Characterization of Viral Particles Isolated from Primary Cultures of Human Breast Cancer Cells

Abstract

The association of human breast cancer with sequences similar to the mouse mammary tumor virus (MMTV) has been shown, but convincing evidence for the presence of viral particles in breast tumors has been lacking. We have described the complete proviral structure of a retrovirus in human breast cancer. This provirus, designated as human mammary tumor virus (HMTV), was 95% homologous to MMTV and revealed features of a replication-competent virus. We have therefore investigated the production of viral particles in primary cultures of human breast cancer (MSSM). Cells isolated from ascites or pleural effusions of patients with metastatic breast cancer contained viral sequences in their DNA, expressed Env protein, and showed retroviral particles by electron microscopy. Viral particles from culture media exhibited morphologic features of B-retroviruses sedimenting at buoyant densities of 1.12 to 1.18 g/mL in sucrose gradients and showed reverse transcriptase activity. cDNA sequences from virion RNA were synthesized, amplified, and sequenced and all the virion genes were detected and 70% of the virion RNA was sequenced. The sequence homologies were, respectively, 85% to 95% compared with the MMTV and HMTV proviruses we have previously described. These results clearly show that breast cancer cells in primary cultures produced HMTV viral particles that are similar to the mouse virus and which may play a role in human breast cancer pathogenesis.

The Signal Peptide of Mouse Mammary Tumor Virus-Env: A Phosphoprotein Tumor Modulator

“…Indeed, it has recently been reported (9) that inflammatory breast cancers from American women show a higher incidence of HMTV-related viral sequences (71%) than sporadic breast cancers, and that the presence of MMTV-env-like exogenous sequences is strictly related to tumor progression (10). Additional support for possible involvement of the virus in breast cancer comes from reports claiming that MMTV can infect and spread through a culture of human mammary cells (11,12); the MMTV env protein, through a specific sequence motif, can initiate changes that are consistent with transformation of mammary epithelial cells (13); cells isolated from ascites or pleural effusions of patients with metastatic breast cancer contained MMTV sequences in their DNA, expressed the MMTV Env protein and showed b-retroviral particles by electron microscopy, similar to the mouse virus (14). The same group recently reported detection of HMTV proteins in human breast cancer cells (15).…”

Mouse Mammary Tumor Virus–like Sequences in Human Breast Cancer

“…Seventy percent of the complete MMTV-like virus genome, identified in human breast cancer specimens and viral particles from human breast metastases, have been sequenced and shown to display 91% to 99% homology to MMTV from mouse mammary tumors (4)(5)(6). In a recent study, env and long terminal repeat sequences with >98% homology to those of MMTV have been identified in breast cancers that had occurred in a mother, father, and daughter of the same family, living under the same roof (7).…”

Human RNA “Rumor” Viruses: the Search for Novel Human Retroviruses in Chronic Disease

“…Similarly, retrovirus-like particles (RVLP) have been found in body fluids from patients with rheumatoid arthritis (RA) (469), SLE (480), and epidemic neuropathy (418) and in cell cultures established from individuals with testicular tumors (46,71) and chronic fatigue syndrome (192). In addition, RVLPs have also been observed in healthy human tissues and fluids including placenta (241,504) and breast milk (138,338) and in cultured cells (126,328,348). Although reported as being retroviruses, such particles do not always have the expected morphology or size of retroviruses; for example, particles described in RA (469) have an apparent diameter of 200 nm, which is roughly double that expected for retroviruses, and their identity as such is therefore doubtful.…”

“…Positive results could be taken as evidence of the presence of a retrovirus, but so far, none of these examples has been confirmed as a genuine infectious virus. In some studies (11,99,189,328,385), RT assays have been performed on virus preparations that have been purified on sucrose density gradients or by ultracentrifugation, procedures thought to remove cellular contaminants. However, such preparations may also contain high levels of cellular material ( Fig.…”

“…The purpose of the gradient procedure is to physically separate encapsidated viral RNA (which migrates through the gradient) from unpackaged soluble RNA (typically defective HERV RNA), which is not expected to enter the gradient to an appreciable extent. Similar purifications have been employed prior to the detection of RT activity using the PERT assay (11,99,189,328). Nuclease treatment can also be used to degrade extraneous cellular DNA and RNA prior to amplification (164,440), because in theory, encapsidated RNA genomes should be protected.…”