What is the “sprinter gene”? Can one gene make or break athletic potential? Can genetics outweigh factors like training and nutrition when it comes to athleticism?

In recent years, these questions have surfaced in both the scientific and commercial worlds. As genetic testing becomes more accessible to the general public, companies like the Athletic Talent Laboratory Analysis System (ATLAS) emerged at an increasing frequency. ATLAS is one of many companies to offer a test for the ACTN3 gene, commonly known as the “sprinter gene” or the “gene for speed,” which is marketed as a way to distinguish a child’s athletic potential based on the ACTN3 genotype. Parents test their children as young as 2 years old, hoping for any insight into whether they should enroll their toddler in speed or power sports (like sprinting or football) or endurance sports (like long distance running).

The ATLAS test is based on the ACTN3, the gene responsible for producing the protein ɑ-actinin-3, a type of protein that is present only in type-II (fast-twitch) muscle fibers. ɑ-actinin-3 contributes to the muscles’ ability to perform rapid contractions, which are employed in fast-twitch movements like sprinting. The ACTN3 genotype can be composed of two different alleles. The difference between these alleles stems from a common polymorphism, ACTN3-r577x. This polymorphism results from a simple C-T base pair SNP, yielding a stop codon instead of the amino acid arginine. Therefore, the allele containing the C base pair, called the R allele, is capable of producing ɑ-actinin-3, while the allele with the T base pair, called the X allele, cannot produce protein. The RR combination is common in sprinters or power sport athletes and accounts for 30% of the general population. RX, present in about 50% of the population, results in a mix of muscle types. XX accounts for only 18% of the population, and has been found by some studies to be common in endurance athletes; it is true that this genotype is disadvantageous for fast-twitch functions, but it is still unclear whether it confers a significant increase in endurance.

In our lab, we aimed to genotype the ACTN3 gene in our own DNA and in four samples of other people’s DNA. To do this, we followed a protocol from Ines Schadock’s article “Simple Method to Genotype the ACTN3 r577x Polymorphism.” We created primer mixes using four sets of pre-ordered PCR primers: internal forward (INT_F), internal reverse (INT_R), external forward (EXT_F), and external reverse (EXT_R). These primers were diluted to 5mM and combined to form a master mix with a ratio of 4 parts EXT_F, 4 parts EXT_R, 1 part INT_F, and 2 parts INT_R. We extracted our DNA by swabbing the inside of our cheek with a flat-edged toothpick and putting it in DPX buffer, and obtained the four other samples from the Milton Academy science department. Two different primer mixes were created: one contained 11ul of master mix, 2ul of DNA, and 12ul of sterilized water; and the other contained 22ul of master mix, 2ul of DNA, and 1ul of sterilized water. These mixes were then run through PCR. We then ran a gel for each set of DNA samples for 45 minutes, after which we were able to evaluate the results for the presence of ACTN3.

The gel showed that each of the science department’s DNA samples had the RX genotypes, as indicated by bands at 318 and 413 bp, which represent the X and R alleles, respectively. Unfortunately, our own DNA extractions yielded no results. These findings tell us that each individual of the four external DNA samples has a mix of fast and slow twitch muscle fibers. This combination of muscle types does not make this individual significantly more fit for sprinting or power sports; however, one study by Korkut Ulucan suggests that this genotype may confer an advantage for dancers. Our results also align with the statistic that the RX genotype is the most common phenotype within the general population.

Although ACTN3 has earned the moniker of the “sprinter gene,” there is still much unknown about the magnitude of its influence on athletic performance. It likely interacts with over 200 other known genes that impact athleticism, and factors like environment, training, nutrition, and luck significantly affect how ACTN3 manifests in performance. The lack of research and other uncertainties lead many to claim that tests like ATLAS’ are no more than snake oil. Some experts urge parents to not judge their children's athletic abilities on this test, because while the gene may impact top tier athletes, little evidence points to its affecting Johnny’s performance in his t-ball game.

Whatever effect ACTN3 truly plays on our athletic abilities, its rising popularity in genetic testing dredges up a host of additional questions. Which plays a bigger role, nature or nurture? Can our abilities (and lives, perhaps) be predicted from a simple genetic test? And is that something we really want to know?

Gel electrophoresis was run for about 45 minutes with 8 different DNA samples. Lane 1 shows a ladder with bands at 100, 300, 500, 1000, and 2000 bp. Lanes 2-5 shows samples of our own DNA: lanes 2 and 3 show Eliza's DNA, while 4 and 5 show Gracie's DNA. Lanes 6-9 show samples of other people's DNA: labelled TG, 7, KB2+, and 6, respectively. These samples were run in two different gels: samples in gel 1 were combined with 11ul of primer mix and samples in gel 2 were combined with 22ul of primer mix.

The results for our own DNA samples did not show clear bands; however, the other four DNA samples showed clearer results. All four seemed to show the RX genotype, judging by the bands around 318 and 413 bp. This shows that all of these people have a mix of muscle types, which is the most common ACTN3 phenotype. The control band created by the external primers at 690 bp can also be seen on all of these samples, suggesting successful PCR and gel electrophoresis for these runs.

When compared to the RR or RX genotypes, the XX genotype seems to confer only detrimental effects on the muscular system. It has been found to negatively affect athletic performance through structural, metabolic, and signalling alterations, and to cause lower resistance to intense exercise. Research that points to an advantage in endurance sports is still mostly inconclusive. So why has this allele survived natural selection, and why does around 18% of the world population have the XX genotype?

Previous studies have found a link between cooler temperatures and the presence of the X allele, suggesting that this allele correlates with cold tolerance. Positive selection of the X allele has been found in European and East Asian populations, which span colder regions than the warm origin of Africa. Some theories suggest that as humans migrated out of Africa and into colder climates, the prevalence of the X allele increased, resulting in the three genotypes we observe today. Although the underlying biological reasoning is still unclear, some studies have proposed reasoning for this correlation. A study showed that mice with ACTN3 knockout ( mice without “sprinter” gene activity) shivered less in the cold than did mice with the ACTN3 allele, suggesting that the X allele helps the body combat cold more efficiently and makes organisms more fit to survive in a cold environment.

The allele does this by manipulating the activity of calcium in the body. A calcium leak is caused by a modification of the RyR receptor protein complexes. The leak of calcium requires the consequent uptake of calcium, and this uptake triggers ATP hydrolysis which generates heat for the organism and conducts thermogenesis. So as the X allele affects the RyR protein, calcium activity is increased and generates heat. Also, the R allele promotes fast twitch muscle while the X allele promotes slow twitch, and the slow twitch muscles couples to a change in neurological muscle activation to increase muscle tone rather than shivering.

Although the X allele once made humans more fit for survival as they migrated to Europe and Asia, it has little effect on humans today. It is not known to cause disease nor inhibit survival in any way, so neither the R nor X allele is subject to significant selective forces. As a result, all three genotypes of RR, RX, and XX are all present in today's human population.

Although parents might be disappointed that little Suzy with her XX genotype will not grow up to be the next elite sprinter, the presence of the X allele and the resulting variety of genotypes is a direct result of an evolutionary history that enabled humans to survive in cold climates. The ACTN3 gene directly influenced this evolution, giving this gene a scope of impact far greater than just the sports world.

"rs1815739 SNP" (SNPedia): https://www.snpedia.com/index.php/Rs1815739

"ACTN3" (ScienceDirect): https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/actn3

"ACTN3: More than Just a Gene for Speed" (Frontiers in Physiology): https://www.frontiersin.org/articles/10.3389/fphys.2017.01080/full

"The ACTN3 sports gene test: what can it really tell you? (Wired): https://www.wired.com/2008/11/the-actn3-sports-gene-test-what-can-it-really-tell-you/

"Born to Run? Little Ones Get Test for Sports Gene" (New York Times): https://www.nytimes.com/2008/11/30/sports/30genetics.html

"ɑ-actinin-3: Why Gene Loss is an Evolutionary Gain" (NCBI): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4295838/

"Is evolutionary loss our gain? The role of ACTN3 p.Arg577Ter (R577X) genotype in athletic performance, ageing, and disease" (NIH): https://pubmed.ncbi.nlm.nih.gov/30281865/

"Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation" (ScienceDirect): https://www.sciencedirect.com/science/article/pii/S0002929721000136

"Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation" (BioRXIV): https://www.biorxiv.org/content/10.1101/2020.10.03.323964v1.full

"Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation" (ScienceDirect): https://www.sciencedirect.com/science/article/pii/S0002929721000136

"Effect of ACTN3 Genotype on Sports Performance, Exercise-Induced Muscle Damage, and Injury Epidemiology" (MDPI): Downloadable file

"Alpha-actinin-3 R577X Polymorphism Profile of Turkish Professional Hip-Hop and Latin Dancers" (ResearchGate): https://www.researchgate.net/publication/313087029_Alpha-actinin-3_R577X_Polymorphism_Profile_of_Turkish_Professional_Hip-Hop_and_Latin_Dancers

"Simple Method to Genotype the ACTN3 r577x Polymorphism": https://drive.google.com/file/d/1u3jH3a_k9C5OF1sgJPAg5xSpRTyUg2xY/view?ts=6075e7b3