Serum lipase levels pre and post Lundh meal: evaluation of exocrine pancreatic status in cystic fibrosis

Determination of pancreatic function is essential in cystic fibrosis. The most-reliable method is by measuring pancreatic enzymes in the duodenum following intravenous or oral stimulation. However, this is invasive, time consuming, and expensive. Indirect tests are non-invasive but lack accuracy. This study examines a simple test which combines pancreatic stimulation by Lundh meal and sequential serum lipase measurements. The test was performed on three groups: group A, 36 cystic fibrosis patients carrying two mutations associated with severe disease and pancreatic insufficiency (delta F508, W1282X, G542X, N1303K, S549R); group B, 8 compound heterozygote cystic fibrosis patients carrying one mutation causing mild disease with pancreatic sufficiency (3849 + 10 kb C-->T); group C, 17 healthy individuals. Basal lipase levels were 2-16.5, 16.4-73, and 8.5-27.8 U/l in groups A, B, and C, respectively, with some overlapping between groups. There were three patterns of lipase activity (1) consistently low levels (group A) suggested a severely affected insufficient pancreas; (2) normal basal levels followed by a linear rise peaking 30 min after the meal (found in 16 of 17 healthy individuals and 3 patients of group B) reflecting an unaffected sufficient pancreas; (3) elevated lipase levels not influenced by the meal (5 patients of group B). This reflects an ongoing destructive process in the pancreas which will eventually result in conversion from pancreatic sufficiency to pancreatic insufficiency. Hence serum lipase activity prior to and 30 min after Lundh meal is a good indicator of pancreatic status allowing categorization of cystic fibrosis patients as pancreatic insufficient, pancreatic sufficient, or pancreatic sufficient with late conversion to insufficiency.

The significance of sweat Cl/Na ratio in patients with borderline sweat test

Recently a few cystic fibrosis (CF) patients with borderline or normal sweat tests have been reported. These patients present a diagnostic challenge. We aimed to study the sweat Cl/Na ratio in cystic fibrosis patients and to assess whether this ratio could be used as a diagnostic criteria. The mean sweat Cl/Na ratio of 3 groups was compared: Group A: 71 CF patients carrying 2 mutations known to be associated with severe disease presentation (delta F508, W1282X, G542X, N1303K, 1717-1G --> A). Group B: 10 compound heterozygous patients who carry one mutation associated with mild clinical disease (3849 + 10 kb --> T). Group C: 142 normal subjects. Sweat chloride levels higher than those of sodium were found in 96% of patients in Group A as compared to 3% of patients in Group C. In Group B 40% of the patients had sweat chloride levels higher than or equal to sodium levels. The mean Cl/Na ratio of Group A (1.2 +/- 0.1) differed significantly from that of Group B (0.94 +/- 0.1) and both groups had significant higher mean Cl/Na ratio compared to Group C (0.7 +/- 0.4) (P < 0.001). Thus in individuals with a borderline sweat test and a Cl/Na ratio > or = 1 the diagnosis of CF should be considered. However, a Cl/Na ratio < 1 does not exclude CF, since patients carrying mild mutations may have sweat sodium levels higher than those of chloride. Our findings suggest that the sweat Cl/Na ratio in CF is genetically determined and it may be of help in establishing the diagnosis of CF in patients with a borderline sweat test.

Congenital bilateral absence of vas deferens in the absence of cystic fibrosis

The high frequency of mutations in the cystic fibrosis gene in patients with congenital bilateral absence of vas deferens (CBAVD) has raised the question whether all of them have a genital form of cystic fibrosis. We investigated 47 CBAVD patients by ultrasonography, 10 (21%) had renal malformations and 37 (79%) did not. In the former group, no cystic fibrosis mutations were found and sweat chloride concentrations were normal. In the latter group, 18 patients (49%) carried at least one cystic fibrosis mutation and sweat chloride was high in 17 of 26 tested (65%). Our findings suggest that CBAVD patients with renal malformations do not necessarily have cystic fibrosis.

Preimplantation diagnosis of cystic fibrosis by simultaneous detection of the W1282X and delta F508 mutations

W1282X (W) and delta F508 (delta) are the two most common mutations of the cystic fibrosis Israeli population. Patients who are homozygotes (WW and delta delta) as well as compound heterozygotes (W delta) present a severe phenotype of the disease. In the present study, we have developed a polymerase chain reaction (PCR)-based method for the detection of both mutations simultaneously in a single blastomere. Unfertilized human oocytes and single polyspermic blastomeres were subjected to a two-round PCR amplification: a first round of multiplex PCR followed by a second round of nested PCR, done separately at each locus. Clear signals at both loci were obtained in 51% (47/65) of oocytes and 69% (24/35) of blastomeres. The genotype of the single cell analysed was determined by endonuclease digestion of the W products and by heteroduplex formation of the delta F products. This diagnostic system will allow the identification of affected embryos (WW, delta delta, W delta) as well as phenotypically normal carriers (W+, +delta), and therefore may be used for cystic fibrosis preimplantation diagnosis in families who carry either or both mutations.

Cost-benefit analysis of a national screening programme for cystic fibrosis in an Israeli population

The recently acquired ability to identify 97% of CF carriers in an Israeli Ashkenazi population, prompts an evaluation of a nationwide screening programme. In 1993, the programme would first screen and counsel 9,261 parents, then 396 spouses of carrier parents and finally screen 16.5 fetuses where both parents are carriers. Assuming 92% of screened parents choose abortion of fetus screened positive, 2.33 cases of CF will be prevented in 1993 at a direct cost of $781,000. The $326,000 direct costs of preventing a CF case, exceed the lifetime excess direct costs per case of $297,000. However, benefits of screening also accrue to subsequent pregnancies, resulting in a direct benefit ($14.45 million) to cost ($10.39 million) ratio of 1.39/1 for the period 1993-2032. When benefits and costs resulting from mortality changes, work absences and transport costs are included, the benefit ($15.95 million) to cost ($13.88 million) ratio falls to 1.15/1. Benefit-cost ratios are lower for other ethnic groups in Israel, due to lower carrier rates and lower mutation detection abilities. A CF screening programme will increase the freedom of individuals choice, but should be carried out carefully in order to minimize stigmatization and even discrimination against CF carriers.

Extended haplotype analysis of cystic fibrosis mutations and its implications for the selective advantage hypothesis

The major cystic fibrosis (CF) mutation, delta F508, is associated with one haplotype (B) determined by the two polymorphic markers, XV2C and KM19. This haplotype is rare (15%) among non-CF chromosomes. Its frequency among non-delta F508 CF chromosomes is 50% with variation between populations. One hypothesis for the high frequency of CF haplotype B chromosomes suggests that there was a selective advantage for CF mutations on this specific "background" as a result of epistatic selection at other closely linked loci. Since the XV2C and KM19 markers are located 200 kb 5' to the CF gene and span only 60 kb, an extended haplotype analysis was needed to test this hypothesis. Haplotypes were determined for 183 CF and 120 non-CF Israeli chromosomes at the XV2C and KM19 loci and at three intragenic polymorphic sites (GATT in intron 6A, TUB18 in intron 19, and 24M in exon 24). Among the studied chromosomes the frequency of non-delta F508 CF chromosomes associated with haplotype B was 70% (88% among Ashkenazi CF chromosomes). Nine mutations (delta F508, W1282X, G542X, N1303K, 3849 + 10 kb C-->T, Q359K/T360K, S549I, S549R, and 1717-1G-->A) were identified among the studied chromosomes. These mutations accounted for 96% of CF chromosomes of Ashkenazi origin. Haplotype B was associated with seven of these (delta F508, W1282X, G542X, N1303K, Q359K/T360K, S549R, and 1717-1G-->A). The extended haplotype analysis revealed that in five of the seven mutations associated with the haplotype B, 97% of the chromosomes shared the same intragenic haplotype, 212. The variation found in 3% of the chromosomes was only in the GATT repeat. Two mutations, W1282X and 1717-1G-->A, were associated with a completely different intragenic haplotype, 121. The results of this study indicate that grouping of CF chromosome by haplotype analysis spanning a small extragenic region might not be sufficient. In addition, the results of the extended haplotype analysis indicate that all the studied CF chromosomes that carry the same mutation derived from the same origin. Furthermore, the results indicate that the majority of the CF mutations are associated with the same extended haplotype, supporting the selective advantage hypothesis.

Mild cystic fibrosis and normal or borderline sweat test in patients with the 3849 + 10 kb C-->T mutation

Different mutations in the cystic fibrosis (CF) gene appear to contribute to heterogeneity of the CF phenotype. We investigated 15 patients with CF who have the 3849 + 10 kb C-->T mutation. All were Ashkenazi Jews. Their clinical features were compared with those of CF patients with the delta F508/delta F508, W1282X/W1282X, W1282X/delta F508 mutations, which are known to be associated with a severe disease. Patients with the 3849 + 10 kb mutation were older, had been diagnosed as having CF at a more advanced age, and were in a better nutritional state. Sweat chloride values were normal (below 60 mmol/L) in 5 3849 + 10 kb patients (33%). 4 of these patients and 6 others (total 66%) had normal pancreatic function. However, age-adjusted pulmonary function did not differ between the two groups. None of the patients with 3849 + 10 kb C-->T had had meconium ileus or had liver disease or diabetes mellitus. We conclude that this mutation is associated with a mild type of CF.

[Screening for carriers of cystic fibrosis mutations in Ashkenazi volunteers]

309 DNA samples obtained from healthy volunteers were tested for the cystic fibrosis mutations DF508 and W1282X. 14 carriers were identified, 7 of each mutation. Since the 2 mutations account for only 80% of CF mutations, the actual number of carriers is 1 in 18. In spite of the fact that this is only a pilot study, the results suggest that screening for CF carriers is feasible and that it identifies unambiguously those who carry the CF genes. When testing for CF carriers becomes available for the general public, it will undoubtedly contribute in reducing significantly the incidence of children born with the disease.

The relation between genotype and phenotype in cystic fibrosis--analysis of the most common mutation (delta F508)

BACKGROUND AND METHODS

Both the clinical manifestations of cystic fibrosis and the genotypes of patients are heterogeneous, but the associations between the two are not known. We therefore studied blood samples from 293 patients with cystic fibrosis for the presence of the most common disease-causing mutation (delta F508) on chromosome 7 and compared the results with the clinical manifestations of the disease.

RESULTS

The prevalence of the delta F508 allele in the cohort was 71 percent; 52 percent of the patients were homozygous for the mutation, 40 percent were heterozygous, and 8 percent had other, undefined mutations. The patients who were homozygous for the mutation had received a diagnosis of cystic fibrosis at an earlier age and had a greater frequency of pancreatic insufficiency; pancreatic insufficiency was present in 99 percent of the homozygous patients, but in 72 percent of the heterozygous patients and only 36 percent of the patients with other genotypes. The patients with pancreatic insufficiency in all three genotype groups had similar clinical characteristics, reflected by an early age at diagnosis, similar sweat chloride values at diagnosis, similar severity of pulmonary disease, and similar percentiles for weight. In contrast, the patients in the heterozygous-genotype and other-genotype groups who did not have pancreatic insufficiency were older and had milder disease. They had lower sweat chloride values at diagnosis, normal nutritional status, and better pulmonary function after adjustment for age.

CONCLUSIONS

The variable clinical course in patients with cystic fibrosis can be attributed at least in part to specific genotypes at the locus of the cystic fibrosis gene.

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.

Rapid nonradioactive detection of the major cystic fibrosis mutation

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DNA marker haplotype association with pancreatic sufficiency in cystic fibrosis

Patients with cystic fibrosis (CF) generally suffer from chronic obstructive lung disease, pancreatic insufficiency (PI), and a number of other exocrine malfunctions. Approximately 15% of CF patients are, however, pancreatic sufficient. To investigate whether the two clinical subgroups, PI and pancreatic sufficiency (PS), are caused by different CF mutant alleles, we have performed linkage disequilibrium and haplotype association analysis with three DNA markers that are tightly linked to the CF locus. The study showed that the allelic and haplotype distributions for these RFLPs are significantly different between the two groups. The data suggest that most of the CF-PI patients are probably descendants of a single mutational event at the CF locus and that the CF-PS patients resulted from multiple, different mutations. While final interpretation of these data awaits molecular cloning of the CF gene, the information on haplotype association in CF may be useful in genetic counseling and disease prognosis, in identifying the gene itself, and in defining the mutations.

Identification and regional localization of DNA markers on chromosome 7 for the cloning of the cystic fibrosis gene

To facilitate mapping of the cystic fibrosis locus (CF) and to isolate the corresponding gene, we have screened a flow-sorted chromosome 7-specific library for additional DNA markers in the 7q31-q32 region. Unique ("single-copy") DNA segments were selected from the library and used in hybridization analysis with a panel of somatic cell hybrids containing various portions of human chromosome 7 and patient cell lines with deletion of this chromosome. A total of 258 chromosome 7-specific single-copy DNA segments were identified, and most of them localized to subregions. Fifty three of these corresponded to DNA sequences in the 7q31-q32 region. Family and physical mapping studies showed that two of the DNA markers, D7S122 and D7S340, are in close linkage with CF. The data also showed that D7S122 and D7S340 map between MET and D7S8, the two genetic markers known to be on opposite sides of CF. The study thus reaffirms the general strategy in approaching a disease locus on the basis of chromosome location.

DNase I sensitivity of Microtus agrestis active, inactive and reactivated X chromosomes in mouse-Microtus cell hybrids

We isolated Microtus agrestis-mouse somatic cell hybrid clones which had retained either the active or the inactive M. agrestis X chromosome. In both hybrid clones the X chromosomes retained their original chromatin conformation as studied by the in situ nick translation technique--the active X chromosome retained its high sensitivity to DNase I while the inactive one remained insensitive. A clone in which the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene had been spontaneously reactivated was isolated from the hybrid containing the inactive X chromosome. The in situ nick translation technique was used to study possible DNA conformation changes in the euchromatin of the inactive X chromosome with special reference to the reactivated HPRT locus. We found that the euchromatin in this X chromosome exhibited the same low sensitivity to DNase I as is characteristic of the inactive X chromosome.

Regions of active chromatin conformation in 'inactive' male meiotic sex chromosomes of the mouse

The sensitivity to DNase I of the meiotic sex chromosomes of the male mouse was determined by in situ nick translation. At pachytene and diakinesis-metaphase I, six segments, four at the ends of the X and Y chromosomes and two at internal sites on the X chromosome, were found to be more sensitive than the other parts of these chromosomes. The sensitive segments presumably reflect an active or potentially active chromatin conformation which is maintained in the sex chromosomes despite the earlier reported, almost complete cessation of uridine incorporation. The distribution of regions which are sensitive to DNase I corresponds to that of early DNA replication bands. Active conformation patterns like those figured here, probably exist in the sex chromosomes of other mammals as well.

DNase I sensitivity in facultative and constitutive heterochromatin

In situ nick translation allows the detection of DNase I sensitive and insensitive regions in fixed mammalian mitotic chromosomes. We have determined the difference in DNase I sensitivity between the active and inactive X chromosomes in Microtus agrestis (rodent) cells, along both their euchromatic and constitutive heterochromatic regions. In addition, we analysed the DNase I sensitivity of the constitutive heterochromatic regions in mouse chromosomes. In Microtus agrestis female cells the active X chromosome is sensitive to DNase I along its euchromatic region while the inactive X chromosome is insensitive except for an early replicating region at its distal end. The late replicating constitutive heterochromatic regions, however, in both the active and inactive X chromosome are sensitive to DNase I. In mouse cells on the other hand, the constitutive heterochromatin is insensitive to DNase I both in mitotic chromosomes and interphase nuclei.

Mapping of DNAase I sensitive regions on mitotic chromosomes

We have shown that in fixed mitotic chromosomes from female G. gerbillus cells the inactive X chromosome is distinctly less sensitive to DNAase I than the active X chromosome, as demonstrated by in situ nick translation. These results indicated that the specific chromatin conformation that renders potentially active genes sensitive to DNAase I is maintained in fixed mitotic chromosomes. We increased the sensitivity and accuracy of in situ nick translation using biotinylated dUTP and a specific detection and staining procedure instead of radioactive label and autoradiography and now show that in both human and CHO chromosomes, the DNAase I sensitive and insensitive chromosomal regions form a specific dark and light banding pattern. The DNAase I sensitive dark D-bands usually correspond to the light G-bands, but not all light G-bands are DNAase I sensitive. Identifiable regions of inactive constitutive heterochromatin are in a DNAase I insensitive conformation. Our methodology provides a new and important tool for studying the structural and functional organization of chromosomes.

In situ nick-translation distinguishes between active and inactive X chromosomes

Template-active regions of chromatin are structurally distinct from nontranscribing segments of the genome. Recently, it was suggested that the conformation of active genes which renders them sensitive to DNase I may be maintained even in fixed mitotic chromosomes. We have developed a technique of mitotic cell fixation and DNase I-directed nick-translation which distinguishes between active and inactive X chromosomes. We report here that Gerbillus gerbillus (rodent) female cells contain easily identified composite X chromosomes each of which includes the original X chromosome flanked by two characteristic autosomal segments. After nick-translation the active X chromosome in each cell is labelled specifically in both the autosomal and X-chromosomal regions. The inactive X chromosome is labelled only in the autosomal regions and in a small early replicating band within the late replicating 'original X' chromosome. Our technique opens the possibility of following the kinetics of X-chromosome inactivation and reactivation during embryogenesis, studying active genes in the inactive X chromosome and mapping tissue-specific gene clusters.