MicroPET imaging of Cre–loxP-mediated conditional activation of a herpes simplex virus type 1 thymidine kinase reporter gene

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Abstract
Site-specific recombination tools such as the Cre–loxP system are used to create animal models where conditional gene deletion/activation studies are required. In the current proof of principle study, we have demonstrated that a PET reporter gene (PRG), the herpes simplex virus type 1 thymidine kinase (HSV1-tk), can be made to remain silent and can be activated by Cre–loxP-mediated recombination in cell culture and in living mice. An adenovirus carrying a silent HSV1-tk was tail-vein injected (1x109 PFU) in six transgenic mice that express Cre recombinase in their liver (Cre+) and in four control mice (Cre−). The liver-specific expression of the PRG in Cre+ mice was detected in the microPET following injection of the reporter probe, 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([18F]-FHBG). The [18F]-FHBG accumulation in the liver in terms of percent-injected dose per gram of tissue was 7.72±1.13 for the Cre+ mice and 0.10±0.02 for the Cre− mice (Pin vitro assays for herpes simplex virus type 1 thymidine kinase enzyme activity. Thus by using the Cre–loxP system it is possible to modulate a PRG and noninvasively monitor the extent of Cre–loxP-mediated gene activation by imaging in a microPET scanner.