DNAJC13 mutations in Parkinson disease

Leucine-Rich Repeat Kinase (LRRK2) Genetics and Parkinson's Disease

The discovery of LRRK2 mutations as a cause of Parkinson's disease (PD), including the sporadic late-onset form, established the decisive role of genetics in the field of PD research. Among LRRK2 mutations, the G2019S, mostly lying in a haplotype originating from a common Middle Eastern ancestor, has been identified in different populations worldwide. The G2385R and R1628P variants represent validated risk factors for PD in Asian populations. Here, we describe in detail the origin, the present worldwide epidemiology, and the penetrance of LRRK2 mutations. Furthermore, this chapter aims to characterize other definitely/probably pathogenic mutations and risk variants of LRRK2. Finally, we provide some general guidelines for a LRRK2 genetic testing and counseling. In summary, LRRK2 discovery revolutionized the understanding of PD etiology and laid the foundation for a promising future of genetics in PD research.

Models of LRRK2-Associated Parkinson's Disease

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic causes of Parkinson's disease (PD) and also one of the strongest genetic risk factors in sporadic PD. The LRRK2 protein contains a GTPase and a kinase domain and several protein-protein interaction domains. Both in vitro and in vivo assays in different model systems have provided tremendous insights into the molecular mechanisms underlying LRRK2-induced dopaminergic neurodegeneration. Among all the model systems, animal models are crucial tools to study the pathogenesis of human disease. How do the animal models recapitulate LRRK2-induced dopaminergic neuronal loss in human PD? To answer this question, this review focuses on the discussion of the animal models of LRRK2-associated PD including genetic- and viral-based models.

Absence of Hikeshi, a nuclear transporter for heat-shock protein HSP70, causes infantile hypomyelinating leukoencephalopathy

Genetic leukoencephalopathies are a heterogeneous group of central nervous system disorders with white matter involvement. In a Finnish patient, we identified a novel homozygous disease-causing variant in HIKESHI, c.11G>C, p.(Cys4Ser), leading to hypomyelinating leukoencephalopathy with periventricular cysts and vermian atrophy. A founder Ashkenazi-Jewish disease-causing variant recently linked Hikeshi and its heat-shock protective function to leukoencephalopathy. In our patient, clinical features of lower limb spasticity, optic atrophy, nystagmus, and severe developmental delay were similar to reported patients. Additional features included vermian atrophy, epileptic seizures, and an ovarian tumor. Structural modeling and protein analyses revealed that modified interactions inside Hikeshi's hydrophobic pockets induce protein instability. The patient's cells showed impaired nuclear translocation of HSP70 during heat shock, and decreased ERO1-Lα, an endoplasmic reticulum (ER) oxidoreductase. Overall, we show that: (1) the clinical spectrum associated with Hikeshi deficiency extends to leukoencephalopathy with vermian atrophy and epilepsy; (2) the cellular disease process involves both nuclear chaperone and ER functions.

Mitochondrial Biology and Neurological Diseases

Mitochondria are extremely active organelles that perform a variety of roles in the cell including energy production, regulation of calcium homeostasis, apoptosis, and population maintenance through fission and fusion. Mitochondrial dysfunction in the form of oxidative stress and mutations can contribute to the pathogenesis of various neurodegenerative diseases such as Parkinson’s (PD), Alzheimer’s (AD), and Huntington’s diseases (HD). Abnormalities of Complex I function in the electron transport chain have been implicated in some neurodegenerative diseases, inhibiting ATP production and generating reactive oxygen species that can cause major damage to mitochondria

Mutations in both nuclear and mitochondrial DNA can contribute to neurodegenerative disease, although the pathogenesis of these conditions tends to focus on nuclear mutations. In PD, nuclear genome mutations in the PINK1 and parkin genes have been implicated in neurodegeneration [1], while mutations in APP, PSEN1 and PSEN2 have been implicated in a variety of clinical symptoms of AD [5]. Mutant htt protein is known to cause HD [2]. Much progress has been made to determine some causes of these neurodegenerative diseases, though permanent treatments have yet to be developed. In this review, we discuss the roles of mitochondrial dysfunction in the pathogenesis of these diseases.

Defects in trafficking bridge Parkinson's disease pathology and genetics

Parkinson's disease is a debilitating, age-associated movement disorder. A central aspect of the pathophysiology of Parkinson's disease is the progressive demise of midbrain dopamine neurons and their axonal projections, but the underlying causes of this loss are unclear. Advances in genetics and experimental model systems have illuminated an important role for defects in intracellular transport pathways to lysosomes. The accumulation of altered proteins and damaged mitochondria, particularly at axon terminals, ultimately might overwhelm the capacity of intracellular disposal mechanisms. Cell-extrinsic mechanisms, including inflammation and prion-like spreading, are proposed to have both protective and deleterious functions in Parkinson's disease.

What lysosomes actually tell us about Parkinson's disease?

Parkinson's disease is a common neurodegenerative disorder of unknown origin mainly characterized by the loss of neuromelanin-containing dopaminergic neurons in the substantia nigra pars compacta and the presence of intraneuronal proteinaceous inclusions called Lewy bodies. Lysosomes are dynamic organelles that degrade, in a controlled manner, cellular components delivered via the secretory, endocytic, autophagic and phagocytic membrane-trafficking pathways. Increasing amounts of evidence suggest a central role of lysosomal impairment in PD aetiology. This review provides an update on how genetic evidence support this connection and highlights how the neuropathologic and mechanistic evidence might relate to the disease process in sporadic forms of Parkinson's disease. Finally, we discuss the influence of ageing on lysosomal impairment and PD aetiology and therapeutic strategies targeting lysosomal function.

Novel gene (TMEM230) linked to Parkinson's disease

Mutations in six genes are known to cause Parkinson’s disease (PD) (autosomal dominant: alpha-synuclein, LRRK2, VPS35 and autosomal recessive: Parkin, PINK1 and DJ1) and number of other genes are implicated. In a recent article Deng and colleagues studied a large four generation American family of European descent and linked mutations in a novel gene, transmembrane-protein 230 gene (TMEM230) with lewy body confirmed PD. The authors demonstrated that pathogenic TMEM230 variants in primary mouse neurons affected movement of synaptic vesicles suggesting that TMEM230 may slow vesicular transport. Further experiments in HEK293 cells (carrying the pathogenic TMEM230 variants) showed increased alpha-synuclein levels. This study indicated that the impaired vesicular trafficking may contribute to the pathogenesis of PD. Understanding the various cellular mechanisms leading to PD may lead to the development of novel, much needed therapeutic options. These mechanisms could include: enhanced clearance of damaged mitochondria, development of kinase inhibitors, VPS35/retromer function enhancers or now the possibility of vesicular transport modification.

Genetics of Parkinson's disease

Almost two decades after the identification of SNCA as the first causative gene in Parkinson's disease (PD) and the subsequent understanding that genetic factors play a substantial role in PD development, our knowledge of the genetic architecture underlying this disease has vastly improved. Approximately 5-10% of patients suffer from a monogenic form of PD where autosomal dominant mutations in SNCA, LRRK2, and VPS35 and autosomal recessive mutations in PINK1, DJ-1, and Parkin cause the disease with high penetrance. Furthermore, recent whole-exome sequencing have described autosomal recessive DNAJC6 mutations in predominately atypical, but also cases with typical PD. In addition, several other genes have been linked to atypical Parkinsonian phenotypes. However, the vast majority of PD is genetically complex, i.e. it is caused by the combined action of common genetic variants in concert with environmental factors. By the application of genome-wide association studies, 26 PD risk loci have been established to date. Similar to other genetically complex diseases, these show only moderate effects on PD risk. Increasing this etiologic complexity, many of the involved genetic and environmental risk factors likely interact in an intricate fashion. This article aims to provide a comprehensive overview of the current knowledge in PD genetics.

The genetic background of Parkinson's disease: current progress and future prospects

Almost two decades of genetic research in Parkinson's disease (PD) have remarkably increased our knowledge regarding the genetic basis of PD with numerous genes and genetic loci having been found to cause familial PD or affect the risk for PD. Approximately 5-10% of PD patients have monogenic forms of the disease, exhibiting a classical Mendelian type of inheritance, however, the majority PD cases are sporadic, probably caused by a combination of genetic and environmental risk factors. Nowadays, six genes, alpha synuclein, LRRK2, VPS35, Parkin, PINK1 and DJ-1, have definitely been associated with an autosomal dominant or recessive PD mode of inheritance. The advent of genome-wide association studies (GWAS) and the implementation of new technologies, like next generation sequencing (NGS) and exome sequencing has undoubtedly greatly aided the identification on novel risk variants for sporadic PD. In this review, we will summarize the current progress and future prospects in the field of PD genetics.

The discovery of LRRK2 p.R1441S, a novel mutation for Parkinson's disease, adds to the complexity of a mutational hotspot

Mutations in the LRRK2 gene result in autosomal dominant, late onset Parkinson's disease (PD). Three such mutations (p.R1441C, p.R1441G, and p.R1441H) are known to occur within codon 1441, and haplotype analyses indicate that each one has arisen independently on multiple occasions. We sequenced the entire coding region of 18 casual genes for PD or other parkinsonian neurodegenerative disorders in the proband of a family with autosomal dominant PD. We discovered a new missense mutation in the LRRK2 gene, c.4321C>A (p.R1441S). The mutation was predicted to be highly deleterious in silico (Combined Annotation Dependent Depletion score of 25.5) and segregated with disease in the pedigree. The clinical characteristics of affected family members were similar to those described in PD families with other mutations in LRRK2 codon 1441 and included resting tremor, rigidity, bradykinesia, unilateral onset, and a good response to levodopa. Age at onset ranged from 41 to 76. Two of the affected members of the pedigree underwent detailed, longitudinal neuropsychological testing, and both displayed evidence of mild cognitive deficits at or slightly preceding the onset of motor symptoms. LRRK2 p.R1441S represents the fourth pathogenic mutation observed within codon 1441 and its discovery adds to the remarkable complexity of a mutational hotspot within the ROC domain of the LRRK2 protein. © 2016 Wiley Periodicals, Inc.

Systematic analysis of genetic variants in Han Chinese patients with sporadic Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders. Accumulated evidence confirms that genetic factors play a considerable role in PD pathogenesis. To examine whether point variants or haplotypes are associated with PD development, genotyping of 35 variants in 22 PD-related genes was performed in a well-characterized cohort of 512 Han Chinese PD patients and 512 normal controls. Both Pearson's χ2 test and haplotype analysis were used to evaluate whether variants or their haplotypes were associated with PD in this cohort. The only statistically significant differences in genotypic and allelic frequencies between the patients and the controls were in the DnaJ heat shock protein family (Hsp40) member C10 gene (DNAJC10) variant rs13414223 (P = 0.004 and 0.002, respectively; odds ratio = 0.652, 95% confidence interval: 0.496-0.857). No other variants or haplotypes exhibited any significant differences between these two groups (all corrected P > 0.05). Our findings indicate that the variant rs13414223 in the DNAJC10 gene, a paralog of PD-related genes DNAJC6 and DNAJC13, may play a protective role in PD. This suggests it may be a PD-associated gene.

Conjugal parkinsonism - Clinical, pathology and genetic study. No evidence of person-to-person transmission

INTRODUCTION

Neurodegeneration is known basis of several different Parkinson syndromes. The most common Parkinson syndrome is the Parkinson's disease. Distinction between different Parkinson syndromes is based on pathology or genetic findings. Recent studies indicate that several major variants of PS have some characteristics of a prion disease and may therefore be transmissible. Married couples offer a unique opportunity to study person-to-person transmission and the role of shared environments as the cause of parkinsonism.

METHODS

Autopsy is offered to patients seen at the Movement Disorders Clinic Saskatchewan at no cost. Five couples seen in our clinic, where each spouse had a clinical diagnosis of parkinsonism, came to autopsy.

RESULTS

Median duration of marriage was 42 years before the Parkinson syndrome first manifested in a spouse. Three couples were pathologically or genetically discordant for Parkinson variant. Each spouse in the other two couples had Parkinson's disease. One couple had onset separated by 20 years and one partner had a strong family history of Parkinson's disease.

CONCLUSION

Our data indicate that neither of the Parkinson's disease, Progressive Supranuclear Palsy and Multiple System Atrophy are transmitted by sexual or other intimate contact. The data also indicate against shared environments as the cause of these disorders.

Altered Function of the DnaJ Family Cochaperone DNJ-17 Modulates Locomotor Circuit Activity in a Caenorhabditis elegans Seizure Model

The highly conserved cochaperone DnaJ/Hsp40 family proteins are known to interact with molecular chaperone Hsp70, and can regulate many cellular processes including protein folding, translocation, and degradation. In studies of Caenorhabditis elegans locomotion mutants, we identified a gain-of-function (gf) mutation in dnj-17 closely linked to the widely used e156 null allele of C. elegans GAD (glutamic acid decarboxylase) unc-25. dnj-17 encodes a DnaJ protein orthologous to human DNAJA5. In C. elegansDNJ-17 is a cytosolic protein and is broadly expressed in many tissues. dnj-17(gf) causes a single amino acid substitution in a conserved domain, and behaves as a hypermorphic mutation. The effect of this dnj-17(gf) is most prominent in mutants lacking GABA synaptic transmission. In a seizure model caused by a mutation in the ionotropic acetylcholine receptor acr-2(gf), dnj-17(gf) exacerbates the convulsion phenotype in conjunction with absence of GABA. Null mutants of dnj-17 show mild resistance to aldicarb, while dnj-17(gf) is hypersensitive. These results highlight the importance of DnaJ proteins in regulation of C. elegans locomotor circuit, and provide insights into the in vivo roles of DnaJ proteins in humans.

Usefulness of Genetic Testing in PD and PD Trials: A Balanced Review

An increasing proportion of the individual and population risk to develop Parkinson’s disease (PD) can be explained by genetic variants of different effect strength, forming a continuum from rare high penetrance gain or loss of function mutations to relatively common genetic risk variants that only mildly modify disease risk. In the coming years, further advances in molecular genetic technologies, in particular the increasing use of next generation sequencing, is likely to generate a wealth of new knowledge about the genetic basis of PD. Although specific treatments for PD based on the underlying genetic etiology will probably not be available in the near future, genetic testing is therefore likely to play an increasing role, both in the counselling of individual patients and their families with respect to the expected disease course and recurrence risks, and in the stratification of patient groups in clinical trials. Thus, the usefulness of genetic testing strongly depends on question asked and needs to be considered within each particular setting.

The Evolution of Genetics: Alzheimer's and Parkinson's Diseases

Genetic discoveries underlie the majority of the current thinking in neurodegenerative disease. This work has been driven by the significant gains made in identifying causal mutations; however, the translation of genetic causes of disease into pathobiological understanding remains a challenge. The application of a second generation of genetics methods allows the dissection of moderate and mild genetic risk factors for disease. This requires new thinking in two key areas: what constitutes proof of pathogenicity, and how do we translate these findings to biological understanding. Here we describe the progress and ongoing evolution in genetics. We describe a view that rejects the tradition that genetic proof has to be absolute before functional characterization and centers on a multi-dimensional approach integrating genetics, reference data, and functional work. We also argue that these challenges cannot be efficiently met by traditional hypothesis-driven methods but that high content system-wide efforts are required.

Identification of TMEM230 mutations in familial Parkinson's disease

Parkinson’s disease is the second most common neurodegenerative disorder without effective treatment. It is generally sporadic with unknown etiology. However, genetic studies of rare familial forms have led to the identification of mutations in several genes, which are linked to typical Parkinson’s disease or parkinsonian disorders. The pathogenesis of Parkinson’s disease remain largely elusive. Here, we report a novel genetic locus for an autosomal dominant, clinically typical and Lewy body confirmed Parkinson’s disease on the short arm of chromosome 20 (20pter-p12) and TMEM230 as the disease-causing gene. We show that TMEM230 encodes a transmembrane protein of secretory/recycling vesicles, including synaptic vesicles in neurons. The disease-linked TMEM230 mutants impair synaptic vesicle trafficking. Our data provide the first genetic evidence that a mutant transmembrane protein of synaptic vesicles in neurons is etiologically linked to Parkinson’s disease, with novel implications in understanding the pathogenic mechanism of Parkinson’s disease and for developing rational therapies.

DNAJC13 p.Asn855Ser mutation screening in Parkinson's disease and pathologically confirmed Lewy body disease patients

BACKGROUND

Recently, a novel mutation in exon 24 of DNAJC13 gene (p.Asn855Ser, rs387907571) has been reported to cause autosomal dominant Parkinson's disease (PD) in a multi-incident Mennonite family.

METHODS

In the present study the mutation containing exon of the DNAJC13 gene has been sequenced in a Caucasian series consisting of 1938 patients with clinical PD and 838 with pathologically diagnosed Lewy body disease (LBD).

RESULTS

Our sequence analysis did not identify any coding variants in exon 24 of DNAJC13. Two previously described variants in intron 23 (rs200204728 and rs2369796) were observed.

CONCLUSION

Our results indicate that the region surrounding the DNAJC13 p.Asn855Ser substitution is highly conserved and mutations in this exon are not a common cause of PD or LBD among Caucasian populations.

α-Synuclein and Lewy pathology in Parkinson's disease

PURPOSE OF REVIEW

The role of protein aggregates, such as Lewy body inclusions, in the molecular pathogenesis of Parkinson's disease has remained controversial and elusive. The protein α-synuclein is a major component of these inclusions but it can exist in many alternate conformations. Here we review advances in our understanding of the roles of Lewy pathology and α-synuclein in Parkinson's disease.

RECENT FINDINGS

Recent work has demonstrated that certain α-synuclein conformations are directly toxic to neurons and may also propagate Lewy pathology within the nervous system. Investigation into clinicopathological correlates in rare genetic forms of Parkinson's disease has revealed that Lewy pathology is associated with nonmotor features but may not contribute to motor symptoms in Parkinson's disease.

SUMMARY

These recent findings open up new avenues of investigation into the molecular pathogenesis of Parkinson's disease. Future work will need to identify the most toxic conformations of α-synuclein and define their relationship to Lewy pathology. This work will be necessary to be able to develop novel therapeutic strategies that target specific pathogenic forms of α-synuclein for disease modification in Parkinson's disease.

Endocytic membrane trafficking and neurodegenerative disease

Neurodegenerative diseases are amongst the most devastating of human disorders. New technologies have led to a rapid increase in the identification of disease-related genes with an enhanced appreciation of the key roles played by genetics in the etiology of these disorders. Importantly, pinpointing the normal function of disease gene proteins leads to new understanding of the cellular machineries and pathways that are altered in the disease process. One such emerging pathway is membrane trafficking in the endosomal system. This key cellular process controls the localization and levels of a myriad of proteins and is thus critical for normal cell function. In this review we will focus on three neurodegenerative diseases; Parkinson disease, amyotrophic lateral sclerosis, and hereditary spastic paraplegias, for which a large number of newly discovered disease genes encode proteins that function in endosomal membrane trafficking. We will describe how alterations in these proteins affect endosomal function and speculate on the contributions of these disruptions to disease pathophysiology.

Genetic Profile, Environmental Exposure, and Their Interaction in Parkinson's Disease

The discovery of causative mutations for Parkinson's disease (PD) as well as their functional characterization in cellular and animal models has provided crucial insight into the pathogenesis of this disorder. Today, we know that PD pathogenesis involves multiple related processes including mitochondrial dysfunction, oxidative and nitrative stress, microglial activation and inflammation, and aggregation of α-synuclein and impaired autophagy. However, with the exception of a few families with Mendelian inheritance, the cause of PD in most individuals is yet unknown and the identified genetic susceptibility factors have only small effect size. Epidemiologic studies have found increased risk of PD associated with exposure to environmental toxicants such as pesticides, organic solvents, metals, and air pollutants, while reduced risk of PD associated with smoking cigarettes and coffee consumption. The role of environmental exposure, as well as the contribution of single genetic risk factors, is still controversial. In most of PD cases, disease onset is probably triggered by a complex interplay of many genetic and nongenetic factors, each of which conveys a minor increase in the risk of disease. This review summarizes the current knowledge on causal mutation for PD, susceptibility factors increasing disease risk, and the genetic factors that modify the impact of environmental exposure.

Association of the MTHFR rs1801131 and rs1801133 variants in sporadic Parkinson's disease patients

Parkinson's disease (PD) is a common age-dependent neurodegenerative movement disorder related to multiple factors, and genetic factors play an important role in the pathogenesis of PD. Variants in the methylenetetrahydrofolate reductase gene (MTHFR), a gene encoding a folate-dependent enzyme that is involved in homocysteine metabolism, have been reported to be associated with PD. To explore the role of the MTHFR gene in the development of PD in Chinese Han population, we analyzed two MTHFR variants (rs1801131 and rs1801133) in a patient cohort consisting of 512 patients with PD from mainland China and a control cohort consisting of 512 age, gender and ethnicity matched normal subjects. Statistically significant differences in genotypic and allelic frequencies were detected in the MTHFR variant rs1801133 (P=0.022 and 0.007, respectively; odds ratio=0.780, 95% confidence interval=0.651-0.934). In addition, the A-T haplotype of rs1801131-rs1801133 showed a protective role against PD development (P=0.007, odds ratio=0.779, 95% confidence interval=0.650-0.933). Our results suggested that the T allele of rs1801133 variant and A-T haplotype of rs1801131-rs1801133 in the MTHFR gene may decrease the risk of developing PD in Chinese Han population from mainland China.

Targeting the Autophagy/Lysosomal Degradation Pathway in Parkinson's Disease

Autophagy is a cellular quality control mechanism crucial for neuronal homeostasis. Defects in autophagy are critically associated with mechanisms underlying Parkinson's disease (PD), a common and debilitating neurodegenerative disorder. Autophagic dysfunction in PD can occur at several stages of the autophagy/lysosomal degradative machinery, contributing to the formation of intracellular protein aggregates and eventual neuronal cell death. Therefore, autophagy inducers may comprise a promising new therapeutic approach to combat neurodegeneration in PD. Several currently available FDA-approved drugs have been shown to enhance autophagy, which may allow for their repurposing for use in novel clinical conditions including PD. This review summarizes our current knowledge of deficits in the autophagy/lysosomal degradation pathways associated with PD, and highlight current approaches which target this pathway as possible means towards novel therapeutic strategies.

Functional Evaluations of Genes Disrupted in Patients with Tourette's Disorder

Tourette's disorder (TD) is a highly heritable neurodevelopmental disorder with complex genetic architecture and unclear neuropathology. Disruptions of particular genes have been identified in subsets of TD patients. However, none of the findings have been replicated, probably due to the complex and heterogeneous genetic architecture of TD that involves both common and rare variants. To understand the etiology of TD, functional analyses are required to characterize the molecular and cellular consequences caused by mutations in candidate genes. Such molecular and cellular alterations may converge into common biological pathways underlying the heterogeneous genetic etiology of TD patients. Herein, we review specific genes implicated in TD etiology, discuss the functions of these genes in the mammalian central nervous system and the corresponding behavioral anomalies exhibited in animal models, and importantly, review functional analyses that can be performed to evaluate the role(s) that the genetic disruptions might play in TD. Specifically, the functional assays include novel cell culture systems, genome editing techniques, bioinformatics approaches, transcriptomic analyses, and genetically modified animal models applied or developed to study genes associated with TD or with other neurodevelopmental and neuropsychiatric disorders. By describing methods used to study diseases with genetic architecture similar to TD, we hope to develop a systematic framework for investigating the etiology of TD and related disorders.

Whole-Exome Sequencing in Familial Parkinson Disease

IMPORTANCE

Parkinson disease (PD) is a progressive neurodegenerative disease for which susceptibility is linked to genetic and environmental risk factors.

OBJECTIVE

To identify genetic variants contributing to disease risk in familial PD.

DESIGN, SETTING, AND PARTICIPANTS

A 2-stage study design that included a discovery cohort of families with PD and a replication cohort of familial probands was used. In the discovery cohort, rare exonic variants that segregated in multiple affected individuals in a family and were predicted to be conserved or damaging were retained. Genes with retained variants were prioritized if expressed in the brain and located within PD-relevant pathways. Genes in which prioritized variants were observed in at least 4 families were selected as candidate genes for replication in the replication cohort. The setting was among individuals with familial PD enrolled from academic movement disorder specialty clinics across the United States. All participants had a family history of PD.

MAIN OUTCOMES AND MEASURES

Identification of genes containing rare, likely deleterious, genetic variants in individuals with familial PD using a 2-stage exome sequencing study design.

RESULTS

The 93 individuals from 32 families in the discovery cohort (49.5% [46 of 93] female) had a mean (SD) age at onset of 61.8 (10.0) years. The 49 individuals with familial PD in the replication cohort (32.6% [16 of 49] female) had a mean (SD) age at onset of 50.1 (15.7) years. Discovery cohort recruitment dates were 1999 to 2009, and replication cohort recruitment dates were 2003 to 2014. Data analysis dates were 2011 to 2015. Three genes containing a total of 13 rare and potentially damaging variants were prioritized in the discovery cohort. Two of these genes (TNK2 and TNR) also had rare variants that were predicted to be damaging in the replication cohort. All 9 variants identified in the 2 replicated genes in 12 families across the discovery and replication cohorts were confirmed via Sanger sequencing.

CONCLUSIONS AND RELEVANCE

TNK2 and TNR harbored rare, likely deleterious, variants in individuals having familial PD, with similar findings in an independent cohort. To our knowledge, these genes have not been previously associated with PD, although they have been linked to critical neuronal functions. Further studies are required to confirm a potential role for these genes in the pathogenesis of PD.

Insights from late-onset familial parkinsonism on the pathogenesis of idiopathic Parkinson's disease

Disease-modifying therapies that slow or halt the progression of Parkinson's disease are an unmet clinical need. Many hypotheses have been put forward to explain the pathogenesis of the disease, but none has led to the development of disease-modifying drugs. Here we focus on familial forms of late-onset parkinsonism that most closely resemble idiopathic Parkinson's disease and present a synthesis of emerging molecular advances. Genetic discoveries and mechanistic investigations have highlighted early alterations to synaptic function, endosomal maturation, and protein sorting that might lead to an intracellular proteinopathy. We propose that these cellular processes constitute one pathway to pathogenesis and suggest that neuroprotection, as an adjunct to current symptomatic treatments, need not remain an elusive goal.

The RAB39B p.G192R mutation causes X-linked dominant Parkinson's disease

Objective

To identify the causal gene in a multi-incident U.S. kindred with Parkinson’s disease (PD).

Methods

We characterized a family with a classical PD phenotype in which 7 individuals (5 males and 2 females) were affected with a mean age at onset of 46.1 years (range, 29-57 years). We performed whole exome sequencing on 4 affected and 1 unaffected family members. Sanger-sequencing was then used to verify and genotype all candidate variants in the remainder of the pedigree. Cultured cells transfected with wild-type or mutant constructs were used to characterize proteins of interest.

Results

We identified a missense mutation (c.574G > A; p.G192R) in the RAB39B gene that closely segregated with disease and exhibited X-linked dominant inheritance with reduced penetrance in females. The mutation occurred in a highly conserved amino acid residue and was not observed among 87,725 X chromosomes in the Exome Aggregation Consortium dataset. Sequencing of the RAB39B coding region in 587 familial PD cases yielded two additional mutations (c.428C > G [p.A143G] and c.624_626delGAG [p.R209del]) that were predicted to be deleterious in silico but occurred in families that were not sufficiently informative to assess segregation with disease. Experiments in PC12 and SK-N-BE(2)C cells demonstrated that p.G192R resulted in mislocalization of the mutant protein, possibly by altering the structure of the hypervariable C-terminal domain which mediates intracellular targeting.

Conclusions

Our findings implicate RAB39B, an essential regulator of vesicular-trafficking, in clinically typical PD. Further characterization of normal and aberrant RAB39B function might elucidate important mechanisms underlying neurodegeneration in PD and related disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0045-4) contains supplementary material, which is available to authorized users.

Parkinson's disease

Parkinson's disease is a neurological disorder with evolving layers of complexity. It has long been characterised by the classical motor features of parkinsonism associated with Lewy bodies and loss of dopaminergic neurons in the substantia nigra. However, the symptomatology of Parkinson's disease is now recognised as heterogeneous, with clinically significant non-motor features. Similarly, its pathology involves extensive regions of the nervous system, various neurotransmitters, and protein aggregates other than just Lewy bodies. The cause of Parkinson's disease remains unknown, but risk of developing Parkinson's disease is no longer viewed as primarily due to environmental factors. Instead, Parkinson's disease seems to result from a complicated interplay of genetic and environmental factors affecting numerous fundamental cellular processes. The complexity of Parkinson's disease is accompanied by clinical challenges, including an inability to make a definitive diagnosis at the earliest stages of the disease and difficulties in the management of symptoms at later stages. Furthermore, there are no treatments that slow the neurodegenerative process. In this Seminar, we review these complexities and challenges of Parkinson's disease.

Gene expression profiling for human iPS-derived motor neurons from sporadic ALS patients reveals a strong association between mitochondrial functions and neurodegeneration

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease that leads to widespread motor neuron death, general palsy and respiratory failure. The most prevalent sporadic ALS form is not genetically inherited. Attempts to translate therapeutic strategies have failed because the described mechanisms of disease are based on animal models carrying specific gene mutations and thus do not address sporadic ALS. In order to achieve a better approach to study the human disease, human induced pluripotent stem cell (hiPSC)-differentiated motor neurons were obtained from motor nerve fibroblasts of sporadic ALS and non-ALS subjects using the STEMCCA Cre-Excisable Constitutive Polycistronic Lentivirus system and submitted to microarray analyses using a whole human genome platform. DAVID analyses of differentially expressed genes identified molecular function and biological process-related genes through Gene Ontology. REVIGO highlighted the related functions mRNA and DNA binding, GTP binding, transcription (co)-repressor activity, lipoprotein receptor binding, synapse organization, intracellular transport, mitotic cell cycle and cell death. KEGG showed pathways associated with Parkinson's disease and oxidative phosphorylation, highlighting iron homeostasis, neurotrophic functions, endosomal trafficking and ERK signaling. The analysis of most dysregulated genes and those representative of the majority of categorized genes indicates a strong association between mitochondrial function and cellular processes possibly related to motor neuron degeneration. In conclusion, iPSC-derived motor neurons from motor nerve fibroblasts of sporadic ALS patients may recapitulate key mechanisms of neurodegeneration and may offer an opportunity for translational investigation of sporadic ALS. Large gene profiling of differentiated motor neurons from sporadic ALS patients highlights mitochondrial participation in the establishment of autonomous mechanisms associated with sporadic ALS.

Rme-8 depletion perturbs Notch recycling and predisposes to pathogenic signaling

The retromer-associated DNAJ protein Rme-8 is necessary for normal Notch recycling, and reductions in Rme-8 sensitize cells so that additional loss-of-sorting retromer or ESCRT-0 components have catastrophic effects.

Notch signaling is a major regulator of cell fate, proliferation, and differentiation. Like other signaling pathways, its activity is strongly influenced by intracellular trafficking. Besides contributing to signal activation and down-regulation, differential fluxes between trafficking routes can cause aberrant Notch pathway activation. Investigating the function of the retromer-associated DNAJ protein Rme-8 in vivo, we demonstrate a critical role in regulating Notch receptor recycling. In the absence of Rme-8, Notch accumulated in enlarged tubulated Rab4-positive endosomes, and as a consequence, signaling was compromised. Strikingly, when the retromer component Vps26 was depleted at the same time, Notch no longer accumulated and instead was ectopically activated. Likewise, depletion of ESCRT-0 components Hrs or Stam in combination with Rme-8 also led to high levels of ectopic Notch activity. Together, these results highlight the importance of Rme-8 in coordinating normal endocytic recycling route and reveal that its absence predisposes toward conditions in which pathological Notch signaling can occur.

Receptor-mediated Endocytosis 8 Utilizes an N-terminal Phosphoinositide-binding Motif to Regulate Endosomal Clathrin Dynamics

Receptor-mediated endocytosis 8 (RME-8) is a DnaJ domain containing protein implicated in translocation of Hsc70 to early endosomes for clathrin removal during retrograde transport. Previously, we have demonstrated that RME-8 associates with early endosomes in a phosphatidylinositol 3-phosphate (PI(3)P)-dependent fashion. In this study, we have now identified amino acid determinants required for PI(3)P binding within a region predicted to adopt a pleckstrin homology-like fold in the N terminus of RME-8. The ability of RME-8 to associate with PI(3)P and early endosomes is largely abolished when residues Lys(17), Trp(20), Tyr(24), or Arg(26) are mutated resulting in diffuse cytoplasmic localization of RME-8 while maintaining the ability to interact with Hsc70. We also provide evidence that RME-8 PI(3)P binding regulates early endosomal clathrin dynamics and alters the steady state localization of the cation-independent mannose 6-phosphate receptor. Interestingly, RME-8 endosomal association is also regulated by the PI(3)P-binding protein SNX1, a member of the retromer complex. Wild type SNX1 restores endosomal localization of RME-8 W20A, whereas a SNX1 variant deficient in PI(3)P binding disrupts endosomal localization of wild type RME-8. These results further highlight the critical role for PI(3)P in the RME-8-mediated organizational control of various endosomal activities, including retrograde transport.

A defect in the retromer accessory protein, SNX27, manifests by infantile myoclonic epilepsy and neurodegeneration

The composition of the neuronal cell surface dictates synaptic plasticity and thereby cognitive development. This remodeling of the synapses is governed by the endocytic network which internalize transmembrane proteins, then sort them back to the cell surface or carry them to the lysosome for degradation. The multi-protein retromer complex is central to this selection, capturing specific transmembrane proteins and remodeling the cell membrane to form isolated cargo-enriched transport carriers. We investigated a consanguineous family with four patients who presented in infancy with intractable myoclonic epilepsy and lack of psychomotor development. Using exome analysis, we identified a homozygous deleterious mutation in SNX27, which encodes sorting nexin 27, a retromer cargo adaptor. In western analysis of patient fibroblasts, the encoded mutant protein was expressed at an undetectable level when compared with a control sample. The patients' presentation and clinical course recapitulate that reported for the SNX27 knock-out mouse. Since the cargo proteins for SNX27-mediated sorting include subunits of ionotropic glutamate receptors and endosome-to-cell surface synaptic insertion of AMPA receptors is severely perturbed in SNX27(-/-) neurons, it is proposed that at least part of the neurological aberrations observed in the patients is attributed to defective sorting of ionotropic glutamate receptors. SNX27 deficiency is now added to the growing list of neurodegenerative disorders associated with retromer dysfunction.

Retromer-mediated endosomal protein sorting: The role of unstructured domains

The retromer complex is a key element of the endosomal protein sorting machinery that is conserved through evolution and has been shown to play a role in diseases such as Alzheimer's disease and Parkinson's disease. Through sorting various membrane proteins (cargo), the function of retromer complex has been linked to physiological processes such as lysosome biogenesis, autophagy, down regulation of signalling receptors and cell spreading. The cargo-selective trimer of retromer recognises membrane proteins and sorts them into two distinct pathways; endosome-to-Golgi retrieval and endosome-to-cell surface recycling and additionally the cargo-selective trimer functions as a hub to recruit accessory proteins to endosomes where they may regulate and/or facilitate retromer-mediated endosomal proteins sorting. Unstructured domains present in cargo proteins or accessory factors play key roles in both these aspects of retromer function and will be discussed in this review.

Linking the VPS35 and EIF4G1 pathways in Parkinson's disease

Elucidating the underlying pathogenic pathways in Parkinson's disease will be critical for targeted drug development. In this issue of Neuron, Dhungel et al. (2015) utilize a yeast model to establish a link between VPS35 and EIF4G1 in α-synuclein-related neurodegeneration.

Molecular chaperones and neuronal proteostasis

Protein homeostasis (proteostasis) is essential for maintaining the functionality of the proteome. The disruption of proteostasis, due to genetic mutations or an age-related decline, leads to aberrantly folded proteins that typically lose their function. The accumulation of misfolded and aggregated protein is also cytotoxic and has been implicated in the pathogenesis of neurodegenerative diseases. Neurons have developed an intrinsic protein quality control network, of which molecular chaperones are an essential component. Molecular chaperones function to promote efficient folding and target misfolded proteins for refolding or degradation. Increasing molecular chaperone expression can suppress protein aggregation and toxicity in numerous models of neurodegenerative disease; therefore, molecular chaperones are considered exciting therapeutic targets. Furthermore, mutations in several chaperones cause inherited neurodegenerative diseases. In this review, we focus on the importance of molecular chaperones in neurodegenerative diseases, and discuss the advances in understanding their protective mechanisms.

The biology of proteostasis in aging and disease

Loss of protein homeostasis (proteostasis) is a common feature of aging and disease that is characterized by the appearance of nonnative protein aggregates in various tissues. Protein aggregation is routinely suppressed by the proteostasis network (PN), a collection of macromolecular machines that operate in diverse ways to maintain proteome integrity across subcellular compartments and between tissues to ensure a healthy life span. Here, we review the composition, function, and organizational properties of the PN in the context of individual cells and entire organisms and discuss the mechanisms by which disruption of the PN, and related stress response pathways, contributes to the initiation and progression of disease. We explore emerging evidence that disease susceptibility arises from early changes in the composition and activity of the PN and propose that a more complete understanding of the temporal and spatial properties of the PN will enhance our ability to develop effective treatments for protein conformational diseases.

Saskatchewan movement disorders program

We review the Saskatchewan Movement Disorders Program, which started in 1968 and has had the dual goals of patient care and research. The clinics are structured to collect research-worthy data including videos, longitudinal follow-up, and autopsy studies of patients seen in the clinics. At every clinic visit, the patient is evaluated by one or both authors. A total of 25% to 30% of the deceased come to autopsy. Frozen half-brain and formalin-fixed remnants from autopsy are preserved in our laboratories. Patients not seen in our clinic are not included in research, which makes it different from brain banks. So far, 515 cases have come to autopsy. So far, there have been 17 collaborating scientific teams from Canada, the United States, Europe, and Japan. The collaborators are not charged for access to our resources. This program offers a unique opportunity to study multiple aspects of movement disorder patients seen in clinical practice.