Incomplete rescue of cystic fibrosis transmembrane conductance regulator deficient mice by the human CFTR cDNA

We have used a mouse model to study the ability of human CFTR to correctthe defect in mice deficient of the endogenous protein. In this model,expression of the endogenous Cftr gene was disrupted and replaced with ahuman CFTR cDNA by a gene targeted 'knock-in' event. Animals homozygous forthe gene replacement failed to show neither improved intestinal pathologynor survival when compared to mice completely lacking CFTR. RNA analysesshowed that the human CFTR sequence was transcribed from the targetedallele in the respiratory and intestinal epithelial cells. Furthermore, invivo potential difference measurements showed that basal CFTR chloridechannel activity was present in the apical membranes of both nasal andrectal epithelial cells in all homozygous knock-in animals examined. Ussingchamber studies showed, however, that the cAMP-mediated chloride channelfunction was impaired in the intestinal tract among the majority ofhomozygous knock-in animals. Hence, failure to correct the intestinalpathology associated with loss of endogenous CFTR was related toinefficient functional expression of the human protein in mice. Theseresults emphasize the need to understand the tissue- specific expressionand regulation of CFTR function when animal models are used in gene therapystudies.