The molecular basis of disease variability among cystic fibrosis patients carrying the 3849+10 kb C-->T mutation

Disease severity varies among cystic fibrosis (CF) patients carrying the same CFTR genotype. Here we studied the mechanism underlying disease variability in individuals carrying a splicing CFTR mutation, 3849+10 kb C-->T. This mutation was shown to produce both correctly and aberrantly spliced CFTR transcripts containing an additional cryptic exon. Semiquantitative nondifferential RT-PCR showed considerable variability in the level (0-28%) of aberrantly spliced RNA transcribed from the 3849+10 kb C-->T mutation in nasal epithelium from 10 patients. A significant inverse correlation was found between the level of the aberrantly spliced CFTR transcripts and pulmonary function, expressed as FEV1 (r = 0.92, P < 0.0001). Patients with normal pulmonary function (FEV1 > 80% predicted) had lower levels of aberrantly spliced CFTR RNA (0 to 3%) than those with FEV1 < 80%, (9 to 28% aberrantly spliced RNA). Only aberrantly spliced CFTR RNA was detected in the lung of a patient with severe lung disease who underwent lung transplantation. Our results show that the severity of CF lung disease correlates with insufficiency of normal CFTR RNA. Thus, the regulation of alternative splice site selection may be an important mechanism underlying partial penetrance in CF. Further understanding of this regulation will contribute to potential therapy for patients carrying splicing mutations in human disease genes.

Screening of CFTR mutations in an isolated population: identification of carriers and patients

One important application of the identification of disease-causing mutations is carrier screening in the general population. Such a project requires a simple accurate test by which a large proportion of the mutations can be identified. This study describes screening for CFTR mutations in an isolated Israeli Arab village. Two mutations, G85E and delta F508, accounted for all the CF alleles of these patients. The screening program tested for these two mutations, as well as the 5T allele, which has recently been shown to down-regulate the CFTR expression and cause variable phenotype. The screened population comprised 497 students from one school, which all the children of the village attend. The results revealed high carrier frequency, 8.5%, for the two CFTR mutations, G85E and delta F508, and a carrier frequency of 12% for the 5T allele. Two compound heterozygotes for the CFTR mutations, delta F508/G85E and G85E/5T, were identified. Both of these students had not been diagnosed previously as having CF since their disease presentation was not typical of CF. The CF incidence in this village was found to be extremely high, 1:72 life births. The screening results were reported to the physicians of the village to be used, upon request, for genetic counselling. This study emphasizes the importance of such programs for the identification of non-classical patients and for carrier detection.

A cystic fibrosis transmembrane conductance regulator splice variant with partial penetrance associated with variable cystic fibrosis presentations

Some patients express various features of cystic fibrosis (CF) even though essential characteristics of the disease might be absent. Such patients may suffer from respiratory disease without pancreatic insufficiency and normal sweat chloride levels. Others may present as male infertility because of congenital bilateral aplasia of the vas deferens (CBAVD) with no other signs of CF. The 5T allele, a DNA variant in a noncoding region of the cystic fibrosis transmembrane conductance regulator (CFTR) gene that reduces the level of the normal CFTR transcripts, was found in increased frequency among male patients with CBAVD. The purpose of this study was to investigate the possibility that the 5T allele is associated with dysfunction of organs other than the male reproductive system, leading to CF or atypical CF. Analysis of the 5T allele was performed on 148 subjects (29 with CF, 61 with atypical CF, and 58 with CBAVD) carrying 232 chromosomes with unidentified CFTR mutations, and on 142 non-CF chromosomes from healthy subjects of Ashkenazi origin. The frequency of the 5T allele among chromosomes from patients of Jewish Ashkenazi origin with CF and atypical CF (six of 33; 18%) was significantly higher than the frequency in the normal Ashkenazi population (eight of 142; 6%; p = 0.03). Analysis of the clinical presentation of the five patients with CF and the 12 patients with atypical CF carrying the 5T allele indicated that most patients suffered from respiratory disease presenting as asthma like symptoms, nasal polyposis, chronic sinusitis, chronic bronchitis, or bronchiectasis. Six patients had pancreatic insufficiency, two with meconium ileus. Sweat Cl- levels ranged from normal to elevated. Of the six male patients with respiratory disease who were old enough to be evaluated for fertility status, five were fertile and one had pancreatic insufficiency. Among male patients with CBAVD, 41% suffered from respiratory symptoms. Thus, the 5T allele is a variant with partial penetrance causing disease with an extreme variability of clinical presentation: from normal healthy fertile subjects or male patients with CBAVD to those with atypical or typical clinical phenotype of CF.