SMA Type II Skeletal Muscle Treated with Nusinersen shows SMN Restoration but Mitochondrial Deficiency.

Spinal muscular atrophy (SMA) is a rare autosomal recessive developmental disorder caused by the genetic loss or mutation of the gene SMN1 (Survival of Spinal Motor Neuron 1). SMA is classically characterized by neuromuscular symptoms, including muscular atrophy, weakness of the proximal muscles, especially those of the lower extremities, and hypotonia. Although originally thought of as a purely motor neuron disease, current research has shown that most, if not all, tissues are affected, including the muscle. Until recently, muscle problems in SMA were predominantly considered a consequence of denervation due to the motor neuron death. However, recent work using muscle specific mouse models of SMN loss, as well as skeletal stem cell specific models have shown that there are tissue specific problems in muscle due to SMN deficiency. Several years ago, SMA treatment underwent a radical transformation, with the approval of three different SMN-dependent disease modifying therapies. This includes two SMN2 splicing therapies, Risdiplam and Nusinersen, which can be administered by Type II patients that have symptom onset later in age. One main challenge for Type II SMA patients treated with Risdiplam and Nusinersen is ongoing muscle fatigue, limited mobility, and other skeletal problems, including hip dysplasia and scoliosis. To date, few molecular studies have been conducted on SMA patient derived tissues after treatment, limiting our understanding how different organ systems react to the therapies, and what additional combination therapies may be beneficial. With this goal in mind, we collected paravertebral muscle from the surgical discard in a cohort of 8 SMA Type II patients undergoing spinal surgery for scoliosis, as well as 7 non SMA controls with scoliosis and used RNA sequencing to characterize their molecular profiles. We observed that despite a restoration of the SMN mRNA and protein levels in these patients, at levels at or above the controls, a subset of patients continued to have alterations in mitochondrial metabolism and other markers of cellular stress.