Redefinition of exon 7 in the COL1A1 gene of type I collagen by an intron 8 splice-donor-site mutation in a form of osteogenesis imperfecta:: Influence of intron splice order on outcome of splice-site mutation

Most splice-site mutations lead to a limited array of products, including exon skipping, use of cryptic splice-acceptor or -donor sites, and intron inclusion. At the intron 8 splice-donor site of the COL1A1 gene, we identified a G+1-->A transition that resulted in the production of several splice products from the mutant allele. These included one in which the upstream exon 7 was extended by 96 nt, others in which either intron 8 or introns 7 and 8 were retained, one in which exon 8 was skipped, and one that used a cryptic donor site in exon 8. To determine the mechanism by which exon-7 redefinition might occur, we examined the order of intron removal in the region of the mutation by using intron/exon primer pairs to amplify regions of the precursor nuclear mRNA between exon 5 and exon 10. Removal of introns 5, 6, and 9 was rapid. Removal of intron 8 usually preceded removal of intron 7 in the normal gene, although, in a small proportion of copies, the order was reversed. The proportion of abnormal products suggested that exon 7 redefinition, intron 7 plus intron 8 inclusion, and exon 8 skipping all represented products of the impaired rapid pathway, whereas the intron-8 inclusion product resulted from use of the slow intron 7-first pathway. The very low-abundance cryptic exon 8 donor site product could have arisen from either pathway. These results suggest that there is commitment of the pre-mRNA to the two pathways, independent of the presence of the mutation, and that the order and rate of intron removal are important determinants of the outcome of splice-site mutations and may explain some unusual alterations.