Canine transmissible venereal tumor (CTVT) is the oldest known somatic cell lineage. It is a transmissible cancer that propagates naturally in dogs. We sequenced the genomes of two CTVT tumors and found that CTVT has acquired 1.9 million somatic substitution mutations and bears evidence of exposure to ultraviolet light. CTVT is remarkably stable and lacks subclonal heterogeneity despite thousands of rearrangements, copy-number changes, and retrotransposon insertions. More than 10,000 genes carry nonsynonymous variants, and 646 genes have been lost. CTVT first arose in a dog with low genomic heterozygosity that may have lived about 11,000 years ago. The cancer spawned by this individual dispersed across continents about 500 years ago. Our results provide a genetic identikit of an ancient dog and demonstrate the robustness of mammalian somatic cells to survive for millennia despite a massive mutation burden.
Breaking Tumor Dogma
Canine transmissible venereal tumor (CTVT) is an unusual form of cancer because the infectious agent is not a virus or bacterium but the tumor cells themselves, which are passed from one dog to another during coitus. To explore the molecular features of the tumor and its possible origins, Murchison et al. (p. 437; see the Perspective by Parker and Ostrander) sequenced the genomes of two CTVTs and their host dogs, one from Australia and one from Brazil. Although CTVT has acquired a massive number of genomic alterations, including hundreds of times more somatic mutations than are normally found in human cancers, the tumor cell genome has remained diploid and stable. Indeed, CTVT may first have arisen in a dog that lived more than 10,000 years ago.