Development 113, 305-310 (1991) Printed in Great Britain © The Company of Biologists Limited 1991 305 Expression of the cystic fibrosis gene in human development ANN HARRIS*, GILLIAN CHALKLEY, SHELLEY GOODMAN and LINDSAY COLEMAN Paedimric Research Unit, Division of Medical & Molecular Genetics, United Medical & Dental Schools of Guy's & St Thomas's Hospitals, 7th floor Guy's Tower, London Bridge, London SEl 9RT, UK * Author for correspondence: Paediatric Molecular Genetics, Institute of Molecular Medicine, John Radcliffe Hospital. Oxford OX3 9DU, UK Summary The specialised epithelia lining the respiratory tract, pancreatic ducts, male genital ducts and sweat gland ducts are defective in the severe inherited disease, cystic fibrosis (CF). We have looked at the expression of the CF gene in human fetal tissues to throw light on the development of function in specialised ductal epithelia and to determine the age of onset of the CF disease process. The CF gene is already seen to be transcribed in mid-trimester fetal lung, pancreas and male genital ducts. Hence, by this developmental stage, and before they are fully differen- tiated, these epithelia have the capability to perform important transport functions. Epithelial cell cultures derived from fetal pancreas and male genital ducts maintain expression of the CF gene in vitro and so form good models for analysing CF gene function and differentiation of these specialised epithelia. Key words: ductal epithelium, development, cystic fibrosis. Introduction Cystic fibrosis (CF) is the most common autosomal recessive disease in Caucasian races. The disease affects four main organ systems: in the sweat glands, the disease causes increased levels of sodium chloride in the sweat; in the lungs, poor mucus clearance results in recurrent lung infections; in the pancreas, blockage of pancreatic ducts with inspissated secretions causes gradual autolysis of the organ and acini are replaced by cystic spaces; in the male genital ducts, the vas deferens is usually absent and the epididymis structurally abnormal, causing sterility in 97% of affected males; this structural defect probably results from an early blockage of the vas deferens with secretory deposits. Though the CF gene has recently been cloned (Rommens et al. 1989; Riordan et al. 1989; Kerem et al. 1989), we do not yet know its exact function. From phenotypic characteristics it appears that the basic defect in CF affects the regulation of chloride ion transport. This abnormality is seen in the specialized epithelial cells that line the sweat gland duct, the airways and, presumably, the pancreatic and genital ducts (Quinton, 1983; Sato and Sato, 1984; Widdi- combe et al. 1985; Knowles et al. 1983; Welsh and Liedtke, 1986; Frizzell et al. 1986; Gray et al. 1989, 1990). The precise defect at the cellular level has not yet been elucidated. The protein product predicted from the cDNA sequence of the CF gene (Riordan et al. 1989) would appear to have 2 membrane-spanning domains, 2 nucleotide (ATP) binding folds and a cytoplasmic domain in which are located multiple potential sub- strate sites for protein kinase A and protein kinase C. How exactly this protein might influence the regulation of Cl~ movements across the apical membrane of these specialised epithelial cells remains unclear. However, it is possible that the cystic fibrosis transmembrane conductance regulator (CFTR) is not itself a chloride ion channel (Hyde et al. 1990). The tissue-specific expression of the CF gene postnatally has been shown to correlate well with the pathology of the disease. High levels of expression are seen in pancreas and nasal polyps, and lower levels in the lung, colon and sweat glands. Detection of the 6.5 kb CF gene message on northern blots of total RNA has not been possible for skin fibroblasts, lymphoblas- toid cell lines or peripheral blood lymphocytes (Rior- dan et al. 1989). There is already good evidence from the pathology of mid-trimester fetuses, terminated following a diagnosis of CF, that some tissues show certain features that are likely to represent the onset of later degenerative changes (Boue etal. 1986; Ornoy etal. 1987). However, to date the pattern of CF gene expression in human fetal tissues has not been examined. This question is of fundamental importance in understanding the disease process for the following reasons. First, if the CF gene is expressed in early fetal development then the effects of its mutation might well cause tissue and organ damage