Mutations in CFTR associated with mild-disease-form Cl- channels with altered pore properties

Correlation of sweat chloride concentration with classes of the cystic fibrosis transmembrane conductance regulator gene mutations

OBJECTIVE

To compare differences in epithelial chloride conductance according to class of mutation of the cystic fibrosis transmembrane conductance regulator (CFTR) gene.

METHODS

We evaluated the relationship between the functional classes of CFTR mutations and chloride conductance using the first diagnostic sweat chloride concentration in a large cystic fibrosis (CF) population.

RESULTS

There was no difference in sweat chloride value value between classes of CFTR mutations that produce no protein (class I), fail to reach the apical membrane because of defective processing (class II), or produce protein that fails to respond to cyclic adenosine monophosphate (class III). Those mutations that produce a cyclic adenosine monophosphate-responsive channel with reduced conductance (class IV) were associated with a significantly lower, intermediate sweat chloride value. However, patients with the mutations that cause reduced synthesis or partially defective processing of normal CFTR (class V) had sweat chloride concentrations similar to those in classes I to III.

CONCLUSION

Studies of differences in chloride conductance between functional classes of CFTR mutations provide insight into phenotypic expression of the disease.

Effects of lovastatin on trafficking of cystic fibrosis transmembrane conductance regulator in human tracheal epithelium

Genetic defects in the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel, cause cystic fibrosis. Most defective forms of CFTR show improper intracellular trafficking. Because isoprenylated, small GTP-binding proteins are involved in the vesicular trafficking of other integral membrane proteins, we have investigated the role of isoprenylation in the trafficking of CFTR to the apical membranes of primary cultures of human airway epithelium and of Calu-3 cells, a human lung carcinoma cell line. CFTR function was measured as short circuit current, 125I efflux, and conductance of cell sheets with permeabilized basolateral membranes. Lovastatin, an inhibitor of isoprenyl lipid biosynthesis, markedly inhibited all measures of CFTR function. The lovastatin-induced declines in CFTR function were corrected by the simultaneous addition of mevalonate or the isoprenyl lipids geranylgeranyl and farnesyl but not cholesterol. Lovastatin reduced total cellular CFTR as assessed by immunoprecipitation. Mevalonate or isoprenyl lipids protected CFTR levels from the actions of lovastatin. Together, these results suggest a role for isoprenyl lipids, presumably through the actions of small GTP-binding proteins, in the trafficking of CFTR to the apical membrane of human airway epithelium.

Pyrophosphate stimulates wild-type and mutant cystic fibrosis transmembrane conductance regulator Cl- channels

A unique feature of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is regulation by ATP through the two cytoplasmic nucleotide-binding domains (NBDs). To better understand this process, we asked how channel activity is affected by inorganic pyrophosphate (PPi), a compound that binds to NBDs in other proteins. PPi and three nonhydrolyzable PPi analogs reversibly stimulated the activity of phosphorylated channels. Kinetic modeling of single channel data demonstrated that PPi affected two distinct steps in channel regulation. First, PPi increased the rate at which channels opened. Second, once channels were open, PPi delayed their closure. PPi could only stimulate channels when it was applied in the presence of ATP. PPi also increased the photolabeling of CFTR by an ATP analog. These two findings suggest that PPi modifies the activity of ATP-dependent CFTR channel gating. Based on these and previous data, we speculate that the effects of PPi are mediated by binding of PPi to NBD2 where it regulates channel opening by NBD1, and then, because it is not hydrolyzed, it slows the rate of NBD2-mediated channel closing. Because PPi stimulated wild-type channels, we tested its effect on CFTR containing the cystic fibrosis mutations: delta F508, R117H, and G551S. PPi stimulated all three. PPi also stimulated endogenous CFTR in the apical membrane of permeabilized T-84 epithelia. These results suggest that PPi or an analog might be of value in the development of new approaches to the treatment of cystic fibrosis.

A specific cystic fibrosis mutation (T3381) associated with the phenotype of isolated hypotonic dehydration

We carried out molecular screening for mutations in the cystic fibrosis transmembrane regulator (CFTR) gene in eight children of Sardinian descent seen because of hypotonic dehydration associated with hyponatremia, hypochloremia, hypokalemia, and metabolic alkalosis; none had pulmonary or pancreatic involvement. All the patients had the T3381 mutation either in homozygosity or compound heterozygosity with another CF mutation. The T3381 mutation was not detected in patients with CF who had classic symptoms or in healthy persons of the same descent. These data suggest that the T3381 mutation is associated with a specific mild CF phenotype.

Molecular mimicry in channel-protein structure

The approach to probing the sequence-structure relationship of ion-channel proteins using small peptides stems from the abundance of sequence information and the virtual absence of structures at atomic resolution. It is anticipated that model peptides may fold predictably into stable structures and reproduce functional properties of specific proteins. Model peptides are well suited to the application of NMR methods to determine protein structure in a membrane environment or to high-resolution X-ray diffraction analysis. It is timely to ask what we have learned through this strategy and where it may lead in our quest to understand the sequence-structure determinism.

A cystic fibrosis mutation associated with mild lung disease

BACKGROUND

Cystic fibrosis is the most common lethal autosomal recessive disorder among whites. Among Dutch patients with cystic fibrosis, delta F508 is the most common mutation and A455E the second most common mutation of the cystic fibrosis transmembrane conductance regulator gene on chromosome 7. A455E is associated with preserved pancreatic function and residual secretion of chloride across membranes. We investigated whether it is also associated with less severe pulmonary disease in patients with cystic fibrosis.

METHODS

A total of 33 patients with compound heterozygosity for the A455E mutation were matched according to age and sex with patients who were homozygous for the delta F508 mutation. The pairs were analyzed with respect to the following outcome variables: age at diagnosis, pulmonary-function values, and the frequency of pseudomonas colonization, pancreatic sufficiency, and diabetes mellitus.

RESULTS

Cystic fibrosis was diagnosed at a later age in the patients with the A455E mutation than in the delta F508 homozygotes (mean age at diagnosis, 15.0 vs. 3.1 years; P < 0.001). Fewer patients with the A455E mutation had pancreatic insufficiency (21.2 percent vs. 93.9 percent, P < 0.001), and none had diabetes mellitus (0 percent vs. 27.3 percent, P = 0.004). Forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) were significantly higher in the patients with the A455E mutation (mean FEV1, 73.9 percent of the predicted value vs. 54.3 percent of the predicted value; P = 0.002; mean FVC, 88.7 percent of the predicted value vs. 76.3 percent of the predicted value; P = 0.04). Fewer patients with the A455E mutation were colonized with Pseudomonas aeruginosa (33.3 percent vs. 60.6 percent, P = 0.02).

CONCLUSIONS

A455E is a common mutation causing cystic fibrosis in the Netherlands. Although several mutations are known to be associated with less severe pancreatic disease, our findings demonstrate a correlation between the A455E mutation and mild pulmonary disease. Because mortality in this disease depends primarily on the progression of pulmonary disease, patients with the A455E mutation have a better prognosis than patients who are homozygous for the delta F508 mutation.

Cyclic AMP-related and cation-affected human platelet chloride transport regulation

Cystic fibrosis has been characterized as a defect in the regulation of cyclic AMP-dependent transepithelial chloride transport. The activation of cyclic AMP-dependent protein kinase A by cyclic AMP occurs normally in cystic fibrosis cells, but they fail to transport chloride ions in response to protein kinase A stimulation. Defective chloride secretion and abnormal electrolyte transport occurs in several organs including the lung, sweat glands, intestine and pancreas. The present work was aimed at exploring whether the same or similar regulatory systems are functional in platelets, and if they are altered or deficient in individuals with cystic fibrosis. Chloride transport in platelets from normal subjects and from cystic fibrosis patients was measured by cell sizing techniques where chloride permeability is the limiting factor. In platelets from healthy volunteers, the chloride channel blocker, 5-nitro-2-(3-phenylpropylamino) benzoic acid, inhibits the transport in a dose-dependent manner. The preservation of chloride transport capability is shown to be dependent upon the presence of either Ca2+ or two divalent cation substitutes, Cd2+ or Cu2+. It is also shown that in normal subjects 0.1 mumol/l prostaglandin E1, which elevates cyclic AMP 6 times and abolishes platelet aggregation, significantly enhances the rate constant of the transport. Furthermore, in five out of nine cystic fibrosis patients studied, platelet chloride transport did not respond to stimulation by prostaglandin E1.

Alternate translation initiation codons can create functional forms of cystic fibrosis transmembrane conductance regulator

To evaluate the function of transmembrane domain 1 (TMD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) and the methionines that function in translation initiation, a series of progressive 5' truncations in TMD1 were created to coincide with residues that might serve as translation initiation codons. Expression of the mutants in Xenopus oocytes demonstrated that internal sites in TMD1 can function as initiation codons. In addition, all of the mutants that progressively removed the first four transmembrane segments (M1-M4) of TMD1 expressed functional cAMP-regulated Cl- channels with ion selectivity identical to wild-type CFTR but with reduced open probability and single channel conductance. Further removal of transmembrane segments did not produce functional Cl- channels. These data suggest that segments M1-M4 are not essential components of the conduction pore or the selectivity filter of CFTR.

Lung disease in the cystic fibrosis mouse exposed to bacterial pathogens

Lung disease is the major cause of death in cystic fibrosis (CF), but there is no evidence for overt lung involvement at birth. We show here that the same is true for the gene targeted cftrm1HGU mutant mouse. Furthermore, this CF mouse model demonstrates an impaired capacity to clear Staphylococcus aureus and Burkholderia (Pseudomonas) cepacia, two opportunistic lung pathogens closely associated with lung disease in CF subjects. The cftrm1HGU homozygotes display mucus retention and frank lung disease in response to repeated microbial exposure. Thus, lung disease in the cftrm1HGU mouse develops in response to bacterial infection, establishing a model to dissect the pathogenesis of CF pulmonary disease and providing a clinically relevant end point to assess the efficacy of pharmacologic or genetic interventions.

Regulation of CFTR Cl- channel gating by ADP and ATP analogues

The cystic fibrosis gene product (CFTR) is a chloride channel which, once phosphorylated, is regulated by nucleotide phosphates (Anderson, M. P., and M. J. Welsh. 1992. Science. 257:1701-1704; Venglarik, C. J., B. D. Schultz, R. A. Frizzell, and R. J. Bridges. 1994. Journal of General Physiology. 104:123-146). Nucleotide triphosphates initiate channel activity, while nucleotide diphosphates and nonhydrolyzable ATP analogues do not. To further characterize the role of these compounds on CFTR channel activity we examined their effects on chloride channel currents in excised inside-out membrane patches from CFTR transfected mouse L cells. ADP competitively inhibited ATP-dependent CFTR channel gating with a Ki of 16 +/- 9 microM. AMP neither initiated CFTR channel gating nor inhibited ATP-dependent CFTR channel gating. Similarly, ATP analogues with substitutions in the phosphate chain, including AMPCPP, AMPPCP, AMPPNP, and ATP gamma S failed to support CFTR channel activity when present at the cytoplasmic face of the membrane and none of these analogues, when present at three to 10-fold excess of ATP, detectably altered ATP-dependent CFTR channel gating. These data suggest that none of these ATP analogues interact with the ATP regulatory site of CFTR which we previously characterized and, therefore, no inference regarding a requirement for ATP hydrolysis in CFTR channel gating can be made from their failure to support channel activity. Furthermore, the data indicate that this nucleotide regulatory site is exquisitely sensitive to alterations in the phosphate chain of the nucleotide; only a nonsubstituted nucleotide di- or triphosphate interacts with this regulatory site. Alternative recording conditions, such as the presence of kinase and a reduction in temperature to 25 degrees C, result in a previously uncharacterized kinetic state of CFTR which may exhibit distinctly different nucleotide dependencies.

Intracellular pH modulates the activity of chloride channels in isolated lacrimal gland acinar cells

The effects of intracellular pH (pHi) on Ca(2+)-activated Cl- currents in rat lacrimal gland acinar cells were examined. Cl- currents were recorded by conventional whole cell patch-clamp methods using K(+)-free and Na(+)-free solutions. pHi was varied by using electrode solutions with pH at 6.8, 7.3, or 7.8, and Ca2+ activity was buffered at 100 nM with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Increasing pH from 6.8 to 7.8 was found to increase whole cell currents. The currents observed exhibited time-dependent activation at depolarizing potentials and time-dependent inactivation at hyperpolarizing potentials (pH 7.8). This behavior is characteristic of Ca(2+)-activated Cl- channels in lacrimal gland cells. The selectivity of the current was examined at pH 7.8 by removing Cl- from the bath solution. This maneuver caused a positive shift in the reversal potential, as expected for a Cl(-)-selective current. Thus increasing pHi appears to activate Ca(2+)-activated Cl- channels. The possibility that an increase in pHi may help sustain Cl- channel activity during secretory activity is discussed.

Identification and characterization of a second mouse Nramp gene

The Nramp gene was isolated as a candidate for the host resistance locus Bcg/Ity/Lsh, which controls natural resistance of mice to several types of infections. We have isolated by cross-hybridization cDNA clones corresponding to a second mouse Nramp gene, which we designate Nramp2. Nucleotide and predicted amino acid sequence analyses of full-length cDNA clones for Nramp2 indicate that this novel Nramp protein is closely homologous to the previously described Nramp and that the two genes form part of a small gene family. The two Nramp proteins encode integral membrane proteins that share 63% identical residues and an overall homology of 78%. They share very similar secondary structure, including identical hydropathy profiles and predicted membrane organization, with a minimum of 10 and most probably 12 transmembrane domains, a cluster of predicted N-linked glycosylation sites, and a consensus transport motif. Analysis of the distribution of Nramp2 mRNA transcripts in normal mouse tissues by Northern blotting revealed that the Nramp2 gene produces several mRNAs, including prominent 3.3- and 2.3-kb species generated by the use of alternative polyadenylation signals. In contrast to the previously described macrophage-specific Nramp gene, Nramp2 mRNAs were found to be expressed at low levels in all tissues tested. Using a polymorphic (GT)26 dinucleotide repeat identified in the 3' untranslated region of the mRNA, we have mapped the Nramp2 gene to the distal part of mouse chromosome 15 between markers D15Mit41 and D15Mit15, with the gene order and intergene distance (in cM): centromere-56.1-D15Mit41-(1 +/- 1)-Nramp2-(5 +/- 2)-D15Mit15.

Transcript analysis of CFTR nonsense mutations in lymphocytes and nasal epithelial cells from cystic fibrosis patients

The mutational effects at the mRNA level were investigated by RT-PCR analysis of nine different nonsense mutations (Q39X, E60X, R75X, G542X, L719X, Y1092X, R1162X, S1196X, W1282X) and one frameshift mutation (1078delT) within the CFTR gene. With the exception of mutation R1162X, reduced mRNA levels ranging from 30% to less than 5% of the wild type have been observed. In case of the R75X and E60X mutations, the mRNA reduction was accompanied by the appearance of atypical CFTR isoforms. Single exon 3 skipping, as well as joint exon 2 and 3 skipping, was observed in lymphocyte and nasal epithelial mRNA derived from R75X alleles. The analysis of mRNA transcribed from E60X alleles revealed skipping of exon 3 (lymphocytes and nasal epithelial cells) or skipping of exons 3 and 4 (nasal epithelial cells). With the exception of the E60X mutation, no obvious tissue-specific differences in the splicing pattern and ratios of mutation to wild-type transcripts were detected between lymphocytes and nasal epithelial cells. In addition to aberrant splicing, the reduction of transcripts is the most common effect of nonsense and frameshift mutations within the CFTR gene.

Pancreatic insufficiency and pulmonary disease in German and Slavic cystic fibrosis patients with the R347P mutation

Cystic fibrosis (CF) is caused by mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR) that codes for a cAMP-regulated chloride channel. The R347P is a missense mutation located within the first membrane spanning domain (MSD1) of the CFTR protein. This mutation occurs with an overall worldwide frequency of about 0.2%. The patients, originally described with this mutation were compound heterozygotes with the delta F508 mutation and had a very mild course of CF, suggesting that R347P, similar to other missense mutations affecting the MSD1 domain, causes a mild phenotype. We report here a group of 19 CF patients with the R347P mutation of German, Bulgarian, Czech, and Slovak origin, including two homozygotes. Most patients presented with early disease onset, pancreas insufficiency (PI), and early pulmonary involvement, suggesting that this mutation can lead to a severe course of CF. Most R347P alleles in the group studied share a common polymorphic haplotype. In addition, these analyses gave evidence for recurrence of the mutation in two CF patients of German and Czech origin.

Treatment of cystic fibrosis based on understanding CFTR

In the five years since the gene associated with cystic fibrosis was isolated, rapid progress has been made in understanding the structure and function of CFTR, the gene product. Based on the knowledge that CFTR is a cAMP-regulated chloride channel in the apical surface of epithelia in many tissues, new approaches to treating CF disease have emerged. These include development of agents to open alternative chloride channels, to relocate mutant CFTR, which commonly is mislocalized because of a trafficking defect, and to deliver CFTR using protein or gene therapy. Although the underlying basis of each of these approaches is now well understood, major practical problems face each of them. Nevertheless, it is difficult not to be optimistic that new therapies will emerge.

Development of mouse models for cystic fibrosis

Using gene targeting in embryonal stem cells it is now possible to create accurate genetic models of inherited human disease in the mouse. The value of an animal model of cystic fibrosis is in providing clarification of disease pathogenesis, genotype-phenotype correlation, the identification of other relevant genetic factors, and as the optimal test system for novel therapeutic intervention. Correction of the basic defect by a somatic gene therapy approach is an attractive approach to disease treatment. CF mouse models have been described which display the characteristic electrophysiological defect and thus both safety and efficacy can be monitored in these animals. Modulation of Cftr levels in transgenic animals and the results on disease phenotype give some indication of the level of gene expression necessary to give clinical effect.

Diagnostic criteria for cystic fibrosis in men with congenital absence of the vas deferens

The high frequency of cystic fibrosis (CF) mutations in males with absence of vas deferens supported the hypothesis of a primarily genital phenotype of CF disease. To consider the idea of an attenuated form of CF, we investigated 14 men with congenital bilateral aplasia of the vasa deferentia. All patients were consulting for infertility and none was known to have CF. The median age was 30.5 years (range, 20-38 yr). DNA analysis for 22 CF mutations showed at least 1 mutation in 10 patients (71%), whereas the CF carrier frequency is only 4% in the general population. Three compound heterozygotes were identified, all carriers of the R117H mutation. The sweat test was considered positive in 6 patients (43%), and a high frequency of radiologic evidence of sinus disease (8 patients) and of elevated antibodies to Pseudomonas (8 patients) was found. Only 2 patients were free of all these criteria for CF disease. This study strengthens the hypothesis that absence of vas deferens is an attenuated form of CF. We propose a combination of tests including DNA study, computerized tomographic scan of the paranasal sinuses, and testing of anti-Pseudomonas antibodies when the sweat test is inconclusive.

Regulation of CFTR channel gating

The debilitating symptoms of cystic fibrosis stem from the reduced Cl- permeability of epithelial cells owing to mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. In cells with normal CFTR channels, receptor-mediated activation of cyclic-AMP-dependent protein kinase causes phosphorylation of several serines in the regulatory domain of CFTR, permitting channel opening and closing via cycles involving ATP hydrolysis. Cellular phosphatases rapidly dephosphorylate the channels, inactivating them. Here we discuss recent advances in our understanding of this complex mechanism for regulating channel gating.

Phosphate stimulates CFTR Cl- channels

Cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels appear to be regulated by hydrolysis of ATP and are inhibited by a product of hydrolysis, ADP. We assessed the effect of the other product of hydrolysis, inorganic phosphate (P(i)), on CFTR Cl- channel activity using the excised inside-out configuration of the patch-clamp technique. Millimolar concentrations of P(i) caused a dose-dependent stimulation of CFTR Cl- channel activity. Single-channel analysis demonstrated that the increase in macroscopic current was due to an increase in single-channel open-state probability (po) and not single-channel conductance. Kinetic modeling of the effect of P(i) using a linear three-state model indicated that the effect on po was predominantly the result of an increase in the rate at which the channel passed from the long closed state to the bursting state. P(i) also potentiated activity of channels studied in the presence of 10 mM ATP and stimulated Cl- currents in CFTR mutants lacking much of the R domain. Binding studies with a photoactivatable ATP analog indicated that Pi decreased the amount of bound nucleotide. These results suggest that P(i) increased CFTR Cl- channel activity by stimulating a rate-limiting step in channel opening that may occur by an interaction of P(i) at one or both nucleotide-binding domains.

Phosphatase inhibitors activate normal and defective CFTR chloride channels

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is regulated by phosphorylation and dephosphorylation at multiple sites. Although activation by protein kinases has been studied in some detail, the dephosphorylation step has received little attention. This report examines the mechanisms responsible for the dephosphorylation and spontaneous deactivation ("rundown") of CFTR chloride channels excised from transfected Chinese hamster ovary (CHO) and human airway epithelial cells. We report that the alkaline phosphatase inhibitors bromotetramisole, 3-isobutyl-1-methylxanthine, theophylline, and vanadate slow the rundown of CFTR channel activity in excised membrane patches and reduce dephosphorylation of CFTR protein in isolated membranes. It was also found that in unstimulated cells, CFTR channels can be activated by exposure to phosphatase inhibitors alone. Most importantly, exposure of mammalian cells to phosphatase inhibitors alone activates CFTR channels that have disease-causing mutations, provided the mutant channels are present in the plasma membrane (R117H, G551D, and delta F508 after cooling). These results suggest that CFTR dephosphorylation is dynamic and that membrane-associated phosphatase activity may be a potential therapeutic target for the treatment of cystic fibrosis.

Novel pore-lining residues in CFTR that govern permeation and open-channel block

The cystic fibrosis transmembrane conductance regulator (CFTR) is both a member of the ATP-binding cassette superfamily and a Cl(-)-selective ion channel. We investigated the permeation pathway of human CFTR with measurements on conduction and open-channel blockade by diphenylamine-2-carboxylic acid (DPC). We used site-directed mutagenesis and oocyte expression to locate residues in transmembrane domain (TM) 6 and TM 12 that contact DPC and control rectification and single-channel conductances. Thus, TM 12 and the previously investigated TM 6 line the CFTR pore. In each TM, residues in contact with DPC are separated by two turns of an alpha helix. The contributions of TM 6 and TM 12 to DPC block and Cl- permeation, however, are not equivalent. The resulting structural model for the conduction pathway may guide future studies of permeation in other Cl- channels and ATP-binding cassette transporters.

Molecular physiology of anion channels

Anion channels have diverse functions, ranging from regulation of cell volume to transepithelial transport and control of excitability. Three well established structural classes of plasma membrane chloride channels now exist: the ligand-gated chloride channels, the cAMP-stimulated cystic fibrosis transmembrane conductance regulator channel, and the voltage-gated (or swelling-activated) members of the CLC chloride channel family. Genetic defects leading to inherited disease are known for each of these classes. A combination of mutagenesis and biophysical analysis has been used to correlate their structure with function. Recently, the role of several molecules has been questioned; rather than being chloride channels themselves, they may be activators of endogenous channels in the cells used for expression.

Long-term survival of the exon 10 insertional cystic fibrosis mutant mouse is a consequence of low level residual wild-type Cftr gene expression

Recently we have created a mouse model of cystic fibrosis (CF) by insertional gene targeting to exon 10. In common with CF subjects, this model displays a low incidence of meconium ileus. This contrasts strikingly with the very high level of fatal intestinal obstruction in the three other CF mouse models so far described. We investigate here the molecular basis of this difference in phenotype. We show that the partial duplication consequent upon insertional gene targeting allows exon skipping and aberrant splicing to produce normal Cftr mRNA, but at levels greatly reduced compared with wild-type mice. Furthermore, instead of the predicted mutant Cftr transcript, a novel mRNA is produced that utilizes cryptic splice sites in the disrupting plasmid sequence. However, we have previously shown that these mice display the ion transport defect characteristic of CF, and mutant animals can be distinguished from their normal littermates on this basis. Consistent with this, residual CFTR function has recently been observed for several "mild" mutations in CF individuals who display pancreatic sufficiency but still develop lung disease. We conclude that (i) residual wild-type mRNA in the exon 10 insertional mutant mouse ameliorates the severity of the intestinal phenotype observed in the absolute "null" CF mice, (ii) the presence of low-level residual wild-type Cftr mRNA does not correct the CF ion transport defect, and (iii) the long-term survival of this insertional mutant mouse provides the opportunity to address the factors important in development of lung disease.

Structural basis of ion channel permeation and selectivity

There has been rapid progress in understanding the structural basis of ion selectivity and permeation in both ligand- and voltage-gated channels. Recognition of similarities in overall architecture within a channel class has led to an increasing focus on the specific molecular determinants that endow a channel with its own distinctive character. It has been possible in some cases to identify individual amino acids essential for ion selectivity.

Effect of ATP concentration on CFTR Cl- channels: a kinetic analysis of channel regulation

Phosphorylated cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels require nucleoside triphosphates, such as ATP, to open. As the concentration of intracellular ATP increases, the probability of the channel being open (Po) increases. To better understand how ATP regulates the channel, we studied excised inside-out membrane patches that contained single, phosphorylated CFTR Cl- channels and examined the kinetics of gating at different concentrations of ATP. As the ATP concentration increased from 0.1 to 3 mM the mean closed time decreased, but mean open time did not change. Analysis of the data using histograms of open- and closed-state durations, the maximum likelihood method, and the log-likelihood ratio test suggested that channel behavior could be described by a model containing one open and two closed states (C1<==>C2<==>O). ATP regulated phosphorylated channels at the transition between the closed states C1 and C2: as the concentration of ATP increased, the rate of transition from C1 to C2 (C1-->C2) increased. In contrast, transitions from C2 to C1 and between C2 and the open state (O) were not significantly altered by ATP. Addition of ADP in the presence of ATP decreased the transition rate from C1 to C2 without affecting other transition rates. These data suggest that ATP regulates CFTR Cl- channels through an interaction that increases the rate of transition from the closed state to a bursting state in which the channel flickers back and forth between an open and a closed state (C2). This transition may reflect ATP binding or perhaps a step subsequent to binding.

Activation of Ca(2+)-dependent K+ and Cl- currents by UTP and ATP in CFPAC-1 cells

Activation of Cl- and K+ conductances by nucleotide receptor-operated mobilization of intracellular Ca2+ was investigated in CFPAC-1 cells with the perforated-patch technique. Adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) caused a dose-dependent fast and transient membrane hyperpolarization. UTP was more effective than ATP. In voltage-clamped cells, two currents with different ionic permeability and kinetics were activated by the nucleotides. The first one was carried by Cl- ions, peaked in the first few seconds after addition of nucleotides, and lasted for 1 +/- 0.3 min. Its amplitude was about 2.7 nA at -100 mV with 100 mumol/l of either ATP or UTP. The second current was carried by K+ ions and was blocked by Cs+. This current peaked more slowly and had a mean duration of 4.6 +/- 0.7 min. Its amplitude was 0.9 nA and 0.5 nA at -20 mV with 100 mumol/l UTP and ATP, respectively. Activation of the nucleotide receptor caused a transient increase in intracellular Ca2+ concentration ([Ca2+]i) that was similar in the presence or absence of extracellular Ca2+. The ED50 for UTP was 24 mumol/l and that for ATP was 94 mumol/l. Depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by thapsigargin prevented both the nucleotide-induced [Ca2+]i increase and the activation of membrane currents. Addition of 2 mmol/l Ca2+ to thapsigargin-treated cells produced a sustained increase of Cl- and K+ currents, which was reversed by Ca2+ removal. The present study demonstrates that CFPAC-1 cells respond to nucleotide receptor activation with a transient increase in [Ca2+]i that stimulates Ca(2+)-dependent Cl- and K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel exon in the cystic fibrosis transmembrane conductance regulator gene activated by the nonsense mutation E92X in airway epithelial cells of patients with cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. We report on a novel nonsense mutation that leads to exon skipping and the activation of a cryptic exon. Screening of genomic DNA from 700 German patients with CF uncovered four cases with the nonsense mutation E92X, a G-->T transversion that creates a termination codon and affects the first base of exon 4 of the CFTR gene. Lymphocyte RNA of two CF patients heterozygous for E92X was found to contain the wild type sequence and a differentially spliced isoform lacking exon 4. In RNA derived from nasal epithelial cells of E92X patients, a third fragment of longer size was observed. Sequencing revealed the presence of E92X and an additional 183-bp fragment, inserted between exons 3 and 4. The 183-bp sequence was mapped to intron 3 of the CFTR gene. It is flanked by acceptor and donor splice sites. We conclude that the 183-bp fragment in intron 3 is a cryptic CFTR exon that can be activated in epithelial cells by the presence of the E92X mutation. E92X abolishes correctly spliced CFTR mRNA and leads to severe cystic fibrosis.

The amino-terminal portion of CFTR forms a regulated Cl- channel

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel consists of two motifs (each containing a membrane-spanning domain [MSD] and a nucleotide-binding domain [NBD]) linked by an R domain. We tested the hypothesis that one MSD-NBD motif could form a Cl- channel. The amino-terminal portion of CFTR (D836X, which contains MSD1, NBD1, and the R domain) formed Cl- channels with conductive properties identical to those of CFTR. However, channel regulation differed. Although phosphorylation increased activity, channels opened without phosphorylation. MgATP stimulated D836X more potently than CFTR and may interact at more than one site. These data and migration of D836X on sucrose density gradients suggest that D836X may function as a multimer. Thus, the amino-terminal portion of CFTR contains all of the structures required to build a regulated Cl- channel.

Gates of Janus: cystic fibrosis and diarrhea

Heat-stable enterotoxin, produced by Escherichia coli, binds to particulate guanylate cyclase to increase cyclic GMP in intestinal cells. This in turn stimulates the cyclic-GMP- or cyclic-AMP-dependent protein kinase, activating the same chloride channel that is defective in cystic fibrosis. It is possible that the relatively high prevalence of cystic fibrosis in humans results from its protective effect against diarrhea.

Identification of an ion channel-forming motif in the primary structure of CFTR, the cystic fibrosis chloride channel

Synthetic peptides with sequences representing putative transmembrane (M) segments of CFTR (the cystic fibrosis transmembrane conductance regulator) were used as tools to identify the involvement of such segments in forming the ionic pore of the CFTR Cl- channel. Peptides with sequences corresponding to M2 and M6 form anion-selective channels after reconstitution in lipid bilayers. In contrast, peptides with the sequences of M1, M3, M4, and M5, or peptides of the same amino acid composition as M2 and M6 but with scrambled sequences, do not form channels. Conductive heterooligomers of M2 and M6 exhibit a single channel conductance of 8 pS (in 0.15 M KCl) and a 95% selectivity for anions over cations, properties that emulate both the conductance and the selectivity of the authentic CFTR channel. The identification of sequence-specific motifs that account for key functional attributes of the CFTR channel suggests that such modules may represent fundamental units of function and are plausible constituents of the pore-forming structure of the CFTR Cl- channel.

A transmembrane domain of the putative channel subunit MEC-4 influences mechanotransduction and neurodegeneration in C. elegans

Aberrant ion channel activity plays a causative role in several human disorders. Inappropriately regulated channel activity also appears to be the basis for neurodegeneration induced by dominant mutations of Caenorhabditis elegans mec-4 (mec-4(d)), a member of the degenerin gene family postulated to encode a subunit of a mechanosensory channel. The degenerin gene family has been defined by two C. elegans genes, mec-4 and deg-1, which can mutate to gain-of-function alleles that induce degeneration of specific groups of neurons. A related mammalian gene, rat alpha-rENaC, induces an amiloride-sensitive Na+ current when introduced to Xenopus oocytes, strongly suggesting that degenerin genes encode ion channel proteins. Deduced amino-acid sequences of the degenerins include two predicted membrane-spanning domains. Here we show that conserved amino acids within the second membrane-spanning domain (MSDII) are critical for MEC-4 activity and that specific substitutions within MSDII, whether encoded in cis or in trans to a mec-4(d) mutation, block or delay the onset of degeneration. Remarkably, MSDII from two other family members, C. elegans deg-1 and rat alpha-rENaC, can functionally substitute for MEC-4 MSDII in chimaeric proteins. Our results support a structural model for a mechanosensory channels in which multiple MEC-4 subunits are oriented such that MSDII lines the channel pore, and a neurodegeneration model in which aberrant ion flow through this channel is a key event.

Determinants of mild clinical symptoms in cystic fibrosis patients. Residual chloride secretion measured in rectal biopsies in relation to the genotype

Previous Ussing chamber measurements of secretagogue-provoked changes in short circuit current in rectal suction biopsies of cystic fibrosis (CF) patients showed that in a minority of patients chloride secretion in response to cholinergic agonists is reduced but not completely absent. To assess a possible relationship between this phenomenon and both the genotype and the phenotype, we performed Ussing chamber experiments on rectal suction biopsies of 51 CF patients. The CF mutation was identified in 89 out of 102 CF alleles. No apparent chloride secretion was found in 30 CF patients (group I). Low residual chloride secretion was found in 11 CF patients (group II), while a relatively high residual secretion appeared in 10 CF patients (group III). Pancreatic function was preserved more frequently in CF patients displaying residual secretion: 0% in group I, 27% in group II, and 60% in group III (P < 0.001). The age at diagnosis (mean +/- SEM) in group III (18.4 +/- 6.6) was significantly different from group I (1.2 +/- 0.4, P < 0.01) and group II (3.5 +/- 1.4, P = 0.05). Residual chloride secretion was found in some of the 28 dF508 homozygous patients (three in group II, and one in group III), disclosing that other factors than the CF gene defect itself affect the transepithelial chloride transport. The age at diagnosis correlates significantly with the magnitude of the secretory response, even within the dF508 homozygous patients (r = 0.4, P < 0.05). We conclude that residual chloride secretion in CF is the pathophysiological basis of preserved pancreatic function and delayed presentation of the disease, which is not exclusively determined by the CF genotype.

CFTR and calcium-activated chloride currents in pancreatic duct cells of a transgenic CF mouse

We have studied the cystic fibrosis transmembrane conductance regulator (CFTR) and calcium-activated chloride currents in pancreatic duct cells isolated from a transgenic cf/cf mouse created by targeted insertional mutagenesis. Adenosine 3',5'-cyclic monophosphate (cAMP)-activated CFTR chloride currents were detected in 78% (29/37) of wild-type cells, in 81% (35/43) of heterozygote cells, and in 61% (29/47) of homozygous cf/cf duct cells (P > 0.05, cf/cf vs. wild-type and heterozygote). The CFTR current density measured at membrane potentials of +/- 60 mV averaged 22-26 pA/pF in wild-type and heterozygote groups but only 13 pA/pF in cells derived from cf/cf animals (P < 0.05, cf/cf vs. wild-type and cf/cf vs. heterozygotes). In contrast, duct cells from animals of all three genotypic groups exhibited calcium-activated chloride currents that were of similar magnitude and up to 11-fold larger than the CFTR currents. We speculate that these transgenic insertional null mice do not develop the pancreatic pathology that occurs in cystic fibrosis patients because their duct cells contain 1) some wild-type CFTR generated by exon skipping and aberrant splicing and 2) a separate anion secretory pathway mediated by calcium-activated chloride channels.

Detection of 98.5% of the mutations in 200 Belgian cystic fibrosis alleles by reverse dot-blot and sequencing of the complete coding region and exon/intron junctions of the CFTR gene

We have previously shown that about 85% of the mutations in 194 Belgian cystic fibrosis alleles could be detected by a reverse dot-blot assay. In the present study, 50 Belgian chromosomes were analyzed for mutations in the cystic fibrosis transmembrane conductance regulator gene by means of direct solid phase automatic sequencing of PCR products of individual exons. Twenty-six disease mutations and 14 polymorphisms were found. Twelve of these mutations and 3 polymorphisms were not described before. With the exception of one mutant allele carrying two mutations, these mutations were the only mutations found in the complete coding region and their exon/intron boundaries. The total sensitivity of mutant CF alleles that could be identified was 98.5%. Given the heterogeneity of these mutations, most of them very rare, CFTR mutation screening still remains rather complex in our population, and population screening, whether desirable or not, does not appear to be technically feasible with the methods currently available.

Multi-ion pore behaviour in the CFTR chloride channel

Cystic fibrosis transmembrane conductance regulator (CFTR) is a non-rectifying, low-conductance channel regulated by ATP and phosphorylation, which mediates apical chloride conductance in secretory epithelia and malfunctions in cystic fibrosis (CF). Mutations at Lys 335 and Arg 347 in the sixth predicted transmembrane helix of CFTR alter its halide selectivity in whole-cell studies and its single channel conductance, but the physical basis of these alterations is unknown and permeation in CFTR is poorly understood. Here we present evidence that wild-type CFTR can contain more than one anion simultaneously. The conductance of CFTR passes through a minimum when channels are bathed in mixtures of two permeant anions. This anomalous mole fraction effect can be abolished by replacing Arg 347 with an aspartate and can be toggled on or off by varying the pH after the same residue is replaced with a histidine. Thus the CFTR channel should provide a convenient model in which to study multi-ion pore behaviour and conduction. The loss of multiple occupancy may explain how naturally occurring CF mutations at this site cause disease.

A mutation in CFTR produces different phenotypes depending on chromosomal background

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene but the association between mutation (genotype) and disease presentation (phenotype) is not straightforward. We have been investigating whether variants in the CFTR gene that alter splicing efficiency of exon 9 can affect the phenotype produced by a mutation. A missense mutation, R117H, which has been observed in three phenotypes, was found to occur on two chromosome backgrounds with intron 8 variants that have profoundly different effects upon splicing efficiency. A close association is shown between chromosome background of the R117H mutation and phenotype. These findings demonstrate that the genetic context in which a mutation occurs can play a significant role in determining the type of illness produced.

Correlation between genotype and phenotype in patients with cystic fibrosis

BACKGROUND

Cystic fibrosis is the most common lethal autosomal recessive disorder among whites. Seventy-two percent of patients with this disease are homozygotes or compound heterozygotes for eight mutations of the cystic fibrosis transmembrane conductance regulator gene on chromosome 7: delta F508, G542X, R553X, W1282X, N1303K, 621 + 1G-->T, 1717-1G-->A, and R117H. We studied the relation between genotype and phenotype in patients from 14 countries.

METHODS

Each of 399 patients who were compound heterozygotes for delta F508 and one other mutation was matched with the delta F508 homozygote of the same sex who was the closest in age from the same center. A paired analysis was performed of the following outcome variables: age at diagnosis, sweat chloride concentration, growth percentiles, pulmonary-function values, chest-film score, pseudomonas colonization, nasal polyps, pancreatic sufficiency, pancreatitis, diabetes mellitus, meconium ileus, distal intestinal obstruction syndrome, rectal prolapse, cirrhosis, and gallbladder disease.

RESULTS

The compound heterozygotes having the genotype R117H/delta F508 clearly differed from the age- and sex-matched delta F508 homozygotes: they more often had pancreatic sufficiency (87 percent vs. 4 percent, P < 0.001), were older when the diagnosis was first made (mean [+/- SD] age, 10.2 +/- 10.5 vs. 2.5 +/- 4.3 years; P = 0.002), and had lower sweat chloride concentrations (80 +/- 18 vs. 108 +/- 14 mmol per liter, P < 0.001). There were no statistically significant differences between delta F508 homozygotes and other compound heterozygotes with regard to any variable tested.

CONCLUSIONS

Prenatal and prognostic counseling for patients with the R117H/delta F508 genotype should include the likelihood that they will have long-term pancreatic sufficiency. Patients with the other genotypes should expect the early onset of pancreatic insufficiency. For none of the genotypes studied can predictions be made about the occurrence of common complications or the severity or course of pulmonary disease.

Transporters, channels and human disease

The past year has seen significant advances in our understanding of the molecular biology of ion channels and transporters and their role in human disease. The star of the show has to be the cystic fibrosis chloride channel about which an enormous amount of information has been accumulated, and the functional effects of some of the mutations found in cystic fibrosis patients have been characterized.