Linking C9orf72 Repeat Expansions with Neurodegeneration in ALS
Introduction. Amyotrophic lateral sclerosis (ALS) is a rare and ultimately fatal neurodegenerative disorder. Typical findings of ALS are progressive and asymmetrical and include degeneration of the lateral and anterior corticospinal tracts along with the loss of motor neurons from the anterior horn of the spinal cord1. Many genetic mutations have been linked to ALS, including mutations in superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 (TDP-43) and RNA-binding protein FUS, and C9orf723,4. The most frequent cause of familial ALS, and FTD, are hexanucleotide expansions (GGGGCC) in the noncoding region of the C9orf72 gene, linked to chromosome C9orf723. The C9orf72 protein be a part of a larger family of the differentially expressed in normal and neoplastic cells (DENN) domain containing proteins, which function as guanine-nucleotide exchange factors (GEFs) and play roles in regulating intracellular membrane trafficking and neural differentiatoin1. Recently, research has lead to more information about the consequences of C9orf72 mutations and its role in ALS. Methods. Researchers generated a line of mice carrying a bacterial artificial chromosome containing exons 1 to 6 of the human C9orf72 gene carrying around 500 repeats of the GGGGCC motif5. Histological and behavioral comparisons were conducted between humans with ALS and these mutated mice2,5. Results. The higher number of GGGGCC repeats found in the C9orf72 gene correlated with an earlier age of onset of the disease, increased DNA methylation and silencing, and thus more transcriptional downregulation, which lead to an increased amount of phenotypes seen2,5. Ultimately decreased C9orf72 protein expression leads to the accumulation of misfolded repeat-containing RNAs into structures that sequester RNA-binding proteins located throughout the CNS, including the spinal cord, cerebellum, frontal cortex, and hippocampus2,5,7. Conclusions. Studies have demonstrated several genetic mutations that are linked to ALS, most notably hexanucleotide expansions (GGGGCC) in the noncoding region of the C9orf72 gene1. Normally, the C9orf72 has several different functions that aid and regulate neural differentiation and axon guidance6. The C9orf72 protein regulates intracellular membrane trafficking, directs the clearance of aggregated proteins, and plays a role in lysosome biogenesis6. However, studies have found that hexanucleotide expansions of the promotor region of this gene results in transcriptional silencing and the production of misfolded repeat-containing RNAs into structures that sequester RNA-binding proteins4,7. Ultimately, research has shown that the higher number of nucleotide expansions in the C9orf72 correlates with significantly more neurodegeneration in samples taken from humans and mice5. The mutated C9orf72 gene is one of several genetic mutations that have been linked to ALS, and these genetic mutations and their interactions with each other remain an important area of research for ALS.
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