Consensus Growing for SV40 Role in Some Mesotheliomas
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, Stockholm.
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 meeting."
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 osteosarcoma patients.
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.