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Sep 14
2020

Key link uncovered between C9orf72 mutation and TDP-43 pathology in ALS/FTD

ALS Headlines, Packard Center News
In a new study, a team of Packard scientists identified a key intersection between two major pathways of pathology in ALS and frontotemporal dementia (FTD)

In a new study in Science Translational Medicine, a team of Packard scientists identified a key intersection between two major pathways of pathology in ALS and frontotemporal dementia (FTD). The work, led by biochemist James Shorter of the University of Pennsylvania and neuroscientist Leonard Petrucelli at the Mayo Clinic, revealed that toxic proteins produced by the repeat expansion in the C9orf72 gene (the most common genetic cause of ALS/FTD) trigger mislocalization and aggregation in the DNA-binding protein TDP-43. The results are a breakthrough not only in understanding how the C9orf72 mutation leads to disease but in the development of potential therapeutics.

Scientists have identified several potential mechanisms through which the C9 repeat expansion can cause ALS. The repeat of six nucleotides (GGGGCC) hundreds, even thousands, of times can confuse the cell’s standard RNA- and protein-making machinery. As a result, the cell makes abnormal mRNA and toxic proteins called dipeptide repeats, or DPRs. Although all of the five DPRs produced by the C9 repeat expansion can be damaging, the DPR known as poly(GR) stands out as being especially toxic. Previous studies have found poly(GR) frequently associated with clumps of TDP-43 in the cytoplasm of neurons in the motor cortex, but how and why these two proteins are associated remained unclear. The goal of the new study was to use biochemistry and cellular and animal models to untangle these links.

When the scientists incubated synthesized poly(GR) with TDP-43 protein, they found that this DPR triggered the formation of TDP-43 aggregates. The other DPR they tested, known as poly(GA), didn’t do this. Detailed protein biochemistry analysis revealed that poly(GR) enhances TDP-43 misfolding and aggregation. Other studies using immuno-electron and immuno-fluorescent microscopy showed that poly(GR) recruited cytoplasmic TDP-43 into toxic clumps in cultured cells, and is sufficient to promote the aggregation of wild-type TDP-43 in mice. In mouse studies, the researchers found that these clumps of poly(GR) also sequester some of the proteins that help large molecules move between the cytoplasm and nucleus in a process called nucleocytoplasmic transport. This disruption contributes to the mislocalization of TDP-43 from the nucleus to the cytoplasm, which further increases ALS/FTD molecular pathology.

The links between C9 DPRs and TDP-43 indicates that any therapeutics specifically targeting the repeat expansion also need to prevent or reverse the impacts on TDP-43 to be effective. For ALS/FTD caused by the C9 repeat expansion, Ionis Therapeutics has developed an antisense oligonucleotide (ASO), a type of molecular therapy that binds to a specific RNA sequence to mediate these RNAs degradation. Although previous studies showed that it can decrease DPR expression, no one knew whether it would also reduce TDP-43 pathology. Using a C9 mouse model, Shorter, Petrucelli, and colleagues showed that the ASO not only reduced DPR production, it also lowered the number of TDP-43 aggregates. The mice also had lower neuronal loss and a reduction in markers indicating neuronal injury.

Taken together, this work shows that C9-linked poly(GR) directly increases the formation of dense TDP-43 aggregates, and that targeting the C9 repeat expansion with an ASO not only reduces DPRs but also TDP-43 pathology. The results provide a direct link between two of the most common factors associated with ALS/FTD.

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