Variation in cytokine genes can contribute to severity of acetabular osteolysis and risk for revision in patients with ABG 1 total hip arthroplasty: a genetic association study

Total hip arthroplasty (THA) is a procedure for alleviating end-stage osteoarthritis with significant impact on the quality of life in these patients [1]. However, the life-in-service of joint implants is limited predominantly by aseptic loosening and periprosthetic osteolysis both causally linked to wear debris that are generated continuously by THA [2]. Phagocytosed wear particles provoke macrophages, fibroblasts and other cells to release proinflammatory cytokines and mediators that attract other precursor cells. As a result, chronic inflammatory milieu and a foreign body granuloma develop [3]. This, together with direct inhibition of osteoblast function by wear particles [4], distorts homeostasis in bone in favour of excessive bone resorption, i.e. osteolysis, which can lead to aseptic loosening or periprosthetic fracture.

The concept of cytokines as initiators and perpetuators of particle disease is supported by reports on proinflammatory cytokine (IL-1, IL-6, TNF-α) up-regulation in macrophages and fibroblasts in response to wear particles [5, 6]. These pleiotropic cytokines substantially promote the recruitment and maturation of osteoclast precursors at the bone-prosthesis interface [7]. Particle disease/osteolysis might be also facilitated by down-regulation of immunomodulatory cytokines with anti-osteoclastogenic properties (e.g. IL-4, IL-10, IFN-γ) as has been already demonstrated in inflammatory joint conditions [8, 9].

However, so called "particle disease" cannot sufficiently explain either the variable degrees of osteolysis found in patients with similar wear rates (and thus exposure to wear particles) or differences in THA survivorship even in the case of the same implant and similar wear rate [10]. For this reason, the concept of individual susceptibility to osteolysis must play a role. Generally, the interaction between the prosthesis and host can be influenced by several less or well known mechanisms including the allergic hypersensitivity, non-allergic and toxic response to a material constituent of the implant [11, 12]. Hypothetically, variation in genes for cytokines can alter gene function and/or expression which may affect the individual's resistance/susceptibility to severe osteolysis [13]. In support of this concept, Wilkinson et al. reported increased rate of osteolysis in patients carrying the A allele of the TNF-238 single nucleotide polymorphism (SNP), [14]. Other studies have addressed variation in single genes for other candidate molecules in relationship with aseptic loosening [15‐17]. Surprisingly, only one study reported the variation in the axis of RANKL/RANK/OPG (at position RANK+575*T) that is considered the single most influencing regulator of osteoclastogenesis [18]. Interesting results were published by Gordon et al. on the association of periprosthetic osteolysis and polymorphism in genes for Wnt canonical pathway (FRZB 200Trp, FRZB 200Arg: 324Arg haplotype), [19]. Recently, the same team identified the association between carriage of IL1RN +2018*C allele and a decreased risk of osteolysis after THA [20]. Taken together, none of the reported findings has been replicated in independent samples thus still being considered as preliminary. Therefore, researchers in the field are strongly encouraged to perform well-organized replication studies to enable a meta-analysis [21].

This study was conducted to investigate the contribution of genetic variation in proinflammatory/immunomodulatory cytokine genes to the risk of development of severe periprosthetic osteolysis. In addition, this study addressed the question whether there is an association between particular cytokine gene variants and risk of THA failure.

This comprehensive study investigated the association between extent of acetabular osteolysis in THA and polymorphic variants across a wide spectrum of genes encoding for cytokines/cytokine receptors with inflammatory and/or immunomodulatory properties. In particular, the study suggests that SNPs in genes for proinflammatory cytokines TNF-α and IL-6 may play a role in the pathogenesis of osteolysis. In addition, a SNP in the gene for the Th1 cytokine IL-2 was found to be a factor associated with severity of acetabular osteolysis and risk for premature THA failure.

In this study, carriage of the TNF-238*A allele was associated with severe acetabular osteolysis and risk for premature failure in contrast to the TNF-238 "GG" genotype. Individuals carrying the TNF-238*A were six times more prone to develop severe osteolysis than patients with the GG genotype. Accordingly, the striking risk for severe acetabular osteolysis associated with TNF-238*A carriage could be detected here at statistically significant level though this variant is relatively rare (see power calculations in Additional file 1). Our study, therefore, confirms that TNF-238 SNP plays a role in the development of osteolysis as suggested by Wilkinson et al. [14]. Concerning another SNP in TNF gene promoter, our findings are concordant to those of Wilkinson et al. and Kolundzic et al. [17], who found no relationship between the carriage of the TNF-308*A allele and risk for osteolysis development or premature prosthetic failure.

To properly evaluate the contribution of SNPs in the TNF gene to osteolysis development and prognosis of THA, it should be acknowledged that the minor TNF-238*A allele occurs rarely in the normal population, limiting its contribution to the overall "population" risk of severe acetabular osteolysis (PAR value, 5.2%). This SNP may, however, serve as a marker for other causative polymorphism(s) in the TNF locus or neighbouring polymorphic major histocompatibility complex (MHC) genes. This interpretation would be also in accordance with equivocal data from in vitro/ex vivo analyses of the functional influence of TNF-238 and also TNF-308 polymorphisms on TNF gene transcription and subsequent cytokine expression, processes which seem to be highly cell or stimulus specific [38, 39].

A SNP in the IL6 gene promoter was another polymorphism implicated in the pathogenesis of OL in this study. IL-6 is a multifunctional cytokine also involved in the regulation of bone metabolism; therefore its participation in the processes occurring at the bone-prosthesis interface is plausible [6]. Here, IL6-174*G allele carriage was associated with the increased risk of severe osteolysis according to both the univariate and multivariate analyses. However, this SNP in gene for IL-6 did not influence the risk for cumulative hazard of THA failure. Association of IL6-174*G with severe osteolysis could not currently be supported by straightforward "mechanistic" explanation because its functional role on IL-6 expression is controversial. The allele IL6-174*G was originally associated with higher induced expression of IL-6 in vitro [40], however this was not subsequently confirmed [41]. Importantly, Terry et al. revealed that effect of IL6 genetic variation on IL-6 expression is tissue-specific and dependent on other environmental factors [41, 42]. This observation may explain the opposite roles of IL6-174 SNP observed in different clinical conditions [40, 43, 44]. Our results, therefore, allow us to speculate that complex local response around THA provides adequate conditions/stimuli promoting "penetrance" of the IL6-174 variant in terms of its effect on IL-6 expression. Concerning previous findings on IL6 gene in THA outcome, Malik et al. [16] like us, reported no association of IL6-174 SNP with the risk of premature failure of cemented THA. The association of the IL6 SNP (positions -597 and -572) with the earlier prosthetic failure has been reported by Kolundzic et al. [17]. However, their study had a small number of patients and included both hips in the same patient into the analysis.

Based on our data the IL2-330*G genetic variant may function protectively against severe osteolysis which could result in significantly lower cumulative hazard of THA failure. Interestingly, Campos et al. [45] found that polymorphism in the IL2-330*G was not associated with premature failure of dental implants. Due to biological and mechanical similarities between THA and dental implants, both studies suggest a protective role of IL2 SNP on the prosthetic-bone interface. IL-2 regulates both survival and death of regulatory T cells which might affect the osteoclast life cycle [46]. However, the functional relevance of the investigated IL2 SNP is not clear [47, 48] and thus the role of IL-2, and therefore also Th1 lymphocytes in the pathogenesis of periprosthetic osteolysis has yet to be elucidated [11]. Whatever the mechanism behind possible IL-2 participation in pathological processes around THA, if proven in other centres/populations, the protective effects of the IL2-330*G would have clear implications because the G allele is carried by more than 50% of Europeans [29, 49].

While interpreting the results of the current study, one should consider the theory of multifactorial susceptibility to complex disease and current issues of genetic association studies (GAS). Any individual genome contains many functional variants and many of these can influence the development of osteolysis in conjunction with other biological, mechanical and material factors. Furthermore, one may also conceivably argue that other genetic "elements" than those identified in this study may be involved in the complex process of particular disease via phenomenon of linkage disequilibrium [50]. Future investigations should be directed at detailed analyses of specified candidate regions on chromosomes 4q, 6p and 7q in order to search for further genetic markers of osteolysis developing after THA. Finally, based on current experience showing that the great proportion of GAS has not been replicated in independent samples, further validation through replication studies is strictly recommended [51]. In this regard, it should be emphasised that our study replicated the findings of Wilkinson et al. for TNF-238*A [14].

All three authors are with the Faculty of Medicine and Dentistry, Palacky University, Czech Republic and in parallel with the Faculty Hospital Olomouc, Czech Republic. Associated Professor Jiri Gallo, MD, PhD is the chief of Department of Orthopaedics; Assistant Professor Frantisek Mrazek, MD, PhD is at the Department of Immunology and leads the DNA section of the Tissue Typing laboratory, Faculty Hospital; Professor Martin Petrek, MD, PhD, is the acting head of the Department of Clinical Chemistry & Immunogenetics, the Principal Investigator at the Laboratory of Immunogenomics & Proteomics and the Director of the Tissue Typing Laboratory.