CHICAGOMultiple independent laboratories have verified the
presence of simian virus 40 (SV40) DNA and proteins in human mesotheliomas,
brain tumors, and bone tumors, using a variety of methods of detection. This
was the consensus reached by a panel of scientists at an international
conference hosted by the University of Chicago.
The panel was chaired by Carlo Croce, MD, director, Kimmel
Cancer Center, Jefferson Medical School, Philadelphia, and George Klein, MD,
DSc, of the Microbiology & Tumor Biology Center, Karolinska Institute,
Although initial reports about the presence of SV40 in human
tumors were conflicting, a considerable body of evidence about the association
has since accumulated, said Michele Carbone, MD, PhD, associate professor of
pathology, Loyola University, who organized the conference along with Nicholas
Vogelzang, MD, director of the University of Chicago Cancer Research Center.
Dr. Carbone said that 62 reports from 30 independent
laboratories have confirmed the presence of SV40 in mesotheliomas and other
human tumors. At least three studies, however, have failed to detect SV40 in
human tumors, and some investigators contend that positive detection of the
virus is a result of intra- and/or interlaboratory sample contamination with a
laboratory strain of SV40 DNA.
However, the researchers forming the consensus believe that the
variability of detectable SV40 DNA in human tumors can be explained by
inadequate DNA extraction methods, the quality of tissue samples, inadequate
sampling of tumor specimens, and the relatively small number of cases analyzed
in each study.
Scientists from all over the world, representing laboratories
that have reported positive and negative data, gathered in Chicago to present
their data before the expert panel.
Contaminated Poliovirus Vaccines
SV40 is a monkey virus that may have entered the human
population through contaminated poliovirus vaccines from 1955 to 1963. However,
SV40 has been detected in individuals born after 1963 and, therefore, at no
risk of having received contaminated poliovirus vaccines.
Jeffrey Kopp, MD, of the National Institutes of Health,
presented data demonstrating the presence of SV40 in the renal tubular cells
and in the urine of immuno-compromised patients. His findings suggest that
SV40, regardless of the initial source of human infection, may have established
a permanent infection in some humans who may spread the virus.
"Because in some areas of the world, such as Turkey, SV40
is not detected in mesothelioma, SV40 should not be considered a necessary
factor for the development of this cancer," Dr. Croce said. However, Dr.
Klein commented, "the causative association of SV40 with human
mesotheliomas has been considerably strengthened by the data presented at the
Dr. Croce pointed to the microdissection experiments of Adi
Gazdar, MD, professor of pathology, University of Texas Southwestern Medical
Center, Dallas, as some of the most convincing evidence about the specificity
of the association of SV40 with mesothelial cells and mesothelioma.
Dr. Gazdar said he was initially very skeptical about the
association of SV40 with mesothelioma, but changed his mind based on PCR
analyses showing the presence of SV40 only in mesothelioma cells and not in
nearby reactive fibroblasts microdissected from the same specimen.
Dr. Klein also pointed to the work of Maurizio Bocchetta, PhD,
of Dr. Carbone’s laboratory, showing the molecular mechanisms that make
mesothelial cells unusually susceptible to SV40 infection and transformation.
Of further relevance, Dr. Klein said, is the work of David
Schrump, MD, PhD, National Cancer Institute, who has used an antisense approach
to show that the SV40 large tumor antigen is required for the maintenance of
the malignant phenotype of mesothelioma cells in culture.
Dr. Carbone told ONI that a workshop organized by the National
Cancer Institute, and held a few weeks before the Chicago conference, reached
similar conclusions: that SV40 is present in human tumors and tissues. That
meeting was chaired by Satvir Tevethia, PhD, professor of microbiology and
immunology, Pennsylvania State University.
Challenge to the Consensus
The strongest challenge to the consensus that SV40 plays a role
in mesothelioma came from Keerti Shah, MD, DrPH, professor of molecular
microbiology and immunology, Johns Hopkins School of Public Health. Dr. Shah
stated that "it is difficult to see how a highly species-specific
polyomavirus would cross the species boundary and circulate freely in humans
without any adaptive changes to its genome."
According to Dr. Shah, the SV40 sequences isolated from human
tissue are, remarkably, almost indistinguishable from those isolated from
rhesus monkeys, without any human-specific alterations in the viral genome.
Dr. Shah also pointed to the fact that several laboratories
failed to detect SV40 sequences in human urine samples and that neutralizing
antibodies to SV40 were not demonstrable in sera from mesothelioma and
Since the conference, the International SV40 Working Group, of
which Dr. Shah was a member, has published a report of a nine-laboratory
multicenter investigation designed to assess the sensitivity, specificity, and
reproducibility of assays for detection of SV40 DNA (Cancer Epidemiology
Biomarkers & Prevention 10:523-532, 2001).
Each lab was given, in a masked fashion, paired replicate DNA
samples extracted from 25 fresh frozen mesotheliomas and one from each of 25
normal human lungs. Interspersed within these slides were masked positive and
negative control samples.
The Working Group reported that "a high level of
specificity and reproducibility was found among the PCR assays performed in
most laboratories. However, none of the selected normal human lung tissue or
the 25 mesothelioma tumor specimens obtained from archival samples at a single
center was reproducibly positive for the presence of SV40 DNA."
Extraction Method Questioned
In an interview with ONI, Carl W. Miller, MD, of UCLA
Cedars-Sinai Research Institute, and a collaborator in the Working Group study,
said that the negative study results could be due to the method the researchers
used to extract SV40 DNA from the test samples.
"The fact that laboratories that had previously detected
SV40 did not detect it in these DNA samples does not invalidate their previous
findings," he said. It only supports the fact that SV40 DNA was not
present in the samples in this study, he said.
Dr. Miller pointed out that it is very difficult to prove the
absence of SV40 DNA in humans, but since many different laboratories have
detected it, and these specimens are distinct from laboratory SV40 strains, he
believes "it is hard to reject its presence in humans." Whether it is
associated with human cancers is another issue, he said.