The canine transmissible venereal tumor (CTVT) is a cancer lineage that arose several millennia ago and survives by “metastasizing” between hosts through cell transfer. The somatic mutations in this cancer record its phylogeography and evolutionary history. We constructed a time-resolved phylogeny from 546 CTVT exomes and describe the lineage’s worldwide expansion. Examining variation in mutational exposure, we identify a highly context-specific mutational process that operated early in the cancer’s evolution but subsequently vanished, correlate ultraviolet-light mutagenesis with tumor latitude, and describe tumors with heritable hyperactivity of an endogenous mutational process. CTVT displays little evidence of ongoing positive selection, and negative selection is detectable only in essential genes. We illustrate how long-lived clonal organisms capture changing mutagenic environments, and reveal that neutral genetic drift is the dominant feature of long-term cancer evolution.

Canine transmissible venereal tumour (CTVT) is a clonally transmissible cancer that originated approximately 11,000 years ago and affects dogs worldwide. Despite the clonal origin of the CTVT nuclear genome, CTVT mitochondrial genomes (mtDNAs) have been acquired by periodic capture from transient hosts. We sequenced 449 complete mtDNAs from a global population of CTVTs, and show that mtDNA horizontal transfer has occurred at least five times, delineating five tumour clades whose distributions track two millennia of dog global migration. Negative selection has operated to prevent accumulation of deleterious mutations in captured mtDNA, and recombination has caused occasional mtDNA re-assortment. These findings implicate functional mtDNA as a driver of CTVT global metastatic spread, further highlighting the important role of mtDNA in cancer evolution.DOI: http://dx.doi.org/10.7554/eLife.14552.001

Osteosarcoma (OSA) is a malignant heterogeneous primary bone tumor responsible for up to 90% of all primary bone tumors in dogs. In this study, osteocalcin (OC) and osteonectin (ON) immunoreactivity was evaluated in 23 canine OSAs, 4 chondrosarcomas, 4 fibrosarcomas, 2 hemangiosarcomas, and 4 histiocytic sarcomas. The effects of three different decalcification agents (ethylenediaminetetraetic acid [EDTA], formic acid and hydrochloric acid [HCl]) on the immunoreactivity for OC and ON was also assessed. Immunoreactivity to OC was present in 19/23 (83%) cases of OSA and all cases of chondrosarcoma. In three OSAs the extracellular matrix showed immunoreactivity to OC. None of the fibrosarcomas, histiocytic sarcomas or hemangiosarcomas showed immunoreactivity to OC. The sensitivity and specificity for OC in canine OSA in this study was 83% and 71% respectively. For ON, 100% of both OSAs (23/23) and non-OSAs (14/14) showed cytoplasmic immunoreactivity to this antibody, giving a sensitivity of 100% but a complete lack of specificity. There were no significant differences in immunoreactivity for OC and ON between the different decalcification agents used. In conclusion, OC showed high sensitivity for identifying OSA but it failed to distinguish between OSA and chondrosarcoma, and the osteoid produced by neoplastic cells in most cases did not show immunoreactivity to OC. These factors may limit the practical utility of OC in the diagnosis of OSA in dogs when chondrosarcoma is a differential diagnosis. ON showed no specificity in detecting OSA and has little practical application for the diagnosis of OSA in dogs.

Autonomous replication and segregation of mitochondrial DNA (mtDNA) creates the potential for evolutionary conflict driven by emergence of haplotypes under positive selection for 'selfish' traits, such as replicative advantage. However, few cases of this phenomenon arising within natural populations have been described. Here, we survey the frequency of mtDNA horizontal transfer within the canine transmissible venereal tumour (CTVT), a contagious cancer clone that occasionally acquires mtDNA from its hosts. Remarkably, one canine mtDNA haplotype, A1d1a, has repeatedly and recently colonised CTVT cells, recurrently replacing incumbent CTVT haplotypes. An A1d1a control region polymorphism predicted to influence transcription is fixed in the products of an A1d1a recombination event and occurs somatically on other CTVT mtDNA backgrounds. We present a model whereby 'selfish' positive selection acting on a regulatory variant drives repeated fixation of A1d1a within CTVT cells.

Outbreaks of humeral fractures in dairy cows have been reported in New Zealand for several years. Gross, histologic, and histomorphometric findings in the humerus from primiparous cows with spontaneous humeral fracture were compared to age-matched control cows. Affected cows had a complete nonarticular spiral fracture of the humerus. Histologically affected humeri had a thicker growth plate with abnormal architecture, thinner cortex with increased abnormal resorption, increased resorption in the distal humerus, decreased trabecular density, abnormal trabecular architecture, presence of growth arrest lines and woven bone formation. Histomorphometry showed reduction in bone volume, trabecular perimeter, and trabecular width. Cows grazed on fodder beet had thicker growth plates with an abnormal appearance compared with cows grazed on pasture, and cows with low/marginal liver copper concentration had more resorption cavities in the distal humerus and thinner cortical bone compared with cows with adequate liver copper concentration. Decreased trabecular density (OR = 249.5), abnormal cortical resorption (OR = 54.2), presence of woven bone formation in the proximal metaphysis (OR = 37.2), and the number of resorption cavities in the distal humerus were significantly associated with a high probability of fracture. Ribs had enlargement of the costochondral junction with fractures in different stages of healing. Histology of the ribs revealed abnormal growth plate appearance, presence of fracture lines, callus tissue, fibrosis, and microfractures. Cows with humeral fracture have osteoporosis due to decreased bone formation and increased bone resorption, likely associated with inadequate feed quality and perhaps copper deficiency leading to a reduction in bone strength and fracture.