MicroRNA-Based Therapeutic Perspectives in Myotonic Dystrophy

Abstract

Myotonic dystrophy involves two types of chronically debilitating rare neuromuscular diseases: type 1 (DM1) and type 2 (DM2). Both share similarities in molecular cause, clinical signs, and symptoms with DM2 patients usually displaying milder phenotypes. It is well documented that key clinical symptoms in DM are associated with a strong mis-regulation of RNA metabolism observed in patient's cells. This mis-regulation is triggered by two leading DM-linked events: the sequestration of Muscleblind-like proteins (MBNL) and the mis-regulation of the CUGBP RNA-Binding Protein Elav-Like Family Member 1 (CELF1) that cause significant alterations to their important functions in RNA processing. It has been suggested that DM1 may be treatable through endogenous modulation of the expression of MBNL and CELF1 proteins. In this study, we analyzed the recent identification of the involvement of microRNA (miRNA) molecules in DM and focus on the modulation of these miRNAs to therapeutically restore normal MBNL or CELF1 function. We also discuss additional prospective miRNA targets, the use of miRNAs as disease biomarkers, and additional promising miRNA-based and miRNA-targeting drug development strategies. This review provides a unifying overview of the dispersed data on miRNA available in the context of DM.

Keywords: CELF1; MBNL proteins; alternative splicing; antisense oligonucleotides; miRNA-based drug; miRNA-targeting drug; microRNA; myotonic dystrophy.

Figure 1

Micro RNA (miRNA) mechanisms for mRNA translation, regulation, and therapeutic intervention. (A) miRNA normal function. In mammals, the interaction between miRNA (in blue) and mRNA targets trigger different mechanisms for transcript regulation through the RISC complex in order to achieve normal cellular protein levels (pink circles) [36,37]. (B) Causes of miRNA dysregulation. Alterations in miRNA biogenesis, editing, or in its biological stability may cause pathological upregulation or downregulation, which leads to decreased (green arrow) or increased (red arrow) target transcript translation regulation and final protein levels (pink circles), respectively [36]. (C) Illustration of miRNA-based technologies [36,37,59]. There are two main strategies of miRNA intervention depending on what is needed with regard to miRNA level correction. (Upper panel in C) When a miRNA is upregulated, inhibition is conducted by using antimiR products (in red) after miRNA-targeting drug development. Different types of antimiR products exist based on their mechanism of action. AntagomiR synthetic molecules are antisense oligonucleotides (ASOs) perfectly complementary to the specific miRNA target. A second strategy is the use of blockmiRs, which are designed to have a sequence that is complementary to one of the mRNA sequences that serve as a binding site for a microRNA. Upon binding, blockmiRs sterically block the microRNA from binding to the same site, which prevents degradation or transcription inhibition of the target. A third approach for direct miRNA binding and enhanced levels of inhibition involves the use of miRNA sponges. Sponges contain several tandemly arranged miRNA target sequences (same or different ones) usually embedded in the 3’UTR of a reporter gene for assessing the activity [60]. (Lower panel in C) miRNA replacement is conducted to restore its function by introducing a miRNA mimic product (in green) and, thus, following miRNA-based drug development. Micro-RNA mimics are synthetic double-stranded biomolecules that contain one strand with the same sequence and chemistry of the lacking miRNA and a second complementary strand that contains chemical modifications used for the delivery and protection of the mimic.

Figure 2

Therapeutic proof-of-concept approaches for DM based on the modulation of miRNA levels. Currently, three different model systems have been used for evaluation: cells* (human and murine lines), flies, and mice. In vivo miRNA interventions were performed in disease backgrounds to directly assess the therapeutic potential in DM. * Cell approaches used DM and non-disease lines indistinctly for therapeutic evaluation and for conceptual modulation of DM-related targets, respectively. As direct miRNA intervention technologies, antagomiRs, sponges, and miRNA mimic products, able to directly bind to miRNA targets, have been used [49,51,61,62,63,64,65,66]. But miRNA modulation has been achieved by indirect approaches such as the recovering of MEF2C levels in DM1 [48] or removal of the expanded CTG by CRISPR technology [53].