Genotype and phenotype in cystic fibrosis

Genetic influences on cystic fibrosis lung disease severity

Understanding the causes of variation in clinical manifestations of disease should allow for design of new or improved therapeutic strategies to treat the disease. If variation is caused by genetic differences between individuals, identifying the genes involved should present therapeutic targets, either in the proteins encoded by those genes or the pathways in which they function. The technology to identify and genotype the millions of variants present in the human genome has evolved rapidly over the past two decades. Originally only a small number of polymorphisms in a small number of subjects could be studied realistically, but speed and scope have increased nearly as dramatically as cost has decreased, making it feasible to determine genotypes of hundreds of thousands of polymorphisms in thousands of subjects. The use of such genetic technology has been applied to cystic fibrosis (CF) to identify genetic variation that alters the outcome of this single gene disorder. Candidate gene strategies to identify these variants, referred to as “modifier genes,” has yielded several genes that act in pathways known to be important in CF and for these the clinical implications are relatively clear. More recently, whole-genome surveys that probe hundreds of thousands of variants have been carried out and have identified genes and chromosomal regions for which a role in CF is not at all clear. Identification of these genes is exciting, as it provides the possibility for new areas of therapeutic development.

Impact of gene patents and licensing practices on access to genetic testing for long QT syndrome

Genetic testing for long QT syndrome exemplifies patenting and exclusive licensing with different outcomes at different times. Exclusive licensing from the University of Utah changed the business model from sole provider to two US providers of long QT syndrome testing. Long QT syndrome is associated with mutations in many genes, 12 of which are now tested by two competing firms in the United States, PGxHealth and GeneDx. Until 2009, PGxHealth was the sole provider, based largely on exclusive rights to patents from the University of Utah and elsewhere. University of Utah patents were initially licensed to DNA Sciences, whose patent rights were acquired by Genaissance, and then by Clinical Data, Inc., which owns PGxHealth. In 2002, DNA Sciences, Inc., "cleared the market" by sending cease-and-desist patent enforcement letters to university and reference laboratories offering long QT syndrome genetic testing. There was no test on the market for a 1- to 2-year period. From 2005-2008, most long QT syndrome-related patents were controlled by Clinical Data, Inc., and its subsidiary PGxHealth. Bio-Reference Laboratories, Inc., secured countervailing exclusive patent rights starting in 2006, also from the University of Utah, and broke the PGxHealth monopoly in early 2009, creating a duopoly for genetic testing in the United States and expanding the number of genes for which commercial testing is available from 5 to 12.

Three-dimensional reconstruction of human cystic fibrosis transmembrane conductance regulator chloride channel revealed an ellipsoidal structure with orifices beneath the putative transmembrane domain

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a membrane-integral protein that belongs to an ATP-binding cassette superfamily. Mutations in the CFTR gene cause cystic fibrosis in which salt, water, and protein transports are defective in various tissues. Here we expressed wild-type human CFTR as a FLAG-fused protein in HEK293 cells heterologously and purified it in three steps: anti-FLAG and wheat germ agglutinin affinity chromatographies and size exclusion chromatography. The stoichiometry of the protein was analyzed using various biochemical approaches, including chemical cross-linking, blue-native PAGE, size exclusion chromatography, and electron microscopy (EM) observation of antibody-decorated CFTR. All these data support a dimeric assembly of CFTR. Using 5,039 automatically selected particles from negatively stained EM images, the three-dimensional structure of CFTR was reconstructed at 2-nm resolution assuming a 2-fold symmetry. CFTR, presumably in a closed state, was shown to be an ellipsoidal particle with dimensions of 120 x 106 x 162 A. It comprises a small dome-shaped extracellular and membrane-spanning domain and a large cytoplasmic domain with orifices beneath the putative transmembrane domain. EM observation of CFTR.anti-regulatory domain antibody complex confirmed that two regulatory domains are located around the bottom end of the larger oval cytoplasmic domain.

Characterization of a 7,8-benzoflavone double effect on CFTR Cl(-) channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of adenosine triphosphate (ATP)-binding cassette (ABC) transporter ATPases. This protein forms a Cl(-) channel with a complex regulation; gene mutations cause cystic fibrosis disease. We investigated the interaction between the protein and the flavone UCCF-029 using the patch-clamp technique in the excised inside-out configuration in order to study the molecular mechanism of action for this potentiator on completely phosphorylated channel (25 U/ml protein kinase A) and a relatively low level of ATP (0.3 mM: ). Low concentrations of UCCF-029 (<50 nM: ) increase the open probability (p (o)), favoring the channel transition to an activated state, while high UCCF-029 (>50 nM: ) levels determine inhibition of the CFTR by a reduction of the total open time. Our data suggest that this drug can potentiate CFTR by binding to a specific site on the nucleotide binding domain, promoting dimer formation. The response of CFTR to variable concentrations of ATP is not modified by application of the potentiator UCCF-029 at either low, activatory, concentration or high, inhibitory, levels. Hence, we conclude that the potentiator may not interfere with binding of ATP but probably acts at an independent site in the protein, interacting directly with CFTR to modulate channel activity.

G551D and G1349D, two CF-associated mutations in the signature sequences of CFTR, exhibit distinct gating defects

Mutations in the gene encoding cystic fibrosis transmembrane conductance regulator (CFTR) result in cystic fibrosis (CF). CFTR is a chloride channel that is regulated by phosphorylation and gated by ATP binding and hydrolysis at its nucleotide binding domains (NBDs). G551D-CFTR, the third most common CF-associated mutation, has been characterized as having a lower open probability (Po) than wild-type (WT) channels. Patients carrying the G551D mutation present a severe clinical phenotype. On the other hand, G1349D, also a mutant with gating dysfunction, is associated with a milder clinical phenotype. Residues G551 and G1349 are located at equivalent positions in the highly conserved signature sequence of each NBD. The physiological importance of these residues lies in the fact that the signature sequence of one NBD and the Walker A and B motifs from the other NBD form the ATP-binding pocket (ABP1 and ABP2, named after the location of the Walker A motif) once the two NBDs dimerize. Our studies show distinct gating characteristics for these mutants. The G551D mutation completely eliminates the ability of ATP to increase the channel activity, and the observed activity is approximately 100-fold smaller than WT-CFTR. G551D-CFTR does not respond to ADP, AMP-PNP, or changes in [Mg(2+)]. The low activity of G551D-CFTR likely represents the rare ATP-independent gating events seen with WT channels long after the removal of ATP. G1349D-CFTR maintains ATP dependence, albeit with a Po approximately 10-fold lower than WT. Interestingly, compared to WT results, the ATP dose-response relationship of G1349D-CFTR is less steep and shows a higher apparent affinity for ATP. G1349D data could be well described by a gating model that predicts that binding of ATP at ABP1 hinders channel opening. Thus, our data provide a quantitative explanation at the single-channel level for different phenotypes presented by patients carrying these two mutations. In addition, these results support the idea that CFTR's two ABPs play distinct functional roles in gating.

No detectable improvements in cystic fibrosis transmembrane conductance regulator by nasal aminoglycosides in patients with cystic fibrosis with stop mutations

Cystic fibrosis (CF) is an autosomal recessive disorder caused by many types of genetic defects, including premature stop codons. Gentamicin can suppress stop mutations in CF transmembrane conductance regulator (CFTR) in vitro and in vivo, leading to improvements in CFTR-dependent ion transport and protein localization to the apical surface of respiratory epithelial cells. The primary objective of this study was to test whether nasally administered gentamicin or tobramycin could suppress premature stop mutations in CFTR, resulting in full-length, functional protein. A secondary objective was to obtain data to aid in the design of multicenter trials using the nasal potential difference as a study endpoint. A multicenter study was conducted in two cohorts of patients with CF, those heterozygous for stop mutations in the CFTR gene and those without nonsense mutations, to investigate the effects of both gentamicin and tobramycin administered over a 28-d period on sequential nasal potential difference and airway cell immunofluorescence endpoints. Eleven patients with CF with stop mutations were enrolled in a randomized, double-blinded, crossover fashion to receive each drug, while 18 subjects with CF without stop mutations were randomized 1:1 in a parallel fashion to receive one drug. After demonstration of drug delivery, neither aminoglycoside produced detectable changes in nasal ion transport or CFTR localization in brushed cells from either study group. These results with first-generation suppressive agents suggest the need for improved drug delivery methods and/or more potent suppressors of nonsense mutations to confer CFTR correction in subjects with CF heterozygous for nonsense mutations. The study provides valuable information on parameters of the nasal potential difference measurements for use in future multicenter clinical trials.

Cystic fibrosis lung disease: genetic influences, microbial interactions, and radiological assessment

Cystic fibrosis (CF) is a multiorgan disease caused by mutation of the CF transmembrane conductance regulator (CFTR) gene. Obstructive lung disease is the predominant cause of morbidity and mortality; thus, most efforts to improve outcomes are directed toward slowing or halting lung-disease progression. Current therapies, such as mucolytics, airway clearance techniques, bronchodilators, and antibiotics, aim to suppress airway inflammation and the processes that stimulate it, namely, retention and infection of mucus plaques at the airway surface. New approaches to therapy that aim to ameliorate specific CFTR mutations or mutational classes by restoring normal expression or function are being investigated. Because of its sensitivity in detecting changes associated with early airway obstruction and regional lung disease, high-resolution CT (HRCT) complements pulmonary function testing in defining disease natural history and measuring response to both conventional and experimental therapies. In this review, perspectives on the genetics and microbiology of CF provide a context for understanding the increasing importance of HRCT and other imaging techniques in assessing CF therapies.

Genotype-phenotype correlation for pulmonary function in cystic fibrosis

BACKGROUND

Since the CFTR gene was cloned, more than 1000 mutations have been identified. To date, a clear relationship has not been established between genotype and the progression of lung damage. A study was undertaken of the relationship between genotype, progression of lung disease, and survival in adult patients with cystic fibrosis (CF).

METHODS

A prospective cohort of adult patients with CF and two CFTR mutations followed up in an adult cystic fibrosis unit was analysed. Patients were classified according to functional effects of classes of CFTR mutations and were grouped based on the CFTR molecular position on the epithelial cell surface (I-II/I-II, I-II/III-V). Spirometric values, progression of lung disease, probability of survival, and clinical characteristics were analysed between groups.

RESULTS

Seventy four patients were included in the study. Patients with genotype I-II/I-II had significantly lower current spirometric values (p < 0.001), greater loss of pulmonary function (p < 0.04), a higher proportion of end-stage lung disease (p < 0.001), a higher risk of suffering from moderate to severe lung disease (odds ratio 7.12 (95% CI 1.3 to 40.5)) and a lower probability of survival than patients with genotype I-II/III, I-II/IV and I-II/V (p < 0.001).

CONCLUSIONS

The presence of class I or II mutations on both chromosomes is associated with worse respiratory disease and a lower probability of survival.

Chloride Transport in Nasal Ciliated Cells of Cystic Fibrosis Heterozygotes

Studying subjects heterozygous for mutations of the cystic fibrosis (CF) gene may help clarify the impact on disease onset of CF transmembrane conductance regulator protein (CFTR-)-dependent chloride secretion. CFTR-mediated chloride transport was evaluated in 52 heterozygous subjects, 32 healthy control subjects, and 77 patients with CF with class I or II mutations. We measured the change in nasal potential difference in response to chloride-free isoproterenol solution for each subject and used a video-imaging fluorescent dye assay to assess the percentage of nasal ciliated cells with cAMP-dependent anion conductance. Our findings did not confirm the standard assumption that heterozygosity implies 50% of normal CFTR function. Half the heterozygous subjects had CFTR-mediated chloride transport levels below 50% of the normal range, and one-third had levels similar to those of the patients with CF. This reduced CFTR function was not associated with an elevated prevalence of CF-like symptoms in heterozygous subjects but was highly related to respiratory status in the patients with CF. These data suggest that CFTR-dependent chloride conductance does not directly modulate disease severity but may be part of a more global defect in patients with CF involving other CFTR functions or currently unknown modulatory factors.

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.

CFTR Cl- channel function in native human colon correlates with the genotype and phenotype in cystic fibrosis

BACKGROUND & AIMS

Cystic fibrosis (CF) is caused by over 1000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and presents with a widely variable phenotype. Genotype-phenotype studies identified CFTR mutations that were associated with pancreatic sufficiency (PS). Residual Cl- channel function was shown for selected PS mutations in heterologous cells. However, the functional consequences of most CFTR mutations in native epithelia are not well established.

METHODS

To elucidate the relationships between epithelial CFTR function, CFTR genotype, and patient phenotype, we measured cyclic adenosine monophosphate (cAMP)-mediated Cl- secretion in rectal biopsy specimens from 45 CF patients who had at least 1 non-DeltaF508 mutation carrying a wide spectrum of CFTR mutations. We compared CFTR genotypes and clinical manifestations of CF patients who expressed residual CFTR-mediated Cl- secretion with patients in whom Cl- secretion was absent.

RESULTS

Residual anion secretion was detected in 40% of CF patients, and was associated with later disease onset (P < 0.0001), higher frequency of PS (P < 0.0001), and less severe lung disease (P < 0.05). Clinical outcomes correlated with the magnitude of residual CFTR activity, which was in the range of approximately 12%-54% of controls.

CONCLUSIONS

Specific CFTR mutations confer residual CFTR function to rectal epithelia, which is related closely to a mild disease phenotype. Quantification of rectal CFTR-mediated Cl- secretion may be a sensitive test to predict the prognosis of CF disease and identify CF patients who would benefit from therapeutic strategies that would increase residual CFTR activity.

Capsaicin potentiates wild-type and mutant cystic fibrosis transmembrane conductance regulator chloride-channel currents

To examine the effects of capsaicin on cystic fibrosis transmembrane conductance regulator (CFTR), we recorded wild-type and mutant CFTR chloride-channel currents using patch-clamp methods. The effects of capsaicin were compared with those of genistein, a well-characterized CFTR activator. In whole-cell experiments, capsaicin potentiates cAMP-stimulated wild-type CFTR currents expressed in NIH 3T3 cells or Chinese hamster ovary cells in a dose-dependent manner with a maximal response approximately 60% of that with genistein and an apparent Kd of 48.4 +/- 6.8 microM. In cell-attached recordings, capsaicin alone fails to activate CFTR in cells that show negligible basal CFTR activity, indicating that capsaicin does not stimulate the cAMP cascade. The magnitude of potentiation with capsaicin depends on the channel activity before drug application; the lower the prestimulated Po, the higher the potentiation. Single-channel kinetic analysis shows that capsaicin potentiates CFTR by increasing the opening rate and decreasing the closing rate of the channel. Capsaicin may act as a partial agonist of genistein because the maximally enhanced wild-type CFTR currents with genistein are partially inhibited by capsaicin. Capsaicin increases DeltaR-CFTR, a protein kinase A (PKA)-independent, constitutively active channel, in cell-attached patches. In excised inside-out patches, capsaicin potentiates the PKA-phosphorylated, ATP-dependent CFTR activity. Both capsaicin and genistein potentiate the cAMP-stimulated G551D-CFTR, DeltaF508-CFTR, and 8SA mutant channel currents. The binding site for capsaicin is probably located at the cytoplasmic domain of CFTR, because pipette application of capsaicin fails to potentiate CFTR activity. In conclusion, capsaicin is a partial agonist of genistein in activation of the CFTR chloride channel. Both compounds affect ATP-dependent gating of CFTR.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

Pathophysiology and management of pulmonary infections in cystic fibrosis

This comprehensive State of the Art review summarizes the current published knowledge base regarding the pathophysiology and microbiology of pulmonary disease in cystic fibrosis (CF). The molecular basis of CF lung disease including the impact of defective cystic fibrosis transmembrane regulator (CFTR) protein function on airway physiology, mucociliary clearance, and establishment of Pseudomonas aeruginosa infection is described. An extensive review of the microbiology of CF lung disease with particular reference to infection with P. aeruginosa is provided. Other pathogens commonly associated with CF lung disease including Staphylococcal aureus, Burkholderia cepacia, Stenotrophomonas maltophilia, Achromobacter xylosoxidans and atypical mycobacteria are also described. Clinical presentation and assessment of CF lung disease including diagnostic microbiology and other measures of pulmonary health are reviewed. Current recommendations for management of CF lung disease are provided. An extensive review of antipseudomonal therapies in the settings of treatment for early P. aeruginosa infection, maintenance for patients with chronic P. aeruginosa infection, and treatment of exacerbation in pulmonary symptoms, as well as antibiotic therapies for other CF respiratory pathogens, are included. In addition, the article discusses infection control policies, therapies to optimize airway clearance and reduce inflammation, and potential future therapies.

Stable dimeric assembly of the second membrane-spanning domain of CFTR (cystic fibrosis transmembrane conductance regulator) reconstitutes a chloride-selective pore

Structural information is required to define the molecular basis for chloride conduction through CFTR (cystic fibrosis transmembrane conductance regulator). Towards this goal, we expressed MSD2, the second of the two MSDs (membrane-spanning domains) of CFTR, encompassing residues 857-1158 in Sf9 cells using the baculovirus system. In Sf9 plasma membranes, MSD2 migrates as expected for a dimer in non-dissociative PAGE, and confers the appearance of an anion permeation pathway suggesting that dimeric MSD2 mediates anion flux. To assess directly the function and quaternary structure of MSD2, we purified it from Sf9 cells by virtue of its polyhistidine tag and nickel affinity. Reconstitution of MSD2 into liposomes conferred a 4,4'-di-isothiocyanostilbene-2,2'-disulphonate-inhibitable, chloride-selective electrodiffusion pathway. Further, this activity is probably mediated directly by MSD2 as reaction of its single cysteine residue (Cys866) with the thiol modifying reagent, N(alpha)(3-maleimidylpropionyl)biocytin, inhibited chloride flux. Only MSD2 dimers were labelled by N(alpha)(3-maleimidylpropionyl)biocytin, supporting the idea that only dimeric MSD2 can mediate anion flux. As a further test of this hypothesis, we conducted a second purification procedure, wherein purified dimeric and monomeric MSD2 proteins were reconstituted separately. Only proteoliposomes containing stable MSD2 dimers mediated chloride electrodiffusion, providing direct evidence that dimeric MSD2 mediates chloride channel function. In summary, we have shown that the second membrane domain of CFTR can be purified and functionally reconstituted as a chloride channel, providing a tool for probing the structural basis of chloride conduction through CFTR.

Dimeric cystic fibrosis transmembrane conductance regulator exists in the plasma membrane

CFTR (cystic fibrosis transmembrane conductance regulator) mediates chloride conduction across the apical membrane of epithelia, and mutations in CFTR lead to defective epithelial fluid transport. Recently, there has been considerable interest in determining the quaternary structure of CFTR at the cell surface, as such information is a key to understand the molecular basis for pathogenesis in patients harbouring disease-causing mutations. In our previous work [Ramjeesingh, Li, Kogan, Wang, Huan and Bear (2001) Biochemistry 40, 10700-10706], we showed that monomeric CFTR is the minimal functional form of the protein, yet when expressed in Sf 9 cells using the baculovirus system, it also exists as dimers. The purpose of the present study was to determine if dimeric CFTR exists at the surface of mammalian cells, and particularly in epithelial cells. CFTR solubilized from membranes prepared from Chinese-hamster ovary cells stably expressing CFTR and from T84 epithelial cells migrates as predicted for monomeric, dimeric and larger complexes when subjected to sizing by gel filtration and analysis by non-dissociative electrophoresis. Purification of plasma membranes led to the enrichment of CFTR dimers and this structure exists as the complex glycosylated form of the protein, supporting the concept that dimeric CFTR is physiologically relevant. Consistent with its localization in plasma membranes, dimeric CFTR was labelled by surface biotinylation. Furthermore, dimeric CFTR was captured at the apical surface of intact epithelial cells by application of a membrane-impermeable chemical cross-linker. Therefore it follows from the present study that CFTR dimers exist at the surface of epithelial cells. Further studies are necessary to understand the impact of dimerization on the cell biology of wild-type and mutant CFTR proteins.

Asymptomatic cystic fibrosis diagnosed in an adult evaluated for hematuria

We report a case of cystic fibrosis in an asymptomatic man evaluated for hematuria with infertility. The ever-broadening spectrum of atypical adult presentations of cystic fibrosis should prompt physicians to have a lower threshold for ordering genetic screening for cystic fibrosis transmembrane regulator (CFTR) gene mutations.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Effect of meconium ileus on the clinical prognosis of patients with cystic fibrosis

The objective of the present study was to determine the possible prognostic factors which may explain the difference in the survival of patients with cystic fibrosis (CF) with and without meconium ileus. Over a period of 20 years, 127 patients with CF, whose diagnosis was confirmed by typical clinical characteristics and altered sweat chloride levels, were studied retrospectively. The patients were divided into two groups: group 1 consisted of patients who presented CF and meconium ileus (N = 9), and group 2 consisted of patients with CF without meconium ileus (N = 118). The characteristics studied were based on data obtained upon admission of the patients using a specific protocol. Demographic, clinical, nutritional and laboratory data were obtained. The genotype was determined in 106 patients by PCR. Survival was analyzed using the Kaplan-Meier method. The median follow-up period was 44 months. A statistically significant difference was observed between the groups studied regarding the following variables: age at diagnosis and weight and height z scores. The presence of meconium ileus was associated with an earlier diagnosis; these patients had greater deficits in height and weight at the time of diagnosis and at the end of the study. The estimated probability of survival for patients with CF without meconium ileus was 62 +/- 14% and for those with meconium ileus 32 +/- 18%. Patients with CF and meconium ileus presented a poor nutritional status at diagnosis and a lower survival rate compared to the general CF population.

Pharmacotherapy of the ion transport defect in cystic fibrosis

1. More than 1300 different mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis (CF), a disease characterized by deficient epithelial Cl- secretion and enhanced Na+ absorption. The clinical course of the disease is determined by the progressive lung disease. Thus, novel approaches in pharmacotherapy are based primarily on correction of the ion transport defect in the airways. 2. The current therapeutic strategies try to counteract the deficiency in Cl- secretion and the enhanced Na+ absorption. A number of compounds have been identified, such as genistein and xanthine derivatives, which directly activate mutant CFTR. Other compounds may activate alternative Ca2+-activated Cl- channels or basolateral K+ channels, which supply the driving force for Cl- secretion. Apart from that, Na+ channel blockers, such as phenamil and benzamil, are being explored, which counteract the hyperabsorption of NaCl in CF airways. 3. Clinical trials are under way using purinergic compounds such as the P2Y(2) receptor agonist INS365. Activation of P2Y(2) receptors has been found to both activate Cl- secretion and inhibit Na+ absorption. 4. The ultimate goal is to recover Cl- channel activity of mutant CFTR by either enhancing synthesis and expression of the protein or by activating silent CFTR Cl- channels. Strategies combining these drugs with compounds facilitating Cl- secretion and inhibiting Na+ absorption in vivo may have the best chance to counteract the ion transport defect in cystic fibrosis.

CFTR: interacting with everything?

More than 1,300 different mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) are the cause for cystic fibrosis. CFTR is in charge of proper secretion and absorption of electrolytes, and thus the disease is characterized by defective epithelial Cl(-) secretion and enhanced Na(+) absorption. Recent studies show that CFTR interacts with other proteins via PDZ domains.

Chronic pancreatitis: diagnosis, classification, and new genetic developments

The utilization of recent advances in molecular and genomic technologies and progress in pancreatic imaging techniques provided remarkable insight into genetic, environmental, immunologic, and pathobiological factors leading to chronic pancreatitis. Translation of these advances into clinical practice demands a reassessment of current approaches to diagnosis, classification, and staging. We conclude that an adequate pancreatic biopsy must be the gold standard against which all diagnostic approaches are judged. Although computed tomography remains the initial test of choice for the diagnosis of chronic pancreatitis, the roles of endoscopic retrograde pancreatography, endoscopic ultrasonography, and magnetic resonance imaging are considered. Once chronic pancreatitis is diagnosed, proper classification becomes important. Major predisposing risk factors to chronic pancreatitis may be categorized as either (1) toxic-metabolic, (2) idiopathic, (3) genetic, (4) autoimmune, (5) recurrent and severe acute pancreatitis, or (6) obstructive (TIGAR-O system). After classification, staging of pancreatic function, injury, and fibrosis becomes the next major concern. Further research is needed to determine the clinical and natural history of chronic pancreatitis developing in the context of various risk factors. New methods are needed for early diagnosis of chronic pancreatitis, and new therapies are needed to determine whether interventions will delay or prevent the progression of the irreversible damage characterizing end-stage chronic pancreatitis.

Amelioration of intestinal disease severity in cystic fibrosis mice is associated with improved chloride secretory capacity

The variability in intestinal disease severity in patients with cystic fibrosis (CF) has been associated with the expression of secondary modifier genes. The locus containing these modifier genes in CF patients is syntenic with a modifier locus previously associated with survival in CF transmembrane conductance regulator-knockout mice. These previous studies showed that the proportion of CF mice that survive weaning (mildly affected mice) versus those that succumb to obstruction of the small intestine (severely affected) is related to their genetic background and the expression of modifier genes. In the present work, we show that the basal transepithelial chloride transport measured across jejuna obtained from mice of mixed genetic backgrounds segregates into two groups, some mice having low and others having high, near normal chloride transport. Further, we report that the segregation of mice with respect to intestinal chloride transport correlates with their predicted segregation on the basis of genotype at the "modifier locus." These findings support the hypothesis that intestinal disease modification in CF mice correlates with improved chloride transport through non-CF transmembrane conductance regulator chloride channels.

Effect of genistein on native epithelial tissue from normal individuals and CF patients and on ion channels expressed in Xenopus oocytes

The flavonoid genistein has been shown to activate a Cl(-) conductance in various cell types expressing CFTR. We examined if similar effects can be observed when genistein is applied to native ex vivo tissues from human respiratory tract and rectum. We further compared the effects when genistein was applied to oocytes of Xenopus laevis expressing CFTR. In oocytes, both wtCFTR and DeltaF508-CFTR were activated by genistein while both cyclic AMP (K(v)LQT1) and Ca(2+) (SK4) activated K(+) channels were inhibited at high concentrations of genistein. Biopsies from nasal polyps and rectal mucosa were obtained from normal individuals (non-CF) and CF patients and in the presence of amiloride (10 micromol l(-1); mucosal side) the effects of genistein were assessed using a perfused Ussing chamber. In non-CF airway epithelia, genistein (50 micromol l(-1); mucosal side) increased lumen negative I(sc) but had no additional effects on tissues pre-stimulated with IBMX and forskolin (100 micromol l(-1) and 1 micromol l(-1); both sides). In non-CF rectal biopsies, in the presence of amiloride (10 micromol l(-1); mucosal side) and indomethacin (10 micromol l(-1); basolateral side), genistein increased lumen negative I(sc) and enabled cholinergic (carbachol; CCH, 100 micromol l(-1); basolateral side) stimulation of Cl(-) secretion indicating activation of luminal CFTR Cl(-) channels. However, after stimulation with IBMX/forskolin, genistein induced opposite effects and significantly inhibited CCH activated I(sc). In CF airway and intestinal tissues genistein failed to induce Cl(-) secretion. Thus, genistein is able to activate luminal CFTR Cl(-) conductance in non-CF tissues and mutant CFTR in oocytes. However, additional inhibitory effects on basolateral K(+) conductance and missing effects in native CF tissues do not support the use for pharmacological intervention in CF.

Genetic predispositions to acute and chronic pancreatitis

Advances in molecular genetics have provided the powerful tools necessary to identify the key molecules and mechanisms that underly the disease process. Continued work in this area promises to reveal new insights as new disease genes are discovered. This article focuses on the insights into the cause of acute and chronic pancreatitis gained by investigation of the HP genes, the diagnosis of the known mutations, the fascinating observation of nonpenetrance, and a look at future directions.

Multiplex PCR of polymorphic markers flanking the CFTR gene; a general approach for preimplantation genetic diagnosis of cystic fibrosis

Cystic fibrosis (CF) is the first monogenic disorder for which single cell preimplantation genetic diagnosis (PGD) has been successfully applied. The spectrum of mutations in CF is extremely heterogeneous, and hence, the development of mutation-specific PGD protocols is impracticable. The current study reports the development and evaluation of a general multiplex marker polymerase chain reaction (PCR) protocol for PGD of CF. Four closely linked highly polymorphic (CA)(n) repeat markers D7S523, D7S486, D7S480 and D7S490, flanking the cystic fibrosis transmembrane regulator (CFTR) gene, were used. In 99% of the single cells tested (100 leukocytes and 50 blastomeres), multiplex PCR results were obtained and the overall allelic drop out (ADO) rate varied from 2 to 5%. After validation for the presence of ADO and additional alleles, 95% of the multiplex PCR results were accepted to construct the marker genotypes. Depending on the genotype of the couple, and taking into account the embryos lost for transfer due to validation criteria (5%), ADO (0-2%) and single recombination (1.1-3%), in general >90% of the embryos could be reliably genotyped by PGD using a single blastomere. The risk of misdiagnosis equals the chance of a double recombination between informative flanking markers and is <0.05%. Therefore, this polymorphic and multi-allelic marker system is a reliable and generally applicable alternative for mutation-directed PGD protocols. Furthermore, it provides a test for the origin of the detected genotype and also gives an indication of the chromosomal ploidy status of the blastomere tested.

Human Genome Project and cystic fibrosis--a symbiotic relationship

When Watson and Crick determined the structure of DNA in 1953, a biological revolution began. One result of this revolution is the Human Genome Project. The primary goal of this international project is to obtain the complete nucleotide sequence of the human genome by the year 2005. Although molecular biologists and geneticists are most enthusiastic about the Human Genome Project, all areas of clinical medicine and fields of biology will be affected. Cystic fibrosis is the most common, inherited, lethal disease of white persons. In 1989, researchers located the cystic fibrosis gene on the long arm of chromosome 7 by a technique known as positional cloning. The most common mutation (a 3-base pair deletion) of the cystic fibrosis gene occurs in 70% of patients with cystic fibrosis. The knowledge gained from genetic research on cystic fibrosis will help researchers develop new therapies (e.g., gene) and improve standard therapies (e.g., pharmacologic) so that a patient's life span is increased and quality of life is improved. The purpose of this review is twofold. First, the article provides an overview of the Human Genome Project and its clinical significance in advancing interdisciplinary care for patients with cystic fibrosis. Second, the article includes a discussion of the genetic basis, pathophysiology, and management of cystic fibrosis.

Newly diagnosed cystic fibrosis in adults: pattern and distribution of bronchiectasis in 12 cases

OBJECTIVE

The aim of this study was to analyse the type and distribution of bronchiectasis at computed tomography (CT) in adults with recently diagnosed cystic fibrosis.

METHODS

The CT examinations of 12 consecutive patients diagnosed with cystic fibrosis in adulthood (six male, six female; age range at diagnosis 25-63 years) were analysed retrospectively using a modified Bhalla scoring system. Bronchiectasis was catergorized by Reid type and by location within the affected lobe (peripheral, central or mixed).

RESULTS

Bronchiectasis was identified in 70 of 71 lobes. Two or more bronchopulmonary segments were involved in 67 of 71 lobes. The majority of lobes demonstrated cylindrical bronchiectasis as the predominant type (n = 51), with varicose (n = 11) and cystic (n = 8) less commonly seen. Bronchiectasis was mainly central in 16 lobes, and both central and peripheral in 54 lobes. The severity of bronchial dilatation was greatest for the upper lobes (mean score, 1.75 right, 1.58 left) compared with the middle lobe/lingula (mean score, 1.42 right, 1.17 left) and lower lobes (mean score, 1.09 right, 1.17 left).

CONCLUSION

Bronchiectasis in patients with cystic fibrosis diagnosed in adulthood is usually widespread, predominantly cylindrical, and is more severe in the upper lobes. The diagnosis of cystic fibrosis should be considered in adults with 'idiopathic' bronchiectasis showing these features on CT.

The cystic fibrosis transmembrane conductance regulator and its function in epithelial transport

CF is a well characterized disease affecting a variety of epithelial tissues. Impaired function of the cAMP activated CFTR Cl- channel appears to be the basic defect detectable in epithelial and non-epithelial cells derived from CF patients. Apart from cAMP-dependent Cl- channels also Ca2+ and volume activated Cl- currents may be changed in the presence of CFTR mutations. This is supported by recent additional findings showing that different intracellular messengers converge on the CFTR Cl- channel. Analysis of the ion transport in CF airways and intestinal epithelium identified additional defects in Na+ transport. It became clear recently that mutations of CFTR may also affect the activity of other membrane conductances including epithelial Na+ channels, KvLQT-1 K+ channels and aquaporins (Fig. 7). Several additional, initially unexpected effects of CFTR on cellular functions, such as exocytosis, mucin secretion and regulation of the intracellular pH were reported during the past. Taken together, these results clearly indicate that CFTR not only acts as a cAMP regulated Cl- channel, but may fulfill several other cellular functions, particularly by regulating other membrane conductances. Failure in CFTR dependent regulation of these membrane conductances is likely to contribute to the defects observed in CF. Currently, no general concept is available that can explain how CFTR controls this variety of cellular functions. Further studies will have to verify whether direct protein interaction, specific effects on membrane turnover, changes of the intracellular ion concentration or additional proteins are involved in these regulatory loops. At the end of this review one cannot share the provocative and reassuring title "CFTR!" of a review written a few years ago [114]. Today one might rather finish with the statement "CFTR?".

Lack of correlation between CFTR expression, CFTR Cl- currents, amiloride-sensitive Na+ conductance, and cystic fibrosis phenotype

Cystic fibrosis (CF) is characterized by defective Cl- and enhanced Na+ conductance, both due to malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) protein in airway epithelial cells. In the present study we examined whether expression of CFTR mRNA (CFTR messenger ribonucleic acid) is different in airway epithelia derived from either CF patients or healthy volunteers. Moreover, we tried to correlate differences in epithelial Cl- and Na+ conductance with the level of CFTR mRNA expression and studied whether these properties correlate to the clinical phenotype of CF patients. To that end, CFTR mRNA was determined by means of quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and cyclic adenosine monophosphate (cAMP)-activated Cl- and epithelial Na+ conductances were examined in airway epithelial cells using microelectrode techniques. Complementary in vitro data were obtained from cultured CF and non-CF airway epithelial cell lines. Genotype and Shwachman score were assessed for each patient. We found variable levels of CFTR mRNA expression in airway cells of both CF patients and healthy volunteers. As expected, epithelial Na+ conductance was enhanced and CFTR Cl- conductance was absent in airway cells from CF patients. However, CFTR mRNA expression did not correlate with either electrophysiological properties or Shwachman scores obtained from CF patients. In addition, CFTR mRNA expression did not correlate to Cl- conductance in cultured CF and non-CF airway epithelial cells. These results indicate a lack of correlation between levels of CFTR mRNA and CFTR function, and that only small amounts of CFTR are required for expression of the CFTR Cl- conductance.

Clinical implications of cystic fibrosis transmembrane conductance regulator mutations

Cystic fibrosis (CF) phenotypes are determined by mutations in the CF gene, genetic background, and environment. The nature of the cystic fibrosis transmembrane conductance regulator (CFTR) mutation determines the extent of protein function. CFTR mutations that abolish protein function are associated with severe CF phenotypes. Mutants that retain partial function of CFTR are associated with mild phenotypes. The effect of CFTR dysfunction is variable in different tissues. Atypical phenotypes caused by mutations in the CF gene may be revealed by CFTR mutation analysis and family studies. These phenotypes help to define the spectrum of clinical manifestations caused by CFTR mutations.