Germ cell specific promoter drives ectopic transgene expression during embryogenesis

In this study, we used the male germ cell-specific phosphoglycerate kinase 2 (Pgk2) promoter to generate Pgk2Cre transgenic mice to allow investigation of genes critically involved in meiosis. The Pgk2 promoter had been used previously to target transgene expression to spermatocytes and spermatids in several laboratories including ours. In several Cre targeting experiments using other promoters, ectopic Cre expression had been observed, but the timing and extent of this expression was not analyzed. We demonstrate that in adult mice the Pgk2Cre transgene is expressed specifically in spermatocytes and spermatids, as expected. However, in offspring from matings of Pgk2Cre mice and an H19loxP indicator strain, we discovered that recombination events had occurred in several, but not all, tissues to varying extents. The lacZ-loxP transgenic indicator strain was next used to uncover ectopic Cre expression even in single cells, which indicated that the Pgk2Cre transgene is expressed between days 11 and 15 during embryogenesis in several tissues and organs. Using an RT PCR assay we were unable to detect endogenous Pgk2 mRNA during embryogenesis or in adult tissues other than testis. In conclusion, the Pgk2 promoter is a valid choice for targeting gene expression to meiotic male germ cells, since transient ectopic expression is unlikely to have a discernable effect in most studies, but it may be inappropriate for utilization with Cre recombinase.