Clinical, paraclinical, and genetic profile of patients with cystic fibrosis from Colombian Caribbean

Background

Cystic fibrosis (CF) is a serious autosomal recessive disorder. Early diagnosis, comorbidity prevention, and control are cornerstones for a quality life and for improving life expectancy. In Colombian Caribbean, where there is a genetically admixed population, CF is an orphan disease affecting children and adults, and it remains a challenging issue to be addressed carefully. This work describes the genetic, clinical, and paraclinical profiles of CF patients from Cartagena de Indias, Colombia.

Methods

Thirty-six patients were included in the study. The subjects were identified and evaluated through the Regional Program for CF patients. CFTR gene mutations, anthropometric parameters, microbiological infections, and pulmonary function were analyzed. Data on demographic parameters, pharmacological treatments, and comorbidities were reported. Frequency and percentages were established for the categorical variables and mean or median for the quantitative variables. In addition, comparisons were made by sex.

Results

The average age of the patients was 11.9 ± 5.3 years and the median age at diagnosis was 14 months. 55.5% were women and 44.5% were men. The mean values for weight, height, and body mass index were 35 ± 17.6 kg, 139.9 ± 28 cm, and 16.5 ± 2.9 kg/m², respectively. The clinical manifestations that occurred more frequently were steatorrhea (65.4%) and recurrent pneumonia (46.2%). Chronic airway infection with Pseudomonas aeruginosa was identified in 71.4% of the cases and the p.F508del mutation was found in 47.2% of the subjects.

Conclusion

The current profile of CF patients from the Colombian Caribbean showed some concerning features, such as nutritional status; however, progress in early diagnosis and clinical follow-up could contribute to improve the general conditions of patients. It is necessary to continue efforts to increase the life expectancy and quality of life of the patients.

CFTR Protein: Not Just a Chloride Channel?

Cystic fibrosis (CF) is a recessive genetic disease caused by mutations in a gene encoding a protein called Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). The CFTR protein is known to acts as a chloride (Cl-) channel expressed in the exocrine glands of several body systems where it also regulates other ion channels, including the epithelial sodium (Na+) channel (ENaC) that plays a key role in salt absorption. This function is crucial to the osmotic balance of the mucus and its viscosity. However, the pathophysiology of CF is more challenging than a mere dysregulation of epithelial ion transport, mainly resulting in impaired mucociliary clearance (MCC) with consecutive bronchiectasis and in exocrine pancreatic insufficiency. This review shows that the CFTR protein is not just a chloride channel. For a long time, research in CF has focused on abnormal Cl- and Na+ transport. Yet, the CFTR protein also regulates numerous other pathways, such as the transport of HCO3-, glutathione and thiocyanate, immune cells, and the metabolism of lipids. It influences the pH homeostasis of airway surface liquid and thus the MCC as well as innate immunity leading to chronic infection and inflammation, all of which are considered as key pathophysiological characteristics of CF.

Genetics of cystic fibrosis: Basics

Cystic fibrosis (CF) is an autosomal recessive genetic disorder whose responsible gene - the CFTR gene - was discovered 30 years ago by a positional cloning strategy. This gene, which encodes a chloride channel, contains more than 2,000 mutations including a major one (p.Phe508del). This discovery has led to considerable progress in the understanding of the pathophysiology of CF as well as in the management of patients and their families. It has also paved the way for the development of specific therapies for the disease. From an epidemiological point of view, the incidence of CF, which shows loco-regional variations, is now estimated at 1/4,700 live births in France. The face of CF has dramatically changed over the past decades: CF has gradually become a disease of the adult with, today, more than 50% of the patients being 18 years old or more and a median predicted survival age that exceeds 45 years. © 2020 French Society of Pediatrics. Published by Elsevier Masson SAS. All rights reserved.

Chloride and sodium ion concentrations in saliva and sweat as a method to diagnose cystic fibrosis

OBJECTIVE

Cystic fibrosis diagnosis is dependent on the chloride ion concentration in the sweat test (≥60mEq/mL - recognized as the gold standard indicator for cystic fibrosis diagnosis). Moreover, the salivary glands express the CFTR protein in the same manner as sweat glands. Given this context, the objective was to verify the correlation of saliva chloride concentration and sweat chloride concentration, and between saliva sodium concentration and sweat sodium concentration, in patients with cystic fibrosis and healthy control subjects, as a tool for cystic fibrosis diagnosis.

METHODS

There were 160 subjects enrolled: 57/160 (35.70%) patients with cystic fibrosis and two known CFTR mutations and 103/160 (64.40%) healthy controls subjects. Saliva ion concentration was analyzed by ABL 835 Radiometer^® equipment and, sweat chloride concentration and sweat sodium concentration, respectively, by manual titration using the mercurimetric procedure of Schales & Schales and flame photometry. Statistical analysis was performed by the chi-squared test, the Mann-Whitney test, and Spearman's correlation. Alpha=0.05.

RESULTS

Patients with cystic fibrosis showed higher values of sweat chloride concentration, sweat sodium concentration, saliva chloride concentration, and saliva sodium concentration than healthy controls subjects (p-value<0.001). The correlation between saliva chloride concentration and sweat chloride concentration showed a positive Spearman's Rho (correlation coefficient)=0.475 (95% CI=0.346 to 0.587). Also, the correlation between saliva sodium concentration and sweat sodium concentration showed a positive Spearman's Rho=0.306 (95% CI=0.158 to 0.440).

CONCLUSIONS

Saliva chloride concentration and saliva sodium concentration are candidates to be used in cystic fibrosis diagnosis, mainly in cases where it is difficult to achieve the correct sweat amount, and/or CFTR mutation screening is difficult, and/or reference methods for sweat test are unavailable to implement or are not easily accessible by the general population.

Functional Assays Are Essential for Interpretation of Missense Variants Associated with Variable Expressivity

Missense DNA variants have variable effects upon protein function. Consequently, interpreting their pathogenicity is challenging, especially when they are associated with disease variability. To determine the degree to which functional assays inform interpretation, we analyzed 48 CFTR missense variants associated with variable expressivity of cystic fibrosis (CF). We assessed function in a native isogenic context by evaluating CFTR mutants that were stably expressed in the genome of a human airway cell line devoid of endogenous CFTR expression. 21 of 29 variants associated with full expressivity of the CF phenotype generated <10% wild-type CFTR (WT-CFTR) function, a conservative threshold for the development of life-limiting CF lung disease, and five variants had moderately decreased function (10% to ∼25% WT-CFTR). The remaining three variants in this group unexpectedly had >25% WT-CFTR function; two were higher than 75% WT-CFTR. As expected, 14 of 19 variants associated with partial expressivity of CF had >25% WT-CFTR function; however, four had minimal to no effect on CFTR function (>75% WT-CFTR). Thus, 6 of 48 (13%) missense variants believed to be disease causing did not alter CFTR function. Functional studies substantially refined pathogenicity assignment with expert annotation and criteria from the American College of Medical Genetics and Genomics and Association for Molecular Pathology. However, four algorithms (CADD, REVEL, SIFT, and PolyPhen-2) could not differentiate between variants that caused severe, moderate, or minimal reduction in function. In the setting of variable expressivity, these results indicate that functional assays are essential for accurate interpretation of missense variants and that current prediction tools should be used with caution.

Drug-induced dyspnea versus cystic fibrosis exacerbation: a diagnostic dilemma

Cystic fibrosis (CF) is a disease caused by a mutation in the cystic fibrosis transmembrane conductance regulator protein in the epithelial membrane, and affects at least 30,000 people in the USA. There are between 900 and 1000 new cases diagnosed every year. Traditionally, CF has been treated symptomatically with pancreatic enzymes, bronchodilators, hypertonic saline, and pulmozyme. In July 2015, the US Food and Drug Administration approved Orkambi (lumacaftor/ivacaftor), a combination drug that works on reversing the effects of the defective cystic fibrosis transmembrane conductance regulator protein. Orkambi and mucolytics decrease the viscosity of mucous secretions, leading to an accumulation of hypoviscous fluid in the alveoli, resulting in dyspnea. This presentation can be mistaken for an infective exacerbation. We present a case in which a young female with CF recently started on Orkambi therapy presented to her primary care physician with dyspnea and increased respiratory secretions and was admitted to the hospital for 2 weeks of intravenous and inhaled antibiotic therapy for a presumed CF exacerbation. We highlight this case to bring awareness and educate patients and clinicians of the side-effect profile of Orkambi therapy with an intent to avoid unnecessary hospitalizations, inpatient antibiotics, and other costly medical services.

Effect of Ionophores on Activity of Na+,Cl-(HCO 3- )-ATPase

We studied the effects of ionophores on activity of Na⁺,Cl^-(HCO ₃^- )-ATPase. The most significant effect on the activity of this enzyme was produced by protonophore 2,4-dinitrophenol. The effect of this drug largely depended on the cation and anion composition and pH of the incubation medium and its pH. Activity of Na⁺,Cl^-(HCO ₃^- )-ATPase increased at neutral and weakly alkaline pH and decreased at pH below 6.5-6.7. In control animals (without histamine injection) with very weak or absent Na⁺,Cl^-(HCO ₃^- )-ATPase activity, the observed effect of the protonophore on ATPase activity was also virtually absent. The stimulatory effect of other ionophores (monensine, valinomycin, and A₂₃₁₈₇) was significantly weaker and depended on pH of the incubation medium, its cationic and anionic composition, and concentration of these ionophores.

Substrate Specificity of Na+,Cl-(HCO3-)-ATPase

We studied substrate specificity of Na⁺,Cl^-(HCO₃^-)-ATPase. In most cases, replacement of ATP for other phosphate-containing substances resulted in not only pronounced suppression of phosphohydrolase reactions, but also dramatic changes of their responsiveness to the stimulating effect of monovalent ions. The data showed that Na⁺,Cl^-(HCO₃^-)-ATPase is a highly specific enzyme for ATP.

Cystic fibrosis transmembrane conductance regulator protein (CFTR) expression in the developing human brain: comparative immunohistochemical study between patients with normal and mutated CFTR

Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein has recently been shown to be expressed in the human adult central nervous system (CNS). As CFTR expression has also been documented during embryonic development in several organs, such as the respiratory tract, the intestine and the male reproductive system, suggesting a possible role during development we decided to investigate the expression of CFTR in the human developing CNS. In addition, as some, although rare, neurological symptoms have been reported in patients with CF, we compared the expression of normal and mutated CFTR at several fetal stages. Immunohistochemistry was performed on brain and spinal cord samples of foetuses between 13 and 40 weeks of gestation and compared with five patients with cystic fibrosis (CF) of similar ages. We showed in this study that CFTR is only expressed in neurons and has an early and widespread distribution during development. Although we did not observe any cerebral abnormality in patients with CF, we observed a slight delay in the maturation of several brain structures. We also observed different expression and localization of CFTR depending on the brain structure or the cell maturation stage. Our findings, along with a literature review on the neurological phenotypes of patients with CF, suggest that this gene may play previously unsuspected roles in neuronal maturation or function.

The human CFTR protein expressed in CHO cells activates aquaporin-3 in a cAMP-dependent pathway: study by digital holographic microscopy

The transmembrane water movements during cellular processes and their relationship to ionic channel activity remain largely unknown. As an example, in epithelial cells it was proposed that the movement of water could be directly linked to cystic fibrosis transmembrane conductance regulator (CFTR) protein activity through a cAMP-stimulated aqueous pore, or be dependent on aquaporin. Here, we used digital holographic microscopy (DHM) an interferometric technique to quantify in situ the transmembrane water fluxes during the activity of the epithelial chloride channel, CFTR, measured by patch-clamp and iodide efflux techniques. We showed that the water transport measured by DHM is fully inhibited by the selective CFTR blocker CFTRinh172 and is absent in cells lacking CFTR. Of note, in cells expressing the mutated version of CFTR (F508del-CFTR), which mimics the most common genetic alteration encountered in cystic fibrosis, we also show that the water movement is profoundly altered but restored by pharmacological manipulation of F508del-CFTR-defective trafficking. Importantly, whereas activation of this endogenous water channel required a cAMP-dependent stimulation of CFTR, activation of CFTR or F508del-CFTR by two cAMP-independent CFTR activators, genistein and MPB91, failed to trigger water movements. Finally, using a specific small-interfering RNA against the endogenous aquaporin AQP3, the water transport accompanying CFTR activity decreased. We conclude that water fluxes accompanying CFTR activity are linked to AQP3 but not to a cAMP-stimulated aqueous pore in the CFTR protein.