Loss of Tmem106b is unable to ameliorate frontotemporal dementia-like phenotypes in an AAV mouse model of C9ORF72-repeat induced toxicity

Loss-of-function mutations in progranulin (GRN) and a non-coding (GGGGCC)n hexanucleotide repeat expansions in C9ORF72 are the two most common genetic causes of frontotemporal lobar degeneration with aggregates of TAR DNA binding protein 43 (FTLD-TDP). TMEM106B encodes a type II transmembrane protein with unknown function. Genetic variants in TMEM106B associated with reduced TMEM106B levels have been identified as disease modifiers in individuals with GRN mutations and C9ORF72 expansions. Recently, loss of Tmem106b has been reported to protect the FTLD-like phenotypes in Grn-/- mice. Here, we generated Tmem106b-/- mice and examined whether loss of Tmem106b could rescue FTLD-like phenotypes in an AAV mouse model of C9ORF72-repeat induced toxicity. Our results showed that neither partial nor complete loss of Tmem106b was able to rescue behavioral deficits induced by the expression of (GGGGCC)66 repeats (66R). Loss of Tmem106b also failed to ameliorate 66R-induced RNA foci, dipeptide repeat protein formation and pTDP-43 pathological burden. We further found that complete loss of Tmem106b increased astrogliosis, even in the absence of 66R, and failed to rescue 66R-induced neuronal cell loss, whereas partial loss of Tmem106b significantly rescued the neuronal cell loss but not neuroinflammation induced by 66R. Finally, we showed that overexpression of 66R did not alter expression of Tmem106b and other lysosomal genes in vivo, and subsequent analyses in vitro found that transiently knocking down C9ORF72, but not overexpression of 66R, significantly increased TMEM106B and other lysosomal proteins. In summary, reducing Tmem106b levels failed to rescue FTLD-like phenotypes in a mouse model mimicking the toxic gain-of-functions associated with overexpression of 66R. Combined with the observation that loss of C9ORF72 and not 66R overexpression was associated with increased levels of TMEM106B, this work suggests that the protective TMEM106B haplotype may exert its effect in expansion carriers by counteracting lysosomal dysfunction resulting from a loss of C9ORF72.

Modifiers of GRN-Associated Frontotemporal Lobar Degeneration

Heterozygous loss-of-function (LOF) mutations in the human progranulin gene (GRN) cause frontotemporal lobar degeneration (FTLD) by a mechanism of haploinsufficiency. Patients present most frequently with frontotemporal dementia, which is the second most common neurodegenerative dementia at young age. Currently, no disease-modifying therapies are available for these patients. Stimulating GRN protein expression or inhibiting its breakdown is an obvious therapeutic strategy, and is indeed the focus of current preclinical research and clinical trials. Multiple studies have demonstrated the heterogeneity in clinical presentation and wide variability in age of onset in patients carrying a GRN LOF mutation. Recently, this heterogeneity became an opportunity to identify disease modifiers, considering that these might constitute suitable targets for developing disease-modifying or disease-delaying therapies.

[Frontotemporal dementia: clinical features, genetics, pathogenesis and treatment]

Frontotemporal dementia (FTD) is the second most common cause of young onset dementia after Alzheimer's disease. FTD presents with progressive changes in behavior, personality and language deficits and it is a clinically, pathologically and genetically heterogeneous disorder. Histopathological heterogeneity includes various intraneuronal inclusions, mostly positive for tau or ubiquitin, with TDP-43 positive, and more rarely FUS positive. About 30-50% of FTD patients are familial with an autosomal dominant pattern of inheritance. Mutations in a number of genes are associated with FTD, most commonly in MAPT or GRN genes, and in the C90RF72, and more rarely in VCP, TARDBP or FUS. Recently, a new pathological classification of FTD was developed according to the precipitate proteins denoted frontotemporal lobe degeneration (FTLD) including FTLD-Tau, FTLD-TDP43, FTLD-FUS, FTLD-UBS and more. There is a correlation between the clinical type and pathological findings and this correlation is important for understanding the mechanism as well as the intervention. Unfortunately, there is no effective treatment for FTD but we hope that recent developments will make advances towards finding effective therapy.

Incorporating new diagnostic schemas, genetics, and proteinopathy into the evaluation of frontotemporal degeneration

PURPOSE OF REVIEW

Within the continuously growing body of knowledge in the field of dementia, frontotemporal degeneration stands out in importance as the second most common cause of early-onset dementia after Alzheimer disease. Neurologists, neuropsychologists, and speech pathologists are particularly involved in the diagnosis and recognition of etiologies for patients with deficits in frontal lobe function and language.

RECENT FINDINGS

The recent discovery of a novel mutant gene (C9ORF72) and the new nomenclature adopted for subclassification have significantly promoted our understanding of this disorder.

SUMMARY

This article relates the most recent consensus criteria for diagnosis of the two forms of frontotemporal degeneration (ie, behavioral and primary progressive aphasia variants) to basic neurologic principles and remind clinicians of the neuropsychiatric and neuroradiologic components that clarify frontotemporal degeneration diagnoses and guide management.

Frontotemporal lobar degeneration: epidemiology, pathophysiology, diagnosis and management

Frontotemporal lobar degeneration (FTLD) is a clinically and pathologically heterogeneous syndrome, characterized by progressive decline in behaviour or language associated with degeneration of the frontal and anterior temporal lobes. While the seminal cases were described at the turn of the 20th century, FTLD has only recently been appreciated as a leading cause of dementia, particularly in patients presenting before the age of 65 years. Three distinct clinical variants of FTLD have been described: (i) behavioural-variant frontotemporal dementia, characterized by changes in behaviour and personality in association with frontal-predominant cortical degeneration; (ii) semantic dementia, a syndrome of progressive loss of knowledge about words and objects associated with anterior temporal neuronal loss; and (iii) progressive nonfluent aphasia, characterized by effortful language output, loss of grammar and motor speech deficits in the setting of left perisylvian cortical atrophy. The majority of pathologies associated with FTLD clinical syndromes include either tau-positive (FTLD-TAU) or TAR DNA-binding protein 43 (TDP-43)-positive (FTLD-TDP) inclusion bodies. FTLD overlaps clinically and pathologically with the atypical parkinsonian disorders corticobasal degeneration and progressive supranuclear palsy, and with amyotrophic lateral sclerosis. The majority of familial FTLD cases are caused by mutations in the genes encoding microtubule-associated protein tau (leading to FTLD-TAU) or progranulin (leading to FTLD-TDP). The clinical and pathological heterogeneity of FTLD poses a significant diagnostic challenge, and in vivo prediction of underlying histopathology can be significantly improved by supplementing the clinical evaluation with genetic tests and emerging biological markers. Current pharmacotherapy for FTLD focuses on manipulating serotonergic or dopaminergic neurotransmitter systems to ameliorate behavioural or motor symptoms. However, recent advances in FTLD genetics and molecular pathology make the prospect of biologically driven, disease-specific therapies for FTLD seem closer than ever.

[Relearning vocabulary. A comparative analysis between a case of dementia and Alzheimer's disease with predominant compromise of language]

INTRODUCTION

Semantic dementia is characterised by a progressive loss of semantic content that initially affects the capacity to name things, and is associated with asymmetric atrophy of the anterior temporal lobes. In Alzheimer's disease (AD) with predominant compromise of language, anomia is also the main symptom. The study examined the capacity to relearn vocabulary of two patients, each exhibiting one of these two forms of degenerative anomia.

CASE REPORTS

The two cases presented similar ages, gender, levels of schooling and degree of compromise. Their capacity to name a list of 40 pictures was evaluated at baseline, following 20 sessions of relearning, at one month and at six months. The patient with semantic dementia named 25/40 objects at baseline, 40/40 after relearning, 35/40 at one month and 27/40 at six months. The patient with AD named 29/40 at baseline, 30/40 after relearning, 29/40 at one month and 32/40 at six months. No intrusions were observed following relearning.

CONCLUSIONS

The patient with semantic dementia was able to relearn all the vocabulary she was shown, even though she lost everything she had acquired after treatment was interrupted. The AD patient did not improve her naming capacity with therapy. These differences suggest that the learning and consolidation circuits are affected in different ways. Subjects with semantic dementia, but not those with AD, could benefit from word relearning strategies with this method.

[Care for the patients with frontotemporal lobar degeneration]

In frontotemporal lobar degeneration (FTLD), which primarily involves the front of the brain, characteristic psychiatric symptoms and disordered behaviors, such as disinhibition and stereotypical behaviors, often appear in the foreground at the initial stages of the disease. It is therefore an extremely difficult disease to treat. However, unlike diseases such as Alzheimer's disease, which involve general dementia, there is a clear dissociation between the conserved and disordered functions; therefore, it is possible to enhance the conserved functions and simultaneously modify the disordered functions into adaptive behaviors in order to provide care and increase the quality of life (QOL). In this paper, we report the use of routinizing therapy, which is an occupational therapeutic approach, involving speech therapy for semantic dementia (SD), drug therapy for stereotypic behaviors and psychiatric symptoms of FTLD, family education, etc.