Title: CGG repeat associated translation in Fragile X-associated Tremor/Ataxia Syndrome. Abstract: Dominantly inherited nucleotide repeat expansion disorders are thought to elicit neurodegeneration in one of two ways: 1) The repeat as RNA can bind to and sequester specific proteins, preventing them from performing their normal functions;or 2) If the repeat is translated into protein, the repetitive amino acid expansion can trigger toxicity through a variety of mechanisms including protein misfolding and aggregation. Traditionally, the dominant contribution of each pathogenic mechanism has been suggested by the repeat's location within the disease gene, with exonic repeats exerting toxicity primarily as protein and non-exonic repeats presumably acting via RNA-mediated mechanisms. Recent data, however, indicate that repeats in "non-coding" regions of transcripts can be aberrantly translated into proteins through Repeat Associated Non- AUG initiated (RAN) translation. In light of this new finding, defining the relative contributions of RNA- and protein-mediated toxic processes in each repeat expansion disorder has surfaced as a critical issue in the field. Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) is an inherited neurodegenerative disorder that results from a CGG repeat expansion at the beginning of the fragile X gene, FMR1. It is characterized pathologically by the formation of proteinaceous inclusions in the brains of patients. Work to date suggests that the repeat is toxic as RNA, but our group recently showed that the CGG repeat expansion also elicits RAN translation ("CGG RAN translation") to produce an aggregation-prone, homopolymeric polyglycine containing protein. This protein aggregates in model systems and is present in inclusions in FXTAS disease brain. In this proposal, we will determine whether the CGG repeat in FXTAS triggers neurodegeneration as RNA, as a toxic protein, or both, and then interrogate how this newly discovered RAN translation occurs mechanistically. To address these questions, we will utilize new fly models of FXTAS to determine the relative abilities of CGG repeats as RNA and as RAN translated proteins to elicit neurodegeneration. We will then extend these findings to pathological and behavioral assessments of two knock-in mouse models of FXTAS that differ in their ability to support CGG RAN translation. In parallel, we will employ a series of biochemical and cell-based approaches to explore the mechanisms underlying CGG RAN translation. These studies should provide critical insight into FXTAS pathogenesis while offering a relevant case-study for other repeat expansion disorders, and in the process facilitate the identification of proximal therapeutic targets based on improved understanding of disease mechanisms.

Public Health Relevance

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS) is a progressive degenerative disease of the brain that results from a genetic mutation. This research project addresses the surprising finding that this mutation triggers production of a toxic protein whose existence was not previously recognized. We will explore how this toxic protein contributes to FXTAS symptoms and work to understand what factors regulate its production, with a long term goal of developing novel therapies for patients.

Agency
National Institute of Health (NIH)
Type
Research Project (R01)
Project #
1R01NS086810-01
Application #
8670071
Study Section
Cellular and Molecular Biology of Neurodegeneration Study Section (CMND)
Program Officer
Riddle, Robert D
Project Start
Project End
Budget Start
Budget End
Support Year
1
Fiscal Year
2014
Total Cost
Indirect Cost
Name
University of Michigan Ann Arbor
Department
Neurology
Type
Schools of Medicine
DUNS #
City
Ann Arbor
State
MI
Country
United States
Zip Code
48109
Kearse, Michael G; Todd, Peter K (2014) Repeat-associated non-AUG translation and its impact in neurodegenerative disease. Neurotherapeutics 11:721-31
He, Fang; Krans, Amy; Freibaum, Brian D et al. (2014) TDP-43 suppresses CGG repeat-induced neurotoxicity through interactions with HnRNP A2/B1. Hum Mol Genet 23:5036-51