Progranulin mutations in Dutch familial frontotemporal lobar degeneration

Preclinical Interventions in Mouse Models of Frontotemporal Dementia Due to Progranulin Mutations

Heterozygous loss-of-function mutations in progranulin (GRN) cause frontotemporal dementia (FTD), a leading cause of early-onset dementia characterized clinically by behavioral, social, and language deficits. There are currently no FDA-approved therapeutics for FTD-GRN, but this has been an active area of investigation, and several approaches are now in clinical trials. Here, we review preclinical development of therapies for FTD-GRN with a focus on testing in mouse models. Since most FTD-GRN-associated mutations cause progranulin haploinsufficiency, these approaches focus on raising progranulin levels. We begin by considering the disorders associated with altered progranulin levels, and then review the basics of progranulin biology including its lysosomal, neurotrophic, and immunomodulatory functions. We discuss mouse models of progranulin insufficiency and how they have been used in preclinical studies on a variety of therapeutic approaches. These include approaches to raise progranulin expression from the normal allele or facilitate progranulin production by the mutant allele, as well as approaches to directly increase progranulin levels by delivery across the blood-brain barrier or by gene therapy. Several of these approaches have entered clinical trials, providing hope that new therapies for FTD-GRN may be the next frontier in the treatment of neurodegenerative disease.

Neurodegenerative Disease Risk in Carriers of Autosomal Recessive Disease

Genetics has driven significant discoveries in the field of neurodegenerative diseases (NDDs). An emerging theme in neurodegeneration warrants an urgent and comprehensive update: that carrier status of early-onset autosomal recessive (AR) disease, typically considered benign, is associated with an increased risk of a spectrum of late-onset NDDs. Glucosylceramidase beta (GBA1) gene mutations, responsible for the AR lysosomal storage disorder Gaucher disease, are a prominent example of this principle, having been identified as an important genetic risk factor for Parkinson disease. Genetic analyses have revealed further examples, notably GRN, TREM2, EIF2AK3, and several other LSD and mitochondria function genes. In this Review, we discuss the evidence supporting the strikingly distinct allele-dependent clinical phenotypes observed in carriers of such gene mutations and its impact on the wider field of neurodegeneration.

Genotype-phenotype links in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) represents a group of neurodegenerative brain diseases with highly heterogeneous clinical, neuropathological and genetic characteristics. This high degree of heterogeneity results from the presence of several different underlying molecular disease processes; consequently, it is unlikely that all patients with FTLD will benefit from a single therapy. Therapeutic strategies for FTLD are currently being explored, and tools are urgently needed that enable the selection of patients who are the most likely to benefit from a particular therapy. Definition of the phenotypic characteristics in patients with different FTLD subtypes that share the same underlying disease processes would assist in the stratification of patients into homogeneous groups. The most common subtype of FTLD is characterized by TAR DNA-binding protein 43 (TDP43) pathology (FTLD-TDP). In this group, pathogenic mutations have been identified in four genes: C9orf72, GRN, TBK1 and VCP. Here, we provide a comprehensive overview of the phenotypic characteristics of patients with FTLD-TDP, highlighting shared features and differences among groups of patients who have a pathogenic mutation in one of these four genes.

Mutation Frequency of the Major Frontotemporal Dementia Genes, MAPT, GRN and C9ORF72 in a Turkish Cohort of Dementia Patients

‘Microtubule-associated protein tau’ (MAPT), ‘granulin’ (GRN) and ‘chromosome 9 open reading frame72’ (C9ORF72) gene mutations are the major known genetic causes of frontotemporal dementia (FTD). Recent studies suggest that mutations in these genes may also be associated with other forms of dementia. Therefore we investigated whether MAPT, GRN and C9ORF72 gene mutations are major contributors to dementia in a random, unselected Turkish cohort of dementia patients. A combination of whole-exome sequencing, Sanger sequencing and fragment analysis/Southern blot was performed in order to identify pathogenic mutations and novel variants in these genes as well as other FTD-related genes such as the ‘charged multivesicular body protein 2B’ (CHMP2B), the ‘FUS RNA binding protein’ (FUS), the ‘TAR DNA binding protein’ (TARDBP), the ‘sequestosome1’ (SQSTM1), and the ‘valosin containing protein’ (VCP). We determined one pathogenic MAPT mutation (c.1906C>T, p.P636L) and one novel missense variant (c.38A>G, p.D13G). In GRN we identified a probably pathogenic TGAG deletion in the splice donor site of exon 6. Three patients were found to carry the GGGGCC expansions in the non-coding region of the C9ORF72 gene. In summary, a complete screening for mutations in MAPT, GRN and C9ORF72 genes revealed a frequency of 5.4% of pathogenic mutations in a random cohort of 93 Turkish index patients with dementia.

Phenotypic Heterogeneity of Monogenic Frontotemporal Dementia

Frontotemporal dementia (FTD) is a genetically and pathologically heterogeneous disorder characterized by personality changes, language impairment, and deficits of executive functions associated with frontal and temporal lobe degeneration. Different phenotypes have been defined on the basis of presenting clinical symptoms, i.e., the behavioral variant of FTD, the agrammatic variant of primary progressive aphasia, and the semantic variant of PPA. Some patients have an associated movement disorder, either parkinsonism, as in progressive supranuclear palsy and corticobasal syndrome, or motor neuron disease (FTD-MND). A family history of dementia is found in 40% of cases of FTD and about 10% have a clear autosomal-dominant inheritance. Genetic studies have identified several genes associated with monogenic FTD: microtubule-associated protein tau, progranulin, TAR DNA-binding protein 43, valosin-containing protein, charged multivesicular body protein 2B, fused in sarcoma, and the hexanucleotide repeat expansion in intron 1 of the chromosome 9 open reading frame 72. Patients often present with an extensive phenotypic variability, even among different members of the same kindred carrying an identical disease mutation. The objective of the present work is to review and evaluate available literature data in order to highlight recent advances in clinical, biological, and neuroimaging features of monogenic frontotemporal lobar degeneration and try to identify different mechanisms underlying the extreme phenotypic heterogeneity that characterizes this disease.

A network of RNA and protein interactions in Fronto Temporal Dementia

Frontotemporal dementia (FTD) is a neurodegenerative disorder characterized by degeneration of the fronto temporal lobes and abnormal protein inclusions. It exhibits a broad clinicopathological spectrum and has been linked to mutations in seven different genes. We will provide a picture, which connects the products of these genes, albeit diverse in nature and function, in a network. Despite the paucity of information available for some of these genes, we believe that RNA processing and post-transcriptional regulation of gene expression might constitute a common theme in the network. Recent studies have unraveled the role of mutations affecting the functions of RNA binding proteins and regulation of microRNAs. This review will combine all the recent findings on genes involved in the pathogenesis of FTD, highlighting the importance of a common network of interactions in order to study and decipher the heterogeneous clinical manifestations associated with FTD. This approach could be helpful for the research of potential therapeutic strategies.

Therapeutic and diagnostic challenges for frontotemporal dementia

In the search for therapeutic modifiers, frontotemporal dementia (FTD) has traditionally been overshadowed by other conditions such as Alzheimer's disease (AD). A clinically and pathologically diverse condition, FTD has been galvanized by a number of recent discoveries such as novel genetic variants in familial and sporadic forms of disease and the identification of TAR DNA binding protein of 43 kDa (TDP-43) as the defining constituent of inclusions in more than half of cases. In combination with an ever-expanding knowledge of the function and dysfunction of tau-a protein which is pathologically aggregated in the majority of the remaining cases-there exists a greater understanding of FTD than ever before. These advances may indicate potential approaches for the development of hypothetical therapeutics, but FTD remains highly complex and the roles of tau and TDP-43 in neurodegeneration are still wholly unclear. Here the challenges facing potential therapeutic strategies are discussed, which include sufficiently accurate disease diagnosis and sophisticated technology to deliver effective therapies.

Targeted manipulation of the sortilin-progranulin axis rescues progranulin haploinsufficiency

Progranulin (GRN) mutations causing haploinsufficiency are a major cause of frontotemporal lobar degeneration (FTLD-TDP). Recent discoveries demonstrating sortilin (SORT1) is a neuronal receptor for PGRN endocytosis and a determinant of plasma PGRN levels portend the development of enhancers targeting the SORT1–PGRN axis. We demonstrate the preclinical efficacy of several approaches through which impairing PGRN's interaction with SORT1 restores extracellular PGRN levels. Our report is the first to demonstrate the efficacy of enhancing PGRN levels in iPSC neurons derived from frontotemporal dementia (FTD) patients with PGRN deficiency. We validate a small molecule preferentially increases extracellular PGRN by reducing SORT1 levels in various mammalian cell lines and patient-derived iPSC neurons and lymphocytes. We further demonstrate that SORT1 antagonists and a small-molecule binder of PGRN_588–593, residues critical for PGRN–SORT1 binding, inhibit SORT1-mediated PGRN endocytosis. Collectively, our data demonstrate that the SORT1–PGRN axis is a viable target for PGRN-based therapy, particularly in FTD-GRN patients.

Activity-dependent secretion of progranulin from synapses

The secreted growth factor progranulin (PGRN) has been shown to be important for regulating neuronal survival and outgrowth, as well as synapse formation and function. Mutations in the PGRN gene that result in PGRN haploinsufficiency have been identified as a major cause of frontotemporal dementia (FTD). Here we demonstrate that PGRN is colocalized with dense-core vesicle markers and is co-transported with brain-derived neurotrophic factor (BDNF) within axons and dendrites of cultured hippocampal neurons in both anterograde and retrograde directions. We also show that PGRN is secreted in an activity-dependent manner from synaptic and extrasynaptic sites, and that the temporal profiles of secretion are distinct in axons and dendrites. Neuronal activity is also shown to increase the recruitment of PGRN to synapses and to enhance the density of PGRN clusters along axons. Finally, treatment of neurons with recombinant PGRN is shown to increase synapse density, while decreasing the size of the presynaptic compartment and specifically the number of synaptic vesicles per synapse. Together, this indicates that activity-dependent secretion of PGRN can regulate synapse number and structure.

Progranulin: an emerging target for FTLD therapies

Frontotemporal lobar degeneration (FTLD), a neurodegenerative disease primarily affecting the frontal and temporal lobes, is one of the most common types of dementia. While the majority of FTLD cases are sporadic, approximately 10-40% of patients have an inherited form of FTLD. Mutations in the progranulin gene (GRN) have recently been identified as a major cause of FTLD with ubiquitin positive inclusions (FTLD-U). Because over 70 disease-linked GRN mutations cause abnormal deficiencies in the production of PGRN, a protein that plays a crucial role in embryogenesis, cell growth and survival, wound repair and inflammation, researchers now aim to design therapies that would increase PGRN levels in affected individuals, thereby alleviating the symptoms associated with disease. Several compounds and genetic factors, as well as PGRN receptors, have recently been identified because of their ability to regulate PGRN levels. Strict quality control measures are needed given that extreme PGRN levels at either end of the spectrum - too low or too high - can lead to neurodegeneration or cancer, respectively. The aim of this review is to highlight what is known regarding PGRN biology; to improve understanding of the mechanisms involved in regulating PGRN levels and highlight studies that are laying the groundwork for the development of effective therapeutic modulators of PGRN. This article is part of a Special Issue entitled RNA-Binding Proteins.

Progranulin deficiency decreases gross neural connectivity but enhances transmission at individual synapses

Frontotemporal dementia (FTD) has been linked to mutations in the progranulin gene (GRN) that lead to progranulin (PGRN) haploinsufficiency. Thus far, our understanding of the effects of PGRN depletion in the brain has been derived from investigation of gross pathology, and more detailed analyses of cellular function have been lacking. We report that knocking down PGRN levels in rat primary hippocampal cultures reduces neural connectivity by decreasing neuronal arborization and length as well as synapse density. Despite this, the number of synaptic vesicles per synapse and the frequency of mEPSCs are increased in PGRN knockdown cells, suggesting an increase in the probability of release at remaining synapses. Interestingly, we demonstrate that the number of vesicles per synapse is also increased in postmortem brain sections from FTD patients with PGRN haploinsufficiency, relative to controls. Our observations show that PGRN knockdown severely alters neuronal connectivity in vitro and that the synaptic vesicle phenotype observed in culture is consistent with that observed in the hippocampus of FTD patients.

A thorough assessment of benign genetic variability in GRN and MAPT

Mutations in APP, PSEN1, MAPTand GRNare the most common genetic causes of dementia. The previous miss-assignment of pathogenicity to benign variants in these genes stresses the importance of discerning between disease causing mutations and benign variants with no pathogenic effect on the function of the respective protein. In this study we sequenced GRNand MAPTin 282 samples from the Centre d'Etude du Polymorphisme Humain - Human Genome Diversity Cell Line Panel, in order to identify benign variants that could otherwise be mistaken for pathogenic mutations. We found sixteen different non-synonymous changes, eleven of which are novel variants.

The spectrum of mutations in progranulin: a collaborative study screening 545 cases of neurodegeneration

BACKGROUND

Mutation in the progranulin gene (GRN) can cause frontotemporal dementia (FTD). However, it is unclear whether some rare FTD-related GRN variants are pathogenic and whether neurodegenerative disorders other than FTD can also be caused by GRN mutations.

OBJECTIVES

To delineate the range of clinical presentations associated with GRN mutations and to define pathogenic candidacy of rare GRN variants.

DESIGN

Case-control study.

SETTING

Clinical and neuropathology dementia research studies at 8 academic centers.

PARTICIPANTS

Four hundred thirty-four patients with FTD, including primary progressive aphasia, semantic dementia, FTD/amyotrophic lateral sclerosis (ALS), FTD/motor neuron disease, corticobasal syndrome/corticobasal degeneration, progressive supranuclear palsy, Pick disease, dementia lacking distinctive histopathology, and pathologically confirmed cases of frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U); and 111 non-FTD cases (controls) in which TDP-43 deposits were a prominent neuropathological feature, including subjects with ALS, Guam ALS and/or parkinsonism dementia complex, Guam dementia, Alzheimer disease, multiple system atrophy, and argyrophilic grain disease.

MAIN OUTCOME MEASURES

Variants detected on sequencing of all 13 GRN exons and at least 80 base pairs of flanking introns, and their pathogenic candidacy determined by in silico and ex vivo splicing assays.

RESULTS

We identified 58 genetic variants that included 26 previously unknown changes. Twenty-four variants appeared to be pathogenic, including 8 novel mutations. The frequency of GRN mutations was 6.9% (30 of 434) of all FTD-spectrum cases, 21.4% (9 of 42) of cases with a pathological diagnosis of FTLD-U, 16.0% (28 of 175) of FTD-spectrum cases with a family history of a similar neurodegenerative disease, and 56.2% (9 of 16) of cases of FTLD-U with a family history.

CONCLUSIONS

Pathogenic mutations were found only in FTD-spectrum cases and not in other related neurodegenerative diseases. Haploinsufficiency of GRN is the predominant mechanism leading to FTD.

Brain progranulin expression in GRN-associated frontotemporal lobar degeneration

Frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) is characterized by progressive decline in behavior, executive function, and language. Progranulin (GRN) gene mutations are pathogenic for FTLD-TDP, and GRN transcript haploinsufficiency is the proposed disease mechanism. However, the evidence for this hypothesis comes mainly from blood-derived cells; we measured progranulin expression in brain. We characterized mRNA and protein levels of progranulin from four brain regions (frontal cortex, temporal cortex, occipital cortex, and cerebellum) in FTLD-TDP patients with and without GRN mutations, as well as neurologically normal individuals. Moreover, we performed immunohistochemistry to evaluate the degree of TDP-43 pathology and microglial infiltration present in these groups. In most brain regions, patients with GRN mutations showed mRNA levels comparable to normal controls and to FTLD-TDP without GRN mutations. However, GRN transcript levels in a brain region severely affected by disease (frontal cortex) were increased in mutation-bearing patients. When compared with normal individuals, GRN mutation-bearing cases had a significant reduction in the amount of progranulin protein in the cerebellum and occipital cortex, but not in the frontal and temporal cortices. In GRN mutant cases, GRN mRNA originated from the normal allele, and moderate microglial infiltration was observed. In conclusion, GRN mutation carriers have increased levels of mRNA transcript from the normal allele in brain, and proliferation of microglia likely increases progranulin levels in affected regions of the FTLD-TDP brain, and whether or not these findings underlie the accumulation of TDP-43 pathology in FTLD-TDP linked to GRN mutations remains to be determined.

Frequency of ubiquitin and FUS-positive, TDP-43-negative frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is a clinically, genetically and pathologically heterogeneous disorder. Within FTLD with ubiquitin-positive inclusions (FTLD-U), a new pathological subtype named FTLD-FUS was recently found with fused in sarcoma (FUS) positive, TDP-43-negative inclusions, and striking atrophy of the caudate nucleus. The aim of this study was to determine the frequency of FTLD-FUS in our pathological FTLD series, and to describe the clinical, neuroimaging and neuropathological features of FTLD-FUS, especially caudate atrophy. Demographic and clinical data collected prospectively from 387 patients with frontotemporal dementia (FTD) yielded 74 brain specimens. Immunostaining was carried out using a panel of antibodies, including AT-8, ubiquitin, p62, FUS, and TDP-43. Cortical and caudate atrophy on MRI (n = 136) was rated as normal, mild-moderate or severe. Of the 37 FTLD-U cases, 33 were reclassified as FTLD-TDP and four (0.11, 95%: 0.00-0.21) as FTLD-FUS, with ubiquitin and FUS-positive, p62 and TDP-43-negative neuronal intranuclear inclusions (NII). All four FTLD-FUS cases had a negative family history, behavioural variant FTD (bvFTD), and three had an age at onset <or=40 years. MRI revealed mild-moderate or severe caudate atrophy in all, with a mean duration from onset till MRI of 63 months (range 16-119 months). In our total clinical FTD cohort, we found 11 patients (0.03; 95% CI: 0.01-0.05) with bvFTD, negative family history, and age at onset <or=40 years. Caudate atrophy was present in 10 out of 136 MRIs, and included all four FUS-cases. The newly identified FTLD-FUS has a frequency of 11% in FTLD-U, and an estimated frequency of three percent in our clinical FTD cohort. The existence of this pathological subtype can be predicted with reasonable certainty by age at onset <or=40 years, negative family history, bvFTD and caudate atrophy on MRI.

Review: Recent progress in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is a highly familial condition and is increasingly being recognized as an important form of dementia. The literature published on this disease is often difficult to collate due to the wide range in nomenclature used. Thankfully, consensus recommendations have now been published to address this issue and hopefully the community will adopt these as intended. Much progress has been made in our understanding of the clinical, pathological and genetic understanding of FTLD in recent years. Progranulin and TDP-43 have recently been identified as new important proteins involved in the pathophysiology of FTLD and this latter protein may have potential as a biomarker of this disease. However, much remains before we have a full picture of the genes that cause FTLD and the biological pathways in which they function. The purpose of this review is to summarize the current concepts and recent advances in our knowledge of this disease.

Comprehensive mRNA expression profiling distinguishes tauopathies and identifies shared molecular pathways

Background

Understanding the aetiologies of neurodegenerative diseases such as Alzheimer's disease (AD), Pick's disease (PiD), Progressive Supranuclear Palsy (PSP) and Frontotemporal dementia (FTD) is often hampered by the considerable clinical and molecular overlap between these diseases and normal ageing. The development of high throughput genomic technologies such as microarrays provide a new molecular tool to gain insight in the complexity and relationships between diseases, as they provide data on the simultaneous activity of multiple genes, gene networks and cellular pathways.

Methodology/Principal Findings

We have constructed genome wide expression profiles from snap frozen post-mortem tissue from the medial temporal lobe of patients with four neurodegenerative disorders (5 AD, 5 PSP, 5 PiD and 5 FTD patients) and 5 control subjects. All patients were matched for age, gender, ApoE-ε and MAPT (tau) haplotype. From all groups a total of 790 probes were shown to be differently expressed when compared to control individuals. The results from these experiments were then used to investigate the correlations between clinical, pathological and molecular findings. From the 790 identified probes we extracted a gene set of 166 probes whose expression could discriminate between these disorders and normal ageing.

Conclusions/Significance

From genome wide expression profiles we extracted a gene set of 166 probes whose expression could discriminate between neurological disorders and normal ageing. This gene set can be further developed into an accurate microarray-based classification test. Furthermore, from this dataset we extracted a disease specific set of genes and identified two aging related transcription factors (FOXO1A and FOXO3A) as possible drug targets related to neurodegenerative disease.

A decade of genetic counseling in frontotemporal dementia affected families: few counseling requests and much familial opposition to testing

A decade of genetic counseling of frontotemporal dementia (FTD) affected families has generated two important observations. First, the uptake rate for presymptomatic testing for FTD is low in our department of Clinical Genetics at the Erasmus Medical Center in the Netherlands. Second, FTD at-risk counselees reported substantial familial opposition to genetic testing, which is distinct from the attitude in Huntington Disease affected families. We hypothesize that the low acceptance for FTD genetic counseling is consequential to the familial opposition and explain this within the theoretical framework of separation-individuation. Furthermore, we hypothesize that separation-individuation problems do not similarly influence the acceptance of HD genetic counseling, due to the educative role of the well-organised patient organization for HD in the Netherlands. We offer counseling recommendations that serve to facilitate the individuation of the counselee with respect to the FTD genetic test.

Frequency of progranulin mutations in a German cohort of 79 frontotemporal dementia patients

Mutations of the progranulin gene lead to progranulin haploinsufficiency and to frontotemporal lobar degeneration (FTD) with TDP-43 positive inclusions. It is assumed that unknown genetic, epigenetic and environmental factors are responsible for the observed marked degree of phenotypic variability among mutation carriers. This is the first published series of German FTD cases screened for progranulin mutations. Mean age at onset was 62 years, 19 patients (24%) had a positive family history of dementia, and 11 patients (14%) had a positive family history for probable FTD. Data on FTD subtypes are presented. Two mutations were identified (3%), one of which has been described previously. Clinically, both patients showed the frontal-behavioural variant type of FTD. Remarkably, a sibling of one case presented with progressive nonfluent aphasia, clinically distinct from the brother. We also performed quantitative PCR analyses to detect potential whole progranulin gene and exon deletions. Here, results were negative.

Novel progranulin mutation: screening for PGRN mutations in a Portuguese series of FTD/CBS cases

Mutations in the progranulin (PGRN) gene were recently described as the cause of ubiquitin positive frontotemporal dementia (FTD) in many families. Different frequencies of these genetic changes have been reported in diverse populations leading us to determine if these mutations were a major cause of FTD in the Portuguese population. The entire coding sequence plus exon 0 of PGRN were sequenced in a consecutive series of 46 FTD/CBS Portuguese patients. Two mutations were found: a novel pathogenic insertion (p.Gln300GlnfsX61) and a previously described point variant (p.T182M) of unclear pathogenicity. Pathogenic mutations in the PGRN gene were found in one of the 36 probands studied (3% of the probands in our series) who had a corticobasal syndrome presentation, indicating that in the Portuguese population, mutations in this gene are not a major cause of FTD.

Granulin mutations associated with frontotemporal lobar degeneration and related disorders: an update

Mutations in the gene encoding granulin (HUGO gene symbol GRN, also referred to as progranulin, PGRN), located at chromosome 17q21, were recently linked to tau-negative ubiquitin-positive frontotemporal lobar degeneration (FTLDU). Since then, 63 heterozygous mutations were identified in 163 families worldwide, all leading to loss of functional GRN, implicating a haploinsufficiency mechanism. Together, these mutations explained 5 to 10% of FTLD. The high mutation frequency, however, might still be an underestimation because not all patient samples were examined for all types of loss-of-function mutations and because several variants, including missense mutations, have a yet uncertain pathogenic significance. Although the complete phenotypic spectrum associated with GRN mutations is not yet fully characterized, it was shown that it is highly heterogeneous, suggesting the influence of modifying factors. A role of GRN in neuronal survival was suggested but the exact mechanism by which neurodegeneration and deposition of pathologic brain inclusions occur still has to be clarified.

Update on recent molecular and genetic advances in frontotemporal lobar degeneration

Great strides have been made in the last 2 years in the field of frontotemporal lobar degeneration (FTLD), particularly with respect to the genetics and molecular biology of FTLD with ubiquitinated inclusions. It is now clear that most cases of familial FTLD with ubiquitinated inclusions have mutations in the progranulin gene, located on chromosome 17. It is also clear that most ubiquitinated inclusions in FTLD with ubiquitinated inclusions are composed primarily of TAR DNA-binding protein-43. Thus, FTLDs can be separated into 2 major groups (i.e. tauopathies and ubiquitinopathies), and most of the ubiquitinopathies can now be defined as TAR DNA-binding protein-43 proteinopathies. Many of the familial FTLDs are linked to chromosome 17, including both the familial tauopathies and the familial TAR DNA-binding protein-43 proteinopathies with progranulin mutations. This review highlights the neuropathologic features and the most important discoveries of the last 2 years and places these findings into the historical context of FTLD.

Mutations in progranulin (GRN) within the spectrum of clinical and pathological phenotypes of frontotemporal dementia

BACKGROUND

Frontotemporal dementia (FTD) is predominantly a presenile disorder that is characterised by behavioural changes and cognitive impairment, particularly in language and executive functions, and is associated with neurodegeneration in the frontal or temporal cortices, or both. Research into FTD has made many advances over the past 20 years that have important implications for clinical practice. Different clinical variants (ie, behavioural, aphasic, and motor neuron disease variants) are now recognised as part of the clinical spectrum of FTD. Neuropathologically, the disease can be divided into two main pathological subtypes: frontotemporal lobar degeneration (FTLD) with neuronal and glial tau inclusions (FTLD-tau); and FTLD with neuronal inclusions that are positive for ubiquitin (FTLD-U). 20-30% of cases of FTD follow an autosomal dominant pattern of inheritance, and half of which are caused by defects in MAPT, CHMP2B, and VCP.

RECENT DEVELOPMENTS

Mutations in the gene that encodes progranulin (GRN) on chromosome 17q21-22 have been identified in patients with hereditary FTD who have tau-negative, ubiquitin-positive inclusions. The recognition of the clinical phenotype associated with more than 50 different mutations in GRN has expanded the clinical knowledge of FTD to include presentations that resemble Alzheimer's disease, Lewy body disease, and corticobasal syndrome, with a variable age at onset (35-89 years) within families. Another recent breakthrough is the identification of the TAR DNA-binding protein (TARDBP; also known as TDP-43) as the main constituent of FTLD-U with mutations in GRN and with mutations in VCP, as well as in FTLD with amyotrophic lateral sclerosis. WHERE NEXT?: To develop therapeutic strategies to prevent FTD or delay its progression we must understand whether the loss of progranulin leads to the accumulation of TARDBP. In this Rapid Review, we focus on the clinical and pathological phenotypes associated with mutations in GRN, and distinguish those from other forms of hereditary FTD. In addition, we discuss the potential association of mutations in GRN on the pathophysiology of FTD with the accumulation of TARDBP.

Novel exon 1 progranulin gene variant in Alzheimer's disease

BACKGROUND AND PURPOSE

Progranulin (PGRN) expression is increased in activated microglia in Alzheimer's disease (AD) brain, suggesting a potential role in this pathology.

METHODS

A mutation scanning of exons and flanking regions of PGRN was carried out in 120 patients with sporadic frontotemporal lobar degeneration and 145 with sporadic AD.

RESULTS

Amongst variants not yet deposited, a novel allelic variant was identified in Exon 1 (g100169G > A). It leads to an amino acidic change (p.Gly35Arg) and was observed in a patient with late onset AD. In silico analysis predicted that this mutation is possibly damaging. A second variant (g.100165C > T), resulting in a silent mutation (pAsp33Asp), was found in a patient with semantic dementia and in another with early onset AD. Both variants were absent in 226 controls. In addition, two rare non-pathogenic variants lying very close to PGRN splice-site regions (IVS2 + 7-->G > A and IVS7 + 7-->G > A) were observed. Transcriptional analysis in peripheral blood mononuclear cells from patients demonstrated they do not affect exon splicing.

CONCLUSIONS

A novel putative PGRN mutation leading to an amino acidic substitution was identified in a patient with clinical AD.

Parietal lobe deficits in frontotemporal lobar degeneration caused by a mutation in the progranulin gene

OBJECTIVE

To describe the clinical, neuropsychologic, and radiologic features of a family with a C31LfsX35 mutation in the progranulin gene CCDS11483.1).

DESIGN

Case series.

PATIENTS

A large British kindred (DRC255) with a PGRN mutation was assessed. Affected individuals presented with a mean age of 57.8 years (range, 54-67 years) and a mean disease duration of 6.1 years (range, 2-11 years).

RESULTS

All patients exhibited a clinical and radiologic phenotype compatible with frontotemporal lobar degeneration based on current consensus criteria. However, unlike sporadic frontotemporal lobar degeneration, parietal deficits, consisting of dyscalculia, visuoperceptual /visuospatial dysfunction, and/or limb apraxia, were a common feature, and brain imaging showed posterior extension of frontotemporal atrophy to involve the parietal lobes. Other common clinical features included language output impairment with either dynamic aphasia or nonfluent aphasia and a behavioral syndrome dominated by apathy.

CONCLUSION

We suggest that parietal deficits may be a prominent feature of PGRN mutations and that these deficits may be caused by disruption of frontoparietal functional pathways.

Progranulin functions as a neurotrophic factor to regulate neurite outgrowth and enhance neuronal survival

Recently, mutations in the progranulin (PGRN) gene were found to cause familial and apparently sporadic frontotemporal lobe dementia (FTLD). Moreover, missense changes in PGRN were identified in patients with motor neuron degeneration, a condition that is related to FTLD. Most mutations identified in patients with FTLD until now have been null mutations. However, it remains unknown whether PGRN protein levels are reduced in the central nervous system from such patients. The effects of PGRN on neurons also remain to be established. We report that PGRN levels are reduced in the cerebrospinal fluid from FTLD patients carrying a PGRN mutation. We observe that PGRN and GRN E (one of the proteolytic fragments of PGRN) promote neuronal survival and enhance neurite outgrowth in cultured neurons. These results demonstrate that PGRN/GRN is a neurotrophic factor with activities that may be involved in the development of the nervous system and in neurodegeneration.

A distinct clinical, neuropsychological and radiological phenotype is associated with progranulin gene mutations in a large UK series

Mutations in the progranulin gene (GRN) are a major cause of frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U) but the distinguishing clinical and anatomical features of this subgroup remain unclear. In a large UK cohort we found five different frameshift and premature termination mutations likely to be causative of FTLD in 25 affected family members. A previously described 4-bp insertion mutation in GRN exon 2 comprised the majority of cases in our cohort (20/25), with four novel mutations being identified in the other five affected members. Additional novel missense changes were discovered, of uncertain pathogenicity, but deletion of the entire gene was not detected. The patient collection was investigated by a single tertiary referral centre and is enriched for familial early onset FTLD with a high proportion of patients undergoing neuropsychological testing, MRI and eventual neuropathological diagnosis. Age at onset was variable, but four mutation carriers presented in their 40s and when analysed as a group, the mean age at onset of disease in GRN mutation carriers was later than tau gene (MAPT) mutation carriers and duration of disease was shorter when compared with both MAPT and FTLD-U without mutation. The most common clinical presentation seen in GRN mutation carriers was behavioural variant FTLD with apathy as the dominant feature. However, many patients had language output impairment that was either a progressive non-fluent aphasia or decreased speech output consistent with a dynamic aphasia. Neurological and neuropsychological examination also suggests that parietal lobe dysfunction is a characteristic feature of GRN mutation and differentiates this group from other patients with FTLD. MR imaging showed evidence of strikingly asymmetrical atrophy with the frontal, temporal and parietal lobes all affected. Both right- and left-sided predominant atrophy was seen even within the same family. As a group, the GRN carriers showed more asymmetry than in other FTLD groups. All pathologically investigated cases showed extensive type 3 TDP-43-positive pathology, including frequent neuronal cytoplasmic inclusions, dystrophic neurites in both grey and white matter and also neuronal intranuclear inclusions. Finally, we confirmed a modifying effect of APOE-E4 genotype on clinical phenotype with a later onset in the GRN carriers suggesting that this gene has distinct phenotypic effects in different neurodegenerative diseases.

Progranulin locus deletion in frontotemporal dementia

Ubiquitin-positive, tau-negative, frontotemporal dementia (FTD) is caused by null mutations in progranulin (PGRN; HUGO gene symbol GRN), suggesting a haploinsufficiency mechanism. Since whole gene deletions also lead to the loss of a functional allele, we performed systematic quantitative analyses of PGRN in a series of 103 Belgian FTD patients. We identified in one patient (1%) a genomic deletion that was absent in 267 control individuals. The deleted segment was between 54 and 69 kb in length and comprised PGRN and two centromeric neighboring genes RPIP8 (HUGO gene symbol RUNDC3A) and SLC25A39. The patient presented clinically with typical FTD without additional symptoms, consistent with haploinsufficiency of PGRN being the only gene contributing to the disease phenotype. This study demonstrates that reduced PGRN in absence of mutant protein is sufficient to cause neurodegeneration and that previously reported PGRN mutation frequencies are underestimated.

Frequency and clinical characteristics of progranulin mutation carriers in the Manchester frontotemporal lobar degeneration cohort: comparison with patients with MAPT and no known mutations

Two hundred and twenty-three consecutive patients fulfilling clinical diagnostic criteria for frontotemporal lobar degeneration (FTLD), and 259 patients with motor neuron disease (MND), for whom genomic DNA was available, were investigated for the presence of mutations in tau (MAPT) and progranulin (PGRN) genes. All FTLD patients had undergone longitudinal neuropsychological and clinical assessment, and in 44 cases, the diagnosis had been pathologically confirmed at post-mortem. Six different PGRN mutations were found in 13 (6%) patients with FTLD. Four apparently unrelated patients shared exon Q415X 10 stop codon mutation. However, genotyping data revealed all four patients shared common alleles of 15 SNPs from rs708386 to rs5848, defining a 45.8-kb haplotype containing the whole PGRN gene, suggesting they are related. Three patients shared exon 11 R493X stop codon mutation. Four patients shared exon 10 V452WfsX38 frameshift mutation. Two of these patients were siblings, though not apparently related to the other patients who in turn appeared unrelated. One patient had exon 1 C31LfsX34 frameshift mutation, one had exon 4 Q130SfsX130 frameshift mutation and one had exon 10 Q468X stop codon mutation. In addition, two non-synonymous changes were detected: G168S change in exon 5 was found in a single patient, with no family history, who showed a mixed FTLD/MND picture and A324T change in exon 9 was found in two cases; one case of frontotemporal dementia (FTD) with a sister with FTD+MND and the other in a case of progressive non-fluent aphasia (PNFA) without any apparent family history. MAPT mutations were found in 17 (8%) patients. One patient bore exon 10 + 13 splice mutation, and 16 patients bore exon 10 + 16 splice mutation. When PGRN and MAPT mutation carriers were excluded, there were no significant differences in either the allele or genotype frequencies, or haplotype frequencies, between the FTLD cohort as a whole, or for any clinical diagnostic FTLD subgroup, and 286 controls or between MND cases and controls. However, possession of the A allele of SNP rs9897526, in intron 4 of PGRN, delayed mean age at onset by approximately 4 years. Patients with PGRN and MAPT mutations did not differ significantly from other FTLD cases in terms of gender distribution or total duration of illness. However, a family history of dementia in a first-degree relative was invariably present in MAPT cases, but not always so in PGRN cases. Onset of illness was earlier in MAPT cases compared to PGRN and other FTLD cases. PNFA, combined with limb apraxia was significantly more common in PGRN mutation cases than other FTLD cases. By contrast, the behavioural disorder of FTD combined with semantic impairment was a strong predictor of MAPT mutations. These findings complement recent clinico-pathological findings in suggesting identifiable associations between clinical phenotype and genotype in FTLD.

The tauopathy associated with mutation +3 in intron 10 of Tau: characterization of the MSTD family

Multiple system tauopathy with presenile dementia (MSTD) is an inherited disease caused by a (g) to (a) transition at position +3 in intron 10 of Tau. It belongs to the spectrum of frontotemporal dementia and parkinsonism linked to chromosome 17 with mutations in Tau (FTDP-17T). Here we present the longitudinal clinical, neuropsychological, neuroimaging, neuropathological, biochemical and genetic characterization of the MSTD family. Presenting signs were consistent with the behavioural variant of frontotemporal dementia in 17 of 21 patients. Two individuals presented with an atypical form of progressive supranuclear palsy and two others with either severe postural imbalance or an isolated short-term memory deficit. Memory impairment was present at the onset in 15 patients, with word finding difficulties and stereotyped speech also being common. Parkinsonism was first noted 3 years after the onset of symptoms. Neuroimaging showed the most extensive grey matter loss in the hippocampus, parahippocampal gyrus and frontal operculum/insular cortex of the right hemisphere and, to a lesser extent, in the anterior cingulate gyrus, head of the caudate nucleus and the posterolateral orbitofrontal cortex and insular cortex bilaterally. Neuropathologically, progressive nerve cell loss, gliosis and coexistent neuronal and/or glial deposits consisting mostly of 4-repeat tau were present in frontal, cingulate, temporal and insular cortices, white matter, hippocampus, parahippocampus, basal ganglia, selected brainstem nuclei and spinal cord. Tau haplotyping indicated that specific haplotypes of the wild-type allele may act as modifiers of disease presentation. Quantitative neuroimaging has been used to analyse the progression of atrophy in affected individuals and for predicting disease onset in an asymptomatic mutation carrier. This multidisciplinary study provides a comprehensive description of the natural history of disease in one of the largest known families with FTDP-17T.

Frontotemporal lobar degeneration with ubiquitin-positive inclusions: a molecular genetic update

Frontotemporal lobar degeneration (FTLD) is a clinically, pathologically and genetically highly complex disorder. In the last few years enormous progress has been made in dissecting the genetic etiology of FTLD. Mutations have been identified in the progranulin gene (PGRN), the charged multivesicular body protein 2B gene (CHMP2B) and the valosin-containing protein gene (VCP). Mutations in these genes all lead to FTLD pathology characterized by ubiquitin-immunoreactive neuronal cytoplasmic and intranuclear lentiform inclusions (FTLD-U). The similar pathology suggests that these genes may be connected trough a common disease pathway leading to neurodegeneration and the formation of these pathognomic inclusions. This review focuses on the molecular genetic processes underlying FTLD-U pathology.

The genetics of frontotemporal lobar degeneration

The clinical disorders associated with frontotemporal lobar degeneration (FTLD) are increasingly recognized as an important cause of early-onset dementia. Patients usually present with progressive changes in personality, behavior, or language, progressing to general cognitive impairment and ultimately death. In the past decade, improved clinical and histopathologic characterization uncovered extensive heterogeneity, and multiple clinical and pathologic FTLD subtypes were defined. Simultaneously, the discovery of four causal FTLD genes emphasized the genetic complexity associated with FTLD. More recently, the field of FTLD has gained increased attention as a result of two major findings. First, mutations in the progranulin gene (PGRN) were recognized as a major cause of FTLD with ubiquitin-positive and tau-negative inclusions (FTLD-U), and subsequently the TAR DNA-binding protein-43 (TDP-43) was identified as a key protein within the ubiquitinated inclusions in FTLD-U and amyotrophic lateral sclerosis (ALS). In this report, we outline the progress made in the study of the genetic etiologies and neuropathologic substrates in FTLD.

The molecular genetics and neuropathology of frontotemporal lobar degeneration: recent developments

The past year has seen a number of significant advances in our understanding of the neuropathological and molecular genetic basis of frontotemporal lobar degeneration (FTLD). Whereas, in the past, most attention focused on FTLD associated with tau-based pathology and microtubule associated protein tau gene (MAPT) mutations, there has recently been greater attention paid to non-tau FTLD. FTLD with tau-negative, ubiquitinated inclusions (FTLD-U) is now recognized as the most common pathology associated with clinical FTLD. Mutations in the progranulin gene (PGRN) have been identified as the cause of FTLD-U linked to chromosome 17. A rapidly growing number of PGRN mutations have been identified, and to date, all appear to cause FTLD by reducing the amount of functional PGRN protein (haploinsufficiency). The neuropathology associated with each of the known non-MAPT FTLD genes and loci (PGRN, valosin-containing protein gene, CHMP2B and 9p), has been shown to be a specific subtype of FTLD-U. The ubiquitinated pathological protein in FTLD-U has been identified as TAR deoxyribonucleic acid-binding protein with M (r) 43 kDa (TDP-43). Immunohistochemical and biochemical studies of TDP-43 have helped to clarify the relationship between different sub-types of FTLD-U and related conditions. It is anticipated that these discoveries will facilitate the development of new diagnostic tests and therapeutics.

Progranulin null mutations in both sporadic and familial frontotemporal dementia

Frontotemporal dementia (FTD) is the second most frequent type of neurodegenerative dementias. Mutations in the progranulin gene (GRN, PGRN) were recently identified in FTDU-17, an FTD subtype characterized by ubiquitin-immunoreactive inclusions and linkage to chromosome 17q21. We looked for PGRN mutations in a large series of 210 FTD patients (52 familial, 158 sporadic) to accurately evaluate the frequency of PGRN mutations in both sporadic and familial FTD, and FTD with associated motoneuron disease (FTD-MND), as well as to study the clinical phenotype of patients with a PGRN mutation. We identified nine novel PGRN null mutations in 10 index patients. The relative frequency of PGRN null mutations in FTD was 4.8% (10/210) and 12.8% (5/39) in pure familial forms. Interestingly, 5/158 (3.2%) apparently sporadic FTD patients carried a PGRN mutation, suggesting the possibility of de novo mutations or incomplete penetrance. In contrast, none of the 43 patients with FTD-MND had PGRN mutations, supporting that FTDU-17 and FTD-MND are genetically distinct. The clinical phenotype of PGRN mutation carriers was particular because of the wide range in onset age and the frequent occurrence of early apraxia (50%), visual hallucinations (30%), and parkinsonism (30%) during the course of the disease. This study supports that PGRN null mutations represent a more frequent cause of FTD than MAPT mutations (4.8% vs. 2.9%) but are not responsible for FTD-MND. It also demonstrates that half of the patients with a PGRN mutation in our series had no apparent family history of dementia. Taking this into account, genetic testing should be now considered more systematically, even in patients without obvious familial history of FTD.

The neuropathology and clinical phenotype of FTD with progranulin mutations

Mutations in the progranulin gene (PGRN), on chromosome 17q21, have recently been identified as a major cause of familial frontotemporal dementia (FTD). These cases have a characteristic pattern of neuropathology that is a distinct subtype of frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U), with lentiform neuronal intranuclear inclusions being a consistent feature. There is no abnormal accumulation of PGRN protein in the brain and immunohistochemical and biochemical analysis indicates that the ubiquitinated pathological protein is TDP-43. In these families, FTD is inherited in an autosomal dominant fashion with high penetrance. The clinical phenotype is usually a combination of behavioural abnormality and language disturbance that is most often a form of primary progressive aphasia. Mild parkinsonism is common but motor neuron disease is notably rare. Marked variation in the disease course and clinical features are common, not only between families with different mutations, but also within individual families. This degree of clinical variability makes it difficult to predict which cases of familial FTD will turn out to have a PGRN mutation.

Progranulin and frontotemporal lobar degeneration

Frontotemporal lobar degeneration is the term used to describe the non-Alzheimer clinical syndromes of frontotemporal dementia, semantic dementia and progressive non-fluent aphasia, regardless of the underlying neuropathological features. Considerable progress has been made in recent years in our understanding of the aetiology of this disorder, notably the identification of mutations in tau and progranulin genes, both on chromosome 17q21. Mutations in tau appear to affect the ability of tau to bind microtubules and/or increase this protein's ability to form fibrils. In contrast, progranulin mutations cause haploinsufficiency leading to TDP-43 accumulation. These genes collectively account for 10-20% of FTLD. However, it is clear that much remains to be discovered before our knowledge of this heterogeneous condition is complete.

The complex aetiology of frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is now a widely recognised form of dementia. This heterogeneous disease has been of particular interest to geneticists due to its high rate of heritability with up to 40% of patients reporting a family history of the disease in at least one extra family member. There have been several chromosome loci linked to this disorder and three genes have already been identified. Remarkably, it has been recently demonstrated that 2 of these are only 1.7 Mb from one another on chromosome 17q21, these being tau and progranulin. The identification of these genes has contributed greatly to our understanding of the differing neuropathologies associated with FTLD. Furthermore, the discovery that TDP-43 is a component of the neuronal inclusions seen in the most common neuropathological subtype has also helped expand the biochemical pathways that are the focus of much FTLD research. Nevertheless, other genes causing FTLD remain to be identified and their biology elucidated before we have a complete understanding of the complex aetiology of this disease.