Mechanisms of degeneration in skeletal muscle and spinal cord in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a fatal human genetic disease, caused by mutations in the Survival of Motor Neuron (SMN) gene. It is characterized by progressive symmetrical limb and trunk paralysis and muscle atrophy. We focused on SMA cell death mechanisms at the whole animal level to identify systemic pathology, unlike previous studies that examined motor neurons (MNs) only. We hypothesized that the mechanisms of SMA pathology are not restricted to MNs, but involve primary disease in the musculoskeletal system, which is initiated prenatally by DNA damage accumulation. Using the “Taiwanese” mouse model of SMA, we assessed cell death in the spinal cord and skeletal muscle at multiple embryonic and neonatal time points and in primary cultured myocytes. SMA mouse skeletal muscles exhibited DNA fragmentation and cell death at birth. In the same SMA mice, no DNA fragmentation or cell death was detected in spinal cord ventral horn MNs as late as P5. There was no loss of MNs in the ventral horn of SMA mice at embryonic days 13 and 15.5, or P5. We determined that skeletal muscle cells in SMA mice die by apoptosis, and that satellite cells and differentiated myotubes are affected. Although SMA mice had smaller total skeletal muscle mass than their control littermates, there was no significant reduction in individual myofiber area or in myofiber density. Cultured SMA muscle satellite cells differentiated normally in culture and expressed stem cell and satellite cell markers at levels similar to control cultures. Differentiated SMA myotubes normally clustered acetyl choline receptors in response to agrin stimulation. SMA myotubes responded to and repaired DNA damage similarly to control myotubes. We conclude that in this mouse model, systemic reduction of SMN protein results in skeletal muscle-specific primary pathology. Skeletal muscle cells die by apoptosis before cell death in MNs in the spinal cord. Reduced levels of SMN protein do not interfere with satellite cell survival, differentiation into myotubes, or DNA repair in this mouse model. Given our evidence of primary skeletal muscle disease, we conclude that SMA therapies should be administered systemically and should target multiple tissue types, including skeletal muscle