Multiple mutations in highly conserved residues are found in mildly affected cystic fibrosis patients

Expression of the cystic fibrosis transmembrane conductance regulator gene in cells of non-epithelial origin

Consistent with the fact that the clinical disorder cystic fibrosis (CF) is manifested on epithelial surfaces, active transcription of the CF transmembrane conductance regulator (CFTR) gene and CFTR mRNA transcripts are detectable in a variety of epithelial cells, suggesting CFTR gene expression might be epithelial cell-specific. However, analysis of the CFTR gene promoter suggests it is a housekeeping gene, implying more widespread expression than only in epithelial cells. To evaluate the latter hypothesis, various human cells of non-epithelial origin, including lung fibroblasts, U-937 histiocytic lymphoma cells, K-562 erythroleukemia cells, HL-60 promyelocytic leukemia cells as well as freshly isolated blood lymphocytes, neutrophils, monocytes, and alveolar macrophages were examined for CFTR gene expression. Although Northern analysis failed to show CFTR mRNA transcripts in these cells, amplification of mRNA (after conversion to cDNA) by polymerase chain reaction combined with Southern analysis demonstrated the presence of CFTR mRNA transcripts at low levels in all cells evaluated except HL-60 cells. Comparative quantitative analysis showed fibroblasts contained 200-400 fold less CFTR mRNA transcripts than the T84 and HT-29 colon carcinoma epithelial cell lines, but had similar levels of CFTR transcripts to those of other epithelial cell lines. Nuclear transcription run-on analyses demonstrated very low level CFTR gene transcription in fibroblasts and U-937 cells, similar to that of other epithelial cells, but lower than the T84 and HT-29 colon carcinoma cell lines. Interestingly, while chromatin DNA of fibroblasts had no DNase I hypersensitivity sites in the 5' flanking region of the CFTR gene, HT-29 chromatin DNA exhibited four DNase I accessible sites in the same region, suggesting that these sites may be related to more active transcription of the CFTR gene in the intestinal epithelial cells than in fibroblasts.

A bacterial system for investigating transport effects of cystic fibrosis--associated mutations

LIV-I, a high-affinity system that transports neutral, branched-chain amino acids into Escherichia coli, has two components, LivG and LivF, that are homologous to the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). CF-associated mutations of human CFTR were introduced into corresponding regions of LivG, and their effects on leucine transport could be grouped into three classes. Mutations were found that (i) abolished LIV-I--directed transport, (ii) retained about a quarter of wild-type activity at the Michaelis-Menten constant (KM), and (iii) had minimal activity at the KM. A mutation equivalent to a benign polymorphism had no effect on transport. The correlation of these mutational phenotypes in LivG and CFTR suggests that the LIV-I prokaryotic transporter is functionally similar to the CF protein and that this similarity can be exploited to clarify the properties of the nucleotide-binding fold in this superfamily of proteins.

Sequence analysis of the cystic fibrosis gene in patients with disseminated bronchiectatic lung disease. Application in the identification of a cystic fibrosis patient with atypical clinical course

The diagnosis of classical cystic fibrosis (CF) is easily made by clinical assessment alone, but may be missed or delayed in cases with an atypical clinical course. In a recent major study the age at diagnosis varied between 2 months and 47 years. For diagnostic purposes we have investigated the cystic fibrosis transmembrane regulator (CFTR) gene in 10 adult patients (age 18 to 45 years) with chronic obstructive pulmonary disease since childhood or adolescence and bronchiectases disseminated through both lungs. Only one subject (a 29-year-old male) had exocrine pancreatic insufficiency (PI); all others were pancreatic-sufficient (PS). The first nucleotide (ATP)-binding fold of the CFTR was analyzed by direct sequencing of polymerase chain reaction (PCR)-amplified genomic DNA in these cases. Two patients with different phenotypes (one PI, one PS) were found to be homozygous for the common delta F508 mutation of the CFTR gene, which proved the diagnosis of cystic fibrosis in their cases and allowed genetic counselling. The PS patient had normal sweat tests and had not previously been recognized as having CF. Four other patients were heterozygous for delta F508, with no other mutation in exons 10 or 11 of the gene, and four patients had normal sequences of these exons. Because only about 70% of all CF chromosomes carry delta F508, the unexpectedly high frequency (4/8 = 50%) of heterozygosity for delta F508 among the non-delta F508/delta F508 patients with bronchiectases suggests that some of these might also have unrecognized CF with rare genotypes and mutations in any of the 22 exons not sequenced.(ABSTRACT TRUNCATED AT 250 WORDS)

Nine mutations in the cystic fibrosis (CF) gene account for 80% of the CF chromosomes in French patients

Thirteen mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have been screened in a French sample of 185 cystic fibrosis (CF) patients, together with their respective associated RFLP haplotypes at the linked D7S23 locus (XV2C and KM19 markers). The respective frequencies of the mutations showed that 9 of them account for 80% of the CF chromosomes. Implications for prenatal diagnosis and heterozygote detection are defined and discussed. The well-known great excess of RFLP B marker within CF chromosomes is partially explained by two already characterized mutations highly associated with haplotype B: delta F508 and G542X. Similarly, the excess of haplotype D within CF chromosomes is partially explained by the association between delta I507 and this haplotype. These results may suggest the existence of two still untested or uncharacterized mutations, whose frequencies could be near 1%, one which would be associated with haplotype B and a second which would be associated with haplotype D. The possible cause of the specific association between most of the main different CF mutations and the RFLP haplotype B is discussed.

cAMP-inducible chloride conductance in mouse fibroblast lines stably expressing the human cystic fibrosis transmembrane conductance regulator

A cAMP-inducible chloride permeability has been detected in mouse fibroblast (L cell) lines upon stable integration of a full-length cDNA encoding the human cystic fibrosis transmembrane conductance regulator (CFTR). As indicated by a Cl(-)-indicator dye, the Cl- permeability of the plasma membrane increases by 10- to 30-fold within 2 min after treatment of the cells with forskolin, an activator of adenylyl cyclase. The properties of the conductance are similar to those described in secretory epithelial cells; the whole-cell current-voltage relationship is linear and there is no evidence of voltage-dependent inactivation or activation. In contrast, this cAMP-dependent Cl- flux is undetectable in the untransfected cells or cells harboring defective cDNA constructs, including one with a phenylalanine deletion at amino acid position 508 (delta F508), the most common mutation causing cystic fibrosis. These observations are consistent with the hypothesis that the CFTR is a cAMP-dependent Cl- channel. The availability of a heterologous (nonepithelial) cell type expressing the CFTR offers an excellent system to understand the basic mechanisms underlying this CFTR-associated ion permeability and to study the structure and function of the CFTR.

Identification and characterization of the familial adenomatous polyposis coli gene

DNA from 61 unrelated patients with adenomatous polyposis coli (APC) was examined for mutations in three genes (DP1, SRP19, and DP2.5) located within a 100 kb region deleted in two of the patients. The intron-exon boundary sequences were defined for each of these genes, and single-strand conformation polymorphism analysis of exons from DP2.5 identified four mutations specific to APC patients. Each of two aberrant alleles contained a base substitution changing an amino acid to a stop codon in the predicted peptide; the other mutations were small deletions leading to frameshifts. Analysis of DNA from parents of one of these patients showed that his 2 bp deletion is a new mutation; furthermore, the mutation was transmitted to two of his children. These data have established that DP2.5 is the APC gene.

High conservation of sequences involved in cystic fibrosis mutations in five mammalian species

Several mutations have been identified in the first nucleotide binding fold (NBF) of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene. We have analyzed the DNA sequences of exons 10 and 11 in five different mammalian species, marmoset, mouse, cow, pig, and sheep; the amino acid conservation studied for nine disease mutations; and two "benign" mutations. For exon 10, 87% homology at the DNA level and 93.5% at the amino acid level were found for these species. For exon 11, the lowest homology (70%) was found in mouse and the highest in marmoset (93%), whereas the amino acid sequence conservation ranged from 82.5 to 100%. All codons involved in CF mutations are highly conserved throughout evolution.

Cystic fibrosis gene analysis: recent diagnostic applications

Cystic fibrosis (CF) is the most common severe genetic disease of Caucasians. Recent work by several laboratories has resulted in the identification of the CF gene and its major mutation. These findings have greatly facilitated carrier detection and prenatal diagnosis for members of families with a history of CF. This review briefly considers the historical background of CF testing, the basic features of the CF gene, and the methodologies for CF gene analysis. Examples of the application of recent technologies for analysis of CF families are discussed. In addition to family analysis, CF carrier detection in the general population is becoming more feasible. The possibility of carrier screening for CF has been the topic of much discussion recently with arguments presented both for and against. Some basic issues pertaining to population testing are presented.

Demonstration that CFTR is a chloride channel by alteration of its anion selectivity

Expression of the cystic fibrosis transmembrane conductance regulator (CFTR) generates adenosine 3',5'-monophosphate (cAMP)-regulated chloride channels, indicating that CFTR is either a chloride channel or a chloride channel regulator. To distinguish between these possibilities, basic amino acids in the putative transmembrane domains were mutated. The sequence of anion selectivity of cAMP-regulated channels in cells containing either endogenous or recombinant CFTR was bromide greater than chloride greater than iodide greater than fluoride. Mutation of the lysines at positions 95 or 335 to acidic amino acids converted the selectivity sequence to iodide greater than bromide greater than chloride greater than fluoride. These data indicate that CFTR is a cAMP-regulated chloride channel and that lysines 95 and 335 determine anion selectivity.

Effect of deleting the R domain on CFTR-generated chloride channels

The cystic fibrosis transmembrane conductance regulator (CFTR), which forms adenosine 3',5'-monophosphate (cAMP)-regulated chloride channels, is defective in patients with cystic fibrosis. This protein contains two putative nucleotide binding domains (NBD1 and NBD2) and an R domain. CFTR in which the R domain was deleted (CFTR delta R) conducted chloride independently of the presence of cAMP. However, sites within CFTR other than those deleted also respond to cAMP, because the chloride current of CFTR delta R increased further in response to cAMP stimulation. In addition, deletion of the R domain suppressed the inactivating effect of a mutation in NBD2 (but not NBD1), a result which suggests that NBD2 interacts with the channel through the R domain.

CA/GT microsatellite alleles within the cystic fibrosis transmembrane conductance regulator (CFTR) gene are not generated by unequal crossingover

The gene responsible for cystic fibrosis (CF) has recently been identified, and a three-nucleotide deletion (delta F508 mutation) that results in the loss of a phenylalanine residue in the first putative ATP-binding domain of the predicted protein (CF transmembrane conductance regulator, CFTR) has been found to be the major CF mutation. Although several other mutations have been identified in the CFTR gene, most of them are very rare, making their application to genetic diagnosis difficult. While characterizing the genomic region encompassing the CF locus, we have identified three CA/GT blocks that flank exon 9 of the CF gene. One of the CA/GT blocks exhibits a highly informative variable number of dinucleotide repeats (VNDR) polymorphism. This intragenic VNDR microsatellite should, by itself, provide full information for genetic analysis in approximately 80% of CF families and will help elucidate the associations between DNA polymorphism haplotypes and specific gene mutations. Haplotype analyses of CF chromosomes with and without the delta F508 mutation suggest that the different alleles are generated by slipped-strand mispairing within the dinucleotide repeat during DNA replication, rather than by unequal crossingover within a recombination hot spot.

Medical genetics

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Molecular cloning and sequence analysis of the murine cDNA for the cystic fibrosis transmembrane conductance regulator

We have cloned the mouse homolog of the human cystic fibrosis transmembrane conductance regulator (CFTR) using clones isolated from a mouse lung cDNA library and using amplification of cDNA to isolate specific regions. The cDNA was 6304 bp in length and encoded a polypeptide of 1476 amino acids. Comparison of the deduced amino acid sequence showed that the mouse protein has high homology to the human protein; overall identity was 78.3%. The amino acid identity was high for both transmembrane domains (first transmembrane domain, 86.7%; second transmembrane domain, 81.1%) and for both ATP-binding folds (first ATP-binding fold, 80.5%; second ATP-binding fold, 83.9%), suggesting the functional importance of these regions. On the other hand, the R domain was less well conserved (68.9% identity). All of the published missense mutation sites and the site of the common delta F508 mutation were conserved between human and mouse.

Non-radioactive single strand conformation polymorphism (SSCP) using the Pharmacia 'PhastSystem'

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Progress towards completing the human linkage map

Genetic linkage maps order polymorphic loci (markers) along a chromosome and provide a measure of distance that is an inherently statistical measure of the frequency of meiotic recombination and has no simple relationship to DNA length in Mb. Few accurate and dense maps now exist. Maps with markers spaced every 2 cM providing accurate estimates of distance should be available in the next few years, however.

Probing the basic defect in cystic fibrosis

The concurrent developments in electrophysiology studies and the identification of the cystic fibrosis transmembrane conductance regulator (CFTR) gene has provided a unique opportunity to probe the basic cellular defect underlying cystic fibrosis. Various properties of the CFTR protein have been deduced from its primary sequence, the variety of mutations in patients and genotype-phenotype correlations, as well as the results of more recent DNA transfection studies. The most exciting observation is the fact that CFTR acts like a cAMP-regulated Cl- channel.

Mammalian genome mapping: lessons and prospects

The recent emphasis on human genome mapping has stimulated the development of gene maps in close to thirty mammalian species. Animal gene maps provide an invaluable resource for genetic analysis and manipulation of phenotypic characters, as well as a retrospective glimpse at the patterns and processes of genome evolution. An empirical strategy for developing new gene maps in mammals by emphasizing two important classes of index or anchor marker loci is presented.

Cystic fibrosis--the way forward from the gene

Cloning of the cystic fibrosis (CF) gene and elucidation of the physiological functions of the encoded protein is a triumph, not only for molecular biology, but also for people affected by CF. For them, not only is there now the possibility of screening for the commonest mutations, but they may also look forward to the prospect of improved therapies being developed.

Genomic DNA sequence of the cystic fibrosis transmembrane conductance regulator (CFTR) gene

The gene responsible for cystic fibrosis, the most common severe autosomal recessive disorder, is located on the long arm of human chromosome 7, region q31-q32. The gene has recently been identified and shown to be approximately 250 kb in size. To understand the structure and to provide the basis for a systematic analysis of the disease-causing mutations in the gene, genomic DNA clones spanning different regions of the previously reported cDNA were isolated and used to determine the coding regions and sequences of intron/exon boundaries. A total of 22,708 bp of sequence, accounting for approximately 10% of the entire gene, was obtained. Alignment of the genomic DNA sequence with the cDNA sequence showed perfect colinearity between the two and a total of 27 exons, each flanked by consensus splice signals. A number of repetitive elements, including the Alu and Kpn families and simple repeats, such as (GT)17, (GATT)7, and (TA)14, were detected in close vicinity of some of the intron/exon boundaries. At least three of the simple repeats were found to be polymorphic in the population. Although an internal amino acid sequence homology could be detected between the two halves of the predicted polypeptide, especially in the regions of the two putative nucleotide-binding folds (NBF1 and NBF2), the lack of alignment of the nucleotide sequence as well as the different positions of the exon/intron boundaries does not seem to support the hypothesis of a recent gene duplication event. To facilitate detection of mutations by direct sequence analysis of genomic DNA, 28 sets of oligonucleotide primers were designed and tested for their ability to amplify individual exons and the immediately flanking sequences in the introns.

The incidence of different cystic fibrosis mutations in the Scottish population: effects on prenatal diagnosis and genetic counselling

We present an analysis of the frequency of 16 different cystic fibrosis (CF) mutant alleles in the Scottish population. Each allele was detected in DNA amplified by the polymerase chain reaction (PCR) either directly on polyacrylamide gels, on agarose gels after restriction enzyme digestion, or by using allele specific oligonucleotides. Among 506 CF chromosomes, of predominantly Scottish origin, the frequencies of the different mutations were delta F508 0.71, G551D 0.05, G542X 0.04, R117H 0.01, 1717-1G----A 0.01, A455E + delta I507 + R553X + R560T + W1282X + 621 + 1G----T combined 0.03, unpublished 0.01, and unknown 0.13. No examples of D110H, R347P, S549N, S549I, or 2566ins AT mutations were found. The relevance of this type of analysis for both prenatal diagnosis and heterozygote screening is discussed.

Identification of mutations in exons 1 through 8 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene

Five different mutations have been identified in the gene causing cystic fibrosis (CF) through sequencing regions encompassing exons 1-8, including the 5' untranslated leader. Two of these apparent mutations are missense mutations, one in exon 3 (Gly to Glu at position 85; G85E) and another in exon 5 (Gly to Arg at 178; G178R), both causing significant changes in the corresponding amino acids in the encoded protein--cystic fibrosis transmembrane conductance regulator (CFTR). Two others affect the highly conserved RNA splice junction flanking the 3' end of exons 4 and 5 (621 + 1G----T, 711 + 1G----T), resulting in a probable splicing defect. The last mutation is a single-basepair deletion in exon 4, causing a frameshift. These five mutations account for the 9 of 31 non-delta F508 CF chromosomes in our Canadian CF family collection and they are not found in any of the normal chromosomes. Three of the mutations, 621 + 1G----T, 711 + 1G----T, and G85E, are found in the French-Canadian population, with 621 + 1G----T being the most abundant (5/7). There are two other sequence variations in the CFTR gene; one of them (129G----C) is located 4 nucleotides upstream of the proposed translation initiation codon and, although present only on CF chromosomes, it is not clear whether it is a disease-causing mutation; the other (R75Q) is most likely a sequence variation within the coding region.

The search for south European cystic fibrosis mutations: identification of two new mutations, four variants, and intronic sequences

The major mutation in the cystic fibrosis (CF) gene is a 3-bp deletion (delta F508) in exon 10. About 50% of the CF chromosomes in Southern Europe carry this mutation, while other previously described mutations account for less than 4%. To identify other common mutations in CF patients from the Mediterranean area, we have sequenced, exon by exon, 16 chromosomes that did not show the delta F508 deletion from a selected panel of eight unrelated CF patients. We describe here one missense and one nonsense mutation, and four sequence polymorphisms. We have also found two previously reported mutations in three chromosomes. Overall, these mutations may account for about 20% of CF alleles in the Italian and Spanish populations. No other mutations were detected in 10 out of 16 CF chromosomes after analyzing about 90% of the coding region of the CF gene, and 39 out of 54 intron/exon boundaries. Therefore, about 26% of CF mutations remain to be identified. In addition we provide the intron/exon boundary sequences for exons 4 to 9. These results together with previously reported linkage data suggest that in the Mediterranean populations further mutations may lie in the promoter region, or in intron sequences not yet analyzed.

Detection of three rare frameshift mutations in the cystic fibrosis gene in an African-American (CF444delA), an Italian (CF2522insC), and a Soviet (CF3821delT)

We have identified three new frameshift mutations in the CFTR gene in patients with cystic fibrosis (CF). The first one involves the deletion of an adenine nucleotide in exon 4 in an African-American patient (CF444delA), the second involves the insertion of a cytosine nucleotide in exon 13 in an Italian patient (CF2522insC), and the third results from the deletion of a thymidine nucleotide in exon 19 in a Soviet patient (CF3821delT). Each mutation is predicted to result in premature termination of the CFTR protein.

Two-temperature PCR and heteroduplex detection: application to rapid cystic fibrosis screening

We describe a rapid two-temperature PCR protocol for amplification of genomic DNA applied to the region of the most common mutation (delta F508) of the cystic fibrosis gene. Amplification products are detected as homo- or heteroduplexes on polyacrylamide gels as previously described. Data using two-temperature PCR show complete concordance with allele-specific hybridization after classical three-temperature PCR in 105 normal, carrier and affected individuals. Clinical application is demonstrated in a family which was uninformative by traditional RFLP linkage analysis. Two-temperature PCR may offer advantages of speed and specificity over three-temperature PCR in many clinical and research applications.

Identifying genes determining trait differences in behavior and drug response: a comment on 'Quantitative trait loci and psychopharmacology'

In Plomin, McClearn and Gora-Maslak's target article (see also Science 248: 183-188, 1990), reverse genetic approaches are emphasized for locating genes determining behavioral and pharmacogenetic traits. Furthermore, prospects for such an undertaking are presented pessimistically in that behavioral traits are asserted to be polygenic (due to the simultaneous action of variant alleles at multiple loci) and are conceptualized as being determined in large part by unshared environmental factors. We disagree with Plomin et al. in three major areas and argue the following:(1) Forward genetic approaches involving candidate locus analysis and detailed analysis of the phenotype are of primary importance for isolating genes for behavioral traits, as for other genetic traits.(2) Virtually all physiologic processes and metabolic pathways involve sets of genes, resulting in genetic heterogeneity (multiple genetic origins for a trait). However, polygenicity is approximately as unusual for behavioral traits as for other traits.(3) Heritability analyses underestimate the extent to which behavioral traits are amenable to genetic analysis and have been misinterpreted to overestimate the importance of environmental factors.

Identification of point mutations by mispairing PCR as exemplified in MERRF disease

The point mutation in the tRNA(Lys) gene of mitochondrial DNA (mtDNA) from patients with myoclonic epilepsy and ragged red fibers (MERRF) was quantitatively analyzed after digestion with the restriction endonuclease Nae I of the PCR amplified DNA. Since the point mutation is not part of a restriction site for a commonly available restriction endonuclease, the Nae I restriction site was introduced by PCR using a mispairing primer. The percentage of mutated mtDNA was determined in a few hairs of five members of an affected family by counting the radioactivity of the fragments after PCR amplification with labelled dATP.

Defective intracellular transport and processing of CFTR is the molecular basis of most cystic fibrosis

The gene associated with cystic fibrosis (CF) encodes a membrane-associated, N-linked glycoprotein called CFTR. Mutations were introduced into CFTR at residues known to be altered in CF chromosomes and in residues believed to play a role in its function. Examination of the various mutant proteins in COS-7 cells indicated that mature, fully glycosylated CFTR was absent from cells containing delta F508, delta 1507, K464M, F508R, and S5491 cDNA plasmids. Instead, an incompletely glycosylated version of the protein was detected. We propose that the mutant versions of CFTR are recognized as abnormal and remain incompletely processed in the endoplasmic reticulum where they are subsequently degraded. Since mutations with this phenotype represent at least 70% of known CF chromosomes, we argue that the molecular basis of most cystic fibrosis is the absence of mature CFTR at the correct cellular location.

Independent genetic determinants of pancreatic and pulmonary status in cystic fibrosis

The concordance of pancreatic and pulmonary status among siblings with cystic fibrosis, the cystic fibrosis genotype, and non-genetic factors were analysed in adult patients with cystic fibrosis. Genetic factors were more important than non-genetic factors in affecting the severity of pancreatic and lung disease. The genetic factors were independent of each other and did not invariably cosegregate with specific mutations at the cystic fibrosis locus.

Identification of mutations in regions corresponding to the two putative nucleotide (ATP)-binding folds of the cystic fibrosis gene

Additional mutations in the cystic fibrosis (CF) gene were identified in the regions corresponding to the two putative nucleotide (ATP)-binding folds (NBFs) of the predicted polypeptide. The patient cohort included 46 Canadian CF families with well-characterized DNA marker haplotypes spanning the disease locus and several other families from Israel. Eleven mutations were found in the first NBF, 2 were found in the second NBF, but none was found in the R-domain. Seven of the mutations were of the missense type affecting some of the highly conserved amino acid residues in the first NBF; 3 were nonsense mutations; 2 would probably affect mRNA splicing; 2 corresponded to small deletions, including another 3-base-pair deletion different from the major mutation (delta F508), which could account for 70% of the CF chromosomes in the population. Nine of these mutations accounted for 12 of the 31 non-delta F508 CF chromosomes in the Canadian families. The highly heterogeneous nature of the remaining CF mutations provides important insights into the structure and function of the protein, but it also suggests that DNA-based genetic screening for CF carrier status will not be straightforward.

Correction of the cystic fibrosis defect in vitro by retrovirus-mediated gene transfer

We have used retrovirus-mediated gene transfer to demonstrate complementation of the cystic fibrosis (CF) defect in vitro. Amphotropic retroviruses were used to transduce a functional cystic fibrosis transmembrane conductance regulator (CFTR) cDNA into CFPAC-1, a pancreatic adenocarcinoma cell line derived from a patient with CF that stably expresses the chloride transport abnormalities characteristic of CF. CFPAC-1 cells were exposed to control virus (PLJ) and CFTR-expressing virus (PLJ-CFTR); viral-transduced clones were isolated and subjected to molecular and physiologic analysis. RNA analysis detected a viral-derived CFTR transcript in all of the PLJ-CFTR clones that contained unrearranged proviral sequences. Agents that increase intracellular cAMP stimulated 125I efflux in PLJ-CFTR clones but not PLJ clones. Whole-cell patch-clamp performed on three responding clones showed that the anion efflux responses were due to cAMP stimulation of Cl conductance. Our findings indicate that expression of the normal CFTR gene confers cAMP-dependent Cl channel regulation on CF epithelial cells.

A major segment of the neurofibromatosis type 1 gene: cDNA sequence, genomic structure, and point mutations

Overlapping cDNA clones from the translocation breakpoint region (TBR) gene, recently discovered at the neurofibromatosis type 1 locus and found to be interrupted by deletions and a t(17;22) translocation, have been sequenced. A 4 kb sequence of the transcript of the TBR gene has been compared with sequences of genomic DNA, identifying a number of small exons. Identification of splice junctions and a large open reading frame indicates that the gene is oriented with its 5' end toward the centromere, in opposition to the three known active genes in the region. PCR amplification of a subset of the exons, followed by electrophoresis of denatured product on native gels, identified six variant conformers specific to NF1 patients, indicating base pair changes in the gene. Sequencing revealed that one mutant allele contains a T----C transition changing a leucine to a proline; another NF1 allele harbors a C----T transition changing an arginine to a stop codon. These results establish the TBR gene as the NF1 gene and provide a description of a major segment of the gene.