Parkinson's disease (PD) is characterized by the progressive loss of neurons that produce a chemical messenger called "dopamine", which is critical in the control of movement. Therefore, dopamine levels decrease progressively in PD brains as these neurons die. One approach to replace the lost dopamine, known as "L-dopa therapy", can be quite effective in controlling symptoms in patients. However, over time, this treatment often leads to side-effects that can be as debilitating as the PD symptoms; and L-dopa does not stop the destruction of dopamine neurons, and thus does not halt disease progression. Over the last decades, scientists have identified many genetic mutations in families affected by PD. But they represent only a small subset of patients, leaving 90% of the cases due to unknown causes; called "sporadic" or "idiopathic" PD. We do not know why these persons become sick; but because some occupations, such as working in the military, lead to a higher risk of developing PD, we believe they may have been exposed to substances or chemicals toxic for their brain. Metals, such as lead and manganese that are found in ammunitions, for instance, are among the prime candidates suspected to cause PD. Excessive exposure to manganese is known to cause a disease closely resembling PD in smelters and welders. Other toxic chemicals presumed to have a causal role in PD were found to contaminate military base drinking water systems. These toxic metals and chemicals are well known to accumulate in the brain over time. However, because PD does not develop in every person who is exposed to these toxic metals and chemicals, it is thought that the combination of individual vulnerability (e.g., a genetic pre-disposition) and toxic exposure can cause the disease. This is through a phenomenon called "gene-environment interaction" with respect to PD risk, that our proposed work relates to the FY21 NETP Focus Area(s).

Exposure to toxic metals and chemicals may not only have a role in "sporadic/idiopathic" PD but also in familial PD, as many people who carry a mutation causing PD in their family never develop the disease, known as incomplete penetrance. This is often the case for the mutations that we will study in this proposal in the gene LRRK2. Our central hypothesis is that exposure to, and brain accumulation of metals/organic toxicants, interacts with genetic mutations in LRRK2 to modulate the risk of developing PD. To test this hypothesis, we will combine investigations in patient blood samples (specific aim 1), on human dopamine neurons produced from patient skin cells by novel cell reprogramming technologies (specific aim 2), and in “humanized” genetic mouse models of PD (specific aim 3).

Specific Aim 1: In clinical PD, there is a lack of clear biological markers or "biomarkers" indicative of disease state and patients' brain exposure to environmental toxic factors. Here we will analyze the content of very small capsules, called extracellular vesicles (EVs) that are produced in the brain and exported to the blood to eliminate waste and communicate with the rest of the body, as a new source of biomarkers. The content of these capsules will open a window on PD patient brains by revealing the levels of metals and toxicants to which they were exposed and changes in key proteins such as LRRK2 that are associated with disease progression. We will measure metal, toxicant, and LRRK2 levels and potential dysfunction in brain EVs from control, idiopathic PD, LRRK2 PD patients, and LRRK2 carriers unaffected by PD. We will first test whether metal/toxicant levels in brain EVs can allow us to distinguish between the different types of patients, and their disease stage. Then, we will test whether the elimination of metals/toxicants in EVs is impaired in patients as disease progresses, and whether they are correlated with LRRK2 dysfunction.

Specific Aim 2: We will screen the toxicity of metals and chemicals suspected for their role in PD in human neurons derived from control, or patients with mutations in LRRK2 affected or not by PD. We will determine whether PD and mutated LRRK2 neurons are more vulnerable to metals/chemicals than control cells, revealing a "gene-environment interaction" critical in dopamine neuron loss. Finally, we will test whether EVs can propagate cell death between affected and unaffected cells and if therapeutic strategies can prevent this.

Specific Aim 3: We will initiate a chronic exposure to the metals and toxicants identified in PD patients in a "humanized" mouse model carrying the same LRRK2 mutation. We will monitor the motor performance of these mice and determine whether they develop symptoms and dopamine neuron loss like in PD.

Taken together, we anticipate that our studies will help elucidate the link(s) between exposure to metals/toxicants, the dysfunction of a key protein involved in PD, and the modulation of PD risk itself. We will validate novel EV biomarkers, which could become critical tools for PD diagnosis, but also in clinical trials to determine the benefit of new drugs. Ultimately, confirming the importance of toxicant-gene interactions in PD could promote the need for both environmental exposure and genetic counseling in PD prevention.