John Cornelis Van Swieten

Pathogenic germline variants in TARDBP can cause amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP). Recently, we identified somatic TARDBP variants in the brains of two individuals with semantic dementia and FTLD-TDP type C pathology. These two somatic variants (L41F and R42H) were identified in the N-terminal domain of TARDBP , whereas the vast majority of functional studies so far have focused on C-terminal variants, in which other known pathogenic germline variants are located. We assessed the pathogenicity of the somatic N-terminal variants in vivo using zebrafish in direct comparison to germline TARDBP variants causing ALS and FTLD-TDP (P112H, K263E, and I383V). In line with in silico predictions, we show that somatic variant L41F is poorly tolerated in in vivo toxicity assays, resulting in a toxic gain-of-function when ubiquitously expressed. Furthermore, L41F showed substantial loss-of-function in rescue experiments performed in tardbp -deficient background. Germline variant K263E showed similar outcomes, while somatic variant R42H and germline variant I383V demonstrated lower levels of toxicity and loss-of-function in these assays. An increase in microglia was detected with overexpression of R42H mRNA compared to wild-type, indicating a potential immunogenic effect. Additionally, we created a novel germline N-terminal tardbp single amino acid deletion (G37del) in zebrafish, which caused severe morphological abnormalities. In conclusion, these data provide evidence of the pathogenicity of N-terminal variants, including L41F and R42H, found as somatic variants in FTLD-TDP C, and suggest that possibly germline variants in this domain may not be compatible with life. This might contribute to the absence of pathogenic N-terminal variants in patients, and further reinforces the importance of considering somatic contributions in FTLD-TDP.

Neurodegenerative and psychiatric brain disorders show substantial heterogeneity in clinical and neuropathological manifestations, possibly reflecting both distinct and shared pathogenic mechanisms. Our understanding of brain disorder heterogeneity is limited by the scarcity of deeply phenotyped, multi-modal post-mortem brain bank cohorts. Here, we integrated genotype, clinical and neuropathological data from 2,553 Netherlands Brain Bank donors to investigate how genetic risk contributes to disease manifestations. Disease-specific polygenic risk scores revealed extensive cross-disorder enrichment of genetic risk, suggesting shared pathogenic mechanisms beyond diagnostic boundaries. Donors with frontotemporal lobar degeneration carrying C9orf72 repeat expansions showed elevated polygenic risk for multiple disorders, indicating that genetic variation may modify monogenic disease expression. Neuropsychiatric symptoms were associated with distinct personality-trait polygenic risk profiles across disorders, highlighting diagnosis-dependent genetic contributions to clinical heterogeneity. Together, our findings demonstrate the value of this unique resource and reveal a dynamic interplay of genetic risk across brain disorders.

Background and Objectives: Converging evidence hints at neurodevelopmental effects in genetic frontotemporal degeneration (FTD). In cross-sectional studies, for some genes, young adult FTD variant carriers show differences in brain volumes and cognition compared to familial non-carriers. However, longitudinal trajectories may more sensitively capture FTD-related neurodevelopmental vs. neurodegenerative changes than cross-sectional approaches. This study examined longitudinal trajectories of brain volumes, executive function, and plasma biomarkers in young adult carriers compared to familial non-carriers, as measures of neurodevelopmental and neurodegenerative outcomes of FTD-causing variants. Methods: This longitudinal cohort study comprised participants, aged 18-30 years, from the FTD Prevention Initiative across Europe, Canada, and the USA. Genetic groups included C9orf72 (47%), MAPT (30%), and GRN (23%). Linear mixed-effects models were computed to assess longitudinal outcomes across age between groups, controlling for sex, scanner (for brain volumes), and education (for executive function); random effects accounted for between-subject variability nested within family membership. Results: Variant carriers ( n =147) and familial non-carriers ( n =113) did not differ in age (mean±SD, 25.9±3.2 years), sex (53% female), or number of visits (2.1±1.7). Young adult C9orf72 repeat expansion carriers exhibited smaller thalamic volumes than non-carriers at the reference age of 26 years ( b =-982.8mm ³ , SE=317.0, p= 0.0046, f ² =0.32), with relatively stable trajectories across ages 18-30 (i.e., no change over time). Trajectories of rostral anterior cingulate volumes differed in C9orf72 carriers and non-carriers across age, where carriers showed relatively stable trajectories and non-carriers showed age-appropriate declines ( b =64.4mm ³ , SE=29.9, p= 0.035, f ² =0.07). For MAPT and GRN , there were little to no differences in total brain, cortical, or subcortical volumes between groups and over time. No longitudinal differences were observed between carriers and non-carriers in executive function, or plasma NfL or GFAP for any genetic group. Discussion: C9orf72 repeat expansions were linked to smaller average thalamic volumes and stable trajectories between ages 18 to 30, supporting potential neurodevelopmental origins. The modest evidence supporting an absence of difference in neurodegenerative biomarkers and executive function suggests minimal early neurodegeneration and functional preservation in young adulthood.

Iron accumulation is known to be involved in frontotemporal lobar degeneration (FTLD) and possibly with a different spatial pattern in FTLD with tau (FTLD-tau) versus TDP-43 (FTLD-TDP) pathology. In this study, we aimed to visualize the spatial distribution of iron in ex vivo brain tissue with FTLD-tau or with FTLD-TDP using both histology and MRI. High resolution multi-echo T2*-weighted 7T MRI was performed on ex vivo tissue of the frontal and temporal cortex of 14 FTLD cases (6 FTLD-tau, 8 FTLD-TDP) and 11 healthy controls (HC) to obtain T2*-weighted images and quantitative susceptibility maps (QSM). These tissue blocks were then stained for iron. The spatial iron distribution was assessed visually by different scoring features on the three modalities (T2*-weighted MRI, QSM, and histology) and analyzing cortical layer profiles of the signal intensity. We found more iron accumulation in the temporal cortex of FTLD cases compared to HC, displayed by higher visual ratings and lower signal intensity values on cortical layer profiles. Histology showed a good correlation with T2*-weighted MRI. QSM offered complementary information compared to T2*-weighted MRI, particularly for identifying distinct histological features of iron accumulation within the subcortical U-fibers. We conclude that iron accumulation is involved in the disease process of FTLD and that T2*-weighted MRI and QSM can be used as a noninvasive imaging modality to study cortical and subcortical iron accumulation in FTLD.

The majority of individuals at 50% risk of carrying a pathogenic variant causing frontotemporal dementia (FTD) chooses not to undergo presymptomatic genetic testing. To explore reasons of individuals at risk of genetic FTD for and against genetic testing, we conducted a secondary analysis of semi-structured interviews with 28 Dutch individuals at risk of genetic FTD: 11 carriers, three non-carriers and 14 individuals at 50% risk. We analyzed the data using inductive thematic analysis. Reasons in favor of presymptomatic genetic testing included wanting to know and relieving uncertainty, hoping to test negative, relevance for life planning, reproductive decision-making, explaining future symptoms, and contributing to the common good. Reasons against presymptomatic genetic testing were fearing psychological impact, fearing effects on employment, insurability or mortgage, maintaining hope, and no immediate relevance for life planning, partly due to uncertainty surrounding age of onset. Reasons were similar in all groups, but were weighed differently. Decision-making processes were influenced by (expectations of) psychological burden before and after testing, personal experiences with FTD in the family, and whether genetic status was perceived as actionable. Perceived relevance was partly influenced by age (in relation to age of onset), life circumstances and research participation. When supporting individuals at risk of FTD in decision-making regarding genetic testing, genetic counselors should be aware of and tailor counseling to the individual’s age and lived experience with being at risk, experiences with FTD and related disorders in the family, and factors influencing the perceived relevance of genetic status for reproductive or life decision-making.

Background: As understanding of biomarkers for genetic frontotemporal dementia (FTD) advances, there is a need to develop onset-predictive biomarker tests (OPBTs) to detect changes before the onset of symptoms. OPBTs can be used to recruit carriers or individuals at 50% risk of carriership into clinical trials of investigational therapies targeting the preclinical and prodromal phases of FTD. OPBT results should be disclosed as part of the informed consent process, with positive results indicating that symptom onset is likely in the next few years. This information can be psychologically burdensome, especially in individuals at 50% risk, for whom a positive OPBT result would reveal their genetic status. There is a need for ethical guidance for disclosure processes to help researchers implement disclosure of OPBT results responsibly at their study sites. Methods: Existing literature on disclosure of genetic and biomarker results in neurodegenerative conditions informed the design of this disclosure process for OPBT in FTD. Drafts were discussed with the multidisciplinary research team, scientific and clinical FTD experts across European countries, and other stakeholders and revised accordingly. Results: The suggested disclosure process provides guidance for first-time or repeated disclosure of OPBT results to carriers or individuals at 50% risk of genetic FTD in research settings. Conclusions: Researchers involved in clinical trials using OPBTs can adopt this disclosure process as a framework for responsible communication of OPBT results at their study site. The process was designed for international applicability and facilitates the alignment of disclosure processes for clinical trial recruitment across European countries.

Background: Diagnosis of Frontotemporal dementia (FTD) and its specific underlying neuropathologies (frontotemporal lobar degeneration; FTLD-Tau and FTLD-TDP) are challenging, and thus, fluid biomarkers are needed to improve diagnostic accuracy. Methods: We used proximity extension assays to analyze 665 proteins in cerebrospinal fluid (CSF) samples from a multicenter cohort, which included patients with FTD ( n = 189), Alzheimer’s Disease dementia (AD; n = 232), and cognitively unimpaired individuals ( n = 196). In a subset, FTLD neuropathology was determined based on phenotype or genotype (FTLD-Tau = 87 and FTLD-TDP = 67). Differences in protein expression profiles were analyzed using nested linear models. Penalized generalized linear modeling was used to identify classification protein panels, which were translated to custom multiplex assays and validated in two clinical cohorts (cohort 1: n = 161; cohort 2: n = 162), one autopsy-confirmed cohort ( n = 100), and one genetic cohort ( n = 55). Results: Forty-three proteins were differentially regulated in FTD compared to controls and AD, reflecting axon development, regulation of synapse assembly, and cell-cell adhesion mediator activity pathways. Classification analysis identified a 14- and 13-CSF protein panel that discriminated FTD from controls (FTD diagnostic panel, AUC: 0.96) or AD (FTD differential diagnostic panel, AUC: 0.91). Custom multiplex panels confirmed the strong discriminative performancen between FTD and controls (AUCs > 0.96) and between FTD and AD (AUCs > 0.88) across three validation cohorts, including one with autopsy confirmation (AUCs > 0.90). Validation in genetic FTD (including C9orf72 , GRN , and MAPT mutation carriers) revealed high accuracy of the FTD diagnostic panel in identifying both the presymptomatic (AUCs > 0.95) and symptomatic (AUC: 1) stages. Six proteins were differentially regulated between FTLD-TDP and FTLD-Tau. However, a reproducible classification model could not be generated (AUC: 0.80). Conclusions: Overall, this study introduces novel FTD-specific biomarker panels with potential use in diagnostic settings.

Executive dysfunction is a core feature of frontotemporal dementia (FTD). While there has been extensive research into such impairments in sporadic FTD, there has been little research in the familial forms. Seven hundred fifty-two individuals were recruited in total: 214 C9orf72; 205 progranulin (GRN) and 86 microtubule associated protein tau (MAPT) mutation carriers, stratified into asymptomatic, prodromal, and fully symptomatic; and 247 mutation-negative controls. Attention and executive function were measured using the Weschler Memory Scale-Revised (WMS-R) Digit Span Backwards (DSB), Wechsler Adult Intelligence Scale-Revised Digit Symbol task, Trail Making Test Parts A and B, and the Delis-Kaplan Executive Function System Color Word Interference Test. Linear regression models with bootstrapping were used to assess differences between groups. Correlation of task score with disease severity was also performed, as well as an analysis of the neuroanatomical correlates of each task. Fully symptomatic C9orf72, GRN, and MAPT mutation carriers were significantly impaired on all tasks compared with controls (all p < 0.001), except on the WMS-R DSB in the MAPT mutation carriers (p = 0.147). While asymptomatic and prodromal C9orf72 individuals also demonstrated differences compared with controls, neither the GRN or MAPT asymptomatic or prodromal mutation carriers showed significant deficits. All tasks were significantly correlated with disease severity in each of the genetic groups (all p < 0.001). Some individuals with C9orf72 mutations show difficulties with executive function from very early on in the disease and this continues to deteriorate with disease severity. By contrast, similar difficulties occur only in the later stages of the disease in GRN and MAPT mutation carriers. This differential performance across the genetic groups will be important in neuropsychological task selection in upcoming clinical trials.

Background: In the last decade, the importance of DNA methylation in the functioning of the central nervous system has been highlighted through associations between methylation changes and differential expression of key genes involved in aging and neurodegenerative diseases. In frontotemporal lobar degeneration (FTLD), aberrant methylation has been reported in causal disease genes including GRN and C9orf72; however, the genome-wide contribution of epigenetic changes to the development of FTLD remains largely unexplored. Methods: We performed reduced representation bisulfite sequencing of matched pairs of post-mortem tissue from frontal cortex (FCX) and cerebellum (CER) from pathologically confirmed FTLD patients with TDP-43 pathology (FTLD-TDP) further divided into five subtypes and including both sporadic and genetic forms (N = 25 pairs per group), and neuropathologically normal controls (N = 42 pairs). Case-control differential methylation analyses were performed, both at the individual CpG level, and in regions of grouped CpGs (differentially methylated regions; DMRs), either including all genomic locations or only gene promoters. Gene Ontology (GO) analyses were then performed using all differentially methylated genes in each group of sporadic patients. Finally, additional datasets were queried to prioritize candidate genes for follow-up. Results: Using the largest FTLD-TDP DNA methylation dataset generated to date, we identified thousands of differentially methylated CpGs (FCX = 6,520; CER = 7,134) and several hundred DMRs in FTLD-TDP brains (FCX = 134; CER = 219). Of these, less than 10% are shared between pathological subgroups. Combining additional datasets, we identified, validated and replicated hypomethylation of CAMTA1 in TDP-A potentially also impacting additional genes in the locus. GO analysis further implicated DNA methylation in myelination and developmental processes, as well as important disease-relevant mechanisms with subtype specificity such as protein phosphorylation and DNA damage repair in TDP-A, cholesterol biosynthesis in TDP-B, and protein localization in TDP-C. Conclusions: We identify methylation changes in all FTLD-TDP patient groups and show that most changes are unique to a specific pathological FTLD-TDP subtype, suggesting that these subtypes not only have distinct transcriptomic and genetic signatures, but are also epigenetically distinct. Our study constitutes an invaluable resource to the community and highlights the need for further studies to profile additional epigenetic layers within each FTLD-TDP pathological subtype.