In 1965, Sir Austin Bradford-Hill (1897-1991), a British medical statistician, established nine widely used criteria to determine the strength of an association between a disease and its supposed causative agent. These criteria are used as a way of determining the causal link between a specific factor (e.g., cigarette smoking, presence of SV40) and a disease (such as cancer).
Below are two examples of in which the Bradford-Hill criteria are applied to SV40. The first example focuses on mesothelioma and the second one focuses on brain cancer.
Bradford-Hill Criteria and Mesothelioma
In the article SV40 and Human Tumours: Myth, Association or Causality? the authors, Drs. Gazdar, Butel, and Carbone include a table in which they applied the Bradford-Hill criteria to SV40 and human papillomavirus (HPV). The HPV comparison is included because it represents a relationship that is universally accepted. This Table supports the position that the SV40 association with some human mesothelioma cancers is causative.
Table 1: SV40 and human mesothelioma – association or causation?
| Bradford-Hill Criterion | HPV and cervical cancer | SV40 and mesothelioma |
| Strength and association | The strength of association between HPV and cervical cancer is considered one of the strongest for a human cancer. Recent studies have shown that HPV (all types combined) is present in >90% of cervical cancers | Several studies have found evidence of SV40 in ~50% of human mesotheliomas, both in the United States and in some European countries |
| Consistency | The presence of HPV in cervical cancer is consistent among a large number of studies, regardless of the HPV testing system used. There are no published studies with negative observations that challenge the association of HPV and cervical cancer | At least 30 studies have reported SV40 in human mesotheliomas using a variety of techniques, whereas four have failed to find an association |
| Specificity | Specific cancers are related to the presence of HPV. HPV type is also important in the development of specific cancers. HPV is present in the tumour cells. Viral oncogene expression (E6 and E7) occurs in tumour material, but not in stromal cells | SV40 is present in a highly specific group of human tumours. Furthermore, injection of the virus into hamsters produces the same tumours. In human mesotheliomas, SV40 is found only in tumour cells and not in the surrounding non-malignant tissue |
| Temporality | HPV infections precede pre-cancerous cervical lesions and cervical cancer by years to decades | Little is known, although the virus has been found in preneoplastic lesions. The tremendous increase in the incidence of mesotheliomas over the past several decades was preceded by the administration of SV40-contaminated poliovirus vaccines, as well as by an increased exposure to asbestos |
| Biological gradient (dose-response) | Unclear, but early studies show that cervical cancer is associated with high viral loads | Unknown. Because humans are permissive to SV40, millions of SV40 particles are produced from few infected cells |
| Biological plausibility | HPV is a powerful carcinogen that immortalizes human keratinocytes in vitro. There are no animal models in which a sexually transmitted PV produces cervical cancer. HPV is present in cervical cancer, where it expresses the oncogenic proteins E6 and E7 that inactivate the host regulatory proteins p53 and RB, respectively. Epidemiological studies support a role for HPV in cervical cancer | SV40 is a powerful carcinogen that, in vitro, transforms human mesothelial cells. It induces mesothelioma development in hamsters. SV40 also expresses the oncogenic protein T Ag in human mesotheliomas, which inactivates p53 and RB. Definitive epidemiological studies are lacking |
| Biological coherence | The association does not conflict with what is known about the natural history of cervical cancer development | The association does not conflict with what is known about the natural history of mesothelioma, and it might explain why those with no history of asbestos exposure can develop the disease |
| Experimental evidence | In vitro and in vivo evidence supports a causal role for HPV in the development of cervical cancer | In vitro and in vivo evidence indicates a causal role for SV40 in mesothelioma development |
| Analogy | Other DNA tumour viruses can induce cancers in humans, and species-specific papillomaviruses can induce cancers in animals | Other DNA tumour viruses can induce cancers in humans, and SV40 induces mesotheliomas in animals |
Bradford-Hill Criteria and Brain Cancer
This section was written by one of us (Michael Horwin, MA, JD). His methodology was simply to apply what was published in the peer-reviewed medical literature to the nine Bradford-Hill criteria in respect to medulloblastoma and other brain cancers. The fifty-four citations that form the basis of this analysis can be found below.
Introduction
1. Consistency of findings across studies conducted with different methodologies and in different settings.
Although the data is limited because so few studies have been funded, every study that has looked for SV40 in human medulloblastomas has found it. In addition, different labs in different geographic regions of the world including the U.S., France and China have independently confirmed the presence of SV40 DNA in medulloblastoma.
Furthermore, SV40 has been found in every type of primary pediatric brain tumor examined including: meningioma, astrocytoma, choroid plexus papillomas and ependymomas. Thus, the available evidence indicates that SV40 is consistently involved in pediatric brain tumors including medulloblastoma.
2. Specificity in that the exposure precedes the effect and causes a particular disease, (e.g. the observation that cigarette smoking often precedes squamous cell carcinoma of the respiratory tract or Hepatitis B infection often precedes hepatocarcinoma).
Five epidemiological studies have demonstrated that exposure to SV40 precedes the increase in pediatric brain tumors including medulloblastoma. In each of these studies, the association between exposure to SV40 was statistically significant relative to the increase in pediatric brain tumors. These studies include the following:
In 1968, Innis found a significant association between immunization with the SV40 contaminated Salk vaccine and subsequent childhood malignancy.
In 1973, Heinonen et al. demonstrated an increased rate of tumors of neural origin in children exposed to SV40. This study reported a 12.7 times greater chance of a child getting cancer if his/her mother was vaccinated with killed polio vaccine containing SV40 while the child was in utero.
In 1979 and 1984 Farwell et al. demonstrated a higher number of medulloblastoma in children exposed to SV40. She reported that among medulloblastoma patients, 10 of 15 were exposed to SV40 and that this rate of exposure is higher and is significantly greater than among controls. In 1984 she reported that “…an excessive number of children born in the period 1954 to 1958 have developed medulloblastomas. A relationship to polio vaccine contaminated with SV40 may exist”.
In 1990, Geissler et al reported that patients with medulloblastoma had a greater likelihood of being exposed to SV40 from vaccine. His study demonstrated an increase of almost 30% in the number of medulloblastomas in a cohort of approximately 800,000 people.
In 1999, Fisher et al used SEER data to determine that there were increased rates of ependymomas (37%), mesotheliomas (90%) and other cancers in cohorts exposed to SV40 versus those who were not exposed.
In addition, PCR data suggests that SV40 DNA infection occurs early in tumor development. Using PCR-Southern hybridization, one study has demonstrated that in individual patients, the SV40 status of high-grade glioblastomas is similar to the status of the original tumors (lower grade astrocytomas) that gave rise to the glioblastoma. This suggests that SV40 DNA infection occurs “during early stages of tumor development rather than being associated with glioma progression”. Another study using transgenic mice has demonstrated that SV40 antigen is present before detectable pathology of SV40 transformed cells. This also suggests that exposure to SV40 precedes the genesis of the tumor.
3. Strength of the association examines what proportion of tumors are shown to contain the virus.
As mentioned above, although limited studies have been conducted, every PCR or immunohistochemistry test conducted on various human medulloblastoma tissues samples has found the presence of SV40 in a significant percentage (from 100% (1 out of 1) to 29%). In addition, every type of pediatric brain tumor tested has been demonstrated to be SV40 positive. As Table 2 indicates, these percentages, although varying, are significant:
| Table 2 | % positive (reference) | % positive (reference) | % positive (reference) | % positive (reference) |
| medulloblastoma | 29% | 33% | 100% | 50% |
| ependymomas | 56% | 91% | 100% | 75% |
| choroid plexus | 38% | 50% | 83% | |
| astrocytomas | 73% | 47% | ||
| meningioma | 43% | 31% | 100% | 19% |
| oligodendrogliomas | 10% | 33% | ||
| mix of pediatric brain tumors | 82% |
4. Temporal relationship considers if there is a relationship in time between the virus and the tumor.
The increasing incidence of medulloblastoma and other pediatric brain cancers in the United States has been preceded by the administration of SV40 contaminated polio vaccines. According to one published report the “rising childhood cancer rate represents a far more serious problem in the United States than previous reports have suggested”. Furthermore, SV40 has infected humans mainly through contaminated polio vaccines. Finally, significant differences in the incidence of medulloblastoma has been associated with exposure to SV40.
5. Dose-response relationship examines the relationship between the dose of the virus and the response of the host.
Uniform doses of SV40 produce higher percentage of tumors in younger animals. For example, the younger the animal was when exposed to SV40 the greater the percentage of tumors. In one study, over 95% of newborn and less than 2-day-old suckling hamsters developed tumors after being injected with SV40. When hamsters were inoculated at 7 or 8 days of age, about 60% still developed tumors. The animals remained susceptible to the oncogenic potency of SV40 up to 3 weeks of age, but the incidence of tumors was substantially lower in the older groups. According to this researcher, “The observation that a latent virus indigenous in monkey kidney cells, and apparently harmless for its carrier host species, can induce a high incidence of malignant, progressively growing tumors when inoculated into newborn animals of another species is striking”.
6. Coherence of epidemiological evidence: the geographically widespread association of SV40 DNA with human medulloblastoma is consistent with the widespread vaccination of large numbers of children with SV40 contaminated polio vaccines.
7. Biological plausibility: the well-recognized oncogenic properties of the SV40 virus in a variety of animal bio-assays (see below) and in vitro human cell culture studies, its capacity to survive and replicate in infected human subjects for prolonged periods, its ability to create brain tumors via subcutaneous injections in animals, its capacity to be carried through the blood and infect the brain, and its particular affinity for human brain cells, support its potential to cause tumors in susceptible children.
SV40 can transform human cells in vitro, causing chromosomal aberrations, aneuploidy and point mutations: SV40 is an oncogenic virus capable of transforming cells of different species, including human cells in vitro. This has been known for over 40 years. In 1962, “Koprowski and his colleagues…inoculated organ cultures prepared from human skin and buccal mucosa, with SV40. A transformation could be observed in the infected cell cultures 8 to 14 weeks after exposure to the virus…these cells eventually spread and overgrew the normal cell population. Distinct chromosomal abnormalities accompanied this cell transformation”. In addition, a number of other studies have independently demonstrated this association or cited this well-known capacity. Furthermore a recent report published by the NCI demonstrated that an adenoviral vector expressing an antisense transcript to SV40 inhibited T antigen expression and mediated “significant growth inhibition and apoptosis in malignant pleural mesotheliomas.” This effect was not observed in mesotheliomas containing no SV40 sequences. The authors concluded that these data “suggest that SV40 oncoproteins contribute to the malignant phenotype of pleural mesotheliomas…”
SV40 has the capacity to replicate in humans. Human exposure to SV40 and subsequent excretion of the virus for several weeks indicates that the virus has the ability to replicate in humans. This has been known for nearly 40 years.
SV40 creates brain tumors via subcutaneous injection in animals. SV40 has been demonstrated to create brain tumors with significant consistency (80%) in laboratory animals when injected via subcutaneous inoculation without the development of either subcutaneous or visceral neoplasms resulting from such an inoculation.
SV40 is carried through the blood to infect target organs such as the brain in humans. Experimental evidence suggests that human peripheral blood cells and B and T-lymphocytes are vectors for the transfer of SV40 to other human tissues of the host.
SV40 has a particular affinity for human brain cells. SV40 is known to replicate in certain human glial cells and this suggests that these cells have a functional SV40 receptor.
8. Reasoning by analogy: SV40 is a proven cancer virus, based on experimental animal model systems, analogous tumor systems, in vitro transformation studies involving human cells (see above), and its well-known capacity to bind suppressor genes (see below).
SV40 can create medulloblastoma in animals. SV40 has been shown to cause medulloblastoma-like PNET’s in rats using retrovirally transduced SV40 large Tag.
SV40 has also been demonstrated to cause primitive neuroectodermal type tumors (PNET’s) in transgenic mice.
Analogous tumor systems. SV40 may act as a cofactor in human carcinogenesis particularly medulloblastomas, much like EBV in Burkitt’s lymphoma and nasopharynx carcinoma or hepatitis B virus in hepatocarcinoma. In addition, HPV and JC viruses, which share capacity to bind and inhibit p53 and Rb proteins, have been causally linked to cervical cancer and brain tumors, respectively.
9. Experimental evidence demonstrates that SV40 can grow better in human fetal brain cells than other human cells, affect changes in human chromosomes, bind human tumor suppressor genes, and is detected in the tumor cells but the adjacent normal tissue is devoid of viral DNA. In addition, there is genetic and pathological evidence to support the hypothesis that human pre-cursor brain cells may be infected by SV40 and produce medulloblastoma and that SV40 DNA infection occurs early in brain tumor development.
SV40 grows better in human fetal brain cells than human fetal kidney cells or fibroblasts. In a study that examined the infection rates of SV40 in human fibroblasts, human embryonic kidney cells, and human fetal brain cells, it was demonstrated that the most “robust” growth took place in brain cells. The infection rate was as productive as that found in the highly related human BK virus.
SV40 can affect changes in human chromosomes. It has been demonstrated that SV40 Tag can induce chromosomal aberrations in normal human cells.
SV40 binds human tumor suppressor genes. SV40 binds the products of p53 and p105 RB tumor suppressor genes leading to their functional inactivation which causes or contributes to oncogenic transformation.
Adjacent normal tissue devoid of viral DNA. SV40 large T antigen sequences have been found in the brain tumors investigated but not in the adjacent healthy brain tissue.
Pre-cursor brain cells may be infected by SV40 and produce medulloblastoma. It is thought that large Tag induced PNET’s (i.e. medulloblastomas) arise from a subpopulation of precursor cells which are susceptible to transformation by Tag. Through inactivation of Rb, these cells retain their proliferative and migratory capability and that inactivation of p53 allows for the establishment and accumulation of genetic alterations that prevent apoptosis leading to medulloblastoma.
Conclusion
The epidemiological, biological, genetic, and experimental data present a strong case for the role of SV40 in causing or contributing to the transformation and oncogenic progression of human brain tumors. There is more data on the plausible carcinogenic role for SV40 than for many other substances and agents that have been deemed anticipated or known human carcinogens by IARC, EPA and OSHA.
Currently, there are no comprehensive studies in the U.S. examining the role SV40 may play in pediatric brain cancer or any brain cancer for any age group. This is due to a lack of governmental interest in funding this research perhaps because the virus was introduced into the human population via contaminated vaccines. The fear of liability on the part of the government and the vaccine manufacturer’s has resulted in the virtual paralysis of research in this area.
Orthodox cancer therapy is the only treatment legally permitted to be administered to children with cancer. If parents desire other modalities, at the oncologist’s behest, children may be taken from their parents so that cytotoxic therapies (radiation and chemotherapy) can be administered without parental permission. Beyond, the moral and ethical questions raised by such actions, there is compelling scientific evidence to suggest that such conduct is medically irresponsible. As cited above, SV40 is known to induce oncogenic transformation by forming complexes with critical tumor suppressor genes such as p53. However, it is these very tumor suppressor genes that cytotoxic therapy depends on in order to initiate cellular apoptosis. If the cells’ tumor suppressor genes and their corresponding proteins can not be expressed appropriately then cell-killing therapy is significantly handicapped. The presence of SV40 can ensure that chemotherapy and radiation will have negligible efficacy. The fact that a significant percentage of pediatric brain tumors are positive for SV40 and a significant percentage of pediatric brain tumor patients will reap no benefit from orthodox cytotoxic therapy may not be coincidental. There is strong scientific evidence that oncologists are sentencing thousands of children to ineffective, debilitating and toxic cancer therapies that do nothing more than insure their death and fill their last months alive with needless misery and pain.
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