Advances in the genetics of Parkinson disease

Transcription

1 Advances in the genetics of Parkinson disease Joanne Trinh and Matt Farrer Abstract Parkinson disease (PD) is a multifactorial neurodegenerative disease that was long considered the result of environmental factors. In the past 15 years, however, a genetic aetiology for PD has begun to emerge. Here, we review results from linkage and next-generation sequencing studies of familial parkinsonism, as well as candidate gene and genome-wide association findings in sporadic PD. In these studies, many of the genetic findings overlap, despite different designs and study populations, highlighting novel therapeutic targets. The molecular results delineate a sequence of pathological events whereby deficits in synaptic exocytosis and endocytosis, endosomal trafficking, lysosome-mediated autophagy and mitochondrial maintenance increase susceptibility to PD. These discoveries provide the rationale, molecular insight and research tools to develop neuroprotective and disease-modifying therapies. Trinh, J. & Farrer, M. Nat. Rev. Neurol. advance online publication 16 July 2013; doi: /nrneurol REVIEWS Introduction Parkinson disease (PD) is the most common neurodegenerative movement disorder that has an agedependent prevalence. Approximately 1% of the pop ulation is affected at 65 years, increasing to 4 5% in 85-year-old individuals. 1 The mean age of onset is 70 years, although 4% of patients develop early-onset disease, before the age of Whereas one in seven patients with idio pathic PD has a first-degree relative with parkinsonism, large multi-incident Mendelian pedigrees are relatively rare. Less than 10% of familial disease is currently ascribed to a single monogenic mutation. PD is generally considered a multifactorial disorder that arises owing to a combination of genes and environmental factors. Parkinsonism is characterized clinically by motor dysfunction that manifests as resting tremor, bradykinesia, rigidity and postural instability. 3 5 Nonmotor features such as dysautonomia, depression, sensory loss and sleep disturbance are difficult to treat and often manifest before motor disability. 6 Mild cognitive impairment and subsequent dementia is common. 7,8 Dopamine replacement therapy improves motor symptoms but does not slow disease progression and is associated with adverse effects such as motor fluctuations and dyskinesias. 9 For a neuropathological diagnosis of PD, neuronal loss in the substantia nigra pars compacta must be evident, accompanied by Lewy body pathology in surviving neurons. 10,11 Although the majority of idiopathic PD has Lewy body pathology at autopsy, this feature is neither exclusive to this disease nor common to all cases (Table 1). 12 Until 1997, the notion that heritability has a role in PD was contentious; indeed, PD was considered to be the archetypal nongenetic disorder. 13 The disease was thought to be caused by environmental factors largely due to the epidemic of postencephalitic parkinsonism Competing interests The authors declare no competing interests (PEP) after the First World War, 14 the discovery of 1 methyl 4-phenyl 1,2,5,6-tetrahydropyridine (MPTP)- induced parkinsonism 15 and the lack of diseaseconcordance in monozygotic twin studies. 16,17 However, a genetic basis for several forms of PD and related parkinsonian disorders has since been elucidated. This Review focuses on genetic findings in the more typical late-onset, levodopa-responsive Lewy body PD, as the majority of patients seen in movement disorder clinics have this phenotype. Molecular findings from Parkinson-plus syndromes, a group of heterogeneous neurological disorders with parkinsonian features, are included. The clinical features of such disorders include poor response to levodopa, distinct pathology and a poor prognosis that differs from classical PD. Understanding of these disorders could inform studies of idiopathic PD. Past discoveries, especially in monogenic families, seem to converge on three interconnected cellular processes: first, synaptic transmission (exo cytosis and endocytosis) and endosomal receptor sorting and recycling; second, lysosome-mediated autophagy; and third, mitochondrial quality control and stress response (Figure 1). The emerging synthesis provides a molecular foundation for hypothesis testing, pharmaceutical development and future clinical trials aimed at disease modification. Implicated genes Many genes have been implicated in PD, through three key approaches. The methods employed include linkage analysis, genome sequencing and genetic association, and each approach has consequences for how the results can be interpreted and applied (Box 1). SNCA SNCA encodes α synuclein, the key component of Lewy body inclusions, which are the hallmark of PD. Linkage Department of Medical Genetics, UBC Pavillion Hospital, 2 nd floor, Room S 132 Koerner Building, VCHRI, 2211 Wesbrook Mall, University of British Columbia, Vancouver, British Columbia V6T 2B5, Canada (J. Trinh, M. Farrer). Correspondence to: M. Farrer mfarrer@can.ubc.ca NATURE REVIEWS NEUROLOGY ADVANCE ONLINE PUBLICATION 1

2 Key points Parkinson disease (PD) is a multifactorial disorder that, in most cases, results from genetic and environmental factors Approaches to study the genetics of PD include linkage analysis, genome sequencing and association studies Many genes, mutations and polymorphisms have implicated in PD pathogenesis Genetic discoveries highlight biological processes and pathways that are consistently perturbed in idiopathic PD, although individual results are seldom useful for a patient diagnosis Identification of such genetic variability may inform the design of clinical trials and future therapeutic strategies analysis studies have identified PD associated mutations in SNCA that include the dominantly inherited substitutions Ala30Pro, Glu46Lys, His50Gln, Gly51Asp and Ala53Thr and genomic multiplications (specifically, duplications) in familial, late-onset Lewy body PD (Table 1) Additional SNCA copies lead to earlieronset and more fulminant Lewy body disease in which dementia is a prominent feature. 24,29 Association studies have found evidence for a link between single nucleotide polymorphisms (SNPs) in SNCA and idiopathic PD in all populations tested, despite population distinct haplotype frequencies and patterns of linkage disequilibrium (the association of two or more alleles with one another) Metaanalysis of several small association studies has provided convincing support for an association between PD and a mixed dinucleotide repeat (termed REP1) in the 5ʹ promoter of SNCA 35 that influences gene transcription in cell lines and in vivo Nevertheless, the 5ʹ REP1 association is unlikely to underlie associations observed between PD and SNPs in intron 4 and the 3ʹ untranslated region of SNCA, as the loci involved are in linkage equilibrium (that is, the loci are not correlated). Overall, findings suggest that additional functional variability, albeit in nonconding regions but possibly involved in regulation of gene expression, has yet to be discovered. Evidence for SNCA association extends to other neuro degenerative diseases in Table 1 Phenotypes associated with selected genes implicated in late-onset Lewy body PD Disease OMIM identifier Gene Mutations Age at onset (range) Dominantly inherited late-onset PD SNCA Missense: Ala30Pro, Glu46Lys, His50Gln, Gly51Asp, Ala53Thr 60 years (30 80) SNCA Locus duplication (and triplication) years (24 48) LRRK2 Missense: Asn1437His, Arg1441Cys/Gly/His, Tyr1699Cys, Gly2019Ser, Ile2020Thr Common polymorphisms: Ala419Val, Arg1628Pro, Gly2385Arg (Asia) Protective haplotype: Asn551Lys Arg1398His Lys1423Lys 60 years (32 79) VPS35 Missense: Asp620Asn 53 years (40 68) Juvenile and early-onset recessively inherited parkinsonism PARK2 Numerous missense, exon deletion and duplication mutations PINK1 Missense: Gln129X, Gln129fsX157, Pro196Leu, Gly309Asp Trp437X, Gly440Glu, Gln456X Rare: locus and exon deletion DJ 1 Missense: Glu163Lys, Leu166Pro Exon 1 5 deletion, g dup ATP13A2 Missense: Phe182Leu, Gly504Arg, Gly877Arg, 1019GfsX1021 Exon G>A Exon bp deletion <45 years (12 58) Typically <45 years (18 56) <40 years (24 39) <20 years (10 33) Synopsis of clinical features Predominant pathology References Levodopa-responsive parkinsonism Diffuse LBD 18 21,23 Levodopa-responsive parkinsonism, cognitive decline, autonomic dysfunction and dementia; progression more rapid in SNCA triplication cases Levodopa-responsive parkinsonism consistent with sporadic PD; occasionally dystonia, amyotrophy, gaze palsy and dementia Tremor-dominant levodopa-responsive parkinsonism, dyskinesia and dystonia, occasionally dementia Levodopa-responsive parkinsonism, often juvenile and typically slowly progressive Levodopa-responsive parkinsonism, often akinetic with postural instability/gait disturbance with slow progression; sleep benefit Levodopa-responsive parkinsonism, psychological and behavioural disturbances, amyotrophy and cognitive impairment Levodopa-responsive atypical parkinsonism associated with supranuclear gaze palsy, spasticity and dementia Abbreviations: fs, frameshift; LBD, Lewy body disease; OMIM, Online Mendelian Inheritance in Man; PD, Parkinson disease; X, stop codon. Diffuse LBD, with prominent nigral and hippocampal (CA2 3) neuronal loss Brainstem LBD, neurofibrillary tangle or TDP 43 pathology and/or nigral neuronal loss Inconclusive, possibly without LBD Predominantly nigral neuronal loss, occasionally with synuclein or tau pathology ,43,55 71,85 87, 89,90,159 75,78 One case with LBD 77,79,80 Unknown 81,82 Neuroradiological atrophy with iron accumulation in basal ganglia 115,116 2 ADVANCE ONLINE PUBLICATION

3 1 2 3 α-synuclein LRRK2 VPS GBA Parkin PINK DJ 1 ATP13A2 MAPT Presynaptic glutamatergic axon (excitatory) 9 Cortex or thalamus Nucleus Peroxisome 4 8 Endosome Lysosome Mitochondrion Presynaptic dopaminergic axon (inhibitory) 2 5 Glutamate Receptor 1 2 Clathrin 2 Medium spiny neuron Early endosome Golgi apparatus Multivesicular body 4 9 Microtubule 1 Dopamine Endosome Dendritic spine Figure 1 Key molecular processes implicated in parkinsonism by genetic findings and extrapolated in models of disease. An axon of a presynaptic glutamatergic cortical neuron (blue), a dopaminergic substantia nigra neuron (green), and a dendritic spine of a medium spiny neuron (yellow) are illustrated. In presynaptic terminals, α synuclein (1) promotes exocytosis and can play a part in endocytosis. 138, Postsynaptically, LRRK2 (2) regulates phosphorylation of endophilin A (and the release of clathrin-coated endocytic vesicles), 163,164 neuronal polarity and arborization. LRRK2 also has roles in chaperone-mediated autophagy, MAPT phosphorylation and microtubule stabilization. VPS35 (3) is an integral part of the retromer, a complex that mediates cargo recognition of early endosomes and membrane recruitment to form a clathrin-independent carrier Alternatively, cargoes may be destined for lysosomal degradation or exosome secretion. 68,165,168 VPS35 mediates recycling from endosomes to the Golgi apparatus or plasma membrane, 170 and vesicle transport between mitochondria and peroxisomes. 169 Lysosomal acid hydrolases, including GBA (4), also require the retromer for receptor recycling. Loss-of-function mutations in Parkin (5), PINK1 (6) and DJ 1 (7) affect mitochondrial biogenesis and induction of autophagy. Parkin is involved in ubiquitination and proteasomal function, and PINK1 and Parkin are involved in mitochondrial maintenance. ATP13A2 (8) has a role in lysosome-mediated autophagy. MAPT (9) helps to regulate cargo trafficking and delivery, primarily in axons. Abbreviations: GBA, glucocerebrosidase; LRRK2, leucine-rich repeat kinase 2; VPS35, vacuolar protein sorting 35. which α synucleinopathy is a feature, highlighting its importance as a therapeutic target. 11,39,40 LRRK2 Leucine-rich repeat kinase 2 (LRRK2) has well-defined GTPase and kinase functions, and multiple biological roles in striatal neurotransmission, neuronal arborization, endocytosis, autophagy and immunity. 41 Dominantly inherited PD associated mutations in LRRK2 include Asn1437His, Arg1441Cys/Gly/His, Tyr1699Cys, Gly2019Ser and Ile2020Thr. Patients with mutations in this gene have a clinical phenotype that most closely resembles idiopathic PD LRRK2 Gly2019Ser is especially frequent in PD patients of Ashkenazi Jewish or North African Arab-Berber origin, accounting for 13% and 30% of cases in these populations, respectively. 46,47 At autopsy, patients with LRRK2 parkinsonism typically have Lewy body or neurofibrillary tangle pathology, with nigral neuronal loss and gliosis. Pleomorphic pathology can be evident even within families with the same mutation. 42,48,49 Intracellular Lewy bodies and Lewy neurities, by definition the pathological hallmark of PD, are largely comprised of aggregated α synuclein. 11 Hence, LRRK2 parkinsonism challenges the dogma that pathogenesis should be defined according to end-stage neuropathology. Genome-wide association studies (GWAS) have highlighted and replicated LRRK2 as a PD associated locus, 50,51 although the indirect evidence from studies using haplotype tagging SNPs is less compelling than that for SNCA or microtubule-associated protein tau (MAPT, discussed below; Table 2). LRRK2 is a large, 51-exon gene close to the centromere, extensive sequen cing of which has revealed many synonymous and non-synonymous amino acid polymorphisms. 52,53 Results of direct genotyping and combined pooled analysis of these LRRK2 variants has recently been published by the Genetic Epidemiology of Parkinson s Disease consortium and Asian consortia, and shows that many variants are associated with PD. 54,55 LRRK2 Gly2385Arg provides a good illustration: this NATURE REVIEWS NEUROLOGY ADVANCE ONLINE PUBLICATION 3

4 Box 1 Approaches to genetic studies in Parkinson disease Linkage analysis Linkage analysis is the classic study of genetic markers and recombination events in multi-incident families (pedigrees) with disease. 163 The likelihood that a region of the genome (locus) and genetic markers segregate with disease is calculated. Sequencing of the linked locus is used to identify the specific gene mutation responsible. These results can inform a diagnosis in mutation carriers, and can be used to create animal models with the same defect to facilitate therapeutic advances. Historically, loci linked to Parkinson disease (PD) were given a PARK designation until the mutant gene was identified. Next-generation sequencing Exome and whole-genome sequencing technologies provide a reliable, expeditious and cost-effective approach to identify genetic variability in disease. It is accomplished by computational alignment and annotation of short-sequence reads, generally bases. Depending on the method of target-dna capture and extent of sequence coverage, exome sequencing generates ,000 nucleotide differences (2 5 variants per kb) from the human genome reference sequence. Studies include identification of genetic variants underlying Mendelian disorders and/or GWAS loci. 164 Alignment of short sequences to the reference genome presents challenges in assessing repetitive elements and copy number changes. For example, diseases associated with intermediate repeat expansions in several genes notably fragile X tremor/ataxia syndrome 165 and the spinocerebellar ataxias can manifest as PD. Association studies Association studies generally test the hypothesis that common variants cause common diseases, by comparing the frequencies of specific alleles between patients and controls. 125 Such genetic variability is not selected against, so the expected contribution of each variant to disease is low; consequently the sample sizes required to see effects must be large. Several genome-wide association studies of PD (Table 2) have replicated loci containing SNCA, MAPT, LRRK2, HLA- DRA, GAK DGKQ, RAB7L1 and BST1. 40,50,51,58,106, ,169 For SNCA, MAPT and LRRK2 the results concur with previous linkage and candidate gene studies. 170 Further investigation is typically required to identify the cause of risk. variant is specific to Asian populations, largely originating from one ancestral founder, and is found in 8% of patients with PD. Carriers of this variant have double the risk of PD compared with noncarriers. 54,56 Overall, among populations throughout the world, GWAS findings from the various consortia demonstrate that LRRK2 genetic variability confers significant risk of PD at the population level. The results also highlight inverse associations, whereby common LRRK2 genetic variants seem to contribute to healthy ageing of the basal ganglia. 54,55 MAPT Association studies have provided compelling evidence for a link between MAPT genetic variability and Table 2 Selected genome-wide association studies in Parkinson disease Gene Chromosome Population References SNCA 4q21 USA, UK, France, Japan 34,50,51,58, ,131, 160,161 MAPT 17q21.1 USA, UK, France 34,50,51,58, ,131, 161,162 LRRK2 12q12 USA, Japan 50,51 HLA-DRA 6q21.3 USA, UK 51,58,105,106 GAK DGKQ 4p16 USA, UK 34,51,58, PARK16 1q32 USA, UK, Japan 50,107,131 BST1 4p15 France, USA 50,107,161 idiopathic PD. 57,58 There are two major MAPT haplotypes: H1 and H2. The major H1 allele results from an ancestral paracentric inversion (where DNA breakpoints are in one arm of a chromosome) and spans a 1.5 Mb haplotype that is overrepresented in patients with PD compared with controls in white populations. 59,60 Importantly, the most significant associations of MAPT H1 and an H1-subtype (H1c; defined by the major allele of rs242557) in neuro degeneration are with progressive supranuclear palsy, cortico basal degeneration and Parkinson dementia complex of Guam These disorders are rare forms of parkinsonism defined by their primary neurofibrillary tangle pathologies consisting of hyperphosphorylated 4R tau a tau protein isoform with four microtubule-binding domains that results from alternative gene splicing and inclusion of MAPT exon 10. Postmortem studies of Lewy body disease in which patients had a longitudinal clinical diagnosis of PD have also observed a MAPT H1 association. 65 Surprisingly, in Alzheimer disease, which involves neurofibrillary tangle pathology associated with 3R and 4R tau, the evidence for a MAPT association is less compelling. 66,67 As for the pleomorphic pathology observed in LRRK2 parkinsonism, MAPT might have distinct and/or alternative roles in pathogenesis and pathology, and might depend on other modifiers that a GWAS of postmortem progressive supranuclear palsy may yet illuminate. 64 In addition, MAPT is not the only neuronally expressed gene affected by the H1 haplo type; several others that are within or directly flank the 5ʹ and 3ʹ breakpoints might yet be implicated in these disparate diagnoses. 68 EIF4G1 A dominantly inherited Arg1205His mutation in eukaryotic translation initiation factor 4 gamma 1 (EIF4G1) has been linked to late-onset Lewy body PD. 69 However, several unaffected carriers of this mutation have since been identified, including one aged 86 years. 70,71 Potential explanations for this discrepancy include age-associated and/or incomplete penetrance of the mutation; EIF4G1 Arg1205His could be a rare variant that is simply in close proximity to the disease-associated mutation; or the original linkage assignment could be in error. Support for the role of EIF4G1 variation in PD remains equivocal, and further studies to confirm or refute the assignment are warranted. PINK1, PARK2 and PARK7 PINK1 and parkin (also known as PARK2) are important in mitophagy, whereas DJ 1 (also known as PARK7) is a redox sensor of oxidative stress. PINK1, a cytoplasmic but mitochondrially associated protein kinase, acts upstream of Parkin, an E3 ubiquitin ligase that facilitates the degradation of damaged mitochondria Early-onset ( 45 years of age at diagnosis) or juvenile parkinsonism ( 20 years of age at diagnosis) accounts for less than 4% of PD in the community, for which lossof-function of parkin explains a considerable fraction (~15%) of sporadic cases. 75 At autopsy, patients with PARK2-associated PD have nigral neuronal loss, and are 4 ADVANCE ONLINE PUBLICATION

5 mostly without Lewy body pathology, but compound heterozygotes (that is, carriers of two different diseaseassociated alleles at a given locus) can have Lewy body or tau pathology. 30,76 To date, the only autopsy case of PINK1-related parkinsonism had Lewy body disease, 77 and the pathology associated with PARK7-related PD has yet to be reported. By linkage analysis and Sanger sequencing, recessively inherited mutations (homozygous or compound heterozygous loss of function) were identified most frequently in PARK2, 75,78 occasionally in PINK1 77,79,80 and rarely in PARK7. 81,82 VPS35 Vacuolar protein sorting 35 (VPS35) is a critical component of the retromer system that mediates intracellular retrograde transport of endosomes to the trans-golgi network. 83,84 VPS35 Asp620Asn has been shown, via next-generation sequencing (Box 1), to cause lateonset autosomal dominantly inherited parkinsonism. Comparative analysis of exomes from pairs of affected cousins was used to shortlist shared, albeit rare, heterozygous coding substitutions for validation by Sanger sequencing. Only VPS35 Asp620Asn was found to segregate with PD in a Mendelian dominant fashion. 85,86 Nevertheless, the genetic burden of proof required several more thousands of PD and control individuals to be assessed, the discovery of mutation(s) in additional affected probands rather than controls, and segregation of the mutation with disease within those pedigrees. 85 Whereas VPS35 Asp620Asn is inherited with disease within families, between families this mutation seems to have arisen de novo DNAJ genes Next-generation sequencing was used to identify a pathogenic mutation (Asn855Ser) in DNAJC13 in a Dutch German Russian Mennonite kindred with late-onset Lewy body PD. The pedigree is extensive, with affected carriers in at least five Canadian provinces, but proof of pathogenicity relies on support from bioinformatic and functional studies. Further work is needed to identify additional families harbouring DNAJC13 mutations. Until then, some caution is warranted in interpreting the existing evidence, as the mutation could reflect a unique, benign variant originating from a common founder that does not influence disease. Through a combination of homozygosity mapping and next generation sequencing, recessively inherited loss-of-function mutations in juvenile onset parkinsonism have been identified in auxilin (DNAJC6), 91 and the assignment subsequently validated in other families. 87,92,93 GBA and GBAP1 Gaucher disease is largely a consequence of homozygous or compound heterozygous inheritance of Asn370Ser or Leu444Pro mutations in glucocerebrosidase (GBA). 94,95 GBA Asn370Ser is common in Ashkenazi Jews, with a carrier frequency of 5%. 96 Detailed clinical and pathological studies of Gaucher disease type III often noted parkinsonism and Lewy body pathology. 97 Combined, pooled analysis of variability in the GBA locus subsequently provided unequivocal support for an association, 98,99 and the findings have now been extended to postmortem Lewy body disease. 100,101 The neighbouring pseudogene (GBAP1) typically has Ser370 and Pro444 alleles, which might reduce the signal to noise ratio in GWAS, potentially explaining why GBA has not been identified as a PD associated gene in such studies. Glucosidase activity can modulate ceramide metabolism, α synuclein processing and potentially α synucleinopathy, and has quickly become an attractive therapeutic target. 102 HLA-DR genes Neuroinflammation and innate immunity have long been postulated to have a role in dopaminergic neuronal degeneration, but genetic support has been lacking. 103 HLA-DRA and HLA-DRB encode the α and β chains, respectively, of major histocompatibility complex class II DR a cell-surface molecule that interacts with T cell receptors and has been implicated in auto immune diseases. 103 Further highlighting the potential role of inflammation in PD, HLA-DRA and HLA-DRB were found to be associated with PD through GWAS. 104,105 Membrane-associated genes The cyclin G associated kinase diaclyglycerol kinase (GAK DGKQ) locus has been nominated by several independent GWAS in PD. 34,51, The genomic region consists of one linkage disequilibrium block containing three genes for which the genetic and biological evidence of involvement in PD is equally compelling. GAK is the widely expressed homologue of neuronal auxillin (DNAJC6) a protein that is involved in clathrinmediated endocytosis. DGKQ and phosphatidylinositol kinase (PIK3CD) were nominated in a GWAS of familial PD. 34,108 Both of the genes encode proteins that are central to membrane curvature and signalling, suggesting impairment to these processes could be involved in PD. The PARK16 locus contains five genes in which variability is associated with susceptibility to PD. Within this locus, RAB7L1 has been most studied owing to its expression in the brain and alternative exon splicing (Table 2). Notably, RAB7L1 has been shown to interact with LRRK2 and VPS35, and seems to play a part in endosomal lysosomal trafficking. 109 Bone marrow stromal cell antigen 1 (BST1) is a cell-surface glycosylphosphatidylinositol anchored glycoprotein that mediates neutrophil adhesion and migration. A 15-kb linkage disequilibrium block was nominated in a GWAS study, and BST1 is the only gene identified. 50 Further studies are needed to elucidate the role of BST1 in PD and how it may interact in a common disease pathway. Gene environment interactions Few genome-wide studies have investigated the joint contribution of genetic and environmental risk factors such as smoking, consumption of caffeine and alcohol (all inversely associated with PD risk) or exposure to herbicides or pesticides to development of PD. Initial genome-wide interaction studies have highlighted SV2C NATURE REVIEWS NEUROLOGY ADVANCE ONLINE PUBLICATION 5

6 variability and smoking in PD, whereby the risk associated with smoking varied according to SV2C genotype. 110 SV2C encodes a synaptic-vesicle protein in the substantia nigra that is involved in storage and release of neurotransmitters including dopamine. GRIN2A encodes an N methyl d-aspartate-glutamate receptor subunit that regulates excitatory neurotransmission. Stratification of patients according to coffee consumption highlighted an interaction between GRIN2A genetic variability and PD. 111 Genetics of Parkinson-plus syndromes Through linkage analysis, the genetic bases of several Parkinson-plus syndromes have also been found. Worthy of note are dominant mutations in MAPT in parkinsonism and frontotemporal dementia, 112 and dynactin 1 CAP-Gly mutations in Perry syndrome 113 a TDP-proteinopathy formerly linked to familial motor neuron disease. 114 Other recessively inherited conditions in which parkinsonism is prominent include Kufor Rakeb syndrome due to loss of ATP13A2, 115,116 neuroaxonal dystrophy from loss of PLA2G6 117,118 and juvenile-onset pallido-pyramidal parkinsonism due to loss of FBXO7. 119,120 Loss of ATP6A2 has been identified in males with X linked parkinsonism with spasticity. 121 Molecular advances Disease modification and neuroprotection to slow or halt PD progression, or to affect a cure, are likely to require a detailed understanding of molecular aetiology and pathogenesis. In PD, as in most other brain disorders, genetic analysis represents a relatively un biased and noninvasive method to elucidate risk factors. Genetic results provide an unequivocal foundation for subsequent translational neuroscience and therapeutic development. Past heritability estimates in PD, however, largely refute a genetic contribution to disease. 16,17,122 Heritability is usually defined as the proportion of total phenotypic variation that is due to additive genetic factors, the other major component of variance being typically ascribed to environmental influences. In cross-sectional twin studies involving a clinical diagnosis of PD, monozygotic pairs were only concordant in a minority of cases in which disease onset began before the age of 50 years. 16 By contrast, longitudinal studies with or without neuroimaging suggested genetic factors have a greater role. 17,123,124 Heritability estimates from various GWAS concur, 51 although many SNPs with suggestive association are overlooked, as they do not meet stringent genome-wide significance, or may not adequately tag intervening genomic variability. 125 These limitations call for larger sample sizes, genome sequencing rather than GWAS, and a longitudinal clinical diagnosis of PD and its trait components (which might also be associated with specific genetic variability). Lessons from monogenetic PD In approximately 1% of patients, rare monogenetic mutations have been identified that greatly enhance disease risk for late-onset Lewy body PD (Figure 1). Although penetrance remains a function of age, in certain communities such as Ashkenazi Jews and Arab-Berbers, mutation carrier status can be determined and risk estimated in a substantial proportion. On a defined genetic background the influence of environmental agents, including prescribed and novel drug therapies, might be more meaningfully assessed. 126 PD genes and the proteins they encode are immediate targets for intervention strategies, as illustrated by studies on α synuclein: research findings can contribute to accurate diagnosis 18,24,28,127 that is relevant to idiopathic disease, 10,40,50, development of biomarkers for early disease detection, 10,132 model generation, and design of primary prevention strategies. 136,137 Similarly, genetic studies can inform clinical trial design, involving fewer patients at lower cost, by targeting genetically defined subgroups of patients who are most likely to benefit. 138 An integrated approach Results from genetic studies may be more relevant and generalizable to idiopathic PD than are findings from toxin-induced lesion models alone. An integrated molecular synthesis could prove insightful, but some caution is warranted. PD varies with age, and environmental factors are important contributors to disease risk. Several monogenic risk factors have yet to be elucidated, and those identified are extremes contributing to background susceptibility rather than the cause. Nevertheless, an important role for interconnected processes including protein homeostasis, autophagy lysosomal pathways, and mitochondrial quality control and stress response in PD has become apparent. 72,139,140 Autophagy lysosomal pathways Several genetic findings have highlighted an association between PD and autophagy lysosomal pathways, which remove abnormal proteins. Pathogenic mutations in ATP13A2, a lysosomal ATPase, lead to impaired protein degradation. Similarly, GBA deficiency reduces lysosomal function and leads to accumulation of α synuclein. 141 LRRK2 Gly2019Ser has been shown to interfere with chaperone-mediated autophagy in neurons, and to enhance co-localization of α synuclein with LAMP Moreover, mutations in ATP6AP2 a gene required for receptor-mediated endocytosis, membrane trafficking and lysosomal degradation cause X linked parkinsonism with spasticity. 121 Whether increased autophagy is caused by PD or represents a compensatory mechanism to enhance degradation of abnormal proteins remains to be elucidated. Mutant PARK7, 81 PINK1 79 and PARK2 78 contribute to mitochondrial dysfunction and oxidative stress. PINK1 and Parkin work in a common pathway to regulate mitophagy, 72 whereas DJ 1 is a redox sensor. 73 In addition, Parkin has been shown to ubiquitinate many proteins involved in signal transduction at synapses 74 or proteins bound for proteosomal degradation. 143 Key pathways in late-onset PD In late-onset Lewy body PD, dysfunction in synaptic transmission (exocytosis and endocytosis) and sorting and recycling of endosomal receptors seems to be a convergent 6 ADVANCE ONLINE PUBLICATION

7 theme from available, albeit limited, genetic findings. Genes implicated in juvenile and/or early-onset parkinsonism and more atypical Parkinson syndromes may impinge on the same pathway. Mutant proteins only subtly perturb the cellular ecology, promoting susceptibility to insidious but chronically progressive neuro degeneration. The selective vulnerability of dopaminergic neurons may reflect the recycling pathways of specific proteins and vesicular cargoes, such as dopamine receptors, and the dense axonal arbor that dopaminergic neurons must maintain. 144 Notably, however, each protein could have multiple functions and work together in a network, as vesicle formation and trafficking is crucial to numerous cellular processes. Furthermore, caution must be exercised in extrapolating from study findings in the field, as much of the functional analysis is nonphysiological, being derived from non-neuronal cells and model systems. Nevertheless, with each major gene discovered in PD, the emerging synthesis is providing important insights into the normal proteins required for healthy ageing of the basal ganglia. Moreover, these discoveries can identify genetically predisposed individuals in whom investigations might best inform future clinical trials and novel intervention strategies aimed at disease modification. Conclusion Over the past 15 years, the identification of risk loci, genes and mutations in PD has provided new insights into disease aetiology and highlighted new study approaches. In sporadic parkinsonism and familial PD, many genetic findings concur, suggesting that they could be generalizable to the wider population. Several biological processes involved in PD pathogenesis have been highlighted, and the discovery of novel PD associated genes in families with Mendelian disease has been particularly informative in this regard. Each discovery has defined a major theme for translational neuroscience. For example, the discovery of α synuclein as a key component of Lewy bodies highlighted protein aggregation and propagation, 11,145 and the discovery of parkin highlighted protein ubiquitination and the proteosome. Each discovery generally led to a change and/or replacement of focus. Recently, some pathways have emerged that relate to mitochondrial metabolism (PINK1, PARK2) and lysosomal-autophagy (ATP13A2, GBA, LRRK2). Nevertheless, the ultimate focus must be on late-onset Lewy body PD as, clinically and pathologically, this phenotype describes the vast majority of patients. On the basis of the most recent genetic findings on DNAJC6, DNAJC13, VPS35, SNCA and LRRK2, we postulate a unifying synthesis whereby deficits in synaptic exocytosis and endocytosis and in endosomal trafficking place demands on autophagy lysosomal pathways and mitochondrial metabolism, leading to neuronal dysfunction and loss. With the advent of nextgeneration sequencing, we anticipate genetic advances in PD will continue to flourish, and our understanding of the molecular mechanisms underlying susceptibility, progression and response to treatment will continue to evolve. The emerging synthesis of those findings will provide the rationale for therapeutics aimed at neuroprotection. Review criteria We searched the US National Library of Medicine for articles published from January 1996 to March 2013, using search terms including Parkinson, genetic, atypical Parkinsonism, Lewy body, GWAS, LRRK2, SNCA, MAPT, GBA, VPS35, PINK1, PRKN and DJ1. Only English-language articles were considered. Abstracts included were peer-reviewed and presented at the Movement Disorders Society and American Society of Human Genetics meetings. 1. de Lau, L. M. & Breteler, M. M. Epidemiology of Parkinson s disease. Lancet Neurol. 5, (2006). 2. Schrag, A. & Schott, J. M. Epidemiological, clinical, and genetic characteristics of earlyonset parkinsonism. Lancet Neurol. 5, (2006). 3. Lang, A. E. & Lozano, A. M. Parkinson s disease. Second of two parts. N. Engl. J. Med. 339, (1998). 4. Lang, A. E. & Lozano, A. M. Parkinson s disease. First of two parts. N. Engl. J. Med. 339, (1998). 5. Fahn, S. Description of Parkinson s disease as a clinical syndrome. Ann. N. Y. Acad. Sci. 991, 1 14 (2003). 6. Langston, J. W. The Parkinson s complex: parkinsonism is just the tip of the iceberg. Ann. Neurol. 59, (2006). 7. Hely, M. A., Reid, W. G., Adena, M. A., Halliday, G. M. & Morris, J. G. The Sydney multicenter study of Parkinson s disease: the inevitability of dementia at 20 years. Mov. Disord. 23, (2008). 8. Litvan, I. et al. Diagnostic criteria for mild cognitive impairment in Parkinson s disease: Movement Disorder Society Task Force guidelines. Mov. Disord. 27, Fahn, S. The spectrum of levodopa-induced dyskinesias. Ann. Neurol. 47, S2 S9 (2000). 10. Spillantini, M. G. et al. α-synuclein in Lewy bodies. Nature 388, (1997). 11. Goedert, M., Spillantini, M. G., Del Tredici, K. & Braak, H. 100 years of Lewy pathology. Nat. Rev. Neurol. 9, (2013). 12. Halliday, G. M., Holton, J. L., Revesz, T. & Dickson, D. W. Neuropathology underlying clinical variability in patients with synucleinopathies. Acta Neuropathol. 122, (2011). 13. Farrer, M. J. Genetics of Parkinson disease: paradigm shifts and future prospects. Nat. Rev. Genet. 7, (2006). 14. Casals, J., Elizan, T. S. & Yahr, M. D. Postencephalitic parkinsonism a review. J. Neural Transm. 105, (1998). 15. Langston, J. W., Ballard, P., Tetrud, J. W. & Irwin, I. Chronic Parkinsonism in humans due to a product of meperidine-analog synthesis. Science 219, (1983). 16. Tanner, C. M. et al. Parkinson disease in twins: an etiologic study. JAMA 281, (1999). 17. Wirdefeldt, K., Gatz, M., Reynolds, C. A., Prescott, C. A. & Pedersen, N. L. Heritability of Parkinson disease in Swedish twins: a longitudinal study. Neurobiol. Aging 32, 1923.e e8 (2011). 18. Polymeropoulos, M. H. et al. Mutation in the α-synuclein gene identified in families with Parkinson s disease. Science 276, (1997). 19. Kruger, R. et al. Ala30Pro mutation in the gene encoding α-synuclein in Parkinson s disease. Nat. Genet. 18, (1998). 20. Zarranz, J. J. et al. The new mutation, E46K, of α-synuclein causes Parkinson and Lewy body dementia. Ann. Neurol. 55, (2004). 21. Proukakis, C. et al. A novel α-synuclein missense mutation in Parkinson disease. Neurology 80, (2013). 22. Appel-Cresswell, S. et al. Alpha-synuclein p.h50q, a novel pathogenic mutation for Parkinson disease. Mov. Disord. dx.doi.org/ /mds Lesage, S. et al. G51D α-synuclein mutation causes a novel parkinsonian-pyramidal syndrome. Ann. Neurol. 73, (2013). 24. Singleton, A. B. et al. α-synuclein locus triplication causes Parkinson s disease. Science 302, 841 (2003). 25. Chartier-Harlin, M. C. et al. α-synuclein locus duplication as a cause of familial Parkinson s disease. Lancet 364, (2004). 26. Ibáñez, P. et al. Causal relation between α-synuclein gene duplication and familial NATURE REVIEWS NEUROLOGY ADVANCE ONLINE PUBLICATION 7

8 Parkinson s disease. Lancet 364, (2004). 27. Fuchs, J. et al. Phenotypic variation in a large Swedish pedigree due to SNCA duplication and triplication. Neurology 68, (2007). 28. Nishioka, K. et al. Expanding the clinical phenotype of SNCA duplication carriers. Mov. Disord. 24, (2009). 29. Ross, O. A. et al. Genomic investigation of α-synuclein multiplication and parkinsonism. Ann. Neurol. 63, (2008). 30. Farrer, M. et al. Lewy bodies and parkinsonism in families with parkin mutations. Ann. Neurol. 50, (2001). 31. Pals, P. et al. α-synuclein promoter confers susceptibility to Parkinson s disease. Ann. Neurol. 56, (2004). 32. Winkler, S. et al. α-synuclein and Parkinson disease susceptibility. Neurology 69, (2007). 33. Rajput, A. et al. α-synuclein polymorphisms are associated with Parkinson s disease in a Saskatchewan population. Mov. Disord. 24, (2009). 34. Pankratz, N. et al. Genomewide association study for susceptibility genes contributing to familial Parkinson disease. Hum. Genet. 124, (2009). 35. Maraganore, D. M. et al. Collaborative analysis of α-synuclein gene promoter variability and Parkinson disease. JAMA 296, (2006). 36. Chiba-Falek, O., Touchman, J. W. & Nussbaum, R. L. Functional analysis of intraallelic variation at NACP-Rep1 in the α-synuclein gene. Hum. Genet. 113, (2003). 37. Chiba-Falek, O., Kowalak, J. A., Smulson, M. E. & Nussbaum, R. L. Regulation of α-synuclein expression by poly (ADP ribose) polymerase 1 (PARP 1) binding to the NACP-Rep1 polymorphic site upstream of the SNCA gene. Am. J. Hum. Genet. 76, (2005). 38. Cronin, K. D. et al. Expansion of the Parkinson disease-associated SNCA-Rep1 allele upregulates human α-synuclein in transgenic mouse brain. Hum. Mol. Genet. 18, (2009). 39. Nishioka, K. et al. Association of α-, β-, and γ-synuclein with diffuse Lewy body disease. Arch. Neurol. 67, (2010). 40. Scholz, S. W. et al. SNCA variants are associated with increased risk for multiple system atrophy. Ann. Neurol. 65, (2009). 41. Cookson, M. R. Cellular effects of LRRK2 mutations. Biochem. Soc. Trans. 40, Zimprich, A. et al. Mutations in LRRK2 cause autosomal-dominant parkinsonism with pleomorphic pathology. Neuron 44, (2004). 43. Paisán-Ruíz, C. et al. Cloning of the gene containing mutations that cause PARK8-linked Parkinson s disease. Neuron 44, (2004). 44. Kachergus, J. et al. Identification of a novel LRRK2 mutation linked to autosomal dominant parkinsonism: evidence of a common founder across European populations. Am. J. Hum. Genet. 76, (2005). 45. Aasly, J. O. et al. Novel pathogenic LRRK2 p.asn1437his substitution in familial Parkinson s disease. Mov. Disord. 25, (2010). 46. Ozelius, L. J. et al. LRRK2 G2019S as a cause of Parkinson s disease in Ashkenazi Jews. N. Engl. J. Med. 354, (2006). 47. Lesage, S. et al. G2019S LRRK2 mutation in French and North African families with Parkinson s disease. Ann. Neurol. 58, (2005). 48. Ross, O. A. et al. Lrrk2 and Lewy body disease. Ann. Neurol. 59, (2006). 49. Ujiie, S. et al. LRRK2 I2020T mutation is associated with tau pathology. Parkinsonism Relat. Disord. 18, Satake, W. et al. Genome-wide association study identifies common variants at four loci as genetic risk factors for Parkinson s disease. Nat. Genet. 41, (2009). 51. Do, C. B. et al. Web-based genome-wide association study identifies two novel loci and a substantial genetic component for Parkinson s disease. PLoS Genet. 7, e (2011). 52. Mata, I. F. et al. Lrrk2 pathogenic substitutions in Parkinson s disease. Neurogenetics 6, (2005). 53. Rubio, J. P. et al. Deep sequencing of the LRRK2 gene in 14,002 individuals reveals evidence of purifying selection and independent origin of the p.arg1628pro mutation in Europe. Hum. Mutat. 33, Tan, E. K. et al. Multiple LRRK2 variants modulate risk of Parkinson disease: a Chinese multicenter study. Hum. Mutat. 31, (2010). 55. Ross, O. A. et al. Association of LRRK2 exonic variants with susceptibility to Parkinson s disease: a case control study. Lancet Neurol. 10, (2011). 56. Farrer, M. J. et al. Lrrk2 G2385R is an ancestral risk factor for Parkinson s disease in Asia. Parkinsonism Relat. Disord. 13, (2007). 57. Vandrovcova, J. et al. Disentangling the role of the tau gene locus in sporadic tauopathies. Curr. Alzheimer Res. 7, (2010). 58. Pankratz, N. et al. Meta-analysis of Parkinson s disease: identification of a novel locus, RIT2. Ann. Neurol. 71, Skipper, L. et al. Linkage disequilibrium and association of MAPT H1 in Parkinson disease. Am. J. Hum. Genet. 75, (2004). 60. Zody, M. C. et al. Evolutionary toggling of the MAPT 17q21.31 inversion region. Nat. Genet. 40, (2008). 61. Baker, M. et al. Association of an extended haplotype in the tau gene with progressive supranuclear palsy. Hum. Mol. Genet. 8, (1999). 62. Pittman, A. M. et al. Linkage disequilibrium fine mapping and haplotype association analysis of the tau gene in progressive supranuclear palsy and corticobasal degeneration. J. Med. Genet. 42, (2005). 63. Sundar, P. D. et al. Two sites in the MAPT region confer genetic risk for Guam ALS/PDC and dementia. Hum. Mol. Genet. 16, (2007). 64. Hoglinger, G. U. et al. Identification of common variants influencing risk of the tauopathy progressive supranuclear palsy. Nat. Genet. 43, (2011). 65. Williams-Gray, C. H. et al. The distinct cognitive syndromes of Parkinson s disease: 5 year follow-up of the CamPaIGN cohort. Brain 132, (2009). 66. Naj, A. C. et al. Common variants at MS4A4/ MS4A6E, CD2AP, CD33 and EPHA1 are associated with late-onset Alzheimer s disease. Nat. Genet. 43, (2011). 67. Hollingworth, P. et al. Common variants at ABCA7, MS4A6A/MS4A4E, EPHA1, CD33 and CD2AP are associated with Alzheimer s disease. Nat. Genet. 43, (2011). 68. Bekpen, C., Tastekin, I., Siswara, P., Akdis, C. A. & Eichler, E. E. Primate segmental duplication creates novel promoters for the LRRC37 gene family within the 17q21.31 inversion polymorphism region. Genome Res. 22, Chartier-Harlin, M. C. et al. Translation initiator EIF4G1 mutations in familial Parkinson disease. Am. J. Hum. Genet. 89, (2011). 70. Schulte, E. C. et al. Variants in eukaryotic translation initiation factor 4G1 in sporadic Parkinson s disease. Neurogenetics 13, Nuytemans, K. et al. Whole exome sequencing of rare variants in EIF4G1 and VPS35 in Parkinson disease. Neurology 80, (2013). 72. Narendra, D., Walker, J. E. & Youle, R. Mitochondrial quality control mediated by PINK1 and Parkin: links to parkinsonism. Cold Spring Harbor Perspect. Biol. 4, a Kahle, P. J., Waak, J. & Gasser, T. DJ 1 and prevention of oxidative stress in Parkinson s disease and other age-related disorders. Free Radic. Biol. Med. 47, (2009). 74. Fallon, L. et al. A regulated interaction with the UIM protein Eps15 implicates parkin in EGF receptor trafficking and PI(3)K-Akt signalling. Nat. Cell Biol. 8, (2006). 75. Abbas, N. et al. A wide variety of mutations in the parkin gene are responsible for autosomal recessive parkinsonism in Europe. Hum. Mol. Genet. 8, (1999). 76. van de Warrenburg, B. P. et al. Clinical and pathologic abnormalities in a family with parkinsonism and parkin gene mutations. Neurology 56, (2001). 77. Samaranch, L. et al. PINK1-linked parkinsonism is associated with Lewy body pathology. Brain 133, (2010). 78. Kitada, T. et al. Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism. Nature 392, (1998). 79. Valente, E. M. et al. Hereditary early-onset Parkinson s disease caused by mutations in PINK1. Science 304, (2004). 80. Ishihara-Paul, L. et al. PINK1 mutations and parkinsonism. Neurology 71, (2008). 81. Bonifati, V. et al. Mutations in the DJ 1 gene associated with autosomal recessive early-onset parkinsonism. Science 299, (2003). 82. Annesi, G. et al. DJ 1 mutations and parkinsonism dementia amyotrophic lateral sclerosis complex. Ann. Neurol. 58, (2005). 83. Attar, N. & Cullen, P. J. The retromer complex. Adv. Enzyme Regul. 50, (2010). 84. McGough, I. J. & Cullen, P. J. Recent advances in retromer biology. Traffic 12, (2011). 85. Vilarino-Guell, C. et al. VPS35 mutations in Parkinson disease. Am. J. Hum. Genet. 89, (2011). 86. Zimprich, A. et al. A mutation in VPS35, encoding a subunit of the retromer complex, causes lateonset Parkinson disease. Am. J. Hum. Genet. 89, (2011). 87. Lesage, S. et al. Identification of VPS35 mutations replicated in French families with Parkinson disease. Neurology 78, Sharma, M. et al. A multi-centre clinico-genetic analysis of the VPS35 gene in Parkinson disease indicates reduced penetrance for diseaseassociated variants. J. Med. Genet. 49, Ando, M. et al. VPS35 mutation in Japanese patients with typical Parkinson s disease. Mov. Disord. 27, Kumar, K. R. et al. Frequency of the D620N mutation in VPS35 in Parkinson disease. Arch. Neurol. 69, Edvardson, S. et al. A deleterious mutation in DNAJC6 encoding the neuronal-specific clathrin- 8 ADVANCE ONLINE PUBLICATION

9 uncoating co-chaperone auxilin, is associated with juvenile parkinsonism. PLoS ONE 7, e Köroğlu, C., Baysal, L., Cetinkaya, M., Karasoy, H. & Tolun, A. DNAJC6 is responsible for juvenile parkinsonism with phenotypic variability. Parkinsonism Relat. Disord. 19, Vauthier, V. et al. Homozygous deletion of an 80 kb region comprising part of DNAJC6 and LEPR genes on chromosome 1P31.3 is associated with early onset obesity, mental retardation and epilepsy. Mol. Genet. Metab. 106, Diaz, A. et al. Gaucher disease: the N370S mutation in Ashkenazi Jewish and Spanish patients has a common origin and arose several thousand years ago. Am. J. Hum. Genet. 64, (1999). 95. Tsuji, S. et al. A mutation in the human glucocerebrosidase gene in neuronopathic Gaucher s disease. N. Engl. J. Med. 316, (1987). 96. Horowitz, M. et al. Prevalence of glucocerebrosidase mutations in the Israeli Ashkenazi Jewish population. Hum. Mutat. 12, (1998). 97. Sidransky, E. & Lopez, G. The link between the GBA gene and parkinsonism. Lancet Neurol. 11, Aharon-Peretz, J., Rosenbaum, H. & Gershoni- Baruch, R. Mutations in the glucocerebrosidase gene and Parkinson s disease in Ashkenazi Jews. N. Engl. J. Med. 351, (2004). 99. Sidransky, E. et al. Multicenter analysis of glucocerebrosidase mutations in Parkinson s disease. N. Engl. J. Med. 361, (2009) Farrer, M. J. et al. Glucosidase-beta variations and Lewy body disorders. Parkinsonism Relat. Disord. 15, (2009) Tsuang, D. et al. GBA mutations increase risk for Lewy body disease with and without Alzheimer disease pathology. Neurology 79, Sardi, S. P., Singh, P., Cheng, S. H., Shihabuddin, L. S. & Schlossmacher, M. G. Mutant GBA1 expression and synucleinopathy risk: first insights from cellular and mouse models. Neurodegener. Dis. 10, Tansey, M. G. & Goldberg, M. S. Neuroinflammation in Parkinson s disease: its role in neuronal death and implications for therapeutic intervention. Neurobiol. Dis. 37, (2010) Ahmed, I. et al. Association between Parkinson s disease and the HLA-DRB1 locus. Mov. Disord. 27, Hamza, T. H. et al. Common genetic variation in the HLA region is associated with late-onset sporadic Parkinson s disease. Nat. Genet. 42, (2010) Simón-Sánchez, J. et al. Genome-wide association study confirms extant PD risk loci among the Dutch. Eur. J. Hum. Genet. 19, (2011) UK Parkinson s Disease Consortium & Wellcome Trust Case Control Consortium. Dissection of the genetics of Parkinson s disease identifies an additional association 5ʹ of SNCA and multiple associated haplotypes at 17q21. Hum. Mol. Genet. 20, (2011) Lee, D. W., Wu, X., Eisenberg, E. & Greene, L. E. Recruitment dynamics of GAK and auxilin to clathrin-coated pits during endocytosis. J. Cell Sci. 119, (2006) Macleod, D. A. et al. RAB7L1 interacts with LRRK2 to modify intraneuronal protein sorting and Parkinson s disease risk. Neuron 77, (2013) Hill-Burns, E. M. et al. A genetic basis for the variable effect of smoking/nicotine on Parkinson s disease. Pharmacogenomics J Hamza, T. H. et al. Genome-wide geneenvironment study identifies glutamate receptor gene GRIN2A as a Parkinson s disease modifier gene via interaction with coffee. PLoS Genet. 7, e (2011) Hutton, M. et al. Association of missense and 5ʹ splice site mutations in tau with the inherited dementia FTDP 17. Nature 393, (1998) Farrer, M. J. et al. DCTN1 mutations in Perry syndrome. Nat. Genet. 41, (2009) Puls, I. et al. Mutant dynactin in motor neuron disease. Nat. Genet. 33, (2003) Ramirez, A. et al. Hereditary parkinsonism with dementia is caused by mutations in ATP13A2, encoding a lysosomal type 5 P type ATPase. Nat. Genet. 38, (2006) Di Fonzo, A. et al. ATP13A2 missense mutations in juvenile parkinsonism and young onset Parkinson disease. Neurology 68, (2007) Morgan, N. V. et al. PLA2G6, encoding a phospholipase A2, is mutated in neurodegenerative disorders with high brain iron. Nat. Genet. 38, (2006) Paisán-Ruíz, C. et al. Characterization of PLA2G6 as a locus for dystonia-parkinsonism. Ann. Neurol. 65, (2009) Shojaee, S. et al. Genome-wide linkage analysis of a Parkinsonian-pyramidal syndrome pedigree by 500 K SNP arrays. Am. J. Hum. Genet. 82, (2008) Di Fonzo, A. et al. FBXO7 mutations cause autosomal recessive, early-onset parkinsonianpyramidal syndrome. Neurology 72, (2009) Korvatska, O. et al. Altered splicing of ATP6AP2 causes X linked parkinsonism with spasticity (XPDS). Hum. Mol. Genet /hmg/ddt Simon, D. K., Lin, M. T. & Pascual-Leone, A. Nature versus nurture and incompletely penetrant mutations. J. Neurol. Neurosurg. Psychiatry 72, (2002) Dickson, D. et al. Pathology of PD in monozygotic twins with a 20-year discordance interval. Neurology 56, (2001) Piccini, P., Burn, D. J., Ceravolo, R., Maraganore, D. & Brooks, D. J. The role of inheritance in sporadic Parkinson s disease: evidence from a longitudinal study of dopaminergic function in twins. Ann. Neurol. 45, (1999) Goldstein, D. B. Common genetic variation and human traits. N. Engl. J. Med. 360, (2009) Ginsburg, G. S. & Willard, H. F. Genomic and personalized medicine: foundations and applications. Transl. Res. 154, (2009) Farrer, M. et al. Comparison of kindreds with parkinsonism and α-synuclein genomic multiplications. Ann. Neurol. 55, (2004) Farrer, M. et al. α-synuclein gene haplotypes are associated with Parkinson s disease. Hum. Mol. Genet. 10, (2001) Brooks, J. et al. Parkin and PINK1 mutations in early-onset Parkinson s disease: comprehensive screening in publicly available cases and control. J. Med. Genet. 46, (2009) Nalls, M. A. et al. Measures of autozygosity in decline: globalization, urbanization, and its implications for medical genetics. PLoS Genet. 5, e (2009) Simón-Sánchez, J. et al. Genome-wide association study reveals genetic risk underlying Parkinson s disease. Nat. Genet. 41, (2009) Mollenhauer, B., El-Agnaf, O. M., Marcus, K., Trenkwalder, C. & Schlossmacher, M. G. Quantification of α-synuclein in cerebrospinal fluid as a biomarker candidate: review of the literature and considerations for future studies. Biomark. Med. 4, (2010) Burre, J. et al. α-synuclein promotes SNAREcomplex assembly in vivo and in vitro. Science 329, (2010) Cooper, A. A. et al. α-synuclein blocks ER Golgi traffic and Rab1 rescues neuron loss in Parkinson s models. Science 313, (2006) Masliah, E. et al. Dopaminergic loss and inclusion body formation in α-synuclein mice: implications for neurodegenerative disorders. Science 287, (2000) Lewis, J. et al. In vivo silencing of α -synuclein using naked sirna. Mol. Neurodegener. 3, 19 (2008) McCormack, A. L. et al. α-synuclein suppression by targeted small interfering RNA in the primate substantia nigra. PLoS ONE 5, e12122 (2010) Paez, J. G. et al. EGFR mutations in lung cancer: correlation with clinical response to gefitinib therapy. Science 304, (2004) Ebrahimi-Fakhari, D., Wahlster, L. & McLean, P. J. Molecular chaperones in Parkinson s disease present and future. J. Parkinsons Dis. 1, (2011) Plowey, E. D. & Chu, C. T. Synaptic dysfunction in genetic models of Parkinson s disease: a role for autophagy? Neurobiol. Dis. 43, (2011) Cuervo, A. M. Autophagy: many paths to the same end. Mol. Cell Biochem. 263, (2004) Orenstein, S. J. et al. Interplay of LRRK2 with chaperone-mediated autophagy. Nat. Neurosci. 16, (2013) Cook, C., Stetler, C. & Petrucelli, L. Disruption of protein quality control in Parkinson s disease. Cold Spring Harbor Perspect. Med. 2, a Matsuda, W. et al. Single nigrostriatal dopaminergic neurons form widely spread and highly dense axonal arborizations in the neostriatum. J. Neurosci. 29, (2009) Luk, K. C. et al. Pathological α-synuclein transmission initiates Parkinson-like neurodegeneration in nontransgenic mice. Science 338, Kasai, H., Takahashi, N. & Tokumaru, H. Distinct initial SNARE configurations underlying the diversity of exocytosis. Physiol. Rev. 92, Jahn, R. & Fasshauer, D. Molecular machines governing exocytosis of synaptic vesicles. Nature 490, Burgoyne, R. D. & Morgan, A. Chaperoning the SNAREs: a role in preventing neurodegeneration? Nat. Cell Biol. 13, 8 9 (2011) Westphal, C. H. & Chandra, S. S. Monomeric synucleins generate membrane curvature. J. Biol. Chem. 288, (2013) McPherson, P. S. et al. A presynaptic inositol 5 phosphatase. Nature 379, (1996) Milosevic, I. et al. Recruitment of endophilin to clathrin-coated pit necks is required for efficient NATURE REVIEWS NEUROLOGY ADVANCE ONLINE PUBLICATION 9