Genetic interactions between RAD51 and its paralogues for centrosome fragmentation and ploidy control, independently of the sensitivity to genotoxic stresses

A Link between Replicative Stress, Lamin Proteins, and Inflammation

Double-stranded breaks (DSB), the most toxic DNA lesions, are either a consequence of cellular metabolism, programmed as in during V(D)J recombination, or induced by anti-tumoral therapies or accidental genotoxic exposure. One origin of DSB sources is replicative stress, a major source of genome instability, especially when the integrity of the replication forks is not properly guaranteed. To complete stalled replication, restarting the fork requires complex molecular mechanisms, such as protection, remodeling, and processing. Recently, a link has been made between DNA damage accumulation and inflammation. Indeed, defects in DNA repair or in replication can lead to the release of DNA fragments in the cytosol. The recognition of this self-DNA by DNA sensors leads to the production of inflammatory factors. This beneficial response activating an innate immune response and destruction of cells bearing DNA damage may be considered as a novel part of DNA damage response. However, upon accumulation of DNA damage, a chronic inflammatory cellular microenvironment may lead to inflammatory pathologies, aging, and progression of tumor cells. Progress in understanding the molecular mechanisms of DNA damage repair, replication stress, and cytosolic DNA production would allow to propose new therapeutical strategies against cancer or inflammatory diseases associated with aging. In this review, we describe the mechanisms involved in DSB repair, the replicative stress management, and its consequences. We also focus on new emerging links between key components of the nuclear envelope, the lamins, and DNA repair, management of replicative stress, and inflammation.

The ATPase activity of E. coli RecA prevents accumulation of toxic complexes formed by erroneous binding to undamaged double stranded DNA

The Escherichia coli RecA protein catalyzes the central step of homologous recombination using its homology search and strand exchange activity. RecA is a DNA-dependent ATPase, but its homology search and strand exchange activities are largely independent of its ATPase activity. ATP hydrolysis converts a high affinity DNA binding form, RecA-ATP, to a low affinity form RecA-ADP, thereby supporting an ATP hydrolysis-dependent dynamic cycle of DNA binding and dissociation. We provide evidence for a novel function of RecA's dynamic behavior; RecA's ATPase activity prevents accumulation of toxic complexes caused by direct binding of RecA to undamaged regions of dsDNA. We show that a mutant form of RecA, RecA-K250N, previously shown to be toxic to E. coli, is a loss-of-function ATPase-defective mutant. We use a new method for detecting RecA complexes involving nucleoid surface spreading and immunostaining. The method allows detection of damage-induced RecA foci; STED microscopy revealed these to typically be between 50 and 200 nm in length. RecA-K250N, and other toxic variants of RecA, form spontaneous DNA-bound complexes that are independent of replication and of accessory proteins required to load RecA onto tracts of ssDNA in vivo, supporting the hypothesis that RecA's expenditure of ATP serves an error correction function.

High levels of RAD51 perturb DNA replication elongation and cause unscheduled origin firing due to impaired CHK1 activation

In response to replication stress ATR signaling through CHK1 controls the intra-S checkpoint and is required for the maintenance of genomic integrity. Homologous recombination (HR) comprises a series of interrelated pathways that function in the repair of DNA double strand breaks and interstrand crosslinks. In addition, HR, with its key player RAD51, provides critical support for the recovery of stalled forks during replication. High levels of RAD51 are regularly found in various cancers, yet little is known about the effect of the increased RAD51 expression on intra-S checkpoint signaling. Here, we describe a role for RAD51 in driving genomic instability caused by impaired replication and intra-S mediated CHK1 signaling by studying an inducible RAD51 overexpression model as well as 10 breast cancer cell lines. We demonstrate that an excess of RAD51 decreases I-Sce-I mediated HR despite formation of more RAD51 foci. Cells with high RAD51 levels display reduced elongation rates and excessive dormant origin firing during undisturbed growth and after damage, likely caused by impaired CHK1 activation. In consequence, the inability of cells with a surplus of RAD51 to properly repair complex DNA damage and to resolve replication stress leads to higher genomic instability and thus drives tumorigenesis.

Slow Replication Fork Velocity of Homologous Recombination-Defective Cells Results from Endogenous Oxidative Stress

Replications forks are routinely hindered by different endogenous stresses. Because homologous recombination plays a pivotal role in the reactivation of arrested replication forks, defects in homologous recombination reveal the initial endogenous stress(es). Homologous recombination-defective cells consistently exhibit a spontaneously reduced replication speed, leading to mitotic extra centrosomes. Here, we identify oxidative stress as a major endogenous source of replication speed deceleration in homologous recombination-defective cells. The treatment of homologous recombination-defective cells with the antioxidant N-acetyl-cysteine or the maintenance of the cells at low O₂ levels (3%) rescues both the replication fork speed, as monitored by single-molecule analysis (molecular combing), and the associated mitotic extra centrosome frequency. Reciprocally, the exposure of wild-type cells to H₂O₂ reduces the replication fork speed and generates mitotic extra centrosomes. Supplying deoxynucleotide precursors to H₂O₂-exposed cells rescued the replication speed. Remarkably, treatment with N-acetyl-cysteine strongly expanded the nucleotide pool, accounting for the replication speed rescue. Remarkably, homologous recombination-defective cells exhibit a high level of endogenous reactive oxygen species. Consistently, homologous recombination-defective cells accumulate spontaneous γH2AX or XRCC1 foci that are abolished by treatment with N-acetyl-cysteine or maintenance at 3% O₂. Finally, oxidative stress stimulated homologous recombination, which is suppressed by supplying deoxynucleotide precursors. Therefore, the cellular redox status strongly impacts genome duplication and transmission. Oxidative stress should generate replication stress through different mechanisms, including DNA damage and nucleotide pool imbalance. These data highlight the intricacy of endogenous replication and oxidative stresses, which are both evoked during tumorigenesis and senescence initiation, and emphasize the importance of homologous recombination as a barrier against spontaneous genetic instability triggered by the endogenous oxidative/replication stress axis.

Endogenous stress is an important stress because it challenges cells daily. However, endogenous stress is difficult to apprehend. Replication forks are routinely hindered by different endogenous stresses. Because homologous recombination plays a pivotal role in the reactivation of arrested replication forks, defects in homologous recombination reveal the initial endogenous stress(es). Here we identify endogenous oxidative stress among the different potential endogenous stresses as being responsible for spontaneous replication defects in homologous recombination-defective cells. Therefore, oxidative and replication stresses, which are both evoked during tumorigenesis and senescence initiation, are linked, and homologous recombination acts as a barrier against spontaneous genetic instability triggered by endogenous oxidative/replication stress.

Elevated estrogen receptor-α in VHL-deficient condition induces microtubule organizing center amplification via disruption of BRCA1/Rad51 interaction

Since loss of VHL is frequently detected early phase genetic event in human renal cell carcinoma, pVHL is assumed to be indispensable for suppression of tumor initiation step. However, induction of HIF-1α, target of pVHL E3 ligase, is more adequate to angiogenesis step after tumor mass formation. Concerning this, it has been reported that pVHL is involved in centrosome location during metaphase and regulates ER-α signaling. Here, we provide the evidences that pVHL-mediated ER-α suppression is critical for microtubule organizing center (MTOC) maintaining and elevated ER-α promotes MTOC amplification through disruption of BRCA1-Rad51 interaction. In fact, numerous MTOC in VHL- or BRCA1-deficient cells are reduced by Fulvestrant, inhibitor of ER-α expression as well as antagonist. In addition, we reveal that activation of ER signaling can increase γ-tubulin, core factor of TuRC and render the resistance to Taxol. Thus, Fulvestrant but not Tamoxifen, antagonist against ER-α, can restore the Taxol sensitivity in VHL- or BRCA1-deficient cells. Our results suggest that pVHL-mediated ER-α suppression is important for regulation of MTOC as well as drug resistance.

Replication stress in Mammalian cells and its consequences for mitosis

The faithful transmission of genetic information to daughter cells is central to maintaining genomic stability and relies on the accurate and complete duplication of genetic material during each cell cycle. However, the genome is routinely exposed to endogenous and exogenous stresses that can impede the progression of replication. Such replication stress can be an early cause of cancer or initiate senescence. Replication stress, which primarily occurs during S phase, results in consequences during mitosis, jeopardizing chromosome segregation and, in turn, genomic stability. The traces of replication stress can be detected in the daughter cells during G1 phase. Alterations in mitosis occur in two types: 1) local alterations that correspond to breaks, rearrangements, intertwined DNA molecules or non-separated sister chromatids that are confined to the region of the replication dysfunction; 2) genome-wide chromosome segregation resulting from centrosome amplification (although centrosomes do not contain DNA), which amplifies the local replication stress to the entire genome. Here, we discuss the endogenous causes of replication perturbations, the mechanisms of replication fork restart and the consequences for mitosis, chromosome segregation and genomic stability.

Is homologous recombination really an error-free process?

Homologous recombination (HR) is an evolutionarily conserved process that plays a pivotal role in the equilibrium between genetic stability and diversity. HR is commonly considered to be error-free, but several studies have shown that HR can be error-prone. Here, we discuss the actual accuracy of HR. First, we present the product of genetic exchanges (gene conversion, GC, and crossing over, CO) and the mechanisms of HR during double strand break repair and replication restart. We discuss the intrinsic capacities of HR to generate genome rearrangements by GC or CO, either during DSB repair or replication restart. During this process, abortive HR intermediates generate genetic instability and cell toxicity. In addition to genome rearrangements, HR also primes error-prone DNA synthesis and favors mutagenesis on single stranded DNA, a key DNA intermediate during the HR process. The fact that cells have developed several mechanisms protecting against HR excess emphasize its potential risks. Consistent with this duality, several pro-oncogenic situations have been consistently associated with either decreased or increased HR levels. Nevertheless, this versatility also has advantages that we outline here. We conclude that HR is a double-edged sword, which on one hand controls the equilibrium between genome stability and diversity but, on the other hand, can jeopardize the maintenance of genomic integrity. Therefore, whether non-homologous end joining (which, in contrast with HR, is not intrinsically mutagenic) or HR is the more mutagenic process is a question that should be re-evaluated. Both processes can be "Dr. Jekyll" in maintaining genome stability/variability and "Mr. Hyde" in jeopardizing genome integrity.

Spontaneous slow replication fork progression elicits mitosis alterations in homologous recombination-deficient mammalian cells

Homologous recombination deficient (HR(-)) mammalian cells spontaneously display reduced replication fork (RF) movement and mitotic extra centrosomes. We show here that these cells present a complex mitotic phenotype, including prolonged metaphase arrest, anaphase bridges, and multipolar segregations. We then asked whether the replication and the mitotic phenotypes are interdependent. First, we determined low doses of hydroxyurea that did not affect the cell cycle distribution or activate CHK1 phosphorylation but did slow the replication fork movement of wild-type cells to the same level than in HR(-) cells. Remarkably, these low hydroxyurea doses generated the same mitotic defects (and to the same extent) in wild-type cells as observed in unchallenged HR(-) cells. Reciprocally, supplying nucleotide precursors to HR(-) cells suppressed both their replication deceleration and mitotic extra centrosome phenotypes. Therefore, subtle replication stress that escapes to surveillance pathways and, thus, fails to prevent cells from entering mitosis alters metaphase progression and centrosome number, resulting in multipolar mitosis. Importantly, multipolar mitosis results in global unbalanced chromosome segregation involving the whole genome, even fully replicated chromosomes. These data highlight the cross-talk between chromosome replication and segregation, and the importance of HR at the interface of these two processes for protection against general genome instability.

AKT1/BRCA1 in the control of homologous recombination and genetic stability: the missing link between hereditary and sporadic breast cancers

Endogenous replicative stress could be one trigger leading to tumor initiation: indeed, activation of the DNA damage response (DDR), considered the result of replicative stress, is observed in pre-cancerous cells; moreover, in hereditary breast cancers, almost all of the genes affected relate to the DDR. The most frequently mutated gene in hereditary breast cancers, BRCA1, is essential for homologous recombination (HR), a fundamental process for maintaining genome stability that permits the reactivation of blocked replication forks . Recent studies have established links between DDR and the oncogenic kinase AKT1, which is upregulated in about 50% of sporadic breast cancers. More specifically, the activation of AKT1 shows a deficient phenotype in BRCA1 and HR, revealing molecular similarities between hereditary and sporadic breast cancers. However, these results reveal a paradox regarding the physiological role of AKT1: in non-tumor cells, AKT1 promotes cellular proliferation, but consequently endangers genome integrity during replication if HR is inhibited. Since HR could itself lead to genetic instability, we propose that, under physiological conditions, moderate activation of AKT1 does not inhibit but prevents an excess of HR. The regulation of AKT1 would represent a fine transitory system for controlling HR and maintaining genomic integrity.

Interactions among Trypanosoma brucei RAD51 paralogues in DNA repair and antigenic variation

Homologous recombination in Trypanosoma brucei is used for moving variant surface glycoprotein (VSG) genes into expression sites during immune evasion by antigenic variation. A major route for such VSG switching is gene conversion reactions in which RAD51, a universally conserved recombinase, catalyses homology-directed strand exchange. In any eukaryote, RAD51-directed strand exchange in vivo is mediated by further factors, including RAD51-related proteins termed Rad51 paralogues. These appear to be ubiquitously conserved, although their detailed roles in recombination remain unclear. In T. brucei, four putative RAD51 paralogue genes have been identified by sequence homology. Here we show that all four RAD51 paralogues act in DNA repair, recombination and RAD51 subnuclear dynamics, though not equivalently, while mutation of only one RAD51 paralogue gene significantly impedes VSG switching. We also show that the T. brucei RAD51 paralogues interact, and that the complexes they form may explain the distinct phenotypes of the mutants as well as observed expression interdependency. Finally, we document the Rad51 paralogues that are encoded by a wide range of protists, demonstrating that the Rad51 paralogue repertoire in T. brucei is unusually large among microbial eukaryotes and that one member of the protein family corresponds with a key, conserved eukaryotic Rad51 paralogue.

The relative efficiency of homology-directed repair has distinct effects on proper anaphase chromosome separation

Homology-directed repair (HDR) is essential to limit mutagenesis, chromosomal instability (CIN) and tumorigenesis. We have characterized the consequences of HDR deficiency on anaphase, using markers for incomplete chromosome separation: DAPI-bridges and Ultra-fine bridges (UFBs). We show that multiple HDR factors (Rad51, Brca2 and Brca1) are critical for complete chromosome separation during anaphase, while another chromosome break repair pathway, non-homologous end joining, does not affect chromosome segregation. We then examined the consequences of mild versus severe HDR disruption, using two different dominant-negative alleles of the strand exchange factor, Rad51. We show that mild HDR disruption is viable, but causes incomplete chromosome separation, as detected by DAPI-bridges and UFBs, while severe HDR disruption additionally results in multipolar anaphases and loss of clonogenic survival. We suggest that mild HDR disruption favors the proliferation of cells that are prone to CIN due to defective chromosome separation during anaphase, whereas, severe HDR deficiency leads to multipolar divisions that are prohibitive for cell proliferation.

Homologous recombination proteins are associated with centrosomes and are required for mitotic stability

In response to DNA damage, cells need robust repair mechanisms to complete the cell cycle successfully. Severe forms of DNA damage are repaired by homologous recombination (HR), in which the XRCC2 protein plays a vital role. Cells deficient in XRCC2 also show disruption of the centrosome, a key component of the mitotic apparatus. We find that this centrosome disruption is dynamic and when it occurs during mitosis it is linked directly to the onset of mitotic catastrophe in a significant fraction of the XRCC2-deficient cells. However, we also show for the first time that XRCC2 and other HR proteins, including the key recombinase RAD51, co-localize with the centrosome. Co-localization is maintained throughout the cell cycle, except when cells are finishing mitosis when RAD51 accumulates in the midbody between the separating cells. Taken together, these data suggest a tight functional linkage between the centrosome and HR proteins, potentially to coordinate the deployment of a DNA damage response at vulnerable phases of the cell cycle.

AKT1/BRCA1 in the control of homologous recombination and genetic stability: the missing link between hereditary and sporadic breast cancers

Endogenous replicative stress could be one trigger leading to tumor initiation: indeed, activation of the DNA damage response (DDR), considered the result of replicative stress, is observed in pre-cancerous cells; moreover, in hereditary breast cancers, almost all of the genes affected relate to the DDR. The most frequently mutated gene in hereditary breast cancers, BRCA1, is essential for homologous recombination (HR), a fundamental process for maintaining genome stability that permits the reactivation of blocked replication forks . Recent studies have established links between DDR and the oncogenic kinase AKT1, which is upregulated in about 50% of sporadic breast cancers. More specifically, the activation of AKT1 shows a deficient phenotype in BRCA1 and HR, revealing molecular similarities between hereditary and sporadic breast cancers. However, these results reveal a paradox regarding the physiological role of AKT1: in non-tumor cells, AKT1 promotes cellular proliferation, but consequently endangers genome integrity during replication if HR is inhibited. Since HR could itself lead to genetic instability, we propose that, under physiological conditions, moderate activation of AKT1 does not inhibit but prevents an excess of HR. The regulation of AKT1 would represent a fine transitory system for controlling HR and maintaining genomic integrity.

Detection of centrosome aberrations in disease-unrelated cells from patients with tumor treated with tyrosine kinase inhibitors

OBJECTIVES

Tyrosine kinase inhibitors (TKIs) target various pathways associated with proliferation of aberrant clones in malignant diseases. Despite good response and acceptable tolerability, little is known concerning long-term toxicity. Furthermore, the influence of these inhibitors on disease-unrelated cells is not investigated yet.

METHODS

Centrosome aberrations are hallmarks of various cancers. We sought to evaluate the effect of TKIs on centrosomes of disease-unrelated cells. We examined cells of the oral mucosa (OM) and fibroblasts of patients with chronic myeloid leukemia (CML) treated with dasatinib and bosutinib. Results were compared with data from patients with CML treated with imatinib or nilotinib and with data from patients suffering from renal and hepatocellular carcinomas (RCC/HCC) treated with sorafenib or sunitinib. Cells of healthy donors served as controls.

RESULTS

OM cells (n = 12) and fibroblasts (n = 7) of patients with CML treated with dasatinib and OM cells of three patients with CML treated with bosutinib showed centrosomal alterations (mean, 14%) compared with 16 (10 OM and 6 fibroblasts) controls (mean, 3%). OM cells of five patients with CML and one patient with systemic mastocytosis treated with imatinib or nilotinib and of eight patients with RCC or HCC treated with sorafenib or sunitinib showed centrosome defects in a mean of 15%.

CONCLUSIONS

Our data have shown that TKI treatment of tumor patients may influence centrosomes in disease-unrelated cells or tissues. This may be important with regard to various observed side effects.

The ATR-Chk1 pathway plays a role in the generation of centrosome aberrations induced by Rad51C dysfunction

Rad51C is a central component of two complexes formed by five Rad51 paralogs in vertebrates. These complexes are involved in repairing DNA double-strand breaks through homologous recombination. Despite accumulating evidence suggesting that the paralogs may prevent aneuploidy by controlling centrosome integrity, Rad51C's role in maintaining chromosome stability remains unclear. Here we demonstrate that Rad51C deficiency leads to both centrosome aberrations in an ATR-Chk1-dependent manner and increased aneuploidy in human cells. While it was reported that Rad51C deficiency did not cause centrosome aberrations in interphase in hamster cells, such aberrations were observed in interphase in HCT116 cells with Rad51C dysfunction. Caffeine treatment and down-regulation of ATR, but not that of ATM, reduced the frequency of centrosome aberrations in the mutant cells. Silencing of Rad51C by RNA interference in HT1080 cells resulted in similar aberrations. Treatment with a Chk1 inhibitor and silencing of Chk1 also reduced the frequency in HCT116 mutants. Accumulation of Chk1 at the centrosome and nuclear foci of γH2AX were increased in the mutants. Moreover, the mutant cells had a higher frequency of aneuploidy. These findings indicate that the ATR-Chk1 pathway plays a role in increased centrosome aberrations induced by Rad51C dysfunction.

AKT1 represses gene conversion induced by different genotoxic stresses and induces supernumerary centrosomes and aneuploidy in hamster ovary cells

The oncogenic kinase AKT1 is frequently overexpressed or activated in sporadic breast and ovarian cancers. In human breast tumors, we have previously shown that AKT1 represses homologous recombination (HR) induced by one double-strand break (DSB). To further analyze the impact of AKT1 on HR, we ectopically expressed wild-type or mutant forms of AKT1 in a hamster ovary cell line containing an intrachromosomal substrate for monitoring HR. In this cell line, AKT1 repressed HR induced by different genotoxic stresses including ionizing radiation, UV-C and one single DSB introduced into the intrachromosomal substrate. Consistently, AKT1 disrupted RAD51 foci formation, showing that AKT1 specifically affects gene conversion. Concomitantly, AKT1 represses both BRCA1 foci formation and HR stimulation resulting from BRCA1 overexpression, showing that AKT1 affects BRCA1-mediated HR functions, also in another species (hamster) and in another type of cell tissue (ovary cells). Finally, consistent with the HR defects, active AKT1 expression induces supernumerary centrosomes and aneuploidy. In addition to its impact on cell proliferation and apoptosis, the present data propose a novel oncogenic function for AKT1, by producing genomic instability as a consequence of HR repression.

Mitotic crisis: the unmasking of a novel role for RPA

Mitotic DNA damage is a constant threat to genomic integrity, yet understanding of the cellular responses to this stress remain incomplete. Recent work by Anantha et al. (2008; PNAS 105:12903-8) has found surprising evidence that RPA, the primary eukaryotic single-stranded DNA-binding protein, can stimulate the ability of cells to exit mitosis into a 2N G(1) phase. Along with providing additional discussion of this study, we review evidence suggesting that DNA replication and repair factors can modulate mitotic transit by acting through Polo-like kinase-1 (Plk1) and the centrosome. 'A crisis unmasks everyone.'-Mason Cooley, U.S. aphorist.

Homologous recombination is involved in the repair response of mammalian cells to low doses of tritium

Radioactive compounds incorporated in tissues can have biological effects resulting from energy deposition in subcellular compartments. We addressed the genetic consequences of [(3)H] or [(14)C]thymidine incorporation into mammalian DNA. Low doses of [(3)H]thymidine in CHO cells led to enhanced sensitivity compared with [(14)C]thymidine. Compared with wild-type cells, homologous recombination (HR)-deficient cells were more sensitive to lower doses of [(3)H]thymidine but not to any dose of [(14)C]thymidine. XRCC4-defective cells, however, were sensitive to both low and high doses of [(3)H] and [(14)C]thymidine, suggesting introduction of DNA double-strand breaks, which were confirmed by gamma-H2AX focus formation. While gamma rays induced measurable HR only at toxic doses, sublethal levels of [(3)H] or [(14)C]thymidine strongly induced HR. The level of stimulation was in an inverse relationship to the emitted energies. The RAD51 gene conversion pathway was involved, because [(3)H]thymidine induced RAD51 foci, and [(3)H]thymidine-induced HR was abrogated by expression of dominant negative RAD51. In conclusion, both HR and non-homologous end-joining pathways were involved after labeled nucleotide incorporation (low doses); genetic effects were negatively correlated with the energy emitted but were positively correlated with the energy deposited in the nucleus, suggesting that low-energy beta-particle emitters, at non-toxic doses, may induce genomic instability.

JAK2 stimulates homologous recombination and genetic instability: potential implication in the heterogeneity of myeloproliferative disorders

The JAK2V617F mutation is frequently observed in classical myeloproliferative disorders, and disease progression is associated with a biallelic acquisition of the mutation occurring by mitotic recombination. In this study, we examined whether JAK2 activation could lead to increased homologous recombination (HR) and genetic instability. In a Ba/F3 cell line expressing the erythropoietin (EPO) receptor, mutant JAK2V617F and, to a lesser extent, wild-type (wt) JAK2 induced an increase in HR activity in the presence of EPO without modifying nonhomologous end-joining efficiency. Moreover, a marked augmentation in HR activity was found in CD34+-derived cells isolated from patients with polycythemia vera or primitive myelofibrosis compared with control samples. This increase was associated with a spontaneous RAD51 foci formation. As a result, sister chromatid exchange was 50% augmented in JAK2V617F Ba/F3 cells compared with JAK2wt cells. Moreover, JAK2 activation increased centrosome and ploidy abnormalities. Finally, in JAK2V617F Ba/F3 cells, we found a 100-fold and 10-fold increase in mutagenesis at the HPRT and Na/K ATPase loci, respectively. Together, this work highlights a new molecular mechanism for HR regulation mediated by JAK2 and more efficiently by JAK2V617F. Our study might provide some keys to understand how a single mutation can give rise to different pathologies.

P53, cyclin-dependent kinase and abnormal amplification of centrosomes

Centrosomes play a critical role in formation of bipolar mitotic spindles, an essential event for accurate chromosome segregation into daughter cells. Numeral abnormalities of centrosomes (centrosome amplification) occur frequently in cancers, and are considered to be the major cause of chromosome instability, which accelerates acquisition of malignant phenotypes during tumor progression. Loss or mutational inactivation of p53 tumor suppressor protein, one of the most common mutations found in cancers, results in a high frequency of centrosome amplification in part via allowing the activation of the cyclin-dependent kinase (CDK) 2-cyclin E (as well as CDK2-cyclin A) which is a key factor for the initiation of centrosome duplication. In this review, the role of centrosome amplification in tumor progression, and mechanistic view of how centrosomes are amplified in cells through focusing on loss of p53 and aberrant activities of CDK2-cyclins will be discussed.

Requirements for ATP binding and hydrolysis in RecA function in Escherichia coli

RecA is essential for recombination, DNA repair and SOS induction in Escherichia coli. ATP hydrolysis is known to be important for RecA's roles in recombination and DNA repair. In vitro reactions modelling SOS induction minimally require ssDNA and non-hydrolyzable ATP analogues. This predicts that ATP hydrolysis will not be required for SOS induction in vivo. The requirement of ATP binding and hydrolysis for SOS induction in vivo is tested here through the study of recA4159 (K72A) and recA2201 (K72R). RecA4159 is thought to have reduced affinity for ATP. RecA2201 binds, but does not hydrolyse ATP. Neither mutant was able to induce SOS expression after UV irradiation. RecA2201, unlike RecA4159, could form filaments on DNA and storage structures as measured with RecA-GFP. RecA2201 was able to form hybrid filaments and storage structures and was either recessive or dominant to RecA(+), depending on the ratio of the two proteins. RecA4159 was unable to enter RecA(+) filaments on DNA or storage structures and was recessive to RecA(+). It is concluded that ATP hydrolysis is essential for SOS induction. It is proposed that ATP binding is essential for storage structure formation and ability to interact with other RecA proteins in a filament.

Dictyostelium Sun-1 connects the centrosome to chromatin and ensures genome stability

The centrosome-nucleus attachment is a prerequisite for faithful chromosome segregation during mitosis. We addressed the function of the nuclear envelope (NE) protein Sun-1 in centrosome-nucleus connection and the maintenance of genome stability in Dictyostelium discoideum. We provide evidence that Sun-1 requires direct chromatin binding for its inner nuclear membrane targeting. Truncation of the cryptic N-terminal chromatin-binding domain of Sun-1 induces dramatic separation of the inner from the outer nuclear membrane and deformations in nuclear morphology, which are also observed using a Sun-1 RNAi construct. Thus, chromatin binding of Sun-1 defines the integrity of the nuclear architecture. In addition to its role as a NE scaffold, we find that abrogation of the chromatin binding of Sun-1 dissociates the centrosome-nucleus connection, demonstrating that Sun-1 provides an essential link between the chromatin and the centrosome. Moreover, loss of the centrosome-nucleus connection causes severe centrosome hyperamplification and defective spindle formation, which enhances aneuploidy and cell death significantly. We highlight an important new aspect for Sun-1 in coupling the centrosome and nuclear division during mitosis to ensure faithful chromosome segregation.

A homologous recombination defect affects replication-fork progression in mammalian cells

Faithful genome transmission requires a network of pathways coordinating DNA replication to DNA repair and recombination. Here, we used molecular combing to measure the impact of homologous recombination (HR) on the velocity of DNA replication forks. We used three hamster cell lines defective in HR either by overexpression of a RAD51 dominant-negative form, or by a defect in the RAD51 paralogue XRCC2 or the breast tumor suppressor BRCA2. Irrespectively of the type or extent of HR alteration, all three cell lines exhibited a similar reduction in the rate of replication-fork progression, associated with an increase in the density of replication forks. Importantly, this phenotype was completely reversed in complemented derivatives of Xrcc2 and Brca2 mutants. These data reveal a novel role for HR, different from the reactivation of stalled replication forks, which may play an important role in genome stability and thus in tumor protection.

Centrosome control of the cell cycle

Early observations of centrosomes, made a century ago, revealed a tiny dark structure surrounded by a radial array of cytoplasmic fibers. We now know that the fibers are microtubules and that the dark organelles are centrosomes that mediate functions far beyond the more conventional role of microtubule organization. More recent evidence demonstrates that the centrosome serves as a scaffold for anchoring an extensive number of regulatory proteins. Among these are cell-cycle regulators whose association with the centrosome is an essential step in cell-cycle control. Such studies show that the centrosome is required for several cell-cycle transitions, including G(1) to S-phase, G(2) to mitosis and metaphase to anaphase. In this review (which is part of the Chromosome Segregation and Aneuploidy series), we discuss recent data that provide the most direct links between centrosomes and cell-cycle progression.

Haploinsufficiency of RAD51B causes centrosome fragmentation and aneuploidy in human cells

The Rad51-like proteins, Rad51B, Rad51C, Rad51D, XRCC2, and XRCC3, have been shown to form two distinct complexes and seem to assist Rad51 in the early stages of homologous recombination. Although these proteins share sequence similarity with Rad51, they do not show functional redundancy. Among them, Rad51B is unique in that the gene maps to the human chromosome 14q23-24, the region frequently involved in balanced chromosome translocations in benign tumors particularly in uterine leiomyomas. Despite accumulating descriptive evidence of altered Rad51B function in these tumors, the biological significance of this aberration is still unknown. To assess the significance of reduced Rad51B function, we deleted the gene in the human colon cancer cell line HCT116 by gene targeting. Here, we show that haploinsufficiency of RAD51B causes mild hypersensitivity to DNA-damaging agents, a mild reduction in sister chromatid exchange, impaired Rad51 focus formation, and an increase in chromosome aberrations. Remarkably, haploinsufficiency of RAD51B leads to centrosome fragmentation and aneuploidy. In addition, an approximately 50% reduction in RAD51B mRNA levels by RNA interference also leads to centrosome fragmentation in the human fibrosarcoma cell line HT1080. These findings suggest that the proper biallelic expression of RAD51B is required for the maintenance of chromosome integrity in human cells.

Induction of centrosome and chromosome aberrations by imatinib in vitro

Imatinib (STI571, Gleevec/Glivec) is a potent selective tyrosine kinase inhibitor and is used successfully in the treatment of chronic myeloid leukemia (CML). While karyotype alterations, in addition to the Philadelphia chromosome, are a common phenomenon of progressing CML, the observation of BCR-ABL-negative leukemic clones with distinct aberrant karyotypes under an imatinib regimen is not yet understood. Here we test the hypothesis that such tumor clones may be induced de novo from normal cells by imatinib. In vitro experiments with varying drug concentrations (5-20 microM) were performed on normal human dermal fibroblasts (NHDF), Chinese hamster embryonal and Indian muntjak fibroblasts. After 3 weeks of treatment, analysis of cell cultures by centrosome immunostaining and conventional cytogenetics revealed that imatinib induced centrosome and chromosome aberrations in all cultures in a significant dose-dependent and species-independent manner. Moreover, the results of NHDF long-term culture experiments demonstrated that aberrant phenotypes, emerging under imatinib treatment for 12 weeks, were not reversible after prolonged propagation omitting the drug. These observations suggest a causative role of imatinib in the origin of centrosome and karyotype aberrations (genetic instability) and thus may explain the emergence of clonal chromosomal abnormalities in BCR-ABL-negative progenitor cells under imatinib therapy.