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Dorota Sands
Cystic Fibrosis Centre,
Institute of Mother and Child, Warsaw, Poland
The main underlying physiologic abnormality in cystic fibrosis (CF) is dysfunction of the CF transmembrane conductance
regulator (CFTR), which results in abnormal transport of sodium and chloride across epithelial surfaces. CFTR function
could be tested in vivo using measurements of nasal transepithelial potential difference (PD). Nasal measurements show
characteristic features of CF epithelia, including hyperpolarized baseline readings (basal PD), excessive depolarization in
response to sodium channel inhibitors, such as amiloride (∆Amiloride), and little or no chloride (Cl–) secretion in response to
isoproterenol in a chloride-free solution (∆Cl– free-isoproterenol). PD test is applied for CF diagnosis and monitoring of new
therapeutic modulations and corrections.
Key words: cystic fibrosis, nasal transepithelial potential difference (PD)
There is great heterogeneity in the clinical manifestations of cystic fibrosis (CF). Some patients may have all the Cystic fibrosis (CF) is the most common severe classical manifestations of CF from infancy and have autosomal recessive disorder among Caucasians. a relatively poor prognosis, while others have much CF results from mutations in the gene that encodes milder disease manifestations. According to the European a protein-the cystic fibrosis transmembrane regulator Diagnostic Working Group terminology patients are (CFTR). CFTR is a chloride (Cl–) channel located at the diagnosed with classic or typical CF if they have one or apical membrane of exocrine epithelial cells (1). As an ion more phenotypic characteristics and a sweat chloride transporter, it promotes chloride efflux and secondarily concentration of >60 mmol/l (3). Patients with classic CF inhibits sodium influx via the epithelial sodium channel (ENaC). Its dysfunction in CF leads to increased sodium can have exocrine pancreatic insufficiency or pancreatic absorption and impaired chloride secretion. The resulting sufficiency. The disease can have a severe course with mucus dehydration compromises mucociliary clearance. CF rapid progression of symptoms or a milder course airways become obstructed, more vulnerable to infection with very little deterioration over time. Patients with and inflammation ultimately leading to their occlusion non-classic or atypical CF have a CF phenotype in at and destruction. Obstruction of pancreatic ducts, the least one organ system and a normal (<30 mmol/l) or biliary tract, and the vas deferens can occur. CF patients borderline (30-60 mmol/l) sweat chloride level. In these typically develop persistent pulmonary infections and often patients confirmation of the diagnosis of CF requires have pancreatic insufficiency, diminished weight, chronic detection of two disease causing mutations or direct hepatobiliary inflammation, and male infertility. Respiratory quantification of CFTR dysfunction by nasal potential failure is the most common cause of death.
difference measurement (PD). Non-classic CF includes At present, treatment of CF is mainly symptomatic, patients with multiorgan or single organ involvement. aimed at improving mucociliary clearance and treating Most of these patients have exocrine pancreatic sufficiency infection (1, 2).
Most men with CF are infertile due to obstructive azoospermia, which in its severest form presents as congenital bilateral absence of the vas deferens (CBAVD) (4, 5). CBAVD The diagnosis of cystic fibrosis (CF) is usually made also occurs in 1 to 2% of infertile males who are otherwise by clinical findings or newborn screening, sweat chloride healthy, the majority of whom carry CFTR gene mutations concentration and genetic analysis.
on one or both alleles (6, 7). Higher frequency of CFTR gene mutations have been also observed in patients with other Intestinal current measurements (ICM) is one of
single organ diseases, like idiopathic chronic pancreatitis, the alternative methods for demonstration of defective chronic rhinosinusitis, disseminated bronchiectasis (8). ion transport. It can be carried out on suction biopsy These are clinical entities associated with CFTR dysfunction tissue obtained from the colonic mucosa. Biopsy tissue named CFTR-related disorders (CFTR-RD), where the is mounted in an Ussing chamber, the transepithelial diagnosis of CF cannot be unambiguously established. electrical activity, expressed as the short-circuit current They are all listed on WHO diagnostic list for single organ (SCC), measured and the response to secretagogues disease phenotypes associated with CFTR mutations (9). tested. The normal intestine generates a rise in SCC, It is likely that this classification will need further revision a reflection of increased Cl– secretion, when secretagogues in the future as our knowledge and understanding of these are added, but this response is impaired in CF tissues conditions increase.
Nasal potential difference (NPD) measurement
is an electrophysiological test that was developed by The heterogeneity of cystic fibrosis (CF) disease is partially Knowles et al. (15) and is a part of the criteria for CF explained by this high number of different mutations in the diagnosis according to the CFF consensus statement , CFTR gene (10). More than 1900 sequence variations have also in European consensus cited above (16). This test been reported in the CFTR gene including missense, small assesses CFTR activity by the change in the transepithelial deletion or insertion, frameshift, and nonsense mutations, potential difference as a reaction to superinfusion of the often with geographic or ethnic variations in frequency nasal mucosa by several solutions (17).
(2). The classification of CFTR gene mutations in five It is used to demonstrate abnormal function of the classes according to their functional effects on CFTR CFTR protein, establish a diagnosis of CF in patients protein production and function is based on functional with atypical presentation and as a surrogate outcome studies: severe vs non-severe.
measure for new therapies that correct the airway ion When patients who are homozygous, or compound transport defect. CF nose and lower airway demonstrate heterozygous for a class I-III CFTR mutation, are compared similar abnormalities of ion transport.
to patients who carry at least one class IV-V CFTR mutation, the latter group tends to have less severe disease (11).
However, it is only partly predictive of individual outcomes, because many CFTR mutations have different functional consequences and cannot be assigned to one Patients with CF have a characteristic pattern of particular class and only a limited number of mutations have bioelectric properties which reflects defective cAMP- been studied. The CF phenotype is affected by the CFTR mediated (CFTR) Cl– secretion and accelerated rate of Na+ genotype as well as by other genetic and environmental transport across ENaC. Increased sodium absorption is reflected by a basal hyperpolarization of the nasal mucosa Extensive mutation analysis cannot guarantee detection (a large negative PD) together with an increased response of the two disease-causing mutations in all patients; 1-5% to amiloride, which blocks ENaC. The defect in chloride of alleles remain undetermined in CF patients with the transport is demonstrated by a reduced or abolished classical form and even more in patients with atypical repolarization in response to both an electrochemical gradient favorable to chloride efflux during perfusion The limitations of genetic testing include an inability of the mucosa with chloride free solution and addition to detect mutations in the noncoding and promoter of isoprenaline, a cAMP agonist (18). Five aspects of bioelectric properties in the nasal Mutation analysis is not the answer to every diagnostic epithelium usually tested include: 1) the basal PD; 2) the inhibition of PD with perfusion of amiloride; 3) the The sweat test has been used for more than 50 years basal chloride conductance, as indexed by the chloride
for the diagnosis of cystic fibrosis (CF) and remains an diffusion PD in response to perfusion of solution containing important diagnostic test in the genomic era. It has been amiloride but no chloride; 4) the increase in the chloride documented that sweat chloride is related to the abnormal diffusion PD with the addition of isoproterenol to the function of CFTR and shows good discrimination for perfusate; and 5) the hyperpolarization associated with the diagnosis of CF (12). However, most studies did not the addition of ATP to the perfusate (19).
include a ‘healthy’ control group, were performed prior to the availability of the CFTR gene mutation analysis Technical aspectsand did not comply with the currently accepted sweat Nasal potential difference can be measured by a high test method. There is no definitive study that quantitates impedance voltmeter between two electrodes: an exploring the sweat chloride and sodium concentration in a truly electrode placed on the surface of the epithelium and a normal population. Further work is required to re-establish reference electrode (subcutaneous needle or abrasion) sweat electrolyte reference intervals, using the current in contact with interstitial milieu.
standardized sweat test, CFTR gene mutation analysis A fluid-filled bridge or a solid agar bridge were used, to exclude carriers, and correct statistical analyses in referenced to either a subcutaneous electrode or an healthy participants with ages spanning from infancy abraded skin electrode system. However, recent studies to adulthood.
showed superior reliability of the agar nasal catheter approach compared to the perfusion nasal catheter dysfunction of transepithelial transport was evaluated method. Measurements under the inferior turbinate (23). This comprehensive, prospective study of healthy and the floor of the nasal cavity are valid, repeatabilities males and men with a variety of CFTR-associated of both methods are similar (20). The region under the disorders demonstrated a wide spectrum of CFTR-inferior turbinate or on the floor is explored for the site of mediated abnormalities of transepithelial transport, most negative voltage and the probe is stabilized at that which showed a relation to the number and functional point, manually or physically with tape. The solutions severity of CFTR gene mutations. The men with CBAVD are then sequentially perfused over the nasal mucosa showed a wide range of transepithelial measurements, usually through the second lumen of a double-barreled which correlated with the number of identified CFTR catheter and changed after a minimum of 3 min. With mutations and overlapped with those in the healthy each solution, there is an attempt to obtain a steady, control subjects.
noise-free voltage plateau for the terminal 30 sec.
A number of different studies have examined the relationship between the extent of ion transport abnormality and the severity of disease in CF (21, 24, 25).
Baseline voltage is -18.2±8.3 mV (mean ± SD) for Studies of Fajac and Leal showed a correlation of normals, and -45.3±11.4 mV for CF patients. There is a lack NPD with clinical picture.
of response to perfusion with zero chloride +isoproterenol Fajac et al. study has shown that in adult CF patients (+3.2 ±3.5 mV) vs that in normals (-23.7±10.2 mV) with basal nasal PD in the normal range, respiratory (17).
function was less impaired than in CF patients with more Perfusion with agents that increase cAMP (like negative nasal PD (24). Moreover, among CF patients with isoproterenol) are then likely to induce a greater degree of two “severe” mutations, a similar relationship between Cl– secretion. Consequently, the larger changes in PD may nasal PD and respiratory function was found: in patients then allow clearer quantification of the CF bioelectric defects with normal nasal PD, respiratory function was less (18). Taken together, the net response to zero-chloride impaired than in patients with higher nasal PD. Nasal plus isoproterenol provides the clearest discrimination PD in a given patient had good reproducibility.
between CF and normal subjects; the Cl– conductance However, there was no significant relationship between is absent (or very low) in CF patients.
nasal potential difference and the severity of the genotype. There is overlap of the baseline values and in the responses Nasal epithelial ion transport in cystic fibrosis was linked to amiloride between CF and normal subjects.
to the clinical expression of the disease. The pancreatic status appeared to be mostly related to the defect in epithelial chloride secretion whereas the respiratory There are many variations between the protocols used status was mostly related to abnormal sodium transport in different centers, like: warming of the solutions, type (24).
of catheter, perfusion speed, composition of solutions.
Leal et al. data also indicated that nutritional status, Method is time consuming and need experience lung function, age, and chronic P. aeruginosa infection operator, which limits larger use of this awaited insight were co-dependent variables related to both regulatory into CFTR function. Variability of NPD measurements can (sodium channel) and primary (chloride channel) CFTR also reduce the power of studies using NPD as outcome function (21). Multiple inter-relationships between ion measures. It might be caused by the following: the material transport defects and clinical variables were confirmed used (catheters, electrodes); the offset of the electrodes; in this study. the positioning and fixation of the catheter; stability From a practical point of view these findings encourage of the measure, duration of the experiment. However, the measurement of nasal PD not only for diagnostic when performed in a single center, NPD could be a purposes in difficult cases but also to gain more insight reproducible test for CF patients.
into the degree of residual chloride conductance and The basal PD can be lowered by nonspecific damage of sodium abnormality in correlation with severity of to the tissue, such as abrasion or inflammation, f.ex. the disease.
The nasal PD test is a delicate in vivo procedure used to help diagnosis of CF in the presence of atypical clinical Airway ion transport abnormalities in CF can be features and equivocal sweat test results.
assessed in vivo by measuring the transepithelial nasal It has already established position as a part of the criteria for CF diagnosis according to the American and This technique may be useful in assisting in the diagnosis European consensus statements (16). of cystic fibrosis, as well as for monitoring the effect of It is used to demonstrate abnormal function of the CFTR pharmacological agents and gene transfer approaches to protein, establish a diagnosis of CF in patients with atypical correct the abnormalities of ion transport (22).
presentation and in asymptomatic patients suspected to have CF in CF Newborn Screening Programmes (25, The relation between the number and severity of The test is not diagnostic on its own, but is a very CFTR gene mutations and the degree of CFTR-mediated helpful indicative tool.
in the CFTR gene. The primary outcome had three components: change in CFTR-mediated total chloride Using nasal PD measurements researchers showed , that transport; proportion of patients who responded to distinct components of the CF ion transport profile were treatment; and normalization of chloride transport. associated with characteristic phenotypic expression. A better Nasal PD was used to assess whether PTC124 could understanding of the mechanism of dysfunction had direct overcome the effects of a nonsense mutation by restoring relevance to therapeutic strategies aimed at improving the CF the functional activity of CFTR and increasing total phenotype by either restoring reduced/absent chloride transport chloride transport. The results showed that patients or by normalizing exacerbated sodium transport.
responded to treatment with PTC124, as assessed by The test has been used as a surrogate endpoint for clinical an increase in total chloride transport, indicated by a trials on therapeutic modulation of CF basic defects.
change of − 5 mV or more electrically negative and other Gene therapy clinical trials in CF patient included clinical improvements.
viral and non-viral gene transfer to both the nasal and In many patients, PTC124 shifted total chloride Most early trials focused on the nasal epithelium as a These trials exemplified the concept of direct assessment surrogate for the lung to allow for easy access and sampling, of protein function in vivo and a use of nasal PD as a and, importantly, to ensure safety. Once an acceptable sensitive method to assess the activity of full-length, safety profile had been established, gene transfer agents functional CFTR protein on epithelial cell surfaces, and were administered directly into the lung (29).
CFTR-mediated chloride ion transport in nasal mucosa PD was one of the endpoint assays in the assessment as a surrogate for lower airway epithelium. The link of gene transfer. Partial (approximately 20% of non-CF) between changes in NPD towards normal values and correction of chloride transport but not of the sodium, clinical improvement still need to be demonstrated.
has been reported in some studies. However, it is still unclear, if gene transfer efficiency using currently available vectors is sufficient to change more clinically relevant endpoint assays such as lung function, inflammation or Thanks to NPD is possible to observe pathophysiological events leading from CFTR mutation to CF phenotype. CFTR nonsense mutations are mutations causing a It correlates to some extent with genetic abnormalities premature termination signal resulting in the formation of and disease severity.
truncated or unstable protein and subsequently producing It is helpful as diagnostic test and as an endpoint of little or no CFTR chloride channels (30). The aminoglycoside new disease modifying therapies.
antibiotics, in addition to their antimicrobial activity, can There are still a lot of practical issues to be solved, like increase the frequency of a readthrough of the premature standardization and large variability of measurements (34). stop codon, thereby permitting protein translation to There is an unresolved question of correction which is needed continue to the normal end of the gene (31).
for clinical effect as the biggest challenge for this interesting The aim of Wilschanski and coworkers study was to technique still remains our understanding of the impact of determine if gentamicin can, in vivo, induce expression potential changes on clinical course of the disease.
of CFTR in CF patients carrying stop mutations (30). Acknowledgements
They used the nasal potential difference (PD) to measure sodium and chloride transport before and after topical Dr François Vermeulen (Catholic University of Leuven, application of gentamicin drops on the nasal epithelium. Belgium) for manuscript revisions and consultations.
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Received: 05.02.2013 r.
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Comparision of nasal potential difference measurements done at the nasal floor and under the inferior turbinate: Published online
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