Huntington Disease and the Huntingtin Protein

Hope Center for Neurological Diseases, Knight-Alzheimer Disease Research Center, Department of Neurology, Washington University in St. Louis, St. Louis, Missouri, USA.
Progress in molecular biology and translational science (Impact Factor: 3.49). 01/2012; 107:189-214. DOI: 10.1016/B978-0-12-385883-2.00010-2
Source: PubMed


Huntington disease (HD) is a devastating neurodegenerative disease that derives from CAG repeat expansion in the huntingtin gene. The clinical syndrome consists of progressive personality changes, movement disorder, and dementia and can develop in children and adults. The huntingtin protein is required for human development and normal brain function. It is subject to posttranslational modification, and some events, such as phosphorylation, can play an enormous role in regulating toxicity of the huntingtin protein. The function of huntingtin in the cell is unknown, and it may play a role as a scaffold. Multiple mouse models of HD have now been created with fragments and full-length protein. The models show variable fidelity to the disease in terms of genetics, pathology, and rates of progression. Pathogenesis of HD involves cleavage of the protein and is associated with neuronal accumulation of aggregated forms. The potential mechanisms of neurodegeneration are myriad, including primary effects of protein homeostasis, gene expression, and mitochondrial dysfunction. Specific therapeutic approaches are similarly varied and include efforts to reduce huntingtin gene expression, protein accumulation, and protein aggregation.

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    • "Elucidating the relationship of different forms of mHtt aggregates to toxicity and to disease progression is thus an important step in the pathway to therapeutic interventions. In order to study HD pathomechanisms considerable effort has been invested in the development of in vitro and in vivo model systems [6] [9]. One way to study the fate of proteins in living cells is to use fluorescent protein (FP) fusions. "
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    ABSTRACT: Huntington's disease is a hereditary movement disorder that is characterized by progressive neuronal cell death mainly in the cortex and striatum of the brain. It is caused by an unstable CAG repeat extension in the first exon of the IT-15 gene which encodes a protein called huntingtin (Htt). The trinucleotide expansion translates into an elongated polyglutamine (polyQ) stretch. A polyQ length of more than 35 glutamine residues is associated with the appearance of huntingtin aggregates and the development of the disease. The process of aggregation is not fully understood but its inhibition and its modulation provide an insight into the mechanisms leading to aggregate formation which might be a target for the treatment of the disease. Using CFP-and YFP-tagged huntingtin exon 1 fragments we established a cellular model that visualizes the process of huntingtin aggregation and in which the aggregates could be specifically detected by FRET microscopy (acceptor photobleaching and fluorescence lifetime microscopy). The time course of the aggregation process was investigated by image analysis. 1. Role of huntingtin aggregates in Huntington's disease Huntington's disease (HD) (OMIM 143100) is a autosomal dominant, age-at-onset, progressive neurodegenerative disorder caused by an expanded (CAG)n repeat in the exon 1 of the huntingtin gene IT-15 located on chromosome 4 [1]. The expansion above the normal range of 6–35 CAG repeats leads to an elongated poly-glutamine (polyQ) tract that causes misfolding and aberrant protein-protein interactions thereby conferring a multifaceted toxic gain of function to the widely expressed huntingtin protein. The age of onset is most critically determined by and inversely correlated with the length of the expanded CAG repeat. HD is characterized by neurodegeneration and formation of neuronal intranuclear and cytoplasmic accumulation of aggregated mutant huntingtin, particularly in the striatum and cortex but also extended to other brain regions. The resulting clinical phenotype summarizes progressive movement dysfunction, cognitive impairments, psychiatric symptoms, and ultimately death. Currently, no cure or therapy for delaying HD-associated symptoms is available. HD belongs to a set of ten, dominantly inherited neurodegenerative disorders, the polyglutamine (polyQ) diseases, each caused by expanded polyglutamine (polyQ) tracts in otherwise unrelated proteins [2, 3]. HD pathophysiological processes are multiple, complex and variable including impairments of transcription, axonal transport, ubiquitin proteasome system and of mitochondrial function. A key feature in HD pathogenesis is the poly(Q) dependent self-association and aggregation of mutant huntingtin proteins and of N-terminal toxic htt peptides generated by proteolytic cleavage. Thereby, aggregates in the nucleus (nuclear inclusions) but also in the cytoplasm, e.g. neuropils, are formed [4, 5]. The mutant huntingtin can undergo different conformations including aberrantly folded monomeric forms, a wide-range of oligomeric species, fibril states, and larger insoluble aggregates [6]. The role of mutant huntingtin aggregation in the pathogenesis of HD as well as the toxic impact of different forms of mutant Htt is intensely discussed. Aggregation-mediated sequestration of proteins with essential cellular functions could be harmful to the cell, whereas a protective mechanism resulting from sequestration of the toxic Htt moiety or other cellular proteins which stimulate mutant Htt clearance would be beneficial. Furthermore, different structures of mHtt aggregates seem to determine the nature of proteins being trapped. Thus, the resulting toxic effects are also driven by whichever cell-specific proteins are present. Altogether, this may account for selective dysfunction and degeneration in HD. As a consequence, the modulation of mHtt aggregation could have beneficial effects on overall toxicity or specific cellular pathways deregulated in HD. This has been successfully shown by the interaction of a specific intrabody with mutant huntingtin leading to increased ubiquitination and clearance of cytoplasmic mHtt as well as a subsequent prevention of mHtt accumulation in neuronal processes and a reduced neurotoxicity [7]. Modulation of the mHtt aggregation process by shifting the equilibrium toward soluble huntingtin was achieved by reducing the level of histone deacetylase HDAC4. The resulted delay in cytoplasmic mHtt aggregation alleviated disease progression and led to improvement of neurological phenotypes in an HD mouse model. These data clearly indicate that cytoplasmic aggregation mechanisms contribute to HD-related neurodegenerative phenotypes [8]. Elucidating the relationship of different forms of mHtt aggregates to toxicity and to disease progression is thus an important step in the pathway to therapeutic interventions. In order to study HD pathomechanisms considerable effort has been invested in the development of in vitro and in vivo model systems [6, 9]. One way to study the fate of proteins in living cells is to use fluorescent protein (FP) fusions. Proteins tagged with FPs often retain their biochemical properties and allow the functional analysis of proteins in living cells. In combination with microscopic techniques FP tags are ideally suited to analyse spatio-temporal processes such Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.)
    Full-text · Chapter · Sep 2014
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    • "In these diseases the predominant hypothesis has been that the expanded polyQ track confers detrimental properties to the protein, that compromise cell homeostasis. The consequences of polyQ expansion in the HTT protein have been systematically characterized, with detrimental effects in transcriptional activity, vesicle trafficking, mitochondrial function and proteasome activity (Zheng and Diamond, 2012). However, the view of a protein-based toxocity in polyQ disorders has been challenged, as recent findings point to an additional toxic effect of the expanded CAG in the exon 1 of HTT mRNA (Banez-Coronel et al., 2012). "
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    ABSTRACT: Trinucleotide-repeat expansion diseases (TREDs) are a group of inherited human genetic disorders normally involving late-onset neurological/neurodegenerative affectation. Trinucleotide-repeat expansions occur in coding and non-coding regions of unique genes that typically result in protein and RNA toxic gain of function, respectively. In polyglutamine (polyQ) disorders caused by an expanded CAG repeat in the coding region of specific genes, neuronal dysfunction has been traditionally linked to the long polyQ stretch. However, a number of evidences suggest a detrimental role of the expanded/mutant mRNA, which may contribute to cell function impairment. In this review we describe the mechanisms of RNA-induced toxicity in TREDs with special focus in small-non-coding RNA pathogenic mechanisms and we summarize and comment on translational approaches targeting the expanded trinucleotide-repeat for disease modifying therapies.
    Full-text · Article · Dec 2013 · Frontiers in Molecular Neuroscience
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    • "Why is it not manifested in macrophages, in which case the disease would have been dominant? If, alternatively, the dominance results from gain of function, then its development depends on accumulation of enough deleterious product (mutant GCase, in our case), as in the case of Alzheimer disease, which displays age dependent accumulation of β-amyloid and tau, or Huntington disease, which exhibits accumulation of huntingtin [72-74]. Our results suggest the gain of function alternative. "
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    ABSTRACT: In Gaucher disease (GD), resulting from mutations in the GBA gene, mutant beta-glucocerebrosidase (GCase) molecules are recognized as misfolded in the endoplasmic reticulum (ER). They are retrotranslocated to the cytoplasm, where they are ubiquitinated and undergo proteasomal degradation in a process known as the ER Associated Degradation (ERAD). We have shown in the past that the degree of ERAD of mutant GCase correlates with GD severity.Persistent presence of mutant, misfolded protein molecules in the ER leads to ER stress and evokes the unfolded protein response (UPR). We investigated the presence of UPR in several GD models, using molecular and behavioral assays. Our results show the existence of UPR in skin fibroblasts from GD patients and carriers of GD mutations. We could recapitulate UPR in two different Drosophila models for carriers of GD mutations: flies heterozygous for the endogenous mutant GBA orthologs and flies expressing the human N370S or L444P mutant GCase variants. We encountered early death in both fly models, indicating the deleterious effect of mutant GCase during development. The double heterozygous flies, and the transgenic flies, expressing mutant GCase in dopaminergic/serotonergic cells developed locomotion deficit. Our results strongly suggest that mutant GCase induces the UPR in GD patients as well as in carriers of GD mutations and leads to development of locomotion deficit in flies heterozygous for GD mutations.
    Full-text · Article · Sep 2013 · Orphanet Journal of Rare Diseases
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