**5.4 ncRNA biomarkers in amyolateral sclerosis**

In a series of studies Freischmidt et al. identified a number of miRNAs differentially expressed in the serum of familial (miR-143-5p/3p, miR-132-5p/3p and miR-574-5p/3p) and sporadic (miR-1234-3p and miR-1825) ALS, noting that miRNA targets in familial ALS were TDP-43 binding RNAs and that the miRNA signature in sporadic ALS was highly heterogeneous [155, 156]. A subsequent study determined

increased miR-374b-5p, and decreased miR-206 and miR-143-3p in sporadic ALS patient serum [157]. Finally, increased expression of miR-424 and miR-206 in sporadic ALS patient plasma has been shown to correlate with clinical deterioration over time [111].

Recent work has identified 5'tiRNAVal−CAC as a potential biomarker for ALS. This tiRNA was found to be increased in the spinal cord of SOD1G93A mice and is significantly increased in the serum of patients with slow progressing ALS [120].

#### **5.5 ncRNA biomarkers in epilepsy**

Circulating miRNAs have been found to be dysregulated in the serum of epilepsy patients compared with healthy controls. Validation experiments identified an upregulation of let-7d-5p, miR-106b-5p, miR-130a-3p and miR-146a-5p, and a downregulation of miR-15a-5p and miR-194-5p. The highest diagnostic value was found in the upregulated miR-106b-5p [158]. Further studies also revealed miRNAs differentially expressed in treatment-resistant compared to treatment-responsive and control samples. The expression of miR-194-5p, miR-301a-3p, miR-30b-5p, miR-342-5p and miR-4446-3p were altered in drug-resistant epilepsy serum samples, with miR-301a-3p showing the highest sensitivity [159]. Finally, sequencing analysis and RT-qPCR validation identified miR-27a-3p, miR-328-3p and miR-654-3p as differentially expressed in the plasma of epilepsy patients compared to control. Importantly, these miRNAs were detected using a prototype point-of-care device that would greatly improve diagnostic capability in-clinic [160].

Recent sequencing analysis has identified three circulating tRFs that are increased in the plasma of epilepsy patients. The differential expression of 5′AlaTGC, 5′GluCTC and 5′GlyGCC was validated by RT-qPCR in an independent cohort and detected in resected hippocampal and cortical tissue indicating a possible source. Finally, the generation and release of these tRFs was shown to be activity-related in mouse hippocampal neuronal cultures [135].

### **6. ncRNA as a therapeutic target**

The direct involvement of miRNA and tsRNA in normal cellular activity, their dysregulation during disease pathogenesis and ability to target multiple genes within a particular pathway have made ncRNA an attractive and viable therapeutic target for the treatment of many neurological diseases. Therapeutic intervention strategies include the inhibition of overexpressed ncRNA and the restoration of repressed ncRNA. Small interfering RNA (siRNA) and antisense oligonucleotides (ASO) are the most common methods of miRNA inhibition. siRNA are short (20–25 nts) double-stranded RNA molecules that use the RNA interference RISC pathway to degrade target RNA. ASOs, also known as antimiRs or antagomiRs, are short single stranded oligonucleotides that hybridize with the target RNA and sterically interfere with its functionality. Recent advancements include the development of locked nucleic acid technology that increases the stability of ASO and siRNA [161]. The restoration miRNA expression suppressed in a given pathology through the delivery of synthetic double-stranded miRNA mimics, designed to mimic endogenous miRNAs, so far has primarily been used in gain-of-function studies to elucidate miRNA functions and mechanisms [98].

While a considerable amount of progress has been made with a number of miRNAs entering clinical trials, the development of RNA-based therapeutics has not been without issue. Double- and single-stranded RNA are recognized by the immune system, particularly the Toll-like receptors. To combat this, 2'O-methylation

#### *Emerging Roles of Non-Coding RNA in Neuronal Function and Dysfunction DOI: http://dx.doi.org/10.5772/intechopen.101327*

and the neutralization of RNA molecules significantly reduces the immunogenicity of RNA-based therapeutics [162–164]. Delivery systems to aid passage across the cell membrane and the targeting of specific organs and cells types have also been developed. Lipid and metal-based nanoparticles as well as polymer vectors such as polyethylene imine, polylactic-co-glycolic acid and poly-amidoamine have improved the delivery of RNA-based therapeutics [165]. Furthermore, artificial manipulation of miRNAs with the delivery of miRNA mimics *in vivo* is associated with difficult to predict off-target non-specific and unintended alterations in gene expression, and toxicity, off-setting potential for therapeutic efficacy [166].

To date the Federal Drug Administration and the European Medicines Agency have approved a number of RNA-based therapeutics [167]. Notably the 18-mer ASO Nusinersen is an intrathecal administered therapeutic for the treatment of spinal muscular atrophy. Phase II and III clinical trials are also ongoing for RNA-based therapeutics for the treatment of Huntington's disease [165].
