Cellular and physiological functions of C9ORF72 and implications for ALS/FTD

The hexanucleotide repeat expansion (HRE) in the C9ORF72 gene is the main cause of two tightly linked neurodegenerative diseases, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). HRE leads to not only a gain of toxicity from RNA repeats and dipeptide repeats but also reduced levels of C9ORF72 protein. However, the cellular and physiological functions of C9ORF72 were unknown until recently. Through proteomic analysis, Smith–Magenis chromosome regions 8 (SMCR8) and WD repeat‐containing protein (WDR41) were identified as binding partners of C9ORF72. These three proteins have been shown to form a tight complex, but the exact functions of this complex remain to be characterized. Both C9ORF72 and SMCR8 contain a DENN domain, which has been shown to regulate the activities of small GTPases. The C9ORF72 complex has been implicated in many cellular processes, including vesicle trafficking, lysosome homeostasis, mTORC1 signaling , and autophagy. C9ORF72 deficiency in mice results in exaggerated inflammatory responses and human patients with C9ORF72 mutations have neuroinflammation phenotype. Recent studies indicate that C9ORF72 regulates trafficking and lysosomal degradation of inflammatory mediators, including toll‐like receptors (TLRs) and STING, to affect inflammatory outputs. Further exploration of cellular and physiological functions of C9ORF72 will help dissect the pathological mechanism of ALS/FTD caused by C9ORF72 mutations. Hexanucleotide repeat expansion in the C9ORF72 gene is a leading cause of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). Haploinsufficiency of C9ORF72 is one of the disease mechanisms. During the past decade, studies have revealed the cellular and physiological functions of C9ORF72. The C9ORF72 protein forms a stable complex with SMCR8 and WDR41 and regulates several small GTPases to modulate autophagy activities, endosome–lysosome trafficking, and mTORC1 signaling. Loss‐of‐function of C9ORF72 leads to increased endosomal TLR and STING signaling and enhanced microglial activation, as well as decreased neuronal survival due to endolysosomal trafficking defects.