Genotype-phenotype relationships in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The disease is characterized by a wide variability of clinical expression. The cloning of the CFTR gene and the identification of its mutations has promoted extensive research into the association between genotype and phenotype. Several studies showed that there are mutations, like the ΔF508 (the most common mutation worldwide), which are associated with a severe phenotype and there are mutations associated with a milder phenotype. However, there is a substantial variability in disease expression among patients carrying the same mutation. This variability involves also the severity of lung disease. Furthermore, increased frequencies of mutations are found among patients with incomplete CF expression which includes male infertility due to congenital bilateral absence of the vas deferens. In vitro studies of the CFTR function suggested that different mutations cause different defects in protein production and function. The mechanisms by which mutations disrupt CFTR function are defective protein production, processing, channel regulation, and conductance. In addition, reduced levels of the normal CFTR mRNA are associated with the CF disease. These mutations are associated with a highly variable phenotype from healthy individuals or infertile males to a typical CF disease. This variability in disease expression is associated with different levels of normally spliced transcripts. Further understanding the mechanisms of CFTR dysfunction may suggest different therapeutic strategies for each class of mutations.

The molecular basis for disease variability in cystic fibrosis.

Mutations in the cystic fibrosis (CF) conductance transmembrane regulator (CFTR) gene have been detected in patients with CF and in males with infertility attributable to congenital bilateral absence of the vas deferens (CBAVD). Thirty individuals with CBAVD and 10 with congenital unilateral absence of the vas deferens (CUAVD) were analyzed by single-strand conformation analysis and denaturing gradient gel electrophoresis for mutations in most of the CFTR gene. All 40 individuals were pancreatic sufficient, but twenty patients had recurrent or sporadic respiratory infections, asthma/asthmatic bronchitis, and/or rhino-sinusitis. Agenesia or displasia of one or both seminal vesicles was detected in 30 men and other urogenital malformations were present in six subjects. Among the 40 samples, we identified 13 different CFTR mutations, two of which were previously unknown. One new mutation in exon 4 was the deletion of glutamic acid at codon 115 (ΔE115). A second new mutation was found in exon 17b, viz., an A→C substitution at position 3311, changing lysine to threonine at codon 1060 (K1060T). CFTR mutations were detected in 22 out of 30 (73.3%) CBAVD patients and in one out of 10 (10%) CUAVD individuals, showing a significantly lower incidence of CFTR mutations in CBAVD/CUAVD patients (P ≪ 0.0001), compared with that found in the CF patient population. Only three CBAVD patients were found with more than one CFTR mutation (ΔF508/L206W, ΔF508/R74W+D1270N, Rl 17H/712-1G→T), highlighting L206W, R74W/ D1270N, and R117H as benign CF mutations. Sweat electrolyte values were increased in 76.6% of CBAVD patients, but three individuals without CFTR mutations had normal sweat electrolyte levels (10% of the total CBAVD patients), suggesting that factors other than CFTR mutations are involved in CBAVD. The failure to identify a second mutation in exons and their flanking regions of the CFTR gene suggests that these mutations could be located in introns or in the promoter region of CFTR. Such mutations could result in CFTR levels below the minimum 6%–10% necessary for normal protein function.

Extensive analysis of 40 infertile patients with congenital absence of the vas deferens: in 50% of cases only one CFTR allele could be detected

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Screening for cystic fibrosis.

Clinical implications of cystic fibrosis transmembrane conductance regulator mutations

We present the genotype/phenotype correlation analysis for 16 cystic fibrosis (CF) patients who carry mutation R334W. Current age and age of diagnosis was significantly higher in the R334W/any-mutation group (P < 0.05 and P < 0.01), compared with the Δ508/Δ508 group. A slightly, but not significantly, worse lung function was found in the R334W/any-mutation group, when compared with the Δ508/Δ508 patients. The proportion of patients with lung colonization with bacterial pathogens was slightly, but not significantly, higher in the R334W/any-mutation group (71.4%), compared with the Δ508/Δ508 or R334W/Δ508 groups (55.5%). None of the R334W patients had meconium ileus but 60% were pancreatic insufficient (PI), a significantly lower proportion (P ≪ 0.001) than Δ508/Δ508 patients. Two R334W/N1303K compound heterozygous sisters were PI but discrepant for lung function. Two groups of three sibs with genotype R334W/Δ508 showed interfamilial discordant clinical data for lung and pancreatic function. The data provided here for mutation R334W demonstrate that this mutation is responsible for a less severe form of CF than Δ508. Interfamilial differences for PI and lung function suggest that other factors, viz. genetic, environmental and medical, contribute to the wide spectrum of clinical differences observed in CF patients with the same CF transmembrane conductance regulator genotypes.

Clinical characteristics of 16 cystic fibrosis patients with the missense mutation R334W, a pancreatic insufficiency mutation with variable age of onset and interfamilial clinical differences.

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Clinical characteristics of 16 cystic fibrosis patients with the missense mutation R334W, a pancreatic insufficiency mutation with variable age of onset and interfamilial clinical differences.