Published in Proceedings of the Royal Society B, a new study on September 18th, 2024 was the first experimental examination of evolutionary change in transmissible tumors within individual Hydra oligactis, a fresh water organism. The study yields essential information about how cancers that spread between animals evolves, for which only a handful of examples had been known until now. Knowing how such a process works in an organism like Hydra could illuminate the way cancer spreads in other creatures, including humans.
What Are Transmissible Tumors?
In contrast to most cancers, which arise in a single individual, transmissible tumors can transmit from one individual to another. Such cancers are very rare, with only 14 such cases that occur in nature, namely the facial tumor disease from the Tasmanian devil and a type of clam cancer. Just like infections, they spread through populations through a “pathogen-like” mode of transmission. This is the first study that researchers could observe experimental evolution of transmissible tumors in Hydra, providing new information about how such cancers emerge and spread.
The Hydra Model: Simple, Yet Powerful
Hydra oligactis is a freshwater organism which has large regenerative capacities and it can be considered as an excellent model for biological research. Laboratory cultivation of hydras can lead to the development of spontaneous tumors, especially when they are excessively force fed. These tumors form from an overgrowth of stem cells, similar to how cancers arise in more complex animals. Hydras reproduce by asexual budding, a process in which the new individual grows out of the side of the body wall. It was this process that made tracking how tumors spread across generations another essential element studied by the researchers.
Tracking Tumor Transmission Across Generations
The study identified individual hydras that had spontaneously developed tumors and then tracked how those tumors spread from parent to offspring. Transmission of tumors was more than doubled by the 4th generation (from 35 to 84%) This rapid growth shows that transmissible tumors can evolve quickly over short timescales if the conditions are right.
The researchers also observed that once the tumors reached this biological point of visibility, or “pathological,” transmission among them grew exponentially. Moreover, the pathological phase of these hydra would transplant tumors into their offspring at an extremely high rate compared to those that were in the early stages of tumor development. Most notable of all, the rate of tumor transmission was significantly lower when hydras were provided with less food — a factor that reminds us about the environmental flexibility of cancer metastasis.
Life-History Changes in Tumor-Bearing Hydra
The scientists noted dramatic and unusual alterations in the life-cycle characteristics of the cancerous hydras. At first the hydras generated reproductive buds at a greater rate than controls. Despite this their rate of reproduction was severely reduced once the tumours had established themselves. This decrease in size did not translate to decreased survival of the host, leading to the idea that tumor growth and host life-history traits may interact in complex ways.
Another fascinating observation was the behavior of hydras born from tumor-bearing parents but without tumors themselves. These offspring, referred to as “tumor-free hydras from tumoral parents” (TFTP), exhibited increased budding early on, much like their tumor-bearing parents, but without the subsequent decline in reproduction.
Could Transmissible Cancer Occur in Humans?
While transmissible cancers have been reported in some species such as Hydra, Tasmanian devils and clams, contagious cancer is an extremely rare event in humans. Cancer is not generally considered to be transmissible between humans — few exceptions are known. For example, if a donor has undetected cancer, his or her infected cancer cells can be passed onto the recipient during an organ transplant. There have been rare cases of a mother with cancer spreading cancer cells to her fetus. Some viruses, such as HPV or Hepatitis B, can also up cancer risk in a few ways, but the transmission is always of the virus and never of the cancer itself. The likelihood of human-to-human transmission incidents are very low, and these require highly specific circumstances so the direct transfer of cancer from person to person is quite diffrent to what you see in something like a transmissible tumor within animals such as hydras.
Implications for Cancer Evolution
The idea that transmissible cancers are rare because they require to be able to evolve is not supported by this study. Rather, the research argues that transmissibility is able to evolve rapidly but succeed only under very specific environmental conditions such as food availability or population density.
The discovery that a transmissible tumor can acquire novel mutations and spread within an animal indicates that such cancers could potentially arise in other species, underscoring the importance of developing a broader insight into the ecological characteristics that might enable these cancer clones to evolve. Understanding how and why these trends have shifted over the years can inform conservation strategies in ways that could translate to human well-being as well.
Source: https://royalsocietypublishing.org/doi/10.1098/rspb.2024.1636