Ryan Betts

Introduction: Spinal muscular atrophy (SMA) is a heterogenous group of disorders that is characterized by the destruction of lower motor neurons and subsequent muscular atrophy.¹ The most prevalent etiology of this disease is an autosomal recessive mutation in the SMN1 gene.² Decreased copy numbers of the related SMN2 gene constitutes a severe form of SMA, SMA type 1. Individuals who are affected by this disease usually die by the age of 2 without treatment.¹ The SMN2 gene differs from the SMN1 gene by a single nucleotide transition at position 6 of exon 7 in a splicing modulator, which results in the exclusion of exon 7 and production of a truncated, non-functional protein.² Current therapeutic strategies target the splicing of SMN2 or use a vector to deliver the intact SMN1 gene. These have been successful in terms of alleviating the progressive motor weakness, allowing patients to reach motor milestones and reducing mortality. However, a major drawback of this therapy involves the cost and feasibility of extended treatment.² Despite the capabilities of CRISPR/Cas9 gene editing technologies to correct mutations, there are inherent risks of off-target editing, so it is limited in clinical use.² Currently there is a phase 1 clinical trial utilizing CRISPR/Cas9 in the treatment of transthyretin amyloidosis (NCT04601051).³. Results: Remarkably, recent studies in mice have revealed that knockouts of mi-RNA29b⁴, are able to rescue mice models of denervation atrophy, the downstream consequence of the loss of motor neurons that is seen in SMA1. Researchers also were able to increase incorporation of exon 7 by producing knockouts in the splicing regulatory elements ISS-N1 and ISS+100 in stem cells that were transfected with the SMN2 gene.⁵ Additionally, SRSF10 was identified as an additional regulator of the splicing of SMN2.⁶ Finally, a new CRISPR system that targets hnRNA was created, allowing the researchers to induce splicing in RNA transcripts. This CASFx system was shown to induce inclusion of exon 7 in the final RNA transcript in SMA phenotype fibroblasts.⁷ Conclusion: The early successes of the transthyretin amyloidosis clinical trial are the reduction of the pathogenic protein and the ease of infusion This shows that CRISPR/Cas9 is being translated to the clinical setting and that producing knockout mutations is an effective treatment strategy. The targets and technologies discussed represent an exciting avenue of therapy that may produce a more lasting effect than the currently available therapies while also decreasing the burden on patients.

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