Determinants that regulate splicing of SMN

Lorson, Christian

Abstract

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that is the leading genetic cause of infantile death. SMA is the most common inherited motor neuron disease and occurs in approximately 1:6,000 live births. The gene responsible for SMA is called survival motor neuron-1 (SMN1). Interestingly, a human-specific copy gene is present on the same region of chromosome 5q called SMN2. SMN2 is nearly identical to SMN1, however, mutations in SMN2 have no clinical consequence if SMN1 is retained. The reason why SMN2 cannot prevent disease development in the absence of SMN1 is that the majority of SMN2-derived transcripts are alternatively spliced, resulting in a truncate and unstable protein. The alternatively spliced form lacks exon 7, which is normally the final coding exon, and is referred to as SMN exon 7. Importantly, SMN2 does produce low levels of fully functional SMN protein, and therefore, SMA is caused by severely reduced levels of SMN, not the complete absence. SMN2 has been viewed as an essential target for therapeutic development since it is retained in all SMA patients and it has the capacity to encode a fully functional SMN protein. Therefore, strategies that modulate SMN2 exon 7 inclusion and promote full- length expression have been at the forefront of SMA translational research. A variety of modified oligonucleotides, anti-sense sequences, and trans-splicing RNAs have been developed that efficiently increase exon 7 inclusion in SMA patient fibroblasts and more recently in SMA animal models. Understanding the molecular determinants that regulate SMN exon 7 RNA splicing has been fundamentally important from a biological perspective and has provided outstanding targets for therapeutic development. A complex and dynamic collection of cis- and trans-acting factors have been identified that detail the regulatory mechanisms that govern SMN exon 7 inclusion/exclusion. Based upon these molecular targets, SMA is well positioned to benefit from strategies that re-direct SMN2 exon skipping. In this proposal, we leverage our recent findings that identified RNAs that modulate SMN2 expression and move them forward and examine the activity of these RNAs in animal models of SMA.

Public Health Relevance

Spinal Muscular Atrophy is a leading genetic cause of infantile death and is the second most common autosomal recessive disease. With a carrier frequency of ~1:35 and an incidence of ~1/6000 live births, SMA is a relatively common orphan disease. In all SMA patients, a partially functioning back-up gene exists called SMN2. We are proposing to analyze several novel RNAs that upregulate SMN expression from SMN2 as a means to prevent disease development.