Mechanism of Splicing Regulation of Spinal Muscular Atrophy Genes

Abstract

Spinal muscular atrophy (SMA) is one of the major genetic disorders associated with infant mortality. More than 90% cases of SMA result from deletions or mutations of Survival Motor Neuron 1 (SMN1) gene. SMN2, a nearly identical copy of SMN1, does not compensate for the loss of SMN1 due to predominant skipping of exon 7. However, correction of SMN2 exon 7 splicing has proven to confer therapeutic benefits in SMA patients. The only approved drug for SMA is an antisense oligonucleotide (Spinraza™/Nusinersen), which corrects SMN2 exon 7 splicing by blocking intronic splicing silencer N1 (ISS-N1) located immediately downstream of exon 7. ISS-N1 is a complex regulatory element encompassing overlapping negative motifs and sequestering a cryptic splice site. More than 40 protein factors have been implicated in the regulation of SMN exon 7 splicing. There is evidence to support that multiple exons of SMN are alternatively spliced during oxidative stress, which is associated with a growing number of pathological conditions. Here, we provide the most up to date account of the mechanism of splicing regulation of the SMN genes.

Mapping genetic effects on cellular phenotypes with “cell villages”

“…An important therapeutic approach is to coax SMN2 to splice in an SMN1-like manner, generating a more stable and/or more effective protein. The clinical candidate LMI070 (Branaplam) was identified in a screen for modulators of SMN2 splicing (Cheung et al, 2018, Singh andSingh, 2018). LMI070 appears to interact with the splicing enhancer in exon 7 and to increase levels of SMN protein in cells in a concentration-dependent manner (Figure 6A).…”

RNA-Targeted Therapies and High-Throughput Screening Methods

“…The drug is a 2 -O-(2-methoxyethyl) (MOE) phosphorothioate-modified ASO. The modification endows the ASO drug with an improved in vivo stability [75]. The discovery of Spinraza started with a two-step ASO walk strategy by the Krainer group aiming to find modulators of SMN2 exon 7 alternative splicing [76].…”

Mechanisms of Neuronal Alternative Splicing and Strategies for Therapeutic Interventions

“…Humans have another gene, SMN2, which is an almost identical copy of SMN1. Due to the predominant skipping of exon 7 during splicing, SMN2 produces low levels of SMN (for review, see Khoo and Krainer, 2009;Singh and Singh, 2018). Importantly, promoting inclusion of this exon has been exploited for rescuing SMA phenotypes (Hua et al, 2008(Hua et al, , 2010(Hua et al, , 2011(Hua et al, , 2015Passini et al, 2011;Rigo et al, 2012;Sahashi et al, 2013).…”

“…Nusinersen (Spinraza), the first drug approved by the FDA for treatment of SMA, is an antisense oligonucleotide that promotes SMN2 exon 7 inclusion (Singh et al, 2017a). Nusinersen blocks the Intronic Splicing Silencer N1 (ISS-N1) located downstream of SMN2 exon 7 (Singh et al, 2006;Singh and Singh, 2018). Of note, ISS-N1 has been shown to be a highly inhibitory sequence for SMN2 exon 7 usage (Singh et al, 2006).…”