrophoresis demonstrated time-dependent breakage of chromosomal DNA within cells. Although ionizing radiation-induced DSBs were largely repaired within 4h, no repair of PvuII-induced breaks could be detected in diploid cells, even after arrest in G2/M. Rare survivors of PvuII expression had an increased frequency of chromosome XII deletions, an indication that a fraction of the induced DSBs could be repaired by an error-prone process. These results indicate that, unlike DSBs with complementary single-stranded DNA overhangs, blunt-ended DSBs in yeast chromosomes are poor substrates for repair by either NHEJ or recombination.

Mechanisms of the formation of radiation-induced chromosomal aberrations

Although much is now known about the mechanisms of radiation-induction of DNA double-strand breaks (DSB), there is less known about the conversion of DSB into chromosomal aberrations. In particular the induction and 'rejoining' of chromatid breaks has been a controversial topic for many years. However, its importance becomes clear in the light of the wide variation in the chromatid break response of human peripheral blood lymphocytes from different individuals when exposed to ionizing radiation, and the elevation of the frequency of radiation-induced chromatid breaks in stimulated peripheral blood lymphocytes of around 40% of breast cancer cases. A common assumption has been that chromatid breaks are merely expansions of initiating DSB, although the classic 'breakage-first' hypothesis (Sax, Ref. 44) was already challenged in the 50's by Revell [30] who maintained that chromatid breaks were formed as a result of an incomplete exchange process initiated by two interacting lesions of an unspecified nature. Here we argue that both these models of chromatid break formation are flawed and we suggest an alternative hypothesis, namely that a radiation-induced DSB initiates an indirect mechanism leading to a chromatid break. This mechanism we suggest involves the nuclear enzyme topoisomerase IIalpha and we present evidence from topoisomerase IIalpha expression variant human cell lines and from siRNA treatment of human cells that supports this hypothesis.

Nonhomologous end-joining of site-specific but not of radiation-induced DNA double-strand breaks is reduced in the presence of wild-type p53

Nonhomologous end-joining (NHEJ) of DNA double-strand breaks (DSBs) entails two principal mechanisms: modification of DNA ends prior to ligation (error-prone rejoining) or precise ligation without modification if the DNA ends are complementary (error-free repair). Error-prone rejoining is mutagenic, because it can lead to destruction of coding sequence or to chromosomal aberrations, and therefore must be tightly regulated. Previous studies on the role of the p53 tumor suppressor in the regulation of NHEJ have yielded conflicting results, but a rigorous analysis of NHEJ proficiency and fidelity in a purely chromosomal context has not been carried out. To this end, we created novel repair plasmid substrates that integrate into the genome. DSBs generated by the I-SceI endonuclease within these substrates were repaired by either error-prone rejoining or precise ligation. We found that the expression of wild-type p53 inhibited any repair-associated DNA sequence deletion, including a more than 250-fold inhibition of error-prone rejoining events compared to p53-null cells, while any promoting effect of p53 on precise ligation could not be directly evaluated. The role of p53 in NHEJ appeared to involve a direct transactivation-independent mechanism, possibly restricting DNA end-modification by blocking the annealing of single strands along flanking stretches of microhomology. The inhibition of error-prone rejoining by p53 did not apply to the rejoining of DSBs induced by ionizing radiation. In conclusion, our data suggest that p53 restricts the mutagenic effects of NHEJ without compromising repair proficiency or cell survival, thereby maintaining genomic stability.

Different G2/M accumulation in M059J and M059K cells after exposure to DNA double-strand break-inducing agents

PURPOSE

To investigate and compare the cell cycle progression in relation to cell death in the human glioma cell lines, M059J and M059K, after exposure to DNA double-strand break-inducing agents.

METHODS AND MATERIALS

The M059J and M059K cells, deficient and proficient in the catalytic subunit of the DNA-dependent protein kinase, respectively, were exposed to 1 and 4 Gy of photons or accelerated nitrogen ions. In addition, M059J and M059K cells were treated with 10 and 40 mug/mL of bleomycin for 30 min, respectively. Cell cycle progression, monitored by DNA flow cytometry, was measured up to 72 h after treatment.

RESULTS

M059J, but not M059K, cells displayed G(2)/M accumulation after low linear energy transfer irradiation. High linear energy transfer radiation exposure however, resulted in a substantial increase of M059K cells in the G(2)/M phase detected at 48 h. At 72 h, the number of cells in the G(2)/M phase was equivalent to its control. M059J cells accumulated mainly in S phase after high linear energy transfer irradiation. In contrast to M059K, M059J cells were still blocked at 72 h. Bleomycin induced G(2)/M accumulation for both M059J and M059K cells detected 24 h after treatment. At 48 h, the percentage of bleomycin-treated M059J cells in G(2)/M phase remained high, and the number of M059K cells had decreased to control levels. Neither cell line showed cell cycle arrest (< or =10 h) after exposure to these agents.

CONCLUSION

Distinct cell cycle block and release is dependent on the complexity of the induced DNA damage and the presence of the DNA-dependent protein kinase catalytic subunit.

Repair and chromosomal damage

Chromosomal aberrations in somatic cells link DNA damage with radiation-induced cell killing and individual susceptibility to oncogenesis, and are also potential markers of cancer susceptibility. While there is general acceptance that the DNA double-strand break (DSB) is the principal initiating lesion the complexity of the relationship between the induced frequency and the rates of repair and misjoining of DSB, and the production of chromosome and chromatid aberrations has led to much controversy. The principal models of chromosome aberrations are: the classical 'breakage-and-reunion' or 'breakage-first' model of Sax [Genetics 25 (1940) 41-68], the 'mis-recombination' model of Chadwick and Leenhouts [Mutat Res 404 (1998) 113-117] and the 'transcription-based' model of Radford [Int J Radiat Biol 78 (2002) 1081-1093]. Chromatid aberrations have also been variously interpreted on the 'breakage-first model', Revell's 'exchange' model [Proc R Soc B 150 (1959) 563-589] and the 'signal' model [Int J Radiat Biol 73 (1998) 243-251]. Recent evidence argues strongly for different mechanisms for chromosome (formed in G1 or Go) and chromatid (formed in G2) aberrations, i.e. there is little or no correspondence in the relative frequencies between chromosome and chromatid aberrations. The balance of evidence indicates that chromosome aberrations may be formed by a breakage-first type mechanism. Elevated frequencies of chromosomal aberrations occur to various extents in cell lines mutated in genes involved in both non-homologous DSB end-joining and homologous recombinational rejoining of DSB. Chromatid breaks, seem to be formed by a more complex mechanism since there is a lack of correspondence between the rates of DSB rejoining and chromatid break 'disappearance' (assumed by some to represent DSB repair). Thus, a model based on the dissociation of DSB rejoining from chromatid break rejoining is required to explain these data. A substantial proportion (approximately 20%) of both spontaneous and induced chromatid breaks visibly involve inter-chromatid rearrangements (determined using harlequin staining of chromatids). It is postulated that the remaining proportion may also involve rearrangements, but within a single chromatid (i.e. intra-chromatid rearrangements). Disappearance of chromatid breaks with time is postulated to result from the completion of rearrangements, i.e. rather than simply from repair of DSB.

Progress towards understanding the nature of chromatid breakage

The wide range of sensitivities of stimulated T-cells from different individuals to radiation-induced chromatid breakage indicates the involvement of several low penetrance genes that appear to link elevated chromatid breakage to cancer susceptibility. The mechanisms of chromatid breakage are not yet fully understood. However, evidence is accumulating that suggests chromatid breaks are not simply expanded DNA double-strand breaks (DSB). Three models of chromatid breakage are considered. The classical breakage-first and the Revell "exchange" models do not accord with current evidence. Therefore a derivative of Revell's model has been proposed whereby both spontaneous and radiation-induced chromatid breaks result from DSB signaling and rearrangement processes from within large looped chromatin domains. Examples of such rearrangements can be observed by harlequin staining whereby an exchange of strands occurs immediately adjacent to the break site. However, these interchromatid rearrangements comprise less than 20% of the total breaks. The rest are thought to result from intrachromatid rearrangements, including a very small proportion involving complete excision of a looped domain. Work is in progress with the aim of revealing these rearrangements, which may involve the formation of inversions adjacent to the break sites. It is postulated that the disappearance of chromatid breaks with time results from the completion of such rearrangements, rather than from the rejoining of DSB. Elevated frequencies of chromatid breaks occur in irradiated cells with defects in both nonhomologous end-joining (NHEJ) and homologous recombination (HR) pathways, however there is little evidence of a correlation between reduced DSB rejoining and disappearance of chromatid breaks. Moreover, at least one treatment which abrogates the disappearance of chromatid breaks with time leaves DSB rejoining unaffected. The I-SceI DSB system holds considerable promise for the elucidation of these mechanisms, although the break frequency is relatively low in the cell lines so far derived. Techniques to study and improve such systems are under way in different cell lines. Clearly, much remains to be done to clarify the mechanisms involved in chromatid breakage, but the experimental models are becoming available with which we can begin to answer some of the key questions.

Endopolyploid cells produced after severe genotoxic damage have the potential to repair DNA double strand breaks

p53 mutant tumour cells respond to genotoxic insults by bypassing G1 arrest and halting in G2. Following release from G2 arrest they undergo mitotic catastrophe, whereby mitotic cycling is suppressed, delayed apoptosis begins and endopolyploid cells are produced. The ability of these endopolyploid cells to participate in the restitution process is controversial. To facilitate recovery, these endopolyploid cells must repair the extensive DNA damage induced. DNA damage and its resolution were studied by observing the kinetics of gamma-H2AX foci formation and by comet assay analysis. Subsequently, the kinetics and distribution of Rad51 foci were studied as a measure of homologous recombination. Here we present evidence of the resolution of DNA damage in endopolyploid cells through a decrease of tail moment by comet assay and in the number of cells expressing gamma-H2AX foci. Rad51 foci expression reached a maximum in endopolyploid cells on days 5-6 after irradiation, when delayed apoptosis was maximal, indicating that cells were being selected for survival at this time. Furthermore, the proportion of Annexin-V-positive polyploid cells decreased as they continued ongoing rounds of DNA replication, suggesting endoreduplication is involved in selecting cells resistant to apoptosis. Our findings suggest that after severe genotoxic insult endopolyploid cells have a transient survival advantage that may contribute to radioresistance of tumours that undergo mitotic catastrophe.

Induction of chromatid breaks by carbon K-shell ultrasoft X rays

Chromatid breaks have previously been shown to be induced in G2-phase cells after exposure to ionizing radiation (X and gamma rays) as a linear function of dose, consistent with a single-event mechanism. DNA double-strand breaks (DSBs) are thought to be the initiating lesion, and experiments with a genetically engineered cell line containing a single DSB site also indicate that a single DSB is sufficient to induce a chromatid break. Although the precise mechanism of conversion of an isolated DSB into a chromatid break is not yet understood, it is known that a proportion of chromatid breaks result from rearrangements between sister chromatids. Here we report further evidence for the single-event hypothesis for the formation of chromatid breaks. The evidence derives from experiments in which chromatid breaks have been induced by exposure of Chinese hamster cells to ultrasoft carbon K-shell X rays. Since the energy of carbon K-shell X rays is not sufficient for the secondary electrons to span more than one DNA double helix, we conclude that single traversals, and hence single (complex) DSBs, are responsible for the formation of chromatid breaks. We find that, as for 60Co gamma rays, around 10% of the carbon K-shell X-ray-induced chromatid breaks have associated color switches at breakpoints, indicating that they arise through sister chromatid rearrangements.

Mitotic death: a mechanism of survival? A review

Mitotic death is a delayed response of p53 mutant tumours that are resistant to genotoxic damage. Questions surround why this response is so delayed and how its mechanisms serve a survival function. After uncoupling apoptosis from G1 and S phase arrests and adapting these checkpoints, p53 mutated tumour cells arrive at the G2 compartment where decisions regarding survival and death are made. Missed or insufficient DNA repair in G1 and S phases after severe genotoxic damage results in cells arriving in G2 with an accumulation of point mutations and chromosome breaks. Double strand breaks can be repaired by homologous recombination during G2 arrest. However, cells with excessive chromosome lesions either directly bypass the G2/M checkpoint, starting endocycles from G2 arrest, or are subsequently detected by the spindle checkpoint and present with the features of mitotic death. These complex features include apoptosis from metaphase and mitosis restitution, the latter of which can also facilitate transient endocycles, producing endopolyploid cells. The ability of cells to initiate endocycles during G2 arrest and mitosis restitution most likely reflects their similar molecular environments, with down-regulated mitosis promoting factor activity. Resulting endocycling cells have the ability to repair damaged DNA, and although mostly reproductively dead, in some cases give rise to mitotic progeny. We conclude that the features of mitotic death do not simply represent aberrations of dying cells but are indicative of a switch to amitotic modes of cell survival that may provide additional mechanisms of genotoxic resistance.

Status of chromosome breaks and gaps in breast cancer. a follow-up study

Genetic susceptibility and environmental factors are believed to be responsible for chromosomal instabilities and higher incidence of breast cancer. We conducted a follow-up study to find the levels of chromosome breaks and gaps in 20 premenopausal women with breast cancer before surgery, 1 month after surgery, and 3 years after surgery with respect to 20 age- and gender-matched controls. The mean level of chromosome breaks and gaps was found to be significantly higher (P<0.001) in breast cancer patients (before surgery) as compared with the controls. The chromosome breaks and gaps after 1 month of surgery were observed significantly decreased (P<0.005) when compared with that of patients before the surgery. Further significant increase in chromosome breaks and gaps was found after 3 years of surgery as compared with both the patients after 1 month of surgery (P<0.05) and controls (P<0.005). The significant increase in chromosome breaks and gaps in breast cancer patients (before surgery) may be due to the effects of genetic susceptibility to environmental carcinogens and endogenous factors. However, the decrease in this level after 1 month of surgery may be due to the removal of cancerous tissues, which in turn removes the effect of mutagens and clastogenic factors. Further increase in chromosome breaks and gaps after 3 years of surgery may be due to the long-term effects of therapeutic agents and genetic susceptibility to environmental carcinogens in the patients. The study furthermore suggests that the high level of chromosome breaks and gaps after 3 years of surgery may be a risk factor for the development of secondary tumor in patients.

Chromosomal radiosensitivity in G2-phase lymphocytes identifies breast cancer patients with distinctive tumour characteristics

A substantial proportion of women with breast cancer exhibit an abnormally high radiosensitivity as measured by the frequency of chromatid breaks induced in G2-phase, PHA stimulated lymphocytes. Chromatid break frequencies were compared for a cohort of previously untreated sporadic breast cancer patients and hospital outpatient controls. In the breast cancer group 46% showed high radiosensitivity compared to 14% of controls (P< 0.001). Comparison of those breast cancer patients with a high G2 radiosensitivity (G2RS) versus those with a low G2RS showed no difference in menopausal status or age but the high G2RS group had on average a lower score on the Nottingham Prognostic Index. Predicted survival in the high G2RS group at 15 years was 55% compared to 36% for the low G2RS group. Furthermore, 81% of tumours from the high G2RS were oestrogen receptor positive compared to 45% from the low G2RS group. Thus high G2RS identifies a sub-population of patients with distinctive tumour characteristics and with a predicted improved prognosis as compared with those in the low G2RS group. Our findings imply that besides influencing risk of breast cancer the genetic factors determining G2 radiosensitivity also influence the tumour characteristics and prognosis in these patients.