Introduction

Some gene polymorphisms are candidates to explain individual variations in muscle phenotypes. The myostatin (MSTN or growth differentiation factor 8, GDF8 [MIM601788]) gene [1] is receiving growing attention in the last years. The MSTN gene encodes myostatin, a skeletal muscle-specific secreted peptide that functions mainly to modulate myoblast proliferation and thus muscle mass and strength [2]. Variants of the MSTN gene are associated with muscle hypertrophy phenotypes in a range of mammalian species, most notably cattle [3], [4], dogs [5] and mice [2]. A MSTN polymorphism was recently associated with sprinting ability and racing stamina in thoroughbred horses [6]. The myostatin-null mouse model also provides insights into the physiological role of this protein. Besides its function in reducing sarcopenia [7], it appears that myostatin also regulates the structure and function of tendon tissues, as the stiffness of tendons is 14 times higher in myostatin-deficient mice than in their wild type controls [8].

Variations in the MSTN gene, as well as myostatin inhibition, can also have functional consequences in humans (see below). The potential association between MSTN variations and muscle mass phenotypes is best exemplified in the study by Schuelke et al. [9]. They reported the case of 4-year old child with both copies of the MSTN gene carrying a mutation (g.IVS1+5g→a transition in the splice donor site in intron 1) that results in a premature stop codon and failure to synthesize a mature, functioning protein. The child exhibited extraordinary muscle development for his age and precocious physical prowess. Systemic treatment with the myostatin inhibitor MYO-029 provides an adequate safety margin and can induce improvements in the muscle strength/function of adult patients with muscular distrophies [10]. As this type of treatment would be likely to also stimulate muscle growth in healthy humans, myostatin manipulation could be among the next generation of doping in elite sports [11].

Of the identified MSTN variations in humans, the Lys(K)153Arg(R) polymorphism located in exon 2 (rs1805086, 2379 A>G replacement) is one candidate to influence skeletal muscle phenotypes [12]. The Lys(K)153Arg(R) aminoacid replacement is found within the active mature peptide of the myostatin protein; it could theoretically influence proteolytic processing with its propeptide, or affinity to bind with the extracellular activin type II receptor (ActRIIB). The latter results in intracellular activation the SMAD pathway, through which myostatin induces myoblast proliferation [13] and differentiation [14], and thus muscle mass [15]. The frequency of the mutant R allele is of about 3–4% among Caucasians, with a frequency of mutant homozygotes (RR) below 1% [12], [15], [16]. Such low allelic frequency certainly limits the possibility of studying large groups of people carrying the R variant. To date, published data on the MSTN K153R polymorphism and human muscle phenotypes have yield controversial results, partly attributable to inter-ethnic and gender differences. Kostek et al. [15] recently found an association between the variant MSTN 153R allele and maximal isometric contraction of the elbow muscle flexors in African-American young adults of both genders, yet not in Caucasians. Previous studies reported no significant effect of MSTN variants on the muscle mass response to strength training of either Caucasians or African Americans of both genders, including World-class body-builders and elite power lifters [12]. Although, in another study MSTN genotypes did not explain differences in the hypertrophic response to strength training in adults of both genders, when women were analysed separately, the 153R allele was associated with a greater muscle hypertrophic response to training [17]. The MSTN K153R polymorphism can also affect muscle phenotypes in the elderly [16], [18], [19]. For instance, Seibert et al. reported lower muscle strength in old African American women (n = 54, 70–79 years) who carried the variant 153R allele [18].

No study has yet assessed the association between MSTN genotypes and muscle power during naturally occurring movements, e.g. jumping and sprinting tasks. It was the purpose of our study to examine the association between the MSTN K153R polymorphism and ‘explosive’ leg power of non-athletic young adults, as assessed during specific jumping and sprint tests. We hypothesized that the 153R allele is associated with decreased performance in the aforementioned tests. We also genotyped the MSTN exonic variants E164K (rs35781413), I225T and P198A because they also seem to cause amino acid replacements in the gene product (myostatin) expressed in human skeletal muscle [20].

Methods

Results

We detected no failures in sample collection and DNA acquisition. Genotyping success rate was >99.29% (two missing data, one man and one woman).

Genotype distributions met Hardy-Weinberg equilibrium (P = 0.59). No subject carried the variant alleles E164K, I225T, or P198A. We found only one woman with the KR genotype; thus, we present the results only for men. Table 1 shows the association between the K153R polymorphism and study phenotypes in men. We observed that men with the KR genotype had a worse performance in vertical jump (SJ and CMJ) compared with those with the KK genotype. The results persisted after adjusting for multiple comparisons. The variance explained ranged from 5 to 10%. Effect size statistics, as measured by the Cohen's d, indicated a medium effect size. Performance in sprint tests did not differ by K153R genotypes.

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Table 1. Mean estimates of study phenotypes by genotypes of the K153R (rs1805086) polymorphism in the GDF8 gene in men.

https://doi.org/10.1371/journal.pone.0016323.t001

Discussion

The main, novel finding of our study was that the variant 153R allele of the MSTN K153R polymorphism is associated with decreased jumping performance in young non-athletic men. Sprinting (running) ability was however unaffected by the MSTN K153R genotypes. Although more research is needed, and while keeping in mind that exercise-related phenotypes are likely polygenic, our data give support for a role of the MSTN K153R polymorphism in explaining, at least partly, individual variations in the humans' capacity for muscle ‘peak’ power generation. In contrast, in the present cohort of subjects we previously found no association between performance in the jump/sprint tests and the R577X polymorphism in the gene (ACTN3) encoding α-actinin-3 [21]. This variation is thought to play an important role in the muscles' ability to produce high power, at least in elite athletes [27].

We assessed ‘explosive’ muscle power by means of jumping and sprinting tests, which are naturally occurring multi-joint movements in humans that involve the coordinated participation of the majority of lower limb muscles [28], [29]. We believe this is in fact a strength of our study versus previous research in the field of genetics and exercise-related phenotypes that used other tests for muscle power assessment, for instance, maximal concentric muscle work during single-joint movements (e.g. flexor elbow contractions) at relatively low angular velocities (≤120°·s⁻¹) [30]. However, during actual natural high muscle power actions such as the sprint and jumps performed by our subjects, angular velocities at the hip or knee joints can approach 800–1000°·s⁻¹ [31]. To note is that our findings are partly limited by the fact that we did not assess muscle mass in our cohort, and therefore we could not determine whether the influence of MSTN K153R genotypes on muscle power is mediated by its expected effects on muscle mass. Finally, the finding that the MSTN K153R polymorphism was associated with vertical jump performance but not with sprint performance warrants further investigation. Although both tests are thought to determine muscle power performance, stationary jumps and running sprints are determined by different factors. The critical factor during running sprints, owing to the short duration of the foot contact on the ground, is the rate of force development, which in turn is determined by many factors such as muscle fibre type, synchronization of motor units, tendon stiffness, or lean mass of lower extremities [32]. In contrast, the ability of leg muscles (quadriceps) to produce power during the concentric phase of muscle contraction is the main factor affecting stationary vertical jumps as the ones we used here [33]. The elastic properties of tendons can also influence jump ability, at least in the case of CMJ. Compared with a stiffer muscle tendon complex (MTC), people with a more compliant MTC should be more efficient in utilizing elastic strain energy during jumps [34], [35]. The fact that our KR subjects showed worst jumping performance than their wild-type KK counterparts could be associated, at least partly, with a potential role of myostatin in tendon structure. Myostatin-deficient mice showed indeed 14 times higher tendon stiffness than wild-type mice [8]. Further research is needed to determine the possible association between MSTN polymorphisms and tendon characteristics in humans. Up to date, published data on the MSTN K153R polymorphism and human muscle phenotypes (at baseline or in response to training) have yield controversial results, at least in adults of young or medium age. Inter-ethnic differences in allele frequencies, gender-related differences and the low allelic frequency of the 153R allele (limiting the possibility of studying large groups of people carrying the R variant) are important reasons for controversy. Kostek et al. [15] recently found an association between the MSTN 153R allele and maximal isometric contraction of the elbow muscle flexors in a group of 23 African-American young adults of both genders, yet this association was not corroborated in a much larger cohort of Caucasian young adults (n = 509, also men and women). Maximal dynamic contraction (one repetition maximum) was also unaffected by MSTN genotypes in both cohorts. Ferrell et al. [12] reported no significant effect of the MSTN variants we studied here on the muscle mass response to strength training in either Caucasians or African Americans (n = 153 men and women). In another study [17], MSTN genotypes did not explain differences in the hypertrophic response to strength training in 32 adults (age range: 21–75 years) of both genders studied as a group; yet, when women were analyzed separately the 153R allele was associated with a 68% larger increase in muscle volume in response to training. Thomis et al. [30] reported similar values in elbow flexor strength at baseline or in response to training in a young adult with the 153KR genotype compared with those with the 153KK genotype. Evidence for the putative influence of the MSTN K153R polymorphism on muscle phenotypes is probably stronger in the elderly [16], [18], [19]. Notably, in a cohort of old African American women (n = 54, 70–79 years). Seibert et al. [18] reported lower muscle strength (hip and knee flexion and handgrip strength combined) in those who carried the 153R allele. We recently reported lower muscle mass/function in a very old woman (age 96 years) with the very rare MSTN 153RR genotype compared to her age-matched referents with the 153KK genotype [19].

Although more research is needed, the putative effect of the K153R polymorphism on muscle phenotypes is due to its potential to alter the function of the MSTN gene [12]. Myostatin enters the bloodstream as a latent precursor protein; it then undergoes a proteolytic process to become a mature peptide (free from the propeptide) that binds to extracellular activin type II receptor (ActRIIB) [15]. Binding of myostatin to ActRIIB induces intracellular activation of SMAD proteins and, through the SMAD pathway, myostatin modulates myoblast proliferation [13] and differentiation [14], and thus ultimately muscle mass [15]. The Lys(K)153Arg(R) aminoacid replacement is found within the active mature peptide of the myostatin protein, and could theoretically influence (i) proteolytic processing with its propeptide or (ii) affinity to bind with ActRIIB [36], [37]. This in turn would result in inability of myostatin to modulate muscle mass/power [15].

In summary, the MSTN K153R polymorphism is associated with the ability to produce ‘peak’ power during muscle contractions, as assessed with vertical jump tests, in young non-athletic men. Thus, although more research is still needed, this polymorphism is among the numerous candidates to explain, alone or in combination with other polymorphisms, individual variations in muscle phenotypes.