Scientists at UCSF have identified about two dozen genes controlling the fate of embryonic stem cells. They've done so using RNAi techology. “The genes we identified are necessary for embryonic stem cells to maintain a memory of who they are,” said Barbara Panning, PhD, associate professor of biochemistry and biophysics at University of California at San Francisco. “Without them the cell doesn’t know whether it should remain a stem cell or differentiate into a specialized cell.”
Panning and colleagues used a powerful technique known as RNA interference, or RNAi, to screen more than 1,000 genes for their role in mouse embryonic stem cells. RNAi refers to the process of using synthetic small interfering RNA (siRNA) molecules to silence specific genes in a cell. The technique produces results quickly.
"The biggest advantage of using RNAi, not only in embryonic stem cells but also in other cells, is the speed at which you can deplete the protein product," said Frank Buchholz, a group leader at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany.
The UCSF work is important not only because of it's import for understanding embryonic stem cells, but for its potential in reprogramming cells to create Induced Pluripotent Stem Cells. By way of brief background, induced pluripotent stem cells were produced with human skin cells in November of last year in what may turn out to be one of the major accomplishments in the short history of stem cell research. Human cells were reprogrammed by Yamanka and Thomson, working separately, and the work was duplicated by at least one other lab (UCLA).
In every case so far, a combination of four genes (interestingly, not the same four) was used to either activate or silence the expression of downstream genes. These genes were introduced to the adult cell using retroviruses which run the risk of also introducing cancer. For Induced Pluripotent Stem Cells to reach the stage of use within humans, the threat of cancer must be eliminated.
At this point it is impossible to say whether the specifically targeted and rapid 'gene knockdown' provided by RNAi can replace the retrovirus approach to reprogramming. So far, RNAi-based gene silencing is often not as precise or thorough as traditional gene deletion. It turns out that RNAi floats freely in the cell rather than targeting a particular genetic focal point. As a result it often binds not only with its primary target but with other messenger RNAs as well. These off-target effects often produce spurious results. We'll post additional findings as they develop.
Adapted from the UCSF announcement and other sources.
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