Patrick M. Abou-Sleiman

Expanding insights of mitochondrial dysfunction in Parkinson's disease

The quest to disentangle the aetiopathogenesis of Parkinsons disease has been heavily influenced by the genes associated with the disease. The α-synuclein-centric theory of protein aggregation with the adjunct of parkin-driven proteasome deregulation has, in recent years, been complemented by the discovery and increasing knowledge of the functions of DJ1, PINK1 and OMI/HTRA2, which are all associated with the mitochondria and have been implicated in cellular protection against oxidative damage. We critically review how these genes fit into and enhance our understanding of the role of mitochondrial dysfunction in Parkinsons disease, and consider how oxidative stress might be a potential unifying factor in the aetiopathogenesis of the disease.

A common LRRK2 mutation in idiopathic Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been shown to cause autosomal dominant Parkinsons disease. Few mutations in this gene have been identified. We investigated the frequency of a common heterozygous mutation, 2877510 g-->A, which produces a glycine to serine aminoacid substitution at codon 2019 (Gly2019 ser), in idiopathic Parkinsons disease. We assessed 482 patients with the disorder, of whom 263 had pathologically confirmed disease, by direct sequencing for mutations in exon 41 of LRRK2. The mutation was present in eight (1.6%) patients. We have shown that a common single Mendelian mutation is implicated in sporadic Parkinsons disease. We suggest that testing for this mutation will be important in the management and genetic counselling of patients with Parkinsons disease.

The role of pathogenic DJ‐1 mutations in Parkinson's disease

Mutations in DJ‐1 (PARK7) have been reported in two consanguineous families with young‐onset Parkinsons disease (YOPD). This study aims to confirm the presence of pathogenic DJ‐1 mutations and determine their contribution in young‐onset and more typical later onset Parkinsons disease (PD). The entire open reading frame of the DJ‐1 gene was screened by direct sequencing in 185 unrelated YOPD patients and a separate cohort of 190 pathologically proven cases of PD. Ethnically matched controls were screened for all mutations identified. We report a low frequency of pathogenic DJ‐1 mutations in our cohort of patients. One homozygous missense mutation and one heterozygous mutation were found in two YOPD samples. In addition, several variants were found in the coding sequence of the gene, which are likely to represent polymorphisms. In one case, the polymorphism was population specific. The reported 14Kbp deletion was not found in any of our samples or controls. We confirm the presence of pathogenic DJ‐1 mutations in YOPD and estimate their frequency at approximately 1%. No mutations were found in our cohort of later onset sporadic pathologically confirmed cases, suggesting that DJ‐1 mutations may only rarely contribute to the cause of this more typical sporadic form of the disease.

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.

Linkage disequilibrium fine mapping and haplotype association analysis of the tau gene in progressive supranuclear palsy and corticobasal degeneration

Background: The haplotype H1 of the tau gene, MAPT, is highly associated with progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Objective: To investigate the pathogenic basis of this association. Methods: Detailed linkage disequilibrium and common haplotype structure of MAPT were examined in 27 CEPH trios using validated HapMap genotype data for 24 single nucleotide polymorphisms (SNPs) spanning MAPT. Results: Multiple variants of the H1 haplotype were resolved, reflecting a far greater diversity of MAPT than can be explained by the H1 and H2 clades alone. Based on this, six haplotype tagging SNPs (htSNPs) that capture 95% of the common haplotype diversity were used to genotype well characterised PSP and CBD case–control cohorts. In addition to strong association with PSP and CBD of individual SNPs, two common haplotypes derived from these htSNPs were identified that are highly associated with PSP: the sole H2 derived haplotype was underrepresented and one of the common H1 derived haplotypes was highly associated, with a similar trend observed in CBD. There were powerful and highly significant associations with PSP and CBD of haplotypes formed by three H1 specific SNPs. This made it possible to define a candidate region of at least ∼56 kb, spanning sequences from upstream of MAPT exon 1 to intron 9. On the H1 haplotype background, these could harbour the pathogenic variants. Conclusions: The findings support the pathological evidence that underlying variations in MAPT could contribute to disease pathogenesis by subtle effects on gene expression and/or splicing. They also form the basis for the investigation of the possible genetic role of MAPT in Parkinson’s disease and other tauopathies, including Alzheimer’s disease.

A heterozygous effect for PINK1 mutations in Parkinson's disease?

To investigate the significance of PINK1 mutations in sporadic Parkinsons disease (PD).

Transcriptional repression of p53 by parkin and impairment by mutations associated with autosomal recessive juvenile Parkinson's disease

Mutations of the ubiquitin ligase parkin account for most autosomal recessive forms of juvenile Parkinsons disease (AR-JP). Several studies have suggested that parkin possesses DNA-binding and transcriptional activity. We report here that parkin is a p53 transcriptional repressor. First, parkin prevented 6-hydroxydopamine-induced caspase-3 activation in a p53-dependent manner. Concomitantly, parkin reduced p53 expression and activity, an effect abrogated by familial parkin mutations known to either abolish or preserve its ligase activity. ChIP experiments indicate that overexpressed and endogenous parkin interact physically with the p53 promoter and that pathogenic mutations abolish DNA binding to and promoter transactivation of p53. Parkin lowered p53 mRNA levels and repressed p53 promoter transactivation through its Ring1 domain. Conversely, parkin depletion enhanced p53 expression and mRNA levels in fibroblasts and mouse brains, and increased cellular p53 activity and promoter transactivation in cells. Finally, familial parkin missense and deletion mutations enhanced p53 expression in human brains affected by AR-JP. This study reveals a ubiquitin ligase-independent function of parkin in the control of transcription and a functional link between parkin and p53 that is altered by AR-JP mutations.

Altered cleavage and localization of PINK1 to aggresomes in the presence of proteasomal stress

Following our identification of PTEN‐induced putative kinase 1 (PINK1) gene mutations in PARK6‐linked Parkinsons disease (PD), we have recently reported that PINK1 protein localizes to Lewy bodies (LBs) in PD brains. We have used a cellular model system of LBs, namely induction of aggresomes, to determine how a mitochondrial protein, such as PINK1, can localize to aggregates. Using specific polyclonal antibodies, we firstly demonstrated that human PINK1 was cleaved and localized to mitochondria. We demonstrated that, on proteasome inhibition with MG‐132, PINK1 and other mitochondrial proteins localized to aggresomes. Ultrastructural studies revealed that the mechanism was linked to the recruitment of intact mitochondria to the aggresome. Fractionation studies of lysates showed that PINK1 cleavage was enhanced by proteasomal stress in vitro and correlated with increased expression of the processed PINK1 protein in PD brain. These observations provide valuable insights into the mechanisms of LB formation in PD that should lead to a better understanding of PD pathogenesis.

Tau gene and Parkinson’s disease: a case–control study and meta-analysis

Objective: To investigate whether the tau H1 haplotype is a genetic risk factor in Parkinson’s disease and to report a meta-analysis on all previously published data Methods and results: In a sample of 580 patients with Parkinson’s disease and 513 controls there was an increased risk of Parkinson’s disease for both the tau H1 haplotype (p⩽0.0064; odds ratio (OR) 1.34 (95% confidence interval (CI), 1.04 to 1.72)) and the H1H1 genotype (p⩽0.0047; OR 1.42 (1.1 to 1.83)). Under a fixed effect model, the summary OR for this showed that individuals homozygous for the H1 allele were 1.57 times more likely to develop Parkinson’s disease than individuals carrying the H2 allele (95% CI 1.33 to 1.85; p<0.00001). The population attributable risk for the tau variant, for the main comparison of H1H1 against H2 carriers, was 24.8% for all studies combined. Conclusions: Homozygosity for the tau H1 is associated with an increased risk of Parkinson’s disease. This adds to the growing body of evidence that common genetic variation contributes to the pathogenesis of this disorder.

PINK1 (PARK6) associated Parkinson disease in Ireland

Mutations in the PINK1 gene have recently been shown to cause autosomal recessive Parkinson disease (PD). The authors assessed the prevalence of PINK1 gene mutations in 290 well-characterized early- and late-onset PD patients from Ireland. In a 51-year-old PD patient with a family history of PD, the authors identified a novel heterozygous mutation (R147H) in exon 2 of the PINK1 gene. Overall, these data indicate that PINK1 mutations are a rare cause of PD in Ireland.