Expansion of a novel CAG trinucleotide repeat in the 5' region of PPP2R2B is associated with SCA12

The diagnostic value of cerebrospinal fluid tau protein in dementing and nondementing neuropsychiatric disorders

Cerebrospinal fluid (CSF) total tau protein (tauT) is increased in Alzheimer's disease (AD) and may be of some help in the diagnostic work-up of demented patients. The aim of the present study was to investigate the diagnostic aid and the additional help (over that of clinical criteria) of tauT in different clinical situations. Double-sandwich enzyme-linked immunosorbent assay was used to quantify tauT in 61 healthy controls and 241 patients with various neuropsychiatric diseases. Our results suggest that CSF tauT offers significant additional information over that of clinical criteria of AD, for the discrimination of AD from normal aging, depression, synucleinopathy, and possibly vascular dementia. However, for the differential diagnosis from frontotemporal dementia, corticobasal ganglionic degeneration, and secondary dementia, the diagnostic value is inadequate.

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.

The wide spectrum of spinocerebellar ataxias (SCAs)

Spinocerebellar ataxias (SCAs) are a clinically heterogeneous group of disorders. Current molecular classification corresponds to the order of gene description (SCA1-SCA 25). The prevalence of SCAs is estimated to be 1-4/100,000. Patients exhibit usually a slowly progressive cerebellar syndrome with various combinations of oculomotor disorders, dysarthria, dysmetria/kinetic tremor, and/or ataxic gait. They can present also with pigmentary retinopathy, extrapyramidal movement disorders (parkinsonism, dyskinesias, dystonia, chorea), pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioral symptoms), peripheral neuropathy. SCAs are also genetically heterogeneous and the clinical diagnosis of subtypes of SCAs is complicated by the salient overlap of the phenotypes between genetic subtypes. The following clinical features have some specific values for predicting a gene defect: slowing of saccades in SCA2, ophthalmoplegia in SCA1, SCA2 and SCA3, pigmentary retinopathy in SCA7, spasticity in SCA3, dyskinesias associated with a mutation in the fibroblast growth factor 14 (FGF 14) gene, cognitive impairment/behavioral symptoms in SCA17 and DRPLA, seizures in SCA10, SCA17 and DRPLA, peripheral neuropathy in SCA1, SCA2, SCA3, SCA4, SCA8, SCA18 and SCA25. Neurophysiological findings are compatible with a dying-back axonopathy and/or a neuronopathy. Three patterns of atrophy can be identified on brain MRI: a pure cerebellar atrophy, a pattern of olivopontocerebellar atrophy, and a pattern of global brain atrophy. A remarkable observation is the presence of dentate nuclei calcifications in SCA20, resulting in a low signal on brain MRI sequences. Several identified mutations correspond to expansions of repeated trinucleotides (CAG repeats in SCA1, SCA2, SCA3, SCA6, SCA7, SCA17 and DRPLA, CTG repeats in SCA8). A pentanucleotide repeat expansion (ATTCT) is associated with SCA10. Missense mutations have also been found recently. Anticipation is a main feature of SCAs, due to instability of expanded alleles. Anticipation may be particularly prominent in SCA7. It is estimated that extensive genetic testing leads to the identification of the causative gene in about 60-75 % of cases. Our knowledge of the molecular mechanisms of SCAs is rapidly growing, and the development of relevant animal models of SCAs is bringing hope for effective therapies in human.

The Anp32 family of proteins containing leucine-rich repeats

Herein we describe the characteristic features of the Anp32 family represented by the cerebellar leucine-rich repeat protein (Lanp) and the cerebellar developmental-regulated protein 1 (Cpd1). The Anp32 family consists of 32 evolutionarily-conserved proteins and is included within the superfamily of leucine-rich repeat (LRR) proteins characterized by the presence of tandem arrays of a LRR, a structural motif implicated in the mediation of protein-protein interactions. We describe three novel human Anp32 proteins, reveal the evolutionary relationships of the members of the Anp32 family, provide insights into their biochemical and structural properties, and review their macromolecular interactions, substrate specificities, tissue distribution/expression patterns, and physiological and pathological roles. Recent findings indicate a conserved role of members of the Anp32 family during evolution in the modulation of cell signalling and transduction of gene expression to regulate the morphology and dynamics of the cytoskeleton, cell adhesion, neural development or cerebellar morphogenesis.

Spinocerebellar ataxia with mental retardation (SCA13)

Spinocerebellar ataxia 13 is a slowly progressive and relatively pure autosomal dominant cerebellar ataxia with childhood onset and mental deficiency. The responsible gene has been assigned to a 5.2 Mbases interval on chromosome 19q in a single French family.

Genetic and molecular aspects of spinocerebellar ataxias

The group of spinocerebellar ataxias (SCAs) includes more than 20 subgroups based only on genetic research. The "ataxia genes" are autosomal; the "disease-alleles" are dominant, and many of them, but not all, encode a protein with an abnormally long polyglutamine domain. In DNA, this domain can be detected as an elongated CAG repeat region, which is the basis of genetic diagnostics. The polyglutamine tails often tend to aggregate and form inclusions. In some cases, protein-protein interactions are the key to understanding the disease. Protein partners of ataxia proteins include phosphatases and components of chromatin and the transcriptional machinery. To date, investigation of spinocerebellar ataxias involves population genetics, molecular methods, and studying model organisms. However, there is still no efficient therapy for patients. Here, we review the genetic and molecular data gained on spinocerebellar ataxias.

Unfolding-resistant translocase targeting: a novel mechanism for outer mitochondrial membrane localization exemplified by the Bbeta2 regulatory subunit of protein phosphatase 2A

Heterotrimeric serine/threonine protein phosphatase 2A (PP2A) consists of scaffolding (A), catalytic (C), and variable (B, B', and B'') subunits. Variable subunits dictate subcellular localization and substrate specificity of the PP2A holoenzyme. The Bbeta regulatory subunit gene is mutated in spinocerebellar ataxia type 12, and one of its splice variants, Bbeta2, targets PP2A to mitochondria to promote apoptosis in PC12 cells (Dagda, R. K., Zaucha, J. A., Wadzinski, B. E., and Strack, S. (2003) J. Biol. Chem. 278, 24976-24985). Here, we report that Bbeta2 is localized to the outer mitochondrial membrane by a novel mechanism, combining a cryptic mitochondrial import signal with a structural arrest domain. Scanning mutagenesis demonstrates that basic and hydrophobic residues mediate mitochondrial association and the proapoptotic activity of Bbeta2. When fused to green fluorescent protein, the N terminus of Bbeta2 acts as a cleavable mitochondrial import signal. Surprisingly, full-length Bbeta2 is not detectably cleaved and is retained at the outer mitochondrial membrane, even though it interacts with the TOM22 import receptor, as shown by luciferase complementation in intact cells. Mutations that open the C-terminal beta-propeller of Bbeta2 facilitate mitochondrial import, indicating that this rigid fold acts as a stop-transfer domain by resisting the partial unfolding step prerequisite for matrix translocation. Because hybrids of prototypical import and beta-propeller domains recapitulate this behavior, we predict the existence of other similarly localized proteins and a selection against highly stable protein folds in the mitochondrial matrix. This unfolding-resistant targeting to the mitochondrial translocase is necessary but not sufficient for the proapoptotic activity of Bbeta2, which also requires association with the rest of the PP2A holoenzyme.

Recent advances in hereditary spinocerebellar ataxias

In recent years, molecular genetic research has unraveled a major part of the genetic background of autosomal dominant and recessive spinocerebellar ataxias. These advances have also allowed insight in (some of) the pathophysiologic pathways assumed to be involved in these diseases. For the clinician, the expanding number of genes and genetic loci in these diseases and the enormous clinical heterogeneity of specific ataxia subtypes complicate management of ataxia patients. In this review, the clinical and neuropathologic features of the recently identified spinocerebellar ataxias are described, and the various molecular mechanisms that have been demonstrated to be involved in these disorders are discussed.

Spinocerebellar ataxia type 26 maps to chromosome 19p13.3 adjacent to SCA6

The dominantly inherited spinocerebellar ataxias (SCA) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by progressive gait ataxia, upper limb incoordination, and dysarthria. We studied a six-generation kindred of Norwegian ancestry with pure cerebellar ataxia inherited in an autosomal dominant pattern. All affected family members had a slowly progressive cerebellar ataxia, with an age of onset range from 26 to 60 years. Brain magnetic resonance imaging study of 11 affected patients showed that atrophy was confined to the cerebellum. After excluding all the known SCAs using linkage analysis or direct mutation screen, we conducted a genomewide genetic linkage scan. With the aid of a novel linkage analysis strategy, we found linkage between the disease locus and marker D19S591 and D19S1034. Subsequent genetic and clinical analysis identified a critical region of 15.55cM interval on chromosome 19p13.3, flanked by markers D19S886 and D19S894, and have established a new genetic locus designated SCA26. The SCA26 locus is adjacent to the genes for Cayman ataxia and SCA6. The region consists of 3.3 million base pairs (Mb) of DNA sequences with approximately 100 known and predicted genes. Identification of the responsible gene for SCA26 ataxia will provide further insight into mechanisms of neurodegeneration.

The pathogenic agent in Drosophila models of 'polyglutamine' diseases

A substantial body of evidence supports the identity of polyglutamine as the pathogenic agent in a variety of human neurodegenerative disorders where the mutation is an expanded CAG repeat. However, in apparent contradiction to this, there are several human neurodegenerative diseases (some of which are clinically indistinguishable from the 'polyglutamine' diseases) that are due to expanded repeats that cannot encode polyglutamine. As polyglutamine cannot be the pathogenic agent in these diseases, either the different disorders have distinct pathogenic pathways or some other common agent is toxic in all of the expanded repeat diseases. Recently, evidence has been presented in support of RNA as the pathogenic agent in Fragile X-associated tremor/ataxia syndrome (FXTAS), caused by expanded CGG repeats at the FRAXA locus. A Drosophila model of FXTAS, in which 90 copies of the CGG repeat are expressed in an untranslated region of RNA, exhibits both neurodegeneration and similar molecular pathology to the 'polyglutamine' diseases. We have, therefore, explored the identity of the pathogenic agent, and specifically the role of RNA, in a Drosophila model of the polyglutamine diseases by the expression of various repeat constructs. These include expanded CAA and CAG repeats and an untranslated CAG repeat. Our data support the identity of polyglutamine as the pathogenic agent in the Drosophila models of expanded CAG repeat neurodegenerative diseases.

The occurrence of dominant spinocerebellar ataxias among 251 Finnish ataxia patients and the role of predisposing large normal alleles in a genetically isolated population

OBJECTIVES

Frequency and distribution of dominant ataxias caused by dynamic mutations may vary in different populations, which has been explained on the basis of relative frequency of predisposing normal alleles. The aim of the study was to evaluate the occurrence of spinocerebellar ataxias (SCAs) and dentatorubral-pallidoluysian atrophy (DRPLA) in Finland, and to investigate the role of predisposing normal alleles in a genetically homogenous population.

MATERIAL AND METHODS

Mutation analyses for SCA1, 2, 3, 6, 7, 8, 10, 12, 17, and DRPLA and frataxin genes were performed for 251 unrelated Finnish patients who presented with progressive ataxia disorder.

RESULTS

Expansions of SCA1, SCA2, SCA6, SCA7, SCA8, and SCA17 genes were detected in 2, 1, 1, 7, 22, and 1 patients, respectively. Altogether, 39 and 7% of dominant and sporadic SCA patients, respectively, harboured expansions at some of the investigated loci. Normal variation, collected from 477 to 502 chromosomes at each disease loci, revealed that Finns were different from the Japanese but largely similar to other Caucasians.

CONCLUSIONS

Lack of SCA3 and excess of SCA8 are characteristic to the Finnish population. Homozygosity for the SCA8 expansion increases penetrance. Frequencies of large normal alleles at the SCA loci predict poorly prevalence of the respective diseases in Finland. Prioritization in DNA testing, based on ethnic origin and geographical location, is recommendable in Finland, and analogous approach may be applied to other countries as well.

Gene Expression Profile Following Stable Expression of the Cellular Prion Protein

1. The cellular prion protein (PrPC) is expressed widely in neural and nonneural tissues at the highest level in neurons in the central nervous system (CNS). 2. Recent studies indicated that transgenic mice with the cytoplasmic accumulation of PrPC exhibited extensive neurodegeneration in the cerebellum, although the underlying mechanism remains unknown. To identify the genes whose expression is controlled by over-expression of PrPC in human cells, we have established a stable PrPC-expressing HEK293 cell line designated P1 by the site-specific recombination technique. 3. Microarray analysis identified 33 genes expressed differentially between P1 and the parent PrPC-non-expressing cell line among 12,814 genes examined. They included 18 genes involved in neuronal and glial functions, 5 related to production of extracellular matrix proteins, and 2 located in the complement cascade. 4. Northern blot analysis verified marked upregulation in P1 of the brain-specific protein phosphatase 2A beta subunit (PPP2R2B), a causative gene of spinocerebellar ataxia 12, and the cerebellar degeneration-related autoantigen (CDR34) gene associated with development of paraneoplastic cerebellar degeneration. 5. These results indicate that accumulation of PrPC in the cell caused aberrant regulation of a battery of the genes important for specific neuronal function. This represents a possible mechanism underlying PrPC-mediated selective neurodegeneration.

CAG repeats containing CAA interruptions form branched hairpin structures in spinocerebellar ataxia type 2 transcripts

Spinocerebellar ataxia type 2 (SCA2), one of the hereditary human neurodegenerative disorders, is caused by the expansion of the CAG tandem repeats in the translated sequence of the SCA2 gene. In a normal population the CAG repeat is polymorphic not only in length but also in the number and localization of its CAA interruptions. The aim of this study was to determine the structure of the repeat region in the normal and mutant SCA2 transcripts and to reveal the structural basis of its normal function and dysfunction. We show here that the properties of the CAA interruptions are major determinants of the CAG repeat folding in the normal SCA2 transcripts. We also show that the uninterrupted repeats in mutant transcripts form slippery hairpins, whose length is further reduced by the base pairing of the repeat portion with a specific flanking sequence. The structural organization of the repeat interruption systems present in other human transcripts, such as SCA1, TBP, FOXP2, and MAML2, are also discussed.

A pathogenetic classification of hereditary ataxias: is the time ripe?

Harding's classification takes credits for defining the homogeneous phenotypes that have been essential for the genetic linkage studies and it is still useful for didactic purposes. The advances in pathogenetic knowledge make it now possible to modify Harding's classification. Five main pathogenetic mechanisms may be distinguished: 1) mitochondrial; 2) metabolic; 3) defective DNA repair; 4) abnormal protein folding and degradation; 5) channelopathies. The present attempt to classify ataxia disorders according to their pathogenetic mechanism is a work in progress, since the pathogenesis of several disorders is still unknown. A pathogenetic classification may be useful in clinical practice and when new therapeutic strategies become available.

Regional features of autosomal-dominant cerebellar ataxia in Nagano: clinical and molecular genetic analysis of 86 families

The frequency of autosomal-dominant cerebellar ataxia (ADCA) subtypes was examined in 86 unrelated families originating from Nagano prefecture. In Nagano, the prevalence of spinocerebellar degeneration (SCD) was approximately 22 per 100,000 population. Among ADCA families, SCA6 was the most prevalent subtype (16 families, 19%), followed by DRPLA (nine families, 10%), SCA3/MJD (three families, 3%), SCA1 (two families, 2%), and SCA2 (one family, 1%). No families with SCA7, SCA12, or SCA17 were detected. Compared with other districts in Japan, the prevalence of SCA3/MJD was very low in Nagano. More interestingly, the ratio of genetically undetermined ADCA families was much higher in Nagano (55 families, 65%) than in other districts in Japan. These families tended to accumulate in geographically restricted areas such as Kiso, Saku, and Ina, indicating that the founder effect might be responsible for the high frequency of ADCA in these areas. Most patients clinically showed slowly progressive pure cerebellar ataxia of late-onset (ADCA III). In the case of 36 patients from 36 genetically undetermined ADCA III families, however, no one was completely consistent with the founder allele proposed for 16q-ADCA. These results indicate that there might be genetically distinct ADCA subtypes in Nagano.

Pathogenic RNA repeats: an expanding role in genetic disease

Fragile X mental retardation and Friedreich's ataxia were among the first pathogenic trinucleotide repeat disorders to be described in which noncoding repeat expansions interfere with gene expression and cause a loss of protein production. Invoking a similar loss-of-function hypothesis for the CTG expansion causing myotonic dystrophy type 1 (DM1) located in the 3' noncoding portion of a kinase gene was more difficult because DM is a dominantly inherited multisystemic disorder in which the second copy of the gene is unaffected. However, the discovery that a transcribed but untranslated CCTG expansion causes myotonic dystrophy type 2 (DM2), along with other discoveries on DM1 and DM2 pathogenesis, indicate that the CTG and CCTG expansions are pathogenic at the RNA level. This review will detail recent developments on the molecular mechanisms of RNA pathogenesis in DM, and the growing number of expansion disorders that might involve similar pathogenic RNA mechanisms.

Fine mapping of 16q-linked autosomal dominant cerebellar ataxia type III in Japanese families

The autosomal dominant cerebellar ataxias (ADCAs) are a clinically and genetically heterogeneous group of disorders. To date, at least 11 genes and 13 additional loci have been identified in ADCAs. Despite phenotypic differences, spinocerebellar ataxia 4 (SCA4) and Japanese 16q-linked ADCA type III map to the same region of 16q22.1. We report four Japanese families with pure cerebellar ataxia and a disease locus at 16q22.1. Our families yielded a peak lod score of 6.01 at marker D16S3141. To refine the candidate region, we carried out genetic linkage studies in four pedigrees with a high density set of DNA markers from chromosome 16q22.1. Our linkage data suggest that the disease locus for 16q-ADCA type III is within the 1.25-Mb interval delineated by markers 17msm and CTTT01. We screened for mutations in 36 genes within the critical region. Our critical region lies within the linkage interval reported for SCA4 and for Japanese 16q-ADCA type III. These data suggest that the ADCA that we have characterized is allelic with SCA4 and Japanese 16q-linked ADCA type III.

Prevalence of inherited ataxias in the province of Padua, Italy

Few population studies are available on epidemiological indexes of hereditary ataxias. An investigation on the prevalence rate of these movement disorders is in progress for the Veneto region, the main area of northeast Italy with a population of 4,490,586 inhabitants. The first results of this epidemiological survey concern the province of Padua, which numbers 845,203 residents (January 1, 2002). The prevalence rate of inherited ataxias has been estimated at 93.3 cases per million inhabitants. The most common types appeared to be the autosomal dominant forms, namely spinocerebellar ataxia type 1 and 2, with a prevalence of 24 per 1,000,000. In the same population, with a prevalence rate of 6 per 1,000,000, Friedreich's ataxia was defined as the prominent recessive autosomal form. There were very rare cases of ataxia telangiectasia, ataxia with vitamin E deficiency and cerebellar ataxia with congenital muscular dystrophy, a recently identified autosomal recessive disease.

Characterization of ataxias with magnetic resonance imaging and spectroscopy

A wide variety of autosomal transmitted ataxias exist and their ultimate characterization requires genetic testing. Common clinical characteristics among different ataxia types complicate the choice of the appropriate genetic test. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) generally show cerebellar or cerebral atrophy and perturbed metabolite levels which differ between ataxias. In order to help the clinician accurately identify the ataxia type, reported MRI and MRS data in different brain regions are summarized for more than 60 different types of autosomal inherited and sporadic ataxias.

Protein phosphatase 2A as a new target for morphogenetic studies in the chick limb

The family of ser/thr protein phosphatases 2A (PP2A) is a major regulator of cell proliferation and cell death and is critically involved in the maintenance of homeostasis. In order to analyse the importance of PP2A proteins in apoptotic and developmental processes, this review focuses on previous studies concerning the role of PP2A in morphogenesis. We first analyse wing formation in Drosophila, a model for invertebrates, then chick limb bud, a model for vertebrates. We also present a pioneer experiment to illustrate the potential relevance of PP2A studies in BMP signalling during chicken development and we finally discuss the BMP downstream signalling pathways.

Spinocerebellar ataxia type 8: molecular genetic comparisons and haplotype analysis of 37 families with ataxia

We reported elsewhere that an untranslated CTG expansion causes the dominantly inherited neurodegenerative disorder spinocerebellar ataxia type 8 (SCA8). SCA8 shows a complex inheritance pattern with extremes of incomplete penetrance, in which often only one or two affected individuals are found in a given family. SCA8 expansions have also been found in control chromosomes, indicating that separate genetic or environmental factors increase disease penetrance among SCA8-expansion-carrying patients with ataxia. We describe the molecular genetic features and disease penetrance of 37 different families with SCA8 ataxia from the United States, Canada, Japan, and Mexico. Haplotype analysis using 17 STR markers spanning an approximately 1-Mb region was performed on the families with ataxia, on a group of expansion carriers in the general population, and on psychiatric patients, to clarify the genetic basis of the reduced penetrance and to investigate whether CTG expansions among different populations share a common ancestral background. Two major ancestrally related haplotypes (A and A') were found among white families with ataxia, normal controls, and patients with major psychosis, indicating a common ancestral origin of both pathogenic and nonpathogenic SCA8 expansions among whites. Two additional and distinct haplotypes were found among a group of Japanese families with ataxia (haplotype B) and a Mexican family with ataxia (haplotype C). Our finding that SCA8 expansions on three independently arising haplotypes are found among patients with ataxia and cosegregate with ataxia when multiple family members are affected further supports the direct role of the CTG expansion in disease pathogenesis.

Autosomal dominant cerebellar ataxias: clinical features, genetics, and pathogenesis

Autosomal dominant cerebellar ataxias are hereditary neurodegenerative disorders that are known as spinocerebellar ataxias (SCA) in genetic nomenclature. In the pregenomic era, ataxias were some of the most poorly understood neurological disorders; the unravelling of their molecular basis enabled precise diagnosis in vivo and explained many clinical phenomena such as anticipation and variable phenotypes even within one family. However, the discovery of many ataxia genes and loci in the past decade threatens to cause more confusion than optimism among clinicians. Therefore, the provision of guidance for genetic testing according to clinical findings and frequencies of SCA subtypes in different ethnic groups is a major challenge. The identification of ataxia genes raises hope that essential pathogenetic mechanisms causing SCA will become more and more apparent. Elucidation of the pathogenesis of SCA hopefully will enable the development of rational therapies for this group of disorders, which currently can only be treated symptomatically.

Somatic and germline instability of the ATTCT repeat in spinocerebellar ataxia type 10

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant disorder characterized by ataxia, seizures, and anticipation. It is caused by an expanded ATTCT pentanucleotide repeat in intron 9 of a novel gene, designated "SCA10." The ATTCT expansion in SCA10 represents a novel class of microsatellite repeat and is one of the largest found to cause human diseases. The expanded ATTCT repeat is unstably transmitted from generation to generation, and an inverse correlation has been observed between size of repeat and age at onset. In this multifamily study, we investigated the intergenerational instability, somatic and germline mosaicism, and age-dependent repeat-size changes of the expanded ATTCT repeat. Our results showed that (1) the expanded ATTCT repeats are highly unstable when paternally transmitted, whereas maternal transmission resulted in significantly smaller changes in repeat size; (2) blood leukocytes, lymphoblastoid cells, buccal cells, and sperm have a variable degree of mosaicism in ATTCT expansion; (3) the length of the expanded repeat was not observed to change in individuals over a 5-year period; and (4) clinically determined anticipation is sometimes associated with intergenerational contraction rather than expansion of the ATTCT repeat.

Detection of large pathogenic expansions in FRDA1, SCA10, and SCA12 genes using a simple fluorescent repeat-primed PCR assay

At least 18 human genetic diseases are caused by expansion of short tandem repeats. Here we describe a successful application of a fluorescent PCR method for the detection of expanded repeats in FRDA1, SCA10, and SCA12 genes. Although this test cannot give a precise estimate of the size of the expansion, it is robust, reliable, and inexpensive, and can be used to screen large series of patients. It proved useful for confirming the presence of large expansions in the Friedreich ataxia gene following an ambiguous result of long-range PCR, as well as rapid pre-screening for large repeat expansions associated with Friedreich ataxia and SCA10 and the shorter repeat expansions associated with SCA12.

Analysis of trinucleotide repeats in different SCA loci in spinocerebellar ataxia patients and in normal population of Taiwan

OBJECTIVE

To identify various subtypes of spinocerebellar ataxias (SCAs) among autosomal dominant cerebellar ataxia (ADCA) patients referred to our research center, SCA1, SCA2, SCA3/MJD (Machado-Joseph disease), SCA6, SCA7, SCA8 and SCA12 loci were assessed for expansion of trinucleotide repeats.

PATIENTS AND METHODS

A total of 211 ADCA patients, including 202 patients with dominantly inherited ataxia from 81 Taiwanese families and nine patients with sporadic ataxia, were included in this study and subjected to polymerase chain reaction (PCR) analysis. The amplified products of all loci were analyzed on both 3% agarose gels and 6% denaturing urea-polyacrylamide gels. PCR-based Southern blots were also applied for the detection of SCA7 locus.

RESULTS

The SCA1 mutation was detected in six affected individuals from one family (1.2%) with expanded alleles of 50-53 CAG repeats. Fourteen individuals from nine families (11%) had a CAG trinucleotide repeat expansion at the SCA2 locus, while affected SCA2 alleles have 34-49 CAG repeats. The SCA3/MJD CAG trinucleotide repeat expansion in 60 affected individuals from 26 families (32%) was expanded to 71-85 CAG repeats. As for the SCA7 locus, there were two affected individuals from one family (1.2%) possessed 41 and 100 CAG repeats, respectively. However, we did not detect expansion in the SCA6, SCA8 and SCA12 loci in any patient.

CONCLUSIONS

The SCA3/MJD CAG expansion was the most frequent mutation among the SCA patients. The relative prevalence of SCA3/MJD in Taiwan was higher than that of SCA2, SCA1 and SCA7.

Common psychiatric diseases and human genetic variation

OBJECTIVE

A better understanding of human genetic variation is important in assessing disease epidemiology and phenotypic variation, and may be critical in evaluating genetic aspects of common genetic diseases, such as schizophrenia, bipolar disease and Parkinson's. These diseases are particularly difficult to investigate as there are few peripheral markers, and although a genetic aetiology has long been suspected, robust findings have been hard to establish.

METHODS

Variations in alleles at 13 tri-nucleotide gene loci expressed in the brain and implicated in several neurodegenerative diseases, as well as certain other loci, were examined in the Indian population for comparison with other major ethnic groups.

RESULTS AND CONCLUSION

In the Indian population, the distribution of alleles at the Machado-Joseph disease locus was similar to the Western European pattern of distribution. Analysis of haplotypes at the locus for Huntington's disease suggested multiple origins, and possible effects of population admixture because of the recent history of the country. At other alleles of neuropsychiatric interest (dopamine receptor, serotonin receptor, serotonin transporter, alcohol dehydrogenase), allele frequencies in the Indian population differed from other populations. Interspecies comparison suggests a gradual expansion in repeat size, with the exception of the CLOCK gene, which displays a contraction of CAG repeat numbers. World-wide differences in disease phenotypes need to be explored, and an appreciation of their genetic basis may provide a window of opportunity for improving our knowledge of the underlying genetic mechanisms.

The Spinocerebellar Ataxia 8 Noncoding RNA Causes Neurodegeneration and Associates with Staufen in Drosophila

Spinocerebellar Ataxia 8 (SCA8) appears unique among triplet repeat expansion-induced neurodegenerative diseases because the predicted gene product is a noncoding RNA. Little is currently known about the normal function of SCA8 in neuronal survival or how repeat expansion contributes to neurodegeneration. To investigate the molecular context in which SCA8 operates, we have expressed the human SCA8 noncoding RNA in Drosophila. SCA8 induces late-onset, progressive neurodegeneration in the Drosophila retina. Using this neurodegenerative phenotype as a sensitized background for a genetic modifier screen, we have identified mutations in four genes: staufen, muscle-blind, split ends, and CG3249. All four encode neuronally expressed RNA binding proteins conserved in Drosophila and humans. Although expression of both wild-type and repeat-expanded SCA8 induce neurodegeneration, the strength of interaction with certain modifiers differs between the two SCA8 backgrounds, suggesting that CUG expansions alter associations with specific RNA binding proteins. Our demonstration that SCA8 can recruit Staufen and that the interaction domain maps to the portion of the SCA8 RNA that undergoes repeat expansion in the human disease suggests a specific mechanism for SCA8 function and disease. Genetic modifiers identified in our SCA8-based screens may provide candidates for designing therapeutic interventions to treat this disease.

Genetic testing in spinocerebellar ataxia in Taiwan: expansions of trinucleotide repeats in SCA8 and SCA17 are associated with typical Parkinson's disease

DNA tests in normal subjects and patients with ataxia and Parkinson's disease (PD) were carried out to assess the frequency of spinocerebellar ataxia (SCA) and to document the distribution of SCA mutations underlying ethnic Chinese in Taiwan. MJD/SCA3 (46%) was the most common autosomal dominant SCA in the Taiwanese cohort, followed by SCA6 (18%) and SCA1 (3%). No expansions of SCA types 2, 10, 12, or dentatorubropallidoluysian atrophy (DRPLA) were detected. The clinical phenotypes of these affected SCA patients were very heterogeneous. All of them showed clinical symptoms of cerebellar ataxia, with or without other associated features. The frequencies of large normal alleles are closely associated with the prevalence of SCA1, SCA2, MJD/SCA3, SCA6, and DRPLA among Taiwanese, Japanese, and Caucasians. Interestingly, abnormal expansions of SCA8 and SCA17 genes were detected in patients with PD. The clinical presentation for these patients is typical of idiopathic PD with the following characteristics: late onset of disease, resting tremor in the limbs, rigidity, bradykinesia, and a good response to levodopa. This study appears to be the first report describing the PD phenotype in association with an expanded allele in the TATA-binding protein gene and suggests that SCA8 may also be a cause of typical PD.

Novel CAG/CTG repeat expansion mutations do not contribute to the genetic risk for most cases of bipolar disorder or schizophrenia

The possible presence of anticipation in bipolar affective disorder and schizophrenia has led to the hypothesis that repeat expansion mutations could contribute to the genetic etiology of these diseases. Using the repeat expansion detection (RED) assay, we have systematically examined genomic DNA from 100 unrelated probands with schizophrenia and 68 unrelated probands with bipolar affective disorder for the presence of CAG/CTG repeat expansions. Our results show that 28% of the probands with schizophrenia and 30% of probands with bipolar disorder have a CAG/CTG repeat in the expanded range, but that each expansion could be explained by one of three nonpathogenic repeat expansions known to exist in the general population. We conclude that novel CAG/CTG repeat expansions are not a common genetic risk factor for bipolar disorder or schizophrenia.

Protein phosphatase 2A inhibition induces cerebellar long-term depression and declustering of synaptic AMPA receptor

Phosphorylation of synaptic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA) receptors (AMPARs) is an essential component of cerebellar long-term depression (LTD), a form of synaptic plasticity involved in motor learning. Here, we report that protein phosphatase 2A (PP-2A) plays a specific role in controlling synaptic strength and clustering of AMPARs at synapses between granule cells and Purkinje cells. In 22- to 35-day cerebellar cultures, specific inhibition of postsynaptic PP-2A by fostriecin (100 nM) or cytostatin (10-60 microM) induced a gradual and use-dependent decrease of synaptic current evoked by the stimulation of a single granule cell, without altering receptor kinetics nor passive electrical properties. By contrast, PP-2A inhibition had no effect on immature Purkinje cells (12-15 days). Concurrent PP-2A inhibition and AMPAR stimulation induced a reduction of miniature synaptic currents and a reduction of AMPAR density at synapses. Either PP-2A inhibitor alone or AMPA stimulation alone had no significant effect. Inhibition of PP-1 by inhibitor 1 (10-27 units/microl) had no effect on synaptic current. Synaptic depression induced by PP-2A inhibition occluded subsequent induction of LTD by conjunctive stimulation and was abolished by a calcium chelator or a protein kinase inhibitor, suggesting a shared molecular pathway and involvement of PP-2A in LTD induction.

The hereditary spinocerebellar ataxias in Japan

In Japan, multiple system atrophy (MSA) accounts for 40% of all spinocerebellar ataxias (SCAs) and hereditary disorders account for 30%. Among the latter, autosomal dominant disorders are common and recessive ataxias are rare. Although the frequency of SCA genotypes differs between geographic regions throughout Japan, SCA6, SCA3/MJD, and DRPLA are the three major disorders, while SCA7, SCA8, SCA10, SCA12, and SCA17 are infrequent or almost undetected. SCA1 predominantly occurs in the northern part of Japan. Overall, 20-40% of dominant SCAs are due to unknown mutations. From this cluster, pure cerebellar ataxias linked with the SCA4, SCA14, and SCA16 locus have been isolated. Among the recessive SCAs, patients with AVED and EAOH have been detected. However, FRDA associated with GAA repeat expansion in the frataxin gene has not been reported so far.

Why is SCA12 different from other SCAs?

Spinocerebellar ataxia type 12 (SCA12), now described in European-American and Asian (Indian) pedigrees, is unique among the SCAs from clinical, pathological, and molecular perspectives. Clinically, the distinguishing feature is early and prominent action tremor with variability in other signs. Pathologically, brain MRIs also suggest variability, with prominent cortical as well as cerebellar atrophy. Genetically, SCA12 is caused by a CAG repeat expansion that does not encode polyglutamine; we speculate that the mutation may affect expression of the gene PPP2R2B, which encodes a brain-specific regulatory subunit of the protein phosphatase PP2A.

Clinical features and neuropathology of autosomal dominant spinocerebellar ataxia (SCA17)

Autosomal dominant spinocerebellar ataxias (SCAs) are a group of neurodegenerative disorders clinically characterized by late-onset ataxia and variable other manifestations. Genetically and clinically, SCA is highly heterogeneous. Recently, CAG repeat expansions in the gene encoding TATA-binding protein (TBP) have been found in a new form of SCA, which has been designated SCA17. To estimate the frequency of SCA17 among white SCA patients and to define the phenotypic variability, we determined the frequency of SCA17 in a large sample of 1,318 SCA patients. In total, 15 patients in four autosomal dominant SCA families had CAG/CAA repeat expansions in the TBP gene ranging from 45 to 54 repeats. The clinical features of our SCA17 patients differ from other SCA types by manifesting with psychiatric abnormalities and dementia. The neuropathology of SCA17 can be classified as a "pure cerebellar" or "cerebello-olivary" form of ataxia. However, intranuclear neuronal inclusion bodies with immunoreactivity to anti-TBP and antipolyglutamine were much more widely distributed throughout the brain gray matter than in other SCAs. Based on clinical and genetic data, we conclude that SCA17 is rare among white SCA patients. SCA17 should be considered in sporadic and familial cases of ataxia with accompanying psychiatric symptoms and dementia.

Activation of the ERK and JNK signaling pathways caused by neuron-specific inhibition of PP2A in transgenic mice

A reduced activity of protein phosphatase 2A (PP2A) has been shown in brains of patients with Alzheimer's disease (AD), a neurodegenerative disorder characterized histopathologically by amyloid plaques and neurofibrillary tangles. Tau, as the principal component of neurofibrillary tangles, can be hyperphosphorylated by a reduced activity of PP2A in vitro and by pharmacological approaches, suggesting a crucial role of PP2A in tangle formation. To dissect the role of PP2A in vivo, we previously generated transgenic mice with chronically reduced PP2A activity by expressing a dominant-negative mutant form of the PP2A catalytic subunit Calpha, L199P, under the control of a neuron-specific promoter. In these mice, endogenous tau is phosphorylated at the epitopes Ser202/Thr205 and Ser422. In vitro, these tau phospho-epitopes can be phosphorylated by the kinases ERK and JNK, and the kinases themselves are negatively regulated by PP2A. In this study, we show that chronic inhibition of PP2A activity in L199P transgenic mice causes the activation of ERK and JNK as demonstrated by the phosphorylation and nuclear accumulation of the ERK and JNK substrates, Elk-1 and c-Jun. TUNEL staining revealed that activated JNK signaling was not associated with cell death. Our findings imply that PP2A is a negative regulator of the ERK and JNK signaling pathways in vivo, suggesting that in AD, tau hyperphosphorylation may be caused in part by PP2A dysfunction.

Dominant ataxias and Friedreich ataxia: an update

PURPOSE OF REVIEW

The present review covers recent developments in inherited ataxias. The discovery of new loci and genes has led to improved understanding of the breadth and epidemiology of inherited ataxias. This has resulted also in more rational classification schemes. Research on identified loci has begun to yield insights into the pathogenesis of neuronal dysfunction and neurodegeneration in these diseases.

RECENT FINDINGS

There are a plethora of inherited ataxias due to a variety of mutational mechanisms involving numerous loci. While ataxia and other aspects of cerebellar dysfunction are the core features of these diseases, rational classification has been impeded by the simultaneous variety of associated clinical features and considerable overlap in clinical features among diseases involving different loci. Inherited ataxias can be classified according to mode of inheritance and mechanism of mutations. Dominantly inherited ataxias (spinocerebellar ataxias) are one major group of ataxias. Spinocerebellar ataxias can be subdivided into expanded exonic CAG repeat (polyglutamine; polyQ) disorders, dominantly inherited ataxias with mutations in non-coding regions, and dominantly inherited ataxias with chromosomal localizations but unidentified loci. Another group of dominantly inherited ataxias are episodic ataxias due to ion channel mutations. Recessive ataxias constitute a more heterogeneous group due to loss-of-function effects in numerous loci. A number of these loci have now been identified. Progress has been made in investigating the pathogenesis of neuronal dysfunction/neurodegeneration in several inherited ataxias. Convergent evidence suggests that transcriptional dysregulation is an important component of neurodegeneration in polyQ disorders. Mitochondrial dysfunction is central to pathogenesis of the most common recessive ataxia, Friedreich ataxia.

SUMMARY

Mapping of additional ataxia loci and identification of novel ataxia genes continues unabated. Genetic classification enables typology of inherited ataxias. Identification of the affected loci and the mutational mechanisms has allowed the first glimmers of understanding of the pathogenesis of several inherited ataxias.

Mutation at the SCA17 locus is not a common cause of parkinsonism

Spinocerebellar ataxia (SCA) 17 is a dominant, progressive, neurodegenerative disorder. The disease is caused by a triplet repeat expansion mutation within TATA-binding protein (TBP). Ataxia, dementia, parkinsonism and dystonia are common features. We have previously shown in several pedigrees that SCA-2 and SCA-3 can cause both parkinsonism and typical Parkinson's disease in the absence of prominent ataxia; a finding which has been confirmed by others. Given these previous findings and the description of parkinsonism as a common feature of SCA-17 we examined this locus in a series of probands from families with 2 or more members affected with parkinsonism (n=51) and a group of sporadic parkinsonism patients (n=59). We did not find any repeat sizes in the pathogenic range. The repeats we observed ranged from 29 to 41 (mean 36.8; median 37). We conclude that SCA-17 repeat expansion mutations are not a common cause of familial parkinsonism.

A developmentally regulated, neuron-specific splice variant of the variable subunit Bbeta targets protein phosphatase 2A to mitochondria and modulates apoptosis

Heterotrimeric protein phosphatase 2A (PP2A) is a major Ser/Thr phosphatase composed of catalytic, structural, and regulatory subunits. Here, we characterize Bbeta2, a novel splice variant of the neuronal Bbeta regulatory subunit with a unique N-terminal tail. Bbeta2 is expressed predominantly in forebrain areas, and PP2A holoenzymes containing Bbeta2 are about 10-fold less abundant than those containing the Bbeta1 (previously Bbeta) isoform. Bbeta2 mRNA is dramatically induced postnatally and in response to neuronal differentiation of a hippocampal progenitor cell line. The divergent N terminus of Bbeta2 does not affect phosphatase activity but encodes a subcellular targeting signal. Bbeta2, but not Bbeta1 or an N-terminal truncation mutant, colocalizes with mitochondria in neuronal PC12 cells. Moreover, the Bbeta2 N-terminal tail is sufficient to target green fluorescent protein to this organelle. Inducible or transient expression of Bbeta2, but neither Bbeta1, Bgamma, nor a Bbeta2 mutant defective in holoenzyme formation, accelerates apoptosis in response to growth factor deprivation. Thus, alternative splicing of a mitochondrial localization signal generates a PP2A holoenzyme involved in neuronal survival signaling.

Spinocerebellar ataxia type 15 (sca15) maps to 3p24.2-3pter: exclusion of the ITPR1 gene, the human orthologue of an ataxic mouse mutant

We have studied a large Australian kindred with a dominantly inherited pure cerebellar ataxia, SCA15. The disease is characterised by a very slow rate of progression in some family members, and atrophy predominantly of the superior vermis, and to a lesser extent the cerebellar hemispheres. Repeat expansion detection failed to identify either a CAG/CTG or ATTCT/AGAAT repeat expansions segregating with the disease in this family. A genome-wide scan revealed significant evidence for linkage to the short arm of chromosome 3. The highest two-point LOD score was obtained with D3S3706 (Z = 3.4, theta = 0.0). Haplotype analysis identified recombinants that placed the SCA15 locus within an 11.6-cM region flanked by the markers D3S3630 and D3S1304. The mouse syntenic region contains two ataxic mutants, itpr1-/- and opt, affecting the inositol 1,4,5-triphosphate type 1 receptor, ITPR1 gene. ITPR1 is predominantly expressed in the cerebellar Purkinje cells. Mutation analysis from two representative affected family members excluded the coding region of the ITPR1 gene from being involved in the pathogenesis of SCA15. Thus, the itpr1-/- and opt ITPR1 mouse mutants, which each result in ataxia, are not allelic to the human SCA15 locus.

Polyglutamines placed into context

Nine inherited neurodegenerative disorders result from polyglutamine expansions. Two recently published papers on spinocerebellar ataxia type 1, together with studies on spinobulbar muscular atrophy last year, indicate that host protein context is the key arbiter of polyglutamine disease protein toxicity. This insight may represent the most important development in the field since the recognition of nuclear inclusions or the propensity of polyglutamine to aggregate. Indeed, an intimate and inextricable relationship may exist between polyglutamine neurotoxicity and the normal interactions, domains, modifications, and functions of the respective disease proteins.

Anticipation and CAG*CTG repeat expansion in schizophrenia and bipolar affective disorder

The genetic contribution to the etiologies of schizophrenia and bipolar affective disorder (BPAD) has been considered for many decades, with twin, family, and adoption studies indicating consistently that the familial clustering of affected individuals is accounted for mainly by genetic factors. Despite the strong evidence for a genetic component, very little is understood about the underlying genetic and molecular mechanisms for schizophrenia and BPAD. In the early 1990s, after the discovery of "dynamic mutation" or "unstable DNA" as a molecular basis for the genetic anticipation observed in Huntington's disease, myotonic dystrophy, and many others, and the recently rediscovered, albeit still controversial, evidence for genetic anticipation in major psychoses, the genetic epidemiology of schizophrenia and BPAD was re-evaluated to demonstrate strong endorsement for the unstable DNA model. Many of the non-Mendelian genetic features of schizophrenia and BPAD could be explained by the behaviour of unstable DNA, and several molecular genetic approaches became available for testing the unstable DNA hypothesis. However, despite promising findings in the mid-1990s, no trinucleotide repeat expansion has yet been identified as a cause of idiopathic schizophrenia or BPAD.

A novel autosomal dominant spinocerebellar ataxia (SCA22) linked to chromosome 1p21-q23

The autosomal dominant cerebellar ataxias (ADCA) are a clinically, pathologically and genetically heterogeneous group of disorders. Ten responsible genes have been identified for spinocerebellar ataxia types SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA10, SCA12 and SCA17, and dentatorubral pallidoluysian atrophy (DRPLA). The mutation is caused by an expansion of a CAG, CTG or ATTCT repeat sequence of these genes. Six additional loci, SCA4, SCA5, SCA11, SCA13, SCA14 and SCA16 have also been mapped. The growing heterogeneity of the autosomal dominant forms of these diseases shows that the genetic aetiologies of at least 20% of ADCA have yet to be elucidated. We ascertained and clinically characterized a four-generation Chinese pedigree segregating an autosomal dominant phenotype for cerebellar ataxia. Direct mutation analysis, linkage analysis for all known SCA loci and a genome-wide linkage study were performed. Direct mutation analysis excluded SCA1, 2, 3, 6, 7, 8, 10, 12, 17 and DRPLA, and genetic linkage analysis excluded SCA4, 5, 11, 13, 14 and 16. The genome-wide linkage study suggested linkage to a locus on chromosome 1p21-q23, with the highest two-point LOD score at D1S1167 (Zmax = 3.46 at theta = 0.00). Multipoint analysis and haplotype reconstruction traced this novel SCA locus (SCA22) to a 43.7-cM interval flanked by D1S206 and D1S2878 (Zmax = 3.78 under four liability classes, and 2.67 using affected-only method). The age at onset ranged from 10 to 46 years. All affected members had gait ataxia with variable features of dysarthria and hyporeflexia. Head MRI showed homogeneous atrophy of the cerebellum without involvement of the brainstem. In six parent-child pairs, median onset occurred 10 years earlier in offspring than in their parents, suggesting anticipation. This family is distinct from other families with SCA and is characterized by a slowly progressive, pure cerebellar ataxia.

The unstable trinucleotide repeat story of major psychosis

New hopes for cloning susceptibility genes for schizophrenia and bipolar affective disorder followed the discovery of a novel type of DNA mutation, namely unstable DNA. One class of unstable DNA, trinucleotide repeat expansion, is the causal mutation in myotonic dystrophy, fragile X mental retardation, Huntington disease and a number of other rare Mendelian neurological disorders. This finding has led researchers in psychiatric genetics to search for unstable DNA sites as susceptibility factors for schizophrenia and bipolar affective disorder. Increased severity and decreased age at onset of disease in successive generations, known as genetic anticipation, was reported for undifferentiated psychiatric diseases and for myotonic dystrophy early in the twentieth century, but was initially dismissed as the consequence of ascertainment bias. Because unstable DNA was demonstrated to be a molecular substrate for genetic anticipation in the majority of trinucleotide repeat diseases including myotonic dystrophy, many recent studies looking for genetic anticipation have been performed for schizophrenia and bipolar affective disorder with surprisingly consistent positive results. These studies are reviewed, with particular emphasis placed on relevant sampling and statistical considerations, and concerns are raised regarding the interpretation of such studies. In parallel, molecular genetic investigations looking for evidence of trinucleotide repeat expansion in both schizophrenia and bipolar disorder are reviewed. Initial studies of genome-wide trinucleotide repeats using the repeat expansion detection technique suggested possible association of large CAG/CTG repeat tracts with schizophrenia and bipolar affective disorder. More recently, three loci have been identified that contain large, unstable CAG/CTG repeats that occur frequently in the population and seem to account for the majority of large products identified using the repeat expansion detection method. These repeats localize to an intron in transcription factor gene SEF2-1B at 18q21, a site named ERDA1 on 17q21 with no associated coding region, and the 3' end of a gene on 13q21, SCA8, that is believed to be responsible for a form of spinocerebellar ataxia. At present no strong evidence exists that large repeat alleles at either SEF2-1B or ERDA1 are involved in the etiology of schizophrenia or bipolar disorder. Preliminary evidence suggests that large repeat alleles at SCA8 that are non-penetrant for ataxia may be a susceptibility factor for major psychosis. A fourth, but much more infrequently unstable CAG/CTG repeat has been identified within the 5' untranslated region of the gene, MAB21L1, on 13q13. A fifth CAG/CTG repeat locus has been identified within the coding region of an ion transporter, KCNN3 (hSKCa3), on 1q21. Although neither large alleles nor instability have been observed at KCNN3, this repeat locus has been extensively analyzed in association and family studies of major psychosis, with conflicting findings. Studies of polyglutamine containing genes in major psychosis have also shown some intriguing results. These findings, reviewed here, suggest that, although a major role for unstable trinucleotides in psychosis is unlikely, involvement at a more modest level in a minority of cases cannot be excluded, and warrants further investigation.