miR-184 is highly expressed in the central corneal epithelial basal and suprabasal cells as well as in the lens epithelium; its expression was also detected in the corneal endothelium as documented by primary miRNA expression studies in the adult mouse eye [16, 17]. Previous studies found that miR-184 is part of a group of miRNAs that are downregulated in the ischemic retina and that had the potential to reduce retinal neovascularization after intraocular injection [18]. A mutation altering the seed region of miR-184 is responsible for severe familial keratoconus combined with early-onset anterior polar cataract [19]. The same single-base substitution miR-184(+57C > T) is also described in a syndrome characterized by endothelial dystrophy, iris hypoplasia, congenital cataract, and stromal thinning (EDICT syndrome) [14]. Slit-lamp examination of individuals with EDICT syndrome shows corneal haze and a FECD-like beaten-metal appearance of the endothelium [20, 21]. Corneal topography demonstrates nonectatic thinning and uniform steepening of the cornea [20, 21]. Histopathologically, corneas exhibit prominent posterior nodules and attenuated endothelium characteristic of FECD [20, 21]. Pathogenic mechanisms involved in EDICT syndrome may include the miR-184(+57C > T) variant leading to alterations in DICER-binding or RISC assembly reducing expression or activity of mature miR-184 or even resulting in a completely different mature miRNA [14]. The failure of the defective miR-184(+57C > T) variant to sufficiently compete with another miRNA, miR-205, and avoid miR-205 related knockdown of the gene INPPL1 may lead to dysregulation of the Akt signaling pathway and defects in epithelial-mesenchymal transition (EMT) [14, 19, 22]. EMT describes the transition of cells with a polarized character to a migratory phenotype. It is suggested that defective EMT may represent a unifying pathway for corneal endothelial disorders like FECD and EDICT syndrome by irregular migration and replacement of cells particularly from the corneal endothelial periphery to the corneal endothelial center [22].

A comparative analysis of miRNA expression in corneal endothelial cells from FECD and normal individuals demonstrated downregulation of miRNA levels in FECD endothelium: 87 miRNAs exhibit significantly decreased expression levels in FECD corneal endothelial cells [15]. Such unidirectional alterations in miRNA expression have previously been found in other pathologies including cancer tissue, human and rat cigarette smoke-exposed lung tissue, and cortical tissue affected by neuropsychiatric disorders [15, 23–28], and they may be explained by changes in transcription or epigenetic regulation of primary miRNA transcripts, changes in miRNA biogenesis, alterations in Ago protein expression, or changes in miRNA turnover [15, 23]. miR-29a is among the three most downregulated miRNAs in FECD endothelium, and the spectrum of downregulated mature miRNA transcripts encompasses a total of three members of the miR-29 family [15]. More detailed analyses of putative miR-29 target genes on transcriptional and translational level confirmed increased expression of collagen I, collagen IV, and laminin in FECD endothelial samples [15], suggesting impact of altered miR-29 regulation on subendothelial accumulation of these and other extracellular matrix-associated components in FECD [15]. MiR-29 family members are key modulators of ECM homeostasis and inhibit a variety of ECM-related transcripts and proteins. All miR-29 family members cover a similar spectrum of target genes due to similarities in their sequences and due to their identical seed sequences [29]. Aberrant miR-29 expression is described in hepatic, renal, cardiac, and pulmonary fibrosis [29–34]. These studies in non-ocular tissues are supplemented by an increasing number of reports in ocular tissues like Tenon’s capsule or the trabecular meshwork emphasizing the role of miR-29 in ECM modulation [35–37]. Corneal endothelial cells derived from FECD patients exhibit overexpression of EMT-inducing genes ZEB1 and SNAIL1, and it was proposed that ZEB1 and SNAIL1 overexpression may be responsible for increased responsiveness of corneal endothelial cells in FECD to TGF-beta [38]. Activation of TGF-beta pathways causes suppressed miR-29 expression in numerous other ocular and non-ocular tissues [35, 37]. However, the effect of TGF-beta on the expression levels of miR-29 in corneal endothelial cells has so far not been investigated.

The expression of miRNAs is altered in human tissues and body fluids under various pathological conditions, and miRNAs represent promising diagnostic and prognostic biomarkers in a multitude of non-ocular diseases including inflammatory diseases, cardiovascular diseases, neurodegenerative diseases, and cancer [39–43]. Future studies need to clarify if these results from studies in non-ocular diseases are transferable also to ocular disorders and confirm practicability of the approach. miRNA expression is not limited to the ocular tissue itself but tear fluid and aqueous and vitreous humor [39, 44–46] also exhibiting miRNA expression represent additional potential sources for diagnostic and prognostic miRNA analysis.

The avascular clear cornea offers the features of an easily accessible tissue which can be clinically examined without difficulties. This allows for the topical application of drugs and a generally uncomplicated clinical evaluation of their therapeutic effects. Alternative modes to topical external application of miRNA modulators by eye drops include subconjunctival, intracameral, or intravitreal injection or even systemic administration. Changes in endothelial miRNA expression or function suggest that miRNAs could be valuable future therapeutic targets in corneal endothelial diseases. Sequence complementarity of miRNAs and their mRNA targets provides the potential to specifically modulate individual miRNAs and genes. However, it must be considered that individual miRNAs have a multitude of target genes, and thus, modulation of a single miRNA may affect numerous cellular processes and pathways. This either beneficial or disadvantageous effect may be even further augmented by individual therapeutics targeting more than one miRNA. miRNA replenishment can be achieved by gene transfer inducing miRNA expression or by administration of miRNA mimics. miRNA mimics feature a guide strand identical to the miRNA of interest and a passenger strand which may be subject to further molecular modifications for optimized functionality [47]. The spectrum of inhibitory miRNA modulators includes antisense oligonucleotides (single-stranded RNAs complementary to target miRNAs), small-molecule inhibitors (small-molecule chemical compounds interfering with miRNA maturation or biogenesis), and miRNA sponges (overexpression of mRNAs with multiple artificial binding sites to a miRNA of interest). Despite the apparently great potential of miRNA therapy for corneal endothelial disorders, future studies will need to refine and increase the spectrum of target miRNAs and of their target genes. Developing therapeutic approaches will bring about challenges including optimizing tropism of therapeutics for corneal endothelial cells, delivery of sufficient therapeutic levels into the corneal endothelial cytosol, and avoidance of off-target effects and unwanted local unphysiological miRNA levels.