Tasmanian Devils May Overcome Transmissible Cancer That Nearly Wiped Them Out

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You might have heard of the plight of the tasmanian devil. These carnivorous marsupials have been afflicted with a transmissible facial tumor a cancer that jumps from one devil to another when they bite each other in the face not as uncommon as you might think and as a result their populations have gone into steep decline despite these extreme population losses. The devils have been holding on and may even be turning a corner andrew store for and colleagues wrote about this potential turnaround in this week science. Hi andrew hi. Let's start with the downfall of the devils. This facial tumor disease. Dft when did arise and what has been. It's trend in it. Spread among the tasmanian devils. The devil facial tumor disease according to our new study originated. Probably in the late. Nineteen eighty s. It was actually discovered in the mid nineteen ninety s in far northeastern. Tasmania where people started discovering devils with large facial lesions that seem to grow. Initially that wasn't much of a concern because they've seen these types of lesions new places back since the nineteen seventies however when animals started dying in large numbers. People really started paying attention that this was some sort of transmissible disease shortly thereafter. A genetic study was done that showed that these lesions were actually a clone oil transmissible cancer. They were genetically close to identical so since its discovery the disease has actually swept across tasmania and sort of an east to west us. Fred and is now just reached. Some of the last uninfected populations on the west coast has media. Cancer is different from pathogens like bacteria or viruses. How's that affected our ability to understand the transmission of this disease in tasmanian devil. One of the big challenges has been its large genome size so studying. The genome of a virus is fairly straightforward because viruses like sars he'll be to only about eighteen to twenty eight thousand base pairs long. However the tasmanian devil genome is three point two billion bases about the size of the human genome so trying to track. The spread of the disease is much more challenging. In terms of computational power and genomic sequencing methods which weren't really available at the scale they are now when the disease was discovered. So in the study that we're talking about today. You apply to technique that has mainly in the past been used for viruses. What did you have to do differently to get this to work for transmissible cancer. We did complete genome sequencing of fifty one tumors than that being in our final analysis finding parts of the genome that are measurably evolving that is evolving at a regular rate or what we would call them. Killer clock is pretty challenging and in the end we screened about eleven thousand genes which took several months on the computer and found twenty that were measurably evolving clock like fashion. And the reason you want to do that is then you can look at how these or changing over time as the disease has spread and this allows you to estimate epidemiological parameters like the transmission rate and the proportion of the population. That gets infected overtime. So this is how you figured out that it came out in the eighties right. The disease likely originated in the eighties which is consistent with its discovery in the bid nineties because when a disease like this i started taking off. In a wildlife population it might be at fairly low prevalence and so people don't really notice it until it really takes off and our epidemiological parameter which is called our savvy which is equivalent to the transmission rate started to peak in our analysis just before the discovery which makes sense because the disease was rapidly increasing or exponentially increasing in the population. Let's talk a little bit about that transmission rate. That's something that you know using this analysis you're able to show it changed over time. Can you describe that trajectory we identify tumor lineages and in wanted around the mid nineteen ninety s in the other it peaked around two thousand and the really encouraging results of the study showed that in both major tumor lineages that seemed to be across. Tasmania now that the transmission rate declined to just about one at present and this indicates that the disease has reached some sort of stable state that is for every devil that's infected only one additional devil is infected and thus the main conclusion of the paper which is the disease is transitioning from an epidemic state in which it's exponentially moving across populations to an end state where it's just kind of at replacement is the kind of analysis you did hear able to tell you what has changed. Is something different about the tumor. Cells are the devils different. What's going on. We did find some mutations that seem to explain variation in transmission rates among the different tumor lineages and these are related to other types of cancer however these are candidates for downstream discovery at this point so they serve as good hypotheses for future research. And what's different about the devils. We have some other studies that show that also the devils seemed to be evolving in response to cancer. We see changes in the devil genome. That are also seem to be associated with possibly disease resistance. We've also seen spontaneous tumor regression in an increasing number of devils wild populations and we show that that might be related to regulatory changes in the in the devil so perhaps some jeans are up or down regulated in response to the tumor and we also found a mutation in the tumor. That seems to cause the tumor to shrink so a mutation actually when turning on a gene and this gene is implicated in human prostate and colon. Cancer the tumor growth actually slows in laboratory studies when we transact wild type tumors. Don't have this gene with the. Does this suggest that the disease will go away or that. It will coexist peacefully with tasmanian. Devils people really thought that tasmanian devils or on their way to extinction. I think this provides us cautious optimism about the future of the tasmanian devil. This transmissible cancer which is nearly one hundred percent lethal has caused a dramatic decline of this species of process. Entire range has me so they are certainly not out of the woods yet. However because this disease is socially transmitted early models predicted disease extinction because even if you have low densities of individuals the disease can still be transmitted because devils congregate and bite one another. Which is the way. The disease is transmitted for purposes of mayday or scavenging food however a growing number of studies from our group has shown through ecological modeling for example devils are predicted to persist on most scenarios. Some of those involve lower population densities than where they were initially but yet the devils will persist and in the subset of those cases. The tumor will also persist and we may see sort of endemic dynamics where there's populations like this study adds to the growing number of studies that the disease itself also seems to be evolving perhaps lower transmission rate because there may be evolutionary pressure on disease to be less lethal to the devils. Are people trying to figure out how to preserve the devils and will that strategy change with this understanding of the switch from epidemic too endemic. We certainly hope that our new study will help. Influence hearth rations strategy so when devil started declining there was captive breeding to maximize genetic variability in captive populations which are maintained in wildlife parks and some zoos and actually an island offshore from tasmania album. Riot island. where there's a free roaming population of not quite captive devils but devils were introduced there. These were referred to as captive insurance operations with the intent that if devils went extinct on the mainland they could be reintroduced or if devil population sizes or inbreeding reached really high levels. They could perhaps be introduced using a technique called genetic rescued increase genetic diversity in the wild populations

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