Genetic Modifiers of Cystic Fibrosis: Sibling Study

BACKGROUND:

CF is a highly variable but inevitably fatal single gene disorder. Several lines of evidence
suggest that genetic background contributes to the variability of cystic fibrosis
phenotypes. The study will develop CF as a model for the identification of modifier genes by
capitalizing on the availability of a large motivated population of affected twins and
siblings.

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.

DESIGN NARRATIVE:

The study has four aims: 1. To identify heritable CF phenotypes by twin study. Intrapair and
interpair variance will be determined for selected CF phenotypes, and interclass
correlations (monozygotic versus dizygotic) will be performed to identify CF phenotypes with
a substantial heritable component. 2. To determine the contribution of genetic and other
factors to the variability of CF phenotypes by analysis of affected sibs. Variance component
methods will be used to evaluate the CF phenotypes that appear to be heritable based upon
other studies or the results of aim 1. 3. To identify biologic phenotypes that correlate
with heritable CF phenotypes by clinical study of twins and sibs. Multivariate analysis will
be used to find biologic phenotypes associated with CF phenotypes. 4. To identify modifier
genes and loci responsible for heritable CF phenotypes by linkage approaches. Identity by
descent and transmission disequilibrium methods will be used to test linkage between
candidate genes/loci and heritable CF phenotypes. To identify novel loci, genome-wide scans
will be performed upon sib pairs selected for extreme concordance or discordance for
heritable traits.