Cancer and Alzheimer’s disease are two of the most feared diagnoses in medicine, but they rarely strike the same person. For years, epidemiologists have noticed that people with cancer seem less likely to develop Alzheimer’s, and those with Alzheimer’s are less likely to get cancer, but nobody could explain why.
A new study in mice suggests a surprising possibility: certain cancers may actually send a protective signal to the brain that helps clear away the toxic protein clumps linked to Alzheimer’s disease.
Alzheimer’s is characterised by sticky deposits of a protein called amyloid beta that build up between nerve cells in the brain. These clumps, or plaques, interfere with communication between nerve cells and trigger inflammation and damage that slowly erodes memory and thinking.
In the new study, scientists implanted human lung, prostate and colon tumours under the skin of mice bred to develop Alzheimer‑like amyloid plaques. Left alone, these animals reliably develop dense clumps of amyloid beta in their brains as they age, mirroring a key feature of the human disease.
But when the mice carried tumours, their brains stopped accumulating the usual plaques. In some experiments, the animals’ memory also improved compared with Alzheimer‑model mice without tumours, suggesting that the change was not just visible under the microscope.
The team traced this effect to a protein called cystatin‑C that was being pumped out by the tumours into the bloodstream. The new study suggests that, at least in mice, cystatin‑C released by tumours can cross the blood–brain barrier – the usually tight border that shields the brain from many substances in the circulation.
Once inside the brain, cystatin‑C appears to latch on to small clusters of amyloid beta and mark them for destruction by the brain’s resident immune cells, called microglia. These cells act as the brain’s clean‑up crew, constantly patrolling for debris and misfolded proteins.
In Alzheimer’s, microglia seem to fall behind, allowing amyloid beta to accumulate and harden into plaques. In the tumour‑bearing mice, cystatin‑C activated a sensor on microglia known as Trem2, effectively switching them into a more aggressive, plaque‑clearing state.
Surprising trade-offs
At first glance, the idea that a cancer could “help” protect the brain from dementia sounds almost perverse. Yet biology often works through trade-offs, where a process that is harmful in one context can be beneficial in another.
In this case, the tumour’s secretion of cystatin‑C may be a side‑effect of its own biology that happens to have a useful consequence for the brain’s ability to handle misfolded proteins. It does not mean that having cancer is good, but it does reveal a pathway that scientists might be able to harness more safely.
The study slots into a growing body of research suggesting that the relationship between cancer and neurodegenerative diseases is more than a statistical quirk. Large population studies have reported that people with Alzheimer’s are significantly less likely to be diagnosed with cancer, and vice versa, even after accounting for age and other health factors.
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This has led to the idea of a biological seesaw, where mechanisms that drive cells towards survival and growth, as in cancer, may push them away from the pathways that lead to brain degeneration. The cystatin‑C story adds a physical mechanism to that picture.
However, the research is in mice, not humans, and that distinction matters. Mouse models of Alzheimer’s capture some features of the disease, particularly amyloid plaques, but they do not fully reproduce the complexity of human dementia.
We also do not yet know whether human cancers in real patients produce enough cystatin‑C, or send it to the brain in the same way, to have meaningful effects on Alzheimer’s disease risk. Still, the discovery opens intriguing possibilities for future treatment strategies.
One idea is to develop drugs or therapies that mimic the beneficial actions of cystatin‑C without involving a tumour at all. That could mean engineered versions of the protein designed to bind amyloid beta more effectively, or molecules that activate the same pathway in microglia to boost their clean‑up capacity.
The research also highlights how interconnected diseases can be, even when they affect very different organs. A tumour growing in the lung or colon might seem far removed from the slow build up of protein deposits in the brain, yet molecules released by that tumour can travel through the bloodstream, cross protective barriers and change the behaviour of brain cells.
For people living with cancer or caring for someone with Alzheimer’s today, this work will not change treatment immediately. But the study does offer a more hopeful message: by studying even grim diseases like cancer in depth, scientists can stumble on unexpected insights that point towards new ways to keep the brain healthy in later life.
Perhaps the most striking lesson is that the body’s defences and failures are rarely simple. A protein that contributes to disease in one organ may be used as a clean‑up tool in another, and by understanding these tricks, researchers may be able to use them safely to help protect the ageing human brain.
Justin Stebbing does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
