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

G-quadruplexes induce apoptosis in tumor cells

Several G-rich oligodeoxynucleotides (ODNs), which are capable of forming G-quadruplexes, have been shown to exhibit antiproliferative activity against tumor cell lines and antitumor activity in nude mice carrying prostate and breast tumor xenografts. However, the molecular basis for their antitumor activity remains unclear. In the current study, we showed that a variety of telomeric G-tail oligodeoxynucleotides (TG-ODNs) exhibited antiproliferative activity against many tumor cells in culture. Systematic mutational analysis of the TG-ODNs suggests that the antiproliferative activity depends on the G-quadruplex conformation of these TG-ODNs. TG-ODNs were also shown to induce poly(ADP-ribose) polymerase-1 cleavage, phosphatidylserine flipping, and caspase activation, indicative of induction of apoptosis. TG-ODN-induced apoptosis was largely ataxia telangiectasia mutated (ATM) dependent. Furthermore, TG-ODN-induced apoptosis was inhibited by the c-Jun NH(2)-terminal kinase (JNK) inhibitor SP600125. Indeed, TG-ODNs were shown to activate the JNK pathway in an ATM-dependent manner as evidenced by elevated phosphorylation of JNK and c-Jun. Interestingly, a number of G-quadruplex ODNs (GQ-ODN) derived from nontelomeric sequences also induced ATM/JNK-dependent apoptosis, suggesting a possible common mechanism of tumor cell killing by GQ-ODNs.

Identification of carboxyl-terminal MCM3 phosphorylation sites using polyreactive phosphospecific antibodies

The functionally related ATM (ataxia telangiectasia-mutated) and ATR (ATM-Rad3-related) protein kinases are critical regulators of DNA damage responses in mammalian cells. ATM and ATR share highly overlapping substrate specificities and show a strong preference for the phosphorylation of Ser or Thr residues followed by Gln. In this report we used a polyreactive phosphospecific antibody (alpha-pDSQ) that recognizes a subset of phosphorylated Asp-Ser-Gln sequences to purify candidate ATM/ATR substrates. This led to the identification of phosphorylation sites in the carboxyl terminus of the minichromosome maintenance protein 3 (MCM3), a component of the hexameric MCM DNA helicase. We show that the alpha-DSQ antibody recognizes tandem DSQ phosphorylation sites (Ser-725 and Ser-732) in the carboxyl terminus of murine MCM3 (mMCM3) and that ATM phosphorylates both sites in vitro. ATM phosphorylated the carboxyl termini of mMCM3 and human MCM3 in vivo and the phosphorylated form of MCM3 retained association with the canonical MCM complex. Although DNA damage did not affect steady-state levels of chromatin-bound MCM3, the ATM-phosphorylated form of MCM3 was preferentially localized to the soluble, nucleoplasmic fraction. This finding suggests that the carboxyl terminus of chromatin-loaded MCM3 may be sequestered from ATM-dependent checkpoint signals. Finally, we show that ATM and ATR jointly contribute to UV light-induced MCM3 phosphorylation, but that ATM is the predominant UV-activated MCM3 kinase in vivo. The carboxyl-terminal ATM phosphorylation sites are conserved in vertebrate MCM3 orthologs suggesting that this motif may serve important regulatory functions in response to DNA damage. Our findings also suggest that DSQ motifs are common phosphoacceptor motifs for ATM family kinases.

Dimerization of CtIP may stabilize in vivo interactions with the Retinoblastoma-pocket domain

CtIP is a tumor suppressor that interacts with Retinoblastoma protein (Rb) to regulate the G1/S-phase transition of the cell cycle. Despite its large size (897 residues) CtIP has few known structured regions. Rather it contains several linear motifs that interact with known binding partners, including an LXCXE motif that binds the pocket domain of Rb-family proteins. This LXCXE motif lies at the C-terminus of the only known structured domain, an N-terminal coiled-coil dimerization domain (DD; residues 45-160). Yeast two-hybrid (Y2H) and GST-pulldown analyses showed that CtIP requires the LXCXE motif to bind the Rb-pocket. Although isothermal titration calorimetry data indicates that the LXCXE motif is the sole determinant of binding affinity for the Rb-pocket domain (K(A) approximately 10(6)M(-1)), Y2H data indicates that the DD is required to stabilize the interaction in vivo. Thus dimerization may increase the apparent stability of the proteins and/or the lifetime of the complexes.

Poly(ADP-ribose) polymerase-1 plays a role in suppressing mammary tumourigenesis in mice

The DNA strand break-binding molecule, poly(ADP-ribose) polymerase-1 (PARP-1), plays a role in DNA repair, chromosomal stability, transcription and cell death. Accumulating evidence suggests that dysfunction of PARP-1 contributes to tumorigenesis. Here, we report that PARP-1 deficiency causes mammary carcinoma formation in female mice, and that the introduction of Trp53 mutations accelerates the onset and shortens the latency of mammary tumorigenesis. We show that PARP-1 deficiency results in chromosomal aneuploidy and centrosome amplification, which are substantiated by the inactivation of Trp53 in primary mammary epithelial (PME) cells. In addition, PARP-1 deficiency compromises p53 activation and impairs BRCA1 recruitment to the sites of DNA damage in PME cells. PARP-1 complementation partly rescues the defective DNA damage response mediated by p53 and BRCA1. The present study thus identifies a role of PARP-1 in suppressing mammary tumorigenesis in vivo and suggests that dysfunction of PARP-1 may be a risk factor for breast cancer in humans.

BRCA1 ubiquitylation of CtIP: Just the tIP of the iceberg?

Ubiquitylation is an important regulatory mechanism of many cellular processes. The breast and ovarian cancer-specific tumour suppressor BRCA1 is well acknowledged to be a RING/E3 ubiquitin ligase, however, identification of its physiological substrates has proved elusive. Recently published data have shown that the BRCA1-interacting protein CtIP is in fact ubiquitylated by BRCA1, and opens new avenues for the isolation of other substrate proteins.

The role of BRCA1 in transcriptional regulation and cell cycle control

The exact functions of BRCA1 have not been fully described but it now seems apparent that it has roles in DNA damage repair, transcriptional regulation, cell cycle control and most recently in ubiquitylation. These functions of BRCA1 are most likely interdependent but this review will focus on the role of BRCA1 in relation to transcriptional regulation and in particular how this impacts upon cell cycle control. We will (i) describe the structure of BRCA1 and how it may contribute to its transcription function; (ii) describe the interaction of BRCA1 with the core transcriptional machinery (RNA polII); (iii) describe how BRCA1 may regulate transcription at an epigenetic level through chromatin modification; (iv) discuss the role of BRCA1 in modulating transcription through its association with sequence-specific transcription factors. Finally, we will discuss the possible effects of BRCA1 transcriptional regulation on downstream targets with known roles in cell cycle control.

The role of ATM in breast cancer development

Complete or partial inability to sense and repair DNA damage increases the risk of developing cancer. The ataxia telangiectasia mutated (ATM) protein kinase has a crucial role in response to DNA double-strand breaks. Hereditary mutations in the ATM gene are the cause of a rare genomic instability syndrome ataxia telangiectasia (AT) characterized, among others, by elevated cancer risk. Although clear in homozygotes, numerous studies have failed to find a link between heterozygotes and cancer. However, there is increasing evidence that ATM heterozygotes have an increased risk of developing breast cancer. First, epidemiological studies conferred an increased risk of breast cancer among AT relatives. Second, in vitro studies of heterozygous cells provide strong evidence of hyperradiosensitivity. Third, some clinical studies found an increased frequency of ATM mutations among high-risk breast cancer families.

BRCA1 at the crossroad of multiple cellular pathways: approaches for therapeutic interventions

Approximately 10% of the cases of breast cancer and invasive ovarian cancer are hereditary, occurring predominantly in women with germ-line mutations in the BRCA1 or BRCA2 genes. Low expression of these genes in sporadic tumors extends their significance to sporadic breast and ovarian cancers as well. For over a decade since its identification, extensive research has been directed toward understanding the function of the breast and ovarian tumor suppressor gene BRCA1. The long-term goal has been to identify the biochemical pathways reliant on BRCA1 that can be exploited for developing targeted therapies and benefit mutation carriers. To date, no one specific role has been identified, but rather it is clear that BRCA1 has significant roles in multiple fundamental cellular processes, including control of gene expression, chromatin remodeling, DNA repair, cell cycle checkpoint control, and ubiquitination, and overall is important for maintenance of genomic stability. Major findings and potential BRCA1-dependent therapies will be discussed.

Functional assays for BRCA1 and BRCA2

Genetic testing for the two major breast cancer susceptibility genes has now been available for several years with more than 70,000 people tested in the USA alone. While the current genetic testing identifies many sequence alterations there are problems with both sensitivity and specificity of the assay. In particular, the genetic testing is limited in its ability to determine which of the many missense mutations identified in BRCA1 and BRCA2 actually predispose to cancer and which are simply neutral alterations. Here we will focus on the limitations in test result interpretation and we will explore how biochemistry and cell biology can help to clarify these issues. Although we limit our discussion to genetic testing of BRCA1 and BRCA2, the problem is common to an expanding group of genes, including ATM and MSH2, in which germ-line missense mutations may also confer increased risk of cancer. Here we advocate the use of functional assays to complement genetic data in the analysis of unclassified missense mutations and propose a set of standards to conduct and interpret these assays.

Computational exploration of the activated pathways associated with DNA damage response in breast cancer

Molecular signaling events regulate cellular activity. Cancer stimulating signals trigger cellular responses that evade the regulatory control of cell development. To understand the mechanism of signaling regulation in cancer, it is necessary to identify the activated pathways in cancer. We have developed RepairPATH, a computational algorithm that explores the activated signaling pathways in cancer. The RepairPATH integrates RepairNET, an assembled protein interaction network associated with DNA damage response, with the gene expression profiles derived from the microarray data. Based on the observation that cofunctional proteins often exhibit correlated gene expression profiles, it identifies the activated signaling pathways in cancer by systematically searching the RepairNET for proteins with significantly correlated gene expression profiles. Analyzing the gene expression profiles of breast cancer, we found distinct similarities and differences in the activated signaling pathways between the samples from the patients who developed metastases and the samples from the patients who were disease free within 5 years. The cellular pathways associated with the various DNA repair mechanisms and the cell-cycle checkpoint controls are found to be activated in both sample groups. One of the most intriguing findings is that the pathways associated with different cellular processes are functionally coordinated through BRCA1 in the disease-free sample group, whereas such functional coordination is absent in the samples from patients who developed metastases. Our analysis revealed the potential cellular pathways that regulate the signaling events in breast cancer.

Removal of BRCA1/CtIP/ZBRK1 repressor complex on ANG1 promoter leads to accelerated mammary tumor growth contributed by prominent vasculature

BRCA1 exerts transcriptional repression through interaction with CtIP in the C-terminal BRCT domain and ZBRK1 in the central domain. A dozen genes, including angiopoietin-1 (ANG1), a secreted angiogenic factor, are corepressed by BRCA1 and CtIP based on microarray analysis of mammary epithelial cells in 3D culture. BRCA1, CtIP, and ZBRK1 form a complex that coordinately represses ANG1 expression via a ZBRK1 recognition site in the ANG1 promoter. Impairment of this complex upregulates ANG1, which stabilizes endothelial cells that form a capillary-like network structure. Consistently, Brca1-deficient mouse mammary tumors exhibit accelerated growth, pronounced vascularization, and overexpressed ANG1. These results suggest that, besides its role in maintaining genomic stability, BRCA1 directly regulates the expression of angiogenic factors to modulate the tumor microenvironment.

CtIP activates its own and cyclin D1 promoters via the E2F/RB pathway during G1/S progression

Cell cycle progression from G(1) to S phase is mainly controlled by E2F transcription factors and RB family proteins. Previously we showed that the presence of CtIP is essential for G(1)/S transition in primary mouse blastocysts, as well as in NIH 3T3 cells. However, how CtIP executes this function remains to be elucidated. Here we show that in NIH 3T3 cells the expression of CtIP is regulated by the E2F/RB pathway during late G(1) and S phases. The presence of wild-type CtIP, but not the E157K mutant form, which failed to interact with RB, enhanced its own promoter activity. Chromatin immunoprecipitation analysis indicated that the recruitment of CtIP to its promoter occurs concomitantly with TFIIB, a component of the RNA polymerase II complex, and with dissociation of RB from the promoter during late G(1) and G(1)/S transition. Similar positive regulation of cyclin D1 expression by CtIP was also observed. Consistently, cells expressing the CtIP(E157K) protein alone exhibited growth retardation, an increase in the G(1) population, and a decrease in the S-phase population. Taken together, these results suggest that, contrary to the postulated universal corepressor role, CtIP activates a subset of E2F-responsive promoters by releasing RB-imposed repression and therefore promotes G(1)/S progression.

ATM Activation by Ionizing Radiation Requires BRCA1-associated BAAT1

ATM (ataxia telangiectasia mutated) is required for the early response to DNA-damaging agents such as ionizing radiation (IR) that induce DNA double-strand breaks. Cells deficient in ATM are extremely sensitive to IR. It has been shown that IR induces immediate phosphorylation of ATM at Ser(1981), leading to catalytic activation of the protein. We recently isolated a novel BRCA1-associated protein, BAAT1 (BRCA1-associated protein required for ATM activation-1), by yeast two-hybrid screening and found that BAAT1 also binds to ATM, localizes to double-strand breaks, and is required for Ser(1981) phosphorylation of ATM. Small interfering RNA-mediated stable or transient reduction of BAAT1 resulted in decreased phosphorylation of both ATM at Ser(1981) and CHK2 at Thr(68). Treatment of BAAT1-depleted cells with okadaic acid greatly restored phosphorylation of ATM at Ser(1981), suggesting that BAAT1 is involved in the regulation of ATM phosphatase. Protein phosphatase 2A-mediated dephosphorylation of ATM was partially blocked by purified BAAT1 in vitro. Significantly, acute loss of BAAT1 resulted in increased p53, leading to apoptosis. These results demonstrate that DNA damage-induced ATM activation requires a coordinated assembly of BRCA1, BAAT1, and ATM.

Ataxia telangiectasia mutated and checkpoint kinase 2 regulate BRCA1 to promote the fidelity of DNA end-joining

Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are the two mechanisms responsible for repairing DNA double-strand breaks (DSBs) and act in either a collaborative or competitive manner in mammalian cells. DSB repaired by NHEJ may be more complicated than the simple joining of the ends of DSB, because, if nucleotides were lost, it would result in error-prone repair. This has led to the proposal that a subpathway of precise NHEJ exists that can repair DSBs with higher fidelity; this is supported by recent findings that the expression of the HR gene, BRCA1, is causally linked to in vitro and in vivo precise NHEJ activity. To further delineate this mechanism, the present study explored the connection between NHEJ and the cell-cycle checkpoint proteins, ataxia telangiectasia mutated (ATM) and checkpoint kinase 2 (Chk2), known to be involved in activating BRCA1, and tested the hypothesis that ATM and Chk2 promote precise end-joining by BRCA1. Support for this hypothesis came from the observations that (a) knockdown of ATM and Chk2 expression affected end-joining activity; (b) in BRCA1-defective cells, precise end-joining activity was not restored by a BRCA1 mutant lacking the site phosphorylated by Chk2 but was restored by wild-type BRCA1 or a mutant mimicking phosphorylation by Chk2; (c) Chk2 mutants lacking kinase activity or with a mutation at a site phosphorylated by ATM had a dominant negative effect on precise end-joining in BRCA1-expressing cells. These results suggest that the other two HR regulatory proteins, ATM and Chk2, act jointly to regulate the activity of BRCA1 in controlling the fidelity of DNA end-joining by precise NHEJ.

C-terminal binding proteins: Emerging roles in cell survival and tumorigenesis

Within a cell, the levels and activity of multiple pro- and anti-apoptotic molecules act in concert to regulate commitment to apoptosis. Whilst the balance between survival and death can be tipped by the effects of single molecules, cellular apoptosis control pathways very often incorporate key transcription factors that co-ordinately regulate the expression of multiple apoptosis control genes. C-terminal binding proteins (CtBPs), which were originally identified through their binding to the Adenovirus E1A oncoprotein, have been described as such transcriptional regulators of the apoptosis program. Specifically, CtBPs function as transcriptional co-repressors, and have been demonstrated to promote cell survival by suppressing the expression of several pro-apoptotic genes. In this review we summarize the evidence supporting a key role for CtBP proteins in cell survival. We also describe the known mechanisms of transcriptional control by CtBPs, and review the multiplicity of intracellular signaling and transcriptional control pathways with which they are known to be involved. Finally we consider these findings in the context of additional known roles of CtBP molecules, and the potential implications that this combined knowledge may have for our comprehension of diseases of cell survival, notably cancer.

An estrogen receptor-alpha/p300 complex activates the BRCA-1 promoter at an AP-1 site that binds Jun/Fos transcription factors: repressive effects of p53 on BRCA-1 transcription

One of the puzzles in cancer predisposition is that women carrying BRCA-1 mutations preferentially develop tumors in epithelial tissues of the breast and ovary. Moreover, sporadic breast tumors contain lower levels of BRCA-1 in the absence of mutations in the BRCA-1 gene. The problem of tissue specificity requires analysis of factors that are unique to tissues of the breast. For example, the expression of estrogen receptor-alpha (ER alpha) is inversely correlated with breast cancer risk, and 90% of BRCA-1 tumors are negative for ER alpha. Here, we show that estrogen stimulates BRCA-1 promoter activity in transfected cells and the recruitment of ER alpha and its cofactor p300 to an AP-1 site that binds Jun/Fos transcription factors. The recruitment of ER alpha/p300 coincides with accumulation in the S-phase of the cell cycle and is antagonized by the antiestrogen tamoxifen. Conversely, we document that overexpression of wild-type p53 prevents the recruitment of ER alpha to the AP-1 site and represses BRCA-1 promoter activity. Taken together, our findings support a model in which an ER alpha/AP-1 complex modulates BRCA-1 transcription under conditions of estrogen stimulation. Conversely, the formation of this transcription complex is abrogated in cells overexpressing p53.

RNF11 is a multifunctional modulator of growth factor receptor signalling and transcriptional regulation

Our laboratory has found that the 154aa RING finger protein 11 (RNF11), has modular domains and motifs including a RING-H2 finger domain, a PY motif, an ubiquitin interacting motif (UIM), a 14-3-3 binding sequence and an AKT phosphorylation site. RNF11 represents a unique protein with no other known immediate family members yet described. Comparative genetic analysis has shown that RNF11 is highly conserved throughout evolution. This may indicate a conserved and non-redundant role for the RNF11 protein. Molecular binding assays using RNF11 have shown that RNF11 has important roles in growth factor signalling, ubiquitination and transcriptional regulation. RNF11 has been shown to interact with HECT-type E3 ubiquitin ligases Nedd4, AIP4, Smurf1 and Smurf2, as well as with Cullin1, the core protein in the multi-subunit SCF E3 ubiquitin ligase complex. Work done in our laboratory has shown that RNF11 is capable of antagonizing Smurf2-mediated inhibition of TGFbeta signalling. Furthermore, RNF11 is capable of degrading AMSH, a positive regulator of both TGFbeta and EGFR signalling pathways. Recently, we have found that RNF11 can directly enhance TGFbeta signalling through a direct association with Smad4, the common signal transducer and transcription factor in the TGFbeta, BMP, and Activin pathways. Through its association with Smad4 and other transcription factors, RNF11 may have a role in direct transcriptional regulation. Our laboratory and others have found nearly 80 protein interactions for RNF11, placing RNF11 at the cross-roads of cell signalling and transcriptional regulation. RNF11 is highly expressed in breast tumours. Deregulation of RNF11 function may prove to be harmful to patient therapeutic outcomes. RNF11 may therefore provide a novel target for cancer therapeutics. The purpose of this review is to discuss the role of RNF11 in cell signalling and transcription factor modulation with special attention given to the ubiquitin-proteasomal pathway, TGFbeta pathway and EGFR pathway.

CtIP, a candidate tumor susceptibility gene is a team player with luminaries

CtIP is a nuclear protein conserved among vertebrates that was discovered as a cofactor of the transcriptional corepressor CtBP. CtIP also interacts with the tumor suppressors such as BRCA1 and the pRb family members through binding sites that are frequently mutated in human cancers. CtIP is a target for BRCA1-dependent phosphorylation by the ATM kinase induced by DNA double strand breakage. CtIP plays a role in DNA-damage-induced cell cycle checkpoint control at the G2/M transition. Homozygous inactivation of the Ctip gene causes very early embryonic lethality during mouse development. The Ctip(-/-) embryo cells are arrested in G1 and do not enter S phase. Depletion of Ctip in established mouse embryo fibroblasts arrests cells in G1 and results in an accumulation of hypophosphorylated Rb and the Cdk inhibitor p21, suggesting that CtIP is also a critical regulator of G1/S transition of the cell cycle. The Ctip gene contains a mononucleotide (A9) repeat and one of the alleles is mutated at a high frequency in colon cancers with microsatellite instability. The Ctip(+/-) mice develop multiple types of tumors suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Among the various tumor types observed in Ctip(+/-) heterozygous mice, large lymphomas are prevalent. Recent studies raise the possibility that Ctip may itself be a tumor susceptibility gene and suggest that it might be important for the activities of tumor suppressors BRCA1, pRb family proteins and Ikaros family members.

High mobility group A2 potentiates genotoxic stress in part through the modulation of basal and DNA damage-dependent phosphatidylinositol 3-kinase-related protein kinase activation

The high mobility group A2 (HMGA2) protein belongs to the architectural transcription factor HMGA family, playing a role in chromosomal organization and transcriptional regulation. We and others have previously reported that ectopic HMGA2 expression is associated with neoplastic transformation and anchorage-independent cell proliferation. Here, we reported a correlation between increased HMGA2 expression and enhanced chemosensitivity towards topoisomerase II inhibitor, doxorubicin, in breast cancer cells. Using cells exhibiting differential HMGA2 expression and small interfering RNA technique, we showed that HMGA2 expression modulates cellular response to the genotoxicity of DNA double-strand breaks. Notably, HMGA2 enhances doxorubicin-elicited cell cycle delay in sub-G1 and G2-M and augments cell cycle dysregulation on cotreatment of doxorubicin and caffeine. We further reported that HMGA2 induces a persistent Ser139 phosphorylation of histone 2A variant X, analogous to the activation by doxorubicin-mediated genotoxic stress. Moreover, this HMGA2-dependent enhancement of cytotoxicity is further extended to other double-strand breaks elicited by cisplatin and X-ray irradiation and is not restricted to one cell type. Together, we postulated that the enhanced cytotoxicity by double-strand breaks in HMGA2-expressing cells is mediated, at least in part, through the signaling pathway of which the physiologic function is to maintain genome integrity. These findings should contribute to a greater understanding of the role of HMGA2 in promoting tumorigenesis and conveying (chemo)sensitivity towards doxorubicin and other related double-strand breaks.

SQ/TQ cluster domains: concentrated ATM/ATR kinase phosphorylation site regions in DNA-damage-response proteins

ATM/ATR-like protein kinases play central roles in the maintenance of genome stability and phosphorylate numerous substrates in response to DNA damage, preferentially on SQ or TQ motifs. ATM/ATR substrates often contain several closely spaced SQ/TQ motifs in regions that have been termed SQ/TQ cluster domains (SCDs). SCDs are now considered a structural hallmark of DNA-damage-response proteins. Mutational analyses of a number of SCD-containing proteins indicate that multisite phosphorylation of SQ/TQ motifs is required for normal DNA-damage responses, most commonly by mediating protein-protein interactions in the formation of DNA-damage-induced complexes. SCD sequences are highly diverse and these domains may be largely unfolded in their native state rather than adopting a common three-dimensional fold. Structural disorder of SCDs could be advantageous for efficient phosphorylation by ATM/ATR kinases and also enable them to be molded into distinct conformations to facilitate flexible interactions with multiple binding partners.

Interactions between BRCT repeats and phosphoproteins: tangled up in two

The C-terminal region of the breast-cancer-associated protein BRCA1 contains a pair of tandem BRCA1 C-terminal (BRCT) repeats that are essential for the tumour suppressor function of the protein. Similar repeat sequences have been identified in many proteins that seem to mediate cellular mechanisms for dealing with DNA damage. The BRCT domain in BRCA1 has been recently shown to constitute a module for recognizing phosphorylated (phospho-) peptides, with a recognition groove that spans both BRCT repeats. The fact that many other BRCT-containing proteins have phospho-peptide binding activity suggests that BRCT repeats might mediate phosphorylation-dependent protein-protein interactions in processes that are central to cell-cycle checkpoint and DNA repair functions.

Evidence for complex multigenic inheritance of radiation AML susceptibility in mice revealed using a surrogate phenotypic assay

The mapping of genes which affect individual cancer risk is an important but complex challenge. A surrogate assay of susceptibility to radiation-induced acute myeloid leukaemia (AML) in the mouse based on chromosomal radiosensitivity has been developed and validated. This assay was applied to the mapping of radiation-induced AML risk modifier loci by association with microsatellite markers. A region on chromosome (chr) 18 with strong association is identified and confirmed by backcross analysis. Additional loci on chrs 8 and 13 show significant association. A key candidate gene Rbbp8 on chr18 is identified. Rbbp8 is shown to be upregulated in response to X-irradiation in the AML sensitive CBA strain but not AML resistant C57BL/6 strain. This study demonstrates the strength of utilizing surrogate endpoints of cancer susceptibility in the mapping of mouse loci and identifies additional loci that may affect radiation cancer risk.

Molecular basis of ataxia telangiectasia and related diseases

Ataxia telangiectasia (AT) is a rare human disease characterized by extreme cellular sensitivity to radiation and a predisposition to cancer, with a hallmark of onset in early childhood. Several human diseases also share similar symptoms with AT albeit with different degrees of severity and different associated disorders. While all AT patients contain mutations in the AT-mutated gene (ATM), most other AT-like disorders are defective in genes encoding an MRN protein complex consisting of Mre11, Rad50 and Nbs1. Both ATM and MRN function as cellular sensors to DNA double-strand breaks, which lead to the recruitment and phosphorylation of an array of substrate proteins involved in DNA repair, apoptosis and cell-cycle checkpoints, as well as gene regulation, translation initiation and telomere maintenance. ATM is a member of the family of phosphatidylinositol 3-kinase-like protein kinases (PIKK), and the discovery of many ATM substrates provides the underlying mechanisms of heterologous symptoms among AT patients. This review article focuses on recent findings related to the initial recognition of double-strand breaks by ATM and MRN, as well as a DNA-dependent protein kinase complex consisting of the heterodimer Ku70/Ku80 and its catalytic subunit DNA-PKcs, another member of PIKK. This possible interaction suggests that a much greater complex is involved in sensing, transducing and co-ordinating cellular events in response to genome instability.

The GADD45, ZBRK1 and BRCA1 pathway: quantitative analysis of mRNA expression in colon carcinomas

GADD45 is a growth arrest-associated gene that is induced in response to DNA damage. This gene is a target for coordinate regulation by both ZBRK1 and BRCA1. A sequence within intron 3 of GADD45 supports specific assembly of the ZBRK1/BRCA1 complex. In this study, the relationships between GADD45, ZBRK1, and BRCA1 expression were investigated in colon carcinomas. mRNA expression of these three genes was analysed in 116 colon carcinomas by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Genetic and epigenetic changes that could alter expression of these genes were studied. Possible relationships between expression levels of GADD45, ZBRK1, and BRCA1, and a series of clinicopathological parameters classically associated with poor prognosis, were also examined. ZBRK1 showed a tendency towards underexpression, while GADD45 and BRCA1 were generally overexpressed. A direct relationship between these three genes was observed, with the exception of BRCA1 expression levels, similar to normal tissues, which showed a tendency to be associated with low levels of GADD45 mRNA. Concomitantly altered expression of ZBRK1 and BRCA1 was associated with GADD45 mRNA expression. Promoter hypermethylation was not observed in GADD45 or BRCA1, and no mutations in GADD45 or ZBRK1 were found in regions involved in the interaction between the GADD45 gene and the ZBRK1 and BRCA1 proteins. No clinicopathological parameter was correlated with altered GADD45 or ZBRK1 expression but there was a statistically significant relationship between BRCA1 levels and the sex of patients. In conclusion, these results suggest that this pathway, involved in the response to DNA damage, is deregulated in colon carcinomas, and concomitantly altered expression of ZBRK1 and BRCA1 has an additive effect on GADD45 regulation. This is the first study in human carcinomas to analyse the relationships between expression of GADD45, ZBRK1, and BRCA1 mRNA.

Inactivation of CtIP leads to early embryonic lethality mediated by G1 restraint and to tumorigenesis by haploid insufficiency

CtIP interacts with a group of tumor suppressor proteins including RB (retinoblastoma protein), BRCA1, Ikaros, and CtBP, which regulate cell cycle progression through transcriptional repression as well as chromatin remodeling. However, how CtIP exerts its biological function in cell cycle progression remains elusive. To address this issue, we generated an inactivated Ctip allele in mice by inserting a neo gene into exon 5. The corresponding Ctip(-/-) embryos died at embryonic day 4.0 (E4.0), and the blastocysts failed to enter S phase but accumulated in G(1), leading to a slightly elevated cell death. Mouse NIH 3T3 cells depleted of Ctip were arrested at G(1) with the concomitant increase in hypophosphorylated Rb and Cdk inhibitors, p21. However, depletion of Ctip failed to arrest Rb(-/-) mouse embryonic fibroblasts (MEF) or human osteosarcoma Saos-2 cells at G(1), suggesting that this arrest is RB dependent. Importantly, the life span of Ctip(+/-) heterozygotes was shortened by the development of multiple types of tumors, predominantly, large lymphomas. The wild-type Ctip allele and protein remained detectable in these tumors, suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Taken together, this finding uncovers a novel G(1)/S regulation in that CtIP counteracts Rb-mediated G(1) restraint. Deregulation of this function leads to a defect in early embryogenesis and contributes, in part, to tumor formation.

The haploinsufficient tumor suppressor p18 upregulates p53 via interactions with ATM/ATR

p18 was first identified as a factor associated with a macromolecular tRNA synthetase complex. Here we describe the mouse p18 loss-of-function phenotype and a role for p18 in the DNA damage response. Inactivation of both p18 alleles caused embryonic lethality, while heterozygous mice showed high susceptibility to spontaneous tumors. p18 was induced and translocated to the nucleus in response to DNA damage. Expression of p18 resulted in elevated p53 levels, while p18 depletion blocked p53 induction. p18 directly interacted with ATM/ATR in response to DNA damage. The activity of ATM was dependent on the level of p18, suggesting the requirement of p18 for the activation of ATM. Low p18 expression was frequently observed in different human cancer cell lines and tissues. These results suggest that p18 is a haploinsufficient tumor suppressor and a key factor for ATM/ATR-mediated p53 activation.

COBRA1 inhibits AP-1 transcriptional activity in transfected cells

Mutations in the breast cancer susceptibility gene (BRCA1) account for a significant proportion of hereditary breast and ovarian cancers. Cofactor of BRCA1 (COBRA1) was isolated as a BRCA1-interacting protein and exhibited a similar chromatin reorganizing activity to that of BRCA1. However, the biological role of COBRA1 remains largely unexplored. Here, we report that ectopic expression of COBRA1 inhibited activator protein 1 (AP-1) transcriptional activity in transfected cells in a dose-dependent manner, whereas reduction of endogenous COBRA1 with a small interfering RNA significantly enhanced AP-1-mediated transcriptional activation. COBRA1 physically interacted with the AP-1 family members, c-Jun and c-Fos, and the middle region of COBRA1 bound to c-Fos. Lack of c-Fos binding site in the COBRA1 completely abolished the COBRA1 inhibition of AP-1 trans-activation. These findings suggest that COBRA1 may directly modulate AP-1 pathway and, therefore, may play important roles in cell proliferation, differentiation, apoptosis, and oncogenesis.

The emerging role of E2F-1 in the DNA damage response and checkpoint control

Genotoxic stress triggers a myriad of cellular responses including cell cycle arrest, stimulation of DNA repair and apoptosis. A central role for the E2F-1 transcription factor in the DNA damage response pathway is gaining support. E2F-1 is phosphorylated by DNA damage responsive protein kinases, which leads to E2F-1 accumulation and the induction of apoptosis. In addition, emerging information suggests that E2F-1 may play a role in the detection and subsequent repair of damaged DNA.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. It has been proposed that the occurrence and severity of the adverse reactions to radiation therapy are also influenced by such genetic susceptibility. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation offering a better likelihood of cure for malignant tumours. This paper, the first of two parts, reviews the main mechanisms involved in cell response to ionising radiation. DNA repair machinery and cell signalling pathways are considered and their role in radiosensitivity is analysed. The implication of non-targeted and delayed effects in radiosensitivity is also discussed.

Role of BRCA1 and BRCA2 as regulators of DNA repair, transcription, and cell cycle in response to DNA damage

BRCA1 (BReast-CAncer susceptibility gene 1) and BRCA2 are tumor suppressor genes, the mutant phenotypes of which predispose to breast and ovarian cancers. Intensive research has shown that BRCA proteins are involved in a multitude of pivotal cellular processes. In particular, both genes contribute to DNA repair and transcriptional regulation in response to DNA damage. Recent studies suggest that BRCA proteins are required for maintenance of chromosomal stability, thereby protecting the genome from damage. New data also show that BRCAs transcriptionally regulate some genes involved in DNA repair, the cell cycle, and apoptosis. Many of these functions are mediated by a large number of cellular proteins that interact with BRCAs. The functions of BRCA proteins are also linked to distinct and specific phosphorylation events; however, the extent to which phosphorylation-activated molecular pathways contribute to tumor suppressor activity remains unclear. Finally, the reasons why mutations in BRCA genes lead to the development of breast and ovarian cancers are not clearly understood. Elucidation of the precise molecular functions of BRCAs is expected to improve our understanding of hereditary as well as sporadic mammary carcinogenesis.

DNA damage-induced cell cycle checkpoint control requires CtIP, a phosphorylation-dependent binding partner of BRCA1 C-terminal domains

The BRCA1 C-terminal (BRCT) domain has recently been implicated as a phospho-protein binding domain. We demonstrate here that a CTBP-interacting protein CtIP interacts with BRCA1 BRCT domains in a phosphorylation-dependent manner. The CtIP/BRCA1 complex only exists in G(2) phase and is required for DNA damage-induced Chk1 phosphorylation and the G(2)/M transition checkpoint. However, the CtIP/BRCA1 complex is not required for the damage-induced G(2) accumulation checkpoint, which is controlled by a separate BRCA1/BACH1 complex. Taken together, these data not only implicate CtIP as a critical player in cell cycle checkpoint control but also provide molecular mechanisms by which BRCA1 controls multiple cell cycle transitions after DNA damage.

Prediction of hormone sensitivity by DNA microarray

Endocrine-therapy continues to be extensively developed for treatment of breast cancer, and accurate therapeutic prediction of this hormone-associated cancer is strongly desired. Moreover, the role of estrogen and its receptor on the estrogen-dependent growth of breast cancer cells has not been clarified hitherto. Thus, to develop a new diagnostic tool for endocrine-therapy, and to address the molecular mechanism of estrogen-dependent breast carcinogenesis, we investigated the gene expression profile of estrogen-responsive genes in breast cancer using DNA microarray technique. We first comprehensively analyzed the profile of estrogen responsiveness among several estrogen receptor (ER)-positive cancer cell lines by a large-scale DNA microarray. Based on the obtained information, a total of 138 genes which showed high induction or repression of the expression by estrogen stimulation were selected and provided for custom microarray. The results of the custom microarray analysis were consistent with those of large-scale microarray analysis, and revealed that they were clearly categorized into early- or late-response types. Further analysis of these genes may provide new clues in the elucidation of the estrogen-dependent growth mechanisms of cancer. Furthermore, the custom microarray analysis of ER-positive breast cancer tissues also showed similar but not identical profiles to those of cell lines, indicating the potential of this custom microarray to predict the response to endocrine-therapy in the breast cancer. Moreover, in order to discover the new predictive factors for endocrine therapy in breast cancer patients, several candidate genes were selected and their expressions in breast cancer tissues were analyzed by real-time RT-PCR and by immunohistochemical technique. These studies could provide new clues for elucidation of the estrogen-dependent mechanisms of cancer and clinical benefit for patients.

A role for the Fanconi anemia C protein in maintaining the DNA damage-induced G2 checkpoint

Fanconi anemia (FA) is a complex, heterogeneous genetic disorder composed of at least 11 complementation groups. The FA proteins have recently been found to functionally interact with the cell cycle regulatory proteins ATM and BRCA1; however, the function of the FA proteins in cell cycle control remains incompletely understood. Here we show that the Fanconi anemia complementation group C protein (Fancc) is necessary for proper function of the DNA damage-induced G2/M checkpoint in vitro and in vivo. Despite apparently normal induction of the G2/M checkpoint after ionizing radiation, murine and human cells lacking functional FANCC did not maintain the G2 checkpoint as compared with wild-type cells. The increased rate of mitotic entry seen in Fancc-/-mouse embryo fibroblasts correlated with decreased inhibitory phosphorylation of cdc2 kinase on tyrosine 15. An increased inability to maintain the DNA damage-induced G2 checkpoint was observed in Fancc -/-; Trp53 -/-cells compared with Fancc -/-cells, indicating that Fancc and p53 cooperated to maintain the G2 checkpoint. In contrast, genetic disruption of both Fancc and Atm did not cooperate in the G2 checkpoint. These data indicate that Fancc and p53 in separate pathways converge to regulate the G2 checkpoint. Finally, fibroblasts lacking FANCD2 were found to have a G2 checkpoint phenotype similar to FANCC-deficient cells, indicating that FANCD2, which is activated by the FA complex, was also required to maintain the G2 checkpoint. Because a proper checkpoint function is critical for the maintenance of genomic stability and is intricately related to the function and integrity of the DNA repair process, these data have implications in understanding both the function of FA proteins and the mechanism of genomic instability in FA.

Role of cell cycle in mediating sensitivity to radiotherapy

Multiple pathways are involved in maintaining the genetic integrity of a cell after its exposure to ionizing radiation. Although repair mechanisms such as homologous recombination and nonhomologous end-joining are important mammalian responses to double-strand DNA damage, cell cycle regulation is perhaps the most important determinant of ionizing radiation sensitivity. A common cellular response to DNA-damaging agents is the activation of cell cycle checkpoints. The DNA damage induced by ionizing radiation initiates signals that can ultimately activate either temporary checkpoints that permit time for genetic repair or irreversible growth arrest that results in cell death (necrosis or apoptosis). Such checkpoint activation constitutes an integrated response that involves sensor (RAD, BRCA, NBS1), transducer (ATM, CHK), and effector (p53, p21, CDK) genes. One of the key proteins in the checkpoint pathways is the tumor suppressor gene p53, which coordinates DNA repair with cell cycle progression and apoptosis. Specifically, in addition to other mediators of the checkpoint response (CHK kinases, p21), p53 mediates the two major DNA damage-dependent cellular checkpoints, one at the G(1)-S transition and the other at the G(2)-M transition, although the influence on the former process is more direct and significant. The cell cycle phase also determines a cell's relative radiosensitivity, with cells being most radiosensitive in the G(2)-M phase, less sensitive in the G(1) phase, and least sensitive during the latter part of the S phase. This understanding has, therefore, led to the realization that one way in which chemotherapy and fractionated radiotherapy may work better is by partial synchronization of cells in the most radiosensitive phase of the cell cycle. We describe how cell cycle and DNA damage checkpoint control relates to exposure to ionizing radiation.

The DNA double-strand break response pathway: becoming more BRCAish than ever

Breast carcinoma is the leading cause of cancer incidence, and second in cancer mortality to lung cancer, in women of the Western hemisphere. Germ line mutations in the breast cancer susceptibility gene, BRCA1, is responsible for half of all cases of hereditary breast cancer, which constitutes about 5-10% of all cases of breast cancer. Current hypothesis has ascribed a role for Brca1 in maintaining genomic stability, through its involvement in cellular response pathway to the DNA double-strand breaks (DSB). DNA DSB, which are the most deleterious form of DNA damage, are repaired through a series of coordinated steps embedded in a signal transduction pathway that ultimately ensure the elimination of potentially harmful mutations to the genome. This pathway can be crudely divided into a primary and secondary phase. The primary response phase is initiated by sensor proteins that activate transducer protein kinases Atm and Atr, which target downstream effector proteins, such as Chk1 and Chk2, to elicit the secondary response phase. Brca1 has been intimately linked with various aspects of this signaling pathway. However, the precise role of Brca1 in this process remains unclear. In this review, we will provide a simple model in an attempt to clarify the role of Brca1 during cellular response to DNA DSB.

ATM-dependent CHK2 activation induced by anticancer agent, irofulven

Irofulven (6-hydroxymethylacylfulvene, HMAF, MGI 114) is one of a new class of anticancer agents that are semisynthetic derivatives of the mushroom toxin illudin S. Preclinical studies and clinical trials have demonstrated that irofulven is effective against several tumor types. Mechanisms of action studies indicate that irofulven induces DNA damage, MAPK activation, and apoptosis. In this study we found that in ovarian cancer cells, CHK2 kinase is activated by irofulven while CHK1 kinase is not activated even when treated at higher concentrations of the drug. By using GM00847 human fibroblast expressing tetracycline-controlled, FLAG-tagged kinase-dead ATR (ATR.kd), it was demonstrated that ATR kinase does not play a major role in irofulven-induced CHK2 activation. Results from human fibroblasts proficient or deficient in ATM function (GM00637 and GM05849) indicated that CHK2 activation by irofulven is mediated by the upstream ATM kinase. Phosphorylation of ATM on Ser(1981), which is critical for kinase activation, was observed in ovarian cancer cell lines treated with irofulven. RNA interference results confirmed that CHK2 activation was inhibited after introducing siRNA for ATM. Finally, experiments done with human colon cancer cell line HCT116 and its isogenic CHK2 knockout derivative; and experiments done by expressing kinase-dead CHK2 in an ovarian cancer cell line demonstrated that CHK2 activation contributes to irofulven-induced S phase arrest. In addition, it was shown that NBS1, SMC1, and p53 were phosphorylated in an ATM-dependent manner, and p53 phosphorylation on serine 20 is dependent on CHK2 after irofulven treatment. In summary, we found that the anticancer agent, irofulven, activates the ATM-CHK2 DNA damage-signaling pathway, and CHK2 activation contributes to S phase cell cycle arrest induced by irofulven.

The functions of budding yeast Sae2 in the DNA damage response require Mec1- and Tel1-dependent phosphorylation

DNA damage checkpoint pathways sense DNA lesions and transduce the signals into appropriate biological responses, including cell cycle arrest, induction of transcriptional programs, and modification or activation of repair factors. Here we show that the Saccharomyces cerevisiae Sae2 protein, known to be involved in processing meiotic and mitotic double-strand breaks, is required for proper recovery from checkpoint-mediated cell cycle arrest after DNA damage and is phosphorylated periodically during the unperturbed cell cycle and in response to DNA damage. Both cell cycle- and DNA damage-dependent Sae2 phosphorylation requires the main checkpoint kinase, Mec1, and the upstream components of its pathway, Ddc1, Rad17, Rad24, and Mec3. Another pathway, involving Tel1 and the MRX complex, is also required for full DNA damage-induced Sae2 phosphorylation, that is instead independent of the downstream checkpoint transducers Rad53 and Chk1, as well as of their mediators Rad9 and Mrc1. Mutations altering all the favored ATM/ATR phosphorylation sites of Sae2 not only abolish its in vivo phosphorylation after DNA damage but also cause hypersensitivity to methyl methanesulfonate treatment, synthetic lethality with RAD27 deletion, and decreased rates of mitotic recombination between inverted Alu repeats, suggesting that checkpoint-mediated phosphorylation of Sae2 is important to support its repair and recombination functions.

Common ataxia telangiectasia mutated haplotypes and risk of breast cancer: a nested case-control study

INTRODUCTION

The ataxia telangiectasia mutated (ATM) gene is a tumor suppressor gene with functions in cell cycle arrest, apoptosis, and repair of DNA double-strand breaks. Based on family studies, women heterozygous for mutations in the ATM gene are reported to have a fourfold to fivefold increased risk of breast cancer compared with noncarriers of the mutations, although not all studies have confirmed this association. Haplotype analysis has been suggested as an efficient method for investigating the role of common variation in the ATM gene and breast cancer. Five biallelic haplotype tagging single nucleotide polymorphisms are estimated to capture 99% of the haplotype diversity in Caucasian populations.

METHODS

We conducted a nested case-control study of breast cancer within the Nurses' Health Study cohort to address the role of common ATM haplotypes and breast cancer. Cases and controls were genotyped for five haplotype tagging single nucleotide polymorphisms. Haplotypes were predicted for 1309 cases and 1761 controls for which genotype information was available.

RESULTS

Six unique haplotypes were predicted in this study, five of which occur at a frequency of 5% or greater. The overall distribution of haplotypes was not significantly different between cases and controls (chi2 = 3.43, five degrees of freedom, P = 0.63).

CONCLUSION

There was no evidence that common haplotypes of ATM are associated with breast cancer risk. Extensive single nucleotide polymorphism detection using the entire genomic sequence of ATM will be necessary to rule out less common variation in ATM and sporadic breast cancer risk.

Missense Mutations in the BRCT Domain of BRCA-1 from High-Risk Women Frequently Perturb Strongly Hydrophobic Amino Acids Conserved among Mammals

Inherited missense mutations in the tumor suppressor gene, BRCA-1, may predispose to breast or ovarian cancer, but the exact effects on the protein are generally unknown. The COOH-terminal region of BRCA-1 encodes two BRCT repeats, which are partially conserved in mammalian species (human, dog, rat, and mouse; 60% amino acid identity). A bioinformatic analysis was conducted to evaluate 246 BRCT missense mutations from high-risk breast and/or ovarian cancer patients (reported in the NIH Breast Cancer Information Core database). It was hypothesized that amino acids conserved in evolution would be disproportionately targeted by the mutations and that conserved amino acids with strongly hydrophobic side chains would be disproportionately perturbed. A statistical model was developed, and χ2 tests were used to determine whether missense mutations are randomly distributed throughout the BRCT repeats or whether they disproportionately target certain amino acids. The results showed that missense mutations disproportionately target amino acids that are identical in all four mammals (χ2 = 46.01, P < 0.001). In addition, missense mutations disproportionately perturb conserved amino acids with strongly hydrophobic side chains (χ2 = 68.57, P < 0.001) and alter the strongly hydrophobic property. The two most frequently observed known cancer-predisposing missense mutations in the BRCT repeats, M1775R and A1708E, conform to this pattern. These results suggest that missense mutations affecting highly conserved amino acids with strongly hydrophobic side chains can disturb important features of the BRCA-1 protein and may play a role in breast and ovarian cancer formation.

Collaboration of Brca1 and Chk2 in tumorigenesis

Disruption of Brca1 results in cellular demise or tumorigenesis depending on cellular context. Inactivation of p53 contributes to Brca1-associated tumor susceptibility. However the activation of p53-dependent checkpoint/apoptotic signaling in the absence of Brca1 is poorly understood. Here, we show that Chk2 inactivation is partially equivalent to p53 inactivation, in that Chk2 deficiency facilitates the development, survival, and proliferation of Brca1-deficient T cells at the expense of genomic integrity. Brca1 deficiency was found to result in Chk2 phosphorylation and the Chk2-dependent accumulation and activation of p53. Furthermore, inactivation of Chk2 and Brca1 was cooperative in breast cancer. Our findings identify a critical role for Chk2 as a component of the DNA damage-signaling pathway activated in response to Brca1 deficiency.

Dimerization of CtIP, a BRCA1- and CtBP-interacting protein, is mediated by an N-terminal coiled-coil motif

CtIP is a transcriptional co-regulator that binds a number of proteins involved in cell cycle control and cell development, such as CtBP (C terminus-binding protein), BRCA1 (breast cancer-associated protein-1), and LMO4 (LIM-only protein-4). The only recognizable structural motifs within CtIP are two putative coiled-coil domains located near the N and C termini of the protein. We now show that the N-terminal coiled coil (residues 45-160), but not the C-terminal coiled coil, mediates homodimerization of CtIP in mammalian 293T cells. The N-terminal coiled coil did not facilitate binding to LMO4 and BRCA1 proteins in these cells. A protease-resistant domain (residues 27-168) that minimally encompasses the putative N-terminal coiled coil was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. This region is predicted to contain two smaller coiled-coil regions. The CtIP-(45-160) dimerization domain is helical and dimeric, indicating that the domain does form a coiled coil. The two smaller domains, CtIP-(45-92) and CtIP-(93-160), also formed dimers of lower binding affinity, but with less helical content than the longer peptide. The hydrodynamic radius of CtIP-(45-160) is the same as those of CtIP-(45-92) and CtIP-(93-160), implying that CtIP-(45-160) does not form a single long coiled coil, but a more compact structure involving homodimerization of the two smaller coiled coils, which fold back as a four-helix bundle or other compact structure. These results suggest a specific model for CtIP homodimerization via its N terminus and contribute to an improved understanding of how this protein might assemble other factors required for its role as a transcriptional corepressor.

Alteration of the ATM gene occurs in gastric cancer cell lines and primary tumors associated with cellular response to DNA damage

Ataxia telangiectasia mutated (ATM) is the gene mutated in the genetic disorder ataxia telangiectasia (AT), the symptoms of which include sensitivity to radiation and an increased risk of cancer. ATM is a kinase involved in activating the appropriate damage-response pathway, leading to either cell-cycle arrest or apoptosis, and is therefore a key checkpoint molecule in regulating cell-cycle response to DNA damage and responsible for maintenance of genome integrity. However, little is known about the association of ATM mutations with human gastric cancer (HGC). In order to determine the mutation and mRNA expression changes of the ATM gene in HGC, we performed analyses by denaturing high-performance liquid chromatography (DHPLC), DNA sequencing and RT-PCR technique on 13 human gastric tumor cell lines and 30 cases of fresh tumor specimens matched normal tissue. We compared the potential effect of the ATM gene mutation and cell behavior including cell-cycle arrest and induction of apoptosis in the tumor cell lines MGC803 and BGC823 with and without ionizing radiation (IR) exposure. Our data show that frequent variations were observed at 10 exons and 2 cDNA fragments which covered 8 other exons of the ATM gene as 5 out of 13 on the cell lines (38.5%) and 2 out of 30 cases in the tissue specimens (6.7%). All point mutations were confirmed as base substitutions (5982T-C; 6620A-G; 8684G-G/A; 9389C-G) and deletions (1079delC) by use of DNA sequencing. Among the mutations, one was reported previously in breast cancer, the other five have not yet been reported. The expression of ATM was significantly lower in five cell lines (MGC803; MKN45; SGC7901; GES and SUN-1) than in two others (BGC823 and RF48). G2/M cell-cycle arrest and apoptosis were observed in ATM-deficient MGC803 cells challenged with IR. A transient up-regulation of p53 occurred 1h post-IR in BGC823 cells but not in MGC803 cells. Our findings suggest that ATM mutations might be a pathogenic factor for an increased risk of gastric cancer, and the dysfunction of ATM may lead to a hypersensitivity to ionizing radiation in gastric cancer cells, possibly by a p53-dependent pathway.

Modulation of oncogenic transformation by the human adenovirus E1A C-terminal region

The E1A oncogene of human adenoviruses cooperates with other viral and cellular oncogenes in oncogenic transformation of primary and established cells. The N-terminal half of E1A proteins that form specific protein complexes with pRb family and p300/CBP transcriptional regulators is essential for the transforming activities of E1A. Although the C-terminal half of E1A is dispensable for the transforming activities, it negatively modulates the oncogenic activities of the N-terminal region. Mutants of E1A lacking the C-terminal half or a short C-terminal region exhibit a hyper-transforming phenotype in cooperative transformation assays with the activated ras oncogene. The E1A C-terminal region implicated in the oncogenesis-restraining activity interacts with a 48-kDa cellular phosphoprotein, CtBP, that functions as a transcriptional corepressor. It appears that the C-terminal region of E1A may suppress E1A-mediated oncogenic transformation by a dual mechanism of relieving repression cellular genes by CtBP, and also by antagonizing the oncogenic activities of the N-terminal half of E1A.

SIAH-1 interacts with CtIP and promotes its degradation by the proteasome pathway

SIAH-1 and SIAH-2 are the human members of an evolutionary highly conserved E3 ligase family. SIAH-1 is a p53 and p21(Waf-1/Cip-1) induced gene during apoptosis and tumor suppression. In stable-transfected clones of MCF-7 cells, SIAH-1 overexpression was associated with apoptosis, mitotic alterations and p21(Waf-1/Cip-1) induction of expression. Using a two-hybrid screening, we identified here the transcriptional corepressor CtBP-interacting protein (CtIP) as a SIAH-1-interacting protein. CtIP has been proposed as a regulator of p21(Waf-1/Cip-1) gene transcription through a protein complex involving BRCA1. We demonstrate that SIAH-1 associates with CtIP both in vitro and in vivo. This interaction led to CtIP degradation by the ubiquitin-proteasome pathway. As expected, SIAH-1 induced p21(Waf-1/Cip-1) transcription in Jurkat-T cell. Surprisingly, a SIAH protein deleted of its RING finger, SIAH-1DeltaN, which is able to interact with CtIP but does not promote its degradation, also induced transcription from the p21(Waf-1) promoter in a similar extent as did SIAH-1. Our results suggest that p21(Waf-1/Cip-1) induction by SIAH-1 could not be mediated by CtIP degradation.

Hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites

Hyperoxia has been shown to cause DNA damage resulting in growth arrest of cells in p53-dependent, as well as p53-independent, pathways. Although H2O2 and other peroxides have been shown to induce ataxia telangiectasia-mutated (ATM)-dependent p53 phosphorylation in response to DNA damage, the signal transduction mechanisms in response to hyperoxia are currently unknown. Here we demonstrate that hyperoxia phosphorylates the Ser15 residue of p53 independently of ATM. Hyperoxia phosphorylated p53 (Ser15) in DNA-dependent protein kinase null (DNA-PK-/-) cells, indicating that it may not depend on DNA-PK for phosphorylation of p53 (Ser15). We show that Ser37 and Ser392 residues of p53 are also phosphorylated in an ATM-independent manner in hyperoxia. In contrast, H2O2 did not phosphorylate Ser37 in either ATM+/+ or ATM-/- cells. Furthermore, H2O2 failed to phosphorylate Ser15 in ATM-/- cells. Additionally, overexpression of kinase-inactive ATM-and-Rad3-related (ATR) in HEK293T cells diminished Ser15, Ser37, and Ser392 phosphorylation compared with vector-only transfected cells. In contrast, wild-type ATR overexpression did not diminish Ser15, Ser37, or Ser392 phosphorylation. We also show that checkpoint kinase 1 (Chk1) is phosphorylated on Ser345 in response to hyperoxia, which could be inhibited by caffeine or wortmannin, potent inhibitors of phosphoinositide 3-kinase-related kinases. Hyperoxia also phosphorylated Chk1 in ATM+/+ as well as in ATM-/- cells, demonstrating an ATM-independent mechanism in Chk1 phosphorylation. Together, our data suggest that hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites in an ATM-independent manner, which is different from other forms of oxidative stress such as H2O2 or UV light.

Ataxia telangiectasia mutated expression and activation in the testis

Ionizing radiation (IR) and consequent induction of DNA double-strand breaks (DSBs) causes activation of the protein ataxia telangiectasia mutated (ATM). Normally, ATM is present as inactive dimers; however, in response to DSBs, the ATM dimer partners cross-phosphorylate each other on serine 1981, and kinase active ATM monomers are subsequently released. We have studied the presence of both nonphosphorylated as well as active serine 1981 phosphorylated ATM (pS1981-ATM) in the mouse testis. In the nonirradiated testis, ATM was present in spermatogonia and spermatocytes until stage VII of the cycle of the seminiferous epithelium, whereas pS1981-ATM was found only to be present in the sex body of pachytene spermatocytes. In response to IR, ATM became activated by pS1981 cross-phosphorylation in spermatogonia and Sertoli cells. Despite the occurrence of endogenous programmed DSBs during the first meiotic prophase and the presence of ATM in both spermatogonia and spermatocytes, pS1981 phosphorylated ATM did not appear in spermatocytes after treatment with IR. These results show that spermatogonial ATM and ATM in the spermatocytes are differentially regulated. In the mitotically dividing spermatogonia, ATM is activated by cross-phosphorylation, whereas during meiosis nonphosphorylated ATM or differently phosphorylated ATM is already active. ATM has been shown to be present at the synapsed axes of the meiotic chromosomes, and in the ATM knock-out mice spermatogenesis stops at pachytene stage IV of the seminiferous epithelium, indicating that indeed nonphosphorylated ATM is functional during meiosis. Additionally, ATM is constitutively phosphorylated in the sex body where its continued presence remains an enigma.

Functional dissection of transcription factor ZBRK1 reveals zinc fingers with dual roles in DNA-binding and BRCA1-dependent transcriptional repression

The breast- and ovarian-specific tumor suppressor BRCA1 has been implicated in both activation and repression of gene transcription by virtue of its direct interaction with sequence-specific DNA-binding transcription factors. However, the mechanistic basis by which BRCA1 mediates the transcriptional activity of these regulatory proteins remains largely unknown. To clarify this issue, we have examined the functional interaction between BRCA1 and ZBRK1, a BRCA1-dependent KRAB eight zinc finger transcriptional repressor. We report here the identification and molecular characterization of a portable BRCA1-dependent transcriptional repression domain within ZBRK1 composed of zinc fingers 5-8 along with sequences in the unique ZBRK1 C terminus. This C-terminal repression domain functions in a BRCA1-, histone deacetylase-, and promoter-specific manner and is thus functionally distinguishable from the N-terminal KRAB repression domain in ZBRK1, which exhibits no BRCA1 dependence and broad promoter specificity. Significantly, we also find that the BRCA1-dependent transcriptional repression domain on ZBRK1 includes elements that modulate its sequence-specific DNA binding activity. These findings thus reveal the presence within ZBRK1 of functionally bipartite zinc fingers with dual roles in sequence-specific DNA-binding and BRCA1-dependent transcriptional repression. We discuss the implications of these findings for the role of BRCA1 as ZBRK1 co-repressor.

Degradation of transcription repressor ZBRK1 through the ubiquitin-proteasome pathway relieves repression of Gadd45a upon DNA damage

Induction of gene expression in response to DNA damage is important for repairing damaged DNA for cell survival. Previously, we identified a novel zinc finger protein, ZBRK1, which contains a KRAB domain at the N terminus, eight zinc fingers at the center, and a BRCA1-binding region at the C terminus. In a BRCA1-dependent manner, ZBRK1 represses Gadd45a transcription through binding to a specific sequence in intron 3. In addition, ZBRK1-binding sequences are located at the regulatory region of many DNA damage-inducible genes, suggesting that ZBRK1 may have a role in DNA damage response. However, it is unclear how transcription repression by ZBRK1 is relieved subsequent to DNA damage. Here we report that ZBRK1 is rapidly degraded upon treatment with the DNA-damaging agents UV and methyl methanesulfonate. Specific proteasome inhibitors block DNA damage-induced degradation of ZBRK1, and the polyubiquitinated form of ZBRK1 is detectable, suggesting that the ubiquitin-proteasome pathway mediates the degradation of ZBRK1. In both BRCA1-proficient and -deficient cells, ZBRK1 is degraded with similar efficiencies independent of BRCA1 E3 ligase activity. By analysis of a series of ZBRK1 mutants, a 44-amino-acid element located between the N-terminal KRAB domain and the eight zinc fingers was found to be sufficient for the DNA damage-induced degradation of ZBRK1. Cells expressing a ZBRK1 mutant lacking the 44-amino-acid element are hypersensitive to DNA damage and are compromised for Gadd45a derepression. These results indicate that ZBRK1 is a novel target for DNA damage-induced degradation and provide a mechanistic explanation of how ZBRK1 is regulated in response to DNA damage.

BRCA1 interacts with FHL2 and enhances FHL2 transactivation function

Germ-line mutations in BRCA1 are associated with an increased lifetime risk of developing breast and/or ovarian tumors. The BRCA1 gene product is a 220-kDa protein that contains a tandem of two BRCA1 C-terminal (BRCT) domains required for transcription. In an attempt to understand how BRCA1 exerts its function through BRCT domains, we search for partners of the BRCT domains of BRCA1. Using the yeast two-hybrid system, we identified the four and a half LIM-only protein 2 (FHL2) as a novel BRCA1 interacting protein. We demonstrate that BRCA1 and FHL2 can physically associate in vitro, in yeast, and in human cells. BRCA1 interacted with FHL2 through its second BRCT domain and the interaction of FHL2 with BRCA1 requires the last three LIM domains of FHL2. BRCA1 enhanced FHL2-mediated transcriptional activity in transient transfections. Tumor-derived transactivation-deficient BRCA1 mutants showed a reduced ability to enhance transactivation by FHL2. Lack of BRCA1 binding sites in the FHL2 completely abolished the FHL2 transactivation function. Reverse transcription polymerase chain reaction analysis showed that FHL2 mRNA levels may be downregulated in many breast cancer cell lines. These results suggest that the BRCA1-FHL2 interaction may be involved in transcriptional regulation and play a significant role in cancer cell growth.