Cystic fibrosis illustrates how our understanding of this disorder has changed through molecular genetics. What was previously a ''disease'' is now a syndrome because DNA-based knowledge has shown that a number of sporadic conditions are part of the cystic fibrosis spectrum. These include meconium ileus in the fetus, male infertility related to congenital bilateral absence of vas deferens (CBAVD), some forms of disseminated bronchiectasis, and chronic idiopathic pancreatitis. Evidence that the aforementioned are mild forms of cystic fibrosis has come from population studies in which it was shown that mutations in the cystic fibrosis gene known as CFTR (cystic fibrosis transmembrane conductance regulator) were more commonly found in the patient group compared with controls. For example, 13% of 134 patients with chronic pancreatitis had one CFTR mutation compared with the population frequency of 5%. In addition, 10% of the patients (twice the expected frequency) had 5T splicing abnormality, which is associated with impaired function of CFTR. A similar story is seen in disseminated bronchiectasis, with increased finding of 5T defect and a greater number of CFTR mutations compared with what would be expected in the normal population. In CBAVD, the 5T allele and a CFTR mutation (R117H) are often detected. In up to 50% of patients with CBAVD, it is possible to detect two CFTR mutations.
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