Founder mutation p.R1441C in the leucine-rich repeat kinase 2 gene in Belgian Parkinson's disease patients

The Evolution of Genetic Variability at the LRRK2 Locus

Leucine-rich repeat kinase 2 (LRRK2) c.6055G>A (p.G2019S) is a frequent cause of Parkinson's disease (PD), accounting for >30% of Tunisian Arab-Berber patients. LRRK2 is widely expressed in the immune system and its kinase activity confers a survival advantage against infection in animal models. Here, we assess haplotype variability in cis and in trans of the LRRK2 c.6055G>A mutation, define the age of the pathogenic allele, explore its relationship to the age of disease onset (AOO), and provide evidence for its positive selection.

Endemic parkinsonism: clusters, biology and clinical features

The term 'endemic parkinsonism' refers to diseases that manifest with a dominant parkinsonian syndrome, which can be typical or atypical, and are present only in a particular geographically defined location or population. Ten phenotypes of endemic parkinsonism are currently known: three in the Western Pacific region; two in the Asian-Oceanic region; one in the Caribbean islands of Guadeloupe and Martinique; and four in Europe. Some of these disease entities seem to be disappearing over time and therefore are probably triggered by unique environmental factors. By contrast, other types persist because they are exclusively genetically determined. Given the geographical clustering and potential overlap in biological and clinical features of these exceptionally interesting diseases, this Review provides a historical reference text and offers current perspectives on each of the 10 phenotypes of endemic parkinsonism. Knowledge obtained from the study of these disease entities supports the hypothesis that both genetic and environmental factors contribute to the development of neurodegenerative diseases, not only in endemic parkinsonism but also in general. At the same time, this understanding suggests useful directions for further research in this area.

Overview of the Impact of Pathogenic LRRK2 Mutations in Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) is a large protein kinase that physiologically phosphorylates and regulates the function of several Rab proteins. LRRK2 is genetically implicated in the pathogenesis of both familial and sporadic Parkinson's disease (PD), although the underlying mechanism is not well understood. Several pathogenic mutations in the LRRK2 gene have been identified, and in most cases the clinical symptoms that PD patients with LRRK2 mutations develop are indistinguishable from those of typical PD. However, it has been shown that the pathological manifestations in the brains of PD patients with LRRK2 mutations are remarkably variable when compared to sporadic PD, ranging from typical PD pathology with Lewy bodies to nigral degeneration with deposition of other amyloidogenic proteins. The pathogenic mutations in LRRK2 are also known to affect the functions and structure of LRRK2, the differences in which may be partly attributable to the variations observed in patient pathology. In this review, in order to help researchers unfamiliar with the field to understand the mechanism of pathogenesis of LRRK2-associated PD, we summarize the clinical and pathological manifestations caused by pathogenic mutations in LRRK2, their impact on the molecular function and structure of LRRK2, and their historical background.

Review of the epidemiology and variability of LRRK2 non-p.Gly2019Ser pathogenic mutations in Parkinson’s disease

Parkinson’s disease (PD) is a heterogenous neurodegenerative disorder. Genetic factors play a significant role, especially in early onset and familial cases. Mutations are usually found in the LRRK2 gene, but their importance varies. Some mutations, such as p.Arg1441Cys or other alterations in the 1441 codon, show clear correlation with PD, whereas others are risk factors found also in healthy populations or have neglectable consequences. They also exhibit various prevalence among different populations. The aim of this paper is to sum up the current knowledge regarding the epidemiology and pathogenicity of LRRK2 mutations, other than the well-established p.Gly2019Ser. We performed a review of the literature using PubMed database. 103 publications met our inclusion criteria. p.Arg1441Cys, p.Arg1441Gly, p.Arg1441His, p.Arg1441Ser are the most common pathogenic mutations in European populations, especially Hispanic. p.Asn1437His is pathogenic and occurs mostly in the Scandinavians. p.Asn1437Ser and p.Asn1437Asp have been reported in German and Chinese cohorts respectively. p.Ile2020Thr is a rare pathogenic mutation described only in a Japanese cohort. p.Met1869Thr has only been reported in Caucasians. p.Tyr1699Cys, p.Ile1122Val have only been found in one family each. p.Glu1874Ter has been described in just one patient. We found no references concerning mutation p.Gln416Ter. We also report the first case of a Polish PD family whose members carried p.Asn1437His.

Molecular mechanisms defining penetrance of LRRK2-associated Parkinson's disease

Mutations in Leucine-rich repeat kinase 2 (LRRK2) are the most frequent cause of dominantly inherited Parkinson's disease (PD). LRRK2 mutations, among which p.G2019S is the most frequent, are inherited with reduced penetrance. Interestingly, the disease risk associated with LRRK2 G2019S can vary dramatically depending on the ethnic background of the carrier. While this would suggest a genetic component in the definition of LRRK2-PD penetrance, only few variants have been shown to modify the age at onset of patients harbouring LRRK2 mutations, and the exact cellular pathways controlling the transition from a healthy to a diseased state currently remain elusive. In light of this knowledge gap, recent studies also explored environmental and lifestyle factors as potential modifiers of LRRK2-PD. In this article, we (i) describe the clinical characteristics of LRRK2 mutation carriers, (ii) review known genes linked to LRRK2-PD onset and (iii) summarize the cellular functions of LRRK2 with particular emphasis on potential penetrance-related molecular mechanisms. This section covers LRRK2's involvement in Rab GTPase and immune signalling as well as in the regulation of mitochondrial homeostasis and dynamics. Additionally, we explored the literature with regard to (iv) lifestyle and (v) environmental factors that may influence the penetrance of LRRK2 mutations, with a view towards further exposomics studies. Finally, based on this comprehensive overview, we propose potential future in vivo, in vitro and in silico studies that could provide a better understanding of the processes triggering PD in individuals with LRRK2 mutations.

Prevalence of ten LRRK2 variants in Parkinson's disease: A comprehensive review

INTRODUCTION

Variants in the leucine-rich repeat kinase 2 gene (LRRK2) are risk factors for Parkinson's disease (PD), but their prevalence varies geographically, reflecting the locations of founder events and dispersion of founders' descendants.

METHODS

A comprehensive literature review was conducted to identify studies providing prevalence estimates for any of ten variants in LRRK2 (G2019S, R1441C, R1441G, R1441H, I2020T, N1437H, Y1699C, S1761R, G2385R, R1628P) among individuals with PD globally. We calculated crude country-specific variant prevalence estimates and, when possible, adjusted estimates for ethno-racial composition. For clinic-based studies, probands were used over other familial cases, whereas for population-based studies, all PD cases were used.

RESULTS

The analysis included 161 articles from 52 countries yielding 581 prevalence estimates across the ten variants. G2019S was the most common variant, exceeding 1.0% in 26 of 51 countries with estimates. The other variants were far less common. G2385R and R1628P were observed almost exclusively in East Asian countries, where they were found in ∼5-10% of cases. All prevalence estimates adjusted for ethno-racial composition were lower than their unadjusted counterparts, although data permitting this adjustment was only available for six countries.

CONCLUSIONS

Except for G2019S, the LRRK2 variants covered in this review were uncommon in most countries studied. However, there were countries with higher prevalence for some variants, reflecting the uneven geographic distribution of LRRK2 variants. The fact that ethno-racial group‒adjusted estimates were lower than crude estimates suggests that estimates derived largely from clinic-based studies may overstate the true prevalence of some LRRK2 variants in PD.

Are genetic and idiopathic forms of Parkinson's disease the same disease?

Genetic forms represent a small fraction of Parkinson's disease (PD) but their discovery has revolutionized research in the field, putting α-synuclein in the spotlight, and uncovering other key neuropathological mechanisms of the disease. The question of whether genetic and idiopathic PD (iPD) correspond to a same disease entity is not simply philosophical, has implications for the discovery of the biological background of PD and for the development of novel therapeutic strategies that may also be applicable to the larger iPD group. Here, we review the current landscape of what has been labeled genetic PD and critically discuss the rational for merging or separating genetic and idiopathic forms of PD as the same or different disease entities. We conclude by addressing the potential implications for future research.

Genetic analysis of Mendelian mutations in a large UK population-based Parkinson's disease study

Our objective was to define the prevalence and clinical features of genetic Parkinson's disease in a large UK population-based cohort, the largest multicentre prospective clinico-genetic incident study in the world. We collected demographic data, Movement Disorder Society Unified Parkinson's Disease Rating Scale scores, and Montreal Cognitive Assessment scores. We analysed mutations in PRKN (parkin), PINK1, LRRK2 and SNCA in relation to age at symptom onset, family history and clinical features. Of the 2262 participants recruited to the Tracking Parkinson's study, 424 had young-onset Parkinson's disease (age at onset ≤ 50) and 1799 had late onset Parkinson's disease. A range of methods were used to genotype 2005 patients: 302 young-onset patients were fully genotyped with multiplex ligation-dependent probe amplification and either Sanger and/or exome sequencing; and 1701 late-onset patients were genotyped with the LRRK2 'Kompetitive' allele-specific polymerase chain reaction assay and/or exome sequencing (two patients had missing age at onset). We identified 29 (1.4%) patients carrying pathogenic mutations. Eighteen patients carried the G2019S or R1441C mutations in LRRK2, and one patient carried a heterozygous duplication in SNCA. In PRKN, we identified patients carrying deletions of exons 1, 4 and 5, and P113Xfs, R275W, G430D and R33X. In PINK1, two patients carried deletions in exon 1 and 5, and the W90Xfs point mutation. Eighteen per cent of patients with age at onset ≤30 and 7.4% of patients from large dominant families carried pathogenic Mendelian gene mutations. Of all young-onset patients, 10 (3.3%) carried biallelic mutations in PRKN or PINK1. Across the whole cohort, 18 patients (0.9%) carried pathogenic LRRK2 mutations and one (0.05%) carried an SNCA duplication. There is a significant burden of LRRK2 G2019S in patients with both apparently sporadic and familial disease. In young-onset patients, dominant and recessive mutations were equally common. There were no differences in clinical features between LRRK2 carriers and non-carriers. However, we did find that PRKN and PINK1 mutation carriers have distinctive clinical features compared to young-onset non-carriers, with more postural symptoms at diagnosis and less cognitive impairment, after adjusting for age and disease duration. This supports the idea that there is a distinct clinical profile of PRKN and PINK1-related Parkinson's disease. We estimate that there are approaching 1000 patients with a known genetic aetiology in the UK Parkinson's disease population. A small but significant number of patients carry causal variants in LRRK2, SNCA, PRKN and PINK1 that could potentially be targeted by new therapies, such as LRRK2 inhibitors.

STXBP1 encephalopathy

De novo pathogenic variants in STXBP1 encoding syntaxin1-binding protein (STXBP1, also known as Munc18-1) lead to a range of early-onset neurocognitive conditions, most commonly early infantile epileptic encephalopathy type 4 (EIEE4, also called STXBP1 encephalopathy), a severe form of epilepsy associated with developmental delay/intellectual disability. Other neurologic features include autism spectrum disorder and movement disorders. The progression of neurologic symptoms has been reported in a few older affected individuals, with the appearance of extrapyramidal features, reminiscent of early onset parkinsonism. Understanding the pathologic process is critical to improving therapies, as currently available antiepileptic drugs have shown limited success in controlling seizures in EIEE4 and there is no precision medication approach for the other neurologic features of the disorder. Basic research shows that genetic knockout of STXBP1 or other presynaptic proteins of the exocytic machinery leads to widespread perinatal neurodegeneration. The mechanism that regulates this effect is under scrutiny but shares intriguing hallmarks with classical neurodegenerative diseases, albeit appearing early during brain development. Most critically, recent evidence has revealed that STXBP1 controls the self-replicating aggregation of α-synuclein, a presynaptic protein involved in various neurodegenerative diseases that are collectively known as synucleinopathies, including Parkinson disease. In this review, we examine the tantalizing link among STXBP1 function, EIEE, and the neurodegenerative synucleinopathies, and suggest that neural development in EIEE could be further affected by concurrent synucleinopathic mechanisms.

Crystal structure of the WD40 domain dimer of LRRK2

Leucine-rich repeat kinase 2 (LRRK2) is a large multidomain protein with both a Ras of complex (ROC) domain and a kinase domain (KD) and, therefore, exhibits both GTPase and kinase activities. Human genetics studies have linked LRRK2 as a major genetic contributor to familial and sporadic Parkinson's disease (PD), a neurodegenerative movement disorder that inflicts millions worldwide. The C-terminal region of LRRK2 is a Trp-Asp-40 (WD40) domain with poorly defined biological functions but has been implicated in microtubule interaction. Here, we present the crystal structure of the WD40 domain of human LRRK2 at 2.6-Å resolution, which reveals a seven-bladed WD40 fold. The structure displays a dimeric assembly in the crystal, which we further confirm by measurements in solution. We find that structure-based and PD-associated disease mutations in the WD40 domain including the common G2385R polymorphism mainly compromise dimer formation. Assessment of full-length LRRK2 kinase activity by measuring phosphorylation of Rab10, a member of the family of Rab GTPases known to be important kinase substrates of LRRK2, shows enhancement of kinase activity by several dimerization-defective mutants including G2385R, although dimerization impairment does not always result in kinase activation. Furthermore, mapping of phylogenetically conserved residues onto the WD40 domain structure reveals surface patches that may be important for additional functions of LRRK2. Collectively, our analyses provide insights for understanding the structures and functions of LRRK2 and suggest the potential utility of LRRK2 kinase inhibitors in treating PD patients with WD40 domain mutations.

Genetic analysis of LRRK2 in Parkinson's disease in Han Chinese population

Mutations in Leucine-rich repeat kinase 2 (LRRK2) are recognized as the most frequent genetic factors contributing to Parkinson's disease (PD). The aim of our study was to explore LRRK2 variants in PD patients within the mainland Han Chinese population. The whole coding regions of LRRK2 from 296 PD patients were sequenced by targeted regions sequencing and exome sequencing. Eighteen rare variants were identified in 27 PD patients, and 13 of them (M100T, L153W, A459S, S722N, R792K, C925Y, R981K, S1007T, V1447M, R1677S, N2308D, N2313S, and S2350I) were firstly reported in PD. We also tried to explore the genotype-phenotype associations of LRRK2 variants in our data and found that PD with common and rare LRRK2 variants was more likely to have motor fluctuation and nonmotor symptoms. The identification of novel variants in LRRK2 suggests that this gene plays an important role in the pathogenesis and phenotype of PD in Han Chinese population, and our data also rang the alarm bell-more attention should be paid to the whole coding regions of LRRK2.

Leucine-Rich Repeat Kinase (LRRK2) Genetics and Parkinson's Disease

The discovery of LRRK2 mutations as a cause of Parkinson's disease (PD), including the sporadic late-onset form, established the decisive role of genetics in the field of PD research. Among LRRK2 mutations, the G2019S, mostly lying in a haplotype originating from a common Middle Eastern ancestor, has been identified in different populations worldwide. The G2385R and R1628P variants represent validated risk factors for PD in Asian populations. Here, we describe in detail the origin, the present worldwide epidemiology, and the penetrance of LRRK2 mutations. Furthermore, this chapter aims to characterize other definitely/probably pathogenic mutations and risk variants of LRRK2. Finally, we provide some general guidelines for a LRRK2 genetic testing and counseling. In summary, LRRK2 discovery revolutionized the understanding of PD etiology and laid the foundation for a promising future of genetics in PD research.

The discovery of LRRK2 p.R1441S, a novel mutation for Parkinson's disease, adds to the complexity of a mutational hotspot

Mutations in the LRRK2 gene result in autosomal dominant, late onset Parkinson's disease (PD). Three such mutations (p.R1441C, p.R1441G, and p.R1441H) are known to occur within codon 1441, and haplotype analyses indicate that each one has arisen independently on multiple occasions. We sequenced the entire coding region of 18 casual genes for PD or other parkinsonian neurodegenerative disorders in the proband of a family with autosomal dominant PD. We discovered a new missense mutation in the LRRK2 gene, c.4321C>A (p.R1441S). The mutation was predicted to be highly deleterious in silico (Combined Annotation Dependent Depletion score of 25.5) and segregated with disease in the pedigree. The clinical characteristics of affected family members were similar to those described in PD families with other mutations in LRRK2 codon 1441 and included resting tremor, rigidity, bradykinesia, unilateral onset, and a good response to levodopa. Age at onset ranged from 41 to 76. Two of the affected members of the pedigree underwent detailed, longitudinal neuropsychological testing, and both displayed evidence of mild cognitive deficits at or slightly preceding the onset of motor symptoms. LRRK2 p.R1441S represents the fourth pathogenic mutation observed within codon 1441 and its discovery adds to the remarkable complexity of a mutational hotspot within the ROC domain of the LRRK2 protein. © 2016 Wiley Periodicals, Inc.

Evaluating LRRK2 genetic variants with unclear pathogenicity

Mutations in the leucine-rich repeat kinase 2 (LRRK2) have been known to be a major genetic component affecting Parkinson's disease (PD). However, the pathogenicity of many of the LRRK2 variants is unclear because they have been detected in single patients or also in patients and controls. Here, we selected 5 exonic variants (L1165P, T1410M, M1646T, L2063X, and Y2189C) from each of the protein domain of LRRK2 and analysed their possible association with pathogenicity using in vitro functional assays. Point mutations representing each of these variants were incorporated into the LRRK2 gene, and functional aspects such as the percentage of cell survival upon application of stress and kinase activity were measured. Our results showed that all 5 variants had a significantly negative effect on the survival of cells, in both presence and absence of stress, as compared to the wild-type. In addition, there was also a slight increase in kinase activity in most of the variants in comparison to the wild-type. A negative correlation between cell survival and kinase activity was observed. These data suggest that most of the variants despite being located in different domains of LRRK2 appear to exert a potential pathogenic effect possibly through an increased kinase activity, supporting a gain of function mechanism.

Cognitive and behavioral symptoms in Parkinson's disease patients with the G2019S and R1441G mutations of the LRRK2 gene

OBJECTIVE

To compare the cognitive and psychiatric status of patients with Parkinson's disease related to the G2019S and the R1441G mutations of the LRRK2 gene (LRRK2-PD) and idiopathic Parkinson's disease (iPD) patients.

METHODS

We examined cognition and psychiatric symptoms in 27 patients with LRRK2-PD (12 G2019S and 15 R1441G) and 27 iPD patients.

RESULTS

The groups were similar in age, education, disease duration, levodopa equivalent daily dose, and Unified Parkinson's Disease Rating Scale (UPDRS) II-IV; however, the LRRK2-PD showed less impairment on UPDRS-I (2.0 ± 1.7 vs. 4.2 ± 2.8, p = 0.003). The LRRK2-PD presented less frequent subjective cognitive complaints (18.5% vs. 63.0%, p = 0.002), and mild cognitive impairment or dementia (25.9% vs. 59.2%, p = 0.027). They also showed less impairment on scales for general cognition (Mattis dementia rating scale 131.2 ± 10.9 vs. 119 ± 24.0, p = 0.022), episodic verbal memory (Rey's auditory verbal learning test, immediate recall 39.2 ± 9.5 vs. 27.6 ± 12.8 p < 0.001, delayed recall 7.2 ± 3.7 vs. 4.7 ± 4.0 p = 0.022), and the Neuropsychiatric Inventory (9.7 ± 9.2 vs. 20.5 ± 14.3, p = 0.004, significant differences for apathy and hallucinations). The LRRK2-PD subjects were less frequently treated with antipsychotic medication (0% vs. 25.9%, p = 0.010). There were no significant differences between G2019S and R1441G mutation carriers.

CONCLUSIONS

Mutations of the LRRK2 gene might cause PD associated with less cognitive and neuropsychiatric impairment as compared to iPD.

Advances in the Genetics of Parkinson's Disease: A Guide for the Clinician

Over the last 16 years, insights in clinical and genetic characteristics of Parkinson's disease (PD) have increased substantially. We summarize the clinical, genetic, and pathological findings of autosomal dominant PD linked to mutations in SNCA, leucine-rich repeat kinase 2, vacuolar protein sorting-35, and eukaryotic translation initiation factor 4 gamma 1 and autosomal recessive PD linked to parkin,PINK1, and DJ-1, as well as autosomal recessive complicated parkinsonian syndromes caused by mutations in ATP13A2,FBXO7,PLA2G6,SYNJ1, and DNAJC6. We also review the advances in high- and low-risk genetic susceptibility factors and present multisystem disorders that may present with parkinsonism as the major clinical feature and provide recommendations for prioritization of genetic testing. Finally, we consider the challenges of future genetic research in PD.

A multicenter study of glucocerebrosidase mutations in dementia with Lewy bodies

IMPORTANCE

While mutations in glucocerebrosidase (GBA1) are associated with an increased risk for Parkinson disease (PD), it is important to establish whether such mutations are also a common risk factor for other Lewy body disorders.

OBJECTIVE

To establish whether GBA1 mutations are a risk factor for dementia with Lewy bodies (DLB). DESIGN We compared genotype data on patients and controls from 11 centers. Data concerning demographics, age at onset, disease duration, and clinical and pathological features were collected when available. We conducted pooled analyses using logistic regression to investigate GBA1 mutation carrier status as predicting DLB or PD with dementia status, using common control subjects as a reference group. Random-effects meta-analyses were conducted to account for additional heterogeneity.

SETTING

Eleven centers from sites around the world performing genotyping.

PARTICIPANTS

Seven hundred twenty-one cases met diagnostic criteria for DLB and 151 had PD with dementia. We compared these cases with 1962 controls from the same centers matched for age, sex, and ethnicity.

MAIN OUTCOME MEASURES

Frequency of GBA1 mutations in cases and controls. RESULTS We found a significant association between GBA1 mutation carrier status and DLB, with an odds ratio of 8.28 (95% CI, 4.78-14.88). The odds ratio for PD with dementia was 6.48 (95% CI, 2.53-15.37). The mean age at diagnosis of DLB was earlier in GBA1 mutation carriers than in noncarriers (63.5 vs 68.9 years; P < .001), with higher disease severity scores.

CONCLUSIONS AND RELEVANCE

Mutations in GBA1 are a significant risk factor for DLB. GBA1 mutations likely play an even larger role in the genetic etiology of DLB than in PD, providing insight into the role of glucocerebrosidase in Lewy body disease.

The GTPase function of LRRK2

LRRK2 (leucine-rich repeat kinase 2) is a large protein encoding multiple functional domains, including two catalytically active domains, a kinase and a GTPase domain. The LRRK2 GTPase belongs to the Ras-GTPase superfamily of GTPases, more specifically to the ROC (Ras of complex proteins) subfamily. Studies with recombinant LRRK2 protein purified from eukaryotic cells have confirmed that LRRK2 binds guanine nucleotides and catalyses the hydrolysis of GTP to GDP. LRRK2 is linked to PD (Parkinson's disease) and GTPase activity is impaired for several PD mutants located in the ROC and COR (C-terminal of ROC) domains, indicating that it is involved in PD pathogenesis. Ras family GTPases are known to function as molecular switches, and several studies have explored this possibility for LRRK2. These studies show that there is interplay between the LRRK2 GTPase function and its kinase function, with most data pointing towards a role for the kinase domain as an upstream regulator of ROC. The GTPase function is therefore a pivotal functionality within the LRRK2-mediated signalling cascade which includes partners encoded by other LRRK2 domains as well as other cellular signalling partners. The present review examines what is known of the enzymatic properties of the LRRK2 GTPase, the interplay between ROC and other LRRK2 domains, and the interplay between ROC and other cellular proteins with the dual goal to understand how LRRK2 GTPase affects cellular functions and point to future research venues.

LRRK2: cause, risk, and mechanism

In 2004 it was first shown that mutations in LRRK2 can cause Parkinson's disease. This initial discovery was quickly followed by the observation that a single particular mutation is a relatively common cause of Parkinson's disease across varied populations. Further genetic investigation has revealed a variety of genetic ties to Parkinson's disease across this gene. These include common alleles with quite broad effects on risk, likely through both alterations at the protein sequence level, and in the context of expression. A great deal of functional characterization of LRRK2 and disease-causing mutations in this protein has occurred over the last 9 years, and considerable progress has been made. Particular attention has been paid to the kinase activity of LRRK2 as a therapeutic target, and while it is no means certain that this is viable target it is likely that this hypothesis will be tested in clinical trials sooner rather than later. We believe that the future goals for LRRK2 research are, while challenging, relatively clear and that the next 10 years of research promises to be perhaps more exciting than the last.

Redox proteomics analyses of the influence of co-expression of wild-type or mutated LRRK2 and Tau on C. elegans protein expression and oxidative modification: relevance to Parkinson disease

AIMS

The human LRRK2 gene has been identified as the most common causative gene of autosomal-dominantly inherited and idiopathic Parkinson disease (PD). The G2019S substitution is the most common mutation in LRRK2. The R1441C mutation also occurs in cases of familial PD, but is not as prevalent. Some cases of LRRK2-based PD exhibit Tau pathology, which suggests that alterations on LRRK2 activity affect the pathophysiology of Tau. To investigate how LRRK2 might affect Tau and the pathophysiology of PD, we generated lines of C. elegans expressing human LRRK2 [wild-type (WT) or mutated (G2019S or R1441C)] with and without V337M Tau. Expression and redox proteomics were used to identify the effects of LRRK2 (WT and mutant) on protein expression and oxidative modifications.

RESULTS

Co-expression of WT LRRK2 and Tau led to increased expression of numerous proteins, including several 60S ribosomal proteins, mitochondrial proteins, and the V-type proton ATPase, which is associated with autophagy. C. elegans expressing mutant LRRK2 showed similar changes, but also showed increased protein oxidation and lipid peroxidation, the latter indexed as increased protein-bound 4-hydroxy-2-nonenal (HNE).

INNOVATION

Our study brings new knowledge about the possible alterations induced by LRRK2 (WT and mutated) and Tau interactions, suggesting the involvement of G2019S and R1441C in Tau-dependent neurodegenerative processes.

CONCLUSION

These results suggest that changes in LRRK2 expression or activity lead to corresponding changes in mitochondrial function, autophagy, and protein translation. These findings are discussed with reference to the pathophysiology of PD.

Is inhibition of kinase activity the only therapeutic strategy for LRRK2-associated Parkinson's disease?

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are a common cause of familial Parkinson's disease (PD). Variation around the LRRK2 locus also contributes to the risk of sporadic PD. The LRRK2 protein contains a central catalytic region, and pathogenic mutations cluster in the Ras of complex protein C terminus of Ras of complex protein (mutations N1437H, R1441G/C and Y1699C) and kinase (G2019S and I2020T) domains. Much attention has been focused on the kinase domain, because kinase-dead versions of mutant LRRK2 are less toxic than kinase-active versions of the same proteins. Furthermore, kinase inhibitors may be able to mimic this effect in mouse models, although the currently tested inhibitors are not completely specific. In this review, we discuss the recent progress in the development of specific LRRK2 kinase inhibitors. We also discuss non-kinase-based therapeutic strategies for LRRK2-associated PD as it is possible that different approaches may be needed for different mutations.

The curious case of phenocopies in families with genetic Parkinson's disease

Monogenic forms of Parkinson's disease account for ∼3% of all "idiopathic" Parkinson's disease. With reduced penetrance in dominant forms and manifesting heterozygotes in recessive forms of Parkinson's disease, it has been well recognized that inheritance patterns do not always follow classic Mendelian genetics. A novel twist to the puzzle is the presence of phenocopies (i.e., family members with the same clinical syndrome as the mutation carriers, but lacking the familial mutation). We reviewed all pedigrees published between 1997 and 2009 with α-synuclein, leucine-rich repeat kinase 2, Parkin, or PTEN-induced kinase 1 mutations with at least 2 affected individuals and known genetic status for the possible presence of phenocopies. Of 537 patients with clinical Parkinson's disease in 160 families meeting our inclusion criteria, 27 patients (5.0%) from 23 families (14.4%) were phenocopies. Phenocopies represented 3.8% of all blood relatives reported in the pedigrees containing phenocopies and an estimated 1.3% of all blood relatives in all pedigrees included. Both of these rates exceeded age-specific prevalences of Parkinson's disease. In 4 families, the phenocopy was explained by another known mutation: In 2 pedigrees, a monogenic cause was likely; in another 2, secondary parkinsonism was suspected; and in the remaining 15 families, "sporadic Parkinson's disease" was suggested as the cause of disease in the phenocopy. The unexpectedly high number of phenocopies of mostly unknown origin within families with a seemingly known etiology of Parkinson's disease adds another level of complexity to genetic research of Parkinson's disease, as well as to the interpretation of genetic testing results in the clinical diagnostic setting.

TMEM106B a novel risk factor for frontotemporal lobar degeneration

Recently, the first genome-wide association (GWA) study in frontotemporal lobar degeneration (FTLD) identified common genetic variability at the TMEM106B gene on chromosome 7p21.3 as a potential important risk-modifying factor for FTLD with pathologic inclusions of TAR DNA-binding protein (FTLD-TDP), the most common pathological subtype in FTLD. To gather additional evidence for the implication of TMEM106B in FTLD risk, multiple replication studies in geographically distinct populations were set up. In this review, we revise all recent replication and follow-up studies of the FTLD-TDP GWA study and summarize the growing body of evidence that establish TMEM106B as a bona fide risk factor for FTLD. With the TMEM106B gene, a new player has been identified in the pathogenic cascade of FTLD which could hold important implications for the future development of disease-modifying therapies.

Genetic etiology of Parkinson disease associated with mutations in the SNCA, PARK2, PINK1, PARK7, and LRRK2 genes: a mutation update

To date, molecular genetic analyses have identified over 500 distinct DNA variants in five disease genes associated with familial Parkinson disease; alpha-synuclein (SNCA), parkin (PARK2), PTEN-induced putative kinase 1 (PINK1), DJ-1 (PARK7), and Leucine-rich repeat kinase 2 (LRRK2). These genetic variants include approximately 82% simple mutations and approximately 18% copy number variations. Some mutation subtypes are likely underestimated because only few studies reported extensive mutation analyses of all five genes, by both exonic sequencing and dosage analyses. Here we present an update of all mutations published to date in the literature, systematically organized in a novel mutation database (http://www.molgen.ua.ac.be/PDmutDB). In addition, we address the biological relevance of putative pathogenic mutations. This review emphasizes the need for comprehensive genetic screening of Parkinson patients followed by an insightful study of the functional relevance of observed genetic variants. Moreover, while capturing existing data from the literature it became apparent that several of the five Parkinson genes were also contributing to the genetic etiology of other Lewy Body Diseases and Parkinson-plus syndromes, indicating that mutation screening is recommendable in these patient groups.

Clinical features of LRRK2 parkinsonism

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene were initially identified in large families with autosomal dominant Parkinson disease (PD). These mutations (p.R1441C, p.R1441G, p.Y1699C and p.I2020T) revealed that genetic mutations could cause clinically typical, late-onset PD. Subsequently, the p.G2019S mutation was found to be a frequent cause of both autosomal dominant and "sporadic" PD, particularly in populations in North Africa or the Middle East. Two Lrrk2 protein substitutions (p.R1628P and p.G2385R) have since been associated with susceptibility to PD in Asian populations. More than a hundred variants have been identified in the LRRK2 gene, but pathogenicity is most convincing for the p.R1441H substitution. The role in PD remains unknown for other variants because segregation with disease has not been shown. Screening these variants in very large patient-control series may help clarify their role in PD. Lrrk2 is a large, multidomain protein with pathogenic mutations occurring in several functional domains. Cell biological experiments have shown that the p.G2019S mutation increase kinase activity. This is consistent with the observation that homozygous p.G2019S carriers do not have earlier disease onset or more severe disease compared with heterozygous carries. It is now necessary to identify the regulators and substrates of Lrrk2 in order to understand the effect of each LRRK2 mutation. The identification of a large number of presymptomatic LRRK2 mutation carriers provides a unique possibility for future studies on neuroprotection. However, more insight into the basic function of Lrrk2 is needed in order to exploit this potential for translational research.

From genes to proteins in mendelian Parkinson's disease: an overview

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. A progressive movement disorder typified by the production of bradykinesia, tremor, rigidity, and impairment of postural reflexes, PD is characterized by a depletion of dopamine in the striatum. For the last decade, several Mendelian forms of PD have been identified. Mutations in these genes potentially lead to autosomal dominant (alpha-synuclein and LRRK2), or autosomal recessive PD (Parkin, PINK1, DJ1, and ATP13A2). This article will spotlight these six distinct genes unambiguously associated with Mendelian PD and the function of their encoded proteins.

Zeroing in on LRRK2-linked pathogenic mechanisms in Parkinson's disease

The frequency and potency of mutations in the LRRK2 gene redefine the role of genetic susceptibility in Parkinson's disease. Dominant missense mutations that fulfill initial criteria for potential gain of function mechanisms coupled with enzymatic activity likely amenable to small molecule inhibition position LRRK2 as a promising therapeutic target. Herein, key observations from the clinic to the test tube are highlighted together with points of contention and outstanding critical issues. Resolution of the critical issues will expedite the development of therapies that exploit LRRK2 activity for neuroprotection strategies.

Lrrk2 R1441C parkinsonism is clinically similar to sporadic Parkinson disease

OBJECTIVE

Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common cause of Parkinson disease (PD). Several dominantly inherited pathogenic substitutions have been identified in different domains of the Lrrk2 protein. Herein, we characterize the clinical and genetic features associated with Lrrk2 p.R1441C.

METHODS

We identified 33 affected and 15 unaffected LRRK2 c.4321C>T (p.R1441C) mutation carriers through an international consortium originating from three continents. The age-specific cumulative incidence of PD was calculated by Kaplan-Meier analysis.

RESULTS

The clinical presentation of Lrrk2 p.R1441C carriers was similar to sporadic PD and Lrrk2 p.G2019S parkinsonism. The mean age at onset for parkinsonism was 60 years, range 30-79 years; fewer than 20% of the patients had symptoms before the age 50 years, while by 75 years >90% of them had developed symptoms. Haplotype analysis suggests four independent founders for the p.R1441C mutation.

CONCLUSIONS

The distribution in age at onset and clinical features in Lrrk2 p.R1441C patients are similar to idiopathic and Lrrk2 p.G2019S parkinsonism. Several independent founders of the p.R1441C substitution suggest this site is prone to recurrent mutagenesis.