The variability in activity of the universally expressed human cytomegalovirus immediate early gene 1 enhancer/promoter in transgenic mice

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Abstract
Transcriptional control regions which direct transgene expression to all tissues in transgenic animals can be useful tools for gain-of-function experiments in transgenic animals. A candidate for this purpose is the regulatory region of the human cytomegalovirus immediate early 1 gene (HCMVIE1) which is highly expressed in many lines of tissue culture cells. Here we analyzed the activity of the HCMVIE1 enhancer/promoter using a sensitive reporter gene, the bacterial chloramphenicol acetyltransferase (CAT) gene. Three lines of transgenic mice with an intact transgene were established. All 3 lines expressed the transgene in all 28 tissues analyzed; however, levels of expression between the three lines varied up to 100,000 fold. In addition, expression levels in the high expressing line varied over a 10,000 fold continuum, while expression levels between tissues was almost uniform in the lowest expressing line. The transgene was well expressed in the high expressing line; CAT activity in the highest expressing tissues was equivalent to levels previously reported for tissue specific CAT transgenes active only in a limited number of tissues. These data support the utilization of the HCMVIE1 enhancer/promoter as a means of expressing a transgene in all tissues, but indicate that lines with substantially different overall levels of expression may be generated, and that markedly different levels of tissue specific expression may be found when the overall level of transgene expression is high.