Nusinersen: the antisense oligonucleotide at the forefront of spinal muscular atrophy treatment

Spinal muscular atrophy (SMA) is the first human disease to be treated with an antisense oligonucleotide (ASO) that restores the full coding sequence of a mRNA through splicing modulation. The therapeutic ASO nusinersen (marketed as SpinrazaTM) targets intronic splicing silencer N1 (ISS-N1) located downstream of the predominantly skipped exon 7 of Survival Motor Neuron 2 (SMN2) gene. The full-length transcript of SMN2 codes for SMN, an essential housekeeping protein with a prominent role in RNA metabolism. The success of nusinersen could be attributed at least in part to the accessibility of ISS-N1 that was found to have a strong inhibitory effect on splicing of SMN2 exon 7. Nusinersen has saved thousands of lives affected by SMA. However, limitations of an ASO-based therapy continue to emerge. Here we describe lessons learned from ASO-mediated splicing corrections in general and nusinersen in particular. Specific focus of this review is to discuss how information gleaned from the off-target effects of nusinersen could be utilized to develop next generation of ASO-based therapies with improved efficacies.