Large expansion of the ATTCT pentanucleotide repeat in spinocerebellar ataxia type 10

[Molecular and genetic analysis of spinocerebellar ataxia type 10 (SCA10)]

Spinocerebellar ataxia type 10 (SCA10) is a dominantly inherited neurodegenerative disease caused by expansion of the ATTCT pentanucleotide repeat in intron 9 of a novel gene, ATXN10, on chromosome 22q13.3. It is clinically characterized by progressive ataxia, seizures, and anticipation, which can vary within and between families. The length of the expanded ATTCT repeats is highly unstable on paternal transmission and shows a variable degree of somatic and germline instabilty, revealing complex SCA10 genetic mechanisms. How this untranslated ATTCT expansion leads to neurodegeneration has been still controversial. Growing number of evidence indicates a gain-of-function RNA mechanism, similar to the myotonic dystrophies caused by non-coding CTG or CCTG repeat expansions.

Therapeutic prospects for spinocerebellar ataxia type 2 and 3

Spinocerebellar ataxia type 2 (SCA2) and type 3 (SCA3) are autosomal-dominant neurodegenerative disorders. SCA2 primarily affects cerebellar Purkinje neurons. SCA3 primarily affects dentate and pontine nuclei and substantia nigra. Both disorders belong to a class of polyglutamine (polyQ) expansion disorders. SCA2 is caused by a polyQ expansion in the amino-terminal region of a cytosolic protein ataxin-2 (Atxn2). SCA3 is caused by a polyQ expansion in the carboxy-terminal portion of a cytosolic protein ataxin-3 (Atxn3). Both disorders are found worldwide, but SCA2 is common among people of Cuban decent and SCA3 is common among people of Portuguese decent. No effective treatment exist for SCA2, SCA3 or any other polyQ-expansion disorder. Based on anecdotal evidence, a number of small scale clinical trials have been attempted previously for SCA2 and SCA3. These trials were underpowered and did not yield any promising results so far. A number of pathogenic mechanisms have been proposed to explain neuronal dysfunction and degeneration in SCA2 and SCA3. Knockdown of mutant Atxn2 and Atxn3 protein by RNAi or similar approach is most promising avenue of therapeutic development in the long term, but translation of this approach to clinic faces very serious technical challenges. Recent preclinical studies in SCA2 and SCA3 genetic mouse model suggested that abnormal neuronal calcium (Ca²⁺) signaling may play an important role in SCA2 and SCA3 pathology. These studies also suggested that dantrolene and other Ca²⁺ signaling inhibitors and stabilizers may have a therapeutic value for treatment of SCA2 and SCA3. Controlled clinical evaluation of dantrolene, memantine, riluzole, dihydropyridines, CoQ10, creatine or other Ca²⁺ blockers and stabilizers in SCA2 and SCA3 patients is necessary to test clinical importance of these ideas. The EUROSCA consortium provides a potential framework for such clinical evaluation.

Thermodynamics of unstable DNA structures from the kinetics of the microgene PCR

The microgene polymerization reaction (MPR) generates head-to-tail tandem repeats from homoduplexes (HDs). In MPR initiation, one HD putatively aligns two others in the proximity required to form a nucleation complex, thus allowing the DNA polymerase to skip the intertemplate gap and generate an initial doublet (ID) prone to repeat propagation. The current investigation refines this stage by additional thermodynamic considerations and elucidates the fundamental mechanism underlying propagation. Four different HD types were designed to extend the range of melting temperatures and to simultaneously modify the stabilities of their secondary structures. Following the propagation kinetics with these, using real-time PCR at different temperatures revealed a new stage in the MPR, amplification of an ID by an original HD, and enabled us to decipher the biphasic kinetics of the process. This amplification merges with the propagation stage if the lifetime of the staggered conformation of the ID is sufficiently long for DNA polymerase to fill in the overhangs. The observed increase with temperature of thermodynamically unfavorable conformations of singlet and doublet HDs that underlies, respectively, MPR initiation and propagation is well correlated with simulations by UNAFold.

The biological effects of simple tandem repeats: lessons from the repeat expansion diseases

Tandem repeats are common features of both prokaryote and eukaryote genomes, where they can be found not only in intergenic regions but also in both the noncoding and coding regions of a variety of different genes. The repeat expansion diseases are a group of human genetic disorders caused by long and highly polymorphic tandem repeats. These disorders provide many examples of the effects that such repeats can have on many biological processes. While repeats in the coding sequence can result in the generation of toxic or malfunctioning proteins, noncoding repeats can also have significant effects including the generation of chromosome fragility, the silencing of the genes in which they are located, the modulation of transcription and translation, and the sequestering of proteins involved in processes such as splicing and cell architecture.

[Molecular and genetic analysis of spinocerebellar ataxia type 10 (SCA10)]

Spinocerebellar ataxia type 10 (SCA10) is a dominantly inherited neurodegenerative disease caused by expansion of the ATTCT pentanucleotide repeat in intron 9 of a novel gene, ATXN10, on chromosome 22q13.3. It is clinically characterized by progressive ataxia, seizures, and anticipation, which can vary within and between families. The length of the expanded ATTCT repeats is highly unstable on paternal transmission and shows a variable degree of somatic and germline instabilty, revealing complex SCA10 genetic mechanisms. Moreover, the purity of the expanded repeat element may be a disease modifier. ATTCT repeats have been recently shown to form unpaired DNA structure and may serve as an aberrant DNA replication origin, potentially contributing to repeat instability and cell death. How this untranslated ATTCT expansion leads to neurodegeneration has been still controversial. We discuss several possible pathogenic mechanisms for SCA10, and growing number of evidence indicates a gain-of-function RNA mechanism, similar to the myotonic dystrophies caused by non-coding CTG or CCTG repeat expansions.

Experience and outcome of 3 years of a European EQA scheme for genetic testing of the spinocerebellar ataxias

The European Molecular Genetics Quality Network (EMQN) has been organizing an external quality assessment (EQA) scheme for molecular genetic testing of trinucleotide repeat mutations in the spinocerebellar ataxias (SCAs) since 2004. DNA samples were validated by at least two independent labs and two different methods. Together with mock clinical case descriptions and requests for specific SCA gene analyses, these were sent to registered participants each year. Laboratories were asked to use their routine procedures and protocols. A panel of assessors reviewed the final returns, including genotype results and reports, to assess the quality of (1) genotyping and (2) interpretation and reporting. A description of methods and raw data were also requested and were very useful for the final analysis. Altogether, during 3 years, 239 reports were received from the laboratories. Overall genotype error rate ranged 1.1-5.2%, a significant cause of concern. Scores for interpretation and reporting also showed that there is still much room for progress, although performance has improved over this period of assessment. The consequences of suboptimal laboratory practices, genotyping errors and misdiagnosis and of incorrect or incomplete interpretation and reporting have wide implications for patient lives, as well as for health management and counselling of relatives. EQA schemes are an important part of quality assurance in molecular genetic laboratories, and their use should become a routine part of laboratory diagnostic practice. Current evidence shows also that it is important that laboratories participate on a yearly basis and that this becomes mandatory for reference laboratories.

The history of spinocerebellar ataxia type 10 in Brazil: travels of a gene

The authors report the history of spinocerebellar ataxia 10 (SCA10), since its first report in a large Portuguese-ancestry family with autosomal dominant pure cerebellar ataxia, till the final identification of further families without Mexican ancestry. These families present a quite different phenotype from those SCA10 families described in Mexico.

Trinucleotide repeat disorders

The discovery that expansion of unstable repeats can cause a variety of neurological disorders has changed the landscape of disease-oriented research for several forms of mental retardation, Huntington disease, inherited ataxias, and muscular dystrophy. The dynamic nature of these mutations provided an explanation for the variable phenotype expressivity within a family. Beyond diagnosis and genetic counseling, the benefits from studying these disorders have been noted in both neurobiology and cell biology. Examples include insight about the role of translational control in synaptic plasticity, the role of RNA processing in the integrity of muscle and neuronal function, the importance of Fe-S-containing enzymes for cellular energy, and the dramatic effects of altering protein conformations on neuronal function and survival. It is exciting that within a span of 15 years, pathogenesis studies of this class of disorders are beginning to reveal pathways that are potential therapeutic targets.

Unstable spinocerebellar ataxia type 10 (ATTCT*(AGAAT) repeats are associated with aberrant replication at the ATX10 locus and replication origin-dependent expansion at an ectopic site in human cells

Spinocerebellar ataxia type 10 (SCA10) is associated with expansion of (ATTCT)n repeats (where n is the number of repeats) within the ataxin 10 (ATX10/E46L) gene. The demonstration that (ATTCT)n tracts can act as DNA unwinding elements (DUEs) in vitro has suggested that aberrant replication origin activity occurs at expanded (ATTCT)n tracts and may lead to their instability. Here, we confirm these predictions. The wild-type ATX10 locus displays inefficient origin activity, but origin activity is elevated at the expanded ATX10 loci in patient-derived cells. To test whether (ATTCT)n tracts can potentiate origin activity, cell lines were constructed that contain ectopic copies of the c-myc replicator in which the essential DUE was replaced by ATX10 DUEs with (ATTCT)n. ATX10 DUEs containing (ATTCT)27 or (ATTCT)48, but not (ATTCT)8 or (ATTCT)13, could substitute functionally for the c-myc DUE, but (ATTCT)48 could not act as an autonomous replicator. Significantly, chimeric c-myc replicators containing ATX10 DUEs displayed length-dependent (ATTCT)n instability. By 250 population doublings, dramatic two- and fourfold length expansions were observed for (ATTCT)27 and (ATTCT)48 but not for (ATTCT)8 or (ATTCT)13. These results implicate replication origin activity as one molecular mechanism associated with the instability of (ATTCT)n tracts that are longer than normal length.

Spectrum and prevalence of autosomal dominant spinocerebellar ataxia in Hokkaido, the northern island of Japan: a study of 113 Japanese families

Autosomal dominant cerebellar ataxia (ADCA) is a genetically heterogeneous group of neurodegenerative disorders. To shed further light on the clinical and genetic spectrum of ADCA in Japan, we conducted a study to determine the frequency of a new variety of different subtypes of SCAs among ADCA patients. This current study was carried out from April 1999 to December 2006 on the basis of patients with symptoms and signs of ADCA disorders. PCR and/or direct sequencing were evaluated in a total of 113 families. Among them, 35 families were found to have the mutation associated with SCA6, 30 with SCA3, 11 with SCA1, five with SCA2, five with DRPLA, and one with SCA14. We also detected the heterozygous -16C --> T single nucleotide substitution within the puratrophin-1 gene responsible for 16q22.1-linked ADCA in ten families. In this study, unusual varieties of SCA, including 27, 13, 5, 7, 8, 12, 17, and 16 were not found. Of the 113 patients, 14% had as yet unidentified ADCA mutations. The present study validates the prevalence of genetically distinct ADCA subtypes based on ethnic origin and geographical variation, and shows that 16q-linked ADCA has strong hereditary effects in patients with ADCAs in Japan.

Dynamic mutations as digital genetic modulators of brain development, function and dysfunction

A substantial portion of the human genome has been found to consist of simple sequence repeats, including microsatellites and minisatellites. Microsatellites, tandem repeats of 1-6 nucleotides, form the template for dynamic mutations, which involve heritable changes in the lengths of repeat sequences. In recent years, a large number of human disorders have been found to be caused by dynamic mutations, the most common of which are trinucleotide repeat expansion diseases. Dynamic mutations are common to numerous nervous system disorders, including Huntington's disease, various spinocerebellar ataxias, fragile X syndrome, fragile X tremor/ataxia syndrome, Friedreich ataxia and other neurodegenerative disorders. The involvement of dynamic mutations in brain disorders will be reviewed, with a focus on the large group caused by CAG/glutamine repeat expansions. We will also outline a proposed role of tandem repeat polymorphisms (TRPs), with unique 'digital' genetic distributions, in modulating brain development and normal function, so as to generate additional mutational diversity upon which natural selection may act.

Spinocerebellar ataxias: an update

PURPOSE OF REVIEW

Here we discuss recent advances regarding the molecular genetic basis of dominantly inherited ataxias.

RECENT FINDINGS

Important recent observations include insights into the mechanisms by which expanded polyglutamine causes cerebellar degeneration; new findings regarding how noncoding expansions may cause disease; the discovery that conventional (i.e. nonrepeat) mutations underlie recently identified ataxias; and growing recognition that multiple biological pathways, when perturbed, can cause cerebellar degeneration.

SUMMARY

The dominant ataxias, also known as spinocerebellar ataxias, continue to grow in number. Here we review the major categories of spinocerebellar ataxias: expanded polyglutamine ataxias; noncoding repeat ataxias; and ataxias caused by conventional mutations. After discussing features shared by these disorders, we present recent evidence supporting a toxic protein mechanism for the polyglutamine spinocerebellar ataxias and the recognition that both protein misfolding and perturbations in nuclear events represent key events in pathogenesis. Less is known about pathogenic mechanisms in spinocerebellar ataxias due to noncoding repeats, though a toxic RNA effect remains possible. Newly discovered, conventional mutations in spinocerebellar ataxias suggest a wide range of biological pathways can be disrupted to cause progressive ataxia. Finally, we discuss how new mechanistic insights can drive the push toward preventive treatment.

Clinical and genetic epidemiological study of 16q22.1-linked autosomal dominant cerebellar ataxia in western Japan

OBJECTIVE

Autosomal dominant cerebellar ataxia (ADCA) is a heterogeneous neurodegenerative disorder. A single nucleotide substitution in the puratrophin-1 gene is associated with 16q22.1-linked ADCA showing pure cerebellar ataxia. We screened patients with spinocerebellar degeneration (SCD) to investigate the frequency and clinical features of 16q22.1-linked ADCA.

MATERIALS AND METHODS

We examined 91 SCD patients from a 1998 community-based prevalence study of Tottori Prefecture in western Japan. We also analyzed samples from 176 patients with SCD collected from a 1996 to 2006 laboratory-based study.

RESULTS

In the community-based study, the prevalence of spinocerebellar ataxia 6 (SCA6) and 16q22.1-linked ADCA was 2.4 and 1.12 per 100,000 individuals, respectively. In the laboratory-based study, the frequency of SCA6 and 16q22.1-linked ADCA was 28% and 17%, respectively. We found two cases of 16q22.1-linked ADCA among 26 SCD patients with no family history.

CONCLUSION

In this area in Japan, 16q22.1-linked ADCA was the second most common type of hereditary SCD.

Redefining the disease locus of 16q22.1-linked autosomal dominant cerebellar ataxia

The 16q22.1-linked autosomal dominant cerebellar ataxia (16q-ADCA; Online Mendelian Inheritance in Man [OMIN] #117210) is one of the most common ADCAs in Japan. Previously, we had reported that the patients share a common haplotype by founder effect and that a C-to-T substitution (-16C>T) in the puratrophin-1 gene was strongly associated with the disease. However, recently, an exceptional patient without the substitution was reported, indicating that a true pathogenic mutation might be present elsewhere. In this study, we clarified the disease locus more definitely by the haplotype analysis of families showing pure cerebellar ataxia. In addition to microsatellite markers, the single nucleotide polymorphisms (SNPs) that we identified on the disease chromosome were examined to confirm the borders of the disease locus. The analysis of 64 families with the -16C>T substitution in the puratrophin-1 gene revealed one family showing an ancestral recombination event between SNP04 and SNP05 on the disease chromosome. The analysis of 22 families without identifiable genetic mutations revealed another family carrying the common haplotype centromeric to the puratrophin-1 gene, but lacking the -16C>T substitution in this gene. We concluded that the disease locus of 16q-ADCA was definitely confined to a 900-kb genomic region between the SNP04 and the -16C>T substitution in the puratrophin-1 gene in 16q22.1.

Noncoding RNAs and RNA Editing in Brain Development, Functional Diversification, and Neurological Disease

The progressive maturation and functional plasticity of the nervous system in health and disease involve a dynamic interplay between the transcriptome and the environment. There is a growing awareness that the previously unexplored molecular and functional interface mediating these complex gene-environmental interactions, particularly in brain, may encompass a sophisticated RNA regulatory network involving the twin processes of RNA editing and multifaceted actions of numerous subclasses of non-protein-coding RNAs. The mature nervous system encompasses a wide range of cell types and interconnections. Long-term changes in the strength of synaptic connections are thought to underlie memory retrieval, formation, stabilization, and effector functions. The evolving nervous system involves numerous developmental transitions, such as neurulation, neural tube patterning, neural stem cell expansion and maintenance, lineage elaboration, differentiation, axonal path finding, and synaptogenesis. Although the molecular bases for these processes are largely unknown, RNA-based epigenetic mechanisms appear to be essential for orchestrating these precise and versatile biological phenomena and in defining the etiology of a spectrum of neurological diseases. The concerted modulation of RNA editing and the selective expression of non-protein-coding RNAs during seminal as well as continuous state transitions may comprise the plastic molecular code needed to couple the intrinsic malleability of neural network connections to evolving environmental influences to establish diverse forms of short- and long-term memory, context-specific behavioral responses, and sophisticated cognitive capacities.

Replication fork stalling at natural impediments

Accurate and complete replication of the genome in every cell division is a prerequisite of genomic stability. Thus, both prokaryotic and eukaryotic replication forks are extremely precise and robust molecular machines that have evolved to be up to the task. However, it has recently become clear that the replication fork is more of a hurdler than a runner: it must overcome various obstacles present on its way. Such obstacles can be called natural impediments to DNA replication, as opposed to external and genetic factors. Natural impediments to DNA replication are particular DNA binding proteins, unusual secondary structures in DNA, and transcription complexes that occasionally (in eukaryotes) or constantly (in prokaryotes) operate on replicating templates. This review describes the mechanisms and consequences of replication stalling at various natural impediments, with an emphasis on the role of replication stalling in genomic instability.

Distinct distribution of autosomal dominant spinocerebellar ataxia in the Mexican population

Dominant ataxias show wide geographic variation. We analyzed 108 dominant families and 123 sporadic ataxia patients from Mexico for mutations causing SCA1-3, 6-8, 10, 12, 17 and DRPLA. Only 18.5% of dominant families remained undiagnosed; SCA2 accounted for half (45.4%), followed by SCA10 (13.9%), SCA3 (12%), SCA7 (7.4%), and SCA17 (2.8%). None had SCA1, 6, 8, 12 or DRPLA. Among sporadic cases, 6 had SCA2 (4.9%), and 2 had SCA17 (1.6%). In the SCA2 patients we identified 6 individuals with the rare (CAG)(33) allele, 2 of whom showed early onset ataxia. The distribution of dominant ataxia mutations in Mexicans is distinct from other populations.

Detection of an unstable non-coding tandem repeat in the ZNF291 gene

Repeat instability is an important form of mutation that is responsible for several neurological, neurodegenerative and neuromuscular disorders. In this study we identified an unstable [CCTG](n) repeat in the second intron of the ZNF291 gene, on chromosome 15q21-24. The repeat number is polymorphic in normal population and the ZNF291 transcript is expressed in different areas of human brain, skeletal muscle and heart. These findings suggest that ZNF291 gene should be taken in consideration as an attractive candidate for neuromuscular expansion related diseases mapping in this locus.

Replication fork regression in repetitive DNAs

Among several different types of repetitive sequences found in the human genome, this study has examined the telomeric repeat, necessary for the protection of chromosome termini, and the disease-associated triplet repeat (CTG)·(CAG)_n. Evidence suggests that replication of both types of repeats is problematic and that a contributing factor is the repetitive nature of the DNA itself. Here we have used electron microscopy to investigate DNA structures formed at replication forks on large model DNAs containing these repeat sequences, in an attempt to elucidate the contributory effect that these repetitive DNAs may have on their replication. Visualization of the DNA revealed that there is a high propensity for a paused replication fork to spontaneously regress when moving through repetitive DNAs, and that this results in a four-way chickenfoot intermediate that could present a significant block to replication in vivo, possibly leading to unwanted recombination events, amplifications or deletions.

On autosomal dominant cerebellar ataxia (ADCA) other than polyglutamine diseases, with special reference to chromosome 16q22.1-linked ADCA

Autosomal dominant cerebellar ataxia (ADCA) is a group of heterogeneous conditions. More than 20 genes or gene loci have been identified that are responsible for ADCA. Although expansions of the trinucleotide (CAG) repeat that encode polyglutamine are known to cause some forms of ADCA, growing knowledge about the genetic basis of ADCA indicates that many subtypes of ADCA are caused by mutations other than the CAG repeat/polyglutamine expansion. In this paper, we review ADCA caused by mutations other than polyglutamine expansions (i.e. "non-polyglutamine diseases"). We also describe the neuropathology of chromosome 16q22.1-linked ADCA, which appears to be the most common non-polyglutamine disease in Japan. What we find to be characteristic on the chromosome 16q22.1-linked ADCA brain is the presence of atrophic Purkinje cells surrounded by the formation of amorphous material, the latter composed of the Purkinje cell dendrites stemming from the cell bodies, the presynaptic terminals innervated by certain neurons, and the astroglial processes. Such neuropathological findings seem to be unique for this disease.

The role of ataxin 10 in the pathogenesis of spinocerebellar ataxia type 10

BACKGROUND

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant disorder characterized by cerebellar ataxia and seizures. SCA10 is caused by an expansion of an ATTCT pentanucleotide repeat in intron 9 of the ataxin 10 (ATXN10) gene encoding an approximately 55-kd protein of unknown function. However, how this mutation leads to SCA10 is unknown.

METHODS

In an effort to understand the pathogenic mechanism of SCA10, the authors conducted a series of experiments to address the effect of repeat expansion on the transcription and RNA processing of the ATXN10 gene. In addition, we generated Sca10 (mouse ataxin 10 homolog)-null mice and addressed the role of Sca10 gene dosage on the cerebellum.

RESULTS

Mutant ATXN10 allele is transcribed at the normal level, and the pre-mRNA containing an expanded repeat is processed normally in patient-derived cells. Sca10-null mice exhibited embryonic lethality. Heterozygous mutants were overtly normal and did not develop SCA10 phenotype

CONCLUSION

A simple gain of function or loss of function of ATXN10 is unlikely to be the major pathogenic mechanism contributing to the spinocerebellar ataxia type 10 phenotype.

An improved method for Southern DNA and Northern RNA blotting using a Mupid®-2 Mini-Gel electrophoresis unit

An improved method for Southern DNA and Northern RNA blotting using the Mupid-2 Mini-Gel System is described. We get sharp and clear bands in Southern and Northern blotting after only 30 min short gel electrophoresis instead of the several hours large gel electrophoresis of conventional methods. The high electrical voltage with a pulse-like current of the Mupid-2 Mini-Gel System also allows reduction of the amount of formaldehyde, a harmful reagent, from the gel running buffer in RNA blotting. This minor modification of DNA and RNA blotting technique enables us to perform the complete experimental procedure more quickly economically in less space, than conventional Southern and Northern blotting, as well as using an extremely small amount of formaldehyde in RNA blotting.

RNA-MEDIATED NEUROMUSCULAR DISORDERS

Myotonic dystrophy type 1 (DM1) is caused by a CTG expansion mutation located in the 3' untranslated portion of the dystrophica myotonin protein kinase gene. The identification and characterization of RNA-binding proteins that interact with expanded CUG repeats and the discovery that a similar transcribed but untranslated CCTG expansion in an intron causes myotonic dystrophy type 2 (DM2) have uncovered a new type of mechanism in which microsatellite expansion mutations cause disease through an RNA gain-of-function mechanism. This review discusses RNA pathogenesis in DM1 and DM2 and evidence that similar mechanisms may play a role in a growing number of dominant noncoding expansion disorders, including fragile X tremor ataxia syndrome (FXTAS), spinocerebellar ataxia type 8 (SCA8), SCA10, SCA12, and Huntington's disease-like 2 (HDL2).

Non-coding RNAs in the nervous system

Increasing evidence suggests that the development and function of the nervous system is heavily dependent on RNA editing and the intricate spatiotemporal expression of a wide repertoire of non-coding RNAs, including micro RNAs, small nucleolar RNAs and longer non-coding RNAs. Non-coding RNAs may provide the key to understanding the multi-tiered links between neural development, nervous system function, and neurological diseases.

DM2 CCTG•CAGG Repeats are Crossover Hotspots that are More Prone to Expansions than the DM1 CTG•CAG Repeats in Escherichia coli

Myotonic dystrophy type 2 (DM2) is caused by the extreme expansion of the repeating tetranucleotide CCTG*CAGG sequence from <30 repeats in normal individuals to approximately 11,000 for the full mutation in certain patients. This repeat is in intron 1 of the zinc finger protein 9 gene on chromosome 3q21. Since prior work demonstrated that CTG*CAG and GAA*TTC triplet repeats (responsible for DM1 and Friedreich's ataxia, respectively) can expand by genetic recombination, we investigated the capacity of the DM2 tetranucleotide repeats to also expand during this process. Both gene conversion and unequal crossing over are attractive mechanisms to effect these very large expansions. (CCTG*CAGG)n (where n=30, 75, 114 or 160) repeats showed high recombination crossover frequencies (up to 27-fold higher than the non-repeating control) in an intramolecular plasmid system in Escherichia coli. Furthermore, a distinct orientation effect was observed where orientation II (CAGG on the leading strand template) was more prone to recombine. Expansions of up to double the length of the tetranucleotide repeats were found. Also, the repeating tetranucleotide sequence was more prone to expansions (to give lengths longer than a single repeating tract) than deletions as observed for the CTG*CAG and GAA*TTC repeats. We determined that the DM2 tetranucleotide repeats showed a lower thermodynamic stability when compared to the DM1 trinucleotide repeats, which could make them better targets for DNA repair events, thus explaining their expansion-prone behavior. Genetic studies in SOS-repair mutants revealed high frequencies of recombination crossovers although the SOS-response itself was not induced. Thus, the genetic instabilities of the CCTG*CAGG repeats may be mediated by a recombination-repair mechanism that is influenced by DNA structure.

DNA structures, repeat expansions and human hereditary disorders

Expansions of simple DNA repeats are responsible for more than two dozen hereditary disorders in humans, including fragile X syndrome, myotonic dystrophy, Huntington's disease, various spinocerebellar ataxias, Friedreich's ataxia and others. During the past decade, it became clear that unusual structural features of expandable repeats greatly contribute to their instability and could lead to their expansion. Furthermore, DNA replication, repair and recombination are implicated in the formation of repeat expansions, as shown in various experimental systems. The replication model of repeat expansion stipulates that unusual structures of expandable repeats stall replication fork progression, whereas extra repeats are added during replication fork restart. It also explains the bias toward repeat expansion or contraction that was observed in different organisms.

Ataxin-10 interacts with O-linked beta-N-acetylglucosamine transferase in the brain

Modification by O-GlcNAc involves a growing number of eucaryotic nuclear and cytosolic proteins. Glycosylation of intracellular proteins is a dynamic process that in several cases competes with and acts as a reciprocal modification system to phosphorylation. O-Linked beta-N-acetylglucosamine transferase (OGT) levels are highest in the brain, and neurodegenerative disorders such as Alzheimer disease have been shown to involve abnormally phosphorylated key proteins, probably as a result of hypoglycosylation. Here, we show that the neurodegenerative disease protein ataxin-10 (Atx-10) is associated with cytoplasmic OGT p110 in the brain. In PC12 cells and pancreas, this association is competed by the shorter OGT p78 splice form, which is down-regulated in brain. Overexpression of Atx-10 in PC12 cells resulted in the reconstitution of the Atx-10-OGT p110 complex and enhanced intracellular glycosylation activity. Moreover, in an in vitro enzyme assay using PC12 cell extracts, Atx-10 increased OGT activity 2-fold. These data indicate that Atx-10 might be essential for the maintenance of a critical intracellular glycosylation level and homeostasis in the brain.

Ataxin 10 induces neuritogenesis via interaction with G-protein beta2 subunit

Spinocerebellar ataxia type 10 (SCA10) is a dominantly inherited disorder caused by an intronic ATTCT pentanucleotide repeat expansion. The ATXN10 gene encodes a novel protein, ataxin 10, known previously as E46L, which is widely expressed in the brain. Ataxin 10 deficiency has been shown recently to cause increased apoptosis in primary cerebellar cultures, thus implicated in SCA10 pathogenesis. The biologic functions of ataxin 10 remain largely unknown. By using yeast-two-hybrid screening of a human brain cDNA library, we identified the G-protein beta2 subunit (Gbeta2) as an ataxin 10 binding partner, and the interaction was confirmed by coimmunoprecipitation and colocalization in mammalian cells in culture. Overexpression of ataxin 10 in PC12 cells induced neurite extension and enhanced neuronal differentiation induced by nerve growth factor (NGF). Moreover, coexpression of ataxin 10 and Gbeta2 potently activated the Ras-MAP kinase-Elk-1 cascade. Dominant negative Ras or inhibitor of MEK-1/2 (U0126) aborted this activation, and blocked morphologic changes, whereas inhibition of TrkA receptor by K252a had no effects. Our data suggest that the ataxin 10-Gbeta2 interaction represents a novel mechanism for inducing neuritogenesis in PC12 cells by activating the Ras-MAP kinase-Elk-1 cascade.

A -16C>T substitution in the 5' UTR of the puratrophin-1 gene is prevalent in autosomal dominant cerebellar ataxia in Nagano

The molecular bases of autosomal dominant cerebellar ataxia (ADCA) have been increasingly elucidated, but 17-50% of ADCA families still remain genetically undefined in Japan. In this study we investigated 67 genetically undefined ADCA families from the Nagano prefecture, and found that 63 patients from 51 families possessed the -16C>T change in the puratrophin-1 gene, which was recently found to be pathogenic for 16q22-linked ADCA. Most patients shared a common haplotype around the puratrophin-1 gene. All patients with the -16C>T change had pure cerebellar ataxia with middle-aged or later onset. Only one patient in a large, -16C>T positive family did not have this change, but still shared a narrowed haplotype with, and was clinically indistinguishable from, the other affected family members. In Nagano, 16q22-linked ADCA appears to be much more prevalent than either SCA6 or dentatorubral-pallidoluysian atrophy (DRPLA), and may explain the high frequency of spinocerebellar ataxia.

Non-B DNA structure-induced genetic instability

Repetitive DNA sequences are abundant in eukaryotic genomes, and many of these sequences have the potential to adopt non-B DNA conformations. Genes harboring non-B DNA structure-forming sequences increase the risk of genetic instability and thus are associated with human diseases. In this review, we discuss putative mechanisms responsible for genetic instability events occurring at these non-B DNA structures, with a focus on hairpins, left-handed Z-DNA, and intramolecular triplexes or H-DNA. Slippage and misalignment are the most common events leading to DNA structure-induced mutagenesis. However, a number of other mechanisms of genetic instability have been proposed based on the finding that these structures not only induce expansions and deletions, but can also induce DNA strand breaks and rearrangements. The available data implicate a variety of proteins, such as mismatch repair proteins, nucleotide excision repair proteins, topoisomerases, and structure specific-nucleases in the processing of these mutagenic DNA structures. The potential mechanisms of genetic instability induced by these structures and their contribution to human diseases are discussed.

Chemical modifiers of unstable expanded simple sequence repeats: what goes up, could come down

A mounting number of inherited human disorders, including Huntington disease, myotonic dystrophy, fragile X syndrome, Friedreich ataxia and several spinocerebellar ataxias, have been associated with the expansion of unstable simple sequence DNA repeats. Despite a similar genetic basis, pathogenesis in these disorders is mediated by a variety of both loss and gain of function pathways. Thus, therapies targeted at downstream pathology are likely to be disease specific. Characteristically, disease-associated expanded alleles in these disorders are highly unstable in the germline and somatic cells, with a tendency towards further expansion. Whereas germline expansion accounts for the phenomenon of anticipation, tissue-specific, age-dependent somatic expansion may contribute towards the tissue-specificity and progressive nature of the symptoms. Thus, somatic expansion presents as a novel therapeutic target in these disorders. Suppression of somatic expansion should be therapeutically beneficial, whilst reductions in repeat length could be curative. It is well established that both cis- and trans-acting genetic modifiers play key roles in the control of repeat dynamics. Importantly, recent data have revealed that expanded CAG.CTG repeats are also sensitive to a variety of trans-acting chemical modifiers. These data provide an exciting proof of principle that drug induced suppression of somatic expansion might indeed be feasible. Moreover, as our understanding of the mechanism of expansion is refined more rational approaches to chemical intervention in the expansion pathway can be envisioned. For instance, the demonstration that expansion of CAG.CTG repeats is dependent on the Msh2, Msh3 and Pms2 genes, highlights components of the DNA mismatch repair pathway as therapeutic targets. In addition to potential therapeutic applications, the response of expanded simple repeats to genotoxic assault suggests such sequences could also have utility as bio-monitors of environmentally induced genetic damage in the soma.

Adult onset spinocerebellar ataxia in a Canadian movement disorders clinic

BACKGROUND

The spinocerebellar ataxias (SCAs) are a genetically and clinically heterogeneous group of neurodegenerative disorders. Relative frequencies vary within different ethnic groups and geographical locations.

OBJECTIVES

1) To determine the frequencies of hereditary and sporadic adult onset SCAs in the Movement Disorders population; 2) to assess if the fragile X mental retardation gene 1 (FMR1) premutation is found in this population.

METHODS

A retrospective chart review of individuals with a diagnosis of adult onset SCA was carried out. Testing for SCA types 1, 2, 3, 6, 7, and 8, Dentatorubral-pallidoluysian atrophy (DRPLA), Friedreich ataxia and the FMR1 expansion was performed.

RESULTS

A total of 69 patients in 60 families were identified. Twenty-one (35%) of the families displayed autosomal dominant and two (3.3%) showed autosomal recessive (AR) pattern of inheritance. A positive but undefined family history was noted in nine (15%). The disorder appeared sporadic in 26 patients (43.3%). In the AD families, the most common mutation was SCA3 (23.8%) followed by SCA2 (14.3%) and SCA6 (14.3%). The SCA1 and SCA8 were each identified in 4.8%. FA was found in a pseudodominant pedigree, and one autosomal recessive pedigree. One sporadic patient had a positive test (SCA3).Dentatorubral-pallidoluysian atrophy and FMR1 testing was negative.

CONCLUSION

A positive family history was present in 53.3% of our adult onset SCA patients. A specific genetic diagnosis could be given in 61.9% of dominant pedigrees with SCA3 being the most common mutation, followed by SCA2 and SCA6. The yield in sporadic cases was low. The fragile X premutation was not found to be responsible for SCA.

Frequency analysis and clinical characterization of different types of spinocerebellar ataxia in Serbian patients

The relative frequencies of different spinocerebellar ataxias (SCAs) vary widely among different ethnic groups, presumably due to a founder effect. We investigated the relative prevalence of SCA1-3, 6-8, 12, 17; dentate-rubro-pallidoluysian atrophy; and Friedreich's ataxia (FRDA) in Serbian patients with adult-onset (>20 years of age) hereditary and sporadic SCAs, and compared clinical features of patients with genetically confirmed SCAs. A total of 108 patients from 54 families (38 apparently dominant [ADCA] and 16 apparently recessive) with adult-onset hereditary ataxia and 75 apparently sporadic patients were assessed. Of 38 families with ADCA, 13 (34%) were positive for an expansion in an SCA1 and 5 families (13%) for an expansion in an SCA2 allele. In 20 families (53%), no expansions have been identified in any of the analyzed genes. Gaze palsy, spasticity, and hyperreflexia were significantly more common in SCA1, whereas slow saccades, hypotonia, hyporeflexia, and dystonia prevailed in SCA2 patients. Among the 16 families with an apparently recessive mode of ataxia inheritance, 4 (25%) were identified as having the FRDA mutation. Ataxia-causing mutations were identified in 8 (10.6%) of patients with apparently sporadic adult-onset ataxia.

Interruptions in the expanded ATTCT repeat of spinocerebellar ataxia type 10: repeat purity as a disease modifier?

Spinocerebellar ataxia type 10 (SCA10) is one of numerous genetic disorders that result from simple repeat expansions. SCA10 is caused by expansion of an intronic ATTCT pentanucleotide repeat tract. It is clinically characterized by progressive ataxia, seizures, and anticipation, which can vary within and between families. We report two SCA10 families showing distinct frequencies of seizures and correlations of repeat length with age at onset. One family displayed uninterrupted ATTCT expansions, whereas the other showed multiple interruptions of the repeat by nonconsensus repeat units, which differed both in the length and/or sequence of the repeat unit. Disease-causing microsatellite expansions have been assumed to be composed of uninterrupted pure repeats. Our findings for SCA10 challenge this convention and suggest that the purity of the expanded repeat element may be a disease modifier.

[Mutation mechanisms and their consequences]

The identification of mutations leading to human genetic diseases has grown into an intensive research field during the last few years. Through novel DNA analysis progress, it is now possible to determine the mutational spectrum for a given genetic disease and international databases are now available online. Genetic diagnosis of hereditary diseases has become an essential tool in genetic counselling and prenatal diagnosis. The knowledge of the deleterious mutation type and the molecular associated mechanism is fundamental in order to devise the optimal molecular diagnosis strategy. This review aims to present the various mutation categories involved in genetic diseases (single base-pair substitutions, small deletions or insertions, dynamic mutations, gross DNA lesions...) and to summarize our current knowledge about the main molecular mechanisms responsible for these mutations. Their deleterious consequences on gene expression, including transcription and transcript maturation, and protein loss or gain of function are also discussed in this review.

Ataxin-10 interacts with O-GlcNAc transferase OGT in pancreatic β cells

Several nuclear and cytoplasmic proteins in metazoans are modified by O-linked N-acetylglucosamine (O-GlcNAc). This modification is dynamic and reversible similar to phosphorylation and is catalyzed by the O-linked GlcNAc transferase (OGT). Hyperglycemia has been shown to increase O-GlcNAc levels in pancreatic beta cells, which appears to interfere with beta-cell function. To obtain a better understanding of the role of O-linked GlcNAc modification in beta cells, we have isolated OGT interacting proteins from a cDNA library made from the mouse insulinoma MIN6 cell line. We describe here the identification of Ataxin-10, encoded by the SCA10 (spinocerebellar ataxia type 10) gene as an OGT interacting protein. Mutations in the SCA10 gene cause progressive cerebellar ataxias and seizures. We demonstrate that SCA10 interacts with OGT in vivo and is modified by O-linked glycosylation in MIN6 cells, suggesting a novel role for the Ataxin-10 protein in pancreatic beta cells.

Absence of spinocerebellar ataxia type 3/Machado-Joseph disease within ataxic patients in the Czech population

Although spinocerebellar ataxia type 3 (SCA3)/Machado-Joseph disease is the most common type of SCA worldwide, we did not identify any cases of the disease amongst SCA patients in the Czech population. It has been proposed that the prevalence of large normal alleles correlates with the frequency of various types of SCA. We have therefore attempted to resolve the absence of SCA3 in our population by investigating, within 204 normal chromosomes, the frequency and nature of CAG repeats as well as two intragenic polymorphisms. We found that large normal alleles with more than 33 CAG repeats were observed at a frequency of only 0.49%. Whereas most of the expanded alleles worldwide have the CA haplotype, this was the least common (5.4%) variant observed in our study, although it was associated with a larger mean CAG repeat length (26.9). We postulate that the absence of SCA3 in the Czech population might be explained by the lack of large normal alleles and consequently a relatively small reservoir for aberrant CAG expansions at the SCA3 locus.

Diseases of unstable repeat expansion: mechanisms and common principles

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

Spinocerebellar ataxia type 6 in Mainland China: Molecular and clinical features in four families

The hereditary spinocerebellar ataxias (SCAs) are a clinically and genetically heterogeneous group of neurodegenerative disorders. The genes causing 11 of these diseases have been identified. To date, there is no report of SCA type 6 (SCA6) in Mainland Chinese. Using a molecular approach, we investigated SCA6 as well as other SCA subtype in 120 Mainland Chinese families with dominantly inherited ataxias and in 60 Mainland Chinese patients with sporadic ataxias. Clinical and molecular features of SCA6 were further characterized in 13 patients from 4 families. We found that SCA3/MJD was the most common type of autosomal dominant SCA in Mainland Chinese, accounting for 83 patients from 59 families (49.2%), followed by SCA2 (8 [6.7%]), SCA1 (7 [5.8%]), SCA6 (4 [3.3%]), SCA7 (1 [0.8%]), SCA8 (0%), SCA10 (0%), SCA12 (1 [0.8%]), SCA14 (0%), SCA17 (0%) and DRPLA (0%). The genes responsible for 40 (33.3%) of dominantly inherited SCA families remain to be determined. Among the 60 patients with sporadic ataxias in the present series, 3 (5.0%) were found to harbor SCA3 mutations, whereas none were found to harbor SCA6 mutations. In the 4 families with SCA6, we found significant anticipation in the absence of genetic instability on transmission. This is the first report of geographic cluster of families with SCA6 subtype in Mainland China.

An autosomal dominant cerebellar ataxia linked to chromosome 16q22.1 is associated with a single-nucleotide substitution in the 5' untranslated region of the gene encoding a protein with spectrin repeat and Rho guanine-nucleotide exchange-factor domains

Autosomal dominant cerebellar ataxia (ADCA) is a group of heterogeneous neurodegenerative disorders. By positional cloning, we have identified the gene strongly associated with a form of degenerative ataxia (chromosome 16q22.1-linked ADCA) that clinically shows progressive pure cerebellar ataxia. Detailed examination by use of audiogram suggested that sensorineural hearing impairment may be associated with ataxia in our families. After restricting the candidate region in chromosome 16q22.1 by haplotype analysis, we found that all patients from 52 unrelated Japanese families harbor a heterozygous C-->T single-nucleotide substitution, 16 nt upstream of the putative translation initiation site of the gene for a hypothetical protein DKFZP434I216, which we have called "puratrophin-1" (Purkinje cell atrophy associated protein-1). The full-length puratrophin-1 mRNA had an open reading frame of 3,576 nt, predicted to contain important domains, including the spectrin repeat and the guanine-nucleotide exchange factor (GEF) for Rho GTPases, followed by the Dbl-homologous domain, which indicates the role of puratrophin-1 in intracellular signaling and actin dynamics at the Golgi apparatus. Puratrophin-1--normally expressed in a wide range of cells, including epithelial hair cells in the cochlea--was aggregated in Purkinje cells of the chromosome 16q22.1-linked ADCA brains. Consistent with the protein prediction data of puratrophin-1, the Golgi-apparatus membrane protein and spectrin also formed aggregates in Purkinje cells. The present study highlights the importance of the 5' untranslated region (UTR) in identification of genes of human disease, suggests that a single-nucleotide substitution in the 5' UTR could be associated with protein aggregation, and indicates that the GEF protein is associated with cerebellar degeneration in humans.