In this study, we used a two-sample MR approach for the first time to investigate the genetic causality between iron status and IVDD. Although we used GWAS data for multiple iron statuses and performed a meta-analysis of MR analysis results from different sources, our results suggest that there is no genetic causality between ferritin, iron, transferrin, TSAT, and TIBC with IVDD. Although the presence of horizontal pleiotropy in ferritin and TIBC was still detected in the MR-PRESSO global test after removal of outliers, the MR-PRESSO distortion test did not detect outliers, and the results of the horizontal pleiotropy test of MR‒Egger showed no horizontal pleiotropy in ferritin and TIBC, so our results remain robust. There are two possible reasons for this. First, the lack of sample size leads to variation in different analyses. Second, there may be other small-scale pleiotropic problems in the data that are not easy to detect. These pleiotropic problems do not show obvious outliers, but their cumulative effects are sufficient to affect the global test of MR-PRESSO 41.
Iron, an essential trace element, is critical for many biochemical processes 42. Although past studies have found an association between iron status and IVDD, the evidence is mixed. A retrospective study published in 2006 investigated lumbar disc degeneration in patients with thalassemia, which can lead to iron overload in the body as a result of regular blood transfusions, and found that lumbar discs in patients with thalassemia showed significant degeneration on MRI and X-ray evaluations when compared to age- and gender-matched controls, with a higher degree of degeneration in patients who received chelation therapy. In addition, the patient developed other spine-related pathologies, such as osteoporosis and vertebral abnormalities. These results suggest that iron overload may be associated with lumbar disc degeneration. However, the small sample size of this study may have biased the results 43. By evaluating the serum concentrations of selected elements in patients with disc degeneration and comparing them with those in healthy volunteers, Staszkiewicz et al.. 44 found that iron concentrations were significantly lower in healthy volunteers than in patients with disc degeneration. However the research group of this study only confirmed the absence of lumbar disc degeneration by statement, without imaging. In addition, this study was a single-center study in a Polish population, so the results may not be generalizable. Interestingly, a clinical study that included 217 patients with lumbar disc degeneration revealed that the degree of disc degeneration was negatively correlated with serum ferritin and not statistically significantly different from iron, TSAT, UIBC, and TIBC by assessing the severity of the patients' IVDD and by measuring the levels of iron metabolism markers such as serum ferritin, TSAT, and TIBC. However, this study has many shortcomings, such as a small sample size, lack of control for other potential factors, and lack of long-term follow-up 12.
It is also possible that these studies, which differed from our findings, were influenced by inflammatory factors. Interleukin 6 (IL-6) serves a crucial function as a proinflammatory cytokine in the progression of IVDD through its facilitation of ECM degradation 45. IL-6 activates signal transducer, activator of transcription factor 3 (STAT3), in the NP and AF cells in the intervertebral disc, which promotes the expression of matrix metalloproteinases (MMPs), which leads to an increase in the decomposition of the ECM of the intervertebral disc, which further contributes to the onset of IVDD and its progression 46. In addition, Bin et al. 47 demonstrated in a series of experiments that IL-6 induced lipid peroxidation and abnormal accumulation of intracellular iron in the CEP cells of the intervertebral disc. Observational studies usually involve a very large number of potential variables, so their results are confounded by reverse causality and confounding factors, which can be effectively avoided in MR studies 48.
Similarly, experimental studies on iron status and IVDD have yielded different conclusions. In an experimental study, researchers found that knockdown and overexpression of the PolE gene had an effect on apoptosis, and that iron deficiency led to a decrease in PolE protein levels and affected the stability of the DNA polymerase ε complex, which in turn accelerated the process of disc degeneration 13. Although this study innovatively found that the stability of the DNA polymerase ε complex was affected by iron deficiency, it also failed to provide a comprehensive consideration of malnutrition, inflammation, and other factors that may affect disc degeneration 47. In contrast, several experimental studies have reported that iron overload promotes disc degeneration 49–51. Under normal physiological conditions, iron is absorbed as divalent iron ions and stored in endosomes after translocation to peripheral tissues. Trivalent iron in endosomes is reduced to divalent iron and stored in ferritin and unstable iron pools, and in this process, ferritin light and heavy chains play a role in storing and mineralizing iron 52. When iron metabolism homeostasis is disrupted, excess iron generates reactive oxygen species via the Fenton or Haber-Weiss reaction, causing lipid peroxide accumulation and inducing the development of ferroptosis in NP cells, AF cells 53, and CEP cells 49, ultimately leading to IVDD 54. However, there is always a gap between in vitro cell experiments and in vivo animal experiments and human conditions 55.
There are limitations to our study. First, the number of SNPs on iron status in the study was small, although we extracted iron status data from two separate GWAS studies, and subsequent MR studies with larger-scale iron status GWAS data are needed to improve the ability to test for associations. Second, our study only included participants of European ancestry, and our conclusions may not be applicable for non-European populations. More MR studies, including studies in other ethnic groups, are needed to verify causality. Finally, our study did not stratify the causal relationship between iron status and IVDD according to age or gender, despite studies suggesting that age or gender may by an effect on iron status and IVDD 56,57. However, there are some advantages of our study. First, we obtained data on iron status from multiple databases to perform MR analysis with IVDD. Second, we performed a meta-analysis of the MR analysis results from different sources. Finally, we have further validated the MR analysis results. All these advantages will make our MR analysis results more accurate.
In conclusion, our MR analysis did not find a clear causal relationship between the five indicators related to iron status and IVDD in terms of genetic factors, suggesting that elevated iron status indicators may not imply an increased risk of IVDD in European individuals. Deeper and more extensive studies are needed to explore the relationship between iron status and IVDD.