Epstein–Barr Virus Infection of Mammary Epithelial Cells Promotes Malignant Transformation

• Mammary epithelial cells, but not breast cancer cells, express CD21 and can be infected by EBV.
• EBV infection of MECs rewires mitogenic signaling with activation of the MET pathway and gene expression changes (EBVness).
• EBV infection of MECs lowers the threshold for malignant transformation in xenografts.
• Human tumors with ‘EBVness’ are associated with poor prognosis and presence of remnant EBV DNA (but not RNA).Research in context:This study shows that Epstein–Barr Virus (EBV) infection occurs in breast epithelial cells (MECs), but not in breast cancer cells. EBV infection of MECs leads to changes in gene expression, activation of oncogenic signaling via c-MET and lowers the threshold for cancerous transformation. In human breast cancer, these gene expression changes, termed ‘EBVness’, are associated with high grade and the finding of remnant EBV DNA. EBV infection of breast epithelial cells may increase the odds of later breast cancer formation in predisposed individuals, which could potentially be avoided by a childhood vaccine against EBV.

Whether the human tumor virus, Epstein–Barr Virus (EBV), promotes breast cancer remains controversial and a potential mechanism has remained elusive. Here we show that EBV can infect primary mammary epithelial cells (MECs) that express the receptor CD21. EBV infection leads to the expansion of early MEC progenitor cells with a stem cell phenotype, activates MET signaling and enforces a differentiation block. When MECs were implanted as xenografts, EBV infection cooperated with activated Ras and accelerated the formation of breast cancer. Infection in EBV-related tumors was of a latency type II pattern, similar to nasopharyngeal carcinoma (NPC). A human gene expression signature for MECs infected with EBV, termed EBVness, was associated with high grade, estrogen-receptor-negative status, p53 mutation and poor survival. In 11/33 EBVness-positive tumors, EBV-DNA was detected by fluorescent in situ hybridization for the viral LMP1 and BXLF2 genes. In an analysis of the TCGA breast cancer data EBVness correlated with the presence of the APOBEC mutational signature. We conclude that a contribution of EBV to breast cancer etiology is plausible, through a mechanism in which EBV infection predisposes mammary epithelial cells to malignant transformation, but is no longer required once malignant transformation has occurred.