Abstract
Objective: Glucocerebrosidase (GBA) gene mutations represent a strong risk factor for Parkinson disease (PD). PD penetrance in GBA mutation carriers, which represents a key issue for genetic counseling, especially for relatives of patients with Gaucher disease (GD), is unknown. Our objective was to estimate PD penetrance in a familial study of GBA mutation carriers.
Methods: Probands with familial PD were recruited through the French Parkinson Disease Genetic Study Group. All GBA exons were sequenced in probands and their relatives. To estimate the age-specific cumulative PD risk (i.e., penetrance) in GBA mutation carriers, we used the proband's phenotype exclusion likelihood method and corrected for selection of familial cases by considering the status of one affected relative per family as unknown.
Results: Of 525 probands with familial PD, 24 (4.6%) were GBA mutation carriers. Of their 256 relatives, 43 (16.8%) had PD and 26 of 32 affected relatives tested for GBA mutations were mutation carriers; 213 relatives did not have PD and 31 of 71 of unaffected relatives tested for GBA mutations were mutation carriers. Under a dominant model, penetrance was estimated as 7.6%, 13.7%, 21.4%, and 29.7% at 50, 60, 70, and 80 years, respectively. There was no significant difference in penetrance at 70 years between N370S carriers, L444P carriers, and carriers of rarer mutations.
Conclusion: The relatively high penetrance estimate in GBA carriers obtained in this study should lead to consideration of GBA as a dominant causal gene with reduced penetrance and should be taken into account for genetic counseling in relatives of patients with GD and patients with GBA-associated PD.
GLOSSARY
- GBA=
- glucocerebrosidase;
- GD=
- Gaucher disease;
- PD=
- Parkinson disease
Gaucher disease (GD) and Parkinson disease (PD) were initially connected because of the observation of parkinsonism and Lewy body pathology in patients with GD. It is now clear that mutations in the glucocerebrosidase (GBA) gene, which encodes the enzyme deficient in GD, represent a strong genetic risk factor for PD in different populations.1,–,4 Although heterozygous GBA mutation carriers are more likely than noncarriers to have affected relatives,4 segregation analysis in 21 multiplex families revealed that 46% of the unaffected relatives were GBA mutation carriers.3 PD penetrance in GBA mutation carriers, which represents a key issue for genetic counseling, especially for relatives of patients with GD, is unknown. We estimated PD penetrance in GBA mutation carriers using the proband's phenotype exclusion likelihood method while accounting for ascertainment bias.5,6
METHODS
Patients with PD were recruited through the French Parkinson Disease Genetic Study Group.3 All probands with familial PD (at least one affected relative in addition to the proband) were consecutively included in the study independently of the number of affected relatives, whereas patients with sporadic cases are not systematically included, thus resulting in an overrepresentation of familial cases (57%) compared with sporadic cases (43%). We therefore restricted our analyses to probands with familial PD and their relatives and corrected for ascertainment bias as explained below. In probands, PD was defined by at least 3 of 4 cardinal signs (bradykinesia, rigidity, rest tremor, and asymmetry of signs at onset), good response to dopaminergic treatment, and absence of other causes of parkinsonism7; age at onset was defined as the age at first symptom(s). Detailed pedigrees were constructed for each family. DNA samples were available for all probands (n = 525).
First-degree relatives were identified based on the pedigrees and examined in person, and blood was sampled when possible; the same diagnostic criteria as in probands were used. If they were not examined in person, their disease status (and age at onset if relevant) was obtained through structured interviews with the probands or, less frequently, the relatives. All probands were of European ancestry, and the majority of them were of French origin (88%). Patients with Parkin, PINK1, DJ1, and LRRK2 G2019S mutation–associated PD were excluded. All GBA exons were sequenced in the probands and their affected (n = 32) and unaffected relatives (n = 71) with DNA available.3 Peripheral blood was collected from each participant, and DNA was extracted from leukocytes according to standard procedures. Exons and flanking intronic regions of GBA were sequenced in all participants. To avoid amplifying and sequencing the neighboring pseudogene, GBA was amplified in 3 large fragments (a 2,972 bp-fragment encompassing exons 1–5, a 2,049-bp fragment encompassing exons 5–7, and a 1,682-bp fragment encompassing exons 8–11), using previously described primers and a unique 64°C to 54°C touchdown PCR program (table e-1 on the Neurology® Web site at www.neurology.org). PCR products were sequenced with internal primers, adjacent to coding exons and exon-intron boundaries, using the Big Dye Terminator Cycle Sequencing Ready Reaction Kit (Applied Biosystems). Sequencing products were purified using the Big Dye XTerminator Purification Kit (Applied Biosystems) and then electrophoresed on an ABI 3730 automated sequencer and analyzed with DNA Sequencing Analysis (version 5.1) and SeqScape (version 2.1.1) software (Applied Biosystems).
To estimate the age-specific cumulative PD risk (i.e., penetrance), we used the phenotype exclusion likelihood method, a maximum likelihood method that uses phenotypic and genotypic information of family members and corrects for ascertainment bias by excluding the proband from the analysis.5,6 Phenotypic information consists of the disease status from birth through one of the following events, whichever occurred first: PD onset, last contact alive, or death without PD. The additional bias due to inclusion of familial cases was corrected by excluding one affected relative per family (i.e., by considering his or her status as unknown), specifically the closest affected relative of the proband. This method requires the allele frequency of the deleterious mutations, which was set to 0.0051,3 and is implemented in GENERISK software.6 We first estimated PD penetrance for all GBA mutations combined and then distinguished the 2 main mutations (N370S and L444P) from rarer mutations. Risk of PD in noncarriers was defined on the basis of incidence estimates from a Caucasian population.8
Standard protocol approvals, registrations, and patient consents.
The ethics committee of the Salpêtrière hospital approved the study. All participants gave written informed consent.
RESULTS
Of 525 probands with familial PD, 24 (4.6%) carried at least one GBA mutation (1, n = 20; 2, n = 4); 109 of their 256 first-degree relatives were examined. The characteristics of the probands and their relatives are presented in tables 1 and 2. Forty-three (16.8%) relatives had PD (direct examination, n = 35; interview, n = 8).
Characteristics of the probands
Characteristics of the relatives of the probands (n = 256)
Among 256 relatives, 103 (32 affected) were tested for GBA mutations; 57 (55.3%) were GBA mutation carriers (1, n = 50; 2, n = 7), and 26 (45.6%) of the mutation carriers were affected (1, n = 23; 2, n = 3). Six tested affected relatives did not carry GBA mutations; although they had a higher median age at onset (62 years, range 48–76) than 26 affected relatives who carried GBA mutations (55 years, range 16–73), the difference was not statistically significant (p = 0.14). Among affected relatives who were GBA mutation carriers, PD started at ≤40 years in 3 (12%), between 41 and 50 years in 6 (23%), between 51 and 60 years in 10 (39%), and at >60 years in 7 (27%) of them. Eleven subjects (4 probands and 7 relatives) carried 2 GBA mutations, and 2 of them were diagnosed with GD3; all the other affected patients had typical PD.
To estimate penetrance, the status of one affected relative per family was considered as unknown; there were therefore 20 affected relatives from 12 families, of whom 15 were tested for GBA mutations: 12 were mutation carriers (1, n = 11, 2, n = 1). Under a dominant model, the cumulative PD risk was estimated as 7.6%, 13.7%, 21.4%, and 29.7% at 50, 60, 70, and 80 years, respectively, in GBA mutation carriers (figure). Under a codominant model, the corresponding figures were 7.2%, 12.3%, 18.9%, and 27.1% in carriers of 1 GBA mutation and 9.1%, 35.4%, 46.6%, and 46.6% in carriers of 2 mutations. Although there was a trend toward higher penetrance in carriers of 2 mutations compared with carriers of 1 mutation, the codominant model was not significantly better than the dominant model (p = 0.26). There was no evidence in favor of different penetrance in men and women (data not shown). There was no significant difference (p = 0.44) in penetrance at 70 years between N370S carriers (19%), L444P carriers (30%), and carriers of rarer mutations (21%). PD risk in noncarriers was 0%, 0.4%, 1.2%, and 2.5% at 50, 60, 70, and 80 years of age, respectively.
Estimated age-specific cumulative risk (penetrance) of PD in glucocerebrosidase gene mutation carriers (red line; 95% confidence interval, black lines) compared with the cumulative risk in noncarriers (blue line) derived from incidence figures in a Caucasian population.7
DISCUSSION
We estimated that PD penetrance in GBA mutation carriers is 29.7% at 80 years under a dominant model. This relatively high penetrance estimate should lead to consideration of GBA as a PD causal dominant gene. Moreover, it should be taken into account for genetic counseling in relatives of patients with GD or with GBA-associated PD. This penetrance estimate is similar to that of the LRRK2 G2019S mutation, which is considered a dominant PD gene.9,10 As for the LRRK2 G2019S mutation, the incomplete penetrance of PD in GBA carriers limits the predictive value of presymptomatic testing, which would be relevant if disease modifier PD treatments were available.
Parkinsonism is a frequent feature of GD. Compared with the general population, patients with GD type 1 who carry 2 GBA mutations have a 20-fold increased lifetime risk of developing PD11; this finding suggests that there may be a gene dosage effect. Although we observed a trend toward a higher penetrance in carriers of 2 mutations compared with carriers of 1 mutation, there was no statistically significant difference between the dominant and codominant models. This is likely to be due to the few carriers of 2 mutations in our sample, which resulted in limited statistical power to detect a significant difference between both models; larger studies will be necessary to address this issue. In addition, we found no difference in penetrance according to the type of GBA mutation, but larger studies will also be needed to address this question.
The risk of PD in GBA mutation carriers was higher than that in noncarriers at all ages and the median age at PD onset was lower in affected relatives who were GBA mutation carriers than in noncarriers. The difference was not statistically significant, but the number of affected relatives who were noncarriers was small. PD started at >50 years in approximately two-thirds of affected relatives who carried GBA mutations; therefore, in the majority of cases, GBA mutations do not lead to young-onset PD.
The main strength of our study was the complete sequencing of all GBA exons in a large sample of consecutive patients with well-characterized PD. One limitation is that ∼40% of the relatives were examined in person, but PD status and age at onset were determined based on probands' interviews in the remaining relatives, which may have induced some degree of diagnostic misclassification. In addition, we estimated PD penetrance based on a sample of familial cases of PD; however, we corrected for ascertainment bias by considering one affected relative per family as unaffected and by using an ascertainment-adjusted method that allows estimation of asymptotically unbiased age-specific cumulative risks of diseases associated with deleterious mutations based on families in which such mutations have been identified.
AUTHOR CONTRIBUTIONS
Dr. Anheim: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, study supervision or coordination. Dr. Elbaz: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, statistical analysis, study supervision or coordination. Dr. Lesage: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, study supervision or coordination. Dr. Durr: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, study supervision or coordination. C. Condroyer: analysis or interpretation of data, acquisition of data. Dr. Viallet: drafting/revising the manuscript for content, including medical writing for content, acquisition of data. Dr. Pollak: drafting/revising the manuscript for content, including medical writing for content, analysis or interpretation of data, acquisition of data. B. Bonaïti: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, statistical analyses, study supervision or coordination. Dr. Bonaïti-Pellié: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, statistical analyses, study supervision or coordination. Dr. Brice: drafting/revising the manuscript for content, including medical writing for content, study concept or design, analysis or interpretation of data, acquisition of data, study supervision or coordination.
DISCLOSURE
Dr. Anheim reports no disclosures. Dr. Elbaz receives research support from Agence Nationale de la Recherche (France). Dr. Lesage, Dr. Durr, C. Condroyer, Dr. Viallet, Dr. Pollak, B. Bonaïti, and Dr. Bonaïti-Pellié report no disclosures. Dr. Brice receives research support from Agence Nationale de la Recherche (France).
Footnotes
French Parkinson Disease Genetic Group Coinvestigators are listed on the Neurology® Web site at www.neurology.org.
Study funding: Supported by the Agence Nationale de la Recherche-Eranet Neuron (ANR-08-NEUR-004-01/R08200DS) to A.B.
-
Supplemental data at www.neurology.org
- Received July 8, 2011.
- Accepted September 29, 2011.
- Copyright © 2012 by AAN Enterprises, Inc.
Letters: Rapid online correspondence
- Gaucher and Parkinsonism: Blurred boundaries Between Mendelian and Complex Disorders
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