The Landscape of Monogenic Parkinson's Disease in Populations of Non-European Ancestry: A Narrative Review.

INTRODUCTION: There has been a bias in the existing literature on Parkinson's disease (PD) genetics as most studies involved patients of European ancestry, mostly in Europe and North America. Our target was to review published research data on the genetic profile of PD patients of non-European or mixed ancestry. METHODS: We reviewed articles published during the 2000-2023 period, focusing on the genetic status of PD patients of non-European origin (Indian, East and Central Asian, Latin American, sub-Saharan African and Pacific islands). RESULTS: There were substantial differences regarding monogenic PD forms between patients of European and non-European ancestry. The G2019S Leucine Rich Repeat Kinase 2 (LRRK2) mutation was rather scarce in non-European populations. In contrast, East Asian patients carried different mutations like p.I2020T, which is common in Japan. Parkin (PRKN) variants had a global distribution, being common in early-onset PD in Indians, in East Asians, and in early-onset Mexicans. Furthermore, they were occasionally present in Black African PD patients. PTEN-induced kinase 1 (PINK1) and PD protein 7 (DJ-1) variants were described in Indian, East Asian and Pacific Islands populations. Glucocerebrosidase gene variants (GBA1), which represent an important predisposing factor for PD, were found in East and Southeast Asian and Indian populations. Different GBA1 variants have been reported in Black African populations and Latin Americans. CONCLUSIONS: Existing data reveal a pronounced heterogeneity in the genetic background of PD. A number of common variants in populations of European ancestry appeared to be absent or scarce in patients of diverse ethnic backgrounds. Large-scale studies that include genetic screening in African, Asian or Latin American populations are underway. The outcomes of such efforts will facilitate further clinical studies and will possibly contribute to the identification of either new pathogenic mutations in already described genes or novel PD-related genes.

Comprehensive Evaluation of Psychotic Features and Their Clinical Correlates in Early Parkinson's Disease.

BACKGROUND: Some reports suggest that psychotic features may occur in the early stages of Parkinson's disease (PD), but sensitive tools have not been utilized. OBJECTIVE: The aim was to evaluate the presence of psychotic symptoms using detailed scales and to assess the association with clinical characteristics. METHODS: Healthy controls and patients within three years of PD onset were recruited. Participants were examined for psychotic symptoms using two different instruments: the Comprehensive Assessment of At-Risk Mental States (CAARMS) and a 10 question PD specific psychosis severity scale (10PDQ). In the PD group, medication use, motor and non-motor symptoms were documented. RESULTS: Based on CAARMS and 10PDQ scales, psychotic features were present in 39% (27/70) of patients and 4% (3/74) of controls. The prevalence of passage hallucinations and illusions was significantly higher in PD compared to the control group. The presence of PD-associated psychotic features was not significantly affected by medication, motor severity or global cognitive status. Higher prevalence of overall non-motor manifestations, REM sleep behavior disorder (RBD) and depressive symptoms was significantly associated with the manifestation of psychotic features in PD [(adjusted OR:1.3; 95% CI:1.1-1.6; p = 0.003), (adjusted OR:1.3; 95% CI:1.0-1.6; p = 0.023), and (adjusted OR:1.2; 95% CI:1.0-1.4;p = 0.026)]. CONCLUSIONS: Psychotic phenomena mainly of minor nature are highly common in early PD. Cumulative non-motor symptoms, RBD and depressive features are associated with the presence of psychotic symptoms in this non-demented, early-stage PD population. More studies are needed to clarify the mechanisms that contribute to the onset of psychotic features in early PD.

Jumping to conclusions bias, psychosis and impulsivity in early stages of Parkinson's disease.

OBJECTIVES: The aim was to explore the correlations between Jumping to Conclusions (JtC) tendency and neuropsychiatric features in patients with early Parkinson's disease (PD). BACKGROUND: According to few reports, PD patients with impulsive-compulsive behaviors (ICBs) are prone to working memory difficulties including JtC bias. The correlation of psychotic features and JtC tendency remains still unclear. METHODS: Healthy controls and patients within 3 years of PD onset were recruited. Participants were examined for psychotic symptoms using a 10 question PD-specific psychosis severity scale. JtC was measured by a probalistic reasoning scenario (beads task). In PD group, medication use, motor and non-motor symptoms were documented. Impulsivity was evaluated using the Questionnaire for Impulsive-Compulsive Disorders in PD (QUIP). RESULTS: The prevalence of JtC bias was 9% (6/70) in healthy individuals, compared to 32% (22/68) of PD group [p = 0.001]. No association was detected between the presence of JtC tendency and PD-associated psychosis (p = 0.216). Patients with JtC had shorter duration of PD, more tremor-dominant PD subtype and higher QUIP scores, regardless of the dopaminergic therapy (p = 0.043, p = 0.015, p = 0.007, respectively). A trend towards attention and inhibition control deficit was noticed in JtC patients. CONCLUSIONS: We found a high prevalence of JtC bias in early, cognitively intact PD population and a potential link between subthreshold ICBs and poor performance on beads task. Additional studies are needed to confirm our results and elaborate on the mechanisms that correlate impulsivity with JtC tendency, which are likely to be different from those mediating psychotic features in early PD.

Embracing Monogenic Parkinson's Disease: The MJFF Global Genetic PD Cohort.

BACKGROUND: As gene-targeted therapies are increasingly being developed for Parkinson's disease (PD), identifying and characterizing carriers of specific genetic pathogenic variants is imperative. Only a small fraction of the estimated number of subjects with monogenic PD worldwide are currently represented in the literature and availability of clinical data and clinical trial-ready cohorts is limited. OBJECTIVE: The objectives are to (1) establish an international cohort of affected and unaffected individuals with PD-linked variants; (2) provide harmonized and quality-controlled clinical characterization data for each included individual; and (3) further promote collaboration of researchers in the field of monogenic PD. METHODS: We conducted a worldwide, systematic online survey to collect individual-level data on individuals with PD-linked variants in SNCA, LRRK2, VPS35, PRKN, PINK1, DJ-1, as well as selected pathogenic and risk variants in GBA and corresponding demographic, clinical, and genetic data. All registered cases underwent thorough quality checks, and pathogenicity scoring of the variants and genotype-phenotype relationships were analyzed. RESULTS: We collected 3888 variant carriers for our analyses, reported by 92 centers (42 countries) worldwide. Of the included individuals, 3185 had a diagnosis of PD (ie, 1306 LRRK2, 115 SNCA, 23 VPS35, 429 PRKN, 75 PINK1, 13 DJ-1, and 1224 GBA) and 703 were unaffected (ie, 328 LRRK2, 32 SNCA, 3 VPS35, 1 PRKN, 1 PINK1, and 338 GBA). In total, we identified 269 different pathogenic variants; 1322 individuals in our cohort (34%) were indicated as not previously published. CONCLUSIONS: Within the MJFF Global Genetic PD Study Group, we (1) established the largest international cohort of affected and unaffected individuals carrying PD-linked variants; (2) provide harmonized and quality-controlled clinical and genetic data for each included individual; (3) promote collaboration in the field of genetic PD with a view toward clinical and genetic stratification of patients for gene-targeted clinical trials. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Clinical differences between early-onset and mid-and-late-onset Parkinson's disease: Data analysis of the Hellenic Biobank of Parkinson's disease.

BACKGROUND: Age at onset is one of the most critical factors contributing to the clinical heterogeneity of Parkinson's disease (PD), and available evidence is rather conflicting. OBJECTIVE: The aim of this study is to investigate the clinical differences between early-onset PD (EOPD) and mid-and-late-onset PD (MLOPD) in the Greek population, based on the existing data of the Hellenic Biobank of PD (HBPD). METHODS: HBPD contains information of PD cases from two centers in Greece during 2006-2017. Patients with the A53T mutation in the SNCA gene or mutations in the GBA1 gene were excluded. Associations between clinical characteristics (motor and non-motor symptoms, side of onset, first symptom, motor complications) and MLOPD versus EOPD were explored with a single logistic regression model adjusting for gender, family history of PD, disease and dopaminergic therapy duration, disease severity (UPDRS III), levodopa equivalent daily dose, as well as each of the other clinical characteristics. RESULTS: 675 patients (129 EOPD, 546 MLOPD) were included. EOPD was more frequently associated with dystonia (OR 0.19, 95% CI 0.08-0.50, p < 0.01) and motor complications (0.23, 0.07-0.76, 0.02), compared to MLOPD. Bilateral onset (9.38, 1.05-84.04, 0.045) and autonomic dysfunction (2.31, 1.04-5.11, 0.04) were more frequently associated with MLOPD. CONCLUSIONS: EOPD and MLOPD display distinct clinical profiles, regarding motor and non-motor symptoms, side of onset and motor complications in the Greek population. These differences may reflect diverse pathophysiological backgrounds, potentially attributed to genetic or age-related epigenetic influences.

Apathy: An underestimated feature in GBA and LRRK2 non-manifesting mutation carriers.

INTRODUCTION: Higher prevalence of motor and non-motor features has been observed in non-manifesting mutation carriers of Parkinson's Disease (PD) compared to Healthy Controls (HC). The aim was to detect the differences between GBA and LRRK2 mutation carriers without PD and HC on neuropsychiatric symptoms. METHODS: This is a cross-sectional retrospective study of non-manifesting GBA and LRRK2 mutation carriers and HC enrolled into Parkinson's Progression Markers Initiative (PPMI). Data extracted from the PPMI database contained: demographics and performance in MoCA scale and MDS-UPDRS scale part 1A (neuropsychiatric symptoms). All six features were treated as both continuous (MDS-UPDRS individual scores) and categorical variables (MDS-UPDRS individual score>0 and MDS-UPDRS individual score = 0). Logistic regression analyses were applied to evaluate the association between mutation carrying status and neuropsychiatric symptoms. RESULTS: In this study, the neuropsychiatric evaluation was performed in 285 GBA non-manifesting carriers, 369 LRRK2 non-manifesting carriers and 195 HC. We found that GBA non-manifesting mutation carriers were 2.6 times more likely to present apathy compared to HC, even after adjustment for covariates (adjusted OR = 2.6, 95% CI = 1.1-6.3, p = 0.031). The higher percentage of apathy for LRRK2 carriers compared to HC was marginally non-significant. GBA carriers were 1.5 times more likely to develop features of anxiety compared to LRRK2 carriers (adjusted OR = 1.5, 95% CI = 1.1-2.2, p = 0.015). Other neuropsychiatric symptoms, such as psychotic or depressive manifestations, did not differ between groups. CONCLUSION: Symptoms of apathy could be present in the prediagnostic period of non-manifesting mutation carriers, especially, GBA. Longitudinal data, including detailed neuropsychiatric evaluation and neuroimaging, would be essential to further investigate the pathophysiological basis of this finding.

Serum Uric Acid in LRRK2 Related Parkinson's Disease: Longitudinal Data from the PPMI Study.

BACKGROUND: Previous studies have highlighted serum uric acid as a putative idiopathic Parkinson's disease (iPD) biomarker. Only one study, so far, showed higher levels of serum uric acid in leucine-rich repeat kinase 2 (LRRK + 2) carriers compared to those who developed PD, however a longitudinal comparison between LRRK2 + PD and healthy controls (HC) has not been performed. OBJECTIVE: The aim of this study was to determine whether there are longitudinal differences in serum uric acid between iPD, LRRK2 + PD and HC and their association with motor and non-motor features. METHODS: Longitudinal data of uric acid of 282 de novo iPD, 144 LRRK2 + PD patients, and 195 age-matched HC were obtained from the Parkinson's Progression Markers Initiative (PPMI) database. We also used longitudinal Montreal Cognitive Assessment (MoCA), Movement Disorder Society-Unified Parkinson's Disease Rating Scale part III (MDS-UPDRS-III), Geriatric Depression Scale (GDS) scores, and DaTSCAN striatal binding ratios (SBRs). RESULTS: Longitudinal uric acid measurements were significantly lower in LRRK2 + PD patients compared to HC up to 5 years follow-up. There was no significant impact or correlation of adjusted or unadjusted uric acid levels with MoCA, MDS-UPDRS III, or GDS scores, the presence of RBD or DAT-SCAN SBRs. CONCLUSION: LRRK2 + PD group had significantly lower uric acid concentrations compared to HC after adjusting for age, sex and baseline BMI up to 5 years follow-up. There were no significant associations between uric acid levels and indices of disease severity. These findings identify serum uric acid as a marker linked to LRRK2 + PD.

A Novel SNCA A30G Mutation Causes Familial Parkinson's Disease.

BACKGROUND: The SNCA gene encoding α-synuclein (αSyn) is the first gene identified to cause autosomal-dominant Parkinson's disease (PD). OBJECTIVE: We report the identification of a novel heterozygous A30G mutation of the SNCA gene in familial PD and describe clinical features of affected patients, genetic findings, and functional consequences. METHODS: Whole exome sequencing was performed in the discovery family proband. Restriction digestion with Bbvl was used to screen SNCA A30G in two validation cohorts. The Greek cohort included 177 familial PD probands, 109 sporadic PD cases, and 377 neurologically healthy controls. The German cohort included 136 familial PD probands, 380 sporadic PD cases, and 116 neurologically healthy controls. We also conducted haplotype analysis using 13 common single nucleotide variants around A30G to determine the possibility of a founder effect for A30G. We then used biophysical methods to characterize A30G αSyn. RESULTS: We identified a novel SNCA A30G (GRCh37, Chr4:90756730, c.89 C>G) mutation that co-segregated with the disease in five affected individuals of three Greek families and was absent from controls. A founder effect was strongly suggested by haplotype analysis. The A30G mutation had a local effect on the intrinsically disordered structure of αSyn, slightly perturbed membrane binding, and promoted fibril formation. CONCLUSION: Based on the identification of A30G co-segregating with the disease in three families, the absence of the mutation in controls and population databases, and the observed functional effects, we propose SNCA A30G as a novel causative mutation for familial PD. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Serum uric acid level as a putative biomarker in Parkinson's disease patients carrying GBA1 mutations: 2-Year data from the PPMI study.

INTRODUCTION: Blood uric acid represents an important biomarker in sporadic Parkinson's disease (PD). Whether uric acid levels change in genetic forms of PD is beginning to be assessed. The aim of the present study was to evaluate differences in serum uric acid level among PD patients harboring mutations in the glucocerebrosidase (GBA1) gene, sporadic PD, and healthy controls followed longitudinally. METHODS: Longitudinal 2-year serum uric acid measurement data of 120 GBA-PD patients have been downloaded from the Parkinson's Progression Markers Initiative (PPMI) database. This cohort was compared with 369 de novo sporadic PD patients and 195 healthy controls enrolled in the same study. RESULTS: Following adjustment for age, sex and BMI the GBA-PD cohort exhibited lower 2-year longitudinal uric acid level as compared to the controls (p = 0.016). Baseline uric acid measurements showed only a marginal difference (p = 0.119), but year 2 uric acid levels were lower in the GBA-PD cohort (p < 0.001). There was no difference in baseline, year 2 and 2-year longitudinal serum uric acid in the GBA-PD cohort as compared to sporadic PD (p = 0.664, p = 0.117 and p = 0.315). CONCLUSIONS: This is the first study to assess serum uric acid in a GBA-PD cohort. Our findings suggest that low serum uric acid might be a progression biomarker in GBA-PD. However, more studies (ideally longitudinal) on the association between low serum uric acid and clinical data in GBA-PD are needed. These results are consistent with data from previous reports assessing uric acid as a biomarker in other genetic forms of PD.

Peripheral alpha-synuclein levels in patients with genetic and non-genetic forms of Parkinson's disease.

BACKGROUND: Variations of α-synuclein levels have been reported in serum and plasma in Parkinson's Disease (PD) Patients. METHODS: Serum and plasma were obtained from PD patients without known mutations (GU-PD, n = 124)), carriers of the A53T/G209A point mutation in the α-synuclein gene (SNCA) (n = 29), and respective age-/sex-matched controls. Levels of total α-synuclein were assessed using an in-house ELISA assay. RESULTS: A statistically significant increase of α-synuclein levels was found in serum, but not plasma, from GU-PD patients compared to healthy controls. A statistically significant decrease of α-synuclein levels was found in serum and plasma from symptomatic A53T mutation carriers compared to healthy controls. Plasma α-synuclein levels were modestly negatively correlated with UPDRS part III score and disease duration in A53T-PD patients. CONCLUSION: Increased α-synuclein levels in serum of GU-PD patients suggest a systemic deregulation of α-synuclein homeostasis in PD. The opposite results in A53T-PD highlight the complexity of α-synuclein homeostatic regulation in PD, and suggest the possibility of reduced expression of the mutant allele.

Serum Uric Acid Level as a Biomarker in Idiopathic and Genetic (p.A53T Alpha-Synuclein Carriers) Parkinson's Disease: Data from the PPMI Study.

BACKGROUND: Blood uric acid level represents an emerging biomarker in Parkinson's disease (PD). Whether uric acid levels change in genetic forms of PD is just beginning to be explored. OBJECTIVE: The aim of the present study was to assess differences in serum uric acid level among PD patients harboring the p.A53T mutation in the alpha-synuclein gene, idiopathic PD, and healthy controls. METHODS: Longitudinal 5-year serum uric acid measurement data of 369 de novo idiopathic PD patients and 174 age- and gender-matched healthy controls have been downloaded from the Parkinson's Progression Markers Initiative (PPMI) database. Furthermore, we assessed baseline serum uric acid measurements of 24 p.A53T alpha-synuclein PD patients enrolled in PPMI and followed in our site as compared to 24 age-, gender- and disease duration-matched sporadic PD patients and 24 healthy controls. RESULTS: Longitudinal serum uric acid measurements did not differ statistically between idiopathic PD patients and healthy controls (despite a trend for lower uric acid in the PD group) (p = 0.879). This was also true when male and female subgroups were assessed separately. The p.A53T SNCA mutation carrier PD group exhibited lower baseline serum uric acid level as compared to their matched healthy controls (p = 0.025). CONCLUSION: In the present study we did not replicate the established lower serum uric acid measurements in PD patients as compared to controls using PPMI data, possibly due to the fact that PD patients in baseline visit were de novo and the average disease duration was shorter than that observed in most epidemiological PD studies. The faster progression rate and increased disease severity in p.A53T PD possibly correlate with the lower serum uric acid observed in this subgroup.

Circulating Brain-enriched MicroRNAs for detection and discrimination of idiopathic and genetic Parkinson's disease.

BACKGROUND: A minimally invasive test for early detection and monitoring of Parkinson's disease (PD) is a highly unmet need for drug development and planning of patient care. Blood plasma represents an attractive source of biomarkers. MicroRNAs (miRNAs) are conserved noncoding RNA molecules that serve as posttranscriptional regulators of gene expression. As opposed to ubiquitously expressed miRNAs that control house-keeping processes, brain-enriched miRNAs regulate diverse aspects of neuron development and function. These include neuron-subtype specification, axonal growth, dendritic morphogenesis, and spine density. Backed by a large number of studies, we now know that the differential expression of neuron-enriched miRNAs leads to brain dysfunction. OBJECTIVES: The aim was to identify subsets of brain-enriched miRNAs with diagnostic potential for familial and idiopathic PD as well as specify the molecular pathways deregulated in PD. METHODS: Initially, brain-enriched miRNAs were selected based on literature review and validation studies in human tissues. Subsequently, real-time reverse transcription polymerase chain reaction was performed in the plasma of 100 healthy controls and 99 idiopathic and 53 genetic (26 alpha-synucleinA53T and 27 glucocerebrosidase) patients. Statistical and bioinformatics analyses were carried out to pinpoint the diagnostic biomarkers and deregulated pathways, respectively. RESULTS: An explicit molecular fingerprint for each of the 3 PD cohorts was generated. Although the idiopathic PD fingerprint was different from that of genetic PD, the molecular pathways deregulated converged between all PD subtypes. CONCLUSIONS: The study provides a group of brain-enriched miRNAs that may be used for the detection and differentiation of PD subtypes. It has also identified the molecular pathways deregulated in PD. © 2019 International Parkinson and Movement Disorder Society.

The relationship between environmental factors and different Parkinson's disease subtypes in Greece: Data analysis of the Hellenic Biobank of Parkinson's disease.

INTRODUCTION: The aim of this study is to investigate the association between environmental factors (smoking, coffee, pesticide exposure) and Parkinson's disease (PD) subtypes (early-onset, mid-and-late onset, familial and sporadic) in the Greek population. METHODS: The Hellenic Biobank of PD recorded information of PD cases and controls from two centers in Greece during 2006-2017. Patients with the A53T mutation in SNCA or GBA mutations were excluded. Associations of environmental factors with PD overall (and PD subtypes) versus controls were explored with logistic regression models adjusting for age, gender and each environmental factor. RESULTS: 686 patients and 356 controls were included. Smoking was associated with a reduced risk of PD overall (OR 0.48, 95% CI 0.35-0.67), mid-and-late onset (0.46, 0.32-0.66), familial (0.53, 0.34-0.83) and sporadic (0.46, 0.32-0.65), but not early-onset PD. There was an inverse linear association with pack-years of smoking, except for early-onset PD. Early-onset PD was the only PD subtype inversely associated with coffee consumption when dichotomously treated. Compared to never-coffee drinkers, only those at the upper tertile had lower odds for PD overall (0.52, 0.29-0.91), early-onset (0.16, 0.05-0.53) and familial PD (0.36, 0.17-0.75). No associations were found between pesticides and PD. CONCLUSIONS: Our study shows that the well-known negative association of smoking with PD occurs across all PD subtypes in the Greek population, apart from early-onset PD. Early-onset PD was also most strongly inversely associated with coffee consumption, highlighting a potential distinct underlying physiopathology in this PD subset that may involve specific gene-environment interactions.

Phenotypic Characteristics in GBA-Associated Parkinson's Disease: A Study in a Greek Population.

We compared phenotypic characteristics in 35 Greek patients with Parkinson's disease (PD), carriers of GBA1 mutations (GBA-PD), with 35 Genetically Unidentified PD patients (GU-PD). We found a previously reported higher prevalence of cognitive impairment and a little appreciated more frequent bilateral onset of the disease in GBA-PD vs GU-PD. As far as the exposure to environmental factors, linked to PD, is concerned, our study hints to the possibility that pesticide exposure may be more common in GBA-PD patients, and possibly act synergistically with the mutation carrier status to trigger the disease.

Evaluation of the interaction between LRRK2 and PARK16 loci in determining risk of Parkinson's disease: analysis of a large multicenter study.

A recent study MacLeod et al. has shown that an interaction between variants at the LRRK2 and PARK16 loci influences risk of development of Parkinson's disease (PD). Our study examines the proposed interaction between LRRK2 and PARK16 variants in modifying PD risk using a large multicenter series of PD patients (7715) and controls (8261) from sites participating in the Genetic Epidemiology of Parkinson's Disease Consortium. Our data does not support a strong direct interaction between LRRK2 and PARK16 variants; however, given the role of retromer and lysosomal pathways in PD, further studies are warranted.

Large-scale assessment of polyglutamine repeat expansions in Parkinson disease.

OBJECTIVES: We aim to clarify the pathogenic role of intermediate size repeat expansions of SCA2, SCA3, SCA6, and SCA17 as risk factors for idiopathic Parkinson disease (PD). METHODS: We invited researchers from the Genetic Epidemiology of Parkinson's Disease Consortium to participate in the study. There were 12,346 cases and 8,164 controls genotyped, for a total of 4 repeats within the SCA2, SCA3, SCA6, and SCA17 genes. Fixed- and random-effects models were used to estimate the summary risk estimates for the genes. We investigated between-study heterogeneity and heterogeneity between different ethnic populations. RESULTS: We did not observe any definite pathogenic repeat expansions for SCA2, SCA3, SCA6, and SCA17 genes in patients with idiopathic PD from Caucasian and Asian populations. Furthermore, overall analysis did not reveal any significant association between intermediate repeats and PD. The effect estimates (odds ratio) ranged from 0.93 to 1.01 in the overall cohort for the SCA2, SCA3, SCA6, and SCA17 loci. CONCLUSIONS: Our study did not support a major role for definite pathogenic repeat expansions in SCA2, SCA3, SCA6, and SCA17 genes for idiopathic PD. Thus, results of this large study do not support diagnostic screening of SCA2, SCA3, SCA6, and SCA17 gene repeats in the common idiopathic form of PD. Likewise, this largest multicentered study performed to date excludes the role of intermediate repeats of these genes as a risk factor for PD.

Protective effect of LRRK2 p.R1398H on risk of Parkinson's disease is independent of MAPT and SNCA variants.

The best validated susceptibility variants for Parkinson's disease are located in the α-synuclein (SNCA) and microtubule-associated protein tau (MAPT) genes. Recently, a protective p.N551K-R1398H-K1423K haplotype in the leucine-rich repeat kinase 2 (LRRK2) gene was identified, with p.R1398H appearing to be the most likely functional variant. To date, the consistency of the protective effect of LRRK2 p.R1398H across MAPT and SNCA variant genotypes has not been assessed. To address this, we examined 4 SNCA variants (rs181489, rs356219, rs11931074, and rs2583988), the MAPT H1-haplotype-defining variant rs1052553, and LRRK2 p.R1398H (rs7133914) in Caucasian (n = 10,322) and Asian (n = 2289) series. There was no evidence of an interaction of LRRK2 p.R1398H with MAPT or SNCA variants (all p ≥ 0.10); the protective effect of p.R1398H was observed at similar magnitude across MAPT and SNCA genotypes, and the risk effects of MAPT and SNCA variants were observed consistently for LRRK2 p.R1398H genotypes. Our results indicate that the association of LRRK2 p.R1398H with Parkinson's disease is independent of SNCA and MAPT variants, and vice versa, in Caucasian and Asian populations.

Assessment of Parkinson's disease risk loci in Greece.

Genome-wide association studies (GWAS) have been shown to be a powerful approach to identify risk loci for neurodegenerative diseases. Recent GWAS in Parkinson's disease (PD) have been successful in identifying numerous risk variants pointing to novel pathways potentially implicated in the pathogenesis of PD. Contributing to these GWAS efforts, we performed genotyping of previously identified risk alleles in PD patients and control subjects from Greece. We showed that previously published risk profiles for Northern European and American populations are also applicable to the Greek population. In addition, although our study was largely underpowered to detect individual associations, we replicated 5 of 32 previously published risk variants with nominal p values <0.05. Genome-wide complex trait analysis revealed that known risk loci explain disease risk in 1.27% of Greek PD patients. Collectively, these results indicate that there is likely a substantial genetic component to PD in Greece, similarly to other worldwide populations, that remains to be discovered.

Population-specific frequencies for LRRK2 susceptibility variants in the Genetic Epidemiology of Parkinson's Disease (GEO-PD) Consortium.

BACKGROUND: Variants within the leucine-rich repeat kinase 2 gene are recognized as the most frequent genetic cause of Parkinson's disease. Leucine-rich repeat kinase 2 variation related to disease susceptibility displays many features that reflect the nature of complex, late-onset sporadic disorders like Parkinson's disease. METHODS: The Genetic Epidemiology of Parkinson's Disease Consortium recently performed the largest genetic association study for variants in the leucine-rich repeat kinase 2 gene across 23 different sites in 15 countries. RESULTS: Herein, we detail the allele frequencies for the novel risk factors (p.A419V and p.M1646T) and the protective haplotype (p.N551K-R1398H-K1423K) nominated in the original publication. Simple population allele frequencies not only can provide insight into the clinical relevance of specific variants but also can help genetically define patient groups. CONCLUSIONS: Establishing individual patient-based genomic susceptibility profiles that incorporate both risk factors and protective factors will determine future diagnostic and treatment strategies.