Genetic Analysis of Prosaposin, the Lysosomal Storage Disorder Gene in Parkinson's Disease

GBA1-Associated Parkinson's Disease Is a Distinct Entity

GBA1-associated Parkinson's disease (GBA1-PD) is increasingly recognized as a distinct entity within the spectrum of parkinsonian disorders. This review explores the unique pathophysiological features, clinical progression, and genetic underpinnings that differentiate GBA1-PD from idiopathic Parkinson's disease (iPD). GBA1-PD typically presents with earlier onset and more rapid progression, with a poor response to standard PD medications. It is marked by pronounced cognitive impairment and a higher burden of non-motor symptoms compared to iPD. Additionally, patients with GBA1-PD often exhibit a broader distribution of Lewy bodies within the brain, accentuating neurodegenerative processes. The pathogenesis of GBA1-PD is closely associated with mutations in the GBA1 gene, which encodes the lysosomal enzyme beta-glucocerebrosidase (GCase). In this review, we discuss two mechanisms by which GBA1 mutations contribute to disease development: 'haploinsufficiency,' where a single functional gene copy fails to produce a sufficient amount of GCase, and 'gain of function,' where the mutated GCase acquires harmful properties that directly impact cellular mechanisms for alpha-synuclein degradation, leading to alpha-synuclein aggregation and neuronal cell damage. Continued research is advancing our understanding of how these mechanisms contribute to the development and progression of GBA1-PD, with the 'gain of function' mechanism appearing to be the most plausible. This review also explores the implications of GBA1 mutations for therapeutic strategies, highlighting the need for early diagnosis and targeted interventions. Currently, small molecular chaperones have shown the most promising clinical results compared to other agents. This synthesis of clinical, pathological, and molecular aspects underscores the assertion that GBA1-PD is a distinct clinical and pathobiological PD phenotype, necessitating specific management and research approaches to better understand and treat this debilitating condition.

Meta-analysis of the association of prosaposin polymorphisms rs4747203 and rs885828 with risk of Parkinson's disease

BACKGROUND

Previous research has established a connection between polymorphisms rs4747203 and rs885828 in the prosaposin (PSAP) gene and an increased risk of Parkinson's disease (PD). However, other studies have found no significant difference in risk compared to the general population.

METHODS

To evaluate the current evidence linking rs4747203 and rs885828 to PD risk, we conducted a comprehensive search of PubMed, the Web of Science, Embase, and the Cochrane Library for relevant studies up until May 2023. In addition, we analyzed data from the publicly available "PD Variant Browser". We performed a meta-analysis using Stata 17.0 to synthesize the findings from the selected studies.

RESULTS

Our meta-analysis, which included data from six published studies and the public database, revealed no significant association between PD risk and either rs4747203 [OR (95% CI) = 0.99 (0.93-1.05), I2 = 90.3%, P = 0.635] or rs885828 [OR (95% CI) = 1.01 (0.95-1.07), I2 = 90.7%, P = 0.773]. These results remained consistent when examining subgroups of individuals within or outside of Asia.

CONCLUSION

The available evidence does not support an association between the genotype at rs4747203 or rs885828 and the risk of PD.

Prosaposin variants in sporadic, familial, and early-onset Parkinson's disease: a Taiwanese case-control study and meta-analysis

Polymorphisms in the PSAP gene, which encodes prosaposin and is involved in the lysosomal function, yielded conflicting results regarding the association with Parkinson's disease (PD). Therefore, this study aims to investigate the role of PSAP in familial PD (FPD), early onset PD (EOPD) with age at onset before 50 years old, and sporadic PD (SPD) among Taiwanese population, and summarize relevant studies via meta-analysis. By sequencing exon 1 to 14 in 183 FPD and 219 EOPD, two novel exonic variants were found in EOPD, including p.A146E (c.437C > A) on exon 5 and p.Y248C (c.743A > G) on exon 7. Furthermore, four previously reported intronic variants (rs142614739/rs74733861), rs749823, rs4747203 and rs885828) in intron 11 and 12 were analyzed in 485 SPD and 712 in-hospital controls, in addition to the aforementioned FPD and EOPD groups. The adjusted odd ratios (ORs) by age and sex, only rs142614739 was significantly associated with higher risk of EOPD (OR = 1.85, 95% CI = 1.33-2.58). The risk effect was further confirmed by the meta-analysis of the association between rs142614739 and the risk of PD in both common effect (OR = 1.29, 95% CI = 1.11-1.50) and random effect (OR = 1.29, 95% CI = 1.11-1.50). Our findings suggest that the PSAP rs142614739 variant is associated with the risk of EOPD. Further functional studies are warranted to elucidate the biochemical mechanisms.

Choroid plexus mis-splicing and altered cerebrospinal fluid composition in myotonic dystrophy type 1

Myotonic dystrophy type 1 is a dominantly inherited multisystemic disease caused by CTG tandem repeat expansions in the DMPK 3' untranslated region. These expanded repeats are transcribed and produce toxic CUG RNAs that sequester and inhibit activities of the MBNL family of developmental RNA processing factors. Although myotonic dystrophy is classified as a muscular dystrophy, the brain is also severely affected by an unusual cohort of symptoms, including hypersomnia, executive dysfunction, as well as early onsets of tau/MAPT pathology and cerebral atrophy. To address the molecular and cellular events that lead to these pathological outcomes, we recently generated a mouse Dmpk CTG expansion knock-in model and identified choroid plexus epithelial cells as particularly affected by the expression of toxic CUG expansion RNAs. To determine if toxic CUG RNAs perturb choroid plexus functions, alternative splicing analysis was performed on lateral and hindbrain choroid plexi from Dmpk CTG knock-in mice. Choroid plexus transcriptome-wide changes were evaluated in Mbnl2 knockout mice, a developmental-onset model of myotonic dystrophy brain dysfunction. To determine if transcriptome changes also occurred in the human disease, we obtained post-mortem choroid plexus for RNA-seq from neurologically unaffected (two females, three males; ages 50-70 years) and myotonic dystrophy type 1 (one female, three males; ages 50-70 years) donors. To test that choroid plexus transcriptome alterations resulted in altered CSF composition, we obtained CSF via lumbar puncture from patients with myotonic dystrophy type 1 (five females, five males; ages 35-55 years) and non-myotonic dystrophy patients (three females, four males; ages 26-51 years), and western blot and osmolarity analyses were used to test CSF alterations predicted by choroid plexus transcriptome analysis. We determined that CUG RNA induced toxicity was more robust in the lateral choroid plexus of Dmpk CTG knock-in mice due to comparatively higher Dmpk and lower Mbnl RNA levels. Impaired transitions to adult splicing patterns during choroid plexus development were identified in Mbnl2 knockout mice, including mis-splicing previously found in Dmpk CTG knock-in mice. Whole transcriptome analysis of myotonic dystrophy type 1 choroid plexus revealed disease-associated RNA expression and mis-splicing events. Based on these RNA changes, predicted alterations in ion homeostasis, secretory output and CSF composition were confirmed by analysis of myotonic dystrophy type 1 CSF. Our results implicate choroid plexus spliceopathy and concomitant alterations in CSF homeostasis as an unappreciated contributor to myotonic dystrophy type 1 CNS pathogenesis.

Saposin C, Key Regulator in the Alpha-Synuclein Degradation Mediated by Lysosome

Lysosomal dysfunction has been proposed as one of the most important pathogenic molecular mechanisms in Parkinson disease (PD). The most significant evidence lies in the GBA gene, which encodes for the lysosomal enzyme β-glucocerebrosidase (β-GCase), considered the main genetic risk factor for sporadic PD. The loss of β-GCase activity results in the formation of α-synuclein deposits. The present study was aimed to determine the activity of the main lysosomal enzymes and the cofactors Prosaposin (PSAP) and Saposin C in PD and healthy controls, and their contribution to α-synuclein (α-Syn) aggregation. 42 PD patients and 37 age-matched healthy controls were included in the study. We first analyzed the β-GCase, β-galactosidase (β-gal), β-hexosaminidase (Hex B) and Cathepsin D (CatD) activities in white blood cells. We also measured the GBA, β-GAL, β-HEX, CTSD, PSAP, Saposin C and α-Syn protein levels by Western-blot. We found a 20% reduced β-GCase and β-gal activities in PD patients compared to controls. PSAP and Saposin C protein levels were significantly lower in PD patients and correlated with increased levels of α-synuclein. CatD, in contrast, showed significantly increased activity and protein levels in PD patients compared to controls. Increased CTSD protein levels in PD patients correlated, intriguingly, with a higher concentration of α-Syn. Our findings suggest that lysosomal dysfunction in sporadic PD is due, at least in part, to an alteration in Saposin C derived from reduced PSAP levels. That would lead to a significant decrease in the β-GCase activity, resulting in the accumulation of α-syn. The accumulation of monohexosylceramides might act in favor of CTSD activation and, therefore, increase its enzymatic activity. The evaluation of lysosomal activity in the peripheral blood of patients is expected to be a promising approach to investigate pathological mechanisms and novel therapies aimed to restore the lysosomal function in sporadic PD.

The mutation spectrum of PD-related genes in early-onset Parkinson's disease in ethnic Chinese

BACKGROUND

Recent genetic progress has shown many causative/risk genes linked to Parkinson's disease (PD), mainly in patients of European ancestry. The study aimed to investigate the PD-related genes and determine the mutational spectrum of early-onset PD in ethnic Chinese.

METHODS

In this study, whole-exome sequencing and/or gene dosage analysis were performed in 704 early-onset PD (EOPD) patients (onset age ≤ 45 years) and 1866 controls. Twenty-six PD-related genes and 20 other genes linked to neurodegenerative and lysosome diseases were analyzed.

RESULTS

We identified 82 (11.6%, 82/704) EOPD patients carrying rare pathogenic/ likely pathogenic variants in PD-related genes. The mutation frequency in autosomal recessive inheritance EOPD (AR-EOPD, 42.9%, 27/63) was much higher than that in autosomal dominant inheritance EOPD (AD-EOPD, 0.9%, 12/110) or sporadic EOPD (8.1%, 43/531). Bi-allelic mutations in PRKN were the most frequent, accounting for 5.1% of EOPD cases. Three common pathogenic variants, p.A53V in SNCA, p.G284R in PRKN, and p.P53Afs*38 in CHCHD2, occur exclusively in Asians. The putative damaging variants from GBA, PRKN, DJ1, PLA2G6, and GCH1 contributed to the collective risk for EOPD. Notably, the protein-truncating variants in CHCHD2 were enriched in EOPD, especially for p.P53Afs*38, which was also found in three patients from an independent cohort of patients with late-onset PD (n=1300). Functional experiments confirmed that truncated CHCHD2 variants cause loss of function and are linked to mitochondrial dysfunction.

CONCLUSIONS

Our study reveals that the genetic spectrum EOPD in Chinese, which may help develop genetic scanning strategies, provided more evidence supporting CHCHD2 in PD.

Loss of Drosophila NUS1 results in cholesterol accumulation and Parkinson's disease-related neurodegeneration

NgBR is the Nogo-B receptor, encoded by NUS1 gene. As NgBR contains a C-terminal domain that is similar to cis-isoprenyltransferase (cis-IPTase), NgBR was speculated to stabilize nascent Niemann-Pick type C 2 (NPC2) to facilitate cholesterol transport out of lysosomes. Mutations in the NUS1 were known as risk factors for Parkinson's disease (PD). In our previous study, it was shown that knockdown of Drosophila NUS1 orthologous gene tango14 causes decreased climbing ability, loss of dopaminergic neurons, and decreased dopamine contents. In this study, tango14 mutant flies were generated with a mutation in the C-terminal enzyme activity region using CRISPR/Cas9. Tango14 mutant showed a reduced lifespan with locomotive defects and cholesterol accumulation in Malpighian tubules and brains, especially in dopaminergic neurons. Multilamellar bodies were found in tango14 mutants using electron microscopy. Neurodegenerative-related brain vacuolization was also detected in tango14 knockdown flies in an age-dependent manner. In addition, tango14 knockdown increased α-synuclein (α-syn) neurotoxicity in α-syn-overexpressing flies, with decreased locomotive activities, dopamine contents, and the numbers of dopaminergic neurons in aging flies. Thus, these observations suggest a role of NUS1, the ortholog of tango14, in PD-related pathogenesis.

Rare PSAP Variants and Possible Interaction with GBA in REM Sleep Behavior Disorder

BACKGROUND

PSAP encodes saposin C, the co-activator of glucocerebrosidase, encoded by GBA. GBA mutations are associated with idiopathic/isolated REM sleep behavior disorder (iRBD), a prodromal stage of synucleinopathy.

OBJECTIVE

To examine the role of PSAP mutations in iRBD.

METHODS

We fully sequenced PSAP and performed Optimized Sequence Kernel Association Test in 1,113 iRBD patients and 2,324 controls. We identified loss-of-function (LoF) mutations, which are very rare in PSAP, in three iRBD patients and none in controls (uncorrected p = 0.018).

RESULTS

Two variants were stop mutations, p.Gln260Ter and p.Glu166Ter, and one was an in-frame deletion, p.332_333del. All three mutations have a deleterious effect on saposin C, based on in silico analysis. In addition, the two carriers of p.Glu166Ter and p.332_333del mutations also carried a GBA variant, p.Arg349Ter and p.Glu326Lys, respectively. The co-occurrence of these extremely rare PSAP LoF mutations in two (0.2%) GBA variant carriers in the iRBD cohort, is unlikely to occur by chance (estimated co-occurrence in the general population based on gnomAD data is 0.00035%). Although none of the three iRBD patients with PSAP LoF mutations have phenoconverted to an overt synucleinopathy at their last follow-up, all manifested initial signs suggestive of motor dysfunction, two were diagnosed with mild cognitive impairment and all showed prodromal clinical markers other than RBD. Their probability of prodromal PD, according to the Movement Disorder Society research criteria, was 98% or more.

CONCLUSION

These results suggest a possible role of PSAP variants in iRBD and potential genetic interaction with GBA, which requires additional studies.

Are Lysosomes Potential Therapeutic Targets for Parkinson's Disease?

Parkinson´s Disease (PD) is the second most common neurodegenerative disorder, affecting ~2-3% of the population over 65 years old. In addition to progressive degeneration of nigrostriatal neurons, the histopathological feature of PD is the accumulation of misfolded α-synuclein protein in abnormal cytoplasmatic inclusions, known as Lewy Bodies (LBs). Recently, Genome-Wide Association Studies (GWAS) have indicated a clear association of variants within several lysosomal genes with risk for PD. Newly evolving data have been shedding light on the relationship between lysosomal dysfunction and alpha-synuclein aggregation. Defects in lysosomal enzymes could lead to the insufficient clearance of neurotoxic protein materials, possibly leading to selective degeneration of dopaminergic neurons. Specific modulation of lysosomal pathways and their components could be considered a novel opportunity for therapeutic intervention for PD. The purpose of this review is to illustrate lysosomal biology and describe the role of lysosomal dysfunction in PD pathogenesis. Finally, the most promising novel therapeutic approaches designed to modulate lysosomal activity, as a potential disease-modifying treatment for PD will be highlighted.

Identification of Potential Core Genes in Parkinson's Disease Using Bioinformatics Analysis

Purpose

This study aimed to explore new core genes related to the occurrence of Parkinson's disease (PD) and core genes that can lead to the progression of PD.

Methods

The expression profile data of GSE42966, which contained six substantia nigra tissues isolated from normal individuals and nine substantia nigra tissues isolated from patients with PD, were obtained from Gene Expression Omnibus. Differentially expressed genes (DEGs) were identified, followed by functional enrichment analysis and protein-protein interaction (PPI) network construction. We then identified 10 hub genes and analyzed their expression in different Braak stages.

Results

A total of 773 DEGs were identified that were significantly enriched in metabolic pathways. Ten hub genes were identified through the PPI network, namely, GNG3, MAPK1, FPR1, ATP5B, GNG2, PRKACA, HRAS, HSPA8, PSAP, and GABBR2. The expression of HRAS was different in patients with PD with Braak stages 3 and 4.

Conclusion

These 10 hub genes and the metabolic pathways they are enriched in may be involved in the pathogenesis of PD. HRAS may have potential value in predicting the progression of PD.

The Role of Protein S-Nitrosylation in Protein Misfolding-Associated Diseases

Abnormal and excessive nitrosative stress contributes to neurodegenerative disease associated with the production of pathological levels of misfolded proteins. The accumulated findings strongly suggest that excessive NO production can induce and deepen these pathological processes, particularly by the S-nitrosylation of target proteins. Therefore, the relationship between S-nitrosylated proteins and the accumulation of misfolded proteins was reviewed. We particularly focused on the S-nitrosylation of E3-ubiquitin-protein ligase, parkin, and endoplasmic reticulum chaperone, PDI, which contribute to the accumulation of misfolded proteins. In addition to the target proteins being S-nitrosylated, NOS, which produces NO, and GSNOR, which inhibits S-nitrosylation, were also suggested as potential therapeutic targets for protein misfolding-associated diseases.

Rare Variants Analysis of Lysosomal Related Genes in Early-Onset and Familial Parkinson's Disease in a Chinese Cohort

BACKGROUND

Genetic studies have indicated that variants in several lysosomal genes are risk factors for idiopathic Parkinson's disease (PD). However, the role of lysosomal genes in PD in Asian populations is largely unknown.

OBJECTIVE

This study aimed to analyze rare variants in lysosomal related genes in Chinese population with early-onset and familial PD.

METHODS

In total, 1,136 participants, including 536 and 600 patients with sporadic early-onset PD (SEOPD) and familial PD, respectively, underwent whole-exome sequencing to assess the genetic etiology. Rare variants in PD were investigated in 67 candidate lysosomal related genes (LRGs), including 15 lysosomal function-related genes and 52 lysosomal storage disorder genes.

RESULTS

Compared with the autosomal dominant PD (ADPD) or SEOPD cohorts, a much higher proportion of patients with multiple rare damaging variants of LRGs were found in the autosomal recessive PD (ARPD) cohort. At a gene level, rare damaging variants in GBA and MAN2B1 were enriched in PD, but in SCARB2, MCOLN1, LYST, VPS16, and VPS13C were much less in patients. At an allele level, GBA p. Leu483Pro was found to increase the risk of PD. Genotype-phenotype correlation showed no significance in the clinical features among patients carrying a discrepant number of rare variants in LRGs.

CONCLUSION

Our study suggests rare variants in LRGs might be more important in the pathogenicity of ARPD cases compared with ADPD or SEOPD. We further confirm rare variants in GBA are involve in PD pathogenecity and other genes associated with PD identified in this study should be supported with more evidence.

miR-637 Prevents Glioblastoma Progression by Interrupting ZEB2/WNT/β-catenin Cascades

Glioblastomas (GBMs) are the most frequent primary malignancies in the central nervous system. Aberrant activation of WNT/β-catenin signaling pathways is critical for GBM malignancy. However, the regulation of WNT/β-catenin signaling cascades remains unclear. Presently, we observed the increased expression of ZEB2 and the decreased expression of miR-637 in GBM. The expression of miR-637 was negatively correlated with ZEB2 expression. miR-637 overexpression overcame the ZEB2-enhanced cell proliferation and G1/S phase transition. Besides, miR-637 suppressed the canonical WNT/β-catenin pathways by targeting WNT7A directly. Gain- and loss-of-function experiments with U251 mice demonstrated that miR-637 inhibited cell proliferation and arrested the G1/S phase transition, leading to tumor growth suppression. The collective findings suggest that ZEB2 and WNT/β-catenin cascades merge at miR-637, and the ectopic expression of miR-637 disturbs ZEB2/WNT/β-catenin-mediated GBM growth. The findings provide new clues for improving β-catenin-targeted therapy against GBM.