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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 is 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 HD-hESC lines as well as 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. Furthermore, it provides a new human-specific target for drug screening in Huntington’s disease.

ORGANISM(S): Homo sapiens

PROVIDER: GSE66769 | GEO | 2015/06/10

SECONDARY ACCESSION(S): PRJNA277912

REPOSITORIES: GEO

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Discovery of novel isoforms of Huntingtin reveals a new hominid-specific exon

Project description: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 is 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 HD-hESC lines as well as 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. Furthermore, it provides a new human-specific target for drug screening in Huntington’s disease. We performed RNAseq of human embryonic stem cells in pluripotency conditions to check expression of multiple HTT isoforms.

2015-06-10 | E-GEOD-66769 | biostudies-arrayexpress

In vivo CRISPR base editing for treatment of Huntington’s disease

Project description:Huntington's disease (HD) is an inherited and ultimately fatal neurodegenerative disorder caused by an expanded polyglutamine-encoding CAG repeat within exon 1 of the huntingtin (HTT) gene, which produces a mutant protein that destroys striatal and cortical neurons.Importantly, a critical event in the pathogenesis of HD is the proteolytic cleavage of the mutant HTT protein by caspase-6, which generates fragments of the N-terminal domain of the protein that form highly toxic aggregates. Given the role that proteolysis of the mutant HTT protein plays in HD, strategies forpreventing this process hold potential for treating the disorder.By screening 141 CRISPR base editor variants targeting splice elements in the HTT gene, we identified platforms capable of producing HTT protein isoforms resistant to caspase-6-mediated proteolysis via editing of the splice acceptor sequence for exon 13. When delivered to the striatum of a rodent HD model, these base editors induced efficient exon skipping and decreased the formation of the N-terminal fragments, which in turn reduced HTT protein aggregation and attenuatedstriatal and cortical atrophy.Collectively, these results illustrate the potential for CRISPR base editing to decrease the toxicity of the mutant HTT protein for HD.

2024-07-12 | GSE272134 | GEO

PTMs of HTT protein - Post-translational modifications (PTMs), identified on endogenous huntingtin, cluster within proteolytic domains between HEAT repeats

Project description:Post-translational modifications (PTMs) of proteins regulate various cellular processes. PTMs of polyglutamine-expanded huntingtin (Htt) protein, causative of Huntington’s disease (HD), are likely modulators of HD pathogenesis. Previous studies have identified and characterized several PTMs on exogenously expressed Htt fragments, however none of these studies were designed to systematically characterize PTMs on the endogenous full-length Htt protein.We found that full-length endogenous Htt, immunoprecipitated from HD knock-in mouse and human post mortem brain, is suitable for detection of PTMs by mass spectrometry. Using label-free mass spectrometry, we identified around 40 PTMs, of which half are novel. These findings will be instrumental in the further assembling the Htt PTM framework, and validate PTMs of Htt as promising therapeutic targets for HD.

2017-06-29 | PXD005753 | Pride

ChIP-seq of HTT protein in mouse cortex following HTT knockdown

Project description:Progressive striatal gene expression changes and epigenetic alterations are a prominent feature of Huntington’s disease (HD), but direct relationships between the huntingtin (HTT) protein and chromatin remain poorly described. Here, using chromatin immunoprecipitation and sequencing (ChIP-seq), we show that HTT reproducibly occupies specific locations in the mouse genome, including thousands of genomic loci that are differentially occupied in striatal tissue from a knock-in mouse model of the HD mutation (B6.HttQ111/+) versus wildtype controls. ChIP-seq of histone modifications, generated in parallel, revealed genotype-specific colocalization of HTT with trimethylation of histone 3 lysine 27 (H3K27me3), a repressive chromatin mark. Near genes that are differentially regulated in HD, greater HTT occupancy in HttQ111/+ vs. wildtype mice predicted increased H3K27me3, reduced histone 3 lysine 4 (H3K4me3), a marker of poised and active promoters, and down-regulated gene expression. Altered huntingtin-chromatin interactions may therefore play a direct role in driving transcriptional dysregulation in HD.

2023-05-16 | GSE150748 | GEO

ChIP-seq of histone modifications and EZH2 in the striatum of four-month-old HttQ111/+ and wildtype mice

2023-05-16 | GSE150747 | GEO

Truncation of mutant huntingtin in knock-in mice via CRISPR-Cas9 uncovers exon1 huntingtin as a key pathogenic form

Project description:It remains unknown how polyglutamine expansion in widely expressed proteins can cause selective neurodegeneration. In Huntington’s disease (HD), proteolytic processing generates toxic N-terminal huntingtin (HTT) fragments that preferentially kill striatal neurons. Considerable efforts have been devoted to investigating how HTT is cleaved and whether blocking its cleavage is therapeutically beneficial. However, using CRISPR-Cas9 to truncate full-length mutant Htt in HD140Q knock-in (KI) mice, we found that exon1 Htt is stably present in the brain, regardless of truncation sites in full-length Htt. This N-terminal Htt led to similar HD phenotypes and age-dependent Htt accumulation in striatum in different KI mice. Exon1 Htt is constantly generated but its selective accumulation in the striatum is caused by the age-dependent expression of striatum-enriched HspBP1, a chaperone inhibitory protein. Our findings suggest that tissue-specific chaperone function accounts for the selective neuropathology in HD and highlight therapeutic importance in regulating this function.

2020-04-21 | GSE145879 | GEO

Small molecule activation of a pseudoexon triggers huntingtin-lowering (RNA-Seq)

Project description:Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by abnormal expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the huntingtin gene (HTT). The resultant mutant protein is ubiquitously expressed and drives pathogenesis of HD through a toxic gain-of-function mechanism. Animal models of HD have demonstrated that reducing huntingtin protein (HTT) levels alleviates HD-associated motor and neuropathological abnormalities, confirming the importance of huntingtin-lowering as a therapeutic approach. Several therapies in development repress HTT transcription or translation, including antisense oligonucleotides, virally-delivered microRNAs, and zinc finger protein transcription factors. However, they all require invasive procedures to reach the central nervous system (CNS) and do not distribute evenly to target areas in the brain. Systemically distributed therapeutics are needed to address the CNS and peripheral dysfunctions associated with HD. Here we report the discovery of small molecule splicing modifiers that lower HTT expression by selective modulation of pre-mRNA splicing. These compounds promote the inclusion of a pseudoexon containing a premature termination codon triggering HTT mRNA degradation and a reduction of HTT protein levels in vitro and in vivo. These orally bioavailable small molecules represent a non-invasive treatment option for HD and our findings support their continued development for the treatment of HD.

2021-07-01 | GSE162813 | GEO

Small molecule activation of a pseudoexon triggers huntingtin-lowering (AmpliSeq)

2021-07-01 | GSE162812 | GEO

Depletion of p62 reduces nuclear inclusions and paradoxically ameliorates disease phenotypes in Huntington’s model mice

Project description:Huntington’s disease (HD) is a dominantly inherited genetic disease caused by mutant huntingtin (htt) protein with expanded polyglutamine tracts. A neuropathological hallmark of HD is the presence of neuronal inclusions of mutant htt. p62 is an important regulatory protein in selective autophagy, a process by which aggregated proteins are degraded, and it is associated with several neurodegenerative disorders including HD. Here we investigated the effect of p62 depletion in three HD model mice: R6/2, HD190QG and HD120QG mice. We found that loss of p62 in these models led to longer lifespans and reduced nuclear inclusions, although cytoplasmic inclusions increased with polyglutamine length. In mouse embryonic fibroblasts (MEFs) with or without p62, mutant htt with a nuclear localization signal (NLS) showed no difference in nuclear inclusion between the two MEF types. In the case of mutant htt without NLS, however, p62 depletion increased cytoplasmic inclusions. Furthermore, to examine the effect of impaired autophagy in HD model mice, we crossed R6/2 mice with Atg5 conditional knockout mice. These mice also showed decreased nuclear inclusions and increased cytoplasmic inclusions, similar to HD mice lacking p62. These data suggest that the genetic ablation of p62 in HD model mice enhances cytoplasmic inclusion formation by interrupting autophagic clearance of polyQ inclusions. This reduces polyQ nuclear influx and paradoxically ameliorates disease phenotypes by decreasing toxic nuclear inclusions. Gene expression profiles were analyzed to examine the effects of p62 depletion in mouse with or without mutant huntingtin exon 1

2014-10-10 | GSE62210 | GEO

Experimental replacement therapy counters decreased IGF-1 levels in Huntington’s disease

Project description:Huntington’s disease (HD) is a monogenetic neurodegenerative disorder caused by the expansion of a polyglutamine (polyQ) stretch in huntingtin (htt). Here we show that mutant htt reduces the transcription of insulin-like growth factor 1 (IGF-1) and leads to loss of IGF-1 in HD brains, HD mouse models and mutant htt-transgenic microglial cells. IGF-1 replacement therapy by transplantation of genetically engineered mouse neuronal precursor cells (mNPCs) in a mouse model of HD reverted the motor phenotype and countered striatal neuronal loss.

2018-10-01 | GSE19291 | GEO

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