This SNP, in the ACTN3 gene, encodes a premature stop codon in a muscle protein called alpha-actinin-3. The polymorphism alters position 577 of the alpha-actinin-3 protein.
In the (C;C) genotype is often called RR. The truncated (T;T) is often called XX.
[PMID 18043716] (T;T) is under-represented in elite strength athletes, consistent with previous reports indicating that alpha-actinin-3 deficiency appears to impair muscle performance.
The most common nucleotide at this position, (C), encodes an arginine (amino acid code R), the alternative T allele encodes a stop codon (X). Hence, the SNP is referred to as R577X, with homozygotes being either RR or XX and heterozygotes being RX. XX individuals completely lack the expression of alpha-actinin-3.
An earlier report studying a relatively small number of Australian elite (i.e. ~Olympic) athletes found that, at least in females, the R allele (ie rs1815739(C)) is associated with sprinters, while the X allele (rs1815739(T)) is associated with endurance athletes. No female or Olympic-level sprinters were XX homozygotes (rs1815739(T;T)). The association tended the same way but was statistically weaker in males. [PMID 12879365]
There have been several subsequent studies, but few with large sample sizes and thus few with much statistical power. An example of a typical study: no increase in endurance ability was associated with the X allele in elite male cyclists. [PMID 16612741]
An extensive blog post from one of the original authors of this research.
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[PMID 22224919] ACTN3 R577X polymorphism and performance phenotypes in young Chinese male soldiers.
|qualified_impact||Insufficiently evaluated not reviewed|