COMPANIES in the US and Britain are working on ways to get human embryonic
stem cells without destroying viable embryos, 麻豆传媒 has
learned.
Their goal is to create compatible tissue for transplant without falling foul
of the legal and ethical objections to getting stem cells from normal or cloned
embryos. But will their approach work鈥攁nd does it really bypass the
ethical issues?
Stem cells that can grow into all sorts of specialised tissues are thought to
have enormous potential in medicine. The most versatile and useful are embryonic
stem cells (ESCs), which you get from the ball of cells that forms a few days
after fertilisation.
Advertisement
The trouble is that to get ESCs you have to destroy an embryo that could
become a child. This has led to fierce opposition from some quarters, even
though most ESCs come from spare IVF embryos that would otherwise have been
discarded.
Therapeutic cloning, where you create compatible tissue for transplant by
taking ESCs from a cloned embryo, is even more controversial. This will become a
crime in the US if the Senate approves an anti-cloning bill later this year.
Now, however, at least two groups in the US are working with embryos that
never have the potential to become a person. But these clumps of cells sometimes
develop far enough for researchers to extract the equivalent of embryonic stem
cells.
The research carried out at Advanced Cell Technology (ACT) in Massachusetts
involves a process known as parthenogenesis. Normally, fertilised egg cells get
one set of chromosomes from the mother and one from the father. But in
parthenogenesis, the egg cell duplicates one set of maternal chromosomes and
develops as if it had been fertilised (see graphic).
Some insects, lizards and even birds can reproduce asexually via
parthenogenesis. In mammals, though, having two sets of maternal chromosomes
causes such severe problems that parthenogenic embryos never develop into a
normal fetus.
Despite this, ACT has managed to harvest ESC-like cells from parthenogenic
monkey embryos, according to a patent application seen by 麻豆传媒.
Like normal ESCs, the cells stayed undifferentiated for four months when kept
with mouse cells called feeder cells. When the researchers took away the feeder
cells, the parthenogenic cells spontaneously differentiated into what appeared
to be more specialised types, such as skin cells and beating heart cells.
The patent is the first report of ESC-like cells being derived from primates
via parthenogenesis. If it works in humans too, it means ACT may have found a
way to get ESCs without destroying a potential person.
What鈥檚 more, ACT thinks that this technique could also make therapeutic
cloning unnecessary. For example, when creating transplant tissue for a woman,
parthenogenic cells derived from one of her egg cells would have half of her
genetic material, so any tissue created from such stem cells would be a close
match. In the case of a man, the patent application suggests that one set of his
chromosomes could be transferred to an egg that has had its nucleus removed.
Then the egg could be induced to undergo parthenogenesis (this could also be
done for a woman).
ACT wouldn鈥檛 comment on the research. 鈥淭he patent speaks for itself,鈥� says
chief executive Michael West. 鈥淲e鈥檙e not going to be releasing any more
information until we publish the data in a peer-reviewed journal.鈥�
But ACT is not alone. In Los Angeles, Jerry Hall of the Institute of
Reproductive Medicine and Genetic Testing is also working on parthenogenesis. He
wouldn鈥檛 reveal any details but was confident it would work in humans. 鈥淣ot only
are we optimistic that parthenogenesis in humans would lead us to the same
results, I would be surprised if they didn鈥檛.鈥�
However, much remains to be done to show that the technique is feasible. It
is unclear how versatile parthenogenic stem cells will be鈥攐r how safe.
There is much concern that normal ESC implants could become cancerous, and some
human ovarian cancers are formed by parthenogenesis.
But parthenogenesis isn鈥檛 the only option. Another method that may yield
ESC-like cells without using viable embryos is the transfer of cytoplasm from an
egg cell into ordinary adult cells. The egg cytoplasm seems to turn the
specialised adult cells back into an undifferentiated state.
Several companies are working on this ooplasmic transfer approach, but it is
unclear how successful it has been. PPL Therapeutics in Scotland reported this
year that cow skin cells injected with cytoplasm from cow eggs dedifferentiated
into cells that looked like ESCs, and that the addition of certain growth
factors turned them into beating heart cells. The company hopes to try it with
human cells, says research director Alan Colman.
For companies that can find a way round the proposed ban on therapeutic
cloning in the US, the rewards could be great. To satisfy the ethical concerns
of critics, however, they will have to prove that none of their techniques could
ever create a viable embryo.
鈥淚t鈥檚 a positive thing if people are working on technologies that would not
damage a human embryo,鈥� says Douglas Johnson, legislative director of the
National Right to Life Committee. 鈥淏ut we want to see that it鈥檚 not a word
驳补尘别.鈥�