Myotonic dystrophy type 1 (DM1) is a relatively common form of muscular dystrophy. The genetic basis is an expansion of CTG repeats in the 3' untranslated region of DMPK, a gene encoding a protein kinase expressed in skeletal, cardiac, and smooth muscle, and in neurons. This unusual mutation gives rise to RNA dominance, in which expression of RNA containing an expanded CUG (CUG^'') repeat leads cell dysfunction. The mutant RNA accumulates in nuclear foci and initiates a complex cascade of downstream events, such as, defects in the regulation of RNA splicing. An attractive therapeutic approach, therefore, is to attack the problem at its root cause, by accelerating the clearance of the toxic RNA. To this end, we are proposing to develop RNase H-active antisense oligonucleotides (ASOs) targeting the mutant human DMPK {mut-hDMPK) mRNA. Previous studies have indicated that biodistribution and activity of ASOs in skeletal and candiac muscle is low. In contrast, we have found that systemically-delivered ASOs are highly active in muscle of wild-type mice, when targeted against a transcript that is retained in the nucleus - presumably because this is the compartment in which RNase H is active. We therefore postulated that CUG-expanded transcripts may also show efficient knockdown in muscle, because they also are retained in the nucleus. Consistent with this idea, subcutaneous administration of ASO for four weeks caused highly effective knockdown of CUG(R)'' transcripts in muscle, reversal of RNA splicing derangements, and rescue of myotonia in transgenic mice. We seek now to develop optimally-effective, systemically-active ASOs that target mut- hDMPK transcripts for cleavage in skeletal and cardiac muscle. Development plans call for identification of ASOs that are highly active in cells, selection of ASOs that show optimal mut-hDMPK knockdown in transgenic mice, elimination of ASOs that show unacceptable toxicity in rodents and monkeys, and then IND- enabling toxicology/pharmacology studies using GMP-manufactured drug. To accomplish these goals we have formed an academic-commercial collaboration that includes all of the scientific, clinical, regulatory, and manufacturing expertise that is needed to bring a drug treatment for DM1 to the clinic.