Quantitative imaging of gene induction in living animals

Methods to repeatedly, non-invasively, and quantitatively image gene expres sion in living animals are rapidly emerging and should fundamentally change studies of gene expression in vivo. We previously developed assays utilizi ng positron emission tomography (PET) to image reporter gene expression. In this paper we: (1) describe a new bi-directional, tetracycline-inducible s ystem that can be used to pharmacologically induce target gene expression a nd to quantitatively image induced expression by using a PET reporter gene; (2) demonstrate the potential of this system in transient and stable cell transfection assays; and (3) demonstrate the ability to repetitively and qu antitatively image tetracycline and tetracycline analog induction of gene e xpression in living animals. We utilize the dopamine type-2 receptor (D2R) and the mutant herpes-simplex virus type 1 thymidine kinase (HSV1-sr39tk) r eporter genes to validate this system. We utilize microPET technology to sh ow that quantitative tomographic imaging of gene induction is possible. We find a high correlation (r(2) = 0.98) between 'target' and reporter gene ex pression. This work establishes a new technique for imaging time-dependent variation of gene expression both from vectors with inducible promoters and in transgenic animals in which pharmacologic induction of gene expression must be monitored. These techniques may be applied both in gene therapy and for the study of gene expression in transgenic animals.