title: Genetic influence on the age at onset of asthma: a twin study.
abstract: BACKGROUND
  Although the genetics of asthma susceptibility have been frequently explored, little is known about genetic factors that influence the age at onset of asthma.
  
  
  OBJECTIVE
  To study the variation in the age at onset of asthma attributable to genetic and environmental factors.
  
  
  METHODS
  Data on the age at onset and predictors of asthma were collected in 2002 via a multidisciplinary questionnaire study of 34,782 Danish twins 20 to 71 years of age. Survival analytic methods were applied to partition variation in the age at onset of asthma into genetic and environmental components.
  
  
  RESULTS
  Sex, hay fever, atopic dermatitis, smoking, and exposure to passive smoking in childhood were significant risk factors, whereas BCG vaccination was protective for asthma. The risk of asthma in the co-twin of an affected twin was higher in monozygotic than in dizygotic twins (hazard ratio, 2.59; 95% CI, 1.83-3.68; P < .001). The risk of asthma in the co-twin decreased with increasing age at onset of asthma in the index twin (hazard ratio per ten years, 0.86; 95% CI, 0.76-0.98; P = .019). The effect was attenuated in dizygotic twins relative to monozygotic twins (P = .005). Genetic factors explained 34% of the variation in the age at onset of asthma, and environmental factors accounted for 66%.
  
  
  CONCLUSIONS
  Host-related differences in genetic makeup cause different individuals to develop asthma at different ages. Better understanding of the causes for variation in the age at onset of disease may ultimately lead the way to targeted treatments.
authors.affiliation: Department of Respiratory Medicine, Bispebjerg Hospital, Copenhagen, Denmark. sft@city.dk
authors.forename: Simon Francis
authors.initials: SF
authors.lastname: Thomsen
authors.forename: David Lorenzo
authors.initials: DL
authors.lastname: Duffy
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Vibeke
authors.initials: V
authors.lastname: Backer
mesh_terms: D000328:Adult; D017668:Age of Onset; D000368:Aged; D001249:Asthma / Q000235:genetics*; D003718:Denmark / Q000453:epidemiology; D004777:Environment; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D008875:Middle Aged; D012044:Regression Analysis; D012307:Risk Factors; D011795:Surveys and Questionnaires; D055815:Young Adult