Cystic Fibrosis Heterozygotes Research Articles

The Impact of the CFTR Gene Discovery on Cystic Fibrosis Diagnosis, Counseling, and Preventive Therapy.

Discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene was the long-awaited scientific advance that dramatically improved the diagnosis and treatment of cystic fibrosis (CF). The combination of a first-tier biomarker, immunoreactive trypsinogen (IRT), and, if high, DNA analysis for CF-causing variants, has enabled regions where CF is prevalent to screen neonates and achieve diagnoses within 1–2 weeks of birth when most patients are asymptomatic. In addition, IRT/DNA (CFTR) screening protocols simultaneously contribute important genetic data to determine genotype, prognosticate, and plan preventive therapies such as CFTR modulator selection. As the genomics era proceeds with affordable biotechnologies, the potential added value of whole genome sequencing will probably enhance personalized, precision care that can begin during infancy. Issues remain, however, about the optimal size of CFTR panels in genetically diverse regions and how best to deal with incidental findings. Because prospects for a primary DNA screening test are on the horizon, the debate about detecting heterozygote carriers will likely intensify, especially as we learn more about this relatively common genotype. Perhaps, at that time, concerns about CF heterozygote carrier detection will subside, and it will become recognized as beneficial. We share new perspectives on that issue in this article.

Paranasal sinus size is decreased in CFTR heterozygotes with chronic rhinosinusitis.

Cystic fibrosis (CF) heterozygotes with a single mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene are at significantly higher risk to develop chronic rhinosinusitis (CRS). However the reasons why remain unknown. We tested the hypothesis that CFTR heterozygotes would have smaller sinus volumes than healthy controls. To exclude sinus disease as a confounding factor we also assessed paranasal sinus volume in those with CRS, but without known CFTR mutations. A total of 131 adults of white Northern European and Latino origin were recruited: 81 diagnosed with CRS and 50 healthy controls. Subjects were genotyped for 9 common CFTR mutations covering >80% of mutation prevalence. Those with CRS were separated by CFTR mutational status and matched demographically to healthy controls. Three-dimensional sinus volume, mucosal opacification, and skull volume were quantified to obtain the percentage of pneumatization and extent of mucosal disease in each sinus. Twenty-item Sino-Nasal Outcome Test (SNOT-20) and endoscopy scores were also analyzed. In individuals diagnosed with CRS we identified 7 CFTR heterozygotes (8.64%); no CFTR mutations were identified in our healthy controls. There were no significant differences between the 3 matched groups other than sinus pneumatization. The frontal and maxillary sinuses were significantly smaller in CFTR heterozygotes with CRS compared to CFTR wild-type subjects with or without disease. CFTR heterozygotes with CRS have significantly smaller frontal and maxillary sinus size compared to those without mutations, irrespective of disease state. This sinus hypoplasia may contribute to impaired mucus clearance and chronic sinus disease development.

Impact of cystic fibrosis disease on archaea and bacteria composition of gut microbiota.

Cystic fibrosis is often associated with intestinal inflammation due to several factors, including altered gut microbiota composition. In this study, we analyzed the fecal microbiota among patients with cystic fibrosis of 10–22 years of age, and compared the findings with age-matched healthy subjects. The participating patients included 14 homozygotes and 14 heterozygotes with the delF508 mutation, and 2 heterozygotes presenting non-delF508 mutations. We used PCR-DGGE and qPCR to analyze the presence of bacteria, archaea and sulfate-reducing bacteria. Overall, our findings confirmed disruption of the cystic fibrosis gut microbiota. Principal component analysis of the qPCR data revealed no differences between homozygotes and heterozygotes, while both groups were distinct from healthy subjects who showed higher biodiversity. Archaea were under the detection limit in all homozygotes subjects, whereas methanogens were detected in 62% of both cystic fibrosis heterozygotes and healthy subjects. Our qPCR results revealed a low frequency of sulfate-reducing bacteria in the homozygote (13%) and heterozygote (13%) patients with cystic fibrosis compared with healthy subjects (87.5%). This is a pioneer study showing that patients with cystic fibrosis exhibit significant reduction of H2-consuming microorganisms, which could increase hydrogen accumulation in the colon and the expulsion of this gas through non-microbial routes.

Risk of asthma in heterozygous carriers for cystic fibrosis: A meta-analysis

BackgroundPatients with cystic fibrosis (CF) have a higher prevalence of asthma than the background population, however, it is unclear whether heterozygous CF carriers are susceptible to asthma. Given this, a meta-analysis is necessary to determine the veracity of the association of CF heterozygosity with asthma. MethodsWe screened the medical literature from 1966 to 2015 and performed a meta-analysis to determine the risk of asthma in CF heterozygotes vs. non-carriers. ResultsAggregating data from 15 studies, the odds ratio for asthma in CF heterozygotes compared with non-carriers was significantly elevated at 1.61 (95% CI: 1.18–2.21). When analyzing the studies considered of high quality in which asthma was diagnosed by a physician, the patients were >18years, or study size was ≥500, the trend remained the same, that heterozygous carriers of CF had elevated risk for asthma. ConclusionsThe results show that heterozygous carriers for CF have a higher risk of asthma than non-carriers.

Role of Cystic Fibrosis Transmembrane Conductance Regulator in Patients With Chronic Sinopulmonary Disease

Previous studies report a high frequency of mutations in the cystic fibrosis (CF) transmembrane conductance regulator gene (CFTR) in patients with idiopathic bronchiectasis. However, most studies have based their findings on preselected patient groups or have performed limited testing for CF transmembrane conductance regulator (CFTR) dysfunction. The objective of our study was to evaluate the prevalence of CFTR gene mutations and/or CFTR-related ion channel abnormalities among subjects with idiopathic chronic sinopulmonary disease and the prevalence of CF or a CFTR-related disorder in this population. We evaluated 72 prospectively enrolled patients from 1995 to 2005 at the Hospital for Sick Children and St. Michael’s Hospital with idiopathic chronic sinopulmonary disease for evidence of CFTR-mediated abnormalities. We performed CFTR genotyping and assessed CFTR function using sweat testing and nasal potential difference testing. The results were compared with data from healthy control subjects, CF heterozygotes, and patients with CF. The CFTR functional tests in idiopathic sinopulmonary patients showed a continuous spectrum, ranging from normal to values typically seen in individuals with CF. Forty-eight patients (66%) demonstrated CFTR mutations and/or abnormalities of CFTR function. Twenty-two (31%) fulfilled criteria for a diagnosis of CF and 26 (36%) for a CFTR-related disorder with a strong female preponderance. Functional tests, more than genotyping, were instrumental in establishing a CF diagnosis. Clinical features failed to distinguish subjects with CF from those with CFTR-related or idiopathic disease. The high prevalence of CF and CFTR dysfunction among patients with idiopathic chronic sinopulmonary disease underscores the need for extensive diagnostic evaluation for CF.

Activation of CFTR-specific T Cells in Cystic Fibrosis Mice Following Gene Transfer

Gene therapy for cystic fibrosis (CF) airway disease has emerged as a potentially successful therapy, because expression of the CF gene would be expected to restore the electrophysiological function of the airway epithelium to normalcy. Although, cellular and humoral immune responses to viral gene transfer vectors have been studied extensively, there has been no evaluation of T cell-mediated responses to the therapeutic human CF gene product. Using an adenovirus vector we demonstrated that T cells against human CF gene protein are elicited in CF gene knockout (KO), heterozygote (Het), and wild-type (wt) mice. A dominant CD8 T cell epitope found in CF gene KO, Het, and wt mice was mapped to NTYLRYITV. In CF gene KO mice we also identified (to a conserved region of the CF gene CSQFSWIMPGTIKEN), a minor T cell epitope that did not show any activity in the Het or wt mice.

Cystic fibrosis heterozygotes do not have increased platelet activation

Introduction We have previously demonstrated platelet hyperreactivity in cystic fibrosis (CF) patients. Carriers of one CF mutation (heterozygotes) have been shown to have abnormalities related to the presence of only one-half the normal amount of CF transmembrane conductance regulator protein. Platelet hyperreactivity in CF heterozygotes would be an important cardiovascular risk factor, since approximately 1 in 25 Caucasians is a CF carrier. Materials and methods We used highly sensitive assays of platelet activation to assess the difference between 16 CF heterozygotes and 16 age- and sex-matched healthy controls without CF mutations. Results We found no difference in platelet activation between CF heterozygotes and controls. Conclusions The 50% reduction in the CF transmembrane conductance regulator protein in heterozygotes is insufficient to cause platelet activation.

Composition of nasal airway surface liquid in cystic fibrosis and other airway diseases determined by X-ray microanalysis

The ionic composition of the airway surface liquid (ASL) in healthy individuals and in patients with cystic fibrosis (CF) has been debated. Ion transport properties of the upper airway epithelium are similar to those of the lower airways and it is easier to collect nasal ASL from the nose. ASL was collected with ion exchange beads, and the elemental composition of nasal fluid was determined by X-ray microanalysis in healthy subjects, CF patients, CF heterozygotes, patients with rhinitis, and with primary ciliary dyskinesia (PCD). In healthy subjects, the ionic concentrations were approximately isotonic. In CF patients, CF heterozygotes, rhinitis, and PCD patients, [Na] and [Cl] were significantly higher compared when compared with those in controls. [K] was significantly higher in CF and PCD patients compared with that in controls. Severely affected CF patients had higher ionic concentrations in their nasal ASL than in patients with mild or moderate symptoms. Female CF patients had higher levels of Na, Cl, and K than male patients. As higher salt concentrations in the ASL are also found in other patients with airway diseases involving chronic inflammation, it appears likely that inflammation-induced epithelial damage is important in determining the ionic composition of the ASL.

Composition of Airway Surface Liquid Determined by X-ray Microanalysis

The composition of the airway surface liquid, a thin layer of fluid covering the airway wall, has been debated. Two new techniques to determine the ionic composition of the airway surface liquid are presented. In the first technique, pieces of the airway were shock-frozen and analyzed by X-ray microanalysis in the frozen state in the scanning electron microscope.In the second technique, the airway surface liquid was collected with the help of dextran beads that were allowed to absorb the fluid. The beads were collected in silicon oil, cleaned, dried, and analyzed. Airway surface liquid from pig airways was isotonic to lightly hypertonic, whereas airway surface liquid from mouse and rat airways was hypotonic. The ionic composition of airway surface liquid from rodent airways could be changed by pharmacological stimulation of fluid transport. Transgenic mice with cystic fibrosis (CF) had significantly higher Na and Cl concentrations in the airway surface liquid than normal mice. Nasal fluid was also collected from humans.In CF patients, CF heterozygotes, and rhinitis patients, the levels of Na and Cl in the nasal fluid were significantly higher than in healthy controls. In CF patients K levels were also significantly higher than in healthy controls. The ionic concentrations in fluid collected from patients with primary ciliary dyskinesia (PCD) were not different from normal. Females with CF had significantly higher concentrations of Na, Cl and K in their nasal fluid compared to male patients. The dextran bead technique was also used to determine the ionic composition of the apical fluid in cultures of respiratory epithelial cells from healthy controls and CF patients. In the healthy controls, the fluid was hypotonic. In the CF cell cultures, the apical fluid had a higher Na and Cl concentration than in the controls.

The cystic fibrosis transmembrane conductance regulator gene and ion channel function in patients with idiopathic pancreatitis

Cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations are associated with cystic fibrosis (CF)-related monosymptomatic conditions, including idiopathic pancreatitis. We evaluated prospectively enrolled patients who had idiopathic recurrent acute pancreatitis or idiopathic chronic pancreatitis, healthy controls, CF heterozygotes, and CF patients (pancreatic insufficient or sufficient) for evidence of CFTR gene mutations and abnormalities of ion transport by sweat chloride and nasal potential difference testing. DNA samples from anonymous blood donors were controls for genotyping. At least one CFTR mutation or variant was carried in 18 of 40 patients (45%) with idiopathic chronic pancreatitis and in 6 of 16 patients (38%) with idiopathic recurrent acute pancreatitis but in only 11 of the 50 controls (22%, P=0.005). Most identified mutations were rare and would not be identified in routine genetic screening. CFTR mutations were identified on both alleles in six patient (11%). Ion transport measurements in patients with pancreatitis showed a wide range of results, from the values in patients with classically diagnosed CF to those in the obligate heterozygotes and healthy controls. In general, ion channel measurements correlated with the number and severity of CFTR mutations. Twelve of 56 patients with pancreatitis (21%) fulfilled current clinical criteria for the diagnosis of CF, but CFTR genotyping alone confirmed the diagnosis in only two of these patients. We concluded that extensive genotyping and ion channel testing are useful to confirm or exclude the diagnosis of CF in the majority of patients with idiopathic pancreatitis.

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.

Cystic fibrosis in Greece: molecular diagnosis, haplotypes, prenatal diagnosis and carrier identification amongst high-risk individuals.

Cystic fibrosis (CF) mutation analysis on 437 CF patients, characterized 80 different mutations (20 so far specific to our population) accounting for 91% of CF genes and generating 103 different genotypes. Eight mutations were common [F508del (53.4%), 621+1G>T (5.7%), G542X (3.9%), N1303K (2.6%), 2789+5G>A (1.7%), 2183AA>G (1.4%), E822X (1.4%), R1158X (1%)], 12 showed frequencies between 0.5% and 1%, while the remaining (60) were very rare (1 to 3 alleles). Denaturing gradient gel electrophoresis (DGGE) screening of 12 exons (3, 4, 7, 10, 11, 13, 14b, 16, 17b 20 and 21) detected 85.5% of CF alleles. Haplotypes for eight diallelic and three microsatellite markers have been characterized for the common, a few rare and novel Greek mutations. Results of 165 prenatal diagnoses (including 49 due to bowel hyperechogenicity), testing a total of 41 different parental genotypes, are reported. One hundred and sixteen prenatal tests resulted in 22 affected, 59 heterozygous, 34 normal fetuses and one incomplete diagnosis. Of the 49 echogenic bowel fetuses, 3 were heterozygotes. Carrier screening was initiated, with emphasis on individuals and couples in high-risk groups - with a family history of CF, one partner with CF, and couples with male infertility seeking in vitro fertilization (IVF). Mutation analysis on 672 individuals (120 couples, 91 unaffected CF siblings, 283 CF family relatives and 58 general population subjects), identified a total of 176 heterozygotes and 7 couples where both partners were CF heterozygotes. Prenatal diagnosis was performed in 4 cases and 3 were counseled on the availability of a prenatal test.

Loci of intestinal distress in cystic fibrosis knockout mice.

The strain-dependent survival of cystic fibrosis (CF) knockout mice has been used to map a modifier of CF, Cfm1, in mice and, subsequently, in humans. To identify additional modifiers of the CF phenotype, in this study, the survival of F2 CF mice derived from a cross between congenic C57BL/6J CF and BALB/cJ CF heterozygotes was followed up to 12 wk of age. A genome-wide linkage scan completed in F2 CF mice revealed a chromosome 10 locus (P = 1.2 x 10(-4)) to predict for intestinal distress in CF male mice. An X chromosome locus for which non-Mendelian inheritance favoring B6 alleles in the surviving CF mice and BALB alleles in mice of a control population, was identified. The survival of female mice, both F2 CF and F2 control, was linked to loci on chromosomes 3 and 5. The identification of additional putative CF modifier loci may permit further genetic dissection of the complex CF phenotype.

Inclusion of heterozygotes for cystic fibrosis in the egg donor pool

Objective: To describe and discuss our experience with cystic fibrosis (CF) carrier testing in a donor egg program. Design: Retrospective review. Setting: Community hospital-based assisted reproductive technology (ART) program. Patient(s): Forty anonymous white oocyte donor applicants. Intervention(s): Testing with a DNA mutation analysis panel. Main Outcome Measure(s): Frequency of heterozygotes for CF mutation among the donor applicants and the likelihood of carriers and noncarriers being selected by recipients. Result(s): Five of 40 egg donor applicants (12.5%) were found to be heterozygous for a CF mutation; 35 women (87.5%) tested negative. Two of the five CF carriers (40.0%) were selected by five recipient couples and underwent four donation cycles after the recipients’ male partners tested negative. Twenty-nine of the 35 noncarrier donors (82.9%) were matched and underwent 81 egg donation cycles. The likelihood of being selected was lower for CF carriers than for noncarriers. Conclusion(s): Our experience strongly supports the recommendation of routine CF testing of prospective white egg donors. Whereas heterozygosity lowers the probability of a donor being matched, it need not exclude her from the donor pool provided the recipient’s partner is not a carrier. Empowering recipients to choose their own donors, focused patient education, and genetic counseling with precise determination of residual risk are important prerequisites for inclusion of CF carriers.

Salivary electrolyte concentrations are associated with cystic fibrosis transmembrane regulator genotypes.

It is estimated that about one in 24 Belgian Caucasians is a cystic fibrosis (CF) heterozygote. Until now, CF heterozygotes can only be identified by genetic decoding (e.g. Inno-Lipa CF2 test), as they cannot be phenotypically distinguished from the general population. The aim of this study was to evaluate differences in salivary electrolyte concentrations (calcium, bicarbonate, chloride, potassium, sodium and phosphate) and salivary osmolarity between CF homozygotes (n=41), CF heterozygotes (n=56) and healthy controls (n=65). Differences between the three groups were investigated by means of non-parametric tests. Several significant differences between the three study groups and among, as well as between, the different cystic fibrosis transmembrane regulator (CFTR) genotypes were observed. Significant differences in salivary electrolyte concentrations between individuals with delta F508 mutation and those without, and between CF homozygotes and heterozygotes were observed. The concentrations of several salivary electrolytes, and salivary osmolarity were significantly higher in CFTR genotype individuals. Differences in salivary electrolyte concentrations may partly explain differences in caries between the three study groups.

Mutans Streptococci, Lactobacilli and Caries Experience in Cystic Fibrosis Homozygotes, Heterozygotes and Healthy Controls

Mutans streptococci (MS) and lactobacillus (LB) counts (CRT Bacteria chair–side test) were compared and correlated with the respective caries experience (DMF) of cystic fibrosis (CF) homozygotes (n = 20), CF heterozygotes (n = 20) and healthy controls (n = 20). Each group of subjects was divided into two equal age groups: group A <20 years and group B >20 years. No significant differences were found regarding DMF between the three groups. CF heterozygotes had a significantly higher DMF–S than CF homozygotes (p = 0.040). This was also found in age group B (p<0.02), where it also counted for DMF–T (p<0.01). When in age group B, CF heterozygotes and controls were combined as one group, it was found that they had a significantly higher DMF–T (p = 0.031) and DMF–S (p = 0.008) than CF homozygotes. No significant differences in LB counts between the three groups were observed. CF homozygotes and those in age group A had significantly (p = 0.018 and p = 0.015) more low MS counts. LB and MS counts were positive and significantly correlated with DMF–T/–S in CF homozygotes and in the combined group of CF heterozygotes with controls (p≤0.05 and ≤0.001). No correlations between bacterial counts and DMF were observed in CF heterozygotes. It can be concluded that despite the fact that CF homozygotes behave as high caries risk individuals, they did not have significantly more dental decay. More research is required in order to explore salivary, dietary and medication influences.

A Pilot Survey of Cystic Fibrosis Clinical Manifestations in CFTR Mutation Heterozygotes

A cystic fibrosis (CF) heterozygote incidence higher than in the general population has been repeatedly reported in conditions which include clinical features found in CF, like pancreatitis, disseminated bronchiectasis, and allergic bronchopulmonary aspergillosis. Some cases may be explained by an unidentified compound heterozygosity; others could be manifesting heterozygotes. This study was aimed at detecting the incidence of CF-related clinical features in a population of carriers. A group of 261 obligate heterozygotes (mean age, 44 years) and a control group, composed of 201 individuals negative for a standard mutation panel (mean age, 36 years), were surveyed for possibly CF-related conditions (asthma, bronchiectasis, pneumothorax, allergic bronchopulmonary aspergillosis, sinusitis, nasal polyps, gallstones, liver cirrhosis, diabetes, pancreatitis, bone fractures, plus hypertension). A questionnaire was administered, and the accuracy of the statements was evaluated by phone interviews. There was no difference between heterozygotes and controls, with the exception of hypertension (carriers 28/261, controls 7/201, p = 0.004), and, in males, nasal polyps (carriers 7/126, controls 0/102, p value = 0.0178), and, again, hypertension (carriers 17/126, controls 5/102, p value = 0.0407). To avoid age bias, 126 heterozygotes matched to controls of the same gender and age were separately processed: these two groups showed no significant differences. CF-related clinical manifestations in obligate CFTR mutation heterozygotes are not more represented than in individuals with a low risk of being carriers.

Age-related Changes in Neuroendocrine System of the Gut

The composition of the airway surface liquid, a thin layer of fluid covering the airway wall, has been debated. Two new techniques to determine the ionic composition of the airway surface liquid are presented. In the first technique, pieces of the airway were shock-frozen and analyzed by X-ray microanalysis in the frozen state in the scanning electron microscope.In the second technique, the airway surface liquid was collected with the help of dextran beads that were allowed to absorb the fluid. The beads were collected in silicon oil, cleaned, dried, and analyzed. Airway surface liquid from pig airways was isotonic to lightly hypertonic, whereas airway surface liquid from mouse and rat airways was hypotonic. The ionic composition of airway surface liquid from rodent airways could be changed by pharmacological stimulation of fluid transport. Transgenic mice with cystic fibrosis (CF) had significantly higher Na and Cl concentrations in the airway surface liquid than normal mice. Nasal fluid was also collected from humans.In CF patients, CF heterozygotes, and rhinitis patients, the levels of Na and Cl in the nasal fluid were significantly higher than in healthy controls. In CF patients K levels were also significantly higher than in healthy controls. The ionic concentrations in fluid collected from patients with primary ciliary dyskinesia (PCD) were not different from normal. Females with CF had significantly higher concentrations of Na, Cl and K in their nasal fluid compared to male patients. The dextran bead technique was also used to determine the ionic composition of the apical fluid in cultures of respiratory epithelial cells from healthy controls and CF patients. In the healthy controls, the fluid was hypotonic. In the CF cell cultures, the apical fluid had a higher Na and Cl concentration than in the controls.

Active Intestinal Chloride Secretion in Human Carriers of Cystic Fibrosis Mutations: An Evaluation of the Hypothesis That Heterozygotes Have Subnormal Active Intestinal Chloride Secretion

To explain the very high frequency of cystic fibrosis (CF) mutations in most populations of European descent, it has been proposed that CF heterozygotes have a survival advantage when infected with Vibrio cholerae or Escherichia coli, the toxins of which induce diarrhea by stimulation of active intestinal chloride secretion. Two assumptions underlie this hypothesis: (1) chloride conductance by the CF transmembrane conductance regulator (CFTR) is the rate-limiting step for active intestinal chloride secretion at all levels of expression, from approximately zero in patients with CF to normal levels in people who are not carriers of a mutation; and (2) heterozygotes have smaller amounts of functional intestinal CFTR than do people who are not carriers, and heterozygotes therefore secrete less chloride when exposed to secretagogues. The authors used an intestinal perfusion technique to measure in vivo basal and prostaglandin-stimulated jejunal chloride secretion in normal subjects, CF heterozygotes, and patients with CF. Patients with CF had essentially no active chloride secretion in the basal state, and secretion was not stimulated by a prostaglandin analogue. However, CF heterozygotes secreted chloride at the same rate as did people without a CF mutation. If heterozygotes are assumed to have less-than-normal intestinal CFTR function, these results mean that CFTR expression is not rate limiting for active chloride secretion in heterozygotes. The results do not support the theory that the very high frequency of CF mutations is due to a survival advantage that is conferred on heterozygotes who contract diarrheal illnesses mediated by intestinal hypersecretion of chloride.

A simplified cyclic adenosine monophosphate–mediated sweat rate test for quantitative measure of cystic fibrosis transmembrane regulator (CFTR) function

Objective: Sweat production is stimulated by both cholinergic and β-adrenergic pathways in the sweat gland secretory coil. β-Adrenergic pathway–mediated sweating is absent in cystic fibrosis (CF) because cyclic adenosine monophosphate (cAMP)–mediated chloride transport through the cystic fibrosis transmembrane regulator (CFTR) is disrupted. We report the development of a rapid, reproducible, macroscopic, and quantitative methodology to test the hypothesis that β-adrenergic sweat rate discriminates among 3 different CFTR phenotypes—CF, heterozygote CF carriers, and non-CF. Study design: Intradermal injection of a mixture of 50 μmol/L isoproterenol, 5 mmol/L aminophylline (to potentiate the β-adrenergic stimulation), and 140 μmol/L atropine (to block potential cholinergic stimulation) in lactated Ringer’s solution was performed in duplicate on one forearm. A single injection of 0.5 mmol/L methacholine to stimulate sweat production by the cholinergic pathway was performed on the other forearm. Sweat rate was determined as the amount of sweat collected on filter paper over 20 minutes. Results and conclusions: Median cAMP-mediated sweat rates were 1.45 mg/20 min (CF, n = 29), 2.55 mg/20 min (CF heterozygote carriers, n = 30), and 3.65 mg/20 min (non-CF, n = 30) and were significantly different in all 3 groups (P =.0001, Kruskal-Wallis test). Methacholine-stimulated sweat rates were similar for all 3 groups. The cAMP-mediated sweat rate test may be a useful endpoint for studies of new agents to increase the function of CFTR. (J Pediatr 2000;137:849-55)