MicroRNA and diabetic retinopathy-biomarkers and novel therapeutics

Abstract

Diabetic retinopathy (DR) accounts for ~80% of legal blindness in persons aged 20-74 years and is associated with enormous social and health burdens. Current therapies are invasive, non-curative, and in-effective in 15-25% of DR patients. This review outlines the potential utility of microRNAs (miRNAs) as biomarkers and potential therapy for diabetic retinopathy. miRNAs are small noncoding forms of RNA that may play a role in the pathogenesis of DR by altering the level of expression of genes via single nucleotide polymorphism and regulatory loops. A majority of miRNAs are intracellular and specific intracellular microRNAs have been associated with cellular changes associated with DR. Some microRNAs are extracellular and called circulatory microRNAs. Circulatory miRNAs have been found to be differentially expressed in serum and bodily fluid in patients with diabetes mellitus (DM) with and without retinopathy. Some miRNAs have been associated with the severity of DR, and future studies may reveal whether circulatory miRNAs could serve as novel reliable biomarkers to detect or predict retinopathy progression. Therapeutic strategies can be developed utilizing the natural miRNA/long noncoding RNA (lncRNA) regulatory loops. miRNAs and lncRNAs are two major families of the non-protein-coding transcripts. They are regulatory molecules for fundamental cellular processes via a variety of mechanisms, and their expression and function are tightly regulated. The recent evidence indicates a cross-talk between miRNAs and lncRNAs. Therefore, dysregulation of miRNAs and lncRNAs is critical to human disease pathogenesis, such as diabetic retinopathy. miRNAs are long-distance communicators and reprogramming agents, and they embody an entirely novel paradigm in cellular and tissue signaling and interaction. By targeting specific miRNAs, whole pathways implicated in the pathogenesis of DR may potentially be altered. Understanding the endogenous roles of miRNAs in the pathogenesis of diabetic retinopathy could lead to novel diagnostic and therapeutic approaches to managing this frequently blinding retinal condition.

Keywords: DR therapy; MicroRNA (miRNA); diabetes mellitus (DM); diabetic retinopathy (DR).

Figure 1

Regulation of gene expression with miRNAs. MiRNA regulates gene expression at the level of translation (protein synthesis) by binding to messenger RNA (mRNA). Genetic variations, mostly single-nucleotide polymorphisms (SNPs), can cause imperfect pairing with target mRNA as well as structural mismatches in hairpin loops, resulting in dysregulation of miRNA/mRNA binding. Long non-coding RNAs (lncRNAs) are long RNA transcripts that do not encode proteins. In case when lncRNAs may regulate target gene expression via sequestering miRNA, they are known as competing endogenous RNA (ceRNA). Circular RNAs (circRNAs) are new endogenous non-coding RNA family members that arise during pre-mRNA splicing. CircRNA acts as a kind of ceRNA to play a role in regulating miRNA.

Figure 2

miRNA/lncRNA regulatory loops in DR. Ingenuity Pathway Analysis of the pathways affected by dysregulated miRNA in DR. The therapeutic strategies can be borrowed from natural regulatory loops to plan targeted therapy. Here are several examples of this type of regulatory network identified in diabetes and relevant to DR. AGO2, argonaute RISC catalytic component 2; AGO2-MIRLET7, Argonaut2/MIRLET7; Akt, AKT1/2/3; AR, androgen receptor; ATG3, autophagy related 3; ATG7, autophagy related 7; ATXN1, ataxin 1; BPA, 4'-bisphenol A; BRD4, bromodomain containing 4; CD44, CD44 molecule (Indian blood group); CDH1, cadherin 1; CDKN1A, cyclin dependent kinase inhibitor 1A; collagen type 1, Collagen I; CTNNB1, catenin beta 1; DES, DES disodium salt, 4,4'-(1,2-diethyl-1,2-ethenediyl)bis-, (E)-; DNMT3B, DNA methyltransferase 3 beta; DZIP3, DAZ interacting zinc finger protein 3; ELAV1, ELAV like RNA binding protein 1; ER, estrogen receptor; ERBB2, epidermal growth factor receptor 2; Estradiol, 17-beta-estradiol; FOXA1, forkhead box A1; FOXM1, forkhead box M1; GRB2, growth factor receptor bound protein 2; HIF1A, hypoxia inducible factor 1 subunit alpha; Histone H3, Histone H3B; HOTAIR, HOX transcript antisense RNA; HOXD10, homeobox D10; HSF1, heat shock transcription factor 1; ICAM1, intercellular adhesion molecule 1; IL6, interleukin 6; IRF1, interferon regulatory factor 1; JAM2, junctional adhesion molecule 2; KDM1A, lysine demethylase 1A; KMT2A, lysine methyltransferase 2A; KMT2C, lysine methyltransferase 2C; LAMTOR5, late endosomal/lysosomal adaptor, MAPK and MTOR activator 5; let-7, microRNA let-7i; LPS, lipopolysaccharides ; MALAT1, metastasis associated lung adenocarcinoma transcript 1; MDM2, MDM2 proto-oncogene; MET, MET proto-oncogene, receptor tyrosine kinase; MEX3B, mex-3 RNA binding family member B; mir-183, microRNA 183; miR-203a-3p, hsa-miR-203a-3p; miR-216a-5p, hsa-miR-216a-5p; mir-320, microRNA 320a; MIR130A, microRNA 130a; MIR141, microRNA 141; MIR148A, microRNA 148a; MIR29B, microRNA 29a; MIR320, microRNA 320; MIR331, hsa-miR-331; MIR7, microRNA 7-1; MIR9, microRNA 9-1MIRLET7, microRNA LET7; Mmp, Matrix Metalloproteinase; MRTFA, myocardin related transcription factor A; MYC, MYC proto-oncogene, bHLH transcription factor; NFκB, transcription factor nuclear factor κ-b; NFKBIA, NFKB inhibitor alpha; P300/CBP, p300/CBP; PCDH10, protocadherin 10; PCDHB5, protocadherin beta 5; PI3K, Phosphatidylinositol 3 kinase; PRC2, Polycomb Repressive Complex 2; PTEN, phosphatase and tensin homolog; RBM38, RNA binding motif protein 38; RHOC, ras homolog family member C; Rock, Rho Kinase, ROKs; SASP, senescence-associated secretory phenotype; SETDB1, SET domain bifurcated histone lysine methyltransferase 1; SHC, SHC adaptor protein 1; SIRT1, sirtuin 1; SNAI2, snail family transcriptional repressor 2; SNUPN, snurportin 1; SOS, son of sevenless; SPP1, secreted phosphoprotein 1; SRF, serum response factor; STAT3, signal transducer and activator of transcription 3; TCF/LEF, LEF/TCF; TGFβ1, transforming growth factor beta 1; TLR4, toll like receptor 4; TP53, tumor protein p53; VEGFA, vascular endothelial growth factor A; VEGFR, vascular endothelial growth factor receptor; VIM, vimentin; WIF1, WNT inhibitory factor 1; Wnt, wingless-related integration site; XIAP, X-linked inhibitor of apoptosis; CP, Canonical Pathways.

Figure 3

Exosomes as therapeutic vehicles for miRNA delivery. The high relative stability of miRNA in common clinical tissues and biofluids (e.g., plasma, serum, vitreous, etc.) results from their release from cells in the protective encapsulated form within microvesicles or exosomes, and bound to proteins such as argonaute 2 or packaged within lipid particles, exosomes. Such exosomes represent a promising delivery vehicle for novel drugs and gene therapy. Exosomes can be loaded with miRNA, they are easily taken up by cells, they are non-immunogenic, and they can cross blood/brain and blood/retinal barriers. There is a strong interest for exosomes to be engineered for effective miRNA delivery as a nanosized delivery system.

Figure 4

MiRNA as Targeted Therapy. MiRNAs as therapeutic molecules come in many different flavors, and can be custom designed: miRNA mimics, antagomiRs or anti-miRs, miR-mask that mask miR binding sequence on mRNA, and miRNA sponges. Several miRNA delivery methods are used in practice, such as viral vectors, plasmid, piggybacks expression vectors, nanoparticles, liposomes and exosomes. Different therapeutic miRNA strategies, delivery vehicles, and therapeutic strategies for targeted therapy are presented.