Combining SMN2 splicing modifiers with HDAC6 inhibition greatly improves muscle function and survival in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a rare, progressive and severe neuromuscular disease. It is mostly caused by mutations in the SMN gene, which lead to the death of spinal cord motor neurons. In the absence of treatment, more than half of affected children die before the age of two. Recently, groundbreaking gene therapies were developed, allowing children to survive. However, a new clinical presentation of the disease has emerged in treated patients, characterized by ongoing functional deficits and a disability mainly due to persistent muscle atrophy. Over the last years, treatments of various animal models of neuromuscular disorders have shown the ability of inhibitors of the non-conventional histone deacetylase 6 (HDAC6) to reduce inflammation, fibrosis and muscle atrophy, and to ameliorate acetylcholine receptor distribution at the neuromuscular junction, microtubule network and mitochondrial transport in axons, indicating potential interest for the treatment of neuromuscular disorders. The present study was designed to properly characterize the effect of HDAC6 inhibition on muscle cells proliferation and differentiation and to evaluate in vivo if HDAC6 inhibition combined with the new standard of care treatments of SMA could ameliorate skeletal muscle and general status of a SMA mouse model. Here, we report that HDAC6 and tubulin acetylation controls myotube formation and maturation in vitro. In particular, HDAC6 inhibition increases the size of SMA patients-derived muscle primary myotubes. In vivo, when combined with ASOs inducing exon 7 inclusion in SMN2 RNA in motoneurons, HDAC6 systemic inhibition strongly improved muscle strength, mass, function, and longevity of the Smn[Delta]7/[Delta]7; hSMN2+/- mouse model of SMA. These findings provide evidence that in muscle cells, HDAC6 is the only tubulin deacetylase and that selective inhibition of HDAC6 improves myogenic progression and that HDAC6 inhibitors are good candidates to ameliorate persisting symptoms of SMA patients treated with the new standard of care.