Cross validation of gene expression changes in C9orf72 FTD using Drosophila and human model systems

FTD is the second most common neurodegenerative dementia after Alzheimer’s disease (AD) in adults under the age of 65. Pathologically, 45% of FTD cases are characterized by cytoplasmic protein aggregates containing TAR DNA-binding protein 43 (TDP-43). Genetically, up to 43% of FTD patients have a family history of dementia or related neurodegenerative diseases, with mutations in the C9orf72 (C9) gene representing the most common genetic abnormality in FTD (10-30% of FTD cases). C9 mutations are expansions of GGGGCC (G4C2) hexanucleotide repeats (HR) within its non-coding first intron. When affected by dementia, individuals with C9 FTD most frequently show clinical features of behavioral variant (bv) FTD accompanied by memory impairment. To begin addressing the mechanism of cognitive impairments in C9 FTD/ALS patients, the Sattler group has recently used single nuclei RNA seq (snRNA seq) and, by comparing the transcriptomes of neuronal nuclei containing TDP-43 associated cryptic exons (CEs) to those of nuclei containing canonical splice junctions, identified significant changes in gene expression linked to TDP-43 pathology. Independently, using RNA immunoprecipitations in a Drosophila model of dementia based on TDP-43 overexpression in mushroom bodies (MBs), a neuronal circuit controlling dementia relevant behaviors, the Zarnescu group identified a subset of TDP-43 candidate mRNA targets overlapping with differentially expressed genes in neuronal nuclei with TDP-43 pathology in FTD patients, referred herein as “C9 FTD-TDP targets”. These findings led us to hypothesize that “C9 FTD-TDP targets” mediate dementia relevant phenotypes in Drosophila models of C9orf72 FTD and exhibit altered expression in FTD patient derived neurons and tissues. To test this hypothesis we will cross-validate our C9 FTD-TDP targets between novel Drosophila models of dementia based on G4C2 HRE expression in MBs and human experimental models including iPSC cortical neurons and post-mortem tissues. First, we will use molecular and genetic approaches to identify C9 FTD-TDP targets that are altered in Drosophila models of C9orf72 FTD and modify C9orf72 dependent axonal degeneration, polyGR accumulation, working memory, sleep and lifespan. Second, we will validate C9 FTD-TDP targets in patient cortical neurons and post-mortem tissues. These cross-validation experiments will identify physiologically significant targets of G4C2 HR expansions in neurons and will highlight potentially novel therapeutic approaches for C9 FTD.