Abstract
Mutations in the glucocerebrosidase (GBA1) gene, the most common genetic contributor to Parkinson’s disease (PD), are associated with an increased risk of PD in heterozygous and homozygous carriers. While glucocerebrosidase enzyme (GCase) activity is consistently low in Gaucher disease, there is a range of leukocyte GCase activity in healthy heterozygous GBA1 mutation carriers. To determine whether GCase activity may be a marker for PD with heterozygous GBA1 mutations (GBA1 mutation PD, GBA PD), GBA PD patients (n=15) were compared to PD patients without heterozygous GBA1 mutations (idiopathic PD; n=8), heterozygous GBA1 carriers without PD (asymptomatic carriers; n=4), and biallelic mutation carriers with PD (Gaucher disease with PD, GD1 PD; n=3) in a pilot study. GCase activity (nmol/mg protein/hour) in GD1 PD (median [interquartile range]; minimum–maximum: 6.4 [5.7]; 5.3–11) was lower than that of GBA PD (p=0.01), while GCase activity in GBA PD (16.0 [7.0]; 11–40) was lower than idiopathic PD (28.5 [15.0]; 16–56) (p=0.01) and asymptomatic carriers (25.5 [2.5]; 23–27) (p=0.04). Therefore, GCase activity appears to be a possible marker of heterozygous GBA1 mutation PD, and larger studies are warranted. Prospective studies are also necessary to determine whether lower GCase activity precedes development of PD.
Keywords: Biomarker, Gaucher, GBA, GBA1, GBA enzyme activity, Glucocerebrosidase, Parkinson’s disease
1. Introduction
Heterozygous glucocerebrosidase enzyme (GCase) mutations (from the glucocerebrosidase gene [GBA1]) encode lysosomal GCase and increase the risk of Parkinson’s disease (PD) and dementia with Lewy bodies [1,2]. However the penetrance of GBA1 mutations is low [1,2]. While GCase activity is consistently decreased in type 1 Gaucher disease (GD1), a range of GCase activity in heterozygous GBA1 mutation carriers exists [3,4]. To address the relationship between GCase activity and PD, this pilot study assesses GCase activity in GBA1 mutation carriers, with and without PD, and non-carriers with PD (idiopathic PD).
2. Methods
Participants were recruited from a genetics study of PD in Ashkenazi Jews [6]. GCase activity from fresh blood was assayed using 4-methylumbelliferyl-β-D-glucopyranoside leukocyte assay [4] for 15 PD patients with heterozygous mutations (GBA PD) (12 N370S, one of each 84GG, R496H, and E326), eight idiopathic PD patients, four asymptomatic GBA1 carriers (N370S, 84GG, R496H, and E326K) and three GD1 patients with PD (GD1 PD) (two previously reported; N370S/N370S, N370S/R496H, and N370S/A456P) [6]. Groups were compared using Mann–Whitney and Fisher’s exact tests with Bonferroni correction. The internal review board approved this study.
3. Results
GCase activity (nmol/mg protein/hour) in GBA PD patients (median, interquartile range: 16.0, 7.0) was lower than idiopathic PD (28.5, 15.0) (p=0.01) and asymptomatic carriers (25.5, 2.5) (p=0.04), although the latter difference was not significant using a correction of p<0.017 (Fig. 1, Table 1). GCase activity in GD1 PD patients (6.4, 5.7) was lower than GBA PD patients (p=0.01).
Fig. 1. Glucocerebrosidase enzyme (GCase) activity.
Asymptomatic carriers = GBA1 mutation carriers without Parkinson’s disease, GBA1 mutation PD = monoallelic GBA1 mutation carriers with Parkinson’s disease, GD1 PD = biallelic GBA1 mutation carriers with Parkinson’s disease, idiopathic PD = Parkinson’s disease participants without GBA1 mutations.
** GCase activity in GD1 PD (median 6.4, interquartile range 5.7) was lower than GBA1 mutation PD (p=0.01). Each point represents the GCase activity measured for an individual.
Table 1.
Summary of clinical features and glucocerebrosidase enzyme activity in Parkinson’s disease and GBA1 mutation groups
| p value |
|||||||
|---|---|---|---|---|---|---|---|
| GD1 PD (n=3) |
GBA1 PD (n=15) |
IPD (n=8) |
Non-PD carriers (n=4) |
GBA1 PD vs. IPD |
GBA1 PD vs. non-PD carriers |
IPD vs. non-PD carriers |
|
| Women, % (n) | 66.7 (2) | 40.0 (6) | 25.0 (2) | 50.0 (2) | 0.66 | 0.57 | 0.55 |
| Age, years | 58.0 (7.0) | 67.0 (10.0) | 63.5 (11.5) | 64.0 (4.5) | 0.92 | 0.76 | 0.67 |
| Onset age, years | 53.0 (10.0) | 59.0 (17.0) | 57.5 (5.5) | --- | 0.90 | --- | --- |
| Duration, years | 5.0 (3.0) | 10.5 (8.0) | 5.0 (8.0) | --- | 0.67 | --- | --- |
| H&Y stage | 2.0 (1.5) | 2.0 (1.0) | 2.0 (0.0) | --- | 0.17 | --- | --- |
| Activity a | 6.4 (5.7) | 16.0 (7.0) | 28.5 (15.0) | 25.5 (2.5) | 0.01 | 0.04 | 0.73 |
Data are presented as median (interquartile range) unless otherwise indicated. GBA1 PD = monoallelic GBA1 mutation carriers with Parkinson’s disease, GD1 PD = biallelic GBA1 mutation carriers with Parkinson’s disease, H&Y = Hoehn and Yahr, IPD = idiopathic Parkinson’s disease participants without GBA1 mutations, Non-PD carriers: GBA1 mutation carriers without Parkinson’s disease, vs. = versus.
nmol/mg protein/hour.
4. Discussion
Lower peripheral GCase activity is associated with GBA PD, suggesting that reduced peripheral GCase activity has potential as a marker of GBA PD. While the causal mechanism for GCase to produce synucleinopathies has not been fully explicated, a bi-directional loop has been reported, where deficiency of GCase promotes α-synuclein via excess substrate, resulting in stabilization of α-synuclein into oligomers, and greater GCase dysfunction [5]. This is supported by our finding of decreased GCase activity in GBA PD. Gain of function has also been proposed, as GCase co-localizes with α-synuclein in Lewy bodies in GBA PD [7]. Alternatively, gain and loss of function mechanisms may co-exist. Additional study including a larger sample, a greater distribution of mutation types and asymptomatic carriers, and assessment of sphingolipid substrate and products may lend insight into leukocyte GCase activity as a marker and its mechanism.
Highlights.
1. As glucocerebrosidase mutations increase the risk of Parkinson Disease (PD), glucocerebrosidase enzyme activity in different groups was evaluated.
The primary focus of this pilot study was on PD patients with and without GBA1 mutations.
Glucocerebrosidase enzyme activity was lower in GBA1 heterozygous mutation carriers with PD when compared to non-GBA1 mutation PD.
Larger prospective studies are warranted determine whether enzyme activity is associated with the development of PD.
Acknowledgements
This study was supported by the Bigglesworth Family Foundation, Empire State Clinical Research Training Program, the Marcled Foundation, and NIH-NINDS NS073836.We are grateful to the study participants, who donated their time and energy for this study, as well as to Mariel Pullman and Amanda Glickman for their work. Funding: Bigglesworth Family Foundation, Empire State Clinical Research Training Program, Marcled Foundation, NIH- NINDS NS073836.
Sources of Support: the Bigglesworth Family Foundation, Empire State Clinical Research Training Program, the Marcled Foundation, and NIH-NINDS NS073836.
Footnotes
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Conflicts of Interest/Disclosures
Dr. Severt has received personal compensation for activities with Teva Neuroscience, Allergan Inc., Merz Pharma, and UCB Pharma. Dr. Severt has received research support from Teva Neuroscience and Merz Pharma. Susan Bressman has received a royalty, license fee or contractual rights payment from Athena Diagnostics. The other authors declare that they have no financial or other conflicts of interest in relation to this research and its publication.
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