The Role of the Cerebellum in Huntington's Disease: a Systematic Review

Prodromal Cognitive Changes as a Prognostic Indicator of Forthcoming Huntington's Disease Severity: A Retrospective Longitudinal Study

BACKGROUND

Cognitive changes in Huntington's disease (HD) precede motor manifestations. ENROLL-HD platform includes four cognitive measures of information processing speed (IPS). Our group is eager to seek clinical markers in the life stage that is as close as possible to the age of onset (ie, the so called prodromal HD phase) because this is the best time for therapeutic interventions.

OBJECTIVES

Our study aimed to test whether cognitive scores in prodromal ENROLL-HD mutation carriers show the potential to predict the severity of motor and behavioral changes once HD became fully manifested.

METHODS

From the global ENROLL-HD cohort of 21,343 participants, we first selected a premanifest Cohort#1 (ie, subjects with Total Motor Score (TMS) <10 and Diagnostic Confidence Level (DCL) <4, N = 1.222). From this cohort, we then focused on a prodromal Cohort#2 of subjects who were ascertained to phenoconvert into manifest HD at follow-up visits (ie, subjects from 6 ≤ TMS≤9 and DCL <4 to TMS≥10 and DCL = 4, n = 206).

RESULTS

The main results of our study showed that low IPS before phenoconversion in Cohort#2 predicted the severity of motor and behavioral manifestations. By combining the four IPS cognitive measures (eg, the Categorical Verbal Fluency Test; Stroop Color Naming Test; Stroop Word Reading; Symbol Digit Modalities Test), we generated a Composite Cognition Score (CCS). The lower the CCS score the higher the TMS and the apathy scores in the same longitudinally followed-up patients after phenoconversion.

CONCLUSIONS

CCS might represent a clinical instrument to predict the prognosis of mutation carriers who are close to manifesting HD.

Neurophysiological hallmarks of Huntington's disease progression: an EEG and fMRI connectivity study

Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can provide corroborative data on neurophysiological alterations in Huntington's disease (HD). However, the alterations in EEG and fMRI resting-state functional connectivity (rsFC), as well as their interrelations, at different stages of HD remain insufficiently investigated. This study aimed to identify neurophysiological alterations in individuals with preclinical HD (preHD) and early manifest HD (EMHD) by analyzing EEG and fMRI rsFC and examining their interrelationships. We found significant differences in EEG power between preHD individuals and healthy controls (HC), with a decrease in power in a specific frequency range at the theta-alpha border and slow alpha activity. In EMHD patients, in addition to the decrease in power in the 7-9 Hz range, a reduction in power within the classic alpha band compared to HC was observed. The fMRI analysis revealed disrupted functional connectivity in various brain networks, particularly within frontal lobe, putamen-cortical, and cortico-cerebellar networks, in individuals with the HD mutation compared to HC. The analysis of the relationship between EEG and fMRI rsFC revealed an association between decreased alpha power, observed in individuals with EMHD, and increased connectivity in large-scale brain networks. These networks include putamen-cortical, DMN-related and cortico-hippocampal circuits. Overall, the findings suggest that EEG and fMRI provide valuable information for monitoring pathological processes during the development of HD. A decrease in inhibitory control within the putamen-cortical, DMN-related and cortico-hippocampal circuits, accompanied by a reduction in alpha and theta-alpha border oscillatory activity, could potentially contribute to cognitive decline in HD.

Social cognition in hyperkinetic movement disorders: a systematic review

Numerous lines of research indicate that our social brain involves a network of cortical and subcortical brain regions that are responsible for sensing and controlling body movements. However, it remains unclear whether movement disorders have a systematic impact on social cognition. To address this question, we conducted a systematic review examining the influence of hyperkinetic movement disorders (including Huntington disease, Tourette syndrome, dystonia, and essential tremor) on social cognition. Following the PRISMA guidelines and registering the protocol in the PROSPERO database (CRD42022327459), we analyzed 50 published studies focusing on theory of mind (ToM), social perception, and empathy. The results from these studies provide evidence of impairments in ToM and social perception in all hyperkinetic movement disorders, particularly during the recognition of negative emotions. Additionally, individuals with Huntington's Disease and Tourette syndrome exhibit empathy disorders. These findings support the functional role of subcortical structures (such as the basal ganglia and cerebellum), which are primarily responsible for movement disorders, in deficits related to social cognition.

Non-Invasive Neuromodulation Methods to Alleviate Symptoms of Huntington's Disease: A Systematic Review of the Literature

Huntington's disease (HD) is a progressive and debilitating neurodegenerative disease. There is growing evidence for non-invasive neuromodulation tools as therapeutic strategies in neurodegenerative diseases. This systematic review aims to investigate the effectiveness of noninvasive neuromodulation in HD-associated motor, cognitive, and behavioral symptoms. A comprehensive literature search was conducted in Ovid MEDLINE, Cochrane Central Register of Clinical Trials, Embase, and PsycINFO from inception to 13 July 2021. Case reports, case series, and clinical trials were included while screening/diagnostic tests involving non-invasive neuromodulation, review papers, experimental studies on animal models, other systematic reviews, and meta-analyses were excluded. We have identified 19 studies in the literature investigating the use of ECT, TMS, and tDCS in the treatment of HD. Quality assessments were performed using Joanna Briggs Institute's (JBI's) critical appraisal tools. Eighteen studies showed improvement of HD symptoms, but the results were very heterogeneous considering different intervention techniques and protocols, and domains of symptoms. The most noticeable improvement involved depression and psychosis after ECT protocols. The impact on cognitive and motor symptoms is more controversial. Further investigations are required to determine the therapeutic role of distinct neuromodulation techniques for HD-related symptoms.

Cerebellar granule neurons induce Cyclin D1 before the onset of motor symptoms in Huntington's disease mice

Although Huntington's disease (HD) is classically defined by the selective vulnerability of striatal projection neurons, there is increasing evidence that cerebellar degeneration modulates clinical symptoms. However, little is known about cell type-specific responses of cerebellar neurons in HD. To dissect early disease mechanisms in the cerebellum and cerebrum, we analyzed translatomes of neuronal cell types from both regions in a new HD mouse model. For this, HdhQ200 knock-in mice were backcrossed with the calm 129S4 strain, to constrain experimental noise caused by variable hyperactivity of mice in a C57BL/6 background. Behavioral and neuropathological characterization showed that these S4-HdhQ200 mice had very mild behavioral abnormalities starting around 12 months of age that remained mild up to 18 months. By 9 months, we observed abundant Huntingtin-positive neuronal intranuclear inclusions (NIIs) in the striatum and cerebellum. The translatome analysis of GABAergic cells of the cerebrum further confirmed changes typical of HD-induced striatal pathology. Surprisingly, we observed the strongest response with 626 differentially expressed genes in glutamatergic neurons of the cerebellum, a population consisting primarily of granule cells, commonly considered disease resistant. Our findings suggest vesicular fusion and exocytosis, as well as differentiation-related pathways are affected in these neurons. Furthermore, increased expression of cyclin D1 (Ccnd1) in the granular layer and upregulated expression of polycomb group complex protein genes and cell cycle regulators Cbx2, Cbx4 and Cbx8 point to a putative role of aberrant cell cycle regulation in cerebellar granule cells in early disease.

Case report: Cerebellar sparing in juvenile Huntington's disease

Juvenile Huntington's disease is an early-onset variant of Huntington's disease, generally associated with large CAG repeats and distinct clinical symptoms. The role of the cerebellum in Huntington's disease has been reevaluated, based on the presence of ataxia and findings on the impact of the disease on cerebellar volume. Recent studies showed a hyperconnectivity between the cerebellum and the basal ganglia in premanifest children with expanded CAG repeats, as well as an enlargement of the cerebellum in adolescence-onset Huntington's disease. We report a 21-year-old Brazilian female with Huntington's disease (age at disease onset 16 years) with Parkinsonism and no ataxic features. There was no reduction of cerebellar volume over 3 years of follow-up, despite the brain atrophy in other regions and clinical worsening. Furthermore, the cerebellar volume of the patient was similar to age- and sex-matched controls. These findings support the existence of compensatory mechanisms involving the cerebellum in individuals with a moderate-to-high number of CAG repeats (50-100 copies) in the early stages of life.

Selective transduction of cerebellar Purkinje and granule neurons using delivery of AAV-PHP.eB and AAVrh10 vectors at axonal terminal locations

Adeno-associated virus (AAV)-based brain gene therapies require precision without off-targeting of unaffected neurons to avoid side effects. The cerebellum and its cell populations, including granule and Purkinje cells, are vulnerable to neurodegeneration; hence, conditions to deliver the therapy to specific cell populations selectively remain challenging. We have investigated a system consisting of the AAV serotypes, targeted injections, and transduction modes (direct or retrograde) for targeted delivery of AAV to cerebellar cell populations. We selected the AAV-PHP.eB and AAVrh10 serotypes valued for their retrograde features, and we thoroughly examined their cerebellar transduction pattern when injected into lobules and deep cerebellar nuclei. We found that AAVrh10 is suitable for the transduction of neurons in the mode highly dependent on placing the virus at axonal terminals. The strategy secures selective transduction for granule cells. The AAV-PHP.eB can transduce Purkinje cells and is very selective for the cell type when injected into the DCN at axonal PC terminals. Therefore, both serotypes can be used in a retrograde mode for selective transduction of major neuronal types in the cerebellum. Moreover, our in vivo transduction strategies are suitable for pre-clinical protocol development for gene delivery to granule cells by AAVrh10 and Purkinje cells by AAV-PHPeB.

Huntington Disease Gene Expression Signatures in Blood Compared to Brain of YAC128 Mice as Candidates for Monitoring of Pathology

While the genetic cause of Huntington disease (HD) is known since 1993, still no cure exists. Therapeutic development would benefit from a method to monitor disease progression and treatment efficacy, ideally using blood biomarkers. Previously, HD-specific signatures were identified in human blood representing signatures in human brain, showing biomarker potential. Since drug candidates are generally first screened in rodent models, we aimed to identify HD signatures in blood and brain of YAC128 HD mice and compare these with previously identified human signatures. RNA sequencing was performed on blood withdrawn at two time points and four brain regions from YAC128 and control mice. Weighted gene co-expression network analysis was used to identify clusters of co-expressed genes (modules) associated with the HD genotype. These HD-associated modules were annotated via text-mining to determine the biological processes they represented. Subsequently, the processes from mouse blood were compared with mouse brain, showing substantial overlap, including protein modification, cell cycle, RNA splicing, nuclear transport, and vesicle-mediated transport. Moreover, the disease-associated processes shared between mouse blood and brain were highly comparable to those previously identified in human blood and brain. In addition, we identified HD blood-specific pathology, confirming previous findings for peripheral pathology in blood. Finally, we identified hub genes for HD-associated blood modules and proposed a strategy for gene selection for development of a disease progression monitoring panel.

Non-Cell Autonomous and Epigenetic Mechanisms of Huntington's Disease

Huntington's disease (HD) is a rare neurodegenerative disorder caused by an expansion of CAG trinucleotide repeat located in the exon 1 of Huntingtin (HTT) gene in human chromosome 4. The HTT protein is ubiquitously expressed in the brain. Specifically, mutant HTT (mHTT) protein-mediated toxicity leads to a dramatic degeneration of the striatum among many regions of the brain. HD symptoms exhibit a major involuntary movement followed by cognitive and psychiatric dysfunctions. In this review, we address the conventional role of wild type HTT (wtHTT) and how mHTT protein disrupts the function of medium spiny neurons (MSNs). We also discuss how mHTT modulates epigenetic modifications and transcriptional pathways in MSNs. In addition, we define how non-cell autonomous pathways lead to damage and death of MSNs under HD pathological conditions. Lastly, we overview therapeutic approaches for HD. Together, understanding of precise neuropathological mechanisms of HD may improve therapeutic approaches to treat the onset and progression of HD.

Neurodegeneration and Sensorimotor Function

Sensorimotor integration is an essential function for both motor control and learning. Over recent decades, a growing body of evidence has emerged in support of the role of altered sensorimotor integration in the pathophysiology of various neurological conditions and movement disorders, particularly bradykinesia, tremor, and dystonia. However, the various causes and mechanisms underlying altered sensorimotor integration in movement disorders are still not entirely understood. The lack of complete insight into the pathophysiological role of altered sensorimotor integration in movement disorders is certainly due to the heterogeneity of movement disorders as well as to the variable occurrence of neurodegenerative phenomena, even in idiopathic movement disorders, which contribute to pathophysiology in a complex and often not easily interpretable way. Clarifying the possible relationship between neurodegenerative phenomena and sensorimotor deficits in movement disorders and other neurological conditions may guide the development of a more detailed disease prognosis and lead, perhaps, to the implementation of novel and individualized therapeutic interventions.