Tumors have been independently identified in 2 different species of Hydra, a freshwater polyp related to the jellyfish and coral family. They arise through the same pathways as human tumors, suggesting that innate errors in the cellular machinery that cause cancer in all animals can never be prevented, researchers report.
"If you want to understand complex disease, you have to understand the evolutionary context," said Thomas Bosch, PhD, an evolutionary biologist at the Zoological Institute, Kiel University, in Germany. "It's important to go back in time and investigate tissues that are at the very beginning of multicellularity," he told Medscape Medical News.
"We are trying to understand the basics of multicellular life by using a model system that is as simple as possible but that offers insight into complex questions, he explained.
The report by Dr. Bosch and his colleagues was published in the June issue of Nature Communications.
Tumor-bearing Hydra polyp (right) next to a healthy animal (Photo/copyright: Klimovich)
Hydra may be a primordial species, but they are deceptively complex. They are comprised of only 3 stem cell lineages, and "are the first animal on earth that are truly multicellular," Dr. Bosch said. "They have a true epithelium, a nerve cell system, an innate immune system, and they are all very similar and are not much less complex than those in vertebrates, including humans," he explained.
The team found that the tumors identified in 2 different Hydra species originate from 1 of the 3 cell lineages. This is a multipotent stem cell lineage that gives rise to somatic cells, as well as gametes, Dr. Bosch noted.
"Our detailed molecular analysis showed that when this stem cell makes a decision to become a female gamete — it could also make a decision to become a male gamete — but if it makes a decision to become a female gamete, the step allowing the female gamete to develop into a mature egg is blocked," Dr. Bosch said. When that happens, precursor cells accumulate, creating tissue bulk.
Normally, apoptosis eliminates the excessive accumulation of cells, but in this tumor tissue, a gene that inhibits apoptosis is upregulated. "So now there are 2 problems," Dr. Bosch explained. "The stem cell cannot make the mature final end product, meaning you have an accumulation of precursor cells, and these precursor cells cannot be removed from the tissue because the normal functioning machinery of cell death is inhibited."
This, in fact, is one of the hallmarks of cancer in humans, he noted. In nearly all human cancers, the machinery leading to programmed cell death has been disarmed, and if cells with the potential to turn cancerous cannot be removed, they propagate, proliferate, and form a tumor.
Why would it be desirable, from an evolutionary perspective, for organisms to be able to develop cancer? That's simply not how Mother Nature works, Dr. Bosch told Medscape Medical News.
"Nature and evolution work by tinkering around and producing forms and shapes and mechanisms, but nothing is foolproof. I think our study has shown that in designing this complex machinery, which cells need in order to make multicellular tissue, mistakes occasionally happen, and they can cause trouble," he said.
"From a medical point of view, our study also shows that there is no way we can eliminate making mistakes. They will always happen; they've been happening over 560 million years during evolution, and will go on. Medicine and cell biology and biochemistry must work very hard to improve our ability to eradicate cancer, but any animal on earth can develop cancer. It's an intrinsic property in complex biochemical machinery, and it is something we can never prevent," Dr. Bosch explained.
Nat Commun. 2014;5:4222. Abstract
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Cite this: Tumors in Primordial Animals Suggest Cancer Can't Be Prevented - Medscape - Aug 26, 2014.
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