RNAi Screening of the Kinome Identifies PACT as a Novel Genetic Modifier of Foci Integrity in Myotonic Dystrophy type 1

Myotonic Dystrophy type 1 (DM1), the most common form of adult muscular dystrophy (∼1:8000) currently has no effective treatment. In DM1, expansion of a tri-nucleotide repeat in the 3' UTR of the DMPK gene results in DMPK mRNA hairpin structures, aggregating as insoluble ribonuclear foci. The resulting mis-regulation of important splicing factors, causes the inclusion of fetal exons in dozens of transcripts that contribute to the disease phenotype (ClC-1: myotonia, IR: insulin resistance, and muscle wasting). In order to identify novel gene targets and kinase signalling pathways for potential therapeutics we have performed a high-throughput RNAi screen using an siRNA library targeting 518 protein kinases. RNA foci were visualized by in-situ hybridization of a fluorescently tagged probe to the expanded DMPK mRNA and the surface area and number of foci per nuclei were recorded. From our screen, we have identified a novel gene, PACT, as a modulator of foci integrity and that PACT knockdown can induce MBNL1 protein levels. The identified signalling complex represents a valid target for DM1 therapeutics considering its role in the modulation of the disease phenotype. Our data further emphasizes the utility of RNAi screens in identifying disease-associated genes.