But because only a very low percentage of cells complete reversion to the embryonic stem cell state with this technique, and because the cells are situated among millions of cells in the culture dish that do not complete the transformation, the scientists had a difficult time identifying the fully reprogrammed cells. Thus, they developed the technique of inserting the foreign drug-resistance gene into the mouse skin cells. This gene was designed to only switch on in cells that completed the reversion to the embryonic stem cell state. With addition of the drug to the culture dish, the vast majority of cells, those that had not reverted to embryonic stem cells, died. Only those that had reverted survived and could then be expanded.
With the alternative technique developed by the UCSF team, the efficiency of embryonic stem cell production remained low. However, the mouse skin cells that did start to revert to embryonic stem cells could readily be identified by their form and structure in the absence of any drug. The researchers went on to show that these cells indeed behaved like embryonic stem cells and could give rise to all cell types of the body.
Separately, the team demonstrated that reprogramming could be achieved when one of the four genes over-expressed to initiate reprogramming -- c-myc -- was replaced with a related gene, known as n-myc. These genes are involved in the formation of different tumors, so by beginning to replace genes in this method the researchers may find combinations of reprogramming genes that are safer, says Blelloch.
Studies should address the relative efficacy of n-myc versus c-myc in reprogram
|Contact: Jennifer O'Brien|
University of California - San Francisco