Cystic fibrosis transmembrane conductance regulator splice variants are not conserved and fail to produce chloride channels

Variation in CFTR-dependent ‘β-sweating’ among healthy adults

The genetic disease cystic fibrosis (CF) results when mutations in the gene for the anion channel CFTR reduce CFTR’s activity below a critical level. CFTR activity = N·P_O·γ (number of channels x open probability x channel conductance). Small molecules are now available that partially restore CFTR function with dramatic improvements in health of CF subjects. Continued evaluation of these and other compounds in development will be aided by accurate assessments of CFTR function. However, measuring CFTR activity in vivo is challenging and estimates vary widely. The most accurate known measure of CFTR activity in vivo is the ‘β/M’ ratio of sweat rates, which is produced by stimulation with a β-adrenergic agonist cocktail referenced to the same individual’s methacholine-stimulated sweat rate. The most meaningful metric of CFTR activity is to express it as a percent of normal function, so it is critical to establish β/M carefully in a population of healthy control subjects. Here, we analyze β/M from a sample of 50 healthy adults in which sweat rates to cholinergic and β-adrenergic agonists were measured repeatedly (3 times) in multiple, (~50) identified sweat glands from each individual (giving ~20,000 measurements). The results show an approximately 7-fold range, 26–187% of the WT average set to 100%. These provide a benchmark against which other measures of CFTR activity can be compared. Factors contributing to β/M variation in healthy controls are discussed.

Theratyping cystic fibrosis in vitro in ALI-culture and organoid models generated from patient-derived nasal epithelial Conditionally Reprogrammed Stem Cells

QUESTION

Cystic Fibrosis (CF) is due to pathogenic variants in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. Recent improvement enabled pharmacologic therapy aiming at restoring mutated CFTR expression and function. CFTR "modulators" have revolutionised the CF therapeutic landscape, particularly the last approved Trikafta. This drug-combination is indicated by FDA and very recently by EMA for genotypes carrying at least one copy of CFTR with F508del pathogenic variant. However, several genotypes, are not eligible for Trikafta treatment, yet.

MATERIALS/PATIENTS AND METHODS

We exploited an innovative cellular approach allowing highly efficient in vitro-expansion of airway epithelial stem cells (AESC) through conditional reprogramming (CRC) from nasal brushing of CF patients. This approach, coupled to development of AESC-derived personalised disease models, as organoids and air liquid interface (ALI) cultures, revealed highly suitable for CFTR pharmacological-testing.

RESULTS AND ANSWER TO THE QUESTION

We fully validated the experimental models and implemented the CFTR functional assays and biochemical CFTR protein characterisation, that allowed to evaluate the efficacy of clinically available modulators in restoring CFTR maturation and function of each patient-derived "avatar" (theratyping). F508del homozygous genotypes, used as controls, confirmed the higher clinical activity of Trikafta in comparison with older modulators. Trikafta showed its efficacy also on three rare genotypes previously not eligible for modulators-treatment, opening the way to clinical translation. Finally, encouraging results for innovative drug combinations were also obtained.

Novel mutation c.1210-3C > G in cis with a poly-T tract of 5T affects CFTR mRNA splicing in a Chinese patient with cystic fibrosis

Cystic fibrosis (CF) is a rare autosomal recessive disease with only one pathogenic gene cystic fibrosis transmembrane conductance regulator (CFTR). To identify the potential pathogenic mutations in a Chinese patient with CF, we conducted Sanger sequencing on the genomic DNA of the patient and his parents and detected all 27 coding exons of CFTR and their flanking intronic regions. The patient is a compound heterozygote of c.2909G > A, p.Gly970Asp in exon 18 and c.1210-3C > G in cis with a poly-T of 5T (T5) sequence, 3 bp upstream in intron 9. The splicing effect of c.1210-3C > G was verified via minigene assay in vitro, indicating that wild-type plasmid containing c.1210-3C together with T7 sequence produced a normal transcript and partial exon 10-skipping-transcript, whereas mutant plasmid containing c.1210-3G in cis with T5 sequence caused almost all mRNA to skip exon 10. Overall, c.1210-3C > G, the newly identified pathogenic mutation in our patient, in combination with T5 sequence in cis, affects the CFTR gene splicing and produces nearly no normal transcript in vitro. Moreover, this patient carries a p.Gly970Asp mutation, thus confirming the high-frequency of this mutation in Chinese patients with CF.

Elucidating clinical phenotypic variability associated with the polyT tract and TG repeats in CFTR

Biallelic pathogenic variants in CFTR manifest as cystic fibrosis (CF) or other CFTR-related disorders (CFTR-RDs). The 5T allele, causing alternative splicing and reduced protein activity, is modulated by the adjacent TG repeat element, though previous data have been limited to small, selective cohorts. Here, the risk and spectrum of phenotypes associated with the CFTR TG-T5 haplotype variants (TG11T5, TG12T5, and TG13T5) in the absence of the p.Arg117His variant are evaluated. Individuals who received physician-ordered next-generation sequencing of CFTR were included. TG[11-13]T5 variant frequencies (biallelic or with another CF-causing variant [CFvar]) were calculated. Clinical information reported by the ordering provider or the individual was examined. Among 548,300 individuals, the T5 minor allele frequency (MAF) was 4.2% (TG repeat distribution: TG11 = 68.1%, TG12 = 29.5%, TG13 = 2.4%). When present with a CFvar, each TG[11-13]T5 variant was significantly enriched in individuals with a high suspicion of CF or CFTR-RD (personal/family history of CF/CFTR-RD) compared to those with a low suspicion for CF or CFTR-RD (hereditary cancer screening, CFTR not requisitioned). Compared to CFvar/CFvar individuals, those with TG[11-13]T5/CFvar generally had single-organ involvement, milder symptoms, variable expressivity, and reduced penetrance. These data improve our understanding of disease risks associated with TG[11-13]T5 variants and have important implications for reproductive genetic counseling.

Congenital absence of the vas deferens: Cystic fibrosis transmembrane regulatory gene mutations

Congenital absence of the vas deferens (CAVD) is a rare genetic condition first discovered in the mid-18th century related to mutations in the cystic fibrosis transmembrane regulatory genes. The condition is typically found during work-up of male infertility, and the majority of cases can be diagnosed with complete history and physical examination and pertinent investigations. The condition can be separated into three subcategories, and genetic advances have led to a much better understanding behind the disease, its pathogenesis, and options for treatment. In this review, we discuss the genetics, pathogenesis, embryology, and diagnosis of treatment of CAVD. Future work in this area likely will aim to better understand the epigenetic factors that influence the development of the condition in order to identify potential upstream therapeutic targets.

Synchronous Periampullary Tumors in a Patient With Pancreas Divisum and Neurofibromatosis Type 1

INTRODUCTION

In this study, we describe for the first time a Neurofibromatosis type 1 patient with pancreas divisum, multiple periampullary tumors and germline pathogenic variants in NF1 and CFTR genes.

CASE REPORT

A 62-year-old female NF1 patient presented with weakness, choluria, nausea, and diffuse abdominal pain to an emergency room service. Magnetic resonance imaging revealed an abdominal mass involving the periampullary region and pancreas divisum. After surgical resection, three synchronous neoplasms were detected including two ampullary tumors (adenocarcinoma of the major ampulla and a neuroendocrine tumor of the minor ampulla) and a gastrointestinal stromal tumor (GIST). Germline multigene panel testing (MGPT) identified two pathogenic heterozygous germline variants: NF1 c.838del and CFTR c.1210-34TG[12]T[5].

CONCLUSION

This is the first report of a Neurofibromatosis type 1 patient with pancreas divisum and multiple periampullary tumors harboring pathogenic germline variants in NF1 and CFTR genes. The identification of two germline variants and a developmental anomaly in this patient may explain the unusual and more severe findings and underscores the importance of comprehensive molecular analyses in patients with complex phenotypes.

Frequency of Cystic Fibrosis Transmembrane Conductance Regulator Variants in Individuals Evaluated for Primary Ciliary Dyskinesia

OBJECTIVE

To evaluate whether cystic fibrosis transmembrane conductance regulator (CFTR) variants are more common among individuals tested for primary ciliary dyskinesia (PCD) compared with controls.

STUDY DESIGN

Data were studied from 1021 individuals with commercial genetic testing for suspected PCD and 91 777 controls with genetic testing at the same company (Invitae) for symptoms/diseases unrelated to PCD or CFTR testing. The prevalence of CFTR variants was compared between controls and each of 3 groups of individuals tested for PCD (PCD-positive, -uncertain, and -negative molecular diagnosis).

RESULTS

The prevalence of 1 pathogenic CFTR variant was similar among the individual groups. When combining the PCD-uncertain and PCR-negative molecular diagnosis groups, there was a higher prevalence of single pathogenic CFTR variants compared with controls (P = .03). Importantly, >1% of individuals who had negative genetic testing results for PCD had 2 pathogenic CFTR variants (8 of 723), and the incidence of cystic fibrosis (CF) (2 pathogenic variants) is roughly 1 in 3000 individuals of Caucasian ethnicity (∼0.03%). This incidence was also greater than that of 2 pathogenic CFTR variants in the control population (0.09% [84 of 91 777]; P = 9.60 × 10^-16). These variants correlate with mild CFTR-related disease.

CONCLUSIONS

Our results suggest that a single pathogenic CFTR variant is not likely to be a PCD-mimetic, but ongoing studies are needed in individuals in whom PCD is suspected and genetic testing results are uncertain or negative. Furthermore, CF may be misdiagnosed as PCD, reflecting phenotypic overlap. Among individuals evaluated for PCD, CF should be considered in the differential even in the CF newborn screening era.

Compound heterozygous mutations in CFTR causing CBAVD in Chinese pedigrees

BACKGROUND

Congenital bilateral absence of the vas deferens (CBAVD) is an important cause of obstructive azoospermia and male infertility. Mutations of CFTR caused the majority of CBAVD cases, and ADGRG2 was recently identified as a new pathogenic gene. Yet, most of the genetic evidence came from sporadic cases, and only one mutation in CFTR can be found in patients.

METHODS

In present study, we collected two CBAVD pedigrees, each having two affected male siblings. We performed whole exome sequencing on all patients and validated all potential variants by Sanger sequencing.

RESULTS

We excluded ADGRG2 variants but identified compound heterozygous variants of CFTR in both families (NM_000492.3:c.1210-33_1210-6GT[13]T[5] and c.4056G>C;p.Gln1352Cys in pedigree 1, c.592G>C;p.Ala198Pro and c.3717G>A;p.Arg1239= in pedigree 2), which were subsequently validated by direct sequencing. c.1210-33_1210-6GT[13]T[5] (also known as IVS8-T5-TG13) was a known disease-causing variant causing the skipping of exon 9 of CFTR and inherited from the proband's mother. p.Gln1352Cys and Ala198Pro were rare or novel in public databases and predicted to be deleterious. The p.Arg1239= was a synonymous variant but located at the end of an exon, which was predicted to alter the splicing pattern.

CONCLUSION

Our study, in which compound heterozygous variants were identified in two pedigrees, provides more familial evidence that only recessive variants (homozygous or compound heterozygous) in CFTR cause CBAVD. Furthermore, whole exome sequencing may be utilized as a useful tool for mutation screening of genes causing CBAVD.

Congenital bilateral absence of the vas deferens as an atypical form of cystic fibrosis: reproductive implications and genetic counseling

Congenital bilateral absence of the vas deferens (CBAVD) is found in 1% to 2% of males with infertility and is present in 6% of obstructive azoospermia cases. Nearly 95% of men with cystic fibrosis (CF, an autosomal recessive disorder) have CBAVD. There are genetic links between CBAVD and CF. Some mutations in the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR) can lead to CBAVD as a monosymptomatic form of CF. With the use of assisted reproductive techniques (ART), especially testicular or epididymal sperm aspiration, intracytoplasmic sperm injection, and in vitro fertilization, it is possible that men with CBAVD can produce offspring. Therefore, genetic counseling should be offered to couples undergoing ART to discuss the probability of having offspring that carry CFTR gene mutations. The aim of this review was to present the main cause of CBAVD, to call attention to its implications for assisted reproduction, and to show the importance of genetic counseling for couples where men have CBAVD, as they can have offspring with a lethal disease.

The CFTR M470V, intron 8 poly-T, and 8 TG-repeats detection in Chinese males with congenital bilateral absence of the vas deferens

Purpose. To evaluate the significance of molecular detection of cystic fibrosis transmembrane conductance regulator (CFTR) M470V, intron 8 poly-T, and intron 8 TG-repeats in congenital bilateral absence of the vas deferens (CBAVD). Methods. Eighty-nine male patients with CBAVD and 103 healthy males were included in this study. Polymerase chain reaction was performed to amplify the polymorphic regions using primers from conserved regions. M470V was genotyped using real-time PCR by cycling probe. The exon 9 DNA sequence was determined using an automated sequencer. TG-repeats and poly-T were identified by direct sequencing analysis. Results. The 5T allele distribution was 0.32, 0.66 for 7T, and 0.02 for 9T in CBAVD males, respectively. In contrast, the 5T allele distribution was 0.03, 0.96 for 7T, and 0.01 for 9T in healthy control. Study of the polymorphisms of the upstream of exon 9 revealed a higher frequency of 5T allele in the CBAVD males. All cases with TG13T5 haplotype and TG12T5 homozygous led to CBAVD. The CFTR TG12T5-V470 variant haplotype was associated with CBAVD. Conclusion. The 5T allele of intron 8 of CFTR has clinically significant association with CBAVD. TG13T5 and TG12T5 homozygously led to CBAVD, and TG12T5-V470 may also lead to CBAVD.

Disease-relevant proteostasis regulation of cystic fibrosis transmembrane conductance regulator

Mismanaged protein trafficking by the proteostasis network contributes to several conformational diseases, including cystic fibrosis, the most frequent lethal inherited disease in Caucasians. Proteostasis regulators, as cystamine, enable the beneficial action of cystic fibrosis transmembrane conductance regulator (CFTR) potentiators in ΔF508-CFTR airways beyond drug washout. Here we tested the hypothesis that functional CFTR protein can sustain its own plasma membrane (PM) stability. Depletion or inhibition of wild-type CFTR present in bronchial epithelial cells reduced the availability of the small GTPase Rab5 by causing Rab5 sequestration within the detergent-insoluble protein fraction together with its accumulation in aggresomes. CFTR depletion decreased the recruitment of the Rab5 effector early endosome antigen 1 to endosomes, thus reducing the local generation of phosphatidylinositol-3-phosphate. This diverts recycling of surface proteins, including transferrin receptor and CFTR itself. Inhibiting CFTR function also resulted in its ubiquitination and interaction with SQSTM1/p62 at the PM, favoring its disposal. Addition of cystamine prevented the recycling defect of CFTR by enhancing BECN1 expression and reducing SQSTM1 accumulation. Our results unravel an unexpected link between CFTR protein and function, the latter regulating the levels of CFTR surface expression in a positive feed-forward loop, and highlight CFTR as a pivot of proteostasis in bronchial epithelial cells.

Association of cystic fibrosis transmembrane conductance regulator gene variants with acute lung injury in African American children with pneumonia*

OBJECTIVES

The cystic fibrosis transmembrane conductance regulator regulates fluid balance in alveolar epithelial cells and appears to modulate the inflammatory response. To determine whether more severe lung injury in children who develop community-acquired pneumonia is associated with variations known to affect function in the gene coding for cystic fibrosis transmembrane conductance regulator.

DESIGN

A prospective cohort genetic association study of lung injury in children with community-acquired pneumonia.

SETTING

Three major tertiary care children's hospitals.

SUBJECTS

Caucasian and African American children with community-acquired pneumonia either evaluated in the emergency department or admitted to the hospital.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Caucasian and African American children with pneumonia were genotyped for the most common variants reported to affect cystic fibrosis transmembrane conductance regulator function, the p.508del mutation, the (TG)mTn variable repeat region, and the M470V polymorphism in the cystic fibrosis transmembrane conductance regulator gene. Genotypes and haplotypes were determined, and the association of high-risk alleles or high-risk haplotypes (defined as the presence of at least one variant known to decrease the level of functional cystic fibrosis transmembrane conductance regulator) with the need for mechanical ventilation or the development of acute lung injury was evaluated. Forty-two children in the Caucasian cohort (n = 304) required mechanical ventilation; 32 developed acute lung injury. Forty-three children in the African American cohort (n = 474) required mechanical ventilation; 29 developed acute lung injury. In African American children, high-risk (TG)mTn alleles known to result in decreased levels of functional cystic fibrosis transmembrane conductance regulator were associated with the need for mechanical ventilation (p = .0013) and the development of acute lung injury (p = .0061). Multivariable analysis demonstrated that high-risk (TG)mTn alleles were independently associated with mechanical ventilation (odds ratios = 3.19; 95% confidence interval, 1.63-6.26) and acute lung injury (odds ratios = 3.36; 95% confidence interval, 1.50-7.53) in African American children.

CONCLUSION

Genetic variation in cystic fibrosis transmembrane conductance regulator is associated with acute lung injury in African American children with community-acquired pneumonia.

Recommendations for the classification of diseases as CFTR-related disorders

Several diseases have been clinically or genetically related to cystic fibrosis (CF), but a consensus definition is lacking. Here, we present a proposal for consensus guidelines on cystic fibrosis transmembrane conductance regulator (CFTR)-related disorders (CFTR-RDs), reached after expert discussion and two dedicated workshops. A CFTR-RD may be defined as "a clinical entity associated with CFTR dysfunction that does not fulfil diagnostic criteria for CF". The utility of sweat testing, mutation analysis, nasal potential difference, and/or intestinal current measurement for the differential diagnosis of CF and CFTR-RD is discussed. Algorithms which use genetic and functional diagnostic tests to distinguish CF and CFTR-RDs are presented. According to present knowledge, congenital bilateral absence of vas deferens (CBAVD), acute recurrent or chronic pancreatitis and disseminated bronchiectasis, all with CFTR dysfunction, are CFTR-RDs.

CELF proteins regulate CFTR pre-mRNA splicing: essential role of the divergent domain of ETR-3

Cystic fibrosis is a prominent genetic disease caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Among the many disease-causing alterations are pre-mRNA splicing defects that can hamper mandatory exon inclusion. CFTR exon 9 splicing depends in part on a polymorphic UG(m)U(n) sequence at the end of intron 8, which can be bound by TDP-43, leading to partial exon 9 skipping. CELF proteins, like CUG-BP1 and ETR-3, can also bind UG repeats and regulate splicing. We show here that ETR-3, but not CUG-BP1, strongly stimulates exon 9 skipping, although both proteins bind efficiently to the same RNA motif as TDP-43 and with higher affinity. We further show that the skipping of this exon may be due to the functional antagonism between U2AF65 and ETR-3 binding onto the polymorphic U or UG stretch, respectively. Importantly, we demonstrate that the divergent domain of ETR-3 is critical for CFTR exon 9 skipping, as shown by deletion and domain-swapping experiments. We propose a model whereby several RNA-binding events account for the complex regulation of CFTR exon 9 inclusion, with strikingly distinct activities of ETR-3 and CUG-BP1, related to the structure of their divergent domain.

Functional properties and evolutionary splicing constraints on a composite exonic regulatory element of splicing in CFTR exon 12

In general, splicing regulatory elements are defined as Enhancers or Silencers depending on their positive or negative effect upon exon inclusion. Often, these sequences are usually present separate from each other in exonic/intronic sequences. The Composite Exonic Splicing Regulatory Elements (CERES) represent an extreme physical overlap of enhancer/silencer activity. As a result, when CERES elements are mutated the consequences on the splicing process are difficult to predict. Here, we show that the functional activity of the CERES2 sequence in CFTR exon 12 is regulated by the binding, in very close proximity to each other, of several SR and hnRNP proteins. Moreover, our results show that practically the entire exon 12 sequence context participate in its definition. The consequences of this situation can be observed at the evolutionary level by comparing changes in conservation of different splicing elements in different species. In conclusion, our study highlights how it is increasingly difficult to define many exonic sequences by simply breaking them down in isolated enhancer/silencer or even neutral elements. The real picture is close to one of continuous competition between positive and negative factors where affinity for the target sequences and other dynamic factors decide the inclusion or exclusion of the exon.

Markers of early disease and prognosis in COPD

COPD is a complex disease with multiple pathological components, which we unfortunately tend to ignore when spirometry is used as the only method to evaluate the disorder. Additional measures are needed to allow a more complete and clinically relevant assessment of COPD. The earliest potential risk factors of disease in COPD are variations in the genetic background. Genetic variations are present from conception and can determine lifelong changes in enzyme activities and protein concentrations. In contrast, measurements in blood, sputum, exhaled breath, broncho-alveolar lavage, and lung biopsies may vary substantially over time. This review explores potential markers of early disease and prognosis in COPD by examining genetic markers in the α₁-antitrypsin, cystic fibrosis transmembrane conductance regulator (CFTR), and MBL-2 genes, and by examining the biochemical markers fibrinogen and C-reactive protein (CRP), which correlate with degree of pulmonary inflammation during stable conditions of COPD. Chronic lung inflammation appears to contribute to the pathogenesis of COPD, and markers of this process have promising predictive value in COPD. To implement markers for COPD in clinical practice, besides those already established for the α₁-antitrypsin gene, further research and validation studies are needed.

Structural determinants of the cellular localization and shuttling of TDP-43

TDP-43 (also known as TARDBP) regulates different processes of gene expression, including transcription and splicing, through RNA and DNA binding. Moreover, recent reports have shown that the protein interacts with the 3'UTRs of specific mRNAs. The aberrant cellular distribution and aggregation of TDP-43 were recently reported in neurodegenerative diseases, namely frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). A detailed description of the determinants for cellular localization has yet to emerge, including information on how the known functions of TDP-43 and cellular targeting affect each other. We provide the first experimental evidence that TDP-43 continuously shuttles between nucleus and cytoplasm in a transcription-dependent manner. Furthermore, we investigate the role of the functional TDP-43 domains in determining cellular targeting through a combination of immunofluorescence and biochemical fractionation methods. Our analyses indicate that the C-terminus is essential for solubility and cellular localization, because its deletion results in the formation of large nuclear and cytoplasmic aggregates. Disruption of the RNA-recognition domain required for RNA and DNA binding, however, alters nuclear distribution by decreasing TDP-43 presence in the nucleoplasm. Our findings suggest that TDP-43 solubility and localization are particularly sensitive to disruptions that extend beyond the newly found nuclear localization signal and depend on a combination of factors that are closely connected to the functional properties of this protein.

Consensus on the use and interpretation of cystic fibrosis mutation analysis in clinical practice

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.

Analysis of Y chromosome microdeletions and CFTR gene mutations as genetic markers of infertility in Serbian men

BACKGROUND/AIM

Impaired fertility of a male partner is the main cause of infertility in up to one half of all infertile couples. At the genetic level, male infertility can be caused by chromosome aberrations or gene mutations. The presence and types of Y chromosome microdeletions and cystic fybrosis transmembrane conductance regulator (CFTR) gene mutations as genetic cause of male infertility was tested in Serbian men. The aim of this study was to analyze CFTR gene mutations and Y chromosome microdelations as potential causes of male infertility in Serbian patients, as well as to test the hypothesis that CFTR mutations in infertile men are predominantly located in the several last exons of the gene.

METHODS

This study has encompassed 33 men with oligo- or azoospermia. The screening for Y chromosome microdeletions in the azoospermia factor (AZF) region was performed by multiplex PCR analysis. The screening of the CFTR gene was performed by denaturing gradient gel electrophoresis (DGGE) method.

RESULTS

Deletions on Y chromosome were detected in four patients, predominantly in AZFc region (four of total six deletions). Mutations in the CFTR gene were detected on eight out of 66 analyzed chromosomes of infertile men. The most common mutation was F508del (six of total eight mutations).

CONCLUSION

This study confirmed that both Y chromosome microdeletions and CFTR gene mutations played important role in etiology of male infertility in Serbian infertile men. Genetic testing for Y chromosome microdeletions and CFTR gene mutations has been introduced in routine daignostics and offered to couples undergoing assisted reproduction techniques. Considering that both the type of Y chromosome microdeletion and the type of CFTR mutation have a prognostic value, it is recomended that AZF and CFTR genotyping should not only be performed in patients with reduced sperm quality before undergoing assisted reproduction, but also for the purpose of preimplantation and prenatal diagnostics in couples in which in vitro fertilization has been performed successfully.

CFTR 5T variant has a low penetrance in females that is partially attributable to its haplotype

PURPOSE

The study's purpose was to understand the molecular basis for different clinical phenotypes of the 5T variant, a tract of 5 thymidines in intron 8 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which disrupts processing of CFTR mRNA and reduces synthesis from the corresponding CFTR alleles.

METHOD

We analyzed the polymorphic TG dinucleotide repeat adjacent to the 5T variant in intron 8 and the codon 470 in exon 10. Patients selected for this study were positive for both the 5T variant and the major cystic fibrosis mutation, Delta F508. Almost all Delta F508 mutation alleles occur in a 10TG-9T-470M haplotype. Therefore, it is possible to determine the haplotype of the 5T variant in trans.

RESULTS

Of the 74 samples analyzed, 41 (55%) were 11TG-5T-470M, 31 (42%) were 12TG-5T-470V, and 2 (3%) were 13TG-5T-470M. Of the 49 cases for which we had clinical information, 17.6% of females (6/34) and 66.7% of males (10/15) showed symptoms resembling atypical cystic fibrosis. The haplotype with the highest penetrance in females (42% or 5/12) and more than 80% (5/6) in males is 12TG-5T-470V. We also evaluated 12 males affected with congenital bilateral absence of vas deferens and positive for the 5T variant; 10 of 12 had the 12TG-5T-470V haplotype.

CONCLUSION

Overall, the 5T variant has a milder clinical consequence than previously estimated in females. The clinical presentations of the 5T variant are associated with the 5T-12TG-470M haplotype.

Differential use of signal peptides and membrane domains is a common occurrence in the protein output of transcriptional units

Membrane organization describes the orientation of a protein with respect to the membrane and can be determined by the presence, or absence, and organization within the protein sequence of two features: endoplasmic reticulum signal peptides and alpha-helical transmembrane domains. These features allow protein sequences to be classified into one of five membrane organization categories: soluble intracellular proteins, soluble secreted proteins, type I membrane proteins, type II membrane proteins, and multi-spanning membrane proteins. Generation of protein isoforms with variable membrane organizations can change a protein's subcellular localization or association with the membrane. Application of MemO, a membrane organization annotation pipeline, to the FANTOM3 Isoform Protein Sequence mouse protein set revealed that within the 8,032 transcriptional units (TUs) with multiple protein isoforms, 573 had variation in their use of signal peptides, 1,527 had variation in their use of transmembrane domains, and 615 generated protein isoforms from distinct membrane organization classes. The mechanisms underlying these transcript variations were analyzed. While TUs were identified encoding all pairwise combinations of membrane organization categories, the most common was conversion of membrane proteins to soluble proteins. Observed within our high-confidence set were 156 TUs predicted to generate both extracellular soluble and membrane proteins, and 217 TUs generating both intracellular soluble and membrane proteins. The differential use of endoplasmic reticulum signal peptides and transmembrane domains is a common occurrence within the variable protein output of TUs. The generation of protein isoforms that are targeted to multiple subcellular locations represents a major functional consequence of transcript variation within the mouse transcriptome.

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.

Human, Drosophila, and C.elegans TDP43: Nucleic Acid Binding Properties and Splicing Regulatory Function

TAR DNA binding protein (TDP43), a highly conserved heterogeneous nuclear ribonucleoprotein, was found to down-regulate splicing of the exon 9 cystic fibrosis transmembrane conductance regulator (CFTR) through specific binding to a UG-rich polymorphic region upstream of the 3' splice site. Despite the emergence of new information regarding the protein's nuclear localization and splicing regulatory activity, TDP43's role in cells remains elusive. To investigate the function of human TDP43 and its homologues, we cloned and characterized the proteins from Drosophila melanogaster and Caenorhabditis elegans. The proteins from human, fly, and worm show striking similarities in their nucleic acid binding specificity. We found that residues at two different positions, which show a strong conservation among TDP43 family members, are linked to the tight recognition of the target sequence. Our three-dimensional model of TDP43 in complex with a (UG)(m) sequence predicts that these residues make amino acid side-chain to base contacts. Moreover, our results suggest that Drosophila TDP43 is comparable to human TDP43 in regulating exon splicing. On the other hand, C.elegans TDP43 has no effect on exon recognition. TDP43 from C.elegans lacks the glycine-rich domain found at the carboxy terminus of the other two homologues. Mutants of human and fly TDP43 devoid of the C-terminal domain are likewise unable to affect splicing. Our studies suggest that the glycine-rich domain is essential for splicing regulation by human and fly TDP43.

Synonymous mutations in CFTR exon 12 affect splicing and are not neutral in evolution

It is well established that exonic sequences contain regulatory elements of splicing that overlap with coding capacity. However, the conflict between ensuring splicing efficiency and preserving the coding capacity for an optimal protein during evolution has not been specifically analyzed. In fact, studies on genomic variability in fields as diverse as clinical genetics and molecular evolution mainly focus on the effect of mutations on protein function. Synonymous variations, in particular, are assumed to be functionally neutral both in clinical diagnosis and when measuring evolutionary distances between species. Using the cystic fibrosis transmembrane conductance regulator (CFTR) exon 12 splicing as a model, we have established that about one quarter of synonymous variations result in exon skipping and, hence, in an inactive CFTR protein. Furthermore, comparative splicing evaluation of mammalian sequence divergences showed that artificial combinations of CFTR exon 12 synonymous and nonsynonymous substitutions are incompatible with normal RNA processing. In particular, the combination of the mouse synonymous with the human missense variations causes exon skipping. It follows that there are two sequential levels at which evolutionary selection of genomic variants take place: splicing control and protein function optimization.

CFTR mutations and polymorphisms in male infertility

Apart from cystic fibrosis, mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene are also involved in congenital bilateral absence of the vas deferens (CBAVD). A mutation is identified in about 80% of the CFTR genes derived from CBAVD patients; the genetic defect in the remainder is yet unknown. In contrast to CF patients, when CFTR is involved, at least one of the mutant CFTR genes of CBAVD patients harbors a mild mutation. A polyvariant mutant CFTR gene is the most frequent CBAVD causing mutant CFTR gene. Here, combinations of particular alleles at several polymorphic loci yield insufficient functional CFTR. The fact that most CBAVD patients, that carry mutations on both CFTR genes, have no lung disease is most probably explained by tissue specific alternative splicing, which is increased in vas deferens compared to bronchial tissue. It has also been reported that CBAVD may be involved in other forms of infertility than CBAVD, however this has not always been confirmed in other studies. Because of techniques such as intracytoplasmic sperm injection, CBAVD patients are now able to father children, however such couples have an increased risk of having a child with cystic fibrosis, and therefore genetic testing and counselling should be provided.

Molecular pathology of the CFTR locus in male infertility

Congenital bilateral absence of the vas deferens (CBAVD) is a form of infertility with an autosomal recessive genetic background in otherwise healthy males. CBAVD is caused by cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations on both alleles in approximately 80% of cases. Striking CFTR genotypic differences are observed in cystic fibrosis (CF) and in CBAVD. The 5T allele is a CBAVD mutation with incomplete penetrance. Recent evidence confirmed that a second polymorphic locus exists and is a major CFTR modifier. The development of minigene models have led to results suggesting that CFTR exon 9 is skipped in humans because of unusual suboptimal 5' splice sites. An extremely rare T3 allele has been reported and it has recently been confirmed that the T3 allele dramatically increases exon 9 skipping and should be considered as a 'CF' mutation. Routine testing for the most prevalent mutations in the CF Caucasian population will miss most CFTR gene alterations, which can be detected only through exhaustive scanning of CFTR sequences. Finally, a higher than expected frequency of CFTR mutations and/or polymorphisms is now found in a growing number of monosymptomatic disorders, which creates a dilemma for setting nosologic boundaries between CF and diseases related to CFTR.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

The role of common single-nucleotide polymorphisms on exon 9 and exon 12 skipping in nonmutated CFTR alleles

Classic cystic fibrosis (CF) is caused by two loss-of-function mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, whereas patients with nonclassic CF have at least one copy of a mutant gene that retains partial function of the CFTR protein. In addition, there are several other phenotypes associated with CFTR gene mutations, such as idiopathic chronic pancreatitis. In CFTR-associated disorders and in nonclassic CF, often only one CFTR mutation or no CFTR mutations can be detected. In this study, we screened 23 patients with CFTR-associated disorders for CFTR mutations by complete gene testing and quantitative transcript analysis. Mutations were found in 10 patients. In cells from respiratory epithelium, we detected aberrant splicing of CFTR mRNA in all investigated individuals. We observed a highly significant association between the presence of coding single-nucleotide polymorphisms (coding SNPs, or cSNPs) and increased skipping of exon 9 and 12. This association was found both in patients and in normal individuals carrying the same cSNPs. The cSNPs c.1540A>G, c.2694T>G, and c.4521G>A may have affected pre-mRNA splicing by changing regulatory sequence motifs of exonic splice enhancers, leading to lower amounts of normal transcripts. The analysis of CFTR exons indicated that less frequent and weak exonic splicing enhancer (ESE) motifs make exon 12 vulnerable to skipping. The number of splice variants in individuals with cSNPs was similar to previously reported values for the T5 allele, suggesting that cSNPs may enhance susceptibility to CFTR related diseases. In addition, cSNPs may be responsible for variation in the phenotypic expression of CFTR mutations. Quantitative approaches rather than conventional genomic analysis are required to interpret the role of cSNPs.

Alternative splicing of the ovine CFTR gene

Alternative splicing of the human CFTR gene was studied previously and shown not to generate functional CFTR-like chloride ion channels. However, it is possible that some of the alternatively spliced forms may encode CFTR proteins with different functions. The ovine CFTR gene is very similar to the human gene and has regulatory mechanisms in common. To evaluate whether the alternatively spliced forms of human CFTR are conserved in the sheep, the splice forms of the ovine CFTR gene were examined. A transcript lacking exon 9 was observed in the sheep, but unlike the human exon 9-transcript, it did not result from a polymorphic intron 8 splice acceptor site. Sheep CFTR transcripts lacking exon 17b were seen and have also been described in the human. Transcripts lacking 98 bp of the 5' end of exon 13, the whole of exon 13, and both exons 14b and 15 respectively were seen in sheep but have not been reported in human. Splice site donor and acceptor sequences were isolated, and alternative transcripts were shown to result from a combination of aberrant sites and competition of 5' splice donor sequences.

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.

Missense, nonsense, and neutral mutations define juxtaposed regulatory elements of splicing in cystic fibrosis transmembrane regulator exon 9

Exonic sequence variations may induce exon inclusion or exclusion from the mature mRNA by disrupting exonic regulatory elements and/or by affecting a nuclear reading frame scanning mechanism. We have carried out a systematic study of the effect on cystic fibrosis transmembrane regulator exon 9 splicing of natural and site-directed sequence mutations. We have observed that changes in the splicing pattern were not related to the creation of premature termination codons, a fact that indicates the lack of a significant nuclear check of the reading frame in this system. In addition, the splice pattern could not be predicted by available Ser/Arg protein matrices score analysis. An extensive site-directed mutagenesis of the 3' portion of the exon has identified two juxtaposed splicing enhancer and silencer elements. The study of double mutants at these regulatory elements showed a complex regulatory activity. For example, one natural mutation (146C) enhances exon inclusion and overrides all of the downstream silencing mutations except for a C to G transversion (155G). This unusual effect is explained by the creation of a specific binding site for the inhibitory splicing factor hnRNPH. In fact, on the double mutant 146C-155G, the silencing effect is dominant. These results indicate a strict dependence between the two juxtaposed enhancer and silencer sequences and show that many point mutations in these elements cause changes in splicing efficiency by different mechanisms.

Fusion of FIG to the receptor tyrosine kinase ROS in a glioblastoma with an interstitial del(6)(q21q21)

The transmembrane proto-oncogene receptor tyrosine kinase (RTK) ROS is an orphan receptor that is aberrantly expressed in neoplasms of the central nervous system. Here, we report the fusion of its carboxy-terminal kinase domain to the amino-terminal portion of a protein called FIG (Fused in Glioblastoma) in a human glioblastoma multiforme (GBM). By characterizing both FIG and ROS genes in normal and in U118MG GBM cells, we determined that an intra-chromosomal homozygous deletion of 240 kilobases on 6q21 is responsible for the formation of the FIG-ROS locus. The FIG-ROS transcript is encoded by 7 FIG exons and 9 ROS-derived exons. We also demonstrate that the FIG-ROS locus encodes for an in-frame fusion protein with a constitutively active kinase activity, suggesting that FIG-ROS may act as an oncogene. This is the first example of a fusion RTK protein that results from an intra-chromosomal deletion, and it represents the first fusion RTK protein isolated from a human astrocytoma.

Atypical 5' splice sites cause CFTR exon 9 to be vulnerable to skipping

The molecular basis of the skipping of constitutive exons in many messenger RNAs is not fully understood. A well-studied example is exon 9 of the human cystic fibrosis transmembrane conductance regulator gene (CFTR), in which an abbreviated polypyrimidine tract between the branch point A and the 3' splice site is associated with increased exon skipping and disease. However, many exons, both in CFTR and in other genes and have short polypyrimidine tracts in their 3' splice sites, yet they are not skipped. Inspection of the 5' splice sites immediately up- and downstream of exon 9 revealed deviations from consensus sequence, so we hypothesized that this exon may be inherently vulnerable to skipping. To test this idea, we constructed a CFTR minigene and replicated exon 9 skipping associated with the length of the polypyrimidine tract upstream of exon 9. We then mutated the flanking 5' splice sites and determined the effect on exon skipping. Conversion of the upstream 5' splice site to consensus by replacing a pyrimidine at position +3 with a purine resulted in increased exon skipping. In contrast, conversion of the downstream 5' splice site to consensus by insertion of an adenine at position +4 resulted in a substantial reduction in exon 9 skipping, regardless of whether the upstream 5' splice site was consensus or not. These results suggested that the native downstream 5' splice site plays an important role in CFTR exon 9 skipping, a hypothesis that was supported by data from sheep and mouse genomes. Although CFTR exon 9 in sheep is preceded by a long polypyrimidine tract (Y(14)), it skips exon 9 in vivo and has a nonconsensus downstream 5' splice site identical to that in humans. On the other hand, CFTR exon 9 in mice is preceded by a short polypyrimidine tract (Y(5)) but is not skipped in vivo. Its downstream 5' splice site differs from that in humans by a 2-nt insertion, which, when introduced into the human CFTR minigene, abolished exon 9 skipping. Taken together, these observations place renewed emphasis on deviations at 5' splice sites in nucleotides other than the invariant GT, particularly when such changes are found in conjunction with other altered splicing sequences, such as a shortened polypyrimidine tract. Thus, careful inspection of entire 5' splice sites may identify constitutive exons that are vulnerable to skipping.

[Mutations and polymorphisms in CFTR genes in infertile men with oligospermia or azoospermia]

INTRODUCTION

Impaired infertility of the male partner is causative or contributOry to in up to one half of all couples unable to conceive spontaneously. A considerable number of genes are now known that have an essential function in human reproduction and which, when deleted or mutated, can cause pathologic changes in the male reproductive system. Congenital bilateral absence of the vas deferens (CBAVD) is an important cause of obstructive azoospermia in otherwise healthy men. It is also present in 95% of men with an autosomal recessive systematic disease--cystic fibrosis. However, clinically affected CF patients present a spectrum of genital phenotypes ranging from normal fertility to severely impaired spermatogenesis and CBAVD. Cystic fibrosis and most cases of CBAVD are caused by mutations in CFTR (cystic fibrosis transmembrane conductance regulator) gene. The aim of this study was to test the possible involvement of the CFTR gene in the aetiology of male infertility other than CBAVD.

METHODS

Twenty one infertile men with oligo or azoospermia were analysed for the presence of mutations and polymorphisms in the CFTR gene. Patients were divided in two groups according to the spermatogram: 1) patients with obstructive azoospermia (V < 2 mL, pH < 7.2, low level of a-glucosidase and fructose and absence of spermatozoa; 2) patients with impaired spermatogenesis or sperm maturation. We performed direct detection for the following mutations: delta F508 and delta 1507 (heteroduplex analysis), 621 + 1 G-->T, and N1303K (PSM--PCRmediated site-specific mutagenesis), A455E, 1717-1 G-->A, S549N, R560T, W1282X, R334W, R347P, R117H, 3849 + 10 kb C-->T and Tn, F508C, 1507V, 1506V polymorphisms (reverse dot blot method). G542X, R553X and GSS1D mutations were tested by SSCP (Single Strand Conformation Polymorphism). We also performed indirect detection of mutations and polymorphisms in 3, 5, 6a, 8, 9, 11, 12, 14a, 14b, 15, 17b, 18, 20, 21 and 23 exons by DGGE (Denaturant Gradient Gel Electrophoresis). Differences between frequencies were tested by chi-square statistic, p values of less than 0.05 were considered statistically significant.

RESULTS

Among 42 chromosomes from infertile men with oligo or azoospermia we detected 7 mutations in CFTR gene (16.7%), which was significantly (p = 0.0319) more frequent than in general population (2%). Frequency of 5T allele in analysed group was high (11.9%) compared to general population (5%), but not statistically significant (0.0938). The most common mutation in the group of 10 men with obstructive azoospermia was delta F508. It was detected on one chromosome in five patients. In three of these patients with 4F508 mutation on the other chromosome we found 5T allele on polymorphic Tn locus. In one patient, heterozygous for delta F508 mutation, 711 + 3 A-->G mutation on the other chromosome was detected. In the group of 11 infertile men with impaired spermatogenesis or sperm maturation we detected one mutation--delta F508. Two patients from this group had 5T variant on one chromosome.

DISCUSSION

We analysed 21 infertile men with oligo or azoospermia not caused by endocrine or inflammatory character, or chromosome mutations. Within this group frequency of CFTR mutations was increased compared to general population (p = 0.0319), suggesting that CFTR gene may be involved in the aetiology of infertility in men with oligo or azOospermia. In the group of patients with obstructive azoospermia 50% had at least one mutation, but only 10% had mutations in both chromosomes. One of the possible explanations would be that mutations are in the promoter region, introns or exons that were not included in analyses. The second explanation could be that some cases of obstructive azoospermia are only partially (or not) related to CFTR gene. In the group of patients with impaired spermatogenesis or sperm maturation, the frequencies of CFTR mutations and 5T allele were also increased compared to general population, but lower than in the group with obstructive azoospermia. This fact could mean that the influence of some other genes is higher in this condition than in the case of obstructive azoospermia.

CONCLUSION

We concluded that CFTR gene plays a role in the aetiology of obstructive azoospermia and that is also could be involved in some cases of impaired spermatogenesis and sperm maturation. Due to the high incidence of CFTR mutations in patients with obstructive azoospermia we suggest screening of CFTR mutations before assisted reproduction.

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.

Characterization and functional implications of the RNA binding properties of nuclear factor TDP-43, a novel splicing regulator of CFTR exon 9

Variations in a polymorphic (TG)m sequence near exon 9 of the human CFTR gene have been associated with variable proportions of exon skipping and occurrence of disease. We have recently identified nuclear factor TDP-43 as a novel splicing regulator capable of binding to this element in the CFTR pre-mRNA and inhibiting recognition of the neighboring exon. In this study we report the dissection of the RNA binding properties of TDP-43 and their functional implications in relationship with the splicing process. Our results show that this protein contains two fully functional RNA recognition motif (RRM) domains with distinct RNA/DNA binding characteristics. Interestingly, TDP-43 can bind a minimum number of six UG (or TG) single-stranded dinucleotide stretches, and binding affinity increases with the number of repeats. In particular, the highly conserved Phe residues in the first RRM region play a key role in nucleic acid recognition.

Identification of an alternative transcript of ABCA1 gene in different human cell types

We have observed two ABCA1 gene transcripts in human skin fibroblasts. The RT-PCR amplification of the exon 3-exon 8 region generated a normal fragment (740 bp) and an abnormal fragment (600 bp) in a ratio ranging from 3:1 to 8/9:1. These two transcripts were present in other cells such as leukemia T-cells, endothelial and smooth muscle cells as well human hepatoma cells (HepG2). Restriction enzyme analysis and sequencing indicated that in the abnormal fragment exon 3 was followed by exon 5. The complete skipping of exon 4 leads to a premature stop and a predicted translation product of 74 amino acids. The ratio between the normal and alternative transcript is not affected by variation in ABCA1 gene expression induced by incubating cells in serum-free medium and in the presence of cholesterol. It is possible that this alternative splicing represents as mechanism that regulates the ABCA1 content in tissues.

Nuclear factor TDP-43 and SR proteins promote in vitro and in vivo CFTR exon 9 skipping

Alternative splicing of human cystic fibrosis transmembrane conductance regulator (CFTR) exon 9 is regulated by a combination of cis-acting elements distributed through the exon and both flanking introns (IVS8 and IVS9). Several studies have identified in the IVS8 intron 3' splice site a regulatory element that is composed of a polymorphic (TG)m(T)n repeated sequence. At present, no cellular factors have been identified that recognize this element. We have identified TDP-43, a nuclear protein not previously described to bind RNA, as the factor binding specifically to the (TG)m sequence. Transient TDP-43 overexpression in Hep3B cells results in an increase in exon 9 skipping. This effect is more pronounced with concomitant overexpression of SR proteins. Antisense inhibition of endogenous TDP-43 expression results in increased inclusion of exon 9, providing a new therapeutic target to correct aberrant splicing of exon 9 in CF patients. The clinical and biological relevance of this finding in vivo is demonstrated by our characterization of a CF patient carrying a TG10T9(DeltaF508)/TG13T3(wt) genotype leading to a disease-causing high proportion of exon 9 skipping.

Genetic, andrological and clinical characteristics of patients with congenital bilateral absence of the vas deferens

The possibility of retrieving spermatozoa from the epididymis allows patients with congenital bilateral absence of the vas deferens (CBAVD) to father a child by means of assisted reproduction techniques. This has, however, increased the chance of transmitting a mutated allele of the cystic fibrosis transmembrane conductance regulator (CFTR) gene which increases the risk of generating offspring with cystic fibrosis (CF). Because of the increased heterogeneity of the CFTR locus, the study of a discrete number of mutations, as usually carried out in a diagnostic work-up, is unable to ascertain the presence of a mutation in a relatively high proportion of the patients screened. In an attempt to increase the chance of detecting the presence of CFTR gene abnormalities, 37 patients with CBAVD and one patient with congenital unilateral agenesis of the vas deferens (CUAVD) underwent an enlarged diagnostic protocol, which included screening for the most expected mutations of the CFTR gene in our population, evaluation of the five thymidine (5T) allelic variant, sweat test, respiratory function tests, evaluation of steatocrit, and an accurate evaluation of the history of the patient to search for symptoms commonly found in patients with CF. A single CFTR gene mutation was found in 18 patients (48.6%) with CBAVD and in the patient with CUAVD. The most frequent mutation observed was the Delta F508. Eleven patients (45.8%) had the 5T variant and in five of them it was not associated with any detectable mutation of the CFTR gene. Two female partners were found to be carriers of a mutation, whereas 5 (18.5%) had the 5T variant. As many as 71% of CBVAD patients had the simultaneous presence of at least two signs and/or symptoms suggestive of CF, albeit they were of mild intensity and the patients felt fit and healthy. In conclusion, these results suggested that some patients with CBAVD without CFTR gene mutation or 5T variant, even when their sweat test is negative, may show clinical suspicion of carrying a CFTR gene mutation and therefore are at risk of generating children affected by CF if the partner carries a mutation as well. The screening for mutations and a careful clinical examination may contribute to better identification of patients with CFTR-related CBAVD.

Nramp2 analysis in hemochromatosis probands

The mechanism that leads to iron overload in hereditary hemochromatosis is not yet fully understood and genes other than HFE may be involved. Nramp2 is an intestinal iron transporter, upregulated by dietary iron deficiency, which also colocalizes with transferrin in recycling endosomes. The purpose of the present study was to analyze the coding region of the Nramp2 gene in 14 hemochromatosis probands which did not carry any HFE mutations on both chromosomes. We confirmed the existence of a polymorphism (1254 T --> C), which presumably is not associated with hereditary hemochromatosis, but we did not find any mutation. On the other hand, we identified 17 splice variants of the Nramp2 mRNA. Eight corresponded to activation of cryptic splicing sequences between exons 3 and 4. They were observed in a majority of hemochromatosis probands and control subjects. This indicates the existence of an important splicing instability in this region. At this stage, the biological significance of these variants is unclear. Our study did not find evidence for the involvement of the Nramp2 gene in hereditary hemochromatosis. The remaining question is whether hemochromatosis probands in our study have iron overload because of environmental factors or due to mutation in gene(s) other than HFE and Nramp2.

Splicing factors induce cystic fibrosis transmembrane regulator exon 9 skipping through a nonevolutionary conserved intronic element

In monosymptomatic forms of cystic fibrosis such as congenital bilateral absence of vas deferens, variations in the TG(m) and T(n) polymorphic repeats at the 3' end of intron 8 of the cystic fibrosis transmembrane regulator (CFTR) gene are associated with the alternative splicing of exon 9, which results in a nonfunctional CFTR protein. Using a minigene model system, we have previously shown a direct relationship between the TG(m)T(n) polymorphism and exon 9 splicing. We have now evaluated the role of splicing factors in the regulation of the alternative splicing of this exon. Serine-arginine-rich proteins and the heterogeneous nuclear ribonucleoprotein A1 induced exon skipping in the human gene but not in its mouse counterpart. The effect of these proteins on exon 9 exclusion was strictly dependent on the composition of the TG(m) and T(n) polymorphic repeats. The comparative and functional analysis of the human and mouse CFTR genes showed that a region of about 150 nucleotides, present only in the human intron 9, mediates the exon 9 splicing inhibition in association with exonic regulatory elements. This region, defined as the CFTR exon 9 intronic splicing silencer, is a target for serine-arginine-rich protein interactions. Thus, the nonevolutionary conserved CFTR exon 9 alternative splicing is modulated by the TG(m) and T(n) polymorphism at the 3' splice region, enhancer and silencer exonic elements, and the intronic splicing silencer in the proximal 5' intronic region. Tissue levels and individual variability of splicing factors would determine the penetrance of the TG(m)T(n) locus in monosymptomatic forms of cystic fibrosis.

Suppressive interactions between mutations located in the two nucleotide binding domains of CFTR

The S1235R locus in CFTR was studied in combination with alleles found at the M470V and G628R loci. While R628 caused a maturational defect, R1235 did not. The impact of R1235 was found to be influenced by the alleles present at the G628R and M470V loci. At the single channel level, R1235-V (R1235 on a V470 background) was characterized by an open probability significantly higher than V470-wildtype CFTR. M470, which on its own increases CFTR chloride transport activity when compared to V470-wildtype CFTR, suppressed the activity of R1235 in such a way that a protein with an open probability not significantly different from V470-wildtype CFTR was obtained. While R628-V CFTR had similar current densities as V470-wildtype CFTR in Xenopus laevis oocytes, R1235-V resulted in current densities that were more than twofold higher than those of V470-wildtype CFTR. However, the current densities generated by R1235/R628-V (R1235 and R628 on a V470 background) CFTR were significant lower than R1235-V or R628-V CFTR.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Functional analysis of cis-acting elements regulating the alternative splicing of human CFTR exon 9

The rate of exon 9 exclusion from the cystic fibrosis transmembrane conductance regulator (CFTR) mRNA is associated with monosymptomatic forms of cystic fibrosis. Exon 9 alternative splicing is modulated by a polymorphic polythymidine tract within its 3' splice site. We have generated a minigene carrying human CFTR exon 9 with its flanking intronic sequences and set up an in vivo model to study the cis-acting DNA elements which modulate its splicing. Transfections into human cell lines showed that T5, but not T9 or T7 alleles, significantly increases the alternative splicing of exon 9. Moreover, we found that another polymorphic locus juxtaposed upstream of the T tract, and constituted by (TG)(n)repeats, can further modulate exon 9 skipping but only when activated by the T5 allele. Then, we extended our studies to the mouse CFTR exon 9 which does not show alternative splicing. Comparison of human and mouse introns 8 and 9 revealed a low homology between the two sequences and the absence of the human polymorphic loci within the mouse intron 3' splice site. We have tested a series of constructs where the whole human exon 9 with its flanking intronic sequences was replaced partially or completely by the murine counterpart. The transfections of these constructs in human and murine cell lines reveal that also sequences of the downstream intron 9 affect exon 9 definition and co-modulate, with the UG/U 3' splice site sequences, the extent of exon 9 skipping in CFTR mRNA.

Fluorescent chloride indicators to assess the efficacy of CFTR cDNA delivery

Cl(-)-sensitive fluorescent indicators have been used extensively in cell culture systems to measure the Cl(-)-transporting function of the cystic fibrosis transmembrane conductance regulator protein CFTR. These indicators have been used in establishing a surrogate end point to assess the efficacy of CFTR cDNA delivery in human gene therapy trials. The ability to measure Cl- transport with high sensitivity in small and heterogeneous tissue samples makes the use of Cl- indicators potentially attractive in gene delivery studies. In this review article, the important technical aspects of Cl- transport measurements by fluorescent indicators such as SPQ are described, applications of Cl- indicators to assay CFTR function are critically evaluated, and new methodological developments are discussed. The available Cl- indicators have been effective in quantifying Cl- transport rates in cell culture models and in vitro systems such as isolated membrane vesicles and liposomes. However, the imperfect photophysical properties of existing Cl- indicators limit their utility in performing measurements in airway tissues, where gene transfer vectors are delivered in CF gene therapy trials. The low efficiency of gene transfer and the cellular heterogeneity in airway samples pose substantial obstacles to functional measurements of CFTR expression. Significant new developments in generating long-wavelength and dual-wavelength halide indicators are described, and recommendations are proposed for the use of the indicators in gene therapy trials.

Regulation of recombinant cardiac cystic fibrosis transmembrane conductance regulator chloride channels by protein kinase C

We investigated the regulation of cardiac cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels by protein kinase C (PKC) in Xenopus oocytes injected with cRNA encoding the cardiac (exon 5-) CFTR Cl- channel isoform. Membrane currents were recorded using a two-electrode voltage clamp technique. Activators of PKC or a cAMP cocktail elicited robust time-independent Cl- currents in cardiac CFTR-injected oocytes, but not in control water-injected oocytes. The effects of costimulation of both pathways were additive; however, maximum protein kinase A (PKA) activation occluded further activation by PKC. In oocytes expressing either the cardiac (exon 5-) or epithelial (exon 5+) CFTR isoform, Cl- currents activated by PKA were sustained, whereas PKC-activated currents were transient, with initial activation followed by slow current decay in the continued presence of phorbol esters, the latter effect likely due to down-regulation of endogenous PKC activity. The specific PKA inhibitor, adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS), and various protein phosphatase inhibitors were used to determine whether the stimulatory effects of PKC are dependent upon the PKA phosphorylation state of cardiac CFTR channels. Intraoocyte injection of 1,2-bis(2-aminophenoxy)ethane-N,N, N,N-tetraacetic acid (BAPTA) or pretreatment of oocytes with BAPTA-acetoxymethyl-ester (BAPTA-AM) nearly completely prevented dephosphorylation of CFTR currents activated by cAMP, an effect consistent with inhibition of protein phosphatase 2C (PP2C) by chelation of intracellular Mg2+. PKC-induced stimulation of CFTR channels was prevented by inhibition of basal endogenous PKA activity, and phorbol esters failed to stimulate CFTR channels trapped into either the partially PKA phosphorylated (P1) or the fully PKA phosphorylated (P1P2) channel states. Site-directed mutagenesis of serines (S686 and S790) within two consensus PKC phosphorylation sites on the cardiac CFTR regulatory domain attentuated, but did not eliminate, the stimulatory effects of phorbol esters on mutant CFTR channels. The effects of PKC on cardiac CFTR Cl- channels are consistent with a simple model in which PKC phosphorylation of the R domain facilitates PKA-induced transitions from dephosphorylated (D) to partially (P1) phosphorylated and fully (P1P2) phosphorylated channel states.

Structure and function of the CFTR chloride channel

Structure and Function of the CFTR Chloride Channel. Physiol. Rev. 79, Suppl.: S23-S45, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) is a unique member of the ABC transporter family that forms a novel Cl- channel. It is located predominantly in the apical membrane of epithelia where it mediates transepithelial salt and liquid movement. Dysfunction of CFTR causes the genetic disease cystic fibrosis. The CFTR is composed of five domains: two membrane-spanning domains (MSDs), two nucleotide-binding domains (NBDs), and a regulatory (R) domain. Here we review the structure and function of this unique channel, with a focus on how the various domains contribute to channel function. The MSDs form the channel pore, phosphorylation of the R domain determines channel activity, and ATP hydrolysis by the NBDs controls channel gating. Current knowledge of CFTR structure and function may help us understand better its mechanism of action, its role in electrolyte transport, its dysfunction in cystic fibrosis, and its relationship to other ABC transporters.