Solvent May Boost Parkinson’s Risk

A study of Parkinson’s disease in twins shows that long-term occupational exposure to the solvent trichloroethylene (TCE) is associated with a risk of developing the movement disorder that is six times that for unexposed people(Ann. Neurol., DOI: 10.1002/ana.22629).

The investigation, led by Samuel M. Goldman and Caroline M. Tanner at the Parkinson’s Institute, in Sunnyvale, Calif., is the first to nail down in an entire population a long-suspected link between TCE and Parkinson’s disease.

Until now, associations between solvent exposure and Parkinson’s disease had been largely anecdotal. The work “takes an important step” by clearly implicating TCE, says Michael A. Schwarzschild, a neurologist at Harvard Medical School and MassGeneral Institute for Neurodegenerative Disease.

“If these findings could be confirmed in the general population, the public health implications will be considerable,” agrees Honglei Chen, an epidemiologist at the National Institute of Environmental Health Sciences.

The work also bolsters the case for the so-called TaClo hypothesis, which posits that reaction of the amino-acid derivative tryptamine with TCE creates 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo), a neurotoxin that destroys dopamine-producing cells. TaClo structurally resembles other molecules known to induce Parkinson’s symptoms in people and in animal models.

Goldman and Tanner’s team examined the occupations, hobbies, and environments of 99 pairs of twins, in which one twin had Parkinson’s and the other did not. The researchers estimated the twins’ long-term occupational exposure to six organic solvents, including TCE and perchloroethylene (PERC).

The study design allowed the researchers not only to account for genetic factors, but also to reduce other confounding factors, such as household environment, demographics, and education.

Exposure to n-hexane, toluene, and xylene did not appear to increase the risk of developing Parkinson’s, but exposure to TCE multiplied the risk six times. PERC and CCl₄ were also associated with increased risk, although it was not statistically significant.

TCE is now ubiquitous in the environment, especially in groundwater. It was recently classified as a human carcinogen by EPA (C&EN, Oct. 3, page 26).

Goldman and Tanner’s group are hoping to expand their studies to larger populations and to be able to quantify exposure doses, Goldman says.