Danuta Markiewicz

The Relation between Genotype and Phenotype in Cystic Fibrosis — Analysis of the Most Common Mutation (ΔF508)

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.

Detection of a cystic fibrosis modifier locus for meconium ileus on human chromosome 19q13

Detection of a cystic fibrosis modifier locus for meconium ileus on human chromosome 19q13

Correlation of sweat chloride concentration with classes of the cystic fibrosis transmembrane conductance regulator gene mutations

OBJECTIVE To compare differences in epithelial chloride conductance according to class of mutation of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. METHODS We evaluated the relationship between the functional classes of CFTR mutations and chloride conductance using the first diagnostic sweat chloride concentration in a large cystic fibrosis (CF) population. RESULTS There was no difference in sweat chloride value value between classes of CFTR mutations that produce no protein (class I), fail to reach the apical membrane because of defective processing (class II), or produce protein that fails to respond to cyclic adenosine monophosphate (class III). Those mutations that produce a cyclic adenosine monophosphate-responsive channel with reduced conductance (class IV) were associated with a significantly lower, intermediate sweat chloride value. However, patients with the mutations that cause reduced synthesis or partially defective processing of normal CFTR (class V) had sweat chloride concentrations similar to those in classes I to III. CONCLUSION Studies of differences in chloride conductance between functional classes of CFTR mutations provide insight into phenotypic expression of the disease.

Skipping of exon 12 as a consequence of a point mutation (1898 + 5G T) in the cystic fibrosis transmembrane conductance regulator gene found in a consanguineous Chinese family

A point mutation (1898+5G→T) located five base pairs downstream from the donor splice site in intron 12 of the CFTR gene has been identified in a consanguineous CF patient of Chinese origin. To determine if this nucleotide substitution could affect mRNA splicing, PCR analysis was performed with RNA isolated from the lymphoblastoid cell line of the mother of the deceased patient. While exon 12‐minus transcript was detected in this sample, it was also found in individuals without 1898+5G→T, albeit in a smaller proportion. Using a sequence polymorphism associated with each of the two alleles in the mother, however, we showed that mutant transcript was almost exclusively produced by the 1898+5G→T allele. Skipping of exon 12 would result in the deletion of 29 amino acids from the first nucleotide binding domain of CFTR, rendering the protein non‐functional. The possibility of a low level (2.5%) of normal transcript from the mutant allele cannot be excluded and it may explain the pancreatic sufficient phenotype of the patient. The 1898+5G→T mutation was found in two other CF patients of Chinese origin, but it was not detected in 192 CF chromosomes of Caucasian origin and 30 other chromosomes from Chinese individuals without a family history of CF.

Spectrum of mutations in the CFTR gene of patients with classical and atypical forms of cystic fibrosis from southwestern Sweden: Identification of 12 novel mutations

Cystic fibrosis (CF) is caused by mutations in the CFTR gene. The spectrum of CFTR mutations varies between populations and depends on different factors, such as ethnic background and geographical location. The extensive CFTR mutation screening of 129 patients with classical or atypical CF from the south-western region of Sweden revealed the presence of 37 CFTR mutations, including 12 novel alleles. The overall mutation detection rate in this study population was 92%, the highest among all tested regions in Sweden. Eight mutations with a frequency above 1% (DeltaF508, 394delTT, R117C, 3659delC, E60X, 1112delT, R764X, and 621 + 1G --> T) accounted for 78% of CF chromosomes and have been recommended for inclusion in the CFTR mutation screening panel for molecular diagnosis of CF in this region. The multiple occurrence of specific CFTR alleles less common than the predominant DeltaF508 mutation (394delTT, R117C, 3659delC) allowed for genotype-phenotype comparisons and revealed consistent relationships between these mutations and disease severity.

Linkage of cystic fibrosis to the proα2(I) collagen gene, COL1A2, on chromosome 7

A linkage has been detected between the locus for cystic fibrosis (CF) and the pro alpha 2(I) collagen gene (COL1A2) which is located in the region q21.3----q22.1 of chromosome 7. Based on the combined linkage data derived from 50 informative two-generation nuclear families collected in Canada and Denmark, the distance between COL1A2 and CF is estimated to be 19 centiMorgans. Close linkage has also been detected between COL1A2 and the DNA marker D7S15 (formerly D0CRI-917) and the serum enzyme activity marker paraoxonase (PON), both of which have previously been found linked to CF. The results of the two-point and three-point linkage analyses indicate that the most probable order of these four genetic loci is COL1A2-D7S15 - PON - CF.

Molecular genetics of cystic fibrosis.

The gene for cystic fibrosis (CF) has recently been isolated (Rommens et al., 1989a; Riordan et al., 1989; Kerem et al., 1989) rendering it possible for the first time to study the elusive symptoms of the disease. In this brief review, we try to summarize our current understanding of the CF gene and our attempts to study the gene product and its function. We have also included a short discussion on the correlation between genotype and phenotype in CF patients.

Two novel frameshift deletions (1924del7, 2055del9→A) in the CFTR gene in Mexican cystic fibrosis patients

Lorena Orozco,* Julian Zielenski, Danuta Markiewicz, Teresa Villarreal, Lap-Chee Tsui, Jose Luis Lezana, and Rosa M del Angel Molecular Biology Laboratory, Human Genetics Department, Instituto Nacional de Pediatria, Mexico City, Mexico Department of Genetics, Hospital for Sick Children, Toronto, Canada Department of Molecular and Medical Genetics, University of Toronto, Toronto, Canada Asociacion Mexicana de Fibrosis Quistica, Mexico City, Mexico Department of Experimental Pathology, CINVESTAV-IPN, Mexico City, Mexico