Human sporadic breast carcinoma histotypes driven by the Human Betaretrovirus homologous to Mouse Mammary Tumor Virus

Abstract

The viral hypothesis for human sporadic breast carcinoma is based on the murine model of Mouse Mammary Tumor Virus (MMTV)-induced mammary tumors. Known risk factors like estrogens, obesity, and alcohol do not play a direct causal role. The Human Betaretrovirus (HBRV), also called Human Mammary Tumor Virus (HMTV), is the human homolog of MMTV, implicated in sporadic breast carcinoma (80% of ductal carcinoma in situ and 40% of invasive tumors). In contrast, hereditary breast carcinomas lack viral sequences. Murine mammary tumor histotypes are determined by specific viral strains activating definite molecular pathways via insertional mutagenesis. Similarly, the diverse histotypes observed in human invasive breast carcinoma may be influenced by a viral etiology. A study of 253 invasive breast carcinoma cases, representing 15 histotypes, detected HBRV/MMTV-ENV sequences in 20%, consistent with international literature. All histotypes tested positive except those linked to hereditary syndromes, such as medullary, apocrine, and metaplastic carcinoma. This distinction reinforces the reported lack of association between HBRV/HMTV and hereditary breast cancer, while supporting a viral etiology for sporadic carcinoma. Relevant characteristics of sporadic histotypes align with the "hit and run" hypothesis of viral carcinogenesis. Histotype differences may result from molecular pathways activated by Int genes, though mechanism beyond insertional mutagenesis and the possibility of specific HBRV strains cannot be ruled out. The potential for detected viral sequences to originate in human tumors from endogenous MMTV or contamination with murine material is critically examined.

Keywords: breast cancer etiology; breast cancer histotype; human betaretrovirus‐HBRV; human mammary tumor virus‐HMTV; mouse mammary tumor virus‐MMTV.

FIGURE 1

Graphic representation of HBRV distribution among human breast carcinoma histotypes. It is worth noting: (A) tubular carcinoma exhibits a very high positivity rate (59%), likely due to its high degree of differentiation. (B) Apocrine, medullary, and metaplastic carcinomas, all associated with hereditary cancer syndromes, show absent or nearly absent positivity.

FIGURE 2

Example of PCR detection of an MMTV/HBRV ENV sequence positive case. Representative capillary electrophoresis image obtained via fluorescent fragment analysis, showing a positive case for ENV sequence amplification. The expected amplicon is 191 bp (red asterisk) after the semi‐nested PCR. Yellow peaks represent the ROX‐labeled size standard.

FIGURE 3

Confirmation of viral sequence by Sanger sequencing. A portion of the amplified fragment sequence is shown, confirming alignment with GenBank accession number AF243039. This validates that the amplified PCR product corresponds to the MMTV‐ENV region of the MMTV virus. The analysis was performed using the Sanger sequencing assay.

FIGURE 4

Absence of contaminating murine DNA, electrophoresis gel of murine DNA amplification. (A) Murine IAP—Intracisternal A Particles LTRs. DNA from human HBRV‐ENV positive cases tested negative for IAP DNA. The expected amplicon is 250 bp. S1–S8: Representative negative human cases. P: Murine positive control. N: Negative PCR control. A 50 bp DNA ladder was used as the molecular size marker. (B) Murine Mitochondrial DNA. DNA from human HBRV‐ENV positive cases tested negative for Murine Mitochondrial DNA. The expected amplicon is 153 bp. S1–S8: representative negative human cases. P: murine positive control. N: negative PCR control. A 50 bp DNA ladder was used as the molecular size marker.

FIGURE 5

Immunohistochemical analysis of p14 protein. (A–C) Two cases of ENV‐positive NST invasive carcinomas showing p14 positivity; C is an enlargement of B. (D) negative control of A, showing the absence of antibody staining. (E) an ENV‐negative NST invasive carcinoma, also negative for p14. Positivity is limited to cancer cells.