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Introduction
Huntington's disease (HD) is a devastating, autosomal dominant neurodegenerative disorder caused by a Cytosine
Adenine Guanine (CAG) trinucleotide repeat expansion within exon 1 of the huntingtin (Htt)
gene[1]. This disease is associated with the selective degeneration of striatal GABAergic projection neurons and cortical pyramidal neurons and is
characterized by choreiform movements, cognitive deficits and psychiatric disturbances. The genetic mutation underlying HD was
discovered in 1993. Normally, the number of CAG-repeats in the polyglutamine (polyQ) tract near the N-terminus of the Htt
gene is between 19 and 35. In the diseased state, this number increases to more than 36 repeats. Complete penetration is seen
with CAG-repeat lengths of 42 or
more[2]. This disease affects about 5 out of 100 000 people in Western countries. Although
the onset of HD symptoms generally occur at the age of 40 or 50, the disease can start at any time from early childhood to old
age, with a mean duration of 15_20 years.
The pathogenesis of HD has not yet been fully under-stood. Several hypotheses have been proposed to elucidate the
mechanisms of HD pathogenesis, including excitotoxicity, oxidative stress, and impaired energy
metabolism[3], abnormal protein aggregation, transcription dysregu-lation and abnormal protein
interactions[4]. Because of
incomplete understanding of the mechanisms of HD patho-genesis, treatment to delay the onset or slowdown progression
remains unavailable at present. Animal models which closely mimic the neurobiological and clinical symptoms of the disease
may provide an alternative approach for the study of HD molecular pathogenesis, the refinement of
existing treatments and the development of novel therapies for HD. Therefore, animal models are a crucial part of this rapidly
advancing field of HD research. In this paper, a variety of animal models generated to date are reviewed, and their differences,
the implications in uncovering pathogenic mechanisms and developing therapies are discussed.
Clinical symptoms and neuropathology in HD patients
There are 3 main groups of symptoms including movement disorder, cognitive impairment and psychiatric disturbance, of
which the most characteristic is the choreic movement. Generally, clinical symptoms develop very rapidly after onset and
compose a 3-part picture with motor symptoms, characterized by hyperkinesia evolving to hypokinesia. Initially, patients
demonstrate personality changes and develop small involuntary movements. Typically, the earliest motor signs are eye
movement abnormalities, followed by the progressive appearance of orofacial dyskinesias, involving the head, neck, trunk
and arms, before becoming chorea. As the disease progresses, the movement disorder becomes more pronounced. Severity
may vary from restlessness with mild, intermittent exaggeration of gesture and expression, fidgeting movements of the hands,
unstable, dance-like gait, to a continuous flow of disabling, violent movements. Even though the typical movement disorder
is chorea, virtually any type of movement disorder can be seen, including dystonia, rigidity, myoclonus, and athetosis. As
the disease progresses, choreiform movements may be reduced in intensity or frequency; the initial hyperkinetic syndrome
being progressively replaced by a more hypokinetic syndrome in which bradykinesia, rigidity and dystonia dominate. With
the movement disorder, cognitive deficits as well as psychiatric disturbances occur. Cognitive dysfunction includes
dementia and difficulties with executive functioning. Psychiatric disturbances most commonly manifest as apathy and depression,
but obsessive-compulsive disorder, psychosis, paranoia, and substance abuse also occur. Weight loss is another common
feature of the disease. The patients rapidly require constant care and progressively lose their autonomy, dying on average
17 years from the onset of the disease. Death usually results from aspiration pneumonia secondary to dysphagia, or from
complications resulting from falls or chronic
illness[5].
The juvenile form of HD is caused by CAG-repeat lengths exceeding 60. Clinical manifestations are more severe in
presentation and progress more rapidly. Juvenile HD is most often inherited through paternal transmission. The movement
disorder tends to be more parkinsonian than the adult form and is characterized by bradykinesia, rigidity, resting tremor and
seizure. Juvenile patients have a more severe course of the disease with an average duration of 5 to 20
years[6,7].
HD brains are significantly lighter at death than normal brains. The most striking neuropathological feature of HD is a
marked atrophy of the caudate and putamen within the basal ganglia. In addition, the neuronal loss in the striatum is
accompanied by pronounced gliosis. The pathology of HD has been classified by
Vonsattel et al from grade 0 (no changes) to grade 4 depending on the degree of striatal atrophy (caudate and putamen reduced to a rim of
tissue)[8]. The striatum is comprised of several neuronal subtypes including medium spiny projection neurons and interneurons. The latter category
includes the medium-sized aspiny reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase-positive,
neuronal nitric oxide synthase (nNOS)-positive neurons and large aspiny cholinergic neurons. Interestingly, many studies
have consistently shown that not all striatal cells are equally affected by the degenerative process. HD preferentially affects
the GABAergic medium-sized spiny neurons, leaving the other subpopulations of striatal neurons largely unaffected in the
early stage[9_13]. The factors that render striatal projection neurons more susceptible to damage are unclear. Within the
subpopulation of striatal GABAergic medium-sized spiny neurons, not all neurons are similarly affected by HD. A double
gradient of striatal degeneration has been described in the HD striatum, one progressing in a dorsolateral to ventral direction
and another in a caudo-rostral
direction[14].
Although the striatum is the most profoundly affected region in HD, neuronal loss may also occur in the cortex, thalamus,
zona reticulata of the substantia nigra, superior olive, lateral tuberal nucleus of the hypothalamus and deep cerebellar
nuclei[15-17]. Therefore, the changes in the striatum reflect a relatively selective vulnerability to cell death. Inter-estingly, a common
feature to all these areas is that, with the exception of the cortex, they all belong to the basal ganglia circuitry, and as such, are
directly or indirectly connected to the striatum. The cerebral cortex is also markedly atrophied in the later stages of the
disease. In the cortex, large neurons in layer VI are the most affected, with smaller amounts of degeneration seen in layers III
and V. As for the other structures belonging to the basal ganglia circuitry, whether cortex degeneration follows, accompanies
or precedes that of the striatum is still
controversial[18].
Up to now, the exact mechanism of HD pathogenesis still remains unclear. Since the genetic cause of HD was uncovered
in 1993, several hypotheses have been put forward to elucidate its molecular pathogenesis, including excitotoxicity, oxidative
stress, impaired energy metabolism, and abnormal protein_protein interactions—which in turn may cause transcriptional
dysregulation and altered gene expression. Central to all hypotheses is the attempt to understand the role of huntingtin (Htt)
in normal physiology and in the diseased state.
Htt is encoded by the IT15 gene. Its amino acid sequence does not resemble that of any other known proteins. The 5' end
of the IT15 gene contains a polymorphic trinucleotide CAG-repeat which encodes a series of glutamines. Normal people have
CAG-repeat lengths of 7_35. The CAG-repeat is expanded and unstable in HD patients. By virtue of having an expanded
polyQ, HD becomes a member of neurodegen-erative disorders known as polyQ disorders. The level of expression and the
regional distribution of the mutated huntingtin (mHtt) in the brain as well as in the
peripheral tissues were found to be rather similar to those of the normal
protein[19]. Htt is normally distributed predominantly
within the cytoplasm; for mHtt, however, nuclear
localization is increased[20_22]. Htt is ubiquitously expressed, but despite extensively overlapping expression patterns, the
neuronal cell death is relatively specific and can differ markedly. The normal function of Htt is poorly understood at present,
but it has been proposed to play roles in neurogenesis, apoptosis, and vesicle trafficking. Some of the features of HD may
be the result of loss of function of wild-type huntingtin
(wHtt)[23, 24].
HD is a protein misfolding disease. The bio-hallmark of HD is the formation of intranuclear inclusions (NII) and
cytoplasmic aggregates in neurons in vulnerable brain areas. Aggregates of mHtt have been detected both in postmortem tissue from
patients affected by the disease and in mouse models of HD. The inclusions and aggregates are usually formed by small
N-terminal Htt fragments and are co-localized with other cellular proteins involved in proteolysis, vesicle trafficking and protein
degradation. Whether these aggregates are deleterious, protective or incidental remains unclear. Caspases and calpain have
been implicated in the cleavage of both mHtt and
wHtt[25_28]. Cleavage of Htt by caspases results in the production and
accumulation of small N-terminal fragments which are prone to form NII and cytoplasmic aggregates and induce apoptosis.
Thus N-terminal Htt plays an important role in the pathogenesis of HD. While the traditional view is that the mHtt is cleared
by the ubiquitin-proteasome pathway, it is proved that the autophagy-lysosomal route is also involved in the degradation of
mHtt[29_33].
In vivo and in vitro
models of HD
Many animal models mimic HD symptoms or pathology. Characterization of multiple animal models is necessary for
understanding the pathogenesis and the effects of potential therapies.
Excitotoxic lesion
models Excitotoxicity refers to the
deleterious effects produced on neuronal cells by
relatively high concentrations of glutamate interacting with its selective
membrane receptors. As the striatum receives large glutama-tergic input from corticostriatal afferents, it is a structure at risk
of glutamate-mediated excitotoxic injury. The theory of excitotoxicity as a pathogenic mechanism in HD has emerged in the
last few decades, beginning with the observation that injections of excitatory amino acids into the striatum of rodents and
non-human primates led to neuronal depletion and a neurological phenotype that was similar to HD. The initial reports in
1976 demonstrating that a direct intrastriatal injection of kainate, a
non-N-methyl-D-aspartate (NMDA)glutamate agonist,
could mimic the axon-sparing striatal lesions observed in HD, was the starting point of a large number of literature on the use
of glutamate analogues as neurotoxic compounds in HD
research[34]. Nevertheless, intrastriatal injections of kainate do not
perfectly reproduce the histological hallmarks of HD, since both projection neurons and NADPH-positive interneurons are
killed by this excitotoxin, as opposed to the relative sparing of the striatal interneurons in
HD[35,36]. In contrast, intrastriatal
injections of quinolinate, a NMDA-selective glutamate agonist and an endogenous metabolite of trypophan, induces a
preferential degeneration of GABAergic neurons and a relative sparing of NADPH diaphorase- and cholinergic inter-neurons,
suggesting that a selective activation of the NMDA receptors is required to mimic HD striatal
pathology[37].
Excitotoxin-induced destruction of striatal neurons involves the process of
apoptosis[38]. Further studies have indicated
that the nuclear factor-κB (NF-κB) activation contributes to the excitotoxin-induced death of striatal
neurons[39]. Liang et al evaluated the potential contribution of some apoptosis regulatory proteins such as Bcl-2, p53 and c-Myc to the differential
vulnerability of striatal neurons to the NMDA receptor agonist quinolinic acid. They found that selective vulnerability of
striatal medium-sized spiny neurons to degeneration in a rodent model of HD appeared to correlate with their low levels of
Bcl-2 immunoreactivity and high levels of induced p53 and c-Myc
immunoreactivity[40].
Excitotoxic striatal lesions can replicate some of the
behavioral aspects of HD including hyperkinesias, impaired motor skills, and deficits in spatial maze learning and executive
function in rats. However, the behavioral symptoms fail to include dyskinesias or chorea-like
movements[41]. The excitotoxic lesion models have led to the use of glutamate antagonists in HD treatment. For example, NMDA receptor antagonists such
as riluzole and amantadine are often used clinically. Some data suggest that there are transient antichoreatic effects and more
sustained effects of riluzole on psychomotor speed and behavior in HD
patients[42].
Indirect excitotoxic lesion models (or metabolic toxins models)
The injection of various mitochondrial toxins
(amino-oxyacetate, rotenone, MPP+, malonate,
Mn2+, 3-acetyl-pyridine) into the rat striatum has been shown to produce increased
lactate formation, adenosine triphosphate (ATP) depletion and a delayed neuronal degeneration by the mechanism of
disrupting mitochondrial energy metabo
lism and secondary excitotoxicity. The common nature of these lesions was exemplified by the marked degeneration of
GABAergic neurons and the relative sparing of cholinergic and NADPH diaphorase-positive interneurons.
The initial identification of 3-nitropropionic acid (3-NP), a metabolite of 3-nitropropanol, as a toxic agent responsible for
livestock poisoning, was first made in western USA. Animals intoxicated with leguminous plants presented various motor
abnormalities consisting of general weakness and uncoordination of the hind limbs evolving to
paralysis[43]. The early controlled studies in animals indicated that 3-NP injection could produce hypoxic-like cerebral lesions, preferentially
affecting the basal ganglia. In vitro biochemical studies have established that 3-NP is a suicide inhibitor of succinate dehydrogenase,
an enzyme located in the mitochondrial inner membrane and responsible for the oxidation of succinate to
fumarate[44]. Because of the histochemical and pathological similarities between the 3-NP animal model and HD, the 3-NP model has been
proposed as an alternative HD model.
Many studies have indicated that acute and large doses of 3-NP administration can not replicate HD pathology. Acute
3-NP toxicity was observed after either 1 or several injections in a short period of time
(1-5 d). Depending on the dose administered, the neurological deficits can develop rapidly, ranging from general uncoordination, drowsiness and general
weakness to hind limb paralysis without rigidity and finally recumbency and
death[45]. The histological features observed in
the striatum of rats subjected to acute 3-NP protocol are quite different from those observed in HD patients. In 3-NP-lesioned
brains, the central area of the striatal lesion is totally neuron depletion and there is only a very limited transition zone between
the core of the lesion and the normal striatal tissue. Nevertheless, acute 3-NP toxicity has been frequently associated with
extra-striatal cerebral lesions involving various brain structures such as the pallidum, the hippocampus, the thalamus and the
substantia nigra pars reticulata[46,47].
Chronic intoxication with 3-NP was initially tested in adult rats using a chronic low dose
(10-12 mg·kg_1·d_1, 1 month)
regimen of administration[47,48]. This experimental protocol produced a partial, steady-state metabolic impairment, similar to
that found in HD patients. In 30%_40% of the treated animals, chronic 3-NP delivery induced motor abnormalities and
selective striatal lesions. Behavioral studies with the chronic 3-NP model indicated that the HD-like striatal lesions were also
associated with symptoms and persistent abnormal movements, in many ways analogous to HD motor deficits. In a study
performed in animals repeatedly treated with 3-NP (10
mg·kg_1 every 4 d for 28 d), a quantitative analysis of spontaneous
locomotor behavior showed that this protocol was associated with an early phase of hyperkinesia (first and second week of
treatment), followed by a later phase of
hypokinesia[49,50].
The animals subjected to the chronic 3-NP intoxication always displayed bilateral and symmetrical dorso-lateral striatal
lesions. In their most rostral part, these lesions appeared restricted to the dorso-lateral aspect of the caudate-putamen,
whereas a more ventral localization is usually observed caudally. Contrasting with lesions observed in more acute paradigms,
the chronic lesion presented a more diffuse cell loss, progressively increasing from the unaffected striatum to the center of
the lesion. Within the core of the lesion, an obvious, but partial neuronal loss was noted, accompanied by a moderate
astrogliosis, a decrease in cytochrome oxidase activity and a relative sparing of the NADPH diaphorase-positive
interneurons and the dopaminergic striatal
afferents[46,47,51].
As mentioned previously, the deterioration of cognitive functions in HD is paralleled by the progression of motor effects
from a choreic dyskinesia to a more disabling akinetic and parkinsonian-like syndrome. Excitotoxic animal models have not
characterized the "progressive" behavioral pathology of HD. In contrast, the progressive locomotor alterations in rats can
be obtained when some dosing regimen of 3-NP is administered. Initially, systemically 3-NP-treated animals exhibited
significant hyperactivity during the first 2 injections, reaching a plateau after the third injection, and then displaying
hypoactivity after the fourth
injection[52]. The study by Mettler indicates that small lesions are visible at the onset of
hypoactivity, but at the end of the long-term course of 3-NP administration, larger lesions were noted; there is no visible
striatal degeneration at the period of
hyperactivity[53_55].
Despite chronic 3-NP rat models replicating some of the features of HD, this model still has some limitations. Primarily,
there is a very different repertoire of movements in the primate compared to the rat. Secondly, the organization of the basal
ganglia in primates is quite different from that in rats. In primate animals, but not in rats, the striatum is structurally divided
into 2 parts: the caudate nucleus and the putamen. This difference between the rodent and primate models is exemplified by
the behavioral response to the dopamine agonist apomorphine observed in non-human primates with excitotoxic-induced
striatal lesions. In non-human primates with chronic 3-NP treatment, a variety of abnormal movements are highly reminiscent
to those seen in HD patients; these types of movements have never been observed in rats in the same experimental conditions.
As discussed earlier, rats chronically treated with 3-NP did not show clearly identifiable dyskinetic movements resembling
chorea even though hyperlocomotor activity has been reported early in the course of intoxication, as well as the presence of
dystonia, bradykinesia and gait
abnormalities[46,49]. Therefore, it may be that the dyskinetic component of HD
symptomatology is part of a motor repertoire that can only be expressed in primates.
The non-human primate HD model also shows the `progressive' characteristics of HD. Hantraye
et al treated 2 adolescent baboons for 16 weeks with 3-NP at an initial dose of 10
mg·kg_1·d_1, which was progressively increased to
28 mg·kg_1 d_1 (1
mg·kg_1 increment at weekly intervals). During the first 6 weeks of the protocol, no spontaneous or even
apomorphine-induced abnormal movements were observed. The animals were still in the `non-symptomatic' phase. After
8-10 weeks of treatment, apomorphine administration induced choreiform movements in all 3-NP-treated animals, indicating
that at this stage they had entered into the `presymptomatic' phase, which is characterized by the absence of spontaneous
abnormal movements and the presence of frontal deficits and of apomorphine-induced abnormal movements. The severity of
motor abnormalities after apomorphine administration increased as the 3-NP intoxication progressed. After 3 months of
intoxication, all animals showed spontaneous foot dyskinesia and dystonia, which means they entered into the `symptomatic
phase'. At this stage the animals were killed for pathological examination. Results of postmortem evaluation showed the
presence of the bilateral striatal lesions without detectable extra-striatal lesions. Therefore, animal models which mimic the
adult-onset form of HD were obtained with this
protocol[56].
In summary, only chronic systemic administration of
3-NP produces motor dysfunctions and striatal lesions that mimic many histological and neurochemical features of HD.
These models also offer the flexibility to investigate HD at different stages. Moreover, the efficacy of experimental treatments
can be tested at various times of disease progression. The use of mitochondrial toxin lesion models have led to the clinical
administration of mitochondria function protectors, including coenzyme Q10 and creatine for the treatment of HD. However,
many studies have shown that these mitochondria function protectors only have limited beneficial effects for HD
patients[42]. Since the mutant gene that causes HD was identified, numerous genetic animal models have been generated. Because
genetic models can mimic the pathology of HD more accurately, chemical lesion models are now considered outdated.
However, these models still have some validation. For example, excitotoxin and mitochondrial toxin are often used today in
transgenic HD models and other in vitro models to study the sensitivity of genetic models to these
toxins[57].
Transgenic mouse models Since the mutant gene that causes HD was identified in 1993, one of the most important
advances in HD research has been the generation of various genetic mouse models. Genetic mouse models of HD mainly
include transgenic, knock-in, knock-out, and virally-inserted mutated polyQ tract models.
In transgenic mouse models, the mutant gene, or part of it, is inserted randomly into the mouse genome, leading to the
expression of a mutant protein in addition to the endogenous, normal Htt. Several transgenic mouse models of HD now exist
and fall into 2 broad categories. The first category is mice expressing Htt N-terminal fragments, usually the first 1 or 2 exons
of the human Htt gene that contain the polyQ expansion (in addition to both alleles of murine wHtt, Hdh). The second
category is transgenic mice expressing the full-length human HD gene with an expanded polyQ tract (plus the murine Hdh).
All these models share some features with human HD. The characteristics in transgenic mice are described by Hickey
MA et al. In general, it seems that the shorter the transgene, the longer the CAG repeat, and the higher the expression levels, the
more severe the phenotype[58].
Transgenic mice with Htt fragments: The first successful transgenic mouse model of HD was termed the R6 mouse. These
mice were generated by overexpressing exon 1 of the human Htt gene with long (141_157) CAG-repeat expansions. To date,
6 lines have been established, in which four lines contain expanded CAG repeats of a size larger than that generally
associated with the juvenile HD patients, namely R6/1,
(CAG) 115; R6/2, (CAG) 145; R6/5,
(CAG) 135_156; and R6/0, (CAG)
142. In all cases, except line R6/0 in which the transgene was probably silenced by the site of integration, the transgene protein showed
a ubiquitous tissue expression profile. On the basis of home cage behavior, the onset ages are approximately 2 months in line
R6/2 and 4_5 months in line R6/1. The movement disorder includes an irregular gait, stereotypic grooming movements, rapid
shudders and a tendency to clasp the hind limbs and forelimbs together when suspended by the tail. In addition to the
movement disorder, the mice exhibit a progressive weight loss. The phenotype progresses rapidly and the mice rarely
survive beyond 12 weeks of age[59].
Of the many lines generated, the R6/2 model is the most extensively studied and is readily available commercially. The fast
course of the R6/2 makes it a less expensive model to study than others. The R6/2 mice have a well-characterized progressive
phenotype with moderate variability such that experimental groups can contain as few as 10 mice to detect 10% of differences
in many outcome measures. It is possible to perform survival studies in approximately 3 months which is lifespan of most
R6/2 mice. Motor behavioral deficits could be measured in these mice as early as 5_6 weeks of age. However, overt behavioral
anomalies did not appear until 8 weeks, and these were followed by an early death at 10_13 weeks. The mice had a severe
phenotype with low weight, diabetes, clasping, tremor and con-vulsions. At autopsy, brain weight was markedly reduced,
but neuronal death was minimal and delayed compared with behavioral symptoms. In contrast, HD is characterized by a
massive loss of striatal neurons in humans. Although it is likely that cell death follows a long phase of neuronal dysfunction
in both mice and humans, it remains puzzling that this transgenic model did not show overt cell death until the last stages of
the disease. An explanation might be the short lifespan of the mouse, which might be caused by general metabolic disease
(eg diabetes and severe weight loss) and seizures. Alternatively, because R6/2 mice are relatively resistant to kainic acid
in vivo, they might have protective mechanisms that are not present in
humans[60-63]. Another special mouse model is the
N-171-82Q mouse, which has a longer N-terminal fragment of Htt (exon 1 and 2) with 82 polyQ. These mice have a less well-defined
neurobehavioral phenotype than that of R6/2 mice. Their neuropathological features are more similar to human HD in that Htt
aggregates are more prominent in the cortex than in the striatum, and neurodegeneration is more prominent and seems more
selective for the striatum. However, the phenotype is more variable than that of the R6/2 mice and therefore, a much larger
number of mice (more than 20 in each treatment group) are necessary to provide better accuracy in detecting statistical
significance of changes[64].
Although these mice show rather little overt cell death, it was in this strain that the NII were first demonstrated,
suggesting that the phenotype is an expression of abnormal function of cells expressing the mutant protein rather than a
result of cell death. The discovery of NII in the brains of these transgenic mice came at a time when similar features were
reported in models of other CAG-repeat diseases,
suggesting a common pathological mechanism of these
diseases[65-68]. This triggered a flurry of studies aimed at identifying
the components of inclusions, the mechanism of Htt aggre-gation, the role of NII in neurotoxicity and ways to prevent Htt
aggregation.
Although the transgenic mice with fragments display many of the behavioral and neuropathological features observed in
HD patients, it is not a perfect genetic and neuropathological match to that observed in humans. For example, the R6/2 mice
have more extensive Htt aggregate formation than what occurs in HD, they are resistant to excitotoxicity and neuronal loss
is less selective and dramatic. In addition, the pathogenic mechanism of the truncated Htt in these mouse models is
questionable. The major problem is all these fragments chosen have no physiological ground because these fragments may
not be produced in the human HD brain. Since Htt is a big protein with many potential functional domains, its conformation
and function may show variable changes and thus, create artificial properties when Htt is cut into different sizes of fragments.
These would make many findings in transgenic HD mice expressing N-terminal fragments hard to relate to HD pathogenic
mechanisms[64].
Transgenic mice with full-length Htt:
Several mouse models have been generated using a full-length human
IT15 gene as the transgene. Unfortunately, some of these models are not successful. For instance, Goldberg
et al generated transgenic mice that carried full-length human
Htt with 44 CAG repeats. No protein expression was detected and there was no indication
of neuronal loss, neurodegeneration or behavioral
abnormalities[69], but with the CAG repeats increased, the symptoms and
pathology of transgenic mice models become more significant. For instance, mice that express a full-length
IT15 cDNA clone with either 48 or 89 repeats driven by the cytomcgalovirus promoter (CMV) showed a progressive motor phenotype, and
more importantly, neuronal loss in the striatum. Surprisingly, NII were extremely rare in these mice. A similar feature is shared
by a yeast artificial chromosome (YAC) mouse expressing a full length IT15 gene with 72 repeats. Although these mice did not
have NII, much smaller aggregates of Htt were observed in the nucleus. Disease progression was slow in the YAC mice,
which correlates with a smaller repeat length and a much lower level of transgene expression (30%_50%). An attractive
feature of the YAC mice is that cell loss is limited to the striatum, thus recapitulating to some extent the regional selectivity of
HD[70,71].
Jeremy et al also studied the selective degeneration of neurons in YAC128 mice. They found that these mice exhibit
selective atrophy and neuronal loss in a pattern similar to human HD. Although the striatum does not show increased mHtt
expression, nuclear localization of mHtt occurs earlier and to a greater extent in the striatum, suggesting the possibility that
selective nuclear localization of mHtt may contribute to the selective neurodegeneration in HD. Furthermore, the appearance
of mHtt in the nucleus coincides with the onset of behavioral abnormalities, suggesting that this may contribute to neuronal
dysfunction. A comparison to R6/1 mice reveals non-selective nuclear detection of mHtt in these mouse models and suggests
that the expression of full-length mHtt may be important in modeling the selective neuropathology in
HD[72]. Tanaka et al have generated an inducible mouse model of HD expressing full-length Htt with 148Q using a doxycycline-regulated promoter.
In inducible trans-genic mice, Htt was expressed widely in the brain under the control of the Tet-transactivator driven by the
prion promoter. They found that there were prominent NII in the cortex and striatum, as well as cytoplasmic aggregates. Their
further studies showed that the distribution of the inclusions and other aggregates were more widespread than that typically
seen in HD patients, with inclusions prominent not only in the cortex and striatum, but also in the hippocampus, brain stem
and cerebellum. There may be 2 reasons for this: First, their constructs have a very long polyQ repeat (with 148Q); HD
patients with longer polyQ repeats also tend to have more widespread pathology. Second, expression is driven
not by the Htt promoter, but by the prion protein
promoter[73].
The prominent difference between transgenic mice with full-length HD and human HD is the relative lack of marked
neuronal cell death in mice striatum. There is increased glial fibrillary acidic protein (GFAP) labeling, characteristic of
astrogliosis, and ventricular enlargement, consistent with neurodegeneration, but there does not appear to be massive
neuronal loss in the striatum. In this respect, transgenic mice models with full-length mHtt are similar to other genetic mouse
models of HD. While there has been some degree of degeneration and astrogliosis in several of these models, none of them
reproduce the massive selective neuronal cell death of up to 95% of medium spiny neurons that can be seen in advanced HD
patients[73].
Compared to transgenic mice with truncated gene, the neuropathology of transgenic mice with full-length Htt has greater
fidelity with human disease. However, the N-terminal fragment mice generally have a more obvious behavioral and
pathological phenotype than mice expressing full-length Htt. The variability and the slow phenotype development of models
expressing full-length Htt may hinder its use for the research of potential therapeutic compounds. In contrast, the efficiency and
clear experimental endpoints are the major advantages of N-terminal fragment mice and are also the reasons for its widespread
use in HD research.
Knock-in mouse models In theory, knock-in models are the most faithful genetic models of the human HD condition,
since knock-in mice carry the mutation in its appropriate murine genomic and protein context and under the endogenous Hdh
promoter. However, these models were disappointing initially because the mice showed either no behavioral phenotypes or
anomalies that apparently did not involve movement disorders. This initial disappointment was later dispelled after closer
analysis and generation of additional
models[74,75]. Homozygous knock-in mice have now been shown to develop very early
behavioral anomalies prior to any detectable neuropathology. These mice carried 94 CAG repeats and behavioral data
suggested that cellular dysfunction (prior to mHtt aggregation) was responsible for the initial symptoms of HD. The mice
showed abnormal activity levels that were biphasic and mirrored the progression of motor symptoms in HD. Therefore,
knock-in mouse models represent a valid model of HD, with the added advantage of a slower progression of phenotype and
pathology, thus allowing more detailed
analysis[76,77].
So far, the knock-in mice do not have sufficient expression of disease to use progressive morbidity and survival as an
endpoint; however, they have a variety of measurable neuropathological and behavioral phenotypes that could be validated
as potential endpoints in therapeutic studies. A common feature of different knock-in mice models is the presence of nuclear
staining and microaggregates of Htt in the brains of mice at 2_6 months of age, which is relatively early in the course of the
disease. By contrast, NII are only observed when the mice are older (10_18 months of age), and cell death has not been
reported. Overt neuronal loss and gliosis, even in older animals, is absent. However, molecular alterations (decrease in
mRNA encoding enkephalin in the striatum) and cellular alterations (increased sensitivity to NMDA) similar to those
observed in the R6/2 mice were present in the 94-CAG-repeat knock-in mice. The presence of these clear anomalies further
suggests that neuronal dysfunction precedes cell death in HD and might be primarily responsible for early functional deficits.
This correlates with the finding that subtle motor deficits precede by many years the appearance of overt symptoms and
striatal atrophy in HD patients. Further crucial information obtained from knock-in mice is evidence of an age-related
instability of expanded CAG repeats in neurons, despite the fact that mature neurons do not divide. Importantly, this
instability is region-specific, with larger increases in CAG numbers found in the striatum and cortex. Because there is a clear
relationship between the number of CAG repeats and the toxicity of mHtt, this could be a reason for the regional selectivity
of neuronal loss in adult-onset
HD[78,79].
In summary, genetic mouse models provide insight into the pathogenesis of HD and are invaluable tools for the
evaluation of potential therapeutic approaches. The widespread use of genetic mice has led to some important discoveries in HD
molecular pathogenesis, including misfolding and aggregation, abnormal protein interactions and dysregulation of
transcription conferred on mHtt. Meanwhile, genetic mice have also led to the emergence of many potential chemicals which may
correct these acquired properties. These compounds may inhibit mHtt aggregation, transglutaminase, protease or histon
deacetylase in animal experiments[42].
The knock-out mice model
Soon after the discovery of the gene mutation that causes HD, it was found that homozygous
gene knock-out in mice was embryonic lethal, which contrasts with the late onset of the human disease. Thus, these early
knock-out mice are not good HD models, but they indicate that Htt has an essential role in embryonic development. Furthermore,
mHtt can rescue the knock-out phenotype, which indicates that the effect of the mutation is not primarily due to loss of
function[80-83]. Htt contains a polymorphic stretch of polyQ near its N-terminus. HD results when the polyQ stretch is
expanded beyond 37Q. However, the role of the normal polyQ stretch in the function of Htt is unknown. Clabough
et al deleted the CAG triplet repeat encoding 7Q in the mouse HD gene
(HdhDQ) to determine the contribution of the polyQ stretch
to normal Htt function. They found these Hdh(DQ/DQ) mice are born with normal Mendelian frequency and exhibit no gross
phenotypic differences in comparison to control littermates, suggesting that the polyQ stretch is not essential for Htt's
functions during embryonic development. However, adult mice commit more errors initially in the Barnes circular maze
learning and memory test and perform slightly better than wild-type controls in the accelerating rotarod test for motor
coordination. The polyQ deletion results is only a subtle phenotype
in vivo, thus, it is likely that the polyQ stretch is not
required for an essential function of Htt, but instead, may modulate a normal function of
Htt[84]
Mouse models with virally-inserted mutated polyQ
tracts Viral-vector aided insertion in genes, whether in full or partial,
has the advantage of precise localization of injection and gene expression. There are 2 types of viral vectors currently used.
These include adeno-associated and lentiviral, both of which integrate into the host genome. These kinds of models are more
labor-intensive than transgenic or knock-in mice. However, this technology is applicable to non-rodent species in a way that
is not currently possible with transgene or knock-in technology. These models can be used to study the effects of discrete
amounts of the mHtt protein and can also be used to study load, temporal and spatial effects of the mutation in more
controlled circumstances than in transgenic or knock-in animals. In addition, the effect of different viral vector inserts can be
compared, potentially in the same animal. This may be important since genetically identical animals with the same mutant
polyQ transgene develop very different amounts of aggregates of the mHtt
protein[85,86].
Senut et al examined the effect of a polyQ (97Q) repeat expression in the context of an adeno-associated viral vector
(AAVV). High levels of expression of the polyQ tract led to development of nuclear and cytoplasmic aggregates as early as
5 d post infection. This indicates that expression of the polyQ alone is highly associated with the development of inclusion
bodies. Aggregates were also observed at a distance from the injection site, indicating that anterograde and retrograde
transport of the vector had taken place. In theory, lentiviral vectors are capable of carrying larger transgenes than AAVVs.
de Almeida et al also found that expression levels of the transgene were proportional to the severity of neuropathology. In
addition, increased expression led to the production of NII, whereas neuritic and nuclear aggregates were observed when
expression was driven by a weaker promoter. Longer CAG-repeat lengths correlated with increased aggregate formation.
Loss of staining for aggregated mHtt near the injection site was attributed to cell death and proportional to high expression
levels. Importantly, choline acetyltransferase (ChAT)and
NADPH-d interneurones were relatively preserved as the same
feature of human
HD[85,87].
Fly (Drosophila) models
The fly is one of the best invertebrates for modeling higher organisms. Comparative
genome analysis reveals that at least 50% of fly genes are similar to those of
humans[88]. Among those human genes known
to be associated with disease, 75% have a
Drosophila ortholog[89]. Analysis of the genomic and cDNA
sequences indicates that the Drosophila HD gene has 29 exons, compared with the 67 exons present in vertebrate HD genes,
and that Drosophila Htt lacks the polyQ and polyproline stretches presented in its mammalian
counterparts[90]. The fly is also an excellent choice for modeling neurodegenerative diseases because it contains a fully functional nervous system with
an architecture that separates specialized functions such
as vision, olfaction, learning and memory. Further, the compound
eye of a fruit fly is made up of hundreds of repeating constellations of photoreceptor neurons such that any perturbation in
the pattern is quite evident. Most importantly, in
Drosophila, foreign genes can be engineered to be expressed in
tissue-specific and temporally regulated patterns and an impressive array of genetic tools are
available[91].
Foreign genes are expressed using a bipartite gene expression system in which genes inserted behind the yeast upstream
activator sequence (UAS) are activated by the yeast Gal4 protein. Genes fused to UAS and injected into embryos with a
helper element integrate into the chromosome producing transgenic lines carrying the UAS transgene. Many measures of
neuronal dysfunction are possible, with some of the most common ones being climbing ability or integrity of photoreceptor
cells of the eye[92,93]. In all models studied, it has been found that pathology exhibits a polyQ length dependency similar to
that of humans. Drosophila embryogenesis spans approximately 1 d, and neurogenesis begins at about 5 h and concludes
by about 15 h. To our knowledge, no evidence of neurodegeneration has previously been described early in the larval
stages, but clear evidence of degeneration occurs in mature larvae, in pupae and in aging adults. Thus, by every measure,
flies expressing mutant human genes present with pathology that mimics the human disease in every important
way[94,95]. How to make
Drosophila models more amenable to high-throughput and automated screening for therapeutics is an
important issue. In this regard, practical hurdles to be overcome are the automated manipulation and scoring of flies and the fact
that flies are not accessible to externally administered drugs.
Caenorhabditis
elegans models The nematode Caenorhabditis
elegans (C elegans) is an established model for
developmental biology. Parker et al expressed the first 57 amino acids of human Htt with normal and expanded polyQ fused to a
fluorescent protein marker in C elegans touch receptor neurons by using the mec-3 promoter (Pmec-3). Pmec-3 is active in 6
touch receptor neurons needed for gentle touch. Because
C elegans does not contain a Htt homolog nor long polyQ tracts,
transgenic phenotypes in worms can be attributed to polyQ
transgenes[96-98]. In C
elegans, expanded polyQ produces mild nose touch abnormalities and low penetration of dye-filling defects, and causes cell death when expressed in sensitized ASH
sensory neurons under the control of the osm-10 promoter. Perinuclear protein accumulation was observed in tail
mechanosensory neurons, the phospholemman (PLM) cells, but did not correlate with polyQ length or the mechanosensory
defective (Mec) phenotype. Animals expressing 128 Gln residues (Glns) were strongly Mec at the tail and more likely to have
aggregates in PLM neuronal processes. Additionally, neurons appeared to show selective susceptibility to polyQ-mediated
degeneration. The difference in penetrance between anterior and posterior touch responsiveness may reflect neuronal
susceptibility to polyQ toxicity. These observations are consistent with another observation that mechanosen-sory neurons
of the Drosophila eye are resistant to polyQ-induced toxicity, whereas photoreceptor neurons are highly sensitive. Most
notably, PLM neuronal processes in these animals also display morphological abnormalities, and neuronal dysfunction
occurs in the absence of cell death. Importantly, studies of these animals also indicate that polyQ-mediated neuronal
dysfunction is independent of cell body aggregates and partially correlates with aggregation in
neuronal processes and abnormal morphology of
axons[99,100].
Yeast HD models Although several transgenic animal models exist for studying the functions of Htt, none are as readily
amenable to genetic analysis as yeast. Yeast models of HD have been created primarily by transgene approaches using
glutamine-encoding trinucleotide expansions. To provide a genetically tractable model system for the study of Htt, Krobitsch
et al engineered yeast cells to express an N-terminal fragment of Htt with different polyQ repeat lengths. Homopolymeric
tracts of CAG, the naturally occurring glutamine codon in Htt, are inherently unstable, and particularly so in yeast. To reduce
this problem, the CAG and Cytosine Adenine Adenine (CAA)-mixed codon polyQ repeats were performed in their
experiments according to the fact that glutamine is encoded by both CAG and CAA and that mixed-codon repeats are considerably
more stable[101_103]. The results indicate that the extent of aggregation varies with the length of the polyQ repeat. At the 2
extremes, most HtQ103 protein coalesced into a single large cytoplasmic aggregate, whereas HtQ25 exhibited no sign of
aggregation. Further-more, the polyQ fragments of Htt exhibited minimal toxicity in yeast, whether they were present in the
aggregated or soluble state[104].
In humans, Htt proteins with the longer repeat expansions not only produce disease earlier, but also expand the cell-type
distribution of toxicity. Lack of toxicity for the aggregated and soluble Htt fragments in yeast is advantageous for 3 reasons.
First, it provides an opportunity to study natural cellular factors that control the fate of misfolded polyQ proteins and to
search for potential therapeutic agents that affect misfolding, aggregation and degradation, without the complication of
deciphering the contributions of toxicity to the outcome of the assay. Second, polyQ-expanded proteins may perturb the
distribution of other cellular proteins by providing novel interaction
surfaces[105,106]. The nature of these interactions is most
readily and rapidly tested in an environment where the aggregation state of the polyQ protein can readily be manipulated.
Finally, the greatest conundrum of the polyQ diseases may be that most of the proteins are ubiquitously expressed, yet each
manifests its toxicity in a unique and distinct set of neurons. Yeast screens with different polyQ proteins may provide an
opportunity to identify the cell type-specific factors that contribute to the unique spectrum of toxicities and subsequently, to
search for factors that ameliorate their
effects[106].
Cell culture models of
HD Gene expression studies conducted with HD animal models have revealed profound
modifications in gene transcription. However, the complexity of
in vivo tissue hampers definition of very early transcriptional
modifications and does not allow discrimination between cell-autonomous changes and those resulting from intercellular
activity processes. An inducible, clonally derived, cell line expressing mHtt can offer a stable and con
trolled genetic and transcriptional background in which to perform gene expression studies. In such a system, biological and
experimental variability can be greatly reduced. Several cell models of HD are available. One of them is a mouse neuron
which has been fused with mouse teratoma cells, and the resulting hybrids have been stably transfected with various
polyQ-containing peptides. Another available model is based on stably transfected, temperature-sensitive, immortalized mouse
striatal neurons[107,108].
Stable, inducible PC12 models of HD expressing wild-type and mutant Htt, either in the context of an exon 1 fragment or
the full-length protein, have been employed. These models exhibit normal morphology and growth patterns which are
indistinguishable from the parental PC12 Tet-On line, together with high transgene inducibility and low background. These
cell lines expressed similar levels of endogenous and exogenous mHtt, analogous to the heterozygous condition. Compared
with the exon 1 model, these full-length cell lines exhibited very low levels of cell death in cycling cells. The mutant polyQ
lines also exhibited time-dependent aggregate formation, in both the nucleus and cytoplasm. ST14A cells, derived from rat
embryonic striatum, were also used to generate inducible cell lines expressing the N-terminal fragment of Htt. The advantage
of these models is that we can study events without the complications of cell death, because cell death in the mutant, mitotic
lines is very low and is no higher than that of the wild type
lines[109,110].
Cell models to elucidate the molecular pathogenesis of HD have been used. Many hypotheses have been produced
based on cell models studies in vitro. Some investigators have explored the role of autophagy in Htt processing in 3 cell
lines: clonal striatal cells, PC12 cells and rodent embryonic cells lacking cathepsin D. Results suggest that autophagy plays
a critical role in the degradation of N-terminal Htt. Blocking autophagy raises levels of exogenously expressed Htt, reduces
cell viability and increases the number of cells bearing mHtt aggregates. Stimulating autophagy promotes Htt degradation,
including the breakdown of caspase cleaved N-terminal Htt fragments. They also found that Htt expression increases levels
of the lysosomal enzyme cathepsin D by an autophagy-dependent pathway. These results indicate that altered processing
of mHtt by autophagy and cathepsin D may contribute to HD
pathogenesis[111]. The mammalian target of rapamycin is a
kinase which can inhibit autophagy in cells from yeast to humans. Rapamycin is a specific inhibitor of mammalian target of
rapamycin (mTOR) which can stimulate autophagy. Rapamycin is lipophilic and demonstates good blood-brain barrier
penetration. These results indicate that rapamycin is a good chemical candidate for use in treating HD. Ravikumar
et al found that the ability of rapamycin to inhibit mTOR activity may be impaired after prolonged Htt expression and thus
increased aggregate formation. Therefore, early treatment with rapamycin may attenuate
HD[112].
Classification and evaluation of HD models
As described earlier, there are many animal models which mimic some aspects of HD symptoms or pathology.
Characterization of multiple animal models is necessary to understand the mechanism of pathogenesis and the effects of potential
therapies. According to the different methods of classification, HD models can be divided into several respective categories.
According to the mechanism of replication, HD models can be divided into chemically-induced animal models and genetic
animal models. The chemically-induced HD models are the earliest models which were widely used before the identification
of the HD gene, including excitotoxic lesion models (glutamate, kainic acid, quinolinic acid-induced) and mitochondrial
dysfunction models (3-NP, malonate-induced). Genetic animal models, which are the most recent of HD models, can be
subdivided into transgenic (expressing the full-length or fragment of HD gene), knock-in, knock-out and virally-inserted
mutated polyQ tract models. It is well known that HD is a hereditary neurodegenerative disorder known as one of the CAG
trinucleotide repeat disorders. Therefore, genetic models mimic the molecular pathogenesis of HD more closely than that of
chemical lesion models.
According to the species of model animals, HD models can be divided into cell-free and cell culture models, lower
organisms (such as yeast, Drosophila, C
elegans, zebrafish), rodent models (mouse, rat), and non-human primate models
(monkey and baboon). The outcome measure of cell culture or cell-free models is cytotoxicity or some structural or
biochemical measure related to pathogenesis which is especially suitable for high-throughput screening. Lower organisms can
provide a more salient context for the genetic mutation and opportunity for genetic analyses. The disease expression and
response to potential therapies of the mammalian models can be quite close to that occurring in humans. Rodent and
non-human primates are particularly useful to elucidate aspects of the disease that are related to neuronal circuitry, a feature not
reproduced in in vitro models or lower animals.
After classification, the next crucial question is how to evaluate these models. Are there some advantages and limitations
of each model? Which model is the best model to closely mimic the symptoms and pathology of HD? In order to answer these
questions, we should first establish a standard. The ideal mouse model would have the following characteristics: (1) a robust
phenotype, well-defined neuro-behavioral abnormalities which are easy to be quantified; (2) neuropathological findings
which accurately mirror human patients; (3) rapid disease onset and progression; and (4) limited variability to enhance study
accuracy[113].
To date, there is no model which satisfies all the standards described above. Among all the HD models, including the
3-NP lesion model, R6/2 and R6/1 mice, N171 mice and YAC72 mice models studied in the most detail, it was established that
each model has its own inevitable advantages and limitations. For example, chronic systemic administration of 3-NP
produces motor dysfunctions and striatal lesions that share many of the characteristic histological and neurochemical
features of HD, therefore, this kind of model has attracted much attention over the last decades. However, although
metabolic toxin models may provide a higher level of neuropathological validity, the interanimal variability and the incidence
of gross nonspecific striatal damage are higher than that of excitotoxin lesion models and it requires very slow chronic
titration of delivery to achieve an acceptable level of specificity.
Another example is the genetic mouse model. Lots of genetic models have been created and all of these models share
some features with human HD. Both strains of R6 mice exhibit little cell death and neuritic pathology, but widespread NII.
N171 mice show striatal cell death and widespread NII. YAC72 mice, which contain the entire human gene and the human
promoter, show variable amounts of hyperactivity at about 1 year and then become less active, displaying striatal pathology
and evidence of apoptotic cell
death[114]. In general, the more similar to the human HD, the more closely the model
reproduces the exact neuropathological and molecular conditions for HD. Unfortunately, the more genetically accurate the model
(for example, YAC 128, YAC72 mice), the more variable and subtle the phenotype. Thus, it has so
far been much more feasible to use the fragment models
(R6/2, R6/1 mice) for therapeutics research because the outcomes are more prevalent and
definable[113]. An emerging strategy is to use the fragment models for most experiments and full-length transgenic or
knock-in mice for confirmatory studies once the potential of a compound has been established. Until there is more feedback from
human therapeutic experiments, it will not be known whether the fragment mouse models stand on their own as predictors of
responses to therapies or whether the full-length models are even necessary.
Implications of HD models in developing
candidate therapies
Animal models provide an opportunity to test potential treatments and explore their promise for translation to humans.
In the last several years, animal models of HD have been extensively used to test potential therapies. In order to get effective
and safe medicines from thousands to millions of small molecules, high-throughput screening should first be carried out.
When the lead compounds arise, proof of efficacy in mammalian models is considered a prerequisite before considering
possible testing in humans.
High-throughput screening techniques are essential for the identification of potential treatments. Cell-free, cultured cell
and other lower organism models are often used in the process of high-throughput screening, in which to explore the effect
of small molecules on some specific molecular targets in the disease process. Potential targets for high-throughput chemical
screenings include molecular cha-perones, caspases, the ubiquitin/proteasome proteases, transcription factors, and the
offending polyglutamine-containing proteins themselves. Much attention has been focused on screening for drugs that
prevent aggregation of Htt with the expanded polyQ
tract[42]. Since aggregates are such ubiquitous hallmarks of
neurodegenerative diseases, polyQ aggregates are a tempting target for high-throughput screening of pharmacological
agents that might block or disrupt aggregate formation. Many cell-free and cell-based screens have been
developed[115_118]. However, the potential efficacy of aggregate preventing compounds in relieving pathology must be addressed
in vivo.
Live animal screens exhibit inherently lower throughput than cell or cell-free-based screens. However, they can filter out
a large number of false leads in the early phases of screening. Efforts to automate and improve the throughput of live animal
screens are underway. High-throughput screening is also being planned using transgenic HD mice. Hickey
et al have identified early behavioral deficits in tests of motor function that are amenable to cost effective automated analysis in R6/2
mice. Running wheel activity and climbing behavior were reduced in R6/2 mice from as early as 4.5 weeks of age, at a time
when rotarod performance and grip strength were still normal. Power calculations showed that the running wheel test could
detect improvement with manageable group sizes. This test can be automated and requires little manual input. Therefore, the
running wheel test is appropriate for efficient, high-throughput drug screening at an early
age[119].
Identification of lead compounds from high-throughput screening then proceeds to animal testing where a large number
drop out, either because they do not ameliorate the disease process
in vivo or because of some unwanted side effects.
Therefore, this process of drug development is slow and expensive. It could proceed more rapidly and at lower cost by using
non-vertebrate organisms that can be genetically engineered and have short generation times that allow rapid identification
of the most promising strategies.
Drosophila models have proven effective in rapidly allowing the efficacy of various
pharmacological and synthetic peptide agents on neuropathology to be
tested[118,120].
Apostol et al have developed inducible PC12 cell-culture models to screen for loss of visible aggregates. To test the
validity of this approach, compounds that inhibit aggregation in the PC12 cell-based screen were tested in a
Drosophila model. The disruption of aggregation in PC12 cells strongly correlates with the suppression of neuronal degeneration in
Drosophila. Thus, the engineered PC12 cells coupled with the
Drosophila model provide a rapid and effective method to
screen and validate compounds[121]. The concordance of compounds that are effective in both fly and mouse models of HD
underscores the utility of using fly models of human disease to screen for target pathways. It also argues that wider use of
invertebrate systems to screen directly for compounds that lead to functional neurological improvement may be
effective[122-124].
Summary and conclusion
As described earlier, there are many animal models closely mimicking HD symptoms or pathology. They include
chemical-induced HD animal models, genetic animal HD models in which cell-free and cell culture, lower organisms (such as yeast,
Drosophila, C. elegans, zebrafish), rodent (mouse, rat) and non-human primates are involved. These animal models provide
some accessible systems in which to study molecular pathogenesis and to test potential treatments. Among these models,
3-NP induced and R6/2 transgenic mouse models are most commonly used. Because HD is an inherited disorder caused by
gene mutation, genetic models more faithfully replicate the human condition than chemical lesion models. Among all the
genetic models, knock-in mice have the advantage of carrying the mutation in the appropriate protein context, the full-length
Huntington protein and under the endogenous promoter which makes knock-in mice the most valuable models at present.
Currently, screening for drugs and therapeutics takes place in many laboratories. However, the development of more
effective therapies may not come until better animal models are available for evaluation of a drug's efficacy. The usefulness
of genetic models is invaluable and the seminal multidisciplinary work promises a very exciting future for understanding the
pathological mechanisms of HD and for devising new avenues for treatment. Despite this promising beginning, none of the
models replicate the massive cell loss of striatal neurons occurring in human patients. Much remains to be done to create
mechanistically significant, full models of this disease. A combination of new genetic approaches and closer mimicking of the
environment in which the disease develops in humans is required.
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