A blog about microbes, diseases and biomedical research advances. Posts in English or German.
A single cell, originally a part of one animal’s body, goes awry and becomes an infectious agent for the whole species – Does this unique feature make contagious cancers new organisms, equal to parasitic bacteria or protists?
This week, the discovery of a contagious leukemic cancer in clams was announced in the journal Cell – making it the third example of direct transmission of cancer cells between animals (the others being a sexually transmitted cancer in dogs and a facial cancer in Tasmanian devils, that spreads through bites [paywalled]). These cancerous cells have a terrifying characteristic: They can outlive their original host by colonising new hosts – similar to parasitic, disease-causing organisms. This unique feature raises an interesting question: Should we consider these cancers as new life forms that evolved from their host’s bodies to become distinct organisms?
To answer this question, we first have to agree on what we understand as life – and that proves difficult. ‘Life’ is a concept, and even today – after centuries of discussion among biologists and philosophers – its many definitions are not undisputed. In biology, we commonly consider a living thing to display a set of criteria; it uses energy to maintain a state of homeostasis and organisation, and additionally it can grow and reproduce, while being able to adapt and react to stimuli.
All of these criteria apply to the contagious cancer cells; like other cells, they are capable of regulating internal processes and responding to external signals. They convert energy through their metabolism, and they generate more biomass and offspring through cell division. And they evolve – even though all clam leukaemia cells seem to derive from one common ancestor cell, they have diverged into two distinct molecular subgroups on the US and the Canadian Atlantic coast.
Realising that these cancers can be categorised as life forms might initially baffle us. It feels counter-intuitive: We often think of evolution as a process that leads to more complex, multicellular organisms, and not a process that gives rise to a cancer cell that is able to break free from a whole animal. The ‘evolution from simple to complex’ view is misleading; Darwinian evolution is based on natural selection and the survival of the fittest. These cancer cells have achieved just that; through mutations in their DNA they have gained a degree of independence that allows them to survive even when their host dies. Based on what we know, they probably still need hosts to live in the long run – but so do many other parasitic life forms, such as the malaria-causing protist Plasmodium or bacteria of the genus Chlamydia. We categorize these parasites as living beings; accordingly, we could describe contagious cancer cells as organisms.
The origin of the first clam leukaemia cell is still a mystery. Some clues point towards viral mechanism: The clam cancer cells possess a great number (about 30 times more copies than normal clam tissue cells) of certain retrovirus-like DNA sequences termed Steamer. We don’t know if these sequences drive the cancer development or if they are unrelated to it, but we do know that several viruses can cause cancer in animals, including in humans (e.g. HPV is responsible for most human cervical cancers).
Together with this clam leukaemia report, we now have evidence for transmissible cancers in at least three organisms. But could a similar event also happen in humans? It certainly seems possible. The self-foreign recognitions system of dogs and Tasmanian devils, based on MHC molecules, is similar to ours – and their contagious cancers evolved to override these defence mechanisms. Evolution of cancerous organisms might occur any time: The contagious cancer in Tasmanian devils came up only recently – probably less than 30 years ago – while the dog cancer dates back more than 10 000 years. Scary as the thought may be, we will probably encounter more of these cancer-derived organisms. And they could already exist or arise in a wide range of animals – even humans.