Artificial Chromosome Passed On
"It's the first time an artificial chromosome has ever been shown to be inherited in any mammal," says Eileen Utterson, VP of corporate development. Chromos plans to use the technology to create herds of genetically modified animals whose milk will contain pharmaceuticals.
Researchers have been creating transgenic animals by injecting genes into a newly fertilized embryo. However, often the gene doesn't get incorporated into the embryo's genome. Even when it does, the gene may insert into an inactive region of the chromosome, or worse yet, it could disrupt necessary genes, causing developmental abnormalities.
While occasional miscues are tolerated in animal experiments, the danger of causing congenital defects prevents researchers from applying germline gene therapy to people. Instead of trying to correct genetic mutations at the start of life by adding genes to embryos, gene therapists treat people with genetic disorders by adding therapeutic genes to specific tissues in the hopes that they will correct the defect.
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's experiments with mice suggest that this might be possible. "Because the artificial chromosome is separate, it doesn't interfere with the cell's own genetic machinery," says Utterson.
Creating an Artificial Chromosome
Chromos's scientists start with natural chromosomes that have one long and one short arm containing no working genes. Using DNA replication enzymes, the researchers duplicate the short arms, extending them with satellite DNA. The full-length arms drop off, leaving an artificial chromosome containing only the elements it needs to survive and copy itself (see figure). These elements include telomeres, repeating sequences at the tips of the chromosomes, and a centromere.
Finally, the researchers weave interesting genes, as well as promoters to control their expression, into the chromosome's backbone of satellite DNA.
Using fluorescent dyes that bind to the different parts of the chromosome, Chromos's 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' 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 because they can carry much more DNA than is possible with existing methods, which use viruses or plasmids.
But the company says it won't let its technology be used for human germline engineering. "We are in control of the technology, and we don't want to engage in germline gene therapy," 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.
"This is obviously going to open up the debate again in the field of germline gene therapy," says Norman Nevin of Belfast City Hospital, who chairs Britain's Gene Therapy Advisory Committee.
However, scientific advisors remain cautious. Claudia Mickleson of the Massachusetts Institute of Technology, who chairs the National Institutes of Health's Recombinant DNA Advisory Committee, says that her committee wouldn't approve a germline trial without extensive preclinical information on safety. And given concerns about the technology being used to create "designer babies," Mickleson also says trials would not proceed without "intense discussion" with the public.
For more information: Eileen Utterson, VP of Corporate Development, Chromos Molecular Systems Inc., 6660 NW Marine Dr., Vancouver, BC, Canada V6T 1Z4. Tel: 604-718-6400. Fax: 604-718-6424.