An apparently sporadic case with parkin gene mutation in a Korean woman

Contribution of rare variant associations to neurodegenerative disease presentation

Genetic factors contribute to neurodegenerative diseases, with high heritability estimates across diagnoses; however, a large portion of the genetic influence remains poorly understood. Many previous studies have attempted to fill the gaps by performing linkage analyses and association studies in individual disease cohorts, but have failed to consider the clinical and pathological overlap observed across neurodegenerative diseases and the potential for genetic overlap between the phenotypes. Here, we leveraged rare variant association analyses (RVAAs) to elucidate the genetic overlap among multiple neurodegenerative diagnoses, including Alzheimer's disease, amyotrophic lateral sclerosis, frontotemporal dementia (FTD), mild cognitive impairment, and Parkinson's disease (PD), as well as cerebrovascular disease, using the data generated with a custom-designed neurodegenerative disease gene panel in the Ontario Neurodegenerative Disease Research Initiative (ONDRI). As expected, only ~3% of ONDRI participants harboured a monogenic variant likely driving their disease presentation. Yet, when genes were binned based on previous disease associations, we observed an enrichment of putative loss of function variants in PD genes across all ONDRI cohorts. Further, individual gene-based RVAA identified significant enrichment of rare, nonsynonymous variants in PARK2 in the FTD cohort, and in NOTCH3 in the PD cohort. The results indicate that there may be greater heterogeneity in the genetic factors contributing to neurodegeneration than previously appreciated. Although the mechanisms by which these genes contribute to disease presentation must be further explored, we hypothesize they may be a result of rare variants of moderate phenotypic effect contributing to overlapping pathology and clinical features observed across neurodegenerative diagnoses.

Molecular imaging in hereditary forms of parkinsonism

The development of in vivo molecular imaging to evaluate the dopamine (DA) system with positron-emission tomography and single photon emission computed tomography has been of key importance on monitoring in vivo nigrostriatal neuronal loss in Parkinson's disease (PD), mostly through assessments of pre- and post-synaptic DA receptors. The discoveries of genes related to hereditary forms of parkinsonism (PARK1, PARK2, PARK6, PARK7 and PARK8) have increased our understanding either of distinct subtypes of clinical expression in PD or its etiology. This article revises current data on molecular neuroimaging of genetic forms of parkinsonism comparing and contrasting its main features with the classical sporadic forms. Awareness of the spectrum variance in the genotype and its respective PD phenotype are useful to distinguish different pathophysiological mechanisms of PD.

The role of ubiquitin-protein ligases in neurodegenerative disease

Alzheimer's disease and Parkinson's disease are the most common neurodegenerative conditions associated with the ageing process. The pathology of these and other neurodegenerative disorders, including polyglutamine diseases, is characterised by the presence of inclusion bodies in brain tissue of affected patients. In general, these inclusion bodies consist of insoluble, unfolded proteins that are commonly tagged with the small protein, ubiquitin. Covalent tagging of proteins with chains of ubiquitin generally targets them for degradation. Indeed, the ubiquitin/proteasome system (UPS) is the major route through which intracellular proteolysis is regulated. This strongly implicates the UPS in these disease-associated inclusions, either due to malfunction (of specific UPS components) or overload of the system (due to aggregation of unfolded/mutant proteins), resulting in subsequent cellular toxicity. Protein targeting for degradation is a highly regulated process. It relies on transfer of ubiquitin molecules to the target protein via an enzyme cascade and specific recognition of a substrate protein by ubiquitin-protein ligases (E3s). Recent advances in our knowledge gained from the Human Genome Mapping Project have revealed the presence of potentially hundreds of E3s within the human genome. The discovery that parkin, mutations in which are found in at least 50% of patients with autosomal recessive juvenile parkinsonism, is an E3 further highlights the importance of the UPS in neurological disease. To date, parkin is the only E3 confirmed to have a direct causal role in neurodegenerative disorders. However, a number of other (putative) E3s have now been identified that may cause disease directly or interact with neurological disease-associated proteins. Many of these are either lost or mutated in a given disease or fail to process disease-associated mutant proteins correctly. In this review, we will discuss the role(s) of E3s in neurodegenerative disorders.

Early-onset Parkinson's disease in a Chinese population: 99mTc-TRODAT-1 SPECT, Parkin gene analysis and clinical study

Early Onset Parkinson's Disease (EOPD) is characterized by selective degeneration of nigrostriatal dopaminergic neurons and a marked response to levodopa. However, at present, few methods are available as diagnostic tools for EOPD except for 18F-DOPA PET. In addition, little is known about the correlation between clinical severity, neuroimaging grading and genetic susceptibility. In the present study, 99mTc-TRODAT-1 SPECT and brain MRI were used to identify 30 cases of non-familial EOPD from a Chinese cohort of 230. All 30 PD patients had an age of onset of less than 55 years (mean age at onset, 41.5+/-9.3 years). Each of the 30 EOPD cases was sub-classified into one of five stages based on the 99mTc-TRODAT-1 SPECT findings. In the early stages of PD (stages 1 and 2), a lower uptake of 99mTc-TRODAT-1 in the putamen was found, while uptake in the caudate nucleus was normal. In the latter stages (stages 3, 4, 5), 24 patients revealed a diffuse and uniform loss of 99mTc-TRODAT-1 uptake in the putamen and the caudate nucleus. Further, in conventional genetic studies of the 30 patients, six novel mutations were found in the Parkin gene, and these included five heterozygous point mutations (C441R, Q311H, V258M, C212G, and S193I) and one homozygous deletion (exon 10-12). Known polymorphisms (Ser167Asn, Val380Leu) were also found in a number of patients. However, gene dosage analysis did not reveal any compound heterozygous mutations in these 30 patients using quantitative duplex PCR. This is the first study to examine EOPD patients of Chinese ethnic background (not exhibiting a definite familial trait), to offer a complete genetic analysis of the Parkin gene, and to correlate clinical stages of the disease with dopamine re-uptake.

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.

Diagnostic considerations in juvenile parkinsonism

Juvenile parkinsonism (JP) describes patients in whom the clinical features of parkinsonism manifest before 21 years of age. Many reported cases that had a good response to levodopa have proved to have autosomal recessive juvenile parkinsonism (AR-JP) due to mutations in the parkin gene. With the exception of parkin mutations and dopa-responsive dystonia, most causes are associated with the presence of additional neurological signs, resulting from additional lesions outside of the basal ganglia. Lewy body pathology has only been reported in one case, suggesting that a juvenile form of idiopathic Parkinson's disease may be extremely rare.

Parkinson's disease: imaging update

PURPOSE OF REVIEW

We aim to review recent neuroimaging contributions to our understanding of the cause and pathogenesis of Parkinson's disease, as well as treatment-related complications of disease, with a focus on functional anatomy and neurochemistry.

RECENT FINDINGS

Recent reports describe altered dopaminergic activity in extrastriatal regions, as well as changes in other monoaminergic systems, such as serotonin. Attempts to correlate altered dopaminergic function with personality traits have also been described in the last year. The role of different markers of presynaptic dopaminergic integrity in the assessment of disease progression is discussed, as is the role of biomarkers in detection of preclinical disease. Cerebral activation studies not only confirm altered function of cortico-striatal-thalamo-cortical loops in Parkinson's, but also emphasize the importance of networks involving the cerebellum. The ability to detect changes in synaptic availability of dopamine using positron emission tomography with [(11)C]raclopride is reviewed, including the application to detect altered levels of dopamine in response to pharmacological, mechanical and behavioral stimuli. Such studies have been used to identify altered patterns associated with the development of motor fluctuations, as well as a biochemical substrate underlying the placebo effect in Parkinson's.

SUMMARY

Functional imaging studies can provide novel insights into the etiopathogenesis of Parkinson's disease, as well as the mechanisms that contribute to complications of long-term therapy. They also shed light on the mechanisms that may underly behavioral changes and benefit derived from surgical interventions.