|Scanning electron micrograph of|
human embryonic stem cells (source)
|Very simplified mechanisms for|
the two types of stem cell creation
There are two methods of creating stem cells from adult somatic cells: somatic cell nuclear transfer (SCNT), in which genetic material from an adult cell is transferred into an egg cell (these are called NT-ES cells); and induced pluripotent stem cells (iPS), in which adult somatic cells are reverted back into stem cells by artificially turning on targeted genes - this is also known as a transcription factor-mediated manner of making stem cells.
By examining DNA methylation and other epigenetic effects, which can have a profound impact on gene expression, the authors found that each method of creating stem cells (embryonic, NT-ES, and iPS) resulted in different gene expression signatures. The DNA methylation and gene expression patterns in NT-ES cells more closely resembled those of the embryonic stem cells than the iPS cells did. The reprogramming of the cells in the production of iPS is presumed to have caused this problem. Since SCNT does not require cell reprogramming, epigenetic modifications were less likely to occur. The epigenetic modifications in iPS cells have the potential to impact the utility of these cells in regenerative medicine, and also in the in vitro modeling of various diseases.
The authors of the study conclude that SCNT is the superior technique for creating stem cells from adult somatic cells used in cell replacement therapies. However, this method is very difficult, and ethical issues in the U.S. prevent funding of the practice. Nevertheless, Dr. Shoukhrat Mitalipov of Oregon Health & Science University, one of the co-principal investigators of this study, is confident that SCNT will someday help us cure and treat a number of diseases that leave us stumped today.