Screening for F508del as a first step in the molecular diagnosis of cystic fibrosis

OBJECTIVE:

To determine the relevance of screening for the F508del mutation of the cystic fibrosis transmembrane conductance regulator gene as a first step in the genetic diagnosis of cystic fibrosis (CF) by associating the genotype with various clinical variables.

METHODS:

We evaluated 180 CF patients regarding the F508del mutation. The clinical data were obtained from the medical records of the patients and from interviews with their parents or legal guardians.

RESULTS:

Of the 180 patients studied, 65 (36.1%) did not carry the F508del mutation (group 0 [G0]), 67 (37.2%) were F508del heterozygous (G1), and 48 (26.7%) were F508del homozygous (G2). All three groups showed associations with the clinical variables. Homozygosis was associated with younger patients, younger age at CF diagnosis, and younger age at the first isolation of Pseudomonas aeruginosa (PA), as well as with higher prevalence of pancreatic insufficiency (PI) and non-mucoid PA (NMPA) colonization. In comparison with G1+G2 patients, G0 patients were older; first experienced clinical symptoms, digestive disease, and pulmonary disease at an older age; were older at CF diagnosis and at first PA isolation; and had a lower prevalence of PI and meconium ileus, as well as of colonization by NMPA, mucoid PA, and Burkholderia cepacia. In G1 patients, values were intermediate for age at CF diagnosis; age at first PA isolation, first pulmonary symptoms, and first clinical manifestations; MPA colonization; and OR for PI.

CONCLUSIONS:

The identification of F508del in 63.9% of the patients studied showed that this can be a useful tool as a first step in the genetic diagnosis of CF. The F508del genotype was associated with clinical severity of the disease, especially with the variables related to CF onset.     

Molecular genetics principles in cystic fibrosis. An example of genetic illness in pneumology

The generalized exocrinopathy cystic fibrosis (CF) is caused by molecular lesions in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The basic defect of this autosomal-recessive disorder manifests in decreased permeability for chloride ions across the apical epithelial membrane. Of the more than 1,000 known CFTR mutations the most frequent mutation F508del occurs on about 70% of North- and Mideuropean CF chromosomes. CFTR mutations are also causatively involved in male infertility, pancreatitis and several airway diseases like disseminated bronchiectasis. The differential diagnosis between CF, other CFTR-opathies and diseases of unrelated etiology can be achieved by the assessment of clinical symptoms, CFTR mutation analysis and electrophysiological bioassays (sweat test, nasal potential difference, intestinal current measurements).     

Pathophysiology of the pancreas in cystic fibrosis.

Pancreatic insufficiency occurs in the majority of cystic fibrosis (CF) patients. Deficient fluid secretion is apparent at all levels of pancreatic function and leads to pancreatic protein hypersecretion which may in turn result in protein precipitation and ductal plugging. An impaired chloride and bicarbonate secretion appears to account for this fluid secretion deficit. A minority of CF patients have sufficient preservation of pancreatic function to prevent steatorrhoea. These patients are diagnosed at a later age, experience milder symptoms and have a far superior overall prognosis than patients with pancreatic insufficiency (PI). Patients who are homozygous for delta F508 have a high frequency of PI (99%), whereas patients with other genotypes are more often pancreatic sufficient (PS). In 538 patients with CF DNA analysis was performed and related with pancreatic function. The most striking observation was that nearly all given genotypes were associated with either PI or PS and not with both. In addition, we were able to classify mutations as "severe" and "mild" with respect to pancreatic function.     

Cystic fibrosis transmembrane conductance regulator mutations at a referral center for cystic fibrosis

OBJECTIVE:

To determine the frequency of six mutations (F508del, G542X, G551D, R553X, R1162X, and N1303K) in patients with cystic fibrosis (CF) diagnosed, at a referral center, on the basis of abnormal results in two determinations of sweat sodium and chloride concentrations.

METHODS:

This was a cross-sectional study involving 70 patients with CF. The mean age of the patients was 12.38 ± 9.00 years, 51.43% were female, and 94.29% were White. Mutation screening was performed with polymerase chain reaction (for F508del), followed by enzymatic digestion (for other mutations). Clinical analysis was performed on the basis of gender, age, ethnicity, pulmonary/gastrointestinal symptoms, and Shwachman-Kulczycki (SK) score.

RESULTS:

All of the patients showed pulmonary symptoms, and 8 had no gastrointestinal symptoms. On the basis of the SK scores, CF was determined to be mild, moderate, and severe in 22 (42.3%), 17 (32.7%), and 13 (25.0%) of the patients, respectively. There was no association between F508del mutation and disease severity by SK score. Of the 140 alleles analyzed, F508del mutation was identified in 70 (50%). Other mutations (G542X, G551D, R553X, R1162X, and N1303K) were identified in 12 (7.93%) of the alleles studied. In F508del homozygous patients with severe disease, the OR was 0.124 (95% CI: 0.005-0.826).

CONCLUSIONS:

In 50% of the alleles studied, the molecular diagnosis of CF was confirmed by identifying a single mutation (F508del). If we consider the analysis of the six most common mutations in the Brazilian population (including F508del), the molecular diagnosis was confirmed in 58.57% of the alleles studied.     

The significance of sweat Cl/Na ratio in patients with borderline sweat test

Recently a few cystic fibrosis (CF) patients with borderline or normal sweat tests have been reported. These patients present a diagnostic challenge. We aimed to study the sweat CINa ratio in cystic fibrosis patients and to assess whether this ratio could be used as a diagnostic criteria. The mean sweat CINa ratio of 3 groups was compared: Group A: 71 CF patients carrying 2 mutations known to be associated with severe disease presentation (ΔF508, W1282X, G542X, N1303K, 1717-1G → A). Group B: 10 compound heterozygous patients who carry one mutation associated with mild clinical disease (3849 + 10 kb C → T). Group C: 142 normal subjects. Sweat chloride levels higher than those of sodium were found in 96% of patients in Group A as compared to 3% of patients in Group C. In Group B 40% of the patients had sweat chloride levels higher than or equal to sodium levels. The mean Cl/Na ratio of Group A (1.2 ± 0.1) differed significantly from that of Group B (0.94 ± 0.1) and both groups had significant higher mean CINa ratio compared to Group C (0.7 ± 0.4) (P < 0.001). Thus in individuals with a borderline sweat test and a Cl/Na ratio < 1 the diagnosis of CF should be considered. However, a Cl/Na ratio < 1 does not exclude CF, since patients carrying mild mutations may have sweat sodium levels higher than those of chloride. Our findings suggest that the sweat CINa ratio in CF is genetically determined and it may be of help in establishing the diagnosis of CF in patients with a borderline sweat test. Pediatr Pulmonol. 1995; 20:369–371 . © 1995 Wiley-Liss, Inc.     

Isolated liver disease in a patient with a CFTR genotype F508del/12TG-5T and 470MV: A new face of an old disease

Today the knowledge of genotype-phenotype correlation in cystic fibrosis is enriched by the growing discoveries of new mutations of the CFTR gene. Although the combination of two severe mutations usually leads to the classic disease (pulmonary and pancreatic insufficiency, sterility, nasal polyposis), the presence of a complex genotype characterized by severe and milder mutations or polymorphism can cause a hidden disease, which is often asymptomatic at early ages. We report on a case of a 15 years old boy, in whom the only clinical signs of CF were chronic hypertransaminasemia and hyperbilirubinemia, and in whom it was demonstrated the presence of the mutations F508del associated with TG11-9T-470M in one allele and TG12-5T-470V in the other allele. Although a clear genotype-phenotype correlation for liver disease is still missing for CF patients, it is possible to state that this isolated clinical presentation could represent an unusual phenotype of CF, related to a complex genotype characterized by a severe mutation and one (or more) polymorphism.     

Genotype-phenotype correlations in cystic fibrosis: clinical severity of mutation S549R(T-->G).

With a view to assessing genotype-to-phenotype correlations in cystic fibrosis (CF), the clinical presentation of CF children from the United Arab Emirates (UAE) who were homozygous for cystic fibrosis transmembrane conductance regulator (CFTR) mutation S549R(T-->G was investigated. This mutation is localized in intron 11 (nucleotide binding domain 1 of the CFTR protein) and had so far been described as a private mutation only. The associations between the R549/R549 genotype and 20 outcome variables, including age at diagnosis, sweat chloride concentrations, growth percentiles, meconium ileus, pancreatic sufficiency, pulmonary disease, associated complications and micro-organism colonization were examined in a group of 15 CF children (9 females and 6 males). Mean current age and age at diagnosis were both low (5.4+/-3.5 and 1.0+/-1.1 yrs, respectively). Although none of the 15 CF patients had presented with meconium ileus at birth, all were pancreatic insufficient and had very severe lung disease, with a high rate of Pseudomonas aeruginosa and Staphylococcus aureus. Two patients died during the course of this investigation (one was 5 months and the other, 6 yrs old). The clinical presentation associated with S549R(T-->G) homozygosity in the United Arab Emirates is quite homogeneous and shows an extreme degree and course of cystic fibrosis severity.     

Molecular repair of a defective CFTR protein in cystic fibrosis

We analyse a paper, which reports an entirely novel approach to the treatment of cystic fibrosis, consisting in "repairing" the defective mutant protein. Patients with cystic fibrosis have a mutation of the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial chloride channel involved in salt and fluid transport in multiple organs, including the lungs and pancreas. The mutations have different effects on the CFTR protein, such as misfolding for the ΔF508 mutation (the most common), or defective opening of the chloride channel for the G551D-CFTR mutation, found in 4 to 5% of the patients. The authors of this work have shown that VX-770, an agent known previously to increase the activity of wild-type and G551D-CFTR cell surface protein in vitro, was able, when given orally to 39 cystic fibrosis patients during 14 and 28 days, to partially restore chloride conductance, as measured by nasal epithelium potential difference. Similarly, the agent partially restored chloride transport in sweat glands, as measured by the sweat chloride concentration. Clinically, VX-770 increased the forced expiratory volume per second (FEVi). Side effects included macular skin rash, elevation of blood glucose concentration and glycosuria. All side effects resolved after discontinuation of the drug. VX-770 has also been shown to increase the activity of ΔF508-CFTR channels in vitro, provided they reach the cell surface. This study appears to be a milestone in the treatment of cystic fibrosis and possibly other genetic diseases.     

Molekulargenetische Grundlagen der zystischen Fibrose als Beispiel genetischer Erkrankungen in der Pneumologie

The generalized exocrinopathy cystic fibrosis (CF) is caused by molecular lesions in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The basic defect of this autosomal-recessive disorder manifests in decreased permeability for chloride ions across the apical epithelial membrane. Of the more than 1,000 known CFTR mutations the most frequent mutation F508del occurs on about 70% of North- and Mideuropean CF chromosomes. CFTR mutations are also causatively involved in male infertility, pancreatitis and several airway diseases like disseminated bronchiectasis. The differential diagnosis between CF, other CFTR -opathies and diseases of unrelated etiology can be achieved by the assessment of clinical symptoms, CFTR mutation analysis and electrophysiological bioassays (sweat test, nasal potential difference, intestinal current measurements).     

Evaluation of fecal pancreatic elastase-1 as a measure of pancreatic exocrine function in children with cystic fibrosis

Pancreatic elastase-1 (EL-1) is a specific human protease synthesised by the acinar cells. It is stable, unaffected by exogenous pancreatic enzyme treatment, and correlates well with stimulated pancreatic function tests. We report our experience of EL-1 measurements in 142 patients from a large cystic fibrosis (CF) clinic. The median patient age was 7.7 years (range, 0.1-20.8 years), 93 were homozygous and 38 heterozygous for DeltaF508, and 11 had other or unidentified mutations. There were 85 non-CF control subjects. Seven were pancreatic sufficient (PS). The median (quartile 1-quartile 3) fecal EL-1 of the 135 pancreatic insufficient (PI) patients was 10 microg/g stool (2.5-33); of the 7 PS patients, 698 microg/g stool (400.5-824.5), and of the non-CF controls, 615 microg/g stool (420-773). Using the Mann-Whitney U test, there was a statistically significant difference for fecal EL-1 activity between the PS and PI patients (P = 0.0001) and the PI and control group (P < 0.0001), but not between the control and PS groups (P = 0.63). Median (quartile 1-quartile 3) fecal EL-1 in the pancreatic insufficient DeltaF508 homozygotes was 10 microg/g stool (2-33), and in the heterozygotes 12 microg/g stool (4-39) (not significant, P = 0.62). We now use fecal EL-1 as evidence of PI in screened CF infants (reliable over the age of 2 weeks); in older CF patients at diagnosis; for confirming the need for pancreatic enzymes in patients referred to the clinic already taking enzymes; for annual monitoring of PS patients to detect the onset of PI; and as supporting evidence when excluding the diagnosis of CF in patients attending the pediatric gastroenterology clinic. The low values in the first 2 weeks in some normal and premature infants, and the persisting normal values in PS infants, make the fecal EL-1 test unsuitable for neonatal CF screening.     

Revertant mutants G550E and 4RK rescue cystic fibrosis mutants in the first nucleotide-binding domain of CFTR by different mechanisms

The revertant mutations G550E and 4RK [the simultaneous mutation of four arginine-framed tripeptides (AFTs): R29K, R516K, R555K, and R766K] rescue the cell surface expression and function of F508del-cystic fibrosis (CF) transmembrane conductance regulator (-CFTR), the most common CF mutation. Here, we investigate their mechanism of action by using biochemical and functional assays to examine their effects on F508del and three CF mutations (R560T, A561E, and V562I) located within a conserved region of the first nucleotide-binding domain (NBD1) of CFTR. Like F508del, R560T and A561E disrupt CFTR trafficking. G550E rescued the trafficking defect of A561E but not that of R560T. Of note, the processing and function of V562I were equivalent to that of wild-type (wt)-CFTR, suggesting that V562I is not a disease-causing mutation. Biochemical studies revealed that 4RK generates higher steady-state levels of mature CFTR (band C) for wt- and V562I-CFTR than does G550E. Moreover, functional studies showed that the revertants rescue the gating defect of F508del-CFTR with different efficacies. 4RK modestly increased F508del-CFTR activity by prolonging channel openings, whereas G550E restored F508del-CFTR activity to wt levels by altering the duration of channel openings and closings. Thus, our data suggest that the revertants G550E and 4RK might rescue F508del-CFTR by distinct mechanisms. G550E likely alters the conformation of NBD1, whereas 4RK allows F508del-CFTR to escape endoplasmic reticulum retention/retrieval mediated by AFTs. We propose that AFTs might constitute a checkpoint for endoplasmic reticulum quality control.     

The cystic fibrosis gene and relationships to clinical status.

Cystic fibrosis (CF) is a disease characterized by significant variability in both presentation and clinical course. The genetic and environmental factors responsible for the phenotypic expression of CF are most likely legion, and their relationship to the disease complex. Therefore, it was not until the isolation and characterization of the CF gene and the ability to genotype individual patients that further elucidation of the genotype-phenotype relationship in CF was possible. Currently, the pancreatic status of CF patients appears to be primarily determined by genetic factors and patients homozygous for the most common mutation, delta F508 are, as a rule, pancreatic insufficient. Other specific alleles that confer pancreatic sufficiency have been identified. Patients having these alternative alleles remain pancreatic sufficient, are diagnosed later, have lower sweat chloride values, milder respiratory disease, and a better prognosis than patients with alleles associated with pancreatic insufficiency. Other clinical manifestations, including meconium ileus and the presence of liver disease, appear to be associated with pancreatic insufficiency. The variability in the pulmonary course for homozygous delta F508 patients suggests that genetic heterogeneity at other loci or environmental factors are important. Therefore, the CF genotype does not precisely predict pulmonary status.     

Relation of sweat chloride concentration to severity of lung disease in cystic fibrosis

In cystic fibrosis (CF), sweat chloride concentration has been proposed as an index of CFTR function for testing systemic drugs designed to activate mutant CFTR. This suggestion arises from the assumption that greater residual CFTR function should lead to a lower sweat chloride concentration, as well as protection against severe lung disease. This logic gives rise to the hypothesis that the lower the sweat chloride concentration, the less severe the lung disease. In order to test this hypothesis, we studied 230 patients homozygous for the DeltaF508 allele, and 34 patients with at least one allele associated with pancreatic sufficiency, born since January 1, 1955, who have pulmonary function data and sweat chloride concentrations recorded in our CF center database, and no culture positive for B. cepacia. We calculated a severity index for pulmonary disease, using an approach which takes into account all available pulmonary function data as well as the patient's current age and survival status. Patients with alleles associated with pancreatic sufficiency had significantly better survival (P = 0.0083), lower sweat chloride concentration (81.4 +/- 23.8 vs. 103.2 +/- 14.2 mEq/l, P < 0.0001), slower rate of decline of FEV(1) % predicted (-0.75 +/- 0.34 vs. -2.34 +/- 0.17% predicted per year), and a better severity index than patients homozygous for the DeltaF508 allele (median 73rd percentile vs. median 55th percentile, P = 0.0004). However, the sweat chloride concentration did not correlate with the severity index, either in the population as a whole, or in the population of patients with alleles associated with pancreatic sufficiency, who are thought to have some residual CFTR function. These data suggest that, by itself, sweat chloride concentration does not necessarily predict a milder pulmonary course in patients with cystic fibrosis.     

Genotype analysis and phenotypic manifestations of children with intermediate sweat chloride test results

STUDY OBJECTIVES: Cystic fibrosis (CF) is one of the most common inherited diseases among whites. Since the cloning of the CF transmembrane conductance regulator (CFTR) gene, a number of studies have focused on associations between the genotype and phenotype in CF. This had led to the progressive identification of new groups of patients, including those who have mild lung disease and those who have normal sweat chloride values (< 60 mEq/L). The aim of the present work was to provide information on the genotype and the phenotypic characteristics of children with intermediate-range sweat chloride test results. PATIENTS AND RESULTS: We focused on children referred to the pulmonary department for various types of pulmonary disease and who had several sweat chloride test results with median values in the range of 40 to 60 mEq/L. Twenty-four patients over a 10-year period were enrolled (mean age, 4.8 years). Respiratory manifestations at initial evaluation included recurrent bronchitis, wheezing, chronic cough, and pneumonia. The duration of the follow-up ranged from 0.5 to 10.5 years. Sputum cultures revealed the presence of Haemophilus influenzae (10 children), Staphylococcus aureus (4 children), and Pseudomonas aeruginosa (3 children). Pancreatic insufficiency was found in two patients. Analysis of the entire coding sequence allowed identification of 16 known mutations in CFTR gene. Fifteen chromosomes (31.2%) carried a mutation in CFTR gene and one allele carried two mutations. Three patients were homozygous or double heterozygous (DeltaF508/DeltaF508, DeltaF508/3849 + 10 kb C-->T, S1235R/G551D). The 5-thymidine allele was identified in four children. CONCLUSION: These results indicate an higher frequency of CFTR gene mutations in patients with borderline sweat chloride test results, compared to data reported in the general population. They lead to the recommendations for complete pulmonary and GI investigations in this group of patients, as well as assiduous care and medical follow-up.     

Prevalencia e importancia de micobacterias no tuberculosas en pacientes adultos con fibrosis quística en un hospital de Madrid

IntroductionThe role of non-tuberculous mycobacteria (NTM) among cystic fibrosis (CF) patients, on occasion, remains unknown. The aim of our study is to evaluate the prevalence and clinical/microbiological characteristics of CF adult patients colonized by NTM, highlighting Mycobacterium abscessus (M. abscessus).MethodsA retrospective study was conducted with 92 CF adult patients: including a control group of 64 patients, not colonized by NTM, and a study group of 28 patients, colonized by NTM. We have analyzed variables such as age, F508del mutation, lung function, pancreatic involvement, auramine staining and co-colonizations between both groups.ResultsThe prevalence of NTM found was 30.4%. The most prevalent was Mycobacterium avium complex followed by M. abscessus. For M. abscessus, in the comparative study with patients colonized by other NTM, significant results were obtained for variables age.DiscussionWe have found a high prevalence of NTM among adult patients with CF, and we associated the presence of M. asbcessus with ages less than 30 years and F508del. Due to the pathogenic role of NTM, especially M. asbcessus, multicenter studies are required within the population suffering from CF.     

Serum lipase levels as a diagnostic marker in cystic fibrosis patients with normal or borderline sweat tests

Patients with normal or borderline sweat test present a diagnostic challenge. In spite of the availability of different methods such as genetic analysis and measurements of nasal potential difference, uncertainty in diagnosing cystic fibrosis (CF) in some patients still exists. Neonates with CF have high serum lipase levels, which decline over time in pancreatic-insufficient patients, whereas pancreatic-sufficient patients demonstrate high serum lipase levels beyond infancy. Because patients with borderline or normal sweat test are almost always pancreatic sufficient, this study was aimed to assess whether serum lipase levels may be of help in establishing the diagnosis of CF in these patients. Serum lipase levels were measured in 100 CF patients and in 17 healthy individuals. Patients were grouped according to their genotype. Group A patients (n = 70) carried two mutations previously found to be associated with a pathologic sweat test and pancreatic insufficiency (delta F508, W1282X, G542X, N1303K, S549R). Group B (n = 30) were compound heterozygote patients who carried one mutation known to cause mild disease with borderline or normal sweat tests and pancreatic sufficiency (3849+10kb C-->T, 5T). Group C included 17 healthy controls. Serum lipase levels ranged between 2 and 104.4 U/L (mean +/- SD 16.9 +/- 14.7), 6.1-200 U/L (mean +/- SD 53.9 +/- 47.9), and 8.5-27.8 U/L (mean +/- SD 16.9 +/- 5.1) in Groups A, B, and C, respectively, with some overlapping between groups. The distribution of lipase levels was significantly different in Group B vs Groups A and C (P < 0.01). High lipase levels were found in 63.3% (19/30) of Group B patients, but in only 4.3% (3/70) and 0% (0/17) of Group A and C, respectively. Lipase levels were found to be inversely related to sweat chloride concentrations (r = -0.19, P < 0.05). Patients with borderline or normal sweat tests had high lipase levels, whereas low lipase levels were associated with pathologic sweat tests. Our findings indicate that the serum lipase level is genetically determined and that it has a useful role in the diagnosis of CF. Thus, in patients with borderline sweat tests and high lipase levels, the diagnosis of CF should be considered.     

Correction of the F508del-CFTR protein processing defect in vitro by the investigational drug VX-809

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene that impair the function of CFTR, an epithelial chloride channel required for proper function of the lung, pancreas, and other organs. Most patients with CF carry the F508del CFTR mutation, which causes defective CFTR protein folding and processing in the endoplasmic reticulum, resulting in minimal amounts of CFTR at the cell surface. One strategy to treat these patients is to correct the processing of F508del-CFTR with small molecules. Here we describe the in vitro pharmacology of VX-809, a CFTR corrector that was advanced into clinical development for the treatment of CF. In cultured human bronchial epithelial cells isolated from patients with CF homozygous for F508del, VX-809 improved F508del-CFTR processing in the endoplasmic reticulum and enhanced chloride secretion to approximately 14% of non-CF human bronchial epithelial cells (EC(50), 81 ± 19 nM), a level associated with mild CF in patients with less disruptive CFTR mutations. F508del-CFTR corrected by VX-809 exhibited biochemical and functional characteristics similar to normal CFTR, including biochemical susceptibility to proteolysis, residence time in the plasma membrane, and single-channel open probability. VX-809 was more efficacious and selective for CFTR than previously reported CFTR correctors. VX-809 represents a class of CFTR corrector that specifically addresses the underlying processing defect in F508del-CFTR.     

Clinical outcomes in infants with cystic fibrosis transmembrane conductance regulator (CFTR) related metabolic syndrome

An unavoidable outcome of cystic fibrosis newborn screening (CF NBS) programs is the detection of infants with an indeterminate diagnosis. The United States CF Foundation recently proposed the term cystic fibrosis transmembrane conductance regulator related metabolic syndrome (CRMS) to describe infants with elevated immunoreactive trypsinogen (IRT) on NBS who do not meet diagnostic criteria for CF. The objective of this study was to describe the clinical outcomes of infants with CRMS identified through an IRT/DNA algorithm. We reviewed the records of all infants with CRMS diagnosed at our CF Center from 2002 to 2010. We identified 12 infants, and compared them to 27 infants diagnosed with CF by NBS. Compared to CF patients, CRMS patients were more likely to be pancreatic sufficient as assessed by fecal elastase measurement (100% vs. 8%, P < 0.01). Their weight for age percentile was normal from birth. A positive oropharyngeal (OP) culture for Pseudomonas aeruginosa (Pa) was found in 25% of CRMS patients. One patient with the F508del/R117H/7T genotype was reassigned the diagnosis of CF after he had a positive OP culture for Pa, and his follow up sweat Cl at 1 year of life was 73 mmol/L. CF patients were more likely to receive oral antibiotics and be hospitalized for pulmonary symptoms. Our results indicate that CRMS patients can develop signs of CF disease, but have a milder clinical course than CF infants. Close initial monitoring of these patients is warranted. Pediatr. Pulmonol. 2011; 46:1079–1084. © 2011 Wiley Periodicals, Inc.