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Huntington disease: pathogenesis, biomarkers, and approaches to experimental therapeutics
Parkinsonism and Related Disorders 15S3 (2009) S135–S138
Contents lists available at ScienceDirect
Parkinsonism and Related Disorders journal homepage: www.elsevier.com/locate/parkreldis
Huntington disease: pathogenesis, biomarkers, and approaches to experimental therapeutics Christopher A. Rossa, *, Ira Shoulsonb a Professor
of Psychiatry, Neurology, Pharmacology and Neuroscience, Johns Hopkins University School of Medicine, CMSC 8–121, 600 North Wolfe Street, Baltimore, MD 21287, USA C. Lasagna Professor in Experimental Therapeutics, Professor of Neurology, Medicine, Pharmacology & Physiology, University of Rochester, 1351 Mount Hope Ave, Suite 218, Rochester, NY 14620, USA
b Louis
article info
summary
Keywords: huntingtin MRI Coenzyme Q10 striatum BDNF Rhes HDAC TRACK-HD PREDICT-HD PHAROS
Huntington disease (HD) is characterized by motor, cognitive and behavioral abnormalities that typically emerge in adulthood in persons who have inherited the mutant gene. HD has a single genetic cause, a well-defined neuropathology, and informative pre-manifest predictive genetic testing. Thus, it has been possible to develop imaging biomarkers of HD progression, not just in the period of manifest illness, but also in the prodromal or “premanifest” period. Striatal atrophy is the most studied, and shows steady progression beginning in the prodromal period beginning up to 15 years before predicted onset, and continuing through the period of manifest illness. Therapeutic targets for HD include the huntingtin protein itself, either by reducing its levels with antisense oligonucleotides or siRNA, or potentially by intervening via posttranslational modifications such as phosphorylation, acetylation, SUMOylation, or proteolytic cleavage. Other strategies involve bolstering the cell’s ability to deal with abnormal proteins, either via chaperones or protein degradation machinery. It may be possible to counteract the abnormal transcription caused by mutant huntingtin, with histone deacetylase inhibitors, or to enhance relevant gene products such as Brain Derived Neurotrophic Factor (BDNF). Another tactic is to enhance cellular metabolic defenses, such as with creatine or Coenzyme Q10. Strategies are being devised to use biomarkers, and administer therapeutic agents which can be given safely for long periods of time during the proodromal period, with a goal not just to slow progression, but to delay, or conceivably even prevent, the onset of clinical HD. © 2009 Elsevier Ltd. All rights reserved.
1. Clinical and genetic features Huntington disease (HD) is characterized by motor, cognitive and behavioral abnormalities that typically emerge in adulthood in persons who have inherited the mutant gene. Transmission of the HD gene occurs by the autosomal dominant inheritance, such that offspring of an affected parent bear a nominal 50:50 risk of inheriting the mutant gene. The HD gene, huntingtin, is located near the telomere of chromosome 4 and the mutation is an expansion of the cytosine–adenine–guanine (CAG) trinucleotide repeat within the first exon. The length of the expanded CAG repeat is inversely correlated with the age at clinical onset of HD and accounts for roughly 50–70% of the variance in onset age. Individuals normally have fewer than 27 CAG repeat units. Among offspring of an affected HD parent, a CAG repeat length between 27 and 35 is considered ‘high normal’. Although these slightly expanded repeat lengths do not result in HD, they may rarely expand, especially with paternal inheritance, and can result in * Corresponding author. Christopher A. Ross. Tel.: +1 410 614 0011; fax +1 410 614 0013. E-mail address: [email protected] (C.A. Ross). 1353-8020/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
the appearance of HD in subsequent generations [1]. There is age-dependent penetrance for short CAG expansions. CAG repeat lengths between 36 and 39 are considered to be variably penetrant because full manifestations of HD may or may not occur in the individual’s lifetime. Expanded CAG repeats of 40 or more are considered fully penetrant; such individuals carry a 100% lifetime risk of manifesting HD. The motor, cognitive and behavioral clinical characteristics of HD are caused by selective neuronal degeneration that preferentially targets neurons in the neostriatum and cerebral cortex, but as the disease progresses, involves many other regions including the thalamus, hippocampus, amygdala, other brainstem areas, and cerebral white matter. Motor and cognitive impairments develop in all HD patients, but their nature and severity can vary. Behavioral disorders are more variable and episodic in different people at different stages of the illness. The emergence of an otherwise unexplained, progressive extrapyramidal movement disorder has been the gold standard of clinical diagnosis of HD, especially if associated with ocular motor impairment or cognitive decline. These clinical features are usually sufficient for confirming the diagnosis in the setting
S136
C.A. Ross, I. Shoulson / Parkinsonism and Related Disorders 15S3 (2009) S135–S138
of a positive family history of HD. Striatal atrophy by head CT or MRI is helpful, but may not be clearly detectable in a clinical scan, especially during the early stages. Chorea is the most characteristic and conspicuous movement disorder of HD, but a wide range of other abnormalities may appear early including impairments of ocular motility, rapid alternating movements and fine motor coordination. Dystonia, bradykinesia, rigidity, incoordination, myoclonus, dystaxia, dysarthria and dysphagia may develop and become increasingly disabling as neural degeneration and clinical manifestations progress. The intellectual impairment of HD may occur early, and can sometimes be problematic even before clear motor abnormalities develop. The behavioral abnormalities of HD are varied, manifold and often episodic, including affective illness, impulse-control disorders, irritability, apathy and psychosis. In very early and ‘premanifest’ HD, behavioral disorders are not sufficiently specific to distinguish expanded and non-expanded individuals. As HD progresses, personality change, including apathy, irritability, perseverative behaviors, dysinihibition, and other behaviors may emerge, though are not universal. Since 1993, the ability to detect the mutant HD gene and the number of expanded CAG repeat units has provided the opportunity for adults at risk for HD to learn of their gene-carrier status. While a relatively small proportion of at-risk adults do choose presymptomatic testing, most adults at high risk for HD in the United Staes have chosen not to be tested, perhaps because of the current unavailability of disease-modifying treatments and the preferences of these individuals to live with uncertainty [2]. Two major observational studies have shed light on the genetic, biological and psychosocial outcomes among healthy adults at risk for HD, either who have decided not to undergo pre-symptomatic testing [3] or who have elected to be tested and learn of their HD gene-carrier status [4]. Both longitudinal studies, together encompassing more than 2000 research participants, indicate that the early clinical signs of HD emerge very gradually and that subtle cognitive and motor precursors develop over the course of many years preceding the traditional motor diagnosis of HD based on an otherwise unexplained extrapyramidal movement disorder [5,6]. Initial data from the PHAROS and PREDICT-HD studies indicate that cognitive abnormalities can be detected very early, perhaps months or even years before extrapyramidal motor abnormalities appear. The PHAROS data suggest that impairments on the Stroop test and lack of learning effects may distinguish individuals with expanded CAG repeats from their non-expanded at-risk counterparts. 2. Biomarkers Huntington disease has a single genetic cause, a well-defined neuropathology, and informative pre-manifest predictive genetic testing. Thus, it has been possible to develop imaging biomarkers of HD progression, not just in the period of manifest illness, but also in the prodromal or “premanifest” period. Striatal atrophy is the most studied, and shows progression beginning in the prodromal period and continuing through the period of manifest illness. Longitudinal studies indicate that striatal atrophy in manifest illness proceeds steadily, and use of striatal volumes as an outcome measure could improve informativeness and reduce the number of subjects needed in an interventional therapeutic trial [7]. In addition, cross-sectional data from both the multi-center PREDICT-HD study and the multi-center European TRACK-HD study indicate that striatal atrophy may begin up to 15 years prior to predicted onset of HD, and striatal volumes correlate with years to predicted motor onset [4,8]. Furthermore, longitudinal data from the prodromal period indicate that striatal volumes may be a useful outcome measure. An initial small study [9] suggested that longitudinal change could be detected at least 10
years prior to predicted onset of HD and that subjects have lost approximately 50% of striatal volume at the time of onset. The much larger PREDICT-HD study [4] is beginning to yield longitudinal data which closely match these initial observations (Aylward et al submitted). Striatal atrophy begins at least 15 years prior to predicted onset, and proceeds quite steadily through the prodromal period. Interestingly, preliminary cross-sectional and longitudinal data suggest that white matter loss may also be a highly sensitive measure of disease progression even in the prodromal period. Other gray matter regions studied to date, including cortical gray matter, do not have the same degree of volumetric loss. Cortical thickness however may prove to be useful, as cortical thinning can be detected during the premanifest period, and appears to correlate with clinical features [8,10,11]. Another potentially useful imaging measures of disease progression is diffusion tensor imaging [12,13] which shows white matter atrophy, perhaps due in part to the circuits undergoing neuronal degeneration. Functional imaging may also be a useful measure. fMRI can detect abnormalities during the premanifest period [14,15]. Magnetic resonance spectroscopy, especially with newer high field strength magnets [16], may also provide new opportunities for detecting and following novel chemical biomarkers. It would be highly desirable to have peripheral biomarkers of HD progression. Some studies have shown increased sensitivity to neurotoxic stress in HD lymphoblasts [17], though the extent to which this could be used for following individual patients is not clear. A marker of oxidative nucleic acid damage (8-hydroxydeoxyguanosine or 8-OHdG) may indicate oxidative injury [18], and is being further in larger research cohorts. Levels of the mutant protein huntingtin and other chemical candidates are also being examined in order to detect potential state biomarkers of pre-manifest HD. 3. Pathogenesis in comparison to PD While HD is a relatively rare disorder, it serves as a paradigm for studying other neurodegenerative diseases, and shares some features with Parkinson disease (PD). These include selective neuronal vulnerability, delayed onset despite wide expression of disease-related proteins beginning early in life, abnormal protein processing and aggregation, including deposition of abnormal proteins into inclusion bodies in neurons, and cell toxicity involving both intra-cellular and inter-cellular mechanisms. In PD the identification of several disease-causing genes has made it possible to begin to put together a pathogenic pathway [19,20]. In HD there is only the single huntingtin gene which is causative; however, modifier genes are beginning to be identified, and may contribute to pathogenic understanding. These include huntingtinassociated protein 1 (HAP1), which points to a role of cell transport mechanisms in pathogenesis and the glutamate receptor GluR6, which points toward a role for excitotoxicity. Shared features of pathogenesis also include excitotoxicity and metabolic toxicity. It has long been known that excitotoxicity (excessive stimulation of excitatory amino acid receptors) is a mechanism which can cause selective striatal vulnerability, and which has increasingly been implicated in HD pathogenesis [21]. Inflammatory mechanisms may also contribute to both HD and PD. Recent evidence has been found for widespread innate immune activation, both centrally and peripherally in HD patients and mouse models [22]. Metabolic stress may also play an important role in HD as shown by the ability of 3-nitropropionic acid (3-NPA) to cause selective striatal degeneration [23] and the more recent identification of ATP depletion in HD cells [24]. These mechanisms may be even more relevant for PD wherein environmental stress such as pesticide exposure may contribute.
C.A. Ross, I. Shoulson / Parkinsonism and Related Disorders 15S3 (2009) S135–S138
4. Pathogenesis and therapeutic targets Most of the available data regarding HD pathogenesis suggest that the disease arises predominantly due to a toxic gain of function from an abnormal conformation of the mutant huntingtin protein, though loss of function may also contribute [25,26]. The normal functions of the huntingtin protein are still only partly understood, and may include regulation of gene transcription, RNA biogenesis and vesicle trafficking. It is predominately cytosolic and widely expressed throughout the body, and in highest concentration in the brain and testis. Huntingtin is a very large protein (3144 amino acids) which appears to act as a scaffold and engages many protein interaction partners. Knockouts in mice are early embryonic lethal, indicating a critical role for the normal protein in embryogenesis. The expanded CAG coding of excessive poly-glutamine tracts within mutant huntingtin may confer abnormal protein folding and protein-protein interactions that lead eventually to the pathological and clinical consequences of HD. Mutant huntingtin is present in inclusion bodies in HD post-mortem brain consistent with the idea of abnormal conformation. However, the inclusion bodies, themselves, appear to be a cellular response rather than directly pathogenic. Many different hypotheses have been proposed regarding HD pathogenesis that have led to therapeutic strategies. So far there has not been a clear way to target the abnormal conformation of the mutant huntingtin protein. However, there are a number of potential strategies including antisense oligos and RNAi which can reduce levels of huntingtin message and thus overall levels of the mutant protein. Some of these can also be more selective for the mutant than wild-type allele [27,28]. It may also be possible to anhance the cell’s ability to refold or degrade mutant huntingtin, by enhancing chaperone activity or autophagy. It may be possible to target post-translational modifications of huntingtin, which have a strong influence on the formation of the abnormal conformation and cell toxicity. Phosphorylation at serine 421 reduces toxicity in vitro, though this has not yet been confirmed in vivo. The N-terminal of the huntingtin protein appears to be especially susceptible to several different forms of post-translational modification. Phosphorylation at threonine 3, serine 13 and serine 16 appear to be protective, and may provide therapeutic targets. Preferential binding of huntingtin by the Rhes (Ras homolog enriched in the striatum) protein [29] is especially interesting, and may help explain the selective regional vulnerability in HD. Rhes binds strongly to mutant Htt and elicits SUMOylation, a process increasing soluble mutant huntingtin, reducing inclusions and other aggregates, and decreasing cytotoxicity. Another post-translational modification is proteolytic cleavage. There are many potential sites, but the best characterized is a putative cleavage event at a predicted caspase 6 cleavage sequence including position 586. Transgenic mice expressing fulllength mutant huntingtin with this sequence altered showed a substantially ameliorated phenotype compared with transgenic mice expressing mutant huntingtin with this sequence intact [30]. Caspase 6 activity can cleave DJ1 has recently been implicated in in the pathogenesis of PD [31]. Thus, caspase 6 inhibition may be a widely applicable therapeutic target. Interference with gene transcription may mediate HD cellular toxicity [32]. A number of molecular mechanisms have been proposed including interference with transcription mediated by CBP, Sp1 and several components of the core transcriptional machinery. This area has stimulated a great deal of therapeutic interest. Some of the transcriptional abnormalities may be ameliorated by inhibition of histone deacetylase enzymes (HDACs) which cause transcriptional repression and thus, when inhibited, may relieve the abnormal transcriptional repression due to mutant
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huntingtin. Several therapeutic trials of HDAC inhibitors have been conducted in HD mouse or fly models [33,34] and a safety and tolerability study of the HDAC inhibitor phenylbutyrate has been conducted in HD patients. BDNF is one of the target genes reduced in HD. In addition, BDNF transport can be altered by mutant huntingtin, suggesting a rationale for therapy targeted at BDNF. Agents that act via different mechanisms to enhance BDNF activity have been reported to have therapeutic effects in mouse models of [35–37]. Much therapeutic activity in HD has been directed at the metabolic and oxidative and excitotoxic stress believed to underlie part of mutant huntingtin’s effects. A large multi-center randomized controlled trial in early HD patients, the CARE-HD study, included coenzyme Q10 which acts in part to enhance mitochondrial antioxidative and free-radical scavenger mechanisms [38]. While it did not reach the pre-specified efficacy end point, coenzyme Q10 at 600 mg/day caused slowing of functional and cognitive outcome measures of HD progression over 30 months of observation. The glutamate receptor blocker remacemide, which was also administered in the CARE-HD trial, lessened chorea but had no effect on function or cognition. The 2-CARE study is a larger trial involving dosages of coenzyme Q10 2400 mg/day over 5 years of placebo-controlled treatment [ClinicalTrials.gov registration NCT00608881]. This therapeutic strategy in HD has also been undertaken in PD [“QE3” NCT00740714]. Creatine is also under study in HD [“CREST-E” NCT00712426]. One substantial difference between HD and PD is the potential in HD for developing therapeutic trials based on predictive genetic testing. Progress in understanding the pathogenesis of HD and the ability to identify pre-manifest HD gene carrier have provided an opportunity to develop and pursue therapeutic strategies aimed at postponing or even preventing the clinical onset of HD. A multicenter phase II safety, tolerability and feasibility study in prodromal HD has been funded by the NINDS [“PREQUEL” NCT00920699] to test several different doses of coenzyme Q10, including the dose used in the 2-CARE study. The goal will be to identify a dosage which is safe and well tolerated in this otherwise healthy population. A single site study of creatine is also under way [“PRECREST” NCT00592995]. The ultimate goal of the research is to design strategies to use biomarkers, and administer therapeutic agents which can be given safely for long periods of time during the proodromal period, in order not just to slow progression, but to delay, or conceivably even prevent, the onset of manifest HD. Acknowledgements The authors thank Mary Slough and Deb Pollard for assistance with manuscript preparation. Conflict of interests There are no conflicts of interest related to this paper for either author. References 1. Hendricks AE, Latourelle JC, Lunetta KL, Cupples LA, Wheeler V, Macdonald ME, et al. Estimating the probability of de novo HD cases from transmissions of expanded penetrant CAG alleles in the Huntington disease gene from male carriers of high normal alleles (27–35 CAG). Am J Med Genet 2009 Jul;149A(7):1375–81. 2. Quaid K, Sims S, Swenson M, Harrison J, Moskowitz C, Stepanov N, et al. and the Huntington Study Group PHAROS Investigators. Living at risk: Concealing risk and preserving hope in Huntington’s disease. Neurotherapeutics 2008;5(2): 368–9. 3. The Huntington Study Group PHAROS Investigators. At risk for Huntington disease: The PHAROS (Prospective Huntington At Risk Observational Study) cohort enrolled. Arch Neurol 2006 Jul;63(7):991–6.
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4. Paulsen JS, Hayden M, Stout JC, Langbehn DR, Aylward E, Ross CA, et al. and the PREDICT-HD Investigators of the Huntington Study Group. Preparing for preventive clinical trials: the PREDICT-HD study. Arch Neurol 2006 Jun;63(6):883–90. 5. Biglan KM, Ross CA, Langbehn DR, Aylward EH, Stout JC, Queller S, et al. and the PREDICT-HD Investigators of the Huntington Study Group. Motor abnormalities in premanifest persons with Huntington’s disease: the PREDICT-HD study. Mov Disord 2009 Sep 15;24(12):1763–72. 6. Kayson E and the Huntington Study Group PHAROS Investigators. Five years of monitoring the safety and feasibility of PHAROS (prospective Huntington at risk observational study). Neurotherapeutics 2008;5(2):366. 7. Aylward EH, Rosenblatt A, Field K, Yallapragada V, Kieburtz K, McDermott M, et al. Caudate volume as an outcome measure in clinical trials for Huntington’s disease: a pilot study. Brain Res Bull 2003 Dec 15;62(2):137–41. 8. Tabrizi SJ, Langbehn DR, Leavitt BR, Roos RA, Durr A, Craufurd D, et al. Biological and clinical manifestations of Huntington’s disease in the longitudinal TRACK-HD study: cross-sectional analysis of baseline data. Lancet Neurol 2009 Sep;8(9):791–801. 9. Aylward EH, Sparks BF, Field KM, Yallapragada V, Shpritz BD, Rosenblatt A, et al. Onset and rate of striatal atrophy in preclinical Huntington disease. Neurology 2004 Jul 13;63(1):66–72. 10. Rosas HD, Salat DH, Lee SY, Zaleta AK, Pappu V, Fischl B, et al. Cerebral cortex and the clinical expression of Huntington’s disease: complexity and heterogeneity. Brain 2008 Apr;131(Pt 4):1057–68. 11. Rosas HD, Hevelone ND, Zaleta AK, Greve DN, Salat DH, Fischl B. Regional cortical thinning in preclinical Huntington disease and its relationship to cognition. Neurology 2005 Sep 13;65(5):745–7. 12. Reading SA, Yassa MA, Bakker A, Dziorny AC, Gourley LM, Yallapragada V, et al. Regional white matter change in pre-symptomatic Huntington’s disease: a diffusion tensor imaging study. Psychiatry Res 2005 Oct 30;140(1):55–62. 13. Douaud G, Behrens TE, Poupon C, Cointepas Y, Jbabdi S, Gaura V, et al. In vivo evidence for the selective subcortical degeneration in Huntington’s disease. NeuroImage 2009 Jul 15;46(4):958–66. 14. Reading SA, Dziorny AC, Peroutka LA, Schreiber M, Gourley LM, Yallapragada V, et al. Functional brain changes in presymptomatic Huntington’s disease. Ann Neurol 2004 Jun;55(6):879–83. 15. Paulsen JS. Functional imaging in Huntington’s disease. Exp Neurol 2009 Apr;216(2):272–7. 16. Martin WR, Wieler M, Hanstock CC. Is brain lactate increased in Huntington’s disease? J Neurol Sci 2007 Dec 15;263(1–2):70–4. 17. Sawa A, Wiegand GW, Cooper J, Margolis RL, Sharp AH, Lawler JF, Jr., et al. Increased apoptosis of Huntington disease lymphoblasts associated with repeat length-dependent mitochondrial depolarization. Nat Med 1999 Oct;5(10):1194–8. 18. Hersch SM, Gevorkian S, Marder K, Moskowitz C, Feigin A, Cox M, et al. Creatine in Huntington disease is safe, tolerable, bioavailable in brain and reduces serum 8OH2’dG. Neurology 2006 Jan 24;66(2):250–2. 19. Hardy J, Cai H, Cookson MR, Gwinn-Hardy K, Singleton A. Genetics of Parkinson’s disease and parkinsonism. Ann Neurol 2006 Oct;60(4):389–98. 20. Ross CA, Smith WW. Gene–environment interactions in Parkinson’s disease. Parkinsonism Relat Disord 2007;13(Suppl 3):S309–15. 21. Schwarcz R, Guidetti P, Sathyasaikumar KV, Muchowski PJ. Of mice, rats and
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Contents lists available at ScienceDirect
Parkinsonism and Related Disorders journal homepage: www.elsevier.com/locate/parkreldis
Huntington disease: pathogenesis, biomarkers, and approaches to experimental therapeutics Christopher A. Rossa, *, Ira Shoulsonb a Professor
of Psychiatry, Neurology, Pharmacology and Neuroscience, Johns Hopkins University School of Medicine, CMSC 8–121, 600 North Wolfe Street, Baltimore, MD 21287, USA C. Lasagna Professor in Experimental Therapeutics, Professor of Neurology, Medicine, Pharmacology & Physiology, University of Rochester, 1351 Mount Hope Ave, Suite 218, Rochester, NY 14620, USA
b Louis
article info
summary
Keywords: huntingtin MRI Coenzyme Q10 striatum BDNF Rhes HDAC TRACK-HD PREDICT-HD PHAROS
Huntington disease (HD) is characterized by motor, cognitive and behavioral abnormalities that typically emerge in adulthood in persons who have inherited the mutant gene. HD has a single genetic cause, a well-defined neuropathology, and informative pre-manifest predictive genetic testing. Thus, it has been possible to develop imaging biomarkers of HD progression, not just in the period of manifest illness, but also in the prodromal or “premanifest” period. Striatal atrophy is the most studied, and shows steady progression beginning in the prodromal period beginning up to 15 years before predicted onset, and continuing through the period of manifest illness. Therapeutic targets for HD include the huntingtin protein itself, either by reducing its levels with antisense oligonucleotides or siRNA, or potentially by intervening via posttranslational modifications such as phosphorylation, acetylation, SUMOylation, or proteolytic cleavage. Other strategies involve bolstering the cell’s ability to deal with abnormal proteins, either via chaperones or protein degradation machinery. It may be possible to counteract the abnormal transcription caused by mutant huntingtin, with histone deacetylase inhibitors, or to enhance relevant gene products such as Brain Derived Neurotrophic Factor (BDNF). Another tactic is to enhance cellular metabolic defenses, such as with creatine or Coenzyme Q10. Strategies are being devised to use biomarkers, and administer therapeutic agents which can be given safely for long periods of time during the proodromal period, with a goal not just to slow progression, but to delay, or conceivably even prevent, the onset of clinical HD. © 2009 Elsevier Ltd. All rights reserved.
1. Clinical and genetic features Huntington disease (HD) is characterized by motor, cognitive and behavioral abnormalities that typically emerge in adulthood in persons who have inherited the mutant gene. Transmission of the HD gene occurs by the autosomal dominant inheritance, such that offspring of an affected parent bear a nominal 50:50 risk of inheriting the mutant gene. The HD gene, huntingtin, is located near the telomere of chromosome 4 and the mutation is an expansion of the cytosine–adenine–guanine (CAG) trinucleotide repeat within the first exon. The length of the expanded CAG repeat is inversely correlated with the age at clinical onset of HD and accounts for roughly 50–70% of the variance in onset age. Individuals normally have fewer than 27 CAG repeat units. Among offspring of an affected HD parent, a CAG repeat length between 27 and 35 is considered ‘high normal’. Although these slightly expanded repeat lengths do not result in HD, they may rarely expand, especially with paternal inheritance, and can result in * Corresponding author. Christopher A. Ross. Tel.: +1 410 614 0011; fax +1 410 614 0013. E-mail address: [email protected] (C.A. Ross). 1353-8020/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
the appearance of HD in subsequent generations [1]. There is age-dependent penetrance for short CAG expansions. CAG repeat lengths between 36 and 39 are considered to be variably penetrant because full manifestations of HD may or may not occur in the individual’s lifetime. Expanded CAG repeats of 40 or more are considered fully penetrant; such individuals carry a 100% lifetime risk of manifesting HD. The motor, cognitive and behavioral clinical characteristics of HD are caused by selective neuronal degeneration that preferentially targets neurons in the neostriatum and cerebral cortex, but as the disease progresses, involves many other regions including the thalamus, hippocampus, amygdala, other brainstem areas, and cerebral white matter. Motor and cognitive impairments develop in all HD patients, but their nature and severity can vary. Behavioral disorders are more variable and episodic in different people at different stages of the illness. The emergence of an otherwise unexplained, progressive extrapyramidal movement disorder has been the gold standard of clinical diagnosis of HD, especially if associated with ocular motor impairment or cognitive decline. These clinical features are usually sufficient for confirming the diagnosis in the setting
S136
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of a positive family history of HD. Striatal atrophy by head CT or MRI is helpful, but may not be clearly detectable in a clinical scan, especially during the early stages. Chorea is the most characteristic and conspicuous movement disorder of HD, but a wide range of other abnormalities may appear early including impairments of ocular motility, rapid alternating movements and fine motor coordination. Dystonia, bradykinesia, rigidity, incoordination, myoclonus, dystaxia, dysarthria and dysphagia may develop and become increasingly disabling as neural degeneration and clinical manifestations progress. The intellectual impairment of HD may occur early, and can sometimes be problematic even before clear motor abnormalities develop. The behavioral abnormalities of HD are varied, manifold and often episodic, including affective illness, impulse-control disorders, irritability, apathy and psychosis. In very early and ‘premanifest’ HD, behavioral disorders are not sufficiently specific to distinguish expanded and non-expanded individuals. As HD progresses, personality change, including apathy, irritability, perseverative behaviors, dysinihibition, and other behaviors may emerge, though are not universal. Since 1993, the ability to detect the mutant HD gene and the number of expanded CAG repeat units has provided the opportunity for adults at risk for HD to learn of their gene-carrier status. While a relatively small proportion of at-risk adults do choose presymptomatic testing, most adults at high risk for HD in the United Staes have chosen not to be tested, perhaps because of the current unavailability of disease-modifying treatments and the preferences of these individuals to live with uncertainty [2]. Two major observational studies have shed light on the genetic, biological and psychosocial outcomes among healthy adults at risk for HD, either who have decided not to undergo pre-symptomatic testing [3] or who have elected to be tested and learn of their HD gene-carrier status [4]. Both longitudinal studies, together encompassing more than 2000 research participants, indicate that the early clinical signs of HD emerge very gradually and that subtle cognitive and motor precursors develop over the course of many years preceding the traditional motor diagnosis of HD based on an otherwise unexplained extrapyramidal movement disorder [5,6]. Initial data from the PHAROS and PREDICT-HD studies indicate that cognitive abnormalities can be detected very early, perhaps months or even years before extrapyramidal motor abnormalities appear. The PHAROS data suggest that impairments on the Stroop test and lack of learning effects may distinguish individuals with expanded CAG repeats from their non-expanded at-risk counterparts. 2. Biomarkers Huntington disease has a single genetic cause, a well-defined neuropathology, and informative pre-manifest predictive genetic testing. Thus, it has been possible to develop imaging biomarkers of HD progression, not just in the period of manifest illness, but also in the prodromal or “premanifest” period. Striatal atrophy is the most studied, and shows progression beginning in the prodromal period and continuing through the period of manifest illness. Longitudinal studies indicate that striatal atrophy in manifest illness proceeds steadily, and use of striatal volumes as an outcome measure could improve informativeness and reduce the number of subjects needed in an interventional therapeutic trial [7]. In addition, cross-sectional data from both the multi-center PREDICT-HD study and the multi-center European TRACK-HD study indicate that striatal atrophy may begin up to 15 years prior to predicted onset of HD, and striatal volumes correlate with years to predicted motor onset [4,8]. Furthermore, longitudinal data from the prodromal period indicate that striatal volumes may be a useful outcome measure. An initial small study [9] suggested that longitudinal change could be detected at least 10
years prior to predicted onset of HD and that subjects have lost approximately 50% of striatal volume at the time of onset. The much larger PREDICT-HD study [4] is beginning to yield longitudinal data which closely match these initial observations (Aylward et al submitted). Striatal atrophy begins at least 15 years prior to predicted onset, and proceeds quite steadily through the prodromal period. Interestingly, preliminary cross-sectional and longitudinal data suggest that white matter loss may also be a highly sensitive measure of disease progression even in the prodromal period. Other gray matter regions studied to date, including cortical gray matter, do not have the same degree of volumetric loss. Cortical thickness however may prove to be useful, as cortical thinning can be detected during the premanifest period, and appears to correlate with clinical features [8,10,11]. Another potentially useful imaging measures of disease progression is diffusion tensor imaging [12,13] which shows white matter atrophy, perhaps due in part to the circuits undergoing neuronal degeneration. Functional imaging may also be a useful measure. fMRI can detect abnormalities during the premanifest period [14,15]. Magnetic resonance spectroscopy, especially with newer high field strength magnets [16], may also provide new opportunities for detecting and following novel chemical biomarkers. It would be highly desirable to have peripheral biomarkers of HD progression. Some studies have shown increased sensitivity to neurotoxic stress in HD lymphoblasts [17], though the extent to which this could be used for following individual patients is not clear. A marker of oxidative nucleic acid damage (8-hydroxydeoxyguanosine or 8-OHdG) may indicate oxidative injury [18], and is being further in larger research cohorts. Levels of the mutant protein huntingtin and other chemical candidates are also being examined in order to detect potential state biomarkers of pre-manifest HD. 3. Pathogenesis in comparison to PD While HD is a relatively rare disorder, it serves as a paradigm for studying other neurodegenerative diseases, and shares some features with Parkinson disease (PD). These include selective neuronal vulnerability, delayed onset despite wide expression of disease-related proteins beginning early in life, abnormal protein processing and aggregation, including deposition of abnormal proteins into inclusion bodies in neurons, and cell toxicity involving both intra-cellular and inter-cellular mechanisms. In PD the identification of several disease-causing genes has made it possible to begin to put together a pathogenic pathway [19,20]. In HD there is only the single huntingtin gene which is causative; however, modifier genes are beginning to be identified, and may contribute to pathogenic understanding. These include huntingtinassociated protein 1 (HAP1), which points to a role of cell transport mechanisms in pathogenesis and the glutamate receptor GluR6, which points toward a role for excitotoxicity. Shared features of pathogenesis also include excitotoxicity and metabolic toxicity. It has long been known that excitotoxicity (excessive stimulation of excitatory amino acid receptors) is a mechanism which can cause selective striatal vulnerability, and which has increasingly been implicated in HD pathogenesis [21]. Inflammatory mechanisms may also contribute to both HD and PD. Recent evidence has been found for widespread innate immune activation, both centrally and peripherally in HD patients and mouse models [22]. Metabolic stress may also play an important role in HD as shown by the ability of 3-nitropropionic acid (3-NPA) to cause selective striatal degeneration [23] and the more recent identification of ATP depletion in HD cells [24]. These mechanisms may be even more relevant for PD wherein environmental stress such as pesticide exposure may contribute.
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4. Pathogenesis and therapeutic targets Most of the available data regarding HD pathogenesis suggest that the disease arises predominantly due to a toxic gain of function from an abnormal conformation of the mutant huntingtin protein, though loss of function may also contribute [25,26]. The normal functions of the huntingtin protein are still only partly understood, and may include regulation of gene transcription, RNA biogenesis and vesicle trafficking. It is predominately cytosolic and widely expressed throughout the body, and in highest concentration in the brain and testis. Huntingtin is a very large protein (3144 amino acids) which appears to act as a scaffold and engages many protein interaction partners. Knockouts in mice are early embryonic lethal, indicating a critical role for the normal protein in embryogenesis. The expanded CAG coding of excessive poly-glutamine tracts within mutant huntingtin may confer abnormal protein folding and protein-protein interactions that lead eventually to the pathological and clinical consequences of HD. Mutant huntingtin is present in inclusion bodies in HD post-mortem brain consistent with the idea of abnormal conformation. However, the inclusion bodies, themselves, appear to be a cellular response rather than directly pathogenic. Many different hypotheses have been proposed regarding HD pathogenesis that have led to therapeutic strategies. So far there has not been a clear way to target the abnormal conformation of the mutant huntingtin protein. However, there are a number of potential strategies including antisense oligos and RNAi which can reduce levels of huntingtin message and thus overall levels of the mutant protein. Some of these can also be more selective for the mutant than wild-type allele [27,28]. It may also be possible to anhance the cell’s ability to refold or degrade mutant huntingtin, by enhancing chaperone activity or autophagy. It may be possible to target post-translational modifications of huntingtin, which have a strong influence on the formation of the abnormal conformation and cell toxicity. Phosphorylation at serine 421 reduces toxicity in vitro, though this has not yet been confirmed in vivo. The N-terminal of the huntingtin protein appears to be especially susceptible to several different forms of post-translational modification. Phosphorylation at threonine 3, serine 13 and serine 16 appear to be protective, and may provide therapeutic targets. Preferential binding of huntingtin by the Rhes (Ras homolog enriched in the striatum) protein [29] is especially interesting, and may help explain the selective regional vulnerability in HD. Rhes binds strongly to mutant Htt and elicits SUMOylation, a process increasing soluble mutant huntingtin, reducing inclusions and other aggregates, and decreasing cytotoxicity. Another post-translational modification is proteolytic cleavage. There are many potential sites, but the best characterized is a putative cleavage event at a predicted caspase 6 cleavage sequence including position 586. Transgenic mice expressing fulllength mutant huntingtin with this sequence altered showed a substantially ameliorated phenotype compared with transgenic mice expressing mutant huntingtin with this sequence intact [30]. Caspase 6 activity can cleave DJ1 has recently been implicated in in the pathogenesis of PD [31]. Thus, caspase 6 inhibition may be a widely applicable therapeutic target. Interference with gene transcription may mediate HD cellular toxicity [32]. A number of molecular mechanisms have been proposed including interference with transcription mediated by CBP, Sp1 and several components of the core transcriptional machinery. This area has stimulated a great deal of therapeutic interest. Some of the transcriptional abnormalities may be ameliorated by inhibition of histone deacetylase enzymes (HDACs) which cause transcriptional repression and thus, when inhibited, may relieve the abnormal transcriptional repression due to mutant
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huntingtin. Several therapeutic trials of HDAC inhibitors have been conducted in HD mouse or fly models [33,34] and a safety and tolerability study of the HDAC inhibitor phenylbutyrate has been conducted in HD patients. BDNF is one of the target genes reduced in HD. In addition, BDNF transport can be altered by mutant huntingtin, suggesting a rationale for therapy targeted at BDNF. Agents that act via different mechanisms to enhance BDNF activity have been reported to have therapeutic effects in mouse models of [35–37]. Much therapeutic activity in HD has been directed at the metabolic and oxidative and excitotoxic stress believed to underlie part of mutant huntingtin’s effects. A large multi-center randomized controlled trial in early HD patients, the CARE-HD study, included coenzyme Q10 which acts in part to enhance mitochondrial antioxidative and free-radical scavenger mechanisms [38]. While it did not reach the pre-specified efficacy end point, coenzyme Q10 at 600 mg/day caused slowing of functional and cognitive outcome measures of HD progression over 30 months of observation. The glutamate receptor blocker remacemide, which was also administered in the CARE-HD trial, lessened chorea but had no effect on function or cognition. The 2-CARE study is a larger trial involving dosages of coenzyme Q10 2400 mg/day over 5 years of placebo-controlled treatment [ClinicalTrials.gov registration NCT00608881]. This therapeutic strategy in HD has also been undertaken in PD [“QE3” NCT00740714]. Creatine is also under study in HD [“CREST-E” NCT00712426]. One substantial difference between HD and PD is the potential in HD for developing therapeutic trials based on predictive genetic testing. Progress in understanding the pathogenesis of HD and the ability to identify pre-manifest HD gene carrier have provided an opportunity to develop and pursue therapeutic strategies aimed at postponing or even preventing the clinical onset of HD. A multicenter phase II safety, tolerability and feasibility study in prodromal HD has been funded by the NINDS [“PREQUEL” NCT00920699] to test several different doses of coenzyme Q10, including the dose used in the 2-CARE study. The goal will be to identify a dosage which is safe and well tolerated in this otherwise healthy population. A single site study of creatine is also under way [“PRECREST” NCT00592995]. The ultimate goal of the research is to design strategies to use biomarkers, and administer therapeutic agents which can be given safely for long periods of time during the proodromal period, in order not just to slow progression, but to delay, or conceivably even prevent, the onset of manifest HD. Acknowledgements The authors thank Mary Slough and Deb Pollard for assistance with manuscript preparation. Conflict of interests There are no conflicts of interest related to this paper for either author. References 1. Hendricks AE, Latourelle JC, Lunetta KL, Cupples LA, Wheeler V, Macdonald ME, et al. Estimating the probability of de novo HD cases from transmissions of expanded penetrant CAG alleles in the Huntington disease gene from male carriers of high normal alleles (27–35 CAG). Am J Med Genet 2009 Jul;149A(7):1375–81. 2. Quaid K, Sims S, Swenson M, Harrison J, Moskowitz C, Stepanov N, et al. and the Huntington Study Group PHAROS Investigators. Living at risk: Concealing risk and preserving hope in Huntington’s disease. Neurotherapeutics 2008;5(2): 368–9. 3. The Huntington Study Group PHAROS Investigators. At risk for Huntington disease: The PHAROS (Prospective Huntington At Risk Observational Study) cohort enrolled. Arch Neurol 2006 Jul;63(7):991–6.
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