C9ORF72 hexanucleotide repeat expansions in neurodegenerative diseases

Neurodegenerative diseases are a group of devastating disorders that increasingly contribute to our global health burden. The majority have no cure or disease altering treatment available. Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two such neurodegenerative diseases that exist on a spectrum connected by shared neuropathological, clinical, and genetic features. A recent discovery that mutations in the gene C9orf72 are the most common known cause of both ALS and FTD (together c9-ALS/FTD) has spurred tremendous interest in understanding the shared molecular mechanisms underlying these two diseases. c9-ALS/FTD patients harbor a [GGGGCC] hexanucleotide repeat polymorphism in the first intron of C9orf72; neurologically healthy individuals usually contain small numbers of this repeat (typically less than 20), while c9-ALS/FTD patients harbor hundreds or even thousands of these repeat units. Three prominent hypotheses have been proposed to explain the pathogenic mechanisms of the C9orf72 repeat expansion: (1) loss of C9orf72 protein function due to haploinsufficiency (2) repeat-RNA mediated toxicity, and (3) toxicity facilitated by a non-canonical form of translation from the repeat RNA producing dipeptide repeat proteins (DPRs). Accumulating evidence suggests that these DPRs are harmful to neurons in a variety of experimental models. Chapter 2 summarizes a series of genetic modifier screens to help understand how these toxic DPRs can cause neurodegeneration. We uncovered a series of genes that could suppress or enhance the toxicity of these peptides, highlighting a number of cellular mechanisms that might contribute to pathology, including nucleocytoplasmic transport, RNA processing pathways, chromatin modifications, and endoplasmic reticulum (ER) function. One potent modifier gene, TMX2 -- encoding an ER resident protein of unknown function, could mitigate DPR toxicity by modulating the ER stress response in neurons. In ongoing studies (Chapter 4), we are attempting to elucidate the endogenous function of TMX2 and how it relates to c9-ALS/FTD pathogenesis. We also investigated a strategy attempting to prevent any gain of function toxicity mechanism from the C9orf72 repeat expansion by targeting transcription of the repeat itself. An approach that could halt the accumulation of repeat containing RNA transcripts would simultaneously alleviate both repeat RNA- and DPR-mediated toxicity. In Chapter 3, we define a role for the transcription elongation factor, Spt4, in selectively regulating expanded C9orf72 hexanucleotide repeat containing transcripts. We found that reduction of Spt4 in yeast, worms, flies, and patient cells reduced the accumulation of both sense and antisense C9orf72 repeat RNA foci, as well as the DPRs produced from these transcripts. These findings highlight Spt4 as a potential therapeutic target for c9-ALS/FTD. Therapeutic targeting of a single factor to eliminate c9-ALS/FTD pathological features offers advantages over approaches that require targeting sense and antisense repeats separately; however, future studies will also be necessary to fully understand global changes on the transcriptome when reducing Spt4 levels.

Advisor: Gitler Aaron D. 
Bassik Michael 
Reimer Richard J. 
Shen Kang