Frailty and Associated Environmental Factors Only Have Small Effects on Age of Onset in Huntington's Disease.

BACKGROUND: Over one third of age of onset variation in Huntington's disease is unexplained by CAG repeat length. In Alzheimer's disease, frailty partly modulates the relationship between neuropathology and dementia. OBJECTIVE: We investigated whether a multi-domain frailty index, reflecting non-genetic factors in Huntington's disease, similarly modulates the relationship between CAG repeat length and age of onset. METHODS: We created a frailty index assessing comorbidities, substance abuse, polypharmacy, and education. We applied multiple linear regression models to 2,741 subjects with manifest Huntington's disease from the Enroll-HD cohort study, including 729 subjects with late-onset (post-60 years) disease, using frailty index or constituent item scores and CAG repeat length as independent variables. We used actual and "residual" ages of onset (difference between actual and CAG-based predicted onset) as dependent variables, the latter offsetting the increased time available to accumulate comorbidities in older subjects. RESULTS: Higher frailty index scores were associated with significantly lower residual ages of onset in the late-onset subgroup (p = 0.03), though the effect was small (R2 = 0.27 with frailty as a predictor vs. 0.26 without). Number of comorbidities was also associated with significantly lower residual ages of onset in the late-onset subgroup (p = 0.04). Drug abuse and smoking were associated with significantly earlier ages of onset in the whole cohort (p < 0.01, p = 0.02) and late-onset subgroup (p < 0.01, p = 0.03). CONCLUSIONS: The impact of non-genetic factors on age of onset, assessed using a frailty index or separately, in Huntington's disease is limited.

A Phenotypic Atlas for Huntington Disease Based on Data From the Enroll-HD Cohort Study.

Background and Objectives: The variable CAG repeat expansion in the huntingtin gene and its inverse relationship to motor dysfunction onset are fundamental features of Huntington disease (HD). However, the wider phenotype (including non-motor features) at particular CAG lengths, ages, and functional levels is less well-characterized. The large number of participants in the Enroll-HD observational study enables the development of a phenotype atlas that summarizes the range and distribution of HD phenotypes, including outliers and possible clusters, with respect to various CAG repeat lengths, age ranges, and declining functional levels. Methods: Enroll-HD is an ongoing prospective longitudinal observational study that collects natural history data, releasing periodic data sets, in people with HD (PwHD) and controls. Core assessments at annual visits focus on behavioral, cognitive, motor, and functional status. Periodic data set 5, used for the development of the first iteration of the Enroll-HD Phenotype Atlas (EHDPA), included all eligible data collected through October 31, 2020. The atlas is based on subsets (cells) of descriptive data for all motor, cognitive, psychiatric, and functional measures that are routinely collected at most Enroll-HD sites, analyzed by single CAG lengths and 5-year age blocks. Results: Data from 42,840 visits from 15,982 unique PwHD were available for analysis. At baseline, participants had a mean ± SD age of 48.9 ± 13.9 years and CAG repeat length of 43.4 ± 3.6 and 54.1% were female. The EHDPA includes 223 age-by-CAG subsets for CAG repeats between 36 and 69 with five-year age brackets starting from 20-24 years up to 85-89 years. The atlas can be browsed at enroll-hd.org/for-researchers/atlas-of-hd-phenotype/. Discussion: The EHDPA summarizes the spectrum and distribution of HD phenotypes, including outliers and possible clusters, in all domains of disease involvement for the range of CAG repeat lengths, ages, and functional levels. Its availability in an easy-to-use online format will assist clinicians in tracking disease progression in PwHD by identifying phenotypic features most associated with loss of function and enabling conversations related to prognosis. The observable patterns in the EHDPA should also catalyze more formal multidomain characterization of motor, cognitive, and psychiatric progression and their relationships to functional decline and disease modifiers. Trial Registration Information: Enroll-HD is registered with clinicaltrials.gov: NCT01574053.

Apathy and Depression in Huntington's Disease: Distinct Longitudinal Trajectories and Clinical Correlates.

OBJECTIVE: Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease resulting in motor disturbances, dementia, and psychiatric symptoms. Apathy is a common manifestation and rated as one of the most impactful by patients and caregivers. It can often be difficult to distinguish from depression because of shared features and frequent overlap. This study examined the longitudinal trajectories and clinical correlates of apathy and depression. METHODS: Data were drawn from the Cooperative Huntington Observational Research Trial, a prospective, multicenter observational study that recruited 1,082 patients with HD. Measures of cognition, function, neuropsychiatric symptoms, motor function, and medication use were completed annually over 5 years. RESULTS: Overall, 423 patients (39%) showed evidence of apathy at study baseline, and both the prevalence and overall severity of apathy increased over time. Depression, by contrast, affected a similar proportion at baseline, although levels remained relatively stable over the study. Apathy was associated with worse cognition, function, neuropsychiatric symptoms, and motor symptoms. Depression was associated with worse neuropsychiatric symptoms, suicidal ideation, and independence but not other outcomes after control for other variables. CONCLUSIONS: Apathy in HD increased over time and was associated with worse clinical outcomes. These associations were independent of depression and other clinical variables. The findings highlight the need to distinguish between apathy and depression given their distinct implications for prognosis and management.

Greater time in bed and less physical activity associate with poorer cognitive functioning performance in Huntington's disease.

Objective: This study aimed to investigate how sleep and physical activity habits related to cognitive functioning, in naturalistic settings, in early Huntington's disease (HD). Method: Forty-two participants with the expanded HD repeat (20 manifest, 22 premanifest) and 29 healthy controls wore Fitbit One sleep and activity monitors for 7 days and 7 nights. They used a smartphone application to complete daily sleep and activity diaries, sleep and mood inventories, and a brief battery of cognitive tests, which were completed on Day 8 of the study. All data were collected in naturalistic home and community settings. Results: Amongst participants with the expanded HD repeat, greater time spent in bed, measured by Fitbit, was associated with poorer accuracy and response speed on a test of visual memory, whereas lower levels of physical activity, measured by Fitbit, were associated with poorer accuracy on a test involving a working memory component. Neither time in bed nor physical activity is associated with a test of psychomotor speed. Groups were mostly similar across a range of Fitbit and self-report measures of sleep and physical activity, although the Manifest-HD group spent more time in bed than the Premanifest-HD and Healthy Control groups and had better self-reported sleep quality and more self-reported time spent sitting than the Healthy Control group and the Premanifest-HD group, respectively. Conclusions: Sleep timing and physical activity relate to cognitive functioning in HD and may be important targets for management in behavioral intervention studies aimed at improving cognition in HD. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Mendelian Randomization in Cardiovascular Research: Establishing Causality When There Are Unmeasured Confounders.

Mendelian randomization is an epidemiological approach to making causal inferences using observational data. It makes use of the natural randomization that occurs in the generation of an individual's genetic makeup in a way that is analogous to the study design of a randomized controlled trial and uses instrumental variable analysis where the genetic variant(s) are the instrument (analogous to random allocation to treatment group in an randomized controlled trial). As with any instrumental variable, there are 3 assumptions that must be made about the genetic instrument: (1) it is associated (not necessarily causally) with the exposure (relevance condition); (2) it is associated with the outcome only through the exposure (exclusion restriction condition); and (3) it does not share a common cause with the outcome (ie, no confounders of the genetic instrument and outcome, independence condition). Using the example of type II diabetes and coronary artery disease, we demonstrate how the method may be used to investigate causality and discuss potential benefits and pitfalls. We conclude that although Mendelian randomization studies can usually not establish causality on their own, they may usefully contribute to the evidence base and increase our certainty about the effectiveness (or otherwise) of interventions to reduce cardiovascular disease.

Withdrawal or continuation of cholinesterase inhibitors or memantine or both, in people with dementia.

BACKGROUND: Dementia is a progressive syndrome characterised by deterioration in memory, thinking and behaviour, and by impaired ability to perform daily activities. Two classes of drug - cholinesterase inhibitors (donepezil, galantamine and rivastigmine) and memantine - are widely licensed for dementia due to Alzheimer's disease, and rivastigmine is also licensed for Parkinson's disease dementia. These drugs are prescribed to alleviate symptoms and delay disease progression in these and sometimes in other forms of dementia. There are uncertainties about the benefits and adverse effects of these drugs in the long term and in severe dementia, about effects of withdrawal, and about the most appropriate time to discontinue treatment. OBJECTIVES: To evaluate the effects of withdrawal or continuation of cholinesterase inhibitors or memantine, or both, in people with dementia on: cognitive, neuropsychiatric and functional outcomes, rates of institutionalisation, adverse events, dropout from trials, mortality, quality of life and carer-related outcomes. SEARCH METHODS: We searched the Cochrane Dementia and Cognitive Improvement Group's Specialised Register up to 17 October 2020 using terms appropriate for the retrieval of studies of cholinesterase inhibitors or memantine. The Specialised Register contains records of clinical trials identified from monthly searches of a number of major healthcare databases, numerous trial registries and grey literature sources. SELECTION CRITERIA: We included all randomised, controlled clinical trials (RCTs) which compared withdrawal of cholinesterase inhibitors or memantine, or both, with continuation of the same drug or drugs. DATA COLLECTION AND ANALYSIS: Two review authors independently assessed citations and full-text articles for inclusion, extracted data from included trials and assessed risk of bias using the Cochrane risk of bias tool. Where trials were sufficiently similar, we pooled data for outcomes in the short term (up to 2 months after randomisation), medium term (3-11 months) and long term (12 months or more). We assessed the overall certainty of the evidence for each outcome using GRADE methods. MAIN RESULTS: We included six trials investigating cholinesterase inhibitor withdrawal, and one trial investigating withdrawal of either donepezil or memantine. No trials assessed withdrawal of memantine only. Drugs were withdrawn abruptly in five trials and stepwise in two trials. All participants had dementia due to Alzheimer's disease, with severities ranging from mild to very severe, and were taking cholinesterase inhibitors without known adverse effects at baseline. The included trials randomised 759 participants to treatment groups relevant to this review. Study duration ranged from 6 weeks to 12 months. There were too few included studies to allow planned subgroup analyses. We considered some studies to be at unclear or high risk of selection, performance, detection, attrition or reporting bias. Compared to continuing cholinesterase inhibitors, discontinuing treatment may be associated with worse cognitive function in the short term (standardised mean difference (SMD) -0.42, 95% confidence interval (CI) -0.64 to -0.21; 4 studies; low certainty), but the effect in the medium term is very uncertain (SMD -0.40, 95% CI -0.87 to 0.07; 3 studies; very low certainty). In a sensitivity analysis omitting data from a study which only included participants who had shown a relatively poor prior response to donepezil, inconsistency was reduced and we found that cognitive function may be worse in the discontinuation group in the medium term (SMD -0.62; 95% CI -0.94 to -0.31). Data from one longer-term study suggest that discontinuing a cholinesterase inhibitor is probably associated with worse cognitive function at 12 months (mean difference (MD) -2.09 Standardised Mini-Mental State Examination (SMMSE) points, 95% CI -3.43 to -0.75; moderate certainty). Discontinuation may make little or no difference to functional status in the short term (SMD -0.25, 95% CI -0.54 to 0.04; 2 studies; low certainty), and its effect in the medium term is uncertain (SMD -0.38, 95% CI -0.74 to -0.01; 2 studies; very low certainty). After 12 months, discontinuing a cholinesterase inhibitor probably results in greater functional impairment than continuing treatment (MD -3.38 Bristol Activities of Daily Living Scale (BADLS) points, 95% CI -6.67 to -0.10; one study; moderate certainty). Discontinuation may be associated with a worsening of neuropsychiatric symptoms over the short term and medium term, although we cannot exclude a minimal effect (SMD - 0.48, 95% CI -0.82 to -0.13; 2 studies; low certainty; and SMD -0.27, 95% CI -0.47 to -0.08; 3 studies; low certainty, respectively). Data from one study suggest that discontinuing a cholinesterase inhibitor may result in little to no change in neuropsychiatric status at 12 months (MD -0.87 Neuropsychiatric Inventory (NPI) points; 95% CI -8.42 to 6.68; moderate certainty). We found no clear evidence of an effect of discontinuation on dropout due to lack of medication efficacy or deterioration in overall medical condition (odds ratio (OR) 1.53, 95% CI 0.84 to 2.76; 4 studies; low certainty), on number of adverse events (OR 0.85, 95% CI 0.57 to 1.27; 4 studies; low certainty) or serious adverse events (OR 0.80, 95% CI 0.46 to 1.39; 4 studies; low certainty), and on mortality (OR 0.75, 95% CI 0.36 to 1.55; 5 studies; low certainty). Institutionalisation was reported in one trial, but it was not possible to extract data for the groups relevant to this review. AUTHORS' CONCLUSIONS: This review suggests that discontinuing cholinesterase inhibitors may result in worse cognitive, neuropsychiatric and functional status than continuing treatment, although this is supported by limited evidence, almost all of low or very low certainty. As all participants had dementia due to Alzheimer's disease, our findings are not transferable to other dementia types. We were unable to determine whether the effects of discontinuing cholinesterase inhibitors differed with baseline dementia severity. There is currently no evidence to guide decisions about discontinuing memantine. There is a need for further well-designed RCTs, across a range of dementia severities and settings. We are aware of two ongoing registered trials. In making decisions about discontinuing these drugs, clinicians should exercise caution, considering the evidence from existing trials along with other factors important to patients and their carers.

Feasibility and initial validation of 'HD-Mobile', a smartphone application for remote self-administration of performance-based cognitive measures in Huntington's disease.

OBJECTIVE: Smartphone-based cognitive assessment measures allow efficient, rapid, and convenient collection of cognitive datasets. Establishment of feasibility and validity is essential for the widespread use of this approach. We describe a novel smartphone application (HD-Mobile) that includes three performance-based cognitive tasks with four key outcome measures, for use with Huntington's disease (HD) samples. We describe known groups and concurrent validity, test-retest reliability, sensitivity, and feasibility properties of the tasks. METHODS: Forty-two HD CAG-expanded participants (20 manifest, 22 premanifest) and 28 healthy controls completed HD-Mobile cognitive tasks three times across an 8-day period, on days 1, 4, and 8. A subsample of participants had pen-and-paper cognitive task data available from their most recent assessment from their participation in a separate observational longitudinal study, Enroll-HD. RESULTS: Manifest-HD participants performed worse than healthy controls for three of four HD-Mobile cognitive measures, and worse than premanifest-HD participants for two of four measures. We found robust test-retest reliability for manifest-HD participants (ICC = 0.71-0.96) and with some exceptions, in premanifest-HD (ICC = 0.52-0.96) and healthy controls (0.54-0.96). Correlations between HD-Mobile and selected Enroll-HD cognitive tasks were mostly medium to strong (r = 0.36-0.68) as were correlations between HD-Mobile cognitive tasks and measures of expected disease progression and motor symptoms for the HD CAG-expanded participants (r = - 0.34 to - 0.54). CONCLUSIONS: Results indicated robust known-groups, test-retest, concurrent validity, and sensitivity of HD-Mobile cognitive tasks. The study demonstrates the feasibility and utility of HD-Mobile for conducting convenient, frequent, and potentially ongoing assessment of HD samples without the need for in-person assessment.

The complex relationship between genotype, pathology and phenotype in familial dementia.

Causative genes involved in familial forms of dementias, including Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD) and dementia with Lewy bodies (DLB), as well as amyotrophic lateral sclerosis and prion diseases where dementia is present as a significant clinical feature, are associated with distinct proteinopathies. This review summarizes the relationship between known genetic determinants of these dementia syndromes and variations in key neuropathological proteins in terms of three types of heterogeneity: (i) Locus Heterogeneity, whereby mutations in different genes cause a similar proteinopathy, as exemplified by mutations in APP, PSEN1 and PSEN2 leading to AD neuropathology; (ii) Allelic Heterogeneity, whereby different mutations in the same gene lead to different proteinopathies or neuropathological severity, as exemplified by different mutations in MAPT and PRNP giving rise to protein species that differ in their biochemistry and affected cell types; and (iii) Phenotypic Heterogeneity, where identical gene mutations lead to different proteinopathies, as exemplified by LRRK2 p.G2019S being associated with variable Lewy body presence and alternative AD neuropathology or FTLD-tau. Of note, the perceived homogeneity in histologic phenotypes may arise from laboratory-specific assessment protocols which can differ in the panel of proteins screened. Finally, the understanding of the complex relationship between genotype and phenotype in dementia families is highly relevant in terms of therapeutic strategies which range from targeting specific genes, to a broader strategy of targeting a downstream, common biochemical problem that leads to the histopathology.

Circular RNAs: The Brain Transcriptome Comes Full Circle.

Circular RNAs (circRNAs) are a class of RNA molecules with a covalently closed loop structure formed by back-splicing of exon-exon junctions. The detection of circRNAs across many eukaryotic species, often with cell-type- and tissue-type-specific expression, has catalyzed a growing interest in understanding circRNA biogenesis and their potential functions. circRNAs are enriched in the brain, and accumulate upon neuronal differentiation and depolarization, suggesting that these RNAs are an integral component of the brain transcriptome, and may play functional roles. Here, we give an overview of the current understanding of circRNA biogenesis and function, discuss how circRNAs contribute to transcriptome complexity in the brain, and discuss recent data on the functional roles of circRNAs in the brain. We also discuss emerging data on the role of circRNAs in brain disorders and address common challenges of circRNA quantification in postmortem human brain.

Altered serum protein levels in frontotemporal dementia and amyotrophic lateral sclerosis indicate calcium and immunity dysregulation.

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that are considered to be on the same disease spectrum because of overlapping genetic, pathological and clinical traits. Changes in serum proteins in FTD and ALS are poorly understood, and currently no definitive biomarkers exist for diagnosing or monitoring disease progression for either disease. Here we applied quantitative discovery proteomics to analyze protein changes in FTD (N = 72) and ALS (N = 28) patient serum compared to controls (N = 22). Twenty three proteins were significantly altered in FTD compared to controls (increased-APOL1, C3, CTSH, EIF5A, MYH2, S100A8, SUSD5, WDR1; decreased-C1S, C7, CILP2, COMP, CRTAC1, EFEMP1, FBLN1, GSN, HSPG2, IGHV1, ITIH2, PROS1, SHBG, UMOD, VASN) and 14 proteins were significantly altered in ALS compared to controls (increased-APOL1, CKM, CTSH, IGHG1, IGKC, MYH2; decreased-C7, COMP, CRTAC1, EFEMP1, FBLN1, GSN, HSPG2, SHBG). There was substantial overlap in the proteins that were altered in FTD and ALS. These results were validated using western blotting. Gene ontology tools were used to assess functional pathways potentially dysregulated in the two diseases, and calcium ion binding and innate immunity pathways were altered in both diseases. When put together, these results suggest significant overlap in pathophysiological peripheral changes in FTD and ALS. This study represents the first proteomics side-by-side comparison of serum changes in FTD and ALS, providing new insights into under-recognized perturbed pathways and an avenue for biomarker development for FTD and ALS.

Making Decisions About Long-Term Institutional Care Placement Among People With Dementia and Their Caregivers: Systematic Review of Qualitative Studies.

BACKGROUND AND OBJECTIVES: People with dementia become increasingly dependent on others for care as cognition declines. Decision making about placement of people with dementia into long-term institutional care can be emotionally complex. The objective of this review is to describe experiences and perspectives of people with dementia and their family caregivers in making decisions about institutional care placement. RESEARCH DESIGN AND METHODS: MEDLINE, Embase, PsycINFO, and CINAHL were searched from inception to August 2018. Thematic synthesis was used to analyze results. RESULTS: We included 42 studies involving 123 people with dementia and 705 family caregivers from 12 countries. We identified five themes: ensuring safety (avoiding injury due to frailty, protecting against dangerous behaviors, preventing aggressive encounters), reaching breaking point (insufferable burden of caregiving, needs exceeding capabilities, intensifying family conflict, loneliness and isolation, straining under additional responsibilities, making extreme personal sacrifices), vulnerability in lacking support (ill-prepared for crisis, unable to access professional expertise, unpredictable prognostic trajectory, uncertainty navigating health care services, pressured by limited placement opportunities, high cost of placement, resenting loss of autonomy), avoiding guilt of abandonment (sharing accountability, mitigating against disagreement and stigma, reluctance to relinquish caregiving, seeking approval), and seeking reassurance and validation (preserving personhood and former identity, empowerment through engagement, assurance of care quality, acceptance from other care residents). DISCUSSION AND IMPLICATIONS: People with dementia and family caregivers feel vulnerable, disempowered, and guilty in decision making about institutionalization. Person-centered communication and support strategies that foster confidence and reassurance are needed to assist people with dementia and caregivers to make decisions about placement into long-term institutional care settings.

Psychosis and longitudinal outcomes in Huntington disease: the COHORT Study.

OBJECTIVE: Huntington disease (HD) is an autosomal dominant neurodegenerative disease involving motor disturbances, cognitive decline and psychiatric symptoms. Psychotic symptoms occur in a significant proportion of patients. We sought to characterise the clinical outcomes of this group of patients. METHODS: Data were drawn from the Cooperative Huntington Observational Research Trial, a prospective, multi-centre observational study. 1082 patients with HD were recruited. Measures of cognition, function, behavioural disturbance and motor function were completed annually over 5 years. RESULTS: Overall, 190 patients (17.6%) displayed psychotic symptoms. These patients demonstrated worse cognition, function and behavioural disturbances than patients without psychosis over time. Patients with psychosis also demonstrated lower levels of chorea than patients without psychosis, despite adjusting for concurrent antipsychotic and tetrabenazine use. CONCLUSIONS: Psychosis in HD is associated with poorer outcomes in cognition, function and behavioural symptoms. Patients with psychotic symptoms may also have less chorea. Altogether, the findings suggest patients with psychosis have a distinct clinical course.

A Deep Learning-Based Approach for Gait Analysis in Huntington Disease.

Huntington Disease (HD) is a genetic neurodegenerative disease which leads to involuntary movements and impaired balance. These changes have been quantified using footstep pressure sensor mats such as Protokinetics' Zeno Walkway. Drawing from distances between recorded footsteps, patients' disease severity have been measured in terms of high level gait characteristics such as gait width and stride length. However, little attention has been paid to the pressure data collected during formation of individual footsteps. This work investigates the potential of classifying patient disease severity based on individual footstep pressure data using deep learning techniques. Using the Motor Subscale of the Unified HD Rating Scale (UHDRS) as the gold standard, our experiments showed that using VGG16 and similar modules can achieve classification accuracy of 89%. Image pre-processing are key steps for better model performance. This classification accuracy is compared to results based on 3D CNN (82%) and SVM (86.9%).

CNS cell type-specific gene profiling of P301S tau transgenic mice identifies genes dysregulated by progressive tau accumulation.

The microtubule-associated protein tau undergoes aberrant modification resulting in insoluble brain deposits in various neurodegenerative diseases, including frontotemporal dementia (FTD), progressive supranuclear palsy, and corticobasal degeneration. Tau aggregates can form in different cell types of the central nervous system (CNS) but are most prevalent in neurons. We have previously recapitulated aspects of human FTD in mouse models by overexpressing mutant human tau in CNS neurons, including a P301S tau variant in TAU58/2 mice, characterized by early-onset and progressive behavioral deficits and FTD-like neuropathology. The molecular mechanisms underlying the functional deficits of TAU58/2 mice remain mostly elusive. Here, we employed functional genomics (i.e. RNAseq) to determine differentially expressed genes in young and aged TAU58/2 mice to identify alterations in cellular processes that may contribute to neuropathy. We identified genes in cortical brain samples differentially regulated between young and old TAU58/2 mice relative to nontransgenic littermates and by comparative analysis with a dataset of CNS cell type-specific genes expressed in nontransgenic mice. Most differentially-regulated genes had known or putative roles in neurons and included presynaptic and excitatory genes. Specifically, we observed changes in presynaptic factors, glutamatergic signaling, and protein scaffolding. Moreover, in the aged mice, expression levels of several genes whose expression was annotated to occur in other brain cell types were altered. Immunoblotting and immunostaining of brain samples from the TAU58/2 mice confirmed altered expression and localization of identified and network-linked proteins. Our results have revealed genes dysregulated by progressive tau accumulation in an FTD mouse model.

Neuroinflammation in frontotemporal dementia.

Frontotemporal dementia (FTD) refers to a group of progressive neurodegenerative disorders with different pathological signatures, genetic variability and complex disease mechanisms, for which no effective treatments exist. Despite advances in understanding the underlying pathology of FTD, sensitive and specific fluid biomarkers for this disease are lacking. As in other types of dementia, mounting evidence suggests that neuroinflammation is involved in the progression of FTD, including cortical inflammation, microglial activation, astrogliosis and differential expression of inflammation-related proteins in the periphery. Furthermore, an overlap between FTD and autoimmune disease has been identified. The most substantial evidence, however, comes from genetic studies, and several FTD-related genes are also implicated in neuroinflammation. This Review discusses specific evidence of neuroinflammatory mechanisms in FTD and describes how advances in our understanding of these mechanisms, in FTD as well as in other neurodegenerative diseases, might facilitate the development and implementation of diagnostic tools and disease-modifying treatments for FTD.

Recent Developments in TSPO PET Imaging as A Biomarker of Neuroinflammation in Neurodegenerative Disorders.

Neuroinflammation is an inflammatory response in the brain and spinal cord, which can involve the activation of microglia and astrocytes. It is a common feature of many central nervous system disorders, including a range of neurodegenerative disorders. An overlap between activated microglia, pro-inflammatory cytokines and translocator protein (TSPO) ligand binding was shown in early animal studies of neurodegeneration. These findings have been translated in clinical studies, where increases in TSPO positron emission tomography (PET) signal occur in disease-relevant areas across a broad spectrum of neurodegenerative diseases. While this supports the use of TSPO PET as a biomarker to monitor response in clinical trials of novel neurodegenerative therapeutics, the clinical utility of current TSPO PET radioligands has been hampered by the lack of high affinity binding to a prevalent form of polymorphic TSPO (A147T) compared to wild type TSPO. This review details recent developments in exploration of ligand-sensitivity to A147T TSPO that have yielded ligands with improved clinical utility. In addition to developing a non-discriminating TSPO ligand, the final frontier of TSPO biomarker research requires developing an understanding of the cellular and functional interpretation of the TSPO PET signal. Recent insights resulting from single cell analysis of microglial phenotypes are reviewed.