Cystic fibrosis as a genetically complex disease = Cystische Fibrose : eine genetisch komplexe Erkrankung

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An association study on contrasting cystic fibrosis endophenotypes recognizes KRT8 but not KRT18 as a modifier of cystic fibrosis disease severity and CFTR mediated residual chloride secretion

<p>Abstract</p> <p>Background</p> <p>F508del-CFTR, the most frequent disease-causing mutation among Caucasian cystic fibrosis (CF) patients, has been characterised as a mutant defective in protein folding, processing and trafficking. We have investigated the two neighbouring cytokeratin genes <it>KRT8 </it>and <it>KRT18 </it>in a candidate gene approach to ask whether variants in <it>KRT8 </it>and/or <it>KRT18 </it>modify the impaired ion conductance known as the CF basic defect, and whether they are associated with correct trafficking of mutant CFTR and disease severity of CF.</p> <p>Methods</p> <p>We have selected contrasting F508del-<it>CFTR </it>homozygous patient subpopulations stratified for disease severity, comparing 13 concordant mildly affected sib pairs vs. 12 concordant severely affected sib pairs, or manifestation of the CF basic defect in intestinal epithelium, comparing 22 individuals who exhibit CFTR-mediated residual chloride secretion vs. 14 individuals who do not express any chloride secretion, for an association. The <it>KRT8</it>/<it>KRT18 </it>locus was initially interrogated with one informative microsatellite marker. Subsequently, a low density SNP map with four SNPs in KRT8 and two SNPs in KRT18, each selected for high polymorphism content, was used to localize the association signal.</p> <p>Results</p> <p><it>KRT8</it>, but not <it>KRT18</it>, showed an association with CF disease severity (P_best= 0.00131; P_corr= 0.0185) and CFTR mediated residual chloride secretion (P_best= 0.0004; P_corr= 0.0069). Two major four-marker-haplotypes spanning 13 kb including the entire <it>KRT8 </it>gene accounted for 90% of chromosomes, demonstrating strong linkage disequilibrium at that locus. Absence of chloride secretion was associated with the recessive haplotype 1122 at rs1907671, rs4300473, rs2035878 and rs2035875. The contrasting haplotype 2211 was dominant for the presence of CFTR mediated residual chloride secretion. In consistency, the <it>KRT8 </it>haplotype 2211 was associated with mild CF disease while 1122 was observed as risk haplotype. Analysis of microsatellite allele distributions on the SNP background suggests that the mild <it>KRT8 </it>haplotype 2211 is phylogenetically older than its severe counterpart.</p> <p>Conclusions</p> <p>The two opposing <it>KRT8 </it>alleles which have been identified as a benign and as a risk allele in this work are likely effective in the context of epithelial cell differentiation. As the mild <it>KRT8 </it>allele is associated with CFTR mediated residual chloride secretion among F508del-<it>CFTR </it>homozygotes, the KRT8/KRT18 heterodimeric intermediary filaments of the cytoskeleton apparently are an essential component for the proper targeting of CFTR to the apical membrane in epithelial cells.</p

Chloride conductance and genetic background modulate the cystic fibrosis phenotype of ΔF508 homozygous twins and siblings

To investigate the impact of chloride (Cl(–)) permeability, mediated by residual activity of the cystic fibrosis transmembrane conductance regulator (CFTR) or by other Cl(–) channels, on the manifestations of cystic fibrosis (CF), we determined Cl(–) transport properties of the respiratory and intestinal tracts in ΔF508 homozygous twins and siblings. In the majority of patients, cAMP and/or Ca(2+)-regulated Cl(–) conductance was detected in the airways and intestine. Our finding of cAMP-mediated Cl(–) conductance suggests that, in vivo, at least some ΔF508 CFTR can reach the plasma membrane and affect Cl(–) permeability. In respiratory tissue, the expression of basal CFTR-mediated Cl(–) conductance, demonstrated by 30% of ΔF508 homozygotes, was identified as a positive predictor of milder CF disease. In intestinal tissue, 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid–insensitive (DIDS-insensitive) Cl(–) secretion, which is indicative of functional CFTR channels, correlated with a milder phenotype, whereas DIDS-sensitive Cl(–) secretion was observed mainly in more severely affected patients. The more concordant Cl(–) secretory patterns within monozygous twins compared with dizygous pairs imply that genes other than CFTR significantly influence the manifestation of the basic defect

Differential decay of parent-of-origin-specific genomic sharing in cystic fibrosis-affected sib pairs maps a paternally imprinted locus to 7q34

Cystic fibrosis (CF) is a monogenic disease characterized by a high variability of disease severity and outcome that points to the role of environmental factors and modulating genes that shape the course of this multiorgan disease. We genotyped families of cystic fibrosis sib pairs homozygous for F508del-CFTR who represent extreme clinical phenotypes at informative microsatellite markers spanning a 38 Mb region between CFTR and 7qtel. Recombination events on both parental chromosomes were compared between siblings with concordant clinical phenotypes and siblings with discordant clinical phenotypes. Monitoring parent-of-origin-specific decay of genomic sharing delineated a 2.9-Mb segment on 7q34 in which excess of recombination on paternal chromosomes in discordant pairs was observed compared with phenotypically concordant sibs. This 2.9-Mb core candidate region was enriched in imprinting-related elements such as predicted CCCTC-binding factor consensus sites and CpG islands dense in repetitive elements. Moreover, allele frequencies at a microsatellite marker within the core candidate region differed significantly comparing mildly and severely affected cystic fibrosis sib pairs. The identification of this paternally imprinted locus on 7q34 as a modulator of cystic fibrosis disease severity shows that imprinted elements can be identified by straightforward fine mapping of break points in sib pairs with informative contrasting phenotypes