Naturally occurring transmissible cancers
Transmissible cancers have been identified as spreading within two vertebrates, dogs (
Canis lupus familiaris) and Tasmanian devils (
Sarcophilus harrisii), and also multiple independent lineages of transmissible cancers in several species of bivalves.
Canine transmissible venereal tumour
Canine transmissible venereal tumour (CTVT) was first described by a veterinarian in London in 1810. It was also experimentally transplanted between dogs in 1876. However, it was not identified as a contagious cancer (descended from a single lineage) until 2006.
Previous studies have proposed that the cell line arose 11,000 years ago. Most recently, the source has been placed in a population of Native American dogs.
CTVT spreads as a sexually transmitted infection in feral dog populations throughout the world. It infects animals in at least 90 countries on all continents except Antarctica. The cancer is mostly localised on the external genitalia, but in rare cases it can also manifest on the face. It typically regresses on its own but can be treated easily using chemotherapy drugs such as vincristine.
The concept that this tumour is naturally transmissible came from three important observations:
- CTVTs can only be experimentally induced by transplanting living tumour cells
- The tumour karyotype is aneuploid but has characteristic marker chromosomes in all tumours collected in different geographic regions
- LINE-1 insertion near c-myc has been found in all tumours examined so far
The Tasmanian devil facial tumour disease
The first observation of a Tasmanian devil suffering from devil facial tumour disease (DFTD) was in 1996 in north eastern Tasmania. This first lineage is often now referred to as DFT1. The second and independently risen lineage (DFT2) was discovered in 2014 in D’Entrecasteaux Peninsula. Both diseases present as large ulcerating tumours around the face and jaws, but DFT2 tumours often spread to other parts of the body.
Similarly to CTVT, both diseases spread from physical contact when devils bite each other. DFT1 and DFT2 cells are genetically, chromosomally and histologically different. The presence of chromosome Y in DFT2 and remnants of an X chromosome in DFT1 indicate that the older cell line emerged in a female devil and the younger from a male. Transcriptomic analysis has revealed that classical DTFD likely originated in Schwann cells. The cell type origin of DFTD2 is unknown.
Bivalve transmissible neoplasias
Abnormal proliferation of cells in bivalve haemolymph (circulatory fluid) has been widely described since the 1960s. Various disseminated neoplasia have been reported in the soft-shelled clam (
Mya arenaria), the mussel (
Mytilus trossulus), the cockle species (
Cerastoderma edule) and the golden carpet-shell clam (
Poltitapes aureus).
As multiple lineages of transmissible cancers are spreading through multiple bivalve species, these diseases are known as bivalve transmissible neoplasias (BTN). The origin of disseminated neoplsias is unclear. However, researchers have suggested that the malignant cells stem from haemocyte cells that reside in the animals’ circulatory system.
The distribution of transmissible BTN across continents most likely has been facilitated by a transmission mode that does not require direct contact between individuals. For example, human interference via marine transportation. Neoplastic haemocytes in bivalves are also often aneuploid with an increased DNA content indicating high chromosomal instability. Neoplastic diseases in bivalves appear at their highest frequency in polluted areas.