NEARBY STOP CODONS IN EXONS OF THE NEUROFIBROMATOSIS TYPE-1 GENE ARE DISPARATE SPLICE EFFECTORS

Stop mutations are known to disrupt gene function in different ways. T hey both give rise to truncated polypeptides because of the premature- termination codons (PTCs) and frequently affect the metabolism of the corresponding mRNAs. The analysis of neurofibromin transcripts from di fferent neurofibromatosis type 1 (NF1) patients revealed the skipping of exons containing PTCs. The phenomenon of exon skipping induced by n onsense mutations has been described for other disease genes, includin g the CFTR (cystic fibrosis transmembrance conductance regulator) gene and the fibrillin gene. We characterized several stop mutations local ized within a few base pairs in exons 7 and 37 and noticed complete sk ipping of either exon in some cases. Because skipping of exon 7 and of exon 37 does not lead to a frameshift, PTCs are avoided in that way. Nuclear-scanning mechanisms for PTCs have been postulated to trigger t he removal of the affected exons from the transcript. However, other s top mutations that we found in either NF1 exon did not lead to a skip, although they were localized within the same region. Calculations of minimum-free-energy structures of the respective regions suggest that both changes in the secondary structure of the mRNA and creation or di sruption of exonic sequences relevant for the splicing process might i n fact cause these different splice phenomena observed in the NF1 gene .