Do polymorphisms in the familial Parkinsonism genes contribute to risk for sporadic Parkinson's disease?

Toxic interactions between dopamine, α-synuclein, monoamine oxidase, and genes in mitochondria of Parkinson's disease

Parkinson's disease is characterized by its distinct pathological features; loss of dopamine neurons in the substantia nigra pars compacta and accumulation of Lewy bodies and Lewy neurites containing modified α-synuclein. Beneficial effects of L-DOPA and dopamine replacement therapy indicate dopamine deficit as one of the main pathogenic factors. Dopamine and its oxidation products are proposed to induce selective vulnerability in dopamine neurons. However, Parkinson's disease is now considered as a generalized disease with dysfunction of several neurotransmitter systems caused by multiple genetic and environmental factors. The pathogenic factors include oxidative stress, mitochondrial dysfunction, α-synuclein accumulation, programmed cell death, impaired proteolytic systems, neuroinflammation, and decline of neurotrophic factors. This paper presents interactions among dopamine, α-synuclein, monoamine oxidase, its inhibitors, and related genes in mitochondria. α-Synuclein inhibits dopamine synthesis and function. Vice versa, dopamine oxidation by monoamine oxidase produces toxic aldehydes, reactive oxygen species, and quinones, which modify α-synuclein, and promote its fibril production and accumulation in mitochondria. Excessive dopamine in experimental models modifies proteins in the mitochondrial electron transport chain and inhibits the function. α-Synuclein and familiar Parkinson's disease-related gene products modify the expression and activity of monoamine oxidase. Type A monoamine oxidase is associated with neuroprotection by an unspecific dose of inhibitors of type B monoamine oxidase, rasagiline and selegiline. Rasagiline and selegiline prevent α-synuclein fibrillization, modulate this toxic collaboration, and exert neuroprotection in experimental studies. Complex interactions between these pathogenic factors play a decisive role in neurodegeneration in PD and should be further defined to develop new therapies for Parkinson's disease.

SMetABF: A rapid algorithm for Bayesian GWAS meta-analysis with a large number of studies included

Bayesian methods are widely used in the GWAS meta-analysis. But the considerable consumption in both computing time and memory space poses great challenges for large-scale meta-analyses. In this research, we propose an algorithm named SMetABF to rapidly obtain the optimal ABF in the GWAS meta-analysis, where shotgun stochastic search (SSS) is introduced to improve the Bayesian GWAS meta-analysis framework, MetABF. Simulation studies confirm that SMetABF performs well in both speed and accuracy, compared to exhaustive methods and MCMC. SMetABF is applied to real GWAS datasets to find several essential loci related to Parkinson's disease (PD) and the results support the underlying relationship between PD and other autoimmune disorders. Developed as an R package and a web tool, SMetABF will become a useful tool to integrate different studies and identify more variants associated with complex traits.

Lack of Association Between DJ-1 Gene Promoter Polymorphism and the Risk of Parkinson's Disease

Low DJ-1 protein level caused by DJ-1 gene mutation leads to autosomal recessive Parkinson's disease (PD) due to impaired antioxidative activity. In sporadic PD patients, although mutations were rarely found, lower DJ-1 protein level was also reported. Dysregulation of DJ-1 gene expression might contribute to low DJ-1 protein level. Since the promoter is the most important element to initiate gene expression, whether polymorphisms in the DJ-1 promoter result in the dysregulation of gene expression, thus leading to low protein level and causing PD, is worth exploring. The DJ-1 promoter region was sequenced in a Chinese cohort to evaluate possible links between DJ-1 promoter polymorphisms, PD risk and clinical phenotypes. Dual-luciferase reporter assay was conducted to evaluate the influence of promoter polymorphisms on DJ-1 transcriptional activity. Related information in an existing genome-wide association studies (GWAS) database were looked up, meta-analysis of the present study and other previous reports was conducted, and expression quantitative trait loci (eQTL) analysis was performed to further explore the association. Three single nucleotide polymorphisms (SNPs) (rs17523802, rs226249, and rs35675666) and one 18 bp deletion (rs200968609) were observed in our cohort. However, there was no significant association between the four detected genetic variations and the risk of PD either in allelic or genotype model, in single-point analysis or haplotype analysis. This was supported by the meta-analysis of this study and previous reports as well as that of GWAS database PDGene. Dual luciferase reporter assay suggested these promoter polymorphisms had no influence on DJ-1 transcriptive activity, which is consistent with the eQTL analysis results using the data from GTEx database. Thus, DJ-1 promoter polymorphisms may play little role in the dysregulation of DJ-1 expression and PD susceptibility in sporadic PD.

SNCA REP1 and Parkinson's disease

REP1 is a polymorphic dinucleotide repeat sequence located in the promoter region of the SNCA gene (OMIM 163890). Opinions regarding the interaction between the various REP1 alleles and Parkinson's disease (PD) or its phenotypes have been inconsistent and have thus far not been comprehensively analyzed. In this study, we searched Medline, Embase and Cochrane databases as well as the Chinese-language Wanfang and CNKI databases using strict inclusion and exclusion criteria and conducted our analysis using Revman 5.3 software. Our search produced 28 articles describing REP1 alleles and their associated PD risks and 8 articles which discussed the relationship between REP1 variation and PD phenotypes. We found that the 265-, 269-, and 271-bp alleles of REP1 (using the nomenclature established by Xia et al.) increased the risk of PD (OR: 1.81, 1.05, 1.17; p: 0.0002, 0.003, 0.002) while the 267-bp allele decreased PD risk (OR: 0.86, p: <0.00001) when taking all populations into account. By ethnicity, we observed an obvious population heterogeneity in the effects of various alleles, where the 269-, 271-, and 273-bp alleles increased PD risk (OR: 1.06, 1.22, 1.89; p: 0.001, 0.003, 0.001) and the 267-bp allele decreased PD risk (OR: 0.85; p: <0.00001) in Caucasian populations, and the 263- and 265-bp alleles increased the risk of PD (OR: 2.22, 2.03; p: 0.03, 0.0002) and the 267- and 273-bp alleles decreased PD risk (OR: 0.90, 0.78; p: 0.02, 0.03) in Asian populations. We also determined that the 267-, 269-, and 271-bp alleles occurred the most frequently, although the frequency distribution varied among different ethnicities. Phenotypic analysis demonstrated that PD patients carrying the 271-bp allele were prone to early onset PD (OR: 1.75, p: 0.02) while the 267-bp had the opposite effect (OR: 0.81; p: 0.01).

Pipeline to gene discovery - Analysing familial Parkinsonism in the Queensland Parkinson's Project

INTRODUCTION

Family based study designs provide an informative resource to identify disease-causing mutations. The Queensland Parkinson's Project (QPP) has been involved in numerous genetic screening studies; however, details of the families enrolled into the register have not been comprehensively reported. This article characterises the families enrolled in the QPP and summarises monogenic forms of hereditary Parkinsonism found in the register.

METHOD

The presence of pathogenic point mutations and copy number variations (CNVs) were, generally, screened in a sample of over 1000 PD patients from the total of 1725. Whole exome sequencing (WES) was performed on eighteen probands from multiplex families.

RESULTS

The QPP contains seventeen incidences of confirmed monogenic forms of PD, including LRRK2 p.G2019S, VPS35 p.D620N, SNCA duplications and PARK2 p.G430D (hom) & exon 4 deletion (hom). Of these seventeen, five belong to multi-incident families, while another eight have a family history of at least one other case of PD. In additional families, WES did not identify known forms of monogenic Parkinsonism; however, three heterozygous mutations in PARK2, p.R275W, p.Q34fs, and a 40bp deletion in exon 3 were identified. Of these three mutations, only the 40bp deletion segregated with disease in a dominant inheritance pattern.

CONCLUSION

Eighteen probands have screened negative for known CNVs and mutations that cause clear monogenic forms of PD. Each family is a candidate for further genetic analysis to identify genetic variants segregating with disease. The families enrolled in the QPP provide a useful resource to aid in identifying novel forms of monogenic PD.

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.

NR4A2 genetic variation and Parkinson's disease: Evidence from a systematic review and meta-analysis

INTRODUCTION

The homo sapiens nuclear receptor subfamily 4, group A (NR4A2) genetic variation has been implicated as a risk factor for Parkinson's disease (PD). Nevertheless, the results are inconclusive. We conducted a comprehensive systematic review and meta-analysis to quantify the impact of NR4A2 variation on the risk of PD.

METHODS

All eligible case-control studies published up to June 2016 by searching Pubmed, OVID, EBSCO, PsycINFO, ISI Web of Knowledge, Chinese Biomedical Literature Database and China Academic Journals Database were identified. Pooled odds ratio (OR) with 95% confidence interval (CI) were used to access the strength of the association in fixed- or random-effects model.

RESULTS

Eighteen studies reported 24 genetic variants with a total of 6150 cases and 5919 controls were included. Twelve studies for NR4A2 rs35479735 polymorphism and 4 studies for rs12803 were available for meta-analysis. A significant association was observed for rs35479735 under the homozygous model (OR=1.31, 95% CI: 1.10-1.56, P=0.003), whereas no significant association for rs12803 was detected. In subgroup analysis stratified by ethnicity, age onset and familial history, we found no significant association except one in sporadic PD subgroup under the recessive (OR=3.30, 95% CI: 1.23-8.84, P=0.02) and homozygous model (OR=3.43, 95% CI: 1.26-9.33, P=0.02) for rs35479735.

CONCLUSION

The study comprehensively evaluated the association of NR4A2 variation with PD, and the results failed to demonstrate that the NR4A2 polymorphisms significantly associated with PD except for rs35479735, suggesting that more studies are needed to elucidate if NR4A2 is a risk of PD.

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.

Generation of mouse lines with conditionally or constitutively inactivated Snca gene and Rosa26-stop-lacZ reporter located in cis on the mouse chromosome 6

α-Synuclein is involved in many important molecular processes in neuronal cells and their synapses, and its malfunction has been linked to the development of Parkinson’s and certain other neurodegenerative diseases. Animal models allowing tightly monitored conditional inactivation of the encoding gene, Snca, are indispensible for studies aimed at understanding normal function of α-synuclein in various neuronal populations and its role in pathogenesis of neurodegenerative diseases. We have recently reported the production of several novel mouse lines for manipulating expression of the endogenous Snca gene, including a line for Cre-recombinase-driven conditional inactivation of the gene (mice with floxed Snca) and a new line with a constitutive knockout of α-synuclein. Rosa26-stop-lacZ reporter cassette is commonly used for monitoring efficiency of Cre-recombination but in mouse genome Snca and Rosa26 loci are located on the same chromosome. Here we describe production of lines with a modified Snca locus, either floxed or constitutively inactivated and the Rosa26-stop-lacZ reporter cassette located in cis on the mouse chromosome 6. These new mouse lines are invaluable for fast identification of cells with inactivation of Snca by Cre-recombination and represent useful tools for in vivo studies of α-synuclein function and dysfunction.

Combinational losses of synucleins reveal their differential requirements for compensating age-dependent alterations in motor behavior and dopamine metabolism

Synucleins are involved in multiple steps of the neurotransmitter turnover, but the largely normal synaptic function in young adult animals completely lacking synucleins suggests their roles are dispensable for execution of these processes. Instead, they may be utilized for boosting the efficiency of certain molecular mechanisms in presynaptic terminals, with a deficiency of synuclein proteins sensitizing to or exacerbating synaptic malfunction caused by accumulation of mild alterations, which are commonly associated with aging. Although functional redundancy within the family has been reported, it is unclear whether the remaining synucleins can fully compensate for the deficiency of a lost family member or whether some functions are specific for a particular member. We assessed several structural and functional characteristics of the nigrostriatal system of mice lacking members of the synuclein family in every possible combination and demonstrated that stabilization of the striatal dopamine level depends on the presence of α-synuclein and cannot be compensated by other family members, whereas β-synuclein is required for efficient maintenance of animal's balance and coordination in old age.

A novel resource for studying function and dysfunction of α-synuclein: mouse lines for modulation of endogenous Snca gene expression

Pathological modification of α-synuclein is believed to be an important event in pathogenesis of Parkinson’s disease and several other neurodegenerative diseases. In normal cells this protein has been linked to many intracellular processes and pathways. However, neither normal function of α-synuclein in neuronal and certain other types of cells nor its exact role in the disease pathogenesis is well understood, which is largely due to limitations of animal models used for studying this protein. We produced and validated several novel mouse lines for manipulating expression of the endogenous Snca gene coding for α-synuclein. These include a line for conditional Cre-recombinase-driven inactivation of the gene; a line for conditional Flp-driven restoration of a neo-cassete-blocked α-synuclein expression; a new line with a “clean” constituent knockout of the gene as well as a line carrying this knockout locus and Rosa26-stop-lacZ reporter locus linked at the same mouse chromosome 6. Altogether these lines represent a set of new useful tools for studies of α-synuclein normal function and the role of this protein in disease pathogenesis.

Combination therapies: The next logical Step for the treatment of synucleinopathies?

Currently there are no disease-modifying alternatives for the treatment of most neurodegenerative disorders. The available therapies for diseases such as Parkinson's disease (PD), PD dementia (PDD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), in which the protein alpha-synuclein (α-Syn) accumulates within neurons and glial cells with toxic consequences, are focused on managing the disease symptoms. However, using strategic drug combinations and/or multi-target drugs might increase the treatment efficiency when compared with monotherapies. Synucleinopathies are complex disorders that progress through several stages, and toxic α-Syn aggregates exhibit prion-like behavior spreading from cell to cell. Therefore, it follows that these neurodegenerative disorders might require equally complex therapeutic approaches to obtain significant and long-lasting results. Hypothetically, therapies aimed at reducing α-Syn accumulation and cell-to-cell transfer, such as immunotherapy against α-Syn, could be combined with agents that reduce neuroinflammation with potential synergistic outcomes. Here we review the current evidence supporting this type of approach, suggesting that such rational therapy combinations, together with the use of multi-target drugs, may hold promise as the next logical step for the treatment of synucleinopathies.

Comprehensive assessment of genetic sequence variants in the antioxidant 'master regulator' NRF2 in idiopathic Parkinson's disease

Parkinson’s disease (PD) is a complex neurodegenerative disorder influenced by a combination of genetic and environmental factors. The molecular mechanisms that underlie PD are unknown; however, oxidative stress and impairment of antioxidant defence mechanisms have been implicated as major contributors to disease pathogenesis. Previously, we have reported a PD patient-derived cellular model generated from biopsies of the olfactory mucosa, termed hONS cells, in which the NRF2-mediated antioxidant response pathway genes were among the most differentially-expressed. To date, few studies have examined the role of the NRF2 encoding gene, NFE2L2, and PD. In this study, we comprehensibly assessed whether rare and common NFE2L2 genetic variations modify susceptibility to PD using a large Australian case-control sample (PD=1338, controls=1379). We employed a haplotype-tagging approach that identified an association with the tagging SNP rs2364725 and PD (OR = 0.849 (0.760-0.948), P = 0.004). Further genetic screening in hONS cell lines produced no obvious pathogenic variants in the coding regions of NFE2L2. Finally, we investigated the relationship between xenobiotic exposures and NRF2 function, through gene-environment interactions, between NFE2L2 SNPs and smoking or pesticide exposure. Our results demonstrated a significant interaction between rs2706110 and pesticide exposure (OR = 0.597 (0.393-0.900), P = 0.014). In addition, we were able to identify some age-at-onset modifying SNPs and replicate an ‘early-onset’ haplotype that contains a previously identified ‘functional promoter’ SNP (rs6721961). Our results suggest a role of NFE2L2 genetic variants in modifying PD susceptibility and onset. Our findings also support the utility of testing gene-environment interactions in genetic studies of PD.

Alpha-synuclein (SNCA) polymorphisms and susceptibility to Parkinson's disease: a meta-analysis

It has been reported that single nucleotide polymorphisms (SNPs) of Alpha-synuclein (SNCA) are associated with Parkinson's disease (PD). Some researchers have attempted to validate this finding in various ethnic populations. The results of studies concerning SNCA polymorphisms and PD susceptibility remain conflicting. To evaluate the association between these SNPs and PD, the authors conducted a series of meta-analyses using a predefined protocol. Databases including PubMed, MEDLINE and PD gene were searched to identify relevant studies. Odds ratios (ORs) with 95% confidence intervals (CIs) were used to assess the strength of the association. All analyses were calculated using STATA11.0. A total of 19 studies on the SNPS rs181489, rs356186, rs356219, rs894278, rs2583988, rs2619363, rs2619364, rs2737029, rs10005233 and rs11931074 were included. This meta-analysis showed that eight out of these 10 candidate SNPs may be associated with PD risk. Significant association was found between PD and the following SNPs: rs181489, rs356186, rs356219, rs894278 rs2583988, rs2619364, rs10005233 and rs11931074. Among these SNPs, rs356186 was found to be the only SNP that may play a protective role in Parkinson's disease. These results suggest that the SNCA gene may be associated with PD.

Pooled analysis of iron-related genes in Parkinson's disease: association with transferrin

Pathologic features of Parkinson's disease (PD) include death of dopaminergic neurons in the substantia nigra, presence of α-synuclein containing Lewy bodies, and iron accumulation in PD-related brain regions. The observed iron accumulation may be contributing to PD etiology but it also may be a byproduct of cell death or cellular dysfunction. To elucidate the possible role of iron accumulation in PD, we investigated genetic variation in 16 genes related to iron homeostasis in three case-control studies from the United States, Australia, and France. After screening 90 haplotype tagging single nucleotide polymorphisms (SNPs) within the genes of interest in the US study population, we investigated the five most promising gene regions in two additional independent case-control studies. For the pooled data set (1289 cases, 1391 controls) we observed a protective association (OR=0.83, 95% CI: 0.71-0.96) between PD and a haplotype composed of the A allele at rs1880669 and the T allele at rs1049296 in transferrin (TF; GeneID: 7018). Additionally, we observed a suggestive protective association (OR=0.87, 95% CI: 0.74-1.02) between PD and a haplotype composed of the G allele at rs10247962 and the A allele at rs4434553 in transferrin receptor 2 (TFR2; GeneID: 7036). We observed no associations in our pooled sample for haplotypes in SLC40A1, CYB561, or HFE. Taken together with previous findings in model systems, our results suggest that TF or a TF-TFR2 complex may have a role in the etiology of PD, possibly through iron misregulation or mitochondrial dysfunction within dopaminergic neurons.

Knowledge and attitudes towards genetic testing in those affected with Parkinson's disease

Advances in genetic tests provide valuable information for clinicians and patients around risks and inheritance of Parkinson's Disease (PD); however, questions arise whether those affected or at risk of PD will want genetic testing, particularly given that there are no preventive or disease-modifying therapies currently available. This study sought to determine knowledge and attitudes toward genetic testing for those affected with PD. A cross-sectional study was undertaken using a standardized questionnaire with six multi-choice genetic knowledge and 17 multi-choice attitude items. Participants were selected from a registry of people affected with PD living in Queensland, Australia. Half of the selected index cases had a family history of PD. Ordinal regression was used to evaluate the association between support for genetic testing and demographic, knowledge, and other attitudinal factors. The level of genetic knowledge was relatively low (37 % correct responses). The vast majority supported diagnostic testing (97 %) and 90 % would undertake a genetic test themselves. Support for predictive was lower (78 %) and prenatal genetic testing had the least support (58 %). Benefits of testing were identified as the ability to know the child's risk, seek therapies, and helping science with finding a cure. Concerns about genetic testing included potential emotional reactions and test accuracy. Genetic knowledge was not significantly associated with attitudes towards genetic testing. Patients with PD have strong interest in genetic testing for themselves with support for diagnostic testing but less support for predictive and prenatal testing. Genetic knowledge was unrelated to testing attitudes.

Immunotherapy for neurodegenerative diseases: focus on α-synucleinopathies

Immunotherapy is currently being intensively explored as much-needed disease-modifying treatment for neurodegenerative diseases. While Alzheimer's disease (AD) has been the focus of numerous immunotherapeutic studies, less attention has been paid to Parkinson's disease (PD) and other neurodegenerative disorders. The reason for this difference is that the amyloid beta (Aβ) protein in AD is a secreted molecule that circulates in the blood and is readably recognized by antibodies. In contrast, α-synuclein (α-syn), tau, huntingtin and other proteins involved in neurodegenerative diseases have been considered to be exclusively of intracellular nature. However, the recent discovery that toxic oligomeric versions of α-syn and tau accumulate in the membrane and can be excreted to the extracellular environment has provided a rationale for the development of immunotherapeutic approaches for PD, dementia with Lewy bodies, frontotemporal dementia, and other neurodegenerative disorders characterized by the abnormal accumulation of these proteins. Active immunization, passive immunization, and T cell-mediated cellular immunotherapeutic approaches have been developed targeting Aβ, α-syn and tau. Most advanced studies, including results from phase III clinical trials for passive immunization in AD, have been recently reported. Results suggest that immunotherapy might be a promising therapeutic approach for neurodegenerative diseases that progress with the accumulation and propagation of toxic protein aggregates. In this manuscript we provide an overview on immunotherapeutic advances for neurodegenerative disorders, with special emphasis on α-synucleinopathies.

New evidence for, and challenges in, linking small CGG repeat expansion FMR1 alleles with Parkinson's disease

We recently reported a significant increase in the frequency of carriers of grey zone (GZ) alleles of FMR1 gene in Australian males with Parkinson's disease (PD) from Victoria and Tasmania. Here, we report data comparing an independent sample of 817 PD patients from Queensland to 1078 consecutive Australian male newborns from Victoria. We confirmed the earlier finding by observing a significant excess of GZ alleles in PD (4.8%) compared to controls (1.5%). Although both studies provided evidence in support of an association between GZ-carrier status and increased risk for parkinsonism, the existing evidence in the literature from screening studies remains equivocal and we discuss the need for alternative approaches to resolve the issue.

α-Synuclein genetic variants predict faster motor symptom progression in idiopathic Parkinson disease

Currently, there are no reported genetic predictors of motor symptom progression in Parkinson's disease (PD). In familial PD, disease severity is associated with higher α-synuclein (SNCA) expression levels, and in postmortem studies expression varies with SNCA genetic variants. Furthermore, SNCA is a well-known risk factor for PD occurrence. We recruited Parkinson's patients from the communities of three central California counties to investigate the influence of SNCA genetic variants on motor symptom progression in idiopathic PD. We repeatedly assessed this cohort of patients over an average of 5.1 years for motor symptom changes employing the Unified Parkinson's Disease Rating Scale (UPDRS). Of 363 population-based incident PD cases diagnosed less than 3 years from baseline assessment, 242 cases were successfully re-contacted and 233 were re-examined at least once. Of subjects lost to follow-up, 69% were due to death. Adjusting for covariates, risk of faster decline of motor function as measured by annual increase in motor UPDRS exam score was increased 4-fold in carriers of the REP1 263bp promoter variant (OR 4.03, 95%CI:1.57-10.4). Our data also suggest a contribution to increased risk by the G-allele for rs356165 (OR 1.66; 95%CI:0.96-2.88), and we observed a strong trend across categories when both genetic variants were considered (p for trend = 0.002). Our population-based study has demonstrated that SNCA variants are strong predictors of faster motor decline in idiopathic PD. SNCA may be a promising target for therapies and may help identify patients who will benefit most from early interventions. This is the first study to link SNCA to motor symptom decline in a longitudinal progression study.

Genome-wide association studies: Where we are heading?

We have witnessed tremendous success in genome-wide association studies (GWAS) in recent years. Since the identification of variants in the complement factor H gene on the risk of age-related macular degeneration, GWAS have become ubiquitous in genetic studies and have led to the identification of genetic variants that are associated with a variety of complex human diseases and traits. These discoveries have changed our understanding of the biological architecture of common, complex diseases and have also provided new hypotheses to test. New tools, such as next-generation sequencing, will be an important part of the future of genetics research; however, GWAS studies will continue to play an important role in disease gene discovery. Many traits have yet to be explored by GWAS, especially in minority populations, and large collaborative studies are currently being conducted to maximize the return from existing GWAS data. In addition, GWAS technology continues to improve, increasing genomic coverage for major global populations and decreasing the cost of experiments. Although much of the variance attributable to genetic factors for many important traits is still unexplained, GWAS technology has been instrumental in mapping over a thousand genes to hundreds of traits. More discoveries are made each month and the scale, quality and quantity of current work has a steady trend upward. We briefly review the current key trends in GWAS, which can be summarized with three goals: increase power, increase collaborations and increase populations.

Factors associated with depression in Parkinson's disease

BACKGROUND

Depression is common in Parkinson's disease (PD) and contributes significantly to a reduced quality of life in PD patients. The determinants of depression in PD are complex and poorly understood. We investigated the factors associated with depression in PD.

METHODS

PD patients were recruited from Neurology clinics. A validated method was used to screen for a lifetime history of depression. 'Depressed' patients were identified by a score of >6 in the Geriatric Depression Scale (GDS-15) or by having had prescribed treatment for depression. 'Never depressed' patients were recognised by a score of <5 in the GDS-15 with no signs of a history of depression. A newly developed and validated questionnaire was used to collect other information.

RESULTS

Depression was identified in 66% of the 639 PD patients who met the inclusion criteria. Depression was associated with an increased severity of illness as evidenced by higher Unified PD Rating Scale scores and a higher Hoehn and Yahr stage. Other clinical factors associated with disease severity were also more frequently observed in depressed patients. Similar to findings in non-PD samples, depressed PD patients were more likely to have a lower education level, a history of smoking and to regularly use non-aspirin based NSAIDs or analgesics. Comorbidities such as anxiety, memory problems, hallucinations, sleep disturbances and postural hypotension were more common in depressed PD patients.

LIMITATIONS

To avoid patient exhaustion of over-surveying, some factors within the psychological domain were not examined.

CONCLUSION

Our results provide a focus for future intervention strategies.

Family-based association analysis of the MAPT gene in Parkinson disease

The MAPT gene has been shown to be associated with several neurodegenerative disorders, including forms of parkinsonism and Parkinson disease (PD), but the results reveal population differences. We investigated the association of 10 single-nucleotide polymorphisms (SNPs) in the region of MAPT on chromosome 17q21 with PD and age at onset, by using 443 discordant sib pairs in PD from a public dataset (Mayo-Perlegen LEAPS Collaboration). Association with PD was assessed by the FBAT using generalized estimating equations (FBAT-GEE), while the association with age at onset as a quantitative trait was evaluated using the FBAT-logrank statistic. Five SNPs were significantly associated with PD (P &lt; 0.05) in an additive model, and 9 SNPs were associated with PD (P &lt; 0.05) in dominant and recessive models. Interestingly, 8 PD-associated SNPs were also associated with age at onset of PD (P &lt; 0.05) in dominant and recessive models. The SNP most significantly associated with PD and age at onset was rs17649641 (P = 0.015 and 0.021, respectively). Two-SNP haplotypes inferred from rs17563965 and rs17649641 also showed association with PD (P = 0.018) and age at onset (P = 0.026). These results provide further support for the role of MAPT in development of PD.

Probabilistic subgroup identification using Bayesian finite mixture modelling: a case study in Parkinson's disease phenotype identification

This article explores the use of probabilistic classification, namely finite mixture modelling, for identification of complex disease phenotypes, given cross-sectional data. In particular, if focuses on posterior probabilities of subgroup membership, a standard output of finite mixture modelling, and how the quantification of uncertainty in these probabilities can lead to more detailed analyses. Using a Bayesian approach, we describe two practical uses of this uncertainty: (i) as a means of describing a person's membership to a single or multiple latent subgroups and (ii) as a means of describing identified subgroups by patient-centred covariates not included in model estimation. These proposed uses are demonstrated on a case study in Parkinson's disease (PD), where latent subgroups are identified using multiple symptoms from the Unified Parkinson's Disease Rating Scale (UPDRS).

Genome-wide association study confirms BST1 and suggests a locus on 12q24 as the risk loci for Parkinson's disease in the European population

We performed a three-stage genome-wide association study (GWAS) to identify common Parkinson's disease (PD) risk variants in the European population. The initial genome-wide scan was conducted in a French sample of 1039 cases and 1984 controls, using almost 500 000 single nucleotide polymorphisms (SNPs). Two SNPs at SNCA were found to be associated with PD at the genome-wide significance level (P < 3 × 10(-8)). An additional set of promising and new association signals was identified and submitted for immediate replication in two independent case-control studies of subjects of European descent. We first carried out an in silico replication study using GWAS data from the WTCCC2 PD study sample (1705 cases, 5200 WTCCC controls). Nominally replicated SNPs were further genotyped in a third sample of 1527 cases and 1864 controls from France and Australia. We found converging evidence of association with PD on 12q24 (rs4964469, combined P = 2.4 × 10(-7)) and confirmed the association on 4p15/BST1 (rs4698412, combined P = 1.8 × 10(-6)), previously reported in Japanese data. The 12q24 locus includes RFX4, an isoform of which, named RFX4_v3, encodes brain-specific transcription factors that regulate many genes involved in brain morphogenesis and intracellular calcium homeostasis.

Lipid classes and fatty acid patterns are altered in the brain of γ-synuclein null mutant mice

The well-documented link between α-synuclein and the pathology of common human neurodegenerative diseases has increased attention to the synuclein protein family. The involvement of α-synuclein in lipid metabolism in both normal and diseased nervous system has been shown by many research groups. However, the possible involvement of γ-synuclein, a closely-related member of the synuclein family, in these processes has hardly been addressed. In this study, the effect of γ-synuclein deficiency on the lipid composition and fatty acid patterns of individual lipids from two brain regions has been studied using a mouse model. The level of phosphatidylserine (PtdSer) was increased in the midbrain whereas no changes in the relative proportions of membrane polar lipids were observed in the cortex of γ-synuclein-deficient compared to wild-type (WT) mice. In addition, higher levels of docosahexaenoic acid were found in PtdSer and phosphatidylethanolamine (PtdEtn) from the cerebral cortex of γ-synuclein null mutant mice. These findings show that γ-synuclein deficiency leads to alterations in the lipid profile in brain tissues and suggest that this protein, like α-synuclein, might affect neuronal function via modulation of lipid metabolism.

Genetic animal models of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is characterized by the degeneration of dopamine (DA) and non-DA neurons, the almost uniform presence of Lewy bodies, and motor deficits. Although the majority of PD is sporadic, specific genetic defects in rare familial cases have provided unique insights into the pathogenesis of PD. Through the creation of animal and cellular models of mutations in LRRK2 and alpha-synuclein, which are linked to autosomal-dominant PD, and mutations in parkin, DJ-1, and PINK1, which are responsible for autosomal-recessive PD, insight into the molecular mechanisms of this disorder are leading to new ideas about the pathogenesis of PD. In this review, we discuss the animal models for these genetic causes of PD, their limitations, and value. Moreover, we discuss future directions and potential strategies for optimization of the genetic models.

MicroRNAs and deregulated gene expression networks in neurodegeneration

Neurodegeneration is characterized by the progressive loss of neuronal cell types in the nervous system. Although the main cause of cell dysfunction and death in many neurodegenerative diseases is not known, there is increasing evidence that their demise is a result of a combination of genetic and environmental factors which affect key signaling pathways in cell function. This view is supported by recent observations that disease-compromised cells in late-stage neurodegeneration exhibit profound dysregulation of gene expression. MicroRNAs (miRNAs) introduce a novel concept of regulatory control over gene expression and there is increasing evidence that they play a profound role in neuronal cell identity as well as multiple aspects of disease pathogenesis. Here, we review the molecular properties of brain cells derived from patients with neurodegenerative diseases, and discuss how deregulated miRNA/mRNA expression networks could be a mechanism in neurodegeneration. In addition, we emphasize that the dysfunction of these regulatory networks might overlap between different cell systems and suggest that miRNA functions might be common between neurodegeneration and other disease entities.

Gene therapy in Parkinson's disease: rationale and current status

Parkinson's disease is the second most common age-related neurodegenerative disorder, typified by the progressive loss of substantia nigra pars compacta dopamine neurons and the consequent decrease in the neurotransmitter dopamine. Patients exhibit a range of clinical symptoms, with the most common affecting motor function and including resting tremor, rigidity, akinesia, bradykinesia and postural instability. Current pharmacological interventions are palliative and largely aimed at increasing dopamine levels through increased production and/or inhibition of metabolism of this key neurotransmitter. The gold standard for treatment of both familial and sporadic Parkinson's disease is the peripheral administration of the dopamine precursor, levodopa. However, many patients gradually develop levodopa-induced dyskinesias and motor fluctuations. In addition, dopamine enhancement therapies are most useful when a portion of the nigrostriatal pathway is intact. Consequently, as the number of substantia nigra dopamine neurons and striatal projections decrease, these treatments become less efficacious. Current translational research is focused on the development of novel disease-modifying therapies, including those utilizing gene therapeutic approaches. Herein we present an overview of current gene therapy clinical trials for Parkinson's disease. Employing either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors, these clinical trials are focused on three overarching approaches: augmentation of dopamine levels via increased neurotransmitter production; modulation of the neuronal phenotype; and neuroprotection. The first two therapies discussed in this article focus on increasing dopamine production via direct delivery of genes involved in neurotransmitter synthesis (amino acid decarboxylase, tyrosine hydroxylase and GTP [guanosine triphosphate] cyclohydrolase 1). In an attempt to bypass the degenerating nigrostriatal pathway, a third clinical trial utilizes rAAV2 to deliver glutamic acid decarboxylase to the subthalamic nucleus, converting a subset of excitatory neurons to GABA-producing cells. In contrast, the final clinical trial is aimed at protecting the degenerating nigrostriatum by striatal delivery of rAAV2 harbouring the neuroprotective gene, neurturin. Based on preclinical studies, this gene therapeutic approach is posited to slow disease progression by enhancing neuronal survival. In addition, we discuss the outcome of each clinical trial and discuss the potential rationale for the marginal yet incremental clinical advancements that have thus far been realized for Parkinson's disease gene therapy.

Genome-wide association study confirms SNPs in SNCA and the MAPT region as common risk factors for Parkinson disease

Parkinson disease (PD) is a chronic neurodegenerative disorder with a cumulative prevalence of greater than one per thousand. To date three independent genome-wide association studies (GWAS) have investigated the genetic susceptibility to PD. These studies implicated several genes as PD risk loci with strong, but not genome-wide significant, associations. In this study, we combined data from two previously published GWAS of Caucasian subjects with our GWAS of 604 cases and 619 controls for a joint analysis with a combined sample size of 1752 cases and 1745 controls. SNPs in SNCA (rs2736990, p-value = 6.7 x 10(-8); genome-wide adjusted p = 0.0109, odds ratio (OR) = 1.29 [95% CI: 1.17-1.42] G vs. A allele, population attributable risk percent (PAR%) = 12%) and the MAPT region (rs11012, p-value = 5.6 x 10(-8); genome-wide adjusted p = 0.0079, OR = 0.70 [95% CI: 0.62-0.79] T vs. C allele, PAR%= 8%) were genome-wide significant. No other SNPs were genome-wide significant in this analysis. This study confirms that SNCA and the MAPT region are major genes whose common variants are influencing risk of PD.

DJ-1 is a Parkinson's disease susceptibility gene in southern Italy

Mutations in the gene DJ-1 have been shown to be a rare cause of early-onset Parkinson's disease (EOPD). Since DJ-1 mutations have been found in patients with Parkinson's disease (PD) from southern Italy, we aimed to investigate whether polymorphisms within the DJ-1 gene could represent a risk factor for sporadic PD. First, we genotyped 294 patients with PD and 298 controls coming from southern Italy to assess the distribution of the insertion/deletion (Ins/Del) polymorphism. In a second phase, we identified five single-nucleotide polymorphisms (SNPs) useful to delimit a region potentially involved and genotyped all patients and controls for these markers. All the markers analyzed were significantly associated with PD at both allelic and genotypic level. The most significant association with the disease was found at the Ins/Del polymorphism (p = 0.0001; adjusted odds ratio (OR ) = 2.05; confidence interval (CI ) = 1.36-3.08). When we considered a three-marker sliding window, we found a highly significant association between the disease and the haplotypes including markers rs17523802, Ins/Del, and rs3766606 (p = 0.0007) and markers Ins/Del, rs3766606 and rs7517357 (p = 0.0054). Our results indicate that polymorphisms located in a region spanning 3535 bp from the promoter to the intron 2 of the DJ-1 gene confer risk to sporadic PD in southern Italy.