ONE sheep looks much like another, so why clone them?
As Ian Wilmut will tell anyone who cares to listen, the main aim of his team
at the Roslin Institute in Scotland was to transform the genetic engineering of
farm animals (sheep first, then cows) from a hit-and-miss experimental procedure
into a robust technology. Cloning was just a welcome by-product; Dolly, the
extra cheese on the pizza.
Genetic engineering has long promised to revolutionise medicine and
agriculture. A few sheep and cows, for instance, have already been engineered to
produce human proteins such as clotting factors in their milk. And in future,
their cousins could have their genes altered so that they develop human genetic
diseases like cystic fibrosis, or even heart disease.
A sheep with cystic fibrosis sounds gruesome, but Wilmut believes the
advantages would outweigh the cost in animal suffering. Cystic fibrosis is one
of the most common, serious heritable diseases. At the moment, drugs to treat
the disease can only be tested in mice. Sheep lungs are far more like our
own.
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A more distant possibility is that genetic engineering could turn out farm
animals with organs that look human, immunologically speaking, providing an
endless stock of organs for transplant. But the idea is controversial because of
the risk of passing diseases from animals to humans. On the agricultural front,
cows could be engineered so that they produce lots of milk without being
susceptible to mastitis, a painful inflammation of the udder. Then there’s the
possibility of tweaking genes to make meat lean, or to get low-fat milk straight
from the cow.
None of this was feasible on a large scale until the Roslin team’s
breakthrough. Before then, the only way to genetically engineer sheep and cows
was to inject a gene directly into a newly fertilised egg. This is horribly
inefficient. You have to inject thousands of fertilised eggs to get one animal
that makes it to adulthood with the new gene active in the correct tissues. And
even if you succeed once, there is no guarantee that you will be able to do it
again.
To get round those problems, Wilmut and his colleagues wanted to be able to
do their genetic manipulations in sheep cells growing in a flask. This makes
genetic engineering far more efficient because you can insert and remove genes,
or even create tiny mutations in the DNA, and then allow the cell population to
grow. From this population you can take out a few cells to check that you have
changed the genes correctly, and still have thousands of identical copies left
to play with. But it does mean you need a way of turning the cells back into
sheep. This is where the Roslin team made their biggest contribution. They
transferred the nuclei of lab-grown cells from very young sheep embryos, and
then from older sheep fetuses, into eggs that had had their own genetic material
removed, triggering the development of an embryo and eventually a whole
sheep—that is, a clone of the animal that donated the original cells.
Dolly came along when they tried the same thing using adult sheep cells grown
in a flask. And since Dolly has come Polly —proof positive that you can
combine genetic engineering (she’ll secrete a human protein in her milk) and the
cloning of lab-grown fetal sheep cells.
Dolly also raises the possibility of a whole new way of streamlining genetic
engineering. Once wrinkles in the technique have been ironed out, researchers
will be able to clone directly from genetically modified adult farm animals that
have already proved their value as biofactories or food producers.
–Rachel Nowak
Why start with an egg to make a clone of an adult?
“The most remarkable substance on the planet” is how developmental biologist
Keith Latham of Temple University School of Medicine in Philadelphia describes
the egg cytoplasm, the goo surrounding the nucleus.
Latham’s enthusiasm is understandable. After all, Dolly is alive and well
because an egg reprogrammed a cell designed for the very adult task of being
part of an udder.
The resurrecting proteins of the egg’s cytoplasm—most of them still
unidentified—are there to prepare the sperm’s nucleus for its union with
the egg’s genetic material, and to orchestrate the first cell divisions that
turn the embryo into a ball of eight or 16 cells. Only at that point do the new
embryo’s genes take the helm.
As the embryo grows further and cells start to specialise, genes that are
needed only for the early stages of development become cloaked in proteins
called histones and are turned off, while the genes a particular cell needs to
perform its adult functions are turned on by proteins called transcription
factors. To send an udder cell back to its unspecialised state, the egg had to
turn off the adult genes, and reactivate the embryonic ones.
“In reprogramming donor cells, the egg is mimicking what it does to the sperm
nucleus,” says Keith Campbell, one of Dolly’s creators, who is based at PPL
Therapeutics in Edinburgh. The sperm, too, is a specialised cell, although its
task is a pretty simple one. It’s designed for the express purpose of carrying
the father’s genes from testes to egg, and to do that the adult genes necessary
for the trip must be turned on.
To reprogram sperm genes, the egg cytoplasm contains large amounts of a
protein called MPF that dissolves nuclear membranes, condenses chromosomes, and
loosens the grips of transcription factors. It contains another enzyme that
clips specific bonds between proteins known as protamines, which help package
the sperm chromosomes. Immersion in the huge bath of cytoplasm—the egg is
a very big cell—also helps to slough the proteins from the
chromosomes.
Remarkably, mouse eggs come equipped with a means of removing histones too,
according to as yet unpublished research by developmental biologist Lawrence
Smith of the University of Montreal in Canada. For cloning researchers that
might turn out to be an important quirk of egg design—sperm may not
contain histones, but the cells used in cloning do.
—Philip Cohen
Could somebody clone you?
Suppose someone wants to have a little baby copy of you (technology
permitting). Maybe it’s an old boyfriend or ex-wife who is still smitten, or
some scary new-style stalker. Whoever they are, they would have little problem
finding some of your precious cells to try to clone from—and the chances
are you’d never even know it.
“The average person doesn’t realise they leave tissue around all the time,”
says Robin Alta Charo, a law professor at the University of Wisconsin, Madison.
If you have ever given blood, had a biopsy or surgery, then a piece of you may
still be tucked away clearly labelled in a pathologist’s freezer. In many US
states and European countries, blood is routinely taken from newborns for
genetic screening and sometimes stored for decades.
But for one-stop shopping for a well-documented fragment of human tissue it’s
hard to beat the Armed Forces Institute of Pathology in Washington DC, a tissue
bank used by pathologists and forensic scientists. It holds about 92 million
tissue bits from civilians and military personnel around the world, and millions
more are added each year. Increasingly, such samples are given or sold to
academics and biotechnology companies for genetics research.
And if the would-be cloners can’t find a scientific institution to front for
them? They could try surreptitiously swabbing your wine glass, or plucking some
of your hairs—that might just provide enough cells to generate a
clone.
—Philip Cohen
Isn’t there a law against it?
If you discovered you were the victim of body banditry, calling the police
wouldn’t necessarily help, says Lori Andrews of the Chicago-Kent College of Law
in Illinois. Cloning humans is banned in only a few places—Britain, but
not in most states in the US and Australia. What’s more, “there isn’t much law
that lets us control our tissues once they are outside our bodies,” says
Andrews. She cites, for example, the 1980 lawsuit brought by John Moore, who
discovered that researchers had used a sample of his spleen to make a new drug
worth billions of dollars. The California Supreme Court found that Moore had no
legal rights to share in that bounty.
Even your right to assume parentage of your cloned offspring would be
ambiguous at best. In most European countries, the law defines the mother of a
child as the woman who has carried the pregnancy; the father is variously
defined as the partner of the woman who gestates the child or the sperm
provider. In the US, some states rely on genetic tests to establish parenthood
in case of disputes.
But if a child was secretly cloned, say, from your blood you will have
contributed neither sperm nor eggs, and you won’t have been involved in the
gestation. Routine maternity or paternity genetic testing would only add to the
confusion by showing that you are too closely related to be a parent, and were
instead a twin.
Legislators are unlikely to be in any hurry to address the dilemmas of human
cloning for fear of tacitly condoning a controversial idea. But eventually the
law will need to deal with the new realities, says Alta Charo. “To protect
people’s rights, we’ll have to revisit the logic of how we view our bodies and
relationships like mother, father, sister, brother. Right now we are kind of
˛ą»ĺ°ůľ±´ÚłŮ.”
—Philip Cohen
Would it be weird to be a clone of your Mum or Dad?
FOR identical twins life is bittersweet. They have enviably close
relationships. Yet they also have trouble “developing an identity, developing a
sense of their own agency in the world”, says Ricardo Ainslie, a psychologist at
the University of Texas in Austin, who studies twins. “It’s an issue that most
of us take for granted. But identical twins grapple with it all the time.”
In an effort to forge their identities, twins can pigeonhole
themselves—one may become overly artsy, the other too practical. “It can
rob you of opportunities to use your talents,” Ainslie says. Cloned children
could have similar identity conflicts with their parent, he says, and one
possible outcome is a double dose of teenage rebellion as the child attempts to
establish his or her own identity.
Of course, a lot depends on the parent. “Parenting is partly narcissistic,”
says Ainslie. “It allows us to be selfless, but it also means that you sometimes
insert your own interests in the place of the child.” From pressuring their
children to join the family firm, to disapproval if they support a rival
football team, normal parents’ narcissistic tendencies could be amplified by
clonehood, Ainslie fears. The child could “feel like nothing but an extension of
the parent’s needs”.
Let’s not get carried away, says Renee Garfinkel, a psychologist at the
Adoption Studies Institute in Washington DC. Every factor that helps a child
create a healthy sense of self has an impact, from birth order and family style
to a child’s talents, physical appearance and whether or not he or she is clone.
“But no one factor is ever decisive,” she says.
Garfinkel also argues that fears of sexual interest from the non-genetic
parent are unfounded. Sure, the child will bear an uncanny physical resemblance
to the other parent as he or she reaches adulthood, but this counts for little.
According to studies of adopted children and unrelated children raised together,
the aversion to incest is created during rearing, and is not dependent on who
you are genetically related to, or physical appearance, Garfinkel says.
—Rachel Nowak
What about a Manhattan Project for human cloning?
Picture this. Dolly’s birth is announced and the world cheers. There are no
doubts, no wariness on the part of researchers, religious leaders, politicians
or the public. They speak with one voice: we want human clones!
In response, nations unite to create a lavishly funded International
Institute on Cloning, charged with copying adult humans as quickly and safely as
possible. Its first priority is to improve cloning efficiency. Dolly took more
than 400 attempts, including 28 miscarriages. Such a high failure rate is
unacceptable for humans.
One theory is that adult cells are more set in their ways than, say, the
cells of a fetus or an embryo, which are easier to clone. Since the cytoplasm of
the egg does the reprogramming magic, IIC cell biologists try passing the
nucleus of an adult cell through not one egg, but two. They also try fusing
nuclei with less mature eggs, which typically stall at the reprogramming step,
giving extra hours in which to remould the nucleus.
A second explanation for the inefficiency of current cloning techniques is
that most adult cells are damaged beyond the egg’s ability to repair them. As a
cell ages it collects genetic scars: its chromosome ends shorten, and DNA
mutations accumulate. The damage may not be obvious. An udder cell would
function perfectly well no matter how mangled its heart genes have become,
because they are switched off. But if the cell was used in cloning, the embryo
would develop cardiovascular problems and be aborted.
To get to grips as quickly as possible with the problem of genetic damage,
IIC geneticists focus on mice, reasoning that their genetic make-up is well
known, and it’s possible to study several generations in a matter of months.
With those mice, they identify the types of cells that are least prone to
age-related damage, and develop efficient new ways to screen for genetic damage,
and even to reverse it.
In another wing of the sprawling IIC, reproductive technologists start on the
human end of the project. They offer women made infertile by defects in their
egg cytoplasm the option of having their egg’s nuclear material transferred into
a donor egg with a healthy cytoplasm, and then fertilised.
Meanwhile, IIC tissue engineers transfer the nuclei of cells taken from
patients with Parkinson’s disease into eggs that have had their own nuclei
removed, so creating an embryo clone. Every cell in the very young embryo is
pluripotent—capable of generating any type of adult cell. They put the
embryo cells in a flask and try adding different nutrients and growth factors at
different times in an attempt to grow new nerve cells. Once they succeed, they
will have a stock of healthy nerve cells that are a perfect match for the
Parkinson’s patients who donated the originals—ready to be transplanted
back.
So far the IIC team hasn’t tried to create a human clone that makes it to
adulthood. But they are getting a feel for nuclear transfer with human cells,
the technique that underpins cloning. It will stand them in good stead come the
fateful day when they make their first attempt to copy a human.
—Philip Cohen
But real life’s not like that… is it?
It’s not as different as you might think. Despite widespread misgivings about
human cloning, researchers are pursuing the very same avenues of investigation
as the fictional IIC. That’s because every technique that brings human cloning
closer also holds great promise for medicine and agriculture. In the past year,
calves have been born that were created from fetal cell nuclei that had been
passed through two eggs, or inserted into a younger stalled egg.
Already infertile women have been treated by injecting healthy cytoplasm into
their eggs (The Lancet, vol 350, p 186). Now, there are plans afoot to
use nuclear transfer to treat infertility caused by problems in the egg
cytoplasm. Meanwhile, Parkinson’s patients routinely receive transplants of
nerve cells from fetuses. Using cloning to grow those or other types of human
cells for transplant is a much talked about prospect, and the concept has just
been tested using cloned cow cells in rats (see this month’s Nature
Medicine,vol 4, p 569).
Don Wolf of the Oregon Regional Primate Research Center in Beaverton, who is
working on primate cloning for AIDS research, says that human cloning might be
feasible within as little as five years. “Step by step,” he says, “the cloning
industry will get better at what they do.”
—Philip Cohen
