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Editors Selection IGR 16-4

Epidemiology: To what extent is IOP heritable?

Comment by Tin Aung & Monisha Nongpiur on:

56146 Heritabilities of intraocular pressure in the population of Korea: the Korean National Health and Nutrition Examination Survey 2008-2009, Kim NR; Park HJ; Suh YJ et al., JAMA ophthalmology, 2014; 132: 278-285

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Glaucoma is frequently associated with abnormally elevated intraocular pressure (IOP), which results from impaired drainage of aqueous humour through outflow pathways. IOP remains the only major modifiable risk factor for glaucoma to date. Glaucoma is generally considered a complex trait, most likely resulting from the interactions of multiple genetic factors as well as environmental exposures.

In this study, Kim and colleagues investigated the familial correlations and heritability of IOP in an Asian population. The authors found significant correlation coefficient estimates for IOP between parent-offspring pairs (0.19, P < .001), sibling pairs (0.31, P = 0.001), and also between spouse pairs (0.29, P < .001). A heritability estimate of 0.345 (adjusted for age, sex, refractive error, body mass index, systolic blood pressure, and fasting serum glucose and total cholesterol levels) was obtained and the total variance in IOPs were explained by the additive genetic factor (36% [95% CI, 32%-40%]) and unique environment factors (64% [95% CI, 60%-68%]). The authors additionally evaluated the impact of parents' IOP on offspring's IOP. For this analysis, the subjects were stratified into groups: those with IOP19 mmHg and those with IOP < 19 mmHg, based on the mean plus two standard deviation IOP value of the entire study population. A greater risk of high IOP (i.e., 19 mmHg) in offspring was observed when both parents had high IOP (odds ratio, 9.76 [95% CI, 2.16- 44.12]) compared to offspring with no parents having high IOP. This was a well-conducted study, however, the findings of a higher correlation between spouse pairs compared to parent-offspring pairs is rather surprising. The authors proposed shared environmental effects since marriage and time of marriage as the reasons for the findings; however the other likely possibility could be the effect of age, as spouse pairs are more inclined to be of a similar age group. Interestingly, Klein et al.¹ in their evaluation in a Caucasian population noted no significant correlation for IOP between spouse pairs.

A greater risk of high IOP (i.e., ≥ 19 mmHg) in offspring was observed when both parents had high IOP (odds ratio, 9.76 [95% CI, 2.16-44.12]) compared to offspring with no parents having high IOP

The heritability estimates of IOP inferred from this study is consistent with others conducted in Caucasian populations,^1-3 and supports the rationale for IOP to be considered an endophenotype for glaucoma. However, when assessing IOP, it is important to keep in mind that it is a variable trait, which is affected by several intrinsic factors such as corneal thickness, physiological diurnal fluctuations, as well as external factors such as the type of instrument for measurement of IOP, examiner experience, and patient posture. It would be interesting to know if the genes/genetic variants that govern IOP also confer similar susceptibility to glaucoma in different ethnic groups.

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