The genetics of endurance: Frequency of the ACTN3 R577X variant in Ironman World Championship athletes

Abstract

Objectives

To investigate the frequency of the ACTN3 R577X polymorphism in elite endurance triathletes, and whether ACTN3 R577X is significantly associated with performance time.

Design

Cross-sectional study.

Methods

Saliva samples, questionnaires, and performance times were collected for 196 elite endurance athletes who participated in the 2008 Kona Ironman championship triathlon. Athletes were of predominantly North American, European, and Australian origin. A one-way analysis of variance was conducted to compare performance times between genotype groups. Multiple linear regression analysis was performed to model the effect of questionnaire variables and genotype on performance time. Genotype and allele frequencies were compared to results from different populations using the chi-square test.

Results

Performance time did not significantly differ between genotype groups, and age, sex, and continent of origin were significant predictors of finishing time (age and sex: p < 5 × 10⁻⁶; continent: p = 0.003) though genotype was not. Genotype and allele frequencies obtained (RR 26.5%, RX 50.0%, XX 23.5%, R 51.5%, X 48.5%) were found to be not significantly different from Australian, Spanish, and Italian endurance athletes (p > 0.05), but were significantly different from Kenyan, Ethiopian, and Finnish endurance athletes (p < 0.01).

Conclusions

Genotype and allele frequencies agreed with those reported for endurance athletes of similar ethnic origin, supporting previous findings for an association between 577X allele and endurance. However, analysis of performance time suggests that ACTN3 does not alone influence endurance performance, or may have a complex effect on endurance performance due to a speed/endurance trade-off.

Introduction

Genetic factors are known to be large contributors to endurance performance, as many endurance phenotype traits show high heritability estimates.¹ Currently, 12 genes have been reported as being significantly associated with elite endurance athlete status by candidate gene studies investigating endurance athletes of various types. These are outlined in the series of “Human Gene Map for Performance and Health-related Fitness Phenotypes” (first installment published 2001²). Of these genes, the ACTN3 gene has been studied most extensively in endurance athletes from various populations around the world3, 4, 5; however, study conclusions have been varied.

The ACTN3 gene codes for the α-actinin-3 protein, which belongs to a group of tissue-specific actin-binding proteins called α-actinins 1–4. While α-actinin-1 and -4 are non-sarcomeric and assist in cytoskeletal anchoring, α-actinin-2 and -3 are expressed in muscle fibres and help anchor sarcomere actin filaments in the Z-discs of skeletal muscle during contractions, contributing to tensile strength.⁶ Though α-actinin-2 and α-actinin-3 are ∼80% identical, α-actinin-3 expression is limited to type II muscle fibres (50% of type IIa fibres and 100% of type IIx fibres), while α-actinin-2 is expressed in both type I and type IIa and IIx fibres.6, 7 Type IIx fibres are known as fast-twitch fibres due to their increased maximal shortening velocity and rate of force development compared with slow-twitch type I fibres. Additionally, type IIx fibres have reduced capacity to generate energy from oxidative processes and are more reliant on anaerobic glycolytic processes to generate energy compared to type I fibres. Therefore, α-actinin-3 expression is predominantly associated with the anaerobic rapidly contracting type IIx fibres.⁶ This is supported by studies with α-actinin-3 deficient knockout mice, which showed that α-actinin-3 deficiency results in a decrease in size of type IIx fibres, a decrease in muscle strength and contractability,⁸ and a shift toward aerobic energy-generating processes in type II fibres, making muscle fibres more fatigue-resistant.⁶

In humans, such an α-actinin-3 deficiency results from the ACTN3 (arg) R577X (ter) nonsense mutation (C>T SNP rs1815739), which results in a truncated, non-functional protein.⁷ The ACTN3 577R allele has been associated with increased sprinting ability in sprint/power athletes, and the 577X allele has been investigated in relation to endurance performance. However, while some studies show an association between ACTN3 and endurance,4, 9 others have failed to show a significant association in their population.10, 11 A recent meta-analysis examining ACTN3 studies on sprint/power athletes and endurance athletes found an association for the R allele with sprint/power athletes in European Caucasians but none for West Africans, and no association for the X allele in endurance athletes in Caucasians or East Africans.¹² This analysis did note significant heterogeneity in both analyses of the European athletes, though this may be due to the fact that the Caucasian groups originated from quite geographically separate nations (Australia, Finland, Spain, Russia, Israel, etc.). It is well known that allele frequencies range from approximately 0.1 to 0.5 in different populations.⁸ This divergence in results may also in part be due to the difficulty in defining elite endurance athlete status as a phenotype, as it has been estimated that as little as 2.5 min separates the all-time top 50 performers amongst marathon runners.¹³ Some studies overcome this difficulty by sampling athletes competing at extreme levels in endurance events, such as Olympic-level competitors.

To investigate the ACTN3 R577X polymorphism in elite endurance athletes, we collected DNA samples from a population of athletes who had qualified for participation in the Kona Hawaii Ironman championship triathlon. In addition to being an ‘ultra’ distance event – more than double Olypmic marathon distances per leg – the Ironman championship is considered one of the most extreme endurance events in the world due to the qualifying requirements and the severe environmental conditions encountered during the race. Qualifying athletes can therefore be considered among the elite of worldwide endurance triathletes. These race participants represent athletes with an extremely high level of endurance ability and present a valuable opportunity to investigate ACTN3 R577X frequencies in elite endurance athletes.