The French-Canadian population in the Saguenay-Lac St. Jean region of northeastern Quebec has an elevated frequency of cystic fibrosis (CF). The average incidence of cystic fibrosis was 1 in 891 births and the prevalence of CF carriers was estimated to be 1 in 15. We tested for 10 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene in 133 French-Canadian CF families from Quebec. Ninety-one families were from the Saguenay-Lac St. Jean region and 42 families were referred from other regions of Quebec. We detected the CFTR mutation in 93 and 92% of the CF chromosomes in the Saguenay-Lac St. Jean and the major-urban Quebec families, respectively. The two groups of French-Canadian families were significantly different for the proportions of CFTR mutations. The three most common mutations in the Saguenay-Lac St. Jean families were delta F508 (58%), 621 + 1G----T (23%), and A455E (8%); and in the major-urban Quebec families were delta F508 (71%), 711 + 1G----T (9%), and 621 + 1G----T (5%). These results provide evidence for the role of founder effect in the elevated incidence of cystic fibrosis in the Saguenay-Lac St. Jean population.

Cystic fibrosis (CF) has a high incidence in the French-Canadian population of Saguenay Lac-Saint-Jean (Quebec). The A455E mutation accounts for 8.3% of the CF chromosomes. This mutation was shown to be associated with a milder lung disease in the Dutch population. Twenty two CF patients distributed in 17 families and compound heterozygotes for the A455E mutation have been followed at the Clinique de Fibrose Kystique de Chicoutimi. Fourteen patients also carried the delta F508 mutation while the remaining eight patients had the 621 + 1G-->T mutation. Each patient was matched by sex and age to a patient homozygous for the delta F508 mutation. The pairs were analyzed for several clinical and laboratory variables. The A455E compound heterozygotes were diagnosed at a later age (P = 0.003) and had chloride concentrations at the sweat test lower than those homozygous for the delta F508 mutation (P = 0.007). More patients were pancreatic sufficient (P = 0.004). They had a higher Shwachman score (P = 0.001) and better pulmonary function tests (P < 0.02). CF patients compound heterozygous for the A455E mutation have a milder pancreatic and lung disease than the delta F508 homozygotes. Therefore, the A455E should be associated with a better prognosis.

Patients with cystic fibrosis (CF) display defects in airway ion transport, but the influence of airway transport phenotype on improved prognosis is not known. We studied airway bioelectric properties in five CF patients with the rare A455E mutation that is associated with mild pulmonary disease. We also evaluated five patients possessing premature truncation mutations (G542X and R553X) for which an association with mild pulmonary disease has not been as well established. We found no evidence in vivo that a mild lung disease mutation in the CF transmembrane regulator gene (CFTR) led to correction or partial correction of: (1) unstimulated Cl- secretion; (2) beta-agonist-activated Cl- secretion; (3) basal sodium reabsorption; or (4) amiloride-sensitive airway sodium transport. Early phase therapeutic trials in CF, including human gene transfer trials, rely heavily on improvements in airway potential difference to identify promising interventions and an improved prognosis. Based on our findings in a naturally occurring group of CF patients with an improved pulmonary prognosis (A455E), one can argue that marked clinical benefit might be possible without any improvement whatsoever in airway bioelectric phenotype. Moreover, if genetic modifiers exist that influence the severity of a particular CFTR mutation (e.g., A455E), these may be independent of human airway Cl-secretion in vivo, since we detected minimal Cl--secretory responses in patients with A455E.

We have analyzed the CFTR mRNA populations in a cystic fibrosis patient heterozygous for the 621 + 1G-->T and 711 + 1G-->T mutations. Total RNA isolated from the nasal epithelial cells and Epstein-Barr virus-transformed lymphoblasts derived from this patient was reversely transcribed and a region extending from exon 3 to exon 7 of the gene was amplified by the polymerase chain reaction and analyzed. Three abnormal products were identified, suggesting the presence of three aberrant transcripts, and their profiles were identical in both cell types. Two of the products were found to be missing either exon 4 or exon 5 as anticipated from the transcripts from the 621 + 1G-->T or 711 + 1G-->T alleles, respectively. The third product was apparently derived from an alternatively spliced mRNA species in the absence of the nominal splice site (in 621 + 1G-->T) through the use of a cryptic splice donor sequence (TT528/GTGAGG) within exon 4. Although reading frames appeared to be preserved in all three putative transcripts, significant portions of the presumed first and second transmembrane spans as well as the immediately following cytoplasmic domain would be deleted from the mutant CFTR polypeptides, if made. These observations are consistent with a loss of CFTR function in this cystic fibrosis patient.

Over the past few years, we have conducted a systematic study of 230 cystic fibrosis (CF) chromosomes in the Saguenay Lac-Saint-Jean (SLSJ) population which has a high CF incidence (1/936 live births). We identified 11 mutations accounting for 100% of the CF chromosomes found in patients born in SLSJ. Our results indicate that denaturing gradient gel electrophoresis (DGGE) is a powerful method of identifying CF mutations. They have also considerable implications for genetic counselling and molecular characterization of doubtful patients. They make carrier screening technically feasible in this population.