Living Scars, Living Cells
In the months following the atomic bombing of Hiroshima, many survivors with severe burns developed thick, raised scars called keloids. These formations appeared most frequently in those exposed within two kilometers of the hypocenter, becoming most prominent between six and 14 months after the blast. While most keloids eventually shrank or healed, the tissue samples preserved from them became an invaluable biological archive. These samples contain fibroblasts, the cells responsible for forming connective tissue, that hold a direct genetic record of the damage inflicted by a massive dose of ionizing radiation.
The study of these cells falls to organizations like the Radiation Effects Research Foundation (RERF), a joint U.S.-Japan institution established to study the long-term health effects of radiation on survivors. Originally founded as the Atomic Bomb Casualty Commission (ABCC) in 1947, RERF has conducted follow-up studies on a cohort of over 120,000 survivors. The biological samples, including keloid tissue, donated by survivors and their families are the basis of this research, allowing scientists to investigate how radiation alters human DNA at a molecular level.
Atomic-Scale Damage
Analysis of tissues from survivors reveals specific patterns of DNA damage, known as mutational signatures. Ionizing radiation is a potent carcinogen primarily because it damages DNA. The blast's radiation caused DNA double-strand breaks, and errors during the cells' subsequent repair processes led to lasting mutations. These are not inherited genetic traits but somatic mutations, changes within the body's cells that are not passed to offspring.
Studies of radiation-exposed tissues show a high frequency of distinct genomic alterations. Two characteristic signatures are small deletions of 1 to 100 base pairs and an increase in balanced inversions, where a segment of a chromosome is reversed end to end. Compared to non-exposed tissues, radiation-associated cells carry a median of 201 extra deletions across the genome. These mutations are distributed randomly, unlike other types of DNA damage that correlate with factors like chromatin structure or replication timing. This specific pattern of widespread, small deletions and inversions is the signature of ionizing radiation exposure, a genetic scar left by the 1945 bombing that persists in survivor cells more than 75 years later. These findings from Hiroshima survivors form the bedrock of global radiation protection standards.