Role of the TE-cGAS-STING pathway in C9orf72-ALS/FTD pathogenesis

Neurodegenerative diseases are characterized by genomic instability, abnormal chromatin relaxation, and transposable element (TE) mobilization, which can harm neuronal function. However, the understanding of these processes and their impact on neuronal physiology remains limited. In a recent study on Alzheimer's disease

(AD), we discovered a novel mechanism involving the upregulation and hyper-phosphorylation of the c-JUN subunit of the AP-1 transcriptional activator. This hyperactive AP-1 complex, in conjunction with the BAF chromatin remodeling complex, led to the inappropriate de-repression and activation of TE-derived chromatin

regions in AD patient-derived induced pluripotent stem cells (iPSCs). The aberrant TE de-repression resulted in the accumulation of TE-derived RNA::DNA hybrids, triggering cGAS/STING activation and apoptosis. The c-JUN/JNK pathway is also implicated in C9orf72-associated with amyotrophic lateral sclerosis and

frontotemporal dementia (C9-ALS/FTD), where haploinsufficiency of the C9orf72 gene results in impaired autophagy and the accumulation of damaged cellular components. This accumulation triggers an innate immune response, leading to neuroinflammation and neuronal death. Given the shared involvement of the c-JUN/JNK

pathway, inflammation, and cell death in AD and related dementias (ADRD), we propose to investigate the c- JUN/TE/cGAS/STING axis in the context of C9-ALS/FTD by developing the following aims: Aim 1 seeks to identify cell-type specific patterns of transposon mobilization in C9-ALS/FTD by profiling TE

mobilization in brain organoids derived from patient iPSC lines. Furthermore, analysis of post-mortem brain tissues using ATAC-seq will identify cell-specific patterns of TE activation, integrating data from organoids and post-mortem tissues to unravel transposon activity across different brain cell types.

Aim 2 seeks to elucidate the activation of the c-JUN/TEs/cGAS/STING pathway in C9-ALS/FTD. Patient-derived iPSC lines will be used to generate brain organoids and study the accumulation of TE-derived RNA::DNA hybrids and activation of the cGAS/STING pathway. Findings will also be validated in CNS post-mortem tissue from C9-

ALS/FTD patients. Aim 3 will explore the therapeutic potential of inhibiting the c-JUN/TE/cGAS/STING pathway. Targeting the pathway in C9-ALS/FTD organoids with small molecule inhibitors could mitigate pathological consequences associated with its activation, providing potential therapeutic strategies for ALS/FTD.

In summary, this project aims to investigate the involvement of the c-JUN/TE/cGAS/STING pathway in C9- ALS/FTD and evaluate its therapeutic potential. By deciphering the mechanisms underlying TE mobilization, identifying cell-specific patterns, and assessing the efficacy of pathway inhibition, this study will contribute to our

understanding of C9-ALS/FTD pathogenesis and provide insights into potential therapeutic interventions.