Development of a PIKFYVE suppressing antisense oligonucleotide treatment for FTD Project Summary Frontotemporal dementia (FTD) is a complex disease that results from many diverse genetic etiologies. There are no drugs that slow the progression of FTD. Although therapeutic strategies that target specific causal mutations (e.g. C9ORF72 ASOs) may prove effective against individual forms of FTD, these approaches cannot address the vast majority of cases that have unknown genetic etiology. Moreover, given the large number of different genes that likely contribute to FTD and the fact that each genetic form is relatively rare, this strategy may be difficult to implement for all cases. Thus, there is a pressing need for new therapeutic strategies that rescue multiple forms of FTD, particularly those with unknown genetic etiologies. 45% of FTD patients display cytosolic aggregates of TDP-43 in cortical neurons, while another 45% harbor tau aggregates. Studies suggest that these neuronal TDP-43 and tau aggregates drive neurodegeneration. Thus, to identify new therapeutic targets for FTD, we used cellular reprogramming to generate induced cortical neurons (iNs) from C9ORF72 FTD patients, who display TDP-43 aggregates, as well as MAPT FTD patients, who harbor tau aggregates. We then performed chemical screens to identify targets that rescue the degeneration of both C9ORF72 and MAPT FTD iNs. Inhibitors of PIKFYVE kinase were among the most potent compounds on both C9ORF72 and MAPT FTD iNs. Antisense oligonucleotide (ASO)-mediated suppression of PIKFYVE confirmed that blocking PIKFYVE activity rescues FTD iN survival. In contrast to small molecules, antisense oligonucleotides (ASOs) provide a facile approach to targeting the CNS because they can be injected directly into the spinal cord, achieve sustained target engagement throughout the CNS, and are less likely to cause peripheral toxicity. Thus, we are pursuing ASO-mediated suppression of PIKFYVE as a therapeutic approach for diverse forms of ALS. We have screened hundreds of human PIKFYVE ASOs and identified ten lead ASOs with potent PIKFYVE knockdown in vitro . We have tested hundreds of human PIKFYVE ASOs and identified three promising leads for development. The objective of this Direct to Phase 2 proposal is to further characterize the efficacy of PIKFYVE suppression, and the safety of the lead ASOs to select a bona fide development candidate for advancement in GLP toxicity studies. Our discovery of secretory autophagy as a therapeutic approach in neurodegeneration is high impact for the field because activating the proteasome and autophagy has had mixed results in neurodegeneration models.