Osteoarthritis (OA) is a common joint disease characterized by loss of articular cartilage, remodeling of the synovitis, and alterations of periarticular structures (Castañeda and Vicente, 2017). Any joint can develop OA, but symptoms linked to OA most commonly affect the knees, hips, hands, and feet (Katz et al., 2021). It is reported that persons with knee or hip OA have excess mortality compared with age-matched controls (Nüesch et al., 2011; Katz et al., 2021). Currently, there is no curative drug for OA. According to the estimation of the World Health Organization, there are about 300 million OA patients worldwide, and the prevalence of OA can reach up to 10%–20% due to the increasing obese and longevous population (GBD, 2017 Disease and Injury Incidence and Prevalence Collaborators, 2018). Given the high health economic burden, a better understanding of the risk factors associated with the occurrence of OA, especially modifiable factors, is needed.

Iron is an essential mineral to various biochemical processes, including DNA synthesis, ATP generation, and oxygen transport, where its absence or depletion can cause abnormal metabolization (Abbaspour et al., 2014). However, excess iron can also be toxic as it would be deposited into organs forming free radicals (Abbaspour et al., 2014). Therefore, disorders of iron homeostasis are involved in a wide scope of diseases (Bogdan et al., 2016). In humans, systemic iron status can be measured by clinical biomarkers: Serum iron, transferrin saturation, ferritin, and total iron-binding capacity (Bell et al., 2021). High serum iron, transferrin saturation, and ferritin signify high iron, while high total iron-binding capacity signifies low iron (Winter et al., 2014). Increasing evidence has found that systemic iron status is associated with OA. For example, high synovial iron was associated with a faster progression of OA in a murine model (Camacho et al., 2016). In vitro, iron overload was found to have detrimental effects on various joint components, such as synovium, cartilage, and subchondral bone, leading to synovial hyperplasia and inflammation, abnormal osteoblast function, and chondrocyte apoptosis (van Vulpen et al., 2015). Besides, some epidemiological evidence has shown the relationship between iron and OA risk. In a 2-year longitudinal study with 127 OA patients, radiographic findings showed that higher ferritin was significantly associated with narrower baseline joint space width and higher risks in the prediction of Kellgren-Lawrence grade severity (Kennish et al., 2014). Among 40 subjects with symptomatic knee OA, elevated serum ferritin levels were found to be associated with faster progression of cartilage damage assessed by arthroscopy (Nugzar et al., 2018). However, whether high iron status is a causal factor of OA is unclear because of the inherent defects of observational studies, such as residual confounding and reverse causality.

Mendelian randomization (MR) is a study design that can strengthen the causal inference on exposure-outcome relationships by minimizing the effect of confounding and excluding the potential reverse causality based on the use of genetic variants as instruments (Sheehan et al., 2008). The rationale of the causality assessment in MR is that genetic variants solely associated with the exposure are randomly assorted so that genetic effects on the outcome cannot be affected by potential confounders. Also, genetic variants are not modified by the occurrence or development of any diseases where reverse causality is impossible. For these reasons, MR represents robust indirect evidence of a causal relationship between exposure and outcome if any effect of the selected instruments on diseases is entirely mediated through the exposure (Sheehan et al., 2008).

Therefore, we conducted a 2-sample MR (Lawlor, 2016) study to examine the causal effect of four clinical iron biomarkers on four OA phenotypes, including knee OA, hip OA, total knee replacement, and total hip replacement.

A 2-sample MR study was conducted to investigate the potential causal relationship between systemic iron status and knee OA, hip OA, total knee replacement, and total hip replacement by using summary-level data. Systemic iron status was comprehensively represented by serum iron, transferrin saturation, ferritin, and total iron-binding capacity. Figure 1 showed the overview of the exposures, the outcomes, and the three assumptions of the genetic instruments for the MR design. Since this study is based on existing publications and public databases, both ethical approval and participant consent have been received by each relevant institutional review committee.

Tables 1–4 show the associations of the four systemic iron status biomarkers and knee OA, hip OA, knee replacement, and hip replacement using the three instrument sets. In the IVW analyses using liberal instruments, genetically predicted serum iron and transferrin saturation were found to be significantly associated with hip OA and hip replacement, and the other associations were not evident (Tables 1–4; Figure 2). The odds ratios of hip OA were 1.18 (95% CI: 1.07–1.30) per one standard deviation increment in serum iron and 1.15 (95% CI: 1.07–1.24) in transferrin saturation (Tables 1–4; Figure 2).

Similar results were seen in analyses using sensitivity instruments. In the analyses using conservative instruments, genetically predicted total iron-binding capacity was negatively associated with hip OA and hip replacement with ORs ranging from 0.80 to 0.82 (Tables 2, 4). However, statistical evidence of heterogeneity across the MR estimates was found especially for hip OA and hip replacement (Tables 2, 4; Figure 2). For example, in the MR estimates of ferritin on hip OA and hip replacement, the I² of the heterogeneity was 64.8%–91.1%. When we used the Wald ratio method to assess the MR estimates of four individual SNPs (rs57659670, rs855791, rs1799945, and rs1800562) with hip OA and hip replacement, rs1800562 was the SNP that was significantly associated with hip OA in serum iron (OR = 1.48; 95% CI: 1.29-1.68), transferrin saturation (OR = 1.57; 95% CI: 1.17-1.37), ferritin (OR = 2.24; 95% CI: 1.71-2.94), and total-iron binding capacity (OR = 0.79; 95% CI: 0.73-0.86), and hip replacement in serum iron (OR = 1.45; 95% CI: 1.25-1.69), transferrin saturation (OR = 1.25; 95% CI: 1.14-1.37), ferritin (OR = 1.37; 95% CI: 1.58-2.99), and total-iron binding capacity (OR = 0.80; 95% CI: 0.73-0.88). The other three SNPs did not show evident associations with hip OA or hip replacement (data not shown). Also, the leave-one-out analyses demonstrated that the causal effect of systemic iron status on hip OA and total hip replacement was mainly resulted from rs1800562 (Supplementary Table S5). We repeated the MR estimates of the four systemic iron status biomarkers on knee OA and hip OA in the second-largest meta-analysis of OA GWAS, and similar results were shown in Supplementary Table S6. That is, serum iron, transferrin saturation, and total iron-binding capacity were associated with hip OA, and the other associations were not evident.

The present 2-sample MR study used the largest public dataset to date to comprehensively evaluate the causal role of systemic iron status for knee OA, hip OA, total knee replacement, and total hip replacement. In a pattern concordant with an effect on systemic iron status, we found that genetically predicted clinical iron biomarkers were significantly associated with hip OA and total hip replacement. Given the heterogeneity across the genetic instrumental variables, these associations were mainly resulted from rs1800562, a missense mutation of the HFE gene.

Our findings are in line with previous conclusions from observational and experimental studies that iron is involved in the development of OA. In an observational study, synovial fluid iron concentrations determined by the colorimetric method were significantly higher in OA patients compared to the controls (Yazar et al., 2005). Another 2-year follow-up study reported that higher iron status was associated with more severe radiographic progression in knee OA patients (Kennish et al., 2014). In murine models, cellular iron accumulation in the knee joint resulted from systemic iron overload could induce the early onset of OA and accelerate OA progression via compromising chondrocyte metabolism and over-expressing local inflammatory mediators (Camacho et al., 2016; Simão et al., 2019; Burton et al., 2020). Several studies using the MR method also assessed the association between iron and OA, though the results were inconsistent. Using HFE genotype (rs1800562 & rs1799945) instrumented transferrin saturation as the iron load, Pilling et al. (2019) found that homozygotes of rs1800562 or compound heterozygotes of rs1800562 & rs1799945 were positively associated with incident OA in both men and women. Using rs800562, rs1799945, and rs855791 as the instruments of serum iron, no significant association was observed between iron levels and overall OA in UK Biobank (Zhou et al., 2020; Zhou et al., 2021), but per standard deviation increment in iron was associated with increased risk of OA in males (Zhou et al., 2021). Using rs800562, rs1799945, rs855791, and rs8177240 as the instruments of nutritional iron and summary data from the UK Biobank and arcOGEN cohorts, no significant association was observed between the iron and knee OA, hip OA, or overall OA, but the positive causal effect of iron levels on overall OA was observed in females (Qu et al., 2020). Using the instruments from the Genetics of Iron Status Consortium and the summary-level data of outcomes from the UK Biobank and arcOGEN cohorts, Xu et al. (2022) found that transferrin saturation was positively associated with both knee OA and hip OA, transferrin was negatively associated with hip OA, but serum iron and ferritin did not show a prominent effect on OA outcomes. In our study, we assessed the iron status using four biomarkers with three instrument sets while the previous studies used one biomarker with three or four instruments. Our study used the largest meta-analysis data for OA while the others included the relatively smaller size of OA patients. The inconsistencies between the previous studies with the present study can be partly explained by the different biomarkers with non-specific instruments and underpowered samples. Thus, the conclusion from our study may be more representative of the association between iron status and OA.

Our findings only suggest causal associations among hip OA and total hip replacement, but not knee OA or total knee replacement. This could be explained by the fact that genetic heritability contributes more to hip OA than knee OA [14.7% for knee OA and 51.9% for hip OA (Tachmazidou et al., 2019)], and the exposures in the MR study are genetically predicted. In the main analyses and the repeated analyses, we only found causal associations between hip OA & total hip replacement and serum iron, transferrin saturation, & total iron-binding capacity, but not ferritin. This can be partly explained by the heterogeneity among the variants in the analyses (I² = 91% in hip OA and I² = 83% in the total hip replacement). Another possible explanation for this is that ferritin is a representative biomarker of body iron stores in a non-inflammatory state (Adams, 2015; Bell et al., 2021), while OA is regarded as an inflammatory disease (Dainese et al., 2022), and thus, there might be a weak relationship between ferritin and OA. A potential concern for MR conclusions relates to the horizontal pleiotropy of the instruments. Our MR study does not have a such concern, because the findings were robust to analyses using sensitivity instruments that exclude variants associated with potential confounders. Though rs1800562 from the HFE gene is associated with potential confounders glycosylated hemoglobin, low-density lipoprotein, and total cholesterol, we did not exclude this variant in our analyses. This is because the HFE protein encoded by the HFE gene is the central regulator of systemic iron homeostasis and has no other well-established roles (Barton et al., 2015), any effect of the second phenotype may probably be acting downstream of iron status, rather than independent of it. Therefore, rs1800562 may have a vertical pleiotropy with the outcome which does not violate the assumption of MR (Davies et al., 2018). However, there is still a possibility that rs1800562 has other horizontal effects on the outcomes independent of iron.

Besides the causal evidence of the MR relationships, the biological plausibility adds further support for the possibility of causality. Hereditary hemochromatosis is a common disease in Europeans characterized by iron overload in multiple organs where the mutation of rs1800562 is the main contributor (Husar-Memmer et al., 2014). It is reported that people with hemochromatosis are at increased risk of OA and OA is one of the most frequent complications in people with hemochromatosis (Elmberg et al., 2013; Husar-Memmer et al., 2014). The missense mutation rs1800562 from the HFE gene affects the synthesis of HFE protein which is associated with the regulation of the circulating iron by regulating the interaction of the transferrin receptor with transferrin. Studies showed that excess iron could act as a catalyst to produce a large number of reactive oxygen species which are deleterious agents involved in OA cartilage degradation (Henrotin et al., 2003; Yang et al., 2018). Also, evidence showed that iron homeostasis dysregulation could lead to the secretion of pro-inflammatory cytokines which promote chondrocytes apoptosis and subsequent OA cartilage degeneration (Simão et al., 2019; Jing et al., 2021).

Since high iron status is a treatable condition, our findings have important clinical and public health implications. It is reported that phlebotomy, oral chelation, and dietary changes are options for people with iron overload (Palmer et al., 2018). There may be a benefit to the careful reduction of iron indices for reducing OA risk in people with high iron status, especially among hemochromatosis patients or people with the mutation rs1800562. However, the interpretation of the MR finding that iron decrement might be a benefit for OA patients requires justification, because MRs are the effects of lifelong exposures while interventions consider short-term pharmacological treatments. Taken together, iron decrement could be an attractive new target for hip OA treatment which needs to be validated in randomized controlled trials in the future.

The major strength of the present study is the 2-sample MR design, which minimizes the confounding and reverse causality seen in observational studies. We comprehensively investigated the causation of the systemic iron status in four representative biomarkers to knee OA, hip OA, total knee replacement, and total hip replacement using the largest summary-level data. Also, we used three instrument sets to obtain robust conclusions. Furthermore, we repeated our analyses in the second largest OA GWAS dataset with European descent, thereby diminishing bias from population stratification. However, we must acknowledge some potential limitations. First, rs1800562 was the only SNP associated with OA which needs to be verified in another dataset independent from the UK Biobank study and the arcOGEN consortium. Second, the findings of our study relate to patterns of iron status largely within the normal range, and thus, cannot be used to make inferences on the effect of abnormally high or low serum iron levels. Third, our study does not offer insight into whether the estimates are equally applicable to both men and women. Despite these limitations, the results of this work show consistent and biologically plausible effects.

In summary, the present MR findings suggest that high iron status might be a causal factor of hip OA and total hip replacement where rs1800562 is the main contributor. Given the modifiable nature of the iron status, further clinical trials are warranted to validate the therapeutic role of iron decrement in people with hip OA, especially among those with the mutation rs1800562.