Effect of genotype on phenotype and mortality in cystic fibrosis: a retrospective cohort study

Background Over 1000 mutations of the cystic fibrosis transmembrane conductance regulator gene (CFTR) that cause cystic fibrosis have been identified. We examined the effect of CFTR genotype on mortality and disease phenotype. Methods Using the US Cystic Fibrosis Foundation National Registry, we did a retrospective cohort study to compare standardised mortality rates for the 11 most common genotypes heterozygous for ΔF508 with those homozygous for ΔF508. Of the 28455 patients enrolled in the registry at the time of our analysis, 17 853 (63%) were genotyped. We also compared the clinical phenotype, including lung function, age at diagnosis, and nutritional measures, of 22 ΔF508 heterozygous genotypes. Mortality rates and clinical phenotype were also compared between genotypes classified into six classes on the basis of their functional effect on CFTR production. Findings Between 1991 and 1999, genetic and clinical data were available for 17 853 patients with cystic fibrosis, which was 63% of the total cohort. There were 1547 deaths during the 9 years of follow-up. In the analysis of the 11 most common genotypes, ΔF508/R117H, Δ F508/AI507, Δ F508/3849+10 kbC←T, and ΔF508/2789+5G←A had a significantly lower mortality rate (4•7, 8•0, 11•9, and 4•4, respectively) than the genotype homozygous for ΔF508 (21•8, p=0•0060). ΔF508/R117H, ΔF508/AI507, ΔF508/ 3849+10 kbC←T, ΔF508/2789+5G←A, and ΔF508/A455E have a milder clinical phenotype. Outcomes for all functional classes were compared with that of class II (containing δF508 homozygotes) and classes IV and V had a significantly lower mortality rate and milder clinical phenotype. Interpretation Patients with cystic fibrosis have distinct genetic subgroups that are associated with mild clinical manifestations and low mortality. These differences in phenotype are also related to the functional classification of CFTR genotype.