Huntington Diseases and the Huntingtin Protein

Huntington Disease and theHuntingtin Protein

Progress in Molecular Biologyand Translational Science, Vol. 107 189DOI: 10.1016/B978-0-12-385883-2.00010-2

Zhiqiang Zheng andMarc I. Diamond

Hope Center for Neurological Diseases,Knight-Alzheimer Disease Research Center,Department of Neurology, WashingtonUniversity in St. Louis, St. Louis, Missouri,USA

I.
I ntroduction ...... .. ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... ..
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G enetic and Clinical Features...... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 90 III. S tructural Features of the Huntingtin Protein .... ... .. ... .. .. ... .. ... .. ... .. ... .. 1 92 IV. H tt Function(s) .... ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 93 V. A nimal Models of Huntington Disease.... ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 95
A. R
6/1 and R6/2 .... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 95 B. N 171-82Q ...... ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 96 C. Y AC Transgenic HD Models..... ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 96 D. B ACHD ...... .. ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 96 E. H D Knock-in ..... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 96 F. O ther Mammalian HD Models..... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 97
VI.
P athogenesis of HD ...... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 97 A. P roteolytic Cleavage ..... ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 97 B. M isfolding and Aggregation of Htt .... ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 97 C. T ranscriptional Dysregulation ...... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 1 99 D. M itochondrial Dysfunction..... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 00 E. D efects in Htt Clearance: UPS and Autophagy (See Chapter ‘‘Protein
Quality Control in Neurodegenerative Disease’’) .... .. .. ... .. ... .. ... .. ... ..
201 VII. T herapeutic Targets for HD ........ .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 01
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ranscription ..... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 02 B. M itochondrial Dysfunction..... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 02 C. T argeting Mutant Htt Protein ...... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 03
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III. C onclusion..... ... .. ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 04 R eferences..... ... .. ... .. ... .. .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. .. ... .. ... .. ... .. ... .. 2 05
Huntington disease (HD) is a devastating neurodegenerative disease thatderives from CAG repeat expansion in the huntingtin gene. The clinicalsyndrome consists of progressive personality changes, movement disorder,and dementia and can develop in children and adults. The huntingtin proteinis required for human development and normal brain function. It is subject toposttranslational modification, and some events, such as phosphorylation, canplay an enormous role in regulating toxicity of the huntingtin protein. Thefunction of huntingtin in the cell is unknown, and it may play a role as ascaffold. Multiple mouse models of HD have now been created with fragmentsand full-length protein. The models show variable fidelity to the disease in

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190 ZHENG AND DIAMOND

terms of genetics, pathology, and rates of progression. Pathogenesis of HDinvolves cleavage of the protein and is associated with neuronal accumulationof aggregated forms. The potential mechanisms of neurodegeneration aremyriad, including primary effects of protein homeostasis, gene expression,and mitochondrial dysfunction. Specific therapeutic approaches are similarlyvaried and include efforts to reduce huntingtin gene expression, protein accu-mulation, and protein aggregation.

I. Introduction

Huntington disease (HD) is an autosomal dominant neurodegenerativedisorder caused by the expansion of the CAG codon repeat within the hun-tingtin gene. It affects approximately 7 per 100,000.1 CAG encodes the aminoacid glutamine, so the expansion results in an abnormally long glutamine tractwithin the N-terminus of the huntingtin protein (Htt). In addition to HD, eightother dominantly inherited neurodegenerative diseases have been described,each caused by CAG expansion. These disorders are collectively termed ‘‘poly-glutamine’’ diseases. They are prototypical protein misfolding disorders—in allcases studied, the expanded polyglutamine tracts destabilize the native proteinconformation. Penetrance is virtually 100%, with the exception of spinobulbarmuscular atrophy, a motor neuron disease and sensory neuronopathy caused bypolyglutamine expansion in the androgen receptor.2–4 This disease afflicts onlymales because dihydrotestosterone initiates pathogenesis.5

II. Genetic and Clinical Features

Normal individuals have fewer than 36 CAG repeats in the huntingtin geneand most commonly about 15–25 repeats. Repeats between 27 and 35 areunstable during meiosis, which can lead to new expansion of CAG repeats intothe disease range in subsequent generations. When repeats are present in thepathogenic range, repeat expansion inherited by children leads to ‘‘geneticanticipation,’’ or earlier disease onset than was observed in the parent. This isusually seen when fathers pass the mutant gene to their children, which may bedue to the fact that repeat instability is observed during spermatogenesis.6

Virtually all individuals with 38 or more CAG repeats are predicted to developHD symptoms if they live long enough. However, individuals with fewer than40 repeats may only manifest symptoms at a late age.7 The length of the

HUNTINGTON DISEASE AND THE HUNTINGTIN PROTEIN 191

expanded CAG repeats correlates inversely with the age at symptom onset.8,9

Extremely long CAG repeats (>60) are highly associated with early onset HD,also known as juvenile HD. It is estimated that CAG repeat length accounts forapproximately 70% of the variance in age of onset,8,10 which means that othergenetic and environmental factors also play an important role in determiningpathogenesis.

HD is particularly devastating because of its multiple cognitive and motorfeatures, which have a relentless course over 10–20 years. Juvenile HD is veryrapid, and the affected children seldom survive into adulthood. The generalfeatures of each stage of disease are detailed below, but the timing andconstellation of features are quite variable among patients. Early HD symp-toms relate to frontal/executive dysfunction and include progressive person-ality changes, for example, irritability, and difficulty with multitasking andprioritization of activities. Psychiatric disturbances, especially depression, aswell as slight involuntary movements and poor coordination may also bepresent.1 In the middle phase of the disease, in addition to progressivecognitive disturbances, a movement disorder becomes more prominent,with frequent involuntary twitching, and writhing movements of the face,trunk, and extremities termed ‘‘chorea.’’ Disturbances of gait manifest, withfrequent falls.11 A thought disorder may also appear, with bizarre ideationthat can merge into frank psychosis.12 In the later stages of the disease, choreacan become quite severe, and a hypermetabolic state produces progressiveweight loss.13 Difficulty in swallowing leads to choking and coughing epi-sodes. At this point, the cognitive disturbance is severe enough to producedisability. Patients are unable to maintain their usual employment, and oftenrequire significant caretaking, if not institutionalization.1 In the final stages,patients become bedbound and usually succumb to wasting and aspirationpneumonia.

The Htt protein is widely expressed and causes pathology throughout thebrain. This has been readily observed using serial MRI in presymptomatic andsymptomatic patients.14 Striking atrophy occurs in the striatum, especially thecaudate and putamen. Pathology here involves vulnerable GABAergicmedium-sized spiny neurons.15 Striatal atrophy develops long before symptomonset, with �50% reduction in striatal volume at the time of diagnosis.16,17 Yetit is misleading pathologically to look solely to the striatum. HD also affectsother brain regions, including cerebral cortex (layers III, V, and VI), hippocam-pus, thalamus, globus pallidus, subthalamic nucleus, substantia nigra, whitematter, and cerebellum.7,18 The involvement of so many brain structures helpsexplain the highly variable clinical presentations of the disease. The cloning ofthe Htt gene made it possible to recognize that Htt fragments accumulate asintracellular inclusions in multiple brain regions,19 highlighting the role ofprotein misfolding and aggregation in this disorder.


III. Structural Features of the Huntingtin Protein

Htt and its homologs are evolutionarily conserved among many vertebratesand invertebrates. For example, the coding region of Htt from humans and theFugu fish is 69% identical at the nucleotide level.20 The gene-encoding humanHtt spans over 180kb, with 67 exons. Human Htt is ubiquitously expressed,with the highest levels in the central nervous system.21 The human Htt geneconsists of two alternatively polyadenylated forms, which differ in size by 3kb.The longer transcript is predominantly expressed in brain, whereas the shorterone is more widely expressed.22,23 Within the cell, Htt is associated withmultiple organelles, including the nucleus, plasma membrane, endocytic andautophagic vesicles, endoplasmic reticulum, endosome, Golgi, and mitochon-dria.24 This may reflect a role as a scaffold protein that links multiple factorsand is consistent with its large, multidomain structure. Htt is required fornormal embryonic development.25,26

Full-length Htt is composed of �3114 amino acids, producing a protein of�350kDa. Due to its size, it has not yet been possible to generate full-lengthHtt crystals. However, the structures of several domains have been described inrelative detail. To elucidate the role of the N-terminal polyglutamine tract innormal Htt function, an ablation of the seven CAG repeats in the mouse genewas performed.27 Development was normal (unlike knockout of the Htt gene,which is embryonic lethal), and adult mice exhibited only subtle defects inlearning and memory. This suggests that the polyglutamine stretch contributesto the subtle regulation of neural function by Htt but is not required for Htt’sessential functions.

Htt also contains HEAT (Htt, elongation factor 3, the PR65/A subunit ofprotein phosphatase 2A and the lipid kinase Tor) repeats. HEAT domainsconsist of �50 amino acids and comprise two antiparallel a-helices forming ahairpin that is normally involved in protein–protein interactions. A total of 16HEAT repeats have been identified in Htt, organized into four clusters.28,29

The biological role of HEAT domains in Htt function is unknown.Wild-type Htt is normally found in the cytoplasm, but mutant Htt accu-

mulates within the nucleus, especially as N-terminal truncation frag-ments.19,21,30–33 A conserved nuclear export signal (NES) in the C-terminushas been identified.34 Another potential NES exists in the N-terminal 17 aminoacid region that has been demonstrated to interact with the nuclear poreprotein (TPR), which is involved in the export of proteins from nucleus intocytoplasm.35

Multiple proteolytic cleavage sites have been identified in Htt, which likelygenerate a wide range of N-terminal fragments. Putative Htt proteases includecaspases, calpain, aspartic endopeptidases, and matrix metalloproteinases. Httis cleaved by caspase 3 in vitro at amino acids 513 and 552,36,37 caspase 2 at


amino acids 552,38 and caspase 6 at amino acids 586.37 Two putative calpainsites are located at amino acids 469 and 536.39,40 The matrix metalloproteinase,MMP-10, which is active in HD mice, cleaves Htt at amino acid 402.41 Httfragments with mutated cleavage sites, which are resistant to the cleavage bysome of these enzymes, exhibit less toxicity in vitro and in vivo, suggesting thatproteolytic processing of full-length Htt is probably an important event in theinitiation of HD pathogenesis.36,42,43

Htt also has many posttranslational modifications, including ubiquitination,phosphorylation, acetylation, sumoylation, and palmitoylation. These could reg-ulate its stability, localization, and function. Ubiquitination targets Htt for degra-dation.30,44 Three lysines in its N-terminus, K6, K9 and K15, have beenidentified as sumoylation sites.45 While ubiquitination of Htt may serve a protec-tive role by increasing its clearance, Htt sumoylation at these lysines exacerbatespolyglutamine-mediated neurodegeneration in a Drosophila model of HD.45

Many phosphorylation sites in the Htt N-terminus also have been identified.Phosphorylation of threonine 3 may affect Htt aggregation and toxicity.46 It hasalso been observed that the IkappaB kinase (IKK) complex phosphorylates Htt atserine 13 and 16, which can promote the modification of the adjacent lysines andenhance Htt clearance. Expansion of the polyglutamine tract may reduce Httphosphorylation, helping promote accumulation of mutant Htt.47 Additionally,the importance of these residues has been clearly shown in a full-length Httmouse model based on bacterial artificial chromosome (BAC) expression. In thiswork, phosphomimic aspartate substitutions at serines 13 and 16 completelyprevented polyglutamine-mediated toxicity, indicating that these amino acidsmay be very important determinants of pathogenesis.48 Several other phosphor-ylation sites in Htt have been identified, including S421 by Akt; S434, S1181, andS1201 by Cdk5t.49–52 Further studies, especially using phosphomimetic muta-tions in full-length mouse models, will be needed to clarify the role of phosphor-ylation at these sites in controlling Htt function and toxicity. Additionally, it hasbeen shown that Htt acetylation at lysine 444 is required for degradation by themacroautophagy pathway.53 Finally, palmitoylation at C214 by HIP14 may alterHtt aggregation. Polyglutamine tract expansion decreases the interaction be-tween HIP14 and Htt, resulting in decreased palmitoylation, increased inclu-sions, and enhanced neurotoxicity (Fig. 1).54

IV. Htt Function(s)

Htt probably plays multiple roles in development and physiology. It isclearly essential for embryonic development, as Htt gene knockout is lethalin mice by embryonic day 8.5.25,26 Increased cellular death in Htt knockoutanimals may indicate an anti-apoptotic role for Htt, which is supported by the

N17 polyQ polyP NESHEATs

Proteolyic cleavage

HEATs HEATs

114–413 672–969 2667–2938400–600 2397

Phosphorylation:T3, S13, S16Ubiquitination/sumoyalation: K6, K9, K15NES: L4, L7, F11, L14

Phosphorylation421/434

Palmitoylation214

Phosphorylation1181/1201

Acetylation444

FIG. 1. Diagram of regions of the Htt protein. Full-length htt is composed of �3144 aminoacids, with a molecular weight of 350kDa. The N-terminal 17 amino acid (N17) region is highlyconserved and contains multiple posttranslational modifications, such as phosphorylation andubiquitination, which could strongly affect the pathogenesis. Following the N17 region are thepolyglutamine (polyQ) and polyproline (polyP) regions. The function of polyproline is still largelyunclear. Studies have suggested a hot spot of proteolyic cleavage exists between amino acids 400and 600. Putative proteases include caspases 2, 3, 6, calpain, and matrix metalloproteinase 10. Up to16 HEAT repeats with unknown functions have been identified in Htt. Multiple regions ofphosphorylation have been identified. The C-terminus contains a leucine-rich nuclear export signal(NES), which may target full-length Htt to cytoplasm. Another leucine-rich NES could exist in theN17 region.


findings that overexpression of full-length wild-type Htt in cultured striatalneurons protects them from apoptosis, whereas depletion of wild-type Httmakes neurons more sensitive to apoptotic insults.55–57 The molecular mecha-nism of the anti-apoptotic function of Htt is unclear. One possible mechanismis that Htt can bind and prevent the formation of the pro-apoptotic Hip1-HIPPI (Hip1 protein interactor) complex.58 Another possible mechanism isthat Htt can directly block the activation caspase 3 and 9.55,59

Many studies have implicated Htt in transcription. Htt interacts withvarious transcription factors and transcriptional regulatory proteins. For exam-ple, Htt has been proposed to regulate the production of the brain-derivedneurotrophic factor (BDNF).60,61 BDNF transcription is suppressed by thebinding of the repressor element-1 silencing transcription factor (REST)/neuro-restrictive silencer factor (NRSF) to its upstream DNA elementneuron-restrictive silencer element.62,63 Htt acts as positive regulator ofBDNF transcription by preventing the recruitment of REST/NRSF to itsDNA response element. Evidence suggests Htt may not interact directly withREST/NRSF, but rather forms a complex with HAP1 and RILP, which candirectly bind REST/NRSF.63 It is unclear which, if any, of the HD symptomsmight result from the effect on BDNF.

By interacting with a variety of endocytic/trafficking proteins, such as a-adaptin, HIP1, HIP14, HAP1, HAP40, PACSIN1 and SH3GL3, clathrin, anddynamin, Htt also may participate in long- and short-range axonal transport and


vesicle trafficking. For example, in addition to the role of transcriptionalregulation of BDNF, full-length wild-type Htt also stimulates BDNF vesiculartrafficking in neurons,64 while knockdown of Htt reduces such transport.65 Httis enriched at synaptic terminals and interacts with cytoskeletal and synapticvesicle proteins to regulate synaptic activity in neurons. By interacting with theSH3 domain of PSD95, a key protein in synaptic transmission, Htt mightregulate synaptic plasticity.66

V. Animal Models of Huntington Disease

With the cloning of the HD gene, multiple model organisms, such asCaenorhabditis elegans (worm), Drosophila (fruit fly), and Danio rerio (zebra-fish), have now been used to model polyglutamine toxicity.67–71 These areadvantageous in their rapid development of pathology and low cost. Theyalso exhibit many features of the human disease, such as progressive neuronalcell death, reduced lifespan, aggregation of Htt, and formation of inclusions.These models are especially useful in the identification of pathways involved inHD and the screening of potential suppressors of HD pathology. However, it isstill uncertain to what extent these will predict effects of chemical and geneticmodifiers of pathology in mammals. Thus they are probably best used toidentify candidates for further study in mammalian systems, rather than tovet candidates identified in mammalian systems.

Many mouse HD models now exist, both transgenic and knock-in. A listfollows of the commonly used models.

A. R6/1 and R6/2
The transgenic R6/1 and R6/2 HD mouse lines contain �1kb of human
HD promoter region followed by exon 1 of the Htt gene, carrying 115–156CAG repeats and 262bp of intron 1.19,72 R6/2 mice develop progressive neuro-logical phenotypes, such as reduced brain and striatal volume, extensive intra-neuronal inclusion formation, and relatively little cell death. Mice exhibit aresting tremor and shuddering movements as early as 5weeks of age. They haveprogressive motor deficits and seizures. The phenotype is rapidly progressive,and these animals die between 12 and 18 weeks, depending on the laboratorysetting and strain background. Their predictable onset and rapid course hasmade them a mainstay of preclinical drug trials. However, their distribution ofpathology is not anatomically faithful to human disease, and thus they mayserve best as a model of ‘‘polyglutaminopathy’’ (which is still very useful), ratherthan true HD. The R6/1 line has a more slowly progressive phenotype than theR6/2 line.


B. N171-82Q
The N171-82Q transgenic HD mouse line was generated by expressing a
cDNA encoding an N-terminal fragment (171 amino acids) of huntingtin with82 glutamines under the control of the mouse prion promoter.73 Starting at theage of 10 weeks, these mice develop behavioral abnormalities including coor-dination loss, tremors, hypokinesis, and abnormal gait. Intranuclear inclusionbodies and neuritic aggregates are also observed in multiple types of neurons.

C. YAC Transgenic HD Models
Yeast artificial chromosome (YAC) transgenic HD mice were generated by
expressing the full-length human Htt gene with varying numbers of CAGrepeats.74 YAC46 and YAC72 were first created. YAC72 mice show earlyelectrophysiological abnormalities and selective degeneration of mediumspiny neurons by 12 months. Interestingly, neurodegeneration in these miceoccurs in the absence of obvious Htt inclusion formation. To create more rapiddisease onset and reduce inter-animal variability, YAC128 was generated (128CAG repeats).33,75 This line exhibits uniform phenotypes as early as 3 monthsof age. A progressive motor deficit on the rotarod occurs at the age of 6 months.Severe neurodegeneration occurs, with striatal atrophy at 9 months, and corti-cal atrophy at 12 months of age.

D. BACHD
Recently, new full-length transgenic HD mice have been generated via the
BAC technique. Like the YACmodels, the BAC transgenic allows expression ofthe human Htt gene within the context of the surrounding genomic regulatoryelements. The BAC readily enables modification of the Htt gene within thiscontext, which facilitates the study of various mutants. In BACDHDmice, full-length human Htt with 97 CAG repeats is expressed under the control ofendogenous Htt regulatory elements. These mice exhibit progressive motordeficits in rotarod performance starting as early as 2 months of age, neuronalsynaptic dysfunction, and late-onset selective neurodegeneration in both stria-tum and cortex.76 This model has also been used with great success to definethe effect of specific N-terminal mutations in the Htt gene.48

E. HD Knock-in
In transgenic mice, the gene of interest is randomly inserted into the
genome. This may interfere with the activity of other genes. Also, overexpres-sion of the transgene may produce artifactual results. To overcome thesedrawbacks, ‘‘knock-in’’ HDmice have been generated in which the endogenousmouse Htt gene is replaced by a mutated Htt that carries expanded CAGrepeats. The knock-in models are generally considered the most ‘‘faithful’’ tothe human disease in that mutated Htt is placed in the appropriate context


within the mouse genome and expression is controlled by the endogenous Httpromoter. However, knock-in HD mice generally exhibit subtle phenotypes.Neuropathology is observed at a very old age, even with the presence of a verylong polyglutamine tract.77–79 While these animals may be less useful forpreclinical studies, they may be very useful for studies of more subtle HDphenotypes, for example, network dysfunction or early metabolic changes.

F. Other Mammalian HD Models
A transgenic rat HD model has been generated by expressing a truncated
Htt fragment carrying 51 CAG repeats under the control of the endogenous ratHtt promoter.80 This model exhibits adult-onset neurological phenotypes,slowly progressive motor dysfunction, and formation of intracellular inclusionsin neurons. In vivo imaging has demonstrated significant striatal shrinkage,suggesting this model could be useful for preclinical imaging studies. A trans-genic monkey model has been developed by expressing Htt exon 1 with 84CAG repeats fused to the green fluorescent protein (GFP). This model showsextensive neuronal inclusions and mimics some important clinical features ofHD, such as dystonia and chorea (Table I).81

VI. Pathogenesis of HD

A. Proteolytic Cleavage
The proteolysis of full-length of Htt, which results in N-terminal fragments
containing the polyglutamine tract, appears to be a crucial step for progressionof HD. The truncated N-terminal fragments tend to display more aggregationand neuronal toxicity than full-length Htt. The presence of these truncatedN-terminal fragments in both HDmice and patients has been shown.31,40,82 Sofar, a variety of proteases that cleave Htt have been identified, includingcaspases 2, 3, 6, 7, calpains, and aspartic endopeptidases. Some proteaseinhibitors, or mutations that block Htt cleavage, have been demonstrated toslow HD progression in mice.37,42 Cleavage of Htt could produce a repertoireof fragments, each with a distinct propensity to cause disease. Further work willdetermine the specific role of these different truncated N-terminal fragmentsin HD progression, and to what extent a single protease can be targeted to slowpathogenesis.

B. Misfolding and Aggregation of Htt
The role of protein aggregation in the pathogenesis of HD is a confusing
and controversial area. Htt inclusions are formed within cells. They comprisemacromolecular aggregates of Htt, but the distribution of aggregate types andstructures is poorly understood. Aggregates of many types have been

TABLE IMOUSE MODELS OF HUNTINGTON DISEASE

Model Promoter ConstructPolyglutaminelength Age of onset Neuropathology Behavior phenotype

R6/1 Human Htt �1.9kb fragment of the50-human Htt gene

115 for R6/1 5 months forR6/1

Intranuclear and neuropilaggregates throughout thebrain; global brainatrophy; minimal celldeath

Tremors and gaitabnormalities; rotaroddeficit; clasping behavior;learning deficit

R6/2 144 for R6/2 2 months forR6/2

N171-82Q Mouse prion N-terminal 171 aminoacids of human Htt

82 5 months Inclusions in striatum,cortex, hippocampus andamygdala; degeneration instriatum

Tremors and gaitabnormalities; rotaroddeficit; loss ofcoordination; hypokinesis

YAC128 Human Htt YAC expressing full-length human Htt

128 3 months Inclusions in striatum;neuron loss in striatum.

Rotarod deficit; clasping; gaitabnormalities; circlingbehavior

BACHD Human Htt BAC expressing full-length human Htt

97 3 months Synaptic dysfunction;cortical and striatalatrophy.

Rotarod deficit

HdhQ92–111 Mouse Htt Replace mouse Htt exon1 with a humanmutant exon 1

92–111 20 months No brain atrophy andneuronal loss; gliosis instriatum

No obvious behavior deficit

HdhQ140 Mouse Htt Replace the endogenouspolyglutamine withexpandedpolyglutamine

140 12 months Nuclear and neuropilinclusions in striatum,cortex, nucleusaccumbens, and olfactorytubercle

Increased locomotor activityand rearing at 1 month ofage; hypoactivity and gaitabnormalities later

HdhQ150 Mouse Httpromoter

Replace the endogenouspolyglutamine withexpandedpolyglutamine

150 4 months Nuclear inclusions instriatum; striatal gliosis

Clasping behavior; gaitabnormalities; rotaroddeficit; hypoactivity


described. Fibrils are long, ordered, para-crystalline polymers of Htt peptidesthat are readily sedimented by centrifugation. Oligomers are small, orderedstructures that may range in size from two to possibly hundreds of mono-mers. Other ordered structures also may exist (e.g., annuli). In addition,amorphous aggregates form that lack ordered structure. There is also likelyto be considerable conformational diversity even among these structures. Anemerging consensus holds that oligomers, rather than large fibrils or intracel-lular inclusions, are the toxic agents in HD. It is clear that Htt formsinclusions in patients, but their prevalence within affected regions is notcompletely certain.19,83–85

Expanded polyglutamine Htt is aggregation-prone in vitro and in vivo, anda similar polyglutamine length threshold exists for in vitro aggregation and forin vivo toxicity in patients.86 Aggregation of Htt fragments depends highly onthe length of polyglutamine, fragment size, and intracellular protein interac-tors.87–90 Polyglutamine expansion could promote aggregation by facilitating aconformational transition to a novel aggregation-prone form of Htt. Either amonomer or an oligomer then could serve to nucleate (seed) aggregate growth,producing structures rich in b-sheets.91

It is very difficult to dissociate Htt cellular toxicity from its ability toaggregate using site-directed mutagenesis—mutations in Htt that block aggre-gation also block its toxicity.92 Yet, it is clearly possible to dissociate inclusionformation from toxicity.93 Although in vivo studies have shown an association ofinclusions with HD progression,72,94 several lines of evidence also suggest littlecorrelation between inclusions and behavioral dysfunction or neuronal loss.74–76

In addition, in some HD animal models, little inclusion formation is observedduring HD progression, which has caused some to argue that aggregation maynot be required for HD pathogenesis. However, it is likely that ‘‘micro-aggregates,’’ or oligomers, are present before the appearance of large proteindeposits. Emerging in vitro and in vivo evidence has shown the existence ofsuch Htt oligomers, which appear to be more toxic than Htt fibrils, and are thuslikely to be a principal pathogenic agent.83,84,95,96

C. Transcriptional Dysregulation
A number of studies have documented alterations of gene expression that
are associated with HD pathogenesis. The challenge has been to sort outpathogenic changes, that is, those that, if corrected, would alter the course ofdisease, from epiphenomena which simply reflect metabolic changes in sickcells. Htt with expanded polyglutamine has been reported to bind and changethe activity of many transcription factors and cofactors, including CREB-binding protein (CBP), p300/CBP-associated factor (p/CAF), p53, sp1,TAFII130, and PQBO1.97,98 Htt binding may sequester these transcriptionalfactors/cofactors away from their normal DNA elements. For example, binding


of mutated Htt to CBP leads to relocation of CBP from the nucleus into Httinclusions.99 Another well-characterized transcription factor, REST/NRSF,binds the NRSE within the BDNF promoter to suppress its transcription.62,63

Wild-type Htt binds cytosolic REST/NRSF and prevents its translocation tothe nucleus, whereas Htt with expanded polyglutamine interacts with REST/NRSF less effectively, resulting in the accumulation of REST/NRSF in thenucleus. Mutated Htt also occupies the promoter region of PGC-1a by asso-ciating with the CREB–TAF4 complex. This is reported to suppress expressionof PGC-1a. Reduced PGC-1a mRNA expression has been found in bothmouse and human HD brains and may partly account for the impaired energymetabolism observed in HD.100 So far, it has not been possible to demonstratea single gene or group of genes whose elimination or overexpression couldreplicate the HD phenotype, so that the question of whether HD truly derivesfrom a primary genetic disturbance remains uncertain.

D. Mitochondrial Dysfunction
As with transcriptional dysregulation, it is not yet clear whether the mitochon-
drial dysfunction found inHD is a cause, or simply an epiphenomenon, of neuronaltoxicity. Indeed, mitochondrial dysfunction and oxidative stress have been impli-cated in a variety of neurodegenerative diseases.101,102 Mitochondria are an impor-tant source of reactive oxygen species (ROS) and also are a key target for ROSdamage. ROS can cause peroxidation of lipids and oxidative damage of DNA byforming 8-hydroxy-20-deoxyguanosine (8-OHdG). Increased striatal lipid peroxi-dation is found to parallel the neurological phenotype in HD transgenic mice.103

Oxidative damage to DNA also is found in both HD patients and transgenicmice.104,105 Mutated Htt has been reported to damage mitochondria by multiplemechanisms, including reduction of energy production and membrane potential,alteration in calcium homeostasis, enhancement of oxidative stress, and triggeringof the caspase-mediated apoptotic pathways.106–109 These mitochondrial defectsmight be caused by an interaction betweenmutantHtt and the outermembrane ofmitochondria.109–111Htt could also indirectly impactmitochondrial biogenesis andfunction through effects on PGC-1a gene expression.

It has been demonstrated that mitochondrial energy metabolism is im-paired in both HD patients and HDmouse models.112,113 Decreased activity ofseveral important enzymes involved in energy metabolism has been reported,including respiratory chain complexes I, II, III, and IV.114–116 Reduction ATPproduction and energy failure could make striatal cells more vulnerable toother pathological insults. Mitochondria isolated from cells expressing expand-ed Htt also have decreased membrane potential. Neuronal cells derived fromknock-in HD mice HdhQ111 exhibit high sensitivity to the calcium-inducedpermeability transition pore opening. It has also been demonstrated that anexpanded N-terminal Htt fragment, but not wild-type Htt, significantly


decreases the calcium concentration threshold necessary to trigger the mito-chondrial permeability transition pore opening and increases the propensity forcalcium-induced cytochrome c release.110 Mitochondria from clonal striatalcells expressing mutated Htt also show a reduced buffering capacity for calci-um uptake.117

E. Defects in Htt Clearance: UPS and Autophagy(See Chapter ‘‘Protein Quality Control inNeurodegenerative Disease’’)
The ubiquitin–proteasome system (UPS) mediates attachment of ubiquitin
to target proteins and their subsequent degradation by the proteasome. Dys-function of the UPS in HD has been demonstrated.30,118,119 This may be linkedmore broadly to general defects in protein quality control that are induced by asingle misfolded species.120 Htt aggregates are modified with ubiquitin andcolocalize with proteasome subunits, suggesting that aggregated or misfoldedHtt may be targeted for degradation by the UPS. It has been proposed thatsequestration of UPS components into Htt inclusions alters their subcellularlocalization and thus the efficiency of the UPS.19,30 Alternatively, polyglutamine-containing proteins may not be degraded efficiently within the proteasome, andexpanded polyglutamine sequences trapped in the proteasome may block theentry of other substrates into the barrel of its catalytic core.121 Finally, it has beenproposed that inefficient degradation of expanded polyglutamine tracts may leadto the production of aggregation-prone fragments.119,122 The importance of Httdegradation in the aggregation process is also linked to its effect on protein levels.Proteasome inhibitors increase the formation of Htt inclusions, whereas en-hancement of proteasome degradation by overexpression of heat shock proteinsor chaperones decreases Htt aggregation.123–125

Multiple studies have now shown that autophagy also plays a role in theclearance of huntingtin aggregates.126,127 Activation of autophagy, either bychemical treatments (e.g., rapamycin) or by overexpression of autophagy com-ponents, increases the clearance of mutated Htt and reduces its aggrega-tion.126,128 Posttranslational modification of the mutant Htt by acetylation atlysine residue 444 (K444) also enhances its clearance by increasing Htt deliveryto the autophagosome.53

VII. Therapeutic Targets for HD

There are no effective therapies for HD, and treatments are limited tocontrolling psychiatric symptoms and chorea. Various therapeutic strategies arenow proposed but are still in their experimental stages.


A. Transcription
Alterations in gene expression in animal models of HD have led some to
propose that correcting these deficits will rescue neuronal dysfunction anddegeneration. BDNF is perhaps the most compelling target.60,129,130 Likewise,disruption of PGC1-a transcription by Htt has also been proposed as a mecha-nism for defective mitochondrial function.100,131,132 For most transcriptionalchanges, however, it remains unclear whether they represent a cause or conse-quence of HD pathology. In certain cases, gene expression alterations might infact be adaptive. Mutant Htt has been proposed to disrupt the activity of manykey transcriptional factors or transcription-regulatory proteins, generally lead-ing to the downregulation of target gene expression. A number of compoundshave been proposed to restore normal transcription activity in HD animalmodels. For example, HDAC inhibitors such as suberoylanilide hydroxamicacid (SAHA), sodium butyrate, and phenylbutyrate have been administeredto experimental animals,133–137 but the enzymatic targets of these compoundsremain elusive, and may not be acetyl transferases that are directly related totranscription. Indeed, some studies suggest that HDAC inhibitors may beneuroprotective by increasing acetylation of other nonhistone proteins, suchas tubulin,138 or by increasing levels of heat shock proteins.139 Other effectsof HDAC inhibitors, such as prevention of Htt aggregation, anti-inflammation, and anti-apoptotic properties, may also mediate their neuro-protection in HD.140,141 Further studies are needed to clarify the contribu-tions of epigenetic modifications and nonhistone effects when applyingHDAC inhibitors as HD therapies. HDAC inhibitors are of limited potentialbecause of their high toxicity, and thus identification of more specific targetscould improve their utility.

B. Mitochondrial Dysfunction
Mutant Htt, directly or indirectly, is associated with mitochondrial dys-
function, resulting in impaired energy production, increased intracellularcalcium, oxidative stress, and apoptosis. Creatine, which stimulates mito-chondrial respiration and has antioxidant properties, has alleviated symptomsin mouse HD models.142,143 Coenzyme Q10, which promotes energy produc-tion, is also neuroprotective in R6/2 HD mice.144,145 However, clinical trialswith these compounds did not show significant benefits in HD patients.146,147

While PGC1-a, a transcriptional regulator of mitochondrial biogenesis andrespiration, has been implicated as a potential target of HD therapy,100,131,132

it is not clear yet how this pathway could be targeted specifically with smallmolecules.


C. Targeting Mutant Htt Protein1. PROTEOLYSIS

If Htt proteolysis initiates pathogenesis, then compounds that decrease itscleavage could have therapeutic value. Caspase 6 has been implicated as apotential therapeutic target, since mutation of a cleavage site for this proteasereduced toxicity of the full-length protein.42 No inhibitors of this enzyme areyet in clinical trials. Minocycline had small effects when administered to R6/2mice, improving lifespan.148,149 This compound may inhibit caspase activity,although it has been shown to produce benefits only in a mouse model thatexpresses an Htt fragment, and thus it is unlikely to function via preventing Httcleavage. Nonetheless, a clinical trial with minocycline is currently beingconducted (www.huntington-study-group.com) because this is an FDA-approved drug. If specific proteases are clearly implicated in Htt proteolysis,this may create new opportunities to design more targeted therapies.

2. PROTEIN CLEARANCE

Activation of autophagy may enhance clearance of aggregated Htt and offerprotection in HD.150 A principal challenge in developing therapies will be toactivate components of this pathway that are selective for protein aggregation,without inducing more general toxicity. The mTOR inhibitor rapamycin, whichinduces autophagy, has been shown to be beneficial in cell, Drosophila, andmouse HD models.126,151,152 However, strong side effects, especially immuno-suppression, prevent its chronic use in HD patients. Recently, new compoundswere described to selectively stimulate the clearance of mutant Htt; however,the underlying molecular mechanisms remain elusive.153–155 Given the wide-spread role of protein aggregation in neurodegenerative diseases, this mode oftherapy could be applicable more generally.

3. PROTEIN AGGREGATION

Much effort has been devoted to the discovery of compounds that inhibitHtt aggregation. Many compounds that block Htt aggregation directly orindirectly have been identified in cell-based assays and in Drosophila mod-els.156–159 However, effects of these compounds in HDmouse models are so farmodest, at best.156,157,160,161 This may be due to poor bioavailability, and in thecase of compounds that stoichiometrically inhibit Htt aggregation via directinteraction, it may be especially difficult to achieve sufficient brain concentra-tions without concurrent toxicity. Induction of molecular chaperones, such asHsp70, may promote protein refolding or degradation of mutant Htt.162–164

It remains unclear, however, whether global induction of chaperones can beaccomplished safely on a chronic basis. A better understanding of protein

http://www.huntington-study-group.com


interactions and regulatory pathways that control Htt misfolding and aggrega-tion could help create more effective therapies using this strategy, as the targetsnow are somewhat limited.

4. GENE THERAPY

Gene therapy using RNA interference (RNAi) or antisense oligonucleo-tides is promising for a variety of genetic disorders. Indeed, a phase I clinicaltrial using antisense oligonucleotides to treat patients suffering from amyo-trophic lateral sclerosis due to mutation in SOD1 is now under way (http://www.alsa.org/patient/drug.cfm?id¼1592). The use of shRNA- or siRNA-basedknockdown of mutant Htt expression has been validated in HD mouse mod-els,165,166 as has the use of antisense oligonucleotides.167 Several issues must beaddressed before these approaches can be applied to patients. Off-targeteffects are probably more widespread than generally appreciated.168,169 Deliv-ery of siRNA or antisense oligonucleotides requires chronic administration intothe cerebrospinal fluid (CSF) via catheter and whole brain penetration is notfully established for large animals. Finally, the safety of virus-based geneknockdown in the brain is not yet readily controlled or proven safe. Althoughthere are still significant hurdles, the potential of genetic therapy in HD isnonetheless promising.

5. ARTIFICIAL PEPTIDES OR INTRABODIES

Intrabodies are recombinant antibody fragments that target intracellularHtt.170–173 The protein specificity created by a peptide has allowed targeting ofvarious domains of the Htt molecule, including the N-terminal region and thepolyglutamine tract, to reduce aggregation and promote protein clearance.174

This therapeutic strategy is still in a very early stage and will need more work tosolve technical issues such as stability and specificity of intrabodies and optimaldelivery methods, yet it could offer highly selective modulation of intracellularHtt clearance and aggregation.

VIII. Conclusion

Genetic and experimental evidence suggests that most polyglutamine pro-teins, including Htt, exert toxic effects as a result of an abnormal conformationinduced by the polyglutamine expansion. This is likely distinct from theirnormal function and linked to aggregation and abnormal protein interactions.This contrasts other types of disease-causing mutations that accentuate orinhibit normal enzymatic activities. It is still unknown what Htt’s crucial cellularfunctions are, and indeed, it is likely that the protein has several, given its largesize and multiple conserved regions. Despite the fact that Htt’s disease-causing

http://www.alsa.org/patient/drug.cfm?id=1592




activities represent an apparent gain of toxic function, its normal interactionswithin the cell will hold important clues to how this pathogenic activity isreleased, for example, via proteolysis, posttranslational modifications, or pro-tein contacts that prevent or promote misfolding or regulate clearance. This hasobvious implications for Htt-specific therapy. Genetic therapies will becomemore feasible as delivery systems evolve, while nonspecific therapies aimed atbolstering general cellular resistance to protein misfolding and metabolicdefects may also play a role. The plethora of mouse models will facilitatepreclinical testing, especially as our understanding develops about how bestto use them. Thus, the treatment of HD is likely to depend on a detailed andevolving knowledge of Htt biology, coupled with interventions that targetcrucial events in the pathogenic cascade.

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Huntington Diseases and the Huntingtin Protein