Reference
Complete the bibliographic reference for the article according to AJE format.

Category of HuGE information
Specify the types of information (from the list below) available in the article:

1. Prevalence of gene variant
2. Gene-disease association
3. Gene-environment interaction
4. Gene-gene interaction
5. Genetic test evaluation/monitoring

1. Prevalence of gene variant
2. Gene-disease association

Study hypotheses or purpose
The authors study hypotheses or main purpose for conducting the study

Gene(s)
Identification of the following:

1. Gene name
2. Chromosome location
3. Gene product/function
4. Alleles
5. OMIM #
6. GDPInfo link

1. Gene name: GBA, Acid Beta Glucosidase, Glucocerebrosidase

2. Chromosome location: 1q21

3. Gene product/function: Glucocerebrosidase is a lysosomal enzyme that hydrolyzes glucocerebroside to glucose and ceramide. Deficiency of glucocerebrosidase results in accumulation of glucocerebroside in macrophages, causing multiorgan involvement.

4. Alleles: The N370S and 84GG alleles are the most frequent mutations in the GBA gene among Ashkenazi Jews, with rates 1:17.5 for N370S and 1:400 for 84GG; these alleles are associated with mild and severe Gaucher disease, respectively.¹ N370S is a single base substitution (A-to-G) in exon 9 which results in a substitution of serine for asparagine at amino acid 370 of the protein.² 84GG is a single base insertion of a guanine (G) at cDNA position 84, resulting in a frameshift mutation.³ Other rare GBA variants identified in Ashkenazi Jews include L444P, IVS2+1G-->A, V394L, and R496H. L444P is a single base substitution (T-to-C) in exon 10 which results in a substitution of proline for leucine at amino acid 444 of the protein.⁴ IVS2+1G--> A is a single nucleotide substitution (G-to-A) at the first position of the splice donor site of intron 2, resulting in aberrant mRNA splicing.⁵ V394L is a missense mutation resulting in a substitution of leucine for valine at amino acid 394 of the protein.⁶ R496H is a single base substitution (G-to-A) at nucleotide 1604 of the cDNA which results in a substitution of histidine for arginine at amino acid 496 of the protein.⁷

5. OMIM #: 606463

6. Go to GDPInfo Genes A-Z result

Environmental factor(s)
Identification of the major environmental factors studied (infectious, chemical, physical, nutritional, and behavioral)

None

Health outcome(s)
Identification of the major health outcome(s) studied

1. Case-control
2. Cohort 
3. Cross-sectional
4. Descriptive or case series
5. Clinical trial
6. Population screening

1. Case Control

Case definition
For study designs 2, 3, and 6, the following are defined, where available:

1. Case selection criteria
2. Exclusion criteria
3. Gender
4. Race/ethnicity
5. Age
6. Time period
7. Geographic location
8. Number of participants

1. Disease case definition: Parkinson's disease was diagnosed according to the United Kingdom brain-bank criteria;⁸ patients underwent a physical, neurobehavioral, and neurologic examination that incorporated the Unified Parkinson's Disease Rating Scale.⁹
2. Exclusion criteria: A history of neurologic or psychiatric conditions other than Parkinson's disease.
3. Gender: 55 men and 44 women
4. Race/ethnicity: All of Ashkenazi Jewish background based on family history assessments.
5. Age: Not specified
6. Time period: February 21, 2002 to April 30, 2004
7. Geographic location: Patients at the Cognitive and Movement Disorder Unit at the Rambam Medical Center, Haifa, Israel
8. Number of participants: 99 (% of total eligible was not specified)

Control definition  
For study design 1, the following are defined, if available. 

1. Control selection criteria
2. Matching variables
3. Exclusion criteria 
4. Gender
5. Race/ethnicity
6. Age
7. Time period
8. Geographic location
9. Number of participants

1. Control selection criteria: Alzheimer's disease patients, serving as a comparison group, met the criteria for dementia of the Alzheimer's type according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition,¹⁰ and the criteria for probable Alzheimer's disease of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association.¹¹
2. Matching variables: None
3. Exclusion criteria: None specified
4. Gender: 42 men and 32 women
5. Race/ethnicity: All of Ashkenazi Jewish background based on family history assessments.
6. Age: Not specified
7. Time period: Not specified
8. Geographic location: Patients at the Cognitive and Movement Disorder Unit at the Rambam Medical Center, Haifa, Israel
9. Number of participants: 74 (% of total eligible was not specified)

1. Control selection criteria: Healthy Ashkenazi Jews from the same geographic area who were undergoing testing to identify heterozygosity for certain recessive diseases.
2. Matching variables: None
3. Exclusion criteria: None specified
4. Gender: None specified
5. Race/ethnicity: All of Ashkenazi Jewish background based on family history assessments.
6. Age: Not specified
7. Time period: Not specified
8. Geographic location: Controls were reportedly from the same geographic location as patients (Haifa, Israel).
9. Number of participants: 1543 (% of total eligible was not specified)

1. Environmental factor
2. Exposure assessment
3. Exposure definition
4. Number of participants with exposure data (%
   of total eligible)

1. Environmental factor: None
2. Exposure assessment: Not applicable
3. Exposure definition:  Not applicable
4. Number of Participants with exposure data: Not applicable

1. Gene
2. DNA source
3. Methodology
4. Number of participants genotyped (% of total eligible) 

1. Gene: GBA

2. DNA source: Not stated, although presumed to be peripheral blood

3. Genotyping method: PCR amplification of genomic segments flanking each of the 6 mutation sites, followed by digestion with the appropriate restriction enzyme to distinguish the wild type from the mutant allele.¹² All mutant allele profiles were confirmed by a sequence analysis of the product from an independent PCR assay.

4. Number of participants genotyped: All 99 patients with Parkinson's disease, all 74 patients with Alzheimer's disease, and all 1543 controls (100% of total eligible).

The authors analyzed all patients and controls for the 6 mutations described above. When an individual was found to be a heterozygote, a homozygote, or a compound heterozygote for a mutation, that individual was defined as a "carrier." Differences in "carrier" rates between the Parkinson's disease, Alzheimer's disease, and normal control groups were analyzed by the chi-square test. Differences in clinical characteristics were compared between "carriers" and noncarriers by an independent-sample t-test for age, and a chi-square test for family history.

1. Prevalence of gene variant
2. Gene-disease association
3. Gene-environment interaction
4. Gene-gene interaction
5. Genetic test evaluation/monitoring

1. Prevalence of gene variant
   Among the 1543 control subjects, 95 were heterozygous for a Gaucher disease-causing mutation (6.2%); none were homozygous or compound heterozygous for a mutation. Of the carriers, 92 were heterozygous for N370S and 3 were heterozygous for 84GG; these results were consistent with a carrier rate of 1 in 16.7 for the N370S mutation and 1 in 514 for the 84GG mutation (previous estimates of the carrier frequency for these mutations were 1 in 17.5 and 1 in 400, respectively¹).
   
2. Gene-disease association
   The rates of "carriage of Gaucher disease" (heterozygotes plus homozygotes for any mutation) were calculated for patients with Parkinson's disease, Alzheimer's disease, and control subjects:

The frequency of the mutant N370S GBA allele in the Parkinson's disease patients was 5 times that among the healthy Ashkenazi control subjects (p<0.001). The frequency of the mutant 84GG GBA allele in the Parkinson's disease patients was 21 times that among the healthy Ashkenazi control subjects (p<0.001). Three patients with Parkinson's disease were homozygous for the N370S mutation (nonpenetrant for Gaucher disease), while none of the 1543 control subjects were homozygous.

Upon comparing the 31 Parkinson's disease "carriers" of a GBA gene mutation to the 68 noncarriers, Parkinson's "carriers" were significantly younger at age of onset [60.0+/-14.2 years versus 64.2 +/-11.7 years; p=0.04]. There was no significant difference between "carriers" and non-carriers with regard to (1) presence of a family history of Parkinson's disease in a first or second degree relative, (2) initial motor manifestations, or (3) initial response to medications.

The authors also showed that patients with Parkinson's disease had significantly greater odds for being "carriers" of Gaucher disease than did patients with Alzheimer's disease (odds ratio, 10.8; 95% confidence interval, 3.0-46.6; p<0.001) or control subjects (odds ratio, 7.0; 95% confidence interval, 4.2-11.4; p<0.001). In addition, the rate of "carriage" of a GBA gene mutation among patients with Alzheimer's disease did not differ significantly from healthy controls (odds ratio, 0.6; 95% confidence interval, 0.2-2.2; p=0.62). This result indicated that the association between the GBA gene mutations analyzed here and Parkinson's disease is specific, and that GBA gene mutations are not associated another common neurodegenerative process, namely Alzheimer's disease.

The authors' overall conclusions from the study were (1) the prevalence of GBA mutations in the population of Ashkenazi Jews with Parkinson's disease is much higher than the reported prevalence of mutations in other susceptibility genes, such as parkin and alpha-synuclein, and (2) mutations in the GBA gene emerge as a strong genetic determinant predisposing people to Parkinson's disease.

Comments
Provide additional insight, including methodologic issues and/or concerns about the study

The authors did not evaluate the effects of specific alleles, or the contribution of homozygosity for a mutant allele versus heterozygosity, on the risk for developing Parkinson's disease in this patient population. In order to determine the allelic contribution, the Parkinson's disease and healthy control groups can be compared as shown:

^#"+" refers to the wild type or non-mutant allele
*The expected number of controls with this genotype (1.3713 controls) was calculated (assuming Hardy-Weinberg equilibrium--see below), which was then used to determine the odds ratio.

This analysis showed that (1) N370S homozygotes were approximately 47 times more likely than people without this mutation to have Parkinson's disease, but accounted for only 3 of 98 cases, (2) N370S heterozygotes were approximately 5 times more likely to have Parkinson's disease, and (3) 84GG heterozygotes were approximately 28 times more likely to have Parkinson's disease, but accounted for only 4 out of 98 cases.

*The assumption that the genotypes are in Hardy-Weinberg equilibrium for the control population was shown to be valid, as calculated below:

Chi-square = 1.5566 (3 degrees of freedom) p = 0.500-0.750

In order to determine how strong the association is between GBA gene mutations and Parkinson's disease, attributable fractions were calculated to quantify the contribution of each of these mutations to the occurrence of Parkinson's disease:

This analysis showed that although homozygosity for N370S and heterozygosity for 84GG are strong risk factors for Parkinson's disease, they account for only a small proportion of cases. Although the association with Parkinson's disease was stronger for N370S homozygotes, heterozygotes were much more prevalent among cases, contributing to a larger estimated attributable fraction.

To calculate the absolute risk for developing Parkinson's disease for members of the Ashkenazi Jewish population, the absolute risk was calculated for individuals with each genotype. First, the allele and genotype frequencies were calculated (allele frequencies derived from the population genetics expressions of p + q + r = 1 and [(p x p) + (2p x q) + (q x q) + (2p x r) + (2q x r) + (r x r) = 1], which assume that both the N370S and 84GG alleles are in Hardy-Weinberg equilibrium, which was shown above to be the case):

Genotype Frequencies:
+/+ = 0.93939
+/N370S = 0.02889 x 2 = 0.05778
+/84GG = 0.00094 x 2 = 0.00188
N370S/N370S = 0.00089

The population wide incidence (sum of the genotype specific incidence rates) and the absolute risk were calculated by summing the products of the genotype frequencies and the odds ratios (assuming an estimated prevalence of Parkinson's disease of 1/100 (0.01), which was the prevalence quoted by Aharon-Peretz et al.):

Prevalence = 0.01 = 1.3399x= 0.0074 [absolute risk]

The risk for developing Parkinson's disease for individuals with each genotype was then determined by multiplying the baseline absolute risk (x) by the odds ratio:

These results indicate that there are significant risks for developing Parkinson's disease, over the baseline population risk of 1%, for both N370S and 84GG carriers, as well as for N370S homozygotes. However, are these risks important from a population-based public health perspective? Would it be beneficial to perform population-based screening of Ashkenazi Jews for the N370S mutation--the mutation with the highest incidence in the population--in order to identify those individuals with an increased risk of developing Parkinson's disease?

Screening 1000 Ashkenazi Jews will identify approximately 59 individuals who are N370S heterozygotes or homozygotes:

(1000)(0.05778) + (1000)(.00089)
57.8 + 0.9 = approximately 59 individuals

Based on the risks that individuals with each genotype will develop Parkinson's disease, of the 59 identified individuals, approximately 3 would be expected to develop the condition:

(1000)(.05778)(0.039) + (1000)(.00089)(0.35)
2.25 + 0.31 = approximately 3 individuals

Therefore, 95% of identified individuals are not expected to develop Parkinson's disease. This finding is in contrast to the authors' conclusion that "mutations in the GBA gene emerge as a strong genetic determinant predisposing people to Parkinson's disease." This statement was probably based on the fact that mutations in the GBA gene, particularly in the Ashkenazi Jewish population, are much more common than mutations in other genes implicated in the genetic susceptibility to Parkinson's disease (parkin, alpha-synuclein, synphilin-1, etc.). However, the individual risk of developing Parkinson's disease for carriers of a mutation in the GBA gene (particularly the N370S mutation) is still very low. Since there is no means of determining which N370S heterozygotes or homozygotes will develop Parkinson's disease in relation to those who won't, and there are no clear lifestyle modifications or medical treatments to lower the risk, it is unreasonable to test individuals to determine their carrier status for this mutation, since N370S carrier identification would cause needless anxiety about the uncertain potential of a major future health problem and provide no direct health benefit.

1. Horowitz M et al. Prevalence of glucocerebrosidase mutations in the Israeli Ashkenazi Jewish population. Hum Mutat. 1998;12:240-4.
2. Tsuji S et al. Genetic heterogeneity in type I Gaucher disease: multiple genotypes in Ashkenazic and non-Ashkenazic individuals. Proc Nat Acad Sci. 1988;85:2349-52.
3. Beutler E et al. Identification of the second common Jewish Gaucher disease mutation makes possible population-based screening for the heterozygous state. Proc Nat Acad Sci. 1991;88:10544-7.
4. Tsuji S et al. A mutation in the human glucocerebrosidase gene in neuronopathic Gaucher's disease. New Eng J Med. 1987;316:570-5.
5. Beutler E et al. Mutations in Jewish patients with Gaucher disease. Blood. 1992;79:1662-6.
6. Latham T et al. Complex alleles of the acid beta-glucosidase gene in Gaucher disease. Am J Hum Genet. 1990;47:79-86.
7. Beutler E et al. Identification of six new Gaucher disease mutations. Genomics. 1993;15:203-5.
8. Hughes AJ et al. Accuracy of clinical diagnosis of idiopathic Parkinson's disease: a clinico-pathological study of 100 cases. J Neurol Neurosurg Psychiatry. 1992;55:181-4.
9. Fahn S and Elton RL. Members of the unified Parkinson's disease rating scale development committee. In Fahn S et al, eds. Recent Developments in Parkinson's Disease. New York :Macmillan, 1987:153-63.
10. Diagnostic and Statistical Manual of Mental Disorders, 4th ed.:DSM-IV. Washington , DC :American Psychiatric Association, 1994.
11. McKhann G et al. Clinical diagnosis of Alzheimer's disease: report of the NINCDS-ADRDA work group under the auspices of Department of Health and Human Services Task Force on Alzheimer's Disease. Neurology. 1984;34:939-44.
12. Beutler E et al. The facile detection of the nt 1226 mutation of glucocerebrosidase by 'mismatched' PCR. Clin Chim Acta. 1990;194:161-6.