A pathogenic mechanism in Huntington's disease involves small CAG-repeated RNAs with neurotoxic activity

Area of Science:

• Neuroscience
• Genetics
• Molecular Biology

Background:

• Huntington's disease (HD) is an inherited neurodegenerative disorder.
• It is caused by expanded CAG repeats in the Huntingtin (HTT) gene, leading to a toxic mutant HTT protein.
• The precise mechanisms of HTT-mediated toxicity, particularly at the RNA level, require further elucidation.

Purpose of the Study:

• To investigate the role of mutant Huntingtin (HTT) CAG repeats in cellular toxicity at the RNA level.
• To identify and characterize small CAG-repeated RNAs (sCAGs) in Huntington's disease pathogenesis.
• To explore the therapeutic potential of targeting sCAGs in HD.

Main Methods:

• Utilized human neuronal cells expressing expanded HTT exon-1 mRNA.
• Quantified small CAG-repeated RNAs (sCAGs) using Dicer-dependent assays.
• Assessed neuronal viability and employed Ago2-dependent mechanisms.
• Conducted luciferase-reporter assays to study gene silencing.
• Tested the efficacy of anti-miRs specific for sCAGs.

Main Results:

• Expanded HTT mRNA with CAG repeat lengths ≥40 induced cell death and increased sCAG levels.
• sCAG generation and HTT mRNA toxicity correlated with CAG expansion length.
• Small RNAs from mutant HTT cells and HD brains reduced neuronal viability via Ago2.
• Anti-miRs targeting sCAGs effectively blocked HTT-mediated toxicity.
• Expanded HTT was shown to silence CTG-containing genes, which are downregulated in HD.

Conclusions:

• Mutant HTT CAG repeats interfere with neuronal cell viability primarily through RNA-level mechanisms.
• sCAGs play a critical role in the toxicity associated with Huntington's disease.
• Aberrant activation of RNA silencing pathways, including siRNA/miRNA machinery, may contribute to HD pathogenesis.
• Targeting sCAGs presents a promising therapeutic strategy for Huntington's disease.