Concise review: transmissible animal tumors as models of the cancer stem-cell process.

Tasmanian devil facial tumor disease (DFTD) and canine transmissible venereal tumor (CTVT) are highly unusual cancers capable of being transmitted between animals as an allograft. The concept that these tumors represent a cancer stem-cell process has never been formally evaluated. For each, evidence of self-renewal is found in the natural history of these tumors in the wild, tumor initiation in recipient animals, and serial transplantation studies. Additional data for stem-cell-specific genes and markers in DFTD also exist. Although both tumor types manifest as undifferentiated cancers, immunocytohistochemistry supports a histiocytic phenotype for CTVT and a neural crest origin, possibly a Schwann-cell phenotype, for DFTD. In these data, differential expression of lineage markers is seen which may suggest some capacity for differentiation toward a heterogeneous variety of cell types. It is proposed that DFTD and CTVT may represent and may serve as models of the cancer stem-cell process, but formal investigation is required to clarify this. Appreciation of any such role may act as a stimulus to ongoing research in the pathology of DFTD and CTVT, including further characterization of their origin and phenotype and possible therapeutic approaches. Additionally, they may provide valuable models for future studies of their analogous human cancers, including any putative CSC component.