DNA repair and mutagenesis in mammalian cells.

— Mutations may result from imperfect DNA replication, unrepaired DNA damage, and errorprone DNA repair. DNA damage may induce any or all of these effects. Numerous physical and chemical agents damage DNA, and the repair of such damage may be either incomplete, error-free, or error-prone. It is assumed that correctly repaired DNA damage has no deleterious biological consequence. Unrepaired or misrepaired DNA damage has been related to such physiological changes as cell death, division suppression, gene repression and derepression, altered transcription, elevated cAMP levels, predisposition of the cell to viral transformation, decreased cellular respiration, and mutation. Defective DNA repair has been linked in man to cancer, birth defects, arteriosclerosis, high blood pressure, aging, and neurological dysfunction. Various clinical syndromes including xeroderma pigmentosum (XP), ataxia telangiectasia (AT), Fanconi's anemia (FA), progeria, actinic keratosis (AK), and possibly Cockayne syndrome (CS) have been described as being defective in at least one of the various forms of DNA repair. There are at least four general types of enzymatic repair systems in mammalian cells (excision, strand break, postreplication, and photoreactivation). Each of these, except the latter, is composed of multiple pathways. The two most intensely studied of these repair systems are excision and postreplication repair. Excision repair presumably involves the error-free removal of damaged DNA by a complex of enzymes. The damaged segment is removed and replaced with newly synthesized DNA using the opposite strand as the template. A measure of excision repair may be made by assaying: (1) removal of known products; (2) production of single-strand breaks following treatment of cellular DNA with damage-specific endonuclease preparations; or (3) measurement of repair synthesis. Since (XP) individuals who are defective in the excision of ultraviolet (UV) light induced lesions are also defective in the repair of certain forms of chemically-induced DNA damage, and since the size of repaired regions for both acetoxy-acetylaminofluorene (AAAF) and UV-induced DNA damage were believed to be similar, it was assumed that similar mechanisms for the repair of both physically and chemically-induced DNA damage existed. Recent evidence, however, suggests that significant differences may exist in the repair of DNA damage induced by agents producing similar patch sizes. Excision repair capacity also appears to vary as a function of species, organ, state of differentiation, and pattern of chromatin association with DNA. Although there appear to be several enzymes specific for the recognition of specific forms of DNA damage, once recognition (nicking) has occurred, the subsequent sequence of events for most types of excision repair are presumed to be similar. Postreplication repair, which presumably may be either error-free or error-prone, is a process in which DNA, newly-synthesized from a defective or damaged template, is repaired. Postreplication repair is usually studied by observing that the newly-synthesized DNA from damaged templates contains gaps that disappear with time. In human cells, the number of gaps approximately equals the number of lesions. This system is important in actively dividing cells and cells that have been induced to divide. XP “variant” cells, which are normal for excision repair, have been shown to be defective in postreplication repair. However, in cell cultures that have been treated with either physical or chemical mutagens and DNA replication blocked following such treatment, the mutation frequency per dose decreases as a function of the duration of mitotic blockage. These data thus suggest a role in mammalian cell mutagenesis for an error-prone postreplication repair system as well as misinsertion during DNA replication. This paper will discuss various mammalian cell DNA repair processes, the role of specific forms of DNA damage in cellular mutagenesis, and mammalian repair defective mutant cell lines. An attempt will be made to correlate these items plus what is known in bacterial mutagenesis to potential mechanisms for mutagenesis in mammalian cells.