Engagement with genetic discrimination: concerns and experiences in the context of Huntington disease

It has been over 20 years since the inception of predictive testing for Huntington disease (HD), yet the social implications of knowing one's genetic risk for HD have not been fully explored. Genetic discrimination (GD) is a potential risk associated with predictive testing. Although anecdotal reports of GD have been documented, there is a paucity of research on the nature and experiences of GD in the context of HD. The purpose of this study was to describe the concerns and experiences of GD in the HD community. Semistructured interviews were conducted with 45 genetically tested and 10 untested individuals and analyzed using grounded theory methods. Our findings demonstrate that a majority of individuals were concerned about (37/55) and experienced GD (32/55) across a variety of contexts that extend beyond the traditionally examined contexts of insurance and employment to include family, social, government, and health-care domains. We describe a process of engagement with GD in which individuals formed meaningful interpretations of GD and personalized its risk and consequences in their lives. Our findings provide an insight into some of the specific processes and factors influencing engagement with GD. These results help identify areas where more education and support is needed and provide direction to genetic professionals supporting their clients as they confront issues of GD and genetic testing.

Genotypic Approaches to Therapy in Children: A National Active Surveillance Network (GATC) to Study the Pharmacogenomics of Severe Adverse Drug Reactions in Children

A striking failure of modern medicine is the debilitating and lethal consequences of adverse drug reactions (ADRs), which rank as one of the top 10 leading causes of death and illness in the developed world with direct medical costs of 137-177 billion annually US dollars in the USA. Although many factors influence the effect of medications (e.g., age, organ function, drug interactions), genetic factors account for 20% to 95% of drug response variability and play a significant role in the incidence and severity of ADRs. The field of pharmacogenomics seeks to identify genetic factors responsible for individual differences in drug efficacy and ADRs. Pharmacogenomics has led to several genetic tests that provide clinical dosing recommendations. The Genetic Approaches to Therapy in Children (GATC) project is a national project established in Canada to identify novel predictive genomic markers for severe ADRs in children. An ADR surveillance network has been established in eight of Canada's major children's hospitals, serving up to 75% of all Canadian children. The goal of the project is to identify patients experiencing specific ADRs and matched controls, collect DNA samples, and apply genomics-based technologies to identify ADR-associated genetic markers with the goal of preventing serious ADRs in susceptible children.

Regulation of huntingtin palmitoylation and its role in Huntington Diseases

Huntington’s Disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, cognitive, and psychiatric deficits and selective neuronal cell death. The causative mutation in HD is an expansion of the N-terminal polyglutamine tract in huntingtin (htt), which results in altered trafficking of mutant htt and enhanced toxicity to striatal neurons. Post-translational modification by the lipid palmitate has been shown to play a critical role in the trafficking and function of many proteins, including htt. It has been previously demonstrated that huntingtin-interacting protein 14 (HIP14) is a palmitoyl transferase that palmitoylates htt. Previous characterization of HIP14 demonstrated a reduced interaction with mutant htt resulting in reduced palmitoylation, suggesting that palmitoylation may play a role in the pathogenesis of HD. Most recently, we have identified cysteine 214 as a major site of htt palmitoylation in the N-terminus of htt, close to the site of polyglutamine expansion. It was demonstrated that mutation of this site, rendering htt palmitoylation-resistant, results in increased neuronal toxicity, enhanced inclusion formation, and in altered trafficking of htt. Remarkably, mutation of the palmitoylation site in wild type htt also resulted in enhanced toxicity similar to that seen in mutant htt. Together, these previous studies suggest a critical role of palmitoylation in htt trafficking and function. Based on this preliminary work, we are characterizing the enzymatic regulation of huntingtin palmitoylation. Exploring htt palmitoylation in a number of existing and new mouse models imparts key insights into how this process is regulated in vivo. We are also exploring the relationship between palmitoylation and other post-translational modifications of htt. These studies will lead to an understanding of the regulation of palmitoylation of huntingtin in vivo, as well as setting the precedent to understand the general role of palmitoylation in a wide range of other human diseases. Ultimately, this may lead to identification of new therapeutic targets and treatments for patients. F.B.J.Y. is supported by a Canadian Institutes of Health Research Walter and Jessie Boyd & Charles Scriver - Child and Family Research Institute - UBC MD/PhD Studentship Award. She also receives funding from the Michael Smith Foundation for Health Research as a Junior Trainee.

Beyond disgust: impaired recognition of negative emotions prior to diagnosis in Huntington's disease

Previous studies of emotion recognition suggest that detection of disgust relies on processing within the basal ganglia and insula. Research involving individuals with symptomatic and pre-diagnostic Huntington's disease (HD), a disease with known basal ganglia atrophy, has generally indicated a relative impairment in recognizing disgust. However, some data have suggested that recognition of other emotions (particularly fear and anger) may also be affected in HD, and a recent study found fear recognition deficits in the absence of other emotion-recognition impairments, including disgust. To further examine emotion recognition in HD, we administered a computerized facial emotion recognition task to 475 individuals with the HD CAG expansion and 57 individuals without. Logistic regression was used to examine associations of emotion recognition performance with estimated proximity to clinical diagnosis (based on CAG repeat length and current age) and striatal volumes. Recognition of anger, disgust, fear, sadness and surprise (but not happiness) was associated with estimated years to clinical diagnosis; performance was unrelated to striatal volumes. Compared to a CAG-normal control group, the CAG-expanded group demonstrated significantly less accurate recognition of all negative emotions (anger, disgust, fear, sadness). Additionally, participants with more pronounced motor signs of HD were significantly less accurate at recognizing negative emotions than were individuals with fewer motor signs. Findings indicate that recognition of all negative emotions declines early in the disease process, and poorer performance is associated with closer proximity to clinical diagnosis. In contrast to previous results, we found no evidence of relative impairments in recognizing disgust or fear, and no evidence to support a link between the striatum and disgust recognition.

Cholesterol biosynthesis pathway is disturbed in YAC128 mice and is modulated by huntingtin mutation

Our recent analyses of the cholesterol biosynthetic pathway in Huntington's disease (HD) cells, in the R6/2 huntingtin-fragment mouse model of HD as well as in human tissues have provided the first evidence of altered activity of this pathway in genetically identifiable HD samples. Here we report that these changes also occur in the full-length-huntingtin YAC128 (yeast artificial chromosome) mouse model, which shows a consistent reduction in the activity or levels of multiple components of the cholesterogenic pathway. We also show that this phenotype is progressive and is specific for the brain region most affected in HD. Mice over-expressing the wild-type protein with 18 CAG (YAC18 mice) show the opposite phenotype with higher activity of the cholesterol biosynthetic pathway compared with littermate mice. Finally, we report that plasma levels of cholesterol, its precursors and its brain-derived catabolite 24-S-hydroxycholesterol in YAC mice mirror brain biosynthetic levels supporting further investigation of their potential as peripheral biomarkers in HD.

Managing genetic discrimination: strategies used by individuals found to have the Huntington disease mutation

The introduction of predictive testing for Huntington disease (HD) over 20 years ago has led to the advent of a new group of individuals found to have the HD mutation that are currently asymptomatic, yet destined in all likelihood to become affected at some point in the future. Genetic discrimination, a social risk associated with predictive testing, is the differential treatment of individuals based on genotypic difference rather than physical characteristics. While evidence for genetic discrimination exists, little is known about how individuals found to have the HD mutation cope with the potential for or experiences of genetic discrimination. The purpose of this study was to explore how individuals found to have the HD mutation manage the risk and experience of genetic discrimination. Semi-structured individual interviews were conducted with 37 individuals who were found to have the HD mutation and analysed using grounded theory methods. The findings suggest four main strategies: "keeping low", minimizing, pre-empting and confronting genetic discrimination. Strategies varied depending on individuals' level of engagement with genetic discrimination and the nature of the experience (actual experience of genetic discrimination or concern for its potential). This exploratory framework may explain the variation in approaches and reactions to genetic discrimination among individuals living with an increased risk for HD and may offer insight for persons at risk for other late-onset genetic diseases to cope with genetic discrimination.

Glutamate receptor abnormalities in the YAC128 transgenic mouse model of Huntington's disease

A yeast artificial chromosome (YAC) mouse model of Huntington's disease (YAC128) develops motor abnormalities, age-dependent striatal atrophy and neuronal loss. Alteration of neurotransmitter receptors, particularly glutamate and dopamine receptors, is a pathological hallmark of Huntington's disease. We therefore analyzed neurotransmitter receptors in symptomatic YAC128 Huntington's disease mice. We found significant increases in N-methyl-d-aspartate, AMPA and metabotropic glutamate receptor binding, which were not due to increases in receptor subunit mRNA expression levels. Subcellular fractionation analysis revealed increased levels of glutamate receptor subunits in synaptic membrane fractions from YAC128 mice. We found no changes in dopamine, GABA or adenosine receptor binding, nor did we see alterations in dopamine D1, D2 or adenosine A2a receptor mRNA levels. The receptor abnormalities in YAC128 transgenic mice thus appear limited to glutamate receptors. We also found a significant decrease in preproenkephalin mRNA in the striatum of YAC128 mice, which contrasts with the lack of change in levels of mRNA encoding neurotransmitter receptors. Taken together, the abnormal and selective increases in glutamate receptor subunit expression and binding are not due to increases in receptor subunit expression and may exert detrimental effects. The decrease in preproenkephalin mRNA suggests a selective transcriptional deficit, as opposed to neuronal loss, and could additionally contribute to the abnormal motor symptoms in YAC128 mice.

Testicular degeneration in Huntington disease

Huntington disease (HD) is an adult onset, neurodegenerative disorder that results from CAG expansion in the HD gene. Recent work has demonstrated testicular degeneration in mouse models of HD and alterations in the hypothalamic-pituitary-gonadal (HPG) axis in HD patients. Here, we show that HD patients have specific testicular pathology with reduced numbers of germ cells and abnormal seminiferous tubule morphology. In the YAC128 mouse model, testicular degeneration develops prior to 12 months of age, but at 12 months, there is no evidence for decreased testosterone levels or loss of GnRH neurons in the hypothalamus. This suggests that testicular pathology results from a direct toxic effect of mutant huntingtin in the testis and is supported by the fact that huntingtin is highly expressed in the affected cell populations in the testis. Understanding the pathogenesis of HD in the testis may reveal common critical pathways which lead to degeneration in both the brain and testis.

CAG-encoded polyglutamine length polymorphism in the human genome

Background

Expansion of polyglutamine-encoding CAG trinucleotide repeats has been identified as the pathogenic mutation in nine different genes associated with neurodegenerative disorders. The majority of individuals clinically diagnosed with spinocerebellar ataxia do not have mutations within known disease genes, and it is likely that additional ataxias or Huntington disease-like disorders will be found to be caused by this common mutational mechanism. We set out to determine the length distributions of CAG-polyglutamine tracts for the entire human genome in a set of healthy individuals in order to characterize the nature of polyglutamine repeat length variation across the human genome, to establish the background against which pathogenic repeat expansions can be detected, and to prioritize candidate genes for repeat expansion disorders.

Results

We found that repeats, including those in known disease genes, have unique distributions of glutamine tract lengths, as measured by fragment analysis of PCR-amplified repeat regions. This emphasizes the need to characterize each distribution and avoid making generalizations between loci. The best predictors of known disease genes were occurrence of a long CAG-tract uninterrupted by CAA codons in their reference genome sequence, and high glutamine tract length variance in the normal population. We used these parameters to identify eight priority candidate genes for polyglutamine expansion disorders. Twelve CAG-polyglutamine repeats were invariant and these can likely be excluded as candidates. We outline some confusion in the literature about this type of data, difficulties in comparing such data between publications, and its application to studies of disease prevalence in different populations. Analysis of Gene Ontology-based functions of CAG-polyglutamine-containing genes provided a visual framework for interpretation of these genes' functions. All nine known disease genes were involved in DNA-dependent regulation of transcription or in neurogenesis, as were all of the well-characterized priority candidate genes.

Conclusion

This publication makes freely available the normal distributions of CAG-polyglutamine repeats in the human genome. Using these background distributions, against which pathogenic expansions can be identified, we have begun screening for mutations in individuals clinically diagnosed with novel forms of spinocerebellar ataxia or Huntington disease-like disorders who do not have identified mutations within the known disease-associated genes.

Mutant huntingtin's effects on striatal gene expression in mice recapitulate changes observed in human Huntington's disease brain and do not differ with mutant huntingtin length or wild-type huntingtin dosage

To test the hypotheses that mutant huntingtin protein length and wild-type huntingtin dosage have important effects on disease-related transcriptional dysfunction, we compared the changes in mRNA in seven genetic mouse models of Huntington's disease (HD) and postmortem human HD caudate. Transgenic models expressing short N-terminal fragments of mutant huntingtin (R6/1 and R6/2 mice) exhibited the most rapid effects on gene expression, consistent with previous studies. Although changes in the brains of knock-in and full-length transgenic models of HD took longer to appear, 15- and 22-month CHL2(Q150/Q150), 18-month Hdh(Q92/Q92) and 2-year-old YAC128 animals also exhibited significant HD-like mRNA signatures. Whereas it was expected that the expression of full-length huntingtin transprotein might result in unique gene expression changes compared with those caused by the expression of an N-terminal huntingtin fragment, no discernable differences between full-length and fragment models were detected. In addition, very high correlations between the signatures of mice expressing normal levels of wild-type huntingtin and mice in which the wild-type protein is absent suggest a limited effect of the wild-type protein to change basal gene expression or to influence the qualitative disease-related effect of mutant huntingtin. The combined analysis of mouse and human HD transcriptomes provides important temporal and mechanistic insights into the process by which mutant huntingtin kills striatal neurons. In addition, the discovery that several available lines of HD mice faithfully recapitulate the gene expression signature of the human disorder provides a novel aspect of validation with respect to their use in preclinical therapeutic trials.

Altered NMDA receptor trafficking in a yeast artificial chromosome transgenic mouse model of Huntington's disease

Overactivation of NMDA receptors (NMDARs) is believed to play a role in degeneration of striatal medium-sized spiny neurons (MSNs) in Huntington's disease (HD). This hereditary disorder is caused by an expansion >35 in the polyglutamine (polyQ) region of the protein huntingtin (htt). Previous work has shown that NMDAR current, calcium signaling, and/or toxicity are enhanced in striatal MSNs in a variety of transgenic mice and cellular models of HD, but whether the enhancement is specific for MSNs or correlated with mutant htt (mhtt) polyQ length is not known. Furthermore, the mechanism underlying the increase in NMDAR activity has not been elucidated. Here we report polyQ length-dependent enhancement of peak NMDAR current density by mhtt in cultured MSNs, but not cortical neurons, from the yeast artificial chromosome (YAC) transgenic HD mouse model. We also observed a shift of NMDAR subunits NR1 and NR2B from internal pools to the plasma membrane and a significantly faster rate of NMDAR insertion to the surface in YAC72 MSNs. In comparing YAC72 with wild-type striatal tissue, subcellular fractionation revealed a relative enrichment of NR1 C2'-containing NMDARs in the vesicle/microsome-enriched fraction, and coimmunoprecipitation experiments demonstrated an increased proportion of NR1 C2' isoforms associated with NR2 subunits, which may contribute to faster forward trafficking of these receptors. Our results suggest that altered NMDAR trafficking may underlie potentiation of NMDAR-mediated current and toxicity in the YAC72 HD mouse model. This polyQ length-dependent, neuronal-specific change in NMDAR activity induced by mhtt may contribute to selective neuronal degeneration in HD.

Physiologically regulated transgenic ABCA1 does not reduce amyloid burden or amyloid-β peptide levels in vivo

ABCA1-deficient mice have low levels of poorly lipidated apolipoprotein E (apoE) and exhibit increased amyloid load. To test whether excess ABCA1 protects from amyloid deposition, we crossed APP/PS1 mice to ABCA1 bacterial artificial chromosome (BAC) transgenic mice. Compared with wild-type animals, the ABCA1 BAC led to a 50% increase in cortical ABCA1 protein and a 15% increase in apoE abundance, demonstrating that this BAC supports modest ABCA1 overexpression in brain. However, this was observed only in animals that do not deposit amyloid. Comparison of ABCA1/APP/PS1 mice with APP/PS1 controls revealed no differences in levels of brain ABCA1 protein, amyloid, Abeta, or apoE, despite clear retention of ABCA1 overexpression in the livers of these animals. To further investigate ABCA1 expression in the amyloid-containing brain, we then compared ABCA1 mRNA and protein levels in young and aged cortex and cerebellum of APP/PS1 and ABCA1/APP/PS1 animals. Compared with APP/PS1 controls, aged ABCA1/APP/PS1 mice exhibited increased ABCA1 mRNA, but not protein, selectively in cortex. Additionally, ABCA1 mRNA levels were not increased before amyloid deposition but were induced only in the presence of extensive Abeta and amyloid levels. These data suggest that an induction of ABCA1 expression may be associated with late-stage Alzheimer's neuropathology.

Loss-of-function mutations in the Nav1.7 gene underlie congenital indifference to pain in multiple human populations

Congenital indifference to pain (CIP) is a rare condition in which patients have severely impaired pain perception, but are otherwise essentially normal. We identified and collected DNA from individuals from nine families of seven different nationalities in which the affected individuals meet the diagnostic criteria for CIP. Using homozygosity mapping and haplotype sharing methods, we narrowed the CIP locus to chromosome 2q24-q31, a region known to contain a cluster of voltage-gated sodium channel genes. From these prioritized candidate sodium channels, we identified 10 mutations in the SCN9A gene encoding the sodium channel protein Nav1.7. The mutations completely co-segregated with the disease phenotype, and nine of these SCN9A mutations resulted in truncation and loss-of-function of the Nav1.7 channel. These genetic data further support the evidence that Nav1.7 plays an essential role in mediating pain in humans, and that SCN9A mutations identified in multiple different populations underlie CIP.

NMDA receptor function and NMDA receptor-dependent phosphorylation of huntingtin is altered by the endocytic protein HIP1

Huntingtin-interacting protein 1 (HIP1) is an endocytic adaptor protein that plays a role in clathrin-mediated endocytosis and the ligand-induced internalization of AMPA receptors (AMPARs) (Metzler et al., 2003). In the present study, we investigated the role of HIP1 in NMDA receptor (NMDAR) function by analyzing NMDA-dependent transport and NMDA-induced excitotoxicity in neurons from HIP1-/- mice. HIP1 colocalizes with NMDARs in hippocampal and cortical neurons and affinity purifies with NMDARs by GST (glutathione S-transferase) pull down and coimmunoprecipitation. A profound decrease in NMDA-induced AMPAR internalization of 75% occurs in neurons from HIP1-/- mice compared with wild type, using a quantitative single-cell-based internalization assay. This defect in NMDA-dependent removal of surface AMPARs is in agreement with the observed defect in long-term depression induction in hippocampal brain slices of HIP1-/- mice and supports a role of HIP1 in AMPAR internalization in vivo. HIP1-/- neurons are partially protected from NMDA-induced excitotoxicity as assessed by LDH (lactate dehydrogenase) release, TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling) and caspase-3 activation assays, which points to a role of HIP1 in NMDA-induced cell death. Interestingly, phosphorylation of Akt and its substrate huntingtin (htt) decreases during NMDA-induced excitotoxicity by 48 and 31%, respectively. This decrease is significantly modulated by HIP1, resulting in 94 and 48% changes in P-Akt and P-htt levels in HIP1-/- neurons, respectively. In summary, we have shown that HIP1 influences important NMDAR functions and that both HIP1 and htt participate in NMDA-induced cell death. These findings may provide novel insights into the cellular mechanisms underlying enhanced NMDA-induced excitotoxicity in Huntington's disease.

Pharmacogenomics and its implications for autoimmune disease

A striking failure of modern medicine is the debilitating and lethal consequences of adverse drug reactions (ADRs) which rank as one of the top ten leading causes of death and illness in the developed world with direct medical costs of 137-177 billion US dollars annually in the USA. Although many factors influence the effect of medications (i.e. age, organ function, drug interactions), genetic factors account for 20-95% of drug response variability and play a significant role in the incidence and severity of ADRs. The field of pharmacogenomics seeks to identify genetic factors responsible for individual differences in drug efficacy and adverse drug reactions. Pharmacogenomics has led to several genetic tests that provide clinical dosing recommendations. For autoimmune disease, pharmacogenomics has led to several DNA-based tests to improve drug selection, optimize dosing, and minimize the risk of toxicity. The 'GATC' project is a nation-wide project established in Canada to identify novel predictive genomic markers of severe ADRs in children. An ADR surveillance network has been established in all of Canada's major children's hospitals, serving up to 75% of all Canadian children. The goal of the project is to identify patients experiencing specific ADRs, collect DNA samples, and apply genomics-based technologies to identify ADR-associated genetic markers.

Beta-cell ABCA1 influences insulin secretion, glucose homeostasis and response to thiazolidinedione treatment

Type 2 diabetes is characterized by both peripheral insulin resistance and reduced insulin secretion by beta-cells. The reasons for beta-cell dysfunction in this disease are incompletely understood but may include the accumulation of toxic lipids within this cell type. We examined the role of Abca1, a cellular cholesterol transporter, in cholesterol homeostasis and insulin secretion in beta-cells. Mice with specific inactivation of Abca1 in beta-cells had markedly impaired glucose tolerance and defective insulin secretion but normal insulin sensitivity. Islets isolated from these mice showed altered cholesterol homeostasis and impaired insulin secretion in vitro. We found that rosiglitazone, an activator of the peroxisome proliferator-activated receptor-gamma, which upregulates Abca1 in beta-cells, requires beta-cell Abca1 for its beneficial effects on glucose tolerance. These experiments establish a new role for Abca1 in beta-cell cholesterol homeostasis and insulin secretion, and suggest that cholesterol accumulation may contribute to beta-cell dysfunction in type 2 diabetes.

Verbal episodic memory declines prior to diagnosis in Huntington's disease

Previous studies of verbal episodic memory in pre-diagnostic Huntington's disease (HD) have yielded mixed results; some evidence suggests that memory decline is evident prior to the onset of pronounced neurological signs of HD, whereas other data indicate that memory function remains normal throughout the pre-diagnostic period. This study examines verbal episodic memory in a sample of CAG expanded individuals who have not yet been clinically diagnosed, and who represent a wide range of points along the continuum from health to disease. The Hopkins Verbal Learning Test-Revised (HVLT-R) was administered to 479 participants (428 with the HD CAG expansion and 51 without), and performance was compared to neurobiological indices of disease progression, including a DNA-based estimate of proximity to clinical diagnosis, magnetic resonance imaging (MRI) measures of striatal volume, and neurologist ratings of motor signs. Lower HVLT-R scores were associated with closer proximity to clinical diagnosis and smaller striatal volumes; these relationships were found even in groups with no neurological signs of HD. The CAG expanded groups, including those with only minimal neurological signs, had significantly lower HVLT-R scores than the control group, and performance was worse in sub-groups that had more neurological signs consistent with HD. These findings indicate that verbal episodic memory is affected in early pre-diagnostic HD and may decline as striatal volumes decrease and individuals approach the motor diagnostic threshold.

Chapter 15 Juvenile amyotrophic lateral sclerosis

Several forms of genetically defined juvenile amy-otrophic lateral sclerosis (ALS) have now been charac-terized and discussion of these conditions will form the basis for this chapter. ALS2 is an autosomal recessive form of ALS with a juvenile onset and very slow progression that mapped to chromosome 2q33. Nine different mutations have been identified in the ALS2 gene that result in premature stop codons, suggesting a loss of function in the gene product, alsin. The alsin protein is thought to function as a guanine-nucleotide exchange factor for GTPases and may play a role in vesicle transport or membrane trafficking processes. ALS4 is an autosomal dominant form of juvenile onset ALS associated with slow progression, severe muscle weakness and pyramidal signs, in the absence of bulbar and sensory abnormalities. Mutations in the SETX gene cause ALS4, and the SETX gene product senataxin may have DNA and RNA helicase activity and play a role in the regulation of RNA and/or DNA in the cell. A third form of juvenile-onset ALS (ALS5) is associated with slowly progressing lower motor neuron signs (weak-ness and atrophy) initially of the hands and feet, with eventual bulbar involvement. Progressive upper motor neuron disease becomes more obvious with time. ALS5 has been linked to a 6 cM region of chromosome 15q15.1-q21.1, but the causative gene mutation for ALS5 has yet to be identified. The high degree of clin-ical and genetic heterogeneity in the various forms of juvenile ALS can make differential diagnosis difficult, other genetic disorders that must be considered include: spinal muscular atrophy, hereditary spastic paraplegia, SBMA, GM2 gangliosidosis and the hereditary motor neuronopathies/motor forms of Charcot-Marie-Tooth disease. Acquired disorders that must also be consid-ered include heavy metal intoxications (especially lead), multifocal motor neuropathy, paraneoplastic syndromes, vitamin deficiencies (B12) and infections (HTLV-II, HIV and poliomyelitis).

Expression of LPL in Endothelial-Intact Artery Results in Lipid Deposition and Vascular Cell Adhesion Molecule-1 Upregulation in Both LPL and ApoE-Deficient Mice

Objective— Overexpression of lipoprotein lipase (LPL) in deendothelialized artery led to profound localized lipid deposition. In this study the role of LPL in atherogenesis in endothelial-intact carotid arteries was assessed in genetically hyperlipidemic LPL- and ApoE-deficient mice.

Methods and Results— Human wild-type LPL (hLPLwt), catalytically inactive LPL (hLPL194), or control alkaline phosphatase (hAP) were expressed in endothelial-intact carotid arteries via adenoviral vectors. Compared with Ad-hAP, lipid deposition in the arterial wall increased 10.0- and 5.1-fold for Ad-hLPLwt and Ad-hLPL194 in LPL-deficient mice, and 10.6- and 6.2-fold in ApoE-deficient mice, respectively. Vascular cell adhesion molecule-1 (VCAM-1) was upregulated in Ad-hLPLwt and Ad-hLPL194 transferred arteries.

Conclusions— Endothelial cell associated LPL, either active or inactive, in the arterial wall is a strong proatherosclerotic factor in both LPL- and ApoE-deficient mice.

Phenotypic abnormalities in the YAC128 mouse model of Huntington disease are penetrant on multiple genetic backgrounds and modulated by strain

The YAC128 mouse model of Huntington disease (HD) exhibits motor abnormalities, cognitive dysfunction and selective neuropathology which are similar to the human disease. Backcrossing YAC128 mice from the FVB/N strain onto the C57BL/6 strain and the 129 strain revealed that striatal volume loss and motor dysfunction are penetrant on all three genetic backgrounds. The severity of HD-like phenotypes in these mice is modulated by strain and this variation is not accounted for by differences in mutant huntingtin expression. In contrast, nuclear localization of mutant htt is modulated by strain and is correlated with the severity of neuropathology. Differences in phenotypic severity between the strains provide the opportunity to identify modifier genes which could impact the pathogenesis of HD. Importantly, the demonstration of penetrance across all three strains permits examining the effect of specific genes on the phenotypic severity in YAC128 mice without necessarily backcrossing onto the FVB/N strain background.

Predictive testing for Huntington disease: interpretation and significance of intermediate alleles

Direct mutation analysis for Huntington disease (HD) became possible in 1993 with the identification of an expanded CAG trinucleotide repeat as the mutation underlying the disease. Expansion of CAG length beyond 35 repeats may be associated with the clinical presentation of HD. HD has never been seen in a person with a CAG size of <36 repeats. Intermediate alleles are defined as being below the affected CAG range but have the potential to expand to >35 CAG repeats within one generation. Thus, children of intermediate allele carriers have a low risk of developing HD. Currently, the intermediate allele range for HD is between 27 and 35 CAG repeats. In this study, we review the current knowledge on intermediate alleles for HD including the CAG repeat range, the intermediate allele frequency, and the clinical implications of an intermediate allele predictive test result. The factors influencing CAG repeat expansion, including the CAG size of the intermediate allele, the sex and age of the transmitting parent, the family history, and the HD gene sequence and haplotype, will also be reviewed.

Mitochondrial-dependent Ca2+ handling in Huntington's disease striatal cells: effect of histone deacetylase inhibitors

Evidence suggests that neuronal dysfunction in Huntington's disease (HD) striatum involves deficits in mitochondrial function and in Ca2+ handling. However, the relationship between mitochondria and Ca2+ handling has been incompletely studied in intact HD striatal cells. Treatment with histone deacetylase (HDAC) inhibitors reduces cell death in HD models, but the effects of this promising therapy on cellular function are mostly unknown. Here, we use real-time functional imaging of intracellular Ca2+ and mitochondrial membrane potential to explore the role of in situ HD mitochondria in Ca2+ handling. Immortalized striatal (STHdh) cells and striatal neurons from transgenic mice, expressing full-length mutant huntingtin (Htt), were used to model HD. We show that (1) active glycolysis in STHdh cells occludes the mitochondrial role in Ca2+ handling as well as the effects of mitochondrial inhibitors, (2) STHdh cells and striatal neurons in the absence of glycolysis are critically dependent on oxidative phosphorylation for energy-dependent Ca2+ handling, (3) expression of full-length mutant Htt is associated with deficits in mitochondrial-dependent Ca2+ handling that can be ameliorated by treatment with HDAC inhibitors (treatment with trichostatin A or sodium butyrate decreases the proportion of STHdh cells losing Ca2+ homeostasis after Ca2+-ionophore challenging, and accelerates the restoration of intracellular Ca2+ in striatal neurons challenged with NMDA), and (4) neurons with different response patterns to NMDA receptor activation exhibit different average somatic areas and are differentially affected by treatment with HDAC inhibitors, suggesting subpopulation or functional state specificity. These findings indicate that neuroprotection induced by HDAC inhibitors involves more efficient Ca2+ handling, thus improving the neuronal ability to cope with excitotoxic stimuli.

Huntingtin inhibits caspase-3 activation

Huntington's disease results from a mutation in the HD gene encoding for the protein huntingtin. The function of huntingtin, although beginning to be elucidated, remains largely unclear. To probe the prosurvival function of huntingtin, we modulate levels of wild-type huntingtin in a number of cellular and in vivo models. Huntingtin depletion resulted in caspase-3 activation, and overexpression of huntingtin resulted in caspase-3 inhibition. Additionally, we demonstrate that huntingtin physically interacts with active caspase-3. Interestingly, mutant huntingtin binds active caspase-3 with a lower affinity and lower inhibitory effect on active caspase-3 than does wild-type huntingtin. Although reduction of huntingtin levels resulted in caspase-3 activation in all conditions examined, the cellular response was cell-type specific. Depletion of huntingtin resulted in either overt cell death, or in increased vulnerability to cell death. These data demonstrate that huntingtin inhibits caspase-3 activity, suggesting a mechanism whereby caspase-mediated huntingtin depletion results in a detrimental amplification cascade leading to further caspase-3 activation, resulting in cell dysfunction and cell death.

Selective degeneration in YAC mouse models of Huntington disease

Huntington disease (HD) is one of at least nine polyglutamine disorders caused by a CAG expansion in the coding region of a disease-causing gene. These disorders are characterized by selective degeneration of different regions of the brain, which is not explained by the expression pattern of the mutant protein. In HD, degeneration primarily occurs in the striatum and cortex. To examine the mechanisms responsible for the selective neuronal loss in HD, we have generated yeast artificial chromosome (YAC) transgenic models of HD that express full length mutant huntingtin (htt) from a YAC. These mice have appropriate tissue-specific and temporal expression of mutant htt and accordingly recapitulate the motor deficits, cognitive impairment and selective degeneration of HD. As in human patients, mutant htt expression is not increased in the affected regions of the brain. In contrast, detection of mutant htt in the nucleus is earliest and greatest in the striatum, the region most affected in HD, suggesting that selective nuclear localization of mutant htt may contribute to the region specific atrophy in these mice. Selective phosphorylation of mutant htt on serine 421 may also contribute, as phosphorylation of mutant htt reduces its toxicity and is decreased in the striatum compared to other regions of the brain. Finally, the fact that mutant htt expression increases the susceptibility of striatal neurons to excitotoxicity but not neurons from the cerebellum, suggests that altered sensitization to excitotoxic death may also contribute to selective degeneration in YAC mice. Overall, YAC mice recapitulate the region specific damage that occurs in HD and provide a suitable model for examining the mechanisms underlying of selective degeneration.