Project Summary/Abstract Duchenne muscular dystrophy (DMD), an X-linked inherited neuromuscular disorder, has a worldwide incidence of one in ~3,500-5,000 live male births, making it the most common muscular dystrophy. DMD is caused by mutations in the dystrophin gene, resulting in a progressive muscle-wasting disorder due to loss of skeletal and cardiac muscle. It is an early lethal disease, and most afflicted males die in their 20’s or 30’s of cardiac or respiratory complications. There is thus an urgent need for novel therapeutic avenues for the treatment of DMD as the current treatments have only limited efficacy. The molecular mechanisms mediating deleterious effects downstream of dystrophin loss remain unclear. We note that the expression of the miR-128-1 microRNA is elevated in the muscle and circulation of human DMD patients, and in muscle of mouse and zebrafish DMD models. Moreover, the miR-128-1 genomic locus is markedly linked to weak grip strength and poor lung muscle function in the UK Biobank (>300,000 individuals). Our preliminary studies from the zebrafish and mouse models of DMD have found that inhibition of miR-128-1 using locked nucleic acid (LNA) antisense oligos (ASO) dramatically mitigates the DMD phenotypes, including muscle atrophy and exercise intolerance. Furthermore, our preliminary studies have revealed that miR-128-1 inhibition largely rescues the expression of a suite of key genes involved in skeletal mitochondrial health and energy homeostasis, accompanied by improved mitochondrial biogenesis and function in vitro and in vivo. In this application, we propose studies to test the hypothesis that miR-128-1 represents a crucial disease modifier that orchestrates the deleterious effects of dystrophin loss by regulating a set of key target genes that are important for mitochondrial health and putative target genes that are critical for muscle metabolism. In the first Aim, we will investigate what roles miR-128-1 play in mediating mitochondrial abnormalities in DMD and assess whether improving mitochondrial function by combining miR-128-1 inhibition and pharmacological activators of mitochondrial function can synergistically ameliorate muscle dysfunction in the mdx5cv mouse DMD model and in human DMD patient-derived muscle cells. In addition, we will comprehensively identify miR-128-1 target genes in mdx5cv mice by performing transcriptomic analysis and assess human conservation. In the second Aim, we will evaluate the therapeutic efficacy of miR-128-1 inhibition in the mouse mdx5cv DMD model using LNA ASOs, as well as conditional mouse KO of miR-128-1 and muscle-targeted adeno-associated virus approaches. Successful completion of the proposed studies will determine whether miR-128-1 may indeed represent a powerful therapeutic target in DMD, and reveal the downstream mechanism whereby miR-128-1 mediates the DMD pathologies.