Specific tau variants in the brains of patients with myotonic dystrophy

Myotonic dystrophy, when simple repeats reveal complex pathogenic entities: new findings and future challenges

Expanded, non-coding RNAs can exhibit a deleterious gain-of-function causing human disease through abnormal interactions with RNA-binding proteins. Myotonic dystrophy (DM), the prototypical example of an RNA-dominant disorder, is mediated by trinucleotide repeat-containing transcripts that deregulate alternative splicing. Spliceopathy has therefore been a major focus of DM research. However, changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. The exciting finding of bidirectional transcription and non-conventional RNA translation of trinucleotide repeat sequences points to a new scenario, in which DM is not mediated by one single expanded RNA transcript, but involves multiple pathogenic elements and pathways. The study of the growing number of human diseases associated with toxic repeat-containing transcripts provides important insight into the understanding of the complex pathways of RNA toxicity. This review describes some of the recent advances in the understanding of the molecular mechanisms behind DM and other RNA-dominant disorders.

Cognition and event-related potentials in adult-onset non-demented myotonic dystrophy type 1

OBJECTIVE

To clarify the cognitive and event-related potentials (ERPs) profiles of adult-onset genetically-proven non-demented myotonic dystrophy type 1 (DM1).

METHODS

Fourteen DM1 patients and matched 14 normal controls were enrolled. DM1 patients were compared with normal controls, using a variety of neuropsychological tests; an auditory "oddball" counting paradigm for the ERPs, and low-resolution brain electromagnetic tomography (LORETA). For patients, ERPs and neuropsychological parameters were correlated with CTG repeat size, duration of illness, grip strength, and arterial blood gas analysis.

RESULTS

Frontal lobe dysfunction, prolonged N1 latency, and attenuated N2/P3 amplitudes were observed in DM1. Longer CTG repeat size was associated with fewer categories achieved on Wisconsin Card Sorting Test. Greater grip strength was associated with better scores on color-word "interference" of Stroop test. P3 latency was negatively correlated with PaO(2). LORETA revealed significant hypoactivities at the orbitofrontal and medial temporal lobe, cingulate, and insula. There was no correlation between ERPs and CTG expansion.

CONCLUSIONS

Adult-onset non-demented DM1 presented frontal lobe dysfunction. Absence of correlations between CTG repeat size and objective ERP parameters suggested CTG expansion in lymphocytes does not directly contribute to cognitive dysfunction.

SIGNIFICANCE

CTG expansion in lymphocytes does not directly contribute to cognitive dysfunction of adult-onset non-demented DM1.

Mis-splicing of Tau exon 10 in myotonic dystrophy type 1 is reproduced by overexpression of CELF2 but not by MBNL1 silencing

Tau is the proteinaceous component of intraneuronal aggregates common to neurodegenerative diseases called Tauopathies, including myotonic dystrophy type 1. In myotonic dystrophy type 1, the presence of microtubule-associated protein Tau aggregates is associated with a mis-splicing of Tau. A toxic gain-of-function at the ribonucleic acid level is a major etiological factor responsible for the mis-splicing of several transcripts in myotonic dystrophy type 1. These are probably the consequence of a loss of muscleblind-like 1 (MBNL1) function or gain of CUGBP1 and ETR3-like factor 1 (CELF1) splicing function. Whether these two dysfunctions occur together or separately and whether all mis-splicing events in myotonic dystrophy type 1 brain result from one or both of these dysfunctions remains unknown. Here, we analyzed the splicing of Tau exons 2 and 10 in the brain of myotonic dystrophy type 1 patients. Two myotonic dystrophy type 1 patients showed a mis-splicing of exon 10 whereas exon 2-inclusion was reduced in all myotonic dystrophy type 1 patients. In order to determine the potential factors responsible for exon 10 mis-splicing, we studied the effect of the splicing factors muscleblind-like 1 (MBNL1), CUGBP1 and ETR3-like factor 1 (CELF1), CUGBP1 and ETR3-like factor 2 (CELF2), and CUGBP1 and ETR3-like factor 4 (CELF4) or a dominant-negative CUGBP1 and ETR-3 like factor (CELF) factor on Tau exon 10 splicing by ectopic expression or siRNA. Interestingly, the inclusion of Tau exon 10 is reduced by CUGBP1 and ETR3-like factor 2 (CELF2) whereas it is insensitive to the loss-of-function of muscleblind-like 1 (MBNL1), CUGBP1 and ETR3-like factor 1 (CELF1) gain-of-function, or a dominant-negative of CUGBP1 and ETR-3 like factor (CELF) factor. Moreover, we observed an increased expression of CUGBP1 and ETR3-like factor 2 (CELF2) only in the brain of myotonic dystrophy type 1 patients with a mis-splicing of exon 10. Taken together, our results indicate the occurrence of a mis-splicing event in myotonic dystrophy type 1 that is induced neither by a loss of muscleblind-like 1 (MBNL1) function nor by a gain of CUGBP1 and ETR3-like factor 1 (CELF1) function but is rather associated to CUGBP1 and ETR3-like factor 2 (CELF2) gain-of-function.

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

Neuropathology does not Correlate with Regional Differences in the Extent of Expansion of CTG Repeats in the Brain with Myotonic Dystrophy Type 1

Myotonic dystrophy (DM1) is known to be an adult-onset muscular dystrophy caused by the expansion of CTG repeats within the 3' untranslated region of the dystrophin myotonin protein kinase (DMPK) gene. The clinical features of DM1 include CNS symptoms, such as cognitive impairment and personality changes, the pathogenesis of which remains to be elucidated. We hypothesized that the distribution of neuropathological changes might be correlated with the extent of the length of the CTG repeats in the DMPK genes in DM1 patients. We studied the neuropathological changes in the brains of subjects with DM1 and investigated the extent of somatic instability in terms of CTG repeat expansion in the different brain regions of the same individuals by Southern blot analysis. The neuropathological changes included état criblé in the cerebral deep white matter and neurofibrillary tangles immunoreactive for phosphorylated tau in the hippocampus and entorhinal cortex, both of which were compatible with the subcortical dementia in DM1 patients. However, the length of the CTG repeats did not correlate with the regional differences in the extent of neuropathological changes. Our data suggested that pathomechanisms of dementia in DM1 might be more multifactorial rather than a toxic gain-of-function due to mutant RNA.

White matter abnormalities and neurocognitive correlates in children and adolescents with myotonic dystrophy type 1: a diffusion tensor imaging study

Diffusion tensor imaging was used to evaluate cerebral white matter in eight patients (ages 10-17), with myotonic dystrophy type 1 (3 congenital-onset, 5 juvenile-onset) compared to eight controls matched for age and sex. Four regions of interest were examined: inferior frontal, superior frontal, supracallosal, and occipital. The myotonic dystrophy group showed white matter abnormalities compared to controls in all regions. All indices of white matter integrity were abnormal: fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity. With no evidence of regional variation, correlations between whole cerebrum white matter fractional anisotropy and neurocognitive functioning were examined in the patients. Strong correlations were observed between whole cerebrum fractional anisotropy and full-scale intelligence and a measure of executive functioning. Results indicate that significant white matter abnormality is characteristic of young patients with myotonic dystrophy type 1 and that the white matter abnormality seen with neuroimaging has implications for cognitive functioning.

[Molecular actors in Alzheimer's disease: which diagnostic and therapeutic consequences?]

Alzheimer's disease is a neurodegenerative disorder characterized by neuropathological lesions: amyloid deposits and neurofibrillary degeneration. However, the links between these two brain hallmarks are still poorly understood. Until now, mainly amyloid pathology has been targeted un many clinical trials without any success. Both new therapeutic strategies and diagnosis improvement are needed.

From tau phosphorylation to tau aggregation: what about neuronal death?

Tau pathology is characterized by intracellular aggregates of abnormally and hyperphosphorylated tau proteins. It is encountered in many neurodegenerative disorders, but also in aging. These neurodegenerative disorders are referred to as tauopathies. Comparative biochemistry of the tau aggregates shows that they differ in both tau isoform phosphorylation and content, which enables a molecular classification of tauopathies. In conditions of dementia, NFD (neurofibrillary degeneration) severity is correlated to cognitive impairment and is often considered as neuronal death. Using tau animal models, analysis of the kinetics of tau phosphorylation, aggregation and neuronal death in parallel to electrophysiological and behavioural parameters indicates a disconnection between cognition deficits and neuronal cell death. Tau phosphorylation and aggregation are early events followed by cognitive impairment. Neuronal death is not observed before the oldest ages. A sequence of events may be the formation of toxic phosphorylated tau species, their aggregation, the formation of neurofibrillary tangles (from pre-tangles to ghost tangles) and finally neuronal cell death. This sequence will last from 15 to 25 years and one can ask whether the aggregation of toxic phosphorylated tau species is a protection against cell death. Apoptosis takes 24 h, but NFD lasts for 24 years to finally kill the neuron or rather to protect it for more than 20 years. Altogether, these data suggest that NFD is a transient state before neuronal death and that therapeutic interventions are possible at that stage.

The role of CELF proteins in neurological disorders

CELF (CUG-BP and ETR-3-like factors) proteins are structurally related RNA-binding proteins involved in various aspects of RNA processing including splicing and mRNA stability. The first member of the family, CELF1/CUG-BP1, was identified through its role in myotonic dystrophy, type 1. Several recent studies have uncovered the recurrent implication, to various extents, of CELF proteins or of the functionally related muscleblind-like 1 protein in a number of neurological conditions. This is particularly clear for inherited neurodegenerative disorders caused by expansions of translated or untranslated triplet repeats in the causative gene. Here we review the role played by CELF proteins, at least as modifiers of the pathological phenotype, in a number of neurological diseases. The involvement of CELF proteins suggest that individual pathogenic pathways in a number of neurological conditions overlap at the level of RNA processing.

Tau as a biomarker of neurodegenerative diseases

The microtubule-associated protein Tau is mainly expressed in neurons of the CNS and is crucial in axonal maintenance and axonal transport. The rationale for Tau as a biomarker of neurodegenerative diseases is that it is a major component of abnormal intraneuronal aggregates observed in numerous tauopathies, including Alzheimer's disease. The molecular diversity of Tau is very useful when analyzing it in the brain or in the peripheral fluids. Immunohistochemical and biochemical characterization of Tau aggregates in the brain allows the postmortem classification and differential diagnosis of tauopathies. As peripheral biomarkers of Alzheimer's disease in the cerebrospinal fluid, Tau proteins are now validated for diagnosis and predictive purposes. For the future, the detailed characterization of Tau in the brain and in peripheral fluids will lead to novel promising biomarkers for differential diagnosis of dementia and monitoring of therapeutics.

Biochemistry of Tau in Alzheimer's disease and related neurological disorders

Microtubule-associated Tau proteins belong to a family of factors that polymerize tubulin dimers and stabilize microtubules. Tau is strongly expressed in neurons, localized in the axon and is essential for neuronal plasticity and network. From the very beginning of Tau discovery, proteomics methods have been essential to the knowledge of Tau biochemistry and biology. In this review, we have summarized the main contributions of several proteomic methods in the understanding of Tau, including expression, post-translational modifications and structure, in both physiological and pathophysiological aspects. Finally, recent advances in proteomics technology are essential to develop further therapeutic targets and early predictive and discriminative diagnostic assays for Alzheimer's disease and related disorders.

Myotonic dystrophy type I in childhood Long-term evolution in patients surviving the neonatal period

In a retrospective study, 32 patients with myotonic dystrophy, including congenital (n=17) and infantile/juvenile forms (n=15) were studied during a long follow-up lasting 7-28 years (median: 17 years). The clinical presentation was extremely variable; however, a continuum did exist between severe and less severe congenital forms, and later-onset forms, without genotype-phenotype correlation. We observed some unusual presentations, such as 3 cases of isolated club-feet during the neonatal period, and 7 patients (23%) with a completely isolated mental deficiency, language delay and school failure, who only completed the clinical picture several years later. Wechsler scale testing was performed in all cases, and repeated with 8 patients. It demonstrated a decrease in intellectual abilities in 5 patients, suggesting the possibility of a degenerative cerebral process occurring in these children. This decrease has also been reported in some adult cases. This study illustrates the extremely heterogeneous clinical presentation of myotonic dystrophy in childhood.

Myotonic dystrophy 1 in the nervous system: from the clinic to molecular mechanisms

Myotonic dystrophy type 1 (DM1) is a dominant neuromuscular disorder caused by the expansion of trinucleotide CTG repeats in the 3'-untranslated region (3'-UTR) of the DMPK gene. Prominent features of classical DM1 are muscle wasting and myotonia, whereas mental retardation is distinctive for congenital DM1. The main nervous system symptoms of DM1 are cognitive impairment, neuroendocrine dysfunction, and personality and behavior abnormalities. It is thought that expansion of CTG repeats causes DM1 pathology through different molecular mechanisms; however, a growing body of evidence indicates that an RNA gain-of-function mechanism plays a major role in the disease development. At the skeletal muscle level, three main molecular events can be distinguished in this model: 1) formation of nuclear foci that are composed at least of mutant DMPK mRNA and recruited RNA-binding proteins, such as splicing regulators and transcription factors; 2) disturbance of alternative splicing of specific genes; and 3) impairment of cell differentiation. Contrasting with the substantial advances in understanding DM1 muscle pathology, the molecular basis of DM1 in the nervous system has just started to be revealed. This review focuses in the DM1 nervous system pathology and provides an overview of the genetic and molecular studies analyzing the effects of the DMPK gene CUG expanded repeats on cell function in neuronal systems. A comparison between the molecular mechanisms of DM1 in the skeletal muscle and those identified in DM1 nervous system models is provided. Finally, future directions in the study of DM1 in the nervous system are discussed.

Overexpression of MBNL1 fetal isoforms and modified splicing of Tau in the DM1 brain: two individual consequences of CUG trinucleotide repeats

Neurofibrillary degeneration is often observed in the brain of patients with type 1 myotonic dystrophy (DM1). It consists principally of the aggregation of Tau isoforms that lack exon 2/3 encoded sequences, and is the consequence of the modified splicing of Tau pre-mRNA. In experimental models of DM1, the splicing of several transcripts is modified due to the loss of Muscleblind-like 1 (MBNL1) function. In the present study, we demonstrate that the MBNL1 protein is also present in the human brain, and consists of several isoforms, as shown by RT-PCR and sequencing. In comparison with controls, we show that the adult DM1 brain exhibits modifications in the splicing of MBNL1, with the preferential expression of long MBNL1 isoforms--a splicing pattern similar to that seen in the fetal human brain. In cultured HeLa cells, the presence of long CUG repeats, such as those found in the DM1 mutation, leads to similar changes in the splicing pattern of MBNL1, and the localization of MBNL1 in nuclear RNA foci. Long CUG repeats also reproduce the repression of Tau exon 2/3 inclusion, as in the human disease, suggesting that their effect on MBNL1 expression may lead to changes in Tau splicing. However, while an overall reduction in the expression of MBNL1 mimics the effect of the DM1 mutation, none of the MBNL1 isoforms tested so far modulates the endogenous splicing of Tau. The modified splicing of Tau thus results from a possibly CUG-mediated loss of function of MBNL1, but not from changes in the MBNL1 expression pattern.

Cloning and embryonic expression patterns of the chicken CELF family

The CUG-BP and ETR-3-like factor (CELF) protein family has been implicated in the regulation of pre-mRNA alternative splicing, mRNA stability, and translation. Here we discuss the evolution and radiation of the CELF protein subfamilies, and report the cloning of the chicken CELF family members. In this study, we examined the embryonic expression patterns of the CELF family in the chick by in situ hybridization. We found that the tissue specificity reported for CELF proteins in the adult is established early during embryogenesis. Members of one subfamily, CUG-BP1 and ETR-3, are broadly expressed in the early embryo, while members of the second subfamily, CELF4-6, are restricted primarily to the nervous system. Expression patterns of individual CELF genes in several tissues, including the heart, liver, eye, and neural tube, exhibit distinct, yet overlapping, expression patterns. This suggests that different members of the CELF family play distinct functional roles during embryogenesis.

RNA in brain disease: no longer just "the messenger in the middle"

RNA research has made great progress in recent years. A variety of unforeseen complexities have been identified, many with relevance to human brain disease. For example, neurologic illnesses may arise because of perturbations in distinct but interrelated tiers of RNA-based genetic regulation: pre-mRNA splicing; nonsplicing RNA modifications; and mRNA translational regulation. Furthermore, there is poor correlation between mRNA levels and protein levels in mammalian cells, due partly to complicated post-transcriptional regulation by hitherto unknown noncoding RNAs. Some noncoding RNAs have been shown to be involved in human brain diseases. Diseases potentially mediated by alterations in RNA processes include tauopathies, myotonic dystrophy, Alzheimer disease, brain cancer, and many others. Here we present an overview of new research highlighting functions for RNA that far surpass the "messenger in the middle" role and that identify RNA molecules as important agents in the human brain in health and in disease states.

Ribonuclear foci at the neuromuscular junction in myotonic dystrophy type 1

In myotonic dystrophy type 1 (DM1) the muscle fibers express RNA containing an expanded CUG repeat (CUG(exp)). The CUG(exp) RNA is retained in the nucleus, forming ribonuclear foci. Splicing factors in the muscleblind (MBNL) family are sequestered in ribonuclear foci, resulting in abnormal regulation of alternative splicing. In extrajunctional nuclei, these effects on splicing regulation lead to reduced chloride conductance and altered insulin receptor signaling. Here we show that CUG(exp) RNA is also expressed in subsynaptic nuclei of muscle fibers and in motor neurons in DM1, causing sequestration of MBNL1 protein in both locations. In a transgenic mouse model, expression of CUG(exp) RNA at high levels in extrajunctional nuclei replicates many features of DM1, but the toxic RNA is poorly expressed in subsynaptic nuclei and the mice fail to develop denervation-like features of DM1 myopathology. Our findings indicate that subsynaptic nuclei and motor neurons are at risk for DM1-induced spliceopathy, which may affect function or stability of the neuromuscular junction.

Cerebral involvement in myotonic dystrophies

Myotonic dystrophy types 1 (DM1) and 2 (DM2) are similar yet distinct autosomal-dominant disorders characterized by muscle weakness, myotonia, cataracts, and multiple organ involvement, including the brain. One key difference between DM1 and DM2 is that a congenital form has been described for DM1 only. Expression of RNA transcripts containing pathogenic repeat lengths produces defects in alternative splicing of multiple RNAs, sequesters specific repeat-binding proteins, and ultimately leads to developmentally inappropriate splice products for a particular tissue. Whether brain pathology in its entirety in adult DM1 and DM2 is caused by interference in RNA processing remains to be determined. This review focuses on the similarities and differences between DM1 and DM2 with respect to neuropsychological, neuropathological, and neuroimaging data relating to cerebral involvement, with special emphasis on the clinical relevance and social consequences of such involvement.

Gene expression analysis in myotonic dystrophy: indications for a common molecular pathogenic pathway in DM1 and DM2

An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins. An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins.

Brain 1H magnetic resonance spectroscopic differences in myotonic dystrophy type 2 and type 1

To evaluate cerebral metabolism and intergroup differences in closely matched patients with myotonic dystrophy type 2 (DM2, n = 15) and type 1 (DM1, n = 14), we performed (1)H magnetic resonance spectroscopic (MRS) analyses of the occipital and temporoparietal cortical regions as well as of subcortical frontal white matter. Relative to healthy subjects, the concentration of N-acetylaspartate was significantly reduced in all tested brain regions in both disease groups. In the DM1 patients we also observed a concomitant depletion of creatine and choline levels, particularly in the frontal white matter. A discriminant analysis based on the (1)H-MRS data distinguished between the DM2, DM1, and control groups with an overall accuracy of 88%. (1)H-MRS indicates that neurochemical alterations involving gray and white matter occur in patients with DM2 and DM1. Although structural abnormalities (cerebral atrophy, white matter lesions) are similar in DM2 and DM1, changes in cerebral metabolites can differentiate these disease groups, suggesting that the diseases differ in their neurocellular pathology.

Cortical damage in brains of patients with adult-form of myotonic dystrophy type 1 and no or minimal MRI abnormalities

OBJECTIVE

To evaluate, by using quantitative MRI metrics, subtle cortical changes in brains of patients with the adult form of myotonic dystrophy type I (DM1) who showed no or minimal abnormalities on MRI.

BACKGROUND

DM1 is an autosomal dominant multisystem disorder caused by the expansion of CTG repeats in the myotonic dystrophy-protein kinase gene. Mild to severe involvement of the CNS can be part of the clinical features of the disease. Several MRI studies have demonstrated that both focal white matter (WM) lesions and diffuse grey matter atrophy can be found in the brains of DM1 patients. However, whether these two processes are related or may occur independently is not clear.

DESIGN/METHODS

Ten genetically-proven DM1 patients who showed no or minimal abnormalities on MRI underwent a new brain MRI examination to obtain computerized measures of total and regional brain volumes normalized to head size and regional measurements of the magnetization transfer ratio (MTr).

RESULTS

Normalized brain volumes (NBV) were significantly (p < 0.0001) lower in DM1 subjects than in a group of age- and sex-matched normal controls. Normalized cortical volumes (NCV) also were lower (p = 0.003) in DM1 subjects than in normal controls, whereas normalized WM volumes were not different between the two groups (p = 0.3). In agreement with this, values of MTr in the neocortex (cortical-MTr) were significantly (p = 0.006) lower in DM1 patients than in normal controls and this difference was not found in the WM tissue (p = 0.8).

CONCLUSIONS

Neocortical damage seems to be evident in the absence of visible WM lesions suggesting that a neocortical pathology, unrelated to WM lesion formation, occurs in DM1 brains.

Neurofibrillary tangles and deposition of oxidative products in the brain in cases of myotonic dystrophy

Myotonic dystrophy (MyD) is a neuromuscular degenerative disorder that is neuropathologically characterized by minor changes, such as the presence of neurofibrillary tangles (NFT), thalamic inclusions and functional brainstem lesions. In the current study, we conducted an immunohistochemical analysis to examine the distribution of NFT and formation of oxidative products in the brain specimens of 12 patients with MyD. Neurofibrillary tangles were found in the limbic system and/or the brainstem of all the cases examined but there were no senile plaques. The density of distribution of the NFT was not significantly correlated with clinicopathological findings, although cases with fewer NFTin the brain frequently showed sleep disturbances and lack of spontaneity. Nuclear and cytoplasmic immunoreactivities for 8-hydroxy-2'-deoxyguanosine and advanced glycation end products were observed in the glial cells and/or neurons in the brainstem, but not in the cerebral cortex. On the other hand, 10 out of the 12 cases showed cytoplasmic immunoreactivity for 4-hydroxy-2-nonenal-modified protein (4-HNE) in neurons of the temporal cortex and raphe nucleus. Deposition of 4-HNE was also recognized in the hippocampus and mesencephalic central gray matter, but not in the subiculum. The distribution pattern of the immunoreactivity for 4-HNE showed no clear correlation with either the psychological disturbances or the distribution of the NFT. Altered expression of monoaminergic neurons in the brainstem of MyD patients has already been reported, and it is worth noting that most of our cases showed NFT in the brainstem. The selective deposition of 4-HNE in the limbic system and brainstem suggests that lipid peroxidation may be involved in the neurodegenerative process in MyD. Using immunohistochemical analysis to determine the distribution of neurotransmitters in the mesencephalic central gray matter and/or pontine raphe nucleus may help elucidate the relationship between the clinical abnormalities, distribuion of NFT, and 4-HNE deposition in the brain in patients with MyD.

Effect of the [CCTG]n repeat expansion on ZNF9 expression in myotonic dystrophy type II (DM2)

Myotonic dystrophy is caused by two different mutations: a (CTG)n expansion in 3' UTR region of the DMPK gene (DM1) and a (CCTG)n expansion in intron 1 of the ZNF9 gene (DM2). The most accredited mechanism for DM pathogenesis is an RNA gain-of-function. Other findings suggest a contributory role of DMPK-insufficiency in DM1. To address the issue of ZNF9 role in DM2, we have analyzed the effects of (CCTG)n expansion on ZNF9 expression in lymphoblastoid cell lines (n=4) from DM2 patients. We did not observe any significant alteration in ZNF9 mRNA and protein levels, as shown by QRT-PCR and Western blot analyses. Additional RT-PCR experiments demonstrated that ZNF9 pre-mRNA splicing pattern, which includes two isoforms, is unmodified in DM2 cells. Our results indicate that the (CCTG)n expansion in the ZNF9 intron does not appear to have a direct consequence on the expression of the gene itself.

Facial emotion recognition in myotonic dystrophy type 1 correlates with CTG repeat expansion

OBJECTIVE

To investigate the ability of patients with myotonic dystrophy type 1 to recognise basic facial emotions. We also explored the relationship between facial emotion recognition, neuropsychological data, personality, and CTG repeat expansion data in the DM-1 group.

METHODS

In total, 50 patients with DM-1 (28 women and 22 men) participated, with 41 healthy controls. Recognition of facial emotional expressions was assessed using photographs of basic emotions. A set of tests measured cognition and personality dimensions, and CTG repeat size was quantified in blood lymphocytes.

RESULTS

Patients with DM-1 showed impaired recognition of facial emotions compared with controls. A significant negative correlation was found between total score of emotion recognition in a forced choice task and CTG repeat size. Furthermore, specific cognitive functions (vocabulary, visuospatial construction ability, and speed) and personality dimensions (reward dependence and cooperativeness) correlated with scores on the forced choice emotion recognition task.

CONCLUSION

These findings revealed a CTG repeat dependent facial emotion recognition deficit in the DM-1 group, which was associated with specific neuropsychological functions. Furthermore, a correlation was found between facial emotional recognition ability and personality dimensions associated with sociability. This adds a new clinically relevant dimension in the cognitive deficits associated with DM-1.

ETR-3 represses Tau exons 2/3 inclusion, a splicing event abnormally enhanced in myotonic dystrophy type I

Altered splicing of transcripts, including the insulin receptor (IR) and the cardiac troponin (cTNT), is a key feature of myotonic dystrophy type I (DM1). CELF and MBNL splicing factor members regulate the splicing of those transcripts. We have previously described an alteration of Tau exon 2 splicing in DM1 brain, resulting in the favored exclusion of exon 2. However, the factors required for alternative splicing of Tau exon 2 remain undetermined. Here we report a decreased expression of CELF family member and MBNL transcripts in DM1 brains as assessed by RT-PCR. By using cellular models with a control- or DM1-like splicing pattern of Tau transcripts, we demonstrate that ETR-3 promotes selectively the exclusion of Tau exon 2. These results together with the analysis of Tau exon 6 and IR exon 11 splicing in brain, muscle, and cell models suggest that DM1 splicing alteration of several transcripts involves various factors.

Misregulation of alternative splicing causes pathogenesis in myotonic dystrophy

Myotonic dystrophy (DM), the most common form of adult onset muscular dystrophy, affects skeletal muscle, heart, and the central nervous system (CNS). Mortality results primarily from muscle wasting and cardiac arrhythmias. There are two forms of the disease: DM1 and DM2. DM1, which constitutes 98% of cases, is caused by a CTG expansion in the 3' untranslated region (UTR) of the DMPK gene. DM2 is caused by a CCTG expansion in the first intron of the ZNF9 gene. RNA containing CUG- or CCUG-expanded repeats are transcribed but are retained in the nucleus in foci. Disease pathogenesis results primarily from a gain of function of the expanded RNAs, which alter developmentally regulated alternative splicing as well as pathways of muscle differentiation. The toxic RNA has been implicated in sequestration of splicing regulators and transcription factors thereby causing specific symptoms of the disease. Here we review the proposed mechanisms for the toxic effects of the expanded repeats and discuss the molecular mechanisms of splicing misregulation and disease pathogenesis.

Cognitive impairment in neuromuscular disorders

Several studies have suggested the presence of central nervous system involvement manifesting as cognitive impairment in diseases traditionally confined to the peripheral nervous system. The aim of this review is to highlight the character of clinical, genetic, neurofunctional, cognitive, and psychiatric deficits in neuromuscular disorders. A high correlation between cognitive features and cerebral protein expression or function is evident in Duchenne muscular dystrophy, myotonic dystrophy (Steinert disease), and mitochondrial encephalomyopathies; direct correlation between tissue-specific protein expression and cognitive deficits is still elusive in certain neuromuscular disorders presenting with or without a cerebral abnormality, such as congenital muscular dystrophies, congenital myopathies, amyotrophic lateral sclerosis, adult polyglucosan body disease, and limb-girdle muscular dystrophies. No clear cognitive deficits have been found in spinal muscular atrophy and facioscapulohumeral dystrophy.

Myotonic dystrophy expanded CUG repeats disturb the expression and phosphorylation of τ in PC12 cells

Mental retardation is a main feature of the congenital form of myotonic dystrophy (DM1), however, the molecular mechanisms underlying the central nervous system symptoms of DM1 are poorly understood. We have established a PC12 cell line-based model expressing the DM1 expanded CUG repeats (CTG90 cells) to analyze the effects of this mutation on neuronal functions. Previously, we have reported that CTG90 cells displayed impaired NGF-induced neuronal differentiation. Because disruption of normal expression of the microtubule associated protein tau and neuronal aggregates of hyperphosphorylated tau have been associated with DM1, this study analyzes the behavior of tau in the CTG90 cells. Several alterations of tau were observed in the PC12 cells that express expanded CUG repeats, including a subtle but reproducible reduction in the expression of the tau mRNA splicing isoform containing exon 10, decreased expression of tau and hyperphosphorylation of both tau and high molecular weight tau as well as abnormal nuclear localization of tau phosphorylated at Ser396/404. Interestingly, phosphorylation regulates negatively the activity of tau as microtubule-associated protein. In addition, impaired activity of the Akt/GSK3beta pathway, which phosphorylates tau, was also identified in the CTG90 cells. Besides tau phosphorylation, the Akt/GSK3beta signaling pathway regulates other key processes of PC12 cells, such as apoptosis and neuronal differentiation. Our results indicate that defective neuronal differentiation exhibited by the PC12 cells expressing expanded CUG repeats could be the result of combinatory effects derived from the altered behavior of tau and the impaired activation of the Akt/GSK3beta signaling pathway.

Brain-specific change in alternative splicing of Tau exon 6 in myotonic dystrophy type 1

Alternative splicing is altered in myotonic dystrophy of type 1 (DM1), a syndrome caused by an increase of CTG triplet repeats in the 3' untranslated region of the myotonic dystrophy protein kinase gene. Previously, we reported the preferential skipping of Tau exon 2 in DM1 brains. In this study, we analyze the alternative splicing of Tau exon 6 which can be inserted in three different forms (c, p and d) depending on the 3' splice site used. In fact, inclusion of exon 6c decreases in DM1 brains compared to control brains whereas inclusion of 6d increases. Alteration of exon 6 splicing was not observed in DM1 muscle although this exon was inserted in RNAs from normal muscle and DM1 splicing alterations were first described in this organ. In contrast, alteration of exon 2 of Tau mRNA was observed in both muscle and brain. However, co-transfections of a minigene containing exon 6 with CELF or MBNL1 cDNAs, two splicing factor families suspected to be involved in DM1, showed that they influence exon 6 splicing. Altogether, these results show the importance of determining all the exons and organs targeted by mis-splicing to determine the dysregulation mechanisms of mis-splicing in DM1.

Unique Tauopathy in Fukuyama-Type Congenital Muscular Dystrophy

Fukuyama-type congenital muscular dystrophy (FCMD) is characterized by muscular dystrophy and cortical dysgenesis of the cerebrum and cerebellum. We investigated the extent and nature of tauopathy in the brains of 7 postfetal (14-34 years of age) and 2 fetal (18- and 20-week gestational age) FCMD cases. In all postfetal cases, tauopathy was found in the areas of cortical dysgenesis in the cerebrum, in addition to predictable sites such as the hippocampus. In fetal cases, the neuropil of malformed cerebral cortex was diffusely immunostained with anti-aberrantly phosphorylated tau antibodies. By immunoelectron microscopy, the epitope of the antibodies was associated with microtubule-like bundles within cellular processes protruding through disrupted glia limitans. In Western blot analysis, a unique 50-kDa band of tau was detected in a fetal and a postfetal case. In addition, 3 to 4 tau bands of 60 to 68 kD, similar to tau in Alzheimer disease, were also detected in the latter. After dephosphorylation, the insoluble tau from the fetal and the postfetal cases showed highly similar immunoblotting patterns. This anomalous phosphorylation of tau may be related to the development of the cortical dysgenesis in FCMD and may shed light on the biologic function of tau in the development of the central nervous system.

Tau-Positive Fine Granules in the Cerebral White Matter: A Novel Finding Among the Tauopathies Exclusive to Parkinsonism-Dementia Complex of Guam

We examined the autopsied brains of cases of 6 types of tauopathy: parkinsonism-dementia complex of Guam (PDC), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), Pick disease, Alzheimer disease (AD), and myotonic dystrophy together with Guamanian controls. Light microscopy sections of these brains were examined using anti-tau antibodies. Tau-positive fine granules (TFGs) were globe-shaped, and 3 to 6 mum in diameter, were observed predominantly in the frontal white matter in 30 of the 35 patients with PDC. However, no TFGs were found in association with PSP, myotonic dystrophy, Pick disease, AD, or CBD. Western blot analysis of frozen brain tissue taken from the PDC cases revealed that the frontal cortex was hyperphosphorylated and contained 6 tau isoforms (3R+4R tau). However, in the present study, it was revealed that the novel TFGs in the white matter of patients with PDC was composed of 4R tau. Western blot analysis of sarkosyl-insoluble tau from the white matter of the PDC cases showed 2 major bands of 60 and 64 kDa and one minor band of 67 kDa. After dephosphorylation, these bands resolved into one major band of 4-repeat (4R) tau isoform and 3 minor bands of 3-repeat (3R) and 4R tau isoforms. Moreover, the TFGs observed in cases in which the number of neurofibrillary tangles (NFTs) was higher than the threshold level were not correlated with the presence of cortical NFTs. In conclusion, these novel TFGs were found almost exclusively in PDC brains and could therefore be considered as a characteristic neuropathologic marker of this particular tauopathy. The TFGs were hyperphosphorylated tau-positive structures that may be formed by a different mechanism from that used to produce cortical NFTs.

Clinical and molecular aspects of the myotonic dystrophies: a review

Type 1 myotonic dystrophy or DM1 (Steinert's disease), which is the commonest muscular dystrophy in adults, has intrigued physicians for over a century. Unusual features, compared with other dystrophies, include myotonia, anticipation, and involvement of other organs, notably the brain, eyes, smooth muscle, cardiac conduction apparatus, and endocrine system. Morbidity is high, with a substantial mortality relating to cardiorespiratory dysfunction. More recently a second form of multisystem myotonic disorder has been recognized and variously designated as proximal myotonic myopathy (PROMM), proximal myotonic dystrophy (PDM), or DM2. For both DM1 and DM2 the molecular basis is expansion of an unstable repeat sequence in a noncoding part of a gene (DMPK in DM1 and ZNF9 in DM2). There is accumulating evidence that the basic molecular mechanism is disruption of mRNA metabolism, which has far-reaching effects on many other genes, in part through the induction of aberrant splicing, explaining the multisystemic nature of the disease. The unstable nature of the expansion provides a molecular explanation for anticipation. This review emphasizes the clinical similarities and differences between DM1 and DM2. It examines current views about the molecular basis of these disorders, and contrasts them with other repeat expansion disorders that have increasingly been recognized as a cause of neurological disease.

Brain magnetic resonance image changes in a family with congenital and classic myotonic dystrophy

We present the clinical manifestations, brain magnetic resonance images (MRI), and genetic analysis of a family with 2 siblings with congenital myotonic dystrophy type 1 (DM1) and 4 patients with classic DM1. These 2 patients with congenital DM1 had severe mental retardation and a characteristic feature of hyperintensity of white matter at the posterior-superior trigone (HWMPST), in addition to ventricular dilatation in T2-weighted images (T2WI) of brain MRI. In 2 of the 4 classic DM1 patients, brain T2WI MRI showed hyperintensity lesions in the bilateral frontal and/or temporal regions, which were absent in congenital DM1. In conclusion, we suggest that the HWMPST in brain MRI is a characteristic finding in congenital DM1, and that the severe cognitive impairments are not only attributable to the subcortical white matter lesions. In congenital DM1, the cognitive function is a diffuse impairment, which is different from that in classic DM1.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. 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. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

Diffusion tensor imaging of cerebral white matter in patients with myotonic dystrophy

PURPOSE

To determine whether myotonic dystrophy (MyD) patients have diffusion tensor abnormalities suggestive of microstructural changes in normal-appearing white matter (NAWM).

MATERIAL AND METHODS

Conventional and diffusion tensor magnetic resonance images of the brain were obtained in 19 MyD patients and 19 age-matched normal control subjects. Fractional anisotropy (FA) and mean diffusivity (MD) values were calculated in white matter lesions (WMLs) and NAWM in MyD patients and in the white matter of normal control subjects. Differences between WML and NAWM values and between MyD patient and control subject values were analyzed statistically.

RESULTS

Significantly lower FA and higher MD values were found in all regions of interest in the NAWM of MyD patients than in the white matter of control subjects (P<0.01), as well as significantly lower FA and higher MD values in WMLs than in NAWM of MyD patients (P < 0.05). There was no significant correlation of mean FA or MD values in NAWM with patient age, age at onset, or duration of illness (P>0.1).

CONCLUSION

Diffusion tensor imaging analysis suggests the presence of diffuse microstructural changes in NAWM of MyD patients that may play an important role in the development of disability.

Temperament and character in patients with classical myotonic dystrophy type 1 (DM-1)

This study was designed to investigate personality in classical Myotonic Dystrophy (DM-1). Forty-six patients with DM-1 (25 women and 21 men), 31 healthy controls and 37 subjects in a contrast group, consisting of patients with other muscle disorders (spinal muscular atrophy, facioscapulohumeral dystrophy and limb girdle muscular dystrophy), completed the Temperament and Character Inventory (TCI) (Cloninger, 1994). We aimed to establish whether CTG triplet repeat size correlated with ratings of personality dimensions in the TCI. The DM-1 patients scored significantly higher on the TCI dimension Harm avoidance and lower on Persistence, Self-directedness and Cooperativeness. Signs of a personality disorder were found in 20% of the DM-1 patients. No correlation was found between the number of CTG repeats and scores in the TCI. This study indicates deviant personality in classical DM-1 regarding temperament and character, both in comparison to healthy controls and to patients with other muscle disorders with no known brain disorder.

Myotonic dystrophy type 1 is associated with nuclear foci of mutant RNA, sequestration of muscleblind proteins and deregulated alternative splicing in neurons

Myotonic dystrophy type 1 (DM1) is caused by expansion of a CTG repeat in the DMPK gene. In skeletal muscles, DM1 may involve a novel, RNA-dominant disease mechanism in which transcripts from the mutant DMPK allele accumulate in the nucleus and compromise the regulation of alternative splicing. Here we show evidence for a similar disease mechanism in brain. Examination of post-mortem DM1 tissue by fluorescence in situ hybridization indicates that the mutant DMPK mRNA, with its expanded CUG repeat in the 3'-untranslated region, is widely expressed in cortical and subcortical neurons. The mutant transcripts accumulate in discrete foci within neuronal nuclei. Proteins in the muscleblind family are recruited into the RNA foci and depleted elsewhere in the nucleoplasm. In parallel, a subset of neuronal pre-mRNAs show abnormal regulation of alternative splicing. These observations suggest that CNS impairment in DM1 may result from a deleterious gain-of-function by mutant DMPK mRNA.

CELF6, a Member of the CELF Family of RNA-binding Proteins, Regulates Muscle-specific Splicing Enhancer-dependent Alternative Splicing

We previously described a family of five RNA-binding proteins: CUG-binding protein, embryonic lethal abnormal vision-type RNA-binding protein 3, and the CUG-binding protein and embryonic lethal abnormal vision-type RNA-binding protein 3-like factors (CELFs) 3, 4, and 5. We demonstrated that all five of these proteins specifically activate exon inclusion of cardiac troponin T minigenes in vivo via muscle-specific splicing enhancer (MSE) sequences. We also predicted that a sixth family member, CELF6, was located on chromosome 15. Here, we describe the isolation and characterization of CELF6. Like the previously described CELF proteins, CELF6 shares a domain structure containing three RNA-binding domains and a divergent domain of unknown function. CELF6 is strongly expressed in kidney, brain, and testis and is expressed at very low levels in most other tissues. In the brain, expression is widespread and maintained from the fetus to the adult. CELF6 activates exon inclusion of a cardiac troponin T minigene in transient transfection assays in an MSE-dependent manner and can activate inclusion via multiple copies of a single element, MSE2. These results place CELF6 in a functional subfamily of CELF proteins that includes CELFs 3, 4, and 5. CELF6 also promotes skipping of exon 11 of insulin receptor, a known target of CELF activity that is expressed in kidney.

Myotonic syndromes

PURPOSE OF REVIEW

To highlight recent advances in understanding the clinical manifestations and molecular genetics of myotonic syndromes, with particular emphasis on the myotonic dystrophies.

RECENT FINDINGS

Myotonic syndromes include the non-dystrophic myotonias, caused by mutations in genes encoding the chloride or sodium channels that are specific to skeletal muscle, and the myotonic dystrophies. Previous studies have shown that myotonic dystrophy type 1 is caused by the expansion of a CTG repeat in the gene. Recently, it was discovered that myotonic dystrophy type 2 (proximal myotonic myopathy) is also caused by a DNA expansion mutation. In both types of myotonic dystrophy the expanded repeat is transcribed and the RNA produced from the mutant allele is retained in nuclear inclusions. Recent studies suggest that the mutant RNA has a toxic effect on muscle fibers by interfering with the essential functions of the myonucleus, such as RNA processing.

SUMMARY

It now appears likely that myotonic dystrophy is the first instance of a genetic disease in which the harmful effect of a mutation involves the production of a pathogenic RNA. However, the exact mechanism is not understood, and it is unclear whether this RNA-mediated disease process is also responsible for the manifestations of myotonic dystrophy in non-muscle tissues.

Brain MRI features of congenital- and adult-form myotonic dystrophy type 1: case-control study

To compare and characterize the magnetic resonance imaging (MRI) of brain in the congenital and adult form of myotonic dystrophy type 1, we evaluated five patients with congenital dystrophy type 1, 10 age- and 10 disease duration-matched patients with adult-form dystrophy type 1 and 20 age-matched healthy volunteers. The ventricular enlargement was evaluated by the ventricular:brain ratio, the signal intensity of white matter posterosuperior to trigones by reference to standard images and the white matter lesions by a semiquantitative method. In the congenital dystrophy type 1, MRI was characterized by ventriculomegaly and moderate/severe hyperintensity of white matter posterosuperior to trigones, which showed no correlation with the age. MRI in the adult-form dystrophy type 1 was strictly related to disease duration and varied between normal findings, except for temporo-polar white matter lesions, in age-matched patients and ventriculomegaly with white matter hyperintensities in disease duration-matched patients. These results suggest that the origin of MRI abnormalities in myotonic dystrophy type 1 is mainly developmental for the congenital form and mainly degenerative for the adult form.

Magnetization transfer measurements of cerebral white matter in patients with myotonic dystrophy

To determine whether patients with myotonic dystrophy (MyD) have structural changes in the cerebral white matter, we performed magnetization transfer (MT) imaging of the cerebral white matter in 14 MyD patients and 11 age-matched normal controls. We calculated MT ratios in both the white matter lesions (WMLs) and the normal-appearing white matter (NAWM) of MyD patients using region of interest (ROI) analysis. MT ratios in WMLs were markedly decreased, and all ROIs in NAWM also showed significantly lower MT ratios in MyD patients than in normal controls. The average MT ratio of all ROIs in WMLs and NAWM in each patient showed a significant negative correlation with duration of illness, but not with the patient's age or age at onset. The results of the present study indicate not only the presence of pathological changes in WMLs but also the widespread involvement of NAWM in MyD patients. The results also suggest that structural changes in the white matter may be progressive during the clinical course of MyD.

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.

Widespread expression of alpha-synuclein and tau immunoreactivity in Hallervorden-Spatz syndrome with protracted clinical course

Hallervorden-Spatz syndrome (HSS) is a rare autosomal recessive disorder clinically characterized by extrapyramidal signs and progressive dementia. In a typical case, the clinical symptoms become apparent during late childhood, and usually the course is protracted over a decade or more. We recently had an opportunity to study the brains of two cases of HSS with a clinical course of over 30 years. Case 1 was a 44-year-old female and case 2 was a 37-year-old male. Grossly, the brains showed severe fronto-temporal lobar atrophy with abundant spheroids and mild iron deposits in the globus pallidus, associated with features of motor neuron disease. In addition, there was diffuse sponginess in the atrophic cortex as well as widespread Alzheimer's neurofibrillary tangles (NFTs) and Lewy bodies (LBs) in the cortical and subcortical regions, including the spinal cord. Ultrastructurally, NFTs were composed of paired helical filaments, and LBs of central dense cores with radiating fibrils. Discrete immunostaining was demonstrated in NFTs and neuropil threads with various antibodies against phosphorylated tau, and in LBs with antibody against alpha-synuclein. In addition, diffuse, overlapping immunoreactivity of alpha-synuclein and phosphorylated tau was seen within the cytoplasm of many neurons. However, when LBs and NFTs coexisted within the same neurons, they were clearly segregated. The findings of our present cases as well as those reported in the literature may indicate that simultaneous and extensive occurrence of abnormal phosphorylation of tau and accumulation of alpha-synuclein may constitute cardinal pathological features of HSS with protracted clinical course.

Comparative biochemistry of tau in progressive supranuclear palsy, corticobasal degeneration, FTDP-17 and Pick's disease

Neurodegenerative disorders referred to as tauopathies have cellular hyperphosphorylated tau protein aggregates in the absence of amyloid deposits. Comparative biochemistry of tau aggregates shows that they differ in both phosphorylation and content of tau isoforms. The six tau isoforms found in human brain contain either three (3R) or four microtubule-binding domains (4R). In Alzheimer's disease, all six tau isoforms are abnormally phosphorylated and aggregate into paired helical filaments. They are detected by immunoblotting as a major tau triplet (tau55, 64 and 69). In corticobasal degeneration and progressive supranuclear palsy, only 4R-tau isoforms aggregate into twisted and straight filaments respectively. They appear as a major tau doublet (tau64 and 69). Finally, in Pick's disease, only 3R-tau isoforms aggregate into random coiled filaments. They are characterized by another major tau doublet (tau55 and 64). These differences in tau isoforms may be related to either the degeneration of particular cell populations in a given disorder or aberrant cell trafficking of particular tau isoforms. Finally, recent findings provide a direct link between a genetic defect in tau and its abnormal aggregation into filaments in fronto-temporal dementia with Parkinsonism linked to chromosome 17, demonstrating that tau aggregation is sufficient for nerve cell degeneration. Thus, tau mutations and polymorphisms may also be instrumental in many neurodegenerative disorders.

REVIEW: tau protein pathology in Alzheimer's disease and related disorders

Abundant neurofibrillary lesions made of hyperphosphorylated microtubule-associated protein tau constitute one of the defining neuropathological features of Alzheimer's disease. However, tau containing filamentous inclusions in neurones and/or glial cells also define a number of other neurodegenerative disorders clinically characterized by dementia and/or motor syndromes. All these disorders, therefore, are grouped under the generic term of tauopathies. In the first part of this review we outline the morphological and biochemical features of some major tauopathies, e. g. Alzheimer's disease, argyrophilic grain disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration. The impact of the recent finding of tau gene mutations in familial frontotemporal dementia and parkinsonism linked to chromosome 17 on other tauopathies is discussed in the second part. The review closes with a look towards a new understanding of neurodegenerative disorders characterized by filamentous nerve cell inclusions. The recent identification of the major protein component of their respective inclusions led to a surprising convergence of seemingly unrelated disorders. The new findings now allow us to classify neurodegenerative disorders with filamentous nerve cell inclusions into four main categories: (i) the tauopathies; (ii) the alpha-synucleinopathies; (iii) the polyglutamine disorders; and (iv) the iquitin disorders'. Within the proposed classification scheme, tauopathies constitute the most frequent type of disorder.