Functional link of BRCA1 and ataxia telangiectasia gene product in DNA damage response

p53-independent induction of Gadd45 by histone deacetylase inhibitor: coordinate regulation by transcription factors Oct-1 and NF-Y

Histone deacetylase (HDAC) inhibitors cause growth arrest at the G1 and/or G2/M phases, and induce differentiation and/or apoptosis in a wide variety of tumour cells. The growth arrest at G1 phase by HDAC inhibitors is thought to be highly dependent on the upregulation of p21/WAF1, but the precise mechanism by which HDAC inhibitors cause G2/M arrest or apoptosis in tumour cells is unknown. Gadd45 causes cell cycle arrest at the G2/M phase transition and participates in genotoxic stress-induced apoptosis. We show here that it is also induced by a typical HDAC inhibitor, trichostatin A (TSA), through its promoter, in a p53-independent manner. To identify the mechanism of activation of the gadd45 promoter, we performed luciferase reporter analyses and electrophoretic mobility shift assays. These revealed that both the Oct-1 and CCAAT sites are needed for the full activation by TSA. We also found that the transcription factors Oct-1 and NF-Y specifically bind to each site. Thus, HDAC inhibitors can induce Gadd45 through its promoter without the need for functional p53, and both the Oct-1 and NF-Y concertedly participate in TSA-induced activation of the gadd45 promoter.

E2F6 negatively regulates BRCA1 in human cancer cells without methylation of histone H3 on lysine 9

E2F6 contains a DNA binding domain that is very similar to that of the other members of the E2F family of transcriptional regulators. However, E2F6 cannot bind to all promoters that contain consensus E2F-binding sites. Therefore, we used a combination of chromatin immunoprecipitation and genomic microarrays to identify promoters bound by E2F6 in human cells. Although most of the identified promoters were bound by multiple E2F family members, one promoter was bound only by E2F6. To determine which of the newly identified promoters were regulated by E2F6, we reduced the level of E2F6 by using RNA interference technology. We found that mRNA transcribed from promoters bound by E2F6 was increased after reduction of the amount of E2F6 protein in the cell. Interestingly, many of the E2F6-regulated genes encoded functions involved in tumor suppression and the maintenance of chromatin structure. Specifically, our results suggest that E2F6 represses transcription of the brca1, ctip, art27, hp1alpha, and the rbap48 genes. E2F6 has been postulated to mediate transcriptional repression by recruiting a histone H3 methyltransferase to the DNA. However, we found that the E2F6-regulated promoters did not contain histone H3 methylated at lysine 9. To determine the mechanism by which E2F6 regulates transcription, we performed chromatin immunoprecipitation before and after the introduction of small inhibitory ribonucleic acids specific to E2F6. We found that depletion of E2F6 resulted in the recruitment of E2F1 to the target promoters. In summary, we have identified 48 endogenous target genes of E2F6 and have shown that E2F6 can repress target promoters in a manner that does not require histone H3 methylation at lysine 9.

ATM's leucine-rich domain and adjacent sequences are essential for ATM to regulate the DNA damage response

The ATM protein kinase regulates the DNA damage response by phosphorylating proteins involved in cell cycle checkpoints and DNA repair. We report here on the function of the predicted leucine zipper (LZ) motif, and sequences adjacent to this, in regulating ATM activity. The predicted LZ sequence was deleted from ATM, generating ATMDeltaLZ, and expressed in an ATM-negative AT cell line. ATM increased cell survival following exposure to ionizing radiation, whereas expression of ATMDeltaLZ failed to increase cell survival. ATMDeltaLZ retained in vitro kinase activity, but was unable to phosphorylate p53 in vivo. Leucine zippers mediate homo- and heterodimerization of proteins. However, the predicted LZ of ATM did not mediate the formation of ATM dimers. We examined if the predicted LZ of ATM was a dominant-negative inhibitor of ATM function in SW480 cells. Expression of amino acids 769-1436 of ATM, including the predicted LZ, sensitized SW480 cells to ionizing radiation, but did not inhibit ATM's kinase activity or its ability to phosphorylate Brca1. Further, this dominant-negative activity was not dependent on the predicted LZ domain. The central region of the ATM protein therefore contains multiple sequences which regulate cell survival following DNA damage.

BRCA1 and p53: compensatory roles in DNA repair

The BRCA1 breast cancer susceptibility gene has been implicated in many cellular processes, yet its specific mechanism of tumor suppression remains unclear. BRCA1 plays a role in several DNA repair pathways including nucleotide excision repair (NER). Loss of the p53 tumor suppressor gene, a key regulator of NER, is an important and necessary event in the pathogenesis of BRCA1-mutated tumors. Here we discuss the role of BRCA1 and NER in breast cancer and the interactions of BRCA1 with p53 in breast tumorigenesis and suggest approaches for risk assessment and chemotherapeutic management of BRCA1-related breast cancer.

Regulation and mechanisms of mammalian double-strand break repair

The double-strand break (DSB) is believed to be one of the most severe types of DNA damage, and if left unrepaired is lethal to the cell. Several different types of repair act on the DSB. The most important in mammalian cells are nonhomologous end-joining (NHEJ) and homologous recombination repair (HRR). NHEJ is the predominant type of DSB repair in mammalian cells, as opposed to lower eucaryotes, but HRR has recently been implicated in critical cell signaling and regulatory functions that are essential for cell viability. Whereas NHEJ repair appears constitutive, HRR is regulated by the cell cycle and inducible signal transduction pathways. More is known about the molecular details of NHEJ than HRR in mammalian cells. This review focuses on the mechanisms and regulation of DSB repair in mammalian cells, the signaling pathways that regulate these processes and the potential crosstalk between NHEJ and HRR, and between repair and other stress-induced pathways with emphasis on the regulatory circuitry associated with the ataxia telangiectasia mutated (ATM) protein.

DNA damage checkpoint control in cells exposed to ionizing radiation

Damage induced in the DNA after exposure of cells to ionizing radiation activates checkpoint pathways that inhibit progression of cells through the G1 and G2 phases and induce a transient delay in the progression through S phase. Checkpoints together with repair and apoptosis are integrated in a circuitry that determines the ultimate response of a cell to DNA damage. Checkpoint activation typically requires sensors and mediators of DNA damage, signal transducers and effectors. Here, we review the current state of knowledge regarding mechanisms of checkpoint activation and proteins involved in the different steps of the process. Emphasis is placed on the role of ATM and ATR, as well on CHK1 and CHK2 kinases in checkpoint response. The roles of downstream effectors, such as P53 and the CDC25 family of proteins, are also described, and connections between repair and checkpoint activation are attempted. The role of checkpoints in genomic stability and the potential of improving the treatment of cancer by DNA damage inducing agents through checkpoint abrogation are also briefly outlined.

657del5 mutation in the gene for Nijmegen breakage syndrome (NBS1) in a cohort of Russian children with lymphoid tissue malignancies and controls

Nijmegen breakage syndrome (NBS, OMIM 251260) is a rare hereditary disease, characterized by immune deficiency, microcephaly, and an extremely high incidence of lymphoid tissue malignancies. The gene mutated in NBS, NBS1, was recently cloned from its location on chromosome 8q21. The encoded protein, nibrin (p95), together with hMre11 and hRad50, is involved in the double-strand DNA break repair system. We screened two Russian cohorts for the 657del5 NBS1 mutation and found no carriers in 548 controls and two carriers in 68 patients with lymphoid malignancies: one with acute lymphoblastic leukemia (ALL) and one with non-Hodgkin lymphoma (NHL). Several relatives of the second patient, who were carriers of the same mutation, had cancer (ALL, breast cancer, GI cancers). These preliminary data suggest that NBS1 mutation carriers can be predisposed to malignant disorders.

Toward maintaining the genome: DNA damage and replication checkpoints

DNA checkpoints play a significant role in cancer pathology, perhaps most notably in maintaining genome stability. This review summarizes the genetic and molecular mechanisms of checkpoint activation in response to DNA damage. The major checkpoint proteins common to all eukaryotes are identified and discussed, together with how the checkpoint proteins interact to induce arrest within each cell cycle phase. Also discussed are the molecular signals that activate checkpoint responses, including single-strand DNA, double-strand breaks, and aberrant replication forks. We address the connection between checkpoint proteins and damage repair mechanisms, how cells recover from an arrest response, and additional roles that checkpoint proteins play in DNA metabolism. Finally, the connection between checkpoint gene mutation and genomic instability is considered.

BRCA1 gene in breast cancer

The BRCA1 gene was identified and cloned in 1994 based its linkage to early onset breast cancer and breast-ovarian cancer syndromes in women. While inherited mutations of BRCA1 are responsible for about 40-45% of hereditary breast cancers, these mutations account for only 2-3% of all breast cancers, since the BRCA1 gene is rarely mutated in sporadic breast cancers. However, BRCA1 expression is frequently reduced or absent in sporadic cancers, suggesting a much wider role in mammary carcinogenesis. Since BRCA1 was cloned in 1994, its molecular function has been the subject of intense investigation. These studies have revealed multiple functions of the BRCA1 that may contribute to its tumor suppressor activity, including roles in: cell cycle progression, several highly specialized DNA repair processes, DNA damage-responsive cell cycle check-points, regulation of a set of specific transcriptional pathways, and apoptosis. Many of these functions are linked to protein:protein interactions involving different portions of the 1,863 amino acid (aa) BRCA1 protein. BRCA1 functions in cell cycle progression and the DNA damage response appear to be regulated by distinct and specific phosphorylation events, but the molecular pathways activated by these phosphorylations are only beginning to be unraveled. In addition, the reason that BRCA1 mutation carriers develop specific tumor types (breast and ovarian cancers in women and possibly prostate cancers in men) is not clearly understood. Elucidation of the precise molecular functions of the BRCA1 gene product will greatly enhance our understanding of the pathogenesis of hereditary as well as sporadic mammary carcinogenesis.

BRCA1 shifts p53-mediated cellular outcomes towards irreversible growth arrest

The tumor suppressor protein BRCA1 has been shown to enhance p53 transcription, whereas activated p53 represses BRCA1 transcription. To further understand the functional interaction of these proteins, we investigated the role of BRCA1 in p53-induced phenotypes. We found that BRCA1 when subjected to forced expression acts synergistically with wild-type p53, resulting in irreversible growth arrest, as shown by VhD mouse fibroblast cells expressing a temperature-sensitive mutant of p53. Furthermore, reintroduction of both BRCA1 and p53 into BRCA1(-/-)/p53(-/-) mouse embryonic fibroblasts markedly increased the senescence phenotype compared to that induced by p53 alone. In particular, we found that BRCA1 expression attenuated p53-mediated cell death in response to gamma-irradiation. Moreover, microarray screening of 11 000 murine genes demonstrated that a set of genes upregulated by p53 is enhanced by coexpression of BRCA1 and p53, suggesting that BRCA1 and p53 exert a promoter selectivity leading to a specific phenotype. Taken together, our results provide evidence that BRCA1 is involved in p53-mediated growth suppression rather than apoptosis.

A subset of ATM- and ATR-dependent phosphorylation events requires the BRCA1 protein

BRCA1 is a central component of the DNA damage response mechanism and defects in BRCA1 confer sensitivity to a broad range of DNA damaging agents. BRCA1 is required for homologous recombination and DNA damage-induced S and G(2)/M phase arrest. We show here that BRCA1 is required for ATM- and ATR-dependent phosphorylation of p53, c-Jun, Nbs1 and Chk2 following exposure to ionizing or ultraviolet radiation, respectively, and is also required for ATM phosphorylation of CtIP. In contrast, DNA damage-induced phosphorylation of the histone variant H2AX is independent of BRCA1. We also show that the presence of BRCA1 is dispensable for DNA damage-induced phosphorylation of Rad9, Hus1 and Rad17, and for the relocalization of Rad9 and Hus1. We propose that BRCA1 facilitates the ability of ATM and ATR to phosphorylate downstream substrates that directly influence cell cycle checkpoint arrest and apoptosis, but that BRCA1 is dispensable for the phosphorylation of DNA-associated ATM and ATR substrates.

Designing and implementing quality control for multi-center screening of mutations in the ATM gene among women with breast cancer

Epidemiologic studies of breast and other cancers are increasingly turning toward large, multi-center designs in order to obtain adequate power to detect low-penetrance susceptibility alleles. The size of such studies often makes it necessary to distribute the genetic screening efforts to multiple sites. Careful standardization of screening methodology and quality control across sites is required for such multi-center screening designs to be efficient. In this report, we illustrate our approach to these challenges in the context of the WECARE (Women's Environment, Cancer and Radiation Epidemiology) Study, a multi-center population-based genetic epidemiologic study of women with unilateral and bilateral breast cancer. We provide optimized conditions for screening the ataxia-telangiectasia gene (ATM) for variation by denaturing high-performance liquid chromatography (DHPLC) and describe the results of two independent quality control studies at four international centers employing these conditions. Finally, we report novel mutations in the ATM gene identified both in patients with ataxia-telangiectasia and in patients with unilateral or bilateral breast cancer.

Breast cancer gene 1 (BRCA1): role in cell cycle regulation and DNA repair--perhaps through transcription

Mutations of BRCA1 gene are associated with more than half the cases of hereditary breast cancer. Breast cancer formation in BRCA1 mutation carriers is generally accompanied by loss of the wild-type allele, suggesting that BRCA1 protein may function as a tumor suppressor. The human BRCA1 gene encodes a nuclear protein of 1863 amino acids. Although several lines of evidences suggest that BRCA1 protein may have a role to play in cell cycle regulation, DNA repair, and other processes, the exact mechanism of functioning by BRCA1 protein is not clear. Recent evidences from several laboratories suggest that BRCA1 may regulate the expression of many genes like p21(WAF1/CIP1), Gadd45, Cyclin B1, DBB2, XPC, 14-3-3ó and others at the level of transcription. These BRCA1-regulated gene products have been implicated directly or indirectly in cell cycle regulation and DNA repair. Thus a plausible model is proposed in which BRCA1 protein may bring its effects on cell cycle and DNA repair through its ability to modulate gene expression at the level of transcription.

ATM and related protein kinases: safeguarding genome integrity

Maintenance of genome stability is essential for avoiding the passage to neoplasia. The DNA-damage response--a cornerstone of genome stability--occurs by a swift transduction of the DNA-damage signal to many cellular pathways. A prime example is the cellular response to DNA double-strand breaks, which activate the ATM protein kinase that, in turn, modulates numerous signalling pathways. ATM mutations lead to the cancer-predisposing genetic disorder ataxia-telangiectasia (A-T). Understanding ATM's mode of action provides new insights into the association between defective responses to DNA damage and cancer, and brings us closer to resolving the issue of cancer predisposition in some A-T carriers.

NFBD1, a novel nuclear protein with signature motifs of FHA and BRCT, and an internal 41-amino acid repeat sequence, is an early participant in DNA damage response

Efficient repair of DNA double-strand breaks depends on the intact signaling cascade, comprising molecules involved in DNA damage signal pathways and checkpoints. Budding yeast Rad9 (scRad9) is required for activation of scRad53 (mammalian homolog Chk2) and transduction of the signal further downstream in this pathway. In the search for a mammalian homolog, three proteins in the human data base, including BRCA1, 53BP1, and nuclear factor with BRCT domains protein 1 (NFBD1), were found to share significant homology with the BRCT motifs of scRad9. Because BRCA1 and 53BP1 are involved in DNA damage responses, a similar role for NFBD1 was tested. We show that NFBD1 is a 250-kDa nuclear protein containing a forkhead-associated motif at its N terminus, two BRCT motifs at its C terminus, and 13 internal repetitions of a 41-amino acid sequence. Five minutes after gamma-irradiation, NFBD1 formed nuclear foci that colocalized with the phosphorylated form of H2AX and Chk2, two phosphorylation events known to be involved in early DNA damage response. NFBD1 foci are also detected in response to camptothecin, etoposide, and methylmethanesulfonate treatments. Deletion of the forkhead-associated motif or the internal repeats of NFBD1 has no effect on DNA damage-induced NFBD1 foci formation. Conversely, deletion of the BRCT motifs abrogates damage-induced NFBD1 foci. Ectopic expression of the BRCT motifs reduced damage-induced NFBD1 foci and compromised phosphorylated Chk2- and phosphorylated H2AX-containing foci. These results suggest that NFBD1, like BRCA1 and 53BP1, participates in the early response to DNA damage.

Characterization of Arabidopsis thaliana ortholog of the human breast cancer susceptibility gene 1: AtBRCA1, strongly induced by gamma rays

hBRCA1 is involved in 20-45% of inherited breast cancer cases and is implicated in many mechanisms involved in response to DNA damage. To date, BRCA1 orthologs have been characterized in vertebrate genomes only. We have identified the ortholog of BRCA1 in Arabidopsis thaliana. AtBRCA1 is a 5.5 kb part of the locus At4g21070. The corresponding mRNA of 3.5 kb is composed of 14 exons and encodes a 941 amino acid protein (104 kDa). AtBRCA1, which has one N-terminal RING finger, two C-terminal BRCT and the p300/CBP interacting domain, shows a high similarity to hBRCA1 in these motifs and has the same characteristic molecular organization. We have also identified a putative ortholog in rice (OsBRCA1). With 941 and 968 amino acids, respectively, AtBRCA1 and OsBRCA1 are the shortest members of the BRCA1 family, and may represent a plant specificity. AtBRCA1 is expressed ubiquitously in plant tissues, at levels depending on organ type, with highest levels in flower buds and exponentially growing cell cultures. Increase of mRNA levels in all plant tissues 1 h after irradiation with the highest induction level of approximately 150 times for a 100 Gy dose is consistent with a putative role of AtBRCA1 in DNA repair and in cell-cycle control.

Regulation of the Fanconi anemia pathway by monoubiquitination

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome characterized by multiple congenital anomalies, bone marrow failure, and cellular sensitivity to mitomycin C (MMC). To date, six FA genes have been cloned, and the encoded proteins function in a novel pathway. The FA pathway is required for the normal cellular response to DNA damage. Following DNA damage, the pathway is activated, leading to monoubiquitination of the FA protein, FANCD2, and its targeting to subnuclear foci. Disruption of the FA pathway results in the absence of FANCD2 nuclear foci, leading to the cellular and clinical abnormalities of FA. Here, we review the recent studies describing the regulated monoubiquitination of the FANCD2 protein and discuss the interaction of the FA pathway with other DNA damage response pathways.

Cell cycle differences in DNA damage-induced BRCA1 phosphorylation affect its subcellular localization

Phosphorylation of BRCA1 tumor suppressor protein is regulated during the cell cycle and in response to DNA damage. Several Ser/Thr kinases have been implicated in BRCA1 phosphorylation, including ATM/ATR, cdk2, and hChk2 kinases. In this study, phospho-Ser-specific antibodies recognizing Ser-988, -1423, -1497, and -1524 residues of BRCA1 were employed to study BRCA1 phosphorylation during the S and G2/M phases under conditions of DNA damage. We observed that IR (ionizing radiation) treatment induced phosphorylation of Ser-988/Ser-1524 during the S phase and of Ser-988/Ser-1423 during the G2/M phase. UV treatment induced phosphorylation of Ser-988 during the S phase and of Ser-1423 during the G2/M phase. Phosphorylation of serines 1423 and -1524 was not induced in HCC1937 breast cancer cells, which contain mutant BRCA1 protein. Confocal microscopy revealed that unphosphorylated BRCA1 localizes on chromosomes from metaphase through telophase, whereas Ser-988-phosphorylated BRCA1 resides in the inner chromosomal structure, centrosome, and the cleavage furrow during prophase through telophase. We also found that Ser-988-phosphorylated BRCA1 relocalizes to the perinuclear region when cells are subjected to IR or UV radiation in the S phase. These results reinforce a model wherein phosphorylation of specific residues of BRCA1 after DNA damage affects its localization and function.

Extracellular matrix enhances heregulin-dependent BRCA1 phosphorylation and suppresses BRCA1 expression through its C terminus

Germ line mutations in the breast cancer susceptibility gene BRCA1 account for the increased risk of early onset of familial breast cancer, whereas overexpression of the ErbB family of receptor tyrosine kinases has been linked to the development of nonfamilial or sporadic breast cancer. To analyze whether there is a link between these two regulatory molecules, we studied the effects of ErbB-2 activation by heregulin (HRG) on BRCA1 function. It was previously demonstrated that HRG induced the phosphorylation of BRCA1, which was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Since altered interaction between cells and the surrounding extracellular matrix (ECM) is a common feature in a variety of tumors and since ECM modulates intracellular signaling, we hypothesized that ECM may affect the expression and HRG-dependent phosphorylation of BRCA1. Following stimulation by HRG, a strong increase in [(3)H]thymidine incorporation was observed in human T47D breast cancer cells seeded on plastic (PL). When T47D cells were seeded on laminin (LAM) or Matrigel, HRG induced a significantly higher proliferation than it did in cells seeded on PL. T47D cells seeded on poly-L-lysine had an abrogated mitogenic response, indicating the involvement of integrins in this process. HRG treatment induced a transient phosphorylation of BRCA1 that was enhanced in T47D cells grown on LAM. LAM-enhanced BRCA1 phosphorylation was mediated through alpha(6) integrin upon HRG stimulation. Accordingly, T47D cells grown on LAM had the greatest increase in ErbB-2 activation, PI3K activity, and phosphorylation of Akt. A similar pattern of BRCA1 mRNA expression was observed when T47D cells were seeded on PL, LAM, or COL4. There was a significant decrease in the steady state of the BRCA1 mRNA level on both the LAM and COL4 matrices compared to that for cells seeded on PL. In addition, HRG stimulation caused a significant decrease in BRCA1 mRNA expression that was dependent on protein synthesis. Pretreatment with both the calpain inhibitor ALLN (N-acetyl-Leu-Leu-norleucinal) and the proteosome inhibitor lactacystin inhibited the HRG-induced down-regulation of BRCA1 mRNA expression. Likewise, there was a strong decrease in the protein level of BRCA1 in T47D cells 4 h after treatment with HRG compared to its level in control nontreated T47D cells. Pretreatment with the proteosome inhibitors ALLN, lactacystin, and PSI [N-benzyloxycarbonyl-Ile-Glu-(O-t-butyl)-Ala-leucinal] inhibited also the HRG-induced down-regulation of BRCA1 protein in breast cancer cells. Interestingly, BRCA1 mRNA expression in HCC-1937 breast cancer cells, which express C-terminally truncated BRCA1, was not affected by either LAM or CL4. No phosphorylation of BRCA1 from HCC-1937 cells was observed in response to HRG. While Cdk4 phosphorylated wild-type BRCA1 in response to HRG in T47D cells, Cdk4 failed to phosphorylate the truncated form of BRCA1 in HCC-1937 cells. Furthermore, overexpression of wild-type BRCA1 in HCC-1937 cells resulted in the phosphorylation of BRCA1 and decreased BRCA1 expression upon HRG stimulation while overexpression of truncated BRCA1 in T47D cells resulted in a lack of BRCA1 phosphorylation and restoration of BRCA1 expression. These findings suggest that ECM enhances HRG-dependent BRCA1 phosphorylation and that ECM and HRG down-regulate BRCA1 expression in breast cancer cells. Furthermore, ECM suppresses BRCA1 expression through the C terminus of BRCA1.

AtATM is essential for meiosis and the somatic response to DNA damage in plants

In contrast to yeast or mammalian cells, little is known about the signaling responses to DNA damage in plants. We previously characterized AtATM, an Arabidopsis homolog of the human ATM gene, which is mutated in ataxia telangiectasia, a chromosome instability disorder. The Atm protein is a protein kinase whose activity is induced by DNA damage, particularly DNA double-strand breaks. The phosphorylation targets of Atm include proteins involved in DNA repair, cell cycle control, and apoptosis. Here, we describe the isolation and functional characterization of two Arabidopsis mutants carrying a T-DNA insertion in AtATM. Arabidopsis atm mutants are hypersensitive to gamma-radiation and methylmethane sulfonate but not to UV-B light. In correlation with the radiation sensitivity, atm mutants failed to induce the transcription of genes involved in the repair and/or detection of DNA breaks upon irradiation. In addition, atm mutants are partially sterile, and we show that this effect is attributable to abundant chromosomal fragmentation during meiosis. Interestingly, the transcription of DNA recombination genes during meiosis was not dependent on AtATM, and meiotic recombination occurred at the same rate as in wild-type plants, raising questions about the function of AtAtm during meiosis in plants. Our results demonstrate that AtATM plays a central role in the response to both stress-induced and developmentally programmed DNA damage.

Two nonconsensus sites in the Epstein-Barr virus oncoprotein EBNA3A cooperate to bind the co-repressor carboxyl-terminal-binding protein (CtBP)

CtBP (carboxyl-terminal binding protein) has been shown to be a highly conserved co-repressor of transcription that is important in development, cell cycle regulation, and transformation. Viral proteins E1A and EBNA3C and all the various Drosophila and vertebrate transcription factors to which CtBP has been reported to bind contain a conserved "PXDLS" CtBP-interaction domain. Here we show that EBNA3A binds CtBP both in vitro and in vivo but that this interaction does not require a near consensus (98)PLDLR(102) motif in the NH(2) terminus of EBNA3A. However, further deletion and mutation analysis revealed that CtBP interacts with this viral protein through a cryptic, bipartite motif located in the COOH terminus of EBNA3A. The two components of this binding domain are similar to the canonical PXDLS motif but do not include the highly conserved, and normally critical, first proline residue. These nonconsensus sites, (857)ALDLS(861) and (886)VLDLS(890), synergize to produce very efficient binding to CtBP. Interaction with CtBP was shown to be important in the repression of transcription by EBNA3A and in the ability of EBNA3A to cooperate with activated Ras to immortalize and transform primary rat embryo fibroblasts. Similar bipartite sequences can be found in other viral and cellular proteins that can interact with CtBP, including the retinoblastoma-interacting protein-methyltransferase RIZ, the oncoprotein EVI1, and Marek's disease virus transforming protein Meq.

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.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

Screening for microsatellite instability target genes in colorectal cancers

BACKGROUND

Defects in the DNA repair system lead to genetic instability because replication errors are not corrected. This type of genetic instability is a key event in the malignant progression of HNPCC and a subset of sporadic colon cancers and mutation rates are particularly high at short repetitive sequences. Somatic deletions of coding mononucleotide repeats have been detected, for example, in the TGFbetaRII and BAX genes, and recently many novel target genes for microsatellite instability (MSI) have been proposed. Novel target genes are likely to be discovered in the future. More data should be created on background mutation rates in MSI tumours to evaluate mutation rates observed in the candidate target genes.

METHODS

Mutation rates in 14 neutral intronic repeats were evaluated in MSI tumours. Bioinformatic searches combined with keywords related to cancer and tumour suppressor or CRC related gene homology were used to find new candidate MSI target genes. By comparison of mutation frequencies observed in intronic mononucleotide repeats versus exonic coding repeats of potential MSI target genes, the significance of the exonic mutations was estimated.

RESULTS

As expected, the length of an intronic mononucleotide repeat correlated positively with the number of slippages for both G/C and A/T repeats (p=0.0020 and p=0.0012, respectively). BRCA1, CtBP1, and Rb1 associated CtIP and other candidates were found in a bioinformatic search combined with keywords related to cancer. Sequencing showed a significantly increased mutation rate in the exonic A9 repeat of CtIP (25/109=22.9%) as compared with similar intronic repeats (p< or =0.001).

CONCLUSIONS

We propose a new candidate MSI target gene CtIP to be evaluated in further studies.

Transcription corepressor CtBP is an NAD(+)-regulated dehydrogenase

Transcriptional repression is based on the selective actions of recruited corepressor complexes, including those with enzymatic activities. One well-characterized developmentally important corepressor is the C-terminal binding protein (CtBP). Although intriguingly related in sequence to D2 hydroxyacid dehydrogenases, the mechanism by which CtBP functions remains unclear. We report here biochemical and crystallographic studies which reveal that CtBP is a functional dehydrogenase. In addition, both a cofactor-dependent conformational change, with NAD(+) and NADH being equivalently effective, and the active site residues are linked to the binding of the PXDLS consensus recognition motif on repressors, such as E1A and RIP140. Together, our data suggest that CtBP is an NAD(+)-regulated component of critical complexes for specific repression events in cells.

S-phase-specific interaction of the Fanconi anemia protein, FANCD2, with BRCA1 and RAD51

Fanconi anemia (FA) is a human autosomal recessive cancer susceptibility disorder characterized by cellular sensitivity to mitomycin C and defective cell-cycle progression. Six FA genes (corresponding to subtypes A, C, D2, E, F, and G) have been cloned, and the encoded FA proteins interact in a common pathway. DNA damage activates this pathway, leading to monoubiquitination of the downstream FANCD2 protein and targeting to nuclear foci containing BRCA1. In the current study, we demonstrate that FANCD2 also undergoes monoubiquitination during S phase of the cell cycle. Monoubiquitinated FANCD2 colocalizes with BRCA1 and RAD51 in S-phase-specific nuclear foci. Monoubiquitination of FANCD2 is required for normal cell-cycle progression following cellular exposure to mitomycin C. Our data indicate that the monoubiquitination of FANCD2 is highly regulated, and they suggest that FANCD2/BRCA1 complexes and FANCD2/RAD51 complexes participate in an S-phase-specific cellular process, such as DNA repair by homologous recombination.

Gene expression in inherited breast cancer

Large proportions of hereditary breast cancers are due to mutations in the two breast cancer susceptibility genes BRCA1 and BRCA2. Considerable effort has gone into studying the function(s) of these tumor suppressor genes, both in attempts to better understand why individuals with these inherited mutations acquire breast (and ovarian) cancer and to potentially develop better treatment strategies. The advent of tools such as cDNA microarrays has enabled researchers to study global gene expression patterns in, for example, primary tumors, thus providing more comprehensive overviews of tumor development and progression. Our recent study (Hedenfalk et al., 2001) strongly supports the principle that genomic approaches to classification of hereditary breast cancers are possible, and that further studies will likely identify the most significant genes that discriminate between subgroups and may influence prognosis and treatment. A large number of hereditary breast cancer cases cannot be accounted for by mutations in these two genes and are believed to be due to as yet unidentified breast cancer predisposition genes (BRCAx). Subclassification of these non-BRCA1/2 breast cancers using cDNA microarray-based gene expression profiling, followed by linkage analysis and/or investigation of genomic alterations, may help in the recognition of novel breast cancer predisposition loci. To summarize, gene expression-based analysis of hereditary breast cancer can potentially be used for classification purposes, as well as to expand upon our knowledge of differences between different forms of hereditary breast cancer. Initial studies indicate that a patient's genotype does in fact leave an identifiable trace on her/his cancer's gene expression profile.

Ikaros-CtIP interactions do not require C-terminal binding protein and participate in a deacetylase-independent mode of repression

Ikaros and Aiolos are Kruppel zinc finger proteins that play key roles in hemo-lymphoid development and homeostasis. We have previously shown that they can repress transcription through the recruitment of histone deacetylases (HDACs). Here, we provide the first functional evidence that these proteins can also repress gene function in a manner that does not require deacetylase activity. This functionality can be attributed in part to Ikaros interactions with the HDAC-independent corepressor, C-terminal binding protein (CtBP). However, mutations that block Ikaros-CtBP interactions do not abolish Ikaros's repression activity, implicating the involvement of additional corepressors. Consistent with this expectation, we show that Ikaros can interact with a CtBP-interacting protein (CtIP), which has also been linked to a deacetylase-independent strategy of repression. Despite being a CtBP interactor, CtIP's association with Ikaros does not require CtBP but instead relies upon its Rb interaction domain. Significantly, Ikaros can interact with Rb, which itself can repress gene function in a deacetylase-independent manner. A mutation in Ikaros that abrogates CtIP interactions significantly reduces repression, and a double mutation that prevents interaction with both CtIP and CtBP even further alleviates repression. Finally, we show that CtIP and CtBP can interact with the general transcription factors, TATA binding protein and transcription factor IIB, which suggests a possible mechanism for their deacetylase-independent mode of repression.

Adenovirus-5 E1A: paradox and paradigm

The adenovirus early region 1A (E1A) proteins were described originally as immortalizing oncoproteins that altered transcription in rodent cells. Surprisingly, the 243-amino-acid form of adenovirus-5 E1A was found subsequently to reverse-transform many human tumour cells. Tumour suppression apparently results from the ability of E1A to re-programme transcription in tumour cells, and the molecular basis of this intriguing effect is now beginning to emerge. These discoveries have provided a tool with which to study the regulation of fundamental cellular processes.

Constitutive association of BRCA1 and c-Abl and its ATM-dependent disruption after irradiation

BRCA1 plays an important role in mechanisms of response to double-strand breaks, participating in genome surveillance, DNA repair, and cell cycle checkpoint arrests. Here, we identify a constitutive BRCA1-c-Abl complex and provide evidence for a direct interaction between the PXXP motif in the C terminus of BRCA1 and the SH3 domain of c-Abl. Following exposure to ionizing radiation (IR), the BRCA1-c-Abl complex is disrupted in an ATM-dependent manner, which correlates temporally with ATM-dependent phosphorylation of BRCA1 and ATM-dependent enhancement of the tyrosine kinase activity of c-Abl. The BRCA1-c-Abl interaction is affected by radiation-induced modification to both BRCA1 and c-Abl. We show that the C terminus of BRCA1 is phosphorylated by c-Abl in vitro. In vivo, BRCA1 is phosphorylated at tyrosine residues in an ATM-dependent, radiation-dependent manner. Tyrosine phosphorylation of BRCA1, however, is not required for the disruption of the BRCA1-c-Abl complex. BRCA1-mutated cells exhibit constitutively high c-Abl kinase activity that is not further increased on exposure to IR. We suggest a model in which BRCA1 acts in concert with ATM to regulate c-Abl tyrosine kinase activity.

The amino-terminus of the E2F-1 transcription factor inhibits DNA replication of autonomously replicating plasmids in mammalian cells

The E2F1 transcription factor plays a pivotal role in driving cells out of a quiescent state and into the S phase of the cell cycle, in part by transactivating genes needed for DNA replication including DHFR, thymidine kinase, and DNA Polymerase alpha. E2F1 has also been implicated in regulating an S phase checkpoint, however its role in this checkpoint is not well defined. To determine how E2F1 affects such a checkpoint, we utilized an in vivo replication assay employing a plasmid based SV40 origin of replication, transfected into cells expressing SV40 large T antigen. Here we show that expression of full length E2F1, or only its N terminus, represses replication from plasmids containing the SV40 origin, while N terminal deletions of E2F1 do not. E2F1 appears to inhibit the elongation phase of replication and not the initiation phase since it does not affect the replication of other cotransfected plasmids containing only the SV40 origin. Further, inhibition of replication is dependent on both the amino-terminus of the E2F1 protein and on a DNA sequence that is contained within the 3' end of the E2F1 cDNA. Additionally, both full-length E2F1, or just its N-terminus, form protein complexes with two portions of the 3' end of the E2F1 cDNA. These data provide a clue to the mechanism by which E2F1 regulates transit through the S phase checkpoint, by acting on a specific DNA sequence via its amino-terminal region, to inhibit elongation of DNA replication.

Myeloid differentiation (MyD) primary response genes in hematopoiesis

Myeloid Differentiation (MyD) primary response and Growth Arrest DNA-Damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha MyD118/Gadd45beta, GADD45gamma, MyD116/Gadd34) genes, that have been cloned by virtue of there being co-ordinately induced upon the onset of terminal myeloid differentiation. This review delineates the role MyD genes play in blood cell development, where they function as positive regulators of terminal differentiation, lineage specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis.

Increased error-prone non homologous DNA end-joining--a proposed mechanism of chromosomal instability in Bloom's syndrome

BS is an inherited cancer predisposition disorder caused by inactivation of the RecQ family helicase, BLM. One of the defining features of cells from BS individuals is chromosomal instability, characterized by elevated sister chromatid exchanges (SCEs), as well as chromosomal breaks, deletions, and rearrangements. Although the basis for chromosomal instability is poorly understood, there is evidence that chromosomal abnormalities can arise through an alteration in the efficiency or fidelity of DNA double strand break (DSB) repair. Here, we show that BS cells demonstrate aberrant DSB repair mediated by the non-homologous end-joining (NHEJ) pathway for DNA repair, one of the two main pathways for the repair of DSBs in mammalian cells. Through a comparison of BS cell lines, and a derivative in which the BS phenotype has been reverted by expression of the BLM cDNA, we show that BS cells display aberrant end-joining of DSBs. Importantly, DNA end-joining in BS cells is highly error-prone and frequently results in DNA ligation at distant sites of microhomology, creating large DNA deletions. This aberrant repair is dependent upon the presence of the Ku70/86 heterodimer, a key component in the NHEJ pathway. We propose that aberrant NHEJ is a candidate mechanism for the generation of chromosomal instability in BS.

Early induction of CDKN1A (p21) and GADD45 mRNA by a low dose of ionizing radiation is due to their dose-dependent post-transcriptional regulation

Previous studies have shown that induction of some genes by low-dose radiation has a different dependence on the time after irradiation than induction by high doses. To examine the mechanisms underlying this phenomenon, we investigated the changes in the time course of the rates of transcription of genes in cells of the human myeloblastic leukemia cell line ML-1 by a nuclear run-on assay. It is possible that the more rapid induction of the mRNA of the CDKN1A and GADD45 genes after exposure to 50 cGy of X rays than after 20 Gy is due to a lower level of stabilization of the mRNA of these genes after 50 cGy. In addition, our results show that 50 cGy of X rays increases the transcription rates of the CDKN1A and GADD45 genes, with a maximum induction at 0.5 to 1 h after irradiation, much earlier than the maximum accumulation of stabilized TP53 protein. We suggest the involvement of BRCA1 protein in the early induction of transcription of these two genes.

Myeloid differentiation (MyD)/growth arrest DNA damage (GADD) genes in tumor suppression, immunity and inflammation

Myeloid differentiation (MyD) primary response and growth arrest DNA damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha, MyD118/Gadd45beta, GADD45gamma, MyD116/ Gadd34) genes, that have been cloned by virtue of being co-ordinately induced upon the onset of terminal myeloid differentiation and following exposure of cells to stress stimuli. In recent years it has become evident that MyD/Gadd play a role in blood cell development, where they function as positive regulators of terminal differentiation, lineage-specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis. MyD/Gadd are also involved in inflammatory responses to invading micro-organisms, and response to environmental stress and physiological stress, such as hypoxia, which results in ischemic tissue damage. An intricate network of interactions among MyD/GADD genes and gene products appears to control their diverse functions. Deregulated growth, increased cell survival, compromised differentiation and deficiencies in DNA repair are hallmarks of malignancy and its progression. Thus, the role MyD/Gadd play in negative growth control, including cell cycle arrest and apoptosis, and in DNA repair, make them attractive molecular targets for tumor suppression. The role MyD/Gadd play in innate immunity and host response to hypoxia also make these genes and gene products attractive molecular targets to treat immunity and inflammation disorders, such as septic shock and ischemic tissue damage.

Targeted disruption of NBS1 reveals its roles in mouse development and DNA repair

Nijmegen breakage syndrome (NBS) is an autosomal recessive hereditary disease that shares some common defects with ataxia-telangiectasia. The gene product mutated in NBS, named NBS1, is a component of the Mre11 complex that is involved in DNA strand-break repair. To elucidate the physiological roles of NBS1, we disrupted the N-terminal exons of the NBS1 gene in mice. NBS1(m/m) mice are viable, growth retarded and hypersensitive to ionizing radiation (IR). NBS1(m/m) mice exhibit multiple lymphoid developmental defects, and rapidly develop thymic lymphoma. In addition, female NBS1(m/m) mice are sterile due to oogenesis failure. NBS1(m/m) cells are impaired in cellular responses to IR and defective in cellular proliferation. Most systematic and cellular defects identified in NBS1(m/m) mice recapitulate those in NBS patients, and are essentially identical to those observed in Atm(-/-) mice. In contrast to Atm(-/-) mice, spermatogenesis is normal in NBS1(m/m) mice, indicating that distinct roles of ATM have differential requirement for NBS1 activity. Thus, NBS1 and ATM have overlapping and distinct functions in animal development and DNA repair.

ATM: genome stability, neuronal development, and cancer cross paths

One of the cornerstones of the web of signaling pathways governing cellular life and differentiation is the DNA damage response. It spans a complex network of pathways, ranging from DNA repair to modulation of numerous processes in the cell. DNA double-strand breaks (DSBs), which are formed as a result of genotoxic stress or normal recombinational processes, are extremely lethal lesions that rapidly mobilize this intricate defense system. The master controller that pilots cellular responses to DSBs is the ATM protein kinase, which turns on this network by phosphorylating key players in its various branches. ATM is the protein product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, sterility, genomic instability, cancer predisposition, and radiation sensitivity. The clinical and cellular phenotype of A-T attests to the numerous roles of ATM, on the one hand, and to the link between the DNA damage response and developmental processes on the other hand. Recent studies of this protein and its effectors, combined with a thorough investigation of animal models of A-T, have led to new insights into the mode of action of this master controller of the DNA damage response. The evidence that ATM is involved in signaling pathways other than those related to damage response, particularly ones relating to cellular growth and differentiation, reinforces the multifaceted nature of this protein, in which genome stability, developmental processes, and cancer cross paths.

Reovirus-induced alterations in gene expression related to cell cycle regulation

Mammalian reovirus infection results in perturbation of host cell cycle progression. Since reovirus infection is known to activate cellular transcription factors, we investigated alterations in cell cycle-related gene expression following HEK293 cell infection by using the Affymetrix U95A microarray. Serotype 3 reovirus infection results in differential expression of 10 genes classified as encoding proteins that function at the G(1)-to-S transition, 11 genes classified as encoding proteins that function at G(2)-to-M transition, and 4 genes classified as encoding proteins that function at the mitotic spindle checkpoint. Serotype 1 reovirus infection results in differential expression of four genes classified as encoding proteins that function at the G(1)-to-S transition and three genes classified as encoding proteins that function at G(2)-to-M transition but does not alter any genes classified as encoding proteins that function at the mitotic spindle checkpoint. We have previously shown that serotype 3, but not serotype 1, reovirus infection induces a G(2)-to-M transition arrest resulting from an inhibition of cdc2 kinase activity. Of the differentially expressed genes encoding proteins regulating the G(2)-to-M transition, chk1, wee1, and GADD45 are known to inhibit cdc2 kinase activity. A hypothetical model describing serotype 3 reovirus-induced inhibition of cdc2 kinase is presented, and reovirus-induced perturbations of the G(1)-to-S, G(2)-to-M, and mitotic spindle checkpoints are discussed.

Requirement of ATM in UVA-induced signaling and apoptosis

Solar UVA, but not UVC, reaches the earth's surface and therefore is an important etiological factor for the induction of human skin cancer. ATM kinase is an important regulator of cell survival and cell cycle checkpoints. Here, we observe that UVA, unlike UVC, triggers ATM kinase activity, and the activation may occur through reactive oxygen species produced after irradiation of cells with UVA. We also show that ATM activation is involved in the apoptotic response to UVA but not UVC. Furthermore, we provide evidence that ATM-dependent p53 and c-Jun N-terminal kinase (JNK) pathways are linked to UVA-induced apoptosis. On the other hand, UVC-induced apoptosis occurs through ATR-dependent p53 phosphorylation as well as the JNK pathway. Therefore, these results suggest that ATM, like p53, is involved in the UVA-induced apoptosis to suppress carcinogenesis.

CtBP, an unconventional transcriptional corepressor in development and oncogenesis

CtBP family proteins are conserved among vertebrates and invertebrates and function as transcriptional corepressors. They repress transcription in a histone deacetylase-dependent or -independent manner. CtBPs play important roles during development and oncogenesis. In this review, their unusual properties, the mechanisms of transcriptional repression, regulation, and their biological functions are discussed.

Cancer susceptibility and the functions of BRCA1 and BRCA2

Inherited mutations in BRCA1 or BRCA2 predispose to breast, ovarian, and other cancers. Their ubiquitously expressed protein products are implicated in processes fundamental to all cells, including DNA repair and recombination, checkpoint control of cell cycle, and transcription. Here, I examine what is known about the biological functions of the BRCA proteins and ask how their disruption can induce susceptibility to specific types of cancer.

A DNA damage-regulated BRCT-containing protein, TopBP1, is required for cell survival

BRCA1 carboxyl-terminal (BRCT) motifs are present in a number of proteins involved in DNA repair and/or DNA damage-signaling pathways. Human DNA topoisomerase II binding protein 1 (TopBP1) contains eight BRCT motifs and shares sequence similarity with the fission yeast Rad4/Cut5 protein and the budding yeast DPB11 protein, both of which are required for DNA damage and/or replication checkpoint controls. We report here that TopBP1 is phosphorylated in response to DNA double-strand breaks and replication blocks. TopBP1 forms nuclear foci and localizes to the sites of DNA damage or the arrested replication forks. In response to DNA strand breaks, TopBP1 phosphorylation depends on the ataxia telangiectasia mutated protein (ATM) in vivo. However, ATM-dependent phosphorylation of TopBP1 does not appear to be required for focus formation following DNA damage. Instead, focus formation relies on one of the BRCT motifs, BRCT5, in TopBP1. Antisense Morpholino oligomers against TopBP1 greatly reduced TopBP1 expression in vivo. Similar to that of ataxia telangiectasia-related protein (ATR), Chk1, or Hus1, downregulation of TopBP1 leads to reduced cell survival, probably due to increased apoptosis. Taken together, the data presented here suggest that, like its putative counterparts in yeast species, TopBP1 may be involved in DNA damage and replication checkpoint controls.

CHK2 kinase--a busy messenger

Checkpoint kinase 2 (Chk2) is emerging as a key mediator of diverse cellular responses to genotoxic stress, guarding the integrity of the genome throughout eukaryotic evolution. Recent studies show the fundamental role of Chk2 in the network of genome-surveillance pathways that coordinate cell-cycle progression with DNA repair and cell survival or death. Defects in Chk2 contribute to the development of both hereditary and sporadic human cancers, and earmark this kinase as a candidate tumour suppressor and an attractive target for drug discovery.

Mammalian G1- and S-phase checkpoints in response to DNA damage

The ability to preserve genomic integrity is a fundamental feature of life. Recent findings regarding the molecular basis of the cell-cycle checkpoint responses of mammalian cells to genotoxic stress have converged into a two-wave concept of the G1 checkpoint, and shed light on the so-far elusive intra-S-phase checkpoint. Rapidly operating cascades that target the Cdc25A phosphatase appear central in both the initiation wave of the G1 checkpoint (preceding the p53-mediated maintenance wave) and the transient intra-S-phase response. Multiple links between defects in the G1/S checkpoints, genomic instability and oncogenesis are emerging, as are new challenges and hopes raised by this knowledge.

Effect of DNA damage on a BRCA1 complex

The tumour-suppressor protein BRCA1 mediates its biological functions by interacting with cellular factors such as the CtIP polypeptide, a substrate for the ATM (for 'ataxia telangiectasia mutated') protein kinase. Li et al. report that the BRCA1-CtIP interaction is disrupted by ionizing radiation and by other genotoxic stresses that induce phosphorylation of CtIP by ATM kinase, and that this dissociation of the BRCA1-CtIP complex in turn modulates the transcription of DNA-damage-response genes. We have shown that the BRCA1-binding domain of CtIP (amino-acid residues 133-369) is distal to the sites that are phosphorylated by ATM kinase (residues S664 and S745). We now show that the BRCA1-CtIP complex is stable in irradiated cells, and that the phosphorylated isoforms of CtIP that are induced by ionizing radiation still interact in vivo with BRCA1. We conclude that disruption of the BRCA1-CtIP complex cannot account for induction of DNA-damage-response genes in the way proposed by Li et al.

ATM, a central controller of cellular responses to DNA damage

Mutations in the ATM gene lead to the genetic disorder ataxia-telangiectasia. ATM encodes a protein kinase that is mainly distributed in the nucleus of proliferating cells. Recent studies reveal that ATM regulates multiple cell cycle checkpoints by phosphorylating different targets at different stages of the cell cycle. ATM also functions in the regulation of DNA repair and apoptosis, suggesting that it is a central regulator of responses to DNA double-strand breaks.

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.