Aberrant splicing of HTT generates the pathogenic exon 1 protein in Huntington disease

Abstract

Huntington disease (HD) is a devastating, late-onset, inherited neurodegenerative disorder that manifests with personality changes, movement disorders, and cognitive decline. It is caused by a CAG repeat expansion in exon 1 of the HTT gene that translates to a polyglutamine tract in the huntingtin protein (HTT). The formation of HTT fragments has been implicated as an essential step in the molecular pathogenesis of HD and several proteases that cleave HTT have been identified. However, the importance of smaller N-terminal fragments has been highlighted by their presence in HD postmortem brains and by the fact that nuclear inclusions are only detected by antibodies to the N terminus of HTT. Despite an intense research effort, the precise length of these fragments and the mechanism by which they are generated remains unknown. Here we show that CAG repeat lengthdependent aberrant splicing of exon 1 HTT results in a short polyadenylated mRNA that is translated into an exon 1 HTT protein. Given that mutant exon 1 HTT proteins have consistently been shown to be highly pathogenic in HD mouse models, the aberrant splicing of HTT mRNA provides a mechanistic basis for the molecular pathogenesis of HD. RNA-targeted therapeutic strategies designed to lower the levels of HTT are under development. Many of these approaches would not prevent the production of exon 1 HTT and should be reviewed in light of our findings.exon 1 huntingtin | huntingtin fragment | mis-splicing | SRSF6

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).…”

Unmasking the roles of N- and C-terminal flanking sequences from exon 1 of huntingtin as modulators of polyglutamine aggregation

“…Neuronal intranuclear inclusions are the pathological hallmarks of HD, and N-terminal fragments (NTFs) spanning exon 1 of the Htt protein are the main constituents of these inclusions (2). The Htt gene with expanded CAG tracts can undergo erroneous splicing, and the resultant aberrant messenger RNA is translated into a mutant exon 1 version of Htt that is similar to toxic NTFs found in neuronal intranuclear inclusions (3). Exon 1 spanning NTFs typically include a polyglutamine tract that is flanked on its N terminus by an amphipathic 17-residue stretch (MATLEKLMKAFESLKSF) denoted as N17 and by a 38-residue proline-rich stretch on its C terminus (P 11 -QLPQPPPQAQPLLPQPQ-P 10 ) denoted as C38.…”

Spontaneous self-assembly of pathogenic huntingtin exon 1 protein into amyloid structures

“…Such protein fragments are generated in HD patients through the aberrant splicing of HTT mRNA [11]. HTTex1 rapidly self-assembles into protein aggregates in cell-free as well as cell-based assays and induces toxicity in various in vivo disease model systems [9;10].…”