Reversibility of IVS 2 missplicing in a mutant human beta-globin gene.

We have studied the aberrant splicing of a human beta thalassemia globin gene by expression of the cloned gene in HeLa cells and oligomer-directed mutagenesis. A mutation 705 nucleotides into the large intervening sequence (IVS 2) of this gene leads to missplicing in which IVS 2 is incompletely removed, via two aberrant splices, from the vast majority of transcripts. One splice is from the 5' end of IVS 2 to a normal sequence 580 nucleotides into IVS 2 and another is from the mutated site 705 nucleotides into IVS 2 to the 3' end of the IVS. To study the splicing of this gene further, a mutation was introduced into the cryptic 3' splice site at position 580. This results in the complete removal of IVS 2 despite the presence of the thalassemia mutation at 705. The reversal of abnormal splicing by a change in the cryptic splice site suggests that the two abnormal splices are subtly interdependent. Thus, single base changes within IVS 2 can drastically alter the pattern of splicing in a human beta-globin gene.