Background Age-related cataracts are a major public health problem.The relative importance of genes and environment in the causationof nuclear cataracts, the most common form of age-related cataracts,is not known.
Methods We studied 506 pairs of female twins (226 monozygoticand 280 dizygotic) who were 50 to 79 years old (mean, 62). Theamount of nuclear cataract in the right and left eyes was determinedobjectively by analysis of Scheimpflug lens photographs (yieldingthree measures) and subjectively with use of the Oxford ClinicalCataract Classification and Grading System (yielding one measure).All eight measures (four in each eye) were subsequently combinedin one summary measure of nuclear cataract for each woman. Aunivariate maximum-likelihood model was used to estimate thevariance of the genetic and environmental contributions to eachof the measures.
Results The different measures of cataract formation were highlycorrelated (correlation coefficients, 0.71 to 0.94). The meanscores were similar for the right and left eyes and for monozygoticand dizygotic twins. Quantitative genetic modeling of each ofthe nuclear-cataract scores invariably resulted in a best-fittingmodel that involved additive genetic effects, unique environmentaleffects, and age. The common environmental and dominant geneticeffects could be removed from the models without significantloss of fit. The overall heritability in the combined nuclear-cataractscore (the proportion of the variance explained by genetic factors)was 48 percent (95 percent confidence interval, 42 to 54 percent);age accounted for 38 percent of the variance (95 percent confidenceinterval, 31 to 44 percent) and unique environmental effectsfor 14 percent (95 percent confidence interval, 12 to 18 percent).
Conclusions Genetic effects are important even in such a clearlyage-related disease as nuclear cataract, explaining almost 50percent of the variation in the severity of this disease.
Source Information
From the Twin Research and Genetic Epidemiology Unit, St. Thomas' Hospital (C.J.H., H.S., T.D.S.), and the Department of Preventive Ophthalmology, Institute of Ophthalmology (C.J.H., C.E.G.) both in London.
Address reprint requests to Dr. Hammond at the Twin Research and Genetic Epidemiology Unit, St. Thomas' Hospital, Lambeth Palace Rd., London SE1 7EH, United Kingdom, or at ch{at}twin-research.ac.uk.
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