When a brain is forming, new neurons have to squeeze through very tight spaces to reach the outer layer called the cerebral cortex. While they squeeze, the cells stretch and bend.
A recent study in Nature found that this squeezing often causes a special kind of damage called double‑strand DNA breaks. Both strands of the DNA helix get cut, which looks scary but is actually part of normal brain development.
Professor Mineko Kengaku from Kyoto University says the young brain is built to handle and quickly fix this damage. Understanding how the brain repairs DNA could help explain many brain disorders.
What the Scientists Did
The team built tiny channels that copy the cramped paths neurons travel in a real brain. They let neurons move through these channels while shining fluorescent lights on their DNA.
They saw many double‑strand breaks appear as the cells moved. After the neurons reached the other side, the breaks faded away. Most were fixed within a day, and the cells kept working as usual.
Why the Breaks Happen
The culprit is an enzyme called Topoisomerase IIβ. Normally it cuts DNA for a moment, untangles it, and then reseals it—much like cutting a tangled rope to straighten it. When a neuron is squeezed, the enzyme can get stuck halfway, leaving the DNA cut.
The cell then uses a repair system called non‑homologous end joining to stitch the ends back together.
Neurons vs. Cancer Cells
Neurons differ from cancer cells that were also tested in the same channels. Cancer cells get random DNA breaks that often kill them. Neuron breaks, however, happen mainly in parts of the genome that aren’t needed for essential genes, so the cells stay healthy.
When Repair Fails
To see what happens without proper repair, the researchers made mice that lacked the enzyme Ligase 4, which is needed for fixing breaks. The mice grew normally at first, but as adults they developed mild balance problems—similar to some human cerebellar disorders linked to unstable genomes.
What This Means for Brain Health
The study suggests that DNA breaking and fixing are built into brain development. Small differences created by this process might make each neuron slightly unique and could affect neurodevelopmental or neurodegenerative diseases later in life.
"All neurons start with the same DNA, but the mechanical journey can write tiny changes into the genome," says Professor Kengaku.
This work involved scientists from Kyoto University, the University of Tokyo, Osaka University, the National University of Singapore, and the Tokyo Metropolitan Institute of Medical Science.