Rika Kusumoto

The XPV (xeroderma pigmentosum variant) gene encodes human DNA polymerase eta.

Xeroderma pigmentosum variant (XP-V) is an inherited disorder which is associated with increased incidence of sunlight-induced skin cancers. Unlike other xeroderma pigmentosum cells (belonging to groups XP-A to XP-G), XP-V cells carry out normal nucleotide-excision repair processes but are defective in their replication of ultraviolet-damaged DNA,. It has been suspected for some time that the XPV gene encodes a protein that is involved in trans-lesion DNA synthesis, but the gene product has never been isolated. Using an improved cell-free assay for trans-lesion DNA synthesis, we have recently isolated a DNA polymerase from HeLa cells that continues replication on damaged DNA by bypassing ultraviolet-induced thymine dimers in XP-V cell extracts. Here we show that this polymerase is a human homologue of the yeast Rad30 protein, recently identified as DNA polymerase η (ref. 4). This polymerase and yeast Rad30 are members of a family of damage-bypass replication proteins which comprises the Escherichia coli proteins UmuC and DinB and the yeast Rev1 protein. We found that all XP-V cells examined carry mutations in their DNA polymerase η gene. Recombinant human DNA polymerase η corrects the inability of XP-V cell extracts to carry out DNA replication by bypassing thymine dimers on damaged DNA. Together, these results indicate that DNA polymerase η could be the XPV gene product.

Mechanisms of accurate translesion synthesis by human DNA polymerase η

The XPV (xeroderma pigmentosum variant) gene encodes human DNA polymerase η (pol η), which is involved in the replication of damaged DNA. Pol η catalyzes efficient and accurate translesion synthesis past cis‐syn cyclobutane di‐thymine lesions. Here we show that human pol η can catalyze translesion synthesis past an abasic (AP) site analog, N‐2‐acetylaminofluorene (AAF)‐modified guanine, and a cisplatin‐induced intrastrand cross‐link between two guanines. Pol η preferentially incorporated dAMP and dGMP opposite AP, and dCMP opposite AAF‐G and cisplatin‐GG, but other nucleotides were also incorporated opposite these lesions. However, after incorporating an incorrect nucleotide opposite a lesion, pol η could not continue chain elongation. In contrast, after incorporating the correct nucleotide opposite a lesion, pol η could continue chain elongation, whereas pol α could not. Thus, the fidelity of translesion synthesis by human pol η relies not only on the ability of this enzyme to incorporate the correct nucleotide opposite a lesion, but also on its ability to elongate only DNA chains that have a correctly incorporated nucleotide opposite a lesion.

Xeroderma pigmentosum variant (XP‐V) correcting protein from HeLa cells has a thymine dimer bypass DNA polymerase activity

Xeroderma pigmentosum variant (XP‐V) represents one of the most common forms of this cancer‐prone DNA repair syndrome. Unlike classical XP cells, XP‐V cells are normal in nucleotide excision repair but defective in post‐replication repair. The precise molecular defect in XP‐V is currently unknown, but it appears to be a protein involved in translesion synthesis. Here we established a sensitive assay system using an SV40 origin‐based plasmid to detect XP‐V complementation activity. Using this system, we isolated a protein from HeLa cells capable of complementing the defects in XP‐V cell extracts. The protein displays novel DNA polymerase activity which replicates cyclobutane pyrimidine dimer‐containing DNA templates. The XPV polymerase activity was dependent on MgCl2, sensitive to NEM, moderately sensitive to KCl, resistant to both aphidicolin and ddTTP, and not stimulated by PCNA. In glycerol density gradients, the activity co‐sedimented with a 54 kDa polypeptide at 3.5S, indicating that the monomeric form of this polypeptide was responsible for the activity. The protein factor corrected the translesion defects of extracts from three XPV cell strains. Bypass DNA synthesis by the XP‐V polymerase occurred only in the presence of dATP, indicating that it can incorporate only dATP to bypass a di‐thymine lesion.

Diversity of the damage recognition step in the global genomic nucleotide excision repair in vitro

The XPC-HR23B complex, a mammalian factor specifically involved in global genomic nucleotide excision repair (NER) has been shown to bind various forms of damaged DNA and initiate DNA repair in cell-free reactions. To characterize the binding specificity of this factor in more detail, a method based on immunoprecipitation was developed to assess the relative affinity of XPC-HR23B for defined lesions on DNA. Here we show that XPC-HR23B preferentially binds to UV-induced (6-4) photoproducts (6-4PPs) as well as to cholesterol, but not to the cyclobutane pyrimidine dimer (CPD), 8-oxoguanine (8-oxo-G), O6-methylguanine (O6-Me-G), or a single mismatch. Human whole cell extracts could efficiently excise 6-4PPs and cholesterol in an XPC-HR23B-dependent manner, but not 8-oxo-G, O6-Me-G or mismatches. Thus, there was good correlation between the binding specificity of XPC-HR23B for certain types of lesion and the ability of human cell extracts to excise these lesions, supporting the model that XPC-HR23B initiates global genomic NER. Although, XPC-HR23B does not preferentially bind to CPDs, the excision of CPDs in human whole cell extracts was found to be absolutely dependent on XPC-HR23B, in agreement with the in vivo observation that CPDs are not removed from the global genome in XP-C mutant cells. These results suggest that, in addition to the excision repair pathway initiated by XPC-HR23B, there exists another sub-pathway for the global genomic NER that still requires XPC-HR23B but is not initiated by XPC-HR23B. Possible mechanisms will be discussed.

DNA binding properties of human DNA polymerase η: implications for fidelity and polymerase switching of translesion synthesis

The human XPV (xeroderma pigmentosum variant) gene is responsible for the cancer–prone xeroderma pigmentosum syndrome and encodes DNA polymerase η (pol η), which catalyses efficient translesion synthesis past cis‐syn cyclobutane thymine dimers (TT dimers) and other lesions. The fidelity of DNA synthesis by pol η on undamaged templates is extremely low, suggesting that pol η activity must be restricted to damaged sites on DNA. Little is known, however, about how the activity of pol η is targeted and restricted to damaged DNA. Here we show that pol η binds template/primer DNAs regardless of the presence of TT dimers. Rather, enhanced binding to template/primer DNAs containing TT dimers is only observed when the 3′‐end of the primer is an adenosine residue situated opposite the lesion. When two nucleotides have been incorporated into the primer beyond the TT dimer position, the pol η‐template/primer DNA complex is destabilized, allowing DNA synthesis by DNA polymerases α or δ to resume. Our study provides mechanistic explanations for polymerase switching at TT dimer sites.

Werner Protein Cooperates with the XRCC4-DNA Ligase IV Complex in End-Processing†

Werner syndrome is a rare human disease characterized by the premature onset of aging-associated pathologies, cancer predisposition, and genomic instability. The Werner protein (WRN), which is defective in Werner syndrome ( WS) patients, belongs to the RecQ family helicases and interacts with several DNA metabolic proteins, including DNA repair factors and telomere associated proteins. Nonhomologous end-joining (NHEJ) is an important pathway in the repair of DNA double strand breaks (DSBs), and the DNA-PK complex, composed of the heterodimer Ku 70/86 and the DNA-PK catalytic subunit (DNA-PKcs), together with the XRCC4-DNA ligase IV complex (X4L4), are major factors. One of the most prominent protein interactions of WRN is with Ku 70/86, and it is possible that WRN is involved in NHEJ via its associations with Ku 70/86 and DNA-PKcs. This study demonstrates that WRN physically interacts with the major NHEJ factor, X4L4, which stimulates WRN exonuclease but not its helicase activity. The human RecQ helicase, BLM, which possesses only helicase activity, does not bind to X4L4, and its helicase activity is not affected by X4L4. In a DNA end-joining assay, we find that a substrate, which is processed by WRN, is ligated by X4L4, thus further supporting the significance of their functional interaction.

Reconstitution of damage DNA excision reaction from SV40 minichromosomes with purified nucleotide excision repair proteins

We previously constructed the cell-free nucleotide excision repair (NER) assay system with UV-irradiated SV40 minichromosomes to analyze the mechanism of NER reaction on chromatin DNA. Here we investigate the factor that acts especially on nucleosomal DNA during the damage excision reaction, and reconstitute the damage excision reaction on SV40 minichromosomes. NER-proficient HeLa whole cell extracts were fractionated, and the amounts of known NER factors involved in the column fractions were determined by immunoblot analyses. The column fractions were quantitatively and systematically replaced by highly purified NER factors. Finally, damage DNA excision reaction on SV40 minichromosomes was reconstituted with six highly purified NER factors, XPA, XPC-HR23B, XPF-ERCC1, XPG, RPA and TFIIH, as those essential for the reaction with naked DNA. Further analysis showed that the damages on chromosomal DNA were excised as the same efficiency as those on naked DNA for short incubation. At longer incubation time, however, the damage excision efficiency on nucleosomal DNA was decreased whereas naked DNA was still vigorously repaired. These observations suggest that although the six purified NER factors have a potential to eliminate the damage DNA from SV40 minichromosomes, the chromatin structure may still have some repressive effects on NER.

Identification and characterization of an intermediate in the alkali degradation of (6-4) photoproduct-containing DNA.

The (6-4) photoproduct formed by ultraviolet light is known as an alkali-labile DNA lesion. Strand breaks occur at (6-4) photoproducts when UV-irradiated DNA is treated with hot alkali. We have analyzed the degradation reaction of this photoproduct under alkaline conditions using synthetic oligonucleotides. A tetramer, d(GT(6-4)TC), was prepared, and its degradation in 50 mm KOH at 60 °C was monitored by high performance liquid chromatography. A single peak with a UV absorption spectrum similar to that of the starting material was detected after the reaction, and this compound was regarded as an intermediate before the strand break. The formation of this intermediate was compared with intermediates from the degradation of other alkali-labile lesions such as the abasic site, thymine glycol, and 5,6-dihydrothymine. The results strongly suggested that the first step of the alkali degradation of the (6-4) photoproduct was the hydrolysis between the N3 and C4 positions of the 5′-pyrimidine component. Analyses by NMR spectroscopy and mass spectrometry supported the chemical structure of this product. Assays of the complex formation with XPC·HR23B and the translesion synthesis by DNA polymerase η revealed that the biochemical properties are indistinguishable between the intact and hydrolyzed photoproducts.