Nocturnal asthma and the importance of race/ethnicity and genetic ancestry.

RATIONALE: Nocturnal asthma is a common presentation and is associated with a more severe form of the disease. However, there are few epidemiologic studies of nocturnal asthma, particularly in minority populations. OBJECTIVES: To identify factors associated with nocturnal asthma, including the contribution of self-identified race/ethnicity and genetic ancestry. METHODS: The analysis included individuals from the Study for Asthma Phenotypes and Pharmacogenomic Interactions by Race-ethnicity (SAPPHIRE) cohort. Nocturnal asthma symptoms were assessed by questionnaire. Genome-wide genotype data were used to estimate genetic ancestry in a subset of African American participants. Logistic regression was used evaluate the association of various factors with nocturnal asthma, such as self-identified race/ethnicity and genetic ancestry. MEASUREMENT AND MAIN RESULTS: The study comprised 3,380 African American and 1,818 European Americans individuals with asthma. After adjusting for other potential explanatory variables, including controller medication use, African Americans were more than twice as likely (odds ratio, 2.56; 95% confidence interval, 2.24-2.93) to report nocturnal asthma when compared with European American individuals. Among the subset of African American participants with genome-wide genotype data (n = 1,040), estimated proportion of African ancestry was also associated with an increased risk of nocturnal asthma (P = 0.007). Differences in lung function explained a small, but statistically significant (P = 0.02), proportion of the relationship between genetic ancestry and nocturnal asthma symptoms. CONCLUSIONS: Both self-identified race/ethnicity and African ancestry appear to be independent predictors of nocturnal asthma. The mechanism by which genetic ancestry contributes to population-level differences in nocturnal asthma appears to be largely independent of lung function.