We often hear on the news of recent outbreaks of a virus somewhere or discussions of the impending Flu season. Viral outbreaks are particularly difficult. Unlike bacteria there is no antibiotic for a virus, because they replicate inside of our cells. Those features which make outbreaks difficult to control also make viruses good candidates to be used by scientists to treat other diseases.
In the modern age of health care, there is a drug for just about everything. However, if you suffer from a genetic disease, the treatment options may not be as bright. Unlike treating someone’s symptoms with a drug, how do you change someone’s genetic makeup. One answer: gene therapy. Still in its infancy, gene therapy is the insertion of genes into an individual’s cells to treat a disease. For example, sickle cell anemia is a genetic disorder in which the hemoglobin in your blood is defective, causing the cell to have a different shape and not carry oxygen as well. But if the DNA in the bone marrow could be replaced with functional DNA, then the bone marrow would produce fully functional red blood cells and the patient would be cured. This is the potential of gene therapy. But how is this replacement performed? In tissue culture, this process can be accomplished relatively easily, using chemicals which can fuse with the cellular membrane releasing encapsulated DNA into the cell, but changing the DNA of a person is much more difficult. To accomplish this task, scientists use viruses to target specific organs and then deliver a specific gene directly into the target cell, where it can be used by the host to produce the correct protein or be incorporated directly into the genome permanently. So how does this work?
Unlike bacteria, viruses are intracellular parasites. Bacteria are free living organisms capable of living and replicating on their own outside of a human host. While a virus is simply a capsule of genetic material (DNA or RNA) covered in a protein coat. They do not contain the enzymes or machinery to perform any metabolism outside of their host cell. Because of this tight relationship, it is very difficult to produce antibiotics that affect the virus but not the cell. So when a virus comes in contact with its host cell, it is taken up by the cell whereupon the virus takes over control of the cellular machinery and redirects its resources for normal activity to the production of new viruses. With the completion of this cycle, the newly formed viruses are released from the cell back into the body to search out a new host. During this process, if the host cell does not spontaneously die, the virus will kill it.
This process has been modified so that once the virus has been taken up by the cell, the DNA is released or integrated into the genome. The virus never starts replicating and the cell does not die. It is the ability to deliver DNA directly into the cell and the ability to control which tissues get the gene that makes viruses so useful to gene therapy. Current research is involved in being able to better package DNA and control more tightly which cells a virus infects, but the future potential of gene therapy is tremendous. All using microorganisms that were once a source of disease, but now controlled for use in the treatment of disease