Arrhythmia and sudden death associated with elevated cardiac chloride channel activity.

The identification and analysis of several cationic ion channels and their associated genes have greatly improved our understanding of the molecular and cellular mechanisms of cardiac arrhythmia. Our objective in this study was to examine the involvement of anionic ion channels in cardiac arrhythmia. We used a transgenic mouse model to overexpress the human cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a cAMP-regulated chloride channel. We used RNase protection and in situ hybridization assays to determine the level of CFTR expression, and radiotelemetry and in vivo electrophysiological study in combination with pharmacological intervention to analyse the cardiac function. Cardiac CFTR overexpression leads to stress-related sudden death in this model. In vivo intracardiac electrophysiological studies performed in anaesthetized mice showed no significant differences in baseline conduction parameters including atrial-His bundle (AH) or His bundle-ventricular (HV) conduction intervals, atrioventricular (AV) Wenckebach or 2:1 AV block cycle length and AV nodal functional refractory period. However, following isoproterenol administration, there was marked slowing of conduction parameters, including high-grade AV block in transgenic mice, with non-sustained ventricular tachycardia easily inducible using programmed stimulation or burst pacing. Our sudden death mouse model can be a valuable tool for investigation of the role of chloride channels in arrhythmogenesis and, potentially, for future evaluation of novel anti-arrhythmic therapeutic strategies and pharmacological agents.

Do common in silico tools predict the clinical consequences of amino-acid substitutions in the CFTR gene?

Computational methods are used to predict the molecular consequences of amino-acid substitutions on the basis of evolutionary conservation or protein structure, but their utility in clinical diagnosis or prediction of disease outcome has not been well validated. We evaluated three popular computer programs, namely, PANTHER, SIFT and PolyPhen, by comparing the predicted clinical outcomes for a group of known CFTR missense mutations against the diagnosis of cystic fibrosis (CF) and clinical manifestations in cohorts of subjects with CF-disease and CFTR-related disorders carrying these mutations. Owing to poor specificity, none of tools reliably distinguished between individual mutations that confer CF disease from mutations found in subjects with a CFTR-related disorder or no disease. Prediction scores for CFTR mutations derived from PANTHER showed a significant overall statistical correlation with the spectrum of disease severity associated with mutations in the CFTR gene. In contrast, PolyPhen- and SIFT-derived scores only showed significant differences between CF-causing and non-CF variants. Current computational methods are not recommended for establishing or excluding a CF diagnosis, notably as a newborn screening strategy or in patients with equivocal test results.

Nonsteroidal anti-inflammatory drugs upregulate function of wild-type and mutant CFTR.

Small-scale clinical trials show that treatment of cystic fibrosis (CF) patients with ibuprofen, a nonsteroidal anti-inflammatory drug, improves the symptoms of CF and slows down the decline of lung function. Paradoxically, ibuprofen inhibits ligand-stimulated CF transmembrance conductance regulator (CFTR) activity. The aim of the present study was to investigate the effects of ibuprofen on CFTR function under different conditions. Patch-clamp recordings were performed in two lines of human airway epithelial cells: IB3-8-3-7 cells, which express wild-type CFTR; and IB3-1 cells, which express the variant CFTR with deletion of phenylalanine 580 (DeltaF580CFTR). Addition of ibuprofen to the extracellular solution caused a rapid inhibition of CFTR activity in IB3-8-3-7 cells in the presence of a high intracellular concentration of cAMP, whereas ibuprofen enhanced the CFTR conductance at low levels of cAMP. Introducing ibuprofen into the interior of cells occluded the enhancing effect of ibuprofen. Notably, the variant CFTR-mediated conductance was detected in IB3-1 cells treated with myoinositol and was enhanced by ibuprofen at endogenous levels of cAMP. In summary, nonsteroidal anti-inflammatory drugs increase the function of both wild-type cystic fibrosis transmembrane conductance regulator and the phenylalanine 580 deletion in cultured human airway epithelial cells at endogenous levels of cAMP.

The prevalence and clinical characteristics of cystic fibrosis in South Asian Canadian immigrants.

BACKGROUND: Cystic fibrosis (CF) is considered to be rare among individuals from the Indian subcontinent. Furthermore, affected individuals are reported to experience a more severe clinical course. AIMS: It was hypothesised that CF is under diagnosed in people of South Asian origin and therefore the prevalence may be higher than previously estimated. METHODS: The prevalence of CF in the South Asian and in the general population living in the same geographic region (Metropolitan Toronto) were compared between 1996 and 2001. Population data were obtained from the Canadian census survey. CF phenotype and genotype data were obtained from the Toronto CF database. RESULTS: Among 381 patients with CF, 15 were of South Asian descent. The age related prevalence of CF among the South Asian and general populations was: 0-14 years, 1:9200 versus 1:6600; 15-24 years, 1:13,200 versus 1:7600; older than 25 years, 1:56,600 versus 1:12,400. Age at diagnosis, duration and severity of symptoms at diagnosis, current nutritional status, and FEV(1) were similar in the two groups. While not significant, FEV1 tended to be lower (48% versus 57% predicted) among adult South Asians, compared to the general CF population. Also, the percentage with pancreatic sufficiency was higher (27% versus 16%) and the frequency of DeltaF508 allele was lower (50% versus 65.1%). CONCLUSIONS: These data suggest that the prevalence and natural history of CF in South Asians is similar to that among individuals of European origin. The relatively lower prevalence among older South Asians may reflect an improving recognition of CF in this ethnic subgroup.

A family exhibiting arterial tortuosity syndrome displays homozygosity for markers in the arterial tortuosity locus at chromosome 20q13.

Arterial tortuosity associated with hyperextensible skin and hypermobility of joints, features that are characteristics of Ehlers-Danlos syndrome (EDS), has been described in several families. An arterial tortuosity locus has recently been mapped to chromosome 20q13. Here, we report a consanguineous Kurdish family in which an affected child manifested elongation and severe tortuosity of the aorta, carotid, and other arteries. Additional clinical symptoms include loose skin, hypermobile joints, hernias, and facial features that resemble EDS individuals. To examine whether the arterial tortuosity locus was involved in this child, homozygosity analysis was performed using microsatellite markers on 20q13. The affected child was found homozygous, whereas the unaffected parents and three siblings were heterozygous. Additional typing defined the genomic interval to a 37-cm region within which the arterial tortuosity locus is located. Three functional candidate genes (B4GALT5, KCNB1, and PTGIS) were sequenced. No mutations were discovered in the coding regions of these three genes and the promoter regions of B4GALT5 and KCNB1 genes. Moreover, the B4GALT5 mRNA expression was unaltered in patient-derived lymphoblastoid cells. In the PTGIS gene promoter, the affected child was homozygous for eight variable number of tandem repeats, while parents and unaffected siblings carried six repeats.

Fine mapping of the X-linked split-hand/split-foot malformation (SHFM2) locus to a 5.1-Mb region on Xq26.3 and analysis of candidate genes.

Split-hand/split-foot malformation (SHFM) is a genetically heterogeneous disorder, with five known loci, that causes a lack of median digital rays, syndactyly, and aplasia or hypoplasia of the phalanges, metacarpals, and metatarsals. In the only known SHFM2 family, affected males and homozygous females exhibit monodactyly or bidactyly of the hands and lobster-claw feet. This family (1) was revisited to include additional subjects and genealogical data. All 39 affected males and three females fully expressed the SHFM, while 13 carrier females examined exhibited partial expression of SHFM. We narrowed the previously linked 22-Mb genetic interval on Xq24-q26 (2), by analyzing additional family members and typing additional markers. The results define a 5.1-Mb region with a new centromeric boundary at DXS1114 and a telomeric boundary at DXS1192. We did not identify mutations in the exons and exon/intron boundaries of 19 candidate genes. These data suggest that the mutation may lie in a regulatory region of one of these candidate genes or in another gene within the SHFM2 region with unclear role in limb development.

Novel mutations in the EXT1 gene in two consanguineous families affected with multiple hereditary exostoses (familial osteochondromatosis).

Multiple hereditary exostoses (HME) is an autosomal dominant developmental disorder exhibiting multiple osteocartilaginous bone tumors that generally arise near the ends of growing long bones. Here, we report two large consanguineous families from Pakistan, who display the typical features of HME. Affected individuals also show a previously unreported feature--bilateral overriding of single toes. Analysis using microsatellite markers for each of the known EXT loci, EXT1, EXT2, and EXT3 showed linkage to EXT1. In the first family, mutation analysis of the EXT1 gene revealed that affected individuals were heterozygous for an in-frame G-to-C transversion at the conserved splice donor site in intron 1. This mutation is predicted to disrupt splicing of the first intron and produce a frameshift that leads to a premature termination codon. In the second family, an insertion of an A in exon 8 is predicted to produce a frameshift at codon 555 followed by a premature termination, a further 10 codons downstream. In both families, an increased number of affected male subjects were observed. In affected females in family 2, phenotypic variability and incomplete penetrance were noted.

The locus responsible for horizontal gaze palsy/progressive scoliosis and brainstem hypoplasia is refined to a 9-cM region on chromosome 11q23.

Horizontal gaze palsy associated with progressive scoliosis (HGPS) is a rare autosomal recessive condition that has been recently mapped to a 30-cM region on chromosome 11q23-25. In this report, we describe a consanguineous family in which three of five sibs are affected with HGPS. In two of the affected sibs, there was significant cognitive delay in addition to congenital horizontal gaze palsy and childhood onset scoliosis. In all three affected sibs, magnetic resonance imaging (MRI) scans revealed brainstem hypoplasia, a finding that has recently been associated with HGPS. Clinical examination of the family showed no significant dysmorphic features, while karyotyping, EMG, nerve conduction, and muscle biopsies were unrevealing. Homozygosity mapping was performed to narrow the disease locus on 11q23-25. A recombination event was observed in one affected sib between markers D11S1345 and D11S4464, which further refined the region to a 9-cM interval. Since the MRI results provide support for the theory that maldevelopment of neurons in the abducens nuclei and caudal longitudinal fascicle is the cause for HGPS, we speculate on the existence of a gene in this 9-cM interval on chromosome 11q23, which is critical for brainstem development.

Novel mutation in the gene encoding c-Abl-binding protein SH3BP2 causes cherubism.

Cherubism is a rare autosomal dominant inherited condition caused by mutations in the c-Abl-binding protein SH3BP2. It is characterized by multiple cystic giant cell lesions of the jaw appearing in early childhood with stabilization and remission after puberty. In the present study, we used direct sequence analysis of the SH3BP2 gene of several individuals from a family with cherubism to search for additional SH3BP2 mutations resulting in cherubism. In affected relatives, we found a previously unreported G to A transition in exon 9 leading to a Gly to Arg substitution at amino acid position 420. G420R has been reported previously with a G to C transversion. To date there have been no disease causing mutations outside exon 9. Therefore, the amino acid sequence from positions 415 to 420 may represent a specific protein domain which, when disrupted, leads to the cherubism phenotype.

Molecular consequences of cystic fibrosis transmembrane regulator (CFTR) gene mutations in the exocrine pancreas.

BACKGROUND AND AIMS: We tested the hypothesis that the actual or predicted consequences of mutations in the cystic fibrosis transmembrane regulator gene correlate with the pancreatic phenotype and with measures of quantitative exocrine pancreatic function. METHODS: We assessed 742 patients with cystic fibrosis for whom genotype and clinical data were available. At diagnosis, 610 were pancreatic insufficient, 110 were pancreatic sufficient, and 22 pancreatic sufficient patients progressed to pancreatic insufficiency after diagnosis. RESULTS: We identified mutations on both alleles in 633 patients (85.3%), on one allele in 95 (12.8%), and on neither allele in 14 (1.9%). Seventy six different mutations were identified. The most common mutation was DeltaF508 (71.3%) followed by G551D (2.9%), G542X (2.3%), 621+1G-->T (1.2%), and W1282X (1.2%). Patients were categorized into five classes according to the predicted functional consequences of each mutation. Over 95% of patients with severe class I, II, and III mutations were pancreatic insufficient or progressed to pancreatic insufficiency. In contrast, patients with mild class IV and V mutations were consistently pancreatic sufficient. In all but four cases each genotype correlated exclusively with the pancreatic phenotype. Quantitative data of acinar and ductular secretion were available in 93 patients. Patients with mutations belonging to classes I, II, and III had greatly reduced acinar and ductular function compared with those with class IV or V mutations. CONCLUSION: The predicted or known functional consequences of specific mutant alleles correlate with the severity of pancreatic disease in cystic fibrosis.

Mutation in the cartilage-derived morphogenetic protein-1 (CDMP1) gene in a kindred affected with fibular hypoplasia and complex brachydactyly (DuPan syndrome).

The present authors have previously described a consanguineous Pakistani family with fibular hypoplasia and complex brachydactyly (DuPan syndrome) inherited as an autosomal recessive trait. All affected individuals showed either reductions or absence of bones in the limbs, and appendicular bone dysmorphogenesis with unaffected axial bones. Obligate heterozygote parents were phenotypically normal. Mutations in the cartilage-derived morphogenetic protein 1 (CDMP1) gene have been reported in two acromesomelic chondrodysplasias (i.e. Hunter-Thompson type and Grebe type) which are phenotypically related to DuPan syndrome. CDMP1, a member of the transforming growth factor beta super-family of secreted signalling molecules, has been reported to regulate limb patterning and distal bone growth. Therefore, the present authors examined genomic DNA from the family with DuPan syndrome for mutations in the CDMP1 gene. Affected individuals were homozygous for a missense mutation, T1322C, in the coding region of the CDMP1 gene. This mutation was not found in 44 control subjects of Pakistani origin. The T1322C change predicts a leu441pro substitution in the mature domain of the CDMP1 protein. This is likely to cause a conformational change in the CDMP1 protein that influences the expression of genes which are required for normal bone development. This finding extends the spectrum of phenotypes produced by defects in the CDMP1 gene.

Complete screening of the CFTR gene in Argentine cystic fibrosis patients.

In order to establish the nature and the distribution of mutations causing cystic fibrosis (CF) in 220 unrelated Argentine families, the present authors conducted an extensive molecular analysis of the CF transmembrane regulator (CFTR) gene. First, a direct mutation analysis of 13 common mutations was done, enabling the detection of 319 out of 440 CF alleles (72.52%). Then an exhaustive screening of the entire coding region and the adjacent sequences of the CFTR gene was performed in all patients carrying at least one unidentified CF allele using the multiplex heteroduplex analysis assay followed by direct DNA sequencing. Thirty-nine different CF mutations, including five previously undescribed mutations (i.e. L6V, Y362X, 1353insT, 2594delGT and 2686insT) and two novel polymorphisms (i.e. 1170G/C and 3315A/C) were identified. As a result, the overall detection rate increased by up to 83.45%. Besides DeltaF508, only five mutations showed frequencies higher than 1%. In addition, a total of 49% of the mutations were rare because they were found in only one CF family. This wide spectrum of CF mutations is in agreement with the heterogeneous ethnic origin of the Argentine population. The data obtained here may have important consequences for the development of adequate strategies for the molecular diagnosis of CF in Argentina.

Cystic fibrosis in Uruguay

We conducted clinical and genetic analyses of 52 cystic fibrosis (CF) patients in Uruguay, which is about half of the known affected individuals in the country. A relatively high proportion had a mild presentation, characterized by pancreatic sufficiency (28), a strong pulmonary component (97), and borderline sweat electrolyte measurements (25). Mutational analysis of CF chromosomes demonstrated a relatively low incidence of the DeltaF508 allele (40) and a large number of other cystic fibrosis conductance regulator mutations, with an overall detection rate of about 71. Fifteen different mutations were detected in our patients: DeltaF508, G542X, R1162X, G85E, N1303K, R334W, R75Q, R74W, D1270N, W1282X, DeltaI507, 2789+5G-->A, R1066C, -816C/T, R553X, as well as RNA splicing variant IVS8-5T. This group of Uruguayan CF patients has some characteristics in common with other populations of similar origin (Hispanics), as well as some unique characteristics

Identification of the human cortactin-binding protein-2 gene from the autism candidate region at 7q31.

Human chromosome 7q31 contains putative susceptibility loci for autism (AUTS1) and speech and language disorder (SPCH1). We report here the identification and characterization of a novel gene encoding cortactin-binding protein-2 (CORTBP2), which is located 45 kb telomeric to the cystic fibrosis transmembrane conductance regulator gene (CFTR) at 7q31.3. The full-length (5975-bp) gene was isolated and found to be composed of 23 exons encompassing 170 kb of DNA. In addition to being a positional candidate for AUTS1, CORTBP2 was expressed at highest levels in the brain, as shown by northern blot analysis. Subsequent mutation analysis of CORTBP2 in 90 autistic patients identified two polymorphisms, including a leucine to valine change caused by a T to G substitution in exon 15. However, comparison of allele frequencies between autistic and control populations (n=96) showed no significant difference, suggesting that this variant is not a susceptibility factor for autism.

A 1.5 million-base pair inversion polymorphism in families with Williams-Beuren syndrome.

Williams-Beuren syndrome (WBS) is most often caused by hemizygous deletion of a 1.5-Mb interval encompassing at least 17 genes at 7q11.23 (refs. 1,2). As with many other haploinsufficiency diseases, the mechanism underlying the WBS deletion is thought to be unequal meiotic recombination, probably mediated by the highly homologous DNA that flanks the commonly deleted region. Here, we report the use of interphase fluorescence in situ hybridization (FISH) and pulsed-field gel electrophoresis (PFGE) to identify a genomic polymorphism in families with WBS, consisting of an inversion of the WBS region. We have observed that the inversion is hemizygous in 3 of 11 (27%) atypical affected individuals who show a subset of the WBS phenotypic spectrum but do not carry the typical WBS microdeletion. Two of these individuals also have a parent who carries the inversion. In addition, in 4 of 12 (33%) families with a proband carrying the WBS deletion, we observed the inversion exclusively in the parent transmitting the disease-related chromosome. These results suggest the presence of a newly identified genomic variant within the population that may be associated with the disease. It may result in predisposition to primarily WBS-causing microdeletions, but may also cause translocations and inversions.

Cystic fibrosis mutations and associated haplotypes in Turkish cystic fibrosis patients.

Identification of mutations causing cystic fibrosis (CF) in the Turkish population is essential for assessment of the molecular basis of CF in Turkey and the development of strategies for prenatal diagnosis and genetic counseling. Here, we present an updated report of mutations found in the Turkish CF population from an extensive screening study of the entire coding region, including exon-intron boundaries and the promoter region. Cases for which mutations could not be identified were also screened for previously defined large alterations and (TG)mTn-M470V loci. This study revealed a total of 27 different mutations accounting for almost 60% of disease genes in the Turkish population. In this study, we also identified the haplotypes associated with 17 mutations and those associated with unknown mutations. The mutation spectrum of CF in Turkey and its associated haplotypes indicated the presence of a major Mediterranean component in the contemporary Turkish population.

Regulated expression of the human CFTR gene in epithelial cells.

We developed an epithelium-specific, inducible cystic fibrosis transmembrane conductance regulator (CFTR) expression system. In this system we used a human cytokeratin 18 expression cassette to drive epithelium-specific expression of the reverse tetracycline transactivator (rtTA), which turns on CFTR expression from a Tet-inducible promoter in the presence of doxycycline. CFTR expression was monitored by reverse-transcription polymerase chain reaction, immunostaining, and Western blotting. We confirmed that protein expression was dose-dependent in double stable transfected cell lines, with no detectable protein in the absence of doxycycline. However, low levels of CFTR mRNA could be detected in the uninduced state. When clones capable of inducing high levels of CFTR expression were analyzed, we observed a decrease in cell proliferation, consistent with reports in other cell lines (NIH3T3 and BTS). We generated transgenic mice expressing rtTA from the K18 expression cassette and demonstrated that the system retained its tissue specificity for lacZ reporter expression in vivo. When mice were induced with doxycycline, high levels of expression were found in the trachea, upper bronchi, and submucosal glands. Therefore, this inducible system can improve our understanding of the role of CFTR in the lung and should help in the design of safe and effective CF therapies.

Comparative analysis of the gene-dense ACHE/TFR2 region on human chromosome 7q22 with the orthologous region on mouse chromosome 5.

Chromosome 7q22 has been the focus of many cytogenetic and molecular studies aimed at delineating regions commonly deleted in myeloid leukemias and myelodysplastic syndromes. We have compared a gene-dense, GC-rich sub-region of 7q22 with the orthologous region on mouse chromosome 5. A physical map of 640 kb of genomic DNA from mouse chromosome 5 was derived from a series of overlapping bacterial artificial chromosomes. A 296 kb segment from the physical map, spanning ACHE: to Tfr2, was compared with 267 kb of human sequence. We identified a conserved linkage of 12 genes including an open reading frame flanked by ACHE: and Asr2, a novel cation-chloride cotransporter interacting protein Cip1, Ephb4, Zan and Perq1. While some of these genes have been previously described, in each case we present new data derived from our comparative sequence analysis. Adjacent unfinished sequence data from the mouse contains an orthologous block of 10 additional genes including three novel cDNA sequences that we subsequently mapped to human 7q22. Methods for displaying comparative genomic information, including unfinished sequence data, are becoming increasingly important. We supplement our printed comparative analysis with a new, Web-based program called Laj (local alignments with java). Laj provides interactive access to archived pairwise sequence alignments via the WWW. It displays synchronized views of a dot-plot, a percent identity plot, a nucleotide-level local alignment and a variety of relevant annotations. Our mouse-human comparison can be viewed at http://web.uvic.ca/~bioweb/laj.html. Laj is available at http://bio.cse.psu.edu/, along with online documentation and additional examples of annotated genomic regions.