Transmissible cancer in marine bivalves: Implications for mode of cancer transmission

Cancer spread away from the primary tumour towards other parts of the body is what is most feared in cancer treatment, and is the leading cause of relapse for patients who are in remission after initial rounds of treatment with surgery, radiation or chemotherapy. Typically more aggressive than the primary tumour, secondary tumours developed through the process of metastasis are seeded by circulating tumour cells that migrate away from solid tumours to distant body sites. But increasingly, there is another method by which cancer is able to spread: transmission between members of the same or different species. Dangerous for the implications that it portends, the intra- and interspecies transmission of cancer highlights the potential infectious nature of the disease of neoplastic cell growth, and the implications it may have on transmission of cancer through the food chain, even though the latter link find no verifiable reported episode, but which remains a possibility to be vigilant for. Currently, transmissible cancers have been detected in dogs, Tasmanian devils, and molluscs. In a recent report in Nature, Goff and coworkers reported the widespread transmission of independent cancer lineages within and between marine bivalves species (“Widespread transmission of independent cancer lineage within multiple bivalve species”). Field studies conducted along the Pacific Northwest coast led to the identification of neoplastic bivalves, which upon sequencing of the mitochondrial cytochrome c oxidase gene proffer a set of single nucleotide polymorphisms (SNP) that could be clustered into distinct cancer lineages in a phylogenetic tree. Using sequence information as guide and histology observations as clues, clear evidence of cancer transmission between and within bivalves species were confirmed, with affected bivalves showing clear phenotypic neoplasms. Given that bivalves are a source of nutrients and vitamins in the delicacy segment of Asian cuisine, possible transmission of cancer between bivalves and humans as well as mammals such as bears that feed on them is an important direction for future research.