NACP-REP1 polymorphism is not involved in Parkinson's disease: a case-control study in a population sample from southern Italy

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).

Advancing Stem Cell Models of Alpha-Synuclein Gene Regulation in Neurodegenerative Disease

Alpha-synuclein (non A4 component of amyloid precursor, SNCA, NM_000345.3) plays a central role in the pathogenesis of Parkinson's disease (PD) and related Lewy body disorders such as Parkinson's disease dementia, Lewy body dementia, and multiple system atrophy. Since its discovery as a disease-causing gene in 1997, alpha-synuclein has been a central point of scientific interest both at the protein and gene level. Mutations, including copy number variants, missense mutations, short structural variants, and single nucleotide polymorphisms, can be causative for PD and affect conformational changes of the protein, can contribute to changes in expression of alpha-synuclein and its isoforms, and can influence regulation of temporal as well as spatial levels of alpha-synuclein in different tissues and cell types. A lot of progress has been made to understand both the physiological transcriptional and epigenetic regulation of the alpha-synuclein gene and whether changes in transcriptional regulation could lead to disease and neurodegeneration in PD and related alpha-synucleinopathies. Although the histopathological changes in these neurodegenerative disorders are similar, the temporal and spatial presentation and progression distinguishes them which could be in part due to changes or disruption of transcriptional regulation of alpha-synuclein. In this review, we describe different genetic alterations that contribute to PD and neurodegenerative conditions and review aspects of transcriptional regulation of the alpha-synuclein gene in the context of the development of PD. New technologies, advanced gene engineering and stem cell modeling, are on the horizon to shed further light on a better understanding of gene regulatory processes and exploit them for therapeutic developments.

The genetics of Parkinson disease

About 15% of patients with Parkinson disease (PD) have family history and 5-10% have a monogenic form of the disease with Mendelian inheritance. To date, at least 23 loci and 19 disease-causing genes for parkinsonism have been found, but many more genetic risk loci and variants for sporadic PD phenotype have been identified in various association studies. Investigating the mutated protein products has uncovered potential pathogenic pathways that provide insights into mechanisms of neurodegeneration in familial and sporadic PD. To commemorate the 200th anniversary of Parkinson's publication of An Essay on the Shaking Palsy, we provide a comprehensive and critical overview of the current clinical, neuropathological, and genetic understanding of genetic forms of PD. We also discuss advances in screening for genetic PD-related risk factors and how they impact genetic counseling and contribute to the development of potential disease-modifying therapies.

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.

Deregulation of α-synuclein in Parkinson's disease: Insight from epigenetic structure and transcriptional regulation of SNCA

Understanding regulation of α-synuclein has long been a central focus for Parkinson's disease (PD) researchers. Accumulation of this protein in the Lewy body or neurites, mutations in the coding region of the gene and strong association of α-synuclein encoding gene multiplication (duplication/triplication) with familial form of PD have indicated the importance of this molecule in pathogenesis of the disease. Several years of research identified many potential faulty pathways associated with accumulation of α-synuclein inside dopaminergic neurons and its transmission to neighboring ones. Concurrently, an appreciable body of research is growing to understand the epigenetic and genetic deregulation of α-synuclein that might contribute to the disease pathology. Completion of the ENCODE (Encyclopedia of DNA Elements) project and recent advancement made in the epigenetic and trans factor mediated regulation of each gene, has tremendously accelerated the need to carefully understand the epigenetic structure of the gene (SNCA) encoding α-synuclein protein in order to decipher the regulation and contribution of α-synuclein to the pathogenesis of PD. We have also analyzed the detailed epigenetic structure of this gene with knowledge from ENCODE database, which may open new avenues in α-synuclein research. Interestingly, we have found that the gene contains several transcriptionally activate histone modifications and associated potential transcription factor binding sites in the non-coding areas that strongly suggest alternative regulatory pathways. Altogether this review will provide interesting insight of α-synuclein gene regulation from epigenetic, genetic and post-transcriptional perspectives and their potential implication in the PD pathogenesis.

Genetic variants and animal models in SNCA and Parkinson disease

Parkinson disease (PD; MIM 168600) is the second most common progressive neurodegenerative disorder characterized by a variety of motor and non-motor features. To date, at least 20 loci and 15 disease-causing genes for parkinsonism have been identified. Among them, the α-synuclein (SNCA) gene was associated with PARK1/PARK4. Point mutations, duplications and triplications in the SNCA gene cause a rare dominant form of PD in familial and sporadic PD cases. The α-synuclein protein, a member of the synuclein family, is abundantly expressed in the brain. The protein is the major component of Lewy bodies and Lewy neurites in dopaminergic neurons in PD. Further understanding of its role in the pathogenesis of PD through various genetic techniques and animal models will likely provide new insights into our understanding, therapy and prevention of PD.

Mutations in PRKN and SNCA Genes Important for the Progress of Parkinson's Disease

Although Parkinson’s disease (PD) was first described almost 200 years ago, it remains an incurable disease with a cause that is not fully understood. Nowadays it is known that disturbances in the structure of pathological proteins in PD can be caused by more than environmental and genetic factors. Despite numerous debates and controversies in the literature about the role of mutations in the SNCA and PRKN genes in the pathogenesis of PD, it is evident that these genes play a key role in maintaining dopamine (DA) neuronal homeostasis and that the dysfunction of this homeostasis is relevant to both familial (FPD) and sporadic (SPD) PD with different onset. In recent years, the importance of alphasynuclein (ASN) in the process of neurodegeneration and neuroprotective function of the Parkin is becoming better understood. Moreover, there have been an increasing number of recent reports indicating the importance of the interaction between these proteins and their encoding genes. Among others interactions, it is suggested that even heterozygous substitution in the PRKN gene in the presence of the variants +2/+2 or +2/+3 of NACP-Rep1 in the SNCA promoter, may increase the risk of PD manifestation, which is probably due to ineffective elimination of over-expressed ASN by the mutated Parkin protein. Finally, it seems that genetic testing may be an important part of diagnostics in patients with PD and may improve the prognostic process in the course of PD. However, only full knowledge of the mechanism of the interaction between the genes associated with the pathogenesis of PD is likely to help explain the currently unknown pathways of selective damage to dopaminergic neurons in the course of PD.

[Role of genetics in the etiology of synucleinopathies]

The protein family known as synucleins is composed of α-, β- and γ-synuclein. The most widely studied is the α-synuclein protein due to its participation in essential processes of the central nervous system. Neurotoxicity of this protein is related to the presence of multiplications (duplications and triplications) and point mutations in the gene sequence of the α-synuclein gene (SNCA), differential expression of its isoforms and variations in post-transductional modifications. Neurotoxicity is also related to cytoplasmic inclusions known as Lewy bodies (LBs) and Lewy neurites (LNs), which are also present in α-synucleinopathies. In general, the β-synuclein protein, codified by the SNCB gene, acts as a regulator of processes triggered by α-synuclein and its function is altered by variations in the gene sequence, while γ-synuclein, codified by the SNCG gene, seems to play a major role in certain tumoral processes.

Genetic variability in SNCA and Parkinson's disease

Over the last decades, increasing knowledge about the genetic architecture of Parkinson's disease has provided novel insights into the pathogenesis of the disorder, generating hypotheses for further research. Characterizing the role of SNCA, encoding the α-synuclein protein, has been a particularly important aspect of this development. The identification of SNCA as the first gene implicated in monogenic parkinsonism led to the recognition of α-synuclein as a key protein in the pathogenesis and a major component of pathological hallmark lesions. An association between common variants in SNCA and risk of sporadic Parkinson's disease has been established through numerous studies. We review our current understanding of SNCA variability contributing to Parkinson's disease, highlighting the characterization of functionally relevant susceptibility alleles as a major future challenge. We argue that new strategies will be needed to pinpoint the variants that are ultimately responsible for the signals detected in association studies, where targeted resequencing may represent an attractive initial approach.

Genome-wide association study confirms extant PD risk loci among the Dutch

In view of the population-specific heterogeneity in reported genetic risk factors for Parkinson's disease (PD), we conducted a genome-wide association study (GWAS) in a large sample of PD cases and controls from the Netherlands. After quality control (QC), a total of 514,799 SNPs genotyped in 772 PD cases and 2024 controls were included in our analyses. Direct replication of SNPs within SNCA and BST1 confirmed these two genes to be associated with PD in the Netherlands (SNCA, rs2736990: P = 1.63 × 10(-5), OR = 1.325 and BST1, rs12502586: P = 1.63 × 10(-3), OR = 1.337). Within SNCA, two independent signals in two different linkage disequilibrium (LD) blocks in the 3' and 5' ends of the gene were detected. Besides, post-hoc analysis confirmed GAK/DGKQ, HLA and MAPT as PD risk loci among the Dutch (GAK/DGKQ, rs2242235: P = 1.22 × 10(-4), OR = 1.51; HLA, rs4248166: P = 4.39 × 10(-5), OR = 1.36; and MAPT, rs3785880: P = 1.9 × 10(-3), OR = 1.19).

Lack of association between G-protein coupled receptor kinase 5 gene and Parkinson's disease

The major component of Lewy Bodies (LB), the pathological hallmark of Parkinson's disease (PD) is α-synuclein, most prominently phosphorylated at serine 129. G-protein coupled receptor kinase 5 (GRK5) has been reported to phosphorylate α-synuclein in vitro, enhancing the α-synuclein toxicity to dopaminergic neurons in Drosophila model. Moreover, GRK5 was found in LBs from brain of PD patients. A genetic association study performed in the Japanese population revealed haplotypic association of the GRK5 gene with susceptibility to sporadic PD. We aimed at investigating whether four polymorphisms within the GRK5 gene (rs871196, rs2420616, rs7069375, rs4752293) could represent a risk factor for sporadic PD in Southern Italy. We genotyped 446 patients with PD and 450 controls for these markers and did not find any significant association with the disease at allelic, genotypic and haplotypic level. Our results indicate that the GRK5 gene does not confer risk to sporadic PD in our sample from Southern Italy.

Lack of replication of a previously reported association between polymorphism in the 3'UTR of the alpha-synuclein gene and Parkinson's disease in Chinese subjects

Recent studies have implicated polymorphisms in the 3' untranslated region (3'UTR) of the alpha-synuclein (SNCA) gene in the development of Parkinson's disease (PD). Single nucleotide polymorphism (SNP) rs356165 is one of polymorphisms located in the 3'UTR and its association with PD has been reported but remains controversial. Herein, we conducted a case-control study to further evaluate the possible association between SNP rs356165 and PD in Chinese. All subjects (330 PD patients and 300 normal controls) were successfully genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. No statistically significant difference in genotype frequency between cases and controls was observed (P=0.863), suggesting no association of SNP rs356165 with PD in our population. Thus, it may be premature to conclude an association between the 3'UTR of the SNCA gene and PD, and this association should be further examined in different ethnic populations.

The impact of genetic research on our understanding of Parkinson's disease

Until recently, genetics was thought to play a minor role in the development of Parkinson's disease (PD). Over the last decade, a number of genes that definitively cause PD have been identified, which has led to the generation of disease models based on pathogenic gene variants that recapitulate many features of the disease. These genetic studies have provided novel insight into potential mechanisms underlying the aetiology of PD. This chapter will provide a profile of the genes conclusively linked to PD and will outline the mechanisms of PD pathogenesis implicated by genetic studies. Mitochondrial dysfunction, oxidative stress and impaired ubiquitin-proteasome system function are disease mechanisms that are particularly well supported by genetic studies and are therefore the focus of this chapter.

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

Recent whole genome association studies provided little evidence that polymorphisms at the familial Parkinsonism loci influence the risk for Parkinson's disease (PD). However, these studies are not designed to detect the types of subtle effects that common variants may impose. Here, we use an alternative targeted candidate gene approach to examine common variation in 11 genes related to familial Parkinsonism. PD cases (n = 331) and unaffected control subjects (n = 296) were recruited from three specialist movement disorder clinics in Brisbane, Australia and the Australian Electoral Roll. Common genetic variables (76 SNPs and 1 STR) were assessed in all subjects and haplotype, genotype, and allele associations explored. Modest associations (uncorrected P < 0.05) were observed for common variants around SNCA, UCHL1, MAPT, and LRRK2 although none were of sufficient magnitude to survive strict statistical corrections for multiple comparisons. No associations were seen for PRKN, PINK1, GBA, ATP13A2, HTRA2, NR4A2, and DJ1. Our findings suggest that common genetic variables of selected PD-related loci contribute modestly to PD risk in Australians.

Genetic susceptibility in Parkinson's disease

It is hoped that an understanding of the genetic basis of Parkinson's disease (PD) will lead to an appreciation of the molecular pathogenesis of disease, which in turn will highlight potential points of therapeutic intervention. It is also hoped that such an understanding will allow identification of individuals at risk for disease prior to the onset of motor symptoms. A large amount of work has already been performed in the identification of genetic risk factors for PD and some of this work, particularly those efforts that focus on genes implicated in monogenic forms of PD, have been successful, although hard won. A new era of gene discovery has begun, with the application of genome wide association studies; these promise to facilitate the identification of common genetic risk loci for complex genetic diseases. This is the first of several high throughput technologies that promise to shed light on the (likely) myriad genetic factors involved in this complex, late-onset neurodegenerative disorder.

Multiple alpha-synuclein gene polymorphisms are associated with Parkinson's disease in a Norwegian population

OBJECTIVES

Previous studies have found associations between Parkinson's disease (PD) and polymorphisms located within both the alpha-synuclein gene (SNCA) promoter and other gene regions. Our aim was to study SNCA gene markers in a closely matched Norwegian PD population to examine the genetic relationship between different polymorphisms associated with the disease.

METHODS

We genotyped seven single nucleotide polymorphisms (SNPs) located in the SNCA promoter and two SNPs in the 3' gene region and seven microsatellite markers located across the gene in a closely matched series of 236 PD patients and 236 controls. Linkage disequilibrium (LD) structure was examined, and association of single markers and gene haplotypes analyzed.

RESULTS

Several markers located across the SNCA gene were associated with PD, including marker alleles associated with disease in previous studies (Rep1 263-bp allele, rs356165 and rs356219).

CONCLUSION

LD between associated marker alleles located across the SNCA gene suggests that a single genetic effect might explain the previous reported association in the promoter and 3' regions.

Genetic association between alpha-synuclein and idiopathic Parkinson's disease

Point mutations and copy number variations in SNCA, the gene encoding alpha-synuclein, cause familial Parkinson's disease (PD). A dinucleotide polymorphism (REP1) in the SNCA promoter may be a risk factor for common forms of PD. We studied 1,802 PD patients and 2,129 controls from the NeuroGenetics Research Consortium, using uniform, standardized protocols for diagnosis, subject recruitment, data collection, genotyping, and data analysis. Three common REP1 alleles (257, 259, and 261 bp, with control frequencies of 0.28, 0.65, and 0.06) and several rare alleles (combined frequency <0.01) were detected. We confirmed association of REP1 with PD risk [odds ratio (OR) = 0.86, P = 0.006 for 257-carriers; OR = 1.25, P = 0.022 for 261-carriers]. Using a normalization procedure, we showed that the 257 and 261 alleles are both independently associated with PD risk (for 257, P = 0.002 in overall data, 0.003 in non-familial PD, 0.001 in early-onset PD; for 261, P = 0.056 in overall data, 0.024 in non-familial PD, 0.052 in early-onset PD). The 257-associated risk was consistent with a dominant model [hazard ratio (HR) = 0.99, P = 0.91 for 257/257 vs. 257/X where X denotes all other common alleles; HR = 1.16, P = 0.004 for X/X vs. 257/X]. The 261-associated risk was consistent with a recessive model (HR = 1.89, P = 0.026 for 261/261 vs. 261/X; HR = 0.95, P = 0.42 for X/X vs. 261/X). Genotype-specific mean onset ages (+/-SD) ranged from 54.8 +/- 12.1 for 261/261 to 59.4 +/- 11.5 for 257/257, displaying a trend of decreasing onset age with increasing allele size (P = 0.055). Genetic variation in SNCA and its regulatory regions play an important role in both familial and sporadic PD.

Controlling the mass action of alpha-synuclein in Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disease with unknown etiology. Growing evidence from genetic, pathologic, animal modeling, and biochemical studies strongly support the theory that abnormal aggregation of alpha-synuclein plays a critical role in the pathogenesis of PD. Protein aggregation is an alternative folding process that competes with the native folding pathway. Whether or not a protein is subject to the aggregation process is determined by the concentration of the protein as well as thermodynamic properties inherent to each polypeptide. An increase in cellular concentration of alpha-synuclein has been associated with the disease in both familial and sporadic forms of PD. Thus, maintenance of the intraneuronal steady state levels of alpha-synuclein below the critical concentration is a key challenge neuronal cells are facing. Expression of the alpha-synuclein gene is under the control of environmental factors and aging, the two best-established risk factors for PD. Studies also suggest that the degradation of this protein is mediated by proteasomal and autophagic pathways, which are two mechanisms that are related to the pathogenesis of PD. Recently, vesicle-mediated exocytosis has been suggested as a novel mechanism for disposal of neuronal alpha-synuclein. Relocalization of the protein to specific compartments may be another method for increasing its local concentration. Regulation of the neuronal steady state levels of alpha-synuclein has significant implications in the development of PD, and understanding the mechanism may disclose potential therapeutic targets for PD and other related diseases.

Nurr1 transcriptionally regulates the expression of alpha-synuclein

Parkinson's disease is one of the most common neurodegenerative disorders and still remains incurable. The condition is linked to mutations and alterations in expression in several genes, in particular that encoding alpha-synuclein. Mutations in Nurr1 leading to a reduction in expression were also found to lead to Parkinson's disease. In view of the importance of gene regulation in Parkinson's disease, we examined the effect of changes in Nurr1 expression on alpha-synuclein expression. Nurr1 was shown to be involved in the regulation of alpha-synuclein, as decreased expression of Nurr1, which has been found in Parkinson's disease patients with Nurr1 mutations, was shown to transcriptionally increase alpha-synuclein expression.

Alpha-synuclein promoter haplotypes and dementia in Parkinson's disease

Dementia is a common complication of Parkinson's disease (PD). It correlates significantly with the presence of cortical, limbic or nigral Lewy bodies, mainly constituted of alpha-synuclein. Mutations of the alpha-synuclein gene (SNCA) have been linked to rare familial forms of PD, while association studies on the promoter polymorphisms have given conflicting results in sporadic patients. We have performed a case control study to investigate whether genetic variability in the promoter of the alpha-synuclein gene could predispose to dementia in PD. A total of 114 demented patients and 114 non-demented patients with sporadic PD were included in the study. Six polymorphic loci (including the Rep1 microsatellite) in the promoter of the SNCA gene were examined. Each marker, taken individually, did not show association to dementia and no significant differences were observed in the inferred haplotype frequencies of demented and non-demented patients. Our data suggest the lack of involvement of the SNCA promoter in the pathogenesis of dementia in PD. Further studies in other populations are needed to confirm these results.

Association of alpha-synuclein gene haplotypes with Parkinson's disease

In a previous study, we detected an association between a dinucleotide repeat (Rep1) in the alpha-Synuclein (SNCA) gene and sporadic Parkinson's disease (PD). To extend our previous finding in a larger sample and further determine the role of SNCA in the development of PD, we screened a sample of 194 familial PD (FPD), 327 sporadic PD (SPD), and 215 controls with the Rep1 marker and 2 single nucleotide polymorphisms (SNPs) (770 and int4) in the SNCA gene. There was significant difference in allele frequency between African American and American Indian groups for Rep1 marker (p=0.03). These two samples were excluded from further analysis because of sample size. Comparison of allele frequency differences between PD and controls for the single-locus was significant only for Rep1 and SPD (p=0.017). The global case control association was highly significant for the three loci haplotypes comparisons. Our results indicate that Rep1 locus may be in linkage disequilibrium (LD) with a mutation in the gene or itself could be a risk factor for SPD.

A novel pathway for transcriptional regulation of alpha-synuclein

Alpha-synuclein is an abundant neuronal protein that has been linked to both normal synaptic function and neurodegeneration--in particular, Parkinson's disease (PD). Uncovering mechanisms that control alpha-synuclein transcription is therefore critical for PD pathogenesis and synaptic function. We previously reported that in PC12 cells and primary neurons, alpha-synuclein is transcriptionally up-regulated after application of growth factors. In the current work we have characterized the pathway involved in this regulation in PC12 cells. The MAP/ERK pathway, and in particular Ras, is both sufficient and necessary for the NGF and basic fibroblast growth factor (bFGF) -mediated response. Significantly, response elements for this pathway, including a putative occult promoter, lie within intron 1, a hitherto unappreciated regulatory region of the gene that may be utilized in this or other settings. The PI3 kinase pathway is also involved in alpha-synuclein regulation, but response elements for this pathway appear to lie primarily outside of intron 1. These findings indicate that NGF- and bFGF-mediated signal transduction via the MAP/ERK and PI3 kinase pathways, and in part via regulatory regions within intron 1, may be involved in alpha-synuclein transcriptional regulation. Targeting of these pathways may serve to modulate alpha-synuclein so that it achieves desirable levels within neuronal cells.

Gene-environment interactions in sporadic Parkinson's disease

Much has been learned in recent years about the genetics of familial Parkinson's disease. However, far less is known about those malfunctioning genes which contribute to the emergence and/or progression of the vast majority of cases, the 'sporadic Parkinson's disease', which is the focus of our current review. Drastic differences in the reported prevalence of Parkinson's disease in different continents and countries suggest ethnic and/or environmental-associated multigenic contributions to this disease. Numerous association studies showing variable involvement of multiple tested genes in these distinct locations support this notion. Also, variable increases in the risk of Parkinson's disease due to exposure to agricultural insecticides indicate complex gene-environment interactions, especially when genes involved in protection from oxidative stress are explored. Further consideration of the brain regions damaged in Parkinson's disease points at the age-vulnerable cholinergic-dopaminergic balance as being involved in the emergence of sporadic Parkinson's disease in general and in the exposure-induced risks in particular. More specifically, the chromosome 7 ACHE/PON1 locus emerges as a key region controlling this sensitive balance, and animal model experiments are compatible with this concept. Future progress in the understanding of the genetics of sporadic Parkinson's disease depends on globally coordinated, multileveled studies of gene-environment interactions.

How genetics research in Parkinson's disease is enhancing understanding of the common idiopathic forms of the disease

PURPOSE OF REVIEW

Rapid progress in genetics has meant that there are now five genes identified for 'Parkinson's disease'. The detailed phenotypes vary, but generally the dominant genes cause a Lewy body disease spectrum whereas recessive genes cause a milder parkinsonism with variable inclusion body pathology. The subject of this review is to highlight these discoveries and to discuss their relationships to idiopathic Parkinson's disease.

RECENT FINDINGS

In January 2004, mutations in PINK1, coding for a mitochondrial kinase, were found to be causal for recessive parkinsonism. Subsequently, several studies have found additional mutations associated with early onset parkinsonism. Some cases have been described with a phenotype much closer to idiopathic Parkinson's disease, but it does not appear that PINK1 is a major risk factor for the sporadic disease. Later in the same year, the LRRK2 gene was shown to cause a dominant disease with a broader phenotype. The protein product was named dardarin and contains GTPase and kinase domains. Lewy bodies have been reported in LRRK2 cases, potentially linking this gene with sporadic Parkinson's disease. One mutation, G2019S, is found in a significant percentage of cases, including sporadic Parkinson's disease.

SUMMARY

Mutations in these two genes, along with previously described Mendelian variants, are beginning to yield important information about loss of specific neuronal groups or to protein inclusion pathology. How this relates to sporadic Parkinson's disease, however, is not yet fully defined. There are clear phenotypic overlaps with genetic and sporadic Parkinson's disease, especially for the dominant genes, suggesting that common facets of pathogenesis may exist.

Association of α-synuclein Rep1 polymorphism and Parkinson's disease: Influence of Rep1 on age at onset

The alpha-synuclein Rep1 polymorphism was studied in patients and controls in an ethnic Greek population. There was an association of allele 2 with risk of Parkinson's disease (PD; adjusted odd ratio = 3.25; 95% CI = 1.80-5.87). Survival analyses (Cox proportional hazards models) were employed to explore the influence of genotypes on age at onset of PD. Age at onset of carriers of at least one Rep1 allele 2 was earlier (3.6 years) compared to noncarriers (adjusted hazard ratio = 2.21; 95% CI = 1.58-3.10). Kaplan-Meier analysis also supported a dosage effect of Rep1 allele 2 on age at onset. For Rep1 allele 1, there was neither association with risk of PD nor influence on age at onset. This is the first study showing an influence of Rep1 polymorphism on age at onset of PD.

Molecular genetic pathways in Parkinson's disease: a review

Major progress has been made in the last decade in understanding the genetic basis of PD (Parkinson's disease) with five genes unequivocally associated with disease. As a result, multiple pathways have been implicated in the pathogenesis of PD, including proteasome impairment and mitochondrial dysfunction. Although Mendelian genetics has been successful in establishing a genetic predisposition for familial PD, this has not been reiterated in the sporadic form. In fact no genetic factors have been unequivocally associated with increased risk for sporadic PD. The difficulty in identifying susceptibility factors in PD has not only been because of numerous underpowered studies, but we have been unable to dissect out the genetic component in a multifactorial disease. This review aims to summarize the genetic findings within PD.

Regulation of alpha-synuclein expression by poly (ADP ribose) polymerase-1 (PARP-1) binding to the NACP-Rep1 polymorphic site upstream of the SNCA gene

Alleles at NACP-Rep1, the polymorphic microsatellite repeat located approximately 10 kb upstream of the alpha -synuclein gene (SNCA), are associated, in some reports, with differing risks of sporadic Parkinson disease (PD). We showed previously that NACP-Rep1 acts as a negative modulator of SNCA transcription, with an effect that varied threefold among different NACP-Rep1 alleles. Given that duplications and triplications of SNCA have been implicated in familial Parkinson disease (PD), even a 1.5-2-fold increase in alpha -synuclein expression may, over many decades, contribute to PD. Thus, the association of different NACP-Rep1 alleles with PD may be a consequence of polymorphic differences in transcriptional regulation of SNCA. Here we aimed to identify the factor(s) that bind to NACP-Rep1 and potentially contribute to SNCA transcriptional modulation, by pulling down proteins that bind to NACP-Rep1 and identifying them by mass spectrometry. One of these proteins was poly-(ADP-ribose) transferase/polymerase-1 (PARP-1), a DNA-binding protein and transcriptional regulator. Electrophoresis mobility shift and chromatin immunoprecipitation assays showed specific binding of PARP-1 to NACP-Rep1. Inhibition of PARP-1's catalytic domain increased the endogenous SNCA mRNA levels in cultured SH-SY5Y cells. Furthermore, PARP-1 binding to NACP-Rep1 specifically reduced the transcriptional activity of the SNCA promoter/enhancer in luciferase reporter assays. This down-regulation effect of PARP-1 depended on NACP-Rep1 being present in the construct and was abrogated by inhibiting PARP-1's catalytic activity with 3-aminobenzamide. The association of different NACP-Rep1 alleles with PD may be mediated, in part, by the effect of PARP-1, as well as other factors, on SNCA expression.

Joint analysis of the NACP-REP1 marker within the alpha synuclein gene concludes association with alcohol dependence

Various studies have linked alcohol dependence phenotypes to chromosome 4. One candidate gene is NACP (non-amyloid component of plaques), coding for alpha synuclein. Recently, it has been shown that alpha synuclein mRNA is increased in alcohol-dependent patients within withdrawal state. This increase is significantly associated with craving, especially obsessive craving. On the basis of these observations, the present study analysed two polymorphic repeats within the NACP gene. We found highly significant longer alleles of NACP-REP1 in alcohol-dependent patients compared with healthy controls (Kruskal-Wallis test, chi(2)=99.5; df=3, P<0.001). In addition, these lengths significantly correlate with levels of expressed alpha synuclein mRNA (chi(2)=8.83; df=2, P=0.012). The present results point to a novel approach for a genetic determination of craving, a key factor in the genesis and maintenance not only of alcoholism but also of addiction in general.

alpha-Synuclein promoter confers susceptibility to Parkinson's disease

Familial Parkinson's disease (PD) has been linked to missense and genomic multiplication mutations of the alpha-synuclein gene (SNCA). Genetic variability within SNCA has been implicated in idiopathic PD in many populations. We now confirm and extend these findings, within a Belgian sample, using a high-resolution map of genetic markers across the SNCA locus. Our study implicates the SNCA promoter in susceptibility to PD, and more specifically defines a minimum promoter haplotype, spanning approximately 15.3kb of sequence, which is overrepresented in patients. Our findings represent a biomarker for PD and may have implications for patient diagnosis, longitudinal evaluation, and treatment.

Australian data and meta-analysis lend support for alpha-synuclein (NACP-Rep1) as a risk factor for Parkinson's disease

It remains unclear whether genetic variants in SNCA (the alpha-synuclein gene) alter risk for sporadic Parkinson's disease (PD). The polymorphic mixed sequence repeat (NACP-Rep1) in the promoter region of SNCA has been previously examined as a potential susceptibility factor for PD with conflicting results. We report genotype and allele distributions at this locus from 369 PD cases and 370 control subjects of European Australian ancestry, with alleles designated as -1, 0, +1, +2, and +3 as previously described. Allele frequencies designated (0) were less common in Australian cases compared to controls (OR=0.80, 95% CI 0.62-1.03). Combined analysis including all previously published ancestral European Rep1 data yielded a highly significant association between the 0 allele and a reduced risk for PD (OR=0.79, 95% CI 0.70-0.89, p=0.0001). Further study must now proceed to examine in detail this interesting and biologically plausible genetic association.

Polymorphism in candidate genes: implications for the risk and treatment of idiopathic Parkinson's disease

Idiopathic Parkinson's disease (IPD) is a progressive neurodegenerative disorder for which no restorative or neuroprotective therapy is available. Interest has recently been directed to association studies on polymorphisms of various genes, mainly those related to dopamine metabolism and transport, and their effect on response to PD, which includes primarily levodopa and dopaminomimetics. Approximately 15-20% of patients with PD do not respond to levodopa, and the majority of those who do respond develop adverse fluctuations in motor response, primarily levodopa-induced dyskinesias. This review summarizes the influence of polymorphisms in various genes on the relative risk of IPD and on levodopa efficacy. It focuses on the importance of well-designed polymorphism studies that include large samples of patients with IPD and tightly matched controls and use identical methodologies. Valid data on such polymorphisms might increase the efficacy of levodopa, decrease its side effects, and reduce the occurrence of levodopa-induced dyskinesias. They might also provide a novel diagnostic tool for PD.