MicrobiologyBytes: Virology: Viruses & gene therapy Updated: January 28, 2007 Search

Virus Vectors & Gene Therapy:
Problems, Promises & Prospects

  1. Introduction: Viruses as Vectors
  2. Retrovirus Vectors
  3. Adenovirus Vectors
  4. Adeno-Associated Virus
  5. Herpes Simplex Virus
  6. Non-Viral Methods of DNA Transfer
  7. Comparison of Vectors
  8. Strategies for Gene Therapy: Cancer & Clinical Trials
  9. References

 

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Introduction:

The cumulative research directed towards understanding the molecular biology underlying all aspects of disease & development has yielded a wealth of information. As gene products & their interactions with the cellular environment have been characterised, so the possibility of treating disease by using DNA as a drug has arisen. All proteins are coded for by DNA, & diseases ultimately result from the expression of one or more abberent proteins, e.g. an oncogene or pathogen protein, or the lack of a functional form. In theory therefore, all diseases could be treated by expression of the appropriate protein in the effected cells. Conceptually the most simple disease to treat would be a monogenic recessively inherited disease, such as haemophilia (Snyder et al, 1997), whereby the functional form of the gene would be added to the cell restoring it to a normal phenotype. However, research is underway to treat monogenic dominantly inherited diseases such as hypercholesteroleamia (Gerad & Collen, 1997), & acquired genetic diseases such as cancers (Roth & Cristiano, 1997). Regulation of cellular proliferation e.g. to prevent atherosclerosis following angioplasty (Kim et al, 1997), or to promote of cellular repair following trauma to the CNS (Federoff et al, 1992) & protection from infectious disease (Caruso & Bank, 1997) are also currently being investigated.

Gene therapy potentially represents one of the most important developments to occur in medicine, but before this can be realised certain technical problems common to all methods of gene delivery must be overcome. In order to modify a specific cell type or tissue, the therapeutic gene must be efficiently delivered to the cell, in such a way that the gene can be expressed at the appropriate level & for a sufficient duration. Two broad approaches have been used to deliver DNA to cells, namely viral vectors & non-viral vectors, which have different advantages as regards efficiency, ease of production & safety. This paper will review these methods & then discuss the genetic strategies used to achieve prolonged tissue specific expression of the therapeutic gene.

Viruses as Vectors:

Viruses are obligate intra-cellular parasites, designed through the course of evolution to infect cells, often with great specificity to a particular cell type. They tend to be very efficient at transfecting their own DNA into the host cell, which is expressed to produced new viral particles. By replacing genes that are needed for the replication phase of their life cycle (the non-essential genes) with foreign genes of interest, the recombinant viral vectors can transduce the cell type it would normally infect. To produce such recombinant viral vectors the non-essential genes are provided in trans, either integrated into the genome of the packaging cell line or on a plasmid. As viruses have evolved as parasites, they all elicit a host immune system response to some extent. Though a number of viruses have been developed, interest has centred on four types; retroviruses (including lentiviruses), adenoviruses, adeno-associated viruses & herpes simplex virus type 1.

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