Question

Composition of influence on children

Answer 1

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We read with interest the article by de Jong and colleagues (Gut 2003; 52: 547–51) reporting studies of genetic associations between DNA polymorphisms in xenobiotic metabolising genes and Crohn’s disease (CD). The authors employed a case control study design to test seven polymorphisms in five candidate genes for disease association. Evidence was found for a significant association of a single nucleotide polymorphism (SNP), Tyr113His (348T. C), in the microsomal epoxide hydrolase 1 gene (EPHX1), with CD. Homozygosity for the T (Tyr 113) allele was significantly higher in cases than in healthy controls (x2= 23.7, p, 0.0001, odds ratio 2.9). The observed frequency of the T allele in controls was 41%, which is outside the range of frequencies (58–94%) reported in other control populations (reviewed in de Jong et al). Its frequency in CD cases was 67%. In view of the strength of reported association, we sought to replicate this observation. We genotyped the Tyr113His SNP (ref SNP ID rs1051740) in 307 independent sporadically ascertained cases of CD and 344 ethnically matched healthy control subjects. 1 This compared with 151 cases and 149 controls typed by de Jong et al. Our study design provided 80% power to detect a significant difference (p, 0.05) in allele frequency of> 7.5% between cases and controls compared with the difference of 26% observed in the published study. Our power calculations were based on an observed minor (C) allele frequency of 30.2% in our control cohort, the common (T) allele frequency being 69.8%.

We used TaqMan chemistry (Applied Biosystems) to genotype DNA from cases and controls with an Applied Biosystems 7700 Sequence Detection System. Preoptimised primers and fluorescent probes were obtained from Applied Biosystems (SNP assay ID C_14938_1). All cases and controls were previously genotyped for three confirmed disease susceptibility alleles for CD (R702W, G908R, L1007fs) in CARD15, 2–4 permitting stratification of data by CARD15 mutational status to identify potential

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