A Model for Epigenetic Inhibition via Transvection in the Mouse

Transvection is broadly defined as the ability of one locus to affect its homologous locus in trans. Although it was first discovered in the 1950s, there are only two known cases in mammals. Here, we report another instance of mammalian transvection induced by the Cre/LoxP system, which is widely used for conditional gene targeting in the mouse. We attempted to use the germline-expressed Vasa-Cre transgene to engineer a mouse mutation, but observe a dramatic reduction of LoxP recombination in mice that inherit an already deleted LoxP allele in trans. A similar phenomenon has previously been observed with another Cre that is expressed during meiosis: Sycp-1-Cre. This second example of LoxP inhibition in trans reinforces the conclusion that certain meiotically expressed Cre alleles can initiate transvection in mammals. However, unlike the previous example, we find that the inhibition of LoxP recombination is not due to DNA methylation. In addition, we demonstrate that LoxP inhibition is easily alleviated by adding an extra generation to our crossing scheme. This finding confirms that the LoxP sites are inhibited via an epigenetic mechanism, and provides a method for the use of other Cre transgenes associated with a similar LoxP inhibition event. Furthermore, the abrogation of LoxP inhibition by the simple addition of an extra generation in our crosses establishes a unique mouse system for future studies to uncover the mechanism of transvection in mammals.