Physiological modulation of endogenous BRCA1 p220 abundance suppresses DNA damage during the cell cycle

A transcription-independent mechanism determines rapid periodic fluctuations of BRCA1 expression

BRCA1 expression is highly regulated to prevent genomic instability and tumorigenesis. Dysregulation of BRCA1 expression correlates closely with sporadic basal-like breast cancer and ovarian cancer. The most significant characteristic of BRCA1 regulation is periodic expression fluctuation throughout the cell cycle, which is important for the orderly progression of different DNA repair pathways throughout the various cell cycle phases and for further genomic stability. However, the underlying mechanism driving this phenomenon is poorly understood. Here, we demonstrate that RBM10-mediated RNA alternative splicing coupled to nonsense-mediated mRNA decay (AS-NMD), rather than transcription, determines the periodic fluctuations in G1/S-phase BRCA1 expression. Furthermore, AS-NMD broadly regulates the expression of period genes, such as DNA replication-related genes, in an uneconomical but more rapid manner. In summary, we identified an unexpected posttranscriptional mechanism distinct from canonical processes that mediates the rapid regulation of BRCA1 as well as other period gene expression during the G1/S-phase transition and provided insights into potential targets for cancer therapy.

WWOX binds MERIT40 and modulates its function in homologous recombination, implications in breast cancer

The tumor suppressor gene WWOX is localized in an unstable chromosomal region and its expression is decreased or absent in several types of cancer. A low expression of WWOX is associated with a poor prognosis in breast cancer (BC). It has recently been shown that WWOX contributes to genome stability through its role in the DNA damage response (DDR). In breast cancer cells, WWOX inhibits homologous recombination (HR), and thus promotes the repair of DNA double-stranded breaks (DSBs) by non-homologous end joining (NHEJ). The fine-tuning modulation of HR activity is crucial. Its under or overstimulation inducing genome alterations that can induce cancer. MERIT40 is a positive regulator of the DDR. This protein is indispensable for the function of the multi-protein complex BRCA1-A, which suppresses excessive HR activity. MERIT40 also recruits Tankyrase, a positive regulator of HR, to the DSBs to stimulate DNA repair. Here, we identified MERIT40 as a new molecular partner of WWOX. We demonstrated that WWOX inhibited excessive HR activity induced by overexpression of MERIT40. We showed that WWOX impaired the MERIT40-Tankyrase interaction preventing the role of the complex on DSBs. Furthermore, we found that MERIT40 is overexpressed in BC and that this overexpression is associated to a poor prognosis. These results strongly suggest that WWOX, through its interaction with MERIT40, prevents the deleterious impact of excessive HR on BC development by inhibiting MERIT40-Tankyrase association. This inhibitory effect of WWOX would oppose MERIT40-dependent BC development.

BRCA1 degradation in response to mitochondrial damage in breast cancer cells

BRCA1 is a well-studied tumor suppressor involved in the homologous repair of DNA damage, whereas PINK1, a mitochondrial serine/threonine kinase, is known to be involved in mitochondrial quality control. Genetic mutations of PINK1 and Parkin cause autosomal recessive early-onset Parkinson's disease. We found that in breast cancer cells, the mitochondrial targeting reagents, which all induce mitochondrial depolarization along with PINK1 upregulation, induced proteasomal BRCA1 degradation. This BRCA1 degradation was dependent on PINK1, and BRCA1 downregulation upon mitochondrial damage caused DNA double-strand breaks. BRCA1 degradation was mediated through the direct interaction with the E3 ligase Parkin. Strikingly, BRCA1 and PINK1/Parkin expression were inversely correlated in cancerous mammary glands from breast cancer patients. BRCA1 knockdown repressed cancer cell growth, and high BRCA1 expression predicted poor relapse-free survival in breast cancer patients. These observations indicate a novel mechanism by which mitochondrial damage is transmitted to the nucleus, leading to BRCA1 degradation.

BRCA1 and RNAi factors promote repair mediated by small RNAs and PALB2-RAD52

Strong connections exist between R-loops (three-stranded structures harbouring an RNA:DNA hybrid and a displaced single-strand DNA), genome instability and human disease^1-5. Indeed, R-loops are favoured in relevant genomic regions as regulators of certain physiological processes through which homeostasis is typically maintained. For example, transcription termination pause sites regulated by R-loops can induce the synthesis of antisense transcripts that enable the formation of local, RNA interference (RNAi)-driven heterochromation⁶. Pause sites are also protected against endogenous single-stranded DNA breaks by BRCA1⁷. Hypotheses about how DNA repair is enacted at pause sites include a role for RNA, which is emerging as a normal, albeit unexplained, regulator of genome integrity⁸. Here we report that a species of single-stranded, DNA-damage-associated small RNA (sdRNA) is generated by a BRCA1-RNAi protein complex. sdRNAs promote DNA repair driven by the PALB2-RAD52 complex at transcriptional termination pause sites that form R-loops and are rich in single-stranded DNA breaks. sdRNA repair operates in both quiescent (G0) and proliferating cells. Thus, sdRNA repair can occur in intact tissue and/or stem cells, and may contribute to tumour suppression mediated by BRCA1.

BRCA1 protects cardiac microvascular endothelial cells against irradiation by regulating p21-mediated cell cycle arrest

AIMS

Microvascular endothelial cell dysfunction is a leading cause of radiation-induced heart disease (RIHD). BRCA1 plays an important role in DNA damage repair. The study aims to explore the effect of BRCA1 in endothelial cells involved in RIHD.

MATERIALS AND METHODS

BRCA1 and p21 expression were detected in human umbilical vein endothelial cells (HUVECs) and in mouse heart tissue after irradiation exposure. The effects of BRCA1 on cell proliferation, cell cycle and radiosensitivity were determined in HUVECs with overexpression and knockdown of BRCA1. A mouse model of RIHD was established. Heart damage was detected in C57BL/6J mice and endothelial cell specific knockout BRCA1 mice (EC-BRCA1^-/-).

KEY FINDINGS

BRCA1 and p21 expression was significantly increased both in vitro and vivo response to irradiation. BRCA1 overexpression in endothelial cells enhanced cell growth and G1/S phase arrest, and the opposite results were observed in BRCA1 knockdown endothelial cells. BRCA1 downregulated endothelial cell cycle-related genes cyclin A, cyclin D1, cyclin E and p-Rb through increasing p21 expression, and HUVECs with BRCA1 gene knockdown were more sensitive to radiation. In vivo, a decrease in cardiac microvascular density, as well as cardiomyocyte hypoxia and apoptosis were observed in a time-dependent manner. EC-BRCA1^-/- mice were more prone to severe RIHD than EC-BRCA1^+/- mice after 16Gy radiation exposure due to endothelial dysfunction caused by loss of BRCA1, and p21 was declined in EC-BRCA1^-/- mice heart.

SIGNIFICANCE

These findings indicate that BRCA1 plays a protective role in RIHD by regulating endothelial cell cycle arrest mediated by p21 signal.

RAP80 and BRCA1 PARsylation protect chromosome integrity by preventing retention of BRCA1-B/C complexes in DNA repair foci

Normally, BRCA1 promotes physiological, error-free homologous recombination repair (HRR) of damaged DNA and genome stability. In contrast, excessive, deregulated HRR can lead to genome instability. The BRCA1-binding protein RAP80 restricts HRR amplitude and genome instability, at least in part by manifesting polyubiquitin and poly-ADP-ribose binding activities in postdamage nuclear foci. Although how these processes operate in detail remains unknown, we find that simultaneous defects in RAP80/BRCA1 complex formation and in BRCA1 poly-ADP-ribosylation result in the persistent accumulation of BRCA1-containing complexes in nuclear foci that also contain CtIP and BACH1. These effects lead to excessive HRR, chromosomal hyper-recombination, and gross chromosomal abnormalities.

BRCA1 promotes error-free, homologous recombination-mediated repair (HRR) of DNA double-stranded breaks (DSBs). When excessive and uncontrolled, BRCA1 HRR activity promotes illegitimate recombination and genome disorder. We and others have observed that the BRCA1-associated protein RAP80 recruits BRCA1 to postdamage nuclear foci, and these chromatin structures then restrict the amplitude of BRCA1-driven HRR. What remains unclear is how this process is regulated. Here we report that both BRCA1 poly-ADP ribosylation (PARsylation) and the presence of BRCA1-bound RAP80 are critical for the normal interaction of BRCA1 with some of its partners (e.g., CtIP and BACH1) that are also known components of the aforementioned focal structures. Surprisingly, the simultaneous loss of RAP80 and failure therein of BRCA1 PARsylation results in the dysregulated accumulation in these foci of BRCA1 complexes. This in turn is associated with the intracellular development of a state of hyper-recombination and gross chromosomal disorder. Thus, physiological RAP80-BRCA1 complex formation and BRCA1 PARsylation contribute to the kinetics by which BRCA1 HRR-sustaining complexes normally concentrate in nuclear foci. These events likely contribute to aneuploidy suppression.

Ftx non coding RNA-derived miR-545 promotes cell proliferation by targeting RIG-I in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third cause of cancer-related death worldwide. Accumulating studies have demonstrated that aberrant expression of several lncRNAs was found to be involved in the hepatocarcinogenesis. In this study, a lncRNA Ftx was chosen to investigate its effects on HCC cells, and clarify the possible mechanism. We demonstrated that the lncRNA Ftx and Ftx-derived miR-545 were up-regulated in both HCC tissues and cells. MiR-545 was positively correlated with lncRNA Ftx expression. Notably, clinical association analysis revealed that the high expression of lncRNA Ftx and miR-545 was associated with poor prognostic features, and conferred a reduced 5-year overall survival (OS) and disease-free survival (DFS) of HCC patients. We found that miR-545 was a pivotal mediator in Ftx-induced promotion of HCC cell growth. Subsequently, we identified RIG-I as a direct target of miR-545. The expression of RIG-I was downregulated in HCC tissues and was inversely correlated with miR-545 expression. Our data revealed that ectopic expression of RIG-I abrogated the effects of lncRNA Ftx or miR-545 on HCC cells. LncRNA Ftx/miR-545-mediated downregulation of RIG-I led to increased Akt phosphorylation in vitro and in vivo. Inhibition of Akt phosphorylation abolished the effects of lncRNA Ftx/miR-545 on HCC cells. In conclusion, our study demonstrates that the novel pathway lncRNA Ftx/miR-545/RIG-I promotes HCC development by activating PI3K/Akt signaling, and it may serve as a novel prognostic biomarker and therapeutic target for HCC.

Dysregulation of the BRCA1/long non-coding RNA NEAT1 signaling axis contributes to breast tumorigenesis

Dysregulation of long non-codng RNA (lncRNA) expression has been found to contribute to tumorigenesis. However, the roles of lncRNAs in BRCA1-related breast cancer remain largely unknown. In this study, we delineate the role of the novel BRCA1/lncRNA NEAT1 signaling axis in breast tumorigenesis. BRCA1 inhibits NEAT1 expression potentially through binding to its genomic binding site upstream of the NEAT1 gene. BRCA1 deficiency in human normal/cancerous breast cells and mouse mammary glands leads to NEAT1 overexpression. Our studies show that NEAT1 upregulation resulting from BRCA1 deficiency stimulates in vitro and in vivo breast tumorigenicity. We have further identified molecular mediators downstream of the BRCA1/NEAT1 axis. NEAT1 epigenetically silences miR-129-5p expression by promoting the DNA methylation of the CpG island in the miR-129 gene. Silencing of miR-129-5p expression by NEAT1 results in upregulation of WNT4 expression, a target of miR-129-5p, which leads to activation of oncogenic WNT signaling. Our functional studies indicate that this NEAT1/miR-129-5p/WNT4 axis contributes to the tumorigenic effects of BRCA1 deficiency. Finally our in silico expression correlation analysis suggests the existence of the BRCA1/NEAT1/miR-129-5p axis in breast cancer. Our findings, taken together, suggest that the dysregulation of the BRCA1/NEAT1/miR-129-5p/WNT4 signaling axis is involved in promoting breast tumorigenesis.

The BRCA1/2-directed miRNA signature predicts a good prognosis in ovarian cancer patients with wild-type BRCA1/2

Ovarian cancer patients carrying alterations (i.e., germline mutations, somatic mutations, hypermethylations and/or deletions) of BRCA1 or BRCA2 (BRCA1/2) have a better prognosis than BRCA1/2 alteration non-carriers. However, patients with wild-type BRCA1/2 may also have a favorable prognosis as a result of other mechanisms that remain poorly elucidated, such as the deregulation of miRNAs. We therefore sought to identify BRCA1/2-directed miRNA signatures that have prognostic value in ovarian cancer patients with wild-type BRCA1/2 and study how the deregulation of miRNAs impacts the prognosis of patients treated with platinum-based chemotherapy. By analyzing multidimensional datasets of ovarian cancer patients from the TCGA data portal, we identified three miRNAs (hsa-miR-146a, hsa-miR-148a and hsa-miR-545) that target BRCA1/2 and were associated with overall survival and progression-free survival in patients with wild-type BRCA1/2. By analyzing the expression profiles and Gene Ontology functional enrichment, we found that carriers of BRCA1/2 alterations and patients with miRNA deregulation shared a common mechanism, regulation of the DNA repair-related pathways, that affects the prognosis of ovarian cancer patients. Our work highlights that a proportion of patients with wild-type BRCA1/2 ovarian cancers benefit from platinum-based chemotherapy and that the patients who benefit could be predicted from BRCA1/2-directed miRNA signature.

Platinum and PARP Inhibitor Resistance Due to Overexpression of MicroRNA-622 in BRCA1-Mutant Ovarian Cancer

High-grade serous ovarian carcinomas (HGSOCs) with BRCA1/2 mutations exhibit improved outcome and sensitivity to double-strand DNA break (DSB)-inducing agents (i.e., platinum and poly(ADP-ribose) polymerase inhibitors [PARPis]) due to an underlying defect in homologous recombination (HR). However, resistance to platinum and PARPis represents a significant barrier to the long-term survival of these patients. Although BRCA1/2-reversion mutations are a clinically validated resistance mechanism, they account for less than half of platinum-resistant BRCA1/2-mutated HGSOCs. We uncover a resistance mechanism by which a microRNA, miR-622, induces resistance to PARPis and platinum in BRCA1 mutant HGSOCs by targeting the Ku complex and restoring HR-mediated DSB repair. Physiologically, miR-622 inversely correlates with Ku expression during the cell cycle, suppressing non-homologous end-joining and facilitating HR-mediated DSB repair in S phase. Importantly, high expression of miR-622 in BRCA1-deficient HGSOCs is associated with worse outcome after platinum chemotherapy, indicating microRNA-mediated resistance through HR rescue.

Serine/arginine-rich splicing factor 3 (SRSF3) regulates homologous recombination-mediated DNA repair

Background

Our previous work found that serine/arginine-rich splicing factor 3 (SRSF3) was overexpressed in human ovarian cancer and the overexpression of SRSF3 was required for ovarian cancer cell growth and survival. The mechanism underlying the role of SRSF3 in ovarian cancer remains to be addressed.

Methods

We conducted microarray analysis to profile the gene expression and splicing in SRSF3-knockdown cells and employed quantitative PCR and western blotting to validate the profiling results. We used chromatin immunoprecipitation to study transcription and the direct repeat green fluorescent protein reporter assay to study homologous recombination-mediated DNA repair (HRR).

Results

We identified 687 genes with altered expression and 807 genes with altered splicing in SRSF3-knockdown cells. Among expression-altered genes, those involved in HRR, including BRCA1, BRIP1 and RAD51, were enriched and were all downregulated. We demonstrated that the downregulation of BRCA1, BRIP1 and RAD51 expression was caused by decreased transcription and not due to increased nonsense-mediated mRNA decay. Further, we found that SRSF3 knockdown impaired HRR activity in the cell and increased the level of γ-H2AX, a biomarker for double-strand DNA breaks. Finally, we observed that SRSF3 knockdown changed splicing pattern of KMT2C, a H3K4-specific histone methyltransferase, and reduced the levels of mono- and trimethylated H3K4.

Conclusion

These results suggest that SRSF3 is a new regulator of HRR process, which possibly regulates the expression of HRR-related genes indirectly through an epigenetic pathway. This new function of SRSF3 not only explains why overexpression of SRSF3 is required for ovarian cancer cell growth and survival but also offers a new insight into the mechanism of the neoplastic transformation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0422-1) contains supplementary material, which is available to authorized users.

Cell cycle regulation of human DNA repair and chromatin remodeling genes

Maintenance of a genome requires DNA repair integrated with chromatin remodeling. We have analyzed six transcriptome data sets and one data set on translational regulation of known DNA repair and remodeling genes in synchronized human cells. These data are available through our new database: www.dnarepairgenes.com. Genes that have similar transcription profiles in at least two of our data sets generally agree well with known protein profiles. In brief, long patch base excision repair (BER) is enriched for S phase genes, whereas short patch BER uses genes essentially equally expressed in all cell cycle phases. Furthermore, most genes related to DNA mismatch repair, Fanconi anemia and homologous recombination have their highest expression in the S phase. In contrast, genes specific for direct repair, nucleotide excision repair, as well as non-homologous end joining do not show cell cycle-related expression. Cell cycle regulated chromatin remodeling genes were most frequently confined to G1/S and S. These include e.g. genes for chromatin assembly factor 1 (CAF-1) major subunits CHAF1A and CHAF1B; the putative helicases HELLS and ATAD2 that both co-activate E2F transcription factors central in G1/S-transition and recruit DNA repair and chromatin-modifying proteins and DNA double strand break repair proteins; and RAD54L and RAD54B involved in double strand break repair. TOP2A was consistently most highly expressed in G2, but also expressed in late S phase, supporting a role in regulating entry into mitosis. Translational regulation complements transcriptional regulation and appears to be a relatively common cell cycle regulatory mechanism for DNA repair genes. Our results identify cell cycle phases in which different pathways have highest activity, and demonstrate that periodically expressed genes in a pathway are frequently co-expressed. Furthermore, the data suggest that S phase expression and over-expression of some multifunctional chromatin remodeling proteins may set up feedback loops driving cancer cell proliferation.

BRCA1 functions as a novel transcriptional cofactor in HIV-1 infection

Background

Viruses have naturally evolved elegant strategies to manipulate the host’s cellular machinery, including ways to hijack cellular DNA repair proteins to aid in their own replication. Retroviruses induce DNA damage through integration of their genome into host DNA. DNA damage signaling proteins including ATR, ATM and BRCA1 contribute to multiple steps in the HIV-1 life cycle, including integration and Vpr-induced G₂/M arrest. However, there have been no studies to date regarding the role of BRCA1 in HIV-1 transcription.

Methods

Here we performed various transcriptional analyses to assess the role of BRCA1 in HIV-1 transcription by overexpression, selective depletion, and treatment with small molecule inhibitors. We examined association of Tat and BRCA1 through in vitro binding assays, as well as BRCA1-LTR association by chromatin immunoprecipitation.

Results

BRCA1 was found to be important for viral transcription as cells that lack BRCA1 displayed severely reduced HIV-1 Tat-dependent transcription, and gain or loss-of-function studies resulted in enhanced or decreased transcription. Moreover, Tat was detected in complex with BRCA1 aa504-802. Small molecule inhibition of BRCA1 phosphorylation effector kinases, ATR and ATM, decreased Tat-dependent transcription, whereas a Chk2 inhibitor showed no effect. Furthermore, BRCA1 was found at the viral promoter and treatment with curcumin and ATM inhibitors decreased BRCA1 LTR occupancy. Importantly, these findings were validated in a highly relevant model of HIV infection and are indicative of BRCA1 phosphorylation affecting Tat-dependent transcription.

Conclusions

BRCA1 presence at the HIV-1 promoter highlights a novel function of the multifaceted protein in HIV-1 infection. The BRCA1 pathway or enzymes that phosphorylate BRCA1 could potentially be used as complementary host-based treatment for combined antiretroviral therapy, as there are multiple potent ATM inhibitors in development as chemotherapeutics.

miR-19, a component of the oncogenic miR-17∼92 cluster, targets the DNA-end resection factor CtIP

MicroRNA-19 (miR-19) was recently identified as the key oncogenic component of the polycistronic miR-17∼92 cluster, also known as oncomiR-1, which is frequently upregulated or amplified in multiple tumor types. However, the gene targets and the pathways underlying the tumor-promoting activity of miR-19 still remain largely elusive. CtIP/RBBP8 promotes DNA-end resection, a critical step in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR), and is considered to function as a tumor suppressor. In this study, we show that miR-19 downregulates CtIP expression by binding to two highly conserved sequences located in the 3'-untranslated region of CtIP mRNA. We further demonstrate that CtIP expression is repressed by miR-19 during continuous genotoxic stress in a p53-dependent manner. Finally, we report that miR-19 impairs CtIP-mediated DNA-end resection, which results in reduced HR levels and DNA damage hypersensitivity. By downregulating CtIP, miR-19 overexpression suppresses the faithful repair of DSBs that is crucial for genome maintenance. Our findings thus provide new mechanistic insight into the oncogenic role of the miR-17∼92 cluster.

PARP1-driven poly-ADP-ribosylation regulates BRCA1 function in homologous recombination-mediated DNA repair

BRCA1 promotes homologous recombination-mediated DNA repair (HRR). However, HRR must be tightly regulated to prevent illegitimate recombination. We previously found that BRCA1 HRR function is regulated by the RAP80 complex, but the mechanism was unclear. We have now observed that PARP1 interacts with and poly-ADP-ribosylates (aka PARsylates) BRCA1. PARsylation is directed at the BRCA1 DNA binding domain and downmodulates its function. Moreover, RAP80 contains a poly-ADP-ribose-interacting domain that binds PARsylated BRCA1 and helps to maintain the stability of PARP1-BRCA1-RAP80 complexes. BRCA1 PARsylation is a key step in BRCA1 HRR control. When BRCA1 PARsylation is defective, it gives rise to excessive HRR and manifestations of genome instability. BRCA1 PARsylation and/or RAP80 expression is defective in a subset of sporadic breast cancer cell lines and patient-derived tumor xenograft models. These observations are consistent with the possibility that such defects, when chronic, contribute to tumor development in BRCA1+/+ individuals.

SIGNIFICANCE

We propose a model that describes how BRCA1 functions to both support and restrict HRR. BRCA1 PARsylation is a key event in this process, failure of which triggers hyper-recombination and chromosome instability. Thus, hyperfunctioning BRCA1 can elicit genomic abnormalities similar to those observed in the absence of certain BRCA1 functions.

TGFβ induces "BRCAness" and sensitivity to PARP inhibition in breast cancer by regulating DNA-repair genes

Transforming growth factor beta (TGFβ) proteins are multitasking cytokines, in which high levels at tumor sites generally correlate with poor prognosis in human patients with cancer. Previously, it was reported that TGFβ downregulates the expression of ataxia telangiectasia-mutated (ATM) and mutS homolog 2 (MSH2) in breast cancer cells through an miRNA-mediated mechanism. In this study, expression of a panel of DNA-repair genes was examined, identifying breast cancer 1, early onset (BRCA1) as a target downregulated by TGFβ through the miR181 family. Correlations between the expression levels of TGFβ1 and the miR181/BRCA1 axis were observed in primary breast tumor specimens. By downregulating BRCA1, ATM, and MSH2, TGFβ orchestrates DNA damage response in certain breast cancer cells to induce a "BRCAness" phenotype, including impaired DNA-repair efficiency and synthetic lethality to the inhibition of poly (ADP-ribose) polymerase (PARP). Xenograft tumors with active TGFβ signaling exhibited resistance to the DNA-damaging agent doxorubicin but increased sensitivity to the PARP inhibitor ABT-888. Combination of doxorubicin with ABT-888 significantly improved the treatment efficacy in TGFβ-active tumors. Thus, TGFβ can induce "BRCAness" in certain breast cancers carrying wild-type BRCA genes and enhance the responsiveness to PARP inhibition, and the molecular mechanism behind this is characterized.

IMPLICATIONS

These findings enable better selection of patients with sporadic breast cancer for PARP interventions, which have exhibited beneficial effects in patients carrying BRCA mutations.

MicroRNAs down-regulate homologous recombination in the G1 phase of cycling cells to maintain genomic stability

Homologous recombination (HR)-mediated repair of DNA double-strand break (DSB)s is restricted to the post-replicative phases of the cell cycle. Initiation of HR in the G1 phase blocks non-homologous end joining (NHEJ) impairing DSB repair. Completion of HR in G1 cells can lead to the loss-of-heterozygosity (LOH), which is potentially carcinogenic. We conducted a gain-of-function screen to identify miRNAs that regulate HR-mediated DSB repair, and of these miRNAs, miR-1255b, miR-148b*, and miR-193b* specifically suppress the HR-pathway in the G1 phase. These miRNAs target the transcripts of HR factors, BRCA1, BRCA2, and RAD51, and inhibiting miR-1255b, miR-148b*, and miR-193b* increases expression of BRCA1/BRCA2/RAD51 specifically in the G1-phase leading to impaired DSB repair. Depletion of CtIP, a BRCA1-associated DNA end resection protein, rescues this phenotype. Furthermore, deletion of miR-1255b, miR-148b*, and miR-193b* in independent cohorts of ovarian tumors correlates with significant increase in LOH events/chromosomal aberrations and BRCA1 expression.

DOI:http://dx.doi.org/10.7554/eLife.02445.001

The DNA in a cell is damaged thousands of times every day. One of the most serious types of damage involves something breaking both of the strands in the double helix. Such a double-strand break can delete genes or even kill the cell. In fact, conventional cancer therapy kills cancer cells by causing irreparable double-strand breaks. Conversely, a normal cell that is constantly exposed to DNA damaging agents can become a tumor if double-strand breaks are incorrectly repaired. An efficient and accurate double-strand break repair system needs to be in place to prevent this transformation. Therefore, an in-depth understanding of double-strand break repair and the factors involved are important for both gaining insight into the cause of cancer and to improve cancer therapy.

Cells have evolved several different ways to detect and repair double-strand breaks. A method called homologous recombination, for example, uses an undamaged DNA molecule as a template that can be copied to make new DNA. Since it needs a readily available DNA template, this method only works in phases of the cell growth cycle where there are many copies of DNA—that is, in the post-DNA replication phases. In particular, homologous recombination does not work during the pre-replication, G1 phase. If homologous recombination is attempted during G1, it will block the other methods employed by cells to repair broken strands of DNA.

An important challenge is to understand how homologous recombination is restricted to particular parts of the cell cycle. Although certain proteins associated with the early stages of double-strand repair are thought to determine the type of DNA repair that occurs, the details of this process are not fully understood.

One group of molecules that are thought to be involved are microRNAs, which normally limit the number of proteins produced from certain genes. However, since a single microRNA molecule can be associated with several proteins, and since a single protein can be associated with several microRNA molecules, it has proved difficult to establish the exact effects of a specific microRNA molecule.

Choi et al. now show that seven microRNA molecules can control homologous recombination, and three microRNAs in particular restrict homologous recombination during the G1 phase of the cell cycle. If these microRNAs are inhibited during the G1 phase, which allows homologous recombination to start, and counter-intuitively more double-stranded breaks are seen. However, if a gene involved in starting homologous repair–called CtIP—is silenced while the microRNAs are inhibited, then the DNA breaks are repaired. Exactly, how the microRNA molecules produce different effects during different phases of the cell cycle will be need to be investigated by future studies.

DOI:http://dx.doi.org/10.7554/eLife.02445.002