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Report:
Breeding vaccines for
Dengue
Investigator: Juha
Punnonen
Thursday Jul
26th, 2001
by Melissa Mertl
Juha Punnonen, the director of the vaccine
division at biotech company Maxygen of Redwood City, California, presented
new data on how DNA shuffling and screening has faired in in vitro
experiments when applied to vaccine development for Dengue. Maxygen is one
of several companies pursuing the approach known as directed molecular
evolution - they see DNA shuffling and screening as a powerful tool in
vaccine development.
Punnonen compared the technique to breeding a dog for a better snout.
"The power of sexual recombination was developed by nature, but man
has also learned how to use breeding to come up with new kinds of plant and
animal species," Punnonen said. One can select for traits and
characteristics without even having to know the molecular mechanism that
regulate the desired trait. After cutting up and randomly splicing together
sections of a gene that are closely located, the resulting library of chimeric
genes can be screened for altered or improved phenotypes. Those that are
selected can then be shuffled and selected again, repeating the process as
many times as desired.
Punnonen reported today on work to develop a Dengue vaccine using the
technique. To breed a better vaccine, Maxygen has generated libraries of
chimeric genes by shuffling and screening DNA from the envelope genes of
the four different strains of Dengue.
The four strains of this mosquito-born virus are related but are
antigenically distinct. Antibodies against one do not protect against the
other, and infection from any strain generally weakens immunity to the
other types. "It's widely agreed that a tetravalent vaccine is needed
- one that protects against all four serotypes," Punnonen said.
"Our goal was to . . . combine all of the dengue virus antigens to
develop a single [vaccine] that would provide a cross-protective
response," he said. There is currently no Dengue vaccine available.
First, the DNA from the envelope genes was split into fragments, and the
fragments denatured into single strands. A PCR-like reaction then
reassembled the fragments. "The reassembly works because of the
homology of the starting genes," said Punnonen. "Nature has
already tested the sequences, so it's not totally random," he said.
Mice were injected with the novel DNA sequences from the first round of
shuffling. Punnonen randomly selected clones to analyze the level of
chimerism from the four Dengue antigen libraries produced. "It turns
out that all of these fragments contained fragments of all four parent
genes, indicating very efficient DNA shuffling," he said.
While not all chimeras gave cross immunity to all four Dengue serotypes,
a few did. Analysis from the clones selected for a second round of
reshuffling is ongoing. "A number of clones show improved immune
response," said Punnonen.
They have yet to give the mice a shot of Dengue to see how they fare,
prompting delegates to question Punnonen's enthusiasm. "It is an
interesting technology," said one, "[but Maxygen] has a long way
to go."
"We don't make a claim that we have a Dengue vaccine," said
Punnonen. "The next step will be to analyze the level of protective
immune response."
He announced that the company has just formed a collaboration with an
unnamed academic center to carry out these studies.
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