Determinants of melanoma in a case-control study of twins (United States)

Objectives: To estimate the relative contribution of environmental and genetic factors disposing towards the development of melanoma. Methods: We investigated risk factors for melanoma in a case-control study conducted among 185 North American twin pairs in which one was diagnosed with melanoma and the other (the co-twin of the case) was not. We considered monozygous (MZ) and dyzygous (DZ) twins separately. Results: While greater risk of melanoma was associated with number of large nevi in DZ twins (adjusted OR = 26.6 (4.2-170.8) for three or more large moles), this was not the case for MZ twins (adjusted OR = 1.4 (0.6-3.2) for three or more large moles). Elevated risks of developing melanoma with site-specific sun exposures resulting in sunburn also appeared to be confined to DZ twins. Despite the number of identical twins reporting a difference in mole prevalence, we observed only a modest and inconsistent increase in melanoma risk attributable to that factor, whereas the increase within fraternal twin pairs was larger by an order of magnitude. Conclusions: If confirmed, this indicates that the significance of mole prevalence as a risk factor for melanoma is largely as a genetic, rather than an environmental, factor. We noticed an increased risk of developing melanoma among DZ twins who drank moderate amounts of beer, but we believe this result may be due to the sun-exposure activities of those people most likely to drink moderate amounts of beer.