Parkinson's genetics: molecular insights for the new millennium

Polymorphisms of Interleukin-6 and Interleukin-8 Are Not Associated with Parkinson's Disease in Taiwan

Background: Studies have suggested that cytokines are crucial mediators in the pathogenesis of Parkinson’s disease (PD). The multifunctional cytokine interleukin (IL)-6 and its single nucleotide polymorphisms (SNPs) were found to have an impact on the development of PD. However, different studies in associations of IL-6 genetic variants with PD showed inconsistent results and it has never been explored in a Taiwanese population. Both IL-1α and IL-8 contribute to the same inflammation pathway. IL-1α genetic polymorphism has an effect on late-onset PD in Taiwan, whereas the associations of IL-8 genetic variants with PD in Taiwan remain to be investigated. Methods: This study examined the frequencies of polymorphisms within the critical promoter areas of the proinflammatory cytokine genes: IL-6 G-174C (rs1800795) and IL-8 A-251T (rs4073) in Taiwanese PD patients compared with age-and gender-matched healthy subjects. Comparisons were also made in genotype and allele frequencies of IL-6 G-174C (rs1800795) and IL-8 A-251T (rs4073) among different populations in previous studies. Results: In total, 1120 subjects, including 509 PD patients (female/male: 259/250) and 511 control subjects (female/male: 252/259), were recruited. We found no statistically significant differences in IL-6 G-174C (rs1800795) or IL-8 A-251T (rs4073) genotypic and allelic distribution between PD and controls, even after being stratified by age at onset and gender. Conclusions: The results did not demonstrate any association of IL-6 G-174C (rs1800795) or IL-8 A-251T (rs4073) with PD in a Taiwanese population. Despite the negative results, this is the first study in associations of IL-6 G-174C (rs1800795) and IL-8 A-251T (rs4073) with PD in Taiwan. The relevance of genetic variants of IL-6 G-174C (rs1800795) or IL-8 A-251T (rs4073) on PD susceptibility warrants further investigation.

Parkinson's disease: genetics and beyond

Parkinson's disease (PD) is characterized clinically by resting tremor, rigidity, bradykinesia and postural instability due to progressive and selective loss of dopamine neurons in the ventral substantia nigra, with the presence of ubiquitinated protein deposits called Lewy bodies in the neurons. The pathoetiology of cell death in PD is incompletely understood and evidence implicates impaired mitochondrial complex I function, altered intracellular redox state, activation of proapoptotic factors and dysfunction of ubiquitinproteasome pathway. Now it is believed that genetic aberration, an environmental toxin or combination of both leads to a cascade of events culminating in the destruction of myelinated brainstem catecholaminergic neurons. Also the role of production of significant levels of abnormal proteins, which may misfold, aggregate and interfere with intracellular processes causing cytotoxicity has recently been hypothesized. In this article, the diverse pieces of evidence that have linked the various factors responsible for the pathophysiology of PD are reviewed with special emphasis to various candidate genes and proteins. Furthermore, the present therapeutic strategies and futuristic approaches for the pharmacotherapy of PD are critically discussed.

Parkin transcript variants in rat and human brain

Alternative splicing has an important role in expanding protein diversity. We have identified complementary DNA species from adult rat and fetal human brain encoding seven new splice variants of parkin, a gene mutated in autosomal recessive juvenile parkinsonism (ARJP). Alternative splicing affects almost all previously characterized exons, plus 3 new exons of 72, 156, and 180 nucleotides. This creates the potential to express hundreds of different isoforms. The encoded parkin isoforms have different amino acid composition, post-translational modifications, and, most important, molecular architectures. They diverge for the presence or absence of the ubiquitin-like domain, one or two C3HC4 ring fingers, the in-between ring fingers (IBR) domain, and a thiol proteases active site, which has not been previously characterized. Distinct expression patterns occur in primary cultures of neuronal and glial cells. Extensive splicing of parkin produces regional and structural diversity and may have important implications for the pathogenetic mechanisms underlying ARJP.

Lack of mutations in DJ-1 in a cohort of Taiwanese ethnic Chinese with early-onset parkinsonism

Recently, mutations in DJ-1 (PARK7) were described as a novel cause of early-onset parkinsonism. We analysed the DJ-1 gene in a cohort of patients originating from Taiwan with early-onset Parkinson's disease; 41 subjects were clinically and genetically examined. These patients were evaluated previously for the presence of parkin mutations (PARK2) and were found to be negative. The entire DJ-1 open-reading frame was amplified from cDNA, analysed for size alterations indicative of mutations affecting splice motifs, and sequenced to identify coding variants. In addition, we developed quantitative polymerase chain reaction assays to examine the genomic copy number of DJ-1 exons. No potential splice site mutations, coding sequence alterations, or exon deletion/duplications were detected. Our results and previous studies suggest that alterations to DJ-1 are not a common cause of early-onset Parkinson's disease and other causes, genetic and/or environmental, remain to be identified.

Functional promoter region polymorphism of the proinflammatory chemokine IL-8 gene associates with Parkinson's disease in the Irish

Parkinson's disease (PD) is one of the most prevalent neurodegenerative disorders and is characterized by the progressive loss of dopamine neurons in the substantia nigra. There is increasing evidence to suggest the inflammatory response of the brain contributes to the pathogenesis of PD. This study investigated the frequency of polymorphism located in the critical promoter region of the proinflammatory cytokine genes: interleukin (IL)-2, IL-6, IL-8, and tumor necrosis factor alpha (TNF-alpha) within a cohort of patients with PD in comparison to a group of healthy elderly individuals. No association was observed for single nucleotide polymorphism in the promoter regions of the IL-2, IL-6, and TNF-alpha genes. The single nucleotide polymorphism in the chemokine IL-8 gene was observed to associate with PD and appeared to be independent of age at onset. This association further supports the theory that the proinflammatory response in the brains of patients with PD plays a role in the pathogenesis of the disease and warrants further investigation into the role of chemokines in the brain, and a more detailed analysis of the genetics involved in the immune response of the brain.

Genetics of parkin-linked disease

Research into Parkinson's disease (PD), once considered the archetypical non-genetic neurodegenerative disorder, has been revolutionized by the identification of a number of genes, mutations of which underlie various familial forms of the disease. Whereas such mutations appear to exist in a relatively small number of individuals from a few families, the study of the function of these genes promises to reveal the fundamental disease pathogenesis, not only of familial forms of the disease, but also of the much more common sporadic PD. The observation that mutations in the second identified PD locus (parkin) are common in juvenile- and early-onset PD and increasing evidence supporting a direct role for parkin in late-onset disease make this gene a particularly compelling candidate for intensified investigation. The determination of the frequency and effect of parkin mutations in various subsets of PD will be crucial for understanding the way in which parkin is related to neurodegenerative mechanisms, and whether these subsets might be effectively identified and treated. In addition, many aspects of parkin-linked disease, originally thought to be well defined, have now been obscured both by genetic studies that preclude a simple model of disease transmission and by clinical and pathological studies that demonstrate broad variability in cases with parkin mutations. Future studies that address the issues in question should have a far-reaching impact in downstream biochemical studies and our understanding of parkin's role in PD.

The PARK8 locus in autosomal dominant parkinsonism: confirmation of linkage and further delineation of the disease-containing interval

Recently, a new locus (PARK8) for autosomal dominant parkinsonism has been identified in one large Japanese family. Linkage has been shown to a 16-cM centromeric region of chromosome 12, between markers D12S1631 and D12S339. We tested 21 white families with Parkinson disease and an inheritance pattern compatible with autosomal dominant transmission for linkage in this region. Criteria for inclusion were at least three affected individuals in more than one generation. A total of 29 markers were used to saturate the candidate region. One hundred sixty-seven family members were tested (84 affected and 83 unaffected). Under the assumption of heterogeneity and through use of an affecteds-only model, a maximum multipoint LOD score of 2.01 was achieved in the total sample, with an estimated proportion of families with linkage of 0.32. This LOD score is significant for linkage in a replication study and corresponds to a P value of.0047. Two families (family A [German Canadian] and family D [from western Nebraska]) reached significant linkage on their own, with a combined maximum multipoint LOD score of 3.33, calculated with an affecteds-only model (family A: LOD score 1.67, P=.0028; family D: LOD score 1.67, P=.0028). When a penetrance-dependent model was calculated, the combined multipoint LOD score achieved was 3.92 (family A: LOD score 1.68, P=.0027; family D: LOD score 2.24, P=.0007). On the basis of the multipoint analysis for the combined families A and D, the 1-LOD support interval suggests that the most likely disease location is between a CA repeat polymorphism on genomic clone AC025253 (44.5 Mb) and marker D12S1701 (47.7 Mb). Our data provide evidence that the PARK8 locus is responsible for the disease in a subset of families of white ancestry with autosomal dominant parkinsonism, suggesting that it could be a more common locus.

It's a double knock-out! The quaking mouse is a spontaneous deletion of parkin and parkin co-regulated gene (PACRG)

Mutations in the parkin gene (PRKN) are the commonest cause of juvenile and early-onset parkinsonism. However, the pathogenic mechanism by which loss of parkin protein results in degeneration of dopaminergic neurons remains elusive. Animal models provide a useful tool for the study of development and disease, and the recent production of transgenic fly and mouse parkin deficient models allows investigation of the molecular role of parkin in dopamine regulation and nigrostriatal function. We have identified the mouse mutant Quaking as a spontaneously occurring PRKN knockout. The quaking mutation is a deletion of approximately 1.17 Mb of mouse chromosome 17, resulting in the deletion of the entire promoter and first five coding exons of PRKN In addition, the recently described Parkin Co-Regulated Gene (PACRG) is completely deleted. Homozygous Quaking mice show a complete loss of PRKN and PACRG mRNA and protein. These mice will constitute a useful additional model for studies of the molecular role of parkin and PACRG in neurodegeneration.

Looking Backward to Move Forward: Early Detection of Neurodegenerative Disorders

Early detection of neurodegenerative disorders would provide clues to the underlying pathobiology of these diseases and would enable more effective diagnosis and treatment of patients. Recent advances in molecular neuroscience have begun to provide the tools to detect diseases like Alzheimer's disease, Parkinson's disease, and others early in their course and potentially even before the development of clinical manifestations of disease. These genetic, imaging, clinical, and biochemical tools are being validated in a number of studies. Early detection of these slowly progressive diseases offers the promise of presymptomatic diagnosis and, ultimately, of disease-modifying medications for use early in disease and during the presymptomatic period.

Impact of genetic analysis on Parkinson's disease research

Genetic analysis is changing our view of Parkinson's disease: not only is it challenging the long-cherished views about the diagnosis of the disease, it is also starting to suggest a biochemical pathway to disease pathogenesis. These developments are reviewed in the context of three known (synuclein, parkin, and DJ-1) and one suspected (ubiquitin hydrolase) genes for the disease.

Co-ordinate transcriptional regulation of dopamine synthesis genes by alpha-synuclein in human neuroblastoma cell lines

Abnormal accumulation of alpha-synuclein in Lewy bodies is a neuropathological hallmark of both sporadic and familial Parkinson's disease (PD). Although mutations in alpha-synuclein have been identified in autosomal dominant PD, the mechanism by which dopaminergic cell death occurs remains unknown. We investigated transcriptional changes in neuroblastoma cell lines transfected with either normal or mutant (A30P or A53T) alpha-synuclein using microarrays, with confirmation of selected genes by quantitative RT-PCR. Gene products whose expression was found to be significantly altered included members of diverse functional groups such as stress response, transcription regulators, apoptosis-inducing molecules, transcription factors and membrane-bound proteins. We also found evidence of altered expression of dihydropteridine reductase, which indirectly regulates the synthesis of dopamine. Because of the importance of dopamine in PD, we investigated the expression of all the known genes in dopamine synthesis. We found co-ordinated downregulation of mRNA for GTP cyclohydrolase, sepiapterin reductase (SR), tyrosine hydroxylase (TH) and aromatic acid decarboxylase by wild-type but not mutant alpha-synuclein. These were confirmed at the protein level for SR and TH. Reduced expression of the orphan nuclear receptor Nurr1 was also noted, suggesting that the co-ordinate regulation of dopamine synthesis is regulated through this transcription factor.