Spinocerebellar Ataxia Type 17 (SCA17)

Cognitive dysfunction, social behavior disorder, cerebellar ataxia, and atypical brain FDG-PET presentation in spinocerebellar ataxia 17: a case report

BACKGROUND

Spinocerebellar ataxia 17 (SCA17) is a rare autosomal dominant form of inherited ataxia, caused by heterozygous trinucleotide repeat expansions encoding glutamine in the TATA box-binding protein (TBP) gene.

CASE DESCRIPTION

We describe the clinical history, neuropsychological, and neuroimaging findings of a 42-year-old patient who presented for medical attention showing prevalent behavioral and cognitive problems along with progressively worsening gait disturbances. The patient's family history indicated the presence of SCA17 in the maternal lineage. Genetic analysis confirmed a heterozygous 52-CAG pathological expansion repeat in TBP (normal interval, 25-40 CAG. Brain 18-fluorodeoxyglucose positron emission tomography (FDG-PET) showed bilateral hypometabolism in the sensorimotor cortex, with a slight predominance on the right, as well as in the striatal nuclei and thalamic hypermetabolism, a finding similar to what is observed in Huntington's disease. The patient also underwent neuropsychological evaluation, which revealed mild cognitive impairment and difficulties in social interaction and understanding other's emotions (Faux Pas Test and Reading the Mind in the Eyes Test).

CONCLUSION

Our report emphasizes the importance of considering SCA17 as a possible diagnosis in patients with a prevalent progressive cognitive and behavioral disorders, even with a pattern of FDG-PET hypometabolism not primarily indicative of this disease.

Polyglutamine disorders: Pathogenesis and potential drug interventions

Polyglutamine/poly(Q) diseases are a group nine hereditary neurodegenerative disorders caused due to abnormally expanded stretches of CAG trinucleotide in functionally distinct genes. All human poly(Q) diseases are characterized by the formation of microscopically discernable poly(Q) positive aggregates, the inclusion bodies. These toxic inclusion bodies are responsible for the impairment of several cellular pathways such as autophagy, transcription, cell death, etc., that culminate in disease manifestation. Although, these diseases remain largely without treatment, extensive research has generated mounting evidences that various events of poly(Q) pathogenesis can be developed as potential drug targets. The present review article briefly discusses the key events of disease pathogenesis, model system-based investigations that support the development of effective therapeutic interventions against pathogenesis of human poly(Q) disorders, and a comprehensive list of pharmacological and bioactive compounds that have been experimentally shown to alleviate poly(Q)-mediated neurotoxicity. Interestingly, due to the common cause of pathogenesis, all poly(Q) diseases share etiology, thus, findings from one disease can be potentially extrapolated to other poly(Q) diseases as well.

A Review of Ocular Movement Abnormalities in Hereditary Cerebellar Ataxias

Cerebellar ataxias are a wide heterogeneous group of disorders that may present with fine motor deficits as well as gait and balance disturbances that have a significant influence on everyday activities. To review the ocular movements in cerebellar ataxias in order to improve the clinical knowledge of cerebellar ataxias and related subtypes. English papers published from January 1990 to May 2022 were selected by searching PubMed services. The main search keywords were ocular motor, oculomotor, eye movement, eye motility, and ocular motility, along with each ataxia subtype. The eligible papers were analyzed for clinical presentation, involved mutations, the underlying pathology, and ocular movement alterations. Forty-three subtypes of spinocerebellar ataxias and a number of autosomal dominant and autosomal recessive ataxias were discussed in terms of pathology, clinical manifestations, involved mutations, and with a focus on the ocular abnormalities. A flowchart has been made using ocular movement manifestations to differentiate different ataxia subtypes. And underlying pathology of each subtype is reviewed in form of illustrated models to reach a better understanding of each disorder.

A Chinese Family with Digenic TBP/STUB1 Spinocerebellar Ataxia

Spinocerebellar ataxias (SCAs) are inherited neurodegenerative diseases characterized by loss of balance, coordination, and slurred speech. Recently, a digenic mode of inheritance of TBP/STUB1 contributing to SCA was demonstrated. The clinical manifestations of SCATBP/STUB1 include not only ataxia but also obvious cognitive and behavioral impairment. Here, we describe a Chinese family with SCATBP/STUB1 and performed a literature search for similar cases. We identified a Chinese family with SCATBP/STUB1 and compare our clinical findings with other cases described in the literature so far. Four individuals in this family have been found to carry SCATBP/STUB1, of which three have clinical manifestations. A heterozygous deletion mutation in the STIP1-homologous and U-box containing protein 1 (STUB1) gene, NM_005861.4:c433_435del(p.K145del), was identified. The proband is a 34-year-old female with progressive dementia and dysarthria. The mother and uncle of the proband first presented with motor abnormalities and gradually developed cognitive impairment. The proband and her uncle showed cerebellar atrophy on MRI. The proband's brother carried digenic variants but was asymptomatic. SCATBP/STUB1 is a novel SCA subtype. The main clinical manifestations are motor, cognitive, and behavioral abnormalities. Brain MRI shows significant cerebellar atrophy and cortical thinning. The independent segregation of TBP and STUB1 alleles should be considered when evaluating patients with cognitive impairment and ataxia.

The molecular mechanisms of spinocerebellar ataxias for DNA repeat expansion in disease

Spinocerebellar ataxias (SCAs) are a heterogenous group of neurodegenerative disorders which commonly inherited in an autosomal dominant manner. They cause muscle incoordination due to degeneration of the cerebellum and other parts of nervous system. Out of all the characterized (>50) SCAs, 14 SCAs are caused due to microsatellite repeat expansion mutations. Repeat expansions can result in toxic protein gain-of-function, protein loss-of-function, and/or RNA gain-of-function effects. The location and the nature of mutation modulate the underlying disease pathophysiology resulting in varying disease manifestations. Potential toxic effects of these mutations likely affect key major cellular processes such as transcriptional regulation, mitochondrial functioning, ion channel dysfunction and synaptic transmission. Involvement of several common pathways suggests interlinked function of genes implicated in the disease pathogenesis. A better understanding of the shared and distinct molecular pathogenic mechanisms in these diseases is required to develop targeted therapeutic tools and interventions for disease management. The prime focus of this review is to elaborate on how expanded 'CAG' repeats contribute to the common modes of neurotoxicity and their possible therapeutic targets in management of such devastating disorders.

Dilemma in Differentiation of Spinocerebellar Ataxia Type 17 from Huntington's Disease：Comorbidity or Independent Disease?

Both Huntington's disease (HD) and Spinocerebellar ataxia 17 (SCA17) mutations showed expanded CAG repeats, with overlapping clinical manifestation: motor disorders, psychiatric symptoms and cognitive impairments. Therefore, SCA17 is also called Huntington like disease (HD-like, HDL) type 4. In this paper, we reported that one patient had 47 CAG repeats in HTT gene and 42 CAG repeats in TBP gene. There is a dilemma in differentiation of SCA 17 from HD in one patient, never been reported before. Is the diagnosis comorbidity of HD with SCA17 or HD only?

Phenotypic defects from the expression of wild-type and pathogenic TATA-binding proteins in new Drosophila models of Spinocerebellar Ataxia Type 17

Spinocerebellar Ataxia Type 17 (SCA17) is the most recently identified member of the polyglutamine (polyQ) family of disorders, resulting from abnormal CAG/CAA expansion in the TATA box-binding protein (TBP), an initiation factor essential for of all eukaryotic transcription. A largely autosomal dominant inherited disease, SCA17, is unique in both its heterogeneous clinical presentation and low incidence of genetic anticipation, the phenomenon in which subsequent generations inherit longer polyQ expansions that yield earlier and more severe symptom onset. Like other polyQ disease family members, SCA17 patients experience progressive ataxia and dementia, and treatments are limited to preventing symptoms and increasing quality of life. Here, we report 2 new Drosophila models that express human TBP with polyQ repeats in either wild-type or SCA17 patient range. We find that TBP expression has age- and tissue-specific effects on neurodegeneration, with polyQ-expanded SCA17 protein expression generally having more severe effects. In addition, SCA17 model flies accumulate more aggregation-prone TBP, with a greater proportion localizing to the nucleus. These new lines provide a new resource for the biochemical characterization of SCA17 pathology and the future identification of therapeutic targets.

Late-onset hereditary ataxias with dementia

PURPOSE OF REVIEW

Late-onset genetic cerebellar ataxias are clinically heterogenous with variable phenotypes. Several of these conditions are commonly associated with dementia. Recognition of the relationship between ataxia and dementia can guide clinical genetic evaluation.

RECENT FINDINGS

Spinocerebellar ataxias often present with variable phenotypes that may include dementia. Genomic studies have begun to identify links between incomplete penetrance and such variable phenotypes in certain hereditary ataxias. Recent studies evaluating the interaction of TBP repeat expansions and STUB1 sequence variants provide a framework to understand how genetic interactions influence disease penetrance and dementia risk in spinocerebellar ataxia types 17 and 48. Further advances in next generation sequencing methods will continue to improve diagnosis and create new insights into the expressivity of existing disorders.

SUMMARY

The late-onset hereditary ataxias are a clinically heterogenous group of disorders with complex presentations that can include cognitive impairment and/or dementia. Genetic evaluation of late-onset ataxia patients with dementia follows a systemic testing approach that often utilizes repeat expansion testing followed by next-generation sequencing. Advances in bioinformatics and genomics is improving both diagnostic evaluation and establishing a basis for phenotypic variability. Whole genome sequencing will likely replace exome sequencing as a more comprehensive means of routine testing.

Genetic screening for Huntington disease phenocopies in Sweden: A tertiary center case series focused on short tandem repeat (STR) disorders

OBJECTIVE

To perform a screening for Huntington disease (HD) phenocopies in a Swedish cohort.

METHODS

Seventy-three DNA samples negative for HD were assessed at a tertiary center in Stockholm. The screening included analyses for C9orf72-frontotemporal dementia/amyotrophic lateral sclerosis (C9orf72-FTD/ALS), octapeptide repeat insertions (OPRIs) in PRNP associated with inherited prion diseases (IPD), Huntington's disease-like 2 (HDL2), spinocerebellar ataxia-2 (SCA2), spinocerebellar ataxia 3 (SCA3) and spinocerebellar ataxia-17 (SCA17). Targeted genetic analysis was carried out in two cases based on the salient phenotypic features.

RESULTS

The screening identified two patients with SCA17, one patient with IPD associated with 5-OPRI but none with nucleotide expansions in C9orf72 or for HDL2, SCA2 or SCA3. Furthermore, SGCE-myoclonic-dystonia 11 (SGCE-M-D) and benign hereditary chorea (BHC) was diagnosed in two sporadic cases. WES identified VUS in STUB1 in two patients with predominant cerebellar ataxia.

CONCLUSIONS

Our results are in keeping with previous screenings and suggest that other genes yet to be discovered are involved in the etiology of HD phenocopies.

Spinocerebellar Ataxia type 17 presenting with progressive myoclonic epilepsy

•SCA17 should be included in the differential diagnoses of PMEs.•SCA17 is characterized by cerebellar features, myoclonic epilepsy, cognitive decline, psychiatric features, and chorea.•Subtle clinical signs like chorea can provide additional diagnostic clues to SCA17(HDL4), a Huntington disease phenocopy.

Motor band sign in a Huntington disease phenocopy

We report a 68-year-old lady who presented with Huntington phenocopy with generalized chorea and was genetically proven to have Spinocerebellar ataxia (SCA)17. MRI Brain demonstrated motor band sign, which is most commonly reported in motor neuron disease. This is the first case of motor band sign with SCA 17 and highlights the widening spectrum of radiological signs in SCA 17.

ConvNets for automatic detection of polyglutamine SCAs from brain MRIs: state of the art applications

Polyglutamine spinocerebellar ataxias (polyQ SCAs) are a group of neurodegenerative diseases, clinically and genetically heterogeneous, characterized by loss of balance and motor coordination due to dysfunction of the cerebellum and its connections. The diagnosis of each type of polyQ SCA, alongside with genetic tests, includes medical images analysis, and its automation may help specialists to distinguish between each type. Convolutional neural networks (ConvNets or CNNs) have been recently used for medical image processing, with outstanding results. In this work, we present the main clinical and imaging features of polyglutamine SCAs, and the basics of CNNs. Finally, we review studies that have used this approach to automatically process brain medical images and may be applied to SCAs detection. We conclude by discussing the possible limitations and opportunities of using ConvNets for SCAs diagnose in the future.

Neurogenetic motor disorders

Advances in the field of neurogenetics have practical applications in rapid diagnosis on blood and body fluids to extract DNA, obviating the need for invasive investigations. The ability to obtain a presymptomatic diagnosis through genetic screening and biomarkers can be a guide to life-saving disease-modifying therapy or enzyme replacement therapy to compensate for the deficient disease-causing enzyme. The benefits of a comprehensive neurogenetic evaluation extend to family members in whom identification of the causal gene defect ensures carrier detection and at-risk counseling for future generations. This chapter explores the many facets of the neurogenetic evaluation in adult and pediatric motor disorders as a primer for later chapters in this volume and a roadmap for the future applications of genetics in neurology.

A survey of protein interactions and posttranslational modifications that influence the polyglutamine diseases

The presence and aggregation of misfolded proteins has deleterious effects in the nervous system. Among the various diseases caused by misfolded proteins is the family of the polyglutamine (polyQ) disorders. This family comprises nine members, all stemming from the same mutation—the abnormal elongation of a polyQ repeat in nine different proteins—which causes protein misfolding and aggregation, cellular dysfunction and disease. While it is the same type of mutation that causes them, each disease is distinct: it is influenced by regions and domains that surround the polyQ repeat; by proteins with which they interact; and by posttranslational modifications they receive. Here, we overview the role of non-polyQ regions that control the pathogenicity of the expanded polyQ repeat. We begin by introducing each polyQ disease, the genes affected, and the symptoms experienced by patients. Subsequently, we provide a survey of protein-protein interactions and posttranslational modifications that regulate polyQ toxicity. We conclude by discussing shared processes and pathways that bring some of the polyQ diseases together and may serve as common therapeutic entry points for this family of incurable disorders.

Cognitive Syndromes Associated With Movement Disorders

PURPOSE OF REVIEW

This article reviews the recognition and management of cognitive syndromes in movement disorders, including those with parkinsonism, chorea, ataxia, dystonia, and tremor.

RECENT FINDINGS

Cognitive and motor syndromes are often intertwined in neurologic disorders, including neurodegenerative diseases such as Parkinson disease, atypical parkinsonian syndromes, Huntington disease, and other movement disorders. Cognitive symptoms often affect attention, working memory, and executive and visuospatial functions preferentially, rather than language and memory, but heterogeneity can be seen in the various movement disorders. A distinct cognitive syndrome has been recognized in patients with cerebellar syndromes. Appropriate recognition and screening for cognitive changes in movement disorders may play a role in achieving accurate diagnoses and guiding patients and their families regarding progression and management decisions.

SUMMARY

In the comprehensive care of patients with movement disorders, recognition of cognitive syndromes is important. Pharmacologic treatments for the cognitive syndromes, including mild cognitive impairment and dementia, in these movement disorders lag behind the therapeutics available for motor symptoms, and more research is needed. Patient evaluation and management require a comprehensive team approach, often linking neurologists as well as neuropsychologists, psychologists, psychiatrists, social workers, and other professionals.

Oxidative Stress and Neurodegeneration: Interconnected Processes in PolyQ Diseases

Neurodegenerative polyglutamine (polyQ) disorders are caused by trinucleotide repeat expansions within the coding region of disease-causing genes. PolyQ-expanded proteins undergo conformational changes leading to the formation of protein inclusions which are associated with selective neuronal degeneration. Several lines of evidence indicate that these mutant proteins are associated with oxidative stress, proteasome impairment and microglia activation. These events may correlate with the induction of inflammation in the nervous system and disease progression. Here, we review the effect of polyQ-induced oxidative stress in cellular and animal models of polyQ diseases. Furthermore, we discuss the interplay between oxidative stress, neurodegeneration and neuroinflammation using as an example the well-known neuroinflammatory disease, Multiple Sclerosis. Finally, we review some of the pharmaceutical interventions which may delay the onset and progression of polyQ disorders by targeting disease-associated mechanisms.

Juvenile Huntington's Disease and Other PolyQ Diseases, Update on Neurodevelopmental Character and Comparative Bioinformatic Review of Transcriptomic and Proteomic Data

Polyglutamine (PolyQ) diseases are neurodegenerative disorders caused by the CAG repeat expansion mutation in affected genes resulting in toxic proteins containing a long chain of glutamines. There are nine PolyQ diseases: Huntington’s disease (HD), spinocerebellar ataxias (types 1, 2, 3, 6, 7, and 17), dentatorubral-pallidoluysian atrophy (DRPLA), and spinal bulbar muscular atrophy (SBMA). In general, longer CAG expansions and longer glutamine tracts lead to earlier disease presentations in PolyQ patients. Rarely, cases of extremely long expansions are identified for PolyQ diseases, and they consistently lead to juvenile or sometimes very severe infantile-onset polyQ syndromes. In apparent contrast to the very long CAG tracts, shorter CAGs and PolyQs in proteins seems to be the evolutionary factor enhancing human cognition. Therefore, polyQ tracts in proteins can be modifiers of brain development and disease drivers, which contribute neurodevelopmental phenotypes in juvenile- and adult-onset PolyQ diseases. Therefore we performed a bioinformatics review of published RNAseq polyQ expression data resulting from the presence of polyQ genes in search of neurodevelopmental expression patterns and comparison between diseases. The expression data were collected from cell types reflecting stages of development such as iPSC, neuronal stem cell, neurons, but also the adult patients and models for PolyQ disease. In addition, we extended our bioinformatic transcriptomic analysis by proteomics data. We identified a group of 13 commonly downregulated genes and proteins in HD mouse models. Our comparative bioinformatic review highlighted several (neuro)developmental pathways and genes identified within PolyQ diseases and mouse models responsible for neural growth, synaptogenesis, and synaptic plasticity.

Ophthalmic Manifestations and Genetics of the Polyglutamine Autosomal Dominant Spinocerebellar Ataxias: A Review

Spinocerebellar ataxia (SCA) is a part of the cerebellar neurodegenerative disease group that is diverse in genetics and phenotypes. It usually shows autosomal dominant inheritance. SCAs, always together with the cerebellar degeneration, may exhibit clinical deficits in brainstem or eye, especially retina or optic nerve. Interestingly, autosomal dominant SCAs share a common genetic mechanism; the length of the glutamine chain is abnormally expanded due to the increase in the cytosine–adenine–guanine (CAG) repeats of the disease causing gene. Studies have suggested that the mutant ataxin induces alteration of protein conformation and abnormal aggregation resulting in nuclear inclusions, and causes cellular loss of photoreceptors through a toxic effect. As a result, these pathologic changes induce a downregulation of genes involved in the phototransduction, development, and differentiation of photoreceptors such as CRX, one of the photoreceptor transcription factors. However, the exact mechanism of neuronal degeneration by mutant ataxin restricted to only certain type of neuronal cell including cerebellar Purkinje neurons and photoreceptor is still unclear. The most common SCAs are types 1, 2, 3, 6, 7, and 17 which contain about 80% of autosomal dominant SCA cases. Various aspects of eye movement abnormalities are evident depending on the degree of cerebellar and brainstem degeneration in SCAs. In addition, certain types of SCAs such as SCA7 are characterized by both cerebellar ataxia and visual loss mainly due to retinal degeneration. The severity of the retinopathy can vary from occult macular photoreceptor disruption to extensive retinal atrophy and is correlated with the number of CAG repeats. The value of using optical coherence tomography in conjunction with electrodiagnostic and genetic testing is emphasized as the combination of these tests can provide critical information regarding the etiology, morphological evaluation, and functional significances. Therefore, ophthalmologists need to recognize and differentiate SCAs in order to properly diagnose and evaluate the disease. In this review, we have described and discussed SCAs showing ophthalmic abnormalities with particular attention to their ophthalmic features, neurodegenerative mechanisms, genetics, and future perspectives.

Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Polyglutamine spinocerebellar ataxias (polyQ SCAs) include SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 and constitute a group of adult onset neurodegenerative disorders caused by the expansion of a CAG repeat sequence located within the coding region of specific genes, which translates into polyglutamine tract in the corresponding proteins. PolyQ SCAs are characterized by degeneration of the cerebellum and its associated structures and lead to progressive ataxia and other diverse symptoms. In recent years, gene and epigenetic deregulations have been shown to play a critical role in the pathogenesis of polyQ SCAs. Here, we provide an overview of the functions of wild type and pathogenic polyQ SCA proteins in gene regulation, describe the extent and nature of gene expression changes and their pathological consequences in diseases, and discuss potential avenues to further investigate converging and distinct disease pathways and to develop therapeutic strategies.

Molecular Mechanisms and Therapeutics for SCA17

Spinocerebellar ataxia type 17 (SCA17) is caused by polyglutamine (polyQ) expansion in the TATA box-binding protein (TBP), which functions as a general transcription factor. Like other polyQ expansion-mediated diseases, SCA17 is characterized by late-onset and selective neurodegeneration, despite the disease protein being ubiquitously expressed in the body. To date, the pathogenesis of polyQ diseases is not fully understood, and there are no effective treatments for these devastating disorders. The well-characterized function of TBP and typical neurodegeneration in SCA17 give us opportunities to understand how polyQ expansion causes selective neurodegeneration and to develop effective therapeutics. In this review, we discuss the molecular mechanisms behind SCA17, focusing on transcriptional dysregulation as its major cause. Mounting evidence suggests that reversing transcriptional alterations induced by mutant TBP and reducing the expression of mutant TBP are promising strategies to treat SCA17.

Complexity of the Genetics and Clinical Presentation of Spinocerebellar Ataxia 17

Spinocerebellar ataxia type 17 (SCA17) is a rare autosomal dominant neurodegenerative disease caused by a CAG repeat expansion in the TATA-box binding protein gene (TBP). The disease has a varied age at onset and clinical presentation. It is distinct from other SCAs for its association with dementia, psychiatric symptoms, and some patients presenting with chorea. For this reason, it is also called Huntington’s disease-like 4 (HDL-4). Here we examine the distribution of SCA17 allele repeat sizes in a United Kingdom-based cohort with ataxia and find that fully penetrant pathogenic alleles are very rare (5 in 1,316 chromosomes; 0.38%). Phenotype-genotype correlation was performed on 30 individuals and the repeat structure of their TBP genes was examined. We found a negative linear correlation between total CAG repeat length and age at disease onset and, unlike SCA1, there was no correlation between the longest contiguous CAG tract and age at disease onset. We were unable to identify any particular phenotypic trait that segregated with particular CAG/CAA repeat tract structures or repeat lengths. One individual within the cohort was homozygous for variable penetrance range SCA17 alleles. This patient had a similar age at onset to heterozygotes with the same repeat sizes, but also presented with a rapidly progressive dementia. A pair of monozygotic twins within the cohort presented 3 years apart with the sibling with the earlier onset having a more severe phenotype with dementia and chorea in addition to the ataxia observed in their twin. This appears to be a case of variable expressivity, possibly influenced by other environmental or epigenetic factors. Finally, there was an asymptomatic father with a severely affected child with an age at onset in their twenties. Despite this, they share the same expanded allele repeat sizes and sequences, which would suggest that there is marked difference in the penetrance of this 51-repeat allele. We therefore propose that the variable penetrance range extend from 48 repeats to incorporate this allele. This study shows that there is variability in the presentation and penetrance of the SCA17 phenotype and highlights the complexity of this disorder.

Novel Imaging Biomarkers for Huntington's Disease and Other Hereditary Choreas

PURPOSE OF THE REVIEW

Imaging biomarkers for neurodegenerative disorders are primarily developed with the goal to aid diagnosis, to monitor disease progression, and to assess the efficacy of disease-modifying therapies in support to clinical outcomes that may either show limited sensitivity or need extended time for their evaluation. This article will review the most recent concepts and findings in the field of neuroimaging applied to Huntington's disease and Huntington-like syndromes. Emphasis will be given to the discussion of potential pharmacodynamic biomarkers for clinical trials in Huntington's disease (HD) and of neuroimaging tools that can be used as diagnostic biomarkers in HD-like syndromes.

RECENT FINDINGS

Several magnetic resonance (MR) and positron emission tomography (PET) molecular imaging tools have been identified as potential pharmacodynamic biomarkers and others are in the pipeline after preclinical validation. MRI and 18F-fluorodeoxyglucose PET can be considered useful supportive diagnostic tools for the differentiation of other HD-like syndromes. New trials in HD have the primary goal to lower mutant huntingtin (mHTT) protein levels in the brain in order to reduce or alter the progression of the disease. MR and PET molecular imaging markers have been developed as tools to monitor disease progression and to evaluate treatment outcomes of disease-modifying trials in HD. These markers could be used alone or in combination for detecting structural and pharmacodynamic changes potentially associated with the lowering of mHTT.