Human saliva as route of inter-human infection for mouse mammary tumor virus

Abstract

Etiology of human breast cancer is unknown, whereas the Mouse Mammary Tumor Virus (MMTV) is recognized as the etiologic agent of mouse mammary carcinoma. Moreover, this experimental model contributed substantially to our understanding of many biological aspects of the human disease. Several data strongly suggest a causative role of MMTV in humans, such as the presence of viral sequences in a high percentage of infiltrating breast carcinoma and in its preinvasive lesions, the production of viral particles in primary cultures of breast cancer, the ability of the virus to infect cells in culture. This paper demonstrates that MMTV is present in human saliva and salivary glands. MMTV presence was investigated by fluorescent PCR, RT-PCR, FISH, immunohistochemistry, and whole transcriptome analysis. Saliva was obtained from newborns, children, adults, and breast cancer patients. The saliva of newborns is MMTV-free, whereas MMTV is present in saliva of children (26.66%), healthy adults (10.60%), and breast cancer patients (57.14% as DNA and 33.9% as RNA). MMTV is also present in 8.10% of salivary glands. RNA-seq analysis performed on saliva of a breast cancer patient demonstrates a high expression of MMTV RNA in comparison to negative controls. The possibility of a contamination by murine DNA was excluded by murine mtDNA and IAP LTR PCR. These findings confirm the presence of MMTV in humans, strongly suggest saliva as route in inter-human infection, and support the hypothesis of a viral origin for human breast carcinoma.

Keywords: Pathology Section; breast cancer; breast cancer etiology; mouse mammary tumor virus (MMTV); saliva.

Figure 1. Whole transcriptome analysis of human saliva

Reads count mapped to MMTV genome. A. MMTVels positive sample for a breast cancer patient. B and C. MMTVels negative human breast tissues used as negative control. A strong difference between A and B/C is easily evident.

Figure 2. FISH analysis of MMTVels in human saliva

MMTV signals in the cytoplasm of a possible epithelial exfoliated cell. In the nucleus a positive p53 signal is evident as control.

Figure 3. Immunohistochemical analysis of p14 protein expression in human salivary glands

A. a cytoplasmic positivity for p14 is well evident in a sample positive for MMTVels. B. higher magnification of A; C. absence of p14 in a sample negative for MMTVels.

Figure 4. Murine mitochondrial DNA and Intracisternal A Particles long terminal repeats (IAP LTRs) PCR in human saliva

A) mtDNA: MMTVels positive saliva samples (lanes 1–12); A and C. are PCR and nested PCR negative controls; B. is mouse DNA. All human samples result negative. B) IAP LTRs: MMTVels positive saliva samples (lanes 1–12); D. is a PCR negative control; E. is mouse DNA. All human samples result negative.