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71 (); 259-268
doi:
10.1016/j.jor.2025.08.049

Identification of key pathways and biomarkers in rheumatoid arthritis synovial tissue through comprehensive transcriptomic integration

Division of Spinal Surgery, The First Affiliated Hospital of Guangxi Medical University, Shuangyong Road 6, Nanning, Guangxi Zhuang Autonomous Region, 530021, People's Republic of China
Department of Orthopedic, Shenzhen Hengsheng Hospital, 20 Yintian Rd, Shenzhen, Guangdong, 518102, People's Republic of China
Department of Spinal Surgery, Zhujiang Hospital of Southern Medical University, Guangzhou, 510280, People's Republic of China

⁎Corresponding author: Jinzhou Luo. l025690@163.com

⁎⁎Corresponding author: Mao-Lin He. hemaolin@stu.gxmu.edu.cn

Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Abstract

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovitis and joint destruction. Although our understanding of its pathogenesis has deepened, the molecular mechanisms underlying changes in synovial tissue remain incompletely elucidated.

We integrated transcriptomic datasets from the GEO database, including gene expression microarray datasets and RNA-seq datasets (GSE1919, GSE12021, GSE55235, GSE55457, GSE77298, GSE89408), to identify differentially expressed genes (DEGs) in RA synovial tissue. Functional enrichment (GO, KEGG) and gene set enrichment analysis (GSEA) were performed to explore key pathways. CIBERSORT was used to assess immune cell infiltration. We applied a comprehensive machine learning approach using 113 algorithms to screen for core diagnostic genes, which were subsequently validated across multiple datasets.

In the training set (GSE89408), we identified 9204 DEGs that were significantly enriched in immune-related processes (leukocyte migration, cytokine activity) and pathways (cytokine-cytokine receptor interaction, chemokine signaling). GSEA confirmed the activation of these pathways (NES >1, FDR <0.001). Immune infiltration analysis showed a significant increase in plasma cells in RA synovium (with no plasma cells in the control group). Machine learning identified 9 core genes (AIM2, AKR1B10, CXCL10, CXCL13, IGLC1, IL2RG, LRRC15, SDC1, and IGHG1), which demonstrated robust diagnostic performance.

Cytokine-cytokine receptor interactions, chemokine signaling pathways, and plasma cell infiltration may play critical roles in synovial pathogenesis of rheumatoid arthritis. Among the 9 core genes identified, 8 have been experimentally validated for their functions in RA, while the unverified IGHG1 may serve as an important biomarker for the pathogenesis of RA synovium.

Keywords

Rheumatoid arthritis (RA)
Synovial tissue
113 machine learning
IGHG1
Biomarkers
1

1 Introduction

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation, proliferation of synovial cells, and subsequent destruction of cartilage and bone. The pathogenesis of RA involves complex interactions among immune cells and persistent inflammation.1–4 According to epidemiological data from 2021, the global age-standardized incidence rate (ASIR) of RA among adolescents and young adults is 9.46 per 100,000 (95 % UI: 5.92 to 13.54), with an age-standardized prevalence (ASPR) of 104.35 per 100,000 (77.44–137.84), and an age-standardized mortality rate (ASMR) of 0.016 per 100,000 (0.013–0.019). Alarmingly, both the incidence and prevalence of RA are on the rise worldwide.5

Multiple factors contribute to the etiology of RA, including abnormalities in the immune system, alterations in cellular behavior, epigenetic regulation, angiogenesis, and the interactions between various signaling pathways. Together, these mechanisms lead to the chronic inflammatory state and joint destruction commonly observed in RA.6–11 Patients frequently present with generalized fatigue, joint swelling and tenderness, as well as morning stiffness in the early stages of the disease. If not treated promptly and effectively, patients may develop severe systemic manifestations during disease progression, including pleural effusions, pulmonary nodules, and interstitial lung disease.12,13

Despite these insights, the pathogenesis of local synovial tissue in RA patients remains incompletely understood. This study aims to integrate transcriptome datasets from RA synovial tissues to further investigate the infiltration characteristics of immune cells and to identify key functional pathways and core genes. The findings of this study will provide a theoretical foundation for elucidating the pathological mechanisms underlying RA synovial tissue and will contribute to a deeper understanding of the disease's pathogenesis.

2

2 Materials and methods

2.1

2.1 Data download and processing

The technical roadmap for this study is illustrated in Fig. 1. We retrieved transcriptome datasets related to RA from the Gene Expression Omnibus (GEO) database (https://www.ncbi.nlm.nih.gov/geo/) using the search keyword “Rheumatoid Arthritis.” The screening criteria included “Expression profiling by array,” “Expression profiling by high throughput sequencing,” and “Homo sapiens.” Additionally, the selected datasets had to contain both healthy controls and RA groups, with a minimum of 5 samples in each group. Ultimately, six datasets were included: GSE1919, GSE12021, GSE55235, GSE55457, GSE77298, and GSE89408. Detailed information about the datasets is provided in Table 1. Notably, GSE89408 consists of next-generation sequencing (high-throughput sequencing) data, while the other five datasets are derived from first-generation sequencing. For data analysis, GSE89408 was utilized as the training set, and the other microarray datasets served as validation sets.

Flowchart of the comprehensive analysis process of the transcriptome in synovial tissues of rheumatoid arthritis.
Fig. 1 Flowchart of the comprehensive analysis process of the transcriptome in synovial tissues of rheumatoid arthritis.
Table 1 Sample information of the rheumatoid arthritis-related datasets.
Dataset Platform ID Normal Samples RA Samples Species
GSE1919 GPL91 5 5 Human
GSE12021 GPL96 9 12 Human
GSE55235 GPL96 10 10 Human
GSE55457 GPL96 10 13 Human
GSE77298 GPL570 7 16 Human
GSE89408 GPL570 23 150 Human
2.2

2.2 Differential gene analysis

Differentially expressed gene (DEG) analysis was conducted on all datasets using the “limma” R package, with screening criteria set at a corrected p-value <0.05 and |log fold change (FC)| ≥ 1. The “ggplot2″ and “pheatmap” R packages were employed to create volcano plots and heat maps to visualize the expression of differentially expressed genes.

2.3

2.3 Enrichment analysis of GO and KEGG

Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed using the “clusterProfiler” R package. Statistical significance was determined at a corrected p-value <0.05, and the enrichment of differential genes in cell components, biological processes, molecular functions, and pathways was analyzed.

2.4

2.4 GSEA analysis

Gene Set Enrichment Analysis (GSEA) was performed using the “clusterProfiler” R package, applying a filtering criterion of corrected p-values <0.05. Pathways with a normalized enrichment score (NES) greater than 1 were considered significantly activated, while those with NES less than 1 were deemed significantly inhibited. GSEA analysis was utilized to identify pathways activated or inhibited during the development of the disease.14

2.5

2.5 Immune cell infiltration analysis

The deconvolution algorithm and the “CIBERSORT” R package were employed to infer the infiltration of various immune cells in the synovial tissues of normal individuals and rheumatoid arthritis patients. Bar charts were generated to illustrate the composition of immune cells in RA samples compared to normal samples, and violin plots were employed to visually depict differences in the expression levels of different immune cells between RA and normal samples.

2.6

2.6 Deep screening of core genes using machine learning

Based on the differentially expressed genes identified in the training set, we applied a total of 113 machine learning algorithms for an in-depth analysis across all datasets. These algorithms were arranged and combined using 12 basic algorithms, including LASSO, SVM, Random Forest (RF), glmBoost, Partial Least Squares Regression (plsRglm), Stepglm, Elastic Net (Enet), Linear Discriminant Analysis (LDA), Gradient Boosting Machine (GBM), Ridge Regression, XGBoost, and Naive Bayes. The R packages utilized included: randomForestSRC, glmnet, plsRglm, gbm, caret, mboost, e1071, MASS, xgboost, and stats. The area under the receiver operating characteristic (ROC) curve (AUC) for the training set and five validation sets was calculated for each of the 113 machine learning algorithms, with model results displayed as heat maps. Genes screened by the algorithm yielding the largest average AUC value were designated as core genes. Finally, the ROC curve and mRNA expression of core genes across all datasets were plotted.

3

3 Results

3.1

3.1 Identification of differentially expressed genes

The differential gene expression across all datasets is illustrated in Fig. 2A and B. A total of 9204 DEGs were identified in the training set, of which 4015 were up-regulated and 5189 were down-regulated. In GSE1919, 459 DEGs were identified, with 234 up-regulated and 225 down-regulated. GSE12021 revealed 253 DEGs, including 140 up-regulated and 113 down-regulated genes. In GSE55235, a total of 1108 DEGs were identified, with 619 up-regulated and 489 down-regulated genes. GSE55457 identified 341 DEGs, comprised of 210 up-regulated and 131 down-regulated genes. Lastly, GSE77298 included 468 DEGs, with 260 up-regulated and 208 down-regulated genes.

(A) Volcano plot of the training and validation sets; red points indicate upregulated genes in RA compared to normal individuals, while green points indicate downregulated genes in RA. (B) Heatmap of the training and validation sets showing the top 50 upregulated and downregulated genes between RA and normal tissues, with red indicating upregulation and blue indicating downregulation.
Fig. 2 (A) Volcano plot of the training and validation sets; red points indicate upregulated genes in RA compared to normal individuals, while green points indicate downregulated genes in RA. (B) Heatmap of the training and validation sets showing the top 50 upregulated and downregulated genes between RA and normal tissues, with red indicating upregulation and blue indicating downregulation.
3.2

3.2 GO and KEGG enrichment analysis of aggregated immune responses

The GO enrichment analysis circle diagram shows the entries of these differentially expressed genes that were significantly enriched in biological process (BP), cell composition (CC) and molecular function (MF) (Fig. 3A). However, the circle plot could not show all the entries with significant differences, so we compiled the simultaneously enriched entries from all datasets into Table 2. These genes were mainly involved in several key biological processes, such as the migration, regulation and chemical orientation of various types of leukocytes (including lymphocytes, monocytes and neutrophils), cell-cell interactions, and various immune responses (leukocyte-mediated immune response, adaptive immune response and regulatory immune effector processes). In terms of cellular composition, these genes are involved in the outer surface of the plasma membrane; In terms of molecular function, it focuses on cytokine activity and CXCR chemokine receptor binding. KEGG enrichment analysis showed that the pathways that were common to all the data sets were the cytokine-cytokine receptor interaction pathway and the chemokine signaling pathway (Fig. 3B).

(A) GO analysis circle plot of the training and validation sets, including biological processes, cellular components, and molecular functions. (B) Bar chart of the KEGG analysis results of the training and validation sets. The cytokine-cytokine receptor interaction pathway and chemokine signaling pathway are significantly enriched across all datasets and are emphasized in red text in the figure.
Fig. 3 (A) GO analysis circle plot of the training and validation sets, including biological processes, cellular components, and molecular functions. (B) Bar chart of the KEGG analysis results of the training and validation sets. The cytokine-cytokine receptor interaction pathway and chemokine signaling pathway are significantly enriched across all datasets and are emphasized in red text in the figure.
Table 2 GO terms simultaneously enriched in all datasets.
ONTOLOGY ID Description
BP GO:0022407 regulation of cell-cell adhesion
BP GO:0050900 leukocyte migration
BP GO:0002443 leukocyte mediated immunity
BP GO:0002449 lymphocyte mediated immunity
BP GO:0030595 leukocyte chemotaxis
BP GO:0060326 cell chemotaxis
BP GO:0001909 leukocyte mediated cytotoxicity
BP GO:0071674 mononuclear cell migration
BP GO:1990266 neutrophil migration
BP GO:0097529 myeloid leukocyte migration
BP GO:0002460 adaptive immune response based on somatic recombination of immune receptors built from immunoglobulin superfamily domains
BP GO:0030593 neutrophil chemotaxis
BP GO:0097530 granulocyte migration
BP GO:0072676 lymphocyte migration
BP GO:0071621 granulocyte chemotaxis
BP GO:0002697 regulation of immune effector process
BP GO:0002685 regulation of leukocyte migration
BP GO:1990868 response to chemokine
BP GO:1990869 cellular response to chemokine
BP GO:0034612 response to tumor necrosis factor
BP GO:0071675 regulation of mononuclear cell migration
BP GO:0050853 B cell receptor signaling pathway
BP GO:0006816 calcium ion transport
CC GO:0009897 external side of plasma membrane
MF GO:0005125 cytokine activity
MF GO:0045236 CXCR chemokine receptor binding
3.3

3.3 GSEA analysis showed that the significantly activated pathways were highly consistent with KEGG

A gene-set enrichment analysis of the classic 180 KEGG pathways showed that only two pathways were co-activated or inhibited in all data sets: the cytokine-cytokine receptor interaction pathway and the chemokine signaling pathway (Fig. 4A and B). Both pathways showed significant activation signatures in all data sets, with NES greater than 1 and an adjusted p value of less than 0.001.

GSEA analysis results of the training and validation sets. (A) The cytokine-cytokine receptor interaction pathway is significantly activated in all datasets, with NES values greater than 1.5 and P values less than 0.001. (B) The chemokine signaling pathway is also significantly activated in all datasets, with NES values greater than 1.9 and P values less than 0.001.
Fig. 4 GSEA analysis results of the training and validation sets. (A) The cytokine-cytokine receptor interaction pathway is significantly activated in all datasets, with NES values greater than 1.5 and P values less than 0.001. (B) The chemokine signaling pathway is also significantly activated in all datasets, with NES values greater than 1.9 and P values less than 0.001.

More notably, the previous KEGG analysis results also showed that differentially expressed genes were significantly enriched in these two pathways. This high concordance further emphasizes the importance of cytokine-cytokine receptor interaction pathway and chemokine signaling pathway in the development of RA.

3.4

3.4 Immune infiltration analysis revealed significant infiltration of plasma cells in RA synovial tissue

The histogram illustrates the ratio of 22 immune cell types infiltrating normal versus RA synovial tissue (Fig. 5A). Violin plots provide a visual representation of the significant differences in immune cell infiltration between the normal and RA groups. While variations in immune cell infiltrations were observed across different datasets, a consistent finding emerged: the proportion of plasma cells in the normal group was nearly absent, whereas a significant infiltration of plasma cells was observed in the RA group (Fig. 5B). This suggests a marked increase in plasma cell populations within the synovial tissue of RA patients.

Immune infiltration analysis results of the training and validation sets. (A) Percentage of immune cell infiltration in the normal group and RA group. (B) Differences in immune cell infiltration between the normal group and RA group. The normal group is marked in green, while the RA group is marked in red. The proportion of plasma cells in the normal group is nearly 0, while the infiltration ratio of plasma cells is significantly increased in the RA group, with P values all less than 0.05.
Fig. 5 Immune infiltration analysis results of the training and validation sets. (A) Percentage of immune cell infiltration in the normal group and RA group. (B) Differences in immune cell infiltration between the normal group and RA group. The normal group is marked in green, while the RA group is marked in red. The proportion of plasma cells in the normal group is nearly 0, while the infiltration ratio of plasma cells is significantly increased in the RA group, with P values all less than 0.05.
3.5

3.5 Screening of nine core genes using 113 machine learning algorithms

Based on 113 machine learning algorithms, we conducted an in-depth analysis of all datasets to identify the best predictive model. Multiple models showed excellent performance, with AUC values exceeding 0.95, demonstrating significant predictive power (Fig. 6A). In particular, the average AUC of the plsRglm model reached an impressive 0.968, so we selected this model as the best model and screened nine core genes: AIM2, AKR1B10, CXCL10, CXCL13, IGHG1, IGLC1, IL2RG, LRRC15, and SDC1. We then plotted ROC curves and their mRNA expression profiles for the nine core genes (Fig. 6B). The results showed that the AUC values of these core genes were more than 0.70 in the training set and all validation sets. Among them, AIM2, CXCL10, CXCL13, IL2RG and SDC1 had AUCs greater than 0.8 in all datasets. The mRNA expression levels of these nine genes were shown by boxplots (Fig. 7A–F), and the results showed that they were able to significantly distinguish the normal group from the RA group (all p values < 0.05). AKR1B10 was significantly down-regulated in the RA group compared with the normal group, while the remaining core genes were significantly up-regulated in the RA group.

(A) In-depth analysis of 113 machine learning algorithms to screen core genes across all datasets, with the plsRglm model yielding the highest average AUC value. (B) The plsRglm algorithm identified 9 core genes, and ROC curves for each gene in the training set and 5 validation sets were generated; the AUC values of these 9 core genes exceed 0.7 in all datasets.
Fig. 6 (A) In-depth analysis of 113 machine learning algorithms to screen core genes across all datasets, with the plsRglm model yielding the highest average AUC value. (B) The plsRglm algorithm identified 9 core genes, and ROC curves for each gene in the training set and 5 validation sets were generated; the AUC values of these 9 core genes exceed 0.7 in all datasets.
Expression levels of the 9 core genes in the normal group and RA group across different datasets: (A) Training set, (B) GSE1919, (C) GSE12021, (D) GSE55235, (E) GSE55457, (F) GSE77298. In all datasets, AKR1B10 is significantly downregulated in the RA group compared to the normal group, while the other core genes are significantly upregulated in the RA group.
Fig. 7 Expression levels of the 9 core genes in the normal group and RA group across different datasets: (A) Training set, (B) GSE1919, (C) GSE12021, (D) GSE55235, (E) GSE55457, (F) GSE77298. In all datasets, AKR1B10 is significantly downregulated in the RA group compared to the normal group, while the other core genes are significantly upregulated in the RA group.
4

4 Discussion

In this study, we conducted a comprehensive transcriptome analysis of RA synovial tissue, integrating both first-generation and next-generation sequencing technologies to explore the molecular characteristics of RA. Based on GO analysis, we found that the migration, regulation, and chemotaxis of various leukocytes, cell-cell interactions, and multiple immune responses are closely related to the occurrence and progression of RA. Moreover, KEGG and GSEA consistently demonstrated significant activation of the cytokine-cytokine receptor interaction pathway and chemokine signaling pathway during the development of RA. Immune infiltration analysis revealed significant infiltration of plasma cells within RA synovial tissues. By applying 113 machine learning algorithms, we successfully identified nine key core genes (AIM2, AKR1B10, CXCL10, CXCL13, IGHG1, IGLC1, IL2RG, LRRC15, and SDC1), which exhibited strong diagnostic performance in distinguishing normal controls from RA patients. Thus, we propose that these nine core genes may serve as important biomarkers influencing RA synovial tissue.

This study has notable advantages over previous investigations. First, regarding sequencing technology, we integrated both first-generation and second-generation sequencing data of RA synovial tissue, while most prior studies relied solely on first-generation sequencing (microarray data), which lacked comprehensiveness. Second, our study incorporated a large sample size, totaling 270 samples— including 64 healthy controls and 206 RA samples—compared to smaller sample sets used in previous studies, some of which lacked appropriate normal or disease samples. We discarded datasets with sample sizes less than five to mitigate potential biases.15–19 Third, our analysis highlights commonalities among transcriptomic studies conducted by different researchers in varied experimental settings and time points. This is crucial, as differences in clinical sampling techniques may lead to disparate analysis results. By integrating diverse datasets, we aimed to draw conclusions with broader applicability. Finally, our unique approach utilizes a combination of 113 algorithms, whereas previous studies typically employed only 2–3 algorithms for intersection. Given that each algorithm has its strengths and weaknesses, using a more extensive combination can help avoid overlooking potentially important genes.20–23

Our KEGG and GSEA analyses underscore the critical roles of the cytokine-cytokine receptor interaction pathway and chemokine signaling pathway in RA. Although several prior studies have indicated that various pathways may be significantly activated or inhibited in RA pathogenesis, we found that certain pathways did not demonstrate substantial changes across these studies (p < 0.05). Furthermore, discrepancies exist among researchers regarding the degree of immune cell infiltration in RA synovial tissue. In contrast, our findings revealed a consistently higher proportion of plasma cells in RA synovial tissue when compared to normal controls across all datasets, with normal synovial tissue almost entirely devoid of plasma cells. This observation suggests that plasma cells may play a pivotal role in the pathogenesis of RA, warranting further investigation in future studies.

We observed significantly increased expression levels of AIM2 in the synovial tissue and fibroblast-like cells of RA patients compared to healthy controls. Previous studies have indicated that AIM2 knockdown impairs the proliferation, migration, and invasion of fibroblast-like synoviocytes.24,25 AKR1B10 functions as an inflammation regulator, stimulating its own expression.26 In rats, the mRNA expression level of AKR1B10 in synovial tissue was markedly lower than that in controls.27 Moreover, low expression of AKR1B10 in gastric cancer tissues has been associated with M2 macrophage polarization, promoting disease progression.28 Thus, it is plausible that AKR1B10 may also influence macrophage polarization in synovial tissues. Additionally, we confirmed the significantly higher expression of CXCL13 and CXCL10 in RA synovium, with JAK inhibitor tofacitinib effectively inhibiting JAK1-STAT signaling and reducing mRNA expression levels of these chemokines.29,30

L2RG is integral to the development, differentiation, and function of T cells, B cells, and NK cells, with its deletion leading to severe immune system defects.31Compared to healthy individuals, IL2RG expression was significantly up-regulated.32 Studies indicate that overexpression of LRRC15 correlates with enhanced proliferation, migration, invasion, and angiogenesis of RA synovial fibroblasts. Inhibiting LRRC15 expression can significantly curtail synovial proliferation and bone invasion, thereby potentially mitigating joint destruction.33,34 Furthermore, serum levels of SDC-1 were significantly elevated in RA patients compared to normal controls.35 However, the role of SDC-1 remains controversial, as some research suggests its loss may exacerbate the inflammatory features of RA, indicating the need for further experimental validation.36 In terms of gene expression, IGLC1 mRNA levels were notably increased in RA patients relative to healthy samples, a trend corroborated at the protein level.37 Although no study has directly investigated IGHG1 expression in RA synovium, evidence suggests it may promote disease development and metastasis.38–40

In summary, our study makes three significant contributions. First, in terms of pathway analysis, we identified two activated signaling pathways. Second, we highlighted the significant infiltration of plasma cells in RA synovium regarding immune cell infiltration. Finally, we identified nine key core genes in RA synovial tissue, eight of which have been experimentally validated.

Nevertheless, some limitations remain in this study. The activation of the identified pathways and the novel gene IGHG1 have yet to be validated through in vitro and animal models. Therefore, we anticipate that our findings can provide new insights and directions for future research on RA synovial tissue.

5

5 Conclusions

In this study, KEGG and GSEA analyses identified significant up-regulation of cytokine-cytokine receptor interaction pathway and chemokine signaling pathway in RA synovium. In addition, immune infiltration analysis revealed significant infiltration of plasma cells. Nine core genes (AIM2, AKR1B10, CXCL10, CXCL13, IGHG1, IGLC1, IL2RG, LRRC15, SDC1) were identified and validated using 113 machine learning algorithms, providing new insights into the understanding of the molecular mechanisms of RA.

Author statement

Qifan Chen: Conceptualization; Methodology; Formal analysis; Writing - Original Draft.

Chu-Song Zhou: Conceptualization; Methodology.

Hanhua Wu: Software; Validation.

Bufan Li: Investigation; Data Curation.

Yu-Nan Man: Data Curation; Visualization.

Mao-Lin He: Writing - Review & Editing.

Jinzhou Luo: Writing - Review & Editing.

Ethical statement

The data for this study were sourced from the GEO database, which is a public repository for gene expression data. The GEO database ensures that all data submissions comply with relevant guidelines and regulations for the collection and sharing of biological data. Therefore, this study does not require additional ethical or moral statements.

Funding

This work was supported by the National Natural Science Foundation of China under Grant [grant number 82160536].

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