Sequence of the murine Huntington disease gene: evidence for conservation, alternate splicing and polymorphism in a triplet (CCG) repeat [corrected]

The evolutionary history of the polyQ tract in huntingtin sheds light on its functional pro-neural activities

Huntington's disease is caused by a pathologically long (>35) CAG repeat located in the first exon of the Huntingtin gene (HTT). While pathologically expanded CAG repeats are the focus of extensive investigations, non-pathogenic CAG tracts in protein-coding genes are less well characterized. Here, we investigated the function and evolution of the physiological CAG tract in the HTT gene. We show that the poly-glutamine (polyQ) tract encoded by CAGs in the huntingtin protein (HTT) is under purifying selection and subjected to stronger selective pressures than CAG-encoded polyQ tracts in other proteins. For natural selection to operate, the polyQ must perform a function. By combining genome-edited mouse embryonic stem cells and cell assays, we show that small variations in HTT polyQ lengths significantly correlate with cells' neurogenic potential and with changes in the gene transcription network governing neuronal function. We conclude that during evolution natural selection promotes the conservation and purity of the CAG-encoded polyQ tract and that small increases in its physiological length influence neural functions of HTT. We propose that these changes in HTT polyQ length contribute to evolutionary fitness including potentially to the development of a more complex nervous system.

A Fresh Look at Huntingtin mRNA Processing in Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by a mutation that expands the polyglutamine (CAG) repeat in exon 1 of the huntingtin (HTT) gene. Wild-type HTT protein interacts with other proteins to protect cells against toxic stimuli, mediate vesicle transport and endocytosis, and modulate synaptic activity. Mutant HTT protein disrupts autophagy, vesicle transport, neurotransmitter signaling, and mitochondrial function. Although many of the activities of wild-type HTT protein and the toxicities of mutant HTT protein are characterized, less is known about the activities of HTT mRNA. Most putative HD therapies aim to target mutant HTT mRNA before it is translated into the protein. Therefore, it is imperative to learn as much as we can about how cells handle both wild-type and mutant HTT mRNA so that effective therapies can be designed. Here, we review the structure of wild-type and mutant HTT mRNA, with emphasis on their alternatively polyadenylated or spliced isoforms. We then consider the abundance of HTT mRNA isoforms in HD and discuss the potential implications of these findings. Evidence in the review should be used to guide future research aimed at developing mRNA-lowering therapies for HD.

Huntington Disease as a Neurodevelopmental Disorder and Early Signs of the Disease in Stem Cells

Huntington disease (HD) is a dominantly inherited disorder caused by a CAG expansion mutation in the huntingtin (HTT) gene, which results in the HTT protein that contains an expanded polyglutamine tract. The adult form of HD exhibits a late onset of the fully symptomatic phase. However, there is also a long presymptomatic phase, which has been increasingly investigated and recognized as important for the disease development. Moreover, the juvenile form of HD, evoked by a higher number of CAG repeats, resembles a neurodevelopmental disorder and has recently been the focus of additional interest. Multiple lines of data, such as the developmental necessity of HTT, its role in the cell cycle and neurogenesis, and findings from pluripotent stem cells, suggest the existence of a neurodevelopmental component in HD pathogenesis. Therefore, we discuss the early molecular pathogenesis of HD in pluripotent and neural stem cells, with respect to the neurodevelopmental aspects of HD.

Distinct cellular toxicity of two mutant huntingtin mRNA variants due to translation regulation

Huntington's disease (HD) is a neurodegenerative disorder caused by CAG repeat expansion within exon1 of the HTT gene. The gene generates two mRNA variants that carry either a short or long 3' untranslated region (3'UTR) while encoding the same protein. It remains unknown whether the two mRNA variants play distinct roles in HD pathogenesis. We found that the long HTT 3'UTR was capable of guiding mRNA to neuronal dendrites, suggesting that some long-form HTT mRNA is transported to dendrites for local protein synthesis. To assay roles of two HTT mRNA variants in cell bodies, we expressed mRNA harboring HTT exon1 containing 23x or 145x CAGs with the short or long 3'UTR. We found that mutant mRNA containing the short 3'UTR produced more protein aggregates and caused more apoptosis in both cultured neurons and HEK293 cells, compared with mutant mRNA containing the long 3'UTR. Although the two 3'UTRs did not affect mRNA stability, we detected higher levels of protein synthesis from mRNA containing the short 3'UTR than from mRNA containing the long 3'UTR. These results indicate that the long HTT 3'UTR suppresses translation. Thus, short-form mutant HTT mRNA will be more efficient in producing toxic protein than its long-form counterpart.

Deletion of the huntingtin proline-rich region does not significantly affect normal huntingtin function in mice

The N-terminus of Huntingtin, the protein encoded by the Huntington's disease gene, contains a stretch of polyglutamine residues that is expanded in Huntington's disease. The polyglutamine stretch is flanked by two conserved protein domains in vertebrates: an N1-17 domain, and a proline-rich region (PRR). The PRR can modulate the structure of the adjacent polyglutamine stretch, and is a binding site for several interacting proteins. To determine the role of the PRR in Huntingtin function, we have generated a knock-in allele of the mouse Huntington's disease gene homolog that expresses full-length normal huntingtin lacking the PRR. Mice that are homozygous for the huntingtin PRR deletion are born at the normal Mendelian frequency, suggesting that the PRR is not required for essential huntingtin functions during embryonic development. Moreover, adult homozygous mutants did not exhibit any significant differences from wild-type controls in general motor function and motor learning. However, 18 month-old male, but not female, homozygous PRR deletion mutants exhibited deficits in the Morris water task, suggesting that age-dependent spatial learning and memory may be affected in a sex-specific fashion by the huntingtin PRR deletion.

PolyQ 2.0: an improved version of PolyQ, a database of human polyglutamine proteins

Proteins with expanded polyglutamine (polyQ) repeats are involved in human neurodegenerative diseases, via a gain-of-function mechanism of neuronal toxicity involving protein conformational changes that result in the formation and deposition of β-sheet-rich aggregates. Aggregation is dependent on the context and properties of the host protein, such as domain context and location of the repeat tract. In order to explore this relationship in greater detail, here we describe PolyQ 2.0, an updated database that provides a comprehensive knowledgebase for human polyQ proteins. Compared with the previous PolyQ database, our new database provides a variety of substantial updates including detailed biological annotations and search options. Biological annotations in terms of domain context information, protein structural and functional annotation, single point mutations, predicted disordered regions, protein-protein interaction partners, metabolic/signaling pathways, post-translational modification sites and evolutionary information are made available. Several new database functionalities have also been provided, including search using multiple/combinatory keywords, and submission of new data entries. Also, several third-party plug-ins are employed to enhance data visualization in PolyQ 2.0. In PolyQ 2.0 the proteins are reclassified into 3 new categories and contain 9 reviewed disease-associated polyQ proteins, 105 reviewed non-disease polyQ proteins and 146 un-reviewed polyQ proteins (reviewed by UniProt curators). We envisage that this updated database will be a useful resource for functional and structural investigation of human polyQ proteins. Database URL: http://lightning.med.monash.edu/polyq2/.

Critical evaluation of in silico methods for prediction of coiled-coil domains in proteins

Coiled-coils refer to a bundle of helices coiled together like strands of a rope. It has been estimated that nearly 3% of protein-encoding regions of genes harbour coiled-coil domains (CCDs). Experimental studies have confirmed that CCDs play a fundamental role in subcellular infrastructure and controlling trafficking of eukaryotic cells. Given the importance of coiled-coils, multiple bioinformatics tools have been developed to facilitate the systematic and high-throughput prediction of CCDs in proteins. In this article, we review and compare 12 sequence-based bioinformatics approaches and tools for coiled-coil prediction. These approaches can be categorized into two classes: coiled-coil detection and coiled-coil oligomeric state prediction. We evaluated and compared these methods in terms of their input/output, algorithm, prediction performance, validation methods and software utility. All the independent testing data sets are available at http://lightning.med.monash.edu/coiledcoil/. In addition, we conducted a case study of nine human polyglutamine (PolyQ) disease-related proteins and predicted CCDs and oligomeric states using various predictors. Prediction results for CCDs were highly variable among different predictors. Only two peptides from two proteins were confirmed to be CCDs by majority voting. Both domains were predicted to form dimeric coiled-coils using oligomeric state prediction. We anticipate that this comprehensive analysis will be an insightful resource for structural biologists with limited prior experience in bioinformatics tools, and for bioinformaticians who are interested in designing novel approaches for coiled-coil and its oligomeric state prediction.

Evidence of Extensive Alternative Splicing in Post Mortem Human Brain HTT Transcription by mRNA Sequencing

Despite 20 years since its discovery, the gene responsible for Huntington’s Disease, HTT, has still not had its function or transcriptional profile completely characterized. In response to a recent report by Ruzo et al. of several novel splice forms of HTT in human embryonic stem cell lines, we have analyzed a set of mRNA sequencing datasets from post mortem human brain from Huntington’s disease, Parkinson’s disease, and neurologically normal control subjects to evaluate support for previously observed and to identify novel splice patterns. A custom analysis pipeline produced supporting evidence for some of the results reported by two previous studies of alternative isoforms as well as identifying previously unreported splice patterns. All of the alternative splice patterns were of relatively low abundance compared to the canonical splice form.

Discovery of novel isoforms of huntingtin reveals a new hominid-specific exon

Huntington’s disease (HD) is a devastating neurological disorder that is caused by an expansion of the poly-Q tract in exon 1 of the Huntingtin gene (HTT). HTT is an evolutionarily conserved and ubiquitously expressed protein that has been linked to a variety of functions including transcriptional regulation, mitochondrial function, and vesicle transport. This large protein has numerous caspase and calpain cleavage sites and can be decorated with several post-translational modifications such as phosphorylations, acetylations, sumoylations, and palmitoylations. However, the exact function of HTT and the role played by its modifications in the cell are still not well understood. Scrutiny of HTT function has been focused on a single, full length mRNA. In this study, we report the discovery of 5 novel HTT mRNA splice isoforms that are expressed in normal and HTT-expanded human embryonic stem cell (hESC) lines as well as in cortical neurons differentiated from hESCs. Interestingly, none of the novel isoforms generates a truncated protein. Instead, 4 of the 5 new isoforms specifically eliminate domains and modifications to generate smaller HTT proteins. The fifth novel isoform incorporates a previously unreported additional exon, dubbed 41b, which is hominid-specific and introduces a potential phosphorylation site in the protein. The discovery of this hominid-specific isoform may shed light on human-specific pathogenic mechanisms of HTT, which could not be investigated with current mouse models of the disease.

Huntington disease and the huntingtin protein

Huntington disease (HD) is a devastating neurodegenerative disease that derives from CAG repeat expansion in the huntingtin gene. The clinical syndrome consists of progressive personality changes, movement disorder, and dementia and can develop in children and adults. The huntingtin protein is required for human development and normal brain function. It is subject to posttranslational modification, and some events, such as phosphorylation, can play an enormous role in regulating toxicity of the huntingtin protein. The function of huntingtin in the cell is unknown, and it may play a role as a scaffold. Multiple mouse models of HD have now been created with fragments and full-length protein. The models show variable fidelity to the disease in terms of genetics, pathology, and rates of progression. Pathogenesis of HD involves cleavage of the protein and is associated with neuronal accumulation of aggregated forms. The potential mechanisms of neurodegeneration are myriad, including primary effects of protein homeostasis, gene expression, and mitochondrial dysfunction. Specific therapeutic approaches are similarly varied and include efforts to reduce huntingtin gene expression, protein accumulation, and protein aggregation.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

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.

Mouse models of Huntington disease: variations on a theme

An accepted prerequisite for clinical trials of a compound in humans is the successful alleviation of the disease in animal models. For some diseases, however, successful translation of drug effects from mouse models to the bedside has been limited. One question is whether the current models accurately reproduce the human disease. Here, we examine the mouse models that are available for therapeutic testing in Huntington disease (HD), a late-onset neurodegenerative disorder for which there is no effective treatment. The current mouse models show different degrees of similarity to the human condition. Significant phenotypic differences are seen in mouse models that express either truncated or full-length human, or full-length mouse, mutant huntingtin (mHTT). These differences in phenotypic expression may be attributable to the influences of protein context, mouse strain and a difference in regulatory sequences between the mouse Htt and human HTT genes.

Trends in the molecular pathogenesis and clinical therapeutics of common neurodegenerative disorders

The term neurodegenerative disorders, encompasses a variety of underlying conditions, sporadic and/or familial and are characterized by the persistent loss of neuronal subtypes. These disorders can disrupt molecular pathways, synapses, neuronal subpopulations and local circuits in specific brain regions, as well as higher-order neural networks. Abnormal network activities may result in a vicious cycle, further impairing the integrity and functions of neurons and synapses, for example, through aberrant excitation or inhibition. The most common neurodegenerative disorders are Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis and Huntington’s disease. The molecular features of these disorders have been extensively researched and various unique neurotherapeutic interventions have been developed. However, there is an enormous coercion to integrate the existing knowledge in order to intensify the reliability with which neurodegenerative disorders can be diagnosed and treated. The objective of this review article is therefore to assimilate these disorders’ in terms of their neuropathology, neurogenetics, etiology, trends in pharmacological treatment, clinical management, and the use of innovative neurotherapeutic interventions.

The Hdh(Q150/Q150) knock-in mouse model of HD and the R6/2 exon 1 model develop comparable and widespread molecular phenotypes

The identification of the Huntington's disease (HD) mutation as a CAG/polyglutamine repeat expansion enabled the generation of transgenic rodent models and gene-targeted mouse models of HD. Of these, mice that are transgenic for an N-terminal huntingtin fragment have been used most extensively because they develop phenotypes with relatively early ages of onset and rapid disease progression. Although the fragment models have led to novel insights into the pathophysiology of HD, it is important that models expressing a mutant version of the full-length protein are analysed in parallel. We have generated congenic C57BL/6 and CBA strains for the HdhQ150 knock-in mouse model of HD so that homozygotes can be analysed on an F1 hybrid background. Although a significant impairment in grip strength could be detected from a very early age, the performance of these mice in the quantitative behavioural tests most frequently used in preclinical efficacy trials indicates that they are unlikely to be useful for preclinical screening using a battery of conventional tests. However, at 22 months of age, the Hdh(Q150/Q150) homozygotes showed unexpected widespread aggregate deposition throughout the brain, transcriptional dysregulation in the striatum and cerebellum and decreased levels of specific chaperones, all well-characterised molecular phenotypes present in R6/2 mice aged 12 weeks. Therefore, when strain background and CAG repeat length are controlled for, the knock-in and fragment models develop comparable phenotypes. This supports the continued use of the more high-throughput fragment models to identify mechanisms of pathogenesis and for preclinical screening.

Huntingtin gene evolution in Chordata and its peculiar features in the ascidian Ciona genus

Background

To gain insight into the evolutionary features of the huntingtin (htt) gene in Chordata, we have sequenced and characterized the full-length htt mRNA in the ascidian Ciona intestinalis, a basal chordate emerging as new invertebrate model organism. Moreover, taking advantage of the availability of genomic and EST sequences, the htt gene structure of a number of chordate species, including the cogeneric ascidian Ciona savignyi, and the vertebrates Xenopus and Gallus was reconstructed.

Results

The C. intestinalis htt transcript exhibits some peculiar features, such as spliced leader trans-splicing in the 98 nt-long 5' untranslated region (UTR), an alternative splicing in the coding region, eight alternative polyadenylation sites, and no similarities of both 5' and 3'UTRs compared to homologs of the cogeneric C. savignyi. The predicted protein is 2946 amino acids long, shorter than its vertebrate homologs, and lacks the polyQ and the polyP stretches found in the the N-terminal regions of mammalian homologs. The exon-intron organization of the htt gene is almost identical among vertebrates, and significantly conserved between Ciona and vertebrates, allowing us to hypothesize an ancestral chordate gene consisting of at least 40 coding exons.

Conclusion

During chordate diversification, events of gain/loss, sliding, phase changes, and expansion of introns occurred in both vertebrate and ascidian lineages predominantly in the 5'-half of the htt gene, where there is also evidence of lineage-specific evolutionary dynamics in vertebrates. On the contrary, the 3'-half of the gene is highly conserved in all chordates at the level of both gene structure and protein sequence. Between the two Ciona species, a fast evolutionary rate and/or an early divergence time is suggested by the absence of significant similarity between UTRs, protein divergence comparable to that observed between mammals and fishes, and different distribution of repetitive elements.

Genomic structure, promoter activity, and developmental expression of the mouse homologue of the Machado-Joseph disease (MJD) gene

Machado-Joseph disease (MJD) is a neurodegenerative disorder, caused by the expansion of the (CAG)n tract in the MJD gene. This encodes the protein ataxin-3, of unknown function. The mouse Mjd gene has a structure similar to that of its human counterpart and it also contains a TATA-less promoter. Its 5' flanking region contains conserved putative binding regions for transcription factors Sp1, USF, Arnt, Max, E47, and MyoD. Upon differentiation of P19 cells, the Mjd gene promoter is preferentially activated in endodermal and mesodermal derivatives, including cardiac and skeletal myocytes; and less so in neuronal precursors. Mouse ataxin-3 is ubiquitously expressed during embryonic development and in the adult, with strong expression in regions of the CNS affected in MJD. It is particularly abundant in all types of muscle and in ciliated epithelial cells, suggesting that it may be associated with the cytoskeleton and may have an important function in cell structure and/or motility.

Mouse Huntington's disease homolog mRNA levels: variation and allele effects

Huntington's disease homolog (Hdh) mRNA levels in mice with different Hdh alleles were measured. Brain Hdh mRNA levels varied up to threefold in genetically identical wild-type mice, indicating nongenetic factors influence Hdh expression. Striatal Hdh mRNA levels from an allele with a repeat expanded to 150 CAGs were diminished compared with wild-type and showed variation that might contribute to phenotypic variability in the Hdh(CAG)150 knock-in mouse model. To determine whether Hdh mRNA levels are tightly regulated, we assessed these levels in mice heterozygous for a deletion of the Hdh promoter. The loss of one allele reduced Hdh mRNA levels in most tissues, suggesting mechanisms to maintain Hdh mRNA levels are not in effect and should not impede therapies designed to destroy mutant huntingtin mRNA. Finally, we found a correlation between tissue mRNA levels and the susceptibility of the Hdh locus to Cre-mediated deletion. The two tissues with the highest levels of Hdh mRNA, testes and brain, were the only tissues susceptible to Cre-mediated recombination between loxP sites at Hdh locus. In contrast, the same Cre-expressing line caused recombination in every tissue for loxP sites at another genomic location. The pattern of Cre susceptibility at Hdh suggests a correlation between chromatin accessibility and high levels of Hdh expression in testes and brain.

Cloning and expression analysis of the murine homolog of the spinocerebellar ataxia type 7 (SCA7) gene

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disease caused by the expansion of a polyglutamine tract in the protein ataxin-7, a protein of unknown function. In order to analyze the expression pattern of wild type ataxin-7 in detail, the murine SCA7 gene homolog was cloned and the expression pattern in mice analyzed. The SCA7 mouse and human gene exhibit a high degree of identity at both DNA (88.2%) and protein (88.7%) level. The CAG repeat region, known to be polymorphic in man, is conserved in mouse but contained only five repeats in all mouse strains analyzed. The arrestin homology domain and the nuclear localization signal found in human ataxin-7 is also conserved in the murine homolog. Expression of ataxin-7 was detected during mouse embryonic development and in all adult mouse tissues examined by northern and western blots. In brain, immunohistological staining revealed an ataxin-7 expression pattern similar to that in human, with ataxin-7 expression in cerebellum, several brainstem nuclei, cerebral cortex and hippocampus. Our data show high conservation of ataxin-7 both structurally and at the level of expression, suggesting a conserved role for the protein in mice and humans.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

RAI1 is a novel polyglutamine encoding gene that is deleted in Smith-Magenis syndrome patients

The human chromosomal band 17p11.2 is a genetically unstable interval. It has been shown to be deleted in patients suffering from Smith-Magenis syndrome. Previous efforts of physical and transcriptional mapping in 17p11.2 and subsequent genomic sequencing of the candidate interval allowed the identification of new genes that might be responsible for the Smith-Magenis syndrome. In this report, one of these genes named RAI1, the human homologue of the mouse Rai1 gene, has been investigated for its contribution to the syndrome. Expression analysis on different human adult and fetal tissues has shown the existence of at least three splice variants. Moreover, the most interesting feature of the gene is the presence of a polymorphic CAG repeat coding for a polyglutamine stretch in the amino terminal domain of the protein.

Huntington's disease: from gene to potential therapy

Huntington's disease (HD) is a progressive, late-onset neurodegenerative illness with autosomal dominant inheritance that affects one in 10 000 individuals in Western Europe. The disease is caused by a polyglutamine repeat expansion located in the N-terminal region of the huntingtin protein. The mutation is likely to act by a gain of function, but the molecular mechanisms by which it leads to neuronal dysfunction and cell death are not yet known. The normal function of huntingtin in cell metabolism is also unclear. There is no therapy for HD. Research on HD should help elucidate the pathogenetic mechanism of this illness in order to develop successful treatments to prevent or slow down symptoms. This article presents new results in HD research focusing on in vivo and in vitro model systems, potential molecular mechanisms of HD, and the development of therapeutic strategies.

Wild-type huntingtin reduces the cellular toxicity of mutant huntingtin in vivo

We have developed yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 or YAC72) human huntingtin (htt), in a developmental- and tissue-specific manner, that is identical to endogenous htt. YAC72 mice develop selective degeneration of medium spiny projection neurons in the lateral striatum, similar to what is observed in Huntington disease. Mutant human htt expressed by YAC transgenes can compensate for the absence of endogenous htt and can rescue the embryonic lethality that characterizes mice homozygous for targeted disruption of the endogenous Hdh gene (-/-). YAC72 mice lacking endogenous htt (YAC72 -/-) manifest a novel phenotype characterized by infertility, testicular atrophy, aspermia, and massive apoptotic cell death in the testes. The testicular cell death in YAC72 -/- mice can be markedly reduced by increasing endogenous htt levels. YAC72 mice with equivalent levels of both wild-type and mutant htt (YAC72 +/+) breed normally and have no evidence of increased testicular cell death. Similar findings are seen in YAC46 -/- mice compared with YAC46 +/+ mice, in which wild-type htt can completely counteract the proapoptotic effects of mutant htt. YAC18 -/- mice display no evidence of increased cellular apoptosis, even in the complete absence of endogenous htt, demonstrating that the massive cellular apoptosis observed in YAC46 -/- mice and YAC72 -/- mice is polyglutamine-mediated toxicity from the mutant transgene. These data provide the first direct in vivo evidence of a role for wild-type htt in decreasing the cellular toxicity of mutant htt.

Identification and characterization of the miniature pig Huntington's disease gene homolog: evidence for conservation and polymorphism in the CAG triplet repeat

Huntington's disease (HD) is associated with a significant expansion of a CAG trinucleotide repeat, which results in a lengthened polyglutamine tract in the single gene product, huntingtin, on human 4p16.3. We isolated cDNA clones that encompassed the entire coding sequence of the miniature pig HD gene (Sus HD) from two porcine testis cDNA libraries. The cDNA contig revealed a 12,749-nucleotide transcript coding for a 345-kDa protein (3139 amino acid residues), which exhibited 96% peptide sequence homology to human huntingtin. Northern blot analysis revealed that the Sus HD gene was ubiquitously expressed as two large transcripts of approximately 11 and 13 kb in size in all the tested tissues, much like the human HD gene. The CAG trinucleotide repeat was found to be interrupted by CAA triplets and to encode 17 or 18 consecutive glutamine residues. In our laboratory stock of miniature pig, three allotypes in the triplet repeat sequence were found. Thus, the Sus HD gene closely resembles its human counterpart in terms of sequence and expression pattern. In particular, human-miniature pig similarities in the normal length of the CAG triplet repeat as well as its repeat-number polymorphism may indicate that miniature pig would provide a good animal model for Huntington's disease.

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.

Glutamine repeats and neurodegeneration

A growing number of neurodegenerative diseases have been found to result from the expansion of an unstable trinucleotide repeat. Over the past 6 years, researchers have focused on identifying the mechanism by which the expanded polyglutamine tract renders a protein toxic to a subset of vulnerable neurons. In this review, we summarize the clinicopathologic features of these disorders (spinobulbar muscular atrophy, Huntington disease, and the spinocerebellar ataxias, including dentatorubropallidoluysian atrophy), describe the genes involved and what is known about their products, and discuss the model systems that have lent insight into pathogenesis. The review concludes with a model for pathogenesis that illuminates the unifying features of these polyglutamine disorders. This model may prove relevant to other neurodegenerative disorders as well.

A putative Drosophila homolog of the Huntington's disease gene

The Huntington's disease (HD) gene encodes a protein, huntingtin, with no known function and no detectable sequence similarity to other proteins in current databases. To gain insight into the normal biological role of huntingtin, we isolated and sequenced a cDNA encoding a protein that is a likely homolog of the HD gene product in Drosophila melanogaster. We also determined the complete sequence of 43 125 contiguous base pairs of genomic DNA that encompass the Drosophila HD gene, allowing the intron-exon structure and 5'- and 3'-flanking regions to be delineated. The predicted Drosophila huntingtin protein has 3583 amino acids, which is several hundred amino acids larger than any other previously characterized member of the HD family. Analysis of the genomic and cDNA sequences indicates that Drosophila HD has 29 exons, compared with the 67 exons present in vertebrate HD genes, and that Drosophila huntingtin lacks the polyglutamine and polyproline stretches present in its mammalian counterparts. The Drosophila HD mRNA is expressed in a broad range of developmental stages and in the adult, a temporal pattern of expression similar to that observed for mammalian HD transcripts. We can discern five regions of high similarity from multiple sequence alignments between Drosophila and vertebrate huntingtins. These regions may define functionally important domains within the protein.

Expression of the Huntington's disease gene is regulated in astrocytes in the arcuate nucleus of the hypothalamus of postpartum rats

Huntington's disease (HD) is one of a number of neurodegenerative disorders caused by expansion of polyglutamine-encoding CAG repeats within specific genes. Huntingtin, the protein product of the HD gene, is widely expressed in neural and nonneural human and rodent tissue. The function of the wild-type or mutated form of huntingtin is currently unknown. We have observed that relative to naive and male animals, huntingtin protein was significantly increased in the arcuate nucleus of postpartum rats. Using an oligonucleotide probe, in situ and Northern blot hybridization confirmed the expression of huntingtin mRNA. Quantification of the in situ hybridization signal in the arcuate nucleus revealed an approximate sevenfold increase in the expression of huntingtin mRNA in postpartum, lactating animals compared with naive female or male animals. Emulsion autoradiography and immunohistochemistry revealed that the cells with elevated huntingtin expression had a stellate conformation that morphologically resembled astrocytes. Dual label immunofluorescence immunohistochemistry demonstrated the colocalization of huntingtin and glial fibrillary acidic protein in these cells, confirming that they were astrocytes. Astrocytes expressing huntingtin were consistently found in close apposition to neuronal soma, suggesting interactions between these cell types. During the perinatal and postnatal period, the hypothalamus undergoes alterations in metabolic function. Our results support the idea of glia-induced metabolic changes in the hypothalamus. These results provide the first demonstration of naturally occurring changes in the expression of the Huntington's disease gene in the brain and suggest that huntingtin may play an important role in the processes that regulate neuroendocrine function.

Neuronal degeneration in the basal ganglia and loss of pallido-subthalamic synapses in mice with targeted disruption of the Huntington's disease gene

Huntington's disease (HD) is a progressive neurodegenerative disorder associated with CAG repeat expansion within a novel gene (IT15). We have previously created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homologue of the HD gene. Homozygotes for the Hdhex5 mutation exhibit embryolethality before embryonic day 8.5, while heterozygotes survive to adulthood and display increased motor activity and cognitive deficits. Detailed morphometric and stereological analyses of the basal ganglia in adult heterozygous mice were performed by light and electron microscopy. Morphometric analyses demonstrated a significant loss of neurons from both the globus pallidus (29%) and the subthalamic nucleus (51%), with a normal complement of neurons in the caudate-putamen and substantia nigra. The ultrastructural appearance of sporadic degenerating neurons in these regions indicated apoptosis. The highest frequency of apoptotic neurons was observed in the globus pallidus and subthalamic nucleus. Stereological analyses in the subthalamic nucleus revealed a significant decrease in the numerical density of symmetric synapses (43%), suggesting a relatively selective loss of inhibitory pallido-subthalamic afferents. Immunohistochemistry using antibodies against enkephalin and substance-P was unremarkable in heterozygotes, indicating a normal complement of enkephalin-immunoreactive striatopallidal afferents and substance-P-immunoreactive striatopeduncular and striatonigral afferents in these animals. These findings show that loss of an intact huntingtin protein is associated with significant morphological alterations in the basal ganglia of adult mice, indicating an important role for this protein during development of the central nervous system.

Characterization of the Huntington's disease (HD) gene homologue in the zebrafish Danio rerio

The Huntington's disease (HD) gene contains a trinucleotide repeat that is expanded and unstable in patients with the disease (HDCRG, 1993). As the first step toward investigating a potential role for this gene in early vertebrate development, we isolated the homolog of the Huntington's disease (ZHD) cDNA in zebrafish. This cDNA encodes a predicted protein product of 3121 amino acids with 70% identity to human huntingtin. The first exon is predicted to encode four glutamines, followed by only one proline, demonstrating that the polymorphic polyproline stretch found in mammalian HD sequences is absent in the fish. We sequenced approximately 900bp upstream from the predicted start codon and found that it lacks a TATA box, CCAAT box, or Sp1 binding sites. Western blot analysis revealed that the protein is expressed at a high level in late embryonic development and at moderate levels in the adult head.

Aberrant processing of the Fugu HD (FrHD) mRNA in mouse cells and in transgenic mice

The puffer fish ( Fugu rubripes ) has a compact genome of 400 Mbp which is approximately 7.5-fold smaller than the human genome. It contains a similar number of genes but is deficient in intergenic, intronic and dispersed repetitive sequences. Fugu is becoming established as the model vertebrate genome for the identification and characterisation of novel human genes and conserved regulatory sequences. It has also been proposed that Fugu genes may provide natural mini-genes for the production of transgenic mice. We have used the Fugu homologue of the Huntington's disease (HD) gene to test this possibility. The human and Fugu HD genes cover 170 kb and 23 kb respectively and have previously been sequenced in their entirety. In Fugu tissue, the Fugu HD gene was found to be expressed as predicted from the gene sequence but three differentially spliced forms were also detected. Despite the absence of conserved promoter sequences, the Fugu promoter was found to be functional in mouse cells. We have generated mice transgenic for the Fugu HD gene and conducted a detailed expression analysis across the entire 10 kb transcript. This revealed the presence of many aberrant splice forms which would be incompatible with the production of the Fugu huntingtin protein. The Fugu HD gene is incorrectly processed in mouse cells both in vitro and in vivo which sheds doubt on the usefulness of Fugu genes for transgenesis.

Transgenic mouse models of neurodegenerative disease caused by CAG/polyglutamine expansions

CAG/polyglutamine expansion is the mutational mechanism that causes a number of late-onset neurodegenerative diseases. Expanded CAG repeats are unstable: they vary in size between tissues and change in size upon transmission from parent to offspring. These mutations are thought to impart a dominant gain of function to the proteins in which they are located. Recent reports describing the first mouse models of these diseases promise to shed light on the molecular mechanisms underlying CAG-repeat instability, the pathways by which polyglutamine expansion causes cell death and the factors that determine the specificity of the neurodegeneration.

Huntington disease: advances in molecular and cell biology

Huntington disease is an inherited neurodegeneration, for which the associated mutation was isolated in 1993. The mutation is an expansion of a CAG trinucleotide repeat, which translates to give a polyglutamine tract at the N-terminus of a large protein, huntingtin. Neither the normal nor the pathogenic functions of this protein have been identified, but it is clear that pathogenesis is mediated through the expanded polyglutamine tract within the protein, and that polyglutamine is toxic to cells. A number of proteins which interact with the N-terminal region of huntingtin have been isolated, but this has not, so far, yielded a rationale for pathogenesis. Huntingtin is found in areas of the brain that degenerate in this disease but is also associated with pathogenic inclusions in Alzheimer disease and Pick disease. It is possible that Huntington disease has pathogenic mechanisms in common with these other neurodegenerative diseases, and that the mechanism may relate to the formation of abnormal, cytoskeletal-associated, inclusions within cells.

Carbonic anhydrase II and carbonic anhydrase-related protein in the cerebellar cortex of normal and lurcher mice

The developmental profiles of carbonic anhydrase II (CA-II) and a carbonic anhydrase related protein (CARP) were studied in rat and mouse cerebella. Enzyme histochemistry, immunohistochemistry, in situ hybridisation and Western blotting were used to study the synthesis and expression of these enzymes in cerebellar sections from age matched control, CA-II deficient and lurcher mice, the latter being characterised by Purkinje cell degeneration. Both CA-II and CARP were first found to be expressed in the Purkinje cells in the 9 day old mouse, and the immunoreactivity of both peptides increased with time. Immunohistochemistry showed more intense staining of CARP than of CA-II in Purkinje cells throughout the developmental profile of the mouse, and this was mirrored by the mRNA levels determined by in situ hybridisation. Immunohistochemistry of CA-II and CARP also demonstrated the progressive dendritic growth of the mouse and rat Purkinje cells. CA-II and CARP immunoreactivity ceased by the end of cerebellar maturation. The onset of Purkinje cell degeneration was detected at day 10 in the lurcher mouse, with concomitant marked decrease in CA-II level: however CARP expression was found to be unchanged. By postnatal day 16 neither CA-II mRNA, protein, nor activity was detectable in contrast to CARP which remained at a decreased level unit the Purkinje cells population had completely degenerated. Our findings suggest a role of CA-II in the degenerative processes of the lurcher Purkinje cells, with CARP playing an important role in the development and maturation of the cerebellar cortex.

Identification of a long huntingtin mRNA transcript in mouse brain

Two mRNA species of the Huntington disease (HD) gene that share identical protein coding sequences but differ in their 3' untranslated region have been identified in human. Although a similar situation has been suggested to occur in mouse, only one cDNA has been isolated to date. We report the isolation of a novel partial cDNA of the mouse HD gene that is identical in its protein coding sequence to the previously reported cDNA, although it differs in the distal portion of the 3' untranslated region. Northern blotting assays indicate that this mRNA transcript is preferentially expressed in brain, with highest levels in cerebellum, cerebral cortex and striatum.

Glutamine repeats and inherited neurodegenerative diseases: molecular aspects

Several dominantly inherited, late onset, neurodegenerative diseases are due to expansion of CAG repeats, leading to expansion of glutamine repeats in the affected proteins. These proteins are of very different sizes and, with one exception, show no sequence homology to known proteins or to each other; their functions are unknown. In some, the glutamine repeat starts near the N-terminus, in another near the middle and in another near the C-terminus, but regardless of these differences, no disease has been observed in individuals with fewer than 37 repeats, and absence of disease has never been found in those with more than 41 repeats. Protein constructs with more than 41 repeats are toxic to E. coli and to CHO cells in culture, and they elicit ataxia in transgenic mice. These observations argue in favour of a distinct change of structure associated with elongation beyond 37-41 glutamine repeats. The review describes experiments designed to find out what these structures might be and how they could influence the properties of the proteins of which they form part. Poly-L-glutamines form pleated sheets of beta-strands held together by hydrogen bonds between their amides. Incorporation of glutamine repeats into a small protein of known structure made it associate irreversibly into oligomers. That association took place during the folding of the protein molecules and led to their becoming firmly interlocked by either strand- or domain-swapping. Thermodynamic considerations suggest that elongation of glutamine repeats beyond a certain length may lead to a phase change from random coils to hydrogen-bonded hairpins. Possible mechanisms of expansion of CAG repeats are discussed in the light of looped DNA model structures.

Huntington's disease: translating a CAG repeat into a pathogenic mechanism

The specific pattern of neuronal cell death in Huntington's disease (HD) is triggered by an abnormal version of the huntingtin protein, which is produced by translation of the HD gene defect, an expanded CAG repeat in a novel 4p16.3 gene. The extended amino-terminal polyglutamine segment may act via the protein's inherent activity, increasing it or decreasing it in a graded fashion, or, alternatively, it may confer the ability to interact with a completely different set of cellular pathways, focusing attention on the HD protein's normal and abnormal physiological functions.

Simple sequences and the expanding genome

Recent analysis of the contribution of replication slippage to genome evolution shows that it has played a significant role in all species from eubacteria to humans. The overall level of repetition in genomes is related to genome size and to the degree of repetition that can be measured within individual ribosomal RNA genes, suggesting that the entire genome accepts simple sequences in a concerted manner when its size increases. Although coding sequences accept simple sequences much less readily than non-coding sequences, they accept some repeats, particularly (CAG)n, preferentially. This may have consequences for the evolution of the genes involved in trinucleotide expansion diseases and the transcriptional networks of which they may form a part.

Codon repeats in genes associated with human diseases: fewer repeats in the genes of nonhuman primates and nucleotide substitutions concentrated at the sites of reiteration

Five human diseases are due to an excessive number of CAG repeats in the coding regions of five different genes. We have analyzed the repeat regions in four of these genes from nonhuman primates, which are not known to suffer from the diseases. These primates have CAG repeats at the same sites as in human alleles, and there is similar polymorphism of repeat number, but this number is smaller than in the human genes. In some of the genes, the segment of poly(CAG) has expanded in nonhuman primates, but the process has advanced further in the human lineage than in other primate lineages, thereby predisposing to diseases of CAG reiteration. Adjacent to stretches of homogeneous present-day codon repeats, previously existing codons of the same kind have undergone nucleotide substitutions with high frequency. Where these lead to amino acid substitutions, the effect will be to reduce the length of the original homopolymeric stretch in the protein.

Cloning and developmental expression analysis of the murine homolog of the spinocerebellar ataxia type 1 gene (Sca1)

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by the expansion of a CAG trinucleotide repeat which encodes glutamine in the novel protein ataxin-1. In order to characterize the developmental expression pattern of SCA1 and to identify putative functional domains in ataxin-1, the murine homolog (Sca1) was isolated. Cloning and characterization of the murine Sca1 gene revealed that the gene organization is similar to that of the human gene. The murine and human ataxin-1 are highly homologous but the CAG repeat is virtually absent in the mouse sequence suggesting that the polyglutamine stretch is not essential for the normal function of ataxin-1 in mice. Cellular and developmental expression of the murine homolog was examined using RNA in situ hybridization. During cerebellar development, there is a transient burst of Sca1 expression at postnatal day 14 when the murine cerebellar cortex becomes physiologically functional. There is also marked expression of Sca1 in mesenchymal cells of the intervertebral discs during development of the spinal column. These results suggest that the normal Sca1 gene, has a role at specific stages of both cerebellar and vertebral column development.

Huntington's disease. Their loss is our gain?

'Knockout' mice have been developed that lack the Huntington's disease gene, in an effort to gain insight into the disorder and into the pathophysiological effects of tri-nucleotide repeat expansion.

Huntington's disease: CAG genetics expands neurobiology

Huntington's disease, with its progressive uncontrolled movements and characteristic selective neuropathology, has represented a baffling enigma to geneticists and neurobiologists alike. Discovery of the HD gene and its defect has demystified the genetic aspects of the disorder, but has not yet explained its pathogenesis. Attempts to explore this issue suggest that the defect acts as a gain of function, conferring a new deleterious property on the huntingtin protein, and that the gene's normal function may be irrelevant to the disease process.

The identification of a functional nuclear localization signal in the Huntington disease protein

Positional cloning has shown that the Huntington disease (HD) mutation is an expanded trinucleotide repeat in the IT15 gene. Although this mutation clearly produces the HD phenotype, the function of the Huntington disease protein remains undefined. One recent immunocytochemical study suggested that the IT15 protein preferentially localizes to the nucleus of affected neuronal cells. If this result is accurate, it could link the biochemical function of this protein to nuclear activities such as gene regulation. To examine the nuclear transport of the Huntington disease protein, we searched for basic peptide motifs that could produce nuclear localization. One peptide (RRKGKEK) was identified that is highly homologous to a consensus nuclear localization signal. When fused to the cytoplasmic reporter protein, beta-galactosidase, nuclear localization was observed in stably transformed human cell lines. In a complementary study, an anti-peptide polyclonal antibody, raised against a sequence adjacent to the putative nuclear localization sequence, detected the IT15 protein in the nucleus of human cells. These results extend and confirm the previous localization studies and identify an IT15 peptide motif that can function for nuclear localization.

Inactivation of the mouse Huntington's disease gene homolog Hdh

Huntington's disease (HD) is a dominant neurodegenerative disorder caused by expansion of a CAG repeat in the gene encoding huntingtin, a protein of unknown function. To distinguish between "loss of function" and "gain of function" models of HD, the murine HD homolog Hdh was inactivated by gene targeting. Mice heterozygous for Hdh inactivation were phenotypically normal, whereas homozygosity resulted in embryonic death. Homozygotes displayed abnormal gastrulation at embryonic day 7.5 and were resorbing by day 8.5. Thus, huntingtin is critical early in embryonic development, before the emergence of the nervous system. That Hdh inactivation does not mimic adult HD neuropathology suggests that the human disease involves a gain of function.

Targeted disruption of the Huntington's disease gene results in embryonic lethality and behavioral and morphological changes in heterozygotes

Huntington's disease (HD) is an incurable neuropsychiatric disease associated with CAG repeat expansion within a widely expressed gene that causes selective neuronal death. To understand its normal function, we have created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homolog of the HD gene. Homozygotes die before embryonic day 8.5, initiate gastrulation, but do not proceed to the formation of somites or to organogenesis. Mice heterozygous for the Hdhex5 mutation display increased motor activity and cognitive deficits. Neuropathological assessment of two heterozygous mice shows significant neuronal loss in the subthalamic nucleus. These studies show that the HD gene is essential for postimplantation development and that it may play an important role in normal functioning of the basal ganglia.

Widespread expression of Huntington's disease gene (IT15) protein product

Huntington's Disease (HD) is caused by expansion of a CAG repeat within a putative open reading frame of a recently identified gene, IT15. We have examined the expression of the gene's protein product using antibodies developed against the N-terminus and an internal epitope. Both antisera recognize a 350 kDa protein, the predicted size, indicating that the CAG repeat is translated into polyglutamine. The HD protein product is widely expressed, most highly in neurons in the brain. There is no enrichment in the striatum, the site of greatest pathology in HD. Within neurons, the protein is diminished in nuclei and mitochondria and is present in the soluble cytoplasmic compartment, as well as loosely associated with membranes or cytoskeleton, in cell bodies, dendrites, and axons. It is concentrated in nerve terminals, including terminals within the caudate and putamen. Thus, the normal HD gene product may be involved in common intracellular functions, and possibly in regulation of nerve terminal function. The product of the expanded allele is expressed, consistent with a gain of function mechanism for HD at the protein level.