RAN Translation of the Expanded CAG Repeats in the SCA3 Disease Context

Area of Science:

• Neurogenetics
• Molecular Biology
• Cellular Toxicology

Background:

• Spinocerebellar ataxia type 3 (SCA3) is a progressive neurodegenerative disorder linked to CAG repeat expansion in the ATXN3 gene.
• The precise pathogenic mechanisms driving SCA3, particularly the role of expanded repeats, remain incompletely understood.

Purpose of the Study:

• To investigate the toxicity induced by expanded CAG repeats in SCA3 through comprehensive analysis of repeat-associated non-ATG (RAN) translation.
• To elucidate the mechanistic insights into SCA3 RAN translation, including the influence of flanking sequences and cellular stress responses.

Main Methods:

• Utilized various cellular models expressing ATXN3 sequences with varying CAG repeat lengths.
• Investigated the initiation and efficiency of RAN translation for polyglutamine (polyQ) and polyalanine (polyA) proteins.
• Examined the impact of RAN proteins on nuclear integrity and apoptosis, and the role of integrated stress response activation.

Main Results:

• Two SCA3 RAN proteins, polyQ and polyA, were detected exclusively with pathogenic CAG repeat lengths.
• RAN polyQ and polyA proteins were frequently coexpressed within the same cells, leading to impaired nuclear integrity and apoptosis.
• ATXN3 sequence context upstream of the repeat region influences RAN translation initiation and efficiency, with polyQ translation initiating upstream and polyA translation likely within the repeats.
• Activation of the integrated stress response enhanced SCA3 RAN translation.

Conclusions:

• The ATXN3 sequence context is a critical factor in triggering SCA3 RAN translation.
• SCA3 RAN proteins, polyQ and polyA, contribute to cellular toxicity through mechanisms including nuclear impairment and apoptosis.
• Understanding RAN translation provides new mechanistic insights into SCA3 pathogenesis.