Cutaneous melanoma: how does ultraviolet light contribute to melanocyte transformation?

Different genetic mechanisms mediate spontaneous versus UVR-induced malignant melanoma

Genetic variation conferring resistance and susceptibility to carcinogen-induced tumorigenesis is frequently studied in mice. We have now turned this idea to melanoma using the collaborative cross (CC), a resource of mouse strains designed to discover genes for complex diseases. We studied melanoma-prone transgenic progeny across seventy CC genetic backgrounds. We mapped a strong quantitative trait locus for rapid onset spontaneous melanoma onset to Prkdc, a gene involved in detection and repair of DNA damage. In contrast, rapid onset UVR-induced melanoma was linked to the ribosomal subunit gene Rrp15. Ribosome biogenesis was upregulated in skin shortly after UVR exposure. Mechanistically, variation in the 'usual suspects' by which UVR may exacerbate melanoma, defective DNA repair, melanocyte proliferation, or inflammatory cell infiltration, did not explain melanoma susceptibility or resistance across the CC. Instead, events occurring soon after exposure, such as dysregulation of ribosome function, which alters many aspects of cellular metabolism, may be important.

Multiparameter analysis of naevi and primary melanomas identifies a subset of naevi with elevated markers of transformation

Here we have carried out a multiparameter analysis using a panel of 28 immunohistochemical markers to identify markers of transformation from benign and dysplastic naevus to primary melanoma in three separate cohorts totalling 279 lesions. We have identified a set of eight markers that distinguish naevi from melanoma. None of markers or parameters assessed differentiated benign from dysplastic naevi. Indeed, the naevi clustered tightly in terms of their immunostaining patterns whereas primary melanomas showed more diverse staining patterns. A small subset of histopathologically benign lesions had elevated levels of multiple markers associated with melanoma, suggesting that these represent naevi with an increased potential for transformation to melanoma.

Controversial issues on melanoma

In this chapter, we try to debate two of the most several controversial points about melanoma: the role of the ultraviolet (UV) ray exposure and the position of the sentinel node biopsy in the dermatological daily activity. It has been demonstrated as a direct relationship between UV exposure and the risk of developing melanoma, but it is also true that a chronic continuous UV ray exposure can develop a protective action. Nodal evaluation is one of the most important prognostic indicators to be considered for the patient outcome. The aims for which sentinel node biopsy is so often adopted can be summarized in three points: a detailed nodal staging, a regional disease control, and a possible overall improved survival. At present, many authors do not think that it let the overall survival grow; therefore, they suggest its use only to stage regional lymph nodes and accurately identify patients who could benefit through an early complete regional lymphadenectomy.

Recent advances in sunlight-induced carcinogenesis using the Xiphophorus melanoma model

Unlike breast and prostate cancers, the nature and sequence of critical genetic and epigenetic events involved in the initiation and progression of melanoma are not well understood. A contributing factor to this dilemma, especially given our current understanding of the importance of UV light in melanoma etiology, is the lack of quality UV-inducible melanoma animal models. In this study we elaborate on the capability of UV light to induce cutaneous malignant melanomas (CMM) in Xiphophorus fishes, which were previously found to develop melanomas after acute neonatal UVB irradiation. In two separate tumorigenesis experiments, we exposed adult Xiphophorus hybrids to either acute UVB irradiations (5 consecutive daily treatments) or chronic solar irradiations (continuous UVA/UVB treatment for 9 months). Acute adult UVB irradiation resulted in the significant induction of melanomas, and moreover, this induction rate is equivalent to that of animals exposed to acute neonatal UVB irradiation. This study represents the first evidence that acute adult UVB irradiation, in the absence of any early life exposures, induces CMM. Similar to the findings conducted on other divergent melanoma models, including HGF/SF transgenic mice and Monodelphis domestica, prolonged chronic solar UV was not a factor in melanomagenesis.

Chemoprevention of melanoma

Despite advances in drug discovery programs and molecular approaches for identifying drug targets, incidence and mortality rates due to melanoma continue to rise at an alarming rate. Existing preventive strategies generally involve mole screening followed by surgical removal of the benign nevi and abnormal moles. However, due to lack of effective programs for screening and disease recurrence after surgical resection, there is a need for better chemopreventive agents. Although sunscreens have been used extensively for protecting from UV-induced melanomas, results of correlative population-based studies are controversial, with certain studies suggest increased skin cancer risk in sunscreen users. Therefore, these studies require further authentication to conclusively confirm the chemoprotective efficacy of sunscreens. This chapter reviews the current understanding regarding melanoma chemoprevention and the various strategies used to accomplish this objective.

The specificity of UVA-induced DNA damage in human melanocytes

Exposure to solar UV radiation is the origin of most skin cancers, including deadly melanomas. Melanomas are quite different from keratinocyte-derived tumours and exhibit a different mutation spectrum in the activated oncogenes, possibly arising from a different class of DNA damage. In addition, some data suggest a role for UVA radiation in melanomagenesis. To get further insight into the molecular mechanisms underlying induction of melanoma, we quantified a series of UV-induced DNA damage in primary cultures of normal human melanocytes. The results were compared with those obtained in keratinocytes from the same donors. In the UVB range, the frequency and the distribution of pyrimidine dimers was the same in melanocytes and keratinocytes. UVA was also found to produce thymine cyclobutane dimer as the major DNA lesion with an equal efficiency in both cell types. In contrast, following UVA-irradiation a large difference was found for the yield of 8-oxo-7,8-dihydroguanine; the level of this product was 2.2-fold higher in melanocytes than in keratinocytes. The comet assay showed that the induction of strand breaks was equally efficient in both cell types but that the yield of Fpg-sensitive sites was larger in melanocytes. Our data show that, upon UVA irradiation, oxidative lesions contribute to a larger extent to DNA damage in melanocytes than in keratinocytes. We also observed that the basal level of oxidative lesions was higher in the melanocytes, in agreement with a higher oxidative stress that may be due to the production of melanin. The bulk of these results, combined with qPCR and cell survival data, may explain some of the differences in mutation spectrum and target genes between melanomas and carcinomas arising from keratinocytes.

Malignant melanomas on head/neck and foot: differences in time and latitudinal trends in Norway

BACKGROUND

Cutaneous malignant melanoma (CMM) incidence continues to increase in many parts of the world. Solar ultraviolet (UV) radiation is the main environmental risk factor for CMM. Different body locations are subjected to different doses and exposure patterns of solar UV. Time and latitudinal trends of CMMs on shielded and exposed skin give valuable information about the aetiology of these cancers. In this study, we have compared the time and latitudinal trends of CMM incidence on skin areas which are chronically (head and neck) and rarely (foot) exposed to UV radiation, to gain more information about the relationship between sun doses, exposure patterns and melanomagenesis.

METHODS

We have analysed epidemiological data from the Cancer Registry of Norway, for foot and head and neck CMM for two time periods: 1966-1986 and 1987-2007.

RESULTS

Cutaneous malignant melanoma incidence rate on head and neck has increased with time, while incidence rates of foot CMM have remained almost constant with time in Norway. There is a large north-south gradient in incidence rates of CMM on head and neck in Norway, while there is almost no north-south gradient for CMM incidence on foot.

CONCLUSIONS

Comparisons of time trends and latitudinal trends of the incidence rates of CMM on head/neck and on foot indicate that solar radiation plays a role in the induction of the former CMM but probably not for the latter.

An experimental population study of nucleotide excision repair as a risk factor for UVB-induced melanoma

Nucleotide excision repair (NER) is the primary defense against the DNA damage implicit in skin cancer formation and is negatively affected by chronic exposure to UVB radiation. However, in situ and in vitro studies consistently yield equivocal results when addressing individual DNA repair capacity and melanoma susceptibility. The primary objective of this study was to determine if individual global NER capacity is a risk factor for melanoma formation in a prominent UVB-inducible melanoma model, hybrid Xiphophorus fishes. After neonatal UVB irradiation, adult tumor-bearing and tumor-free fish were given a challenge UVB dose and (6-4) photoproduct repair was quantified in individual fish at 24 h using radioimmunoassay. Despite considerable inter-individual variation in repair capacity, ranging from 13% to 91%, we found no difference in mean NER capacity between fish with and without melanomas, thus detaching global NER from melanomagenesis. Furthermore, despite epidemiological data indicating that sex and age are important risk factors underlying melanoma susceptibility, we found no difference in mean NER rates among the sexes or as a function of age. We conclude with a discussion of the apparent paradox of how inter-individual variation in NER is not a risk factor given the clear evidence that DNA damage underlies melanoma susceptibility.

Ultraviolet-B phototoxicity and hypothetical photomelanomagenesis: intraocular and crystalline lens photoprotection

PURPOSE

Ultraviolet-B (UV-B) radiation can cause phototoxic macular injuries in young people who have been sunbathing but not sungazing and in welders. Welders have a reportedly increased risk of uveal melanoma. We analyze phakic and pseudophakic risks for solar and welding arc UV-B exposure.

DESIGN

Optical radiation measurement, analysis, and perspective.

METHODS

Spectral transmittances were measured for UV-transmitting, UV-blocking, and blue-blocking intraocular lenses (IOLs). The photoprotective performances of crystalline and intraocular lenses were analyzed using relevant epidemiologic and laboratory data and action spectra for acute retinal phototoxicity and melanoma photocarcinogenesis.

RESULTS

Crystalline lens UV-B retinal protection is deficient in children and young adults, increasing their potential susceptibility to acute retinal phototoxicity and hypothetical photomelanomagenesis. UV-B radiation has sufficient energy/photon to induce primary melanomagenic DNA lesions, unlike blue light or UV-A radiation. UV-blocking and blue-blocking IOLs have negligible UV-B transmittance. UV-transmitting IOL transmittance of UV-B radiation is equivalent to that of a 15-year-old crystalline lens.

CONCLUSIONS

If optical radiation exposure is responsible for welders' increased risk of uveal melanoma, then UV-B radiation is the most probable causative agent and spectacle wear is a potential confounding factor in epidemiologic studies of ocular melanoma. Welders under 30 years of age are at greater risk for welding maculopathy than older welders. Children, adults under 30 years of age, and pseudophakic individuals with UV-transmitting IOLs should wear sunglasses in bright environments because of the UV-B window in their crystalline lenses or IOLs.

DNA repair gene polymorphisms and risk of cutaneous melanoma: a systematic review and meta-analysis

Polymorphisms of DNA repair-related genes might modulate cancer predisposition. We performed a systematic review and meta-analysis of the available evidence regarding the relationship between these polymorphisms and the risk of developing cutaneous melanoma. Relevant studies were searched using PubMed, Medline, Embase, Cancerlit, Cochrane and ISI Web of Knowledge databases. Data were gathered according to the Meta-analysis Of Observational Studies in Epidemiology (MOOSE) guidelines. The model-free approach was adopted to perform the meta-analysis of the retrieved data. We identified 20 original reports that describe the relationship between melanoma risk and the single-nucleotide polymorphisms (SNPs) of 16 genes (cases = 4195). For seven SNPs considered in at least two studies, the findings were heterogeneous. Data were suitable for meta-analysis only in the case of the XPD/ERCC2 SNP rs13181 (cases = 2308, controls = 3698) and demonstrated that the variant C allele is associated with increased melanoma risk (odds ratio = 1.12, 95% confidence interval = 1.03-1.21, P = 0.01; population attributable risk = 9.6%). This is the first meta-analysis suggesting that XPD/ERCC2 might represent a low-penetrance melanoma susceptibility gene. Much work is still to be done before definitive conclusions can be drawn on the role of DNA repair alterations in melanomagenesis since for the other genes involved in this highly complex process, the available information is scarce or null.