Alternative processing of human HTT mRNA with implications for Huntington's disease therapeutics

Area of Science:

• Neuroscience
• Genetics
• Molecular Biology

Background:

• Huntington's disease (HD) stems from a CAG repeat expansion in the huntingtin gene (HTT).
• Previously, we identified HTT1a, an exon 1-only transcript, produced by incomplete splicing of expanded HTT mRNA.
• This HTT1a transcript yields a pathogenic, aggregation-prone exon 1 HTT protein.

Purpose of the Study:

• To investigate HTT1a transcript and exon 1 HTT protein expression in a humanized mouse model of HD.
• To assess the role of HTT1a in the aggregation process and evaluate therapeutic strategies.

Main Methods:

• Utilized YAC128 transgenic mice expressing human expanded repeat HTT.
• Employed RNAscope for single-molecule visualization of HTT transcripts.
• Used homogeneous time-resolved fluorescence for protein analysis.

Main Results:

• Detected HTT1a transcripts throughout YAC128 mouse brains.
• Observed HTT1a and full-length HTT in nuclear RNA clusters and cytoplasmic transcripts.
• Confirmed translation of HTT1a into exon 1 HTT protein, correlating with aggregation.

Conclusions:

• Exon 1 HTT protein, derived from HTT1a, likely initiates HD aggregation.
• Current huntingtin-lowering therapies may not reduce pathogenic exon 1 HTT.
• Development of HTT1a-targeting agents and understanding transcript localization are crucial for HD therapeutics.