Duchenne Muscular Dystrophy (DMD) is a dystrophinopathy characterized by progressive muscle degeneration and weakness that affects approximately 1 in 3500 live male births. Despite development of promising therapeutics, there is currently no cure. Transforming Growth Factor β (TGFβ) is a multifunctional cytokine that is aberrantly activated in muscular dystrophies, including DMD. TGFβ recruits macrophages, neutrophils, and fibroblasts to sites of muscle injury and exacerbates disease progression by promoting inflammation and fibrosis in muscle with chronic injury. Genome-wide association studies identified both risk and protective variant alleles encoding TGFβ regulators, Latent TGFβ Binding Protein 4 (LTBP4) and Thrombospondin 1 (THBS1), that correlated with age to loss of ambulation. LTBP4 sequesters TGFβ in the latency associated peptide complex (LAP) and THBS1 is a multifunctional protein that activates TGFβ by releasing it from the LAP. Protective LTBP4 and THBS1 alleles result in reduced TGFβ signaling while risk alleles cause the opposite. We are utilizing chemical and genetic approaches in the zebrafish DMD (dmd) disease model to investigate how Tgfβ signaling influences the dmd phenotype. Transient inhibition of Tgfβ receptors before significant muscle degeneration begins results in significant and sustained rescue of dmd mutant muscle integrity and ultrastructure at later stages when untreated controls show severe damage. We have generated and are characterizing loss-of-function alleles of ltbp4, thbs1a, and thbs1b (zebrafish have two copies of THBS1) to mimic protective decreased expression alleles described in humans. Double ltbp4;dmd mutants show partial rescue of dmd mutant muscle integrity and increased survivorship compared to dmd single mutant siblings. A long-term goal of our work is to further define the cell-specific regulatory mechanisms that control TGFβ signaling through LTBP4 and THBS1 to potentially identify novel therapeutic targets.