UCHL1 is associated with Parkinson's disease: A case-unaffected sibling and case-unrelated control study

Role of Deubiquitinases in Parkinson's Disease-Therapeutic Perspectives

Parkinson's disease (PD) is a neurodegenerative disorder that has been associated with mitochondrial dysfunction, oxidative stress, and defects in mitophagy as well as α-synuclein-positive inclusions, termed Lewy bodies (LBs), which are a common pathological hallmark in PD. Mitophagy is a process that maintains cellular health by eliminating dysfunctional mitochondria, and it is triggered by ubiquitination of mitochondrial-associated proteins-e.g., through the PINK1/Parkin pathway-which results in engulfment by the autophagosome and degradation in lysosomes. Deubiquitinating enzymes (DUBs) can regulate this process at several levels by deubiquitinating mitochondrial substrates and other targets in the mitophagic pathway, such as Parkin. Moreover, DUBs can affect α-synuclein aggregation through regulation of degradative pathways, deubiquitination of α-synuclein itself, and/or via co-localization with α-synuclein in inclusions. DUBs with a known association to PD are described in this paper, along with their function. Of interest, DUBs could be useful as novel therapeutic targets against PD through regulation of PD-associated defects.

Increased levels of UCHL1 are a compensatory response to disrupted ubiquitin homeostasis in spinal muscular atrophy and do not represent a viable therapeutic target

AIM

Levels of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) are robustly increased in spinal muscular atrophy (SMA) patient fibroblasts and mouse models. We therefore wanted to establish whether changes in UCHL1 contribute directly to disease pathogenesis, and to assess whether pharmacological inhibition of UCHL1 represents a viable therapeutic option for SMA.

METHODS

SMA mice and control littermates received a pharmacological UCHL1 inhibitor (LDN-57444) or DMSO vehicle. Survival and weight were monitored daily, a righting test of motor performance was performed, and motor neurone loss, muscle fibre atrophy and neuromuscular junction pathology were all quantified. Ubiquitin-like modifier activating enzyme 1 (Uba1) was then pharmacologically inhibited in neurones in vitro to examine the relationship between Uba1 levels and UCHL1 in SMA.

RESULTS

Pharmacological inhibition of UCHL1 failed to improve survival, motor symptoms or neuromuscular pathology in SMA mice and actually precipitated the onset of weight loss. LDN-57444 treatment significantly decreased spinal cord mono-ubiquitin levels, further exacerbating ubiquitination defects in SMA mice. Pharmacological inhibition of Uba1, levels of which are robustly reduced in SMA, was sufficient to induce accumulation of UCHL1 in primary neuronal cultures.

CONCLUSION

Pharmacological inhibition of UCHL1 exacerbates rather than ameliorates disease symptoms in a mouse model of SMA. Thus, pharmacological inhibition of UCHL1 is not a viable therapeutic target for SMA. Moreover, increased levels of UCHL1 in SMA likely represent a downstream consequence of decreased Uba1 levels, indicative of an attempted supportive compensatory response to defects in ubiquitin homeostasis caused by low levels of SMN protein.

Association between ubiquitin carboxy-terminal hydrolase-L1 S18Y variant and risk of Parkinson's disease: the impact of ethnicity and onset age

The Ubiquitin carboxy-terminal hydrolase-L1 (UCHL1) is a candidate risk gene for Parkinson' disease (PD), and a function SNP (rs5030732) in the coding region of this gene has been studied for the association with the disease extensively among worldwide populations, but the results were inconsistent and controversial. Here, to estimate the association between UCHL1 S18Y polymorphism and risk of PD in general population, we conducted a systematic meta-analysis by combining all available case-control subjects in Asian, European, and American populations, with a total of 7742 PD cases and 8850 healthy controls, and the pooled odds ratios (ORs) and 95% confidence intervals (95% CIs) for UCHL1 S18Y polymorphism and PD were calculated using the Mantel-Haenszel method with a fixed- or random-effects model. Subgroup analysis was also performed in different onset age-matched groups. Among high-quality studies, UCHL1 S18Y polymorphism was moderately associated with the risk of PD (allele contrasts, OR = 1.063, 95% CI 1.008-1.122; p = 0.024; regressive genetic model, OR = 1.078, 95% CI 1.005-1.157; p = 0.035). When stratifying for ethnicity, none association were observed in subgroups. Analysis of early-onset PD (EOPD) and late-onset PD (LOPD) revealed that the polymorphism was not associated with the risk of PD. In conclusion, our meta-analysis suggests that UCHL1 S18Y polymorphism is moderately associated with susceptibility to PD, and more studies are needed to confirm our conclusion.

UCHL1 S18Y variant is a risk factor for Parkinson's disease in Japan

Background

A recent meta-analysis on the UCHL1 S18Y variant and Parkinson’s disease (PD) showed a significant inverse association between the Y allele and PD; the individual studies included in that meta-analysis, however, have produced conflicting results. We examined the relationship between UCHL1 S18Y single nucleotide polymorphism (SNP) and sporadic PD in Japan.

Methods

Included were 229 cases within 6 years of onset of PD, defined according to the UK PD Society Brain Bank clinical diagnostic criteria. Controls were 357 inpatients and outpatients without neurodegenerative disease. Adjustment was made for sex, age, region of residence, smoking, and caffeine intake.

Results

Compared with subjects with the CC or CA genotype of UCHL1 S18Y SNP, those with the AA genotype had a significantly increased risk of sporadic PD: the adjusted OR was 1.57 (95 % CI: 1.06 − 2.31). Compared with subjects with the CC or CA genotype of UCHL1 S18Y and the CC or CT genotype of SNCA SNP rs356220, those with the AA genotype of UCHL1 S18Y and the TT genotype of SNP rs356220 had a significantly increased risk of sporadic PD; the interaction, however, was not significant. Our previous investigation found significant inverse relationships between smoking and caffeine intake and PD in this population. There were no significant interactions between UCHL1 S18Y and smoking or caffeine intake affecting sporadic PD.

Conclusions

This study reveals that the UCHL1 S18Y variant is a risk factor for sporadic PD. We could not find evidence for interactions affecting sporadic PD between UCHL1 S18Y and SNCA SNP rs356220, smoking, or caffeine intake.

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.

Case-control study of the UCH-L1 S18Y variant in sporadic Parkinson's disease in the Chinese population

The ubiquitin carboxy-terminal hydrolase L1 gene (UCH-L1) has been implicated in the etiology of Parkinson's disease (PD). In several previous studies, an S18Y (C54A) polymorphism in exon 3 of the UCH-L1 gene has been found to be protective against PD. We performed polymerase chain reaction-restriction fragment length polymorphism analysis for DNA samples from 408 Chinese patients with PD and 398 Chinese healthy controls. For the S18Y variant, there was no significant difference either in the individual allele or genotype frequencies between cases and control subjects. Possession of the S18Y variant did not alter the risk of developing PD (odds ratio: 0.827; 95% confidence interval=0.596-1.147). There was no statistically significant difference in terms of age or sex distribution between the patients and controls (p>0.05). Overall, considering our present results together with those of our previous studies, we now have access to data from more than 1000 patients from different regions of China, supporting the conclusion that the S18Y polymorphism may not have a protective effect against PD in the Chinese population.

Ubiquitin carboxyl-terminal esterase L1 (UCHL1) S18Y polymorphism in patients with cataracts

Background: Cataract is characterized by light-scattering protein aggregates. The ubiquitin-proteasome system has been proposed a role in proteolytic removal of these protein aggregates. Ubiquitin carboxyl-terminal esterase L1 (UCHL1) is a de-ubiquitinating enzyme with

important functions in recycling of ubiquitin. A protective role of the p.S18Y polymorphism of the UCHL1 gene has been shown in Parkinson's disease. The current study aimed to examine possible effects on cataract formation.

Methods: Patients with cataract (n = 493) and controls (n = 142) were analyzed for the UCHL1 p.S18Y polymorphism using dynamic allele-specific hybridization.

Results: Significant differences were observed in allele and genotype frequencies of the p.S18Y polymorphism between controls and cataract patients, where a positive UCHL1 allele A carrier status was associated with the cataract diagnosis (adjusted OR 1.7 [95% CI = 1.1-2.6] p = 0.02). No significant differences were seen in genotype distribution when stratifying for type of cataract. Nor did the mean age at cataract surgery differ between genotypes.

Conclusion: The current study does not support a protective role for the UCHL1 S18Y polymorphism in cataract development, but may instead suggest a disease-promoting effect.

Common variants in PARK loci and related genes and Parkinson's disease

Rare mutations in PARK loci genes cause Parkinson's disease (PD) in some families and isolated populations. We investigated the association of common variants in PARK loci and related genes with PD susceptibility and age at onset in an outbred population. A total of 1,103 PD cases from the upper Midwest, USA, were individually matched to unaffected siblings (n = 654) or unrelated controls (n = 449) from the same region. Using a sequencing approach in 25 cases and 25 controls, single nucleotide polymorphisms (SNPs) in species-conserved regions of PARK loci and related genes were detected. We selected additional tag SNPs from the HapMap. We genotyped a total of 235 SNPs and two variable number tandem repeats in the ATP13A2, DJ1, LRRK1, LRRK2, MAPT, Omi/HtrA2, PARK2, PINK1, SNCA, SNCB, SNCG, SPR, and UCHL1 genes in all 2,206 subjects. Case-control analyses were performed to study association with PD susceptibility, while cases-only analyses were used to study association with age at onset. Only MAPT SNP rs2435200 was associated with PD susceptibility after correction for multiple testing (OR = 0.74, 95% CI = 0.64-0.86, uncorrected P < 0.0001, log additive model); however, 16 additional MAPT variants, seven SNCA variants, and one LRRK2, PARK2, and UCHL1 variants each had significant uncorrected P-values. There were no significant associations for age at onset after correction for multiple testing. Our results confirm the association of MAPT and SNCA genes with PD susceptibility but show limited association of other PARK loci and related genes with PD.

A system-based comparison of gene expression reveals alterations in oxidative stress, disruption of ubiquitin-proteasome system and altered cell cycle regulation after exposure to cadmium and methylmercury in mouse embryonic fibroblast

Environmental and occupational exposures to heavy metals such as methylmercury (MeHg) and cadmium (Cd) pose significant health risks to humans, including neurotoxicity. The underlying mechanisms of their toxicity, however, remain to be fully characterized. Our previous studies with Cd and MeHg have demonstrated that the perturbation of the ubiquitin-proteasome system (UPS) was associated with metal-induced cytotoxicity and apoptosis. We conducted a microarray-based gene expression analysis to compare metal-altered gene expression patterns with a classical proteasome inhibitor, MG132 (0.5 microM), to determine whether the disruption of the UPS is a critical mechanism of metal-induced toxicity. We treated mouse embryonic fibroblast cells at doses of MeHg (2.5 microM) and Cd (5.0 microM) for 24 h. The doses selected were based on the neutral red-based cell viability assay where initial statistically significant decreases in variability were detected. Following normalization of the array data, we employed multilevel analysis tools to explore the data, including group comparisons, cluster analysis, gene annotations analysis (gene ontology analysis), and pathway analysis using GenMAPP and Ingenuity Pathway Analysis (IPA). Using these integrated approaches, we identified significant gene expression changes across treatments within the UPS (Uchl1 and Ube2c), antioxidant and phase II enzymes (Gsta2, Gsta4, and Noq1), and genes involved in cell cycle regulation pathways (ccnb1, cdc2a, and cdc25c). Furthermore, pathway analysis revealed significant alterations in genes implicated in Parkinson's disease pathogenesis following metal exposure. This study suggests that these pathways play a critical role in the development of adverse effects associated with metal exposures.

Association between the ubiquitin carboxyl-terminal esterase L1 gene (UCHL1) S18Y variant and Parkinson's Disease: a HuGE review and meta-analysis

The ubiquitin carboxyl-terminal esterase L1 gene, UCHL1, located on chromosome 4p14, has been studied as a potential candidate gene for Parkinson's disease risk. The authors conducted a Human Genome Epidemiology review and meta-analysis of published case-control studies of the UCHL1 S18Y variant and Parkinson's disease in Asian and Caucasian samples. The meta-analysis of studies in populations of Asian ancestry showed a statistically significant association between the Y allele and reduced risk of Parkinson's disease under a recessive model (odds ratio (OR) for YY vs. SY + SS = 0.79, 95% confidence interval (CI): 0.67, 0.94; P = 0.006). For a dominant model, the association was not significant in Asian populations (OR for YY + SY vs. SS = 0.88, 95% CI: 0.68, 1.14; P = 0.33). For populations of European ancestry, the meta-analysis showed a significant association between the Y allele and decreased risk of Parkinson's disease under a dominant model (OR = 0.89, 95% CI: 0.81, 0.98; P = 0.02) but not under a recessive model (OR = 0.92, 95% CI: 0.66, 1.30; P = 0.65). Using the Venice criteria, developed by the Human Genome Epidemiology Network Working Group on the assessment of cumulative evidence, the authors concluded that moderate evidence exists for an association between the S18Y variant and Parkinson's disease.

alpha-Synuclein: a therapeutic target for Parkinson's disease?

Parkinson's disease is a progressive age-related neurodegenerative disease with invariant loss of substantia nigra dopamine neurons and striatal projections. This disorder is well known for the associated motoric symptoms including resting tremor and the inability to initiate movement. However, it is now apparent that Parkinson's disease is a multisystem disorder with patients exhibiting symptoms derived from peripheral nervous system and extra-nigral dysfunctions in addition to the prototypical nigrostriatal damage. Although the etiology for sporadic Parkinson's disease is unknown, information gleaned from both familial forms of the disease and animal models places misfolded alpha-synuclein at the forefront. The disease is currently without a cure and most therapies target the motoric symptoms relying on increasing dopamine tone. In this review, the role of alpha-synuclein in disease pathogenesis and as a potential therapeutic target focusing on toxic conformers of this protein is considered. The addition of protofibrillar/oligomer-directed neurotherapeutics to the existing armamentarium may extend the symptom-free stage of Parkinson's disease as well as alleviate pathogenesis.

Lack of evidence for association of a UCH-L1 S18Y polymorphism with Parkinson's disease in a Han-Chinese population

Mutation in UCH-L1 has been reported as a rare cause of autosomal dominant Parkinson's disease (PD). A S18Y polymorphism in the same gene has been associated with sporadic PD. We investigated the frequency of this polymorphism among the Han-Chinese ethnic population in a case-control study. A total of 600 patients with PD and 334 unrelated healthy controls were genotyped using PCR-restriction fragment length polymorphism analysis. We did not observe any difference in allele or genotype frequencies between the cases and the controls (P>0.05). Our results do not support a role for this variant in sporadic PD.

Lack of evidence for an association between UCHL1 S18Y and Parkinson's disease

UCHL1 has been proposed as a candidate gene for Parkinson's disease (PD). A meta-analysis of white and Asian subjects reported an inverse association between the non-synonymous UCHL1 S18Y polymorphism and PD risk. However, this finding was not replicated in a large case-control study and updated meta-analysis restricted to white subjects. We performed a case-control study of 1757 PD patients recruited from movement disorder clinics and 2016 unrelated controls from four regions of the United States. All subjects self-reported as white. We did not observe evidence for an association between S18Y genotypes and PD (overall P-value for association: P = 0.42). After adjustment for age, sex, and recruitment region, the odds ratio for Y/S versus S/S was 0.91 (95% CI: 0.78-1.06) and for Y/Y versus S/S was 0.87 (95% CI: 0.58-1.29). We also did not observe a significant association for recessive or dominant models of inheritance, or after stratification by age at onset, age at blood draw, sex, family history of PD, or recruitment region. Our results suggest that UCHL1 S18Y is not a major susceptibility factor for PD in white populations although we cannot exclude the possibility that the S18Y variant exerts weak effects on risk, particularly in early-onset disease.

The role of common genetic risk variants in Parkinson disease

Despite the discovery of at least five pathogenic genes in Parkinson disease (PD), the genetic etiology in the vast majority of PD remains to be clarified. Common genetic variants could act as susceptibility risk factors. Our previous meta-analysis of PD genetic association studies, over a 30-year period yielded four genes (N-acetylcysteine 2, monoamine oxidase B, glutathione transferase, and mitochondrial tRNA), as their common variants were found to be associated with PD. More recently, international collaborative studies and meta-analysis have identified the S18Y variant of ubiquitin carboxy-terminal hydrolase L1, Rep 1 variant of alpha-synuclein and tau H1 haplotype to be genetic susceptibility risk/protective factors. However, the most significant, common genetic risk factor in PD has been its association with the leucine-rich repeat kinase-2 (LRRK2) G2385R variant. We conducted an analysis of independent studies involving 2205 PD and 1817 controls and found the average carrier rate of G2385R variant to be about 9% in PD and 4% in controls (p < 0.001; odds ratio: 2.27; 95% confidence interval: 1.78-2.9). A higher frequency of G2385R carriers has been observed in familial PD when compared with sporadic patients. Based on current evidence, certain common genetic variants are likely to modulate the risk of PD.

The functions of UCH-L1 and its relation to neurodegenerative diseases

Parkinson's disease (PD) and Alzheimer's disease (AD), the most common neurodegenerative diseases, are caused by both genetic and environmental factors. Ubiquitin carboxy-terminal hydrolase L1 (UCH-L1) is a deubiquitinating enzyme that is involved in the pathogenesis of both of these neurodegenerative diseases. Several functions of UCH-L1, other than as an ubiquitin hydrolase, have been proposed; these include acting as an ubiquitin ligase and stabilizing mono-ubiquitin. This review focuses on recent findings on the functions and the regulation of UCH-L1, in particular those that relate to PD and AD.

Reversible monoubiquitination regulates the Parkinson disease-associated ubiquitin hydrolase UCH-L1

Deubiquitinating enzymes (DUBs) are negative regulators of protein ubiquitination and play an important role in ubiquitin-dependent processes. Recent studies have found that diverse cellular mechanisms are employed to control the activity of DUBs. Ubiquitin C-terminal hydrolase-L1 (UCH-L1) is a highly expressed neuronal DUB linked to Parkinson disease; however, little is known about its specific functions or modes of regulation. Here, we demonstrate that UCH-L1 is post-translationally modified by monoubiquitin in cells, at lysine residues near the active site. This modification restricts enzyme activity by preventing binding to ubiquitinated targets, and permanent monoubiquitination, as mimicked by a ubiquitin-UCH-L1 fusion, inhibits UCH-L1 in its capacity to increase free ubiquitin levels in cells. Interestingly, UCH-L1 catalyzes its own deubiquitination in an intramolecular manner, thereby regulating the lifetime of this modification. Our results illustrate monoubiquitination as a reversible regulatory mechanism for DUB activity involving auto-deubiquitination.

Case–control study of UCHL1 S18Y variant in Parkinson's disease

A recent meta-analysis observed a greater significant inverse association of the ubiquitin carboxy-terminal hydrolase L1 (UCHL1) S18Y variant with Parkinson's disease (PD) for Asian (predominantly Japanese) populations compared with Caucasian populations. We performed an independent case-control study in 335 PD and 341 control subjects with data from a Chinese population to investigate the age-of-onset effect of the UCHL1 variant in PD. The Y/Y and Y/S genotypes were less frequent in the PD young-onset group than in controls and the frequency of the Y alleles was higher in young controls compared to young-onset PD (age at examination <or= 65 years; P = 0.003). Multivariate analysis revealed the Y/Y genotype was significantly lower (P = 0.008) in the young-onset PD (Y/Y vs. S/S: odds ratio [OR]: 0.42; 95% confidence interval [CI]: 0.24, 0.74; S/Y vs. S/S: OR: 0.66, 95% CI: 0.41, 1.08) compared with controls, but this difference was not seen for the late-onset PD. Kaplan-Meier analysis carried out on PD subjects demonstrated that the Y/Y genotype was associated with a later onset of PD than Y/S plus S/S genotypes (P = 0.05). We provided an independent confirmation of the protective effect of the UCHL1 S18Y variant (Y/Y genotype) against PD in young Chinese subjects. Further functional studies of the S18Y variant in both cell and animal models will be of interest.