Despite recent advancements in the diagnosis of MS, the exploration of non-invasive biomarkers could effectively increase the accuracy of diagnosis. To this end, in an integrated microarray study of peripheral blood mononuclear cells from MS patients and control subjects, potential biomarkers were sought. A total of 106 genes were found to be differentially expressed which were functionally enriched in well-established pathways involved in the pathogenesis of MS. Gene ontology analysis reflected the contribution of DEGs to various aspects of immune system functions. Highly enriched biological processes were predominantly engaged in lymphocyte differentiation, production of immune mediators, and regulatory signaling pathways. Most significantly, immune response-regulating cell surface receptor signaling pathways, such as TLR (18), JAK/STAT (19), PD-1/PD-L1 (20), and TGFβ (21) are implicated in the immune responses in MS.
Similarly, the DEGs were associated with numerous immune-related components and complexes. The most significant cellular component term, the external side of the plasma membrane, has been reported to be enriched with common DEGs in neurodegenerative disorders (22). In line with the ranking of blood microparticles as the second most significant term in this category, elevated levels of blood microparticles originating from endothelial and platelet cells were observed in different types of MS, disrupting the barrier function of endothelial cells in vitro (23). In the molecular function category, tetrapyrrole binding and cytokine activity were the most prominent terms, followed by terms mainly associated with immune response and molecular signaling. Tetrapyrrole, heme, and haptoglobin binding-related terms were similarly enriched with genes of hemoglobin complex, including HBA1, HBD, HBG1, and HBB. Although the exact role of these genes in MS pathogenesis has yet to be elucidated, recent advancements in the field have hypothesized that these genes might be involved in functions beyond oxygen transport, such as the role of HBD in counteracting oxidative stress (24). KEGG pathways meeting the criterion of an adjusted p-value of < 0.05 were further enriched in pathways involved in autoimmune disorders such as type I diabetes mellitus, rheumatoid arthritis, inflammatory bowel disease, graft versus host disease, and allograft rejection, as well as infectious diseases, specifically Epstein-Barr virus infection, leishmaniasis, measles, and malaria. Moreover, immune-related signaling pathways highlighted the pivotal role of lymphocytes in MS, particularly Th1, Th2, Th17, and B cells. The flow cytometry analysis of peripheral blood mononuclear cells from MS patients identified MAPK signaling, the third most enriched pathway in this study, as a key pathway responsible for the multiplication and survival of immune cells, particularly B-lymphocytes (25).
Neurodegeneration in the CNS is influenced by miRNAs, which affect the development of lymphocytes and, in turn, play a key role in MS pathogenesis (26). Therefore, screening of key miRNAs was conducted by the construction of miRNA-mRNA regulatory network. In the interaction network, CD69, a conventionally acknowledged marker for T-cell activation expressed shortly after TCR and cytokine stimulation (27), exhibited the highest number of interactions. This transmembrane protein is expressed in various peripheral blood mononuclear cells, such as B- and T-lymphocytes, NK cells, and monocytes (28). Considering high connectivity as a measure of hub gene identification, CD69, PAX5, DUSP1, FASLG, and IFNG were the hub genes in the regulatory network, being in the top 10% of connectivity.
Compared to healthy controls, miR-493-5p elevation was observed in the peripheral blood leucocytes of untreated RRMS patients (29). Other key miRNAs, each interacting with four mRNAs, included miR-124-3p, miR-181a-5p, miR-181b-5p, miR-181c-5p, miR-181d-5p, miR-450b-5p, and miR-506-3p. Dysregulation of miR-124-3p has been observed in a wide spectrum of neurodegenerative disorders, including MS and Parkinson's disease. It has also been found to have therapeutic potential, as upregulation of miR-124-3p can inhibit the activation of macrophages and microglia, which are involved in the neuroinflammation in MS (30, 31).
Four members of miR-181 Family, including miR-181a-5p, miR-181b-5p, miR-181c-5p, and miR-181d-5p were of the key regulators of the DEGs. These miRNAs targeted CD69 and PAX5, the two genes with the highest number of interactions and considered hub genes in the network. The current body of literature demonstrates the altered expression of the miR-181 family in neurodegenerative disorders, including MS, Parkinson's disease, and Alzheimer's disease (32). Quantitative real-time PCR (qPCR) of PBMCs identified miR-181a-5p and miR-181b-5p as differentially downregulated and upregulated miRNAs, respectively, in MS patients (33, 34). Moreover, targeting genes involved in neuroinflammation, such as MAP2K1, CREB1, ATXN1, and ATXN3, by miR-181a-5p reflects its therapeutic potential (34). The expression of miR-181c-5p is reported to be misregulated in PBMCs, cerebrospinal fluid (CSF), and serum of relapsing-remitting multiple sclerosis (RRMS) patients (35). A study on sporadic and familial forms of MS found miR-181d-5p to be downregulated in PBMCs, as revealed by qPCR analysis (36).
Assessment of miR-450b-5p serum levels in RRMS and SPMS patients revealed its reduction and suggested it as a potential biomarker for disease progression (37). Evaluation of miR-506-3p expression levels in the peripheral blood of RRMS individuals using qPCR following fingolimod treatment, an immune-regulating therapy approved for these patients, revealed no significant alterations in its levels in MS patients and responders compared to healthy controls and non-responders, respectively (38).
Assigning 13,237 genes into 15 modules using WGCNA resulted in the identification of a set of co-expressed genes, Module 7, which was negatively correlated with MS (correlation = -0.53, p-value < 0.01) and highly enriched in neurological disorders. Functional enrichment analysis of 23 highly connected genes in the PPI network of this module demonstrated that mitochondrial functioning and pathways of neurodegenerative disorders were fully enriched with these genes.
Out of 23 highly connected genes in this module, 11 were selected as hub genes, including COX5B, UQCRQ, NDUFS3, UQCR10, NDUFA4, COX7B, NDUFA7, NDUFS7, NDUFA2, NDUFA1, and NDUFB2, based on the criteria of module membership > 0.8 and gene significance > 0.2. These hub genes encode different subunits of electron transport chain complexes, including NADH oxidoreductase (Complex I), ubiquinol-cytochrome c reductase (Complex III), and cytochrome c oxidase (Complex IV), supporting the evidence of defect in mitochondrial functioning in PBMCs of MS patients.
Mitochondrial dysfunctions have been widely established as a contributor to MS (39). However, at the peripheral level, the reduction of mitochondrial proteins and impairment of this organelle have been reported relatively recently (40). According to the current literature on MS, the downregulation of COX5B (41), NDUFS3, and NDUFA4 (42), has been documented in blood samples, while a downward trend in the expression of COX7B (43), NDUFA4, NDUFB2, and UQCRQ (44), has been reported only in brain lesions. A genome-wide association study (GWAS), reported NDUFA7 as a differentially methylated gene in both immune cells and neurons (45). Another association study found NDUFA7, NDUFS5, and NDUFS7 genes to be associated with MS (46).
Among the 59 genes selected from coordinated results of differential expression analysis and WGCNA (Fig. 3E), COPG1, RPN1, and KDM3B were selected as potential biomarkers based on the converging results of machine-based learning methods and ROC analysis (Figs. 5 and 6).
COPG1 is the gamma subunit encoding gene of the COPI complex, which is involved in vesicle transport from the Golgi to the endoplasmic reticulum (ER) and intra-Golgi traffic (47). In a bioinformatics study on MS, COPG1 was ascertained as one of the five hub mRNAs in the circRNA-miRNA-hub mRNA network that met the criterion of having a non-zero coefficient in LASSO regression analysis. Moreover, the expression level of COPG1 in MS was markedly lower than that of the control group (48). Similarly, the differential expression analysis of GSE21942 identified COPG1 as one of the most downregulated genes in PBMCs of MS patients (49). In a study on COPA syndrome, which is caused by mutations in the alpha subunit of the COPI complex, applying CRISPR/Cas9 to induce COPG1 deficiency in Th-1 cells resulted in the transcription of inflammatory genes (50). This observation suggests a potential link between COPG1 downregulation and cellular inflammation in PBMCs of MS patients.
RPN1 encodes a regulatory subunit of the 26S proteasome, which is responsible for the ATP-dependent degradation of ubiquitin-marked proteins (51). In neurons, defects in this central catalytic component of the ubiquitin-proteasome system (UPS) lead to the accumulation of misfolded proteins and subsequent neurodegeneration in the CNS (52). Meanwhile, the 26S proteasome is also involved in oxidative stress, transcription of genes, and the release of neurotransmitters. Moreover, it can indirectly affect T-cell development and migration by regulating dopamine release (53). Despite these key regulatory activities of the 26S proteasome, its consideration as a therapeutic target in MS has remained controversial. On the one hand, free proteasomes were detected in the blood specimens of MS patients, and the 26S proteasome was observed to damage the myelin sheath (54). On the other hand, 26S proteasome inhibitors result in the exacerbation of neurodegeneration as a consequence of ubiquitinated protein aggregation (53).
KDM3B encodes lysine demethylase 3B, which is responsible for demethylating histone 3 lysine 9 (H3K9) and is predominantly considered to create a transcriptionally active site (55). KDM3B downregulation found by differential expression analysis in the present study, is in accord with various reports of hypermethylation in PBMCs of MS patients (56). By conducting exome sequencing, pathogenic variants of KDM3B were reported to be associated with a range of cognitive impairments including behavior problems, attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and epilepsy (55). Moreover, KDM3B plays a crucial role in mediation of autophagy through the regulation of autophagy-related genes (57). Autophagy, in turn, is involved in different aspects of MS pathogenesis, notably oxidative stress, myelination, and the activation of immune cells. Meanwhile, it's controversial whether inhibition or amplification of autophagy holds therapeutic potential, as its activation in different cell types is followed by contrasting consequences (58).
In conclusion, the results of the present study demonstrated COPG1, RPN1, and KDM3B to have an acceptable diagnostic efficacy across the analyzed datasets. However, further investigations are required to shed light on the mechanisms through which these genes contribute to MS pathogenesis.