Guidance on Gene Replacement Therapy in Spinal Muscular Atrophy: A Canadian Perspective

Spinal muscular atrophy (SMA) is the second most common autosomal recessive disorder in childhood. It is caused by deletions or mutations in the survival motor neuron 1 (SMN1) gene and characterized by degeneration of motor neurons leading to progressive muscle weakness and atrophy. A similar backup gene called survival motor neuron 2 gene (SMN2) acts as a disease modifier, with increasing number of SMN2 copies associated with milder phenotype.

Considerable advances in the last 10 years have led to the development of disease-modifying therapies in SMA which have transformed outcomes. Therapeutic approaches use different mechanisms of action, routes of administration, and dose scheduling. The core treatments are SMN-dependent and include SMN1 gene replacement or SMN2 pre-mRNA splicing modulation.

With all therapies, the highest efficacy is seen when administered pre-symptomatically, prompting the need for SMA newborn screening programs. Nusinersen was the first drug approved for SMA treatment in Canada in June 2017. It is administered intrathecally at 4 monthly intervals after 4 initial loading doses. Another SMN2 splicing modulator is risdiplam which is administered orally on a daily basis.