Infographic: When Cancers Become Parasites
Infographic: When Cancers Become Parasites

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.

Apr 1, 2019
Katarina Zimmer

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 speciesDomestic 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)
CancerCanine 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 cancers are typically fatal, and have driven devils to the brink of extinction.
Marisa Yonemitsu and Rachael Giersch, Pacific Northwest Research Institute
These leukemia cells clog bivalves’  internal organs. While the disease has triggered mass die-offs of soft-shelled clams in the past, today it doesn’t seem to cause severe problems.
OriginRecent studies have estimated that CTVT arose in one of the first dog populations to inhabit North America some 8,000 years ago. It is believed to stem from a macrophage or another kind of immune cell.DFT1 is thought to have originated in a devil that lived some 20 years ago. DFT2 is thought to be much younger than DFT1, but its exact age is unknown. Researchers think the cancers stem from Schwann cells, which protect neurons and help repair them after damage.Not much is known about the origin of disseminated neoplasia in clams, but researchers have suggested that the malignant cells stem from hemocyte cells that reside in the animals’ circulatory system.
Mode of transmission
©istock.com, Oleg_0
CTVT spreads through sexual intercourse.
©istock.com, Parfenov yurii
Both devil cancers are thought to spread when the animals bite each other, for instance, during the mating season.
@istock.com, hailshadow
Dead individuals are thought to release cancer cells that drift through seawater, infecting other individuals that likely pick them up through filter feeding.

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.

© MESA SCHUMACHER



1Cancer cells that form DFT1 tumors  overproduce transmembrane enzymes known as ERBB receptors.
2When 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.
3In the nucleus, Stat3 drives the production of genes such as MMP2 that are known to trigger metastasis in humans.
4Stat3 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.
© MESA SCHUMACHER
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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