An ALS Neuron Screen Separates RNA-Foci Clearance from Repeat-Protein and Growth Responses

A Patient-Neuron Screen with Several Layers of Hit Filtering

Expanded G4C2 repeats in C9ORF72 can generate nuclear RNA foci and abnormal dipeptide-repeat proteins. A Novartis-led study in Nucleic Acids Research asked whether small molecules could alter the RNA-foci phenotype in patient-derived cells, then investigated what happened to the repeat RNA after the foci disappeared. The resulting experiments distinguish an accessible screening endpoint from downstream biological consequences.

The researchers generated induced pluripotent stem-cell-derived neurons using Neurogenin-2 induction. The primary screen used one ALS line, ALS80/5a, while broader characterization included repeat-expansion lines from ALS and frontotemporal-dementia donors and healthy controls. RNA-foci levels varied between donors and even between clones from the same donor, without an apparent relationship to repeat length.

The screen tested 96,200 compounds at 8 μM for 48 hours in a 1,536-well format, with one initial replicate per compound. Fluorescence in situ hybridization, or FISH, visualized repeat-RNA foci for automated counting relative to nuclei. Healthy-donor and ALS vehicle-treated neurons supplied the low and high reference signals on each plate.

Of 2,952 compounds selected for concentration–response testing, 261 confirmed a dose-dependent foci effect: 223 decreased foci and 38 increased them. Counterassays removed 122 compounds that broadly interfered with gene expression. Further testing of the remaining candidates in C9ORF72 lines from three independent donors yielded 82 final validated hits. These hits modulated the assay phenotype in either direction; they were not a set of established neuroprotective drugs.

Spliceosome Modulation Clears Foci but Differs from Target Depletion

A chemical series termed splice-modulator-like, or SML, resembled known ligands of SF3B1, a component of the machinery that processes pre-messenger RNA. Follow-up selected SML-3, also known as FR901464. It cleared foci with an IC50 of about 2 nM in the tested fibroblast and induced-neuron assays. A closely related compound, SML-4, was inactive and provided a useful control.

Other SF3B1 modulators, including pladienolide B, also reduced foci in patient fibroblasts, induced neurons and separately derived motor neurons. Lowering SF3B1 protein by RNA interference produced the opposite result: foci increased. The pharmacological effect therefore could not be interpreted simply as equivalent to reducing the amount of SF3B1.

Reporter experiments showed that foci could disappear independently of the native C9ORF72 gene context. Although the active compounds affected expression and splicing of many transcripts, the repeat-reporter RNA did not fall in parallel with its foci. Kinetic and transcription-control experiments supported altered handling of existing repeat RNA rather than simple elimination through transcription shutdown.

Protein Production and Cell Growth Were Tested in Reporter Cells

The investigators could not robustly detect endogenous dipeptide-repeat proteins in their patient-derived induced or motor neurons with the available antibodies. They therefore used engineered HEK293 reporter cells to examine repeat-associated noncanonical translation, often called RAN translation. These constructs allowed tagged poly-GA and poly-GP repeat proteins to be measured.

After 24 hours of 100 nM SML-3 or pladienolide B treatment, poly-GA and poly-GP signals increased. A corresponding poly-GR increase was not detected, although its baseline signal was much lower. In a growth-tracking comparison over around four days at 100 nM, combining repeat-protein induction with the active compounds reduced cell proliferation more than either condition alone. The inactive SML-4 did not reproduce that effect.

Higher repeat-protein levels and reduced growth were associated in this system. The study did not establish that the repeat proteins themselves caused the additional toxicity, and contributions from the reporter tags could not be excluded. These HEK-cell findings also remain distinct from a demonstration of increased repeat-protein production or neuronal death in the patient-derived cultures.

Export Provides a Mechanistic Explanation and a New Endpoint Question

Single-molecule RNA imaging in another HEK reporter system showed movement of repeat transcripts from the nucleus to the cytoplasm after pladienolide B treatment. Protein-interaction experiments identified increased recruitment of SRSF1, a splicing factor that also helps export RNA. Binding assays supported direct interaction between SRSF1 and folded repeat RNA.

Together, these observations support a model in which spliceosome modulation releases repeat RNA from nuclear foci and facilitates its export for translation. The authors propose that foci can act as a storage compartment that limits access to potentially harmful protein production. That remains a mechanistic hypothesis whose relevance depends on cellular context; broad splicing effects complicate attribution of every phenotype to this pathway alone.

For drug evaluation, the result makes the endpoint choice concrete. A smaller nuclear-foci signal can coexist with redistribution of the same RNA and increased reporter-protein production. The study’s compounds provide tools for testing those alternative RNA fates, while interpretation of a favorable screening image requires accompanying RNA-location, protein and functional measurements.