Human papilloma virus has already been linked to a number of cancers including cervical, skin, nasopharyngeal, prostate and breast cancer. Now it appears that there may be a second candidate for a human breast cancer virus.
Evidence in Favor of the Existence of Human Breast Cancer Virus
The existence in some women of a virus that is related to themouse mammary tumor virus is evidenced by the following. TypeB particles are occasionally found in human milk. A complicationis the fact that most human milks destroy or damage the murine,mammary tumor virus (MuMTV) and its RNA-directed DNA polymerase(RDDP) when the mouse virus is added to the human milk. Humanmilk presumably has the same effect on any human RNA virionspresent. RDDP is found in many human milks although becauseof the destroying factors (not present in mouse or cow's milk),its presence usually depends on the freshness and/or amountof destroying factors in human milk; RDDP is associated withparticles having the same buoyant density as MuMTV (1.18 to1.22 g/ml versus 1.15 to 1.17 g/ml for leukemia viruses). RDDPis associated with a 35 S or 70 S RNA which is a characteristicof RNA tumor viruses. Human RDDP responds to magnesium ionsfor activity with a synthetic template, polyribocytidylic·deoxyguanylic(12 to 18 nucleotides long), in agreement with MuMTV, whereasmouse leukemia virus RDDP responds to manganese ions. Hybridizationstudies indicate a relationship between MuMTV RNA and humanbreast cancer RNA. (Using the S1 nuclease treatment and moststringent procedures, the RNA from 8 out of 22 human breasttumors hybridized from 18 to 77% of the MuMTV DNA probe; nonehybridized with Mason-Pfizer monkey virus probes.) Using themigration inhibition factor test, positive responses of humanleukocytes to homologous in situ breast cancer tissue are correlatedwith responsiveness to MuMTV. Many human sera completely neutralizeMuMTV. (The neutralizing activity resided in the globulin fraction,was absorbed by RIII milk, but was not absorbed by MTV-freeC57BL milk.) Some human breast cancer patients' sera containmaterial that precipitates specifically on the membrane of buddingMuMTV virions as demonstrated with peroxidase-coupled anti-humanglobulin. Slices of mouse tumors rich in MuMTV react, in immunofluorescencetests, with sera from some women with breast cancer or fibrocysticmastopathy.
The means of virus transfer (route of infection) in humans remainsunknown. RNA sequences homologous to MuMTV probes are foundin human breast cancers, but to date no homologous DNA sequenceshave been found. The virus does not seem to be endogenous inhumans. This study was conducted under USPHS Contract NO1-CP-33339 withinthe Virus Cancer Program of the National Cancer Institute; GrantCA 08740 from the National Cancer Institute; General ResearchSupport Grant FR-5582 from the Division of Research Facilitiesand Resources; and Grant in Aid M-43 from the State of New Jersey.