Cell-specific imaging of reporter gene expression using a two-step transcriptional amplification strategy

INTRODUCTION The monitoring of reporter gene expression allows measurement of the location(s), magnitude, and time variation of gene transcription in living animals and humans. Several imaging modalities can be employed for repetitive, noninvasive monitoring of reporter gene expression. The most common methods include positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), and optical imaging by bioluminescence (e.g., Firefly luciferase, Fluc, or luc) or fluorescence (e.g., green fluorescent protein, GFP). The strengths of each imaging modality are reviewed in Chapters 1–4. Noninvasive imaging has been applied extensively to monitor gene therapy, to detect cell migration and metastasis, and finally to monitor endogenous gene expression (by the use of transgenic mice expressing a reporter gene). A common and successful means to target imaging of reporter gene expression to a particular tissue is to employ a transcriptional targeting strategy. Transcriptional targeting refers to the use of a cell-specific regulatory element (promoter or promoter/enhancer) to restrict gene expression to a particular tissue or cell type. A pitfall in using tissue- or tumor-specific promoters (TSPs) is that the relatively weak transcriptional activity of a cellular promoter could in principle greatly limit imaging sensitivity due to low levels of reporter gene expression in vivo. This contrasts with the potent but non-tissue–specific viral promoters like the simian virus 40 (SV40) early promoter or the cytomegalovirus (CMV) enhancer/promoter.