There's some good news for people who have had certain kinds of skin cancer: A new study suggests that their odds of developing Alzheimer's disease may be significantly lower than it is for others.
People who had non-melanoma skin cancer were nearly 80% less likely to develop Alzheimer's disease than people who did not. The association was not found with other types of dementia.
To understand the possible association between skin cancer and Alzheimer's, it is important to know that people have a combination of cells that are multiplying and others that are dying, explained study author Dr Richard Lipton, a professor of neurology, epidemiology and population health at the Albert Einstein College of Medicine in New York City. The research was published in the journal Neurology.
Identifying a chronic disease
"When cell division gets out of control, we call that cancer. And when specific populations of brain cells die, we call that Alzheimer's," Lipton said. "So, there is a balance between cell division [growth] and cell death. If you have an individual with an increased risk of cell division over cell death, that may be linked to a decreased risk of Alzheimer's."
The finding was intriguing to one expert.
"It's fascinating that we can get clues about what's going on in the brain by looking at the periphery [skin]," said Terrence Town, a professor in the physiology and biophysics department at the Keck School of Medicine at the University of Southern California.
Non-melanoma skin cancer is the most common cancer in the United States; there were more than 2 million new cases in 2012, according to the US National Cancer Institute.
What could be causing the possible association between skin cancer and a reduced risk of Alzheimer's?
"Either developing skin cancer is a marker for some biological process that protects against Alzheimer's or environmental factors may play a role," Lipton said. Genetics could be a factor, as could lots of outdoor physical activity and exercise, although Lipton cautioned people to avoid too much sun exposure and wear sunscreen.
Lowered immune response of skin cells
Others think the link may be directly related to how the lowered immune response of skin cells in skin cancer corresponds to a similar immune response in the brain.
"This research is another piece of evidence that tells us that peripheral inflammation [in the skin] is very important in Alzheimer's disease," Town said. He thinks that people who develop non-melanoma skin cancers don't have an immune response in their skin, and thus develop skin cancers, because an immune response may be critical to fighting skin cancer. But that benefits them when it comes to developing Alzheimer's disease.
"This reduced inflammatory response that was permissive to the skin cancer was perhaps beneficial in the brain," said Town.
Town thinks the study suggests a fascinating and important concept: skin cancer may be a biomarker for resistance to Alzheimer's disease. That means, for example, that it may be possible that drugs that dampen the inflammatory response, such as a TNF-alpha inhibitor, could potentially be used to help prevent Alzheimer's disease.
TNF-alpha inhibitors block TNF-alpha, a protein that is present in larger quantities in people who have certain inflammatory conditions, such as rheumatoid arthritis, Crohn's disease and psoriasis. TNF-alpha inhibitors include adalimumab (Humira), etanercept (Enbrel) and infliximab (Remicade).
The latest research involved 1 102 people in New York City, whose average age was 79 when they enrolled in the study. None of the participants had dementia at the beginning of the study. Every year, a team tested them for memory, concentration, language, planning abilities and other factors. During the average four-year follow-up, they were asked annually whether they had developed non-melanoma skin cancer.
Participants with cancer history
At the start of the study, 109 people reported that they had been diagnosed with skin cancer in the past. During the study, 32 people developed skin cancer and 126 people developed dementia, 100 of those with Alzheimer's. Of the 141 people with skin cancer, only two developed Alzheimer's disease, compared to 98 of the 961 people without skin cancer.
Alzheimer's is the most common form of dementia, defined by declines in memory and other thinking abilities.
"For a long time, we didn't even know inflammation was important, a key factor in the evolution of Alzheimer's disease," said Town. "Now [this paper suggests that] we can start to think more broadly; maybe it's inflammation in the blood or the skin that might be important factors."
Although the study found an association between certain non-melanoma skin cancers and lower risk of Alzheimer's, it did not prove a cause-and-effect relationship.
Learn more about skin cancer from the U.S. National Cancer Institute.