Did you know there is a “sprinter” gene?

ACTN3, the genetic secret behind the Jamaican sprint team

Jamaica left Rio 2016 with 11 Olympic medals. Every single one of them was secured in a sprint event, the 100m, 200m or the 400m dash. Not only that, five of the twenty fastest 100m sprinters are Jamaicans. Obviously, there’s a bit of speculation about this sprinting prowess, such as performance-enhancing substances. However, the world’s fastest man, Usain Bolt, attributes their country’s almost insurmountable success to having “a system” that recognizes and nurtures the skills of young athletes. Indeed, superior training and dedication has much to do with excelling in sport. However, it’s also possible that Jamaicans are genetically better suited to become world class sprinters. This is the story of a variant of ACTN3 known as the “sprinter” gene. This variant provides an advantage in activities that require sudden bursts of speed, strength or power, like sprinting and weight lifting.

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Our muscles are made up of an array of fibres, each containing filaments made of two proteins, actin and myosin that are arranged in parallel. Muscle contractions are caused by these filaments sliding past each other, which means one side of the filament needs to be anchored. Actin is anchored via two proteins, one of which is alpha-actinin-3 encoded by the ACTN3 gene. Alpha-actinin-3 is found specifically in “fast-twitch” muscle fibres required for sprinting and weight lifting. However, not all of us make alpha-actinin-3. There are two common versions or genetic variants of ACTN3. There is the functional or the sprinter version of the gene called 577R and the ‘defective’ version called 577X (or rs1815739), which doesn’t produce a functional protein. People who inherit two defective copies (577XX) do not make any alpha-actinin-3 at all. Luckily they don’t suffer from any muscle disorders, because a similar protein, alpha-actinin-2 is thought to compensate for the lack of alpha-actinin-3.

Multiple studies have consistently observed a significant under-representation of 577XX in sprint/power athletes. Elite Greek track and field athletes were twice as likely to carry two functional versions of ACTN3 (577RR) compared to the general population, a difference that was even more pronounced when they only compared sprinters to non-athletes. The defective 577XX genes are also significantly less prevalent in elite-level strength athletes in the US, Russian power-oriented athletes, and Australian sprint/power athletes. These differences can be at least partly explained by the greater number of fast-twitch fibres found in individuals with two functional copies of ACTN3. 

It’s true that the “sprinter” version of ACTN3 is more common among Jamaican sprinters. However, at the Olympic level, many of their competitors also have this same genetic speed advantage. This means ACTN3 alone can’t explain the Jamaican sprinting prowess. The more interesting question perhaps is why are there approximately a billion people around the world who no longer make any alpha-actinin-3? Just what is their advantage?

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