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Genetic Traits

Muscle Performance

Could you tell if your daughter or son has the potential to be a good athlete based on genetic testing?  The answer is maybe.  If we took a look at those recent dominant Olympic champions: Jamaican sprinters and Kenyan long distance runners, we would likely come away with the notion that genetics has something to do with athletic performance.  Indeed, many studies have revealed many Performance Enhancing Polymorphisms (PEPs). Muscle is the mechanic device animals use to create force and motion.  After millions of years of evolution, specialized muscle in human body are doing great things for our daily life without us to even think about it.  For example, in an average life time, a human’s heart works continuously without stop and the amount of blood it pumps would be equivalent to the volume of water coming out a fully open faucet for 45 years.  The heart would generate enough power to drive a truck from the earth to the moon and back (see more interesting facts about heart [1]).  While the heart is the extreme example of endurance, it only produces 1-5 watts of power.  On the other hand, quadriceps muscle produces >100 watts of power, but can do it only for a short duration of time.  From these examples, we can see that two key performance characters of muscle are endurance and power/strength.  These two characters are usually not optimized in the same muscle.  In human skeletal muscle, there are two types of fibers: Type I fiber (slow twitch fiber) and Type II fiber (fast twitch fiber). Type I fiber is optimized for endurance performance and it uses aerobic process to generate energy, while producing slow, lower power for long durations of time (in hours). Type II fiber uses anaerobic process to produce energy and it produces faster, larger power but fatigues very quickly (in minutes) [2]. 
Every individual is born with his/her own a composition of fast and slow twitch muscles fibers.  This native composition is mostly determined by one’s genetic background.   Power sports like sprint and high/long jump that utilize short bursts of power need good fast twitch muscle; endurance sports such as long distance running, on other the hand, would benefit from good slow twitch muscles. One would expect that elite athletes coming out of stiff selection processes --- world level sporting competitions, would have a combination of a favorable genetic background and training. Indeed, now people have found many performance enhancing polymorphisms (PEPs), some of which specifically affects muscle performance. 

α-Actinin-3 (ACTN3) polymorphism:

ACTN3 gene is highly conserved through evolution.  ACTN3 protein is found on the Z disk along muscle fiber, expressed only in type II myofibers.   A common polymorphism of ACTN3 is R577X where the 577th amino acid of ACTN3 can be an arginine (577R) or a stop codon (577X).  The homozygous individuals with 577X have no detectable ACTN3 protein.  In 2003, ACTN3 R577X was the 1st identified PEP [3]. Yang and his colleagues found that among 429 elite white athletes from 14 different sports, sprint athletes has high frequency of R genotype and low frequency of X genotype (table 1 reference [3]).

Angiotensin-converting enzyme (ACE):

This is an example of a non-muscle related genetic polymorphism that affects muscle performance.  The ACE gene is a part of human endocrine system known as rennin-angiotensin system.  Its primary function is to regulate body fluid level and blood pressure.  ACE is also involved in other performance related processes such as respiratory drive, red blood cell supply, tissue oxygenation and the regulation of skeletal muscle efficiency.  The most common polymorphism of the human ACE gene is in the form of presence (allele I form) or absence (allele D form) of a 287 base intron.  Genetic studies found that I allele is found in higher frequency in elite endurance athletes (such as mountaineers in figure a) and people with II genotype showed better improvement of endurance after 10 weeks of training (figure b reference 4).

Since the discovery of the 1st PEP ACTN3 in 2003, more that 200 PEPs were identified on both autosomes and X-chromosome.  As we can see from the two examples discussed here, those are the polymorphisms affecting not just muscle performance but also other performance related functions such as response to training, energy utilization efficiency, oxygen capacity and utilization, blood vessel formation, connective tissue and psychological aptitude.  With multiple PEPs, people now are studying if favorable polygenic profile of PEPs can be used to identify ideal candidate for athletic performance.

References

[1] 36 Interesting Facts About The Human Heart. http://facts.randomhistory.com/human-heart-facts.html
[2] Wikipedia: Muscle.  http://en.wikipedia.org/wiki/Muscle; Skeletal Striated Muscle. http://en.wikipedia.org/wiki/Skeletal_striated_muscle
[3] Yang et al 2003. ACTN3 Genotype Is Associated with Human Elite Athletic Performance.  Am. J. Hum. Genet. 73:627–631, 2003
[4] EA Ostrander et al. 2009. Genetics of Athletic Performance. Annu. Rev. Genomics Hum. Genet. 2009.
10:407–29

 

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