Analysis of alpha-synuclein, parkin, tau, and UCH-L1 in a Japanese family with autosomal dominant parkinsonism

Molecular genetics of Parkinson's disease: Contributions and global trends

Parkinson's disease (PD) is a neurodegenerative disorder primarily characterized by motor dysfunction. Aging is the greatest risk factor for developing PD. Recent molecular genetic studies have revealed that genetic factors, in addition to aging and environmental factors, play an important role in the development of the disorder. Studies of familial PD have identified approximately 20 different causative genes. PRKN is the most frequently detected causative gene in Japan. The PRKN gene is located at a common fragile site, and both copy number variants as well as single nucleotide variants are frequently detected. The location and variety of variant types makes an accurate genetic diagnosis difficult with conventional genetic testing. In sporadic PD, genome-wide association studies have revealed more than 200 genes that are potential drivers for the development of PD. Many of these studies have been conducted in Caucasian populations alone, which has limited the identification of all genetic risk factors for sporadic PD, particularly as genetic backgrounds vary widely by race. The Global Parkinson's Genetics Program is a global undertaking meant to address the issue of regional differences in genetic studies of PD.

Familial parkinsonism: study of original Sagamihara PARK8 (I2020T) kindred with variable clinicopathologic outcomes

BACKGROUND

Since the causative gene linked to PARK8 parkinsonism was identified as LRRK2, LRRK2 gene mutations have been found to occur in about 4% of patients with hereditary Parkinson disease (PD); this percentage is even higher in certain populations. Moreover, no clear clinical differences between PARK8-linked parkinsonism and sporadic PD have been identified. Neuropathologic findings have been diverse in PARK8 parkinsonism, but few of the clinicopathologic examinations have been performed in the same family tree. We aimed to describe PET and neuropathologic findings in members of the same family tree with PARK8 parkinsonism.

METHODS

We conducted PET of 2 subjects and neuropathologically examined 8 subjects in the same family from the Sagamihara district, the original source of PARK8-linked parkinsonism (I2020T mutation).

RESULTS

The results of the PET scans were virtually identical to those seen in sporadic PD. The neuropathologic study results showed pure nigral degeneration with no Lewy bodies in 6 cases. One case, however, showed the presence of Lewy bodies and was similar neuropathologically to conventional PD with Lewy bodies. Another case had multiple system atrophy pathology.

CONCLUSIONS

Our study of PARK8-linked parkinsonism affecting several members of the same pedigree shows that the same gene mutation can induce diverse neuropathologies, even if the clinical picture and PET findings are virtually identical.

An LRRK2 mutation as a cause for the parkinsonism in the original PARK8 family

We detected a missense mutation in the kinase domain of the LRRK2 gene in members with autosomal dominant Parkinson's disease of the Japanese family (the Sagamihara family) who served as the basis for the original defining of the PARK8 Parkinson's disease locus. The results of the Sagamihara family, in combination with the unique pathological features characterized by pure nigral degeneration without Lewy bodies, provided us with valuable information for elucidating the protein structure-pathogenesis relationship for the gene product of LRRK2. We did not detect this mutation or other known mutations of the LRRK2 gene in Japanese patients with sporadic Parkinson's disease.

Genetic contributions to Parkinson's disease

Sporadic Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by the loss of midbrain dopamine neurons and Lewy body inclusions. It is thought to result from a complex interaction between multiple predisposing genes and environmental influences, although these interactions are still poorly understood. Several causative genes have been identified in different families. Mutations in two genes [alpha-synuclein and nuclear receptor-related 1 (Nurr1)] cause the same pathology, and a third locus on chromosome 2 also causes this pathology. Other familial PD mutations have identified genes involved in the ubiquitin-proteasome system [parkin and ubiquitin C-terminal hydroxylase L1 (UCHL1)], although such cases do not produce Lewy bodies. These studies highlight critical cellular proteins and mechanisms for dopamine neuron survival as disrupted in Parkinson's disease. Understanding the genetic variations impacting on dopamine neurons may illuminate other molecular mechanisms involved. Additional candidate genes involved in dopamine cell survival, dopamine synthesis, metabolism and function, energy supply, oxidative stress, and cellular detoxification have been indicated by transgenic animal models and/or screened in human populations with differing results. Genetic variation in genes known to produce different patterns and types of neurodegeneration that may impact on the function of dopamine neurons are also reviewed. These studies suggest that environment and genetic background are likely to have a significant influence on susceptibility to Parkinson's disease. The identification of multiple genes predisposing to Parkinson's disease will assist in determining the cellular pathway/s leading to the neurodegeneration observed in this disease.

A new locus for Parkinson's disease (PARK8) maps to chromosome 12p11.2-q13.1

We performed genomewide linkage analysis of a Japanese family with autosomal dominant parkinsonism, which exhibits clinical features compatible with those of common Parkinson's disease. Parametric two-point linkage analysis yielded a highest log odds (LOD) score of 4.32 at D12S345 (12p11.21). Parametric multipoint linkage analysis of the 13.6cM interval around this marker yielded LOD scores almost uniformly of >4.0 with a Z(max) of 4.71 at D12S85 (12q12). Haplotype analysis detected two obligate recombination events at D12S1631 and D12S339 and defined the disease-associated haplotype in the 13.6cM interval in 12p11.2-q13.1. This haplotype was shared by all the patients and by some unaffected carriers, suggesting that disease penetration in this family is incomplete. This low penetrance suggests that environmental or other genetic factors modify expression of the disease. Nonparametric two-point and multipoint linkage analyses, which are penetrance-independent, yielded Z(max) LOD scores of 14.2 and 24.9 at D12S345, respectively, strongly supporting the mapping of the parkinsonism locus in this family to 12p11.23-q13.11. This chromosome region is different from any known locus for hereditary parkinsonism, in keeping with the unique genetic features of the parkinsonism in this family. The nomenclature of PARK8 was assigned to the new locus.