Abstract : Myotonic Dystrophy type I (DM1) is caused by an abnormal expansion of CTG triplets in the 3' UTR of the DMPK gene, leading to the aggregation of the mutant transcript in nuclear RNA foci. Based on structural studies on short CUG repeats, it has been proposed that expanded CUG repeats fold into an imperfect hairpin structure that interferes with the activities of RNA binding proteins and alters their normal cellular function. The muscleblind-like 1 protein (MBNL1) was identified by its ability to bind to CUG repeats. It has been shown that the expanded mutant transcript promotes the sequestration of the MBNL1 splicing factor in nuclear RNA foci. CUGBP1 is another splicing factor that is involved in DM1. Instead of being sequestered by the repeats, the steady-state level of CUGBP1 is increased in DM1 tissues, leading to a gain of activity of the protein. The sequestration of MBNL1 and the up-regulation of CUGBP1 in DM1 results in the misregulation of alternative splicing of a subset of muscle and brain-specific transcripts, leading to the re-expression of fetal isoforms in adult tissues. However, several recent studies suggest that factors or signaling pathways other than MBNL1 and CUGBP1 could be involved in DM1 pathogenesis.The aim of this work was to isolate new factors that bind to CUG repeats. Using an affinity chromatography strategy with an RNA containing 95 pure CUG repeats, we identified the RNA helicase p68 (DDX5). p68 is a prototype of DEAD-box RNA helicase proteins. This family is characterized by a conserved core, consisting of nine conserved motifs including the DEAD signature, which gives rise to the name to these proteins. p68 is involved in many aspects of RNA metabolism including transcription, RNA processing, RNA export, translation and mRNA degradation. We showed that p68 colocalized with RNA foci in cells expressing the 3'UTR of the DMPK gene containing expanded CTG repeats. We found that p68 increased MBNL1 binding onto pathological repeats and the stem-loop structure regulatory element within the cardiac Troponin T (TNNT2) pre-mRNA, splicing of which is misregulated in DM1. Mutations in the helicase core of p68 prevented both the stimulatory effect of the protein on MBNL1 binding and the colocalization of p68 with CUG repeats, suggesting that remodeling of RNA secondary structure through a ATP-dependant manner by p68 facilitates MBNL1 binding. We also found that the competence of p68 for regulating TNNT2 exon 5 inclusion depended on the integrity of MBNL1 binding sites.We propose that p68 acts as a modifier of MBNL1 activity on splicing targets and pathogenic RNA.