The purpose of this study is to examine genetic modifiers of the severity of cystic fibrosis
Cystic Fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance
regulator (CFTR) gene resulting in impaired chloride transport across epithelial cells. While
many organs are involved, infection, inflammation and destruction of the lungs ultimately
result in morbidity and mortality. There is an association between residual CFTR function and
severity of disease, however there is great variability within specific mutations suggesting
gene modifiers. Even though there are over 900 mutations in CFTR that are related to CF lung
disease, F508 the most common one is represented in 70 percent of the American CF population.
Thus, establishing a phenotype/genotype correlation using homozygote F508 patients is likely
to identify genes that are responsible for a mild form of disease. Why is this important?
Whereas since the identification of the gene CFTR a significant amount of knowledge has been
accumulated on CFTR function and CF pathogenesis, the cure for CF (treated as a monogenic
disease) has been elusive. Identification of genetic modifiers (that may explain why 10
percent of CF patients died before the age of 10, 1/3 before the age of 20 while 50 percent
live over 32 years of age) should expand the therapeutic targets that may lead to shifting of
the severe phenotypes to milder ones. Moreover, the approach outlined in this study may also
result in a better understanding of CFTR and delta F508 biogenesis and function, as it may
identify genes directly related to CFTR.
The study is in response to a Request for Applications titled "Genetic Modifiers of Single
Gene Defect Diseases" released in August, 2000 and co-sponsored by the National Institute of
Diabetes, Digestive, and Kidney Diseases.
Patients with cystic fibrosis (CF) display a wide range of disease severity, particularly in
pulmonary phenotype. Although some of this variability can be attributed to specific
mutations within the CFTR gene (allelic heterogeneity), much of this variability has not been
adequately explained. The central hypothesis of the study is that much of the "severity" (or
"mildness") of CF lung disease reflects the influence of non-CFTR "modifier" alleles (genes).
The study is designed to identify associations between non-CFTR genes and the pulmonary
phenotype. To accomplish this goal, studies will be conducted on 600 CF patients who have the
same CFTR genetic background, i.e., homozygous deltaF508, and who are at the extremes of
pulmonary phenotype, i.e., the most severe and mildest lung disease. Pulmonary disease
severity (or mildness) will be quantitated by longitudinal lung function analysis with
informative censoring. The overall strategy will be to test for the association of candidate
modifier alleles (genes) with the severity (or mildness) of pulmonary disease. Key clinical
features (gender; age-at-diagnosis; sweat chloride; nutrition; and respiratory microbiology)
will be important variables in the overall analysis. Initially, the study will test candidate
genes (n=200) that have been implicated in the pathophysiology of CF lung disease. A pooling
strategy will be used to expedite the first rounds of testing. After pooling DNA from the
"severe" patients, and pooling DNA from the mild patients, those genes (alleles) can be
identified with the greatest association with phenotype. Follow-up genotyping in individual
subjects will allow subgroup analyses (gender; age-at-diagnosis; nutrition; respiratory
microbiology) for each gene, as well as more complex analyses to search for interaction among
different alleles. Subsequent studies will involve genome-wide testing with single nucleotide
polymorphisms (SNPs) to identify loci (and genes) that are not present in the initial list of
candidate genes. Identification of genes that modulate the severity of the pulmonary
phenotype will improve understanding of the pathophysiology of CF lung disease, and identify
new targets for therapeutic intervention.
- Diagnosed with CF