Cerebral glucose hypermetabolism in Friedreich's ataxia detected with positron emission tomography

Determinant of the cerebellar cognitive affective syndrome in Friedreich's ataxia

BACKGROUND

Individuals with Friedreich's ataxia (FRDA) display significantly lower performances in many cognitive domains with a pattern of impairment that falls within the cerebellar cognitive affective syndrome (CCAS).

OBJECTIVE

To assess in a large cohort of individuals with FRDA, the main determinant of the CCAS using multiple variable regression models.

METHODS

This is a monocentric observational study that included 39 individuals with FRDA. Ataxic motor symptoms were evaluated with the SARA and cognitive functions with the CCAS-Scale (CCAS-S). Age, SARA, GAA1, Age of symptoms onset (ASO), Age and disease duration (DD) were chosen as covariates in a linear regression model to predict CCAS-S failed items and covariates in a logistic regression model to predict definite CCAS.

RESULTS

Patients mean age, SARA score, ASO, DD and GAA1 were respectively of 29 ± 14, 22 ± 10, 14 ± 11, 15 ± 9 and 712 ± 238 (4 point-mutations). Mean CCAS-S raw score was of 86 ± 16, mean number of failed items was 2.9 ± 1.6. Twenty-three individuals had definite CCAS. The multiple linear regression model with age, SARA, ASO, DD & GAA1 as covariates was statistically significant to predict CCAS-S failed items. The SARA was the only significant coefficient in regression models for predicting CCAS-S failed items number and the definite CCAS occurrence.

CONCLUSIONS

CCAS is highly prevalent in adult individuals with FRDA. CCAS is predicted by ataxic motor symptoms severity. This finding supports common core cerebellar pathophysiology in both cognitive and motor symptoms in FRDA and warrants screening for CCAS, especially in patients with SARA > 20.

Cerebellar cognitive affective syndrome after acute cerebellar stroke

Introduction

The cerebellum modulates both motor and cognitive behaviors, and a cerebellar cognitive affective syndrome (CCAS) was described after a cerebellar stroke in 1998. Yet, a CCAS is seldom sought for, due to a lack of practical screening scales. Therefore, we aimed at assessing both the prevalence of CCAS after cerebellar acute vascular lesion and the yield of the CCAS-Scale (CCAS-S) in an acute stroke setting.

Materials and methods

All patients admitted between January 2020 and January 2022 with acute onset of a cerebellar ischemic or haemorrhagic first stroke at the CUB-Hôpital Erasme and who could be evaluated by the CCAS-S within a week of symptom onset were included.

Results

Cerebellar acute vascular lesion occurred in 25/1,580 patients. All patients could complete the CCAS-S. A definite CCAS was evidenced in 21/25 patients. Patients failed 5.2 ± 2.12 items out of 8 and had a mean raw score of 68.2 ± 21.3 (normal values 82–120). Most failed items of the CCAS-S were related to verbal fluency, attention, and working memory.

Conclusion

A definite CCAS is present in almost all patients with acute cerebellar vascular lesions. CCAS is efficiently assessed by CCAS-S at bedside tests in acute stroke settings. The magnitude of CCAS likely reflects a cerebello-cortical diaschisis.

Neuroimaging Applications in Chronic Ataxias

Magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT) and positron emission tomography (PET) are the main instruments for neuroimaging investigation of patients with chronic ataxia. MRI has a predominant diagnostic role in the single patient, based on the visual detection of three patterns of atrophy, namely, spinal atrophy, cortical cerebellar atrophy and olivopontocerebellar atrophy, which correlate with the aetiologies of inherited or sporadic ataxia. In fact spinal atrophy is observed in Friedreich ataxia, cortical cerebellar atrophy in Ataxia Telangectasia, gluten ataxia and Sporadic Adult Onset Ataxia and olivopontocerebellar atrophy in Multiple System Atrophy cerebellar type. The 39 types of dominantly inherited spinocerebellar ataxias show either cortical cerebellar atrophy or olivopontocerebellar atrophy. T2 or T2* weighted MR images can contribute to the diagnosis by revealing abnormally increased or decreased signal with a characteristic distribution. These include symmetric T2 hyperintensity of the posterior and lateral columns of the cervical spinal cord in Friedreich ataxia, diffuse and symmetric hyperintensity of the cerebellar cortex in Infantile Neuro-Axonal Dystrophy, symmetric hyperintensity of the peridentate white matter in Cerebrotendineous Xanthomatosis, and symmetric hyperintensity of the middle cerebellar peduncles and peridentate white matter, cerebral white matter and corpus callosum in Fragile X Tremor Ataxia Syndrome. Abnormally decreased T2 or T2* signal can be observed with a multifocal distribution in Ataxia Telangectasia and with a symmetric distribution in the basal ganglia in Multiple System Atrophy. T2 signal hypointensity lining diffusely the outer surfaces of the brainstem, cerebellum and cerebrum enables diagnosis of superficial siderosis of the central nervous system. The diagnostic role of nuclear medicine techniques is smaller. SPECT and PET show decreased uptake of radiotracers investigating the nigrostriatal system in Multiple System Atrophy and in patients with Fragile X Tremor Ataxia Syndrome. Semiquantitative or quantitative MRI, SPECT and PET data describing structural, microstructural and functional changes of the cerebellum, brainstem, and spinal cord have been widely applied to investigate physiopathological changes in patients with chronic ataxias. Moreover they can track diseases progression with a greater sensitivity than clinical scales. So far, a few small-size and single center studies employed neuroimaging techniques as surrogate markers of treatment effects in chronic ataxias.

Nuclear medicine of the cerebellum

Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) with different radiotracers enable regional evaluation of blood flow and glucose metabolism, of receptors and transporters of several molecules, and of abnormal deposition of peptides and proteins in the brain. The cerebellum has been used as a reference region for different radiotracers in several disease conditions. Whole-brain voxel-wise analysis is not affected by a priori knowledge bias and should be preferred. SPECT and PET have contributed to establishing the cerebellum role in motion, cognition, and emotion control in physiologic and pathophysiologic conditions. The basic abnormal imaging findings include decreased or increased uptake of flow or metabolism tracers in the cerebellum alone or as part of a network. Decreased uptake is generally observed in primary structural damage of the cerebellum, but can also represent a distant effect of cerebral damage (crossed diaschisis). Increased uptake can be observed in Freidreich ataxia, inflammatory or immune-mediated diseases of the cerebellum, and in status epilepticus. The possibility is also recognized that primary structural damage of the cerebellum might determine distance effects on other brain structures (reversed diaschisis). So far, SPECT and PET have been predominantly used in clinical studies to investigate cerebellar changes in neurologic and psychiatric diseases and in connection with pharmacologic, transcranial magnetic stimulation, deep-brain stimulation, or surgical treatments.

Insulin Resistance and Alzheimer's Disease: Bioenergetic Linkages

Metabolic dysfunction is a well-established feature of Alzheimer's disease (AD), evidenced by brain glucose hypometabolism that can be observed potentially decades prior to the development of AD symptoms. Furthermore, there is mounting support for an association between metabolic disease and the development of AD and related dementias. Individuals with insulin resistance, type 2 diabetes mellitus (T2D), hyperlipidemia, obesity, or other metabolic disease may have increased risk for the development of AD and similar conditions, such as vascular dementia. This association may in part be due to the systemic mitochondrial dysfunction that is common to these pathologies. Accumulating evidence suggests that mitochondrial dysfunction is a significant feature of AD and may play a fundamental role in its pathogenesis. In fact, aging itself presents a unique challenge due to inherent mitochondrial dysfunction and prevalence of chronic metabolic disease. Despite the progress made in understanding the pathogenesis of AD and in the development of potential therapies, at present we remain without a disease-modifying treatment. In this review, we will discuss insulin resistance as a contributing factor to the pathogenesis of AD, as well as the metabolic and bioenergetic disruptions linking insulin resistance and AD. We will also focus on potential neuroimaging tools for the study of the metabolic dysfunction commonly seen in AD with hopes of developing therapeutic and preventative targets.

Consensus paper: radiological biomarkers of cerebellar diseases

Hereditary and sporadic cerebellar ataxias represent a vast and still growing group of diseases whose diagnosis and differentiation cannot only rely on clinical evaluation. Brain imaging including magnetic resonance (MR) and nuclear medicine techniques allows for characterization of structural and functional abnormalities underlying symptomatic ataxias. These methods thus constitute a potential source of radiological biomarkers, which could be used to identify these diseases and differentiate subgroups of them, and to assess their severity and their evolution. Such biomarkers mainly comprise qualitative and quantitative data obtained from MR including proton spectroscopy, diffusion imaging, tractography, voxel-based morphometry, functional imaging during task execution or in a resting state, and from SPETC and PET with several radiotracers. In the current article, we aim to illustrate briefly some applications of these neuroimaging tools to evaluation of cerebellar disorders such as inherited cerebellar ataxia, fetal developmental malformations, and immune-mediated cerebellar diseases and of neurodegenerative or early-developing diseases, such as dementia and autism in which cerebellar involvement is an emerging feature. Although these radiological biomarkers appear promising and helpful to better understand ataxia-related anatomical and physiological impairments, to date, very few of them have turned out to be specific for a given ataxia with atrophy of the cerebellar system being the main and the most usual alteration being observed. Consequently, much remains to be done to establish sensitivity, specificity, and reproducibility of available MR and nuclear medicine features as diagnostic, progression and surrogate biomarkers in clinical routine.

Milestones in Friedreich ataxia: more than a century and still learning

Friedreich ataxia (FRDA) is the most common autosomal recessive ataxia worldwide. This review highlights the main clinical features, pathophysiological mechanisms, and therapeutic approaches for FRDA patients. The disease is characterized by a combination of neurological involvement (ataxia and neuropathy), cardiomyopathy, skeletal abnormalities, and glucose metabolism disturbances. FRDA is caused by expanded guanine-adenine-adenine (GAA) triplet repeats in the first intron of the frataxin gene (FXN), resulting in reduction of messenger RNA and protein levels of frataxin in different tissues. The molecular and metabolic disturbances, including iron accumulation, lead to pathological changes characterized by spinal cord and dorsal root ganglia atrophy, dentate nucleus atrophy without global cerebellar volume reduction, and hypertrophic cardiomyopathy. DNA analysis is the hallmark for the diagnosis of FRDA. There is no specific treatment to stop the disease progression in FRDA patients. However, a number of drugs are under investigation. Therapeutic approaches intend to improve mitochondrial functioning and to increase FXN expression.

Cognitive deficits in Friedreich ataxia correlate with micro-structural changes in dentatorubral tract

Atrophy of the dentate nucleus is one of the major neuropathological changes in Friedreich ataxia (FRDA). Neuroimaging studies demonstrated white matter (WM) degeneration in FRDA. In this study, we used advanced tractography techniques to quantitatively measure WM changes in the dentato-thalamic and dentato-rubral tracts, and correlated these changes with cognitive profiles of FRDA. We also analysed diffusivity changes of the thalamo-cortical tract to assess whether neurological degeneration of WM extends beyond the primary site of involvement in FRDA. Twelve genetically proven individuals with FRDA and 14 controls were recruited. Sixty directions diffusion tensor images were acquired. The WM bundles from the dentate nucleus were estimated using a constrained spherical deconvolution method and the diffusivity characteristics measured. The Simon task was used to assess cognitive profile of FRDA. The dentato-rubral, dentato-thalamic and thalamo-cortical tracts manifested significantly lower fractional anisotropy, higher mean diffusivity and increased radial diffusivity in FRDA compared with controls. There was no difference in axial diffusivity between the two groups. The mean and radial diffusivity of the dentato-rubral tract was positively correlated with choice reaction time, congruent reaction time, incongruent reaction time and Simon effect reaction time and negatively with the larger GAA repeat. Significant changes in diffusivity characteristics were observed in the dentato-thalamic and thalamo-cortical tracts, suggesting extensive WM degeneration and affected WM structures in FRDA. Correlation of WM changes in the dentato-rubral tract with the cognitive assessment suggested that this tract is an important contributor to cognitive disturbances in FRDA.

A longitudinal VBM study monitoring treatment with erythropoietin in patients with Friedreich ataxia

Background

Recombinant human erythropoietin (rhuEPO) has received considerable attention because of its neuroprotective properties. It has recently been reported that rhuEPO increases frataxin levels in combination with clinical improvement in rhuEPO treated patients with Friedreich ataxia (FRDA).

Purpose

To determine possible therapy dependent intracranial volume changes after treatment with rhuEPO using voxel-based morphometry (VBM).

Material and Methods

Nine FRDA patients were scanned on the same 1.5-Tesla MRI scanner before and after treatment with rhuEPO. FRDA patients received 5000 IU rhuEPO thrice weekly subcutaneously for a time period of 8 weeks followed by 2000 IU thrice weekly over 6 months. To test for re-test reliability a control group of 12 healthy volunteers were scanned twice on the same scanner without rhuEPO treatment. Neurological state was defined by the Friedreich Ataxia Rating Scale (FARS) and the Scale for the Assessment and Rating of Ataxia (SARA). Statistical parametric mapping software was used for image processing and statistical analysis.

Results

When comparing follow-up scans after rhuEPO treatment with baseline scans (P <0.001 uncorrected) an increase of gray matter volume was observed bilaterally in the Pulvinar and the posterior parietal cortex. Moreover, clinical improvement detected using specific Ataxia scores correlated with VBM results in the pulvinar.

Conclusion

Given the limitation of a small sample size, our study confirms previous findings that MRI may serve as reliable biomarker in neurodegenerative diseases as well as in monitoring of microstructural changes representing disease progression and/or therapy effects.

White matter changes in patients with friedreich ataxia after treatment with erythropoietin

BACKGROUND AND PURPOSE

Erythropoietin (EPO) has received growing attention because of its neuroregenerative properties. Preclinical and clinical evidence supports its therapeutic potential in brain conditions like stroke, multiple sclerosis, and schizophrenia. Also, in Friedreich ataxia, clinical improvement after EPO therapy was shown. The aim of this study was to assess possible therapy-associated brain white matter changes in these patients.

METHODS

Nine patients with Friedreich ataxia underwent Diffusion Tensor Imaging (DTI) before and after EPO treatment. Tract-based spatial statistics was used for longitudinal comparison.

RESULTS

We detected widespread longitudinal increase in fractional anisotropy and axial diffusivity (D||) in cerebral hemispheres bilaterally (P < .05, corrected), while no changes were observed within the cerebellum, medulla oblongata, and pons.

CONCLUSIONS

To the best of our knowledge, this is the first DTI study to investigate the effects of EPO in a neurodegenerative disease. Anatomically, the diffusivity changes appear disease unspecific, and their biological underpinnings deserve further study.

Cerebello-cerebral connectivity deficits in Friedreich ataxia

Brain pathology in Friedreich ataxia is characterized by progressive degeneration of nervous tissue in the brainstem, cerebellum and cerebellar peduncles. Evidence of cerebral involvement is however equivocal. This brain imaging study investigates cerebello-cerebral white matter connectivity in Friedreich ataxia with diffusion MRI and tractography performed in 13 individuals homozygous for a GAA expansion in intron one of the frataxin gene and 14 age- and gender-matched control participants. New evidence is presented for disrupted cerebello-cerebral connectivity in the disease, leading to secondary effects in distant cortical and subcortical regions. Remote regions affected by primary cerebellar and brainstem pathology include the supplementary motor area, cingulate cortex, frontal cortices, putamen and other subcortical nuclei. The connectivity disruptions identified provide an explanation for some of the non-ataxic symptoms observed in the disease and support the notion of reverse cerebellar diaschisis. This is the first study to comprehensively map white matter connectivity disruptions in Friedreich ataxia using tractography, connectomic techniques and super-resolution track density imaging.

A functional MRI study of motor dysfunction in Friedreich's ataxia

Friedreich's ataxia (FRDA) is the most common form of hereditary ataxia. In addition to proximal spinal cord and brain stem atrophy, mild to moderate atrophy of the cerebellum has been reported in advanced FRDA. The aim of this study was to examine dysfunction in motor-related areas involved in the execution of finger tapping tasks in individuals with FRDA, and to investigate functional re-organization of cortico-cerebellar, cortico-striatal and parieto-frontal loops as a result of the cerebellar pathology. Thirteen right-handed individuals with FRDA and fourteen right-handed controls participated. Functional MRI images were acquired during four different finger tapping tasks consisting of visually cued regular and irregular single finger tapping tasks, a self-paced regular finger tapping task, and a visually cued multi-finger tapping task. Both groups showed significant activation of the motor-related network including the pre-central cortex and supplementary motor area bilaterally; the left primary motor cortex, somatosensory cortex and putamen; and the right cerebellum. During the visually cued regular finger tapping task, the right hemisphere of the cerebellar cortex, bilateral supplementary motor areas and right inferior parietal cortex showed higher activation in the healthy control group, while in individuals with FRDA the left premotor cortex, left somatosensory cortex and left inferior parietal cortex were more active. In addition, during the visually cued irregular finger tapping task, the right middle temporal gyrus in the control group and the right superior parietal lobule and left superior and middle temporal gyri in the individuals with FRDA showed higher activation. During visually cued multi-finger tapping task, the control group showed higher activation in the bilateral middle frontal gyri, bilateral somatosensory cortices, bilateral inferior parietal lobules, left premotor cortex, left supplementary area, right superior frontal gyrus and right cerebellum, while individuals with FRDA showed increased activity in the left inferior parietal lobule, left primary motor cortex, left middle occipital gyrus, right somatosensory cortex and the left cerebellum. Only the right crus I/II of the cerebellum showed higher activation in individuals with FRDA during the self-paced regular finger tapping task, whereas wide-spread regions including the left superior frontal gyrus, left central opercular cortex, left somatosensory cortex, left putamen, right cerebellum, bilateral primary motor cortices, bilateral inferior parietal lobules and the left insula were more active in the control group. Although the pattern of the BOLD signal from the putamen was different during the self-paced regular finger tapping task to the other tasks in controls, in individuals with FRDA there was no distinction of the signal between the tasks suggesting that primary cerebellar pathology may cause secondary basal ganglia dysregulation. While individuals with FRDA tapped at a slightly lower rate (0.59Hz) compared with controls (0.74Hz) they showed significantly decreased activity of the SMA and the inferior parietal lobule, which may suggest disruption to the fronto-parietal connections. These findings suggest that the motor impairments in individuals with FRDA result from dysfunction extending beyond the spinal cord and cerebellum to include sub-cortical and cortical brain regions.

Magnetic resonance and nuclear medicine imaging in ataxias

Imaging techniques including computed tomography (CT), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and positron emission tomography (PET) have been widely applied to the investigation of patients with acute or chronic ataxias. Fundamentally, CT has a role in the emergency evaluation of the patient with acute ataxia to ascertain brainstem or cerebellar hemorrhage and to exclude a mass lesion in the posterior cranial fossa. Conventional MRI is the most frequently performed imaging investigation in patients with ataxia. It can support the diagnosis of acute cerebellitis and Wernicke encephalopathy by revealing T2 signal changes with a typical distribution. In patients with inherited or sporadic chronic ataxia it reveals three fundamental patterns of atrophy of the brainstem, cerebellum, and spinal cord which match the gross neuropathological descriptions. These are represented by olivopontocerebellar atrophy (OPCA), cortical cerebellar atrophy (CCA), and spinal atrophy (SA). A substantial correspondence exists among these patterns of atrophy shown by MRI and the etiological classification of inherited or acquired chronic ataxias. This, along with demonstration of T2 signal changes characteristic of some diseases, makes conventional MRI potentially useful for the diagnostic work-up of the single patient, especially in the case of a sporadic disease. Non-conventional MR techniques including diffusion MR, spectroscopy, and functional MR have been used in patients with acute or chronic ataxia, but their exact role in the evaluation of the single patient is not established yet. They are currently investigated as potential tools to monitor progression of neurodegeneration in chronic ataxia and to serve as "surrogate markers" in clinical trials. Several radiotracers have been utilized in combination with SPECT and PET in patients with ataxia. Perfusion SPECT can reveal cerebellar blood flow abnormalities early in the course of cerebellitis. It has also been utilized to investigate perfusion of the brain in several inherited or sporadic chronic ataxic diseases, contributing to improved understanding of the pathophysiology of these conditions. Recently, perfusion SPECT has been tested as a "surrogate marker" to verify the effects of newly developed therapies in patients with a variety of chronic ataxias. Whole-body FDG-PET is recommended in patients with suspected paraneoplastic cerebellar degeneration to detect the primary malignancy. Brain FDG-PET has provided important information on the pathophysiology of several acquired and inherited conditions. PET and SPECT with radiotracers able to assess the nigrostriatal system or the density of D2 dopamine receptors in the striatum are increasingly used in patients with adult-onset sporadic ataxia for the differential diagnosis between multiple system atrophy in which overt striatal abnormalities are found and idiopathic late-onset cerebellar ataxia in which no abnormality is detected.

Characterising the neuropathology and neurobehavioural phenotype in Friedreich ataxia: a systematic review

Friedreich ataxia (FRDA), the most common of the hereditary ataxias, is an autosomal recessive, multisystem disorder characterised by progressive ataxia, sensory symptoms, weakness, scoliosis and cardiomyopathy. FRDA is caused by a GAA expansion in intron one of the FXN gene, leading to reduced levels of the encoded protein frataxin, which is thought to regulate cellular iron homeostasis. The cerebellar and spinocerebellar dysfunction seen in FRDA has known effects on motor function; however until recently slowed information processing has been the main feature consistently reported by the limited studies addressing cognitive function in FRDA. This chapter will systematically review the current literature regarding the neuropathological and neurobehavioural phenotype associated with FRDA. It will evaluate more recent evidence adopting systematic experimental methodologies that postulate that the neurobehavioural phenotype associated with FRDA is likely to involve impairment in cerebello-cortico connectivity.

Friedreich ataxia: the clinical picture

Friedreich ataxia (FRDA) is a rare autosomal recessive hereditary disorder that affects approximately 1 in 50,000 Caucasians. It is caused by hyperexpansion of GAA repeats in the first intron of the frataxin gene. Initial symptoms of FRDA usually appear around the beginning of the second decade of life. In addition to neuropathological disabilities such as ataxia, sensory loss, and muscle weakness, common signs are scoliosis, foot deformity, and hypertrophic cardiomyopathy. Approximately 10 % of patients with FRDA develop diabetes. The neuronopathy in the dorsal root ganglia, accompanied by the loss of peripheral sensory nerve fibres and the degeneration of posterior columns of the spinal cord, is a hallmark of the disease and is responsible for the typical combination of signs and symptoms specific to FRDA. Variation in neurophysiological abnormalities is correlated with the size of the GAA repeat expansion and likely accounts for individual variation in the progression of FRDA. Despite a range of disease severity, most patients will lose their ability to walk, stand, or sit without support within 10 to 15 years of disease onset. In addition to a review of the clinicopathological features of FRDA, a discussion of recent advances in our understanding of the underlying molecular mechanisms is provided.

Towards an understanding of cognitive function in Friedreich ataxia

There is limited documentation regarding cognitive function in individuals with Friedreich ataxia (FRDA), possibly because FRDA is widely held to predominantly affect the spinal cord, peripheral sensory nerves and cerebellum and not to affect cognition. Traditionally, the cerebellum has been thought to coordinate voluntary movement and motor tone, posture and gait. However, recent studies have implicated the cerebellum in a range of cognitive functions including executive function, visuospatial organisation and memory. We review the available data on cognitive function and neuroimaging in FRDA and the role of the cerebellum in cognitive function. We conclude with recommendations for future research including correlating cognitive function in individuals with FRDA with possible determinants of disease severity, such as age of onset and the causative genetic mutation.

Hypo-excitability of cortical areas in patients affected by Friedreich ataxia: A TMS study

The aim of the study was to explore excitability of a motor and a non-motor (visual) area in patients affected by Friedreich ataxia and to correlate neurophysiological data with clinical parameters. Seven patients (3M/4F) and ten healthy controls (5M/5F) participated in the study. The hot-spot for activation of right abductor pollicis brevis was checked by means of a figure-of-eight coil and the motor threshold (MT) on this point was recorded. The phosphene threshold (PT) was measured by means of a focal coil over the occipital cortex as the lower intensity of magnetic stimulation able to induce the perception of phosphenes. The patients showed a significantly higher mean PT (p<.03) and MT values (p<.001) than controls. In all but one patient unable to perceive phosphenes (42% vs. 50% of controls), TMS at 100% intensity did not elicit motor response at rest. The difference in percentage of patients (57.1%) and controls (100%) with motor responses was nearly significant. The size of GAA1 expansion showed significant correlations with PT and MT values. The results of our study showed that FA patients had reduced cortical activation, involving both the motor and the visual cortex. The cortical involvement in these patients seems to be mainly genetically determined. The study provides the first evidence of cortical dysfunction in patients with genetically defined Friedreich ataxia.

Friedreich ataxia

There has been rapid progress in the understanding of several aspects of Friedreich's ataxia (FA) since the gene mutation was identified in 1996. At the clinical level, now it is possible to confirm that the majority of patients fullfilling clinical criteria for classic FA have the FA gene mutation but some do not, indicating genetic heterogeneity. Also, the phenotype associated with the FA mutation is much wider than that defined by clinical criteria and includes ataxia with retained or brisk reflexes as well as late onset ataxia with or without retained reflexes. It is now clear that the unstable GAA expansion that underlies FA causes a deficiency of the mitochondrial protein frataxin, leading to potentially harmful oxidative injury associated with excessive iron deposits in mitochondria. In addition, pathogenesis may involve a primary defect in synthesis of iron-sulfur cluster containing enzymes. Therapeutic attempts are already using anti-oxidant strategies and such efforts are likely to be enhanced by the rapid availability of animal models of the disease.

PET studies in dementia

Measurement of local cerebral glucose metabolism (lCMRGlc) by positron emission tomography (PET) and 18F-2-fluoro-2-deoxy-D-glucose (FDG) has become a standard technique during the past 20 years and is now available at many university hospitals in all highly developed countries. Many studies have documented a close relation between lCMRGlc and localized cognitive functions, such as language and visuoconstructive abilities. Alzheimer's disease (AD) is characterized by regional impairment of cerebral glucose metabolism in neocortical association areas (posterior cingulate, temporoparietal and frontal multimodal association cortex), whereas primary visual and sensorimotor cortex, basal ganglia, and cerebellum are relatively well preserved. In a multicenter study comprising 10 PET centers (Network for Efficiency and Standardisation of Dementia Diagnosis, NEST-DD) that employed an automated voxel-based analysis of FDG PET images, the distinction between controls and AD patients was 93% sensitive and 93% specific, and even in very mild dementia (at MMSE 24 or higher) sensitivity was still 84% at 93% specificity. Significantly abnormal metabolism in mild cognitive deficit (MCI) indicates a high risk to develop dementia within the next two years. Reduced neocortical glucose metabolism can probably be detected with FDG PET in AD on average one year before onset of subjective cognitive impairment. In addition to glucose metabolism, specific tracers for dopamine synthesis (18F-F-DOPA) and for (11C-MP4A) are of interest for differentiation among dementia subtypes. Cortical acetylcholine esterase activity (AChE) activity is significantly lower in patients with AD or with dementia with Lewy bodies (DLB) than in age-matched normal controls. In LBD there is also impairment of dopamine synthesis, similar to Parkinson disease.

Relative sparing of the parietal cortex in cerebellar ataxia documented by positron emission tomography

With the intention to assess remote effects of cerebellar dysfunction, 23 patients with inherited or idiopathic cerebellar ataxia were studied with positron emission tomography (PET) and 2[18F]fluoro-2-deoxy-D-glucose (FDG). Eight patients (group 1) suffered from early onset cerebellar ataxia (EOCA, age of symptom onset <20 years), nine patients (group 2) from late onset cerebellar ataxia (LOCA, symptom onset between the ages of 20 and 50), and six patients (group 3) experienced symptom onset beyond the age of 50 years. The pattern of cerebral glucose metabolism in cerebellar ataxia was compared to the results in a control group of 16 healthy subjects. In all patients, a reduction in relative (EOCA, group 1) or absolute (LOCA, groups 2 and 3) values of regional cerebral glucose metabolism (rCMR(glu)) occurred in both cerebellar hemispheres as well as the vermis and both dentate nuclei. In patients from all groups presenting with a clinical syndrome of pure cerebellar ataxia, impairment of regional glucose metabolism also extended to the pontine and brainstem regions. In contrast to this infratentorial reduction of rCMR(glu) in all patients, in those with LOCA, a significant relative increase in rCMR(glu) was present in distinct supratentorial cortical regions, namely the cuneus, the pre-cuneus and the gyrus supramarginalis in the patients of group 2. In group 3, this significant relative increase in rCMR(glu) was restricted to the cuneus. Thus, FDG-PET in patients suffering from cerebellar ataxia shows distinct patterns of altered glucose metabolism which exceed pure cerebellar impairment. Most importantly, FDG-PET yields insight into the influence of cerebellar disease on supratentorial glucose metabolism and documents impairment of supratentorial neuronal function with relative sparing of the parietal cortex.

Hereditary ataxias

There are many causes of hereditary ataxia. These can be grouped into categories of autosomal recessive, autosomal dominant, and X-linked. Molecularly, many of them are due to trinucleotide repeat expansions. In Friedreich ataxia, the trinucleotide repeat expansions lead to a "loss of function." In the dominant ataxias, the expanded repeats lead to a "gain of function," most likely through accumulation of intranuclear (and less commonly cytoplasmic) polyglutamine inclusions. Channelopathies can also lead to ataxia, especially episodic ataxia. Although phenotypic characteristics are an aid to the clinician, a definitive diagnosis is usually made only through genotypic or molecular studies. Genetic counseling is necessary for the testing of symptomatic and asymptomatic individuals. No effective treatment is yet available for most ataxic syndromes, except for ataxia with isolated vitamin E deficiency and the episodic ataxias.

Regional brain variations of cytochrome oxidase activity and motor coordination in hot-foot mutant mice

Hot-foot mutant mice are characterized by defective arborization of Purkinje cell dendrites, resulting in ataxia of gait and deficits of equilibrium. Regional brain variations of cytochrome oxidase (CO) activity were analysed for the purpose of identifying those brain regions with abnormal metabolic activity as a secondary consequence to the cerebellar alteration. In addition, the possible relation between CO activity and motor deficits was evaluated. By comparison to normal littermate controls of the same background strain, hot-foot mutants had higher CO activity in the molecular layer of the cerebellum, the ventrolateral and midline thalamic nuclei, as well as in the frontal eye field. There was no other alteration of CO activity in the hot-foot brain. No linear correlation was discerned between CO activity in the molecular layer of the cerebellum and the ventrolateral and midline nuclei on one hand, and motor coordination performance on the other. These results indicate regionally selective abnormalities of metabolic activity in a cerebellar mutant with defective dendritic arborization of the Purkinje cell.

Positron emission tomography in asymptomatic gene carriers of Machado-Joseph disease

OBJECTIVES

The metabolic changes in the brain of symptomatic subjects affected with Machado-Joseph disease have been previously documented using PET with fluorine-18-fluorodeoxyglucose (FDG). The aim of this study was to evaluate these changes in asymptomatic Machado-Joseph disease gene carriers.

METHODS

Seven asymptomatic Machado-Joseph disease gene carriers, identified using a molecular test, and 10 normal control subjects were recruited for PET studies using FDG. Regional uptake ratios of FDG were calculated from the radioactivity of the cerebellar hemispheres, brainstem, and the temporal, parietal and occipital cortices, divided by the activity in the thalamus.

RESULTS

In comparison with data obtained from normal control subjects, there was significantly decreased FDG utilisation in the cerebellar hemispheres, brainstem, and occipital cortex, and increased FDG metabolism in the parietal and temporal cortices of asymptomatic Machado-Joseph disease gene carriers, suggesting preclinical disease activity. Discriminant analysis of regional FDG uptake correctly classified genetic status (Machado-Joseph disease mutation carriers v mutation negative subjects) in 25 of 25 subjects (100% sensitivity and 100% specificity), and clinical status (asymptomatic mutation carriers v symptomatic patients) in 14 of 15 subjects (100% sensitivity and 85.7% specificity).

CONCLUSION

Subclinical changes of FDG consumption, as measured by noninvasive PET, can act as an objective marker of preclinical disease activity in Machado-Joseph disease.

Molecular genetics of the hereditary ataxias

One of us (MP) learned about the mapping of Huntington disease gene to chromosome 4 from the late Dr. Anita Harding. She got the news over the phone from her London office during a visit to Italy for a meeting on hereditary ataxias. In Britain, they receive Nature at least a week earlier than us. Dr. Harding was very excited, and she immediately said that that was the way to go if we wanted to understand the causes of hereditary ataxias, classify these diseases in a rational way, and eventually find a treatment. At that time, the challenge seemed, and indeed was, formidable. No clue was then available about the genetic basis of what Dr. Harding aptly called "hereditary ataxias of unknown cause," their classification was confused and controversial, and all attempts to find specific biochemical abnormalities had failed. Fourteen years later, the success of the molecular genetic studies is astounding. The defective genes have been identified for Friedreich ataxia, the major recessive "hereditary ataxia of unknown cause," and for five dominantly inherited "hereditary ataxias of unknown cause." Three more dominant ataxia genes have been mapped. The molecular pathogenesis of the dominant ataxias begins to be unraveled and animal models have been and are being developed. Information is also quickly accumulating about the defective protein in Friedreich ataxia. Direct molecular diagnosis is now possible. Classification has been revolutionized. Diagnostic criteria are being redefined in the light of the molecular discoveries. The goal of this review, dedicated to the memory of the late Dr. Harding, is to offer a concise summary of current knowledge about the molecular genetics of some of the hereditary ataxias that used to be classified as of "unknown cause."

Glutamate in neurologic diseases

Excitotoxicity has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases. Cerebral ischemia, head and spinal cord injury, and prolonged seizure activity are associated with excessive release of glutamate into the extracellular space and subsequent neurotoxicity. Accumulating evidence suggests that impairment of intracellular energy metabolism increases neuronal vulnerability to glutamate which, even when present at physiologic concentrations, can damage neurons. This mechanism of slow excitotoxicity may be involved in neuronal death in chronic neurodegenerative diseases such as the mitochondrial encephalomyopathies, Huntington's disease, spinocerebellar degeneration syndromes, and motor neuron diseases. If so, glutamate antagonists in combination with agents that selectively inhibit the multiple steps downstream of the excitotoxic cascade or help improve intracellular energy metabolism may slow the neurodegenerative process and offer a therapeutic approach to treat these disorders.

Frataxin gene of Friedreich's ataxia is targeted to mitochondria

Friedreich's ataxia is caused by a triplet repeat expansion in intron 1, a noncoding region of the frataxin gene (X25). We have generated a chimeric gene composed of the frataxin gene fused with the green fluorescent protein (GFP) gene as a reporter. Transfection of the fusion construct into living COS cells revealed that the frataxin-GFP construct localizes to organelles that double-label with 8-(4'-chloromethyl) phenyl-2,3,5,6,11,12,14,15-octahydro-1H,4H,10H-13H-diquinolizin o-8H-xanthylium chloride (CMXRos), a novel mitochondrial dye. Thus, frataxin appears to be a nuclear-encoded mitochondrial protein.

Acute effects of alcohol administration on regional cerebral blood flow: the role of acetate

The effect of alcohol intoxication on regional cerebral blood flow (rCBF) is unclear. The few published studies provide a mixed picture of alcohol effects, suggesting that blood flow increases at low doses and decreases at higher doses of alcohol. Using the cerebral blood flow agent 99mtechnetium hexamethyl-propylene-amine-oxime (HMPAO) and image reconstruction with single photon emission computed tomography (SPECT), we evaluated the effect of an oral dose of ethanol (0.6 g/kg) on rCBF in two age-stratified groups of healthy, nonalcoholic men (12 age 22-37 and 12 age 63-77). Intoxication was associated with a significant 4% increase in global cortical CBF (t = 2.54, p = 0.02). Changes in HMPAO uptake were negatively correlated to ethanol levels in the entire group (r = -0.47, p = 0.04). This relationship was seen in the older subgroup (r = -0.70, p = 0.05), but not in the younger group (r = -0.34, p = 0.26). In contrast, the younger group showed a significant positive correlation between rCBF and increases in acetate levels (r = 0.71, p < 0.01), which was not seen in the older group (r = -0.02, p = 0.96). These findings suggest that both acetate and alcohol contribute to the changes in CBF seen in the intoxication syndrome and that their relative influence is age-dependent.

Metabolic and cognitive changes in hereditary ataxia

Fourteen subjects (affected, n = 7; at risk, n = 7) from one well-known kindred with adult onset autosomal dominant olivopontocerebellar atrophy (OPCA), were studied with [18F]-2-deoxy-D-glucose (FDG) positron emission tomography (PET), magnetic resonance imaging (MRI), cognitive testing and scored neurological examination, and compared with normal controls. The neurological examination, MRI and cognitive tests showed no significant differences between at risk and normal control subjects. Mild cognitive deficits were seen in affected subjects; the degree of cognitive change appeared to relate to the severity of the illness. MRI demonstrated cerebellar and brainstem atrophy in all affected subjects. PET studies showed higher global metabolic rates (mean [SD]) in at risk subjects (10.5 [1.5] mg per min per 100 g) as compared to affected (9.0 [0.8] mg per min per 100 g) and normal control subjects (9.1 [1.5] mg per min per 100 g). Normalized (region/global average) regional metabolic rates were reduced in cerebellar hemispheres and vermis, and in frontal and prefrontal areas of affected subjects in comparison to at risk and normal control subjects. These findings indicate that functional changes in some forms of autosomal dominant hereditary cerebellar ataxia may extend beyond the cerebellum and brainstem to involve other parts of the neuraxis.

Uses and limitations of positron emission tomography in clinical pharmacokinetics/dynamics (Part II)

Positron emission tomography (PET) involves imaging the biodistribution and tissue localisation of small amounts of radiolabelled biomolecules or drugs. In Part I of this article, which appeared in the previous issue of the Journal, the applications of pharmacokinetics in PET were discussed in order to derive quantitative measures of physiological function. Part II examines the use of PET imaging as a tool to study the pharmacokinetics and pharmacodynamics of specific drugs.

Does impairment of energy metabolism result in excitotoxic neuronal death in neurodegenerative illnesses?

The etiology of nerve cell death in neuronal degenerative disease is unknown, but it has been hypothesized that excitotoxic mechanisms may play a role. Such mechanisms may play a role in diseases such as Huntington's disease, Parkinson's disease, amyotropic lateral sclerosis, and Alzheimer's disease. In these illnesses, the slowly evolving neuronal death is unlikely to be due to a sudden release of glutamate, such as occurs in ischemia. One possibility, however, is that a defect in mitochondrial energy metabolism could secondarily lead to slow excitotoxic neuronal death, by making neurons more vulnerable to endogenous glutamate. With reduced oxidative metabolism and partial cell membrane depolarization, voltage-dependent N-methyl-D-aspartate (NMDA) receptor ion channels would be more easily activated. In addition, several other processes involved in buffering intracellular calcium may be impaired. Recent studies in experimental animals showed that mitochondrial toxins can result in a pattern of neuronal degeneration closely resembling that seen in Huntington's disease, which can be blocked with NMDA antagonists. NMDA antagonists also block neuronal degeneration induced by 1-methyl-4-phenylpyridium, which has been implicated in experimental models of Parkinson's disease. The delayed onset of neurodegenerative illnesses could be related to the progressive impairment of mitochondrial oxidative phosphorylation, which accompanies normal aging. If defective mitochondrial energy metabolism plays a role in cell death in neurodegenerative disorders, potential therapeutic strategies would be to use excitatory amino acid antagonists or agents to bypass bioenergetic defects.