Chromosome stability, DNA recombination and the BRCA2 tumour suppressor

Balancing act: BRCA2's elaborate management of telomere replication through control of G-quadruplex dynamicity

In billion years of evolution, eukaryotes preserved the chromosome ends with arrays of guanine repeats surrounded by thymines and adenines, which can form stacks of four-stranded planar structure known as G-quadruplex (G4). The rationale behind the evolutionary conservation of the G4 structure at the telomere remained elusive. Our recent study has shed light on this matter by revealing that telomere G4 undergoes oscillation between at least two distinct folded conformations. Additionally, tumor suppressor BRCA2 exhibits a unique mode of interaction with telomere G4. To elaborate, BRCA2 directly interacts with G-triplex (G3)-derived intermediates that form during the interconversion of the two different G4 states. In doing so, BRCA2 remodels the G4, facilitating the restart of stalled replication forks. In this review, we succinctly summarize the findings regarding the dynamicity of telomeric G4, emphasize its importance in maintaining telomere replication homeostasis, and the physiological consequences of losing G4 dynamicity at the telomere.

A comprehensive analysis of BRCA2 gene: focus on mechanistic aspects of its functions, spectrum of deleterious mutations, and therapeutic strategies targeting BRCA2-deficient tumors

BRCA2is the main susceptibility gene known to be involved in the pathogenesis of breast cancer. It plays an important role in maintaining the genome stability by homologous recombination through DNA double-strand breaks repairing, by interacting with various other proteins including RAD51, DSS1, RPA, MRE11, PALB2, and p53. BRCA2-deficient cells show the abnormalities of chromosome number. BRCA2 is also found to be involved in centrosome duplication specifically in the metaphase to anaphase transition. Inactivation or depletion of BRCA2 leads to centrosome amplification that results in unequal separation of chromosomes. BRCA2 localizes with central spindle and midbody during telophase and cytokinesis. Inactivation or depletion of BRCA2 leads to multinucleation of cell. Around 2000 mutations have been reported in BRCA2 gene. BRCA2-deficient tumors are being taking into consideration for targeted cancer therapy by using different inhibitors like poly ADP-ribose polymerase and thymidylate synthase. The present review focusses on the role of BRCA2 in various critical cellular processes based on the mechanistic approaches. Mutations reported in the BRCA2 gene in various ethnic groups till date have also been compiled with an insight into the functional aspects of these alterations. The therapeutic strategies for targeting BRCA2-deficient tumors have also been targeted.

NEK8 regulates DNA damage-induced RAD51 foci formation and replication fork protection

Proteins essential for homologous recombination play a pivotal role in the repair of DNA double strand breaks, DNA inter-strand crosslinks and replication fork stability. Defects in homologous recombination also play a critical role in the development of cancer and the sensitivity of these cancers to chemotherapy. RAD51, an essential factor for homologous recombination and replication fork protection, accumulates and forms immunocytochemically detectable nuclear foci at sites of DNA damage. To identify kinases that may regulate RAD51 localization to sites of DNA damage, we performed a human kinome siRNA library screen, using DNA damage-induced RAD51 foci formation as readout. We found that NEK8, a NIMA family kinase member, is required for efficient DNA damage-induced RAD51 foci formation. Interestingly, knockout of Nek8 in murine embryonic fibroblasts led to cellular sensitivity to the replication inhibitor, hydroxyurea, and inhibition of the ATR kinase. Furthermore, NEK8 was required for proper replication fork protection following replication stall with hydroxyurea. Loading of RAD51 to chromatin was decreased in NEK8-depleted cells and Nek8-knockout cells. Single-molecule DNA fiber analyses revealed that nascent DNA tracts were degraded in the absence of NEK8 following treatment with hydroxyurea. Consistent with this, Nek8-knockout cells showed increased chromosome breaks following treatment with hydroxyurea. Thus, NEK8 plays a critical role in replication fork stability through its regulation of the DNA repair and replication fork protection protein RAD51.

High levels of BRC4 induced by a Tet-On 3G system suppress DNA repair and impair cell proliferation in vertebrate cells

•

The Tet-On 3G system is useful for conditional gene overexpression studies in DT40.

•

The Tet-On-I-SceI effectively induces DSB formation in vertebrate cells.

•

BRC4 overexpression induces chromosomal breaks and G2-arrest.

•

BRC4 cytotoxicity is mediated by endogenous BRCA2, but independent of NHEJ.

•

BRC4 inhibits cancer cell proliferation and exacerbates the effects of chemotherapy.

Transient induction or suppression of target genes is useful to study the function of toxic or essential genes in cells. Here we apply a Tet-On 3G system to DT40 lymphoma B cell lines, validating it for three different genes. Using this tool, we then show that overexpression of the chicken BRC4 repeat of the tumor suppressor BRCA2 impairs cell proliferation and induces chromosomal breaks. Mechanistically, high levels of BRC4 suppress double strand break-induced homologous recombination, inhibit the formation of RAD51 recombination repair foci, reduce cellular resistance to DNA damaging agents and induce a G2 damage checkpoint-mediated cell-cycle arrest. The above phenotypes are mediated by BRC4 capability to bind and inhibit RAD51. The toxicity associated with BRC4 overexpression is exacerbated by chemotherapeutic agents and reversed by RAD51 overexpression, but it is neither aggravated nor suppressed by a deficit in the non-homologous end-joining pathway of double strand break repair. We further find that the endogenous BRCA2 mediates the cytotoxicity associated with BRC4 induction, thus underscoring the possibility that BRC4 or other domains of BRCA2 cooperate with ectopic BRC4 in regulating repair activities or mitotic cell division. In all, the results demonstrate the utility of the Tet-On 3G system in DT40 research and underpin a model in which BRC4 role on cell proliferation and chromosome repair arises primarily from its suppressive role on RAD51 functions.

Replication stress-induced genome instability: the dark side of replication maintenance by homologous recombination

Homologous recombination (HR) is an evolutionary-conserved mechanism involved in a subtle balance between genome stability and diversity. HR is a faithful DNA repair pathway and has been largely characterized in the context of double-strand break (DSB) repair. Recently, multiple functions for the HR machinery have been identified at arrested forks. These are evident across different organisms and include replication fork-stabilization and fork-restart functions. Interestingly, a DSB appears not to be a prerequisite for HR-mediated replication maintenance. HR has the ability to rebuild a replisome at inactivated forks, but perhaps surprisingly, the resulting replisome is liable to intrastrand and interstrand switches leading to replication errors. Here, we review our current understanding of the replication maintenance function of HR. The error proneness of these pathways leads us to suggest that the origin of replication-associated genome instability should be re-evaluated.

Homologous recombination as a replication fork escort: fork-protection and recovery

Homologous recombination is a universal mechanism that allows DNA repair and ensures the efficiency of DNA replication. The substrate initiating the process of homologous recombination is a single-stranded DNA that promotes a strand exchange reaction resulting in a genetic exchange that promotes genetic diversity and DNA repair. The molecular mechanisms by which homologous recombination repairs a double-strand break have been extensively studied and are now well characterized. However, the mechanisms by which homologous recombination contribute to DNA replication in eukaryotes remains poorly understood. Studies in bacteria have identified multiple roles for the machinery of homologous recombination at replication forks. Here, we review our understanding of the molecular pathways involving the homologous recombination machinery to support the robustness of DNA replication. In addition to its role in fork-recovery and in rebuilding a functional replication fork apparatus, homologous recombination may also act as a fork-protection mechanism. We discuss that some of the fork-escort functions of homologous recombination might be achieved by loading of the recombination machinery at inactivated forks without a need for a strand exchange step; as well as the consequence of such a model for the stability of eukaryotic genomes.

High levels of wild-type BRCA2 suppress homologous recombination

Endogenous levels of the BRCA2 (breast cancer susceptibility 2) protein promote homologous recombination by regulating the essential strand exchange protein RAD51. To examine BRCA2 function in homologous recombination, we expressed human BRCA2 in control mouse hybridoma cells, as well as those that were depleted of endogenous Brca2 by small interfering RNA. With moderate human BRCA2 expression, homologous recombination was stimulated. Conversely, a higher level of BRCA2 reduced homologous recombination and DNA-damage-induced Rad51 foci formation. Cells expressing high levels of BRCA2 feature normal growth, increased sensitivity to mitomycin C, and increased illegitimate recombination. BRCA2-overexpressing cells are also characterized by suppression of p53 transcriptional regulation and a corresponding reduction in the expression of the p53-responsive genes Noxa and p21. Notably, in cells expressing high levels of BRCA2, small interfering RNA depletion of human BRCA2 or ectopic expression of Rad51 increases homologous recombination and decreases illegitimate recombination. Thus, high levels of wild-type BRCA2 perturb Rad51-mediated homologous recombination, and relatively normal recombination responses can be restored by rebalancing recombination factors.

DNA double-strand break repair in Caenorhabditis elegans

Faithful repair of DNA double-strand breaks (DSBs) is vital for animal development, as inappropriate repair can cause gross chromosomal alterations that result in cellular dysfunction, ultimately leading to cancer, or cell death. Correct processing of DSBs is not only essential for maintaining genomic integrity, but is also required in developmental programs, such as gametogenesis, in which DSBs are deliberately generated. Accordingly, DSB repair deficiencies are associated with various developmental disorders including cancer predisposition and infertility. To avoid this threat, cells are equipped with an elaborate and evolutionarily well-conserved network of DSB repair pathways. In recent years, Caenorhabditis elegans has become a successful model system in which to study DSB repair, leading to important insights in this process during animal development. This review will discuss the major contributions and recent progress in the C. elegans field to elucidate the complex networks involved in DSB repair, the impact of which extends well beyond the nematode phylum.

Recovery of deficient homologous recombination in Brca2-depleted mouse cells by wild-type Rad51 expression

The BRCA2 tumor suppressor is important in maintaining genomic stability. BRCA2 is proposed to control the availability, cellular localization and DNA binding activity of the central homologous recombination protein, RAD51, with loss of BRCA2 resulting in defective homologous recombination. Nevertheless, the roles of BRCA2 in regulating RAD51 and how other proteins implicated in RAD51 regulation, such as RAD52 and RAD54 function relative to BRCA2 is not known. In this study, we tested whether defective homologous recombination in Brca2-depleted mouse hybridoma cells could be rectified by expression of mouse Rad51 or the Rad51-interacting mouse proteins, Rad52 and Rad54. In the Brca2-depleted cells, defective homologous recombination can be restored by over-expression of wild-type mouse Rad51, but not mouse Rad52 or Rad54. Correction of the homologous recombination defect requires Rad51 ATPase activity. A sizeable fraction ( approximately 50%) of over-expressed wild-type Rad51 is nuclear localized. The restoration of homologous recombination in the presence of a low (i.e., non-functional) level of Brca2 by wild-type Rad51 over-expression is unexpected. We suggest that Rad51 may access the nuclear compartment in a Brca2-independent manner and when Rad51 is over-expressed, the normal requirement for Brca2 control over Rad51 function in homologous recombination is dispensable. Our studies support loss of Rad51 function as a critical underlying factor in the homologous recombination defect in the Brca2-depleted cells.

Identification of Rad51 regulation by BRCA2 using Caenorhabditis elegans BRCA2 and bimolecular fluorescence complementation analysis

BRCA2 is involved in double-stranded DNA break repair by binding and regulating Rad51-mediated homologous recombination. Insights as to how BRCA2 regulates Rad51-mediated DNA repair arose from in vitro biochemical studies on fragments of BRCA2. However, the large 400-kDa BRCA2 protein has hampered our ability to understand the entire process by which full-length BRCA2 regulates Rad51. Here, we show that CeBRC-2, which is only one tenth the size of mammalian BRCA2, complemented BRCA2-deficiency in Rad51 regulation. CeBRC-2 was able to bind to mammalian Rad51 (mRad51) and form distinct nuclear foci when they interacted. In our bimolecular fluorescence complementation analysis (BiFC), we show that the strength of the interaction between CeBRC-2 and mRad51 increased markedly after DNA damage. The BRC motif of CeBRC-2 was responsible for binding mRad51, but without the OB fold, the complex was unable to target damaged DNA. When CeBRC-2 was introduced into BRCA2-deficient cells, it restored Rad51 foci after DNA damage. Our study suggests a mode of action for BRCA2 with regard to DNA repair.

Regulation of bacterial RecA protein function

The RecA protein is a recombinase functioning in recombinational DNA repair in bacteria. RecA is regulated at many levels. The expression of the recA gene is regulated within the SOS response. The activity of the RecA protein itself is autoregulated by its own C-terminus. RecA is also regulated by the action of other proteins. To date, these include the RecF, RecO, RecR, DinI, RecX, RdgC, PsiB, and UvrD proteins. The SSB protein also indirectly affects RecA function by competing for ssDNA binding sites. The RecO and RecR, and possibly the RecF proteins, all facilitate RecA loading onto SSB-coated ssDNA. The RecX protein blocks RecA filament extension, and may have other effects on RecA activity. The DinI protein stabilizes RecA filaments. The RdgC protein binds to dsDNA and blocks RecA access to dsDNA. The PsiB protein, encoded by F plasmids, is uncharacterized, but may inhibit RecA in some manner. The UvrD helicase removes RecA filaments from RecA. All of these proteins function in a network that determines where and how RecA functions. Additional regulatory proteins may remain to be discovered. The elaborate regulatory pattern is likely to be reprised for RecA homologues in archaeans and eukaryotes.

Analysis of DNA repair and recombination responses in mouse cells depleted for Brca2 by SiRNA

The tumor suppressor BRCA2 is considered to play an important role in the maintenance of genome integrity through the repair of DNA lesions by homologous recombination. A mechanistic understanding of BRCA2 has been complicated by the embryonic lethality of mice bearing allelic knockouts of Brca2, and by variation in the DNA damage response in cells bearing BRCA2 deficiencies. It would be advantageous to develop approaches that avoid the cell lethality associated with complete inactivation of the gene, or the use of established tumor cell lines in which other genes in addition to BRCA2 may be mutant. In this study, SiRNA was used in stable transformation assays to knockdown Brca2 in mouse hybridoma cells by at least 75%. The Brca2-depleted cells were analyzed with respect to cell growth, sensitivity to DNA damaging agents (mitomycin C, methylmethane sulfonate, or ionizing radiation), intrachromosomal homologous recombination and gene targeting. Although the effect of Brca2-depletion on cell growth and sensitivity to DNA damaging agents was modest, the Brca2-depleted cells did show a significant shift in homologous recombination from gene conversion to single-strand annealing and a significant decrease in the efficiency of gene targeting. Both of these phenotypes are consistent with the proposed role of Brca2 in DNA repair and recombination.

Involvement of homologous recombination in carcinogenesis

DNA alterations of every type are associated with the incidence of carcinogenesis, often on the genomic scale. Although homologous recombination (HR) is an important pathway of DNA repair, evidence is accumulating that deleterious genomic rearrangements can result from HR. It therefore follows that HR events may play a causative role in carcinogenesis. HR is elevated in response to carcinogens. HR may also be increased or decreased when its upstream regulation is perturbed or components of the HR machinery itself are not fully functional. This chapter summarizes research findings that demonstrate an association between HR and carcinogenesis. Increased or decreased frequencies of HR have been found in cancer cells and cancer-prone hereditary human disorders characterized by mutations in genes playing a role in HR, such as ATM, Tp53, BRCA, BLM, and WRN genes. Another evidence linking perturbations in HR and carcinogenesis is provided by studies showing that exposure to carcinogens results in an increased frequency of HR resulting in DNA deletions in yeast, human cells, or mice.

Genomic instability and cancer: networks involved in response to DNA damage

A new approach to cancer and new methods in examining rare human chromosome breakage syndromes have brought to light complex interactions between different pathways involved in damage response, cell cycle checkpoint control and DNA repair. The genes affected in these different syndromes are involved in networks of processes that respond to DNA damage and prevent chromosomal aberrations during the cell cycle. The genes involved include the ATM, ATR, FA-associated genes, NBS1 and the cancer susceptibility genes BRCA1 and BRCA2. Chromosomal instability is a common feature of many human cancers and most of the instability syndromes, characterized by sensitivity to different types of DNA damage, also show increased cancer susceptibility. Better understanding of these syndromes and their links with familial cancer provide new insight into associations between defects in DNA damage response, cell cycle control, DNA repair and cancer. Understanding the damage response repair networks that these studies are revealing will have important implications for the development of cancer management and treatment.

The RAD51 gene family, genetic instability and cancer

Inefficient repair or mis-repair of DNA damage can cause genetic instability, and defects in some DNA repair genes are associated with rare human cancer-prone disorders. In the last few years, homologous recombination has been found to be a key pathway in human cells for the repair of severe DNA damage such as double-strand breaks. The RAD51 family of genes, including RAD51 and the five RAD51-like genes (XRCC2, XRCC3, RAD51L1, RAD51L2, RAD51L3) are known to have crucial non-redundant roles in this pathway. Current knowledge of the functions of the RAD51 gene family is reviewed, as well as the evidence for extensive genetic instability arising from loss of their activity. Reports of potential associations between variants of RAD51 family genes and specific forms of cancer are summarized, but it is seen that many of these studies have relatively low statistical power. As yet these data provide only tantalizing suggestions of modified cancer risks arising from polymorphisms, mutations, or changes in expression of the RAD51 gene family, and there is still a lot to learn before firm conclusions can be made.

The XRCC genes: expanding roles in DNA double-strand break repair

Functional analysis of the XRCC genes continues to make an important contribution to the understanding of mammalian DNA double-strand break repair processes and mechanisms of genetic instability leading to cancer. New data implicate XRCC genes in long-standing questions, such as how homologous recombination (HR) intermediates are resolved and how DNA replication slows in the presence of damage (intra-S checkpoint). Examining the functions of XRCC genes involved in non-homologous end joining (NHEJ), paradoxical roles in repair fidelity and telomere maintenance have been found. Thus, XRCC5-7 (DNA-PK)-dependent NHEJ commonly occurs with fidelity, perhaps by aligning ends accurately in the absence of sequence microhomologies, but NHEJ-deficient mice show reduced frequencies of mutation. NHEJ activity seems to be involved in both mitigating and mediating telomere fusions; however, defective NHEJ can lead to telomere elongation, while loss of HR activity leads to telomere shortening. The correct functioning of XRCC genes involved in both HR and NHEJ is important for genetic stability, but loss of each pathway leads to different consequences, with defects in HR additionally leading to mitotic disruption and aneuploidy. Confirmation that these responses are likely to contribute to cancer induction and/or progression, is given by studies of humans and mice with XRCC gene disruptions: those affecting NHEJ show increased lymphoid tumours, while those affecting HR lead to breast cancer and perhaps to gynaecological tumours.

Parathyroid carcinoma

Parathyroid carcinoma is an uncommon malignancy. It accounts for less than 1% of cases of primary hyperparathyroidism (HPT). It is manifested by severe hypercalcemia and up to 50% of patients will have concomitant kidney or bone disease. The etiology of parathyroid carcinoma is unknown, however, the recently discovered HRPT2 gene, a tumor suppressor gene encoding for the protein parafibromin, has been implicated in the pathogenesis. Identification of inactivating germ-line mutations in HRPT2 has significant implications for diagnosis and management. This article summarizes the genetic aspects of parathyroid carcinoma, reviews its clinical manifestations, and outlines the principles of surgical therapy, the indications for adjuvant therapy, and the use of bisphosphonate and calcimimetic agents for management of hypercalcemia.

Increased rates of spontaneous sister chromatid exchange in lymphocytes of BRCA2+/- carriers of familial breast cancer clusters

Heterozygous carriers of germ-line mutations of the BRCA2 breast cancer susceptibility gene are predisposed to breast, ovarian, pancreatic and other cancers. The BRCA2 protein is implicated in the maintenance of chromosome stability through its essential function in double-strand DNA repair and recombination. Our previous studies had revealed multiple intrachromosomal rearrangements, duplications, inversions and deletions on 9p23-24 in lymphocytes and fibroblasts of BRCA2+/- members from independently ascertained familial breast cancer clusters. In pursuit of evaluating if there is a subtle genomic instability in BRCA2+/- individuals, we have determined frequencies of spontaneous sister chromatid exchanges (SCEs) in BRCA2 wild-types and BRCA2 mutation carriers of two familial breast cancer clusters. Here, we demonstrate an average increase of 65% of spontaneous SCEs in BRCA2+/- versus BRCA2+/+ family members. In one cluster, the number of metaphases with multiple SCEs was 5-times higher in BRCA2+/- compared to wild-type members, while in the second cluster BRCA2+/- members had 8.9% of metaphases with multiple SCEs compared to a level below detection in BRCA2 wild types. To investigate the correlation between SCE and genomic instability in 9p, we performed fluorescence detection of SCEs and FISH analysis with 9p probes. The frequency of SCE in 9p of BRCA2 mutation carriers was 3-4 fold (P = 0.005) higher compared to BRCA2 wild-types. Collectively, the increased rates of SCE in BRCA2 heterozygous mutation carriers indicate a BRCA2 haploinsufficiency, which might be an important factor for the accumulation of structural chromosomal alterations with the consequence of damage in as yet unidentified genes.

Cytogenetic changes in nonmalignant breast tissue

Cytogenetic changes are common in breast cancer and have also been described in fibroadenomas and fibrocystic disease, but not in histologically normal breast tissue. Cytogenetic analysis was performed on nonmalignant breast tissue from benign breast lumps (n = 8), reduction mammoplasties (n = 31), and grossly nontumorous tissue from cancerous breasts (n = 84), using standard techniques and G-banding. All samples were reviewed histologically. Clonal chromosomal changes were found in three of eight benign breast tumors (38%). Of the reduction mammoplasties, 17 samples contained nonproliferative changes, and three of these (18%) showed a clonal deletion of 3p. No pathology was identified in the other 14 samples, of which one (7%) contained two clonal changes, apparently balanced translocations. Of nontumorous tissues from cancerous breasts, 15 (18%) showed clonal chromosomal abnormalities. Five of these samples were histologically normal. Two clones were identical to those found in the corresponding cancer. In 18 additional samples, single cells were detected with the same change as that seen in clones or single cells in the cancer. Only 4 of these 20 samples contained detectable cancer cells. Clonal abnormalities found in two or more samples included trisomies X, 7, and 20 and monosomies 19 and 18. Clonal changes were not significantly more frequent in proliferative than in nonproliferative lesions. The Icelandic BRCA2 founder mutation, 999del5, was detected in four samples, all histologically normal, two of which had clonal chromosomal abnormalities. In conclusion, clonal chromosomal changes are not infrequent in nonmalignant breast tissue and can be detected even in the absence of histological abnormalities.

Transcriptional regulation of the murine brca2 gene by CREB/ATF transcription factors

The brca2 gene encodes a nuclear protein which is mainly involved in DNA repair and, when mutated, is responsible for some of the hereditary breast cancers. However, brca2 expression is also deregulated in sporadic breast tumors. In the mouse brca2 gene we had earlier identified a region of 148bp upstream of the transcription start site sufficient to activate its expression. In the present report, we show that the -92 to -40bp region is essential for the transcription of brca2 in murine mammary cells and that this nucleotide sequence contains one putative CREB/ATF consensus site (cAMP responsive element: CRE). We demonstrated that the mutation of this binding site led to a highly significant reduction of the mouse brca2 transcription, and that CREB, CREM, and/or ATF-1 functionally bound to and regulated this promoter. Therefore, the regulation of the promoter of the mouse brca2 gene is driven by this family of transcription factors.

A delayed chemically induced tumorigenesis in Brca2 mutant mice

BRCA2 is a breast cancer susceptibility gene. Germline mutations of BRCA2 account for about 10-30% of familial breast cancer cases. Consistent with its tumor-suppressor activity, BRCA2 plays an important role in DNA repair. To assess the susceptibility of carriers of mutant BRCA2 to tumorigenesis induced by DNA-damaging carcinogens, we generated a Brca2 knockout mouse strain and studied its susceptibility to chemically induced tumorigenesis. Similar to previously reported Brca2 knockout mice, our Brca2-/- embryos die at E8.5-9.5, while the Brca2+/- mice are tumor-free and fertile. Unexpectedly, Brca2+/- mice developed tumors slower than did their wild-type littermates when treated with a potent carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). In vitro experiments showed that Brca2+/- mouse cells and Capan-1 cells, a human pancreatic cancer cell line deficient of BRCA2, were more sensitive to DMBA-induced apoptosis, than were Brca2+/+ mouse cells and a derivative of Capan-1 cells that expressed exogenous wild-type BRCA2, respectively. Our results suggest that enhanced sensitivity of Brca2 mutant cells to DMBA-induced apoptosis at the dose of DMBA we used contributes to the delayed tumorigenesis of Brca2+/- animals. This suggestion may also provide a rational explanation for a previous unexpected finding that cigarette smoking appears to reduce the breast cancer risk of BRCA2 mutation carriers.

Role of the insulin-like growth factor I/insulin receptor substrate 1 axis in Rad51 trafficking and DNA repair by homologous recombination

The receptor for insulin-like growth factor I (IGF-IR) controls normal and pathological growth of cells. DNA repair pathways represent an unexplored target through which the IGF-IR signaling system might support pathological growth leading to cellular transformation. However, this study demonstrates that IGF-I stimulation supports homologous recombination-directed DNA repair (HRR). This effect involves an interaction between Rad51 and the major IGF-IR signaling molecule, insulin receptor substrate 1 (IRS-1). The binding occurs within the cytoplasm, engages the N-terminal domain of IRS-1, and is attenuated by IGF-I-mediated IRS-1 tyrosine phosphorylation. In the absence of IGF-I stimulation, or if mutated IGF-IR fails to phosphorylate IRS-1, localization of Rad51 to the sites of damaged DNA is diminished. These results point to a direct role of IRS-1 in HRR and suggest a novel role for the IGF-IR/IRS-1 axis in supporting the stability of the genome.

M phase-specific phosphorylation of BRCA2 by Polo-like kinase 1 correlates with the dissociation of the BRCA2-P/CAF complex

BRCA2 is a breast tumor susceptibility gene encoding a 390-kDa protein with functions in maintaining genomic stability and cell cycle progression. Evidence has been accumulated to support the concept that BRCA2 has a critical role in homologous recombination of DNA double-stranded breaks by interacting with RAD51. In addition, BRCA2 may have chromatin modifying activity through interaction with a histone acetyltransferase protein, p300/CBP-associated factor (P/CAF). To explore how the functions of BRCA2 may be regulated, the post-translational modifications of BRCA2 throughout the cell cycle were examined. We found that BRCA2 is hyperphosphorylated specifically in M phase and becomes dephosphorylated as cells exit M phase and enter interphase. This specific phosphorylation of BRCA2 was not observed in cells treated with DNA-damaging agents. Systematic mapping of the potential mitosis specific phosphorylation sites revealed the N-terminal 284 amino acids of BRCA2 (BR-N1) as the major region of phosphorylation and mass spectrometric analysis identified two phosphopeptides that contain "phosphorylation consensus motifs" for Polo-like kinase 1 (Plk1). Phosphorylation of BR-N1 with Plk1 recapitulated the electrophoretic mobility change as seen in BR-N1 isolated from M phase cells. Plk1 interacts with BRCA2 in vivo, and mutation of Ser193, Ser205/206, and Thr203/207 to Ala in BR-N1 abolished Plk1 phosphorylation, suggesting that BRCA2 is the substrate of Plk1. Furthermore, both the hyperphosphorylated and hypophosphorylated forms of BRCA2 bind to RAD51, whereas the M phase hyperphosphorylated form of BRCA2 no longer associates with the P/CAF, suggesting that the dissociation of P/CAF-BRCA2 complex is regulated by phosphorylation. Taken together, these results implicate a potential role of BRCA2 in modulating M phase progression.

Mutational analyses of RB and BRCA2 as candidate tumour suppressor genes in parathyroid carcinoma

OBJECTIVE

Strong evidence indicates that at least one key tumour suppressor gene important for the development of malignant parathyroid tumours is located on chromosome 13, but the critical target gene remains unknown. Importantly, the region of acquired DNA loss includes two established tumour suppressor genes, the retinoblastoma gene, RB (RB1) and BRCA2. Resolution of whether RB or BRCA2 is the critical 13q tumour suppressor gene in parathyroid cancer requires analysis of these genes' sequences for intragenic inactivating mutations. Therefore, RB and BRCA2 were analysed in a group of parathyroid carcinomas in which mutations of these genes should be most readily detectable.

PATIENTS AND DESIGN

Six parathyroid carcinomas from four patients which showed loss of heterozygosity (LOH) at the RB locus and/or 13q loss by comparative genomic hybridazation (CGH) were selected from a CGH/LOH-screened panel of 16 carcinoma specimens from 10 patients. These tumours were examined for mutations by direct sequencing of the complete 27-exon coding region, intron-exon boundaries and promoter of RB. The 26 coding exons and intron-exon boundaries of BRCA2 were also directly sequenced in seven parathyroid carcinomas with loss in the BRCA2 region.

RESULTS

No microdeletions, insertions, or point mutations were detected in either RB or BRCA2 in any of the carcinomas.

CONCLUSION

The absence of tumour-specific somatic mutations in RB and BRCA2 suggests that they are unlikely to act as classic tumour suppressor genes in the pathogenesis of parathyroid carcinomas. While decreased expression of these genes might contribute to parathyroid carcinomatosis in a secondary fashion and 13q loss warrants further study as a diagnostic marker for parathyroid carcinoma, the putative 13q tumour suppressor awaits identification.

The mammalian XRCC genes: their roles in DNA repair and genetic stability

Analysis of the XRCC genes has played an important part in understanding mammalian DNA repair processes, especially those involved in double-strand break (DSB) repair. Most of these genes were identified through their ability to correct DNA damage hypersensitivity in rodent cell lines, and they represent components of several different repair pathways including base-excision repair, non-homologous end joining, and homologous recombination. We document the phenotypic effects of mutation of the XRCC genes, and the current state of our knowledge of their functions. In addition to their continuing importance in discovering mechanisms of DNA repair, analysis of the XRCC genes is making a substantial contribution to the understanding of specific human disorders, including cancer.

Hereditary breast cancer and handling of patients at risk

The identification of BRCA1 and BRCA2, the two known genes causing a dominantly inherited susceptibility for breast and ovarian cancer has allowed genetic testing and identification of high risk individuals in a proportion of breast cancer families. In the future, when both the surveillance methods and prophylactic measures will be further developed this will have even more important clinical value in the management of breast cancer families. To date, as prophylactic mastectomy and/or oophorectomy have been shown to offer a significant risk reduction, these should be considered at least for known mutation carriers. Before considering this, patients should be referred for genetic counseling including risk assessment and genetic testing. Identification of a mutation in the family facilitates carrier detection by allowing predictive testing of healthy individuals. In mutation positive families, a negative test result for an individual has great value as it releases from coping with high risk of cancer and from intensive surveillance. When prophylactic surgery is considered, young age is an important determinant. A skin-sparing mastectomy with implant or autologous tissue transfer is the reconstruction method of choice. Other options like surveillance or chemoprevention can be accepted, but their uncertainty should be pointed out.

Highlight: BRCA1 and BRCA2 proteins in breast cancer

Heterozygous carriers of loss-of-function germline mutations in the BRCA1 or BRCA2 breast cancer susceptibility genes have a predisposition to breast and ovarian cancer. Multiple functions have been ascribed to the products of these genes, linking them to pathways that inhibit progression to neoplasia. Various investigators have assigned roles for these tumor suppressor gene products in the cell functions of genome repair, transcription, and growth control. There is emerging evidence that BRCA1 may participate in ubiquitin E3 ligase activity. BRCA1 and BRCA2 have each been implicated in chromatin remodeling dynamics via protein partnering. Ubiquitin ligase and chromatin remodeling activities need not be mutually exclusive and both may function in DNA repair, transcriptional regulation, or cell cycle control. Here we highlight certain recent findings and currently unanswered questions regarding BRCA1 and BRCA2 in breast cancer.

Role of RAD52 epistasis group genes in homologous recombination and double-strand break repair

The process of homologous recombination is a major DNA repair pathway that operates on DNA double-strand breaks, and possibly other kinds of DNA lesions, to promote error-free repair. Central to the process of homologous recombination are the RAD52 group genes (RAD50, RAD51, RAD52, RAD54, RDH54/TID1, RAD55, RAD57, RAD59, MRE11, and XRS2), most of which were identified by their requirement for the repair of ionizing-radiation-induced DNA damage in Saccharomyces cerevisiae. The Rad52 group proteins are highly conserved among eukaryotes, and Rad51, Mre11, and Rad50 are also conserved in prokaryotes and archaea. Recent studies showing defects in homologous recombination and double-strand break repair in several human cancer-prone syndromes have emphasized the importance of this repair pathway in maintaining genome integrity. Although sensitivity to ionizing radiation is a universal feature of rad52 group mutants, the mutants show considerable heterogeneity in different assays for recombinational repair of double-strand breaks and spontaneous mitotic recombination. Herein, I provide an overview of recent biochemical and structural analyses of the Rad52 group proteins and discuss how this information can be incorporated into genetic studies of recombination.

Human Mus81-associated endonuclease cleaves Holliday junctions in vitro

Mus81, a protein with homology to the XPF subunit of the ERCC1-XPF endonuclease, is important for replicational stress tolerance in both budding and fission yeast. Human Mus81 has associated endonuclease activity against structure-specific oligonucleotide substrates, including synthetic Holliday junctions. Mus81-associated endonuclease resolves Holliday junctions into linear duplexes by cutting across the junction exclusively on strands of like polarity. In addition, Mus81 protein abundance increases in cells following exposure to agents that block DNA replication. Taken together, these findings suggest a role for Mus81 in resolving Holliday junctions that arise when DNA replication is blocked by damage or by nucleotide depletion. Mus81 is not related by sequence to previously characterized Holliday junction resolving enzymes, and it has distinct enzymatic properties that suggest it uses a novel enzymatic strategy to cleave Holliday junctions.

Functions of BRCA1 and BRCA2 in the biological response to DNA damage

Inheritance of one defective copy of either of the two breast-cancer-susceptibility genes, BRCA1 and BRCA2, predisposes individuals to breast, ovarian and other cancers. Both genes encode very large protein products; these bear little resemblance to one another or to other known proteins, and their precise biological functions remain uncertain. Recent studies reveal that the BRCA proteins are required for maintenance of chromosomal stability in mammalian cells and function in the biological response to DNA damage. The new work suggests that, although the phenotypic consequences of their disruption are similar, BRCA1 and BRCA2 play distinct roles in the mechanisms that lead to the repair of DNA double-strand breaks.