In a procedure called ex vivo gene therapy, cells from the target tissue are removed and grown in vitro where they can be infected with the altered retro viral vector. The transgenic cells are then transplanted back into the patient where they express the desired gene product. Longer gene expression has been observed in other systems, and the solution may involve finding the correct enhancer-promoter combination for a given cell type. Lentiviruses are viruses that belong to the retrovirus family, but can infect both dividing and non-dividing cells. Lentiviruses can be manipulated in a similar fashion to the retroviruses, and appear to sustain expression for over six-months.
Adenoviruses are a family of DNA viruses that can infect both dividing and non-dividing cells.
The replication process of these viruses can be disabled and the genome manipulated so that the virus expresses a desirable gene much like the vectors that have been discussed previously. However, in contrast to the retroviruses the Adenoviruses can infect cells in vivo and express the trans gene products at high levels. Moreover, a large percentage of the human population already has antibodies for Adenoviruses because of prior naturally occurring infections. One final family of viruses that warrants mention as potential vectors in gene therapy is the Adeno-associated viruses. Many of the trials have resulted in short term improvement in patients receiving treatments, but effects have not been sustained.