A growth-factor-activated lysosomal K+ channel regulates Parkinson's pathology.

Lysosomes have fundamental physiological roles and have previously been implicated in Parkinson's disease1-5. However, how extracellular growth factors communicate with intracellular organelles to control lysosomal function is not well understood. Here we report a lysosomal K+ channel complex that is activated by growth factors and gated by protein kinase B (AKT) that we term lysoKGF. LysoKGF consists of a pore-forming protein TMEM175 and AKT: TMEM175 is opened by conformational changes in, but not the catalytic activity of, AKT. The minor allele at rs34311866, a common variant in TMEM175, is associated with an increased risk of developing Parkinson's disease and reduces channel currents. Reduction in lysoKGF function predisposes neurons to stress-induced damage and accelerates the accumulation of pathological α-synuclein. By contrast, the minor allele at rs3488217-another common variant of TMEM175, which is associated with a decreased risk of developing Parkinson's disease-produces a gain-of-function in lysoKGF during cell starvation, and enables neuronal resistance to damage. Deficiency in TMEM175 leads to a loss of dopaminergic neurons and impairment in motor function in mice, and a TMEM175 loss-of-function variant is nominally associated with accelerated rates of cognitive and motor decline in humans with Parkinson's disease. Together, our studies uncover a pathway by which extracellular growth factors regulate intracellular organelle function, and establish a targetable mechanism by which common variants of TMEM175 confer risk for Parkinson's disease.

L1CAM is not associated with extracellular vesicles in human cerebrospinal fluid or plasma.

L1CAM is a transmembrane protein expressed on neurons that was presumed to be found on neuron-derived extracellular vesicles (NDEVs) in human biofluids. We developed a panel of single-molecule array assays to evaluate the use of L1CAM for NDEV isolation. We demonstrate that L1CAM is not associated with extracellular vesicles in human plasma or cerebrospinal fluid and therefore recommend against its use as a marker in NDEV isolation protocols.

GPNMB Biomarker Levels in GBA1 Carriers with Lewy Body Disorders.

BACKGROUND: The GPNMB single-nucleotide polymorphism rs199347 and GBA1 variants both associate with Lewy body disorder (LBD) risk. GPNMB encodes glycoprotein nonmetastatic melanoma protein B (GPNMB), a biomarker for GBA1-associated Gaucher's disease. OBJECTIVE: The aim of this study was to determine whether GPNMB levels (1) differ in LBD with and without GBA1 variants and (2) associate with rs199347 genotype. METHODS: We quantified GPNMB levels in plasma and cerebrospinal fluid (CSF) from 124 individuals with LBD with one GBA1 variant (121 plasma, 14 CSF), 631 individuals with LBD without GBA1 variants (626 plasma, 41 CSF), 9 neurologically normal individuals with one GBA1 variant (plasma), and 2 individuals with two GBA1 variants (plasma). We tested for associations between GPNMB levels and rs199347 or GBA1 status. RESULTS: GPNMB levels associate with rs199347 genotype in plasma (P = 0.022) and CSF (P = 0.007), but not with GBA1 status. CONCLUSIONS: rs199347 is a protein quantitative trait locus for GPNMB. GPNMB levels are unaltered in individuals carrying one GBA1 variant. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Omics in Neurodegenerative Disease: Hope or Hype?

The past 15 years have seen a boom in the use and integration of 'omic' approaches (limited here to genomic, transcriptomic, and epigenomic techniques) to study neurodegenerative disease in an unprecedented way. We first highlight advances in and the limitations of using such approaches in the neurodegenerative disease literature, with a focus on Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal lobar degeneration (FTLD), and amyotrophic lateral sclerosis (ALS). We next discuss how these studies can advance human health in the form of generating leads for downstream mechanistic investigation or yielding polygenic risk scores (PRSs) for prognostication. However, we argue that these approaches constitute a new form of molecular description, analogous to clinical or pathological description, that alone does not hold the key to solving these complex diseases.

Plasma MIA, CRP, and Albumin Predict Cognitive Decline in Parkinson's Disease.

OBJECTIVE: Using a multi-cohort, discovery-replication-validation design, we sought new plasma biomarkers that predict which individuals with Parkinson's disease (PD) will experience cognitive decline. METHODS: In 108 discovery cohort PD individuals and 83 replication cohort PD individuals, we measured 940 plasma proteins on an aptamer-based platform. Using proteins associated with subsequent cognitive decline in both cohorts, we trained a logistic regression model to predict which patients with PD showed fast (> = 1 point drop/year on Montreal Cognitive Assessment [MoCA]) versus slow (< 1 point drop/year on MoCA) cognitive decline in the discovery cohort, testing it in the replication cohort. We developed alternate assays for the top 3 proteins and confirmed their ability to predict cognitive decline - defined by change in MoCA or development of incident mild cognitive impairment (MCI) or dementia - in a validation cohort of 118 individuals with PD. We investigated the top plasma biomarker for causal influence by Mendelian randomization (MR). RESULTS: A model with only 3 proteins (melanoma inhibitory activity protein [MIA], C-reactive protein [CRP], and albumin) separated fast versus slow cognitive decline subgroups with an area under the curve (AUC) of 0.80 in the validation cohort. The individuals with PD in the validation cohort in the top quartile of risk for cognitive decline based on this model were 4.4 times more likely to develop incident MCI or dementia than those in the lowest quartile. Genotypes at MIA single nucleotide polymorphism (SNP) rs2233154 associated with MIA levels and cognitive decline, providing evidence for MIA's causal influence. CONCLUSIONS: An easily obtained plasma-based predictor identifies individuals with PD at risk for cognitive decline. MIA may participate causally in development of cognitive decline. ANN NEUROL 2022;92:255-269.

The Post-GWAS Era: From Association to Function.

During the past 12 years, genome-wide association studies (GWASs) have uncovered thousands of genetic variants that influence risk for complex human traits and diseases. Yet functional studies aimed at delineating the causal genetic variants and biological mechanisms underlying the observed statistical associations with disease risk have lagged. In this review, we highlight key advances in the field of functional genomics that may facilitate the derivation of biological meaning post-GWAS. We highlight the evidence suggesting that causal variants underlying disease risk often function through regulatory effects on the expression of target genes and that these expression effects might be modest and cell-type specific. We moreover discuss specific studies as proof-of-principle examples for current statistical, bioinformatic, and empirical bench-based approaches to downstream elucidation of GWAS-identified disease risk loci.

TMEM106B modifies TDP-43 pathology in human ALS brain and cell-based models of TDP-43 proteinopathy.

The neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TAR DNA-binding protein-43 (TDP-43) inclusions (FTLD-TDP) share the neuropathological hallmark of aggregates of TDP-43. However, factors governing the severity and regional distribution of TDP-43 pathology, which may account for the divergent clinical presentations of ALS and FTLD-TDP, are not well understood. Here, we investigated the influence of genotypes at TMEM106B, a locus associated with risk for FTLD-TDP, and hexanucleotide repeat expansions in C9orf72, a known genetic cause for both ALS and FTLD-TDP, on global TDP-43 pathology and regional distribution of TDP-43 pathology in 899 postmortem cases from a spectrum of neurodegenerative diseases. We found that, among the 110 ALS cases, minor (C)-allele homozygotes at the TMEM106B locus sentinel SNP rs1990622 had more TDP-43 pathology globally, as well as in select brain regions. C9orf72 expansions similarly associated with greater TDP-43 pathology in ALS. However, adjusting for C9orf72 expansion status did not affect the relationship between TMEM106B genotype and TDP-43 pathology. To elucidate the direction of causality for this association, we directly manipulated TMEM106B levels in an inducible cell system that expresses mislocalized TDP-43 protein. We found that partial knockdown of TMEM106B, to levels similar to what would be expected in rs1990622 C allele carriers, led to development of more TDP-43 cytoplasmic aggregates, which were more insoluble, in this system. Taken together, our results support a causal role for TMEM106B in modifying the development of TDP-43 proteinopathy.

Neurofilament Light Chain as a Biomarker for Cognitive Decline in Parkinson Disease.

BACKGROUND: Neurofilament light chain protein (NfL) is a promising biomarker of neurodegeneration. OBJECTIVES: To determine whether plasma and CSF NfL (1) associate with motor or cognitive status in Parkinson's disease (PD) and (2) predict future motor or cognitive decline in PD. METHODS: Six hundred and fifteen participants with neurodegenerative diseases, including 152 PD and 200 healthy control participants, provided a plasma and/or cerebrospinal fluid (CSF) NfL sample. Diagnostic groups were compared using the Kruskal-Wallis rank test. Within PD, cross-sectional associations between NfL and Unified Parkinson's Disease Rating Scale Part III (UPDRS-III) and Mattis Dementia Rating Scale (DRS-2) scores were assessed by linear regression; longitudinal analyses were performed using linear mixed-effects models and Cox regression. RESULTS: Plasma and CSF NfL levels correlated substantially (Spearman r = 0.64, P < 0.001); NfL was highest in neurocognitive disorders. PD participants with high plasma NfL were more likely to develop incident cognitive impairment (HR 5.34, P = 0.005). CONCLUSIONS: Plasma NfL is a useful prognostic biomarker for PD, predicting clinical conversion to mild cognitive impairment or dementia. © 2021 International Parkinson and Movement Disorder Society.

A Dementia-Associated Risk Variant near TMEM106B Alters Chromatin Architecture and Gene Expression.

Neurodegenerative diseases pose an extraordinary threat to the world's aging population, yet no disease-modifying therapies are available. Although genome-wide association studies (GWASs) have identified hundreds of risk loci for neurodegeneration, the mechanisms by which these loci influence disease risk are largely unknown. Here, we investigated the association between common genetic variants at the 7p21 locus and risk of the neurodegenerative disease frontotemporal lobar degeneration. We showed that variants associated with disease risk correlate with increased expression of the 7p21 gene TMEM106B and no other genes; co-localization analyses implicated a common causal variant underlying both association with disease and association with TMEM106B expression in lymphoblastoid cell lines and human brain. Furthermore, increases in the amount of TMEM106B resulted in increases in abnormal lysosomal phenotypes and cell toxicity in both immortalized cell lines and neurons. We then combined fine-mapping, bioinformatics, and bench-based approaches to functionally characterize all candidate causal variants at this locus. This approach identified a noncoding variant, rs1990620, that differentially recruits CTCF in lymphoblastoid cell lines and human brain to influence CTCF-mediated long-range chromatin-looping interactions between multiple cis-regulatory elements, including the TMEM106B promoter. Our findings thus provide an in-depth analysis of the 7p21 locus linked by GWASs to frontotemporal lobar degeneration, nominating a causal variant and causal mechanism for allele-specific expression and disease association at this locus. Finally, we show that genetic variants associated with risk of neurodegenerative diseases beyond frontotemporal lobar degeneration are enriched in CTCF-binding sites found in brain-relevant tissues, implicating CTCF-mediated gene regulation in risk of neurodegeneration more generally.

TMEM106B Effect on cognition in Parkinson disease and frontotemporal dementia.

OBJECTIVE: Common variants near TMEM106B associate with risk of developing frontotemporal dementia (FTD). Emerging evidence suggests a role for TMEM106B in neurodegenerative processes beyond FTD. We evaluate the effect of TMEM106B genotype on cognitive decline across multiple neurogenerative diseases. METHODS: We longitudinally followed 870 subjects with diagnoses of Parkinson disease (PD; n = 179), FTD (n = 179), Alzheimer disease (AD; n = 300), memory-predominant mild cognitive impairment (MCI; n = 75), or neurologically normal control subjects (NC; n = 137) at the University of Pennsylvania (UPenn). All participants had annual Mini-Mental State Examination (MMSE; median follow-up duration = 3.0 years) and were genotyped at TMEM106B index single nucleotide polymorphism rs1990622. Genotype effects on cognition were confirmed by extending analyses to additional cognitive instruments (Mattis Dementia Rating Scale-2 [DRS-2] and Montreal Cognitive Assessment [MoCA]) and to an international validation cohort (Parkinson's Progression Markers Initiative [PPMI], N = 371). RESULTS: The TMEM106B rs1990622T allele, linked to increased risk of FTD, associated with greater MMSE decline over time in PD subjects but not in AD or MCI subjects. For FTD subjects, rs1990622T associated with more rapid decrease in MMSE only under the minor-allele, rs1990622C , dominant model. Among PD patients, rs1990622T carriers from the UPenn cohort demonstrated more rapid longitudinal decline in DRS-2 scores. Finally, in the PPMI cohort, TMEM106B risk allele carriers demonstrated more rapid longitudinal decline in MoCA scores. INTERPRETATION: Irrespective of cognitive instrument or cohort assessed, TMEM106B acts as a genetic modifier for cognitive trajectory in PD. Our results implicate lysosomal dysfunction in the pathogenesis of cognitive decline in 2 different proteinopathies. ANN NEUROL 2019;85:801-811.

ADNC-RS, a clinical-genetic risk score, predicts Alzheimer's pathology in autopsy-confirmed Parkinson's disease and Dementia with Lewy bodies.

Growing evidence suggests overlap between Alzheimer's disease (AD) and Parkinson's disease (PD) pathophysiology in a subset of patients. Indeed, 50-80% of autopsy cases with a primary clinicopathological diagnosis of Lewy body disease (LBD)-most commonly manifesting during life as PD-have concomitant amyloid-beta and tau pathology, the defining pathologies of AD. Here we evaluated common genetic variants in genome-wide association with AD as predictors of concomitant AD pathology in the brains of people with a primary clinicopathological diagnosis of PD or Dementia with Lewy Bodies (DLB), diseases both characterized by neuronal Lewy bodies. In the first stage of our study, 127 consecutive autopsy-confirmed cases of PD or DLB from a single center were assessed for AD neuropathological change (ADNC), and these same cases were genotyped at 20 single nucleotide polymorphisms (SNPs) found by genome-wide association study to associate with risk for AD. In these 127 training set individuals, we developed a logistic regression model predicting the presence of ADNC, using backward stepwise regression for model selection and tenfold cross-validation to estimate performance. The best-fit model generated a risk score for ADNC (ADNC-RS) based on age at disease onset and genotype at three SNPs (APOE, BIN1, and SORL1 loci), with an area under the receiver operating curve (AUC) of 0.751 in our training set. In the replication stage of our study, we assessed model performance in a separate test set of the next 81 individuals genotyped in our center. In the test set, the AUC was 0.781, and individuals with ADNC-RS in the top quintile had four-fold increased likelihood of having AD pathology at autopsy compared with those in each of the lowest two quintiles. Finally, in the validation stage of our study, we applied our ADNC-RS model to 70 LBD individuals from 20 Alzheimer's Disease Research Centers (ADRC) whose autopsy and genetic data were available in the National Alzheimer's Coordinating Center (NACC) database. In this validation set, the AUC was 0.754. Thus, in patients with autopsy-confirmed PD or DLB, a simple model incorporating three AD-risk SNPs and age at disease onset substantially enriches for concomitant AD pathology at autopsy, with implications for identifying LBD patients in which targeting amyloid-beta or tau is a therapeutic strategy.

AAV-Mediated Progranulin Delivery to a Mouse Model of Progranulin Deficiency Causes T Cell-Mediated Toxicity.

Adeno-associated virus-mediated gene replacement is emerging as a safe and effective means of correcting single-gene mutations affecting the CNS. AAV-mediated progranulin gene (GRN) delivery has been proposed as a treatment for GRN-deficient frontotemporal dementia and neuronal ceroid lipofuscinosis, and recent studies using intraparenchymal AAV-Grn delivery to brain have shown moderate success in histopathologic and behavioral rescue in mouse models. Here, we used AAV9 to deliver GRN to the lateral ventricle to achieve widespread expression in the Grn null mouse brain. We found that, despite a global increase in progranulin, overexpression resulted in dramatic and selective hippocampal toxicity and degeneration affecting neurons and glia. Hippocampal degeneration was preceded by T cell infiltration and perivascular cuffing. GRN delivery with an ependymal-targeting AAV for selective secretion of progranulin into the cerebrospinal fluid similarly resulted in T cell infiltration, as well as ependymal hypertrophy. Interestingly, overexpression of GRN in wild-type animals also provoked T cell infiltration. These results call into question the safety of GRN overexpression in the CNS, with evidence for both a region-selective immune response and cellular proliferative response. Our results highlight the importance of careful consideration of target gene biology and cellular response to overexpression prior to progressing to the clinic.

Characterization of Parkinson's disease using blood-based biomarkers: A multicohort proteomic analysis.

BACKGROUND: Parkinson's disease (PD) is a progressive neurodegenerative disease affecting about 5 million people worldwide with no disease-modifying therapies. We sought blood-based biomarkers in order to provide molecular characterization of individuals with PD for diagnostic confirmation and prediction of progression. METHODS AND FINDINGS: In 141 plasma samples (96 PD, 45 neurologically normal control [NC] individuals; 45.4% female, mean age 70.0 years) from a longitudinally followed Discovery Cohort based at the University of Pennsylvania (UPenn), we measured levels of 1,129 proteins using an aptamer-based platform. We modeled protein plasma concentration (log10 of relative fluorescence units [RFUs]) as the effect of treatment group (PD versus NC), age at plasma collection, sex, and the levodopa equivalent daily dose (LEDD), deriving first-pass candidate protein biomarkers based on p-value for PD versus NC. These candidate proteins were then ranked by Stability Selection. We confirmed findings from our Discovery Cohort in a Replication Cohort of 317 individuals (215 PD, 102 NC; 47.9% female, mean age 66.7 years) from the multisite, longitudinally followed National Institute of Neurological Disorders and Stroke Parkinson's Disease Biomarker Program (PDBP) Cohort. Analytical approach in the Replication Cohort mirrored the approach in the Discovery Cohort: each protein plasma concentration (log10 of RFU) was modeled as the effect of group (PD versus NC), age at plasma collection, sex, clinical site, and batch. Of the top 10 proteins from the Discovery Cohort ranked by Stability Selection, four associations were replicated in the Replication Cohort. These blood-based biomarkers were bone sialoprotein (BSP, Discovery false discovery rate [FDR]-corrected p = 2.82 × 10-2, Replication FDR-corrected p = 1.03 × 10-4), osteomodulin (OMD, Discovery FDR-corrected p = 2.14 × 10-2, Replication FDR-corrected p = 9.14 × 10-5), aminoacylase-1 (ACY1, Discovery FDR-corrected p = 1.86 × 10-3, Replication FDR-corrected p = 2.18 × 10-2), and growth hormone receptor (GHR, Discovery FDR-corrected p = 3.49 × 10-4, Replication FDR-corrected p = 2.97 × 10-3). Measures of these proteins were not significantly affected by differences in sample handling, and they did not change comparing plasma samples from 10 PD participants sampled both on versus off dopaminergic medication. Plasma measures of OMD, ACY1, and GHR differed in PD versus NC but did not differ between individuals with amyotrophic lateral sclerosis (ALS, n = 59) versus NC. In the Discovery Cohort, individuals with baseline levels of GHR and ACY1 in the lowest tertile were more likely to progress to mild cognitive impairment (MCI) or dementia in Cox proportional hazards analyses adjusting for age, sex, and disease duration (hazard ratio [HR] 2.27 [95% CI 1.04-5.0, p = 0.04] for GHR, and HR 3.0 [95% CI 1.24-7.0, p = 0.014] for ACY1). GHR's association with cognitive decline was confirmed in the Replication Cohort (HR 3.6 [95% CI 1.20-11.1, p = 0.02]). The main limitations of this study were its reliance on the aptamer-based platform for protein measurement and limited follow-up time available for some cohorts. CONCLUSIONS: In this study, we found that the blood-based biomarkers BSP, OMD, ACY1, and GHR robustly associated with PD across multiple clinical sites. Our findings suggest that biomarkers based on a peripheral blood sample may be developed for both disease characterization and prediction of future disease progression in PD.

Increased expression of the frontotemporal dementia risk factor TMEM106B causes C9orf72-dependent alterations in lysosomes.

Frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) is an important cause of dementia in individuals under age 65. Common variants in the TMEM106B gene were previously discovered by genome-wide association to confer genetic risk for FTLD-TDP (p = 1 × 10-11, OR = 1.6). Furthermore, TMEM106B may act as a genetic modifier affecting age at onset and age at death in the Mendelian subgoup of FTLD-TDP due to expansions of the C9orf72 gene. Evidence suggests that TMEM106B variants increase risk for developing FTLD-TDP by increasing expression of Transmembrane Protein 106B (TMEM106B), a lysosomal protein. To further understand the functional role of TMEM106B in disease pathogenesis, we investigated the cell biological effects of increased TMEM106B expression. Here, we report that increased TMEM106B expression results in the appearance of a vacuolar phenotype in multiple cell types, including neurons. Concomitant with the development of this vacuolar phenotype, cells over-expressing TMEM106B exhibit impaired lysosomal acidification and degradative function, as well as increased cytotoxicity. We further identify a potential lysosomal sorting motif for TMEM106B and demonstrate that abrogation of sorting to lysosomes rescues TMEM106B-induced defects. Finally, we show that TMEM106B-induced defects are dependent on the presence of C9orf72, as knockdown of C9orf72 also rescues these defects. In sum, our results suggest that TMEM106B exerts its effects on FTLD-TDP disease risk through alterations in lysosomal pathways. Furthermore, TMEM106B and C9orf72 may interact in FTLD-TDP pathophysiology.

APOE, thought disorder, and SPARE-AD predict cognitive decline in established Parkinson's disease.

BACKGROUND: People with PD are at high risk of developing cognitive impairment and dementia. Cross-sectional studies have identified candidate biomarkers associated with cognitive decline. However, longitudinal studies on this topic are rarer, and few have investigated the use of biomarker panels encompassing multiple modalities. The objective of this study was to find baseline predictors of cognitive decline in longitudinally followed, nondemented Parkinson's disease patients. METHODS: We performed a prospective cohort study of 100 PD patients with a median disease duration of 6.4 years. All participants were nondemented at baseline. We examined 16 baseline biomarkers from clinical, genetic, biochemical, and MRI-based imaging modalities for their association with longitudinal cognitive decline for up to 8 years. We investigated biomarkers individually, as well as in a multivariate linear mixed-effects model encompassing multimodal biomarkers, with change in the Mattis Dementia Rating Scale-2 over time as the primary outcome. Annual consensus process-derived cognitive diagnosis was used for Cox proportional hazards modeling of risk for cognitive decline. RESULTS: In multivariate analysis, the presence of the APOE E4 allele, thought disorder, and an Alzheimer's disease pattern of brain atrophy (spatial pattern of abnormality for recognition of early Alzheimer's disease index) best predicted cognitive decline, with APOE E4 genotype exerting the greatest effect. The presence of the APOE E4 allele was associated with a 3.5 times higher risk of worsening cognitive diagnosis over time (HR, 3.53; 95% CI, 1.52-8.24; P < 0.05). The APOE genotype effect was not specific to any Mattis Dementia Rating Scale-2 domain. CONCLUSIONS: Our results confirm the importance of Alzheimer's disease biomarkers as risk factors for cognitive decline in established Parkinson's disease. © 2017 International Parkinson and Movement Disorder Society.

Cerebrospinal fluid α-synuclein contributes to the differential diagnosis of Alzheimer's disease.

INTRODUCTION: The ability of Alzheimer's disease (AD) cerebrospinal fluid (CSF) biomarkers (amyloid ß peptide 1-42, total tau, and phosphorylated tau) to discriminate AD from related disorders is limited. Biomarkers for other concomitant pathologies (e.g., CSF α-synuclein [α-syn] for Lewy body pathology) may be needed to further improve the differential diagnosis. METHODS: CSF total α-syn, phosphorylated α-syn at Ser129, and AD CSF biomarkers were evaluated with Luminex immunoassays in 367 participants, followed by validation in 74 different neuropathologically confirmed cases. RESULTS: CSF total α-syn, when combined with amyloid ß peptide 1-42 and either total tau or phosphorylated tau, improved the differential diagnosis of AD versus frontotemporal dementia, Lewy body disorders, or other neurological disorders. The diagnostic accuracy of the combined models attained clinical relevance (area under curve ∼0.9) and was largely validated in neuropathologically confirmed cases. DISCUSSION: Combining CSF biomarkers representing AD and Lewy body pathologies may have clinical value in the differential diagnosis of AD.

Vitamin D in the Parkinson Associated Risk Syndrome (PARS) study.

BACKGROUND: Lower vitamin D levels have been associated with manifest Parkinson's disease, prompting the hypothesis that vitamin D insufficiency or deficiency may increase risk for PD. OBJECTIVES: To evaluate vitamin D levels in a population at risk for developing PD. METHODS: Plasma vitamin D levels were measured in the Parkinson Associated Risk Syndrome Study, a cohort of asymptomatic individuals, some of whom are at high risk for PD. Vitamin D levels were compared between subjects at high risk for PD (hyposmia and dopamine transporter scan deficit) versus all others and examined for correlations with dopaminergic system integrity. RESULTS: Mean vitamin D levels did not differ between groups, with a level of 27.8 ng/mL (standard deviation = 12.0) in the high-risk group versus 24.7 ng/mL (standard deviation = 9.0) in all others (P = 0.09). Vitamin D levels did not associate with putaminal dopamine transporter uptake. CONCLUSIONS: Our data from the asymptomatic Parkinson Associated Risk Syndrome cohort do not support the hypothesis that chronic vitamin D insufficiency threatens dopaminergic system integrity, contributing to PD pathogenesis. © 2017 International Parkinson and Movement Disorder Society.