Positron emission tomography imaging of conditional gene activation in the heart

The Cre-loxP system has been routinely used for conditional activation and deletion of gene expression. However, the spatiotemporal manner of these events in the heart has not yet been defined by in vivo imaging. Adenovirus (1 × 109 pfu) carrying the silent positron emission tomography (PET) reporter gene, herpes simplex virus type 1 thymidine kinase (HSV1-tk), was injected into the left ventricular wall of male transgenic mice (n = 15) or FVB controls (n = 8). Transgenic mice expressed Cre recombinase driven by a cardiac-specific α-myosin heavy chain (α-MHC) promoter. Following injection of the 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([18F]-FHBG; 137 ± 25 μCi) reporter probe, microPET imaging was used to assess the expression of HSV1-tk reporter gene in the myocardium. Two days following adenoviral injection, cardiac HSV1-tk gene activation resulted in tracer uptake of 3.20 ± 0.51%ID/g for α-MHC-Cre and 0.05 ± 0.02%ID/g for control mice (P < 0.01). The in vivo results were confirmed by RT-PCR and Western blot analysis. Similar transfections were evaluated in both cardiac-specific and non-cardiac-specific cell lines. Enzyme activity showed a robust correlation (r2 = 0.82) between in vivo molecular imaging technique and traditional in vitro enzyme assays. With further development and validation, PET imaging will likely play an important role in the noninvasive, repetitive, and quantitative measurement of conditional gene activation in the future.