PROJECT SUMMARY Amyotrophic lateral sclerosis (ALS), an adult onset motor neuron degenerative disease, is increasingly recognized to have clinical, pathological and genetic overlaps with Frontotemporal dementia (FTD). Dysfunction of RNA metabolisms has emerged to play crucial roles in disease etiology. Pathological inclusions and/or genetic mutations in several RNA-binding proteins are widely found in the two diseases. Alternatively, the hexanucleotide repeat expansion in the non- coding region of C9ORF72 gene could also induce toxicity from the repeat RNA-derived products. This supports the susceptibility of neurons to the dysfunction of RNA processing and the importance of RNA homeostasis in preserving neuronal integrity. RNA modifications have recently emerged to play important roles in posttranscriptional gene regulation. N6-methyladenosine (m6A) is by far the most abundant internal RNA modification of eukaryotic cells. m6A modification is dynamic and reversible, providing an additional layer of regulation on RNA. It is noted that m6A is most enriched and the methylome is highly specific in the nervous system compared to other tissues. Emerging studies indicate the important roles of m6A in regulating brain function, from development to synaptic plasticity, learning and memory, and neurodegeneration. But it has not been explored in ALS/FTD or other neurodegenerative diseases. The hexanucleotide GGGGCC repeat expansion located in the first intron of the C9ORF72 gene is the most common cause of both ALS and FTD. The leading hypothesis for the disease mechanism is gain of toxicity from the expanded repeats, with two non-mutually exclusive mechanisms: 1) RNA foci formed by repeats that could sequester RNA binding proteins and disrupt RNA processing; and 2) accumulation of dipeptide repeat proteins (DPRs) produced by repeat-associated non-AUG translation (RAN translation). Our lab recently identified that m6A RNA methylation can affect the DPR levels produced from the repeats. Furthermore, we also found that the m6A pathway is profoundly reduced in C9ORF72-ALS/FTD patient neurons. Therefore, it is important to determine the molecular mechanisms on how the m6A dysregulation changes the repeat RNA metabolisms and disturbs the global mRNA processing, and how this contributes to the neuronal dysfunction and degeneration in C9ORF72-ALS/FTD. Furthermore, we will also determine whether targeting specific components in the m6A pathway can rescue the disease-related phenotypes. This study addresses an emerging theme of RNA regulation that has not been explored in neurodegeneration. The findings will help understanding the etiology of the disease and developing novel therapeutic strategies for ALS and FTD.