Association of CBP/p300 acetylase and thymine DNA glycosylase links DNA repair and transcription

Chromatin remodeling and the maintenance of genome integrity

DNA damage of any type is threatening for a cell. If lesions are left unrepaired, genomic instability can arise, faithful transmission of genetic information is greatly compromised eventually leading the cell to undergo apoptosis or carcinogenesis. In order to access/detect and repair these damages, repair factors must circumvent the natural repressive barrier of chromatin. This review will present recent progress showing the intricate link between chromatin, its remodeling and the DNA repair process. Several studies demonstrated that one of the first events following specific types of DNA damage is the phosphorylation of histone H2A. This mark or the damage itself are responsible for the association of chromatin-modifying complexes near damaged DNA. These complexes are able to change the chromatin structure around the wounded DNA in order to allow the repair machinery to gain access and repair the lesion. Chromatin modifiers include ATP-dependent remodelers such as SWI/SNF and Rad54 as well as histone acetyltransferases (HATs) like SAGA/NuA4-related complexes and p300/CBP, which have been shown to facilitate DNA accessibility and repair in different pathways leading to the maintenance of genome integrity.

An invariant aspartic acid in the DNA glycosylase domain of DEMETER is necessary for transcriptional activation of the imprinted MEDEA gene

Helix-hairpin-helix DNA glycosylases are typically small proteins that initiate repair of DNA by excising damaged or mispaired bases. An invariant aspartic acid in the active site is involved in catalyzing the excision reaction. Replacement of this critical residue with an asparagine severely reduces catalytic activity but preserves enzyme stability and structure. The Arabidopsis DEMETER (DME) gene encodes a large 1,729-aa polypeptide with a 200-aa DNA glycosylase domain. DME is expressed primarily in the central cell of the female gametophyte. DME activates maternal allele expression of the imprinted MEDEA (MEA) gene in the central cell and is required for seed viability. We mutated the invariant aspartic acid at position 1304 in DME to asparagine (D1304N) to determine whether the catalytic activity of the DNA glycosylase domain is required for DME function in vivo. Transgenes expressing wild-type DME in the central cell rescue seed abortion caused by a mutation in the endogenous DME gene and activate maternal MEA:GFP transcription. However, transgenes expressing the D1304N mutant DME do not rescue seed abortion or activate maternal MEA:GFP transcription. Whereas ectopic expression of the wild-type DME polypeptide in pollen is sufficient to activate ectopic paternal MEA and MEA:GUS expression, equivalent expression of the D1304N mutant DME in pollen failed to do so. These results show that the conserved aspartic acid residue is necessary for DME to function in vivo and suggest that an active DNA glycosylase domain, normally associated with DNA repair, promotes gene transcription that is essential for gene imprinting.

A novel protein with similarities to Rb binding protein 2 compensates for loss of Chk1 function and affects histone modification in fission yeast

The conserved protein kinase Chk1 mediates cell cycle progression and consequently the ability of cells to survive when exposed to DNA damaging agents. Cells deficient in Chk1 are hypersensitive to such agents and enter mitosis in the presence of damaged DNA, whereas checkpoint-proficient cells delay mitotic entry to permit time for DNA repair. In a search for proteins that can improve the survival of Chk1-deficient cells exposed to DNA damage, we identified fission yeast Msc1, which is homologous to a mammalian protein that binds to the tumor suppressor Rb (RBP2). Msc1 and RBP2 each possess three PHD fingers, domains commonly found in proteins that influence the structure of chromatin. Msc1 is chromatin associated and coprecipitates a histone deacetylase activity, a property that requires the PHD fingers. Cells lacking Msc1 have a dramatically altered histone acetylation pattern, exhibit a 20-fold increase in global acetylation of histone H3 tails, and are readily killed by trichostatin A, an inhibitor of histone deacetylases. We postulate that Msc1 plays an important role in regulating chromatin structure and that this function modulates the cellular response to DNA damage.

Single-Stranded Breaks in DNA but Not Oxidative DNA Base Damages Block Transcriptional Elongation by RNA Polymerase II in HeLa Cell Nuclear Extracts

Transcription and repair of many DNA helix-distorting lesions such as cyclobutane pyrimidine dimers have been shown to be coupled in cells across phyla from bacteria to humans. The signal for transcription-coupled repair appears to be a stalled transcription complex at the lesion site. To determine whether oxidative DNA lesions can block correctly initiated human RNA polymerase II, we examined the effect of site-specifically introduced oxidative damages on transcription in HeLa cell nuclear extracts. We found that transcription was blocked by single-stranded breaks, common oxidative DNA lesions, when present in the transcribed strand of the transcription template. Cyclobutane pyrimidine dimers, which have been previously shown to block transcription both in vitro and in vivo, also blocked transcription in the HeLa cell nuclear transcription assay. In contrast, the oxidative DNA base lesions, 8-oxoguanine, 5-hydroxycytosine, and thymine glycol did not inhibit transcription, although pausing was observed with the thymine glycol lesion. Thus, DNA strand breaks but not oxidative DNA base damages blocked transcription by RNA polymerase II.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

A role for p300/CREB binding protein genes in promoting cancer progression in colon cancer cell lines with microsatellite instability

Our manipulation of the nonsense-mediated decay pathway in microsatellite unstable colon cancer cell lines identified the p300 gene as a potential tumor suppressor in this subtype of cancer. Here, we have demonstrated that not only the p300 gene but also the highly homologous cAMP-response element-binding protein (CREB) binding protein (CBP) gene together are mutated in >85% of microsatellite instability (MSI)+ colon cancer cell lines. A limited survey of primary tumors with MSI+ shows that p300 is also frequently mutated in these cancers, demonstrating that these mutations are not consequences of in vitro growth. The mutations in both genes occur frequently in mononucleotide repeats that generate premature stop codons. Reintroduction of p300 into MSI colon cancer cells could only be supported in the presence of an inactivated CBP gene, suggesting the idea that one or the other function must be inactivated for cancer cell viability. p300 is known to acetylate p53 in response to DNA damage, and when MSI+ cells null for p300 activity are forced to reexpress exogenous p300 cells show slower growth and a flatter morphology. p53 acetylation is increased upon reexpression of p300, suggesting that MSI+ cells constitutively activate the DNA damage response pathway in the absence of DNA-damaging agents. In support of this hypothesis, c-ABL kinase, which is also activated in response to DNA damage, shows higher levels of basal kinase activity in MSI+ cells. These observations suggest that there is a selective growth/survival advantage to mutational inactivation of p300/CBP in cells with inactivated mismatch repair capabilities.

Role of acetylated human AP-endonuclease (APE1/Ref-1) in regulation of the parathyroid hormone gene

The human AP-endonuclease (APE1/Ref-1), a multifunctional protein central to repairing abasic sites and single-strand breaks in DNA, also plays a role in transcriptional regulation. Besides activating some transcription factors, APE1 is directly involved in Ca2+-dependent downregulation of parathyroid hormone (PTH) expression by binding to negative calcium response elements (nCaREs) present in the PTH promoter. Here we show that APE1 is acetylated both in vivo and in vitro by the transcriptional co-activator p300 which is activated by Ca2+. Acetylation at Lys6 or Lys7 enhances binding of APE1 to nCaRE. APE1 stably interacts with class I histone deacetylases (HDACs) in vivo. An increase in extracellular calcium enhances the level of acetylated APE1 which acts as a repressor for the PTH promoter. Moreover, chromatin immunoprecipitation (ChIP) assay revealed that acetylation of APE1 enhanced binding of the APE1-HDACs complex to the PTH promoter. These results indicate that acetylation of APE1 plays an important role in this key repair protein's action in transcriptional regulation.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

A role for p300/CREB binding protein genes in promoting cancer progression in colon cancer cell lines with microsatellite instability

Our manipulation of the nonsense-mediated decay pathway in microsatellite unstable colon cancer cell lines identified the p300 gene as a potential tumor suppressor in this subtype of cancer. Here, we have demonstrated that not only the p300 gene but also the highly homologous cAMP-response element-binding protein (CREB) binding protein (CBP) gene together are mutated in >85% of microsatellite instability (MSI)+ colon cancer cell lines. A limited survey of primary tumors with MSI+ shows that p300 is also frequently mutated in these cancers, demonstrating that these mutations are not consequences of in vitro growth. The mutations in both genes occur frequently in mononucleotide repeats that generate premature stop codons. Reintroduction of p300 into MSI colon cancer cells could only be supported in the presence of an inactivated CBP gene, suggesting the idea that one or the other function must be inactivated for cancer cell viability. p300 is known to acetylate p53 in response to DNA damage, and when MSI+ cells null for p300 activity are forced to reexpress exogenous p300 cells show slower growth and a flatter morphology. p53 acetylation is increased upon reexpression of p300, suggesting that MSI+ cells constitutively activate the DNA damage response pathway in the absence of DNA-damaging agents. In support of this hypothesis, c-ABL kinase, which is also activated in response to DNA damage, shows higher levels of basal kinase activity in MSI+ cells. These observations suggest that there is a selective growth/survival advantage to mutational inactivation of p300/CBP in cells with inactivated mismatch repair capabilities.

Characterization of DNA demethylation in normal and cancerous cell lines and the regulatory role of cell cycle proteins in human DNA demethylase activity

DNA methylation/demethylation constitutes a major consequence in all biological processes involving transcription, differentiation, development, DNA repair, recombination, and chromosome organization. Our earlier studies established that demethylation of CpG rich sequence by human DNA demethylase activity (5-methylcytosine-DNA glycosylase (5MeC-DNA glycosylase)) resembles "base excision DNA repair activity" and creates single-strand breaks on DNA that is associated with proliferating cell nuclear antigen (PCNA). Here in this report, we have identified differential DNA demethylation targets (hemi-methylated vs. fully-methylated) in normal cell lines and cancerous cell lines, and a shortened G(0)/G(1) resting time in cancerous cell lines than the normal cell lines. We have identified that in normal HFL1 fibroblast cell line, DNA demethylase activity targets hemi-methylated CpG specific sites on DNA. This normal cell line DNA demethylase activity associates with PCNA immune complex that is inhibited by CDKI proteins p21(waf1)/Gadd45alpha and Gadd45beta. While in cancerous LnCap and BT20 cell lines DNA demethylase activity targets fully-methylated CpG specific sites on DNA. This cancer cell line DNA demethylase activity is not associated with PCNA immune complex and is not inhibited by CDKI proteins p21(waf1)/Gadd45alpha and Gadd45beta. We have also identified that the fully-methylated CpG specific DNA demethylase activity from cancerous cell lines to associate with p300/CBP protein. These significant observations of variable targets of DNA demethylation and alternate partner proteins for DNA demethylase activity in cancerous cell lines are discussed in terms of double-strand DNA breaks versus single-strand DNA breaks and their role in the exit of G(1)/G(2) cell cycle stages. Also, the inability of cell cycle regulatory proteins like PCNA, p21(waf1), and Gadd45 to control DNA demethylase activity in cancerous cell lines is discussed in terms of accelerated G(1)/G(2) cell cycle stage exit to facilitate unregulated cellular proliferation, loss of control of chromosomal organization, and the development of oncogenesis in cancerous cell lines.

Mammalian DNA base excision repair proteins: their interactions and role in repair of oxidative DNA damage

The DNA base excision repair (BER) is a ubiquitous mechanism for removing damage from the genome induced by spontaneous chemical reaction, reactive oxygen species (ROS) and also DNA damage induced by a variety of environmental genotoxicants. DNA repair is essential for maintaining genomic integrity. As we learn more about BER, a more complex mechanism emerges which supersedes the classical, simple pathway requiring only four enzymatic reactions. The key to understand the complete BER process is to elucidate how multiple proteins interact with one another in a coordinated process under specific physiological conditions.

The human proliferating Cell nuclear antigen regulates transcriptional coactivator p300 activity and promotes transcriptional repression

Chromatin structure plays an important role in DNA replication, repair, and transcription. p300 is a transcriptional coactivator with protein acetyltransferase activity, and proliferating cell nuclear antigen (PCNA) plays important roles in DNA replication and repair. It has been shown recently that p300 is necessary for DNA synthesis and repair. However, it is not known whether human PCNA, in a reciprocal manner, can regulate the enzymatic activity and transcriptional regulatory properties of p300. Here we show that human PCNA associates with p300 and potently inhibits the acetyltransferase activity and transcriptional activation properties of p300. Surprisingly, PCNA fails to inhibit p300/CBP-associated factor (PCAF) acetyltransferase function as well as PCAF-dependent transcription. Additionally, PCNA potently represses transcription when targeted to chromatin in vivo. Consistent with these observations, using chromatin immunoprecipitation assays, we demonstrate that PCNA recruitment to promoters causes hypoacetylation of chromatin. Together, our results demonstrate for the first time a novel role for human PCNA in transcriptional repression and in modulating chromatin modification. The reciprocal modulation of p300 and PCNA activities by each other provides an example of integrative regulatory cross-talk among chromatin-based processes such as DNA transcription, repair, and synthesis.

Methylated DNA-binding domain 1 and methylpurine-DNA glycosylase link transcriptional repression and DNA repair in chromatin

The methyl-CpG dinucleotide containing a symmetrical 5-methylcytosine (mC) is involved in gene regulation and genome stability. We report here that methylation-mediated transcriptional repressor methylated DNA-binding domain 1 (MBD1) interacts with methylpurine-DNA glycosylase (MPG), which excises damaged bases from substrate DNA. MPG itself actively represses transcription and has a synergistic effect on gene silencing together with MBD1. Chromatin immunoprecipitation analysis reveals the molecular movement of MBD1 and MPG in vivo: (i) The MBD1-MPG complex normally exists on the methylated gene promoter; (ii) treatment of cells with alkylating agent methylmethanesulfonate (MMS) induces the dissociation of MBD1 from the methylated promoter, and MPG is located on both methylated and unmethylated promoters; and (iii) after completion of the repair, the MBD1-MPG complex is restored on the methylated promoter. Mobility-shift and structural analyses show that the MBD of MBD1 binds a methyl-CpG pair (mCpG x mCpG) but not the methyl-CpG pair containing a single 7-methylguanine (N) (mCpG x mCpN) that is known as one of the major lesions caused by MMS. We further demonstrate that knockdown of MBD1 by specific small interfering RNAs significantly increases cell sensitivity to MMS. These data suggest that MBD1 cooperates with MPG for transcriptional repression and DNA repair. We hypothesize that MBD1 functions as a reservoir for MPG and senses the base damage in chromatin

T:G mismatch-specific thymine-DNA glycosylase potentiates transcription of estrogen-regulated genes through direct interaction with estrogen receptor alpha

Nuclear receptors (NR) classically regulate gene expression by stimulating transcription upon binding to their cognate ligands. It is now well established that NR-mediated transcriptional activation requires the recruitment of coregulator complexes, which facilitate recruitment of the basal transcription machinery through direct interactions with the basal transcription machinery and/or through chromatin remodeling. However, a number of recently described NR coactivators have been implicated in cross-talk with other nuclear processes including RNA splicing and DNA repair. T:G mismatch-specific thymine DNA glycosylase (TDG) is required for base excision repair of deaminated methylcytosine. Here we show that TDG is a coactivator for estrogen receptor alpha (ERalpha). We demonstrate that TDG interacts with ERalpha in vitro and in vivo and suggest a separate role for TDG to its established role in DNA repair. We show that this involves helix 12 of ERalpha. The region of interaction in TDG is mapped to a putative alpha-helical motif containing a motif distinct from but similar to the LXXLL motif that mediates interaction with NR. Together with recent reports linking TFIIH in regulating NR function, our findings provide new data to further support an important link between DNA repair proteins and nuclear receptor function.

Identification and characterization of BCL-3-binding protein: implications for transcription and DNA repair or recombination

A putative oncogene bcl-3 was originally identified and cloned at the breakpoint in the recurring chromosome translocation t(14;19) found in some cases of B cell chronic lymphocytic leukemia. Studies of bcl-3-deficient mice demonstrated a critical role for bcl-3 in the development of a normal immune response and the formation of germinal centers in secondary lymphoid organs. However, the molecular mechanism that underlies B cell leukemogenesis and the knockout mouse phenotype remains unclear. Here we have identified and characterized BCL-3-binding protein (B3BP) as a protein interacting specifically with the bcl-3 gene product (BCL-3) by a yeast two-hybrid screen. We found that B3BP associates with not only BCL-3 but also p300/CBP histone acetyltransferases. The N-terminal region of B3BP that contains the ATP-binding site is important for the interaction with BCL-3 and p300/CBP. Homology searches indicate that the ATP-binding region of B3BP, which contains a typical Walker-type ATP-binding P-loop, most resembles that of 2',3'-cyclic nucleotide 3'-phosphodiesterase of mammals and polynucleotide kinase of T4 bacteriophage. In fact B3BP shows intrinsic ATP binding and hydrolyzing activity. Furthermore, we demonstrated that B3BP is a 5'-polynucleotide kinase. We also found a small MutS-related domain, which is thought to be involved in the DNA repair or recombination reaction, in the C-terminal region of B3BP, and it shows nicking endonuclease activity. These observations might help to gain new insights into the function of BCL-3 and p300/CBP, especially the coupling of transcription with repair or recombination.

The versatile thymine DNA-glycosylase: a comparative characterization of the human, Drosophila and fission yeast orthologs

Human thymine-DNA glycosylase (TDG) is well known to excise thymine and uracil from G.T and G.U mismatches, respectively, and was therefore proposed to play a central role in the cellular defense against genetic mutation through spontaneous deamination of 5-methylcytosine and cytosine. In this study, we characterized two newly discovered orthologs of TDG, the Drosophila melanogaster Thd1p and the Schizosaccharomyces pombe Thp1p proteins, with an objective to address the function of this subfamily of uracil-DNA glycosylases from an evolutionary perspective. A systematic biochemical comparison of both enzymes with human TDG revealed a number of biologically significant facts. (i) All eukaryotic TDG orthologs have broad and species-specific substrate spectra that include a variety of damaged pyrimidine and purine bases; (ii) the common most efficiently processed substrates of all are uracil and 3,N4- ethenocytosine opposite guanine and 5-fluorouracil in any double-stranded DNA context; (iii) 5-methylcytosine and thymine derivatives are processed with an appreciable efficiency only by the human and the Drosophila enzymes; (iv) none of the proteins is able to hydrolyze a non-damaged 5'-methylcytosine opposite G; and (v) the double strand and mismatch dependency of the enzymes varies with the substrate and is not a stringent feature of this subfamily of DNA glycosylases. These findings advance our current view on the role of TDG proteins and document that they have evolved with high structural flexibility to counter a broad range of DNA base damage in accordance with the specific needs of individual species.

Impairment of the DNA binding activity of the TATA-binding protein renders the transcriptional function of Rvb2p/Tih2p, the yeast RuvB-like protein, essential for cell growth

In Saccharomyces cerevisiae, two highly conserved proteins, Rvb1p/Tih1p and Rvb2p/Tih2p, have been demonstrated to be major components of the chromatin-remodeling INO80 complex. The mammalian orthologues of these two proteins have been shown to physically associate with the TATA-binding protein (TBP) in vitro but not clearly in vivo. Here we show that yeast proteins interact with TBP under both conditions. To assess the functional importance of these interactions, we examined the effect of mutating both TIH2/RVB2 and SPT15, which encodes TBP, on yeast cell growth. Intriguingly, only those spt15 mutations that affected the ability of TBP to bind to the TATA box caused synthetic growth defects in a tih2-ts160 background. This suggests that Tih2p might be important in recruiting TBP to the promoter. A DNA microarray technique was used to identify genes differentially expressed in the tih2-ts160 strain grown at the restrictive temperature. Only 34 genes were significantly and reproducibly affected; some up-regulated and others down-regulated. We compared the transcription of several of these Tih2p target genes in both wild type and various mutant backgrounds. We found that the transcription of some genes depends on functions possessed by both Tih2p and TBP and that these functions are substantially impaired in the spt15/tih2-ts160 double mutants that confer synthetic growth defects.

The main role of human thymine-DNA glycosylase is removal of thymine produced by deamination of 5-methylcytosine and not removal of ethenocytosine

Metabolites of vinyl chloride react with cytosine in DNA to form 3,N(4)-ethenocytosine. Recent studies suggest that ethenocytosine is repaired by the base excision repair pathway with the ethenobase being removed by thymine-DNA glycosylase. Here single turnover kinetics have been used to compare the excision of ethenocytosine by thymine-DNA glycosylase with the excision of thymine. The effect of flanking DNA sequence on the excision of ethenocytosine was also investigated. The 34-bp duplexes studied here fall into three categories. Ethenocytosine base-paired with guanine within a CpG site (i.e. CpG.(epsilon)C-DNA) was by far the best substrate having a specificity constant (k(2)/K(d)) of 25.1 x 10(6) m(-1) s(-1). The next best substrates were DNA duplexes containing TpG.(epsilon)C, GpG.(epsilon)C, and CpG.T. These had specificity constants 45-130 times smaller than CpG.(epsilon)C-DNA. The worst substrates were DNA duplexes containing ApG.(epsilon)C and TpG.T, which had specificity constants, respectively, 1,600 and 7,400 times lower than CpG.(epsilon)C-DNA. DNA containing ethenocytosine was bound much more tightly than DNA containing a G.T mismatch. This is probably because thymine-DNA glycosylase can flip out ethenocytosine from a G.(epsilon)C base pair more easily than it can flip out thymine from a G.T mismatch. Because thymine-DNA glycosylase has a larger specificity constant for the removal of ethenocytosine, it has been suggested its primary purpose is to deal with ethenocytosine. However, these results showing that thymine-DNA glycosylase has a strong sequence preference for CpG sites in the excision of both thymine and ethenocytosine suggest that the main role of thymine-DNA glycosylase in vivo is the removal of thymine produced by deamination of 5-methylcytosine at CpG sites.

Xeroderma pigmentosum group C protein interacts physically and functionally with thymine DNA glycosylase

The XPC-HR23B complex recognizes various helix-distorting lesions in DNA and initiates global genome nucleotide excision repair. Here we describe a novel functional interaction between XPC-HR23B and thymine DNA glycosylase (TDG), which initiates base excision repair (BER) of G/T mismatches generated by spontaneous deamination of 5-methylcytosine. XPC-HR23B stimulated TDG activity by promoting the release of TDG from abasic sites that result from the excision of mismatched T bases. In the presence of AP endonuclease (APE), XPC-HR23B had an additive effect on the enzymatic turnover of TDG without significantly inhibiting the subsequent action of APE. Our observations suggest that XPC-HR23B may participate in BER of G/T mismatches, thereby contributing to the suppression of spontaneous mutations that may be one of the contributory factors for the promotion of carcinogenesis in xeroderma pigmentosum genetic complementation group C patients.

Enzymology of the repair of free radicals-induced DNA damage

A number of intrinsic and extrinsic mutagens induce structural damage in cellular DNA. These DNA damages are cytotoxic, miscoding or both and are believed to be at the origin of cell lethality, tissue degeneration, ageing and cancer. In order to counteract immediately the deleterious effects of such lesions, leading to genomic instability, cells have evolved a number of DNA repair mechanisms including the direct reversal of the lesion, sanitation of the dNTPs pools, mismatch repair and several DNA excision pathways including the base excision repair (BER) nucleotide excision repair (NER) and the nucleotide incision repair (NIR). These repair pathways are universally present in living cells and extremely well conserved. This review is focused on the repair of lesions induced by free radicals and ionising radiation. The BER pathway removes most of these DNA lesions, although recently it was shown that other pathways would also be efficient in the removal of oxidised bases. In the BER pathway the process is initiated by a DNA glycosylase excising the modified and mismatched base by hydrolysis of the glycosidic bond between the base and the deoxyribose of the DNA, generating a free base and an abasic site (AP-site) which in turn is repaired since it is cytotoxic and mutagenic.

Acetylation regulates the DNA end-trimming activity of DNA polymerase beta

We describe a novel regulatory mechanism for DNA polymerase beta (Polbeta), a protein involved in DNA base excision repair (BER). Polbeta colocalized in vivo and formed a complex with the transcriptional coactivator p300. p300 interacted with Polbeta through distinct domains and acetylated Polbeta in vitro. Polbeta acetylation was furthermore observed in vivo. Lysine 72 of Polbeta was identified as the main target for acetylation by p300. Interestingly, acetylated Polbeta showed a severely reduced ability to participate in a reconstituted BER assay. This was due to an impairment of the dRP-lyase activity of Polbeta. Acetylation of Polbeta thus acts as an intranuclear regulatory mechanism and implies that p300 plays a critical regulatory role in BER.

Estradiol receptor potentiates, in vitro, the activity of 5-methylcytosine DNA glycosylase

At a concentration of 5 x 10(-9) M of hemi-methylated DNA (one order of magnitude below the K(m)), MCF-7 (a human breast carcinoma cell line) nuclear extracts potentiate the activity of 5-methylcytosine DNA glycosylase (5-MCDG, alias G/T mismatch DNA glycosylase). Depending on the ratio between MCF-7 nuclear extracts and 5-MCDG, there is an up to 10-fold increase in 5-MCDG activity. The potentiation of 5-MCDG by MCF-7 nuclear extracts requires an estradiol response element adjacent to the hemi-methylated site. Depletion of the estradiol receptor from MCF-7 nuclear extracts with specific antibodies abolishes the potentiation of 5-MCDG activity. The estradiol receptor present in MCF-7 nuclear extracts can be precipitated with antibodies directed against 5-MCDG. Reciprocally, antibodies directed against the estradiol receptor precipitate 5-MCDG. The results indicate the formation of a complex between the estradiol receptor and 5-MCDG.

DEMETER, a DNA glycosylase domain protein, is required for endosperm gene imprinting and seed viability in arabidopsis

We isolated mutations in Arabidopsis to understand how the female gametophyte controls embryo and endosperm development. For the DEMETER (DME) gene, seed viability depends only on the maternal allele. DME encodes a large protein with DNA glycosylase and nuclear localization domains. DME is expressed primarily in the central cell of the female gametophyte, the progenitor of the endosperm. DME is required for maternal allele expression of the imprinted MEDEA (MEA) Polycomb gene in the central cell and endosperm. Ectopic DME expression in endosperm activates expression of the normally silenced paternal MEA allele. In leaf, ectopic DME expression induces MEA and nicks the MEA promoter. Thus, a DNA glycosylase activates maternal expression of an imprinted gene in the central cell.