Spinal Muscular Atrophy, Approved Therapies and Future Prospects

Abstract

Spinal muscular atrophy (SMA) is a complex congenital myoneural disorder, noted for gradual muscle atrophy leading to increasing muscular weakness and significant disability. SMA manifests broad array of clinical severity and its clinical traits can be graded into four primary phenotypes -SMA type I, II, III and IV mainly based on the age of onset of symptoms and highest motor milestones attained. Over the past few years, numerous feasible interventions found on the commonly accepted precept of augmenting the translation of the survival motor neuron (SMN) protein have been tried. Gene therapies focusing on SMN include splicing modulation of SMN2 gene (nusinersen) and replacing SMN1 gene (onasemnogene abeparvovec), whereas therapies focusing SMN-independent factors involve treatments improving muscular function, treatment of spinal deformities and neuroprotection. A noted milestone in history of SMA was the approval of Nusinersen in 2016. It was followed by approval in 2019 and in 2020 to onasemnogene abeparvovec and risdiplam respectively. These three approved therapies offered hope to the patients and their families globally. But some of the weaknesses are also associated with these therapies. Deficiency of phase 4 clinical trial data, high price tag, nonavailability of guidelines for the selection of therapies for specific phenotypes and lack of data form clinical trial targeted to compare safety and efficacy of currently approved therapies are some of the issues that licensed therapies for SMA are suffering. In the face such challenges, initiation of intervention at an early age along with the utilization of adjunct therapies seems to be a sound approach to treat SMA patients. The review discusses the classification, clinical description, genetics, diagnostics and treatment of SMA along with SMN independent therapeutic targets that can utilised for the development of new therapies.