The tumor suppressor p53 also controls somatic-cell reprogramming, researchers at the Salk Institute for Biological Studies and Center for Regenerative Medicine in Barcelona have discovered. They found that p53 prevents differentiated cells from returning to an embryonic stem cell like state.
“Although we have been able to reprogram specialized cells for a while now, there had been nothing known about the control mechanisms that prevent it from happening spontaneously in the body and why it has been so hard to change their fate in a Petri dish,” points out Juan-Carlos Izpisa Belmonte, Ph.D., a professors in the gene expression laboratory. His team’s findings are published in the August 9 advance online edition of Nature.
The most widely used technology to reprogram differentiated cells into induced pluripotent stem cells (iPSCs) involves the forced expression of four transcription factors—Oct4, Sox2, Klf4, and c-Myc—in fully committed adult cells.
“Unfortunately, Klf4 and c-Myc are oncogenes, and adding them carries the risk of inducing cancer,” Dr. Belmonte notes. Additionally, this method changes a tiny fraction of cells into iPSCs that look and act like embryonic stem cells.
The Salk group thus began to research whether what they were doing to get the cells to reprogram induced a response that stopped the cells from growing. “We knew that c-Myc and some of the other genes that are required for reprogramming activate the tumor suppressor p53, and we wondered whether it had any part in it,” explains Geoffrey M. Wahl, Ph.D., also a professor in the gene expression laboratory.
Their experiments revealed that adding the reprogramming factors c-Myc and Klf4 alone or in various combinations activated the p53 pathway. As a first-responder, p53 then turns on genes that halt cell division to allow time for repairs or when all rescue attempts prove futile, order the cell to stop dividing.
In cells genetically engineered to lack p53, reprogramming efficiency was increased at least 10-fold compared to control cells, demonstrating that p53 clearly played an important role in reigning in cells trying to revert into a stem-like state.
Because iPSCs generated with the full complement of reprogramming factors run the risk to turn malignant, Dr. Belmonte and his team wanted to know whether mouse cells lacking p53 could be reprogrammed using only two factors, Oct4 and Sox2. The cells readily converted into iPSCs and gave rise to healthy, full term mice that were able to reproduce, passing the ultimate test for pluripotent embryonic stem cells.