Horizontal transfer of molecules, which include signal proteins, bioactive lipids, and genetic material, through microvesicles or exosomes (EVs) is a recognized method of intercellular communication implemented in numerous physiological and pathological processes1,2,3,4,5,6,7,8,9,10,11,12.
In the last few years, the role of EVs in cancer genesis has been subjected to intense study due to recent discoveries on their role in cancer development, progression and metastatic niche formation13,14,15,16,17. Recently our group has demonstrated that exosomes isolated from the sera of cancer patients are able to transfer malignant traits to immortalized cells such as HEK293 (Human Embryonic Kidney cells) and oncosuppressor-mutated cells (OMCs) such as BRCA1-mutated human fibroblasts. Cancer exosomes would trigger their transformation into cancer cells as proved by the cancer masses that the treated OMCs generate when transplanted into NOD-SCID mice18,19,20,21. More important, we demonstrated that BRCA1-mutated fibroblasts turned into cancer cells, which exhibited phenotypical and immunohistochemical differentiation comparable to the cancer of the donor patients18,19. These discoveries, for the first time, confirmed the legitimacy of the hypothesis that the metastatic process might be secondary to an horizontal transfer of malignant traits rather than to circulating cells. According to this theory, metastatic disease might be independent from cell relocation and be a process reproducible at distance through the uptake of circulating cancer factors by primed cells located in target organs and their subsequent transformation into cancer cells with phenotypes analogous to the donor primary cancer cell22,23,24,25.
The ability of the OMCs to “sense” neoplastic factors circulating in the serum, uptake them, turn into malignant cells and give cancer masses when transplanted into NOD-SCID mice was further investigated by our group to verify if this peculiar feature could be incorporated into a biological platform and be used as a liquid biopsy test for cancer screening. This first biological based cancer-screening liquid biopsy test was called MATER-D platform (Metastatic and Transforming Elements Released Discovery platform) and preliminary evidence of its effectiveness in differentiating benign from malignant lesions and in detecting solid and hematological neoplasias was reported in a pilot study26.
In the present work, we aimed to verify if a different type of OMCs such as the human epithelial breast cell line MCF10A27,28 with a PTEN deletion was as effective in sensing oncogenic factors circulating in the sera of cancer patients as the OMCs we previously studied18,19.
The tumor suppressor gene PTEN (Phosphatase and tensin homolog) is a central negative regulator of the PI3K/AKT signaling cascade that influences multiple cellular functions including cell growth, survival, proliferation and migration in a context-dependent manner. Loss of PTEN function (due to mutations, deletions, or epigenetic silencing) is involved in many solid and hematological human malignancies29,30.
In addition to the above objective, we sought to strengthen the validity of the MATER-D platform and assess its effectiveness in detecting precancerous and early cancer lesions such as dysplasia and carcinoma in situ arising in different organs.
When exposed to cancer patient’s sera, MCF10A cells with a deletion of the PTEN gene turned malignant, as confirmed by the cancer masses obtained in NOD-SCID mice after xenotransplantation. PTEN-deficient MCF10A cells exposed to sera of a cohort of healthy patients, used as a control group, never gave any cancer masses when transplanted in NOD-SCID mice. These findings confirmed the concept that different types of cells with different oncosuppressor mutations can uptake circulating cancer factors, turn malignant and may be used in biological based liquid biopsy tests.
The MATER-D platform was able to detect dysplasia and carcinomas in situ lesions in the five different organs that were investigated (pancreas, colon, gallbladder, breast and skin). Surprisingly enough, circulating factors were detected in patients even years after the removal of precancerous and non-invasive lesions strengthening the validity of the hypothesis that the metastatic process might occur before cancer cells invasion of the basal membrane and be independent from cell migration.
Immunohistochemistry analysis of the xenotransplants obtained in mice after injection of OMCs treated with different cancer sera revealed that the immunohistochemistry patterns of malignant differentiation in BRCA1-mutated fibroblasts and PTEN-deficient MCF10A cells were always identical regardless of the cancer sera type that the cells were exposed to, confirming that horizontal transfer of malignant traits is a concept applicable also to epithelial cells (MCF10A) with a different oncosuppressor mutations (PTEN) than the ones previously investigated (BRCA1, P16 and P53)19,20,25. Furthermore, this unique discovery showed that malignant differentiation of target cells located in different organs, through horizontal transfer of oncofactors might be dependent on the histology of the target cells rather than the type of circulating cancer messengers.
Taken together, these observations shed new insights on the mechanisms behind the horizontal transfer of malignant traits and corroborate the validity of the claim that oncosuppressor-mutated cell-based liquid biopsy tests might be regarded as promising platforms for cancer screening, which deserve further study in properly designed clinical trials.