LONG-TERM SURVIVAL OF THE EXON-10 INSERTIONAL CYSTIC-FIBROSIS MUTANT MOUSE IS A CONSEQUENCE OF LOW-LEVEL RESIDUAL WILD-TYPE CFTR GENE-EXPRESSION

Recently we have created a mouse model of cystic fibrosis (CF) by inse rtional gene targeting to exon 10. In common with CF subjects, this mo del displays a low incidence of meconium ileus. This contrasts strikin gly with the very high level of fatal intestinal obstruction in the th ree other CF mouse models so far described. We investigate here the mo lecular basis of this difference in phenotype. We show that the partia l duplication consequent upon insertional gene targeting allows exon s kipping and aberrant splicing to produce normal Cftr mRNA, but at leve ls greatly reduced compared with wild-type mice. Furthermore, instead of the predicted mutant Cftr transcript, a novel mRNA is produced that utilizes cryptic splice sites in the disrupting plasmid sequence. How ever, we have previously shown that these mice display the ion transpo rt defect characteristic of CF, and mutant animals can be distinguishe d from their normal littermates on this basis. Consistent with this, r esidual CFTR function has recently been observed for several ''mild'' mutations in CF individuals who display pancreatic sufficiency but sti ll develop lung disease. We conclude that (i) residual wild-type mRNA in the exon 10 insertional mutant mouse ameliorates the severity of th e intestinal phenotype observed in the absolute ''null'' CF mice, (ii) the presence of low-level residual wild-type Cftr mRNA does not corre ct the CF ion transport defect, and (iii) the long-term survival of th is insertional mutant mouse provides the opportunity to address the fa ctors important in development of lung disease.