Genetic analysis of SCA2, 3 and 17 in idiopathic Parkinson's disease

Genetic Movement Disorders Commonly Seen in Asians

The increasing availability of molecular genetic testing has changed the landscape of both genetic research and clinical practice. Not only is the pace of discovery of novel disease-causing genes accelerating but also the phenotypic spectra associated with previously known genes are expanding. These advancements lead to the awareness that some genetic movement disorders may cluster in certain ethnic populations and genetic pleiotropy may result in unique clinical presentations in specific ethnic groups. Thus, the characteristics, genetics and risk factors of movement disorders may differ between populations. Recognition of a particular clinical phenotype, combined with information about the ethnic origin of patients could lead to early and correct diagnosis and assist the development of future personalized medicine for patients with these disorders. Here, the Movement Disorders in Asia Task Force sought to review genetic movement disorders that are commonly seen in Asia, including Wilson's disease, spinocerebellar ataxias (SCA) types 12, 31, and 36, Gerstmann-Sträussler-Scheinker disease, PLA2G6-related parkinsonism, adult-onset neuronal intranuclear inclusion disease (NIID), and paroxysmal kinesigenic dyskinesia. We also review common disorders seen worldwide with specific mutations or presentations that occur frequently in Asians.

Movement disorders and neuropathies: overlaps and mimics in clinical practice

Movement disorders as well as peripheral neuropathies are extremely frequent in the general population; therefore, it is not uncommon to encounter patients with both these conditions. Often, the coexistence is coincidental, due to the high incidence of common causes of peripheral neuropathy, such as diabetes and other age-related disorders, as well as of Parkinson disease (PD), which has a typical late onset. Nonetheless, there is broad evidence that PD patients may commonly develop a sensory and/or autonomic polyneuropathy, triggered by intrinsic and/or extrinsic mechanisms. Similarly, some peripheral neuropathies may develop some movement disorders in the long run, such as tremor, and rarely dystonia and myoclonus, suggesting that central mechanisms may ensue in the pathogenesis of these diseases. Although rare, several acquired or hereditary causes may be responsible for the combination of movement and peripheral nerve disorders as a unique entity, some of which are potentially treatable, including paraneoplastic, autoimmune and nutritional aetiologies. Finally, genetic causes should be pursued in case of positive family history, young onset or multisystemic involvement, and examined for neuroacanthocytosis, spinocerebellar ataxias, mitochondrial disorders and less common causes of adult-onset cerebellar ataxias and spastic paraparesis. Deep phenotyping in terms of neurological and general examination, as well as laboratory tests, neuroimaging, neurophysiology, and next-generation genetic analysis, may guide the clinician toward the correct diagnosis and management.

Neuropathology of genetic synucleinopathies with parkinsonism: Review of the literature

Clinical-pathological studies remain the gold-standard for the diagnosis of Parkinson's disease (PD). However, mounting data from genetic PD autopsies challenge the diagnosis of PD based on Lewy body pathology. Most of the confirmed genetic risks for PD show heterogenous neuropathology, even within kindreds, which may or may not include Lewy body pathology. We review the literature of genetic PD autopsies from cases with molecularly confirmed PD or parkinsonism and summarize main findings on SNCA (n = 25), Parkin (n = 20, 17 bi-allelic and 3 heterozygotes), PINK1 (n = 5, 1 bi-allelic and 4 heterozygotes), DJ-1 (n = 1), LRRK2 (n = 55), GBA (n = 10 Gaucher disease patients with parkinsonism), DNAJC13, GCH1, ATP13A2, PLA2G6 (n = 8 patients, 2 with PD), MPAN (n = 2), FBXO7, RAB39B, and ATXN2 (SCA2), as well as on 22q deletion syndrome (n = 3). Findings from autopsies of heterozygous mutation carriers of genes that are traditionally considered recessively inherited are also discussed. Lewy bodies may be present in syndromes clinically distinctive from PD (eg, MPAN-related neurodegeneration) and absent in patients with clinical PD syndrome (eg, LRRK2-PD or Parkin-PD). Therefore, the authors can conclude that the presence of Lewy bodies are not specific to the diagnosis of PD and that PD can be diagnosed even in the absence of Lewy body pathology. Interventions that reduce alpha-synuclein load may be more justified in SNCA-PD or GBA-PD than in other genetic forms of PD. The number of reported genetic PD autopsies remains small, and there are limited genotype-clinical-pathological-phenotype studies. Therefore, larger series of autopsies from genetic PD patients are required. © 2017 International Parkinson and Movement Disorder Society.

The Multiple Faces of Spinocerebellar Ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is among the most common forms of autosomal dominant ataxias, accounting for 15% of the total families. Occurrence is higher in specific populations such as the Cuban and Southern Italian. The disease is caused by a CAG expansion in ATXN2 gene, leading to abnormal accumulation of the mutant protein, ataxin‐2, in intracellular inclusions. The clinical picture is mainly dominated by cerebellar ataxia, although a number of other neurological signs have been described, ranging from parkinsonism to motor neuron involvement, making the diagnosis frequently challenging for neurologists, particularly when information about the family history is not available. Although the functions of ataxin‐2 have not been completely elucidated, the protein is involved in mRNA processing and control of translation. Recently, it has also been shown that the size of the CAG repeat in normal alleles represents a risk factor for ALS, suggesting that ataxin‐2 plays a fundamental role in maintenance of neuronal homeostasis.

Search for SCA2 blood RNA biomarkers highlights Ataxin-2 as strong modifier of the mitochondrial factor PINK1 levels

Ataxin-2 (ATXN2) polyglutamine domain expansions of large size result in an autosomal dominantly inherited multi-system-atrophy of the nervous system named spinocerebellar ataxia type 2 (SCA2), while expansions of intermediate size act as polygenic risk factors for motor neuron disease (ALS and FTLD) and perhaps also for Levodopa-responsive Parkinson's disease (PD). In view of the established role of ATXN2 for RNA processing in periods of cell stress and the expression of ATXN2 in blood cells such as platelets, we investigated whether global deep RNA sequencing of whole blood from SCA2 patients identifies a molecular profile which might serve as diagnostic biomarker. The bioinformatic analysis of SCA2 blood global transcriptomics revealed various significant effects on RNA processing pathways, as well as the pathways of Huntington's disease and PD where mitochondrial dysfunction is crucial. Notably, an induction of PINK1 and PARK7 expression was observed. Conversely, expression of Pink1 was severely decreased upon global transcriptome profiling of Atxn2-knockout mouse cerebellum and liver, in parallel to strong effects on Opa1 and Ghitm, which encode known mitochondrial dynamics regulators. These results were validated by quantitative PCR and immunoblots. Starvation stress of human SH-SY5Y neuroblastoma cells led to a transcriptional phasic induction of ATXN2 in parallel to PINK1, and the knockdown of one enhanced the expression of the other during stress response. These findings suggest that ATXN2 may modify the known PINK1 roles for mitochondrial quality control and autophagy during cell stress. Given that PINK1 is responsible for autosomal recessive juvenile PD, this genetic interaction provides a concept how the degeneration of nigrostriatal dopaminergic neurons and the Parkinson phenotype may be triggered by ATXN2 mutations.

Large-scale assessment of polyglutamine repeat expansions in Parkinson disease

OBJECTIVES

We aim to clarify the pathogenic role of intermediate size repeat expansions of SCA2, SCA3, SCA6, and SCA17 as risk factors for idiopathic Parkinson disease (PD).

METHODS

We invited researchers from the Genetic Epidemiology of Parkinson's Disease Consortium to participate in the study. There were 12,346 cases and 8,164 controls genotyped, for a total of 4 repeats within the SCA2, SCA3, SCA6, and SCA17 genes. Fixed- and random-effects models were used to estimate the summary risk estimates for the genes. We investigated between-study heterogeneity and heterogeneity between different ethnic populations.

RESULTS

We did not observe any definite pathogenic repeat expansions for SCA2, SCA3, SCA6, and SCA17 genes in patients with idiopathic PD from Caucasian and Asian populations. Furthermore, overall analysis did not reveal any significant association between intermediate repeats and PD. The effect estimates (odds ratio) ranged from 0.93 to 1.01 in the overall cohort for the SCA2, SCA3, SCA6, and SCA17 loci.

CONCLUSIONS

Our study did not support a major role for definite pathogenic repeat expansions in SCA2, SCA3, SCA6, and SCA17 genes for idiopathic PD. Thus, results of this large study do not support diagnostic screening of SCA2, SCA3, SCA6, and SCA17 gene repeats in the common idiopathic form of PD. Likewise, this largest multicentered study performed to date excludes the role of intermediate repeats of these genes as a risk factor for PD.

Parkinsonism in spinocerebellar ataxia

Spinocerebellar ataxia (SCA) presents heterogeneous clinical phenotypes, and parkinsonism is reported in diverse SCA subtypes. Both levodopa responsive Parkinson disease (PD) like phenotype and atypical parkinsonism have been described especially in SCA2, SCA3, and SCA17 with geographic differences in prevalence. SCA2 is the most frequently reported subtype of SCA related to parkinsonism worldwide. Parkinsonism in SCA2 has unique genetic characteristics, such as low number of expansions and interrupted structures, which may explain the sporadic cases with low penetrance. Parkinsonism in SCA17 is more remarkable in Asian populations especially in Korea. In addition, an unclear cutoff of the pathologic range is the key issue in SCA17 related parkinsonism. SCA3 is more common in western cohorts. SCA6 and SCA8 have also been reported with a PD-like phenotype. Herein, we reviewed the epidemiologic, clinical, genetic, and pathologic features of parkinsonism in SCAs.

Psychotic-affective symptoms and multiple system atrophy expand phenotypes of spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is a progressive neurodegenerative disorder, characterised by ataxic gait, slow saccades and peripheral neuropathy. Levodopa-responsive parkinsonism could be a clinical phenotype of SCA2, especially those of Chinese origin. In addition to these motor symptoms, SCA2 has been associated with depression and cognitive dysfunction, with only rare reports of psychosis. The authors report the presence of severe psychosis, major depression and multiple system atrophy in affected subjects of a Taiwanese family with intermediate CAG repeats within the ATXN2 gene. The identification of this rare and distinctive SCA2 phenotype expands the current knowledge of the phenotypic variability of SCA2 and suggests that modifier genes could influence the clinical phenotype of SCA2.

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

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

Movement disorders in spinocerebellar ataxias

Autosomal dominant spinocerebellar ataxias (SCAs) can present with a large variety of noncerebellar symptoms, including movement disorders. In fact, movement disorders are frequent in many of the various SCA subtypes, and they can be the presenting, dominant, or even isolated disease feature. When combined with cerebellar ataxia, the occurrence of a specific movement disorder can provide a clue toward the underlying genotype. There are reasons to believe that for some coexisting movement disorders, the cerebellar pathology itself is the culprit, for example, in the case of cortical myoclonus and perhaps dystonia. However, movement disorders in SCAs are more likely related to extracerebellar pathology, and imaging and neuropathological data indeed show involvement of other parts of the motor system (substantia nigra, striatum, pallidum, motor cortex) in some SCA subtypes. When confronted with a patient with an isolated movement disorder, that is, without ataxia, there is currently no reason to routinely screen for SCA gene mutations, the only exceptions being SCA2 in autosomal dominant parkinsonism (particularly in Asian patients) and SCA17 in the case of a Huntington's disease-like presentation without an HTT mutation.

Relative contribution of SCA2, SCA3 and SCA17 in Korean patients with parkinsonism and ataxia

We examined the relative significance of SCA2, SCA3 and SCA17 in Koreans patients with parkinsonism and ataxia. We recruited patients with either parkinsonism (n = 524; PD = 386 and MSA = 138) or ataxia (n = 44) as their main clinical feature for two years. These patients were screened for SCA2, SCA3 and SCA17. Six cases carried SCA2; one, SCA3; and eight, SCA17. In SCA2 patients, one patient exhibited MSA-P phenotype, and the other five exhibited ataxia. The single patient with SCA3 showed ataxia. In SCA17 patients, one patient presented ataxia, the other seven patients showed parkinsonism (three PD and four MSA-P). Dopamine transporter (DAT) imaging was performed in a subset of ataxic or parkinsonian SCA2 or SCA17, all of whom showed decreased DAT binding. In Korean population, the mutation frequencies of SCA2 and SCA17 were similar. SCA2 was a more significant cause of ataxia, whereas SCA17 was a more significant cause of parkinsonism. Contribution of SCA3 to parkinsonism was insignificant.

Parkinson's disease: from monogenic forms to genetic susceptibility factors

Research in Parkinson's disease (PD) genetics has been extremely prolific over the past decade. More than 13 loci and 9 genes have been identified, but their implication in PD is not always certain. Point mutations, duplications and triplications in the alpha-synuclein (SNCA) gene cause a rare dominant form of PD in familial and sporadic cases. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are a more frequent cause of autosomal dominant PD, particularly in certain ethnic groups. Loss-of-function mutations in Parkin, PINK1, DJ-1 and ATP13A2 cause autosomal recessive parkinsonism with early-onset. Identification of other Mendelian forms of PD will be a main challenge for the next decade. In addition, susceptibility variants that contribute to PD have been identified in several populations, such as polymorphisms in the SNCA, LRRK2 genes and heterozygous mutations in the beta-glucocerebrosidase (GBA) gene. Genome-wide associations and re-sequencing projects, together with gene-environment interaction studies, are expected to further define the causal role of genetic determinants in the pathogenesis of PD, and improve prevention and treatment.

Spinocerebellar ataxia type 12 was not found in Korean Parkinsonian patients

BACKGROUND

Parkinsonism (PD) is occasionally seen in several types of spinocerebellar ataxia (SCA). Mutations in SCA gene have been reported in the patients of parkinsonism without ataxia.

METHODS

We examined spinocerebellar ataxia type 12 mutation in 877 PD and 199 multiple system atrophy (MSA) patients.

RESULTS AND CONCLUSIONS

No patients showed abnormal SCA12 expansion. It suggests that PD and MSA are not associated with SCA12 and it is not necessary to screen SCA12 in PD and MSA patients.

Are interrupted SCA2 CAG repeat expansions responsible for parkinsonism?

BACKGROUND

Autosomal dominant parkinsonism (ADP) is caused in a large percentage of familial and sporadic cases by mutations in the LRRK2 gene, particularly G2019S. It is also caused by mutations in genes associated with autosomal dominant cerebellar ataxia (ADCA), notably CAG/CAA repeat expansions in SCA2.

METHODS

We screened 164 families with ADP for expansions in the SCA2, 3, and 17 genes and for the G2019S mutation in LRRK2. The SCA2 CAG/CAA repeat expansion was sequenced to determine its structure. The phenotypes of patients with ADP caused by the SCA2, LRRK2, and unknown mutations were compared, as well as those of SCA2 patients with interrupted or uninterrupted expansions of the same size.

RESULTS

Three French ADP families had SCA2 mutations. The expansions ranged from 37 to 39 repeats and were interrupted and stable upon transmission. All patients (n = 9) had levodopa-responsive parkinsonism without cerebellar signs. They had significantly more symmetric signs and less rigidity than ADP caused by the G2019S mutation in LRRK2 or by unknown mutations. Interestingly, two sisters carrying both the SCA2 and the G2019S LRRK2 mutations had markedly earlier onset than their mother with only SCA2. In contrast, similar-sized but uninterrupted repeats were associated with ADCA in which cerebellar ataxia was constant and associated only rarely with one or more mild parkinsonian signs.

CONCLUSION

These results suggest that the configuration of the SCA2 CAG/CAA repeat expansions plays an important role in phenotype variability. Uninterrupted SCA2 repeat expansions found in families with autosomal dominant cerebellar ataxia result in somatic mosaicism and produce large hairpin RNAs, which may interact with double-stranded RNA-binding proteins. These characteristics are modified by interruption of the SCA2 repeat expansion as found in families with autosomal dominant parkinsonism.

Lack of mutations in spinocerebellar ataxia type 2 and 3 genes in a Taiwanese (ethnic Chinese) cohort of familial and early-onset parkinsonism

Recent reports suggest that CAG triplet expansions of spinocerebellar ataxia type 2 and 3 (SCA2 and SCA3) genes are the cause of typical levodopa-responsive Parkinson's disease (PD) in familial cases, several of which were ethnic Chinese. To investigate the role of SCA2 and SCA3 mutations in Chinese familial and early-onset PD patients, we analyzed CAG triplet repeat expansions of SCA2 and SCA3 genes in a cohort of 73 Taiwanese/Ethnic Chinese familial and early-onset PD patients [mean age at onset 42.70 +/- 7.17 years (mean +/- SD)]. Thirteen of them (17.8%) had positive family history. All patients received comprehensive clinical evaluation including a thorough neurological examination, laboratory tests, and neuroimaging studies to exclude secondary causes and atypical parkinsonism. The CAG repeat length in these genes was determined using polymerase chain reaction polyacrylamide gel electrophoresis. SCA2 gene CAG repeats ranged from 15 to 26 repeats with a median of 20, and SCA3 gene CAG repeats ranged from 15 to 40 with a median of 15. No long pathogenic repeats were found in either SCA2 or SCA3, although borderline CAG repeat number was detected in the SCA3 gene of four patients. Thus, mutations of SCA2 or SCA3 did not play a major role in familial or early-onset PD in our study cohort. PD patients without autosomal dominant family history or obvious cerebellar ataxia should not be candidates for routine screening of SCA2 or SCA3 mutations for cost-effectiveness.