The Clinical Expression and Implication of Plasma miRNAs in Patients with Atrial Fibrillation

Introduction: circulating miRNA were identified as potential biomarkers in cardiac arrhythmias. It is intended to evaluate, in patients with atrial fibrillation (AF), the expression of miRNAs in plasma, their clinical repercussion and their prognostic factor. Materials and Methods: PubMed, Cochrane and Google Scholar platforms were used as a data source. Eligibility criteria: miRNAs as direct biomarkers in the clinical expression of AF; all languages. Last search: 19/09/2019; studies involving other comorbidities were excluded; pharmacological focus; in animals, in patients already submitted to cardiac procedures and review articles. Plasma collection and storage, RNA isolation, data analysis, real-time PCR, miRNA target prediction and statistical analysis were performed in the studies. Results: The studies generated n = 332 patients: with AF and control group. In patients with AF there was an increase in the expression of miR-1266, miR-4279, miR-892a, miR-3149, miR-155, miR-146b5p, miR-19b and a reduction in miR-3171, miR-3664-5p and miRNA -150. Discussion : it was noted that the regulation of inflammatory mediators, ion channels and cardiac morphophysiology are related to the expression of miRNA. Conclusion: Therefore, the levels of changes in miRNAs may reflect the severity of the clinical and pathophysiological progression of AF, and can be used as predictors of AF.


INTRODUCTION
Atrial fibrillation (AF) is a supraventricular arrhythmia in which a complete disorganization in the atrial electric activity occurs. It occurs when a diffuse and disorganized pattern of electrical activity in the atria suppresses or replaces the normal sinus mechanism.
This disorder is one of the main causes of morbidity, mortality and public health spending 1 .
In the last two decades, AF has become a major public health problem at great cost to health resources. It has an important impact on life quality, especially due to its clinical consequences, thromboembolic phenomena and cognitive alterations 1 . In this regard, studies involving microRNAs (miRNAs) appear as a new perspective in early AF diagnosis.
Atrial fibrillation is the most frequent cardiac arrhythmia in clinical practice and its prevalence in the general population was estimated between 0.5 and 1%.
In the United States, 2.3 million people have fibrillation, and that number is expected to rise to 5.6 million by 2050. It is also associated with a five-fold increased risk of stroke and is estimated to cause 15% of all strokes 1  This led to a paradigm shift from an "electrical" problem to something more "structural" 4 . Thus, miRNAs are understood as potential biomarkers for the development and maintenance of this arrhythmia.

OBJECTIVES
The objective of this study was to evaluate the expression of miRNAs in plasma, their clinical repercussion and their prognostic factor in AF patients.
This review contributes to the understanding of the process that triggers AF by encouraging new studies to evaluate the application of these molecules as biomarkers of this disease.

METHODOLOGY
This is a systematic review according to PRISMA recommendation in order to analyze studies on the expression of miRNAs in plasma, their clinical repercussion and their prognostic factor in AF patients.
There was no language selection or time delimitation, and the last search was made on September 19, 2019. Studies involving other comorbidities, with a pharmacological focus, in animals, in patients already submitted to heart procedures, and review articles were excluded. Thus, studies that brought miRNAs as biomarkers in the clinical expression of AF were prioritized, allowing the analysis of the direct correlation between these molecules and the disease.
The search for articles occurred until September 19, 2019 using the keywords "miRNAs", "atrial fibrillation" and "biomarkers" (always in additive character), in the platforms: PubMed and Cochrane, where 55 and 100 articles were screened respectively. At PubMed, it was typed in the search bar: miRNAs AND atrial fibrillation AND biomarkers. There was no language selection, specific period or specification of publication type.
The search was made until September 19, 2019, and it generated 55 articles. Google Scholar was also used as a search platform, however, by following the same strategy of the other platforms, the number of articles exceeded 1,600, interfering in the quality of this review. Thus, a periodization was stipulated (selecting articles from the year 2015 until the date of the last search of the other platforms) using the keywords "miRNAs expression", "atrial fibrillation" and "biomarkers", which resulted in 112 articles, totaling an n = 267 articles screened for this review. According to Risk of Bias in Systematic Reviews (ROBIS), the review is evaluated as low bias potential.

RESULTS
The study of Xu et al. 5  It was found that the plasma level of miRNA-150 in AF patients was substantially lower than in healthy people.
As a result, the reduced miRNA-150 in circulation was significantly associated with AF. As a next step, the study sought to determine whether miRNA-150 was linked to AF pathogenesis. The study sought to identify correlations in clinical samples between miR-150 expression levels and important protein-coding genes involved in AF pathogenesis. A target search for miR-150 was performed using four databases (TargetScan, miRanda, Starbase Clip-seq and miRDB). It was found that at least 18 genes were related to AF. Among them, 11 genes were related to the inflammatory response system, 4 played a role in calcium ion channels, 2 were involved in apoptosis and 1 related to fibrosis of the heart tissue.
The inflammation was not only a marker of the incident AF, but is also mechanically involved in inducing the fibrillation process. The predicted target genes (IL-6, IL-18, TNF-α and TGF-β) increased significantly in AF patients. Cytokines such as these are produced by activated cells, usually monocytes and macrophages that also secrete miRNA-150 in response to inflammatory stimuli, with the objective of reducing inflammation. They are fundamental in the activation of the inflammatory cascade and in the production of acute phase proteins. One such acute phase protein that is the focus of much research is the C-reactive protein (CRP). These findings suggest a negative correlation between the expression of inflammatory cytokines and miR-150. Thus, the study found that plasma levels of CRP in patients with persistent AF were higher than in patients with paroxysmal AF, and the levels in both groups of AF were higher than in the control group.
MiRNA-150 had a high negative correlation with the inflammatory response and, consequently, with CRP 6 .
Liu et al. 6   The miR-19b participates in the regulation of nuclear factor kappa-B (NF-kB) activity, which is highly related to inflammatory processes in cardiomyocytes and, consequently, to AF.
Circulating miRNAs in AF patients were sequenced by Liu et al. 6 and the expression levels of miR-146a, miR-150 were remarkably reduced. The authors concluded that a reduction in circulating miR-150 was significantly associated with AF. In addition, they found that plasma CRP levels were negatively correlated to plasma levels of miR-150, which was also evidenced by Xu et al. 5 .
According to Korantzopoulos et al. 8    • ParoAF compared to the PersAF group did not show significance, but there was one difference.

Liu et al. (2012) 6
• It was found that the plasma level of miRNA-150 in AF patients was substantially lower than in healthy people.
• 18 genes were related to AF. Among them, 11 genes were related to the inflammatory response system, 4 played a role in the calcium ion channels, 2 were involved in apoptosis and 1 was related to fibrosis of the heart tissue.
• In the population evaluated with AF, significantly lower levels of expression of miRNA-150 were found, and thus it was identified as a predictor of the disease.  proven in other studies, such as that of Korantzopoulos et al. 8 , in which miRNA are highly related to the stress of the body, participating in the regulation of inflammatory processes. Thus, new studies evaluating the role of miRNA in processes that promote aggressions to the organism should be conducted in order to evaluate their correlations not only with AF, but with several pathologies, as shown in Fig. 1.

CONCLUSION
In short, it is understood that miRNAs are in fact positively or negatively correlated to atrial fibrillation, acting directly on morphology and/or cardiac function.
Thus, the union of these studies reinforces the idea of the importance of these molecules as biomarkers of the disease, giving new directions to research on the diagnosis, development, maintenance and prognosis of atrial fibrillation.
However, the ability of this review to confirm the diagnostic power of miRNAs is still limited, given the small number of individuals in the reviewed studies.
Another important point that needs to be addressed is the pathophysiological mechanisms to which these molecules are involved, allowing a more punctual and effective diagnostic approach. Artigos revisados e selecionados para o estudo: (n=3) Critérios de exclusão -Artigos de revisão sistemática; de pesquisa desenvolvidas em animais; apenas com resultados preliminares.