Gene suppression strategies for dominantly inherited neurodegenerative diseases: lessons from Huntington's disease and spinocerebellar ataxia

Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances

Spinocerebellar ataxia is a phenotypically and genetically heterogeneous group of autosomal dominant-inherited degenerative disorders. The gene mutation spectrum includes dynamic expansions, point mutations, duplications, insertions, and deletions of varying lengths. Dynamic expansion is the most common form of mutation. Mutations often result in indistinguishable clinical phenotypes, thus requiring validation using multiple genetic testing techniques. Depending on the type of mutation, the pathogenesis may involve proteotoxicity, RNA toxicity, or protein loss-of-function. All of which may disrupt a range of cellular processes, such as impaired protein quality control pathways, ion channel dysfunction, mitochondrial dysfunction, transcriptional dysregulation, DNA damage, loss of nuclear integrity, and ultimately, impairment of neuronal function and integrity which causes diseases. Many disease-modifying therapies, such as gene editing technology, RNA interference, antisense oligonucleotides, stem cell technology, and pharmacological therapies are currently under clinical trials. However, the development of curative approaches for genetic diseases remains a global challenge, beset by technical, ethical, and other challenges. Therefore, the study of the pathogenesis of spinocerebellar ataxia is of great importance for the sustained development of disease-modifying molecular therapies.

Gene Therapy for Neurofibromatosis Type 2-Related Schwannomatosis: Recent Progress, Challenges, and Future Directions

Neurofibromatosis type 2 (NF2)-related schwannomatosis is a rare autosomal dominant monogenic disorder caused by mutations in the NF2 gene. The hallmarks of NF2-related schwannomatosis are bilateral vestibular schwannomas (VS). The current treatment options for NF2-related schwannomatosis, such as observation with serial imaging, surgery, radiotherapy, and pharmacotherapies, have shown limited effectiveness and serious complications. Therefore, there is a critical demand for novel effective treatments. Gene therapy, which has made significant advancements in treating genetic diseases, holds promise for the treatment of this disease. This review covers the genetic pathogenesis of NF2-related schwannomatosis, the latest progress in gene therapy strategies, current challenges, and future directions of gene therapy for NF2-related schwannomatosis.

Exploring molecular mechanisms, therapeutic strategies, and clinical manifestations of Huntington's disease

Huntington's disease (HD) is a paradigm of a genetic neurodegenerative disorder characterized by the expansion of CAG repeats in the HTT gene. This extensive review investigates the molecular complexities of HD by highlighting the pathogenic mechanisms initiated by the mutant huntingtin protein. Adverse outcomes of HD include mitochondrial dysfunction, compromised protein clearance, and disruption of intracellular signaling, consequently contributing to the gradual deterioration of neurons. Numerous therapeutic strategies, particularly precision medicine, are currently used for HD management. Antisense oligonucleotides, such as Tominersen, play a leading role in targeting and modulating the expression of mutant huntingtin. Despite the promise of these therapies, challenges persist, particularly in improving delivery systems and the necessity for long-term safety assessments. Considering the future landscape, the review delineates promising directions for HD research and treatment. Innovations such as Clustered regularly interspaced short palindromic repeats associated system therapies (CRISPR)-based genome editing and emerging neuroprotective approaches present unprecedented opportunities for intervention. Collaborative interdisciplinary endeavors and a more insightful understanding of HD pathogenesis are on the verge of reshaping the therapeutic landscape. As we navigate the intricate landscape of HD, this review serves as a guide for unraveling the intricacies of this disease and progressing toward transformative treatments.

Antisense therapy: a potential breakthrough in the treatment of neurodegenerative diseases

Neurodegenerative diseases are a group of disorders characterized by the progressive degeneration of neurons in the central or peripheral nervous system. Currently, there is no cure for neurodegenerative diseases and this means a heavy burden for patients and the health system worldwide. Therefore, it is necessary to find new therapeutic approaches, and antisense therapies offer this possibility, having the great advantage of not modifying cellular genome and potentially being safer. Many preclinical and clinical studies aim to test the safety and effectiveness of antisense therapies in the treatment of neurodegenerative diseases. The objective of this review is to summarize the recent advances in the development of these new technologies to treat the most common neurodegenerative diseases, with a focus on those antisense therapies that have already received the approval of the U.S. Food and Drug Administration.

Novel systemic delivery of a peptide-conjugated antisense oligonucleotide to reduce α-synuclein in a mouse model of Alzheimer's disease

Neurodegenerative disorders of aging are characterized by the progressive accumulation of proteins such as α-synuclein (α-syn) and amyloid beta (Aβ). Misfolded and aggregated α-syn has been implicated in neurological disorders such as Parkinson's disease, and Dementia with Lewy Bodies, but less so in Alzheimer's Disease (AD), despite the fact that accumulation of α-syn has been confirmed in over 50% of postmortem brains neuropathologically diagnosed with AD. To date, no therapeutic strategy has effectively or consistently downregulated α-syn in AD. Here we tested the hypothesis that by using a systemically-delivered peptide (ApoB11) bound to a modified antisense oligonucleotide against α-syn (ASO-α-syn), we can downregulate α-syn expression in an AD mouse model and improve behavioral and neuropathologic phenotypes. Our results demonstrate that monthly systemic treatment with of ApoB11:ASO α-syn beginning at 6 months of age reduces expression of α-synuclein in the brains of 9-month-old AD mice. Downregulation of α-syn led to reduction in Aβ plaque burden, prevented neuronal loss and astrogliosis. Furthermore, we found that AD mice treated with ApoB11:ASO α-syn had greatly improved hippocampal and spatial memory function in comparison to their control counterparts. Collectively, our data supports the reduction of α-syn through use of systemically-delivered ApoB11:ASO α-syn as a promising future disease-modifying therapeutic for AD.

Therapeutic Strategies for Spinocerebellar Ataxia Type 1

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder that affects one or two individuals per 100,000. The disease is caused by an extended CAG repeat in exon 8 of the ATXN1 gene and is characterized mostly by a profound loss of cerebellar Purkinje cells, leading to disturbances in coordination, balance, and gait. At present, no curative treatment is available for SCA1. However, increasing knowledge on the cellular and molecular mechanisms of SCA1 has led the way towards several therapeutic strategies that can potentially slow disease progression. SCA1 therapeutics can be classified as genetic, pharmacological, and cell replacement therapies. These different therapeutic strategies target either the (mutant) ATXN1 RNA or the ataxin-1 protein, pathways that play an important role in downstream SCA1 disease mechanisms or which help restore cells that are lost due to SCA1 pathology. In this review, we will provide a summary of the different therapeutic strategies that are currently being investigated for SCA1.

Advances in gene therapy hold promise for treating hereditary hearing loss

Gene therapy focuses on genetic modification to produce therapeutic effects or treat diseases by repairing or reconstructing genetic material, thus being expected to be the most promising therapeutic strategy for genetic disorders. Due to the growing attention to hearing impairment, an increasing amount of research is attempting to utilize gene therapy for hereditary hearing loss (HHL), an important monogenic disease and the most common type of congenital deafness. Several gene therapy clinical trials for HHL have recently been approved, and, additionally, CRISPR-Cas tools have been attempted for HHL treatment. Therefore, in order to further advance the development of inner ear gene therapy and promote its broad application in other forms of genetic disease, it is imperative to review the progress of gene therapy for HHL. Herein, we address three main gene therapy strategies (gene replacement, gene suppression, and gene editing), summarizing the strategy that is most appropriate for particular monogenic diseases based on different pathogenic mechanisms, and then focusing on their successful applications for HHL in preclinical trials. Finally, we elaborate on the challenges and outlooks of gene therapy for HHL.

Therapeutic targeting of Huntington's disease: Molecular and clinical approaches

Huntington's disease (HD) is an autosomal dominant ailment that affects a larger population. Due to its complex pathology operating at DNA, RNA, and protein levels, it is regarded as a protein-misfolding disease and an expansion repeat disorder. Despite the availability of early genetic diagnostics, disease-modifying treatments are still missing. Importantly, potential therapies are starting to make their way through clinical trials. Still, clinical trials are ongoing to discover potential drugs to relieve HD symptoms. However, now being aware of the root cause, the clinical studies are focused on molecular therapies to target it. The road to success has not been without bumps since a big phase III trial of tominersen was unexpectedly discontinued due to exceeding risks than drug's benefit to the patients. Although the trial's conclusion was disappointing, there is still cause to be optimistic about what this technique may achieve. We have reviewed the present disease-modifying therapies in clinical development for HD and examined the current landscape of developing clinical therapies. We further investigated the pharmaceutical development of Huntington's medicine in the pharma industries and addressed the existing challenges in their therapeutic success.

Altering gene expression using antisense oligonucleotide therapy for hearing loss

Hearing loss affects more than 430 million people, worldwide, and is the third most common chronic physical condition in the United States and Europe (GBD Hearing Loss Collaborators, 2021; NIOSH, 2021; WHO, 2021). The loss of hearing significantly impacts motor and cognitive development, communication, education, employment, and overall quality of life. The inner ear houses the sensory organs for both hearing and balance and provides an accessible target for therapeutic delivery. Antisense oligonucleotides (ASOs) use various mechanisms to manipulate gene expression and can be tailor-made to treat disorders with defined genetic targets. In this review, we discuss the preclinical advancements within the field of the highly promising ASO-based therapies for hereditary hearing loss disorders. Particular focus is on ASO mechanisms of action, preclinical studies on ASO treatments of hearing loss, timing of therapeutic intervention, and delivery routes to the inner ear.

Recent approaches on Huntington's disease (Review)

Huntington's disease (HD) is a neurodegenerative disorder characterized by severe motor, cognitive and psychiatric symptoms. Patients of all ages can present with a dysfunction of the nervous system, which leads to the progressive loss of movement control and disabilities in speech, swallowing, communications, etc. The molecular basis of the disease is well-known, as HD is related to a mutated gene, a trinucleotide expansion, which encodes to the huntingtin protein. This protein is linked to neurogenesis and the loss of its function leads to neurodegenerative disorders. Although the genetic cause of the disorder has been known for decades, no effective treatment is yet available to prevent onset or to eliminate the progression of symptoms. Thus, the present review focused on the development of novel methods for the timely and accurate diagnosis of HD in an aim to aid the development of therapies which may reduce the severity of the symptoms and control their progression. The majority of the therapies include gene-silencing mechanisms of the mutated huntingtin gene aiming to suppress its expression, and the use of various substances as drugs with highly promising results. In the present review, the latest approaches on the diagnosis of HD are discussed along with the need for genetic counseling and an up-to-date presentation of the applied treatments.

Mutant huntingtin messenger RNA forms neuronal nuclear clusters in rodent and human brains

Mutant messenger RNA (mRNA) and protein contribute to the clinical manifestation of many repeat-associated neurological disorders, with the presence of nuclear RNA clusters being a common pathological feature. Yet, investigations into Huntington's disease-caused by a CAG repeat expansion in exon 1 of the huntingtin (HTT) gene-have primarily focused on toxic protein gain-of-function as the primary disease-causing feature. To date, mutant HTT mRNA has not been identified as an in vivo hallmark of Huntington's disease. Here, we report that, in two Huntington's disease mouse models (YAC128 and BACHD-97Q-ΔN17), mutant HTT mRNA is retained in the nucleus. Widespread formation of large mRNA clusters (∼0.6-5 µm³) occurred in 50-75% of striatal and cortical neurons. Cluster formation was independent of age and driven by expanded repeats. Clusters associate with chromosomal transcriptional sites and quantitatively co-localize with the aberrantly processed N-terminal exon 1-intron 1 mRNA isoform, HTT1a. HTT1a mRNA clusters are observed in a subset of neurons from human Huntington's disease post-mortem brain and are likely caused by somatic expansion of repeats. In YAC128 mice, clusters, but not individual HTT mRNA, are resistant to antisense oligonucleotide treatment. Our findings identify mutant HTT/HTT1a mRNA clustering as an early, robust molecular signature of Huntington's disease, providing in vivo evidence that Huntington's disease is a repeat expansion disease with mRNA involvement.

Haplotype-specific insertion-deletion variations for allele-specific targeting in Huntington's disease

Huntington's disease (HD) is a dominantly inherited neurodegenerative disease caused by an expanded CAG repeat in huntingtin (HTT). Given an important role for HTT in development and significant neurodegeneration at the time of clinical manifestation in HD, early treatment of allele-specific drugs represents a promising strategy. The feasibility of an allele-specific antisense oligonucleotide (ASO) targeting single-nucleotide polymorphisms (SNPs) has been demonstrated in models of HD. Here, we constructed a map of haplotype-specific insertion-deletion variations (indels) to develop alternative mutant-HTT-specific strategies. We mapped indels annotated in the 1000 Genomes Project data on common HTT haplotypes, revealing candidate indels for mutant-specific HTT targeting. Subsequent sequencing of an HD family confirmed candidate sites and revealed additional allele-specific indels. Interestingly, the most common normal HTT haplotype carries indels of big allele length differences at many sites, further uncovering promising haplotype-specific targets. When patient-derived cells carrying the most common HTT diplotype were treated with ASOs targeting the mutant alleles of candidate indels (rs772629195 or rs72239206), complete mutant specificity was observed. In summary, our map of haplotype-specific indels permits the identification of allele-specific targets in HD subjects, potentially contributing to the development of safe HTT-lowering therapeutics that are suitable for early treatment in HD.

Role of siRNA-based nanocarriers for the treatment of neurodegenerative diseases

Neurodegenerative disorders (NDs) lead to the progressive degeneration of the structural and physiological functions of the central and peripheral nervous systems, resulting in lifelong cognitive and motor dysfunction. Although comprehensive treatment of NDs is lacking, small interfering (si)RNA has shown therapeutic utility in the form of cellular nuclease-driven downregulation of mRNA levels. Various nanotechnologies have been used to modulate crucial physicochemical and biopharmaceutical properties of siRNA to provide protection and to enhance biomembrane interactions, residence times, tissue absorption, and cellular internalization for improved cytoplasm and/or nucleus interactions. In this review, we highlight advances in, and the role of, siRNA-based nanocarriers for the treatment of various NDs.

Nucleic Acid-Based Therapeutic Approach for Spinal and Bulbar Muscular Atrophy and Related Neurological Disorders

The recent advances in nucleic acid therapeutics demonstrate the potential to treat hereditary neurological disorders by targeting their causative genes. Spinal and bulbar muscular atrophy (SBMA) is an X-linked and adult-onset neurodegenerative disorder caused by the expansion of trinucleotide cytosine-adenine-guanine repeats, which encodes a polyglutamine tract in the androgen receptor gene. SBMA belongs to the family of polyglutamine diseases, in which the use of nucleic acids for silencing a disease-causing gene, such as antisense oligonucleotides and small interfering RNAs, has been intensively studied in animal models and clinical trials. A unique feature of SBMA is that both motor neuron and skeletal muscle pathology contribute to disease manifestations, including progressive muscle weakness and atrophy. As both motor neurons and skeletal muscles can be therapeutic targets in SBMA, nucleic acid-based approaches for other motor neuron diseases and myopathies may further lead to the development of a treatment for SBMA. Here, we review studies of nucleic acid-based therapeutic approaches in SBMA and related neurological disorders and discuss current limitations and perspectives to apply these approaches to patients with SBMA.

Innovative Therapeutic Approaches for Huntington's Disease: From Nucleic Acids to GPCR-Targeting Small Molecules

Huntington’s disease (HD) is a fatal neurodegenerative disorder due to an extraordinarily expanded CAG repeat in the huntingtin gene that confers a gain-of-toxic function in the mutant protein. There is currently no effective cure that attenuates progression and severity of the disease. Since HD is an inherited monogenic disorder, lowering the mutant huntingtin (mHTT) represents a promising therapeutic strategy. Huntingtin lowering strategies mostly focus on nucleic acid approaches, such as small interfering RNAs (siRNAs) and antisense oligonucleotides (ASOs). While these approaches seem to be effective, the drug delivery to the brain poses a great challenge and requires direct injection into the central nervous system (CNS) that results in substantial burden for patients. This review discusses the topics on Huntingtin lowering strategies with clinical trials in patients already underway and introduce an innovative approach that has the potential to deter the disease progression through the inhibition of GPR52, a striatal-enriched class A orphan G protein-coupled receptor (GPCR) that represents a promising therapeutic target for psychiatric disorders. Chemically simple, potent, and selective GPR52 antagonists have been discovered through high-throughput screening and subsequent structure-activity relationship studies. These small molecule antagonists not only diminish both soluble and aggregated mHTT in the striatum, but also ameliorate HD-like defects in HD mice. This therapeutic approach offers great promise as a novel strategy for HD therapy, while nucleic acid delivery still faces considerable challenges.

Characterization of Lifestyle in Spinocerebellar Ataxia Type 3 and Association with Disease Severity

BACKGROUND

Lifestyle could influence the course of hereditary ataxias, but representative data are missing.

OBJECTIVE

The objective of this study was to characterize lifestyle in spinocerebellar ataxia type 3 (SCA3) and investigate possible associations with disease parameters.

METHODS

In a prospective cohort study, data on smoking, alcohol consumption, physical activity, physiotherapy, and body mass index (BMI) were collected from 243 patients with SCA3 and 119 controls and tested for associations with age of onset, disease severity, and progression.

RESULTS

Compared with controls, patients with SCA3 were less active and consumed less alcohol. Less physical activity and alcohol abstinence were associated with more severe disease, but not with progression rates or age of onset. Smoking, BMI, or physiotherapy did not correlate with disease parameters.

CONCLUSION

Differences in lifestyle factors of patients with SCA3 and controls as well as associations of lifestyle factors with disease severity are likely driven by the influence of symptoms on behavior. No association between lifestyle and disease progression was detected. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Secreted therapeutics: monitoring durability of microRNA-based gene therapies in the central nervous system

The preclinical development of microRNA-based gene therapies for inherited neurodegenerative diseases is accompanied by translational challenges. Due to the inaccessibility of the brain to periodically evaluate therapy effects, accessible and reliable biomarkers indicative of dosing, durability and therapeutic efficacy in the central nervous system are very much needed. This is particularly important for viral vector-based gene therapies, in which a one-time administration results in long-term expression of active therapeutic molecules in the brain. Recently, extracellular vesicles have been identified as carriers of RNA species, including microRNAs, and proteins in all biological fluids, whilst becoming potential sources of biomarkers for diagnosis. In this study, we investigated the secretion and potential use of circulating miRNAs associated with extracellular vesicles as suitable sources to monitor the expression and durability of gene therapies in the brain. Neuronal cells derived from induced pluripotent stem cells were treated with adeno-associated viral vector serotype 5 carrying an engineered microRNA targeting huntingtin or ataxin3 gene sequences, the diseases-causing genes of Huntington disease and spinocerebellar ataxia type 3, respectively. After treatment, the secretion of mature engineered microRNA molecules was confirmed, with extracellular microRNA levels correlating with viral dose and cellular microRNA expression in neurons. We further investigated the detection of engineered microRNAs over time in the CSF of non-human primates after a single intrastriatal injection of adeno-associated viral vector serotype 5 carrying a huntingtin-targeting engineered microRNA. Quantifiable engineered microRNA levels enriched in extracellular vesicles were detected in the CSF up to two years after brain infusion. Altogether, these results confirm the long-term expression of adeno-associated viral vector serotype 5-delivered microRNAs and support the use of extracellular vesicle-associated microRNAs as novel translational pharmacokinetic markers in ongoing clinical trials of gene therapies for neurodegenerative diseases.

Gene Therapy for Polyglutamine Spinocerebellar Ataxias: Advances, Challenges, and Perspectives

Polyglutamine spinocerebellar ataxias (SCAs) comprise a heterogeneous group of six autosomal dominant ataxias caused by cytosine-adenine-guanine repeat expansions in the coding region of single genes. Currently, there is no curative or disease-slowing treatment for these disorders, but their monogenic inheritance has informed rationales for development of gene therapy strategies. In fact, RNA interference strategies have shown promising findings in cellular and/or animal models of SCA1, SCA3, SCA6, and SCA7. In addition, antisense oligonucleotide therapy has provided encouraging proofs of concept in models of SCA1, SCA2, SCA3, and SCA7, but they have not yet progressed to clinical trials. On the contrary, the gene editing strategies, such as the clustered regularly interspaced short palindromic repeat (CRISPR/Cas9), have been introduced to a limited extent in these disorders. In this article, we review the available literature about gene therapy in polyglutamine SCAs and discuss the main technological and ethical challenges toward the prospect of their use in future clinical trials. Although antisense oligonucleotide therapies are further along the path to clinical phases, the recent failure of three clinical trials in Huntington's disease may delay their utilization for polyglutamine SCAs, but they offer lessons that could optimize the likelihood of success in potential future clinical studies. © 2021 International Parkinson and Movement Disorder Society.

Widespread and sustained target engagement in Huntington's disease minipigs upon intrastriatal microRNA-based gene therapy

Huntingtin (HTT)-lowering therapies hold promise to slow down neurodegeneration in Huntington's disease (HD). Here, we assessed the translatability and long-term durability of recombinant adeno-associated viral vector serotype 5 expressing a microRNA targeting human HTT (rAAV5-miHTT) administered by magnetic resonance imaging-guided convention-enhanced delivery in transgenic HD minipigs. rAAV5-miHTT (1.2 × 10¹³ vector genome (VG) copies per brain) was successfully administered into the striatum (bilaterally in caudate and putamen), using age-matched untreated animals as controls. Widespread brain biodistribution of vector DNA was observed, with the highest concentration in target (striatal) regions, thalamus, and cortical regions. Vector DNA presence and transgene expression were similar at 6 and 12 months after administration. Expression of miHTT strongly correlated with vector DNA, with a corresponding reduction of mutant HTT (mHTT) protein of more than 75% in injected areas, and 30 to 50% lowering in distal regions. Translational pharmacokinetic and pharmacodynamic measures in cerebrospinal fluid (CSF) were largely in line with the effects observed in the brain. CSF miHTT expression was detected up to 12 months, with CSF mHTT protein lowering of 25 to 30% at 6 and 12 months after dosing. This study demonstrates widespread biodistribution, strong and durable efficiency of rAAV5-miHTT in disease-relevant regions in a large brain, and the potential of using CSF analysis to determine vector expression and efficacy in the clinic.

Current Status of Antisense Oligonucleotide-Based Therapy in Neuromuscular Disorders

Neuromuscular disorders include a wide range of diseases affecting the peripheral nervous system, which are primarily characterized by progressive muscle weakness and wasting. While there were no effective therapies until recently, several therapeutic approaches have advanced to clinical trials in the past few years. Among these, the antisense technology aiming at modifying RNA processing and function has remarkably progressed and a few antisense oligonucleotides (ASOs) have now been approved. Despite these recent clinical successes, several ASOs have also failed and clinical programs have been suspended, in most cases when the route of administration was systemic, highlighting the existing challenges notably with respect to effective ASO delivery. In this review we summarize the recent advances and current status of antisense based-therapies for neuromuscular disorders, using successful as well as unsuccessful examples to highlight the variability of outcomes depending on the target tissue and route of administration. We describe the different ASO-mediated therapeutic approaches, including splice-switching applications, steric-blocking strategies and targeted gene knock-down mediated by ribonuclease H recruitment. In this overview, we discuss the merits and challenges of the current ASO technology, and discuss the future of ASO development.

Mutant Ataxin-3-Containing Aggregates (MATAGGs) in Spinocerebellar Ataxia Type 3: Dynamics of the Disorder

Spinocerebellar ataxia type 3 (SCA3) is the most common type of SCA worldwide caused by abnormal polyglutamine expansion in the coding region of the ataxin-3 gene. Ataxin-3 is a multi-faceted protein involved in various cellular processes such as deubiquitination, cytoskeletal organisation, and transcriptional regulation. The presence of an expanded poly(Q) stretch leads to altered processing and misfolding of the protein culminating in the production of insoluble protein aggregates in the cell. Various post-translational modifications affect ataxin-3 fibrillation and aggregation. This review provides an exhaustive assessment of the various pathogenic mechanisms undertaken by the mutant ataxin-3-containing aggregates (MATAGGs) for disease induction and neurodegeneration. This includes in-depth discussion on MATAGG dynamics including their formation, role in neuronal pathogenesis, and the debate over the toxic v/s protective nature of the MATAGGs in disease progression. Additionally, the currently available therapeutic strategies against SCA3 have been reviewed. The shift in the focus of such strategies, from targeting the steps that lead to or reduce aggregate formation to targeting the expression of mutant ataxin-3 itself via RNA-based therapeutics, has also been presented. We also discuss the intriguing promise that various growth and neurotrophic factors, especially the insulin pathway, hold in the modulation of SCA3 progression. These emerging areas show the newer directions through which SCA3 can be targeted including various preclinical and clinical trials. All these advances made in the last three decades since the discovery of the ataxin-3 gene have been critically reviewed here.

Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains

Interactome maps are valuable resources to elucidate protein function and disease mechanisms. Here, we report on an interactome map that focuses on neurodegenerative disease (ND), connects ∼5,000 human proteins via ∼30,000 candidate interactions and is generated by systematic yeast two-hybrid interaction screening of ∼500 ND-related proteins and integration of literature interactions. This network reveals interconnectivity across diseases and links many known ND-causing proteins, such as α-synuclein, TDP-43, and ATXN1, to a host of proteins previously unrelated to NDs. It facilitates the identification of interacting proteins that significantly influence mutant TDP-43 and HTT toxicity in transgenic flies, as well as of ARF-GEP₁₀₀ that controls misfolding and aggregation of multiple ND-causing proteins in experimental model systems. Furthermore, it enables the prediction of ND-specific subnetworks and the identification of proteins, such as ATXN1 and MKL1, that are abnormally aggregated in postmortem brains of Alzheimer's disease patients, suggesting widespread protein aggregation in NDs.

Adeno-Associated Virus Vector for Central Nervous System Gene Therapy

The past several years have witnessed significant advances in the development of therapeutic gene delivery for neurological disorders of the central nervous system (CNS). In particular, genome-wide sequencing analysis has deepened our understanding of mutations that underlie many monogenic disorders, which in turn has contributed to clinical advances involving adeno-associated virus (AAV) vector delivery of replacement genes to treat recessive disorders. Moreover, gene therapy has been further bolstered with advances in genome editing tools that allow researchers to silence, repair, and amend endogenous genes. However, despite strong preclinical and clinical progress, challenges remain, including delivery and safety. Here, we discuss advances in AAV engineering, recent developments in cargo design, and translation of these technologies towards clinical progress.

Current Status of Gene Therapy Research in Polyglutamine Spinocerebellar Ataxias

Polyglutamine spinocerebellar ataxias (PolyQ SCAs) are a group of 6 rare autosomal dominant diseases, which arise from an abnormal CAG repeat expansion in the coding region of their causative gene. These neurodegenerative ataxic disorders are characterized by progressive cerebellar degeneration, which translates into progressive ataxia, the main clinical feature, often accompanied by oculomotor deficits and dysarthria. Currently, PolyQ SCAs treatment is limited only to symptomatic mitigation, and no therapy is available to stop or delay the disease progression, which culminates with death. Over the last years, many promising gene therapy approaches were investigated in preclinical studies and could lead to a future treatment to stop or delay the disease development. Here, we summed up the most promising of these therapies, categorizing them in gene augmentation therapy, gene silencing strategies, and gene edition approaches. While several of the reviewed strategies are promising, there is still a gap from the preclinical results obtained and their translation to clinical studies. However, there is an increase in the number of approved gene therapies, as well as a constant development in their safety and efficacy profiles. Thus, it is expected that in a near future some of the promising strategies reviewed here could be tested in a clinical setting and if successful provide hope for SCAs patients.

Clinical, Imaging, and Laboratory Markers of Premanifest Spinocerebellar Ataxia 1, 2, 3, and 6: A Systematic Review

Background and Purpose

Premanifest mutation carriers with spinocerebellar ataxia (SCA) can exhibit subtle abnormalities before developing ataxia. We summarized the preataxic manifestations of SCA1, -2, -3, and -6, and their associations with ataxia onset.

Methods

We included studies of the premanifest carriers of SCA published between January 1998 and December 2019 identified in Scopus and PubMed by searching for terms including ‘spinocerebellar ataxia’ and several synonyms of ‘preataxic manifestation’. We systematically reviewed the results obtained in studies categorized based on clinical, imaging, and laboratory markers.

Results

We finally performed a qualitative analysis of 48 papers. Common preataxic manifestations appearing in multiple SCA subtypes were muscle cramps, abnormal muscle reflexes, instability in gait and posture, lower Composite Cerebellar Functional Severity scores, abnormalities in video-oculography and transcranial magnetic stimulation, and gray-matter loss and volume reduction in the brainstem and cerebellar structures. Also, decreased sensory amplitudes in nerve conduction studies were observed in SCA2. Eotaxin and neurofilament light-chain levels were revealed as sensitive blood biomarkers in SCA3. Concerning potential predictive markers, hyporeflexia and abnormalities of somatosensory evoked potentials showed correlations with the time to ataxia onset in SCA2 carriers. However, no longitudinal data were found for the other SCA gene carriers.

Conclusions

Our results suggest that preataxic manifestations vary among SCA1, -2, -3, and -6, with some subtypes sharing specific features. Combining various markers into a standardized index for premanifest carriers may be useful for early screening and assessing the risk of disease progression in SCA carriers.

Huntingtin-Lowering Therapies for Huntington Disease: A Review of the Evidence of Potential Benefits and Risks

Huntington disease (HD) is caused by a cytosine-adenine-guanine trinucleotide repeat expansion in the huntingtin gene, HTT, that results in expression of variant (mutant) huntingtin protein (HTT). Therapeutic strategies that reduce HTT levels are currently being pursued to slow or stop disease progression in people with HD. These approaches are supported by robust preclinical data indicating that reducing variant huntingtin protein is associated with decreased HD pathology. However, the risk-benefit profile of reducing either variant HTT or both variant and wild-type HTT is currently an open question that is being addressed in ongoing clinical trials. This review aims to examine the current data available regarding altered HTT in humans, normal animals, and animal models of HD. Studies indexed in PubMed were searched using the MeSH term Huntington disease or the text words huntington or huntingtin from August 31, 1999, to August 31, 2019, with no language restrictions. Additional studies were included from the reference lists of relevant studies and the authors' personal files. Articles describing at least 1 aspect of HTT reduction were included, prioritizing those published within the last 10 years. In vivo studies were also prioritized, with a focus on studies that examined the consequences of wild-type HTT reduction in adults. In a recently completed phase 1/2a study of RG6042 in 46 adults with early manifest HD, antisense oligonucleotide-mediated partial reduction of HTT was reported to be generally safe and well tolerated over the course of 4-monthly RG6042 doses. In case studies of people with rare genetic variations in huntingtin alleles, the loss of 1 wild-type allele was not associated with HD. People with homozygous cytosine-adenine-guanine expansions developed normally until the onset of HD, although they may have experienced a more aggressive disease course. In mouse models of HD, partial reduction of HTT was beneficial, with improvements in motor, cognitive, and behavioral phenotypes. The partial reduction of wild-type HTT in normal adult rodents and nonhuman primates was generally safe and well tolerated. The body of evidence reviewed in this article indicates a positive risk-benefit profile for the partial reduction of either variant HTT alone or both variant and wild-type HTT. These strategies target the underlying cause of HD and are currently being tested in several investigational clinical trials.

Neurons under genetic control: What are the next steps towards the treatment of movement disorders?

Since the implementation of deep-brain stimulation as a therapy for movement disorders, there has been little progress in the clinical application of novel alternative treatments. Movement disorders are a group of neurological conditions, which are characterised with impairment of voluntary movement and share similar anatomical loci across the basal ganglia. The focus of the current review is on Parkinson's disease and Huntington's disease as they are the most investigated hypokinetic and hyperkinetic movement disorders, respectively. The last decade has seen enormous advances in the development of laboratory techniques that control neuronal activity. The two major ways to genetically control the neuronal function are: 1) expression of light-sensitive proteins that allow for the optogenetic control of the neuronal spiking and 2) expression or suppression of genes that control the transcription and translation of proteins. However, the translation of these methodologies from the laboratories into the clinics still faces significant challenges. The article summarizes the latest developments in optogenetics and gene therapy. Here, I compare the physiological mechanisms of established electrical deep brain stimulation to the experimental optogenetical deep brain stimulation. I compare also the advantages of DNA- and RNA-based techniques for gene therapy of familial movement disorders. I highlight the benefits and the major issues of each technique and I discuss the translational potential and clinical feasibility of optogenetic stimulation and gene expression control. The review emphasises recent technical breakthroughs that could initiate a notable leap in the treatment of movement disorders.

Polyglutamine spinocerebellar ataxias: emerging therapeutic targets

INTRODUCTION

Six of the most frequent dominantly inherited spinocerebellar ataxias (SCAs) worldwide - SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 - are caused by an expansion of a polyglutamine (polyQ) tract in the corresponding proteins. While the identification of the causative mutation has advanced knowledge on the pathogenesis of polyQ SCAs, effective therapeutics able to mitigate the severe clinical manifestation of these highly incapacitating disorders are not yet available.

AREAS COVERED

This review provides a comprehensive and critical perspective on well-established and emerging therapeutic targets for polyQ SCAs; it aims to inspire prospective drug discovery efforts.

EXPERT OPINION

The landscape of polyQ SCAs therapeutic targets and strategies includes (1) the mutant genes and proteins themselves, (2) enhancement of endogenous protein quality control responses, (3) abnormal protein-protein interactions of the mutant proteins, (4) disturbed neuronal function, (5) mitochondrial function, energy availability and oxidative stress, and (6) glial dysfunction, growth factor or hormone imbalances. Challenges include gaining a clearer definition of therapeutic targets for the drugs in clinical development, the discovery of novel drug-like molecules for challenging key targets, and the attainment of a stronger translation of preclinical findings to the clinic.

Therapeutic Update on Huntington's Disease: Symptomatic Treatments and Emerging Disease-Modifying Therapies

Huntington's disease (HD) is a monogenic neurodegenerative disorder that presents with progressive motor, behavior, and cognitive symptoms leading to early disability and mortality. HD is caused by an expanded CAG repeats in exon 1 of the huntingtin (HTT) gene. The corresponding genetic test allows a clinical, definite diagnosis in life and the identification of a fully penetrant mutation carrier in a premanifest stage. In addition to the development of symptomatic treatments that attempt to address unmet care needs such as apathy, irritability, and cognition, novel therapies that target pathways specific to HD biology are being developed with the intent of slowing disease progression. Among these approaches, HTT protein lowering therapies hold great promise. There are currently active programs using antisense oligonucleotides (ASOs), RNA interference, small-molecule splicing modulators, and zinc-finger protein transcription factor. Except for ASOs and RNA interference approaches, the remaining therapeutic strategies are at a preclinical stage of development. While the current therapeutic landscape in HD may bring an unparalleled change in the lives of people with HD and their families with the first-ever disease-modifying therapy, the evaluation of these therapies requires novel tools that enable a more efficient and expedited discovery and evaluative process. Examples are biomarkers targeting the HTT protein to measure target engagement or disease progression and rating scales more sensitive to the earliest clinical changes. These tools will be instrumental in the next phase of disease-modifying clinical trials in HD likely to target the phenoconversion period of the disease, including the prodromal HD stage.

Antisense Oligonucleotide Therapies for Neurodegenerative Diseases

Antisense oligonucleotides represent a novel therapeutic platform for the discovery of medicines that have the potential to treat most neurodegenerative diseases. Antisense drugs are currently in development for the treatment of amyotrophic lateral sclerosis, Huntington's disease, and Alzheimer's disease, and multiple research programs are underway for additional neurodegenerative diseases. One antisense drug, nusinersen, has been approved for the treatment of spinal muscular atrophy. Importantly, nusinersen improves disease symptoms when administered to symptomatic patients rather than just slowing the progression of the disease. In addition to the benefit to spinal muscular atrophy patients, there are discoveries from nusinersen that can be applied to other neurological diseases, including method of delivery, doses, tolerability of intrathecally delivered antisense drugs, and the biodistribution of intrathecal dosed antisense drugs. Based in part on the early success of nusinersen, antisense drugs hold great promise as a therapeutic platform for the treatment of neurological diseases.

Positive Attitudes and Therapeutic Misconception Around Hypothetical Clinical Trial Participation in the Huntington's Disease Community

Background:

New therapies that could modify the disease course of Huntington’s disease (HD) are entering clinical trials. However, conceptions about clinical research from the HD community are unknown. This knowledge could help inform patient-clinician discussions surrounding clinical trial participation.

Objective:

The purpose of this study was to assess clinical trial attitudes and understanding in the HD community.

Methods:

We developed a survey incorporating two measures of trial understanding and attitudes and the impact of therapeutic route of administration on hypothetical trial participation. The survey was distributed via emails, flyers, and social media through HD-related organizations.

Results:

There were 73 responses. Individuals self-reported as clinically diagnosed with HD, gene positive but asymptomatic, or primary caregivers. Respondents viewed clinical trials positively and generally viewed trials as safe. Individuals with prior HD-related research experience were less likely to have negative expectations about trials than those without research experience (p = 0.002), and women had higher information needs than men (p = 0.001). Individuals with HD were more likely than the other groups to experience therapeutic misconception (p = 0.002). All respondents were able to appraise risks and benefits of research but exhibited optimism about trial outcomes. Willingness to participate was highest when the route of administration was minimally invasive.

Conclusions:

While the HD community views clinical trials positively, patients with HD are at high risk for therapeutic misconception and all groups are optimistic about trial outcomes. Limitations of this study include a small sample that may be inclined to view research positively given past trial participation and interest in participating in HD surveys. However, the findings from this study can be used to strengthen informed consent during HD clinical trial recruitment.

Systemic Brain Delivery of Antisense Oligonucleotides across the Blood-Brain Barrier with a Glucose-Coated Polymeric Nanocarrier

Current antisense oligonucleotide (ASO) therapies for the treatment of central nervous system (CNS) disorders are performed through invasive administration, thereby placing a major burden on patients. To alleviate this burden, we herein report systemic ASO delivery to the brain by crossing the blood-brain barrier using glycemic control as an external trigger. Glucose-coated polymeric nanocarriers, which can be bound by glucose transporter-1 expressed on the brain capillary endothelial cells, are designed for stable encapsulation of ASOs, with a particle size of about 45 nm and an adequate glucose-ligand density. The optimized nanocarrier efficiently accumulates in the brain tissue 1 h after intravenous administration and exhibits significant knockdown of a target long non-coding RNA in various brain regions, including the cerebral cortex and hippocampus. These results demonstrate that the glucose-modified polymeric nanocarriers enable noninvasive ASO administration to the brain for the treatment of CNS disorders.

Therapeutic Antisense Targeting of Huntingtin

Antisense oligonucleotides (ASOs) are a relatively new therapeutic entity that utilizes short chemically modified strands of DNA in targeted interactions with RNA to modulate the type or amount of resultant protein. This brief review summarizes the preclinical, translational, and early clinical development of an ASO designed to reduce the production of the disease-causing protein in Huntington's disease, an inherited neurodegenerative disease.

Nuclear bodies formed by polyQ-ataxin-1 protein are liquid RNA/protein droplets with tunable dynamics

A mutant form of the ataxin-1 protein with an expanded polyglutamine (polyQ) tract is the underlying cause of the inherited neurodegenerative disease spinocerebellar ataxia 1 (SCA1). In probing the biophysical features of the nuclear bodies (NBs) formed by polyQ-ataxin-1, we defined ataxin-1 NBs as spherical liquid protein/RNA droplets capable of rapid fusion. We observed dynamic exchange of the ataxin-1 protein into these NBs; notably, cell exposure to a pro-oxidant stress could trigger a transition to slower ataxin-1 exchange, typical of a hydrogel state, which no longer showed the same dependence on RNA or sensitivity to 1,6-hexanediol. Furthermore, we could alter ataxin-1 exchange dynamics either through modulating intracellular ATP levels, RNA helicase inhibition, or siRNA-mediated depletion of select RNA helicases. Collectively, these findings reveal the tunable dynamics of the liquid RNA/protein droplets formed by polyQ-ataxin-1.

Cerebellar contribution to the cognitive alterations in SCA1: evidence from mouse models

Spinocerebellar ataxia type 1 (SCA1) is a fatal neurodegenerative disease caused by abnormal expansion of glutamine (Q) encoding CAG repeats in the gene Ataxin-1 (ATXN1). Although motor and balance deficits are the core symptoms of SCA1, cognitive decline is also commonly observed in patients. While mutant ATXN1 is expressed throughout the brain, pathological findings reveal severe atrophy of cerebellar cortex in SCA1 patients. The cerebellum has recently been implicated in diverse cognitive functions, yet to what extent cerebellar neurodegeneration contributes to cognitive alterations in SCA1 remains poorly understood. Much of our understanding of the mechanisms underlying pathogenesis of motor symptoms in SCA1 comes from mouse models. Reasoning that mouse models could similarly offer important insights into the mechanisms of cognitive alterations in SCA1, we tested cognition in several mouse lines using Barnes maze and fear conditioning. We confirmed cognitive deficits in Atxn1154Q/2Q knock-in mice with brain-wide expression of mutant ATXN1 and in ATXN1 null mice. We found that shorter polyQ length and haploinsufficiency of ATXN1 do not cause significant cognitive deficits. Finally, ATXN1[82Q ] transgenic mice-with cerebellum limited expression of mutant ATXN1-demonstrated milder impairment in most aspects of cognition compared to Atxn1154Q/2Q mice, supporting the concept that cognitive deficits in SCA1 arise from a combination of cerebellar and extra-cerebellar dysfunctions.

Disrupted Calcium Signaling in Animal Models of Human Spinocerebellar Ataxia (SCA)

Spinocerebellar ataxias (SCAs) constitute a heterogeneous group of more than 40 autosomal-dominant genetic and neurodegenerative diseases characterized by loss of balance and motor coordination due to dysfunction of the cerebellum and its efferent connections. Despite a well-described clinical and pathological phenotype, the molecular and cellular events that underlie neurodegeneration are still poorly undaerstood. Emerging research suggests that mutations in SCA genes cause disruptions in multiple cellular pathways but the characteristic SCA pathogenesis does not begin until calcium signaling pathways are disrupted in cerebellar Purkinje cells. Ca²⁺ signaling in Purkinje cells is important for normal cellular function as these neurons express a variety of Ca²⁺ channels, Ca²⁺-dependent kinases and phosphatases, and Ca²⁺-binding proteins to tightly maintain Ca²⁺ homeostasis and regulate physiological Ca²⁺-dependent processes. Abnormal Ca²⁺ levels can activate toxic cascades leading to characteristic death of Purkinje cells, cerebellar atrophy, and ataxia that occur in many SCAs. The output of the cerebellar cortex is conveyed to the deep cerebellar nuclei (DCN) by Purkinje cells via inhibitory signals; thus, Purkinje cell dysfunction or degeneration would partially or completely impair the cerebellar output in SCAs. In the absence of the inhibitory signal emanating from Purkinje cells, DCN will become more excitable, thereby affecting the motor areas receiving DCN input and resulting in uncoordinated movements. An outstanding advantage in studying the pathogenesis of SCAs is represented by the availability of a large number of animal models which mimic the phenotype observed in humans. By mainly focusing on mouse models displaying mutations or deletions in genes which encode for Ca²⁺ signaling-related proteins, in this review we will discuss the several pathogenic mechanisms related to deranged Ca²⁺ homeostasis that leads to significant Purkinje cell degeneration and dysfunction.

Universal RNAi Triggers for the Specific Inhibition of Mutant Huntingtin, Atrophin-1, Ataxin-3, and Ataxin-7 Expression

The expansion of CAG repeats within the coding region of associated genes is responsible for nine inherited neurodegenerative disorders including Huntington’s disease (HD), spinocerebellar ataxias (SCAs), and dentatorubral-pallidoluysian atrophy (DRPLA). Despite years of research aimed at developing an effective method of treatment, these diseases remain incurable and only their symptoms are controlled. The purpose of this study was to develop effective and allele-selective genetic tools for silencing the expression of mutated genes containing expanded CAG repeats. Here we show that repeat-targeting short hairpin RNAs preferentially reduce the levels of mutant huntingtin, atrophin-1, ataxin-3, and ataxin-7 proteins in patient-derived fibroblasts and may serve as universal allele-selective reagents for polyglutamine (polyQ) diseases.

Transcriptional Regulation of the Huntingtin Gene

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide expansion in the HTT gene, which encodes for an abnormal polyglutamine tract in the huntingtin protein (HTT). This review examines the known mechanisms of HTT gene regulation. We discuss HTT expression patterns, features of the HTT promoter, regulatory regions of the HTT promoter with functional significance, and HTT regulators located outside of the proximal promoter region. The factors that influence HTT expression in the brain and the mechanisms of HTT transcriptional regulation are currently poorly understood, despite continuing research. Expanding knowledge of HTT regulation will inform future studies investigating HTT function. Improving understanding of HTT expression and control may also uncover novel therapeutic approaches for HD through the development of methods to modulate mHTT levels.

MicroRNAs Dysregulation and Metabolism in Multiple System Atrophy

Multiple system atrophy (MSA) is an adult onset, fatal disease, characterized by an accumulation of alpha-synuclein (α-syn) in oligodendroglial cells. MicroRNAs (miRNAs) are small non-coding RNAs involved in post-translational regulation and several biological processes. Disruption of miRNA-related pathways in the central nervous system (CNS) plays an important role in the pathogenesis of neurodegenerative diseases, including MSA. While the exact mechanisms underlying miRNAs in the pathogenesis of MSA remain unclear, it is known that miRNAs can repress the translation of messenger RNAs (mRNAs) that regulate the following pathogenesis associated with MSA: autophagy, neuroinflammation, α-syn accumulation, synaptic transmission, oxidative stress, and apoptosis. In this review, the metabolism of miRNAs and their functional roles in the pathogenesis of MSA are discussed, thereby highlighting miRNAs as potential new biomarkers for the diagnosis of MSA and in increasing our understanding of the disease process.

Molecular Mechanisms and Therapeutics for SCA17

Spinocerebellar ataxia type 17 (SCA17) is caused by polyglutamine (polyQ) expansion in the TATA box-binding protein (TBP), which functions as a general transcription factor. Like other polyQ expansion-mediated diseases, SCA17 is characterized by late-onset and selective neurodegeneration, despite the disease protein being ubiquitously expressed in the body. To date, the pathogenesis of polyQ diseases is not fully understood, and there are no effective treatments for these devastating disorders. The well-characterized function of TBP and typical neurodegeneration in SCA17 give us opportunities to understand how polyQ expansion causes selective neurodegeneration and to develop effective therapeutics. In this review, we discuss the molecular mechanisms behind SCA17, focusing on transcriptional dysregulation as its major cause. Mounting evidence suggests that reversing transcriptional alterations induced by mutant TBP and reducing the expression of mutant TBP are promising strategies to treat SCA17.

Application of 2'-OMethylRNA' Antisense Oligomer to Control Candida albicans EFG1 Virulence Determinant

Antisense oligomers and their analogs have been successfully utilized to silence gene expression for the treatment of many human diseases; however, the control of yeast’s virulence determinants has never been exploited before. In this sense, this work is based on the key hypothesis that if a pathogen’s genetic sequence is a determinant of virulence, it will be possible to synthesize a nucleic acid mimic based on antisense therapy (AST) that will bind to the mRNA produced, blocking its translation into protein and, consequently, reducing the pathogen virulence phenotype. EFG1 is an important determinant of virulence that is involved in the regulation of the Candida albicans switch from yeast to filamentous form. Thus, our main goal was to design and synthesize an antisense oligonucleotide (ASO) targeting the EFG1 mRNA and to validate its in vitro applicability. The results show that the anti-EFG1 2′-OMethylRNA (2′OMe) oligomer was able to significantly reduce the levels of EFG1 gene expression and of Efg1p protein translation (both approximately 60%), as well as effectively prevent filamentation of C. albicans cells (by 80%). Moreover, it was verified that anti-EFG1 2′OMe keeps the efficacy in different simulated human body fluids. Undeniably, this work provides potentially valuable information for future research into the management of Candida infections, regarding the development of a credible and alternative method to control C. albicans infections, based on AST methodology.

Spinocerebellar ataxias: prospects and challenges for therapy development

The spinocerebellar ataxias (SCAs) comprise more than 40 autosomal dominant neurodegenerative disorders that present principally with progressive ataxia. Within the past few years, studies of pathogenic mechanisms in the SCAs have led to the development of promising therapeutic strategies, especially for SCAs caused by polyglutamine-coding CAG repeats. Nucleotide-based gene-silencing approaches that target the first steps in the pathogenic cascade are one promising approach not only for polyglutamine SCAs but also for the many other SCAs caused by toxic mutant proteins or RNA. For these and other emerging therapeutic strategies, well-coordinated preparation is needed for fruitful clinical trials. To accomplish this goal, investigators from the United States and Europe are now collaborating to share data from their respective SCA cohorts. Increased knowledge of the natural history of SCAs, including of the premanifest and early symptomatic stages of disease, will improve the prospects for success in clinical trials of disease-modifying drugs. In addition, investigators are seeking validated clinical outcome measures that demonstrate responsiveness to changes in SCA populations. Findings suggest that MRI and magnetic resonance spectroscopy biomarkers will provide objective biological readouts of disease activity and progression, but more work is needed to establish disease-specific biomarkers that track target engagement in therapeutic trials. Together, these efforts suggest that the development of successful therapies for one or more SCAs is not far away.

Clinical Characteristics and Possible Drug Targets in Autosomal Dominant Spinocerebellar Ataxias

Background & Objective:

The autosomal dominant spinocerebellar ataxias (SCAs) belong to a large and expanding group of neurodegenerative disorders. SCAs comprise more than 40 subtypes characterized by progressive ataxia as a common feature. The most prevalent diseases among SCAs are caused by CAG repeat expansions in the coding-region of the causative gene resulting in polyglutamine (polyQ) tract formation in the encoded protein. Unfortunately, there is no approved therapy to treat cerebellar motor dysfunction in SCA patients. In recent years, several studies have been conducted to recognize the clinical and pathophysiological aspects of the polyQ SCAs more accurately. This scientific progress has provided new opportunities to develop promising gene therapies, including RNA interference and antisense oligonucleotides.

Conclusion:

The aim of the current work is to give a brief summary of the clinical features of SCAs and to review the cardinal points of pathomechanisms of the most common polyQ SCAs. In addition, we review the last few year’s promising gene suppression therapies of the most frequent polyQ SCAs in animal models, on the basis of which human trials may be initiated in the near future.

Full sequence of mutant huntingtin 3'-untranslated region and modulation of its gene regulatory activity by endogenous microRNA

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat in the first exon of the huntingtin gene (HTT). Since the entire course of the disease starts from this dominant gain-of-function mutation, lowering total or mutant huntingtin mRNA/protein has emerged as an appealing therapeutic strategy. We reasoned that endogenous mechanisms underlying HTT gene regulation may inform strategies to target the source of the disease. As part of our investigation to understand how the expression of HTT is controlled, we performed (1) complete sequencing analysis for mutant HTT 3'-UTR and (2) unbiased screening assays to identify naturally-occurring miRNAs that could lower the HTT mRNA levels. By sequencing HD families inheriting the major European mutant haplotype, we determined the full sequence of HTT 3'-UTRs of the most frequent mutant (i.e., hap.01) and normal (i.e., hap.08) haplotypes, revealing 5 sites with alternative alleles. In subsequent miRNA activity assays using the full-length hap.01 and hap.08 3'-UTR reporter vectors and follow-up validation experiments, hsa-miR-4324 and hsa-miR-4756-5p significantly reduced HTT 3'-UTR reporter activity and endogenous HTT protein levels. However, those miRNAs did not show strong haplotype-specific effects. Nevertheless, our data highlighting full sequences of HTT 3'-UTR haplotypes, effects of miRNAs on HTT levels, and potential interaction sites provide rationale and promising targets for total and mutant-specific HTT lowering intervention strategies using endogenous and artificial miRNAs, respectively.

Systemic peptide mediated delivery of an siRNA targeting α-syn in the CNS ameliorates the neurodegenerative process in a transgenic model of Lewy body disease

Neurodegenerative disorders of the aging population are characterized by progressive accumulation of neuronal proteins such as α-synuclein (α-syn) in Parkinson's Disease (PD) and Amyloid ß (Aß) and Tau in Alzheimer's disease (AD) for which no treatments are currently available. The ability to regulate the expression at the gene transcription level would be beneficial for reducing the accumulation of these proteins or regulating expression levels of other genes in the CNS. Short interfering RNA molecules can bind specifically to target RNAs and deliver them for degradation. This approach has shown promise therapeutically in vitro and in vivo in mouse models of PD and AD and other neurological disorders; however, delivery of the siRNA to the CNS in vivo has been achieved primarily through intra-cerebral or intra-thecal injections that may be less amenable for clinical translation; therefore, alternative approaches for delivery of siRNAs to the brain is needed. Recently, we described a small peptide from the envelope protein of the rabies virus (C2-9r) that was utilized to deliver an siRNA targeting α-syn across the blood brain barrier (BBB) following intravenous injection. This approach showed reduced expression of α-syn and neuroprotection in a toxic mouse model of PD. However, since receptor-mediated delivery is potentially saturable, each allowing the delivery of a limited number of molecules, we identified an alternative peptide for the transport of nucleotides across the BBB based on the apolipoprotein B (apoB) protein targeted to the family of low-density lipoprotein receptors (LDL-R). We used an 11-amino acid sequence from the apoB protein (ApoB11) that, when coupled with a 9-amino acid arginine linker, can transport siRNAs across the BBB to neuronal and glial cells. To examine the value of this peptide mediated oligonucleotide delivery system for PD, we delivered an siRNA targeting the α-syn (siα-syn) in a transgenic mouse model of PD. We found that ApoB11 was effective (comparable to C2-9r) at mediating the delivery of siα-syn into the CNS, co-localized to neurons and glial cells and reduced levels of α-syn protein translation and accumulation. Delivery of ApoB11/siα-syn was accompanied by protection from degeneration of selected neuronal populations in the neocortex, limbic system and striato-nigral system and reduced neuro-inflammation. Taken together, these results suggest that systemic delivery of oligonucleotides targeting α-syn using ApoB11 might be an interesting alternative strategy worth considering for the experimental treatment of synucleinopathies.

RNA delivery biomaterials for the treatment of genetic and rare diseases

Genetic and rare diseases (GARDs) affect more than 350 million patients worldwide and remain a significant challenge in the clinic. Hence, continuous efforts have been made to bridge the significant gap between the supply and demand of effective treatments for GARDs. Recent decades have witnessed the impressive progress in the fight against GARDs, with an improved understanding of the genetic origins of rare diseases and the rapid development in gene therapy providing a new avenue for GARD therapy. RNA-based therapeutics, such as RNA interference (RNAi), messenger RNA (mRNA) and RNA-involved genome editing technologies, demonstrate great potential as a therapy tool for treating genetic associated rare diseases. In the meantime, a variety of RNA delivery vehicles were established for boosting the widespread applications of RNA therapeutics. Among all the RNA delivery platforms which enable the systemic applications of RNAs, non-viral RNA delivery biomaterials display superior properties and a few biomaterials have been successfully exploited for achieving the RNA-based gene therapies on GARDs. In this review article, we focus on recent advances in the development of novel biomaterials for delivery of RNA-based therapeutics and highlight their applications to treat GARDs.

The Emerging Role of microRNAs in Polyglutamine Diseases

MicroRNAs (miRNAs) are small non-coding molecules that regulate a large amount of post-transcriptional repressor genes by recognizing semi-complementary target sequences that are normally located in the 3' UTR of the mRNA. Altered expression of miRNA has been related to several pathological processes, including polyglutamine (Poly Q) diseases. Specific expression patterns in the circulating fluids and brain parenchyma have been speculated as potential biomarkers for Poly Q disease diagnosis and prognosis. Several miRNAs have been consistently identified in diseases including Huntington's disease (HD) and spinocerebellar ataxia (SCA). In our review, we describe the emerging role of miRNAs in Poly Q diseases and provide an overview on general miRNA biology, implications in pathophysiology, and their potential roles as future biomarkers and applications for therapy.