Nusinersen: The First Option Beyond Supportive Care for Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by degeneration of spinal motor neurons and poses significant adverse outcome in affected population. Survival motor neuron 1 (SMN1) protein encoded by SMN1 gene located on 5q¹³ is critical for survival and functioning of motor neurons. Almost identical gene SMN2, present on the same chromosome, produces a small truncated protein (SMN2) because of skipping of exon 7 from translation due to translation silent C6U substitution in exon 7 of SMN2 pre-mRNA transcript. Only 10% of the SMN2 mRNAs produce full length SMN2 protein by including exon 7 in healthy individuals. A large deletion or sometimes a point mutation in SMN1 gene is responsible for SMA. In this case the number of copies of SMN2 genes in an individual determines the severity of disease (the more the number of copies the less severe the disease). Nusinersen (ISIS 396443) binds to intron splicing silencer-N1 (ISS-N1; a site present ten nucleotides down to the junction of exon 7 and intron 7), modulating the splicing of SMN2 pre-mRNA transcript to increase the inclusion of exon 7, thereby increasing the production of full length SMN2 protein. Major evidence of its efficacy came from a sham controlled phase 3 clinical study ENDEAR. The study was stopped early based on significantly favorable results in interim analysis and all the patients were transitioned to receive nusinersen in an ongoing open-label, phase 3 study, SHINE, which will evaluate the long-term efficacy, safety and tolerability of the drug. Nusinersen is globally the first drug approved (by the US FDA) for treatment of SMA in children and adults.