Genetics and Parkinson's disease

GLP-1 Receptor Agonists in Neurodegeneration: Neurovascular Unit in the Spotlight

Defects in brain energy metabolism and proteopathic stress are implicated in age-related degenerative neuronopathies, exemplified by Alzheimer’s disease (AD) and Parkinson’s disease (PD). As the currently available drug regimens largely aim to mitigate cognitive decline and/or motor symptoms, there is a dire need for mechanism-based therapies that can be used to improve neuronal function and potentially slow down the underlying disease processes. In this context, a new class of pharmacological agents that achieve improved glycaemic control via the glucagon-like peptide 1 (GLP-1) receptor has attracted significant attention as putative neuroprotective agents. The experimental evidence supporting their potential therapeutic value, mainly derived from cellular and animal models of AD and PD, has been discussed in several research reports and review opinions recently. In this review article, we discuss the pathological relevance of derangements in the neurovascular unit and the significance of neuron–glia metabolic coupling in AD and PD. With this context, we also discuss some unresolved questions with regard to the potential benefits of GLP-1 agonists on the neurovascular unit (NVU), and provide examples of novel experimental paradigms that could be useful in improving our understanding regarding the neuroprotective mode of action associated with these agents.

Diagnosis of Parkinson's disease by investigating the inhibitory effect of serum components on P450 inhibition assay

Parkinson's disease (PD) is the second most common neurodegenerative disease, and diagnostic methods and biomarkers for patients without subjective motor symptoms have not yet been established. Previously, we developed a cytochrome P450 inhibition assay that detects alterations in metabolite levels associated with P450s caused by inflammation and exposure to endogenous or exogenous substances. However, it is unknown whether the P450 inhibition assay can be applied in PD diagnosis. Here, we determined whether the P450 inhibition assay can discriminate sera between patients with PD and healthy individuals. The results of the assay revealed that the P450 inhibition assay can discriminate PD with an area under the receiver operating characteristic curve (AUC) value of 0.814-0.914 in rats and an AUC value of 0.910 in humans. These findings demonstrate that the P450 inhibition assay can aid in the future development of liquid biopsy-based diagnostic methods for PD.

PARK2 and PARK7 Gene Polymorphisms as Risk Factors Associated with Serum Element Concentrations and Clinical Symptoms of Parkinson's Disease

Besides clinical and imaging techniques, there is a lack of molecular makers for the diagnosis of Parkinson's disease (PD). There is an immense need to develop biomarkers associated with the phenotypes which may be valuable for individualized treatment. Single-nucleotide polymorphisms (PARK2: Ser167Asn (G>A) and Val380Leu (G>C); PARK7: IVS4 + 46G>A and IVS4 + 30T>G) in PD-related genes were examined to elucidate its relationship with concentration of serum elements and clinical symptoms of PD. A total of 214 PD patients and 213 controls from Indian population were genotyped using PCR and DNA sequencing methods. The serum element concentrations were detected and clinical symptoms were determined based on UPDRS scale and recorded at the time of sample collection. The IVS4 + 30T>G, Ser167Asn (G>A) and Val380Leu (G>C) polymorphisms appeared to alter element concentrations in PD. The patients with Ser167Asn polymorphism showed significant association with copper, iron and zinc that reinforces the role of A allele as a factor for change in the concentrations of elements, than those patients with G allele. In particular, patients with A allele of Ser167Asn have risk of having high serum iron concentration (OR 11.55, 95% CI 5.59-23.85), which are associated with dementia and postural imbalance. Similar results were observed for Val380Leu (G>C) and IVS4 + 30T>G polymorphisms which suggest their role in element concentration and neurological symptoms. Overall, our study demonstrates the influence of polymorphisms of PD genes on element concentrations and clinical symptoms. Results of this study may be taken into account when considering the contributing factors for PD symptoms.

Abundance of Synaptic Vesicle-Related Proteins in Alpha-Synuclein-Containing Protein Inclusions Suggests a Targeted Formation Mechanism

Proteinaceous α-synuclein-containing inclusions are found in affected brain regions in patients with Parkinson's disease (PD), Dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). These appear in neurons as Lewy bodies in both PD and DLB and as glial cytoplasmic inclusions (GCIs) in oligodendrocytes in MSA. The role they play in the pathology of the diseases is unknown, and relatively little is still known about their composition. By purifying the inclusions from the surrounding tissue and comprehensively analysing their protein composition, vital clues to the formation mechanism and role in the disease process may be found. In this study, Lewy bodies were purified from postmortem brain tissue from DLB cases (n = 2) and GCIs were purified from MSA cases (n = 5) using a recently improved purification method, and the purified inclusions were analysed by mass spectrometry. Twenty-one percent of the proteins found consistently in the GCIs and LBs were synaptic-vesicle related. Identified proteins included those associated with exosomes (CD9), clathrin-mediated endocytosis (clathrin, AP-2 complex, dynamin), retrograde transport (dynein, dynactin, spectrin) and synaptic vesicle fusion (synaptosomal-associated protein 25, vesicle-associated membrane protein 2, syntaxin-1). This suggests that the misfolded or excess α-synuclein may be targeted to inclusions via vesicle-mediated transport, which also explains the presence of the neuronal protein α-synuclein within GCIs.

Association between Glutathione S-transferase M1/Glutathione S-transferase T1 polymorphisms and Parkinson's disease: a meta-analysis

The Glutathione S-transferase M1 (GSTM1) and Glutathione S-transferase T1 (GSTT1) genes have been studied extensively as potential candidate genes for the risk of Parkinson's disease (PD). However, direct evidence from genetic association studies remains inconclusive. In order to address this issue, we performed an updated and refined meta-analysis to determine the effect of GSTM1 and GSTT1 polymorphisms on Parkinson's disease. A fixed-effect model was utilized to calculate the combined odds ratio (OR), OR of different ethnicities, and 95% confidence intervals (CIs). Potential publication bias was estimated. Homogeneity of the included studies was also evaluated. The pooled OR was 1.13 [95% CI (1.03, 1.24)] and 0.96 [95% CI (0.82, 1.12)] for GSTM1 and GSTT1 polymorphisms, respectively. Analysis according to different races found no association between GSTM1/GSTT1 polymorphisms and PD risks except for GSTM1 variant in Caucasians, which showed a weak correlation (OR 1.16 [95% CI (1.04, 1.29), I squared=6.2%, p=0.384]). Neither publication bias nor heterogeneity was found among the included studies. The results of this meta-analysis suggest that GSTM1 polymorphism is weakly associated with the risk of PD in Caucasians whereas GSTT1 polymorphism is not a PD risk factor.

Genetics of Parkinson's disease - a clinical perspective

Discovering genes following Medelian inheritance, such as autosomal dominant-synuclein and leucine-rich repeat kinase 2 gene, or autosomal recessive Parkin, P-TEN-induced putative kinase 1 gene and Daisuke-Junko 1 gene, has provided great insights into the pathogenesis of Parkinson's disease (PD). Genes found to be associated with PD through investigating genetic polymorphisms or via the whole genome association studies suggest that such genes could also contribute to an increased risk of PD in the general population. Some environmental factors have been found to be associated with genetic factors in at-risk patients, further implicating the role of gene-environment interactions in sporadic PD. There may be confusion for clinicians facing rapid progresses of genetic understanding in PD. After a brief review of PD genetics, we will discuss the insight of new genetic discoveries to clinicians, the implications of ethnic differences in PD genetics and the role of genetic testing for general clinicians managing PD patients.

Absence of commonly reported leucine-rich repeat kinase 2 mutations in Eastern Indian Parkinson's disease patients

BACKGROUND

Pathogenic mutations in leucine-rich repeat kinase 2 (LRRK2; PARK8) encoding dardarin, implicated in patients with autosomal dominant and sporadic Parkinson's disease (PD) among different ethnic groups (Ashkenazi Jews, North African Arabs, Basques) might be of some help in diagnostic screening and genetic counseling.

AIM OF THE STUDY

We investigated the seven common mutations spanning exons 31, 35, and 41 reported in the LRRK2 gene among Eastern Indian patients with PD.

METHODS

Mutations R1441G, R1441C, R1441H, G2019S, Y1699C, I2020T, and I2012T were screened in 320 individuals (PD, 150 and controls, 170) by direct sequencing.

RESULTS

We did not observe any of these abovementioned mutations in our studied individuals.

CONCLUSION

We conclude that these mutations are rare causes of PD in the Eastern Indian population and, therefore, of little help for genetic counseling and diagnostic purposes.

Dementia with Lewy bodies in an elderly Greek male due to alpha-synuclein gene mutation

We report the case of an elderly man of Greek background who presented with progressive cognitive decline and motor parkinsonism on a background of a strong family history of Parkinson's disease. Associated symptoms included visual hallucinations, excessive daytime drowsiness, recurrent falls, orthostatic hypotension and urinary incontinence. His major clinical symptoms and signs fulfilled consensus criteria for a clinical diagnosis of dementia with Lewy bodies. An alpha-synuclein gene mutation analysis for the G209A substitution was positive. We conclude that the alpha-synuclein (G209A) gene mutation genotype should be considered in the differential diagnosis of dementia with Lewy bodies, particularly in patients with European ancestry and a family history of Parkinson's disease.

Oxidative damage of DJ-1 is linked to sporadic Parkinson and Alzheimer diseases

Mutations in DJ-1 cause an autosomal recessive, early onset familial form of Parkinson disease (PD). However, little is presently known about the role of DJ-1 in the more common sporadic form of PD and in other age-related neurodegenerative diseases, such as Alzheimer disease (AD). Here we report that DJ-1 is oxidatively damaged in the brains of patients with idiopathic PD and AD. By using a combination of two-dimensional gel electrophoresis and mass spectrometry, we have identified 10 different DJ-1 isoforms, of which the acidic isoforms (pI 5.5 and 5.7) of DJ-1 monomer and the basic isoforms (pI 8.0 and 8.4) of SDS-resistant DJ-1 dimer are selectively accumulated in PD and AD frontal cortex tissues compared with age-matched controls. Quantitative Western blot analysis shows that the total level of DJ-1 protein is significantly increased in PD and AD brains. Mass spectrometry analyses reveal that DJ-1 is not only susceptible to cysteine oxidation but also to previously unsuspected methionine oxidation. Furthermore, we show that DJ-1 protein is irreversibly oxidized by carbonylation as well as by methionine oxidation to methionine sulfone in PD and AD. Our study provides new insights into the oxidative modifications of DJ-1 and indicates association of oxidative damage to DJ-1 with sporadic PD and AD.

Parkin expression in human skeletal muscle

Parkin is known to be present in human neurons and peripheral nerves. Using an antibody against parkin protein we have now demonstrated that parkin is also expressed in the sarcoplasm and sarcolemmal region of human skeletal muscle fibres. We have also found different age-related patterns of expression with increase in intensity and organization of distribution at older ages. These findings suggest a change in the functional role of parkin in skeletal muscle with ageing and may contribute to understanding the mechanisms of muscle aging.

Potential occupational risks for neurodegenerative diseases

BACKGROUND

Associations between occupations and neurodegenerative diseases (NDD) may be discernable in death certificate data.

METHODS

Hypotheses generated from 1982 to 1991 study were tested in data from 22 states for the years 1992-1998. Specific occupations and exposures to pesticides, solvents, oxidative stressors, magnetic fields, and welding fumes were evaluated.

RESULTS

About one third (26/87) of the occupations hypothesized with neurodegenerative associations had statistically significant elevated mortality odds ratios (MOR) for the same outcome. Occupations with the largest MORs were (a) for presenile dementia (PSD)-dentists, graders/sorters (non-agricultural), and clergy; (b) for Alzheimer's disease (AD)-bank tellers, clergy, aircraft mechanics, and hairdressers; (c) for Parkinson's disease (PD)-biological scientists, clergy, religious workers, and post-secondary teachers; and (d) for motor neuron disease (MND)-veterinarians, hairdressers, and graders and sorters (non-agricultural). Teachers had significantly elevated MORs for all four diseases, and hairdressers for three of the four. Non-horticultural farmers below age 65 had elevated PD (MOR = 2.23, 95% CI = 1.47-3.26), PSD (MOR = 2.22, 95% CI = 1.10-4.05), and AD (MOR = 1.76, 95% CI = 1.04-2.81). Sixty hertz magnetic fields exhibited significant exposure-response for AD and, below age 65, for PD (MOR = 1.87, 95% CI = 1.14-2.98) and MND (MOR = 1.63, 95% CI = 1.10-2.39). Welding had elevated PD mortality below age 65 (MOR = 1.77, 95% CI = 1.08-2.75).

CONCLUSIONS

Support was observed for hypothesized excess neurodegenerative disease associated with a variety of occupations, 60 Hz magnetic fields and welding.

Analysis of the trinucleotide CAG repeat from the DNA polymerase γ gene (POLG) in patients with Parkinson's disease

The human gene for the catalytic subunit of the mitochondrial DNA (mtDNA) polymerase (POLG) contains a trinucleotide CAG repeat encoding a polyglutamine tract near the amino-terminus of the protein. Expansions of similar polyglutamine-encoding CAG microsatellite repeats in other genes are known to cause neurodegenerative disorders. As mitochondrial dysfunction has been implicated in the aetiology of Parkinson's disease, we determined the POLG CAG repeat length in DNA samples extracted from 22 idiopathic Parkinson's disease patients and 31 control subjects. The distribution of the POLG CAG repeat length in the control samples matched the distribution reported for control samples by others. Comparison between the CAG repeat length distribution of control and Parkinson's disease samples revealed no evidence of either germ line or somatic POLG CAG repeat instability in Parkinson's disease patients. Our results rule out POLG CAG repeat instability as a common pathogenic mechanism in idiopathic Parkinson's disease.

The Proteomics of Neurodegeneration

The continuing improvement and refinement of proteomic and bioinformatic tools has made it possible to obtain increasing amounts of structural and functional information about proteins on a global scale. The emerging field of neuroproteomics promises to provide powerful strategies for further characterizing neuronal dysfunction and cell loss associated with neurodegenerative diseases. Neuroproteomic studies have thus far revealed relatively comprehensive quantitative changes and post-translational modifications (mostly oxidative damage) of high abundance proteins, confirming deficits in energy production, protein degradation, antioxidant protein function, and cytoskeletal regulation associated with neurodegenerative diseases such as Alzheimer and Parkinson disease. The identification of changes in low-abundance proteins and characterization of their functions based on protein-protein interactions still await further development of proteomic methodologies and more dedicated application of these technologies by neuroscientists. Once accomplished, however, the resulting information will certainly provide a truly comprehensive view of neurodegeneration-associated changes in protein expression, facilitating the identification of novel biomarkers for the early detection of neurodegenerative diseases and new targets for therapeutic intervention.