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SV40, Polio Vaccine, and Cancer: Now
Beyond Coincidence?
Researchers have been discussing accusations that contaminated polio
vaccine stocks are to blame for certain cancers, based on the publication
a month ago of two high-profile papers linking the simian virus SV40 to
human lymphomas.
Less than a week after the papers were published in March, the US National
Cancer Institute contacted the researchers to establish plans to send
blinded results to three independent labs.
Researchers scanned 99 lymphomas, 235 epithelial tumors and 40 control
tissues for the virus. They found the virus in 43% of non-Hodgkin's
lymphomas, 9% of Hodgkin's lymphomas, and in none of the control tissues.
A second team independently found the virus in 42% of non-Hodgkin's
lymphomas, almost unbelievable agreement.
"These are very respectable labs with basically identical results," said
Michele Carbone, associate professor of pathology at Loyola University in
Chicago. The "clear clustering of positives" is "no accident."
This is not the first time scientists have linked SV40 to human cancers.
Researchers suggested for years that millions of vials of polio vaccine,
contaminated with SV40, infected individuals between 1953 and 1963 and
caused human tumors. Until recently, they were inevitably met with
skepticism, even contempt - and some NCI researchers published directly
contradictory results.
In 1997, the US National Institutes of Health, with other organizations,
organized an international conference to review the SV40 literature and
address the possibility that the virus causes human tumors. At the
meeting, Carbone, presented his then-controversial data linking the virus
to mesotheliomas. (Since then, more than 30 independent reports have
confirmed his results).
After the meeting, Carbone says, a conscientious Chicago public health
official contacted Carbone and gave him the last remaining stocks of polio
vaccine from the 1950s. In her paper, Butel isolated a strain of SV40 from
three patients that closely matches the strain Carbone sequenced from the
polio vaccine vials.
The evidence proves Butel's results are no artifact, Carbone says. "You
cannot contaminate with something that doesn't exist," he said. "This
thing only exists in my freezer."
Since publication of their research in the Lancet last month, Gazdar and
his colleagues have been investigating rarer subtypes like leukemia and
multiple myelomas. The experiments have not been proceeding as fast as
they would like, Gazdar says, partly because "there's no government
funding" for the research. "The lymphoma story might force them to fund
it."
An important next step, Gazdar says, is to prove that the SV40 virus
causes lymphomas and isn't just a "passenger" in the cells. That is no
easy task, since researchers have only been able to isolate the virus in
rare instances. For the most part, they believe, the virus launches a
"hit-and-run" attack, initiating a cascade of tumorigenic events before it
is destroyed by the body.
Still, it is critical that this research continue, Gazdar says, because
molecular and immunologic data suggest those born after 1963 have also
been exposed to the virus, via horizontal or vertical transmission, or
through sexual contact.
The rates of mesotheliomas, lymphomas and brain tumors have also all gone
up "dramatically" in the last 30 years. "Coincidence or not, we have to
find out," he said. "It's something to think about."
American Association of Cancer Research San Francisco, CA April 10, 2002
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