Transfer of Viral and Mitochondrial Nucleic Acids through Microvesicles: a Possible Mechanism of Genetic Intra-Tumor Heterogeneity in Breast Cancer

Microvesicles (MVs) are small extracellular vesicles divided in exosomes (40-100nm), shedding microvesicles (100nm–1µm), and apoptotic bodies (1-5µm). MVs contain bioactive molecules, nucleic acids and proteins and are physiologically secreted by cells in order to guarantee cell-to-cell communication. The presence of viral miRNA in exosomes and viral DNA in apoptotic bodies was demonstrated. Viral DNA sequences like EBV and HPV have been identified in numerous cancers tissues, like in breast cancer, in the absence of a primary infection. Two meta-analyses studies analyzed the presence of HPV and EBV sequences in breast cancer tissues worldwide, and identified the viral nucleic acids in the 23% and 29% of the samples, respectively. However, the mechanisms of viral DNA acquisition in this cancer have not been investigated. This paper demonstrates the presence of viral nucleic acids in MVs coming from cell line models of onco-viral dependent transformation, including cervical carcinoma (HPV) and lymphoma (EBV), and their transfer through MVs to viral negative epithelial cells and fibroblasts. The presence and transfer of murine mitochondrial DNA in MVs coming from murine cancer-associated fibroblasts is also demonstrated, as well as its transfer to human recipient cells. This study also shows the presence of viral HPV16/18/31 DNA in the circulating MVs from two different cohorts: one of newly diagnosed breast cancer patients, collected in Bologna, Italy, and the other one of patients affected by hormonal therapy-resistant breast cancer, collected in New York, USA. The presence of DNaseI-resistant DNA in the RNA fraction prompted us to study the physical-chemical status of the viral DNA sequences packaged in the MVs coming from viral positive cell lines; the DNA is found in the form of DNA:RNA hybrids (R-loops), which are known to be more stable structures, mostly synthesized in hypoxic conditions.