| Cystic fibrosis (CF), the most common lethal single gene disorder in Caucasians, is due to mutations in the CFTR gene. Twin and sibling analysis indicates that modifier genes substantially contribute to the variability in CF lung disease and the neonatal intestinal obstruction, meconium ilcus (MI), independently of CFTR. We used a family-based approach to test for association between genetic variants and these phenotypes in subjects from the CF Twin and Sibling Study (CFTSS). Transmission analysis of 472 CF trios revealed that two functional SNPs (rs1800469, "-509" and rs1800470, "codon 10") in the 5' region of transforming growth factor-beta1 (TGFB1), a putative CF modifier gene, showed significant transmission distortion when patients were stratified by CFTR genotype. Additional tagging SNPs that capture most of the diversity in TGFB1 were also genotyped but none showed association with variation in lung function. However, a haplotype composed of the -509 C and codon 10 T alleles along with the C allele of the 3' SNP rs8179181 was highly associated with increased lung function in non-F508del homozygous patients. We also performed a critical replication analysis of IFRD1 and CEBPA/CEBPG , two putative genetic modifiers of lung disease severity. Transmission testing in CFTSS trios revealed that a SNP in IFRD1 (rs7817) and haplotypes in the CEBP genes were significantly associated with variation in cross-sectional and longitudinal measures of lung function. Similarly to the TGFB1 haplotype, the effect of the CEBP haplotypes was enhanced in non-F508de1 homozygotes, suggesting gene-gene interaction between CFTR and these modifier genes. Finally, we tested for association between MI and 2896 SNPs in a previously reported MI linkage region at Chr8p23.1. Transmission analysis in 133 families from the CFTSS in which at least one child had MI (n=166 with MI, 104 without MI) revealed that a SNP (rs614197) 5' of the methionine sulfoxide reductase A (MSRA) gene was highly associated with MI (P=8.35x10-6). Transmission testing of a 2-Mb region flanking rs614197 identified a 3-SNP haplotype (rs10903323 T - rs4840475 G - rs17151637 A) spanning a 3.5 kb region in intron 3 of MSRA as being protective against MI (P=1.08x10-6). Association of this haplotype with MI was replicated in two independent CF populations using case/control analysis. The MSRA haplotype was under-represented in 161 cases of MI compared to 165 controls without MI from the CF Genetic Modifier Study (P=0.016) and was associated with reduced risk of MI in 220 cases and 1163 controls from the Canadian Consortium for CF Gene Studies (P=0.04). These studies support a role for the use of haplotypes in identifying genetic variation that modifies CF phenotypes and suggest that the interaction of modifier genes with the disease-causing gene may enhance their potential to modulate CF disease severity. |
