MicroRNAs are tiny RNA molecules (about 22 nucleotides long) that regulate gene expression after transcription. They can bind to mRNA molecules and either block them from being translated into protein or trigger their destruction. Think of them as the cell's "volume control" for individual genes.
MUSC's RNA therapeutic approach: The MUSC team chose micro-RNA therapeutics because they can be designed to specifically target and modulate the expression of genes downstream of MEF2C — essentially trying to "turn up the volume" on the genes that MEF2C should be activating. This is more targeted than delivering a whole new gene (like gene therapy) but doesn't fix the underlying genetic problem.