Key words BRCA11 tumor DNA double-strand break (DNA DSB homologous recombination nonhomologous end joining

    In the entire life cycle of the organisms in the presence of DNA damage. These injuries are caused by exogenous factors such as ionizing radiation (ionizing radiation, IR), UV (ultraviolet, UV) and chemotherapy drugs also caused by internal factors, such as gametogenesis in DNA replication and meiotic recombination error. The study six major DNA damage repair pathways in human cells are homologous recombination repair (homologous recombination repair, HRR), nucleoside excision repair (nucleotide excision repair, NER), non-homologous end joining (nonhomologous end  joining, NHEJ), nucleotide excision repair (base excision repair, BER), mismatch repair (mismatch repari, MMR) and Fanconi anemia pathway. Has identified over 100 types of genes with six DNA damage repair pathways [1]. 1994, scientists using positional cloning techniques successfully cloned the first gene associated with familial breast and ovarian cancer BRCA1 (, breast cancer susceptibility gene 1) [2]. BRCA1 germline mutations make women prone to breast cancer and ovarian cancer. The BRCA1 exon contains 24 outer, human encoding contains 1863 amino acid residues of the protein, while the mouse coding for 1812 amino acid residues. Since the BRCA1 found a growing number of studies have shown that the protein in HRR, NHEJ, NER and Fanconi anemia DNA damage repair pathway plays an important role. Therefore, this article on the recent years the role of BRCA1 in DNA damage repair and tumor suppressor mechanism are reviewed.

    1 BRCA1 defective breast tumors

    BRCA1 tumor suppressor gene is a determined, it’s easy germline mutations to women suffering from breast cancer and ovarian cancer. BRCA1 breast cancer mouse model helps to the tumor mechanism for tumor prevention and treatment research. Most BRCA1 mutation Department of death during pregnancy, so only those individuals carrying tissue-specific or sub-allele mutation can grow into adult [3  6].

    In the the conditional blocking the mouse mammary BRCA1 gene study, Xu et al [7] found that breast tumors after long-term latent lower frequency of occurrence. The most tumor p53 transcriptional also changed suggests that the tumor suppressor BRCA1 tumor formation. The study found that the p53 + / – mutations introduced can significantly accelerate tumorigenesis, but breast tumors appear randomly in the form to show that in addition to p53, there are multiple factors involved in this process. Mammary tumors in highly different histopathological types also support the conclusion [4,6]. At the same time, the expression study also revealed that a large number of genetic or molecular changes that erbB  2, and c  Myc, p27 and Cyclin D1 overexpression in most tumor [6]. From Brca1 conditions blocking tumor in mice showed a high degree of chromosomal abnormalities, and this is precisely one of the main characteristics of the cancer cells. Promote the occurrence of cancer caused by increasing the specific mutation of the malignant phenotype. Chromosomal abnormalities in most of the cases reflect the weakened mitotic functions can not be moved, including the uneven distribution of chromosomes and cytoplasm, resulting in the formation of aneuploid cells. This mutant mouse tumor is a viewpoint after the genomic instability and Brca1 consistent.

    2 BRCA1 and DNA double-strand breaks (DSB) repair

    The current study showed that BRCA1 plays an important role in the cell’s DNA damage response the receptors mediated injury and repair effector. Cell DNA damage factor treatment DSB, BRCA1 hyperphosphorylation and quickly re-positioning its new site in the replication fork, proliferating cell nuclear antigen labeled [7]. In addition, a large nuclear proteins composite was also found that the BRCA1, is named for the BRCA1  related monitoring of composite objects (the BRCA1  Associated Surveillance CompleX, the BASC), the composite is considered in monitoring genomic damage and Preparation protein signal passed the sensing role [8].

    There are two main types: HRR and the NHEJ DSB repair. HRR is a typical error-free repair damage by a non-homologous sequences to repair the damage of the double-stranded by gene conversion. NHEJ usually associated with the changes in the sequence of the breaking point. The degree of sequence modifications from loss or delete twelve bases to a wide range of very different. Reconnect the mutagenic potential of this pathway is more determined by the quality of repair, rather than the breaking point. If the breaking point sequence deletions, which induced connection is usually mediated by micro-homologous regions from a size 8 bases. HRR occasionally also in the sequence introduced by exchanging error, but the exchange of the product in mammalian cells in mitosis is rapidly suppressed.

    3 BRCA1 and homologous recombination repair (HRR)

    There are two possible roles in mammalian cells HR: one is to repair DNA the DSB, the other is in the replication fork reversal and cross damage repair can not be carried out by a specific polymerase, reboot caused by the damage to the single-stranded replication fork stagnation. Although initially thought that the the BRCA1 genome protection may be achieved indirectly by controlling the cell cycle [9], but the cell cycle from the repair defective genetic solution to make this idea become impossible. BRCA1 has a direct role in the response to DSB can raise BRCA2, thus contributing to the Rad51 protein of the filamentous form [10]. Rad51 is a DNA recombinase homologous bacterial RecA protein, an enzyme involved in mitosis and meiosis, homologous recombination and DNA double-strand break repair, and co-localization with BRCA1, which is required for homologous recombination. BRCA1, Rad51 subnuclear assembly required, and is also necessary for cell survival, even after treatment of cells with DNA cross-linking agent cisplatin [18]. Have reported direct interaction of in BRCA2  Rad51 and BRCA2  BRCA1 protein, Rad51 and BRCA1 is likely to be indirect through BRCA2 in DNA damage sites associated.

    BRCA1 and other DNA damage response associated protein Rad50, Rad50 and Mre11 and Nijmegen breakage syndrome gene 1 protein to form a tight complex. Mre11/Rad50/Nijmegen breakage syndrome gene 1 (MRN) complex with homologous recombination and NHEJ. Mre11 or Rad50 protein polymers will be raised to the DSB sites, but of Rad51 the polymer in the same cells and in the same time but never observed [11]. Rad50 and Rad51 after DNA damage of positioning in the phosphorylation of H2AX (γ  H2AX) polymer [12]. BRCA1 can also be co-localization with the γ  H2AX before Rad50 or Rad51 raised to these loci, which indicates that the BRCA1 can decide Rad50 or Rad51 raised. However, MRN early in H2AX presence or absence of a condition to raise the DSB sites. It can be speculated that the delay in the response of BRCA1 and DSB or replication fork, and can control the follow-up the repair proteins MRN or BRCA2/Rad51 of the raised. MRN complex homologous recombination in human cells is unclear, but speculated that the MRN complex may play a role solve the replication fork delay, and reported Nijmegen [13] breakage syndrome gene 1 protein in DT40 cells can promote homologous recombination. Nijmegen breakage syndrome gene destruction can lead to gene conversion and sister chromatid exchange diminished capacity has not been affected, but Rad51 polymer formation induced by γ radiation (6Gy). Therefore, BRCA1 is how to raise the BRCA2 and Rad51, but also affect the the MRN complex further study. Zhang [14] and other studies have shown that homologous recombination, BRCA1 functions missing can produce more Mre11 polymer and BRCA1 supplement inhibit Mre11 polymer formation.

    The defective BLM helicase in the delay replication forks and prevent an excessive number of homologous recombination in Bloom syndrome functional. BLM from a biochemical point of view, inverted Holliday connection formed. When the the BLM missing sister chromatid exchange more frequent, indicating that the BLM is preventing Rad51-mediated recombination. Although the relationship between the BLM function and BRCA1 have not been fully elucidated, but two different ways of BLM and BRCA1 solve the replication fork delay has become a model. The replication fork delay can result in a “chicken" structure (replication fork reversal, that is formed between a four-chain structure of the newly synthesized strand annealing) is formed [15]. BLM is the reversal “chicken" structure having a specific activity helicase. Therefore, to prevent the replication fork to advance the damage can be replication fork reversal or transformed avoid template, then by the reverse replication fork structure BLM restart replication function. Delayed replication forks can also be a specific endonuclease (e.g. Mus81) cutting, to form a double-stranded DNA termini, thus starting a chain exchange reaction. Promote delay replication forks to the cutting structure, the conversion can be assumed BRCA1, thereby starting Rad51-dependent strand exchange. However, the precise details of these processes is not yet been elucidated, BRCA1 and BLM in this model are expected to appear in the damage monitoring composite and BLM is considered to be part of the the BRCA1 associated monitoring complexes. Further, the same with BRCA1 BLM after ionizing radiation, through ATM phosphorylation and forming a polymer and Rad51.

    4 BRCA1 and non-homologous end joining (NHEJ)

    NHEJ-related gene polymorphism with high-risk breast cancer [16]. BRCA1 can affect the risk of breast cancer and DNA DSB end connectivity, the results show that NHEJ related to the occurrence of breast cancer associated with BRCA1. However, the results of BRCA1 in NHEJ function reports there are some contradictions, these results reflect the possible different methods for measuring NHEJ NHEJ different sub-path.

    The NHEJ subtype is present in mammalian cells, including the end connections of error-free and error-prone. Error-free end of the connection is by using a cell extract or cell-mediated recovery in vitro to the linearized plasmid circular DNA reassociation experiments detected. The end of the most physical connection is error-prone, such as V (D) J rearrangements encoded connection. One or more component defects When dependence Ku repair pathway, the sequence will appear large or small changes (such as delete). Recently, there is evidence that different genetic need error-free and error-prone end joining [17], there is more than one NHEJ path. The current study supports the BRCA1 NHEJ error-free form of the necessary, but NHEJ regulator mechanism is unclear.

    Strongly support the evidence obtained from the yeast and mammalian cells rely Ku NHEJ is an error-prone steps [18]. BRCA1 can also adjust the end of the error-prone connections: large sequences of the BRCA1 inactivation and DSB sites deleted related. Physical role between BRCA1 and human Rad50 has been observed, but the functional results of this interaction is unclear. Recently, Hopfner et al [19] through the the crystal inspection apparatus obtained evidence to show that Rad50 a hook-like structure is formed, in its helical region. Thus, the interaction with the two Mre11 protein DNA binding domain of the globular head of the RAD50 protein by the hook-like structure, the two independent MRN complex linking, and thus the end of the two DNA linking. This mechanism sister chromosomes can be connected together to homologous recombination or fracture end joining together to promote the NHEJ irrelevant. Therefore, MRN complex can play a structural support function and Mre11 nuclease function. Wang [14], etc. The study showed that the BRCA1 can inhibit the MRN complex, thereby reducing the fallibility the NHEJ. In addition, they also observed that BRCA1 defective cells tend to micro-homology mediated end joining, which have been reported in yeast Mre11 role similar. BRCA1 biochemical results support the in vitro inhibition Mre11 dissolved nucleation activity of wild-type BRCA1 Mre11 activity can be suppressed thereby preventing the easy wrong terminal connection assumptions. WRN and Artemis nuclease having a potentially functional in the end connection, and thus is likely to exist a different the Mre11 unknown protein, this protein may be of BRCA1 suppression.

    The functional relationships between the 5 BRCA1 and Fanconi anemia proteins

    Studies show FANCD2 protein ubiquitination is a critical step of the Fanconi anemia path, and relies on the function of the BRCA1. BRCA1 defective cells with DNA damage-induced FANCD2 subnuclear polymer missing monoubiquitinated FANCD2 related. Several evidence to suggest that FANCD2 ubiquitination, BRCA1 E3 ligase activity a target, but none of these studies pointed out that FANCD2 is a direct substrate BRCA1/BARD1 ubiquitin ligase. In contrast, recent studies have found that human cells BRCA1 small interfering RNA-mediated knock cause FANCD2 targeting to sites of DNA damage defects in addition to, but not FANCD2 ubiquitination defects. Similarly, mutant BRCA1 and BARD1 were knockout DT40 cells is very sensitive to the DNA damage, but the cells in FANCD2 ubiquitination no defect. Therefore, the purpose of the BRCA1-mediated ubiquitination, the target protein, and this ubiquitin ligase enzyme activities to the role of BRCA1 in homologous recombination or tumor suppressor is still unknown. Recent reports indicate that BRCA1-associated helicase BACH1 role in homologous recombination, and seems to be the Fanconi anemia gene product FANCJ. BACH1 were taken the FANCJ and FANCD1 be treated as BRCA2 the results show that the presence of a variety of contact [20] between the BRCA1 and Fanconi anemia pathway.

    BRCA1 and nucleotide excision repair (NER)

    NER is divided into two ways: whole genome repair (GCR), capable of damage from the removal of the entire genome; transcription coupled repair (TCR), from the expression of genes transcribed strand priority removal of the damage. Gowen and other studies have shown that the BRCA1 defective mouse ES cells are unable to perform the repair of oxidative DNA damage, and hypersensitivity to ionizing radiation and hydrogen peroxide. These results show and BRCA1 directly or indirectly involved in oxidative DNA damage TCR.

    Recently, Hartman and Ford are two kinds of people in U2OS osteosarcoma cell line UBR60 and E621 [21]. UBR60 carrying wild-type p53, E621 comes from UBR60. Lymphoma due to human mastoid E6 stably expressing the p53 deletion (because E6 is based on p53 for degradation object). These two cell lines expressed low levels of endogenous BRCA1, exogenous transfection of BRCA1 by tetracycline regulation control. These cells is the study of BRCA1 and p53 on NER role model. Their research shows that tetracycline existing conditions, the efficiency of GGR p53 deficient cells than in the p53 wild-type cells was significantly low. This result suggests that the absence of p53 function causes GGR defects. The E621 cells BRCA1 expression p53 defects can GGR recovery, DNA repair BRCA1 induced 3% up to 34% within 24 hours. BRCA1 induction of p53 wild-type cells UBR cells enables the GGR’s ability to rise from 28% to 42% [21]. These results indicate the important role of BRCA1 in GGR.

    Shortly thereafter, they studied the the TCSs the BRCA1 whether these cells affect UV radiation. With an expression of the gene (DHFR), single-chain-specific primers, BRCA1 mutations affect only the repair of the non-transcribed strand, and without affecting the UV-induced transcription chain. Although previous studies have shown that BRCA1 have a certain role in the TCR of oxidative DNA damage, but does not affect the TCSs induced by UV radiation.

    7 BRCA1 after phosphorylation of the transcriptional regulator

    BRCA1 has been identified as the target of several nuclear phosphatidylinositol 3-kinase-related, such as ATM and ATR.

    ATM allows BRCA1 of Ser1387, Ser1423, Ser1457 and Ser1524 in vivo or in vitro phosphorylation, ATR can be phosphorylated in vitro so that the same ATM Ser1423 six serine or threonine residue. Recently found, is considered to be a tumor suppressor protein kinase (Chk2) after ionizing radiation with BRCA1 in discrete core polymer interactions and BRCA1 in Ser988 phosphorylation sites. However, few reports on how these different phosphorylation events acting on functional repair pathway. Zhang [22] and other studies have shown that Chk2-dependent phosphorylation sites Ser988 regulation of BRCA1 in homologous recombination and non-homologous recombination in the main channel, the channel is established between Chk2 and DSB repair adjustment Contact. Chk2 mediated BRCA1 phosphorylation then is how to reconcile the DSB repair is unclear. Biochemical studies have shown that BRCA1 452-1079 amino acids at the presence of a DNA-binding domain, which makes the BRCA1 can inhibit Mre11 in vitro nucleic acid exonuclease activity of [19]. At the same time, the study also shows a Rad51 interaction domain is positioned in the amino acid fragment of BRCA1 758-1064. Zhang [22] and other studies of BRCA1 Ser988 sites is not only responsible for the control of homologous recombination, the connection is very critical of the suppression of the end of the error-prone. Therefore, Ser988 mutations will change the function of the DNA binding domain. Further, the point mutation can affect the interaction between the BRCA1 and Rad51, but direct in vivo evidence has yet to be elucidated.

    In addition to DNA repair, the BRCA1-mediated cell cycle checkpoint effect can also be protein kinases. S1423A BRCA1 mutations destroy the ability to produce G2  M phase arrest, but still retains its DNA repair capacity. In contrast, rely on ATM-ionizing radiation-induced S-phase checkpoint requires BRCA1 Ser1387 sites rather than Ser1423 sites. Proven homologous recombination in the cell cycle when sister chromatids can be used as a template, S and G2  M play an important role. But the ability to maintain the S and G2 ~ M cell cycle checkpoint that is not dependent on homologous recombination. BRCA1 mutation of S988A hinder its role in homologous recombination, but the S-phase checkpoint no effect; Instead, S1423/1524A of mutations with normal homologous recombination G2 ~ M phase checkpoint defects.

    MDC1 is another important BRCA1 phosphorylation dependent on the ATM adjustment factors [23]. This protein is NH2 end nuclear factor FHA associated domain and BRCT domain tandem repeat. These domains in DNA repair or DNA damage response path several protein. MDC1 is required to activate the mammalian cellular DNA damage response proteins Chk2. When MDC1 cells negative adjustment of ionizing radiation can not induce BRCA1 polymer. Therefore, MDC1 role of BRCA1-dependent homologous recombination, cell cycle checkpoint needs further investigation.

    BRCA1 is located on the cross point of the DSB response path of the key. Phosphorylation of BRCA1 transcriptional regulation established a link between BRCA1 and Chk2, ATR and ATM, and provides a clue to understanding the relationship between BRCA1 and Mre11 and Rad51. BRCA1 mutations, Chk2, ATR or ATM activity changes can promote genetic instability and increased susceptibility to breast cancer and other tumor. Considerable evidence that ATM mutations can increase susceptibility to breast cancer. Epidemiological research has Chk2 as a path to the same kinds of breast cancer and BRCA1. The MRN complex genetic mutation has been identified in breast cancer [24]. The other possibility is that the ATR or Rad51 defects may be associated with the constitution of the risk for breast cancer.

    8 BRCA1 deletion of genetic instability

    DNA damage repair gene deletion is a direct consequence of genetic instability. Shen et al [25] analyzed the chromosome from the the missing embryos E8.5  9.5BRCA1 isolated by spectral karyotype analysis (spectral karyotyping, SKY), found BRCA1 missing embryos have abnormal chromosome number and structure. In the BRCA1 mutant MEFs, breast tumors and Brca1 mutant mouse lymphoma the complete chromosomal damage. Through comparative genomic hybridization (comparative genome hybridization CGH) and SKY of Brca1 conditions blocking in-depth analysis of mammary tumors in mice found, Brca1 missing tumors similar to human breast cancer showed chromosomal gain and loss [26]. Most tumors (9/15) showed the end of chromosome 11 obtained this region with human chromosome 17q11  qter similar map location erbB  2. There are four tumors also shows the proximal end of the same number of chromosome 11 deletions proximal chromosome 11 and human chromosome 17p similar. The eight kinds of tumors observed in the chromosome 15 of the whole or parts obtain concentrated in 15D2  D3, similar to the human chromosome 8q24, i.e. c  myc gene in map location. The six tumor showed a part of chromosome 14, or all of the deletion, including 14D3, i.e. Rb1 map location. These observations show that the in Brca1 conditional blocking mouse mammary tumors erbB-2, increased expression of c  Myc and p53 deletion.

    Brca1 mutant cells also showed missing the abnormal centrosome expand and G2  M cell cycle checkpoint control [27]. These abnormalities probably is not dependent on DNA damage and can promote genetic instability. Therefore, Brca1 mutations in somatic cell genetic instability may be the result of the role of these defects.

    9 Conclusion

    BRCA1 plays an important role in DNA damage repair and tumor. BRCA1 defects can weaken the HRR, the NHEJ Fanconi anemia pathway and NER. Abnormal DNA damage repair defects and G2 / M cell cycle checkpoint the common genetic instability caused by BRCA1-deficient cells. Accumulation of DNA damage can activate the p53 tumor suppressor gene and induces cell cycle arrest and apoptosis, causing a series of physiological responses, including defects in cell proliferation, differentiation, and transcriptional regulation defects. These abnormal the BRCA1 mutant embryonic lethal. On the other hand, BRCA1 defects also can increase the mutation rate of all genes, including of p53. p53 deletion or mutation of DNA damage in cells can survive, and undergone clonal expansion, and ultimately produce cancerous.

    Cancer is a multi-step process. BRCA1 germline mutations, other genetic events necessary for breast cancer is unclear. For example, the p53 gene was found in over 90% of breast cancer-associated cancers inactivation, but the reason for this result is unclear role in DNA repair. The usual explanation is that the damage-induced apoptosis missing damaged cells will be able to survive in order to promote mutation. It is unclear why BRCA1 mutations cause individuals only breast cancer and ovarian cancer, but the research direction in the study of the mechanism of tumorigenesis and maintenance of the Women’s Health has far-reaching significance.