For decades, researchers have noted that cancer and Alzheimer's disease are rarely found in the same person, fuelling speculation that one condition might offer some degree of protection from the other. Nature: Now, a study in mice provides a possible molecular solution to the medical mystery: a protein produced by cancer cells seems to infiltrate the brain, where it helps to break apart clumps of misfolded proteins that are often associated with Alzheimer's disease. The study, which was 15 years in the making, was published on 22 January in Cell and could help researchers to design drugs to treat Alzheimer's disease. "They have a piece of the puzzle," says Donald Weaver, a neurologist and chemist at the Krembil Research Institute at the University of Toronto in Canada, who was not involved in the study. "It's not the full picture by any stretch of the imagination. But it's an interesting piece." [...] A 2020 meta-analysis of data from more than 9.6 million people found that cancer diagnosis was associated with an 11% decreased incidence of Alzheimer's disease. It has been a difficult relationship to unpick: researchers must control for a variety of external factors. For example, people might die of cancer before they are old enough to develop symptoms of Alzheimer's disease, and some cancer treatments can cause cognitive difficulties, which could obscure an Alzheimer's diagnosis.

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