From preschool wheeze to childhood asthma : the role of genetics and environmental factors

Abstract

Background: Respiratory illness with wheeze is very common at preschool age and is often the first clinical manifestation of asthma. Two-thirds of preschool wheezers outgrow their symptoms by their sixth birthday whereas one third of these children develop asthma at school age. Although the role of environmental and host-specific factors in asthma pathogenesis has been explored by numerous studies the pathogenesis of asthma is only partly understood, and the factors that predict the persistence or remission of preschool wheeze remain unclear. Better understanding of etiological architecture of asthma would help us to predict persistent wheeze at school age and focus on specific treatment in order to prevent, reverse or slow down its development. The aim of this thesis was to evaluate risk factors for asthma at school age among children with a history of preschool wheeze, recruited in the ‘Gene Expression in Wheezing and Asthmatic Children’ (GEWAC) cohort.

Methods: The ‘Gene Expression in Wheezing and Asthmatic Children’ (GEWAC) cohort comprises 156 children aged 6 to 48 months (cases) who were enrolled at Astrid Lindgren’s Children’s Hospital in Stockholm when seeking hospital care for an acute episode of wheeze, from 2008 to 2012. During the same period, 102 age-matched healthy controls were recruited at the Surgical Daycare Ward, Astrid Lindgren’s Children’s Hospital. The studies included in this thesis are mainly based on a case-cohort design; the cases were followed at a first revisit 2-3 months after enrolment and thereafter annually until 7 years of age. The controls have been evaluated at enrolment and at 7 years of age. Both cases and controls that attended the 7 years’ follow-up were assessed regarding asthma diagnosis. Nasal swab tests for virus and bacteria detection were obtained from the cases at inclusion and at the 3-month revisit. Blood samples were obtained from cases and controls at each visit. Standardized questionnaires concerning airway symptoms, life quality, hereditary and environmental factors were answered by legal guardians at the first revisit of cases and at inclusion of controls. Analyses of blood samples included complete blood count, allergen-specific Immunoglobulin E (IgE) antibodies against a mix of common food allergens, and a mix of common airborne allergens, ISAC (Thermo Fisher Scientific) analysis of 112 allergen molecules, levels of vitamin D (25(OH)D), genotyping for VDR (coding for Vitamin D receptor), VDBP (Vitamin D binding protein), and multiple genes at the 17q21 locus. In addition, lung function measurement was performed at 7 years of age.

Results: STUDY I: Eighty of 113 cases (70.8%) who were assessed at 7 years were diagnosed with asthma whereas 1 of 54 healthy controls had asthma at the same age (p < 0.001). Cases with asthma at 7 years had a higher prevalence of rhinovirus infection at inclusion (p=0.011), had been admitted to hospital more often (p = 0.024) and spent more days admitted (p = 0.01) during the year following inclusion compared to cases without asthma. STUDY II: We found that the more the number of allergen molecules with IgE reactivity increased between preschool age and the age of 7, the more likely it was to have an asthma diagnosis at 7 years (p=0.039, OR = 1.25, 95%CI 1.01, 1.54). Sensitization to the major birch pollen allergen Bet v 1 (20.8%) at 7 years was overrepresented among cases with asthma compared to those without asthma at 7 years (N=14 (28.6%) vs N=1 (4.3%), p=0.027). Moreover, the number of IgE reactivities per case at 7 years (6.5 (1-21)) was significantly associated with asthma at the same age (p = 0.043, OR 1.20, 95%CI 1.02, 1.42). STUDY III: Vitamin D levels (nmol/L) at preschool age (p=0.276) or at 7 years (p=0.854) did not differ between cases with and without asthma at 7 years. The VDR-rs2228570 was significantly associated with asthma at 7 years (GG+AG vs AA; p=0.009, OR=6.125, 95%CI 1.57-23.97, AG vs GG+AA; p=0.003, OR=5.28, 95%CI 1.68-16.63). STUDY IV: Among the studied genetic variants in the 17q21 locus, rs8076131 (AA vs GG) was related to increased risk of preschool wheeze (p=0.001, OR=3.50), and asthma at 7 years (p=0.002, OR=8.55). Rs12603332 (CC vs TT), was associated with asthma at 7 years regardless of rhinovirus infection at inclusion or current signs of airborne allergy (p=0.016, aOR=7.17). A significant association of rs6967330-AA/AG (CDHR3) with asthma at 7 years was restricted to children with specific genotypes in the 17q21 locus (rs8076131-AA, rs8079416-CC, and rs3859192-TT). Rhinovirus infection at inclusion was significantly related to asthma in strata of children with specific genotypes for rs8079416 (CC) and/or rs3859192 (TT).

Conclusion: Polysensitization and molecular spreading from preschool to school age, are related to asthma diagnosis at 7 years in children with a history of preschool wheeze. Rs2228570 in VDR, is related to asthma at school age among preschool wheezers, irrespective of vitamin D levels. The severity of wheezing symptoms and rhinovirus infection with wheeze at preschool age are factors associated with asthma at 7 years. Our results point to genetic variation in the 17q21 locus as a considerable risk factor for asthma and suggest early rhinovirus induced wheeze as marker of asthma susceptibility. The effect of some genetic variants in the 17q21 locus or the CDHR3-rs6967330, might be age-, or phenotypespecific and modified by gene-gene or gene-environmental interactions. More longitudinal studies in larger, well-characterized child cohorts are warranted to map these interactions.