ACTN3

ACTN3 is a gene that encodes a structural protein, alpha-actinin 3, associated with muscle fibers.

Fast twitch versus slow twitch muscle fibers
Skeletal muscle is composed of long cylindrical cells called muscle fibers. There are two types of muscle fibers, slow twitch or muscle contraction (type I) and fast twitch (type II). Slow twitch fibers are more efficient in using oxygen to generate energy whilst fast twitch fibers are less efficient. However, fast twitch fibers fire more rapidly and generate more force.

Muscle fiber composition in elite athletes
On average each person has an even percentage of each fiber type but Olympic sprinters tend to have around 80% fast twitch fibers. Conversely, Olympic marathon runners tend to have around 80% slow twitch. There is controversy whether training may alter the percentage of fiber type percentage over time therefore heredity or genetics should be considered as playing the greatest role.

ACTN3 in muscle fiber
Each muscle fiber is composed of long tubes called myofibrils which in turn are composed of filaments. There are two types of filaments: actin (thin filaments) and myosin (thick filaments) which are arranged in parallel. A muscle contraction involves these filaments sliding past each other.

Actin filaments are stabilized by actin binding proteins known as actinins of which there are two main types, type 2 and type 3. Each of these is encoded by a specific gene, ACTN2 and ACTN3 respectively.

ACTN2 is expressed in all skeletal muscle fibers whereas ACTN3 is expressed only in fast twitch fibers.

The rs1815739 mutation
A mutation (rs1815739) has been identified in the ACTN3 gene which results in a deficiency of alpha-actinin 3 in a significant proportion of the population. Based on ethnicity the deficiency is found in 20-50% of people. Generally, African Americans have the lowest incidence of the mutation whilst Asians have the highest. Scientists believe that variations in this gene evolved to accommodate the energy expenditure requirements of people in various parts of the world.

Studies have linked the fiber twitch type with ACTN3, i.e. fast twitch fiber abundant individuals carry the non-mutant gene version. Also, studies in elite athletes have shown that the ACTN3 gene may influence athletic performance. Whilst the non-mutant version of the gene is associated with sprint performance, the mutant version is associated with endurance.