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

Characterization of Progranulin Gene Mutations in Portuguese Patients with Frontotemporal Dementia

In Portugal, heterozygous loss-of-function mutations in the progranulin (GRN) gene account for approximately half of the genetic mediated forms of frontotemporal dementia (FTD). GRN mutations reported thus far cause FTD through a haploinsufficiency disease mechanism. Herein, we aim to unveil the GRN mutation spectrum, investigated in 257 FTD patients and 19 family members from the central/north region of Portugal using sequencing methods. Seven different pathogenic variants were identified in 46 subjects including 40 patients (16%) and 6 relatives (32%). bvFTD was the most common clinical presentation among the GRN mutation patients, who showed a global pattern of moderate-to-severe frontotemporoparietal deficits in the neuropsychological evaluation. Interestingly, two mutations were novel (p.Thr238Profs*18, p.Leu354Profs*16), and five were previously described, although three of them only in the Portuguese population, suggesting a population-specific GRN mutational spectrum. The subjects harboring a GRN mutation showed a significant reduction in serum PGRN levels, supporting the pathogenic nature of these variants. This work broadens the mutation spectrum of GRN and the identification of the underlying GRN mutations provided an accurate genetic counselling and allowed the enrolment of subjects with GRN mutations (both asymptomatic and symptomatic) in ongoing clinical trials, which is essential to test new drugs for the disease.

Rare exonic variant affects GRN splicing and contributes to frontotemporal lobar degeneration

Heterozygous loss-of-function (LOF) mutations in the progranulin gene (GRN) cause frontotemporal lobar degeneration (FTLD) by a mechanism of haploinsufficiency. For most missense mutations, the contribution to FTLD is however unclear. We studied the pathogenicity of rare GRN missense mutations using patient biomaterials. We identified a new mutation in GRN, c.1178 A>C, in a patient with a diagnosis of primary progressive aphasia. Neuropathological examination of autopsied brain showed FTLD with TAR DNA-binding protein 43 (FTLD-TDP) type A pathology with concomitant Alzheimer's disease pathology. Serum progranulin protein levels were reduced to levels comparable to known LOF mutations. The mutation is in the last codon of exon 10, in the splice donor sequence. Our data provide evidence that the mutation leads to aberrant splicing, resulting in a frameshift (p.(Glu393AlafsTer31)) and consequently nonsense-mediated mRNA decay. Our finding demonstrates that carefully examining sequencing data around splice sites is needed since this mutation was annotated as a missense mutation. Unraveling the pathogenicity of variants of unknown significance is important for clinical diagnosis and genetic counseling.

Progranulin loss results in sex-dependent dysregulation of the peripheral and central immune system

Introduction

Progranulin (PGRN) is a secreted glycoprotein, the expression of which is linked to several neurodegenerative diseases. Although its specific function is still unclear, several studies have linked it with lysosomal functions and immune system regulation. Here, we have explored the role of PGRN in peripheral and central immune system homeostasis by investigating the consequences of PGRN deficiency on adaptive and innate immune cell populations.

Methods

First, we used gene co-expression network analysis of published data to test the hypothesis that Grn has a critical role in regulating the activation status of immune cell populations in both central and peripheral compartments. To investigate the extent to which PGRN-deficiency resulted in immune dysregulation, we performed deep immunophenotyping by flow cytometry of 19-24-month old male and female Grn-deficient mice (PGRN KO) and littermate Grn-sufficient controls (WT).

Results

Male PGRN KO mice exhibited a lower abundance of microglial cells with higher MHC-II expression, increased CD44 expression on monocytes in the brain, and more CNS-associated CD8+ T cells compared to WT mice. Furthermore, we observed an increase in CD44 on CD8+ T cells in the peripheral blood. Female PGRN KO mice also had fewer microglia compared to WT mice, and we also observed reduced expression of MHC-II on brain monocytes. Additionally, we found an increase in Ly-6Chigh monocyte frequency and decreased CD44 expression on CD8+ and CD4+ T cells in PGRN KO female blood. Given that Gpnmb, which encodes for the lysosomal protein Glycoprotein non-metastatic melanoma protein B, has been reported to be upregulated in PGRN KO mice, we investigated changes in GPNMB protein expression associated with PGRN deficits and found that GPNMB is modulated in myeloid cells in a sex-specific manner.

Discussion

Our data suggest that PGRN and GPNMB jointly regulate the peripheral and the central immune system in a sex-specific manner; thus, understanding their associated mechanisms could pave the way for developing new neuroprotective strategies to modulate central and peripheral inflammation to lower risk for neurodegenerative diseases and possibly delay or halt progression.

Genotype-phenotype correlation in the spectrum of frontotemporal dementia-parkinsonian syndromes and advanced diagnostic approaches

The term frontotemporal dementia (FTD) refers to a group of progressive neurodegenerative disorders characterized mainly by atrophy of the frontal and anterior temporal lobes. Based on clinical presentation, three main clinical syndromes have traditionally been described: behavioral variant frontotemporal dementia (bvFTD), non-fluent/agrammatic primary progressive aphasia (nfPPA), and semantic variant PPA (svPPA). However, over the last 20 years, it has been recognized that cognitive phenotypes often overlap with motor phenotypes, either motor neuron diseases or parkinsonian signs and/or syndromes like progressive supranuclear palsy (PSP) and cortico-basal syndrome (CBS). Furthermore, FTD-related genes are characterized by genetic pleiotropy and can cause, even in the same family, pure motor phenotypes, findings that underlie the clinical continuum of the spectrum, which has pure cognitive and pure motor phenotypes as the extremes. The genotype-phenotype correlation of the spectrum, FTD-motor neuron disease, has been well defined and extensively investigated, while the continuum, FTD-parkinsonism, lacks a comprehensive review. In this narrative review, we describe the current knowledge about the genotype-phenotype correlation of the spectrum, FTD-parkinsonism, focusing on the phenotypes that are less frequent than bvFTD, namely nfPPA, svPPA, PSP, CBS, and cognitive-motor overlapping phenotypes (i.e. PPA + PSP). From a pathological point of view, they are characterized mainly by the presence of phosphorylated-tau inclusions, either 4 R or 3 R. The genetic correlate of the spectrum can be heterogeneous, although some variants seem to lead preferentially to specific clinical syndromes. Furthermore, we critically review the contribution of genome-wide association studies (GWAS) and next-generation sequencing (NGS) in disentangling the complex heritability of the FTD-parkinsonism spectrum and in defining the genotype-phenotype correlation of the entire clinical scenario, owing to the ability of these techniques to test multiple genes, and so to allow detailed investigations of the overlapping phenotypes. Finally, we conclude with the importance of a detailed genetic characterization and we offer to patients and families the chance to be included in future randomized clinical trials focused on autosomal dominant forms of FTLD.

Cerebrospinal Fluid Biomarker Profile in TDP-43-Related Genetic Frontotemporal Dementia

Cerebrospinal fluid (CSF) biomarkers, namely total tau, phospho-tau and amyloid beta peptides, have received much attention specifically regarding Alzheimer’s disease (AD), since they can detect the biochemical fingerprint of AD and serve as a diagnostic tool for accurate and early diagnosis during life. In the same way, biomarkers for other neurodegenerative disease pathologies are also needed. We present a case series of six patients with genetic frontotemporal dementia (FTD), with TDP-43 underlying proteinopathy, in an attempt to assess TDP-43 as a novel biomarker alone and in combination with established AD biomarkers for this specific patient group, based on the principles of personalized and precision medicine. Our results indicate that genetic TDP-43-FTD is characterized by increased CSF TPD-43 and increased TDP-43 × τΤ/τP-181 combination. Hence, TDP-43 combined with tau proteins could be a useful tool for the diagnosis of genetic FTD with TDP-43 underling histopathology, supplementing clinical, neuropsychological and imaging data.

Frontotemporal Dementia and Glucose Metabolism

Frontotemporal dementia (FTD), hallmarked by antero-temporal degeneration in the human brain, is the second most common early onset dementia. FTD is a diverse disease with three main clinical presentations, four different identified proteinopathies and many disease-associated genes. The exact pathophysiology of FTD remains to be elucidated. One common characteristic all forms of FTD share is the dysregulation of glucose metabolism in patients’ brains. The brain consumes around 20% of the body’s energy supply and predominantly utilizes glucose as a fuel. Glucose metabolism dysregulation could therefore be extremely detrimental for neuronal health. Research into the association between glucose metabolism and dementias has recently gained interest in Alzheimer’s disease. FTD also presents with glucose metabolism dysregulation, however, this remains largely an unexplored area. A better understanding of the link between FTD and glucose metabolism may yield further insight into FTD pathophysiology and aid the development of novel therapeutics. Here we review our current understanding of FTD and glucose metabolism in the brain and discuss the evidence of impaired glucose metabolism in FTD. Lastly, we review research potentially suggesting a causal relationship between FTD proteinopathies and impaired glucose metabolism in FTD.

New insights and therapeutic opportunities for progranulin-deficient frontotemporal dementia

Frontotemporal dementia (FTD) is the second most common form of dementia. It affects the frontal and temporal lobes of the brain and has a highly heterogeneous clinical representation with patients presenting with a wide range of behavioral, language, and executive dysfunctions. Etiology of FTD is complex and consists of both familial and sporadic cases. Heterozygous mutations in the GRN gene, resulting in GRN haploinsufficiency, cause progranulin (PGRN)-deficient FTD characterized with cytoplasmic mislocalization of TAR DNA-binding protein 43 kDa (TDP-43) aggregates. GRN codes for PGRN, a secreted protein that is also localized in the endolysosomes and plays a critical role in regulating lysosomal homeostasis. How PGRN deficiency modulates immunity and causes TDP-43 pathology and FTD-related neurodegeneration remains an active area of intense investigation. In the current review, we discuss some of the significant progress made in the past two years that links PGRN deficiency with microglial-associated neuroinflammation, TDP-43 pathology, and lysosomal dysfunction. We also review the opportunities and challenges toward developing therapies and biomarkers to treat PGRN-deficient FTD.

Microglia regulate brain progranulin levels through the endocytosis/lysosomal pathway

Genetic variants in Granulin (GRN), which encodes the secreted glycoprotein progranulin (PGRN), are associated with several neurodegenerative diseases, including frontotemporal lobar degeneration, neuronal ceroid lipofuscinosis, and Alzheimer's disease. These genetic alterations manifest in pathological changes due to a reduction of PGRN expression; therefore, identifying factors that can modulate PGRN levels in vivo would enhance our understanding of PGRN in neurodegeneration and could reveal novel potential therapeutic targets. Here, we report that modulation of the endocytosis/lysosomal pathway via reduction of Nemo-like kinase (Nlk) in microglia, but not in neurons, can alter total brain Pgrn levels in mice. We demonstrate that Nlk reduction promotes Pgrn degradation by enhancing its trafficking through the endocytosis/lysosomal pathway, specifically in microglia. Furthermore, genetic interaction studies in mice showed that Nlk heterozygosity in Grn haploinsufficient mice further reduces Pgrn levels and induces neuropathological phenotypes associated with PGRN deficiency. Our results reveal a mechanism for Pgrn level regulation in the brain through the active catabolism by microglia and provide insights into the pathophysiology of PGRN-associated diseases.

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.

Frontotemporal dementia non-sense mutation of progranulin rescued by aminoglycosides

Frontotemporal dementia (FTD) is an early onset dementia characterized by progressive atrophy of the frontal and/or temporal lobes. FTD is highly heritable with mutations in progranulin accounting for 5-26% of cases in different populations. Progranulin is involved in endocytosis, secretion and lysosomal processes, but its functions under physiological and pathological conditions remains to be defined. Many FTD-causing non-sense progranulin mutations contain a premature termination codon (PTC), thus progranulin haploinsufficiency has been proposed as a major disease mechanism. Currently, there is no effective FTD treatment or therapy. Aminoglycosides are a class of antibiotics that possess a less-known function to induce eukaryotic ribosomal readthrough of PTCs to produce a full-length protein. The aminoglycoside-induced readthrough strategy has been utilized to treat multiple human diseases caused by PTCs. In this study, we tested the only clinically approved readthrough small molecule PTC124 and 11 aminoglycosides in a cell culture system on four PTCs responsible for FTD or a related neurodegenerative disease amyotrophic lateral sclerosis. We found that the aminoglycosides G418 and gentamicin rescued the expression of the progranulin R493X mutation. G418 was more effective than gentamicin (~50% rescue versus <10%), and the effect was dose- and time-dependent. The progranulin readthrough protein displayed similar subcellular localization as the wild-type progranulin protein. These data provide an exciting proof-of-concept that aminoglycosides or other readthrough-promoting compounds are a therapeutic avenue for familial FTD caused by progranulin PTC mutations.

Frequency of frontotemporal dementia-related gene variants in Turkey

Just as its clinical heterogeneity, genetic basis of Frontotemporal dementia (FTD) is also diverse and multiple molecular pathways are thought to be involved in disease pathogenesis. In the present study, FTD- related genes were evaluated in a Turkish cohort of 175 index FTD patients with a gene panel including GRN, MAPT, TARDBP, FUS, CHMP2B and VCP genes. Potential genetic associations were prospected in 16 patients (9.1%); five variants (p.(Gly35Glufs) and p.(Cys253Ter) in GRN; p.(Arg95Cys) in VCP; p.(Met405Val) in TARDBP and p.(Pro636Leu) in MAPT) were classified as pathogenic (P) or likely pathogenic (LP), in four familial and one sporadic patients. Three novel variants in MAPT, CHMP2B and FUS were also identified in familial cases. The most common pathogenic variants were observed in the GRN gene with a frequency of 1.14% (2/175) and this rate was 4.57% (8/175), including variants of uncertain significance (VUS). In this study with the largest cohort of Turkish FTD patients, GRN and MAPT variants were identified as the most common genetic associations; and rare causes like VCP, TARDBP, CHMP2B and FUS variants are recommended to be considered in patients with compatible clinical findings.

Modelling frontotemporal dementia using patient-derived induced pluripotent stem cells

Frontotemporal dementia (FTD) describes a group of clinically heterogeneous conditions that frequently affect people under the age of 65 (Le Ber et al., 2013). There are multiple genetic causes of FTD, including coding or splice-site mutations in MAPT, GRN mutations that lead to haploinsufficiency of progranulin protein, and a hexanucleotide GGGGCC repeat expansion in C9ORF72. Pathologically, FTD is characterised by abnormal protein accumulations in neurons and glia. These aggregates can be composed of the microtubule-associated protein tau (observed in FTD with MAPT mutations), the DNA/RNA-binding protein TDP-43 (seen in FTD with mutations in GRN or C9ORF72 repeat expansions) or dipeptide proteins generated by repeat associated non-ATG translation of the C9ORF72 repeat expansion. There are currently no disease-modifying therapies for FTD and the availability of in vitro models that recapitulate pathologies in a disease-relevant cell type would accelerate the development of novel therapeutics. It is now possible to generate patient-specific stem cells through the reprogramming of somatic cells from a patient with a genotype/phenotype of interest into induced pluripotent stem cells (iPSCs). iPSCs can subsequently be differentiated into a plethora of cell types including neurons, astrocytes and microglia. Using this approach has allowed researchers to generate in vitro models of genetic FTD in human cell types that are largely inaccessible during life. In this review we explore the recent progress in the use of iPSCs to model FTD, and consider the merits, limitations and future prospects of this approach.

Progranulin mutations result in impaired processing of prosaposin and reduced glucocerebrosidase activity

Frontotemporal dementia (FTD) is a common neurogenerative disorder characterized by progressive degeneration in the frontal and temporal lobes. Heterozygous mutations in the gene encoding progranulin (PGRN) are a common genetic cause of FTD. Recently, PGRN has emerged as an important regulator of lysosomal function. Here, we examine the impact of PGRN mutations on the processing of full-length prosaposin to individual saposins, which are critical regulators of lysosomal sphingolipid metabolism. Using FTD-PGRN patient-derived cortical neurons differentiated from induced pluripotent stem cells, as well as post-mortem tissue from patients with FTLD-PGRN, we show that PGRN haploinsufficiency results in impaired processing of prosaposin to saposin C, a critical activator of the lysosomal enzyme glucocerebrosidase (GCase). Additionally, we found that PGRN mutant neurons had reduced lysosomal GCase activity, lipid accumulation and increased insoluble α-synuclein relative to isogenic controls. Importantly, reduced GCase activity in PGRN mutant neurons is rescued by treatment with saposin C. Together, these findings suggest that reduced GCase activity due to impaired processing of prosaposin may contribute to pathogenesis of FTD resulting from PGRN mutations.

Premature termination codon readthrough upregulates progranulin expression and improves lysosomal function in preclinical models of GRN deficiency

Background

Frontotemporal lobar degeneration (FTLD) is a devastating and progressive disorder, and a common cause of early onset dementia. Progranulin (PGRN) haploinsufficiency due to autosomal dominant mutations in the progranulin gene (GRN) is an important cause of FTLD (FTLD-GRN), and nearly a quarter of these genetic cases are due to a nonsense mutation. Premature termination codons (PTC) can be therapeutically targeted by compounds allowing readthrough, and aminoglycoside antibiotics are known to be potent PTC readthrough drugs. Restoring endogenous PGRN through PTC readthrough has not previously been explored as a therapeutic intervention in FTLD.

Methods

We studied whether the aminoglycoside G418 could increase PGRN expression in HEK293 and human induced pluripotent stem cell (hiPSC)-derived neurons bearing the heterozygous S116X, R418X, and R493X pathogenic GRN nonsense mutations. We further tested a novel substituted phthalimide PTC readthrough enhancer in combination with G418 in our cellular models. We next generated a homozygous R493X knock-in hiPSC isogenic line (R493X^−/− KI), assessing whether combination treatment in hiPSC-derived neurons and astrocytes could increase PGRN and ameliorate lysosomal dysfunction relevant to FTLD-GRN. To provide in vivo proof-of-concept of our approach, we measured brain PGRN after intracerebroventricular administration of G418 in mice expressing the V5-tagged GRN nonsense mutation R493X.

Results

The R418X and R493X mutant GRN cell lines responded to PTC readthrough with G418, and treatments increased PGRN levels in R493X^−/− KI hiPSC-derived neurons and astrocytes. Combining G418 with a PTC readthrough enhancer increased PGRN levels over G418 treatment alone in vitro. PGRN deficiency has been shown to impair lysosomal function, and the mature form of the lysosomal protease cathepsin D is overexpressed in R493X^−/− KI neurons. Increasing PGRN through G418-mediated PTC readthrough normalized this abnormal lysosomal phenotype in R493X^−/− KI neuronal cultures. A single intracerebroventricular injection of G418 induced GRN PTC readthrough in 6-week-old AAV-GRN-R493X-V5 mice.

Conclusions

Taken together, our findings suggest that PTC readthrough may be a potential therapeutic strategy for FTLD caused by GRN nonsense mutations.

Genetic architecture of neurodegenerative dementias

Molecular genetics has been an invaluable tool to help understand the molecular basis of neurodegenerative dementias. In this review, we provide an overview of the genetic architecture underlying some of the most prevalent causes of dementia, including Alzheimer's dementia, frontotemporal lobar degeneration, Lewy body dementia, and prion diseases. We also discuss the complexity of the human genome and how the novel technologies have revolutionized and accelerated the way we screen the variety of our DNA. Finally, we also provide some examples about how this genetic knowledge is being transferred into the clinic through personalized medicine. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

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.

The Missing Heritability of Sporadic Frontotemporal Dementia: New Insights from Rare Variants in Neurodegenerative Candidate Genes

Frontotemporal dementia (FTD) is a common form of dementia among early-onset cases. Several genetic factors for FTD have been revealed, but a large proportion of FTD cases still have an unidentified genetic origin. Recent studies highlighted common pathobiological mechanisms among neurodegenerative diseases. In the present study, we investigated a panel of candidate genes, previously described to be associated with FTD and/or other neurodegenerative diseases by targeted next generation sequencing (NGS). We focused our study on sporadic FTD (sFTD), devoid of disease-causing mutations in GRN, MAPT and C9orf72. Since genetic factors have a substantially higher pathogenetic contribution in early onset patients than in late onset dementia, we selected patients with early onset (<65 years). Our study revealed that, in 50% of patients, rare missense potentially pathogenetic variants in genes previously associated with Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis and Lewy body dementia (GBA, ABCA7, PARK7, FUS, SORL1, LRRK2, ALS2), confirming genetic pleiotropy in neurodegeneration. In parallel, a synergic genetic effect on FTD is suggested by the presence of variants in five different genes in one single patient. Further studies employing genome-wide approaches might highlight pathogenic variants in novel genes that explain the still missing heritability of FTD.

Genetic screen in a large series of patients with primary progressive aphasia

INTRODUCTION

Primary progressive aphasia (PPA) is a neurological syndrome, associated with both frontotemporal dementia and Alzheimer's disease, in which progressive language impairment emerges as the most salient clinical feature during the initial stages of disease.

METHODS

We screened the main genes associated with Alzheimer's disease and frontotemporal dementia for pathogenic and risk variants in a cohort of 403 PPA cases.

RESULTS

In this case series study, 14 (3.5%) cases carried (likely) pathogenic variants: four C9orf72 expansions, nine GRN, and one TARDBP mutation. Rare risk variants, TREM2 R47H and MAPT A152T, were associated with a three- to seven-fold increase in risk for PPA.

DISCUSSION

Our results show that while pathogenic variants within the most common dementia genes were rarely associated with PPA, these were found almost exclusively in GRN and C9orf72, suggesting that PPA is more TDP43- than tau-related in our series. This is consistent with the finding that PPA frequency in dominantly inherited dementias is the highest in kindreds with GRN variants.

Genetics and molecular mechanisms of frontotemporal lobar degeneration: an update and future avenues

Frontotemporal lobar degeneration (FTLD) is the second most common form of dementia after Alzheimer's disease. The study and the dissection of FTLD is complex due to its clinical, pathological, and genetic heterogeneity. In this review, we survey the state-of-the-art genetics of familial FTLD and recapitulate our current understanding of the genetic architecture of sporadic FTLD by summarizing results of genome-wide association studies performed in FTLD to date. We then discuss the challenges of translating these heterogeneous genetic features into the understanding of the molecular underpinnings of FTLD pathogenesis. We particularly highlight a number of susceptibility processes that appear to be conserved across familial and sporadic cases (e.g., and the cellular waste disposal pathways, and immune system signaling) and finally describe cutting-edge approaches, based on mathematical prediction tools, highlighting novel intriguing risk pathways such as DNA damage response as an emerging theme in FTLD.

The lysosomal function of progranulin, a guardian against neurodegeneration

Progranulin (PGRN), encoded by the GRN gene in humans, is a secreted growth factor implicated in a multitude of processes ranging from regulation of inflammation to wound healing and tumorigenesis. The clinical importance of PGRN became especially evident in 2006, when heterozygous mutations in the GRN gene, resulting in haploinsufficiency, were found to be one of the main causes of frontotemporal lobar degeneration (FTLD). FTLD is a clinically heterogenous disease that results in the progressive atrophy of the frontal and temporal lobes of the brain. Despite significant research, the exact function of PGRN and its mechanistic relationship to FTLD remain unclear. However, growing evidence suggests a role for PGRN in the lysosome-most striking being that homozygous GRN mutation leads to neuronal ceroid lipofuscinosis, a lysosomal storage disease. Since this discovery, several links between PGRN and the lysosome have been established, including the existence of two independent lysosomal trafficking pathways, intralysosomal processing of PGRN into discrete functional peptides, and direct and indirect regulation of lysosomal hydrolases. Here, we summarize the cellular functions of PGRN, its roles in the nervous system, and its link to multiple neurodegenerative diseases, with a particular focus dedicated to recent lysosome-related mechanistic developments.

Murine knockin model for progranulin-deficient frontotemporal dementia with nonsense-mediated mRNA decay

Mutations in the GRN gene cause frontotemporal dementia, a devastating neurological disease. The majority of these GRN mutations are nonsense and frameshift mutations. Here, we generated a knockin mouse model with a Grn mutation corresponding to the most prevalent human disease mutation, GRN^R493X. We show that mice harboring this mutation phenocopy progranulin-deficient mice, and that the mutation triggers mRNA decay and, as a consequence, low production of Grn. However, the truncated mutant protein that would be produced from this allele is functional, suggesting inhibiting mRNA decay as a therapeutic approach for individuals with progranulin-deficient frontotemporal dementia caused by nonsense mutations.

Frontotemporal dementia (FTD) is the most common neurodegenerative disorder in individuals under age 60 and has no treatment or cure. Because many cases of FTD result from GRN nonsense mutations, an animal model for this type of mutation is highly desirable for understanding pathogenesis and testing therapies. Here, we generated and characterized Grn^R493X knockin mice, which model the most common human GRN mutation, a premature stop codon at arginine 493 (R493X). Homozygous Grn^R493X mice have markedly reduced Grn mRNA levels, lack detectable progranulin protein, and phenocopy Grn knockout mice, with CNS microgliosis, cytoplasmic TDP-43 accumulation, reduced synaptic density, lipofuscinosis, hyperinflammatory macrophages, excessive grooming behavior, and reduced survival. Inhibition of nonsense-mediated mRNA decay (NMD) by genetic, pharmacological, or antisense oligonucleotide-based approaches showed that NMD contributes to the reduced mRNA levels in Grn^R493X mice and cell lines and in fibroblasts from patients containing the GRN^R493X mutation. Moreover, the expressed truncated R493X mutant protein was functional in several assays in progranulin-deficient cells. Together, these findings establish a murine model for in vivo testing of NMD inhibition or other therapies as potential approaches for treating progranulin deficiency caused by the R493X mutation.

Extended FTLD pedigree segregating a Belgian GRN-null mutation: neuropathological heterogeneity in one family

BACKGROUND

In this paper, we describe the clinical and neuropathological findings of nine members of the Belgian progranulin gene (GRN) founder family. In this family, the loss-of-function mutation IVS1 + 5G > C was identified in 2006. In 2007, a clinical description of the mutation carriers was published that revealed the clinical heterogeneity among IVS1 + 5G > C carriers. We report our comparison of our data with the published clinical and neuropathological characteristics of other GRN mutations as well as other frontotemporal lobar degeneration (FTLD) syndromes, and we present a review of the literature.

METHODS

For each case, standardized sampling and staining were performed to identify proteinopathies, cerebrovascular disease, and hippocampal sclerosis.

RESULTS

The neuropathological substrate in the studied family was compatible in all cases with transactive response DNA-binding protein (TDP) proteinopathy type A, as expected. Additionally, most of the cases presented also with primary age-related tauopathy (PART) or mild Alzheimer's disease (AD) neuropathological changes, and one case had extensive Lewy body pathology. An additional finding was the presence of cerebral small vessel changes in every patient in this family.

CONCLUSIONS

Our data show not only that the IVS1 + 5G > C mutation has an exclusive association with FTLD-TDP type A proteinopathy but also that other proteinopathies can occur and should be looked for. Because the penetrance rate of the clinical phenotype of carriers of GRN mutations is age-dependent, further research is required to investigate the role of co-occurring age-related pathologies such as AD, PART, and cerebral small vessel disease.

Frontotemporal dementia: latest evidence and clinical implications

BACKGROUND

Frontotemporal dementia (FTD) describes a cluster of neurocognitive syndromes that present with impairment of executive functioning, changes in behavior, and a decrease in language proficiency. FTD is the second most common form of dementia in those younger than 65 years and is expected to increase in prevalence as the population ages. This goal in our review is to describe advances in the understanding of neurobiological pathology, classification, assessment, and treatment of FTD syndromes.

METHODS

PubMed was searched to obtain reviews and studies that pertain to advancements in genetics, neurobiology, neuroimaging, classification, and treatment of FTD syndromes. Articles were chosen with a predilection to more recent preclinical/clinical trials and systematic reviews.

RESULTS

Recent reviews and trials indicate a significant advancement in the understanding of molecular and neurobiological clinical correlates to variants of FTD. Genetic and histopathologic markers have only recently been discovered in the past decade. Current therapeutic modalities are limited, with most studies reporting improvement in symptoms with nonpharmacological interventions. However, a small number of studies have reported improvement of behavioral symptoms with selective serotonin reuptake inhibitor (SSRI) treatment. Stimulants may help with disinhibition, apathy, and risk-taking behavior. Memantine and cholinesterase inhibitors have not demonstrated efficacy in ameliorating FTD symptoms. Antipsychotics have been used to treat agitation and psychosis, but safety concerns and side effect profiles limit utilization in the general FTD population. Nevertheless, recent breakthroughs in the understanding of FTD pathology have led to developments in pharmacological interventions that focus on producing treatments with autoimmune, genetic, and molecular targets.

CONCLUSION

FTD is an underdiagnosed group of neurological syndromes comprising multiple variants with distinct neurobiological profiles and presentations. Recent advances suggest there is an array of potential novel therapeutic targets, although data concerning their effectiveness are still preliminary or preclinical. Further studies are required to develop pharmacological interventions, as there are currently no US Food and Drug administration approved treatments to manage FTD syndromes.

Genetics of FTLD: overview and what else we can expect from genetic studies

Frontotemporal lobar degeneration (FTLD) comprises a highly heterogeneous group of disorders clinically associated with behavioral and personality changes, language impairment, and deficits in executive functioning, and pathologically associated with degeneration of frontal and temporal lobes. Some patients present with motor symptoms including amyotrophic lateral sclerosis. Genetic research over the past two decades in FTLD families led to the identification of three common FTLD genes (microtubule-associated protein tau, progranulin, and chromosome 9 open reading frame 72) and a small number of rare FTLD genes, explaining the disease in almost all autosomal dominant FTLD families but only a minority of apparently sporadic patients or patients in whom the family history is less clear. Identification of additional FTLD (risk) genes is therefore highly anticipated, especially with the emerging use of next-generation sequencing. Common variants in the transmembrane protein 106 B were identified as a genetic risk factor of FTLD and disease modifier in patients with known mutations. This review summarizes for each FTLD gene what we know about the type and frequency of mutations, their associated clinical and pathological features, and potential disease mechanisms. We also provide an overview of emerging disease pathways encompassing multiple FTLD genes. We further discuss how FTLD specific issues, such as disease heterogeneity, the presence of an unclear family history and the possible role of an oligogenic basis of FTLD, can pose challenges for future FTLD gene identification and risk assessment of specific variants. Finally, we highlight emerging clinical, genetic, and translational research opportunities that lie ahead. Genetic research led to the identification of three common FTLD genes with rare variants (MAPT, GRN, and C9orf72) and a small number of rare genes. Efforts are now ongoing, which aimed at the identification of rare variants with high risk and/or low frequency variants with intermediate effect. Common risk variants have also been identified, such as TMEM106B. This review discusses the current knowledge on FTLD genes and the emerging disease pathways encompassing multiple FTLD genes.

Neurotrophic effects of progranulin in vivo in reversing motor neuron defects caused by over or under expression of TDP-43 or FUS

Progranulin (PGRN) is a glycoprotein with multiple roles in normal and disease states. Mutations within the GRN gene cause frontotemporal lobar degeneration (FTLD). The affected neurons display distinctive TAR DNA binding protein 43 (TDP-43) inclusions. How partial loss of PGRN causes TDP-43 neuropathology is poorly understood. TDP-43 inclusions are also found in affected neurons of patients with other neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. In ALS, TDP-43 inclusions are typically also immunoreactive for fused in sarcoma (FUS). Mutations within TDP-43 or FUS are themselves neuropathogenic in ALS and some cases of FTLD. We used the outgrowth of caudal primary motor neurons (MNs) in zebrafish embryos to investigate the interaction of PGRN with TDP-43 and FUS in vivo. As reported previously, depletion of zebrafish PGRN-A (zfPGRN-A) is associated with truncated primary MNs and impaired motor function. Here we found that depletion of zfPGRN-A results in primary MNs outgrowth stalling at the horizontal myoseptum, a line of demarcation separating the myotome into dorsal and ventral compartments that is where the final destination of primary motor is assigned. Successful axonal outgrowth beyond the horizontal myoseptum depends in part upon formation of acetylcholine receptor clusters and this was found to be disorganized upon depletion of zfPGRN-A. PGRN reversed the effects of zfPGRN-A knockdown, but a related gene, zfPGRN-1, was without effect. Both knockdown of TDP-43 or FUS, as well as expression of humanTDP-43 and FUS mutants results in MN abnormalities that are reversed by co-expression of hPGRN mRNA. Neither TDP-43 nor FUS reversed MN phenotypes caused by the depletion of PGRN. Thus TDP-43 and FUS lie upstream of PGRN in a gene complementation pathway. The ability of PGRN to override TDP-43 and FUS neurotoxicity due to partial loss of function or mutation in the corresponding genes may have therapeutic relevance.

Clinical and genetic analyses of familial and sporadic frontotemporal dementia patients in Southern Italy

INTRODUCTION

We investigated the clinical differences between familial and sporadic frontotemporal dementia (FTD), screening for mutations in known FTD genes.

METHODS

We diagnosed 22 affected individuals belonging to eight families and 43 sporadic cases with FTD in Apulia, Southern Italy, in 2 years. Mutations in common causative FTD genes (GRN, MAPT, VCP, and TARDBP) and C9ORF72 expansions were screened.

RESULTS

Behavioral variant of FTD was the most common clinical subtype (50% and 69% in familial and sporadic cases, respectively). Social conduct impairment/disinhibition, loss of insight, and inflexibility were the most frequent clinical features observed at onset. One new mutation was identified in GRN in family A.

DISCUSSION

Disease onset in sporadic FTD was more frequently characterized by a clustering of behavioral symptoms with apathy and loss of personal hygiene. Mutations in common causative FTD genes are not a major cause of familial and sporadic FTD in the Southern Italian population.

EphA2 is a functional receptor for the growth factor progranulin

The receptor for the growth factor progranulin has remained unclear. Neill et al. show that the Ephrin receptor tyrosine kinase EphA2 is a functional signaling receptor for progranulin and mediates its effects in capillary morphogenesis and autoregulation.

Although the growth factor progranulin was discovered more than two decades ago, the functional receptor remains elusive. Here, we discovered that EphA2, a member of the large family of Ephrin receptor tyrosine kinases, is a functional signaling receptor for progranulin. Recombinant progranulin bound with high affinity to EphA2 in both solid phase and solution. Interaction of progranulin with EphA2 caused prolonged activation of the receptor, downstream stimulation of mitogen-activated protein kinase and Akt, and promotion of capillary morphogenesis. Furthermore, we found an autoregulatory mechanism of progranulin whereby a feed-forward loop occurred in an EphA2-dependent manner that was independent of the endocytic receptor sortilin. The discovery of a functional signaling receptor for progranulin offers a new avenue for understanding the underlying mode of action of progranulin in cancer progression, tumor angiogenesis, and perhaps neurodegenerative diseases.

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.

Progranulin and granulin-like protein as novel VEGF-independent angiogenic factors derived from human mesothelioma cells

Malignant mesothelioma is an aggressive tumor arising from the mesothelial cells of serous membranes and is associated with tumor angiogenesis, which is a prerequisite for tumor progression. Vascular endothelial growth factors (VEGFs) including VEGF-A have a crucial role in tumor angiogenesis. However, bevacizumab, a monoclonal antibody to VEGF-A, has recently been reported not to improve the progression-free survival of patients with malignant mesothelioma. Cell culture supernatant contains extracellular components such as serum, which can mask the existence of unknown cell-derived factors in the supernatant and make it difficult to detect the factors by subsequent protein analysis. We tried using serum-free culture for human mesothelioma cell lines, NCI-H28, NCI-H2452 and NCI-H2052, and only NCI-H2052 cells adapted to serum-free culture. We found that serum-free culture supernatant derived from NCI-H2052 cells induces the formation of capillary-like tube structures (tube formation) in three-dimensional culture, in which endothelial cells sandwiched between two layers of collagen or embedded in collagen are incubated with various angiogenic inducers. However, neither neutralization of VEGF-A nor RNA interference of VEGF receptor 2 (VEGFR2) suppressed the supernatant-induced tube formation. Using mass spectrometry, we identified a total of 399 proteins in the supernatant, among which interleukin-8 (IL-8), growth-regulated α-protein, midkine, IL-18, IL-6, hepatoma-derived growth factor, clusterin and granulin (GRN), also known as progranulin (PGRN), were included as a candidate protein inducing angiogenesis. Neutralizing assays and RNA interference showed that PGRN, but not the above seven candidate proteins, caused the supernatant-induced tube formation. We also found that NCI-H28 and NCI-H2452 cells express PGRN. Furthermore, we demonstrate that not only PGRN but also GRN-like protein have an important role in the supernatant-induced tube formation. Thus, mesothelioma-derived GRNs induce VEGF-independent angiogenesis.

Genetic Determination of Serum Levels of Diabetes-Associated Adipokines

Adipose tissue secretes an abundance of proteins. Some of these proteins are known as adipokines and adipose-derived hormones which have been linked with metabolic disorders, including type 2 diabetes, and even with cancer. Variance in serum adipokine concentration is often closely associated with an increase (obesity) or decrease (lipodystrophy) in fat tissue mass, and it is affected by age, gender, and localization of the adipose tissue. However, there may be genetic variants which, in consequence, influence the serum concentration of a certain adipokine, and thereby promote metabolic disturbances or, with regard to the "protective" allele, exert beneficial effects. This review focuses on the genetic determination of serum levels of the following adipokines: adiponectin, chemerin, leptin, progranulin, resistin, retinol binding protein 4, vaspin, adipsin, apelin, and omentin. The article reports on the latest findings from genome-wide association studies (GWAS) and candidate gene studies, showing variants located in/nearby the adipokine genes and other (non-receptor) genes. An extra chapter highlights adipokine-receptor variants. Epigenetic studies on adipokines are also addressed.

Increased Wnt Signaling and Reduced Viability in a Neuronal Model of Progranulin-Deficient Frontotemporal Lobar Degeneration

Progranulin (PGRN) deficiency is considered the major cause of frontotemporal lobar degeneration with TDP-43 protein inclusions (FTLD-TDP). Recent work unveiled a relationship between Wnt signaling and PGRN in cellular models of FTLD and cells of patients carrying loss-of-function GRN mutations. This study was undertaken to explore the relationship between PGRN deficit and Wnt signaling in the regulation of survival of GRN knockdown neuroblastoma SH-SY5Y cells (GRN KD). We report here that both canonical and noncanonical Wnt signaling cascades are overactivated in GRN KD cells. We detected increased expression levels of Wnt1 and Wnt5a ligands of the Frizzled receptors, as well as evidence for increased signaling of the Wnt/β-catenin and Wnt/Ca²⁺ cascades in PGRN deficient cells, such as increased nuclear content of β-catenin and higher levels of cyclin D1, or increased levels of the active form of the NFAT1 transcription factor, respectively. Upregulation of either Wnt/β-catenin or Wnt/Ca²⁺ signaling in GRN KD cells leads to the stimulation of BrdU incorporation into DNA, hyperphosphorylation of the pRb family of proteins and reduced cell viability over time. Blocking the Wnt cascades by specific canonical or noncanonical inhibitors of Wnt-dependent signaling, normalized the rate of DNA synthesis, and what it is more important restored the viability of GRN KD cells. Our results suggest an important role of Wnt activation inducing cell cycle disturbance-mediated neuronal loss in the pathogenesis of PGRN deficiency-linked FTLD-TDP. Therefore, it is plausible that modulation of Wnt signaling could be a promising strategy for developing of new disease-modifying treatments for FTLD-TDP.

Clinical features of TBK1 carriers compared with C9orf72, GRN and non-mutation carriers in a Belgian cohort

We identified in a cohort of patients with frontotemporal dementia (n = 481) or amyotrophic lateral sclerosis (n = 147), 10 index patients carrying a TBK1 loss of function mutation reducing TBK1 expression by 50%. Here, we describe the clinical and pathological characteristics of the 10 index patients and six of their affected relatives carrying a TBK1 mutation. Six TBK1 carriers were diagnosed with frontotemporal dementia, seven with amyotrophic lateral sclerosis, one with both clinical phenotypes and two with dementia unspecified. The mean age at onset of all 16 TBK1 carriers was 62.1 ± 8.9 years (range 41–73) with a mean disease duration of 4.7 ± 4.5 years (range 1–13). TBK1 carriers with amyotrophic lateral sclerosis had shorter disease duration than carriers with frontotemporal dementia. Six of seven TBK1 carriers were diagnosed with the behavioural variant of frontotemporal dementia, presenting predominantly as disinhibition. Memory loss was an important associated symptom in the initial phase of the disease in all but one of the carriers with frontotemporal dementia. Three of the patients with amyotrophic lateral sclerosis exhibited pronounced upper motor neuron symptoms. Overall, neuroimaging displayed widespread atrophy, both symmetric and asymmetric. Brain perfusion single-photon emission computed tomography or fluorodeoxyglucose-positron emission tomography showed asymmetric and predominantly frontotemporal involvement. Neuropathology in two patients demonstrated TDP-43 type B pathology. Further, we compared genotype–phenotype data of TBK1 carriers with frontotemporal dementia (n = 7), with those of frontotemporal dementia patients with a C9orf72 repeat expansion (n = 65) or a GRN mutation (n = 52) and with frontotemporal dementia patients (n = 259) negative for mutations in currently known causal genes. TBK1 carriers with frontotemporal dementia had a later age at onset (63.3 years) than C9orf72 carriers (54.3 years) (P = 0.019). In clear contrast with TBK1 carriers, GRN carriers were more often diagnosed with the language variant than the behavioural variant, and presented in case of the diagnosis of behavioural variant, more often than TBK1 carriers with apathy as the predominant characteristic (P = 0.004). Also, TBK1 carriers exhibited more often extrapyramidal symptoms than C9orf72 carriers (P = 0.038). In conclusion, our study identified clinical differences between the TBK1, C9orf72 and GRN carriers, which allows us to formulate guidelines for genetic diagnosis. After a negative result for C9orf72, patients with both frontotemporal dementia and amyotrophic lateral sclerosis should be tested first for mutations in TBK1. Specifically in frontotemporal dementia patients with early memory difficulties, a relatively late age at onset or extrapyramidal symptoms, screening for TBK1 mutations should be considered.

Progranulin mutation analysis: Identification of one novel mutation in exon 12 associated with frontotemporal dementia

Progranulin (PGRN) mutations account for an average of 15% of familial frontotemporal dementia (FTD) cases and 20% of total FTD cases worldwide. Here, we investigated the frequency of PGRN mutations in FTD patients (n = 116) from a clinical cohort of south India and detected one novel mutation located on exon 12 in a familial behavioral variant FTD patient (accounting for ∼1% of total FTD cases and 6% of familial FTD cases). This mutation was found to introduce a premature termination codon and the prematurely terminated messenger RNA may probably undergo nonsense-mediated decay. In enzyme-linked immunosorbent assay, the proband showed significantly reduced level of plasma PGRN (28 ng/mL) compared with controls (150 ± 38 ng/mL), which implicates haploinsufficiency as the pathogenic mechanism.

Clinical Subtypes of Frontotemporal Dementia

Frontotemporal dementia (FTD) was one of the lesser known dementias until the recent advancements revealing its genetic and pathological foundation. This common neurodegenerative disorder has three clinical subtypes- behavioral, semantic and progressive non fluent aphasia. The behavioral variant mostly exhibits personality changes, while the other two encompass various language deficits. This review discusses the basic pathology, genetics, clinical and histological presentation and the diagnosis of the 3 subtypes. It also deliberates the different therapeutic modalities currently available for frontotemporal dementia and the challenges faced by the caregivers. Lastly it explores the scope of further research into the diagnosis and management of FTD.

Frontotemporal dementia

Frontotemporal dementia is an umbrella clinical term that encompasses a group of neurodegenerative diseases characterised by progressive deficits in behaviour, executive function, or language. Frontotemporal dementia is a common type of dementia, particularly in patients younger than 65 years. The disease can mimic many psychiatric disorders because of the prominent behavioural features. Various underlying neuropathological entities lead to the frontotemporal dementia clinical phenotype, all of which are characterised by the selective degeneration of the frontal and temporal cortices. Genetics is an important risk factor for frontotemporal dementia. Advances in clinical, imaging, and molecular characterisation have increased the accuracy of frontotemporal dementia diagnosis, thus allowing for the accurate differentiation of these syndromes from psychiatric disorders. As the understanding of the molecular basis for frontotemporal dementia improves, rational therapies are beginning to emerge.

Microarray gene and miRNA expression studies: looking for new therapeutic targets for frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) encompasses a spectrum of neurodegenerative disorders characterized by behavioral, executive and language impairment, with a common overlap with parkinsonism and motor-neuron disease. Despite an increased understanding of its genetic background and molecular pathophysiology, FTLD is still an orphan disorder and there are currently no effective therapies available. In this brief overview we report the results obtained by several high-throughput and bioinformatic studies aimed at discovering impairment in the transcriptional profiles in brain and peripheral tissues from FTLD patients and in animal models. Taken together, all these results provide an interesting but still fragmentary list of genes and miRNAs whose role in FTLD should be thoroughly investigated.

Increasing progranulin levels and blockade of the ERK1/2 pathway: upstream and downstream strategies for the treatment of progranulin deficient frontotemporal dementia

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disorder marked by mild-life onset and progressive changes in behavior, social cognition, and language. Loss-of-function progranulin gene (GRN) mutations are the major cause of FTLD with TDP-43 protein inclusions (FTLD-TDP). Disease-modifying treatments for FTLD-TDP are not available yet. Mounting evidence indicates that cell cycle dysfunction may play a pathogenic role in neurodegenerative disorders including FTLD. Since cell cycle re-entry of posmitotic neurons seems to precede neuronal death, it was hypothesized that strategies aimed at preventing cell cycle progression would have neuroprotective effects. Recent research in our laboratory revealed cell cycle alterations in lymphoblasts from FTLD-TDP patients carrying a null GRN mutation, and in PGRN deficient SH-SY5Y neuroblastoma cells, involving overactivation of the ERK1/2 signaling pathway. In this work, we have investigated the effects of PGRN enhancers drugs and ERK1/2 inhibitors, in these cellular models of PGRN-deficient FTLD. We report here that both restoring the PGRN content, by suberoylanilide hydroxamic acid (SAHA) or chloroquine (CQ), as blocking ERK1/2 activation by selumetinib (AZD6244) or MEK162 (ARRY-162), normalized the CDK6/pRb pathway and the proliferative activity of PGRN deficient cells. Moreover, we found that SAHA and selumetinib prevented the cytosolic TDP-43 accumulation in PGRN-deficient lymphoblasts. Considering that these drugs are able to cross the blood-brain barrier, and assuming that the alterations in cell cycle and signaling observed in lymphoblasts from FTLD patients could be peripheral signs of the disease, our results suggest that these treatments may serve as novel therapeutic drugs for FTLD associated to GRN mutations.

Progranulin Deficiency Reduces CDK4/6/pRb Activation and Survival of Human Neuroblastoma SH-SY5Y Cells

Null mutations in GRN are associated with frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP). However, the influence of progranulin (PGRN) deficiency in neurodegeneration is largely unknown. In neuroblastoma cells, silencing of GRN gene causes significantly reduced cell survival after serum withdrawal. The following observations suggest that alterations of the CDK4/6/retinoblastoma protein (pRb) pathway, secondary to changes in PI3K/Akt and ERK1/2 activation induced by PGRN deficiency, are involved in the control of serum deprivation-induced apoptosis: (i) inhibiting CDK4/6 levels or their associated kinase activity by sodium butyrate or PD332991 sensitized control SH-SY5Y cells to serum deprivation-induced apoptosis without affecting survival of PGRN-deficient cells; (ii) CDK4/6/pRb seems to be downstream of the PI3K/Akt and ERK1/2 signaling pathways since their specific inhibitors, LY294002 and PD98059, were able to decrease CDK6-associated kinase activity and induce death of control SH-SY5Y cells; (iii) PGRN-deficient cells show reduced stimulation of PI3K/Akt, ERK1/2, and CDK4/6 activities compared with control cells in the absence of serum; and (iv) supplementation of recombinant human PGRN was able to rescue survival of PGRN-deficient cells. These observations highlight the important role of PGRN-mediated stimulation of the PI3K/Akt-ERK1/2/CDK4/6/pRb pathway in determining the cell fate survival/death under serum deprivation.

Dual cell protective mechanisms activated by differing levels of oxidative stress in HT22 murine hippocampal cells

Oxidative stress is recognized as one of the pathogenic mechanisms involved in neurodegenerative disease. However, recent evidence has suggested that regulation of cellular fate in response to oxidative stress appears to be dependent on the stress levels. In this study, using HT22 cells, we attempted to understand how an alteration in the oxidative stress levels would influence neuronal cell fate. HT22 cell viability was reduced with exposure to high levels of oxidative stress, whereas, low levels of oxidative stress promoted cell survival. Erk1/2 activation induced by a low level of oxidative stress played a role in this cell protective effect. Intriguingly, subtoxic level of H2O2 induced expression of a growth factor, progranulin (PGRN), and exogenous PGRN pretreatment attenuated HT22 cell death induced by high concentrations of H2O2 in Erk1/2-dependent manner. Together, our study indicates that two different cell protection mechanisms are activated by differing levels of oxidative stress in HT22 cells.

Early neuropsychological characteristics of progranulin mutation carriers

Mutations in the progranulin gene (GRN) are a common cause of familial frontotemporal dementia. We used a comprehensive neuropsychological battery to investigate whether early cognitive changes could be detected in GRN mutation carriers before dementia onset. Twenty-four at-risk members from six families with known GRN mutations underwent detailed neuropsychological testing. Group differences were investigated by domains of attention, language, visuospatial function, verbal memory, non-verbal memory, working memory and executive function. There was a trend for mutation carriers (n=8) to perform more poorly than non-carriers (n=16) across neuropsychological domains, with significant between group differences for visuospatial function (p<.04; d=0.92) and working memory function (p<.02; d=1.10). Measurable cognitive differences exist before the development of frontotemporal dementia in subjects with GRN mutations. The neuropsychological profile of mutation carriers suggests early asymmetric, right hemisphere brain dysfunction that is consistent with recent functional imaging data from our research group and the broader literature.

Parkinsonian syndrome in familial frontotemporal dementia

Parkinsonism in frontotemporal dementia (FTD) was first described in families with mutations in the microtubule-associated protein tau (MAPT) and progranulin (PRGN) genes. Since then, mutations in several other genes have been identified for FTD with parkinsonism, including chromosome 9 open reading frame 72 (C9ORF72), chromatin modifying protein 2B (CHMP2B), valosin-containing protein (VCP), fused in sarcoma (FUS) and transactive DNA-binding protein (TARDBP). The clinical presentation of patients with familial forms of FTD with parkinsonism is highly variable. The parkinsonism seen in FTD patients is usually characterized by akinetic-rigid syndrome and is mostly associated with the behavioral variant of FTD (bvFTD); however, some cases may present with classical Parkinson's disease. In other cases, atypical parkinsonism resembling progressive supranuclear palsy (PSP) or corticobasal syndrome (CBS) has also been described. Although rare, parkinsonism in FTD may coexist with motor neuron disease. Structural neuroimaging, which is crucial for the diagnosis of FTD, shows characteristic patterns of brain atrophy associated with specific mutations. Structural neuroimaging is not helpful in distinguishing among patients with parkinsonian features. Furthermore, dopaminergic imaging that shows nigrostriatal neurodegeneration in FTD with parkinsonism cannot discriminate parkinsonian syndromes that arise from different mutations. Generally, parkinsonism in FTD is levodopa unresponsive, but there have been cases where a temporary benefit has been reported, so dopaminergic treatment is worth trying, especially, when motor and non-motor manifestations can cause significant problems with daily functioning. In this review, we present an update on the clinical and genetic correlations of FTD with parkinsonism.

Disease-related mutations among Caribbean Hispanics with familial dementia

Pathogenic mutations in the three known genes – the amyloid precursor protein (APP), presenilin 1 (PSEN1), presenilin 2 (PSEN2) – are known to cause familial Alzheimer's disease (AD) and tend to be associated with early-onset AD. However, the frequency and risk associated with these mutations vary widely. In addition, mutations in the frontotemporal lobar degeneration (FTLD) genes – the microtubule-associated protein tau (MAPT), granulin (GRN) – have also been found to be associated with clinical AD. Here, we conducted targeted resequencing of the exons in genes encoding APP, PSEN1, PSEN2, GRN, and MAPT in 183 individuals from families with four or more affected relatives, presumed to be AD, and living in the Dominican Republic and Puerto Rico. We then performed linkage and family-based association analyses in carrier families, and genotyped 498 similarly aged unrelated controls from the same ethnic background. Twelve potentially pathogenic mutations were found to be associated with disease in 53 individuals in the five genes. The most frequently observed mutation was the p.Gly206Ala variant in PSEN1 present in 30 (57%) of those sequenced. In the combined linkage and association analyses several rare variants were associated with dementia. In Caribbean Hispanics with familial AD, potentially pathogenic variants were present in 29.2%, four were novel mutations, while eight had been previously observed. In addition, some family members carried variants in the GRN and MAPT genes which are associated with FTLD.

Common pathobiochemical hallmarks of progranulin-associated frontotemporal lobar degeneration and neuronal ceroid lipofuscinosis

Heterozygous loss-of-function mutations in the progranulin (GRN) gene and the resulting reduction of GRN levels is a common genetic cause for frontotemporal lobar degeneration (FTLD) with accumulation of TAR DNA-binding protein (TDP)-43. Recently, it has been shown that a complete GRN deficiency due to a homozygous GRN loss-of-function mutation causes neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disorder. These findings suggest that lysosomal dysfunction may also contribute to some extent to FTLD. Indeed, Grn(-/-) mice recapitulate not only pathobiochemical features of GRN-associated FTLD-TDP (FTLD-TDP/GRN), but also those which are characteristic for NCL and lysosomal impairment. In Grn(-/-) mice the lysosomal proteins cathepsin D (CTSD), LAMP (lysosomal-associated membrane protein) 1 and the NCL storage components saposin D and subunit c of mitochondrial ATP synthase (SCMAS) were all found to be elevated. Moreover, these mice display increased levels of transmembrane protein (TMEM) 106B, a lysosomal protein known as a risk factor for FTLD-TDP pathology. In line with a potential pathological overlap of FTLD and NCL, Ctsd(-/-) mice, a model for NCL, show elevated levels of the FTLD-associated proteins GRN and TMEM106B. In addition, pathologically phosphorylated TDP-43 occurs in Ctsd(-/-) mice to a similar extent as in Grn(-/-) mice. Consistent with these findings, some NCL patients accumulate pathologically phosphorylated TDP-43 within their brains. Based on these observations, we searched for pathological marker proteins, which are characteristic for NCL or lysosomal impairment in brains of FTLD-TDP/GRN patients. Strikingly, saposin D, SCMAS as well as the lysosomal proteins CTSD and LAMP1/2 are all elevated in patients with FTLD-TDP/GRN. Thus, our findings suggest that lysosomal storage disorders and GRN-associated FTLD may share common features.

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.

Progranulin peripheral levels as a screening tool for the identification of subjects with progranulin mutations in a Portuguese cohort

BACKGROUND

Progranulin (PGRN) mutations are associated with different clinical phenotypes, including frontotemporal lobar degeneration (FTLD), corticobasal syndrome (CBS) and Alzheimer's disease (AD). As all pathogenic PGRN mutations identified so far cause disease through haploinsufficiency, determination of PGRN levels has been proposed as a reliable method to identify mutation carriers.

OBJECTIVE

To evaluate the accuracy of peripheral PGRN levels in the identification of the PGRN mutation carriers detected thus far in our Portuguese cohort.

METHODS

Serum PGRN levels were measured in 244 subjects (124 patients in the spectrum of FTLD, 2 asymptomatic descendants of a FTLD patient, 56 AD patients and 64 controls) by a novel commercial ELISA kit.

RESULTS

Low PGRN levels were detected in 7 individuals (5 behavioral variant frontotemporal dementia, 1 CBS, and 1 still clinically unaffected) that constituted the group of the null PGRN mutation carriers previously identified in our molecular diagnostic laboratory. The pathogenic mutations found consisted of 4 insertion-deletions, causing frameshifts resulting in premature stop codons, 3 of which were novel. In addition, a normal PGRN level was found in a patient harboring a novel missense variant. For this novel ELISA kit, we established a PGRN cut-off level that identified with 100% accuracy the pathogenic mutation carriers.

CONCLUSION

This study supports the use of a novel assay for the determination of PGRN levels as a screening procedure to identify patients harboring null PGRN mutations. This approach would significantly decrease the required PGRN mutation analysis workload and should be extended to other clinical phenotypes than behavioral variant frontotemporal dementia and to apparently sporadic cases.

Plasma progranulin levels in cortical dementia phenotypes with asymmetric perisylvian atrophy

BACKGROUND AND PURPOSE

Decreased plasma progranulin levels are a very specific marker for the diagnosis of frontotemporal lobar degeneration (FTLD) caused by mutations in the progranulin gene (GRN). A frequent neuroimaging pattern in this type of dementia is asymmetric cortical atrophy. The aim of this study was to screen for GRN-linked FTLD in cases with different cortical dementia phenotypes and asymmetric perisylvian atrophy.

METHODS

Progranulin plasma levels were analyzed in a variety of FTLD phenotypes (n = 71), dementia of the Alzheimer type (DAT) (n = 22) and probable Lewy body dementia (n = 8), both latter groups presented with asymmetric perisylvian atrophy. A group of elderly controls (n = 29) and DAT cases with symmetric atrophy (n = 33) were also analyzed. The GRN gene was sequenced in cases with lower plasma levels.

RESULTS

Four cases with clinical FTLD phenotypes and plasma levels below 70 ng/ml were found to carry different GRN mutations: M1?, C139R, a point mutation in the splice donor site of intron 3 (A89VfsX41), and a deletion in exon 9 (A303AfsX57), this latter one being a new mutation. Thirteen cases with levels between 72 and 85 ng/ml did not show pathogenic changes in the GRN gene. None of the cases with asymmetric atrophy and clinical phenotypes other than FTLD had GRN mutations.

CONCLUSIONS

Asymmetric perisylvian atrophy is not likely to predict progranulin-linked FTLD unless it is associated with a consistent FTLD clinical phenotype.

Dissociation of frontotemporal dementia-related deficits and neuroinflammation in progranulin haploinsufficient mice

Frontotemporal dementia (FTD) is a neurodegenerative disease with hallmark deficits in social and emotional function. Heterozygous loss-of-function mutations in GRN, the progranulin gene, are a common genetic cause of the disorder, but the mechanisms by which progranulin haploinsufficiency causes neuronal dysfunction in FTD are unclear. Homozygous progranulin knock-out (Grn(-/-)) mice have been studied as a model of this disorder and show behavioral deficits and a neuroinflammatory phenotype with robust microglial activation. However, homozygous GRN mutations causing complete progranulin deficiency were recently shown to cause a different neurological disorder, neuronal ceroid lipofuscinosis, suggesting that the total absence of progranulin may have effects distinct from those of haploinsufficiency. Here, we studied progranulin heterozygous (Grn(+/-)) mice, which model progranulin haploinsufficiency. We found that Grn(+/-) mice developed age-dependent social and emotional deficits potentially relevant to FTD. However, unlike Grn(-/-) mice, behavioral deficits in Grn(+/-) mice occurred in the absence of gliosis or increased expression of tumor necrosis factor-α. Instead, we found neuronal abnormalities in the amygdala, an area of selective vulnerability in FTD, in Grn(+/-) mice. Our findings indicate that FTD-related deficits resulting from progranulin haploinsufficiency can develop in the absence of detectable gliosis and neuroinflammation, thereby dissociating microglial activation from functional deficits and suggesting an important effect of progranulin deficiency on neurons.