“Gene doping” is the term used to describe the potential abuse of gene therapy as a performance-enhancing agent. Gene doping would apply the techniques used in gene therapy to provide altered expression of genes that would promote physical superiority. For example, insulin-like growth factor 1 (IGF-1) is a primary target for growth hormone; overexpression of IGF-1 can lead to increased muscle mass and power. Although gene doping is still largely theoretical, its implications for sports, health, ethics, and medical genetics are significant.
As genetic technologies and breakthroughs continue to progress at a rapid pace, so does the potential misuse of these advancements. Although “conventional” doping technologies are also evolving over time, one of the most intriguing and potentially destructive performance-enhancing concepts has arisen in the form of “gene doping.” Gene therapy has been established as a technique with the potential to correct nonworking genes that lead to disease, however, with only some success in human trials. Adapting the principles of gene therapy to supply athletes with a competitive advantage is the (as of yet theoretical) goal of gene doping.
The World Anti-Doping Agency (WADA) categorizes gene doping as a “prohibited method” in its 2010 Prohibited List and defines it as “1 – The transfer of cells or genetic elements (eg, DNA, RNA); 2 – The use of pharmacologic or biologic agents that alter gene expression… with the potential to enhance athletic performance.” Although to date, confirmation of gene doping in competition has not occurred, WADA has acknowledged both the potential for misuse of gene therapy in this regard and the vigilance necessary to be ready to address this threat to fair play once gene doping moves from the realm of possibility to probability. The allure of gene doping for an athlete looking to cheat is multifaceted but largely involves the inherent difficulty in detection. However, although the unknowns associated with gene doping suggest detection difficulties, these same unknowns underscore the serious potential threat to the health and safety of the doping subjects.
Methods of gene therapy or doping
The general goal of gene therapy is to promote expression of a functional gene in an unhealthy individual to correct a disease caused by an underlying genetic mutation. The ideal gene therapy candidate is a monogenic condition caused by a nonfunctional or aberrant gene product, such as in Duchenne muscular dystrophy (DMD). In DMD, mutations in the dystrophin gene lead to absent, decreased, or dysfunctional dystrophin protein production. “Classic” gene therapy in this case would be the mechanism to deliver the dystrophin gene to an affected individual to produce functional dystrophin. This is the most common approach to gene therapy for monogenic conditions. “Nonclassic” gene therapy is the term used to describe this procedure when the goal of treatment is to control the expression of genes or the effects of gene expression, such as in cancer. Gene therapy is a promising tool in the treatment of genetic disease as it establishes treatment at the source of the underlying defect, and if continuous cell expression is achieved, it allows for a more constant administration of the gene product.
To facilitate the introduction of a gene into the cells of the recipient, a delivery vehicle is required. These delivery systems can be classified into 3 main types: biologic, physical, and chemical. The use of a biologic vector is the most common delivery mode for gene therapy. Viral vectors such as retroviruses, adenoviruses, and adeno-associated viruses (AAVs) are commonly used as they function to integrate into host cells and use this cell to replicate their own genetic material. In gene therapy, these viruses are modified to reduce the potential for viral infection while carrying the ability to be delivered to specific cells for expression. Plasmid DNA (pDNA) is an alternative biologic vector but differs from viral vectors in that they are synthetic and may be grown in bacteria, then purified. Although more inefficient than viral vectors, pDNA has the advantage of avoiding a possible immune response. Liposomes can assist in the penetration of cell membranes. RNA interference is another method of manipulating and controlling gene expression to enhance or manage gene therapy techniques that are under investigation.
Gene transfer can be accomplished by direct physical injection or enhanced by physical methods, such as electroporation, ultrasound, laser, and magnetic particles.
Gene expression can occur in vivo or ex vivo. Both techniques have pros and cons, inside and outside of the laboratory ( Table 1 ).
| Method | Advantages | Disadvantages | |
|---|---|---|---|
| Ex vivo | Cells from patient treated in culture, then administered to the patient |
|
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| In vivo | Gene is delivered via vector or direct physical route in to patient |
|
|
Of importance is the difference between somatic and germline gene therapies. Somatic gene therapy is limited to adult cells, and the effects are not a permanent change to an individual’s DNA. Germline gene therapy, or transgenesis, is the process used in many animal studies and involves changing the genetic makeup of the animal permanently, including gametes, thereby making this genetic change present in all body cells and also heritable. Many of the animal studies in gene therapy that report the most significant results have induced germline mutations.
The fundamental difference, physically, biochemically, and ethically, between gene therapy and gene doping is that the goal of gene doping is not to replace an absent or dysfunctional protein in an unhealthy individual but rather to artificially alter gene expression in an otherwise healthy individual. The evolution of gene therapy from a strictly medical tool to a performance-enhancement mechanism has significant ramifications both in the competitive sports world and in the general population.
Candidates for gene doping
What makes a gene a good candidate for doping? Obviously, the targets for gene doping would depend on the desired effect. Overexpression or underexpression of the gene product should enhance traits that are desirable for peak athletic performance. For endurance sports, such as long distance running or swimming, genes that bolster oxygen production or usage and delay fatigue would be the likely candidates. For sports in which strength or agility provide the competitive advantage, genes involved in muscle mass stimulation and injury recovery are the more likely targets. Research into gene therapy for disease treatment has led to a bounty of information that could theoretically be incorporated into gene doping programs.
Genes for Endurance
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Erythropoietin (EPO) : EPO is a hormone produced in response to decreased oxygen levels in the blood that signals the body to increase hemoglobin production. EPO-stimulating agents have long been a part of performance-enhancing doping. Overexpression of EPO by gene doping would increase endogenous hemoglobin production and thereby oxygen distribution to muscles.
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Peroxisome proliferator-activated receptor delta (PPAR-δ) : PPAR-δ and its family of hormones are involved in changing type I (fast twitch) skeletal muscle fibers to type II (slow twitch) muscle fibers. Upregulation of this gene could produce an increase in the number of type II muscle fibers desired for endurance sports, even in the absence of endurance training. The WADA 2010 Prohibited List bans PPAR-δ agonists (eg, GW1516) and PPAR-δ–adenosine monophosphate–activated protein kinase axis agonists (eg, AICAR), the only genes specifically mentioned under the gene doping section.
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Phosphoenolpyruvate carboxykinase (PEPCK) : the role of PEPCK in skeletal muscle is somewhat unclear, but overexpression in mice increases endurance and longevity and leads to decreased body fat.
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Vascular endothelial growth factor : this growth factor is instrumental in the development of new blood vessels and also appears to be important in some injury-healing molecular pathways.
Genes for Strength
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Insulinlike growth factor 1 (IGF-1) : IGF-1 is the primary target of growth hormone action. Increased gene expression leads to increased muscle mass and power. In addition to promoting muscle hypertrophy, IGF-1 also hastens muscle repair.
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Myostatin : unlike many other candidate genes for gene doping, myostatin would be targeted to promote decreased expression of this gene. Myostatin is a negative regulator of muscle growth, and by impeding its actions, increased muscle mass would be expected.
Genes for Tissue Repair/Other
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Bone morphogenetic protein (BMP) : the BMP family of growth factors enhance bone repair and would theoretically shorten recovery time from injury. In the absence of an injury, these growth factors have the potential to increase bone, cartilage, or tendon strength in an effort to stave off potential career-ending injuries.
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Endorphins : Endorphins are important components of pain management, fatigue delay, and endurance. Genes that increase endorphins would increase pain threshold both acutely during competition by reducing lactic acid–related pain and chronically by dulling the effects of prior injury. These effects make genes related to endorphin production, expression, and release reasonable targets for gene doping.
This is by no means a complete list of gene doping targets but an overview of prime candidates due to their cellular function. As more genes are identified and characterized with regard to athletic potential, the list of potential gene doping candidates is sure to expand as well.
Candidates for gene doping
What makes a gene a good candidate for doping? Obviously, the targets for gene doping would depend on the desired effect. Overexpression or underexpression of the gene product should enhance traits that are desirable for peak athletic performance. For endurance sports, such as long distance running or swimming, genes that bolster oxygen production or usage and delay fatigue would be the likely candidates. For sports in which strength or agility provide the competitive advantage, genes involved in muscle mass stimulation and injury recovery are the more likely targets. Research into gene therapy for disease treatment has led to a bounty of information that could theoretically be incorporated into gene doping programs.
Genes for Endurance
- •
Erythropoietin (EPO) : EPO is a hormone produced in response to decreased oxygen levels in the blood that signals the body to increase hemoglobin production. EPO-stimulating agents have long been a part of performance-enhancing doping. Overexpression of EPO by gene doping would increase endogenous hemoglobin production and thereby oxygen distribution to muscles.
- •
Peroxisome proliferator-activated receptor delta (PPAR-δ) : PPAR-δ and its family of hormones are involved in changing type I (fast twitch) skeletal muscle fibers to type II (slow twitch) muscle fibers. Upregulation of this gene could produce an increase in the number of type II muscle fibers desired for endurance sports, even in the absence of endurance training. The WADA 2010 Prohibited List bans PPAR-δ agonists (eg, GW1516) and PPAR-δ–adenosine monophosphate–activated protein kinase axis agonists (eg, AICAR), the only genes specifically mentioned under the gene doping section.
- •
Phosphoenolpyruvate carboxykinase (PEPCK) : the role of PEPCK in skeletal muscle is somewhat unclear, but overexpression in mice increases endurance and longevity and leads to decreased body fat.
- •
Vascular endothelial growth factor : this growth factor is instrumental in the development of new blood vessels and also appears to be important in some injury-healing molecular pathways.
Genes for Strength
- •
Insulinlike growth factor 1 (IGF-1) : IGF-1 is the primary target of growth hormone action. Increased gene expression leads to increased muscle mass and power. In addition to promoting muscle hypertrophy, IGF-1 also hastens muscle repair.
- •
Myostatin : unlike many other candidate genes for gene doping, myostatin would be targeted to promote decreased expression of this gene. Myostatin is a negative regulator of muscle growth, and by impeding its actions, increased muscle mass would be expected.
Genes for Tissue Repair/Other
- •
Bone morphogenetic protein (BMP) : the BMP family of growth factors enhance bone repair and would theoretically shorten recovery time from injury. In the absence of an injury, these growth factors have the potential to increase bone, cartilage, or tendon strength in an effort to stave off potential career-ending injuries.
- •
Endorphins : Endorphins are important components of pain management, fatigue delay, and endurance. Genes that increase endorphins would increase pain threshold both acutely during competition by reducing lactic acid–related pain and chronically by dulling the effects of prior injury. These effects make genes related to endorphin production, expression, and release reasonable targets for gene doping.
This is by no means a complete list of gene doping targets but an overview of prime candidates due to their cellular function. As more genes are identified and characterized with regard to athletic potential, the list of potential gene doping candidates is sure to expand as well.
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