Human Somatic Cell Nuclear Transfer For Patient Specific Stem Cells: Will It Work?
What is Somatic Cell Nuclear Transfer (SCNT)?
SCNT is a phrase coined by scientists to describe the process of injecting the nucleus (which contains the chromosomes) from another cell in the body into a human egg. Last week a team of Oregon scientists reported creating four unique stem cells by this process (Cell, June, 2013 (pdf)). This work is a follow-up to studies originally reported by Bedford Research Foundation scientists, Jose Cibelli and Ann Kiessling, in 2001 (Somatic Cell Nuclear Transfer in Humans: Pronuclear and Early Embryonic Development. J of Regenerative Medicine (pdf)).
Why are the new stem cells important?
Stem cell-based treatments, termed regenerative medicine, are being developed to replace defective tissues and organs such as heart and kidney failure, spinal cord injury and disease, diabetes, AIDS, Parkinson’s and Alzheimer’s diseases, and degenerative joints. The source of the stem cells is key. If they can be harvested from the patient, there will be no problems with tissue rejection, such as can happen with kidney or heart or bone marrow transplants from donors. SCNT provides a powerful method to create stem cells with the patient’s own chromosomes, thus a perfect tissue match. SCNT had been accomplished in many species, but not human.
 The human egg is a huge cell, that is released from the ovary surrounded by cells that support it. The blue areas shown above are the DNA, the genetic information contained within each cell. The cluster of blue within the egg is the egg’s chromosomes, half of which will be expelled when the sperm enters, so the developing embryo will have genetic information from both the mother and the father.For parthenogenesis, the egg chromosomes alone guide development, for SCNT, the chromosomes are usually removed with a tiny pipette so the only genetic information remaining is from the transferred nucleus. |
Background to SCNT
SCNT was first performed several decades ago to test whether or not every cell in the body contains all the genetic information of the individual. For example, does a liver cell contain all the genetic information to form every other type of cell, but “non-liver” genes are silenced? Or does a liver cell lose other genes and only retain “liver” genes?
Dr. John Gordon received the Nobel Prize in 2012 for demonstrating in 1958 that tadpole cell nuclei contained all the chromosomes necessary to form new frogs following transfer into frog eggs whose own chromosomes had been removed. The new frogs were genetically identical to the tadpole. Frogs were the first cloned animals.
For nearly three decades, it was believed that although successful with amphibia, mammals could not be cloned by transferring a nucleus into an egg, although other types of studies had confirmed that all cells contain all the genetic information needed to form a new being.
In 1997, to the surprise of the scientific community, Dolly the sheep was cloned by SCNT into a sheep egg, thus proving that like frogs, mammals could also be cloned from a single cell (Ian Wilmut at the Activated Egg Symposium (video)). Since then, many species have been similarly cloned, including mice, rats, rabbits, dogs, goats and cattle.
What is the magic contained within an egg?
Perhaps partly because of its enormous size, an egg has a unique capacity to remodel the structure of chromosomes, including its own and those of sperm. The remodeling allows expression of all the genetic information needed for stem cell pluripotency. Without being fertilized by sperm, eggs can be activated to generate stem cells, parthenote stem cells (1 min video about Parthenotes), using their own chromosomes. Parthenote stem cells have all the pluripotency characteristics of stem cells derived from human embryos (1 min video about hES). These studies suggested that SCNT would be highly successful in human eggs.
Why no human SCNT cells before now?
The inability to derive a line of stem cells from SCNT into human eggs has been a mystery, thought to be related to the status of the egg itself.
Thirteen years ago, Bedford Research scientists (JRM, 2001 (pdf)) reported side by side experiments activating human eggs parthenogenetically and by SCNT. Although 12 out of 22 parthenote eggs did divide and progress to the blastocyst stage, only 3 out of 19 SCNT eggs divided once or twice, and none progressed to the blastocyst stage (schematic). These results led to the current ongoing work by Bedford Research scientists to fully characterize gene expression in normal 8-Cell human embryos to begin to understand the problems with human parthenotes and SCNT eggs (JARG 2010; JARG 2009).
 8-cell human embryo |
A few years later, a large South Korean scientific team, well experienced in SCNT and animal cloning, reported the successful generation of several stem cell lines from SCNT into human eggs. The reports were quickly dispelled by other South Korean scientists as being totally fraudulent. This was a huge blow to the integrity of the stem cell scientific community.
Nonetheless, the lesson learned from the South Korean reports is that they had access to approximately 2,000 human eggs and had been unsuccessful in SCNT. This revealed loud and clear that there was a fundamental difference between human eggs and those of other mammals.
Two years ago, a New York scientific team headed up by Dieter Egli (AES 2011, Time 2011) reported deriving SCNT stem cells from human eggs whose chromosomes had not been removed (Nature, Vol 478). There was something helpful about leaving the human egg chromosomes in the egg during the activation process. This is regarded as an important breakthrough even though the resulting stem cells contain twice as many chromosomes as normal stem cells.
And now comes the Oregon scientific team reporting the successful derivation of SCNT stem cell lines by utilizing advances in egg culture, activation, and chromosome removal. Nonetheless, that unknown egg-specific factors play a role, however, was shown by marked differences in SCNT success rates.
Unfortunately, there are irregularities in the manuscript reported in Cell (Science Magazine, May 23, 2013), thus once again casting a shadow over the work. Other scientific teams are currently investigating the cell lines to determine the accuracy of the report.
Are SCNT eggs actually embryos?
Accuracy of language for new technologies involving human eggs will provide the balanced framework necessary for everyone to evaluate the value — and the threat — of SCNT technology (CT law review). If “embryo” is reserved for the unique union of egg and sperm, and if other activities of human eggs have more accurate terms, such as parthenote for eggs activated with their own chromosomes, and SCNT-egg (or “ovasome”) for eggs activated following SCNT, there will be far less confusion, especially by the non-scientific community.
Whether or not SCNT-eggs should be assigned the same status as embryos since there is the potential — however small — that such an entity could develop further if transferred into a uterus (“human cloning”) is a separate consideration from whether or not they should be termed “embryos.” New technologies need new terms.