Multicenter analysis of glucocerebrosidase mutations in Parkinson's disease

Large-scale screening of the Gaucher's disease-related glucocerebrosidase gene in Europeans with Parkinson's disease

Pathogenic variants in the glucocerebrosidase gene (GBA) encoding the enzyme deficient in Gaucher's disease (GD) are associated with Parkinson's disease (PD). To investigate the sequence variants, their association with PD and the related phenotypes in a large cohort of European, mostly French, patients and controls, we sequenced all exons of GBA in 786 PD patients from 525 unrelated multiplex families, 605 patients with apparently sporadic PD and 391 ethnically matched controls. GBA mutations were significantly more frequent (odds ratio=6.98, 95% confidence interval 2.54-19.21; P=0.00002) in the PD patients (76/1130=6.7%) than in controls (4/391=1.0%) and in patients with family histories of PD (8.4%) than in isolated cases (5.3%). Twenty-eight different mutations were identified in patient and control groups, including seven novel variants. N370S and L444P accounted for 70% of all mutant alleles in the patient group. PD patients with GBA mutations more frequently had bradykinesia as the presenting symptom and levodopa-induced dyskinesias. The phenotype was similar in patients with one, two or complex GBA mutations, although the two patients with c.1263del+RecTL and N370S/RecΔ55 mutations had signs of GD. Segregation analyses in 21 multiplex families showed that 17% of the affected relatives did not carry GBA mutations found in the given family, indicating heterogeneity of the aetiology, but 46% of the unaffected relatives were GBA mutation carriers. These genotype and clinical analyses on the largest homogeneous sample of European patients studied to date confirmed that GBA mutations are the most common genetic risk factor for PD, particularly in familial forms.

Towards a complete resolution of the genetic architecture of disease

After years of linear gains in the genetic dissection of human disease we are now in a period of exponential discovery. This is particularly apparent for complex disease. Genome-wide association studies (GWAS) have provided myriad associations between common variability and disease, and have shown that common genetic variability is unlikely to explain the entire genetic predisposition to disease. Here we detail how one can expand on this success and systematically identify genetic risks that lead or predispose to disease using next-generation sequencing. Geneticists have had for many years a protocol to identify Mendelian disease. A similar set of tools is now available for the identification of rare moderate-risk loci and common low-risk variants. Whereas major challenges undoubtedly remain, particularly regarding data handling and the functional classification of variants, we suggest that these will be largely practical and not conceptual.

Novel ATP13A2 (PARK9) homozygous mutation in a family with marked phenotype variability

Mutations in the ATP13A2 (PARK9) and FBXO7 (PARK15) genes are linked to different forms of autosomal recessive juvenile-onset neurodegenerative diseases with overlapping phenotypes, including levodopa-responsive parkinsonism, pyramidal disturbances, cognitive decline, and supranuclear gaze disturbance. However, the associated genotypes and phenotypes are poorly characterized due to the small number of patients described. Here, we report clinical, instrumental, and genetic findings in an Italian family with novel PARK9 and PARK15 mutations. The proband developed a severe progressive phenotype including juvenile-onset parkinsonism, pyramidal disturbances, cognitive decline, and oculomotor abnormalities. On the contrary, his brother only shows mild abnormalities (pyramidal, cognitive, and oculomotor) on the neurological examination at the age of 31 years. These two brothers both carry a novel homozygous PARK9 missense (p.G877R) and a novel heterozygous PARK15 mutation (p.R481C). The PARK9 mutation replaces a crucial residue for the ATPase activity, and is therefore most likely a loss-of-function mutation and disease-causing in homozygous state. The pathogenic significance of the PARK15 single heterozygous mutation remains unclear. In both sibs, DaTSCAN single photon emission computed tomography showed marked nigrostriatal dopaminergic defects, and transcranial magnetic stimulation detected prolonged central motor conduction time. MRI, including T2*-weighted imaging, detected no evidence of brain iron accumulation. This family, the third reported with homozygous PARK9 mutations and the first with mutations in two genes for atypical juvenile parkinsonism, illustrates that PARK9-linked disease might display wide intra-familial clinical variability and milder phenotypes, suggesting the existence of strong, still unknown, modifiers.

Mutational analysis of GIGYF2, ATP13A2 and GBA genes in Brazilian patients with early-onset Parkinson's disease

In the last decade, several genes have been linked to Parkinson's disease (PD), including GIGYF2, ATP13A2 and GBA. To explore whether mutations in these genes contribute to development of PD in the Brazilian population, we screened 110 patients with early-onset PD. No clearly pathogenic mutations were identified in ATP13A2 and GIGYF2. In contrast, we identified a significantly higher frequency of known pathogenic mutations in GBA gene among the PD cases (6/110=5.4%) when compared to the control group (0/155) (P=0.0047). Our results strongly support an association between GBA gene mutations and an increased risk of PD. Mutations in GIGYF2 and ATP13A2 do not seem to represent a risk factor to the development of PD in the Brazilian population. Considering the scarcity of studies on GIGYF2, ATP13A2 and GBA mutation frequency in Latin American countries, we present significant data about the contribution of these genes to PD susceptibility.

C. elegans as a model organism to investigate molecular pathways involved with Parkinson's disease

Parkinson's disease (PD) is an age-related movement disorder resulting, in part, from selective loss of dopaminergic neurons. Both invertebrate and mammalian models have been developed to study the cellular mechanisms altered during disease progression; nevertheless there are limitations within each model. Mammalian models remain invaluable in studying PD, but are expensive and time consuming. Here, we review genetic and environmental factors associated with PD, and describe how the nematode roundworm, Caenorhabditis elegans, has been used as a model organism for studying various aspects of this neurodegenerative disease. Both genetic and chemical screens have been conducted in C. elegans to identify molecular pathways, proteins, and small molecules that can impact PD pathology. Lastly, we highlight future areas of investigation, in the context of emerging fields in biology, where the nematode can be exploited to provide mechanistic insights and potential strategies to accelerate the path toward possible therapeutic intervention for PD.

Genetic variability at the PARK16 locus

Parkinson's disease (PD) is a complex neurodegenerative disease which is clinically heterogeneous and pathologically consists of loss of dopaminergic neurons in the substantia nigra and intracytoplasmic neuronal inclusions containing alpha-synuclein aggregations known as Lewy bodies. Although the majority of PD is idiopathic, pathogenic mutations in several mendelian genes have been successfully identified through linkage analyses. To identify susceptibility loci for idiopathic PD, several genome-wide association studies (GWAS) within different populations have recently been conducted in both idiopathic and familial forms of PD. These analyses have confirmed SNCA and MAPT as loci harboring PD susceptibility. In addition, the GWAS identified several other genetic loci suggestively associated with the risk of PD; among these, only one was replicated by two different studies of European and Asian ancestries. Hence, we investigated this novel locus known as PARK16 for coding mutations in a large series of idiopathic pathologically proven PD cases, and also conducted an association study in a case-control cohort from the United Kingdom. An association between a novel RAB7L1 mutation, c.379-12insT, and disease (P-value=0.0325) was identified. Two novel coding variants present only in the PD cohort were also identified within the RAB7L1 (p.K157R) and SLC41A1 (p.A350V) genes. No copy number variation analyses have yet been performed within this recently identified locus. We concluded that, although both coding variants and risk alleles within the PARK16 locus seem to be rare, further molecular analyses within the PARK16 locus and within different populations are required in order to examine its biochemical role in the disease process.

Genetics of Parkinson disease and essential tremor

PURPOSE OF REVIEW

Elucidating the genetic background of Parkinson disease and essential tremor is crucial to understand the pathogenesis and improve diagnostic and therapeutic strategies.

RECENT FINDINGS

A number of approaches have been applied including familial and association studies, and studies of gene expression profiles to identify genes involved in susceptibility to Parkinson disease. These studies have nominated a number of candidate Parkinson disease genes and novel loci including Omi/HtrA2, GIGYF2, FGF20, PDXK, EIF4G1 and PARK16. A recent notable finding has been the confirmation for the role of heterozygous mutations in glucocerebrosidase (GBA) as risk factors for Parkinson disease. Finally, association studies have nominated genetic variation in the leucine-rich repeat and Ig containing 1 gene (LINGO1) as a risk for both Parkinson disease and essential tremor, providing the first genetic evidence of a link between the two conditions.

SUMMARY

Although undoubtedly genes remain to be identified, considerable progress has been achieved in the understanding of the genetic basis of Parkinson disease. This same effort is now required for essential tremor. The use of next-generation high-throughput sequencing and genotyping technologies will help pave the way for future insight leading to advances in diagnosis, prevention and cure.

New insights into the role of mitochondrial dysfunction and protein aggregation in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative movement disorder that affects increasing number of elderly in the world population. The disease is caused by a selective degeneration of dopaminergic neurons in the substantia nigra pars compacta with the molecular mechanism underlying this neurodegeneration still not fully understood. However, various studies have shown that mitochondrial dysfunction and abnormal protein aggregation are two of the major contributors for PD. In fact this notion has been supported by recent studies on genes that are linked to familial PD (FPD). For instance, FPD linked gene products such as PINK1 and parkin have been shown to play critical roles in the quality control of mitochondria, whereas α-synuclein has been found to be the major protein aggregates accumulated in PD patients. These findings suggest that further understanding of how dysfunction of these pathways in PD will help develop new approaches for the treatment of this neurodegenerative disorder.

Interaction between parkin and mutant glucocerebrosidase variants: a possible link between Parkinson disease and Gaucher disease

Gaucher disease (GD), a sphingolipidosis characterized by impaired activity of the lysosomal enzyme glucocerebrosidase (GCase), results from mutations in the GCase-encoding gene, GBA. We have shown that mutant GCase variants present variable degrees of endoplasmic reticulum (ER) retention and undergo ER-associated degradation (ERAD) in the proteasome. Furthermore, the degree of ERAD of mutant GCase variants correlates with and is one of the factors that determine GD severity. An association between GD and Parkinson disease (PD) has been demonstrated by the concurrence of PD in GD patients and the identification of GCase mutations in probands with sporadic PD. One of the genes involved in PD is PARK2, encoding the E3 ubiquitin ligase parkin. Parkin functions in the ERAD of misfolded ER proteins, and it is upregulated by unfolded protein response. Loss of parkin function leads to the accumulation of its substrates, which is deleterious to dopaminergic neurons in PD. We, therefore, tested the possibility that the association between GD and PD reflects the fact that parkin acts as an E3 ligase of mutant GCase variants. Our results showed that mutant GCase variants associate with parkin. Normal parkin, but not its RING finger mutants, affects the stability of mutant GCase variants. Parkin also promotes the accumulation of mutant GCase in aggresome-like structures and is involved in K48-mediated polyubiquitination of GCase mutants, indicating its function as its E3 ligase. We suggest that involvement of parkin in the degradation of mutant GCase explains the concurrence of GD and PD.

Missing pieces in the Parkinson's disease puzzle

Parkinson's disease is a neurodegenerative process characterized by numerous motor and nonmotor clinical manifestations for which effective, mechanism-based treatments remain elusive. Here we discuss a series of critical issues that we think researchers need to address to stand a better chance of solving the different challenges posed by this pathology.

Multiplexed resequencing analysis to identify rare variants in pooled DNA with barcode indexing using next-generation sequencer

We have recently found that multiple rare variants of the glucocerebrosidase gene (GBA) confer a robust risk for Parkinson disease, supporting the 'common disease-multiple rare variants' hypothesis. To develop an efficient method of identifying rare variants in a large number of samples, we applied multiplexed resequencing using a next-generation sequencer to identification of rare variants of GBA. Sixteen sets of pooled DNAs from six pooled DNA samples were prepared. Each set of pooled DNAs was subjected to polymerase chain reaction to amplify the target gene (GBA) covering 6.5 kb, pooled into one tube with barcode indexing, and then subjected to extensive sequence analysis using the SOLiD System. Individual samples were also subjected to direct nucleotide sequence analysis. With the optimization of data processing, we were able to extract all the variants from 96 samples with acceptable rates of false-positive single-nucleotide variants.

Molecular and clinical prodrome of Parkinson disease: implications for treatment

The development of interventions to slow or prevent progression represents an important aim for current research into Parkinson disease (PD). General agreement prevails that success in this endeavor will depend on a clearer understanding of etiology and pathogenesis, and several important advances have recently been made, particularly in defining the genetic causes of PD. Studies of the biochemical consequences of the mutations that cause familial PD, and postmortem brain studies of idiopathic, sporadic PD, have highlighted mitochondrial dysfunction, oxidative stress, and protein metabolism by the ubiquitin-proteasomal and autophagy systems as being central to pathogenesis. In parallel with advances in etiopathogenesis, a clearer perception has developed of the clinical prodrome of PD, offering an opportunity to identify individuals who are at risk of PD, as well as those in the earliest clinical phase of the disease that might even precede the onset of motor symptoms. These populations are potentially the most suitable in which to test new protective therapies, and to study potential peripheral markers of disease progression. The awareness of the early symptomatic period of PD also raises the possibility of providing treatments that not only improve motor function but might also favorably modify outcome.

The role of glucocerebrosidase mutations in Parkinson disease and Lewy body disorders

Mutations in the gene encoding glucocerebrosidase (GBA), the enzyme deficient in the lysosomal storage disorder Gaucher disease, are associated with the development of Parkinson disease and other Lewy body disorders. In fact, GBA variants are currently the most common genetic risk factor associated with parkinsonism, and identified subjects with Parkinson disease are more than five times more likely to carry mutations in GBA. The mechanisms underlying this association are not known, but proposed theories include enhanced protein aggregation, alterations in lipid levels, and autophagy-lysosomal dysfunction promoting the retention of undegraded proteins. We review the genetic studies linking GBA to parkinsonism, as well as several of the mechanisms postulated to explain the association of GBA mutations and the synucleinopathies, which demonstrate how studies of a rare mendelian disease may provide insights into our understanding of a common complex disorder.

False-positive results using a Gaucher diagnostic kit--RecTL and N370S

The Pronto Gaucher kits and Pronto Gaucher RecTL Amplification Mix, marketed to identify mutations in the gene for glucocerebrosidase, are widely used for the diagnosis of Gaucher disease. Subjects genotyped using this kit have been reported with an allele including both the common N370S mutation and RecTL, a previously described Gaucher mutation arising from recombination between the glucocerebrosidase gene and pseudogene. Using direct sequencing and real-time PCR, we show that the RecTL, N370S allele is a false positive result, demonstrating possible pitfalls of diagnostic kits.

Genetics of neurodegenerative diseases: insights from high-throughput resequencing

During the past three decades, we have witnessed remarkable advances in our understanding of the molecular etiologies of hereditary neurodegenerative diseases, which have been accomplished by 'positional cloning' strategies. The discoveries of the causative genes for hereditary neurodegenerative diseases accelerated not only the studies on the pathophysiologic mechanisms of diseases, but also the studies for the development of disease-modifying therapies. Genome-wide association studies (GWAS) based on the 'common disease-common variants hypothesis' are currently undertaken to elucidate disease-relevant alleles. Although GWAS have successfully revealed numerous susceptibility genes for neurodegenerative diseases, odds ratios associated with risk alleles are generally low and account for only a small proportion of estimated heritability. Recent studies have revealed that the effect sizes of the disease-relevant alleles that are identified based on comprehensive resequencing of large data sets of Parkinson disease are substantially larger than those identified by GWAS. These findings strongly argue for the role of the 'common disease-multiple rare variants hypothesis' in sporadic neurodegenerative diseases. Given the rapidly improving technologies of next-generation sequencing next-generation sequencing (NGS), we expect that NGS will eventually enable us to identify all the variants in an individual's personal genome, in particular, clinically relevant alleles. Beyond this, whole genome resequencing is expected to bring a paradigm shift in clinical practice, where clinical practice including diagnosis and decision-making for appropriate therapeutic procedures is based on the 'personal genome'. The personal genome era is expected to be realized in the near future, and society needs to prepare for this new era.

Lysosomal function in macromolecular homeostasis and bioenergetics in Parkinson's disease

The pathological changes occurring in Parkinson's and several other neurodegenerative diseases are complex and poorly understood, but all clearly involve protein aggregation. Also frequently appearing in neurodegeneration is mitochondrial dysfunction which may precede, coincide or follow protein aggregation. These observations led to the concept that protein aggregation and mitochondrial dysfunction either arise from the same etiological factors or are interactive. Understanding the mechanisms and regulation of processes that lead to protein aggregation or mitochondrial dysfunction may therefore contribute to the design of better therapeutics. Clearance of protein aggregates and dysfunctional organelles is dependent on macroautophagy which is the process through which aged or damaged proteins and organelles are first degraded by the lysosome and then recycled. The macroautophagy-lysosomal pathway is essential for maintaining protein and energy homeostasis. Not surprisingly, failure of the lysosomal system has been implicated in diseases that have features of protein aggregation and mitochondrial dysfunction. This review summarizes 3 major topics: 1) the current understanding of Parkinson's disease pathogenesis in terms of accumulation of damaged proteins and reduction of cellular bioenergetics; 2) evolving insights into lysosomal function and biogenesis and the accumulating evidence that lysosomal dysfunction may cause or exacerbate Parkinsonian pathology and finally 3) the possibility that enhancing lysosomal function may provide a disease modifying therapy.

The risk of Parkinson's disease in type 1 Gaucher disease

In Gaucher disease, defective lysosomal glucocerebrosidase due to mutations in the GBA1 gene results in lysosomal accumulation of glucocerebroside in mononuclear phagocytes and a multisystemic phenotype. Observations of occurrence of Parkinson's disease in some patients with non-neuronopathic type 1 Gaucher disease (GD1) and their first degree relatives has led to the identification of GBA1 heterozygous mutations as a genetic risk factor for idiopathic Parkinson's disease (PD). However, the magnitude of risk of PD in patients with known GD1 has not been determined, and it is not known whether GD1/PD represents a specific sub-phenotype of GD1 with distinctive genotype/phenotype characteristics. We estimated the risk of PD in a cohort of 444 consecutively evaluated patients with GD1 compared to that in the general population. Eleven patients developed parkinsonian syndrome during a 12-year follow-up period. The adjusted life-time risk ratio of PD in GD1 compared to that in the general population was 21.4 [95% confidence interval (95% CI) 10.7-38.3], with a higher risk in men compared to women. In our cohort, GD1/Parkinson's disease phenotype (GD1/PD) was characterized by higher GD1 severity score, due to higher incidence of avascular osteonecrosis. The clinical spectrum of PD varied from mild to potentially life-threatening disease. All but one patient with GD1/PD phenotype had at least one N370S GBA1 allele. In conclusion, compared to the general population, patients with GD1 have an almost 20-fold increased life-time risk of developing PD.

Biomarkers: Parkinson disease with dementia and dementia with Lewy bodies

Dementia is a common feature in Parkinson disease (PD), the time of onset determining how patients are classified. Those patients where dementia develops prior to parkinsonism or during the first year of disease are designated as having dementia with Lewy bodies (DLB). In those where dementia develops over a year after the onset of motor signs, the condition is known as Parkinson's disease with dementia (PDD). While this seems at first sight to be a definitive way to distinguish these conditions, reality is rather different. The overlap between them is considerable, and there is much uncertainty associated with patients who have both motor symptoms and early cognitive impairment. The diagnosis is still based on medical history and clinical evaluation. It is not even certain that they can be accurately distinguished at autopsy. For this reason, the data concerning these entities have been reviewed, to examine various markers employed or measured in clinical, neuropathological, neuroimaging, and biochemical investigations. The concept of PDD and DLB being separate conditions is comparatively new, and the most promising tools with which to separate them at present are cerebrospinal fluid (CSF) markers and positron emission tomography (PET) scanning that indicate increased amyloid-beta burden in DLB compared to PDD. However as yet there are no markers that unequivocally distinguish between PDD and DLB.

The ceramide transporter and the Goodpasture antigen binding protein: one protein--one function?

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPDelta26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPDelta26 knock-out mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis.

Glucocerebrosidase as a genetic modifier influencing susceptibility and phenotype of Parkinson’s disease

Evaluation of: Sidransky E, Nalls MA, Aasly JO et al.: Multicenter analysis of glucocerebrosidase mutations in Parkinson’s disease. N. Engl. J. Med. 361, 1651–1661 (2009). This study aimed to substantiate the role of glucocerebrosidase (GBA) mutations in Parkinson’s disease (PD) by pooling clinical and genotypic data for 5691 PD patients and 4898 unaffected individuals from 16 international centers. Genotyping was performed for either two mutations (N370S and L444P) or for three to eight GBA mutations. One of the N370S and L444P mutations was found in 15 and 3% of PD patients of Ashkenazi and non-Ashkenazi Jewish origin, respectively. Whole gene sequencing performed in a subgroup of these patients revealed a combined odds ratio of carrying any GBA mutation of 5.43. Genotype–phenotype correlations verified that GBA mutation carriers were more likely to develop PD at an earlier age, have affected relatives and show increased incidence of cognitive changes compared with PD patients without mutations. This study provides evidence that heterozygosity for GBA mutations is the most common susceptibility factor for the onset of PD symptoms to date. It also highlights the need for further studies using large numbers of patients and controls in order to elucidate the effect(s) of GBA mutations on PD clinical manifestations and their role in the molecular mechanisms of PD pathogenesis.