7,8-Dihydro-8-oxoadenine, a highly mutagenic adduct, is repaired by Escherichia coli and human mismatch-specific uracil/thymine-DNA glycosylases.

Hydroxyl radicals predominantly react with the C(8) of purines forming 7,8-dihydro-8-oxoguanine (8oxoG) and 7,8-dihydro-8-oxoadenine (8oxoA) adducts, which are highly mutagenic in mammalian cells. The majority of oxidized DNA bases are removed by DNA glycosylases in the base excision repair pathway. Here, we report for the first time that human thymine-DNA glycosylase (hTDG) and Escherichia coli mismatch-specific uracil-DNA glycosylase (MUG) can remove 8oxoA from 8oxoA•T, 8oxoA•G and 8oxoA•C pairs. Comparison of the kinetic parameters of the reaction indicates that full-length hTDG excises 8oxoA, 3,N(4)-ethenocytosine (εC) and T with similar efficiency (k(max) = 0.35, 0.36 and 0.16 min(-1), respectively) and is more proficient as compared with its bacterial homologue MUG. The N-terminal domain of the hTDG protein is essential for 8oxoA-DNA glycosylase activity, but not for εC repair. Interestingly, the TDG status had little or no effect on the proliferation rate of mouse embryonic fibroblasts after exposure to γ-irradiation. Nevertheless, using whole cell-free extracts from the DNA glycosylase-deficient murine embryonic fibroblasts and E. coli, we demonstrate that the excision of 8oxoA from 8oxoA•T and 8oxoA•G has an absolute requirement for TDG and MUG, respectively. The data establish that MUG and TDG can counteract the genotoxic effects of 8oxoA residues in vivo.

A novel missense mutation in the high mobility group domain of SRY drastically reduces its DNA-binding capacity and causes paternally transmitted 46,XY complete gonadal dysgenesis.

OBJECTIVE: To investigate the familial segregation, role, and function of a novel SRY missense mutation c.347T>C in two half-sisters affected by 46,XY complete gonadal dysgenesis (CDG) compatible with a successful pregnancy outcome. DESIGN: Phenotypic, mutational, and functional study. SETTING: Academic research unit. PATIENT(S): Two half-sisters, their common father, and 100 healthy control individuals. INTERVENTION(S): Chromosome, molecular cytogenetic analysis, and Sanger sequencing of the SRY gene in blood lymphocytes of the proband, her affected half-sister, and in inflammatory tissue of the father postmortem. Cloning and expression of high mobility group box carboxy-terminal domains of Sry and electrophoretic mobility shift assay were performed. MAIN OUTCOME MEASURE(S): Not applicable. RESULT(S): A novel SRY missense mutation c.347T>C (p.Leu116Ser) was identified in two half-sisters and segregates with the CGD phenotype. It is present in the common healthy father in a mosaic state. Functional analyses demonstrate the pathogenic effect of the mutation by a strong reduction of DNA affinity for the mutant p.Leu116Ser SRY protein. CONCLUSION(S): The missense mutation c.347T>C in the high mobility group domain of SRY causes 46,XY CGD. Paternal gonadal mosaicism is likely to explain the familial occurrence of 46,XY CGD suggesting a de novo mutational event during the early stages of embryonic development. This novel mutation is compatible with a successful pregnancy outcome.

Embryonic lethal phenotype reveals a function of TDG in maintaining epigenetic stability.

Thymine DNA glycosylase (TDG) is a member of the uracil DNA glycosylase (UDG) superfamily of DNA repair enzymes. Owing to its ability to excise thymine when mispaired with guanine, it was proposed to act against the mutability of 5-methylcytosine (5-mC) deamination in mammalian DNA. However, TDG was also found to interact with transcription factors, histone acetyltransferases and de novo DNA methyltransferases, and it has been associated with DNA demethylation in gene promoters following activation of transcription, altogether implicating an engagement in gene regulation rather than DNA repair. Here we use a mouse genetic approach to determine the biological function of this multifaceted DNA repair enzyme. We find that, unlike other DNA glycosylases, TDG is essential for embryonic development, and that this phenotype is associated with epigenetic aberrations affecting the expression of developmental genes. Fibroblasts derived from Tdg null embryos (mouse embryonic fibroblasts, MEFs) show impaired gene regulation, coincident with imbalanced histone modification and CpG methylation at promoters of affected genes. TDG associates with the promoters of such genes both in fibroblasts and in embryonic stem cells (ESCs), but epigenetic aberrations only appear upon cell lineage commitment. We show that TDG contributes to the maintenance of active and bivalent chromatin throughout cell differentiation, facilitating a proper assembly of chromatin-modifying complexes and initiating base excision repair to counter aberrant de novo methylation. We thus conclude that TDG-dependent DNA repair has evolved to provide epigenetic stability in lineage committed cells.

Base excision by thymine DNA glycosylase mediates DNA-directed cytotoxicity of 5-fluorouracil.

5-Fluorouracil (5-FU), a chemotherapeutic drug commonly used in cancer treatment, imbalances nucleotide pools, thereby favoring misincorporation of uracil and 5-FU into genomic DNA. The processing of these bases by DNA repair activities was proposed to cause DNA-directed cytotoxicity, but the underlying mechanisms have not been resolved. In this study, we investigated a possible role of thymine DNA glycosylase (TDG), one of four mammalian uracil DNA glycosylases (UDGs), in the cellular response to 5-FU. Using genetic and biochemical tools, we found that inactivation of TDG significantly increases resistance of both mouse and human cancer cells towards 5-FU. We show that excision of DNA-incorporated 5-FU by TDG generates persistent DNA strand breaks, delays S-phase progression, and activates DNA damage signaling, and that the repair of 5-FU-induced DNA strand breaks is more efficient in the absence of TDG. Hence, excision of 5-FU by TDG, but not by other UDGs (UNG2 and SMUG1), prevents efficient downstream processing of the repair intermediate, thereby mediating DNA-directed cytotoxicity. The status of TDG expression in a cancer is therefore likely to determine its response to 5-FU-based chemotherapy.

Cell cycle regulation as a mechanism for functional separation of the apparently redundant uracil DNA glycosylases TDG and UNG2.

Human Thymine-DNA Glycosylase (TDG) is a member of the uracil DNA glycosylase (UDG) superfamily. It excises uracil, thymine and a number of chemical base lesions when mispaired with guanine in double-stranded DNA. These activities are not unique to TDG; at least three additional proteins with similar enzymatic properties are present in mammalian cells. The successful co-evolution of these enzymes implies the existence of non-redundant biological functions that must be coordinated. Here, we report cell cycle regulation as a mechanism for the functional separation of apparently redundant DNA glycosylases. We show that cells entering S-phase eliminate TDG through the ubiquitin-proteasome system and then maintain a TDG-free condition until G2. Incomplete degradation of ectopically expressed TDG impedes S-phase progression and cell proliferation. The mode of cell cycle regulation of TDG is strictly inverse to that of UNG2, which peaks in and throughout S-phase and then declines to undetectable levels until it appears again just before the next S-phase. Thus, TDG- and UNG2-dependent base excision repair alternates throughout the cell cycle, and the ubiquitin-proteasome pathway constitutes the underlying regulatory system.

The enigmatic thymine DNA glycosylase.

When it was first isolated from extracts of HeLa cells in Josef Jiricny's laboratory, the thymine DNA glycosylase (TDG) attracted attention because of its ability to remove thymine, i.e. a normal DNA base, from G.T mispairs. This implicated a function of DNA base excision repair in the restoration of G.C base pairs following the deamination of a 5-methylcytosine. TDG turned out to be the founding member of a newly emerging family of mismatch-directed uracil-DNA glycosylases, the MUG proteins, that act on a comparably broad spectrum of base lesion including G.U as the common, most efficiently processed substrate. However, because of its apparent catalytic inefficiency, some have considered TDG a poor DNA repair enzyme without an important biological function. Others have reported 5-meC DNA glycosylase activity to be associated with TDG, thrusting the enzyme into limelight as a possible DNA demethylase. Yet others have found the glycosylase to interact with transcription factors, implicating a function in gene regulation, which appears to be critically important in developmental processes. This article reviews all these developments in view of possible biological functions of this multifaceted DNA glycosylase.

Meiotic recombination: sealing the partnership at the junction.

Crossovers ensure proper chromosome segregation in meiosis. A heterodimer of MutS proteins, hMSH4-hMSH5, has recently been found to interact with recombination intermediates in a manner that suggests a mechanism for directing meiotic DNA double strand break repair towards a crossover pathway.

Repair of damaged and mismatched DNA by the XPC homologues Rhp41 and Rhp42 of fission yeast.

Rhp41 and Rhp42 of Schizosaccharomyces pombe are homologues of human XPC, which is involved in nucleotide excision repair (NER) of damaged DNA. Inactivation of rhp41 caused moderate sensitivity to ultraviolet (UV) radiation. In addition, an increase of mitotic mutation rates was observed in the rhp41 mutant, which was dependent on active translesion polymerase Z. UV sensitivity and mutation rates were not different between rhp42 and wild type, but compared to rhp41 were further increased in rhp41 rhp42 cells. Transcription of the fbp1 gene (induced in vegetative cells) and of the SPBC1289.14 gene (induced during meiosis) was strongly blocked by UV-induced damages in the rhp41 mutant, but not, or only slightly, reduced in rhp42 background. NER-dependent short-patch repair of mismatches formed during meiosis was slightly affected in rhp41, moderately affected in rhp42, and absent in rhp41 rhp42. Epistasis analysis with rhp7 and rhp26 indicates that Rhp41 and Rhp42 are both involved in the global genome and transcription-coupled repair subpathways of NER. Rhp41 plays a major role in damage repair and Rhp42 in mismatch repair.

Mutagenesis of the HMGB (high-mobility group B) protein Cmb1 (cytosine-mismatch binding 1) of Schizosaccharomyces pombe: effects on recognition of DNA mismatches and damage.

Cmb1 (cytosine-mismatch binding 1) is a high-mobility group (HMG) protein of Schizosaccharomyces pombe, which consists of 223 amino acids and has a single HMG domain at the C-terminal end. We have created several mutant and deletion forms of the Cmb1 protein and studied the effects on general DNA binding and specific binding to DNA mismatches and damaged DNA. Cmb1Delta41 (i.e. Cmb1 from which the 41 N-terminal amino acids have been deleted) bound specifically to cytosine-containing mismatches, to the cisplatin-induced intrastrand cross-links cis -GG and cis -AG and to an O (6)-methylguanine lesion. DNA binding was not affected when the 45 N-terminal amino acids were deleted, but was abolished in the absence of the 50 N-terminal amino acids, and was reduced when Cmb1 was truncated by between five and eleven C-terminal amino acids. Cmb1, both with and without the C-terminal truncations, retained its DNA binding affinity after heating at 95 degrees C. The cmb1 gene was induced when S. pombe cells were treated with cisplatin. Mitotic mutation rates were increased in a S. pombe cmb1 null mutant and in a cmb1-(1-212) mutant, which encodes a Cmb1 protein lacking the 11 C-terminal amino acids. We conclude that mutation avoidance by Cmb1 is distinct from Msh2-dependent mismatch repair, but related to nucleotide excision repair.

DNA mismatch repair and mutation avoidance pathways.

Unpaired and mispaired bases in DNA can arise by replication errors, spontaneous or induced base modifications, and during recombination. The major pathway for correction of mismatches arising during replication is the MutHLS pathway of Escherichia coli and related pathways in other organisms. MutS initiates repair by binding to the mismatch, and activates together with MutL the MutH endonuclease, which incises at hemimethylated dam sites and thereby mediates strand discrimination. Multiple MutS and MutL homologues exist in eukaryotes, which play different roles in the mismatch repair (MMR) pathway or in recombination. No MutH homologues have been identified in eukaryotes, suggesting that strand discrimination is different to E. coli. Repair can be initiated by the heterodimers MSH2-MSH6 (MutSalpha) and MSH2-MSH3 (MutSbeta). Interestingly, MSH3 (and thus MutSbeta) is missing in some genomes, as for example in Drosophila, or is present as in Schizosaccharomyces pombe but appears to play no role in MMR. MLH1-PMS1 (MutLalpha) is the major MutL homologous heterodimer. Again some, but not all, eukaryotes have additional MutL homologues, which all form a heterodimer with MLH1 and which play a minor role in MMR. Additional factors with a possible function in eukaryotic MMR are PCNA, EXO1, and the DNA polymerases delta and epsilon. MMR-independent pathways or factors that can process some types of mismatches in DNA are nucleotide-excision repair (NER), some base excision repair (BER) glycosylases, and the flap endonuclease FEN-1. A pathway has been identified in Saccharomyces cerevisiae and human that corrects loops with about 16 to several hundreds of unpaired nucleotides. Such large loops cannot be processed by MMR.