SEQUENCES CONTAINING THE 2ND-INTRON ENHANCER ARE ESSENTIAL FOR TRANSCRIPTION OF THE HUMAN APOLIPOPROTEIN-B GENE IN THE LIVERS OF TRANSGENICMICE

To identify DNA sequence elements from the human apolipoprotein B (apo B) gene required for high-level, correct tissue-specific expression in transgenic mice, we made several constructs that included one or more of the key regulatory elements that were previously characterized wit h cultured liver-derived and intestine-derived cell lines. Our data sh ow that the apoB promoter alone (-898 to +121) is not sufficient to di rect transcription in transgenic mice. An enhancer located in the seco nd intron is absolutely required to specify transcription by the homol ogous apoB promoter in the livers of transgenic mice; this enhancer do es not direct transcription in the small intestines. Thus, the element s controlling transcriptional activation of the apoB gene in the liver and the intestine in vivo are distinct and separable. Analysis of the DNase I hypersensitivity of the integrated human transgenes in variou s lines of expressing and nonexpressing mice suggests that the formati on of DH4, a strong hypersensitive site in intron 2, may be a prerequi site for hepatic expression of the apoB gene. Nuclear matrix associati on regions (MARs) of the apoB gene may play a role in transgene expres sion. Constructs including MAR sequences displayed higher levels of ex pression than those lacking them. However, these MARs did not complete ly insulate the associated transgenes from position effects.