Genetic and environmental factors in the pathogenesis of Huntington's disease

Defective Mitochondrial Dynamics and Protein Degradation Pathways Underlie Cadmium-Induced Neurotoxicity and Cell Death in Huntington's Disease Striatal Cells

Exposure to heavy metals, including cadmium (Cd), can induce neurotoxicity and cell death. Cd is abundant in the environment and accumulates in the striatum, the primary brain region selectively affected by Huntington's disease (HD). We have previously reported that mutant huntingtin protein (mHTT) combined with chronic Cd exposure induces oxidative stress and promotes metal dyshomeostasis, resulting in cell death in a striatal cell model of HD. To understand the effect of acute Cd exposure on mitochondrial health and protein degradation pathways, we hypothesized that expression of mHTT coupled with acute Cd exposure would cooperatively alter mitochondrial bioenergetics and protein degradation mechanisms in striatal STHdh cells to reveal novel pathways that augment Cd cytotoxicity and HD pathogenicity. We report that mHTT cells are significantly more susceptible to acute Cd-induced cell death as early as 6 h after 40 µM CdCl₂ exposure compared with wild-type (WT). Confocal microscopy, biochemical assays, and immunoblotting analysis revealed that mHTT and acute Cd exposure synergistically impair mitochondrial bioenergetics by reducing mitochondrial potential and cellular ATP levels and down-regulating the essential pro-fusion proteins MFN1 and MFN2. These pathogenic effects triggered cell death. Furthermore, Cd exposure increases the expression of autophagic markers, such as p62, LC3, and ATG5, and reduces the activity of the ubiquitin-proteasome system to promote neurodegeneration in HD striatal cells. Overall, these results reveal a novel mechanism to further establish Cd as a pathogenic neuromodulator in striatal HD cells via Cd-triggered neurotoxicity and cell death mediated by an impairment in mitochondrial bioenergetics and autophagy with subsequent alteration in protein degradation pathways.

Poly (ADP-ribose) polymerase-1 as a promising drug target for neurodegenerative diseases

AIMS

Poly (ADP-ribose) polymerase- (PARP)-1 is predominantly triggered by DNA damage. Overexpression of PARP-1 is known for its association with the pathogenesis of several CNS disorders, such as Stroke, Parkinson's disease (PD), Alzheimer's disease (AD), Huntington (HD) and Amyotrophic lateral sclerosis (ALS). NAD+ depletion resulted PARP related cell death only happened when the trial used extreme high oxidization treatment. Inhibition of PARP1/2 may induce replication related cell death due to un-repaired DNA damage. This review has discussed PARP-1 modulated downstream pathways in neurodegeneration and various FDA approved PARP-1 inhibitors.

MATERIALS AND METHODS

A systematic literature review of PubMed, Medline, Bentham, Scopus and EMBASE (Elsevier) databases was carried out to understand the nature of the extensive work done on mechanistic role of Poly (ADP-ribose) polymerase and its inhibition in Neurodegenerative diseases.

KEY FINDINGS

Several researchers have put forward number of potential treatments, of which PARP-1 enzyme has been regarded as a potent target intended for the handling of neurodegenerative ailments. Targeting PARP using its chemical inhibitors in various neurodegenerative may have therapeutic outcomes by reducing neuronal death mediated by PARPi. Numerous PARP-1 inhibitors have been studied in neurodegenerative diseases but they haven't been clinically evaluated.

SIGNIFICANCE

In this review, the pathological role of PARP-1 in various neurodegenerative diseases has been discussed along with the therapeutic role of PARP-1 inhibitors in various neurodegenerative diseases.

Age is an important independent modifier of SCA3 phenotype severity

OBJECTIVE

This study aimed to investigate factors modulating spinocerebellar ataxia type 3 (SCA3) phenotype severity besides the expanded CAG repeats (ExpCAG) of ATXN3.

METHODS

Data regarding CAG trinucleotide repeats, age at onset (AO), duration, age, sex, transmitting parent, and scale scores of SCA3 patients were collected. Multiple linear regression analysis was performed to identify influential independent variables. Age, AO, ExpCAG, and duration were considered control variables to analyze the correlation between independent variables and scale scores.

RESULTS

Duration, age, and ExpCAG were screened as influential independent variables (P = 0.000). Age had the greatest impact on multiple linear regression models (P<5E-8). ExpCAG and SARA/ICARS/INAS/Barthel index were not correlated (P > 0.05); considering only age as the control, ExpCAG was slightly-to-moderately correlated with all aforementioned scores except INAS (P < 0.05). Age and all scores, except INAS, were positively correlated (P < 0.05); considering duration, AO, or ExpCAG as controls, their correlations did not change significantly. On controlling age, AO was negatively correlated with all scores (P < 0.05), except for the Barthel index (P > 0.05). Furthermore, the interaction model revealed that the interaction between age, duration, and ExpCAG was significantly associated with SCA3 disease severity (P < 0.05).

CONCLUSION

Age is a potentially important modifier of SCA3 phenotype severity, through the interaction between ExpCAG and aging factors.

Ellagic acid rescues motor and cognitive deficits in the R6/2 mouse model of Huntington's disease by lowering mutant huntingtin protein

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by a highly polymorphic CAG trinucleotide repeat expansion encoding an extended polyglutamine (polyQ) tract at the N-terminus of huntingtin protein (HTT). The polyQ tract promotes the formation of toxic oligomers and aggregates of HTT, which leads to neuronal dysfunction and death. Therapies to lower mutant HTT (mHTT) and its aggregates appear to be the most promising strategies. Ellagic acid (EA) has been marketed as a dietary supplement with various claimed benefits and neuroprotective effects on several neurodegenerative disorders, while its effect on mHTT pathology is still unknown. Here we reported that EA significantly attenuated motor and cognitive deficits in R6/2 mice. Moreover, EA significantly lowered mHTT levels, reduced neuroinflammation, rescued synapse loss, and decreased oxidative stress in R6/2 mouse brains. These findings indicated that EA has promising therapeutic potential for HD treatment.

Cerebellar Predominant Increase in mRNA Expression Levels of Sirt1 and Sirt3 Isoforms in a Transgenic Mouse Model of Huntington's Disease

The potential role of Sirt1 and Sirt2 subtypes of Sirtuins (class III NAD⁺-dependent deacetylases) in the pathogenesis of Huntington’s disease (HD) has been extensively studied yielding some controversial results. However, data regarding the involvement of Sirt3 and their variants in HD are considerably limited. The aim of this study was to assess the expression pattern of Sirt1 and three Sirt3 mRNA isoforms (Sirt3-M1/2/3) in the striatum, cortex and cerebellum in respect of the effect of gender, age and the presence of the transgene using the N171-82Q transgenic mouse model of HD. Striatal, cortical and cerebellar Sirt1-Fl and Sirt3-M1/2/3 mRNA levels were measured in 8, 12 and 16 weeks old N171-82Q transgenic mice and in their wild-type littermates. Regarding the striatum and cortex, the presence of the transgene resulted in a significant increase in Sirt3-M3 and Sirt1 mRNA levels, respectively, whereas in case of the cerebellum the transgene resulted in increased expression of all the assessed subtypes and isoforms. Aging exerted minor influence on Sirt mRNA expression levels, both in transgene carriers and in their wild-type littermates, and there was no interaction between the presence of the transgene and aging. Furthermore, there was no difference between genders. The unequivocal cerebellar Sirtuin activation with presumed compensatory role suggests that the cerebellum might be another key player in HD in addition to the most severely affected striatum. The mitochondrially acting Sirt3 may serve as an interesting novel therapeutic target in this deleterious condition.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Huntington's disease in the United States: Variation by demographic and socioeconomic factors

BACKGROUND

Despite extensive research regarding the etiology of Huntington's disease, relatively little is known about the epidemiology of this rare disorder, particularly in the United States where there are no national-scale estimates of the disease.

OBJECTIVES

To provide national-scale estimates of Huntington's disease in a U.S. population and to test whether disease rates are increasing, and whether frequency varies by race, ethnicity, or other factors.

METHODS

Using an insurance database of over 67 million enrollees, we retrospectively identified a cohort of 3,707 individuals diagnosed with Huntington's disease between 2003 and 2016. We estimated annual incidence, annual diagnostic frequency, and tested for trends over time and differences in diagnostic frequency by sociodemographic characteristics.

RESULTS

During the observation period, the age-adjusted cumulative incidence rate was1.22 per 100,000 persons (95% confidence interval: 1.53, 1.65), and age-adjusted diagnostic frequency was 6.52 per 100,000 persons (95% confidence interval: 5.31, 5.66); both rates remained relatively stable over the 14-year period. We identified several previously unreported differences in Huntington's disease frequency by self-reported sex, income, and race/ethnicity. However, racial/ethnic differences were of lower magnitude than have previously been reported in other country-level studies.

CONCLUSIONS

In these large-scale estimates of U.S. Huntington's disease epidemiology, we found stable disease frequency rates that varied by several sociodemographic factors. These findings suggest that disease patterns may be more driven by social or environmental factors than has previously been appreciated. Results further demonstrate the potential utility of administrative Big Data in rare disease epidemiology when other data sources are unavailable. © 2019 International Parkinson and Movement Disorder Society.

Therapeutic effects of stem cells in rodent models of Huntington's disease: Review and electrophysiological findings

The principal symptoms of Huntington's disease (HD), chorea, cognitive deficits, and psychiatric symptoms are associated with the massive loss of striatal and cortical projection neurons. As current drug therapies only partially alleviate symptoms, finding alternative treatments has become peremptory. Cell replacement using stem cells is a rapidly expanding field that offers such an alternative. In this review, we examine recent studies that use mesenchymal cells, as well as pluripotent, cell-derived products in animal models of HD. Additionally, we provide further electrophysiological characterization of a human neural stem cell line, ESI-017, which has already demonstrated disease-modifying properties in two mouse models of HD. Overall, the field of regenerative medicine represents a viable and promising avenue for the treatment of neurodegenerative disorders including HD.

Trinucleotide repeat disorders

Trinucleotide repeat disorders comprise a variable group of inherited neurodegenerative diseases, with a large range in prevalence figures. There is a broad range in clinical presentations, but many of these diseases lead to some form of ataxia or other movement disorders, which are frequently combined with cognitive or psychiatric disturbances. This group can be divided into CAG- versus non-CAG-repeat diseases. Apart from spinocerebellar ataxia type 6 and 12 (SCA6 and SCA12), these CAG-repeat diseases, as well as Huntington disease-like 2 (HDL2) and SCA8, can be neuropathologically identified using 1C2 polyglutamine antibodies. In fragile X-associated tremor and ataxia, SCA6 and SCA12 ubiquitin/p62-positive and 1C2-negative inclusion bodies can be observed. In the other diseases proteinaceous inclusions are not found. For definite diagnosis genetic analysis is necessary.

Rehabilitation training in neural restitution

Over the last decade, neural transplantation has emerged as one of the more promising, albeit highly experimental, potential therapeutics in neurodegenerative disease. Preclinical studies in rat lesion models of Huntington's disease (HD) and Parkinson's disease (PD) have shown that transplanted precursor neuronal tissue from a fetus into the lesioned striatum can survive, integrate, and reconnect circuitry. Importantly, specific training on behavioral tasks that target striatal function is required to encourage functional integration of the graft to the host tissue. Indeed, "learning to use the graft" is a concept recently adopted in preclinical studies to account for unpredicted profiles of recovery posttransplantation and is an emerging strategy for improving graft functionality. Clinical transplant studies in HD and PD have resulted in mixed outcomes. Small sample sizes and nonstandardized experimental procedures from trial to trial may explain some of this variability. However, it is becoming increasingly apparent that simply replacing the lost neurons may not be sufficient to ensure the optimal graft effects. The knowledge gained from preclinical grafting and training studies suggests that lifestyle factors, including physical activity and specific cognitive and/or motor training, may be required to drive the functional integration of grafted cells and to facilitate the development of compensatory neural networks. The clear implications of preclinical studies are that physical activity and cognitive training strategies are likely to be crucial components of clinical cell replacement therapies in the future. In this chapter, we evaluate the role of general activity in mediating the physical ability of cells to survive, sprout, and extend processes following transplantation in the adult mammalian brain, and we consider the impact of general and specific activity at the behavioral level on functional integration at the cellular and physiological level. We then highlight specific research questions related to timing, intensity, and specificity of training in preclinical models and synthesize the current state of knowledge in clinical populations to inform the development of a strategy for neural transplantation rehabilitation training.

Glycation potentiates neurodegeneration in models of Huntington's disease

Protein glycation is an age-dependent posttranslational modification associated with several neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases. By modifying amino-groups, glycation interferes with folding of proteins, increasing their aggregation potential. Here, we studied the effect of pharmacological and genetic manipulation of glycation on huntingtin (HTT), the causative protein in Huntington’s disease (HD). We observed that glycation increased the aggregation of mutant HTT exon 1 fragments associated with HD (HTT72Q and HTT103Q) in yeast and mammalian cell models. We found that glycation impairs HTT clearance thereby promoting its intracellular accumulation and aggregation. Interestingly, under these conditions autophagy increased and the levels of mutant HTT released to the culture medium decreased. Furthermore, increased glycation enhanced HTT toxicity in human cells and neurodegeneration in fruit flies, impairing eclosion and decreasing life span. Overall, our study provides evidence that glycation modulates HTT exon-1 aggregation and toxicity, and suggests it may constitute a novel target for therapeutic intervention in HD.

Analysis of the GGGGCC Repeat Expansions of the C9orf72 Gene in SCA3/MJD Patients from China

Neurodegenerative disorders are a heterogeneous group of chronic progressive diseases and have pathological mechanisms in common. A certain causative gene identified for a particular disease may be found to play roles in more than one neurodegenerative disorder. We analyzed the GGGGCC repeat expansions of C9orf72 gene in patients with SCA3/MJD from mainland China to determine whether the C9orf72 gene plays a role in the pathogenesis of SCA3/MJD. In our study, there were no pathogenic repeats (>30 repeats) detected in either the patients or controls. SCA3/MJD patients with intermediate/intermediate or short/intermediate genotype (short: <7 repeats; intermediate: 7-30 repeats) of the GGGGCC repeats had an earlier onset compared with those with short/short genotype. The presence of the intermediate allele of the GGGGCC repeats in the patients decreased the age at onset by nearly 3 years. Our study firstly demonstrate that the development of SCA3/MJD may involve some physiological functions of the C9orf72 gene and provide new evidence to the hypothesis that a specific mutation identified in one of the neurodegenerative disorders may be a modulator in this class of diseases.

Common dysregulation network in the human prefrontal cortex underlies two neurodegenerative diseases

Using expression profiles from postmortem prefrontal cortex samples of 624 dementia patients and non-demented controls, we investigated global disruptions in the co-regulation of genes in two neurodegenerative diseases, late-onset Alzheimer's disease (AD) and Huntington's disease (HD). We identified networks of differentially co-expressed (DC) gene pairs that either gained or lost correlation in disease cases relative to the control group, with the former dominant for both AD and HD and both patterns replicating in independent human cohorts of AD and aging. When aligning networks of DC patterns and physical interactions, we identified a 242-gene subnetwork enriched for independent AD/HD signatures. This subnetwork revealed a surprising dichotomy of gained/lost correlations among two inter-connected processes, chromatin organization and neural differentiation, and included DNA methyltransferases, DNMT1 and DNMT3A, of which we predicted the former but not latter as a key regulator. To validate the inter-connection of these two processes and our key regulator prediction, we generated two brain-specific knockout (KO) mice and show that Dnmt1 KO signature significantly overlaps with the subnetwork (P = 3.1 × 10⁻¹²), while Dnmt3a KO signature does not (P = 0.017).

Modulation of the age at onset in spinocerebellar ataxia by CAG tracts in various genes

Polyglutamine-coding (CAG)n repeat expansions in seven different genes cause spinocerebellar ataxias. Although the size of the expansion is negatively correlated with age at onset, it accounts for only 50-70% of its variability. To find other factors involved in this variability, we performed a regression analysis in 1255 affected individuals with identified expansions (spinocerebellar ataxia types 1, 2, 3, 6 and 7), recruited through the European Consortium on Spinocerebellar Ataxias, to determine whether age at onset is influenced by the size of the normal allele in eight causal (CAG)n-containing genes (ATXN1-3, 6-7, 17, ATN1 and HTT). We confirmed the negative effect of the expanded allele and detected threshold effects reflected by a quadratic association between age at onset and CAG size in spinocerebellar ataxia types 1, 3 and 6. We also evidenced an interaction between the expanded and normal alleles in trans in individuals with spinocerebellar ataxia types 1, 6 and 7. Except for individuals with spinocerebellar ataxia type 1, age at onset was also influenced by other (CAG)n-containing genes: ATXN7 in spinocerebellar ataxia type 2; ATXN2, ATN1 and HTT in spinocerebellar ataxia type 3; ATXN1 and ATXN3 in spinocerebellar ataxia type 6; and ATXN3 and TBP in spinocerebellar ataxia type 7. This suggests that there are biological relationships among these genes. The results were partially replicated in four independent populations representing 460 Caucasians and 216 Asian samples; the differences are possibly explained by ethnic or geographical differences. As the variability in age at onset is not completely explained by the effects of the causative and modifier sister genes, other genetic or environmental factors must also play a role in these diseases.

Huntington's Disease: Characteristics of Fallers

INTRODUCTION

Individuals with Huntington's disease have a high prevalence of falls, but the specific factors that may increase the risk of falling have not been clearly identified in this clinical population. This study aimed to identify the characteristics of individuals with early to mid-stage Huntington's disease who had a history of falls, compared with a cohort with no history of falls.

METHODS

Twenty-four participants (10 non-fallers and 14 recurrent fallers) with a diagnosis of early to mid-stage Huntington's disease were included in this study. Falls data were collected using retrospective survey analysis. Participants were assessed using measures of balance (Berg Balance Scale), mobility (Rivermead mobility index [RMI]), fear of falling (Activity-specific Balance Confidence Scale) and gait (6-min walk test; 10-m walk test self-paced and dual tasking).

RESULTS

There was no difference in severity of disease state between fallers (Unified Huntington Disease Rating Scale [UHDRS] motor 25.33) and non-fallers (UHDRS motor 25.13) (p = 0.97). The prevalence of falls was high with 66.7% of participants reporting at least one fall and 58.3% reporting two or more falls in the past 12 months. There was no difference in age or gender between recurrent fallers and non-fallers. Recurrent fallers had significantly lower scores on the Activity-specific Balance Confidence Scale (p < 0.01) and the RMI (p < 0.05). The probability of falling increases rapidly with a RMI score of less than 10.

DISCUSSION

Recurrent falls are common in people with Huntington's disease. Individuals with a history of falls were found to have a greater fear of falling and lower functional mobility performance than those who did not have a history of falls. These measures may be useful in the identification of individuals with Huntington's disease who might benefit from a falls prevention programme. Copyright © 2014 John Wiley & Sons, Ltd.

Development of an ELISA assay for the quantification of soluble huntingtin in human blood cells

Background

Huntington’s disease (HD) is a monogenic disorder caused by an aberrant expansion of CAG repeats in the huntingtin gene (HTT). Pathogenesis is associated with expression of the mutant (mHTT) protein in the CNS, with its levels most likely related to disease progression and symptom severity. Since non-invasive methods to quantify HTT in the CNS do not exist, measuring amount of soluble HTT in peripheral cells represents an important step in development of disease-modifying interventions in HD.

Results

An ELISA assay using commercially available antibodies was developed to quantify HTT levels in complex matrices like mammalian cell cultures lysates and human samples. The immunoassay was optimized using a recombinant full-length HTT protein, and validated both on wild-type and mutant HTT species. The ability of the assay to detect significant variations of soluble HTT levels was evaluated using an HSP90 inhibitor that is known to enhance HTT degradation. Once optimized, the bioassay was applied to peripheral blood mononuclear cells (PBMCs) from HD patients, demonstrating good potential in tracking the disease course.

Conclusions

The method described here represents a validated, simple and rapid bio-molecular assay to evaluate soluble HTT levels in blood cells as useful tool in disease and pharmacodynamic marker identification for observational and clinical trials.

Disease-toxicant interactions in manganese exposed Huntington disease mice: early changes in striatal neuron morphology and dopamine metabolism

YAC128 Huntington's disease (HD) transgenic mice accumulate less manganese (Mn) in the striatum relative to wild-type (WT) littermates. We hypothesized that Mn and mutant Huntingtin (HTT) would exhibit gene-environment interactions at the level of neurochemistry and neuronal morphology. Twelve-week-old WT and YAC128 mice were exposed to MnCl₂-4H₂O (50 mg/kg) on days 0, 3 and 6. Striatal medium spiny neuron (MSN) morphology, as well as levels of dopamine (DA) and its metabolites (which are known to be sensitive to Mn-exposure), were analyzed at 13 weeks (7 days from initial exposure) and 16 weeks (28 days from initial exposure). No genotype-dependent differences in MSN morphology were apparent at 13 weeks. But at 16 weeks, a genotype effect was observed in YAC128 mice, manifested by an absence of the wild-type age-dependent increase in dendritic length and branching complexity. In addition, genotype-exposure interaction effects were observed for dendritic complexity measures as a function of distance from the soma, where only YAC128 mice were sensitive to Mn exposure. Furthermore, striatal DA levels were unaltered at 13 weeks by genotype or Mn exposure, but at 16 weeks, both Mn exposure and the HD genotype were associated with quantitatively similar reductions in DA and its metabolites. Interestingly, Mn exposure of YAC128 mice did not further decrease DA or its metabolites versus YAC128 vehicle exposed or Mn exposed WT mice. Taken together, these results demonstrate Mn-HD disease-toxicant interactions at the onset of striatal dendritic neuropathology in YAC128 mice. Our results identify the earliest pathological change in striatum of YAC128 mice as being between 13 to 16 weeks. Finally, we show that mutant HTT suppresses some Mn-dependent changes, such as decreased DA levels, while it exacerbates others, such as dendritic pathology.

Direct-to-Consumer Genetic Testing

Genetics has fascinated societies since ancient times, and references to traits or behaviors that appear to be shared or different among related individuals have permeated legends, literature, and popular culture. Biomedical advances from the past century, and particularly the discovery of the DNA double helix, the increasing numbers of links that were established between mutations and medical conditions or phenotypes, and technological advances that facilitated the sequencing of the human genome, catalyzed the development of genetic testing. Genetic tests were initially performed in health care facilities, interpreted by health care providers, and included the availability of counseling. Recent years have seen an increased availability of genetic tests that are offered by companies directly to consumers, a phenomenon that became known as direct-to-consumer genetic testing. Tests offered in this setting range from the ones that are also provided in health care establishments to tests known as ‘recreational genomics,’ and consumers directly receive the test results. In addition, testing in this context often does not involve the availability of counseling and, when this is provided, it frequently occurs on-line or over the phone. As a field situated at the interface between biotechnology, biomedical research, and social sciences, direct-to-consumer genetic testing opens multiple challenges that can be appropriately addressed only by developing a complex, inter-disciplinary framework.

Mitochondrial and metabolic-based protective strategies in Huntington's disease: the case of creatine and coenzyme Q

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by an expansion of CAG repeats in the HD gene encoding for huntingtin (Htt), resulting in progressive death of striatal neurons, with clinical symptoms of chorea, dementia and dramatic weight loss. Metabolic and mitochondrial dysfunction caused by the expanded polyglutamine sequence have been described along with other mechanisms of neurodegeneration previously described in human tissues and animal models of HD. In this review, we focus on mitochondrial and metabolic disturbances affecting both the central nervous system and peripheral cells, including mitochondrial DNA damage, mitochondrial complexes defects, loss of calcium homeostasis and transcriptional deregulation. Glucose abnormalities have also been described in peripheral tissues of HD patients and in HD animal and cellular models. Moreover, there are no effective neuroprotective treatments available in HD. Thus, we briefly discuss the role of creatine and coenzyme Q10 that target mitochondrial dysfunction and impaired bioenergetics and have been previously used in HD clinical trials.

A tale of two factors: what determines the rate of progression in Huntington's disease? A longitudinal MRI study

Over the past several years, increased attention has been devoted to understanding regionally selective brain changes that occur in Huntington's disease and their relationships to phenotypic variability. Clinical progression is also heterogeneous, and although CAG repeat length influences age of onset, its role, if any, in progression has been less clear. We evaluated progression in Huntington's disease using a novel longitudinal magnetic resonance imaging analysis. Our hypothesis was that the rate of brain atrophy is influenced by the age of onset of Huntington's disease. We scanned 22 patients with Huntington's disease at approximately 1-year intervals; individuals were divided into 1 of 3 groups, determined by the relative age of onset. We found significant differences in the rates of atrophy of cortex, white matter, and subcortical structures; patients who developed symptoms earlier demonstrated the most rapid rates of atrophy compared with those who developed symptoms during middle age or more advanced age. Rates of cortical atrophy were topologically variable, with the most rapid changes occurring in sensorimotor, posterior frontal, and portions of the parietal cortex. There were no significant differences in the rates of atrophy in basal ganglia structures. Although both CAG repeat length and age influenced the rate of change in some regions, there was no significant correlation in many regions. Rates of regional brain atrophy seem to be influenced by the age of onset of Huntington's disease symptoms and are only partially explained by CAG repeat length. These findings suggest that other genetic, epigenetic, and environmental factors play important roles in neurodegeneration in Huntington's disease.

What is the impact of education on Huntington's disease?

Huntington's disease (HD) is a neurodegenerative disease caused by a cytosine adenosine guanine (CAG) expansion in the huntingtin gene. The length of the triplet repeat is the most important factor in determining age of onset and the severity of the disease, but substantial variability of these parameters is attributed to other factors. To investigate the relationship between the years of education and the age at onset and the severity of the phenotype in patients with HD, we applied multiple linear regression analysis to examine the impact of education on the age at onset and the severity of the clinical scores assessed by the Unified Huntington's Disease Rating Scale (UHDRS) of 891 patients with HD from the multinational observational study "Registry" conducted by the European Huntintgton's Disease Network. The model was adjusted for CAG repeat length and age at the time of assessment. Patients with lengthier education exhibited earlier estimated age at onset but less severe clinical scores (motor = -3.6, P = 0.006; cognitive = 27.0, P < 0.001; behavioral = -3.0, P < 0.001; and functional capacity = 1.1 points, P < 0.001) than those with shorter education, after controlling for age and number of CAG repeats. These differences persisted throughout all quartiles of disease severity. An earlier recognition of symptoms and manifestations among the more educated patients could explain the earlier estimated age at onset in this group. The link between better clinical UHDRS scores and higher education might reflect a beneficial effect of education or its covariates on the course of HD.

Emerging drug therapies in Huntington's disease

Huntington's disease (HD) is a relentless neurodegenerative disease that results in profound disability through a triad of motor, cognitive and neuropsychiatric symptoms. At present, there are very few therapeutic interventions available with the exception of a limited number of drugs that offer mild symptomatic relief. Although the genetic basis of the disease has been identified, the mechanisms behind the cellular pathogenesis are still not clear and as a result no candidate drugs with the potential for disease modification have been found clinically until now. One of the major limitations in assessing the usefulness of drug treatments in HD is the lack of well-designed, double-blind, placebo-controlled clinical trials. Most studies have been open-label, using a small number of patients and tend to concentrate on the motor features of the disease, primarily the chorea. This review discusses the treatments now used for HD before evaluating the newer drugs at present being explored in both the clinic and in the laboratory in mouse models of the disease.

Differential susceptibility to striatal neurodegeneration induced by quinolinic acid and kainate in inbred, outbred and hybrid mouse strains

In mice, the genetic background determines susceptibility to hippocampal neurodegeneration induced by the excitotoxin kainic acid (KA). If genetic background plays as significant a role in the striatum, the area most affected in Huntington's disease (HD), it is important to characterize intrinsic differences in neuronal susceptibility in mouse strains used in HD models. This study was performed to investigate whether strain differences of different HD mouse models are determinants of striatal resistance to excitotoxicity. We conducted a survey of the susceptibility of striatal neurons to neurodegeneration induced by quinolinic acid and KA in six inbred, two outbred and two F1 hybrid (resistant*vulnerable) strains. These are the same strains in which we have assessed vulnerability to KA-induced hippocampal neurodegeneration. We found significant strain differences in response to both excitotoxins and, for the most part, the strain-dependent patterns of susceptibility to quinolinic acid and KA were similar and comparable to those previously found with KA-induced hippocampal neurodegeneration. There were some incongruities, suggesting that the genetic determinants may be different for the two forms of excitotoxicity or that there are important interacting factors. For example, the F1 hybrid strains exhibited neurodegeneration similar to their vulnerable parent, indicating that the vulnerable phenotype is dominant. This is in contrast to KA-induced hippocampal neurodegeneration, where F1 hybrids exhibit the resistant phenotype. These results are also of significance with regard to the issue of region-specific vulnerability in the context of different diseases in which genetic modifiers affect age of onset and/or disease progression.

Huntington disease in a 9-year-old boy: clinical course and neuropathologic examination

Huntington disease is a dominantly inherited, neurodegenerative disorder, usually with onset in the fourth to fifth decade of life but in a small proportion of patients before the age of 20 years. The early-onset form, juvenile Huntington disease, is clinically different from that of more common adult-onset forms and includes cognitive decline, parkinsonism, myoclonus, and seizures. We report a case of a boy with juvenile Huntington disease with a very early age at disease onset (3 years). The suspected clinical diagnosis was confirmed by DNA analysis, which revealed (CAG)(n) expansion into the range characteristic of juvenile Huntington disease (95 repeats). The clinical course of the disease was typical for the juvenile form of Huntington disease, but the diagnosis was not so obvious because there was no history of any neurodegenerative disorder in the family. The child died at the age of 11 years. The detailed neuropathologic investigations performed postmortem showed the characteristic features of Huntington disease. As the patient's de novo mutation was very unlikely to occur, genetic counseling and the possibility of predictive testing were proposed to the family. Indirect molecular data indicate the familial character of the disease, with strong anticipation of transmission.

Monozygotic Twins Suffering from Huntington’s Disease Show Different Cognitive and Behavioural Symptoms

Monozygotic male twins, carrying the same number of trinucleotide repeats in the IT 15 Huntington disease (HD) gene, showed a different clinical course. Patient 1 presented with anxiety and chorea at the age of 40. Patient 2 showed persecution paranoia and motor impersistence at the age of 42. Both patients were monitored for 30 months using currently recommended motor and behaviour scales. No differences were observed in motor scoring besides small interevaluation fluctuations. However, on the cognitive and behaviour scales, patient 1 showed a significant worsening when compared with patient 2. Our cases support the belief that the motor symptoms and signs in HD are highly dependent on the trinucleotide expansion. However, the differences in the evolution of mental status in our patients suggest that other still unknown environmental factors are important in the phenotypic expression of Huntington's disease.

Comparison of seizure phenotype and neurodegeneration induced by systemic kainic acid in inbred, outbred, and hybrid mouse strains

We assessed inbred, outbred and hybrid mouse strains for susceptibility to seizures and neurodegeneration induced by systemic administration of kainic acid (KA). Each strain showed a unique pattern of susceptibility to seizures as assessed by the dose necessary to induce continuous tonic clonic seizures, progression through six seizure levels, the number of mice that failed to satisfy seizure criteria, and seizure-induced mortality. In general, the C57BL/6, ICR, FVB/N, and BALB/c strains were resistant to seizures while the C57BL/10, DBA/2 J, and F1 C57BL/6*CBA/J strains were vulnerable. Neuronal cell death was quantified in four subfields of the hippocampus: CA3, the hilus of the dentate gyrus, CA1, and the dentate granule cell layer. Neurodegeneration was also semiquantitatively assessed in other brain regions including the neocortex, striatum, thalamus, hypothalamus and amygdala. Although there was variability in the extent of cell death within strains, there were significant differences in the amount of hippocampal cell death between strains and also different patterns of neurodegeneration in affected brain areas. In general, the C57BL/6, C57BL/10, and F1 C57BL/6*CBA/J strains were resistant to neurodegeneration while the FVB/N, ICR and DBA/2 J strains were vulnerable. The BALB/c strain was unique in that neurodegeneration was confined to the hippocampus. Consistent with previous findings, the resistant neurodegeneration phenotype was dominant in an F1 cross of resistant and vulnerable inbred strains. Our results, using a large number of mouse strains, definitively demonstrate that a mouse strain's seizure phenotype is not related to its neurodegeneration phenotype.

Huntingtin-interacting protein 1-mediated neuronal cell death occurs through intrinsic apoptotic pathways and mitochondrial alterations

Huntingtin interacting protein-1 (Hip1) is known to be associated with the N-terminal domain of huntingtin. Although Hip1 can induce apoptosis, the exact upstream signal transduction pathways have not been clarified yet. In the present study, we examined whether activation of intrinsic and/or extrinsic apoptotic pathways occurs during Hip1-mediated neuronal cell death. Overexpression of Hip1 induced cell death through caspase-3 activation in immortalized hippocampal neuroprogenitor cells. Interestingly, proteolytic processing of Hip1 into partial fragments was observed in response to Hip1 transfection and apoptosis-inducing drugs. Moreover, Hip1 was found to directly bind to and activate caspase-9. This promoted cytosolic release of cytochrome c and apoptosis-inducing factor via mitochondrial membrane perturbation. Furthermore, Hip1 could directly bind to Apaf-1, suggesting that the neurotoxic signals of Hip1 transmit through the intrinsic mitochondrial apoptotic pathways and the formation of apoptosome complex.

Enriched environments, experience-dependent plasticity and disorders of the nervous system

Behavioural, cellular and molecular studies have revealed significant effects of enriched environments on rodents and other species, and provided new insights into mechanisms of experience-dependent plasticity, including adult neurogenesis and synaptic plasticity. The demonstration that the onset and progression of Huntington's disease in transgenic mice is delayed by environmental enrichment has emphasized the importance of understanding both genetic and environmental factors in nervous system disorders, including those with Mendelian inheritance patterns. A range of rodent models of other brain disorders, including Alzheimer's disease and Parkinson's disease, fragile X and Down syndrome, as well as various forms of brain injury, have now been compared under enriched and standard housing conditions. Here, we review these findings on the environmental modulators of pathogenesis and gene-environment interactions in CNS disorders, and discuss their therapeutic implications.

Differential effects of voluntary physical exercise on behavioral and brain-derived neurotrophic factor expression deficits in Huntington's disease transgenic mice

Huntington's disease is a fatal neurodegenerative disorder caused by a mutation of the huntingtin gene and involves progressive motor abnormalities (including chorea), cognitive deficits (dementia) as well as psychiatric symptoms. We have previously demonstrated that environmental enrichment slows the onset and progression of Huntington's disease in transgenic mice. Here, we investigated the effects of enhanced physical exercise on disease progression and brain-derived neurotrophic factor expression. Standard-housed Huntington's disease mice developed phenotypic rear-paw clasping by 16 weeks of age, displayed abnormal rearing behavior, deficits in motor co-ordination and of spatial working memory. Huntington's disease mice with access to running wheels exhibited delayed onset of rear-paw clasping, normalized levels of rearing behavior and amelioration of the cognitive deficits. However, in contrast to our previous environmental enrichment studies, there was no rescue of motor coordination deficits in wheel-running Huntington's disease mice. An abnormal accumulation of brain-derived neurotrophic factor protein in the frontal cortex of Huntington's disease mice was unaffected by running. Striatal and hippocampal brain-derived neurotrophic factor protein levels were unchanged. Brain-derived neurotrophic factor mRNA levels were reduced in the anterior cortex, striatum and hippocampus of Huntington's disease mice, and only striatal deficits were ameliorated by running. Overall, we show that voluntary physical exercise delays the onset of Huntington's disease and the decline in cognitive ability. In addition, our results reveal that some aspects of hippocampal dependent memory are not entirely reliant on sustained hippocampal brain-derived neurotrophic factor expression.

Essential fatty acids and mental illness

The treatment of psychiatric illness requires novel pharmacological strategies. There is a growing body of evidence examining the role of neuronal phospholipid abnormalities in the pathogenesis of psychiatric illness, particularly in schizophrenia. However, work in other conditions like mood disorders are also showing interesting outcomes with EPA supplementation. Diseases that are considered to have a genetic basis may be significantly influenced by environmental factors including dietary supplementation. The suggestion that EFA supplementation may prevent the onset of symptoms of a psychiatric disease or aberrant behaviour needs longitudinal randomized controlled research. In recent years the focus has shifted from omega-6 to omega-3. It is true that western diets have far more omega-6 than omega-3. In the 1980s, there were positive outcomes in research studies using GLA in schizophrenia (Vaddadi et al., 1989). Future research needs to incorporate studies using pure GLA. Research should not be restricted to parent fatty acid (omega-3) supplementation alone but be expanded to include bioactive down-the-chain metabolites. The recent identification of novel omega-3 derived mediators such as resolvins and neuroprotectins, which are a highly bioactive (1-10 nMol range), may well have some role to play in psychiatric disorders; however this remains highly speculative at this stage.

GENE–ENVIRONMENT INTERACTIONS, NEURONAL DYSFUNCTION AND PATHOLOGICAL PLASTICITY IN HUNTINGTON'S DISEASE

Huntington's disease (HD) is a fatal autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. The dynamic mutation that causes the disease is common to numerous other brain disorders, which may share similar pathogenic mechanisms. Much progress has been made in the past decade in understanding how a trinucleotide (CAG) repeat expansion, encoding an expanded polyglutamine tract in the huntingtin protein, induces dysfunction at molecular and cellular levels. The present review integrates various lines of experimental evidence in an attempt to move towards a unifying mechanistic framework, which may explain the pathogenesis of HD, from molecular through to neuronal network and behavioural levels. Recent evidence, using transgenic mouse models, also suggests that environmental factors can modify the onset and progression of HD. The effects of specific environmental manipulations are discussed in the context of gene-environment interactions and experience-dependent plasticity in the healthy and diseased brain, particularly the cerebral cortex.

Cognitive disorders and neurogenesis deficits in Huntington's disease mice are rescued by fluoxetine

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeat encoding an extended polyglutamine tract in the huntingtin protein. Affected individuals display progressive motor, cognitive and psychiatric symptoms (including depression), leading to terminal decline. Given that transgenic HD mice have decreased hippocampal cell proliferation and that a deficit in neurogenesis has been postulated as an underlying cause of depression, we hypothesized that decreased hippocampal neurogenesis contributes to depressive symptoms and cognitive decline in HD. Fluoxetine, a serotonin-reuptake inhibitor commonly prescribed for the treatment of depression, is known to increase neurogenesis in the dentate gyrus of wild-type mouse hippocampus. Here we show that hippocampal-dependent cognitive and depressive-like behavioural symptoms occur in HD mice, and that the administration of fluoxetine produces a marked improvement in these deficits. Furthermore, fluoxetine was found to rescue deficits of neurogenesis and volume loss in the dentate gyrus of HD mice.

Novel therapeutic targets for Huntington’s disease

Huntington's disease (HD) is a fatal autosomal-dominant disorder involving progressive motor, cognitive and psychiatric symptoms. HD is one of a large family of neurodegenerative diseases caused by a trinucleotide (CAG) repeat mutation, encoding an expanded tract of glutamines in the disease protein. HD was one of the first neurological disorders for which accurate transgenic models were created, allowing mechanisms of pathogenesis to be explored at molecular, cellular and behavioural levels. In the last decade, the understanding of molecular and cellular changes which occur in HD prior to onset of symptoms, and at early and late stages of disease progression, has been greatly expanded. A wide range of potential molecular targets for therapeutic intervention have been identified, associated with a variety of cellular processes including gene transcription, protein trafficking, protein degradation, protein-protein interactions, glutamatergic synaptic transmission, presynaptic signalling, postsynaptic signalling, synaptic plasticity, dopaminergic and neurotrophic modulation of synaptic function, experience-dependent neurogenesis, mitochondrial function and oxidative metabolism. Presymptomatic testing for the HD gene mutation necessitates future development of novel therapeutics aimed at delaying onset of symptoms, as well as slowing or reversing disease progression.

Nature, nurture and neurology: gene-environment interactions in neurodegenerative disease. FEBS Anniversary Prize Lecture delivered on 27 June 2004 at the 29th FEBS Congress in Warsaw

Neurodegenerative disorders, such as Huntington's, Alzheimer's, and Parkinson's diseases, affect millions of people worldwide and currently there are few effective treatments and no cures for these diseases. Transgenic mice expressing human transgenes for huntingtin, amyloid precursor protein, and other genes associated with familial forms of neurodegenerative disease in humans provide remarkable tools for studying neurodegeneration because they mimic many of the pathological and behavioural features of the human conditions. One of the recurring themes revealed by these various transgenic models is that different diseases may share similar molecular and cellular mechanisms of pathogenesis. Cellular mechanisms known to be disrupted at early stages in multiple neurodegenerative disorders include gene expression, protein interactions (manifesting as pathological protein aggregation and disrupted signaling), synaptic function and plasticity. Recent work in mouse models of Huntington's disease has shown that enriching the environment of transgenic animals delays the onset and slows the progression of Huntington's disease-associated motor and cognitive symptoms. Environmental enrichment is known to induce various molecular and cellular changes in specific brain regions of wild-type animals, including altered gene expression profiles, enhanced neurogenesis and synaptic plasticity. The promising effects of environmental stimulation, demonstrated recently in models of neurodegenerative disease, suggest that therapy based on the principles of environmental enrichment might benefit disease sufferers and provide insight into possible mechanisms of neurodegeneration and subsequent identification of novel therapeutic targets. Here, we review the studies of environmental enrichment relevant to some major neurodegenerative diseases and discuss their research and clinical implications.

Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent

Previously, we had reported that hippocampal susceptibility to the neurotoxic effects of excitotoxin administration is strain dependent [Schauwecker and Steward, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 4103]. However, it has been unclear whether strain-related gene products may play a similar role in providing protection against drugs that produce striatal lesions. The present series of experiments sought to elucidate whether genetic background alters neuronal viability within the striatum following metabolic or excitotoxic injury. Thus, we have examined the effect of mouse strain on susceptibility to striatal injury using well-characterized animal models of Huntington's disease by examining whether C57BL/6 mice, previously identified as resistant to excitotoxin-induced hippocampal cell death, are resistant to quinolinate, malonate, and 3-nitropropionic acid (3-NP). Intrastriatal injection of either malonate or quinolinate and systemic administration of 3-NP resulted in significantly smaller striatal lesions in C57BL/6 mice as compared to FVB/N mice, previously identified as susceptible to hippocampal excitotoxic injury. The existence of an animal strain with decreased resistance to striatal lesions suggests that there are mediating factors involved in the preferential vulnerability of the striatum to neurotoxic lesioning. The identification of these factors could provide strategies for therapeutic intervention in Huntington's disease.

Genomes for medicine

We have the human genome sequence. It is freely available, accurate and nearly complete. But is the genome ready for medicine? The new resource is already changing genetic research strategies to find information of medical value. Now we need high-quality annotation of all the functionally important sequences and the variations within them that contribute to health and disease. To achieve this, we need more genome sequences, systematic experimental analyses, and extensive information on human phenotypes. Flexible and user-friendly access to well-annotated genomes will create an environment for innovation, and the potential for unlimited use of sequencing in biomedical research and practice.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.