We think of cancer as a chronic condition that results in uncontrollable cell proliferation. Whether is be through UV radiation that causes point mutations or improper signaling cascades resulting in abnormal mitotic division. Cancer is widespread; yet most people would say there is no chance of infecting others. While this is the case for most species, some cases in Tasmanian devil and dog allude to the exception with recent research on clams revealing free-floating contagious cancer cells.
While cancer is not typically thought of as a viral disease, some induced viral tumors are present in two cases, sexually transmitted canine transmissible venereal tumor and Tasmanian devil facial tumor disease through bites. Other than these two cases identified in the 1980’s, no identifiable cancers have been discovered to be contagious until recent research carried out by a group of researchers at Cambridge University identified disseminated neoplasia (similar pathology to leukemia) in soft shell clams. Notable pathology includes a misallocated p53 tumor suppressant gene that codes for a regulatory protein that inhibits mitotic division for damaged cancerous cells. Neoplasia can be induced in soft shell clams by injection of affected hemocytes (blood cells). Through lab experimentation of the soft shell clams, the effected hemocytes displayed reverse transcriptase activity suggesting the presence of a possible cancerous integration site. Through cDNA sequencing LTR-retrotransposon, the sequence code, Steamer, was identified and concurrent with neoplasia. Typical clams display a genome with anywhere from 2 to 10 copies of the endogenous code while affected neoplastic clams show 150-300. With further experimentation analyzing mitochondrial DNA, genotypes of the affected hemocytes do not match host cells suggesting that Steamer sequences stem from other neoplasmic cells carrying the common Steamer sequence concurrent with the rare infectious cancers affecting dogs and Tasmanian devils. In addition, all genotypes of disseminated neoplasia hemocytes are virtually identical alluding to horizontal transmission of infectious cancer cells derived from a single colony of soft shell clam along the Atlantic coast.
Figure A. Representative Clams
Figure B. Hemolymph from normal clam
Figure C. Hemolymph from Neoplasia positive clam
Figure D. Areas of collection
Cancer has for long been classified as a chronic disease with no horizontal transmission except in the rare cases of the dog and Tasmanian devil. This new research extending contagious cancer into the marine ecosystem only adds another dimension to a disease assumed to be non-transmittable. What does this mean for the future of cancer and could humans have to worry about infection in the future? It does present that possibility which is quite terrifying. In developed nations, chronic disease is a more pressing concern thanks to medical advances and a superior healthcare system that has controlled and wiped out many infectious diseases. Considering how difficult cancer is to treat with no know cure on the horizon, adding another dimension, infection, to the picture presents a struggle that would ensue devastation. Also to be kept in mind is that although it may be a while before contagious cancer reaches the human population, there may be an extension to other species that would certainly cause havoc in the ecosystem.
The original research can be found here
The article from the New York Times can be found here