The 3 prime paradigm of the miR-200 family and other microRNAs

The number of predicted human microRNAs in Sanger miRBase currently stands at over 1,000, with each of these in turn predicted to target numerous mRNAs. However, those micro-RNAs for which mRNA targets have been evaluated, verified and reported in the literature are still in the minority and the bulk of microRNA/mRNA interactions are yet to be confirmed. Confirmation of microRNA interaction with predicted mRNA targets represents a considerable undertaking, made more complex by potential synergistic effects of multiple microRNAs and the three possible outcomes (translational repression, degradation or a mixture of both). In addition, contrasting results obtained when either stably expressing or transiently transfecting members of the miR- 200 family illustrate limitations in the verification methods currently in use. In this article we suggest that instead of allowing computational predictions to drive investigation, it would be desirable, when possible, to systematically evaluate microRNA targets using inducible, stable, ectopic expression. The advantage of stable lines ectopically expressing microRNA(s) is that they allow an analysis of changes to both the proteome and the transcriptome. This would allow verification of targets, improve the design of prediction algorithms and greatly increase our understanding of the outcome of microRNA/mRNA interaction.