No association of the single nucleotide polymorphism rs8044769 in the fat mass and obesity-associated gene with knee osteoarthritis risk and body mass index

Article focus

• To date,no previous study has been conducted to explore the possibility that the single nucleotide polymorphism (SNP) rs8044769 in the fat mass and obesity-associated (FTO) gene predisposes patients to osteoarthritis (OA) development in Asian populations. We investigated the association between the SNP rs8044769 in the FTO gene and osteoarthritis in the Chinese population.

Key messages

• The SNP rs8044769 in the FTO gene was not associated with OA risk in the Chinese population. Further studies are warranted to validate and extend our findings.

Strengths and limitations

• Strengths: This is the first report on the association between the SNP rs8044769 in FTO gene and OA diseases in the Chinese population. Our findings showed that SNP rs8044769 in the FTO gene may neither modify individual susceptibility to OA or increased body mass index in the Chinese population.

• Limitations: More studies should be conducted with larger sample sizes and using different ethnic groups to validate and extend our findings.

Introduction

Osteoarthritis (OA) is the most common degenerative joint disease and has emerged as a major public health concern around the world.^1-3 OA is predicted to be the single greatest cause of disability in the general population by 2030.⁴ Similar increasing trends of OA prevalence can also be found in China.⁵ Despite its high prevalence and substantial public health impact, the aetiology of OA is not fully understood. OA may be considered as a polygenic disease, which is the combined interaction of multiple factors including individual genetic origins and environmental components.⁶ Epidemiological evidence suggests that ageing, genetic predisposition, obesity, inflammation and excessive mechanical loading predispose to OA development.^7-9 Furthermore, the identification of candidate genes and pathways would help elucidate the molecular mechanisms of OA, and have the potential to evaluate the risk for OA and lead to the development of OA gene-targeted therapies.¹⁰

Epidemiological studies have shown that OA has strong genetic components,^11-13 and a number of candidate genes have been implicated as susceptibility loci for primary OA, such as type II procollagen gene(COL2A1),¹⁴ oestrogen receptor-α(ESR-α),¹⁵ growth differentiation factor 5(GDF5),¹⁶ double von Willebrand factor A domains (DVWA),¹⁷ interleukin-6(IL-6),¹⁸ the secreted frizzled-related protein (FRZB)^19,20 and Asporin.²¹

Recently, compelling evidence has suggested that obesity is one of the strongest predictive and prognostic factors for OA, particularly in knee joints, and is considered to be moderately associated with hip OA.^22-26 Previous genetic studies of the fat mass and obesity-associated gene (FTO) have principally focused on obesity properties, which are currently considered as the first and the most consistently replicated candidate genes contributing to obesity in multiple populations of different countries.^27-31 The FTO gene, located on chromosome 16q12.2, consists of nine exons and encodes a 2-oxoglutarate-dependent nucleic acid demethylase. FTO was discovered as an obesity susceptibility gene in 2007.³² Subsequently, a cluster of variants of the FTO gene was identified as those carrying the association and predisposition to obesity-related traits in European populations in both adults and children.^27,33 However, these associations remain controversial. To date, more than 100 single nucleotide polymorphisms (SNPs) in the FTO gene have been reported, among which the rs8044769 is a well-established SNP that is associated with fat mass and obesity, and it is in partial linkage disequilibrium (LD) with other reported body mass index (BMI)(mass/height²)-associated SNPs.²⁷ Interestingly, a recent large well powered genome-wide association study (GWAS) has identified the variant SNP rs8044769 as being strongly associated with risk of OA development.³⁴ However, the fact that the effect was thoroughly attenuated with adjustment for BMI indicates that the FTO gene may exert its effect on OA through obesity;³⁵ that is, the strong association between FTO and OA risk was mediated by BMI.

To the best of our knowledge, no previous study has been undertaken to explore the possibility that the SNP rs8044769 in FTO gene predisposes to OA development in Asian populations. The understanding of ethnicity-specific gene effects would be valuable for the human OA population. On this basis, we speculate that the SNP rs8044769 may influence the susceptibility to OA in the Asian population. We focused on one SNP of special interest: cytosine/thyamine (rs8044769) in the promoter region of the FTO gene. The current study tested the following two hypotheses in the Chinese population: that SNP rs8044769 is associated with susceptibility to OA and that SNPrs8044769 is associated with BMI.

Materials and Methods

Results

The distribution of selected characteristics between the 196 OA cases and the 442 controls are summarised in Table I. In brief, the gender distribution between cases and controls was comparable (p >0.05). The mean age of the cases was higher than that of the controls. Compared with controls, the cases had a higher rate of an overweight BMI (47% vs 33%). The smoking and drinking status between cases and controls could be compared (p > 0.05).

The basic information on the SNP rs8044769is shown in the supplementary material (Table i). The success rate for genotyping for this polymorphism is 98.59% in the current study. The observed genotype frequency for the SNP rs8044769 was in agreement with HWE in the controls (p = 0.15). As shown in Table II, the variant T allele showed no significant association for OA risk in all tested models (CT versus CC: OR = 1.19, 95% CI 0.80 to 1.76; TT versus CC: OR = 1.00, 95% CI 0.60 to 1.69; dominant model: OR = 1.13, 95% CI 0.79 to 1.64; additive model: OR = 1.03, 95% CI 0.81 to 1.32).

Furthermore, in the stratification analysis, the association between the SNP rs8044769 and OA risk was evaluated in subgroups based on age, gender, BMI, smoking status and drinking status. As shown in the supplementary material (Table ii), no significant difference between any subgroups was observed for the association between the SNP rs8044769and OA risk.

Because obesity and age are both established risk factors for OA, we further investigated whether the effect of the SNP rs8044769 variant on OA was modified by age and BMI. As shown in Supplementary Table iii, the interaction of the SNP rs8044769 and age had a borderline significant association with OA risk (interaction p=0.062). Individuals over 57 years old, as well as those carrying the CT/TT genotype had a 149% increased risk of developing OA (OR = 2.49, 95% CI 1.37 to 4.52) compared with those under 57 years old carrying the CC genotype. A similar interactive effect was observed between the SNPrs8044769variant and BMI contributing to the risk for OA (interaction p =0.037) (Supplementary Table iv).

Out of interest, we also investigated whether the SNP rs8044769 is associated with BMI in our study cases and controls, respectively. As shown in Table III, the SNP rs8044769showed no significant association with BMI in the OA case group (p = 0.882 in additive model), while a similar result was observed in the OA control group (p = 0.479 in additive model). Not surprisingly, further meta-analysis of the results from the two groups indicated that the SNP rs8044769 shows no significant association with BMI (p_meta=0.483).

Discussion

Genetic polymorphisms are involved in biodiversity and could be influenced by ethnic heritage and geographic localisation.³⁷ It is therefore of interest to explore the geographical and ethnic distribution of genetic polymorphisms. The purpose of this study was to examine the FTOSNP rs8044769 as a potential genetic candidate for knee OA by means of an association study in a Chinese population. We evaluated the association of the SNP rs8044769with OA risk in a case-control study including 196 cases and 442 controls in a northern Chinese population. The variant T allele of the SNP rs8044769 showed no significant association with OA risk. In addition, the SNP rs8044769 showed no significant association with BMI. To the best of our knowledge, this is the first association study of polymorphisms in FTO with OA and BMI in the Chinese population. Although our association findings are negative, replication in larger studies should be considered given the possibility of beta error due to population stratification and small sample size.

Our present study showed no significant association between the FTOSNP rs8044769polymorphism and OA risk. However, in previous studies conducted in the European population, the SNP rs8044769was definitively associated with increased OA risk.³⁸ There are several reasons for this discrepancy. First, the difference may be due to genetic heterogeneity between different ethnicities since the allele frequency of the SNP rs8044769 polymorphism displays differences among the Asian and European populations.³⁸ Secondly, the sample size of our population might not be large enough to reach a convincing conclusion. Thirdly, bias in patient enrolment criteria and differences in OA affected joint sites could also confound study findings.

The present study has several strengths. First, our OA cases came from a systematic screening of OA in a large population-based study conducted in Heilongjiang Province, while the controls also came from the same community, which may have reduced potential selection bias. Secondly, OA phenotypic definitions, which reflect a different subset of OA, have been shown to influence the ability to detect genetic associations. The focus on radiographic features and clinical diagnosis in the definition of OA is a strength because analyses of patients identified by more stringent and clinically relevant criteria have more powerful implications. Thirdly, by exerting much effort in identifying whether the SNP rs8044769 is associated with BMI, we performed analyses on the OA case group and the OA control group, respectively, and further combined the results using a meta-analysis. Thus, our strictly designed analyses were likely to produce reliable results. Additionally, with effort to maximise the success and accuracy rates of genotyping, we used the Sequenom genotyping platform with stringent quality controls.

However, several limitations of our study also need to be addressed. First, OA is a multifactorial disease with a strong genetic component with various heritability estimates depending on different joint sites. We only evaluated the risk of knee OA and the SNP rs8044769 in the FTO gene and the results cannot be generalised to OA in other joints. Secondly, the current sample size (196 OA cases and 442 control subjects) has a statistical power of 46.55% to detect an effect size of 1.50 with an α-level of 0.05 for the association of the SNP rs8044769 with OA risk. Thus, well-conducted larger sample studies are needed to explore further the OA risk associated withthe SNP rs8044769in Chinese populations. Thirdly, we only genotyped the previously-reported SNP rs8044769 in FTO. Further studies are required in order to genotype more SNPs in FTO, and evaluate whether other SNPs in FTO are associated with OA, as well as BMI, in Chinese populations.

In conclusion, our study investigated the role of the SNP rs8044769 in the FTO gene in OA development and BMI in a Chinese population. The results of our study draw attention to one important issue: the SNP rs8044769presentsno distinct association with OA or BMI in a northern Chinese population. The result is different from the recent GWAS study on hip and/or knee OA.³⁹ Replication of FTO association in other larger study samples is essential for clarification as OA merits significant attention in order to develop better prevention strategies and therapies that can be applied globally. Given the fact that genetic factors may vary with different disease patterns, severity, gender and populations, studies in larger and more diverse populations are warranted.