Differential regulation of base and nucleotide excision repair in mammalian cells

In this study, repair mechanisms have been deduced through the use of a sensitive and convenient technique to measure single-strand breaks (SSB) in cellular DNA, named Alkaline DNA Unwinding (ADU). The level of SSB has been followed in human fibroblasts of normal xeroderma pigmentosum origin after exposure to 8 different genotoxic agents. These agents fall into two distinct groups. The first group is represented by uv light and uv-mimetic agents that will introduce bulky DNA adducts, for which xp-cells have a decreased repair capacity and survival. The second group contains simple alkylating agents that will introduce methyl and ethyl-groups on DNA for which xp-cells have more or less normal repair and survival. Lesions produced by the first group are believed to be recognized by direct acting endonucleases by virtue of the local distortion of the DNA helix. Lesions formed by the second group of agents are recognized by a different set of enzymes, glycosylases, that cleave off modified bases, e.g., 3-methyladenine (3-MeAd) and 7-methylguanine (7-MeGu), the major adducts after treatment with dimethylsulphate or methyl methansulphonate. The resulting apurinic sites (AP-sites) are then cut by AP-endonucleases, giving rise to SSBs. This repair pathway, sometimes called base excision repair or short patch repair, has been claimed to produce primarily short repair patches, 1 to 3 inserted deoxynucleotides. Repair of uv-mimetic lesions by nucleotide excision results in patches 25 to 100 deoxynucleotides long, hence the name long patch repair. 25 refs., 8 figs., 1 tab