Aberrantly splicedHTT,a new player in Huntington’s disease pathogenesis

Abstract

This discovery spurred our hypothesis that mis-splicing as opposed to proteolysis could be generating the smallest huntingtin fragment. We demonstrated that mis-splicing of mutant huntingtin intron 1 does indeed occur and results in a short polyadenylated mRNA, which is translated into an exon 1 protein.The exon 1 protein fragment is highly pathogenic. Transgenic mouse models containing just human huntingtin exon 1 develop a rapid onset of HD-like symptoms. Our finding that a small, mis-spliced HTT transcript and corresponding exon 1 protein are produced in the context of an expanded CAG repeat has unraveled a new molecular mechanism in HD pathogenesis. Here we present detailed models of how mis-splicing could be facilitated, what challenges remain in this model, and implications for therapeutic studies.

Structural Mechanisms of Mutant Huntingtin Aggregation Suppression by the Synthetic Chaperonin-like CCT5 Complex Explained by Cryoelectron Tomography

“…In addition, the 46Q construct used here has N17 and C38 flanking regions with a C-terminal His tag, whereas the previous construct was composed of N17 and C36 flanking regions with no C-terminal tag (16,25). Both of these constructs represent the newly discovered aberrant splice variants (7,8), which have been described previously as highly toxic (42). Confirming the previous fibril formation and aggregation observations with a new mHTT construct further solidifies these findings.…”

“…These aggregates contain fragments of mHTT protein, including those composed of only the first exon of Htt, the region where the polyglutamine repeat is located (6). Recent studies show that these highly toxic exon 1 fragments are created through aberrant splicing of mHTT mRNA rather than proteolytic cleavage of the full-length mHTT protein (7,8), making the study of exon 1 protein fragments increasingly important. In cell models, large insoluble aggregates have been found as part of inclusion bodies, whereas smaller, soluble aggregates are ubiquitous throughout the cell (3).…”

Integration-independent Transgenic Huntington Disease Fragment Mouse Models Reveal Distinct Phenotypes and Life Span in Vivo

“…Notably, this fragment did not contain the N terminus perhaps suggesting further processing of the protein at both the C and N terminus of the protein. The putative protease of this fragment remains unknown; but recent work has identified the cause of the exon 1 fragment as an aberrant splicing event (19,52). However, it should be noted that our transgenic mouse models produce a product containing less than 120 amino acids; but as they are derived from cDNA, they do not contain the introns that could cause an aberrant splicing event.…”

“…Proteolysis of the Transgenic Rosa HTTQ148 Fragment Mice-There is growing evidence that protein products are generated from the HTT gene either by aberrant splicing or proteolysis after translation with protein products ending between amino acids 90 and 171 (19,20,51,52,63). Taking advantage of the fact that the HTT cDNA generated to make these new models is not likely to be aberrantly spliced, we analyzed the role of proteolytic processing in these new transgenic mouse models of HD.…”

Does the Mutant CAG Expansion in Huntingtin mRNA Interfere with Exonucleolytic Cleavage of its First Exon?

“…Mis-splicing of the transgenic mutant huntingtin can produce a short form of mutant huntingtin transcripts, facilitated by the presence of exon 1 with poly (A) [19]. The scAAV-U6-miRNA- HTT -GFP we have used in this study induces cleavage in exon 48, so that exonucleolytic activity would not affect this aberrant mutant huntingtin mRNA fragment.…”