Segregation analysis of Parkinson disease revealing evidence for a major causative gene

Plasticity-related gene 3 (LPPR1) and age at diagnosis of Parkinson disease

Objective

To identify modifiers of age at diagnosis of Parkinson disease (PD).

Methods

Genome-wide association study (GWAS) included 1,950 individuals with PD from the NeuroGenetics Research Consortium (NGRC) study. Replication was conducted in the Parkinson's, Genes and Environment study, including 209 prevalent (PAGE_P) and 517 incident (PAGE_I) PD cases. Cox regression was used to test association with age at diagnosis. Individuals without neurologic disease were used to rule out confounding. Gene-level analysis and functional annotation were conducted using Functional Mapping and Annotation of GWAS platform (FUMA).

Results

The GWAS revealed 2 linked but seemingly independent association signals that mapped to LPPR1 on chromosome 9. LPPR1 was significant in gene-based analysis (p = 1E-8). The top signal (rs17763929, hazard ratio [HR] = 1.88, p = 5E-8) replicated in PAGE_P (HR = 1.87, p = 0.01) but not in PAGE_I. The second signal (rs73656147) was robust with no evidence of heterogeneity (HR = 1.95, p = 3E-6 in NGRC; HR = 2.14, p = 1E-3 in PAGE_P + PAGE_I, and HR = 2.00, p = 9E-9 in meta-analysis of NGRC + PAGE_P + PAGE_I). The associations were with age at diagnosis, not confounded by age in patients or in the general population. The PD-associated regions included variants with Combined Annotation Dependent Depletion (CADD) scores = 10–19 (top 1%–10% most deleterious mutations in the genome), a missense with predicted destabilizing effect on LPPR1, an expression quantitative trait locus (eQTL) for GRIN3A (false discovery rate [FDR] = 4E-4), and variants that overlap with enhancers in LPPR1 and interact with promoters of LPPR1 and 9 other brain-expressed genes (Hi-C FDR < 1E-6).

Conclusions

Through association with age at diagnosis, we uncovered LPPR1 as a modifier gene for PD. LPPR1 expression promotes neuronal regeneration after injury in animal models. Present data provide a strong foundation for mechanistic studies to test LPPR1 as a driver of response to damage and a therapeutic target for enhancing neuroregeneration and slowing disease progression.

S100B polymorphisms are associated with age of onset of Parkinson's disease

Background

In this study we investigated the association between SNPs in the S100B gene and Parkinson’s disease (PD) in two independent Swedish cohorts. The SNP rs9722 has previously been shown to be associated with higher S100B concentrations in serum and frontal cortex in humans. S100B is widely expressed in the central nervous system and has many functions such as regulating calcium homeostasis, inflammatory processes, cytoskeleton assembly/disassembly, protein phosphorylation and degradation, and cell proliferation and differentiation. Several of these functions have been suggested to be of importance for the pathophysiology of PD.

Methods

The SNPs rs9722, rs2239574, rs881827, rs9984765, and rs1051169 of the S100B gene were genotyped using the KASPar® PCR SNP genotyping system in a case-control study of two populations (431 PD patients and 465 controls, 195 PD patients and 378 controls, respectively). The association between the genotype and allelic distributions and PD risk was evaluated using Chi-Square and Cox proportional hazards test, as well as logistic regression. Linear regression and Cox proportional hazards tests were applied to assess the effect of the rs9722 genotypes on age of disease onset.

Results

The S100B SNPs tested were not associated with the risk of PD. However, in both cohorts, the T allele of rs9722 was significantly more common in early onset PD patients compared to late onset PD patients. The SNP rs9722 was significantly related to age of onset, and each T allele lowered disease onset with 4.9 years. In addition, allelic variants of rs881827, rs9984765, and rs1051169, were significantly more common in early-onset PD compared to late-onset PD in the pooled population.

Conclusions

rs9722, a functional SNP in the 3’-UTR of the S100B gene, was strongly associated with age of onset of PD.

The M458L missense mutation disrupts the catalytic properties of Parkin

Parkin encodes an E3 ubiquitin ligase, and mutations affecting its catalytic potential are implicated in autosomal recessive Parkinson's disease (PD). The M458L mutation of parkin and its enzymatic effects require characterization. Therefore, we examined the enzymatic activity of Parkin with M458L mutation. We show that the M458L mutant retains its autoubiquitination potential in vitro but not in cells. Fas-associated factor 1 and p38 (substrates of Parkin) are able to bind to the M458L mutant in cells; however, these Parkin substrates are not ubiquitinated and degraded in M458L mutant-transfected cells. Moreover, M458L mutant fails to protect the mitochondria against hydrogen peroxide_, leading to cell death. Considering the role of mitochondrial dysfunction in PD pathogenesis, our results imply a causative role for the M458L mutation in neurodegeneration.

Identification of genetic modifiers of age-at-onset for familial Parkinson's disease

Parkinson’s disease (PD) is the most common cause of neurodegenerative movement disorder and the second most common cause of dementia. Genes are thought to have a stronger effect on age-at-onset of PD than on risk, yet there has been a phenomenal success in identifying risk loci but not age-at-onset modifiers. We conducted a genome-wide study for age-at-onset. We analysed familial and non-familial PD separately, per prior evidence for strong genetic effect on age-at-onset in familial PD. GWAS was conducted in 431 unrelated PD individuals with at least one affected relative (familial PD) and 1544 non-familial PD from the NeuroGenetics Research Consortium (NGRC); an additional 737 familial PD and 2363 non-familial PD were used for replication. In familial PD, two signals were detected and replicated robustly: one mapped to LHFPL2 on 5q14.1 (P_NGRC=3E-8, P_Replication=2E-5, P_{NGRC + Replication}=1E-11), the second mapped to TPM1 on 15q22.2 (P_NGRC=8E-9, P_Replication=2E-4, P_{NGRC + Replication}=9E-11). The variants that were associated with accelerated onset had low frequencies (<0.02). The LHFPL2 variant was associated with earlier onset by 12.33 [95% CI: 6.2; 18.45] years in NGRC, 8.03 [2.95; 13.11] years in replication, and 9.79 [5.88; 13.70] years in the combined data. The TPM1 variant was associated with earlier onset by 15.30 [8.10; 22.49] years in NGRC, 9.29 [1.79; 16.79] years in replication, and 12.42 [7.23; 17.61] years in the combined data. Neither LHFPL2 nor TPM1 was associated with age-at-onset in non-familial PD. LHFPL2 (function unknown) is overexpressed in brain tumours. TPM1 encodes a highly conserved protein that regulates muscle contraction, and is a tumour-suppressor gene.

Polygenic risk of Parkinson disease is correlated with disease age at onset

Objective

We have investigated the polygenic architecture of Parkinson disease (PD) and have also explored the potential relationship between an individual's polygenic risk score and their disease age at onset.

Methods

This study used genotypic data from 4,294 cases and 10,340 controls obtained from the meta‐analysis of PD genome‐wide association studies. Polygenic score analysis was performed as previously described by the International Schizophrenia Consortium, testing whether the polygenic score alleles identified in 1 association study were significantly enriched in the cases relative to the controls of 3 independent studies. Linear regression was used to investigate the relationship between an individual's polygenic score for PD risk alleles and disease age at onset.

Results

Our polygenic score analysis has identified significant evidence for a polygenic component enriched in the cases of each of 3 independent PD genome‐wide association cohorts (minimum p = 3.76 × 10⁻⁶). Further analysis identified compelling evidence that the average polygenic score in patients with an early disease age at onset was significantly higher than in those with a late age at onset (p = 0.00014).

Interpretation

This provides strong support for a large polygenic contribution to the overall heritable risk of PD and also suggests that early onset forms of the illness are not exclusively caused by highly penetrant Mendelian mutations, but can also be contributed to by an accumulation of common polygenic alleles with relatively low effect sizes. Ann Neurol 2015;77:582–591

Cyclin-G-associated kinase modifies α-synuclein expression levels and toxicity in Parkinson's disease: results from the GenePD Study

Although family history is a well-established risk factor for Parkinson's disease (PD), fewer than 5% of PD cases can be attributed to known genetic mutations. The etiology for the remainder of PD cases is unclear; however, neuronal accumulation of the protein α-synuclein is common to nearly all patients, implicating pathways that influence α-synuclein in PD pathogenesis. We report a genome-wide significant association (P = 3.97 × 10(-8)) between a polymorphism, rs1564282, in the cyclin-G-associated kinase (GAK) gene and increased PD risk, with a meta-analysis odds ratio of 1.48. This association result is based on the meta-analysis of three publicly available PD case-control genome-wide association study and genotyping from a new, independent Italian cohort. Microarray expression analysis of post-mortem frontal cortex from PD and control brains demonstrates a significant association between rs1564282 and higher α-synuclein expression, a known cause of early onset PD. Functional knockdown of GAK in cell culture causes a significant increase in toxicity when α-synuclein is over-expressed. Furthermore, knockdown of GAK in rat primary neurons expressing the A53T mutation of α-synuclein, a well-established model for PD, decreases cell viability. These observations provide evidence that GAK is associated with PD risk and suggest that GAK and α-synuclein interact in a pathway involved in PD pathogenesis. The GAK protein, a serine/threonine kinase, belongs to a family of proteins commonly targeted for drug development. This, combined with GAK's observed relationship to the levels of α-synuclein expression and toxicity, suggests that the protein is an attractive therapeutic target for the treatment of PD.

Maternal inheritance and mitochondrial DNA variants in familial Parkinson's disease

Background

Mitochondrial function is impaired in Parkinson's disease (PD) and may contribute to the pathogenesis of PD, but the causes of mitochondrial impairment in PD are unknown. Mitochondrial dysfunction is recapitulated in cell lines expressing mitochondrial DNA (mtDNA) from PD patients, implicating mtDNA variants or mutations, though the role of mtDNA variants or mutations in PD risk remains unclear. We investigated the potential contribution of mtDNA variants or mutations to the risk of PD.

Methods

We examined the possibility of a maternal inheritance bias as well as the association between mitochondrial haplogroups and maternal inheritance and disease risk in a case-control study of 168 multiplex PD families in which the proband and one parent were diagnosed with PD. 2-tailed Fisher Exact Tests and McNemar's tests were used to compare allele frequencies, and a t-test to compare ages of onset.

Results

The frequency of affected mothers of the proband with PD (83/167, 49.4%) was not significantly different from the frequency of affected females of the proband generation (115/259, 44.4%) (Odds Ratio 1.22; 95%CI 0.83 - 1.81). After correcting for multiple tests, there were no significant differences in the frequencies of mitochondrial haplogroups or of the 10398G complex I gene polymorphism in PD patients compared to controls, and no significant associations with age of onset of PD. Mitochondrial haplogroup and 10398G polymorphism frequencies were similar in probands having an affected father as compared to probands having an affected mother.

Conclusions

These data fail to demonstrate a bias towards maternal inheritance in familial PD. Consistent with this, we find no association of common haplogroup-defining mtDNA variants or for the 10398G variant with the risk of PD. However, these data do not exclude a role for mtDNA variants in other populations, and it remains possible that other inherited mitochondrial DNA variants, or somatic mDNA mutations, contribute to the risk of familial PD.

Genomewide association study for onset age in Parkinson disease

Background

Age at onset in Parkinson disease (PD) is a highly heritable quantitative trait for which a significant genetic influence is supported by multiple segregation analyses. Because genes associated with onset age may represent invaluable therapeutic targets to delay the disease, we sought to identify such genetic modifiers using a genomewide association study in familial PD. There have been previous genomewide association studies (GWAS) to identify genes influencing PD susceptibility, but this is the first to identify genes contributing to the variation in onset age.

Methods

Initial analyses were performed using genotypes generated with the Illumina HumanCNV370Duo array in a sample of 857 unrelated, familial PD cases. Subsequently, a meta-analysis of imputed SNPs was performed combining the familial PD data with that from a previous GWAS of 440 idiopathic PD cases. The SNPs from the meta-analysis with the lowest p-values and consistency in the direction of effect for onset age were then genotyped in a replication sample of 747 idiopathic PD cases from the Parkinson Institute Biobank of Milan, Italy.

Results

Meta-analysis across the three studies detected consistent association (p < 1 × 10^-5) with five SNPs, none of which reached genomewide significance. On chromosome 11, the SNP with the lowest p-value (rs10767971; p = 5.4 × 10^-7) lies between the genes QSER1 and PRRG4. Near the PARK3 linkage region on chromosome 2p13, association was observed with a SNP (rs7577851; p = 8.7 × 10^-6) which lies in an intron of the AAK1 gene. This gene is closely related to GAK, identified as a possible PD susceptibility gene in the GWAS of the familial PD cases.

Conclusion

Taken together, these results suggest an influence of genes involved in endocytosis and lysosomal sorting in PD pathogenesis.

Advances in the genetics of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting a significant proportion of the ageing population. The etiology is unknown and it is likely due to a multifactorial interaction of genes and the environment on the background of ageing. Findings in the last decade suggest that the contribution of genetics to familial forms of PD is much greater than previously appreciated. Twelve loci are now associated with highly penetrant autosomal dominant or recessive PD, and causative mutations have been identified in eight genes with mutation carriers often characterized by a phenotype indistinguishable from idiopathic disease. To date, PD pharmacotherapy is symptomatic only and does not slow disease progression. Understanding how genetic mutations cause familial PD is likely to clarify molecular mechanisms underlying PD in general and will provide a guide for the development of novel therapies, both preventative and palliative, applicable to all forms of parkinsonism. This review outlines the advances in the study of the genetic background of PD and their possible clinical implications.

Complex segregation analysis of Parkinson's disease: The Mayo Clinic Family Study

OBJECTIVE

To conduct complex segregation analyses of Parkinson's disease (PD).

METHODS

Data on the familial aggregation of PD remain conflicting. We conducted a historical cohort study of 1,234 relatives of 162 patients with PD representative of people of Olmsted County, MN, and of 3,009 relatives of 411 patients with PD referred to the Mayo Clinic. Relatives were interviewed and screened for parkinsonism either directly or through a proxy, those who screened positive were examined, or a copy of their medical record was obtained to confirm the diagnosis. For subjects who resided in Olmsted County, additional information was obtained from the archives of the Olmsted County Historical Society and from a records-linkage system.

RESULTS

Thirty-two relatives of population-based probands and 69 relatives of referral patients developed PD (101 in total). Combining population-based and referral samples, the model that best explained the familial clustering of PD overall was a major gene with additive effect on the penetrance. This model predicted an average decrease in age at onset of PD of approximately 18 years for each copy of the putative high-risk allele. The best fitting model for younger onset PD (age <or= 59 years) was an autosomal recessive model. The best fitting models for older onset PD (age > 59 years) were a recessive or an additive model.

INTERPRETATION

The familial aggregation of PD may be explained in part by a major gene with additive effect on the penetrance.

Pathophysiology, pleiotrophy and paradigm shifts: genetic lessons from Parkinson's disease

PD (Parkinson's disease) is an aetiologically heterogeneous disorder characterized by a clinical phenotype consisting of resting tremor, rigidity and bradykinesia. Motor symptoms are associated with a progressive loss of dopaminergic neurons, with Lewy body inclusions within surviving neurons. Although heritability studies have shown evidence of familial aggregation, twin studies have provided limited support for a genetic aetiology. Nevertheless, classical linkage methods have nominated 11 regions of the genome and pathogenic mutations have been identified in several genes, including alpha-synuclein, parkin, ubiquitin C-terminal hydrolase L1, oncogene DJ-1, PTEN-induced protein kinase 1 and microtubule-associated protein tau. Most recently, heterozygous mutations in LRRK2 (leucine-rich repeat kinase 2) were found to cause late-onset, autosomal-dominant PD. Despite their consistent clinical phenotype, family members with LRRK2 mutations can have variable alpha-synuclein and tau pathologies. Lrrk2 is a member of the Roc (Ras of complex proteins) family, with Ras GTPase and MAPKKK (mitogen-activated protein kinase kinase kinase) catalytic domains. Thus its discovery highlights vesicle dynamics and secondary-messenger signalling in disease pathophysiology. To diagnose a disease accurately and effectively treat it, requires an understanding of its molecular pathogenesis. Herein, we provide an overview of the genetics of PD, how these discoveries are revolutionizing long-held beliefs and more importantly how this knowledge may be translated into patient therapy.

Increased intracranial volume in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) and multiple system atrophy (MSA) are neurodegenerative diseases that can be difficult to diagnose and distinguish from each other. STUDY AIMS AND METHODS: Patients with PD and MSA and controls were studied with magnetic resonance imaging (MRI) using tissue segmentation and outlining of regions in order to identify regional volume changes that might be useful in the diagnosis of the two diseases.

RESULTS

Patients with PD had significantly larger intracranial volumes (ICVs) and significantly smaller putaminal and sustantia nigra volumes than controls. MSA patients had significantly smaller substantia nigra and caudate volumes than controls but normal intracranial volume. In both patient groups there was a further trend towards smaller amygdala volumes.

DISCUSSION

Increased ICV in PD patients is a new finding that may be explained by genetic factors or compensatory responses to early CNS damage. Atrophy of the amygdala in MSA patients has not been demonstrated with MR before. It might explain why these patients can have hyposmia. The putaminal atrophy found in the PD group may be a trait of the later stages of PD. Segmentation of the substantia nigra can be a useful aid in the diagnosis of PD and MSA.

Familial-associated mutations differentially disrupt the solubility, localization, binding and ubiquitination properties of parkin

Mutations in parkin are largely associated with autosomal recessive juvenile parkinsonism. The underlying mechanism of pathogenesis in parkin-associated Parkinson's disease (PD) is thought to be due to the loss of parkin's E3 ubiquitin ligase activity. A subset of missense and nonsense point mutations in parkin that span the entire gene and represent the numerous inheritance patterns that are associated with parkin-linked PD were investigated for their E3 ligase activity, localization and their ability to bind, ubiquitinate and effect the degradation of two substrates, synphilin-1 and aminoacyl-tRNA synthetase complex cofactor, p38. Parkin mutants vary in their intracellular localization, binding to substrates and enzymatic activity, yet they are ultimately deficient in their ability to degrade substrate. These results suggest that not all parkin mutations result in loss of parkin's E3 ligase activity, but they all appear to manifest as loss-of-function mutants due to defects in solubility, aggregation, enzymatic activity or targeting proteins to the proteasome for degradation.

Differences in age at onset and familial aggregation between clinical types of idiopathic Parkinson's disease

Idiopathic Parkinson's disease (PD) can be subdivided by its patterns of motor symptoms into tremor-dominant (TDT), akinetic-rigid (ART), and mixed type (MT). Our objective was to determine whether age at onset and family history are different in these three types. In total, 366 patients with PD were assigned in a standardized approach to one of the three subtypes. Age at onset and family history were obtained in all patients and all presumably affected family members were examined. Mean ages at disease onset were similar in all three groups, but distribution of age at onset was markedly different: monophasic in TDT with a peak around 60 years, biphasic in ART with two peaks, one in the middle of the sixth decade (earlier onset, ART-EO), another during the first half of the seventh decade (later onset, ART-LO), and increasing with age only in MT patients A positive family history was significantly associated only with TDT (odds ratio = 5.7) and ART-EO (odds ratio = 7.8), but not with MT or ART-LO patients. Segregation analysis suggested an autosomal recessive mode of transmission in ART-EO and an autosomal dominant mode of transmission in TDT.

Genetics of parkin-linked disease

Research into Parkinson's disease (PD), once considered the archetypical non-genetic neurodegenerative disorder, has been revolutionized by the identification of a number of genes, mutations of which underlie various familial forms of the disease. Whereas such mutations appear to exist in a relatively small number of individuals from a few families, the study of the function of these genes promises to reveal the fundamental disease pathogenesis, not only of familial forms of the disease, but also of the much more common sporadic PD. The observation that mutations in the second identified PD locus (parkin) are common in juvenile- and early-onset PD and increasing evidence supporting a direct role for parkin in late-onset disease make this gene a particularly compelling candidate for intensified investigation. The determination of the frequency and effect of parkin mutations in various subsets of PD will be crucial for understanding the way in which parkin is related to neurodegenerative mechanisms, and whether these subsets might be effectively identified and treated. In addition, many aspects of parkin-linked disease, originally thought to be well defined, have now been obscured both by genetic studies that preclude a simple model of disease transmission and by clinical and pathological studies that demonstrate broad variability in cases with parkin mutations. Future studies that address the issues in question should have a far-reaching impact in downstream biochemical studies and our understanding of parkin's role in PD.

Suggestive linkage to chromosome 19 in a large Cuban family with late-onset Parkinson's disease

The identification of disease genes using family-based approaches has provided important insights into the pathogenesis of Parkinson's disease (PD) demonstrating the importance of genetic studies on monogenic forms of the disease. We studied a large Cuban family with typical, late-onset PD and probable autosomal dominant inheritance. Mean age at onset was 61.2 years (+/- 12.53, 45-76). Other phenotypes such as essential tremor and atypical parkinsonism were observed in this family. We carried out a genome-wide scan and linkage analyses. The genetic data were analyzed using a conservative model in which only patients with clinically definite or likely PD were considered affected, other phenotypes were regarded as "unknown." Multipoint analyses yielded a maximum LOD of 2.26 between markers D19S221 and D19S840. Haplotype analysis showed a region on chromosome 19 shared by six of seven PD patients. The essential tremor phenotype and the atypical parkinsonism do not segregate with this haplotype, suggesting a different etiology. Our findings suggest the presence of a novel locus for PD on chromosome 19p13.3-q12. We propose that an oligogenic model with moderate contribution of two or three genes rather than a "pure" monogenic model might explain better the wide range in age at onset, the reduced penetrance and the phenotypical variability observed in PD families.

Identification of an autosomal recessive mode of inheritance in paediatric Behçet's families by segregation analysis

We have conducted a segregation analysis in order to characterise the transmission of Behçet Disease (BD), a multifactorial condition with a strong genetic component. Complete information about BD status and pedigree was obtained on 104 probands from our database. We used the criteria of the International Study Group for BD (ISBD) to delineate the clinical status of the sibs: possible BD (patients meeting two criteria), or ascertained BD (patients meeting at least three criteria). A proband was defined as "paediatric" when he/she completed ISBD criteria before/by the age of 16 years. Families were distinguished as paediatric (n = 67) (ascertained through a paediatric proband), and non-paediatric (n = 37) ones. An Expectation Maximization (EM) algorithm was used to estimate the Mendelian segregation ratio P in nuclear families (two parents and their offspring). The maximum likelihood estimate: Pcirc; = 0.248, calculated in the paediatric data set, was consistent with the theoretical value of P = (1/4) for autosomal recessive inheritance, whereas the Pcirc; value was 0.08 when using the non-paediatric data set. Our work provides the first evidence of genetic heterogeneity in BD, and of the existence of a Mendelian entity in the paediatric BD subgroup. Previous studies failed to show any simple mode of inheritance in BD, probably because they were performed on the whole BD population.

Neurodegenerative disorders of protein aggregation

In recent years, it has become increasingly clear that many neurodegenerative diseases involve aggregation and deposition of misfolded proteins such as amyloid beta, tau, alpha-synuclein and polyglutamine containing proteins. This abnormal deposition of misfolded proteins produce malfunctioning of a distinctive set of neurons. It may also induce oxidative and endoplasmic reticulum stress and proteosomal and mitochondrial dysfunction that ultimately leads to neuronal death. While hereditary forms of disorders are caused by genetic mutations, many sporadic cases are likely to be due to genetic and environmental factors. These disorders are progressive in nature. Therefore, treatment is difficult. However, for some diseases, a growing number of treatment options such as drugs, antioxidants, cell transplantation, surgery, rehabilitation procedures and preimplantation diagnosis is available. It should be noted that many of these treatments produce unacceptable risks or adverse effects and they are of only minimal benefit for patients. In future, an understanding of the causes of protein aggregation and genetic and environmental susceptibility factors of a specific individual (or specific individual determinants) may provide a better opportunity for an effective therapeutic intervention.

PARK3 influences age at onset in Parkinson disease: a genome scan in the GenePD study

Parkinson disease (PD) is a late-onset neurodegenerative disorder. The mean age at onset is 61 years, but the disease can range from juvenile cases to cases in the 8th or 9th decade of life. The parkin gene on chromosome 6q and loci on chromosome 1p35-36 and 1p36 are responsible for some cases of autosomal recessive early-onset parkinsonism, but they do not appear to influence susceptibility or variability of age at onset for idiopathic PD. We have performed a genomewide linkage analysis using variance-component methodology to identify genes influencing age at onset of PD in a population of affected relatives (mainly affected sibling pairs) participating in the GenePD study. Four chromosomal loci showed suggestive evidence of linkage: chromosome 2p (maximum multipoint LOD [MaxLOD] = 2.08), chromosome 9q (MaxLOD = 2.00), chromosome 20 (MaxLOD = 1.82), and chromosome 21 (MaxLOD = 2.21). The 2p and 9q locations that we report here have previously been reported as loci influencing PD affection status. Association between PD age at onset and allele 174 of marker D2S1394, located on 2p13, was observed in the GenePD sample (P=.02). This 174 allele is common to the PD haplotype observed in two families that show linkage to PARK3 and have autosomal dominant PD, which suggests that this allele may be in linkage disequilibrium with a mutation influencing PD susceptibility or age at onset of PD.