Antisense gene therapy for neurodegenerative disease?

Targeted Oligonucleotides for Treating Neurodegenerative Tandem Repeat Diseases

Nucleotide repeat disorders encompass more than 30 diseases, most of which show dominant inheritance, such as Huntington's disease, spinocerebellar ataxias, and myotonic dystrophies. Yet others, including Friedreich's ataxia, are recessively inherited. A common feature is the presence of a DNA tandem repeat in the disease-associated gene and the propensity of the repeats to expand in germ and in somatic cells, with ensuing neurological and frequently also neuromuscular defects. Repeat expansion is the most frequent event in these diseases; however, sequence contractions, deletions, and mutations have also been reported. Nucleotide repeat sequences are predisposed to adopt non-B-DNA conformations, such as hairpins, cruciform, and intramolecular triple-helix structures (triplexes), also known as H-DNA. For gain-of-function disorders, oligonucleotides can be used to target either transcripts or duplex DNA and in diseases with recessive inheritance oligonucleotides may be used to alter repressive DNA or RNA conformations. Most current treatment strategies are aimed at altering transcript levels, but therapies directed against DNA are also emerging, and novel strategies targeting DNA, instead of RNA, are described. Different mechanisms using modified oligonucleotides are discussed along with the structural aspects of repeat sequences, which can influence binding modes and efficiencies.

New developments in exon skipping and splice modulation therapies for neuromuscular diseases

INTRODUCTION

Antisense oligonucleotide (AON) therapy is a form of treatment for genetic or infectious diseases using small, synthetic DNA-like molecules called AONs. Recent advances in the development of AONs that show improved stability and increased sequence specificity have led to clinical trials for several neuromuscular diseases. Impressive preclinical and clinical data are published regarding the usage of AONs in exon-skipping and splice modulation strategies to increase dystrophin production in Duchenne muscular dystrophy (DMD) and survival of motor neuron (SMN) production in spinal muscular atrophy (SMA).

AREAS COVERED

In this review, we focus on the current progress and challenges of exon-skipping and splice modulation therapies. In addition, we discuss the recent failure of the Phase III clinical trials of exon 51 skipping (drisapersen) for DMD.

EXPERT OPINION

The main approach of AON therapy in DMD and SMA is to rescue ('knock up' or increase) target proteins through exon skipping or exon inclusion; conversely, most conventional antisense drugs are designed to knock down (inhibit) the target. Encouraging preclinical data using this 'knock up' approach are also reported to rescue dysferlinopathies, including limb-girdle muscular dystrophy type 2B, Miyoshi myopathy, distal myopathy with anterior tibial onset and Fukuyama congenital muscular dystrophy.

Cell penetrating peptide tethered bi-ligand liposomes for delivery to brain in vivo: Biodistribution and transfection

Targeted nano-particulate systems hold extraordinary potential for delivery of therapeutics across blood brain barrier (BBB). In this work, we investigated the potential of novel bi-ligand (transferrin-poly-l-arginine) liposomal vector for delivery of desired gene to brain, in vivo. The in vivo evaluation of the delivery vectors is essential for clinical translation. We followed an innovative approach of combining transferrin receptor targeting with enhanced cell penetration to design liposomal vectors for improving the transport of molecules into brain. The biodistribution profile of 1, 1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine iodide(DiR)-labeled liposomes was evaluated in adult rats after single intravenous injection at dose of 15.2μmoles of phospholipids/kg body weight. We demonstrated that bi-ligand liposomes accumulated in rat brain at significantly (p<0.05) higher concentrations as compared to the single-ligand (transferrin) or plain liposomes. In addition, the bi-ligand liposomes resulted in increased expression of β-galactosidase(β-gal) plasmid in rat brain tissue in comparison to the single-ligand liposomes. Histological examination of the transfected tissues did not show any signs of tissue necrosis or inflammation. Hemolysis assay further authenticated the biocompatibility of bi-ligand liposomes in blood up to 600 nmoles of phospholipids/1.4×10(7) erythrocytes. The findings of this study provide important and detailed information regarding the distribution of bi-ligand liposomes in vivo and accentuate their ability to demonstrate improved brain penetration and transfection potential over single-ligand liposomes.

Transforming Growth Factor-Beta Signaling in the Neural Stem Cell Niche: A Therapeutic Target for Huntington's Disease

The neural stem cell niches possess the regenerative capacity to generate new functional neurons in the adult brain, suggesting the possibility of endogenous neuronal replacement after injury or disease. Huntington disease (HD) is a neurodegenerative disease and characterized by neuronal loss in the basal ganglia, leading to motor, cognitive, and psychological disabilities. Apparently, in order to make use of the neural stem cell niche as a therapeutic concept for repair strategies in HD, it is important to understand the cellular and molecular composition of the neural stem cell niche under such neurodegenerative conditions. This paper mainly discusses the current knowledge on the regulation of the hippocampal neural stem cell niche in the adult brain and by which mechanism it might be compromised in the case of HD.

Experimental surgical therapies for Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by abnormal movement, cognitive decline, and psychiatric disturbance. HD is caused by a trinucleotide repeat expansion in the HTT gene and a corresponding neurotoxic polyglutamine expansion in the huntingtin protein. There is currently no therapy to modify the progressive course of the disease, and symptomatic treatment options are limited. In this review we describe a diverse set of emerging experimental therapeutic strategies for HD: deep brain stimulation; delivery of neurotrophic factors; cell transplantation; HTT gene silencing using RNA interference or antisense oligonucleotides; and delivery of intrabodies. The common feature of these experimental therapies is that they all require a neurosurgical intervention, either for implantation of an electrode or for brain delivery of molecules, viruses or cells that do not cross the blood-brain barrier upon oral or intravenous administration. We summarize available data on the rationale, safety, efficacy, and intrinsic limitations of each of these approaches, focusing mainly on studies in HD patients and genetic animal models of HD. Although each of these strategies holds significant promise, their efficacy remains to be proven in HD patients.

Characterization of the nanostructure of complexes formed by single- or double-stranded oligonucleotides with a cationic surfactant

We report the use of dynamic light scattering (DLS), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) to characterize the nanostructure of complexes formed by either single- or double-stranded oligonucleotides with a cationic surfactant (cetyltrimethylammonium bromide, CTAB) in aqueous solution (1 mM Li(2)SO(4)). For single-stranded oligonucleotides 5'-A(20)-3' and 5'-CCCCATTCTAGCAGCCCGGG-3', both the appearance of two Bragg peaks (at 0.14 and 0.28 Å(-1)) in SAXS spectra with a spacing of 1:2 and form factor fits to SANS spectra are consistent with the presence of multilamellar vesicles (with, on average, 6-9 layers with a periodicity of 45-48 Å). Some samples showed evidence of an additional Bragg peak (at 0.20 Å(-1)) associated with periodic packing (with a periodicity of 31 Å) of the oligonucleotides within the lamellae of the nanostructure. The nucleotide composition of the single-stranded oligonucleotides was also found to impact the number and size of the complexes formed with CTAB. In contrast to 5'-A(20)-3' and 5'-CCCCATTCTAGCAGCCCGGG-3', 5'-T(20)-3' did not change the state of aggregation of CTAB (globular micelles) over a wide range of oligonucleotide:CTAB charge ratios. These results support the proposition that hydrophobic interactions, as well as electrostatics, play a central role in the formation of complexes between cationic amphiphiles and single-stranded oligonucleotides and thus give rise to nanostructures that depend on nucleotide composition. In contrast to the single-stranded oligonucleotides, for double-stranded oligonucleotides mixed with CTAB, three Bragg peaks (0.13, 0.23, and 0.25 Å(-1)) in SAXS spectra with a spacing ratio of 1:√3:√4 and characteristic changes in SANS spectra indicate formation of a hexagonal nanostructure. Also, the composition of the double-stranded oligonucleotides did not measurably impact the nanostructure of complexes formed with CTAB, suggesting that electrostatic interactions dominate the formation of these complexes. Overall, these results provide insights into the intermolecular interactions that occur between cationic amphiphiles and oligonucleotides and establish that single and double-stranded oligonucleotides form complexes with cationic surfactants that differ in nanostructure. The results also provide guidance for the design of oligonucleotide complexes with cationic amphiphiles.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Pharmaceutical, cellular and genetic therapies for Huntington's disease

HD (Huntington's disease) is a devastating neurodegenerative disorder caused by a polyglutamine expansion in the gene encoding the huntingtin protein. Presently, there is no known cure for HD and existing symptomatic treatments are limited. However, recent advances have identified multiple pathological mechanisms involved in HD, some of which have now become the focus of therapeutic intervention. In this review, we consider progress made towards developing safe and effective pharmaceutical-, cell- and genetic-based therapies, and discuss the extent to which some of these therapies have been successfully translated into clinical trials. These new prospects offer hope for delaying and possibly halting this debilitating disease.

Adenovirus-Mediated Silencing of Huntingtin Expression by shRNA

Huntington's disease (HD) is an inherited autosomal dominant, neurodegenerative disease that is caused by a gain of function mutation characterized by the expansion of a CAG trinucleotide repeat in exon 1 of the huntingtin (htt) gene. Since hairpin small interference RNA (shRNA) technology allows inhibition of specific gene expression in vitro and in vivo, vector-mediated expression of an shRNA directed to htt mRNA could form the basis of a new treatment modality for HD. By initial plasmid transfection of 293 cells, we identified one exon 1-targeted shRNA, which efficiently inhibited expression of an htt exon 1-GFP fusion protein and the endogenous htt gene. A replication-deficient adenovirus (Ad) vector Adie-1-1 was constructed to express this shRNA from the U6 promoter. In A549 cells expressing exon 1 of htt with an expanded CAG allele, Adie- 1-1 efficiently prevented htt exon 1 expression and htt aggregate formation. In addition, in different neuronal and nonneuronal cell lines, Adie-1-1 efficiently inhibited the expression of endogenous htt. Together, this data indicates the delineation of an shRNA strategy that may become the basis for treatment of HD.

Antisense downregulation of mutant huntingtin in a cell model

BACKGROUND

Huntington's disease (HD) is an inherited neurodegenerative disorder which is caused by an expansion of a CAG repeat sequence in the HD gene. The repeat encodes an expanded polyglutamine tract in the protein huntingtin. The still unknown pathological mechanisms leading to death of specific neurons in the brains of HD patients correlate with the expression of mutant huntingtin. Therefore, we have studied whether mutant huntingtin expression can be downregulated by antisense technique.

METHODS

NT2 precursor cells and differentiated postmitotic NT2-N neurons, respectively, were transfected with plasmid constructs containing exon 1 of the HD gene with expanded CAG repeats in frame with the reporter protein EGFP. The transfected cell cultures were treated with a phosphorothioated antisense oligonucleotide (PS-ASHD/20+) or a control oligonucleotide either by cotransfection or by addition to the culture medium.

RESULTS

Expression of the fusion protein containing the mutant huntingtin fragment resulted in diffuse green fluorescence in the cytoplasm and formation of aggregates in some of the NT2 cells and NT2-N neurons. We obtained antisense sequence-specific inhibition of expression of the fusion protein and/or suppression of the aggregate formation in both cell types. In the NT2 cells the antisense effect was dependent on the way of administration of the oligo.

CONCLUSIONS

The PS-antisense oligo is effective in downregulation of mutant huntingtin, and the reduction of aggregate formation is a sensitive biological marker. The findings suggest that antisense knockdown of huntingtin could be a useful strategy for treatment of HD, and could also be suitable for studies of the normal and pathological function of huntingtin in different cellular model systems.

Huntington's disease: new hope for therapeutics

Huntington's disease (HD) is one of eight progressive neurodegenerative disorders in which the underlying mutation is a CAG expansion encoding a polyglutamine tract. There are currently no cures or even effective therapies for HD. Effective strategies have remained elusive because little is known about either the mechanisms of expansion or the mechanism of polyglutamine-mediated neuronal death. However, recent advances in understanding the basic mechanisms of expansion and toxicity have renewed hope that a therapeutic strategy might someday be possible. Strategies effective in the treatment of HD are likely to be relevant in the treatment of a range of neurological and neurodegenerative disorders.

Synthetic oligonucleotides as therapeutics: the coming of age

Synthetic oligonucleotides (ODNs) are short nucleic acid chains that can act in a sequence specific manner to control gene expression. Significant progress has been made in the development of synthetic ODN therapeutics since the first demonstration of gene inhibition by antisense ODNs in a cell culture system two decades ago. This new class of therapeutic agents can potentially target any abnormally expressed genes in a broad range of diseases from viral infections to psychoneurological disorders. A number of "first" generation synthetic ODNs have entered into human clinical trials in the last few years. The eminent approval of the first ODN for the treatment of cytomaglovirus retinitis by the FDA in USA will provide much excitement that this new class of compounds holds great promise as a therapeutic "magic bullet". However, many obstacles still exist in the development of this technology. In this review, the current status of synthetic ODN chemistry, drug delivery methods, mechanisms of ODN action, potential clinical applications and its limitations in a wide range of human disorders will be described.

Inhibition of Huntington synthesis by antisense oligodeoxynucleotides

The Huntington disease gone encodes the protein huntington, which is widely expressed during embryonic development and in mature tissues. In order to elucidate the physiological function of huntington, which so far is unknown, we intend to study the effect of antisense down-regulated huntington expression. We have found an inhibiting effect of a phosphorothioated oligodeoxynucleotide (PS-ODN) added to the culture medium of embryonic teratocarcinoma cells (NT2) and postmitotic neurons (NT2N neurons) differentiated from the NT2 cells. Specific inhibition of expression of endogenous huntington was achieved in NT2N neurons in the concentration range of 1-5 microM PS-ODN, whereas no inhibition was obtained in NT2 cells. We describe in detail the selection of the target sequence for the antisense oligo and the uptake, intracellular distribution, and stability of the antisense PS-ODN in the two cell types. Antisense down-regulation of huntington in this model of human neurons represents a suitable approach to study its normal function.

Neurotrophic factors NGF and FGF-2 alter levels of huntingtin (IT15) in striatal neuronal cell cultures

A mutation of the human IT15 gene is responsible for Huntington's disease (HD) and the causative factor in the major neuronal loss observed in the striatum. The growth factors basic fibroblast growth factor (FGF-2), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) improve survival and promote differentiation of striatal neurons, as well as exert a neuroprotective effect when such neurons are challenged with metabolic toxins or excitatory amino acids. Using Western blotting and striatal cell cultures, we found that FGF-2 increased the level of huntingtin in a dose-dependent fashion, whereas NGF decreased huntingtin expression. The neurotrophic factor-specific, dose-dependent effect on striatal levels of huntingtin may be relevant to understanding the normal role of IT15 and developing new therapies against the disease provoking mutated IT15.

Gene therapy in the CNS

Gene therapy for neurological disorder is currently an experimental concept. The goals for clinical utilization are the relief of symptoms, slowing of disease progression, and correction of genetic abnormalities. Experimental studies are realizing these goals in the development of gene therapies in animal models. Discoveries of the molecular basis of neurological disease and advances in gene transfer systems have allowed focal and global delivery of therapeutic genes for a wide variety of CNS disorders. Limitations are still apparent, such as stability and regulation of transgene expression, and safety of both vector and expressed transgene. In addition, the brain adds several challenges not seen in peripheral gene therapy paradigms, such as post-mitotic cells, heterogeneity of cell types and circuits, and limited access. Moreover, it is likely that several modes of gene delivery will be necessary for successful gene therapies of the CNS. Collaborative efforts between clinicians and basic researchers will likely yield effective gene therapy in the CNS.

Brain as a unique antisense environment

During the last few years, antisense oligodeoxyribonucleotides (asODN) have become a commonly used tool for blocking of gene expression in the mammalian central nervous system. Successful gene inhibition has been reported for such diverse targets as those encoding neurotransmitter receptors, neuropeptides, trophic factors, transcription factors, cytokines, transporters, ion channels, and others. This review presents a discussion of recent studies on ODN in the brain, with a focus on specific approaches taken by the researchers in this field and especially on peculiar features of this organ as a milieu for asODN action. It is concluded that from the presented literature survey no coherent view on how to rationally design ODN for brain studies has emerged.

Therapeutic strategies for Huntington's disease based on a molecular understanding of the disorder

A mutation on chromosome 4p16.3 with an expanded polyglutamine tract has been identified as the cause of Huntington's disease (HD). The neuroscience and clinical community now faces the formidable challenge of using this information to develop a treatment against this fatal and currently untreatable disease. This article reviews the recent literature pertaining to HD and presents an overview of possible intervention strategies against the neurodegenerative process of HD. Because little is known about the physiological function of the HD gene, there are four biological levels at which therapies could be devised. Identification and cloning of the gene might direct novel therapies for HD using the following strategies: interference (1) at the DNA or (2) at the RNA level; (3) blocking the deleterious effect of the protein; and (4) physiological intervention using pharmacological agents or neural cell transplants.