C9orf72 and amyotrophic lateral sclerosis: ALS and FTD represent a clinical spectrum of neurodegenerative disorders that share overlapping pathogenic mechanisms, including the progressive degeneration of motor neurons (MNs) and cortical neurons, disruption of RNA and protein homeostasis, and the involvement of mutations in genes such as TARDBP, SOD1, C9ORF72, and VCP [116,121,122].