PTPA variants and impaired PP2A activity in early-onset parkinsonism with intellectual disability.

The protein phosphatase 2A complex (PP2A), the major Ser/Thr phosphatase in the brain, is involved in a number of signalling pathways and functions, including the regulation of crucial proteins for neurodegeneration, such as alpha-synuclein, tau and LRRK2. Here, we report the identification of variants in the PTPA/PPP2R4 gene, encoding a major PP2A activator, in two families with early-onset parkinsonism and intellectual disability. We carried out clinical studies and genetic analyses, including genome-wide linkage analysis, whole-exome sequencing, and Sanger sequencing of candidate variants. We next performed functional studies on the disease-associated variants in cultured cells and knock-down of ptpa in Drosophila melanogaster. We first identified a homozygous PTPA variant, c.893T>G (p.Met298Arg), in patients from a South African family with early-onset parkinsonism and intellectual disability. Screening of a large series of additional families yielded a second homozygous variant, c.512C>A (p.Ala171Asp), in a Libyan family with a similar phenotype. Both variants co-segregate with disease in the respective families. The affected subjects display juvenile-onset parkinsonism and intellectual disability. The motor symptoms were responsive to treatment with levodopa and deep brain stimulation of the subthalamic nucleus. In overexpression studies, both the PTPA p.Ala171Asp and p.Met298Arg variants were associated with decreased PTPA RNA stability and decreased PTPA protein levels; the p.Ala171Asp variant additionally displayed decreased PTPA protein stability. Crucially, expression of both variants was associated with decreased PP2A complex levels and impaired PP2A phosphatase activation. PTPA orthologue knock-down in Drosophila neurons induced a significant impairment of locomotion in the climbing test. This defect was age-dependent and fully reversed by L-DOPA treatment. We conclude that bi-allelic missense PTPA variants associated with impaired activation of the PP2A phosphatase cause autosomal recessive early-onset parkinsonism with intellectual disability. Our findings might also provide new insights for understanding the role of the PP2A complex in the pathogenesis of more common forms of neurodegeneration.

International Genetic Testing and Counseling Practices for Parkinson's Disease.

BACKGROUND: There is growing clinical and research utilization of genetic testing in Parkinson's disease (PD), including direct-to-consumer testing. OBJECTIVES: The aim is to determine the international landscape of genetic testing in PD to inform future worldwide recommendations. METHODS: A web-based survey assessing current practices, concerns, and barriers to genetic testing and counseling was administered to the International Parkinson and Movement Disorders Society membership. RESULTS: Common hurdles across sites included cost and access to genetic testing, and counseling, as well as education on genetic counseling. Region-dependent differences in access to and availability of testing and counseling were most notable in Africa. High-income countries also demonstrated heterogeneity, with European nations more likely to have genetic testing covered through insurance than Pan-American and Asian countries. CONCLUSIONS: This survey highlights not only diversity of barriers in different regions but also the shared and highly actionable needs for improved education and access to genetic counseling and testing for PD worldwide. © 2023 International Parkinson and Movement Disorder Society.

Genetic Testing in Parkinson's Disease.

Genetic testing for persons with Parkinson's disease is becoming increasingly common. Significant gains have been made regarding genetic testing methods, and testing is becoming more readily available in clinical, research, and direct-to-consumer settings. Although the potential utility of clinical testing is expanding, there are currently no proven gene-targeted therapies, but clinical trials are underway. Furthermore, genetic testing practices vary widely, as do knowledge and attitudes of relevant stakeholders. The specter of testing mandates financial, ethical, and physician engagement, and there is a need for guidelines to help navigate the myriad of challenges. However, to develop guidelines, gaps and controversies need to be clearly identified and analyzed. To this end, we first reviewed recent literature and subsequently identified gaps and controversies, some of which were partially addressed in the literature, but many of which are not well delineated or researched. Key gaps and controversies include: (1) Is genetic testing appropriate in symptomatic and asymptomatic individuals without medical actionability? (2) How, if at all, should testing vary based on ethnicity? (3) What are the long-term outcomes of consumer- and research-based genetic testing in presymptomatic PD? (4) What resources are needed for clinical genetic testing, and how is this impacted by models of care and cost-benefit considerations? Addressing these issues will help facilitate the development of consensus and guidelines regarding the approach and access to genetic testing and counseling. This is also needed to guide a multidisciplinary approach that accounts for cultural, geographic, and socioeconomic factors in developing testing guidelines. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The South African Parkinson's Disease Study Collection.

Parkinson's disease (PD) incidence is increasing in sub-Saharan Africa. We recruited 687 individuals with PD from different ancestral groups across South Africa. More Afrikaner Europeans had early-onset PD than other ancestral groups. More men had PD than women, with a younger age at onset for men (56 years).

Underrepresented Populations in Parkinson's Genetics Research: Current Landscape and Future Directions.

BACKGROUND: Human genetics research lacks diversity; over 80% of genome-wide association studies have been conducted on individuals of European ancestry. In addition to limiting insights regarding disease mechanisms, disproportionate representation can create disparities preventing equitable implementation of personalized medicine. OBJECTIVE: This systematic review provides an overview of research involving Parkinson's disease (PD) genetics in underrepresented populations (URP) and sets a baseline to measure the future impact of current efforts in those populations. METHODS: We searched PubMed and EMBASE until October 2021 using search strings for "PD," "genetics," the main "URP," and and the countries in Latin America, Caribbean, Africa, Asia, and Oceania (excluding Australia and New Zealand). Inclusion criteria were original studies, written in English, reporting genetic results on PD from non-European populations. Two levels of independent reviewers identified and extracted information. RESULTS: We observed imbalances in PD genetic studies among URPs. Asian participants from Greater China were described in the majority of the articles published (57%), but other populations were less well studied; for example, Blacks were represented in just 4.0% of the publications. Also, although idiopathic PD was more studied than monogenic forms of the disease, most studies analyzed a limited number of genetic variants. We identified just nine studies using a genome-wide approach published up to 2021, including URPs. CONCLUSION: This review provides insight into the significant lack of population diversity in PD research highlighting the immediate need for better representation. The Global Parkinson's Genetics Program (GP2) and similar initiatives aim to impact research in URPs, and the early metrics presented here can be used to measure progress in the field of PD genetics in the future. © 2022 International Parkinson and Movement Disorder Society.

Neurexin 2 p.G849D variant, implicated in Parkinson's disease, increases reactive oxygen species, and reduces cell viability and mitochondrial membrane potential in SH-SY5Y cells.

Parkinson's disease (PD) is a neurodegenerative movement disorder, affecting 1-2% of the human population over 65. A previous study by our group identified a p.G849D variant in neurexin 2α (NRXN2) co-segregating with PD, prompting validation of its role using experimental methods. This novel variant had been found in a South African family with autosomal dominant PD. NRXN2α is an essential synaptic maintenance protein with multiple functional roles at the synaptic cleft. The aim of the present study was to investigate the potential role of the translated protein NRXN2α and the observed mutant in PD by performing functional studies in an in vitro model. Wild-type and mutant NRXN2α plasmids were transfected into SH-SY5Y cells to assess the effect of the mutant on cell viability and apoptosis [(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) Assay; ApoTox-Glo™ Triplex Assay)], mitochondrial membrane potential (MMP; MitoProbe™ JC-1 Assay), mitochondrial network analysis (MitoTracker®) and reactive oxygen species (ROS; ROS-Glo™ H2O2 Assay). Cells transfected with the mutant NRXN2α plasmid showed decreased cell viability and MMP. They also exhibited increased ROS production. However, these cells showed no changes in mitochondrial fragmentation. Our findings led us to speculate that the p.G849D variant may be involved in a toxic feedback loop leading to neuronal death in PD. Mitochondrial dysfunction and synaptic dysfunction have been linked to PD. Therefore, findings from this exploratory study are in line with previous studies connecting these two processes and warrants further investigation into the role of this variant in other cellular and animal models.

Movement of prion-like α-synuclein along the gut-brain axis in Parkinson's disease: A potential target of curcumin treatment.

A pathological hallmark of the neurodegenerative disorder, Parkinson's disease (PD), is aggregation of toxic forms of the presynaptic protein, α-synuclein in structures known as Lewy bodies. α-Synuclein pathology is found in both the brain and gastrointestinal tracts of affected individuals, possibly due to the movement of this protein along the vagus nerve that connects the brain to the gut. In this review, we discuss current insights into the spread of α-synuclein pathology along the gut-brain axis, which could be targeted for therapeutic interventions. The prion-like propagation of α-synuclein, and the clinical manifestations of gastrointestinal dysfunction in individuals living with PD, are discussed. There is currently insufficient evidence that surgical alteration of the vagus nerve, or removal of gut-associated lymphoid tissues, such as the appendix and tonsils, are protective against PD. Furthermore, we propose curcumin as a potential candidate to prevent the spread of α-synuclein pathology in the body by curcumin binding to α-synuclein's non-amyloid ß-component (NAC) domain. Curcumin is an active component of the food spice turmeric and is known for its antioxidant, anti-inflammatory, and potentially neuroprotective properties. We hypothesize that once α-synuclein is bound to curcumin, both molecules are subsequently excreted from the body. Therefore, dietary supplementation with curcumin over one's lifetime has potential as a novel approach to complement existing PD treatment and/or prevention strategies. Future studies are required to validate this hypothesis, but if successful, this could represent a significant step towards improved nutrient-based therapeutic interventions and preventative strategies for this debilitating and currently incurable disorder.

Nuclear Genes Associated with Mitochondrial DNA Processes as Contributors to Parkinson's Disease Risk.

Over the past four decades, mitochondrial dysfunction has been a recurring theme in Parkinson's disease (PD) and is hypothesized to play a central role in its disease pathogenesis. Given the instrumental role of mitochondria in cellular energy production, their dysfunction can be detrimental to highly energy-dependent dopaminergic neurons, known to degenerate in PD. Mitochondria harbor multiple copies of their own genomes (mtDNA), encoding critical respiratory chain complexes required for energy production. Consequently, mtDNA has been investigated as a source of mitochondrial dysfunction in PD. As seen in multiple mitochondrial diseases, deleterious mtDNA variation and mtDNA copy number depletion can impede mtDNA protein synthesis, leading to inadequate energy production in affected cells and the onset of a disease phenotype. As such, high burdens of mtDNA defects but also mtDNA depletion, previously identified in the substantia nigra of PD patients, have been suggested to play a role in PD. Genetic variation in nuclear DNA encoding factors required for replicating, transcribing, and translating mtDNA, could underlie these observed mtDNA changes. Herein we examine this possibility and provide an overview of studies that have investigated whether nuclear-encoded genes associated with mtDNA processes may influence PD risk. Overall, pathway-based analysis studies, mice models, and case reports of mitochondrial disease patients manifesting with parkinsonism all implicate genes encoding factors related to mtDNA processes in neurodegeneration and PD. Most notably, cumulative genetic variation in these genes likely contributes to neurodegeneration and PD risk by acting together in common pathways to disrupt mtDNA processes or impair their regulation. © 2021 International Parkinson and Movement Disorder Society © 2021 International Parkinson and Movement Disorder Society.

Targeted next-generation sequencing identifies novel variants in candidate genes for Parkinson's disease in Black South African and Nigerian patients.

BACKGROUND: The prevalence of Parkinson's disease (PD) is increasing in sub-Saharan Africa, but little is known about the genetics of PD in these populations. Due to their unique ancestry and diversity, sub-Saharan African populations have the potential to reveal novel insights into the pathobiology of PD. In this study, we aimed to characterise the genetic variation in known and novel PD genes in a group of Black South African and Nigerian patients. METHODS: We recruited 33 Black South African and 14 Nigerian PD patients, and screened them for sequence variants in 751 genes using an Ion AmpliSeq™ Neurological Research panel. We used bcftools to filter variants and annovar software for the annotation. Rare variants were prioritised using MetaLR and MetaSVM prediction scores. The effect of a variant on ATP13A2's protein structure was investigated by molecular modelling. RESULTS: We identified 14,655 rare variants with a minor allele frequency ≤ 0.01, which included 2448 missense variants. Notably, no common pathogenic mutations were identified in these patients. Also, none of the known PD-associated mutations were found highlighting the need for more studies in African populations. Altogether, 54 rare variants in 42 genes were considered deleterious and were prioritized, based on MetaLR and MetaSVM scores, for follow-up studies. Protein modelling showed that the S1004R variant in ATP13A2 possibly alters the conformation of the protein. CONCLUSIONS: We identified several rare variants predicted to be deleterious in sub-Saharan Africa PD patients; however, further studies are required to determine the biological effects of these variants and their possible role in PD. Studies such as these are important to elucidate the genetic aetiology of this disorder in patients of African ancestry.

Wild-type and mutant (G2019S) leucine-rich repeat kinase 2 (LRRK2) associate with subunits of the translocase of outer mitochondrial membrane (TOM) complex.

Leucine-rich repeat kinase 2 (LRRK2) is important in various cellular processes including mitochondrial homeostasis and mutations in this gene lead to Parkinson's disease (PD). However, the full spectrum of LRRK2's functions remain to be elucidated. The translocase of outer mitochondrial membrane (TOM) complex is essential for the import of almost all nuclear-encoded mitochondrial proteins and is fundamental for cellular survival. Using co-immunoprecipitation, super-resolution structured illumination microscopy (SR-SIM), and 3D virtual reality (VR) assisted co-localization analysis techniques we show that wild-type and mutant (G2019S) LRRK2 associate and co-localize with subunits of the TOM complex, either under basal (dimethyl sulfoxide, DMSO) or stress-induced (carbonyl cyanide m-chlorophenyl hydrazine, CCCP) conditions. Interestingly, LRRK2 interacted with TOM40 under both DMSO and CCCP conditions, and when the PD causing mutation, G2019S was introduced, the association was not altered. Moreover, overexpression of G2019S LRRK2 resulted in the formation of large, perinuclear aggregates that co-localized with the TOM complex. Taken together, this is the first study to show that both WT and mutant LRRK2 associate with the TOM complex subunits. These findings provide additional evidence for LRRK2's role in mitochondrial function which has important implications for its role in PD pathogenesis.

Frequency of the LRRK2 G2019S mutation in South African patients with Parkinson's disease.

G2019S in LRRK2 is the most common mutation associated with Parkinson's disease (PD). Highest frequencies are in North African Arabic (30-41%) and Ashkenazi Jewish (6-30%) populations, mostly due to founder effects. Here, we investigated the frequency of G2019S in 647 unrelated South African PD patients from different ancestral origins. It was found in only 1.2% (8/647) of patients. Notably, none of the 91 individuals of African ancestry had G2019S. It was present in 1.9% (3/154) and 1% (5/493) of early- and late-onset cases, respectively. The frequency of G2019S exhibits ethnic-specific differences and warrants further study in sub-Saharan African populations.

A review of genome-wide transcriptomics studies in Parkinson's disease.

Parkinson's disease (PD) is a progressive and incurable neurodegenerative disorder. Although numerous genetic and environmental factors have been linked to the aetiology of PD the underlying pathobiology remains poorly understood, hampering the development of improved therapies. Transcriptomics has the potential to reveal significant insights into disease processes. In this review, we focused on published transcriptomics studies on PD with the aim of summarizing studies and identifying common biological pathways. A total of 96 articles were identified as follows: 12 meta-analyses, 21 re-analyses of existing data and 63 original studies. Of the 63 original studies, 33 were performed on brain tissue, 26 on blood, three on cerebrospinal fluid and one on skin. In the brain studies, altered pathways identified included those involved in dopamine metabolism, mitochondrial function, oxidative stress, protein degradation, neuroinflammation, vesicular transport and synaptic transmission. Studies on blood samples revealed alterations in pathways involved in immune function, inflammation, RNA processing, protein chaperones, mitochondrial function and programmed cell death. Limitations of these studies include small sample sizes (generally <40 cases/40 controls) and the application of widely varying statistical analysis and parameters. Only eight studies used the RNA-Seq technique. This review highlights the need for harmonization of transcriptomic approaches and the statistical analyses, and for the data to be deposited into publicly available databases in a standardized format for meta-analyses. Notably, the concordance of several pathways such as mitochondrial function, protein degradation and inflammation, identified in both blood and brain tissues, suggests that the disease process is systemic and not restricted to neurological tissues.

PTRHD1 Loss-of-function mutation in an african family with juvenile-onset Parkinsonism and intellectual disability.

BACKGROUND: The genetic bases of PD in sub-Saharan African (SSA) populations remain poorly characterized, and analysis of SSA families with PD might lead to the discovery of novel disease-related genes. OBJECTIVES: To investigate the clinical features and identify the disease-causing gene in a black South African family with 3 members affected by juvenile-onset parkinsonism and intellectual disability. METHODS: Clinical evaluation, neuroimaging studies, whole-exome sequencing, homozygosity mapping, two-point linkage analysis, and Sanger sequencing of candidate variants. RESULT: A homozygous 28-nucleotide frameshift deletion in the PTRHD1 coding region was identified in the 3 affected family members and linked to the disease with genome-wide significant evidence. PTRHD1 was recently nominated as the disease-causing gene in two Iranian families, each containing 2 siblings with similar phenotypes and homozygous missense mutations. CONCLUSION: Together with the previous reports, we provide conclusive evidence that loss-of-function mutations in PTRHD1 cause autosomal-recessive juvenile parkinsonism and intellectual disability. © 2018 International Parkinson and Movement Disorder Society.

Gitelman syndrome in a South African family presenting with hypokalaemia and unusual food cravings.

BACKGROUND: Gitelman syndrome (GS) is an autosomal recessive renal tubular disorder characterised by renal salt wasting with hypokalaemia, metabolic alkalosis, hypomagnesaemia and hypocalciuria. It is caused by mutations in SLC12A3 encoding the sodium-chloride cotransporter on the apical membrane of the distal convoluted tubule. We report a South African family with five affected individuals presenting with hypokalaemia and unusual food cravings. METHODS: The affected individuals and two unaffected first degree relatives were enrolled into the study. Phenotypes were evaluated through history, physical examination and biochemical analysis of blood and urine. Mutation screening was performed by sequencing of SLC12A3, and determining the allele frequencies of the sequence variants found in this family in 117 ethnically matched controls. RESULTS: The index patient, her sister, father and two aunts had a history of severe salt cravings, fatigue and tetanic episodes, leading to consumption of large quantities of salt and vinegar. All affected individuals demonstrated hypokalaemia with renal potassium wasting. Genetic analysis revealed that the pseudo-dominant pattern of inheritance was due to compound heterozygosity with two novel mutations: a S546G substitution in exon 13, and insertion of AGCCCC at c.1930 in exon 16. These variants were present in the five affected individuals, but only one variant each in the unaffected family members. Neither variant was found in any of the controls. CONCLUSIONS: The diagnosis of GS was established in five members of a South African family through clinical assessment, biochemical analysis and mutation screening of the SLC12A3 gene, which identified two novel putative pathogenic mutations.

Evidence for a common biological pathway linking three Parkinson's disease-causing genes: parkin, PINK1 and DJ-1.

Parkinson's disease (PD) is characterised by the loss of dopaminergic neurons in the midbrain. Autosomal recessive, early-onset cases of PD are predominantly caused by mutations in the parkin, PINK1 and DJ-1 genes. Animal and cellular models have verified a direct link between parkin and PINK1, whereby PINK1 phosphorylates and activates parkin at the outer mitochondrial membrane, resulting in removal of dysfunctional mitochondria via mitophagy. Despite the overwhelming evidence for this interaction, few studies have been able to identify a link for DJ-1 with parkin or PINK1. The aim of this review is to summarise the functions of these three proteins, and to analyse the existing evidence for direct and indirect interactions between them. DJ-1 is able to rescue the phenotype of PINK1-knockout Drosophila models, but not of parkin-knockouts, suggesting that DJ-1 may act in a parallel pathway to that of the PINK1/parkin pathway. To further elucidate a commonality between these three proteins, bioinformatics analysis established that Miro (RHOT1) interacts with parkin and PINK1, and HSPA4 interacts with all three proteins. Furthermore, 30 transcription factors were found to be common amongst all three proteins, with many of them being involved in transcriptional regulation. Interestingly, expression of these proteins and their associated transcription factors are found to be significantly down-regulated in PD patients compared to healthy controls. In summary, this review provides insight into common pathways linking three PD-causing genes and highlights some key questions, the answers to which may provide critical insight into the disease process.

Mitochondrial impairment observed in fibroblasts from South African Parkinson's disease patients with parkin mutations.

Parkinson's disease (PD), defined as a neurodegenerative disorder, is characterized by the loss of dopaminergic neurons in the substantia nigra in the midbrain. Loss-of-function mutations in the parkin gene are a major cause of autosomal recessive, early-onset PD. Parkin has been implicated in the maintenance of healthy mitochondria, although previous studies show conflicting findings regarding mitochondrial abnormalities in fibroblasts from patients harboring parkin-null mutations. The aim of the present study was to determine whether South African PD patients with parkin mutations exhibit evidence for mitochondrial dysfunction. Fibroblasts were cultured from skin biopsies obtained from three patients with homozygous parkin-null mutations, two heterozygous mutation carriers and two wild-type controls. Muscle biopsies were obtained from two of the patients. The muscle fibers showed subtle abnormalities such as slightly swollen mitochondria in focal areas of the fibers and some folding of the sarcolemma. Although no differences in the degree of mitochondrial network branching were found in the fibroblasts, ultrastructural abnormalities were observed including the presence of electron-dense vacuoles. Moreover, decreased ATP levels which are consistent with mitochondrial dysfunction were observed in the patients' fibroblasts compared to controls. Remarkably, these defects did not manifest in one patient, which may be due to possible compensatory mechanisms. These results suggest that parkin-null patients exhibit features of mitochondrial dysfunction. Involvement of mitochondria as a key role player in PD pathogenesis will have important implications for the design of new and more effective therapies.

Screening of two indel polymorphisms in the 5'UTR of the DJ-1 gene in South African Parkinson's disease patients.

Mutations in the DJ-1 gene have been implicated in early-onset Parkinson's disease (PD). Two indel variants (g.168_185del and g.-6_+10del) in the 5'UTR of DJ-1 have been described. Genotyping of both variants in 402 South African PD patients of various ethnicities and 528 ethnically matched controls revealed that they are rare in the South African population. Further studies on these variants in other populations are warranted given their possible role in transcriptional regulation and DJ-1's critical role in alleviating oxidative stress.

Understanding monogenic Parkinson's disease at a global scale.

Until recently, about three-quarters of all monogenic Parkinson's disease (PD) studies were performed in European/White ancestry, thereby severely limiting our insights into genotype-phenotype relationships at global scale. The first systematic approach to embrace monogenic PD worldwide, The Michael J. Fox Foundation Global Monogenic PD (MJFF GMPD) Project, contacted authors of publications reporting individuals carrying pathogenic variants in known PD-causing genes. In contrast, the Global Parkinson's Genetics Program's (GP2) Monogenic Network took a different approach by targeting PD centers not yet represented in the medical literature. Here, we describe combining both efforts in a "merger project" resulting in a global monogenic PD cohort with build-up of a sustainable infrastructure to identify the multi-ancestry spectrum of monogenic PD and enable studies of factors modifying penetrance and expression of monogenic PD. This effort demonstrates the value of future research based on team science approaches to generate comprehensive and globally relevant results.

Population-specific frequencies for LRRK2 susceptibility variants in the Genetic Epidemiology of Parkinson's Disease (GEO-PD) Consortium.

BACKGROUND: Variants within the leucine-rich repeat kinase 2 gene are recognized as the most frequent genetic cause of Parkinson's disease. Leucine-rich repeat kinase 2 variation related to disease susceptibility displays many features that reflect the nature of complex, late-onset sporadic disorders like Parkinson's disease. METHODS: The Genetic Epidemiology of Parkinson's Disease Consortium recently performed the largest genetic association study for variants in the leucine-rich repeat kinase 2 gene across 23 different sites in 15 countries. RESULTS: Herein, we detail the allele frequencies for the novel risk factors (p.A419V and p.M1646T) and the protective haplotype (p.N551K-R1398H-K1423K) nominated in the original publication. Simple population allele frequencies not only can provide insight into the clinical relevance of specific variants but also can help genetically define patient groups. CONCLUSIONS: Establishing individual patient-based genomic susceptibility profiles that incorporate both risk factors and protective factors will determine future diagnostic and treatment strategies.