Abstract
Objective
To estimate the risk of Parkinson's disease in individuals with mutations in the Parkin gene.
Design
We assessed point mutations and exon deletions and duplications in the Parkin gene in 247 PD probands with age at onset ≤50 and 104 control probands enrolled in the Genetic Epidemiology of PD study. For each first-degree relative, a consensus diagnosis of PD was established. The probability that each relative carried a mutation was estimated from the proband's Parkin carrier status using Mendelian principles and the relationship of the relative to the proband .
Results
Parkin mutations were identified in 25 PD probands (10.1%), 72% of whom were heterozygotes. One Parkin homozygote reported 2 siblings with PD. The cumulative incidence of PD to age 65 in carrier relatives (age-specific penetrance) was estimated to be 7.0% (95% CI: 0.4-71.9%) compared to 1.7% (95% CI: 0.8-3.4%) in non-carrier relatives of cases (p=0.59) and 1.1% (95% CI: 0.3-3.4%), in relatives of controls ( compared to non-carriers p=0.52).
Conclusions
The cumulative risk of PD to age 65 in a non-carrier relative of a case with AAO ≤50 is not significantly greater than the general population risk among controls. Age specific penetrance among Parkin carriers, in particular heterozygotes, deserves further study.
Keywords: Parkin, Mutations, Parkinson's disease, Kin-cohort study, Early onset
Introduction
Mutations in the Parkin gene (PARK2)1, 2 are associated primarily with early-onset Parkinson's disease (EOPD), defined as age at onset (AAO) ranging from ≤45 to ≤553-7, but have also been described in PD cases with an AAO over 70 years 7-10. In PD cases with AAO ≤ 45 with a mode of inheritance consistent with autosomal recessive transmission, the frequency of Parkin mutations may be as high as 49%3 while in cases without a family history of PD the range is 15-18%.4, 6 AAO is inversely correlated with the frequency of Parkin mutations in both familial3 and sporadic6 cases.
Several studies have compared the AAO of PD in heterozygous, compound heterozygous, and homozygous Parkin mutation carriers 3-10 and found that heterozygous cases, both familial and sporadic, have older AAO. Heterozygous Parkin mutation carriers are more frequently reported among sporadic than familial cases7.
Information on the risk of PD in individuals who carry Parkin mutations in either the homozygous, compound heterozygous, or heterozygous state (or penetrance) is essential for genetic counseling. The penetrance of Parkin mutations has only been reported for isolated families7. Most of the previous study designs sampled PD cases based on family history of PD, which would bias penetrance estimates upwards11, 12. To obtain an unbiased estimate of risk, a population-based random sample would be desirable, but Parkin mutations are so rare in the population that such a sample would have to be extremely large to obtain sufficient precision in penetrance estimates.
To obtain unbiased estimates of the risk of PD in Parkin carriers despite the low population frequency of Parkin mutations 13, we used a kin-cohort study design11, 12 applied to participants in the Genetic Epidemiology of Parkinson's Disease (GEPD) study14. The kin-cohort design is highly efficient for estimating penetrance, because the relatives’ mutation status is not required for the analyses, thus reducing costs for genetic analysis15.
Methods
Subjects
Details of the GEPD study have been previously described 14, 16, 17. Cases were ascertained based on AAO of motor signs ≤50 (EOPD) or >50 (LOPD). In this study we included all 247 PD cases with AAO ≤50. All cases were recruited from the Center for Parkinson's Disease and Other Movement Disorders at Columbia University (CPD), and EOPD cases were oversampled14. One hundred and five controls were randomly selected from 412 controls in the GEPD study for complete sequencing of the Parkin gene.18 The majority of the controls were recruited by random digit dialing, with frequency matching by age, gender, ethnicity and area code/exchange. An additional sample of 40 controls of Hispanic descent, 11 African American (AA) controls and 170 Caucasian controls participating in GEPD were used to examine Parkin variants that have not been previously described. All PD cases and control probands were seen in-person and underwent an identical evaluation14 that included a medical history, Unified Parkinson's Disease Rating Scale (UPDRS)19 and videotape assessment.
Diagnosis of PD in Relatives
Information on the family history of PD in first-degree relatives was obtained by administering a reliable, validated interview to each case, control, and first-degree relative. For relatives who were deceased or otherwise unavailable for interview, the history was obtained by interviewing the most knowledgeable informant16. An algorithm was created to generate a final diagnosis for PD in each first-degree relative based on the family history interview and the direct interview with the relative. For relatives diagnosed with PD, a level of certainty was assigned as definite, probable, possible, uncertain, and unlikely. For first-degree relatives who met criteria for uncertain, possible, probable or definite PD, we tried to obtain additional information in the form of an examination, medical records, or an independent interview by a neurologist. A best estimate diagnosis of PD was assigned for each relative 14. Family history information including age at onset of PD was available for 1330 relatives of 224 PD cases and 638 relatives of 103 controls included in the penetrance analysis. The Institutional Review Board at the College of Physicians and Surgeons, Columbia University approved this study.
Criteria for Inclusion of Mutation Carriers
All sequence variants identified in cases and controls were genotyped in ethnically matched controls. Sequence variants observed in controls but at a frequency of ≤1% were classified as rare variants. The remaining variants were classified as mutations and included in the analysis based on three criteria: 1) The mutation is absent in controls 2) The mutation is recurrent , has been reported in PD cases (unrelated) in more than one study, or the mutation changes an amino acid that is evolutionarily conserved and which is predicted to effect protein function 3) The mutation is located in the coding region and predicted to change the amino acid sequence.
Molecular Genetic Analysis of Probands
Mutation screening was performed in 247 cases and 105 controls to detect point mutations and exon deletions and duplications in the Parkin gene. In a previous study, we sequenced all Parkin exons and screened for exon deletions and duplications by semi-quantitative multiplex PCR in 101 cases and 105 controls18. One control was subsequently found to have a putative splice variant IVS6-14 C>G, that has not been reported in cases or controls. We consider this a rare variant and not a mutation and have excluded it from the analysis. In this study we report data on an additional 146 PD cases from the GEPD study who were screened for Parkin mutations by denaturing high performance liquid chromatography (DHPLC) (WAVE Transgenomic), which has 100% sensitivity and specificity. Primers and DHPLC conditions used for analysis of the Parkin gene have been described previously20. Amplicons were either directly sequenced (n=126) or analyzed using a Parkin genotyping array (n=20)21 in DNA samples with abnormal elution profiles. The genotyping array has excellent sensitivity and specificity for detection of sequence variants when compared to the gold standard of sequencing21. The primers used for PCR amplification of Parkin exons 1-12 and intron-exon boundaries and sequencing have been described previously22. Cycle sequencing was performed on the purified PCR product as per the manufacturer's instructions (BigDye, Applied Biosystems). Products were analyzed on an ABI3700 genetic analyzer. Chromatograms were viewed using Sequencher (Genecodes) and sequence variants determined. All sequence variants identified in cases and controls were confirmed by analysis in a separate PCR followed by bi-directional sequencing. To identify genomic deletions and exon rearrangements in Parkin, semi-quantitative multiplex PCR was performed as previously described18.
In addition to screening for mutations in Parkin, we genotyped five LRRK2 mutations (G2019S, L1114L, I1122V, R1441C and Y1699C) in 247 cases and 104 controls. Results of the analysis of LRRK2 in all participants in the GEPD study, including both early and late onset PD cases, and all controls have been reported.23
Statistical methods
Demographic and clinical characteristics were compared between cases and controls and between mutation carriers and non-carriers. Fisher's exact test for categorical characteristics and Student's t-test for continuous characteristics were used to assess statistical significance. The penetrance of Parkin mutations was estimated using the kin-cohort11 method. In this method, the genotypes of the relatives are first estimated using Mendelian principles and the relationship of the relatives to the proband. Then the observed disease occurrence in the relatives is evaluated in relation to these estimated genotypes. This method assumes that, although the PD case probands were sampled through their AAO, thereby increasing the proportion of carriers among cases and their first-degree relatives, the relatives of these PD cases are representative of randomly chosen individuals with certain genotypes. Hence familial influences on the relatives’ PD risk other than the Parkin genotype are assumed to be negligible.
First, we computed the probability that each relative carries a mutation. Second, we used the kin-cohort method to estimate genotype-specific disease rates, using the consensus diagnoses of PD in the first-degree relatives16, 17. Third, we used Kaplan-Meier survival analysis to compute the cumulative risk of PD in the first-degree relatives of control probands. Kaplan-Meier analysis cannot be directly applied to estimate genotype-specific cumulative incidence in the relatives of cases because the carrier status in these relatives is unknown; however the kin-cohort method allows calculation of cumulative incidence through a method similar to Kaplan–Meier analysis. Confidence intervals for the penetrance and the cumulative incidence were computed using log-log transformation to ensure the lower limits of the confidence intervals were positive 24.
Results
Demographic and Clinical Characteristics of the Case and Control Probands
Demographic and clinical characteristics of the cases and controls are shown in Table 1. The mean AAO of the 247 cases was 41.8 years (SD: 6.8), mean disease duration was 10.7 (SD: 7.6) years, and the total motor score (UPDRS Part III) was 20.3 (SD: 12.8). Nineteen (8.5%) of the 224 cases on whom family history information was available had a diagnosis of PD in a first-degree relative.
Table 1.
Demographic Characteristics of Case and Control Probands
| |
Total (n=351) |
Cases (n=247) |
Controls (n=104) |
Significance Cases vs. Controls |
|---|---|---|---|---|
| % male (n) |
59.7 (209) |
61.1 (151) |
55.8 (58) |
0.40 |
| Age (years) (sd) |
54.9 (10.2) |
52.5 (9.0) |
60.8 (10.5) |
<0.001 |
| % White (n) | 84.3 (296) | 81.0 (200) | 92.3 (96) | |
| % African-American (n) | 2.3 (8) | 2.0 (5) | 2.9 (3) | |
| % Hispanic (n) | 8.0 (28) | 10.1 (25) | 2.9 (3) | |
| % Other (n) |
5.4 (19) |
6.9 (17) |
1.9 (2) |
0.02 |
| Years Education (sd) |
15.5 (3.2) |
15.5 (3.3) |
15.5 (2.9) |
0.97 |
| % with Parkin variants (n) |
7.1 (25) |
10.1 (25) |
0.0 (0) |
<0.001 |
| % with family history of PD* (n) | 7.7 (25) | 8.5 (19/224) | 5.8 (6/103) | 0.50 |
Family history was available for 328 probands (224 cases and 103 controls)
Frequency of Parkin Mutations in Cases and Controls
Twenty-five (10.1%) of the 247 cases had a Parkin mutation; five (20%) were homozygous, two (8%) were compound heterozygotes and eighteen (72%) were heterozygous (Table 2). Twenty two of the mutations have been previously described in other studies 2, 21, 22 Three new mutations were identified that have not been previously published and were not detected in any of our control samples (Iso298Leu, Asp18Asn, and Pro153Arg). Eleven different point mutations (c.81G>T, Gly319Gly, Arg42Pro, Arg275Trp, Met192Leu, Cys253Tyr, Asp280Asn, Iso298Leu, Arg366Gln, Asp18Asn, Pro153Arg) and four different exon rearrangements were identified (Exon 5 del, Exon 3-4del, Exon 3 40bp del and Exon 2 del) (Table 2). Point mutations included nine missense mutations (Arg42Pro, Arg275Trp, Met192Leu, Cys253Tyr, Asp280Asn, Iso298Leu, Arg366Gln, Asp18Asn, Pro153Arg), one synonymous substitution (Gly319Gly), and a non-coding 5’UTR mutation (c.81G>T). Exon deletions were found in four different exons (exons 2, 3, 4 and 5). 40 percent (10/25) of the variants identified in cases were found in exons encoding functional domains including the ubiquitin domain (Exon 2) and RING1 domain (Exon 7). Six cases carried the Exon 3 40bp deletion, five cases carried Arg275Trp and two carried Arg42Pro. We previously reported that the synonymous substitution, Leu261Leu, is a variant rather than a disease associated mutation and thus have not included carriers of Leu261Leu in our estimates23. Among the 25 carriers, three (12%) had AAO ≤20 (2/3 heterozygotes); four (16%) had AAO 21-30 (3/4 heterozygotes); six (24%) had AAO 31-40 (5/6 heterozgygotes); and twelve (48%) had AAO 41-50 (8/12 heterozgyotes). Among all 247 EOPD case probands, carriers represented 75% (3/4) of those with AAO≤20, 36% (4/11) of those with AAO 21-30, 8% (6/73) of those with AAO 31-40, and 8% (12/159) of those with AAO 41-50.
Table 2.
Parkin Mutations identified in cases
| Patient | Exon | Mutation | Zygosity | AAO (years) | Ethnicity |
|---|---|---|---|---|---|
| 1 | 2 | Arg42Pro | Het | 44 | White/non Hispanic |
| 2 | 2 | Arg42Pro | Het | 32 | Hispanic |
| 3 | 2 | Exon 2 deletion | Hom | 42 | White/non Hispanic |
| 4 | 2 | Asp18Asn | Het | 32 | Hispanic |
| 5 | 3 | Exon 3 40bp deletion | Hom | 38 | White/non Hispanic |
| 6 | 3,7 | Exon 3 40bp deletion + Arg275Trp | Comp. Het | 19 | White/non Hispanic |
| 7 | 3 | Exon 3 40bp deletion | Het | 29 | White/non Hispanic |
| 8 | 3 | Exon 3 40bp deletion | Hom | 47 | White/non Hispanic |
| 9 | 3 | Exon 3 40bp deletion | Hom | 45 | White/non Hispanic |
| 10 | 3 | Exon 3 40bp deletion | Hom | 27 | White/non Hispanic |
| 11 | 3,4 | Exon 3-4 deletion | Het | 16 | Hispanic |
| 12 | 4 | Pro153Arg | Het | 42 | Hispanic |
| 13 | 5 | Exon 5 deletion | Het | 48 | White/non Hispanic |
| 14 | 5 | Exon 5 deletion | Het | 50 | White/non Hispanic |
| 15 | 5 | Met192Leu | Het | 36 | Hispanic |
| 16 | 5 | Met192Leu. | Het | 42 | Hispanic |
| 17 | 7 | Cys253Tyr +Asp280Asn | Comp. Het | 47 | White/non Hispanic |
| 18 | 5’UTR | c.81G>T | Het | 28 | Hispanic |
| 19 | 7 | Arg275Trp | Het | 47 | White/non Hispanic |
| 20 | 7 | Arg275Trp | Het | 24 | Other |
| 21 | 7 | Arg275Trp | Het | 35 | White/non Hispanic |
| 22 | 7 | Arg275Trp | Het | 42 | Hispanic |
| 23 | 8 | Iso298Leu1 | Het | 37 | White/non Hispanic |
| 24 | 9 | Gly319Gly | Het | 47 | Hispanic |
| 25 | 10 | Arg366Gln | Het | 17 | White/non Hispanic |
Clinical Characteristics of Case Probands With and Without Parkin Mutations
Demographic and clinical features of the 25 mutation carriers and 222 non-carriers are shown in Table 3. The AAO of PD was significantly younger in carriers (36.5 ± 10.3) than in non-carriers (42.4 ± 6.1) (p=0.01), but did not differ between heterozygotes compared to compound heterozygotes and homozygotes combined. Other comparisons of clinical features were not significant.
Table 3.
Characteristics of Case Probands With and Without Parkin Mutations
| |
Mutation (n=25) |
No mutation (n=222) |
p value |
|---|---|---|---|
| Current age (sd) |
47.2 (9.9) |
53.1 (8.7) |
0.008 |
| Age onset (sd) |
36.5 (10.3) |
42.4 (6.1) |
0.01 |
| % male (n) |
40.0 (10) |
63.5 (141) |
0.03 |
| % Ethnicity: White (n) | 64.0 (16) | 82.3 (184) | |
| African-American (n) | 0.0 (0) | 2.3 (5) | |
| Hispanic (n) | 28.0 (7) | 8.1 (18) | 0.03 |
| Other (n) |
8.0 (2) |
6.8 (15) |
|
| Years education (sd) |
14.1 (3.8) |
15.6 (3.3) |
0.06 |
| Duration PD (yrs)(sd) |
10.6 (7.5) |
10.7 (7.7) |
0.96 |
| Total motor score (UPDRS III)(sd) |
22.1 (15.8) |
20.1 (12.2) |
0.57 |
| Total modified mini mental state (mMMS) (sd) |
52.3 (5.3) |
53.7 (4.6) |
0.24 |
| % Family history PD in first-degree relative* |
4.4 (1/23) |
8.9 (18/201) |
0.72 |
| % Bradykinesia |
88.0 (22) |
82.9 (184) |
0.78 |
| % Rest tremor |
80.0 (20) |
78.2 (172/220) |
1.00 |
| % Rigidity |
96.0 (24) |
98.7 (219) |
0.35 |
| % Postural instability |
44.0 (11) |
45.0 (99/220) |
1.00 |
| % Asymmetry |
96.0 (24) |
88.5 (192/217) |
0.49 |
| % on L dopa |
70.8 (17/24) |
70.2 (153/218) |
1.00 |
| % Improve L dopa |
95.0 (19/20) |
97.4 (150/154) |
0.46 |
| %On-off |
51.7 (15) |
53.6 (113/211) |
1.00 |
| Hoehn and Yahr (sd) |
2.3 (0.9) |
2.1 (0.9) |
0.32 |
| % First symptom†: rest tremor (n) |
44.0 (11) |
48.9 (108) |
0.68 |
| % First symptom: gait (n) |
0.0 |
4.5 (10) |
0.60 |
| % First symptom: rigidity (n) |
8.0 (2) |
9.5 (21) |
1.00 |
| % First symptom: bradykinesia (n) |
4.0 (1) |
4.1 (9) |
1.00 |
| % First symptom: motor (n) |
4.0 (1) |
1.8 (4) |
0.42 |
| % First symptom: poor balance (n) |
4.0 (1) |
0.5 (1) |
0.19 |
| % First symptom: pain (n) | 8.0 (2) | 1.8 (4) | 0.12 |
Family history was available for 224 case probands.
First symptom information was available for 221 out of 222 non-carriers.
The clinical characteristics of first-degree relatives stratified by the probands’ mutation status are presented in Table 4. Information on the family history of PD in first-degree relatives was available for 23/25 carriers and 201/218 non-carriers. One of 23 carrier probands (4.4%) had a family history of PD. This proband is homozygous for an 40 bp deletion in exon 3 and had two affected siblings (AAO 26, 30).
Table 4.
Characteristics of first-degree relatives of case probands stratified by probands’ carrier status
| |
Total Relatives of Case Probands (n=1330) |
Relatives of Homozyogous/Compound Heterozygous Carriers (n=42) |
Relatives of Heterozygous carriers (n=110) |
Relatives of Non-carriers (n=1178) |
Relatives of Control Probands (n=638) |
Significance across probands’ genotypes |
|---|---|---|---|---|---|---|
| % male (n) |
47.7 (634) |
52.4 (22) |
44.6 (49) |
47.8 (563) |
50.9 (325) |
0.09 |
| Age (years) (sd) | 49.7 (22.7) | 47.6 (22.2) | 45.4 (19.7) | 50.2 (22.9) | 53.9 (20.2) | 0.08 |
| Parent (years, sd) | 73.0 (14.0) | 72.8 (9.1) | 65.8 (13.7) | 73.6 (13.9) | 72.6 (13.9) | 0.01 |
| Siblings (years, sd) | 49.6 (11.6) | 46.7 (8.8) | 45.3 (12.1) | 50.2 (11.6) | 55.1 (14.4) | 0.01 |
| Offspring (years, sd) |
24.9 (11.5) |
19.3 (8.8) |
22.8 (9.0) |
25.2 (11.7) |
33.6 (11.1) |
0.14 |
| Ethnicity (%, n) | ||||||
| White | 74.4 (989) | 100 (42) | 42.7 (47) | 76.4 (900) | 91.5 (584) | |
| African-American | 2.9 (39) | 0 | 0 | 3.3 (39) | 1.7 (11) | <0.001 |
| Hispanic | 13.9 (185) | 0 | 47.3 (52) | 11.3 (133) | 3.6 (23) | |
| Other |
8.8 (117) |
0 |
9.4 (11) |
9.0 (106) |
3.1 (20) |
|
| Years Education (sd) |
12.6 (4.9) |
13.4 (4.4) |
11.7(4.9) |
12.7 (4.9) |
13.3 (4.1) |
0.11 |
| Relationship %,( n) | ||||||
| Parents | 32.4 (431) | 30.9 (13) | 28.2 (31) | 32.9 (387) | 31.5 (201) | |
| Siblings | 37.4 (497) | 42.9 (18) | 46.4 (51) | 36.3 (428) | 36.7 (234) | 0.29 |
| Offspring |
30.2 (402) |
26.2 (11) |
25.5 (28) |
30.8 (363) |
31.8 (203) |
|
| Number Affected (%, n) | ||||||
| All relatives | 1.6 (21) | 4.8 (2) | 0 | 1.6 (19) | 0.9 (6) | 0.09 |
| AAO years for affecteds mean,( sd) | 61.3 (16.1) | 27.0 (2.8) | -- | 64.9 (11.9) | 66.5 (12.8) | <0.001 |
| Parents | 3.5 (15) | 0 | 0 | 3.9 (15) | 3.0 (6) | 0.51 |
| AAO years for affecteds mean, (sd) | 69.1 (7.3) | -- | -- | 69.5 (7.7) | 66.5 (12.8) | 0.60 |
| Siblings % (n) | 1.2 (6) | 7.7 (2) | 0 | 1.0 (4) | 0 | 0.07 |
| AAO years for affecteds mean, (sd) | 41.7 (15.3) | 27.0 (2.8) | -- | 49.0 (13.2) | -- | 0.09 |
| Offspring | 0 | 0 | 0 | 0 | 0 |
Penetrance Estimates of Parkin Mutations
The probability of a relative being a carrier, whether or not he/she was actually diagnosed with PD, stratified by the proband's carrier status is presented in Table 5. In the calculation, the population frequency of Parkin mutations, p, was assumed to be 0.03% {Lucking, 2000 #1168; Oliveira, 2003 #2;. We have run a sensitivity analysis by taking p to be 2.8% (the upper limit of the 95% exact confidence interval of the mutation frequency estimated from the controls) and the results did not change, suggesting that the estimates are robust to misspecification to the population frequency of Parkin mutations.
Table 5.
Probability of a relative being a carrier stratified by the proband's mutation status*
| Relatives of homozygous or compound heterozygous probands | |||
|---|---|---|---|
| Probability of the relative being a homozygous carrier | Probability of the relative being a heterozygous carrier | Probability of the relative being a non-carrier | |
| Parent | P | 1- p | 0 |
| Sibling | ¼*(p2-2 p+1) | 1/2*(- p2+1) | 1/4*(1-p)2 |
| Offspring | P | 1- p | 0 |
| Relatives of heterozygous probands | |||
|---|---|---|---|
| Probability of the relative being a homozygous carrier | Probability of the relative being a heterozygous carrier | Probability of the relative being a non-carrier | |
| Parent | l/2*p | 1/2 | 1/2*(1-p) |
| Sibling | 1/4*(p2 +p) | 1/2*(- p2+ p+1) | 1/4*(p2-3p +2) |
| Offspring | 1/2*p | 1/2 | 1/2*(1-p) |
| Relatives of non-carrier probands | |||
|---|---|---|---|
| Probability of the relative being a homozygous carrier | Probability of the relative being a heterozygous carrier | Probability of the relative being a non-carrier | |
| Parent | 0 | p | 1-p |
| Sibling | 1/4*p2 | 1/2(-p2+2p) | 1/4*(p2-4p+4) |
| Offspring | 0 | p | 1-p |
p is the population frequency of the mutation.
The expected genotype distribution in the relatives and the prevalence of a history of PD in relatives predicted to be carriers or noncarriers are shown in Table 6. To obtain these prevalence estimates, we first computed the probability that each of the relatives was a carrier based on the observed carrier status in the probands (see Table 1) and then combined the information on the predicted genotypes in the relatives with the observed PD diagnoses in the relatives. There were 93 relatives expected to be carriers (homozygotes or heterozygotes), among whom 2 had PD (Table 6). Therefore, the prevalence of a history of PD in carrier relatives was estimated at 2/93 [2.2% (CI: 0.3%, 7.6%)]. Among relatives expected to be noncarriers (N=1237), 19 had PD (Table 6); thus the prevalence of a history of PD in noncarrier relatives was estimated as 1.7% (0.9-2.4%). These two prevalence estimates did not differ significantly.
Table 6.
Estimated genotype distribution and prevalence of a history of PD in relatives of cases
| Carriers (Homozygotes and Heterozygotes) | Non-carriers | Total | |||
|---|---|---|---|---|---|
| PD | Non-PD | PD | Non-PD | ||
| Parent | 0 | 29 | 15 | 387 | 431 |
| Siblings | 2 | 38 | 4 | 453 | 497 |
| Offspring | 0 | 25 | 0 | 377 | 402 |
| Total | 2 | 91 | 19 | 1218 | 1330 |
| Prevalence (exact 95% CI) | 2.2% (0.3%, 7.6%) | 1.7% (0.9%, 2.4%) | |||
The cumulative incidence of PD in 1330 relatives of case probands (without regard to carrier status) was estimated to be 1.7% (95% CI: 0.9-3.3%) to age 65, and 5.9% (95% CI: 3.7-9.3%) to age 80. Estimates of the cumulative risk of PD in Parkin carriers and non-carriers and the cumulative risk of PD in 647 relatives of controls are presented in table 7. Among relatives expected to be carriers, the cumulative incidence of PD was 7.0% to age 65 and remained so up to age 80. Among relatives expected to be noncarriers, the cumulative incidence was 1.7% to age 65 and 6.1% to age 80. The ratio of the cumulative incidence to age 80 for carriers versus non-carriers was 1.1 (95% CI: 0.07-18.8). Estimates of risk in carriers and noncarriers did not differ significantly with our sample size. The cumulative incidence in relatives of controls was similar to that in relatives expected to be noncarriers (1.1% to age 65 and 4.1% to age 80).
Table7.
Estimated cumulative incidence of PD in Parkin carrier relatives, non-carrier relatives, and relatives of controls
| Age |
Parkin Carrier (Homozygotes and heterozygotes) Relatives (95% CI) |
Non-carrier Relatives (95% CI) |
Relatives of Controls (95% CI) |
|---|---|---|---|
| 25 |
3.7% | 0 |
0 |
| (1.1, 84.0%) | |||
| 35 |
7.0% | 0 |
0 |
| (0.4,71.9%) | |||
| 45 |
7.0% | 0.2% | 0.2% |
| (0.4,71.9%) |
(0.1, 1.0%) |
(0.03-1.7%) |
|
| 55 |
7.0% | 0.4 | 0.2% |
| (0.4,71.9%) |
(0.1, 1.4%) |
(0.03, 1.7%) |
|
| 60 |
7.0% | 1.1 | 0.6% |
| (0.4,71.9%) |
(0.5, 2.5%) |
(0.1, 2.5%) |
|
| 65 |
7.0% | 1.7% | 1.1% |
| (0.4,71.9%) |
(0.8, 3.4%) |
(0.3, 3.4%) |
|
| 70 |
7.0% | 3.3% | 1.1% |
| (0.4,71.9%) |
(1.9, 5.7%) |
(0.3, 3.4%) |
|
| 75 |
7.0% | 4.6% | 2.8% |
| (0.4,71.9%) |
(2.7, 7.5%) |
(1.1, 7.2%) |
|
| 80 | 7.0% | 6.1% | 4.1% |
| (0.4,71.9%) | (3.8, 9.8%) | (1.7, 9.8%) |
We have reported that LRRK2 mutations may be responsible for familial aggregation in both EOPD and LOPD in the GEPD study{Clark, 2006 #12300}. To separate out the effect of the LRRK2 G2019S mutation (the only observed LRRK2 mutation in the cases), we repeated our analyses using 214 case probands who did not carry a LRRK2 mutation. However, we can not we can not exclude the possibility that they carry ‘other’ mutations in the LRRK2 gene as we did not completely sequence the LRRK2 gene but only genotyped 5 previously reported LRRK2 mutations. There were 1274 relatives included in the analysis. Since these relatives were family members of probands who did not carry any of the five previously identified LRRK2 mutations, it is unlikely they carry any of these mutations. The cumulative incidence to age 80 for relatives expected to be non-carriers of either LRRK2 mutations or Parkin mutations was 5.9%, compared to 6.1% for relatives expected to be noncarriers of Parkin mutations. The cumulative incidence of PD to age 80 in non-Parkin, non-LRRK2 carrier relatives was not significantly different from controls.
Discussion
Using the kin-cohort method, we have shown that the cumulative risk to age 65 in a relative of an EOPD case who is not estimated to carry a Parkin mutation is not significantly greater than the general population risk among controls. We estimated a cumulative risk of PD in carriers of Parkin mutations (age-specific penetrance) of 7.0% (95% CI: 0.4-71.9%) up to age 80. We were unable to examine risk of PD among heterozygotes separately. Parkin heterozgyosity may not be sufficient for the development of PD. A recent paper suggested that Parkin variants were equally common in cases and controls in predominantly non-coding regions25.
The sampling scheme of our study, in which probands were ascertained without regard to their family history of PD, differs markedly from that in other studies that included only families with multiple affected individuals. As noted in the context of estimating the penetrance of mutations in BRCA1 and BRCA2, penetrance estimates are inflated when based on samples selected though family history 12, 15, and hence we believe our estimates are more representative of those in the general population than are those derived from familial samples. While the confidence intervals are extremely wide, the observed low frequency of PD in first-degree relatives of Parkin mutation carriers, 72% of whom are heterozygotes, may be a reflection of the low penetrance in carriers.
The frequency of Parkin mutations was 10.1% (95% CI: 8.0-16.4%) in 247 cases with AAO ≤ 50, which is within the range of other series with primarily sporadic cases (15-18%)6, or population-based series (9%)26. The mean AAO of our case sample was 41.6 years, and 48% of the cases had AAO between 40 and 50. The frequency of Parkin mutations has been reported to decrease with increasing age-at-onset3, 6. While the age-specific frequencies of carriers are similar to other reported series6, the high percentage of older cases in this study may explain why we report a frequency at the lower end of the spectrum.
Limitations
There are three important limitations in this study. Most importantly, the number of relatives of probands who are estimated to carry Parkin mutations is limited, which results in cumulative risk estimates with wide confidence intervals. We have limited our study to only those who participated in the GEPD study for whom we have accurate information on vital status, a necessary criterion for the kin-cohort method. In addition, only 105 controls were completely sequenced for the Parkin gene, which may explain our inability to detect differences between relatives of PD probands and relatives of control probands. The sample size required for a typical kin-cohort study is usually large, due to the low population prevalence of the mutation being studied. Second, the diagnosis in the relatives was a best estimate diagnosis. We have previously reported high sensitivity (95.5%) and specificity (96.2%) of our family history questionnaire based on examination of 141 relatives. Third, we did not have actual genotype data on relatives. We used the kin-cohort method to estimate penetrance in the absence of these data.
Some relatives may have been too young to manifest PD. Our sampling plan was to include all probands with AAO ≤50 regardless of their family history of PD and AAO in the relatives. The relatives of these probands are likely to be younger than the relatives of randomly selected PD patients. In a study of affected sibling pairs, the mean AAO of heterozygous Parkin carriers was reported to be 49.6.10 Forty-nine percent of the relatives in our sample were younger than 49.6 (Table 4). Our estimates of the prevalence of a history of PD in the relatives do not account for this young age distribution, and thus underestimate prevalence of PD according to Parkin genotype in the general population. We accounted for the younger age distribution by computing age-specific cumulative incidence of PD according to genotype.
One assumption required for the kin-cohort analyses to be valid is that risk of PD in relatives within the same family is independent, given the proband's Parkin genotype. The presence of other genetic and environmental risk factors that aggregate in the families may violate this assumption and bias the penetrance estimation15, 27. Due to the possibility of additional unspecified familial risk factors in the relatives of early-onset PD probands, penetrance estimates obtained from our sample can be applied to the population of relatives of the early-onset PD probands but not to the entire population.
We are currently recruiting a larger multi-center sample of early-onset cases and examining and obtaining DNA in relatives of carrier probands. We hope to use this new sample to refine our estimates of penetrance in heterozygous, homozygous, and compound heterozygous carriers.
Acknowledgements
This study was funded by NIH NS36630, RR00645 (KM) and the Parkinson's Disease Foundation (KM and LNC). The authors thank Dr. Paul Greene for his participation.
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