Modulation of disease severity in cystic fibrosis transmembrane conductance regulator deficient mice by a secondary genetic factor

Assignment of the tibial muscular dystrophy locus to chromosome 2q31

Tibial muscular dystrophy (TMD) is a rare autosomal dominant distal myopathy with late adult onset. The phenotype is relatively mild: muscle weakness manifests in the patient's early 40s and remains confined to the tibial anterior muscles. Histopathological changes in muscle are compatible with muscular dystrophy, with the exception that rimmed vacuoles are a rather common finding. We performed a genomewide scan, with 279 highly polymorphic Cooperative Human Linkage Center microsatellite markers, on 11 affected individuals of one Finnish TMD family. The only evidence for linkage emerged from markers in a 43-cM region on chromosome 2q. In further linkage analyses, which included three other Finnish TMD families and which used a denser set of markers, a maximum two-point LOD score of 10.14 (recombination fraction of .05) was obtained with marker D2S364. Multipoint likelihood calculations, combined with the haplotype and recombination analyses, restricted the TMD locus to an approximately 1-cM critical chromosomal region without any evidence of heterogeneity. Since all the affecteds share one core haplotype, the dominance of one ancestor mutation is obvious in the Finnish TMD families. The disease locus that was found represents a novel muscular dystrophy locus, providing evidence for the involvement of one additional gene in the distal myopathy group of muscle disorders.

Complex traits and polygenic inheritance in the mouse

Polygenic inheritance has recently become an increasingly active field of research due to the availability of techniques allowing in-depth screening of genetic markers across the entire genome. The mouse is being used both in its own right and as a model system for certain human traits. The advantages and disadvantages of the mouse for such studies are outlined and in this context, the adequacy of the mouse as a model for polygenic traits in humans is discussed. A detailed overview of the approaches and methods used in the analysis of polygenic inheritance in the mouse is presented.

Comparison of the clinical manifestations of cystic fibrosis in black and white patients

No large-scale studies of the incidence or disease severity of cystic fibrosis (CF) in black patients have been reported to date. In this study, the CF Foundation National Patient Registry was used to establish new incidence figures and to compare the clinical status of U.S. black (n = 601) and white patients (n = 17,755) with CE Results indicate that the incidence of CF is approximately 1 in 3,200 white and 1 in 15,000 black live births in the United States. Black patients with CF are currently, and were at diagnosis, younger and have poorer nutritional status and pulmonary function than white patients with CF. Fewer have meconium ileus, but more have distal intestinal obstruction syndrome. To control for genotype, each black deltaF508 homozygote (n = 47) was compared with four age- and sex-matched white deltaF508 homozygotes. Only the difference in nutritional status remained. The deltaF508 mutation is associated with higher levels of meconium ileus than other genotypes, independent of race. In conclusion, the clinical manifestations of CF are similar in black and white patients except for poorer nutritional status in black patients, which appears to be independent of age and genotype.

Functional expression of a truncated Ca(2+)-activated Cl- channel and activation by phorbol ester

We have isolated a niflumic acid-insensitive, Ca(2+)-activated Cl- channel (CaCC) from bovine trachea that migrates at 38 kDa (p38) on reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, a cloned CaCC isolated from a tracheal cDNA expression library by screening with an antibody raised against p38 has a primary cDNA transcript of 2712 base pairs that codes for a 100-kDa protein and is not susceptible to dithiothreitol reduction. To test the hypothesis that the functional channel may be a much smaller posttranslationally processed form of the 100-kDa protein, we generated a mutant construct (CaCCX, 42.5-kDa protein) truncated at the NH2 and COOH termini. The whole cell currents of wild-type (wt) CaCC and CaCCX expressed in Xenopus oocytes were 10-fold higher than those of water-injected oocytes and were further increased by ionomycin or A-23187 and inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and dithiothreitol. Whole cell currents in wtCaCC- and CaCCX-expressing oocytes could also be activated by phorbol 12-myristate 13-acetate and could be inhibited by chelerythrine chloride, suggesting that the cloned CaCC is regulated by protein kinase C. These results suggest that a smaller form of the full-length CaCC can form a functional channel.

Polyvariant mutant cystic fibrosis transmembrane conductance regulator genes. The polymorphic (Tg)m locus explains the partial penetrance of the T5 polymorphism as a disease mutation

In congenital bilateral absence of the vas deferens patients, the T5 allele at the polymorphic Tn locus in the CFTR (cystic fibrosis transmembrane conductance regulator) gene is a frequent disease mutation with incomplete penetrance. This T5 allele will result in a high proportion of CFTR transcripts that lack exon 9, whose translation products will not contribute to apical chloride channel activity. Besides the polymorphic Tn locus, more than 120 polymorphisms have been described in the CFTR gene. We hypothesized that the combination of particular alleles at several polymorphic loci might result in less functional or even insufficient CFTR protein. Analysis of three polymorphic loci with frequent alleles in the general population showed that, in addition to the known effect of the Tn locus, the quantity and quality of CFTR transcripts and/or proteins was affected by two other polymorphic loci: (TG)m and M470V. On a T7 background, the (TG)11 allele gave a 2.8-fold increase in the proportion of CFTR transcripts that lacked exon 9, and (TG)12 gave a sixfold increase, compared with the (TG)10 allele. T5 CFTR genes derived from patients were found to carry a high number of TG repeats, while T5 CFTR genes derived from healthy CF fathers harbored a low number of TG repeats. Moreover, it was found that M470 CFTR proteins matured more slowly, and that they had a 1.7-fold increased intrinsic chloride channel activity compared with V470 CFTR proteins, suggesting that the M470V locus might also play a role in the partial penetrance of T5 as a disease mutation. Such polyvariant mutant genes could explain why apparently normal CFTR genes cause disease. Moreover, they might be responsible for variation in the phenotypic expression of CFTR mutations, and be of relevance in other genetic diseases.

Pathology of pancreatic and intestinal disorders in cystic fibrosis

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Lung disease in mice with cystic fibrosis

The leading cause of mortality and morbidity in humans with cystic fibrosis is lung disease. Advances in our understanding of the pathogenesis of the lung disease of cystic fibrosis, as well as development of innovative therapeutic interventions, have been compromised by the lack of a natural animal model. The utility of the CFTR-knockout mouse in studying the pathogenesis of cystic fibrosis has been limited because of their failure, despite the presence of severe intestinal disease, to develop lung disease. Herein, we describe the phenotype of an inbred congenic strain of CFTR-knockout mouse that develops spontaneous and progressive lung disease of early onset. The major features of the lung disease include failure of effective mucociliary transport, postbronchiolar over inflation of alveoli and parenchymal interstitial thickening, with evidence of fibrosis and inflammatory cell recruitment. We speculate that the basis for development of lung disease in the congenic CFTR-knockout mice is their observed lack of a non-CFTR chloride channel normally found in CFTR-knockout mice of mixed genetic background.

The genetics of male infertility in relation to cystic fibrosis

Absence, dysfunction or low levels of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein result in a broad range of clinical manifestations with CF with pancreatic insufficiency at the severe end of the phenotypic spectrum and, at the other end relatively mild clinical conditions, including several forms of male infertility. The condition of congenital bilateral absence of the vas deferens (CBAVD) is in 75-80% of the patients associated with defects in the CFTR gene. In the remaining patients, CBAVD is also associated with urinary tract malformations, and this form of CBAVD is not related to CF. Congenital unilateral absence of the vas deferens also seems to be associated with CF except when associated with renal abnormalities at the ipsilateral side of the absent vas. A possible role of the CFTR protein in the aetiology of infertility due to defects in sperm production and maturation has also been suggested recently. In contrast, Young's syndrome is probably not related to CF. The relation between some conditions of male infertility and CF implies appropriate clinical examination of the patients, CFTR mutation analysis and genetic counselling. Because infertility can now in many cases be treated by artificial reproductive technology couples have an increased risk of having children with CF or infertility if the female partner is also a carrier of a CFTR mutation. Couples should be well informed about these risks and risk prevention including pre-implantation diagnosis. Follow-up studies of children born to these couples are mandatory, whether male infertility is linked to CF or not.

Coupling of ATP hydrolysis with channel gating by purified, reconstituted CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel situated on the apical membrane of epithelial cells. Our recent studies of purified, reconstituted CFTR revealed that it also functions as an ATPase and that there may be coupling between ATP hydrolysis and channel gating. Both the ATP turnover rate and channel gating are slow, in the range of 0.2 to 1 s(-1), and both activities are suppressed in a disease-causing mutation situated in a putative nucleotide binding motif. Our future studies using purified protein will be directed toward understanding the structural basis and mechanism for coupling between hydrolysis and channel function.

A missense cystic fibrosis transmembrane conductance regulator mutation with variable phenotype

OBJECTIVE

Cystic fibrosis (CF) has variable clinical presentation. Disease severity is partially associated with the type of mutation. The aim of this study was to report genotype-phenotype analysis of the G85E mutation.

PATIENTS

The phenotype of 12 patients (8 were from the same extended family, and 5 of them were siblings from 2 families) carrying at least one copy of the G85E mutation was evaluated and compared with the phenotype of 40 patients carrying the two severe mutations, W1282X and/or DeltaF508 (group 1), and with 20 patients carrying the splicing mutation, 3849+10kb C->T, which was found to be associated with milder disease (group 2).

RESULTS

A high phenotypic variability was found among the patients carrying the G85E mutation. This high variability was found among patients carrying the same genotype and among siblings. All the studied chromosomes carrying the G85E mutation had the 7T variant in the polythymidine tract at the branch/acceptor site in intron 8. Of the G85E patients, 25% had pancreatic sufficiency and none had meconium ileus, compared with 0% and 32%, respectively, of patients from group 1, and 80% and 0%, respectively, from group 2. Two patients carrying the G85E mutation had sweat chloride levels <60 mmol/L whereas all the others had typically elevated levels >80 mmol/L. Compared with group 2, patients carrying the G85E mutation were diagnosed at an earlier age and had higher sweat chloride levels, with mean values similar to group 1 but significantly more variable. Forced expiratory volume in 1 second (FEV1) was similar in the three groups, with no differences in the slope or in age-adjusted mean values of FEV1. The levels of transcripts lacking exon 9 transcribed from the G85E allele measured in 3 patients were 55%, 49%, and 35% and their FEV1 values were 82%, 83%, and 50% predicated, respectively.

CONCLUSIONS

The G85E mutation shows variable clinical presentation in all clinical parameters. This variability could be seen among patients carrying on the other chromosome the same CFTR mutation, and also among siblings. This variability is not associated with the level of exon 9 skipping. Thus, the G85E mutation cannot be classified either as a severe or as a mild mutation.

A mouse model for X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is a peroxisomal disorder with impaired beta-oxidation of very long chain fatty acids (VLCFAs) and reduced function of peroxisomal very long chain fatty acyl-CoA synthetase (VLCS) that leads to severe and progressive neurological disability. The X-ALD gene, identified by positional cloning, encodes a peroxisomal membrane protein (adrenoleukodystrophy protein; ALDP) that belongs to the ATP binding cassette transporter protein superfamily. Mutational analyses and functional studies of the X-ALD gene confirm that it and not VLCS is the gene responsible for X-ALD. Its role in the beta-oxidation of VLCFAs and its effect on the function of VLCS are unclear. The complex pathology of X-ALD and the extreme variability of its clinical phenotypes are also unexplained. To facilitate understanding of X-ALD pathophysiology, we developed an X-ALD mouse model by gene targeting. The X-ALD mouse exhibits reduced beta-oxidation of VLCFAs, resulting in significantly elevated levels of saturated VLCFAs in total lipids from all tissues measured and in cholesterol esters from adrenal glands. Lipid cleft inclusions were observed in adrenocortical cells of X-ALD mice under the electron microscope. No neurological involvement has been detected in X-ALD mice up to 6 months. We conclude that X-ALD mice exhibit biochemical defects equivalent to those found in human X-ALD and thus provide an experimental system for testing therapeutic intervention.

Complementation of null CF mice with a human CFTR YAC transgene

We have made transgenic mice carrying a 320 kb YAC with the intact human cystic fibrosis transmembrane regulator (CFTR) gene. Mice that only express the human transgene were obtained by breeding with Cambridge null CF mice. One line has approximately two copies of the intact YAC. Mice carrying this transgene and expressing no mouse cftr appear normal and breed well, in marked contrast to the null mice, where 50% die by approximately 5 days after birth. The chloride secretory responses in these mice are as large or larger than in wild-type tissues. Expression of the transgene is highly cell type specific and matches that of the endogenous mouse gene in the crypt epithelia throughout the gut and in salivary gland tissue. However, there is no transgene expression in some tissues, such as the Brunner's glands, where it would be expected. Where there are differences between the mouse and human pattern of expression, the transgene follows the mouse pattern. We have thus defined a cloned fragment of DNA which directs physiological levels of expression in many of the specific cells where CFTR is normally expressed.

Genotype and phenotype in cystic fibrosis

Questions about the function of the disease-related gene are still not fully answered, but correlations are emerging between specific mutations and a patient's clinical condition. The strongest link is for pancreatic failure. A second involves azoospermia. Indeed, mutations are being found in males with infertility as the sole sign of disease. Improved knowledge of such patterns may suggest novel approaches to severe cystic fibrosis.

Genotypes at the GluR6 kainate receptor locus are associated with variation in the age of onset of Huntington disease

Huntington disease (HD) is associated with abnormal expansions of a CAG repeat close to the 5' end of the IT15 gene. We have assembled a set of 293 HD subjects whose ages of onset were known and sized their HD CAG repeats. These repeats accounted for 69% of the variance of age of onset when we used the most parsimonious model, which relates the logarithm of age of onset to a function of CAG repeat number. Since other familial factors have been proposed to influence the age of onset of HD, we have examined a number of candidate loci. The CAG repeat number on normal chromosomes, the delta2642 polymorphism in the HD gene, and apolipoprotein E genotypes did not affect the age of onset of HD. Although mitochondrial energy production defects in HD have led to suggestions that variants in the mitochondrial genome may be associated with clinical variability in HD, this suggestion was not supported by our preliminary experiments that examined the DdeI mitochondrial restriction fragment length polymorphism at position 10,394. Excitotoxicity has been a favored mechanism to explain the cell death in HD, particularly since intrastriatal injection of excitatory amino acids in animals creates HD-like pathology. Accordingly, we investigated the GluR6 kainate receptor. Of the variance in the age of onset of HD that was not accounted for by the CAG repeats, 13% could be attributed to GluR6 genotype variation. These data implicate GluR6-mediated excitotoxicity in the pathogenesis of HD and highlight the potential importance of this process in other polyglutamine repeat expansion diseases.

Gene targeting of a CFTR allele in HT29 human epithelial cells

HT29 cells endogenously express the cystic fibrosis transmembrane conductance regulator (CFTR) and have been used previously as a model to examine cellular regulation of CFTR expression and chloride secretory function. Homologous recombination has been used to specifically disrupt CFTR transcription in the HT29-18-C1 subclone. Experiments demonstrate successful disruption of a CFTR allele by DNA constructs, which target insertion of the neomycin phosphotransferase gene into CFTR exon 1 via homologous recombination. The mutation of one allele is a partial knockout because this cell line has multiple CFTR alleles. The mutation is confirmed by polymerase chain reaction (PCR) and genomic Southern blot analysis. A 52-68% reduction in CFTR mRNA levels is observed in the mutant cell line by both Northern and PCR analysis. However, Western blots show no decrease in total CFTR protein levels. Consistent with the lack of reduction in CFTR protein, the partial knockout mutant does not demonstrate alterations in cyclic AMP or calcium stimulation of chloride efflux or net osmolyte loss. Results suggest that posttranscriptional regulation of CFTR levels may contribute to maintenance of cellular chloride transport function.

Genotype-phenotype relationship in 12 patients carrying cystic fibrosis mutation R334W

We present a phenotype-genotype correlation analysis in 12 patients with cystic fibrosis (CF) carrying the mutation R334W in the CFTR gene. The clinical data obtained for this group were compared with the clinical data of deltaF508/deltaF508 patients. Current age and age at diagnosis were significantly higher in the R334W mutation group (p=0.028 and p=0.0001). We found a lower rate of Pseudomonas aeruginosa colonisation in patients carrying the R334W mutation, although the difference was not found to be statistically significant. However, we found a statistically significant higher age of onset of Pseudomonas aeruginosa colonisation (p=0.0036) in the group of patients with the R334W mutation. Thirty three percent of R334W patients were pancreatic insufficient, significantly lower than the deltaF508/deltaF508 patients (p=0.004). We also found that the weight expressed as a percentage of ideal weight for height was significantly higher in patients with the R334W mutation (p=0.0028).

Deletion and conversion in spinal muscular atrophy patients: is there a relationship to severity?

The spinal muscular atrophy-determining gene, survival motor neuron (SMN), is present in two copies, telSMN and cenSMN, which can be distinguished by base-pair changes in exons 7 and 8. The telSMN gene is often absent in spinal muscular atrophy patients, which could be due to deletion or sequence conversion (telSMN conversion to cenSMN giving rise to two cenSMN genes). To test for conversion events in spinal muscular atrophy, we amplified a 1-kb fragment that spanned exons 7 and 8 of SMN from 5 patients who retained telSMN exon 8 but lacked exon 7. In all patients, sequence analysis demonstrated that cenSMN exon 7 was adjacent to telSMN exon 8, indicating conversion. All 5 patients with this mutation had type II or III spinal muscular atrophy, strongly supporting an association with chronic spinal muscular atrophy. We also identified 3 families in which 2 siblings had no detectable telSMN but presented with markedly different phenotypes. We suggest that sequence conversion is a common event in spinal muscular atrophy and is associated with the milder form of the disease. The severity, however, can be modified in either a positive or negative direction by other factors that influence splicing or expression of the sequence converted SMN gene.

Mapping of a major genetic modifier of embryonic lethality in TGF beta 1 knockout mice

The transforming growth factor beta 1 (TGF beta 1) signalling pathway is important in embryogenesis and has been implicated in hereditary haemorrhagic telangiectasia (HHT), atherosclerosis, tumorigenesis and immunomodulation. Therefore, identification of factors which modulate TGF beta 1 bioactivity in vivo is important. On a mixed genetic background, approximately 50% Tgfb1-/- conceptuses die midgestation from defective yolk sac vasculogenesis. The other half are developmentally normal but die three weeks postpartum. Intriguingly, the vascular defects of Tgfb1-/- mice share histological similarities to lesions seen in HHT patients. It has been suggested that dichotomy in Tgfb1-/- lethal phenotypes is due to maternal TGF beta 1 rescue of some, but not all, Tgfb1-/- embryos12. Here we show that the Tgfb1-/- phenotype depends on the genetic background of the conceptus. In NIH/Ola, C57BL/6J/Ola and F1 conceptuses, Tgfb1-/- lethality can be categorized into three developmental classes. A major codominant modifier gene of embryo lethality was mapped to proximal mouse chromosome 5, using a genome scan for non-mendelian distribution of alleles in Tgfb1-/- neonatal animals which survive prenatal lethality. This gene accounts for around three quarters of the genetic effect between mouse strains and can, in part, explain the distribution of the three lethal phenotypes. This approach, using neonatal DNA samples, is generally applicable to identification of loci that influence the effect of early embryonic lethal mutations, thus furthering knowledge of genetic interactions that occur during early mammalian development in vivo.

Mouse models of human genetic disease: which mouse is more like a man?

There has always been great interest in animal models of human genetic disease, and mice provide the largest number of examples. A mutation in the homologous gene in mice does not always lead to the same phenotype as is found in man, however. Recent studies made it apparent that one mutation can have markedly different phenotypes when placed on different genetic backgrounds. This variation is due to different alleles at modifying loci in various inbred strains. Thus, if one wishes to obtain the optimal mouse model for a human disease, one needs to choose the correct genetic background as well as the correct mutation.

Endoscopic sinus surgery in the treatment of cystic fibrosis with nasal polyposis

We have performed endoscopic sinus surgery on 46 patients with chronic, polypoid rhinosinusitis since 1989. Follow-up ranged from 1 month to 6 years (mean, 28.2 months). Overall, our patients had a 50% chance either of their symptoms returning to preoperative severity or of undergoing a second endoscopic sinus procedure, by 18 to 24 months of postoperative follow-up. Patients with predominantly infective symptoms of mucopurulent rhinorrhea and pain had a significantly better outcome than those with predominantly nasal blockage. The chance of the former group of patients suffering symptom deterioration back to the preoperative state or undergoing a second endoscopic sinus operation was 37% of that of the latter group. The extent of disease on computed tomography scan had no relation to outcome.