PINK1 heterozygous rare variants: prevalence, significance and phenotypic spectrum

The Landscape of Monogenic Parkinson's Disease in Populations of Non-European Ancestry: A Narrative Review

INTRODUCTION

There has been a bias in the existing literature on Parkinson's disease (PD) genetics as most studies involved patients of European ancestry, mostly in Europe and North America. Our target was to review published research data on the genetic profile of PD patients of non-European or mixed ancestry.

METHODS

We reviewed articles published during the 2000-2023 period, focusing on the genetic status of PD patients of non-European origin (Indian, East and Central Asian, Latin American, sub-Saharan African and Pacific islands).

RESULTS

There were substantial differences regarding monogenic PD forms between patients of European and non-European ancestry. The G2019S Leucine Rich Repeat Kinase 2 (LRRK2) mutation was rather scarce in non-European populations. In contrast, East Asian patients carried different mutations like p.I2020T, which is common in Japan. Parkin (PRKN) variants had a global distribution, being common in early-onset PD in Indians, in East Asians, and in early-onset Mexicans. Furthermore, they were occasionally present in Black African PD patients. PTEN-induced kinase 1 (PINK1) and PD protein 7 (DJ-1) variants were described in Indian, East Asian and Pacific Islands populations. Glucocerebrosidase gene variants (GBA1), which represent an important predisposing factor for PD, were found in East and Southeast Asian and Indian populations. Different GBA1 variants have been reported in Black African populations and Latin Americans.

CONCLUSIONS

Existing data reveal a pronounced heterogeneity in the genetic background of PD. A number of common variants in populations of European ancestry appeared to be absent or scarce in patients of diverse ethnic backgrounds. Large-scale studies that include genetic screening in African, Asian or Latin American populations are underway. The outcomes of such efforts will facilitate further clinical studies and will possibly contribute to the identification of either new pathogenic mutations in already described genes or novel PD-related genes.

Genetic Architecture of Parkinson's Disease

Year 2022 marks 25 years since the first mutation in familial autosomal dominant Parkinson's disease was identified. Over the years, our understanding of the role of genetic factors in the pathogenesis of familial and idiopathic forms of Parkinson's disease has expanded significantly - a number of genes for the familial form of the disease have been identified, and DNA markers for an increased risk of developing its sporadic form have been found. But, despite all the success achieved, we are far from an accurate assessment of the contribution of genetic and, even more so, epigenetic factors to the disease development. The review summarizes the information accumulated to date on the genetic architecture of Parkinson's disease and formulates issues that need to be addressed, which are primarily related to the assessment of epigenetic factors in the disease pathogenesis.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

PINK1 kinase dysfunction triggers neurodegeneration in the primate brain without impacting mitochondrial homeostasis

In vitro studies have established the prevalent theory that the mitochondrial kinase PINK1 protects neurodegeneration by removing damaged mitochondria in Parkinson's disease (PD). However, difficulty in detecting endogenous PINK1 protein in rodent brains and cell lines has prevented the rigorous investigation of the in vivo role of PINK1. Here we report that PINK1 kinase form is selectively expressed in the human and monkey brains. CRISPR/Cas9-mediated deficiency of PINK1 causes similar neurodegeneration in the brains of fetal and adult monkeys as well as cultured monkey neurons without affecting mitochondrial protein expression and morphology. Importantly, PINK1 mutations in the primate brain and human cells reduce protein phosphorylation that is important for neuronal function and survival. Our findings suggest that PINK1 kinase activity rather than its mitochondrial function is essential for the neuronal survival in the primate brains and that its kinase dysfunction could be involved in the pathogenesis of PD.

Genotype-Phenotype Correlations in Monogenic Parkinson Disease: A Review on Clinical and Molecular Findings

Parkinson disease (PD) is a complex neurodegenerative disorder, usually with multifactorial etiology. It is characterized by prominent movement disorders and non-motor symptoms. Movement disorders commonly include bradykinesia, rigidity, and resting tremor. Non-motor symptoms can include behavior disorders, sleep disturbances, hyposmia, cognitive impairment, and depression. A fraction of PD cases instead is due to Parkinsonian conditions with Mendelian inheritance. The study of the genetic causes of these phenotypes has shed light onto common pathogenetic mechanisms underlying Parkinsonian conditions. Monogenic Parkinsonisms can present autosomal dominant, autosomal recessive, or even X-linked inheritance patterns. Clinical presentations vary from forms indistinguishable from idiopathic PD to severe childhood-onset conditions with other neurological signs. We provided a comprehensive description of each condition, discussing current knowledge on genotype-phenotype correlations. Despite the broad clinical spectrum and the many genes involved, the phenotype appears to be related to the disrupted cell function and inheritance pattern, and several assumptions about genotype-phenotype correlations can be made. The interest in these assumptions is not merely speculative, in the light of novel promising targeted therapies currently under development.

Parkinsonism and dementia

The aim of the present review is to summarize literature data on dementia in parkinsonian disorders. Cognitive decline and the gradual development of dementia are considered to be key features in the majority of parkinsonian conditions. The burden of dementia in everyday life of parkinsonian patients and their caregivers is vast and can be even more challenging to handle than the motor component of the disease. Common pathogenetic mechanisms involve the aggregation and spreading of abnormal proteins like alpha-synuclein, tau or amyloid in cortical and subcortical regions with subsequent dysregulation of multiple neurotransmitter systems. The degree of cognitive deterioration in these disorders is variable and ranges from mild cognitive impairment to severe cognitive dysfunction. There is also variation in the number and type of affected cognitive domains which can involve either a single domain like executive or visuospatial function or multiple ones. Novel genetic, biological fluid or imaging biomarkers appear promising in facilitating the diagnosis and staging of dementia in parkinsonian conditions. A significant part of current research in Parkinson's disease and other parkinsonian syndromes is targeted towards the cognitive aspects of these disorders. Stabilization or amelioration of cognitive outcomes represents a primary endpoint in many ongoing clinical trials for novel disease modifying treatments in this field. This article is part of the Special Issue "Parkinsonism across the spectrum of movement disorders and beyond" edited by Joseph Jankovic, Daniel D. Truong and Matteo Bologna.

Miro1 Impairment in a Parkinson's At-Risk Cohort

There is a lack of reliable molecular markers for Parkinson’s disease (PD) patients and at-risk individuals. The detection of the pre-symptomatic population of PD will empower more effective clinical intervention to delay or prevent disease onset. We have previously found that the mitochondrial protein Miro1 is resistant to mitochondrial depolarization-induced degradation in fibroblasts from a large number of PD patients and several at-risk individuals. Therefore, Miro1 has the potential to molecularly label PD populations. In order to determine whether Miro1 could serve as a molecular marker for the risk of PD, here we examine the Miro1 response to mitochondrial depolarization by biochemical approaches in induced pluripotent stem cells from a cohort of at-risk individuals. Our results show that the Miro1 phenotype is significantly associated with PD risk. We propose that Miro1 is a promising molecular marker for detecting both PD and at-risk populations. Tracking this Miro1 marker could aid in diagnosis and Miro1-based drug discoveries.

The identified clinical features of Parkinson's disease in homo-, heterozygous and digenic variants of PINK1

To investigate the prevalence and genotype-phenotype correlations of phosphatase and tensin homolog induced putative kinase 1 (PINK1) variants in Parkinson's disease (PD) patients, we analyzed 1700 patients (842 familial PD and 858 sporadic PD patients from Japanese origin). We screened the entire exon and exon-intron boundaries of PINK1 using Sanger sequencing and target sequencing by Ion torrent system. We identified 30 patients with heterozygous variants, 3 with homozygous variants, and 3 with digenic variants of PINK1-PRKN. Patients with homozygous variants presented a significantly younger age at onset than those with heterozygous variants. The allele frequency of heterozygous variants in patients with age at onset at 50 years and younger with familial PD and sporadic PD showed no differences. [¹²³I]meta-iodobenzylguanidine (MIBG) myocardial scintigraphy indicated that half of patients harboring PINK1 heterozygous variants showed a decreased heart to mediastinum ratio (12/23). Our findings emphasize the importance of PINK1 variants for the onset of PD in patients with age at onset at 50 years and younger and the broad spectrum of clinical symptoms in patients with PINK1 variants.

Brain mitochondrial impairment in early-onset Parkinson's disease with or without PINK1 mutation

BACKGROUND

PINK1 mutations are likely to affect mitochondrial function. The objective of this study was to study brain mitochondrial function in patients with early-onset Parkinson's disease, with or without PINK1 mutations.

METHODS

We investigated brain intracellular pH, mitochondrial activity, and energetics with functional magnetic resonance spectroscopy in patients with early-onset Parkinson's disease with PINK1 mutations (n = 10), early-onset Parkinson's disease without PINK1 mutations (n = 10), and healthy sex- and age-matched subjects (n = 20). We measured peak areas of phosphocreatine and beta adenosine triphosphate.

RESULTS

The EOPD- group had normal PCr + βATP contents at rest (P = NS) and under activation (P = NS), but reduced contents during recovery (P < 0.001). The EOPD+ group had abnormal PCr + βATP contents at rest (P < 0.001) and during activation (P < 0.001); during recovery, the contents only partially recovered (P < 0.001). Brain intracellular pH alterations were more severe with EOPD+ than with EOPD-.

CONCLUSIONS

Brain mitochondrial impairments were similar in early-onset Parkinson's disease without PINK1 mutations and late-onset Parkinson's disease. However, mitochondrial impairments were more severe in early-onset Parkinson's disease with PINK1 mutations. © 2020 International Parkinson and Movement Disorder Society.

Hypoxia regulates the mitochondrial activity of hepatocellular carcinoma cells through HIF/HEY1/PINK1 pathway

Hypoxia is commonly found in cancers. Hypoxia, due to the lack of oxygen (O₂) as the electron recipient, causes inefficient electron transfer through the electron transport chain at the mitochondria leading to accumulation of reactive oxygen species (ROS) which could create irreversible cellular damages. Through hypoxia-inducible factor 1 (HIF-1) which elicits various molecular events, cells are able to overcome low O₂. Knowledge about the new molecular mechanisms governed by HIF-1 is important for new therapeutic interventions targeting hypoxic tumors. Using hepatocellular carcinoma (HCC) as a model, we revealed that the HIF-1 and the Notch signaling pathways cross-talk to control mitochondrial biogenesis of cancer cells to maintain REDOX balance. From transcriptome sequencing, we found that HEY1, a transcriptional repressor, in the NOTCH pathway was consistently induced by hypoxia in HCC cell lines. We identified a strong hypoxia response element (HRE) in HEY1 by chromatin immunoprecipitation (ChIP) and luciferase reporter assays. Transcriptome and ChIP sequencing further identified PINK1, a gene essential for mitochondrial biogenesis, as a novel transcriptional target of HEY1. HCC cells with HEY1 knockdown re-expressed PINK1. HEY1 and PINK1 expressions inversely correlated in human HCC samples. Overexpression of HEY1 and under-expression of PINK1 were detected in human HCC and associated with poor clinical outcomes. Functionally, we found that overexpression of HEY1 or knockdown of PINK1 consistently reduced mitochondrial cristae, mitochondrial mass, oxidative stress level, and increased HCC growth.

The Role of Genetic Testing in the Clinical Practice and Research of Early-Onset Parkinsonian Disorders in a Hungarian Cohort: Increasing Challenge in Genetic Counselling, Improving Chances in Stratification for Clinical Trials

The genetic analysis of early-onset Parkinsonian disorder (EOPD) is part of the clinical diagnostics. Several genes have been implicated in the genetic background of Parkinsonism, which is clinically indistinguishable from idiopathic Parkinson's disease. The identification of patient's genotype could support clinical decision-making process and also track and analyse outcomes in a comprehensive fashion. The aim of our study was to analyse the genetic background of EOPD in a Hungarian cohort and to evaluate the clinical usefulness of different genetic investigations. The age of onset was between 25 and 50 years. To identify genetic alterations, multiplex ligation-dependent probe amplification (n = 142), Sanger sequencing of the most common PD-associated genes (n = 142), and next-generation sequencing (n = 54) of 127 genes which were previously associated to neurodegenerative disorders were carried out. The genetic analysis identified several heterozygous damaging substitutions in PD-associated genes (C19orf12, DNAJC6, DNAJC13, EIF4G1, LRRK2, PRKN, PINK1, PLA2G6, SYNJ1). CNVs in PRKN and SNCA genes were found in five patients. In our cohort, nine previously published genetic risk factors were detected in three genes (GBA, LRRK2, and PINK1). In nine cases, two or three coexisting pathogenic mutations and risk variants were identified. Advances of sequencing technologies make it possible to aid diagnostics of PD by widening the scope of analysis to genes which were previously linked to other neurodegenerative disorders. Our data suggested that rare damaging variants are enriched versus neutral variants, among PD patients in the Hungarian population, which raise the possibility of an oligogenic effect. Heterozygous mutations of multiple recessive genes involved in the same pathway may perturb the molecular process linked to PD pathogenesis. Comprehensive genetic assessment of individual patients can rarely reveal monogenic cause in EOPD, although it may identify the involvement of multiple PD-associated genes in the background of the disease and may facilitate the better understanding of clinically distinct phenocopies. Due to the genetic complexity of the disease, genetic counselling and management is getting more challenging. Clinical geneticist should be prepared for counselling of patients with coexisting disease-causing mutations and susceptibility factors. At the same time, genomic-based stratification has increasing importance in future clinical trials.

Long Non-coding RNAs Associated With Neurodegeneration-Linked Genes Are Reduced in Parkinson's Disease Patients

Transcriptome analysis has identified a plethora of long non-coding RNAs (lncRNAs) expressed in the human brain and associated with neurological diseases. However, whether lncRNAs expression levels correlate with Parkinson's disease (PD) pathogenesis remains unknown. Herein, we show that a number of lncRNA genes encompassing transcriptional units in close proximity to PD-linked protein-coding genes, including SNCA, LRRK2, PINK1, DJ-1, UCH-L1, MAPT and GBA1, are expressed in human dopaminergic cells and post-mortem material, such as cortex, Substantia Nigra and cerebellum. Interestingly, these lncRNAs are upregulated during neuronal differentiation of SH-SY5Y cells and of dopaminergic neurons generated from human fibroblast-derived induced pluripotent stem cells. Importantly, six lncRNAs are found under-expressed in the nigra and three in the cerebellum of PD patients compared to controls. Simultaneously, SNCA mRNA levels are increased in the nigra, while LRRK2 and PINK1 mRNA levels are decreased both in the nigra and the cerebellum of PD subjects compared to controls, indicating a possible correlation between the expression profile of the respective lncRNAs with their adjacent coding genes. Interestingly, all dysregulated lncRNAs are also detected in human peripheral blood mononuclear cells and four of them in exosomes derived from human cerebrospinal fluid, providing initial evidence for their potential use as diagnostic tools for PD. Our data raise the intriguing possibility that these lncRNAs may be involved in disease pathogenesis by regulating their neighboring PD-associated genes and may thus represent novel targets for the diagnosis and/or treatment of PD or related diseases.

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.

Heterozygous PLA2G6 Mutation Leads to Iron Accumulation Within Basal Ganglia and Parkinson's Disease

Mutations of PLA2G6 gene are responsible for PARK14, an autosomal recessive L-DOPA responsive dystonia/parkinsonism with early/adult onset. This phenotype possesses an high clinical variability, which consists in the occurrence of cerebral and cerebellar atrophy, iron accumulation in the basal ganglia, and cognitive decline. This report describes a PD patient carrying an heterozygous PLA2G6 mutation, which was identified also in his PD affected sister. This patient is characterized by a L-DOPA responsive typical parkinsonian syndrome without the occurrence of dystonia, a slight cognitive decline, presence of iron accumulation both in neo and paleostriatum while cerebellar atrophy was absent. Clinical and imaging features are compatible with the PARK14 phenotype. Although PARK14 has been previously reported to be inherited as a recessive disorder, clinical and genetic analysis of this proband and his family rise the hypothesis that even heterozygous PLA2G6 mutations may cause PARK14. It remains to be analyzed whether these heterozygous variants may act as dominant mutations, or they merely increase the risk to develop PD by acting within a context of synergistic genetic and/or environmental backgrounds.

Pooled-DNA target sequencing of Parkinson genes reveals novel phenotypic associations in Spanish population

Eighteen loci and several susceptibility genes have been related to Parkinson's disease (PD). However, most studies focus on single genes in small PD series. Our aim was to establish the genetic background of a large Spanish PD sample. Pooled-DNA target sequencing of 7 major PD genes (SNCA, PARK2, PINK1, DJ-1, LRRK2, GBA, and MAPT) was performed in 562 PD cases. Forty-four variants were found among 114 individuals (20.28%, p<0.05). Among these variants, 30 were found in Mendelian genes (68.18%) and 14 in PD susceptibility genes (31.82%). Seven novel variants were identified. Interestingly, most variants were found in PARK2 and PINK1 genes, whereas SNCA and DJ-1 variants were rare. Validated variants were also genotyped in Spanish healthy controls (n = 597). Carriers of heterozygous PARK2 variants presented earlier disease onset and showed dementia more frequently. PD subjects carrying 2 variants at different genes (1.42%) had an earlier age of onset and a predominantly akinetic-rigid PD phenotype (55.6%, p < 0.05), suggesting that the accumulation of genetic risk variants could modify PD phenotype.

PINK1 p.K520RfsX3 mutation identified in a Chinese family with early-onset Parkinson's disease

Parkinson's disease (PD) features selective loss of dopaminergic neurons of the substantia nigra pars compacta accompanied by the accumulation and aggregation of alpha-synuclein in Lewy bodies. PTEN induced putative kinase 1 gene (PINK1) mutations are the second most common genetic cause of autosomal recessive early-onset Parkinson's disease (EOPD). A single nucleotide deletion in PINK1 exon 8 (c.1557delG) was identified in a consanguineous Chinese family with EOPD. The homozygous deletion was co-segregated with disease in the family and resulted in a frameshift after codon 520 with a premature termination at codon 522 (p.K520RfsX3). These findings have significant implications on genetic counseling for the family and may be helpful in considering potential pathogenesis-targeted and disease-modifying strategies which should further improve patient quality of life.

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.

Early Onset Parkinson's Disease in a family of Moroccan origin caused by a p.A217D mutation in PINK1: a case report

BACKGROUND

Bi-allelic mutations in the genes Parkin (PARK2), PINK1 (PARK6) and DJ-1 (PARK7) are established causes of autosomal recessive early-onset Parkinson's Disease (EOPD). PINK1 mutations are the second commonest cause of EOPD. Specific mutations may be relatively common in certain populations because of a founder effect. Homozygous p.A217D PINK1 mutations were previously shown to cause EOPD in a large Sudanese kindred.

CASE PRESENTATION

Here we report the segregation of homozygous PINK1 p.A217D mutations in a family originating in Morocco with a history of parental consanguinity. From the clinical information available for the index case, the phenotype of mild, slowly-progressive Parkinsonism is consistent with previous reports of p.A217D disease and of PINK1 disease phenotype more generally. The reported features of early prominent lower-limb symptoms and gait disturbance with asymmetrical onset are more frequent among PINK1 disease cases.

CONCLUSIONS

Together, reports of p.A217D in families of Moroccan and Sudanese origin suggest that p.A217D is a North African mutation due to a founder effect. Wider genetic analyses of EOPD in North Africa would be useful to estimate the prevalence of Parkinsonism caused by PINK1 p.A217D. In the absence of bi-allelic Parkin mutations, PINK1 mutations should be considered in cases with evidence of autosomal recessive inheritance of EOPD and presentation of atypical features such as early lower-limb symptoms and gait disturbance with asymmetrical onset, which appear to be common in Mendelian EOPD.

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.

Early-Onset Parkinsonism: Case Report and Review of the Literature

BACKGROUND

Early-onset parkinsonism can be caused by PTEN-induced putative kinase 1 (PINK1) gene defects and is usually characterized by an age of onset in the fourth decade of life, slow disease progression, resting tremor, rigidity, bradykinesia, postural instability, and levodopa-induced dyskinesia.

METHODS

We evaluated a child with early-onset symptoms and performed a literature review for previously reported examples of children aged 18 years or less with PINK1 gene defects.

RESULTS

We describe a five-year-old boy with autosomal recessive early-onset parkinsonism caused by a homozygous missense mutation in the PINK1 gene. This is the youngest individual yet reported with early-onset parkinsonism.

CONCLUSION

PINK1-type of early-onset parkinsonism can occur in very young patients, and phenotypic expression of PINK1 mutations may depend on age of onset and ethnicity.

PINK1 in the limelight: multiple functions of an eclectic protein in human health and disease

The gene PINK1 [phosphatase and tensin homologue (PTEN)-induced putative kinase 1] encodes a serine/threonine kinase which was initially linked to the pathogenesis of a familial form of Parkinson's disease. Research on PINK1 has recently unravelled that its multiple functions extend well beyond neuroprotection, implicating this eclectic protein in a growing number of human pathologies, including cancer, diabetes, cardiopulmonary dysfunctions, and inflammation. Extensive studies have identified PINK1 as a crucial player in the mitochondrial quality control pathway, required to label damaged mitochondria and promote their elimination through an autophagic process (mitophagy). Mounting evidence now indicates that PINK1 activities are not restricted solely to mitophagy, and that different subcellular and even sub-mitochondrial pools of PINK1 are involved in distinct signalling cascades to regulate cell metabolism and survival. In this review, we provide a concise overview on the different functions of PINK1 and their potential role in human diseases. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

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.

Parkinson's Disease in Saudi Patients: A Genetic Study

Parkinson’s disease (PD) is one of the major causes of parkinsonism syndrome. Its characteristic motor symptoms are attributable to dopaminergic neurons loss in the midbrain. Genetic advances have highlighted underlying molecular mechanisms and provided clues to potential therapies. However, most of the studies focusing on the genetic component of PD have been performed on American, European and Asian populations, whereas Arab populations (excluding North African Arabs), particularly Saudis remain to be explored. Here we investigated the genetic causes of PD in Saudis by recruiting 98 PD-cases (sporadic and familial) and screening them for potential pathogenic mutations in PD-established genes; SNCA, PARKIN, PINK1, PARK7/DJ1, LRRK2 and other PD-associated genes using direct sequencing. To our surprise, the screening revealed only three pathogenic point mutations; two in PINK1 and one in PARKIN. In addition to mutational analysis, CNV and cDNA analysis was performed on a subset of patients. Exon/intron dosage alterations in PARKIN were detected and confirmed in 2 cases. Our study suggests that mutations in the ORF of the screened genes are not a common cause of PD in Saudi population; however, these findings by no means exclude the possibility that other genetic events such as gene expression/dosage alteration may be more common nor does it eliminate the possibility of the involvement of novel genes.

Novel P-TEN-induced putative kinase 1 (PINK1) variant in Indian Parkinson's disease patient

Loss-of-function mutation in PINK1 is known for causing autosomal recessive early onset Parkinsonism accounting approximately 6.5% of PD cases. Recently, PINK1 has also been shown to cause Parkinson's disease (PD) in eastern India. Present study is aimed to see its contribution in north-Indian PD patients. A total of 106 PD patients and 60 ethnically matched healthy controls were included in the study. All the patients were screened for mutation in PINK1 by direct DNA sequence analysis of the PCR amplicons covering all exons and exon-intron boundaries. Identified novel variant was reconfirmed by DNA sequencing of 10 randomly selected TA clones containing the variant amplicon. In vitro functional assay of the mutant protein was performed by transfecting COS-7 cell line with wild type and mutant (created by site-directed-mutagenesis) cDNA construct of PINK1 fused to N' terminal GFP followed by western blot analysis. Two potentially pathogenic, one being novel (p.Q267X) and 6 other apparently non-pathogenic variants were identified. Western blot analysis reveals production of truncated PINK1 fusion protein of ∼55kDa in p.Q267X mutant instead of 82/93kDa of wild type PINK1 fusion protein (molecular weight of GFP is ∼27kDa). Our study concludes that PINK1 variants are prevalent for causing Parkinson's disease (PD) in India, as revealed by the occurrence of 1.8% (2/106) in PD patients from north Indian population. The novel homozygous variant of PINK1 (c.799C>T) reported here is the plausible cause for disease manifestation in this patient. Future study, however, would be helpful to understand the functional mechanism how this premature PINK1 protein (p.Q267X) responds to cellular stress leading to the PD pathophysiology.

Syndromic parkinsonism and dementia associated with OPA1 missense mutations

Objective

Mounting evidence links neurodegenerative disorders such as Parkinson disease and Alzheimer disease with mitochondrial dysfunction, and recent emphasis has focused on mitochondrial dynamics and quality control. Mitochondrial dynamics and mtDNA maintenance is another link recently emerged, implicating mutations in the mitochondrial fusion genes OPA1 and MFN2 in the pathogenesis of multisystem syndromes characterized by neurodegeneration and accumulation of mtDNA multiple deletions in postmitotic tissues. Here, we report 2 Italian families affected by dominant chronic progressive external ophthalmoplegia (CPEO) complicated by parkinsonism and dementia.

Methods

Patients were extensively studied by optical coherence tomography (OCT) to assess retinal nerve fibers, and underwent muscle and brain magnetic resonance spectroscopy (MRS), and muscle biopsy and fibroblasts were analyzed. Candidate genes were sequenced, and mtDNA was analyzed for rearrangements.

Results

Affected individuals displayed a slowly progressive syndrome characterized by CPEO, mitochondrial myopathy, sensorineural deafness, peripheral neuropathy, parkinsonism, and/or cognitive impairment, in most cases without visual complains, but with subclinical loss of retinal nerve fibers at OCT. Muscle biopsies showed cytochrome c oxidase‐negative fibers and mtDNA multiple deletions, and MRS displayed defective oxidative metabolism in muscle and brain. We found 2 heterozygous OPA1 missense mutations affecting highly conserved amino acid positions (p.G488R, p.A495V) in the guanosine triphosphatase domain, each segregating with affected individuals. Fibroblast studies showed a reduced amount of OPA1 protein with normal mRNA expression, fragmented mitochondria, impaired bioenergetics, increased autophagy and mitophagy.

Interpretation

The association of CPEO and parkinsonism/dementia with subclinical optic neuropathy widens the phenotypic spectrum of OPA1 mutations, highlighting the association of defective mitochondrial dynamics, mtDNA multiple deletions, and altered mitophagy with parkinsonism. Ann Neurol 2015;78:21–38

Ultrasound-based motion analysis demonstrates bilateral arm hypokinesia during gait in heterozygous PINK1 mutation carriers

Carriers of a single heterozygous PINK1 (PTEN-induced putative kinase 1) gene mutation provide an ideal opportunity to study the development of parkinsonian motor signs from the very beginning. Measuring tools that reliably represent mild motor symptoms could also facilitate the assessment of future neuroprotective therapies and early diagnosis of Parkinson's disease (PD). We investigated nine family members carrying a heterozygous PINK1 mutation in comparison with 25 age-matched healthy controls. Arm kinematics were quantified during treadmill walking at four different speeds using ultrasound-based motion analysis. Heterozygous PINK1 mutation carriers showed a bilateral reduction of arm swing amplitudes (P = 0.003) and arm anteversion (P = 0.001), which was more pronounced on the predominantly affected body side but also was present, albeit to a lesser degree, contralaterally (amplitude P = 0.01, anteversion P = 0.002, repeated measures analysis of covariance [rmANCOVA]). Single post-hoc comparisons revealed similar results for all speeds on both body sides (P < 0.05) except for 2.0 km/h on the less affected side. A single heterozygous mutation in the PINK1 gene is associated with a bilateral dopaminergic dysfunction in this family. Ultrasound-based three-dimensional motion analysis of arm swing during gait is a suitable tool to quantify even subtle hypokinesia in mildly affected PINK1 mutation carriers, which tends to be easily overlooked on the less affected body side during clinical examination. Therefore, this technique is a promising application in early stage PD and in at-risk populations for the disease.

Genetic mutations in early-onset Parkinson's disease Mexican patients: molecular testing implications

Mutations in PARK2, PINK1, and DJ-1 have been associated with autosomal recessive early-onset Parkinson's disease. Here, we report the prevalence of sequence and structural mutations in these three main recessive genes in Mexican Mestizo patients. The complete sequences of these three genes were analyzed by homo/heteroduplex DNA formation and direct sequencing; exon dosage was determined by multiplex ligation-dependent probe amplification and real-time PCR in 127 patients belonging to 122 families and 120 healthy Mexican Mestizo controls. All individuals had been previously screened for the three most common LRRK2 mutations. The presence of two mutations in compound heterozygous or homozygous genotypes was found in 16 unrelated patients, 10 had mutations in PARK2, six in PINK1, and none in DJ-1. Two PARK2-PINK1 and one PARK2-LRRK2 digenic cases were observed. Novel mutations were identified in PARK2 and PINK1 genes, including PINK1 duplication for the first time. Exon dosage deletions were the most frequent mutations in PARK2 (mainly in exons 9 and 12), followed by those in PINK1. The high prevalence of heterozygous mutations in PARK2 (12.3%) and the novel heterozygous and homozygous point mutations in PINK1 observed in familial and sporadic cases from various states of Mexico support the concept that single heterozygous mutations in recessive Parkinson's disease genes play a pathogenic role. These data have important implications for genetic counseling of Mexican Mestizo patients with early-onset Parkinson's disease. The presence of digenic inheritance underscores the importance of studying several genes in this disease. A step-ordered strategy for molecular diagnosis is proposed.

Mutant PINK1 upregulates tyrosine hydroxylase and dopamine levels, leading to vulnerability of dopaminergic neurons

PINK1 mutations cause autosomal recessive forms of Parkinson disease (PD). Previous studies suggest that the neuroprotective function of wild-type (WT) PINK1 is related to mitochondrial homeostasis. PINK1 can also localize to the cytosol; however, the cytosolic function of PINK1 has not been fully elucidated. In this study we demonstrate that the extramitochondrial PINK1 can regulate tyrosine hydroxylase (TH) expression and dopamine (DA) content in dopaminergic neurons in a PINK1 kinase activity-dependent manner. We demonstrate that overexpression of full-length (FL) WT PINK1 can downregulate TH expression and DA content in dopaminergic neurons. In contrast, overexpression of PD-linked G309D, A339T, and E231G PINK1 mutations upregulates TH and DA levels in dopaminergic neurons and increases their vulnerability to oxidative stress. Furthermore transfection of FL WT PINK1 or PINK1 fragments with the PINK1 kinase domain can inhibit TH expression, whereas kinase-dead (KD) FL PINK1 or KD PINK1 fragments upregulate TH level. Our findings highlight a potential novel function of extramitochondrial PINK1 in dopaminergic neurons. Deregulation of these functions of PINK1 may contribute to PINK1 mutation-induced dopaminergic neuron degeneration. However, deleterious effects caused by PINK1 mutations may be alleviated by iron-chelating agents and antioxidant agents with DA quinone-conjugating capacity.

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.

PINK1 heterozygous mutations induce subtle alterations in dopamine-dependent synaptic plasticity

Homozygous or compound heterozygous mutations in the phosphatase and tensin homolog-induced putative kinase 1 (PINK1) gene are causative of autosomal recessive, early onset Parkinson's disease. Single heterozygous mutations have been detected repeatedly both in a subset of patients and in unaffected individuals, and the significance of these mutations has long been debated. Several neurophysiological studies from non-manifesting PINK1 heterozygotes have demonstrated the existence of neural plasticity abnormalities, indicating the presence of specific endophenotypic traits in the heterozygous state. We performed a functional analysis of corticostriatal synaptic plasticity in heterozygous PINK1 knockout (PINK1(+/-) ) mice using a multidisciplinary approach and observed that, despite normal motor behavior, repetitive activation of cortical inputs to striatal neurons failed to induce long-term potentiation (LTP), whereas long-term depression was normal. Although nigral dopaminergic neurons exhibited normal morphological and electrophysiological properties with normal responses to dopamine receptor activation, a significantly lower dopamine release was measured in the striatum of PINK1(+/-) mice compared with control mice, suggesting that a decrease in stimulus-evoked dopamine overflow acts as a major determinant for the LTP deficit. Accordingly, pharmacological agents capable of increasing the availability of dopamine in the synaptic cleft restored normal LTP in heterozygous mice. Moreover, monoamine oxidase B inhibitors rescued physiological LTP and normal dopamine release. Our results provide novel evidence for striatal plasticity abnormalities, even in the heterozygous disease state. These alterations might be considered an endophenotype to this monogenic form of Parkinson's disease and a valid tool with which to characterize early disease stage and design possible disease-modifying therapies.

Genetic analysis of PARK2 and PINK1 genes in Brazilian patients with early-onset Parkinson's disease

Parkinson's disease is the second most frequent neurodegenerative disorder in the world, affecting 1-2% of individuals over the age of 65. The etiology of Parkinson's disease is complex, with the involvement of gene-environment interactions. Although it is considered a disease of late manifestation, early-onset forms of parkinsonism contribute to 5-10% of all cases. In the present study, we screened mutations in coding regions of PARK2 and PINK1 genes in 136 unrelated Brazilian patients with early-onset Parkinson's disease through automatic sequencing. We identified six missense variants in PARK2 gene: one known pathogenic mutation, two variants of uncertain role, and three nonpathogenic changes. No pathogenic mutation was identified in PINK1 gene, only benign polymorphisms. All putative pathogenic variants found in this study were in heterozygous state. Our data show that PARK2 point mutations are more common in Brazilian early-onset Parkinson's disease patients (2.9%) than PINK1 missense variants (0%), corroborating other studies worldwide.

Recognizing genetic forms of parkinsonism

SUMMARY It has been generally recognized that, in most cases of Parkinson’s disease (PD), the causes are a result of the complex interaction of genetic and environmental factors. Only approximately 10–15% of PD cases appear to be rare forms with a Mendelian genetic cause. There has been an increasing need and demand in daily clinical work to look for a molecular diagnosis, although a number of neurologists feel uneasy when dealing with the growing number of possible genes and risk factors related to PD that are accessible for diagnosis. Research has highlighted the consequences not only for hereditary but also sporadic forms of PD. This review will provide an overview of the principal aspects of the genetics of PD. It will focus on their differences and similarities, and discuss several useful tools for clinicians, and the role and importance of a neurogeneticist.

Behavioral testing regimens in genetic-based animal models of Parkinson's disease: cogencies and caveats

Although the onset and progression of Parkinson's disease (PD) is fundamentally sporadic, identification of several of the genes implicated in the disease has provided significant insight concerning patho-physiological mechanisms potentially underlying sporadic PD. Moreover, such studies have caused a revolution in the way researchers view the disease. Since single genes responsible for rare familial forms of the disease have only been identified within the past few years, animal models based on these defects have only recently been generated, thereby not leaving a lot of time for their evaluation and subsequent improvement. The current article provides an extensive review of the major motor and non-motor behavioral tests used in genetically-induced Parkinsonian animals. Moreover, we assess the insights concerning the etiopathogenesis of PD generated from use of such tests and how these have improved available treatment strategies for alleviating aspects of sporadic and non-sporadic parkinsonism.

The genetic landscape of high-risk neuroblastoma

Neuroblastoma is a malignancy of the developing sympathetic nervous system that often presents with widespread metastatic disease, resulting in survival rates of less than 50%. To determine the spectrum of somatic mutation in high-risk neuroblastoma, we studied 240 affected individuals (cases) using a combination of whole-exome, genome and transcriptome sequencing as part of the Therapeutically Applicable Research to Generate Effective Treatments (TARGET) initiative. Here we report a low median exonic mutation frequency of 0.60 per Mb (0.48 nonsilent) and notably few recurrently mutated genes in these tumors. Genes with significant somatic mutation frequencies included ALK (9.2% of cases), PTPN11 (2.9%), ATRX (2.5%, and an additional 7.1% had focal deletions), MYCN (1.7%, causing a recurrent p.Pro44Leu alteration) and NRAS (0.83%). Rare, potentially pathogenic germline variants were significantly enriched in ALK, CHEK2, PINK1 and BARD1. The relative paucity of recurrent somatic mutations in neuroblastoma challenges current therapeutic strategies that rely on frequently altered oncogenic drivers.

Role of mendelian genes in "sporadic" Parkinson's disease

The molecular mechanisms underlying neuronal degeneration leading to Parkinson's disease (PD) remain unknown. However, it is becoming increasingly clear that genetic factors contribute to its complex pathogenesis. In the past 15 years, the genetic basis of rare forms of PD with Mendelian inheritance, which represent no more than 10% of the cases, has been investigated. More than 18 loci, identified through linkage analysis or genome wide association studies (GWAS), and eight validated genes have been identified so far [parkin, PTEN-induced kinase 1 (PINK1), DJ-1, ATP13A2, SNCA, Leucine-rich repeat kinase 2 (LRRK2), as well as two recently identified possibly causative genes, vacuolar protein sorting 35 (VPS35) and eukaryotic translation initiation factor 4G1 (EIF4G1)]. Many studies have shed light on their implication not only in familial but also in sporadic forms of PD. Recent GWAS have provided convincing evidence that polymorphic variants in these genes also confer an increased risk for late-onset sporadic PD. In addition, heterozygous mutations in GBA have now been well-validated as susceptibility factors for PD. The role of the most relevant associated genes and risk factors in sporadic PD are discussed in this review.

Gene-environment interactions: key to unraveling the mystery of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease. The gradual, irreversible loss of dopamine neurons in the substantia nigra is the signature lesion of PD. Clinical symptoms of PD become apparent when 50-60% of nigral dopamine neurons are lost. PD progresses insidiously for 5-7 years (preclinical period) and then continues to worsen even under the symptomatic treatment. To determine what triggers the disease onset and what drives the chronic, self-propelling neurodegenerative process becomes critical and urgent, since lack of such knowledge impedes the discovery of effective treatments to retard PD progression. At present, available therapeutics only temporarily relieve PD symptoms. While the identification of causative gene defects in familial PD uncovers important genetic influences in this disease, the majority of PD cases are sporadic and idiopathic. The current consensus suggests that PD develops from multiple risk factors including aging, genetic predisposition, and environmental exposure. Here, we briefly review research on the genetic and environmental causes of PD. We also summarize very recent genome-wide association studies on risk gene polymorphisms in the emergence of PD. We highlight the new converging evidence on gene-environment interplay in the development of PD with an emphasis on newly developed multiple-hit PD models involving both genetic lesions and environmental triggers.

Converging environmental and genetic pathways in the pathogenesis of Parkinson's disease

As a prototypic neurodegenerative disorder Parkinson's disease (PD) is characterized by the progressive loss of specific neuronal subpopulations leading to a late-onset movement disorder. Based on familial forms of PD, to date a significant number of genes were identified that allowed first insight into the molecular pathogenesis of this common movement disorder. These pathways include impaired protein degradation and subsequent aggregation within neuronal cells and impaired mitochondrial function followed by energy depletion due to oxidative stress leading to cell death. The respective disease models were supported by pathoanatomical and biochemical findings in brains of sporadic PD patients without apparent genetic contribution to pathogenesis. Indeed recent genetic and epidemiological studies hint to a complex interplay of genetic susceptibility factors and environmental risk factors to converge to processes of pathological protein accumulation and mitochondrial damage that trigger neurodegeneration in PD. Therefore large-scale geneticoepidemiological studies combining genetic whole genome approaches with a detailed ascertainment of environmental exposures are expected to provide important clues to decipher the complexity of neurodegeneration of this most frequent neurodegenerative movement disorder.

Emerging parkinsonian phenotypes

There is no unique way to define Parkinson's disease (PD) clinically. "Classical parkinsonian features" can be found not only in sporadic idiopathic PD patients, but also in other parkinsonian disorders, such as genetic forms associated with mutations in PARK or in other genes. The present review will describe the parkinsonian phenotypes emerging from the new Mendelian genes which have been linked to PD (such as PARK9 and PARK14), the associated dystonia-parkinsonism disorders (such as the syndromes of neurodegeneration with brain iron accumulation) and the emerging data on heterozygous variants of genes which could influence the risk to develop PD and the PD phenotypes (like PD associated with glucose cerebrosidase mutations).

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.