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From the Department of Ophthalmology and Visual Sciences, University of Wisconsin Medical School, Madison, Wisconsin.
| Abstract |
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METHODS. During the baseline examination of the Beaver Dam Eye Study, standardized measurements of intraocular pressure were performed with a Goldmann applanation tonometer. Stereoscopic photographs of the optic discs were taken of both eyes of each study participant. The eyes were graded for the size of the optic disc and cup according to a standardized protocol, with graders masked to other subject characteristics. Family members who had participated in the examination phase were identified.
RESULTS. Correlations in sibling pairs (n = 1136), parentchild pairs (n = 514), and cousin pairs (n = 1807) for intraocular pressure were 0.17, 0.18, and 0.12, respectively and were all statistically significant, whereas the spouse pair correlation was not. Correlations for sibling, parentchild, and avuncular pairs were higher for vertical optic disc, vertical optic cup, and vertical cup-to-disc ratio than for intraocular pressure. Heritability estimates were 0.36, 0.55, 0.57, and 0.48 for intraocular pressure, optic cup diameter, optic disc diameter, and cup-to-disc ratio, respectively. Correlations for the optic disc parameters were compatible with the amount of gene sharing in relative pairs of different degrees.
CONCLUSIONS. Risk indicators of open-angle glaucoma correlate highly in families, and the patterns are consistent with the hypothesis of genetic determinants of these factors.
Family studies of primary open-angle glaucoma,7 8 9 a study of individuals with the disease,10 and large epidemiologic studies11 12 13 are all consistent with the notion of genetic determinants of this condition. Twin studies14 15 as well as other family studies16 17 support the hypothesis of genetic determinants of intraocular pressure. There are few data in the literature concerning family similarity of optic disc morphology, but there are data indicating that there are genetic determinants of the number of retinal ganglion cells in mice,18 19 which would be likely to affect optic disc morphology in that species.
The clinical diagnosis of glaucoma is not made according to a uniform set of guidelines. The lack of uniformity may lead to phenotypic heterogeneity. This may hamper the search for determinants of this condition, both environmental and genetic. Also, it is likely that identifying affected individuals early in the disease is hard to do with certainty. Waiting for the appearance of late-stage unequivocal disease may impede the search for causes, because the number of persons with overt disease may be small relative to the number with early-onset disease. Therefore, the search for either early markers or strong risk factors may facilitate understanding the genetic mechanisms underlying this disease as it occurs in populations. We sought to investigate the heritability of intraocular pressure and vertical cup and disc diameters in a large, population-based study.
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Persons with one or more siblings who were identified during the initial census of Beaver Dam were grouped into families. The families were expanded to include cousin and avuncular relationships by telephone interviews and obituaries. Family relationships were confirmed by interview and extended pedigrees constructed. There were 2783 persons in these families. There were additional spouses without other relatives in the population.
Statistical analyses were accomplished using FCOR, a procedure for estimating multivariate family correlations, in the SAGE software.23 Logistic regression was used to assess the odds of a person being affected given that a family member was affected, adjusting for age and sex. Generalized estimating equations (GEE) were used to adjust for contributions from multiple family members.24
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| Discussion |
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The correlations for intraocular pressure between family members was lower than for the optic disc and cup measures. This may be a reflection of the quantity and variability of correlates of intraocular pressure which include not only age and sex, but medical intervention by drops or surgery, time of day of measurement (diurnal variation) as well as systolic blood pressure, body mass index, refractive error, serum cholesterol, hematocrit, month of measurement, pulse rate, severity of nuclear sclerosis, diastolic blood pressure, and glycolsylated hemoglobin.25
We present data for the eye with the higher value, because data on glaucoma reflect these values and data from other studies do indicate that genetic factors are important in this disease. We did not find a consistent difference in the patterns of correlations when we analyzed data from like-side eyes (data not shown).
We note that primary open-angle glaucoma occurs in approximately 2% of many adult white22 26 27 28 populations, but the frequency is probably more than twice that in black populations.26 29 It would be of interest to know whether there is a familial tendency to intraocular pressure and to larger cup-to-disc ratios in diverse black populations and whether the correlations are of the same magnitude that we found in whites. It would also be of interest to determine whether the same genes are related to these characteristics across ethnic groups.
| Footnotes |
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Submitted for publication May 23, 2003; revised August 12, 2003; accepted August 16, 2003.
Disclosure: B.E.K. Klein, None; R. Klein, None; K.E. Lee, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Barbara E. K. Klein, Department of Ophthalmology and Visual Sciences, University of Wisconsin, Madison, 610 North Walnut Street, 4th Floor WARF, Madison, WI 53726-2336; kleinb{at}epi.ophth.wisc.edu.
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