EXON-9 OF THE CFTR GENE - SPLICE-SITE HAPLOTYPES AND CYSTIC-FIBROSIS MUTATIONS

The alternatively spliced exon 9 of the cystic fibrosis transmembrane conductance regulator (CFTR) gene codes for the initial part of the am ino-terminal nucleotide-binding fold of CFTR. A unique feature of the acceptor splice site preceding this exon is a variable length polymorp hism within the polypyrimidine tract influencing the extent of exon 9 skipping in CFTR mRNA. We investigated this repeat for its relationshi p to CFTR mutations and intragenic markers on 200 chromosomes from Ger man patients with cystic fibrosis (CF). Four frequent length variation s were strongly associated with the four predominant haplotypes previo usly defined by intragenic marker dimorphisms. One of these alleles di splayed absolute linkage disequilibrium to the major CF mutation Delta F508. Other frequent CFTR mutations were linked to one particular spl ice site haplotype indicating that differential exon 9 skipping contri butes little to the clinical heterogeneity among CF patients with an i dentical mutation. We also identified a novel missense mutation (V456F ) and a novel nonsense mutation (Q414X) within the coding region of ex on 9. The missense mutation V456F adjacent to Walker motif A was prese nt in a pancreas-sufficient CF patient. In contrast, the pancreas-insu fficient Q414X/Delta F508 compound heterozygote suffered from a severe form of the disease, indicating that alternative splicing of exon 9 d oes not overcome the deleterious effect of a stop codon within this ex on.