Fen-Biao GaoHalim, Dilara O2023-06-232023-06-232023-05-3010.13028/caxd-3132https://hdl.handle.net/20.500.14038/52155Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two neurodegenerative disorders without cure. A GGGGCC repeat expansion in the first intron of C9ORF72 is the most common genetic cause of ALS and FTD. Although the mechanism of toxicity in C9ORF72 pathology has not been fully understood, one proposed mechanism is toxic gain of function of dipeptide repeat (DPR) proteins translated from expanded repeat RNA. Five different DPRs can be synthesized from the repeat RNAs. Of these, poly(GR) expression shows a correlation with neurodegeneration in C9ORF72 patient brains. To understand how poly(GR) induces toxicity, our lab previously performed an unbiased screen in a Drosophila model and identified Sec5/EXOC2 as a suppressor of poly(GR) toxicity. Here I aimed to investigate how EXOC2 is involved in neurodegeneration in C9ORF72-ALS/FTD patient induced pluripotent stem cell-derived motor neurons (iPSC-MNs). Using the CRISPR-Cas9 technology, I deleted EXOC2 which encodes an essential exocyst subunit in iPSCs lines derived from C9ORF72-ALS/FTD patients. These cells remained viable due to the expression of a truncated EXOC2, suggesting exocyst function was partially maintained. I found that partial loss of EXOC2 rescues several disease relevant phenotypes including neurite degeneration and neuronal cell death. Surprisingly, partial loss of EXOC2 reduced the levels of expanded repeats containing C9ORF72 transcripts. Treatment of fully differentiated C9ORF72 iPSC-MNs with EXOC2 antisense oligonucleotides also decreased C9ORF72 pre-mRNA and rescued disease-related phenotypes. Moreover, I demonstrated that EXOC2 binds to C9ORF72 promoter in an expanded repeats-dependent manner. These results indicate GGGGCC expanded repeats exhibit gain-of-function properties, reveal a novel function for EXOC2 in transcription and identify EXOC2 as a novel modifier and potential therapeutic target in C9ORF72-ALS/FTD.en-USCopyright © 2023 Dilara O. HalimAll Rights ReservedALSFTDC9orf72exocystiPSCmotor neuronDoctoral Dissertation0009-0007-4417-4087