title: Birth order modifies the effect of IL13 gene polymorphisms on serum IgE at age 10 and skin prick test at ages 4, 10 and 18: a prospective birth cohort study.
abstract: BACKGROUND
  Susceptibility to atopy originates from effects of the environment on genes. Birth order has been identified as a risk factor for atopy and evidence for some candidate genes has been accumulated; however no study has yet assessed a birth order-gene interaction.
  
  
  OBJECTIVE
  To investigate the interaction of IL13 polymorphisms with birth order on allergic sensitization at ages 4, 10 and 18 years.
  
  
  METHODS
  Mother-infant dyads were recruited antenatally and followed prospectively to age 18 years. Questionnaire data (at birth, age 4, 10, 18); skin prick test (SPT) at ages 4, 10, 18; total serum IgE and specific inhalant screen at age 10; and genotyping for IL13 were collected. Three SNPs were selected from IL13: rs20541 (exon 4, nonsynonymous SNP), rs1800925 (promoter region) and rs2066960 (intron 1). Analysis included multivariable log-linear regression analyses using repeated measurements to estimate prevalence ratios (PRs).
  
  
  RESULTS
  Of the 1456 participants, birth order information was available for 83.2% (1212/1456); SPT was performed on 67.4% at age 4, 71.2% at age 10 and 58.0% at age 18. The prevalence of atopy (sensitization to one or more food or aeroallergens) increased from 19.7% at age 4, to 26.7% at 10 and 41.1% at age 18. Repeated measurement analysis indicated interaction between rs20541 and birth order on SPT. The stratified analyses demonstrated that the effect of IL13 on SPT was restricted only to first-born children (p = 0.007; adjusted PR = 1.35; 95%CI = 1.09, 1.69). Similar findings were noted for firstborns regarding elevated total serum IgE at age 10 (p = 0.007; PR = 1.73; 1.16, 2.57) and specific inhalant screen (p = 0.034; PR = 1.48; 1.03, 2.13).
  
  
  CONCLUSIONS
  This is the first study to show an interaction between birth order and IL13 polymorphisms on allergic sensitization. Future functional genetic research need to determine whether or not birth order is related to altered expression and methylation of the IL13 gene.
authors.affiliation: Department of Epidemiology and Biostatistics, Norman J Arnold School of Public Health, University of South Carolina, USA. ike.ogbuanu@gmail.com.
authors.forename: Ikechukwu U
authors.initials: IU
authors.lastname: Ogbuanu
authors.forename: Wilfried J
authors.initials: WJ
authors.lastname: Karmaus
authors.forename: Hongmei
authors.initials: H
authors.lastname: Zhang
authors.forename: Tara
authors.initials: T
authors.lastname: Sabo-Attwood
authors.forename: Susan
authors.initials: S
authors.lastname: Ewart
authors.forename: Graham
authors.initials: G
authors.lastname: Roberts
authors.forename: Syed H
authors.initials: SH
authors.lastname: Arshad
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