Phenotypic variability in cystic fibrosis

Cystic fibrosis (CF) is among the best studied and most common autosomal recessive disorders. Although CF is often described as a simple Mendelian trait, the clinical variability seen among patients hints at genetic or environmental modifiers. In particular, while CFTR, the chloride channel missing in affected individuals, is normally expressed in a number of epithelia in addition to the lung, not all patients with lung disease develop symptoms in the intestine or elsewhere. Some of the phenotypic variability results from the presence of different disease alleles, but, as Bronsveld and colleagues note, a great deal of variability persists even among people carrying a single allele in homozygous form. The authors worked with sets of affected twins and other sib pairs in which both siblings were homozygous for the most common CFTR allele, DF508. This mutation causes the CFTR to be retained and degraded intracellularly, but some molecules apparently reach the cell surface and may explain the ?leakiness? of the phenotype in certain individuals? tissues. Electrophysiological measurements in the respiratory tract and in intestinal biopsies confirm that high residual channel activity characteristic of CFTR is a reasonable predictor of mild disease. Such residual activity is often concordant between identical twins but less so between other siblings, implying that this cellular feature is under independent genetic control and that genes other than CFTR influence the clinical presentation of CF.

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