A partial hprt gene duplication generated by non-homologous recombination in V79 Chinese hamster cells is eliminated by homologous recombination

Zebularine induces replication-dependent double-strand breaks which are preferentially repaired by homologous recombination

Zebularine is a second-generation, highly stable hydrophilic inhibitor of DNA methylation with oral bioavailability that preferentially target cancer cells. It acts primarily as a trap for DNA methyl transferases (DNMTs) protein by forming covalent complexes between DNMT protein and zebularine-substrate DNA. It's well documented that replication-blocking DNA lesions can cause replication fork collapse and thereby to the formation of DNA double-strand breaks (DSB). DSB are dangerous lesions that can lead to potentially oncogenic genomic rearrangements or cell death. The two major pathways for repair of DSB are non-homologous end joining (NHEJ) and homologous recombination (HR). Recently, multiple functions for the HR machinery have been identified at arrested forks. Here we investigate in more detail the importance of the lesions induced by zebularine in terms of DNA damage and cytotoxicity as well as the role of HR in the repair of these lesions. When we examined the contribution of NHEJ and HR in the repair of DSB induced by zebularine we found that these breaks were preferentially repaired by HR. Also we show that the production of DSB is dependent on active replication. To test this, we determined chromosome damage by zebularine while transiently inhibiting DNA synthesis. Here we report that cells deficient in single-strand break (SSB) repair are hypersensitive to zebularine. We have observed more DSB induced by zebularine in XRCC1 deficient cells, likely to be the result of conversion of SSB into toxic DSB when encountered by a replication fork. Furthermore we demonstrate that HR is required for the repair of these breaks. Overall, our data suggest that zebularine induces replication-dependent DSB which are preferentially repaired by HR.

Homologous recombination preferentially repairs heat-induced DNA double-strand breaks in mammalian cells

PURPOSE

Heat shock induces DNA double-strand breaks (DSBs), but the precise mechanism of repairing heat-induced damage is unclear. Here, we investigated the DNA repair pathways involved in cell death induced by heat shock.

MATERIALS AND METHODS

B02, a specific inhibitor of human RAD51 (homologous recombination; HR), and NU7026, a specific inhibitor of DNA-PK (non-homologous end-joining; NHEJ), were used for survival assays of human cancer cell lines with different p53-gene status. Mouse embryonic fibroblasts (MEFs) lacking Lig4 (NHEJ) and/or Rad54 (HR) were used for survival assays and a phosphorylated histone H2AX at Ser139 (γH2AX) assay. MEFs lacking Rad51d (HR) were used for survival assays. SPD8 cells were used to measure HR frequency after heat shock.

RESULTS

Human cancer cells were more sensitive to heat shock in the presence of B02 despite their p53-gene status, and the effect of B02 on heat sensitivity was specific to the G₂ phase. Rad54-deficient MEFs were sensitive to heat shock and showed prolonged γH2AX signals following heat shock. Rad51d-deficient MEFs were also sensitive to heat shock. Moreover, heat shock-stimulated cells had increased HR.

CONCLUSIONS

The HR pathway plays an important role in the survival of mammalian cells against death induced by heat shock via the repair of heat-induced DNA DSBs.

Homologous recombination repairs secondary replication induced DNA double-strand breaks after ionizing radiation

Ionizing radiation (IR) produces direct two-ended DNA double-strand breaks (DSBs) primarily repaired by non-homologous end joining (NHEJ). It is, however, well established that homologous recombination (HR) is induced and required for repair of a subset of DSBs formed following IR. Here, we find that HR induced by IR is drastically reduced when post-DNA damage replication is inhibited in mammalian cells. Both IR-induced RAD51 foci and HR events in the hprt gene are reduced in the presence of replication polymerase inhibitor aphidicolin (APH). Interestingly, we also detect reduced IR-induced toxicity in HR deficient cells when inhibiting post-DNA damage replication. When studying DSB formation following IR exposure, we find that apart from the direct DSBs the treatment also triggers formation of secondary DSBs peaking at 7–9 h after exposure. These secondary DSBs are restricted to newly replicated DNA and abolished by inhibiting post-DNA damage replication. Further, we find that IR-induced RAD51 foci are decreased by APH only in cells replicating at the time of IR exposure, suggesting distinct differences between IR-induced HR in S- and G2-phases of the cell cycle. Altogether, our data indicate that secondary replication-associated DSBs formed following exposure to IR are major substrates for IR-induced HR repair.

Inhibition of poly(ADP-ribose) glycohydrolase (PARG) specifically kills BRCA2-deficient tumor cells

Poly(ADP-ribose) glycohydrolase (PARG), removes poly(ADP-ribose) subunits from proteins that have previously been modified by poly(ADP-ribose) polymerse. This ensures that modification is transient, and it is suggested that removal of poly(ADP-ribose) is essential for some types of DNA repair. Here we show increased γH2AX foci formation and increased homologous recombination when PARG is inhibited. These effects are reduced when replication is inhibited, suggesting that in the absence of PARG activity, replication forks collapse, and homologous recombination is induced for repair. Consistent with this, we show that cells deficient in the homologous recombination protein BRCA2 are sensitive to PARG depletion or inhibition. These data raise the exciting possibility that PARG inhibitors may be used to specifically kill BRCA2 and other homologous recombination-deficient tumors.

DNA repair and replication influence the number of mutations per adduct of polycyclic aromatic hydrocarbons in mammalian cells

Polycyclic aromatic hydrocarbons (PAH) are an important class of environmental contaminants many of which require metabolic activation to DNA-reactive bay or fjord region diolepoxides (DE) in order to exert their mutagenic and carcinogenic effects. In this study, the mutagenicity of the bay region diolepoxides (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and (±)-anti-1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydrodibenzo[a,h]anthracene (DBADE) and the fjord region diolepoxides (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]-pyrene (DBPDE) and (±)-anti-3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo[c]-phenanthrene (BPhDE) was compared in nucleotide excision repair (NER) proficient and deficient hamster cell lines. The (32)P-postlabelling assay was applied to analyze DNA adduct levels and the Hprt gene mutation assay for monitoring mutations. Previously, we found that the mutagenicity per adduct was four times higher for DBPDE compared to BPDE in NER proficient cells. In these same cells, the mutagenicity of DBADE and BPhDE adducts was now found to be significantly lower compared to that of BPDE. In NER deficient cells the highest mutagenicity per adduct was found for BPDE and there was a tenfold and fivefold difference when comparing the BPDE data with the DBADE and BPhDE data, respectively. In order to investigate to what extent the mutagenicity of the different adducts in NER proficient cells was influenced by repair or replication bypass, we measured the overall NER incision rate, the rate of adduct removal, the rate of replication bypass and the frequency of induced recombination in the Hprt gene. Since NER turned out to be an important pathway for the yield of mutations, we further analyzed the role of transcription coupled NER versus global genome NER. However, our data demonstrate that neither of these pathways seems to be the sole factor determining the mutation frequency of the four PAH-DE and that the differences in the repair efficiency of these compounds could not be related to the presence of a bay or fjord region in the parent PAH.

Homologous recombination repair is essential for repair of vosaroxin-induced DNA double-strand breaks

Vosaroxin (formerly voreloxin) is a first-in-class anticancer quinolone derivative that intercalates DNA and inhibits topoisomerase II, inducing site-selective double-strand breaks (DSB), G2 arrest and apoptosis. Objective responses and complete remissions were observed in phase 2 studies of vosaroxin in patients with solid and hematologic malignancies, and responses were seen in patients whose cancers were resistant to anthracyclines. The quinolone-based scaffold differentiates vosaroxin from the anthracyclines and anthracenediones, broadly used DNA intercalating topoisomerase II poisons. Here we report that vosaroxin induces a cell cycle specific pattern of DNA damage and repair that is distinct from the anthracycline, doxorubicin. Both drugs stall replication and preferentially induce DNA damage in replicating cells, with damage in G2 / M > S >> G1. However, detectable replication fork collapse, as evidenced by DNA fragmentation and long tract recombination during S phase, is induced only by doxorubicin. Furthermore, vosaroxin induces less overall DNA fragmentation. Homologous recombination repair (HRR) is critical for recovery from DNA damage induced by both agents, identifying the potential to clinically exploit synthetic lethality.

Homologous recombination repair is essential for repair of vosaroxin-induced DNA double-strand breaks

Vosaroxin (formerly voreloxin) is a first-in-class anticancer quinolone derivative that intercalates DNA and inhibits topoisomerase II, inducing site-selective double-strand breaks (DSB), G2 arrest and apoptosis. Objective responses and complete remissions were observed in phase 2 studies of vosaroxin in patients with solid and hematologic malignancies, and responses were seen in patients whose cancers were resistant to anthracyclines. The quinolone-based scaffold differentiates vosaroxin from the anthracyclines and anthracenediones, broadly used DNA intercalating topoisomerase II poisons. Here we report that vosaroxin induces a cell cycle specific pattern of DNA damage and repair that is distinct from the anthracycline, doxorubicin. Both drugs stall replication and preferentially induce DNA damage in replicating cells, with damage in G2 / M > S >> G1. However, detectable replication fork collapse, as evidenced by DNA fragmentation and long tract recombination during S phase, is induced only by doxorubicin. Furthermore, vosaroxin induces less overall DNA fragmentation. Homologous recombination repair (HRR) is critical for recovery from DNA damage induced by both agents, identifying the potential to clinically exploit synthetic lethality.

Homologous recombination in cancer development, treatment and development of drug resistance

Although DNA double-strand breaks (DSBs) are substrates for homologous recombination (HR) repair, it is becoming apparent that DNA lesions produced at replication forks, for instance by many anticancer drugs, are more significant substrates for HR repair. Cells defective in HR are hypersensitive to a wide variety of anticancer drugs, including those that do not produce DSBs. Several cancers have mutations in or epigenetically silenced HR genes, which explain the genetic instability that drives cancer development. There are an increasing number of reports suggesting that mutation or epigenetic silencing of HR genes explains the sensitivity of cancers to current chemotherapy treatments. Furthermore, there are also many examples of re-expression of HR genes in tumours to explain drug resistance. Emerging data suggest that there are several different subpathways of HR, which can compensate for each other. Unravelling the overlapping pathways in HR showed that BRCA1- and BRCA2-defective cells rely on the PARP protein for survival. This synthetic lethal interaction is now being exploited for selective treatment of BRCA1- and BRCA2-defective cancers with PARP inhibitors. Here, I discuss the diversity of HR and how it impacts on cancer with a particular focus on how HR can be exploited in future anticancer strategies.

Hydroxyurea-stalled replication forks become progressively inactivated and require two different RAD51-mediated pathways for restart and repair

Faithful DNA replication is essential to all life. Hydroxyurea (HU) depletes the cells of dNTPs, which initially results in stalled replication forks that, after prolonged treatment, collapse into DSBs. Here, we report that stalled replication forks are efficiently restarted in a RAD51-dependent process that does not trigger homologous recombination (HR). The XRCC3 protein, which is required for RAD51 foci formation, is also required for replication restart of HU-stalled forks, suggesting that RAD51-mediated strand invasion supports fork restart. In contrast, replication forks collapsed by prolonged replication blocks do not restart, and global replication is rescued by new origin firing. We find that RAD51-dependent HR is triggered for repair of collapsed replication forks, without apparent restart. In conclusion, our data suggest that restart of stalled replication forks and HR repair of collapsed replication forks require two distinct RAD51-mediated pathways.

Amifostine metabolite WR-1065 disrupts homologous recombination in mammalian cells

Repair of DNA damage through homologous recombination (HR) pathways plays a crucial role in maintaining genome stability. However, overstimulation of HR pathways in response to genotoxic stress may abnormally elevate recombination frequencies, leading to increased mutation rates and delayed genomic instability. Radiation-induced genomic instability has been detected after exposure to both low- and high-linear energy transfer (LET) radiations, but the mechanisms responsible for initiating or propagating genomic instability are not known. We have demonstrated that WR-1065, the active metabolite of amifostine, protects against radiation-induced cell killing and delayed genomic instability. We hypothesize that hyperstimulation of HR pathways plays a mechanistic role in radiation-induced genomic instability and that, in part, WR-1065 exerts it radioprotective effect through suppression of the HR pathway. Results of this study demonstrate that WR-1065 treatment selectively protected against radiation-induced cell killing in HR-proficient cell lines compared to an HR-deficient cell line. Further, WR-1065 treatment decreases HR in response to DNA damage using two different mammalian cell systems. This suppression of hyper-recombination is a previously unrecognized mechanism by which WR-1065 effects radioprotection in mammalian cells.

A role for SSRP1 in recombination-mediated DNA damage response

A possible role for structure-specific recognition protein 1 (SSRP1) in replication-associated repair processes has previously been suggested based on its interaction with several DNA repair factors and the replication defects observed in SSRP1 mutants. In this study, we investigated the potential role of SSRP1 in association with DNA repair mediated by homologous recombination (HR), one of the pathways involved in repairing replication-associated DNA damage, in mammalian cells. Surprisingly, over-expression of SSRP1 reduced the number of hprt(+) recombinants generated via HR both spontaneously and upon hydroxyurea (HU) treatment, whereas knockdown of SSRP1 resulted in an increase of HR events in response to DNA double-strand break formation. In correlation, we found that the depletion of SSRP1 in HU-treated human cells elevated the number of Rad51 and H2AX foci, while over-expression of the wild-type SSRP1 markedly reduced HU-induced Rad51 foci formation. We also found that SSRP1 physically interacts with a key HR repair protein, Rad54 both in vitro and in vivo. Further, branch migration studies demonstrated that SSRP1 inhibits Rad54-promoted branch migration of Holliday junctions in vitro. Taken together, our data suggest a functional role for SSRP1 in spontaneous and replication-associated DNA damage response by suppressing avoidable HR repair events.

Specific targeted gene repair using single-stranded DNA oligonucleotides at an endogenous locus in mammalian cells uses homologous recombination

The feasibility of introducing point mutations in vivo using single-stranded DNA oligonucleotides (ssON) has been demonstrated but the efficiency and mechanism remain elusive and potential side effects have not been fully evaluated. Understanding the mechanism behind this potential therapy may help its development. Here, we demonstrate the specific repair of an endogenous non-functional hprt gene by a ssON in mammalian cells, and show that the frequency of such an event is enhanced when cells are in S-phase of the cell cycle. A potential barrier in using ssONs as gene therapy could be non-targeted mutations or gene rearrangements triggered by the ssON. Both the non-specific mutation frequencies and the frequency of gene rearrangements were largely unaffected by ssONs. Furthermore, we find that the introduction of a mutation causing the loss of a functional endogenous hprt gene by a ssON occurred at a similarly low but statistically significant frequency in wild type cells and in cells deficient in single strand break repair, nucleotide excision repair and mismatch repair. However, this mutation was not induced in XRCC3 mutant cells deficient in homologous recombination. Thus, our data suggest ssON-mediated targeted gene repair is more efficient in S-phase and involves homologous recombination.

PARP is activated at stalled forks to mediate Mre11-dependent replication restart and recombination

If replication forks are perturbed, a multifaceted response including several DNA repair and cell cycle checkpoint pathways is activated to ensure faithful DNA replication. Here, we show that poly(ADP-ribose) polymerase 1 (PARP1) binds to and is activated by stalled replication forks that contain small gaps. PARP1 collaborates with Mre11 to promote replication fork restart after release from replication blocks, most likely by recruiting Mre11 to the replication fork to promote resection of DNA. Both PARP1 and PARP2 are required for hydroxyurea-induced homologous recombination to promote cell survival after replication blocks. Together, our data suggest that PARP1 and PARP2 detect disrupted replication forks and attract Mre11 for end processing that is required for subsequent recombination repair and restart of replication forks.

BRCA2-dependent homologous recombination is required for repair of Arsenite-induced replication lesions in mammalian cells

Arsenic exposure constitutes one of the most widespread environmental carcinogens, and is associated with increased risk of many different types of cancers. Here we report that arsenite (As[III]) can induce both replication-dependent DNA double-strand breaks (DSB) and homologous recombination (HR) at doses as low as 5 µM (0.65 mg/l), which are within the typical doses often found in drinking water in contaminated areas. We show that the production of DSBs is dependent on active replication and is likely to be the result of conversion of a DNA single-strand break (SSB) into a toxic DSB when encountered by a replication fork. We demonstrate that HR is required for the repair of these breaks and show that a functional HR pathway protects against As[III]-induced cytotoxicity. In addition, BRCA2-deficient cells are sensitive to As[III] and we suggest that As[III] could be exploited as a therapy for HR-deficient tumours such as BRCA1 and BRCA2 mutated breast and ovarian cancers.

Regional genomic instability predisposes to complex dystrophin gene rearrangements

Mutations in the dystrophin gene (DMD) cause Duchenne and Becker muscular dystrophies and the majority of cases are due to DMD gene rearrangements. Despite the high incidence of these aberrations, little is known about their causative molecular mechanism(s). We examined 792 DMD/BMD clinical samples by oligonucleotide array-CGH and report on the junction sequence analysis of 15 unique deletion cases and three complex intragenic rearrangements to elucidate potential underlying mechanism(s). Furthermore, we present three cases with intergenic rearrangements involving DMD and neighboring loci. The cases with intragenic rearrangements include an inversion with flanking deleted sequences; a duplicated segment inserted in direct orientation into a deleted region; and a splicing mutation adjacent to a deletion. Bioinformatic analysis demonstrated that 7 of 12 breakpoints combined among 3 complex cases aligned with repetitive sequences, as compared to 4 of 30 breakpoints for the 15 deletion cases. Moreover, the inversion/deletion case may involve a stem-loop structure that has contributed to the initiation of this rearrangement. For the duplication/deletion and splicing mutation/deletion cases, the presence of the first mutation, either a duplication or point mutation, may have elicited the deletion events in an attempt to correct preexisting mutations. While NHEJ is one potential mechanism for these complex rearrangements, the highly complex junction sequence of the inversion/deletion case suggests the involvement of a replication-based mechanism. Our results support the notion that regional genomic instability, aided by the presence of repetitive elements, a stem-loop structure, and possibly preexisting mutations, may elicit complex rearrangements of the DMD gene.

Transcription-associated recombination is independent of XRCC2 and mechanistically separate from homology-directed DNA double-strand break repair

It has previously been shown that transcription greatly enhances recombination in mammalian cells. However, the proteins involved in catalysing this process and the recombination pathways involved in transcription-associated recombination (TAR) are still unknown. It is well established that both the BRCA2 protein and the RAD51 paralog protein XRCC2 are required for homologous recombination. Here, we show that the BRCA2 protein is also required for TAR, while the XRCC2 protein is not involved. Expression of the XRCC2 gene in XRCC2 mutated irs1 cells restores the defect in homologous recombination repair of an I-SceI-induced DNA double-strand break, while TAR is unaffected. Interestingly, the XRCC2-deficient irs1 cells are also proficient in recombination induced at slowed replication forks, suggesting that TAR is mechanistically linked with this recombination pathway. In conclusion, we show that TAR depends on BRCA2 but is independent of XRCC2, and that this recombination pathway is separate from that used to repair a two-ended DNA double-strand break.

Cyclobutane pyrimidine dimers do not fully explain the mutagenicity induced by UVA in Chinese hamster cells

UVA generates low levels of cyclobutane pyrimidine dimers (CPDs). Here we asked the question whether CPDs could fully explain the level of mutations induced by UVA. Relative mutagenicities of UVA and UVC were calculated at equal levels of CPDs in cell lines, deficient in different aspects of repair. Survival and gene mutations in the hprt locus were analyzed in a set of Chinese hamster ovary (CHO) cell lines, i.e., wild-type, Cockayne syndrome B protein-deficient (CSB), XRCC3-deficient and XRCC1-deficient adjusted to the same level of CPDs which was analyzed as strand breaks as a result of DNA cleavage by T4 endonuclease V at CPD sites. Induced mutagenicity of UVA was approximately 2 times higher than the mutagenicity of UVC in both wild-type and XRCC1-deficient cells when calculated at equal level of CPDs. Since this discrepancy could be explained by the fact that the TT-dimers, induced by UVA, might be more mutagenic than C-containing CPDs induced by UVC, we applied acetophenone, a photosensitizer previously shown to generate enhanced levels of TT-CPDs upon UVB exposure. The results suggested that the TT-CPDs were actually less mutagenic than the C-containing CPDs. We also found that the mutagenic effect of UVA was not significantly enhanced in a cell line deficient in the repair of CPDs. Altogether this suggests that neither base excision- nor nucleotide excision-repair was involved. We further challenge the possibility that the lesion responsible for the mutations induced by UVA was of a more complex nature and which possibly is repaired by homologous recombination (HR). The results indicated that UVA was more recombinogenic than UVC at equal levels of CPDs. We therefore suggest that UVA induces a complex type of lesion, which might be an obstruction during replication fork progression that requires HR repair to be further processed.

Both replication bypass fidelity and repair efficiency influence the yield of mutations per target dose in intact mammalian cells induced by benzo[a]pyrene-diol-epoxide and dibenzo[a,l]pyrene-diol-epoxide

Mutations induced by polycyclic aromatic hydrocarbons (PAH) are expected to be produced when error-prone DNA replication occurs across unrepaired DNA lesions formed by reactive PAH metabolites such as diol epoxides. The mutagenicity of the two PAH-diol epoxides (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and (+/-)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DBPDE) was compared in nucleotide excision repair (NER) proficient and deficient hamster cell lines. We applied the (32)P-postlabelling assay to analyze adduct levels and the hprt gene mutation assay for monitoring mutations. It was found that the mutagenicity per target dose was 4 times higher for DBPDE compared to BPDE in NER proficient cells while in NER deficient cells, the mutagenicity per target dose was 1.4 times higher for BPDE. In order to investigate to what extent the mutagenicity of the different adducts in NER proficient cells was influenced by repair or replication bypass, we measured the overall NER incision rate, the rate of adduct removal, the rate of replication bypass and the frequency of induced recombination in the hprt gene. The results suggest that NER of BPDE lesions are 5 times more efficient than for DBPDE lesions, in NER proficient cells. However, DBPDE adducts block replication more efficiently and also induce 6 times more recombination events in the hprt gene than adducts of BPDE, suggesting that DBPDE adducts are, to a larger extent, bypassed by homologous recombination. The results obtained here indicate that the mutagenicity of PAH is influenced not only by NER, but also by replication bypass fidelity. This has been postulated earlier based on results using in vitro enzyme assays, but is now also being recognized in terms of forward mutations in intact mammalian cells.

Transcription-associated recombination is dependent on replication in Mammalian cells

Transcription can enhance recombination; this is a ubiquitous phenomenon from prokaryotes to higher eukaryotes. However, the mechanism of transcription-associated recombination in mammalian cells is poorly understood. Here we have developed a construct with a recombination substrate in which levels of recombination can be studied in the presence or absence of transcription. We observed a direct enhancement in recombination when transcription levels through the substrate were increased. This increase in homologous recombination following transcription is locus specific, since homologous recombination at the unrelated hprt gene is unaffected. In addition, we have shown that transcription-associated recombination involves both short-tract and long-tract gene conversions in mammalian cells, which are different from double-strand-break-induced recombination events caused by endonucleases. Transcription fails to enhance recombination in cells that are not in the S phase of the cell cycle. Furthermore, inhibition of transcription suppresses induction of recombination at stalled replication forks, suggesting that recombination may be involved in bypassing transcription during replication.

Caffeine delays replication fork progression and enhances UV-induced homologous recombination in Chinese hamster cell lines

The ability to bypass DNA lesions encountered during replication is important in order to maintain cell viability and avoid genomic instability. Exposure of mammalian cells to UV-irradiation induces the formation of DNA lesions that stall replication forks. In order to restore replication, different bypass mechanisms are operating, previously named post-replication repair. Translesion DNA synthesis is performed by low-fidelity polymerases, which can replicate across damaged sites. The nature of lesions and of polymerases involved influences the resulting frequency of mutations. Homologous recombination represents an alternative pathway for the rescue of stalled replication forks. Caffeine has long been recognized to influence post-replication repair, although the mechanism is not identified. Here, we found that caffeine delays the progress of replication forks in UV-irradiated Chinese hamster cells. The length of this enhanced delay was similar in wild-type cells and in cell deficient in either homologous recombination or nucleotide excision repair. Furthermore, caffeine attenuated the frequency of UV-induced mutations in the hprt gene, whereas the frequency of recombination, monitored in this same gene, was enhanced. These observations indicate that in cells exposed to UV-light, caffeine inhibits the rescue of stalled replication forks by translesion DNA synthesis, thereby causing a switch to bypass via homologous recombination. The biological consequence of the former pathway is mutations, while the latter results in chromosomal aberrations.

Duplications in the DMD gene

The detection of duplications in Duchenne (DMD)/Becker Muscular Dystrophy (BMD) has long been a neglected issue. However, recent technological advancements have significantly simplified screening for such rearrangements. We report here the detection and analysis of 118 duplications in the DMD gene of DMD/BMD patients. In an unselected patient series the duplication frequency was 7%. In patients already screened for deletions and point mutations, duplications were detected in 87% of cases. There were four complex, noncontiguous rearrangements, with two also involving a partial triplication. In one of the few cases where RNA was analyzed, a seemingly contiguous duplication turned out to be a duplication/deletion case generating a transcript with an unexpected single-exon deletion and an initially undetected duplication. These findings indicate that for clinical diagnosis, duplications should be treated with special care, and without further analysis the reading frame rule should not be applied. As with deletions, duplications occur nonrandomly but with a dramatically different distribution. Duplication frequency is highest near the 5' end of the gene, with a duplication of exon 2 being the single most common duplication identified. Analysis of the extent of 11 exon 2 duplications revealed two intron 2 recombination hotspots. Sequencing four of the breakpoints showed that they did not arise from unequal sister chromatid exchange, but more likely from synthesis-dependent nonhomologous end joining. There appear to be fundamental differences therefore in the origin of deletions and duplications in the DMD gene.

Homologous recombination is involved in repair of chromium-induced DNA damage in mammalian cells

Chromium is a potent human carcinogen, probably because of its well-documented genotoxic effects. Chromate (Cr[VI]) causes a wide range of DNA lesions, including DNA crosslinks and strand breaks, presumably due to the direct and indirect effects of DNA oxidation. Homologous recombination repair (HRR) is important for error-free repair of lesions occurring at replication forks. Here, we show that HR deficient cell lines irs1SF and V-C8, deficient in XRCC3 and BRCA2, respectively, are hypersensitive to Cr[VI], implicating this repair pathway in repair of Cr[VI] damage. Furthermore, we find that Cr[VI] causes DNA double-strand breaks and triggers both Rad51 foci formation and induction of HRR. Collectively, these data suggest that HRR is important in repair of Cr[VI]-induced DNA damage. In addition, we find that ERCC1, XRCC1 and DNA-PKcs defective cells are hypersensitive to Cr[VI], indicating that several repair pathways cooperate in repairing Cr[VI]-induced DNA damage.

Inhibition of poly (ADP-ribose) polymerase activates ATM which is required for subsequent homologous recombination repair

Poly (ADP-ribose) polymerase (PARP-1), ATM and DNA-dependent protein kinase (DNA-PK) are all involved in responding to DNA damage to activate pathways responsible for cellular survival. Here, we demonstrate that PARP-1-/- cells are sensitive to the ATM inhibitor KU55933 and conversely that AT cells are sensitive to the PARP inhibitor 4-amino-1,8-napthalamide. In addition, PARP-1-/- cells are shown to be sensitive to the DNA-PK inhibitor NU7026 and DNA-PKcs or Ku80 defective cells shown to be sensitive to PARP inhibitors. We believe PARP inhibition results in an increase in unresolved spontaneous DNA single-strand breaks (SSBs), which collapse replication forks and trigger homologous recombination repair (HRR). We show that ATM is activated following inhibition of PARP. Furthermore, PARP inhibitor-induced HRR is abolished in ATM, but not DNA-PK, inhibited cells. ATM and DNA-PK inhibition together give the same sensitivity to PARP inhibitors as ATM alone, indicating that ATM functions in the same pathways as DNA-PK for survival at collapsed forks, likely in non-homologous end joining (NHEJ). Altogether, we suggest that ATM is activated by PARP inhibitor-induced collapsed replication forks and may function upstream of HRR in the repair of certain types of double-strand breaks (DSBs).

Spontaneous homologous recombination is induced by collapsed replication forks that are caused by endogenous DNA single-strand breaks

Homologous recombination is vital to repair fatal DNA damage during DNA replication. However, very little is known about the substrates or repair pathways for homologous recombination in mammalian cells. Here, we have compared the recombination products produced spontaneously with those produced following induction of DNA double-strand breaks (DSBs) with the I-SceI restriction endonuclease or after stalling or collapsing replication forks following treatment with thymidine or camptothecin, respectively. We show that each lesion produces different spectra of recombinants, suggesting differential use of homologous recombination pathways in repair of these lesions. The spontaneous spectrum most resembled the spectra produced at collapsed replication forks formed when a replication fork runs into camptothecin-stabilized DNA single-strand breaks (SSBs) within the topoisomerase I cleavage complex. We found that camptothecin-induced DSBs and the resulting recombination repair require replication, showing that a collapsed fork is the substrate for camptothecin-induced recombination. An SSB repair-defective cell line, EM9 with an XRCC1 mutation, has an increased number of spontaneous gammaH2Ax and RAD51 foci, suggesting that endogenous SSBs collapse replication forks, triggering recombination repair. Furthermore, we show that gammaH2Ax, DSBs, and RAD51 foci are synergistically induced in EM9 cells with camptothecin, suggesting that lack of SSB repair in EM9 causes more collapsed forks and more recombination repair. Furthermore, our results suggest that two-ended DSBs are rare substrates for spontaneous homologous recombination in a mammalian fibroblast cell line. Interestingly, all spectra showed evidence of multiple homologous recombination events in 8 to 16% of clones. However, there was no increase in homologous recombination genomewide in these clones nor were the events dependent on each other; rather, we suggest that a first homologous recombination event frequently triggers a second event at the same locus in mammalian cells.

Up-Regulation of the Error-Prone DNA Polymerase κ Promotes Pleiotropic Genetic Alterations and Tumorigenesis

It is currently widely accepted that genetic instability is key to cancer development. Many types of cancers arise as a consequence of a gradual accumulation of nucleotide aberrations, each mutation conferring growth and/or survival advantage. Genetic instability could also proceed in sudden bursts leading to a more drastic upheaval of structure and organization of the genome. Genetic instability, as an operative force, will produce genetic variants and the greater the instability, the larger the number of variants. We report here that the overexpression of human DNA polymerase κ, an error-prone enzyme that is up-regulated in lung cancers, induces DNA breaks and stimulates DNA exchanges as well as aneuploidy. Probably as the result of so many perturbations, excess polymerase κ favors the proliferation of competent tumor cells as observed in immunodeficient mice. These data suggest that altered regulation of DNA metabolism might be related to cancer-associated genetic changes and phenotype.

Characterisation of cytotoxicity and DNA damage induced by the topoisomerase II-directed bisdioxopiperazine anti-cancer agent ICRF-187 (dexrazoxane) in yeast and mammalian cells

Background

Bisdioxopiperazine anti-cancer agents are inhibitors of eukaryotic DNA topoisomerase II, sequestering this protein as a non-covalent protein clamp on DNA. It has been suggested that such complexes on DNA represents a novel form of DNA damage to cells. In this report, we characterise the cytotoxicity and DNA damage induced by the bisdioxopiperazine ICRF-187 by a combination of genetic and molecular approaches. In addition, the well-established topoisomerase II poison m-AMSA is used for comparison.

Results

By utilizing a panel of Saccharomyces cerevisiae single-gene deletion strains, homologous recombination was identified as the most important DNA repair pathway determining the sensitivity towards ICRF-187. However, sensitivity towards m-AMSA depended much more on this pathway. In contrast, disrupting the post replication repair pathway only affected sensitivity towards m-AMSA. Homologous recombination (HR) defective irs1SF chinese hamster ovary (CHO) cells showed increased sensitivity towards ICRF-187, while their sensitivity towards m-AMSA was increased even more. Furthermore, complementation of the XRCC3 deficiency in irs1SF cells fully abrogated hypersensitivity towards both drugs. DNA-PK_cs deficient V3-3 CHO cells having reduced levels of non-homologous end joining (NHEJ) showed slightly increased sensitivity to both drugs. While exposure of human small cell lung cancer (SCLC) OC-NYH cells to m-AMSA strongly induced γH2AX, exposure to ICRF-187 resulted in much less induction, showing that ICRF-187 generates fewer DNA double strand breaks than m-AMSA. Accordingly, when yeast cells were exposed to equitoxic concentrations of ICRF-187 and m-AMSA, the expression of DNA damage-inducible genes showed higher levels of induction after exposure to m-AMSA as compared to ICRF-187. Most importantly, ICRF-187 stimulated homologous recombination in SPD8 hamster lung fibroblast cells to lower levels than m-AMSA at all cytotoxicity levels tested, showing that the mechanism of action of bisdioxopiperazines differs from that of classical topoisomerase II poisons in mammalian cells.

Conclusion

Our results point to important differences in the mechanism of cytotoxicity induced by bisdioxopiperazines and topoisomerase II poisons, and suggest that bisdioxopiperazines kill cells by a combination of DNA break-related and DNA break-unrelated mechanisms.

Induction of homologous recombination in the hprt gene of V79 Chinese hamster cells in response to low- and high-LET irradiation

Dense ionization tracks from high linear energy transfer (LET) radiations form multiple damaged sites (MDS), which involve several types of DNA lesions in close vicinity. The primary DNA damage triggers sensor proteins that activate repair processes, cell cycle control or eventually apoptosis in subsequent cellular responses. The question how homologous recombination (HR) and non-homologous end joining (NHEJ) interact in the repair of radiation-induced DNA damage of MDS type has been addressed in different model systems but several questions remain to be answered. We have therefore challenged cells with treatments of ionizing radiation of different qualities to investigate whether primary DNA damages of different complexity are reflected in the processes of repair by HR as well as cell survival. We used the V79 derived SPD8 cell line to determine the induction of HR in the hprt exon 7 and clonogenic assay for survival in response to radiation. SPD8 cells were irradiated with gamma-rays (137Cs 0.5 keV/microm), boron ions (40 and 80 keV/microm) and nitrogen ions (140 keV/microm), with doses up to 5 Gy. Analysis of clonogenic survival showed that B-ions (80 keV/microm) and N-ions were more toxic than gamma-rays, 4.1 and 5.0 times respectively, while B-ions at 40 keV/microm were 2.0 times as toxic as gamma-rays. Homologous recombination in the cells exposed to B-ions (80 keV/microm) increased 2.9 times, a significant response as compared to cells exposed to gamma-rays, while for B-ions (40 keV/microm) and N-ions a nonsignificant increase in HR of 1.2 and 1.4, respectively, was observed. We hypothesize that the high-LET generated formation of MDS is responsible for the enhanced cytotoxicity as well as for the mobilization of the HR machinery.

Inactivating mutations block the tumor necrosis factor-alpha-converting enzyme in the early secretory pathway

The ectodomain of different transmembrane molecules is released by a proteolytic event known as shedding. The metalloprotease disintegrin proTNF-alpha converting enzyme (TACE) is responsible for the shedding of various proteins, including protransforming growth factor-alpha (proTGF-alpha) and amyloid-beta precursor protein (APP). Inactive TACE accumulates in the early secretory pathway of cell mutants (M1 and M2) defective in proTGF-alpha and APP shedding. Although previous evidences indicated that the component mutated in M1 and M2 cells is different from TACE, recent results show the existence of two heterozygous point mutations in TACE from M2 cells. Here, we show that wild-type TACE stably transfected in M2 cells is processed, transported to the cell surface, and rescues the proTGF-alpha and APP shedding-defective phenotype. Furthermore, M1 cells also express mutant TACE and transfection with wild-type TACE restores the wild-type phenotype. Therefore, different inactivating mutations result in the accumulation of TACE in the early secretory pathway, emphasizing the importance of the initial steps in the biosynthesis of TACE.

X-linked ectodermal dysplasia and immunodeficiency caused by reversion mosaicism of NEMO reveals a critical role for NEMO in human T-cell development and/or survival

X-linked ectodermal dysplasia and immunodeficiency (XL-EDA-ID) is an X-linked recessive disease caused by a mutation in the nuclear factor-kappaB (NF-kappaB) essential modulator (NEMO). Here we report an XL-EDA-ID patient with atypical features of very few naive-phenotype T cells and defective mitogen-induced proliferation of peripheral blood mononuclear cells (PBMCs). The patient's NEMO defect was diagnosed by flow cytometric analysis of intracellular NEMO staining. Specific cell lineages (monocytes and neutrophils) expressed reduced levels of NEMO, but 2 populations of T, B, and NK cells were detected with normal and reduced expression of NEMO. Genomic analysis revealed that duplication of a 4.4-kb sequence ranging from intron 3 to exon 6 caused the reduced expression of NEMO. Polymorphism analysis showed that the patient's B- and T-cell lines with reduced and normal expression of NEMO had the same X chromosome, indicating that the somatic mosaicism was not due to fetomaternal transfusion but was most likely due to postzygotic reversion. This XLEDA-ID case adds to our understanding of NEMO biology, indicating that NEMO is critical for T-cell development and/or survival in humans as well as in mice.

RAD51 is involved in repair of damage associated with DNA replication in mammalian cells

The RAD51 protein, a eukaryotic homologue of the Escherichia coli RecA protein, plays an important role in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) in mammalian cells. Recent findings suggest that HR may be important in repair following replication arrest in mammalian cells. Here, we have investigated the role of RAD51 in the repair of different types of damage induced during DNA replication with etoposide, hydroxyurea or thymidine. We show that etoposide induces DSBs at newly replicated DNA more frequently than gamma-rays, and that these DSBs are different from those induced by hydroxyurea. No DSB was found following treatment with thymidine. Although these compounds appear to induce different DNA lesions during DNA replication, we show that a cell line overexpressing RAD51 is resistant to all of them, indicating that RAD51 is involved in repair of a wide range of DNA lesions during DNA replication. We observe fewer etoposide-induced DSBs in RAD51-overexpressing cells and that HR repair of etoposide-induced DSBs is faster. Finally, we show that induced long-tract HR in the hprt gene is suppressed in RAD51-overexpressing cells, although global HR appears not to be suppressed. This suggests that overexpression of RAD51 prevents long-tract HR occurring during DNA replication. We discuss our results in light of recent models suggested for HR at stalled replication forks.

Overexpression of cyclin E does not influence homologous recombination in Chinese hamster cells

Overexpressed cyclin E in tumours is a prognosticator for poor patient outcome. Cells that overexpress cyclin E have been shown to be impaired in S-phase progression and exhibit genetic instability that may drive this subset of cancers. However, the origin for genetic instability caused by cyclin E overexpression is unknown. Homologous recombination plays an important role in S-phase progression and is also regulated by the same proteins that regulate cyclin E-associated kinase activity, i.e., p53 and p21. To test the hypothesis that overexpressed cyclin E causes genetic instability through homologous recombination, we investigated the effect of cyclin E overexpression on homologous recombination in the hprt gene in a Chinese hamster cell line. Although cyclin E overexpression shortened the G1 phase in the cell cycle as expected, we could see no change in neither spontaneous nor etoposide-induced recombination. Also, overexpression of cyclin E did not affect the repair of DNA double-strand breaks and failed to potentiate the cytotoxic effects of etoposide. Our data suggest that genetic instability caused by overexpression of cyclin E is not mediated by aberrant homologous recombination.

Different roles for nonhomologous end joining and homologous recombination following replication arrest in mammalian cells

Homologous recombination (HR) and nonhomologous end joining (NHEJ) play overlapping roles in repair of DNA double-strand breaks (DSBs) generated during the S phase of the cell cycle. Here, we characterized the involvement of HR and NHEJ in the rescue of DNA replication forks arrested or slowed by treatment of hamster cells with hydroxyurea or thymidine. We show that the arrest of replication with hydroxyurea generates DNA fragmentation as a consequence of the formation of DSBs at newly replicated DNA. Both HR and NHEJ protected cells from the lethal effects of hydroxyurea, and this agent also increased the frequency of recombination mediated by both homologous and nonhomologous exchanges. Thymidine induced a less stringent arrest of replication and did not generate detectable DSBs. HR alone rescued cells from the lethal effects of thymidine. Furthermore, thymidine increased the frequency of DNA exchange mediated solely by HR in the absence of detectable DSBs. Our data suggest that both NHEJ and HR are involved in repair of arrested replication forks that include a DSB, while HR alone is required for the repair of slowed replication forks in the absence of detectable DSBs.

ELF magnetic field affects proliferation of SPD8/V79 Chinese hamster cells but does not interact with intrachromosomal recombination

Extremely low-frequency (ELF) magnetic fields have previously been shown to affect conformation of chromatin, cell proliferation, and calcium metabolism. Possible mutagenic and carcinogenic effects of ELF have also been discussed and tested. In this study, intrachromosomal recombination in the hprt gene after exposure to ELF magnetic field was investigated using the SPD8 recombination assay. SPD8 cells, derived from V79 Chinese hamster cells were exposed to ELF at a specific combination of static and ELF magnetic fields, that has been proven to have effects on chromatin conformation in several cell types. The genotoxic agent camptothecin (CPT) was used either as a positive control or simultaneously with ELF. We also analysed the effect of ELF and CPT on chromatin conformation with the anomalous viscosity time dependence (AVTD) technique, cell growth kinetics, and cell survival with clonogenic assay. DNA fragmentation was analysed by pulsed field gel electrophoresis (PFGE). ELF did not induce recombination alone, neither did ELF modify the recombinogenic effect of CPT. Although, there was no effect on cell survival in response to ELF exposure, inhibition of cell growth was observed. On the other hand, ELF exposure partly counteracted the growth inhibition seen with CPT. The data suggest that ELF exposure may stimulate or inhibit cell growth depending on the state of the cells. Although, ELF did not induce recombination, a weak but statistically significant DNA fragmentation comparable with CPT-induced fragmentation was observed with PFGE 48h after exposure to ELF.

DNA double-strand breaks associated with replication forks are predominantly repaired by homologous recombination involving an exchange mechanism in mammalian cells

DNA double-strand breaks (DSB) represent a major disruption in the integrity of the genome. DSB can be generated when a replication fork encounters a DNA lesion. Recombinational repair is known to resolve such replication fork-associated DSB, but the molecular mechanism of this repair process is poorly understood in mammalian cells. In the present study, we investigated the molecular mechanism by which recombination resolves camptothecin (CPT)-induced DSB at DNA replication forks. The frequency of homologous recombination (HR) was measured using V79/SPD8 cells which contain a duplication in the endogenous hprt gene that is resolved by HR. We demonstrate that DSB associated with replication forks induce HR at the hprt gene in early S phase. Further analysis revealed that these HR events involve an exchange mechanism. Both the irs1SF and V3-3 cell lines, which are deficient in HR and non-homologous end joining (NHEJ), respectively, were found to be more sensitive than wild-type cells to DSB associated with replication forks. The irs1SF cell line was more sensitive in this respect than V3-3 cells, an observation consistent with the hypothesis that DSB associated with replication forks are repaired primarily by HR. The frequency of formation of DSB associated with replication forks was not affected in HR and NHEJ deficient cells, indicating that the loss of repair, rather than the formation of DSB associated with replication forks is responsible for the increased sensitivity of the mutant strains. We propose that the presence of DSB associated with replication forks rapidly induces HR via an exchange mechanism and that HR plays a more prominent role in the repair of such DSB than does NHEJ.

RAD51 supports spontaneous non-homologous recombination in mammalian cells, but not the corresponding process induced by topoisomerase inhibitors

The RAD51 protein has been shown to participate in homologous recombination by promoting ATP-dependent homologous pairing and strand transfer reactions. In the present study, we have investigated the possible involvement of RAD51 in non-homologous recombination. We demonstrate that overexpression of CgRAD51 enhances the frequency of spontaneous non-homologous recombination in the hprt gene of Chinese hamster cells. However, the rate of non-homologous recombination induced by the topoisomerase inhibitors campothecin and etoposide was not altered by overexpression of RAD51. These results indicate that the RAD51 protein may perform a function in connection with spontaneous non-homologous recombination that is not essential to or not rate-limiting for non-homologous recombination induced by camptothecin or etoposide. We discuss the possibility that the role played by RAD51 in non-homologous recombination observed here may not be linked to non-homologous end-joining.

Inhibition of DNA synthesis is a potent mechanism by which cytostatic drugs induce homologous recombination in mammalian cells

Recombination is a process thought to be underlying genomic instability involved in carcinogenesis. This report examines the potential of cytostatic drugs to induce intrachromosomal homologous recombination. In order to address this question, the hprt gene of a well-characterized mammalian cell line was employed as a unique endogenous marker for homologous recombination. Commonly used cytostatic drugs with different mode of action were investigated in this context, i.e. bifunctional alkylating agents, inhibitors of DNA synthesis, inhibitors of topoisomerases and a spindle poison. With the exception of the spindle poison, all these drugs were found to induce homologous recombination, with clear differences in their recombination potency, which could be related to their mechanism of action. Bifunctional alkylating agents were the least efficient, whereas inhibitors of DNA synthesis were found to be the most potent inducers of homologous recombination. This raises the question whether these later drugs should be considered for adverse effects in cancer chemotheraphy.

Genomic structure of the mouse Ap3b1 gene in normal and pearl mice

The mouse hypopigmentation mutant pearl is an established model for Hermansky-Pudlak syndrome (HPS), a genetically heterogenous disease with misregulation of the biogenesis/function of melanosomes, lysosomes, and platelet dense granules. The pearl (Ap3b1) gene encodes the beta3A subunit of the AP-3 adaptor complex, which regulates vesicular trafficking. The genomic structure of the normal Ap3b1 gene includes 25 introns and a putative promoter sequence. The original pearl (pe) mutation, which has an unusually high reversion rate on certain strain backgrounds, has been postulated to be caused by insertion of a transposable element. Indeed, the mutation contains a 215-bp partial mouse transposon at the junction point of a large tandem genomic duplication of 6 exons and associated introns. At the cDNA level, three pearl mutations (pearl, pearl-8J, and pearl-9J) are caused by deletions or duplications of a complete exon(s).

Arsenic[III] and heavy metal ions induce intrachromosomal homologous recombination in the hprt gene of V79 Chinese hamster cells

In the present study the carcinogenic metal ions Cd[II], Co[II], Cr[VI], Ni[II], and Pb[II], as well as As[III], were examined for their ability to induce intrachromosomal homologous and nonhomologous recombination in the hprt gene of two V79 Chinese hamster cell lines, SPD8 and Sp5, respectively. With the exception of Pb[II], all of these ions enhanced homologous recombination, the order of potency being Cr>Cd>As>Co>Ni. In contrast, Cr[VI] was the only ion to enhance recombination of the nonhomologous type. In order to obtain additional information on the mechanism of recombination in the SPD8 cell line, individual clones exhibiting metal-induced recombination were isolated, and the sequence of their hprt gene determined. These findings confirmed that all recombinogenic events in this cell line were of the homologous type, involving predominantly a chromatid exchange mechanism. The mechanisms underlying the recombination induced by these ions are discussed in relationship to their genotoxicity, as well as to DNA repair and replication. Induced recombination may constitute a novel mechanism for induction of neoplastic disease.

Splicing mutations of 54-bp exons in the COL11A1 gene cause Marshall syndrome, but other mutations cause overlapping Marshall/Stickler phenotypes

Stickler and Marshall syndromes are dominantly inherited chondrodysplasias characterized by midfacial hypoplasia, high myopia, and sensorineural-hearing deficit. Since the characteristics of these syndromes overlap, it has been argued whether they are distinct entities or different manifestations of a single syndrome. Several mutations causing Stickler syndrome have been found in the COL2A1 gene, and one mutation causing Stickler syndrome and one causing Marshall syndrome have been detected in the COL11A1 gene. We characterize here the genomic structure of the COL11A1 gene. Screening of patients with Stickler, Stickler-like, or Marshall syndrome pointed to 23 novel mutations. Genotypic-phenotypic comparison revealed an association between the Marshall syndrome phenotype and splicing mutations of 54-bp exons in the C-terminal region of the COL11A1 gene. Null-allele mutations in the COL2A1 gene led to a typical phenotype of Stickler syndrome. Some patients, however, presented with phenotypes of both Marshall and Stickler syndromes.

The RAD51 protein supports homologous recombination by an exchange mechanism in mammalian cells

Information concerning the function of recombination proteins in mammalian cells has been obtained from biochemical studies, but little is known about their mechanisms of action in growing cells. The eukaryotic recombination protein RAD51, a homologue of the Escherichia coli RecA protein, has been shown to interact with various proteins, including the p53 protein, the guardian of genomic stability maintenance. Here, the hamster RAD51 protein, CgRAD51, has been overexpressed in the SPD8 cell line, derived from Chinese hamster V79 cells. This cell line offers unique possibilities for studying different mechanisms for homologous recombination on endogenous substrates. We report that the SPD8 cell line contains a mutated p53 gene, which provides new insights into the recombination process in these cells. The present study demonstrates that overexpression of CgRAD51 in these cells results in a two- to threefold increase in endogenous recombination. In addition, sequence analysis indicated that RAD51 promotes homologous recombination by a chromatid exchange mechanism.

Brominated flame retardants induce intragenic recombination in mammalian cells

In the present study we have examined the effects of brominated flame retardants (BFR) and several other environmental contaminants in two in vitro assays for intragenic recombination at an endogenous locus in mammalian cells. A total ten compounds were investigated, i. e., two technical PCB mixtures (Aroclor 1221 and Aroclor 1254), DDT, PCP, tetrabromobisphenol A (TBBPA), 4,4'-bischlorophenyl sulfone (BCPS), hexabromocyclododecane (HBCD) and the three different polybrominated diphenylethers (PBDEs): 2-bromodiphenylether (MBDE), 3,4-dibromodiphenylether (DBDE) and 2,4,2', 4'-tetrabromodiphenylether (TBDE). In the SPD8 assay system statistically significant increases in recombination frequency were observed with Aroclor 1221, BCPS, DBDE, DDT, HBCD, MBDE and TBDE. In the Sp5 assay system, only DBDE, HBCD and MBDE caused statistically significant increases in recombination frequency. In conclusion, our findings indicate that the modern additives to plastic, i.e., HBCD and PBDEs, as well as the plastic monomer BCPS may have the same effect to human health as DDT and PCBs, in terms of inducing genetic recombination, which is known to provoke a number of diseases, including cancer.