This proposal uses innovative approaches to uncover underlying molecular mechanisms and develop a novel treatment strategy for frontal temporal dementia (FTD), the second most common form of young-onset dementia and an Alzheimer-disease related dementia. Unfortunately, there are no treatment options that prevent or slow FTD. A mutation in C9ORF72 (C9) is the most common cause of FTD and amyotrophic lateral sclerosis (ALS), collectively referred to as C9 ALS/FTD. This mutation leads to an abnormal excess of DNA and RNA structures, termed G-quadruplexes. Increased G-quadruplex burden contributes to accumulation of toxic RNAs and proteins leading to neuron degeneration and disease onset. As such, C9 ALS/FTD is fundamentally a ‘G-quadruplex disease’. Within the cell reside helicase enzymes that unwind G-quadruplexes. DHX36 is a major G-quadruplex helicase, accounting for the majority of G-quadruplex helicase activity in human cells. In the previous funding period, it was shown that DHX36 enhances production of toxic C9 proteins via repeat-associated non-AUG (RAN) translation. It was also shown that RAN translation of toxic C9 proteins during cellular stress requires DHX36, linking DHX36, stress, and RAN translation for the first time. DHX36 localizes to stress granules following stress, however it is unknown if the presence of DHX36 in stress granules is directly connected to DHX36 effects on C9 RAN translation. For Aim 1, it is hypothesized that loss of DHX36 reduces C9 protein translation through reduction of stress granule abundance. This hypothesis will be tested using C9 patient- derived cells, mouse primary brain neurons, and in vivo transgenic mouse models. In addition to a cytoplasmic effect of DHX36 on RAN translation, preliminary data suggest that DHX36 may protect C9 ALS/FTD cells from increased DNA damage in the nucleus. These observations suggest that nuclear DHX36 may be protective while cytoplasmic DHX36 may exacerbate C9 ALS/FTD. For Aim 2, it is hypothesized that specifically targeting cytoplasmic DHX36 is a viable therapeutic strategy that reduces toxic C9 protein production while preserving protective effects of nuclear DHX36. This hypothesis will be tested using human C9 ALS/FTD patient-derived neuronal progenitor cells. This research builds on a long-standing collaboration between three research groups led by Dr. Philip Smaldino (Ball State U.), Dr. Yuh-Hwa Wang (U. of Virginia), and Dr. Peter Todd (U. of Michigan), with additional support from Dr. Lindsey Hayes (Johns Hopkins U.) and Dr. Ashley Kalinski (Ball State U.). The proposed studies will be the first to determine the role of DHX36 in stress responses and genomic integrity in C9 ALS/FTD and is poised to identify DHX36 as a novel therapeutic target. Undergraduate and graduate students will be integrated at every stage of the project allowing them to gain authentic experience with innovative cell and mouse technologies applied to a deadly human disease. Completion of this w...