Late health effects and changes in lifestyle factors after cancer in childhood with and without subsequent second primary cancers – the KiKme case-control study

Background

Improved treatments for childhood cancer result in a growing number of long-term childhood cancer survivors (CCS). The diagnosis and the prevalence of comorbidities may, however, influence their lifestyle later in life. Nonetheless, little is known about differences in late effects between CCS of a first primary neoplasm (FPN) in childhood and subsequent second primary neoplasms (SPN) and their impact on lifestyle. Therefore, we aim to investigate associations between the occurrence of FPN or SPN and various diseases and lifestyle in the later life of CCS.

Methods

CCS of SPN (n=101) or FPN (n=340) and cancer-free controls (n=150) were matched by age and sex, and CCS additionally by year and entity of FPN. All participants completed a self-administered questionnaire on anthropometric and socio-economic factors, medical history, health status, and lifestyle. Mean time between FPN diagnosis and interview was 27.3 years for SPN and 26.2 years for FPN CCS. To confirm results from others and to generate new hypotheses on late effects of childhood cancer as well as CCS´ lifestyles, generalized linear mixed models were applied.

Results

CCS were found to suffer more likely from diseases compared to cancer-free controls. In detail, associations with cancer status were observed for hypercholesterinemia and thyroid diseases. Moreover, CCS were more likely to take regular medication compared to controls. A similar association was observed for CCS of SPN compared to CCS of FPN. In contrast to controls, CCS rarely exercise more than 5 hours per week, consumed fewer soft and alcoholic drinks, and were less likely to be current, former, or passive smokers. Additionally, they were less likely overweight or obese. All other exploratory analyses performed on cardiovascular, chronic lung, inflammatory bone, allergic, and infectious diseases, as well as on a calculated health-score revealed no association with tumor status.

Conclusion

CCS were more affected by pathologic conditions and may consequently take more medication, particularly among CCS of SPN. The observed higher disease burden is likely related to the received cancer therapy. To reduce the burden of long-term adverse health effects in CCS, improving cancer therapies should therefore be in focus of research in this area.

Turning end-joining upside down in mitosis

How cells deal with DNA breaks during mitosis is not well understood. While canonical non-homologous end-joining predominates in interphase, it is inhibited in mitosis to avoid telomere fusions. DNA polymerase θ mediated end-joining appears to be repressed in interphase, but promotes break repair in mitosis. The nature and induction time of breaks might determine their fate during mitosis.

POLθ-mediated end joining is restricted by RAD52 and BRCA2 until the onset of mitosis

BRCA2-mutant cells are defective in homologous recombination, making them vulnerable to the inactivation of other pathways for the repair of DNA double-strand breaks (DSBs). This concept can be clinically exploited but is currently limited due to insufficient knowledge about how DSBs are repaired in the absence of BRCA2. We show that DNA polymerase θ (POLθ)-mediated end joining (TMEJ) repairs DSBs arising during the S phase in BRCA2-deficient cells only after the onset of the ensuing mitosis. This process is regulated by RAD52, whose loss causes the premature usage of TMEJ and the formation of chromosomal fusions. Purified RAD52 and BRCA2 proteins both block the DNA polymerase function of POLθ, suggesting a mechanism explaining their synthetic lethal relationships. We propose that the delay of TMEJ until mitosis ensures the conversion of originally one-ended DSBs into two-ended DSBs. Mitotic chromatin condensation might further serve to juxtapose correct break ends and limit chromosomal fusions.

Homologous Recombination Subpathways: A Tangle to Resolve

Homologous recombination (HR) is an essential pathway for DNA double-strand break (DSB) repair, which can proceed through various subpathways that have distinct elements and genetic outcomes. In this mini-review, we highlight the main features known about HR subpathways operating at DSBs in human cells and the factors regulating subpathway choice. We examine new developments that provide alternative models of subpathway usage in different cell types revise the nature of HR intermediates involved and reassess the frequency of repair outcomes. We discuss the impact of expanding our understanding of HR subpathways and how it can be clinically exploited.

One end to rule them all: Non-homologous end-joining and homologous recombination at DNA double-strand breaks

Double-strand breaks (DSBs) represent the most severe type of DNA damage since they can lead to genomic rearrangements, events that can initiate and promote tumorigenic processes. DSBs arise from various exogenous agents that induce two single-strand breaks at opposite locations in the DNA double helix. Such two-ended DSBs are repaired in mammalian cells by one of two conceptually different processes, non-homologous end-joining (NHEJ) and homologous recombination (HR). NHEJ has the potential to form rearrangements while HR is believed to be error-free since it uses a homologous template for repair. DSBs can also arise from single-stranded DNA lesions if they lead to replication fork collapse. Such DSBs, however, have only one end and are repaired by HR and not by NHEJ. In fact, the majority of spontaneously arising DSBs are one-ended and HR has likely evolved to repair one-ended DSBs. HR of such DSBs demands the engagement of a second break end that is generated by an approaching replication fork. This HR process can cause rearrangements if a homologous template other than the sister chromatid is used. Thus, both NHEJ and HR have the potential to form rearrangements and the proper choice between them is governed by various factors, including cell cycle phase and genomic location of the lesion. We propose that the specific requirements for repairing one-ended DSBs have shaped HR in a way which makes NHEJ the better choice for the repair of some but not all two-ended DSBs.

Spontaneous and Radiation-Induced Chromosome Aberrations in Primary Fibroblasts of Patients With Pediatric First and Second Neoplasms

The purpose of the present study was to investigate whether former childhood cancer patients who developed a subsequent secondary primary neoplasm (SPN) are characterized by elevated spontaneous chromosomal instability or cellular and chromosomal radiation sensitivity as surrogate markers of compromised DNA repair compared to childhood cancer patients with a first primary neoplasm (FPN) only or tumor-free controls. Primary skin fibroblasts were obtained in a nested case-control study including 23 patients with a pediatric FPN, 22 matched patients with a pediatric FPN and an SPN, and 22 matched tumor-free donors. Clonogenic cell survival and cytogenetic aberrations in Giemsa-stained first metaphases were assessed after X-irradiation in G1 or on prematurely condensed chromosomes of cells irradiated and analyzed in G2. Fluorescence in situ hybridization was applied to investigate spontaneous transmissible aberrations in selected donors. No significant difference in clonogenic survival or the average yield of spontaneous or radiation-induced aberrations was found between the study populations. However, two donors with an SPN showed striking spontaneous chromosomal instability occurring as high rates of numerical and structural aberrations or non-clonal and clonal translocations. No correlation was found between radiation sensitivity and a susceptibility to a pediatric FPN or a treatment-associated SPN. Together, the results of this unique case-control study show genomic stability and normal radiation sensitivity in normal somatic cells of donors with an early and high intrinsic or therapy-associated tumor risk. These findings provide valuable information for future studies on the etiology of sporadic childhood cancer and therapy-related SPN as well as for the establishment of predictive biomarkers based on altered DNA repair processes.

An Assessment of Radiation Doses From Radon Exposures Using a Mouse Model System

BACKGROUND

Radon and its progenies contribute significantly to the natural background radiation and cause several thousands of lung cancer cases per year worldwide. Moreover, patients with chronic inflammatory joint diseases are treated in radon galleries. Due to the complex nature of radon exposure, the doses associated with radon exposures are difficult to assess. Hence, there is a clear need to directly measure dose depositions from radon exposures to provide reliable risk estimates for radiation protection guidelines.

OBJECTIVES

We aimed to assess tissue-specific radiation doses associated with radon activity concentrations, that deposit similar dose levels as the annual natural radon exposure or radon gallery visits.

METHODS

We exposed mice to defined radon concentrations, quantified the number of 53BP1 foci as a measure of induced DNA damage, and compared it with the number of foci induced by known doses of reference-type radiations. An image-based analysis of the 3-dimensional foci pattern provided information about the radiation type inflicting the DNA damage.

RESULTS

A 1-hour exposure to 440 kBq/m³ radon-induced DNA damage corresponding to a dose of ∼10 mGy in the lung and ∼3.3 mGy in the kidney, heart, and liver. A 1-hour exposure to 44 kBq/m³ provided values consistent with a linear relationship between dose and radon concentration. Two-thirds of the dose in the lung was caused by α-particles. The dose in the kidney, heart, and liver and one-third of the dose in the lung likely resulted from β- and γ-rays.

DISCUSSION

We found that radon exposures mainly lead to α-particle-induced DNA damage in the lung, consistent with the lung cancer risk obtained in epidemiologic studies. Our presented biodosimetric approach can be used to benchmark risk model calculations for radiation protection guidelines and can help to understand the therapeutic success of radon gallery treatments.

AutoFoci, an automated high-throughput foci detection approach for analyzing low-dose DNA double-strand break repair

Double-strand breaks (DSBs) are the most lethal DNA damages induced by ionising radiation (IR) and their efficient repair is crucial to limit genomic instability. The cellular DSB response after low IR doses is of particular interest but its examination requires the analysis of high cell numbers. Here, we present an automated DSB quantification method based on the analysis of γH2AX and 53BP1 foci as markers for DSBs. We establish a combination of object properties, combined in the object evaluation parameter (OEP), which correlates with manual object classification. Strikingly, OEP histograms show a bi-modal distribution with two maxima and a minimum in between, which correlates with the manually determined transition between background signals and foci. We used algorithms to detect the minimum, thus separating foci from background signals and automatically assessing DSB levels. To demonstrate the validity of this method, we analyzed over 600.000 cells to verify results of previous studies showing that DSBs induced by low doses are less efficiently repaired compared with DSBs induced by higher doses. Thus, the automated foci counting method, called AutoFoci, provides a valuable tool for high-throughput image analysis of thousands of cells which will prove useful for many biological screening approaches.

DNA repair synthesis and histone deposition partner during homologous recombination

Chromatin remodeling is critical for the regulation of the DNA damage response. We highlight findings from our recent study showing that the deposition of the histone variant H3.3 by the alpha-thalassemia mental retardation X-linked protein (ATRX) and the death domain associated protein (DAXX) chromatin remodeling complex regulates DNA repair synthesis during homologous recombination.

The pendulum of the Ku-Ku clock

Canonical DNA non-homologous end-joining (c-NHEJ) and homologous recombination (HR), the two major DNA double-strand break (DSB) repair pathways, have long been depicted as competitors, fighting a race to rejoin DSBs. In human cells, Ku, an upstream component of NHEJ, is highly abundant and has exquisite end-binding capacity. Emerging evidence has suggested that Ku is the first protein binding most, if not all, DSBs, and creates a block to resection. Although most c-NHEJ proceeds without resection, recent studies have provided strong evidence for a process of resection-dependent c-NHEJ, that repairs a subset of DSBs. HR also repairs a subset of two-ended DSBs in G2 phase and processes one-ended DSBs that arise following replication fork stalling or collapse to promote replication restart. HR also necessitates end-resection. This raises the question of how end-resection takes place despite Ku's avid end-binding capacity. Insight into this enigma has been gained from the analysis of DSBs generated by Spo11 or TOP2, which create protein-bridged DSBs. The progression of repair by HR or NHEJ requires removal of the end-blocking lesions. The MRE11-RAD50-NBS1 (MRN) complex, CtIP and EXO1 play critical roles in this process. Here, we review our current understanding of how resection arises at lesions blocked by covalently bound Spo11 or TOP2 or following Ku binding, which effectively creates a distinct resection-blocking lesion due to its avid end-binding activity and abundance. Our review reveals that Ku plays an active role in determining pathway choice and exposes similarities yet distinctions in the progression of resection that is suited to the optimal repair pathway choice.

ATRX Promotes DNA Repair Synthesis and Sister Chromatid Exchange during Homologous Recombination

ATRX is a chromatin remodeler that, together with its chaperone DAXX, deposits the histone variant H3.3 in pericentromeric and telomeric regions. Notably, ATRX is frequently mutated in tumors that maintain telomere length by a specific form of homologous recombination (HR). Surprisingly, in this context, we demonstrate that ATRX-deficient cells exhibit a defect in repairing exogenously induced DNA double-strand breaks (DSBs) by HR. ATRX operates downstream of the Rad51 removal step and interacts with PCNA and RFC-1, which are collectively required for DNA repair synthesis during HR. ATRX depletion abolishes DNA repair synthesis and prevents the formation of sister chromatid exchanges at exogenously induced DSBs. DAXX- and H3.3-depleted cells exhibit identical HR defects as ATRX-depleted cells, and both ATRX and DAXX function to deposit H3.3 during DNA repair synthesis. This suggests that ATRX facilitates the chromatin reconstitution required for extended DNA repair synthesis and sister chromatid exchange during HR.

DNA Double-Strand Break Resection Occurs during Non-homologous End Joining in G1 but Is Distinct from Resection during Homologous Recombination

Canonical non-homologous end joining (c-NHEJ) repairs DNA double-strand breaks (DSBs) in G1 cells with biphasic kinetics. We show that DSBs repaired with slow kinetics, including those localizing to heterochromatic regions or harboring additional lesions at the DSB site, undergo resection prior to repair by c-NHEJ and not alt-NHEJ. Resection-dependent c-NHEJ represents an inducible process during which Plk3 phosphorylates CtIP, mediating its interaction with Brca1 and promoting the initiation of resection. Mre11 exonuclease, EXD2, and Exo1 execute resection, and Artemis endonuclease functions to complete the process. If resection does not commence, then repair can ensue by c-NHEJ, but when executed, Artemis is essential to complete resection-dependent c-NHEJ. Additionally, Mre11 endonuclease activity is dispensable for resection in G1. Thus, resection in G1 differs from the process in G2 that leads to homologous recombination. Resection-dependent c-NHEJ significantly contributes to the formation of deletions and translocations in G1, which represent important initiating events in carcinogenesis.

Nek1 Regulates Rad54 to Orchestrate Homologous Recombination and Replication Fork Stability

Never-in-mitosis A-related kinase 1 (Nek1) has established roles in apoptosis and cell cycle regulation. We show that human Nek1 regulates homologous recombination (HR) by phosphorylating Rad54 at Ser572 in late G2 phase. Nek1 deficiency as well as expression of unphosphorylatable Rad54 (Rad54-S572A) cause unresolved Rad51 foci and confer a defect in HR. Phospho-mimic Rad54 (Rad54-S572E), in contrast, promotes HR and rescues the HR defect associated with Nek1 loss. Although expression of phospho-mimic Rad54 is beneficial for HR, it causes Rad51 removal from chromatin and degradation of stalled replication forks in S phase. Thus, G2-specific phosphorylation of Rad54 by Nek1 promotes Rad51 chromatin removal during HR in G2 phase, and its absence in S phase is required for replication fork stability. In summary, Nek1 regulates Rad51 removal to orchestrate HR and replication fork stability.

Polo-like kinase 3 regulates CtIP during DNA double-strand break repair in G1

Plk3 phosphorylates CtIP in G1 in a damage-inducible manner and is required with CtIP for the repair of complex double-strand breaks and regulation of resection-mediated end-joining pathways.

DNA double-strand breaks (DSBs) are repaired by nonhomologous end joining (NHEJ) or homologous recombination (HR). The C terminal binding protein–interacting protein (CtIP) is phosphorylated in G2 by cyclin-dependent kinases to initiate resection and promote HR. CtIP also exerts functions during NHEJ, although the mechanism phosphorylating CtIP in G1 is unknown. In this paper, we identify Plk3 (Polo-like kinase 3) as a novel DSB response factor that phosphorylates CtIP in G1 in a damage-inducible manner and impacts on various cellular processes in G1. First, Plk3 and CtIP enhance the formation of ionizing radiation-induced translocations; second, they promote large-scale genomic deletions from restriction enzyme-induced DSBs; third, they are required for resection and repair of complex DSBs; and finally, they regulate alternative NHEJ processes in Ku^−/− mutants. We show that mutating CtIP at S327 or T847 to nonphosphorylatable alanine phenocopies Plk3 or CtIP loss. Plk3 binds to CtIP phosphorylated at S327 via its Polo box domains, which is necessary for robust damage-induced CtIP phosphorylation at S327 and subsequent CtIP phosphorylation at T847.

Requirement for PBAF in transcriptional repression and repair at DNA breaks in actively transcribed regions of chromatin

Actively transcribed regions of the genome are vulnerable to genomic instability. Recently, it was discovered that transcription is repressed in response to neighboring DNA double-strand breaks (DSBs). It is not known whether a failure to silence transcription flanking DSBs has any impact on DNA repair efficiency or whether chromatin remodelers contribute to the process. Here, we show that the PBAF remodeling complex is important for DSB-induced transcriptional silencing and promotes repair of a subset of DNA DSBs at early time points, which can be rescued by inhibiting transcription globally. An ATM phosphorylation site on BAF180, a PBAF subunit, is required for both processes. Furthermore, we find that subunits of the PRC1 and PRC2 polycomb group complexes are similarly required for DSB-induced silencing and promoting repair. Cancer-associated BAF180 mutants are unable to restore these functions, suggesting PBAF's role in repressing transcription near DSBs may contribute to its tumor suppressor activity.

ATM release at resected double-strand breaks provides heterochromatin reconstitution to facilitate homologous recombination

Non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the two main pathways for repairing DNA double-strand breaks (DSBs). During the G2 phase of the mammalian cell cycle, both processes can operate and chromatin structure is one important factor which determines DSB repair pathway choice. ATM facilitates the repair of heterochromatic DSBs by phosphorylating and inactivating the heterochromatin building factor KAP-1, leading to local chromatin relaxation. Here, we show that ATM accumulation and activity is strongly diminished at DSBs undergoing end-resection during HR. Such DSBs remain unrepaired in cells devoid of the HR factors BRCA2, XRCC3 or RAD51. Strikingly, depletion of KAP-1 or expression of phospho-mimic KAP-1 allows repair of resected DSBs in the absence of BRCA2, XRCC3 or RAD51 by an erroneous PARP-dependent alt-NHEJ process. We suggest that DSBs in heterochromatin elicit initial local heterochromatin relaxation which is reversed during HR due to the release of ATM from resection break ends. The restored heterochromatic structure facilitates HR and prevents usage of error-prone alternative processes.

Double-strand breaks (DSBs) are critical DNA lesions because they can lead to cell death or, which is even more devastating, the formation of genomic rearrangements. Cells are equipped with two main pathways to repair such lesions, homologous recombination (HR) and non-homologous end-joining (NHEJ). HR is an error-free process and completely restores the genetic information, whereas NHEJ has the potential to form genomic rearrangements. We have previously shown that the structure of the chromatin is one important factor which determines the choice between these two pathways, such that DSBs localizing to highly condensed heterochromatic regions are mainly repaired by HR and breaks in more open euchromatic DNA undergo repair by NHEJ. Here, we investigate this aspect of DSB repair pathway choice. We show that DSB end-resection, which channels DSB repair into the process of HR, counteracts the profound local relaxation which initially takes place at the break site and reconstitutes the heterochromatic structure. Cells which are genetically modified, such that they cannot reconstitute the heterochromatic structure at resected DSBs, fail to employ HR and instead repair heterochromatic DSBs by alternative NHEJ mechanisms. Thus, chromatin modifications which occur during the process of end-resection prevent error-prone repair pathways from generating genomic rearrangements.

Glioma-amplified sequence KUB3 influences double-strand break repair after ionizing radiation

Human glioblastomas are characterized by frequent DNA amplifications most often at chromosome regions 7p11.2 and 12q13-15. Although amplification is a well-known hallmark of glioblastoma genetics the function of most amplified genes in glioblastoma biology is not understood. Previously, we cloned Ku70-binding protein 3 (KUB3) from the amplified domain at 12q13-15. Here, we report that glioblastoma cell cultures with endogenous KUB3 gene amplification and with elevated KUB3 protein expression show an efficient double-strand break (DSB) repair after being irradiated with 1 Gy. A significantly less efficient DSB repair was found in glioma cell cultures without KUB3 amplification and expression. Furthermore, we found that a siRNA-mediated reduction of the endogenous KUB3 expression in glioblastoma cells resulted in a reduction of the repair efficiency. HeLa cells transfected with KUB3 showed an increased DSB repair in comparison to untreated HeLa cells. In addition, KUB3 seems to influence DSB efficiency via the DNA-PK-dependent repair pathway as shown by simultaneous inhibition of KUB3 and DNA-PK. The data provide the first evidence for a link between the level of KUB3 amplification and expression in glioma and DSB repair efficiency.

X-ray induced DNA double-strand breaks in coronary CT angiography: comparison of sequential, low-pitch helical and high-pitch helical data acquisition

BACKGROUND

Aim of this study was to compare DNA double-strand breaks (DSBs) in blood lymphocytes of patients undergoing high-pitch helical, low-pitch helical and sequential coronary CT angiography.

METHODS AND RESULTS

66 patients were examined with various scan protocols and modes (low-pitch helical scan: 100-120 kV, 320-438 mAs/rot, pitch 0.18-0.39, with or without ECG-pulsing, n=35; prospectively ECG-triggered high-pitch helical scan: 100-120 kV, 320-456 mAs/rotation, pitch 3.2-3.4, n=19; prospectively ECG-triggered sequential scan: 100-120 kV, 150-300 mAs or 320-370 mAs/rotation, n=12) either using a 64-slice or 128-slice dual-source CT or a 128-slice single source CT scanner. Blood samples were obtained before and 30 min after CT and DSBs were analyzed in isolated lymphocytes using γ-H2AX immunofluorescence microscopy. A significant increase of DSBs was measurable 30 min after CTA (range 0.01-0.71/cell). CT induced DSBs showed a significant correlation with the estimated effective dose (ρ=0.90, p<0.00001). Both prospectively ECG-triggered sequential (0.10 DSBs/cell, 176 mGy cm, p<0.00001) and high-pitch helical scan protocols (0.03 DSBs/cell, 109 mGy cm, p<0.00001) led to a significant reduction of median DLP and DSB levels compared to low-pitch helical scans (0.34 DSBs/cell, 828 mGy cm). A reduction of the tube voltage resulted in significantly lower whereas additional calcium scoring resulted in elevated DLP and DNA damages (p<0.05 each).

CONCLUSION

In coronary CTA, data acquisition protocols have a significant influence on the X-ray induced DSB levels. Using γ-H2AX immunofluorescence microscopy different scan modes in different CT generations can be compared concerning their biological impact.

Elevated radiation-induced γH2AX foci in G2 phase heterozygous BRCA2 fibroblasts

BACKGROUND AND PURPOSE

About 5-10% of all breast cancer cases are associated with heterozygous germ-line mutations in the genes encoding BRCA1 and BRCA2. Carriers of such mutations are highly predisposed for developing breast or ovarian cancer and, thus, are advised to undergo regular radio-diagnostic examinations. BRCA1 and BRCA2 are involved in multiple cellular processes including the repair of ionizing radiation (IR)-induced DNA double-strand breaks (DSBs) and different studies addressing the DSB repair capacity of BRCA1+/- or BRCA2+/- cells led to contradictory results.

MATERIALS AND METHODS

Using the sensitive method of γH2AX foci analysis in combination with cell cycle markers, we specifically measured DSB repair in confluent G0 as well as in exponentially growing G1 and G2 phase primary WT, BRCA1+/- and BRCA2+/- fibroblasts.

RESULTS

Both BRCA1+/- and BRCA2+/- cells displayed normal DSB repair in G0 and in G1. In contrast, in G2, BRCA2+/- but not BRCA1+/- cells exhibited a decreased DSB repair capacity which was in between that of WT and that of a hypomorphic BRCA2-/- cell line.

CONCLUSIONS

The residual amount of normal BRCA1 seems to be sufficient for efficient DSB repair in all cell cycle phases, while the decreased DSB repair capacity of heterozygous BRCA2 mutations suggests gene dosage effects in G2.

Understanding the limitations of radiation-induced cell cycle checkpoints

The DNA damage response pathways involve processes of double-strand break (DSB) repair and cell cycle checkpoint control to prevent or limit entry into S phase or mitosis in the presence of unrepaired damage. Checkpoints can function to permanently remove damaged cells from the actively proliferating population but can also halt the cell cycle temporarily to provide time for the repair of DSBs. Although efficient in their ability to limit genomic instability, checkpoints are not foolproof but carry inherent limitations. Recent work has demonstrated that the G1/S checkpoint is slowly activated and allows cells to enter S phase in the presence of unrepaired DSBs for about 4–6 h post irradiation. During this time, only a slowing but not abolition of S-phase entry is observed. The G2/M checkpoint, in contrast, is quickly activated but only responds to a level of 10–20 DSBs such that cells with a low number of DSBs do not initiate the checkpoint or terminate arrest before repair is complete. Here, we discuss the limitations of these checkpoints in the context of the current knowledge of the factors involved. We suggest that the time needed to fully activate G1/S arrest reflects the existence of a restriction point in G1-phase progression. This point has previously been defined as the point when mitogen starvation fails to prevent cells from entering S phase. However, cells that passed the restriction point can respond to DSBs, albeit with reduced efficiency.

DNA double-strand breaks in heterochromatin elicit fast repair protein recruitment, histone H2AX phosphorylation and relocation to euchromatin

DNA double-strand breaks (DSBs) can induce chromosomal aberrations and carcinogenesis and their correct repair is crucial for genetic stability. The cellular response to DSBs depends on damage signaling including the phosphorylation of the histone H2AX (γH2AX). However, a lack of γH2AX formation in heterochromatin (HC) is generally observed after DNA damage induction. Here, we examine γH2AX and repair protein foci along linear ion tracks traversing heterochromatic regions in human or murine cells and find the DSBs and damage signal streaks bending around highly compacted DNA. Given the linear particle path, such bending indicates a relocation of damage from the initial induction site to the periphery of HC. Real-time imaging of the repair protein GFP-XRCC1 confirms fast recruitment to heterochromatic lesions inside murine chromocenters. Using single-ion microirradiation to induce localized DSBs directly within chromocenters, we demonstrate that H2AX is early phosphorylated within HC, but the damage site is subsequently expelled from the center to the periphery of chromocenters within ∼20 min. While this process can occur in the absence of ATM kinase, the repair of DSBs bordering HC requires the protein. Finally, we describe a local decondensation of HC at the sites of ion hits, potentially allowing for DSB movement via physical forces.

Factors determining DNA double-strand break repair pathway choice in G2 phase

DNA non-homologous end joining (NHEJ) and homologous recombination (HR) function to repair DNA double-strand breaks (DSBs) in G2 phase with HR preferentially repairing heterochromatin-associated DSBs (HC-DSBs). Here, we examine the regulation of repair pathway usage at two-ended DSBs in G2. We identify the speed of DSB repair as a major component influencing repair pathway usage showing that DNA damage and chromatin complexity are factors influencing DSB repair rate and pathway choice. Loss of NHEJ proteins also slows DSB repair allowing increased resection. However, expression of an autophosphorylation-defective DNA-PKcs mutant, which binds DSBs but precludes the completion of NHEJ, dramatically reduces DSB end resection at all DSBs. In contrast, loss of HR does not impair repair by NHEJ although CtIP-dependent end resection precludes NHEJ usage. We propose that NHEJ initially attempts to repair DSBs and, if rapid rejoining does not ensue, then resection occurs promoting repair by HR. Finally, we identify novel roles for ATM in regulating DSB end resection; an indirect role in promoting KAP-1-dependent chromatin relaxation and a direct role in phosphorylating and activating CtIP.

Sister chromatid exchanges occur in G2-irradiated cells

DNA double-strand breaks (DSBs) are arguably the most important lesions induced by ionizing radiation (IR) since unrepaired or mis-repaired DSBs can lead to chromosomal aberrations and cell death. The two major pathways to repair IR-induced DSBs are non-homologous end-joining (NHEJ) and homologous recombination (HR). Perhaps surprisingly, NHEJ represents the predominant pathway in the G1 and G2 phases of the cell cycle, but HR also contributes and repairs a subset of IR-induced DSBs in G2. Following S-phase-dependent genotoxins, HR events give rise to sister chromatid exchanges (SCEs), which can be detected cytogenetically in mitosis. Here, we describe that HR occurring in G2-irradiated cells also generates SCEs in ~50% of HR events. Since HR of IR-induced DSBs in G2 is a slow process, SCE formation in G2-irradiated cells requires several hours. During this time, irradiated S-phase cells can also reach mitosis, which has contributed to the widely held belief that SCEs form only during S phase. We describe procedures to measure SCEs exclusively in G2-irradiated cells and provide evidence that following IR cells do not need to progress through S phase in order to form SCEs.

CtIP and MRN promote non-homologous end-joining of etoposide-induced DNA double-strand breaks in G1

Topoisomerases class II (topoII) cleave and re-ligate the DNA double helix to allow the passage of an intact DNA strand through it. Chemotherapeutic drugs such as etoposide target topoII, interfere with the normal enzymatic cleavage/re-ligation reaction and create a DNA double-strand break (DSB) with the enzyme covalently bound to the 5'-end of the DNA. Such DSBs are repaired by one of the two major DSB repair pathways, non-homologous end-joining (NHEJ) or homologous recombination. However, prior to repair, the covalently bound topoII needs to be removed from the DNA end, a process requiring the MRX complex and ctp1 in fission yeast. CtIP, the mammalian ortholog of ctp1, is known to promote homologous recombination by resecting DSB ends. Here, we show that human cells arrested in G0/G1 repair etoposide-induced DSBs by NHEJ and, surprisingly, require the MRN complex (the ortholog of MRX) and CtIP. CtIP's function for repairing etoposide-induced DSBs by NHEJ in G0/G1 requires the Thr-847 but not the Ser-327 phosphorylation site, both of which are needed for resection during HR. This finding establishes that CtIP promotes NHEJ of etoposide-induced DSBs during G0/G1 phase with an end-processing function that is distinct to its resection function.

Homologous recombination protects mammalian cells from replication-associated DNA double-strand breaks arising in response to methyl methanesulfonate

DNA-methylating agents of the S(N)2 type target DNA mostly at ring nitrogens, producing predominantly N-methylated purines. These adducts are repaired by base excision repair (BER). Since defects in BER cause accumulation of DNA single-strand breaks (SSBs) and sensitize cells to the agents, it has been suggested that some of the lesions on their own or BER intermediates (e.g. apurinic sites) are cytotoxic, blocking DNA replication and inducing replication-mediated DNA double-strand breaks (DSBs). Here, we addressed the question of whether homologous recombination (HR) or non-homologous end-joining (NHEJ) or both are involved in the repair of DSBs formed following treatment of cells with methyl methanesulfonate (MMS). We show that HR defective cells (BRCA2, Rad51D and XRCC3 mutants) are dramatically more sensitive to MMS-induced DNA damage as measured by colony formation, apoptosis and chromosomal aberrations, while NHEJ defective cells (Ku80 and DNA-PK(CS) mutants) are only mildly sensitive to the killing, apoptosis-inducing and clastogenic effects of MMS. On the other hand, the HR mutants were almost completely refractory to the formation of sister chromatid exchanges (SCEs) following MMS treatment. Since DSBs are expected to be formed specifically in the S-phase, we assessed the formation and kinetics of repair of DSBs by γH2AX quantification in a cell cycle specific manner. In the cytotoxic dose range of MMS a significant amount of γH2AX foci was induced in S, but not G1- and G2-phase cells. A major fraction of γH2AX foci colocalized with 53BP1 and phosphorylated ATM, indicating they are representative of DSBs. DSB formation following MMS treatment was also demonstrated by the neutral comet assay. Repair kinetics revealed that HR mutants exhibit a significant delay in DSB repair, while NHEJ mutants completed S-phase specific DSB repair with a kinetic similar to the wildtype. Moreover, DNA-PKcs inhibition in HR mutants did not affect the repair kinetics after MMS treatment. Overall, the data indicate that agents producing N-alkylpurines in the DNA induce replication-dependent DSBs. Further, they show that HR is the major pathway of protection of cells against DSB formation, killing and genotoxicity following S(N)2-alkylating agents.

Effect of CT scan protocols on x-ray-induced DNA double-strand breaks in blood lymphocytes of patients undergoing coronary CT angiography

AIMS

To compare in vivo DNA lesions induced during helical and sequential coronary computed tomography angiography (CTA) and to evaluate the effect of CT parameters on double-strand break (DSB) levels.

METHODS

Thirty-six patients were examined with various CT protocols and modes (helical scan, n = 27; sequential scan, n = 9) either using a 64-slice dual-source or a 128-slice CT system. Blood samples were obtained before and 30 min after CT. Lymphocytes were isolated, stained against the phosphorylated histone variant γ-H2AX, and DSBs were visualised by using fluorescence microscopy.

RESULTS

DSB yields 30 min after CTA ranged from 0.04 to 0.71 per cell and showed a significant correlation to DLP (ρ = 0.81, p < 0.00001). Median DSB yield and median DLP were significantly lower after sequential compared to helical CT examinations (0.11 vs. 0.37 DSBs/cell and 249 vs. 958 mGy cm, p < 0.00001). Additional calcium scoring led to an increase in DLP (p = 0.15) and DSB levels (p = 0.04). DSB levels normalised to the DLP showed a significant correlation to the attenuation of the blood (ρ = 0.53, p = 0.01) and a negative correlation to the body mass index of the patients (ρ = -0.37, p = 0.06).

CONCLUSION

γ-H2AX immunofluorescence microscopy allows one to determine dose-related effects on x-ray-induced DSB levels and to consider individual factors which cannot be monitored by physical dose measurements.

gammaH2AX foci analysis for monitoring DNA double-strand break repair: strengths, limitations and optimization

DNA double-strand breaks (DSBs) represent an important radiation-induced lesion and impaired DSB repair provides the best available correlation with radiosensitivity. Physical techniques for monitoring DSB repair require high, non-physiological doses and cannot reliably detect subtle defects. One outcome from extensive research into the DNA damage response is the observation that H2AX, a variant form of the histone H2A, undergoes extensive phosphorylation at the DSB, creating gammaH2AX foci that can be visualized by immunofluorescence. There is a close correlation between gammaH2AX foci and DSB numbers and between the rate of foci loss and DSB repair, providing a sensitive assay to monitor DSB repair in individual cells using physiological doses. However, gammaH2AX formation can occur at single-stranded DNA regions which arise during replication or repair and thus does not solely correlate with DSB formation. Here, we present and discuss evidence that following exposure to ionizing radiation, gammaH2AX foci analysis can provide a sensitive monitor of DSB formation and repair and describe techniques to optimize the analysis. We discuss the limitations and benefits of the technique, enabling the procedure to be optimally exploited but not misused.

ATM and Artemis promote homologous recombination of radiation-induced DNA double-strand breaks in G2

Homologous recombination (HR) and non-homologous end joining (NHEJ) represent distinct pathways for repairing DNA double-strand breaks (DSBs). Previous work implicated Artemis and ATM in an NHEJ-dependent process, which repairs a defined subset of radiation-induced DSBs in G1-phase. Here, we show that in G2, as in G1, NHEJ represents the major DSB-repair pathway whereas HR is only essential for repair of approximately 15% of X- or gamma-ray-induced DSBs. In addition to requiring the known HR proteins, Brca2, Rad51 and Rad54, repair of radiation-induced DSBs by HR in G2 also involves Artemis and ATM suggesting that they promote NHEJ during G1 but HR during G2. The dependency for ATM for repair is relieved by depleting KAP-1, providing evidence that HR in G2 repairs heterochromatin-associated DSBs. Although not core HR proteins, ATM and Artemis are required for efficient formation of single-stranded DNA and Rad51 foci at radiation-induced DSBs in G2 with Artemis function requiring its endonuclease activity. We suggest that Artemis endonuclease removes lesions or secondary structures, which inhibit end resection and preclude the completion of HR or NHEJ.

DNA double-strand break repair of blood lymphocytes and normal tissues analysed in a preclinical mouse model: implications for radiosensitivity testing

PURPOSE

Radiotherapy is an effective cancer treatment, but a few patients suffer severe radiation toxicities in neighboring normal tissues. There is increasing evidence that the variable susceptibility to radiation toxicities is caused by the individual genetic predisposition, by subtle mutations, or polymorphisms in genes involved in cellular responses to ionizing radiation. Double-strand breaks (DSB) are the most deleterious form of radiation-induced DNA damage, and DSB repair deficiencies lead to pronounced radiosensitivity. Using a preclinical mouse model, the highly sensitive gammaH2AX-foci approach was tested to verify even subtle, genetically determined DSB repair deficiencies known to be associated with increased normal tissue radiosensitivity.

EXPERIMENTAL DESIGN

By enumerating gammaH2AX-foci in blood lymphocytes and normal tissues (brain, lung, heart, and intestine), the induction and repair of DSBs after irradiation with therapeutic doses (0.1-2 Gy) was investigated in repair-proficient and repair-deficient mouse strains in vivo and blood samples irradiated ex vivo.

RESULTS

gammaH2AX-foci analysis allowed to verify the different DSB repair deficiencies; even slight impairments caused by single polymorphisms were detected similarly in both blood lymphocytes and solid tissues, indicating that DSB repair measured in lymphocytes is valid for different and complex organs. Moreover, gammaH2AX-foci analysis of blood samples irradiated ex vivo was found to reflect repair kinetics measured in vivo and, thus, give reliable information about the individual DSB repair capacity.

CONCLUSIONS

gammaH2AX analysis of blood and tissue samples allows to detect even minor genetically defined DSB repair deficiencies, affecting normal tissue radiosensitivity. Future studies will have to evaluate the clinical potential to identify patients more susceptible to radiation toxicities before radiotherapy.

Radiation-induced HPRT mutations resulting from misrejoined DNA double-strand breaks

DNA double-strand breaks (DSBs) are the most severe lesions induced by ionizing radiation, and unrejoined or misrejoined DSBs can lead to cell lethality, mutations and the initiation of tumorigenesis. We have investigated X-ray- and alpha-particle-induced mutations that inactivate the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene in human bladder carcinoma cells and in hTERT-immortalized human fibroblasts. Fifty to 80% of the mutants analyzed exhibited partial or total deletions of the 9 exons of the HPRT locus. The remaining mutants retained unaltered PCR products of all 9 exons but often displayed a failure to amplify the HPRT cDNA. Hybridization analysis of a 2-Mbp NotI fragment spanning the HPRT gene with a probe 200 kbp distal to the HPRT locus indicated altered fragment sizes in most of the mutants with a wild-type PCR pattern. These mutants likely contain breakpoints for genomic rearrangements in the intronic sequences of the HPRT gene that allow the amplification of the exons but prevent HPRT cDNA amplification. Additionally, mutants exhibiting partial and total deletions of the HPRT exons also frequently displayed altered NotI fragments. Interestingly, all mutations were very rarely associated with interchromosomal exchanges analyzed by FISH. Collectively, our data suggest that intrachromosomal genomic rearrangements on the Mbp scale represent the prevailing type of radiation-induced HPRT mutations.

The impact of a negligent G2/M checkpoint on genomic instability and cancer induction

DNA damage responses (DDR) encompass DNA repair and signal transduction pathways that effect cell cycle checkpoint arrest and/or apoptosis. How DDR pathways respond to low levels of DNA damage, including low doses of ionizing radiation, is crucial for assessing environmental cancer risk. It has been assumed that damage-induced cell cycle checkpoints respond to a single double strand break (DSB) but the G2/M checkpoint, which prevents entry into mitosis, has recently been shown to have a defined threshold of 10-20 DSBs. Here, we consider the impact of a negligent G2/M checkpoint on genomic stability and cancer risk.

X-irradiation of cells on glass slides has a dose doubling impact

Immunofluorescence detection of gammaH2AX foci is a widely used tool to quantify the induction and repair of DNA double-strand breaks (DSBs) induced by ionising radiation. We observed that X-irradiation of mammalian cells exposed on glass slides induced twofold higher foci numbers compared to irradiation with gamma-rays. Here, we show that the excess gammaH2AX foci after X-irradiation are produced from secondary radiation particles generated from the irradiation of glass slides. Both 120 kV X-rays and (137)Cs gamma-rays induce approximately 20 gammaH2AX foci per Gy in cells growing on thin ( approximately 2 microm) plastic foils immersed in water. The same yield is obtained following gamma-irradiation of cells growing on glass slides. However, 120 kV X-rays produce approximately 40 gammaH2AX foci per Gy in cells growing on glass, twofold greater than obtained using cells irradiated on plastic surfaces. The same increase in gammaH2AX foci number is obtained if the plastic foil on which the cells are grown is irradiated on a glass slide. Thus, the physical proximity to the glass material and not morphological differences of cells growing on different surfaces accounts for the excess gammaH2AX foci. The increase in foci number depends on the energy and is considerably smaller for 25 kV relative to 120 kV X-rays, a finding which can be explained by known physical properties of radiation. The kinetics for the loss of foci, which is taken to represent the rate of DSB repair, as well as the Artemis dependent repair fraction, was similar following X- or gamma-irradiation, demonstrating that DSBs induced by this range of treatments are repaired in an identical manner.

The DNA-mismatch repair enzyme hMSH2 modulates UV-B-induced cell cycle arrest and apoptosis in melanoma cells

The mechanisms by which the post-replicative DNA mismatch repair (MMR) enzyme MSH2 is involved in the complex response mechanisms to UV damage are yet to be clarified. Here, we show increased levels of MSH2 mRNA in malignant melanoma, metastases of melanoma, and melanoma cell (MeWo) lines as compared with melanocytic nevi or primary cultured benign melanocytes. UV-B treatment modulated MSH2 expression and silencing of MSH2 gene expression using small interfering RNA technology regulated UV-B-induced cell cycle arrest and apoptosis in human MeWo. We show that MSH2-deficient non-malignant mouse fibroblasts (MEF-/-) are partially resistant against UV-B-induced apoptosis and show reduced S-Phase accumulation. In addition, we show that an Msh2 point mutation (MEFGA) that affects MMR does not affect UV-B-induced apoptosis. In conclusion, we demonstrate that MSH2 modulates in human melanocytes both UV-B-induced cell cycle regulation and apoptosis, most likely via independent, uncoupled mechanisms.

An imperfect G2M checkpoint contributes to chromosome instability following irradiation of S and G2 phase cells

DNA double strand break (DSB) repair and checkpoint control represent two major mechanisms that function to reduce chromosomal instability following ionizing irradiation (IR). Ataxia telangiectasia (A-T) cells have long been known to have defective checkpoint responses. Recent studies have shown that they also have a DSB repair defect following IR raising the issue of how ATM's repair and checkpoint functions interplay to maintain chromosomal stability. A-T and Artemis cells manifest an identical and epistatic repair defect throughout the cell cycle demonstrating that ATM's major repair defect following IR represents Artemis-dependent end-processing. Artemis cells show efficient G(2)/M checkpoint induction and a prolonged arrest relative to normal cells. Following irradiation of G(2) cells, this checkpoint is dependent on ATM and A-T cells fail to show checkpoint arrest. In contrast, cells irradiated during S phase initiate a G(2)/M checkpoint which is independent of ATM and, significantly, both Artemis and A-T cells show a prolonged arrest at the G(2)/M checkpoint likely reflecting their repair defect. Strikingly, the G(2)/M checkpoint is released before the completion of repair when approximately 10-20 DSBs remain both for S phase and G(2) phase irradiated cells. This defined sensitivity level of the G(2)/M checkpoint explains the prolonged arrest in repair-deficient relative to normal cells and provides a conceptual framework for the cooperative phenotype between checkpoint and repair functions in maintaining chromosomal stability.