1. Introduction
Noncoding RNA (ncRNA) is a class of molecules that do not encode proteins but are involved in the regulation of gene expression and thereby influence numerous biological processes. From a historical perspective, the central dogma of molecular biology emphasized the role of messenger RNAs in protein synthesis, overshadowing the potential biological functions of noncoding RNAs. At that time, ncRNAs were considered molecular junk or background noise in the cell [
1]. The role of ribosomal RNA and transfer RNA in gene expression was already established in the 1950s, but it was the discovery of microRNA (miRNA) in the early 2000s that accelerated research into the functions of other classes of noncoding RNAs [
2]. It is now known that transcription may involve more than four-fifths of the genomic DNA, while only approximately 1.5% of the transcriptome corresponds to protein-coding sequences, indicating that RNA molecules without coding potential are abundant. Since then, growing evidence has supported their role in gene expression regulation, including transcriptional, post-transcriptional, and epigenetic mechanisms. NcRNAs are involved in various cellular processes such as messenger RNA degradation, alternative splicing, chromatin remodeling, and the maintenance of genomic stability [
1,
3].
NcRNAs can be divided into two classes depending on their length: long ncRNAs, such as long noncoding RNAs (lncRNAs) and circular RNAs, as well as short molecules, including miRNAs, small nuclear RNAs, and small nucleolar RNAs [
3,
4]. MiRNAs are endogenous, small, single-stranded ncRNAs approximately 18–25 nucleotides in length. The biogenesis of miRNAs involves transcription of their sequences by RNA polymerase II, resulting in the formation of primary miRNAs (pri-miRNAs), which adopt a hairpin structure. The maturation of pri-miRNAs is mediated by two RNase III-type enzymes, Drosha and Dicer, which sequentially cleave the hairpin arms [
5]. Cleavage by Drosha produces precursor miRNAs (pre-miRNAs), which are transported by exportin-5 to the cytoplasm for further processing by the Dicer protein [
6,
7,
8]. The mature miRNA is then loaded onto the Argonaute (Ago) protein to form the RNA-induced silencing complex (RISC). In addition to regulating gene expression, miRNAs are involved in several fundamental biological processes, including cell proliferation, apoptosis, and immune system regulation [
9,
10].
Rheumatoid arthritis (RA) is a chronic autoimmune disorder that causes systemic inflammation throughout the body. One of the most common symptoms of the disease is symmetrical joint pain, swelling, and stiffness. In the advanced stages of the disease, there is an overproduction of pro-inflammatory cytokines, such as tumor necrosis factor (TNF) and interleukin-6 (IL-6), which damage synovial cells and contribute to cartilage and bone destruction [
11]. RA is a multifactorial disease in which environmental, genetic, and epigenetic factors contribute to its development. Among epigenetic factors, the main regulatory mechanisms are considered to be DNA methylation, histone protein modifications, and the influence of ncRNAs on gene expression regulation [
12,
13].
MiRNAs in RA are extensively studied in serum, plasma, whole blood, peripheral blood mononuclear cells (PBMCs), and various tissues [
14]. Many of these molecules contribute to the overproduction of pro-inflammatory cytokines and the activation of leukocytes, both of which play a role in the pathogenesis and progression of RA [
15,
16]. Changes in miRNAs expression may serve as potential biomarkers for disease diagnosis, exacerbation, or response to treatment. This is because miRNAs are tissue-specific, easily measurable, and cost-effective, particularly when the polymerase chain reaction (PCR) is used as a method with high specificity and sensitivity [
14,
17,
18].
The aim of this investigation was to evaluate the association between the expression level of selected miRNAs in PBMCs, whose quantitative alterations may be linked to RA severity. Based on a review of the literature, we selected five miRNAs, miRNA 186-5p (miRNA-186), miRNA 654-3p (miRNA-654), miRNA 425-5p (miRNA-425), miRNA 22-3p (miRNA-22), and miRNA106b-5b (miRNA-106b), which have been previously associated with RA [
19,
20,
21,
22].
3. Discussion
In this study, we demonstrated the novel finding that miRNA-186 may serve as a potential molecular marker associated with disease activity exacerbation and may be useful for diagnosing patients with rheumatoid arthritis in whom the ACPA has not been detected.
Previous studies have reported that miRNA-186 may act as a pro-inflammatory molecule. The study conducted by Singh et al. [
23] showed upregulation of miRNA-186 in systemic lupus erythematosus (SLE) and a positive correlation with disease activity. Another study [
24] indicated that miRNA-186 is associated with the development of inflammation via interleukin-1-beta (IL-1β) in osteoarthritis (OA). In vitro stimulation of chondrocytes with IL-1β has been shown to increase the expression of inflammatory mediators such as TNF-α and IL-6, which are also key molecules in the pathogenesis of RA. Moreover, IL-1β was found to upregulate miRNA-186-5p expression in a cell line model. The study by Akbaba et al. [
25], who compared patients with severe systemic autoinflammatory diseases (SAIDs) and mild familial Mediterranean fever (FMF), demonstrated that miRNA-186-5p is associated with inflammation. However, the authors noted that patients with SAIDs exhibited reduced levels of miRNA-186 expression compared to those with FMF. One of the most important molecules linking the pathophysiology of adaptive and innate immunity is IL-1β, which plays a central role in stimulating inflammation. The study by Akbaba et al. [
25] indicated that miRNA-186 is associated with inflammation-related pathways, such as the cytokine-mediated signaling pathway and the cellular response to cytokine stimulus. In our study, we did not confirm a direct correlation between the level of miRNA-186 expression and disease activity. Furthermore, the miRNA-186 level was decreased in patients with RA, and it was also lower in the group of patients with an active form of the disease compared to healthy individuals. The underlying mechanisms of SLE, RA, and OA are fundamentally distinct. RA is a systemic autoimmune disorder primarily driven by pro-inflammatory cytokines such as TNF-α and IL-6. In contrast, SLE is considered a prototypical interferonopathy, characterized by the overexpression of genes within the “interferon signature,” which significantly contributes to disease activity. Type I interferons are known to modulate the expression of numerous miRNAs, potentially explaining the observed upregulation of miRNA-186 in SLE. OA, on the other hand, is predominantly a degenerative joint disease, where inflammation—if present—is typically secondary and low-grade [
26]. These divergent pathogenic landscapes may account for the differential regulation of miRNA-186 across these conditions.
The differential expression of various ncRNAs between various disease entities is not unusual. MiRNA-146a, which is considered an anti-inflammatory molecule in SLE patients, shows a decrease in PBMCs [
23], while in patients with RA, it is increased in PBMCs. The study conducted by Pauley et al. [
27] showed that although the level of miRNA-146a differed between RA and controls, the two main genes responsible for the regulation of TNF-α levels, interleukin-1 receptor-associated kinase 1 (IRAK1) and TNF receptor-associated factor 6 (TRAF6), did not show significant differences in expression between the groups. Other studies confirm that various ncRNAs exhibit differential expression patterns, reflecting their distinct regulatory roles. For instance,
GAS5 is downregulated in the PBMCs of patients with SLE, while its expression is elevated in RA. Notably, in SLE,
GAS5 interacts with the MAPK signaling pathway, whereas in RA, it is associated with the AMPK pathway [
28]. Similar regulatory phenomena are observed with microRNAs (miRNAs). Previous studies on cell lines have shown that knockout of miR-194 results in the dysregulation of thousands of genes—approximately 2,600 downregulated and 2,400 upregulated—highlighting the complexity of miRNA-mediated gene regulation [
29]. Our in silico analysis suggests a set of potential genes that may interact with miRNA-186; however, these findings should be interpreted with caution, as only functional studies can provide definitive insights into its biological role, especially in context of RA pathogenesis.
This showed that the regulation of inflammatory mediator expression occurs at multiple levels and not through a single molecule. There is no information in the literature on the expression level of miRNA-186-5p and its association with established RA; therefore, our results require confirmation. The aim of this study was not to find a direct relationship between the expression level of the tested molecule and the disease state or to conduct a functional study but to explore the possibility of using miRNAs as additional markers that may be helpful in diagnosing RA exacerbation or in identifying RA patients who are negative for serological markers such as the ACPA.
This study confirmed that the use of single molecular markers does not contribute to achieving satisfactory diagnostic value. Similar observations were noted in our previous study on lncRNAs [
30] and in other studies [
31,
32,
33]. The use of a combination of molecular markers may contribute to an increase in diagnostic value. However, even the use of a single molecular marker in combination with established serological markers, such as the ACPA, may be beneficial. Ren et al. [
34] used a combination of two piwi RNAs, for which the AUC value was 0.79. Upon adding additional markers in the form of ACPAs, the AUC increased to 0.9965, while the inclusion of RF in the diagnostic algorithm resulted in an AUC of 0.9932. Interestingly, the use of C-reactive protein (CRP) alone increased the diagnostic value of the marker combination to an AUC of 0.9289. This highlights the significant potential of combining new molecular markers with classical serological markers or general indicators of inflammation. In this study, we also confirmed such a relationship. In our study, we also observed an improvement in the AUC parameter for the miRNA combination, although a similar enhancement in diagnostic value was noted for miRNA-186. Importantly, miRNA-186 also showed differential expression between ACPA-positive and ACPA-negative RA patients. Moreover, miRNA-186 showed a consistent association with all three RA disease activity scales (DAS28, CDAI, and SDAI). Although methodological differences among these scoring systems led to slight shifts in patient distribution across activity categories, the overall trend of association with disease activity remained evident. This suggests that miRNA-186 may serve as a broadly applicable marker of disease exacerbation and diagnosis; however, this finding requires further validation.
The possibility of using a combination of markers as indicators of disease activity seems particularly important. The commonly used DAS28 scale has certain limitations. The DAS28 includes only 28 joints, mainly of the upper limbs, excluding joints such as the foot and ankle, which are frequently affected in RA. This may lead to an underestimation of disease activity. Furthermore, the scale underestimates the SJN, which is a key indicator of active inflammation. Another limitation is the use of CRP as a component in calculating the DAS28 score. CRP strongly influences the DAS28-CRP value, which favors the use of agents such as IL-6 inhibitors and Janus Kinase (JAK) inhibitors, both of which significantly reduce CRP levels regardless of actual clinical improvement. In contrast, other drugs, such as abatacept or rituximab, which do not directly affect CRP, may be unfairly evaluated as less effective. The study also highlighted the importance of including a global assessment of patients (PGA), despite the controversy that may arise from the subjective nature of health status evaluations [
35]. Some studies suggest the possibility of omitting the PGA as a component in remission assessments. The study by Ferreira et al. [
36] indicated that remission criteria based on three components (3V), TJN, SJN, and CRP, encompass a much larger group of patients than the four-component (4V) remission criteria, which include the PGA, and are associated with a lower risk of radiological progression. Studies have shown that some patients do not meet the 4V remission criteria solely due to a high PGA, even though they do not exhibit active inflammation. In this study, we confirmed that the use of three components, TJN, SJN, and the laboratory marker ESR (instead of CRP), may effectively distinguish the active form of the disease from low activity. However, this study also demonstrates that the PGA significantly enhances the ability to differentiate disease severity. Additionally, molecular markers alone, as well as combinations of classical laboratory markers and molecular markers, are less effective. The approach presented in this research suggests the possibility of evaluating disease exacerbation using only blood-based markers and subjective patient assessments, without the need for a clinical visit. Of course, the proposed approach is not as effective as clinical evaluations of disease activity, although it offers a potential alternative. Nevertheless, the results obtained in this study should be considered preliminary and require validation in future research.
This study has some limitations. Firstly, a limitation of this study is the relatively small sample size, particularly in subgroup analyses, which may affect the statistical power and robustness of the conclusions. Moreover, the molecular profiles observed in this cohort may reflect chronic disease processes rather than early immunopathogenic events. Further functional studies are required to elucidate the biological role of miRNA-186 and its potential involvement in the pathogenesis of rheumatoid arthritis. Therefore, caution should be exercised when generalizing these results to patients in the early stages of RA. Secondly, this study regarding the use of miRNA antibodies and ACPAs as diagnostic markers is limited by the disease duration in patients with RA. Our study group had a median disease duration of approximately 12 years; therefore, the conclusions drawn should be validated in newly diagnosed patients and individuals with pre-RA, in whom the presence of ACPAs may precede the clinical symptoms of the disease by several years. Validation should also include patients with other systemic autoimmune diseases or connective tissue disorders. Notably, ACPAs appear in serum long before the onset of clinical symptoms of joint damage. Furthermore, they are stable over time, and the rate of seroconversion from ACPA-negative to ACPA-positive is very low, primarily occurring in patients who are RF-positive [
37]. For this reason, it seems reasonable to consider combining this marker with molecular indicators.
The last limitation is that RA patients were treated with various classes of medications, including combination therapies. Due to the exploratory nature of this study, as well as the limited number of patients in individual treatment subgroups, this study does not allow for definitive conclusions about the influence of therapy on the expression of the analyzed miRNAs. Further research is needed to determine the relationship between miRNA-186 expression and the pharmacological treatment applied.