Mechanisms of asthma and allergic inflammation
Genetic pleiotropy between asthma and obesity in a community-based sample of twins

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Background

Asthma and obesity are common conditions that are strongly associated. This association might be due to shared genetic or environmental causes.

Objective

We sought to determine whether a shared genetic cause is responsible for the association between asthma and obesity and to estimate the magnitude of shared genetic cause.

Methods

The analyses were performed with 1001 monozygotic and 383 dizygotic same-sex twin pairs within the University of Washington Twin Registry. The presence of asthma was determined by self-report of a physician diagnosis of asthma, and body mass index (BMI) was calculated by using self-reported height and weight. Obesity was defined as a BMI of 30 or greater. The association between asthma and BMI was assessed by means of mixed-effects ordinal regression. Twin correlations examined the association of asthma and obesity. Univariate and bivariate structural equation models estimated the components of variance attributable to genetic and environmental effects.

Results

A strong association between asthma and BMI was identified in the sample population (P < .001). Substantial heritability was detected for asthma (53%) and obesity (77%), which is indicative of additive genetic influences on each disorder. The best-fitting model of shared components of variance indicated that 8% of the genetic component of obesity is shared with asthma.

Conclusion

The covariation between obesity and asthma is predominantly caused by shared genetic risk factors for both conditions.

Section snippets

Sample population

The study population consisted of 1001 monozygotic (MZ) and 383 dizygotic (DZ) same-sex twin pairs registered in the University of Washington Twin Registry, which is a community-based sample of twins derived from the driver's license applications of the Washington State Department of Licensing. Unique to Washington State, all new applicants for a driver's license are asked if they are a twin. Because Washington State law allows state agencies to share data, a weekly electronic list of all new

Sample characteristics

The University of Washington Twin Registry has enrolled 1042 pairs of MZ, 406 pairs of same-sex DZ, and 422 pairs of opposite-sex DZ twins. Because of the known sex differences in the association between asthma and obesity,3, 4, 12, 15, 16 the analyses were restricted to 1001 MZ and 383 DZ same-sex twin pairs with complete asthma and BMI information (2768 individuals). The prevalence of physician-diagnosed asthma was 14.1%, and that of obesity was 13.5%. The characteristics of this sample

Discussion

Asthma and obesity are common conditions in the United States and many other countries.1, 2 Several studies have shown that asthma and obesity are associated, especially in women.3, 4, 12, 15, 16 The relationship between asthma and obesity was similarly identified in this community-based sample of twins. The magnitude of the association between asthma and obesity is modest in this and other studies; obesity is only one of the factors that influence the etiologically heterogeneous asthma

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    Supported by National Institutes of Health grants K23HL04231 (TSH), K23HL72923 (MMW), and U19AI38429 (DB).

    Disclosure of potential conflict of interest: T. Hallstrand has received grants from Merck and Co. Medical School grant awarded to the University of Washington in 2000 and is on the speakers' bureau for Merck and Co. All others—none disclosed.

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