About 10% of Parkinson’s disease (PD) cases are caused by single gene mutations. Of these, the most common cause of autosomal dominant PD is mutation of Leucine-Rich Repeat Kinase 2 (LRRK2), which accounts for about 3% of PD cases. While it is generally believed that pathogenic mutations in LRRK2 confer a toxic gain-of-function – increased LRRK2 kinase activity – that is strongly implicated in pathogenesis, the role of wildtype LRRK2 in idiopathic PD (iPD), which accounts for 90% of cases, is less clear. With LRRK2 inhibitors coming to clinical trials, we asked experts in the field to discuss the role of LRRK2 in iPD at the phenotypic level (Dr. Marras) and at the molecular level (Dr. West and Dr. Greenamyre).
Dr. Marras
Assuming that clinical features in PD reflect the underlying biological process, phenotypic similarities or differences between iPD and LRRK2-associated PD may provide insight into the underlying biological relationship between the two groups. Regardless, it would be useful to both clinicians and clinical researchers if clinical clues to the presence of a LRRK2 mutation were identified. This will soon become relevant to practice when LRRK2-specific therapies come to clinical trials.
In a systematic review of reports providing clinical information at the individual level on 724 individuals with LRRK2 mutations reported in the literature a clinical phenotype consistent with iPD was found, with late onset in the majority, good response to levodopa, and common development of motor fluctuations and dyskinesias (Trinh et al, 2018). The clinical phenotypes of LRRK2 PD and iPD have been directly compared in several studies. In these studies, LRRK2 PD has been found to have a phenotype that is largely overlapping that of iPD, but with some differences detectible at a group level. LRRK2 G2019S mutation carriers have been reported to have lower motor Unified Parkinson’s disease Rating Scale scores than iPD (Marras et al, 2016; Ben Romdhan et al, 2018) lower depression scores (Marras et al, 2016) and better smell identification (Marras et al, 2016) after adjusting for disease duration suggesting a more benign phenotype. Somewhat surprising, however, given these findings is that the group with LRRK2 mutations had a higher proportion of postural instability gait disorder (PIGD) phenotype than those with iPD (Marras et al, 2016). The same was found in a study restricted to individuals with early onset PD (Alcalay et al, 2009).
Generalizations related to the phenotype of LRRK2-associated PD may be overly simplistic, however as the clinical features could differ by mutation. A comparison of reported R1441 and G2019S mutations suggested that motor fluctuations may be more common in the latter but still common in both (Trinh et al, 2018). Individuals with the G2385R risk variants have been found to have greater motor severity than iPD or G2019S LRRK2 PD for a given disease duration. Results from several studies would suggest that more rapid disease progression, measured by motor scores or the emergence of motor complications, may be more aggressive in manifesting carriers of risk variants common in Asian populations (Gao et al., 2013; Oosterveld et al., 2015). A recent meta-analysis of Asian case-control studies of G2385R variant carriers compared with non-carriers did not replicate these observations on motor severity, however, and concluded a largely overlapping phenotype with iPD (Di et al., 2018). Conflicting findings may relate in part to population stratification, with regional variation in genetic and environmental influences on the phenotype. In addition, methodological limitations of studies, including different methods of evaluation of cases and controls, may account for variation in results. At this point evidence for important phenotypic differences between LRRK2 mutations is lacking.
It is of considerable interest to know if iPD-related non-motor features can predict the emergence of motor parkinsonism in individuals carrying LRRK2 mutations. If so, there may be an opportunity to intervene with PD-preventive strategies in this high risk group. Identifying such individuals is becoming increasingly relevant as clinical trials targeting LRRK2 kinase activity begin. However, lower RBDSQ scores and better olfaction in individuals with LRRK2 mutations compared with iPD (Marras et al, 2016) raise the hypothesis that non-motor features may be less predictive of future motor manifestations in LRRK2 G2019S PD than in iPD. We currently lack longitudinal studies examining the association between phenotypic features of non-manifesting carriers and future development of PD.
The preceding discussion addresses only the relationship between LRRK2 mutations and a clinical phenotype of PD. Dramatic pathological heterogeneity within a single family carrying LRRK2 R1441C mutations has been previously reported, ranging from typical Lewy body pathology to tau-positive neurofibrillary tangles to nonspecific nigral degeneration (Wszolek et al, 2004). Despite the pathological heterogeneity, the clinical phenotype was reported to be typical of iPD for the most part, with a supranuclear gaze palsy in only one of the four pathologically examined individuals. Several groups have screened cohorts of pathologically proven PSP and MSA (Sanchez-Contreras et al., 2017) or clinically diagnosed atypical parkinsonism (Tan et al., 2006) and found a very low frequency of LRRK2 variants.
Taken together, the literature suggests that while there may be some group-level differences in phenotype that can be detected when large numbers of individuals are studied, at the individual level it is not possible to identify a distinct phenotype suggestive of a LRRK2 mutation. Identifying individuals with LRRK2 mutations will soon become important, as clinical trials directly targeting LRRK2 activity are initiated and enrol LRRK2 mutation carriers. International identification efforts are beginning to be organized, using unselected genetic testing of patients with PD as the identification strategy. This unselected approach is logical considering the non-specific and heterogeneous phenotype of LRRK2 PD.
Dr. West
LRRK2 is linked to iPD in several ways, with each link suggesting the presence of a subpopulation of iPD where LRRK2 might contribute more significantly to disease than in other (most) cases of iPD.
First, there are at least six known pathogenic (large effect) mutations that lack full lifetime penetrance for the development of PD. Because of the lack of full penetrance of LRRK2 mutations, many PD patients have been identified with pathogenic LRRK2 mutations that do not have a family history of disease. In these “sporadic” cases of disease, LRRK2 is unequivocally a key contributor to disease susceptibility. However, we do not know whether LRRK2 is important for disease progression in LRRK2 mutation carriers that already have disease, a nuance that may have critical importance for the success or failure of LRRK2-targeted therapeutics.
Second, a link between iPD and LRRK2 comes from genetic association studies with common genetic variants that are not pathogenic on their own. PD is not typically a heritable disease and has less of a genetic component than many other neurological diseases. Nevertheless, LRRK2 shares a tiny slice of the common genetic susceptibility underlying iPD. In individuals that bear PD-associated variants in LRRK2, the importance of LRRK2 dysfunction may be higher in those iPD cases than in other iPD cases.
Third, biomarker studies in biofluids, brain tissue, and immune cells have highlighted the possibility that LRRK2 is overactive in a subpopulation of iPD. However, changes in disease can be difficult to interpret; for example, protective responses that later morph to detrimental responses, or changes that are reactionary and lack consequence for disease progression.
With these caveats in mind, each of these links between LRRK2 and iPD suggest the presence of a subpopulation of iPD where LRRK2 is more important in disease than in other iPD cases. Thus, LRRK2 targeted therapies will be best applied to groups of iPD cases with one or more direct genetic and/or biochemical links to disease.
Dr. Greenamyre
The role of wildtype LRRK2 in iPD has been unknown, in large part, because LRRK2 is a low abundance protein and its activity has been impossible to assay with a cellular level of resolution. We developed a novel proximity assay for LRRK2 with the ability to determine activity on a cell-by-cell basis (Di Maio et al, 2018). We recently reported that in brains from individuals with iPD wildtype (non-mutated) LRRK2 was abnormally activated in dopamine neurons and this correlated with phosphorylation of its substrate, Rab10. Moreover, this nigrostriatal neuron LRRK2 activation (increased kinase activity) was reproduced in two rodent models: the rotenone rat and viral-mediated α-synuclein over-expression.
What do iPD, the rotenone model and α-synuclein over-expression have in common that might cause LRRK2 activation? One possibility is that they are all associated with oxidative stress. To test whether oxidative mechanisms are responsible, cells were treated with physiological concentrations of H2O2 and assayed for LRRK2 activation. We found that H2O2 activated LRRK2 and this was prevented by co-treatment with the antioxidant α-tocopherol. Thus, it appears that oxidative stress can activate LRRK2 kinase activity.
What are the downstream consequences of aberrant LRRK2 activity? LRRK2 has been implicated in endosomal, lysosomal and autophagic function. In work that is submitted for publication, we first catalogued and characterized the endolysosomal and autophagic defects in nigrostriatal dopamine neurons from iPD patients. Next, we showed that each of these defects was reproduced in the rotenone rat model. Finally, we found that systemic treatment of rotenone rats with a brain-penetrant LRRK2 kinase inhibitor prevented all the abnormalities found in the iPD brains.
Together, these results indicate that (i) LRRK2 is activated in iPD by oxidative mechanisms; (ii) aberrant LRRK2 activity leads to endolysosomal and autophagic defects and pathological accumulation of α-synuclein; and (iii) all of these abnormalities are prevented by LRRK2 kinase inhibition.
Our publication on LRRK2 activation demonstrated that treatment of rats with the pesticide, rotenone, activates LRRK2 – and occupational exposure to rotenone is a risk factor for PD. Therefore, we wondered whether activation of LRRK2 is a common mechanism of environmental toxins linked to PD risk. In another study being prepared for publication, we found that, like rotenone, the PD-linked environmental toxicants, paraquat and trichloroethylene (TCE), also activate LRRK2.
In summary, there is strong evidence that wildtype LRRK2 plays a central role in the pathogenesis of iPD. In this context, there is reason to be optimistic that the use of LRRK2-targeted therapeutics may have utility far beyond the 3% of cases caused by LRRK2 mutations.
References
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