In vivo quantitative noninvasive imaging of gene transfer by single-photon emission computerized tomography

Systems aimed at detecting gene expression noninvasively in vivo are desirable for evaluating the outcome of gene transfer in clinical trials. Several approaches have been exploited using magnetic resonance imaging and spectroscopy (P-31 MRS), positron emission tomography (PET), single-photon emission tomography (SPECT), and detection of bioluminescent signals. An ideal system is based on transfer of a marker gene, the activity of which can be detected against a background from the target tissue without interfering with normal physiology or eliciting an immune response. The majority of approaches described to date use genes encoding a nonmammalian protein that can elicit immune responses or a transmembrane receptor as a marker gene whose ectopic expression may cause aberrant signaling in the target cell through binding to endogenous ligands. The dopamine transporter (DAT) is normally expressed at high levels, mainly in the dopaminergic neurons of the central nervous system. We previously synthesized a radioactive ligand, [Tc-99m]TRODAT-1, that binds with high affinity to the dopamine transporter, allowing for SPECT imaging of the striatum in normal control subjects and individuals affected with Parkinson's disease. Here we describe a strategy to monitor gene transfer based on adeno-associated viral vector (AAV)-mediated transduction of DAT in murine muscle followed by [Tc-99m]TRODAT-1 imaging by SPECT of cells expressing the transgene. We show that quantitative, noninvasive imaging of gene transfer is successfully achieved in vivo, using a single-photon computed tomography camera. This system employs a reporter gene encoding a mammalian protein that is absent in most tissues, has no enzymatic activity, and does not activate intracellular pathways. This should be useful to monitor gene transfer in the settings of gene therapy.