Discovery of Novel Isoforms of Huntingtin Reveals a New Hominid-Specific Exon

Abstract

Huntington’s disease (HD) is a devastating neurological disorder that is caused by an expansion of the poly-Q tract in exon 1 of the Huntingtin gene (HTT). HTT is an evolutionarily conserved and ubiquitously expressed protein that has been linked to a variety of functions including transcriptional regulation, mitochondrial function, and vesicle transport. This large protein has numerous caspase and calpain cleavage sites and can be decorated with several post-translational modifications such as phosphorylations, acetylations, sumoylations, and palmitoylations. However, the exact function of HTT and the role played by its modifications in the cell are still not well understood. Scrutiny of HTT function has been focused on a single, full length mRNA. In this study, we report the discovery of 5 novel HTT mRNA splice isoforms that are expressed in normal and HTT-expanded human embryonic stem cell (hESC) lines as well as in cortical neurons differentiated from hESCs. Interestingly, none of the novel isoforms generates a truncated protein. Instead, 4 of the 5 new isoforms specifically eliminate domains and modifications to generate smaller HTT proteins. The fifth novel isoform incorporates a previously unreported additional exon, dubbed 41b, which is hominid-specific and introduces a potential phosphorylation site in the protein. The discovery of this hominid-specific isoform may shed light on human-specific pathogenic mechanisms of HTT, which could not be investigated with current mouse models of the disease.

Huntington Disease as a Neurodevelopmental Disorder and Early Signs of the Disease in Stem Cells

“…HTT is an ubiquitously expressed protein [ 113 ]; however, its level increases along with brain development [ 114 ] or following the in vitro differentiation of PSC into neural lineages [ 48 , 85 ]. Moreover, multiple non-canonical HTT isoforms that result from alternative splicing have been described in normal human cells and HD ESC [ 115 ]. One alternatively spliced transcript, which excludes HTT posttranslational cleavage-regulating exon 10, has been downregulated during neuronal differentiation, which suggests a role in development.…”

Chromosomal instability during neurogenesis in Huntington's disease

“…For example, the human forebrain is greatly increased in cell number and complexity, and the normal HTT CAG repeat length is expanded from 7 in rodent to ∼20 in humans. In addition, we recently reported that the human HTT locus encodes multiple mRNA isoforms, thus generating multiple HTT proteins, some of which are present only in higher apes and humans (hominids; Ruzo et al, 2015). Thus, HD cannot be fully studied in existing non-human model systems.…”

Regulatory mechanisms of incomplete huntingtin mRNA splicing

“…The mutation is a CAG repeat expansion in exon 1 of the huntingtin gene ( HTT ) 2 , which translates to a polyglutamine (polyQ) track in the huntingtin protein (HTT). In addition to the three full length isoforms 3 – 5 , other isoforms consisting of a combination of the inclusion of cryptic exons 6 , 7 , retention of parts of introns and alternative splice site usages 7 – 9 have been identified. However, these isoforms are subject to nonsense mediated RNA decay and will have no functional consequences.…”