The goal of this project is to define the role of Histone Deacetylase 4 (HDAC4) in Duchenne Muscular Dystrophy (DMD) in order to improve pharmacological treatments presently in use for DMD.
Background/Rationale. DMD is a devastating, genetic disorder characterized by progressive muscle weakness and degeneration. To date, no cure is available for this disease. The general HDAC inhibitor (HDACi) givinostat is presently in phase III clinical trial for the treatment of DMD. However, several limitations are associated with the use of HDACi. HDAC4 is a stress responsive member of class II HDACs that regulates many responses in skeletal muscle, including satellite cell biology and muscle regeneration upon injury. HDAC4 expression is upregulated in skeletal muscle of mdx mice, a murine model for studying DMD, suggesting a role in this disease. However, HDAC4 functions in DMD are uncharacterized yet.
To investigate HDAC4 role in DMD with a genetic approach, we generated mdx mice with a deletion of HDAC4 specifically in skeletal muscle (mdx;HDAC4mKO mice). HDAC4 deletion in skeletal muscle reduced muscle regeneration in mdx mice, hampering satellite cell to properly differentiate in vitro, overall leading to functional deficit. Importantly, HDAC4 mainly localized in the cytoplasm of dystrophic muscles and ectopic expression of the cytoplasmic-restricted form of HDAC4 restores mdx;HDAC4mKO satellite cell differentiation. Here plan to ectopically express the cytoplasmic-restricted form of HDAC4 in vivo in mdx;HDAC4mKO mice, and to evaluate any beneficial effects on muscle regeneration, degeneration and function.
Anticipated output. We speculate that ectopic expression of the cytoplasmic-restricted form of HDAC4 will improve mdx;HDAC4mKO muscle homeostasis, shading new light on the unrecognized role of HDAC4 in skeletal muscle cytoplasm and providing the experimental bases for the development of more effective treatments in combination with pan-HDACi for treating DMD.
The potential progress towards an innovative therapeutic approach for DMD is the long-term aim of this proposal. To date there is no effective cure for muscular dystrophies and more research must be done to further understand the molecular mechanisms underlying these pathologies. The pan-HDAC inhibitor givinostat has been shown to be partially beneficial for dystrophic mice and patients. However, givinostat was not able to improve adult dystrophic mouse phenotype or DMD patient muscle performance. Moreover, as a pan- HDACi, givinostat nonspecifically blocks all HDAC members and long-term treatment with pan-HDACi has been associated with numerous side effects.
Our results indicate that inhibition of the member of class II HDAC, HDAC4, in skeletal muscle is deleterious for skeletal muscle architecture in muscular dystrophy. Limiting the therapeutic approach to the use of class I HDAC inhibitors may still in part influences the acetylation status of HDAC4 targets, since HDAC4 may deacetylate via class I HDACs. The cytosolic localization in mdx skeletal muscles and the finding that the ectopic expression of the cytoplasmic-restricted form of HDAC4 restores KO satellite cell differentiation and survival let us speculate on the importance of preserving HDAC4 cytoplasmic functions in DMD. Further studies on HDAC4 role in the cytoplasm in vivo are necessary to better delineate the molecular functions of HDAC4 in DMD, and specifically in muscle necrosis and regeneration, in order to propose combined pharmacological approaches with HDAC inhibitors, to ameliorate the treatments presently in use.