With the aging of the population and chronic sports injuries, osteoarthritis has become a prominent problem affecting the quality of life of the elderly. Currently, most diagnostic OA methods are only based on images and patients’ symptoms; thus, most OA patients are not diagnosed early, and late and terminal stage of patients can only undergo surgery to rehabilitate their daily activity. The early diagnosis role of the peripheral blood in OA is often ignored by doctors. However, the biomarkers of PBMCs not only affects the progression OA, but can also serve as early diagnostic biomarkers. From this study, we can provide some useful novel early biomarkers for diagnosis even as the therapy targets for OA patients.
Some of the previous OA studies emphasized articular cartilage[17] and synovium[3, 18, 19] ignored the role of PBMCs in the diagnosis of OA. In this study, we tried to establish diagnosis model by identifying the hub genes related to PBMCs in OA. 14 hub genes were screened out by bioinformatics analysis, and the biological functions and signaling pathways involved were analyzed in the present study. We found that these hub genes and the enriched pathways were involved in many immune responses, so we analyzed the immune cell infiltration by the CIBERSORT algorithm method, and the results showed that there was a significant difference between OA PBMCs and normal controls.
As a result, 14 genes, including the human replacement histone H3.3B gene (H3F3B), β2-adrenergic receptor (ADRB2), nuclear factor-kappa-B-inhibitor alpha (NFKBIA) Small Nucleolar RNA, C/D Box 68 (SNORD68), serine and arginine rich splicing factor 5 (SFRS5), 5'-nucleotidase, cytosolic III (NT5C3), small nucleolar RNA, C/D box 13 (SNORD13), neuroblastoma breakpoint family member 10 (NBPF10), LIM zinc finger domain containing 1 (LIMS1), eukaryotic translation initiation factor 1A Y-linked (EIF1AY), G protein nucleolar 3 like (GNL3L), LOC649841, zinc finger protein 148 (ZNF148), regulator of G protein signaling 18 (RGS18) were identified as differentially expressed hub biomarkers.
Gene ADRB2 was the only up-regulated gene in the OA group and was associated with a variety of immune cells. ADRB2 was positively correlated with CD8+ T cells, γδ T cells, activated DC cells, and negatively correlated with monocytes, M0 macrophages, and memory resting CD4+ T cells. On this basis, we wanted to further explore the relationship between this gene and OA. The gene ADRB2 encodes the β2-adrenergic receptor (β2-AR) and β-adrenergic receptor subtypes were detected in various tissues related to OA development such as synovium, cartilage, and subchondral bone[20, 21]. In the temporomandibular joint (TMJ) OA model, activation of the β2-AR resulted in subchondral bone loss[22]. Furthermore, conditional deletion of ADRB2 in mesenchymal stem cells attenuates osteoarthritis-like defects in TMJ[23]. β2-AR plays a major role in OA-related cartilage calcification and subchondral bone changes but the contribution of other AR subtypes should be taken into consideration[24]. Hence, the underlying mechanism of the relationship between ADRB2 and OA needs to explore in the long run.
Among all the hub genes, H3F3B showed the largest AUC with 0.879, which means the gene has a high diagnostic power of OA. Resting mast cells and monocytes showed statistically significant positive correlation with H3F3B, while naive CD4+ T cells were negatively correlated. The first comprehensive exploration of gene expression in peripheral blood linked osteoarthritis with apoptotic pathways[25] and H3F3B was among the top genes identified. miR-10a-5p/H3F3B axis is regulated to suppress chondrocytes apoptosis in OA, and H3F3B were downregulated in OA cartilage tissues[26]. Behjati et al.[27] found H3F3B mutated in 73/77 (95%) cases of chondroblastoma, which is one of two genes encoding histone 3.3. Nevertheless, the association between H3F3B and OA pathogenesis remains unknown and needs to further study.
Through GO analysis, we found that the hub genes were mainly involved in response to TNF and TNF-mediated signaling pathway. Elevated level of TNF-α has been found in the synovial fluid, synovial membrane, subchondral bone and cartilage of OA patients[28, 29], confirming their important roles in OA pathogenesis[30]. It plays a pivotal role in cartilage matrix degradation and bone resorption in OA[31]. Obese people tended to suffer from OA and showed a higher level of serum TNF-α produced by macrophages derived from adipose tissue[32]. TNF-α was the factor released by fat to negatively regulate cartilage directly. Besides, inflammatory mediators, such as TNF-α was highly upregulated in OA joints and induced ROS production and expression of matrix degrading proteases leading to matrix degradation and joint dysfunction[33].
Based on the KEGG database, 14 hub genes were mainly enriched in cAMP signaling pathway, NF-κB signaling pathway and other signaling pathways. cAMP signaling pathway is an anti-aging target which is a multi-unit protein kinase that mediates signal transduction of G-protein-coupled receptors through its activation by adenyl cyclase-mediated cAMP[34]. A systematic evaluation based on Han Chinese population identified significant association signals from NFKBIA with hip OA for the first time[35] and NF-κB signaling pathway could be regulated to prevent cartilage degeneration in OA[36]. Furthermore, aging contributed to the initiation and progression of osteoarthritis and age was statistically negatively correlated with the expression of NFKBIA[37]. Therefore, more studies are needed to define the function of NFKBIA in the pathomechanism of OA. In addition, it has been demonstrated that NF-κB plays an important role in mediating the effects of TNF-α and IL-1β on the expression of type II collagen[38].
From the immune infiltration analysis, we found that there was a significant difference in the relative cell content of CD8+ T cells and resting mast cells between the two groups. In peripheral blood immune cell composition in OA, CD8+ T cells frequency were higher compared to healthy controls, CD8+ memory-like cells were more likely to be found in OA and increased frequency of CD8+ T cell related to inflammation were also present in OA[39]. In accordance with the previous studies, CD8+ T cells infiltrated the synovium after OA induction[40]. CD8+ T cells may act as a primary activator to exacerbate synovitis and/or a secondary regulator to destruct the cartilage[41]. The effects of NE on memory CD8+ T cells were primarily mediated by ADRB2[42]. Memory CD8+ T cells expressed a significantly higher level of ADRB2 compared to naive cells, which had a faster and stronger immune response. The content of mast cells in OA patients was less than that in normal people. Due to little literature reported the association between OA and resting mast cells in peripheral blood, the results in synovial fluids (SF) might have something in common. Despite the low number of MC in SF (range 0-1.2%), there is a growing body of evidence for their crucial role in OA pathogenesis[43].
Nonetheless, despite these findings, the study still has some limitations. First of all, this study was a retrospective analysis based on a public database, so more prospective approaches are required to substantiate the results. Secondly, the results we discovered through bioinformatics are lacking experimental validation. Finally, Mechanistic insight into the hub genes and their connections with related pathways and immune cells should be investigated.