PROJECT SUMMARY/ABSTRACT Expanded GGGGCC hexanucleotide repeats in the C9orf72 gene were recently identified as the most common genetic cause of Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS), two neurodegenerative disorders with clinical, pathological and genetic overlaps. Proposed disease mechanisms include loss of C9orf72 protein function and gain of toxicity from the bidirectionally transcribed sense or antisense repeat-containing RNAs, mediated by sequestration of RNA binding proteins into RNA foci or by production of at least five different aberrant dipeptide repeat (DPR) proteins [GA, GP, GR, PR, and PA] from the intronic sequence through a nonconventional translation mechanism called repeat-associated non-AUG- dependent (RAN) translation. Several studies demonstrated that gain of toxicity from C9orf72 repeat containing RNAs plays a central role in disease pathogenesis. It is yet unknown which specific cell types in the central nervous system is responsible for the disease pathogenesis from gain of toxicity. C9orf72 loss of function alone is insufficient to cause FTD/ALS in mice, but our preliminary data showed it exacerbates diseases together with gain of toxicity. C9orf72 plays an important role in the immune cells as C9orf72 null mice develop splenomegaly and enlarged cervical lymph nodes. However, the function of C9orf72 in microglia and its contribution to FTD/ALS is not determined. In this proposal, will determine 1) molecular pathways altered by C9orf72 loss of function in microglia using genome-wide RNA sequencing and whether such changes in microglia will lead to neuronal toxicity (Aim 1); 2) Whether C9orf72 loss of function in microglia contributes to FTD/ALS pathogenesis caused by gain of toxicity from the repeat containing RNAs in mice (Aim 2); and 3) cell-type-specific toxicity from C9orf72 repeat containing RNAs (Aim 3). If successful, the proposed study will further help us understand the disease mechanisms of C9orf72 repeat expansions in FTD and ALS and identify potential therapeutic interventions.

PROJECT NARRATIVE Hexanucleotide GGGGCC repeat expansion in the C9ORF72 gene is the most frequent cause of two neurodegenerative diseases, Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). Disease mechanisms include loss of C9orf72 function and gain of toxicity from the repeat containing RNAs. This project will determine cell-type-specific toxicity of C9orf72 repeat expansions in FTD and ALS.