Coexpression after electroporation of plasmid mixtures into muscle in vivo

Muscle is perhaps the most frequently considered tissue for non-viral gene therapy, in particular after gene transfer by electroporation. Expression in muscle is stable, but since the cell turnover is so slow incorporation in the host genome is not required. This raises interesting practical and theoretical questions related to the behaviour of the transgenic DNA under such conditions.

We have investigated expression of reporter genes from plasmid mixtures electroporated into the extensor digitorum longus (EDL) muscle in mice in order to assess the degree of coexpression.

Under conditions where the reporter is easily identified the coexpression rate was 100%, as none of 287 fibres from five different muscles expressing blue fluorescent protein (BFP) failed to express green fluorescent protein (GFP). With other reporter combinations the rate was lower, but this we attribute to marginal sensitivity for fluorescent proteins, or from reporter protein degradation for beta-galactosidase.

The high degree of coexpression suggests that a large copy number takes part in the final transcription with this system. The finding also enhances the usefulness of muscle and electroporation for gene therapy and experimental biology.