PINK1 mutation heterozygosity and the risk of Parkinson's disease

Systematic Functional Analysis of PINK1 and PRKN Coding Variants

Loss of either PINK1 or PRKN causes an early onset Parkinson’s disease (PD) phenotype. Functionally, PINK1 and PRKN work together to mediate stress-activated mitochondrial quality control. Upon mitochondrial damage, PINK1, a ubiquitin kinase and PRKN, a ubiquitin ligase, decorate damaged organelles with phosphorylated ubiquitin for sequestration and degradation in lysosomes, a process known as mitophagy. While several genetic mutations are established to result in loss of mitophagy function, many others have not been extensively characterized and are of unknown significance. Here, we analyzed a set of twenty variants, ten in each gene, focusing on understudied variants mostly from the Parkinson’s progressive marker initiative, with sensitive assays to define potential functional deficits. Our results nominate specific rare genetic PINK1 and PRKN variants that cause loss of enzymatic function in line with a potential causative role for PD. Additionally, we identify several variants with intermediate phenotypes and follow up on two of them by gene editing midbrain-derived neuronal precursor cells. Thereof derived isogenic neurons show a stability defect of the rare PINK1 D525N mutation, while the common PINK1 Q115L substitution results in reduced kinase activity. Our strategy to analyze variants with sensitive functional readouts will help aid diagnostics and disease treatment in line with current genomic and therapeutic advances.

Community-based genetic study of Parkinson's disease in Estonia

OBJECTIVE

To examine the genetic variability of Estonian Parkinson's disease (PD) patients using an ongoing epidemiological study in combination with a genetic analysis.

METHODS

This study was a community-based genetic screening study of 189 PD patients, and 158 age- and sex-matched controls screened for potential mutations in 9 PD genes using next-generation sequencing and multiplex ligation-dependent probe amplification method. Different clinimetric scales and questionnaires were used to examine PD patients and assess clinical characteristics and severity of the disease.

RESULTS

The overall frequency of pathogenic PD-causing variants was 1.1% (2/189), and any rare genetic variant was present in 21.2% (40/189) of the patients and in 8.2% (13/158) of the controls (P < .05). Variants of unknown significance accounted for 10.6% (20/189). Frequency of any GBA variant among PD patients was 10.1% (19/189) and in controls 3.8% (6/158). The frequency of any GBA variant in PD compared to controls was significantly higher (P = .035; OR 2.82; CI 95% 1.05-8.87). Burden of rare variants was not different between patients and controls. Also, a novel GBA pathogenic variant p.E10X was detected.

CONCLUSION

Among different genetic variants identified in Estonian PD patients, GBA variants are the most common, while an overall pathogenic variant frequency was 1.1%.

Rare and novel variants of PRKN and PINK1 genes in Vietnamese patients with early-onset Parkinson's disease

Background

Early‐onset Parkinson's disease (EOPD) refers to that of patients who have been diagnosed or had onset of motor symptoms before age 50, accounting for 4% of Parkinson's disease patients. The PRKN and PINK1 genes, both involved in a metabolic pathway, are associated with EOPD.

Methods

To identify variants associated with EOPD, coding region of PARKIN and PINK1 genes in 112 patients and 112 healthy individuals were sequenced. Multiplex ligation‐dependent probe amplification kit was used to determine EOPD patients that carried mutations in PRKN and PINK1 genes.

Results and Conclusion

Three rare and three novel mutations in total of 14 variants of PARKIN and PINK1 were detected in the EOPD cohorts. Mutations of PRKN and PINK1 genes were found in five (4.4%) patients, which were four patients with compound heterozygous variants in the PRKN and one case with a homozygous mutation of the PINK1 gene. The novel mutations might reduce the stability of the PRKN and PINK1 protein molecules. The frequency of homozygous mutant genotype p.A340T of the PINK1 in the EOPD cohort was higher than in control (p = 0.0001, OR = 5.704), suggesting this variant might be a risk factor for EOPD. To the best of our knowledge, this is the first study of PRKN and PINK1 genes conducted on Vietnamese EOPD patients. These results might contribute to the genetic screening of EOPD in Vietnam.

Comprehensive assessment of PINK1 variants in Parkinson's disease

Multiple genes have been associated with monogenic Parkinson's disease and Parkinsonism syndromes. Mutations in PINK1 (PARK6) have been shown to result in autosomal recessive early-onset Parkinson's disease. In the past decade, several studies have suggested that carrying a single heterozygous PINK1 mutation is associated with increased risk for Parkinson's disease. Here, we comprehensively assess the role of PINK1 variants in Parkinson's disease susceptibility using several large data sets totalling 376,558 individuals including 13,708 cases with Parkinson's disease and 362,850 control subjects. After combining these data, we did not find evidence to support a role for heterozygous PINK1 mutations as a robust risk factor for Parkinson's disease.

Long-Term Outcomes of Genetic Parkinson's Disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that affects 1–2% of people by the age of 70 years. Age is the most important risk factor, and most cases are sporadic without any known environmental or genetic causes. Since the late 1990s, mutations in the genes SNCA, PRKN, LRRK2, PINK1, DJ-1, VPS35, and GBA have been shown to be important risk factors for PD. In addition, common variants with small effect sizes are now recognized to modulate the risk for PD. Most studies in genetic PD have focused on finding new genes, but few have studied the long-term outcome of patients with the specific genetic PD forms. Patients with known genetic PD have now been followed for more than 20 years, and we see that they may have distinct and different prognoses. New therapeutic possibilities are emerging based on the genetic cause underlying the disease. Future medication may be based on the pathophysiology individualized to the patient’s genetic background. The challenge is to find the biological consequences of different genetic variants. In this review, the clinical patterns and long-term prognoses of the most common genetic PD variants are presented.

The deglycase activity of DJ-1 mitigates α-synuclein glycation and aggregation in dopaminergic cells: Role of oxidative stress mediated downregulation of DJ-1 in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder associated with the degeneration of dopamine neurons of the substantia nigra pars compacta (SNpc) and the presence of intra-neuronal aggregates of α-synuclein and its post-translational products. Based on emerging reports on the association between glycated α-synuclein and PD; and the newly identified deglycase activity of DJ-1, we sought to find the relevance of deglycase activity of DJ-1 on glycation of α-synuclein and its plausible role in PD. Our results demonstrate that DJ-1 has a higher affinity towards the substrate methylglyoxal (MGO) (Km = 900 mM) as compared to its familial mutant, L166P (Km = 1900 mM). Also, CML α-synuclein (CML-syn) served as a substrate for the deglycase activity of DJ-1. Treatment of cells with Parkinsonian mimetic, 1-methyl-4-phenylpyridinium ion (MPP⁺); oxidants, such as H₂O₂ and methylglyoxal (MGO) lead to a dose-dependent decrease in the levels of DJ-1 with a concomitant increase in CML-syn. Also, MGO induced cytosolic α-synuclein aggregates in cells which stained positive with the anti-CML antibody. Further, unilateral stereotaxic administration of MGO into the SNpc of mice induced α-synuclein aggregates and CML-syn with a concomitant reduction in the number of TH positive neurons, protein levels of TH and DJ-1 at the site of injection. Interestingly, overexpression of DJ-1 enhanced the clearance of preformed CML-syn in cells, mitigated MGO induced CML-syn and intracellular α-synuclein aggregates. Overall, the findings of our present study demonstrate that DJ-1 plays a pivotal role in the glycation and aggregation of α-synuclein. Reduced DJ-1 activity due to mutations or oxidative stress may lead to the accumulation of glycated α-synuclein and its aggregates.

Impact of gene mutation in the development of Parkinson's disease

Parkinson's disease (PD) is the second most common age related neurodegenerative disorder worldwide and presents as a progressive movement disorder. Globally seven million to 10 million people have Parkinson's disease. Parkinsonism is typically sporadic in nature. Loss of dopaminergic neurons from substantia nigra pars compacta (SNpc) and the neuronal intracellular Lewy body inclusions are the major cause of PD. Gene mutation and protein aggregation play a pivotal role in the degeneration of dopamine neurons. But the actual cause of dopamine degeneration remains unknown. However, several rare familial forms of PD are associated with genetic loci, and the recognition of causal mutations has provided insight into the disease process. Yet, the molecular pathways and gene transformation that trigger neuronal susceptibility are inadequately comprehended. The discovery of a mutation in new genes has provided a basis for much of the ongoing molecular work in the PD field and testing of targeted therapeutics. Single gene mutation in a dominantly or recessively inherited gene results a great impact in the development of Parkinson's disease. In this review, we summarize the molecular genetics of PD.

Unravelling the interaction between the DRD2 and DRD4 genes, personality traits and concussion risk

Background

Concussion occurs when biomechanical forces transmitted to the head result in neurological deficits. Personality may affect the balance between safe and dangerous play potentially influencing concussion risk. Dopamine receptor D2 (DRD2) and dopamine receptor D4 (DRD4) genetic polymorphisms were previously associated with personality traits.

Objectives

This case-control genetic association study investigated the associations of (1) DRD2 and DRD4 genotypes with concussion susceptibility and personality, (2) personality with concussion susceptibility and (3) the statistical model of genotype, personality and concussion susceptibility.

Methods

In total, 138 non-concussed controls and 163 previously concussed cases were recruited from high school (n=135, junior), club and professional rugby teams (n=166, senior). Participants were genotyped for DRD2 rs12364283 (A>G), DRD2 rs1076560 (C>A) and DRD4 rs1800955 (T>C) genetic variants. Statistical analyses including structural equation modelling were performed using the R environment and STATA.

Results

The rs1800955 CC genotype (p=0.014) and inferred DRD2 (rs12364283-rs1076560)-DRD4 (rs1800955) A-C-C allele combination (p=0.019) were associated with decreased concussion susceptibility in juniors. The rs1800955 TT and CT genotypes were associated with low reward dependence in juniors (p<0.001) and seniors (p=0.010), respectively. High harm avoidance was associated with decreased concussion susceptibility in juniors (p=0.009) and increased susceptibility in seniors (p=0.001). The model showed that a genetic variant was associated with personality while personality was associated with concussion susceptibility.

Conclusion

These findings highlight the linear relationship between genetics, personality and concussion susceptibility. Identifying a genetic profile of 'high risk' behaviour, together with the development of personalised behavioural training, can potentially reduce concussion risk.

The Effects of Variants in the Parkin, PINK1, and DJ-1 Genes along with Evidence for their Pathogenicity

Early onset Parkinson's disease can be caused by variants in the PINK1, Parkin, and DJ-1 genes. Since their initial discoveries, hundreds of variants have been found in these genes that are associated with a Parkinsonian phenotype. This review will briefly discuss the functions of the protein products of the three genes, then focus on the effects that disease associated variants have on these functions. We will also discuss how experimental findings can help decide whether individual variants are pathogenic or not.

Heterozygous PINK1 p.G411S increases risk of Parkinson's disease via a dominant-negative mechanism

See Gandhi and Plun-Favreau (doi:10.1093/aww320) for a scientific commentary on this article.

Heterozygous mutations in recessive Parkinson’s disease genes have been postulated to increase disease risk. Puschmann et al. report a genetic association between heterozygous PINK1 p.G411S and Parkinson’s disease. They provide structural and functional explanations for a partial dominant-negative effect of the mutant protein, which impairs wild-type PINK1 activity through hetero-dimerization.

See Gandhi and Plun-Favreau (doi:10.1093/aww320) for a scientific commentary on this article.

It has been postulated that heterozygous mutations in recessive Parkinson’s genes may increase the risk of developing the disease. In particular, the PTEN-induced putative kinase 1 (PINK1) p.G411S (c.1231G>A, rs45478900) mutation has been reported in families with dominant inheritance patterns of Parkinson’s disease, suggesting that it might confer a sizeable disease risk when present on only one allele. We examined families with PINK1 p.G411S and conducted a genetic association study with 2560 patients with Parkinson’s disease and 2145 control subjects. Heterozygous PINK1 p.G411S mutations markedly increased Parkinson’s disease risk (odds ratio = 2.92, P = 0.032); significance remained when supplementing with results from previous studies on 4437 additional subjects (odds ratio = 2.89, P = 0.027). We analysed primary human skin fibroblasts and induced neurons from heterozygous PINK1 p.G411S carriers compared to PINK1 p.Q456X heterozygotes and PINK1 wild-type controls under endogenous conditions. While cells from PINK1 p.Q456X heterozygotes showed reduced levels of PINK1 protein and decreased initial kinase activity upon mitochondrial damage, stress-response was largely unaffected over time, as expected for a recessive loss-of-function mutation. By contrast, PINK1 p.G411S heterozygotes showed no decrease of PINK1 protein levels but a sustained, significant reduction in kinase activity. Molecular modelling and dynamics simulations as well as multiple functional assays revealed that the p.G411S mutation interferes with ubiquitin phosphorylation by wild-type PINK1 in a heterodimeric complex. This impairs the protective functions of the PINK1/parkin-mediated mitochondrial quality control. Based on genetic and clinical evaluation as well as functional and structural characterization, we established p.G411S as a rare genetic risk factor with a relatively large effect size conferred by a partial dominant-negative function phenotype.

Ubiquitin phosphorylation in Parkinson's disease: Implications for pathogenesis and treatment

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, characterized primarily by the loss of dopaminergic neurons in substantia nigra. The pathogenic mechanisms of PD remain unclear, and no effective therapy currently exists to stop neurodegeneration in this debilitating disease. The identification of mutations in mitochondrial serine/threonine kinase PINK1 or E3 ubiquitin-protein ligase parkin as the cause of autosomal recessive PD opens up new avenues for uncovering neuroprotective pathways and PD pathogenic mechanisms. Recent studies reveal that PINK1 translocates to the outer mitochondrial membrane in response to mitochondrial depolarization and phosphorylates ubiquitin at the residue Ser65. The phosphorylated ubiquitin serves as a signal for activating parkin and recruiting autophagy receptors to promote clearance of damaged mitochondria via mitophagy. Emerging evidence has begun to indicate a link between impaired ubiquitin phosphorylation-dependent mitophagy and PD pathogenesis and supports the potential of Ser65-phosphorylated ubiquitin as a biomarker for PD. The new mechanistic insights and phenotypic screens have identified multiple potential therapeutic targets for PD drug discovery. This review highlights recent advances in understanding ubiquitin phosphorylation in mitochondrial quality control and PD pathogenesis and discusses how these findings can be translated into novel approaches for PD diagnostic and therapeutic development.

Differential submitochondrial localization of PINK1 as a molecular switch for mediating distinct mitochondrial signaling pathways

Mutations in mitochondrial kinase PINK1 cause Parkinson disease (PD), but the submitochondrial site(s) of PINK1 action remains unclear. Here, we report that three-dimensional structured illumination microscopy (3D-SIM) enables super-resolution imaging of protein submitochondrial localization. Dual-color 3D-SIM imaging analysis revealed that PINK1 resides in the cristae membrane and intracristae space but not on the outer mitochondrial membrane (OMM) of healthy mitochondria. Under normal physiological conditions, PINK1 colocalizes with its substrate TRAP1 in the cristae membrane and intracristae space. In response to mitochondrial depolarization, PINK1, but not TRAP1, translocates to the OMM. The PINK1 translocation to the OMM of depolarized mitochondria is independent of new protein synthesis and requires combined action of PINK1 transmembrane domain and C-terminal region. We found that mitochondrial depolarization-induced PINK1 OMM translocation is required for recruitment of parkin to the OMM of damaged mitochondria. Our findings suggest that differential submitochondrial localization of PINK1 serves as a molecular switch for mediating two distinct mitochondrial signaling pathways in maintenance of mitochondrial homeostasis. Furthermore, our study provides evidence for the involvement of deregulated PINK1 submitochondrial localization in PD pathogenesis.

Genetic insights into sporadic Parkinson's disease pathogenesis

Intensive research over the last 15 years has led to the identification of several autosomal recessive and dominant genes that cause familial Parkinson’s disease (PD). Importantly, the functional characterization of these genes has shed considerable insights into the molecular mechanisms underlying the etiology and pathogenesis of PD. Collectively; these studies implicate aberrant protein and mitochondrial homeostasis as key contributors to the development of PD, with oxidative stress likely acting as an important nexus between the two pathogenic events. Interestingly, recent genome-wide association studies (GWAS) have revealed variations in at least two of the identified familial PD genes (i.e. α-synuclein and LRRK2) as significant risk factors for the development of sporadic PD. At the same time, the studies also uncovered variability in novel alleles that is associated with increased risk for the disease. Additionally, in-silico meta-analyses of GWAS data have allowed major steps into the investigation of the roles of gene-gene and gene-environment interactions in sporadic PD. The emergent picture from the progress made thus far is that the etiology of sporadic PD is multi-factorial and presumably involves a complex interplay between a multitude of gene networks and the environment. Nonetheless, the biochemical pathways underlying familial and sporadic forms of PD are likely to be shared.

Genetic etiology of Parkinson disease associated with mutations in the SNCA, PARK2, PINK1, PARK7, and LRRK2 genes: a mutation update

To date, molecular genetic analyses have identified over 500 distinct DNA variants in five disease genes associated with familial Parkinson disease; alpha-synuclein (SNCA), parkin (PARK2), PTEN-induced putative kinase 1 (PINK1), DJ-1 (PARK7), and Leucine-rich repeat kinase 2 (LRRK2). These genetic variants include approximately 82% simple mutations and approximately 18% copy number variations. Some mutation subtypes are likely underestimated because only few studies reported extensive mutation analyses of all five genes, by both exonic sequencing and dosage analyses. Here we present an update of all mutations published to date in the literature, systematically organized in a novel mutation database (http://www.molgen.ua.ac.be/PDmutDB). In addition, we address the biological relevance of putative pathogenic mutations. This review emphasizes the need for comprehensive genetic screening of Parkinson patients followed by an insightful study of the functional relevance of observed genetic variants. Moreover, while capturing existing data from the literature it became apparent that several of the five Parkinson genes were also contributing to the genetic etiology of other Lewy Body Diseases and Parkinson-plus syndromes, indicating that mutation screening is recommendable in these patient groups.

Targeting the progression of Parkinson's disease

By the time a patient first presents with symptoms of Parkinson's disease at the clinic, a significant proportion (50-70%) of the cells in the substantia nigra (SN) has already been destroyed. This degeneration progresses until, within a few years, most of the cells have died. Except for rare cases of familial PD, the initial trigger for cell loss is unknown. However, we do have some clues as to why the damage, once initiated, progresses unabated. It would represent a major advance in therapy to arrest cell loss at the stage when the patient first presents at the clinic. Current therapies for Parkinson's disease focus on relieving the motor symptoms of the disease, these unfortunately lose their effectiveness as the neurodegeneration and symptoms progress. Many experimental approaches are currently being investigated attempting to alter the progression of the disease. These range from replacement of the lost neurons to neuroprotective therapies; each of these will be briefly discussed in this review. The main thrust of this review is to explore the interactions between dopamine, alpha synuclein and redox-active metals. There is abundant evidence suggesting that destruction of SN cells occurs as a result of a self-propagating series of reactions involving dopamine, alpha synuclein and redox-active metals. A potent reducing agent, the neurotransmitter dopamine has a central role in this scheme, acting through redox metallo-chemistry to catalyze the formation of toxic oligomers of alpha-synuclein and neurotoxic metabolites including 6-hydroxydopamine. It has been hypothesized that these feed the cycle of neurodegeneration by generating further oxidative stress. The goal of dissecting and understanding the observed pathological changes is to identify therapeutic targets to mitigate the progression of this debilitating disease.

GCH1 in early-onset Parkinson's disease

Mutations in GTP-cyclohydrolase 1 (GCH1) cause autosomal dominant dopa-responsive dystonia (DRD), characterized by childhood-onset foot dystonia that later generalizes. DRD patients frequently present with associated Parkinsonism. Conversely, early-onset Parkinson's disease (EOPD) patients commonly display dystonia. Herein, we investigated the frequency of GCH1 mutations in a series of 53 familial EOPD patients (21 with dystonia) and screened them for mutations in PRKN, PINK1, and DJ-1. In addition, we examined a matched EOPD patient-control series for association of common variability at the GCH1 locus and EOPD susceptibility. No GCH1 coding change or copy-number abnormality was identified in familial EOPD patients. A novel 18-bp deletion was found in the proximal promoter (two patients, one control), which is expected to knock out two regulatory elements previously shown to regulate GCH1 transcription. No association was found between GCH1 variability and risk of EOPD. Fourteen (26.4%) familial EOPD patients had homozygous or compound heterozygous PRKN mutations. PRKN-positive patients were 10 years younger than PRKN-negative patients and had a twofold higher prevalence of dystonia. This study does not support a significant role for genetic variation at the GCH1 locus in EOPD. However, our results further highlight the relevance of PRKN screening in familial EOPD.

PINK1: one protein, multiple neuroprotective functions

Mutations in the PINK1 gene are the second most frequent cause of autosomal recessive parkinsonism after Parkin, and can represent a risk factor towards sporadic Parkinson’s disease. The PINK1 protein product has been implicated in several functions, mostly aimed at protecting neuronal cells against different types of stress. Growing evidence indicates that PINK1 interplays with other proteins mutated in familial forms of Parkinson’s disease, such as Parkin, DJ-1 and α-synuclein. These interactions contribute to the definition of a complex scenario in which the mainteinance of mitochondrial homeostasis and the clearance of misfolded proteins and damaged organelles represent key neuroprotective processes. In this review, we summarize the current knowledge on PINK1 characteristics and functions, and analyze the network of functional relationships that link PINK1 to other Parkinson’s disease-related proteins.

Ordered-subset analysis (OSA) for family-based association mapping of complex traits

Association analysis provides a powerful tool for complex disease gene mapping. However, in the presence of genetic heterogeneity, the power for association analysis can be low since only a fraction of the collected families may carry a specific disease susceptibility allele. Ordered-subset analysis (OSA) is a linkage test that can be powerful in the presence of genetic heterogeneity. OSA uses trait-related covariates to identify a subset of families that provide the most evidence for linkage. A similar strategy applied to genetic association analysis would likely result in increased power to detect association. Association in the presence of linkage (APL) is a family-based association test (FBAT) for nuclear families with multiple affected siblings that properly infers missing parental genotypes when linkage is present. We propose here APL-OSA, which applies the OSA method to the APL statistic to identify a subset of families that provide the most evidence for association. A permutation procedure is used to approximate the distribution of the APL-OSA statistic under the null hypothesis that there is no relationship between the family-specific covariate and the family-specific evidence for allelic association. We performed a comprehensive simulation study to verify that APL-OSA has the correct type I error rate under the null hypothesis. This simulation study also showed that APL-OSA can increase power relative to other commonly used association tests (APL, FBAT and FBAT with covariate adjustment) in the presence of genetic heterogeneity. Finally, we applied APL-OSA to a family study of age-related macular degeneration, where cigarette smoking was used as a covariate.

Significance of the parkin and PINK1 gene in Jordanian families with incidences of young-onset and juvenile parkinsonism

BACKGROUND

Parkinson's disease is a progressive neurodegenerative disorder, where most cases are sporadic with a late onset. In rare incidences familial forms of early-onset parkinsonism occur, and when recessively inherited, cases are often explained by mutations in either the parkin (PARK2) or PINK1 (PARK6) gene or on exceptional occasions the DJ-1 (PARK7) or ATP13A2 (PARK9) gene. Recessively inherited deletions/duplications and point mutations in the parkin gene are the most common cause of early-onset parkinsonism known so far, but in an increasing number of studies, genetic variations in the serine/threonine kinase domain of the PINK1 gene are found to explain early-onset parkinsonism.

METHODS

In this study all families were from a population with a high incidence of consanguinity. We investigated 11 consanguineous families comprising 17 affected with recessively inherited young-onset parkinsonism for mutations both in the parkin and PINK1 gene. Exons and flanking regions were sequenced, and segregation patterns of genetic variation were assessed in members of the respective families. An exon dosage analysis was performed for all exons in both genes.

RESULTS

In the parkin gene, a three generation family was identified with an exon 4 deletion segregating with disease. Both affected were homozygous for the deletion that segregated on a haplotype that spanned the gene in a haplotype segregation analysis that was performed using additional markers. Exon dosage analysis confirmed the recessive pattern of inheritance with heterozygous deletions segregating in healthy family members. In the PINK1 gene we identified two novel putative pathogenic substitutions, P416R and S419P, located in a conserved motif of the serine/threonine kinase domain. Both substitutions segregated with disease in agreement with a recessive pattern of inheritance within respective families and both were present as homozygous in two affected each. We also discuss common polymorphisms in the two genes found to be co-segregating within families.

CONCLUSION

Our results further extend on the involvement of PINK1 mutations in recessive early-onset parkinsonism with clinical features similar to carriers of parkin mutations.

Progress in the pathogenesis and genetics of Parkinson's disease

Recent progresses in the pathogenesis of sporadic Parkinson's disease (PD) and genetics of familial PD are reviewed. There are common molecular events between sporadic and familial PD, particularly between sporadic PD and PARK1-linked PD due to alpha-synuclein (SNCA) mutations. In sporadic form, interaction of genetic predisposition and environmental factors is probably a primary event inducing mitochondrial dysfunction and oxidative damage resulting in oligomer and aggregate formations of alpha-synuclein. In PARK1-linked PD, mutant alpha-synuclein proteins initiate the disease process as they have increased tendency for self-aggregation. As highly phosphorylated aggregated proteins are deposited in nigral neurons in PD, dysfunctions of proteolytic systems, i.e. the ubiquitin-proteasome system and autophagy-lysosomal pathway, seem to be contributing to the final neurodegenerative process. Studies on the molecular mechanisms of nigral neuronal death in familial forms of PD will contribute further on the understanding of the pathogenesis of sporadic PD.

Impact of recent genetic findings in Parkinson's disease

PURPOSE OF REVIEW

Parkinson's disease is the second most common age-related neurodegenerative disorder and is characterized clinically by classical parkinsonism and pathologically by selective loss of dopaminergic neurons in the substantia nigra and Lewy bodies. Although for most classical parkinsonism the etiology is unknown, a clear genetic component has been determined in a minority. Mutations in five causative genes combined [alpha-Synuclein (SNCA), Parkin, PTEN-induced kinase 1 (PINK1), DJ-1, Leucine-rich repeat kinase 2 (LRRK2)] account for 2-3% of all cases with classical parkinsonism, often clinically indistinguishable from idiopathic Parkinson's disease.

RECENT FINDINGS

The functional role of PINK1 and LRRK2 as kinases has been clearly established. Further, mutations in the ATP13A2 gene have been linked to Kufor-Rakeb syndrome (PARK9), a form of atypical parkinsonism. ATP13A2 encodes a lysosomal ATPase and shows elevated expression levels in the brains of sporadic patients, suggesting a potential role in the more common idiopathic Parkinson's disease. Finally, first promising pilot studies have been performed to identify differentially expressed genes and proteins as biomarkers for parkinsonism.

SUMMARY

The identification of single genes and their functional characterization has enhanced our understanding of the pathogenesis of parkinsonism, has led to improvement of diagnostic tools for genetic parkinsonism, and allows for the purposeful consideration of novel therapeutic targets.

Mitochondrial disorders

PURPOSE OF REVIEW

Mitochondrial disorders are increasingly acknowledged as a major category in clinical neurology. In this review we highlight the most recent advances in the field, including the characterization of new disease genes, new physiopathological insights, and the role of mitochondrial dysfunction in neurodegeneration.

RECENT FINDINGS

Substantial progress has been made on the genetic basis and pathogenic mechanisms in disorders associated with altered mitochondrial DNA stability and expression. These defects include a wide spectrum of neurological conditions caused by genetic abnormalities of the mitochondrial replication and translation machineries, and of the metabolic pathways controlling the nucleotide supply to organelles, cells and tissues. Another relevant contribution has been given to the molecular dissection of coenzyme Q deficiency, a clinically heterogeneous, potentially treatable condition, thanks to the biochemical and genetic characterization of the first defects in coenzyme Q biosynthesis. Finally, the genetic determinants controlling the penetrance of mitochondrial disorders, as well as the role of mitochondrial dysfunction in neurodegenerative conditions such as Parkinson's and Huntington's diseases, have been investigated in both patients and animal models.

SUMMARY

The dual genetic contribution controlling mitochondrial biogenesis, and the intricacy and universality of the metabolic pathways operating in the mitochondrion explain the complexity of what is now known as 'mitochondrial medicine'.

Mutations in the glucocerebrosidase gene are associated with early-onset Parkinson disease

OBJECTIVE

To evaluate the frequency of glucocerebrosidase (GBA) mutations in cases and controls enrolled in the Genetic Epidemiology of Parkinson's Disease (GEPD) study.

METHODS

We sequenced all exons of the GBA gene in 278 Parkinson disease (PD) cases and 179 controls enrolled in GEPD, with a wide range of age at onset (AAO), and that included a subset of 178 Jewish cases and 85 Jewish controls. Cases and controls were recruited without knowledge of family history of PD, and cases were oversampled in the AAO < 50 years category.

RESULTS

13.7% of PD cases (38/278) carried GBA mutations, compared with 4.5% of controls (8/179) (odds ratio [OR] 3.4, 95% CI 1.5 to 7.4). The frequency of GBA mutations was 22.2% in 90 cases with AAO < or = 50 years, compared with 9.7% in 185 cases with AAO > 50 years (OR 2.7, 95% CI 1.3 to 5.3). Adjusting for age at the time of evaluation, sex, family history of PD, and Jewish ancestry, GBA carriers had a 1.7-year-earlier AAO of PD (95% CI 0.5 to 3.3, p < 0.04) than noncarriers. The average AAO of PD was 2.5 years earlier in carriers with an AAO < or = 50 years compared with noncarriers (95% CI 0.6 to 4.5, p < 0.01) and this was not seen in the AAO > 50 years group. The frequency of GBA mutations was higher in a subset of 178 cases that reported four Jewish grandparents (16.9%) than in cases who did not report Jewish ancestry (8.0%) (p < 0.01). Nine different GBA mutations were identified in PD cases, including 84insGG, E326K, T369M, N370S, D409H, R496H, L444P, RecNciI, and a novel mutation, P175P.

CONCLUSIONS

This study suggests that the Glucocerebrosidase gene may be a susceptibility gene for Parkinson disease and that Glucocerebrosidase mutations may modify age at onset.

Genetics of Parkinson's disease and parkinsonism

The past 10 years has seen a shift in our etiological concepts of Parkinson's disease, moving from a nearly exclusively environmentally mediated disease towards a complex disorder with important genetic contributors. The identification of responsible mutations in certain genes, particularly alpha-synuclein, Parkin, PINK1, DJ-1 and LRRK2, has increased our understanding of the clinical and pathological changes underlying Parkinson's disease, with implications for patient diagnosis, management and future research. This review will outline the specific genetic advances, discuss their implications for clinical practice and hint at future directions for research into this common and disabling disease.