Infographic: When Cancers Become Parasites

Above: © MESA SCHUMACHER

So far, only dogs, Tasmanian devils, and four bivalve species are known to carry transmissible cancers, which have varying effects on their hosts.

Host species	Domestic dog (Canis lupus familiaris)	Tasmanian devil (Sarcophilus harrisii)	Bivalves (the soft-shelled clam, Mya arenaria, the mussel, Mytilus trossulus, and the cockle species, Cerastoderma edule, and the golden carpet-shell clam, Poltitapes aureus)
Cancer	Canine transmissible venereal tumor (CTVT)	Devil’s facial tumor disease (DFT1 and DFT2)	Clam disseminated neoplasia
Outcome	GABRIELE MARINO, UNIVERSITY OF MESSINA, ITALY CTVT manifests as tumors on dogs’ genitals (seen here at base of penis) that rarely result in death. It regresses on its own or can be treated easily using chemotherapy drugs such as vincristine.	CAMILA ESPEJO, UNIVERSITY OF TASMANI DFT1 and DFT2 form tumors around the mouth and face that restrict the devils’ ability to feed. These...

In one of the most extensive studies of devil facial tumor disease (DFT1) to date, an international team of researchers has uncovered a mechanism that drives the cancer’s metastasis and helps it to evade the Tasmanian devils’ immune system.

1	Cancer cells that form DFT1 tumors overproduce transmembrane enzymes known as ERBB receptors.
2	When stimulated by specific proteins—likely EGF and NRG1, which are also overproduced in DFT1 cells—ERBB receptors induce production of a signaling protein and transcription factor called Stat3 and drive its activation.
3	In the nucleus, Stat3 drives the production of genes such as MMP2 that are known to trigger metastasis in humans.
4	Stat3 also binds to and inhibits another transcription factor, Stat1, which normally drives the expression of genes necessary for the generation of MHC class I molecules.

5	MHC class I molecules normally interact with receptors on cytotoxic T cells to discriminate self from foreign cells. By downregulating the production of MHC proteins, DFT1 cells are able to evade detection by the animals’ immune system.
6	Under normal circumstances, cells lacking MHC markers would be detected by natural killer cells (NK), but for reasons that are unclear, devil NK cells don’t react to DFT1.

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Infographic: When Cancers Become Parasites

Only a handful of contagious cancers are known to exist, yet they have attracted an increasing number of researchers worldwide trying to understand how and why they arise.

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