Brain AVMs-Related microRNAs: Machine Learning Algorithm for Expression Profiles of Target Genes

Introduction: microRNAs (miRNAs) are a class of non-coding RNAs playing a myriad of important roles in regulating gene expression. Of note, recent work demonstrated a critical role of miRNAs in the genesis and progression of brain arteriovenous malformations (bAVMs). Accordingly, here we examine miRNA signatures related to bAVMs and associated gene expression. In so doing we expound on the potential prognostic, diagnostic, and therapeutic significance of miRNAs in the clinical management of bAVMs. Methods: A PRISMA-based literature review was performed using PubMed/Medline database with the following search terms: “brain arteriovenous malformations”, “cerebral arteriovenous malformations”, “microRNA”, and “miRNA”. All preclinical and clinical studies written in English, regardless of date, were selected. For our bioinformatic analyses, miRWalk and miRTarBase machine learning algorithms were employed; the Kyoto Encyclopedia of Genes and Genomes (KEGG) database was quired for associated pathways/functions. Results: four studies were ultimately included in the final analyses. Sequencing data consistently revealed the decreased expression of miR-18a in bAVM-endothelial cells, resulting in increased levels of vascular endodermal growth factor (VEGF), Id-1, matrix metalloproteinase, and growth signals. Our analyses also suggest that the downregulation of miR-137 and miR-195* within vascular smooth muscle cells (VSMCs) may foster the activation of inflammation, aberrant angiogenesis, and phenotypic switching. In the peripheral blood, the overexpression of miR-7-5p, miR-629-5p, miR-199a-5p, miR-200b-3p, and let-7b-5p may contribute to endothelial proliferation and nidus development. The machine learning algorithms employed confirmed associations between miRNA-related target networks, vascular rearrangement, and bAVM progression. Conclusion: miRNAs expression appears to be critical in managing bAVMs’ post-transcriptional signals. Targets of microRNAs regulate canonical vascular proliferation and reshaping. Although additional scientific evidence is needed, the identification of bAVM miRNA signatures may facilitate the development of novel prognostic/diagnostic tools and molecular therapies for bAVMs.

The expression profiles of mRNA targets of microRNAs were further investigated via the miRTarBase (http://mirtarbase.cuhk.edu.cn (accessed on 16 July 2022)). miRNAs, putative targets, level of expression in human tissue, and associated pathologies were derived [27].
Functional enrichment analysis of target genes was performed using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database [28].

Literature Review and Data Extraction
Our literature search returned 26 records; after the removal of duplicates and the application of our inclusion criteria, four items were deemed eligible for inclusion within the review. The PRISMA flow chart outlining the search strategy is presented in Figure 1.  Eight miRNAs were detected via genetic investigation(s) of surgically resected tissue and in vitro samples of bAVM-derived endothelial cells (bAVM-ECs), vascular smooth muscle cells (VSMCs), and peripheral blood. Human bAVMs samples were obtained after microsurgical resection. The onset, sporadic nature, or inherited genetic disease was undefined, except for Chen's series. This latter included only patients harboring unruptured bAVMs [29][30][31][32]. Table 1 summarizes the genetics and expression profiles of the miRNAs identified.   . miRNA transcription and mechanism of action. RNA polymerase II (RNA Pol II) drives the transcription of primary microRNA (pri-miRNA) from the miRNA gene. Pri-miRNA is then processed by the Double-Stranded RNA-Specific Endoribonuclease (DROSHA), an RNase III enzyme, into 60-to 110-nucleotides pre-miRNA within the nucleus. The pre-miRNA is transferred to the cytoplasm by the Exportin-5 (XPO5). pre-miRNA is then cleaved by the RNase DICER, combined with the transactivation response element RNA-binding protein (TRBP), an RNA-binding cofactor, into a mature miRNA duplex. The miRNA is loaded on AGO2, an endonucleolytic component of the RISC complex (RNA-induced silencing complex), and split. The miRNA antisense strand binds the target messenger RNA (mRNA), assembling a double-stranded helix. If there is a total complementarity, mRNA undergoes endonucleolytic degradation. Contrastingly, in the case of partial complementarity, miRNA is translationally repressed.

MicroRNA Sequencing Analyses
Robust miRNA sequencing was performed by Ferreira and colleagues in 2014 [29]; they undertook the deep genetic profiling of human bAVM-ECs. ECs own atypical features and intense proliferation pathways are upregulated by the overexpression of endo- Figure 2. miRNA transcription and mechanism of action. RNA polymerase II (RNA Pol II) drives the transcription of primary microRNA (pri-miRNA) from the miRNA gene. Pri-miRNA is then processed by the Double-Stranded RNA-Specific Endoribonuclease (DROSHA), an RNase III enzyme, into 60to 110-nucleotides pre-miRNA within the nucleus. The pre-miRNA is transferred to the cytoplasm by the Exportin-5 (XPO5). pre-miRNA is then cleaved by the RNase DICER, combined with the transactivation response element RNA-binding protein (TRBP), an RNA-binding cofactor, into a Brain Sci. 2022, 12, 1628 5 of 22 mature miRNA duplex. The miRNA is loaded on AGO2, an endonucleolytic component of the RISC complex (RNA-induced silencing complex), and split. The miRNA antisense strand binds the target messenger RNA (mRNA), assembling a double-stranded helix. If there is a total complementarity, mRNA undergoes endonucleolytic degradation. Contrastingly, in the case of partial complementarity, miRNA is translationally repressed.

MicroRNA Sequencing Analyses
Robust miRNA sequencing was performed by Ferreira and colleagues in 2014 [29]; they undertook the deep genetic profiling of human bAVM-ECs. ECs own atypical features and intense proliferation pathways are upregulated by the overexpression of endothelial growth factors [22,33]. Critically, miRNA-18a (miR-18a) was found to be downregulated in bAVM-ECs as compared to ECs from the healthy cortex (i.e., a 2.94-fold decrease). The canonical function of miR-18a centers on its regulatory role in angiogenesis; this is accomplished via the inhibition of proangiogenic factors in part via the silencing of the inhibitor of DNAbinding protein 1 (Id-1), a transcriptional repressor of the thrombospondin-1 (TSP-1) [34,35].
Accordingly, under-expression of miR-18a and TSP-1 results in uncontrolled expression of Id-1, leading to an increased level of the vascular endothelial growth factors (VEGF)-A and -D and aberrant vascular proliferation [13,29]. In line with this, the miR-18a was shown to modulate TSP-1 transcription with associated reductions in Id-1 and VEGFs expression, thereby normalizing bAVMs' growth rate [29].
In 2020, Marín-Ramos et al. confirmed the downregulation of miR-18a in human surgical specimens of bAVM-ECs [30]. As part of this work, they also sought to use miR-18a as a potential non-invasive therapy to restore the normal bECs phenotype(s) and in so doing revert the bAVMs pathogenesis/progression. miR-18a was administered to murine AVM models; Mgp−/− treated mice underwent computed tomography angiography (CTA) which served to demonstrate the effectiveness of the drug in reducing aberrant neoangiogenesis and AVM development [30]. Protein expression analysis confirmed the involvement of miR-18a in the antiangiogenetic pathways impeding the expression of VEGFs through the inhibition of the plasminogen activator inhibitor-1 (PAI-1), bone morphogenetic protein 4 (BMP4), and hypoxia-inducible factor 1α (HIF-1α) [30,36]. miR-18a demonstrated additional roles in the rearrangement of matrix proteins within the microenvironment bordering bAVM via a reduction in the secretion of matrix metalloproteinases (MMP2 and 9) and ADAM metallopeptidase domain 10 (ADAM10) [30] (Figure 3).

miR-137 and miR-195*
The histopathological hypertrophy of bAVM vascular walls provided the theoretical basis for the genetic analysis of the cross-link between VSMCs, biomolecular growth signals, and inflammation pathways [37].
In 2017, Huang et al. explored the miRNA signatures in the bAVM and the effects on the VSMCs phenotype [31]. miR-137 and miR-195* were found to be downregulated, while the related downstream pathways increased.
Assuming the hypothetical role of angiogenesis suppression, miR-137 and miR-195* were then transfected to the bAVM-VSMCs. Results displayed the restoration of normal cellular functions and the inhibition of proliferation [31].
These pieces of evidence suggested the noticeable role of miR-137 and miR-195 in VSMCs phenotypic switching and tube development (Figure 4). as a potential non-invasive therapy to restore the normal bECs phenotype(s) and in so doing revert the bAVMs pathogenesis/progression. miR-18a was administered to murine AVM models; Mgp−/− treated mice underwent computed tomography angiography (CTA) which served to demonstrate the effectiveness of the drug in reducing aberrant neoangiogenesis and AVM development [30]. Protein expression analysis confirmed the involvement of miR-18a in the antiangiogenetic pathways impeding the expression of VEGFs through the inhibition of the plasminogen activator inhibitor-1 (PAI-1), bone morphogenetic protein 4 (BMP4), and hypoxia-inducible factor 1α (HIF-1α) [30,36]. miR-18a demonstrated additional roles in the rearrangement of matrix proteins within the microenvironment bordering bAVM via a reduction in the secretion of matrix metalloproteinases (MMP2 and 9) and ADAM metallopeptidase domain 10 (ADAM10) [30] (Figure 3).

miR-137 and miR-195*
The histopathological hypertrophy of bAVM vascular walls provided the theoretical basis for the genetic analysis of the cross-link between VSMCs, biomolecular growth signals, and inflammation pathways [37]. In 2017, Huang et al. explored the miRNA signatures in the bAVM and the effects on the VSMCs phenotype [31]. miR-137 and miR-195* were found to be downregulated, while the related downstream pathways increased.
Assuming the hypothetical role of angiogenesis suppression, miR-137 and miR-195* were then transfected to the bAVM-VSMCs. Results displayed the restoration of normal cellular functions and the inhibition of proliferation [31].
These pieces of evidence suggested the noticeable role of miR-137 and miR-195 in VSMCs phenotypic switching and tube development ( Figure 4).

Peripheral Blood miRNAs
In 2018, Chen and colleagues performed a wide miRNA next-generation gene sequencing on peripheral blood samples obtained from three patients harboring bAVMs compared to healthy subjects [32]. In the study group, 246 miRNAs were found to be dysregulated. The top five miRNAs, found at high serum levels, were as follows: miR-7-5p, miR-629-5p, miR-199a-5p, miR-200b-3p, and let-7b-5p. The functional enrichment analysis of potential target networks showed the above-mentioned miRNAs were all implicated in the vascular rearrangement. The increased levels of miRNAs regulate multiple pathways implicated in bAVMs pathogenesis, triggering growth factors, vascular adhesion molecules, and inflammatory mediators. Amid the enriched regulatory networks of aberrant angiogenesis, the VEGF signaling cascade was pivotal in supporting tube formation, bECs proliferation, and migration [32].

Peripheral Blood MiRNAs
In 2018, Chen and colleagues performed a wide miRNA next-generation gene sequencing on peripheral blood samples obtained from three patients harboring bAVMs compared to healthy subjects [32]. In the study group, 246 miRNAs were found to be dysregulated. The top five miRNAs, found at high serum levels, were as follows: miR-7-5p, miR-629-5p, miR-199a-5p, miR-200b-3p, and let-7b-5p. The functional enrichment analysis of potential target networks showed the above-mentioned miRNAs were all implicated in the vascular rearrangement. The increased levels of miRNAs regulate multiple pathways implicated in bAVMs pathogenesis, triggering growth factors, vascular adhesion molecules, and inflammatory mediators. Amid the enriched regulatory networks of aberrant angiogenesis, the VEGF signaling cascade was pivotal in supporting tube formation, bECs proliferation, and migration [32].
Furthermore, the bioinformatic analysis on the KEGG database retrieved a functional enrichment appraisal of miRNA-related intracellular signals involved in the pathogenesis of bAVMs [28]. The main pathway, modulated by the miRNAs, was the VEGF, which regulates the aberrant angiogenesis and remodeling of the nidus ( Figure 8A). Furthermore, the bioinformatic analysis on the KEGG database retrieved a functional enrichment appraisal of miRNA-related intracellular signals involved in the pathogenesis of bAVMs [28]. The main pathway, modulated by the miRNAs, was the VEGF, which regulates the aberrant angiogenesis and remodeling of the nidus ( Figure 8A).  miR-18a was proved to downregulate the HIF-1α function. The HIF-1α pathway promotes cell proliferation, metabolism, bECs growth, and vascular tone ( Figure 8B). Figure 9 reports the interaction between the VEGF and HIF-1 signaling pathways. Hypoxia triggers pathological molecular mechanisms underlying the progression of neurovascular pathologies, such as bAVMs (Figure 9). miR-137 and miR-195*, expressed within the bAVM-VSMCs, inhibit the RAS pathway and MAPK/ERK ( Figure 10A), PI3K/Akt ( Figure 10B), and NFkB ( Figure 10C) pathways, which control the cell cycle, spreading, survival, gene expression, and activation of the inflammatory cascade. genase (GAPDH), inducing angiogenesis in response to reduced oxygen availability, cell proliferation, vascular tone, erythropoiesis, iron, and anaerobic metabolism, respectively. miR-18a was proved to downregulate the HIF-1α function. The HIF-1α pathway promotes cell proliferation, metabolism, bECs growth, and vascular tone ( Figure 8B). Figure 9 reports the interaction between the VEGF and HIF-1 signaling pathways. Hypoxia triggers pathological molecular mechanisms underlying the progression of neurovascular pathologies, such as bAVMs (Figure 9).

Figure 9.
Hypoxia-inducible factor 1 (HIF-1α), in normoxia conditions, undergoes hydroxylation at specific prolyl residues. It is identified by the von Hippel-Lindau tumor-suppressor protein (VHL) and this interaction promotes its rapid ubiquitination and degradation. Under hypoxia, the complex HIF-1α/β, combined with the aryl hydrocarbon receptor nuclear translocator (ARNT) and the transcriptional regulator hypoxia-response element (HRE), induces transcription of erythropoietin (EPO), vascular endothelial growth factor (VEGF), and glucose transporter 1 (GLUT1). These pathways regulate, erythropoiesis, cell metabolism, and migration. Furthermore, the high blood level of VEGF promotes endothelial proliferation and angiogenesis. vascular endothelial growth factor receptor (VEGFR). miR-137 and miR-195*, expressed within the bAVM-VSMCs, inhibit the RAS pathway and MAPK/ERK ( Figure 10A), PI3K/Akt ( Figure 10B), and NFkB ( Figure 10C) pathways, which control the cell cycle, spreading, survival, gene expression, and activation of the inflammatory cascade. Figure 9. Hypoxia-inducible factor 1 (HIF-1α), in normoxia conditions, undergoes hydroxylation at specific prolyl residues. It is identified by the von Hippel-Lindau tumor-suppressor protein (VHL) and this interaction promotes its rapid ubiquitination and degradation. Under hypoxia, the complex HIF-1α/β, combined with the aryl hydrocarbon receptor nuclear translocator (ARNT) and the transcriptional regulator hypoxia-response element (HRE), induces transcription of erythropoietin (EPO), vascular endothelial growth factor (VEGF), and glucose transporter 1 (GLUT1). These pathways regulate, erythropoiesis, cell metabolism, and migration. Furthermore, the high blood level of VEGF promotes endothelial proliferation and angiogenesis. vascular endothelial growth factor receptor (VEGFR). and GTPase-activating proteins (GAPs). Activated RAS (RAS-GTP) regulates multiple cellular functions through effectors including Raf, phosphatidylinositol 3-kinase (PI3K), Ral guanine nucleotidedissociation stimulator (RALGDS), calcium signaling pathways, cell endocytosis, migration, and survival. (B) The phosphatidylinositol 3 -kinase (PI3K)-Akt signaling pathway is activated by many types of cellular stimuli or toxic insults and regulates fundamental cellular functions such as transcription, translation, proliferation, growth, and survival. The binding of growth factors to their receptor tyrosine kinase (RTK) or G protein-coupled receptors (GPCR) stimulates class Ia and Ib PI3K isoforms, respectively. PI3K catalyzes the production of phosphatidylinositol-3,4,5-triphosphate (PIP3) at the cell membrane. PIP3 in turn serves as a second messenger that helps to activate Akt. Once active, Akt can control key cellular processes by phosphorylating substrates involved in apoptosis, protein synthesis, glucose metabolism, and cell cycle. (C) Nuclear factor-kappa B (NFkB) is a transcription factor that regulates mechanisms of immunity, inflammation, and cell survival. The canonical pathway of NFkB is activated by the tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), or infections. These signals rely on the IkB kinase (IKK)-mediated IkB-alpha phosphorylation on Ser32 and 36, leading to its degradation. This allows the p50/p65 NF-kB dimer to penetrate the nucleus and activate gene transcription. Atypical IKK-independent pathways rely on the phosphorylation of IkB-alpha on Tyr42 or Ser residues in the IkappaB-alpha PEST domain. The non-canonical pathway is triggered by members of the tumor necrosis factor receptor 1 (TNFR) superfamily, such as lymphotoxin-beta (LT-beta) or B-cell activating factor (BAFF). It involves NIK and IKK-alpha-mediated p100 phosphorylation and processing to p52, resulting in the nuclear translocation of p52/RelB heterodimers, involving in B-cell survival and lymphopoiesis.
All the miRNA-target regulatory networks are interconnected and converge in the modulation of pro-inflammatory, proangiogenetic pathways, and gene transduction aimed at dynamically enhancing bAVMs growth.

Discussion
The present article was aimed at identifying the miRNAs involved in bAVMs pathogenesis, related downstream genetic networks, the potential significance as prognostic biomarkers, and their diagnostic and therapeutic role.
Current shreds of evidence in the literature report the bAVMs as dynamic and mutable lesions, supported by constant vascular remodeling under the stimulus of blood flow [5,38,39]. They consist of enlarged arterial feeders and draining veins, which shunt in a nidus of tangled vessels. The direct communication of arterial and venous blood pressures, without capillary interposition, explains the high blood flow within the nidus and the mild hemorrhage rate ranging from 2% to 4% per year [40,41].
In 2017, Brinjikji and colleagues conducted a systematic review and meta-analysis of the correlation between bAVMs and HHT. They concluded that the bAVMs have a prevalence of approximately 10% in the HHT population, with a greater incidence of HHT1. HHT-related bAVMs are usually symptomatic, Spetzler-Martin grade 2, and have a low risk of rupture [46].
HHT-related genes are related to the TGF-β pathway, which proved to be pivotal in regulating endothelial expansion, differentiation, and matrix remodeling [51,52]. HHTmutations crosstalk even with the VEGF, PTEN, PI3K/AKT, and MAPK/ERK activity, which regulates the physiological mechanism of angiogenesis [53][54][55]. This evidence, also supported by studies on genetic mouse models, supports the hypothetical role of the aberrant pathways underlying the etiology and pathogenesis of hereditary bAVMs [56][57][58].
Moreover, the analyses revealed sporadic aberrations and single nucleotide polymorphisms predisposing to de novo cerebral vascular malformations [13,59,60]. The mechanism of inheritance of sporadic bAVMs is still not well elucidated. The main gene mutations, identified as genetic risk factors for bAVMs occurrence, involve the RAS-related pathways.
In 2022, Wang et al. recruited a cohort of 150 patients with bAVMs and performed whole-exome sequencing on peripheral blood DNA, to investigate the mutational spectrum [70]. Results revealed a strong correlation with the aberration of the RAS-RAF-MEK-ERK pathway, particularly the presence of the RASA1 mutation, an autosomal dominant disorder related to vasculogenesis [64].
These mutations within the endothelial genome are responsible for the steady bAVMs vascular reshaping, also in concurrence with the pulsatility of blood flow. The continuous wall shear stress alters the chromatin assembling, DNA methylome, and endothelial mechanotransduction, leading to bAVM development [71][72][73]. The endothelial dysfunction reflects an increased activation of the inflammatory cascade, proangiogenic mediators, matrix proteins, and growth factors triggering tube proliferation [74][75][76][77][78].
Among these vascular disorders, high flow shunt, dysfunction of vascular cells, inflammatory cell infiltration, and matrix remodeling may cause vessel wall weakness and predispose to bAVMs rupture. The molecular mechanisms underlying the risk of bAVMs breakage are recognizable in the mutations of angiogenesis pathways and inflammation signals.
On these assumptions, the analysis of the miRNAs is intended to deepen the genetic aspects which upstream regulate and support the molecular pathways involved in cerebral vascular malformations.
Our review retrieved four articles sustaining the theoretical role of miRNAs in the bAVMs pathogenesis. Genetic investigations were conducted on the ECs, VSMCs, and peripheral blood [29][30][31][32].
In 2017, Huang and his group investigated the miRNAs' influence on bAVM-VSMCs properties [31]. VSMCs are fundamental in supporting vascular structure, as they sustain the stiffness of vessels wall and resilience against blood flow. VSMCs contribute to the bAVMs' progression by heading the arterialization of the venous drainages and managing the molecular interactions which promote vasculogenesis and inflammation [79]. miR-137 or miR-195* were found under-expressed in the bAVM-VSMCs, while the miRNA-related pathways were upregulated [31].
The increase of target regulatory networks, such as the VEGF, NFkB, PI3K/Akt, and MAPK signals, enhanced bAVM-VSMCs migration and tube formation. Additionally, miR-137 and miR-195* may affect the phenotypic switching of VSMCs, through cytokine and growth factors secretion, resulting in the inhibition of aberrant vascular remodeling.
The bioinformatics analysis we performed using artificial intelligence algorithms confirmed the correlation between miRNA-related target genes and the above-mentioned molecular mechanisms underlying bAVMs growth. The functional enrichment investigation revealed several intracellular signals allegedly involved. These included firstly the VEGF cascade, renowned for angiogenesis; HIF-1α, transcribed in reaction to hypoxia; PI3K/Akt, MAPK/ERK, and NFkB pathways implicated in cell survival; ECs proliferation; and inflammation.
Their conclusions strongly support the significance of miRNAs in the occurrence and hemodynamics of neurovascular malformations [23].
Apart from supporting the pathogenesis of vascular abnormalities, miRNAs should be interpreted as prognostic markers as well as innovative diagnostic tools. Identification of specific miRNAs within the bAVMs microenvironment allows for designing novel therapeutic strategies and also improving the existing practices.

Experimental Strategies and Future Perspectives for bAVMs
Therapeutic management algorithms for bAVMs are constantly improved by the design of hybrid surgical, endovascular, and radiotherapy protocols. The up-to-date guidelines refer to tailored grading systems that stratify the individual risk and allow the choice of the most appropriate treatment [80,81].
Based on the substantial implications of the VEGF pathway in the bAVMs vascular remodeling, it was the first candidate as a therapeutic target. In animal studies, bevacizumab, an anti-VEGF monoclonal antibody, was shown to reduce the progression of cerebral vascular malformations [95,96]. A phase I clinical trial, completed in 2020, tested the bevacizumab in bAVMs treatment. It reported efficacy in reducing the size of lesions and no side effects were reported (#NCT02314377).
Despite the administration of soluble VEGFR has exhibited antiangiogenetic properties in the AVMs mouse model, no further strategies were planned due to the risk of simultaneous inhibition of the proper cerebral vasculature [97,98].
A class of antibiotics, tetracyclines-especially doxycycline-, was tested as bAVM therapy [99,100]. Doxycycline was employed as monotherapy, or combined with minocycline, in three already completed phase I clinical trials, (https://clinicaltrials.gov (accessed on 16 July 2022) #NCT00783523, #NCT00783523, #NCT00243893). It demonstrated significant activity against vascular proliferation and tube formation with a good safety profile. Doxycycline also proved to inhibit MMP remodeling, reducing the risk of rupture [99].
The most recent phase II clinical trial exploits the anti-inflammatory and antiproliferative properties of lovastatin for bAVMs ECs and VSCMs (#NCT04297033). Results are still pending. Furthermore, the intravenous administration of naked miR-18a showed effective results. miR-18a controlled the ECs function via an easy intracellular diffusion, without a need for transfection reagents, resulting in the inhibition of tube development and vascular proliferation [22,29].

Emerging miRNA-Based Therapies
miRNA-based therapeutics can be feasible and effective noninvasive strategies and they are classified as mimics or inhibitors (antimiRs). Mimics are synthetic double-stranded molecules that directly link miRNAs restoring missing expressions, while the antagonists inhibit specific miRNA targets.
Several miRNA-based drugs are under examination in phase I and II clinical trials, revealing potential efficacy for the treatment of tumors, hepatitis C, atherosclerosis, and kidney diseases.
In cancer therapy, miRNAs may control the tumor progression via the expression/repression of target genes aimed at regulation of the cell cycle, metabolism, apoptosis, and immunosuppression. The overexpression of miR-21 in tumors blocks the activity of oncosuppressor genes. The antimiR-21, tested in breast cancer, was demonstrated to inactivate the AKT and MAPK pathways, which are related to cancer cell proliferation, angiogenesis, and chemoresistance [101,102]. The miR-34 is was found to be under-expressed in neuroblastoma, lung cancer, melanomas, and leukemias [103,104]. The oncosuppressive activity of miR-34, exploited via the p53 pathway, was employed as a mimic therapeutic agent and showed promising results [105,106].
The miR-122 has a fundamental role in liver cell metabolism and is involved in the assembly of the hepatitis C virus (HCV) [107,108]. antimiR-122 drugs were tested in clinical and preclinical models, via intravenous administration, showing dose-dependent inhibition of HCV replication [109][110][111]. Unfortunately, a severe side effect was reported, namely the quick reduction in plasma cholesterol levels [112,113].
Furthermore, the miR-33a/b inhibits the expression of the ABCA1, a cholesterol transporter in the liver cells [114]. Inhibitors of miR-33a/b proved to enhance the expression of ABCA1, resulting in the increase of plasma HDL level, an atheroprotective effect, and subsequent plaque regression [114,115].
Numerous microRNAs are involved in the progression of renal fibrosis, including the miR-2. miR-2 is an excellent candidate for therapy because it regulates the redox metabolic pathway and lipid metabolism by the peroxisome proliferator-activated receptor-a (Ppar-a) [116].
miRNA therapeutics are even usable in the treatment of neurovascular malformations, to prevent vascular remodeling and decrease aberrant angiogenesis.
In a clinical study, Marín-Ramos et al. tested the miR-18a, via intravenous and intranasal administration, as a noninvasive therapy for bAVMs. They reported the efficacy of miR-18a in boosting TSP-1 activity, inhibiting VEGF pathways and AVM-BEC dysfunction [30].
Despite the encouraging results of miRNA-based therapies, the poor bioavailability, restricted tissue permeability, payload instability, and choice of delivery systems are still the main concerns [117].
Brain administration is further complicated by the cross of the blood-brain barrier (BBB), through which only lipid-soluble small molecules of less than 400 Daltons can penetrate.
Liposomes and nanoparticles were tested as miRNA vectors, but the results are still forthcoming [117,118]. Another developing strategy was the transient disruption of the BBB via chemical agents or alcohols [119]. In the study conducted by Marín-Ramos, they combined the miR-18a with the NEO100, a perillyl alcohol, aiming to facilitate the miR-18a delivery through the BBB [30].
Although the miRNA-based therapies were not yet approved by the FDA, advances in genetic engineering and translational medicine may lead to identifying new valid miRNA therapeutics to be translated from the bench to the bedside.

Limitations
The present study has several potential limitations: first, the limited number of studies involved; second, the scarcity of the genetic data reported; third, the intrinsic biases of artificial intelligence algorithms.
Additional limitations lie in the relative rarity of cerebral vascular malformations and the lack of certain knowledge about mechanisms underlying bAVMs pathogenesis. Therefore, results should be interpreted with caution.

Conclusions
bAVMs are high-flow vascular disorders that undergo constant remodeling under multifarious genetic and hemodynamic stimuli. miRNAs are important for genetic transcription regulation and modulation of intracellular pathways implicated in the vasculogenesis and proliferation signals.
The downregulation of miRNA-18a on the ECs may justify the lack of TSP-1 transcription and the overexpression of Id-1, proangiogenic factors, PAI-1, BMP4, HIF-1α, and MMPs, which leads to vessels rearrangement and bAVM growth.
On the VSMCs, the downregulation of miR-137 and miR-195* should affect the boost of VEGF, PI3K/Akt, MAPK/ERK, and NFkB pathways. These last induce the VSMCs phenotypic switching, vessel hypertrophy, and activation of the inflammatory cascade.
The increased levels of miR-7-5p, miR-629-5p, miR-199a-5p, miR-200b-3p, and let-7b-5p in the peripheral blood samples triggered remodeling of extracellular matrix, aberrant angiogenesis, and nidus formation. miRNA identification may act as a pivot for designing novel diagnostic and prognostic implements, as well as innovative drug mimetics of the miRNAs. Further advances are needed to implement our knowledge about bAVMs' pathophysiology and refine their treatment regimens.