Parkinson's disease (PD) is a common devastating disease of the nervous system that, aside from the characteristic tremor, results in progressive worsening of mobility, fine motor skills, and cognition, with concomitant loss of independence. PD is defined by the clumping up of a particular protein (alpha-synuclein) in a special type of dopamine-secreting neuron in the brain. About one million Americans live with the disease, incurring an estimated cost to the country of $25 billion annually. Only Alzheimer's disease (AD) is more common among degenerative brain diseases and consumes more resources.

Most cases of PD occur late in life and are unexplained. It is becoming clearer that PD might actually be many different diseases and that a "one size fits all" treatment strategy will not work to prevent or slow down the disease. The variability in PD is thought to arise from two main sources -- the genetic make-up of a patient and the environmental exposures of the patient. Considerable progress has been made in identifying distinct genetic risk factors related to PD, for example, alterations within genes that encode the alpha-synuclein protein itself or the Gaucher protein ("GBA"). But the environmental exposures that confer risks for PD remain far more mysterious. In human studies, we cannot currently shed light on the mechanisms of how a patient's environmental exposures contributed to their PD. Even more challenging is understanding how the genetic differences between persons confer risk for specific environmental exposures. These so-called "gene environment" interactions (GxE) are key to better understanding PD etiology and are the subject of the specific focus area in this application, and the main focus of this grant. We address three major challenges in the field.

First, the epidemiologists working to define quantitative environmental exposure risks for complex chronic diseases like PD do not often closely interact with biologists undertaking mechanistic and cellular studies. In this proposal, we bring together three investigators of different backgrounds to build bridges across disciplines that will be important to understanding GxE exposures in PD: Dr Beate Ritz, a leader in quantitative epidemiology who has made critical contributions to our understanding of how environmental toxicants, including specific pesticides, affect PD risk in populations; Dr. Lee Rubin, a stem cell biologist who has developed methods to create large numbers of dopamine neurons from patient stem cells ("iPSC") and quantitatively screen them against small molecules and potential toxins; Dr. Vik Khurana, a neurologist and protein-folding biologist who studies the effects of alpha-synuclein misfolding in patient-derived neuronal models. This collaboration enables us to test pesticides implicated in Dr. Ritz's rigorous environmental studies for their effects "in the dish." Moreover, we will study these toxins in the context of mutations in the synuclein and GBA genes that are also being analyzed in Dr. Ritz's cohort. Our experiments will enable us to answer whether these key genetic risk factors create sensitivities in patients to particular toxins.

Second, we will ascertain whether gene-toxin interactions play out specifically at the level of the dopamine neuron. It is unclear whether any toxicant implicated in PD is affecting the dopamine neuron, or other cells in the nervous system or beyond. Our method will address this important question for specific toxins identified "in the field." Third, we create an approach that addresses GxE interactions at the level of an individual patient. Currently, we have genetic and environmental studies that create general observations about how these factors alter risk at the level of whole populations. But how do we interpret these data in the context of the individual patient in the clinic? Our approach builds a platform that, ultimately, gives us the ability to determine specific GxE interactions within the patient's own iPSC-derived brain cells.

What are the deliverables? We believe that at the end of three years, our proposal has a strong chance of delivering important, actionable data that will influence our understanding of PD, clinical practice and guidelines, and drug development pipelines. If we show that specific environmental factors trigger specific cellular toxicities, then we will have much stronger evidence to share with patients and regulators regarding specific toxic exposures. Moreover, we will uncover whether two key PD genes -- alpha-synuclein and GBA -- confer specific sensitivity to those toxins. Because our interactions will be uncovered in a tractable cellular model, we will lay the foundation for experimental efforts to better understand the biological mechanisms of these interactions and to develop drugs to counteract them. We also propose an unbiased screening effort in dopamine neurons to uncover new gene-environment interactions in PD, findings that will allow Dr. Ritz and other epidemiologists to broaden their search in the field for novel PD risk factors. Finally, by pinpointing susceptibilities and mechanisms within neurons of patients with specific gene mutations, our study paves the way for future clinical trials and gene-environment interaction studies in groups of patients stratified by specific gene mutations, an important foundation for personalized medicine approaches in PD.