Genetic and environmental factors in age-related nuclear cataracts in monozygotic and dizygotic twins.

Background: Age-related cataracts are a major public health problem. The re lative importance of genes and environment in the causation of nuclear cata racts, the most common form of age-related cataracts, is not known. Methods: We studied 506 pairs of female twins (226 monozygotic and 280 dizy gotic) who were 50 to 79 years old (mean, 62). The amount of nuclear catara ct in the right and left eyes was determined objectively by analysis of Sch eimpflug lens photographs (yielding three measures) and subjectively with u se of the Oxford Clinical Cataract Classification and Grading System (yield ing one measure). All eight measures (four in each eye) were subsequently c ombined in one summary measure of nuclear cataract for each woman. A univar iate maximum-likelihood model was used to estimate the variance of the gene tic and environmental contributions to each of the measures. Results: The different measures of cataract formation were highly correlate d (correlation coefficients, 0.71 to 0.94). The mean scores were similar fo r the right and left eyes and for monozygotic and dizygotic twins. Quantita tive genetic modeling of each of the nuclear-cataract scores invariably res ulted in a best-fitting model that involved additive genetic effects, uniqu e environmental effects, and age. The common environmental and dominant gen etic effects could be removed from the models without significant loss of f it. The overall heritability in the combined nuclear-cataract score (the pr oportion of the variance explained by genetic factors) was 48 percent (95 p ercent confidence interval, 42 to 54 percent); age accounted for 38 percent of the variance (95 percent confidence interval, 31 to 44 percent) and uni que environmental effects for 14 percent (95 percent confidence interval, 1 2 to 18 percent). Conclusions: Genetic effects are important even in such a clearly age-relat ed disease as nuclear cataract, explaining almost 50 percent of the variati on in the severity of this disease. (N Engl J Med 2000;342:1786-90.) (C)200 0, Massachusetts Medical Society.