Cystic Fibrosis Gene Mutation in Two Sisters with Mild Disease and Normal Sweat Electrolyte Levels

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Abstract
CYSTIC fibrosis is the most common fatal recessive disease in whites. In the past the diagnosis has been based on the presence of lung disease, pancreatic exocrine insufficiency, and increased sweat electrolyte concentrations.1 Patients with cystic fibrosis have well-defined abnormalities in epithelial tissue, including defective cyclic AMP—mediated regulation of chloride channels and an increased transepithelial potential difference, which reflects increased sodium absorption and decreased chloride permeability.2 3 4 5 6 The raised sweat chloride values that result from impaired reabsorption of salt in the sweat ducts have been exploited in the pilocarpine iontophoresis test, the cornerstone of diagnosis since 1959.7 Although the vast majority of patients with cystic fibrosis have elevated sweat electrolyte concentrations, some patients have symptoms8 and epithelial bioelectric properties9 consistent with cystic fibrosis, but normal sweat electrolyte concentrations. Until recently, it was impossible to determine whether these patients had a mild form of cystic fibrosis or a genetically distinct disease.