Nasal potential difference in cystic fibrosis diagnosis of very young children

Comparison of Nasal Potential Difference and Intestinal Current Measurements as Surrogate Markers for CFTR Function

OBJECTIVES

Nasal potential difference (NPD) measurement is part of the diagnostic criteria for cystic fibrosis (CF) and now used routinely as an endpoint in clinical trials of correcting the basic defect in CF. Intestinal current measurement (ICM), measured ex vivo on a rectal biopsy, has been used to study cystic fibrosis transmembrane conductance regulator (CFTR) function but has not been compared to NPD in the same subject in adults and children. The aim of the study is to evaluate the potential usefulness of ICM as a marker of CFTR function for treatment studies compared NPD in patients with CF and in healthy control subjects.

METHODS

ICM and NPD were performed on healthy controls and patients with CF. The healthy adults were individuals undergoing routine screening colonoscopy at the Beth Israel Deaconess Medical Center. The healthy children were undergoing colonoscopy for suspicion of inflammation in Hadassah Hebrew University Medical Center. The CF adults were recruited from Boston Children's Hospital CF Center and CF Center Worcester Mass, the children with CF from Hadassah CF Center.

RESULTS

ICM measurements in healthy control subjects (n = 16) demonstrated a mean (±SE) carbachol response of 16.0 (2.2) μA/cm, histamine response of 13.2 (2.1) μA/cm and a forskolin response of 6.3 (2.0) μA/cm. Basal NPD of -15.9 (1.9) and response to Cl free + isoproterenol of -13.8 (2.0). These responses were inverted in CF subjects (n = 12) for ICM parameters with carbachol response of -3.0 (0.5) μA/cm, histamine -1.0 (0.8) μA/cm and a forskolin response of 0.5 (0.3) and also for NPD parameters; basal NPD of -42.2 (4.3) and response to Cl free + isoproterenol of 4.3 (0.7). Pearson correlation test showed the comparability of ICM and NPD in assessing CFTR function.

CONCLUSIONS

ICM is equivalent to NPD in the ability to distinguish patients with CF from controls and could be used as surrogate markers of CFTR activity in treatment protocols.

Multi-physiopathological consequences of the c.1392G>T CFTR mutation revealed by clinical and cellular investigations

This study combines a clinical approach and multiple level cellular analyses to determine the physiopathological consequences of the c.1392G>T (p.Lys464Asn) CFTR exon 10 mutation, detected in a CF patient with a frameshift deletion in trans and a TG(11)T(5) in cis. Minigene experiment, with different TG(m)T(n) alleles, and nasal cell mRNA extracts were used to study the impact of c.1392G>T on splicing in both in cellulo and in vivo studies. The processing and localization of p.Lys464Asn protein were evaluated, in cellulo, by western blotting analyses and confocal microscopy. Clinical and channel exploration tests were performed on the patient to determine the exact CF phenotype profile and the CFTR chloride transport activity. c.1392G>T affects exon 10 splicing by inducing its complete deletion and encoding a frameshift transcript. The polymorphism TG(11)T(5) aggravates the effects of this mutation on aberrant splicing. Analysis of mRNA obtained from parental airway epithelial cells confirmed these in cellulo results. At the protein level the p.Lys464Asn protein showed neither maturated form nor membrane localization. Furthermore, the in vivo channel tests confirmed the absence of CFTR activity. Thus, the c.1392G>T mutation alone or in association with the TG repeats and the poly T tract revealed obvious impacts on splicing and CFTR protein processing and functionality. The c.[T(5); 1392G>T] complex allele contributes to the CF phenotype by affecting splicing and inducing a severe misprocessing defect. These results demonstrate that the classical CFTR mutations classification is not sufficient: in vivo and in cellulo studies of a possible complex allele in a patient are required to provide correct CFTR mutation classification, adequate medical counseling, and adapted therapeutic strategies.

Recommendations for the classification of diseases as CFTR-related disorders

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