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A Gene for Speed

Credit: Wikimedia Commons

Olympic gold medallist Usain Bolt is regarded as the fastest man alive. He is the first man to hold both the 100 and 200 metre world records, as well as the 4 x 100 metre relay. Credit: Wikimedia Commons

By Peter Houweling & Kathryn North

A gene that may have enabled ancient humans to spread to colder climates may also be the difference between power athletes and the rest of us, and play a role in muscle diseases.

The health of our skeletal muscle is crucial for everyday activities. It provides structural support and stability to bones and joints and is also a key powerhouse for energy production and use.

Our research team at the Murdoch Childrens Research Institute focuses on how our genes effect the ability of our skeletal muscle to perform across the spectrum of health, including both inherited and acquired muscle diseases. By examining both athletes and people affected by muscle diseases we are able to look at the contrasting role our genes play under these extremely different settings.

In 1999 we discovered a variant of the ACTN3 gene, which encodes the protein α-actinin-3. This structural protein is found in the fast-twitch skeletal muscle fibres that produce the rapid, powerful movements that set elite sprinters and weightlifters apart from the rest of us.

The ACTN3 gene variant R577X is common, and results in complete deficiency of α-actinin-3 in almost 20% of the general population (or 1.5 billion people worldwide). In contrast, we found that α-actinin-3 deficiency is extremely rare in sprint athletes, suggesting that this protein plays a crucial role in the function of fast-twitch muscle fibres.

The association between ACTN3 and athletic performance has since been replicated in athletes from around the world. The effect in sprint athletes is particularly strong: of the 74 Olympic-level sprint athletes that have so far been tested for ACTN3, not a single one is deficient for α-actinin-3.

Hence α-actinin-3 is commonly referred to as the “gene for speed” and is now one of the best characterised and most frequently studied genes related to sport and exercise performance.

In our laboratory we have generated an ACTN3 gene knockout mouse to determine how α-actinin-3 influences muscle performance and metabolism. We are also interested in its role in health and disease, including its interaction with the inherited muscle disorder Duchenne muscular dystrophy.

Fast or Slow? Long or Strong?

Our skeletal muscle consists of two α-actinin proteins, α-actinin-2 and α-actinin-3. While it was originally thought that these proteins provide structural support to our muscles during contraction, we now know that they interact with many different proteins that have structural, metabolic and signalling roles. Functional differences in these pathways have been identified in α-actinin-3-deficient muscle, and these differences combine to alter muscle function.

α-Actinin-3 is predominately expressed in the fast-twitch fibres of our skeletal muscle. These fibres are responsible for rapid and forceful contractions but they fatigue quickly and are prone to injury. This is the opposite of our slow-twitch muscle fibres, which generate less force but are resistant to fatigue.

The R577X variant plays an important role in our muscles’ ability to generate strength. Through changes in muscle structure, metabolism and cellular signals, people who are α-actinin-3-deficient have a shift in their fast-twitch muscle fibres towards the properties of a slower muscle fibre. Therefore α-actinin-3-deficient muscles generate less power during contractions but recover more quickly from fatigue.

Deficiency of α-actinin-3 does not cause muscle disease, but is a natural variant that influences muscle function and performance.

Athletes Are a Breed Apart

In 2003 we examined 439 elite Australian athletes and 436 unrelated controls to demonstrate that the loss of α-actinin-3 is detrimental to sprint performance in elite athletes. In this study the number of α-actinin-3-deficient sprint/power individuals was significantly lower in both male and female sprinters, with no Olympic sprint athlete being α-actinin-3-deficient.

This landmark study has now been repeated in at least 15 independent studies of athletes from around the world. To date, no elite sprint athlete has been found who is α-actinin-3-deficient. On the other-hand, α-actinin-3 deficiency was found to be higher in female Australian endurance athletes.

In addition, the R577X variant also influences normal muscle function in non-athletes. Both men and women deficient in α-actinin-3 show reduced muscle strength and take longer to complete timed sprints. These findings are consistent with the athlete data, and suggest that α-actinin-3 deficiency has a detrimental effect on sprint/power performance and potentially benefits endurance sports.

In order to study the loss α-actinin-3 in more detail we developed an ACTN3 knockout mouse that doesn’t expresses α-actinin-3 in its skeletal muscle. While the knockout mice look the same as their wild-type littermates, we observed clear differences in muscle function and metabolism.

ACTN3 knockout mice generate less force but run further on a motorised treadmill, mirroring the performance of α-actinin-3-deficient human muscles. Knockout mice also show a shift in the metabolic characteristic of their muscles.

Given the specific expression of α-actinin-3 in fast fibres, the functional effects on sprint and endurance performance and the interaction between the α-actinins and many metabolic proteins, we investigated whether the loss of α-actinin-3 produced changes in skeletal muscle metabolism. We examined the two principal metabolic pathways in skeletal muscle: anaerobic metabolism (predominantly fast-twitch muscles) and the slower, more efficient aerobic metabolism predominantly found in slow-twitch muscles.

The data indicated a shift in the muscle metabolism of ACTN3 knockout fast fibres away from their traditional reliance on anaerobic metabolism to the aerobic metabolism of slow-twitch muscles.

We have also begun to identify the molecular switches that cause these changes in muscle strength and metabolism. In 2013 we demonstrated that calcineurin activity – a key signalling protein that influences the fast-to-slow skeletal muscle fibre type change – is higher in the absence of α-actinin-3. We believe this drives many of the features associated with the loss of α-actinin-3, including reduced muscle strength and increased endurance performance.

It’s important to note that no single gene can be used to determine our athletic ability. Like many physical features, athletic performance is a complex characteristic that involves both our genes and the environment. There are likely to be many genes that contribute to athletic performance, but α-actinin-3 was the first for which a clear association has been demonstrated in many athlete and non-athlete groups.

An Adaptation to Cold?

By studying α-actinin-3-deficient humans and the ACTN3 knockout mouse, significant progress has been made in understanding how ACTN3 expression alters muscle function. This has led to an appreciation of the diverse roles that α-actinin-3 plays in our skeletal muscle. But how did this variation become so common in modern humans?

The ACTN3 gene is estimated to be over one million years old. However, the genetic signatures surrounding the ACTN3 R577X variant suggests recent positive selection for the loss of α-actinin-3 as modern humans migrated out of Africa into colder climates around 15–30,000 years ago. This means that the number of people who possess the R577X allele is at its highest in places with reduced mean annual temperature and food availability. This suggests that the R577X allele may have conferred resistance to cold exposure or famine.

R577X is one of only two known examples in the human genome where a gene variant results in a clear selection advantage. (The other is a variant in the CASP12 gene, which influences our ability to resist serious infection).

As such, there is great interest in understanding how α-actinin-3 deficiency provides an advantage and why the absence of this protein alters human muscle function today. We have already shown that the loss of α-actinin-3 appears to be detrimental to sprint/power performance but may benefit muscle endurance. The current theory for this increase in α-actinin-3 deficiency is that a shift towards slower muscle fibres that use energy more efficiently provides a survival advantage during exposure to cold and famine.

Muscle Diseases

It is now well established that ACTN3 influences performance in elite athletes, but the ultimate goal of our laboratory is to find cures for children suffering from severe muscle diseases. Recently we have started to explore how ACTN3 influences the severity and progression of diseases associated with muscle weakness.

To understand the effect of α-actinin-3 in muscle development and disease we examined our ACTN3 knockout mouse in response to muscle disease. Using a method that is similar to prolonged bed rest in humans, which results in muscle wasting over time, we examined how wild-type and ACTN3 knockout mice responded to immobilisation. Interestingly, we were able to show that α-actinin-3 deficiency protects against muscle breakdown in response to immobilisation. ACTN3 knockout mice resisted muscle wasting compared with wild-type controls.

Currently we are building on the knowledge we have gained by studying elite athletes and the ACTN3 knockout mouse to determine the role of α-actinin-3 in inherited muscle disorders such as Duchenne muscular dystrophy, as well as muscle-wasting conditions seen during ageing some types of cancer. We propose that the changes in muscle strength and metabolism induced by ACTN3 will influence the way individuals respond to and develop different disease conditions.

Australia has a fantastic track record in the science of sport performance, and it would be great if we can build on that knowledge to help find new treatments for inherited muscle diseases and wasting conditions.

Peter Houweling is Senior Research Officer at the Murdoch Childrens Research Institute, where Kathryn North is Group Leader and Director.