The Speed Gene ACTN3
Does having the right alleles to this gene give selective advantage in sports?
This web page was produced as an assignment for an undergraduate course at Davidson College.
Introduction:
Yang et al. linked the gene ACTN3 to athletic performance in 2003 by connecting it with a particular influence on the normal variation in muscle function found in the population. ACTN3 is a highly conserved member of the α-actinin family, which are actin binding proteins. Specifically, ACTN3 is a specialized skeletal muscle actin binding protein that is restricted to fast twitch myofibers. These fast fibers are responsible for rapid force generation. The two alleles variants were R577X and 577R. R577X has a common stop-codon polymorphism that causes a complete deficiency of the ACTN3 protein in homozygous XX people. This deficiency is common in the population and does not cause a disease phenotype because the homologous protein ACTN2 most likely compensates for the loss of ACTN3.
Scientific Paper:
Yang et. al. assert that there are strong genetic influences on athletic performance and have demonstrated that ACTN3 is part of an evolutionary “trade-off” between performance traits for speed and endurance activities (Yang et. al. 2003). The paper demonstrates associations with ACTN3 genotype and athletic performance based on a study of Australian elite athletes who had represented Australia in an international contest.
Methods:
The study required the genotyping of 436 controls (sex was known for 292 females and 134 males) and 429 elite white athletes initially, but due to classification criteria 128 athletes could not be classified as strictly sprinters or endurance runners and were eliminated. There were 107 specialist sprinters/power athletes (72 male and 35 female) competing in track, swimming, judo, short-distance track cyclists, or speed skating. The subset of specialist endurance athletes included 194 subjects (122 male and 72 female) and competed in long-distance cycling, rowing, swimming, track, or cross-country skiing.
Results:
The results showed that the control group did not deviate from Hardy-Weinburg Equilibrium even when the male and female controls were separated. There was also no noteworthy difference between the elite athlete group as a whole compared to the controls. However, when comparing sprint/power athletes and endurance athletes against the control group there was strong evidence of allele frequency variation. Sprint athletes had a lower frequency of the XX genotype, with no female sprinter being XX and no Olympian sprinter being XX genotype either. There was also a higher frequency of RR genotype and a decreased frequency of heterozygotes in the sprinter population. Conversely the endurance athletes had a higher frequency of the XX genotype.
The genotype variation seen in female sprint and endurance atheles was shown to be statistically significant, but was not shown to be significant in males. This suggests that the ACTN3 genotype affects the performance of males and females differently.
Table 1: Number and Frequency (%) of ACTN3 Genotypes and Frequency (%) of ACTN3 Alleles in Controls and Elite Sprint/Power and Endurance Athletes. Figure was taken from Yang et al., 2003. Permission Pending.
Figure 1: ACTN3 genotype frequency in controls, elite sprint/power athletes, and endurance athletes. Shown is a marked decrease in the frequency of the XX genotype in sprint athletes, and in both male and female Olympians there were none that carried the XX genotype. Oppositely, a trend toward an increase in the XX genotype isfound in endurance athlete, although this association only reaches statistical significance in female athletes. Error bars indicate 95% CIs. Figure was taken from Yang et al., 2003.Permission Pending.
Conculsion:
Their findings led them to make the claim that the ACTN3 577R allele provides an advantage for power and sprint activities. ACTN3 is the first structural skeletal-muscle gene for which an association with fitness and performance has been demonstrated, but 73 other genetic loci have also been associated with fitness and performance phenotypes. The functional reason for this effect on athletic performance likely has to do with the ACTN3 gene’s role as the predominant fast fiber isoform and may provide a greater capacity for the absorption or transmission of force at the Z line during rapid contraction. There is also a possibility that ACTN3 alters glucose metabolism or is optimized to minimize damage caused by eccentric muscle contraction. To further study the mechanism of how ACTN3 affects muscle function they have created a ACTN3 knockout.
They also try to answer the question of why the 577XX genotype has been so conserved in human evolution even though the 577RR genotype provides a power-performance advantage. Their best hypothesis stems from the conclusion drawn from their data that the XX genotype enhances endurance performance meaning that the genotype has been acted on by positive natural selection. This would indicate that both alleles provided advantages under different environmental conditions and so are kept at high frequencies by balancing selection. The gene that encodes ACE, or angiotensin-converting enzyme, alleles I and D have also been indicated in providing either a sprinting (D allele) advantage or endurance advantage (I allele) depending on one’s genotype. It is also suggested that there is a “trade-off” between sprint and endurance traits that cause a person to be inherently predisposed toward specialist performance in one area.
Popular Press Article:
The popular press article was from the New York Times in 2008 evaluating a genetic test being offered that identified what alleles were present in the ACTN3 gene, so parents could better identify what sports their children would be good at. The Kua et. al. article asserts that the purpose of scientific writing is to translate science into non-scientific language (2004). The article does cite the Yang et. al., 2003 paper and explains their subject pool and results, but the paper over emphasizes the significance of the data and does not share with the reader that even though there were increased allele frequencies for the different phenotypes in males they were still within Hardy-Weinburg Equilibrium and so they weren’t statistically significant. Kua et. al. says that readers are mostly looking for the evidence that backs up new scientific claims being reported and this article does give the evidence, but the problem here is that they withhold some information. Another common problem found in scientific writing is an overstatement of the generalizability of the data (Kua et al., 2004).The article puts too much emphasis on this ACTN3 gene being the gene that will show you what sports you possess talents for based on sprinting versus endurance. However, while this is the view laid out in the beginning of the article the author does present other viewpoints later in the article. Macur gives testimonies from both sides as to whether professionals think this is a good or bad test, which changes the article from pure statement and fact to more of a report on the progressive discovery of these genes, which Kua et al. says is important in scientific writing (2004). The author also makes an appeal to pathos in the article by asking the reader how they think information from this test will affect the people that get back test results that do not match their expectations or how people might misuse the information and go overboard based on the results. There is also an appeal to logos because the article presents real life examples of how a person’s genotype did not match their strengths. This was based off the Spanish Olympic long jumper whose genotype was XX, meaning they should not have had such explosive power (Kua et al. 2004). This implies that there are always exceptions to this genotype and the article even admits that scientists do not definitively understand what combination of genes creates explosiveness or endurance. There are many more genes than just ACTN3 that affect athletic performance.
Based on the articles content, it does give information on the discovery by Yang et al., but the point behind the article is really a discussion about the use of genetic testing to test for this gene to predict athletic performance. This is a good way to talk about the scientific discovery because it brings in a discussion about the wider context and how it can apply to the reader, which is something the readers look for in scientific writing according to Kua et al. (2004). The article also does a good job with not sensationalizing the discovery by providing the reader with actual facts about both the applications and the restrictions on using ACTN3 as an athletic predictor. According to the guidelines laid out in Kua et al. I think this was a well written scientific article (2004).
References:
Eunice Kua, Michael Reder, and Martha J. Grossel. 2004. Science in the News: A Study of Reporting Genomics. Public Understanding of Science. 13: 309-322.
Nan Yang, Daniel G. MacArthur, Jason P. Gulbin, Allan G. Hahn, Alan H. Beggs, Simon Easteal, and Katherine North. 2003. ACTN3 Genotype is associated with human elite athletic performance. Am. Soc. Hum. Genet. 73: 627-631.
Juliet Macur, " Born to Run? Little Ones Get Test for Sports Gene". The New York Times. 29 Nov. 2008. 29 Jan 2012. http://www.nytimes.com/2008/11/30/sports/30genetics.html?pagewanted=all.
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