CFTR haplotype backgrounds on normal and mutant CFTR genes

CFTR gene mutations and polymorphism are associated with non-obstructive azoospermia: From case-control study

A variety of experimental studies have yielded evidence that the cystic fibrosis transmembrane conductance regulator (CFTR) protein participates in the process of spermatogenesis. However, the association between CFTR gene and non-obstructive azoospermia (NOA) disease remained to be a question. First, we reviewed available data from the PubMed and Embase databases before May 2016 to find the most common mutations of CFTR gene in NOA patients. Second, an original case-control study was conducted on NOA patients (n=100) and a control group consisting of fertile males (n=100), selected from August 2015 to March 2017, to detect CFTR gene mutations and polymorphism. Peripheral blood samples from NOA patients and normal controls were analyzed for the presence of specific sequences of CFTR gene by polymerase chain reaction amplification followed by direct sequencing. From our comprehensive review, 12 case-control studies were found concerning the relation between CFTR gene mutations and polymorphism and NOA disease. Fifty-four mutations were mentioned and IVS8 poly-T, TG repeats, F508del and R117H mutations were the most common ones. Based on that, we detected IVS8 poly-T, TG repeats, F508del, R117H and M470V mutations in our case control study. We found that the T5 allele was present at a significantly higher rate in NOA patients than in the control group (5.00% versus 0.00%, p<0.01) with increased risk having NOA [Odds ratios (OR) 2.05, 95% confidence intervals (CI) 1.85-2.27]. The T5 variant was always accompanied by TG12 (10/10) and V470 allele participated in most TG12T5 haplotypes (8/10). TG12T5-V470 haplotype also enhanced risk of having NOA [OR 2.04, 95% CI 1.84-2.26]. F508del and R117H mutations were not found in either group. In conclusion, the polyvariant mutant genes of CFTR: T5 allele and TG12-T5-V470 genotype are correlated with NOA, but F508del and R117H mutations have low possibility to be associated with NOA.

Epidemiology and genetics of cystic fibrosis in Asia: In preparation for the next-generation treatments

Cystic fibrosis (CF) in the Asian population is less frequently reported due to under-diagnosis and lack of centralized CF patient registries. Clinical studies on CF cases from Asia have documented a severe course of the disease. The spectrum of the cystic fibrosis transmembrane conductance regulator (CFTR) variants in this population is quite heterogeneous. In total, 166 variants have been reported on approximately 3700 Asian CF chromosomes. The frequency of F508del among Asians is low compared with Caucasians. Recent in vitro studies have shown promise of small molecule correction and potentiation of 45 different CFTR variants. Of these variants, 16 (including G551D and F508del) have also been observed among Asian CF individuals. We suggest undertaking molecular studies extensively to annotate CFTR variants that will help Asian CF individuals to benefit from the precision medicine gaining momentum in the Western countries.

Application of Multiplex ARMS and SSCP/HD Analysis in Molecular Diagnosis of Cystic Fibrosis in Indian Patients

BACKGROUND

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. The most severe, DeltaF508, mutation accounts for nearly 70% of CF cases worldwide. Besides DeltaF508, there are other point mutations, namely G542X, G551D, R553X, N1303K, and 621+1(G-->T), which are common among Caucasians. Additionally, a polyT polymorphism in intron 8 is also involved in the pathogenesis of CF. However, neither the prevalence nor the types of mutations causing CF in India are known. In this study, we aimed at estimating the frequency of the above common mutations and polymorphism in clinically suspected CF cases. We also carried out partial analysis of the CFTR gene, limited to exons 10 and 11, to identify other variations in these exons.

METHODS

The multiplex amplification refractory mutation system (ARMS) test was applied for rapid simultaneous analysis of six most common CF mutations, in 100 normal and 39 elevated sweat chloride cases. The scanning of exons 10 and 11 was carried out by single-stranded conformation polymorphism/heteroduplex (SSCP/HD) analysis, followed by DNA sequencing in 50 normal and 37 elevated sweat chloride cases. A single ARMS-polymerase chain reaction assay was used to distinguish the 5T, 7T, and 9T alleles in 100 normal and 33 elevated sweat chloride cases.

RESULTS

The multiplex ARMS analysis identified the DeltaF508 mutation at an allele frequency of 24% in Indian CF cases. However, the other predominant CF mutations were found to be absent. The 7T polyT variant was observed to be the most common allele, followed by the 9T, and 5T, which was the lowest. The DeltaF508 mutation was observed in all instances with the 9T variant. The SSCP/HD and DNA sequencing additionally revealed a known polymorphism (M470V, exon 10) and a known mutation [1525-1(G-->A), intron 9]. The 1525-1(G-->A) mutation, observed in a single 4-year-old male, is predicted to code for a class II defective CFTR protein.

CONCLUSION

The findings of this study suggest a difference in relative frequencies and spectrum of CFTR mutations in Indian CF cases. A larger screening study of the entire CFTR gene in clinically well defined CF cases is required to delineate common mutations in the CFTR gene and enable molecular diagnosis of CF in India.

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) in Chinese patients with congenital bilateral absence of vas deferens

BACKGROUND

Genetic testing of the cystic fibrosis transmembrane conductance (CFTR) gene is currently performed in patients with congenital bilateral absence of vas deferens (CBAVD). This study was conducted to investigate the role of mutations in the CFTR gene in CBAVD-dependent male infertility.

METHODS

73 Chinese patients diagnosed with CBAVD were studied. The entire coding regions and splice sites of 27 exons of the CFTR gene were sequenced in 146 chromosomes from the 73 CBAVD patients. Screening was carried out using PCR, gel electrophoresis and DNA sequencing to identify novel variants of the entire coding regions and boundaries of the 27 exons.

RESULTS

Five novel nonsynonymous mutations, three novel splice site mutations and one deletion were identified by sequencing. Apart from the novel variants, we also found 19 previously reported mutations and polymorphism sites. Thirty-four patients (46.57%) had the 5T variant (6 homozygous and 28 heterozygous) and in two of them it was not associated with any detectable mutation of the CFTR gene. All potential pathogenic mutations are not contained in the 1000 Genome Project database. In total, the present study identified 30 potential pathogenic variations in the CFTR gene, 9 of which had not previously been described.

CONCLUSIONS

Most patients with CBAVD have mutations in the CFTR gene. A mild genotype with one or two mild or variable mutations was observed in all the patients. These findings improve our understanding of the distribution of CFTR alleles in CBAVD patients and will facilitate the development of more sensitive CFTR mutation screening.

Newborn screening for cystic fibrosis in Alberta: Two years of experience

On April 1, 2007, Alberta became the first province in Canada to introduce cystic fibrosis (CF) to its newborn screening program. The Alberta protocol involves a two-tier algorithm involving an immunoreactive trypsinogen measurement followed by molecular analysis using a CF panel for 39 mutations. Positive screens are followed up with sweat chloride testing and an assessment by a CF specialist. Of the 99,408 newborns screened in Alberta during the first two years of the program, 221 had a positive CF newborn screen. The program subsequently identified and initiated treatment in 31 newborns with CF. A relatively high frequency of the R117H mutation and the M1101K mutation was noted. The M1101K mutation is common in the Hutterite population. The presence of the R117H mutation has created both counselling and management dilemmas. The ability to offer CF transmembrane regulator full sequencing may help resolve diagnostic dilemmas. Counselling and management challenges are created when mutations are mild or of unknown clinical significance.

Association between cystic fibrosis transmembrane conductance regulator gene mutations and susceptibility for childhood asthma in Korea

PURPOSE

Classic cystic fibrosis is now known part of cystic fibrosis transmembrane conductance regulator (CFTR)-related disorders. These include a wide spectrum, from multi-system disorders, such as cystic fibrosis, to mono-symptomatic conditions, such as chronic pancreatitis or congenital bilateral absence of the vas deferens. However, respiratory disease is considered typical for the multi system disorder, cystic fibrosis, and is the major cause of morbidity and mortality. The purpose of this study was to evaluate the potential effects of CFTR gene mutations in Korean children with asthma.

MATERIALS AND METHODS

We selected 14 mutations identified in Korea and each of the 48 children with and without asthma were genotyped for the case-control study.

RESULTS

No significant differences were found in genotype and allele frequencies of the 9 polymorphisms observed between the non-asthma and asthma groups. In a haplotype determination based on a Bayesian algorithm, 8 haplotypes were assembled in the 98 individuals tested. However, we also did not find any significant differences in haplotype frequencies between the non-asthma and asthma groups.

CONCLUSION

We have concluded that this study did not show any evidence in support of providing that CFTR genetic variations significantly contribute to the susceptibility of asthma in Korean children.

Mutations in the amiloride-sensitive epithelial sodium channel in patients with cystic fibrosis-like disease

We investigated whether mutations in the genes that code for the different subunits of the amiloride-sensitive epithelial sodium channel (ENaC) might result in cystic fibrosis (CF)-like disease. In a small fraction of the patients, the disease could be potentially explained by an ENaC mutation by a Mendelian mechanism, such as p.V114I and p.F61L in SCNN1A. More importantly, a more than three-fold significant increase in incidence of several rare ENaC polymorphisms was found in the patient group (30% vs. 9% in controls), indicating an involvement of ENaC in some patients by a polygenetic mechanism. Specifically, a significantly higher number of patients carried c.-55+5G>C or p.W493R in SCNN1A in the heterozygous state, with odds ratios (ORs) of 13.5 and 2.7, respectively.The p.W493R-SCNN1A polymorphism was even found to result in a four-fold more active ENaC channel when heterologously expressed in Xenopus laevis oocytes. About 1 in 975 individuals in the general population will be heterozygous for the hyperactive p.W493R-SCNN1A mutation and a cystic fibrosis transmembrane conductance regulator (CFTR) gene that results in very low amounts (0-10%) functional CFTR. These ENaC/CFTR genotypes may play a hitherto unrecognized role in lung diseases.

CLINICAL, ANDROLOGICAL AND GENETIC CHARACTERISTICS OF PATIENTS WITH CONGENITAL BILATERAL ABSENCE OF VAS DEFERENS (CBAVD)

Congenital bilateral absence of the vas deferens (CBAVD) is a form of infertility with an autosomal recessive genetic background in otherwise healthy males. In this study, we examined the clinical and cystic fibrosis transmembrane-conductance regulator (CFTR) gene mutations in sixty patients with bilateral absence of vas deferens that applied to andrology clinic due to male factor infertility. Urogenital anomalies of vas deferens, seminal vesicle and epididymis were detected in our patient group. CFTR gene mutations, which are known to be frequent among cystic fibrosis patients, could not be detected in our patient group with that high frequency. Delta F508 mutations were detected in only 6% of patients. IVS8 polyT alleles were positive in 68% of patients. No 1677delTA mutations and M470V variants were detected in our patient group. However, sperm retrieval is almost always possible from CBAVD patients; secondary pathologies may also result defective spermatogenesis.

Consensus on the use and interpretation of cystic fibrosis mutation analysis in clinical practice. J Cyst Fibros. 2008;7:179-196. [PMID: 18456578 DOI: 10.1016/j.jcf.2008.03.009]

It is often challenging for the clinician interested in cystic fibrosis (CF) to interpret molecular genetic results, and to integrate them in the diagnostic process. The limitations of genotyping technology, the choice of mutations to be tested, and the clinical context in which the test is administered can all influence how genetic information is interpreted. This paper describes the conclusions of a consensus conference to address the use and interpretation of CF mutation analysis in clinical settings. Although the diagnosis of CF is usually straightforward, care needs to be exercised in the use and interpretation of genetic tests: genotype information is not the final arbiter of a clinical diagnosis of CF or CF transmembrane conductance regulator (CFTR) protein related disorders. The diagnosis of these conditions is primarily based on the clinical presentation, and is supported by evaluation of CFTR function (sweat testing, nasal potential difference) and genetic analysis. None of these features are sufficient on their own to make a diagnosis of CF or CFTR-related disorders. Broad genotype/phenotype associations are useful in epidemiological studies, but CFTR genotype does not accurately predict individual outcome. The use of CFTR genotype for prediction of prognosis in people with CF at the time of their diagnosis is not recommended. The importance of communication between clinicians and medical genetic laboratories is emphasized. The results of testing and their implications should be reported in a manner understandable to the clinicians caring for CF patients.

Comparative analysis of common CFTR polymorphisms poly-T, TG-repeats and M470V in a healthy Chinese population

AIM: To investigate the three important cystic fibrosis transmembrane conductance regulator (CFTR) haplotypes poly-T, TG-repeats and the M470V polymorphisms in the Chinese population, and to compare their distribution with that in Caucasians and other Asian populations.

METHODS: Genomic DNA was extracted from blood leukocytes. Exons 9 and 10 of the CFTR gene were obtained through polymerase chain reaction (PCR). Exon 9 DNA sequences were directly detected by an automated sequencer and poly-T and TG-repeats were identified by direct sequence analysis. Pure exon 10 PCR-amplified products were digested by HphI restriction enzyme and the M470V mutation was detected by the AGE photos of digestion products.

RESULTS: T7 was the most common (93.6%) haplotype and the (TG)11 frequency of 57.2% and (TG)12 frequency of 40.9% were dominant haplotypes in the junction of intron 8 (IVS-8) and exon 9. The frequency of T5 was 3.8% and all T5 allele tracts (10 alleles) were joined with (TG)12. Four new alleles of T6 (1.5%) were found in three healthy individuals. In exon 10, the V allele (56.1%) was slightly more frequent than the M allele (43.9%), and the M/V (45.5%) was the dominant genotype in these individuals. The three major haplotypes T7-(TG)11-V470, T7-(TG)12-M470 and T7-TG11-M470 were related to nearly 86.0% of the population.

CONCLUSION: The polymorphisms of poly-T, TG-repeats, and M470V distribution were similar to those in other East Asians, but they had marked differences in frequency from those single haplotype polymorphisms or linkage haplotypes in Caucasians. Thus, they may be able to explain the low incidence of CF and CF-like diseases in Asians.

Cystic fibrosis in a southern Brazilian population: characteristics of 90% of the alleles

Cystic fibrosis (CF) is a genetic disease that frequently leads to death in infancy among Europeans and their descendants. The goals of the present study were to analyze the molecular aspects of CFTR gene characterizing mutations, their frequencies, and the haplotypes formed by four CFTR gene intragenic markers, IVS8-6(T)n, IVS8CA, IVS17bTA and IVS17bCA, in a southern Brazilian population of Caucasian origin. DNA samples from 56 non-related CF patients were analyzed using scanning techniques (single strand conformation polymorphism and denaturing gradient gel electrophoresis), restriction fragment length polymorphism and direct DNA sequencing to identify the mutations. Our results revealed a total of 25 different CF mutations representing nearly 90% of CF alleles, two being novel mutations. Microsatellite haplotypes were defined for CF and normal alleles. The mutational spectrum and the associated haplotypes described for the first time in this study should prove relevant for genetic counselling and CF population screening in Brazil. Moreover, our results suggest the presence of a major Mediterranean component in the contemporary Brazilian CF patient pool.

Genetic and geographical variability in cystic fibrosis: evolutionary considerations

We have studied cystic fibrosis from an evolutionary perspective by focusing on three main points: the high frequency of cystic fibrosis; the different spectrum of mutations that cause the disease in different areas of Europe; and the age of the delta F508 mutation. Several hypotheses have been suggested to explain the high frequency of cystic fibrosis. We argue that the most plausible is heterozygote resistance to cholera and to other dehydrating intestinal diseases. Different population sizes in the past could explain the higher diversity among cystic fibrosis mutations found in southern Europe. Three microsatellite loci are found close to the delta F508 mutation. The reconstruction of the original microsatellite haplotype in which delta F508 arose, and of the genealogy of the microsatellite haplotypes bearing the mutation, allowed us to date the mutation event at around 50,000 years ago.

Characterization of a recurrent novel large duplication in the cystic fibrosis transmembrane conductance regulator gene

Recently, DNA rearrangements in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have been described with increasing frequency. These large DNA rearrangements are not detected using conventional methods of DNA sequencing, single-strand conformational polymorphism, or denaturing high-performance liquid chromatography. We and others have described methods to detect such rearrangements in the CFTR gene. With one exception, all rearrangements reported thus far are single or multiple exon deletions, whereas only one report has described a large duplication. We describe here the detection and characterization of a novel large duplication in the CFTR gene. This duplication, referred to as gIVS6a + 415_IVS10 + 2987Dup26817bp, was detected in a classic CF female patient whose other mutation was DeltaF508. The duplication was inherited paternally. The duplication encompassed exons 6b to 10 and occurred on the IVS8-11TG/IVS8-7T/G1540 haplotype. This large duplication is predicted to result in the production of a truncated CFTR protein lacking the terminal part of NBD1 domain and beyond and thus can be considered a null allele. The combination of the DeltaF508 and gIVS6a + 415_IVS10 + 2987Dup26817bp mutation probably causes the severe CF phenotype in this patient. We designed a simple polymerase chain reaction test to detect the duplication, and we further detected the same duplication from another independent laboratory. The duplication breakpoint is identical in all three patients, suggesting a likely founder mutation.

Highly preferential association of NonF508del CF mutations with the M470 allele

BACKGROUND

On the basis of previous findings on random individuals, we hypothesized a preferential association of CF causing mutations with the M allele of the M470V polymorphic site of the CFTR gene.

METHODS

We have determined the M/V-CF mutation haplotype in a series of 201 North East Italian and 73 Czech CF patients who were not F508del homozygotes, as F508del was already known to be fully associated with the M allele.

RESULTS

Out of 358 not F508del CF genes, 84 carried the V allele and 274 the less common M allele. In the N-E Italian population, MM subjects have a risk of carrying a CF causing mutation 6.9x greater than VV subjects when F508del is excluded and 15.4x when F508del is included. In the Czech population a similar, although less pronounced, association is observed.

CONCLUSIONS

Besides the possible biological significance of this association, the possibility of exploiting it for a pilot screening program has been explored in a local North East Italian population for which CF patients were characterized for their CF mutation. General M470V genotyping followed by common CF mutation screening limited to couples in which each partner carries at least one M allele would need testing only 39% of the couples, which contribute 89% of the total risk, with a cost benefit.

A Frequent Large Rearrangement in theCFTRGene in Cystic Fibrosis Patients from Reunion Island

Reunion Island is a French province, 800 km east of Madagascar and 200 km west of Mauritius. On Reunion Island, the birth prevalence of cystic fibrosis (CF) is particularly high in the population of European origin, approximately 1:1000. In a previous study, we demonstrated that the screening of the 27 exons of the CF transmembrane conductance regulator (CFTR) gene by denaturing high-pressure liquid chromatography (DHPLC) in 114 CF families allowed the detection of about 93% of the molecular defects present on Reunion Island. Unidentified CF mutations may lie in introns or in regulatory regions that are not routinely investigated, or may correspond to gene rearrangements such as large, heterozygous deletions that escape detection using current PCR-based techniques. Using a combination of different methods (such as multiplex ligation-dependent probe amplification), 6 of the 13 unidentified CF alleles (46%) were found to harbor a deletion of 5288 bp, spanning from exon 17a to 18. Identification and examination of the breakpoint sequences showed that this deletion is different from the 3120+1kbdel8.6Kb previously found in the Palestinian Arabs. The chromosomes bearing IVS16+3316_IVS18+644del5288 did not have a common extragenic haplotype. Clinical evaluation of homozygotes (2 unrelated patients) and compound heterozygotes indicated that this deletion represents a severe mutation associated with positive sweat chloride test, pancreatic insufficiency, and early age at diagnosis.

Asthma and COPD in cystic fibrosis intron-8 5T carriers. A population-based study

Background

Carriers of cystic fibrosis intron-8 5T alleles with high exon-9 skipping could have increased annual lung function decline and increased risk for asthma or chronic obstructive pulmonary disease (COPD).

Methods

We genotyped 9131 individuals from the adult Danish population for cystic fibrosis 5T, 7T, 9T, and F508del alleles, and examined associations between 11 different genotype combinations, and annual FEV₁ decline and risk of asthma or COPD.

Results

5T heterozygotes vs. 7T homozygous controls had no increase in annual FEV₁ decline, self-reported asthma, spirometry-defined COPD, or incidence of hospitalization from asthma or COPD. In 5T/7T heterozygotes vs. 7T homozygous controls we had 90% power to detect an increase in FEV₁ decline of 8 ml, an odds ratio for self-reported asthma and spirometry-defined COPD of 1.9 and 1.7, and a hazard ratio for asthma and COPD hospitalization of 1.8 and 1.6, respectively. Both 5T homozygotes identified in the study showed evidence of asthma, while none of four 5T/F508del compound heterozygotes had severe pulmonary disease. 7T/9T individuals had annual decline in FEV₁ of 19 ml compared with 21 ml in 7T homozygous controls (t-test:P = 0.03). 6.7% of 7T homozygotes without an F508del allele in the cystic fibrosis transmembrane conductance regulator gene reported asthma vs. 11% of 7T/9T individuals with an F508del allele (χ²:P = 0.01) and 40% of 7T homozygotes with an F508del allele (P = 0.04). 7T homozygotes with vs. without an F508del allele also had higher incidence of asthma hospitalization (log-rank:P = 0.003); unadjusted and adjusted equivalent hazard ratios for asthma hospitalization were 11 (95%CI:1.5–78) and 6.3 (0.84–47) in 7T homozygotes with vs. without an F508del allele.

Conclusion

Polythymidine 5T heterozygosity is not associated with pulmonary dysfunction or disease in the adult Caucasian population. Furthermore, our results support that F508del heterozygosity is associated with increased asthma risk independently of the 5T allele.

Variations of the CFTR gene in the Hanoi-Vietnamese

In order to investigate polymorphic backgrounds of the cystic fibrosis transmembrane conductance regulator gene (CFTR) in the Vietnamese, we analyzed 495 blood samples of randomly selected healthy individuals in Hanoi for the delta F508 mutation and TG-repeats, poly-T, and M470V polymorphisms. We compared their distributions with those of Caucasians and other Asian populations. No delta F508 mutation was found, being consistent with the extremely low incidence of cystic fibrosis (CF) in Vietnam. Allele frequency of the T5 allele promoting exon 9 skipping was 0.037. Greater number of TG-repeats, which is known to facilitate this aberrant splicing, was a predominant trend in the Vietnamese and other Asians. A "T5-TG12-V470" haplotype was most common (29/37) among T5-bearing haplotypes. Three major haplotypes, T7-TG12-M470, T7-TG11-V470, and T7-TG12-V470, estimated by PHASE program, related to 92% of the population. This is the first study of the CFTR gene among the Vietnamese.

Gene SNPs and mutations in clinical genetic testing: haplotype-based testing and analysis

Haplotype-based analysis using high-density single nucleotide polymorphism (SNP) markers have gained increasing attention in evaluating candidate genes in various clinical situations. For example, haplotype information is useful for predicting the severity and prognosis of certain genetic disorders. The intragenic cis-interactions between the common polymorphisms and the pathogenic mutations of prion protein (PRNP) and cystic fibrosis transmembrane conductance regulator (CFTR) genes greatly influence the phenotypes and the disease penetrance of hereditary Creutzfeldt-Jakob disease and cystic fibrosis. Merits of haplotype study are more evident in the fine mapping of complex diseases and in identifying genetic variations that influence individual's response to drugs. Knowledge-based approaches and/or linkage analyses using SNP tagged haplotypes are effective tools in detecting genetic associations. For example, haplotype studies in the inflammatory bowel disease susceptibility loci revealed diverse cis and trans gene-gene interactions, which can affect the clinical outcomes. Although currently, we have very limited knowledge on haplotype-phenotypic characterizations of most genes, these examples demonstrate that increased understanding of the clinically relevant haplotypes will provide better results in the diagnosis and possibly in the treatment of both monogenic and polygenic diseases.

An exonic splicing enhancer offsets the atypical GU-rich 3' splice site of human apolipoprotein A-II exon 3

Human apolipoprotein A-II (apoA-II) intron 2/exon 3 junction shows a peculiar tract of alternating pyrimidines and purines (GU tract) that makes the acceptor site deviate significantly from the consensus. However, apoA-II exon 3 is constitutively included in mRNA. We have studied this unusual exon definition by creating a construct with the genomic fragment encompassing the whole gene from apoA-II and its regulatory regions. Transient transfections in Hep3B cells have shown that deletion or replacement of the GU repeats at the 3' splice site resulted in a decrease of apoA-II exon 3 inclusion, indicating a possible role of the GU tract in splicing. However, a 3' splice site composed of the GU tract in heterologous context, such as the extra domain A of human fibronectin or cystic fibrosis transmembrane conductance regulator exon 9, resulted in total skipping of the exons. Next, we identified the exonic cis-acting elements that may affect the splicing efficiency of apoA-II exon 3 and found that the region spanning from nucleotide 87 to 113 of human apoA-II exon 3 is essential for its inclusion in the mRNA. Overlapping deletions and point mutations (between nucleotides 91 and 102) precisely defined an exonic splicing enhancer (ESEwt). UV cross-linking assays followed by immunoprecipitation with anti-SR protein monoclonal antibodies showed that ESEwt, but not mutated ESE RNA, was able to bind both alternative splicing factor/splicing factor 2 and SC35. Furthermore, overexpression of both splicing factors enhanced exon 3 inclusion. These results show that this protein-ESE interaction is able to promote the incorporation of exon 3 in mRNA and suggest that they can rescue the splicing despite the noncanonical 3' splice site.

Bronchiectasis in adult patients: an expression of heterozygosity for CFTR gene mutations?

While all patients with cystic fibrosis (CF) have mutations in both CFTR alleles, often only one CFTR change is detected in patients with other lung disorders. The aim of this study was to investigate whether heterozygosity for CFTR mutations could be a determinant risk factor in the development of bronchiectasis in adult patients. We have performed the CFTR gene analysis in a cohort of 55 bronchiectasis adult patients with unknown etiology. The 5T variant (TG)m and the M470V polymorphisms were also analyzed. A general population in which the same molecular analysis was previously performed was used as the control group. The mutational spectrum of patients was also compared with that found in our CF population. CFTR mutations/variants were found in 20 patients (36%), 14 with only one mutant gene (25%). All six patients colonized by Staphylococcus aureus presented with at least one CFTR change (p = 0.001). No statistical significance was observed between patients with and without mutations for other clinical features. The 5T variant was found in four patients. Additionally, 90% of patients with mutations had the more functional M470 allele (p < 0.001). These results suggest the involvement of the CFTR gene in bronchiectasis of unknown etiology in adult patients.

Different CFTR mutational spectrum in alcoholic and idiopathic chronic pancreatitis?

OBJECTIVE

Cystic fibrosis transmembrane conductance regulator (CFTR) mutations are responsible for cystic fibrosis (CF) and have been postulated as a predisposing risk factor to chronic pancreatitis (CP), but controversial results demand additional support. We have therefore investigated the role of the CFTR gene in a cohort of 68 CP patients.

METHODS

We have performed the CFTR gene analysis using 2 screening techniques. Fragments showing abnormal migration patterns were characterized by sequencing. Patients were classified in alcoholic (ACP) (n = 37) and idiopathic (ICP) (n = 31) chronic pancreatitis. Clinical features of CP and CF were evaluated.

RESULTS

Sixteen mutations/variants were identified in 27 patients (40%), most of them (35%) presenting a single CFTR mutant gene. The 1716G/A variant showed the highest frequency accounting for 22% in ICP and 5% in ACP, in contrast with other more common mutations such as F508del found in 8% of ACP and the 5T variant identified in 7% of patients. Acute pancreatitis, abdominal pain, tobacco, pancreatic calcifications, and pancreatic pseudocysts showed significant higher values in ACP than ICP patients. No significant differences were found between patients with and without CFTR mutations.

CONCLUSIONS

Apart from reinforcing previous findings our data highlight the increased susceptibility of CFTR heterozygous to developing CP. Heterozygosity, combined with other factors, places these individuals at greater risk.

A T3 allele in theCFTR gene exacerbates exon 9 skipping in vas deferens and epididymal cell lines and is associated with Congenital Bilateral Absence of Vas Deferens (CBAVD)

The different alleles at the (TG)m(T)n polymorphic loci at the 3' end of the human CFTR intron 8 determine the efficiency by which exon 9 is spliced. We identified a novel TG12T3 allele in a congenital bilateral absence of vas deferens (CBAVD) patient who carries a [TG11T7; p.Phe508Cys; p.Met470Val] haplotype on the other chromosome. To better understand the complex regulation of exon 9 splicing, we analyzed the levels of correctly spliced CFTR transcripts in six CFTR-expressing epithelial cell lines derived from lung, colon, testis, vas deferens, and epididymis transiently transfected with four CFTR minigenes (pTG11T7, pTG12T7, pTG12T5, and pTG12T3). In this work, we show that a decrease in the Ts at the polymorphic locus in a TG12 background determines a cell-type dependent reduction in exon 9+ transcripts that is not related to the basal splicing efficiency in the cell line. These data emphasize the role of the T5 allele in CBAVD and identify the T3 allele as a severe cystic fibrosis (CF) disease-causing mutation. Finally, UV cross-linking experiments demonstrated that tissue-specific trans-acting splicing factors do not contribute to the different patterns of exon 9 splicing found between the cell lines. However, we observed that lower numbers of Ts can alter the binding of TDP-43 (TDP43 or TARDBP) to its specific target ug12 in a tissue-specific manner. Our results support the idea that the ratio of general splicing factors plays a role in the tissue variability of exon 9 alternative splicing.

A haplotype-based molecular analysis of CFTR mutations associated with respiratory and pancreatic diseases

Aberrant membrane transport caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with a wide spectrum of respiratory and digestive diseases as well as cystic fibrosis. Using a gene scanning method, we found 11 polymorphisms and mutations of the CFTR gene in the Korean population. Individual variants at these sites were analyzed by conventional DNA screening in 117 control and 75 patients having bronchiectasis or chronic pancreatitis. In a haplotype determination based on a Bayesian algorithm, 15 haplotypes were assembled in the 192 individuals tested. Several haplotypes, especially with Q1352H, IVS8 T5, and E217G, were found to have disease associations in a case-control study. Notably, a common polymorphism of M470V appears to affect the intensity of the disease association. Among the two haplotypes having IVS8 T5, the T5-V470 haplotype showed higher disease association than the T5-M470 haplotype. In addition, a Q1352H mutation found in a V470 background showed the strongest disease association. The physiological significances of the identified mutations were rigorously analyzed. Non-synonymous E217G and Q1352H mutations in the M470 background caused a 60-80% reduction in CFTR-dependent Cl(-) currents and HCO3(-) -transport activities. Surprisingly, the additional M470V polymorphic variant with the Q1352H mutation completely abolished CFTR-dependent anion transport activities. These findings provide the first evidence on the importance of CFTR mutations in the Asian population. Importantly, the results also reveal that interactions between multiple genetic variants in cis affect the final function of the gene products.

Cystic fibrosis mutation frequencies in an Irish population

The incidence of cystic fibrosis (CF) at birth in Ireland is 1/1461. Neonate CF genetic testing is not routinely performed in Ireland. Currently, screening is only carried out where there is clinical evidence or a family history to suggest disease. Here we report the frequencies of common CF mutations occurring in an Irish population composed of samples collected from western, mid-western and southern regions of Ireland. Rarer CF mutations were also identified in a selected number of CF patients. In addition, a number of polymorphisms were identified, some of which are reported to be functionally and phenotypically important.

Highest heterogeneity for cystic fibrosis: 36 mutations account for 75% of all CF chromosomes in Turkish patients

We analyzed the CFTR locus in 83 Turkish cystic fibrosis patients to identify mutations, haplotypes, and the carrier frequency in the population. We detected 36 different mutations in 125 (75%) of the total 166 CF chromosomes. Seven novel mutations were identified: four missense (K68E, Q493P, E608G, and V1147I), two splice-site (406 -3T > C and 3849 +5G > A), and one deletion (CFTRdele17b,18). The data showed that the Turkish population has the highest genetic heterogeneity at the CFTR locus reported so far. The results of this thorough molecular analysis at the CFTR locus of a population not of European descent shows that CF is not uncommon in all such populations. The large number of mutations present, as well as the high heterogeneity in haplotypes associated with the mutations suggests that most of the mutations have persisted for a long time in the population. Consistently, the carrier frequency is assessed to be high, indicating that the disease in the population is ancient.

Involvement of CFTR gene alterations in obstructive and nonobstructive infertility in men

There have not been many studies concerning CFTR gene alterations in nonobstructive causes of male infertility and subfertility, and in those that have been published, the results reported are not concordant. Therefore, we proposed to determine, in a representative unselected sample of men who were sent for microsurgical epididymal sperm aspiration, if different types of male infertility and impaired fertility were associated with CFTR gene alterations. We screened 80 men with idiopathic azoospermia, 50 men with severe oligozoospermia, 70 men with oligoasthenoteratozoospermia, and 7 men with congenital bilateral absence of the vas deferens (CBAVD), as well as 95 controls from Slovenia, for mutations in 10 CFTR exons that include the majority of the most common cystic fibrosis (CF) disease causing mutations. We also wanted to evaluate the risk for CF in children born after the intracytoplasmic sperm injection (ICSI) method of in vitro fertilization (IVF). No tested individual had mutations in both CFTR alleles. Altogether 13 different nucleotide alterations were identified. The frequencies of both CFTR gene alterations and polymorphisms did not differ significantly between the control group and men with idiopathic nonobstructive azoospermia and subfertility, but were significantly increased in men with CBAVD (DeltaF508, p = 0.039; IVS8-5T, p = 0.006). Our results suggest that CFTR mutations are not associated with errors in spermatogenesis and nonobstructive pathology of urogenital tract in men with any frequency. However, genetic counseling and CFTR mutation screening continue to be recommended for men with obstructive azoospermic conditions and their female partners.

Skipping of exon 9 of human CFTR in YAC-transgenic mice

Exon 9 of the human gene CFTR is skipped in some mRNA transcripts in human tissues. The level of skipping correlates with the number of TG's and T's in the 5' splice acceptor of exon 9. Poorly spliced alleles are associated with mild cystic fibrosis related phenotypes. Here we describe transgenic mice carrying a yeast artificial chromosome (YAC) with the intact human gene CFTR. When the YAC carries 10 TG's and 7 T's at the splice acceptor, there is about 50% skipping of exon 9 in most tissues, whereas 12 TG's and 5 T's give about 90% skipping. The level of skipping is quite uniform over many tissues, except the testis, in which there is a much higher level of correct splicing. These mice confirm that the TG(m)T(n) polymorphism has an effect on splicing and should be valuable for studying this phenomenon.

The molecular basis of cystic fibrosis in South Africa

The spectrum of CFTR mutations in three South African populations is presented. To date. a total of 192 white patients (384 chromosomes) with confirmed CF have been tested. deltaF508 accounts for 76% of the CF chromosomes in this group, with 3272-26A-->G, 394delTT and G542X occurring at the following frequencies: 4, 3.6 and 1.3%, respectively. A further 11 mutations account for 6% of CF chromosomes. A total of 91% of the CF-causing mutations can now be detected in the South African white population. Haplotype analysis suggests a founder effect in South Africans of European origin for the two common CFTR mutations, 3272-26A-->G and 394delTT. The diagnosis of CF has been confirmed in 14 coloured and 12 black CF patients. In the coloured population, both the deltaF508 and 3120 + 1G-->A mutations occur at appreciable frequencies of 43 and 29%, respectively. In the black population, the most common CF-causing mutation, the 3120 + 1G-->A mutation, occurs at an estimated frequency of 46%. Four other mutations have been detected, resulting in the identification of a total of 62.5% of mutations in this population.

Molecular analysis in Brazilian cystic fibrosis patients reveals five novel mutations

We have performed molecular genetic analyses on 160 Brazilian patients diagnosed with cystic fibrosis (CF). Screening of mutations in 320 CF chromosomes was performed through single strand conformation polymorphism (SSCP) and heteroduplex analyses assay followed by DNA sequencing of the 27 exons and exon/intron boundaries of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The frequency of CFTR variants of T-tract length of intron 8 (IVS8 Tn) was also investigated. This analysis enabled the detection of 232/320 CF mutations (72.2%) and complete genotyping of 61% of the patients. The deltaF508 mutation was found in 48.4% of the alleles. Another fifteen mutations (previously reported) were detected: G542X, R1162X, N1303K, R334W, W1282X, G58E, L206W, R553X, 621+1G-->T, V232D, 1717-1G-->A, 2347 delG, R851L, 2789+5G-->A, and W1089X. Five novel mutations were identified, V201M (exon 6a), Y275X (exon 6b), 2686 insT (exon 14a), 3171 delC (exon 17a), and 3617 delGA (exon 19). These results contribute to the molecular characterization of CF in the Brazilian population. In addition, the identification of the novel mutation Y275X allowed prenatal diagnosis in a high-risk fetus.

Heterogeneity for mutations in the CFTR gene and clinical correlations in patients with congenital absence of the vas deferens

Congenital absence of the vas deferens (CAVD) is a heterogeneous disorder, largely due to mutations in the cystic fibrosis (CFTR) gene. Patients with unilateral absence of the vas deferens (CUAVD) and patients with CAVD in association with renal agenesis appear to have a different aetiology to those with isolated CAVD. We have studied 134 Spanish CAVD patients [110 congenital bilateral absence of the vas deferens (CBAVD) and 24 CUAVD], 16 of whom (six CBAVD, 10 CUAVD) had additional renal anomalies. Forty-two different CFTR mutations were identified, seven of them being novel. Some 45% of the CFTR mutations were specific to CAVD, and were not found in patients with cystic fibrosis or in the general Spanish population. CFTR mutations were detected in 85% of CBAVD patients and in 38% of those with CUAVD. Among those patients with renal anomalies, 31% carried one CFTR mutation. Anomalies in seminal vesicles and ejaculatory ducts were common in patients with CAVD. The prevalence of cryptorchidism and inguinal hernia appeared to be increased in CAVD patients, as well as nasal pathology and frequent respiratory infections. This study confirms the molecular heterogeneity of CFTR mutations in CAVD, and emphasizes the importance of an extensive CFTR analysis in these patients. In contrast with previous studies, this report suggests that CFTR might have a role in urogenital anomalies.

Rapid characterization of the variable length polythymidine tract in the cystic fibrosis (CFTR) gene: association of the 5T allele with selected CFTR mutations and its incidence in atypical sinopulmonary disease

The CFTR intron 8 variable length polythymidine tract modulates the cystic fibrosis (CF) phenotype associated with the mutation R117H. To explore whether other mutations reside on multiple intron 8 backgrounds with discernible impacts on phenotype, we developed an allele-specific PCR assay to characterize this locus. Our approach types samples rapidly without the use or radioisotopes. Polythymidine alleles were identified for mutations either associated with a wide range of clinical phenotypes (R117H, R347P, G85E, D1152H, R334W, 2789 + 5 G > A, 3849 + 10kb C > T), and/or located at hypermutable CpG loci (R117H, 3845 + 10kb C > T, R553X, R334W, S945L and R75Q). R117H was detected in cis with each of three alleles (5T, 7T, 9T) at the intron 8 locus. The novel R117H-9T association was detected in a 10-month African-American male with borderline-to-mildly elevated sweat chloride values (approximately 50-66 mEq/L). All other mutations studied were associated with 7T except 3849 + 10kb C > T, which was detected on both 7T and 9T backgrounds, but not 5T. Three individuals with a delta F508/3849 + 10kb C > T genotype were 9T,9T and had pancreatic sufficiency and normal sweat chloride values, whereas 15 others who carried 3849 + 10kb C > T on a 7T background had variable pancreatic function (sufficient, n = 12, insufficient, n = 3), and variable sweat chloride values (normal, n = 12, elevated, n = 3). Surprisingly, when not associated with known CFTR mutations, 5T was detected with elevated frequency among individuals with sinopulmonary disease of ill-defined etiology, but with some characteristics of variant CF. In summary, the 5T allele was not found in cis with CF-causing mutations besides R117H, but an elevated 5T allele frequency in variant CF patients suggests 5T may be associated with disease in some situations.

Cystic fibrosis patients with the 3272-26A-->G mutation have mild disease, leaky alternative mRNA splicing, and CFTR protein at the cell membrane

We characterized the 3272-26A-->G mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, creating an alternative acceptor splice site in intron 17a, that competes with the normal one, although we predict from consensus values, with lower efficiency. We analyzed five Cystic Fibrosis (CF) Portuguese patients with the 3272-26A-->G/F508del genotype. Besides clinical and haplotype characterization of those patients, we report here results from CFTR transcript analysis in nasal brushings from all five patients. RT-PCR analysis supports alternative splicing in all patients and carriers, but not in controls. By sequencing, we determined that the alternative transcript includes 25 nucleotides from intron 17a, which predictively cause frameshift and a premature stop codon. The use of this alternative splice site causes a reduction in the levels of normal transcripts from the allele with this mutation and, most probably, of normal protein as well. By immunocytochemistry of both epithelial primary cell cultures and slices from CF polyps, CFTR protein is detected at the cell membrane, with three different antibodies. Ussing chamber analysis of one nasal polyp shows a high sodium absorption, characteristic of CF. Altogether, the results suggest that the main defect caused by the 3272-26A-->G mutation is a reduction in normal CFTR transcripts and protein and therefore this mutation should be included in class V, according to Zielenski and Tsui.

Complete genomic sequence of the human PK-L/R-gene includes four intragenic polymorphisms defining different haplotype backgrounds of normal and mutant PK-genes

The human pyruvate kinase L/R-gene has been completely sequenced in unrelated normal individuals and in pyruvate kinase-deficient patients by a PCR-based direct genomic sequencing approach and analyzed for polymorphisms. The total length of the gene is 8409 nucleotides. Four polymorphic sites have been detected: C/A1705 and C/T1992 in exon 12, a T-stretch in intron 1 occurring in the two polymorphic forms (T)10 and (T)19 and an (ATT)n microsatellite in intron J which has been found in the variation (ATT)11-17. Haplotype analysis using these four markers has been applied to trace the genetic background in PK-deficiencies. The results support the idea of a single origin of most of the individual PK-mutations.

Polyvariant mutant cystic fibrosis transmembrane conductance regulator genes. The polymorphic (Tg)m locus explains the partial penetrance of the T5 polymorphism as a disease mutation

In congenital bilateral absence of the vas deferens patients, the T5 allele at the polymorphic Tn locus in the CFTR (cystic fibrosis transmembrane conductance regulator) gene is a frequent disease mutation with incomplete penetrance. This T5 allele will result in a high proportion of CFTR transcripts that lack exon 9, whose translation products will not contribute to apical chloride channel activity. Besides the polymorphic Tn locus, more than 120 polymorphisms have been described in the CFTR gene. We hypothesized that the combination of particular alleles at several polymorphic loci might result in less functional or even insufficient CFTR protein. Analysis of three polymorphic loci with frequent alleles in the general population showed that, in addition to the known effect of the Tn locus, the quantity and quality of CFTR transcripts and/or proteins was affected by two other polymorphic loci: (TG)m and M470V. On a T7 background, the (TG)11 allele gave a 2.8-fold increase in the proportion of CFTR transcripts that lacked exon 9, and (TG)12 gave a sixfold increase, compared with the (TG)10 allele. T5 CFTR genes derived from patients were found to carry a high number of TG repeats, while T5 CFTR genes derived from healthy CF fathers harbored a low number of TG repeats. Moreover, it was found that M470 CFTR proteins matured more slowly, and that they had a 1.7-fold increased intrinsic chloride channel activity compared with V470 CFTR proteins, suggesting that the M470V locus might also play a role in the partial penetrance of T5 as a disease mutation. Such polyvariant mutant genes could explain why apparently normal CFTR genes cause disease. Moreover, they might be responsible for variation in the phenotypic expression of CFTR mutations, and be of relevance in other genetic diseases.

Higher proportion of intact exon 9 CFTR mRNA in nasal epithelium compared with vas deferens

The 5-thymidine (5T) variant of the cystic fibrosis transmembrane conductance regulator (CFTR) intron 8 polypyrimidine tract (IVS8-T tract) is the most frequent CFTR gene alteration identified in men with congenital bilateral absence of vas deferens (CBAVD). This alternative splicing variant gives rise to two transcripts, one normal with exon 9 intact and the other with in-frame deletion of exon 9. That CBAVD men usually have none of the other clinical signs of classical cystic fibrosis (CF) suggests less functional CFTR is produced in the reproductive tract than in other CF-associated organs. Nasal epithelia and segments of vas deferens were obtained from healthy, previously vasectomized men who presented for vasectomy reversal. Quantitative RT-PCR was performed on these specimens, with the region of CFTR cDNA spanning exon 9 amplified. For both nasal and vasal tissues, a strong positive correlation was found between the length of the IVS8-T tract and the proportion of mRNA with exon 9 intact. In addition, within the same subject, a significantly higher level of transcripts lacking exon 9 was found in vas deferens than nasal epithelia, regardless of the IVS8-T genotype. These findings suggest that the splicing of CFTR precursor mRNA is less efficient in vasal epithelia compared with respiratory epithelia. Thus, differential splicing efficiency between the various tissues which express CFTR provides one possible explanation for the reproductive tract abnormalities observed in infertile men with CFTR gene alterations but without other clinical manifestations of CF.

CFTR mutations and IVS8-5T variant in newborns with hypertrypsinaemia and normal sweat test

Neonates positive for immunoreactive trypsinogen assay (IRT) and negative for sweat test have formerly been found to carry the major cystic fibrosis (CF) mutation, delta F508, much more frequently than the general population. Among the 716 IRT positive newborns detected by a three tier (IRT, mutation analysis plus meconium lactase assay, sweat test) CF screening programme in north eastern Italy during the period January 1993 to March 1996, we found 45 carriers, a number significantly higher than the expected 17 (p < 0.001). We speculated that some of these heterozygotes could actually be affected by a very mild form of CF, and carry on the other chromosome an undetected CFTR mutation or a DNA variant, such as the 5-thymidine allele in intron 8 of the CFTR gene (IVS8-5T). This hypothesis was tested in four samples; group A (the 45 carriers mentioned above), group B (51 non-carrier, IRT positive neonates), group C (50 IRT negative neonates), and group D (90 CF adult female carriers). Chromosomes with IVS8-5T were seven (7.78%) in group A, seven (6.86%) in group B, five (5%) in group C, and four in group D (2.22%). The 5T prevalence in group A was significantly higher (p < 0.05) compared to group D; similarly, a higher (p < 0.05) 5T frequency in group A compared to group C was detected by considering the chromosomes free from CFTR mutations. This study is consistent with previous papers in finding among neonates with high trypsin levels a CF carrier frequency significantly higher than that expected. It is also suggested that in at least some babies raised trypsin levels at birth could be a phenotypic expression of compound heterozygosity for a major CF mutation plus IVS8-5T.

Haplotype analysis of 94 cystic fibrosis mutations with seven polymorphic CFTR DNA markers

We have analyzed 416 normal and 467 chromosomes carrying 94 different cystic fibrosis (CF) mutations with polymorphic genetic markers J44, IVS6aGATT, IVS8CA, T854, IVS17BTA, IVS17BCA, and TUB20. The number of mutations found with each haplotype is proportional to its frequency among normal chromosomes, suggesting that there is no preferential haplotype in which mutations arise and thus excluding possible selection for specific haplotypes. While many common mutations in the worldwide CF population showed absence of haplotype variation, indicating their recent origins, some mutations were associated with more than one haplotype. The most common CF mutations, delta F508, G542X, and N1303K, showed the highest number of slippage events at microsatellites, suggesting that they are the most ancient CF mutations. Recurrence was probably the case for 9 CF mutations (R117H, H199Y, R347YH, R347P, L558S, 2184insA, 3272-26A-->G, R1162X, and 3849 + 10kbC-->T). This analysis of 94 CF mutations should facilitate mutation screening and provides useful data for studies on population genetics of CF.

Mutations in the cystic fibrosis gene in patients with congenital absence of the vas deferens

BACKGROUND

Congenital bilateral absence of the vas deferens (CBAVD) is a form of male infertility in which mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have been identified. The molecular basis of CBAVD is not completely understood. Although patients with cystic fibrosis have mutations in both copies of the CFTR gene, most patients with CBAVD have mutations in only one copy of the gene.

METHODS

To investigate CBAVD at the molecular level, we have characterized the mutations in the CFTR gene in 102 patients with this condition. None had clinical manifestations of cystic fibrosis. We also analyzed a DNA variant (the 5T allele) in a noncoding region of CFTR that causes reduced levels of the normal CFTR protein. Parents of patients with cystic fibrosis, patients with types of infertility other than CBAVD, and normal subjects were studied as controls.

RESULTS

Nineteen of the 102 patients with CBAVD had mutations in both copies of the CFTR gene, and none of them had the 5T allele. Fifty-four patients had a mutation in one copy of CFTR, and 34 of them (63 percent) had the 5T allele in the other CFTR gene. In 29 patients no CFTR mutations were found, but 7 of them (24 percent) had the 5T allele. In contrast, the frequency of this allele in the general population was about 5 percent.

CONCLUSIONS

Most patients with CBAVD have mutations in the CFTR gene. The combination of the 5T allele in one copy of the CFTR gene with a cystic fibrosis mutation in the other copy is the most common cause of CBAVD: The 5T allele mutation has a wide range of clinical presentations, occurring in patients with CBAVD or moderate forms of cystic fibrosis and in fertile men.