Everyone's blood carries cells that multiply. Now and then one cell picks up a random mutation and starts dividing faster than its neighbors. Its descendants, or clones, gradually fill the blood. In older people this is common: 6% at ages 60-69, 10% at 70-79, and 18% after 90.
Scientists examined 1,091 people three times: at 73, 76, and 79. They measured methylation in the blood and took brain scans. It turned out that people with more clones at 73 had, on average, smaller brains. And over the next six years, their gray matter, the part of the brain that houses nerve cells, shrank faster. Curiously, the regions that suffer most look like the ones hit by Alzheimer's disease. But the hippocampus, which is crucial for memory, stays untouched. This means blood clones are linked to brain aging, though they aren't a direct cause of the disease. More likely, both processes, aging of the blood and aging of the brain, run in parallel.
For now this is a research tool, not a hospital test. But if blood methylation proves to be a reliable way to gauge clones, it could be used for early warning about the risk of neurodegeneration in older people.
The 2% variant allele frequency threshold for a CHIP diagnosis was not chosen at random: below that fraction, it is technically hard to tell true expansion from sequencing noise. But error-corrected studies show that among people over 80, clones with a frequency of 1% or more turn up in 62%, meaning most older adults carry traces of clonal expansion in their blood, even if they do not formally meet the criteria for CHIP.
The idea that mutations in blood cells can affect the brain echoes Greg Egan's novel Quarantine: there, neuron modifications and systemic changes to the body lead to unpredictable cognitive consequences. In a gentler form, the theme of blood as a carrier of aging is played out in the film In Time (2011), where life resources literally flow between people.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.