Suppression of a CFTR premature stop mutation in a bronchial epithelial cell line

Disease genes: flattery and deception

Completion of the human genome project raises the possibility of genetically based treatments for a multitude of human diseases. As yet only a handful of patients have benefited clinically from this approach. Why gene transfer is such a complex issue is discussed in this article. Theoretically, the easiest diseases to treat are single gene recessive diseases, where, presumably, gene delivery to somatic cells is all that is required. Two prime candidates for gene therapy are severe combined immunodeficiency disease (SCID) and cystic fibrosis (CF). Attempts to treat both of these diseases by gene therapy commenced in the late 1980s. Some clinical benefit has been recorded with SCID, but none, as yet, has been recorded with CF.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Stop codons affect 5' splice site selection by surveillance of splicing

Pre-mRNA splicing involves recognition of a consensus sequence at the 5' splice site (SS). However, only some of the many potential sites that conform to the consensus are true ones, whereas the majority remain silent and are not normally used for splicing. We noticed that in most cases the utilization of such a latent intronic 5' SS for splicing would introduce an in-frame stop codon into the resultant mRNA. This finding suggested a link between SS selection and maintenance of an ORF within the mRNA. Here we tested this idea by analyzing the splicing of pre-mRNAs in which in-frame stop codons upstream of a latent 5' SS were mutated. We found that splicing with the latent site is indeed activated by such mutations. Our findings predict the existence of a checking mechanism, as a component of the nuclear pre-mRNA splicing machine, to ensure the maintenance of an ORF. This notion is highly important for accurate gene expression, as perturbations that would lead to splicing at these latent sites are expected to introduce in-frame stop codons into the majority of mRNAs.

Expression of CTNS alleles: subcellular localization and aminoglycoside correction in vitro

Mutations in CTNS result in one of three forms of cystinosis: benign, intermediate, or nephropathic. Homozygosity for a nonsense mutation in CTNS (753G -->A), encoding a premature termination codon (PTC) at amino acid 138 (W138X), results in nephropathic cystinosis. Gentamicin is known to induce PTC readthrough and hence full-length protein production. We demonstrate that addition of gentamicin (300 microg/ml) to cystinotic fibroblasts leads to depletion of intracellular cystine in cell lines with a premature termination codon, but not in those with a large deletion or a deletion leading to a frameshift mutation. Plasmids were constructed with GFP as a C-terminal or N-terminal fusion to CTNS. The normal CTNS protein fused with either N- or C-terminal GFP colocalized with Lysotracker red, a fluorescent stain which selectively accumulates in lysosomes. PTC-GFP, a construct with GFP fused to the C-terminus of CTNS containing a PTC, allowed GFP to serve as a reporter of PTC readthrough. No significant fluorescence was observed in PTC-GFP-transfected cells in the absence of gentamicin but was seen and localized to lysosomes in its presence. A patient with a splice site mutation (IVS11 + 2T -->C) that eliminates the GYDQL lysosomal targeting sequence of cystinosin on one allele, and a PTC mutation (753G -->A) on the other, displays the intermediate phenotype. Transfection of the splice site mutant allele into CTNS null fibroblasts produced cystine depletion. Plasmids with GFP fused to the N-terminus of CTNS containing the splice site mutation (GFP-SS) were constructed. While the normal CTNS-GFP fusion protein was found to colocalize with Lysotracker red almost exclusively, the GFP-SS fusion product was found in the plasma membrane and cytoplasm, as well as lysosomes. A second lysosomal targeting motif in CTNS is present in this sequence, just proximal to the mutation, accounting for the partial lysosomal localization.

A(2) adenosine receptors regulate CFTR through PKA and PLA(2)

We investigated adenosine (Ado) activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro and in vivo. A(2B) Ado receptors were identified in Calu-3, IB-3-1, COS-7, and primary human airway cells. Ado elevated cAMP in Calu-3, IB-3-1, and COS-7 cells and activated protein kinase A-dependent halide efflux in Calu-3 cells. Ado promoted arachidonic acid release from Calu-3 cells, and phospholipase A(2) (PLA(2)) inhibition blocked Ado-activated halide efflux in Calu-3 and COS-7 cells expressing CFTR. Forskolin- and beta(2)-adrenergic receptor-stimulated efflux were not affected by the same treatment. Cytoplasmic PLA(2) (cPLA(2)) was identified in Calu-3, IB-3-1, and COS-7 cells, but cPLA(2) inhibition did not affect Ado-stimulated cAMP concentrations. In cftr(+) and cftr(-/-) mice, Ado stimulated nasal Cl(-) secretion that was CFTR dependent and sensitive to A(2) receptor and PLA(2) blockade. In COS-7 cells transiently expressing DeltaF508 CFTR, Ado activated halide efflux. Ado also activated G551D CFTR-dependent halide efflux when combined with arachidonic acid and phosphodiesterase inhibition. In conclusion, PLA(2) and protein kinase A both contribute to A(2) receptor activation of CFTR, and components of this signaling pathway can augment wild-type and mutant CFTR activity.

Correction of CFTR malfunction and stimulation of Ca-activated Cl channels restore HCO3- secretion in cystic fibrosis bile ductular cells

In view of the occurrence of hepatobiliary disorders in cystic fibrosis (CF) this study addresses the role of the cystic fibrosis transmembrane conductance regulator (CFTR) and of Ca(2+)-activated Cl(-) channels in promoting HCO3- secretion in bile ductular cells. Human cholangiocytes were isolated from control livers and from 1 patient with CF (DeltaF508/G542X mutations). Single channel and whole cell currents were analyzed by patch clamp techniques, and HCO3- secretion was determined by fluorometric analysis of the rate of recovery of intracellular pH following alkaline loading. In control cholangiocytes, both cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) catalytic subunit, activated CFTR Cl(-) channels that exhibited a nonrectifying conductance of 8 pS and appeared in clusters. Activation of Cl(-) current by cAMP was associated with an increase in the rate of HCO3- secretion. The basal rate of HCO3- secretion was lower in CF than in control cholangiocytes. In both control and CF cholangiocytes, raising intracellular Ca(2+) concentrations with ionomycin led to a parallel activation of Cl(-) current and HCO3- secretion. Consistent with reports that premature stop codon mutations (class I; e.g., G542X) can be read over by treatment with aminoglycoside antibiotics, exposure of CF cholangiocytes to gentamicin restored activation by cAMP of Cl(-) current and HCO3- secretion. The observation that activation of Ca(2+)-dependent Cl(-) channels can substitute for cystic fibrosis transmembrane conductance regulator (CFTR) in supporting HCO3- secretion and the efficacy of gentamicin in restoring CFTR function and HCO3- secretion in class I mutations are of potential clinical interest.

Towards the pharmacogenomics of cystic fibrosis

Cystic fibrosis (CF) is the most common lethal recessive genetic disease affecting children in Europe and the US. CF is a multiorgan disease and may present a variety of clinical symptoms, like chronic obstructive lung disease, exocrine pancreatic insufficiency (PI) and elevated sweat chloride concentration. CF mutations have also been found in other related clinical diseases such as congenital bilateral absence of the vas deferens (CBAVD), disseminated bronchiectasis and chronic pancreatitis. These clinical overlaps pose etiopathogenetic, diagnostic and therapeutic questions. Despite stunning advances in genomic technologies and drug discovery, drug therapy often improves disease symptoms but does not cure the disease. One of the main causes of this failure in CF cure may be attributable to genetic variability and to the scarce knowledge of CF biochemistry. Therefore, knowing the genotype of a patient might help improve drug efficacy, reduce toxicity and suggests innovative genomic-based therapy approaches.

Partial correction of defective Cl(-) secretion in cystic fibrosis epithelial cells by an analog of squalamine

Defective cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-mediated Cl(-) transport across the apical membrane of airway epithelial cells is implicated in the pathophysiology of CF lungs. A strategy to compensate for this loss is to augment Cl(-) transport through alternative pathways. We report here that partial correction of this defect could be attained through the incorporation of artificial anion channels into the CF cells. Introduction of GL-172, a synthetic analog of squalamine, into CFT1 cells increased cell membrane halide permeability. Furthermore, when a Cl(-) gradient was generated across polarized monolayers of primary human airway or Fischer rat thyroid cells in an Ussing chamber, addition of GL-172 caused an increase in the equivalent short-circuit current. The magnitude of this change in short-circuit current was ~30% of that attained when CFTR was maximally stimulated with cAMP agonists. Patch-clamp studies showed that addition of GL-172 to CFT1 cells also increased whole cell Cl(-) currents. These currents displayed a linear current-voltage relationship and no time dependence. Additionally, administration of GL-172 to the nasal epithelium of transgenic CF mice induced a hyperpolarization response to perfusion with a low-Cl(-) solution, indicating restoration of Cl(-) secretion. Together, these results demonstrate that in CF airway epithelial cells, administration of GL-172 is capable of partially correcting the defective Cl(-) secretion.

Nonsense-mediated mRNA decay in Saccharomyces cerevisiae

Cell survival depends on the precise and correct production of polypeptides. Eukaryotic cells have evolved conserved proofreading mechanisms to get rid of incomplete and potentially deleterious proteins. The nonsense-mediated mRNA decay (NMD) pathway is an example of a surveillance mechanism that monitors premature translation termination and promotes degradation of aberrant transcripts that code for nonfunctional or even harmful proteins. In this review we will describe our current knowledge of the NMD pathway, analyzing primarily the results obtained from the yeast Saccharomyces cerevisiae, but establishing functional comparisons with those obtained in higher eukaryotes. Based on these observations, we present two related working models to explain how this surveillance pathway recognizes and selectively degrades aberrant mRNAs.

Pharmacological treatment of the biochemical defect in cystic fibrosis airways

The understanding of the biochemical defect in cystic fibrosis (CF) has advanced considerably since discovery of the CF gene in 1989 and characterization of its product. Studies showing that the abnormality in chloride flux could be corrected by transfection of wild-type cystic fibrosis transmembrane conductance regulator (CFTR) complimentary deoxyribonucleic acid (cDNA) have led to gene therapy trials on both sides of the Atlantic. However, gene therapy as a treatment for CF has yet to be realized. Pharmacological manipulation of the biochemical defect may provide an alternative or complementary approach to treatment. This review will discuss pharmacological agents in development which could correct the abnormal ion movement. The mechanisms of action of these pharmacological agents can be divided broadly into drugs which affect the most common CF mutation, deltaF508, which increase trafficking of the mutant CF protein to the apical membrane; drugs which increase chloride secretion; and drugs which reduce sodium reabsorption across the apical membrane. Treatment options for cystic fibrosis have developed rapidly since discovery of the cystic fibrosis gene over a decade ago. The targeting of specific therapies for particular cystic fibrosis genotypes and the use of combination treatments of chloride channel openers with sodium channel blockers are likely to be key advances in the next decade.

Evidence that systemic gentamicin suppresses premature stop mutations in patients with cystic fibrosis

Here we report the effects of gentamicin treatment on cystic fibrosis transmembrane regulator (CFTR) production and function in CF airway cells and patients with CF with premature stop mutations. Using immunocytochemical and functional [6-methoxy-N- (3-sulfopropyl) quinolinium (SPQ)-based] techniques, ex vivo exposure of airway cells from stop mutation CF patients led to the identification of surface-localized CFTR in a dose-dependent fashion. Next, five patients with CF with stop mutations and five CF control subjects were treated with parenteral gentamicin for 1 wk, and underwent repeated in vivo measures of CFTR function (nasal potential difference [PD] measurements and sweat chloride [Cl(-)] testing). During the treatment period, the number of nasal PD readings in the direction of Cl(-) secretion was increased approximately 3-fold in the stop mutation patient group compared with controls (p < 0.001), and four of five stop mutation patients with CF had at least one reading during gentamicin treatment with a Cl(-) secretory response of more than -5 mV (hyperpolarized). A response of this magnitude was not seen in any of the CF control subjects (p < 0.05). In an independent series of experiments designed to test the ability of repeat nasal PDs to detect wild-type CFTR function, evidence of Cl(-) secretion was seen in 88% of control (non-CF) nasal PDs, and 71% were more than -5 mV hyperpolarized. Together, these results suggest that gentamicin treatment can suppress premature stop mutations in airway cells from patients with CF, and produce small increases in CFTR Cl(-) conductance (as measured by the nasal PD) in vivo.

Comprehensive mutation screening in a cystic fibrosis center

OBJECTIVES AND BACKGROUND

The identities of a cystic fibrosis (CF) patient's CFTR mutations can influence therapeutic strategies, but because >800 CFTR mutations exist, cost-effective, comprehensive screening requires a multistage approach. Single-strand conformation polymorphism and heteroduplex analysis (SSCP/HA) can be an important part of mutation detection, but must be calibrated within each laboratory. The sensitivity of a combined commercial-SSCP/HA approach to genotyping in a large, ethnically diverse US center CF population has not been established.

STUDY DESIGN

We screened all 27 CFTR exons in 10 human participants who had an unequivocal CF diagnosis including a positive sweat chloride test and at least 1 unknown allele after commercial testing for the 70 most common mutations by SSCP/HA. These participants were compared with 7 participants who had negative sweat tests but at least 1 other CF-like symptom meriting complete genotyping.

RESULTS

For the 10 CF participants, we detected 11 of 16 unknown alleles (69%) and all 4 of the known alleles (100%), for an overall rate of 75% inpatients not fully genotyped by conventional 70 mutation screen. For 7 participants with negative sweat tests, we confirmed 1 identified mutation in 14 alleles and detected 3 additional mutations. Mutations detected in both groups included 7 missense mutations (S13F, P67L, G98R, S492F, G970D, L1093P, N1303K) and 9 deletion, frameshift, nonsense or splicing mutations (R75X, G542X, DeltaF508, 451-458Delta8 bp, 5T, 663DeltaT, exon 13 frameshift, 1261+1G-->A and 3272-26A-->G). Three of these mutations were novel (G970D, L1093P, and 451-458Delta8 bp(1)). Thirteen other changes were detected, including the novel changes 1812-3 ins T, 4096-278 ins T, 4096-265 ins TG, and 4096-180 T-->G.

CONCLUSION

When combined with the 70 mutation Genzyme test, SSCP/HA analysis allows for detection of >95% of the mutations in an ethnically heterogeneous CF center population. We discuss 5 possible explanations that could account for the few remaining undetected mutations.

Pharmacological treatment of the ion transport defect in cystic fibrosis

Cystic fibrosis (CF) is a lethal monogenetic disease characterised by impaired water and ion transport over epithelia. The lung pathology is fatal and causes death in 95% of CF patients. The genetic basis of the disease is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated chloride channel. The most common mutation, DeltaF508, results in a protein that cannot properly be folded in the endoplasmic reticulum, is destroyed and hence does not reach the apical cell membrane. This paper will discuss those pharmacological approaches that are directed at correcting the defect in ion transport. At present, no clinically effective drug is available, although research has defined areas in which progress might be made. These are the following: (1) the drug 4-phenylbutyrate (4PBA) increases the expression of DeltaF508-CFTR in the cell membrane, probably by breaking the association between DeltaF508-CFTR and a chaperone; (2) a number of xanthines, in particular 8-cyclopentyl-1, 3-dipropylxanthine (CPX), are effective in activating CFTR, presumably by direct binding and also possibly by correcting the trafficking defect; (3) the isoflavone genistein can activate both wild-type and mutant CFTR, probably through direct binding to the channel; (4) purinergic agonists (ATP and UTP) can stimulate chloride secretion via a Ca(2+)-dependent chloride channel and in this way compensate for the defect in CFTR, but stable analogues will be required before this type of treatment has clinical significance; (5) treatment with inhaled amiloride may correct the excessive absorption of Na(+) ions and water by airway epithelial cells that appears connected to the defect in CFTR; although clinical tests have not been very successful so far, amiloride analogues with a longer half-life may give better results. The role of CFTR in bicarbonate secretion has not yet been established with certainty, but correction of the defect in bicarbonate secretion may be important in clinical treatment of the disease. Currently, major efforts are directed at developing a pharmacological treatment of the ion transport defect in CF, but much basic research remains to be done, in particular, with regard to the mechanism by which defective CFTR is removed in the endoplasmic reticulum by the ubiquitin-proteasome pathway, which is a central pathway in protein production and of significance for several other diseases apart from CF.

Clinical and pharmacological experience with LJP-394

LJP-394 is a synthetic biological with immunomodulatory functions. Composed of four double-stranded oligodeoxynucleotides attached to a central branched platform, the drug acts as an anti-"anti-ds-DNA" B-cell toleragen by rendering specific B-lymphocytes unresponsive to immunogen so they do not produce autoantibodies. Extensive animal studies and Phase II clinical trials suggested that the effects of LJP-394 are effective and safe when used as a weekly dose of 100 mg intravenously. Analysis of a multicentre, international Phase II/III clinical trial showed that patients with lupus nephritis and high affinity IgG antibodies to LJP-394 have clinical benefits. This includes increased time to renal flares, reduced number of renal flares, time to institution of high-dose corticosteroids and/or cyclophosphamide and lower anti-ds-DNA levels. A definitive trial is in progress. LJP-394 appears to be free of serious adverse reactions. Though promising, the role of LJP-394 in patients with active, organ-threatening lupus is still not known.

Aminoglycoside-mediated suppression of nonsense mutations in late infantile neuronal ceroid lipofuscinosis

The ability of aminoglycoside antibiotics to promote readthrough of eukaryotic stop codons has attracted interest in these drugs as potential therapeutic agents in human disorders caused by nonsense mutations. One disease for which such a therapeutic strategy may be viable is classical late infantile neuronal ceroid lipofuscinosis (LINCL), a fatal childhood neurodegenerative disorder with currently no effective treatment. Premature stop codon mutations in the gene CLN2 encoding the lysosomal tripeptidyl-peptidase 1 (TPP-I) are associated with disease in approximately half of children diagnosed with LINCL. The aim of this study was to examine the ability of the aminoglycoside gentamicin to restore TPP-I activity in LINCL cell lines. In one patient-derived cell line that was compound heterozygous for a commonly seen nonsense mutation, Arg208Stop and a different rare nonsense mutation, approximately 7% of normal levels of TPP-I were maximally restored with gentamicin treatment. In other cell lines from patients that were compound heterozygous for Arg208Stop and a splice junction mutation, approximately 0.5% of maximal activity was restored. These results suggest that pharmacological suppression of nonsense mutations by aminoglycosides or functionally similar pharmaceuticals may have therapeutic potential in LINCL.

Gene therapy and molecular approaches to the treatment of hereditary muscular disorders

Gene therapy for inherited muscle disease is an active area of research and development. Initial emphasis has been on gene replacement but alternative approaches are increasingly being considered in order to overcome difficulties, such as the immune rejection of transduced cells, the need for appropriate and tissue-specific control of expression, and the requirement for systemic spread in some conditions. However, the most significant obstacles to the clinical success of gene therapy are still the lack of efficiency and accuracy of gene medicine delivery.

Applicability of different antibodies for immunohistochemical localization of CFTR in sweat glands from healthy controls and from patients with cystic fibrosis

The hereditary disease cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Understanding of the consequences of CFTR gene mutations is derived chiefly from in vitro studies on heterologous cell cultures and on cells hyperexpressing CFTR. Data from ex vivo studies on human tissue are scarce and contradictory, a fact which is in part explained by secondary tissue destruction in most affected organs. The purpose of this study was to establish conditions under which wild-type and mutated CFTR can be studied in affected human tissue. Sweat glands carry the basic defect underlying CF and are not affected by tissue destruction and inflammation. Therefore, we used this tissue to test a panel of eight different CFTR antibodies under various fixation techniques. The antibodies were tested on skin biopsy sections from healthy controls, from CF patients homozygous for the most common mutation, DeltaF508, and from patients carrying two nonsense mutations. Of the eight CFTR antibodies, only three-M3A7, MATG 1104, and cc24-met the criteria necessary for immunolocalization of CFTR in sweat glands. The labeling pattern in the CF sweat glands was consistent with the postulated processing defect of DeltaF508 CFTR. The antibodies exhibited different sensitivities for detecting DeltaF508 CFTR.

A pilot study of the effect of gentamicin on nasal potential difference measurements in cystic fibrosis patients carrying stop mutations

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene containing a premature termination signal are expected to produce little or no CFTR chloride channels. It has been shown in vitro, that aminoglycoside antibiotics can increase the frequency of erroneous insertion of nonsense codons hence permitting the translation of CFTR alleles carrying missense mutations to continue reading to the end of the gene. This led to the appearance of functional CFTR channels at the apical plasma membrane. The aim of this research was to determine if topical application of gentamicin to the nasal epithelium of patients with cystic fibrosis (CF) carrying stop mutations can express, in vivo, functional CFTR channels. Nine CF patients carrying stop mutations (mean age 23 +/- 11 yr, range 12 to 46 yr) received gentamicin drops (0.3%, 3 mg/ml) three times daily intranasally for a total of 14 d. Nasal potential difference (PD) was measured before and after the treatment. Before gentamicin application all the patients had abnormal nasal PD typical of CF. After gentamicin treatment, significant repolarization of the nasal epithelium representing chloride transport was increased from -1 +/- 1 mV to -10 +/- 11 mV (p < 0. 001). In conclusion, gentamicin may influence the underlying chloride transport abnormality in patients with CF carrying stop mutations.

Sodium 4-phenylbutyrate downregulates Hsc70: implications for intracellular trafficking of DeltaF508-CFTR

The most common mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), DeltaF508, is a trafficking mutant that has prolonged associations with molecular chaperones and is rapidly degraded, at least in part by the ubiquitin-proteasome system. Sodium 4-phenylbutyrate (4PBA) improves DeltaF508-CFTR trafficking and function in vitro in cystic fibrosis epithelial cells and in vivo. To further understand the mechanism of action of 4PBA, we tested the hypothesis that 4PBA modulates the targeting of DeltaF508-CFTR for ubiquitination and degradation by reducing the expression of Hsc70 in cystic fibrosis epithelial cells. IB3-1 cells (genotype DeltaF508/W1282X) that were treated with 0.05-5 mM 4PBA for 2 days in culture demonstrated a dose-dependent reduction in Hsc70 protein immunoreactivity and mRNA levels. Immunoprecipitation with Hsc70-specific antiserum demonstrated that Hsc70 and CFTR associated under control conditions and that treatment with 4PBA reduced these complexes. Levels of immunoreactive Hsp40, Hdj2, Hsp70, Hsp90, and calnexin were unaffected by 4PBA treatment. These data suggest that 4PBA may improve DeltaF508-CFTR trafficking by allowing a greater proportion of mutant CFTR to escape association with Hsc70.

Update on clinical trials in the treatment of pulmonary disease in patients with cystic fibrosis

Cystic fibrosis is a congenital disease resulting from an abnormality of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. A defect in ion transport leads to poor clearance of viscoelastic secretions and a susceptibility to bacterial infection. This initiates a self-perpetuating cycle of infection and inflammation that accounts for the chronic endobronchial sepsis and pulmonary damage observed in patients with cystic fibrosis. Recent studies have attempted to correct the gene defect, enhance the expression and function of the CFTR protein and correct the ion transport defect. Improving the rheological properties of airway secretions, enhancing host defence and controlling inflammation are the other key strategies.

Aminoglycoside antibiotics restore dystrophin function to skeletal muscles of mdx mice

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene, leading to the absence of the dystrophin protein in striated muscle. A significant number of these mutations are premature stop codons. On the basis of the observation that aminoglycoside treatment can suppress stop codons in cultured cells, we tested the effect of gentamicin on cultured muscle cells from the mdx mouse - an animal model for DMD that possesses a premature stop codon in the dystrophin gene. Exposure of mdx myotubes to gentamicin led to the expression and localization of dystrophin to the cell membrane. We then evaluated the effects of differing dosages of gentamicin on expression and functional protection of the muscles of mdx mice. We identified a treatment regimen that resulted in the presence of dystrophin in the cell membrane in all striated muscles examined and that provided functional protection against muscular injury. To our knowledge, our results are the first to demonstrate that aminoglycosides can suppress stop codons not only in vitro but also in vivo. Furthermore, these results raise the possibility of a novel treatment regimen for muscular dystrophy and other diseases caused by premature stop codon mutations. This treatment could prove effective in up to 15% of patients with DMD.

[Mucoviscidosis: therapeutic strategies are multiplying]

Since the cloning of the defective gene in cystic fibrosis, much has been learned on the function of CFTR and on the mechanisms regulating its expression. Based on the current understanding of the processes involved in lung disease progression, a number of approaches have been developed using gene therapy and pharmacological agents. Several of these agents have been reported to restitute a function to CFTR with specific mutations. Other molecules act on channels other than CFTR, and may be effective by bypassing CFTR itself. In the present review the various therapeutical strategies currently investigated are discussed.

Exocytosis is not involved in activation of Cl- secretion via CFTR in Calu-3 airway epithelial cells

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, which mediates transepithelial Cl- transport in a variety of epithelia, including airway, intestine, pancreas, and sweat duct. In some but not all epithelial cells, cAMP stimulates Cl- secretion in part by increasing the number of CFTR Cl- channels in the apical plasma membrane. Because the mechanism whereby cAMP stimulates CFTR Cl- secretion is cell-type specific, our goal was to determine whether cAMP elevates CFTR-mediated Cl- secretion across serous airway epithelial cells by stimulating the insertion of CFTR Cl- channels from an intracellular pool into the apical plasma membrane. To this end we studied Calu-3 cells, a human airway cell line with a serous cell phenotype. Serous cells in human airways, such as Calu-3 cells, express high levels of CFTR, secrete antibiotic-rich fluid, and play a critical role in airway function. Moreover, dysregulation of CFTR-mediated Cl- secretion in serous cells is thought to contribute to the pathophysiology of cystic fibrosis lung disease. We report that cAMP activation of CFTR-mediated Cl- secretion across human serous cells involves stimulation of CFTR channels present in the apical plasma membrane and does not involve the recruitment of CFTR from an intracellular pool to the apical plasma membrane.