Longitudinal analysis of the behavioural phenotype in Hdh(CAG)150 Huntington's disease knock-in mice

Longitudinal analysis of the behavioural phenotype in Hdh(CAG)150 Huntington's disease knock-in mice

Brain Research Bulletin 88 (2012) 182–188 Contents lists available at ScienceDirect Brain Research Bulletin journal homepage: www.elsevier.com/locat...

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Brain Research Bulletin 88 (2012) 182–188

Contents lists available at ScienceDirect

Brain Research Bulletin journal homepage: www.elsevier.com/locate/brainresbull

Research report

Longitudinal analysis of the behavioural phenotype in Hdh(CAG)150 Huntington’s disease knock-in mice Simon Brooks a,∗ , Gemma Higgs a , Lesley Jones b , Stephen B. Dunnett a a b

Brain Repair Group, School of Biosciences, Cardiff University, Museum Avenue, Cardiff CF10 3AX, Wales, UK Department of Psychological Medicine, 2nd Floor, Henry Wellcome Building, Wales School of Medicine, Cardiff University, Cardiff CF14 4XN, Wales, UK

a r t i c l e

i n f o

Article history: Received 29 January 2010 Received in revised form 8 April 2010 Accepted 2 May 2010 Available online 8 May 2010 Keywords: Huntington’s disease Knock-in HdhQ150 Behaviour Phenotype Cognitive Motor

a b s t r a c t In people with Huntington’s disease, an expanded CAG repeat sequence on the HTT gene confers a toxic gain function resulting in a progressive and fatal neurodegeneration. The Hdh(CAG)Q150 Huntington’s disease mouse line is a knock-in model of the disease that carries ∼150 CAG repeats on the normal mouse Htt locus. To determine that these mice are a useful model of the disease, they were assessed longitudinally for motor and cognitive deficits relevant to the human disease state. Each test was conducted bi-monthly across the lifespan of the animal. The results indicate that the HdhQ150/Q150 mice were impaired on each of the measures used, with deficits appearing on a 3-stage water maze test at 4 months of age and on prepulse inhibition at 6 months of age, both of which were prior to the manifestation of motor abnormalities. Grip strength, as measured by the inverted cage lid test, was reduced in the HdhQ150/Q150 mice from 10 months of age, when the male mice also exhibited weight loss relative to their wildtype littermates. On the accelerating rotarod, deficits in the carrier mice did not appear until they were 21 months old. Our results demonstrate that the Hdh(CAG)150 is a valid model of HD that displays early and progressive cognitive deficits that precede the onset of motor abnormalities. © 2010 Elsevier Inc. All rights reserved.

1. Introduction The neuropathology and associated functional deficits that characterise Huntington’s disease (HD) are caused by an abnormally expanded CAG (polyQ) region on exon 1 of the HTT gene [24] which encodes huntingtin (htt). The neuropathology is primarily characterised by the wholesale loss of striatal medium spiny neurons (MSNs) resulting in enlarged lateral ventricles, but also by widespread cortical atrophy and thinning [1,15–16,24,27]. The hallmark of HD pathology is the aggregation of mutant htt Nterminal fragments which form insoluble protein aggregates which ultimately enter the cell nucleus and form neuronal intranuclear inclusions (NIIs). The NIIs have an unknown role in the disease process, but they do provide a reliable indication of abnormality in the affected neuronal populations. These anatomical changes underlie a broad spectrum of functional abnormalities, the most prominent of which is the choreic limb movements, but the earliest signs of functional abnormality are cognitive and emotional in nature [3,8,20].

∗ Corresponding author at: School of Biosciences, Cardiff University, P.O. Box 911, Museum Avenue, Cardiff CF10 3AX, Wales, UK. Tel.: +44 02920 874115; fax: +44 2920 876749. E-mail address: [email protected] (S. Brooks). 0361-9230/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2010.05.004

Since HD is caused by a single gene mutation, it lends itself well to the development of genetically modified mouse models of the disease [4,10–12,18,19,26,28]. The Hdh(CAG)Q150 mouse has an extended polyQ region of 150 CAG repeats knocked-in to exon 1 of the mouse Htt locus. This mouse displays several of the characteristics of the human disease including Htt aggregation and NIIs [10,23]. Dopamine D1 and D2 receptor levels are also reduced in the striatum of the Hdh(CAG)Q150 mouse [6] and, when crossed on to a mouse line that over expresses the NR2B subunit of the NMDA receptor, these mice display an exacerbated striatal neurodegeneration compared with wildtype animals [5]. Isolated Hdh(CAG)Q150 neurons were also found to be more vulnerable to stimulated NMDA-mediated changes in Ca2+ levels [14]. Gene array analysis with the Hdh(CAG)Q150 mouse found many common disease-related changes in mRNA expression levels when compared not only across other HD mouse lines but also with human HD striatal tissue [9,21,30]. Behavioural deficits have also been identified by 40 weeks of age, including gait, balance and rotarod performance deficits, body clasping and weight loss [6,10]. Early cognitive deficits have also been reported with set-shifting deficits being demonstrated at 24 weeks of age [2]. Characterisation of the Hdh(CAG)Q150 mouse on a C57BL/6J x CBA background found grip strength deficits at 6 months of age (an age when the first signs of aggregation were demonstrated in the striatum and cortex), weight loss at 12 months and rotarod deficits at 18 months of age [30]. The previous behavioural studies focus on motor aspects of behaviour as these are the most

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obvious pathological features of the mouse phenotype, and hence are more readily measured. In the present study we sought to further characterise behaviour in the Hdh(CAG)Q150 mouse line using a broad range of motor and cognitive tasks. To produce a detailed characterisation, time points every 2 months from 4 to 24 months of age were used. The aim of the study was to identify early behavioural changes and sensitive pheno-conversion time points which could subsequently be exploited in therapeutic studies. 2. Materials and methods 2.1. Subjects In total, 153 mice were used split across the two experimental groups, which comprised 81 wildtype animals (38 female and 33 males) and 72 homozygotes (32 females and 40 males), with the inserted construct consisting of an extended CAG sequence (∼150) replacing the normal length CAG sequence in mouse Htt [10]. The mice were run as four separate cohorts. The mice were maintained and run on their original 129/Ola x C57BL6/J background. The animals were bred inhouse and genotypes were determined using tail tip DNA (Laragen Inc., Los Angeles). The animals were housed as sex matched littermates in temperature (21 ± 1 ◦ C)- and humidity (60 ± 1%)-controlled rooms in age and sex matched groups, under 12hour light/dark conditions with lights coming on at 0630 h daily. Throughout the experimental period the mice had ad libitum access to food and water. Experiments were conducted daily between the hours of 0830 and 1700 h. All experiments were run in accordance with the United Kingdom Animals (Scientific Procedures) Act of 1986, and subject to local ethical review. 2.2. Body weight As a general indicator of mouse health, the animals were weighed monthly throughout the test period. In HD mouse lines, progressive weight loss is a sign of the advancement of the disease state and is regarded as a core feature of these models. 2.3. Rotarod The mice were placed on a standard rotarod apparatus (Ugo Basile, Varese, Italy) every 2 months to assess general motor coordination. In the present experiments the accelerating version of the task was used whereby the speed at which the beam rotates increases over 5 min from 0 to a maximum of 44 rpm. The latency of the animal to fall from the beam was recorded as a measure of motor coordination. In order to determine optimum performance for each mouse, the mice were initially trained to asymptote prior to data collection. For each test session the mice were given 3 runs on the beam with at least 20 min between each run. The final 2 runs were collected as data. 2.4. Inverted grid test of grip strength To determine grip strength, each mouse was placed on a standard runged (1 mm diameter) cage lid (43 cm × 26.5 cm), which was then shaken gently inducing the mice to grip the rungs, prior to rotating 180◦ through the horizontal plane. The mice clung on the underside of the cage lid approximately 20 cm above a soft surface until their grip failed, which was recorded as the latency to fall, or for a maximum of 60 s. 2.5. Acoustic startle and prepulse inhibition (PPI) The mice were placed in a standard startle chamber (San Diego Instruments, San Diego, USA) which was programmed to deliver 5 min of 70 dB white noise followed by the startle and PPI trials. All trials were pseudo-randomly presented in blocks of 11, with variable intervals up to 30 s between each trial. Eleven trial types were used based on 2 primary startle stimuli of either 105 or 120 dB and prepulses of 0, 2, 4, 8 and 16 dB above the background white noise, with a further “no startle” baseline condition where no noise was delivered. Each of these trials was delivered 10 times resulting in a total of 110 trials per session. The duration of the prepulse was 20 ms which was followed by with a 10 ms inter-pulse interval, prior to the onset of the 50ms primary startle stimulus. The dependent variable was the peak startle response to each primary stimulus, measured by the force transducer plate in arbitrary units. 2.6. Morris water maze The Morris water maze was used as a measure of spatial and spatial reversal learning. As reversal learning deficits are a feature of the cognitive decline seen in HD, we designed a 3-stage water maze protocol that incorporated a switch in platform position and subsequent reversal back to the original location. The mouse water maze was a standard pool (1 m diameter, 47 cm depth) with white walls. The water (23 ± 1◦ C) was filled such that it was of a depth of ∼2 cm higher than the height of the platform (10 cm in diameter, 28 cm high). The water was whitened

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with pasteurised cow’s milk. The maze was divided digitally into quadrants of equal size each with a target area corresponding to the size of the platform, and swim paths recorded using an animal tracking system (Tracksys Ltd., Nottingham, UK) and Noldus Ethovision software. To train the mice, the platform was placed above the water line and marked with a flag that the animals could use as a navigation aid. Testing did not commence until each animal would swim directly to the platform, climb on to it, and remain there for 5 s. Over subsequent training trials the platform was slowly submerged. At each experimental time point the platform location was pseudo-randomly assigned to the middle of one of the 4 quadrants; hence the initial platform position at each new time point varies depending on where in the 4-position cycle that time point lies. For each trial, the start position for each mouse was pseudo-randomly assigned to one of the four positions designated, North, South, East, West, with each of the four platform positions designated as NE, SW, NW and SE. Each animal was then trained for 8 trials per day for 2 days (the last trial being a probe trial where no platform was present) to learn the initial platform position (e.g., NE). Each trial lasted a maximum of 1 min. For any trial on which the mouse did not find the escape platform within the 1 min, the trial was terminated, the mouse was placed on the platform for 5 s, and a value of 60 s recorded as the escape latency. On day 3 the platform position was switched to the middle of the opposite quadrant (in the example, SW) and the same protocol was followed over 2 days, as for the initial position. On day 5, the platform position was returned back to the initial start position (NE). Again the same trial protocol was followed for a final 2 days. Thus, for each platform position every mouse completed 15 trials plus a probe trial. For every mouse there was a minimum of 15 min between each trial. Performance measures were (i) the distance the mouse swam until it reached the platform (or total distance swam in the 60 s max. trial duration) and (ii) the latency to find the platform. For the probe trials, the number of entries and time spent in the platform area was recorded as the data. 2.7. Statistical analysis Within subject analyses of variance were used throughout, using the Genstat 9 statistical software package (VSN International, Hemel Hempstead, UK). For each of the multifactorial analyses, all of the available factors were included resulting in 3and 4-way ANOVAs, with relevant main and interaction effects. Differences between the two sexes are only reported where there is a significant difference between males and females; otherwise data are collapsed over this factor for reporting results from both sexes combined.

3. Results 3.1. Bodyweight All of the mice gained weight up to around 12 months of age when body gweights reached a plateau (Fig. 1A; F1,159 = 41.32, p < 0.001). Both male and female Hdh(CAG)Q150 mice then commenced to decline in weight, from 14 months of age, which was progressive and more pronounced in the males relative to their wildtype littermates, resulting in a significant 3-way interaction effect (Age × Genotype × Sex: F19,847 = 10.45, p < 0.001). 3.2. Grip strength For both genotypes, grip strength declined with age (Fig. 1B; F10,217 = 24.02, p < 0.001), with no differences between the sexes (p > 0.05, n.s.), and no main effect of genotype (p > 0.05, n.s.). When compared between genotype, sex and across time, the HdhQ150/Q150 mice exhibited a progressive inability to remain clinging to the inverted lid in comparison with the wildtype mice from around 10 months of age (Fig. 1B; Age × Genotype × Sex: F9,217 = 16.95, p < 0.001). 3.3. Rotarod No overall main effect of genotype (p > 0.05, n.s.) was found on rotarod performance, but performance of the HdhQ150/Q150 mice became less proficient than their wildtype littermates at the latest time points from 21 months of age (Fig. 1C; Genotype × Age: F12,218 = 13.62, p < 0.01). Sex was not a major factor in rotarod performance and did not affect the performance of the two genotypes (Sex, Genotype × Sex and Age × Sex, all n.s.).

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Fig. 1. Longitudinal changes in bodyweight (A), grip strength (B) and rotarod assessment (C) in the two groups of Hdh(CAG)Q150 and wildtype control mice. Over the 24 months of the study period, bodyweight of the mice diverged, with the HdhQ150/Q150 mice progressively losing weight relative to their wildtype littermates from 12 months of age in the males, and 14 months in the females (A). Grip strength did not progressively decline in either phenotype until around 21 months of age in the wildtype and 24 months in the transgenic mice (B). Clear differences between the groups were apparent from 10 months of age, with the HdhQ150/Q150 mice demonstrating a clear performance deficit. There were no differences in rotarod performance until 21 months when the HdhQ150/Q150 mice became less able to remain on the rotating beam (C). Vertical bars indicate standard errors of the means for comparisons involving principle main effects and interactions, as labelled.

3.4. Water maze The water maze protocol was a 3-stage procedure. On the first stage the mice were required to locate the platform as in a standard water maze task. When analysed over time, the latencies of the HdhQ150/Q150 mice became progressively longer with age than their wildtype littermates, which remained relatively stable over the 20 months of age experimental period (Fig. 2A; Genotype × Age, F9,113 = 35.45, p < 0.001). When analysed by distance swum, although the HdhQ150/Q150 mice from 12 months of

age swam further to find the platform (Fig. 2B; Genotype × Age, F9,113 = 5.51, p < 0.001), their performance was stable over time. To examine the performance of the mice over trials, the ages of the mice were collapsed together. For the initial platform location, both mouse groups demonstrated an improvement in performance with increased trials, but from 4 months of age the wildtype mice were progressively quicker from the earliest trials (Fig. 2C; Genotype × Trial, F14,2291 = 2.85, p < 0.001). They also swam less distance (Fig. 2D; Genotype × Trial, F14,2257 = 1.81, p < 0.05) to find the platform. The behavioural profile for location of the platform when in the opposite location was very similar to that for the initial location. Analyses of performance by age, confirmed a progressive and marked decline in performance, as demonstrated by lengthening latencies to find the platform, in the HdhQ150/Q150 mice (Fig. 2A; Genotype × Age, F9,112 = 82.22, p < 0.001). This decline began at around 8 months of age, with wildtype performance remaining relatively stable. Whilst the HdhQ150/Q150 mice swam further to find the platform (Fig. 2B; Genotype × Age, F9,113 = 16.42, p < 0.001), performance was relatively stable over time, as it was for the wildtype mice. Analyses of genotype by trial demonstrated a progressive shortening of latencies for both mouse groups with increased trials, but from trial number 8, no improvement was found in the HdhQ150/Q150 mice (Fig. 2C; Genotype × Trial, F14,2310 = 2.12, p < 0.009). Both groups also swam less distance to the platform but group performance diverged from around trial 6 where the distance that the HdhQ150/Q150 mice swam ceased to improve resulting in a significant difference between groups (Fig. 2D; Genotype × Trial, F14,2293 = 2.02, p < 0.013). Over time the latencies to find the platform in the reverse platform position lengthened markedly in the HdhQ150/Q150 mice (Genotype × Age: Fig. 2A: F9,112 = 90.67, p < 0.001) from 18 months of age, but were longer than the wildtype animals from 6 months. As with the previous platform positions, the distance swum was relatively stable over time, but with noticeable differences between the groups with wildtype animals swimming a shorter distance than the HdhQ150/Q150 mice (Fig. 2B; Genotype × Age, F9,112 = 4.94, p < 0.001). Analyses of genotype over trials failed to reach significance for latency (Fig. 2C; Genotype × Trial, p > 0.05, n.s.) or distance swam (Fig. 2D; Genotype × Trial, p > 0.05, n.s.), although clear performance deficits were demonstrated between the groups, with the wildtype animals demonstrating shorter latencies (Fig. 2C; Genotype, F1,165 = 37.16, p < 0.01) and swimming less distance (Fig. 2D; Genotype × Trial, F1,165 = 9.15, p < 0.03) than their HdhQ150/Q150 littermates. An overall analyses of mouse performance incorporating each of the platform positions, with age and across time returned a significant 4-way interaction effect for both latencies (Fig. 2A; Genotype × Age × Platform × Trial, F234,2902 = 14.41, p < 0.001) and distance swam (Fig. 2B; Genotype × Age × Platform × Trial, F252,2945 = 13.38, p < 0.001). Analyses of the 1 min probe trial data demonstrated deficits corresponding to those seen in the acquisition of each new platform position, as described above. The number of entries into the platform zone for the 3 platform locations was reduced in the HdhQ150/Q150 mice relative to their wildtype littermates. For both groups the number of entries fell with age, and the general pattern was variable across time. However, clear performance deficits were identified in the HdhQ150/Q150 mice for the initial, opposite and return locations (Fig. 3A; Genotype × Age, F9,256 = 2.34, p < 0.015; 5.58, p < 0.001; and 2.84, p < 0.003; respectively). Inclusion of platform location as a factor in the analyses returned demonstrated that group performance varied by platform location and over time (Fig. 3A. Genotype × Platform × Age, F18,489 = 4.29, p < 0.001). Similarly the time spent in the target zone was greater for the wildtype animals than the carrier mice for the initial, opposite and return

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Fig. 2. Water maze performance in Hdh(CAG)Q150 and wildtype mice. Since the 4-way interactions involve an excess of values for interpretation, the data are presented as differences between Hdh(CAG)Q150 and wildtype mice across the three training locations at the different ages (collapsed across trials within each training block, A and B) and on the successive trials within each block (collapsed across ages, C and D). The HdhQ150/Q150 mice were slower (A) and swam further (B) to find the platform in each of the locations used. A progressive decline in the latencies to find the platform was demonstrated in the carrier mice. The performance of both mouse groups on each platform location improved with increased trials, although the degree of improvement was significantly less in the HdhQ150/Q150 mice on both latency (C) and distance measures (D). Vertical bars indicate standard errors of the means for comparisons involving principle main effects and interactions, as labelled.

locations (Fig. 3B; Genotype × Age, F9,256 = 3.38, 4.28, and 2.84, respectively, all p < 0.005), although performance for both groups was stable across time. Group performance also varied by platform location as well as over time (Fig. 3B; Genotype × Platform × Age, F18,489 = 4.42, p < 0.001).

3.5. Startle and PPI Differences between startle reactivity with the 105 dB primary startle stimuli and associated prepulses were demonstrated between the mouse genotypes (Fig. 4A; F1,194 = 25.05, p < 0.001)

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Fig. 3. Water maze probe trial performance in the Hdh(CAG)Q150 mouse. For each platform location the wildtype mice made a greater number of entries into the platform zone during the probe trial (A) and spent more time in the areas (B) than their carrier littermates. Vertical bars indicate standard errors of the means for comparisons involving principle main effects and interactions, as labelled.

with the HdhQ150/Q150 mice being less reactive than their wildtype littermates. This difference increased with age (Fig. 4C; Genotype × Age, F10,137 = 29.92, p < 0.001). The two mouse genotypes were also differentially affected by the prepulse (Fig. 4D; Genotype × Prepulse, F5,969 = 21.81, p < 0.001) with the HdhQ150/Q150 mice demonstrating a clear reduction in startle magnitude at all but the 16 dB pulse level. Overall, there were demonstrated differences between the genotypes that varied by age and prepulse level (Fig. 4A; Genotype × Age × Prepulse, F50,685 = 18.36, p < 0.001), from 6 months of age, where the HdhQ150/Q150 mice were less reactive than their wildtype littermates. For the 120 dB stimuli an effect of genotype was found (Fig. 4B; F1,196 = 23.07, p < 0.001) that varied with age (Fig. 4E; Genotype × Age, F10,137 = 39.24, p < 0.001) with the HdhQ150/Q150 demonstrating a marked and progressive decrease in reactivity to the startle stimuli from 6 months of age. The decrease in reactivity was present for the primary startle and at all prepulse levels (Fig. 4F; Genotype × Prepulse, F5,979 = 12.73, p < 0.001). These differences are confirmed with a significant 3-way interaction (Fig. 4B; Genotype × Age × Prepulse, F50,685 = 39.24, p < 0.001), indicating that genotype performance varied with age and prepulse level.

4. Discussion The data presented demonstrate clear motor and cognitive deficits in the HdhQ150/Q150 mouse. The earliest deficit to be seen appears at 4 months of age and is the lengthened latencies to find the platform on the water maze task. This is unlikely to be a purely motor deficit as corresponding differences were identified for the distance that the animals swam suggesting a difficulty in navigating to the platform location. The HdhQ150/Q150 mice also demonstrated

a reduce startle reactivity from 6 months of age, loss of grip strength by 10 months, and a late onset rotarod deficit only at 21 months of age. Weight loss became noticeable from 10 months of age in male and from 15 months in female mice suggesting a more general decline in health status. From the present data, the behavioural profile of the HdhQ150/Q150 mouse, as with HD in humans, is that of an early onset cognitive dysfunction that precedes the motor deficits. That cognitive deficits are the earliest abnormality to be identified is consistent with previous report of early cognitive dysfunction where extra-dimensional set-shifting deficits have been identified in the HdhQ150/Q150 mice at 24 months of age [2]. Further comparisons on the cognitive deficits in these animals are not possible, as we have not been able to find comparable data within the literature so far published with this strain. In the present study the first signs of weight loss in the male HdhQ150/Q150 mice occurred at around 10 months of age, which is earlier than observed previously in an initial longitudinal assessment of these mice [6], but later than observed in the original HdhQ150/Q150 study [10], and female F1 CBA x C57BLK/6 HdhQ150/Q150 mice display weight loss from 12 months of age [30]. The original HdhQ150/Q150 (CHL2) mouse line [10] demonstrated marked rotarod deficits at a much earlier age than demonstrated in the present paper. This may reflect the different methodological approaches in the use of the rotarod. In the present paper the mice were trained until asymptotic performance was attained, and only then were the mice tested. An accelerating rod method was used that can run for a maximum of 5 min. This testing protocol is designed to determine the peak performance of mice and to identify whether they have a purely motor deficit on this task. The original HdhQ150/Q150 study [10] used no training and a fixed low speed (5 rpm) setting, and although the mice were tested repeatedly over several days it is not clear whether asymptotic performance was attained. Essentially these animals may have still been in the acqui-

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Fig. 4. Acoustic startle and prepulse inhibition in the Hdh(CAG)Q150 mouse. The HdhQ150/Q150 mice were less reactive to the startle and PPI than their wildtype littermates in an age dependent manner, on both the 105 dB (A) and the 120 dB (B) stimuli. The age related decline in reactivity is clearly demonstrated when the data were collapsed across time for the 105 dB (C) and 120 dB (E) stimuli. The mouse groups were also differentially sensitive to the stimuli and prepulses levels, with the HdhQ150/Q150 mice failing to attain peak response levels of the wildtype mice with any prepulse level for either the 105 dB (D) or 120 dB (F) stimuli. Vertical bars indicate standard errors of the means for comparisons involving principle main effects and interactions, as labelled.

sition phase of the testing, whereas our animals are well practised and could get no better at the task, and even up to 23 months of age they were able to stay on an accelerating rod for longer than the maximum time permitted in the Lin et al. [10] study. Our findings are in general agreement with previous studies that identified late onset rotarod deficits at 100 weeks of age [6] or in F1 CBA x C57BLK/6 hybrids at 18 months of age [30]. However, in each of the 4 studies different methodology was used making comparison of the results difficult. The general picture of motor dysfunction is further confused as clear gait abnormalities were also demonstrated in HdhQ150/Q150 aged between 15 and 40 weeks in the original study [10], but not until 100 weeks in the more recent paper [6]. Also in the present study grip strength on the inverted cage lid was found to be reduced from 10 months of age, in contrast to no differences being identified in the Heng et al. [6] wire hang test. One point of note is that in the Heng et al. [6] study, the mice were reported to be on a 75–90% C57Blk6/J background and there is some evidence that

the disease may be less penetrant in some behavioural measures on this background than on the 129 strain [25]. Moreover, relatively minor differences in housing conditions can ameliorate the HD phenotype in some mouse models [7]. Clearly motor abnormalities are present, and it may be that with sufficient practise the mice are able to overcome these deficits and perform tasks proficiently given the opportunity. Neuropathology in these animals is principally striatal [10,23]. Diffuse staining for mutant N-terminal fragments is present in the striatum from 7 months of age, with neuronal intranuclear inclusions at 10 months, and by 19 months (75 weeks) inclusions are present in the somatosensory and piriform cortices [6](and Bayram-Weston et al., in this issue). At 100 weeks of age the nuclear inclusions are found throughout the brain [6] as they were in 21month-old F1 CBA x C57BLK/6 HdhQ150/Q150 mice that also exhibited striatal inclusions in 45% of cells [30]. Of interest, though, aggregates were identified in the striatum and the hippocampus of the

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F1 hybrids at 6 months of age, and cortex by 8 months, and present in most brain regions by 10 months [30]. This again raises questions regarding disease penetration on different background strains, or differential antibody sensitivity. In either case, the striatal pathology probably accounts for the motor deficits that are presented in the animals. Evidence also suggests a role for the striatum in prepulse inhibition [22] and spatial learning [13,17,29], although the possibility of neuropathology in the hippocampus cannot be excluded as contributing to these impairments. Taken together, the present results describe a progressive cognitive and motor phenotype in the HdhQ150/Q150 mice, that is probably mediated by striatal neuropathology, and as such this mouse model represents a valid model of Huntington’s disease. Conflict of interest

[11]

[12]

[13]

[14]

[15]

[16]

The authors have no conflicts of interest to declare. [17]

Acknowledgments This work was funded by the Cure Huntington’s Disease Initiative (CHDI), the Hereditary Disease Foundation, and the Medical Research Council. We thank Ali Baird and Lyn Elliston for their assistance.

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