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H. R. Shah, R. Reynolds, J. M. Seddon; Discordant Age-Related Macular Degeneration Characteristics in Monozygotic Twin Pairs. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4536.
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A study of the large US population-based sample of monozygotic (MZ) and dizygotic twin pairs, concordant or discordant for age-related macular degeneration (AMD), demonstrated heritability estimates for AMD ranging from 0.46 to 0.71.1 We evaluated MZ twin pairs with different macular phenotypes (discordant twins) to assess AMD phenotypes and environmental factors, controlling for heredity.
Eight hundred and forty twins from the National Academy of Sciences-National Research Council WW II Veteran Twin Registry were enrolled.1,2 Macular characteristics were based on fundus photographs using scores determined at the Wisconsin Reading Center and a 5-grade clinical age-related maculopathy staging system.3 Of the 210 MZ pairs, 43 were discordant based on last known grade. MZ twin pairs were genotyped for 7 known AMD genetic loci: two CFH variants, C2, CFB, C3, CFI andthe LOC387715/HTRA1 gene region as well as other genes which confirmed monozygosity. Linear regression was used to evaluate associations between ocular and non-ocular status for each twin pair.
Several AMD phenotypes differed within the twin pairs. Discordant features included presence of geographic atrophy (N=10 pairs), increased pigment (N=27 pairs), soft drusen (N=14 pairs), soft drusen size (N=39 pairs), and hard distinct drusen (N=24 pairs). Also, categories of personal and environmental factors differed in some MZ pairs: smoking (N=10 pairs), body mass index (N=19 pairs), and education (N=5 pairs). The twin who was the heavier smoker tended to have more advanced stages of AMD than the twin who smoked less (p = 0.03).Conclusions: Within some MZ twin pairs with identical genotypes, characteristics representing various AMD phenotypes were discordant. Heavier smoking was significantly related to more advanced AMD. Results implicate that factors other than genotype are involved in the etiology of AMD.
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