INTRODUCTION .............................................................. 869 Enzymatic Repair of UV Damage in E. coli ......... .......................... 869 Error-Proof and Error-Prone Pathways of DNA Repair ....... ................. 871 The \"SOS\" Hypothesis: the Regulatory Role of DNA Damage ...... ............ 874 EVIDENCE FOR THE INDUCIBILITY OF ERROR-PRONE REPAIR (\"SOS REPAIR\") ACTIVITY ..................................................... 875 SOS Repair of Bacteriophage DNA ............ ............................... 875 SOS Repair of Bacterial DNA ................ ................................ 876 Evidence from studies of repair-deficient mutants ....... .................... 876 Evidence from studies of postreplication repair ........ ...................... 879 MANIFESTATIONS OF SOS REPAIR AND THEIR SIGNIFICANCE ..... ...... 879 Mutation Frequency Response to Increasing Fluence of UV Radiation ..... ..... 879 The UV Lesion Responsible for Induction of SOS Functions.................... 880 Kinetics of Induction and Decay of SOS Repair Activity ....... ................ 882 Time of Action of SOS Repair in UV Mutagenesis ...... ........................ 882 Cryptic Premutational Lesions Susceptible to SOS Repair ...................... 884 Mutator Effect of SOS Repair on Undamaged DNA ....... ..................... 885 PLASMIDS AND SOS REPAIR ................................................ 886 MECHANISM OF SOS REPAIR ............................................... 886 Mechanism of SOS Repair in Bacteriophage ......... .......................... 886 Mechanism of SOS Repair in Bacteria ........... ............................. 887 IMPLICATIONS OF SOS REPAIR FOR CARCINOGENESIS ................... 888 REGULATION OF SOS REPAIR AND OTHER UV-INDUCIBLE FUNCTIONS . 889 PROTEASES AND THE EXPRESSION OF SOS FUNCTIONS .................. 894 CONCLUSIONS ............................................................... 895 APPENDIX................................................................... 896 LITERATURE CITED ......................................................... 898

Ultraviolet mutagenesis and inducible DNA repair in Escherichia coli.

The limited evidence available suggests that most human cancers are not caused by conventional mutagens but are more likely to be the result of genetic transpositions. Although the molecular biology of transposition is starting to be understood, the external factors that influence its frequency have not yet been studied in any detail.

The origin of human cancers.

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.

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Mechanisms of accurate translesion synthesis by human DNA polymerase η

The analysis of the repair of damage to DNA in mammalian cells leads not only to a knowledge of which environmental agents are deleterious to living creatures, but also to an understanding of which reaction products in DNA are potentially carcinogenic and which tissues are the more sensitive.

Repair deficient human disorders and cancer

Complementation of the xeroderma pigmentosum DNA repair defect in cell-free extracts

PATIENTS with the inherited disease, xeroderma pigmentosum (XP), are subject to multiple carcinomas of the skin on areas exposed to sunlight1. Fibroblasts from the skin of the majority of these patients (classical XP) have been shown to be deficient in excision repair of lesions induced in DNA by ultraviolet radiation1–3. One group of patients, however, has been designated ‘XP variants’ because, although they suffer the clinical manifestations of the disease, they carry on normal excision repair of such ultraviolet-light damage to DNA4–6. Lehmann et al.6 reported that cells from such XP variants are abnormally slow in converting initially low molecular weight DNA, synthesised after ultraviolet irradiation, into high molecular weight DNA similar in size to that produced in unirradiated cells. Although these authors suggest that such abnormal DNA replication might explain why such patients are susceptible to cancer of the skin, they do not propose any mechanism. If the somatic cell mutation hypothesis on the origin of cancer, first suggested by Boveri7, is correct one would expect the frequency of mutations induced by ultraviolet light to be higher in cells derived from skin biopsies from both classical and variant XP patients than in cells from normal persons. To test this hypothesis, we have carried out a series of quantitative investigations comparing the frequency of ultraviolet-light induced mutations to azaguanine resistance in normal human skin fibroblasts with that found in various strains derived from classical and variant XPs. We have found that cells derived from both kinds of XP patients indeed show much higher frequencies than normal cells. The data comparing two classical XP strains with normal cells have been published8,9. Here, we present data obtained with variant strain, XP4BE.

Frequency of ultraviolet light-induced mutations is higher in xeroderma pigmentosum variant cells than in normal human cells

XERODERMA pigmentosum (XP) is an inherited human disease clinically characterised by abnormal pigmentation patterns and the development of multiple skin carcinoma on areas exposed to the Sun1. Cells from most XP patients are defective in excision repair of ultraviolet-induced lesions in DNA1–3, but there is a class of patients—XP variants—who have all the clinical manifestations of the disease but who, until recently, appeared to have normal DNA repair. Cleaver4 and others5,6 have shown that variants have normal rates of excision repair of damage caused by ultraviolet light as measured by ultraviolet-stimulated uptake of labelled thymidine or bromodeoxyuridine. Cleaver also reported that variants XP4BE (his patient 13) and XP13BE (his patient 14) have normal sensitivity to ultraviolet light as measured by survival of colony-forming ability4. His survival data, however, consisted of only two points for each strain. As Fig. 1 shows, we find that the percentage survival of the colony-forming ability of variants XP4BE and XP13BE as well as variant XP7TA and XP30RO is more sensitive to ultraviolet irradiation than that of normal cells. We also showed that the survival of the cloning ability of XP variants is more sensitive than normal to active derivatives of four aromatic amide carcinogens7 and to the K-region epoxides of three polycyclic hydrocarbons8. These two classes of carcinogens bind covalently to DNA9,10 and the bound adducts seem to be recognised by the same repair enzymes which function in the repair of ultraviolet-induced damage in these human cells7–11.

Louis M. Ouellette

Papers

Frequency of ultraviolet light-induced mutations is higher in xeroderma pigmentosum variant cells than in normal human cells

Synergistic effect of caffeine on the cytotoxicity of ultraviolet irradiation and of hydrocarbon epoxides in strains of Xeroderma pigmentosum

Caffeine enhancement of the cytotoxic and mutagenic effect of ultraviolet irradiation in a xeroderma pigmentosum variant strain of human cells.