By Robert Hazen, George Mason University
The different research efforts in the field of gene therapy are not oblivious to the huge financial return that’s possible if we can invent an effective gene therapy for any of the inherited genetic diseases. So, what approaches are taken to develop a cure? Do viruses have a role to play? What are the vectors used in gene therapy?
Approaches to Gene Therapy
In essence, there’s nothing really mysterious about the preferred approaches to gene therapy. At least in principle, it’s quite simple. While the treatment of each disease is going to differ in detail, the basic strategy has been the same in all of the initial trials.
In some instances, we have defective cells—for example, from the blood or bone marrow—and we can actually remove these cells. We can modify large numbers of these cells by trying to insert the correct segment of DNA, or perhaps the complementary DNA, and then we allow that DNA to multiply. As a next step, we reintroduce those cells into the body. That’s possible with blood and bone marrow. Needless to say, it’s not so easy if one has an organ, as it’s not always possible to remove it and put it back into the human body.
And yet, the most difficult part of gene therapy, when we use blood cells or bone marrow cells, is to insert the correct gene in the correct spot. This has to be done with a vector, a vector that actually takes a piece of DNA and then inserts it into cells. The most common vectors are built from retroviruses. These are viruses with RNA genomes. The RNA then uses reverse transcriptase to convert RNA within the cell to DNA, and that DNA can be inserted, at times, into the genome.
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Viral genomes can be engineered; we can actually create viruses that have human genes, human genetic information in them, and then use that virus, as the vector, to introduce the DNA. Remember, retroviruses survive by converting their RNA genomes into DNA. We insert the DNA into the target cell, and then trick the cell into producing the proteins that the virus wants.
Here, we actually carry this a step further: by tricking a cell in our own body into producing a protein that the body should have been producing all along, but we had to tell the virus how to do it. It’s really a clever strategy.
Is It All Just Hype?
Molecular biologists are seriously considering genetic engineering with viral nose sprays. This would be especially effective for diseases of the lungs, like cystic fibrosis.
One actually gives someone a nose spray that’s laced with viruses, which have been engineered to produce a human protein that the individual is unable to produce because of the genetic disease.
Unfortunately, despite all the hype that surrounds gene therapy, not one of the many clinical trials has yet yielded definitive, positive results. Many of these trials, such as proposed cures for cystic fibrosis, have been out-and-out failures—very, very disappointing. In a sobering assessment, the National Institutes of Health issued a report in late 1995. They concluded, “Clinical efficacy has not been definitively demonstrated at this time in any gene therapy protocol, despite anecdotal claims of successful therapy.”
Challenges of the Vector Insertion Technique
One of the major difficulties of gene therapy, of all these insertion techniques, is that with only a few exceptions, in experimental organisms, DNA introduced into a cell’s chromosome is inserted essentially at random. We can introduce a virus, it can introduce a strand of DNA, but where does that DNA go? It can go just about anywhere in humans, in any of the 23 chromosomes.
In the case of bacteria or plants, where millions of individual cells can be altered at one time, we’re probably going to get a few cells in which the DNA goes into a place where it can be expressed. We can then extract those few cells, and let them multiply until we have a pure strain of altered bacteria, or a pure strain of plants. But in animals, one can’t do that; other genes are going to be ignored. They’re going to be attacked by the cell’s complex mechanisms that repair errors in DNA sequence.
Additionally, other DNA inserts are actually going to interrupt a critical cellular function. Indeed, attempting to insert a strand of DNA in a cell can actually kill the cell. Nevertheless, the most successful cells can be separated when one has bacteria and plants; we just can’t do that as easily when we have an animal situation. With animals, the only hope is to modify enough cells so that some will produce the missing enzyme; at the same time, we don’t want to kill off good cells. That’s the dilemma.
Common Questions about How Viral Vectors Can Be Used in Gene Therapy
The most difficult part of gene therapy, when we use blood cells or bone marrow cells, is to insert the correct gene in the correct spot. This has to be done with a vector, a vector that actually takes a piece of DNA and then inserts it into cells.
The most common vectors are built from retroviruses. These are viruses with RNA genomes; the RNA then uses reverse transcriptase to convert RNA within the cell to DNA, and that DNA can be inserted, at times, into the genome.
Viral genomes can be engineered; we can actually create viruses that have human genes, human genetic information in them, and then use that virus, as the vector, to introduce the DNA.