This study tested the hypotheses that the hypertonic environment of the renal medulla regulates the expression of cystic fibrosis transmembrane conductance regulator protein (CFTR) and its natural splice variant, TNR-CFTR. To accomplish this, Madin- Darby canine kidney (MDCK) stable cell lines expressing TNR-CFTR or CFTR were used. The cells were treated with hypertonic medium made with either NaCl or urea or sucrose (480 mOsm/kg or 560 mOsm/kg) to mimic the tonicity of the renal medulla environment. Western blot data showed that CFTR and TNR-CFTR total cell protein is increased by hypertonic medium, but using the surface biotinylation technique, only CFTR was found to be increased in cell plasma membrane. Confocal microscopy showed TNR-CFTR localization primarily at the endoplasmic reticulum and plasma membrane. In conclusion, CFTR and TNR-CFTR have different patterns of distribution in MDCK cells and they are modulated by a hypertonic environment, suggesting their physiological importance in renal medulla.
CFTR; TNR-CFTR; Sodium chloride; Urea Sucrose; MDCK cells; Hypertonic environment Plasma membrane
To quantify the relative contribution of factors other than cystic fibrosis transmembrane conductance regulator genotype and environment on the acquisition of Pseudomonas aeruginosa (Pa) by patients with cystic fibrosis.
Lung infection with Pa and mucoid Pa was assessed using a co-twin study design of 44 monozygous (MZ) and 17 dizygous (DZ) twin pairs. Two definitions were used to establish infection: first positive culture and persistent positive culture. Genetic contribution to infection (ie, heritability) was estimated based on concordance analysis, logistic regression, and age at onset of infection through comparison of intraclass correlation coefficients.
Concordance for persistent Pa infection was higher in MZ (0.83; 25 of 30 pairs) than DZ twins (0.45; 5 of 11 pairs), generating a heritability of 0.76. Logistic regression adjusted for age corroborated genetic control of persistent Pa infection. The correlation for age at persistent Pa infection was higher in MZ twins (0.589; 95% CI, 0.222-0.704) than in DZ twins (0.162; 95% CI, −0.352 to 0.607), generating a heritability of 0.85.
Genetic modifiers play a significant role in the establishment and timing of persistent Pa infection in individuals with cystic fibrosis.
Over 1900 mutations have been reported in the cystic fibrosis transmembrane conductance regulator (CFTR), the gene defective in patients with cystic fibrosis. These mutations have been discovered primarily in individuals who have features consistent with the diagnosis of CF. In some cases, it has been recognized that the mutations are not causative of cystic fibrosis but are responsible for disorders with features similar to CF, and these conditions have been termed CFTR-related disorders or CFTR-RD. There are also mutations in CFTR that do not contribute to any known disease state. Distinguishing CFTR mutations according to their penetrance for an abnormal phenotype is important for clinical management, structure/function analysis of CFTR, and understanding the molecular and cellular mechanisms underlying CF.
More than 1900 mutations in the CFTR gene are known. Functionally characterizing these mutations is important for diagnosing and treating patients with cystic fibrosis and related disorders.
A combined genome-wide association and linkage study was used to identify loci causing variation in CF lung disease severity. A significant association (P=3. 34 × 10-8) near EHF and APIP (chr11p13) was identified in F508del homozygotes (n=1,978). The association replicated in F508del homozygotes (P=0.006) from a separate family-based study (n=557), with P=1.49 × 10-9 for the three-study joint meta-analysis. Linkage analysis of 486 sibling pairs from the family-based study identified a significant QTL on chromosome 20q13.2 (LOD=5.03). Our findings provide insight into the causes of variation in lung disease severity in CF and suggest new therapeutic targets for this life-limiting disorder.
In the past three decades, scientists have had immense success in identifying genes and their variants that contribute to an array of diseases. While the identification of such genetic variants has informed our knowledge of the etiologic bases of diseases, there continues to be a substantial gap in our understanding of the factors that modify disease severity. Monogenic diseases provide an opportunity to identify modifiers as they have uniform etiology, detailed phenotyping of affected individuals, and familial clustering. Cystic fibrosis (CF) is among the more common life-shortening recessive disorders that displays wide variability in clinical features and survival. Considerable progress has been made in elucidating the contribution of genetic and nongenetic factors to CF. Allelic variation in CFTR, the gene responsible for CF, correlates with some aspects of the disease. However, lung function, neonatal intestinal obstruction, diabetes, and anthropometry display strong genetic control independent of CFTR, and candidate gene studies have revealed genetic modifiers underlying these traits. The application of genome-wide techniques holds great promise for the identification of novel genetic variants responsible for the heritable features and complications of CF. Since the genetic modifiers are known to alter the course of disease, their protein products become immediate targets for therapeutic intervention.
genome wide; candidate gene; heritability; variation
To assess the relative contributions of environmental and genetic factors to variation in cystic fibrosis (CF) pulmonary disease.
Genetic and environmental contributions were quantified using intra-pair correlations and differences in CF-specific FEV1 measures from 134 monozygous twins and 272 dizygous twins and siblings while in different living environments (i.e. living with parents vs. living alone) as well as using intra-individual differences in lung function from a separate group of 80 siblings.
Lung function among monozygous twins was more similar than among dizygous twin and sibling pairs, regardless of living environment, affirming the role of genetic modifiers in CF lung function. Regression modeling revealed that genetic factors account for 50% of lung function variation, unique environmental and stochastic factors 36%, and shared environmental factors, 14% (Model p: <0.0001). The intra-individual analysis produced similar estimates for the contributions of the unique and shared environment. The shared environment effects appeared primarily due to living with a sibling with CF (p: 0.003), rather than factors within the parental household (p: 0.310).
Genetic and environmental factors contribute equally to lung function variation in CF. Environmental effects are dominated by unique and stochastic effects rather than common exposures.
cystic fibrosis; heritability, lung disease, genetics; FEV1
Cystic fibrosis (CF) is characterized by recurrent respiratory infections and progressive lung disease. Whereas exercise may contribute to preserving lung function, its benefit is difficult to ascertain given the selection bias of healthier patients being more predisposed to exercise. Our objective was to examine the role of self-reported exercise with longitudinal lung function and body mass index (BMI) measures in CF.
A total of 1038 subjects with CF were recruited through the U.S. CF Twin-Sibling Study. Questionnaires were used to determine exercise habits. Questionnaires, chart review, and U.S. CF Foundation Patient Registry data were used to track outcomes.
Within the study sample 75% of subjects self-reported regular exercise. Exercise was associated with an older age of diagnosis (p = 0.002), older age at the time of ascertainment (p < 0.001), and higher baseline FEV1 (p = 0.001), but not CFTR genotype (p = 0.64) or exocrine pancreatic function (p = 0.19). In adjusted mixed models, exercise was associated with both a reduced decline in FEV1 (p < 0.001) and BMI Z-score (p = 0.001) for adults, but not children aged 10–17 years old.
In our retrospective study, self-reported exercise was associated with improved longitudinal nutritional and pulmonary outcomes in cystic fibrosis for adults. Although prospective studies are needed to confirm these associations, programs to promote regular exercise among individuals with cystic fibrosis would be beneficial.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2466-14-159) contains supplementary material, which is available to authorized users.
Cystic fibrosis; Lung function; FEV1; Body mass index; Exercise
Diabetes is a common age-dependent complication of cystic fibrosis (CF) that is strongly influenced by modifier genes. We conducted a genome-wide association study in 3,059 individuals with CF (644 with CF-related diabetes [CFRD]) and identified single nucleotide polymorphisms (SNPs) within and 5′ to the SLC26A9 gene that associated with CFRD (hazard ratio [HR] 1.38; P = 3.6 × 10−8). Replication was demonstrated in 694 individuals (124 with CFRD) (HR, 1.47; P = 0.007), with combined analysis significant at P = 9.8 × 10−10. SLC26A9 is an epithelial chloride/bicarbonate channel that can interact with the CF transmembrane regulator (CFTR), the protein mutated in CF. We also hypothesized that common SNPs associated with type 2 diabetes also might affect risk for CFRD. A previous association of CFRD with SNPs in TCF7L2 was replicated in this study (P = 0.004; combined analysis P = 3.8 × 10−6), and type 2 diabetes SNPs at or near CDKAL1, CDKN2A/B, and IGF2BP2 were associated with CFRD (P < 0.004). These five loci accounted for 8.3% of the phenotypic variance in CFRD onset and had a combined population-attributable risk of 68%. Diabetes is a highly prevalent complication of CF, for which susceptibility is determined in part by variants at SLC26A9 (which mediates processes proximate to the CF disease-causing gene) and at four susceptibility loci for type 2 diabetes in the general population.
Disease variation can be substantial even in conditions with a single gene etiology such as cystic fibrosis (CF). Simultaneously studying the effects of genes and environment may provide insight into the causes of variation.
To determine whether secondhand smoke exposure is associated with lung function and other outcomes in individuals with CF, whether socioeconomic status affects the relationship between secondhand smoke exposure and lung disease severity, and whether specific gene-environment interactions influence the effect of secondhand smoke exposure on lung function.
Design, Setting, and Participants
Retrospective assessment of lung function, stratified by environmental and genetic factors. Data were collected by the US Cystic Fibrosis Twin and Sibling Study with missing data supplemented by the Cystic Fibrosis Foundation Data Registry. All participants were diagnosed with CF, were recruited between October 2000 and October 2006, and were primarily from the United States.
Main Outcome Measures
Disease-specific cross-sectional and longitudinal measures of lung function.
Of 812 participants with data on secondhand smoke in the home, 188 (23.2%) were exposed. Of 780 participants with data on active maternal smoking during gestation, 129 (16.5%) were exposed. Secondhand smoke exposure in the home was associated with significantly lower cross-sectional (9.8 percentile point decrease; P<.001) and longitudinal lung function (6.1 percentile point decrease; P=.007) compared with those not exposed. Regression analysis demonstrated that socioeconomic status did not confound the adverse effect of secondhand smoke exposure on lung function. Interaction between gene variants and secondhand smoke exposure resulted in significant percentile point decreases in lung function, namely in CFTR non-ΔF508 homozygotes (12.8 percentile point decrease; P=.001), TGFβ1-509 TT homozygotes (22.7 percentile point decrease; P=.006), and TGFβ1 codon 10 CC homozygotes (20.3 percentile point decrease; P=.005).
Any exposure to secondhand smoke adversely affects both cross-sectional and longitudinal measures of lung function in individuals with CF. Variations in the gene that causes CF (CFTR) and a CF-modifier gene (TGFβ1) amplify the negative effects of secondhand smoke exposure.
Patients with cystic fibrosis (CF) manifest a multisystem disease due to deleterious mutations in each gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). However, the role of dysfunctional CFTR is uncertain in individuals with mild forms of CF (ie, pancreatic sufficiency) and mutation in only one CFTR gene.
Eleven pancreatic sufficient (PS) CF patients with only one CFTR mutation identified after mutation screening (three patients), mutation scanning (four patients) or DNA sequencing (four patients) were studied. Bi-directional sequencing of the coding region of CFTR was performed in patients who had mutation screening or scanning. If a second CFTR mutation was not identified, CFTR mRNA transcripts from nasal epithelial cells were analysed to determine if any PS-CF patients harboured a second CFTR mutation that altered RNA expression.
Sequencing of the coding regions of CFTR identified a second deleterious mutation in five of the seven patients who previously had mutation screening or mutation scanning. Five of the remaining six patients with only one deleterious mutation identified in the coding region of one CFTR gene had a pathologic reduction in the amount of RNA transcribed from their other CFTR gene (8.4–16% of wild type).
These results show that sequencing of the coding region of CFTR followed by analysis of CFTR transcription could be a useful diagnostic approach to confirm that patients with mild forms of CF harbour deleterious alterations in both CFTR genes.
Cystic fibrosis (CF) is characterized by recurrent respiratory infections and progressive lung disease. Although environmental factors account for 50% of the variation in CF lung function, few specific exposures have been identified. Studies using small study samples focusing on environmental allergies in CF have had inconsistent results. Our objective was to examine the role of environmental allergies in upper and lower respiratory tract morbidities in CF.
A total of 1321 subjects with CF were recruited through the U.S. CF Twin-Sibling Study. Questionnaires were used to determine the presence/absence of environmental allergies. Questionnaires, chart review, and U.S. CF Foundation Patient Registry data were used to track outcomes.
Within the study sample 14% reported environmental allergies. Environmental allergies were associated with a higher risk of sinus disease (adjusted OR: 2.68; p<0.001) and nasal polyps (adjusted OR: 1.74; p=0.003). Environmental allergies were also associated with a more rapid decline in lung function (additional −1.1%/year; p=0.001). However, allergies were associated with a later median age of acquisition of Pseudomonas aeruginosa (6.6 yo vs. 4.4 yo; log rank p=0.027). The reported use of common allergy medications, anti-histamines and leukotriene inhibitors, did not alter the frequency of respiratory morbidities.
Environmental allergies are associated with an increased risk of sinus disease and nasal polyps and a more rapid decline in CF lung function, but may have a protective effect against the acquisition of P. aeruginosa. Prospective studies are needed to confirm these associations which have implications for more aggressive management of allergies.
Cystic Fibrosis; Lung Function; FEV1; Allergies; Sinus Disease; Nasal Polyps; Pseudomonas aeruginosa
Sarcoidosis is a multi-system inflammatory disease with organ involvement that varies by race and sex. Family studies indicate that genes play a role in the etiology and extent of organ involvement in sarcoidosis. In this study, we evaluated whether 25 variants distributed in 19 genes with a known role in inflammation were associated with erythema nodosum status in 659 sarcoidosis patients and 658 controls from A Case–Control Etiologic Study of Sarcoidosis (ACCESS). We found no association with affectation status; however, a variant in the promoter of tumor necrosis factor (TNF) at position −308 was found to be associated with erythema nodosum in Caucasian sarcoidosis patients (study-wide P = 0.027). When separated by sex, a variant in intron 1 of lymphotoxin-α (LTA), a gene adjacent to TNF, was associated with erythema nodosum in female Caucasian sarcoidosis patients (study-wide P = 0.027). These DNA variants frequently occur together in Caucasians, and each variant has individually been associated with erythema nodosum in sarcoidosis patients. These results confirm that variation in the LTA/TNF gene cluster modifies a major skin manifestation of sarcoidosis and may explain the higher rate of erythema nodosum in females with sarcoidosis.
Patients with cystic fibrosis with the same mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene differ widely in survival suggesting other factors have a substantial role in mortality.
To determine if the genotype distribution of variants in three putative cystic fibrosis modifier genes (tumour necrosis factor α (TNFα), transforming growth factor β1 (TGFβ1) or mannose‐binding lectin (MBL2)) differed among patients with cystic fibrosis grouped according to age and survival status.
Genotypes of four variants (TNFα‐238, TNFα‐308, TGFβ1‐509 and MBL2 O) were determined in three groups of Caucasians from a single medical centre: 101 children with cystic fibrosis (aged <17 years; mean age 9.4 years), 115 adults with cystic fibrosis (aged ⩾17 years; mean age 30.8 years) and 38 non‐surviving adults with cystic fibrosis (21 deceased and 17 lung transplant after 17 years of age). Genotypes of 127 healthy Caucasians in the same geographical region were used as controls. Kaplan–Meier and Cox hazard regression were used to evaluate the genotype effect on cumulative survival.
Genotype frequencies among adults and children with cystic fibrosis differed for TNFα‐238 (G/G vs G/A; p = 0.022) and MBL2 (A/A vs O/O; p = 0.016). When adults with cystic fibrosis were compared to non‐surviving adults with cystic fibrosis, genotype frequencies of both genes differed (TNFα‐238G/G vs G/A; p = 0.0015 and MBL2: A/A vs O/O; p = 0.009). The hazard ratio for TNFα‐238G/G vs G/A was 0.25 (95% CI 0.06 to 1.0, p = 0.04) and for MBL2 O/O vs A/A or A/O was 2.5 (95% CI 1.3 to 4.9, p = 0.007).
TNFα‐238 G/A and MBL2 O/O genotypes appear to be genetic modifiers of survival of cystic fibrosis.
Purpose of review
Cystic fibrosis (CF) is a common, life-limiting monogenic disease, which typically manifests as progressive bronchiectasis, exocrine pancreatic dysfunction, and recurrent sinopulmonary infections. Although the gene responsible for CF (CFTR) was described in 1989, it has become increasingly evident that modifier genes and environmental factors play substantial roles in determining the severity of disease, particularly lung disease. Identifying these factors is crucial in devising therapies and other interventions to decrease the morbidity and mortality associated with this disorder.
Although many genes have been proposed as potential modifiers of CF, only a handful have withstood the test of replication. Several of the replicated findings reveal that genes affecting inflammation and infection response play a key role in modifying CF lung disease severity. Interactions between CFTR genotype, modifier genes, and environmental factors have been documented to influence lung function measures and infection status in CF patients.
Several genes have been demonstrated to affect disease severity in CF. Furthermore, it is likely that gene–gene and gene–environment interactions can explain a substantial portion of the variation of lung disease. Ongoing genome-wide studies are likely to identify novel genetic modifiers. Continued exploration of the role of genetic and nongenetic modifiers of CF is likely to yield new options for combating this debilitating disease.
cystic fibrosis; genome-wide; interaction; modifier gene
We have been investigating the functional consequences of rare disease-associated amino acid substitutions in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Mutations of the arginine residue at codon 1070 have been associated with different disease consequences R1070P and R1070Q have “severe” pancreatic insufficient cystic fibrosis (CF) and R1070W have “mild” pancreatic sufficient CF or congenital bilateral absence of the vas deferens. Intriguingly, CFTR bearing each of these mutations is functional when expressed in non-polarized cells. To determine whether R1070 mutations cause disease by affecting CFTR localization, we created polarized MDCK cell lines that express either wild-type or mutant CFTR from the same genomic integration site. Confocal microscopy and biotinylation studies revealed that R1070P was not inserted into the apical membrane, R1070W was inserted at levels reduced from wild-type while R1070Q was present in the apical membrane at levels comparable to wild-type. The abnormal localization of CFTR bearing R1070P and R1070W was consistent with deleterious consequences in patients however the profile of CFTR R1070Q was inconsistent with a “severe” phenotype. Re-analysis of 16 patients with the R1070Q mutation revealed that 11 carried an in cis nonsense mutation, S466X. All 11 patients carrying the complex allele R1070Q-S466X had severe disease, while 4 of 5 patients with R1070Q had “mild” disease, thereby reconciling the apparent discrepancy between the functional studies of R1070Q and the phenotype of patients bearing this mutation. Our results emphasize that localization studies in relevant model systems are needed for the interpretation of the disease-causing potential of rare missense mutations.
cystic fibrosis; CF; CFTR; polarized cells; complex allele; recombinase-mediated integration
Rationale: Obstructive lung disease, the major cause of mortality in cystic fibrosis (CF), is poorly correlated with mutations in the disease-causing gene, indicating that other factors determine severity of lung disease.
Objectives: To quantify the contribution of modifier genes to variation in CF lung disease severity.
Methods: Pulmonary function data from patients with CF living with their affected twin or sibling were converted into reference values based on both healthy and CF populations. The best measure of FEV1 within the last year was used for cross-sectional analysis. FEV1 measures collected over at least 4 years were used for longitudinal analysis. Genetic contribution to disease variation (i.e., heritability) was estimated in two ways: by comparing similarity of lung function in monozygous (MZ) twins (∼ 100% gene sharing) with that of dizygous (DZ) twins/siblings (∼ 50% gene sharing), and by comparing similarity of lung function measures for related siblings to similarity for all study subjects.
Measurements and Main Results: Forty-seven MZ twin pairs, 10 DZ twin pairs, and 231 sibling pairs (of a total of 526 patients) with CF were studied. Correlations for all measures of lung function for MZ twins (0.82–0.91, p < 0.0001) were higher than for DZ twins and siblings (0.50–0.64, p < 0.001). Heritability estimates from both methods were consistent for each measure of lung function and ranged from 0.54 to 1.0. Heritability estimates generally increased after adjustment for differences in nutritional status (measured as body mass index z-score).
Conclusions: Our heritability estimates indicate substantial genetic control of variation in CF lung disease severity, independent of CFTR genotype.
genetics; pulmonary function
Background & Aims
Neonatal intestinal obstruction (meconium ileus or MI) occurs in 15% of patients with cystic fibrosis (CF). Our aim was to determine the relative contribution of genetic and non-genetic modifiers to the development of this major complication of CF.
Using clinical data and DNA collected by the CF Twin and Sibling Study, 65 monozygous twin pairs, 23 dizygous twin/triplet sets, and 349 sets of siblings with CF were analyzed for MI status, significant covariates, and genome-wide linkage.
Specific mutations in CFTR, the gene responsible for CF, correlated with MI indicating a role for CFTR genotype. Monozygous twins showed substantially greater concordance for MI than dizygous twins and siblings (p=1×10−5) demonstrating that modifier genes independent of CFTR contribute substantially to this trait. Regression analysis revealed that MI was correlated with distal intestinal obstruction syndrome (DIOS; p=8×10−4). Unlike MI, concordance analysis indicated that the risk for development of DIOS in CF patients is primarily due to non-genetic factors. Regions of suggestive linkage (logarithm of the odds of linkage >2.0) for modifier genes that cause MI (chromosomes 4q35.1, 8p23.1, and 11q25) or protect from MI (chromosomes 20p11.22 and 21q22.3) were identified by genome-wide analyses. These analyses did not support the existence of a major modifier gene within the CFM1 region on chromosome 19 that had previously been linked to MI.
The CFTR gene along with two or more modifier genes are the major determinants of intestinal obstruction in newborn CF patients, while intestinal obstruction in older CF patients is primarily due to non-genetic factors.
Twins; siblings; linkage; association; intestinal obstruction; CFM-1
Cystic fibrosis (CF) is caused by mutations in the CFTR gene that predispose the airway to infection. Chronic infection by pathogens such as Pseudomonas aeruginosa leads to inflammation that gradually degrades lung function, resulting in morbidity and early mortality. In a previous study of CF monozygotic twins, we demonstrate that genetic modifiers significantly affect the establishment of persistent P. aeruginosa colonization in CF. Recognizing that bacteria other than P. aeruginosa contribute to the CF microbiome and associated pathology, we used deep sequencing of sputum from pediatric monozygotic twins and nontwin siblings with CF to characterize pediatric bacterial communities and the role that genetics plays in their evolution.
We found that the microbial communities in sputum from pediatric patients living together were much more alike than those from pediatric individuals living apart, regardless of whether samples were taken from monozygous twins or from nontwin CF siblings living together, which we used as a proxy for dizygous twins. In contrast, adult communities were comparatively monolithic and much less diverse than the microbiome of pediatric patients.
Taken together, these data and other recent studies suggest that as patients age, the CF microbiome becomes less diverse, more refractory to treatment and dominated by mucoid P. aeruginosa, as well as being associated with accelerated pulmonary decline. Our studies show that the microbiome of pediatric patients is susceptible to environmental influences, suggesting that interventions to preserve the community structure found in young CF patients might be possible, perhaps slowing disease progression.
Cystic fibrosis; Microbiome; Pseudomonas aeruginosa; Sputum
Allelic heterogeneity in disease-causing genes presents a substantial challenge to the translation of genomic variation to clinical practice. Few of the almost 2,000 variants in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have empirical evidence that they cause cystic fibrosis. To address this gap, we collected both genotype and phenotype data for 39,696 cystic fibrosis patients in registries and clinics in North America and Europe. Among these patients, 159 CFTR variants had an allele frequency of ≥0.01%. These variants were evaluated for both clinical severity and functional consequence with 127 (80%) meeting both clinical and functional criteria consistent with disease. Assessment of disease penetrance in 2,188 fathers of cystic fibrosis patients enabled assignment of 12 of the remaining 32 variants as neutral while the other 20 variants remained indeterminate. This study illustrates that sourcing data directly from well-phenotyped subjects can address the gap in our ability to interpret clinically-relevant genomic variation.
Since the 1989 discovery that mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF), there has been substantial progress toward understanding the molecular basis for CF lung disease, leading to the discovery and development of new therapeutic approaches. However, the earliest impact of the loss of CFTR function on airway physiology and structure and its relationship to initial infection and inflammation are poorly understood. Universal newborn screening for CF in the United States represents an unprecedented opportunity for investigating CF clinical manifestations very early in life. Recently developed animal models with pulmonary phenotypic manifestations also provide a window into the early consequences of this genetic disorder. For these reasons, the National Heart, Lung, and Blood Institute (NHLBI) convened a working group of extramural experts, entitled “Future Research Directions in Early CF Lung Disease” on September 21–22, 2010, to identify future research directions of great promise in CF. The priority areas identified included (1) exploring pathogenic mechanisms of early CF lung disease; (2) leveraging newborn screening to elucidate the natural history of early lung disease; (3) developing a spectrum of biomarkers of early lung disease that reflects CF pathophysiology, clinical outcome, and response to treatment; (4) exploring the role of genetics/genomics (e.g., modifier genes, gene–environmental interactions, and epigenetics) in early CF pathogenesis; (5) defining early microbiological events in CF lung disease; and (6) elucidating the initial airway inflammatory, remodeling, and repair mechanisms in CF lung disease.
cystic fibrosis; airway disease; innate immunity; microbiology; genetics
Genetic studies of lung disease in Cystic Fibrosis are hampered by the
lack of a severity measure that accounts for chronic disease progression and
mortality attrition. Further, combining analyses across studies requires common
phenotypes that are robust to study design and patient ascertainment.
Using data from the North American Cystic Fibrosis Modifier Consortium
(Canadian Consortium for CF Genetic Studies, Johns Hopkins University CF Twin
and Sibling Study, and University of North Carolina/Case Western Reserve
University Gene Modifier Study), the authors calculated age-specific CF
percentile values of FEV1 which were adjusted for CF age-specific mortality
The phenotype was computed for 2061 patients representing the Canadian CF
population, 1137 extreme phenotype patients in the UNC/Case Western study, and
1323 patients from multiple CF sib families in the CF Twin and Sibling Study.
Despite differences in ascertainment and median age, our phenotype score was
distributed in all three samples in a manner consistent with ascertainment
differences, reflecting the lung disease severity of each individual in the
underlying population. The new phenotype score was highly correlated with the
previously recommended complex phenotype, but the new phenotype is more robust
for shorter follow-up and for extreme ages.
A disease progression and mortality adjusted phenotype reduces the need
for stratification or additional covariates, increasing statistical power and
avoiding possible distortions. This approach will facilitate large scale genetic
and environmental epidemiological studies which will provide targeted
therapeutic pathways for the clinical benefit of patients with CF.
Forced Expiratory Volume; Age Effects; Severity of Illness Index
Variants associated with meconium ileus in cystic fibrosis (CF) were identified in 3,763 patients by GWAS. Five SNPs at two loci near SLC6A14 (min P=1.28×10−12 at rs3788766), chr Xq23-24 and SLC26A9 (min P=9.88×10−9 at rs4077468), chr 1q32.1 accounted for ~5% of the phenotypic variability, and were replicated in an independent patient collection (n=2,372; P=0.001 and 0.0001 respectively). By incorporating that disease-causing mutations in CFTR alter electrolyte and fluid flux across epithelia into an hypothesis-driven genome-wide analysis (GWAS-HD), we identified the same SLC6A14 and SLC26A9 associated SNPs, while establishing evidence for the involvement of SNPs in a third solute carrier gene, SLC9A3. In addition, GWAS-HD provided evidence of association between meconium ileus and multiple constituents of the apical plasma membrane where CFTR resides (P=0.0002, testing 155 apical genes jointly and replicated, P=0.022). These findings suggest that modulating activities of apical membrane constituents could complement current therapeutic paradigms for cystic fibrosis.
To determine which features of incomplete or “nonclassic” forms of cystic fibrosis (CF) are associated with deleterious CF transmembrane conductance regulator gene (CFTR) mutations, and to explore other etiologies for features not associated with deleterious CFTR mutations.
Clinical features were compared between 57 patients with deleterious mutations in each CFTR and 63 with no deleterious mutations. The Shwachman Bodian Diamond syndrome gene (SBDS) was sequenced to search for mutations in patients with no deleterious CFTR mutations and steatorrhea to determine if any had unrecognized Shwachman-Diamond syndrome (SDS).
The presence of a common CF-causing mutation, absence of the vas deferens, and Pseudomona aeruginosa in the sputum correlated with the presence of two deleterious CFTR mutations, whereas sweat chloride concentration, diagnostic criteria for CF, and steatorrhea did not. However, sweat chloride concentration correlated with CFTR mutation status in patients infected with P aeruginosa. One patient had disease-causing mutations in each SBDS.
Presence of a common CF-causing mutation, absence of the vas deferens and/or P aeruginosa infection in a patient with features of nonclassic CF are predictive of deleterious mutations in each CFTR, whereas steatorrhea in the same context is likely to have etiologies other than CF transmembrane conductance regulator (CFTR) dysfunction.
Meconium ileus (MI), a life-threatening intestinal obstruction due to meconium with abnormal protein content, occurs in approximately 15 percent of neonates with cystic fibrosis (CF). Analysis of twins with CF demonstrates that MI is a highly heritable trait, indicating that genetic modifiers are largely responsible for this complication. Here, we performed regional family-based association analysis of a locus that had previously been linked to MI and found that SNP haplotypes 5′ to and within the MSRA gene were associated with MI (P = 1.99×10−5 to 1.08×10−6; Bonferroni P = 0.057 to 3.1×10−3). The haplotype with the lowest P value showed association with MI in an independent sample of 1,335 unrelated CF patients (OR = 0.72, 95% CI [0.53–0.98], P = 0.04). Intestinal obstruction at the time of weaning was decreased in CF mice with Msra null alleles compared to those with wild-type Msra resulting in significant improvement in survival (P = 1.2×10−4). Similar levels of goblet cell hyperplasia were observed in the ilea of the Cftr−/− and Cftr−/−Msra−/− mice. Modulation of MSRA, an antioxidant shown to preserve the activity of enzymes, may influence proteolysis in the developing intestine of the CF fetus, thereby altering the incidence of obstruction in the newborn period. Identification of MSRA as a modifier of MI provides new insight into the biologic mechanism of neonatal intestinal obstruction caused by loss of CFTR function.
Cystic fibrosis (CF) is a monogenic disease with considerable phenotypic variability. About 15% of newborns with CF suffer from an intestinal obstruction called meconium ileus (MI), and studies in CF twins have shown that modifier genes play a substantial role in the development of this complication. We used a family-based study design to enrich for genetic modifiers of MI and found that variations in the MSRA gene, represented by combinations of SNPs, or haplotypes, were protective against this manifestation of CF. We investigated association between one of the MSRA haplotypes and MI in an independent sample of CF patients and showed that it had a similar protective effect. Furthermore, CF mice lacking Msra expression had lower mortality due to intestinal obstruction at the time of transitioning to solid food and lived longer than CF mice with normal Msra, thus supporting the protective effect of the haplotype we observed in human CF subjects. The identification of modifiers of MI such as MSRA offers new insight into the mechanism of this life-threatening complication of CF.
It is generally presumed that the Cystic Fibrosis (CF) population is relatively homogeneous, and predominantly of European origin. The complex ethnic make-up observed in the CF patients collected by the North American CF Modifier Gene Consortium has brought this assumption into question, and suggested the potential for population substructure in the three CF study samples collected from North America. It is well appreciated that population substructure can result in spurious genetic associations.
To understand the ethnic composition of the North American CF population, and to assess the need for population structure adjustment in genetic association studies with North American CF patients.
Genome-wide single-nucleotide polymorphisms on 3076 unrelated North American CF patients were used to perform population structure analyses. We compared self-reported ethnicity to genotype-inferred ancestry, and also examined whether geographic distribution and CFTR mutation type could explain the structure observed.
Although largely Caucasian, our analyses identified a considerable number of CF patients with admixed African-Caucasian, Mexican-Caucasian and Indian-Caucasian ancestries. Population substructure was present and comparable across the three studies of the consortium. Neither geographic distribution nor mutation type explained the population structure.
Given the ethnic diversity of the North American CF population, it is essential to carefully detect, estimate and adjust for population substructure to guard against potential spurious findings in CF genetic association studies. Other Mendelian diseases that are presumed to predominantly affect single ethnic groups may also benefit from careful analysis of population structure.
ethnicity; principal component analysis; population substructure; population stratification