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Mosaicism, or DNA differences from cell to cell (not just person to person)

posted by Tim Carmody   May 25, 2018

Science writer Carl Zimmer has a new book on genetics and heredity called She Has Her Mother’s Laugh. The New York Times published an excerpt this week focusing on mosaicism — an unexpected but surprisingly common condition where different cells in the same organism display different DNA (sometimes strikingly, fatally different).

Dr. Walsh and his colleagues have discovered intricate mosaics in the brains of healthy people. In one study, they plucked neurons from the brain of a 17-year-old boy who had died in a car accident. They sequenced the DNA in each neuron and compared it to the DNA in cells from the boy’s liver, heart and lungs.

Every neuron, the researchers found, had hundreds of mutations not found in the other organs. But many of the mutations were shared only by some of the other neurons.

It occurred to Dr. Walsh that he could use the mutations to reconstruct the cell lineages — to learn how they had originated. The researchers used the patterns to draw a sort of genealogy, linking each neuron first to its close cousins and then its more distant relatives.

When they had finished, the scientists found that the cells belonged to five main lineages. The cells in each lineage all inherited the same distinctive mosaic signature.

Even stranger, the scientists found cells in the boy’s heart with the same signature of mutations found in some brain neurons. Other lineages included cells from other organs.

Based on these results, the researchers pieced together a biography of the boy’s brain.

I’ve always been drawn to the idea that each of us are many people, an assembly of mismatched parts, manifesting themselves in different times and contexts. It’s striking to see that reflected, albeit in a refracted way, in our array of possible genomes.