MicroRNAs Role in Non-Communicable Diseases and Link to Multidrug Resistance, Regulation or Alteration

: Discovery of microRNAs (miRNAs) twenty years ago, has advocated a new era of “Mo-lecular Genetics”. About 2000 miRNAs are present, that regulate one third of the genome. MiRNAs dysregulated expression may contribute to several diseases including tumor growth. Their presence in body fluids, reflecting levels alteration in various cancers, merit circulating miRNAs as the “next generation biomarkers” for early stages tumor diagnosis and/or prognosis. Herein, we per-formed a comprehensive literature search focusing on the origin, biosynthesis and role of miRNAs and summarized the foremost studies centering on miRs value as non-invasive biomarkers in different non-communicable diseases, including various cancer types. Moreover, during chemotherapy many miRNAs were linked to multidrug resistance, via modulating numerous biological processes and/or pathways that will be highlighted as well.


MicroRNA; the Protein Short Non-coding RNAs
The human genome reveals that the protein-coding genes can be as few as 25,000 [1]. Despite the fact that the exact number of coding genes, within the human genome, is unknown, protein non-coding genes make up a significant portion of the human genome [2]. Human cells contain several distinguishing sequences of non-coding RNA (ncRNA) that could be categorized into two major classes: long ncRNA (≥ 200 nucleotides length) and short ncRNA (< 200 nucleotides). The short ncRNA group comprises different classes such as small-interfering RNAs (siRNA), small nuclear RNAs (snRNAs), small nucleolar RNAs (snoRNAs), piwi-interacting RNAs (piRNAs) and microRNAs (miRNAs) [3] .

1.2.
MiRNAs are the Essential Gene-Expression Regulatory Molecules MiRNAs regulate one-third of the human genome [4]. These are small single-stranded 17-25 nucleotides, recently known as essential gene-expression regulatory molecules [5].
The parent microRNAs member is lin-4, that has been discovered within a nematode called Caenorhabditis Elegans. Lin-4 was found to play a pivotal role in the transition from a larval stage into another, via suppression of lin-4 gene, concerned with larval development [6].
MiRs are encoded either via separate transcription units within the pre-mRNA introns or via multi-cistronic clusters [7]. MiRNAs organized in clusters within the genome, are sharing the same transcriptional regulatory elements, but are expressed individually, in the event, as if they have their own promoters [8].
MiRs direct major cellular functions such as proliferation, differentiation, maturation, and metabolism [9]. Irregular expression of miRNAs may occur in a range of distinctive pathologies, with striking modifications in tumor tissues [9]. Profiling of miRNAs has contributed to the molecular classification of tumors. The presence of miRNAs in body fluids as urine, serum or plasma, CSF, and tears, permitted non-invasive identification of various cancer types [10] , [11] , [12] , [13] considered as beneficial potential liquid biopsy.

2.1.
In Part I, the review aims to briefly discuss "miRs biosynthetic pathways and down-stream effects upon binding target mRNA".

2.2.
In Part II, the review aims to highlight "the utility of circulating miRNAs as biomarkers for non-communicable diseases (NCDs) and a brief about their role in cancer growth or resistance to treatment".

Review Methodology
An online search in the medical databases PUBMED and NCBI for the following terms: ("Circulating miRNA") AND ("Health and diseases regulation of gene expression") AND ("Role in Carcinogenesis") AND ("Epigenetics") AND ("Future promising biomarkers") was done on September, 2020, with publication date limit since 2015. Priority was given to papers with higher empirical evidence methodology, including clinical guidelines, meta-analysis, randomized clinical studies, systematic review, original papers, and narrative reviews.
Part I.

MiRNAs Biogeny
MiRNAs biogenesis include various coordinated steps and specific cellular mechanisms [8]. Biogenesis of miRNA starts with post-transcriptional or co-transcriptional preparation of RNA polymerase II-III transcripts. Around 50% of the miRNAs recently identified are intragenic and are typically regulated from introns and some protein-coding gene exons. The remaining ones are intergenic, freely transcribed and guided according to their own promoters from a host gene [14]. MiRNAs could be translated as a single long transcript, named clusters [15]. Moreover, miRNAs biogenesis is categorized as either canonical or non-canonical.

Canonical miRNAs Biogeny Pathway
This is the main route by which miRNAs are developed, as shown in Figure (1). In this process, primary-miRNAs (pri-miRNAs) are transcribed from their genes by RNA polymerase II. Which is then handled by a microprocessor complex composed of DiGeorge Syndrome Critical Region 8 (DGCR8); an RNA-binding protein, Drosha, a Class 2 ribonuclease III enzyme into precursor-miRNAs (pre-miRNAs) [16]. In this process, DGCR8 identifies an N6-methyl adenylated GGAC and a different motif within the pri-miRNA[16], while Drosha begins processing inside the nucleus by cutting the stem-loop precursor [17].
For most of the double stranded RNAs (dsRNAs) which are involved in small-RNA production routes, pre-miRNA seems to be a signature motif. This signature is recognized by the Exportin-5 protein; that facilitates the release of pre-miRNAs to the cytoplasm, through nuclear pores, depending on a GTP-GDP gradient [17]. Exported pre-miRNA is transferred to another RNase-III enzyme in the cytoplasm, called Dicer. Dicer, the cytoplasmic RNase-III enzyme, cuts the pre-miRNA to a miRNA duplex, which is un-winded afterwards giving the "Fully Developed Functional miRNA" molecule.
In an ATP-dependent manner, the two strands defined from the resultant miRNA duplex, might be stacked into the protein family Argonaute (AGO) known as AGO1-4 [18]. After miRNA duplex formation, one strand of the miRNA associates with an RNA-induced silencing complex (RISC) forming the "regulatory miRNA-RISC complex". The choice of strands 5p or 3p is dependent on the thermodynamic stability at the 5' untranslated regions (UTRs) at the 1-position of the nucleotide [19]. The unoccupied strand known as the passenger strand is loosened by different components from the loaded strand, named the guide or leading strand, depending on complementarity [6].

Figure 1: Canonical miRNAs Biogeny Pathway
[It is the primary route by which miRNAs are produced. In the nucleus, the gene is transcribed to generate primary miRNA (pri-miRNA) that is cleaved producing precursor miRNA (pre-miRNA), which is then exported to the cytoplasm to be broken, resulting in miRNA duplex. miRNA duplex links to RISC complex resulting in miRNA duplex unwinding to create a mature miRNA. Every mature miRNA binds to its target mRNA, resulting in silencing by cleavage, de-adenylation or repressing translation.]

Non-Canonical miRNAs Biogenic Pathway
Several non-canonical pathways have been illustrated to date, primarily Drosha/DGCR8-independent and dicer-independent pathways [

MiRNAs-Target Binding
Via complementarity between unique sequences, which are 2-7 bases, from the 5' end of the miRNA and certain target mRNA sequences, recognized as "miRNA Response Elements" (MREs), the developed miRNA attaches to its target[21]. 5.2. MiRNA-Target Gene mRNA-Binding Types 5.2.1. Ideal Binding where complete complementarity occurs when miRNA binds its target ORF resulting in an "RNA Decay" or 5.2.2. Imperfect Binding resulting in "Post-Transcriptional Silencing" via mRNA de-stabilization, de-capping, de-adenylation and translational repression [6] , [22].
It is worthy to mention the fundamental multifaced aspect of miRNAs target binding, is that their suppressive role is not limited to one-mRNA, highlighting the "One-mRNA Paradigm" in which multiple mRNA targets can be achieved by one microRNA and multiple microRNAs can hit one mRNA[23].

MiRNA-Target Gene(s) mRNA Silencing Mode(s)
Depending on the degree of MREs complementarity, the target gene(s) mRNA silencing strategies, by miRNAs, could be attained either via target gene mRNA degradation or target gene mRNA translation repression.

Target mRNA Decay
MiRNA-induced silencing complex (miRISC) AGO proteins bind to the GW182 (a protein-containing glycine-tryptophan repeat) to enroll the "de-adenylase complex" and promote de-adenylation of the target gene mRNA poly(A) tail. With the aid of the catalytic de-capping protein-2 (DCP2), after de-adenylation, and in the presence of an additional de-capping activators, miRISC de-caps the de-adenylated gene mRNAs. In the presence of an enhancer of de-capping 4 (EDC4), DCP1 and additional de-capping cofactors, the decay of the target mRNA is aided by the cytoplasmic 5' to 3' exonuclease1 (Xrn1p) [24] 5.3.2. Target mRNA Translation Repression miRNA-mediated target mRNA translational repression can occur before and after translational initiation step, through several mechanisms. 5.3.2.1. miRISC ties to the target mRNA at that point AGO protein interacts with the GW182. This interaction promotes the relocation of poly(A) binding protein from the 3' poly(A)-tail and blocks its binding to the eukaryotic initiation factor 4 complex (eIF4G), interfering with the "translation-initiation step"[25].

5.3.2.2.
Repressing cap-structure recognition by eIF4F complex where the AGO protein separates the eIF4A from the 5' cap binding complex of the target mRNA and therefore, the ribosomal subunit will not be recruited or attached to the mRNA for translation initiation[25].

MiRNA-Target Gene(s) Activation Mode
Activated targeted mRNA expression could be triggered by miRNAs [6], via AGO2 protein and fragile-x-mental retardation related protein-1, rather than GW182. This is achieved via MiR attachment on the target promoter, to induce RNA-Polymerase II recruitment followed by transcription activation [26].
Either "MiR-target gene(s) binding" resulted in an expression silencing or activation, these effects have been witnessed and recorded by researchers to be associated with various disease(s), that will be discussed in the current review Part II. Part II. 6 As listed in Table 1, many miRNAs are linked to ß-cells growth, insulin resistance or sensitivity, insulin production/secretion and insulin signaling, which can influence T2DM disease course [31]. Therefore, diabetes-related nephropathy or retinopathy is also affected by an altered microRNAs expression[32].

MiRNAs Lists in Cardiovascular Diseases
miRNAs regulate the cardiac progenitor cells differentiation and proliferation, controlling cardiac myocytes, endothelial cells, pacemaker cells, as well as smooth muscle cells function. Table 2 shows miRNAs lists dysregulated in various CVDs [33]. For example, miR-208a and miR-208b, encoded within alpha and beta-cardiac muscle myosin heavy chain genes, respectively, were found to be elevated in patients with acute myocardial infarction (AMI). Liu and his co-workers[34] demonstrated a significant predictive value for miR-208, miR-1 and miR-499 in AMI, higher than the traditional cardiac biomarkers, namely, TnT and CPK-MB.

MiRNAs List in Cerebrovascular Diseases
miRNAs are essential to the nervous system's improvement, with few miRNAs having function in developing ischemic cerebrovascular disorders incapacity [35]. Many miRNAs have been associated with post-stroke brain edema and post-stroke cell death, namely, apoptosis.
[36]As listed in Table 3

MiRNAs Involvement in Carcinogenesis via mTOR Signaling
In different types of cancer, the mechanistic target of rapamycin (mTOR); a conserved serine/threonine kinase enzyme involved in cell metabolism, could be hyperactive, leading to an abnormal cell proliferation and eventually cancer [143]. An association was observed between miRNA(s) and the mTOR pathway during cancer growth [143].

mTOR Signaling Pathway link to miRNA Biogenesis
Targeted Raptor mutation, a fundamental component of mTORC1 type, may affect increments in miRNA biogenesis [144]. On the other hand, Mdm2-dependent ubiquitination of Drosha, an RNase assigned to pri-miRNA formation to give pre-miRNA, therefore, mTOR activation widely suppresses miRNA biogenesis [144]. Few specific miRNA(s)-related to cancer are known to be regulated by mTOR signaling, as sketched in Figure (2). However, many miRNAs have been documented to target various mTOR signaling stages in different types of cancer, as shown listed in Table 5.