Histone deacetylase inhibitors reduce polyglutamine toxicity

Inhibition of histone deacetylation protects wild-type but not gelsolin-deficient neurons from oxygen/glucose deprivation

Histone acetylation and deacetylation participate in the epigenetic regulation of gene expression. In this paper, we demonstrate that pre-treatment with the histone deacetylation inhibitor trichostatin A (TSA) enhances histone acetylation in primary cortical neurons and protects against oxygen/glucose deprivation, a model for ischaemic cell death in vitro. The actin-binding protein gelsolin was identified as a mediator of neuroprotection by TSA. TSA enhanced histone acetylation of the gelsolin promoter region, and up-regulated gelsolin messenger RNA and protein expression in a dose- and time-dependent manner. Double-label confocal immunocytochemistry visualized the up-regulation of gelsolin and histone acetylation within the same neuron. Together with gelsolin up-regulation, TSA pre-treatment decreased levels of filamentous actin. The neuroprotective effect of TSA was completely abolished in neurons lacking gelsolin gene expression. In conclusion, we demonstrate that the enhancement of gelsolin gene expression correlates with neuroprotection induced by the inhibition of histone deacetylation.

Mechanisms of disease: Histone modifications in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine repeat expansion within the huntingtin protein. HD is characterized by problems with movement, cognition and behavioral functioning, and there is currently no effective treatment. Although multiple pathologic mechanisms have been proposed, the exact mechanism by which mutant huntingtin causes neuronal dysfunction is not known. Recent studies demonstrating altered messenger RNA expression point to transcriptional dysregulation as a central mechanism. The control of eukaryotic gene expression depends on the modification of histone proteins associated with specific genes, with histone acetylation playing a crucial role. Studies in numerous HD models have shown that mutant huntingtin alters histone acetyltransferase activity, and indicate that aberrant activity of this enzyme might be an underlying mechanism of transcriptional dysregulation in HD. Furthermore, recent studies have shown a therapeutic role for histone deacetylase inhibitors in a number of HD models. In this review, we summarize the current state of knowledge regarding the status of histones in HD. In addition, we discuss how these histone modifications not only lead to pathogenesis, but might also provide a novel therapeutic strategy for treating this devastating disease.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

Differential contributions of Caenorhabditis elegans histone deacetylases to huntingtin polyglutamine toxicity

Expansion of a polyglutamine tract in the huntingtin protein causes neuronal degeneration and death in Huntington's disease patients, but the molecular mechanisms underlying polyglutamine-mediated cell death remain unclear. Previous studies suggest that expanded polyglutamine tracts alter transcription by sequestering glutamine rich transcriptional regulatory proteins, thereby perturbing their function. We tested this hypothesis in Caenorhabditis elegans neurons expressing a human huntingtin fragment with an expanded polyglutamine tract (Htn-Q150). Loss of function alleles and RNA interference (RNAi) were used to examine contributions of C. elegans cAMP response element-binding protein (CREB), CREB binding protein (CBP), and histone deacetylases (HDACs) to polyglutamine-induced neurodegeneration. Deletion of CREB (crh-1) or loss of one copy of CBP (cbp-1) enhanced polyglutamine toxicity in C. elegans neurons. Loss of function alleles and RNAi were then used to systematically reduce function of each C. elegans HDAC. Generally, knockdown of individual C. elegans HDACs enhanced Htn-Q150 toxicity, but knockdown of C. elegans hda-3 suppressed toxicity. Neuronal expression of hda-3 restored Htn-Q150 toxicity and suggested that C. elegans HDAC3 (HDA-3) acts within neurons to promote degeneration in response to Htn-Q150. Genetic epistasis experiments suggested that HDA-3 and CRH-1 (C. elegans CREB homolog) directly oppose each other in regulating transcription of genes involved in polyglutamine toxicity. hda-3 loss of function failed to suppress increased neurodegeneration in hda-1/+;Htn-Q150 animals, indicating that HDA-1 and HDA-3 have different targets with opposing effects on polyglutamine toxicity. Our results suggest that polyglutamine expansions perturb transcription of CREB/CBP targets and that specific targeting of HDACs will be useful in reducing associated neurodegeneration.

HATs and HDACs in neurodegeneration: a tale of disconcerted acetylation homeostasis

Gradual disclosure of the molecular basis of selective neuronal apoptosis during neurodegenerative diseases reveals active participation of acetylating and deacetylating agents during the process. Several studies have now successfully manipulated neuronal vulnerability by influencing the dose and enzymatic activity of histone acetyltransferases (HATs) and histone deacetylases (HDACs), enzymes regulating acetylation homeostasis within the nucleus, thus focusing on the importance of balanced acetylation status in neuronal vitality. It is now increasingly becoming clear that acetylation balance is greatly impaired during neurodegenerative conditions. Herein, we attempt to illuminate molecular means by which such impairment is manifested and how the compromised acetylation homeostasis is intimately coupled to neurodegeneration. Finally, we discuss the therapeutic potential of reinstating the HAT-HDAC balance to ameliorate neurodegenerative diseases.

Abnormalities of germ cell maturation and sertoli cell cytoskeleton in androgen receptor 113 CAG knock-in mice reveal toxic effects of the mutant protein

An unresolved question in the study of the polyglutamine neurodegenerative disorders is the extent to which partial loss of normal function of the mutant protein contributes to the disease phenotype. To address this, we studied Kennedy disease, a degenerative disorder of lower motor neurons caused by a CAG/glutamine expansion in the androgen receptor (Ar) gene. Signs of partial androgen insensitivity, including testicular atrophy and decreased fertility, are common in affected males, although the underlying mechanisms are not well understood. Here, we describe a knock-in mouse model that reproduces the testicular atrophy, diminished fertility, and systemic signs of partial androgen insensitivity that occur in Kennedy disease patients. Using this model, we demonstrate that the testicular pathology in this disorder is distinct from that mediated by loss of AR function. Testes pathology in 113 CAG knock-in mice was characterized by morphological abnormalities of germ cell maturation, decreased solubility of the mutant AR protein, and alterations of the Sertoli cell cytoskeleton, changes that are distinct from those produced by AR loss-of-function mutation in testicular feminization mutant mice. Our data demonstrate that toxic effects of the mutant protein mediate aspects of the Kennedy disease phenotype previously attributed to a loss of AR function.

Intracellular trafficking of histone deacetylase 4 regulates neuronal cell death

Histone deacetylase 4 (HDAC4) undergoes signal-dependent shuttling between the cytoplasm and nucleus, which is regulated in part by calcium/calmodulin-dependent kinase (CaMK)-mediated phosphorylation. Here, we report that HDAC4 intracellular trafficking is important in regulating neuronal cell death. HDAC4 is normally localized to the cytoplasm in brain tissue and cultured cerebellar granule neurons (CGNs). However, in response to low-potassium or excitotoxic glutamate conditions that induce neuronal cell death, HDAC4 rapidly translocates into the nucleus of cultured CGNs. Treatment with the neuronal survival factor BDNF suppresses HDAC4 nuclear translocation, whereas a proapoptotic CaMK inhibitor stimulates HDAC4 nuclear accumulation. Moreover, ectopic expression of nuclear-localized HDAC4 promotes neuronal apoptosis and represses the transcriptional activities of myocyte enhancer factor 2 and cAMP response element-binding protein, survival factors in neurons. In contrast, inactivation of HDAC4 by small interfering RNA or HDAC inhibitors suppresses neuronal cell death. Finally, an increase of nuclear HDAC4 in granule neurons is also observed in weaver mice, which harbor a mutation that promotes CGN apoptosis. Our data identify HDAC4 and its intracellular trafficking as key effectors of multiple pathways that regulate neuronal cell death.

Animal models of Kennedy disease

Since the identification of the polyglutamine repeat expansion responsible for Kennedy disease (KD) more than a decade ago, several laboratories have created animal models for KD. The slowly progressive nature of KD, its X-linked dominant mode of inheritance, and its recently elucidated hormone dependence have made the modeling of this lower motor neuron disease uniquely challenging. Several models have been generated in which variations in specificity, age of onset, and rate of progression have been achieved. Animal models that precisely reproduce the motor neuron specificity, delayed onset, and slow progression of disease may not support preclinical therapeutics testing, whereas models with rapidly progressing symptoms may preclude the ability to fully elucidate pathogenic pathways. Drosophila models of KD provide unique opportunities to use the power of genetics to identify pathogenic pathways at work in KD. This paper reviews the new wealth of transgenic mouse and Drosophila models for KD. Whereas differences, primarily in neuropathological findings, exist in these models, these differences may be exploited to begin to elucidate the most relevant pathological features of KD.

Cytoprotective effect of novel histone deacetylase inhibitors against polyglutamine toxicity

Dentatorubral-pallidoluysian atrophy (DRPLA) is an autosomal dominant neurological disorder caused by a CAG repeat expansion in the DRPLA gene encoding polyglutamine (polyQ). Although previous experimental studies have demonstrated that histone deacetylase (HDAC) inhibitors are therapeutically active, known HDAC inhibitors have considerable adverse effects clinically. To identify new HDAC inhibitors for the treatment of DRPLA, we evaluated a new series of HDAC inhibitors, N-hydroxycarboxamides, with our drug screening system, which uses neuronal PC12 cells stably transfected with a part of the DRPLA gene. We found that two of four N-hydroxycarboxamides significantly reduced polyQ-induced cell death. The essential structure of these compounds is a hydroxamic acid residue, which is shared with trichostatin A, a known HDAC inhibitor. Although our study showed mild neuroprotective effects, further structural modification of compounds that retain this residue may decrease cytotoxicity and increase protective activity against polyQ toxicity.

Regulation of CRE-dependent transcription by presenilins: prospects for therapy of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is characterized by memory loss and other cognitive disabilities. Mutations in the presenilin genes are the major cause of familial AD. Analysis of conditional knockout mice has shown that inactivation of presenilins results in progressive memory impairment and age-dependent neurodegeneration, suggesting that reduced presenilin activity might represent an important pathogenic mechanism. Presenilins positively regulate the transcription of cAMP response element (CRE)-containing genes, some of which are known to be important for memory formation and neuronal survival. Phosphodiesterase 4 and histone deacetylase inhibitors, which can enhance CRE-dependent gene expression, have been shown to ameliorate memory deficits and neurodegeneration in animal models. Thus, modulation of CRE-dependent transcription might be beneficial for the treatment of dementia in AD.

Mutant huntingtin represses CBP, but not p300, by binding and protein degradation

Huntington's disease can be used as a model to study neurodegenerative disorders caused by aggregation-prone proteins. It has been proposed that the entrapment of transcription factors in aggregates plays an important role in pathogenesis. We now report that the transcriptional activity of CBP is already repressed in the early time points by soluble mutant huntingtin, whereas the histone acetylase activity of CBP/p300 is gradually diminished over time. Mutant huntingtin bound much stronger to CBP than normal huntingtin, possibly contributing to repression. Especially at the later time points, CBP protein level was gradually reduced via the proteasome pathway. In sharp contrast, p300 was unaffected by mutant huntingtin. This selective degradation of CBP was absent in spinocerebellar ataxia 3. Thus, mutant huntingtin specifically affects CBP and not p300 both at the early and later time points, via multiple mechanisms. In addition to the reduction of CBP, also the altered ratio of these closely related histone acetyltransferases may affect chromatin structure and transcription and thus contribute to neurodegeneration.

DFT-based ranking of zinc-binding groups in histone deacetylase inhibitors

Histone deacetylases (HDACs) have recently attracted considerable interest as targets in the treatment of cell proliferative diseases such as cancer. In the present work, a general framework is proposed for chemical groups that bind into the HDAC catalytic core. Based on this framework, a series of groups was selected for further investigation. A method was developed to rank the HDAC inhibitory potential of these moieties at the B3LYP/6-31G* level, making use of extra diffuse functions and of the PCM solvation model where appropriate. The resulting binding geometries indicate that very stringent constraints should be satisfied in order to have bidental zinc chelation, and even more so to have a strong binding affinity, which makes it difficult to predict the binding mode and affinity of such zinc-binding groups. The chemical hardness and the pK(a) were identified as important criteria for the binding affinity. Also, the hydrophilicity may have a direct influence on the binding affinity. The calculated binding energies were qualitatively validated with experimental results from the literature, and were shown to be meaningful for the purpose of ranking. Additionally, the insights gained from the present work may be useful for increasing the accuracy of QSAR models by providing a rational basis for selecting descriptors.

Diseases of unstable repeat expansion: mechanisms and common principles

The list of developmental and degenerative diseases that are caused by expansion of unstable repeats continues to grow, and is now approaching 20 disorders. The pathogenic mechanisms that underlie these disorders involve either loss of protein function or gain of function at the protein or RNA level. Common themes have emerged within and between these different classes of disease; for example, among disorders that are caused by gain-of-function mechanisms, altered protein conformations are central to pathogenesis, leading to changes in protein activity or abundance. In all these diseases, the context of the expanded repeat and the abundance, subcellular localization and interactions of the proteins and RNAs that are affected have key roles in disease-specific phenotypes.

The paradigm of Huntington's disease: therapeutic opportunities in neurodegeneration

Despite a relatively small number of affected patients, Huntington's disease (HD) has been a historically important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis, and transcriptional dysregulation. The discovery of the HD gene in 1993 opened the door to the mechanisms of HD pathogenesis. Multiple pathologic mechanisms have been discovered, each one serving as a potential therapeutic target. HD thus continues to serve as a paradigmatic disorder, with basic bench research generating clinically relevant insights and stimulating the development of therapeutic human trials.

Novel therapeutic targets for Huntington’s disease

Huntington's disease (HD) is a fatal autosomal-dominant disorder involving progressive motor, cognitive and psychiatric symptoms. HD is one of a large family of neurodegenerative diseases caused by a trinucleotide (CAG) repeat mutation, encoding an expanded tract of glutamines in the disease protein. HD was one of the first neurological disorders for which accurate transgenic models were created, allowing mechanisms of pathogenesis to be explored at molecular, cellular and behavioural levels. In the last decade, the understanding of molecular and cellular changes which occur in HD prior to onset of symptoms, and at early and late stages of disease progression, has been greatly expanded. A wide range of potential molecular targets for therapeutic intervention have been identified, associated with a variety of cellular processes including gene transcription, protein trafficking, protein degradation, protein-protein interactions, glutamatergic synaptic transmission, presynaptic signalling, postsynaptic signalling, synaptic plasticity, dopaminergic and neurotrophic modulation of synaptic function, experience-dependent neurogenesis, mitochondrial function and oxidative metabolism. Presymptomatic testing for the HD gene mutation necessitates future development of novel therapeutics aimed at delaying onset of symptoms, as well as slowing or reversing disease progression.

Genome-wide expression profiling of human blood reveals biomarkers for Huntington's disease

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion of glutamine repeats in ubiquitously distributed huntingtin protein. Recent studies have shown that mutant huntingtin interferes with the function of widely expressed transcription factors, suggesting that gene expression may be altered in a variety of tissues in HD, including peripheral blood. Affymetrix and Amersham Biosciences oligonucleotide microarrays were used to analyze global gene expression in blood samples of HD patients and matched controls. We identified 322 mRNAs that showed significantly altered expression in HD blood samples, compared with controls (P < 0.0005), on two different microarray platforms. A subset of up-regulated mRNAs selected from this group was able to distinguish controls, presymptomatic individuals carrying the HD mutation, and symptomatic HD patients. In addition, early presymptomatic subjects showed gene expression profiles similar to those of controls, whereas late presymptomatic subjects showed altered expression that resembled that of symptomatic HD patients. These elevated mRNAs were significantly reduced in HD patients involved in a dose-finding study of the histone deacetylase inhibitor sodium phenylbutyrate. Furthermore, expression of the marker genes was significantly up-regulated in postmortem HD caudate, suggesting that alterations in blood mRNAs may reflect disease mechanisms observed in HD brain. In conclusion, we identified changes in blood mRNAs that clearly distinguish HD patients from controls. These alterations in mRNA expression correlate with disease progression and response to experimental treatment. Such markers may provide clues to the state of HD and may be of predictive value in clinical trials.

Interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches--augmentation of transcriptional activation as a potential therapeutic strategy for polyglutamine diseases

On the basis of the hypothesis that the interaction of mutant proteins with expanded polyglutamine stretches with transcriptional co-activator, TAFII130, leads to transcriptional dysregulation, the transcriptional activation of c-Fos and its suppression by expanded polyglutamine stretches was investigated. The phosphorylation of cAMP-responsive element binding protein (CREB) and induction of c-Fos in response to cAMP were strongly suppressed in Neuro2a cells expressing expanded polyglutamine. The suppression of CREB-dependent transcriptional activation was reversibly rescued by increasing the concentration of cAMP. Expanded polyglutamine-induced cytotoxicity was also substantially suppressed by augmenting CREB-dependent transcriptional activation with a high concentration of cAMP. FR901228, a histone deacetylase inhibitor, was also demonstrated as rescuing the expanded polyglutamine-induced suppression of CREB phosphorylation and c-Fos expression. Furthermore, nuclear fragmentation was significantly suppressed by FR901228. The co-expression of dominant-negative CREB vectors considerably abrogated the suppressive effect of cAMP and FR901228 on the expanded polyglutamine-induced nuclear fragmentation, suggesting that these compounds suppress polyglutamine-induced cytotoxicity, largely, via the enhancement of CREB-dependent transcriptional activation. These findings suggest that the interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches is involved in the pathogenetic mechanisms underlying neurodegeneration, and that the augmentation of CREB-dependent transcriptional activation is a potential strategy in treating polyglutamine diseases.

The Unfolded Protein Response Modulates Toxicity of the Expanded Glutamine Androgen Receptor

Kennedy disease, a degenerative disorder caused by an expanded glutamine tract, is mediated by misfolding of the mutant androgen receptor (AR) protein, a process that may disrupt proteasome function. We hypothesized that this might lead to endoplasmic reticulum (ER) stress and induction of the unfolded protein response (UPR), a complex physiologic pathway that regulates cell survival. To test this hypothesis, we used aminoterminal fragments of wild type (AR16Q) or mutant (AR112Q) AR that triggered glutamine length-dependent cell death and activated an ER stress-inducible promoter. To evaluate the role of the UPR, we examined the contributions of three proximal sensors of ER stress: activating transcription factor 6 (ATF6), inositol requiring 1 (IRE1), and PKR-like endoplasmic reticulum kinase (PERK). AR112Q toxicity was significantly increased by a dominant negative ATF6 mutant and significantly decreased by a constitutively active ATF6 mutant, indicating that ATF6 promoted cell survival. In contrast, co-transfection with three separate IRE1alpha dominant negative mutants failed to alter glutamine length-dependent toxicity, suggesting that this arm of the UPR did not significantly affect AR112Q induced cell death. Activation of PERK, an ER transmembrane protein that functions as the third proximal UPR sensor, promoted glutamine length-dependent toxicity. Although nuclear localization sequence- and nuclear export sequence-targeted proteins both activated the UPR, this pathway more potently influenced toxicity when proteins were targeted to the cytoplasm. Taken together, our data demonstrate that the UPR is activated in cells expressing long glutamine tracts and that this pathway modulates polyglutamine toxicity.

Theoretical study revealing the functioning of a novel combination of catalytic motifs in histone deacetylase

Histone deacetylases (HDACs) have recently attracted considerable interest as targets in the treatment of cell proliferative diseases such as cancer. In the present work, the chemical properties of the active site of HDAC were theoretically investigated at a high computational level. Evidence was gathered for a novel catalytic mechanism, which differs from a previous proposal in the native protonation state of the His-Asp dyads, and in the deprotonation of water as a distinct step in the mechanism.

Neuronal intranuclear inclusions and neuropil aggregates in HdhCAG(150) knockin mice

We studied the development of neuronal intranuclear inclusions (NIIs), neuropil aggregates (NAs), and expression of expanded repeat polyglutamine protein in the HdhCAG(150) knockin mouse model of Huntington's disease (HD). Diffuse nuclear localization of huntingtin protein (htt) was noted initially within striatal neurons at approximately 28 weeks, followed by the development of striatal htt immunoreactive NIIs by approximately 40 weeks. Striatal NIIs were observed initially in clusters within the matrix compartment but subsequently became diffusely distributed throughout the striatum. In the oldest animals (107 weeks), NIIs were enlarged and diffuse nuclear htt immunoreactivity reduced. Expression of ubiquitin immunoreactive NIIs paralleled but lagged behind the expression of htt immunoreactive NIIs. Abundant NIIs were found by approximately 75 weeks in layers 3 and 4 of somatosensory cortex and in layer 2 of piriform cortex. In the oldest animals, greater than 100 weeks, some NIIs were found in many brain regions. NAs were found mainly within the globus pallidus and substantia nigra, perhaps reflecting expression in striatal terminals. Cyclic AMP response element binding protein (CBP) was not localized to NIIs, arguing against gross sequestration of this transcriptionally active protein. Comparison of the relative levels of a common polyglutamine epitope in HdhCAG(150) knockin and hprtCAG(146) knockin mice shows greater expression of the polyglutamine epitope in the phenotypically less aggressive HdhCAG(150) knockin line. HdhCAG(150) knockin mice may be a model of early pathologic changes in HD.

Neuroprotective effects of phenylbutyrate against MPTP neurotoxicity

There is increasing evidence that administration of histone deacetylase (HDAC) inhibitors can exert neuroprotective effects by a variety of mechanisms. Phenylbutyrate is a well-known HDAC inhibitor, which increases gene transcription of a number of genes, and also exerts neuroprotective effects. These include several antioxidant enzymes, chaperones, and genes involved in cell survival. We examined whether administration of phenylbutyrate could exert significant neuroprotective effects against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which has been used to model Parkinson's disease. Administration of phenylbutyrate significantly attenuated MPTP-induced depletion of striatal dopamine and loss of tyrosine hydroxylase-positive neurons in the substantia nigra. These findings provide further evidence that administration of phenylbutyrate may be a useful approach for the treatment of neurodegenerative diseases.

Progress in Spinobulbar muscular atrophy research: insights into neuronal dysfunction caused by the polyglutamine-expanded androgen receptor

Spinobulbar muscular atrophy (SBMA, Kennedy's disease) results from the dysfunction and degeneration of specific motor and sensory neurons. The underlying cause of this ligand-dependent neurodegenerative disease is expansion of the CAG trinucleotide repeat in the androgen receptor (AR) gene which leads to lengthening of the polyglutamine tract in the AR protein. Recently, the effects of the polyglutamine-expanded AR have been explored in a number of cellular and animal models. Common themes include research on polyglutamine-containing nuclear inclusions and the effect of molecular chaperone overexpression on their formation. In addition, investigations have highlighted the role that abnormal transcriptional regulation, proteasome dysfunction and altered axonal transport may play in disease pathogenesis. These studies suggest a number of potential treatments for restoring neuronal function. One of the most interesting advances in SBMA research has been the creation of mouse models that recapitulate the key features of SBMA progression in men. Lowering testosterone levels in affected transgenic male mice rescued, and even reversed the polyglutamine-induced neuromuscular phenotype, indicating that manipulating androgen levels in men could be of therapeutic benefit. Although the question of why only a distinct subset of neurons is affected by polyglutamine expansion of the AR remains unsolved, future research will provide further insights into the mechanisms contributing to disease progression in SBMA.

Analysis of the two p97/VCP/Cdc48p proteins of Caenorhabditis elegans and their suppression of polyglutamine-induced protein aggregation

A class of inherited neurodegenerative diseases including Huntington's disease is caused by polyglutamine (polyQ) expansion in the responsible proteins. Pathology is typically associated with polyQ expansions of greater than 40 residues, and the longer the length of the expansion, the earlier the onset of disease. It has been reported that p97/VCP/Cdc48p, a member of AAA family of proteins, can bind to longer polyQ tracts. In Caenorhabditis elegans, two p97/VCP/Cdc48p homologues, C41C4.8 and C06A1.1, have been identified. Our results indicate that these p97/VCP/Cdc48p homologues have essential but redundant functions in C. elegans. To provide a model system for investigating the molecular basis of pathogenesis, we have expressed polyQ expansions fused to green fluorescent protein in the body wall muscle cells of C. elegans. When the repeats are longer than 40, discrete cytoplasmic aggregates are formed and these appear at an early stage of embryogenesis. The formation of aggregates was partially suppressed by co-expression of either C41C4.8 or C06A1.1. These results suggest that these p97/VCP/Cdc48p homologues, AAA chaperones, may play a protective role in polyQ aggregation.

Epigenetics and human disease

Epigenetics is comprised of the stable and heritable (or potentially heritable) changes in gene expression that do not entail a change in DNA sequence. The role of epigenetics in the etiology of human disease is increasingly recognized with the most obvious evidence found for genes subject to genomic imprinting. Mutations and epimutations in imprinted genes can give rise to genetic and epigenetic phenotypes, respectively; uniparental disomy and imprinting defects represent epigenetic disease phenotypes. There are also genetic disorders that affect chromatin structure and remodeling. These disorders can affect chromatin in trans or in cis, as well as expression of both imprinted and nonimprinted genes. Data from Angelman and Beckwith-Wiedemann syndromes and other disorders indicate that a monogenic or oligogenic phenotype can be caused by a mixed epigenetic and genetic and mixed de novo and inherited (MEGDI) model. The MEGDI model may apply to some complex disease traits and could explain negative results in genome-wide genetic scans.

Neuroprotective effects of phenylbutyrate in the N171-82Q transgenic mouse model of Huntington's disease

Huntington's disease (HD) is caused by an expansion of exonic CAG triplet repeats in the gene encoding the huntingtin protein (Htt), however, the means by which neurodegeneration occurs remains obscure. There is evidence that mutant Htt interacts with transcription factors leading to reduced histone acetylation. We report that administration of the histone deacetylase inhibitor phenylbutyrate after onset of symptoms in a transgenic mouse model of HD significantly extends survival and attenuates both gross brain and neuronal atrophy. Administration of phenylbutyrate increased brain histone acetylation and decreased histone methylation levels as assessed by both immunocytochemistry and Western blots. Phenylbutyrate increased mRNA for components of the ubiquitin-proteosomal pathway and down-regulated caspases implicated in apoptotic cell death, and active caspase 3 immunoreactivity in the striatum. These results show that administration of phenylbutyrate, at doses that are well tolerated in man, exerts significant neuroprotective effects in a transgenic mouse model of HD, and therefore represents a very promising therapeutic approach for HD.

Kennedy's disease: pathogenesis and clinical approaches

Kennedy's disease, also known as spinal and bulbar muscular atrophy, is a progressive degenerative condition affecting lower motor neurons. It is one of nine neurodegenerative disorders caused by a polyglutamine repeat expansion. Affecting only men, Kennedy's disease is the only one of these conditions that follows an X-linked mode of inheritance. The causative protein in Kennedy's disease, with a polyglutamine expansion residing in the first N-terminal domain, is the androgen receptor. Research in this field has made significant advances in recent years, and with the increased understanding of pathogenic mechanisms, feasible approaches to treatments are being investigated. In Kennedy's disease research, the most significant issue to emerge recently is the role of androgens in exacerbating the disease process. On the basis of animal experiments, a viable hypothesis is that higher circulating levels of androgens in men could trigger the degeneration of motor neurons causing this disease, and that lower levels in heterozygous and homozygous women are protective. This is a major issue, as treatment of individuals affected by Kennedy's disease with testosterone has been considered a reasonable therapy by some neurologists. The rationale behind this approach relates to the fact that Kennedy's disease is accompanied by mild androgen insensitivity. It was therefore believed that treatment with high doses of testosterone might compensate for this loss of androgen action, with the added benefit of preventing muscle wasting. The current review provides an overview of recent advances in the field of Kennedy's disease research, including approaches to treatment.

Targeting CREB-binding protein (CBP) loss of function as a therapeutic strategy in neurological disorders

Histone acetylation/deacetylation is a master regulation of gene expression. Among the enzymes involved in this process, the CREB-binding protein (CBP) displays important functions during central nervous system development. Increasing evidence shows that CBP function is altered during neurodegenerative processes. CBP loss of function has now been reported in several diseases characterized by neurological disorders such as the Rubinstein-Taybi syndrome or polyglutamine-related pathologies (Huntington's disease). Our recent work suggests that CBP loss of function could also be involved in Alzheimer's disease and amyotrophic lateral sclerosis. In a simplified apoptotic model of primary neurons, we described CBP as a substrate of apoptotic caspases, an alternative to its classical proteasomal degradation. In these neuronal death contexts, histone acetylation levels were decreased as well. Altogether, these data point to a central role of CBP loss of function during neurodegeneration. In order to restore proper acetylation levels, a proposed therapeutic strategy relies on HDAC inhibition. Nevertheless, this approach lacks of specificity. Therefore new drugs targeted at counteracting CBP loss of function could stand as a valid therapeutic approach in neurodegenerative disorders. The challenge will be to respect the fine-tuning between cellular HAT/HDAC activities.

Distinct aggregation and cell death patterns among different types of primary neurons induced by mutant huntingtin protein

Aggregation of disease proteins is believed to be a central event in the pathology of polyglutamine diseases, whereas the relationship between aggregation and neuronal death remains controversial. We investigated this question by expressing mutant huntingtin (htt) with a defective adenovirus in different types of neurons prepared from rat cerebral cortex, striatum or cerebellum. The distribution pattern of inclusions is not identical among different types of primary neurons. On day 2 after infection, cytoplasmic inclusions are dominant in cortical and striatal neurons, whereas at day 4 the ratio of nuclear inclusions overtakes that of cytoplasmic inclusions. Meanwhile, nuclear inclusions are always predominantly present in cerebellar neurons. The percentage of inclusion-positive cells is highest in cerebellar neurons, whereas mutant htt induces cell death most remarkably in cortical neurons. As our system uses htt exon 1 protein and thus aggregation occurs independently from cleavage of the full-length htt, our observations indicate that the aggregation process is distinct among different neurons. Most of the neurons containing intracellular (either nuclear or cytoplasmic) aggregates are viable. Our findings suggest that the process of mutant htt aggregation rather than the resulting inclusion body is critical for neuronal cell death.

Polyglutamine diminishes VEGF; passage to motor neuron death?

Altered gene transcription has been implicated in the pathogenesis of polyglutamine-dependent neurodegeneration. In this issue of Neuron, Sopher et al. demonstrate that androgen receptors containing expanded polyglutamine cause decreased expression of vascular endothelial growth factor (VEGF) by interfering with cAMP response element binding protein binding protein (CBP), thereby contributing to the motor neuron degeneration in spinal and bulbar muscular atrophy.

CRE-mediated transcription is increased in Huntington's disease transgenic mice

Disruption of cAMP response element (CRE)-dependent transcription has been hypothesized to contribute to neuronal death and dysfunction in Huntington's disease (HD) and other polyglutamine repeat disorders. Whether dysregulation of CRE-dependent transcription actually occurs in vivo in response to expression of expanded polyglutamine repeats has not been tested. We directly tested whether CRE-dependent transcription is affected in vivo by cross breeding a transgenic mouse model of HD (line R6/2) with a transgenic mouse that expresses a CRE-regulated reporter gene. Instead of compromised CRE-dependent transcription in HD mice, we found a robust upregulation of CRE-dependent transcription in several brain regions (striatum, hippocampus, cortex). CRE-mediated transcription was also evoked by striatal forskolin infusion and by photic stimulation in HD animals. Increased cAMP response element-binding protein (CREB) phosphorylation and elevated levels of the CREB-regulated gene product, CCAAT/enhancer binding protein beta, were also found in HD mice. Significant alterations in CREB binding protein expression and localization were not observed in symptomatic R6/2 mice. Thus, rather than repressing CRE-mediated transcription, mutant huntingtin appears to facilitate transcription via a CRE-dependent mechanism in vivo.

Ataxin 1, a SCA1 neurodegenerative disorder protein, is functionally linked to the silencing mediator of retinoid and thyroid hormone receptors

Ataxin 1 (Atx1) is a foci-forming polyglutamine protein of unknown function, whose mutant form causes type 1 spinocerebellar ataxia in humans and exerts neurotoxicity in transgenic mouse and fly expressing mutant Atx1. In this study, we demonstrate that Atx1 interacts with the transcriptional corepressor SMRT (silencing mediator of retinoid and thyroid hormone receptors) and with histone deacetylase 3. Atx1 binds chromosomes and mediates transcriptional repression when tethered to DNA. Interaction with SMRT-related factors is a conserved feature of Atx1, because Atx1 also binds SMRTER, a Drosophila cognate of SMRT. Significantly, mutant Atx1 forms aggregates in Drosophila, and such mutant Atx1-mediated aggregates sequester SMRTER. Consistently, the neurodegenerative eye phenotype caused by mutant Atx1 is enhanced by a Smrter mutation and, conversely, is suppressed by a chromosomal duplication that contains the wild type Smrter gene. Together, our results suggest that Atx1 is a transcriptional factor whose mutant form exerts its deleterious effects in part by perturbing corepressor-dependent transcriptional pathways.

General transcriptional repression by polyglutamine disease proteins is not directly linked to the presence of inclusion bodies

By using direct immunocytochemistry of BrU incorporated to RNA in the nuclei, we evaluated the effect of mutant huntingtin and ataxin-1 on general transcription in primary cortical and cerebellar neurons. Our quantitative analyses clearly showed that these mutant polyglutamine disease proteins repress general transcription. In addition, we found that general transcription level was almost similar in inclusion body-positive and -negative neurons. The result suggests that presence of inclusion body is not essential for repressing general transcription in contrast to its reported role for suppressing specific gene transcription in the polyglutamine disease pathology.

Androgen Receptor Acetylation Site Mutations Cause Trafficking Defects, Misfolding, and Aggregation Similar to Expanded Glutamine Tracts

Kennedy's disease is a degenerative disorder of motor neurons caused by the expansion of a glutamine tract near the amino terminus of the androgen receptor (AR). Ligand binding to the receptor is associated with several post-translational modifications, but it is poorly understood whether these affect the toxicity of the mutant protein. Our studies now demonstrate that mutation of lysine residues in wild-type AR that are normally acetylated in a ligand-dependent manner mimics the effects of the expanded glutamine tract on receptor trafficking, misfolding, and aggregation. Mutation of lysines 630 or 632 and 633 to alanine markedly delays ligand-dependent nuclear translocation. The K632A/K633A mutant also undergoes ligand-dependent misfolding and aggregation similar to the expanded glutamine tract AR. This acetylation site mutant exhibits ligand-dependent 1C2 immunoreactivity, forms aggregates that co-localize with Hsp40, Hsp70, and the ubiquitin-protein isopeptide ligase (E3) ubiquitin ligase carboxyl terminus of Hsc70-interacting protein (CHIP), and inhibits proteasome function. Ligand-dependent nuclear translocation of the wild-type receptor and misfolding and aggregation of the K632A/K633A mutant are blocked by radicicol, an Hsp90 inhibitor. These data identify a novel role for the acetylation site as a regulator of androgen receptor subcellular distribution and folding and indicate that ligand-dependent aggregation is dependent upon intact Hsp90 function.

Can flies help humans treat neurodegenerative diseases?

Neurodegenerative diseases are becoming increasingly common as life expectancy increases. Recent years have seen tremendous progress in the identification of genes that cause these diseases. While mutations have been found and cellular processes defined that are altered in the disease state, the identification of treatments and cures has proven more elusive. The process of finding drugs and therapies to treat human diseases can be slow, expensive and frustrating. Can model organisms such as Drosophila speed the process of finding cures and treatments for human neurodegenerative diseases? We pose three questions, (1) can one mimic the essential features of human diseases in an organism like Drosophila, (2) can one cure a model organisms of human disease and (3) will these efforts accelerate the identification of useful therapies for testing in mice and ultimately humans? Here we focus on the use of Drosophila to identify potential treatments for neurodegenerative diseases such as Huntington's and we discuss how well these therapies translate into mammalian systems.

Atrophin 2 recruits histone deacetylase and is required for the function of multiple signaling centers during mouse embryogenesis

Atrophins are evolutionarily conserved proteins that are thought to act as transcriptional co-repressors. Mammalian genomes contain two atrophin genes. Dominant polyglutamine-expanded alleles of atrophin 1 have been identified as the cause of dentatorubralpallidoluysian atrophy, an adult-onset human neurodegenerative disease with similarity to Huntington's. In a screen for recessive mutations that disrupt patterning of the early mouse embryo, we identified a line named openmind carrying a mutation in atrophin 2. openmind homozygous embryos exhibit a variety of patterning defects that first appear at E8.0. Defects include a specific failure in ventralization of the anterior neural plate, loss of heart looping and irregular partitioning of somites. In mutant embryos, Shh expression fails to initiate along the anterior midline at E8.0, and Fgf8 is delocalized from the anterior neural ridge at E8.5, revealing a crucial role for atrophin 2 in the formation and function of these two signaling centers. Atrophin 2 is also required for normal organization of the apical ectodermal ridge, a signaling center that directs limb pattern. Elevated expression of atrophin 2 in neurons suggests it may interact with atrophin 1 in neuronal development or function. We further show that atrophin 2 associates with histone deacetylase 1 in mouse embryos, providing a biochemical link between Atr2 and a chromatin-modifying enzyme. Based on our results, and on those of others, we propose that atrophin proteins act as transcriptional co-repressors during embryonic development.

Histone deacetylase inhibition by sodium butyrate chemotherapy ameliorates the neurodegenerative phenotype in Huntington's disease mice

The precise cause of neuronal death in Huntington's disease (HD) is unknown. Although no single specific protein-protein interaction of mutant huntingtin has emerged as the pathologic trigger, transcriptional dysfunction may contribute to the neurodegeneration observed in HD. Pharmacological treatment using the histone deacetylase inhibitor sodium butyrate to modulate transcription significantly extended survival in a dose-dependent manner, improved body weight and motor performance, and delayed the neuropathological sequelae in the R6/2 transgenic mouse model of HD. Sodium butyrate also increased histone and Specificity protein-1 acetylation and protected against 3-nitropropionic acid neurotoxicity. Microarray analysis showed increased expression of alpha- and beta-globins and MAP kinase phosphatase-1 in sodium butyrate-treated R6/2 mice, indicative of improved oxidative phosphorylation and transcriptional regulation. These findings strengthen the hypothesis that transcriptional dysfunction plays a role in the pathogenesis of HD and suggest that therapies aimed at modulating transcription may target early pathological events and provide clinical benefits to HD patients.

Human neurodegenerative disease modeling using Drosophila

A number of approaches have been taken to recreate and to study the role of genes associated with human neurodegenerative diseases in the model organism Drosophila. These studies encompass the polyglutamine diseases, Parkinson's disease, Alzheimer's disease, and tau-associated pathologies. The findings highlight Drosophila as an important model system in which to study the fundamental pathways influenced by these genes and have led to new insights into aspects of pathogenesis and modifier mechanisms.

Protein aggregation in motor neurone disorders

Toxicity associated with abnormal protein folding and protein aggregation are major hypotheses for neurodegeneration. This article comparatively reviews the experimental and human tissue-based evidence for the involvement of such mechanisms in neuronal death associated with the motor system disorders of X-linked spinobulbar muscular atrophy (SBMA; Kennedy's disease) and amyotrophic lateral sclerosis (ALS), especially disease related to mutations in the superoxide dismutase (SOD1) gene. Evidence from transgenic mouse, Drosophila and cell culture models of SBMA, in common with other trinucleotide repeat expansion disorders, show protein aggregation of the mutated androgen receptor, and intraneuronal accumulation of aggregated protein, to be obligate mechanisms. Strong experimental data link these phenomena with downstream biochemical events involving gene transcription pathways (CREB-binding protein) and interactions with protein chaperone systems. Manipulations of these pathways are already established in experimental systems of trinucleotide repeat disorders as potential beneficial targets for therapeutic activity. In contrast, the evidence for the role of protein aggregation in models of SOD1-linked familial ALS is less clear-cut. Several classes of intraneuronal inclusion body have been described, some of which are invariably present. However, the lack of understanding of the biochemical basis of the most frequent inclusion in sporadic ALS, the ubiquitinated inclusion, has hampered research. The toxicity associated with expression of mutant SOD1 has been intensively studied however. Abnormal protein aggregation and folding is the only one of the four major hypotheses for the mechanism of neuronal degeneration in this disorder currently under investigation (the others comprise oxidative stress, axonal transport and cytoskeletal dysfunctions, and glutamatergic excitotoxicity). Whilst hyaline inclusions, which are strongly immunoreactive to SOD1, are linked to degeneration in SOD1 mutant mouse models, the evidence from human tissue is less consistent and convincing. A role for mutant SOD1 aggregation in the mitochondrial dysfunction associated with ALS, and in potentially toxic interactions with heat shock proteins, both leading to apoptosis, are supported by some experimental data. Direct in vitro data on mutant SOD1 show evidence for spontaneous oligomerization, but the role of such oligomers remains to be elucidated, and therapeutic strategies are less well developed for this familial variant of ALS.

Histone deacetylase inhibition by sodium butyrate chemotherapy ameliorates the neurodegenerative phenotype in Huntington's disease mice

The precise cause of neuronal death in Huntington's disease (HD) is unknown. Although no single specific protein-protein interaction of mutant huntingtin has emerged as the pathologic trigger, transcriptional dysfunction may contribute to the neurodegeneration observed in HD. Pharmacological treatment using the histone deacetylase inhibitor sodium butyrate to modulate transcription significantly extended survival in a dose-dependent manner, improved body weight and motor performance, and delayed the neuropathological sequelae in the R6/2 transgenic mouse model of HD. Sodium butyrate also increased histone and Specificity protein-1 acetylation and protected against 3-nitropropionic acid neurotoxicity. Microarray analysis showed increased expression of alpha- and beta-globins and MAP kinase phosphatase-1 in sodium butyrate-treated R6/2 mice, indicative of improved oxidative phosphorylation and transcriptional regulation. These findings strengthen the hypothesis that transcriptional dysfunction plays a role in the pathogenesis of HD and suggest that therapies aimed at modulating transcription may target early pathological events and provide clinical benefits to HD patients.

Fly models of Huntington's disease

Can Drosophila models be engineered that accurately reflect Huntington's disease (HD) and other neurological diseases and can they contribute to the search for treatments and cures? A number of publications seem to provide a resounding yes to that question. Here we seek to review some of the salient features of these models.

Hprt(CAG)146 mice: age of onset of behavioral abnormalities, time course of neuronal intranuclear inclusion accumulation, neurotransmitter marker alterations, mitochondrial function markers, and susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We reported previously a model of polyglutamine repeat disorders with insertion of 146 CAG repeats into the murine hypoxanthine phosphoribosyl transferase locus (Hprt(CAG)146; Ordway et al. [1997] Cell 91:753-763), which does not normally contain polyglutamine repeats. These mice develop an adult-onset neurologic phenotype of incoordination, involuntary limb clasping, seizures, and premature death. Histologic analysis demonstrates widespread ubiquinated neuronal intranuclear inclusions (NIIs). We now report characterization of the age of onset of behavioral abnormalities, correlated with the time course of occurrence of NIIs in several brain regions, and the occurrence of NIIs in non-neuronal tissues. Onset of behavioral abnormalities occurred at approximately 22 weeks of age. There was variable time course of expression of NIIs in several brain regions. Assessment of several non-neuronal tissues revealed nuclear inclusions in hepatocytes and choroid plexus epithelium. Gamma-aminobutyric acid (GABA)/benzodiazepine receptors, dopamine D1-like and D2-like receptors, and type 2 vesicular monoamine transporter (VMAT2) binding sites were assayed before and after the onset of behavioral abnormalities. GABA/benzodiazepine receptors were unchanged either before or after the onset of behavioral abnormalities in any region analyzed, whereas striatal D1-like and D2-like receptors were diminished after but not before the onset of symptoms. Dorsal striatal VMAT2 binding sites were decreased before the onset of behavioral changes. Mitochondrial electron transport chain components were assayed with histochemical methods before and after the onset of behavioral changes. There was no change in behaviorally presymptomatic or symptomatic animals. Hprt(CAG)146 mice did not exhibit increased susceptibility to the mitochondrial toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Hprt(CAG)146 mice are a useful model for studying polyglutamine repeat disorders.

Histone deacetylase activity is retained in primary neurons expressing mutant huntingtin protein

Perturbation of histone acetyl-transferase (HAT) activity is implicated in the pathology of polyglutamine diseases, and suppression of the counteracting histone deacetylase (HDAC) proteins has been proposed as a therapeutic candidate for these intractable disorders. Meanwhile, it is not known whether mutant polyglutamine disease protein affects the HDAC activity in declining neurons, though the answer is essential for application of anti-HDAC drugs for polyglutamine diseases. Here, we show the effect of mutant huntingtin (htt) protein on the expression and activity of HDAC proteins in rat primary cortical neurons as well as in human Huntington's disease (HD) brains. Our findings indicate that expression and activity of HDAC proteins are not repressed by mutant htt protein. Furthermore, expression of normal and mutant htt protein slightly increased HDAC activity although the effects of normal and mutant htt were not remarkably different. In human HD cerebral cortex, HDAC5 immunoreactivity was increased in the nucleus of striatal and cortical neurons, suggesting accelerated nuclear import of this class II HDAC. Meanwhile, western blot and immunohistochemical analyses showed no remarkable change in the expression of class I HDAC proteins such as HDAC1 and HDCA8. Collectively, retained activity in affected neurons supports application of anti-HDAC drugs to the therapy of HD.

Ca(2+)/CREB/CBP-dependent gene regulation: a shared mechanism critical in long-term synaptic plasticity and neuronal survival

CREB is a transcription factor critical for long-term synaptic plasticity. Intriguingly, recent work has elucidated a role for CREB, as well as upstream CREB kinases, in the control of activity-dependent neuronal survival. Additionally, analysis of the molecular pathology of polyglutamine-repeat diseases suggest that alteration of pCREB-CBP function may underlie, at least in part, the neurodegenerative process. Taken together, these new findings support the idea that Ca(2+)/CREB/CBP-dependent gene regulation might be a shared mechanism critical in both long-term synaptic plasticity and neuronal survival.

Pathogenesis of polyglutamine disorders: aggregation revisited

Expansion of CAG trinucleotide repeats coding for polyglutamine in unrelated proteins causes at least nine late-onset progressive neurodegenerative disorders, including Huntington's disease and a number of spinocerebellar ataxias. Expanded polyglutamine provokes a dominant gain-of-function neurotoxicity, regardless of the specific protein context within which it resides. Nevertheless, the protein context does modulate polyglutamine toxicity, as evidenced by the distinct clinical and pathological features of the various disorders. Importantly, polyglutamine toxicity might derive from its ability to aggregate. Indeed, aggregation probably underlies some defining attributes of the polyglutamine disorders, such as their late onset, progressive nature, and the dependence of onset age on polyglutamine length. However, the central role of aggregation in polyglutamine pathogenesis has been challenged by several studies, which instead argued that the soluble form of the disease proteins is responsible for neuronal damage. Thus, the question whether polyglutamine aggregates are deleterious, harmless or protective remains the most passionately disputed issue in the study of these diseases. In this review, we attempt to reconcile some of these controversies.

Invertebrate models of neurologic disease: insights into pathogenesis and therapy

The search for cures of human diseases can be very slow, expensive, and serendipitous. Roughly five decades of basic research in a handful of model systems has revealed that most animals are quite similar to one another especially at the cellular and molecular levels. The commonalities allow one to use animal models to investigate human disease mechanisms. Here, we review contributions demonstrating the use of invertebrate models to investigate human neurodegenerative diseases. We conclude that the integration of fly and worm models into programs seeking to identify therapeutic strategies for neurodegenerative disease can significantly speed progress toward finding cures for these devastating diseases.