Modifier genes and variation in cystic fibrosis

Transforming Growth Factor-β1 Selectively Recruits microRNAs to the RNA-Induced Silencing Complex and Degrades CFTR mRNA under Permissive Conditions in Human Bronchial Epithelial Cells

Mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene lead to cystic fibrosis (CF). The most common mutation F508del inhibits folding and processing of CFTR protein. FDA-approved correctors rescue the biosynthetic processing of F508del-CFTR protein, while potentiators improve the rescued CFTR channel function. Transforming growth factor (TGF-β1), overexpressed in many CF patients, blocks corrector/potentiator rescue by inhibiting CFTR mRNA in vitro. Increased TGF-β1 signaling and acquired CFTR dysfunction are present in other lung diseases. To study the mechanism of TGF-β1 repression of CFTR, we used molecular, biochemical, and functional approaches in primary human bronchial epithelial cells from over 50 donors. TGF-β1 destabilized CFTR mRNA in cells from lungs with chronic disease, including CF, and impaired F508del-CFTR rescue by new-generation correctors. TGF-β1 increased the active pool of selected micro(mi)RNAs validated as CFTR inhibitors, recruiting them to the RNA-induced silencing complex (RISC). Expression of F508del-CFTR globally modulated TGF-β1-induced changes in the miRNA landscape, creating a permissive environment required for degradation of F508del-CFTR mRNA. In conclusion, TGF-β1 may impede the full benefit of corrector/potentiator therapy in CF patients. Studying miRNA recruitment to RISC under disease-specific conditions may help to better characterize the miRNAs utilized by TGF-β1 to destabilize CFTR mRNA.

The influence of genetics on cystic fibrosis phenotypes

Technological advances in genetics have made feasible and affordable large studies to identify genetic variants that cause or modify a trait. Genetic studies have been carried out to assess variants in candidate genes, as well as polymorphisms throughout the genome, for their associations with heritable clinical outcomes of cystic fibrosis (CF), such as lung disease, meconium ileus, and CF-related diabetes. The candidate gene approach has identified some predicted relationships, while genome-wide surveys have identified several genes that would not have been obvious disease-modifying candidates, such as a methionine sulfoxide transferase gene that influences intestinal obstruction, or a region on chromosome 11 proximate to genes encoding a transcription factor and an apoptosis controller that associates with lung function. These unforeseen associations thus provide novel insight into disease pathophysiology, as well as suggesting new therapeutic strategies for CF.

Innate immunity in cystic fibrosis lung disease

Chronic lung disease determines the morbidity and mortality of cystic fibrosis (CF) patients. The pulmonary immune response in CF is characterized by an early and non-resolving activation of the innate immune system, which is dysregulated at several levels. Here we provide a comprehensive overview of innate immunity in CF lung disease, involving (i) epithelial dysfunction, (ii) pathogen sensing, (iii) leukocyte recruitment, (iv) phagocyte impairment, (v) mechanisms linking innate and adaptive immunity and (iv) the potential clinical relevance. Dissecting the complex network of innate immune regulation and associated pro-inflammatory cascades in CF lung disease may pave the way for novel immune-targeted therapies in CF and other chronic infective lung diseases.

Prostaglandin-endoperoxide synthase genes COX1 and COX2 - novel modifiers of disease severity in cystic fibrosis patients

Cystic fibrosis (CF) is one of the most common autosomal recessive diseases among Caucasians caused by a mutation in the CFTR gene. However, the clinical outcome of CF pulmonary disease varies remarkably even in patients with the same CFTR genotype. This has led to a search for genetic modifiers located outside the CFTR gene. The aim of this study was to evaluate the effect of functional variants in prostaglandin-endoperoxide synthase genes (COX1 and COX2) on the severity of lung disease in CF patients. To the best of our knowledge, it is the first time when analysis of COX1 and COX2 as potential CF modifiers is provided. The study included 94 CF patients homozygous for F508del mutation of CFTR. To compare their clinical condition, several parameters were recorded, e.g. a unique clinical score: disease severity status (DSS). To analyse the effect of non-CFTR genetic polymorphisms on the clinical course of CF patients, the whole coding region of COX1 and selected COX2 polymorphisms were analysed. Statistical analysis of genotype-phenotype associations revealed a relationship between the heterozygosity status of identified polymorphisms and better lung function. These results mainly concern COX2 polymorphisms: -765G>C and 8473T>C. The COX1 and COX2 polymorphisms reducing COX protein levels had a positive effect on all analysed clinical parameters. This suggests an important role of these genes as protective modifiers of pulmonary disease in CF patients, due to inhibition of arachidonic acid conversion into prostaglandins, which probably reduces the inflammatory process.

Modifier genes in Mendelian disorders: the example of cystic fibrosis

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.

The genetic background of osteoporosis in cystic fibrosis: Association analysis with polymorphic markers in four candidate genes

BACKGROUND

Reduced Bone Mass Density (BMD) is frequent in Cystic Fibrosis (CF). Potentially, other genes than the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene may contribute to the bone phenotype variability in CF patients.

METHODS

Four candidate genes likely associated with BMD variability were studied: the vitamin D receptor (VDR) gene, the estrogen receptor alpha (ESR1), the calcitonin receptor (CALCR) and the type I alpha 1 collagen (COL1A1) gene. A complete bone and CF evaluation was obtained for 82 subjects (39 m, 43 f): 15 had normal BMD (group 1), 46 were osteopenic (group 2), and 21 were osteoporotic (group 3).

RESULTS

No statistical difference was found among the three groups for age, sex, pancreatic status, and vertebral fractures, nor for any of the biochemical markers. Weight, Body Mass Index (BMI), and FEV1, scored significantly worse in the two groups with the lowest T score. The CFTR mutations R1162X and F508del were more frequent in patients with lower BMD (p=0.044 and p=0.071). There was no significant difference in the distribution of the five marker genotypes among the 3 groups defined according to the unadjusted or adjusted (BMI and FEV1) BMD T score. No significant correlation was found between the VDR, CALCR, or COL1A1 gene polymorphisms and reduced BMD values. The individual ESR1 PvuII-XbaI haplotype C-A is associated to elevated u-calcium levels whereas the haplotype T-A is associated to lower values (p=0.00251).

CONCLUSIONS

There was no evidence that the genes under study, with the possible exception of ESR1 gene variants, may modulate bone phenotype in CF.

Variants in the glutamate-cysteine-ligase gene are associated with cystic fibrosis lung disease

BACKGROUND

Chronic progressive lung disease is the most serious complication of cystic fibrosis (CF). Glutathione plays an important role in the protection of the CF lung against oxidant-induced lung injury.

OBJECTIVES

We hypothesized that a polymorphism in a novel candidate gene that regulates glutathione synthesis might influence CF lung disease.

METHODS

In a cross-sectional study, subjects were recruited from CF clinics in Seattle and multiple centers in Canada. We tested for an association between CF lung disease and a functional polymorphism in the glutamate-cysteine ligase catalytic subunit (GCLC) gene. Multiple linear regression was used to test for association between polymorphisms of GCLC and severity of CF lung disease while adjusting for age, Pseudomonas aeruginosa infection, and cystic fibrosis transmembrane conductance regulator (CFTR) genotype. Analysis was repeated for patients with CF stratified by CFTR genotype.

MEASUREMENTS AND MAIN RESULTS

A total of 440 subjects with CF participated in the study (51% male; mean [+/- SD] age, 26 +/- 11 yr; mean FEV(1), 62 +/- 28% predicted). In the total population, there was a trend toward an association between GCLC genotypes and CF lung disease (linear regression coefficient [SEM], 1.68 [1.0]; p = 0.097). In the stratified analysis, there was a highly significant association between GCLC genotype and CF lung function in subjects with a milder CFTR genotype (linear regression coefficient [SEM], 5.5 (1.7); p = 0.001).

CONCLUSIONS

In patients with CF with a milder CFTR genotype, there is a strong association between functional polymorphisms of the GCLC gene and CF lung disease severity.

Association of improved pulmonary phenotype in Irish cystic fibrosis patients with a 3' enhancer polymorphism in alpha-1-antitrypsin

Modifier genes other than CFTR are thought to influence lung disease phenotype in cystic fibrosis (CF). In this study, we investigated the relationship between a polymorphism (1237 G --> A) in the 3' enhancer region of the alpha-1-antitrypsin (AAT) gene and pulmonary disease severity in 320 CF patients recruited from two independent adult referral centers in Ireland, and evaluated the in vivo effect of the polymorphism on AAT levels during acute infection. When corrected for confounding variables, the polymorphism was found to make a small but significant contribution to variance in percent predicted forced expired volume in 1 sec (FEV1) (1.1%, P = 0.05), with possession of the A allele being associated with better pulmonary function (AA/AG genotype: percent predicted FEV1, 70.8 +/- 3.9; GG genotype: percent predicted FEV1, 62.0 +/- 1.4). As would be expected of a modifier effect, the influence of the polymorphism was more marked in patient groups traditionally associated with more severe lung disease, contributing 3.2% (P = 0.033) to the variance in percent predicted FEV1 in patients homozygous for DF508, 3.3% (P = 0.007) to those infected with Pseudomonas aeruginosa, and 3% (P = 0.024) in female patients. In each instance, a positive association between possession of the A variant and higher percent predicted FEV1 was observed. We did not, however, find any evidence that possession of the A allele effected upregulation of AAT during acute infection in vivo. This lack of a demonstrable functional effect in vivo suggests that the polymorphism is a marker for a modifying effect on pulmonary phenotype in the Irish CF population by a mechanism that is yet to be explained.

A 96-well formatted method for exon and exon/intron boundary full sequencing of the CFTR gene

Full genotypic characterization of subjects affected by cystic fibrosis (CF) is essential for the definition of the genotype-phenotype correlation as well as for the enhancement of the diagnostic and prognostic value of the genetic investigation. High-sensitivity diagnostic methods, capable of full scanning of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, are needed to enhance the significance of these genetic assays. A method for extensive sequencing of the CFTR gene was optimized. This method was applied to subjects clinically positive for CF and to controls from the general population of central Italy as well as to a single subject heterozygous for a mild mutation and with an uncertain diagnosis. Some points that are crucial for the optimization of the method emerged: a 96-well format, primer project and purification, and amplicon purification. The optimized method displayed a high degree of diagnostic sensitivity; we identified a subset of 13 CFTR mutations that greatly enhanced the diagnostic sensitivity of common methods of mutational analysis. A novel G1244R disease causing mutation, leading to a CF phenotype with pancreatic sufficiency but early onset of pulmonary involvement, was detected in the subject with an uncertain diagnosis. Some discrepancies between our results and previously published CFTR sequence were found.

Les gènes modificateurs dans la mucoviscidose

Cystic fibrosis is the most common lethal autosomal recessive disease among the Caucasian population. It is caused by defects in the CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator). Although over 1600 disease-causing mutations in the CFTR gene have been described, the highly variable disease phenotype in cystic fibrosis cannot be explained on the basis of this gene alone. Both the environment and other non-CFTR genes are likely to be important. The increased understanding of pathophysiological processes in the cystic fibrosis lung has led to several studies on genes in these pathways. One of the major aims of such studies is to produce targets for novel drug developments.

Modifier genetics: cystic fibrosis

Cystic fibrosis (CF) is the most common lethal autosomal recessive disorder in the Caucasian population, affecting about 30,000 individuals in the United States. The gene responsible for CF, the CF transmembrane conductance regulator (CFTR), was identified 15 years ago. Substantial variation in the many aspects of the CF phenotype among individuals with the same CFTR genotype demonstrates that factors independent of CFTR exert considerable influence on outcome in CF. To date, the majority of published studies investigating the cause of disease variability in CF report associations between candidate genes and some aspect of the CF phenotype. However, a definitive modifier gene for CF remains to be identified. Despite the challenges posed by searches for modifier effects, studies of affected twins and siblings indicate that genetic factors play a substantial role in intestinal manifestations. Identifying the factors contributing to variation in pulmonary disease, the primary cause of mortality, remains a challenge for CF research.

Understanding genetic disease in a socio-historical context: a case study of cystic fibrosis

In this article I present a socio-historical analysis of the definition and diagnosis of one particular genetic disease--cystic fibrosis (CF)--in an effort the better to understand its social context both before and after the advent of molecular genetics. I begin my analysis with some background on the history of CF, before moving on to consider the emergence of the notion of classic CF, the development of the sweat test, early approaches to mild or variant forms of CF, the concept of CF as a genetic disease, the concept of CF as a collection of related disorders, and developments in the understanding and diagnosis of CF which came about in the wake of molecular genetics. I highlight a range of technological, professional and patient developments and how these stimulated new research, typologies and clinical tools. I also consider how different notions of CF were mobilised, either to support or undermine a particular approach to diagnosis or treatment, and consider how the dynamic and contextual characteristics of CF were accounted for by scientists and clinicians with an interest in CF. I end by discussing the implications of my analysis for the contemporary sociology of genetics, and related studies in the sociology of medicine more generally.

Cystic fibrosis since 1938

Cystic fibrosis (CF) was distinguished from celiac disease in 1938. Then, it was a pathologic diagnosis, life expectancy was approximately 6 months, and the autosomal recessive disease was believed to arise from abnormal mucus plugging exocrine ducts. Death often occurred from lung infection. Discovery of the sweat electrolyte defect in 1953 and standardization of the sweat test in 1959 allowed identification of milder cases, and CF was no longer considered only a disorder of mucus. In 1955, establishment of centers with programs of aggressive, comprehensive care initiated striking improvement in longevity. The pillars of care established then (attention to nutrition, airway clearance, treatment of lung infection) remain today. In 1983, chloride transport was identified as the basic physiologic CF defect, accompanied by increased sodium reabsorption. In 1980, we learned that inflammation contributes independently to lung disease and constitutes an independent therapeutic target. In 1989, the discovery of the CF gene demonstrated the basic defect to be in a cAMP-regulated chloride channel. This afforded new diagnostic tests, opportunities for research, and prospects for using the gene as therapy. Since then, substantial advances in basic and clinical research catalyzed therapeutic improvements: median survival age now exceeds 30 years. The Cystic Fibrosis Foundation center network provides not only opportunity to conduct clinical trials but also means to disseminate new therapies. In the future, treatments directed at the basic defect can be expected, with concomitant improvements in morbidity and mortality.

Cystic fibrosis lung disease: genetic influences, microbial interactions, and radiological assessment

Cystic fibrosis (CF) is a multiorgan disease caused by mutation of the CF transmembrane conductance regulator (CFTR) gene. Obstructive lung disease is the predominant cause of morbidity and mortality; thus, most efforts to improve outcomes are directed toward slowing or halting lung-disease progression. Current therapies, such as mucolytics, airway clearance techniques, bronchodilators, and antibiotics, aim to suppress airway inflammation and the processes that stimulate it, namely, retention and infection of mucus plaques at the airway surface. New approaches to therapy that aim to ameliorate specific CFTR mutations or mutational classes by restoring normal expression or function are being investigated. Because of its sensitivity in detecting changes associated with early airway obstruction and regional lung disease, high-resolution CT (HRCT) complements pulmonary function testing in defining disease natural history and measuring response to both conventional and experimental therapies. In this review, perspectives on the genetics and microbiology of CF provide a context for understanding the increasing importance of HRCT and other imaging techniques in assessing CF therapies.

Disease modifying genes in cystic fibrosis

The variation in cystic fibrosis (CF) lung disease and development of CF related complications correlates poorly with the genotype of the CF transmembrane regulator (CFTR) and with environmental factors. Increasing evidence suggests that phenotypic variation in CF can be attributed to genetic variation in genes other than the CFTR gene, so-called modifier genes. In recent years, multiple candidate modifier genes have been investigated in CF, especially genes that are involved in the control of infection, immunity and inflammation. Some of these genes have been rather conclusively identified as modifiers of the CF phenotype, whereas associations found in other genes have not been confirmed or are conflicting. Identification of genetic variation in modifier genes, obtained by genotype-phenotype studies in well-defined patient populations, may be used as an aid to prognosis and may provide the possibility of new therapeutic interventions.

No indications for altered essential fatty acid metabolism in two murine models for cystic fibrosis

A deficiency of essential fatty acids (EFA) is frequently described in cystic fibrosis (CF), but whether this is a primary consequence of altered EFA metabolism or a secondary phenomenon is unclear. It was suggested that defective long-chain polyunsaturated fatty acid (LCPUFA) synthesis contributes to the CF phenotype. To establish whether cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction affects LCPUFA synthesis, we quantified EFA metabolism in cftr-/-CAM and cftr+/+CAM mice. Effects of intestinal phenotype, diet, age, and genetic background on EFA status were evaluated in cftr-/-CAM mice, DeltaF508/DeltaF508 mice, and littermate controls. EFA metabolism was measured by 13C stable isotope methodology in vivo. EFA status was determined by gas chromatography in tissues of cftr-/-CAM mice, DeltaF508/DeltaF508 mice, littermate controls, and C57Bl/6 wild types fed chow or liquid diet. After enteral administration of [13C]EFA, arachidonic acid (AA) and docosahexaenoic acid (DHA) were equally 13C-enriched in cftr-/-CAM and cftr+/+CAM mice, indicating similar EFA elongation/desaturation rates. LA, ALA, AA, and DHA concentrations were equal in pancreas, lung, and jejunum of chow-fed cftr-/-CAM and DeltaF508/DeltaF508 mice and controls. LCPUFA levels were also equal in liquid diet-weaned cftr-/-CAM mice and littermate controls, but consistently higher than in age- and diet-matched C57Bl/6 wild types. We conclude that cftr-/-CAM mice adequately absorb and metabolize EFA, indicating that CFTR dysfunction does not impair LCPUFA synthesis. A membrane EFA imbalance is not inextricably linked to the CF genotype. EFA status in murine CF models is strongly determined by genetic background.

Macromolecular Interactions and Ion Transport in Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease caused by autosomal recessive mutations of the CF transmembrane regulator, CFTR. CFTR functions in the plasma membrane of epithelial cells lining the lung, pancreas, liver, intestines, sweat duct, and the epididymis. The primary problem in CF is that mutations in CFTR affect its ability to be made, processed, and trafficked to the plasma membrane and/or its function as a Cl(-) channel and conductance regulator. Many proteins and processes normally interact with normal CFTR throughout its life cycle and mutant CFTR during the disease process. Understanding the function of these proteins and processes is expected to provide a clearer understanding of how normal CFTR is involved in salt movement and how mutant CFTR is handled by the cell and leads to the pathophysiology of CF. Recently, efforts to find therapies that correct defective CFTR have been intensifying. To facilitate our understanding of normal and mutant CFTR and the identification of new drug targets for developing novel therapies, a panel of experts was convened by the National Heart, Lung, and Blood Institute to explore the critical questions, challenges, and current opportunities to highlight new areas of research that would facilitate a integrated understanding of the processes and proteins that impact CFTR. The meeting highlighted the multiple pathways and interacting proteins involved in CFTR folding and biosynthesis, processing, and trafficking. A number of critical areas for future study were identified. Although these therapies are promising, a big question remains as to whether simply correcting defective CFTR will lead to significant improvement in patient health or whether the symptoms manifested in CF will require therapies in addition to those that target defective CFTR specifically.

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.

The phenotypic consequences of CFTR mutations

Cystic fibrosis is a common autosomal recessive disorder that primarily affects the epithelial cells in the intestine, respiratory system, pancreas, gall bladder and sweat glands. Over one thousand mutations have currently been identified in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene that are associated with CF disease. There have been many studies on the correlation of the CFTR genotype and CF disease phenotype; however, this relationship is still not well understood. A connection between CFTR genotype and disease manifested in the pancreas has been well described, but pulmonary disease appears to be highly variable even between individuals with the same genotype. This review describes the current classification of CFTR mutation classes and resulting CF disease phenotypes. Complex disease alleles and modifier genes are discussed along with alternative disorders, such as disseminated bronchiectasis and pancreatitis, which are also thought to result from CFTR mutations.

Newborn screening for cystic fibrosis

Early diagnosis of cystic fibrosis (CF) provides an opportunity to improve disease control and prevent early complications. Of patients with CF in the United States, 10% are identified early through newborn screening (including infants born in Colorado, Massachusetts, New Jersey, New York, Wisconsin, Wyoming, and parts of California, Connecticut, Pennsylvania, and Montana). Successful screening programs in these states have stimulated other states to consider adding CF screening to their newborn programs. Additionally, new technology permits expanded screening for numerous genetic conditions. Genetic screening, such as that used most frequently for CF, creates new challenges for the clinician, including atypical disease presentations and carrier detection. In this review, we examine the many advances in CF newborn screening and early care that were reported during the last few years.

Effect of genotype on phenotype and mortality in cystic fibrosis: a retrospective cohort study

BACKGROUND

Over 1000 mutations of the cystic fibrosis transmembrane conductance regulator gene (CFTR) that cause cystic fibrosis have been identified. We examined the effect of CFTR genotype on mortality and disease phenotype.

METHODS

Using the US Cystic Fibrosis Foundation National Registry, we did a retrospective cohort study to compare standardised mortality rates for the 11 most common genotypes heterozygous for DeltaF508 with those homozygous for DeltaF508. Of the 28455 patients enrolled in the registry at the time of our analysis, 17853 (63%) were genotyped. We also compared the clinical phenotype, including lung function, age at diagnosis, and nutritional measures, of 22 DeltaF508 heterozygous genotypes. Mortality rates and clinical phenotype were also compared between genotypes classified into six classes on the basis of their functional effect on CFTR production.

FINDINGS

Between 1991 and 1999, genetic and clinical data were available for 17853 patients with cystic fibrosis, which was 63% of the total cohort. There were 1547 deaths during the 9 years of follow-up. In the analysis of the 11 most common genotypes, DeltaF508/R117H, DeltaF508/DeltaI507, DeltaF508/3849+10kbC-->T, and DeltaF508/2789+5G-->A had a significantly lower mortality rate (4.7, 8.0, 11.9, and 4.4, respectively) than the genotype homozygous for DeltaF508 (21.8, p=0.0060). DeltaF508/R117H, DeltaF508/DeltaI507, DeltaF508/ 3849+10 kbC-->T, DeltaF508/2789+5G-->A, and DeltaF508/A455E have a milder clinical phenotype. Outcomes for all functional classes were compared with that of class II (containing DeltaF508 homozygotes) and classes IV and V had a significantly lower mortality rate and milder clinical phenotype.

INTERPRETATION

Patients with cystic fibrosis have distinct genetic subgroups that are associated with mild clinical manifestations and low mortality. These differences in phenotype are also related to the functional classification of CFTR genotype.

Normal function of the cystic fibrosis conductance regulator protein can be associated with homozygous (Delta)F508 mutation

Cystic fibrosis (CF) is caused by mutations of the gene encoding for the CFTR (CF transmembrane conductance regulator) protein. The most frequent mutation, the (Delta)F508 mutation, results in a defective cAMP-regulated chloride transport in the epithelial cells. The spectrum of clinical manifestations in patients bearing homozygous (Delta)F508 mutations can vary considerably, suggesting that, in the patients with a mild disease, CFTR could be partly functional. To test this hypothesis, we explored in nasal ciliated epithelial cells (NCC) of 9 control subjects and 23 (Delta)F508 homozygous patients the anion conductive pathway by a halide sensitive fluorescent dye assay SPQ (6-methoxy-N-3'-sulfopropylquinolinium) and the CFTR transcript levels by RT-PCR. As 50% represented the lowest fraction of the control subjects NCC demonstrating a cAMP-dependent conductance, a CF patient was considered as "cAMP responder" if at least 50% of the NCC tested displayed a cAMP-dependent conductive pathway. According to these criteria, 8 of the 23 patients were considered as cAMP responders. They had a significantly less severe disease considering the respiratory function and infectious status. The amount of CFTR mRNA did not differ between the control subjects and the patients. No statistical correlation could be found between the transcript level and the expression of a cAMP conductive pathway. This cAMP-dependent Cl(-) conductance detected in homozygous NCC could be due to a residual CFTR activity and may explain the mild phenotypes observed in some (Delta)F508 homozygous patients.

Beyond Mendel: an evolving view of human genetic disease transmission

Methodological and conceptual advances in human genetics have led to the identification of an impressive number of human disease genes. This wealth of information has also revealed that the traditional distinction between Mendelian and complex disorders might sometimes be blurred. Genetic and mutational data on an increasing number of disorders have illustrated how phenotypic effects can result from the combined action of alleles in many genes. In this review, we discuss how an improved understanding of the genetic basis of multilocus inheritance is catalysing the transition from a segmented view of human genetic disease to a conceptual continuum between Mendelian and complex traits.

Fraser syndrome and cryptophthalmos: review of the diagnostic criteria and evidence for phenotypic modules in complex malformation syndromes

Fraser syndrome is characterised by cryptophthalmos, cutaneous syndactyly, malformations of the larynx and genitourinary tract, craniofacial dysmorphism, orofacial clefting, mental retardation, and musculoskeletal anomalies. The inheritance is autosomal recessive. No diagnostic cytogenetic abnormalities have been documented in affected patients, and no molecular genetic studies have been reported. We have reviewed 117 cases diagnosed as Fraser syndrome or cryptophthalmos published since the comprehensive review of Thomas et al in 1986 in order to validate the published diagnostic criteria and to delineate the phenotype associated with this syndrome. Our series showed more females (57/117) than males and consanguinity was present in 29/119 (24.8%). Eighty-eight patients satisfied the diagnostic criteria for Fraser syndrome (75%). Cryptophthalmos was present in 103/117 (88%), syndactyly in 72/117 (61.5%), and ambiguous genitalia in 20/117 (17.1%). Ear malformations were recorded in 69/117 (59%), and renal agenesis in 53/117 (45.3%). Use of the published diagnostic criteria excluded several patients with cryptophthalmos and one or more physical feature(s) consistent with Fraser syndrome. The frequency of additional anomalies in our series was also higher than previously reported (for example, imperforate anus or anal stenosis were found in 34/117 (29%) compared with 2/124 (2%) in the series of Thomas et al (1986) and choanal stenosis or atresia was present in 7/117 (6%) compared to 0/124. These findings emphasise the clinical variability associated with Fraser syndrome and support genetic heterogeneity of the syndrome. We also noted patterns of anomalies (for example, bicornuate uterus with imperforate anus or anal stenosis and renal malformations) that are found in other syndromes and associations without cryptophthalmos, suggesting that common modifier genes may explain some of the phenotypic variation in Fraser syndrome.