Identification of a novel polymorphism in the promoter region of the tau gene highly associated to progressive supranuclear palsy in humans

iPS Cell-Based Model for MAPT Haplotype as a Risk Factor for Human Tauopathies Identifies No Major Differences in TAU Expression

The H1 haplotype of the microtubule-associated protein tau (MAPT) gene is a common genetic risk factor for some neurodegenerative diseases such as progressive supranuclear palsy, corticobasal degeneration, and Parkinson's disease. The molecular mechanism causing the increased risk for the named diseases, however, remains unclear. In this paper, we present a valuable tool of eight small molecule neural precursor cell lines (smNPC) homozygous for the MAPT haplotypes (four H1/H1 and four H2/H2 cell lines), which can be used to identify MAPT-dependent phenotypes. The employed differentiation protocol is fast due to overexpression of NEUROGENIN-2 and therefore suitable for high-throughput approaches. A basic characterization of all human cell lines was performed, and their TAU and α-SYNUCLEIN profiles were compared during a differentiation time of 30 days. We could identify higher levels of conformationally altered TAU in cell lines carrying the H2 haplotype. Additionally, we found increased expression levels of α-SYNUCLEIN in H1/H1 cells. With this resource, we aim to fill a gap in neurodegenerative disease modeling with induced pluripotent stem cells (iPSC) for sporadic tauopathies.

Meta-analysis of the association between variants in MAPT and neurodegenerative diseases

Microtubule-associated protein tau (MAPT) gene is compelling among the susceptibility genes of neurodegenerative diseases which include Alzheimer’s disease (AD), Parkinson’s disease (PD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Our meta-analysis aimed to find the association between MAPT and the risk of these diseases. Published literatures were retrieved from MEDLINE and other databases, and 82 case-control studies were recruited. Six haplotype tagging single-nucleotide polymorphisms (rs1467967, rs242557, rs3785883, rs2471738, del-In9 and rs7521) and haplotypes (H2 and H1c) were significantly associated with the above diseases. The odds ratios (ORs) and 95 % confidence intervals (CIs) were evaluated by comparison in minor and major allele frequency using the R software. This study demonstrated that different variants in MAPT were associated with AD (rs2471738: OR= 1.04, 95%CI = 1.00 - 1.09; H2: OR = 0.94, 95% CI = 0.91 - 0.97), PD (H2: OR = 0.76, 95% CI = 0.74 - 0.79), PSP (rs242557: OR = 1. 96, 95% CI = 1. 71 - 2.25; rs2471738: OR = 1. 85, 95% CI = 1. 48 - 2.31; H2: OR = 0.20, 95% CI = 0.18 - 0.23), CBD (rs242557: OR = 2.51, 95%CI = 1. 66 -3.78; rs2471738: OR = 2.07, 95%CI = 1. 32 -3.23; H2: OR = OR = 0.30, 95% CI = 0.23 - 0.41) and ALS (H2: OR = 0.92, 95% CI = 0.86 - 0.98) instead of FTD (H2: OR = 1.02, 95% CI = 0.78 - 1.32). In conclusion, MAPT is associated with risk of neurodegenerative diseases, suggesting crucial roles of tau in neurodegenerative processes.

Genome-wide association study of corticobasal degeneration identifies risk variants shared with progressive supranuclear palsy

Corticobasal degeneration (CBD) is a neurodegenerative disorder affecting movement and cognition, definitively diagnosed only at autopsy. Here, we conduct a genome-wide association study (GWAS) in CBD cases (n=152) and 3,311 controls, and 67 CBD cases and 439 controls in a replication stage. Associations with meta-analysis were 17q21 at MAPT (P=1.42 × 10(-12)), 8p12 at lnc-KIF13B-1, a long non-coding RNA (rs643472; P=3.41 × 10(-8)), and 2p22 at SOS1 (rs963731; P=1.76 × 10(-7)). Testing for association of CBD with top progressive supranuclear palsy (PSP) GWAS single-nucleotide polymorphisms (SNPs) identified associations at MOBP (3p22; rs1768208; P=2.07 × 10(-7)) and MAPT H1c (17q21; rs242557; P=7.91 × 10(-6)). We previously reported SNP/transcript level associations with rs8070723/MAPT, rs242557/MAPT, and rs1768208/MOBP and herein identified association with rs963731/SOS1. We identify new CBD susceptibility loci and show that CBD and PSP share a genetic risk factor other than MAPT at 3p22 MOBP (myelin-associated oligodendrocyte basic protein).

Novel mutation in MAPT exon 13 (p.N410H) causes corticobasal degeneration

In order to determine the frequency of microtubule-associated protein tau gene (MAPT) mutations and rare variants in CBD, we performed a systematic sequence analysis of MAPT coding and 3′ untranslated region (3′UTR) in a large cohort of autopsy-confirmed CBD patients (N = 109). This identified a novel MAPT mutation in exon 13, p.N410H, in a case that is neuropathologically indistinguishable from sporadic CBD. On immunoblot, the p.N410H mutation carrier had the same insoluble tau profile as seen in CBD. Additionally, tau expression analysis in brain tissue found a significant increase in the 4R/3R tau mRNA ratio (P = 0.04), indicating that p.N410H disrupts tau isoform homeostasis. Biochemically, recombinant tau protein with p.N410H showed a marked increase in tau filament formation compared to wild-type tau (P < 0.001), had a 19.2% decrease in rate of microtubule assembly (P < 0.05), and a 10.3% reduction in the extent of total microtubule polymerization (P < 0.01). Sequence analysis of the complete MAPT 3′UTR in autopsy-confirmed CBD cases further identified two rare variants with nominally significant association with CBD. An ATC nucleotide insertion (“MAPTv8”) was found in 4.6% of CBD patients compared to 1.2% of controls (P = 0.031, OR = 3.71), and rs186977284 in 4.6% CBD patients, but only 0.9% of controls (P = 0.04, OR = 3.58). Rs186977284 was also present in 2.7% of a large cohort of autopsy-confirmed PSP patients (N = 566) and only 0.9% of an additional control series (P = 0.034, OR = 3.08), extending the association to PSP. Our findings show that mutations in MAPT can cause CBD and MAPT non-coding variants may increase the risk of complex 4R tauopathies.

Assessment of common variability and expression quantitative trait loci for genome-wide associations for progressive supranuclear palsy

Progressive supranuclear palsy is a rare parkinsonian disorder with characteristic neurofibrillary pathology consisting of hyperphosphorylated tau protein. Common variation defining the microtubule associated protein tau gene (MAPT) H1 haplotype strongly contributes to disease risk. A recent genome-wide association study (GWAS) revealed 3 novel risk loci on chromosomes 1, 2, and 3 that primarily implicate STX6, EIF2AK3, and MOBP, respectively. Genetic associations, however, rarely lead to direct identification of the relevant functional allele. More often, they are in linkage disequilibrium with the causative polymorphism(s) that could be a coding change or affect gene expression regulatory motifs. To identify any such changes, we sequenced all coding exons of those genes directly implicated by the associations in progressive supranuclear palsy cases and analyzed regional gene expression data from control brains to identify expression quantitative trait loci within 1 Mb of the risk loci. Although we did not find any coding variants underlying the associations, GWAS-associated single-nucleotide polymorphisms at these loci are in complete linkage disequilibrium with haplotypes that completely overlap with the respective genes. Although implication of EIF2AK3 and MOBP could not be fully assessed, we show that the GWAS single-nucleotide polymorphism rs1411478 (STX6) is a strong expression quantitative trait locus with significantly lower expression of STX6 in white matter in carriers of the risk allele.

Clinicopathologic assessment and imaging of tauopathies in neurodegenerative dementias

Microtubule-associated protein tau encoded by the MAPT gene binds to microtubules and is important for maintaining neuronal morphology and function. Alternative splicing of MAPT pre-mRNA generates six major tau isoforms in the adult central nervous system resulting in tau proteins with three or four microtubule-binding repeat domains. In a group of neurodegenerative disorders called tauopathies, tau becomes aberrantly hyperphosphorylated and dissociates from microtubules, resulting in a progressive accumulation of intracellular tau aggregates. The spectrum of sporadic frontotemporal lobar degeneration associated with tau pathology includes progressive supranuclear palsy, corticobasal degeneration, and Pick’s disease. Alzheimer’s disease is considered the most prevalent tauopathy. This review is divided into two broad sections. In the first section we discuss the molecular classification of sporadic tauopathies, with a focus on describing clinicopathologic relationships. In the second section we discuss the neuroimaging methodologies that are available for measuring tau pathology (directly using tau positron emission tomography ligands) and tau-mediated neuronal injury (magnetic resonance imaging and fluorodeoxyglucose positron emission tomography). Both sections have detailed descriptions of the following neurodegenerative dementias – Alzheimer’s disease, progressive supranuclear palsy, corticobasal degeneration and Pick’s disease.

Rational therapeutic approaches to progressive supranuclear palsy

Progressive supranuclear palsy is a sporadic and progressive neurodegenerative disease, most often presenting as a symmetric, akinetic-rigid syndrome with postural instability, vertical supranuclear gaze palsy and frontal lobe deficits. It belongs to the family of tauopathies and involves both cortical and subcortical structures. Although the exact pathophysiology is not yet fully understood, several lines of evidence point to a crucial contribution from both genetic predisposition and mitochondrial dysfunction. Recently gained insights into the pathophysiology of this disease have led to several hypothesis-driven therapeutic approaches aiming at disease-modification rather than mere symptomatic neurotransmitter-replacement therapy. Agents targeting mitochondrial dysfunction have already shown a positive effect in a phase II study and further studies to verify and expand these results are ongoing. Clinical studies with agents targeting tau dysfunction such as tau-kinase inhibitors, tau-aggregation inhibitors and microtubule stabilizers are in preparation or ongoing. This review presents the current pathophysiological concepts driving these exciting therapeutic developments.

The +347 C promoter allele up-regulates MAPT expression and is associated with Alzheimer's disease among the Chinese Han

The microtubule-associated protein tau has been known to be associated with the pathogenesis of Alzheimer's disease (AD). We identified a novel single nucleotide polymorphism (SNP) in the promoter region of the microtubule-associated protein tau gene (MAPT) in Chinese Han population: 347G/C. The samples we analyzed were all identified as H1/H1 genotype; the 347C allele was over-represented in 252 sporadic AD patients (84.3%, P=0.006) when compared with 197 controls (75.1%). The transcriptional activity of SNP in promoter was further investigated using a luciferase reporter assay in two human cell lines, SH-SY5Y and Hela. We demonstrated that the promoter transcriptional activity of the 347 C/C genotype was significantly higher than that of the 347 G/G genotype (SH-SY5Y, P=0.0321; Hela, P=0.0016). Our data suggest that the 347C polymorphism in the promoter of MAPT gene, which is associated with an up-regulation of the gene expression, is a susceptibility factor in sporadic AD.

From 1997 to 2007: a decade journey through the H1 haplotype on 17q21 chromosome

The H1 haplotype was first identified 10 years ago. Initially, a dinucleotide polymorphism was detected in the tau (MAPT) gene and was subsequently found to be in linkage disequilibrium (LD) with other polymorphisms, forming the MAPT H1 haplotype, a risk factor for many neurological diseases, considered as tauopathies. Genetic and histopathologic data are in agreement that MAPT and its encoded protein have a pivotal role in the normal function of neurons. Currently, the H1 haplotype extends beyond the outer edges of MAPT encompassing multiple genes on chromosome 17 and thus increasing the number of candidate genes implicated in the pathogenesis of tauopathies. This review highlights the milestones and basic events in the journey towards uncovering the significance of the H1 haplotype.

Frontotemporal dementia with tau pathology

Tau is a microtubule-associated protein involved in microtubule assembly and stabilization. Filamentous deposits made of tau constitute a major defining characteristic of several neurodegenerative diseases known as tauopathies including Alzheimer's disease. The involvement of tau in neurodegeneration has been clarified by the identification of genetic mutations in the tau gene in cases with familial frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Although the mechanism through which tau mutations lead to neuronal death is still unresolved, it is clear that tau mutations lead to formation of tau filaments that have a different morphology, contain different types of tau isoforms and produce distinct tau deposits. The range of tau pathology identified in FTDP-17 recapitulates the tau pathology present in sporadic tauopathies and indicates that tau dysfunction plays a major role also in these diseases.

The tau N279K exon 10 splicing mutation recapitulates frontotemporal dementia and parkinsonism linked to chromosome 17 tauopathy in a mouse model

Intracellular tau deposits are characteristic of several neurodegenerative disorders called tauopathies. The tau protein regulates the stability and assembly of microtubules by binding to microtubules through three or four microtubule-binding repeats (3R and 4R). The number of microtubule-binding repeats is determined by the inclusion or exclusion of the second microtubule-binding repeat encoded by exon 10 of the TAU gene. TAU gene mutations that alter the inclusion of exon 10, and hence the 4R:3R ratio, are causal in the tauopathy frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). A mutation located in exon 10 has been identified in several FTDP-17 families that present with increased exon 10 inclusion in both mRNA and protein, parkinsonism, movement disorders, and dementia. We have engineered a human tau minigene construct that was designed to allow alternative splicing of the tau exon 10. Here we demonstrate that transgenic mice expressing human tau protein with this mutation develop neurodegeneration as result of aberrant splicing. The mice recapitulate many of the disease hallmarks that are seen in patients with this mutation, including increased tau exon 10 inclusion in both mRNA and protein, motor and behavioral deficits, and tau protein accumulation in neurons and tufted astrocytes. Furthermore, these mice present with degeneration of the nigrostriatal dopaminergic pathway, suggesting a possible mechanism for parkinsonism in FTDP-17. Additionally, activated caspase-3 immunoreactivity in both neurons and astrocytes implicates the involvement of the apoptotic pathway in the pathology of these mice.

The MAPT H1c risk haplotype is associated with increased expression of tau and especially of 4 repeat containing transcripts

Previously we have shown that the H1c haplotype on the background of the H1 clade of haplotypes at the MAPT locus is associated with increased risk for progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and Alzheimer's disease (AD). Here we replicated the association with AD in an additional autopsy confirmed series. We show that this haplotype increases both the expression of total MAPT transcript as well as specifically increasing the proportion of 4 microtubule binding repeat containing transcripts. We discuss these findings both in terms of the problems facing the dissection of the etiologies of complex traits and the pathogenesis of the tauopathies.

Tau and saitohin gene expression pattern in progressive supranuclear palsy

Deregulation of tau mRNA splicing may contribute to causing progressive supranuclear palsy (PSP). The inclusion of exon 10 produces tau protein isoforms containing all four microtubule-binding repeats (4R). Its exclusion gives rise to isoforms with three microtubule-binding repeats (3R). Alternative splicing of exons 2 and 3 produces the 0N, 1N or 2N protein isoforms. Saitohin (STH) is a nested gene included in intron 9 of tau. It has an unknown function, but could also be involved in the pathological process associated with PSP. We used real-time PCR to investigate the level of expression of tau mRNA isoforms and STH mRNA in the frontal cortex and globus pallidus of PSP patients' brains. mRNA levels were compared with those in the brains of two controls groups: healthy controls and Alzheimer's disease patients (AD). The 4R/3R mRNA ratio was significantly higher in the globus pallidus of PSP patients than in controls. The 0N mRNA isoform levels were statistically higher in the frontal cortex and globus pallidus of AD patients and were borderline higher in the globus pallidus of PSP patients than controls. In addition, when all samples were taken into account (PSP+AD+controls), a significant correlation was found between the 4R/3R mRNA tau ratio and STH expression. This correlation was stronger in the globus pallidus than in the frontal cortex. Our results suggest that abnormalities in the alternative splicing of the tau gene are involved in the molecular mechanism related to PSP pathogenesis. Such abnormalities cause an increase in the 4R/3R ratio and may lead to an overexpression of 0N tau isoforms.

Untangling the tau gene association with neurodegenerative disorders

Pathological tau protein inclusions have long been recognized to define the diverse range of neurodegenerative disorders called the tauopathies, which include Alzheimer's disease (AD), progressive supranuclear palsy (PSP) and frontotemporal lobar degeneration. Mutations in the tau gene, MAPT, cause familial frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), and common variation in MAPT is strongly associated with the risk of PSP, corticobasal degeneration and, to a lesser extent, AD and Parkinson's disease (PD), implicating the involvement of tau in common neurodegenerative pathway(s). This review will discuss recent work towards the unravelling of the functional basis of this MAPT gene association. The region of chromosome 17q21 containing MAPT locus is characterized by the complex genomic architecture, including a large inversion that leads to a bipartite haplotype architecture, an inversion-mediated deletion and multiplications resulting from non-allelic homologous recombination between the MAPT family of low-copy repeats.

No evidence of CRHR1 gene involvement in progressive supranuclear palsy

Several genes have been located in the chromosomal region 17q21 genetically associated with progressive supranuclear palsy (PSP). Corticotropin releasing hormone receptor 1 (CRHR1) is a gene included in this region. In order to investigate the possible involvement of CRHR1 in PSP pathogenesis, we measured the globus pallidus mRNA expression of this gene using real-time PCR in 12 PSP comparing with several control groups composed by 10 Alzheimer's disease, 5 cerebrovascular disease and 6 healthy controls subjects. We furthermore sequenced directly the entire coding region of CRHR1 of two histopathologically confirmed PSP patients. Expression pattern of CRHR1 in globus pallidus was similar in all groups. We did not find any coding non-synonymous mutation in the patients analysed. Our results do not support an involvement of CRHR1 gene in PSP pathogenesis.

Correlation of tau gene polymorphism with age at onset of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease and its prevalence increases with age. The microtubule-associated protein tau (MAPT) is thought to be implicated in the pathogenesis of PD. Association of the MAPT H1 haplotype with PD in Caucasians has been extensively studied, however, the results were inconsistent. In this study, we investigated whether MAPT gene variants contribute to the pathogenesis process including the age at onset in Japanese PD. Promoter region of MAPT gene was analyzed to find polymorphisms in Japanese population. Two single nucleotide polymorphisms (SNPs), C-639T and Del-568TIns, in promoter region were found. C-639T was novel. Unlike Caucasians, the -226C and -45A alleles consisting of the H1 haplotype were monomorphic in Japanese population. Association analysis was performed using 240 PD and 191 controls in these SNPs. No significant association was observed between these SNPs and PD. Haplotype analysis also showed no significant association (P=0.72). However, the age at onset showed significant correlation with the genotypes of Del-568TIns in PD samples when analyzed by Kendall rank correlation test (Kendall tau=-0.098, P=0.0243). These results suggested that MAPT gene variants may modify the pathogenesis process of PD.

Hippocampal sclerosis dementia with tauopathy

In some elderly individuals with dementia, hippocampal sclerosis (HS) is the only remarkable autopsy finding. The cause of HS in this setting is puzzling, since known causes of HS such as seizures or global hypoxic-ischemic episodes are rarely present. We here describe a series of HS cases that have a widespread neuronal and/or glial tauopathy. Of 14 consecutive cases of HS, 12 had been clinically diagnosed with dementia and/or Alzheimer's disease (AD) while 2 were non-demented; 7 cases had also been clinically diagnosed with parkinsonism. In addition to HS, 6 cases also met pathologic diagnostic criteria for AD. Gallyas silver staining and immunohistochemistry with the AT8 antibody revealed a glial and/or neuronal tauopathy in 12 of 14 cases, with frequent positive neurons and/or glial cells in the neocortex, basal ganglia, thalamus and/or limbic regions; in addition, 8 of the 14 cases had argyrophilic grains. Screening for known tau mutations was negative in all cases. Western blots of sarkosyl-insoluble tau protein showed a mixture of 3- and 4-repeat forms. The results suggest that most cases of HS dementia are sporadic multisystem tauopathies; we suggest the term "hippocampal sclerosis dementia with tauopathy" (HSDT) for these.

Linkage disequilibrium fine mapping and haplotype association analysis of the tau gene in progressive supranuclear palsy and corticobasal degeneration

BACKGROUND

The haplotype H1 of the tau gene, MAPT, is highly associated with progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD).

OBJECTIVE

To investigate the pathogenic basis of this association.

METHODS

Detailed linkage disequilibrium and common haplotype structure of MAPT were examined in 27 CEPH trios using validated HapMap genotype data for 24 single nucleotide polymorphisms (SNPs) spanning MAPT.

RESULTS

Multiple variants of the H1 haplotype were resolved, reflecting a far greater diversity of MAPT than can be explained by the H1 and H2 clades alone. Based on this, six haplotype tagging SNPs (htSNPs) that capture 95% of the common haplotype diversity were used to genotype well characterised PSP and CBD case-control cohorts. In addition to strong association with PSP and CBD of individual SNPs, two common haplotypes derived from these htSNPs were identified that are highly associated with PSP: the sole H2 derived haplotype was underrepresented and one of the common H1 derived haplotypes was highly associated, with a similar trend observed in CBD. There were powerful and highly significant associations with PSP and CBD of haplotypes formed by three H1 specific SNPs. This made it possible to define a candidate region of at least approximately 56 kb, spanning sequences from upstream of MAPT exon 1 to intron 9. On the H1 haplotype background, these could harbour the pathogenic variants.

CONCLUSIONS

The findings support the pathological evidence that underlying variations in MAPT could contribute to disease pathogenesis by subtle effects on gene expression and/or splicing. They also form the basis for the investigation of the possible genetic role of MAPT in Parkinson's disease and other tauopathies, including Alzheimer's disease.

Transcriptional regulation of the mouse microtubule-associated protein tau

The microtubule-associated protein (MAP) tau is found primarily in neurons and errors in its regulation are associated with Alzheimer's disease and other neurodegenerative disorders. Tau expression is transcriptionally regulated and tissue-specific. In this study, starting with a approximately 7500-bp fragment from the mouse tau gene, which includes tau exon -1, we define regions preferentially conferring tissue-specific expression. Furthermore, gel shift assays indicate that transcriptional regulators SP-1 and AP-2 are important for basal expression but not necessary for neuron-specific expression of the tau transcript.

Novel haplotypes in 17q21 are associated with progressive supranuclear palsy

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are sporadic neurodegenerative diseases presenting as atypical parkinsonian disorders, characterized by the presence of tau-positive neurofibrillary tangles. Recently, an extended haplotype (H1E) of 787.6 kb that comprises several genes including MAPT showed increased association with PSP. The objective of this study was to determine the size of the H1E haplotype associated with PSP and CBD in different populations and to identify specific subhaplotypes in the background of H1E haplotype. Nineteen single nucleotide polymorphisms (SNPs) in the 17q21 region were genotyped in two case-control samples. The SNPs that were associated with higher risk for the disease in the homozygous state delimit a region of more that 1 Mb. Haplotype analyses in the Spanish sample showed that the most frequent haplotype found among the patients (H1E'), which extends 1.04 Mb and contains several genes such as MAPT, CRHR1, IMP5, Saitohin, WTN3, and NSF. A specific subhaplotype (H1E'A) was present in 16% of PSP patients but was not observed in the controls. Furthermore, the H2E'A haplotype, was rarely present in the disease group suggesting that it plays a protective role. The identification of these specific subhaplotypes that modify risk for PSP/CBD supports the hypothesis that a pathogenic allele exists in a subgroup of PSP patients.

Genetic contributions to Parkinson's disease

Sporadic Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by the loss of midbrain dopamine neurons and Lewy body inclusions. It is thought to result from a complex interaction between multiple predisposing genes and environmental influences, although these interactions are still poorly understood. Several causative genes have been identified in different families. Mutations in two genes [alpha-synuclein and nuclear receptor-related 1 (Nurr1)] cause the same pathology, and a third locus on chromosome 2 also causes this pathology. Other familial PD mutations have identified genes involved in the ubiquitin-proteasome system [parkin and ubiquitin C-terminal hydroxylase L1 (UCHL1)], although such cases do not produce Lewy bodies. These studies highlight critical cellular proteins and mechanisms for dopamine neuron survival as disrupted in Parkinson's disease. Understanding the genetic variations impacting on dopamine neurons may illuminate other molecular mechanisms involved. Additional candidate genes involved in dopamine cell survival, dopamine synthesis, metabolism and function, energy supply, oxidative stress, and cellular detoxification have been indicated by transgenic animal models and/or screened in human populations with differing results. Genetic variation in genes known to produce different patterns and types of neurodegeneration that may impact on the function of dopamine neurons are also reviewed. These studies suggest that environment and genetic background are likely to have a significant influence on susceptibility to Parkinson's disease. The identification of multiple genes predisposing to Parkinson's disease will assist in determining the cellular pathway/s leading to the neurodegeneration observed in this disease.

Tau haplotypes regulate transcription and are associated with Parkinson's disease

A primary haplotype (H1) of the microtubule-associated protein Tau (MAPT) gene is associated with Parkinson's disease (PD). However, the mechanism for disease susceptibility remains unknown. We examined the promoter region of MAPT and identified single nucleotide polymorphisms and insertions of 1 to 11 nucleotides. These polymorphisms corresponded to the previously characterized haplotypes, H1 and H2, as well as a novel variant of the H1 haplotype, H1'. As observed in other studies, we demonstrated a significant association with the H1/H1 promoter genotype and PD in a cohort of 206 idiopathic late-onset cases. This is in contrast with a panel of 13 early-onset PD patients, for whom we did not detect any mutations in MAPT. By examining single nucleotide polymorphisms in adjacent genes, we showed that linkage disequilibrium does not extend beyond the MAPT haplotype to neighboring genes. To define the mechanism of disease susceptibility, we examined the transcriptional activity of the promoter haplotypes using a luciferase reporter assay. We demonstrated in two human cell lines, SK-N-MC and 293, that the H1 haplotype was more efficient at driving gene expression than the H2 haplotype. Our data suggest that an increase in expression of the MAPT gene is a susceptibility factor in idiopathic PD.

Microtubule-associated protein tau gene: a risk factor in human neurodegenerative diseases

Tau is a microtubule-associated protein mainly expressed in neurons of central nervous system, which is crucial in the maintenance of these cells. It has a central role in the polymerization and stabilization of microtubules and in the traffic of organelles along axons and dendrites. Aggregates of hyperphosphorylated forms of tau protein participate in the formation of neurofibrillary tangles, which characterize numerous neurodegenerative disorders named tauopathies. The analysis of tau gene and the study of familial cases of tauopathies have led to the discovery of tau gene mutations that cause inherited dementia designated as Frontotemporal dementia (FTD) with parkinsonism linked to chromosome 17 (FTDP-17). However, these familial cases remain rare compared to the sporadic tauopathies, the later involving both genetic and environmental etiologic factors. As tau pathology represents a primary pathogenic event in various neurodegenerative diseases, the hypothesis that tau genotype could influence the development of these diseases was tested by several groups. This review summarizes advances in the molecular genetics of the tau gene, as well as recent studies addressing the disease incidence of novel tau polymorphisms in different neurodegenerative diseases. Hopefully, the identification of several genetic defects of the tau gene will be helpful in improving our understanding of the role of tau protein in the pathogenesis of various neurodegenerative diseases.

The role of tau (MAPT) in frontotemporal dementia and related tauopathies

Tau is a multifunctional protein that was originally identified as a microtubule-associated protein. In patients diagnosed with frontotemporal dementia and parkinsonism linked to chromosome 17, mutations in the gene encoding tau (MAPT) have been identified that disrupt the normal binding of tau to tubulin resulting in pathological deposits of hyperphosphorylated tau. Abnormal filamentous tau deposits have been reported as a pathological characteristic in several other neurodegenerative diseases, including frontotemporal dementia, Pick Disease, Alzheimer disease, argyrophilic grain disease, progressive supranuclear palsy, and corticobasal degeneration. In the last five years, extensive research has identified 34 different pathogenic MAPT mutations in 101 families worldwide. In vitro, cell-free and transfected cell studies have provided valuable information on tau dysfunction and transgenic mice carrying human MAPT mutations are being generated to study the influence of MAPT mutations in vivo. This mutation update describes the considerable differences in clinical and pathological presentation of patients with MAPT mutations and summarizes the effect of the different mutations on tau functioning. In addition, the role of tau as a genetic susceptibility factor is discussed, together with the genetic evidence for additional causal genes for tau-positive as well as tau-negative dementia.

Tau negative frontal lobe dementia at 17q21: significant finemapping of the candidate region to a 4.8 cM interval

We report the results of a genome-wide search in a four-generation pedigree with autosomal dominant early-onset dementia (mean onset age: 64.9 years, range 53-79 years). In this family we previously excluded the known Alzheimer's disease genes based on linkage analysis and mutation screening of the amyloid precursor protein gene (exons 16 and 17) and the presenilin 1 and 2 genes. In addition we excluded mutations in the prion protein gene and exons 9-13 of the microtubule associated protein tau (MAPT) gene. We obtained conclusive linkage with chromosome 17q21 markers with a maximum multi-point LOD score of 5.51 at D17S951 and identified a candidate region of 4.8 cM between D17S1787 and D17S958 containing MAPT. Recent clinical and neuropathological follow-up of the family showed that the phenotype most closely resembled frontotemporal dementia (FTD) characterized by dense ubiquitin-positive neuronal inclusions that were tau negative. Extensive mutation analysis of MAPT identified 38 sequence variations in exons, introns, untranslated regions and the 5' regulatory sequence, however none was comprised within the disease haplotype. Although our findings do not entirely exclude a mutation in a yet unanalyzed region of MAPT, the apparent absence of MAPT mutations combined with the lack of tau pathology is highly suggestive for another defective gene at 17q21 responsible for FTD in this family.

Tau gene mutations: dissecting the pathogenesis of FTDP-17

Tau is a microtubule-associated protein involved in microtubule assembly and stabilization. Abnormal filamentous tau deposits constitute a major defining characteristic of several neurodegenerative diseases, including Alzheimer's disease. Although the presence of tau pathology correlates with the symptoms of Alzheimer's disease, there was no genetic evidence linking tau to neurodegeneration until recently. However, since 1998, the identification of more than 25 mutations in the tau gene, associated with frontotemporal dementia and parkinsonism linked to chromosome 17, has demonstrated that tau dysfunction can lead to neurodegeneration and the development of clinical symptoms.

Progressive supranuclear palsy: clinical and genetic aspects

PURPOSE OF THE REVIEW

This review focuses on the recent additions to the literature in the clinical and genetic aspects of progressive supranuclear palsy.

RECENT FINDINGS

Clinical features of progressive supranuclear palsy are reasonably well established and known to be quite characteristic. Recent epidemiological studies suggest that the disorder is more common than previously considered and that it is frequently misdiagnosed. New laboratory and novel imaging techniques are being tested and cerebrospinal fluid levels of tau protein have been found helpful in diagnosis. Pathological and biochemical studies in progressive supranuclear palsy brains have shown the predominance of hyperphosphorylated tau isoforms which contain the sequence encoded by exon 10 (4R) aggregated into filaments. Familial tauopathies linked to tau gene mutations showing clinical and neuropathological overlap with sporadic progressive supranuclear palsy have been described. Despite recent discoveries of the strong genetic association of sporadic progressive supranuclear palsy with tau gene polymorphisms, a specific risk allele for developing the palsy has not yet been identified yet.

SUMMARY

Recent clinical studies and clinicopathological correlations are contributing significantly to the delineation of the clinical features of progressive supranuclear palsy. These features and the appropriate use of laboratory tests allow for an earlier identification of the disease and a more accurate premortem diagnosis. However, no specific biological markers for the disorder are available yet, and consequently diagnosis in the early stages or when some of the characteristic signs and symptoms are missing, remains a major challenge. Despite the recent advances in the understanding of genetic factors involved in progressive supranuclear palsy, the cause of the disease still remains unknown. Biochemical studies in brains from progressive supranuclear palsy patients provide a potential helpful instrument to improve the characterization of this disorder.

A polymorphism within intron 11 of the tau gene is not increased in frequency in patients with sporadic Alzheimer's disease, nor does it influence the extent of tau pathology in the brain

There are numerous polymorphisms within the tau gene but these are in complete linkage disequilibrium and exist as two common extended haplotypes H1 and H2. We have investigated the frequency of these haplotypes in 83 cases of sporadic Alzheimer's disease (AD) using the +34 polymorphism in intron 11 of the tau gene as a marker of H1 and H2 haplotypes. The total amount of hyperphosphorylated tau protein (tau load), present as neurofibrillary tangles, neuropil threads or plaque neurites, was quantified in the frontal cortex of these patients and related to tau haplotype. We found no increase in H1H1 haplotype in this autopsy population of cases with AD compared to published control data. Stratification of cases for apolipoprotein E (APO E) genotype showed a slight, but not statistically significant, overrepresentation of epsilon 4 allele amongst bearers of H2 haplotype. There were no overall differences in tau load between haplotype groups though cases within each haplotype group bearing APO E epsilon 4 allele had a significantly higher tau load than those without epsilon 4 allele. Neither age at onset or duration of illness differed according to tau haplotype. We conclude that the frequency of tau gene H1 haplotype is not elevated in AD and possession of this has no impact upon the amount of tau pathology in AD.

Strong association of a novel Tau promoter haplotype in progressive supranuclear palsy

The microtubule associated protein, tau, is found in fibrillar lesions that characterise progressive supranuclear palsy (PSP) and related tauopathies. Mutations in the tau gene in frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and genetic association of the H1 haplotype of the tau gene with PSP has firmly established a direct role for tau in disease pathogenesis. However, the functional significance of the tau genetic association in PSP is unknown. We analysed the tau gene promoter sequence and identified two novel single nucleotide polymorphisms. Here we report the genetic association of a novel tau promoter haplotype with PSP which may influence tau transcription.

Neurodegenerative tauopathies

The defining neuropathological characteristics of Alzheimer's disease are abundant filamentous tau lesions and deposits of fibrillar amyloid beta peptides. Prominent filamentous tau inclusions and brain degeneration in the absence of beta-amyloid deposits are also hallmarks of neurodegenerative tauopathies exemplified by sporadic corticobasal degeneration, progressive supranuclear palsy, and Pick's disease, as well as by hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Because multiple tau gene mutations are pathogenic for FTDP-17 and tau polymorphisms appear to be genetic risk factors for sporadic progressive supranuclear palsy and corticobasal degeneration, tau abnormalities are linked directly to the etiology and pathogenesis of neurodegenerative disease. Indeed, emerging data support the hypothesis that different tau gene mutations are pathogenic because they impair tau functions, promote tau fibrillization, or perturb tau gene splicing, thereby leading to formation of biochemically and structurally distinct aggregates of tau. Nonetheless, different members of the same kindred often exhibit diverse FTDP-17 syndromes, which suggests that additional genetic or epigenetic factors influence the phenotypic manifestations of neurodegenerative tauopathies. Although these and other hypothetical mechanisms of neurodegenerative tauopathies remain to be tested and validated, transgenic models are increasingly available for this purpose, and they will accelerate discovery of more effective therapies for neurodegenerative tauopathies and related disorders, including Alzheimer's disease.

Frontotemporal dementia and tauopathy

The presence of abundant neurofibrillary lesions made of hyperphosphorylated tau proteins is the characteristic neuropathology of a subset of neurodegenerative disorders classified as "tauopathies." The discovery of mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) constitutes convincing evidence that tau proteins play a key role in the pathogenesis of neurodegenerative disorders. Moreover, it now is known that the most common form of sporadic frontotemporal dementia (FTD), which is characterized by frontotemporal neuron loss, gliosis, and microvacuolar change, also is a tauopathy caused by a loss of tau protein expression. Thus, these discoveries have begun to change the classification and the neuropathologic diagnosis of FTD and tauopathies, as well as current understanding of the disease mechanisms underlying them. Although transgenic mice expressing wild-type human tau or variants thereof with an FTDP-17 mutation result in tau pathologies and brain degeneration similar to that seen in human tauopathies, the precise mechanisms leading to the onset and progression of neurodegenerative disorders remain incompletely understood. Here, we review current understanding of human neurodegenerative tauopathies and prospects for translative recent insights about these into therapeutic interventions to prevent or ameliorate them.

Increased risk for frontotemporal dementia through interaction between tau polymorphisms and apolipoprotein E epsilon4

The tau gene has an important role in frontotemporal dementia (FTD) as pathogenic mutations have been found in hereditary forms of the disease. Furthermore, a certain extended tau haplotype has been shown to increase the risk for progressive supranuclear palsy, corticobasal degeneration, Parkinson's disease and, in interaction with the apolipoprotein E (apoE) epsilon4 allele, Alzheimer's disease. By microsatellite analysis we investigated an intronic tau polymorphism, in linkage disequilibrium with the extended tau haplotype, in FTD patients (n = 36) and healthy controls (n = 39). No association between any of the tau alleles/genotypes and FTD was seen, but certain tau alleles and apoE epsilon4 interactively increased the risk of FTD (p = 0.006). We thus propose that this extended tau haplotype in combination with apoE epsilon4 is a genetic risk factor for FTD.

Clinic-based cases with frontotemporal dementia show increased cerebrospinal fluid tau and high apolipoprotein E epsilon4 frequency, but no tau gene mutations

Frontotemporal dementia (FTD) belongs to a group of neurodegenerative disorders known as tauopathies, characterized by intracellular aggregation of hyperphosphorylated tau protein in the brain. Some tauopathies, like Alzheimer's disease (AD), consistently show increased levels of tau protein in cerebrospinal fluid (CSF). However, similar studies in FTD populations have shown variable results, although mutations in the tau gene are identified as causes of disease in certain FTD families. In the present study, a Swedish clinic-based FTD population was investigated with respect to CSF tau levels, apolipoprotein E (APOE) genotype distribution and occurrence of mutations in the tau gene. CSF tau levels were significantly increased among FTD patients (534 +/- 235 pg tau/ml, P < 0.001) (n = 47) compared to controls (316 +/- 137 pg tau/ml) (n = 51). Furthermore, a strong increase in the APOE epsilon4 allele frequency was found in the FTD population, as 52% were epsilon4 carriers, compared to 21% of the controls. However, no mutations in the tau gene were identified. These findings support the present notion of a common pathogenic pathway in the disease processes for several tauopathies, with both APOE epsilon4 and CSF tau being a pathological link between the different disorders. Furthermore, we conclude that mutations in the tau gene are a rare cause of FTD. .

Mitochondrial dysfunction and oxidative stress in aging and neurodegenerative disease

A major risk factor for neurodegenerative diseases such as Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) is aging. Two processes that have been implicated in aging are free radical-induced oxidative damage and mitochondrial dysfunction. A progressive impairment of mitochondrial function and/or increased oxidative damage has been suggested to play critical roles in the pathogenesis of these neurodegenerative diseases. For example, decreased complex I activity, increased oxidative damage and altered activities of antioxidant defense enzymes have been demonstrated in PD. In AD, decrements in complex IV activity and increased oxidative damage have been reported. Reductions in complex II activity, increased cortical lactate levels and oxidative damage have been described in HD. Some familial ALS cases are associated with mutations in the gene for Cu,Zn superoxide dismutase (SOD1) while increased oxidative damage is observed in sporadic ALS. Studies in PSP have demonstrated regionally specific reductions in brain and muscle mitochondrial function, hypofrontality and increased oxidative damage. Altogether, the age-dependent onset and progressive course of these neurodegenerative diseases may ultimately highlight an association between aging, mitochondrial impairment and oxidative stress.

New insights into genetic and molecular mechanisms of brain degeneration in tauopathies

Abundant neurofibrillary lesions consisting of the microtubule associated protein tau and amyloid beta peptide deposits are the defining lesions of Alzheimer's disease. Prominent filamentous tau pathology and brain degeneration in the absence of extracellular amyloid deposition characterize a number of other neurodegenerative disorders (i.e. progressive supranuclear palsy, corticobasal degeneration, Pick's disease) collectively referred to as tauopathies. The discovery of multiple tau gene mutations that are pathogenic for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 in many kindreds, as well as the demonstration that tau polymorphisms are genetic risk factors for sporadic tauopathies, directly implicate tau abnormalities in the onset/progression of neurodegenerative disease. Different tau gene mutations may be pathogenic by impairing the functions of tau or by perturbing the splicing of the tau gene, thereby resulting in biochemically and structurally distinct tau aggregates. However, since specific polymorphisms and mutations in the tau gene lead to diverse phenotypes, it is plausible that additional genetic or epigenetic factors influence the clinical and pathological manifestations of both familial and sporadic tauopathies. Thus, efforts to develop animal models of tau-mediated neurodegeneration should provide further insights into the onset and progression of tauopathies as well as Alzheimer's disease, and they could accelerate research to discover more effective therapies for these disorders.

Mitochondrial dysfunction in cybrid lines expressing mitochondrial genes from patients with progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative movement disorder of unknown etiology. We hypothesized that mitochondrial DNA (mtDNA) aberration could occur in this disease and contribute to its pathogenesis. To address this we created transmitochondrial cytoplasmic hybrid (cybrid) cell lines expressing mitochondrial genes from persons with PSP. The presence of cybrid mtDNA aberration was screened for by biochemical assay of mitochondrial gene products. Relative to a control cybrid set, complex I activity was reduced in PSP cybrid lines (p<0.005). Antioxidant enzyme activities were elevated in PSP cybrid lines. These data suggest that mtDNA aberration occurs in PSP, causes electron transport chain pathology, and can produce oxidative stress. Further study of mitochondrial dysfunction in PSP may yield insights into why neurodegeneration occurs in this disease.

Tau pathology: a marker of neurodegenerative disorders

Tau is not only a basic component of neurofibrillary degeneration, but is also an aetiological factor, as demonstrated by mutations on the tau gene responsible for frontotemporal dementias with parkinsonism linked to chromosome 17. Polymorphisms on the tau gene and the hierarchical invasion of neocortical areas by tau pathology in numerous sporadic neurodegenerative diseases also suggest that tau pathology is a primary pathogenic event in non-familial dementing diseases and a lead for solid diagnostic and therapeutic approaches.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.