Contagious Cancer in Dogs...

An old 2008 post by Ed Young, scary...:

"The disease in question is called canine transmissible venereal tumour (CTVT), or Sticker’s sarcoma and it’s a case of a cancer cell evolving into a global parasite. 

CTVT is transmitted through sex or close contact between infected dogs. It was first described 130 years ago by a German scientist called Novinski and its origins have been debated ever since. Some scientists suggested that it was caused by a virus, much like human papillomavirus (HPV) causes cervical cancer in humans today. But Robin Weiss and Claudio Murgia from University College London put these theories to rest in 2006 with a definitive study.

They and their colleagues analysed tumour samples from 40 dogs across five continents. Despite their disparate origins, all these samples shared identical and distinctive genetic markers that uninfected tissues from the same dogs did not. The explanation was clear – these cancers had not developed in the usual way from the cells of the host animals. The cancer cells themselves were spreading from dog to dog.

These rogue cells have become parasites in their own right, evolving from a single ancestor into a dynasty that has colonised the globe aboard canine vessels. How this process began is still a mystery, but Weiss’s analysis provides some hints as to where and when.

The original cancer cell must have developed in either a wolf or an old Asian dog lineage, such as a Husky. It evolved anywhere between 200 and 2500 years ago and may well have been around for even longer. In fact, the CTVT cancer cell is very likely to be the oldest lineage of mammalian cells still in existence. The cells that Weiss is studying today are most probably direct clone descendants of the same cells that Novinski identified 130 years ago – genetically identical great-granddaughters of the original tumour.

When that original cell gained independence, it became truly immortal, long outliving its original body and lasting for centuries. So far, we don’t know of any human cancer cells that have pulled off a similar trick. But Weiss feels that if they did, the best place to look for them would be in people with weaker immune systems including transplant patients and those with HIV.

His group have found evidence that evading the host’s immune defences is a key part of CTVT’s strategy for finding another dog to infect. The cells accomplish this by switching off some key immune system players – a group of genes collectively called dog leukocyte antigens (DLAs). They also secrete a protein called TGF-β1 that strongly blocks any immune responses.

But slipping past immune sentries would do the cells no good if the host died before infecting another dog. Infection requires sex, which may not happen for some time. So CTVT is a merciful parasite. At the start of infection, it grows rapidly, but within 3-9 months, it regresses of its own accord. By never killing its carrier, the cells ensure that they can spread to as many new hosts as possible."