By John Pawlek, PhD
More than 25 years ago the late Prof. Lynn Margulis at the University of Massachusetts, Amherst sent me a 1911 article by German pathologist Otto Aichel who proposed that metastasis might be initiated by hybridization between macrophages and cancer cells, with the daughter cells “thrown out of the path of the mother cells resulting in an entirely new cell having the characteristics of both mother cells to form what has come to be known as a malignant cell.”
The BMDC-cancer cell fusion hypothesis. A motile BMDC (red), such as a macrophage or stem cell, is drawn to a cancer cell (blue). The outer cell membranes of the two cells become attached. Fusion occurs with the formation of a binucleated heterokaryon having a nucleus from each of the fusion partners. The heterokaryon goes through genomic hybridization creating a melanoma–BMDC hybrid with two gene expression patterns conferring deregulated cell division and metastatic competence to the hybrid (1).
That is, at least some of the hybrids would acquire motility from the macrophage along with the de-regulated cell division of the cancer cell, hence the emergence of a metastatic cancer cell. Margulis had devoted much of her career to the role of “endosymbiosis” in evolution and she recognized that the two situations were analogous. Lynn and I were friends but I never knew why she sent the article since I was studying skin color at the time. However I, along with my colleagues have been working on the concept ever since and I am eternally grateful to Lynn for this. Below is a brief update of the field. All the citations can be found in reference (1).
In his prescient statement Aichel not only provided an explanation for metastasis but he also predicted the science of cancer epigenetics. That is, a new hybrid cell with characteristics of both mother cells in today’s terminology would refer to gene expression patterns from both fusion partners in the same cell. For example, at least some hybrids would express the leukocyte traits of motility, chemotaxis, and homing while at the same time have the uncontrolled cell division of the cancer cell. To investigate this concept, our group has been studying cancer patients who had previously received an allogeneic bone marrow transplant (BMT), usually for leukemia or lymphoma, and then later developed a solid tumor. By analyzing tumor cells for both donor and patient DNA, we reasoned that such cells were likely to be leukocyte-tumor cell hybrids.
In our first case, we studied a primary renal cell carcinoma from a female patient who, two years prior to detection of the tumor, had received a BMT from her son. Due to the male donor–female recipient nature of the BMT, FISH could be used to search for putative BMT-tumor hybrids. Karyotyping revealed that the tumor cells contained a clonal trisomy 17. Using dual-label FISH, the donor Y and three or more copies of chromosome 17 were visualized together in individual nuclei of carcinoma cells, providing direct genetic and morphological evidence for BMT-tumor hybrids (Fig. 1). Panel A shows a cell with three copies of chromosome 17 (green) but no Y chromosome, indicating that this cell was likely not a hybrid, while Panel B shows a trisomy 17 (green) plus the Y chromosome (red), indicating that the cell was a hybrid between a patient and the male donor cell. Such cells were in abundance in an area covering about 10% of the tumor, suggesting a clonal origin of the hybrids.
Figure 1: FISH analysis of formalin-fixed sections of a primary renal cell carcinoma described herein (1).
The first genomic evidence for leukocyte-cancer cell hybrids in a human came from our study of a patient who had received an allogeneic BMT for lymphoma and later developed a melanoma brain metastasis. Tumor DNA was analyzed through forensic genetics for donor and patient alleles at 14 loci. Eight of these loci were informative and indicated the presence of donor-patient hybrids throughout the tumor. The second such evidence came from a man who, eight years following an allogeneic BMT from his brother for treatment of chronic myelogenous leukemia, developed a malignant melanoma on the back with spread to an axillary lymph node. The primary tumor and the nodal metastasis each exhibited alleles with donor and patient DNA at all loci.
In addition to direct genomic evidence for a relationship between leukocyte-cancer fusion and metastasis is the large number of macrophage-like traits expressed by metastatic cancer cells. For example, Shabo et al. showed that macrophage traits in cancer cells are induced by macrophage-cancer cell fusion and cannot be explained simply by cellular interactions. They showed that tumor cell expression of the macrophage marker CD163 is related to poor prognosis in patients with breast cancer, colorectal cancer, and urinary bladder cancer. Pawelek et al. showed that following macrophage-cancer cell fusion, the resultant hybrid cells acquired new abilities to promote angiogenesis, matrix alterations, motility, chemotaxis, and immune signaling pathways. Macrophage-tumor cell fusion could explain the aneuploidy, plasticity, and heterogeneity of malignant melanoma and it could also account for epidermal-mesenchymal transition in tumor progression since macrophages are of mesodermal origin. Several other laboratories have since reported such findings.
There is considerable evidence that fusion and hybridization of phagocytes such as macrophages with cancer cells creates metastatic cells. Our group has demonstrated this in three patients with melanoma and two with renal cell carcinoma. In addition, several labs have made immunological observations that metastatic cancer cells exhibit macrophage traits. Thus it seems safe to say that this is at least one mechanism for metastasis. This confirms the century-old proposal of Prof. Otto Aichel that in retrospect was prescient indeed, especially considering that he had only a microscope with which to work.
For the first time we can glimpse an engine that drives metastasis (Fig: The BMDC-cancer cell fusion hypothesis). This information opens many potential targets for the development of new therapies, for example: a) inhibition of the fusion process itself regarding events such as membrane attachment and heterokaryon formation; b) inhibition of the hybridization processes involving integration of parental fusion partner genes into hybrid genomes; and c) prevention of post-hybridization events involving activation of genes that control cell migration, chemotaxis, intravasation, extravasation, and migration to lymph nodes and distant metastases.
- Greggory S. Laberge, Eric Duvall, Kay Haedicke and John Pawelek. Leukocyte–Cancer Cell Fusion - Genesis of a Deadly Journey. Cells 2019, 8(2), 170; https://doi.org/10.3390/cells8020170:18 February 2019.