Chapter 16 / The Biology of Xeroderma Pigmentosum 291 16 The Biology of Xeroderma Pigmentosum Insights Into the Role of Ultraviolet Light in the Development of Melanoma James E. Cleaver CONTENTS INTRODUCTION AND HYPOTHESIS UV DAMAGE DNA REPAIR MECHANISMS REPLICATION OF DAMAGED DNA UV-INDUCED MUTAGENESIS TARGET PATHWAYS IN MELANOMA ANIMAL MODELS FOR MELANOMA CONCLUSIONS AND PERSPECTIVES REFERENCES Summary Exposure to solar ultraviolet (UV) light is a risk factor for the induction of melanoma, but the precise mechanism remains unclear. The increased incidence of the disease in xeroderma pigmentosum patients, whose cells cannot repair DNA damage caused by defects in nucleotide excision repair, demonstrates that UV-induced DNA damage and mutagenesis can plan a role in melanoma. I propose, therefore, that solar exposure in the UVA and UVB range produces a mixture of DNA photoproducts, some being typical pyrimidine dimers and others the result of photosensitizing reactions with melanin precursors that produce DNA damage that requires the nucleotide excision repair or base excision repair pathways. Not all of these damages necessarily produce typical UV-specific mutations in target genes; the association of melanoma induction with acute burns suggests that high UV doses can overwhelm intracellular anti- oxidant defense mechanisms to produce DNA damage. The progression of melanomas is additionally enhanced by early deletions involving one or more of the nucleotide excision repair (NER) genes that leads to enhanced genomic instability. Key Words: Ultraviolet (UV) light; DNA damage; DNA repair; nucleotide excision repair; transcrip- tion-coupled repair; base excision repair; excision; polymerase; low fidelity; mutation. From: From Melanocytes to Melanoma: The Progression to Malignancy Edited by: V. J. Hearing and S. P. L. Leong © Humana Press Inc., Totowa, NJ 291 292 From Melanocytes to Melanoma INTRODUCTION AND HYPOTHESIS The incidence of melanoma has been increasing worldwide over many decades (1). The reasons for the increased incidence of melanoma are varied, and include increased ultraviolet (UV) exposure, changing leisure activities, change in clothing, environmen- tal factors, and changes in the histological criteria for diagnosing melanoma (2). The role of sun exposure in melanoma induction remains difficult to explain in molecular detail. Melanoma develops from the malignant transformation of melanocytes located in the epidermis, dermis, or mucosa, often occurring in pigmented nevi rather than in isolated melanocytes. Several important genes that act at early stages of melanoma induction have been identified, including p16 and B-RAF. However, the precise molecular mecha- nisms that result in mutagenesis of these genes is still a mystery. Excessive sun exposure may predispose a susceptible individual to the development of melanoma, but the link between UV exposure and melanoma is not as strong as in squamous cell carcinomas, in which p53 mutations with a UV signature are seen (3). Evidence is strong in studies with the marsupial, Monodelphis, and the platyfish, Xiphophorus, that typical UV pho- toproducts can initiate melanomas (4,5). Recently developed mouse models also dem- onstrate that neonatal UV exposure can cause melanoma development (6–8). Some of these studies also indicate that the wavelengths for melanoma induction can involve longer UVA wavelengths than for nonmelanoma skin cancers (9). Intermittent exposure and exposure in an individual’s early years apparently play a greater role than chronic exposure or exposures in later life. For example, the greatest increases in incidence of melanomas are seen on the lower extremities in women and the trunk in men (2). Severe sunburns in childhood or sun exposure in sunny locales during childhood also increase the risk of melanoma (10,11). Melanomas are also more common in light-skinned indi- viduals, particularly those with red or blond hair who freckle easily (12,13). There is an association between the risk of developing melanoma and having specific mutations in the melanocortin-1 receptor (12), which plays a key role in determining the type of melanin produced in melanocytes, eumelanin or pheomelanin. One of the stronger examples of a major role for UVB exposure in melanoma is the increased incidence of the disease in xeroderma pigmentosum (XP) patients who cannot repair UVB damage because of defects in nucleotide excision repair (NER) (14). However, even here, the distribution of melanomas over the body resembles that in the normal population, and is not on the commonly exposed regions of the skin. Taken together, these data suggest that UV does indeed play a role in the development of melanoma, although its exact role is not completely understood. Therefore, I propose the following simple, testable hypothesis for the role of solar exposure in melanoma induction: solar exposure in the UVA and UVB range produces a mixture of DNA photoproducts, some being typical pyrimidine dimers and others the result of photosensitizing reactions with melanin pre- cursors that produce DNA damage that requires the NER or base excision repair (BER) pathways. Not all of these damages necessarily produce typical UV-specific mutations in target genes; the association of melanoma induction with acute burns suggests that high UV doses are required to overwhelm intracellular antioxidant defense mechanisms. The progression of melanomas is additionally enhanced by early deletions involving one or more of the NER genes that leads to enhanced genomic instability. This chapter will expand on this hypothesis and will discuss various mechanisms of DNA damage and repair and evidence from XP and other animal models for the induc- tion of melanoma by solar exposure. Chapter 16 / The Biology of Xeroderma PigmentosumTable 1 293 Human Exinuclease Efficiency Substrate Cleavage Alternative pathway [6-4]photoproduct 3–4% T=T dimer 3–4% AP site 3.9, 5% BER CisPt 3.3% T-HMT mono 1% O-6-mG 0.1% Transferase N-6-mA 0.06% BER G*-A mismatch 0.2% MMR G-A* mismatch 0.1% MMR This table represents the approximate cleavage efficiency using cell-free extracts active for NER on synthetic substrates containing various lesions (17). The cleavage efficiency on T=T dimers represents the maximum activity of the NER extract when assayed in vitro. UV DAMAGE Types of Damage Repaired by NER DNA damage can involve a very large variety of different kinds of chemical- and radiation-induced alterations in DNA bases and polynucleotide chains. In an approxi- mate sense, one can classify DNA damage according to which kind of DNA repair system is required for its repair. Large adducts, such as those produced by solar exposure (UVB and UVC), require excision of single-stranded regions of DNA by the NER system (15,16). Other kinds of damage, involving smaller modifications to DNA bases (alkylation) or DNA breakage, require a different suite of enzymes, many of which are involved with immunoglobulin rearrangements and neurodegeneration. The distinc- tions between the various repair systems are not absolute, however, and there are over- laps in the substrate specificity of these various repair systems and variations in the efficiencies and sites of action on DNA in differing metabolic states (Table 1). The NER system recognizes and repairs DNA damage that consists of photoproducts produced by UV light and large DNA adducts produced by carcinogenic chemicals (Fig. 1) (15,16). The most important wavelengths of UV are those in the UVC (240–280 nm) and the UVB (280–320 nm) ranges, which are strongly absorbed by nucleic acids. The photoproducts include the cyclobutane [5-5], [6-6] pyrimidine dimers (CPDs) and the [6-4] pyrimidine pyrimidinone dimers ([6-4]PDs) that involve both T and C pyrim- idines. The [6-4]PD can further photoisomerase to the Dewar photoproduct at longer UV wavelengths, and cytosine in dimers has an increased propensity to deaminate, contributing a C to thymine T mutagenesis. Chemical adducts include those produced by N-acetoxy-N-acetyl aminofluorene (AAAF), benzo(a)pyrene, aflatoxin, photoacti- vated psoralens, and cis-platinum. An oxidative purine product, 5',8-purine cyclodeo- xynucleoside, which may accumulate in neurological tissue, also requires the NER system for repair. The NER system can even recognize DNA triplexes formed by the binding of short oligonucleotides to double-stranded DNA. Other damage that would not, a priori, be expected to require the NER system can be recognized and cleaved, including apurinic sites, alkylated bases, and mismatches (17). The efficiency with which some of these substrates are repaired, especially for mis- 294 From Melanocytes to Melanoma Fig. 1. The sequence of steps involved in nucleotide excision repair from damage recognition by global genome repair (GGR) or transcription coupled repair (TCR) mechanisms (I), DNA unwinding (II), verification (III), excision (III, IV), and polymerization and ligation (V). (Repro- duced from ref. 16, with permission from Macmillan Publishers.) Chapter 16 / The Biology of Xeroderma Pigmentosum 295 matched bases, is weak, and consequently, NER is only a backup mechanism for other kinds of repair (Table 1). Types of Damage Repaired by BER DNA can be damaged by hydrolysis, deamination, oxidative reactions, or alkylations that occur as the result of naturally occurring intracellular oxidative metabolism, as well as from external ionizing radiation, UV light, carcinogens, and alkylating agents (18,19). These processes create singly modified DNA bases and