Thalamic metabolism and symptom onset in preclinical Huntington's disease

PET/CT in diagnosis of movement disorders

Molecular imaging with PET offers a broad variety of tools supporting the diagnosis of movement disorders. The more widely applied PET imaging techniques have focused on the assessment of neurotransmitter systems, predominantly the pre- and postsynaptic dopaminergic system. Additionally, PET imaging with [(18) F]fluorodeoxyglucose has been extensively used to assess local synaptic activity in the resting state and to highlight local changes in brain metabolism accompanying changes in neural activity in movement disorders. PET imaging has provided us with diagnostic agents as well as tools for evaluation of novel therapeutics, and has served as a powerful means for revealing in vivo changes at different stages of movement disorders and within the course of an individual patient's illness.

Development of biomarkers for Huntington's disease

Huntington's disease is an autosomal dominant, progressive neurodegenerative disorder, for which there is no disease-modifying treatment. By use of predictive genetic testing, it is possible to identify individuals who carry the gene defect before the onset of symptoms, providing a window of opportunity for intervention aimed at preventing or delaying disease onset. However, without robust and practical measures of disease progression (ie, biomarkers), the efficacy of therapeutic interventions in this premanifest Huntington's disease population cannot be readily assessed. Current progress in the development of biomarkers might enable evaluation of disease progression in individuals at the premanifest stage of the disease; these biomarkers could be useful in defining endpoints in clinical trials in this population. Clinical, cognitive, neuroimaging, and biochemical biomarkers are being investigated for their potential in clinical use and their value in the development of future treatments for patients with Huntington's disease.

Longitudinal functional magnetic resonance imaging of cognition in preclinical Huntington's disease

Neuropsychological and functional neuroimaging studies have revealed early changes of cognition and brain function in individuals with the Huntington's disease (HD) gene mutation who are presymptomatic for the motor symptoms of the disease (preHD). However, little is known about whether changes of neural function progress over time. In this study, we used neuropsychological tests of attention, working memory and executive function, functional magnetic resonance imaging and voxel-based analyses of high-resolution structural data to explore the temporal dynamics of potential cognitive, functional and structural biomarkers in far from onset preHD (n=13, mean time to the estimated motor symptom onset=19.5 years) and healthy controls (n=13) followed over a 2-year period. Behavioral measures were similar in preHD individuals and controls at baseline and remained normal 2 years later. At both time points, the left dorsolateral prefrontal cortex was less active in preHD than in controls during working memory performance. The left dorsolateral prefrontal cortex did not exhibit further loss of activity over time. Regions showing less gray matter volume in preHD at baseline did not show further volume loss over time. These data indicate that the activity in brain regions contributing to working memory processing differs consistently in HD expansion mutation carriers while cognitive performance remains normal. However, the present data do not support the notion of a progressive decline of left prefrontal cortex activity in far from onset preHD followed over a 2-year period.

Impaired sequence learning in dystonia mutation carriers: a genotypic effect

Abnormalities in motor sequence learning have been observed in non-manifesting carriers of the DYT1 dystonia mutation. Indeed, motor sequence learning deficits in these subjects have been associated with increased cerebellar activation during task performance. In the current study, we determined whether similar changes are also present in clinically manifesting DYT1 carriers as well as in carriers of other primary dystonia mutations such as DYT6. Additionally, we determined whether sequence learning performance and associated brain activation in these subjects correlate with previously described genotype-related abnormalities of cerebellar pathway integrity and striatal D2 dopamine receptor binding. Nineteen DYT1 carriers (10 non-manifesting DYT1: 51.5±15.1 years; nine manifesting DYT1: 46.1±15.1 years) and 12 healthy control subjects (42.8±15.3 years) were scanned with H2(15)O positron emission tomography while performing controlled sequence learning and reference tasks. Eleven DYT6 carriers (four non-manifesting DYT6: 38.0±22.1; seven manifesting DYT6: 35.3±14.2 years) were evaluated during task performance without concurrent imaging. DYT1 and DYT6 carriers also underwent diffusion tensor magnetic resonance imaging for the assessment of tract integrity and 11C-raclopride positron emission tomography to measure caudate/putamen D2 receptor binding. These imaging measures were correlated with sequence learning performance and associated activation responses. Sequence learning deficits of similar magnitude were observed in manifesting and non-manifesting DYT1 carriers. In contrast, learning deficits were not detected in DYT6 carriers, irrespective of clinical penetrance. Affected DYT1 carriers exhibited significant increases in sequence learning-related activation in the left lateral cerebellar cortex and in the right premotor and inferior parietal regions. Increases in premotor cortical activation observed in the mutation carriers correlated with reductions in cerebellar pathway integrity measured using magnetic resonance diffusion tensor imaging and probabilistic tractography. Additionally, the cerebellar tract changes correlated with reductions in dentate nucleus activation recorded during task performance. Sequence learning performance and task-related activation responses did not correlate with striatal D2 receptor binding. In summary, we found that sequence learning deficits and concomitant increases in cerebellar activation are specific features of the DYT1 genotype. The close relationship between reduced cerebellar pathway integrity and increased learning-related activation of the premotor cortex is compatible with the view of DYT1 dystonia as a neurodevelopmental circuit disorder.

Mitochondrial medicine: to a new era of gene therapy for mitochondrial DNA mutations

Mitochondrial disorders can no longer be ignored in most medical disciplines. Such disorders include specific and widespread organ involvement, with tissue degeneration or tumor formation. Primary or secondary actors, mitochondrial dysfunctions also play a role in the aging process. Despite progresses made in identification of their molecular bases, nearly everything remains to be done as regards therapy. Research dealing with mitochondrial physiology and pathology has >20 years of history around the world. We are involved, as are many other laboratories, in the challenge of finding ways to fight these diseases. However, our main limitation is the scarcety of animal models required for both understanding the molecular mechanisms underlying the diseases and evaluating therapeutic strategies. This is especially true for diseases due to mutations in mitochondrial DNA (mtDNA), since an authentic genetic model of mtDNA mutations is technically a very difficult task due to both the inability of manipulating the mitochondrial genome of living mammalian cells and to its multicopy nature. This has led researchers in the field to consider the prospect of gene therapy approaches that can roughly be divided into three groups: (1) import of wild-type copies or relevant sections of DNA or RNA into mitochondria, (2) manipulation of mitochondrial genetic content, and (3) rescue of a defect by expression of an engineered gene product from the nucleus (allotopic or xenotropic expression). We briefly introduce these concepts and indicate where promising progress has been made in the last decade.

Altered dopamine and serotonin metabolism in motorically asymptomatic R6/2 mice

The pattern of cerebral dopamine (DA) abnormalities in Huntington disease (HD) is complex, as reflected by the variable clinical benefit of both DA antagonists and agonists in treating HD symptoms. In addition, little is known about serotonin metabolism despite the early occurrence of anxiety and depression in HD. Post-mortem enzymatic changes are likely to interfere with the in vivo profile of biogenic amines. Hence, in order to reliably characterize the regional and chronological profile of brain neurotransmitters in a HD mouse model, we used a microwave fixation system that preserves in vivo concentrations of dopaminergic and serotoninergic amines. DA was decreased in the striatum of R6/2 mice at 8 and 12 weeks of age while DA metabolites, 3-methoxytyramine and homovanillic acid, were already significantly reduced in 4-week-old motorically asymptomatic R6/2 mice. In the striatum, hippocampus and frontal cortex of 4, 8 and 12-week-old R6/2 mice, serotonin and its metabolite 5-hydroxyindoleacetic acid were significantly decreased in association with a decreased turnover of serotonin. In addition, automated high-resolution behavioural analyses displayed stress-like behaviours such as jumping and grooming and altered spatial learning in R6/2 mice at age 4 and 6 weeks respectively. Therefore, we describe the earliest alterations of DA and serotonin metabolism in a HD murine model. Our findings likely underpin the neuropsychological symptoms at time of disease onset in HD.

Brain networks in Huntington disease

Recent studies have focused on understanding the neural mechanisms underlying the emergence of clinical signs and symptoms in early stage Huntington disease (HD). Although cell-based assays have focused on cell autonomous effects of mutant huntingtin, animal HD models have revealed alterations in the function of neuronal networks, particularly those linking the cerebral cortex and striatum. These findings are complemented by metabolic imaging studies of disease progression in premanifest subjects. Quantifying metabolic progression at the systems level may identify network biomarkers to aid in the objective assessment of new disease-modifying therapies and identify new regions that merit mechanistic study in HD models.

Abnormal metabolic brain networks in Parkinson's disease from blackboard to bedside

Metabolic imaging in the rest state has provided valuable information concerning the abnormalities of regional brain function that underlie idiopathic Parkinson's disease (PD). Moreover, network modeling procedures, such as spatial covariance analysis, have further allowed for the quantification of these changes at the systems level. In recent years, we have utilized this strategy to identify and validate three discrete metabolic networks in PD associated with the motor and cognitive manifestations of the disease. In this chapter, we will review and compare the specific functional topographies underlying parkinsonian akinesia/rigidity, tremor, and cognitive disturbance. While network activity progressed over time, the rate of change for each pattern was distinctive and paralleled the development of the corresponding clinical symptoms in early-stage patients. This approach is already showing great promise in identifying individuals with prodromal manifestations of PD and in assessing the rate of progression before clinical onset. Network modulation was found to correlate with the clinical effects of dopaminergic treatment and surgical interventions, such as subthalamic nucleus (STN) deep brain stimulation (DBS) and gene therapy. Abnormal metabolic networks have also been identified for atypical parkinsonian syndromes, such as multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). Using multiple disease-related networks for PD, MSA, and PSP, we have developed a novel, fully automated algorithm for accurate classification at the single-patient level, even at early disease stages.

Scaled subprofile modeling of resting state imaging data in Parkinson's disease: methodological issues

Consistent functional brain abnormalities in Parkinson's disease (PD) are difficult to pinpoint because differences from the normal state are often subtle. In this regard, the application of multivariate methods of analysis has been successful but not devoid of misinterpretation and controversy. The Scaled Subprofile Model (SSM), a principal components analysis (PCA)-based spatial covariance method, has yielded critical information regarding the characteristic abnormalities of functional brain organization that underlie PD and other neurodegenerative disorders. However, the relevance of disease-related spatial covariance patterns (metabolic brain networks) and the most effective methods for their derivation has been a subject of debate. We address these issues here and discuss the inherent advantages of proper application as well as the effects of the misapplication of this methodology. We show that ratio pre-normalization using the mean global metabolic rate (GMR) or regional values from a "reference" brain region (e.g. cerebellum) that may be required in univariate analytical approaches is obviated in SSM. We discuss deviations of the methodology that may yield erroneous or confounding factors.

Tetrabenazine: the first approved drug for the treatment of chorea in US patients with Huntington disease

Huntington disease (HD) is a dominantly inherited progressive neurological disease characterized by chorea, an involuntary brief movement that tends to flow between body regions. HD is typically diagnosed based on clinical findings in the setting of a family history and may be confirmed with genetic testing. Predictive testing is available to those at risk, but only experienced clinicians should perform the counseling and testing. Multiple areas of the brain degenerate mainly involving the neurotransmitters dopamine, glutamate, and γ-aminobutyric acid. Although pharmacotherapies theoretically target these neurotransmitters, few well-conducted trials for symptomatic or neuroprotective interventions yielded positive results. Tetrabenazine (TBZ) is a dopamine-depleting agent that may be one of the more effective agents for reducing chorea, although it has a risk of potentially serious adverse effects. Some newer antipsychotic agents, such as olanzapine and aripiprazole, may have adequate efficacy with a more favorable adverse-effect profile than older antipsychotic agents for treating chorea and psychosis. This review will address the epidemiology and diagnosis of HD as background for understanding potential pharmacological treatment options. Because TBZ is the only US Food and Drug Administration-approved medication in the United States for HD, the focus of this review will be on its pharmacology, efficacy, safety, and practical uses. There are no current treatments to change the course of HD, but education and symptomatic therapies can be effective tools for clinicians to use with patients and families affected by HD.

Extrastriatal dopamine D(2) receptor binding in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder, primarily affecting medium spiny neurones in the striatum. The density of striatal dopamine D(2) receptors is reduced in HD but there is little known about this biomarker in brain regions outside the striatum. The primary objective of this study was to compare extrastriatal dopamine D(2) receptor binding, in age-matched control subjects and patients with HD. All subjects were examined using a high-resolution positron emission tomography system and the high-affinity dopamine D(2) receptor radioligand [(11) C]FLB 457. A ROI based analysis was used with an atrophy correction method. Dopamine D(2) receptor binding potential was reduced in the striatum of patients with HD. Unlike the striatum, dopamine D(2) receptor binding in thalamic and cortical subregions was not significantly different from that in control subjects. A partial least square regression analysis which included binding potential values from all investigated cortical and subcortical regions revealed a significant model separating patients from controls, conclusively dependent on differences in striatal binding of the radioligand. Some clinical assessments correlated with striatal dopamine D(2) receptor binding, including severity of chorea and cognitive test performance. Hence, the present study demonstrates that dopamine D(2) receptors extrinsic to the striatum are well preserved in early to mid stage patients with HD. This observation may have implication for the development of therapy for HD.

Seeking brain biomarkers for preventive therapy in Huntington disease

Huntington disease (HD) is a severe incurable nervous system disease that generally has an onset age of around 35-50, and is caused by a dominantly transmitted expansion mutation. A genetic test allows persons at risk, i.e., offspring or siblings of affected individuals, to discover their genetic status. Unaffected mutation-positive subjects will manifest HD sometime during life. Despite major advances in research on pathogenic mechanisms, no studies have yet fully validated preventive therapy or biomarkers for use before the symptoms become clinically manifest. Seeking brain and peripheral biomarkers is a requisite to develop a cure for HD. Changes in the brain can be observed in vivo using methods such as structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), functional MRI (fMRI), and positron emission tomography (PET), detecting volumetric changes, microstructural and connectivity alterations, abnormalities in brain activity in response to specific tasks, and abnormalities in metabolism and receptor distribution. Although all these imaging techniques can detect early markers in asymptomatic HD gene carriers for premanifest screening and pharmacological responses to therapeutic interventions no single modality has yet provided and validated an optimal marker probably because this task requires an integrative multimodal imaging approach. In this article, we review the findings from imaging procedures in the attempt to identify potential brain markers, so-called dry biomarkers, for possible application to further, yet unavailable, neuroprotective preventive therapies for HD manifestations.

Striatal and white matter predictors of estimated diagnosis for Huntington disease

Previous MRI studies with participants prior to manifest Huntington disease have been conducted in small single-site samples. The current study reports data from a systematic multi-national study during the prodromal period of Huntington disease and examines whether various brain structures make unique predictions about the proximity to manifest disease. MRI scans were acquired from 657 participants enrolled at 1 of 32 PREDICT-HD research sites. Only prodromal Huntington disease participants (those not meeting motor criteria for diagnosis) were included and subgrouped by estimated diagnosis proximity (Near, Mid, and Far) based upon a formula incorporating age and CAG-repeat length. Results show volumes of all three subgroups differed significantly from Controls for total brain tissue, cerebral spinal fluid, white matter, cortical gray matter, thalamus, caudate, and putamen. Total striatal volume demonstrated the largest differences between Controls and all three prodromal subgroups. Cerebral white matter offered additional independent power in the prediction of estimated proximity to diagnosis. In conclusion, this large cross-sectional study shows that changes in brain volume are detectable years to decades prior to estimated motor diagnosis of Huntington disease. This suggests that a clinical trial of a putative neuroprotective agent could begin as much as 15 years prior to estimated motor diagnosis in a cohort of persons at risk for but not meeting clinical motor diagnostic criteria for Huntington disease, and that neuroimaging (striatal and white matter volumes) may be among the best predictors of diagnosis proximity.

Abnormalities in metabolic network activity precede the onset of motor symptoms in Parkinson's disease

Imaging studies show that Parkinson's disease (PD) alters the activity of motor- and cognition-related metabolic brain networks. However, it is not known whether the network changes appear at or before symptom onset. In this study, we examined 15 hemiparkinsonian patients who underwent serial metabolic imaging with [(18)F]-fluorodeoxyglucose (FDG) PET at baseline and again 2.1 +/- 0.6 (mean +/- SD) and 3.9 +/- 0.7 years later. We assessed longitudinal changes in network activity in each cerebral hemisphere, focusing specifically on the "presymptomatic" hemisphere--ipsilateral to the initially involved body side. At the network level, the activity of the PD motor-related pattern (PDRP) increased symmetrically in both hemispheres over time (p < 0.001), with significant bilateral elevations at each of the three time points. Hemispheric expression of the PD cognition-related pattern likewise increased symmetrically (p < 0.001), although significant elevations were not evident on either side until 4 years. At the regional level, putamen metabolism contralateral to the initially affected body side was elevated at all three time points, without longitudinal change. In contrast, in the initially presymptomatic hemisphere, putamen metabolic activity increased steadily over time, reaching abnormal levels only at 4 years. Metabolic activity in the contralateral precuneus fell to subnormal levels by the final time point. These findings suggest that abnormal PDRP activity antecedes the appearance of motor signs by approximately 2 years. The timing and laterality of symptom onset relates to focal asymmetric metabolic changes at the putamenal node of this network.

Metabolic networks for assessment of therapy and diagnosis in Parkinson's disease

Neuroimaging and modern computational techniques like spatial covariance analysis have contributed greatly to the understanding of neural system abnormalities in neurodegenerative disorders such as Parkinson's disease (PD). The application of network analysis to metabolic PET data obtained from patients with PD has led to the identification and validation of two distinct spatial covariance patterns associated with the motor and cognitive manifestations of the disease. Quantifying the activity of these patterns in individual subjects has provided an objective tool for the assessment of treatment efficacy and differential diagnosis. We have found that activity of the PD motor-related network is modulated by antiparkinsonian treatments such as dopaminergic therapy, deep brain stimulation (DBS), and subthalamic nucleus (STN) gene therapy. By contrast, the cognitive-related network is not altered by these interventions for PD motor symptoms. This pattern may however change in response to therapies targeting the cognitive symptoms of this disorder. Recent work has focused on the identification of specific network biomarkers for atypical parkinsonian conditions such as multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). These disease-related patterns can potentially be used in an automated imaging-based algorithm to classify patients with these disorders.

FDG PET in the Evaluation of Parkinson's Disease

Network analysis of (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is an innovative approach for the study of in movement disorders, such as Parkinson's disease (PD). Spatial covariance analysis of imaging data acquired from PD patients has revealed characteristic regional patterns associated with the motor and cognitive features of disease. Quantification of pattern expression in individual patients can be used for diagnosis, assessment of disease severity, and evaluation of novel medical and surgical therapies. Identification of disease-specific patterns in other parkinsonian syndromes, such as multiple system atrophy and progressive supranuclear palsy, has improved diagnostic accuracy in patients with difficult to diagnose parkinsonism. Further developments of these techniques are likely to enhance the role of functional imaging in investigating underlying abnormalities and potential new therapies in these neurodegenerative diseases.

Early Detection of Huntington Disease

Huntington disease (HD) is a devastating illness, although its autosomal dominant genetic transmission allows a unique opportunity to study apparently healthy individuals before manifest disease. Attempts to study early disease are not unique in neurology (e.g., Mild Cognitive Impairment, Vascular Cognitive Impairment), but studying otherwise-healthy appearing individuals who will go on with nearly 99% certainty to manifest the symptoms of brain disease does provide distinct but valuable information about the true natural history of the disease. The field has witnessed an explosion of research examining possible early indicators of HD during what is now referred to as the "prodrome" of HD. A NIH study in its ninth year (PREDICT-HD) has offered a glimpse into the transition from an apparently healthy state to an obviously diseased state, and can serve as a model for many other genetic diseases, both neurological and non-neurological.

Magnetization transfer imaging in 'premanifest' Huntington's disease

To investigate whether magnetization transfer imaging (MTI) is a useful detector of diffuse brain abnormalities in ‘premanifest’ carriers of the Huntington’s disease (HD) gene mutation. Furthermore we examined the relations between MTI, clinical measures and CAG repeat length. Sixteen premanifest carriers of the HD gene without motor manifestation and 14 non-carriers underwent a clinical evaluation and a MRI scan. MTI analysis of whole brain, grey matter and white matter was performed producing magnetization transfer ratio (MTR) histograms. A lower peak height of the grey matter MTR histogram in carriers was significantly associated with more UHDRS motor abnormalities. Furthermore, a lower peak height of the whole brain, grey and white matter was strongly associated with a longer CAG repeat length. MTI measures themselves did not differ significantly between carriers and non-carriers. In premanifest HD mutation carriers, a lower MTR peak height, reflecting worse histological brain composition, was related to subtle motor abnormalities and higher CAG repeat length. Although we could not detect altered MTI characteristics in carriers of the HD gene mutation without clinical manifestations, we did provide evidence that the MTR peak height might reflect genetic and subclinical disease burden and may be of value in monitoring further disease progression and provide insight in clinical heterogeneity.

Metabolic brain networks in neurodegenerative disorders: a functional imaging approach

Network analysis of functional brain imaging data is an innovative approach to study circuit abnormalities in neurodegenerative diseases. In Parkinson's disease, spatial covariance analysis of resting-state metabolic images has identified specific regional patterns associated with motor and cognitive symptoms. With functional imaging, these metabolic networks have recently been used to measure system-related progression and to evaluate novel treatment strategies. Network analysis is also being used to characterize specific functional biomarkers for Huntington's disease and Alzheimer's disease. These networks have been particularly helpful in uncovering compensatory mechanisms in genetically at-risk individuals. Ongoing developments in network applications are likely to enhance the role of functional imaging in the investigation of neurodegenerative disorders.

The relationship between impairment of voluntary movements and cognitive impairment in Huntington's disease

The relationship between motor symptoms and cognitive impairment in Huntington's disease (HD) is still discussed. We analysed 45 HD patients in various stages using Unified Huntington's Disease Rating Scale motor subscale (voluntary and involuntary components were evaluated separately), verbal memory and executive functions tests. Partial correlations controlling for HD duration and age were used to estimate the relationships among factor scores for motor and cognitive impairment. Voluntary components of motor performance were found to be significantly correlated with verbal short-term memory disturbances (r = -0.361, P = 0.03), with tests of executive functions more dependent on motor performance (r = 0.640, P < 0.01) and also with tests of executive functions less dependent on motor performance (r = 0.461, P < 0.01). Involuntary components did not correlate significantly with any part of cognitive performance.

Functional compensation of motor function in pre-symptomatic Huntington's disease

Involuntary choreiform movements are a clinical hallmark of Huntington's disease. Studies in clinically affected patients suggest a shift of motor activations to parietal cortices in response to progressive neurodegeneration. Here, we studied pre-symptomatic gene carriers to examine the compensatory mechanisms that underlie the phenomenon of retained motor function in the presence of degenerative change. Fifteen pre-symptomatic gene carriers and 12 matched controls performed button presses paced by a metronome at either 0.5 or 2 Hz with four fingers of the right hand whilst being scanned with functional magnetic resonance imaging. Subjects pressed buttons either in the order of a previously learnt 10-item finger sequence, from left to right, or kept still. Error rates ranged from 2% to 7% in the pre-symptomatic gene carriers and from 0.5% to 4% in controls, depending on the condition. No significant difference in task performance was found between groups for any of the conditions. Activations in the supplementary motor area (SMA) and superior parietal lobe differed with gene status. Compared with healthy controls, gene carriers showed greater activations of left caudal SMA with all movement conditions. Activations correlated with increasing speed of movement were greater the closer the gene carriers were to estimated clinical diagnosis, defined by the onset of unequivocal motor signs. Activations associated with increased movement complexity (i.e. with the pre-learnt 10-item sequence) decreased in the rostral SMA with nearing diagnostic onset. The left superior parietal lobe showed reduced activation with increased movement complexity in gene carriers compared with controls, and in the right superior parietal lobe showed greater activations with all but the most demanding movements. We identified a complex pattern of motor compensation in pre-symptomatic gene carriers. The results show that preclinical compensation goes beyond a simple shift of activity from premotor to parietal regions involving multiple compensatory mechanisms in executive and cognitive motor areas. Critically, the pattern of motor compensation is flexible depending on the actual task demands on motor control.

Abnormal regional brain function in Parkinson's disease: truth or fiction?

Normalization of regional measurements by the global mean is commonly employed to minimize inter-subject variability in functional imaging studies. This practice is based on the assumption that global values do not substantially differ between patient and control groups. In this issue of NeuroImage, Borghammer and colleagues challenge the validity of this assumption. They focus on Parkinson's disease (PD) and use computer simulations to show that lower global values can produce spurious increases in subcortical brain regions. The authors speculate that the increased signal observed in these areas in PD is artefactual and unrelated to localized changes in brain function. In this commentary, we summarize what is currently known of the relationship between regional and global metabolic activity in PD and experimental parkinsonism. We found that early stage PD patients exhibit global values that are virtually identical to those of age-matched healthy subjects. SPM analysis revealed increased normalized metabolic activity in a discrete set of biologically relevant subcortical brain regions. Because of their higher variability, the corresponding absolute regional measures did not differ across the two groups. Longitudinal imaging studies in this population showed that the subcortical elevations in normalized metabolism appeared earlier and progressed faster than did focal cortical or global metabolic reductions. The observed increases in subcortical activity, but not the global changes, correlated with independent clinical measures of disease progression. Multivariate analysis with SSM/PCA further confirmed that the abnormal spatial covariance structure of early PD is dominated by these subcortical increases as opposed to network-related reductions in cortical metabolic activity or global changes. Thus, increased subcortical activity in PD cannot be regarded as a simple artefact of global normalization. Moreover, stability of the normalized measurements, particularly at the network level, makes these metabolic indices suitable as imaging biomarkers of PD progression and the treatment response.

Differential diagnosis of parkinsonian syndromes using PCA-based functional imaging features

In the current paper, we describe methodologies for single subject differential diagnosis of degenerative brain disorders using multivariate principal component analysis (PCA) of functional imaging scans. An automated routine utilizing these methods is applied to positron emission tomography (PET) brain data to distinguish several discrete parkinsonian movement disorders with similar clinical manifestations. Disease specific expressions of voxel-based spatial covariance patterns are predetermined using the Scaled Subprofile Model (SSM/PCA) and a scalar measure of the manifestation of each pattern in prospective subject images is subsequently derived. Scores are automatically compared to reference values generated for each pathological condition in a corresponding set of patient and control scans. Diagnostic outcome is optimized using strategies such as the derivation of patterns in a voxel subspace that reflects contrasting image characteristics between conditions, or by using an independent patient population as controls. The prediction models for two, three and four way classification problems using direct scalar comparison as well as classical discriminant analysis are assessed in a composite training population comprised of three different patient classes and normal controls, and validated in a similar independent test population. Results illustrate that highly accurate diagnosis can often be achieved by simple comparison of scores utilizing optimized patterns.

Mitochondria and Huntington's disease pathogenesis: insight from genetic and chemical models

A mechanistic link between cellular energetic defects and the pathogenesis of Huntington's disease (HD) has long been hypothesized based on the cardinal observations of progressive weight loss in patients and metabolic defects in brain and muscle. Identification of respiratory chain deficits in HD postmortem brain led to the use of mitochondrial complex II inhibitors to generate acute toxicity models that replicate aspects of HD striatal pathology in vivo. Subsequently, the generation of progressive genetic animal models has enabled characterization of numerous cellular and systematic changes over disease etiology, including mitochondrial modifications that impact cerebral metabolism, calcium handling, oxidative damage, and apoptotic cascades. This review focuses on how HD animal models have influenced our understanding of mechanisms underlying HD pathogenesis, concentrating on insight gained into the roles of mitochondria in disease etiology. One outstanding question concerns the hierarchy of mitochondrial alterations in the cascade of events following mutant huntingtin (mhtt)-induced toxicity. One hypothesis is that a direct interaction of mhtt with mitochondria may trigger the neuronal damage and degeneration that occurs in HD. While there is evidence that mhtt associates with mitochondria, deleterious consequences of this interaction have not yet been established. Contrary evidence suggests that a primary nuclear action of mhtt may detrimentally influence mitochondrial function via effects on gene transcription. Irrespective of whether the principal toxic action of mhtt directly or secondarily impacts mitochondria, the repercussions of sufficient mitochondrial dysfunction are catastrophic to cells and may arguably underlie many of the other disruptions in cellular processes that evolve during HD pathogenesis.

Functional imaging in Huntington's disease

Huntington's disease (HD) is a genetic brain disease characterized by loss of capacity in movement control, cognition, and emotional regulation over a period of about 30 years. Since it is well established that clinical impairments and brain atrophy can be detected decades prior to receiving a clinical diagnosis, functional neuroimaging efforts have gained momentum in HD research. In most brain disorders, there is accumulating evidence that the clinical manifestations of disease do not simply depend on the extent of tissue loss, but represent a complex balance among neuronal dysfunction, tissue repair, and circuitry reorganization. Based upon this premise, functional neuroimaging modalities may be more sensitive to the earliest changes in HD than are structural imaging approaches. For this review, PET and fMRI studies conducted in HD samples were summarized. Strengths and limitations of the utilization of functional imaging in HD are discussed and recommendations are offered to facilitate future research endeavors.

Changes in striatal dopamine D2 receptor binding in pre-clinical Huntington's disease

BACKGROUND

Carriers of the Huntington disease (HD) mutation develop a progressive neurodegenerative disorder after a pre-clinical phase. We examined the value of (11)C-raclopride PET (RAC) as a biomarker for pre-clinical HD pathophysiology.

METHODS

In a prospective cohort study with clinical and neuropsychological assessment we collected complete RAC data in 18 pre-clinical mutation carriers (HD-PMC) and 11 controls. Follow-up was 2 years. We calculated striatal RAC binding potential (BP) to measure dopamine D2 receptor availability.

RESULTS

No HD-PMC had overt neuropsychological dysfunction. RAC-BP in putamen was abnormal in up to 44% of HD-PMC. The rate of RAC-BP decline (2.6% per year) was not significantly higher than in controls. Follow-up putaminal BP correlated weakly with predicted distance to onset of clinical HD (P = 0.034), but the rate of decline did not. Three HD-PMC developed motor abnormalities suspect for HD but did not show an increased rate of decline of putaminal BP.

CONCLUSIONS

Many HD-PMC have striatal abnormalities but we found no clearly increased rate of D2 receptor changes around the onset of clinical HD. A longer follow-up of the present study cohort is needed to establish the value of RAC-BP in assessing the risk of clinical conversion from striatal D2 binding data.

Altered frontostriatal coupling in pre-manifest Huntington's disease: effects of increasing cognitive load

BACKGROUND AND PURPOSE

Functional neuroimaging studies have suggested a dysfunction of prefrontal regions in clinically pre-symptomatic individuals with the Huntington's disease (HD) gene mutation (pre-HD) during cognitive processing. The objective of this study was to test the impact of cognitive demand on prefrontal connectivity in pre-HD individuals.

METHODS

Sixteen healthy controls and sixteen pre-HD subjects were studied using functional MRI and a verbal working memory task with increasing cognitive load. Load-dependent functional connectivity of the left dorsolateral prefrontal cortex (DLPFC) was investigated by means of psychophysiological interactions.

RESULTS

In pre-HD subjects, aberrant functional connectivity of the left DLPFC was found at high working memory load levels only. Compared with healthy controls, pre-HD individuals exhibited lower connectivity strength in the left putamen, the right anterior cingulate and the left medial prefrontal cortex. Pre-HD individuals close to the onset of motor symptoms additionally exhibited lower connectivity strength in the right putamen and the left superior frontal cortex. The connectivity strength in the left putamen was associated with several clinical measures including CAG repeat length, Unified Huntington's Disease Rating Scale motor score and predicted years to manifest symptom onset.

CONCLUSION

These findings suggest that early prefrontal connectivity abnormalities in pre-HD individuals are modulated by cognitive demand.

Normalization in PET group comparison studies--the importance of a valid reference region

INTRODUCTION

In positron emission tomography (PET) studies of cerebral blood flow (CBF) and metabolism, the large interindividual variation commonly is minimized by normalization to the global mean prior to statistical analysis. This approach requires that no between-group or between-state differences exist in the normalization region. Given the variability typical of global CBF and the practical limit on sample size, small group differences in global mean easily elude detection, but still bias the comparison, with profound consequences for the physiological interpretation of the results.

MATERIALS AND METHODS

Quantitative [15O]H2O PET recordings of CBF were obtained in 45 healthy subjects (21-81 years) and 14 patients with hepatic encephalopathy (HE). With volume-of-interest (VOI) and voxel-based statistics, we conducted regression analyses of CBF as function of age in the healthy group, and compared the HE group to a subset of the controls. We compared absolute CBF values, and CBF normalized to the gray matter (GM) and white matter (WM) means. In additional simulation experiments, we manipulated the cortical values of 12 healthy subjects and compared these to unaltered control data.

RESULTS

In healthy aging, CBF was shown to be unchanged in WM and central regions. In contrast, with normalization to the GM mean, CBF displayed positive correlation with age in the central regions. Very similar artifactual increases were seen in the HE comparison and also in the simulation experiment.

CONCLUSION

Ratio normalization to the global mean readily elevates CBF in unchanged regions when a systematic between-group difference exists in gCBF, also when this difference is below the detection threshold. We suggest that the routine normalization to the global mean in earlier studies resulted in spurious interpretations of perturbed CBF. Normalization to central WM yields less biased results in aging and HE and could potentially serve as a normalization reference region in other disorders as well.