Original Full Length ArticleRANK/RANKL/OPG pathway: Genetic associations with stress fracture period prevalence in elite athletes
Introduction
Stress fractures arise from the inability of bone to tolerate repeated mechanical loading and are characterised by damage to the bone's micro-architecture [1]. Repeated mechanical loading can cause an uncoupling of osteoblast bone formation and osteoclast bone resorption [1]. This can lead to bone loss and subsequent micro-damage that can result in localised bone weakening, prompting stress fracture development [2]. Stress fracture period prevalence in elite athletes and military recruits ranges from 14 to 21% [3], [4], and most commonly manifests in the lower limbs [5].
In athletes, stress fracture injury is likely to have a complex aetiology involving numerous factors, with, for example, prior training [6] and biomechanical variables (e.g., running kinematics) [7] being implicated in stress fracture risk (for a wider review of this issue please see Bennell et al. [4]). Susceptibility may also have genetic origins, supported by reports of monozygotic twins developing similar stress fracture injuries [8], multiple stress fractures occurring in the same individual [9], stress fractures occurring in some individuals but not in others undertaking identical training protocols [3], [4] and a family history of stress fracture injury acting as a risk factor [10].
Genetic associations with stress fracture period prevalence in military personnel have been investigated using a variety of single nucleotide polymorphisms (SNPs) previously associated with receptors known to influence bone mineralisation, remodelling [11] and endocrine abnormalities [12]. Associations were shown for SNPs and haplotype blocks within the vitamin D receptor [13], [16] and an androgen receptor repeat sequence [14]. However, other studies have shown no association for the same SNPs in similar military populations [15]. The reason for the disparity may be the range of SNPs analysed and small numbers of stress fracture cases in some studies (e.g., n = 64, [16]). There is a need to examine this in a large cohort of elite athletes given that the pathogenesis of stress fracture might be different due to the phenotypic differences, training variables and fitness.
Given that disturbances in bone remodelling and the inability of bone to withstand repeated bouts of mechanical loading are implicated in the development of stress fracture injury [1], SNPs repeatedly shown to be associated with these bone phenotypes in large-scale studies are worthy of focused study. As all previous studies have used military personnel, studies involving alternative groups, such as athletes with a similarly high period prevalence of stress fracture injury, are required to provide further insights into both the aetiology and genetic predisposition of stress fracture injury. Furthermore, no published literature exists in relation to genetic associations with stress fracture injury risk in elite athletes.
The receptor activator of nuclear factor-KB (RANK) and its ligand (RANKL), a member of the tumour necrosis factor (TNF) superfamily, are integral to osteoclastogenesis as they stimulate osteoclast activation, formation and differentiation [17]. Osteoprotegerin (OPG) acts as a decoy receptor for RANKL leading to the prevention of osteoclast precursor development into mature osteoclasts, resulting in the subsequent attenuation of bone resorption [17]. These factors in combination make up the RANK/RANKL/OPG signalling pathway, an important system in the regulation of bone turnover, and in the potential mediation of stress fracture injury development in individuals with a high frequency and/or amplitude of mechanical loading.
The specific mechanisms of how SNPs within the RANK/RANKL/OPG signalling pathway influence bone health remain unknown. Several of these SNPs have been associated with bone phenotypic alterations, including changes in bone mineral density (BMD) [18], [19], [20], [21], bone cross sectional area [22], osteoporotic fracture risk [20] and bone resorption and formation [23], although none of these have been established in stress fracture injury.
This study examined whether SNPs within the RANK/RANKL/OPG signalling pathway were associated with stress fracture injury in elite athletes.
Section snippets
Participants
In total a convenience sample of 518 elite athletes, 449 males and 69 females, were recruited by email and word of mouth from professional sports clubs and elite sporting associations based in North America and the United Kingdom from 2010–2013 to form the Stress Fracture in Elite Athletes (SFEA) cohort (see Table 1 for participant characteristics). Participating elite athletes competed in various sports including, football (n = 218), cricket (n = 156), track and field (n = 67, running events n = 62),
Results
Call rates for RANKL rs1021188, RANKL rs9594738, RANK rs3018362 and OPG rs4355801 were 96.2%, 96%, 95.8% and 95.6%, respectively. All SNPs were within Hardy–Weinberg equilibrium apart from OPG rs4355801 (P = 0.04). The method of genotyping is robust and a high level of internal validation and reliability make errors in genotyping an unlikely reason for the deviance.
Discussion
To our knowledge, this is the first study to examine the genetic associations with stress fracture injury in elite athletes, with all other studies to date being from cohorts of military personnel. This study is the first, from any population, to show that SNPs within the RANK/RANKL/OPG signalling pathway are associated with stress fracture injuries; SNPs rs3018362 and rs1021188, which are not in linkage disequilibrium [19], located near RANK and RANKL were associated with stress fracture
Disclosure statement
The authors have nothing to disclose.
Acknowledgments
We are grateful to all athletes for their participation in the study.
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