Chapter 259

Cystic fibrosis (CF) is a monogenic disorder that presents as a multisystem disease. The first signs and symptoms typically occur in childhood, but about 5% of patients in the United States are diagnosed as adults. Due to improvements in therapy, >46% of patients are now adults (≥18 years old) and 16.4% are past the age of 30. The median survival is >37.4 years for patients with CF; thus, CF is no longer only a pediatric disease, and internists must be prepared to recognize and treat its many complications. CF is characterized by chronic bacterial infection of the airways that leads to bronchiectasis and bronchiolectasis, exocrine pancreatic insufficiency and intestinal dysfunction, abnormal sweat gland function, and urogenital dysfunction.

### Genetic Considerations

CF is an autosomal recessive disease resulting from mutations in the CFTRgene located on chromosome 7. The mutations in the CFTR gene fall into five major classes, as depicted in Fig. 259-1. Classes I–III mutations are considered "severe," as indexed by pancreatic insufficiency and high sweat NaCl values (see below). Class IV and V mutations can be "mild," i.e., associated with pancreatic sufficiency and intermediate/normal sweat NaCl values.

###### Figure 259-1

Schema describing classes of genetic mutations in CFTR gene and effects on CFTR protein/function. Note the ΔF508 mutation is a class II mutation and, like class I mutations, would be predicted to produce no mature CFTR protein in the apical membrane. CFTR, cystic fibrosis transmembrane conductance regulator.

The prevalence of CF varies with the ethnic origin of a population. CF is detected in approximately 1 in 3000 live births in the Caucasian population of North America and northern Europe, 1 in 17,000 live births of African Americans, and 1 in 90,000 live births of the Asian population of Hawaii. The most common mutation in the CFTRgene (~70% of CF chromosomes) is a 3-bp deletion (a class II mutation) that results in an absence of phenylalanine at amino acid position 508 (ΔF508) of the CF gene protein product, known as cystic fibrosis transmembrane conductance regulator (CFTR). The large number (>1500) of relatively uncommon (<2% each) mutations identified in the CFTR gene makes genetic testing challenging.

### CFTR Protein

The CFTR protein is a single polypeptide chain, containing 1480 amino acids, that functions both as a cyclic AMP–regulated Cl channel and a regulator of other ion channels. The fully processed form of CFTR localizes to the plasma membrane in normal epithelia. Biochemical studies indicate that the ΔF508 mutation leads to improper maturation and intracellular degradation of the mutant CFTR protein. Thus, absence of CFTR in the plasma membrane is central to the molecular pathophysiology of the ΔF508 mutation and other classes I–II mutations. Classes III–IV mutations produce CFTR proteins that are fully processed but are nonfunctional or only partially functional ...

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