Alternative splicing attenuates transgenic expression directed by the apolipoprotein e promoter-enhancer based expression vector pLIV11

The plasmid vector pLIV11 is used commonly to achieve liver-specifi c expression of genes of interest in transgenic mice and rabbits. Expression is driven by the human apolipoprotein (apo)E 5 ' proximal promoter, which includes 5 kb of upstream sequence, exon 1, intron 1, and 5 bp of exon 2. A 3.8 kb 3 ' hepatic control region, derived from a region 18 kb downstream of the apoE gene, enhances liver-specifi c expression. Here, we report that cDNA sequences inserted into the multiple cloning site (MCS) of pLIV11, which is positioned just downstream of truncated exon 2, can cause exon 2 skipping. Hence, splicing is displaced to downstream cryptic 3 ' splice acceptor sites causing deletion of cloned 5 ' untranslated mRNA sequences and, in some cases, deletion of the 5 ' end of an open reading frame. To prevent use of cryptic splice sites, the pLIV11 vector was modifi ed with an engineered 3 ' splice acceptor site inserted immediately downstream of truncated apoE exon 2. Presence of this sequence fully shifted splicing of exon 1 from the native intron 1-exon 2 splice acceptor site to the engineered site. This fi nding confi rmed that sequences inserted into the MCS of the vector pLIV11 can affect exon 2 recognition and provides a strategy to protect cloned sequences from alternative splicing and possible attenuation of transgenic expression.