Consistent neurodegeneration and its association with clinical progression in Huntington's disease: a coordinate-based meta-analysis

Gray matter alterations in Huntington's disease: A meta-analysis of VBM neuroimaging studies

Increasing neuroimaging studies have attempted to identify biomarkers of Huntington's disease (HD) progression. Here, we conducted voxel-based meta-analyses of voxel-based morphometry (VBM) studies on HD to investigate the evolution of gray matter volume (GMV) alterations and explore the effects of genetic and clinical features on GMV changes. A systematic review was performed to identify the relevant studies. Meta-analyses of whole-brain VBM studies were performed to assess the regional GMV changes in all HD mutation carriers, in presymptomatic HD (pre-HD), and in symptomatic HD (sym-HD). A quantitative comparison was performed between pre-HD and sym-HD. Meta-regression analyses were used to explore the effects of genetic and clinical features on GMV changes. Twenty-eight studies were included, comparing a total of 1811 HD mutation carriers [including 1150 pre-HD and 560 sym-HD] and 969 healthy controls (HCs). Pre-HD showed decreased GMV in the bilateral caudate nuclei, putamen, insula, anterior cingulate/paracingulate gyri, middle temporal gyri, and left dorsolateral superior frontal gyrus compared with HCs. Compared with pre-HD, GMV decrease in sym-HD extended to the bilateral median cingulate/paracingulate gyri, Rolandic operculum and middle occipital gyri, left amygdala, and superior temporal gyrus. Meta-regression analyses found that age, mean lengths of CAG repeats, and disease burden were negatively associated with GMV atrophy of the bilateral caudate and right insula in all HD mutation carriers. This meta-analysis revealed the pattern of GMV changes from pre-HD to sym-HD, prompting the understanding of HD progression. The pattern of GMV changes may be biomarkers for disease progression in HD.

Systemic Symptoms in Huntington's Disease: A Comprehensive Review

BACKGROUND

Although Huntington's disease (HD) is usually thought of as a triad of motor, cognitive, and psychiatric symptoms, there is growing appreciation of HD as a systemic illness affecting the entire body.

OBJECTIVES

This review aims to draw attention to these systemic non-motor symptoms in HD.

METHODS

We identified relevant studies published in English by searching MEDLINE (from 1966 to September 2023), using the following subject headings: Huntington disease, autonomic, systemic, cardiovascular, respiratory, gastrointestinal, urinary, sexual and cutaneous, and additional specific symptoms.

RESULTS

Data from 123 articles were critically reviewed with focus on systemic features associated with HD, such as cardiovascular, respiratory, gastrointestinal, urinary, sexual and sweating.

CONCLUSION

This systematic review draws attention to a variety of systemic and autonomic co-morbidities in patients with HD. Not all of them correlate with the severity of the primary HD symptoms or CAG repeats. More research is needed to better understand the pathophysiology and treatment of systemic and autonomic dysfunction in HD.

Rapid and high-purity differentiation of human medium spiny neurons reveals LMNB1 hypofunction and subtype necessity in modeling Huntington's disease

BACKGROUND

Different neural subtypes are selectively lost in diverse neurodegenerative diseases. Huntington's disease (HD) is an inherited neurodegenerative disease characterized by motor abnormalities that primarily affect the striatum. The Huntingtin (HTT) mutation involves an expanded CAG repeat, leading to insoluble polyQ, which renders GABA+ medium spiny neurons (MSN) more venerable to cell death. Human pluripotent stem cells (hPSCs) technology allows for the construction of disease-specific models, providing valuable cellular models for studying pathogenesis, drug screening, and high-throughput analysis.

METHODS

In this study, we established a method that allows for rapid and efficient generation of MSNs (> 90%) within 21 days from hPSC-derived neural progenitor cells, by introducing a specific combination of transcription factors.

RESULTS

We efficiently induced several neural subtypes, in parallel, based on the same cell source, and revealed that, compared to other neural subtypes, MSNs exhibited higher polyQ aggregation propensity and overexpression toxicity, more severe dysfunction in BDNF/TrkB signaling, greater susceptibility to BDNF withdrawal, and more severe disturbances in nucleocytoplasmic transport (NCT). We further found that the nuclear lamina protein LMNB1 was greatly reduced in HD neurons and mislocalized to the cytoplasm and axons. Knockdown of HTT or treatment with KPT335, an orally selective inhibitor of nuclear export (SINE), effectively attenuated the pathological phenotypes and alleviated neuronal death caused by BDNF withdrawal.

CONCLUSIONS

This study thus establishes an effective method for obtaining MSNs and underscores the necessity of using high-purity MSNs to study HD pathogenesis, especially the MSN-selective vulnerability.

Fractal Dimension Studies of the Brain Shape in Aging and Neurodegenerative Diseases

The fractal dimension is a morphometric measure that has been used to investigate the changes of brain shape complexity in aging and neurodegenerative diseases. This chapter reviews fractal dimension studies in aging and neurodegenerative disorders in the literature. Research has shown that the fractal dimension of the left cerebral hemisphere increases until adolescence and then decreases with aging, while the fractal dimension of the right hemisphere continues to increase until adulthood. Studies in neurodegenerative diseases demonstrated a decline in the fractal dimension of the gray matter and white matter in Alzheimer's disease, amyotrophic lateral sclerosis, and spinocerebellar ataxia. In multiple sclerosis, the white matter fractal dimension decreases, but conversely, the fractal dimension of the gray matter increases at specific stages of disease. There is also a decline in the gray matter fractal dimension in frontotemporal dementia and multiple system atrophy of the cerebellar type and in the white matter fractal dimension in epilepsy and stroke. Region-specific changes in fractal dimension have also been found in Huntington's disease and Parkinson's disease. Associations were found between the fractal dimension and clinical scores, showing the potential of the fractal dimension as a marker to monitor brain shape changes in normal or pathological processes and predict cognitive or motor function.

Characterizing Heart Rate Variability Response to Maximal Exercise Testing in People with Huntington's Disease

Background

Huntington's disease (HD) is an autosomal dominant, neurodegenerative disease that involves dysfunction in the autonomic nervous system (ANS). Heart rate variability (HRV) is a valid and noninvasive measure for ANS dysfunction, yet no study has characterized HRV response to exercise in people with HD.

Objective

Characterize HRV response to exercise in individuals with HD and explore its implications for exercise prescription and cardiac dysautonomia mechanisms.

Methods

19 participants with HD were recruited as part of a cohort of individuals enrolled in the Physical Activity and Exercise Outcomes in Huntington's Disease (PACE-HD) study at Teachers College, Columbia University (TC). 13 non-HD age- and gender-matched control participants were also recruited from TC. HRV was recorded with a Polar H10 heart rate (HR) monitor before, during, and after a ramp cycle-ergometer exercise test.

Results

Participants with HD showed reduced HR peak (p < 0.01) and HR reserve (p < 0.001) compared with controls. Participants with HD demonstrated reduced root mean square of successive differences between normal-to-normal intervals (RMSSD) and successive differences of normal-to-normal intervals (SDSD) at rest (p < 0.001). Participants with HD also showed differences for low frequency (LF) power (p < 0.01), high frequency (HF) normalized units (nu) (p < 0.05), LF (nu) (p < 0.001), and HF/LF ratio (p < 0.05) compared with controls.

Conclusions

We found reduced aerobic exercise capacity and sympathovagal dysautonomia both at rest and during post-exercise recovery in people with HD, suggesting modified exercise prescription may be required for people with HD. Further investigations focusing on cardiac dysautonomia and underlying mechanisms of sympathovagal dysautonomia in people with HD are warranted.

Measuring cognitive impairment and monitoring cognitive decline in Huntington's disease: a comparison of assessment instruments

BACKGROUND

Progressive cognitive decline is an inevitable feature of Huntington's disease (HD) but specific criteria and instruments are still insufficiently developed to reliably classify patients into categories of cognitive severity and to monitor the progression of cognitive impairment.

METHODS

We collected data from a cohort of 180 positive gene-carriers: 33 with premanifest HD and 147 with manifest HD. Using a specifically developed gold-standard for cognitive status we classified participants into those with normal cognition, those with mild cognitive impairment, and those with dementia. We administered the Parkinson's Disease-Cognitive Rating Scale (PD-CRS), the MMSE and the UHDRS cogscore at baseline, and at 6-month and 12-month follow-up visits. Cutoff scores discriminating between the three cognitive categories were calculated for each instrument. For each cognitive group and instrument we addressed cognitive progression, sensitivity to change, and the minimally clinical important difference corresponding to conversion from one category to another.

RESULTS

The PD-CRS cutoff scores for MCI and dementia showed excellent sensitivity and specificity ratios that were not achieved with the other instruments. Throughout follow-up, in all cognitive groups, PD-CRS captured the rate of conversion from one cognitive category to another and also the different patterns in terms of cognitive trajectories.

CONCLUSION

The PD-CRS is a valid and reliable instrument to capture MCI and dementia syndromes in HD. It captures the different trajectories of cognitive progression as a function of cognitive status and shows sensitivity to change in MCI and dementia.

Investigating the heterogeneity within the somatosensory-motor network and its relationship with the attention and default systems

The somatosensory-motor network (SMN) not only plays an important role in primary somatosensory and motor processing but is also central to many disorders. However, the SMN heterogeneity related to higher-order systems still remains unclear. Here, we investigated SMN heterogeneity from multiple perspectives. To characterize the SMN substructures in more detail, we used ultra-high-field functional MRI to delineate a finer-grained cortical parcellation containing 430 parcels that is more homogenous than the state-of-the-art parcellation. We personalized the new parcellation to account for individual differences and identified multiscale individual-specific brain structures. We found that the SMN subnetworks showed distinct resting-state functional connectivity (RSFC) patterns. The Hand subnetwork was central within the SMN and exhibited stronger RSFC with the attention systems than the other subnetworks, whereas the Tongue subnetwork exhibited stronger RSFC with the default systems. This two-fold differentiation was observed in the temporal ordering patterns within the SMN. Furthermore, we characterized how the distinct attention and default streams were carried forward into the functions of the SMN using dynamic causal modeling and identified two behavioral domains associated with this SMN fractionation using meta-analytic tools. Overall, our findings provided important insights into the heterogeneous SMN organization at the system level and suggested that the Hand subnetwork may be preferentially involved in exogenous processes, whereas the Tongue subnetwork may be more important in endogenous processes.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Structural and metabolic brain correlates of arithmetic word-problem solving in Huntington's disease

Individuals with pre-manifest and early symptomatic Huntington's disease (HD) have shown deficits in solving arithmetic word-problems. However, the neural correlates of these deficits in HD are poorly understood. We explored the structural (gray-matter volume; GMV) and metabolic (18F-FDG PET; SUVr) brain correlates of arithmetic performance using the recently developed HD-word problem arithmetic task (HD-WPA) in seventeen preHD and sixteen HD individuals. Symptomatic participants showed significantly lower scores in the HD-WPA than preHD participants. Lower performance in the HD-WPA was associated with reduced GMV in subcortical, medial frontal, and several posterior-cortical clusters in HD participants. No significant GMV loss was found in preHD participants. 18F-FDG data revealed a widespread pattern of hypometabolism in association with lower arithmetic performance in all participants. In preHD participants, this pattern was restricted to the ventrolateral and orbital prefrontal cortex, the insula, and the precentral gyrus. In HD participants, the pattern extended to several parietal-temporal regions. Word-problem solving arithmetic deficits in HD is subserved by a pattern of asynchronous metabolic and structural compromise across the cerebral cortex as a function of disease stage. In preHD individuals, arithmetic deficits were associated with prefrontal alterations, whereas in symptomatic HD patients, more severe arithmetic deficits are associated with the compromise of several frontal-subcortical and temporo-parietal regions. Our results support the hypothesis that cognitive deficits in HD are not exclusively dominated by frontal-striatal dysfunctions but also involve fronto-temporal and parieto-occipital damage.

Cytology, architecture, development, and connections of the primate striatum: Hints for human pathology

Degeneration of neurons and circuits across the striatum shows stereotyped time-course and spatial topography patterns that are distinct for Huntington's disease, Parkinson's disease, or the Tauopathies. These patterns of neurodegeneration in humans have not yet been systematically related to developmental, connectional, cellular, and chemical factors studied in human and non-human primates, that may underlie potential differences in selective vulnerability across striatal sectors. Relating primate anatomy to human pathology could provide new venues for identifying molecular, cellular, and connectional factors linked to the degeneration of striatal neurons and circuits. This review describes and summarizes several developmental, cellular, structural, and connectional features of the primate striatum in relation to patterns of neurodegeneration in the striatum of humans and of non-human primate models. We review (1) the types of neurons in the primate striatum, (2) the cyto-, myelo-, and chemoarchitecture of the primate striatum, (3) the developmental origin of the striatum in light of modern patterning studies, (4) the organization of corticostriatal projections in relation to cortical types, and (5) the topography and time-course of neuron loss, glial reaction, and protein aggregation induced by neurodegenerative diseases in humans and in non-human primate models across striatal sectors and their corresponding cortical areas. We summarize current knowledge about key aspects of primate striatal anatomy and human pathology and indicate knowledge gaps that should be addressed in future studies. We aim to identify factors for selective vulnerability to neurodegeneration of striatal neurons and circuits and obtain hints that could help elucidate striatal pathology in humans.

Structure and Dynamics of Large-Scale Cognitive Networks in Huntington's Disease

BACKGROUND

Huntington's disease is a neurodegenerative disorder characterized by clinical alterations in the motor, behavioral, and cognitive domains. However, the structure and disruptions to large-scale brain cognitive networks have not yet been established.

OBJECTIVE

We aimed to profile changes in large-scale cognitive networks in premanifest and symptomatic patients with Huntington's disease.

METHODS

We prospectively recruited premanifest and symptomatic Huntington's disease mutation carriers as well as healthy controls. Clinical and sociodemographic data were obtained from all participants, and resting-state functional connectivity data, using both time-averaged and dynamic functional connectivity, was acquired from whole-brain and cognitively oriented brain parcellations.

RESULTS

A total of 64 gene mutation carriers and 23 healthy controls were included; 21 patients with Huntington's disease were classified as premanifest and 43 as symptomatic Huntington's disease. Compared with healthy controls, patients with Huntington's disease showed decreased network connectivity within the posterior hubs of the default-mode network and the medial prefrontal cortex, changes that correlated with cognitive (t = 2.25, P = 0.01) and disease burden scores (t = -2.42, P = 0.009). The salience network showed decreased functional connectivity between insular and supramarginal cortices and also correlated with cognitive (t = 2.11, P = 0.02) and disease burden scores (t = -2.35, P = 0.01). Dynamic analyses showed that network variability was decreased for default-central executive networks, a feature already present in premanifest mutation carriers (dynamic factor 8, P = 0.02).

CONCLUSIONS

Huntington's disease shows an early and widespread disruption of large-scale cognitive networks. Importantly, these changes are related to cognitive and disease burden scores, and novel dynamic functional analyses uncovered subtler network changes even in the premanifest stages.

Arithmetic Word-Problem Solving as Cognitive Marker of Progression in Pre-Manifest and Manifest Huntington's Disease

BACKGROUND

Arithmetic word-problem solving depends on the interaction of several cognitive processes that may be affected early in the disease in gene-mutation carriers for Huntington's disease (HD).

OBJECTIVE

Our goal was to examine the pattern of performance of arithmetic tasks in premanifest and manifest HD, and to examine correlations between arithmetic task performance and other neuropsychological tasks.

METHODS

We collected data from a multicenter cohort of 165 HD gene-mutation carriers. The sample consisted of 31 premanifest participants: 16 far-from (>12 years estimated time to diagnosis; preHD-A) and 15 close-to (≤12 years estimated time to diagnosis; preHD-B), 134 symptomatic patients (early-mild HD), and 37 healthy controls (HC). We compared performance between groups and explored the associations between arithmetic word-problem solving and neuropsychological and clinical variables.

RESULTS

Total arithmetic word-problem solving scores were lower in preHD-B patients than in preHD-A (p < 0.05) patients and HC (p < 0.01). Early-mild HD patients had lower scores than preHD patients (p < 0.001) and HC (p < 0.001). Compared to HC, preHD and early-mild HD participants made more errors as trial complexity increased. Moreover, arithmetic word-problem solving scores were significantly associated with measures of global cognition (p < 0.001), frontal-executive functions (p < 0.001), attention (p < 0.001) visual working memory (p < 0.001), mental rotation (p < 0.001), and confrontation naming (p < 0.05).

CONCLUSION

Arithmetic word-problem solving is affected early in the course of HD and is related to deficient processes in frontal-executive and mentalizing-related processes.

Dissociable Motivational Deficits in Pre-manifest Huntington's Disease

Motivation is characterized by a willingness to overcome both cognitive and physical effort costs. Impairments in motivation are common in striatal disorders, such as Huntington’s disease (HD), but whether these impairments are isolated to particular domains of behavior is controversial. We ask whether HD differentially affects the willingness of individuals to overcome cognitive versus physical effort. We tested 20 individuals with pre-manifest HD and compared their behavior to 20 controls. Across separate trials, participants made choices about how much cognitive or physical effort they were willing to invest for reward. Our key results were that individuals with pre-manifest HD were less willing than controls to invest cognitive effort but were no different in their overall preference for physical effort. These results cannot be explained by group differences in neuropsychological or psychiatric profiles. This dissociation of cognitive- and physical-effort-based decisions provides important evidence for separable, domain-specific mechanisms of motivation.

We examine cognitive and physical effort discounting in pre-manifest HD

Individuals with pre-manifest HD are less cognitively motivated than controls

There are no differences in physical motivation between the two groups

This dissociation is not confounded by neuropsychological or psychiatric factors

Structural brain correlates of dementia in Huntington's disease

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Dementia may occur in the early stages of HD and with independence of disease burden.

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More severe posterior-cortical atrophy is associated with dementia in HD.

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Neuropsychological alterations of dementia in HD extends beyond executive dysfunction.

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CAG-independent neuropathological mechanisms may contribute to dementia in HD.

Background

Huntington’s disease (HD) is a fatal genetic neurodegenerative disorder with no effective treatment currently available. Progressive basal ganglia and whole-brain atrophy and concurrent cognitive deterioration are prototypical aspects of HD. However, the specific patterns of brain atrophy underlying cognitive impairment of different severity in HD are poorly understood. The aim of this study was to investigate the specific structural brain correlates of major cognitive deficits in HD and to explore its association with neuropsychological indicators.

Participants

Thirty-five symptomatic early-to-mild HD patients and 15 healthy controls (HC) with available T1-MRI imaging were included in this study.

Methods

In this cross-sectional study, HD patients were classified as patients with (HD-Dem) and without (HD-ND) major cognitive impairment in the range of dementia. This classification was based on previously validated PD-CRS cutoff scores for HD. Differences in brain atrophy across groups were studied by means of grey-matter volume voxel-based morphometry (GMV-VBM) and cortical thickness (Cth). Voxelwise and vertexwise general linear models were used to assess the group comparisons, controlling for the effects of age, sex, education, CAG repeat length and severity of motor symptoms. Clusters surviving p < 0.05 and family-wise error (FWE) correction were considered statistically significant. In order to characterize the impact on cognitive performance of the observed brain differences across groups, GMV and Cth values in the set of significant regions were computed and correlated with specific neuropsychological tests.

Results

All groups had similar sociodemographic profiles, and the HD groups did not significantly differ in terms of CAG repeat length. Compared to HC, both HD groups exhibited significant atrophy in multiple subcortical and parietal brain regions. However, compared to HC and HD-ND groups, HD-Dem patients showed a more prominent pattern of reduced GMV and cortical thinning. Importantly, this thinning was restricted to regions of the parietal-temporal and occipital cortices. Furthermore, these brain alterations were further associated with poorer cognitive performance in tasks assessing frontal-executive and attention domains as well as memory, language and constructional abilities.

Conclusions

Major cognitive impairment in the range of dementia in HD is associated with brain and cognitive alterations exceeding the prototypical frontal-executive deficits commonly recognized in HD. The observed posterior-cortical damage identified by MRI and its association with memory, language, and visuoconstructive dysfunction suggest a strong involvement of extra-striatal atrophy in the onset of severe cognitive dysfunction in HD patients. Critically, major cognitive impairment in this sample was not associated with CAG repeat length, age or education. This finding could support a possible involvement of additional neuropathological mechanisms aggravating cognitive deterioration in HD.

Neocortical morphometry in Huntington's disease: Indication of the coexistence of abnormal neurodevelopmental and neurodegenerative processes

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We found shallower central, intraparietal and left intermediate frontal sulci in HD.

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Shallow calcarine fissure is further evidence of primary cortical degeneration in HD.

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Healthy subjects show strong asymmetry in length of posterior Sylvian fissure (pSF).

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Absence of pSF asymmetry in HD indicates genetic interplay with neurodevelopment.

Huntington's disease (HD) is an inherited, autosomal dominant disorder that is characteristically thought of as a degenerative disorder. Despite cellular and molecular grounds suggesting HD could also impact normal development, there has been scarce systems-level data obtained from in vivo human studies supporting this hypothesis. Sulcus-specific morphometry analysis may help disentangle the contribution of coexisting neurodegenerative and neurodevelopmental processes, but such an approach has never been used in HD. Here, we investigated cortical sulcal depth, related to degenerative process, as well as cortical sulcal length, related to developmental process, in early-stage HD and age-matched healthy controls. This morphometric analysis revealed significant differences in the HD participants compared with the healthy controls bilaterally in the central and intra-parietal sulcus, but also in the left intermediate frontal sulcus and calcarine fissure. As the primary visual cortex is not connected to the striatum, the latter result adds to the increasing in vivo evidence for primary cortical degeneration in HD. Those sulcal measures that differed between HD and healthy populations were mainly atrophy-related, showing shallower sulci in HD. Conversely, the sulcal morphometry also revealed a crucial difference in the imprint of the Sylvian fissure that could not be related to loss of grey matter volume: an absence of asymmetry in the length of this fissure in HD. Strong asymmetry in that cortical region is typically observed in healthy development. As the formation of the Sylvian fissure appears early in utero, and marked asymmetry is specifically found in this area of the neocortex in newborns, this novel finding likely indicates the foetal timing of a disease-specific, genetic interplay with neurodevelopment.

Utility of the Parkinson's disease-Cognitive Rating Scale for the screening of global cognitive status in Huntington's disease

BACKGROUND

Cognitive impairment is an essential feature of Huntington's disease (HD) and dementia is a predictable outcome in all patients. However, validated instruments to assess global cognitive performance in the field of HD are lacking.

OBJECTIVES

We aimed to explore the utility of the Parkinson's disease-Cognitive Rating Scale (PD-CRS) for the screening of global cognition in HD.

METHODS

A multicenter cohort of 132 HD patients at different disease stages and 33 matched healthy controls were classified as having preserved cognition, mild cognitive impairment (HD-MCI) or dementia (HD-Dem) according to the Clinical Dementia Rating and Functional Independence Score. The PD-CRS and the Mini-Mental State Examination were administered. Receiver operating characteristic curve analysis was used to determine optimal cutoffs to differentiate patients according to their cognitive status.

RESULTS

A PD-CRS cutoff score ≤ 81/82 was optimal to detect HD-MCI (sensitivity = 93%; specificity = 80%; area under the curve (AUC) = 0.940), and ≤ 63/64 was optimal to detect HD-Dem (sensitivity = 90%; specificity = 87%; AUC = 0.933). MMSE scores failed to show robust psychometric properties in this context.

DISCUSSION

The PD-CRS is a valid and reliable instrument to assess global cognition in HD in routine clinical care and clinical trials.

Impaired face-like object recognition in premanifest Huntington's disease

Progressive striatal atrophy has long been considered the pathological hallmark of Huntington's disease (HD), but is it now recognized that malfunction and degeneration of posterior-cortical territories are also prominent characteristics of the disease. The limited knowledge about the functional impact of these posterior-cortical changes could be partially attributed to the lack of sensitive measures to capture them. We hypothesized that early malfunction of specific territories of the ventral visual pathway in premanifest HD would lead to difficulties in the recognition of complex stimuli and to differences in their neurophysiological correlates. To test this idea, we used an object, face and face-like object recognition task to be conducted during an electroencephalographic recording. Compared to healthy-matched controls, premanifest participants showed a significantly increased number of recognition errors in the face-like object condition. Moreover, premanifest participants showed a dramatic decrease in the N170 component elicited for the face-like objects. This N170 decrease was significantly associated with the number of recognition errors and with severity of apathy and global cognitive performance. The lack of differences in other clinical and cognitive measures supports a selective deficit in recognition of face-like objects and their neurophysiological correlates in premanifest HD. These deficits occurred in participants up to 15 years before the estimated time to disease onset and correlated strongly with cognitive and behavioral measures known to be sensitive to HD progression. This finding highlights the existence of selective visuoperceptive deficits years before motor-based onset of HD and emphasizes the need to develop sensitive measures to capture early visual system changes in this population. Assessing the integrity of the visual cortex and its related functions in HD could help to identify early markers of disease progression.

Cortical neurodevelopment in pre-manifest Huntington's disease

Background

The expression of the HTT CAG repeat expansion mutation causes neurodegeneration in Huntington's disease (HD). Objectives: In light of the – mainly in-vitro – evidence suggesting an additional role of huntingtin in neurodevelopment we used 3T MRI to test the hypothesis that in CAG-expanded individuals without clinical signs of HD (preHD) there is evidence for neurodevelopmental abnormalities.

Methods

We specifically investigated the complexity of cortical folding, a measure of cortical neurodevelopment, employing a novel method to quantify local fractal dimension (FD) measures that uses spherical harmonic reconstructions.

Results

The complexity of cortical folding differed at a group level between preHD (n = 57) and healthy volunteers (n = 57) in areas of the motor and visual system as well as temporal cortical areas. However, there was no association between the complexity of cortical folding and the loss in putamen volume that was clearly evident in preHD.

Conclusions

Our results suggest that HTT CAG repeat length may have an influence on cortical folding without evidence that this leads to developmental pathology or was clinically meaningful. This suggests that the HTT CAG-repeat expansion mutation may influence the processes governing cortical neurodevelopment; however, that influence seems independent of the events that lead to neurodegeneration.

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Measures of cortical neurodevelopment in preclinical Huntington's disease (HD) gene carriers differ from healthy volunteers

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The influence on cortical folding of the HD gene was not associated with developmental pathology or clinically meaningful

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The influence of the HD gene on cortical neurodevelopment may differ from that on neurodegeneration

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.

Neurocognitive mechanisms of theory of mind impairment in neurodegeneration: a transdiagnostic approach

Much of human interaction is predicated upon our innate capacity to infer the thoughts, beliefs, emotions, and perspectives of others, in short, to possess a "theory of mind" (ToM). While the term has evolved considerably since its inception, ToM encompasses our unique ability to apprehend the mental states of others, enabling us to anticipate and predict subsequent behavior. From a developmental perspective, ToM has been a topic of keen research interest, with numerous studies seeking to explicate the origins of this fundamental capacity and its disruption in developmental disorders such as autism. The study of ToM at the opposite end of the lifespan, however, is paradoxically new born, emerging as a topic of interest in its own right comparatively recently. Here, we consider the unique insights afforded by studying ToM capacity in neurodegenerative disorders. Arguing from a novel, transdiagnostic perspective, we consider how ToM vulnerability reflects the progressive degradation of neural circuits specialized for an array of higher-order cognitive processes. This mechanistic approach enables us to consider the common and unique neurocognitive mechanisms that underpin ToM dysfunction across neurodegenerative disorders and for the first time examine its relation to behavioral disturbances across social, intimate, legal, and criminal settings. As such, we aim to provide a comprehensive overview of ToM research in neurodegeneration, the resultant challenges for family members, clinicians, and the legal profession, and future directions worthy of exploration.

Metformin reverses early cortical network dysfunction and behavior changes in Huntington's disease

Catching primal functional changes in early, ‘very far from disease onset’ (VFDO) stages of Huntington’s disease is likely to be the key to a successful therapy. Focusing on VFDO stages, we assessed neuronal microcircuits in premanifest Hdh150 knock-in mice. Employing in vivo two-photon Ca²⁺ imaging, we revealed an early pattern of circuit dysregulation in the visual cortex - one of the first regions affected in premanifest Huntington’s disease - characterized by an increase in activity, an enhanced synchronicity and hyperactive neurons. These findings are accompanied by aberrations in animal behavior. We furthermore show that the antidiabetic drug metformin diminishes aberrant Huntingtin protein load and fully restores both early network activity patterns and behavioral aberrations. This network-centered approach reveals a critical window of vulnerability far before clinical manifestation and establishes metformin as a promising candidate for a chronic therapy starting early in premanifest Huntington’s disease pathogenesis long before the onset of clinical symptoms.

Huntington’s disease is a devastating brain disorder that causes severe mood disorders, problems with moving, and dementia. Most people develop the condition between their thirties and fifties, and die a decade or two after the symptoms first appear.

The disease emerges because of a mutation in the gene for the Huntingtin protein, which leads to neurons slowly dying in the brain. While genetic testing can reveal who carries the faulty gene, no treatment addresses the root of the disorder or prevents it from appearing. Instead, most therapies for Huntington’s disease aim to reduce brain damage once the telltale symptoms are already present. However, the disease-causing protein is expressed early during the life of a patient, which could give it time to damage the brain long before neurons die and the disorder reveals itself. Treatments that start after the first signs of the disease may be too late to reverse the damage. Detecting and preventing early brain changes in people that carry the mutation may thus help to stop the disease from progressing.

Here, Arnoux, Willam, Griesche et al. set out to detect the minute changes that the faulty Huntingtin protein may cause in the brain network of young mice with the mutation. State-of-the-art imaging tools helped to examine individual neurons in the brain area that processes visual information. These experiments revealed that a group of brain cells had become hyperactive; once this change had occurred, the mutant animals were less anxious than is typical for mice.

Metformin is a drug used to treat diabetes, but it also interferes with a structure that is required to produce the disease-causing Huntingtin protein. Arnoux et al. therefore explored whether the compound could rescue the early brain alterations observed in mutant mice. Adding metformin in the water of the animals for three weeks halted the production of the mutant protein, reversed the brain changes and stopped the abnormal behavior.

Further work is now required in humans to confirm that Huntington’s disease starts with a change in the activity of networks in the brain, and to verify that metformin can stop the disorder in its track.

Testing a longitudinal compensation model in premanifest Huntington's disease

The initial stages of neurodegeneration are commonly marked by normal levels of cognitive and motor performance despite the presence of structural brain pathology. Compensation is widely assumed to account for this preserved behaviour, but despite the apparent simplicity of such a concept, it has proven incredibly difficult to demonstrate such a phenomenon and distinguish it from disease-related pathology. Recently, we developed a model of compensation whereby brain activation, behaviour and pathology, components key to understanding compensation, have specific longitudinal trajectories over three phases of progression. Here, we empirically validate our explicit mathematical model by testing for the presence of compensation over time in neurodegeneration. Huntington's disease is an ideal model for examining longitudinal compensation in neurodegeneration as it is both monogenic and fully penetrant, so disease progression and potential compensation can be monitored many years prior to diagnosis. We defined our conditions for compensation as non-linear longitudinal trajectories of brain activity and performance in the presence of linear neuronal degeneration and applied our model of compensation to a large longitudinal cohort of premanifest and early-stage Huntington's disease patients from the multisite Track-On HD study. Focusing on cognitive and motor networks, we integrated progressive volume loss, task and resting state functional MRI and cognitive and motor behaviour across three sequential phases of neurodegenerative disease progression, adjusted for genetic disease load. Multivariate linear mixed models were fitted and trajectories for each variable tested. Our conceptualization of compensation was partially realized across certain motor and cognitive networks at differing levels. We found several significant network trends that were more complex than that hypothesized in our model. These trends suggest changes to our theoretical model where the network effects are delayed relative to performance effects. There was evidence of compensation primarily in the prefrontal component of the cognitive network, with increased effective connectivity between the left and right dorsolateral prefrontal cortex. Having developed an operational model for the explicit testing of longitudinal compensation in neurodegeneration, it appears that general patterns of our framework are consistent with the empirical data. With the proposed modifications, our operational model of compensation can be used to test for both cross-sectional and longitudinal compensation in neurodegenerative disease with similar patterns to Huntington's disease.

Surface-based morphometry reveals caudate subnuclear structural damage in patients with premotor Huntington disease

In patients with premotor Huntington's disease (pmHD), literature has reported decreases in caudate volume. However, the regional vulnerability of the caudate nucleus to pmHD remains to be clarified. We aimed to determine whether regional structural damage of the caudate nucleus was present in pmHD and was correlated with clinical profile using a surface-based morphometric technique applied to T1-weighted MRI. The study cohort consisted of 14 volunteers with genetically confirmed pmHD (6 males; 41.8 ± 13.2 years) and 11 age- and sex-matched controls (5 males; 46.2 ± 11.9 years, p > 0.3). On 3-T T1-weighted images, bilateral caudate volumes were manually delineated. The resulting labels were converted to a surface, triangulated with 1002 points equally distributed across subjects using SPHARM-PDM. Displacement vectors were then computed between each individual and a template surface representing the whole cohort. Computing point-wise Jacobian determinants (JD) from these vectors quantified local volumes. We found decreases in bilateral global caudate volumes in the pmHD group compared to controls (t = 3.4; p = 0.002). Point-wise analysis of local volumes mapped caudate atrophy in pmHD primarily onto medial surface (t > 2.7; FDR < 0.05), with most pronounced changes in anteromedial subdivision. In a combined group of patients and controls, volume within the area presenting significant group difference was positively correlated with scores of executive function (r = 0.7; p < 0.001) and working memory (r = 0.6; p = 0.002). In patients, the caudate atrophy was associated with increase in disease burden (r = 0.7; p = 0.005). Caudate subnuclear atrophy measured using our surface-based morphometric technique is evident in pmHD, correlates with clinical variables, and may provide a more sensitive biomarker than global volumes.

Structural and metabolic brain correlates of apathy in Huntington's disease

BACKGROUND

Apathy is the most prevalent and characteristic neuropsychiatric feature of Huntington's disease. Congruent with the main early pathological changes, apathy is primarily associated with subcortical damage in frontal-striatal circuits. However, little is known about its precise subserving mechanisms and the contribution of regions other than the basal ganglia.

OBJECTIVES

We aimed to define the neural correlates of apathy in Huntington's disease based on gray matter volume and PET/CT of ¹⁸ F-fluorodeoxyglucose metabolism.

METHODS

We rated the severity of apathy in 40 mild Huntington's disease participants using the Problem Behaviors Assessment for Huntington's disease. Voxelwise regression analysis was performed, controlling for effects of potential confounders, and PET/CT results were corrected for the effects of gray matter atrophy.

RESULTS

Apathy was strongly associated with decreased gray matter within a spatially distributed cortico-subcortical network, with major compromise of the bilateral amygdala and temporal cortex. PET metabolism was significantly decreased in frontotemporal and parietal regions. Metabolic uptake and gray matter values in the identified clusters showed significant correlations with multiple clinical measures.

CONCLUSIONS

Our findings indicate that apathy in Huntington's disease is not exclusively a consequence of basal ganglia and related frontal-executive alterations. It is subserved by a complex cortico-subcortical network where critical reward and emotional-related prefrontal, temporal, and limbic nodes contribute strongly to its severity. This highlights the contribution of damage in regions other than the basal ganglia to the clinical expression of Huntington's disease. © 2018 International Parkinson and Movement Disorder Society.

Genetics Modulate Gray Matter Variation Beyond Disease Burden in Prodromal Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by an expansion mutation of the cytosine–adenine–guanine (CAG) trinucleotide in the HTT gene. Decline in cognitive and motor functioning during the prodromal phase has been reported, and understanding genetic influences on prodromal disease progression beyond CAG will benefit intervention therapies. From a prodromal HD cohort (N = 715), we extracted gray matter (GM) components through independent component analysis and tested them for associations with cognitive and motor functioning that cannot be accounted for by CAG-induced disease burden (cumulative effects of CAG expansion and age). Furthermore, we examined genetic associations (at the genomic, HD pathway, and candidate region levels) with the GM components that were related to functional decline. After accounting for disease burden, GM in a component containing cuneus, lingual, and middle occipital regions was positively associated with attention and working memory performance, and the effect size was about a tenth of that of disease burden. Prodromal participants with at least one dystonia sign also had significantly lower GM volume in a bilateral inferior parietal component than participants without dystonia, after controlling for the disease burden. Two single-nucleotide polymorphisms (SNPs: rs71358386 in NCOR1 and rs71358386 in ADORA2B) in the HD pathway were significantly associated with GM volume in the cuneus component, with minor alleles being linked to reduced GM volume. Additionally, homozygous minor allele carriers of SNPs in a candidate region of ch15q13.3 had significantly higher GM volume in the inferior parietal component, and one minor allele copy was associated with a total motor score decrease of 0.14 U. Our findings depict an early genetical GM reduction in prodromal HD that occurs irrespective of disease burden and affects regions important for cognitive and motor functioning.

Reduced amygdala volumes are related to motor and cognitive signs in Huntington's disease: The IMAGE-HD study

In Huntington's disease (HD), the presence of neurodegeneration in brain regions other than the striatum has been recently gaining attention. The amygdala is one such area, which has been investigated in only eight structural magnetic resonance imaging studies to date, but with inconsistent findings. This is the largest MRI study to date examining manually traced amygdala volumes in HD participants and the relationship of amygdala volumes to clinical measures of HD. Our study included 35 healthy control participants, and groups of 35 pre-symptomatic, and 36 symptomatic HD participants. When comparing the pre-symptomatic and symptomatic HD groups together against the control group, amygdala volumes were significantly lower in HD than controls and in symptomatic HD than pre-symptomatic HD. When examining relationships between amygdala volumes and clinical measures of HD, significantly smaller amygdala volumes were associated with worse motor and cognitive signs. For pre-symptomatic HD participants who were close to disease onset, smaller amygdala volumes were also associated with higher levels of anxiety symptoms. These findings suggest that the amygdala is affected in pre-symptomatic and symptomatic HD, and that the amygdala is related to the clinical profile of HD before onset of motor symptoms.

Whole-Brain Connectivity in a Large Study of Huntington's Disease Gene Mutation Carriers and Healthy Controls

Huntington's disease (HD) is an inherited brain disorder characterized by progressive motor, cognitive, and behavioral dysfunctions. It is caused by abnormally large trinucleotide cytosine-adenine-guanine (CAG) repeat expansions on exon 1 of the Huntingtin gene. CAG repeat length (CAG-RL) inversely correlates with an earlier age of onset. Region-based studies have shown that HD gene mutation carrier (HDgmc) individuals (CAG-RL ≥36) present functional connectivity alterations in subcortical (SC) and default mode networks. In this analysis, we expand on previous HD studies by investigating associations between CAG-RL and connectivity in the whole brain, as well as between CAG-dependent connectivity and motor and cognitive performances. We used group-independent component analysis on resting-state functional magnetic resonance imaging scans of 261 individuals (183 HDgmc and 78 healthy controls) from the PREDICT-HD study, to obtain whole-brain resting state networks (RSNs). Regression analysis was applied within and between RSNs connectivity (functional network connectivity [FNC]) to identify CAG-RL associations. Connectivity within the putamen RSN is negatively correlated with CAG-RL. The FNC between putamen and insula decreases with increasing CAG-RL, and also shows significant associations with motor and cognitive measures. The FNC between calcarine and middle frontal gyri increased with CAG-RL. In contrast, FNC in other visual (VIS) networks declined with increasing CAG-RL. In addition to observed effects in SC areas known to be related to HD, our study identifies a strong presence of alterations in VIS regions less commonly observed in previous reports and provides a step forward in understanding FNC dysfunction in HDgmc.

Cortical thickness, stance control, and arithmetic skill: An exploratory study in premanifest Huntington disease

BACKGROUND

Huntington disease (HD) is an inherited neurodegenerative disorder most commonly manifesting in adulthood. Identification of biomarkers tracking neurodegeneration before the onset of motor symptoms is important for future interventional studies. Our study aimed to contribute in the phenotypic characterization of the premanifest HD phase.

METHODS

28 premanifest subjects (preHD), 25 age-matched controls, and 12 manifest HD patients were enrolled for the study. The participants underwent a multimodal protocol including cognitive evaluations, arithmetic ability test, posturography, composite cerebellar functional test (CCFS), and brain 3T-MRI. PreHD were divided at the group median for predicted years to expected onset into "far-from-onset" (>15 years, PreHD-far), and "close-to-onset" (≤15 years, preHD-close). Basal ganglia volumes and cortical thickness were computed using FreeSurfer.

RESULTS

PreHD-close showed significantly lower scores than controls in Symbol Digit Modalities Test (p = 0.017), Arithmetic subtraction task (p = 0.04), and MMSE (p < 0.006). At posturography, preHD-close showed increased sway velocity (<0.04) and distance (p < 0.02) compared to controls. PreHD-close had reduced striatum and globus pallidus volumes and left occipital cortical thinning compared to controls. Compared to PreHD far-from-onset, PreHD-close showed bilateral cortical thinning in occipital and parahippocampal regions, inversely correlating with burden score and prognostic index for HD. CCFS only differed between controls and manifest HD. PreHD far-from-onset did not show significant differences in comparison with controls.

CONCLUSIONS

We confirmed that quantitative brain MRI represents a valid biomarker of neurodegeneration in preHD. Posturography and Arithmentic tests seem promising tools for detecting early changes in premanifest HD, but need to be further confirmed in large cohorts.

Bupropion for the treatment of apathy in Huntington's disease: A multicenter, randomised, double-blind, placebo-controlled, prospective crossover trial

OBJECTIVE

To evaluate the efficacy and safety of bupropion in the treatment of apathy in Huntington's disease (HD).

METHODS

In this phase 2b multicentre, double-blind, placebo-controlled crossover trial, individuals with HD and clinical signs of apathy according to the Structured Clinical Interview for Apathy-Dementia (SCIA-D), but not depression (n = 40) were randomized to receive either bupropion 150/300mg or placebo daily for 10 weeks. The primary outcome parameter was a significant change of the Apathy Evaluation Scale (AES) score after ten weeks of treatment as judged by an informant (AES-I) living in close proximity with the study participant. The secondary outcome parameters included changes of 1. AES scores determined by the patient (AES-S) or the clinical investigator (AES-C), 2. psychiatric symptoms (NPI, HADS-SIS, UHDRS-Behavior), 3. cognitive performance (SDMT, Stroop, VFT, MMSE), 4. motor symptoms (UHDRS-Motor), 5. activities of daily function (TFC, UHDRS-Function), and 6. caregiver distress (NPI-D). In addition, we investigated the effect of bupropion on brain structure as well as brain responses and functional connectivity during reward processing in a gambling task using magnetic resonance imaging (MRI).

RESULTS

At baseline, there were no significant treatment group differences in the clinical primary and secondary outcome parameters. At endpoint, there was no statistically significant difference between treatment groups for all clinical primary and secondary outcome variables. Study participation, irrespective of the intervention, lessened symptoms of apathy according to the informant and the clinical investigator.

CONCLUSION

Bupropion does not alleviate apathy in HD. However, study participation/placebo effects were observed, which document the need for carefully controlled trials when investigating therapeutic interventions for the neuropsychiatric symptoms of HD.

TRIAL REGISTRATION

ClinicalTrials.gov 01914965.

Intact sensory-motor network structure and function in far from onset premanifest Huntington's disease

Structural and functional changes attributable to the neurodegenerative process in Huntington's disease (HD) may be evident in HTT CAG repeat expansion carriers before the clinical manifestations of HD. It remains unclear, though, how far from motor onset a consistent signature of the neurodegenerative process in HD can be detected. Twelve far from onset preHD and 22 age-matched healthy control participants underwent volumetric structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), and resting-state functional MRI (11 preHD, 22 controls) as well as electrophysiological measurements (12 preHD, 13 controls). There were no significant differences in white matter macro- and microstructure between far from onset preHD participants and controls. Functional connectivity in a basal ganglia-thalamic and motor networks, all measures of the motor efferent and sensory afferent pathways as well as sensory-motor integration were also similar in far from onset preHD and controls. With the methods used in far from onset preHD sensory-motor neural macro- or micro-structure and brain function were similar to healthy controls. This suggests that any observable structural and functional change in preHD nearer to onset, or in manifest HD, at least using comparable techniques such as in this study, most likely reflects an ongoing neurodegenerative process.

Early grey matter changes in structural covariance networks in Huntington's disease

Background

Progressive subcortical changes are known to occur in Huntington's disease (HD), a hereditary neurodegenerative disorder. Less is known about the occurrence and cohesion of whole brain grey matter changes in HD.

Objectives

We aimed to detect network integrity changes in grey matter structural covariance networks and examined relationships with clinical assessments.

Methods

Structural magnetic resonance imaging data of premanifest HD (n = 30), HD patients (n = 30) and controls (n = 30) was used to identify ten structural covariance networks based on a novel technique using the co-variation of grey matter with independent component analysis in FSL. Group differences were studied controlling for age and gender. To explore whether our approach is effective in examining grey matter changes, regional voxel-based analysis was additionally performed.

Results

Premanifest HD and HD patients showed decreased network integrity in two networks compared to controls. One network included the caudate nucleus, precuneous and anterior cingulate cortex (in HD p < 0.001, in pre-HD p = 0.003). One other network contained the hippocampus, premotor, sensorimotor, and insular cortices (in HD p < 0.001, in pre-HD p = 0.023). Additionally, in HD patients only, decreased network integrity was observed in a network including the lingual gyrus, intracalcarine, cuneal, and lateral occipital cortices (p = 0.032). Changes in network integrity were significantly associated with scores of motor and neuropsychological assessments. In premanifest HD, voxel-based analyses showed pronounced volume loss in the basal ganglia, but less prominent in cortical regions.

Conclusion

Our results suggest that structural covariance might be a sensitive approach to reveal early grey matter changes, especially for premanifest HD.

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Identification of anatomical networks in Huntington's disease (HD).

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Independent component analysis was used to examine structural covariance networks.

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HD patients showed changes in subcortical and cortical covariance networks.

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A network-based approach is sensitive to reveal early grey matter changes.

Patterns of Co-Occurring Gray Matter Concentration Loss across the Huntington Disease Prodrome

Huntington disease (HD) is caused by an abnormally expanded cytosine-adenine-guanine (CAG) trinucleotide repeat in the HTT gene. Age and CAG-expansion number are related to age at diagnosis and can be used to index disease progression. However, observed onset-age variability suggests that other factors also modulate progression. Indexing prodromal (pre-diagnosis) progression may highlight therapeutic targets by isolating the earliest-affected factors. We present the largest prodromal HD application of the univariate method voxel-based morphometry (VBM) and the first application of the multivariate method source-based morphometry (SBM) to, respectively, compare gray matter concentration (GMC) and capture co-occurring GMC patterns in control and prodromal participants. Using structural MRI data from 1050 (831 prodromal, 219 control) participants, we characterize control-prodromal, whole-brain GMC differences at various prodromal stages. Our results provide evidence for (1) regional co-occurrence and differential patterns of decline across the prodrome, with parietal and occipital differences commonly co-occurring, and frontal and temporal differences being relatively independent from one another, (2) fronto-striatal circuits being among the earliest and most consistently affected in the prodrome, (3) delayed degradation in some movement-related regions, with increasing subcortical and occipital differences with later progression, (4) an overall superior-to-inferior gradient of GMC reduction in frontal, parietal, and temporal lobes, and (5) the appropriateness of SBM for studying the prodromal HD population and its enhanced sensitivity to early prodromal and regionally concurrent differences.

Longitudinal Changes in the Motor Learning-Related Brain Activation Response in Presymptomatic Huntington's Disease

Neurocognitive decline, including deficits in motor learning, occurs in the presymptomatic phase of Huntington's disease (HD) and precedes the onset of motor symptoms. Findings from recent neuroimaging studies have linked these deficits to alterations in fronto-striatal and fronto-parietal brain networks. However, little is known about the temporal dynamics of these networks when subjects approach phenoconversion. Here, 10 subjects with presymptomatic HD were scanned with 15O-labeled water at baseline and again 1.5 years later while performing a motor sequence learning task and a kinematically matched control task. Spatial covariance analysis was utilized to characterize patterns of change in learning-related neural activation occurring over time in these individuals. Pattern expression was compared to corresponding values in 10 age-matched healthy control subjects. Spatial covariance analysis revealed significant longitudinal changes in the expression of a specific learning-related activation pattern characterized by increasing activity in the right orbitofrontal cortex, with concurrent reductions in the right medial prefrontal and posterior cingulate regions, the left insula, left precuneus, and left cerebellum. Changes in the expression of this pattern over time correlated with baseline measurements of disease burden and learning performance. The network changes were accompanied by modest improvement in learning performance that took place concurrently in the gene carriers. The presence of increased network activity in the setting of stable task performance is consistent with a discrete compensatory mechanism. The findings suggest that this effect is most pronounced in the late presymptomatic phase of HD, as subjects approach clinical onset.

Novel targets for Huntington's disease: future prospects

Huntington's disease (HD) is an incurable, inherited, progressive, neurodegenerative disorder that is characterized by a triad of motor, cognitive, and psychiatric problems. Despite the noticeable increase in therapeutic trials in HD in the last 20 years, there have, to date, been very few significant advances. The main hope for new and emerging therapeutics for HD is to develop a neuroprotective compound capable of slowing down or even stopping the progression of the disease and ultimately prevent the subtle early signs from developing into manifest disease. Recently, there has been a noticeable shift away from symptomatic therapies in favor of more mechanistic-based interventions, a change driven by a better understanding of the pathogenesis of this disorder. In this review, we discuss the status of, and supporting evidence for, potential novel treatments of HD that are currently under development or have reached the level of early Phase I/II clinical trials.

Social cognition in Huntington's disease: A meta-analysis

Neurocognitive impairment in Huntington's disease (HD) frequently includes deficits in emotion recognition, and recent studies have also provided evidence for deficits in theory of mind (ToM). There have been conflicting reports regarding the extent of emotion recognition and ToM deficits before the onset of motor symptoms in HD. In this meta-analysis, ToM and emotion recognition performances of 2226HD or pre-manifest HD and 998 healthy controls were included in the meta-analysis. Meta-regression analyses were conducted to investigate the relationship between social cognition deficits and demographic, cognitive and clinical features in HD. HD patients were significantly less accurate than controls in ToM and across all emotions in response to both facial and vocal stimuli. ToM (d=1.72) and recognition of negative emotions (d=1.20-1.33), especially anger, disgust and fear (d=1.26-1.52) were severely impaired. Pre-manifest HD was also associated with impairment in social cognition. The severity of emotion recognition impairment was significantly associated with disease burden, proximity of onset of motor symptoms and cognitive impairment. Social cognition impairments are potential biomarkers of disease onset and progression in HD.

Large-scale brain network abnormalities in Huntington's disease revealed by structural covariance

Huntington's disease (HD) is a progressive neurodegenerative disorder that can be diagnosed with certainty decades before symptom onset. Studies using structural MRI have identified grey matter (GM) loss predominantly in the striatum, but also involving various cortical areas. So far, voxel-based morphometric studies have examined each brain region in isolation and are thus unable to assess the changes in the interrelation of brain regions. Here, we examined the structural covariance in GM volumes in pre-specified motor, working memory, cognitive flexibility, and social-affective networks in 99 patients with manifest HD (mHD), 106 presymptomatic gene mutation carriers (pre-HD), and 108 healthy controls (HC). After correction for global differences in brain volume, we found that increased GM volume in one region was associated with increased GM volume in another. When statistically comparing the groups, no differences between HC and pre-HD were observed, but increased positive correlations were evident for mHD, relative to pre-HD and HC. These findings could be explained by a HD-related neuronal loss heterogeneously affecting the examined network at the pre-HD stage, which starts to dominate structural covariance globally at the manifest stage. Follow-up analyses identified structural connections between frontoparietal motor regions to be linearly modified by disease burden score (DBS). Moderator effects of disease load burden became significant at a DBS level typically associated with the onset of unequivocal HD motor signs. Together with existing findings from functional connectivity analyses, our data indicates a critical role of these frontoparietal regions for the onset of HD motor signs.

Non-Verbal and Verbal Fluency in Prodromal Huntington's Disease

BACKGROUND

This study examines non-verbal (design) and verbal (phonemic and semantic) fluency in prodromal Huntington's disease (HD). An accumulating body of research indicates subtle deficits in cognitive functioning among prodromal mutation carriers for HD.

METHODS

Performance was compared between 32 mutation carriers and 38 non-carriers in order to examine the magnitude of impairment across fluency tasks. The predicted years to onset (PYTO) in mutation carriers was calculated by a regression equation and used to divide the group according to whether onset was predicted as less than 12.75 years (HD+CLOSE; n = 16) or greater than 12.75 years (HD+DISTANT; n = 16).

RESULTS

The results indicate that both non-verbal and verbal fluency is sensitive to subtle impairment in prodromal HD. HD+CLOSE group produced fewer items in all assessed fluency tasks compared to non-carriers. HD+DISTANT produced fewer drawings than non-carriers in the non-verbal task. PYTO correlated significantly with all measures of non-verbal and verbal fluency.

CONCLUSION

The pattern of results indicates that subtle cognitive deficits exist in prodromal HD, and that less structured tasks with high executive demands are the most sensitive in detecting divergence from the normal range of functioning. These selective impairments can be attributed to the early involvement of frontostriatal circuitry and frontal lobes.

Emotional dysfunctions in neurodegenerative diseases

Neurodegenerative diseases are characterized primarily by motor signs but are also accompanied by emotional disturbances. Because of the limited knowledge about these dysfunctions, this Review provides an overview of emotional competencies in Huntington's disease (HD), Parkinson's disease (PD), and multiple sclerosis (MS), with a focus on emotion recognition, emotion regulation, and depression. Most studies indicate facial emotion recognition deficits in HD and PD, whereas data for MS are inconsistent. On a neural level, dysfunctions of amygdala and striatum, among others, have been linked to these impairments. These dysfunctions also tap brain regions that are part of the emotion regulation network, suggesting problems in this competency, too. Research points to dysfunctional emotion regulation in MS, whereas findings for PD and HD are missing. The high prevalence of depression in all three disorders emphasizes the need for effective therapies. Research on emotional disturbances might improve treatment, thereby increasing patients' and caregivers' well-being.

The role of the amygdala during emotional processing in Huntington's disease: from pre-manifest to late stage disease

BACKGROUND

Deficits in emotional processing can be detected in the pre-manifest stage of Huntington's disease and negative emotion recognition has been identified as a predictor of clinical diagnosis. The underlying neuropathological correlates of such deficits are typically established using correlative structural MRI studies. This approach does not take into consideration the impact of disruption to the complex interactions between multiple brain circuits on emotional processing. Therefore, exploration of the neural substrates of emotional processing in pre-manifest HD using fMRI connectivity analysis may be a useful way of evaluating the way brain regions interrelate in the period prior to diagnosis.

METHODS

We investigated the impact of predicted time to disease onset on brain activation when participants were exposed to pictures of faces with angry and neutral expressions, in 20 pre-manifest HD gene carriers and 23 healthy controls. On the basis of the results of this initial study went on to look at amygdala dependent cognitive performance in 79 Huntington's disease patients from a cross-section of disease stages (pre-manifest to late disease) and 26 healthy controls, using a validated theory of mind task: "the Reading the Mind in the Eyes Test" which has been previously been shown to be amygdala dependent.

RESULTS

Psychophysiological interaction analysis identified reduced connectivity between the left amygdala and right fusiform facial area in pre-manifest HD gene carriers compared to controls when viewing angry compared to neutral faces. Change in PPI connectivity scores correlated with predicted time to disease onset (r=0.45, p<0.05). Furthermore, performance on the "Reading the Mind in the Eyes Test" correlated negatively with proximity to disease onset and became progressively worse with each stage of disease.

CONCLUSION

Abnormalities in the neural networks underlying social cognition and emotional processing can be detected prior to clinical diagnosis in Huntington's disease. Connectivity between the amygdala and other brain regions is impacted by the disease process in pre-manifest HD and may therefore be a useful way of identifying participants who are approaching a clinical diagnosis. Furthermore, the "Reading the Mind in the Eyes Test" is a surrogate measure of amygdala function that is clinically useful across the entire cross-section of disease stages in HD.

HDAC4-myogenin axis as an important marker of HD-related skeletal muscle atrophy

Skeletal muscle remodelling and contractile dysfunction occur through both acute and chronic disease processes. These include the accumulation of insoluble aggregates of misfolded amyloid proteins that is a pathological feature of Huntington’s disease (HD). While HD has been described primarily as a neurological disease, HD patients’ exhibit pronounced skeletal muscle atrophy. Given that huntingtin is a ubiquitously expressed protein, skeletal muscle fibres may be at risk of a cell autonomous HD-related dysfunction. However the mechanism leading to skeletal muscle abnormalities in the clinical and pre-clinical HD settings remains unknown. To unravel this mechanism, we employed the R6/2 transgenic and HdhQ150 knock-in mouse models of HD. We found that symptomatic animals developed a progressive impairment of the contractile characteristics of the hind limb muscles tibialis anterior (TA) and extensor digitorum longus (EDL), accompanied by a significant loss of motor units in the EDL. In symptomatic animals, these pronounced functional changes were accompanied by an aberrant deregulation of contractile protein transcripts and their up-stream transcriptional regulators. In addition, HD mouse models develop a significant reduction in muscle force, possibly as a result of a deterioration in energy metabolism and decreased oxidation that is accompanied by the re-expression of the HDAC4-DACH2-myogenin axis. These results show that muscle dysfunction is a key pathological feature of HD.

Huntington’s disease (HD) is a neurodegenerative disorder in which the mutation results in an extra-long tract of glutamines that causes the huntingtin protein to aggregate. It is characterized by neurological symptoms and brain pathology, which is associated with nuclear and cytoplasmic protein aggregates and with transcriptional deregulation. Despite the fact that HD has been recognized principally as a neurological disease, there are multiple studies indicating that peripheral pathologies including cardiac dysfunction and skeletal muscle atrophy, contribute to the overall progression of HD. To unravel the cause of the skeletal muscle dysfunction, we applied a wide range of molecular and physiological methods to the analysis of two well established genetic mouse models of this disease. We found that symptomatic animals developed muscle dysfunction characterised by a change in the contractile characteristics of fast twitch muscles and a decrease in twitch and tetanic force of hindlimb muscles. In addition, there is a significant decrease in the number of motor units innervating the EDL muscle, and this motor unit loss progresses during the course of the disease. These changes were accompanied by the re-expression of contractile transcripts and markers of muscle denervation such as the HDAC4-Dach2-myogenin axis, as well as the apparent deterioration in energy metabolism and decreased oxidation. Therefore, we conclude, that the HD-related skeletal muscle atrophy is accompanied by progressive loss of functional motor units.

Functional connectivity modeling of consistent cortico-striatal degeneration in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by a complex neuropsychiatric phenotype. In a recent meta-analysis we identified core regions of consistent neurodegeneration in premanifest HD in the striatum and middle occipital gyrus (MOG). For early manifest HD convergent evidence of atrophy was most prominent in the striatum, motor cortex (M1) and inferior frontal junction (IFJ). The aim of the present study was to functionally characterize this topography of brain atrophy and to investigate differential connectivity patterns formed by consistent cortico-striatal atrophy regions in HD. Using areas of striatal and cortical atrophy at different disease stages as seeds, we performed task-free resting-state and task-based meta-analytic connectivity modeling (MACM). MACM utilizes the large data source of the BrainMap database and identifies significant areas of above-chance co-activation with the seed-region via the activation-likelihood-estimation approach. In order to delineate functional networks formed by cortical as well as striatal atrophy regions we computed the conjunction between the co-activation profiles of striatal and cortical seeds in the premanifest and manifest stages of HD, respectively. Functional characterization of the seeds was obtained using the behavioral meta-data of BrainMap. Cortico-striatal atrophy seeds of the premanifest stage of HD showed common co-activation with a rather cognitive network including the striatum, anterior insula, lateral prefrontal, premotor, supplementary motor and parietal regions. A similar but more pronounced co-activation pattern, additionally including the medial prefrontal cortex and thalamic nuclei was found with striatal and IFJ seeds at the manifest HD stage. The striatum and M1 were functionally connected mainly to premotor and sensorimotor areas, posterior insula, putamen and thalamus. Behavioral characterization of the seeds confirmed that experiments activating the MOG or IFJ in conjunction with the striatum were associated with cognitive functions, while the network formed by M1 and the striatum was driven by motor-related tasks. Thus, based on morphological changes in HD, we identified functionally distinct cortico-striatal networks resembling a cognitive and motor loop, which may be prone to early disruptions in different stages of the disease and underlie HD-related cognitive and motor symptom profiles. Our findings provide an important link between morphometrically defined seed-regions and corresponding functional circuits highlighting the functional and ensuing clinical relevance of structural damage in HD.

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Pre-HD atrophy seeds showed common functional co-activation with a cognitive network.

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Modeling of manifest-HD seeds delineated a segregation of a cognitive and motor loop.

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Behavioral decoding of atrophy seeds confirmed functional segregation of networks.

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Based on morphometric changes in HD distinct corticostriatal networks were identified.

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Findings depict functional and ensuing clinical relevance of structural damage in HD.

Abnormal resting-state connectivity of motor and cognitive networks in early manifest Huntington's disease

BACKGROUND

Functional magnetic resonance imaging (fMRI) of multiple neural networks during the brain's 'resting state' could facilitate biomarker development in patients with Huntington's disease (HD) and may provide new insights into the relationship between neural dysfunction and clinical symptoms. To date, however, very few studies have examined the functional integrity of multiple resting state networks (RSNs) in manifest HD, and even less is known about whether concomitant brain atrophy affects neural activity in patients.

METHOD

Using MRI, we investigated brain structure and RSN function in patients with early HD (n = 20) and healthy controls (n = 20). For resting-state fMRI data a group-independent component analysis identified spatiotemporally distinct patterns of motor and prefrontal RSNs of interest. We used voxel-based morphometry to assess regional brain atrophy, and 'biological parametric mapping' analyses to investigate the impact of atrophy on neural activity.

RESULTS

Compared with controls, patients showed connectivity changes within distinct neural systems including lateral prefrontal, supplementary motor, thalamic, cingulate, temporal and parietal regions. In patients, supplementary motor area and cingulate cortex connectivity indices were associated with measures of motor function, whereas lateral prefrontal connectivity was associated with cognition.

CONCLUSIONS

This study provides evidence for aberrant connectivity of RSNs associated with motor function and cognition in early manifest HD when controlling for brain atrophy. This suggests clinically relevant changes of RSN activity in the presence of HD-associated cortical and subcortical structural abnormalities.

Dysfunction of the CNS-heart axis in mouse models of Huntington's disease

Cardiac remodelling and contractile dysfunction occur during both acute and chronic disease processes including the accumulation of insoluble aggregates of misfolded amyloid proteins that are typical features of Alzheimer's, Parkinson's and Huntington's disease (HD). While HD has been described mainly as a neurological disease, multiple epidemiological studies have shown that HD patients exhibit a high incidence of cardiovascular events leading to heart failure, and that this is the second highest cause of death. Given that huntingtin is ubiquitously expressed, cardiomyocytes may be at risk of an HD-related dysfunction. In mice, the forced expression of an expanded polyQ repeat under the control of a cardiac specific promoter led to severe heart failure followed by reduced lifespan. However the mechanism leading to cardiac dysfunction in the clinical and pre-clinical HD settings remains unknown. To unravel this mechanism, we employed the R6/2 transgenic and HdhQ150 knock-in mouse models of HD. We found that pre-symptomatic animals developed connexin-43 relocation and a significant deregulation of hypertrophic markers and Bdnf transcripts. In the symptomatic animals, pronounced functional changes were visualised by cardiac MRI revealing a contractile dysfunction, which might be a part of dilatated cardiomyopathy (DCM). This was accompanied by the re-expression of foetal genes, apoptotic cardiomyocyte loss and a moderate degree of interstitial fibrosis. To our surprise, we could identify neither mutant HTT aggregates in cardiac tissue nor a HD-specific transcriptional dysregulation, even at the end stage of disease. We postulate that the HD-related cardiomyopathy is caused by altered central autonomic pathways although the pathogenic effects of mutant HTT acting intrinsically in the heart may also be a contributing factor.

Huntington's disease (HD) is a neurodegenerative disorder for which the mutation results in an extra-long tract of glutamines that causes the huntingtin protein to aggregate. It is characterized by neurological symptoms and brain pathology that is associated with nuclear and cytoplasmic aggregates and with transcriptional dysregulation. Despite the fact that HD has been recognized principally as a neurological disease, there are multiple epidemiological studies showing that HD patients exhibit a high rate of cardiovascular events leading to heart failure. To unravel the cause of cardiac dysfunction in HD models, we employed a wide range of molecular and physiological methods using two well established genetic mouse models of this disease. We found that pre-symptomatic animals developed aberrant gap junction channel expression and a significant deregulation of hypertrophic markers that may predispose them to arrhythmia and an overall change in cardiac function. These changes were accompanied by the re-expression of foetal genes, apoptotic cardiomyocyte loss and a moderate degree of interstitial fibrosis in the symptomatic animals. Surprisingly, we could identify neither mutant HTT aggregates in cardiac tissue nor a HD-specific transcriptional dysregulation. Therefore, we conclude that the HD-related cardiomyopathy could be driven by altered central autonomic pathways.