Longevity Briefs: Do Turtles Get Cancer?

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

Cancer is the eventual result of random genetic mutations that are accumulated by cells throughout life. It would seem logical that the more cells an organism has and the longer it lives, the more likely cancer is to occur. However, this is not what we observe in reality – a seeming contradiction known as Peto’s Paradox.

Turtles, renowned for their extreme longevity, are a prime example. Despite larger species living well over 100 years, cancer seems to be exceptionally rare in these reptiles. How do they do it, and is there anything we can learn from them to help prevent or treat cancer in humans? In this study, researchers use post-mortem examinations from zoos to estimate cancer rates in turtles and discuss some of the potential mechanisms for cancer resistance.

The discovery:

The researchers analysed 290 post-mortem examinations from 64 turtle species across eight zoos in Europe and the United States. This dataset covered a wide range of turtle sizes from the tiny black-breasted leaf turtle (150 grams) to the Galapagos giant tortoise (over 300 kilograms). In all of these examinations, there was only one documented case of neoplasia (any abnormal tissue growth, whether benign or malignant) and no detected malignancies (cancerous growths). This reinforces existing evidence that cancer rates in turtles are extremely low – about 0.34% vs a roughly 50% lifetime risk in humans.

How do turtles achieve this exceptionally low risk? Genomic analyses of long-lived species like Galapagos giant tortoises have shown that they possess extra copies of key tumour suppressor genes. These are genes that act as ‘breaks’ that kick in to prevent uncontrolled cell division, and thus mutations in these suppressor genes are usually necessary for cancer to occur. Turtles also appear to have more effective DNA repair mechanisms, higher resistance to oxidative stress (damage to cells and DNA caused by reactive oxygen species, a byproduct of metabolism) and less protein dysregulation (improper functioning of proteins).

Another key factor in the longevity of turtles is a low metabolic rate, which may reduce the production of the aforementioned reactive oxygen species. Studies on Galapagos giant tortoise cells also show an enhanced ability to trigger apoptosis (programmed ‘cell suicide’) to clear damaged cells before they can become cancerous. In other words, cancer resistance in turtles isn’t just due to one factor, but a whole suite of protective mechanisms.

The implications:

It isn’t outside the realm of possibility that some of the cancer resistance mechanisms in turtles (or other long-lived cancer-resistant species) could be applied to humans. Unfortunately, this will remain speculative for the foreseeable future – there’s no precedent for ‘porting’ the biochemistry of another species into humans, though we might be able to use knowledge about how cancer resistance works in turtles as a starting point to develop human therapies.