A longitudinal diffusion tensor imaging study in symptomatic Huntington's disease

Neuroimaging to Facilitate Clinical Trials in Huntington's Disease: Current Opinion from the EHDN Imaging Working Group

 Neuroimaging is increasingly being included in clinical trials of Huntington's disease (HD) for a wide range of purposes from participant selection and safety monitoring, through to demonstration of disease modification. Selection of the appropriate modality and associated analysis tools requires careful consideration. On behalf of the EHDN Imaging Working Group, we present current opinion on the utility and future prospects for inclusion of neuroimaging in HD trials. Covering the key imaging modalities of structural-, functional- and diffusion- MRI, perfusion imaging, positron emission tomography, magnetic resonance spectroscopy, and magnetoencephalography, we address how neuroimaging can be used in HD trials to: 1) Aid patient selection, enrichment, stratification, and safety monitoring; 2) Demonstrate biodistribution, target engagement, and pharmacodynamics; 3) Provide evidence for disease modification; and 4) Understand brain re-organization following therapy. We also present the challenges of translating research methodology into clinical trial settings, including equipment requirements and cost, standardization of acquisition and analysis, patient burden and invasiveness, and interpretation of results. We conclude, that with appropriate consideration of modality, study design and analysis, imaging has huge potential to facilitate effective clinical trials in HD.

Alterations of fractional anisotropy throughout cortico-basal ganglia gray matter in a macaque model of Huntington's Disease

We recently generated a nonhuman primate (NHP) model of the neurodegenerative disorder Huntington's disease (HD) using adeno-associated viral vectors to express a fragment of mutant HTT protein (mHTT) throughout the cortico-basal ganglia circuit. Previous work by our group established that mHTT-treated NHPs exhibit progressive motor and cognitive phenotypes which are accompanied by mild volumetric reductions of cortical-basal ganglia structures and reduced fractional anisotropy (FA) in the white matter fiber pathways interconnecting these regions, mirroring findings observed in early-stage HD patients. Given the mild structural atrophy observed in cortical and sub-cortical gray matter regions characterized in this model using tensor-based morphometry, the current study sought to query potential microstructural alterations in the same gray matter regions using diffusion tensor imaging (DTI), to define early biomarkers of neurodegenerative processes in this model. Here, we report that mHTT-treated NHPs exhibit significant microstructural changes in several cortical and subcortical brain regions that comprise the cortico-basal ganglia circuit; with increased FA in the putamen and globus pallidus and decreased FA in the caudate nucleus and several cortical regions. DTI measures also correlated with motor and cognitive deficits such that animals with increased basal ganglia FA, and decreased cortical FA, had more severe motor and cognitive impairment. These data highlight the functional implications of microstructural changes in the cortico-basal ganglia circuit in early-stage HD.

Progressive alterations in white matter microstructure across the timecourse of Huntington's disease

BACKGROUND

Whole-brain longitudinal diffusion studies are crucial to examine changes in structural connectivity in neurodegeneration. Here, we investigated the longitudinal alterations in white matter (WM) microstructure across the timecourse of Huntington's disease (HD).

METHODS

We examined changes in WM microstructure from premanifest to early manifest disease, using data from two cohorts with different disease burden. The TrackOn-HD study included 67 controls, 67 premanifest, and 10 early manifest HD (baseline and 24-month data); the PADDINGTON study included 33 controls and 49 early manifest HD (baseline and 15-month data). Longitudinal changes in fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity, and radial diffusivity from baseline to last study visit were investigated for each cohort using tract-based spatial statistics. An optimized pipeline was employed to generate participant-specific templates to which diffusion tensor imaging maps were registered and change maps were calculated. We examined longitudinal differences between HD expansion-carriers and controls, and correlations with clinical scores, including the composite UHDRS (cUHDRS).

RESULTS

HD expansion-carriers from TrackOn-HD, with lower disease burden, showed a significant longitudinal decline in FA in the left superior longitudinal fasciculus and an increase in MD across subcortical WM tracts compared to controls, while in manifest HD participants from PADDINGTON, there were significant widespread longitudinal increases in diffusivity compared to controls. Baseline scores in clinical scales including the cUHDRS predicted WM microstructural change in HD expansion-carriers.

CONCLUSION

The present study showed significant longitudinal changes in WM microstructure across the HD timecourse. Changes were evident in larger WM areas and across more metrics as the disease advanced, suggesting a progressive alteration of WM microstructure with disease evolution.

Diffusion Tensor Imaging in Preclinical and Human Studies of Huntington's Disease: What Have we Learned so Far?

BACKGROUND

Huntington's Disease is an irreversible neurodegenerative disease characterized by the progressive deterioration of specific brain nerve cells. The current evaluation of cellular and physiological events in patients with HD relies on the development of transgenic animal models. To explore such events in vivo, diffusion tensor imaging has been developed to examine the early macro and microstructural changes in brain tissue. However, the gap in diffusion tensor imaging findings between animal models and clinical studies and the lack of microstructural confirmation by histological methods has questioned the validity of this method.

OBJECTIVE

This review explores white and grey matter ultrastructural changes associated to diffusion tensor imaging, as well as similarities and differences between preclinical and clinical Huntington's Disease studies.

METHODS

A comprehensive review of the literature using online-resources was performed (Pub- Med search).

RESULTS

Similar changes in fractional anisotropy as well as axial, radial and mean diffusivities were observed in white matter tracts across clinical and animal studies. However, comparative diffusion alterations in different grey matter structures were inconsistent between clinical and animal studies.

CONCLUSION

Diffusion tensor imaging can be related to specific structural anomalies in specific cellular populations. However, some differences between animal and clinical studies could derive from the contrasting neuroanatomy or connectivity across species. Such differences should be considered before generalizing preclinical results into the clinical practice. Moreover, current limitations of this technique to accurately represent complex multicellular events at the single micro scale are real. Future work applying complex diffusion models should be considered.

Diffusion imaging in Huntington's disease: comprehensive review

Huntington's disease (HD) is a monogenic disorder with 100% penetrance. With the advent of genetic testing in adults, disease-related, structural brain changes can be investigated from the earliest, premorbid stages of HD. While examining macrostructural change characterises global neuronal damage, investigating microstructural alterations provides information regarding brain organisation and its underlying biological properties. Diffusion MRI can be used to track the progression of microstructural anomalies in HD decades prior to clinical disease onset, providing a greater understanding of neurodegeneration. Multiple approaches, including voxelwise, region of interest and tractography, have been used in HD cohorts, showing a centrifugal pattern of white matter (WM) degeneration starting from deep brain areas, which is consistent with neuropathological studies. The corpus callosum, longer WM tracts and areas that are more densely connected, in particular the sensorimotor network, also tend to be affected early during premanifest stages. Recent evidence supports the routine inclusion of diffusion analyses within clinical trials principally as an additional measure to improve understanding of treatment effects, while the advent of novel techniques such as multitissue compartment models and connectomics can help characterise the underpinnings of progressive functional decline in HD.

One-year changes in brain microstructure differentiate preclinical Huntington's disease stages

OBJECTIVE

To determine whether brain imaging markers of tissue microstructure can detect the effect of disease progression across the preclinical stages of Huntington's disease.

METHODS

Longitudinal microstructural changes in diffusion imaging metrics (mean diffusivity and fractional anisotropy) were investigated in participants with presymptomatic Huntington's disease (N = 35) stratified into three preclinical subgroups according to their estimated time until onset of symptoms, compared with age- and gender-matched healthy controls (N = 19) over a 1y period.

RESULTS

Significant differences were found over the four groups in change of mean diffusivity in the posterior basal ganglia and the splenium of the corpus callosum. This overall effect was driven by significant differences between the group far-from-onset (FAR) of symptoms and the groups midway- (MID) and near-the-onset (NEAR) of symptoms. In particular, an initial decrease of mean diffusivity in the FAR group was followed by a subsequent increase in groups closer to onset of symptoms. The seemingly counter-intuitive decrease of mean diffusivity in the group furthest from onset of symptoms might be an early indicator of neuroinflammatory process preceding the neurodegenerative phase. In contrast, the only clinical measure that was able to capture a difference in 1y changes between the preclinical stages was the UHDRS confidence in motor score.

CONCLUSIONS

With sensitivity to longitudinal changes in brain microstructure within and between preclinical stages, and potential differential response to distinct pathophysiological mechanisms, diffusion imaging is a promising state marker for monitoring treatment response and identifying the optimal therapeutic window of opportunity in preclinical Huntington's disease.

Huntington's disease: Brain imaging in Huntington's disease

Huntington's disease (HD) gene-carriers show prominent neuronal loss by end-stage disease, and the use of magnetic resonance imaging (MRI) has been increasingly used to quantify brain changes during earlier stages of the disease. MRI offers an in vivo method of measuring structural and functional brain change. The images collected via MRI are processed to measure different anatomical features, such as brain volume, macro- and microstructural changes within white matter and functional brain activity. Structural imaging has demonstrated significant volume loss across multiple white and gray matter regions in HD, particularly within subcortical structures. There also appears to be increasing disorganization of white matter tracts and between-region connectivity with increasing disease progression. Finally, functional changes are thought to represent changes in brain activity underlying compensatory mechanisms in HD. This chapter will provide an overview of the principles of MRI and practicalities associated with using MRI in HD studies, and summarize findings from MRI studies investigating brain structure and function in HD.

Progressive microstructural changes of the occipital cortex in Huntington's disease

In this study we longitudinally investigated the rate of microstructural alterations in the occipital cortex in different stages of Huntington's disease (HD) by applying an automated atlas-based approach to diffusion MRI data. Twenty-two premanifest (preHD), 10 early manifest HD (early HD) and 24 healthy control subjects completed baseline and two year follow-up scans. The preHD group was stratified based on the predicted years to disease onset into a far (preHD-A) and near (preHD-B) to disease onset group. Clinical and behavioral measures were collected per assessment time point. An automated atlas-based DTI analysis approach was used to obtain the mean, axial and radial diffusivities of the occipital cortex. We found that the longitudinal rate of diffusivity change in the superior occipital gyrus (SOG), middle occipital gyrus (MOG), and inferior occipital gyrus (IOG) was significantly higher in early HD compared to both preHD and controls (all p's ≤ 0.005), which can be interpreted as an increased rate of microstructural degeneration. Furthermore, the change rate in the diffusivity of the MOG could significantly discriminate between preHD-B compared to preHD-A and the other groups (all p's ≤ 0.04). Finally, we found an inverse correlation between the Stroop Word Reading task and diffusivities in the SOG and MOG (all p's ≤ 0.01). These findings suggest that measures obtained from the occipital cortex can serve as sensitive longitudinal biomarkers for disease progression in preHD-B and early HD. These could in turn be used to assess potential effects of proposed disease modifying therapies.

Platelet-derived extracellular vesicles in Huntington's disease

The production and release of extracellular vesicles (EV) is a property shared by all eukaryotic cells and a phenomenon frequently exacerbated in pathological conditions. The protein cargo of EV, their cell type signature and availability in bodily fluids make them particularly appealing as biomarkers. We recently demonstrated that platelets, among all types of blood cells, contain the highest concentrations of the mutant huntingtin protein (mHtt)-the genetic product of Huntington's disease (HD), a neurodegenerative disorder which manifests in adulthood with a complex combination of motor, cognitive and psychiatric deficits. Herein, we used a cohort of 59 HD patients at all stages of the disease, including individuals in pre-manifest stages, and 54 healthy age- and sex-matched controls, to evaluate the potential of EV derived from platelets as a biomarker. We found that platelets of pre-manifest and manifest HD patients do not release more EV even if they are activated. Importantly, mHtt was not found within EV derived from platelets, despite them containing high levels of this protein. Correlation analyses also failed to reveal an association between the number of platelet-derived EV and the age of the patients, the number of CAG repeats, the Unified Huntington Disease Rating Scale total motor score, the Total Functional Capacity score or the Burden of Disease score. Our data would, therefore, suggest that EV derived from platelets with HD is not a valuable biomarker in HD.

Huntington Disease: Genetics, Prevention, and Therapy Approaches

Huntington's chorea or Huntington disease (HD) is a late-onset autosomal dominant neurodegenerative disorder caused by a trinucleotide repeat expansion. The multidisciplinary study of HD has been the focus of an international collaborating effort of basic and applied research for several decades. HD was the first human genetic disease mapped using linkage analysis of DNA polymorphisms and became a paradigm for scores of genes mapped in the same manner. Presymptomatic and prenatal testing have been available for HD families in the last 30 years, following genetic counseling and careful bioethical guidelines. Nevertheless, with the cure for the disease still elusive the uptake of predictive testing by at risk individuals is low. Current treatment of HD is mostly symptomatic, but ongoing observational studies, clinical trials and development of new gene silencing technologies have provided hopeful results.

Mapping the order and pattern of brain structural MRI changes using change-point analysis in premanifest Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that progressively affects motor, cognitive, and emotional functions. Structural MRI studies have demonstrated brain atrophy beginning many years prior to clinical onset ("premanifest" period), but the order and pattern of brain structural changes have not been fully characterized. In this study, we investigated brain regional volumes and diffusion tensor imaging (DTI) measurements in premanifest HD, and we aim to determine (1) the extent of MRI changes in a large number of structures across the brain by atlas-based analysis, and (2) the initiation points of structural MRI changes in these brain regions. We adopted a novel multivariate linear regression model to detect the inflection points at which the MRI changes begin (namely, "change-points"), with respect to the CAG-age product (CAP, an indicator of extent of exposure to the effects of CAG repeat expansion). We used approximately 300 T1-weighted and DTI data from premanifest HD and control subjects in the PREDICT-HD study, with atlas-based whole brain segmentation and change-point analysis. The results indicated a distinct topology of structural MRI changes: the change-points of the volumetric measurements suggested a central-to-peripheral pattern of atrophy from the striatum to the deep white matter; and the change points of DTI measurements indicated the earliest changes in mean diffusivity in the deep white matter and posterior white matter. While interpretation needs to be cautious given the cross-sectional nature of the data, these findings suggest a spatial and temporal pattern of spread of structural changes within the HD brain. Hum Brain Mapp 38:5035-5050, 2017. © 2017 Wiley Periodicals, Inc.

Developmental Whole Brain White Matter Alterations in Transgenic Huntington's Disease Monkey

Transgenic Huntington’s disease monkey (HD monkey) model provides great opportunity for studying disease progression that could lead to new insight for developing biomarker, early intervention and novel therapeutics. Whole brain white matter integrity of HD-monkeys was examined longitudinally from 6 to 48 months using diffusion tensor imaging (DTI) and tract-based spatial statistics (TBSS). Progressive developmental white matter alterations in HD monkeys were widespread and were observed not only in fiber bundles connecting cortical areas to the striatum (e.g. striatal bundle and external capsule), but also in long association fiber pathways, commissural fibers, and subcortical fiber bundle. In all fiber tracts, the data indicate an arrest in white matter development around 23 months followed by slight decline until adulthood in HD monkeys. The microstructural changes parallel the progressive motor, memory and cognitive decline previously reported as HD monkeys aged. The findings revealed the widespread progressive temporal-spatial microstructural changes in HD monkey brains from infancy to adulthood, suggesting differentiated degenerations across different brain areas during brain development.

The Complexity of Clinical Huntington's Disease: Developments in Molecular Genetics, Neuropathology and Neuroimaging Biomarkers

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterised by extensive neuronal loss in the striatum and cerebral cortex, and a triad of clinical symptoms affecting motor, cognitive/behavioural and mood functioning. The mutation causing HD is an expansion of a CAG tract in exon 1 of the HTT gene. This chapter provides a multifaceted overview of the clinical complexity of HD. We explore recent directions in molecular genetics including the identification of loci that are genetic modifiers of HD that could potentially reveal therapeutic targets beyond the HTT gene transcript and protein. The variability of clinical symptomatology in HD is considered alongside recent findings of variability in cellular and neurochemical changes in the striatum and cerebral cortex in human brain. We review evidence from structural neuroimaging methods of progressive changes of striatum, cerebral cortex and white matter in pre-symptomatic and symptomatic HD, with a particular focus on the potential identification of neuroimaging biomarkers that could be used to test promising disease-specific and modifying treatments. Finally we provide an overview of completed clinical trials in HD and future therapeutic developments.

Structural imaging in premanifest and manifest Huntington disease

Huntington disease (HD) neuropathology has a devastating effect on brain structure and consequently brain function; neuroimaging provides a means to assess these effects in gene carriers. In this chapter we first outline the unique utility of structural imaging in understanding HD and discuss some of the acquisition and analysis techniques currently available. We review the existing literature to summarize what we know so far about structural brain changes across the spectrum of disease from premanifest through to manifest disease. We then consider how these neuroimaging findings relate to patient function and nonimaging biomarkers, and can be used to predict disease onset. Finally we review the utility of imaging measures for assessment of treatment efficacy in clinical trials.

Diffusion imaging studies of Huntington's disease: A meta-analysis

BACKGROUND

Diffusion tensor imaging (DTI) could detect abnormal brain microstructural alterations. DTI studies of Huntington's Disease(HD) have yielded inconsistent results.

OBJECTIVE

To integrate the existing DTI studies of HD and explore the validity of DTI to detect microstructural damages in HD brain via meta-analysis.

METHODS

Systematic and comprehensive searches of the databases were performed for DTI studies of HD. The data from the studies that met our inclusion criteria were extracted and analyzed using the CMA2 software. Random effect models were utilized to minimize the potential between-study heterogeneity. One-way sensitivity analysis was conducted to test the robustness of the results.

RESULTS

The meta-analysis included 140 pre-symptomatic HD (PreHD), 235 symptomatic HD (SymHD) patients and 302 controls, revealing significantly increased fractional anisotropy (FA) in the caudate, putamen, and globus pallidus, while decreased FA in the corpus callosum of both PreHD and SymHD patients compared with controls. In addition, significantly increased mean diffusivity (MD) was identified in the putamen and thalamus of both PreHD and SymHD patients, and in the caudate of SymHD patients, while no significant difference in MD in the caudate of PreHD patients. In the corpus callosum, there was a significant increase of radial diffusivity and axial diffusivity in SymHD patients compared with controls. Meta-regression showed gender-based difference in MD values of the caudate.

CONCLUSIONS

Our meta-analysis provides further evidence that DTI detects microstructural damage of both white matter and gray matter even in PreHD gene carriers. MD is less sensitive than FA in detecting structural changes in PreHD.

Multimodal imaging biomarkers in premanifest and early Huntington's disease: 30-month IMAGE-HD data

BACKGROUND

The discovery of potential disease-modifying therapies in a neurodegenerative condition like Huntington's disease depends on the availability of sensitive biomarkers that reflect decline across disease stages and that are functionally and clinically relevant.

AIMS

To quantify macrostructural and microstructural changes in participants with premanifest and symptomatic Huntington's disease over 30 months, and to establish their functional and clinical relevance.

METHOD

Multimodal magnetic resonance imaging study measuring changes in macrostructural (volume) and microstructural (diffusivity) measures in 40 patients with premanifest Huntington's disease, 36 patients with symptomatic Huntington's disease and 36 healthy control participants over three testing sessions spanning 30 months.

RESULTS

Relative to controls, there was greater longitudinal atrophy in participants with symptomatic Huntington's disease in whole brain, grey matter, caudate and putamen, as well as increased caudate fractional anisotropy; caudate volume loss was the only measure to differ between premanifest Huntington's disease and control groups. Changes in caudate volume and fractional anisotropy correlated with each other and neurocognitive decline; caudate volume loss also correlated with clinical and disease severity.

CONCLUSIONS

Caudate neurodegeneration, especially atrophy, may be the most suitable candidate surrogate biomarker for consideration in the development of upcoming clinical trials.

Dynamics of the connectome in Huntington's disease: A longitudinal diffusion MRI study

Objectives

To longitudinally investigate the connectome in different stages of Huntington's disease (HD) by applying graph theoretical analysis to diffusion MRI data.

Experimental design

We constructed weighted structural networks and calculated their topological properties. Twenty-two premanifest (preHD), 10 early manifest HD and 24 healthy controls completed baseline and 2 year follow-up scans. We stratified the preHD group based on their predicted years to disease onset into a far (preHD-A) and near (preHD-B) to disease onset group. We collected clinical and behavioural measures per assessment time point.

Principle observations

We found a significant reduction over time in nodal betweenness centrality both in the early manifest HD and preHD-B groups as compared to the preHD-A and control groups, suggesting a decrease of importance of specific nodes to overall network organization in these groups (FDR adjusted ps < 0.05). Additionally, we found a significant longitudinal decrease of the clustering coefficient in preHD when compared to healthy controls (FDR adjusted p < 0.05), which can be interpreted as a reduced capacity for internodal information processing at the local level. Furthermore, we demonstrated dynamic changes to hub-status loss and gain both in preHD and early manifest HD. Finally, we found significant cross-sectional as well as longitudinal relationships between graph metrics and clinical and neurocognitive measures.

Conclusions

This study demonstrates divergent longitudinal changes to the connectome in (pre) HD compared to healthy controls. This provides novel insights into structural correlates associated with clinical and cognitive functions in HD and possible compensatory mechanisms at play in preHD.

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Investigates characteristics of the connectome in Huntington's disease (HD).

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HD patients showed longitudinal changes in their structural connectome.

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Connectome dynamics correlated with changes in clinical and cognitive measures.

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Connectomics provides novel insights into compensatory strategies of the diseased brain.

Diffusion-MRI in neurodegenerative disorders

The ability to image the whole brain through ever more subtle and specific methods/contrasts has come to play a key role in understanding the basis of brain abnormalities in several diseases. In magnetic resonance imaging (MRI), "diffusion" (i.e. the random, thermally-induced displacements of water molecules over time) represents an extraordinarily sensitive contrast mechanism, and the exquisite structural detail it affords has proven useful in a vast number of clinical as well as research applications. Since diffusion-MRI is a truly quantitative imaging technique, the indices it provides can serve as potential imaging biomarkers which could allow early detection of pathological alterations as well as tracking and possibly predicting subtle changes in follow-up examinations and clinical trials. Accordingly, diffusion-MRI has proven useful in obtaining information to better understand the microstructural changes and neurophysiological mechanisms underlying various neurodegenerative disorders. In this review article, we summarize and explore the main applications, findings, perspectives as well as challenges and future research of diffusion-MRI in various neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease and degenerative ataxias.

Progression of microstructural damage in spinocerebellar ataxia type 2: a longitudinal DTI study

BACKGROUND AND PURPOSE

The ability of DTI to track the progression of microstructural damage in patients with inherited ataxias has not been explored so far. We performed a longitudinal DTI study in patients with spinocerebellar ataxia type 2.

MATERIALS AND METHODS

Ten patients with spinocerebellar ataxia type 2 and 16 healthy age-matched controls were examined twice with DTI (mean time between scans, 3.6 years [patients] and 3.3 years [controls]) on the same 1.5T MR scanner. Using tract-based spatial statistics, we analyzed changes in DTI-derived indices: mean diffusivity, axial diffusivity, radial diffusivity, fractional anisotropy, and mode of anisotropy.

RESULTS

At baseline, the patients with spinocerebellar ataxia type 2, as compared with controls, showed numerous WM tracts with significantly increased mean diffusivity, axial diffusivity, and radial diffusivity and decreased fractional anisotropy and mode of anisotropy in the brain stem, cerebellar peduncles, cerebellum, cerebral hemisphere WM, corpus callosum, and thalami. Longitudinal analysis revealed changes in axial diffusivity and mode of anisotropy in patients with spinocerebellar ataxia type 2 that were significantly different than those in the controls. In patients with spinocerebellar ataxia type 2, axial diffusivity was increased in WM tracts of the right cerebral hemisphere and the corpus callosum, and the mode of anisotropy was extensively decreased in hemispheric cerebral WM, corpus callosum, internal capsules, cerebral peduncles, pons and left cerebellar peduncles, and WM of the left paramedian vermis. There was no correlation between the progression of changes in DTI-derived indices and clinical deterioration.

CONCLUSIONS

DTI can reveal the progression of microstructural damage of WM fibers in the brains of patients with spinocerebellar ataxia type 2, and mode of anisotropy seems particularly sensitive to such changes. These results support the potential of DTI-derived indices as biomarkers of disease progression.

Microstructural brain abnormalities in Huntington's disease: A two-year follow-up

OBJECTIVES

To investigate both cross-sectional and time-related changes of striatal and whole-brain microstructural properties in different stages of Huntington's disease (HD) using diffusion tensor imaging.

EXPERIMENTAL DESIGN

From the TRACK-HD study, premanifest gene carriers (preHD), early manifest HD and controls were scanned at baseline and 2-year follow-up. Stratification of the preHD group into a far (preHD-A) and near (preHD-B) to predicted disease onset was performed. Age-corrected histograms of whole-brain white matter (WM), gray matter (GM) and striatal diffusion measures were computed and normalised by the number of voxels in each subject's data set.

PRINCIPLE OBSERVATIONS

Higher cross-sectional mean, axial and radial diffusivities were found in both WM (P ≤ 0.001) and GM (P ≤ 0.001) of the manifest HD compared to the preHD and control groups. In preHD, only WM axial diffusivity (AD) was higher than in controls (P ≤ 0.01). This finding remained valid only in preHD-B (P ≤ 0.001). AD was also higher in the striatum of preHD-B compared to controls and preHD-A (P ≤ 0.01). Fractional anisotropy (FA) lacked sensitivity in differentiating between the groups. Histogram peak heights were generally lower in manifest HD compared to the preHD and control groups. No longitudinal differences were found in the degree of diffusivity change between the groups in the two year follow-up. There was a significant relationship between diffusivity and neurocognitive measures.

CONCLUSIONS

Alterations in cross-sectional diffusion profiles between manifest HD subjects and controls were evident, both in whole-brain and striatum. In the preHD stage, only AD alterations were found, a finding suggesting that this metric is a sensitive marker for early change in HD prior to disease manifestation. The individual diffusivities were superior to FA in revealing pathologic microstructural brain alterations. Diffusion measures were well related to clinical functioning and disease stage.

Basal ganglia-cortical structural connectivity in Huntington's disease

Huntington's disease is an incurable neurodegenerative disease caused by inheritance of an expanded cytosine-adenine-guanine (CAG) trinucleotide repeat within the Huntingtin gene. Extensive volume loss and altered diffusion metrics in the basal ganglia, cortex and white matter are seen when patients with Huntington's disease (HD) undergo structural imaging, suggesting that changes in basal ganglia-cortical structural connectivity occur. The aims of this study were to characterise altered patterns of basal ganglia-cortical structural connectivity with high anatomical precision in premanifest and early manifest HD, and to identify associations between structural connectivity and genetic or clinical markers of HD. 3-Tesla diffusion tensor magnetic resonance images were acquired from 14 early manifest HD subjects, 17 premanifest HD subjects and 18 controls. Voxel-based analyses of probabilistic tractography were used to quantify basal ganglia-cortical structural connections. Canonical variate analysis was used to demonstrate disease-associated patterns of altered connectivity and to test for associations between connectivity and genetic and clinical markers of HD; this is the first study in which such analyses have been used. Widespread changes were seen in basal ganglia-cortical structural connectivity in early manifest HD subjects; this has relevance for development of therapies targeting the striatum. Premanifest HD subjects had a pattern of connectivity more similar to that of controls, suggesting progressive change in connections over time. Associations between structural connectivity patterns and motor and cognitive markers of disease severity were present in early manifest subjects. Our data suggest the clinical phenotype in manifest HD may be at least partly a result of altered connectivity.

Longitudinal change in white matter microstructure in Huntington's disease: The IMAGE-HD study

OBJECTIVE

To quantify 18-month changes in white matter microstructure in premanifest (pre-HD) and symptomatic Huntington's disease (symp-HD). To investigate baseline clinical, cognitive and motor symptoms that are predictive of white matter microstructural change over 18months.

METHOD

Diffusion tensor imaging (DTI) data were analyzed for 28 pre-HD, 25 symp-HD, and 27 controls scanned at baseline and after 18months. Unbiased tract-based spatial statistics (TBSS) methods were used to identify longitudinal changes in fractional anisotropy (FA), radial diffusivity (RD), and axial diffusivity (AD) of white matter. Stepwise linear regression models were used to identify baseline clinical, cognitive, and motor measures that are predictive of longitudinal diffusion changes.

RESULTS

Symp-HD compared to controls showed 18-month reductions in FA in the corpus callosum and cingulum white matter. Symp-HD compared to pre-HD showed increased RD in the corpus callosum and striatal projection pathways. FA in the body, genu, and splenium of the corpus callosum was significantly associated with a baseline clinical motor measure (Unified Huntington's Disease Rating Scale: total motor scores: UHDRS-TMS) across both HD groups. This measure was also the only independent predictor of longitudinal decline in FA in all parts of the corpus callosum across both HD groups.

CONCLUSIONS

We provide direct evidence of longitudinal decline in white matter microstructure in symp-HD. Although pre-HD did not show longitudinal change, clinical symptoms and motor function predicted white matter microstructural changes for all gene positive subjects. These findings suggest that loss of axonal integrity is an early hallmark of neurodegenerative changes which are clinically relevant.

Advanced diffusion MRI fiber tracking in neurosurgical and neurodegenerative disorders and neuroanatomical studies: A review

Diffusion MRI enabled in vivo microstructural imaging of the fiber tracts in the brain resulting in its application in a wide range of settings, including in neurological and neurosurgical disorders. Conventional approaches such as diffusion tensor imaging (DTI) have been shown to have limited applications due to the crossing fiber problem and the susceptibility of their quantitative indices to partial volume effects. To overcome these limitations, the recent focus has shifted to the advanced acquisition methods and their related analytical approaches. Advanced white matter imaging techniques provide superior qualitative data in terms of demonstration of multiple crossing fibers in their spatial orientation in a three dimensional manner in the brain. In this review paper, we discuss the advancements in diffusion MRI and introduce their roles. Using examples, we demonstrate the role of advanced diffusion MRI-based fiber tracking in neuroanatomical studies. Results from its preliminary application in the evaluation of intracranial space occupying lesions, including with respect to future directions for prognostication, are also presented. Building upon the previous DTI studies assessing white matter disease in Huntington's disease and Amyotrophic lateral sclerosis; we also discuss approaches which have led to encouraging preliminary results towards developing an imaging biomarker for these conditions.

Progress and prospects for genetic modification of nonhuman primate models in biomedical research

The growing interest of modeling human diseases using genetically modified (transgenic) nonhuman primates (NHPs) is a direct result of NHPs (rhesus macaque, etc.) close relation to humans. NHPs share similar developmental paths with humans in their anatomy, physiology, genetics, and neural functions; and in their cognition, emotion, and social behavior. The NHP model within biomedical research has played an important role in the development of vaccines, assisted reproductive technologies, and new therapies for many diseases. Biomedical research has not been the primary role of NHPs. They have mainly been used for safety evaluation and pharmacokinetics studies, rather than determining therapeutic efficacy. The development of the first transgenic rhesus macaque (2001) revolutionized the role of NHP models in biomedicine. Development of the transgenic NHP model of Huntington's disease (2008), with distinctive clinical features, further suggested the uniqueness of the model system; and the potential role of the NHP model for human genetic disorders. Modeling human genetic diseases using NHPs will continue to thrive because of the latest advances in molecular, genetic, and embryo technologies. NHPs rising role in biomedical research, specifically pre-clinical studies, is foreseeable. The path toward the development of transgenic NHPs and the prospect of transgenic NHPs in their new role in future biomedicine needs to be reviewed. This article will focus on the advancement of transgenic NHPs in the past decade, including transgenic technologies and disease modeling. It will outline new technologies that may have significant impact in future NHP modeling and will conclude with a discussion of the future prospects of the transgenic NHP model.

The Corticospinal Tract in Huntington's Disease

Huntington's disease (HD) is characterized by progressive motor impairment. Therefore, the connectivity of the corticospinal tract (CST), which is the main white matter (WM) pathway that conducts motor impulses from the primary motor cortex to the spinal cord, merits particular attention. WM abnormalities have already been shown in presymptomatic (Pre-HD) and symptomatic HD subjects using magnetic resonance imaging (MRI). In the present study, we examined CST microstructure using diffusion tensor imaging (DTI)-based tractography in 30-direction DTI data collected from 100 subjects: Pre-HD subjects (n = 25), HD patients (n = 25) and control subjects (n = 50), and T2*-weighted (iron sensitive) imaging. Results show decreased fractional anisotropy (FA) and increased axial (AD), and radial diffusivity (RD) in the bilateral CST of HD patients. Pre-HD subjects had elevated iron in the left CST, regionally localized between the brainstem and thalamus. CAG repeat length in conjunction with age, as well as motor (UHDRS) assessment were correlated with CST FA, AD, and RD both in Pre-HD and HD. In the presymptomatic phase, increased iron in the inferior portion supports the "dying back" hypothesis that axonal damage advances in a retrograde fashion. Furthermore, early iron alteration may cause a high level of toxicity, which may contribute to further damage.

Test-Retest Reliability of Diffusion Tensor Imaging in Huntington's Disease

Diffusion tensor imaging (DTI) has shown microstructural abnormalities in patients with Huntington's Disease (HD) and work is underway to characterise how these abnormalities change with disease progression. Using methods that will be applied in longitudinal research, we sought to establish the reliability of DTI in early HD patients and controls. Test-retest reliability, quantified using the intraclass correlation coefficient (ICC), was assessed using region-of-interest (ROI)-based white matter atlas and voxelwise approaches on repeat scan data from 22 participants (10 early HD, 12 controls). T1 data was used to generate further ROIs for analysis in a reduced sample of 18 participants. The results suggest that fractional anisotropy (FA) and other diffusivity metrics are generally highly reliable, with ICCs indicating considerably lower within-subject compared to between-subject variability in both HD patients and controls. Where ICC was low, particularly for the diffusivity measures in the caudate and putamen, this was partly influenced by outliers. The analysis suggests that the specific DTI methods used here are appropriate for cross-sectional research in HD, and give confidence that they can also be applied longitudinally, although this requires further investigation. An important caveat for DTI studies is that test-retest reliability may not be evenly distributed throughout the brain whereby highly anisotropic white matter regions tended to show lower relative within-subject variability than other white or grey matter regions.

Biomarker development for Huntington's disease

Huntington's disease (HD) is a fatal inherited neurodegenerative disorder, treatment to slow the progression of which has not yet been found. Human clinical trials to test a number of therapeutic strategies are underway or imminent, facilitated in part by the recent development of biomarkers that might be used as surrogate endpoints in such trials. However, although much progress in developing HD biomarkers has been made, ongoing work seeks to improve the sensitivity and reliability of current measures, and to demonstrate that they correspond to clear meaningful benefit to patients. Of particular importance is the identification of state biomarkers that can be used in pre-manifest HD gene carriers to test therapies hoped to delay symptom onset in these individuals. Functional, neuroimaging and biochemical biomarkers continue to be investigated for use in the development of disease-modifying treatments of HD.

Huntington disease: natural history, biomarkers and prospects for therapeutics

Huntington disease (HD) can be seen as a model neurodegenerative disorder, in that it is caused by a single genetic mutation and is amenable to predictive genetic testing, with estimation of years to predicted onset, enabling the entire range of disease natural history to be studied. Structural neuroimaging biomarkers show that progressive regional brain atrophy begins many years before the emergence of diagnosable signs and symptoms of HD, and continues steadily during the symptomatic or 'manifest' period. The continued development of functional, neurochemical and other biomarkers raises hopes that these biomarkers might be useful for future trials of disease-modifying therapeutics to delay the onset and slow the progression of HD. Such advances could herald a new era of personalized preventive therapeutics. We describe the natural history of HD, including the timing of emergence of motor, cognitive and emotional impairments, and the techniques that are used to assess these features. Building on this information, we review recent progress in the development of biomarkers for HD, and potential future roles of these biomarkers in clinical trials.

The evolving role of diffusion magnetic resonance imaging in movement disorders

Significant advances have allowed diffusion magnetic resonance imaging (MRI) to evolve into a powerful tool in the field of movement disorders that can be used to study disease states and connectivity between brain regions. Diffusion MRI is a promising potential biomarker for Parkinson's disease and other forms of parkinsonism, and may allow the distinction of different forms of parkinsonism. Techniques such as tractography have contributed to our current thinking regarding the pathophysiology of dystonia and possible mechanisms of penetrance. Diffusion MRI measures could potentially assist in monitoring disease progression in Huntington's disease, and in uncovering the nature of the processes and structures involved the development of essential tremor. The ability to represent structural connectivity in vivo also makes diffusion MRI an ideal adjunctive tool for the surgical treatment of movement disorders. We review recent studies using diffusion MRI in movement disorders research and present the current state of the science as well as future directions.

Multi-modal neuroimaging in premanifest and early Huntington's disease: 18 month longitudinal data from the IMAGE-HD study

IMAGE-HD is an Australian based multi-modal longitudinal magnetic resonance imaging (MRI) study in premanifest and early symptomatic Huntington's disease (pre-HD and symp-HD, respectively). In this investigation we sought to determine the sensitivity of imaging methods to detect macrostructural (volume) and microstructural (diffusivity) longitudinal change in HD. We used a 3T MRI scanner to acquire T1 and diffusion weighted images at baseline and 18 months in 31 pre-HD, 31 symp-HD and 29 controls. Volume was measured across the whole brain, and volume and diffusion measures were ascertained for caudate and putamen. We observed a range of significant volumetric and, for the first time, diffusion changes over 18 months in both pre-HD and symp-HD, relative to controls, detectable at the brain-wide level (volume change in grey and white matter) and in caudate and putamen (volume and diffusivity change). Importantly, longitudinal volume change in the caudate was the only measure that discriminated between groups across all stages of disease: far from diagnosis (>15 years), close to diagnosis (<15 years) and after diagnosis. Of the two diffusion metrics (mean diffusivity, MD; fractional anisotropy, FA), only longitudinal FA change was sensitive to group differences, but only after diagnosis. These findings further confirm caudate atrophy as one of the most sensitive and early biomarkers of neurodegeneration in HD. They also highlight that different tissue properties have varying schedules in their ability to discriminate between groups along disease progression and may therefore inform biomarker selection for future therapeutic interventions.

Functional magnetic resonance imaging of working memory in Huntington's disease: cross-sectional data from the IMAGE-HD study

We used functional magnetic resonance imaging (fMRI) to investigate spatial working memory (WM) in an N-BACK task (0, 1, and 2-BACK) in premanifest Huntington's disease (pre-HD, n = 35), early symptomatic Huntington's disease (symp-HD, n = 23), and control (n = 32) individuals. Overall, both WM conditions (1-BACK and 2-BACK) activated a large network of regions throughout the brain, common to all groups. However, voxel-wise and time-course analyses revealed significant functional group differences, despite no significant behavioral performance differences. During 1-BACK, voxel-wise blood-oxygen-level-dependent (BOLD) signal activity was significantly reduced in a number of regions from the WM network (inferior frontal gyrus, anterior insula, caudate, putamen, and cerebellum) in pre-HD and symp-HD groups, compared with controls; however, time-course analysis of the BOLD response in the dorsolateral prefrontal cortex (DLPFC) showed increased activation in symp-HD, compared with pre-HD and controls. The pattern of reduced voxel-wise BOLD activity in pre-HD and symp-HD, relative to controls, became more pervasive during 2-BACK affecting the same structures as in 1-BACK, but also incorporated further WM regions (anterior cingulate gyrus, parietal lobe and thalamus). The DLPFC BOLD time-course for 2-BACK showed a reversed pattern to that observed in 1-BACK, with a significantly diminished signal in symp-HD, relative to pre-HD and controls. Our findings provide support for functional brain reorganisation in cortical and subcortical regions in both pre-HD and symp-HD, which are modulated by task difficulty. Moreover, the lack of a robust striatal BOLD signal in pre-HD may represent a very early signature of change observed up to 15 years prior to clinical diagnosis.

Seeking Huntington disease biomarkers by multimodal, cross-sectional basal ganglia imaging

Neurodegeneration of the striatum in Huntington disease (HD) is characterized by loss of medium-spiny neurons, huntingtin nuclear inclusions, reactive gliosis, and iron accumulation. Neuroimaging allows in vivo detection of the macro- and micro-structural changes that occur from presymptomatic stages of the disease (preHD). The aim of our study was to evaluate the reliability of multimodal imaging as an in vivo biomarker of vulnerability and development of the disease and to characterize macro- and micro-structural changes in subcortical nuclei in HD. Macrostructure (T1-weighted images), microstructure (diffusion tensor imaging), and iron content (R 2* relaxometry) of subcortical nuclei and medial temporal lobe structures were evaluated by a 3 T scanner in 17 preHD carriers, 12 early-stage patients and 29 matched controls. We observed a volume reduction and microstructural changes in the basal ganglia (caudate, putamen, and globus pallidus) and iron accumulation in the globus pallidus in both preHD and symptomatic subjects; all these features were significantly more pronounced in patients, in whom degeneration extended to the other subcortical nuclei (i.e., thalamus and accumbens). Mean diffusivity (MD) was the most powerful predictor in models explaining more than 50% of the variability in HD development in the caudate, putamen, and thalamus. These findings suggest that the measurement of MD may further enhance the well-known predictive value of striatal volume to assess disease progression as it is highly sensitive to tissue microimpairment. Multimodal imaging may detect brain changes even in preHD stages.

Recent advances in imaging the onset and progression of Huntington’s disease

SUMMARY Huntington’s disease is a devastating autosomal-dominant neurodegenerative disorder resulting in progressive decline in motor and cognitive function, accompanied by neuropsychiatric disturbances. In vivo imaging can reveal the underlying neuropathological changes that contribute to symptom manifestation. Observational studies of individuals carrying the causative gene have demonstrated that structural and functional brain changes are apparent decades before clinical onset of the disease; imaging measures can predict those individuals who subsequently undergo clinical conversion. Such studies have improved our understanding of neurodegeneration across the disease spectrum and aided the identification of therapeutic targets. Clinical trials of potentially disease-modifying treatments are likely to be investigated in the near future and imaging provides a powerful tool to monitor disease progression and thereby assess therapeutic efficacy.

A novel transgenic rat model for spinocerebellar ataxia type 17 recapitulates neuropathological changes and supplies in vivo imaging biomarkers

Spinocerebellar ataxia 17 (SCA17) is an autosomal-dominant, late-onset neurodegenerative disorder caused by an expanded polyglutamine (polyQ) repeat in the TATA-box-binding protein (TBP). To further investigate this devastating disease, we sought to create a first transgenic rat model for SCA17 that carries a full human cDNA fragment of the TBP gene with 64 CAA/CAG repeats (TBPQ64). In line with previous observations in mouse models for SCA17, TBPQ64 rats show a severe neurological phenotype including ataxia, impairment of postural reflexes, and hyperactivity in early stages followed by reduced activity, loss of body weight, and early death. Neuropathologically, the severe phenotype of SCA17 rats was associated with neuronal loss, particularly in the cerebellum. Degeneration of Purkinje, basket, and stellate cells, changes in the morphology of the dendrites, nuclear TBP-positive immunoreactivity, and axonal torpedos were readily found by light and electron microscopy. While some of these changes are well recapitulated in existing mouse models for SCA17, we provide evidence that some crucial characteristics of SCA17 are better mirrored in TBPQ64 rats. Thus, this SCA17 model represents a valuable tool to pursue experimentation and therapeutic approaches that may be difficult or impossible to perform with SCA17 transgenic mice. We show for the first time positron emission tomography (PET) and diffusion tensor imaging (DTI) data of a SCA animal model that replicate recent PET studies in human SCA17 patients. Our results also confirm that DTI are potentially useful correlates of neuropathological changes in TBPQ64 rats and raise hope that DTI imaging could provide a biomarker for SCA17 patients.

Predictors of phenotypic progression and disease onset in premanifest and early-stage Huntington's disease in the TRACK-HD study: analysis of 36-month observational data

BACKGROUND

TRACK-HD is a multinational prospective observational study of Huntington's disease (HD) that examines clinical and biological findings of disease progression in individuals with premanifest HD (preHD) and early-stage HD. We aimed to describe phenotypic changes in these participants over 36 months and identify baseline predictors of progression.

METHODS

Individuals without HD but carrying the mutant huntingtin gene (classed as preHD-A if ≥10·8 years and preHD-B if <10·8 years from predicted onset), participants with early HD (classed as HD1 if they had a total functional capacity score of 11-13 and HD2 if they had a score of 7-10), and healthy control individuals were assessed at four study sites in the Netherlands, the UK, France, and Canada. We measured 36-month change for 3T MRI, clinical, cognitive, quantitative motor, and neuropsychiatric assessments and examined their prognostic value. We also assessed the relation between disease progression and the combined effect of CAG repeat length and age. All participants were analysed according to their baseline subgroups. Longitudinal results were analysed using a combination of repeated-measure weighted least squares models and, when examining risk of new diagnosis, survival analysis.

FINDINGS

At baseline, 366 participants were enrolled between Jan 17, and Aug 26, 2008, and of these 298 completed 36-month follow-up: 97 controls, 58 participants with preHD-A, 46 with preHD-B, 66 with HD1, and 31 with HD2. In the preHD-B group, several quantitative motor and cognitive tasks showed significantly increased rates of decline at 36 months, compared with controls, whereas few had at 24 months. Of the cognitive measures, the symbol digit modality test was especially sensitive (adjusted mean loss 4·11 points [95% CI 1·49-6·73] greater than controls; p=0·003). Among psychiatric indicators, apathy ratings specifically showed significant increases (0·34 points [95% CI 0·02-0·66] greater than controls; p=0·038). There was little evidence of reliable change in non-imaging measures in the preHD-A group, with the exception of the speeded tapping inter-tap interval (0·01 s [95% CI 0·01-0·02] longer than controls; p=0·0001). Several baseline imaging, quantitative motor, and cognitive measures had prognostic value, independent of age and CAG repeat length, for predicting subsequent clinical diagnosis in preHD. Of these, grey-matter volume and inter-tap interval were particularly sensitive (p=0·013 and 0·002, respectively). Longitudinal change in these two measures was also greater in participants with preHD who received a diagnosis of HD during the study compared with those who did not, after controlling for CAG repeat length and age-related risk (p=0·006 and 0·0003, respectively). In early HD, imaging, quantitative motor, and cognitive measures were predictive of decline in total functional capacity and tracked longitudinal change; also, neuropsychiatric changes consistent with frontostriatal pathological abnormalities were associated with this loss of functional capacity (problem behaviours assessment composite behaviour score p<0·0001). Age and CAG repeat length explained variance in longitudinal change of multimodal measures, with the effect more prominent in preHD.

INTERPRETATION

We have shown changes in several outcome measures in individuals with preHD over 36 months. These findings further our understanding of HD progression and have implications for clinical trial design.

FUNDING

CHDI Foundation.

Diffusion weighted imaging of prefrontal cortex in prodromal Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease with no effective disease-modifying treatments. There is considerable interest in finding reliable indicators of disease progression to judge the efficacy of novel treatments that slow or stop disease onset before debilitating signs appear. Diffusion-weighted imaging (DWI) may provide a reliable marker of disease progression by characterizing diffusivity changes in white matter (WM) in individuals with prodromal HD. The prefrontal cortex (PFC) may play a role in HD progression due to its prominent striatal connections and documented role in executive function. This study uses DWI to characterize diffusivity in specific regions of PFC WM defined by FreeSurfer in 53 prodromal HD participants and 34 controls. Prodromal HD individuals were separated into three CAG-Age Product (CAP) groups (16 low, 22 medium, 15 high) that indexed baseline progression. Statistically significant increases in mean diffusivity (MD) and radial diffusivity (RD) among CAP groups relative to controls were seen in inferior and lateral PFC regions. For MD and RD, differences among controls and HD participants tracked with baseline disease progression. The smallest difference was for the low group and the largest for the high group. Significant correlations between Trail Making Test B (TMTB) and mean fractional anisotropy (FA) and/or RD paralleled group differences in mean MD and/or RD in several right hemisphere regions. The gradient of effects that tracked with CAP group suggests DWI may provide markers of disease progression in future longitudinal studies as increasing diffusivity abnormalities in the lateral PFC of prodromal HD individuals.

Structural MRI in Huntington's disease and recommendations for its potential use in clinical trials

Huntington's disease (HD) results in progressive impairment of motor and cognitive function and neuropsychiatric disturbance. There are no disease-modifying treatments available, but HD research is entering a critical phase where promising disease-specific therapies are on the horizon. Thus, a pressing need exists for biomarkers capable of monitoring progression and ultimately determining drug efficacy. Neuroimaging provides a powerful tool for assessing disease progression. However, in order to be accepted as biomarkers for clinical trials, imaging measures must be reproducible, robust to scanner differences, sensitive to disease-related change and demonstrate a relationship to clinically meaningful measures. We provide a review of the current structural imaging literature in HD and highlight inconsistencies between studies. We make recommendations for the standardisation of reporting for future studies, such as appropriate cohort characterisation and documentation of methodologies to facilitate comparisons and inform trial design. We also argue for an intensified effort to consider issues highlighted here so that we have the best chance of assessing the efficacy of the therapeutic benefit in forestalling this devastating disease.

Regionally selective atrophy of subcortical structures in prodromal HD as revealed by statistical shape analysis

Huntington disease (HD) is a neurodegenerative disorder that involves preferential atrophy in the striatal complex and related subcortical nuclei. In this article, which is based on a dataset extracted from the PREDICT-HD study, we use statistical shape analysis with deformation markers obtained through "Large Deformation Diffeomorphic Metric Mapping" of cortical surfaces to highlight specific atrophy patterns in the caudate, putamen, and globus pallidus, at different prodromal stages of the disease. On the basis of the relation to cortico-basal ganglia circuitry, we propose that statistical shape analysis, along with other structural and functional imaging studies, may help expand our understanding of the brain circuitry affected and other aspects of the neurobiology of HD, and also guide the most effective strategies for intervention.

Evaluation of multi-modal, multi-site neuroimaging measures in Huntington's disease: Baseline results from the PADDINGTON study

Background

Macro- and micro-structural neuroimaging measures provide valuable information on the pathophysiology of Huntington's disease (HD) and are proposed as biomarkers. Despite theoretical advantages of microstructural measures in terms of sensitivity to pathology, there is little evidence directly comparing the two.

Methods

40 controls and 61 early HD subjects underwent 3 T MRI (T1- and diffusion-weighted), as part of the PADDINGTON study. Macrostructural volumetrics were obtained for the whole brain, caudate, putamen, corpus callosum (CC) and ventricles. Microstructural diffusion metrics of fractional anisotropy (FA), mean-, radial- and axial-diffusivity (MD, RD, AD) were computed for white matter (WM), CC, caudate and putamen. Group differences were examined adjusting for age, gender and site. A formal comparison of effect sizes determined which modality and metrics provided a statistically significant advantage over others.

Results

Macrostructural measures showed decreased regional and global volume in HD (p < 0.001); except the ventricles which were enlarged (p < 0.01). In HD, FA was increased in the deep grey-matter structures (p < 0.001), and decreased in the WM (CC, p = 0.035; WM, p = 0.053); diffusivity metrics (MD, RD, AD) were increased for all brain regions (p < 0.001). The largest effect sizes were for putamen volume, caudate volume and putamen diffusivity (AD, RD and MD); each was significantly larger than those for all other metrics (p < 0.05).

Conclusion

The highest performing macro- and micro-structural metrics had similar sensitivity to HD pathology quantified via effect sizes. Region-of-interest may be more important than imaging modality, with deep grey-matter regions outperforming the CC and global measures, for both volume and diffusivity. FA appears to be relatively insensitive to disease effects.

Biomarkers for Huntington's disease: an update

Huntington's disease (HD) is a devastating autosomal-dominant neurodegenerative condition caused by a CAG repeat expansion in the gene encoding huntingtin which is characterised by progressive motor impairment, cognitive decline and neuropsychiatric disturbances. There are currently no disease-modifying treatments available to patients, but a number of therapeutic strategies are currently being investigated, chief among them are nucleotide-based 'gene silencing' approaches, modulation of huntingtin post-translation modification and enhancing clearance of the mutant protein. In 2008, the authors' review highlighted the need to develop and validate biomarkers and provided a systematic head-to-head comparison of such measures. They searched the PubMed database for publications, which covered each of the subheadings mentioned below. They identified from these list studies which had relevance to biomarker development, as defined in their previous review. Building on a tradition of collaborative research in HD, great advances have been made in the field since that time and a range of outcome measures are now being recommended in order to assess efficacy in future therapeutic trials.

Altered diffusion tensor imaging measurements in aged transgenic Huntington disease rats

Rodent models of Huntington disease (HD) are valuable tools for investigating HD pathophysiology and evaluating new therapeutic approaches. Non-invasive characterization of HD-related phenotype changes is important for monitoring progression of pathological processes and possible effects of interventions. The first transgenic rat model for HD exhibits progressive late-onset affective, cognitive, and motor impairments, as well as neuropathological features reflecting observations from HD patients. In this report, we contribute to the anatomical phenotyping of this model by comparing high-resolution ex vivo DTI measurements obtained in aged transgenic HD rats and wild-type controls. By region of interest analysis supplemented by voxel-based statistics, we find little evidence of atrophy in basal ganglia regions, but demonstrate altered DTI measurements in the dorsal and ventral striatum, globus pallidus, entopeduncular nucleus, substantia nigra, and hippocampus. These changes are largely compatible with DTI findings in preclinical and clinical HD patients. We confirm earlier reports that HD rats express a moderate neuropathological phenotype, and provide evidence of altered DTI measures in specific HD-related brain regions, in the absence of pronounced morphometric changes.

Genotype specific age related changes in a transgenic rat model of Huntington's disease

We aimed to characterize the transgenic Huntington rat model with in vivo imaging and identify sensitive and reliable biomarkers associated with early and progressive disease status. In order to do so, we performed a multimodality (DTI and PET) longitudinal imaging study, during which the same TgHD and wildtype (Wt) rats were repetitively scanned. Surprisingly, the relative ventricle volume was smaller but increased faster in TgHD compared to Wt animals. DTI (mean, axial, radial diffusivity) revealed subtle genotype-specific aging effects in the striatum and its surrounding white matter, already in the presymptomatic stage. Using ¹⁸F-FDG and ¹⁸F-Fallypride PET imaging, we were not able to demonstrate genotype-specific aging effects within the striatum. The outcome of this longitudinal study was somewhat surprising as it demonstrated a significant differential aging pattern in TgHD versus Wt animals. Although it seems that the TgHD rat model does not have a sufficient expression of disease yet at the age of 12 months, further validation of this model is highly beneficial since there is still an incomplete understanding of the early disease mechanisms of Huntington's disease.

Connectivity-based segmentation of the striatum in Huntington's disease: vulnerability of motor pathways

The striatum, the primary site of degeneration in Huntington's disease (HD), connects to the cerebral cortex via topographically organized circuits subserving unique motor, associative and limbic functions. Currently, it is not known whether all cortico-striatal circuits are equally affected in HD. We aimed to study the selective vulnerability of individual cortico-striatal circuits within the striatum in HD, and hypothesized that motor cortico-striatal pathways would be most affected, consistent with HD being a primarily motor disorder. Diffusion Tensor Imaging (DTI) tractography was used to identify connections between the striatum and seven major cortical regions in 12 HD patients and 14 matched controls. The striatum of both groups was parcellated into subregions based on connectivity with the cerebral cortex. Volumetric and DTI microstructural measures of Fractional Anisotropy (FA) and Mean Diffusivity (MD) were obtained within each subregion and compared statistically between groups. Tractography demonstrated the topographic organization of cortical connections in the striatum of both controls and HD patients. In HD patients, the greatest difference from controls in volume, FA and MD was observed in M1 and S1 subregions of the caudate and putamen. Motor symptoms correlated with volume and MD in sensorimotor striatal subregions, suggesting that sensorimotor striatal degeneration is closely related to motor dysfunction. DTI tractography provides a novel approach to sensitively examine circuit-specific abnormalities in HD and has identified that the motor cortico-striatal circuit is selectively vulnerable in HD.