A Pathogenic Mechanism in Huntington's Disease Involves Small CAG-Repeated RNAs with Neurotoxic Activity

Abstract

Huntington's disease (HD) is an autosomal dominantly inherited disorder caused by the expansion of CAG repeats in the Huntingtin (HTT) gene. The abnormally extended polyglutamine in the HTT protein encoded by the CAG repeats has toxic effects. Here, we provide evidence to support that the mutant HTT CAG repeats interfere with cell viability at the RNA level. In human neuronal cells, expanded HTT exon-1 mRNA with CAG repeat lengths above the threshold for complete penetrance (40 or greater) induced cell death and increased levels of small CAG-repeated RNAs (sCAGs), of ≈21 nucleotides in a Dicer-dependent manner. The severity of the toxic effect of HTT mRNA and sCAG generation correlated with CAG expansion length. Small RNAs obtained from cells expressing mutant HTT and from HD human brains significantly decreased neuronal viability, in an Ago2-dependent mechanism. In both cases, the use of anti-miRs specific for sCAGs efficiently blocked the toxic effect, supporting a key role of sCAGs in HTT-mediated toxicity. Luciferase-reporter assays showed that expanded HTT silences the expression of CTG-containing genes that are down-regulated in HD. These results suggest a possible link between HD and sCAG expression with an aberrant activation of the siRNA/miRNA gene silencing machinery, which may trigger a detrimental response. The identification of the specific cellular processes affected by sCAGs may provide insights into the pathogenic mechanisms underlying HD, offering opportunities to develop new therapeutic approaches.

Author Summary

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormal CAG expansion in the Huntingtin gene ( HTT), resulting in an expanded polyglutamine track in the HTT protein. Longer CAG expansions correlate with an earlier more severe manifestation of the disease that produces choreic movement, behavioural and psychiatric disturbances, and dementia. Although the causative gene is widely expressed, neuropathology is characterized by striatal and cortical atrophy. HTT interacts with proteins involved in transcription, cell signaling, and transport. The pathogenic role of mutant HTT is not fully understood. This study shows that CAG expanded HTT RNA also contributes to neuronal toxicity. Mutant HTT RNA gives rise to small CAG-repeated RNAs (sCAGs) with neurotoxic activity. These short RNAs interfere with cell functions by silencing the expression of genes that are fully or partially complementary, through a mechanism similar to that of microRNAs. These findings suggest that a small RNA–dependent mechanism may contribute to HD neuronal cell loss. The exhaustive identification of the target genes modulated by sCAGs may lead to a better understanding of HD pathology, allowing the development of new therapeutic strategies.