Somatic mutations reveal the ontogeny of microglia in human ageing

Why it matters

Every cell picks up a few unique DNA mutations as it divides, so cells that share mutations must share an ancestor. This study uses that natural record of somatic mutation to answer a basic question about the aging brain: are its immune cells replaced from the blood over time? The answer changes how we think about brain aging and about links between blood-cell clones and Alzheimer's disease. Microglia clear debris, prune connections between neurons and respond to injury, so where they come from matters for brain health. The finding also shows how somatic mutations, usually discussed as harmful, can be used as a research tool.

What they did

The researchers developed a way to use the set of somatic mutations that marks each clone of cells to track bone-marrow-derived cells entering the human brain. They applied it to brain tissue from 20 older people, used single-cell analysis and lineage tracing with mitochondrial DNA variants to characterise the incoming cells, and analysed human cohort data on clonal hematopoiesis and Alzheimer's disease.

What they found

In mice, microglia settle in the brain before birth and largely maintain themselves. In humans, the team found evidence of bone-marrow-derived cells entering the brain in all 20 older people examined. Single-cell analyses showed that these incoming cells resemble microglia and can make up a large fraction of the microglial pool. Cohort data showed a protective association between most types of clonal hematopoiesis and Alzheimer's disease. The authors conclude that an influx of myeloid cells into the healthy human brain is widespread, contributes to microglia, and becomes common with aging.

Limits

The brain study involved only 20 people, all older, so it cannot show when the influx starts. The Alzheimer's finding is an association in cohort data and does not prove that blood-derived microglia protect the brain. Only the abstract was available to us; the full text is behind a paywall. It also cannot say whether the incoming cells behave exactly like the original microglia over the long term.

Key terms

Somatic mutation

A DNA change acquired during life; shared mutations mark cells from the same ancestor.

Microglia

The brain's resident immune cells.

Lineage tracing

Following which cells descend from which ancestors.

Myeloid cells

A family of white blood cells that includes monocytes and macrophages.

Clonal hematopoiesis

Expansion of blood cells descended from one mutated stem cell.

Bone marrow

The tissue inside bones where blood stem cells make new blood cells.

Alzheimer's disease

The most common cause of dementia, marked by plaques and tangles in the brain.