Parkin-proven disease: common founders but divergent phenotypes

Neuropsychological Profile of Parkin Mutation Carriers with and without Parkinson Disease: The CORE-PD Study

The cognitive profile of early onset Parkinson's disease (EOPD) has not been clearly defined. Mutations in the parkin gene are the most common genetic risk factor for EOPD and may offer information about the neuropsychological pattern of performance in both symptomatic and asymptomatic mutation carriers. EOPD probands and their first-degree relatives who did not have Parkinson's disease (PD) were genotyped for mutations in the parkin gene and administered a comprehensive neuropsychological battery. Performance was compared between EOPD probands with (N = 43) and without (N = 52) parkin mutations. The same neuropsychological battery was administered to 217 first-degree relatives to assess neuropsychological function in individuals who carry parkin mutations but do not have PD. No significant differences in neuropsychological test performance were found between parkin carrier and noncarrier probands. Performance also did not differ between EOPD noncarriers and carrier subgroups (i.e., heterozygotes, compound heterozygotes/homozygotes). Similarly, no differences were found among unaffected family members across genotypes. Mean neuropsychological test performance was within normal range in all probands and relatives. Carriers of parkin mutations, whether or not they have PD, do not perform differently on neuropsychological measures as compared to noncarriers. The cognitive functioning of parkin carriers over time warrants further study.

Genetic analysis of Parkin in early onset Parkinson's disease (PD): Novel intron 9 g > a single nucleotide polymorphism and risk of Taiwanese PD

Early onset Parkinson's disease (PD) has been associated with mutations in Parkin. We screened Parkin mutations in a cohort of Taiwanese early onset PD using direct cDNA sequencing. Two deletions (Ex2-3del and Ex5del), one point mutation (R334C), one 86-bp IVS9 insertion (c.1084intron(+)), and two polymorphisms (S167N and V380L) were identified. The mutations identified are heterozygous and none of the mutation carriers possess two Parkin mutations. The c.1084intron(+) was due to a novel IVS9 g > a change. To assess the association of IVS9 g > a, S167N and V380L with the risk of PD, we conducted a case-control study in a cohort of PD and ethnically matched controls. Although the difference is not significant, the V380L C allele frequency was notably lower in PD patients than the controls and a trend toward decrease in risk of developing PD was evident (odds ratio: 0.71, 95% confidence interval: 0.53-0.97, P = 0.029). Contrarily the IVS9 g > a a allele frequency was notably higher in PD patients than the controls and a trend toward increase in risk of developing PD was also evident (odds ratio: 1.65, 95% confidence interval: 1.06-2.59, P = 0.028). Quantitative real-time PCR showed that the relative Parkin c.1084intron(+) mRNA expression was increased in PD patients with IVS9 ga genotype as compared to gg genotype. Pairwise genotype analysis revealed that IVS9 gg genotype strengthens the negative association of the V380L GC genotype with PD (odds ratio: 0.67, 95% confidence interval: 0.48-0.94, P = 0.021). The results of Parkin mutation/polymorphism screening may contribute to our understanding of PD.

Changes in disease gene frequency over time with differential genotypic fitness and various control strategies

A spreadsheet model was constructed to describe the change in allelic frequency over time for a lethal recessive mutation in an animal population. The model allowed relative fitness to differ between genotypes, between sexes, and over time. Whereas a lethal recessive allele is naturally eliminated very slowly from a population, a small selective disadvantage of the heterozygote results in a large increase in the rate of elimination. With selective advantage of the heterozygote through linkage with a production trait or pleiotropy, the allele is never naturally eliminated but tends toward a stable equilibrium frequency. The model was used to investigate various alternative control programs based on the detection of heterozygotes by genotyping and their exclusion from breeding. The programs (genotyping males only, genotyping males and 50% of females, and genotyping all breeding animals) were modeled for various initial heterozygote frequencies, and the results were described in terms of the number of generations, number of tests, and number of culls required to reduce the heterozygote frequency to a predefined level. The model can be used to compare the feasibility and cost of various control strategies and to illustrate clearly to breeders the expected outcomes, as well as the danger of prematurely terminating a control program when there is a selective advantage of the heterozygote.

Biochemical analysis of Parkinson's disease-causing variants of Parkin, an E3 ubiquitin–protein ligase with monoubiquitylation capacity

Mutations in the parkin gene, encoding an E3 ubiquitin-protein ligase, are a frequent cause of autosomal recessive parkinsonism and are also involved in sporadic Parkinson's disease. Loss of Parkin function is thought to compromise the polyubiquitylation and proteasomal degradation of specific substrates, leading to their deleterious accumulation. Several studies have analyzed the effects of parkin gene mutations on the biochemical properties of the protein. However, the absence of a cell-free system for studying intrinsic Parkin activity has limited the interpretation of these studies. Here we describe the biochemical characterization of Parkin and 10 pathogenic variants carrying amino-acid substitutions throughout the sequence. Mutations in the RING fingers or the ubiquitin-like domain decreased the solubility of the protein in detergent and increased its tendency to form visible aggregates. None of the mutations studied compromised the binding of Parkin to a series of known protein partners/substrates. Moreover, only two variants with substitutions of conserved cysteine residues of the second RING finger were inactive in a purely in vitro ubiquitylation assay, demonstrating that loss of ligase activity is a minor pathogenic mechanism. Interestingly, in this in vitro assay, Parkin catalyzed the linkage of single ubiquitin molecules only, whereas the ubiquitin-protein ligases CHIP and Mdm2 promoted the formation of polyubiquitin chains. Similarly, in mammalian cells Parkin promoted the multimonoubiquitylation of its substrate p38, rather than its polyubiquitylation. Thus, Parkin may mediate polyubiquitylation or proteasome-independent monoubiquitylation depending on the protein context. The discovery of monoubiquitylated Parkin species in cells hints at a novel post-translational modification potentially involved in the regulation of Parkin function.

Parkinson's disease: a rethink of rodent models

Parkinson's disease (PD) is a multifactorial disease with a complex etiology that results from genetic risk factors, environmental exposures and most likely a combination of both. Rodent models of parkinsonism aim to reproduce key pathogenic features of the syndrome including movement disorder induced by the progressive loss of dopaminergic neurons in the substantia nigra, accompanied by the formation of alpha-synuclein containing Lewy body inclusions. Despite the creation of many excellent models, both chemically induced and genetically engineered, there is none that accurately demonstrates these features. Recent pathological staging studies in man have also emphasized the significant non-CNS component of PD that has yet to be tackled. Herein, we summarize rodent models of PD and what they offer to the field, and suggest future challenges and opportunities.

Distribution, type, and origin of Parkin mutations: review and case studies

Early-onset Parkinson's disease (PD) has been associated with different mutations in the Parkin gene (PARK2). To study distribution and type of Parkin mutations, we carried out a comprehensive literature review that demonstrated two prominent types of mutations among 379 unrelated mutation carriers: exon rearrangements involving exon 3, 4, or both, and alterations in exons 2 and 7, suggesting mutational hot spots or founders. To elucidate the origin of 14 recurrent Parkin mutations in our samples, we carried out a detailed haplotype analysis at the PARK2 locus. Thirty-eight mutation-positive individuals, available family members, and 62 mutation-negative individuals were genotyped. We determined allele frequencies and linkage disequilibrium (LD) to evaluate the significance of shared haplotypes. We observed no LD between markers at PARK2. Our data support a common founder for the most frequent Parkin point mutation (924C>T; exon 7) and indicate a mutational hot spot as cause of a common small deletion (255/256delA; exon 2). Furthermore, the most frequent Parkin exon deletion (Ex4del) arose independently in 2 of our subjects. However, it also occurred as the result of a founder mutation in 2 cases that shared identical deletion break points. This study provides evidence for both mutational hot spots and founder mutations as a source of recurrent mutations in Parkin, regardless of the mutation type.

Homozygous partial genomic triplication of the parkin gene in early-onset parkinsonism

Autosomal recessive mutations in the parkin gene are the predominant cause of familial, early-onset parkinsonism; missense mutations involving one or a few nucleotides, exonic deletions and duplications have been described. Here we report a family with two affected brothers. Direct sequencing of parkin did not detect mutations, but semi-quantitative analysis identified a novel exonic rearrangement of exons 2-4. Both patients were homozygous for unique genomic triplications of the parkin gene.

Parkinson's disease in Ireland: clinical presentation and genetic heterogeneity in patients with parkin mutations

Early-onset autosomal recessive parkinsonism is associated with parkin gene mutations. Different parkin mutations occur in many ethnic backgrounds; however, the phenotype may vary. We studied 102 young-onset (age at onset <60 years) Parkinson's disease (PD) patients. From 102 patients, 40 with early-onset PD (<45 years at symptomatic onset) were selected for clinical assessment and parkin gene molecular analysis for duplications/deletions and point mutations. We identified parkin mutations in 7 of 40 early-onset patients; including novel compound heterozygotes and potential splice site changes. The mean age at onset in the 7 parkin mutation-positive patients was 33 +/- 9 years (age range, 18-42 years), marginally lower than that of the 33 parkin-negative early-onset patients, 38 +/- 7 years (age range, 17-45 years). A family history of PD was present in 4 of 7 patients with parkin mutations, compared with 6 of 33 early-onset parkin-negative patients. Overall, parkin mutations were found in 4 of 10 patients with a positive family history and 3 of 30 patients without a family history of PD. Patients with parkin mutations had more dystonia, dyskinesia, and sleep benefit compared with parkin-negative patients. We subsequently identified a single point mutation among the 62 young-onset (age at onset 45 to <60 years). Mutations in the parkin gene may account for approximately 17% of early-onset (age at onset <45 years) parkinsonism in Ireland, in agreement with previous European studies.

Parkinson's disease: mechanisms and models

Parkinson's disease (PD) results primarily from the death of dopaminergic neurons in the substantia nigra. Current PD medications treat symptoms; none halt or retard dopaminergic neuron degeneration. The main obstacle to developing neuroprotective therapies is a limited understanding of the key molecular events that provoke neurodegeneration. The discovery of PD genes has led to the hypothesis that misfolding of proteins and dysfunction of the ubiquitin-proteasome pathway are pivotal to PD pathogenesis. Previously implicated culprits in PD neurodegeneration, mitochondrial dysfunction and oxidative stress, may also act in part by causing the accumulation of misfolded proteins, in addition to producing other deleterious events in dopaminergic neurons. Neurotoxin-based models (particularly MPTP) have been important in elucidating the molecular cascade of cell death in dopaminergic neurons. PD models based on the manipulation of PD genes should prove valuable in elucidating important aspects of the disease, such as selective vulnerability of substantia nigra dopaminergic neurons to the degenerative process.