A new way of restoring potentially adequate levels of survival motor neuron (SMN) — the protein missing in spinal muscular atrophy (SMA) — uses small molecules that target the structure of an intermediate player in this protein’s production.
The study on this approach, “Targeting RNA structure in SMN2 reverses spinal muscular atrophy molecular phenotypes,” was published in the journal Nature Communications.
SMA is caused by mutations in the SMN1 gene, which leads to a reduction in the load of the SMN protein. A second survival motor neuron gene (SMN2), with an identical sequence, can ease the damage done by the mutations but only to a very limited degree.
Genetic information of every organism is stored in its genes, located in strands of DNA. This information is transcribed from DNA into messenger RNA (mRNA) by a process called transcription. However, before the mRNA can be transformed into proteins, certain genetic material must be removed and added, through a process known as alternative splicing, to produce a mature mRNA.
Alternative splicing allows for a single gene to give rise to many different proteins. Just like in a recipe, adding or removing certain key ingredients can result in different dishes.
SMN2, like SMN1, is capable of producing SMN. But a slight difference in its DNA sequence results in alternative splicing of a premature version of its mRNA. This difference causes 90 percent of its resulting SMN protein to be shorter and nonfunctional.
Several approaches that therapeutically target alternative splicing of SMN2 are currently in various stages of development. These range from an approved antisense oligonucleotide — Spinraza (nusinersen) — to small molecules (such as RG7916) shown to promote the correct splicing of SMN2 pre-mRNA, and to increase levels of a working SMN protein.
Collaborative work between researchers in Switzerland, Germany, France, and Spain has now shown that a different approach can also target the underlying molecular deficiency of SMA: modulating SMN2 splicing to increase the expression of a full-length SMN2 mRNA molecule.
The team found small molecules that could target a specific region of the SMN2 gene — while still in its pre-mRNA phase (called terminal stem–loop 2 [TSL2]) — because this region prevents the correct splicing of SMN2.
Nineteen molecules were found to bind to TSL2, and almost half successfully induced correct SMN2 splicing. Among those, marine natural molecule homocarbonyltopsentin (called PK4C9) showed the strongest effect, both in cells from SMA patients grown in the lab and in motor nerve cells of an SMA animal model (fruit fly).
In patients’ cells, PK4C9 administration increased the levels of the full-length SMN2 mRNA by 40%, which resulted in a 1.5-fold increase in SMN protein levels. According to researchers, similar increases were shown to be enough to improve motor function and survival in SMA mouse models, suggesting that small molecules targeting TSL2 may be potential therapies for SMA.
“Our study contributes to the increasing use of small molecules to rationally target RNA, and opens new avenues for rational drug discovery in SMA, setting a precedent for other splicing-mediated disorders, where the relevant RNA structures could be similarly targeted to modify the outcome of the splicing events that they regulate,” the researchers wrote.