The L266V tau mutation is associated with frontotemporal dementia and Pick-like 3R and 4R tauopathy

We report a case of rapidly progressive frontotemporal dementia presenting at age 33 years. At autopsy there was severe atrophy of the frontal and temporal lobes. Tau-positive Pick bodies, which ultrastructurally were composed of straight filaments, were present, accompanied by severe neuronal loss and gliosis. RD3, a tau antibody specific for the three-repeat (3R) isoforms, labeled the Pick bodies. ET3, a four-repeat (4R) isoform-specific tau antibody, did not label Pick bodies, but highlighted rare astrocytes, and threads in white matter bundles in the corpus striatum. Analysis of the tau gene revealed an L266V mutation in exon 9. Analysis of brain tissue from this case revealed elevated levels of exon 10+ tau RNA and soluble 4R tau. However, both 3R and 4R isoforms were present in sarkosyl-insoluble tau fractions with a predominance of the shortest 3R isoform. The L266V mutation is associated with decreased rate and extent of tau-induced microtubule assembly, and a 3R isoform-specific increase in tau self assembly as measured by an in vitro assay. Combined, these data indicate that L266V is a pathogenic tau mutation that is associated with Pick-like pathology. In addition, the results of the RD3 and ET3 immunostains clearly explain for the first time the presence of both 3R and 4R tau isoforms in preparations of insoluble tau from some Pick's disease cases.     

A novel L266V mutation of the tau gene causes frontotemporal dementia with a unique tau pathology

We report a novel mutation of tau (L266V missense mutation in exon 9) which may cause a type of familial frontotemporal dementia. The brain of a patient showed Pick body-like inclusions and unique tau-positive, argyrophilic astrocytes with stout filaments and naked, round, or irregular argyrophilic inclusions with deposits of both three-repeat and four-repeat tau. Recombinant tau with a L266V mutation showed a reduced ability to promote microtubule assembly, which may be the primary effect of the mutation.     

An R5L tau mutation in a subject with a progressive supranuclear palsy phenotype

MAPT, the gene encoding tau, was screened for mutations in 96 progressive supranuclear palsy subjects. A point mutation (R5L) was identified in a single progressive supranuclear palsy subject that was not in the other progressive supranuclear palsy subjects or in 96 controls. Functionally, this mutation alters the ability of tau to promote microtubule assembly. Analysis of soluble tau from different brain regions indicates that the mutation does not affect the ratio of tau isoforms synthesized. Aggregated insoluble tau from subcortical regions was predominantly four-repeat tau with no or one amino terminal insert (0N4R and 1N4R). Insoluble tau from cortical regions also contained 1N3R tau. Thus, the R5L mutation causes a progressive supranuclear palsy phenotype, presumably by a gain-of-function mechanism.     

Overexpression of four-repeat tau mRNA isoforms in progressive supranuclear palsy but not in Alzheimer's disease.

Perturbations in the microtubule-associated protein tau occur in several human neurodegenerative diseases. In Alzheimer's disease and progressive supranuclear palsy (PSP), tau proteins assemble into straight and paired helical filaments that form intraneuronal deposits of neurofibrillary tangles (NFTs). The mechanisms underlying the aberrant assembly of tau into NFTs is unknown. To determine whether alterations in the expression of the carboxyl-terminal variants of tau contribute to NFT formation, we analyzed tau mRNA isoform expression in select regions of control, Alzheimer's disease, and PSP brains. In Alzheimer's disease, there were no alterations in tau mRNA isoform expression. However, in PSP, the levels of tau mRNA isoforms containing four microtubule binding domains were increased in the brainstem but not the frontal cortex or cerebellum. The brainstem in PSP has extensive NFT pathology, whereas the frontal cortex and cerebellum are relatively spared, suggesting that alterations in tau mRNA isoform expression occur in NFT-vulnerable regions in this disease. An increase in the four-repeat tau mRNA may lead to an increase in four-repeat tau protein isoforms and may contribute to the formation of NFTs in PSP. A similar increase in four-repeat tau mRNA has been reported for mutations associated with frontotemporal dementia and parkinsonism linked to chromosome 17.     

Pick's disease associated with the novel Tau gene mutation K369I

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. We describe a K369I missense mutation in exon 12 of Tau in a patient with a pathology typical of sporadic Pick's disease. The proband presented with severe personality changes, followed by loss of cognitive function. Detailed postmortem examination of the brain showed atrophy, which was most pronounced in the temporal lobes; and numerous tau-immunoreactive Pick bodies and Pick cells in the neocortex and the hippocampal formation, as well as in subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. However, immunoblot analysis of sarkosyl-insoluble tau showed three major bands of 60, 64, and 68 kDa, consistent with the presence of 3- and 4-repeat tau isoforms, as in Alzheimer's disease. Isolated tau filaments were irregularly twisted ribbons, with a small number of Alzheimer-type paired helical filaments. In the presence of heparin, tau proteins with the K369I mutation formed short, slender filaments. Biochemically, recombinant tau proteins with the K369I mutation showed reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation by providing a pool of aberrant tau for filament assembly. Taken together, results indicate that the K369I mutation in Tau can cause a dementing disease with a neuropathology like that of Pick's disease.     

An immunohistochemical study of cases of sporadic and inherited frontotemporal lobar degeneration using 3R- and 4R-specific tau monoclonal antibodies

The pathological distinctions between the various clinical and pathological manifestations of frontotemporal lobar degeneration (FTLD) remain unclear. Using monoclonal antibodies specific for 3- and 4-repeat isoforms of the microtubule associated protein, tau (3R- and 4R-tau), we have performed an immunohistochemical study of the tau pathology present in 14 cases of sporadic forms of FTLD, 12 cases with Pick bodies and two cases without and in 27 cases of familial FTLD associated with 12 different mutations in the tau gene (MAPT), five cases with Pick bodies and 22 cases without. In all 12 cases of sporadic FTLD where Pick bodies were present, these contained only 3R-tau isoforms. Clinically, ten of these cases had frontotemporal dementia and two had progressive apraxia. Only 3R-tau isoforms were present in Pick bodies in those patients with familial FTLD associated with L266V, Q336R, E342V, K369I or G389R MAPT mutations. Patients with familial FTLD associated with exon 10 N279K, N296H or +16 splice site mutations showed tau pathology characterised by neuronal neurofibrillary tangles (NFT) and glial cell tangles that contained only 4R-tau isoforms, as did the NFT in P301L MAPT mutation. With the R406W mutation, NFT contained both 3R- and 4R-tau isoforms. We also observed two patients with sporadic FTLD, but without Pick bodies, in whom the tau pathology comprised only of 4R-tau isoforms. We have therefore shown by immunohistochemistry that different specific tau isoform compositions underlie the various kinds of tau pathology present in sporadic and familial FTLD. The use of such tau isoform specific antibodies may refine pathological criteria underpinning FTLD.     

Variable phenotypic expression and extensive tau pathology in two families with the novel tau mutation L315R

Mutations in the tau gene cause familial frontotemporal dementia and parkinsonism linked to chromosome 17. Here, we describe two Dutch families with familial frontotemporal dementia associated with the novel missense mutation L315R in exon 11 of tau. The age at onset of disease showed a large variation within each family, ranging from 25 to 64 years. Incomplete penetrance was established in an 82-year-old mutation carrier with no signs of dementia and appeared probable in two additional subjects. The brains of two affected subjects were studied and showed extensive tau pathology in neurons (Pick-like inclusions) and astroglial cells, particularly in the frontotemporal cortex and the hippocampal formation. Sarkosyl-insoluble tau extracted from the cerebral cortex showed the presence of straight and twisted tau filaments and a pattern of pathological tau bands similar to that of Pick's disease. Upon dephosphorylation, only five of the six brain tau isoforms were observed, with the shortest isoform being undetectable. All six tau isoforms were present in soluble brain tau. Recombinant tau proteins with the L315R mutation showed a reduced ability to promote microtubule assembly.     

Novel G335V mutation in the tau gene associated with early onset familial frontotemporal dementia

Mutations in the tau gene cause familial frontotemporal dementia and parkinsonism linked to chromosome 17. Here we describe a novel missense mutation in exon 12 of the tau gene, G335V, in a German family with frontotemporal dementia of early age at onset, in the third decade of life. Functional analysis of recombinant tau protein with the G335V mutation showed a dramatically reduced ability to promote microtubule assembly and a more rapid and accelerated tau filament formation, suggesting that the primary effect of the mutation might be the provision of a pool of unbound tau making it available for aberrant tau aggregation.M. Neumann and S. Diekmann contributed equally to this work.     

Tau-based treatment strategies in neurodegenerative diseases

Neurofibrillary tangles are a characteristic hallmark of Alzheimer’s and other neurodegenerative diseases, such as Pick’s disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). These diseases are summarized as tauopathies, because neurofibrillary tangles are composed of intracellular aggregates of the microtubule-associated protein tau. The molecular mechanisms of tau-mediated neurotoxicity are not well understood; however, pathologic hyperphosphorylation and aggregation of tau play a central role in neurodegeneration and neuronal dysfunction. The present review, therefore, focuses on therapeutic approaches that aim to inhibit tau phosphorylation and aggregation or to dissolve preexisting tau aggregates. Further experimental therapy strategies include the enhancement of tau clearance by activation of proteolytic, proteasomal, or autophagosomal degradation pathways or anti-tau directed immunotherapy. Hyperphosphorylated tau does not bind microtubules, leading to microtubule instability and transport impairment. Pharmacological stabilization of microtubule networks might counteract this effect. In several tauopathies there is a shift toward four-repeat tau isoforms, and interference with the splicing machinery to decrease four-repeat splicing might be another therapeutic option.     

Familial frontotemporal dementia and parkinsonism with a novel N296H mutation in exon 10 of the tau gene and a widespread tau accumulation in the glial cells

We report a 62-year-old Japanese man with familial frontotemporal dementia and a novel missense mutation (N296H) in exon 10 of the tau gene. The patient presented with frontal signs followed by temporal signs and parkinsonism. The brain showed localized frontotemporal lobe atrophy including the precentral gyrus and discoloration of the substantia nigra, and revealed severe neuronal loss with proliferation of tau-positive protoplasmic astroglia in the affected cerebral cortex, tau-positive coiled bodies and threads in the subcortical white matter, and tau-positive pretangle neurons in the subcortical and brain stem nuclei. There were no tau-positive neurofibrillary tangles, Pick bodies, tuft-shaped astrocytes or astrocytic plaques in the cerebral cortex. Immunoelectron microscopically, phosphorylated tau accumulated in both neurons and glial cells in different modalities, such as glial filaments in protoplasmic astroglia, straight tubules in coiled bodies, and free ribosomes in pretangle neurons. These findings suggest that tau proteins are not always assembled in abnormal filaments such as twisted ribbons, paired helical filaments and straight tubules in neurons and glial cells, which have been shown in previous cases with frontotemporal dementia and parkinsonism linked to chromosome 17. Immunoblotting of sarkosyl-insoluble tau exhibited accumulation of four-repeat tau isoforms in the brain. The N296H mutation may interfere with the ability of mutated tau to bind with microtubules and lead to tau aggregation. Further study is necessary to determine whether this mutation can account for the characteristic tau pathology of this case.     

Tau gene mutation G389R causes a tauopathy with abundant pick body-like inclusions and axonal deposits

Exonic and intronic mutations in Tau cause familial neurodegenerative syndromes characterized by frontotemporal dementia and dysfunction of multiple cortical and subcortical circuits. Here we describe a G389R mutation in exon 13 of Tau. When 38 years old, the proband presented with progressive aphasia and memory disturbance, followed by apathy, indifference, and hyperphagia. Repeated magnetic resonance imaging showed the dramatic progression of cerebral atrophy. Positron emission tomography revealed marked glucose hypometabolism that was most severe in left frontal, temporal, and parietal cortical regions. Rigidity, pyramidal signs and profound dementia progressed until death at 43 years of age. A paternal uncle, who had died at 43 years of age, had presented with similar symptoms. The proband's brain showed numerous tau-immunoreactive Pick body-like inclusions in the neocortex and the fascia dentata of the hippocampus. In addition, large numbers of tau-positive filamentous inclusions were present in axons in the frontal, temporal, and parietal lobes. Immunoblot analysis of sarkosyl-insoluble tau showed 2 major bands of 60 and 64 kDa. Upon dephosphorylation, these bands resolved into 4 bands consisting of three- and four-repeat tau isoforms. Most isolated tau filaments were straight and resembled filaments found in Alzheimer disease and some frontotemporal dementias with tau mutations. A smaller number of twisted filaments was also observed. Biochemically, recombinant tau proteins with the G389R mutation showed a reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation. Taken together, the present findings indicate that the G389R mutation in Tau can cause a dementing condition that closely resembles Pick's disease.     

Familial early onset frontotemporal dementia caused by a novel S356T MAPT mutation,initially diagnosed as schizophrenia

Autosomal dominant frontotemporal dementia (FTD) due to mutations in the MAPT gene is referred to as FTD with parkinsonism linked to chromosome 17 with tau pathology (FTDP-17T). Typically the disease begins in the sixth decade of life. We report a novel exon 12 mutation in MAPT (S356T), in a family with an exceptionally early age at onset (27 and 29 years), causing familial behavioural variant frontotemporal dementia. Both the proband and the proband's father were initially diagnosed as having schizophrenia. Pathological examination showed frontotemporal lobar degeneration with extensive neuronal and glial tau deposition. This mutation is one of a small group of MAPT mutations (including P301S, G335V and S352L) that cause very early onset FTDP-17T. It is likely that the early age at onset reflects a marked pathogenic effect of the mutation involving a disturbance of microtubule binding, tau phosphorylation or a major acceleration of tau aggregation.     

Missense tau mutations identified in FTDP-17 have a small effect on tau-microtubule interactions

Frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) is a group of related disorders frequently characterized by the formation of tau inclusions in neurons and glial cells. To determine whether the formation of tau inclusions in FTDP-17 results from an alteration in the ability of mutant tau to maintain the microtubule (MT) system, we compared wild type four-repeat tau with three FTDP-17 mutants (P301L, V337M and R406W) for their ability to bind MT, promote MT assembly and bundling. According to in vitro binding and assembly assays, P301L is the only mutant that demonstrates a small, yet significant reduction, in its affinity for MT while both P301L and R406W have a small reduction in their ability to promote tubulin assembly. Based on studies of neuroblastoma and CHO cells transfected with GFP-tagged tau DNA constructs, both mutant and wild type tau transfectants were indistinguishable in the distribution pattern of tau in terms of co-localization with MT and generation of MT bundles. These results suggest that missense mutation of tau gene do not have an immediate impact on the integrity of MT system, and that exposure of affected neurons to additional insults or factors (e.g., aging) may be needed to initiate the formation of tau inclusions in FTDP-17.     

Frontotemporal dementia and corticobasal degeneration in a family with a P301S mutation in tau.

The tau gene has been found to be the locus of dementia with rigidity linked to chromosome 17. Exonic and intronic mutations have been described in a number of families. Here we describe a P301S mutation in exon 10 of the tau gene in a new family. Two members of this family were affected. One individual presented with frontotemporal dementia, whereas his son has corticobasal degeneration, demonstrating that the same primary gene defect in tau can lead to 2 distinct clinical phenotypes. Both individuals developed rapidly progressive disease in the third decade. Neuropathologically, the father presented with an extensive filamentous pathology made of hyperphosphorylated tau protein. Biochemically, recombinant tau protein with the P301S mutation showed a greatly reduced ability to promote microtubule assembly.     

Tau gene mutation K257T causes a tauopathy similar to Pick's disease

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. Here we describe a K257T missense mutation in exon 9 of Tau. The proband, a 47-yr-old male, presented with severe personality changes followed by semantic memory loss. A diagnosis of Pick's disease was made. The symptoms progressed until death at age 51. The proband's brain showed a marked frontotemporal atrophy that was most pronounced in the temporal lobes. Numerous tau-immunoreactive Pick bodies were present in the neocortex and the hippocampal formation, as well as in some subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. Diffuse staining for hyperphosphorylated tau was also observed in some nerve cell bodies. Immunoblot analysis of sarkosyl-insoluble tau showed 2 major bands of 60 and 64 kDa and 2 very minor bands of 68 and 72 kDa. Upon dephosphorylation, these bands resolved into 6 bands consisting of 3-repeat and 4-repeat tau isoforms, with an overall preponderance of 3-repeat tau. Isolated tau filaments were narrow, irregularly twisted ribbons. Biochemically, recombinant tau proteins with the K257T mutation showed a reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation. In addition, the K257T mutation was found to stimulate heparin-induced assembly of 3-repeat tau into filaments. Taken together, the present findings indicate that the K257T mutation in Tau can cause a dementing condition similar to Pick's disease.     

A novel tau mutation (N296N) in familial dementia with swollen achromatic neurons and corticobasal inclusion bodies

Familial dementia with swollen achromatic neurons and corticobasal inclusion bodies is a neurodegenerative disease that resembles corticobasal degeneration. It is characterized by the presence of abundant neuronal and glial tau protein deposits. Here we describe a novel silent mutation in exon 10 of tau (N296N) in this familial dementia. By exon trapping, the mutation produced an increase in the splicing in of exon 10, indicating that it probably causes disease through an overproduction of four-repeat tau.     

Increased expression of three-repeat isoforms of tau contributes to tau pathology in a rat model of chronic type 2 diabetes

The imbalance between three-repeat (3R) and four-repeat (4R) tau isoforms produced by the alternative splicing of tau exon 10 leads to neuronal instability and eventual neurodegeneration in tauopathy. However, the role of altered 3R/4R tau ratio in Alzheimer's disease (AD) remains controversial. It has been shown that the expression of 3R tau is modulated by peptide amyloid β (Aβ) and that 3R tau levels increase with the progression of AD. The incidence of AD increases in patients with type 2 diabetes mellitus (T2DM), and the comorbidity of these disorders is closely associated with both aging and disease duration. To investigate whether changes in 3R and 4R tau isoforms are involved in AD pathology pertaining to age-related T2DM, the expression of tau isoforms and their relationship with AD-like tau pathology were examined in a spontaneous T2DM model using aged Otsuka Long-Evans Tokushima Fatty (OLETF) rats with obesity. An AD-like pathology consisting of increased aggregates in the neuronal cytoplasm and a loss of synaptic proteins was observed in these rats. The aggregates were reactive with a 3R tau-specific, but not 4R tau-specific, antibody. In contrast to 4R tau, the level of 3R tau profoundly increased and the proteins were prone to taking toxic phosphorylated and truncated forms. Taken together, these findings suggest that increased 3R tau may contribute to AD-like tau pathology in a chronic T2DM model. Thus, the restoration of normal 3R tau expression should be considered as an important therapeutic strategy in the treatment of AD.     

Differences in the abilities of human tau isoforms to promote microtubule assembly.

Three isoforms of human tau protein were compared for their abilities to induce microtubule assembly. The three isoforms, tau 3 (tau containing three microtubule-binding domains), tau 4 (tau containing four microtubule-binding domains) and tau 4L (tau containing four microtubule binding domains plus a 58-amino-acid insert near the N-terminus) were expressed in E. coli and purified using ammonium sulfate precipitation, ion exchange, and size exclusion chromatography. All three isoforms induced microtubule assembly at micromolar concentrations and showed similar critical concentrations for assembly of 0.4-0.45 microM. However, tau 4 induced microtubule formation at a rate five- to tenfold faster than either tau 3 or tau 4L. The rate of microtubule elongation seen with tau 4 was twofold greater than with tau 3 or tau 4L, suggesting that the faster rate of microtubule assembly seen with tau 4 was due, at least in part, to faster elongation. Tau 4 induced a greater number of microtubules to form at steady state than did tau 3 or tau 4L. The microtubules generated with each tau isoform had similar steady-state length distributions and were equally susceptible to cold-induced disassembly. These results indicate that the additional microtubule-binding domain in tau 4 enhances microtubule assembly, while the 58-amino-acid insert negates the stimulatory effect of the fourth microtubule-binding domain.     

Frontotemporal dementia: tau mutations, deposition, and molecular mechanisms of neuronal cell death]

Exonic and intronic tau mutations have been described in a number of families of frontotemporal dementia and parkinsonism linked to chromosome 17. Most of missense mutations alter the ability of tau to promote microtubule assembly, whereas others influence splicing of exon10 and change the ratio of 3Rtau to 4Rtau isoform. In either case, filamentous hyperphosphorylated tau pathology in neurons and glial cells was observed in affected brains. These observations suggest that the effects of tau mutations may induce its hyperphosphorylation and accumulation, resulting in cell death. In sporadic tauopathies, decreased levels of 3R tau mRNA were detected not only in severely affected cases with progressive supranuclear palsy or corticobasal degeneration but also in cases with Alzheimer's disease or Pick's disease. In addition, levels of 3R tau mRNA were closely correlated with levels of neurofilament mRNA. These results suggest that decreased levels of 3R tau mRNA in sporadic tauopathies may be due to degeneration and loss of neurons that express 3R tau isoforms. In tauopathies, neuronal cell death may occur with multiple defects or abnormalities arisen directly or indirectly from hyperphosphorylation of tau and formation of oligomer or filamentous tau.