Transplanted cells that contain their new host's nuclear DNA could still be rejected by the individual's immune system. This is because of slight differences between the cells and the host — in the genomes of the energy-producing organelles called mitochondria.
Jun-Ichi Hayashi at the University of Tsukuba in Japan, Noriko Toyama-Sorimachi at the National Center for Global Health and Medicine in Tokyo and their colleagues compared tumour cells containing nuclear DNA from one mouse strain and mitochondria from either the same or a different strain.
Despite just a 0.5% difference between the two mitochondrial genomes, cells containing the foreign organelles did not form tumours when transplanted into the strain of mice that the nuclear DNA came from. However, the hybrid cells grew at the same rate as normal tumour cells in mice lacking certain types of immune cell.
Tissues made from a person's stem cells could face rejection when transplanted back into him, the researchers say, because mitochondrial genomes tend to accumulate mutations.