快猫短视频

MICE engineered to carry an extra artificial chromosome have successfully
passed it to their offspring. Although the Canadian company responsible has no
intention of repeating the experiments in people, its work shows that human
germline gene therapy鈥攎aking genetic changes that will be inherited by
future generations鈥攊s becoming a practical possibility.

Chromos Molecular Systems of Burnaby, British Columbia, reported the
breakthrough this week in London at a conference on biotechnology. 鈥淚t鈥檚 the
first time an artificial chromosome has ever been shown to be inherited in any
mammal,鈥� says Eileen Utterson, vice-president of corporate development. Chromos
plans to use the technology to create herds of genetically modified animals
whose milk will contain pharmaceuticals.

Geneticists have been creating transgenic animals for years by injecting
genes into a newly fertilised embryo. When this method works, every cell in the
animal that develops from the embryo will contain the added genes, including its
sperm and eggs. This means that the genes will be inherited by the transgenic
animals鈥� offspring.

It鈥檚 a haphazard process, however. Often the gene doesn鈥檛 get incorporated
into the embryo鈥檚 genome. And when it is, the gene splices itself at random into
one of the animal鈥檚 chromosomes, where it may not work as normal or, worse
still, can disrupt other genes鈥攑otentially causing developmental
abnormalities.

While occasional 鈥渕istakes鈥� are tolerated in animal experiments, the danger
of causing congenital defects is one of the reasons why germline gene therapy
for people has remained taboo. So instead of trying to correct genetic defects
such as the mutations that cause cystic fibrosis at the start of life鈥攂y
adding genes to embryos created by IVF鈥攇ene therapists treat people with
genetic disorders by adding therapeutic genes to specific tissues in the hope
that they will be taken up by enough cells to correct the defect.

While this ensures that sperm and egg cells do not become contaminated by the
added genes, it isn鈥檛 very efficient. As a result, many gene therapy trials have
had only limited success.

However, if genes could be ferried into embryos in an artificial chromosome
that would safely be inherited without interfering with the rest of the genome,
germline gene therapy might not be so risky. Chromos鈥檚 experiments with mice
suggest that this should be possible. 鈥淏ecause the artificial chromosome is
separate, it doesn鈥檛 interfere with the cell鈥檚 own genetic machinery,鈥� says
Utterson.

In London this week, Chromos presented preliminary results of experiments
with mice given an artificial chromosome. By taking cell samples and exposing
them to fluorescent dyes that bind to different parts of the chromosome,
Chromos鈥檚 scientists were able to discover which animals had accepted the
chromosome. When the mice carrying the extra chromosome were crossed with normal
mice, it was inherited in exactly the same way as the animals鈥� natural
chromosomes.

Chromos is also working on human artificial chromosomes for use in
conventional, non-germline gene therapy. Artificial chromosomes will have an
advantage here as well, because they can carry much more DNA than is possible
with existing methods, which use viruses or loops of bacterial DNA known as
plasmids. But the company says it won鈥檛 let its technology be used for human
germline engineering. 鈥淲e are in control of the technology, and we don鈥檛 want to
engage in germline gene therapy,鈥� stresses Utterson.

However, many groups worldwide are also striving to create artificial human
chromosomes. And some geneticists are talking openly of one day using such
chromosomes for germline gene therapy
(快猫短视频, 3 October 1998, p 24).

鈥淭his is obviously going to open up the debate again in the field of germline
gene therapy,鈥� says Norman Nevin of Belfast City Hospital, who chairs Britain鈥檚
Gene Therapy Advisory Committee.

However, scientists advising the world鈥檚 governments remain cautious. Claudia
Mickleson of the Massachusetts Institute of Technology, who chairs the US
National Institutes of Health鈥檚 Recombinant DNA Advisory Committee, says that
her committee wouldn鈥檛 approve a germline trial without extensive preclinical
information on safety. And given concerns about the technology being used to
create 鈥渄esigner babies鈥�, Mickleson also says trials would not proceed without
鈥渋ntense discussion鈥� with the public.

CHROMOS鈥檚 scientists start with natural chromosomes that possess one pair of
full-length arms, and another pair of stubby arms containing no working
genes.

Using DNA-manufacturing enzymes, the researchers duplicate these stubby arms
and extend them with inert 鈥渟atellite DNA鈥�. The full-length arms drop off,
leaving an artificial chromosome containing only the elements it needs to
survive and copy itself.

These elements include telomeres, repeating sequences of DNA that cap the
tips of the chromosomes and stop them merging with other chromosomes. In the
middle is a centromere, the 鈥渒ink鈥� in a chromosome that hooks it to the protein
guide wires along which it moves when a cell divides.

Finally, the researchers weave genes of interest, as well as the switches
needed to control them, into the chromosome鈥檚 backbone of satellite DNA.

Chromos has also invented a technique for injecting the chromosomes into
cells or embryos with a fine needle. This took two years to perfect because the
chromosomes are so much larger than the small quantities of DNA usually inserted
through microinjection.