Wednesday, October 12, 2011

Embryonic stem cells for human therapeutic use?

As many of you will have heard, a form of cloning has recently been used to create personalised embryonic stem cells in humans.
Genetic material was taken from an adult skin cell and transferred into a human egg. This was grown to produce an early embryo. However, the stem cells formed contained chromosomes from both the adult and the egg cells. The technique used - somatic cell nuclear transfer - shot to fame in 1997 when Dolly the sheep, the first mammal to be cloned from an adult cell, was unveiled to the world.
Trials involving removing the genetic material from the egg and replaced it with the chromosomes from a skin cell, proved unsuccessful as the egg divided but failed to go past the 6-12 cell stage. In contrast, when the egg's own genetic material was left in place and the skin chromosomes were added, the egg developed. It reached the blastocyst stage, which can contain up to 100 cells and is the usual source of embryonic stem cells.
In this technique the two adult copies are added to the single copy in the egg meaning a total of three, which can be problematic. Often embryos without the correct number of chromosomes do not develop at all. Down's syndrome is caused by three copies of just one chromosome.
Researchers will need to produce embryonic cells which have only donor DNA, however, once the egg starts to divide the chromosomes are combined in the nucleus and would be near impossible to separate.
This research shows that the traditional approach to somatic cell nuclear transfer is inefficient in humans, whereas leaving the host egg genome in place increases efficiency.
Using embryonic cells does raise significant ethical dilemmas. Recently a different route to stem cells has been used. Instead of using an egg, a chemical bath "reprogrammes" an adult cell into a stem cell. While this is seen as more ethical, there are concerns about whether such cells could be used therapeutically. There are differences between embryonic and "induced" stem cells, with the latter being more prone to expressing cancer causing genes.


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