Two of the most common causes of death in America are Alzheimer’s disease and cancer. To most of us, these seem like completely unrelated diseases. However, this intuition is far from the truth. For reasons that puzzle scientists, having Alzheimer’s actually seems to protect you from cancer, while having cancer may protect you from Alzheimer’s.
This counterintuitive relationship was first discovered in 2005 by the Washington University Alzheimer’s Disease Research Center. This study of 882 elderly participants reported that the people with Alzheimer’s disease were significantly less likely to be diagnosed with cancer during the course of the study, and that those who did develop cancer did so at an older age. In 2012, the Framingham Heart Study reported that participants with Alzheimer’s disease were less than half as likely to be diagnosed with cancer during the course of the study compared to dementia-free subjects of the same age, sex, body mass index, and smoking status. A 2014 meta-analysis summarized the results of six independent papers to conclude that Alzheimer’s patients have a 42% reduced risk of cancer in their lifetime, while cancer survivors have a 37% reduced risk of Alzheimer’s. These differences remained statistically significant even after accounting for the effects of reduced life expectancy and demographic factors.
Interestingly, this inverse relationship does not seem to apply to other diseases of the central nervous system, including vascular dementia, Parkinson’s disease, amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), or Down’s Syndrome. This suggests that the cancer-protective mechanism is specific to Alzheimer’s disease.
Scientists are still unsure as to why cancer and Alzheimer’s are inversely related, although several theories have been suggested. One possibility is that proteins that are needed for cells to divide and reproduce might be under-produced in Alzheimer’s patients, causing a reduction in neurogenesis (the formation of new brain cells, important for learning and memory) while also protecting from cancer by preventing uncontrolled cell division. Another theory is that amyloid beta, the protein that forms toxic clumps in the brains of Alzheimer’s patients, may fight cancer by suppressing the growth of tumors. The APOE4 allele, which increases the risk of Alzheimer’s disease, has also been associated with a reduced risk of certain cancers. Further research is needed to establish a definite mechanism.
One of the main consequences of this relationship applies to clinical trials of drug candidates to treat Alzheimer’s disease (see Where’s our cure to Alzheimer’s disease?). Several trials have had to discontinue prematurely due to an increased risk of cancer among the participants, especially skin cancers. In other words, the drugs that were supposed to be combatting Alzheimer’s disease ended up causing some participants to develop cancer. Though only a subset of trials have reported this problem, it’s worth being aware of the possibility when considering whether to participate in a clinical trial, particularly if your family has a history of skin cancer or other cancers.
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