Arko Ghosh

 Introduction: Spinal muscular atrophy (SMA) is a genetic neurodegenerative disease that is characterized by lower motor neuron loss and muscle denervation, which can result in quadriplegia, respiratory failure and ultimately death^1-2. SMA has an incidence of approximately 1 in 6,000 to 1 in 10,000, and as such, is one of the most common genetic causes of infant death¹. SMA is characterized by the genetic defect of Survival of Motor Neuron 1 (SMN1) protein, which causes an insufficient amount of full-length SMN protein to participate in snRNP formation, mRNA splicing and anterograde neuronal transport². Despite SMA’s high infantile mortality rate, the exact molecular pathogenesis remains unknown^1-2. As such, the search for effective treatment remains a major focus of research. The use of antisense oligonucleotide (ASO) therapy has shown promise in the ability to increase the amount of functional SMN protein available. These results could suggest a potential therapy for SMA³.  Methods: SMN knockout mouse models were used to simulate the SMA pathology. ASOs were transfected into fibroblasts and RNA sequence and proteins were analyzed using RT-PCR and western blotting techniques⁴. Additionally, immunofluorescence imaging techniques were utilized to examine for SMN protein deposition. Significance of changes in SMN expression were compared to control and determined using T-test analysis⁵.  Results: The use of an ASO specific to an intron splicing sequence allowed for near complete exon retention and intron splicing of the SMN gene⁴. In addition to targeting intron splicing sequences, bifunctional ASOs additionally recruit splicing regulatory proteins to retain appropriate splicing activity of the SMN gene. Data confirmed increased amount of functional SMN protein in murine brain and spinal cord tissue, as well as show increased lifespan and weight in SMA mice⁵.  Conclusions: The exact pathological mechanism of SMA remains unclear; however, the use of antisense oligonucleotide has shown therapeutic promise via two main mechanisms. ASOs have shown an ability to block silencer motifs and demonstrate bifunctional RNA activity to improve intron splicing activity and recruitment of splicing proteins. Via these mechanisms, ASOs have shown to increase SMN protein level and ultimately increase survival in a murine model. These data on the efficacy of ASO in treatment of SMA demonstrates its great potential in a clinical setting.

1. Ahmad S, et Molecular Mechanisms of Neurodegeneration in Spinal Muscular Atrophy. Journal of Experimental Neuroscience 2016:10: 39-49.
2. D’Amico A, et Spinal Muscular Atrophy. Orphanet Journal of Rare Diseases 2011, 6:71.
3. Evers M et Antisense oligonucleotides in therapy for neurodegenerative disorders. Advanced Drug Delivery Reviews. 2015; 87: 90-103
4. Mitrpant C et Improved Antisense Oligonucleotide Design to Suppress Aberrant SMN2 Gene Transcript Processing: Towards a Treatment for Spinal Muscular Atrophy. PLoS ONE. 2013; 8(4): e62114.
5. Osman E et Bifunctional RNAs Targeting the Intronic Splicing Silencer N1 Increase SMN Levels and Reduce Disease Severity in an Animal Model of Spinal Muscular Atrophy. Molecular Therapy. 2012; 20(1): 119-126.