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Most gene therapy strategies in research and clinical labs up until now have revolved around harnessing the evolved capabilities of viruses to deliver their viral genomes into cells. This is commonly known as use of a viral vector.
Let’s talk a little bit about viruses. Viruses are particles which can infect cells of living organisms. Viruses are made up of a protein shell encasing viral genetic material. In order to reproduce, viruses attach via their protein shells to cell surface membranes where they inject their genetic material. For normal disease causing viruses, the viral genetic material hijacks the cell’s protein and nucleotide generating machinery to produce more complete virus particles. The cycle continues until the immune system can seek and destroy the viral particles (unless the immune system is the target of the virus; as in the case of HIV). The process by which viruses deliver their viral genomes into cells is referred to as viral transduction.
In order to use a virus as a delivery vector, the viral genetic material basically needs to be removed and replaced with genetic material encoding the desired cellular product.
There are a few different kinds of viruses which can be used for gene transduction. Retroviruses are one kind. Retroviruses store their genetic material in the form of RNA. When a retrovirus infects or transduces a cell, it introduces ins RNA and a few additional enzymes to the cell. The RNA is then copied to DNA inside the cell my an enzyme called reverse transcriptase. The new DNA is then inserted into the cell’s own genome by the integrase enzyme. The viral DNA is now a part of the host cell’s DNA. If the host cell divides, then any daughter cells will share the new DNA. The great thing about that from a gene therapy standpoint is the fact that there would be little or no need to introduce the therapeutic gene more than once. The downsides to it, however, are that:
1) The viral DNA can be incorporated into portions of the cell genome that result in faulty transcription of important genes. This could lead to cancer conditions caused by the gene therapy in the same way that human papillomavirus (HPV) predosiposes women for cervical cancer.
2) If the virus inserts itself into the wrong cell type, the genetic material could be passed on indefinitely within unintended cells for unintended results.
Adenoviruses are very different from retroviruses in that the genomic material which adenoviruses use to hijack a cell starts as DNA. Additionally, the DNA does not incorporate itself in the host cell’s genome. The viral DNA finds its way into the host cell’s nucleus where it is transcribed to RNA in the same way all nuclear DNA is transcribed. However, since the viral genes are not incorporated into the cell’s genome, the gene will not be duplicated and passed on to daughter cells after cell division. In one sense, this is advantageous from a gene therapists standpoint. It means that the gene product will only be produced as long as the transduced cells are alive. Long term side effects are minimal. The downside of this approach, however, is the fact that the virus would likely need to be administered more than once.
Adeno-associated viruses (AAV) are like adenoviruses in that they carry DNA. They are like retroviruses in that the viral genomic material that they carry will be incorporated into the host cell’s genome. Daughter cells will carry the gene, but the genes will not incorporate by integrase into a random portion of the host genome. Instead, AAV always incorporates into chromosome 19. One of the biggest advantages of the AAV, however, has nothing to do with its transduction approach. AAV does not induce an immune response in humans so it can pass through the body as a vector without risk of being destroyed by T cells or macrophages. AAV will not cause fevers or inflammation when administered.
The major downside to AAV is the fact that the viral particles are very small and cannot hold very much genetic material. They would be limited in what gene products they could code for.
So now you know the three types of viruses used for gene therapies. You also know their basic advantages and disadvantages. The next installment in this series will talk about non viral gene delivery techniques. After that, we will summarize the potentially therapeutic gene products being tested in contemporary research labs. We hope you are enjoying the content so far.