Age at onset in LRRK2-associated PD is modified by SNCA variants

LRRK2: Genetic mechanisms vs genetic subtypes

In 2004, the identification of pathogenic variants in the LRRK2 gene across several families with autosomal dominant late-onset Parkinson's disease (PD) revolutionized our understanding of the role of genetics in PD. Previous beliefs that genetics in PD was limited to rare early-onset or familial forms of the disease were quickly dispelled. Currently, we recognize LRRK2 p.G2019S as the most common genetic cause of both sporadic and familial PD, with more than 100,000 affected carriers across the globe. The frequency of LRRK2 p.G2019S is also highly variable across populations, with some regions of Asian or Latin America reporting close to 0%, contrasting to Ashkenazi Jews or North African Berbers reporting up to 13% and 40%, respectively. Patients with LRRK2 pathogenic variants are clinically and pathologically heterogeneous, highlighting the age-related variable penetrance that also characterizes LRRK2-related disease. Indeed, the majority of patients with LRRK2-related disease are characterized by a relatively mild Parkinsonism with less motor symptoms with variable presence of α-synuclein and/or tau aggregates, with pathologic pleomorphism widely described. At a functional cellular level, it is likely that pathogenic variants mediate a toxic gain-of-function of the LRRK2 protein resulting in increased kinase activity perhaps in a cell-specific manner; by contrast, some LRRK2 variants appear to be protective reducing PD risk by decreasing the kinase activity. Therefore, employing this information to define appropriate patient populations for clinical trials of targeted kinase LRRK2 inhibition strategies is very promising and demonstrates a potential future application for PD using precision medicine.

Incidence of Parkinson disease in North America

Parkinson disease (PD) is the second most common age-related neurodegenerative condition diagnosed in North America. We recently demonstrated, using multiple epidemiological data sources, that the prevalence of PD diagnoses was greater than previously reported and currently used for clinical, research, and policy decision-making. Prior PD incidence estimates have varied, for unclear reasons. There is a need for improved estimates of PD incidence, not only for care delivery planning and future policy but also for increasing our understanding of disease risk. The objective of this study was thus to investigate the incidence of Parkinson disease across five epidemiological cohorts in North America in a common year, 2012. The cohorts contained data on 6.7 million person-years of adults ages 45 and older, and 9.3 million person-years of adults ages 65 and older. Our estimates of age-sex-adjusted incidence of PD ranged from 108 to 212 per 100,000 among persons ages 65 and older, and from 47 to 77 per 100,00 among persons ages 45 and older. PD incidence increased with age and was higher among males. We also found persistent spatial clustering of incident PD diagnoses in the U.S. PD incidence estimates varied across our data sources, in part due to case ascertainment and diagnosis methods, but also possibly due to the influence of population factors (prevalence of genetic risk factors or protective markers) and geographic location (exposure to environmental toxins). Understanding the source of these variations will be important for health care policy, research, and care planning.

Genomewide Association Studies of LRRK2 Modifiers of Parkinson's Disease

Objective

The aim of this study was to search for genes/variants that modify the effect of LRRK2 mutations in terms of penetrance and age‐at‐onset of Parkinson's disease.

Methods

We performed the first genomewide association study of penetrance and age‐at‐onset of Parkinson's disease in LRRK2 mutation carriers (776 cases and 1,103 non‐cases at their last evaluation). Cox proportional hazard models and linear mixed models were used to identify modifiers of penetrance and age‐at‐onset of LRRK2 mutations, respectively. We also investigated whether a polygenic risk score derived from a published genomewide association study of Parkinson's disease was able to explain variability in penetrance and age‐at‐onset in LRRK2 mutation carriers.

Results

A variant located in the intronic region of CORO1C on chromosome 12 (rs77395454; p value = 2.5E‐08, beta = 1.27, SE = 0.23, risk allele: C) met genomewide significance for the penetrance model. Co‐immunoprecipitation analyses of LRRK2 and CORO1C supported an interaction between these 2 proteins. A region on chromosome 3, within a previously reported linkage peak for Parkinson's disease susceptibility, showed suggestive associations in both models (penetrance top variant: p value = 1.1E‐07; age‐at‐onset top variant: p value = 9.3E‐07). A polygenic risk score derived from publicly available Parkinson's disease summary statistics was a significant predictor of penetrance, but not of age‐at‐onset.

Interpretation

This study suggests that variants within or near CORO1C may modify the penetrance of LRRK2 mutations. In addition, common Parkinson's disease associated variants collectively increase the penetrance of LRRK2 mutations. ANN NEUROL 2021;90:82–94

Pathogenic LRRK2 requires secondary factors to induce cellular toxicity

Pathogenic mutations in the leucine-rich repeat kinase 2 (LRRK2) gene belong to the most common genetic causes of inherited Parkinson’s disease (PD) and variations in its locus increase the risk to develop sporadic PD. Extensive research efforts aimed at understanding how changes in the LRRK2 function result in molecular alterations that ultimately lead to PD. Cellular LRRK2-based models revealed several potential pathophysiological mechanisms including apoptotic cell death, LRRK2 protein accumulation and deficits in neurite outgrowth. However, highly variable outcomes between different cellular models have been reported. Here, we have investigated the effect of different experimental conditions, such as the use of different tags and gene transfer methods, in various cellular LRRK2 models. Readouts included cell death, sensitivity to oxidative stress, LRRK2 relocalization, α-synuclein aggregation and neurite outgrowth in cell culture, as well as neurite maintenance in vivo. We show that overexpression levels and/or the tag fused to LRRK2 affect the relocalization of LRRK2 to filamentous and skein-like structures. We found that overexpression of LRRK2 per se is not sufficient to induce cellular toxicity or to affect α-synuclein-induced toxicity and aggregate formation. Finally, neurite outgrowth/retraction experiments in cell lines and in vivo revealed that secondary, yet unknown, factors are required for the pathogenic LRRK2 effects on neurite length. Our findings stress the importance of technical and biological factors in LRRK2-induced cellular phenotypes and hence imply that conclusions based on these types of LRRK2-based assays should be interpreted with caution.

Genetics of synucleins in neurodegenerative diseases

The SNCA locus currently has an indisputable role in Parkinson's disease and other synucleinopathies. The role of genetic variability in the other members of the synuclein family (SNCB and SNCG) in disease is far less clear. In this review, we critically assess the pathogenicity, main characteristics, and roles of genetic variants in these genes reported to be causative of synucleinopathies. We also summarize the different association signals identified in the SNCA locus that have been associated with risk for disease. We take a bird's eye view of the variability currently reported in the general population for the three genes and use these data to infer on the potential relationship between each of the genes and human disease.

Genetic and Environmental Factors Influence the Pleomorphy of LRRK2 Parkinsonism

Missense mutations in the LRRK2 gene were first identified as a pathogenic cause of Parkinson's disease (PD) in 2004. Soon thereafter, a founder mutation in LRRK2, p.G2019S (rs34637584), was described, and it is now estimated that there are approximately 100,000 people worldwide carrying this risk variant. While the clinical presentation of LRRK2 parkinsonism has been largely indistinguishable from sporadic PD, disease penetrance and age at onset can be quite variable. In addition, its neuropathological features span a wide range from nigrostriatal loss with Lewy body pathology, lack thereof, or atypical neuropathology, including a large proportion of cases with concomitant Alzheimer's pathology, hailing LRRK2 parkinsonism as the "Rosetta stone" of parkinsonian disorders, which provides clues to an understanding of the different neuropathological trajectories. These differences may result from interactions between the LRRK2 mutant protein and other proteins or environmental factors that modify LRRK2 function and, thereby, influence pathobiology. This review explores how potential genetic and biochemical modifiers of LRRK2 function may contribute to the onset and clinical presentation of LRRK2 parkinsonism. We review which genetic modifiers of LRRK2 influence clinical symptoms, age at onset, and penetrance, what LRRK2 mutations are associated with pleomorphic LRRK2 neuropathology, and which environmental modifiers can augment LRRK2 mutant pathophysiology. Understanding how LRRK2 function is influenced and modulated by other interactors and environmental factors-either increasing toxicity or providing resilience-will inform targeted therapeutic development in the years to come. This will allow the development of disease-modifying therapies for PD- and LRRK2-related neurodegeneration.

LRRK2 and Protein Aggregation in Parkinson's Disease: Insights From Animal Models

Mutations in leucine-rich repeat kinase 2 (LRRK2) instigate an autosomal dominant form of Parkinson’s disease (PD). Despite the neuropathological heterogeneity observed in LRRK2-PD, accumulating evidence suggests that alpha-synuclein and tau pathology are observed in a vast majority of cases. Intriguingly, the presence of protein aggregates spans both LRRK2-PD and idiopathic disease, supportive of a common pathologic mechanism. Thus, it is important to consider how LRRK2 mutations give rise to such pathology, and whether targeting LRRK2 might modify the accumulation, transmission, or toxicity of protein aggregates. Likewise, it is not clear how LRRK2 mutations drive PD pathogenesis, and whether protein aggregates are implicated in LRRK2-dependent neurodegeneration. While animal models have been instrumental in furthering our understanding of a potential interaction between LRRK2 and protein aggregation, the biology is far from clear. We aim to provide a thoughtful overview of the evidence linking LRRK2 to protein aggregation in animal models.

From cell lines to pluripotent stem cells for modelling Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder characterized by loss of dopaminergic (DAergic) neurons in the substantia nigra (SN) that contributes to the main motor symptoms of the disease. At present, even if several advancements have been done in the last decades, the molecular and cellular mechanisms involved in the pathogenesis are far to be fully understood. Accordingly, the establishment of reliable in vitro experimental models to investigate the early events of the pathogenesis represents a key issue in the field. However, to mimic and reproduce in vitro the complex neuronal circuitry involved in PD-associated degeneration of DAergic neurons still remains a highly challenging issue. Here we will review the in vitro PD models used in the last 25 years of research, ranging from cell lines, primary rat or mice neuronal cultures to the more recent use of human induced pluripotent stem cells (hiPSCs) and, finally, the development of 3D midbrain organoids.

LRRK2, alpha-synuclein, and tau: partners in crime or unfortunate bystanders?

The identification of genetic forms of Parkinson's disease (PD) has tremendously expanded our understanding of the players and mechanisms involved. Mutations in the genes encoding for alpha-synuclein (aSyn), LRRK2, and tau have been associated with familial and sporadic forms of the disease. aSyn is the major component of Lewy bodies and Lewy neurites, which are pathognomonic protein inclusions in PD. Hyperphosphorylated tau protein accumulates in neurofibrillary tangles in the brains of Alzheimer's disease patients but is also seen in the brains of PD patients. LRRK2 is a complex multi-domain protein with kinase and GTPase enzymatic activity. Since aSyn and tau are phosphoproteins, we review the possible interplay between the three proteins. Understanding the interplay between LRRK2, aSyn and tau is extremely important, as this may enable the identification of novel targets and pathways for therapeutic intervention.

SNCA but not DNM3 and GAK modifies age at onset of LRRK2-related Parkinson's disease in Chinese population

BACKGROUND

Recently, rs2421947 in DNM3 (dynamin 3) was reported as a genetic modifier of age at onset (AAO) of LRRK2 G2019S-related Parkinson's disease (PD) in a genome-wide association study in Arab-Berber population. Rs356219 in SNCA (α-synuclein) was also reported to regulate the AAO of LRRK2-related PD in European populations, and GAK (Cyclin G-associated kinase) rs1524282 was reported to be associated with an increased PD risk with an interaction with SNCA rs356219. G2019S variant is rare in Asian populations, whereas two other Asian-specific LRRK2 variants, G2385R and R1628P, are more frequent with a twofold increased risk of PD.

METHODS

In this study, we investigated whether rs2421947, rs356219 and rs1524282 modified AAO in LRRK2-related PD patients in Han Chinese population. We screened LRRK2 G2385R and R1628P variants in 732 PD patients and 1992 healthy controls, and genotyped DNM3 rs2421947, SNCA rs356219 and GAK rs1524282 among the LRRK2 carriers.

RESULTS

The SNCA rs356219-G allele was found to increase the risk of PD in LRRK2 carriers (OR 1.50, 95%CI 1.08-2.01, P = 0.016), and the AAO of AG + GG genotypes was 4 years earlier than AA genotype (P = 0.006). Nonetheless, no similar association was found in DNM3 rs2421947 and GAK rs1524282.

CONCLUSIONS

Our results show that SNCA but not DNM3 or GAK is associated with AAO of LRRK2-PD patients in Chinese population.

The unlikely partnership between LRRK2 and α-synuclein in Parkinson's disease

Our understanding of the mechanisms underlying Parkinson's disease, the once archetypical nongenetic neurogenerative disorder, has dramatically increased with the identification of α-synuclein and LRRK2 pathogenic mutations. While α-synuclein protein composes the aggregates that can spread through much of the brain in disease, LRRK2 encodes a multidomain dual-enzyme distinct from any other protein linked to neurodegeneration. In this review, we discuss emergent datasets from multiple model systems that suggest these unlikely partners do interact in important ways in disease, both within cells that express both LRRK2 and α-synuclein as well as through more indirect pathways that might involve neuroinflammation. Although the link between LRRK2 and disease can be understood in part through LRRK2 kinase activity (phosphotransferase activity), α-synuclein toxicity is multilayered and plausibly interacts with LRRK2 kinase activity in several ways. We discuss common protein interactors like 14-3-3s that may regulate α-synuclein and LRRK2 in disease. Finally, we examine cellular pathways and outcomes common to both mutant α-synuclein expression and LRRK2 activity and points of intersection. Understanding the interplay between these two unlikely partners in disease may provide new therapeutic avenues for PD.

Clustering of motor and nonmotor traits in leucine-rich repeat kinase 2 G2019S Parkinson's disease nonparkinsonian relatives: A multicenter family study

OBJECTIVES

The objective of this study was to determine phenotypic features that differentiate nonparkinsonian first-degree relatives of PD leucine-rich repeat kinase 2 (LRRK2) G2019S multiplex families, regardless of carrier status, from healthy controls because nonparkinsonian individuals in multiplex families seem to share a propensity to present neurological features.

METHODS

We included nonparkinsonian first-degree relatives of LRRK2 G2019S familial PD cases and unrelated healthy controls participating in established multiplex family LRRK2 cohorts. Study participants underwent neurologic assessment including cognitive screening, olfaction testing, and questionnaires for daytime sleepiness, depression, and anxiety. We used a multiple logistic regression model with backward variable selection, validated with bootstrap resampling, to establish the best combination of motor and nonmotor features that differentiates nonparkinsonian first-degree relatives of LRRK2 G2019S familial PD cases from unrelated healthy controls.

RESULTS

We included 142 nonparkinsonian family members and 172 unrelated healthy controls. The combination of past or current symptoms of anxiety (adjusted odds ratio, 4.16; 95% confidence interval, 2.01-8.63), less daytime sleepiness (adjusted odds ratio [1 unit], 0.90; 95% confidence interval, 0.83-0.97], and worse motor UPDRS score (adjusted odds ratio [1 unit], 1.4; 95% confidence interval, 1.20-1.67) distinguished nonparkinsonian family members, regardless of LRRK2 G2019S mutation status, from unrelated healthy controls. The model accuracy was good (area under the curve = 79.3%).

CONCLUSIONS

A set of motor and nonmotor features distinguishes first-degree relatives of LRRK2 G2019S probands, regardless of mutation status, from unrelated healthy controls. Environmental or non-LRRK2 genetic factors in LRRK2-associated PD may influence penetrance of the LRRK2 G2019S mutation. The relationship of these features to actual PD risk requires longitudinal observation of LRRK2 familial PD cohorts. © 2018 International Parkinson and Movement Disorder Society.

Generation of iPSCs carrying a common LRRK2 risk allele for in vitro modeling of idiopathic Parkinson's disease

Induced pluripotent stem cells (iPSCs) have recapitulated several aspects of Parkinson’s disease (PD), but most iPSCs are derived from familial cases, which account for only about 15% of patients. Thus, while the emphasis has justifiably been on using iPSCs to model rare familial cases, models for the most common forms of PD are critically lacking. Here, we report the generation of an iPSC-based model of idiopathic PD (iPD) with or without RS1491923, which is a common risk variant in the LRRK2 locus. Consistent with GWA studies, we found large variability in our datasets. However, iPSC-derived neurons carrying the risk allele emerged for displaying subtle disturbances of cellular degradative systems, in line with familial PD models. We also observed that treatment with the LRRK2 inhibitor CZC-25146 slightly reduced a marker of aSYN pathology in all iPD lines. Future iPSC-based studies may need to be structured similarly to large GWA studies in order to obtain relevant statistical power. However, results from this pilot study suggest that iPSC-based modeling represents an attractive way to investigate idiopathic diseases.

α-synuclein (SNCA) but not dynamin 3 (DNM3) influences age at onset of leucine-rich repeat kinase 2 (LRRK2) Parkinson's disease in Spain

OBJECTIVES

A recent study showed that Arab-Berbers GG homozygous at rs2421947(C/G) in the dynamin 3 gene (DNM3) had 12.5 years earlier age at onset of leucine-rich repeat kinase 2 (LRRK2)-associated Parkinson's disease (PD) (L2PD). We explored whether this variant modulates the L2PD age at onset in Spain.

METHODS

We genotyped rs2421947 in 329 participants (210 L2PD patients, 119 L2PD nonmanifesting p.G2019S carriers), and marker rs356219 (A/G) in the α-synuclein gene (SNCA).

RESULTS

By Kaplan Meier and Cox regression analyses, we did not find an association of the DNM3 polymorphism with L2PD age at onset. However, we found an association of the SNCA marker with up to an 11 years difference in the L2PD median age at onset (58 years for GG carriers vs 69 years for AA).

CONCLUSION

Our results indicate that SNCA rs356219 but not dynamin 3 DNM3 rs2421947 modifies the penetrance of the mutation G2019S in the Spanish population by influencing the L2PD age at onset. These findings suggest that different genetic modifiers may influence the L2PD age at onset in different populations. © 2018 International Parkinson and Movement Disorder Society.

Genetic Variants in SNCA and the Risk of Sporadic Parkinson's Disease and Clinical Outcomes: A Review

There is increasing evidence of the contribution of genetic susceptibility to the etiology of Parkinson's disease (PD). Genetic variations in the SNCA gene are well established by linkage and genome-wide association studies. Positive associations of single nucleotide polymorphisms (SNPs) in SNCA and increased risk for PD were found. However, the role of SNCA variants in individual traits or phenotypes of PD is unknown. Here, we reviewed the current literature and identified 57 studies, performed in fourteen different countries, that investigated SNCA variants and susceptibility to PD. We discussed the findings based on environmental factors, history of PD, clinical outcomes, and ethnicity. In conclusion, SNPs within the SNCA gene can modify the susceptibility to PD, leading to increased or decreased risk. The risk associations of some SNPs varied among samples. Of notice, no studies in South American or African populations were found. There is little information about the effects of these variants on particular clinical aspects of PD, such as motor and nonmotor symptoms. Similarly, evidence of possible interactions between SNCA SNPs and environmental factors or disease progression is scarce. There is a need to expand the clinical applicability of these data as well as to investigate the role of SNCA SNPs in populations with different ethnic backgrounds.

Variants in SNCA Gene Are Associated with Parkinson's Disease Risk and Cognitive Symptoms in a Brazilian Sample

Genetic susceptibility contributes to the etiology of sporadic Parkinson's Disease (PD) and worldwide studies have found positive associations of polymorphisms in the alpha-synuclein gene (SNCA) with the risk for PD. However, little is known about the influence of variants of SNCA in individual traits or phenotypical aspects of PD. Further, there is a lack of studies with Latin-American samples. We evaluated the association between SNCA single nucleotide polymorphisms (single nucleotide polymorphisms, SNPs - rs2583988, rs356219, rs2736990, and rs11931074) and PD risk in a Brazilians sample. In addition, we investigated their potential interactions with environmental factors and specific clinical outcomes (motor and cognitive impairments, depression, and anxiety). A total of 105 PD patients and 101 controls participated in the study. Single locus analysis showed that the risk allele of all SNPs were more frequent in PD patients (p < 0.05), and the associations of SNPs rs2583988, rs356219, and rs2736990 with increased PD risk were confirmed. Further, the G-rs356219 and C-rs2736990 alleles were associated with early onset PD. T-rs2583988, G-rs356219 and C-2736990 alleles were significantly more frequent in PD patients with cognitive impairments than controls in this condition. In addition, in a logistic regression model, we found an association of cognitive impairment with PD, and the practice of cognitive activity and smoking habits had a protective effect. This study shows for the first time an association of SNCA polymorphism and PD in a South-American sample. In addition, we found an interaction between SNP rs356219 and a specific clinical outcome, i.e., the increased risk for cognitive impairment in PD patients.

Development of LRRK2 Inhibitors for the Treatment of Parkinson's Disease

Linkage and genome-wide association studies have identified a genetic risk locus for late-onset Parkinson's disease in chromosome 12, originally identified as PARK6. The causative gene was identified to code for a large multifunctional protein, LRRK2 (leucine-rich repeat kinase 2). The combined genetic and biochemical evidence supports a hypothesis in which the LRRK2 kinase function is causally involved in the pathogenesis of sporadic and familial forms of PD, and therefore that LRRK2 kinase inhibitors could be useful for treatment. Although LRRK2 has so far not been crystallised, the use of homology modelling and crystallographic surrogates has allowed the optimisation of chemical structures such that compounds of high selectivity with good brain penetration and appropriate pharmacokinetic properties are now available for understanding the biology of LRRK2 in vitro and in vivo. This chapter reviews LRRK2 biology, the structural biology of LRRK2 and gives an overview of inhibitors of LRRK2.

Leucine-Rich Repeat Kinase (LRRK2) Genetics and Parkinson's Disease

The discovery of LRRK2 mutations as a cause of Parkinson's disease (PD), including the sporadic late-onset form, established the decisive role of genetics in the field of PD research. Among LRRK2 mutations, the G2019S, mostly lying in a haplotype originating from a common Middle Eastern ancestor, has been identified in different populations worldwide. The G2385R and R1628P variants represent validated risk factors for PD in Asian populations. Here, we describe in detail the origin, the present worldwide epidemiology, and the penetrance of LRRK2 mutations. Furthermore, this chapter aims to characterize other definitely/probably pathogenic mutations and risk variants of LRRK2. Finally, we provide some general guidelines for a LRRK2 genetic testing and counseling. In summary, LRRK2 discovery revolutionized the understanding of PD etiology and laid the foundation for a promising future of genetics in PD research.

SNCA Gene, but Not MAPT, Influences Onset Age of Parkinson's Disease in Chinese and Australians

Background. α-Synuclein (SNCA) and microtubule-associated protein tau (MAPT) are the two major genes independently, but not jointly, associated with susceptibility for Parkinson's disease (PD). The SNCA gene has recently been identified as a major modifier of age of PD onset. Whether MAPT gene synergistically influences age of onset of PD is unknown. Objective. To investigate independent and joint effects of MAPT and SNCA on PD onset age. Methods. 412 patients with PD were recruited from the Australian PD Research Network (123) and the Neurology Department, Ruijin Hospital Affiliated to Shanghai Jiaotong University, China (289). MAPT (rs17650901) tagging H1/H2 haplotype and SNCA (Rep1) were genotyped in the Australian cohort, and MAPT (rs242557, rs3744456) and SNCA (rs11931074, rs894278) were genotyped in the Chinese cohort. SPSS regression analysis was used to test genetic effects on age at onset of PD in each cohort. Results. SNCA polymorphisms associated with the onset age of PD in both populations. MAPT polymorphisms did not enhance such association in either entire cohort. Conclusion. This study suggests that, in both ethnic groups, SNCA gene variants influence the age at onset of PD and α-synuclein plays a key role in the disease course of PD.

Link between the SNCA gene and parkinsonism

The groundbreaking discovery of mutations in the SNCA gene in a rare familial form of Parkinson's disease (PD) has revolutionized our basic understanding of the etiology of PD and other related disorders. Genome-wide Association Studies has demonstrated a wide array of single-nucleotide polymorphisms associated with the increasing risk of developing the more common type, sporadic PD, further corroborating the genetic etiology of PD. Among them, SNCA is a gene responsible for encoding α-synuclein, a protein found to be the major component of Lewy body and Lewy neurite, both of these components are the pathognomonic hallmarks of PD. Thus, it has been postulated that this gene plays specific roles in pathogenesis of PD. Here, we summarize the basic biological characteristics of the wild type of the protein (wt-α-synuclein) as well as genetic and epigenetic features of its encoding gene (SNCA) in PD. Based on these characteristics, SNCA may be involved in PD pathogenesis in at least 2 ways: wt-α-synuclein overexpression and its mutation types via different mechanisms. Associations between SNCA mutations and other Lewy body disorders, such as dementia with Lewy bodies and multiple system atrophy, are also mentioned. Finally, it is necessary to explore the influences which SNCA exerts on clinical and neuropathological phenotypes by promoting the transfer of scientific research into practice, such as clinical evaluation, diagnosis, and treatment of the disease. We believe it is promising to target SNCA for developing novel therapeutic strategies for parkinsonism.

α-Synuclein rs356219 polymorphisms in patients with Gaucher disease and Parkinson disease

Mutations in β-glucocerebrosidase, the genetic defect in Gaucher disease (GD), are an important susceptibility factor for Parkinson disease (PD). A PD effector is α-synuclein (SNCA) hypothesized to selectively interact with β-glucocerebrosidase under lysosomal conditions. SNCA polymorphism rs356219 may be associated with early-age-onset PD, common among patients with GD+PD. The objective of this study was to ascertain rs356219 genotypes of GD+PD patients. All GD+PD patients at our Gaucher referral clinic were asked to participate. A GD-only sex-, age-, GD genotype-, and enzyme therapy (ERT)-matched control was found for each GD+PD participant. Student's t-test was used (p-value <0.05 as significant). There were 14 GD+PD patients: all Ashkenazi Jewish; 11 males (78.6%); mean (range) age diagnosed GD 34.2 (5-62) years; 50% N370S homozygous; mild to moderate GD; 3 asplenic and only these have osteonecrosis; 5 received ERT; mean age (range) diagnosed PD was 57.8 (43-70) years; first PD sign was tremor in 9 (64.3%); cognitive dysfunction in all. In GD+PD, frequency for AG+GG (9) was greater than AA (5); in GD only, there was equality (7). Odds Ratio risk for PD increases with number minor alleles: but not significantly greater among GD+PD than GD only; in aggregate, there was no difference between cohorts for frequency of minor alleles. The limitation of this study is few GD+PD, albeit virtually all the GD+PD cohort >500 adult GD patients in our clinic. Nonetheless, as a foray into potential genetic GD susceptibility for a synucleinopathy, this study suggests the need for collaboration to achieve larger sample size.

Investigating human disease using stem cell models

Tractable and accurate disease models are essential for understanding disease pathogenesis and for developing new therapeutics. As stem cells are capable of self-renewal and differentiation, they are ideally suited both for generating these models and for obtaining the large quantities of cells required for drug development and transplantation therapies. Although proof of principle for the use of adult stem cells and embryonic stem cells in disease modelling has been established, induced pluripotent stem cells (iPSCs) have demonstrated the greatest utility for modelling human diseases. Furthermore, combining gene editing with iPSCs enables the generation of models of genetically complex disorders.

Mutant LRRK2 toxicity in neurons depends on LRRK2 levels and synuclein but not kinase activity or inclusion bodies

By combining experimental neuron models and mathematical tools, we developed a "systems" approach to deconvolve cellular mechanisms of neurodegeneration underlying the most common known cause of Parkinson's disease (PD), mutations in leucine-rich repeat kinase 2 (LRRK2). Neurons ectopically expressing mutant LRRK2 formed inclusion bodies (IBs), retracted neurites, accumulated synuclein, and died prematurely, recapitulating key features of PD. Degeneration was predicted from the levels of diffuse mutant LRRK2 that each neuron contained, but IB formation was neither necessary nor sufficient for death. Genetic or pharmacological blockade of its kinase activity destabilized LRRK2 and lowered its levels enough to account for the moderate reduction in LRRK2 toxicity that ensued. By contrast, targeting synuclein, including neurons made from PD patient-derived induced pluripotent cells, dramatically reduced LRRK2-dependent neurodegeneration and LRRK2 levels. These findings suggest that LRRK2 levels are more important than kinase activity per se in predicting toxicity and implicate synuclein as a major mediator of LRRK2-induced neurodegeneration.

Genetic correction of a LRRK2 mutation in human iPSCs links parkinsonian neurodegeneration to ERK-dependent changes in gene expression

The LRRK2 mutation G2019S is the most common genetic cause of Parkinson's disease (PD). To better understand the link between mutant LRRK2 and PD pathology, we derived induced pluripotent stem cells from PD patients harboring LRRK2 G2019S and then specifically corrected the mutant LRRK2 allele. We demonstrate that gene correction resulted in phenotypic rescue in differentiated neurons and uncovered expression changes associated with LRRK2 G2019S. We found that LRRK2 G2019S induced dysregulation of CPNE8, MAP7, UHRF2, ANXA1, and CADPS2. Knockdown experiments demonstrated that four of these genes contribute to dopaminergic neurodegeneration. LRRK2 G2019S induced increased extracellular-signal-regulated kinase 1/2 (ERK) phosphorylation. Transcriptional dysregulation of CADPS2, CPNE8, and UHRF2 was dependent on ERK activity. We show that multiple PD-associated phenotypes were ameliorated by inhibition of ERK. Therefore, our results provide mechanistic insight into the pathogenesis induced by mutant LRRK2 and pointers for the development of potential new therapeutics.

SNCA rs356219 variant increases risk of sporadic Parkinson's disease in ethnic Chinese

Alpha-synuclein gene (SNCA) polymorphisms have been associated with Parkinson's disease (PD). A recently published genome-wide association study (GWAS) meta-analysis from the USA and Europe found a strong association between SNCA rs356219 and PD. Considering the population-specific heterogeneity, we investigated the role of SNCA rs356219 as PD susceptibility in a large Han Chinese population of 685 patients and 569 controls. The SNCA rs356219-G allele was found to increase the risk to develop PD (OR = 1.81, 95% CI: 1.54-2.13, P = 5.71E-13). The meta-analysis revealed that the frequency of AG + GG genotypes higher in PD than in control subjects (OR = 1.85, 95% CI: 1.56-2.19, P = 0.00001) in the Asian population. PD patients with AG + GG genotypes were associated with earlier age at onset compared with those with AA genotype. No such significant association was observed in the clinical presentation for gender, age at onset, and onset symptoms. Our study provides strong support for the susceptibility role of SNCA rs356219 in sporadic PD in a Han Chinese population from mainland China and the meta-analysis also revealed a similar finding in the Asian population.

Divergent α-synuclein solubility and aggregation properties in G2019S LRRK2 Parkinson's disease brains with Lewy Body pathology compared to idiopathic cases

Mutations in LRRK2 are the most common genetic cause of Parkinson's disease (PD). The most prevalent LRRK2 mutation is the G2019S coding change, located in the kinase domain of this complex multi-domain protein. The majority of G2019S autopsy cases feature typical Lewy Body pathology with a clinical phenotype almost indistinguishable from idiopathic PD (iPD). Here we have investigated the biochemical characteristics of α-synuclein in G2019S LRRK2 PD post-mortem material, in comparison to pathology-matched iPD. Immunohistochemistry with pS129 α-synuclein antibody showed that the medulla is heavily affected with pathology in G2019S PD whilst the basal ganglia (BG), limbic and frontal cortical regions demonstrated comparable pathology scores between G2019S PD and iPD. Significantly lower levels of the highly aggregated α-synuclein species in urea–SDS fractions were observed in G2019S cases compared to iPD in the BG and limbic cortex. Our data, albeit from a small number of cases, highlight a difference in the biochemical properties of aggregated α-synuclein in G2019S linked PD compared to iPD, despite a similar histopathological presentation. This divergence in solubility is most notable in the basal ganglia, a region that is affected preclinically and is damaged before overt dopaminergic cell death.

•

We compared α-synuclein biochemistry from LRRK2 G2019S and idiopathic PD brains.

•

We used four G2019S PD post-mortem brains and pathology matched idiopathic PD cases.

•

G2019S PD and idiopathic PD cases show comparable Limbic stage Lewy Body pathology.

•

Minimal SDS-insoluble α-synuclein seen in G2019S PD in contrast to idiopathic PD

•

We propose a divergent nature of α-synuclein pathogenic species in G2019S PD.

SNCA: major genetic modifier of age at onset of Parkinson's disease

Age at onset serves as a predictor of progression and mortality in sporadic Parkinson's disease (PD). Therefore, the identification of genetic modifiers for age at onset might lead to a better understanding of disease pathogenesis. We performed multivariate linear regression analysis in 1396 sporadic PD patients assessing 21 single-nucleotide polymorphisms (SNPs) that have been previously suggested to be associated with sporadic PD. Moreover, a cumulative risk score was assigned to each patient and correlated with age at onset. We identified the rs356219 risk allele in the SNCA gene as significantly contributing to earlier age at onset. Neither one of the other 21 SNPs tested in this analysis nor the cumulative number of risk alleles showed a significant impact on PD onset. Because sequence variants in the SNCA gene are not only associated with autosomal dominantly inherited PD and increased susceptibility for sporadic PD but also have been found to modify the phenotype such as age at onset in both sporadic and various monogenic forms of PD, this gene serves as an outstanding target for further research on PD pathogenesis, which in return might provide potential therapeutic options. © 2013 Movement Disorder Society.

LRRK2: cause, risk, and mechanism

In 2004 it was first shown that mutations in LRRK2 can cause Parkinson's disease. This initial discovery was quickly followed by the observation that a single particular mutation is a relatively common cause of Parkinson's disease across varied populations. Further genetic investigation has revealed a variety of genetic ties to Parkinson's disease across this gene. These include common alleles with quite broad effects on risk, likely through both alterations at the protein sequence level, and in the context of expression. A great deal of functional characterization of LRRK2 and disease-causing mutations in this protein has occurred over the last 9 years, and considerable progress has been made. Particular attention has been paid to the kinase activity of LRRK2 as a therapeutic target, and while it is no means certain that this is viable target it is likely that this hypothesis will be tested in clinical trials sooner rather than later. We believe that the future goals for LRRK2 research are, while challenging, relatively clear and that the next 10 years of research promises to be perhaps more exciting than the last.