Analysis of LRRK2 functional domains in nondominant Parkinson disease

Parkinson's Disease is Predominantly a Genetic Disease

The discovery of a pathogenic variant in the alpha-synuclein (SNCA) gene in the Contursi kindred in 1997 indisputably confirmed a genetic cause in a subset of Parkinson's disease (PD) patients. Currently, pathogenic variants in one of the seven established PD genes or the strongest known risk factor gene, GBA1, are identified in ∼15% of PD patients unselected for age at onset and family history. In this Debate article, we highlight multiple avenues of research that suggest an important - and in some cases even predominant - role for genetics in PD aetiology, including familial clustering, high rates of monogenic PD in selected populations, and complete penetrance with certain forms. At first sight, the steep increase in PD prevalence exceeding that of other neurodegenerative diseases may argue against a predominant genetic etiology. Notably, the principal genetic contribution in PD is conferred by pathogenic variants in LRRK2 and GBA1 and, in both cases, characterized by an overall late age of onset and age-related penetrance. In addition, polygenic risk plays a considerable role in PD. However, it is likely that, in the majority of PD patients, a complex interplay of aging, genetic, environmental, and epigenetic factors leads to disease development.

A Next-Generation Sequencing Study in a Cohort of Sicilian Patients with Parkinson's Disease

Parkinson's disease (PD) is a multisystem and multifactorial disorder and, therefore, the application of modern genetic techniques may assist in unraveling its complex pathophysiology. We conducted a clinical-demographic evaluation of 126 patients with PD, all of whom were Caucasian and of Sicilian ancestry. DNA was extracted from the peripheral blood for each patient, followed by sequencing using a Next-Generation Sequencing system. This system was based on a custom gene panel comprising 162 genes. The sample underwent further filtering, taking into account the allele frequencies of genetic variants, their presence in the Human Gene Mutation Database, and their association in the literature with PD or other movement/neurodegenerative disorders. The largest number of variants was identified in the leucine-rich repeat kinase 2 (LRRK2) gene. However, variants in other genes, such as acid beta-glucosidase (GBA), DNA polymerase gamma catalytic subunit (POLG), and parkin RBR E3 ubiquitin protein ligase (PRKN), were also discovered. Interestingly, some of these variants had not been previously associated with PD. Enhancing our understanding of the genetic basis of PD and identifying new variants possibly linked to the disease will contribute to improved diagnostic accuracy, therapeutic developments, and prognostic insights for affected individuals.

LRRK2 exonic variants are associated with lysosomal hydrolase activities and lysosphingolipid alterations in Parkinson's disease

Last data demonstrated that exonic variants of LRRK2 (p.G2019S, p.M1646T) may affect the catalytic activity of lysosomal enzyme glucocerebrosidase (GCase) probably through the phosphorylation of Rab10 protein. We aimed to evaluate an association of LRRK2 exonic variants previously associated with alteration of phosphorylation levels for Rab10Thr73 with PD risk in Russian population and analyze an impact of p.G2019S mutation and selected LRRK2 variants on lysosomal hydrolase activities. LRRK2 variants were determined by full sequencing of LRRK2 in 508 PD patients and 470 controls from Russian population. Activity of lysosomal enzymes (glucocerebrosidase (GCase), alpha-galactosidase A (GLA), acid sphingomyelinase (ASMase) and concentrations of their corresponded substrates (hexosylsphingosine (HexSph), globotriaosylsphingosine (LysoGb3), lysosphingomyelin (LysoSM), respectively) were estimated in 211 PD patients and 179 controls by liquid chromatography with tandem mass spectrometry (LC-MS-MS) in dry blood spots. p.M1646T and p.N2081D were associated with PD (OR = 2.33, CI 95%: 1.1215 to 4.8253, p = 0.023; OR = 1.89, 95%CI: 1.0727 to 3.3313, p = 0.028, respectively) in Russian population. An increased LysoGb3 concentration was found in p.G2019S and p.N2081D LRRK2 carriers among PD patients compared to both PD patients and controls (p.G2019S: p = 0.00086, p = 0.0004, respectively; p.N2081D: p = 0.012, p = 0.0076, respectively). A decreased ASMase activity in p.G2019S LRRK2 carriers among PD patients (p = 0.014) was demonstrated as well. Our study supported possible involvement of LRRK2 dysfunction in an alteration of sphingolipid metabolism in PD.

[Essential tremor: genetic update]

Essential tremor (ET) is a neurological movement disorder characterised by bilateral limb kinetic/postural tremor, with or without tremor in other body parts including head, voice and lower limbs. Since no causative genes for ET have been identified, it is likely that the disorder occurs as a result of complex genetic factors interacting with various cellular and environmental factors that can result in abnormal function of circuitry involving the cerebello-thalamo-cortical pathway. Genetic analyses have uncovered at least 14 loci and 11 genes that are related to ET, as well as various risk or protective genetic factors. Limitations in ET genetic analyses include inconsistent disease definition, small sample size, varied ethnic backgrounds and many other factors that may contribute to paucity of relevant genetic data in ET. Genetic analyses, coupled with functional and animal studies, have led to better insights into possible pathogenic mechanisms underlying ET. These genetic studies may guide the future development of genetic testing and counselling, and specific, pathogenesis-targeted, therapeutic strategies.

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

Case-control analysis of LRRK2 protective variants in Essential Tremor

Co-existence of Parkinson’s disease (PD) and essential tremor (ET) may reflect overlapping pathophysiology underlying both conditions. Furthermore, PD patients with leucine-rich repeat kinase-2 (LRRK2) mutations may present with ET-like features, suggesting the possibility of common genetic underpinnings. Two common LRRK2 variants, R1398H and N551K, have been shown to be protective in multiple PD cohorts. We hypothesized that R1398H and N551K may show a similar effect in ET. In a case-control study involving 3198 subjects (2680 controls and 518 ET cases), R1398H was detected in 16.6% of ET cases compared to 18.0% in controls (OR = 0.91, 95% CI = 0.71–1.17, p = 0.46); while N551K was detected in 16.5% of ET cases compared to 18.0% of controls (OR = 0.89, 95% CI = 0.69–1.15, p = 0.37). While these results suggest that LRRK2 R1398H or N551K do not appear to modulate the risk of ET, it remains possible that a protective trend for both variants may be present in ET and a much larger sample size is required to identify this.

Role of LRRK2 and SNCA in autosomal dominant Parkinson's disease in Turkey

INTRODUCTION

Mutations in the LRRK2 and alpha-synuclein (SNCA) genes are well-established causes of autosomal dominant Parkinson's disease (PD). However, their frequency differs widely between ethnic groups. Only three studies have screened all coding regions of LRRK2 and SNCA in European samples so far. In Turkey, the role of LRRK2 in Parkinson's disease has been studied fragmentarily, and the incidence of SNCA copy number variations is unknown. The purpose of this study is to determine the frequency of LRRK2 and SNCA mutations in autosomal dominant PD in Turkey.

METHODS

We performed Sanger sequencing of all coding LRRK2 and SNCA exons in a sample of 91 patients with Parkinsonism. Copy number variations in SNCA, PRKN, PINK1, DJ1 and ATP13A2 were assessed using the MLPA method. All patients had a positive family history compatible with autosomal dominant inheritance.

RESULTS

Known mutations in LRRK2 and SNCA were found in 3.3% of cases: one patient harbored the LRRK2 G2019S mutation, and two patients carried a SNCA gene duplication. Furthermore, we found a heterozygous deletion of PRKN exon 2 in one patient, and four rare coding variants of unknown significance (LRRK2: A211V, R1067Q, T2494I; SNCA: T72T). Genetic testing in one affected family identified the LRRK2 R1067Q variant as a possibly pathogenic substitution.

CONCLUSION

Point mutations in LRRK2 and SNCA are a rare cause of autosomal dominant PD in Turkey. However, copy number variations should be considered. The unclassified variants, especially LRRK2 R1067Q, demand further investigation.

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.

Splicing: is there an alternative contribution to Parkinson's disease?

Alternative splicing is a crucial mechanism of gene expression regulation that enormously increases the coding potential of our genome and represents an intermediate step between messenger RNA (mRNA) transcription and protein posttranslational modifications. Alternative splicing occupies a central position in the development and functions of the nervous system. Therefore, its deregulation frequently leads to several neurological human disorders. In the present review, we provide an updated overview on the impact of alternative splicing in Parkinson's disease (PD), the second most common neurodegenerative disorder worldwide. We will describe the alternative splicing of major PD-linked genes by collecting the current evidences about this intricate and not carefully explored aspect. Assessing the role of this mechanism on PD pathobiology may represent a central step toward an improved understanding of this complex disease.

Lrrk2 R1628P variant is a risk factor for essential tremor

Essential tremor (ET) and Parkinson's disease (PD) are two of the most common adult onset movement disorders with overlapping clinical features. PD patients with leucine-rich repeat kinase-2 (LRRK2) mutations may present initially with an ET phenotype. To address the possibility of a common genetic link between ET and PD, we examined the association between a common LRRK2 R1628P gene variant and ET. The LRRK2 R1628P was genotyped in ET cases and matched healthy controls. A total of 1277 subjects comprising of 450 ET cases and 827 controls were included. There were 40 heterozygote (GG to CG) variant out of 450 ET cases (genotypic frequency 8.9%) and 36 heterozygote variant (GG to CG, genotypic frequency 4.3%) and one homozygote variant (GG to CC) out of 827 controls. Subjects carrying the R1628P variant had a twofold increased risk of ET (p = 0.0035, OR = 2.20 and 95% confidence interval is 1.30–3.73). Using a case control methodology, we demonstrated an association between a known PD risk variant, LRRK2 R1628P, with ET. Subjects carrying the R1628P variant had twice the risk of developing ET. The sharing of a similar gene risk variant suggests a possible pathophysiologic link between PD and ET.

Genetic analysis of the fused in sarcoma gene in Chinese Han patients with Parkinson's disease

BACKGROUND AND PURPOSE

Exome sequencing in a large essential tremor (ET) family identified a novel nonsense mutation (p.Q290X) in the fused in sarcoma gene (FUS) as the cause of this family. Because of the clinical overlap between ET and Parkinson's disease (PD), the role of FUS in an independent cohort of PD patients from China mainland was evaluated.

METHODS

The entire coding region of FUS in 508 Chinese Han patients with PD and the identified variants in 633 normal controls were evaluated. A variant was further screened in an additional 382 controls for the frequency in our population.

RESULTS

A novel variant c.696C > T (p.Y232Y) in 2 sporadic patients with PD and six variants (c.52C > A, p.P18T; c.52C > T, p.P18S; c.147C > A, p.G49G; c.291C > T, p.Y97Y; c.684C > T, p.G228G; c.1176G > A, p.M392I) without significant difference in genotypic and allelic distributions in our PD cohort were identified.

CONCLUSION

The FUS gene is not a genetic risk factor for PD in the population of Chinese Han ethnicity.

What genetics tells us about the causes and mechanisms of Parkinson's disease

Parkinson's disease (PD) is a common motor disorder of mysterious etiology. It is due to the progressive degeneration of the dopaminergic neurons of the substantia nigra and is accompanied by the appearance of intraneuronal inclusions enriched in α-synuclein, the Lewy bodies. It is becoming increasingly clear that genetic factors contribute to its complex pathogenesis. Over the past decade, the genetic basis of rare PD forms with Mendelian inheritance, representing no more than 10% of the cases, has been investigated. More than 16 loci and 11 associated genes have been identified so far; genome-wide association studies have provided convincing evidence that polymorphic variants in these genes contribute to sporadic PD. The knowledge acquired of the functions of their protein products has revealed pathways of neurodegeneration that may be shared between inherited and sporadic PD. An impressive set of data in different model systems strongly suggest that mitochondrial dysfunction plays a central role in clinically similar, early-onset autosomal recessive PD forms caused by parkin and PINK1, and possibly DJ-1 gene mutations. In contrast, α-synuclein accumulation in Lewy bodies defines a spectrum of disorders ranging from typical late-onset PD to PD dementia and including sporadic and autosomal dominant PD forms due to mutations in SCNA and LRRK2. However, the pathological role of Lewy bodies remains uncertain, as they may or may not be present in PD forms with one and the same LRRK2 mutation. Impairment of autophagy-based protein/organelle degradation pathways is emerging as a possible unifying but still fragile pathogenic scenario in PD. Strengthening these discoveries and finding other convergence points by identifying new genes responsible for Mendelian forms of PD and exploring their functions and relationships are the main challenges of the next decade. It is also the way to follow to open new promising avenues of neuroprotective treatment for this devastating disorder.

Genetic variants of α-synuclein are not associated with essential tremor

BACKGROUND

Given the overlap between Parkinson's disease and essential tremor, we examined genetic variants in α-synuclein (SNCA) as risk determinants for essential tremor.

METHODS

Samples from 661 essential tremor subjects and 1316 control subjects from 4 participating North American sites were included in this study. Parkinson's disease samples (n = 427) were compared against controls. Twenty variants were selected for association analysis within the SNCA locus. Individual logistic regression analyses against essential tremor diagnosis were run for each variant and then combined using meta-analysis.

RESULTS

Our results do not show a significant association between variants in the SNCA locus and risk of essential tremor, whereas the established association of SNCA variants with Parkinson's disease risk was observed.

CONCLUSIONS

Whereas genetic factors are likely to play a large role in essential tremor pathogenesis, our results do not support a role for common SNCA genetic variants in risk for essential tremor.

Revisiting the relationship between essential tremor and Parkinson's disease

BACKGROUND

The relationship between essential tremor and Parkinson's disease has been a subject of reviews and debates for long time, but there is now growing evidence that the two common movement disorders are pathogenically related, at least in some patient populations.

METHODS

PubMed as well as authors' own files were searched for relevant keywords regarding overlap between the disorders in clinical features as well as on epidemiologic, genetic, imaging, and pathological studies.

RESULTS

New findings in each of these categories are critically reviewed and placed in the context of previously published data.

DISCUSSION

Although we believe that there is compelling evidence for the notion that some patients with "pure" ET evolve into PD, the biologic nature of the association is not well understood. Furthermore, it is not clear what factors predict which ET patients later develop PD and whether patients with PD are more likely to develop ET. Further epidemiologic, clinical, genetic, imaging, and pathological studies are needed to better understand this mixed, ET-PD phenotype.

Comprehensive mutational analysis of LRRK2 reveals variants supporting association with autosomal dominant Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by neurodegeneration, most notably of dopaminergic neurons in the substantia nigra. To date, six causative genes have been identified including LRRK2, whose mutations are the most frequent in autosomal dominant PD (Ad-PD). We conducted a comprehensive mutational analysis of LRRK2 in 30 Ad-PD (11 Japanese and 19 Caucasian) families employing a DNA microarray-based resequencing system and direct nucleotide sequence analysis, and identified 23 variants including two known mutations, p.G2019S and p.I1371V, in three Caucasian families and one Caucasian family, respectively, a novel putative pathogenic mutation, p.N1221K, in one Japanese family, and a known nonsynonymous variant, p.G2385R, in two Japanese families. Detailed analysis of the frequency of p.G2385R among 100 Japanese Ad-PD, 73 sporadic PD (sPD) and 238 controls revealed that the frequency of the p.G2385R variant was significantly higher in Ad-PD than in controls (allele frequency, 9.0 vs 2.1%) (χ(2)=16.32, P=5.34 × 10(-5)). The p.G2385R variant, however, did not show complete cosegregation with PD. In addition, the frequency of p.G2385R was also higher in sPD than in controls, although not significant (allele frequency, 3.4 vs 2.1%) (χ(2)=0.76, P=0.38). These observations support the possibility that p.G2385R is associated with an increased risk of PD.

Development of Parkinson's disease biomarkers

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, affecting over 6 million people worldwide. It is anticipated that the number of affected individuals may increase significantly in the most populous nations by 2030. During the past 20 years, much progress has been made in identifying and assessing various potential clinical, biochemical, imaging and genetic biomarkers for PD. Despite the wealth of information, development of a validated biomarker for PD is still ongoing. It is hoped that reliable and well-validated biomarkers will provide critical clues to assist in the diagnosis and management of Parkinson's disease patients in the near future.

Lingo2 variants associated with essential tremor and Parkinson's disease

LINGO2, a member of LRR gene family, has been linked with both Essential tremor (ET) and Parkinson's disease (PD). However, there is a lack of conclusive evidence regarding the etiologic role of LINGO2 genetic variants. We investigated the association of LINGO2 variants with ET and PD in two independent Asian countries. A total of 1,262 subjects comprising 499 controls, 436 PD patients, and 327 ET patients were included. Eight LINGO2 variants, including four single-nucleotide polymorphisms (SNPs) and four coding variants, were initially analyzed in one Asian population. SNPs that showed positive association were then replicated in the second independent Asian population, and a pooled analysis was carried out. Out of the eight variants, two SNPs (rs7033345 and rs10812774) revealed significant or strong positive trend in the first Asian population, and these were analyzed in the second Asian population. In the pooled analysis, the CC genotype at rs7033345 had a higher risk of developing PD (OR = 1.67, 95% CI = 1.18, 2.35, p = 0.003) and ET (OR = 1.50, 95% CI = 1.02, 2.20, p = 0.04) under a recessive model. The C allele at rs10812774 increased the risk of ET (OR = 1.56 95% CI = 1.10, 2.22, p = 0.01) via a recessive model. The effect size and direction of trend were in the same direction in each of the two populations. Our study demonstrated for the first time that rs7033345 is associated with PD and ET and rs10812774 with ET among Asians, suggesting that LINGO2 might act as a susceptibility gene for both conditions.

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.

LRRK2 R1628P variant is a risk factor of Parkinson's disease among Han-Chinese from mainland China

Mutations in LRRK2, the gene that encodes leucine-rich repeat kinase 2 (LRRK2), are associated with autosomal dominant and sporadic forms of Parkinson's disease (PD) and are the most common genetic causes of PD. Recently, a R1628P variant has been reported as a risk factor for PD in Taiwan and Singapore. To determine the association of this variant and PD in the Han-Chinese population from mainland China, we analyzed its frequency in a cohort of 600 patients and 459 unrelated healthy controls. Forty (6.7%) patients were heterozygous and 3 (0.5%) homozygous for the R1628P variant, which was significantly more frequent than in the controls [2.4% heterozygous and 0.0% homozygous, Odds ratio = 3.14, 95%CI: 1.60-6.17, P < 0.01]. Considering the age at onset, this difference was found only in late-onset PD (older than 50) [Odds ratio = 3.76, 95% CI: 1.90-7.45, P < 0.01]. Our data confirms that the LRRK2 R1628P variant is associated with an increased risk to develop late onset PD in the ethnic Han-Chinese population.

LRRK2 mutations and risk variants in Japanese patients with Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic determinant of Parkinson's disease (PD) in European-derived populations, but far less is known about LRRK2 mutations and susceptibility alleles in Asians. To address this issue, we sequenced the LRRK2 coding region in 36 patients with familial PD, then genotyped variants of interest in an additional 595 PD cases and 1,641 controls who were all of Japanese ancestry. We also performed a meta-analysis of studies on G2385R, a polymorphism previously reported to associate with PD. One pathogenic (G2019S) and one putative pathogenic (R1067Q) mutation were each observed in two patients with sporadic PD. The overall mutation frequency among patients was 0.6%. G2385R was highly associated with PD under a dominant model in our dataset (adjusted OR, 1.83; 95% CI, 1.31-2.54; P = 3.3 x 10(-4)) and similar results were seen in the meta-analysis (summary OR assuming fixed effects, 2.55; 95% CI, 2.10-3.10). G2385R represents the first consistently replicated common PD susceptibility variant in a non-European population and its effect size is substantially greater than that reported for other well-validated genetic risk factors for the disease. However, LRRK2 mutations appear to be rare among Japanese patients with PD.

Parkinson's disease and low frequency alleles found together throughout LRRK2

Mutations within LRRK2, most notably p.G2019S, cause Parkinson's disease (PD) in rare monogenic families, and sporadic occurrences in diverse populations. We investigated variation throughout LRRK2 (84 SNPs; genotype or diplotype found for 49 LD blocks) for 275 cases (European ancestry, onset at age 60 or older) and 275 neurologically healthy control subjects (NINDS Neurogenetics Repository). Three grade-of-membership groups, i.e. genetic risk sets, were identified that exactly matched many subjects (cases: 46, 4, 137; controls: 0, 178, 0), and distinguished 94% of the subjects (i.e. >50% likeness to one set). Set I, affected, carried certain low frequency alleles located in multiple functional domains. Set II was unaffected. Set III, also affected, resembled set II except for slightly elevated frequencies of minor alleles not defining set I. We conclude that certain low frequency alleles distributed throughout LRRK2 are a genetic background to a third of cases, defining a distinct subset.

LRRK2 and neurodegeneration

Mutations in leucine-rich repeat kinase 2 gene (PARK8/LRRK2) encoding the protein Lrrk2 are causative of inherited and sporadic Parkinson's disease (PD) with phenotypic manifestations of frontotemporal lobar degeneration, corticobasal degeneration and associated motor neuron disease in some patients, and with variable penetrance. Neuropathology is characterized by loss of dopaminergic neurons in the substantia nigra pars compacta in all cases with accompanying Lewy pathology, or tau pathology or without intraneuronal inclusions, thus indicating that mutations in LRRK2 are not always manifested as Lewy body disease (LBD) or as alpha-synucleinopathy. Molecular studies have not disclosed clear association between nerve cell degeneration and modifications in the kinase activity of Lrrk2, and the pathogenesis of LRRK2 mutations remains unknown. Several morphological studies have suggested that Lrrk2 is a component of Lewy bodies and aberrant neurites in sporadic PD and Dementia with Lewy bodies, whereas other studies have indicated that Lrrk2 does not participate in Lewy body composition. Likewise, some studies have shown Lrrk2 immunoreactivity in hyper-phosphorylated tau inclusions in Alzheimer's disease (AD) and other tauopathies, whereas other studies did not find Lrrk2 in hyper-phosphorylated tau inclusions. We have used three currently used anti-Lrrk2 antibodies (NB-300-268, NB-300-267 and AP7099b) and concluded that these differences are largely dependent on the antibodies used and, particularly, on the interpretation of the origin of the multiple bands of low molecular weight species, in addition to the band corresponding to full-length Lrrk2, that recognize the majority of these antibodies. A review of the available data and our results indicate that full-length Lrrk2 is not a major component of Lewy bodies in LBDs, and of hyper-phosphorylated tau inclusions in AD and tauopathies. Bands of low molecular weight are probably not the result of post-mortem artefacts as they are also present in cultured cells processed under optimal conditions. Truncated forms of Lrrk2 and additional transcripts related with LRRK2, in the absence of spliced forms of Lrrk2 may account for Lrrk2 immunoreactivity in distinct intraneuronal inclusions.

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.

Evidence of increased odds of essential tremor in Parkinson's disease

In a case control study using a standardized protocol, 600 subjects were evaluated for essential tremor (ET). We demonstrated that ET was significantly more frequent in patients with Parkinson's disease (PD) (12/204, 5.9%) compared to diseased controls (2/206, 1%) and healthy controls (1/190, 0.5%). A regression analysis with ET as outcome and group (either PD or healthy controls or diseased controls) as independent variable (adjusting for age and sex) revealed that PD had higher odds of having ET than diseased controls (OR = 5.43, 95% CI = 1.16, 25.39, P < 0.001) and healthy controls (OR = 10.87, 95% CI = 1.39, 85.15, P < 0.001). The low frequency of ET in our controls was further confirmed in a follow-up study in a group of age and gender matched general medical patients who attended an outpatient clinic (0% frequency). Eight of 204 PD (3.9%) compared to none of diseased (0%) (P = 0.004) and healthy controls (0%) (P = 0.008) had a prior diagnosis of ET. The duration of ET symptoms in patients with PD was 25.1 +/- 19.6 (range 3-60) years. A multivariate analysis demonstrated that a lower dose of levodopa (OR = 0.993, 95%CI for OR = 0.988, 0.997, P < 0.001) and a higher age of onset of disease (OR = 1.108, 95%CI for OR = 1.035, 1.187, P < 0.001) were associated with increased odds of PD with ET, compared to patients with PD without ET. In our Asian population, patients with PD were 5 to 10 times more likely to have ET compared to diseased and healthy controls, suggesting that the association of ET and PD is unlikely to be ethnicity-specific.

The nonsynonymous Thr105Ile polymorphism of the histamine N-methyltransferase is associated to the risk of developing essential tremor

OBJECTIVE

We analyzed in patients with essential tremor (ET) the Thr105Ile polymorphism of the Histamine N-methyltransferase (HNMT) enzyme that is associated to Parkinson's disease (PD) risk.

METHODS

Leukocytary DNA from 204 ET patients and a control group of 295 unrelated healthy individuals was studied for the nonsynonymous HNMT Thr105Ile polymorphism by using amplification-restriction analyses.

RESULTS

Patients with ET showed a higher frequency of homozygous HNMT 105Thr genotypes leading to high metabolic activity (p < 0.015) with a statistically significant gene-dose effect, as compared to healthy subjects. These findings were independent of gender, and of tremor localization, but the association of the HNMT polymorphism is more prominent among patients with late-onset ET (p < 0.007).

CONCLUSION

These results, combined with previous findings indicating alterations in the frequency for the HNMT Thr105Ile polymorphism in patients with PD, suggest that alterations of histamine homeostasis in the SNC are associated with the risk of movement disorders.

Corticobasal syndrome and primary progressive aphasia as manifestations of LRRK2 gene mutations

BACKGROUND

Mutations in the LRRK2 gene are an important cause of familial and nonfamilial parkinsonism. Despite pleomorphic pathology, LRRK2 mutations are believed to manifest clinically as typical Parkinson disease (PD). However, most genetic screens have been limited to PD clinic populations.

OBJECTIVE

To clinically characterize LRRK2 mutations in cases recruited from a spectrum of neurodegenerative diseases.

METHODS

We screened for the common G2019S mutation and several additional previously reported LRRK2 mutations in 434 individuals. A total of 254 patients recruited from neurodegenerative disease clinics and 180 neurodegenerative disease autopsy cases from the University of Pennsylvania brain bank were evaluated.

RESULTS

Eight cases were found to harbor a LRRK2 mutation. Among patients with a mutation, two presented with cognitive deficits leading to clinical diagnoses of corticobasal syndrome and primary progressive aphasia.

CONCLUSION

The clinical presentation of LRRK2-associated neurodegenerative disease may be more heterogeneous than previously assumed.

Striatal leucine-rich repeat kinase 2 mRNA is increased in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned common marmosets (Callithrix jacchus) with L-3, 4-dihydroxyphenylalanine methyl ester-induced dyskinesia

The level of leucine-rich repeat kinase 2 (Lrrk2) mRNA expression was measured by reverse transcription-polymerase chain reaction in anterior striatum from normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets (Callithrix jacchus) that had L-3,4-dihydroxyphenylalanine methyl ester (L-DOPA)-induced dyskinesia. The level of striatal Lrrk2 mRNA was increased in MPTP-treated common marmosets that had L-DOPA-induced dyskinesia compared with normal animals that did not receive l-DOPA. Marmosets that exhibited higher levels of dyskinesia had the greatest increase in striatal Lrrk2 mRNA. Lrrk2 mRNA expression was also measured in human striatum and substantia nigra from control subjects and patients dying with Parkinson's disease. In contrast to marmoset tissue, no alteration in Lrrk2 mRNA expression was found in parkinsonian human brain. However, the brain was from patients who had an overall low level of dyskinesia. The correlation between striatal Lrrk2 mRNA levels in MPTP-treated common marmoset striatum and L-DOPA-induced dyskinesia indicates that LRRK2 may have a role in the molecular alterations that cause L-DOPA-induced dyskinesia.

Frequency of the LRRK2 G2019S Mutation in Siblings with Parkinson’s Disease

BACKGROUND

Mutations in the LRRK2 gene, the most frequent of which is the G2019S mutation in exon 41, cause familial and sporadic Parkinson's disease (PD) with reduced penetrance.

OBJECTIVES

To assess the frequency of the LRRK2 G2019S mutation in families thought to have autosomal recessive PD (siblings but not their parents were affected) and to determine the clinical features of LRRK2 mutation carriers.

METHODS

We sequenced both strands of exon 41 of the LRKK2 gene in 90 index cases from French and North African families in which PD might have been inherited as a recessive trait. Patients with mutations underwent detailed clinical evaluations.

RESULTS

We found heterozygous G2019S mutations in 1 Algerian and 1 French family (overall relative frequency = 2.2%). Four LRRK2-positive patients had typical PD, but their Mini Mental State Examination (MMSE) scores were lower than those of patients without this mutation.

CONCLUSION

The LRRK2 G2019S mutation is as frequent in families with possible autosomal recessive PD (2.2%) as in the sporadic cases published elsewhere (1.9%). The clinical features in the LRRK2-positive patients were those of typical PD, except for lower MMSE scores.

Pathogenicity of the Lrrk2 R1514Q substitution in Parkinson's disease

An increasing number of nonsynonymous LRRK2 variants are being reported as putative pathogenic mutations. We identified one large kindred harboring the Lrrk2 R1514Q substitution; however, the variant did not segregate fully with disease. Combined analyses of three case-control series demonstrate that the R1514Q substitution is not associated with increased risk of disease (OR: 1.3; 95% CI: 0.6-2.8; P = 0.45). These findings highlight the importance of using family-based studies and multiple population screenings when examining the association of these polymorphic LRRK2 gene variants with Parkinson's disease.

Screening for Lrrk2 G2019S and clinical comparison of Tunisian and North American Caucasian Parkinson's disease families

Mutations in the leucine-rich repeat kinase-2 gene (LRRK2) are responsible for some forms of familial as well as sporadic Parkinson's disease (PD). The purpose of this study was to examine the frequency of a single pathogenic mutation (6055G > A) in the kinase domain of this gene in United States and Tunisian familial PD and to compare clinical characteristics between patients with and without the mutation. Standardized case report forms were used for clinical and demographic data collection. We investigated the frequency of the most common substitution of LRRK2 (G2019S, 6055G>A) and its impact on epidemiological and phenotypic features. The frequency of mutations in Tunisian families was 42% (38/91) and in U.S. families 2.6% (1/39), with the unique opportunity to compare homozygous (n = 23) and heterozygous (n = 109) Tunisian carriers of G2019S substitutions. Individuals with G2019S substitutions had an older age at onset but few other differences compared with families negative for the substitution. Patients with LRRK2 mutations had typical clinical features of PD. Comparisons between individuals with heterozygous and homozygous LRRK2 mutations suggested that gene dosage was not correlated with phenotypic differences; however, the estimated penetrance was greater in homozygotes across all age groups.

Exploring the relationship between essential tremor and Parkinson's disease

Although essential tremor (ET) and Parkinson's disease (PD) are considered distinct disorders, there is overlap in some clinical features. In some PD patients, a long-standing postural tremor in the hands may precede the onset of parkinsonian features by several years or decades. Furthermore, large families with both ET and PD phenotypes have been described and autopsy studies have demonstrated Lewy body pathology in brains of ET patients. Functional neuroimaging suggests that some ET patients have dopaminergic deficit. We examine here the evidence for and against an association between ET and PD, and critically review data supporting the notion that a subset of ET patients is predisposed to developing PD.

LRRK2 G2019S founder haplotype in the Chinese population

The G2019S mutation in the LRRK2 (leucine-rich repeat kinase) gene appears very rarely in the Chinese population. Among Chinese subjects who were non-G2019S carriers, we demonstrated the frequency of the LRRK2 G2019S founder haplotype (T-254-A-G-A-154) in Parkinson's disease and controls to be 33% and 30%. This rate is similar to the frequency in European noncarriers, indirectly supporting the association of this haplotype with G2019S carriers. The haplotype is likely to be more ancient than the G2019S mutation because it is also found in a population with a very low carrier rate.

Pathogenic mutations in Parkinson disease

Parkinson disease (PD; Parkinson's) is the second most common neurodegenerative disease, characterized by the progressive loss of dopamine neurons and the accumulation of Lewy bodies. Increasing evidence suggests that deficits in mitochondrial function, oxidative and nitrosative stress, the accumulation of aberrant or misfolded proteins, and ubiquitin-proteasome system (UPS) dysfunction may represent the principal molecular pathways that commonly underlie the pathogenesis. The relative role of genetic and environmental factors has been the focus of research and debate. The recent discovery of a number of disease-causing genes (SNCA, Parkin/PARK2, UCHL1, PINK1, DJ1/PARK7, and LRRK2) in familial and sporadic forms of PD has provided considerable insights into the pathophysiology of this complex disorder. The frequency of these gene mutations may vary according to ethnicity and to the specific gene. A gene dosage effect is observed in some cases, and the phenotype of some of the mutation carriers closely resembles typical PD. Penetrance of some of the recurrent mutations is incomplete and may vary with age. Further research to unravel the etiopathology could identify biochemical or genetic markers for potential neuroprotective trials.

Analysis ofLRRK2 Gly2385Arg genetic variant in non-Chinese Asians

A common LRRK2 missense variant, Gly2385Arg, has been found to be a genetic risk factor for Parkinson's disease (PD) in ethnic Chinese and Japanese. However, the presence of the variant in other non-Chinese Asian patients has not been fully clarified. We performed genetic analysis of the Gly2385Arg variant in 472 non-Chinese Asian subjects in Singapore (comprising of 166 PD and 306 controls of Malay/Indian ethnicity). The frequency of the heterozygous Gly2385Arg genotype was not significantly different in PD compared with controls (1.2% vs. 0.8%, odds ratio = 2.83, 95% CI 0.40, 20.2, P = 0.3). No subjects carried the homozygous genotype. Stratification by Malay and Indian ethnicity revealed that there were two carriers each among 98 (2.0%) Malay PD and 173 (1.2%) Malay controls (odds ratio = 1.78, 95% CI 0.25, 12.8, P = 0.6), but there were no carriers among 66 Indian PD and 133 Indian controls. We demonstrated that the Gly2385Arg variant could be detected in our Malay subjects. However, its frequency was much lower than the 8 to 10% prevalence previously reported in our Singaporean and Taiwanese Chinese PD population. The relevance of Gly2385Arg as a genetic risk factor may be restricted to selected Asian races, and more studies will be needed to confirm our observations.

Analysis of 14 LRRK2 mutations in Parkinson's plus syndromes and late-onset Parkinson's disease

The pleomorphic pathology of postmortem LRRK2-positive patients and the frequent association with late-onset Parkinson's disease (LOPD) symptoms suggest that LRRK2 mutations may play a role in Parkinson's Plus disorders and LOPD. Published studies primarily focus on the common G2019S mutation. Analysis of a spectrum of LRRK2 mutations in Parkinson's Plus disorders has yet to be reported. We investigated 14 leucine-rich repeat kinase 2 (LRRK2) mutations in a cohort of Parkinson's Plus disorders and LOPD. A total of 458 patients with progressive supranuclear palsy (PSP), multiple system atrophy (MSA), corticobasal ganglionic degeneration (CBGD), atypical Parkinsonism (AP), and LOPD were screened for 14 mutations that span exons 19 to 41 of the LRRK2 gene. Among the LOPD cases, 1 patient was found to harbor the R1441C mutation. He presented with typical features of PD at age of 58 years old and responded well to levodopa. We did not detect any of the 14 mutations in PSP, MSA, CBGD, and AP patients. We highlight the first case of LRRK2 R1441C mutation in late onset sporadic PD of non-European ancestry. Furthermore, extensive mutational screen found LRRK2 mutations to be rare among patients who presented with PSP, MSA, CBGD, and AP.

Absence/rarity of commonly reported LRRK2 mutations in Indian Parkinson's disease patients

Recent discovery of pathogenic mutations in the leucine-rich repeat kinase 2 (LRRK2) gene in Parkinson's disease (PD) patients in different ethnic groups have raised a hope of diagnostic screening and genetic counseling. We investigated the six most commonly reported mutations in LRRK2 gene among Indian PD patients, using PCR-RFLP method. Mutations G2019S, R1441C, R1441G, and R1441H were screened in 1012 individuals (PD, 800; controls, 212) while mutations I2012T and I2020T were screened in 748 PD patients. We did not observe any of these six mutations in this study sample except in a single female young onset PD patient who showed a heterozygous G2019S mutation. The absence of mutations was reconfirmed by sequencing of probands from several autosomal dominant PD families. Our observations suggest that these mutations may be a rare cause of PD among Indians and therefore of little help for diagnostic screening and genetic counseling for Indian PD patients.

The LRRK2 Gly2385Arg variant is associated with Parkinson’s disease: genetic and functional evidence

Evidence of LRRK2 haplotypes associated with Parkinson's disease (PD) risk was recently found in the Chinese population from Singapore, and a common LRRK2 missense variant, Gly2385Arg, was independently detected as a putative risk factor for PD in the Chinese population from Taiwan. To test the association between the Gly2385Arg variant in a large case-control sample of Chinese ethnicity from Singapore, and to perform functional studies of the wild type and Gly2385Arg LRRK2 protein in human cell lines. In a case-control study involving 989 Chinese subjects, the frequency of the heterozygous Gly2385Arg genotype was higher in PD compared to controls (7.3 vs. 3.6%, odds ratio = 2.1, 95% CI: 1.1-3.9, P = 0.014); these values yield an estimated population attributable risk (PAR) of approximately 4%. In a multivariate logistic regression analysis with the disease group (PD vs. controls) as the dependent variable and the genotype as an independent factor with adjustments made for the effect of age and gender, the heterozygous Gly2385Arg genotype remained associated with an increased risk of PD compared to wild type genotype (odds ratio = 2.67, 95% CI: 1.43-4.99, P = 0.002). The glycine at position 2385 is a candidate site for N-myristoylation, and the Gly2385Arg variant replaces the hydrophobic glycine with the hydrophilic arginine, and increases the net positive charge of the LRRK2 WD40 domain. In transfection studies, we demonstrated that both the wild type and Gly2385Arg variant LRRK2 protein localize to the cytoplasm and form aggregates. However, under condition of oxidative stress, the Gly2385Arg variant was more toxic and associated with a higher rate of apoptosis. Our study lends support to the contention that the Gly2385Arg is a common risk factor for PD in the Chinese population. Our bioinformatics and in-vitro studies also suggest that the Gly2385Arg variant is biologically relevant and it might act through pro-apoptotic mechanisms.

The LRRK2 I2012T, G2019S and I2020T mutations are not common in patients with essential tremor

Several mutations in the leucine-rich repeat kinase 2 gene (LRRK2) have been identified both in familial and sporadic cases of Parkinson's disease (PD). G2019S, located at a kinase (MAPKKK) domain, is the most common mutation in the LRRK2 gene in PD, Two adjacent mutations (I2012T and I2020T) were mapped to the same domain suggesting shared pathogenic mechanism of these mutations. Since phenotypes of PD overlap with essential tremor (ET), we investigated LRRK2 G2019S, I2012T, and I2020T mutations in a cohort of 272 patients with ET. No mutations were found in our ET cohort and, therefore, we conclude that LRKK2 I2012T, G2019S and I2020T variants are rare causes of Caucasian ET.