title: A study of asthma severity in adult twins.
abstract: BACKGROUND
  The tendency to develop asthma runs in families, but whether the severity of asthma symptoms is inherited is not known.
  
  
  OBJECTIVE
  The aim of this study was to examine whether genetic factors influence the variation in the severity of asthma.
  
  
  METHODS
  Of a sample of 21 133 adult twins from the Danish Twin Registry, a total of 575 subjects (256 intact pairs and 63 single twins) who themselves and/or their co-twins reported a history of asthma at a questionnaire survey were clinically examined. The severity of asthma symptoms was graded according to a clinical interview, and markers of airway impairment and allergy were measured.
  
  
  RESULTS
  After adjusting for confounders, genetic factors explained 24% (10%-37%), P = 0.0004, of the variation in overall asthma symptom severity, whereas non-shared environment accounted for the remaining 76% of the variation. A significant genetic component was also found for the severity of specific asthma symptoms; wheezing 12% (3%-22%), P = 0.007 and shortness of breath 17% (7%-27%), P = 0.0006, but not for chest tightness and cough. Asthma symptom severity correlated weakly with rhinitis severity as well as with objective markers of lung function, airway inflammation, airway responsiveness and allergic sensitization.
  
  
  CONCLUSIONS
  The individual variation in asthma symptom severity is to some degree influenced by genetic factors, but environmental factors explain the main part of the variation. The genetic architectures underlying the severity of asthma symptoms and objectively measured asthma-related traits, respectively, seem to differ.
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: Sophie
authors.initials: S
authors.lastname: van der Sluis
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Vibeke
authors.initials: V
authors.lastname: Backer
mesh_terms: D000328:Adult; D017677:Age Distribution; D001249:Asthma / Q000175:diagnosis / Q000453:epidemiology* / Q000235:genetics* / Q000503:physiopathology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D004781:Environmental Exposure / Q000009:adverse effects; D005260:Female; D020022:Genetic Predisposition to Disease / Q000453:epidemiology*; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D012042:Registries; D012129:Respiratory Function Tests; D018570:Risk Assessment; D012720:Severity of Illness Index; D017678:Sex Distribution; D018433:Twin Studies as Topic; D014429:Twins, Dizygotic / Q000235:genetics* / Q000706:statistics & numerical data; D014430:Twins, Monozygotic / Q000235:genetics* / Q000706:statistics & numerical data; D055815:Young Adult