Antisense Oligonucleotides for the Treatment of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disease affecting similar to 1 in 10,000 live births. The most striking component is the loss of alpha-motor neurons in the ventral horn of the spinal cord, resulting in progressive paralysis and eventually premature death. There is no current treatment paradigm other than supportive care, though the past 15 years has seen a striking advancement in understanding of both SMA genetics and molecular mechanisms. A variety of disease-modifying interventions are rapidly bridging the translational gap from the laboratory to clinical trials, including the application of antisense oligonucleotide (ASO) therapy for the correction of aberrant RNA splicing characteristic of SMA. Survival motor neuron (SMN) is a ubiquitously expressed 38-kD protein. Humans have two genes that produce SMN, SMN1 and SMN2, the former of which is deleted or nonfunctional in the majority of patients with SMA. These two genes are nearly identical with one exception, a C to T transition (C6T) within exon 7 of SMN2. C6T disrupts a modulator of splicing, leading to the exclusion of exon 7 from similar to 90% of the mRNA transcript. The resultant truncated Delta 7SMN protein does not oligomerize efficiently and is rapidly degraded. SMA can therefore be considered a disease of too little SMN protein. A number of cis-acting splice modifiers have been identified in the region of exon 7, the steric block of which enhances the retention of the exon and a resultant full-length mRNA sequence. ASOs targeted to these splice motifs have shown impressive phenotype rescue in multiple SMA mouse models.