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

Objectives: To estimate the relative contribution of environmental and gene tic 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 wi th 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 m oles), 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 t o DZ twins. Despite the number of identical twins reporting a difference in mole prevalence, we observed only a modest and inconsistent increase in me lanoma risk attributable to that factor, whereas the increase within frater nal twin pairs was larger by an order of magnitude. Conclusions: If confirmed, this indicates that the significance of mole pre valence as a risk factor for melanoma is largely as a genetic, rather than an environmental, factor. We noticed an increased risk of developing melano ma among DZ twins who drank moderate amounts of beer, but we believe this r esult may be due to the sun-exposure activities of those people most likely to drink moderate amounts of beer.