Monkey Virus Link to Cancer Grows Stronger
A virus that contaminated early batches of polio vaccine was deemed
safe decades ago, but it keeps turning up in tumors. Now researchers
are figuring out how the virus might imperil cells and are searching
for ways to stop it
The old man had been trying to tell Michele Carbone something for months.
He'd buttonholed the molecular virologist on the last day of a busy conference,
but they kept getting interrupted. He'd sent a pile of journal articles
from the 1950s to Carbone's office at Loyola University Medical Center in
Chicago, but they sat unread on Carbone's desk. He'd called and called again.
Carbone finally relented when he realized that the 89-year-old physician,
named Herbert Ratner, lived in the Chicago suburb of Oak Park, less than
a kilometer from Carbone's house. "At a certain point I thought, 'I have
to go' " see what he wants, Carbone recalls.
At the conference where he first met Ratner, held in 1997 at the National
Institutes of Health (NIH) in Bethesda, Maryland, Carbone and colleagues
reported finding traces of a monkey virus called SV40 in a rare form of
cancer of tissue surrounding the lung. SV40 had been spotted in 1960 in
monkey tissue used to make the polio vaccine and was soon found to cause
four types of tumors in hamsters, sparking widespread alarm. By 1963,
when the polio vaccine supply had been screened to ensure that it was
free of the virus, more than 98 million people in the United States and
hundreds of millions worldwide had been exposed to potentially contaminated
vaccine. Researchers have been arguing ever since about whether that SV40
exposure increased their risk of cancer.
Carbone wanted to know whether the SV40 he'd found in tumors could have
come from the polio vaccine. But a 3-year search among manufacturers,
government officials, and researchers for possibly contaminated samples
of vaccine had come up empty. Over tea at Ratner's home on a spring afternoon,
Ratner explained that he had been director of public health for the town
of Oak Park in 1955, when the polio vaccine was first released. Fearing
that the new vaccine was unsafe, he refused to inject local children.
Newspapers vilified him, but he stuck to his guns. Carbone thanked Ratner
and headed for the door. "He said, 'No, no, wait, I have something for
you,' " Carbone recalls. Ratner returned from his basement with vials
of polio vaccine produced in 1954. He told Carbone that he had kept it
in his refrigerator for 42 years, hoping all along that someone would
test it and prove him right.
The 1997 NIH conference, dubbed a consensus conference, produced nothing
of the kind. Believers and skeptics continued arguing about whether SV40
is present in human tumors and, if so, whether it's contributing to cancer
in people. Skeptics pointed to a lack of epidemiological evidence linking
pre-1963 polio vaccination to cancer and to other evidence suggesting
that laboratory artifacts were responsible for some SV40 sightings in
human tumors. Believers, meanwhile, cited decades of tissue-culture and
animal research showing the virus's carcinogenic powers.
Infiltrator. Hexagonal SV40 particles (magnified at bottom)
dot the inside of a mesothelial cell's nucleus.
CREDITS: M. CARBONE
The battle continues today, but on new ground. Although most experts
agree that SV40 infection is not a widespread public health problem, it
clearly can cause cancer when given to newborn animals. Researchers still
don't know whether the virus disproportionately infects cancer patients.
Studies in the past 5 years have led many--but by no means all--to accept
that SV40 genetic sequences are present in the same four types of human
cancers that the virus causes in hamsters. In humans, however, it's not
clear whether the virus helps cause cancer or just happens to infect cancer
Part of the problem is that none of the tests to detect the virus are
universally trusted. Some teams are working on better tests, which could
help confirm or refute virus sightings in tumors and reveal if and how
far SV40 is spreading in people. Many cancer patients with SV40-laden
tumors were not vaccinated with the early polio vaccine, and researchers
aren't sure how they picked up the virus.
Despite all the uncertainties, circumstantial evidence implicating the
virus has piled up, particularly in the past year. Some groups are testing
ways to block SV40 infection in an attempt to prevent or treat cancer.
Skeptics of the SV40-cancer link remain, but more than a few are starting
to come around. "I used to think it was all artifact," says cancer biologist
Denise Galloway of the Fred Hutchinson Cancer Research Center in Seattle.
"I wouldn't say I'm convinced, but I don't think it deserves to be pooh-poohed
Alarmed by the early reports of SV40's carcinogenic effects in hamsters,
researchers at the National Cancer Institute (NCI) and elsewhere did a
series of epidemiological studies, beginning in the 1960s and 1970s, that
seemed to dispel the worry about human risk. None found any increase in
cancer risk in people who had received potentially tainted vaccines as
infants or children. Studies in the United States and Europe tracked thousands
of people for up to 20 years. However, the studies were not powerful enough
to rule out a rise in rare cancers, says pediatric oncologist Bob Garcea,
now at the University of Colorado School of Medicine in Denver.
The case was reopened in 1992, when Garcea, then at Harvard Medical School,
and colleagues stumbled onto SV40 DNA in childhood brain tumors while
searching for traces of two related human viruses. Soon after, Garcea,
Carbone, and colleagues spotted SV40 DNA sequences in a bone cancer called
osteosarcoma. Then Carbone and Harvey Pass, now at Karmanos Cancer Institute
of Wayne State University in Detroit, spotted it in mesothelioma, a rare
and uniformly fatal cancer of the tissue lining the lungs. That meant
the virus had shown up in three of the four types of cancer that it caused
But other researchers looked for the virus and didn't find it. For instance,
cancer epidemiologist Howard Strickler, then at NCI, and Keerti Shah of
the Johns Hopkins Bloomberg School of Public Health in Baltimore found
no sign of SV40 in osteosarcoma and mesothelioma. At the 1997 consensus
conference, Strickler and other skeptics said the virus sightings could
have come from contamination by lab strains of SV40, which are used widely
by cancer researchers.
To resolve the dispute, two researchers arranged multilab studies on
mesothelioma--and came to opposite conclusions. One, led by Strickler,
involved nine different labs. It found traces of SV40 in a small fraction
of mesothelioma samples--but the virus was just as likely to appear in
control samples of lung tissue. Strickler, who's now at Albert Einstein
College of Medicine in New York City, says the study casts doubt on a
role for SV40 in mesothelioma. He points out that when SV40 DNA is found
in tumors, it's scarce--too scarce to be causing cancer. Furthermore,
the proportion of tumors with SV40 varies greatly from one study to another,
which he says casts doubt on the reliability of the screening methods
Meanwhile, molecular geneticist Joseph Testa of the Fox Chase Cancer
Center in Philadelphia and colleagues reported in 1998 that nine of 12
mesothelioma samples turned up positive when tested in all four participating
labs. Testa says the results mean that lab contamination was not to blame,
but Strickler disagrees, pointing out that the researchers did not test
normal tissue to make sure the assay was working properly.
Since then, several studies have replicated Testa's positive findings
using other methods. Marc Ramael's team at St. Elisabeth General Hospital
in Herentals, Belgium, used fluorescent tags to spot SV40 DNA and protein
inside mesothelioma cells in preserved tumor tissue. And molecular biologist
Adi Gazdar of the University of Texas Southwestern Medical Center in Dallas
and colleagues found SV40 in 50% of the mesotheliomas they dissected from
preserved human tissue. In both cases, no SV40 showed up in nearby normal
lung tissue, ruling out contamination, Gazdar says. At first, "I was skeptical
of the whole thing," Gazdar says, but "there's no question that the sequences
Carrier. SV40 from rhesus monkeys contaminated early batches
of polio vaccine.
CREDIT: WOLFGANG KOEHLER/CORBIS
Recently, SV40 went four for four. Researchers found the virus in humans
with non-Hodgkin's lymphoma, a cancer found in SV40-infected hamsters.
Teams led by Gazdar and Janet Butel of Baylor College of Medicine in Houston
tested hundreds of human tumors. As both groups reported in the 9 March
issue of The Lancet, they found SV40 DNA in about four in 10
samples, but not in normal lymph tissue or blood cells. The results still
need to be confirmed in other studies, Butel says, but the virus "might
account for a large fraction of non-Hodgkin's lymphoma."
When Carbone's team analyzed Ratner's old polio vaccine, they found that
it was contaminated with a variant of SV40, the genome of which differed
from that of common lab strains of the virus. The distinctive strain has
now been found again, in three non-Hodgkin's lymphoma patients, Butel's
team reported in its recent Lancet paper. The patients' SV40
and that extracted from Ratner's vaccine have an identical molecular fingerprint,
a matching DNA sequence in a stretch of an SV40 gene that varies from
strain to strain. "This is the ultimate proof you'd want that at least
some of this virus [infecting cancer patients] came from the polio vaccine,"
Butel and colleagues didn't emphasize the match when they reported their
findings. "I don't want to be responsible for scaring people so that they
are afraid to use the polio vaccine," Butel says. She points to all the
good the vaccine has done: It has practically eradicated the dread disease
in much of the world, saving tens of thousands from paralysis and saving
thousands of lives. And the polio vaccine now used in the United States
is grown in cultured monkey kidney cells uninfected by SV40.
Still, the largest human exposure to SV40 probably came from contaminated
vaccines between 1955 and 1963, many researchers say. But SV40 DNA has
turned up in people who never received possibly contaminated vaccine.
Some skeptics see this as evidence of an alternative source, such as an
endemic strain of SV40 in the population, possibly introduced through
monkey bites. Those who point to the vaccine link say that SV40 from contaminated
vaccines has spread from person to person. SV40 can replicate in people,
Butel and others say, and it can also be excreted in feces and urine.
"It seems to me that there's no way around arguing [that there has been]
transmission of the virus," says Galloway of Fred Hutchinson Cancer Research
It's been difficult to track the virus's progress, however, because there's
no widely trusted test to quickly detect antibodies against SV40 in human
blood. Existing tests are either too slow to screen a large number of
samples, or they have trouble telling SV40 from two of its cousins, JC
virus and BK virus, both of which are common in people. Galloway is "very
optimistic" that an as-yet-unpublished assay her group has developed,
which distinguishes BK from JC, can be modified to distinguish both from
SV40. A reliable test could confirm or refute disputed SV40 sightings
and reveal whether cancer patients have more SV40 infections than healthy
people, Galloway says.
Even if SV40 infection is found in cancer patients, that doesn't mean
it causes cancer. Epidemiological studies continue to show no increased
risk of cancer in people exposed to the polio vaccine, Strickler says.
A 1998 study he co-authored, for example, examined data on millions of
patients from NCI's national cancer registry and found no rise in the
incidence of cancer--rare or common--in people who have been exposed to
Even where the virus seems to be present in tumors, says Strickler, it
does not infect every cancer cell, raising the question of how the virus
could cause cancer in cells it doesn't infect. And unlike known cancer-causing
viruses, which usually infect a single tissue, SV40 seems to turn up in
a wide variety of tumor types. "It's hard to see how a virus can go to
all these tissues and produce all these cancers," Shah of Johns Hopkins
Proponents counter that some of the putative SV40-induced cancers are
so rare that a link to polio vaccination wouldn't reach statistical significance
even in huge epidemiological studies. They also say that if SV40 is indeed
spreading through the population, studies based on polio vaccine exposure
wouldn't have a reliable measure of who's been exposed. Those who suspect
a causal role say the scarcity of DNA can be explained if SV40 causes
cancer via a temporary infection that is later undetectable. Alternatively,
the virus might release cancer-promoting molecules that act at a distance.
Evidence that SV40 acts directly on cells to cause at least one cancer,
mesothelioma, has been growing. As early as 1997, teams led by Carbone
and Antonio Giordano, now at Temple University in Philadelphia, showed
that in animals and lab-grown human mesothelioma cells, an SV40 protein
called the large T antigen turns off two tumor-suppressor proteins, Rb
and p53, removing two separate brakes that keep genetically damaged cells
But according to 1999 work by NCI's David Schrump and colleagues, removing
most of SV40's large T antigen can thwart tumor cells. The strategy restored
the p53 pathway in mesothelioma cells isolated from a human tumor; in
response, the tumor cells stopped growing and self-destructed. George
Klein of the Karolinska Institute in Stockholm, who did pioneering work
linking the Epstein-Barr virus to cancer, says that such experiments are
"the strongest piece of evidence you can obtain to say that the virus
is essential for the growth of the tumor," but he adds that more experiments
are needed to prove a causal role.
Like a bomb. Compared to normal chromosomes (left),
those from a SV40-infected cell are multiplied and often abnormal (right,
CREDIT: J. TESTA
Other studies in mesothelia show that SV40 infection induces several
hallmarks of cancer. Testa's team showed in 1999 that SV40 causes dramatic
chromosome rearrangements and damage. "It looks like you set a bomb off
in these cells," he says. In work published in the 21 February issue of
Oncogene, Carbone, Testa, and colleagues report that SV40 spurs
activity of an enzyme called telomerase that extends the tips of chromosomes,
thus allowing cells to become immortal. And in work that was presented
in April at a meeting of the American Association for Cancer Research,
Gazdar, Carbone, and colleagues show that SV40 infection causes cells
to disable a tumor suppressor gene called RassF1 several weeks
after infection, just when cells are becoming malignant.
Another new study, in the October 2001 issue of the Proceedings of
the National Academy of Sciences, suggests that SV40 doesn't have
to be in every cell to cause cancer. Giovanni Gaudino of the University
of Piemonte Orientale in Novara, Italy, Luciano Mutti of the Maugeri Foundation
in Pavia, and their colleagues showed that SV40 infection caused mesothelium
cells to secrete a growth factor. When it diffuses to neighboring cells,
it activates a gene that starts the cells down the road to cell division.
A clean vaccine
As evidence for SV40's ability to induce mesothelioma builds, some researchers
are trying to disable the virus as a way to treat or prevent this fatal
cancer. Martin Sanda and Michael Imperiale of the University of Michigan
Medical School in Ann Arbor came up with an SV40 vaccine in 1999. They
found that a virus called vaccinia containing a neutered SV40 T antigen
held off SV40-induced mesothelioma in mice and shrunk preexisting tumors.
Others have shown that injecting naked DNA encoding the SV40 T antigen
accomplishes the same thing.
Pass of Wayne State University is planning a phase I clinical trial.
The vaccine could eventually help treat patients whose tumors show signs
of SV40 infection, and it might be given to people already at high risk
of mesothelioma, such as asbestos workers, he says. NCI is looking for
a company to manufacture the vaccine.
But longtime SV40 researchers claim that few other funds have been available
from the government, which oversaw distribution of early batches of contaminated
polio vaccine and vouched for its safety. The lack of funds means that
key questions have remained unanswered for years, many SV40 researchers
say. "The federal government's attitude has been, 'We don't believe this
story; it's not proven; so we're not going to fund it,' " Gazdar says.
May Wong, program director for DNA viruses at NCI, acknowledges that
funding has fallen far short of what SV40 researchers have sought, mostly
because the work was considered "very speculative and controversial."
"I don't think the government was trying to cover up anything. It was
just that the data wasn't there," she says. But the stream of new reports
has begun to change that, she adds: NCI recently funded Carbone and Butel.
And a new NCI program to look at possible cancer-causing microbes, including
SV40, is in the works.
Researchers are itching to get started on a backlog of work, including
studies that track the cancer risk of SV40-infected people. They want
to test whether SV40 assists other carcinogens such as asbestos and see
if blocking an SV40 infection could help treat mesothelioma or other cancers.
Those studies will take money and time, and the answers won't just turn
up in someone's refrigerator.
Jonas Salk introduces polio vaccine. Vaccine is injectible, and contains
Monkey kidney extracts used to make vaccine are shown to cause four
rare types of tumors in hamsters.
SV40 is discovered in monkey kidney extracts.
SV40 is shown to cause four rare tumors in hamsters.
U.S. orders vaccine-makers to eliminate SV40 from Salk vaccine.
Albert Sabin introduces oral polio vaccine; it is not thought to contain
NCI study: Children who received infected vaccine face no increased
NCI study: SV40-exposed children, now teenagers, face no increased
SV40 spotted in childhood brain tumors.
SV40 spotted in rare bone and lung cancers.
NCI researchers and others screen tumors, find no SV40.
NIH-sponsored SV40 consensus conference. Debate still unresolved.
Multilaboratory study confirms SV40 in mesothelioma.
NCI: Still no increased cancer risk after more than 3 decades.
Second consensus conference: SV40 is present in mesothelioma and probably
other tumors; may cause cancer.
SV40 is spotted in non-Hodgkin's lymphoma in two large studies.