The CAG/polyglutamine tract diseases: gene products and molecular pathogenesis

NMR structures and magnetic force spectroscopy studies of small molecules binding to models of an RNA CAG repeat expansion

RNA repeat expansions fold into stable structures and cause microsatellite diseases such as Huntington's disease (HD), myotonic dystrophy type 1 (DM1), and spinocerebellar ataxias (SCAs). The trinucleotide expansion of r(CAG), or r(CAG) exp , causes both HD and SCA3, and the RNA's toxicity has been traced to its translation into polyglutamine (polyQ; HD) as well as aberrant pre-mRNA alternative splicing (SCA3 and HD). Previously, a small molecule, 1 , was discovered that binds to r(CAG) exp and rescues aberrant pre-mRNA splicing in patient-derived fibroblasts by freeing proteins bound to the repeats. Here, we report the structures of single r(CAG) repeat motif (5'C A G/3'G A C where the underlined adenosines form a 1×1 nucleotide internal loop) in complex with 1 and two other small molecules via nuclear magnetic resonance (NMR) spectroscopy combined with simulated annealing. Compound 2 was designed based on the structure of 1 bound to the RNA while 3 was selected as a diverse chemical scaffold. The three complexes, although adopting different 3D binding pockets upon ligand binding, are stabilized by a combination of stacking interactions with the internal loop's closing GC base pairs, hydrogen bonds, and van der Waals interactions. Molecular dynamics (MD) simulations performed with NMR-derived restraints show that the RNA is stretched and bent upon ligand binding with significant changes in propeller-twist and opening. Compound 3 has a distinct mode of binding by insertion into the helix, displacing one of the loop nucleotides into the major groove and affording a rod-like shape binding pocket. In contrast, 1 and 2 are groove binders. A series of single molecule magnetic force spectroscopy studies provide a mechanistic explanation for how bioactive compounds might rescue disease-associated cellular phenotypes.

A new small molecule DoNA binding to CAG repeat RNA

We here report a new molecule DoNA binding to a CAG repeat RNA. DoNA is a dimer of the NA molecule that we previously reported. NA binds with high affinity to a CAG repeat DNA but not significantly to a CAG repeat RNA. Binding analyses using SPR and CSI-TOF MS indicated a significant increase in the affinity of DoNA to a single stranded CAG repeat RNA compared to NA. Systematic investigation of the RNA motifs bound by DoNA using hairpin RNA models revealed that DoNA binds to the CAG units at overhang and terminal positions, and notably, it binds to the structurally flexible internal and hairpin loop region.

Prion-like properties of the mutant huntingtin protein in living organisms: the evidence and the relevance

If theories postulating that pathological proteins associated with neurodegenerative disorders behave similarly to prions were initially viewed with reluctance, it is now well-accepted that this occurs in several disease contexts. Notably, it has been reported that protein misfolding and subsequent prion-like properties can actively participate in neurodegenerative disorders. While this has been demonstrated in multiple cellular and animal model systems related to Alzheimer's and Parkinson's diseases, the prion-like properties of the mutant huntingtin protein (mHTT), associated with Huntington's disease (HD), have only recently been considered to play a role in this pathology, a concept our research group has contributed to extensively. In this review, we summarize the last few years of in vivo research in the field and speculate on the relationship between prion-like events and human HD. By interpreting observations primarily collected in in vivo models, our discussion will aim to discriminate which experimental factors contribute to the most efficient types of prion-like activities of mHTT and which routes of propagation may be more relevant to the human condition. A look back at nearly a decade of experimentation will inform future research and whether therapeutic strategies may emerge from this new knowledge.

Propensity for somatic expansion increases over the course of life in Huntington disease

Recent work on Huntington disease (HD) suggests that somatic instability of CAG repeat tracts, which can expand into the hundreds in neurons, explains clinical outcomes better than the length of the inherited allele. Here, we measured somatic expansion in blood samples collected from the same 50 HD mutation carriers over a twenty-year period, along with post-mortem tissue from 15 adults and 7 fetal mutation carriers, to examine somatic expansions at different stages of life. Post-mortem brains, as previously reported, had the greatest expansions, but fetal cortex had virtually none. Somatic instability in blood increased with age, despite blood cells being short-lived compared to neurons, and was driven mostly by CAG repeat length, then by age at sampling and by interaction between these two variables. Expansion rates were higher in symptomatic subjects. These data lend support to a previously proposed computational model of somatic instability-driven disease.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

Spinocerebellar Ataxia Type 17 (SCA17)

In 1999, a polyglutamine expansion was identified in the transcription factor TATA-binding protein (TBP) in a patient with ataxia with negative family history. Subsequently, CAG/CAA repeat expansions in the TBP gene were identified in families with spinocerebellar ataxia (SCA), establishing this repeat expansion as the underlying mutation in SCA type 17 (SCA17). There are several characteristic differences between SCA17 and other polyglutamine diseases. First, SCA17 shows a complex and variable clinical phenotype, in some cases overlapping that of Huntington's disease. Second, compared to the other SCA subtypes caused by expanded trinucleotide repeats, anticipation in SCA17 kindreds is rare because of the characteristic structure of the TBP gene. And thirdly, SCA17 patients often have diagnostic problems that may arise from non-penetrance. Because the gap between normal and abnormal repeat numbers is very narrow, it is difficult to determine a cutoff value for pathologic CAG repeat number in SCA17. Herein, we review the clinical, genetic and pathologic features of SCA17.

Disease onset in X-linked dystonia-parkinsonism correlates with expansion of a hexameric repeat within an SVA retrotransposon in TAF1

The genetic basis of X-Linked dystonia-parkinsonism (XDP) has been difficult to unravel, in part because all patients inherit the same haplotype of seven sequence variants, none of which has ever been identified in control individuals. This study revealed that one of the haplotype markers, a retrotransposon insertion within an intron of TAF1, has a variable number of hexameric repeats among affected individuals with an increase in repeat number strongly correlated with earlier age at disease onset. These data support a contributing role for this sequence in disease pathogenesis while further suggesting that XDP may be part of a growing list of neurodegenerative disorders associated with unstable repeat expansions.

X-linked dystonia-parkinsonism (XDP) is a neurodegenerative disease associated with an antisense insertion of a SINE-VNTR-Alu (SVA)-type retrotransposon within an intron of TAF1. This unique insertion coincides with six additional noncoding sequence changes in TAF1, the gene that encodes TATA-binding protein–associated factor-1, which appear to be inherited together as an identical haplotype in all reported cases. Here we examined the sequence of this SVA in XDP patients (n = 140) and detected polymorphic variation in the length of a hexanucleotide repeat domain, (CCCTCT)_n. The number of repeats in these cases ranged from 35 to 52 and showed a highly significant inverse correlation with age at disease onset. Because other SVAs exhibit intrinsic promoter activity that depends in part on the hexameric domain, we assayed the transcriptional regulatory effects of varying hexameric lengths found in the unique XDP SVA retrotransposon using luciferase reporter constructs. When inserted sense or antisense to the luciferase reading frame, the XDP variants repressed or enhanced transcription, respectively, to an extent that appeared to vary with length of the hexamer. Further in silico analysis of this SVA sequence revealed multiple motifs predicted to form G-quadruplexes, with the greatest potential detected for the hexameric repeat domain. These data directly link sequence variation within the XDP-specific SVA sequence to phenotypic variability in clinical disease manifestation and provide insight into potential mechanisms by which this intronic retroelement may induce transcriptional interference in TAF1 expression.

Intrinsic Disorder in Proteins with Pathogenic Repeat Expansions

Intrinsically disordered proteins and proteins with intrinsically disordered regions have been shown to be highly prevalent in disease. Furthermore, disease-causing expansions of the regions containing tandem amino acid repeats often push repetitive proteins towards formation of irreversible aggregates. In fact, in disease-relevant proteins, the increased repeat length often positively correlates with the increased aggregation efficiency and the increased disease severity and penetrance, being negatively correlated with the age of disease onset. The major categories of repeat extensions involved in disease include poly-glutamine and poly-alanine homorepeats, which are often times located in the intrinsically disordered regions, as well as repeats in non-coding regions of genes typically encoding proteins with ordered structures. Repeats in such non-coding regions of genes can be expressed at the mRNA level. Although they can affect the expression levels of encoded proteins, they are not translated as parts of an affected protein and have no effect on its structure. However, in some cases, the repetitive mRNAs can be translated in a non-canonical manner, generating highly repetitive peptides of different length and amino acid composition. The repeat extension-caused aggregation of a repetitive protein may represent a pivotal step for its transformation into a proteotoxic entity that can lead to pathology. The goals of this article are to systematically analyze molecular mechanisms of the proteinopathies caused by the poly-glutamine and poly-alanine homorepeat expansion, as well as by the polypeptides generated as a result of the microsatellite expansions in non-coding gene regions and to examine the related proteins. We also present results of the analysis of the prevalence and functional roles of intrinsic disorder in proteins associated with pathological repeat expansions.

Advances in Sequencing Technologies for Understanding Hereditary Ataxias: A Review

Importance

The hereditary progressive ataxias comprise genetic disorders that affect the cerebellum and its connections. Even though these diseases historically have been among the first familial disorders of the nervous system to have been recognized, progress in the field has been challenging because of the large number of ataxic genetic syndromes, many of which overlap in their clinical features.

Observations

We have taken a historical approach to demonstrate how our knowledge of the genetic basis of ataxic disorders has come about by novel techniques in gene sequencing and bioinformatics. Furthermore, we show that the genes implicated in ataxia, although seemingly unrelated, appear to encode for proteins that interact with each other in connected functional modules.

Conclusions and Relevance

It has taken approximately 150 years for neurologists to comprehensively unravel the genetic diversity of ataxias. There has been an explosion in our understanding of their molecular basis with the arrival of next-generation sequencing and computer-driven bioinformatics; this in turn has made hereditary ataxias an especially well-developed model group of diseases for gaining insights at a systems level into genes and cellular pathways that result in neurodegeneration.

Nanoparticulate strategies for the treatment of polyglutamine diseases by halting the protein aggregation process

Polyglutamine (polyQ) diseases are a class of neurodegenerative disorders that cause cellular dysfunction and, eventually, neuronal death in specific regions of the brain. Neurodegeneration is linked to the misfolding and aggregation of expanded polyQ-containing proteins, and their inhibition is one of major therapeutic strategies used commonly. However, successful treatment has been limited to date because of the intrinsic properties of therapeutic agents (poor water solubility, low bioavailability, poor pharmacokinetic properties), and difficulty in crossing physiological barriers, including the blood-brain barrier (BBB). In order to solve these problems, nanoparticulate systems with dimensions of 1-1000 nm able to incorporate small and macromolecules with therapeutic value, to protect and deliver them directly to the brain, have recently been developed, but their use for targeting polyQ disease-mediated protein misfolding and aggregation remains scarce. This review provides an update of the polyQ protein aggregation process and the development of therapeutic strategies for halting it. The main features that a nanoparticulate system should possess in order to enhance brain delivery are discussed, as well as the different types of materials utilized to produce them. The final part of this review focuses on the potential application of nanoparticulate system strategies to improve the specific and efficient delivery of therapeutic agents to the brain for the treatment of polyQ diseases.

How Epigallocatechin-3-gallate and Tetracycline Interact with the Josephin Domain of Ataxin-3 and Alter Its Aggregation Mode

Epigallocatechin-3-gallate (EGCG) and tetracycline are two known inhibitors of amyloid aggregation able to counteract the fibrillation of most of the proteins involved in neurodegenerative diseases. We have recently investigated their effect on ataxin-3 (AT3), the polyglutamine-containing protein responsible for spinocerebellar ataxia type 3. We previously showed that EGCG and tetracycline can contrast the aggregation process and toxicity of expanded AT3, although by different mechanisms. Here, we have performed further experiments by using the sole Josephin domain (JD) to further elucidate the mechanism of action of the two compounds. By protein solubility assays and FTIR spectroscopy we have first observed that EGCG and tetracycline affect the JD aggregation essentially in the same way displayed when acting on the full-length expanded AT3. Then, by saturation transfer difference (STD) NMR experiments, we have shown that EGCG binds both the monomeric and the oligomeric JD form, whereas tetracycline can only interact with the oligomeric one. Surface plasmon resonance (SPR) analysis has confirmed the capability of the sole EGCG to bind monomeric JD, although with a KD value suggestive for a non-specific interaction. Our investigations provide new details on the JD interaction with EGCG and tetracycline, which could explain the different mechanisms by which the two compounds reduce the toxicity of AT3.

Cellular stress responses in protein misfolding diseases

Many human diseases, particularly neurodegenerative diseases, are associated with protein misfolding. Cellular protein quality control includes all processes that ensure proper protein folding and thus prevent the toxic consequences of protein misfolding. The heat shock response (HSR) and the unfolded protein response (UPR) are major stress response pathways within protein quality control that antagonize protein misfolding in the cytosol and the endoplasmic reticulum, respectively. Huntington's disease is an inherited neurodegenerative disease caused by the misfolding of an abnormally expanded polyglutamine (polyQ) region in the protein huntingtin (Htt), polyQHtt. Using Huntington's disease as a paradigm, I review here the central role of both the HSR and the UPR in defining the toxicity associated with polyQHtt in Huntington's disease. These findings may begin to unravel a previously unappreciated cooperation between different stress response pathways in cells expressing misfolded proteins and consequently in neurodegenerative diseases.

Transformation between α-helix and β-sheet structures of one and two polyglutamine peptides in explicit water molecules by replica-exchange molecular dynamics simulations

Aggregation of polyglutamine peptides with β-sheet structures is related to some important neurodegenerative diseases such as Huntington's disease. However, it is not clear how polyglutamine peptides form the β-sheets and aggregate. To understand this problem, we performed all-atom replica-exchange molecular dynamics simulations of one and two polyglutamine peptides with 10 glutamine residues in explicit water molecules. Our results show that two polyglutamine peptides mainly formed helix or coil structures when they are separated, as in the system with one-polyglutamine peptide. As the interpeptide distance decreases, the intrapeptide β-sheet structure sometimes appear as an intermediate state, and finally the interpeptide β-sheets are formed. We also find that the polyglutamine dimer tends to form the antiparallel β-sheet conformations rather than the parallel β-sheet, which is consistent with previous experiments and a coarse-grained molecular dynamics simulation.

A Tale of Two Maladies? Pathogenesis of Depression with and without the Huntington's Disease Gene Mutation

Huntington’s disease (HD) is an autosomal dominant disorder caused by a tandem repeat expansion encoding an expanded tract of glutamines in the huntingtin protein. HD is progressive and manifests as psychiatric symptoms (including depression), cognitive deficits (culminating in dementia), and motor abnormalities (including chorea). Having reached the twentieth anniversary of the discovery of the “genetic stutter” which causes HD, we still lack sophisticated insight into why so many HD patients exhibit affective disorders such as depression at very early stages, prior to overt appearance of motor deficits. In this review, we will focus on depression as the major psychiatric manifestation of HD, discuss potential mechanisms of pathogenesis identified from animal models, and compare depression in HD patients with that of the wider gene-negative population. The discovery of depressive-like behaviors as well as cellular and molecular correlates of depression in transgenic HD mice has added strong support to the hypothesis that the HD mutation adds significantly to the genetic load for depression. A key question is whether HD-associated depression differs from that in the general population. Whilst preclinical studies, clinical data, and treatment responses suggest striking similarities, there are also some apparent differences. We discuss various molecular and cellular mechanisms which may contribute to depression in HD, and whether they may generalize to other depressive disorders. The autosomal dominant nature of HD and the existence of models with excellent construct validity provide a unique opportunity to understand the pathogenesis of depression and associated gene-environment interactions. Thus, understanding the pathogenesis of depression in HD may not only facilitate tailored therapeutic approaches for HD sufferers, but may also translate to the clinical depression which devastates the lives of so many people.

UBE4B: a promising regulatory molecule in neuronal death and survival

Neuronal survival and death of neurons are considered a fundamental mechanism in the regulation of the nervous system during early development of the system and in adulthood. Defects in this mechanism are highly problematic and are associated with many neurodegenerative diseases. Because neuronal programmed death is apoptotic in nature, indicating that apoptosis is a key regulatory process, the p53 family members (p53, p73, p63) act as checkpoints in neurons due to their role in apoptosis. The complexity of this system is due to the existence of different naturally occurring isoforms that have different functions from the wild types (WT), varying from apoptotic to anti-apoptotic effects. In this review, we focus on the role of UBE4B (known as Ube4b or Ufd2a in mouse), an E3/E4 ligase that triggers substrate polyubiquitination, as a master regulatory ligase associated with the p53 family WT proteins and isoforms in regulating neuronal survival. UBE4B is also associated with other pathways independent of the p53 family, such as polyglutamine aggregation and Wallerian degeneration, both of which are critical in neurodegenerative diseases. Many of the hypotheses presented here are gateways to understanding the programmed death/survival of neurons regulated by UBE4B in normal physiology, and a means of introducing potential therapeutic approaches with implications in treating several neurodegenerative diseases.

Androgen receptor polyglutamine repeat number: models of selection and disease susceptibility

Variation in polyglutamine repeat number in the androgen receptor (AR CAGn) is negatively correlated with the transcription of androgen-responsive genes and is associated with susceptibility to an extensive list of human disease. Only a small portion of the heritability for many of these diseases is explained by conventional SNP-based genome-wide association studies, and the forces shaping AR CAGn among humans remains largely unexplored. Here, we propose evolutionary models for understanding selection at the AR CAG locus, namely balancing selection, sexual conflict, accumulation-selection, and antagonistic pleiotropy. We evaluate these models by examining AR CAGn-linked susceptibility to eight extensively studied diseases representing the diverse physiological roles of androgens, and consider the costs of these diseases by their frequency and fitness effects. Five diseases could contribute to the distribution of AR CAGn observed among contemporary human populations. With support for disease susceptibilities associated with long and short AR CAGn, balancing selection provides a useful model for studying selection at this locus. Gender-specific differences AR CAGn health effects also support this locus as a candidate for sexual conflict over repeat number. Accompanied by the accumulation of AR CAGn in humans, these models help explain the distribution of repeat number in contemporary human populations.

An evaluation of oligonucleotide-based therapeutic strategies for polyQ diseases

Background

RNA interference (RNAi) and antisense strategies provide experimental therapeutic agents for numerous diseases, including polyglutamine (polyQ) disorders caused by CAG repeat expansion. We compared the potential of different oligonucleotide-based strategies for silencing the genes responsible for several polyQ diseases, including Huntington's disease and two spinocerebellar ataxias, type 1 and type 3. The strategies included nonallele-selective gene silencing, gene replacement, allele-selective SNP targeting and CAG repeat targeting.

Results

Using the patient-derived cell culture models of polyQ diseases, we tested various siRNAs, and antisense reagents and assessed their silencing efficiency and allele selectivity. We showed considerable allele discrimination by several SNP targeting siRNAs based on a weak G-G or G-U pairing with normal allele and strong G-C pairing with mutant allele at the site of RISC-induced cleavage. Among the CAG repeat targeting reagents the strongest allele discrimination is achieved by miRNA-like functioning reagents that bind to their targets and inhibit their translation without substantial target cleavage. Also, morpholino analog performs well in mutant and normal allele discrimination but its efficient delivery to cells at low effective concentration still remains a challenge.

Conclusions

Using three cellular models of polyQ diseases and the same experimental setup we directly compared the performance of different oligonucleotide-based treatment strategies that are currently under development. Based on the results obtained by us and others we discussed the advantages and drawbacks of these strategies considering them from several different perspectives. The strategy aimed at nonallele-selective inhibiting of causative gene expression by targeting specific sequence of the implicated gene is the easiest to implement but relevant benefits are still uncertain. The gene replacement strategy that combines the nonallele-selective gene silencing with the expression of the exogenous normal allele is a logical extension of the former and it deserves to be explored further. Both allele-selective RNAi approaches challenge cellular RNA interference machinery to show its ability to discriminate between similar sequences differing in either single base substitutions or repeated sequence length. Although both approaches perform well in allele discrimination most of our efforts are focused on repeat targeting due to its potentially higher universality.

Polyglutamine aggregation in Huntington and related diseases

Polyglutamine (polyQ)-expansions in different proteins cause nine neurodegenerative diseases. While polyQ aggregation is a key pathological hallmark of these diseases, how aggregation relates to pathogenesis remains contentious. In this chapter, we review what is known about the aggregation process and how cells respond and interact with the polyQ-expanded proteins. We cover detailed biophysical and structural studies to uncover the intrinsic features of polyQ aggregates and concomitant effects in the cellular environment. We also examine the functional consequences ofpolyQ aggregation and how cells may attempt to intervene and guide the aggregation process.

The expansion of amino-acid repeats is not associated to adaptive evolution in mammalian genes

Background

The expansion of amino acid repeats is determined by a high mutation rate and can be increased or limited by selection. It has been suggested that recent expansions could be associated with the potential of adaptation to new environments. In this work, we quantify the strength of this association, as well as the contribution of potential confounding factors.

Results

Mammalian positively selected genes have accumulated more recent amino acid repeats than other mammalian genes. However, we found little support for an accelerated evolutionary rate as the main driver for the expansion of amino acid repeats. The most significant predictors of amino acid repeats are gene function and GC content. There is no correlation with expression level.

Conclusions

Our analyses show that amino acid repeat expansions are causally independent from protein adaptive evolution in mammalian genomes. Relaxed purifying selection or positive selection do not associate with more or more recent amino acid repeats. Their occurrence is slightly favoured by the sequence context but mainly determined by the molecular function of the gene.

Lack of association between PSA-NCAM expression and migration in the rostral migratory stream of a Huntington's disease transgenic mouse model

Huntington's disease is a neurodegenerative autosomal disorder characterized by selective loss of striatal and cortical neurons. The mammalian brain subventricular zone contains a population of neural precursors involved in postnatal neurogenesis. These newly generated cells migrate from the subventricular zone along the rostral migratory stream and differentiate into mature olfactory bulb neurons throughout adulthood. The establishment of this pathway depends upon a variety of molecules, including polysialylated neural cell adhesion molecule (PSA-NCAM). We used a murine model of Huntington's disease, the R6/2 transgenic mouse, and in vivo bromodeoxyuridine administration to label cells undergoing proliferation and to follow their migration along the rostral migratory stream. Bromodeoxyuridine labeling did not show any significant increase in proliferation of progenitor cells in symptomatic R6/2 mice, but migration of neuroblasts along the rostral migratory stream was significantly diminished. The decrease in neuroblast migration was not due to an alteration in the expression of PSA-NCAM along the rostral migratory stream since immunohistochemical analysis showed no significant differences between R6/2 and wild type mice. In addition, we used Fluoro-Jade C to evaluate apoptosis and demonstrated that the number of apoptotic cells in the rostral migratory stream is similar in affected and wild type animals, suggesting that cell death is not responsible for the differences observed in neuroblast migration. We conclude that in R6/2 mice, progenitor cells have an impaired migration in their route to the olfactory bulb, with accumulation of cells in the caudal rostral migratory stream that does not result from changes in PSA-NCAM expression and/or cell death.

Prominent eosinophilic intranuclear inclusions in melanocytes of a melanocytic nevus: the aftermath of an infection with molluscum contagiosum? A case report

A 65-year-old Latino man presented to his dermatologist for the removal of two melanocytic nevi from the back. The first nevus was removed from the right scapula and contained melanocytes with prominent eosinophilic nuclear inclusion bodies. The second nevus was removed from the paravertebral region, without evidence of inclusion bodies. Ultrastructurally, the inclusions in the first nevus contained dispersed finely granular, homogenous bodies without a limiting membrane. Immunohistochemistry characterized them as ubiquitin-positive material. Reverse transcriptase in situ polymerase chain reaction analysis was positive for molluscum-specific primers, suggesting that the inclusions encountered in the first nevus were secondary to a remote, local molluscum viral infection of melanocytes.

Lentivector-mediated rescue from cerebellar ataxia in a mouse model of spinocerebellar ataxia

Polyglutamine disorders are inherited neurodegenerative diseases caused by the accumulation of expanded polyglutamine protein (polyQ). Previously, we identified a new guanosine triphosphatase, CRAG, which facilitates the degradation of polyQ aggregates through the ubiquitin-proteasome pathway in cultured cells. Because expression of CRAG decreases in the adult brain, a reduced level of CRAG could underlie the onset of polyglutamine diseases. To examine the potential of CRAG expression for treating polyglutamine diseases, we generated model mice expressing polyQ predominantly in Purkinje cells. The model mice showed poor dendritic arborization of Purkinje cells, a markedly atrophied cerebellum and severe ataxia. Lentivector-mediated expression of CRAG in Purkinje cells of model mice extensively cleared polyQ aggregates and re-activated dendritic differentiation, resulting in a striking rescue from ataxia. Our in vivo data substantiate previous cell-culture-based results and extend further the usefulness of targeted delivery of CRAG as a gene therapy for polyglutamine diseases.

Polymorphisms of androgen receptor gene in childhood and adolescent males with first-onset major depressive disorder and association with related symptomatology

This study was designed to explore the association between CAG repeats in AR gene and major depressive disorder (MDD) in male children and adolescents. The results showed that there were differences between adolescent depressive patients and adolescent controls in CAG repeats' length and alleles' distributions, and the severity of depression and anxiety was negatively correlated with the length of CAG repeats in adolescent patients. This suggested that AR gene might be involved in the depressive upset in adolescents, and the age- and sex-related prevalent differences might also be associated to CAG repeats.

The role of ubiquitin-protein ligases in neurodegenerative disease

Alzheimer's disease and Parkinson's disease are the most common neurodegenerative conditions associated with the ageing process. The pathology of these and other neurodegenerative disorders, including polyglutamine diseases, is characterised by the presence of inclusion bodies in brain tissue of affected patients. In general, these inclusion bodies consist of insoluble, unfolded proteins that are commonly tagged with the small protein, ubiquitin. Covalent tagging of proteins with chains of ubiquitin generally targets them for degradation. Indeed, the ubiquitin/proteasome system (UPS) is the major route through which intracellular proteolysis is regulated. This strongly implicates the UPS in these disease-associated inclusions, either due to malfunction (of specific UPS components) or overload of the system (due to aggregation of unfolded/mutant proteins), resulting in subsequent cellular toxicity. Protein targeting for degradation is a highly regulated process. It relies on transfer of ubiquitin molecules to the target protein via an enzyme cascade and specific recognition of a substrate protein by ubiquitin-protein ligases (E3s). Recent advances in our knowledge gained from the Human Genome Mapping Project have revealed the presence of potentially hundreds of E3s within the human genome. The discovery that parkin, mutations in which are found in at least 50% of patients with autosomal recessive juvenile parkinsonism, is an E3 further highlights the importance of the UPS in neurological disease. To date, parkin is the only E3 confirmed to have a direct causal role in neurodegenerative disorders. However, a number of other (putative) E3s have now been identified that may cause disease directly or interact with neurological disease-associated proteins. Many of these are either lost or mutated in a given disease or fail to process disease-associated mutant proteins correctly. In this review, we will discuss the role(s) of E3s in neurodegenerative disorders.

Multiple system atrophy: a sporadic synucleinopathy

Multiple system atrophy (MSA) is a sporadic neurodegenerative disease characterized clinically by varying degrees of Parkinsonism, cerebellar ataxia and autonomic dysfunction and pathologically by degeneration in the substantia nigra, putamen, olivary nucleus, pontine nuclei and cerebellum. In addition to selective neuronal loss, iron pigment accumulation and gliosis, myelin pathology is increasingly recognized. In affected white matter, myelin displays signs of degeneration and oligodendroglia contain argyrophilic inclusion bodies, so-called glial cytoplasmic inclusions (GCI). GCI are composed of 10-15-nm diameter coated filaments that are immunoreactive for ubiquitin and alpha-synuclein. Similar inclusions are occasionally found in neuronal cell bodies and cell processes in MSA. Given the presence of inclusion bodies composed of synuclein, it is reasonable to assume that biochemical alterations would be detected in synuclein in MSA and indeed this is the case. In MSA synuclein has biophysical properties that suggest increasing insolubility such as sedimentation in dense fractions in sucrose gradients and ready extraction into detergents and formic acid. Surprisingly, these biochemical modifications in synuclein are more widespread in the brain that the obvious pathology and suggest a fundamental molecular characteristic of the disorder. Similar neuronal, and less frequently glial, inclusions are detected in Lewy body disease, where there is also evidence for biophysical alterations in synuclein. Thus, MSA and LBD are both synucleinopathies, and they may comprise different poles of a disease spectrum that includes sporadic disorders as well as genetically determined disorders such as familial Lewy body Parkinsonism.

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.

Can ataxin-2 be down-regulated by allele-specific de novo DNA methylation in SCA2 patients?

Spinocerebellar ataxia type 2 (SCA2) is caused by a CAG trinucleotide repeat expansion within the coding region of the ataxin-2 gene. Affected individuals typically have between 34 and 57 CAG repeats. Signs of the disorder generally begin in adulthood and include progressive ataxia, dysarthria, tremor, hyporeflexia, and slow saccades. As with other trinucleotide repeat disorders, SCA2 exhibits an inverse correlation between the size of the CAG repeat and the age at onset of clinically detectable disease, with neonatal cases of SCA2 being reported in individuals harboring over 200 CAG repeats. However, a wide range of age at onset is typically observed, especially in individuals with < 40 CAG repeats. CAG repeat number alone explains approximately 25-80% of the variability. In this paper, we hypothesize that the level of mutant ataxin-2 protein in affected cells contributes to these differences. One of the mechanisms that might influence this protein levels is de novo DNA methylation, which would specifically target the allele with the expanded CAG repeat leading to transcriptional silencing. Consequently, the symptoms of SCA2 would occur later in the patient's life history. Our postulations, as well as those previously reported to account for the phenotype of SCA2, are discussed.

A cell-based screen for drugs to treat Huntington's disease

We have developed a medium-throughput cell-based assay to screen drugs for Huntington's disease (HD). The assay measures the ability of drugs to protect cultured neuronal (PC12) cells from death caused by an expanded polyglutamine (poly Q) form of huntingtin exon 1. Using this assay, we have blindly screened a library of 1040 compounds compiled by the NINDS: the NIH Custom Collection (NCC). Each compound was tested at five concentrations for its ability to protect cells against huntingtin-induced cell death as well as for its toxicity. Of the compounds tested, 18 prevented cell death completely, and 51 partially. Some of these also exhibited toxicity at higher doses. The majority of drugs (81%) were ineffective. Caspase inhibitors and cannabinoids showed reproducible protection in our assay. We believe these compounds, and others in our hit list, are appealing candidates for further investigation. Additionally, this assay is amenable to scaling up to screen additional compounds for treating Huntington's disease.

Somatic instability of the DNA sequences encoding the polymorphic polyglutamine tract of the AIB1 gene

BACKGROUND

AIB1 contains a polymorphic polyglutamine tract (poly Q) that is encoded by a trinucleotide CAG repeat. Previously there have been conflicting results regarding the effect of the poly Q tract length on breast cancer. Since poly Q is not encoded by a perfect CAG repeat, the heterozygous polymorphic alleles need to be resolved, to understand the exact DNA sequences encoding poly Q.

METHODS

Poly Q encoding sequences of AIB1 from 107 DNA samples, including breast cancer cell lines, sporadic primary breast tumours, and blood samples from BRCA1/BRCA2 mutation carriers and the general population, were resolved by PCR/cloning followed by sequencing of each individual clone.

RESULTS

25 distinct poly Q encoding sequence patterns were found. More than two distinct sequence patterns were found in a significantly higher proportion of tumours and cell lines than that of the general population, suggesting somatic instability. A significantly higher proportion of cancer cell lines or primary breast tumours than that of the general population contained rare sequence patterns. The proportion of sporadic breast tumours having at least one allele < or =27 repeats is significantly higher than that in the blood of BRCA1/BRCA2 mutation carrier breast cancer patients or the general population.

CONCLUSION

The poly Q encoding DNA sequences are somatically unstable in tumour tissues and cell lines. A missense mutation and a very short glutamine repeat in primary tumours suggests that AIB1 activity may be modulated through poly Q, which in turn plays a role in the cotransactivation of gene expressions in breast cancers.

Molecular clearance of ataxin-3 is regulated by a mammalian E4

Insoluble aggregates of polyglutamine-containing proteins are usually conjugated with ubiquitin in neurons of individuals with polyglutamine diseases. We now show that ataxin-3, in which the abnormal expansion of a polyglutamine tract is responsible for spinocerebellar ataxia type 3 (SCA3), undergoes ubiquitylation and degradation by the proteasome. Mammalian E4B (UFD2a), a ubiquitin chain assembly factor (E4), copurified with the polyubiquitylation activity for ataxin-3. E4B interacted with, and thereby mediated polyubiquitylation of, ataxin-3. Expression of E4B promoted degradation of a pathological form of ataxin-3. In contrast, a dominant-negative mutant of E4B inhibited degradation of this form of ataxin-3, resulting in the formation of intracellular aggregates. In a Drosophila model of SCA3, expression of E4B suppressed the neurodegeneration induced by an ataxin-3 mutant. These observations suggest that E4 is a rate-limiting factor in the degradation of pathological forms of ataxin-3, and that targeted expression of E4B is a potential gene therapy for SCA3.

Focal dystonia as a presenting sign of spinocerebellar ataxia 17

We report on the clinical manifestation of spinocerebellar ataxia 17 (SCA17) in 3 members of a German family, in whom the pathological repeat expansion in the TATA-binding protein gene ranged from 53 to 55 repeats (normal: 29-42). The main clinical features were focal dystonia as presenting sign, followed by cerebellar ataxia, and, in the later course of one case, dementia and marked spasticity with signs of cerebellar and cerebral atrophy on brain computed tomography (CT) scan. In conclusion, SCA17 mutations should be considered in the differential diagnosis of focal dystonia.

Modulation of age at onset in Huntington's disease and spinocerebellar ataxia type 2 patients originated from eastern India

To identify the genetic modifier(s) that might alter the age at onset in Huntington's disease (HD) we have analyzed variations in GluR6 kainate receptor (GluR6), CA150 gene, Delta2642 and polymorphic CCG repeat variation in huntingtin (htt) gene in 77 HD patients and normal individuals. In addition, variation in the RAI1 gene was analyzed in 30 spinocerebellar ataxia (SCA2) patients and normal individuals to show the possible influence on the age at onset. Multiple regression analysis indicated that variation in GluR6 and CCG repeat genotype might explain 6.2% and 3.1%, respectively, of the variability in the age at onset in HD. Similar analysis with SCA2 patients indicated that RAI1 might explain about 13% of the variability in the age at onset. Specific alleles in GluR6 and CA150 locus were only observed in HD patients.

The nucleus raphe interpositus in spinocerebellar ataxia type 3 (Machado-Joseph disease)

The nucleus raphe interpositus (RIP) plays an important role in the premotor network for saccades. Its omnipause neurons gate the activity of the burst neurons for vertical saccades lying within the rostral interstitial nucleus of the medial longitudinal fascicle and that for horizontal saccades residing in the caudal subnucleus of the pontine reticular formation. In the present study we investigated the RIP in five patients with clinically diagnosed and genetically confirmed spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease. Polyethylene glycol-embedded 100 microm serial sections stained for lipofuscin pigment and Nissl material as well as paraffin-embedded Nissl stained thin sections revealed the hitherto overlooked involvement of this pontine nucleus in the degenerative process underlying SCA3, whereby in four of our SCA3 patients the RIP underwent a conspicuous loss of presumed omnipause neurons. As observed in other affected brain structures, the RIP of all our SCA3 patients displayed reactive astrocytes and activated microglial cells, while some of the few of its surviving neurons harbored an ataxin-3-immunopositive intranuclear inclusion body. The findings of the present pathoanatomical study suggest that (1) neurodegeneration in the brain stem of terminal SCA3 patients is more widespread than previously thought and is not confined to cranial nerve nuclei involved in the generation of saccades but likewise involves the premotor network for saccades and (2) damage to the RIP may contribute to slowing of horizontal saccades in SCA3 patients but is not associated with saccadic oscillations as occasionally speculated.

Guidelines for the pathoanatomical examination of the lower brain stem in ingestive and swallowing disorders and its application to a dysphagic spinocerebellar ataxia type 3 patient

Despite the fact that considerable progress has been made in the last 20 years regarding the three-phase process of ingestion and the lower brain stem nuclei involved in it, no comprehensive descriptions of the ingestion-related lower brain stem nuclei are available for neuropathologists confronted with ingestive malfunctions. Here, we propose guidelines for the pathoanatomical investigation of these nuclei based on current knowledge with respect to ingestion and the nuclei responsible for this process. The application of these guidelines is described by drawing upon the example of the lower brain stem of a male patient with spinocerebellar ataxia type 3, also known as Machado-Joseph disease, who displayed malfunctions during the preparatory phase of ingestion, as well as lingual and pharyngeal phases of swallowing. By way of the representative application of the recommended investigation procedure to 100 microm serial sections through the patient's brain stem stained for lipofuscin pigment and Nissl material, we observed neuronal loss together with astrogliosis in nearly all of the ingestion-related lower brain stem nuclei (motor, principal and spinal trigeminal nuclei; facial nucleus; parvocellular reticular nucleus; ambiguous nucleus, motor nucleus of the dorsal glossopharyngeal and vagal area; gelatinous, medial, parvocellular and pigmented solitary nuclei; hypoglossal nucleus). In view of their known functional role in the three-phase process of ingestion, damage to these nuclei not only offers an explanation of the patient's malfunctions related to the preparatory phase of ingestion and lingual and pharyngeal phases of swallowing, but also suggests that the patient may have suffered from additional esophageal phase swallowing malfunctions not mentioned in his medical records.

Prediction of proteasome cleavage motifs by neural networks

We present a predictive method that can simulate an essential step in the antigen presentation in higher vertebrates, namely the step involving the proteasomal degradation of polypeptides into fragments which have the potential to bind to MHC Class I molecules. Proteasomal cleavage prediction algorithms published so far were trained on data from in vitro digestion experiments with constitutive proteasomes. As a result, they did not take into account the characteristics of the structurally modified proteasomes--often called immunoproteasomes--found in cells stimulated by gamma-interferon under physiological conditions. Our algorithm has been trained not only on in vitro data, but also on MHC Class I ligand data, which reflect a combination of immunoproteasome and constitutive proteasome specificity. This feature, together with the use of neural networks, a non-linear classification technique, make the prediction of MHC Class I ligand boundaries more accurate: 65% of the cleavage sites and 85% of the non-cleavage sites are correctly determined. Moreover, we show that the neural networks trained on the constitutive proteasome data learns a specificity that differs from that of the networks trained on MHC Class I ligands, i.e. the specificity of the immunoproteasome is different than the constitutive proteasome. The tools developed in this study in combination with a predictor of MHC and TAP binding capacity should give a more complete prediction of the generation and presentation of peptides on MHC Class I molecules. Here we demonstrate that such an approach produces an accurate prediction of the CTL the epitopes in HIV Nef. The method is available at www.cbs.dtu.dk/services/NetChop/.

Amino acid sequence requirements of peptides that inhibit polyglutamine-protein aggregation and cell death

Proteins with expanded polyglutamine domains cause eight inherited neurodegenerative diseases including Huntington's disease. In a previous paper, we identified peptides that inhibit polyglutamine protein aggregation and cell death and now describe the amino acid sequence requirements necessary for these activities. The original 11 amino acid polyglutamine (Q) Binding Peptide 1(QBP1; SNWKWWPGIFD) can be shortened to 8 amino acids (WKWWPGIF) without loss of ability to inhibit polyglutamine aggregation. Three determinants are responsible for inhibition: a tryptophan-rich motif (WKWW), a spacer amino acid and the tripeptide GIF. GIF can be replaced by a repeat of the tryptophan-rich motif, but the spacer remains necessary. We also demonstrate concordance between peptide activity in the in vitro assay and a cellular assay of polyglutamine aggregation and cell death. Polyglutamine binding peptides targeted for intracellular delivery by fusion to TAT retain the ability to inhibit polyglutamine aggregation and cell death in transfected COS 7 cells.

Pharmacological reactivation of inactive genes: the fragile X experience

The present review on the pharmacological reactivation of inactive genes focuses on our experience with the fragile X syndrome. The fragile X syndrome of mental retardation is the prototype of a series of inherited neurological disorders caused by abnormal expansion of repeated trinucleotide sequences embedded in various genes. In a number of these disorders, such as Huntington disease and several forms of spinocerebellar ataxias, the expanded CAG repeat is translated, resulting in a polyglutamine-containing protein that indirectly causes neurodegeneration. On the contrary, in the fragile X syndrome, the expanded CGG repeat is contained in the regulatory region of the FMR1 gene and causes transcriptional inactivation. The mutation spares the coding region of the FMR1 gene, which potentially would allow synthesis of a normal protein if transcription could be restored. This prompted us to try and reactivate the gene function with different pharmacological regimens. We discuss our successful results with DNA demethylating and histone hyperacetylating drugs and their implications for future treatments of the fragile X syndrome.

The selective vulnerability of nerve cells in Huntington's disease

It is now more than 7 years since the genetic mutation causing Huntington's disease (HD) was first identified. Unstable CAG expansion in the IT15 gene, responsible for disease, is translated into an abnormally long polyglutamine (polyQ) tract near the N-terminus of the huntingtin protein. The presence of expanded polyQ in the mutant protein leads to its abnormal proteolytic cleavage with liberation of toxic N-terminal fragments that tend to aggregate, probably first in the cytoplasm. Subsequent nuclear translocation of the cleaved mutant huntingtin is associated with formation of intranuclear protein aggregates and neurotoxicity, probably involving apoptotic cascades. These processes, which can be experimentally modelled in cultured neuronal and non-neuronal cells, seem to underlie neurodegeneration in HD, and also other polyQ disorders, such as dentatorubro-pallidoluysian degeneration, spinal and bulbar muscular atrophy and the spinocerebellar ataxias. They do not, however, explain why within the corpus striatum and cerebral cortex certain nerve cells are susceptible to disease and others are not. In the human HD brain, vulnerable pyramidal neurones within the deeper layers of the cerebral cortex frequently contain large intranuclear inclusions composed of N-terminal fragments of huntingtin. Such inclusions are, however, rare within neurones of the striatum, even in the medium spiny neurones preferentially lost from this region. While inclusions per se do not seem to be neurotoxic, they may provide a surrogate marker of molecular pathology. Recent studies indicate that the nuclear accumulation of mutant huntingtin interferes with transcriptional events. Of particular importance may be the effect on the genes encoding neurotransmitter receptor proteins, especially those involved with glutamatergic neurotransmission. Such changes may trigger or facilitate a low-grade, chronic excitotoxicity of the glutamatergic cortical projection neurones on their target cells in the striatum, already partly compromised by the toxic effects of the HD mutation. This combination of insults, for anatomical reasons experienced predominantly by striatal projection neurones, would eventually cause their selective demise.

In vivo transduction of cerebellar Purkinje cells using adeno-associated virus vectors

We investigated whether adenovirus or adeno-associated virus vectors can transduce cerebellar Purkinje cells (PCs) in vivo. Mice were injected in the deep cerebellar nuclei (DCN) with lacZ-transducing adenovirus (Ad.RSV-betagal) or a recombinant AAV serotype 2 (rAAV2) vector (vTR-CMVbeta) mixed with wild-type adenovirus type 5 (Ad5). One week later, Ad.RSV-betagal transduced cells were found throughout the cerebellar white matter in a dose-dependent manner, but few transduced PCs were evident. In contrast, vTR-CMVbeta with Ad5 transduced several hundred PCs throughout the injected hemisphere. Using an rAAV2 vector transducing a CMV-regulated green fluorescent protein gene, we again found PC transduction, but only with Ad5 coinjection. To assess the effect of injection site and to determine whether the apparent requirement for Ad5 coinfection is observed with other promoters, a beta-actin-regulated vector was injected with or without Ad5 to DCN or cerebellar cortical sites. Thousands of transduced PCs were observed under each condition. Cortical injection yielded greater numbers of transduced cells. Injection of rAAV2 without Ad5 led to greater specificity for PC transduction. We conclude that injection of rAAV2 vectors into the cerebellum is an effective means for transferring genes into substantial numbers of Purkinje cells in vivo.

Nuclear translocation and aggregate formation of heat shock cognate protein 70 (Hsc70) in oxidative stress and apoptosis

Recent evidence has shown a role for the heat shock cognate protein Hsc70 in the response to oxidative stress. We have investigated the subcellular distribution of Hsc70 by means of laser scanning confocal microscopy in neuroblastoma NB41A3 cells, in fibroblasts R6 cells and in R6-Bcl-2, an apoptosis-resistant cell line, and its function in oxidative stress and in apoptosis has been evaluated. Endogenous Hsc70 is localised predominantly in the cytoplasm in unstressed cells, whereas oxidative stress but not apoptosis induces its translocation into the nucleus. In transfected cells overexpressing Hsc70 increased nuclear translocation and aggregation of Hsc70 in intracellular speckles is observed after oxidative stress and, to a lesser degree, after exposure to apoptotic agents. Bcl-2 did not influence the movement of Hsc70 nor the formation of Hsc70-containing speckles. Nuclear translocation of Hsc70 can be modulated by the expression of components from a previously described plasma membrane oxidoreductase involved in the cellular response against oxidative stress. Our data may suggest a correlation between differential translocation of Hsc70 with specific functions in apoptosis and a potential role in the protection against reactive oxygen species.

Intracellular processing and toxicity of the truncated androgen receptor: nuclear congophilia-associated cell death

The pathogenesis of the selective motor neuron death in spinal bulbar muscular atrophy (SBMA) is not fully understood. Similar to observations with other mutant polyglutamine (poly Q) expanded proteins, truncated androgen receptor (AR) with expanded poly Q tract cause intracellular aggregates; however, the precise relationship between aggregates and disease pathogenesis is unresolved. In order to have a better understanding of the cellular processing and toxicity of the mutant AR, we focused on a short N-terminal portion of AR containing normal or expanded poly Q repeats, and have carried out biochemical, immunocytochemical, cytochemical and ultrastructural studies of BHK cells at different intervals after transfection. In cells expressing mutant truncated AR, using an anti-AR N-terminal antibody, we observed no immune staining in the nucleus and identified immune negative aggregates surrounded by immunopositive material in the cytoplasm. Congo red staining identified a component of aggregates with a beta-pleated secondary structure in both cytosol and nucleus, while electron microscopy revealed a fibrillary-granular material as the ultrastructural correlate. In addition, acid phosphatase staining and ubiquitin immunocytochemistry demonstrated that in transfected cells, both lysosomal and nonlysosomal degradation systems are actively involved in handling the mutant truncated AR. The temporal relationship of nuclear congophilia to a subsequent massive cell death suggests that entry of proteolytic cleavage products into the nucleus, perhaps the expanded poly Q stretch itself, may play an important role in cell toxicity.

Phenotypic manifestations associated with CAG-repeat expansion in the androgen receptor gene in male patients and heterozygous females: a clinical and molecular study of 30 families

Spinal and bulbar muscular atrophy (Kennedy disease) is an adult form of X-linked motor neuron disease caused by the expansion of a polymorphic CAG-repeat sequence in the first exon of the androgen receptor gene. We studied clinical and molecular features of 36 patients and 19 heterozygous females. Phenotypic manifestations and disease severity broadly varied among our spinal and bulbar muscular atrophy patients. The size of CAG expansion significantly influences the age of disease onset, but neither clinical features nor disease severity. The majority of carrier women presented signs of chronic denervation at neurophysiological examination and, in three cases, low-amplitude sensory action potentials were recorded. Notably, a few women developed mild signs of bulbar motor neuron impairment later in life. The identification of a large number of patients by the use of the molecular test further supports the hypothesis that Kennedy disease had been previously underdiagnosed, probably because of the great variability of clinical presentation. Although an early diagnosis may not be crucial for treatment, given the lack of effective therapy, the molecular testing can be of great relevance for disease prognosis and genetic counseling.

The molecular biology of the autosomal-dominant cerebellar ataxias

Autosomal-dominant cerebellar ataxias (ADCA) may present as progressive or paroxysmal disorders. While the progressive ataxias have been named spinocerebellar ataxias (SCA), the paroxysmal disorders are designated episodic ataxias (EA). Until now, three different mutational mechanisms resulting in distinctive pathogenesis have been identified. The first type of mutation present in SCA1, SCA2, SCA3, and SCA7 is an expanded CAG repeat in genes of unknown function that are translated into proteins with expanded polyglutamine tracts. A common ultrastructural feature of these disorders is the formation of neuronal intranuclear inclusions (NII) harboring the expanded disease proteins and a variety of other proteins. The pathogenic role of these inclusions has yet to be clarified. A second group of disorders is the result of mutations in genes that code for ion channels. In EA-1, a disorder characterized by episodes of ataxia provoked by movement and startle, missense mutations in a potassium channel gene, KCNA1, have been found. Patients with EA-2, another form of paroxysmal ataxia, carry nonsense mutations of the gene encoding the alpha1A voltage-dependent calcium channel subunit, CACNA1A, that are predicted to result in truncated channel proteins. In SCA6, a progressive ataxia, an expanded CAG repeat in the 3' translated region of the CACNA1A gene, has been found. The third type of mutation is an untranslated CTG expansion resembling the mutation found in myotonic dystrophy. It is associated with a progressive ataxia, SCA8.

Characterization of trinucleotide- and tandem repeat-containing transcripts obtained from human spinal cord cDNA library by high-density filter hybridization

In order to identify trinucleotide- and tandem repeat-containing transcripts in human spinal cord, hybridization of a high-density spinal cord cDNA library filter was carried out using a radioactively labeled degenerate oligonucleotide designed to detect different trinucleotide repeats including those known to occur in disease-associated expansions, in a single step. The sequence analysis of the trinucleotide repeat-containing transcripts (TNRTs) revealed 23 known mammalian genes with trinucleotide repeat-containing regions (TNRs), some of which were not previously reported to contain TNRs, and 18 cDNA clones with no or insignificant sequence homology to known genes. Amongst the known genes detected was the fragile X gene (FMR-1) containing (CGG)30. Other genes containing extended TNRs of 9 to 21 repeats were calcium-dependent protease, ATBF1-A, ferritin H chain, and the G protein Gsalpha2. Ten sequences containing perfect TNRs and two sequences containing perfect tandem repeats (derived from 11 TNRTs) were further analyzed for allelic variation using primers flanking the TNR, and five were shown to exhibit two to five alleles per TNR. These transcripts were further investigated for their chromosomal localization where unknown or only partially characterized. The transcripts that were polymorphic in the TNR region were ATBF1-A (a homeodomain protein), clone 390013 on chromosome Xp11, a member of the family of the 14.3.3 protein kinase C regulators, a human translation initiation factor (an isolog of the yeast Suilisol gene 1), and a novel sequence (TR21). Only the first two transcripts showed the presence of rare expanded alleles. Characterization of polymorphic TNRs in novel and even known genes expressed in human spinal cord is likely to help in the identification of new candidates for genes involved in neurodegenerative disorders.

Hereditary ataxias

There are many causes of hereditary ataxia. These can be grouped into categories of autosomal recessive, autosomal dominant, and X-linked. Molecularly, many of them are due to trinucleotide repeat expansions. In Friedreich ataxia, the trinucleotide repeat expansions lead to a "loss of function." In the dominant ataxias, the expanded repeats lead to a "gain of function," most likely through accumulation of intranuclear (and less commonly cytoplasmic) polyglutamine inclusions. Channelopathies can also lead to ataxia, especially episodic ataxia. Although phenotypic characteristics are an aid to the clinician, a definitive diagnosis is usually made only through genotypic or molecular studies. Genetic counseling is necessary for the testing of symptomatic and asymptomatic individuals. No effective treatment is yet available for most ataxic syndromes, except for ataxia with isolated vitamin E deficiency and the episodic ataxias.

Inhibition of polyglutamine protein aggregation and cell death by novel peptides identified by phage display screening

Proteins with expanded polyglutamine domains cause eight inherited neurodegenerative diseases, including Huntington's, but the molecular mechanism(s) responsible for neuronal degeneration are not yet established. Expanded polyglutamine domain proteins possess properties that distinguish them from the same proteins with shorter glutamine repeats. Unlike proteins with short polyglutamine domains, proteins with expanded polyglutamine domains display unique protein interactions, form intracellular aggregates, and adopt a novel conformation that can be recognized by monoclonal antibodies. Any of these polyglutamine length-dependent properties could be responsible for the pathogenic effects of expanded polyglutamine proteins. To identify peptides that interfere with pathogenic polyglutamine interactions, we screened a combinatorial peptide library expressed on M13 phage pIII protein to identify peptides that preferentially bind pathologic-length polyglutamine domains. We identified six tryptophan-rich peptides that preferentially bind pathologic-length polyglutamine domain proteins. Polyglutamine-binding peptide 1 (QBP1) potently inhibits polyglutamine protein aggregation in an in vitro assay, while a scrambled sequence has no effect on aggregation. QBP1 and a tandem repeat of QBP1 also inhibit aggregation of polyglutamine-yellow fluorescent fusion protein in transfected COS-7 cells. Expression of QBP1 potently inhibits polyglutamine-induced cell death. Selective inhibition of pathologic interactions of expanded polyglutamine domains with themselves or other proteins may be a useful strategy for preventing disease onset or for slowing progression of the polyglutamine repeat diseases.

Reversal of neuropathology and motor dysfunction in a conditional model of Huntington's disease

Neurodegenerative disorders like Huntington's disease (HD) are characterized by progressive and putative irreversible clinical and neuropathological symptoms, including neuronal protein aggregates. Conditional transgenic models of neurodegenerative diseases therefore could be a powerful means to explore the relationship between mutant protein expression and progression of the disease. We have created a conditional model of HD by using the tet-regulatable system. Mice expressing a mutated huntingtin fragment demonstrate neuronal inclusions, characteristic neuropathology, and progressive motor dysfunction. Blockade of expression in symptomatic mice leads to a disappearance of inclusions and an amelioration of the behavioral phenotype. We thus demonstrate that a continuous influx of the mutant protein is required to maintain inclusions and symptoms, raising the possibility that HD may be reversible.