Expansion of a non-coding CTG tract in the DMPK gene leads to myotonic dystrophy type 1 (DM1), a dominant autosomal condition clinically characterized by myotonia, muscle waste and heart conduction defects among other symptoms. Transcripts containing long CUG stretches are toxic to the cell by their ability to interfere with gene expression regulation at several levels. Chief molecular alterations are sequestration of MBNL1 and upregulation of CUG-BP1, both alternative splicing regulators, leading to the expression of improper mature proteins. Additional molecular alterations have been suggested to contribute to the elaboration of the disease.We propose that changes in the normal levels of specific miRNAs entail a novel mechanism for DM1. This is supported by recent reports that found miR-206 misregulated in DM1 muscle tissue. Our long term objective is to identify additional miRNA expression alterations and to evaluate their potential as therapeutic targets. To this end, we will make use of two animal models, Drosophila and mice, both transcribing long CTG tracts. First, using complementary approaches of next generation sequencing and commercial real time PCR arrays we will obtain the miRNA expression profile in control and toxic RNA conditions. Second, we will characterize the origin of specific miRNA expression alterations, being suggested misregulated biogenesis, stability loss or direct sequestration by CUG RNA, and will validate the clinical relevance of our findings in DM1 muscle samples. Third, miRNAs relevant to DM1 will be functionally analyzed following loss and gain of function approaches in cell culture and in model animals. Finally, we will evaluate the suppression of toxic CUG RNA phenotypes in DM1 model flies in which expression of altered miRNAs has been artificially restored to assess their potential as therapeutic targets. Although focused in the DM1 human disease, our studies may also lead to important findings in the biology of miRNAs.