A newly identified pathway in the development of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) offers a potential new target for therapeutic intervention, according to research published in the journal Science. The study focuses on the role of dipeptide repeat proteins (DPRs) produced from a genetic mutation, specifically an expansion in the C9ORF72 gene, the most common genetic cause of these devastating neurodegenerative diseases.
For years, scientists have understood that the expansion of a repeating DNA sequence (GGGGCC) within the C9ORF72 gene is linked to ALS and FTD. However, the precise mechanisms by which this genetic abnormality leads to disease have remained unclear. Initial theories centered on the accumulation of RNA “foci” – clumps of RNA containing the repeated sequence – and the production of DPRs through a process called repeat-associated non-AUG (RAN) translation. The new research suggests that the DPRs themselves, rather than the RNA foci, are the primary drivers of toxicity.
The study, conducted in both mouse models and human cells derived from patients with the C9ORF72 mutation, demonstrated that blocking the production of DPRs significantly alleviated disease symptoms. Researchers achieved this by mutating a specific codon (CUG) frequently used to initiate DPR translation. This mutation disrupted DPR synthesis while surprisingly leaving the production of the repeat-containing RNAs largely unaffected.
“To disentangle RNA from DPR toxicity, we mutated a CUG codon predominantly used to initiate DPR translation from all three reading frames,” the study authors explained. The results were striking. Despite the continued presence of RNA foci, the mice exhibited improvements in behavioral deficits and pathological abnormalities. Specifically, the researchers observed a reduction in p-TDP-43 inclusions – abnormal protein aggregates linked to neuronal damage – decreased activation of the STING pathway (an immune response pathway implicated in neurodegeneration), reduced motor neuron loss and lessened neuroinflammation. Plasma neurofilament concentration, a biomarker for neuronal damage, also decreased.
Further supporting these findings, base editing of the CUG codon in neurons derived from patients with the C9ORF72 mutation also led to improved molecular markers and increased cell survival. This suggests that therapeutically targeting DPR production, rather than attempting to clear the RNA foci, may be a more effective strategy.
ALS and FTD are progressive neurodegenerative diseases that affect the nervous system. ALS primarily impacts motor neurons, leading to muscle weakness, paralysis, and eventually, respiratory failure. FTD affects the frontal and temporal lobes of the brain, causing changes in personality, behavior, and language. Both diseases currently have limited treatment options and are ultimately fatal.
The C9ORF72 mutation accounts for a significant proportion of both familial ALS and FTD cases. The expanded GGGGCC repeat is found in the non-coding region of the gene, meaning it doesn’t directly code for a protein. However, the repeat expansion disrupts normal gene function and leads to the production of toxic RNA and DPRs.
Previous research has explored various approaches to combatting C9ORF72-related ALS/FTD, including attempts to reduce RNA foci using CRISPR-Cas9 gene editing. A study published in demonstrated successful excision of the repeat expansion in neurons and mouse models, leading to a reduction in RNA foci and DPRs. Another study, published in , explored the use of a high-fidelity CRISPR-Cas13 system to improve abnormalities associated with the disease. These approaches, while promising, have faced challenges related to delivery and off-target effects.
The current research, however, highlights a potentially more targeted approach. By focusing on blocking DPR translation, researchers may be able to mitigate the toxic effects of the C9ORF72 mutation without directly altering the underlying genetic code or attempting to clear the RNA foci, which may have other, yet unknown, functions. The study suggests that the production of DPRs is a critical step in the disease process, and inhibiting this step could offer a significant therapeutic benefit.
The findings also shed light on the complex interplay of different disease mechanisms in C9ORF72-related ALS/FTD. As noted in a review, the pathogenesis likely involves a combination of both loss of function of the C9ORF72 protein and gain of function from the accumulation of RNA and DPRs. Understanding these mechanisms is crucial for developing effective therapies.
While these findings are encouraging, further research is needed to translate these discoveries into clinical treatments. The next steps will involve developing drugs or therapies that can specifically inhibit DPR translation in humans and evaluating their safety and efficacy in clinical trials. The identification of DPR production as a key driver of toxicity represents a significant advance in our understanding of ALS and FTD and offers a renewed hope for patients and families affected by these devastating diseases.
