Clonal origin and evolution of a transmissible cancer

Abstract

The transmissible agent causing canine transmissible venereal tumor (CTVT) is thought to be the tumor cell itself. To test this hypothesis, we analyzed genetic markers including major histocompatibility (MHC) genes, microsatellites, and mitochondrial DNA (mtDNA) in naturally occurring tumors and matched blood samples. In each case, the tumor is genetically distinct from its host. Moreover, tumors collected from 40 dogs in 5 continents are derived from a single neoplastic clone that has diverged into two subclades. Phylogenetic analyses indicate that CTVT most likely originated from a wolf or an East Asian breed of dog between 200 and 2500 years ago. Although CTVT is highly aneuploid, it has a remarkably stable genotype. During progressive growth, CTVT downmodulates MHC antigen expression. Our findings have implications for understanding genome instability in cancer, natural transplantation of allografts, and the capacity of a somatic cell to evolve into a transmissible parasite.

Figure 1. Specific LINE-1/c-myc and DLA Haplotype Genetic Markers for CTVT Detected by Specific PCR Amplification

(A) For each of 11 dogs (A–M), fresh normal and tumor samples are indicated as N and T, respectively. The panel is assembled from three separate gels visualized by ethidium bromide. The invariant DLA-88 intron sequence serves as a positive control for each of the 22 specimens. (B) PCR amplification of DNA using Cy5-labeled forward primers from 21 microdissected tumor cells from paraffin-embedded specimens. The panel is assembled from four separate gels.

Figure 2. Microsatellite DNA Analysis of 11 Fresh Tumors and Matched Host Samples

Unrooted neighbor-joining tree based on chord distance compiled from 21 microsatellite loci. A similar neighbor-joining tree based on allele sharing is provided in Figure S2.

Figure 3. Analysis of mtDNA in CTVT

Tumor haplotypes from fresh and paraffin-embedded tissues showing two main clusters of mtDNA. Diameter of each circle is proportional to the number of tumor samples. Each branch represents one base pair change, with black dots representing intermediates not found in the tumor samples analyzed. The outlined boxes indicate that tumor samples with homozygous (diploid) and hemizygous (haploid) DQA1 genes coincide with mtDNA clusters, except for tumor 9, which is diploid.

Figure 4. Relationship of CTVT to Wolves and Dog Breeds

(A) Results of a Structure analysis of the canids that appeared most closely related to CTVT (yellow at K = 2 in Figure S6). The clustering was based on the nontumor samples only, and the three tumor samples with nearly complete data were then assigned to the appropriate clusters. (B) Neighbor-joining tree based on pairwise differences among the same set of individuals as in (A). The relationship between wolves and CTVT is similar when the tree is constructed using all dogs (Figure S6).

Figure 5. Average Pairwise Divergence of Microsatellite DNA in Wolves, Dogs, and CTVT

The expected heterozygosity at microsatellite loci in different subsets of canids and CTVT was plotted. Non-CTVT data were derived from Parker et al. (2004). The Australian shepherd represents the most diverse defined breed and the miniature bull terrier the least diverse breed for microsatellite DNA.

Figure 6. MHC Expression in CTVT

(A) Expression of DLA class I (DLA-88) and class II (DRB1) in the penile tumor of dog C by RT-PCR using tumor-cell-specific primers (T) and primers specific to the alleles of this animal for host stromal cells and infiltrating normal cells (N). M = marker lanes. (B) Histopathology of a hematoxylin-and-eosin-stained 4 μm section of the same tumor. Scale bar = 30 μm. Cells with large round nuclei are tumor cells, and mitoses are apparent. A stromal cell is indicated with an arrow.