Viral Vectors

[06/04, 9:41 pm] ‪+234 813 872 7278‬: *Viral Vectors* All viruses attack their hosts and introduce their genetic material into the host cell as part of their replication cycle. This genetic material contains basic 'instructions' of how to produce more copies of these viruses, using the body's normal production machinery to serve the needs of the virus. The host cell will carry out these instructions and produce additional copies of the virus, leading to more and more cells becoming infected. Some types of viruses actually physically insert their genes into the host's genome. This incorporates the genes of that virus among the genes of the host cell for the life span of that cell. Viruses like this could be used as vehicles to carry 'good' genes into a human cell. First, a scientist would remove the genes in the virus that cause disease. Then replace those genes with genes encoding the desired effect (for instance, insulin production in the case of diabetics). This procedure must be done in such a way that the genes which allow the virus to insert its genome into its host's genome are left intact. There are six (6) types of Viral Vectors: (i) Retrovirus / lentivirus (ii) Adenoviruse (iii) Adeno-associated Viruse (iv) Alpha viruse (v) Herpesvirus (iv) Vaccinia virus [06/04, 9:45 pm] ‪+234 813 872 7278‬: _Non-viral methods_ present certain advantages over viral methods, with simple large scale production and low host immunogenicity being just two. Previously, low levels of transfection and expression of the gene held non-viral methods at a disadvantage. However, recent advances in vector technology have yielded molecules and techniques with transfection efficiencies similar to those of viruses. Naked DNA This is the simplest method of non-viral transfection. Clinical trials carried out of intramuscular injection of a naked DNA plasmid have occurred with some success; however, the expression has been very low in comparison to other methods of transfection. In addition to trials with plasmids, there have been trials with naked PCR product, which have had similar or greater success. This success, however, does not compare to that of the other methods, leading to research into more efficient methods for delivery of the naked DNA such as electroporation and the use of a "gene gun", which shoots DNA coated gold particles into the cell using high pressure gas. _Oligonucleotides_ The use of synthetic oligonucleotides in gene therapy is to inactivate the genes involved in the disease process. There are several methods by which this is achieved. One strategy uses antisense specific to the target gene to disrupt the transcription of the faulty gene. Another uses small molecules of RNA called siRNA to signal the cell to cleave specific unique sequences in the mRNA transcript of the faulty gene, disrupting translation of the faulty mRNA, and therefore expression of the gene. A further strategy uses double stranded oligo-deoxynucleotides as a decoy for the transcription factors that are required to activate the transcription of the target gene. The transcription factors bind to the decoys instead of the promoter of the faulty gene, which reduces the transcription of the target gene, lowering expression.