Polymorphism in MIR4697 but not VPS13C, GCH1, or SIPA1L2 is associated with risk of Parkinson's disease in a Han Chinese population

Testing SIPA1L2 as a modifier of CMT1A using mouse models

Charcot-Marie-Tooth disease type 1A (CMT1A) is a demyelinating peripheral neuropathy caused by the duplication of peripheral myelin protein 22 (PMP22), leading to muscle weakness and loss of sensation in the hands and feet. A recent case-only genome-wide association study of CMT1A patients conducted by the Inherited Neuropathy Consortium identified a strong association between strength of foot dorsiflexion and variants in signal induced proliferation associated 1 like 2 (SIPA1L2), indicating that it may be a genetic modifier of disease. To validate SIPA1L2 as a candidate modifier and to assess its potential as a therapeutic target, we engineered mice with deletion of exon 1 (including the start codon) of the Sipa1l2 gene and crossed them to the C3-PMP22 mouse model of CMT1A. Neuromuscular phenotyping showed that Sipa1l2 deletion in C3-PMP22 mice preserved muscular endurance assayed by inverted wire hang duration and changed femoral nerve axon morphometrics such as myelin thickness. Gene expression changes suggest involvement of Sipa1l2 in cholesterol biosynthesis, a pathway that is also implicated in C3-PMP22 mice. Although Sipa1l2 deletion did impact CMT1A-associated phenotypes, thereby validating a genetic interaction, the overall effect on neuropathy was mild.

Testing SIPA1L2 as a modifier of CMT1A using mouse models

Charcot-Marie-Tooth 1A is a demyelinating peripheral neuropathy caused by the duplication of peripheral myelin protein 22 (PMP22), which produces muscle weakness and loss of sensation in the hands and feet. A recent case-only genome wide association study by the Inherited Neuropathy Consortium identified a strong association between variants in signal induced proliferation associated 1 like 2 (SIPA1L2) and strength of foot dorsiflexion. To validate SIPA1L2 as a candidate modifier, and to assess its potential as a therapeutic target, we engineered mice with a deletion in SIPA1L2 and crossed them to the C3-PMP22 mouse model of CMT1A. We performed neuromuscular phenotyping and identified an interaction between Sipa1l2 deletion and muscular endurance decrements assayed by wire-hang duration in C3-PMP22 mice, as well as several interactions in femoral nerve axon morphometrics such as myelin thickness. Gene expression changes suggested an involvement of Sipa1l2 in cholesterol biosynthesis, which was also implicated in C3-PMP22 mice. Though several interactions between Sipa1l2 deletion and CMT1A-associated phenotypes were identified, validating a genetic interaction, the overall effect on neuropathy was small.

The endo-lysosomal system in Parkinson's disease: expanding the horizon

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease, and its prevalence is increasing with age. A wealth of genetic evidence indicates that the endo-lysosomal system is a major pathway driving PD pathogenesis with a growing number of genes encoding endo-lysosomal proteins identified as risk factors for PD, making it a promising target for therapeutic intervention. However, detailed knowledge and understanding of the molecular mechanisms linking these genes to the disease are available for only a handful of them (e.g. LRRK2, GBA1, VPS35). Taking on the challenge of studying poorly characterized genes and proteins can be daunting, due to the limited availability of tools and knowledge from previous literature. This review aims at providing a valuable source of molecular and cellular insights into the biology of lesser-studied PD-linked endo-lysosomal genes, to help and encourage researchers in filling the knowledge gap around these less popular genetic players. Specific endo-lysosomal pathways discussed range from endocytosis, sorting, and vesicular trafficking to the regulation of membrane lipids of these membrane-bound organelles and the specific enzymatic activities they contain. We also provide perspectives on future challenges that the community needs to tackle and propose approaches to move forward in our understanding of these poorly studied endo-lysosomal genes. This will help harness their potential in designing innovative and efficient treatments to ultimately re-establish neuronal homeostasis in PD but also other diseases involving endo-lysosomal dysfunction.

Association between VPS13C rs2414739 polymorphism and Parkinson's disease risk: A meta-analysis

OBJECTIVE

We aimed to estimate the role of vacuolar protein sorting 13C (VPS13C) gene single nucleotide polymorphism (SNP) rs2414739 variant in the risk of PD by meta-analysis.

METHODS

Five eligible case-control studies including 2796 PD cases and 4138 health controls involved in this meta-analysis. The fixed or random effect model was selected based on the heterogeneity of the included studies which detected by I² and Q tests. The association between rs2414739 polymorphism and the risk of PD was evaluated using the pooled odds ratios (OR) and 95 % confidence interval (95 %CI). Sensitivity analysis was used to test the stability of the results. Funnel plot and Begg's test were employed to verified publication bias.

RESULTS

The results of our meta-analysis showed a significant correlation between VPS13C rs2424739 gene polymorphism and PD susceptibility in Allele model (A versus vs. G: OR = 1.14, 95 %CI = 1.05-1.23, p = 0.002), dominant model (GG + AG vs. AA: OR = 0.86, 95 %CI = 0.78-0.95, p = 0.004), heterozygote model (AG vs. AA: OR = 0.87, 95 %CI = 0.77-0.99, p = 0.04), homozygote model (GG vs. AA: OR = 0.76, 95 %CI = 0.60-0.96, p = 0.02). Surprisingly, we did not find a significant statistical difference between VPS13C rs2414739 polymorphism and PD risk in Chinese cohort in the regional stratified analysis.

CONCLUSIONS

This meta-analysis suggests that VPS13C rs2414739 polymorphism might act as a genetic predisposition factor for PD, whereas does not include Chinese population.

Comparative proteomic analysis highlights metabolic dysfunction in α-synucleinopathy

The synaptic protein α-synuclein is linked through genetics and neuropathology to the pathogenesis of Parkinson’s disease and related disorders. However, the mechanisms by which α-synuclein influences disease onset and progression are incompletely understood. To identify pathogenic pathways and therapeutic targets we performed proteomic analysis in a highly penetrant new Drosophila model of α-synucleinopathy. We identified 476 significantly upregulated and 563 significantly downregulated proteins in heads from α-synucleinopathy model flies compared to controls. We then used multiple complementary analyses to identify and prioritize genes and pathways within the large set of differentially expressed proteins for functional studies. We performed Gene Ontology enrichment analysis, integrated our proteomic changes with human Parkinson’s disease genetic studies, and compared the α-synucleinopathy proteome with that of tauopathy model flies, which are relevant to Alzheimer’s disease and related disorders. These approaches identified GTP cyclohydrolase (GCH1) and folate metabolism as candidate mediators of α-synuclein neurotoxicity. In functional validation studies, we found that the knockdown of Drosophila Gch1 enhanced locomotor deficits in α-synuclein transgenic flies, while folate supplementation protected from α-synuclein toxicity. Our integrative analysis suggested that mitochondrial dysfunction was a common downstream mediator of neurodegeneration. Accordingly, Gch1 knockdown enhanced metabolic dysfunction in α-synuclein transgenic fly brains while folate supplementation partially normalized brain bioenergetics. Here we outline and implement an integrative approach to identify and validate potential therapeutic pathways using comparative proteomics and genetics and capitalizing on the facile genetic and pharmacological tools available in Drosophila.

Analysis of common and rare VPS13C variants in late-onset Parkinson disease

Objective

We aimed to study the role of coding VPS13C variants in a large cohort of patients with late-onset Parkinson disease (PD) (LOPD).

Methods

VPS13C and its untranslated regions were sequenced using targeted next-generation sequencing in 1,567 patients with PD and 1,667 controls from 3 cohorts. Association tests of rare potential homozygous and compound heterozygous variants and burden tests for rare heterozygous variants were performed. Common variants were analyzed using logistic regression adjusted for age and sex in each of the cohorts, followed by a meta-analysis.

Results

No biallelic carriers of rare VPS13C variants were found among patients, and 2 carriers of compound heterozygous variants were found in 2 controls. There was no statistically significant burden of rare (minor allele frequency [MAF] <1%) or very rare (MAF <0.1%) coding VPS13C variants in PD. A VPS13C haplotype including the p.R153H-p.I398I-p.I1132V-p.Q2376Q variants was nominally associated with a reduced risk for PD (meta-analysis of the tagging SNP p.I1132V [odds ratio = 0.48, 95% confidence interval = 0.28–0.82, p = 0.0052]). This haplotype was not in linkage disequilibrium with the known genome-wide association study top hit.

Conclusions

Our results do not support a role for rare heterozygous or biallelic VPS13C variants in LOPD. Additional genetic replication and functional studies are needed to examine the role of the haplotype identified here associated with reduced risk for PD.

Common and rare GCH1 variants are associated with Parkinson's disease

GCH1 encodes the enzyme guanosine triphospahte (GTP) cyclohydrolase 1, essential for dopamine synthesis in nigrostriatal cells, and rare mutations in GCH1 may lead to Dopa-responsive dystonia (DRD). While GCH1 is implicated in genomewide association studies in Parkinson's disease (PD), only a few studies examined the role of rare GCH1 variants in PD, with conflicting results. In the present study, GCH1 and its 5' and 3' untranslated regions were sequenced in 1113 patients with PD and 1111 controls. To examine the association of rare GCH1 variants with PD, burden analysis was performed. Three rare GCH1 variants, which were previously reported to be pathogenic in DRD, were found in five patients with PD and not in controls (sequence Kernel association test, p = 0.024). A common haplotype, tagged by rs841, was associated with a reduced risk for PD (OR = 0.71, 95% CI = 0.61-0.83, p = 1.24 × 10^-4), and with increased GCH1 expression in brain regions relevant for PD (www.gtexportal.org). Our results support a role for rare, DRD-related variants, and common GCH1 variants in the pathogenesis of PD.

Application of Precision Medicine in Neurodegenerative Diseases

One of the main challenges for healthcare systems is the increasing prevalence of neurodegenerative pathologies together with the rapidly aging populations. The enormous progresses made in the field of biomedical research and informatics have been crucial for improving the knowledge of how genes, epigenetic modifications, aging, nutrition, drugs and microbiome impact health and disease. In fact, the availability of high technology and computational facilities for large-scale analysis enabled a deeper investigation of neurodegenerative disorders, providing a more comprehensive overview of disease and encouraging the development of a precision medicine approach for these pathologies. On this subject, the creation of collaborative networks among medical centers, research institutes and highly qualified specialists can be decisive for moving the precision medicine from the bench to the bedside. To this purpose, the present review has been thought to discuss the main components which may be part of precise and personalized treatment programs applied to neurodegenerative disorders. Parkinson Disease will be taken as an example to understand how precision medicine approach can be clinically useful and provide substantial benefit to patients. In this perspective, the realization of web-based networks can be decisive for the implementation of precision medicine strategies across different specialized centers as well as for supporting clinical/therapeutical decisions and promoting a more preventive and participative medicine for neurodegenerative disorders. These collaborative networks are essentially addressed to find innovative, sustainable and effective strategies able to provide optimal and safer therapies, discriminate at risk individuals, identify patients at preclinical or early stage of disease, set-up individualized and preventative strategies for improving prognosis and patient's quality of life.

Association analyses of variants of SIPA1L2, MIR4697, GCH1, VPS13C, and DDRGK1 with Parkinson's disease in East Asians

A recent large-scale European-originated genome-wide association data meta-analysis followed by a replication study identified 6 new risk loci for Parkinson's disease (PD), which include rs10797576/SIPA1L2, rs117896735/INPP5F, rs329648/MIR4697, rs11158026/GCH1, rs2414739/VPS13C, and rs8118008/DDRGK1. However, whether these new loci are associated with PD in Asian populations remain elusive. The INPP5F is nonpolymorphic in Asians. The present study aimed to understand the effects of the other 5 new loci in a Han Chinese population comprising 579 sporadic PD patients and 642 controls. Significant associations with PD were observed in the variants of SIPA1L2 (p = 0.001) and VPS13C (p = 0.007), where the T (odd ratio [OR] = 1.484, 95% confidence interval [CI] 1.186-1.858) and A (OR = 1.362, 95% CI 1.087-1.707) alleles serve as the risk alleles, respectively. The genotype distributions in the SIPA1L2 and VPS13C variants were also different between the patients and controls (p = 0.002 and p = 0.023, respectively). In contrast, no significant association with PD was found in the variants of MIR4697, GCH1, and DDRGK1 either in allele or genotype frequencies. Noteworthy, a followed meta-analysis of East Asian studies suggested an association of the GCH1 variant with PD (p = 0.04, OR 1.08, 95% CI 1.00-1.16), while the other results are in line with those of our cohort. In conclusion, our study together with meta-analyses demonstrates that the variants of SIPA1L2 and VPS13C, potentially GCH1, but not of MIR4697 and DDRGK1, are associated with PD susceptibility in East Asians.