Viruses and Gene Therapy

VIRUSES AND GENE THERAPY

Certain viruses or virus components are now used as vectors for the delivery of genes to targeted cells. A number of viruses are being used in gene transfer medicinal products (GTMP) and these include adenoviruses (AAV), poxviruses, retroviruses, lentiviruses, adeno-associated viruses and herpesviruses. Viral vectors for human use are freeze-dried or liquid preparations of recombinant viruses, genetically modified to transfer genetic material to human somatic cells in vivo (i.e. injected directly into the patient’s body) or ex vivo (i.e. transferred into host cells before administration).

There are different approaches for the design and construction of viral vectors. The chosen approach depends upon the type of virus used. The procedure aims to minimize the risk of generating replicating viruses or to eliminate helper viruses when used during production. In addition, a number of stringent controls are performed ensuring the complete genetic and phenotypic characterization of the viral vector is carried out. These include the complete sequence of the genome of the viral vector, verification of genomic integrity of the vector, determination of the concentration of the infectious vector, residual host cell protein and DNA, and residual reagents including antibiotics.

For retro-viridae-derived vectors, genetic modification aims to ensure that the recombinant retroviruses are rendered replication-incompetent. Adeno-associated virus vectors (rAAV) are deficient adenovirus in which certain genes (i.e. cap and rep) necessary for viral replication have been replaced. A helper virus is thus needed during production of the rAAV and needs to be eliminated from the final GTMP. As with other viral vectors, the sequence integrity of the viral genes and expression cassette as well as genetic stability need to be controlled, and the absence of wildtype virus (e.g. AAV) verified.