Regulation of DNA Pigmentosum Cells Repair in Serum-stimulated Xeroderma

The regulation of DNA repair during serum stimulation of quiescent cells was examined in normal human cells, in fibroblasts from three xeroderma pigmentosum complementation groups (A, C, and D), in xeroderma pigmentosum variant cells, and in ataxia telangiectasia cells. The regulation of nucleotide excision repair was examined by exposing cells to ultraviolet irradiation at discrete intervals after cell stimulation. Similarly, base excision repair was quantitated after exposure to methylmethane sulfonate. Wl-38 normal human diploid fibroblasts, xeroderma pigmentosum variant cells, as well as ataxia telangiectasia cells enhanced their capacity for both nucleotide excision repair and for base excision repair prior to their enhancement of DNA synthesis. Further, in each cell strain, the base excision repair enzyme uracil DNA glycosylase was increased prior to the induction of DNA polymerase using the identical cells to quantitate each activity. In contrast, each of the three xeroderma complementation groups that were examined failed to increase their capacity for nucleotide excision repair above basal levels at any interval examined. This result was observed using either unscheduled DNA synthesis in the presence of 10 mM hydroxyurea or using repair replication in the absence of hydroxyurea to quantitate DNA repair. However, each of the three complementation groups normally regulated the enhancement of base excision repair after methylmethane sulfonate exposure and each induced the uracil DNA glycosylase prior to DNA synthesis. These results suggest that there may be a relationship between the sensitivity of xeroderma pigmentosum cells from each complementation group to specific DNA damaging agents and their inability to regulate nucleotide excision repair during cell stimulation. Eucaryotic DNA repair pathways function to conserve the genetic information encoded in nucleotide sequences in cellular DNA. Two major excision repair pathways have been characterized in human cells. In nucleotide excision repair, which is responsible for the removal of pyrimidine dimers and other bulky lesions from DNA, the initial enzymatic action is endonucleolytic (29). Subsequently, the modified nucleotides are removed within an oligonucleotide fragment. In base excision repair, the initial enzymatic action is that of a DNA glycosylase (41). The glycosylase cleaves the base sugar glycosyl linkage removing the modified base and leaving an apurinic or apyrimidinic site in DNA. Recent studies suggest that mammalian cells actively regulate each DNA repair pathway during the defined pattern of gene expression observed during cell proliferation (4, 46). As compared with basal levels in quiescent cells, proliferating cells (a) have a greater capacity for DNA repair synthesis after exposure to ultraviolet irradiation (25, 26, 28, 40, 47, 56), ionizing radiation (39), or alkylating agents (25, 26, 28, 56); (b) have higher specific activities of the DNA repair enzymes uracil DNA glycosylase (1, 10, 19, 23, 58), 3-methyladenine DNA glycosylase (15, 23), and the O6-methylguanine methyltransferase in the absence of cellular insult (49, 50) or during toxicityinduced cellular hyperplasia (48); and (c) increase the rates of excision of DNA adducts as a function of their proliferative state (20, 22, 34). We have begun to examine whether normal human cells regulate nucleotide excision repair and base excision repair as they are stimulated to proliferate. In these studies two basic approaches were used. First, we examined the regulation of specific DNA repair enzymes relative to that induction observed for DNA polymerase during cell stimulation. In these THE JOURNAL OF CELL BIOLOGY • VOLUME 99 OCTOBER 1984 1275-1281 © The Rockefeller University Press • 0021-9525184110/1275•07 $1.0