Overview of Oligonucleotide-Mediated Approaches for SMA

What does this therapeutic approach for SMA involve?

This therapeutic approach for SMA involves the use of small pieces of genetic material, called oligonucleotides, to improve the functioning of the back-up gene for SMA called SMN2.   The methodology is often called anti-sense technology.  It uses an antisense oligonucleotide that binds to the SMN2 pre-RNA and corrects its splicing.  Now the SMN2 gene can produce full length SMN protein, rather than truncated, low-functioning protein.    This leads to enhanced production of SMN protein, which is associated with normal motor function.  See below for a detailed explanation of how the process works.

What progress has been made to date?

Researchers have been working on this approach for SMA for the last 5 years with numerous published papers on the topic. 
Click here to see publications showing proof-of-concept data for the approach in SMA disease models.

In fact, Families of SMA funded very early research demonstrating this approach was a feasible therapeutic strategy for SMA with multiple grant awards to Dr. Ravi Singh from 2003 to 2006. 
Please click here to read more about his research. 

Also, please click here to view a list of FSMA funded grants since 2004, including those involving olignucleotide related projects. 

Isis Pharmaceuticals recently announced that they have added a SMA drug, based on anti-sense technology, to their development pipeline. 
Please click here to read their press release.

The scientists involved in the Isis project have published two papers.
Please click here to read these abstracts.

How does this approach work at the molecular level?

Normally SMN2 fails to compensate for the SMN1 mutation, and thus to protect from development of SMA, because its mRNA undergoes alternative splicing to encode for an unstable SMN protein, known as Δ7SMN.  After transcription of a gene into its corresponding pre-messenger RNA (pre-mRNA), the nuclear splicing machinery is responsible for processing that pre-mRNA into its corresponding messenger RNA (mRNA) by eliminating introns (bits of pre-mRNA unnecessary for protein synthesis) and joining together exons (bits of pre-mRNA coding for protein synthesis). The aberrant production of the truncated Δ7SMN is due to exon 7 being skipped during splicing of the SMN2 pre-mRNA, resulting in insufficient amounts of full length SMN protein and consequently in decreased motor functions in SMA patients. The use of anti-sense oligonucleotides, targeting sequences in the SMN2 pre-mRNA, can correct the spicing defect and promote exon 7 inclusion into the SMN2 mRNA transcript.  This results in increased SMN protein levels.  See the figure below for an illustration of this process.

Figure 1.  Splicing of the SMN Genes.


The major difference between the two SMN gene copies is the C (SMN1) or T (SMN2) nucleotide change in exon 7 of the DNA comprising the two genes. Because of this difference, SMN2 mostly makes mRNA that excludes exon 7 and produces a smaller, unstable SMN protein, while SMN1 makes mRNA that includes exon 7 and makes stable full-length SMN protein. This is due to a defect in mRNA splicing caused by the T nucleotide change in the SMN2 gene.  This process is explained below.


(a) The SMN1 and SMN2 gene organization on chromosome 5.


(b) The SMN genes are turned on by their respective promoters (non-blueprint regions of genes that work to turn genes on and off) in a process call transcription.  Turning a gene on (transcription) results in a preliminary RNA message which contains an intermediate blueprint from which specific proteins can eventually be produced.


(c)  The preliminary RNA message must then be processed in an event called RNA  splicing in order to become a useful blue print for protein production. The process of RNA splicing removes chunks of RNA from the preliminary message, which are not part of the protein blueprint. The non-blueprint regions that must be removed are called introns.  The blueprint regions are called exons.  The splicing process results in the final mRNA message which is the contiguous protein blueprint.  The final mRNA message that results is used as the template for protein production in a process called translation.