A G-]A TRANSITION CREATES A BRANCH POINT SEQUENCE AND ACTIVATION OF ACRYPTIC EXON, RESULTING IN THE HEREDITARY DISORDER NEUROFIBROMATOSIS-2

We describe a G-->A transition within intron 5 of the NF2 gene. This m utation creates a consensus splice branch point sequence. To our knowl edge this is the first report of a mutation that creates a functional branch point sequence in a human hereditary disorder. The new branch p oint sequence is located 18 bp upstream of a consensus splice acceptor site. A consensus splice donor site is found 106 bp 3' of the accepto r site. As a consequence the G-->A transition results in an alternativ ely spliced mRNA containing an additional exon 5a of 106 bp derived fr om intron sequences. We cloned the mutant cDNA and show that due to an in-frame stop codon the cDNA codes for a truncated NF2 protein. The m utation was observed in three affected members of an NF2 family. In a tumour of one of the family members both alternatively spliced and wil d-type mRNA were found, although the wild-type allele of the gene is a bsent due to an interstitial deletion on chromosome 22. We also show t hat immunoprecipitations reveal the presence of full-length wildtype N F2 protein in the tumour lysate. These data support the hypothesis tha t some degree of normal splicing of the mutant precursor RNA is taking place. It is therefore likely that this residual activity of the muta nt allele explains the relatively mild phenotype in the family. These data also indicate that complete inactivation of the gene is not requi red for tumour formation.