Gene Doping: Introductory Notes
By Sarah Parker
The Biology of Gene Doping |
Which genes could be used for gene doping?
Potential Problems and Dangers
Gene doping is done using techniques developed for gene therapy. The most commonly used method is a viral vector, a “delivery vehicle” that does not cause disease, contains the gene of interest, and can be engineered to inject this gene into a specific type of tissue. In the case of certain muscle-enhancing treatments, the virus is injected directly into muscle tissue, where it proceeds to “infect” the muscle cells’ nuclei, replicating the gene and ultimately increasing muscle mass. For more information about gene therapy techniques, click here.
Which genes could potentially be used for gene doping?
Erythropoietin (EPO)
Erythropoietin is a hormone produced by the kidneys that regulates the production of red blood cells. Injection of EPO increases the number of red blood cells and thus enhances oxygen-carrying capacity, making it a sort of “wonder drug” for patients with anemia, AIDS, or cancer (Longman, 2001). However, such injections of the hormone itself have relatively short-lived effects; frequent treatments are necessary to maintain the physical benefits. This conventional method of doping is believed to be in widespread use in endurance sports like cycling and long-distance running.
Gene doping, i.e. injecting a synthetic version of the EPO gene as a permanent source of extra EPO, may not be too far away. Scientists have recently injected the EPO gene into leg muscles of eight rhesus monkeys, causing significantly elevated red blood cell levels within two weeks. In four of the monkeys, however, red blood cell counts soon reached dangerous levels, and the researchers had to inject blood thinners twice a month to keep the monkeys alive. In the other four monkeys, EPO and red blood cell levels began to drop dramatically because of an immune response to both the EPO produced in the muscles by the synthetic gene and the natural EPO produced in the kidneys. Consequently, these monkeys developed severe anemia and had to be euthanized. Dr. Jim Wilson, the leader of this experiment, finds it hard “to believe that any athlete would try to do this” (Zarembo, 2004).
Research on the EPO gene continues, and scientists hope to be able to reproduce the effects of a natural mutation found in the family of Eero Mäntyranta, a Finnish cross-country skier who won two gold medals in the 1964 Winter Olympics. Mäntyranta’s mutation led to an excessive response to EPO, which resulted in very high levels of red blood cells (Sweeney, 2004). This high level of red blood cells and the accompanying increase in oxygen-carrying capacity helped him and several members of his family do well in endurance sports. A synthetic gene that produced the same effect would have obvious appeal to athletes and gene therapy patients alike.
Finnish cross-country skier Eero Mäntyranta had a mutation that caused an excessive response to erythropoietin. Permission pending from Lee Sweeney.
Insulin-like Growth Factor 1 (IGF-1) and Myostatin
When muscle tissue is damaged, as it is during exercise, satellite cells proliferate around the wounded fiber to help the repair process; the repaired muscle fiber is bulkier and the overall muscle is stronger, as seen in the cartoon depiction below.
Muscle fibers contain multiple nuclei (panel 1). When the muscle is damaged, satellite cells multiply and fuse with the wounded fiber, contributing their nuclei to the repair process (panel 2). After repairs, the muscle fiber is bulkier (panel 3). Permission pending from Lee Sweeney.
IGF-1 partly controls the building and repair of muscles by stimulating the proliferation of satellite cells. Another gene that encodes the antigrowth factor myostatin has the opposite effect, halting the proliferation of satellite cells. This myostatin gene is effectively blocked in the Belgian Blue Bull breed of cattle. Belgian Blue Bulls have a mutation in the myostatin gene that produces a truncated, ineffective form of the protein. The absence of myostatin allows unchecked muscle growth and interferes with fat deposition; the result is a lean, “double-muscled” bull (Sweeney, 2004). Theoretically, the overproduction of IGF-1 should have a similar physical effect.
The lean, “double-muscled” Belgian Blue Bull breed has a mutated form of the myostatin gene (Sweeney, 2004). Permission pending from Lee Sweeney.
Physiologist H. Lee Sweeney tested this theory by injecting the IGF-1 gene into the leg muscles of mice and exposing them to an eight-week training protocol. After eight weeks, the IGF-1 mice had leg muscles approximately 20-50 times larger than the control mice did, and they had gained nearly twice the strength of the control mice. The IGF-1 mice also lost strength much more slowly after the training protocol stopped. Dr. Sweeney’s ultimate goal is a treatment for muscular dystrophy; he hopes to continue his studies of IGF-1 in dogs, specifically golden retrievers because that breed is susceptible to a severe form of muscular dystrophy. He also hopes to study myostatin-blocking therapies in dogs as an alternative to IGF-1 (Sweeney, 2004).
These discoveries have obvious appeal to athletes. Indeed, athletes and athletic trainers are already inquiring about Sweeney’s research. “I would say about half the emails I get now are from athletes,” Sweeney said at a conference. “The other half is from patients with muscular dystrophy” (quoted in Amos, 2004).
The leg muscles of mice treated with IGF-1 (right) are bigger than the leg muscles of untreated mice (left). http://news.bbc.co.uk/2/hi/science/nature/3493839.stm
A team of scientists from the U.S. and South Korea have engineered what they call a “marathon” mouse. This mouse has been given an enhanced form of the gene PPAR-Delta, a gene that regulates the expression of several other genes and ultimately enhances “slow-twitch” muscle fibers. Of the two types of skeletal muscle fibers, slow-twitch fibers are more fatigue-resistant than fast-twitch fibers, which are capable of bursts of power but tire relatively quickly.
The scientists found that the enhanced mice can run roughly twice as far as normal mice, and they can run for about an hour longer than the average 90 minutes a normal mouse can run, even without previous exercise. The enhanced mice experienced an increase in slow-twitch muscle fibers and a decrease in fast-twitch fibers, as well as an increase in fat burning in adipose tissues (“ Marathon mouse,” 2004).
These findings could be used not only for muscle enhancement, but also for weight loss. The enhanced mice resisted weight gain even when placed on a high-fat, high-calorie diet that made normal mice obese. For people who are obese and have difficulty losing weight, and whose health is suffering as a result, this research may provide a new method of handling such health problems. Of course, athletes might also be interested in this gene, especially athletes in endurance sports like cycling and long-distance running, as it could “make their exercise more efficient” and help them increase endurance more quickly (“ Marathon mouse”, 2004).
A “marathon” mouse prepares to run on a treadmill. http://news.bbc.co.uk/2/hi/science/nature/3592976.stm
What are some potential problems and dangers?
There are some glaring potential problems with current possible abuses of gene therapy techniques. No human trials have been conducted on any of the synthetic genes studied, so their full effects in humans are unknown. Genes that enlarge muscles, such as an overabundance of IGF-1 or a lack of myostatin, may affect the heart and could be very dangerous if not controlled (“Secrets,” 2004). Rapid muscle growth could also strain tendons and bones, although this may not be likely in healthy people (Sweeney, 2004).
With regards to erythropoietin, there are several dangers observed in Dr. Wilson’s experiments. In some monkeys, the synthetic EPO gene raised red blood cells level so high that blood thinners were necessary to keep the monkeys alive, and in other monkeys, the immune response triggered by the synthetic EPO caused severe anemia (Zarembo, 2004). Either condition is very dangerous to daily living and especially dangerous for strenuous athletic activity. An additional deterrent to athletes seeking this method of gene doping is the possibility of detection. French researchers found that when a synthetic EPO gene was injected directly into the muscle tissue of monkeys, the protein product of the synthetic gene was different enough from the natural protein that it could be detected with DNA screens. They do not yet know if the synthetic protein can be detected in urine tests (Warner, 2004). It is encouraging to think that gene doping might be detectable by a method easier to perform than a muscle biopsy, to which athletes preparing to compete would be unlikely to submit.
Questions or Comments: Email Dr. Verna Case
Davidson College Biology Department
Davidson College
This web page was produced as an assignment for an undergraduate course at Davidson College.
The Biology of Gene Doping |