New developments in frontotemporal dementia and parkinsonism linked to chromosome 17

Pathogenesis/genetics of frontotemporal dementia and how it relates to ALS

One of the most interesting findings in the field of neurodegeneration in recent years is tfche discovery of a genetic mutation in the C9orf72 gene, the most common mutation found to be causative of sporadic and familial frontotemporal lobar degeneration (FTLD), amyotrophic lateral sclerosis (ALS) and concomitant FTD-ALS (DeJesus-Hernandez et al., 2011b; Renton et al., 2011). While clinical and molecular data, such as the identification of TDP-43 being a common pathological protein (Neumann et al., 2006) have hinted at such a link for years, the identification of what was formally known as “the chromosome 9 FTLD-ALS gene” has provided a foundation for better understanding of the relationship between the two. Indeed, it is now recognized that ALS and FTLD-TDP represent a disease spectrum. In this review, we will discuss the current genetic and pathological features of the FTLD-ALS spectrum.

Neuropathology of frontotemporal lobar degeneration: a review

Frontotemporal lobar degeneration (FTLD) is the second most common cause of presenile dementia. Three main clinical variants are widely recognized within the FTLD spectrum: the behavioural variant of frontotemporal dementia (bvFTD), semantic dementia (SD) and progressive non-fluent aphasia (PNFA). FTLD represents a highly heterogeneous group of neurodegenerative disorders which are best classified according to the main protein component of pathological neuronal and glial inclusions. The most common pathological class of FTLD is associated with the TDP-43 protein (FTLD-TDP), while FTLD-Tau is considered slightly less common while the FTLD-FUS (Fused in sarcoma protein) pathology is rare. In this review, these three major pathological types of FTLD are discussed.

Management of frontotemporal dementia: targeting symptom management in such a heterogeneous disease requires a wide range of therapeutic options

There are no US FDA-approved therapies for the management of frontotemporal dementia (FTD). Evidence-based medicine that would support a FDA indication for the treatment of FTD requires large-scale, randomized, double-blind, placebo-controlled trials that do not currently exist. Progress in obtaining approval and therapeutic indications for FTD has been severely hampered by the heterogeneity of clinical and pathological phenotypes seen in various FTD disease states. These issues are often misinterpreted by clinicians, caregivers and patients suggesting that potential treatment options are nonexistent for this devastating disease. This article discusses these issues in the context of recent studies and publications investigating therapeutic options in FTD, and further suggests a rationale for individualized therapy in FTD. Targeting the myriad of symptoms seen in FTD requires recognition of such symptoms that may play primary or secondary roles in the spectrum of deficits that lead to functional disability in FTD, and the availability of a wide range of therapeutic options that may be helpful in alleviating such symptomatology. Fortunately, agents targeting the many cognitive, behavioral, psychiatric and motor symptoms that can be seen in FTD are readily available, having been previously developed and approved for symptomatic benefit in other disease states. In contrast to the widespread belief that beneficial treatments are not available for FTD today, our therapeutic armament is stocked with pharmacological tools that may improve quality of life for those suffering from this devastating and incurable class of degenerative diseases.

Compound heterozygosity of 2 novel MAPT mutations in frontotemporal dementia

Intronic MAPT mutations altering exon 10 splicing lead mainly to an increase of 4Rtau. The objective of this study is to report clinical, genetic, and neuropathological data of an apparently sporadic early onset frontotemporal dementia (FTD) case associated with 2 novel intronic MAPT gene mutations IVS10+4A > C and IVS9-15T > C that increase 3Rtau. Methods and subjects used are clinical, neuroradiological, and neuropathological examination; molecular genetics of MAPT, PGRN, and other relevant genes. Exon 10 splicing tested with minigene constructs. Tau deposits detected by immunohistochemistry. Sarkosyl-insoluble and soluble tau investigated by immunoblotting. Two novel MAPT mutations IVS10+4A > C and the IVS9-15T > C transmitted by the unaffected parents were identified. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) analyses on minigenes and in brain tissue showed that both mutations cause an increase of tau mRNA (messenger ribonucleic acid) transcripts lacking exon 10 only in the patient. Immunohistochemistry and immunoblotting of the patient's brain revealed tau deposits composed mostly of 3Rtau isoforms with a predominance of the shorter 3Rtau isoforms. The compound heterozygosity of the patient increasing 3Rtau seems to be responsible for the disease and furthermore suggests that sporadic cases can be caused by genetic mutations.

Neuroimaging in dementia: in vivo amyloid imaging

Dementia, a progressive cognitive decline, leads to a gradually increasing restriction of daily activities. Alzheimer's disease (AD) is the most common form of dementia. The pathological features of AD include plaques and tangles which are constituted by amyloid beta peptide (A beta) and tau protein. These amyloidogenic molecules have been mechanistically implicated in the pathogenesis of AD and related neurodegenerative dementias. The key strategy for establishment of diagnostic and therapeutic approaches to AD is sensitive and specific detection of the incipient neuropathology characteristics of AD, combined with emerging treatments that counteract molecular processes in AD pathogenesis. Recent advances in molecular imaging research have enabled visualization of brain amyloidosis. The rapid development of different compounds suitable for visualizing amyloid would permit pathology-specific diagnosis of AD at an asymptomatic stage in a noninvasive manner, and could also allow early immunotherapeutic intervention without causing an excessive neuroinflammatory response.

Patient care and management of frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disease that affects frontal and temporal regions of the brain. Two proteins indicated in the pathology are tau and the recently discovered TDP-43. Major manifestations include progressive aphasia and a disorder of social comportment. The diagnosis of a patient includes a detailed cognitive exam, clinical testing, and neuroimaging techniques. The current goal of therapy for FTLD is symptomatic management with medications borrowed from other conditions. Nonpharmacologic management such as behavioral interventions and environmental engineering are also efficacious in optimizing quality of life.

Voxel-based morphometry in tau-positive and tau-negative frontotemporal lobar degenerations

BACKGROUND

The identification of specific, diagnostically useful predictors of protein dysfunction in the frontotemporal lobar degenerations (FTLD) is a problem of great clinical and biological interest. Correlations between regional patterns of tissue loss and specific proteinopathies have not been established.

OBJECTIVE

Specific brain imaging correlates of protein tau dysfunction were sought using voxel-based morphometry in FTLD subgroups with and without tau pathology.

METHODS

Seventeen patients with pathologically or genetically confirmed diagnoses of FTLD who had undergone volumetric brain magnetic resonance imaging (MRI) were identified retrospectively and tau-positive (n = 9) and tau-negative (n = 8) subgroups were defined. MRI data were compared with healthy age- and sex-matched controls using voxel-based morphometry implemented in a statistical parametric mapping software package.

RESULTS

Compared with controls, tau-positive and tau-negative subgroups had extensive common areas of regional brain atrophy predominantly affecting the frontal and anterior temporal lobes. No specific brain imaging features were identified for either subgroup.

CONCLUSION

Patterns of frontotemporal atrophy do not predict the presence or absence of tau pathology; conversely, different immunohistochemical profiles are associated with similar patterns of regional vulnerability to neuronal loss in FTLD.

Young-onset dementia: a practical approach to diagnosis

BACKGROUND

Young-onset dementia is best defined as dementia presenting at age less than 65 years. And, while cognitive impairment in the elderly is dominated by dementia of the Alzheimer type, young-onset dementia has a vast differential diagnosis.

REVIEW SUMMARY

This article reviews an extensive differential diagnosis for young-onset dementia by utilizing different clues in the historical records and laboratory findings to aid with diagnosis. Laboratory testing should be completed in at least 2 stages. In the first stage, referred to as the first "wave," we suggest more routine testing, particularly for treatable causes of dementia. The second "wave," which we also outline, emphasizes more esoteric testing that may require referral to a tertiary care medical facility. The manuscript is divided into 2 parts, with part 1 focusing on clues from the historical data, while part 2 focuses on laboratory abnormalities.

CONCLUSION

Unlike dementia presenting in the elderly, the differential diagnosis in young-onset dementia is vast. A thorough historical review of the symptoms, with special emphasis on the pattern of cognitive impairment, temporal profile of the disease, detailed family history, and extensive but coordinated laboratory and ancillary testing, may yield subtle clues to the diagnosis.

Inducible expression of Tau repeat domain in cell models of tauopathy: aggregation is toxic to cells but can be reversed by inhibitor drugs

We generated several cell models of tauopathy in order to study the mechanisms of neurodegeneration in diseases involving abnormal changes of tau protein. N2a neuroblastoma cell lines were created that inducibly express different variants of the repeat domain of tau (tau(RD)) when exposed to doxycycline (Tet-On system). The following three constructs were chosen: (i) the repeat domain of tau that coincides with the core of Alzheimer paired helical filaments; (ii) the repeat domain with the deletion mutation DeltaK280 known from frontotemporal dementia and highly prone to spontaneous aggregation; and (iii) the repeat domain with DeltaK280 and two proline point mutations that inhibit aggregation. The comparison of wild-type, pro-aggregation, and anti-aggregation mutants shows the following. (a) Aggregation of tau(RD) is toxic to cells. (b) The degree of aggregation and toxicity depends on the propensity for beta-structure. (c) Soluble mutants of tau(RD) that cannot aggregate are not toxic. (d) Aggregation is preceded by fragmentation. (e) Fragmentation of tau(RD) in cells is initially due to a thrombin-like protease activity. (f) Phosphorylation of tau(RD) (at KXGS motifs) precedes aggregation but is not correlated with the degree of aggregation. (g) Aggregates of tau(RD) disappear when the expression is silenced, showing that aggregation is reversible. (h) Aggregation can be prevented by drugs and even pre-formed aggregates can be dissolved again by drugs. Thus, the cell models open up new insights into the relationship between the structure, expression, phosphorylation, aggregation, and toxicity of tau(RD) that can be used to test current hypotheses on tauopathy and to develop drugs that prevent the aggregation and degeneration of cells.

Dementia in first-degree relatives of patients with frontotemporal dementia. A family history study

Several studies have found a clustering of dementia in relatives of patients with frontotemporal dementia (FTD). This study analysed the familial aggregation of FTD specifically as well as the occurrence of dementia in general in first-degree relatives of patients with FTD. A family history study was carried out on 478 first-degree relatives of 74 index patients suffering from FTD. Cases of organic dementia and of FTD were diagnosed according to internationally accepted diagnostic criteria. Age- and sex-specific incidences of organic dementia and of FTD were calculated as was the proportion of FTD in relation to organic dementia in general; comparisons with clinical and population studies were made. There was a tenfold increase in the incidence of FTD in the first-degree relatives of FTD patients compared with the incidence of FTD in a population study. The proportion of FTD in relation to all types of organic dementia was much higher in relatives of FTD patients compared to the corresponding proportions in clinical and population-based studies. There was a small, non-significant difference between the present family history study and the population studies as regards the incidence of organic dementia. The findings suggest that hereditary and/or shared environmental factors are strongly involved in the aetiology of FTD. There were no indications of familial clustering of organic dementia in general in relatives of FTD patients.

Neurogenetics II: complex disorders

The genetic analysis of common neurological disorders will be a difficult and protracted endeavour. Genetics is only one of many disciplines that will be required but it has already thrown considerable light on the aetiology of several major neurological disorders through the analysis of rare inherited subgroups. The identification of individual susceptibility genes with variants of smaller effect will be more difficult but there is no sharp demarcation between large and small genetic effects, so that many new and important insights will emerge using existing and new technologies. The availability of improved neuroimaging, better animal models of disease and new genetic tools, such as high-throughput gene chips, expression microarrays and proteomics, are extending the range of traditional genetic mapping tools. Finally, an understanding of the genetic and epigenetic mechanisms that restrain the differentiation and integration of human neural stem cells into mature neuronal networks could have a major impact on clinical practice. These approaches will be illustrated in the context of Alzheimer disease, Parkinson disease and synucleinopathies, tauopathies, amyotrophic lateral sclerosis and stroke.

The accuracy of clinical criteria for the diagnosis of frontotemporal dementia

The clinical diagnosis of frontotemporal dementia (FTD) can be challenging even to experienced clinicians. In the absence of a definitive clinical test, this diagnosis relies on behavioral criteria. Difficulty applying these criteria arise for four main reasons. First, FTD patients present with social and personality changes that defy the neuropsychological model of dementia. Second, FTD is not a single disorder but a spectrum of clinical syndromes with asymmetric and motor variants. Third, there may be qualitatively different symptoms during stages of FTD. Finally, pathologic and genetic variability contributes to the clinical variability. Future research should refine the clinical criteria for FTD using clinicopathological correlation in addition to working on the development of neurobiological markers.

Frontotemporal dementia: genetics and genetic counseling dilemmas

BACKGROUND

Frontotemporal dementia (FTD) is a neurodegenerative disease with early symptoms of personality change and/or language disorder. Approximately 40% of individuals with FTD have a family history of dementia; however, in our experience, less than 10% have clear autosomal dominant inheritance. Mutations in the microtubule-associated protein tau (MAPT) gene have been reported in up to 50% of hereditary cases, but are unusual except in families with more than 3 individuals with FTD. The genetics of FTD is complicated by clinical heterogeneity, variable expression, phenocopies, misdiagnoses, and lost family histories. The objective of this paper is to enable physicians to recognize hereditary patterns and genetic concerns of FTD families and to understand genetic counseling strategies.

REVIEW SUMMARY

The complexity of FTD genetics and genetic counseling are illustrated using 4 case histories. Case 1 demonstrates the difficulty obtaining a reliable FTD family history. Case 2 illustrates how psychiatric phenocopies can make family linkage studies difficult. The lack of genotype and phenotype correlation and issues of predictive genetic testing within FTD families are the subject of case 3, and case 4 shows how normal aging language difficulties and cognitive changes can be misinterpreted when a family history of dementia is present.

CONCLUSIONS

Physicians seeing patients with possible FTD should be aware of the risk of a genetic etiology. A 3-generation family history should be obtained with attention to neurologic, psychiatric, and behavioral symptoms. Variable expression and phenocopies are confounding factors when assessing a possible genetic etiology. Referral of the patient and family for genetic counseling is recommended.

Required techniques and useful molecular markers in the neuropathologic diagnosis of neurodegenerative diseases

Modern neuropathologic methods and molecular biology have lead to increased understanding of neurodegenerative disorders and biologically based classifications of these disorders. The purpose of this review is to discuss neuropathologic methods that are useful in the characterization of neurodegenerative disorders, with emphasis on disorders of late life that present with dementia or movement disorders. A diagnostic algorithm is suggested for neuropathologic evaluation of neurodegenerative disorders. The importance of clinical information is emphasized in arriving at the most precise and meaningful neuropathologic diagnosis.

Phenotypic variation in frontotemporal dementia and parkinsonism linked to chromosome 17

Hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) caused by mutations in the tau gene shows a wide range in age at onset, several distinct clinical presentations, and a spectrum of tau pathology. Although the clinical and pathological phenotype often correlate with the location of the mutation, there also exists considerable interfamilial and intrafamilial phenotypical variation. Not all families with FTDP-17 do have mutations and deposition of hyperphosphorylated tau in the brain, but show ubiquitin-positive, tau-negative inclusions. Future research should focus on the role of other genetic and environmental factors in this form of FTDP-17, whereas the responsible gene defect(s) has still to be identified for hereditary FTD without tau mutations.

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.

A novel presenilin 1 mutation associated with Pick's disease but not beta-amyloid plaques

Familial forms of frontotemporal dementia (FTD) with tauopathy are mostly caused by mutations in the gene encoding the microtubule-associated protein tau (MAPT). However, rare forms of familial tauopathy without MAPT mutations have been reported, suggesting other tauopathy-related genetic defects. Interestingly, two presenilin 1 (PS1) mutations (Leu113Pro and insArg352) recently have been associated with familial FTD albeit without neuropathological confirmation. We report here a novel PS1 mutation in a patient with Pick-type tauopathy in the absence of extracellular beta-amyloid deposits. The mutation is predicted to substitute Gly-->Val at codon position 183 (Gly183Val) and to affect the splice signal at the junction of the sixth exon and intron. Further clinical-genetic investigation showed a positive family history of FTD-like dementia and suggested that Gly183Val is associated with a phenotypically heterogeneous neurodegenerative disorder. Our results suggest PS1 as a candidate gene for Pick-type tauopathy without MAPT mutations.

Young onset dementia

Young onset dementia is a challenging clinical problem with potentially devastating medical and social consequences. The differential diagnosis is wide, and includes a number of rare sporadic and hereditary diseases. However, accurate diagnosis is often possible, and all patients should be thoroughly investigated to identify treatable processes. This review presents an approach to the diagnosis, investigation, and management of patients with young onset dementia, with particular reference to common and treatable causes.

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.

Molecular evolution and genetics of the Saitohin gene and tau haplotype in Alzheimer's disease and argyrophilic grain disease

A single nucleotide polymorphism that results in an amino acid change (Q7R) has been identified in the Saitohin (STH) gene and was initially found to be over-represented in the homozygous state in subjects with late-onset Alzheimer's disease (AD). More extensive studies provide limited support for the association with AD, but confirm an association of the Q allele with progressive supranuclear palsy and argyrophilic grain disease. A homologous sequence was found in the appropriate location of the rat and mouse tau genes, but there was no open reading frame allowing STH expression in these species, suggesting relatively recent evolution of this gene. In some non-human primates, the STH gene was identified, and this was found to differ from the human gene at two of 128 amino acids. All primates in which the STH gene was identified were homozygous for the R allele of STH, suggesting this is the ancestral allele. This observation was surprising, in that the Q allele is more common in human populations, and raises the possibility that natural selection has operated to favor individuals carrying this allele. The STH polymorphism is part of the tau gene haplotype, of which two major variants exist in human populations, the Q being part of the H1 haplotype and the R part of the H2 haplotype. More detailed studies confirm the H2 haplotype to be the ancestral tau gene. This situation is reminiscent of the evolution of the apolipoprotein (ApoE) gene, another locus that is potentially important for the risk of development of AD.

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.

Familial Frontotemporal Dementia: From Gene Discovery to Clinical Molecular Diagnostics

Genetic testing is important for diagnosis and prediction of many diseases. The development of a clinical genetic test can be rapid for common disorders, but for rare genetic disorders this process can take years, if it occurs at all. We review the path from gene discovery to development of a clinical genetic test, using frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) as an example of a complex, rare genetic condition. An Institutional Review Board-approved multidisciplinary research program was developed to identify patients with familial frontotemporal dementia. Genetic counseling is provided and DNA obtained to identify mutations associated with FTDP-17. In some cases it may be appropriate for individuals to be given the opportunity to learn information from the research study to prevent unnecessary diagnostic studies or the utilization of inappropriate therapies, and to make predictive testing possible. Mutations identified in a research laboratory must be confirmed in a clinical laboratory to be used clinically. To facilitate the development of clinical genetic testing for a rare disorder, it is useful for a research laboratory to partner with a clinical laboratory. Most clinical molecular assays are developed in research laboratories and must be properly validated. We conclude that the transition of genetic testing for rare diseases from the research laboratory to the clinical laboratory requires a validation process that maintains the quality-control elements necessary for genetic testing but is flexible enough to permit testing to be developed for the benefit of patients and families.