In the 1990s, Lee Sweeney, a physiologist at the University of Pennsylvania, demonstrated that inserting the gene encoding insulin-like growth factor 1 (IGF-1) into the muscle cells of mice increased muscle mass and strength.1 Soon, athletes started bombarding him with requests to undergo the experimental gene therapy to enhance their performance.
“I was shocked that they would want to basically be a guinea pig for something that could do them harm rather than do them good,” said Sweeney, now at the University of Florida. “Even when I would explain it to them, they said they didn't care.”
Realizing the inevitability of this new form of doping posing a risk to sports, the World Anti-Doping Agency (WADA), an international agency that monitors sports doping, added gene doping to the list of banned practices in 2003.2 WADA defines gene doping as the non-therapeutic delivery of nucleic acids, most commonly into the muscles, in order to enhance sport performance.3 In contrast to administering drugs or hormones to achieve this, delivering genetic information can help the athlete’s cells produce their own doping substances, which are difficult to distinguish from the body’s naturally-produced molecules. Gene doping can be achieved by transferring exogenous genes encoding a performance-enhancing molecule, silencing genes for proteins that diminish performance, or gene editing for targeted modifications. Since the latter two methods are not completely reliable and safe, the detection of gene doping relies on PCR- or sequencing-based strategies to identify transferred exogenous genes, or transgenes.
Although Sweeney personally does not know any athletes practicing gene doping, he noted that WADA had some documented cases of people trying it. “Whether or not it was successful, I don’t know,” he added. Through his collaborations with anti-doping agencies, he also knows that this is a common practice in racehorses. According to him, authorities have detected and banned horses that had shown evidence of gene doping.
Edward Ryder, an animal geneticist studying gene doping detection at the biotechnology company LGC, agreed that doping is a common concern in racehorses. Unlike people who can consent to experimental and risky treatments, “Horses cannot advocate for themselves and refuse treatment,” he said.
Although they haven’t detected transgenes in horses, gene doping is becoming more accessible, with gene editing being carried out in the germline, said Ryder, who works closely with the British Horseracing Authority to detect gene doping in racehorses. Researchers recently reported the birth of gene-edited horses capable of better athletic performance, highlighting the need for detecting potential genetic modifications.
Muscle biopsies are the most reliable way to identify transgenes in both humans and horses, but scientists wanted to develop less invasive tools. These are based on detecting DNA fragments that leak into the bloodstream after exercise-induced muscle breakdown. As such, testing an athlete or horse’s blood after a competitive event can pinpoint the presence of transgenes.
“We've a created a panel of candidate genes that we believe are targets. [We] designed assays so we can detect those,” said Ryder. These potential doping genes include IGF1 and those encoding growth hormones and erythropoietin, which stimulates red blood cell production to improve endurance.4 Most PCR-based methods target exon-exon junctions that would be present in transgene-specific DNA, thus would exclude the athlete or horse’s native genomic DNA, reducing false positive amplifications.5
Although PCR-based methods can detect these transgenes with a high specificity and sensitivity, they can find only a few targets in one reaction.6 Moreover, people participating in doping can alter transgene designs to escape detection by PCR amplification.
To overcome these shortcomings, scientists, including Ryder's team, have developed next-generation sequencing (NGS)-based assays to detect foreign DNA fragments in a high-throughput manner.7 These methods have proven reliable to find potential doping genes, and researchers aim to expand the panel to detect other genes.
Although the research is relatively new, Ryder hopes that it dissuades people from gene doping, especially keeping horse welfare in mind. “If it's known that we're testing for these events, then hopefully that will deter people from doing this in the first place,” he said.
Sweeney, on the other hand, is not as optimistic. “I would hope that the possibility of being detected would discourage people,” he said. “But from my old conversations with athletes it is not clear [that] anything discourages them.”
- Barton-Davis ER, et al. Viral mediated expression of insulin-like growth factor I blocks the aging-related loss of skeletal muscle function. Proc Natl Acad Sci USA. 1998;95(26):15603-15607.
- Brown J. Genetic doping: WADA we do about the future of ‘cheating’ in sport? Int Sports Law J. 2019;19:258–280.
- Tozaki T, Hamilton NA. Control of gene doping in human and horse sports. Gene Ther. 2022;29(3-4):107-112.
- Beiter T, et al. Direct and long-term detection of gene doping in conventional blood samples. Gene Ther. 2011;18(3):225-231.
- Neuberger EWI, et al. Establishment of two quantitative nested qPCR assays targeting the human EPO transgene. Gene Ther. 2016;23(4):330-339.
- Maniego J, et al. Detection of transgenes in equine dried blood spots using digital PCR and qPCR for gene doping control. Drug Test Anal. 2024;17(5):626-633.
- Maniego J, et al. Screening for gene doping transgenes in horses via the use of massively parallel sequencing. Gene Ther. 2022;29(5):236-246.
