Intragenic copy number variation within filaggrin contributes to the risk of atopic dermatitis with a dose-dependent effect

Abstract

Loss-of-function variants within the filaggrin gene (FLG) increase the risk of atopic dermatitis. FLG also demonstrates intragenic copy number variation (CNV), with alleles encoding 10, 11, or 12 filaggrin monomers; hence, CNV may affect the amount of filaggrin expressed in the epidermis. A total of 876 Irish pediatric atopic dermatitis cases were compared with 928 population controls to test the hypothesis that CNV within FLG affects the risk of atopic dermatitis independently of FLG-null mutations. Cases and controls were screened for CNV and common FLG-null mutations. In this population the 11-repeat allele was most prevalent (allele frequency 51.5%); the 10-repeat allele frequency was 33.9% and the 12-repeat allele frequency was 14.6%. Having excluded FLG mutation carriers, the control group had a significantly higher number of repeats than cases (χ(2) P=0.043), and the odds ratio of disease was reduced by a factor of 0.88 (95% confidence interval 0.78-0.98, P=0.025) for each additional unit of copy number. Breakdown products of filaggrin were quantified in tape-stripped stratum corneum from 31 atopic dermatitis patients and urocanic acid showed a positive correlation with total copy number. CNV within FLG makes a significant, dose-dependent contribution to atopic dermatitis risk, and therefore treatments to increase filaggrin expression may have therapeutic utility.

Figure 1

Filaggrin (FLG) structure and genotyping strategies to identify copy number variants. LD, linkage disequilibrium; SNP, single-nucleotide polymorphism. Diagram adapted from Smith et al. (2006).

Figure 2

Visualization of PCR 1 products showing no evidence of copy number variation (CNV) within the 5′ portion of filaggrin (FLG) exon 3. PCR reactions separated on 0.8% agarose gel. Bioline hyperladder I marker (Bioline, London, UK).

Figure 3

Visualization of PCR 2 products showing product sizes representing 10-, 11-, and 12-repeat alleles from the 3′ portion of filaggrin (FLG) exon 3. PCR reactions separated on 0.8% agarose gel. Bioline hyperladder I marker (Bioline).