Herpes viral vectors carrying complete genes ensure long-term gene expression persistence in animal models of gene therapy trials.

Press release: Herpes viral vectors carrying complete genes ensure long-term gene expression persistence in animal models of gene therapy trials.

Some hereditary diseases are caused by "recessive" mutations which cause either a decrease in the amount of a protein or the production of a defective protein, which is unable to properly perform its function. Gene therapy aims to cure such diseases through the introduction of "healthy" genes allowing the functional replacement of the "defective" genes. In order to introduce such healthy genes into the cells affected by the disease, it is necessary to use appropriate carriers, known as "vectors", which, in most cases are viruses devoid of their more pathogenic components.

The success of this type of gene therapy depends on both the proper distribution of the healthy gene to a sufficient number of affected cells and the persistence of the expression of the "healthy" gene in these cells. To ensure a long-term expression of the “healthy” gene has been a goal more difficult to attain than was anticipated. Thus, on many occasions, the "healthy" gene that had been introduced into a cell, as time went by, became "silent", i.e., it failed to give the instructions for the proper production of the protein, so the disease returned again. It is believed that this "silencing" may be due to the use of a number of "artificial" elements: first, the complete gene was not really used but a simplified version, the so-called cDNA, which contains the protein-coding information but lacks many elements required for the regulation of its production, and, second, a "promoter" of viral origin was used to ensure the expression of this gene. It seems that in the body, over time, these elements are in some way recognized as "foreign” and its “silencing” is triggered. As an alternative to the use of these elements, one could resort to the natural version of the gene, in all its length. This is technically more complicated because the genes are normally very large and do not fit into most viral vectors which are normally used in gene therapy trials (such as adenoviruses , retroviruses and lentiviruses). An interesting exception are herpes viral vectors, derived from herpes virus simplex (HSV-1), which are capable of accommodating up to 150 kb of genetic material.
Javier Diaz-Nido and Filip Lim Groups , at the Universidad Autónoma de Madrid in Spain, in collaboration with the Group of Richard Wade-Martins, of the University of Oxford in the United Kingdom, have been exploring the possibility of using this type of Herpes viral vectors for Friedreich's ataxia gene therapy (a disease caused by the deficiency of one protein called frataxin, and that is characterized by the degeneration of certain neurons in the central and peripheral nervous systems, as well as other alterations in the heart and the pancreas).

In a paper published in 2007 they showed that a herpes viral vector carrying the complete frataxin gene was able to make up for the functional deficiencies that were found in cultured skin cells of patients with Friedreich's ataxia (1).

Now, in a paper just published "online" in the "Gene Therapy" magazine (2) they have been able to demonstrate that vectors carrying the full frataxin gene allow a persistent expression "in vivo" after being injected into the mouse cerebellum. These results reinforce the view that vectors carrying complete genes are not silenced and may therefore be very useful for long term gene therapy. Another advantage of this type of herpes viral vectors is that they persist as stable “episomes” in the nuclei of cells, not integrated into any chromosome, and so they do not cause any alterations in the genome of these cells (unlike what happens with other vectors). Furthermore, herpes viral vectors are well known for their effective targeting of neuronal cells. In view of these data, it appears that herpes viral vectors carrying complete genes may be a safe and effective tool for the therapy of neurological hereditary diseases which are caused by recessive mutations (such as Friedreich's ataxia).

1.-Gomez-Sebastian S, Gimenez-Cassina A, Diaz-Nido J, Lim F, Wade-Martins R.
Infectious delivery and expression of 135 KB FRDA human genomic DNA locus complements Friedreich's ataxia deficiency in human cells.
MOL Ther. 2007 Feb; 15 (2): 248-54.
http://www.ncbi.NLM.NIH.gov/PubMed/17235301
http://www.nature.com/MT/journal/V15/N2/full/6300021a.html
2.-Gimenez-Cassina A, Wade-Martins R, Gomez-Sebastian S, Corona JC, Lim F, Diaz-Nido J.
Infectious delivery and long-term persistence of transgene expression in the brain by a 135-kb iBAC-FXN genomic DNA expression vector.
Gene Ther. 2011 Apr 14. DOI:10.1038/gt.2011.45 [Epub ahead of print]
http://www.ncbi.NLM.NIH.gov/PubMed/21490681
http://www.nature.com/gt/journal/vaop/ncurrent/full/gt201145a.html