QUANTITATIVE-ANALYSIS OF THE EXPRESSION OF A VIP TRANSGENE

We have analyzed the expression of a transgene bearing 2 kilobases of the 5' flanking region of the human vasoactive intestinal polypeptide (VIP) gene coupled to beta-galactosidase. Expression was assayed by be ta-galactosidase histochemistry and by mRNA quantitation using polymer ase chain reaction (PCR)-mediated amplification; we compared beta-gala ctosidase activity against both transgene and endogenous VIP mRNA leve ls. We found that the human 5' flanking sequence in this construct is able to direct tissue-specific expression of beta-galactosidase simila r to the pattern for endogenous VIP. However, the transgene is also ex pressed in smooth muscle and Schwann cells, where VIP mRNA is rare. In various tissues where the transgene and endogenous gene are both acti ve, the ratio between their message levels differs dramatically - tran sgene mRNA is more abundant where VIP is relatively scarce, but is muc h less abundant than the endogenous message at sites where VIP mRNA is most concentrated. These results suggest that sequence elements that may restrict VIP transcription or cause tissue-specific VIP mRNA accum ulation are missing from the transgene. In the testis there is a high level of transgene message but no significant beta-galactosidase activ ity; this discrepancy is caused by transcription from a cryptic promot er within the beta-galactosidase sequence.