Gene therapy in mouse models of huntington disease

Protective Effects of Polysaccharides in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are characterized by progressive degeneration and necrosis of neurons, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease and others. There are no existing therapies that correct the progression of these diseases, and current therapies provide merely symptomatic relief. The use of polysaccharides has received significant attention due to extensive biological activities and application prospects. Previous studies suggest that the polysaccharides as a candidate participate in neuronal protection and protect against NDs. In this review, we demonstrate that various polysaccharides mediate NDs, and share several common mechanisms characterized by autophagy, apoptosis, neuroinflammation, oxidative stress, mitochondrial dysfunction in PD and AD. Furthermore, this review reveals potential role of polysaccharides in vitro and in vivo models of NDs, and highlights the contributions of polysaccharides and prospects of their mechanism studies for the treatment of NDs. Finally, we suggest some remaining questions for the field and areas for new development.

In Vivo Genome Editing as a Therapeutic Approach

Genome editing has been well established as a genome engineering tool that enables researchers to establish causal linkages between genetic mutation and biological phenotypes, providing further understanding of the genetic manifestation of many debilitating diseases. More recently, the paradigm of genome editing technologies has evolved to include the correction of mutations that cause diseases via the use of nucleases such as zinc-finger nucleases (ZFN), transcription activator-like effector nucleases (TALENs), and more recently, Cas9 nuclease. With the aim of reversing disease phenotypes, which arise from somatic gene mutations, current research focuses on the clinical translatability of correcting human genetic diseases in vivo, to provide long-term therapeutic benefits and potentially circumvent the limitations of in vivo cell replacement therapy. In this review, in addition to providing an overview of the various genome editing techniques available, we have also summarized several in vivo genome engineering strategies that have successfully demonstrated disease correction via in vivo genome editing. The various benefits and challenges faced in applying in vivo genome editing in humans will also be discussed.

N-terminal Huntingtin Knock-In Mice: Implications of Removing the N-terminal Region of Huntingtin for Therapy

The Huntington’s disease (HD) protein, huntingtin (HTT), is a large protein consisting of 3144 amino acids and has conserved N-terminal sequences that are followed by a polyglutamine (polyQ) repeat. Loss of Htt is known to cause embryonic lethality in mice, whereas polyQ expansion leads to adult neuronal degeneration. Whether N-terminal HTT is essential for neuronal development or contributes only to late-onset neurodegeneration remains unknown. We established HTT knock-in mice (N160Q-KI) expressing the first 208 amino acids of HTT with 160Q, and they show age-dependent HTT aggregates in the brain and neurological phenotypes. Importantly, the N-terminal mutant HTT also preferentially accumulates in the striatum, the brain region most affected in HD, indicating the importance of N-terminal HTT in selective neuropathology. That said, homozygous N160Q-KI mice are also embryonic lethal, suggesting that N-terminal HTT alone is unable to support embryonic development. Using Htt knockout neurons, we found that loss of Htt selectively affects the survival of developing neuronal cells, but not astrocytes, in culture. This neuronal degeneration could be rescued by a truncated HTT lacking the first 237 amino acids, but not by N-terminal HTT (1–208 amino acids). Also, the rescue effect depends on the region in HTT known to be involved in intracellular trafficking. Thus, the N-terminal HTT region may not be essential for the survival of developing neurons, but when carrying a large polyQ repeat, can cause selective neuropathology. These findings imply a possible therapeutic benefit of removing the N-terminal region of HTT containing the polyQ repeat to treat the neurodegeneration in HD.

The 17 amino acids in the N-terminal region of huntingtin (HTT) are conserved in a wide range of species and are followed by a polyglutamine repeat whose expansion causes selective neurodegeneration in Huntington’s disease (HD). Loss of Htt can affect developing neurons and early embryonic development in mice. Whether N-terminal HTT is important for the survival of developing neurons or contributes mainly to a gain of toxic function in HD remains unknown. In the current study, we generated N-terminal mutant HTT knock-in mice and found that N-terminal HTT with an expanded polyQ repeat is unable to support the early development of mice, but can cause age-dependent neurological phenotypes. Further, we show that a truncated HTT without the N-terminal region can rescue the Htt loss-mediated degeneration of developing neurons. Our studies suggest that removal of the N-terminal region of mutant HTT could be a strategy to abolish the neuronal toxicity of mutant HTT.

Viral vector-based tools advance knowledge of basal ganglia anatomy and physiology

Viral vectors were originally developed to deliver genes into host cells for therapeutic potential. However, viral vector use in neuroscience research has increased because they enhance interpretation of the anatomy and physiology of brain circuits compared with conventional tract tracing or electrical stimulation techniques. Viral vectors enable neuronal or glial subpopulations to be labeled or stimulated, which can be spatially restricted to a single target nucleus or pathway. Here we review the use of viral vectors to examine the structure and function of motor and limbic basal ganglia (BG) networks in normal and pathological states. We outline the use of viral vectors, particularly lentivirus and adeno-associated virus, in circuit tracing, optogenetic stimulation, and designer drug stimulation experiments. Key studies that have used viral vectors to trace and image pathways and connectivity at gross or ultrastructural levels are reviewed. We explain how optogenetic stimulation and designer drugs used to modulate a distinct pathway and neuronal subpopulation have enhanced our mechanistic understanding of BG function in health and pathophysiology in disease. Finally, we outline how viral vector technology may be applied to neurological and psychiatric conditions to offer new treatments with enhanced outcomes for patients.

Design, Characterization, and Lead Selection of Therapeutic miRNAs Targeting Huntingtin for Development of Gene Therapy for Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by accumulation of CAG expansions in the huntingtin (HTT) gene. Hence, decreasing the expression of mutated HTT (mtHTT) is the most upstream approach for treatment of HD. We have developed HTT gene-silencing approaches based on expression cassette-optimized artificial miRNAs (miHTTs). In the first approach, total silencing of wild-type and mtHTT was achieved by targeting exon 1. In the second approach, allele-specific silencing was induced by targeting the heterozygous single-nucleotide polymorphism (SNP) rs362331 in exon 50 or rs362307 in exon 67 linked to mtHTT. The miHTT expression cassette was optimized by embedding anti-HTT target sequences in ten pri-miRNA scaffolds and their HTT knockdown efficacy, allele selectivity, passenger strand activity, and processing patterns were analyzed in vitro. Furthermore, three scaffolds expressing miH12 targeting exon 1 were incorporated in an adeno-associated viral serotype 5 (AAV5) vector and their HTT knock-down efficiency and pre-miHTT processing were compared in the humanized transgenic Hu128/21 HD mouse model. Our data demonstrate strong allele-selective silencing of mtHTT by miSNP50 targeting rs362331 and total HTT silencing by miH12 both in vitro and in vivo. Ultimately, we show that HTT knock-down efficiency and guide strand processing can be enhanced by using different cellular pri-miRNA scaffolds.

Ablation of huntingtin in adult neurons is nondeleterious but its depletion in young mice causes acute pancreatitis

The Huntington's disease (HD) protein, huntingtin (HTT), is essential for early development. Because suppressing the expression of mutant HTT is an important approach to treat the disease, we must first understand the normal function of Htt in adults versus younger animals. Using inducible Htt knockout mice, we found that Htt depletion does not lead to adult neurodegeneration or animal death at >4 mo of age, which was also verified by selectively depleting Htt in neurons. On the other hand, young Htt KO mice die at 2 mo of age of acute pancreatitis due to the degeneration of pancreatic acinar cells. Importantly, Htt interacts with the trypsin inhibitor, serine protease inhibitor Kazal-type 3 (Spink3), to inhibit activation of digestive enzymes in acinar cells in young mice, and transgenic HTT can rescue the early death of Htt KO mice. These findings point out age- and cell type-dependent vital functions of Htt and the safety of knocking down neuronal Htt expression in adult brains as a treatment.

Transgenic animal models for study of the pathogenesis of Huntington's disease and therapy

Huntington’s disease (HD) is caused by a genetic mutation that results in polyglutamine expansion in the N-terminal regions of huntingtin. As a result, this polyQ expansion leads to the misfolding and aggregation of mutant huntingtin as well as age-dependent neurodegeneration. The genetic mutation in HD allows for generating a variety of animal models that express different forms of mutant huntingtin and show differential pathology. Studies of these animal models have provided an important insight into the pathogenesis of HD. Mouse models of HD include transgenic mice, which express N-terminal or full-length mutant huntingtin ubiquitously or selectively in different cell types, and knock-in mice that express full-length mutant Htt at the endogenous level. Large animals, such as pig, sheep, and monkeys, have also been used to generate animal HD models. This review focuses on the different features of commonly used transgenic HD mouse models as well as transgenic large animal models of HD, and also discusses how to use them to identify potential therapeutics. Since HD shares many pathological features with other neurodegenerative diseases, identification of therapies for HD would also help to develop effective treatment for different neurodegenerative diseases that are also caused by protein misfolding and occur in an age-dependent manner.

Novel siRNA delivery strategy: a new "strand" in CNS translational medicine?

RNA interference has been envisaged as a powerful tool for molecular and clinical investigation with a great potential for clinical applications. In recent years, increased understanding of cancer biology and stem cell biology has dramatically accelerated the development of technology for cell and gene therapy in these areas. This paper is a review of the most recent report of innovative use of siRNA to benefit several central nervous system diseases. Furthermore, a description is made of innovative strategies of delivery into the brain by means of viral and non-viral vectors with high potential for translation into clinical use. Problems are also highlighted that might hamper the transition from bench to bed, analyzing the lack of reliable preclinical models with predictive validity and the lack of effective delivery systems, which are able to overcome biological barriers and specifically reach the brain site of action.

Engineered antibody therapies to counteract mutant huntingtin and related toxic intracellular proteins

The engineered antibody approach to Huntington's disease (HD) therapeutics is based on the premise that significantly lowering the levels of the primary misfolded mutant protein will reduce abnormal protein interactions and direct toxic effects of the misfolded huntingtin (HTT). This will in turn reduce the pathologic stress on cells, and normalize intrinsic proteostasis. Intracellular antibodies (intrabodies) are single-chain (scFv) and single-domain (dAb; nanobody) variable fragments that can retain the affinity and specificity of full-length antibodies, but can be selected and engineered as genes. Functionally, they represent a protein-based approach to the problem of aberrant mutant protein folding, post-translational modifications, protein-protein interactions, and aggregation. Several intrabodies that bind on either side of the expanded polyglutamine tract of mutant HTT have been reported to improve the mutant phenotype in cell and organotypic cultures, fruit flies, and mice. Further refinements to the difficult challenges of intraneuronal delivery, cytoplasmic folding, and long-term efficacy are in progress. This review covers published studies and emerging approaches on the choice of targets, selection and engineering methods, gene and protein delivery options, and testing of candidates in cell and animal models. The resultant antibody fragments can be used as direct therapeutics and as target validation/drug discovery tools for HD, while the technology is also applicable to a wide range of neurodegenerative and other diseases that are triggered by toxic proteins.

Therapeutics in Huntington's Disease

OPINION STATEMENT: There is no specific treatment for Huntington's disease (HD). Its many symptoms of motor, psychiatric, and cognitive deterioration are managed with symptomatic relief, rehabilitation, and support. The only drug approved by the US Food and Drug Administration (FDA) for the treatment of HD is an antichoreic agent, tetrabenazine, but this drug is used sparingly because of uneasiness regarding its propensity to cause depression and suicidality in this population, which is already at risk for these complications. Neuroleptics are still first-line treatments for chorea accompanied by comorbid depression and/or behavioral or psychotic symptoms, as is often the case. Psychiatric features, which have a significant impact on a patient's professional and personal life, often become the major focus of management. In addition to neuroleptics, commonly used medications include antidepressants, mood stabilizers, anxiolytics, and psychostimulants. In contrast, few treatment options are available for cognitive impairment in HD; this remains an important and largely unmet therapeutic need. HD patients typically lack insight into their disease manifestations, failing to recognize their need for treatment, and possibly even arguing against it. Multipurpose medications are employed advantageously to simplify the medication regimen, so as to facilitate compliance and not overwhelm the patient. For example, haloperidol can be prescribed for a patient with chorea, agitation, and anorexia, rather than targeting each symptom with a different drug. This approach also limits the potential for adverse effects, which can be difficult to distinguish from the features of the disease itself. With HD's complexity, it is best managed with a multidisciplinary approach that includes a movement disorders specialist, a genetic counselor, a mental health professional, a physical therapist, and a social worker for support and coordination of services. As the disease progresses, there may be need for other specialists, such as a speech and occupational therapist, a nutritionist for weight loss, and ultimately, a palliative care specialist.

Mutant huntingtin, abnormal mitochondrial dynamics, defective axonal transport of mitochondria, and selective synaptic degeneration in Huntington's disease

Huntington's disease (HD) is a progressive, fatal neurodegenerative disease caused by expanded polyglutamine repeats in the HD gene. HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment and emotional disturbances. Research into mutant huntingtin (Htt) and mitochondria has found that mutant Htt interacts with the mitochondrial protein dynamin-related protein 1 (Drp1), enhances GTPase Drp1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. This article summarizes latest developments in HD research and focuses on the role of abnormal mitochondrial dynamics and defective axonal transport in HD neurons. This article also discusses the therapeutic strategies that decrease mitochondrial fragmentation and neuronal damage in HD.

Convergent pathogenic pathways in Alzheimer's and Huntington's diseases: shared targets for drug development

Neurodegenerative diseases, exemplified by Alzheimer's disease and Huntington's disease, are characterized by progressive neuropsychiatric dysfunction and loss of specific neuronal subtypes. Although there are differences in the exact sites of pathology, and the clinical profiles of these two conditions only partially overlap, considerable similarities in disease mechanisms and pathogenic pathways can be observed. These shared mechanisms raise the possibility of exploiting common therapeutic targets for drug development. As Huntington's disease has a monogenic cause, it is possible to accurately identify individuals who carry the Huntington's disease mutation but do not yet manifest symptoms. These individuals could act as a model for Alzheimer's disease to test therapeutic interventions that target shared pathogenic pathways.