Compound heterozygosity for delta F508 and F508C: a cautionary note on the molecular diagnosis of cystic fibrosis

The role of the F508C mutation in congenital bilateral absence of the vas deferens

PURPOSE

Congenital bilateral absence of the vas deferens is a pathologic condition associated with normal spermatogenesis, azoospermia, and lack of both vasa deferentia. A significant association between mutations in the cystic fibrosis transmembrane conductance regulator gene among men with congenital bilateral absence of the vas deferens has been established. The objective of this study was to determine whether the F508C variant in the cystic fibrosis transmembrane conductance regulator gene has a significant effect on congenital bilateral absence of the vas deferens prevalence, when present in conjunction with a second cystic fibrosis transmembrane conductance regulator disease causing mutation.

METHODS AND RESULTS

We compared the frequency of F508C in male subjects submitted for diagnostic testing on suspicion of cystic fibrosis or during cystic fibrosis carrier screening, to men with a clinical diagnosis of congenital bilateral absence of the vas deferens. Although frequencies of F508C did not vary significantly between 850 individuals undergoing cystic fibrosis carrier screening and those submitted for diagnostic testing on suspicion of cystic fibrosis, the frequency of F508C in the congenital bilateral absence of the vas deferens population was significantly higher than expected (chi2 = 6.95, corrected P = 0.0486).

CONCLUSION

We conclude that the F508C variant in cystic fibrosis transmembrane conductance regulator may represent a pathogenic defect and lead to congenital bilateral absence of the vas deferens when combined with a second cystic fibrosis transmembrane conductance regulator mutation.

False-positive results of genetic testing in cystic fibrosis

We describe a patient in whom newborn immunoreactive trypsin screening and mutation analysis suggested a diagnosis of cystic fibrosis; however, the clinical course and sweat test results were not consistent with the diagnosis. Direct sequencing of the patient's genomic DNA showed compound heterozygosity for delta F508 and F508C, a polymorphism not associated with clinical disease.