LOS ANGELES – Parkinson’s disease patients have double the risk of developing potentially lethal melanoma, government researchers reported Tuesday. Researchers have long suspected such a link, but the new study, reported in the journal Neurology, provides the strongest evidence to date.
Researchers are at a loss to explain how the link occurs biologically, but they suspect it may be a combination of environmental exposure and genetic predisposition. The association is particularly strange, experts said, because Parkinson’s patients, in general, have a below-normal risk of developing most types of cancer.
Establishing a link between Parkinson’s and melanoma is difficult because both are relatively rare diseases. About 1 million Americans have a diagnosis of Parkinson’s, and about 70,000 Americans are diagnosed with melanoma each year.
The number with both diseases is therefore relatively small. To increase the chances of finding statistically significant results, Dr. Honglei Chen, a neuroscientist at the National Institute of Environmental Health Sciences in Research Triangle Park, near Durham, N.C., and colleagues combined the results from 12 studies conducted between 1965 and 2010, a process known as a meta-analysis.
Most of the studies had fewer than 10 patients with both conditions, but combining them yielded a number large enough for statistical significance.
Overall, the researchers found that the risk of melanoma was 2.11 times normal in Parkinson’s patients. Only some of the studies broke their results down by gender.
For those studies, the team found that the risk of melanoma was 2.04 times normal for men and 1.52 times normal for women. The risk of diagnosis of melanoma was significantly higher after Parkinson’s had been diagnosed. The team found no link to non-melanoma skin cancers.
Many experts had thought that the increased risk of melanoma was produced by levo-dopa, the most common treatment for Parkinson’s disease. But recent epidemiological results, Chen noted, discount that link. Among other things, the same high risk of melanoma is associated with patients who have not received the drug.
Some studies have shown that the risks of Parkinson’s and melanoma are each increased by exposure to pesticides, and that could be a common factor in the new association, the team said. Pigmentation also may play a role: Lighter hair color is associated with an increased risk of Parkinson’s and of melanoma.
The genes involved may increase concentrations of the natural hormone melanin, which exacerbates the development of melanomas.
The team concluded, however, that much more research is needed to clarify the biological basis of the links.