miRNAs in cardiac disease: sitting duck or moving target?

Emerging findings indicate that cells can produce both micro (mi)RNAs and their messenger (m)RNA targets in multiple processing variants in a tissue- and developmental stage-selective manner. Specifically, we find that cells accumulate a greater range of functional miRNAs than hitherto expected, whereas mRNAs with alternative 3' untranslated regions that include varying numbers of miRNA target sites are also seen to be common. This has important implications for both our understanding of miRNA function in a given biological context and the design of successful strategies for experimental or therapeutic intervention. In this review, we relate these new phenomena to miRNAs in the heart, where they are known to play critical roles during normal function as well as in cardiac disease.