Oliver Bandmann

Rapamycin activation of 4E-BP prevents parkinsonian dopaminergic neuron loss

Mutations in PINK1 and PARK2 cause autosomal recessive parkinsonism, a neurodegenerative disorder that is characterized by the loss of dopaminergic neurons. To discover potential therapeutic pathways, we identified factors that genetically interact with Drosophila park and Pink1. We found that overexpression of the translation inhibitor Thor (4E-BP) can suppress all of the pathologic phenotypes, including degeneration of dopaminergic neurons in Drosophila. 4E-BP is activated in vivo by the TOR inhibitor rapamycin, which could potently suppress pathology in Pink1 and park mutants. Rapamycin also ameliorated mitochondrial defects in cells from individuals with PARK2 mutations. Recently, 4E-BP was shown to be inhibited by the most common cause of parkinsonism, dominant mutations in LRRK2. We also found that loss of the Drosophila LRRK2 homolog activated 4E-BP and was also able to suppress Pink1 and park pathology. Thus, in conjunction with recent findings, our results suggest that pharmacologic stimulation of 4E-BP activity may represent a viable therapeutic approach for multiple forms of parkinsonism.

Mitochondrial function and morphology are impaired in parkin mutant fibroblasts

There are marked mitochondrial abnormalities in parkin‐knock‐out Drosophila and other model systems. The aim of our study was to determine mitochondrial function and morphology in parkin‐mutant patients. We also investigated whether pharmacological rescue of impaired mitochondrial function may be possible in parkin‐mutant human tissue.

Wilson's disease and other neurological copper disorders

The copper metabolism disorder Wilsons disease was first defined in 1912. Wilsons disease can present with hepatic and neurological deficits, including dystonia and parkinsonism. Early-onset presentations in infancy and late-onset manifestations in adults older than 70 years of age are now well recognised. Direct genetic testing for ATP7B mutations are increasingly available to confirm the clinical diagnosis of Wilsons disease, and results from biochemical and genetic prevalence studies suggest that Wilsons disease might be much more common than previously estimated. Early diagnosis of Wilsons disease is crucial to ensure that patients can be started on adequate treatment, but uncertainty remains about the best possible choice of medication. Furthermore, Wilsons disease needs to be differentiated from other conditions that also present clinically with hepatolenticular degeneration or share biochemical abnormalities with Wilsons disease, such as reduced serum ceruloplasmin concentrations. Disordered copper metabolism is also associated with other neurological conditions, including a subtype of axonal neuropathy due to ATP7A mutations and the late-onset neurodegenerative disorders Alzheimers disease and Parkinsons disease.

Mitochondrial impairment in patients with Parkinson disease with the G2019S mutation in LRRK2

Objective: The LRRK2G2019S mutation is the most common identifiable cause for Parkinson disease (PD), but the underlying mechanisms leading to neuronal cell death remain largely unclear. Impaired mitochondrial function and morphology have been described in different in vivo and in vitro model systems of early-onset PD (EOPD) as well as in EOPD patient tissue. The aim of our study was to assess mitochondrial function and morphology in LRRK2G2019S mutant patient tissue to determine whether impaired mitochondrial function and morphology are shared features in early-onset and late-onset PD. Methods: Skin biopsies were taken from 5 patients with PD with the LRRK2G2019S mutation. Assessment of mitochondrial membrane potential and intracellular ATP levels as well as substrate-linked mitochondrial ATP production assays were all carried out on 3 independent cell preparations per patient. Results were compared to 5 age-matched controls. Mitochondrial elongation and interconnectivity was assessed using previously published methods. Results: Both mitochondrial membrane potential and total intracellular ATP levels were decreased in the G2019S mutation carriers. Subsequently undertaken mitochondrial ATP production assays suggested that the observed reduction is at least partially due to impaired mitochondrial function. Mitochondrial elongation and interconnectivity were increased in the LRRK2G2019S patient cohort. Conclusion: Our results provide evidence for impaired mitochondrial function and morphology in LRRK2G2019S mutant patient tissue. Further studies are required to determine whether the impaired mitochondrial function is due to increased LRRK2 kinase activity or other mechanisms such as LRRK2 haploinsufficiency.

p53-dependent neuronal cell death in a DJ-1-deficient zebrafish model of Parkinson's disease

Mutations in DJ‐1 lead to early onset Parkinsons disease (PD). The aim of this study was to elucidate further the underlying mechanisms leading to neuronal cell death in DJ‐1 deficiency in vivo and determine whether the observed cell loss could be prevented pharmacologically. Inactivation of DJ‐1 in zebrafish, Danio rerio, resulted in loss of dopaminergic neurons after exposure to hydrogen peroxide and the proteasome inhibitor MG132. DJ‐1 knockdown by itself already resulted in increased p53 and Bax expression levels prior to toxin exposure without marked neuronal cell death, suggesting subthreshold activation of cell death pathways in DJ‐1 deficiency. Proteasome inhibition led to a further increase of p53 and Bax expression with widespread neuronal cell death. Pharmacological p53 inhibition either before or during MG132 exposure in vivo prevented dopaminergic neuronal cell death in both cases. Simultaneous knockdown of DJ‐1 and the negative p53 regulator mdm2 led to dopaminergic neuronal cell death even without toxin exposure, further implicating involvement of p53 in DJ‐1 deficiency‐mediated neuronal cell loss. Our study demonstrates the utility of zebrafish as a new animal model to study PD gene defects and suggests that modulation of downstream mechanisms, such as p53 inhibition, may be of therapeutic benefit.

Parkinson's Disease: insights from pathways

Parkinsons disease (PD) typically presents in sporadic fashion, but the identification of disease-causing mutations in monogenically inherited PD genes has provided crucial insight into the pathogenesis of this disorder. Mutations in autosomal recessively inherited genes, namely parkin, PINK1 and DJ-1, typically lead to early onset parkinsonism. At least two of these genes (PINK1 and parkin) appear to work in the same pathway related to maintenance of mitochondrial functional integrity under conditions of oxidative stress. Dominantly inherited mutations in leucine-rich repeat kinase 2 (LRRK2) and alpha-synuclein cause late onset PD, generally with Lewy bodies that are characteristic of sporadic PD and there is evidence that these two genes are also in a common pathway. There is also growing evidence from recently undertaken genome-wide association studies that naturally occurring sequence variants in alpha-synuclein and LRRK2, but also Tau, also confer an increased risk for late onset, sporadic PD. Collectively, these results highlight how understanding pathways for inherited PD are starting to impact ideas about the pathogenesis, some of which may also be relevant to the commoner sporadic disease.

Association of slow acetylator genotype for N-acetyltransferase 2 with familial Parkinson's disease

BACKGROUND Epidemiological studies have identified positive family history and exposure to environmental toxins as risk factors for Parkinsons disease (PD). An inherited defect of xenobiotic metabolism could result in increased susceptibility to such toxins. We investigated the frequency of functionally relevant polymorphisms in six detoxification enzymes among patients with PD to elucidate the relation between these polymorphisms and the disease. METHODS We obtained brain-tissue samples from 100 patients with apparently sporadic PD and blood samples from 100 living patients with familial PD. For the control group, we extracted DNA from the tissue of 100 pathologically normal brains. The six enzymes analysed in these three groups were: CYP2D6, CYP2E1, NAD(P)H-menadione reductase, glutathione transferases M1 and T1, and N-acetyltransferase 2. We also investigated N-acetyltransferase 2 in 100 blood samples from patients with genetically proven Huntingtons disease. We used PCR-based methods and restriction-enzyme analysis to detect polymorphisms. FINDINGS The slow acetylator genotype for N-acetyltransferase 2 was more common in the familial PD group (69%) than in all controls (37%). Even after correction for multiple comparisons, this result remained highly significant (p = 0.002) for familial PD compared with normal controls (odds ratio 3.79 [95% CI 2.08-6.90]) and compared with Huntingtons disease (2.45 [1.37-4.38], p = 0.004). The slow acetylator frequency for N-acetyltransferase 2 for sporadic PD was between that for Huntingtons disease and familial PD. The frequencies of all the other polymorphisms were similar in the two study groups and the normal control group. INTERPRETATION We found an association between the slow acetylator genotype for N-acetyltransferase 2 and familial PD. Further studies are needed to investigate the biological relevance of these findings, but slow acetylation could lead to impaired ability of patients with familial PD to handle neurotoxic substances.

The tau gene A0 polymorphism in progressive supranuclear palsy and related neurodegenerative diseases

Progressive supranuclear palsy is characterised pathologically by the deposition of neurofibrillary tangles consisting of tau protein. Patients with the disease have been reported to have a more frequent occurrence of one allele of an intronic polymorphism of thetau gene. Other diseases which may involve tau deposition include frontotemporal dementia and corticobasal degeneration. This polymorphism has been studied in a series of subjects with progressive supranuclear palsy, corticobasal degeneration, frontotemporal dementia, idiopathic Parkinson’s disease, and normal controls to (1) confirm this association in a large series and (2) to investigate a possible role for this association in other disorders which involve tau deposition. The results confirm the finding of an overrepresentation of the A0 allele and the A0/A0 genotype in patients with progressive supranuclear palsy, in the largest series reported to date. The A0 allele was found in 91% of patients with progressive supranuclear palsy as opposed to 73% of controls (p<0.001) and the A0/A0 genotype was seen in 84% of patients as compared with 53% of controls (p<0.01). There was no significant difference between patients with Parkinson’s disease, frontotemporal dementia, or corticobasal degeneration, and controls. The A0 allele may have a direct effect on tau isoform expression in progressive supranuclear palsy or it may be in linkage disequilibrium with an adjacent determinant of tau gene expression. The explanation for this difference between a predisposition factor to progressive supranuclear palsy and the other conditions may lie in the molecular pathology of these diseases.

A genetic study of Wilson’s disease in the United Kingdom

Previous studies have failed to identify mutations in the Wilsons disease gene ATP7B in a significant number of clinically diagnosed cases. This has led to concerns about genetic heterogeneity for this condition but also suggested the presence of unusual mutational mechanisms. We now present our findings in 181 patients from the United Kingdom with clinically and biochemically confirmed Wilsons disease. A total of 116 different ATP7B mutations were detected, 32 of which are novel. The overall mutation detection frequency was 98%. The likelihood of mutations in genes other than ATP7B causing a Wilsons disease phenotype is therefore very low. We report the first cases with Wilsons disease due to segmental uniparental isodisomy as well as three patients with three ATP7B mutations and three families with Wilsons disease in two consecutive generations. We determined the genetic prevalence of Wilsons disease in the United Kingdom by sequencing the entire coding region and adjacent splice sites of ATP7B in 1000 control subjects. The frequency of all single nucleotide variants with in silico evidence of pathogenicity (Class 1 variant) was 0.056 or 0.040 if only those single nucleotide variants that had previously been reported as mutations in patients with Wilsons disease were included in the analysis (Class 2 variant). The frequency of heterozygote, putative or definite disease-associated ATP7B mutations was therefore considerably higher than the previously reported occurrence of 1:90 (or 0.011) for heterozygote ATP7B mutation carriers in the general population (P < 2.2 × 10(-16) for Class 1 variants or P < 5 × 10(-11) for Class 2 variants only). Subsequent exclusion of four Class 2 variants without additional in silico evidence of pathogenicity led to a further reduction of the mutation frequency to 0.024. Using this most conservative approach, the calculated frequency of individuals predicted to carry two mutant pathogenic ATP7B alleles is 1:7026 and thus still considerably higher than the typically reported prevalence of Wilsons disease of 1:30 000 (P = 0.00093). Our study provides strong evidence for monogenic inheritance of Wilsons disease. It also has major implications for ATP7B analysis in clinical practice, namely the need to consider unusual genetic mechanisms such as uniparental disomy or the possible presence of three ATP7B mutations. The marked discrepancy between the genetic prevalence and the number of clinically diagnosed cases of Wilsons disease may be due to both reduced penetrance of ATP7B mutations and failure to diagnose patients with this eminently treatable disorder.

Complex I deficiency and dopaminergic neuronal cell loss in parkin-deficient zebrafish (Danio rerio)

Currently, only symptomatic therapy is available for Parkinsons disease. The zebrafish is a vertebrate animal model ideally suited for high throughput compound screening to identify disease-modifying compounds for Parkinsons disease. We have developed a zebrafish model for Parkin deficiency, the most commonly mutated gene in early onset Parkinsons disease. The zebrafish Parkin protein is 62% identical to its human counterpart with 78% identity in functionally relevant regions. The parkin gene is expressed throughout zebrafish development and ubiquitously in adult zebrafish tissue. Abrogation of Parkin activity leads to a significant decrease in the number of ascending dopaminergic neurons in the posterior tuberculum (homologous to the substantia nigra in humans), an effect enhanced by exposure to MPP+. Both light microscopic analysis and staining with the pan-neuronal marker HuC confirmed that this loss of dopaminergic neurons is not due to general impairment of brain development. Neither serotonergic nor motor neurons were affected, further emphasizing that the effect of parkin knockdown appears to be specific for dopaminergic neurons. Notably, parkin knockdown zebrafish embryos also develop specific reduction in the activity of the mitochondrial respiratory chain complex I, making this the first vertebrate model to share both important pathogenic mechanisms (i.e. complex I deficiency) and the pathological hallmark (i.e. dopaminergic cell loss) with human parkin-mutant patients. The zebrafish model is thus ideally suited for future drug screens and other studies investigating the functional mechanisms underlying neuronal cell death in early onset Parkinsons Disease. Additional electron microscopy studies revealed electron dense material in the t-tubules within the muscle tissue of parkin knockdown zebrafish. T-tubules are rich in L-type calcium channels, therefore our work might also provide a tentative link between genetically determined early onset Parkinsons disease and recent studies attributing an important role to these L-type calcium channels in late onset sporadic Parkinsons disease.