New finding clarifies the cause of SMA

Howard Hughes Medical Institute researchers have made a surprising discovery about the molecular basis underlying spinal muscular atrophy. The findings suggest that there may be a new way to promote survival of neurons.
The disease is nearly always linked to the absence or disruption of a gene known as SMN1. A nearby gene, SMN2, is virtually identical to SMN1, and in principle could produce enough SMN protein to keep neurons healthy but yet somehow fails to do so.
In the March 2010 issue of the journal Genes & Development, investigators Gideon Dreyfuss and Sungchan Cho report on their work solving this mystery. Dreyfuss and Cho found that most of the SMN protein produced from SMN2 is flagged for rapid degradation by a cellular waste-disposal system. Thus, the protein is cleared before it accumulates sufficiently to sustain the health of motor neurons. Blocking this degradation signal could therefore, in theory, be a way to treat SMA, Dreyfuss says.

The SMN2 gene differs from SMN1 by a single letter of DNA, or nucleotide. This slight change appears to confound the cellular systems that turn gene sequences into RNA-based templates for the production of proteins. As a result, about 80 percent of the SMN protein copies produced from SMN2 are missing a segment encoded by part of the gene known as exon 7. This shorter version of SMN protein, termed SMNΔ, is almost undetectable in the cells of patients with SMA. But the shorter protein does seem to work: SMA patients who have more copies of the SMN2 gene have milder signs of disease, and animal studies also suggest that increased levels of SMNΔ can be beneficial. This suggests to scientists that if the level of SMNΔ could be boosted therapeutically, patients might benefit.
To find out why SMNΔ levels are so low in patients’ cells, Cho and Dreyfuss created cells that produce SMNΔ that is linked to luciferase, a light-producing enzyme found in the tails of fireflies. After shutting down new protein synthesis, they could observe how quickly the existing SMNΔ was cleared from cells by observing the rate at which luciferase’s glow faded. Using this system, Cho and Dreyfuss confirmed that SMNΔ disappeared much faster than normal-length SMN.
The researchers then tinkered with SMNΔ, deleting certain segments at the malformed end to see whether this could restore the protein’s stability. They found that the improper junction between the segments that normally flank exon 7 had, by chance, created a signal that caused the entire protein to be degraded quickly. Removing this signal from SMNΔ restored the protein to near-normal stability.
How does this signal cause the mutant protein to disappear so quickly? Cho and Dreyfuss found strong evidence implicating the proteasome system, a set of protein-crunching machines that roam through cells, destroying malformed or otherwise unwanted proteins. By suppressing proteasome activity, they restored SMNΔ to normal stability, whereas suppressing other protein-clearance mechanisms had no effect.
“So ultimately SMNΔ meets a proteasome and that is where it gets degraded,” said Dreyfuss. Proteasomes normally degrade only proteins that have been marked for destruction with special tagging molecules. These tags, in turn, bind to target proteins only after detecting certain molecular signatures they interpret as degradation signals, or degrons. “A degron is like a flag that says ‘take me out,’” said Dreyfuss.
In this case, the experiments showed that the mutated end of SMNΔ happens to form a degron. “Thus we found the cause of the extreme instability of SMNΔ,” Dreyfuss said. “That’s important, because it wasn’t clear why this protein simply vanishes.” Further molecular tinkering by Cho and Dreyfuss revealed that, surprisingly, changing a single amino acid removed the degradation signal, allowing SMNΔ levels to rise. This more stable version of SMNΔ was enough to keep alive cultured cells, which quickly die without SMN.
“We don’t know yet whether the same approach will work in mammalian organisms,” said Dreyfuss. “But there are other reasons to believe that SMNΔ contributes a similar function to that of normal-length SMN.” Dreyfuss and his colleagues want to find out more about the cellular mechanisms that trigger SMNΔ destruction. But they are already thinking about the development of therapies based on their findings. The fact that a single amino-acid substitution stabilizes SMNΔ is encouraging, Dreyfuss says, suggesting that a drug compound might produce a similar effect. Dreyfuss’s lab has been working on very broad drug screening efforts, including a large-scale collaboration with the pharmaceutical company Merck. “With these results we can develop more targeted screens for compounds that stabilize SMNΔ,” Dreyfuss said, “and we are pursuing this vigorously.”

(source: HHMI website)