Xeroderma pigmentosum patients from Germany: repair capacity of 45 XP fibroblast strains of the Mannheim XP Collection as measured by colony-forming ability and unscheduled DNA synthesis following treatment with methyl methanesulfonate and N-methyl-N-nitrosourea

In vivo repair of methylation damage in Aag 3-methyladenine DNA glycosylase null mouse cells

3-Methyladenine (3MeA) DNA glycosylases initiate base excision repair by removing 3MeA. These glycosylases also remove a broad spectrum of spontaneous and environmentally induced base lesions in vitro. Mouse cells lacking the Aag 3MeA DNA glycosylase (also known as the Mpg, APNG or ANPG DNA glycosylase) are susceptible to 3MeA-induced S phase arrest, chromosome aberrations and apoptosis, but it is not known if Aag is solely responsible for repair of 3MeA in vivo. Here we show that in AAG:(-/-) cells, 3MeA lesions disappear from the genome slightly faster than would be expected by spontaneous depurination alone, suggesting that there may be residual repair of 3MeA. However, repair of 3MeA is at least 10 times slower in AAG:(-/-) cells than in AAG:(+/+) cells. Consequently, 24 h after exposure to [(3)H]MNU, 30% of the original 3MeA burden is intact in AAG:(-/-) cells, while 3MeA is undetectable in AAG:(+/+) cells. Thus, Aag is the major DNA glycosylase for 3MeA repair. We also investigated the in vivo repair kinetics of another Aag substrate, 7-methylguanine. Surprisingly, 7-methylguanine is removed equally efficiently in AAG:(+/+) and AAG:(-/-) cells, suggesting that another DNA glycosylase acts on lesions previously thought to be repaired by Aag.

Contribution of base excision repair, nucleotide excision repair, and DNA recombination to alkylation resistance of the fission yeast Schizosaccharomyces pombe

DNA damage is unavoidable, and organisms across the evolutionary spectrum possess DNA repair pathways that are critical for cell viability and genomic stability. To understand the role of base excision repair (BER) in protecting eukaryotic cells against alkylating agents, we generated Schizosaccharomyces pombe strains mutant for the mag1 3-methyladenine DNA glycosylase gene. We report that S. pombe mag1 mutants have only a slightly increased sensitivity to methylation damage, suggesting that Mag1-initiated BER plays a surprisingly minor role in alkylation resistance in this organism. We go on to show that other DNA repair pathways play a larger role than BER in alkylation resistance. Mutations in genes involved in nucleotide excision repair (rad13) and recombinational repair (rhp51) are much more alkylation sensitive than mag1 mutants. In addition, S. pombe mutant for the flap endonuclease rad2 gene, whose precise function in DNA repair is unclear, were also more alkylation sensitive than mag1 mutants. Further, mag1 and rad13 interact synergistically for alkylation resistance, and mag1 and rhp51 display a surprisingly complex genetic interaction. A model for the role of BER in the generation of alkylation-induced DNA strand breaks in S. pombe is discussed.

DNA repair synthesis following irradiation with 254-nm and 312-nm ultraviolet light is not diminished in fibroblasts from patients with dysplastic nevus syndrome

The DNA excision repair capacity of 23 primary fibroblast lines from patients with dysplastic nevus syndrome was investigated and DNA repair synthesis ("unscheduled DNA synthesis") was determined after UV exposure. Seventeen fibroblast lines from normal donors served as controls. The dose/response experiments included up to ten dose levels and two wavelength ranges: UV-C (using a low-pressure mercury lamp emitting predominantly 254-nm light) and UV-B (artificial "sunlamp" radiation centering around 312-nm light). For each dose level, silver grains over fibroblast nuclei were counted by visual inspection. Twelve cell lines were also evaluated for both UV wavelength ranges using a new semi-automatic image analyzing system. This system included components for rapid sequential identification of both fibroblast nuclei and silver grains sited above them. Silver grains over 100 nuclei were determined for each UV dose level. Dose/response curves were established and analyzed by linear regression. As a quantitative term for assessing DNA excision repair capacity of a cell line we calculated the linear increase (G0) in the number of grains per nucleus, when the UV dose was multiplied by the factor e (i.e. 2.72). The sensitivity of grain detection and resolution of overlapping grains was approximately threefold better in visual than in automatic counting, especially when there were more than 70 grains over nuclei. The time required for visual counting, however, was tenfold that of automatic counting. The variance-weighted mean G0v.w of all fibroblast lines from patients with dysplastic nevus syndrome was found to be 79.1 (+/- 1.8- grains/nucleus, that of fibroblast lines from normal donors was 74.2 (+/- 1.7) grains/nucleus. This difference revealed a slightly better repair capability for cell lines from patients but was at the borderline of detection and, therefore, should not be overinterpreted. From the experimental accuracy achieved by determination of the variance-weighted means of the two groups, we would have been able to detect a difference of 7 and more grains [> 2 x (sigma normal+sigma patients)]. The variance-weighted mean G0v.w of all fibroblast lines from patients with dysplastic nevus syndrome was found to be 76.4 (+/- 1.4) grains/nucleus, whereas that of fibroblast lines from normal donors was only 66.6 (+/- 1.8) grains/nucleus. This difference was statistically significant and, contrary to expectation, revealed better, not worse post-UV DNA repair capability in cell lines from patients that in those from normal donors.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced expression of mitochondrial genes in xeroderma pigmentosum fibroblast strains from various complementation groups

cDNA libraries constructed from cytoplasmic RNA of normal and xeroderma pigmentosum (XP) fibroblast strains were screened for differential gene expression. XP fibroblast strains included one representative of the complementation groups A, C, D, and one XP variant strain. The XP lambda gt10 cDNA libraries were differentially screened with in vitro transcripts made from cDNA in the pBluescript vector using both the same XP strain and the normal fibroblast strain. Eight differential clones were detected in the libraries of the XP group A, D, and C strains, which caused stronger signals when probed with transcripts from XP strains than with those from the normal strain. The cDNA clones were sequenced. Seven of the eight clones detected coded for three mitochondrial genes: subunit I of cytochrome c oxidase (complex IV of the respiratory chain), apocytochrome b (subunit of complex III), and 16-S rRNA. Two clones representing essentially (a) subunit I of cytochrome c oxidase and (b) 16-S rRNA diverged from the sequence of the human mitochondrial genome present in the data-base libraries. Clone a exhibited a transition mutation, clone b reflected a transcript of a mitochondrial genome rearranged in the 16-S rRNA gene, including four nucleotides of the adjacent tRNA(Leu) gene. The apparently enhanced expression of mitochondrial genes in XP cells, together with the changes in DNA sequence, seem to indicate that functions of the ATP-generating system were impaired. This defect may have originated from mutations due to lack of DNA repair. The data can be interpreted in the light of mitochondrial changes that cause human neuromyopathies to occur. In analogy to these diseases the neurological symptoms in XP might be explained by abnormal mitochondria.

The XPD complementation group. Insights into xeroderma pigmentosum, Cockayne's syndrome and trichothiodystrophy

The xeroderma pigmentosum complementation group D is defined by more than 30 unrelated individuals of whom less than half show major abnormalities of the central nervous system, once considered to be the hallmark of the group. Fibroblasts from the great majority of these individuals show very considerable sensitivity to UV light in vitro despite the fact that the cells carry out what appears to be substantial excision repair, as judged from repair synthesis and incision activity. This article reviews the XPD group and the defects in cellular DNA repair and examines the lack of correlation between repair and the appearance of neurological abnormalities. The article also discusses the recent awareness that at least some members of two other inherited conditions, trichothiodystrophy and Cockayne's Syndrome, carry mutations in the XPD gene.

DNA repair synthesis in fibroblast strains from patients with actinic keratosis, squamous cell carcinoma, basal cell carcinoma, or malignant melanoma after treatment with ultraviolet light, N-acetoxy-2-acetyl-aminofluorene, methyl methanesulfonate, and N-methyl-N-nitrosourea

Fibroblast strains derived from skin biopsies of patients with actinic keratosis (6), malignant melanoma (18), squamous cell carcinoma (11), and basal cell carcinoma (12) were investigated for DNA repair synthesis, with 16 fibroblast strains for normal donors as controls. Cells were exposed to UV light, the "UV-like" carcinogen (Ac)2ONFln, and the methylating carcinogens MeSO2OMe and MeNOUr. Dose-response experiments, which included 10 dose levels, were performed, the data analyzed by linear regression, and the slope of the regression line (term: G0) used as a measure of DNA repair synthesis. The mean experimental variability of G0 of individual fibroblast strains was 9.5%-15.4%, depending upon exposure. For comparison of all cell strains belonging to the same skin malignancy group with those of the control group, G0 values of the individual strains were combined to yield group-specific weighted mean G0 values. In addition, the capacity to incise UV-damaged DNA was measured in 24 cell strains from patients with skin tumors using the alkaline elution technique. For quantitating DNA-incising capacity, the initial velocities of the elution curves were plotted versus the UV dose, and the slope of the resulting regression line was used to obtain the characteristic value E0. The mean experimental variability of E0 of individual strains was +/- 22%. These E0 values were combined to yield weighted mean values of groups. The fibroblast strains in the groups of patients with actinic keratosis and malignant melanoma were found to have normal mean G0 values when DNA repair synthesis was challenged with UV light or one of the three carcinogens. However, the squamous cell carcinoma group exhibited significantly lower mean G0 values after treatment with UV light (82% that of normal donors), (Ac)2ONFln (70%), MeSO2OMe (70%), and MeNOUr (69%). The basal cell carcinoma group showed significantly diminished repair synthesis upon treatment with UV light (81% that of normal donors) and MeSO2OMe (67%). In contrast to these findings, in no skin malignancy group was post UV DNA-incising capacity (E0) significantly diminished, although it should be noted that group sizes were only half as large as for G0 determinations. These data may be interpreted as indicating that DNA excision repair is impaired in fibroblast strains from patients with squamous cell carcinoma and-to a lesser extent-basal cell carcinoma.(ABSTRACT TRUNCATED AT 400 WORDS)

6-Methylguanine and 6-methylguanosine inhibit colony-forming ability in a malignant xeroderma pigmentosum cell line but not in other xeroderma pigmentosum and normal human fibroblast strains after treatment with 1-(2-chloroethyl)-1-nitroso-3-(2-hydroxyethyl)-urea

The XP cell strain XP29MA, its malignant counterpart XP29MAmal and a normal human fibroblast strain were tested for colony-forming ability after treatment with HECNU in the presence of m6G, m6Gua, and he7G. In XP29MAmal, inhibition of post-HECNU colony-forming ability was 35% when 0.25 mM of either m6G or m6Gua were present, whereas in XP29MA and the normal fibroblast strain no inhibition was detected. The he7G caused a similar but smaller inhibitory effect in XP29MAmal, but failed to do so in XP29MA. HECNU predominantly exerts its killing effect by alkylating O-6 of DNA-bound guanine and causing DNA interstrand crosslinks. Alkylation of O-6 of guanine can be repaired by 6-methylguanine-DNA methyltransferase. From our experiments we conclude that in XP29MAmal this methyltransferase was inhibited in the presence of the 6-alkylguanines, thus leaving more 2-chloroethylated sites in DNA unrepaired. This results in sensitization in terms of decreased colony-forming ability observed only in the malignant cell line.