Gene Summary

Gene:DDB1; damage-specific DNA binding protein 1, 127kDa
Summary:The protein encoded by this gene is the large subunit (p127) of the heterodimeric DNA damage-binding (DDB) complex while another protein (p48) forms the small subunit. This protein complex functions in nucleotide-excision repair and binds to DNA following UV damage. Defective activity of this complex causes the repair defect in patients with xeroderma pigmentosum complementation group E (XPE) - an autosomal recessive disorder characterized by photosensitivity and early onset of carcinomas. However, it remains for mutation analysis to demonstrate whether the defect in XPE patients is in this gene or the gene encoding the small subunit. In addition, Best vitelliform mascular dystrophy is mapped to the same region as this gene on 11q, but no sequence alternations of this gene are demonstrated in Best disease patients. The protein encoded by this gene also functions as an adaptor molecule for the cullin 4 (CUL4) ubiquitin E3 ligase complex by facilitating the binding of substrates to this complex and the ubiquitination of proteins. [provided by RefSeq, May 2012]
Databases:OMIM, VEGA, HGNC, Ensembl, GeneCard, Gene
Protein:DNA damage-binding protein 1
Source:NCBIAccessed: 27 February, 2015


What does this gene/protein do?
Show (18)

Cancer Overview

Research Indicators

Publications Per Year (1990-2015)
Graph generated 27 February 2015 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.

  • Models, Molecular
  • Cullin Proteins
  • Proteolysis
  • Molecular Sequence Data
  • Nuclear Proteins
  • Pyrimidine Dimers
  • DNA, Complementary
  • Xeroderma Pigmentosum Group A Protein
  • Carrier Proteins
  • Base Sequence
  • Protein Binding
  • HeLa Cells
  • Cell Nucleus
  • Ubiquitination
  • Cancer Gene Expression Regulation
  • Dimerization
  • Ubiquitin
  • DNA Repair
  • Amino Acid Substitution
  • Mutation
  • DNA Sequence Analysis
  • Transcription
  • Ultraviolet Rays
  • DNA Damage
  • Transfection
  • Antigens, Nuclear
  • Amino Acid Sequence
  • Cell Line
  • Cricetinae
  • Chromatin
  • Cultured Cells
  • Histones
  • Ubiquitin-Protein Ligases
  • Cloning, Molecular
  • Signal Transduction
  • DNA
  • HEK293 Cells
  • Chromosome 11
  • DNA-Binding Proteins
Tag cloud generated 27 February, 2015 using data from PubMed, MeSH and CancerIndex

Latest Publications: DDB1 (cancer-related)

Zhu YX, Braggio E, Shi CX, et al.
Identification of cereblon-binding proteins and relationship with response and survival after IMiDs in multiple myeloma.
Blood. 2014; 124(4):536-45 [PubMed] Article available free on PMC after 24/07/2015 Related Publications
Cereblon (CRBN) mediates immunomodulatory drug (IMiD) action in multiple myeloma (MM). Using 2 different methodologies, we identified 244 CRBN binding proteins and established relevance to MM biology by changes in their abundance after exposure to lenalidomide. Proteins most reproducibly binding CRBN (>fourfold vs controls) included DDB1, CUL4A, IKZF1, KPNA2, LTF, PFKL, PRKAR2A, RANGAP1, and SHMT2. After lenalidomide treatment, the abundance of 46 CRBN binding proteins decreased. We focused attention on 2 of these-IKZF1 and IKZF3. IZKF expression is similar across all MM stages or subtypes; however, IKZF1 is substantially lower in 3 of 5 IMiD-resistant MM cell lines. The cell line (FR4) with the lowest IKZF1 levels also harbors a damaging mutation and a translocation that upregulates IRF4, an IKZF target. Clinical relevance of CRBN-binding proteins was demonstrated in 44 refractory MM patients treated with pomalidomide and dexamethasone therapy in whom low IKZF1 gene expression predicted lack of response (0/11 responses in the lowest expression quartile). CRBN, IKZF1, and KPNA2 levels also correlate with significant differences in overall survival. Our study identifies CRBN-binding proteins and demonstrates that in addition to CRBN, IKZF1, and KPNA2, expression can predict survival outcomes.

Ben Rekaya M, Laroussi N, Messaoud O, et al.
A founder large deletion mutation in Xeroderma pigmentosum-Variant form in Tunisia: implication for molecular diagnosis and therapy.
Biomed Res Int. 2014; 2014:256245 [PubMed] Article available free on PMC after 24/07/2015 Related Publications
Xeroderma pigmentosum Variant (XP-V) form is characterized by a late onset of skin symptoms. Our aim is the clinical and genetic investigations of XP-V Tunisian patients in order to develop a simple tool for early diagnosis. We investigated 16 suspected XP patients belonging to ten consanguineous families. Analysis of the POLH gene was performed by linkage analysis, long range PCR, and sequencing. Genetic analysis showed linkage to the POLH gene with a founder haplotype in all affected patients. Long range PCR of exon 9 to exon 11 showed a 3926 bp deletion compared to control individuals. Sequence analysis demonstrates that this deletion has occurred between two Alu-Sq2 repetitive sequences in the same orientation, respectively, in introns 9 and 10. We suggest that this mutation POLH NG_009252.1: g.36847_40771del3925 is caused by an equal crossover event that occurred between two homologous chromosomes at meiosis. These results allowed us to develop a simple test based on a simple PCR in order to screen suspected XP-V patients. In Tunisia, the prevalence of XP-V group seems to be underestimated and clinical diagnosis is usually later. Cascade screening of this founder mutation by PCR in regions with high frequency of XP provides a rapid and cost-effective tool for early diagnosis of XP-V in Tunisia and North Africa.

Cooper J, Giancotti FG
Molecular insights into NF2/Merlin tumor suppressor function.
FEBS Lett. 2014; 588(16):2743-52 [PubMed] Article available free on PMC after 19/08/2015 Related Publications
The FERM domain protein Merlin, encoded by the NF2 tumor suppressor gene, regulates cell proliferation in response to adhesive signaling. The growth inhibitory function of Merlin is induced by intercellular adhesion and inactivated by joint integrin/receptor tyrosine kinase signaling. Merlin contributes to the formation of cell junctions in polarized tissues, activates anti-mitogenic signaling at tight-junctions, and inhibits oncogenic gene expression. Thus, inactivation of Merlin causes uncontrolled mitogenic signaling and tumorigenesis. Merlin's predominant tumor suppressive functions are attributable to its control of oncogenic gene expression through regulation of Hippo signaling. Notably, Merlin translocates to the nucleus where it directly inhibits the CRL4(DCAF1) E3 ubiquitin ligase, thereby suppressing inhibition of the Lats kinases. A dichotomy in NF2 function has emerged whereby Merlin acts at the cell cortex to organize cell junctions and propagate anti-mitogenic signaling, whereas it inhibits oncogenic gene expression through the inhibition of CRL4(DCAF1) and activation of Hippo signaling. The biochemical events underlying Merlin's normal function and tumor suppressive activity will be discussed in this Review, with emphasis on recent discoveries that have greatly influenced our understanding of Merlin biology.

Zhao R, Han C, Eisenhauer E, et al.
DNA damage-binding complex recruits HDAC1 to repress Bcl-2 transcription in human ovarian cancer cells.
Mol Cancer Res. 2014; 12(3):370-80 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
UNLABELLED: Elevated expression of the antiapoptotic factor Bcl-2 is believed to be one of the contributing factors to an increased relapse rate associated with multiple cisplatin-resistant cancers. DNA damage-binding protein complex subunit 2 (DDB2) has recently been revealed to play an important role in sensitizing human ovarian cancer cells to cisplatin-induced apoptosis through the downregulation of Bcl-2, but the underlying molecular mechanism remains poorly defined. Here, it is report that DDB2 functions as a transcriptional repressor for Bcl-2 in combination with DDB1. Quantitative ChIP and EMSA analysis revealed that DDB2 binds to a specific cis-acting element at the 5'-end of Bcl-2 P1 promoter. Overexpression of DDB2 resulted in marked losses of histone H3K9,14 acetylation along the Bcl-2 promoter and enhancer regions, concomitant with a local enrichment of HDAC1 to the Bcl-2 P1 core promoter in ovarian cancer cells. Coimmunoprecipitation and in vitro binding analyses identified a physical interaction between DDB1 and HDAC1, whereas downregulation of HDAC1 significantly enhanced Bcl-2 promoter activity. Finally, in comparison with wild-type DDB2, mutated DDB2, which is unable to repress Bcl-2 transcription, mediates a compromised apoptosis upon cisplatin treatment. Taken together, these data support a model wherein DDB1 and DDB2 cooperate to repress Bcl-2 transcription. DDB2 recognizes and binds to the Bcl-2 P1 promoter, and HDAC1 is recruited through the DDB1 subunit associated with DDB2 to deacetylate histone H3K9,14 across Bcl-2 regulatory regions, resulting in suppressed Bcl-2 transcription.
IMPLICATIONS: Increasing the expression of DDB complex may provide a molecular strategy for cancer therapy.

Klampfl T, Milosevic JD, Puda A, et al.
Complex patterns of chromosome 11 aberrations in myeloid malignancies target CBL, MLL, DDB1 and LMO2.
PLoS One. 2013; 8(10):e77819 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Exome sequencing of primary tumors identifies complex somatic mutation patterns. Assignment of relevance of individual somatic mutations is difficult and poses the next challenge for interpretation of next generation sequencing data. Here we present an approach how exome sequencing in combination with SNP microarray data may identify targets of chromosomal aberrations in myeloid malignancies. The rationale of this approach is that hotspots of chromosomal aberrations might also harbor point mutations in the target genes of deletions, gains or uniparental disomies (UPDs). Chromosome 11 is a frequent target of lesions in myeloid malignancies. Therefore, we studied chromosome 11 in a total of 813 samples from 773 individual patients with different myeloid malignancies by SNP microarrays and complemented the data with exome sequencing in selected cases exhibiting chromosome 11 defects. We found gains, losses and UPDs of chromosome 11 in 52 of the 813 samples (6.4%). Chromosome 11q UPDs frequently associated with mutations of CBL. In one patient the 11qUPD amplified somatic mutations in both CBL and the DNA repair gene DDB1. A duplication within MLL exon 3 was detected in another patient with 11qUPD. We identified several common deleted regions (CDR) on chromosome 11. One of the CDRs associated with de novo acute myeloid leukemia (P=0.013). One patient with a deletion at the LMO2 locus harbored an additional point mutation on the other allele indicating that LMO2 might be a tumor suppressor frequently targeted by 11p deletions. Our chromosome-centered analysis indicates that chromosome 11 contains a number of tumor suppressor genes and that the role of this chromosome in myeloid malignancies is more complex than previously recognized.

Kim TY, Jackson S, Xiong Y, et al.
CRL4A-FBXW5-mediated degradation of DLC1 Rho GTPase-activating protein tumor suppressor promotes non-small cell lung cancer cell growth.
Proc Natl Acad Sci U S A. 2013; 110(42):16868-73 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
DLC1 encodes a RhoA GTPase-activating protein and tumor suppressor lost in cancer by genomic deletion or epigenetic silencing and loss of DLC1 gene transcription. We unexpectedly identified non-small cell lung cancer (NSCLC) cell lines and tumor tissue that expressed DLC1 mRNA yet lacked DLC1 protein expression. We determined that DLC1 was ubiquitinated and degraded by cullin 4A-RING ubiquitin ligase (CRL4A) complex interaction with DDB1 and the FBXW5 substrate receptor. siRNA-mediated suppression of cullin 4A, DDB1, or FBXW5 expression restored DLC1 protein expression in NSCLC cell lines. FBXW5 suppression-induced DLC1 reexpression was associated with a reduction in the levels of activated RhoA-GTP and in RhoA effector signaling. Finally, FBXW5 suppression caused a DLC1-dependent decrease in NSCLC anchorage-dependent and -independent proliferation. In summary, we identify a posttranslational mechanism for loss of DLC1 and a linkage between CRL4A-FBXW5-associated oncogenesis and regulation of RhoA signaling.

Pan WW, Zhou JJ, Yu C, et al.
Ubiquitin E3 ligase CRL4(CDT2/DCAF2) as a potential chemotherapeutic target for ovarian surface epithelial cancer.
J Biol Chem. 2013; 288(41):29680-91 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Cullin-RING ubiquitin ligases (CRLs) are the largest family of E3 ligases and require cullin neddylation for their activation. The NEDD8-activating enzyme inhibitor MLN4924 reportedly blocked cullin neddylation and inactivated CRLs, which resulted in apoptosis induction and tumor suppression. However, CRL roles in ovarian cancer cell survival and the ovarian tumor repressing effects of MLN4924 are unknown. We show here that CRL4 components are highly expressed in human epithelial ovarian cancer tissues. MLN4924-induced DNA damage, cell cycle arrest, and apoptosis in ovarian cancer cells in a time- and dose-dependent manner. In addition, MLN4924 sensitized ovarian cancer cells to other chemotherapeutic drug treatments. Depletion of CRL4 components Roc1/2, Cul4a, and DDB1 had inhibitory effects on ovarian cancer cells similar to MLN4924 treatment, which suggested that CRL4 inhibition contributed to the chemotherapeutic effect of MLN4924 in ovarian cancers. We also investigated for key CRL4 substrate adaptors required for ovarian cancer cells. Depleting Vprbp/Dcaf1 did not significantly affect ovarian cancer cell growth, even though it was expressed by ovarian cancer tissues. However, depleting Cdt2/Dcaf2 mimicked the pharmacological effects of MLN4924 and caused the accumulation of its substrate, CDT1, both in vitro and in vivo. MLN4924-induced DNA damage and apoptosis were partially rescued by Cdt1 depletion, suggesting that CRL4(CDT2) repression and CDT1 accumulation were key biochemical events contributing to the genotoxic effects of MLN4924 in ovarian cancer cells. Taken together, these results indicate that CRL4(CDT2) is a potential drug target in ovarian cancers and that MLN4924 may be an effective anticancer agent for targeted ovarian cancer therapy.

Wong WJ, Qiu B, Nakazawa MS, et al.
MYC degradation under low O2 tension promotes survival by evading hypoxia-induced cell death.
Mol Cell Biol. 2013; 33(17):3494-504 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Cells encounter oxygen deprivation (hypoxia) in various physiological and pathological contexts. Adaptation to hypoxic stress occurs in part by suppressing MYC, a key regulator of cellular metabolism, proliferation, and survival. Hypoxia has been reported to inhibit MYC through multiple means, including disruption of MYC transcriptional complexes and decreased MYC protein abundance. Here we identify enhanced proteasomal degradation and cathepsin-mediated proteolysis as important mechanisms for hypoxic MYC inhibition in human colon carcinoma cells. MYC protein levels were similarly reduced in hypoxic primary keratinocytes. Increased MYC turnover at low O2 tension was dependent on the E3 ubiquitin ligases FBXW7 and DDB1, as well as hypoxic induction of cathepsins D and S. Reduced MYC protein levels coincided with hypoxic inhibition of RNA polymerase III-dependent MYC target genes, which MYC regulates independently of its binding partner MAX. Finally, MYC overexpression in hypoxic cells promoted cell cycle progression but also enhanced cell death via increased expression of the proapoptotic genes NOXA and PUMA. Collectively, these results indicate that hypoxic cells promote MYC degradation as an adaptive strategy to reduce proliferation, suppress biosynthetic processes, and promote cell survival under low O2 tension.

Dai ZJ, Gao J, Kang HF, et al.
Targeted inhibition of mammalian target of rapamycin (mTOR) enhances radiosensitivity in pancreatic carcinoma cells.
Drug Des Devel Ther. 2013; 7:149-59 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
The mammalian target of rapamycin (mTOR) is a protein kinase that regulates protein translation, cell growth, and apoptosis. Rapamycin (RPM), a specific inhibitor of mTOR, exhibits potent and broad in vitro and in vivo antitumor activity against leukemia, breast cancer, and melanoma. Recent studies showing that RPM sensitizes cancers to chemotherapy and radiation therapy have attracted considerable attention. This study aimed to examine the radiosensitizing effect of RPM in vitro, as well as its mechanism of action. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and colony formation assay showed that 10 nmol/L to 15 nmol/L of RPM had a radiosensitizing effects on pancreatic carcinoma cells in vitro. Furthermore, a low dose of RPM induced autophagy and reduced the number of S-phase cells. When radiation treatment was combined with RPM, the PC-2 cell cycle arrested in the G2/M phase of the cell cycle. Complementary DNA (cDNA) microarray and reverse transcription polymerase chain reaction (RT-PCR) revealed that the expression of DDB1, RAD51, and XRCC5 were downregulated, whereas the expression of PCNA and ABCC4 were upregulated in PC-2 cells. The results demonstrated that RPM effectively enhanced the radiosensitivity of pancreatic carcinoma cells.

Roy N, Bommi PV, Bhat UG, et al.
DDB2 suppresses epithelial-to-mesenchymal transition in colon cancer.
Cancer Res. 2013; 73(12):3771-82 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Colon cancer is one of the deadliest cancers worldwide because of its metastasis to other essential organs. Metastasis of colon cancer involves a complex set of events, including epithelial-to-mesenchymal transition (EMT) that increases invasiveness of the tumor cells. Here, we show that the xeroderma pigmentosum group E (XPE) gene product, damaged DNA-binding protein (DDB)-2, is downregulated in high-grade colon cancers, and it plays a dominant role in the suppression of EMT of the colon cancer cells. Depletion of DDB2 promotes mesenchymal phenotype, whereas expression of DDB2 promotes epithelial phenotype. DDB2 constitutively represses genes that are the key activators of EMT, indicating that DDB2 is a master regulator of EMT of the colon cancer cells. Moreover, we observed evidence that DDB2 functions as a barrier for EMT induced by hypoxia and TGF-β. Also, we provide evidence that DDB2 inhibits metastasis of colon cancer. The results presented here identify a transcriptional regulatory pathway of DDB2 that is directly linked to the mechanisms that suppress metastasis of colon cancer.

Che Y, Best OG, Zhong L, et al.
Hsp90 Inhibitor SNX-7081 dysregulates proteins involved with DNA repair and replication and the cell cycle in human chronic lymphocytic leukemia (CLL) cells.
J Proteome Res. 2013; 12(4):1710-22 [PubMed] Related Publications
The proteomic effects of the Hsp90 inhibitor, SNX-7081, have been determined on the p53-mutated B-cell chronic lymphocytic leukemia (CLL) cell line, MEC1. Following SNX-7081 treatment (500 nM, 24 h), 51 proteins changed abundance by more than 2-fold (p < 0.05); 7 proteins increased while 44 proteins decreased. Proteins identified as differentially abundant by LC-MS/MS were validated by Western blotting (DDB1, PCNA, MCM2, Hsp90, Hsp70, GRP78, PDIA6, HLA-DR). RT-PCR showed that SNX-7081 unexpectedly modulates a number of these proteins in MEC1 cells at the mRNA level (PCNA, MCM2, Nup155, Hsp70, GRP78, PDIA6, and HLA-DR). Pathway analysis determined that 3 of the differentially abundant proteins (cyclin D1, c-Myc and pRb) were functionally related. p53 levels did not change upon SNX-7081 treatment of p53 wild-type Raji cells or p53-mutated MEC1 and U266 cells, indicating that SNX-7081 has a p53-independent mechanism. The decreases in DDB1, MCM2, c-Myc, and PCNA and increases of pRb and cyclin D1 were confirmed in MEC1, U266, Raji, and p53 null HL60 cells by Western blotting. These data suggest that SNX-7081 arrests the cell cycle and inhibits DNA replication and r epair and provides evidence for the mechanism of the observed synergy between Hsp90 inhibitors and drugs that induce DNA strand breaks.

Li C, Yin M, Wang LE, et al.
Polymorphisms of nucleotide excision repair genes predict melanoma survival.
J Invest Dermatol. 2013; 133(7):1813-21 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Melanoma is the most highly malignant skin cancer, and nucleotide excision repair (NER) is involved in melanoma susceptibility. In this analysis of 1,042 melanoma patients, we evaluated whether genetic variants of NER genes may predict survival outcome of melanoma patients. We used genotyping data of 74 tagging single-nucleotide polymorphisms (tagSNPs) in eight core NER genes from our genome-wide association study (including two in XPA, 14 in XPC, three in XPE, four in ERCC1, 10 in ERCC2, eight in ERCC3, 14 in ERCC4, and 19 in ERCC5) and evaluated their associations with prognosis of melanoma patients. Using the Cox proportional hazards model and Kaplan-Meier analysis, we found a predictive role of XPE rs28720291, ERCC5 rs4150314, XPC rs2470458, and ERCC2 rs50871 SNPs in the prognosis of melanoma patients (rs28720291: AG vs. GG, adjusted hazard ratio (adjHR)=11.2, 95% confidence interval (CI) 3.04-40.9, P=0.0003; rs4150314: AG vs. GG, adjHR=4.76, 95% CI 1.09-20.8, P=0.038; rs2470458: AA vs. AG/GG, adjHR=2.11, 95% CI 1.03-4.33, P=0.040; and rs50871: AA vs. AC/CC adjHR=2.27, 95% CI 1.18-4.35, P=0.015). Patients with an increasing number of unfavorable genotypes had markedly increased death risk. Genetic variants of NER genes, particularly XPE rs28720291, ERCC5 rs4150314, XPC rs2470458, and ERCC2 rs50871, may independently or jointly modulate survival outcome of melanoma patients. Because our results were based on a median follow-up of 3 years without multiple test corrections, additional large prospective studies are needed to confirm our findings.

Malatesta M, Peschiaroli A, Memmi EM, et al.
The Cul4A-DDB1 E3 ubiquitin ligase complex represses p73 transcriptional activity.
Oncogene. 2013; 32(39):4721-6 [PubMed] Related Publications
The Cullin4A (cul4A)-dependent ligase (CDL4A) E3 has been implicated in a variety of biological processes, including cell cycle progression and DNA damage response. Remarkably, CDL4A exerts its function through both proteolytic and non-proteolytic events. Here, we show that the p53 family member p73 is able to interact with the CDL4A complex through its direct binding to the receptor subunit DNA-binding protein 1 (DDB1). As a result, the CDL4A complex is able to monoubiquitylate p73. Modification of p73 by CDL4A-mediated ubiquitylation does not affect p73 protein stability, but negatively regulates p73-dependent transcriptional activity. Indeed, genetic or RNA interference-mediated depletion of DDB1 induces the expression of several p73 target genes in a p53-independent manner. In addition, by exploiting a bioinformatic approach, we found that elevated expression of Cul4A in human breast carcinomas is associated with repression of p73 target genes. In conclusion, our findings add a novel insight into the regulation of p73 by the CDL4A complex, through the inhibition of its transcriptional function.

Chang SW, Su CH, Chen HH, et al.
DDB2 is a novel AR interacting protein and mediates AR ubiquitination/degradation.
Int J Biochem Cell Biol. 2012; 44(11):1952-61 [PubMed] Related Publications
Damaged DNA-binding protein 2 (DDB2), a protein that binds damaged DNA, is a DDB1 and CUL4-associated factor. This study is the first to demonstrate that DDB2 is a novel androgen receptor (AR)-interacting protein; and mediating contact with AR and CUL4A-DDB1 complex for AR ubiquitination/degradation. DNA damage induces both p53 and DDB2 gene expression those two can inhibit AR expression. The former reduces AR via transcription regulation but the latter via proteosome degradation. Thereby DDB2 can inhibit cell growth rate in AR-expressing cells (LNCaP) but not in AR-null cells (PC3). Hence DDB2 may be a potential regimen for prostate cancer treatment, especially in androgen-refractory patients harboring high amount of AR who cannot be cured by androgen ablation.

Fuss JO, Tainer JA
XPB and XPD helicases in TFIIH orchestrate DNA duplex opening and damage verification to coordinate repair with transcription and cell cycle via CAK kinase.
DNA Repair (Amst). 2011; 10(7):697-713 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Helicases must unwind DNA at the right place and time to maintain genomic integrity or gene expression. Biologically critical XPB and XPD helicases are key members of the human TFIIH complex; they anchor CAK kinase (cyclinH, MAT1, CDK7) to TFIIH and open DNA for transcription and for repair of duplex distorting damage by nucleotide excision repair (NER). NER is initiated by arrested RNA polymerase or damage recognition by XPC-RAD23B with or without DDB1/DDB2. XP helicases, named for their role in the extreme sun-mediated skin cancer predisposition xeroderma pigmentosum (XP), are then recruited to asymmetrically unwind dsDNA flanking the damage. XPB and XPD genetic defects can also cause premature aging with profound neurological defects without increased cancers: Cockayne syndrome (CS) and trichothiodystrophy (TTD). XP helicase patient phenotypes cannot be predicted from the mutation position along the linear gene sequence and adjacent mutations can cause different diseases. Here we consider the structural biology of DNA damage recognition by XPC-RAD23B, DDB1/DDB2, RNAPII, and ATL, and of helix unwinding by the XPB and XPD helicases plus the bacterial repair helicases UvrB and UvrD in complex with DNA. We then propose unified models for TFIIH assembly and roles in NER. Collective crystal structures with NMR and electron microscopy results reveal functional motifs, domains, and architectural elements that contribute to biological activities: damaged DNA binding, translocation, unwinding, and ATP driven changes plus TFIIH assembly and signaling. Coupled with mapping of patient mutations, these combined structural analyses provide a framework for integrating and unifying the rich biochemical and cellular information that has accumulated over forty years of study. This integration resolves puzzles regarding XP helicase functions and suggests that XP helicase positions and activities within TFIIH detect and verify damage, select the damaged strand for incision, and coordinate repair with transcription and cell cycle through CAK signaling.

Argiris A, Heron DE, Smith RP, et al.
Induction docetaxel, cisplatin, and cetuximab followed by concurrent radiotherapy, cisplatin, and cetuximab and maintenance cetuximab in patients with locally advanced head and neck cancer.
J Clin Oncol. 2010; 28(36):5294-300 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
PURPOSE: We incorporated cetuximab, a chimeric monoclonal antibody against the epidermal growth factor receptor (EGFR), into the induction therapy and subsequent chemoradiotherapy of head and neck cancer (HNC).
PATIENTS AND METHODS: Patients with locally advanced HNC, including squamous and undifferentiated histologies, were treated with docetaxel 75 mg/m2 day 1, cisplatin 75 mg/m2 day 1, and cetuximab 250 mg/m2 days 1, 8, and 15 (after an initial loading dose of 400 mg/m2), termed TPE, repeated every 21 days for three cycles, followed by radiotherapy with concurrent cisplatin 30 mg/m2 and cetuximab weekly (XPE), and maintenance cetuximab for 6 months. Quality of life (QOL) was assessed using Functional Assessment of Cancer Therapy-Head and Neck. In situ hybridization (ISH) for human papillomavirus (HPV), immunohistochemistry for p16, and fluorescence ISH for EGFR gene copy number were performed on tissue microarrays.
RESULTS: Of 39 enrolled patients, 36 had stage IV disease and 23 an oropharyngeal primary. Acute toxicities during TPE included neutropenic fever (10%) and during XPE, grade 3 or 4 oral mucositis (54%) and hypomagnesemia (39%). With a median follow-up of 36 months, 3-year progression-free survival and overall survival were 70% and 74%, respectively. Eight patients progressed in locoregional sites, three in distant, and one in both. HPV positivity was not associated with treatment efficacy. No progression-free patient remained G-tube dependent. The H&N subscale QOL scores showed a significant decrement at 3 months after XPE, which normalized at 1 year.
CONCLUSION: This cetuximab-containing regimen resulted in excellent long-term survival and safety, and warrants further evaluation in both HPV-positive and -negative HNC.

Choi SH, Wright JB, Gerber SA, Cole MD
Myc protein is stabilized by suppression of a novel E3 ligase complex in cancer cells.
Genes Dev. 2010; 24(12):1236-41 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Rapid Myc protein turnover is critical for maintaining basal levels of Myc activity in normal cells and a prompt response to changing growth signals. We characterize a new Myc-interacting factor, TRPC4AP (transient receptor potential cation channel, subfamily C, member 4-associated protein)/TRUSS (tumor necrosis factor receptor-associated ubiquitous scaffolding and signaling protein), which is the receptor for a DDB1 (damage-specific DNA-binding protein 1)-CUL4 (Cullin 4) E3 ligase complex for selective Myc degradation through the proteasome. TRPC4AP/TRUSS binds specifically to the Myc C terminus and promotes its ubiquitination and destruction through the recognition of evolutionarily conserved domains in the Myc N terminus. TRPC4AP/TRUSS suppresses Myc-mediated transactivation and transformation in a dose-dependent manner. Finally, we found that TRPC4AP/TRUSS expression is strongly down-regulated in most cancer cell lines, leading to Myc protein stabilization. These studies identify a novel pathway targeting Myc degradation that is suppressed in cancer cells.

Suh I, Weng J, Fernandez-Ranvier G, et al.
Antineoplastic effects of decitabine, an inhibitor of DNA promoter methylation, in adrenocortical carcinoma cells.
Arch Surg. 2010; 145(3):226-32 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
HYPOTHESES: Decitabine recovers expression of silenced genes on chromosome 11q13 and has antineoplastic effects in adrenocortical carcinoma (ACC) cells.
DESIGN: NCI-H295R cells were treated with decitabine (0.1-1.0 microM) over 5 days. Cells were evaluated at 24-hour intervals for the effects of decitabine on ACC cell proliferation, cortisol secretion, and cell invasion. Expression was quantified for 6 genes on 11q13 (DDB1, MRPL48, NDUFS8, PRDX5, SERPING1, and TM7SF2) that were previously shown to be underexpressed in ACC.
SETTING: Academic research. Study Specimen Human ACC cell line.
MAIN OUTCOME MEASURES: Adrenocortical carcinoma cell proliferation, cortisol secretion, and cell invasion were measured using immunometric assays. Quantitative reverse transcription-polymerase chain reaction was used to measure gene expression relative to GAPDH.
RESULTS: Decitabine inhibited ACC cell proliferation by 39% to 47% at 5 days after treatment compared with control specimens (P < .001). The inhibitory effect was cytostatic, time dependent, and dose dependent. Decitabine decreased cortisol secretion by 56% to 58% at 5 days after treatment (P = .02) and inhibited cell invasion by 64% at 24 hours after treatment (P = .03). Of 6 downregulated genes on 11q13, decitabine recovered expression of NDUFS8 (OMIM 602141) (P < .001) and PRDX5 (OMIM 606583) (P = .006).
CONCLUSIONS: Decitabine exhibits antitumoral properties in ACC cells at clinically achievable doses and may be an effective adjuvant therapy in patients with advanced disease. Decitabine recovers expression of silenced genes on 11q13, which suggests a possible role of epigenetic gene silencing in adrenocortical carcinogenesis.

de Feraudy S, Boubakour-Azzouz I, Fraitag S, et al.
Diagnosing xeroderma pigmentosum group C by immunohistochemistry.
Am J Dermatopathol. 2010; 32(2):109-17 [PubMed] Related Publications
Xeroderma pigmentosum (XP) is a group of rare inherited human neurocutaneous diseases, and the group C (XPC) is the major group of patients with XP in Europe, North America, and South America. Current molecular diagnostic methods for XP require specialized, expensive, and time-consuming UV sensitivity and DNA repair assays followed by gene sequencing. To determine whether immunohistochemistry (IHC) would be a robust alternative method to diagnose patients with XPC, we stained sections of paraffin-embedded skin biopsies for XPC by IHC, using 69 archived blocks from confirmed or clinically suspect patients with XPA, XPC, XPD, XPE, and without XP. We found that XPC expression was strong in all skin biopsies from patients without (14 of 14) and other patients with XP (4 of 4), whereas XPC expression was lost in all biopsies from confirmed XPC patients (29 of 29). Patches of strong XPC signal could be detected in sun-damaged skin, squamous and basal cell carcinomas from patients with XPC that colocalized with strong expression of p53 and Ki-67. Patients with XPC can therefore be diagnosed by IHC from paraffin-embedded skin biopsies from regions of skin that are without sun damage or sun-induced tumors. IHC is therefore a robust alternative method to diagnose patients with XPC. This fast and inexpensive method should increase the options for the diagnosis of patients with XPC from paraffin-embedded skin biopsies and could be developed for other complementation groups.

Youssif C, Goldenbogen J, Hamoudi R, et al.
Genomic profiling of pediatric ALK-positive anaplastic large cell lymphoma: a Children's Cancer and Leukaemia Group Study.
Genes Chromosomes Cancer. 2009; 48(11):1018-26 [PubMed] Related Publications
Anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma (ALCL) is a T-cell malignancy in which ALK expression is a consequence of the t(2;5) or a variant translocation involving Chromosome 2. For the most part, this disease presents in the pediatric population and most, but not all, patients are successfully treated. Although the t(2;5) product nucleophosmin-ALK has been extensively studied for its transforming properties, very little is known regarding cooperative genetic mutations that may contribute to lymphomagenesis and may predict survival outcome, specifically in a purely pediatric population. We set out to determine the frequency and positions of genomic imbalances in this relatively rare disease. We collected biopsy material from 15 UK-resident children with ALK-expressing ALCL. We performed array comparative genomic hybridization at a resolution of 1 MB using DNA isolated from tumor tissue. Some of the more common genomic gains were confirmed by quantitative PCR. Regions of genomic gain were far more common than losses and were most often detected on chromosomes 1-4, 5-12, 14, and 17, with Chromosome 11 being the most frequent site of genomic imbalances. Patients with 14 or fewer imbalances had a lower overall 3-year survival (87.5-40%, P = 0.14) as did patients with gains in the regions of DDB1 or BIRC5. A range of genomic imbalances exist in ALK-expressing ALCL of a pediatric origin, with a greater number associated with poorer overall survival.

Lord CJ, McDonald S, Swift S, et al.
A high-throughput RNA interference screen for DNA repair determinants of PARP inhibitor sensitivity.
DNA Repair (Amst). 2008; 7(12):2010-9 [PubMed] Related Publications
Synthetic lethality is an attractive strategy for the design of novel therapies for cancer. Using this approach we have previously demonstrated that inhibition of the DNA repair protein, PARP1, is synthetically lethal with deficiency of either of the breast cancer susceptibility proteins, BRCA1 and BRCA2. This observation is most likely explained by the inability of BRCA deficient cells to repair DNA damage by homologous recombination (HR) and has led to the clinical trials of potent PARP inhibitors for the treatment of BRCA mutation-associated cancer. To identify further determinants of PARP inhibitor response, we took a high-throughput genetic approach. We tested each of the genes recognised as having a role in DNA repair using short-interfering RNA (siRNA) and assessed the sensitivity of siRNA transfected cells to a potent PARP inhibitor, KU0058948. The validity of this approach was confirmed by the identification of known genetic determinants of PARP inhibitor sensitivity, including genes involved in HR. Novel determinants of PARP inhibitor response were also identified, including the transcription coupled DNA repair (TCR) proteins DDB1 and XAB2. These results suggest that DNA repair pathways other than HR may determine sensitivity to PARP inhibitors and highlight the likelihood that ostensibly distinct DNA repair pathways cooperate to maintain genomic stability and cellular viability. Furthermore, the identification of these novel determinants may eventually guide the optimal use of PARP inhibitors in the clinic.

Martin-Lluesma S, Schaeffer C, Robert EI, et al.
Hepatitis B virus X protein affects S phase progression leading to chromosome segregation defects by binding to damaged DNA binding protein 1.
Hepatology. 2008; 48(5):1467-76 [PubMed] Related Publications
UNLABELLED: Chronic hepatitis B virus (HBV) infection is a leading cause of hepatocellular carcinoma (HCC), but its role in the transformation process remains unclear. HBV encodes a small protein, known as HBx, which is required for infection and has been implicated in hepatocarcinogenesis. Here we show that HBx induces lagging chromosomes during mitosis, which in turn leads to formation of aberrant mitotic spindles and multinucleated cells. These effects require the binding of HBx to UV-damaged DNA binding protein 1 (DDB1), a protein involved in DNA repair and cell cycle regulation, and are unexpectedly attributable to HBx interfering with S-phase progression and not directly with mitotic events. HBx also affects S-phase and induces lagging chromosomes when expressed from its natural viral context and, consequently, exhibits deleterious activities in dividing, but not quiescent, hepatoma cells.
CONCLUSION: In addition to its reported role in promoting HBV replication, the binding of HBx to DDB1 may induce genetic instability in regenerating hepatocytes and thereby contribute to HCC development, thus making this HBV-host protein interaction an attractive target for new therapeutic intervention.

Huang J, Chen J
VprBP targets Merlin to the Roc1-Cul4A-DDB1 E3 ligase complex for degradation.
Oncogene. 2008; 27(29):4056-64 [PubMed] Related Publications
Inactivation of the neurofibromatosis type 2 (NF2) tumor suppressor gene function has been observed not only in familial schwannomas and other central nervous system tumors, but also in malignant tumors unrelated to the NF2 syndrome, indicating a broader role of NF2 in human tumorigenesis. The NF2-encoded protein Merlin is closely related to the Ezrin-Radixin-Moesin family of membrane/cytoskeleton linker proteins, and has been demonstrated to suppress tumor growth by inhibiting extracellular signal-regulated kinase (ERK) and Rac1 activation. Interestingly, serum deprivation has been shown to regulate Merlin at the protein level, however, exactly how such condition affects Merlin remains elusive. In this study, we provide evidence to show that Merlin is regulated in a Roc1-Cullin4A-DDB1-dependent manner. Following serum stimulation, Merlin is recruited to the E3 ligase complex through a direct interaction with the WD40-containing adaptor protein VprBP. Loading of Merlin to the E3 ubiquitin ligase complex resulted in its polyubiquitination, and consequently its proteasome-mediated degradation. Consistently, VprBP depletion abolished the in vivo interaction of Merlin and Roc1-Cullin4A-DDB1, which resulted in Merlin stabilization and inhibited ERK and Rac activation. Together, our data revealed a novel regulatory mechanism for the tumor suppressor function of Merlin.

Fernandez-Ranvier GG, Weng J, Yeh RF, et al.
Candidate diagnostic markers and tumor suppressor genes for adrenocortical carcinoma by expression profile of genes on chromosome 11q13.
World J Surg. 2008; 32(5):873-81 [PubMed] Related Publications
BACKGROUND: The most common genetic change observed in adrenocortical carcinoma is loss of heterozygozity on chromosome 11q13. As genes on this chromosome may be important in the pathogenesis of adrenocortical carcinoma, we compared their expression profile between benign and malignant adrenocortical tissue.
METHODS: We used the Affymetrix GeneChip (U133 plus 2.0) array in 54 adrenocortical tumors (11 carcinoma and 43 benign). Differential gene expression was defined as a twofold higher or lower gene expression level (p<0.05). Differentially expressed genes on microarray analysis were validated by real-time quantitative reverse-transcriptase polymerase chain reaction (RT-PCR). The area under the receiver operating characteristic (ROC) curve (AUC) was used to determined the diagnostic accuracy of the differently expressed genes for distinguishing benign from malignant tumors.
RESULTS: We found 25 of the 314 genes on chromosome 11q13 to be differentially expressed between adrenocortical carcinoma and benign adrenocortical tumor. All 25 were downregulated in adrenocortical carcinoma by 2-fold to 4.8-fold; 21 were validated to be differentially expressed by RT-PCR (Pearson's coefficient>0.5). Six genes (SERPING1, MRPL48, TM7SF2, DDB1, NDUSF8, PRDX5) validated by RT-PCR were significantly differentially expressed between benign and malignant adrenocortical tumors (p<0.05) with an overall accuracy of 89% for SERPING1, 91% for MRPL48, 87% for TM7SF2, 88% for DDB1, 91% for NDUFS8, and 89% for PRDX5. The AUC was 0.89 for the combination of SERPING1, MRPL48, TM7SF2, DDB1, and NDUFS8.
CONCLUSIONS: We have identified 25 genes located on chromosome 11q13 that are downregulated in adrenocortical carcinoma and may be candidate tumor suppressor genes. Six of these genes were good diagnostic markers for distinguishing adrenocortical carcinoma from adenoma.

Stoyanova T, Yoon T, Kopanja D, et al.
The xeroderma pigmentosum group E gene product DDB2 activates nucleotide excision repair by regulating the level of p21Waf1/Cip1.
Mol Cell Biol. 2008; 28(1):177-87 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
The xeroderma pigmentosum group E gene product DDB2, a protein involved in nucleotide excision repair (NER), associates with the E3 ubiquitin ligase complex Cul4A-DDB1. But the precise role of these interactions in the NER activity of DDB2 is unclear. Several models, including DDB2-mediated ubiquitination of histones in UV-irradiated cells, have been proposed. But those models lack clear genetic evidence. Here we show that DDB2 participates in NER by regulating the cellular levels of p21(Waf1/Cip1). We show that DDB2 enhances nuclear accumulation of DDB1, which binds to a modified form of p53 containing phosphorylation at Ser18 (p53(S18P)) and targets it for degradation in low-dose-UV-irradiated cells. DDB2(-/-) mouse embryonic fibroblasts (MEFs), unlike wild-type MEFs, are deficient in the proteolysis of p53(S18P). Accumulation of p53(S18P) in DDB2(-/-) MEFs causes higher expression p21(Waf1/Cip1). We show that the increased expression of p21(Waf1/Cip1) is the cause NER deficiency in DDB2(-/-) cells because deletion or knockdown of p21(Waf1/Cip1) reverses their NER-deficient phenotype. p21(Waf1/Cip1) was shown to bind PCNA, which is required for both DNA replication and NER. Moreover, an increased level of p21(Waf1/Cip1) was shown to inhibit NER both in vitro and in vivo. Our results provide genetic evidence linking the regulation of p21(Waf1/Cip1) to the NER activity of DDB2.

Lovejoy CA, Lock K, Yenamandra A, Cortez D
DDB1 maintains genome integrity through regulation of Cdt1.
Mol Cell Biol. 2006; 26(21):7977-90 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
DDB1, a component of a Cul4A ubiquitin ligase complex, promotes nucleotide excision repair (NER) and regulates DNA replication. We have investigated the role of human DDB1 in maintaining genome stability. DDB1-depleted cells accumulate DNA double-strand breaks in widely dispersed regions throughout the genome and have activated ATM and ATR cell cycle checkpoints. Depletion of Cul4A yields similar phenotypes, indicating that an E3 ligase function of DDB1 is important for genome maintenance. In contrast, depletion of DDB2, XPA, or XPC does not cause activation of DNA damage checkpoints, indicating that defects in NER are not involved. One substrate of DDB1-Cul4A that is crucial for preventing genome instability is Cdt1. DDB1-depleted cells exhibit increased levels of Cdt1 protein and rereplication, despite containing other Cdt1 regulatory mechanisms. The rereplication, accumulation of DNA damage, and activation of checkpoint responses in DDB1-depleted cells require entry into S phase and are partially, but not completely, suppressed by codepletion of Cdt1. Therefore, DDB1 prevents DNA lesions from accumulating in replicating human cells, in part by regulating Cdt1 degradation.

Wijnhoven SW, Hoogervorst EM, de Waard H, et al.
Tissue specific mutagenic and carcinogenic responses in NER defective mouse models.
Mutat Res. 2007; 614(1-2):77-94 [PubMed] Related Publications
Several mouse models with defects in genes encoding components of the nucleotide excision repair (NER) pathway have been developed. In NER two different sub-pathways are known, i.e. transcription-coupled repair (TC-NER) and global-genome repair (GG-NER). A defect in one particular NER protein can lead to a (partial) defect in GG-NER, TC-NER or both. GG-NER defects in mice predispose to cancer, both spontaneous as well as UV-induced. As such these models (Xpa, Xpc and Xpe) recapitulate the human xeroderma pigmentosum (XP) syndrome. Defects in TC-NER in humans are associated with Cockayne syndrome (CS), a disease not linked to tumor development. Mice with TC-NER defects (Csa and Csb) are - except for the skin - not susceptible to develop (carcinogen-induced) tumors. Some NER factors, i.e. XPB, XPD, XPF, XPG and ERCC1 have functions outside NER, like transcription initiation and inter-strand crosslink repair. Deficiencies in these processes in mice lead to very severe phenotypes, like trichothiodystrophy (TTD) or a combination of XP and CS. In most cases these animals have a (very) short life span, display segmental progeria, but do not develop tumors. Here we will overview the available NER-related mouse models and will discuss their phenotypes in terms of (chemical-induced) tissue-specific tumor development, mutagenesis and premature aging features.

Kapetanaki MG, Guerrero-Santoro J, Bisi DC, et al.
The DDB1-CUL4ADDB2 ubiquitin ligase is deficient in xeroderma pigmentosum group E and targets histone H2A at UV-damaged DNA sites.
Proc Natl Acad Sci U S A. 2006; 103(8):2588-93 [PubMed] Article available free on PMC after 01/03/2015 Related Publications
Xeroderma pigmentosum (XP) is a heritable human disorder characterized by defects in nucleotide excision repair (NER) and the development of skin cancer. Cells from XP group E (XP-E) patients have a defect in the UV-damaged DNA-binding protein complex (UV-DDB), involved in the damage recognition step of NER. UV-DDB comprises two subunits, products of the DDB1 and DDB2 genes, respectively. Mutations in the DDB2 gene account for the underlying defect in XP-E. The UV-DDB complex is a component of the newly identified cullin 4A-based ubiquitin E3 ligase, DDB1-CUL4A(DDB2). The E3 ubiquitin ligases recognize specific substrates and mediate their ubiquitination to regulate protein activity or target proteins for degradation by the proteasomal pathway. In this study, we have addressed the role of the UV-DDB-based E3 in NER and sought a physiological substrate. We demonstrate that monoubiquitinated histone H2A in native chromatin coimmunoprecipitates with the endogenous DDB1-CUL4A(DDB2) complex in response to UV irradiation. Further, mutations in DDB2 alter the formation and binding activity of the DDB1-CUL4A(DDB2) ligase, accompanied by impaired monoubiquitination of H2A after UV treatment of XP-E cells, compared with repair-proficient cells. This finding indicates that DDB2, as the substrate receptor of the DDB1-CUL4A-based ligase, specifically targets histone H2A for monoubiquitination in a photolesion-binding-dependent manner. Given that the loss of monoubiquitinated histone H2A at the sites of UV-damaged DNA is associated with decreased global genome repair in XP-E cells, this study suggests that histone modification, mediated by the XPE factor, facilitates the initiation of NER.

Navaraj A, Mori T, El-Deiry WS
Cooperation between BRCA1 and p53 in repair of cyclobutane pyrimidine dimers.
Cancer Biol Ther. 2005; 4(12):1409-14 [PubMed] Related Publications
DNA repair defects can predispose to cancer development and progression. We previously showed that the breast and ovarian cancer susceptibility gene product BRCA1, through p53, upregulates expression of the XPE gene DDB2 encoding the nucleotide excision repair protein p 48. Both XPE and XPC are p53 target genes containing p53 response elements. To further explore the role of BRCA1 and p53 in repair of photoproducts, we eliminated wild type p53 from U2OS osteosarcoma cells and found that cyclobutane pyrimidine dimer (CPD) repair was markedly impaired following UV damage whereas repair of 6-4 photoproduct (6-4 PP) occurred efficiently. Overexpression of p53 in p53-null Calu-6 cells also enhanced CPD repair. In HCC1937 breast cancer cells, harboring mutant BRCA1 and p53 genes, repair of CPD was markedly impaired. Reintroduction of either p53 or BRCA1 using adenovirus vectors into HCC1937 alone had little effect on repair of CPD whereas the combination of p53 and BRCA1 resulted in efficient repair of CPD. Thus there appears to be a cooperative effect between p53 and BRCA1 that may involve induction of repair proteins, inhibition of p53-induced cell death by BRCA1 with altered p53 selectivity towards repair pathways and/or p53-independent effects of BRCA1 on CPD repair.

Matsuda N, Azuma K, Saijo M, et al.
DDB2, the xeroderma pigmentosum group E gene product, is directly ubiquitylated by Cullin 4A-based ubiquitin ligase complex.
DNA Repair (Amst). 2005; 4(5):537-45 [PubMed] Related Publications
Xeroderma pigmentosum (XP) is a genetic disease characterized by hypersensitivity to UV irradiation and high incidence of skin cancer caused by inherited defects in DNA repair. Mutational malfunction of damaged-DNA binding protein 2 (DDB2) causes the XP complementation group E (XP-E). DDB2 together with DDB1 comprises a heterodimer called DDB complex, which is involved in damaged-DNA binding and nucleotide excision repair. Interestingly, by screening for a cellular protein(s) that interacts with Cullin 4A (Cul4A), a key component of the ubiquitin ligase complex, we identified DDB1. Immunoprecipitation confirmed that Cul4A interacts with DDB1 and also associates with DDB2. To date, it has been reported that DDB2 is rapidly degraded after UV irradiation and that overproduction of Cul4A stimulates the ubiquitylation of DDB2 in the cells. However, as biochemical analysis using pure Cul4A-containing E3 is missing, it is still unknown whether the Cul4A complex directly ubiquitylates DDB2 or not. We thus purified the Cul4A-containing E3 complex to near homogeneity and attempted to ubiquitylate DDB2 in vitro. The ubiquitylation of DDB2 was reconstituted using this pure E3 complex, indicating that DDB-Cul4A E3 complex in itself can ubiquitylate DDB2 directly. We also showed that an amino acid substitution, K244E, in DDB2 derived from a XP-E patient did not affect its ubiquitylation.

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