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

Temozolomide-perillyl alcohol conjugate induced reactive oxygen species accumulation contributes to its cytotoxicity against non-small cell lung cancer

Temozolomide-perillyl alcohol conjugate (TMZ − POH), a novel temozolomide analog, was reported to play a cytotoxic role in triple-negative breast cancer and TMZ-resistant gliomas. In a current study we had demonstrated how TMZ − POH also exhibited its cytotoxicity against non-small cell lung cancer (NSCLC), the most common type of lung cancer, as evidence from cell/tumor proliferation inhibition, G2/M arrest, DNA damage and mitochondrial apoptosis. Importantly, TMZ − POH’s cytotoxicity is closely related to reactive oxygen species (ROS) accumulation because it can be reversed by two ROS scavengers, catalase (CAT) and N-acetyl-L-cysteine (NAC). TMZ − POH induces mitochondrial transmembrane potential (MTP) decrease and ROS accumulation, in turn activates mitogen-activated protein kinase (MAPKs) signaling and mitochondrial apoptosis, and then exerts its cytotoxicity, thus proposing TMZ − POH as a potential therapeutic candidate for NSCLC.

Promoter methylation and expression changes of BRCA1 in cancerous tissues of patients with sporadic breast cancer

BRCA1 is a susceptibility gene that has a genetic predisposition for breast cancer. BRCA1 gene mutation is closely associated with familial hereditary breast cancer, but the BRCA1 gene mutation is rarely found in sporadic breast cancer. According to previous studies, decreased expression of BRCA1 was detected in certain types of sporadic breast cancer. Aberrant methylation of DNA promoter CpG islands is one of the mechanisms by which tumor suppressor gene expression and function is lost. The aim of the present study was to investigate BRCA1 gene expression, methylation status and clinical significance in sporadic types of breast cancer. Quantitative polymerase chain reaction (PCR) and bisulfite sequencing PCR were respectively used to detect expression differences of BRCA1 mRNA and BRCA1 methylation in the 49 cancerous and paired non-cancerous samples from patients with breast cancer. The associations of BRCA1 expression and methylation status with the clinicopathologic characteristics were analysed. BRCA1 mRNA expression levels in the 49 breast cancer tissues were lower than those in the paired non-cancerous tissues. There was a significant statistical difference (P=0.001). BRCA1 mRNA expression was not associated with the main clinicopathologic characteristics. Frequency of the BRCA1 promoter methylation in the breast cancerous tissues was significantly higher than that in the non-cancerous tissues (P=0.007); BRCA1 gene methylation status was negatively correlated with mRNA expression (P=0.029); and BRCA1 methylation exhibited no association with all clinicopathological features. DNA promoter hypermethylation may be the potential mechanism accounting for BRCA1 expression silence in part of sporadic types of breast cancer. Some patients with hypermethylated BRCA1 may display favorable clinicopathological status.

Mitochondrial adaptations evoked with exercise are associated with a reduction in age-induced testicular atrophy in Fischer-344 rats

Mitochondrial dysfunction in various tissues has been associated with numerous conditions including aging. In testes, aging induces atrophy and a decline in male reproductive function but the involvement of mitochondria is not clear. The purpose of this study was to examine whether the mitochondrial profile differed with (1) aging, and (2) 10-weeks of treadmill exercise training, in the testes of young (6 month) and old (24 month) Fischer-344 (F344) animals. Old animals exhibited significant atrophy (30 % decline; P < 0.05) in testes compared to young animals. However, relative mitochondrial content was not reduced with age and this was consistent with the lack of change in the mitochondrial biogenesis regulator protein, peroxisome proliferator-activated receptor gamma coactivator 1-alpha and its downstream targets nuclear respiratory factor-1 and mitochondrial transcription factor A. No effect was observed in the pro- or anti-apoptotic proteins, Bax and Bcl-2, respectively, but age increased apoptosis inducing factor levels. Endurance training induced beneficial mitochondrial adaptations that were more prominent in old animals including greater increases in relative mtDNA content, biogenesis/remodeling (mitofusin 2), antioxidant capacity (mitochondrial superoxide dismutase) and lower levels of phosphorylated histone H2AX, an early marker of DNA damage (P < 0.05). Importantly, these exercise-induced changes were associated with an attenuation of testes atrophy in older sedentary animals (P < 0.05). Our results indicate that aging-induced atrophy in testes may not be associated with changes in relative mitochondrial content and key regulatory proteins and that exercise started in late-life elicits beneficial changes in mitochondria that may protect against age-induced testicular atrophy.

The BRCA1 Breast Cancer Suppressor: Regulation of Transport, Dynamics, and Function at Multiple Subcellular Locations

Inherited mutations in the BRCA1 gene predispose to a higher risk of breast/ovarian cancer. The BRCA1 tumor suppressor is a 1863 amino acid protein with multiple protein interaction domains that facilitate its roles in regulating DNA repair and maintenance, cell cycle progression, transcription, and cell survival/apoptosis. BRCA1 was first identified as a nuclear phosphoprotein, but has since been shown to contain different transport sequences including nuclear export and nuclear localization signals that enable it to shuttle between specific sites within the nucleus and cytoplasm, including DNA repair foci, centrosomes, and mitochondria. BRCA1 nuclear transport and ubiquitin E3 ligase enzymatic activity are tightly regulated by the BRCA1 dimeric binding partner BARD1 and further modulated by cancer mutations and diverse signaling pathways. This paper will focus on the transport, dynamics, and multiple intracellular destinations of BRCA1 with emphasis on how regulation of these events has impact on, and determines, a broad range of important cellular functions.

BRCA1-dependent Chk1 phosphorylation triggers partial chromatin disassociation of phosphorylated Chk1 and facilitates S-phase cell cycle arrest

Chk1 phosphorylation by the PI3-like kinases ATR and ATM is critical for its activation and its role in prevention of premature mitotic entry in response to DNA damage or stalled replication. The breast and ovarian tumor suppressor, BRCA1, is among several checkpoint mediators that are required for Chk1 activation by ATM and ATR. Previously we showed that BRCA1 is necessary for Chk1 phosphorylation and activation following ionizing radiation. BRCA1 has been implicated in S-phase checkpoint control yet its mechanism of action is not well characterized. Here we report that BRCA1 is critical for Chk1 phosphorylation in response to inhibition of replication by either cisplatin or hydroxyurea. While Chk1 phosphorylation of S317 is fully dependent on BRCA1, additional proteins may mediate S345 phosphorylation at later time points. In addition, we show that a subset of phosphorylated Chk1 is released from the chromatin in a BRCA1-dependent manner which may lead to the phosphorylation of Chk1 substrate, Cdc25C, on S216 and to S-phase checkpoint activation. Inhibition of Chk1 kinase by UCN-01 or expression of Chk1 phosphorylation mutants in which the serine residues were substituted with alanine residues abrogates BRCA1-dependent cell cycle arrest in response replication inhibition. These data reveal that BRCA1 facilitates Chk1 phosphorylation and its partial chromatin dissociation following replication inhibition that is likely to be required for S-phase checkpoint signaling.

Mitochondrial genome instability resulting from SUV3 haploinsufficiency leads to tumorigenesis and shortened lifespan

Mitochondrial dysfunction has been a hallmark of cancer. However, whether it has a causative role awaits to be elucidated. Here, using an animal model derived from inactivation of SUV3, a mitochondrial helicase, we demonstrated that mSuv3+/- mice harbored increased mitochondrial DNA (mtDNA) mutations and decreased mtDNA copy numbers, leading to tumor development in various sites and shortened lifespan. These phenotypes were transmitted maternally, indicating the etiological role of the mitochondria. Importantly, reduced SUV3 expression was observed in human breast tumor specimens compared with corresponding normal tissues in two independent cohorts. These results demonstrated for the first time that maintaining mtDNA integrity by SUV3 helicase is critical for cancer suppression.

BRCA1 is required for hMLH1 stabilization following doxorubicin-induced DNA damage

Human DNA mismatch repair (MMR) is involved in the removal of DNA base mismatches that arise either during DNA replication or are caused by DNA damage. In this study, we show that the activation of the MMR component hMLH1 in response to doxorubicin (DOX) treatment requires the presence of BRCA1 and that this phenomenon is mediated by an ATM/ATR dependent phosphorylation of the hMLH1 Ser-406 residue. BRCA1 is an oncosuppressor protein with a central role in the DNA damage response and it is a critical component of the ATM/ATR mediated checkpoint signaling. Starting from a previous finding in which we demonstrated that hMLH1 is able to bind to BRCA1, in this study we asked whether BRCA1 might be the bridge for ATM/ATR dependent phosphorylation of the hMLH1 molecular partner. We found that: (i) the negative modulation of BRCA1 expression is able to produce a remarkable reversal of hMLH1 stabilization, (ii) BRCA1 is required for post-translational modification produced by DOX treatment on hMLH1 which is, in turn, attributed to the ATM/ATR activity, (iii) the serine 406 phosphorylatable residue is critical for hMLH1 activation by ATM/ATR via BRCA1. Taken together, our data lend support to the hypothesis suggesting an important role of this oncosuppressor as a scaffold or bridging protein in DNA-damage response signaling via downstream phosphorylation of the ATM/ATR substrate hMLH1.

Screening for large genomic rearrangements of the BRIP1 and CHK1 genes in Finnish breast cancer families

In search for susceptibility genes that could explain an additional portion of familial breast cancer clustering in Finland, we set out to evaluate the presence of large genomic rearrangements in two candidate genes, BRIP1 and CHK1. BRIP1 is a BRCA1 associated protein that is mutated in a fraction of familial breast cancer and Fanconi anemia cases. To date, the role of large BRIP1 deletions in breast cancer susceptibility is not well-characterized. CHK1 is a critical maintainer of cell cycle checkpoints and genomic stability, and is also involved in the BRCA1 and FA protein signalling pathways. Although CHK1 is a very important protein for cell cycle and DNA integrity maintenance control, no mutations in this gene has yet been associated with predisposition to cancer. For the present study, blood DNA from affected index persons of 111 Northern Finnish breast cancer families was assessed for possible constitutional exonic deletions or amplifications in the BRIP1 and CHK1 genes by using the multiplex ligation-dependent probe amplification method. Our results showed that exonic deletions or amplifications affecting the BRIP1 and CHK1 genes seem not to contribute to hereditary breast cancer susceptibility in the Finnish population. To our knowledge, this is the first attempt to determine the existence of large CHK1 deletions in familial breast cancer or in any disease with a hereditary background.

Antitumor agents 279. Structure-activity relationship and in vivo studies of novel 2-(furan-2-yl)naphthalen-1-ol (FNO) analogs as potent and selective anti-breast cancer agents

In our ongoing modification study of neo-tanshinlactone (1), we discovered 2-(furan-2-yl)naphthalen-1-ol (FNO) derivatives 3 and 4 as a new class of anti-tumor agents. To explore structure-activity relationships (SAR) of this scaffold, 18 new analogs, 6-12 and 14-24, were designed and synthesized. The C11-esters 7 and 12 displayed broad anti-tumor activity (ED(50) 1.1-4.3 μg/mL against seven cancer cell lines), while C11-hydroxymethyl 14 showed unique selectivity against the SKBR-3 breast cancer cell line (ED(50) 0.73 μg/mL). Compounds 15 and 22 displayed potent and selective anti-breast tumor activity (ED(50) 1.7 and 0.85 μg/mL, respectively, against MDA-MB-231). The SAR results demonstrated that the substitutions from the ring-opened lactone ring C of 1 are critical to the anti-tumor potency as well as the apparent tumor-tissue type selectivity. Treatment with 3 in Brca1(f11/f11)p53(f5&6/f5&6)Cre(c) mice models significantly inhibited the proliferation of mammary epithelial cells and branching of mammary glands.

BRCA1 promoter methylation in peripheral blood cells is associated with increased risk of breast cancer with BRCA1 promoter methylation

BRCA1 promoter methylation reportedly plays an important part in the pathogenesis of human breast cancer. In the present study, we investigated whether or not BRCA1 promoter methylation in peripheral blood cells (PBCs) can serve as a risk factor for developing breast cancer. The association of BRCA1 promoter methylation in PBCs with breast cancer risk was examined in a case-control study (200 breast cancer patients and 200 controls). BRCA1 promoter methylation in PBCs and breast tumors was determined with a methylation-specific quantitative PCR assay. BRCA1 promoter methylation in PBCs was seen in 43 (21.5%) of the breast cancer patients and in 27 (13.5%) of the controls. The odds ratio for breast cancer adjusted for other epidemiological risk factors was 1.73 (1.01-2.96) and was statistically significant (P = 0.045). When breast tumors were classified into those with and without BRCA1 promoter methylation, the odds ratio was 0.84 (0.43-1.64) (P = 0.61) for BRCA1 promoter methylation-negative and 17.78 (6.71-47.13) (P < 0.001) for BRCA1 promoter methylation-positive breast tumors. BRCA1 promoter methylation in PBCs is significantly associated with risk of breast cancer with BRCA1 promoter methylation. This seems to indicate that BRCA1 promoter methylation in PBCs may constitute a novel risk factor for breast cancer with BRCA1 promoter methylation.

DNA damage stress response in germ cells: role of c-Abl and clinical implications

Cells experiencing DNA damage undergo a complex response entailing cell-cycle arrest, DNA repair and apoptosis, the relative importance of the three being modulated by the extent of the lesion. The observation that Abl interacts in the nucleus with several proteins involved in different aspects of DNA repair has led to the hypothesis that this kinase is part of the damage-sensing mechanism. However, the mechanistic details underlying the role of Abl in DNA repair remain unclear. Here, I will review the evidence supporting our current understanding of Abl activation following DNA insults, while focusing on the relevance of these mechanisms in protecting DNA-injured germ cells. Early studies have shown that Abl transcripts are highly expressed in the germ line. Abl-deficient mice exhibit multiple abnormalities, increased perinatal mortality and reduced fertility. Recent findings have implicated Abl in a cisplatin-induced signaling pathway eliciting death of immature oocytes. A p53-related protein, TAp63, is an important immediate downstream effector of this pathway. Of note, pharmacological inhibition of Abl protects the ovarian reserve from the toxic effects of cisplatin. This suggests that the extent of Abl catalytic outputs may shift the balance between survival (likely through DNA repair) and activation of a death response. Taken together, these observations are consistent with the evolutionary conserved relationship between DNA damage and activation of the p53 family of transcription factors, while shedding light on the key role of Abl in dictating the fate of germ cells upon genotoxic insults.

Transcriptional regulation of the base excision repair pathway by BRCA1

Inactivation of the breast cancer susceptibility gene BRCA1 plays a significant role in the development of a subset of breast cancers, although the major tumor suppressor function of this gene remains unclear. Previously, we showed that BRCA1 induces antioxidant-response gene expression and protects cells against oxidative stress. We now report that BRCA1 stimulates the base excision repair pathway, a major mechanism for the repair of oxidized DNA, by stimulating the activity of key base excision repair (BER) enzymes, including 8-oxoguanine DNA glycosylase (OGG1), the DNA glycosylase NTH1, and the apurinic endonuclease redox factor 1/apurinic endonuclease 1 (REF1/APE1), in human breast carcinoma cells. The increase in BER enzyme activity appears to be due, primarily, to an increase in enzyme expression. The ability of BRCA1 to stimulate the expression of the three BER enzymes and to enhance NTH1 promoter activity was dependent upon the octamer-binding transcription factor OCT1. Finally, we found that OGG1, NTH1, and REF1/APE1 each contribute to the BRCA1 protection against oxidative stress due to hydrogen peroxide and that hydrogen peroxide stimulates the expression of BRCA1 and the three BER enzymes. These findings identify a novel mechanism through which BRCA1 may regulate the repair of oxidative DNA damage.

Increase in double-stranded DNA break-related foci in early-stage thymocytes of aged mice

Cellular and molecular mechanisms involved in aging are notoriously complex. Aging-related immune decline of T lymphocyte function is partly caused by attrition of thymic T cell development, which involves programmed creation and repair of DNA breaks for generating T cell receptors. Aging also leads to significant alterations in the cellular DNA repair ability. We show that higher levels of gamma-phosphorylated H2AX (pH2AX), which marks DNA double-stranded breaks (DSBs), were detectable in early thymocyte subsets of aged as compared to young mice. Also, while only 1-2 foci of nuclear accumulation of pH2AX were detectable in these early thymocytes from young mice, cells from aged mice showed higher numbers of pH2AX foci. In CD4-CD8- double-negative (DN) thymocytes of aged mice, which showed the highest levels of DSBs, there was a modest increase in levels of the DNA repair protein MRE11, but not of either Ku70, another DNA repair protein, or the cell cycle checkpoint protein p53. Thus, immature thymocytes in aged mice show a marked increase in DNA DSBs with only a modest enhancement of repair processes, and the resultant cell cycle block could contribute to aging-related defects of T cell development.

FANCJ helicase operates in the Fanconi Anemia DNA repair pathway and the response to replicational stress

Fanconi anemia (FA) is an autosomal recessive disorder characterized by multiple congenital anomalies, progressive bone marrow failure, and high cancer risk. Cells from FA patients exhibit spontaneous chromosomal instability and hypersensitivity to DNA interstrand cross-linking (ICL) agents. Although the precise mechanistic details of the FA/BRCA pathway of ICL-repair are not well understood, progress has been made in the identification of the FA proteins that are required for the pathway. Among the 13 FA complementation groups from which all the FA genes have been cloned, only a few of the FA proteins are predicted to have direct roles in DNA metabolism. One of the more recently identified FA proteins, shown to be responsible for complementation of the FA complementation group J, is the BRCA1 Associated C-terminal Helicase (BACH1, designated FANCJ), originally identified as a protein associated with breast cancer. FANCJ has been proposed to function downstream of FANCD2 monoubiquitination, a critical event in the FA pathway. Evidence supports a role for FANCJ in a homologous recombination (HR) pathway of double strand break (DSB) repair. In this review, we will summarize the current knowledge in terms of FANCJ functions through its enzymatic activities and protein interactions. The molecular roles of FANCJ in DNA repair and the response to replicational stress will be discussed.

Single-strand annealing, conservative homologous recombination, nonhomologous DNA end joining, and the cell cycle-dependent repair of DNA double-strand breaks induced by sparsely or densely ionizing radiation

The cell cycle-dependent relative contributions of error-prone single-strand annealing (SSA), error-free conservative homologous recombination (HR), and potentially error-prone nonhomologous DNA end joining (NHEJ) to repair simple (induced by 200 kV X rays) or complex (induced by (241)Am alpha particles) DNA double-strand breaks (DSBs) in Chinese hamster ovary cells are reported for the first time. Cells of the parental cell line AA8 and its derivatives UV41 (SSA-deficient), irs1SF (HR-deficient) and V3 (NHEJ-deficient) were synchronized in G(1) or in S phase, and survival responses after exposure to either type of radiation were measured. It is demonstrated for the first time that in G(1)-phase SSA is negligible for the repair of DSBs of various complexities. HR-deficient cells exposed to X rays or alpha particles in G(1) phase show enhanced radiosensitivity, but this does not necessarily mean that HR is important in G(1) phase. NHEJ appears to be the most important (if not the only) mechanism in G(1) phase acting efficiently on simple DSBs, but complex DSBs are a less preferred target. In contrast to X rays, NHEJ-deficient cells show no cell cycle-dependent variation in sensitivity to alpha particles. Surprisingly, when these cells are exposed to X rays in G(1) phase, they are even more sensitive compared to alpha particles. It is also shown for the first time that in S phase all three mechanisms play a role in the repair of simple and complex DSBs. A defect in SSA confers radiosensitivity to cells in S phase, suggesting that the error-prone SSA mechanism is important for the repair of specific simple and complex DSBs that are not a substrate for HR or NHEJ. The most important mechanism in S phase for the repair of simple and complex DSBs is HR. This is also emphasized by the finding that irs1SF cells, after complementation of their HR defect by human XRCC3 cDNA, show a greater radioresistance than parental cells, whereas resistance to mitomycin C is only partially restored. Complementation confers a greater resistance to alpha particles than X rays, suggesting an important role of HR, especially for the repair of complex DSBs. In S phase, NHEJ is more important than SSA for the repair of simple DSBs, but SSA is more important than NHEJ for the repair of complex DSBs.

The role of nonhomologous DNA end joining, conservative homologous recombination, and single-strand annealing in the cell cycle-dependent repair of DNA double-strand breaks induced by H(2)O(2) in mammalian cells

The purpose of this study was to investigate the cell cycle-dependent role of nonhomologous DNA end joining (NHEJ), conservative homologous recombination (HR), and single-strand annealing (SSA) for the repair of simple DNA double-strand breaks (DSBs) induced by H(2)O(2)-mediated OH radicals in CHO cells. Cells of the cell lines V3 (NHEJ-deficient), irs1SF (HR-deficient) and UV41 (SSA-deficient) and their parental cell line AA8 were exposed to various concentrations of H(2)O(2) in G(1) or S phase of the cell cycle and their colony-forming ability was assayed. In G(1) phase, NHEJ was the most important-if not the only-mechanism to repair H(2)O(2)-mediated DSBs; this was similar to results obtained in a parallel study of more complex DSBs induced by sparsely or densely ionizing radiation. Unlike HR (irs1SF)- and SSA (UV41)-deficient cells, the sensitivity of NHEJ-deficient V3 cells to H(2)O(2) relative to parental AA8 cells in G(1) phase is about 50 times higher compared to 200 kV X rays. This points to a specific role of the catalytic subunit of DNA-PK for efficient NHEJ of H(2)O(2)-mediated DSBs that are located at sites critical for the maintenance of the higher-order structure of cellular DNA, whereas X-ray-induced DSBs are distributed stochastically. Surprisingly, SSA-deficient cells in G(1) phase showed an increased sensitivity to high concentrations of H(2)O(2) relative to the parental wild-type cells and to HR-deficient cells, which may be interpreted in terms of a specific type of H(2)O(2)-induced damage requiring SSA for repair after its transfer into S phase. In S phase, HR is the most important mechanism to repair H(2)O(2)-mediated DSBs, followed by NHEJ. In contrast, the action of error-prone SSA may not be beneficial, since SSA-deficient cells are three times more resistant to H(2)O(2) than wild-type AA8 cells. This is likely due to channeling of DSBs into the error-free HR repair pathway or into the potentially error-prone NHEJ pathway. Cells with or without a defect in DSB repair are considerably more sensitive to H(2)O(2) in S phase compared to G(1) phase. This effect is likely due to the fact that topoisomerase II, which is expressed only in proliferating cells, is a target of H(2)O(2), resulting in enhanced accumulation of DSBs and killing of cells treated in S phase with H(2)O(2). The relative sensitivities to H(2)O(2) differ by orders of magnitude for the four cell lines. This seems to be caused mainly by H(2)O(2)-mediated poisoning of topoisomerase IIalpha rather than by a defect in DSB repair.

Cancer stem cells contribute to cisplatin resistance in Brca1/p53-mediated mouse mammary tumors

The majority of BRCA1-associated breast cancers are basal cell-like, which is associated with a poor outcome. Using a spontaneous mouse mammary tumor model, we show that platinum compounds, which generate DNA breaks during the repair process, are more effective than doxorubicin in Brca1/p53-mutated tumors. At 0.5 mg/kg of daily cisplatin treatment, 80% primary tumors (n = 8) show complete pathologic response. At greater dosages, 100% show complete response (n = 19). However, after 2 to 3 months of complete remission following platinum treatment, tumors relapse and become refractory to successive rounds of treatment. Approximately 3.8% to 8.0% (mean, 5.9%) of tumor cells express the normal mammary stem cell markers, CD29(hi)24(med), and these cells are tumorigenic, whereas CD29(med)24(-/lo) and CD29(med)24(hi) cells have diminished tumorigenicity or are nontumorigenic, respectively. In partially platinum-responsive primary transplants, 6.6% to 11.0% (mean, 8.8%) tumor cells are CD29(hi)24(med); these populations significantly increase to 16.5% to 29.2% (mean, 22.8%; P < 0.05) in platinum-refractory secondary tumor transplants. Further, refractory tumor cells have greater colony-forming ability than the primary transplant-derived cells in the presence of cisplatin. Expression of a normal stem cell marker, Nanog, is decreased in the CD29(hi)24(med) populations in the secondary transplants. Top2A expression is also down-regulated in secondary drug-resistant tumor populations and, in one case, was accompanied by genomic deletion of Top2A. These studies identify distinct cancer cell populations for therapeutic targeting in breast cancer and implicate clonal evolution and expansion of cancer stem-like cells as a potential cause of chemoresistance.

Promotion of BRCA1-associated triple-negative breast cancer by ovarian hormones

PURPOSE OF REVIEW

Mammary epithelial proliferation is controlled by the ovarian hormones estrogen and progesterone. Although BRCA1 (breast cancer 1, early onset) is ubiquitously expressed, women with BRCA1 mutations have a propensity to develop tumors in tissues sensitive to ovarian hormone. An understanding of the tissue-specific function of the BRCA1-encoded protein (BRCA1) provides additional insight that may improve cancer risk reduction in BRCA1 mutation carriers.

RECENT FINDINGS

Studies using mouse models have shown that BRCA1 regulates the abundance of progesterone receptor. The half-life of progesterone receptor is extended in cells harboring mutations in BRCA1. Reduced ubiquitination of progesterone receptor contributes to its stabilization and is correlated with increased cell proliferation in response to progesterone. Treatment of mutant mice with antiprogesterone prevents/delays tumor development. In vitro, BRCA1 and its interacting protein BARD1 (BRCA1-associated RING domain) serve as an ubiquitin ligase for the monoubiquitination of estrogen receptor-alpha, which may lead to alterations in estrogen receptor-alpha activity. Furthermore, the ubiquitin ligase activities of BRCA1/BARD1 may be determined by the ubiquitin-conjugating enzyme E2.

SUMMARY

BRCA1 exerts its tissue-specific function through the regulation of progesterone receptor and estrogen receptor-alpha. Interference with progesterone receptor, in addition to estrogen receptor-alpha, may be effective in reducing cancer risk in BRCA1 mutation carriers.

The management of DNA double-strand breaks in mitotic G2, and in mammalian meiosis viewed from a mitotic G2 perspective

DNA double-strand breaks (DSBs) are extremely hazardous lesions for all DNA-bearing organisms and the mechanisms of DSB repair are highly conserved. In the eukaryotic mitotic cell cycle, DSBs are often present following DNA replication while, in meiosis, hundreds of DSBs are generated as a prelude to the reshuffling of the maternally and paternally derived genomes. In both cases, the DSBs are repaired by a process called homologous recombinational repair (HRR), which utilises an intact DNA molecule as the repair template. Mitotic and meiotic HRR are managed by 'checkpoints' that inhibit cell division until DSB repair is complete. Here we attempt to summarise the substantial recent progress in understanding the checkpoint management of HRR in mitosis (focussing mainly on mammals) and then go on to use this information as a framework for understanding the presumed checkpoint management of HRR in mammalian meiosis.

Genetic variants in the H2AFX promoter region are associated with risk of sporadic breast cancer in non-Hispanic white women aged <or=55 years

The histone protein family member X (H2AFX) is important in maintaining chromatin structure and genetic stability. Genetic variants in H2AFX may alter protein functions and thus cancer risk. In this case-control study, we genotyped four common single nucleotide polymorphisms (i.e., -1654A > G [rs643788], -1420G > A [rs8551], and -1187T > C [rs7759] in the H2AFX promoter region and 1057C > T [rs7350] in the 3' untranslated region (UTR)) in 467 patients with sporadic breast cancer and 488 cancer-free controls. All female subjects were non-Hispanic whites aged <or=55 years. We found that significantly increased risk of breast cancer was associated with variant genotypes in the H2AFX promoter: adjusted odds ratio [OR] = 1.80, 95% confidence interval [CI] = 1.38-2.34 for -1654AG/GG; OR = 1.40, 95% CI = 1.07-1.83 for -1420GA/AA; and OR = 1.65, 95% CI = 1.26-2.16 for -1187TC/CC. Furthermore, the number of variant alleles in the promoter haplotypes was associated with increased risks of breast cancer in a dose-response manner (OR = 6.08, 95% CI = 3.25-11.38; OR = 6.83, 95% CI = 3.83-12.18; and OR = 23.61, 95% CI = 3.95-140.99 for one, two, and three variant alleles, respectively) (P (trend) \ < 0.0001). Age at onset of breast cancer significantly decreased as the number of variant alleles increased (P (trend) = 0.024). However, these effects were not observed in the 3'UTR 1057C > T polymorphism. Therefore, we believe that H2AFX promoter polymorphisms may contribute to the etiology of sporadic breast cancer in young non-Hispanic white women. Larger association studies and related functional studies are warranted to confirm these findings.

Phosphorylation of ATR-interacting protein on Ser239 mediates an interaction with breast-ovarian cancer susceptibility 1 and checkpoint function

The signaling of DNA damage and replication stress involves a multitude of proteins, including the kinases ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR), and proteins with BRCA1 COOH-terminal (BRCT) domains. The BRCT domain-containing proteins facilitate the phosphorylation of ATM/ATR substrates and can be coimmunoprecipitated with ATM or ATR. However, their mode of interaction with the ATM/ATR kinases remains elusive. Here, we show that breast-ovarian cancer susceptibility 1 (BRCA1) interacts directly with ATR-interacting protein (ATRIP), an obligate partner of ATR. The interaction involves the BRCT domains of BRCA1 and Ser(239) of ATRIP, a residue that is phosphorylated in both irradiated and nonirradiated cells. Consistent with a role of BRCA1 in ATR signaling, substitution of Ser(239) of ATRIP with Ala leads to a G(2)-M checkpoint defect. We propose that a direct physical interaction between BRCA1 and ATRIP is required for the checkpoint function of ATR.

CCDC98 targets BRCA1 to DNA damage sites

Breast cancer-1 (BRCA1) participates in the DNA damage response. However, the mechanism by which BRCA1 is recruited to DNA damage sites remains elusive. Recently, we have demonstrated that a ubiquitin-binding protein, RAP80, is required for DNA damage-induced BRCA1 translocation. Here we identify another component, CCDC98, in the BRCA1-RAP80 complex. CCDC98 mediates BRCA1's association with RAP80. Moreover, CCDC98 controls both DNA damage-induced formation of BRCA1 foci and BRCA1-dependent G2/M checkpoint activation. Together, our results demonstrate that CCDC98 is a BRCA1 binding partner that mediates BRCA1 function in response to DNA damage.

Brca1 heterozygous mice have shortened life span and are prone to ovarian tumorigenesis with haploinsufficiency upon ionizing irradiation

BRCA1 mutation carriers have an 85% lifetime risk of breast cancer and 60% for ovarian cancer. BRCA1 facilitates DNA double-strand break repair, and dysfunction of BRCA1 leads to hypersensitivity to DNA damaging agents and consequently genomic instability of cells. In this communication, we have examined the tumor incidence and survival of Brca1 heterozygous female mice. Brca1 heterozygotes appear to have a shortened life span with 70% tumor incidence. Lymphoma, but not ovarian and mammary gland tumors, occurs commonly in these mice. After a whole-body exposure to ionizing radiation, Brca1 heterozygous mice have a 3-5-fold higher incidence specific to ovarian tumors, but not lymphoma, when compared with the Brca1+/+ mice. All the tumors from heterozygous mice examined retain the wild-type allele and the cancer cells express Brca1 protein, precluding the chromosomal mechanism for loss of heterozygosity of Brca1 locus. Although the manifestation of BRCA1 haploinsufficiency may be different between human and mouse, this study suggests that women carrying Brca1 mutations may be more prone to ovarian tumor formation after IR exposure than nonmutation carriers.

Prevention of Brca1-Mediated Mammary Tumorigenesis in Mice by a Progesterone Antagonist

Women with mutations in the breast cancer susceptibility gene BRCA1 are predisposed to breast and ovarian cancers. Why the BRCA1 protein suppresses tumor development specifically in ovarian hormone-sensitive tissues remains unclear. We demonstrate that mammary glands of nulliparous Brca1/p53-deficient mice accumulate lateral branches and undergo extensive alveologenesis, a phenotype that occurs only during pregnancy in wild-type mice. Progesterone receptors, but not estrogen receptors, are overexpressed in the mutant mammary epithelial cells because of a defect in their degradation by the proteasome pathway. Treatment of Brca1/p53-deficient mice with the progesterone antagonist mifepristone (RU 486) prevented mammary tumorigenesis. These findings reveal a tissue-specific function for the BRCA1 protein and raise the possibility that antiprogesterone treatment may be useful for breast cancer prevention in individuals with BRCA1 mutations.

Following the path of the virus: the exploitation of host DNA repair mechanisms by retroviruses

Numerous host cellular cofactors are involved in the life cycle of retroviruses. Importantly, DNA repair machinery of infected cells is activated by retroviruses and retroviral vectors during the process of integration and host cell DNA repair proteins are employed to create a fully integrated provirus. The full delineation of these repair mechanisms that are triggered by retroviruses also has implications outside of the field of retrovirology. It will undoubtedly be of interest to developers of gene therapy and will also further facilitate our understanding of DNA repair and cancer. This review gives a brief summary of the accomplishments in the field of DNA repair and retroviral integration and the opportunities that this area of science provides with regards to the elucidation of repair mechanisms, in the context of retroviral infection.

Valosin-containing protein phosphorylation at Ser784 in response to DNA damage

The response of eukaryotic cells to DNA damage includes the activation of phosphatidylinositol-3 kinase-related kinases (PIKK), such as ATM, ATR, and DNA-dependent protein kinase (DNA-PK). These three kinases have very similar substrate specificities in vitro, but in vivo, their substrates overlap only partially. Several in vivo substrates of ATM and ATR have been identified and almost all of them are involved in DNA damage-induced cell cycle arrest and/or apoptosis. In contrast, few in vivo substrates of DNA-PK have been identified. These include histone H2AX and DNA-PK itself. We identify here valosin-containing protein (VCP) as a novel substrate of DNA-PK and other PIKK family members. VCP is phosphorylated at Ser784 within its COOH terminus, a region previously shown to target VCP to specific intracellular compartments. Furthermore, VCP phosphorylated at Ser784 accumulated at sites of DNA double-strand breaks (DSBs). VCP is a protein chaperone that unfolds and translocates proteins. Its phosphorylation in response to DNA damage and its recruitment to sites of DNA DSBs could indicate a role of VCP in DNA repair.

To die or not to die: DNA repair in neurons

One of the critical emerging problems in modern pathobiology is how cells govern the decision to live or die, and the cost of making such a decision. Nowhere are these questions more poignant than in deciphering the tissue-specific responses to DNA damage. Mutations in DNA repair enzymes, malfunctions in cell cycle regulation, and genetic instability are associated with most somatic cancers. However, in many hereditary diseases arising from mutations in DNA repair proteins, the same dominant mutations that cause cancer in dividing cells are often associated with cell death in terminally differentiated neurons. Context dependent differences in the response to DNA damage are used to make fundamental choices as to cell fate, and are likely to shed light on the mechanisms underlying human disease.

Depletion of BRCA1 impairs differentiation but enhances proliferation of mammary epithelial cells

Cumulative evidence indicates that breast cancer-associated gene 1 (BRCA1) participates in DNA damage repair and cell-cycle checkpoint control, serving as a tumor susceptibility gene to maintain the global genomic stability. However, whether BRCA1 has a direct role in cell proliferation and differentiation, two key biological functions in tumorigenesis, remains unclear. Here we demonstrate BRCA1 mediates differentiation of mammary epithelial cell (MEC) for acinus formation by using the in vitro 3D culture system. Reduction of BRCA1 in MEC by RNA interference impairs the acinus formation but enhances proliferation. Such aberrations can be rescued by expression of wild-type BRCA1 as well as a mutant at the RAD50-binding domain but not at the C-terminal BRCT domain, suggesting that the C-terminal BRCT domain has a critical role in these processes. Consistently, depletion of BRCA1 up-regulates the gene expression for proliferation but down-regulates that for differentiation. Moreover, application of the medium conditioned by differentiating normal MEC can reverse the phenotypes of differentiation-defective breast cancer cells bearing reduced BRCA1 functions. Our observation implies BRCA1 is involved in secretion of certain paracrine/autocrine factors that induce MEC differentiation in response to extracellular matrix signals, providing, in part, an explanation for the etiological basis of either sporadic or familial breast cancer due to the loss or reduction of BRCA1.

Analysis of the DNA substrate specificity of the human BACH1 helicase associated with breast cancer

We have investigated the DNA substrate specificity of BACH1 (BRCA1-associated C-terminal helicase). The importance of various DNA structural elements for efficient unwinding by purified recombinant BACH1 helicase was examined. The results indicated that BACH1 preferentially binds and unwinds a forked duplex substrate compared with a duplex flanked by only one single-stranded DNA (ssDNA) tail. In support of its DNA substrate preference, helicase sequestration studies revealed that BACH1 can be preferentially trapped by forked duplex molecules. BACH1 helicase requires a minimal 5 ' ssDNA tail of 15 nucleotides for unwinding of conventional duplex DNA substrates; however, the enzyme is able to catalytically release the third strand of the homologous recombination intermediate D-loop structure irrespective of DNA tail status. In contrast, BACH1 completely fails to unwind a synthetic Holliday junction structure. Moreover, BACH1 requires nucleic acid continuity in the 5 ' ssDNA tail of the forked duplex substrate within six nucleotides of the ssDNA-dsDNA junction to initiate efficiently DNA unwinding. These studies provide the first detailed information on the DNA substrate specificity of BACH1 helicase and provide insight to the types of DNA structures the enzyme is likely to act upon to perform its functions in DNA repair or recombination.

DNA repair, genome stability, and aging

Aging can be defined as progressive functional decline and increasing mortality over time. Here, we review evidence linking aging to nuclear DNA lesions: DNA damage accumulates with age, and DNA repair defects can cause phenotypes resembling premature aging. We discuss how cellular DNA damage responses may contribute to manifestations of aging. We review Sir2, a factor linking genomic stability, metabolism, and aging. We conclude with a general discussion of the role of mutant mice in aging research and avenues for future investigation.

Hypersensitivity of Brca1-deficient MEF to the DNA interstrand crosslinking agent mitomycin C is associated with defect in homologous recombination repair and aberrant S-phase arrest

Hypersensitivity of Brca1-deficient cells to interstrand crosslinking (ICL) agents such as cisplatin and mitomycin C (MMC) implicates an important role for Brca1 in cellular response to the ICL DNA damage repair. However, the detailed mechanism of how Brca1 is involved in the ICL response remains unclear. In this study, we analysed the cellular response to MMC treatment using isogenic mouse embryonic fibroblasts (MEFs) including wild type, p53-/- and p53-/-Brca1-/-. Marked hypersensitivity of p53-/- Brca1-/- MEFs to MMC was found, and the reconstitution of Brca1 expression in these cells restored resistance to MMC. Upon MMC treatment, wild-type MEF was temporarily arrested at G2/M phase but subsequently resumed a normal cell cycle progression. In contrast, Brca1-deficient MEF exhibited a marked time-dependent accumulation of cells arrested at S phase and a prolonged increase in the G2/M population, followed by extensive cell deaths. Importantly, DNA damage-induced Rad51 foci were not formed in these cells, suggesting a defect in homologous recombination. Such defects are fully rescued by reconstitution of Brca1 expression in Brca1-deficient MEF, suggesting that Brca1 directly plays an essential role in ICL repair, which depends on homologous recombination during S phase.

Nuclear Targeting and Cell Cycle Regulatory Function of Human BARD1

The BARD1 gene is mutated in a subset of breast and ovarian cancers, implicating BARD1 as a potential tumor suppressor. BARD1 gains a ubiquitin E3 ligase activity when heterodimerized with BRCA1, but the only known BRCA1-independent BARD1 function is a p53-dependent proapoptotic activity stimulated by nuclear export to the cytoplasm. We described previously the nuclear-cytoplasmic shuttling of BARD1, and in this study, we identify the transport sequences that target BARD1 to the nucleus and show that they are essential for BARD1 regulation of the cell cycle. We used deletion mapping and mutagenesis to define two active nuclear localization signals (NLSs) present in human BARD1 that are not conserved in rodent BARD1. Site-directed mutagenesis of the primary bipartite NLS abolished BARD1 nuclear import and caused its cytoplasmic accumulation. Using flow cytometry and 5-bromo-2-deoxyuridine incorporation assays, we discovered that transiently expressed BARD1 can elicit a p53-independent cell cycle arrest in G1 phase, and that this was abrogated by mutation of the BARD1 NLS but not by mutation of the nuclear export signal. Thus, BARD1 regulation of the cell cycle is a nuclear event and may be linked to its induced expression during mitosis and its possible involvement in the DNA damage checkpoint.

The cellular response to general and programmed DNA double strand breaks

DNA double strand breaks (DSBs) are among the most dangerous lesions that can occur in the genome of eukaryotic cells. Proper repair of chromosomal DSBs is critical for maintaining cellular viability and genomic integrity and, in multi-cellular organisms, for suppression of tumorigenesis. Thus, eukaryotic cells have evolved specialized and redundant molecular mechanisms to sense, respond to, and repair DSBs. In this chapter, we provide an overview of the progress that has been made over the last decade in elucidating the identity and function of components that participate in the cellular response to chromosomal DSBs. Then, we discuss, in more depth, the response to DSBs that occur in the context of the V(D)J recombination and IgH class switch recombination reactions that occur in cells of the lymphocyte lineage.

Regulation of BRCA1, BRCA2 and BARD1 intracellular trafficking

The subcellular location and function of many proteins are regulated by nuclear-cytoplasmic shuttling. BRCA1 and BARD1 provide an interesting model system for understanding the influence of protein dimerization on nuclear transport and localization. These proteins function predominantly in the nucleus to regulate cell cycle progression, DNA repair/recombination and gene transcription, and their export to the cytoplasm has been linked to apoptosis. Germ-line mutations in the BRCA1/BRCA2 and BARD1 genes predispose to risk of breast/ovarian cancer, and certain mutations impair protein function and nuclear accumulation. BRCA1 and BARD1 shuttle between the nucleus and cytoplasm; however heterodimerization masks the nuclear export signals located within each protein, causing nuclear retention of the BRCA1-BARD1 complex and potentially influencing its role in DNA repair, cell survival and regulation of centrosome duplication. This review discusses BRCA1, BRCA2 and BARD1 subcellular localization with emphasis on regulation of transport by protein dimerization and its functional implications.