An accepted prerequisite for clinical trials of a compound in humans is the successful alleviation of the disease in animal models. For some diseases, however, successful translation of drug effects from mouse models to the bedside has been limited. One question is whether the current models accurately reproduce the human disease. Here, we examine the mouse models that are available for therapeutic testing in Huntington disease (HD), a late-onset neurodegenerative disorder for which there is no effective treatment. The current mouse models show different degrees of similarity to the human condition. Significant phenotypic differences are seen in mouse models that express either truncated or full-length human, or full-length mouse, mutant huntingtin (mHTT). These differences in phenotypic expression may be attributable to the influences of protein context, mouse strain and a difference in regulatory sequences between the mouse Htt and human HTT genes.

The vision: from mechanisms in mice to therapies in humans

The most important rationale for modelling human disorders in a non-human organism is the identification of fundamental pathogenic mechanisms that lead to novel therapeutic targets, and the evaluation of the efficacy and safety of potential new drugs. Historically, mice have been used to model human disease because of their physiological, anatomical and genomic similarities to humans. For neurodegenerative diseases, the mammalian neuronal physiology and the human-like brain anatomy makes mouse models especially useful. Nevertheless, their short life span of approximately 2 years is a serious limitation for late-onset neurodegenerative disease and the nervous system of humans is significantly more similar to that of primates. However, the costs and logistics of performing large-scale therapeutic trials in non-human primates are prohibitive. The question therefore remains about whether any positive outcome from therapeutic trials in mice is sufficient evidence to proceed with studies in humans. An alternative question is whether human trials should commence when a drug against a compelling target has been identified that did not show efficacy in mice.

The reality: the predictive value of therapeutic trials in mice

For amyotrophic lateral sclerosis (ALS), the most widely used mouse model has proven to be of poor predictive value: approximately 3% of all cases of ALS are caused by point mutations in the superoxide dismutase (SOD1) gene, and the most commonly used ALS mouse model expresses human mutant SOD1 (G93A) as a transgene (Gurney et al., 1994). A recent review summarized 78 individual and 12 combination compound trials that have been conducted using this mouse model (Benatar, 2007), which subsequently led to 11 double-blind, placebo-controlled clinical trials in humans, all of which failed (Vincent et al., 2008). Even compounds such as creatine, minocycline or cyclooxygenase-2 inhibitors that were most promising in mouse trials (Klivenyi et al., 1999; Drachman et al., 2002; Zhu et al., 2002) have not yet been shown to be beneficial in human clinical trials (Groeneveld et al., 2003; Shefner et al., 2004; Cudkowicz et al., 2006; Gordon et al., 2007). Riluzole remains the only FDA approved drug for ALS (Miller et al., 2003).

The SOD1 mouse model more closely resembles familial than sporadic ALS and the possibility of biologically relevant differences between these forms of the disorder might limit the usefulness of the model. In addition, treatment in mice is commonly started before the onset of symptoms; this cannot be replicated in predominantly sporadic human diseases, such as ALS. These serious limitations apply to all diseases where familial and sporadic forms exist, such as Alzheimer’s and Parkinson’s diseases.

Therapeutic trials for Huntington disease (HD), however, have significant advantages: the expansion of a polyglutamine tract in the huntingtin HTT protein has been determined as the single cause of the disease (The Huntington’s Disease Collaborative Research Group, 1993). Therefore, it should be easier to generate an accurate mouse model of HD than for other conditions with multiple, often unknown, causes. Furthermore, predictive testing is available to accurately predict whether, and when, a person will be affected (Tibben, 2007), making presymptomatic treatment in humans a feasible option. To assess the efficacy of potential therapeutics in the HD mouse model, multiple endpoints have been established that allow the monitoring of neuropathological and behavioural changes in the YAC128 HD mouse model (Table 1) (Slow et al., 2003; Van Raamsdonk et al., 2005a; Van Raamsdonk et al., 2005b).

HD mouse models: too much choice?

HD is characterised by the progressive deterioration of cognitive and motor functions, and the selective loss of GABAergic medium spiny striatal neurons, as well as glutamatergic cortical neurons that project to the striatum (Albin et al., 1990). Aberrant proteolysis of mutant HTT (mHTT) has been proposed to play a role in HD (Wellington et al., 2000; Gafni et al., 2004; Graham et al., 2006), leading to the accumulation of N-terminal mHTT fragments in intracellular inclusions (DiFiglia et al., 1997).

Table 1.

Neuropathological and behavioural endpoints for evaluation of therapeutic candidates in the YAC128 HD mouse model

Primary endpoints
NeuropathologicalBehavioural Motor function
Striatal volume loss Rotarod abnormalities* 
Striatal neuronal loss  
Primary endpoints
NeuropathologicalBehavioural Motor function
Striatal volume loss Rotarod abnormalities* 
Striatal neuronal loss  
Secondary endpoints
PathologicalBehavioural
NeuropathologyPeripheral pathologyMotor functionCognitive function
Reduction in average volume of striatal neurons Testicular degeneration Open-field activity Pre-pulse inhibition 
Brain weight loss    
Cortical volume loss    
Nuclear translocation of mutant HTT    
Caspase-6 activation    
Secondary endpoints
PathologicalBehavioural
NeuropathologyPeripheral pathologyMotor functionCognitive function
Reduction in average volume of striatal neurons Testicular degeneration Open-field activity Pre-pulse inhibition 
Brain weight loss    
Cortical volume loss    
Nuclear translocation of mutant HTT    
Caspase-6 activation    
*

Deficits on the fixed and accelerating rotarod. References: Slow et al., 2003; Van Raamsdonk et al., 2005a; Van Raamsdonk et al., 2005b.

Huntingtin fragment mouse models

HD is characterised by its late onset, therefore reproducing the human HD phenotype within the life span of a mouse represents a major challenge. Different approaches have been taken to increase the severity of the disease phenotype in various models. The R6/2 and N-171 mouse models use the fact that the expression of N-terminal mHTT fragments causes acute neuronal toxicity, by expressing truncated human mHTT consisting of either the first of 67 exons (Mangiarini et al., 1996) or the N-terminal 171 amino acids (Schilling et al., 1999). The drawbacks of these models are the loss of the natural genomic and protein context of the polyglutamine expansion, which could lead to altered regulation and a loss of potential disease-modifying post-translational modifications and protein interactions. Additionally, it has been shown that mHTT fragments generated by intracellular cleavage have a different subcellular localisation than truncated mHTT (Warby et al., 2008). It is therefore possible that the process of mHTT cleavage is in itself relevant to the pathogenesis of HD. In other words, although certain truncated fragments may cause neuronal toxicity, this might not necessarily recapitulate the disease.

The R6/2 fragment mouse model (Mangiarini et al., 1996) has been used in a number of therapeutic trials, and many compounds that have been effective in R6/2 mice have proceeded to clinical trials, with mixed results. Riluzole, remacemide and CoQ10 have successfully ameliorated the disease phenotype in R6/2 mice (Schilling et al., 2001; Ferrante et al., 2002; Schiefer et al., 2002), but were not beneficial for patients in the corresponding clinical trials (Huntington Study Group, 2001; Landwehrmeyer et al., 2007). The clinical trial of CoQ10 showed a trend towards a slowing in the decline of functional capacity, although it was not statistically significant (Huntington Study Group, 2001).

Full-length huntingtin mouse models

Murine models expressing full-length human mHTT were first reported in 1999 with the generation of the YAC46 and YAC72 models (Hodgson et al., 1999). In an effort to create a mouse model with more robust and earlier behavioural abnormalities that would be better suited for therapeutic trials, an additional YAC transgenic mouse was established in which the mHTT contained 128 glutamines (Slow et al., 2003). These mice show a uniform phenotype with age-dependent striatal and subsequent cortical neurodegeneration (Van Raamsdonk et al., 2005a), and have progressive motor and cognitive deficits (Van Raamsdonk et al., 2005b). A progressive deficit on the rotarod is observed in this line starting at 2–4 months of age, which correlates highly with neuronal loss in the striatum (Slow et al., 2003).

Trials of several candidate therapeutic compounds have been performed in the YAC128 mice: ethyl-EPA showed a modest rescue of the behavioural deficits (Van Raamsdonk et al., 2005d), whereas a trial of cystamine demonstrated protection from neuronal loss but no effect on the motor phenotype (Van Raamsdonk et al., 2005c). In human patients, ethyl-EPA led to a slight improvement of motor dysfunction as well as a reduction in cerebral atrophy in the caudate and thalamus (Puri et al., 2005), which was not seen in the YAC128 mice. Tetrabenazine had positive effects on the behavioural phenotype in mice (Tang et al., 2007), which is in agreement with the results from a clinical trial (Huntington Study Group, 2006). However, the true predictive value of this mouse model will only be established when additional compounds are tested and proceed to clinical trials.

Recently, a bacterial artificial chromosome (BAC) mouse expressing full-length human mHTT was established (BACHD mice) (Gray et al., 2008). The BACHD mice exhibit progressive motor deficits and late-onset selective neurodegeneration in the cortex and striatum, consistent with the findings in the YAC128 animals. Similar to the YAC model, the BACHD mice are also inherently well suited for therapeutic trials.

Fig. 1.

Comparison of phenotypes between transgenic and knock-in mouse models of HD and human patients. Transgenic YAC mice express human mHTT with its regulatory sequences and exhibit an earlier and more severe phenotype than knock-in mice, which express the CAG expansion in the context of the mouse Htt gene. References: *Graham et al., 2006; Slow et al., 2003; #Van Raamsdonk et al., 2005b; §Lerch et al., 2008; **Heng et al., 2007; ††Pouladi et al., 2009; ##Tang et al., 2007.

Fig. 1.

Comparison of phenotypes between transgenic and knock-in mouse models of HD and human patients. Transgenic YAC mice express human mHTT with its regulatory sequences and exhibit an earlier and more severe phenotype than knock-in mice, which express the CAG expansion in the context of the mouse Htt gene. References: *Graham et al., 2006; Slow et al., 2003; #Van Raamsdonk et al., 2005b; §Lerch et al., 2008; **Heng et al., 2007; ††Pouladi et al., 2009; ##Tang et al., 2007.

The knock-in mouse model of HD expresses mHTT in the most appropriate genomic and protein context, since it has a polyQ sequence inserted into the endogenous mouse Htt gene (Shelbourne et al., 1999). These mice display a very late-onset phenotype, which is strongest in the 150 CAG homozygotes (Heng et al., 2007). A rotarod deficit was reported at ~25 months of age and occurs in parallel with a reduction in striatal neuron counts and striatal volume (Heng et al., 2007), representing a protracted time course compared with the behavioural phenotypes observed in the YAC128 and BACHD animals (Fig. 1). mHTT expression levels cannot explain this striking difference in age of onset since the YAC128 mice express mHTT at similar levels to the endogenous mouse protein (Slow et al., 2003). However, a patch/matrix pattern of HTT immunoreactivity, as seen in the human striatum (Ferrante et al., 1997), has not been observed in the mouse (Bhide et al., 1996), which could point to differences in expression patterns between the human and mouse proteins. A lingering question is whether inherent differences between the human and mouse HTT protein-coding or regulatory sequences could result in the earlier and more profound changes in transgenic HD mouse models compared with the knock-in HD mouse models.

Can we attribute the phenotypic differences in full-length transgenic and knock-in models to species-specific sequence differences in huntingtin between mice and humans?

The human HTT protein (NP_002102) is 3144 amino acids long and the mouse protein (NP_034544) contains 3120 amino acids. Excluding the differences in their polyglutamine and polyproline tracts, the human and mouse proteins are 91% identical and 95% similar.

The majority of the functional elements in human and mouse HTT do not differ or have only the most conservative amino acid substitutions and are thus unlikely to account for differences in the disease phenotypes of YAC128 and knock-in mouse models. Conserved functional elements include caspase, calpain and aspartyl protease cleavage sites; palmitoylation, phosphorylation, sumoylation and ubiquitylation sites; nucleocytoplasmic shuttling signals; the HTT membrane-associated domain; and a proteasome-targeting sequence (Table 2). One apparent exception is the calpain cleavage site at residues 468–470 that contributes to the toxicity of mHTT when cleaved (Gafni et al., 2004). The human sequence at this site is Leu-Thr-Ala; in the mouse it is Phe-Ala-Ala. This is consistent with the human site being a more favourable cleavage site than that in the mouse (Tompa et al., 2004), which might be significant in light of the increased calpain activity in caudate tissue from human HD patients (Gafni and Ellerby, 2002).

The genomic organisation of the human HTT gene is well conserved in the mouse. This large gene consists of 67 exons that comprise about 170 kb of the human genome and 150 kb of the mouse genome. Both organisms have two mRNAs that are generated by alternative cleavage and polyadenylation of the primary transcript, producing a long and a short 3′ untranslated region (UTR) that differ by 3 kb (Lin et al., 1993; Lin et al., 1994). In both organisms, the long-UTR transcript is predominantly expressed in the brain, whereas the short-UTR transcript is more widely expressed (Lin et al., 1993). Separate from the alternative 3′UTRs, two HTT/Htt transcripts that differ by ~1.4 kb are seen in both the human and mouse brain (Lin et al., 1994). Although there is experimental evidence for this second pair of transcripts in both the mouse and human, Ensembl predictions only support their existence in the mouse genome, with one transcript missing ~480 amino acids from exons 36 to 43 (see ENSMUSG00000029104 at http://www.ensembl.org) (Hubbard et al., 2007). The corresponding human genomic data do not support this alternative splicing event (ENSG00000197386), which could influence phenotypes in mouse models.

Table 2.

Protein sequence elements that do not differ between human and mouse HTT

Functional siteLocation*References
Caspase-6 cleavage IVLD 586 Wellington et al., 2000  
Caspase-2/3 cleavage DLND 552 Hermel et al., 2004  
Caspase-3 cleavage DVSD 513 Wellington et al., 2000  
Palmitoylation C214 Yanai et al., 2006  
Phosphorylation S421 Warby et al., 2005  
Calpain cleavage VLS 438 Gafni and Ellerby, 2002  
 SSS 537  
Aspartyl protease cleavage HCLT…NIVA 104–114 Lunkes et al., 2002  
 RMVA…LLPC 146–214  
Sumoylation, ubiquitylation K6, K9, R15 Steffan et al., 2004  
Proteasome-targeting sequences FQKLL 127 Chandra et al., 2008  
Nuclear export signal IIIS…RLPL 2397–2406 Xia et al., 2003  
Putative nuclear localisation signal PIRR…KEKE 1182–1191 Bessert et al., 1995  
Membrane-associated domain/cytoplasmic retention signal MATL…LKSF 1–17 Steffan et al., 2004; Truant et al., 2006  
Functional siteLocation*References
Caspase-6 cleavage IVLD 586 Wellington et al., 2000  
Caspase-2/3 cleavage DLND 552 Hermel et al., 2004  
Caspase-3 cleavage DVSD 513 Wellington et al., 2000  
Palmitoylation C214 Yanai et al., 2006  
Phosphorylation S421 Warby et al., 2005  
Calpain cleavage VLS 438 Gafni and Ellerby, 2002  
 SSS 537  
Aspartyl protease cleavage HCLT…NIVA 104–114 Lunkes et al., 2002  
 RMVA…LLPC 146–214  
Sumoylation, ubiquitylation K6, K9, R15 Steffan et al., 2004  
Proteasome-targeting sequences FQKLL 127 Chandra et al., 2008  
Nuclear export signal IIIS…RLPL 2397–2406 Xia et al., 2003  
Putative nuclear localisation signal PIRR…KEKE 1182–1191 Bessert et al., 1995  
Membrane-associated domain/cytoplasmic retention signal MATL…LKSF 1–17 Steffan et al., 2004; Truant et al., 2006  
*

Motif sequence and position of the last residue in the motif in human HTT NP_002102, or sequence range for motifs >6 residues.

The nuclear export signal has two conservative Ile to Val changes between the human and mouse.

Differences in human and mouse huntingtin regulatory sequences

Transcription factor binding sites

YAC128 mice contain ~308 kb of the human HTT region including ~24 kb upstream of the coding sequence (M.R.H. and R. Devon, unpublished) (Bates et al., 1992; Hodgson et al., 1996; Slow et al., 2003). Human-mouse conservation of this region is characterised by a few segments of high nucleotide sequence identity (>80%) interrupted by long regions of very dissimilar sequences, precluding alignment. Since HTT in YAC mice includes all the local conserved regions and CpG islands that are expected to influence HTT regulation (based on ‘Mammal Cons’ and ‘CpG islands’ tracks in the UCSC genome browser, http://genome.ucsc.edu/, March 2006 genome assembly) (Kent et al., 2002), we would expect that it is regulated in the mouse as it is in humans, defined by its native promoter and internal regulatory elements. The inclusion of ~24 kb upstream of the HTT transcription start site plus ~116 kb downstream of the gene makes it highly likely that even distant regulatory elements have been included in the YAC mouse model. BACHD mice are expected to retain a similar range of crucial elements since their human transgene includes ~20 kb upstream and ~50 kb downstream of the HTT gene (Gray et al., 2008). Regulatory elements that are farther than 20 kb from a gene’s transcription start site are less common. However, they cannot be excluded as a possibility for the HD gene and would not be present in either the BACHD or YAC mouse models.

Some experimentally validated sites of transcription factor binding show differences between the mouse and human. For example, human HTT is bound by the transcription factor NRSF/REST (neuron-restrictive silencer factor/repressor element 1 silencing transcription factor) (Johnson et al., 2008), which represses the expression of neuronal genes in non-neuronal cells (Schoenherr and Anderson, 1995). This site is not conserved in the mouse and the relevant Htt region shares no alignable sequence with human HTT in this intronic region. Differences between mouse Htt and human HTT targeting by NRSF might influence pathology.

Table 3.

Human HTT or mouse Htt sequence-specific targeting by miRNAs*

Human sequence-specific miRNAsMouse sequence-specific miRNAs
miR-23 miR-15/miR-16/miR-195/miR-424/miR-497 
miR-24 miR-22 
miR-30-5p miR-24 
miR-103/miR-107 miR-30-3p 
miR-128 miR-31 
miR-148/miR-152 miR-133 
miR-181 miR-223 
miR-204/miR-211 miR-224 
miR-214 miR-331 
miR-370 miR-342 
 miR-378 
 miR-491 
Human sequence-specific miRNAsMouse sequence-specific miRNAs
miR-23 miR-15/miR-16/miR-195/miR-424/miR-497 
miR-24 miR-22 
miR-30-5p miR-24 
miR-103/miR-107 miR-30-3p 
miR-128 miR-31 
miR-148/miR-152 miR-133 
miR-181 miR-223 
miR-204/miR-211 miR-224 
miR-214 miR-331 
miR-370 miR-342 
 miR-378 
 miR-491 
*

These miRNAs function in both humans and mice but are strongly predicted to target either the human HTT or mouse Htt 3′UTR, where that site sequence is not conserved in both organisms.

miRNAs implicated in HD pathology.

Table 4.

Strain differences between HD mouse models

Mouse HD modelCAG sizeStrainReferences
R6/2 model 115–150 CAG CBA C57BL/6 mixed background Mangiarini et al., 1996  
N-171 model 18, 44, 82 CAG C3H/HEJ C57BL/6JF1 mixed background Schilling et al., 1999  
YAC models 18, 46, 72, 128 CAG FVB/N Hodgson et al., 1999; Slow et al., 2003  
BACHD model 97 CAG FVB/NJ Gray et al., 2008  
Knock-in models 72–80 CAG 129/Svx C57BL/6 mixed background Shelbourne et al., 1999  
  129/Svx FVB/N mixed background  
 50, 92, 111 CAG 129/CD1 mixed background Wheeler et al., 2000  
 94, 140 CAG 129/Sv C57BL/6 mixed background Menalled et al., 2002; Menalled et al., 2003  
 150 CAG 129/Ola C57BL/6J mixed background Lin et al., 2001 
Mouse HD modelCAG sizeStrainReferences
R6/2 model 115–150 CAG CBA C57BL/6 mixed background Mangiarini et al., 1996  
N-171 model 18, 44, 82 CAG C3H/HEJ C57BL/6JF1 mixed background Schilling et al., 1999  
YAC models 18, 46, 72, 128 CAG FVB/N Hodgson et al., 1999; Slow et al., 2003  
BACHD model 97 CAG FVB/NJ Gray et al., 2008  
Knock-in models 72–80 CAG 129/Svx C57BL/6 mixed background Shelbourne et al., 1999  
  129/Svx FVB/N mixed background  
 50, 92, 111 CAG 129/CD1 mixed background Wheeler et al., 2000  
 94, 140 CAG 129/Sv C57BL/6 mixed background Menalled et al., 2002; Menalled et al., 2003  
 150 CAG 129/Ola C57BL/6J mixed background Lin et al., 2001 

Putative microRNA targets

Human HTT and mouse Htt may also be differentially regulated by microRNA (miRNA) families that function in both humans and mice, which could provide another basis for the differences between mice expressing the mouse gene versus the human gene. There is a precedent for miRNA modulation of cytotoxicity in the polyglutamine-expansion disorder spinocerebellar ataxia 1: several miRNAs co-regulate ataxin 1 at both the RNA and protein levels and influence cyotoxicity of the polyglutamine-expanded protein (Lee et al., 2008). Using TargetScan 4.2, we identified sites that, owing to sequence differences, would be targeted exclusively in the long 3′ UTRs in either the human or mouse. Context scores in TargetScan can be used to predict site performance (Lewis et al., 2003; Grimson et al., 2007), and we selected only miRNA targets that are likely to be significantly correlated with protein downregulation (Baek et al., 2008). Human HTT is predicted to be targeted by ten miRNAs that are conserved in humans and mice, but whose seed sequences are not conserved in the homologous site in the mouse 3′UTR (Table 3). Conversely, TargetScan predicts that mouse Htt is targeted by 12 miRNAs that are conserved in humans and mice but whose seed sequences are not conserved in the homologous site in the human 3′UTR (Table 3). miRNAs from both columns of Table 3, miR-133, miR-204, miR-30-5p and miR-195, have been implicated in HD pathology and related processes (Kim et al., 2007; Johnson et al., 2008; Mellios et al., 2008), pointing to a complex network of dysregulation. The binding of miRNAs can be strongly influenced by single nucleotide differences in their seed regions (Grimson et al., 2007), so unidentified differences between the HTT/Htt 3′UTRs in different mouse models could exacerbate phenotypic differences. Although YAC and knock-in mouse models express mHTT proteins at similar levels in different tissues (Slow et al., 2003), the potential for differential miRNA regulation suggests that there could be important differences in cell type-specific regulation of HTT. Clearly, further studies are needed to explore whether these might form the basis for different phenotypic manifestations of the mouse Htt and the human HTT gene.

Strain differences between knock-in and YAC or BAC mouse models

Another important issue when comparing different mouse models is the respective background strain of mouse used. Breeding of a mouse model onto another strain can lead to changes in phenotypic severity. An Alzheimer’s disease mouse model expressing mutant amyloid precursor protein is lethal on pure FVB/N or C57BL/6J backgrounds but has a milder phenotype on outbred mouse strains (Carlson et al., 1997). A similar phenomenon was observed in caspase-3 knockout mice, which show perinatal lethality on a 129/B6F1 background, whereas backcrossing to the strain C57BL/6J dramatically improved the survival rate (Zheng et al., 1999; Houde et al., 2004).

The transgenic full-length YAC128 and BACHD models are on a pure FVB/N background, whereas most mHTT fragment and knock-in models are on a C57BL/6 mixed background (Table 4) (Mangiarini et al., 1996; Shelbourne et al., 1999; Slow et al., 2003; Gray et al., 2008). Neuronal toxicity mediated by the activation of glutamate receptors (excitotoxicity) contributes to the selective neurodegeneration observed in HD (Pouladi et al., 2006). Mounting evidence reveals that there is a strong genetic component influencing the susceptibility to excitotoxicity; the C57BL/6 strain background confers more resistance to most of the excitotoxins examined when compared with the FVB/N background (Schauwecker, 2005). This difference in susceptibility to excitotoxic neuronal death might underlie the differences in severity of the HD phenotype that were observed between the full-length YAC and BAC HD models compared with the knock-in mouse models.

Conclusion

The use of genetically modified mice has led to great strides in advancing our understanding of disease owing to their physiological similarity to humans. However, over the last decade, the use of murine models of neurodegenerative diseases in preclinical testing shows that therapeutic success in animal models is not always paralleled by clinical success in patients. The experience with HD mouse models is instructive in this regard, showing that introduction of the polyQ expansion sequence into the mouse Htt gene leads to a less severe phenotype and a more protracted time course when compared with that seen with transgenic expression of the full-length mutant human HTT gene. Furthermore, mouse models expressing a truncated form of mHTT that have been used extensively for preclinical testing had very limited predictive value for therapeutic success in patients, suggesting that success in more than one mouse model should be a prerequisite for embarking on clinical trials. However, for those drugs with appropriate ADME (adsorption, distribution, metabolism, excretion) and safety profiles against a compelling disease-relevant target, human trials could commence as soon as the compound becomes available. This approach could lead to expeditious and parallel preclinical trials that target verified disease-relevant pathways, and could provide hope for HD patients who currently have no therapy to influence the course of the illness, while raising the bar for drugs with less well-defined mechanisms of action.

REFERENCES

Albin
RL
,
Young
AB
,
Penney
JB
,
Handelin
B
,
Balfour
R
,
Anderson
KD
,
Markel
DS
,
Tourtellotte
WW
,
Reiner
A
. (
1990
).
Abnormalities of striatal projection neurons and N-methyl-D-aspartate receptors in presymptomatic Huntington’s disease
.
N. Engl. J. Med
.
322
,
1293
1298
.
Baek
D
,
Villen
J
,
Shin
C
,
Camargo
FD
,
Gygi
SP
,
Bartel
DP
(
2008
).
The impact of microRNAs on protein output
.
Nature
455
,
64
71
.
Bates
GP
,
Valdes
J
,
Hummerich
H
,
Baxendale
S
,
Le Paslier
DL
,
Monaco
AP
,
Tagle
D
,
MacDonald
ME
,
Altherr
M
,
Ross
M
, et al. 
(
1992
).
Characterization of a yeast artificial chromosome contig spanning the Huntington’s disease gene candidate region
.
Nat. Genet
.
1
,
180
187
.
Benatar
M
(
2007
).
Lost in translation: treatment trials in the SOD1 mouse and in human ALS
.
Neurobiol. Dis
.
26
,
1
13
.
Bessert
DA
,
Gutridge
KL
,
Dunbar
JC
,
Carlock
LR
(
1995
).
The identification of a functional nuclear localization signal in the Huntington disease protein
.
Brain Res. Mol. Brain Res
.
33
,
165
173
.
Bhide
PG
,
Day
M
,
Sapp
E
,
Schwarz
C
,
Sheth
A
,
Kim
J
,
Young
AB
,
Penney
J
,
Golden
J
,
Aronin
N
, et al. 
(
1996
).
Expression of normal and mutant huntingtin in the developing brain
.
J. Neurosci
.
16
,
5523
5535
.
Carlson
GA
,
Borchelt
DR
,
Dake
A
,
Turner
S
,
Danielson
V
,
Coffin
JD
,
Eckman
C
,
Meiners
J
,
Nilsen
SP
,
Younkin
SG
, et al. 
. (
1997
).
Genetic modification of the phenotypes produced by amyloid precursor protein overexpression in transgenic mice
.
Hum. Mol. Genet
.
6
,
1951
1959
.
Chandra
S
,
Shao
J
,
Li
JX
,
Li
M
,
Longo
FM
,
Diamond
MI
(
2008
).
A common motif targets huntingtin and the androgen receptor to the proteasome
.
J. Biol. Chem
.
283
,
23950
23955
.
Cudkowicz
ME
,
Shefner
JM
,
Schoenfeld
DA
,
Zhang
H
,
Andreasson
KI
,
Rothstein
JD
,
Drachman
DB
(
2006
).
Trial of celecoxib in amyotrophic lateral sclerosis
.
Ann. Neurol
.
60
,
22
31
.
DiFiglia
M
,
Sapp
E
,
Chase
KO
,
Davies
SW
,
Bates
GP
,
Vonsattel
JP
,
Aronin
N
(
1997
).
Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain
.
Science
277
,
1990
1993
.
Drachman
DB
,
Frank
K
,
Dykes-Hoberg
M
,
Teismann
P
,
Almer
G
,
Przedborski
S
,
Rothstein
JD
(
2002
).
Cyclooxygenase 2 inhibition protects motor neurons and prolongs survival in a transgenic mouse model of ALS
.
Ann. Neurol
.
52
,
771
778
.
Ferrante
RJ
,
Gutekunst
CA
,
Persichetti
F
,
McNeil
SM
,
Kowall
NW
,
Gusella
JF
,
MacDonald
ME
,
Beal
MF
,
Hersch
SM
(
1997
).
Heterogeneous topographic and cellular distribution of huntingtin expression in the normal human neostriatum
.
J. Neurosci
.
17
,
3052
3063
.
Ferrante
RJ
,
Andreassen
OA
,
Dedeoglu
A
,
Ferrante
KL
,
Jenkins
BG
,
Hersch
SM
,
Beal
MF
(
2002
).
Therapeutic effects of coenzyme Q10 and remacemide in transgenic mouse models of Huntington’s disease
.
J. Neurosci
.
22
,
1592
1599
.
Gafni
J
,
Ellerby
LM
(
2002
).
Calpain activation in Huntington’s disease
.
J. Neurosci
.
22
,
4842
4849
.
Gafni
J
,
Hermel
E
,
Young
JE
,
Wellington
CL
,
Hayden
MR
,
Ellerby
LM
(
2004
).
Inhibition of calpain cleavage of huntingtin reduces toxicity: accumulation of calpain/caspase fragments in the nucleus
.
J. Biol. Chem
.
279
,
20211
20220
.
Gordon
PH
,
Moore
DH
,
Miller
RG
,
Florence
JM
,
Verheijde
JL
,
Doorish
C
,
Hilton
JF
,
Spitalny
GM
,
MacArthur
RB
,
Mitsumoto
H
, et al. 
(
2007
).
Efficacy of minocycline in patients with amyotrophic lateral sclerosis: a phase III randomised trial
.
Lancet Neurol
.
6
,
1045
1053
.
Graham
RK
,
Deng
Y
,
Slow
EJ
,
Haigh
B
,
Bissada
N
,
Lu
G
,
Pearson
J
,
Shehadeh
J
,
Bertram
L
,
Murphy
Z
, et al. 
(
2006
).
Cleavage at the caspase-6 site is required for neuronal dysfunction and degeneration due to mutant huntingtin
.
Cell
125
,
1179
1191
.
Gray
M
,
Shirasaki
DI
,
Cepeda
C
,
Andre
VM
,
Wilburn
B
,
Lu
XH
,
Tao
J
,
Yamazaki
I
,
Li
SH
,
Sun
YE
, et al. 
. (
2008
).
Full-length human mutant huntingtin with a stable polyglutamine repeat can elicit progressive and selective neuropathogenesis in BACHD mice
.
J. Neurosci
.
28
,
6182
6195
.
Grimson
A
,
Farh
KK
,
Johnston
WK
,
Garrett-Engele
P
,
Lim
LP
,
Bartel
DP
(
2007
).
MicroRNA targeting specificity in mammals: determinants beyond seed pairing
.
Mol. Cell
27
,
91
105
.
Groeneveld
GJ
,
Veldink
JH
,
van der Tweel
I
,
Kalmijn
S
,
Beijer
C
,
de Visser
M
,
Wokke
JH
,
Franssen
H
,
van den Berg
LH
(
2003
).
A randomized sequential trial of creatine in amyotrophic lateral sclerosis
.
Ann. Neurol
.
53
,
437
445
.
Gurney
ME
,
Pu
H
,
Chiu
AY
,
Dal Canto
MC
,
Polchow
CY
,
Alexander
DD
,
Caliendo
J
,
Hentati
A
,
Kwon
YW
,
Deng
HX
, et al. 
(
1994
).
Motor neuron degeneration in mice that express a human Cu,Zn superoxide dismutase mutation
.
Science
264
,
1772
1775
.
Heng
MY
,
Tallaksen-Greene
SJ
,
Detloff
PJ
,
Albin
RL
(
2007
).
Longitudinal evaluation of the Hdh(CAG)150 knock-in murine model of Huntington’s disease
.
J. Neurosci
.
27
,
8989
8998
.
Hermel
E
,
Gafni
J
,
Propp
SS
,
Leavitt
BR
,
Wellington
CL
,
Young
JE
,
Hackam
AS
,
Logvinova
AV
,
Peel
AL
,
Chen
SF
, et al. 
(
2004
).
Specific caspase interactions and amplification are involved in selective neuronal vulnerability in Huntington’s disease
.
Cell Death Differ
.
11
,
424
438
.
Hodgson
JG
,
Smith
DJ
,
McCutcheon
K
,
Koide
HB
,
Nishiyama
K
,
Dinulos
MB
,
Stevens
ME
,
Bissada
N
,
Nasir
J
,
Kanazawa
I
, et al. 
(
1996
).
Human huntingtin derived from YAC transgenes compensates for loss of murine huntingtin by rescue of the embryonic lethal phenotype
.
Hum. Mol. Genet
.
5
,
1875
1885
.
Hodgson
JG
,
Agopyan
N
,
Gutekunst
CA
,
Leavitt
BR
,
LePiane
F
,
Singaraja
R
,
Smith
DJ
,
Bissada
N
,
McCutcheon
K
,
Nasir
J
, et al. 
(
1999
).
A YAC mouse model for Huntington’s disease with full-length mutant huntingtin, cytoplasmic toxicity, and selective striatal neurodegeneration
.
Neuron
23
,
181
192
.
Houde
C
,
Banks
KG
,
Coulombe
N
,
Rasper
D
,
Grimm
E
,
Roy
S
,
Simpson
EM
,
Nicholson
DW
(
2004
).
Caspase-7 expanded function and intrinsic expression level underlies strain-specific brain phenotype of caspase-3-null mice
.
J. Neurosci
.
24
,
9977
9984
.
Hubbard
TJ
,
Aken
BL
,
Beal
K
,
Ballester
B
,
Caccamo
M
,
Chen
Y
,
Clarke
L
,
Coates
G
,
Cunningham
F
,
Cutts
T
, et al. 
. (
2007
).
Ensembl 2007
.
Nucleic Acids Res
.
35
,
D610
D617
.
Huntington Study Group
(
2001
).
A randomized, placebo-controlled trial of coenzyme Q10 and remacemide in Huntington’s disease
.
Neurology
57
,
397
404
.
Huntington Study Group
(
2006
).
Tetrabenazine as antichorea therapy in Huntington disease: a randomized controlled trial
.
Neurology
66
,
366
372
.
Johnson
R
,
Zuccato
C
,
Belyaev
ND
,
Guest
DJ
,
Cattaneo
E
,
Buckley
NJ
(
2008
).
A microRNA-based gene dysregulation pathway in Huntington’s disease
.
Neurobiol. Dis
.
29
,
438
445
.
Kent
WJ
,
Sugnet
CW
,
Furey
TS
,
Roskin
KM
,
Pringle
TH
,
Zahler
AM
,
Haussler
D
(
2002
).
The human genome browser at UCSC
.
Genome Res
.
12
,
996
1006
.
Kim
J
,
Inoue
K
,
Ishii
J
,
Vanti
WB
,
Voronov
SV
,
Murchison
E
,
Hannon
G
,
Abeliovich
A
(
2007
).
A MicroRNA feedback circuit in midbrain dopamine neurons
.
Science
317
,
1220
1224
.
Klivenyi
P
,
Ferrante
RJ
,
Matthews
RT
,
Bogdanov
MB
,
Klein
AM
,
Andreassen
OA
,
Mueller
G
,
Wermer
M
,
Kaddurah-Daouk
R
,
Beal
MF
(
1999
).
Neuroprotective effects of creatine in a transgenic animal model of amyotrophic lateral sclerosis
.
Nat. Med
.
5
,
347
350
.
Landwehrmeyer
GB
,
Dubois
B
,
de Yebenes
JG
,
Kremer
B
,
Gaus
W
,
Kraus
PH
,
Przuntek
H
,
Dib
M
,
Doble
A
,
Fischer
W
, et al. 
(
2007
).
Riluzole in Huntington’s disease: a 3-year, randomized controlled study
.
Ann. Neurol
.
62
,
262
272
.
Lee
Y
,
Samaco
RC
,
Gatchel
JR
,
Thaller
C
,
Orr
HT
,
Zoghbi
HY
(
2008
).
miR-19, miR-101 and miR-130 co-regulate ATXN1 levels to potentially modulate SCA1 pathogenesis
.
Nat. Neurosci
.
11
,
1137
1139
.
Lerch
JP
,
Carroll
JB
,
Dorr
A
,
Spring
S
,
Evans
AC
,
Hayden
MR
,
Sled
JG
,
Henkelman
RM
(
2008
).
Cortical thickness measured from MRI in the YAC128 mouse model of Huntington’s disease
.
Neuroimage
41
,
243
251
.
Lewis
BP
,
Shih
IH
,
Jones-Rhoades
MW
,
Bartel
DP
,
Burge
CB
(
2003
).
Prediction of mammalian microRNA targets
.
Cell
115
,
787
798
.
Lin
B
,
Rommens
JM
,
Graham
RK
,
Kalchman
M
,
MacDonald
H
,
Nasir
J
,
Delaney
A
,
Goldberg
YP
,
Hayden
MR
(
1993
).
Differential 3′ polyadenylation of the Huntington disease gene results in two mRNA species with variable tissue expression
.
Hum. Mol. Genet
.
2
,
1541
1545
.
Lin
B
,
Nasir
J
,
MacDonald
H
,
Hutchinson
G
,
Graham
RK
,
Rommens
JM
,
Hayden
MR
(
1994
).
Sequence of the murine Huntington disease gene: evidence for conservation, alternate splicing and polymorphism in a triplet (CCG) repeat [corrected]
.
Hum. Mol. Genet
.
3
,
85
92
.
Lin
CH
,
Tallaksen-Greene
S
,
Chien
WM
,
Cearley
JA
,
Jackson
WS
,
Crouse
AB
,
Ren
S
,
Li
XJ
,
Albin
RL
,
Detloff
PJ
(
2001
).
Neurological abnormalities in a knock-in mouse model of Huntington’s disease
.
Hum. Mol. Genet
.
10
,
137
144
.
Lunkes
A
,
Lindenberg
KS
,
Ben-Haiem
L
,
Weber
C
,
Devys
D
,
Landwehrmeyer
GB
,
Mandel
JL
,
Trottier
Y
(
2002
).
Proteases acting on mutant huntingtin generate cleaved products that differentially build up cytoplasmic and nuclear inclusions
.
Mol. Cell
10
,
259
269
.
Mangiarini
L
,
Sathasivam
K
,
Seller
M
,
Cozens
B
,
Harper
A
,
Hetherington
C
,
Lawton
M
,
Trottier
Y
,
Lehrach
H
,
Davies
SW
, et al. 
(
1996
).
Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice
.
Cell
87
,
493
506
.
Mellios
N
,
Huang
HS
,
Grigorenko
A
,
Rogaev
E
,
Akbarian
S
(
2008
).
A set of differentially expressed miRNAs, including miR-30a-5p, act as post-transcriptional inhibitors of BDNF in prefrontal cortex
.
Hum. Mol. Genet
.
17
,
3030
3042
.
Menalled
LB
,
Sison
JD
,
Wu
Y
,
Olivieri
M
,
Li
XJ
,
Li
H
,
Zeitlin
S
,
Chesselet
MF
(
2002
).
Early motor dysfunction and striosomal distribution of huntingtin microaggregates in Huntington’s disease knock-in mice
.
J. Neurosci
.
22
,
8266
8276
.
Menalled
LB
,
Sison
JD
,
Dragatsis
I
,
Zeitlin
S
,
Chesselet
MF
(
2003
).
Time course of early motor and neuropathological anomalies in a knock-in mouse model of Huntington’s disease with 140 CAG repeats
.
J. Comp. Neurol
.
465
,
11
26
.
Miller
RG
,
Mitchell
JD
,
Lyon
M
,
Moore
DH
(
2003
).
Riluzole for amyotrophic lateral sclerosis (ALS)/motor neuron disease (MND)
.
Amyotroph. Lateral Scler. Other Motor Neuron Disord
.
4
,
191
206
.
Pouladi
MA
,
Bezprozvanny
I
,
Raymond
LA
,
Hayden
MR
. (
2006
).
Molecular pathogenesis of Huntington’s disease: the role of excitotoxicity
. In
Genetic Instabilities and Neurological Diseases
(ed.
Wells
RD
,
Ashizawa
T
. ), pp.
251
.
New York
:
Academic Press
Pouladi
MA
,
Graham
RK
,
Karasinska
J
,
Xie
Y
,
Dar Santos
R
,
Petersen
A
,
Hayden
MR
. (
2009
).
Prevention of depressive behaviour in the YAC128 mouse model of Huntington disease by mutation at residue 586 of huntingtin
.
Brain
(
in press
) [doi:10.1093/brain/awp006].
Puri
BK
,
Leavitt
BR
,
Hayden
MR
,
Ross
CA
,
Rosenblatt
A
,
Greenamyre
JT
,
Hersch
S
,
Vaddadi
KS
,
Sword
A
,
Horrobin
DF
, et al. 
(
2005
).
Ethyl-EPA in Huntington disease: a double-blind, randomized, placebo-controlled trial
.
Neurology
65
,
286
292
.
Schauwecker
PE
(
2005
).
Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent
.
Brain Res
.
1040
,
112
120
.
Schiefer
J
,
Landwehrmeyer
GB
,
Luesse
HG
,
Sprunken
A
,
Puls
C
,
Milkereit
A
,
Milkereit
E
,
Kosinski
CM
(
2002
).
Riluzole prolongs survival time and alters nuclear inclusion formation in a transgenic mouse model of Huntington’s disease
.
Mov. Disord
.
17
,
748
757
.
Schilling
G
,
Becher
MW
,
Sharp
AH
,
Jinnah
HA
,
Duan
K
,
Kotzuk
JA
,
Slunt
HH
,
Ratovitski
T
,
Cooper
JK
,
Jenkins
NA
, et al. 
(
1999
).
Intranuclear inclusions and neuritic aggregates in transgenic mice expressing a mutant N-terminal fragment of huntingtin
.
Hum. Mol. Genet
.
8
,
397
407
.
Schilling
G
,
Coonfield
ML
,
Ross
CA
,
Borchelt
DR
(
2001
).
Coenzyme Q10 and remacemide hydrochloride ameliorate motor deficits in a Huntington’s disease transgenic mouse model
.
Neurosci. Lett
.
315
,
149
153
.
Schoenherr
CJ
,
Anderson
DJ
(
1995
).
Silencing is golden: negative regulation in the control of neuronal gene transcription
.
Curr. Opin. Neurobiol
.
5
,
566
571
.
Shefner
JM
,
Cudkowicz
ME
,
Schoenfeld
D
,
Conrad
T
,
Taft
J
,
Chilton
M
,
Urbinelli
L
,
Qureshi
M
,
Zhang
H
,
Pestronk
A
, et al. 
(
2004
).
A clinical trial of creatine in ALS
.
Neurology
63
,
1656
1661
.
Shelbourne
PF
,
Killeen
N
,
Hevner
RF
,
Johnston
HM
,
Tecott
L
,
Lewandoski
M
,
Ennis
M
,
Ramirez
L
,
Li
Z
,
Iannicola
C
, et al. 
(
1999
).
A Huntington’s disease CAG expansion at the murine Hdh locus is unstable and associated with behavioural abnormalities in mice
.
Hum. Mol. Genet
.
8
,
763
774
.
Slow
EJ
,
van Raamsdonk
J
,
Rogers
D
,
Coleman
SH
,
Graham
RK
,
Deng
Y
,
Oh
R
,
Bissada
N
,
Hossain
SM
,
Yang
YZ
, et al. 
(
2003
).
Selective striatal neuronal loss in a YAC128 mouse model of Huntington disease
.
Hum. Mol. Genet
.
12
,
1555
1567
.
Steffan
JS
,
Agrawal
N
,
Pallos
J
,
Rockabrand
E
,
Trotman
LC
,
Slepko
N
,
Illes
K
,
Lukacsovich
T
,
Zhu
YZ
,
Cattaneo
E
, et al. 
(
2004
).
SUMO modification of Huntingtin and Huntington’s disease pathology
.
Science
304
,
100
104
.
Tang
TS
,
Chen
X
,
Liu
J
,
Bezprozvanny
I
(
2007
).
Dopaminergic signaling and striatal neurodegeneration in Huntington’s disease
.
J. Neurosci
.
27
,
7899
7910
.
The Huntington’s Disease Collaborative Research Group
(
1993
).
A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes
.
Cell
72
,
971
983
.
Tibben
A
(
2007
).
Predictive testing for Huntington’s disease
.
Brain Res. Bull
72
,
165
171
.
Tompa
P
,
Buzder-Lantos
P
,
Tantos
A
,
Farkas
A
,
Szilagyi
A
,
Banoczi
Z
,
Hudecz
F
,
Friedrich
P
(
2004
).
On the sequential determinants of calpain cleavage
.
J. Biol. Chem
.
279
,
20775
20785
.
Truant
R
,
Atwal
R
,
Burtnik
A
(
2006
).
Hypothesis: Huntingtin may function in membrane association and vesicular trafficking
.
Biochem. Cell Biol
.
84
,
912
917
.
Van Raamsdonk
JM
,
Murphy
Z
,
Slow
EJ
,
Leavitt
BR
,
Hayden
MR
. (
2005a
).
Selective degeneration and nuclear localization of mutant huntingtin in the YAC128 mouse model of Huntington disease
.
Hum. Mol. Genet
.
14
,
3823
3835
.
Van Raamsdonk
JM
,
Pearson
J
,
Slow
EJ
,
Hossain
SM
,
Leavitt
BR
,
Hayden
MR
. (
2005b
). Cognitive dysfunction precedes neuropathology and motor abnormalities in the YAC128 mouse model of Huntington’s disease.
J. Neurosci
.
25
,
4169
4180
.
Van Raamsdonk
JM
,
Pearson
J
,
Bailey
CD
,
Rogers
DA
,
Johnson
GV
,
Hayden
MR
,
Leavitt
BR
. (
2005c
).
Cystamine treatment is neuroprotective in the YAC128 mouse model of Huntington disease
.
J. Neurochem
.
95
,
210
220
.
Van Raamsdonk
JM
,
Pearson
J
,
Rogers
DA
,
Lu
G
,
Barakauskas
VE
,
Barr
AM
,
Honer
WG
,
Hayden
MR
,
Leavitt
BR
. (
2005d
).
Ethyl-EPA treatment improves motor dysfunction, but not neurodegeneration in the YAC128 mouse model of Huntington disease
.
Exp. Neurol
.
196
,
266
272
.
Vincent
AM
,
Sakowski
SA
,
Schuyler
A
,
Feldman
EL
(
2008
).
Strategic approaches to developing drug treatments for ALS
.
Drug Discov. Today
13
,
67
72
.
Warby
SC
,
Chan
EY
,
Metzler
M
,
Gan
L
,
Singaraja
RR
,
Crocker
SF
,
Robertson
HA
,
Hayden
MR
. (
2005
).
Huntingtin phosphorylation on serine 421 is significantly reduced in the striatum and by polyglutamine expansion in vivo
.
Hum. Mol. Genet
.
14
,
1569
1577
.
Warby
SC
,
Doty
CN
,
Graham
RK
,
Carroll
JB
,
Yang
YZ
,
Singaraja
RR
,
Overall
CM
,
Hayden
MR
(
2008
).
Activated caspase-6 and caspase-6-cleaved fragments of huntingtin specifically colocalize in the nucleus
.
Hum. Mol. Genet
.
17
,
2390
2404
.
Wellington
CL
,
Singaraja
R
,
Ellerby
L
,
Savill
J
,
Roy
S
,
Leavitt
B
,
Cattaneo
E
,
Hackam
A
,
Sharp
A
,
Thornberry
N
, et al. 
(
2000
).
Inhibiting caspase cleavage of huntingtin reduces toxicity and aggregate formation in neuronal and nonneuronal cells
.
J. Biol. Chem
.
275
,
19831
19838
.
Wheeler
VC
,
White
JK
,
Gutekunst
CA
,
Vrbanac
V
,
Weaver
M
,
Li
XJ
,
Li
SH
,
Yi
H
,
Vonsattel
JP
,
Gusella
JF
, et al. 
(
2000
).
Long glutamine tracts cause nuclear localization of a novel form of huntingtin in medium spiny striatal neurons in HdhQ92 and HdhQ111 knock-in mice
.
Hum. Mol. Genet
.
9
,
503
513
.
Xia
J
,
Lee
DH
,
Taylor
J
,
Vandelft
M
,
Truant
R
(
2003
).
Huntingtin contains a highly conserved nuclear export signal
.
Hum. Mol. Genet
.
12
,
1393
1403
.
Yanai
A
,
Huang
K
,
Kang
R
,
Singaraja
RR
,
Arstikaitis
P
,
Gan
L
,
Orban
PC
,
Mullard
A
,
Cowan
CM
,
Raymond
LA
, et al. 
(
2006
).
Palmitoylation of huntingtin by HIP14 is essential for its trafficking and function
.
Nat. Neurosci
.
9
,
824
831
.
Zheng
TS
,
Hunot
S
,
Kuida
K
,
Flavell
RA
(
1999
).
Caspase knockouts: matters of life and death
.
Cell Death Differ
.
6
,
1043
1053
.
Zhu
S
,
Stavrovskaya
IG
,
Drozda
M
,
Kim
BY
,
Ona
V
,
Li
M
,
Sarang
S
,
Liu
AS
,
Hartley
DM
,
Wu
DC
, et al. 
(
2002
).
Minocycline inhibits cytochrome c release and delays progression of amyotrophic lateral sclerosis in mice
.
Nature
417
,
74
78
.

COMPETING INTERESTS

The authors declare no competing financial interests.