Small Molecules Show Potential to Treat SMA by Raising SMN Protein Levels, Study Says

Researchers have unraveled the cellular mechanisms behind two RG7916-like splicing modifiers that increase the levels of SMN — the protein missing in spinal muscular atrophy (SMA) — and suggest these small molecules may indeed lead to treatments of benefit to patients.

The study, “Mechanistic studies of a small-molecule modulator of SMN2 splicing,” was published in the journal Proceedings of the National Academy of Sciences (PNAS).

SMA is caused by mutations in the SMN1 gene, which leads to a reduction in the load of survival motor neuron (SMN) protein.  A second survival motor neuron gene (SMN2), with an identical sequence, can ease the damage done by the mutation but only to a very limited degree.

SMN2, like SMN1, is capable of producing SMN. But a slight difference in its DNA sequence leads to an event called alternative splicing (editing) of a premature version of its messenger RNA (mRNA) — the molecule that guides protein production. 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 shown to promote the correct splicing of SMN2 mRNA and to increase levels of a functional SMN protein.

RG7916 is one of those small molecules, now in three Phase 2 or Phase 2/3 clinical trials for several types of SMA (FIREFISH, SUNFISH, and JEWELFISH). It is an oral treatment intended to bypass the blood-brain barrier and effectively reach the central nervous system, the brain and spinal cord. [RG7916 is being developed Roche and Genentech in collaboration with PTC Therapeutics and the SMA Foundation.]

Latest data from the SUNFISH (NCT02908685) and JEWELFISH (NCT03032172) studies showed that RG7916 increases the protein levels of SMN. An additional study in a SMA mouse model found that molecules structurally similar to RG7916 that can modify SMN2 splicing also induce a comparable increase in SMN protein in the animals’ blood and brain.

Two small molecules structurally similar to RG7916 — called SMN-C2 and SMN-C3 — were found to promote the correct splicing of SMN2, presumably through mechanisms similar to RG7916.

Researchers, looking to better understand the mechanisms behind these molecules and potentially help with the design of future splicing modulators, performed a series of chemical and genetic studies.

SMN-C2 and SMN-C3 were found to directly bind to SMN2 pre-mRNA, inducing conformational changes that increase the binding of two proteins involved in pre-mRNA splicing, FUBP1 and KHSRP. This binding was shown to increase the correct splicing of SMN2.

“These findings underscore the potential of small-molecule drugs to selectively bind RNA and modulate pre-mRNA splicing as an approach to the treatment of human disease,” the researchers wrote.