Alpha-synuclein promoter haplotypes and dementia in Parkinson's disease

A systematic review of associations between common SNCA variants and clinical heterogeneity in Parkinson's disease

There is great heterogeneity in both the clinical presentation and rate of disease progression among patients with Parkinson’s disease (PD). This can pose prognostic difficulties in a clinical setting, and a greater understanding of the risk factors that contribute to modify disease course is of clear importance for optimizing patient care and clinical trial design. Genetic variants in SNCA are an established risk factor for PD and are candidates to modify disease presentation and progression. This systematic review aimed to summarize all available primary research reporting the association of SNCA polymorphisms with features of PD. We systematically searched PubMed and Web of Science, from inception to 1 June 2020, for studies evaluating the association of common SNCA variants with age at onset (AAO) or any clinical feature attributed to PD in patients with idiopathic PD. Fifty-eight studies were included in the review that investigated the association between SNCA polymorphisms and a broad range of outcomes, including motor and cognitive impairment, sleep disorders, mental health, hyposmia, or AAO. The most reproducible findings were with the REP1 polymorphism or rs356219 and an earlier AAO, but no clear associations were identified with an SNCA polymorphism and any individual clinical outcome. The results of this comprehensive summary suggest that, while there is evidence that genetic variance in the SNCA region may have a small impact on clinical outcomes in PD, the mechanisms underlying the association of SNCA polymorphisms with PD risk may not be a major factor driving clinical heterogeneity in PD.

Systematic review of genetic association studies in people with Lewy body dementia

OBJECTIVES

Lewy body dementia (LBD) causes more morbidity, disability, and earlier mortality than Alzheimer disease. Molecular mechanisms underlying neurodegeneration in LBD are poorly understood. We aimed to do a systematic review of all genetic association studies that investigated people with LBD for improving our understanding of LBD molecular genetics and for facilitating discovery of novel biomarkers and therapeutic targets for LBD.

METHODS

We systematically reviewed five online databases (PROSPERO protocol: CRD42018087114) and completed the quality assessment using the quality of genetic association studies tool.

RESULTS

Eight thousand five hundred twenty-one articles were screened, and 75 articles were eligible to be included. Genetic associations of LBD with APOE, GBA, and SNCA variants have been replicated by two or more good quality studies. Our meta-analyses confirmed that APOE-ε4 is significantly associated with dementia with Lewy bodies (pooled odds ratio [POR] = 2.70; 95% CI, 2.37-3.07; P < .001) and Parkinson's disease dementia (POR = 1.60; 95% CI, 1.21-2.11; P = .001). Other reported genetic associations that need further replication include variants in A2M, BCHE-K, BCL7C, CHRFAM7A, CNTN1, ESR1, GABRB3, MAPT, mitochondrial DNA (mtDNA) haplogroup H, NOS2A, PSEN1, SCARB2, TFAM, TREM2, and UCHL1.

CONCLUSIONS

The reported genetic associations and their potential interactions indicate the importance of α-synuclein, amyloid, and tau pathology, autophagy lysosomal pathway, ubiquitin proteasome system, oxidative stress, and mitochondrial dysfunction in LBD. There is a need for larger genome-wide association study (GWAS) for identifying more LBD-associated genes. Future hypothesis-driven studies should aim to replicate reported genetic associations of LBD and to explore their functional implications.

Innate Immunity: A Common Denominator between Neurodegenerative and Neuropsychiatric Diseases

The intricate relationships between innate immunity and brain diseases raise increased interest across the wide spectrum of neurodegenerative and neuropsychiatric disorders. Barriers, such as the blood–brain barrier, and innate immunity cells such as microglia, astrocytes, macrophages, and mast cells are involved in triggering disease events in these groups, through the action of many different cytokines. Chronic inflammation can lead to dysfunctions in large-scale brain networks. Neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and frontotemporal dementia, are associated with a substrate of dysregulated immune responses that impair the central nervous system balance. Recent evidence suggests that similar phenomena are involved in psychiatric diseases, such as depression, schizophrenia, autism spectrum disorders, and post-traumatic stress disorder. The present review summarizes and discusses the main evidence linking the innate immunological response in neurodegenerative and psychiatric diseases, thus providing insights into how the responses of innate immunity represent a common denominator between diseases belonging to the neurological and psychiatric sphere. Improved knowledge of such immunological aspects could provide the framework for the future development of new diagnostic and therapeutic approaches.

Alpha-synuclein gene polymorphism affects risk of dementia in Han Chinese with Parkinson's disease

BACKGROUND

Single-nucleotide polymorphisms (SNPs) in the SNCA gene encoding alpha-synuclein have been shown to affect the PD phenotype. However, whether such polymorphisms can influence risk of dementia in PD remains unclear.

OBJECTIVES

To investigate possible associations between SNCA gene polymorphisms and dementia in patients with PD.

MATERIALS AND METHODS

A consecutive series of 291 PD patients with dementia (n = 45, 15.5%) or without it (n = 246, 84.5%) were genotyped at four SNPs in the SNCA gene. As controls, 615 healthy Han Chinese were also genotyped.

RESULTS

Three SNPs (rs11931074, rs7684318 and rs356219) were in strong linkage disequilibrium. The GG genotype at rs11931074 significantly reduced risk of PD (p = 0.023), but it significantly increased risk of dementia after PD onset (p = 0.015) based on the recessive genetic model. Logistic regression identified the following risk factors for dementia among patients with PD: age ≥65 years (odds ratio [OR] 2.69, 95% confidence interval [CI] 1.25-5.77, p = 0.011), education ≤6 years (OR 4.66, 95% CI 2.21-9.83, p < 0.001), part III score on the Unified Parkinson's Disease Rating Scale ≥40 (OR 5.01, 95% CI 2.40-10.45, p < 0.001), and GG genotype at rs11931074 (OR 2.81, 95% CI 1.16-6.83, p = 0.022).

CONCLUSIONS

PD patients carrying the protective GG genotype at SNCA rs11931074 may be at significantly higher risk of dementia than patients with other genotypes. Our results support the view that SNCA polymorphisms can have opposite effects on preclinical and clinical PD.

SNCA REP1 and Parkinson's disease

REP1 is a polymorphic dinucleotide repeat sequence located in the promoter region of the SNCA gene (OMIM 163890). Opinions regarding the interaction between the various REP1 alleles and Parkinson's disease (PD) or its phenotypes have been inconsistent and have thus far not been comprehensively analyzed. In this study, we searched Medline, Embase and Cochrane databases as well as the Chinese-language Wanfang and CNKI databases using strict inclusion and exclusion criteria and conducted our analysis using Revman 5.3 software. Our search produced 28 articles describing REP1 alleles and their associated PD risks and 8 articles which discussed the relationship between REP1 variation and PD phenotypes. We found that the 265-, 269-, and 271-bp alleles of REP1 (using the nomenclature established by Xia et al.) increased the risk of PD (OR: 1.81, 1.05, 1.17; p: 0.0002, 0.003, 0.002) while the 267-bp allele decreased PD risk (OR: 0.86, p: <0.00001) when taking all populations into account. By ethnicity, we observed an obvious population heterogeneity in the effects of various alleles, where the 269-, 271-, and 273-bp alleles increased PD risk (OR: 1.06, 1.22, 1.89; p: 0.001, 0.003, 0.001) and the 267-bp allele decreased PD risk (OR: 0.85; p: <0.00001) in Caucasian populations, and the 263- and 265-bp alleles increased the risk of PD (OR: 2.22, 2.03; p: 0.03, 0.0002) and the 267- and 273-bp alleles decreased PD risk (OR: 0.90, 0.78; p: 0.02, 0.03) in Asian populations. We also determined that the 267-, 269-, and 271-bp alleles occurred the most frequently, although the frequency distribution varied among different ethnicities. Phenotypic analysis demonstrated that PD patients carrying the 271-bp allele were prone to early onset PD (OR: 1.75, p: 0.02) while the 267-bp had the opposite effect (OR: 0.81; p: 0.01).

Biomarkers for dementia and mild cognitive impairment in Parkinson's disease

Cognitive decline is one of the most frequent and disabling nonmotor features of Parkinson's disease. Around 30% of patients with Parkinson's disease experience mild cognitive impairment, a well-established risk factor for the development of dementia. However, mild cognitive impairment in patients with Parkinson's disease is a heterogeneous entity that involves different types and extents of cognitive deficits. Because it is not currently known which type of mild cognitive impairment confers a higher risk of progression to dementia, it would be useful to define biomarkers that could identify these patients to better study disease progression and possible interventions. In this sense, the identification among patients with Parkinson's disease and mild cognitive impairment of biomarkers associated with dementia would allow the early detection of this process. This review summarizes studies from the past 25 years that have assessed the potential biomarkers of dementia and mild cognitive impairment in Parkinson's disease patients. Despite the potential importance, no biomarker has as yet been validated. However, features such as low levels of epidermal and insulin-like growth factors or uric acid in plasma/serum and of Aß in CSF, reduction of cerebral cholinergic innervation and metabolism measured by PET mainly in posterior areas, and hippocampal atrophy in MRI might be indicative of distinct deficits with a distinct risk of dementia in subgroups of patients. Longitudinal studies combining the existing techniques and new approaches are needed to identify patients at higher risk of dementia. © 2016 International Parkinson and Movement Disorder Society.

How close are we to individualized medicine for Parkinson's disease?

INTRODUCTION

There is a considerable inter-individual heterogeneity in clinical features, disease course, and treatment response in Parkinson's disease (PD), which can be explained not only by disease process and clinical variables, but also by an impact from genetic factors. Evidence-based medicine relies on large randomized control trials and meta-analysis-average medicine, which ignores individual differences. However, we are now in the early phases of a paradigm shift in medicine relating to individuality and variability. The purpose of individualized medicine is to predict patients' responses to targeted therapy using diagnostic tests based on genetics or other molecular mechanisms, thus providing the right drug at the right dose at the right time.

AREAS COVERED

In this article, we outline current state of individualized medicine for PD. Expert Commentary: Pharmacogenomics, an important element of individualized medicine, is just beginning to be considered in PD. To advance the clinical use of pharmacogenomics, big data cohort for genomic research and multidisciplinary team approaches are necessary.

Biomarkers of cognitive decline in Parkinson's disease

Cognitive impairment is a frequent and devastating non-motor symptom of Parkinson's disease (PD). Impaired cognition has a major impact on either quality of life or mortality in patients with PD. Notably, the rate of cognitive decline and pattern of early cognitive deficits in PD are highly variable between individuals. Given that the underlying mechanisms of cognitive decline or dementia associated with PD remain unclear, there is currently no mechanism-based treatment available. Identification of biological markers, including neuroimaging, biofluids and common genetic variants, that account for the heterogeneity of PD related cognitive decline could provide important insights into the pathological processes that underlie cognitive impairment in PD. These combined biomarker approaches will enable early diagnosis and provide indicators of cognitive progression in PD patients. This review summarizes recent advances in the development of biomarkers for cognitive impairments in PD.

Does α-synuclein have a dual and opposing effect in preclinical vs. clinical Parkinson's disease?

α-Synuclein gene (SNCA) multiplications cause familial parkinsonism and allele-length polymorphisms within the SNCA dinucleotide repeat REP1 increase the risk for developing Parkinson's disease (PD). Since SNCA multiplications increase SNCA expression, and REP1 genotypes that increase the risk of developing PD show increased SNCA expression in cell-culture systems, animal models, and human blood and brain, PD therapies seek to reduce SNCA expression. We conducted an observational study of 1098 PD cases to test the hypothesis that REP1 genotypes correlated with reduced SNCA expression are associated with better motor and cognitive outcomes. We evaluated the association of REP1 genotypes with survival free of Hoehn and Yahr stages 4 or 5 (motor outcome) and of Modified Telephone Interview for Cognitive Status score ≤27 or Alzheimer's Disease Dementia Screening Interview score ≥2 (cognitive outcome). Median disease duration at baseline was 3.3 years and median lag time from baseline to follow-up was 7.8 years. Paradoxically, REP1 genotypes associated with increased risk of developing PD and increased SNCA expression were associated with better motor (HR = 0.87, p = 0.046, covariate-adjusted age-scale analysis; HR = 0.85, p = 0.020, covariate-adjusted time-scale analysis) and cognitive outcomes (HR = 0.90, p = 0.12, covariate-adjusted age-scale analysis; HR = 0.85, p = 0.023, covariate-adjusted time-scale analysis). Our findings raise the possibility that SNCA has a dual, opposing, and time-dependent role. This may have implications for the development of therapies that target SNCA expression.

Mutations in PRKN and SNCA Genes Important for the Progress of Parkinson's Disease

Although Parkinson’s disease (PD) was first described almost 200 years ago, it remains an incurable disease with a cause that is not fully understood. Nowadays it is known that disturbances in the structure of pathological proteins in PD can be caused by more than environmental and genetic factors. Despite numerous debates and controversies in the literature about the role of mutations in the SNCA and PRKN genes in the pathogenesis of PD, it is evident that these genes play a key role in maintaining dopamine (DA) neuronal homeostasis and that the dysfunction of this homeostasis is relevant to both familial (FPD) and sporadic (SPD) PD with different onset. In recent years, the importance of alphasynuclein (ASN) in the process of neurodegeneration and neuroprotective function of the Parkin is becoming better understood. Moreover, there have been an increasing number of recent reports indicating the importance of the interaction between these proteins and their encoding genes. Among others interactions, it is suggested that even heterozygous substitution in the PRKN gene in the presence of the variants +2/+2 or +2/+3 of NACP-Rep1 in the SNCA promoter, may increase the risk of PD manifestation, which is probably due to ineffective elimination of over-expressed ASN by the mutated Parkin protein. Finally, it seems that genetic testing may be an important part of diagnostics in patients with PD and may improve the prognostic process in the course of PD. However, only full knowledge of the mechanism of the interaction between the genes associated with the pathogenesis of PD is likely to help explain the currently unknown pathways of selective damage to dopaminergic neurons in the course of PD.

Postural instability/gait disturbance in Parkinson's disease has distinct subtypes: an exploratory analysis

OBJECTIVE

To test the hypothesis that postural instability with falling (PIF) and freezing of gait (FOG) are distinct subtypes of the postural instability/gait disturbance (PIGD) form of Parkinson's disease (PD).

METHODS

499 PD subjects from the NeuroGenetics Research Consortium were studied using logistic regression to examine, in a cross sectional analysis, predictors of FOG and PIF. Potential predictors were from four spheres; demographic, clinical motor, clinical non-motor and genetic.

RESULTS

FOG and PIF were both associated with greater gait subscores and lower tremor subscores on the Unified Parkinson's Disease Rating Scale (p ≤ 0.02). However, they differed with regard to demographic, non-motor and genetic predictors. FOG was associated with greater duration of disease, with ORs of 3.01 (95% CI 1.35 to 6.72) and 4.91 (95% CI 2.29 to 10.54) for third and fourth quartiles of duration, respectively, versus the lowest half of duration. The risk of having psychotic symptoms was also significantly increased (OR 3.02, 95% CI 1.41 to 6.49; p=0.004). FOG was inversely associated with the presence of the CYP2D6*4 allele (OR 0.41, 95% CI 0.21 to 0.80; p=0.009) suggesting a protective effect. PIF was associated with depression (OR 1.08, 95% CI 1.01 to 1.15; p<0.02) and was inversely associated with APOE ε4 (OR 0.21, 95% CI 0.05 to 0.87; p=0.03), again suggesting a protective effect.

CONCLUSION

FOG and PIF have different demographic, non-motor and genetic predictors suggesting that they may be pathophysiologically distinct subtypes of PIGD. These findings have implications in the discovery of therapeutic targets for these disabling features as well as for predicting outcomes of PD.

Rationale for therapeutic silencing of alpha-synuclein in Parkinson's disease

The purpose of this paper is to provide the rationale for therapeutic silencing of the alpha-synuclein gene (SNCA) in Parkinson’s disease (PD). The paper reviews the public health significance of PD; the causal links between rare SNCA variants and familial PD; the association of common SNCA variants and PD susceptibility; the association of SNCA variants also with age at onset and motor and cognitive outcomes in PD; therapeutic strategies targeting SNCA in PD; and preliminary findings and considerations on small interfering RNA-based therapies and PD.

The impact of genetic research on our understanding of Parkinson's disease

Until recently, genetics was thought to play a minor role in the development of Parkinson's disease (PD). Over the last decade, a number of genes that definitively cause PD have been identified, which has led to the generation of disease models based on pathogenic gene variants that recapitulate many features of the disease. These genetic studies have provided novel insight into potential mechanisms underlying the aetiology of PD. This chapter will provide a profile of the genes conclusively linked to PD and will outline the mechanisms of PD pathogenesis implicated by genetic studies. Mitochondrial dysfunction, oxidative stress and impaired ubiquitin-proteasome system function are disease mechanisms that are particularly well supported by genetic studies and are therefore the focus of this chapter.