Parkin variants in North American Parkinson's disease: cases and controls

Case report: Early-onset Parkinson's disease with initial spastic paraparesis and hyperreflexia caused by compound heterozygous PRKN-gene exon 2 and 4 deletions

Pathogenic variants in the Parkin-gene (PRKN) are among the most common genetic causes of early onset Parkinson's disease (EOPD). Patients with EOPD can present with atypical clinical features and misdiagnosis is frequent. Here, we report a clinical phenotype with atypical signs and symptoms of a 35-year-old male patient with EOPD caused by a compound heterozygous PRKN-gene deletion of exons 2 and 4. After the initial diagnosis of stiff person syndrome, the patient was admitted to our department for a second opinion after 8 years of untreated disease progression. The patient presented with prominent spastic paraparesis pronounced on the right side and hyperreflexia as well as Parkinsonism with rigidity predominantly affecting the upper limbs, bradykinesia, and resting tremor. In the diagnostic assessment, magnetic evoked potentials to the anterior tibial muscles showed a low amplitude on the right side, compatible with pyramidal tract disturbance. However, an MRI of the head and the spine did not show any pathologies or atrophy. A [123I] FP-CIT SPECT scan revealed profoundly and left-pronounced reduced striatal uptake suggesting a neurodegenerative Parkinson's syndrome. Even though an acute levodopa challenge did not show marked improvement of symptoms, the chronic levodopa challenge with up to 450 mg/day significantly reduced the rigidity and bradykinesia. Surprisingly, spastic paraparesis and hyperreflexia diminished under dopaminergic treatment. Finally, genetic analysis by next-generation sequencing via copy number variant analysis (CNV) and multiplex ligation-dependent probe amplification (MLPA) confirmed compound heterozygous deletions of exons 2 and 4 in the PRKN-gene. As presented in this case, the awareness of atypical clinical symptoms of EOPD is essential to prevent misdiagnosis in young patients.

Clustered regularly interspaced short palindromic repeats as an advanced treatment for Parkinson's disease

Recently, genome-editing technology like clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 has improved the translational gap in the treatments mediated through gene therapy. The advantages of the CRISPR system, such as, work in the living cells and tissues, candidate this technique for the employing in experiments and the therapy of central nervous system diseases. Parkinson's disease (PD) is a widespread, disabling, neurodegenerative disease induced by dopaminergic neuron loss and linked to progressive motor impairment. Pathophysiological basis knowledge of PD has modified the PD classification model and expresses in the sporadic and familial types. Analyses of the earliest genetic linkage have shown in PD the inclusion of synuclein alpha (SNCA) genomic duplication and SNCA mutations in the familial types of PD pathogenesis. This review analyzes the structure, development, and function in genome editing regulated through the CRISPR/Cas9. Also, it explains the genes associated with PD pathogenesis and the appropriate modifications to favor PD. This study follows the direction by understanding the PD linking analyses in which the CRISPR technique is applied. Finally, this study explains the limitations and future trends of CRISPR service in relation to the genome-editing process in PD patients' induced pluripotent stem cells.

Assessing the relationship between monoallelic PRKN mutations and Parkinson's risk

Biallelic Parkin (PRKN) mutations cause autosomal recessive Parkinson’s disease (PD); however, the role of monoallelic PRKN mutations as a risk factor for PD remains unclear. We investigated the role of single heterozygous PRKN mutations in three large independent case-control cohorts totalling 10 858 PD cases and 8328 controls. Overall, after exclusion of biallelic carriers, single PRKN mutations were more common in PD than controls conferring a >1.5-fold increase in the risk of PD [P-value (P) = 0.035], with meta-analysis (19 574 PD cases and 468 488 controls) confirming increased risk [Odds ratio (OR) = 1.65, P = 3.69E-07]. Carriers were shown to have significantly younger ages at the onset compared with non-carriers (NeuroX: 56.4 vs. 61.4 years; exome: 38.5 vs. 43.1 years). Stratifying by mutation type, we provide preliminary evidence for a more pathogenic risk profile for single PRKN copy number variant (CNV) carriers compared with single nucleotide variant carriers. Studies that did not assess biallelic PRKN mutations or consist of predominantly early-onset cases may be biasing these estimates, and removal of these resulted in a loss of association (OR = 1.23, P = 0.614; n = 4). Importantly, when we looked for additional CNVs in 30% of PD cases with apparent monoallellic PRKN mutations, we found that 44% had biallelic mutations, suggesting that previous estimates may be influenced by cryptic biallelic mutation status. While this study supports the association of single PRKN mutations with PD, it highlights confounding effects; therefore, caution is needed when interpreting current risk estimates. Together, we demonstrate that comprehensive assessment of biallelic mutation status is essential when elucidating PD risk associated with monoallelic PRKN mutations.

Analysis of Heterozygous PRKN Variants and Copy-Number Variations in Parkinson's Disease

BACKGROUND

Biallelic PRKN mutation carriers with Parkinson's disease (PD) typically have an earlier disease onset, slow disease progression, and, often, different neuropathology compared to sporadic PD patients. However, the role of heterozygous PRKN variants in the risk of PD is controversial.

OBJECTIVES

Our aim was to examine the association between heterozygous PRKN variants, including single-nucleotide variants and copy-number variations (CNVs), and PD.

METHODS

We fully sequenced PRKN in 2809 PD patients and 3629 healthy controls, including 1965 late-onset (63.97 ± 7.79 years, 63% men) and 553 early-onset PD patients (43.33 ± 6.59 years, 68% men). PRKN was sequenced using targeted next-generation sequencing with molecular inversion probes. CNVs were identified using a combination of multiplex ligation-dependent probe amplification and ExomeDepth. To examine whether rare heterozygous single-nucleotide variants and CNVs in PRKN are associated with PD risk and onset, we used optimized sequence kernel association tests and regression models.

RESULTS

We did not find any associations between all types of PRKN variants and risk of PD. Pathogenic and likely-pathogenic heterozygous single-nucleotide variants and CNVs were less common among PD patients (1.52%) than among controls (1.8%, false discovery rate-corrected P = 0.55). No associations with age at onset and in stratified analyses were found.

CONCLUSIONS

Heterozygous single-nucleotide variants and CNVs in PRKN are not associated with PD. Molecular inversion probes allow for rapid and cost-effective detection of all types of PRKN variants, which may be useful for pretrial screening and for clinical and basic science studies targeting specifically PRKN patients. © 2020 International Parkinson and Movement Disorder Society.

Negative α-synuclein pathology in the submandibular gland of patients carrying PRKN pathogenic variants

INTRODUCTION

Alpha-synuclein (AS) pathology in the peripheral nervous tissue is a potential pathological biomarker in Parkinson disease (PD). Several studies reported the excellent specificity of the AS pathology of the submandibular gland (SMG) biopsy in PD. PRKN pathogenic variant is one of the major genetic causes of young-onset PD without Lewy pathology in the brain. In this study, we evaluated peripheral AS pathology in the SMG biopsy of patients with PRKN pathogenic variants.

METHODS

We enrolled three young-onset PD patients with PRKN pathogenic variants. Two patients were compound heterozygous for trans-exon 3 and 4 deletions and one patient was heterozygous for an exon 2 duplication. We obtained two submandibular gland tissues with core needle biopsy (18G). The neural structures were identified using neurofilament (NF) immunostaining and the neural tissue in the adjacent section were stained with 129 phophorylated α-synuclein (pAS) antibody.

RESULTS

pAS staining in the SMG was negative in all cases with the PRKN pathogenic variants.

CONCLUSIONS

Our data may support the high specificity of the AS pathology of SMG in α-synuclein associated parkinsonism. Future studies evaluating peripheral neural tissue including the SMG in the elderly healthy population are required to validate the role of peripheral AS pathology as a biomarker in PD.

CRISPR System: A High-throughput Toolbox for Research and Treatment of Parkinson's Disease

In recent years, the innovation of gene-editing tools such as the CRISPR/Cas9 system improves the translational gap of treatments mediated by gene therapy. The privileges of CRISPR/Cas9 such as working in living cells and organs candidate this technology for using in research and treatment of the central nervous system (CNS) disorders. Parkinson's disease (PD) is a common, debilitating, neurodegenerative disorder which occurs due to loss of dopaminergic neurons and is associated with progressive motor dysfunction. Knowledge about the pathophysiological basis of PD has altered the classification system of PD, which manifests in familial and sporadic forms. The first genetic linkage studies in PD demonstrated the involvement of Synuclein alpha (SNCA) mutations and SNCA genomic duplications in the pathogenesis of PD familial forms. Subsequent studies have also insinuated mutations in leucine repeat kinase-2 (LRRK2), Parkin, PTEN-induced putative kinase 1 (PINK1), as well as DJ-1 causing familial forms of PD. This review will attempt to discuss the structure, function, and development in genome editing mediated by CRISP/Cas9 system. Further, it describes the genes involved in the pathogenesis of PD and the pertinent alterations to them. We will pursue this line by delineating the PD linkage studies in which CRISPR system was employed. Finally, we will discuss the pros and cons of CRISPR employment vis-à-vis the process of genome editing in PD patients' iPSCs.

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.

The role of monogenic genes in idiopathic Parkinson's disease

In the past two decades, mutations in multiple genes have been linked to autosomal dominant or recessive forms of monogenic Parkinson's disease (PD). Collectively, these monogenic (often familial) cases account for less than 5% of all PD, the majority being apparently sporadic cases. More recently, large-scale genome-wide association studies have identified over 40 loci that increase risk of PD. Importantly, there is overlap between monogenic and sporadic PD genes, particularly for the loci that contain the genes SNCA and LRRK2, which are mutated in monogenic dominant PD. There have also been reports of idiopathic PD cases with heterozygous variants in autosomal recessive genes suggesting that these mutations may increase risk of PD. These observations suggest that monogenic and idiopathic PD may have shared pathogenic mechanisms. Here, we focus mainly on the role of monogenic PD genes that represent pleomorphic risk loci for idiopathic PD. We also discuss the functional mechanisms that may play a role in increasing risk of disease in both monogenic and idiopathic forms.

Novel compound heterozygous mutations in the PARK2 gene identified in a Chinese pedigree with early-onset Parkinson's disease

OBJECTS

To capture point mutations and short insertions/deletions in 49 previously reported genes associated with Parkinson's disease (PD) in a Chinese pedigree with early-onset Parkinson's disease (EOPD)-affected individuals.

METHODS

Clinical examinations and genomic analysis were performed on 21 subjects belonging to three generations of a Chinese family. Target region capture and high-throughput sequencing were used for screening 49 genes, which were previously reported to be associated with PD. The direct Sanger sequencing method in all subjects further verified the abnormal DNA fragments in the PARK2 gene.

RESULTS

Four family members, including a mother (I-1) and her three children (II-2, II-3, and II-7), were diagnosed with PD by clinical manifestations and/or PET/CT imaging analyses. Novel compound heterozygous mutations, consisting of a fragment deletion in exon 1 to 2 (EX 1-2 del) and a splicing point mutation c.619-1 (G > C) in the 6th intron of the PARK2 gene, were identified in II-2, II-3, and II-7. Individual EX 1-2 del or c.619-1 (G > C) mutations were detected in I-1 and the third generation (III-2, 3, 5, 10, and 11).Other mutations were not detected in the 49 known PD-associated genes.

CONCLUSION

Novel compound heterozygous mutations were identified in a Chinese pedigree and might represent a cause of familial EOPD with autosomal dominant inheritance.

Twenty years since the discovery of the parkin gene

Nearly 20 years have passed since we identified the causative gene for a familial Parkinson's disease, parkin (now known as PARK2), in 1998. PARK2 is the most common gene responsible for young-onset Parkinson's disease. It codes for the protein Parkin RBR E3 ubiquitin-protein ligase (PARK2), which directly links to the ubiquitin-proteasome as a ubiquitin ligase. PARK2 is involved in mitophagy, which is a type of autophagy, in collaboration with PTEN-induced putative kinase 1 (PINK1). The PINK1 gene (previously known as PARK6) is also a causative gene for young-onset Parkinson's disease. Both gene products may be involved in regulating quality control within the mitochondria. The discovery of PARK2 as a cause of young-onset Parkinson's disease has had a major impact on other neurodegenerative diseases. The involvement of protein degradation systems has been implicated as a common mechanism for neurodegenerative diseases in which inclusion body formation is observed. The discovery of the involvement of PARK2 in Parkinson's disease focused attention on the involvement of protein degradation systems in neurodegenerative diseases. In this review, we focus on the history of the discovery of PARK2, the clinical phenotypes of patients with PARK2 mutations, and its functional roles.

An updated review of Parkinson's disease genetics and clinicopathological correlations

Knowledge regarding the pathophysiological basis of Parkinson's disease (PD) has been greatly expanded over the past two decades, with extraordinary contributions from the field of genetics. However, genetic classifications became complex, difficult to follow, and at times misleading, by placing well-established monogenic forms of the disease along with others associated with risk loci, often ill characterized. The present paper summarizes the genetic, clinical, and neuropathological findings of the currently described monogenic forms of PD and also approaches the progress made in determining genetic risk factors for PD. Furthermore, the text incorporates the data into a recently proposed classification system that will hopefully bring a "user-friendly" approach to this issue. This paper also highlights a number of inconsistencies regarding classification of PD as a single, unique clinicopathological entity-in fact, in order to achieve the development of truly innovative therapies, PD should probably be regarded clinically as a "Parkinson's disease cluster", instead of a single disease. In the future, we hope that an in-depth and groundbreaking understanding of PD will allow the development of truly disease-modifying therapies that will target the molecular processes responsible for the cascade of pathological events underlying each form of PD.

Genetics of Parkinson's Disease: Genotype-Phenotype Correlations

Since the first discovery of a specific genetic defect in the SNCA gene, encoding for α-synuclein, as a causative factor for Parkinson's disease 20 years ago, a multitude of other genes have been linked to this disease in rare cases with Mendelian inheritance. Furthermore, the genetic contribution to the much more common sporadic disease has been demonstrated through case control association studies and, more recently, genome-wide association studies. Interestingly, some of the genes with Mendelian inheritance, such as SNCA, are also relevant to the sporadic disease, suggesting common pathogenetic mechanisms. In this review, we place an emphasis on Mendelian forms, and in particular genetic defects which present predominantly with Parkinsonism. We provide details into the particular phenotypes associated with each genetic defect, with a particular emphasis on nonmotor symptoms. For genetic defects for whom a sufficient number of patients has been assessed, there are evident genotype-phenotype correlations. However, it should be noted that patients with the same causative mutation may present with distinctly divergent phenotypes. This phenotypic variability may be due to genetic, epigenetic or environmental factors. From a clinical and genetic point of view, it will be especially interesting in the future to identify genetic factors that modify disease penetrance, the age of onset or other specific phenotypic features.

Heterozygote carriers for CNVs in PARK2 are at increased risk of Parkinson's disease

Together with point mutations, homozygous deletions or duplications in PARK2 are responsible for the majority of autosomal recessive juvenile Parkinsonism. It is debated, however, whether heterozygous carriers of these mutations are at increased risk of Parkinson's disease (PD). Our goal was to determine whether heterozygous carriers of copy number variants (CNVs) affecting exons of the PARK2 gene are at risk of PD that is greater than that of non-carriers. We searched for CNVs affecting exons of PARK2 in a sample of 105 749 genotyped Icelanders. In total, 989 carriers, including 24 diagnosed with PD, were identified. The heterozygous carriers were tested for association in a sample of 1415 PD patients and 40 474 controls ≥65 years of age. PD patients were more often heterozygous carriers of PARK2 CNVs than controls [odds ratio (OR) = 1.69, P = 0.03] and compound heterozygous PD patients for a CNV and a missense mutation were not found. Furthermore, we conducted a meta-analysis of studies reporting on case-control samples screened for heterozygous PARK2 CNVs. Ten studies were included in the final analysis, with 4538 cases and 4213 controls. The pooled OR and P-value for the published and Icelandic results showed significant association between PARK2 CNVs and risk of PD (OR = 2.11, P = 2.54 × 10(-6)). Our analysis shows that heterozygous carriers of CNVs affecting exons of PARK2 have greater risk of PD than non-carriers.

Evaluation of PARKIN gene variants in West Bengal Parkinson's disease patients

Little information is available regarding the molecular pathogenesis of Parkinson's disease (PD) among the Bengalee population in West Bengal, India. This study was undertaken to determine the contribution of Parkin variants in well-defined ethnically identical Bengalee population of India and further to describe the clinical spectrum associated with these mutations. A total of 150 unrelated PD patients and 150 controls were recruited for the study. The entire cohort was screened for mutations in all the 12 exons of the gene along with flanking splice junctions by polymerase chain reaction and DNA sequencing. Eleven nucleotide variants including two novel changes were detected. Cerebrospinal fluid (CSF) parkin protein expression of the novel mutation, Val186Ile (found in heterozygous condition in one patient only) was almost 2.7 folds lower than the controls and other PD patients. Molecular characterization of polymorphisms Ser167Asn and Val380Leu depicted that homozygous Ser167 and Val380 are significantly associated with the disease. We did not find any linkage disequilibrium among the SNPs, the low r(2) for every pair of single-nucleotide polymorphisms (SNPs) indicated that these SNPs cannot be tagged by each other. Another novel intronic change, IVS8+48C>T was present in almost equally in PD patients and controls. Among the ethnically defined Bengalee population of West Bengal, occurrence of Parkin mutation is 4% (6/150) of the PD patient pool supported with decreased folds of expression of CSF PARKIN protein. Parkin polymorphisms, Ser167 and Val380 are risk factors for the progression of the disease, and their frequency is greatly influenced by ethnic origin.

Analysis of Dosage Mutation in PARK2 among Korean Patients with Early-Onset or Familial Parkinson's Disease

Background and Purpose

There is some controversy regarding heterozygous mutations of the gene encoding parkin (PARK2) as risk factors for Parkinson's disease (PD), and all previous studies have been performed in non-Asian populations. Dosage mutation of PARK2, rather than a point mutation or small insertion/deletion mutation, was reported to be a risk factor for familial PD; dosage mutation of PARK2 is common in Asian populations.

Methods

We performed a gene-dosage analysis of PARK2 using real-time polymerase chain reaction for 189 patients with early-onset PD or familial PD, and 191 control individuals. In the case of PD patients with heterozygous gene-dosage mutation, we performed a sequencing analysis to exclude compound heterozygous mutations. The association between heterozygous mutation of PARK2 and PD was tested.

Results

We identified 22 PD patients with PARK2 mutations (11.6%). Five patients (2.6%) had compound heterozygous mutations, and 13 patients (6.9%) had a heterozygous mutation. The phase could not be determined in one patient. Three small sequence variations were found in 30 mutated alleles (10.0%). Gene-dosage mutation accounted for 90% of all of the mutations found. The frequency of a heterozygous PARK2 gene-dosage mutation was higher in PD patients than in the controls.

Conclusions

Heterozygous gene-dosage mutation of PARK2 is a genetic risk factor for patients with early-onset or familial PD in Koreans.

Can parkin be a target for future treatment of Parkinson's disease?

INTRODUCTION

Parkinson's disease (PD) is one of the most common neurodegenerative diseases affecting an increasing number of people worldwide with the ageing society. Although the etiology of PD remains largely unknown, it is now clear that genetic factors contribute to the pathogenesis of the disease. Recently, several causative genes have been identified in mendelian forms of PD. Growing evidence indicates that their gene products play important roles in oxidative stress response, mitochondrial function, and the ubiquitin-proteasome system, which are also implicated in idiopathic PD, suggesting that these gene products share a common pathway to nigral degeneration in both familial and idiopathic PD. However, treatment options are currently limited.

AREAS COVERED

Recently, a possible role of parkin, a gene product of PARK2-liked PD, in neuroprotection has been suggested. To this regard, several investigations have focused on the possible contribution of parkin in neurotoxic insults. In this article, the role of parkin in the pathogenesis of PD and the potential of parkin as a therapeutic target in PD will be discussed.

EXPERT OPINION

There is an urgent need to develop novel therapeutic options to better manage patients with PD. The data discussed in this article provide rationale for parkin as a therapeutic target.

A meta-analysis of the relationship of the Parkin p.Val380Leu polymorphism to Parkinson's disease

Parkinson's disease (PD) is one of the most common movement disorders. Parkin p.Val380Leu polymorphism (c.1239G > C) has been investigated as a potential genetic hallmark of PD, but studies examining the association between the polymorphism and PD have reported conflicting results. Therefore, we conducted a meta-analysis to assess the influence of Parkin p.Val380Leu polymorphism on the susceptibility of PD. Computer and hand searches of the literature were conducted using the MEDLINE, EMBASE, Cochrane Library, and China Academic Journals databases to identify studies addressing the association between the Parkin p.Val380Leu polymorphism and PD risk. We performed analyses of study characteristics, heterogeneity, and funnel plot asymmetry in analyses analogous to additive, dominant, recessive, homozygous, and heterozygous genetic models with the odds ratio (OR) as the measure of association. A total of 11 case-control studies involving 2,073 PD cases and 2,131 controls were included. When all 11 studies were pooled into the analysis, the presence of the Leu allele at the Parkin p.Val389Leu polymorphism was associated with decreased risk for PD in three genetic comparison models: OR in additive model: 0.79, 95% confidence interval (CI) = 0.64-0.98, P = 0.029; OR in recessive model: 0.55, 95% CI = 0.35-0.89, P = 0.014; OR in homozygous model: 0.51, 95% CI = 0.32-0.82, P = 0.005. Begg's funnel plot and Egger's test provided visual and statistical evidences for funnel plot symmetry, without evidence presence of publication bias. We conclude that the presence of the Leu allele at the Parkin p.Val380Leu polymorphism is associated decreased risk for PD.

The genetics and neuropathology of Parkinson's disease

There has been tremendous progress toward understanding the genetic basis of Parkinson's disease and related movement disorders. We summarize the genetic, clinical and pathological findings of autosomal dominant disease linked to mutations in SNCA, LRRK2, ATXN2, ATXN3, MAPT, GCH1, DCTN1 and VPS35. We then discuss the identification of mutations in PARK2, PARK7, PINK1, ATP13A2, FBXO7, PANK2 and PLA2G6 genes. In particular we discuss the clinical and pathological characterization of these forms of disease, where neuropathology has been important in the likely coalescence of pathways highly relevant to typical PD. In addition to the identification of the causes of monogenic forms of PD, significant progress has been made in defining genetic risk loci for PD; we discuss these here, including both risk variants at LRRK2 and GBA, in addition to discussing the results of recent genome-wide association studies and their implications for PD. Finally, we discuss the likely path of genetic discovery in PD over the coming period and the implications of these findings from a clinical and etiological perspective.

Contribution of germline mutations to PARK2 gene inactivation in lung adenocarcinoma

Homozygous germline mutations of the PARK2 gene are responsible for the development of early-onset Parkinson's disease (PD). Homozygous PARK2 mutations have been also detected in lung adenocarcinoma (LADC). However, since heterozygous PARK2 germline mutations are present in a subset of non-PD individuals, the timing for the occurrence of two-hit PARK2 mutations in LADC progression is unclear. Therefore, we comprehensively analyzed mutations, expression and copy number variations of the PARK2 gene in 267 primary LADCs together with the corresponding noncancerous lung cells and 39 LADC cell lines. Heterozygous germline exonic deletions were detected in five patients with LADC, and loss of heterozygosity including the PARK2 locus was detected in 31/267 (11.6%) LADCs. However, homozygous PARK2 inactivation was not detected in any of them, including the five patients with germline mutations. Homozygous PARK2 inactivation was detected in 6/39 (15%) cell lines, two exonic deletions, one exonic duplication, and three point mutations, while heterozygous PARK2 inactivation was detected in two cell lines (both by exonic deletions). These results strongly indicate that somatic PARK2 mutations occur rarely (or do not occur) in LADC development and that germline PARK2 mutations could contribute to LADC progression but not to LADC development.

Lack of association between p.Ser167Asn variant of Parkin and Parkinson's disease: a meta-analysis of 15 studies involving 2,280 cases and 2,459 controls

Previous clinical trials have evaluated the association between Parkin p.Ser167Asn (c.601G>A) variant and Parkinson's disease (PD) risk. However, the results remain conflicting rather than conclusive. Therefore, we performed this meta-analysis to assess whether pooled results show the association. We performed structured literature searches for studies addressing the association between the Parkin p.Ser167Asn variant and PD risk. We conducted analyses of study characteristics, heterogeneity, and funnel plot asymmetry in analyses analogous to additive, dominant, recessive, and general genetic models with the odds ratio (OR) as the measure of association. When 15 eligible studies (n = 4,739 subjects) were pooled into the meta-analysis, there was no evidence for significant association in additive genetic model between Parkin p. Ser167Asn variant and PD risk (OR = 1.02, 95% confidence interval (CI) = 0.83-1.25; P = 0.866). The OR for the dominant model was 1.06 (95% CI = 0.80-1.41) while the OR for the recessive model was 0.90 (95% CI = 0.71-1.14). The OR for the heterozygous was 1.07 (95% CI = 0.80-1.43) while the OR for the homozygotes was 1.19 (95% CI = 0.81-1.74). In the subgroup analysis by ethnicity, no significant association was found in any genetic model. Begg's funnel plot and Egger's test provided visual and statistical evidences for funnel plot symmetry, suggesting no presence of publication bias. In summary, the meta-analysis strongly suggests that Parkin p. Ser167Asn variant is not associated with PD risk.

What genetics tells us about the causes and mechanisms of Parkinson's disease

Parkinson's disease (PD) is a common motor disorder of mysterious etiology. It is due to the progressive degeneration of the dopaminergic neurons of the substantia nigra and is accompanied by the appearance of intraneuronal inclusions enriched in α-synuclein, the Lewy bodies. It is becoming increasingly clear that genetic factors contribute to its complex pathogenesis. Over the past decade, the genetic basis of rare PD forms with Mendelian inheritance, representing no more than 10% of the cases, has been investigated. More than 16 loci and 11 associated genes have been identified so far; genome-wide association studies have provided convincing evidence that polymorphic variants in these genes contribute to sporadic PD. The knowledge acquired of the functions of their protein products has revealed pathways of neurodegeneration that may be shared between inherited and sporadic PD. An impressive set of data in different model systems strongly suggest that mitochondrial dysfunction plays a central role in clinically similar, early-onset autosomal recessive PD forms caused by parkin and PINK1, and possibly DJ-1 gene mutations. In contrast, α-synuclein accumulation in Lewy bodies defines a spectrum of disorders ranging from typical late-onset PD to PD dementia and including sporadic and autosomal dominant PD forms due to mutations in SCNA and LRRK2. However, the pathological role of Lewy bodies remains uncertain, as they may or may not be present in PD forms with one and the same LRRK2 mutation. Impairment of autophagy-based protein/organelle degradation pathways is emerging as a possible unifying but still fragile pathogenic scenario in PD. Strengthening these discoveries and finding other convergence points by identifying new genes responsible for Mendelian forms of PD and exploring their functions and relationships are the main challenges of the next decade. It is also the way to follow to open new promising avenues of neuroprotective treatment for this devastating disorder.

Genetics of Parkinson's disease and essential tremor

PURPOSE OF REVIEW

This review summarizes some key findings of the past few years on the genetics of the two common movement disorders Parkinson's disease and essential tremor.

RECENT FINDINGS

Within the last two years, genome-wide association (GWA) analyses have revealed a number of novel low-risk susceptibility variants for Parkinson's disease, among them HLA-DRB5, BST1, ACMSD, STK39, MCCC1/LAMP3, SYT11, and CCDC62/HIP1R) and have confirmed LINGO1 as risk factor for essential tremor. The identification of copy number variations in the Parkin gene in healthy control individuals suggests no major role of these variations in late onset Parkinson's disease. Drosophila studies on Parkin and Pink1 have uncovered a role in the mitochondrial quality control pathway in the pathogenesis of the disease. LRRK2 has been found to interact with the microRNAs processing protein Argonaut, thereby affecting protein translation. Notably, despite the high familial risk for essential tremor no high-risk gene has been found to date. The possibility of a nonmendelian transmission in some cases is discussed.

SUMMARY

GWA studies and positional cloning approaches have led to the identification of a number of risk genes for Parkinson's disease, which give novel insights into pathogenic pathways of the disease. In contrast, our knowledge of the genetics of essential tremor is scarce. Except for LINGO1, no other risk gene has so far been identified. New technologies such as next generation high throughput sequencing might help to identify more risk genes.

Parkinson's disease: genetics and pathogenesis

Recent investigation into the mechanisms of Parkinson's disease (PD) has generated remarkable insight while simultaneously challenging traditional conceptual frameworks. Although the disease remains defined clinically by its cardinal motor manifestations and pathologically by midbrain dopaminergic cell loss in association with Lewy bodies, it is now recognized that PD has substantially more widespread impact, causing a host of nonmotor symptoms and associated pathology in multiple regions throughout the nervous system. Further, the discovery and validation of PD-susceptibility genes contradict the historical view that environmental factors predominate, and blur distinctions between familial and sporadic disease. Genetic advances have also promoted the development of improved animal models, highlighted responsible molecular pathways, and revealed mechanistic overlap with other neurodegenerative disorders. In this review, we synthesize emerging lessons on PD pathogenesis from clinical, pathological, and genetic studies toward a unified concept of the disorder that may accelerate the design and testing of the next generation of PD therapies.

Mitochondrial defect and PGC-1α dysfunction in parkin-associated familial Parkinson's disease

Mutations in the parkin gene are expected to play an essential role in autosomal recessive Parkinson's disease. Recent studies have established an impact of parkin mutations on mitochondrial function and autophagy. In primary skin fibroblasts from two patients affected by an early onset Parkinson's disease, we identified a hitherto unreported compound heterozygous mutation del exon2-3/del exon3 in the parkin gene, leading to the complete loss of the full-length protein. In both patients, but not in their heterozygous parental control, we observed severe ultrastructural abnormalities, mainly in mitochondria. This was associated with impaired energy metabolism, deregulated reactive oxygen species (ROS) production, resulting in lipid oxidation, and peroxisomal alteration. In view of the involvement of parkin in the mitochondrial quality control system, we have investigated upstream events in the organelles' biogenesis. The expression of the peroxisome proliferator-activated receptor gamma-coactivator 1-alpha (PGC-1α), a strong stimulator of mitochondrial biogenesis, was remarkably upregulated in both patients. However, the function of PGC-1α was blocked, as revealed by the lack of its downstream target gene induction. In conclusion, our data confirm the role of parkin in mitochondrial homeostasis and suggest a potential involvement of the PGC-1α pathway in the pathogenesis of Parkinson's disease. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

A comprehensive analysis of deletions, multiplications, and copy number variations in PARK2

OBJECTIVES

To perform a comprehensive population genetic study of PARK2. PARK2 mutations are associated with juvenile parkinsonism, Alzheimer disease, cancer, leprosy, and diabetes mellitus, yet ironically, there has been no comprehensive study of PARK2 in control subjects; and to resolve controversial association of PARK2 heterozygous mutations with Parkinson disease (PD) in a well-powered study.

METHODS

We studied 1,686 control subjects (mean age 66.1 ± 13.1 years) and 2,091 patients with PD (mean onset age 58.3 ± 12.1 years). We tested for PARK2 deletions/multiplications/copy number variations (CNV) using semiquantitative PCR and multiplex ligation-dependent probe amplification, and validated the mutations by real-time quantitative PCR. Subjects were tested for point mutations previously. Association with PD was tested as PARK2 main effect, and in combination with known PD risk factors: SNCA, MAPT, APOE, smoking, and coffee intake.

RESULTS

A total of 0.95% of control subjects and 0.86% of patients carried a heterozygous CNV mutation. CNV mutations found in 16 control subjects were all in exons 1-4, sparing exons that encode functionally critical protein domains. Thirteen patients had 2 CNV mutations, 5 had 1 CNV and 1 point mutation, and 18 had 1 CNV mutation. Mutations found in patients spanned exons 2-9. In whites, having 1 CNV was not associated with increased risk (odds ratio 1.05, p = 0.89) or earlier onset of PD (64.7 ± 8.6 heterozygous vs 58.5 ± 11.8 normal).

CONCLUSIONS

This comprehensive population genetic study in control subjects fills the void for a PARK2 reference dataset. There is no compelling evidence for association of heterozygous PARK2 mutations, by themselves or in combination with known risk factors, with PD.

Predictors of parkin mutations in early-onset Parkinson disease: the consortium on risk for early-onset Parkinson disease study

BACKGROUND

Mutations in the parkin gene are the most common genetic cause of early-onset Parkinson disease (PD). Results from a multicenter study of patients with PD systematically sampled by age at onset have not been reported to date.

OBJECTIVE

To determine risk factors associated with carrying parkin mutations.

DESIGN

Cross-sectional observational study.

SETTING

Thirteen movement disorders centers.

PARTICIPANTS

A total of 956 patients with early-onset PD, defined as age at onset younger than 51 years.

MAIN OUTCOME MEASURES

Presence of heterozygous, homozygous, or compound heterozygous parkin mutations.

RESULTS

Using a previously validated interview, 14.7% of patients reported a family history of PD in a first-degree relative. Sixty-four patients (6.7%) had parkin mutations (3.9% heterozygous, 0.6% homozygous, and 2.2% compound heterozygous). Copy number variation was present in 52.3% of mutation carriers (31.6% of heterozygous, 83.3% of homozygous, and 81.0% of compound heterozygous). Deletions in exons 3 and 4 and 255delA were common among Hispanics (specifically Puerto Ricans). Younger age at onset (<40 years) (odds ratio [OR], 5.0; 95% confidence interval [CI], 2.8-8.8; P = .001), Hispanic race/ethnicity (OR compared with white non-Hispanic race/ethnicity, 2.7; 95% CI, 1.3-5.7; P = .009), and family history of PD in a first-degree relative (OR compared with noncarriers, 2.8; 95% CI, 1.5-5.3; P = .002) were associated with carrying any parkin mutation (heterozygous, homozygous, or compound heterozygous). Hispanic race/ethnicity was associated with carrying a heterozygous mutation (OR compared with white non-Hispanic race/ethnicity, 2.8; 95% CI, 1.1-7.2; P = .03) after adjustment for covariates.

CONCLUSIONS

Age at onset, Hispanic race/ethnicity, and family history of PD are associated with carrying any parkin mutation (heterozygous, homozygous, or compound heterozygous) and heterozygous mutations alone. The increased odds of carrying a parkin mutation among Hispanics warrants further study.

Neurodegenerative models in Drosophila: polyglutamine disorders, Parkinson disease, and amyotrophic lateral sclerosis

Neurodegenerative diseases encompass a large group of neurological disorders. Clinical symptoms can include memory loss, cognitive impairment, loss of movement or loss of control of movement, and loss of sensation. Symptoms are typically adult onset (although severe cases can occur in adolescents) and are reflective of neuronal and glial cell loss in the central nervous system. Neurodegenerative diseases also are considered progressive, with increased severity of symptoms over time, also reflective of increased neuronal cell death. However, various neurodegenerative diseases differentially affect certain brain regions or neuronal or glial cell types. As an example, Alzheimer disease (AD) primarily affects the temporal lobe, whereas neuronal loss in Parkinson disease (PD) is largely (although not exclusively) confined to the nigrostriatal system. Neuronal loss is almost invariably accompanied by abnormal insoluble aggregates, either intra- or extracellular. Thus, neurodegenerative diseases are categorized by (a) the composite of clinical symptoms, (b) the brain regions or types of brain cells primarily affected, and (c) the types of protein aggregates found in the brain. Here we review the methods by which Drosophila melanogaster has been used to model aspects of polyglutamine diseases, Parkinson disease, and amyotrophic lateral sclerosis and key insights into that have been gained from these models; Alzheimer disease and the tauopathies are covered elsewhere in this special issue.

Familial Parkinsonism and early onset Parkinson's disease in a Brazilian movement disorders clinic: phenotypic characterization and frequency of SNCA, PRKN, PINK1, and LRRK2 mutations

The aim of the study was to evaluate the frequency and to perform phenotypic and genotypic characterization of familial Parkinsonism and early onset Parkinson's disease (EOPD) in a Brazilian movement disorder unit. We performed a standardized clinical assessment of patients followed by sequencing of PRKN, PINK1 in EOPD cases and SNCA, LRRK2 in familial Parkinsonism individuals. During the period of study (January through December, 2006), we examined 575 consecutive patients of whom 226 (39.3%) met the diagnosis of Parkinsonism and idiopathic Parkinson's disease (IPD) was diagnosed in 202 of the latter. Of the IPD cases, 45 (22.3%) had EOPD. The age at onset in the EOPD cases (n = 45) was 34.8 +/- 5.4 years (mean +/- standard deviation). The age at onset in the familial late-onset PD patients (n = 8) was 52.3 +/- 12.2 years. In the early onset cases, we identified five known mutations in PRKN, two single heterozygous and three compound heterozygous (P153R, T240M, 255Adel, W54R, V3I); in addition, we identified one novel mutation in PINK1 (homozygous deletion of exon 7). In the familial cases (late onset), 1 patient had a novel LRRK2 variant, Q923H, but no SNCA mutations were identified. We have demonstrated that EOPD accounts for a high frequency of IPD cases in our tertiary referral center. PRKN was the most commonly mutated gene, but we also identified a novel mutation in PINK1 and a novel variant in LRRK2.

Molecular pathogenesis of Parkinson disease: insights from genetic studies

Over the past few years, genetic findings have changed our views on the molecular pathogenesis of Parkinson disease (PD), as mutations in a growing number of genes have been found to cause monogenic forms of the disorder. These mutations cause neuronal dysfunction and neurodegeneration either by a toxic gain of function, as in the case of the dominant forms of monogenic PD caused by mutations in the genes for alpha-synuclein or LRRK2, or by a loss of an intrinsic protective function, as is likely for the recessive PD genes parkin (PRKN), PINK1 and DJ-1. Evidence is emerging that at least some of the pathways uncovered in the rare monogenic forms of PD may play a direct role in the aetiology of the common sporadic disorder and that variants of the respective genes contribute to the risk of developing the disease. These findings will allow the search for new treatment strategies that focus on the underlying molecular pathophysiology, rather than simply on ameliorating symptoms.

Bacterial artificial chromosome transgenic mice expressing a truncated mutant parkin exhibit age-dependent hypokinetic motor deficits, dopaminergic neuron degeneration, and accumulation of proteinase K-resistant alpha-synuclein

Recessive mutations in parkin are the most common cause of familial early-onset Parkinson's disease (PD). Recent studies suggest that certain parkin mutants may exert dominant toxic effects to cultured cells and such dominant toxicity can lead to progressive dopaminergic (DA) neuron degeneration in Drosophila. To explore whether mutant parkin could exert similar pathogenic effects to mammalian DA neurons in vivo, we developed a BAC (bacterial artificial chromosome) transgenic mouse model expressing a C-terminal truncated human mutant parkin (Parkin-Q311X) in DA neurons driven by a dopamine transporter promoter. Parkin-Q311X mice exhibit multiple late-onset and progressive hypokinetic motor deficits. Stereological analyses reveal that the mutant mice develop age-dependent DA neuron degeneration in substantia nigra accompanied by a significant loss of DA neuron terminals in the striatum. Neurochemical analyses reveal a significant reduction of the striatal dopamine level in mutant mice, which is significantly correlated with their hypokinetic motor deficits. Finally, mutant Parkin-Q311X mice, but not wild-type controls, exhibit age-dependent accumulation of proteinase K-resistant endogenous alpha-synuclein in substantia nigra and colocalized with 3-nitrotyrosine, a marker for oxidative protein damage. Hence, our study provides the first mammalian genetic evidence that dominant toxicity of a parkin mutant is sufficient to elicit age-dependent hypokinetic motor deficits and DA neuron loss in vivo, and uncovers a causal relationship between dominant parkin toxicity and progressive alpha-synuclein accumulation in DA neurons. Our study underscores the need to further explore the putative link between parkin dominant toxicity and PD.

Parkin and PINK1 mutations in early-onset Parkinson's disease: comprehensive screening in publicly available cases and control

BACKGROUND

Mutations in parkin and PTEN-induced protein kinase (PINK1) represent the two most common causes of autosomal recessive parkinsonism. The possibility that heterozygous mutations in these genes also predispose to disease or lower the age of disease onset has been suggested, but currently there is insufficient data to verify this hypothesis conclusively.

OBJECTIVE

To study the frequency and spectrum of parkin and PINK1 gene mutations and to investigate the role of heterozygous mutations as a risk factor for early-onset Parkinson's disease (PD).

METHODS

All exons and exon-intron boundaries of PINK1 and parkin were sequenced in 250 patients with early-onset PD and 276 normal controls. Gene dosage measurements were also performed, using high-density single-nucleotide polymorphism arrays.

RESULTS

In total 41 variants were found, of which 8 have not been previously described (parkin: p.A38VfsX6, p.C166Y, p.Q171X, p.D243N, p.M458L; PINK1: p.P52L, p.T420T, p.A427E). 1.60% of patients were homozygous or compound heterozygous for pathogenic mutations. Heterozygosity for pathogenic parkin or PINK1 mutations was over-represented in patients compared with healthy controls (4.00% vs. 1.81%) but the difference was not significant (p = 0.13). The mean age at disease onset was significantly lower in patients with homozygous or compound heterozygous mutations than in patients with heterozygous mutations (mean difference 11 years, 95% CI 1.4 to 20.6, p = 0.03). There was no significant difference in the mean age at disease onset in heterozygous patients compared with patients without a mutation in parkin or PINK1 (mean difference 2 years, 95% CI -3.7 to 7.0, p = 0.54).

CONCLUSIONS

Our data support a trend towards a higher frequency of heterozygosity for pathogenic parkin or PINK1 mutations in patients compared with normal controls, but this effect was small and did not reach significance in our cohort of 250 cases and 276 controls.

Identification and validation of control cell lines for accurate parkin dosage analysis

Mutation of the parkin gene (parkin) is the most common cause of early-onset Parkinson's disease and to date over 100 different mutations have been described. However, screening of parkin is complicated by its genomic architecture and context. Notably, dosage alterations in parkin account for greater than 50% of mutations detected in some cohort studies. To improve the accuracy and reproducibility of parkin genomic dosage assays we have identified and analysed cell lines with chromosomal abnormalities affecting 6q26. FISH and real-time PCR analysis identified cell lines with reduced or increased copy number spanning the entire parkin locus. These cell lines represent a valuable resource to facilitate accurate copy number determination of any parkin exon. The reagents are easily obtainable and are compatible with current quantitative technologies and platforms.

Frequency of heterozygous Parkin mutations in healthy subjects: need for careful prospective follow-up examination of mutation carriers

The role of single heterozygous mutations in the putatively recessive Parkin gene in Parkinson disease (PD) is a vividly debated issue, partly caused by the largely unknown frequency of these mutations in healthy individuals. We investigated mutations in all 12 Parkin exons in 356 controls from two European populations including individuals from South Tyrol and Germany. None of the controls carried a homozygous or compound heterozygous mutation. Seventeen carriers of rare heterozygous alterations were detected, of which 13 (13/356; 3.7%) are considered to alter protein structure including four different gene dosage alterations, four missense mutations, and two frameshift mutations. Two of the mutations occurred recurrently in the South Tyrolean population. There was no obvious difference in the mutation frequency between the two populations. One of the presumably healthy mutation carrier was available for re-examination at the age of 67 years. He presented with mild signs of parkinsonism but not fulfilling diagnostic criteria for definite PD. To elucidate the role of heterozygosity is important for genetic testing and counseling of mutation carriers. A detailed clinical prospective and follow-up examination of mutation carriers is required for a better understanding of the role of heterozygous Parkin mutations.

Analysis of Nigerians with apparently sporadic Parkinson disease for mutations in LRRK2, PRKN and ATXN3

Several genetic variations have been associated with Parkinson disease in different populations over the past few years. Although a considerable number of worldwide populations have been screened for these variants, results from Sub-Saharan populations are very scarce in the literature. In the present report we have screened a cohort of Parkinson disease patients (n = 57) and healthy controls (n = 51) from Nigeria for mutations in the genes PRKN, LRRK2 and ATXN3. No pathogenic mutations were found in any of the genes. Hence, common pathogenic mutations in these genes, observed in several different populations, are not a frequent cause of Parkinson disease in Nigeria.

Susceptibility genes in movement disorders

During the last years, remarkable progress in our understanding of molecular genetic mechanisms underlying movement disorders has been achieved. The successes of linkage studies, followed by positional cloning, have dominated the last decade and several genes underlying monogenic disorders have been discovered. The pathobiological understanding garnered from these mutations has laid the foundation for much of the search for genetic loci that confer risk for, rather than cause, disease. With the introduction of whole genome association studies as a novel tool to investigate genetic variation underlying common, complex diseases, a new era in neurogenomics has just begun. As the field rapidly moves forward several new challenges and critical questions in clinical care have to be addressed. In this review, we summarize recent advances in the discovery of susceptibility loci underlying major movement disorders, explain the newest methodologies and tools employed for finding and characterizing genes and discuss how insights into the molecular genetic basis of neurological disorders will impact therapeutic concepts in patient care.

PINK1 mutations and parkinsonism

BACKGROUND

PINK1 loss-of-function causes recessive, early-onset parkinsonism. In Tunisia there is a high rate of consanguineous marriage but PINK1 carrier frequency and disease prevalence have yet to be assessed.

OBJECTIVES

The frequency of PINK1 mutations in familial parkinsonism, community-based patients with idiopathic Parkinson disease (PD) (non-familial PD), and control subjects was determined. Demographic and clinical characteristics of individuals with PINK1 homozygous or heterozygous variants, or without PINK1 mutations, were compared.

METHODS

A total of 92 kindreds (with 208 affected and 340 unaffected subjects), 240 nonfamilial PD, and 368 control participants were recruited from the Institut National de Neurologie, Tunis. Clinical examinations included Hoehn &Yahr, UPDRS, and Epworth scales. PINK1 sequencing and dosage analysis was performed in familial index patients, the variants identified screened in all subjects. Parkin and LRRK2 genes were also examined.

RESULTS

Four PINK1 homozygous mutations, three novel (Q129X, Q129fsX157, G440E, and one previously reported; Q456X), segregate with parkinsonism in 46 individuals in 14 of 92 families (15%). Six of 240 patients with nonfamilial PD were found with either homozygous Q456X or Q129X (2.5%) substitutions. In patients with familial disease, PINK1 homozygotes were younger at disease onset (36 +/- 12 years) than noncarriers (57 +/- 15 years) and more often had an akinetic-rigid presentation at examination and slow progression.

CONCLUSIONS

Segregation of PINK1 mutations with parkinsonism within families, and frequency estimates within population controls, suggested only four PINK1 mutations were pathogenic. Several PINK1 sequence variants are potentially benign and there was no evidence that PINK1 heterozygosity increases susceptibility to idiopathic Parkinson disease.