Transmissible Tumors Represent a Unique Evolutionary Paradigm in Multiple Species

Cancer and infectious disease are two of the most important modern health concerns. In some cases, such as human T-lymphotropic virus type 1 and human papilloma virus, transmission of infectious material is known to induce neoplastic transformation of host cells. However, a novel paradigm is emerging across species in which transformed neoplastic cells within a host are able to loose the bonds of traditional cancer to clonally infect another without experimental transplantation. Though cancer is traditionally a self-limiting tissue due to host death, these transmissible cancers are allografts that redefine our understanding of the mechanisms by which cancer may evolve. Canine transmissible venereal tumor (CTVT) has been subject to the evolutionary crucible by host-hopping for up to 10,000 years, by far the oldest known continuously-propagating cell lineage. As such, it has arguably been refined to be an intracellular parasite, able to avoid host immune surveillance due to somatic mutation and genome rearrangement. Tasmanian devil facial tumor is much younger, evolving for less than 100 years. Though showing similar immune-avoidance characteristics as CTVT, it does not have the sheer number of somatic mutation, and is responsible for a reduction in devil populations. Recently, another transmissible cancer system was identified in soft-shell clams. Perhaps due to the lack of a major histocompatibility complex in these species, or the lack of barrier to the dissemination of neoplastic cells; these bivalve transmissible neoplasias have been documented to infect multiple species with the potential for cross-species infection. The continued discovery of novel transmissible tumors and their shared characteristics indicate that researchers are only beginning to uncover these novel paradigms of organismal evaluation and the fundamentals of their inception.