Les génotypes responsables de mucoviscidose ou d’absence bilatérale des canaux déférents ABCD

Over the last decade, the genetic basis for CBAVD has been identified by its association with CFTR gene mutations, and CBAVD is now generally considered to be a mild or incomplete form of CF. In this review, the author summarizes the main results of compilation of CFTR gene analysis conducted in French laboratories for 3,923 patients with CF and 800 males with CABVD. The degree of clinical expression can be affected by several variables, including the molecular mechanisms by which the various CFTR mutations impair or disrupt the function of the CFTR chloride channel. Phenotypic expression of CFTR mutational genotypes varies from severe, progressive pulmonary disease with pancreatic insufficiency (CF-PI), to mild pulmonary disease with pancreatic sufficiency (PS) or singleorgan forms of “CFTR-opathies”. In CF, a total of 310 different CFTR mutations accounting for 94% of 7,846 CF alleles have generated almost 500 different genotypes, comprising 2 severe mutations in 88% of cases (CF-PI), one severe mutation in trans to a mild mutation in 11% (CF-PS), and 2 mild mutations in 1% of identified genotypes. In CBAVD, 137 mutations scattered over the whole gene were identified in 60% of 1,600 CBAVD alleles during the study. Among the 150 characterized mutational CFTR genotypes, compound heterozygosity was the rule, and the most frequent CBAVD combinations were ΔF508/5T (35%), ΔF508/other mutation (30%, including ΔF508/R117H-7T: 5,6%), and 5T/other mutation (17%). No combination of two severe mutations was found in CBAVD (0%); by contrast with the CF population, 88% of genotypes identified in CBAVD comprised a severe mutation in trans to a mild mutation, and 12% consisted of 2 mild mutations. A total of 22 genotypes were shared by both CF and CBAVD. The role of the 5T allele as a splicing variant with variable, incomplete disease penetrance in CBAVD is reviewed. Other haplotype backgrounds, such as the TG12 sequence and the M470V polymorphism, may influence CFTR splicing and/or function. This study confirms the high molecular heterogeneity of CFTR mutations in CBAVD and emphasizes the importance of extensive CFTR analysis in these patients. Longterm follow-up studies of CBAVD patients are necessary in order to predict the phenotypic consequences of numerous CFTR mutational genotypes.     

A large deletion causes apparent homozygosity for the D1152H mutation in the cystic fibrosis transmembrane regulator (CFTR) gene

We report the case of a patient with an apparent homozygosity for the D1152H mutation located in exon 18 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The parents had no personal history of cystic fibrosis (CF) and referred to our laboratory after the diagnosis of fetal bowel hyperechogenicity. The proband presented with meconium ileus and normal sweat chloride test. Sequencing of the CFTR exon 18 together with quantitative genomic assays, such as real-time PCR and the multiplex ligation probe amplification (MLPA) techniques, were performed and revealed that the father was heterozygous for the D1152H mutation and the mother carried a large deletion of the CFTR gene encompassing the genomic sequence including the same mutation. The child inherited D1152H from his father and the large deletion of the CFTR gene from his mother. We suggest that D1152H likely acts as a mild mutation with a dominant effect on the severe deletion of exon 18, considering that after 3 years of clinical examinations the child shows no classical signs and symptoms of CF. Not testing for large deletions in subjects with apparent homozygosity for a mutated CFTR allele could lead to the misidentification of CFTR mutation carrier status.     

Molecular basis of cystic fibrosis in Lithuania: incomplete CFTR mutation detection by PCR-based screening protocols

Mutational analysis of the cystic fibrosis transmembrane regulator (CFTR) gene was performed in 98 unrelated CF chromosomes from 49 Lithuanian CF patients through a combined approach in which the p.F508del mutation was first screened by allele-specific PCR while CFTR mutations in nonp.F508del chromosomes have been screened for by denaturing gradient gel electrophoresis analysis. A CFTR mutation was characterized in 62.2% of CF chromosomes, two of which (2.0%) have been previously shown to carry a large gene deletion CFTRdele2,3(21 kb). The most frequent Lithuanian CF mutation is p.F508del (52.0%). Seven CFTR mutations, p.N1303K (2.0%), p.R75Q (1.0%), p.G314R (1.0%), p.R553X (4.2%), p.W1282X (1.0%), and g.3944delGT (1.0%), accounted for 10.1% of Lithuanian CF chromosomes. It was not possible to characterize 35.8% of the CF Lithuanian chromosomes. Analysis of intron 8 (TG)mTn and M470V polymorphic loci did not permit the characterization of the CFTR dysfunction underlying the CF phenotype in the patients for which no CFTR mutation was identified. Thus, screening of the eight CFTR mutations identified in this study and of the large deletion CFTRdele2,3(21 kb) allows the implementation of an early molecular or confirmatory CF diagnosis for 65% of Lithuanian CF chromosomes.     

394delTT: a Nordic cystic fibrosis mutation

In a systematic screening for mutations in the gene encoding the cystic fibrosis transmembrane regulator among Danish cystic fibrosis (CF) patients, we identified a mutation in exon 3 (394delTT); this mutation was found to be relatively common in Denmark. We therefore screened for 394delTT in Sweden and Norway, where it turned out to be the second most frequent mutation, accounting for 4% of all CF mutations. It also occurs with a high frequency in Finland, but has not been found in larger surveys of mutations in the CFTR gene. Thus, 394delTT seems to be a specific Nordic CF mutation.     

Extensive Sequencing of the CFTR gene: lessons learned from the first 157 patient samples

Cystic fibrosis (CF) is one of the most common monogenic diseases affecting Caucasians and has an incidence of approximately 1:3,300 births. Currently recommended screening panels for mutations in the responsible gene (CF transmembrane regulator gene, CFTR) do not detect all disease-associated mutations. Our laboratory offers extensive sequencing of the CFTR (ABCC7) gene (including the promoter, all exons and splice junction sites, and regions of selected introns) as a clinical test to detect mutations which are not found with conventional screening. The objective of this report is to summarize the findings of extensive CFTR sequencing from our first 157 consecutive patient samples. In most patients with classic CF symptoms (18/24, 75%), extensive CFTR sequencing confirmed the diagnosis by finding two disease-associated mutations. In contrast, only 5 of 75 (7%) patients with atypical CF had been identified with two CFTR mutations. A diagnosis of CF was confirmed in 10 of 17 (58%) newborns with either positive sweat chloride readings or positive immunoreactive trypsinogen (IRT) screen results. We ascertained ten novel sequence variants that are potentially disease-associated: two deletions (c.1641AG>T, c.2949_2853delTACTC), seven missense mutations (p.S158T, p.G451V, p.K481E, p.C491S, p.H949L, p.T1036N, p.F1099L), and one complex allele ([p.356_A357del; p.358I]). We ascertained three other apparently novel complex alleles. Finally, several patients were found to carry partial CFTR gene deletions. In summary, extensive CFTR gene sequencing can detect rare mutations which are not found with other screening and diagnostic tests, and can thus establish a definitive diagnosis in symptomatic patients with previously negative results. This enables carrier detection and prenatal diagnosis in additional family members.     

Increased prevalence of CFTR mutations and variants and decreased chloride secretion in primary sclerosing cholangitis

Primary sclerosing cholangitis (PSC) and cystic fibrosis (CF) are both slowly progressive cholestatic liver diseases characterized by fibro-obliterative inflammation of the biliary tract. We hypothesized that dysfunction of the CF gene product, cystic fibrosis transmembrane conductance regulator (CFTR), may explain why a subset of patients with inflammatory bowel disease develop PSC. We prospectively evaluated CFTR genotype and phenotype in patients with PSC ( n=19) compared with patients with inflammatory bowel disease and no liver disease ( n=18), primary biliary cirrhosis ( n=17), CF ( n=81), and healthy controls ( n=51). Genetic analysis of the CFTR gene in PSC patients compared with disease controls (primary biliary cirrhosis and inflammatory bowel disease) demonstrated a significantly increased number of mutations/variants in the PSC group (37% vs 8.6% of disease controls, P=0.02). None of the PSC patients carried two mutations/variants. Of PSC patients, 89% carried the 1540G-variant-containing genotypes (resulting in decreased functional CFTR) compared with 57% of disease controls ( P=0.03). Only one of 19 PSC patients had neither a CFTR mutation nor the 1540G variant. CFTR chloride channel function assessed by nasal potential difference testing demonstrated a reduced median isoproterenol response of 14 mV in PSC patients compared with 19 mV in disease controls ( P=0.04) and 21 mV in healthy controls ( P=0.003). These data indicate that there is an increased prevalence of CFTR abnormalities in PSC as demonstrated by molecular and functional analyses and that these abnormalities may contribute to the development of PSC in a subset of patients with inflammatory bowel disease.     

A new mutation, 3905insT, accounts for 4.8% of 1173 CF chromosomes in Switzerland and causes a severe phenotype

We have analysed 1173 cystic fibrosis (CF) chromosomes from Switzerland for eight mutations in the CF transmembrane conductance regulator (CFTR) gene. This permitted the identification of 88.5% of all mutations present. A novel insertion mutation in exon 20 of the CFTR gene, 3905insT, was discovered. This mutation accounted for 4.8% of CFTR gene mutations in Switzerland and has since been identified in other populations of probable Swiss descent. It is associated with a highly variable clinical phenotype but always with pancreatic insufficiency. Haplotype analysis with three intragenic microsatellites in the CFTR gene showed that the mutation is associated with a haplotype rarely identified on other CFTR alleles and, therefore, that the frequency of the mutation in Switzerland is explained by a founder effect of a relatively recent mutation event. Received: 17 February 1997 / Accepted: 26 March 1977     

Exon 9 of the CFTR gene: splice site haplotypes and cystic fibrosis mutations

The alternatively spliced exon 9 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene codes for the initial part of the amino-terminal nucleotide-binding fold of CFTR. A unique feature of the acceptor splice site preceding this exon is a variable length polymorphism within the polypyrimidine tract influencing the extent of exon 9 skipping in CFTR mRNA. We investigated this repeat for its relationship to CFTR mutations and intragenic markers on 200 chromosomes from German patients with cystic fibrosis (CF). Four frequent length variations were strongly associated with the four predominant haplotypes previously defined by intragenic marker dimorphisms. One of these alleles displayed absolute linkage disequilibrium to the major CF mutation F508. Other frequent CFTR mutations were linked to one particular splice site haplotype indicating that differential exon 9 skipping contributes little to the clinical heterogeneity among CF patients with an identical mutation. We also identified a novel missense mutation (V456F) and a novel nonsense mutation (Q414X) within the coding region of exon 9. The missense mutation V456F adjacent to Walker motif A was present in a pancreas-sufficient CF patient. In contrast, the pancreas-insufficient Q414X/F508 compound heterozygote suffered from a severe form of the disease, indicating that alternative splicing of exon 9 does not overcome the deleterious effect of a stop codon within this exon.     

The ACE gene D/I polymorphism as a modulator of severity of cystic fibrosis

ABSTRACT: BACKGROUND: Cystic Fibrosis (CF) is a monogenic disease with complex expression because of the action of genetic and environmental factors. We investigated whether the ACE gene D/I polymorphism is associated with severity of CF. METHODS: A cross-sectional study was performed, from 2009 to 2011, at University of Campinas - UNICAMP. We analyzed 180 patients for the most frequent mutations in the CFTR gene, presence of the ACE gene D/I polymorphism and clinical characteristics of CF. RESULTS: There was an association of the D/D genotype with early initiation of clinical manifestations (OR: 1.519, CI: 1.074 to 2.146), bacterium Burkholderia cepacia colonization (OR: 3.309, CI: 1.476 to 6.256) and Bhalla score (BS) (p = 0.015). The association was observed in subgroups of patients which were defined by their CFTR mutation genotype (all patients; subgroup I: no mutation detected; subgroup II: one CFTR allele identified to mutation class I, II or III; subgroup III: both CFTR alleles identified to mutation class I, II and/or III). CONCLUSION: An association between the D allele in the ACE gene and the severity of CF was found in our study.     

Intra- and extragenic marker haplotypes of CFTR mutations in cystic fibrosis families

Summary In order to facilitate the screening for the less common mutations in the cystic fibrosis (CF) gene viz., the CF transmembrane conductance regulator gene (CFTR), marker haplotypes were determined for German nonCF (N) and CF chromosomes by polymerase chain reaction analysis of four polymorphisms upstream of the CF gene (XV-2c, KM.19, MP6-D9, J44) and six intragenic polymorphisms (GATT, TUB9, M470V, T854T, TUB18, TUB20) that span the CFTR gene from exon 6 through exon 21. Novel informative sequence variants of CFTR were detected in front of exons 10 (1525-61 A or G), 19 (3601-65 C or A), and 21 (4006-200 A or G). The CF locus exhibits strong long-range marker-marker linkage disequilibrium with breakpoints of recombination between XV-2c and KM.19, and between exons 10 and 19 of CFTR. Marker alleles of GATT-TUB9 and TUB18-TUB20 were found to be in absolute linkage disequilibrium. Four major haplotypes encompass more than 90% of German N and CF chromosomes. Fifteen CFTR mutations detected on 421 out of 500 CF chromosomes were each identified on one of these four predominant 7-marker haplotypes. Whereas all analysed F508 chromosomes carried the same KM.19-D9-J44-GATT-TUB9-M470V-T854T haplotype, another frequent mutation in Germany, R553X, was identified on two different major haplotypes. Hence, a priori haplotyping cannot exclude a particular CF mutation, but in combination with population genetic data, enables mutations to be ranked by decreasing probability.     

A 96-well formatted method for exon and exon/intron boundary full sequencing of the CFTR gene

Full genotypic characterization of subjects affected by cystic fibrosis (CF) is essential for the definition of the genotype-phenotype correlation as well as for the enhancement of the diagnostic and prognostic value of the genetic investigation. High-sensitivity diagnostic methods, capable of full scanning of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, are needed to enhance the significance of these genetic assays. A method for extensive sequencing of the CFTR gene was optimized. This method was applied to subjects clinically positive for CF and to controls from the general population of central Italy as well as to a single subject heterozygous for a mild mutation and with an uncertain diagnosis. Some points that are crucial for the optimization of the method emerged: a 96-well format, primer project and purification, and amplicon purification. The optimized method displayed a high degree of diagnostic sensitivity; we identified a subset of 13 CFTR mutations that greatly enhanced the diagnostic sensitivity of common methods of mutational analysis. A novel G1244R disease causing mutation, leading to a CF phenotype with pancreatic sufficiency but early onset of pulmonary involvement, was detected in the subject with an uncertain diagnosis. Some discrepancies between our results and previously published CFTR sequence were found.     

Spectrum of CFTR mutations in Mexican cystic fibrosis patients: identification of five novel mutations (W1098C, 846delT, P750L, 4160insGGGG and 297–1G→A)

We have analyzed 97 CF unrelated Mexican families for mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Our initial screening for 12 selected CFTR mutations led to mutation detection in 56.66% of the tested chromosomes. In patients with at least one unknown mutation after preliminary screening, an extensive analysis of the CFTR gene by single stranded conformation polymorphism (SSCP) or by multiplex heteroduplex (mHET) analysis was performed. A total of 34 different mutations representing 74.58% of the CF chromosomes were identified, including five novel CFTR mutations: W1098C, P750L, 846delT, 4160insGGGG and 297-1G-->A. The level of detection of the CF mutations in Mexico is still lower than that observed in other populations with a relatively low frequency of the deltaF508 mutation, mainly from southern Europe. The CFTR gene analysis described here clearly demonstrated the high heterogeneity of our CF population, which could be explained by the complex ethnic composition of the Mexican population, in particular by the strong impact of the genetic pool from southern European countries.     

CA/GT microsatellite alleles within the cystic fibrosis transmembrane conductance regulator (CFTR) gene are not generated by unequal crossingover.

The gene responsible for cystic fibrosis (CF) has recently been identified, and a three-nucleotide deletion (delta F508 mutation) that results in the loss of a phenylalanine residue in the first putative ATP-binding domain of the predicted protein (CF transmembrane conductance regulator, CFTR) has been found to be the major CF mutation. Although several other mutations have been identified in the CFTR gene, most of them are very rare, making their application to genetic diagnosis difficult. While characterizing the genomic region encompassing the CF locus, we have identified three CA/GT blocks that flank exon 9 of the CF gene. One of the CA/GT blocks exhibits a highly informative variable number of dinucleotide repeats (VNDR) polymorphism. This intragenic VNDR microsatellite should, by itself, provide full information for genetic analysis in approximately 80% of CF families and will help elucidate the associations between DNA polymorphism haplotypes and specific gene mutations. Haplotype analyses of CF chromosomes with and without the delta F508 mutation suggest that the different alleles are generated by slipped-strand mispairing within the dinucleotide repeat during DNA replication, rather than by unequal crossingover within a recombination hot spot.     

Effect of genotype on selected clinical features of Polish cystic fibrosis adults

Cystic fibrosis (CF), the most common autosomal recessive disorder of Caucasians, is caused by the mutations in the gene encoding CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) protein. Until now, approximately 1000 mutations of the CFTR gene have been described. The genotype-phenotype relationships in CF are still not completely understood. This study was undertaken in an attempt to characterise the distribution of CFTR mutations and their effect on selected clinical parameters in a group of Polish CF adults. A total number of 38 adult CF patients (mean age 21.6 +/- 6.8); 18 females & 20 males were enrolled in the study. The CFTR gene identification was conducted with the use of PCR & InnoLipa-CF set. The assessed clinical parameters included: age at diagnosis, age, lung function test, X-ray scored in Brasfield score, weight & height. We found that: (1) the genotypes of the studied population were unevenly distributed (65.8%- genotype deltaF508/M), (2) a high percentage of 3849+10kbC-->T was noted, (3) patients homozygous for the deltaF508 mutation were diagnosed significantly earlier and had a lower body mass index, (4) no differences were observed in the patients' length of life or the progression of lung disease. Conclusions: 1. In comparison to other populations, Polish adult CF patients display a relatively higher frequency of mild mutations. 2. Late diagnosis of CF in the studied group may be partially caused by a high percentage of CFTR mutations connected with the mild course of the disease that are difficult to identify. 3. Cystic fibrosis should be more commonly taken into consideration in the differential diagnosis in adult patients with milder symptoms.     

Reduced number of CFTR molecules in erythrocyte plasma membrane of cystic fibrosis patients

Cystic fibrosis (CF), the most common genetic disease among Caucasians, is caused by mutations in the gene encoding CFTR (cystic fibrosis transmembrane conductance regulator). The most frequent mutation, DeltaF508, results in protein misfolding and, as a consequence, prevents CFTR from reaching its final location at the cell surface. CFTR is expressed in various cell types including red blood cells. The functional role of CFTR in erythrocytes is still unclear. Since the number of CFTR copies in a single erythrocyte of healthy donors and CF patients with a homozygous DeltaF508 mutation is unknown, we counted CFTR, localized in erythrocyte plasma membrane, at the single molecule level. A novel experimental approach combining atomic force microscopy with quantum-dot-labeled anti-CFTR antibodies, used as topographic surface markers, was employed to detect individual CFTR molecules. Analysis of erythrocyte plasma membranes taken from healthy donors and CF patients with a homozygous DeltaF508 mutation reveals mean (SEM) values of 698 (12.8) (n=542) and 172 (3.8) (n=538) CFTR molecules per red blood cell, respectively. We conclude that erythrocytes reflect the CFTR status of the organism and that quantification of CFTR in a blood sample could be useful in the diagnosis of CFTR related diseases.     

Genotype-phenotype correlation in cystic fibrosis patients bearing [H939R;H949L] allele

Cystic fibrosis (CF) is caused by CFTR (cystic fibrosis transmembrane conductance regulator) gene mutations. We ascertained five patients with a novel complex CFTR allele, with two mutations, H939R and H949L, inherited in cis in the same exon of CFTR gene, and one different mutation per patient inherited in trans in a wide population of 289 Caucasian CF subjects from South Italy. The genotype-phenotype relationship in patients bearing this complex allele was investigated. The two associated mutations were related to classical severe CF phenotypes.     

Increased folding and channel activity of a rare cystic fibrosis mutant with CFTR modulators

Cystic fibrosis (CF) is a lethal recessive genetic disease caused by mutations in the CFTR gene. The gene product is a PKA-regulated anion channel that is important for fluid and electrolyte transport in the epithelia of lung, gut, and ducts of the pancreas and sweat glands. The most common CFTR mutation, ΔF508, causes a severe, but correctable, folding defect and gating abnormality, resulting in negligible CFTR function and disease. There are also a large number of rare CF-related mutations where disease is caused by CFTR misfolding. Yet the extent to which defective biogenesis of these CFTR mutants can be corrected is not clear. CFTRV232D is one such mutant that exhibits defective folding and trafficking. CFTRΔF508 misfolding is difficult to correct, but defective biogenesis of CFTRV232D is corrected to near wild-type levels by small-molecule folding correctors in development as CF therapeutics. To determine if CFTRV232D protein is competent as a Cl(-) channel, we utilized single-channel recordings from transfected human embryonic kidney (HEK-293) cells. After PKA stimulation, CFTRV232D channels were detected in patches with a unitary Cl(-) conductance indistinguishable from that of CFTR. Yet the frequency of detecting CFTRV232D channels was reduced to ～20% of patches compared with 60% for CFTR. The folding corrector Corr-4a increased the CFTRV232D channel detection rate and activity to levels similar to CFTR. CFTRV232D-corrected channels were inhibited with CFTR(inh-172) and stimulated fourfold by the CFTR channel potentiator VRT-532. These data suggest that CF patients with rare mutations that cause CFTR misfolding, such as CFTRV232D, may benefit from treatment with folding correctors and channel potentiators in development to restore CFTRΔF508 function.     

Identification of a nonframeshift 84-bp deletion in exon 13 of the cystic fibrosis gene.

Cystic fibrosis (CF) is the most frequent autosomal recessive inherited disorder in Caucasian populations. The disease is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. We have identified an 84-bp deletion in exon 13 of the CFTR gene, detected by DNA amplification and direct sequencing of 500 bp of the 5' end of exon 13. The deletion was in the maternal allele of a CF patient bearing the delta F508 deletion in the father's allele. The same 84-bp deletion could also be detected in the patient's mother. The deletion spanned from a four-A cluster in positions 1949-1952 to another four-A cluster in positions 2032-2035, including 84 bp which correspond to codons 607-634 (1949del84). The reported mutation would result in the loss of 28 amino acid residues of the R domain of the CFTR protein.     

Distinct spectrum of CFTR gene mutations in congenital absence of vas deferens

Congenital absence of the vas deferens (CAVD) is a frequent cause for obstructive azoospermia and accounts for 1%–2% of male infertility. A high incidence of mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene has recently been reported in males with CAVD. We have investigated a cohort of 106 German patients with congenital bilateral or unilateral absence of the vas deferens for mutations in the coding region, flanking intron regions and promotor sequences of the CFTR gene. Of the CAVD patients, 75% carried CFTR mutations or disease-associated CFTR variants, such as the “5T” allele, on both chromosomes. The distribution of mutation genotypes clearly differed from that observed in cystic fibrosis. None of the CAVD patients was homozygous for ΔF508 and none was compound heterozygous for ΔF508 and a nonsense or frameshift mutation. Instead, homozygosity was found for a few mild missense or splicing mutations, and the majority of CAVD mutations were missense substitutions. Twenty-one German CAVD patients were compound heterozygous for ΔF508 and R117H, which was the most frequent CAVD genotype in our study group. Haplotype analysis indicated a common origin for R117H in our population, whereas another frequent CAVD mutation, viz. the “5T allele” was a recurrent mutation on different intragenic haplotypes and multiple ethnic backgrounds. We identified a total of 46 different mutations and variants, of which 15 mutations have not previously been reported. Thirteen novel missense mutations and one unique amino-acid insertion may be confined to the CAVD phenotype. A few splice or missense variants, such as F508C or 1716 G→A, are proposed here as possible candidate CAVD mutations with an apparently reduced penetrance. Clinical examination of patients with CFTR mutations on both chromosomes revealed elevated sweat chloride concentrations and discrete symptoms of respiratory disease in a subset of patients. Thus, our collaborative study shows that CAVD without renal malformation is a primary genital form of cystic fibrosis in the vast majority of German patients and links the particular expression of clinical symptoms in CAVD with a distinct subset of CFTR mutation genotypes. Received: 15 April 1997 / Accepted: 29 April 1997