In this study, we aimed to identify the candidate genes associated with SpA. Firstly, we integrated a large scale GWAS summary data of SpA and gene expression weights from two specific tissues to conduct the TWAS analysis. The significant genes were further validated by gene expression profile of SpA. In addition, we used FUMA and Metascape tools to perform functional enrichment and annotation analysis associated with candidate genes. As far as we have known, this is the first study systematically identifying the candidate genes related to SpA by using the TWAS analysis. Our results may provide novel clues to study the genetic mechanism, diagnosis and treatment of SpA in the future.
TWAS identified several genes for SpA, such as SFMBT2, MCM4, KIAA1109, CTNNAL1. SFMBT2 protein is a member of polycomb group (PcG) of proteins. Hussain S et al. found SFMBT2 interference altered the expression of key metabolic genes in chondrocytes, SOX9 and COL2A1 were decreased, whereas MMP13 and ADAMTS4 were increased significantly [19]. Some studies have shown that up-regulation or down-regulation of these genes which altered by the SFMBT2 gene can lead to degeneration of cartilage and further causes the pathogenesis of SpA [20]. In our study, ATRNL1 gene found in this study which can also regulate the expression of SOX9 and significantly highly expressed in cartilage tissues of patients with osteoarthritis[21]. To sum up, SFMBT2 and ATRNL1 have potential genetic mechanisms for the pathogenesis of SpA. we can regard SFMBT2 and ATRNL1 as candidate genes related to SpA and the genetic mechanisms between those genes and SpA need further research.
Another important candidate gene identified in this study is MCM4, MCM4 protein is a DNA replication licensing factor which is essential for DNA replication initiation and elongation in eukaryotic cells. In other words, MCM4 gene acts as an essential regulator of cell cycle. It has been proved that MCM4 can cause many diseases by regulating the cell cycle and inducing cell apoptosis [22, 23]. In addition, previous studies has shown that the pathogenesis mechanisms of SpA included thinning of the cartilage and by cartilage degeneration involving chondrocyte apoptosis and proteoglycan loss [24].
In addition, KIAA1109 was detected in both TWAS and gene expression profile of SpA. A candidate gene approach has shown that a 480 kb block on chromosome 4q27 encompassing KIAA1109/Tenr/IL-2/IL-21 gene cluster is associated with rheumatoid arthritis [25]. Zhernakova A et al. found the KIAA1109/Tenr/IL-2/IL-21 gene cluster is involved in susceptibility to multiple autoimmune diseases, implying that this locus is a general risk factor for multiple autoimmune diseases such as rheumatoid arthritis and celiac disease [26]. SpA is a subset of seronegative rheumatic-related autoimmune diseases and Bowes J et al. has already found the significant evidences for associations with susceptibility to SpA and IL-2,IL-21 genes [27, 28]. As far as we known, no researchers have studied whether KIAA1109 has an direct effect on SpA. So this is the first study exploring the genetic correlation between KIAA1109 and SpA.
CTNNAL1 (cadherin-associated protein, alpha-like 1) was found to be ubiquitously expressed in many tissues including skeletal muscle, pancreas and heart [29]. By comparing with psoriasis patients who did not have psoriatic arthritis and patients with psoriatic arthritis, Patrick MT et al. found significant loci overlapping the regulatory elements encompass genes differentially expressed in differentiated osteoblasts, including genes participating in the Wnt signaling such as RUNX1, FUT8, and CTNNAL1 [30]. The proteins in Wnt/β-catenin signaling play essential roles in the development of SpA. Xie W et al. found the Wnt proteins are critically essential in normal bone homeostasis, particular in osteoblastic new bone formation [31]. Therefore, Wnt proteins may also play roles in the process of new bone formation in ankylosing spondylitis and various components of the Wnt signaling molecules were found to be involved in maintaining bone mass [31]. To sum up, these results could provide the new clues for future study on the genetic mechanism of CTNNAL1.
In our study GO enrichment analysis and KEGG pathway were also conducted to explore the
functions of candidate genes and how they distributed in SpA. For example, mitochondrion organization (GO:0007005) was identified by both TWAS and gene expression profile. Cytochrome c is primarily known for its function in the mitochondria as a key participant in the life-supporting function of ATP synthesis [32]. Recently researchers found cytochrome c can interact with protease, which lead to the activation of apoptosis protease activation factor [33]. It has been demonstrated that this biological signal is responsible for the apoptosis and activation of inflammatory process in the pathogenesis of psoriatic arthritis [33]. Combined with these study, these findings suggest the abnormal mitochondrion organization may play roles in the biological mechanism of SpA.
Axon guidance (KEGG:hsa04360) was also identified as enriched in SpA. Recently researchers found semaphorins as a family originally identified as axonal guidance molecules and semaphorins have affected the pathogenesis of multiple arthritis such as SpA, rheumatoid arthritis, osteoarthritis by regulating of immunity, angiogenesis, bone remodeling, apoptosis, cell migration and invasion [34]. In addition, semaphorins family can regulate the biological pathway of TNF-α/ADAMTS-4, blocking semaphorins can decrease the destruction of cartilage and bone, cell infiltration into the synovium, and production of TNFα and IL-6 [35]. The TWAS analysis and gene expression profile of SpA identified axon guidance as a susceptibility pathway for SpA, which was consistent with existing researches.
The strength of our study is that we conducted TWAS analysis by using the latest GWAS summary data of SpA [10]. The large sample size of GWAS summary data ensures the accuracy of our research results. In addition, we verified the candidate genes by comparing with the gene expression profile. These results may provide new clues for future research on the genetic mechanism of SpA.
This study also has some limitations. Firstly, the GWAS summary data are based on the European ancestry and may not apply to other ancestry studies. Therefore, it should be cautious to apply our results to other populations. Further TWAS analysis on other populations are needed to prove our results. Secondly, our results lacked sufficient mechanism-based experiments. So we need more mechanism-based experiments to further confirm the biological rationality and clarify the biological mechanism of our study results, which expect to participate in the development of SpA. Thirdly, to validate the TWAS results, we compared the significant genes identified by TWAS analysis of SpA with the gene expression profile of jSpA, but jSpA is just one subtype of within SpA. So our results should be interpreted with caution. Further biological studies should be conducted to confirm our findings.