The present invention provides methods for high level, regulated
transgene transcription that is restricted to
cell populations of specific types. The process is designed to work with any inducible expression regulation systems, adapting them to a tissue-specific
expression pattern while simultaneously delivering maximal achievable expression levels. In particular, the invention utilizes
hybrid promoters that contain the
DNA elements for both
cell type-specific and regulated transcription. By placing the
gene of the transcriptional activation factor (TAF) under the control of this tissue-specific /
drug-regulated (TSDR)
promoter, this invention achieves high expression levels of TAF in specific target cells by first initiating TAF expression using
cell-
type specific transcription elements, and subsequently amplifying
transcriptional activity by establishing an autoregulatory
positive feedback loop. In non-target cells,
cell type-specific elements of the TSDR
promoter will be inactive, the TAF expression will not be initiated, and auto-upregulation will not occur. For
cell type-specific promoters with leaky low-
level activity in non-target cells, a variation of this
system has been developed which combines autologous upregulation of TAF with the expression of cross-competing transcriptional silencers (TSi) to achieve a type of eukaryotic “genetic switch”—either shutting off
transgene and TAF expression completely or promoting maximal expression levels, depending on the original
activity level of the specific
promoter in that particular cell.