Genetic clues to the pathogenesis of Parkinson's disease

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  • P E R S P E C T I V E

    S58 JULY 2004 NEURODEGENERATION

    Recent years have seen an explosion in the rate of discovery ofgenetic defects linked to Parkinsons disease. Thesebreakthroughs have not provided a direct explanation for thedisease process. Nevertheless, they have helped transformParkinsons disease research by providing tangible clues to theneurobiology of the disorder.

    Parkinsons disease (PD) is the second most common human neu-rodegenerative disorder, after Alzheimers dementia. This disease isprogressive, with a mean age at onset of 55, and its incidence increasesmarkedly with age1. The primary hallmark of PD is the degenerationof the nigrostriatal dopaminergic pathway, which, in depleting thebrain of dopamine, initiates aberrant motor activity such as tremor atrest, rigidity, slowness of voluntary movement, and postural instabil-ity. As with other neurodegenerative disorders, however, the neu-ropathology of PD is not restricted to one pathway, and histologicalabnormalities also occur in many other dopaminergic and non-dopaminergic cell groups including the locus coeruleus, raphe nucleiand nucleus basalis of Meynert2. Because numerous distinct neuro-logical conditions share the clinical features of PD, a definitive diag-nosis of PD can only be made at autopsy, and it has customarily beenbased not only on the loss of nigrostriatal dopaminergic neurons butalso on the presence of intraneuronal inclusions called Lewy bodies(LBs). These are spherical eosinophilic cytoplasmic aggregates con-taining a variety of proteins, of which -synuclein is a major compo-nent3, and are found in every affected brain region. Whetheridentification of LBs should still be considered necessary for the diag-nosis of PD is controversial, in that individuals with inherited PDlinked to mutations in the gene encoding parkin typically lack LBsand are still regarded as having PD1. Moreover, the role of LBs in thePD neurodegenerative process is a matter of fierce debate.

    The cause of almost all cases of PD remains unknown. PD generallyarises as a sporadic condition but is occasionally inherited as a simplemendelian trait (Table 1). Although sporadic and familial PD are verysimilar, inherited forms of the disease usually begin at earlier ages andare associated with atypical clinical features (Table 1). Until recently,all of the hypotheses regarding the cause and mechanism of PD neu-rodegeneration derived from investigations carried out on autopsytissues from individuals with sporadic PD or in neurotoxic animal

    models such as that produced by the mitochondrial poison 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)1. In the mid-1990s,however, this situation changed with the identification of a mutationin the -synuclein gene associated with PD in an Italian kindred4.Since then, four additional genetic defects underlying PD have beenidentified and linkages have been reported for at least four more(Table 1).

    Here we review what is currently known about these PD-causingmutations. As is the case in Alzheimers disease, these gene defectsseem to operate on a common molecular pathway. Thus, we also dis-cuss this pathway and the directions in which those genes may lead usin regard to the development of genetically based animal models,which are crucial to unraveling the basis of the neurodegenerativeprocesses of PD.

    -Synuclein mutations and dopaminergic neurodegenerationThree missense mutations (A53T, A30P and E46K) in the gene encod-ing -synuclein are linked to a dominantly inherited PD46 (Table 1).None of these mutations has been found in sporadic PD or in individ-uals without the disease.

    Injection of either human wild-type or mutant -synuclein-expressing viral vectors into the rat and monkey nigrostriatal pathwaycauses dopaminergic neurodegeneration associated with synu-clein-containing inclusions7,8. Transgenic overexpression of mutantor wild-type -synuclein in mice or flies has produced equivocalresults1, however, in that intraneuronal proteinaceous inclusions, butnot definite neuronal death, have generally been documented. Still,these results, together with the finding that -synuclein ablation inmice does not cause neurodegeneration9,10, support the notion that-synuclein mutations operate by a toxic gain-of-function mecha-nism. Viral vectormediated overexpression of wild-type -synucleinreproduces PD neuropathology in animals7,8, and genomic multipli-cation of the gene encoding -synuclein is associated with a familialform of PD11,12. It is thus possible that the function gained by themutant protein is not a newly acquired property, but rather a nativeproperty that is enhanced and becomes deleterious.

    How mutant -synuclein variants produce neurotoxicity remainselusive, in part because the proteins function is just beginning to beunderstood. Wild-type -synuclein binds preferentially to plasmamembranes (rather than mitochondrial membranes) in yeasts13 andthis interaction, which is mediated by major conformational changesof the protein14, seems to be crucial to several of its physiologicalfunctions1. Membrane-bound -synuclein has been proposed tomodulate phospholipase D activity15, thereby perhaps influencing theavailability of synaptic vesicles for release. Membrane-bound -synu-

    Genetic clues to the pathogenesis of Parkinsons diseaseMiquel Vila & Serge Przedborski

    Miquel Vila is in the Department of Neurology and Serge Przedborski is in theDepartments of Neurology and Pathology and the Center for Neurobiology andBehavior, Columbia University, New York, New York 10032, USA.e-mail: sp30@columbia.edu

    Published online 1 July 2004; doi: 10.1038/nm1068

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    clein also seems to be in dynamic equilibrium with cytosolic -synu-clein13, which upon accumulation can render endogenous dopaminetoxic16, and to act as a seed promoting the formation of cytosolicinclusions17. Conceivably if these aggregates are not promptly clearedby degradation pathways18, neurotoxicity can ensue.

    Both wild-type and mutant -synuclein form amyloid fibrils akinto those seen in LBs, as well as nonfibrillary oligomers1 termedprotofibrils. Because the pathogenic -synucleinA53T mutant pro-motes the formation of protofibrils19, these oligomers may be thetoxic species of -synuclein. In keeping with this and with the knownassociation of -synuclein with synaptosomes, protofibrils may causetoxicity by permeabilizing synaptic vesicles20, allowing dopamine toleak into the cytoplasm and participate in reactions that generateoxidative stress. Furthermore, the selective vulnerability of nigrostri-atal neurons in PD may derive from the ability of dopamine ordopamine-quinone to stabilize -synuclein protofibrils21. However,protofibrils have only been identified and studied in vitro, and so fur-ther work is required to establish whether they form in neurons andwhether their formation correlates with neurotoxicity.

    Parkin, a protein with many substratesLoss-of-function mutations in the gene encoding parkin cause arecessively inherited form of PD22 (Table 1). The onset of parkin-related PD usually, but not always, occurs before age 30 (ref. 1).Pathologically, this form of familial PD is associated with a loss ofnigrostriatal neurons, but LBs are not typically observed. Parkin-nullmice and flies do not develop degeneration of nigrostriatal dopamin-ergic neurons2325. However, these animals do show functional mito-chondrial deficits2426 suggestive of those seen in sporadic PD1.

    Identification of the normal function of parkin has provided hintsto the pathogenic effects of parkin mutations. Parkin is one of a classof proteins containing two RING-FINGER DOMAINS separated by an in-between RING-finger domain, and like other such proteins, parkinfunctions as an E3 ubiquitin ligase27,28, a component of the ubiquitin

    system. Many mutations affecting parkin abolish its E3 ligase activ-ity1, as does the post-translational modification (S-nitrosylation) ofwild-type parkin29. It is thus conceivable that parkin dysfunction isinvolved in the pathogenesis of both familial and sporadic PD, but theunderlying molecular details remain speculative. Loss of parkin activ-ity may trigger cell death by rendering neurons more susceptible tocytotoxic insults, such as those caused by proteasome inhibition ormutant -synuclein30, or by impairing ubiquitination of cyclin E31, amolecule previously implicated in neuronal apoptosis. In support ofthe latter hypothesis, cyclin E is abundant in the midbrains of individ-uals with parkin-related PD, and overexpression of wild-type parkinattenuates cyclin E accumulation and promotes survival in excito-toxin-treated cultured neurons31. Several studies have shown func-tional interactions between parkin and -synuclein, and havesuggested that these interactions may involve the proteasome1. Otherinvestigations have highlighted the multiplicity of parkin substratesand how these might have a key role in neuronal death1. However,none of the parkin substrates that have been identified seem to bespecifically enriched in dopaminergic neurons. Thus, further studiesare needed to explain the relative specificity of dopaminergic neu-rodegeneration mediated by parkin mutations.

    UCH-L1 dabbles in degenerationUbiquitin C-terminal hydrolase-L1 (UCH-L1) is expressed mainlyin the brain, where it catalyzes the hydrolysis of C-terminal ubiqui-tyl esters. A single dominant mutant form (I93M) of UCH-L1,found in two members of a PD-affected family, has been implicatedin the development of an inherited form of PD32. Conversely, it hasbeen confirmed that a polymorphism (S18Y) of UCH-L1 reducesthe risk of developing sporadic PD, especially in early-onset cases33.The I93M mutation decreases the enzymes activity, suggesting thata loss of function is the culprit in disease development. However,mice carrying a UCH-L1-null mutation do show neurodegenerativechanges, but not in the nigrostriatal dopaminergic pathway34. Upon

    Table 1 Genes and loci linked to familial PD

    Locus Chromosomal location Protein Inheritance pattern Atypical PD features Lewy bodies

    PARK1 4q21 -Synucleina AD Early onset YesLower prevalence of tremor

    PARK2 6q25.2q27 Parkin AR Early or juvenile onset Mostly negativeb

    More frequent dystonia and levodopa-induced dyskinesiasSlower disease progression

    PARK3 2p13 Unknown AD Dementia in some individuals YesRapid progression

    PARK4c 4p15 Unknown AD Early onset YesRapid progressionDementiaAutonomic dysfunctionPostural tremor

    PARK5 4p14 UCH-L1 AD None Unknown

    PARK6 1p36 PINK1 AR Early onset UnknownSlow progression

    PARK7 1p36 DJ-1 AR Early onset UnknownPsychiatric symptomsSlow progression

    PARK8 12p11.2q13.1 Unknown AD None No

    PARK9 1p36 Unknown AR Juvenile onset UnknownSpasticitySupranuclear gaze paralysisDementia

    AD, autosomal dominant; AR, autosomal recessive. aIncluding mutations and wild-type multiplications. bLewy bodies reported in one individual with parkin mutations54. cTheinitial PARK4 linkage to 4p15 could not be confirmed, and the PD phenotype in this family was subsequently linked to a PARK1 variant (-synuclein triplication)11.

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    dimerization, UCH-L1 can also exert an ubiquitin ligase activity,which is decreased by the pathogenic I93M mutation and increasedby the protective S18Y polymorphism35. UCH-L1 was also identi-fied as a protease with specificity for Nedd8 (ref. 36), a small ubiqui-tin-like protein implicated in cell-cycle regulation. Although all ofthese findings are consistent with the hypothesis that PD pathogen-esis involves impairment of the ubiquitin systems, further studiesare needed to elucidate how mutations affecting UCH-L1 killdopaminergic neurons.

    Multiplicity of DJ-1 mutationsDJ-1 is a homodimeric, multifunctional protein ubiquitouslyexpressed in human tissues including the brain. Eleven differentmutations affecting DJ-1these include missense, truncating andsplice-site mutations and large deletionshave been linked to anautosomal recessive form of PD3739. Although DJ-1 mutationsaccount for only a small fraction of early-onset PD, they are the sec-ond most frequent cause of recessive forms of PD after parkin muta-tions. These mutations are found throughout the DJ-1 gene in four ofits seven exons. This suggests that even though linked to a similar phe-notype, the different mutations probably affect different domains ofthe protein with distinct functional andstructural roles. This situation is reminiscentof that in amyotrophic lateral sclerosis, wherethe same paralytic syndrome is caused bymany different mutations affecting super-oxide dismutase-1 (http://alsod.org).

    DJ-1 was discovered as part of a multipro-tein complex that stabilizes mRNA through aninteraction with c-myc and as a proteininvolved in infertility in rodents40. DJ-1 maymodulate mRNA expression through interac-tions with a polypeptide complex comprisingthe androgen receptor and the SUMOYLATIONenzyme PIASx41,42. DJ-1 may also functionboth as a sensor for oxidative stress and as anantioxidant40,43. In addition, structural studiesindicate that DJ-1 shares similarities with bac-terial Hsp31, a stress-inducible chaperone44.Yet none of this information sheds much lightinto the pathogenic mechanisms of DJ-1mutations in PD. Studies of a single DJ-1mutant, L166P, have shown that it hasimpaired folding properties and cannot form ahomodimer45,46. Thus, as a misfolded and lessstable protein, DJ-1L166P may cause cytotoxic-ity by overwhelming the cellular protein degra-dation systems and by undergoing abnormalsubcellular localization, for instance in mito-chondria45. It remains to be established howaccurate this pathogenic scenario is andwhether DJ-1L166P reliably reflects the molecu-lar abnormalities of the other known DJ-1mutations. Furthermore, DJ-1 chiefly localizesin brain glial cells47, suggesting that neuronaldeath mediated by DJ-1 mutations may arisefrom a NON-CELL-AUTONOMOUS process.

    PINK1, the new kid on the blockAUTOZYGOSITY mapping of a large consan-guineous Sicilian family localized the PARK6

    locus, which is linked to an autosomal recessive form of PD, to chromo-some 1p3536 (ref. 48). PARK6 has since been linked to an early-onsetrecessive PD in eight additional families from four European coun-tries49. Sequencing of candidate genes within the PARK6 region inaffected members from each family identified two homozygous muta-tions in the gene encoding PTEN-induced putative kinase-1 (PINK1,also known as BRPK)50. PINK1 is a 581-amino-acid protein with a pre-dicted molecular mass of 62.8 kDa, which shows 95% sequence similar-ity between human and mouse51,52. In adult mice, PINK1 isubiquitously expressed among tissues, with an apparently high expres-sion in the brain51. In both human and mouse PINK1, a serine/threo-nine kinase domain is the sole known functional domain51,52.Presumably PINK1 al...