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A 9-month-old girl presents to her pediatrician for persistent cough, failure to gain weight and a bulging mass from her rectum. Her mother reports that the girl has had two episodes of “pneumonia” requiring hospitalization at the age of 3 and 5 months. Since that time, she has not gained much weight and is noted to be at the 10th percentile for weight and length. On examination, the patient has course breath sounds and wheezes throughout the lung fields, and has rectal prolapse (Figure 51-1). The pediatrician suspects cystic fibrosis and orders a sweat chloride test, which is 120 mEq/L. This confirms the suspected diagnosis, as a result greater than 60 mEq/L is diagnostic for cystic fibrosis. The family is referred to a comprehensive cystic fibrosis center.
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Cystic fibrosis (CF) is an autosomal recessive disorder caused by a mutation in the CF transmembrane conductance regular gene that alters the composition of mucus secreted in the lungs, pancreas, sweat glands, digestive tract, and vas deferens. This leads to obstructive lung disease and pancreatic insufficiency leading to malabsorption and malnutrition in affected children.
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CF, fibrocystic disease of pancreas, mucoviscidosis.
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Incidence of CF is 1 in 3200 newborns in the US.1
CF is the most common fatal inherited disorder among Caucasians.
Current median life expectancy of patients diagnosed with CF in the US is between 30 to 40 years.2
CF is most common among Caucasians; 4 to 5 percent of Caucasians in North America are heterozygous for CF.1,3
CF incidence among Hispanics ranges from 1:9200 to 1:13500 individuals.4
CF has an incidence of 1:15,000 in African Americans and 1:31,000 in Asian Americans.3
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Etiology and Pathophysiology
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CF is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene on chromosome 7.
The CFTR gene is a 250 kb, 27 exon gene encoding an ATP binding cassette transporter found on the apical surface of mucosal epithelial cells. This protein is responsible for regulating chloride entrance into mucosal cells.5
Mutations in CFTR are divided into several classes:3,5
Class 1 mutations—Premature transcription termination signal leading to a defective protein.
Class 2 mutations—Protein misfolding leading to premature degradation of CFTR and absence of CFTR expression at the apical surface of mucosal epithelial cells.
Class 3 mutations—Defective regulation of CFTR at the apical surface despite intact ability to traffic CFTR to the cell surface.
Class 4 mutations—Defective CFTR channel conductance of chloride.
Class 5 mutations—Decreased synthesis of functional CFTR due to splicing abnormalities.
Class 6 ...