Gene Summary

Gene:RBBP8; RB binding protein 8, endonuclease
Summary:The protein encoded by this gene is a ubiquitously expressed nuclear protein. It is found among several proteins that bind directly to retinoblastoma protein, which regulates cell proliferation. This protein complexes with transcriptional co-repressor CTBP. It is also associated with BRCA1 and is thought to modulate the functions of BRCA1 in transcriptional regulation, DNA repair, and/or cell cycle checkpoint control. It is suggested that this gene may itself be a tumor suppressor acting in the same pathway as BRCA1. Three transcript variants encoding two different isoforms have been found for this gene. More transcript variants exist, but their full-length natures have not been determined. [provided by RefSeq, Jul 2008]
Databases:OMIM, HGNC, Ensembl, GeneCard, Gene
Protein:DNA endonuclease RBBP8
Source:NCBIAccessed: 30 August, 2019


What does this gene/protein do?
Show (19)
Pathways:What pathways are this gene/protein implicaed in?
Show (1)

Cancer Overview

Research Indicators

Publications Per Year (1994-2019)
Graph generated 30 August 2019 using data from PubMed using criteria.

Literature Analysis

Mouse over the terms for more detail; many indicate links which you can click for dedicated pages about the topic.

  • Messenger RNA
  • Two-Hybrid System Techniques
  • Chromosome 18
  • Transcription
  • Transcription Factors
  • Genetic Predisposition
  • Poly(ADP-ribose) Polymerase Inhibitors
  • Tumor Suppressor p53-Binding Protein 1
  • Protein Binding
  • Protein Structure, Tertiary
  • Carrier Proteins
  • Apoptosis
  • Knockout Mice
  • Phosphorylation
  • Cancer Gene Expression Regulation
  • DNA Repair
  • Cell Line
  • BRCA1
  • Nuclear Proteins
  • Cell Cycle
  • BRCA1 Protein
  • Cell Proliferation
  • Base Sequence
  • BRCA2 Protein
  • Young Adult
  • Antineoplastic Agents
  • Mutation
  • siRNA
  • DNA Damage
  • Neoplasm Proteins
  • Cell Cycle Proteins
  • RBBP8
  • Double-Stranded DNA Breaks
  • Breast Cancer
  • Basic Helix-Loop-Helix Transcription Factors
  • Biomarkers, Tumor
  • Molecular Sequence Data
  • RB1
  • Amino Acid Sequence
  • DNA-Binding Proteins
  • Ovarian Cancer
Tag cloud generated 30 August, 2019 using data from PubMed, MeSH and CancerIndex

Specific Cancers (2)

Data table showing topics related to specific cancers and associated disorders. Scope includes mutations and abnormal protein expression.

Note: list is not exhaustive. Number of papers are based on searches of PubMed (click on topic title for arbitrary criteria used).

Latest Publications: RBBP8 (cancer-related)

Miao WJ, Yuan DJ, Zhang GZ, et al.
lncRNA CASC2/miR‑18a‑5p axis regulates the malignant potential of nasopharyngeal carcinoma by targeting RBBP8.
Oncol Rep. 2019; 41(3):1797-1806 [PubMed] Related Publications
Nasopharyngeal carcinoma (NPC) is a prevalent head and neck tumor which has a high mortality rate in Southeast Asia, especially in Southern China. Cancer susceptibility candidate 2 (CASC2) is a newly identified long non‑coding RNA (lncRNA) that has been found to play a suppressive role in several types of tumors. However, the expression and functional role of CASC2 in NPC are still unclear. In the present study, using NPC tissues, cells and transplanted mice, we investigated the mechanism of CASC2‑mediated regulation of NPC. We showed that the CASC2 level is reduced in NPC tissues and cells. CASC2 downregulation promoted proliferation and inhibited apoptotic cell death in NPC cells. In contrast, CASC2 upregulation inhibited proliferation and increased apoptosis. There were putative binding sites of microRNA (miR)‑18a‑5p in the promoter of CASC2. The level of miR‑18a‑5p was upregulated in NPC tissues and cells. We further confirmed that CASC2 could directly bind with miR‑18a‑5p and inhibit miR‑18a‑5p expression, using reporter gene and RNA immunoprecipitation assays. miR‑18a‑5p suppressed CASC2 upregulation‑mediated decrease in proliferation and increase in apoptotic cell death. Bioinformatics predicted the putative binding site of miR‑18a‑5p in the 3' untranslated region of C‑terminal binding protein interacting protein (CtIP)/RBBP8. It was further confirmed that miR‑18a‑5p could directly bind with RBBP8 and inhibit RBBP8 expression. Downregulation of RBBP8 inhibited the anti‑miR‑18a‑5p‑mediated increase in apoptosis and decrease in proliferation. Downregulation of CASC2 increased tumor growth, increased the level of miR‑18a‑5p and decreased RBBP8 expression in vivo. In summary, CASC2 regulates NPC malignancy through modulation of RBBP8 via sponging miR‑18a‑5p. Our findings highlight the CASC2/miR‑18a‑5p/RBBP8 axis in NPC pathogenesis and provide new biomarkers and potential targets for the therapy of NPC.

Bonache S, Esteban I, Moles-Fernández A, et al.
Multigene panel testing beyond BRCA1/2 in breast/ovarian cancer Spanish families and clinical actionability of findings.
J Cancer Res Clin Oncol. 2018; 144(12):2495-2513 [PubMed] Related Publications
PURPOSE: Few and small studies have been reported about multigene testing usage by massively parallel sequencing in European cancer families. There is an open debate about what genes should be tested, and the actionability of some included genes is under research.
METHODS: We investigated a panel of 34 known high/moderate-risk cancer genes, including 16 related to breast or ovarian cancer (BC/OC) genes, and 63 candidate genes to BC/OC in 192 clinically suspicious of hereditary breast/ovarian cancer (HBOC) Spanish families without pathogenic variants in BRCA1 or BRCA2 (BRCA1/2).
RESULTS: We identified 16 patients who carried a high- or moderate-risk pathogenic variant in eight genes: 4 PALB2, 3 ATM, 2 RAD51D, 2 TP53, 2 APC, 1 BRIP1, 1 PTEN and 1 PMS2. These findings led to increased surveillance or prevention options in 12 patients and predictive testing in their family members. We detected 383 unique variants of uncertain significance in known cancer genes, of which 35 were prioritized in silico. Eighteen loss-of-function variants were detected in candidate BC/OC genes in 17 patients (1 BARD1, 1 ERCC3, 1 ERCC5, 2 FANCE, 1 FANCI, 2 FANCL, 1 FANCM, 1 MCPH1, 1 PPM1D, 2 RBBP8, 3 RECQL4 and 1 with SLX4 and XRCC2), three of which also carry pathogenic variants in known cancer genes.
CONCLUSIONS: Eight percent of the BRCA1/2 negative patients carry pathogenic variants in other actionable genes. The multigene panel usage improves the diagnostic yield in HBOC testing and it is an effective tool to identify potentially new candidate genes.

Gopalakrishnan V, Dahal S, Radha G, et al.
Characterization of DNA double-strand break repair pathways in diffuse large B cell lymphoma.
Mol Carcinog. 2019; 58(2):219-233 [PubMed] Related Publications
Efficient DNA repair is indispensable for maintaining genomic integrity in humans. Cancer associated deletions and mutations are mainly due to misrepaired DNA double-strand breaks (DSBs). Classical nonhomologous end joining (c-NHEJ) and homologous recombination (HR) are two major DSB repair pathways in humans. An error prone, alternative NHEJ pathway that utilizes microhomology was also reported in cancer cells and to a lesser extent in normal cells. In the present study, we evaluated the efficiency of various DSB repair pathways in the most common lymphoma, the diffuse large B cell lymphoma (DLBCL). Here we show that DNA repair through c-NHEJ pathway is limited in SUDHL8, a cell line derived from a DLBCL patient. Unlike c-NHEJ, microhomology mediated end joining (MMEJ) was predominant at physiological temperature. Consistent with the observation, expression level of repair proteins such as LIGASE I, LIGASE III, PARP1, CtIP, and MRE11 was higher in DLBCL cells when compared to c-NHEJ proteins. Further, inhibition of LIGASE I or MRE11, led to reduction in the efficiency of MMEJ in DLBCL cells. Besides, HR-mediated DSB repair occurring through gene conversion was observed. Thus, our results reveal the predominance of MMEJ over c-NHEJ in repairing DSBs in DLBCL cells, while error-free repair through HR was also evident.

Zhen N, Jin L, Ma J, et al.
Ginsenoside Rg1 impairs homologous recombination repair by targeting CtBP-interacting protein and sensitizes hepatoblastoma cells to DNA damage.
Anticancer Drugs. 2018; 29(8):756-766 [PubMed] Related Publications
The ginsenoside Rg1, the primary pharmacologically active ingredient of the traditional Chinese herb ginseng, is widely used in the clinical treatment of diseases of the immune and nervous systems. Recent studies have shown that it also has an antitumor effect. In this study, we explored the effects of Rg1 on hepatoblastoma (HB) and its underlying mechanisms. We demonstrated that Rg1 significantly inhibited HB cell growth both in vivo and in vitro. Mechanistic studies revealed that Rg1 impaired homologous recombination and triggered double-strand breaks in HB cells by directly targeting CtBP-interacting protein (CtIP), a key double-strand break repair factor, which is highly expressed in HB tissues. Moreover, we also demonstrated that Rg1 sensitized HB cells to DNA-damaging agents both in vitro and in vivo. In conclusion, our data not only demonstrate the potential clinical application of Rg1 as a novel chemotherapeutic candidate but also offer a mechanism-based therapeutic option by which DNA-damaging agents can be used in combination with Rg1 to target HB.

Okonkwo A, Mitra J, Johnson GS, et al.
Heterocyclic Analogs of Sulforaphane Trigger DNA Damage and Impede DNA Repair in Colon Cancer Cells: Interplay of HATs and HDACs.
Mol Nutr Food Res. 2018; 62(18):e1800228 [PubMed] Free Access to Full Article Related Publications
SCOPE: DNA repair inhibitors have broad clinical applications in tumor types with DNA repair defects, including colorectal cancer (CRC). Structural analogs of the anticancer agent sulforaphane (SFN) were investigated as modifiers of histone deacetylase (HDAC) and histone acetyltransferase (HAT) activity, and for effects on DNA damage/repair pertinent to human CRC.
METHODS AND RESULTS: In the polyposis in rat colon (Pirc) model, single oral administration of SFN and structurally related long-chain isothiocyanates (ITCs) decreased histone deacetylase 3 (HDAC3) expression and increased pH2AX levels markedly in adenomatous colon polyps, extending prior observations on HDAC3 inhibition/turnover in cell-based assays. Colon cancer cells at a high initial plating density had diminished cytotoxicity from SFN, whereas novel tetrazole-containing heterocyclic analogs of SFN retained their efficacy. The potent SFN analogs triggered DNA damage, cell cycle arrest, apoptosis, and loss of a key DNA repair regulator, C-terminal binding protein (CtBP) interacting protein (CtIP). These SFN analogs also altered HAT/HDAC activities and histone acetylation status, lowered the expression of HDAC3, P300/CBP-associated factor (PCAF) and lysine acetyltransferase 2A (KAT2A/GCN5), and attenuated homologous recombination (HR)/non-homologous end joining (NHEJ) repair activities in colon cancer cells.
CONCLUSION: Novel tetrazole-containing heterocyclic analogs of SFN provide a new avenue for chemosensitization in colon cancer cells via modulation of HAT/HDAC activities and associated DNA damage/repair signaling pathways.

Cánovas B, Igea A, Sartori AA, et al.
Targeting p38α Increases DNA Damage, Chromosome Instability, and the Anti-tumoral Response to Taxanes in Breast Cancer Cells.
Cancer Cell. 2018; 33(6):1094-1110.e8 [PubMed] Related Publications
Breast cancer is the second leading cause of cancer-related death among women. Here we report a role for the protein kinase p38α in coordinating the DNA damage response and limiting chromosome instability during breast tumor progression, and identify the DNA repair regulator CtIP as a p38α substrate. Accordingly, decreased p38α signaling results in impaired ATR activation and homologous recombination repair, with concomitant increases in replication stress, DNA damage, and chromosome instability, leading to cancer cell death and tumor regression. Moreover, we show that pharmacological inhibition of p38α potentiates the effects of taxanes by boosting chromosome instability in murine models and patient-derived xenografts, suggesting the potential interest of combining p38α inhibitors with chemotherapeutic drugs that induce chromosome instability.

Sun C, Yin J, Fang Y, et al.
BRD4 Inhibition Is Synthetic Lethal with PARP Inhibitors through the Induction of Homologous Recombination Deficiency.
Cancer Cell. 2018; 33(3):401-416.e8 [PubMed] Free Access to Full Article Related Publications
Poly(ADP-ribose) polymerase inhibitors (PARPi) are selectively active in cells with homologous recombination (HR) deficiency (HRD) caused by mutations in BRCA1, BRCA2, and other pathway members. We sought small molecules that induce HRD in HR-competent cells to induce synthetic lethality with PARPi and extend the utility of PARPi. We demonstrated that inhibition of bromodomain containing 4 (BRD4) induced HRD and sensitized cells across multiple tumor lineages to PARPi regardless of BRCA1/2, TP53, RAS, or BRAF mutation status through depletion of the DNA double-stand break resection protein CtIP (C-terminal binding protein interacting protein). Importantly, BRD4 inhibitor (BRD4i) treatment reversed multiple mechanisms of resistance to PARPi. Furthermore, PARPi and BRD4i are synergistic in multiple in vivo models.

Mijnes J, Veeck J, Gaisa NT, et al.
Promoter methylation of DNA damage repair (DDR) genes in human tumor entities:
Clin Epigenetics. 2018; 10:15 [PubMed] Free Access to Full Article Related Publications
Background: Genome-wide studies identified pan-cancer genes and shared biological networks affected by epigenetic dysregulation among diverse tumor entities. Here, we systematically screened for hypermethylation of DNA damage repair (DDR) genes in a comprehensive candidate-approach and exemplarily identify and validate candidate DDR genes as targets of epigenetic inactivation unique to bladder cancer (BLCA), which may serve as non-invasive biomarkers.
Methods: Genome-wide DNA methylation datasets (2755 CpG probes of
Results: Overall, 39 DDR genes were hypermethylated in human cancers. Most exclusively and frequently methylated (37%) in primary BLCA was

Chen G, Chen J, Qiao Y, et al.
ZNF830 mediates cancer chemoresistance through promoting homologous-recombination repair.
Nucleic Acids Res. 2018; 46(3):1266-1279 [PubMed] Free Access to Full Article Related Publications
Homologous recombination (HR), which mediates the repair of DNA double-strand breaks (DSB), is crucial for maintaining genomic integrity and enhancing survival in response to chemotherapy and radiotherapy in human cancers. However, the mechanisms of HR repair in treatment resistance for the improvement of cancer therapy remains unclear. Here, we report that the zinc finger protein 830 (ZNF830) promotes HR repair and the survival of cancer cells in response to DNA damage. Mechanistically, ZNF830 directly participates in DNA end resection via interacting with CtIP and regulating CtIP recruitment to DNA damage sites. Moreover, the recruitment of ZNF830 at DNA damage sites is dependent on its phosphorylation at serine 362 by ATR. ZNF830 directly and preferentially binds to double-strand DNA with its 3' or 5' overhang through the Zinc finger (Znf) domain, facilitating HR repair and maintaining genome stability. Thus, our study identified a novel function of ZNF830 as a HR repair regulator in DNA end resection, conferring the chemoresistance to genotoxic therapy for cancers those that overexpress ZNF830.

Chow LWC, Morita S, Chow CYC, et al.
Neoadjuvant palbociclib on ER+ breast cancer (N007): clinical response and EndoPredict's value.
Endocr Relat Cancer. 2018; 25(2):123-130 [PubMed] Free Access to Full Article Related Publications
The purpose of the study was to test the efficacy of neoadjuvant palbociclib therapy and to evaluate its impact on cell cycle arrest and changes in EndoPredict (EP) scores before and after treatment. Postmenopausal women with histologically proven ER+ve, HER2-ve invasive breast cancer, 2 cm or greater, were enrolled in an open-label, single-arm study. Twenty eligible patients were given letrozole 2.5 mg per day together with palbociclib 125 mg per day for 3 out of 4 weeks in repeated cycles for 16 weeks (4 cycles) before surgery. The primary end points were clinical response rates (cRR) and preoperative endocrine prognostic index (PEPI). The secondary end points were pathologic response and gene expression testing with EP test on collected tumor samples. The following results were obtained. 17 patients showed a clinical response of 50% or more, including 8 complete responses and 9 partial responses. There was significant reduction in area (

Sakasai R, Isono M, Wakasugi M, et al.
Aquarius is required for proper CtIP expression and homologous recombination repair.
Sci Rep. 2017; 7(1):13808 [PubMed] Free Access to Full Article Related Publications
Accumulating evidence indicates that transcription is closely related to DNA damage formation and that the loss of RNA biogenesis factors causes genome instability. However, whether such factors are involved in DNA damage responses remains unclear. We focus here on the RNA helicase Aquarius (AQR), a known R-loop processing factor, and show that its depletion in human cells results in the accumulation of DNA damage during S phase, mediated by R-loop formation. We investigated the involvement of Aquarius in DNA damage responses and found that AQR knockdown decreased DNA damage-induced foci formation of Rad51 and replication protein A, suggesting that Aquarius contributes to homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSBs). Interestingly, the protein level of CtIP, a DSB processing factor, was decreased in AQR-knockdown cells. Exogenous expression of Aquarius partially restored CtIP protein level; however, CtIP overproduction did not rescue defective HR in AQR-knockdown cells. In accordance with these data, Aquarius depletion sensitized cells to genotoxic agents. We propose that Aquarius contributes to the maintenance of genomic stability via regulation of HR by CtIP-dependent and -independent pathways.

Cagnetta A, Soncini D, Orecchioni S, et al.
Depletion of SIRT6 enzymatic activity increases acute myeloid leukemia cells' vulnerability to DNA-damaging agents.
Haematologica. 2018; 103(1):80-90 [PubMed] Free Access to Full Article Related Publications
Genomic instability plays a pathological role in various malignancies, including acute myeloid leukemia (AML), and thus represents a potential therapeutic target. Recent studies demonstrate that SIRT6, a NAD

Bombarde O, Larminat F, Gomez D, et al.
The DNA-Binding Polyamine Moiety in the Vectorized DNA Topoisomerase II Inhibitor F14512 Alters Reparability of the Consequent Enzyme-Linked DNA Double-Strand Breaks.
Mol Cancer Ther. 2017; 16(10):2166-2177 [PubMed] Related Publications
Poisons of topoisomerase II (TOP2) kill cancer cells by preventing religation of intermediate DNA breaks during the enzymatic process and thus by accumulating enzyme-drug-DNA complexes called TOP2 cleavage-complex (TOP2cc). F14512 is a highly cytotoxic polyamine-vectorized TOP2 inhibitor derived from etoposide and currently in clinical trials. It was shown

Qu J, Sun W, Zhong J, et al.
Phosphoglycerate mutase 1 regulates dNTP pool and promotes homologous recombination repair in cancer cells.
J Cell Biol. 2017; 216(2):409-424 [PubMed] Free Access to Full Article Related Publications
Glycolytic enzymes are known to play pivotal roles in cancer cell survival, yet their molecular mechanisms remain poorly understood. Phosphoglycerate mutase 1 (PGAM1) is an important glycolytic enzyme that coordinates glycolysis, pentose phosphate pathway, and serine biosynthesis in cancer cells. Herein, we report that PGAM1 is required for homologous recombination (HR) repair of DNA double-strand breaks (DSBs) caused by DNA-damaging agents. Mechanistically, PGAM1 facilitates DSB end resection by regulating the stability of CTBP-interacting protein (CtIP). Knockdown of PGAM1 in cancer cells accelerates CtIP degradation through deprivation of the intracellular deoxyribonucleotide triphosphate pool and associated activation of the p53/p73 pathway. Enzymatic inhibition of PGAM1 decreases CtIP protein levels, impairs HR repair, and hence sensitizes BRCA1/2-proficient breast cancer to poly(ADP-ribose) polymerase (PARP) inhibitors. Together, this study identifies a metabolically dependent function of PGAM1 in promoting HR repair and reveals a potential therapeutic opportunity for PGAM1 inhibitors in combination with PARP inhibitors.

Patel A, Anderson J, Kraft D, et al.
The Influence of the CTIP Polymorphism, Q418P, on Homologous Recombination and Predisposition to Radiation-Induced Tumorigenesis (mainly rAML) in Mice.
Radiat Res. 2016; 186(6):638-649 [PubMed] Related Publications
Exposure to ionizing radiation increases the incidence of acute myeloid leukemia (AML), which has been diagnosed in Japanese atomic bombing survivors, as well as patients treated with radiotherapy. The genetic basis for susceptibility to radiation-induced AML is not well characterized. We previously identified a candidate murine gene for susceptibility to radiation-induced AML (rAML): C-terminal binding protein (CTBP)-interacting protein (CTIP)/retinoblastoma binding protein 8 (RBBP8). This gene is essential for embryonic development, double-strand break (DSB) resection in homologous recombination (HR) and tumor suppression. In the 129S2/SvHsd mouse strain, a nonsynonymous single nucleotide polymorphism (nsSNP) in Ctip, Q418P, has been identified. We investigated the role of Q418P in radiation-induced carcinogenesis and its effect on CTIP function in HR. After whole-body exposure to 3 Gy of X rays, 11 out of 113 (9.7%) 129S2/SvHsd mice developed rAML. Furthermore, 129S2/SvHsd mouse embryonic fibroblasts (MEFs) showed lower levels of recruitment of HR factors, Rad51 and replication protein A (RPA) to radiation-induced foci, compared to CBA/H and C57BL/6 MEFs, isolated from rAML-sensitive and resistant strains, respectively. Mitomycin C and alpha particles induced lower levels of sister chromatid exchanges in 129S2/SvHsd cells compared to CBA/H and C57BL/6. Our data demonstrate that Q418P nsSNP influences the efficiency of CTIP function in HR repair of DNA DSBs in vitro and in vivo, and appears to affect susceptibility to rAML.

Kari V, Mansour WY, Raul SK, et al.
Loss of CHD1 causes DNA repair defects and enhances prostate cancer therapeutic responsiveness.
EMBO Rep. 2016; 17(11):1609-1623 [PubMed] Free Access to Full Article Related Publications
The CHD1 gene, encoding the chromo-domain helicase DNA-binding protein-1, is one of the most frequently deleted genes in prostate cancer. Here, we examined the role of CHD1 in DNA double-strand break (DSB) repair in prostate cancer cells. We show that CHD1 is required for the recruitment of CtIP to chromatin and subsequent end resection during DNA DSB repair. Our data support a role for CHD1 in opening the chromatin around the DSB to facilitate the recruitment of homologous recombination (HR) proteins. Consequently, depletion of CHD1 specifically affects HR-mediated DNA repair but not non-homologous end joining. Together, we provide evidence for a previously unknown role of CHD1 in DNA DSB repair via HR and show that CHD1 depletion sensitizes cells to PARP inhibitors, which has potential therapeutic relevance. Our findings suggest that CHD1 deletion, like BRCA1/2 mutation in ovarian cancer, may serve as a marker for prostate cancer patient stratification and the utilization of targeted therapies such as PARP inhibitors, which specifically target tumors with HR defects.

Kais Z, Rondinelli B, Holmes A, et al.
FANCD2 Maintains Fork Stability in BRCA1/2-Deficient Tumors and Promotes Alternative End-Joining DNA Repair.
Cell Rep. 2016; 15(11):2488-99 [PubMed] Free Access to Full Article Related Publications
BRCA1/2 proteins function in homologous recombination (HR)-mediated DNA repair and cooperate with Fanconi anemia (FA) proteins to maintain genomic integrity through replication fork stabilization. Loss of BRCA1/2 proteins results in DNA repair deficiency and replicative stress, leading to genomic instability and enhanced sensitivity to DNA-damaging agents. Recent studies have shown that BRCA1/2-deficient tumors upregulate Polθ-mediated alternative end-joining (alt-EJ) repair as a survival mechanism. Whether other mechanisms maintain genomic integrity upon loss of BRCA1/2 proteins is currently unknown. Here we show that BRCA1/2-deficient tumors also upregulate FANCD2 activity. FANCD2 is required for fork protection and fork restart in BRCA1/2-deficient tumors. Moreover, FANCD2 promotes Polθ recruitment at sites of damage and alt-EJ repair. Finally, loss of FANCD2 in BRCA1/2-deficient tumors enhances cell death. These results reveal a synthetic lethal relationship between FANCD2 and BRCA1/2, and they identify FANCD2 as a central player orchestrating DNA repair pathway choice at the replication fork.

Zhang Y, Lai J, Du Z, et al.
Targeting radioresistant breast cancer cells by single agent CHK1 inhibitor via enhancing replication stress.
Oncotarget. 2016; 7(23):34688-702 [PubMed] Free Access to Full Article Related Publications
Radiotherapy (RT) remains a standard therapeutic modality for breast cancer patients. However, intrinsic or acquired resistance limits the efficacy of RT. Here, we demonstrate that CHK1 inhibitor AZD7762 alone significantly inhibited the growth of radioresistant breast cancer cells (RBCC). Given the critical role of ATR/CHK1 signaling in suppressing oncogene-induced replication stress (RS), we hypothesize that CHK1 inhibition leads to the specific killing for RBCC due to its abrogation in the suppression of RS induced by oncogenes. In agreement, the expression of oncogenes c-Myc/CDC25A/c-Src/H-ras/E2F1 and DNA damage response (DDR) proteins ATR/CHK1/BRCA1/CtIP were elevated in RBCC. AZD7762 exposure led to significantly higher levels of RS in RBCC, compared to the parental cells. The mechanisms by which CHK1 inhibition led to specific increase of RS in RBCC were related to the interruptions in the replication fork dynamics and the homologous recombination (HR). In summary, RBCC activate oncogenic pathways and thus depend upon mechanisms controlled by CHK1 signaling to maintain RS under control for survival. Our study provided the first example where upregulating RS by CHK1 inhibitor contributes to the specific killing of RBCC, and highlight the importance of the CHK1 as a potential target for treatment of radioresistant cancer cells.

Reczek CR, Shakya R, Miteva Y, et al.
The DNA resection protein CtIP promotes mammary tumorigenesis.
Oncotarget. 2016; 7(22):32172-83 [PubMed] Free Access to Full Article Related Publications
Many DNA repair factors act to suppress tumor formation by preserving genomic stability. Similarly, the CtIP protein, which interacts with the BRCA1 tumor suppressor, is also thought to have tumor suppression activity. Through its role in DNA end resection, CtIP facilitates DNA double-strand break (DSB) repair by homologous recombination (DSBR-HR) and microhomology-mediated end joining (MMEJ). In addition, however, CtIP has also been implicated in the formation of aberrant chromosomal rearrangements in an MMEJ-dependent manner, an activity that could potentially promote tumor development by increasing genome instability. To clarify whether CtIP acts in vivo to suppress or promote tumorigenesis, we have examined its oncogenic potential in mouse models of human breast cancer. Surprisingly, mice heterozygous for a null Ctip allele did not display an increased susceptibility to tumor formation. Moreover, mammary-specific biallelic CtIP ablation did not elicit breast tumors in a manner reminiscent of BRCA1 loss. Instead, CtIP inactivation dramatically reduced the kinetics of mammary tumorigenesis in mice bearing mammary-specific lesions of the p53 gene. Thus, unlike other repair factors, CtIP is not a tumor suppressor, but has oncogenic properties that can promote tumorigenesis, consistent with its ability to facilitate MMEJ-dependent chromosomal instability. Consequently, inhibition of CtIP-mediated MMEJ may prove effective against tumor types, such as human breast cancer, that display MMEJ-dependent chromosomal rearrangements.

Wu Q, Paul A, Su D, et al.
Structure of BRCA1-BRCT/Abraxas Complex Reveals Phosphorylation-Dependent BRCT Dimerization at DNA Damage Sites.
Mol Cell. 2016; 61(3):434-448 [PubMed] Free Access to Full Article Related Publications
BRCA1 accumulation at DNA damage sites is an important step for its function in the DNA damage response and in DNA repair. BRCA1-BRCT domains bind to proteins containing the phosphorylated serine-proline-x-phenylalanine (pSPxF) motif including Abraxas, Bach1/FancJ, and CtIP. In this study, we demonstrate that ionizing radiation (IR)-induces ATM-dependent phosphorylation of serine 404 (S404) next to the pSPxF motif. Crystal structures of BRCT/Abraxas show that phosphorylation of S404 is important for extensive interactions through the N-terminal sequence outside the pSPxF motif and leads to formation of a stable dimer. Mutation of S404 leads to deficiency in BRCA1 accumulation at DNA damage sites and cellular sensitivity to IR. In addition, two germline mutations of BRCA1 are found to disrupt the dimer interface and dimer formation. Thus, we demonstrate a mechanism involving IR-induced phosphorylation and dimerization of the BRCT/Abraxas complex for regulating Abraxas-mediated recruitment of BRCA1 in response to IR.

Wang J, Ding Q, Fujimori H, et al.
Loss of CtIP disturbs homologous recombination repair and sensitizes breast cancer cells to PARP inhibitors.
Oncotarget. 2016; 7(7):7701-14 [PubMed] Free Access to Full Article Related Publications
Breast cancer is one of the leading causes of death worldwide, and therefore, new and improved approaches for the treatment of breast cancer are desperately needed. CtIP (RBBP8) is a multifunctional protein that is involved in various cellular functions, including transcription, DNA replication, DNA repair and the G1 and G2 cell cycle checkpoints. CtIP plays an important role in homologous recombination repair by interacting with tumor suppressor protein BRCA1. Here, we analyzed the expression profile of CtIP by data mining using published microarray data sets. We found that CtIP expression is frequently decreased in breast cancer patients, and the patient group with low-expressing CtIP mRNA is associated with a significantly lower survival rate. The knockdown of CtIP in breast cancer MCF7 cells reduced Rad51 foci numbers and enhanced f H2AX foci formation after f-irradiation, suggesting that deficiency of CtIP decreases homologous recombination repair and delays DNA double strand break repair. To explore the effect of CtIP on PARP inhibitor therapy for breast cancer, CtIP-depleted MCF7 cells were treated with PARP inhibitor olaparib (AZD2281) or veliparib (ABT-888). As in BRCA mutated cells, PARP inhibitors showed cytotoxicity to CtIP-depleted cells by preventing cells from repairing DNA damage, leading to decreased cell viability. Further, a xenograft tumor model in mice with MCF7 cells demonstrated significantly increased sensitivity towards PARP inhibition under CtIP deficiency. In summary, this study shows that low level of CtIP expression is associated with poor prognosis in breast cancer, and provides a rationale for establishing CtIP expression as a biomarker of PARP inhibitor response, and consequently offers novel therapeutic options for a significant subset of patients.

Liu X, Xu Y, Pang Z, et al.
Knockdown of SUMO-activating enzyme subunit 2 (SAE2) suppresses cancer malignancy and enhances chemotherapy sensitivity in small cell lung cancer.
J Hematol Oncol. 2015; 8:67 [PubMed] Free Access to Full Article Related Publications
BACKGROUND: SUMO-activating enzyme subunit 2 (SAE2) is the sole E1-activating enzyme required for numerous important protein SUMOylation, abnormal of which is associated with carcinogenesis. SAE2 inactivation was recently reported to be a therapeutic strategy in cancers with Myc overexpression. However, the roles of SAE2 in small cell lung cancer (SCLC) are largely unknown.
METHODS: Stably SAE2 knockdown in H446 cells were established with a lentiviral system. Cell viability, cell cycle, and apoptosis were analyzed using MTT assay and flow cytometric assay. Expression of SAE2 mRNA and protein were detected by qPCR, western blotting, and immunohistochemical staining. Cell invasion and migration assay were determined by transwell chamber assay. H446 cells with or without SAE2 knockdown, nude mice models were established to observe tumorigenesis.
RESULTS: SAE2 was highly expressed in SCLC and significantly correlated with tumorigenesis in vivo. Cancer cells with RNAi-mediated reduction of SAE2 expression exhibited growth retardation and apoptosis increasing. Furthermore, down-regulation of SAE2 expression inhibited migration and invasion, simultaneously increased the sensitivity of H446 to etoposide and cisplatin.
CONCLUSIONS: SAE2 plays an important role in tumor growth, metastasis, and chemotherapy sensitivity of H446 and is a potential clinical biomarker and therapeutic target in SCLC with high c-Myc expression.

Saini P, Li Y, Dobbelstein M
Wee1 is required to sustain ATR/Chk1 signaling upon replicative stress.
Oncotarget. 2015; 6(15):13072-87 [PubMed] Free Access to Full Article Related Publications
The therapeutic efficacy of nucleoside analogues, e.g. gemcitabine, against cancer cells can be augmented by inhibitors of checkpoint kinases, including Wee1, ATR, and Chk1. We have compared the chemosensitizing effect of these inhibitors in cells derived from pancreatic cancer, a tumor entity where gemcitabine is part of the first-line therapeutic regimens, and in osteosarcoma-derived cells. As expected, all three inhibitors rendered cancer cells more sensitive to gemcitabine, but Wee1 inhibition proved to be particularly efficient in this context. Investigating the reasons for this potent sensitizing effect, we found that Wee1 inhibition or knockdown not only blocked Wee1 activity, but also reduced the activation of ATR/Chk1 in gemcitabine-treated cells. Combination of several inhibitors revealed that Wee1 inhibition requires Cyclin-dependent kinases 1 and 2 (Cdk1/2) and Polo-like kinase 1 (Plk1) to reduce ATR/Chk1 activity. Through activation of Cdks and Plk1, Wee1 inhibition reduces Claspin and CtIP levels, explaining the impairment in ATR/Chk1 activity. Taken together, these results confer a consistent signaling pathway reaching from Wee1 inhibition to impaired Chk1 activity, mechanistically dissecting how Wee1 inhibitors not only dysregulate cell cycle progression, but also enhance replicative stress and chemosensitivity towards nucleoside analogues.

He X, Riceberg J, Pulukuri SM, et al.
Characterization of the loss of SUMO pathway function on cancer cells and tumor proliferation.
PLoS One. 2015; 10(4):e0123882 [PubMed] Free Access to Full Article Related Publications
SUMOylation is a post-translational ubiquitin-like protein modification pathway that regulates important cellular processes including chromosome structure, kinetochore function, chromosome segregation, nuclear and sub-nuclear organization, transcription and DNA damage repair. There is increasing evidence that the SUMO pathway is dysregulated in cancer, raising the possibility that modulation of this pathway may have therapeutic potential. To investigate the importance of the SUMO pathway in the context of cancer cell proliferation and tumor growth, we applied lentivirus-based short hairpin RNAs (shRNA) to knockdown SUMO pathway genes in human cancer cells. shRNAs for SAE2 and UBC9 reduced SUMO conjugation activity and inhibited proliferation of human cancer cells. To expand upon these observations, we generated doxycycline inducible conditional shRNA cell lines for SAE2 to achieve acute and reversible SAE2 knockdown. Conditional SAE2 knockdown in U2OS and HCT116 cells slowed cell growth in vitro, and SAE2 knockdown induced multiple terminal outcomes including apoptosis, endoreduplication and senescence. Multinucleated cells became senescent and stained positive for the senescence marker, SA-β Gal, and displayed elevated levels of p53 and p21. In an attempt to explain these phenotypes, we confirmed that loss of SUMO pathway activity leads to a loss of SUMOylated Topoisomerase IIα and the appearance of chromatin bridges which can impair proper cytokinesis and lead to multinucleation. Furthermore, knockdown of SAE2 induces disruption of PML nuclear bodies which may further promote apoptosis or senescence. In an in vivo HCT116 xenograft tumor model, conditional SAE2 knockdown strongly impaired tumor growth. These data demonstrate that the SUMO pathway is required for cancer cell proliferation in vitro and tumor growth in vivo, implicating the SUMO pathway as a potential cancer therapeutic target.

Pelosi G, Barbareschi M, Cavazza A, et al.
Large cell carcinoma of the lung: a tumor in search of an author. A clinically oriented critical reappraisal.
Lung Cancer. 2015; 87(3):226-31 [PubMed] Related Publications
Large cell carcinoma (LCC) is a merely descriptive term indicating a subtype of lung cancer with no specific features of small-cell lung cancer (SCLC), adenocarcinoma (ADC) or squamous cell carcinoma (SQC). This diagnosis is allowed on surgical specimens only, whereas its counterpart in biopsy/cytology samples is non-small-cell lung carcinoma (NSCLC), not otherwise specified (NOS). Although these two terms do not fulfill the same concept, they can be interchangeable synonyms at the clinical level, reflecting, in different ways, the inability to define a specific subtype. Immunohistochemistry (IHC), next generation sequencing (NGS) analysis and, historically, electron microscopy have been unveiling diverse cell differentiation lineages in LCC, resulting in LCC-favor ADC, LCC-favor SQC and LCC-favor large-cell neuroendocrine carcinoma (LCNEC), the latter hopefully to be included into the neuroendocrine tumor (NET) group in the future. Paradoxically, however, the interpretation issues of LCC/NSCLC-NOS are not diminishing, but even increasing albeight an accurate diagnosis is oncologically required and crucial. Also, rare LCC/NSCLC-NOS cases exhibiting null/unclear phenotype, are difficult to classify, and this terminology could be maintained for the sake of classification (basically these tumors are serendipitous ADC, as also confirmed by the lack of p40). In this review article, seven relevant issues to LCC have been addressed by using a question-answer methodology, with final key points discussing major interpretation issues. In conclusion, most LCC/NSCLC-NOS may be eventually re-classified and addressed by exploiting IHC and/or molecular testing to satisfy the criteria of precision medicine (the right drug, to the right patient, at the right time).

Xu J, Husain A, Hu W, et al.
APE1 is dispensable for S-region cleavage but required for its repair in class switch recombination.
Proc Natl Acad Sci U S A. 2014; 111(48):17242-7 [PubMed] Free Access to Full Article Related Publications
Activation-induced cytidine deaminase (AID) is essential for antibody diversification, namely somatic hypermutation (SHM) and class switch recombination (CSR). The deficiency of apurinic/apyrimidinic endonuclease 1 (Ape1) in CH12F3-2A B cells reduces CSR to ∼20% of wild-type cells, whereas the effect of APE1 loss on SHM has not been examined. Here we show that, although APE1's endonuclease activity is important for CSR, it is dispensable for SHM as well as IgH/c-myc translocation. Importantly, APE1 deficiency did not show any defect in AID-induced S-region break formation, but blocked both the recruitment of repair protein Ku80 to the S region and the synapse formation between Sμ and Sα. Knockdown of end-processing factors such as meiotic recombination 11 homolog (MRE11) and carboxy-terminal binding protein (CtBP)-interacting protein (CtIP) further reduced the remaining CSR in Ape1-null CH12F3-2A cells. Together, our results show that APE1 is dispensable for SHM and AID-induced DNA breaks and may function as a DNA end-processing enzyme to facilitate the joining of broken ends during CSR.

Hu Y, Chau T, Liu HX, et al.
Bile acids regulate nuclear receptor (Nur77) expression and intracellular location to control proliferation and apoptosis.
Mol Cancer Res. 2015; 13(2):281-92 [PubMed] Free Access to Full Article Related Publications
UNLABELLED: Bile acids (BA) are endogenous agents capable of causing cancer throughout the gastrointestinal (GI) tract. To uncover the mechanism by which BAs exert carcinogenic effects, both human liver and colon cancer cells as well as mouse primary hepatocytes were treated with BAs and assayed for viability, genotoxic stress, and transcriptional response. BAs induced both Nur77 (NR4A1) and proinflammatory gene expression. The intracellular location of BA-induced Nur77 was time dependent; short-term (1-3 hours) exposure induced nuclear Nur77, whereas longer (1-2 days) exposure also increased cytosolic Nur77 expression and apoptosis. Inhibiting Nur77 nuclear export with leptomycin B decreased lithocholic acid (LCA)-induced apoptosis. Extended (7 days) treatment with BA generated resistance to BA with increased nuclear Nur77, viability, and mobility. While, knockdown of Nur77 in BA-resistant cells increased cellular susceptibility to LCA-induced apoptosis. Moreover, in vivo mouse xenograft experiments demonstrated that BA-resistant cells form larger tumors with elevated Nur77 expression compared with parental controls. DNA-binding and gene expression assays identified multiple survival genes (CDK4, CCND2, MAP4K5, STAT5A, and RBBP8) and a proapoptosis gene (BID) as Nur77 targets. Consistently, BA-induced upregulation of the aforementioned genes was abrogated by a lack of Nur77. Importantly, Nur77 was overexpressed in high percentage of human colon and liver cancer specimens, and the intracellular location of Nur77 correlated with elevated serum total BA levels in patients with colon cancer. These data show for the first time that BAs via Nur77 have a dual role in modulating cell survival and death.
IMPLICATIONS: These findings establish a direct link between Nur77 and the carcinogenic effect of BAs.

Choi YE, Pan Y, Park E, et al.
MicroRNAs down-regulate homologous recombination in the G1 phase of cycling cells to maintain genomic stability.
Elife. 2014; 3:e02445 [PubMed] Free Access to Full Article Related Publications
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.

Watanabe Y, Maeda I, Oikawa R, et al.
Aberrant DNA methylation status of DNA repair genes in breast cancer treated with neoadjuvant chemotherapy.
Genes Cells. 2013; 18(12):1120-30 [PubMed] Related Publications
Dysregulation of homologous recombination (HR) DNA repair has been implicated in breast carcinogenesis and chemosensitivity. Here, we investigated the methylation status of sixteen HR genes and analyzed their association with tumor subtypes and responses to neoadjuvant chemotherapy. Core specimens were obtained before neoadjuvant chemotherapy from sixty cases of primary breast cancer of the following four subgroups: luminal breast cancer (LBC) with pathological complete response (pCR), LBC with stable disease, triple-negative breast cancer (TNBC) with pCR and TNBC with poor response. The aberrant DNA methylation status of the following HR related-genes was analyzed using bisulfite-pyrosequencing: BRCA1, BRCA2, BARD1, MDC1, RNF8, RNF168, UBC13, ABRA1, PALB2, RAD50, RAD51, RAD51C, MRE11, NBS1, CtIP and ATM. Among the genes analyzed, only the incidence of BRCA1 and RNF8 methylation was significantly higher in TNBC than that in LBC. Whereas the incidence of BRCA1 methylation was tended to be higher in pCR cases than in poor-response cases in TNBC, that of RNF8 was significantly lower in pCR cases than in poor-response cases. Our results indicate that the methylation status of HR genes was not generally associated with TNBC subtype or chemosensitivity although hypermethylation of BRCA1 is associated with TNBC subtype and may impact chemosensitivity.

Lin ZP, Ratner ES, Whicker ME, et al.
Triapine disrupts CtIP-mediated homologous recombination repair and sensitizes ovarian cancer cells to PARP and topoisomerase inhibitors.
Mol Cancer Res. 2014; 12(3):381-393 [PubMed] Free Access to Full Article Related Publications
UNLABELLED: PARP inhibitors exploit synthetic lethality to target epithelial ovarian cancer (EOC) with hereditary BRCA mutations and defects in homologous recombination repair (HRR). However, such an approach is limited to a small subset of EOC patients and compromised by restored HRR due to secondary mutations in BRCA genes. Here, it was demonstrated that triapine, a small-molecule inhibitor of ribonucleotide reductase, enhances the sensitivity of BRCA wild-type EOC cells to the PARP inhibitor olaparib and the topoisomerase II inhibitor etoposide. Triapine abolishes olaparib-induced BRCA1 and Rad51 foci, and disrupts the BRCA1 interaction with the Mre11-Rad50-Nbs1 (MRN) complex in BRCA1 wild-type EOC cells. It has been shown that phosphorylation of CtIP (RBBP8) is required for the interaction with BRCA1 and with MRN to promote DNA double-strand break (DSB) resection during S and G(2) phases of the cell cycle. Mechanistic studies within reveal that triapine inhibits cyclin-dependent kinase (CDK) activity and blocks olaparib-induced CtIP phosphorylation through Chk1 activation. Furthermore, triapine abrogates etoposide-induced CtIP phosphorylation and DSB resection as evidenced by marked attenuation of RPA32 phosphorylation. Concurrently, triapine obliterates etoposide-induced BRCA1 foci and sensitizes BRCA1 wild-type EOC cells to etoposide. Using a GFP-based HRR assay, it was determined that triapine suppresses HRR activity induced by an I-SceI-generated DSB. These results suggest that triapine augments the sensitivity of BRCA wild-type EOC cells to drug-induced DSBs by disrupting CtIP-mediated HRR.
IMPLICATIONS: These findings provide a strong rationale for combining triapine with PARP or topoisomerase inhibitors to target HRR-proficient EOC cells.

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