Research on Toxic Exposure and Parkinson's Disease

Research Findings and New Directions

During the past five years, researchers have made substantial advances in our understanding of the biological factors involved in PD. They are beginning to decipher the roles environmental factors in PD and to learn how the interplay of these factors can lead to the disease.

Environmental Factors

Many researchers believe that environmental exposures also increase a person's risk of developing the disease. As with many familial cases, exposure to toxins or other environmental factors may influence when symptoms of the disease appear and/or how the disease progresses.

One of the primary pieces of evidence that environmental factors play a role in the development of PD is that the relative risk of the disease is higher in industrialized countries than in less industrialized ones. In addition, studies have found that farmers and other workers who work in fields that are exposed to toxic chemicals have an increased risk of developing PD. Taken together, these studies suggest that toxic chemicals or exposure to other environmental factors present in industrial and agricultural areas might increase the risk of PD.

Another piece of evidence comes from observations of people who have been accidentally poisoned with the toxin MPTP (1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine). MPTP is structurally similar to some pesticides and other toxic chemicals. A breakdown product of MPTP, called MPP+, is toxic to substantia nigra neurons — the neurons that are affected in PD. MPTP produces a severe, permanent parkinsonian syndrome in affected people, and is now used to create animal models of PD. This discovery demonstrated that a toxic substance can damage the brain and produce parkinsonian symptoms.

A study of people in the World War II Veteran Twins Registry has suggested that genetic factors do not play a major role in causing sporadic Parkinson's Disease. A number of other twin studies have found similar results. The chance that two siblings will both have PD is similar for fraternal and identical twins, suggesting that environmental exposures are more important than genetics in determining who will get the disease. Other studies have found that fraternal and identical twins of people with PD often have significant loss of dopamine neurons even when they don't experience any symptoms.

Viruses are another possible environmental trigger for PD. People who developed encephalopathy after a 1918 influenza epidemic were later stricken with severe, progressive Parkinson's-like symptoms. However, these cases showed that viruses can sometimes affect the region of the brain damaged in PD. Other studies have found evidence of activated immune cells and the accumulation of inflammation-associated proteins in PD. These changes might be triggered by viruses in some cases. Firefighters and Emergency Medical personnel are constantly exposed to viruses during medical responses, which is 78% of their work load.

Scientists are continuing to study environmental toxins, supported by the National Institute of Environmental Health Sciences, has shown that other agricultural compounds also can produce abnormalities in cells that are similar to those seen in PD. This research is supported through a program called the Collaborative Centers for Parkinson's Disease Environmental Research (CCPDER) Consortium. This program sponsors a variety of projects to examine how occupational exposure to toxins and use of caffeine and other substances may affect risk, and whether inherited genetic mutations may predispose certain people to developing PD after exposure to certain chemicals.