Indirect monitoring of endogenous gene expression by positron emission tomography (PET) imaging of reporter gene expression in transgenic mice.

PURPOSE Repetitive imaging with microPET of endogenous albumin gene expression by using transgenic mice in which the Herpes Simplex Virus Type 1 thymidine kinase (HSV1-tk) reporter gene is driven by the albumin promoter (AL-HSV1-tk). METHODS Transgenic mice were imaged repeatedly on a microPET scanner with approximately 200 microCi of 9-[4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine (FHBG) (a substrate for HSV1-TK enzyme). Four transgenic mice were monitored for body weight, serum albumin, and imaged at the end of each of three dietary phases (17%, 0%, and 25% protein diet). Each phase last 14-21 days. The 0% protein diet has been reported previously to reduce albumin gene expression in rats. Twenty non-transgenic mice of the same strain followed a similar feeding schedule and were monitored for serum albumin, body weight, and sacrificed at various time points for determination of their GAPDH normalized albumin mRNA levels. RESULTS Transgenic mice showed a relatively high FHBG signal from the liver region as expected. Variation of the mean FHBG signal in two mice with a fixed 17% protein diet over a four-month period was <19% s.d. The mean +/- s.e. FHBG liver standardized uptake value (SUV) in four transgenics went from 4.49 +/- 0.32 to 2.17 +/- 0.52 to 6.21 +/- 0.72 as the mice went through the three diets of 17%, 0%, and 25% sequentially. Non-transgenic mice showed GAPDH normalized albumin mRNA that went from 37.68 +/- 6.04 to 26.41 +/- 4.29 to 52.42 +/- 4.09. The FHBG SUV from transgenics was well correlated with GAPDH normalized albumin mRNA from non-transgenics (r(2) = 0.97) supporting that endogenous gene expression of albumin can be indirectly imaged with FHBG. CONCLUSION Measuring correlated changes in albumin expression in wild type mice and HSV1-TK expression by microPET in transgenic mice in which the reporter gene is driven by the albumin promoter demonstrates that the HSV1-tk gene can be used to monitor, in living animals, modulated expression of transgenes.

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