Molecular genetics of spinal muscular atrophy : insights into various routes of therapeutic intervention

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Spinal Muscular Atrophy is an autosomal recessive neuromuscular disorder that is the leading genetic cause of infant mortality. In humans, a nearly identical copy gene is present called SMN2 that is retained in all SMA patients and encodes an identical protein compared to SMN1. However, SMN1 and SMN2 differ by a silent C to T transition at the 5' end of exon 7. This single nucleotide difference results in the production of an alternatively spliced isoform, called SMN[delta]7, which encodes an unstable and non-functional protein. Therefore, the absence of the short peptide encoded by SMN exon 7 is critical in the disease development process. A cytoplasmic localization signal has been previously identified within SMN exon 7 and can readily transport SMN and heterologous proteins to the cytoplasm. While this peptide is important for SMN protein function, here it is shown heterologous sequences can seemingly compensate for the SMN exon 7 peptide, regarding: SMN protein localization, protein stability, supporting neurite outgrowth, and snRNP assembly. Consistent with this, aminoglycosides that suppress efficient recognition of stop codons, known as "read-through" (Rt), resulted in significantly increased levels of SMN protein in SMA patient fibroblasts. It is also shown in this work these aminoglycosides increase lifespan and gross-motor function in SMA model mice. Collectively, these experiments help to provide insight into therapeutics designed to induce readthrough of the SMN[delta]7 stop codon, as well as other routes of therapeutic intervention to be used in combination with a read-through event.