If all cells had an equal chance of developing cancer, it is reasonable to think that living organisms with more cells and larger mass would be more likely to develop cancer, compared to organisms with less cells and less mass. Yet epidemiologist Richard Peto had observed that cancer incicidence is not correlated with an organism's body mass. Mice are far more prone to cancer than elephants and whales. This apparent contradition is referred to as Peto's Paradox.
Retroviruses are known to cause cancer in animals such as chickens and mice, through an array of tumour-promoting mechanisms. Such mechanisms include causing genetic instability from integrating into the host genome as part of the retroviral life cycle. Could Peto's Paradox be explained by looking at endogenous retroviruses (ERVs) in genomic sequences of animals? Using a combination of paleovirology/bioinformatics methods and mathematical modeling, Katzourakis and colleagues found that the genomes of smaller mammals have far more ERVs than the genomes of larger mammals. For example, the mouse genome is loaded with over 3000 ERVs, whereas the dolphin genome harbours only 55 ERVs. Humans also have relatively few ERVs at 348 total found. The authors suspect that larger-bodied mammals may have immunological mechanisms in place to counter-act the cancer-promoting ability of replicating retroviruses, whereas the smaller mammals do not. This selective pressure against the integration of retroviruses into the genomes of large-bodied animals may have resulted in the underrepresentation of ERVs seen today. And perhaps, intriguingly, this may explain lower cancer incidence as well.
Read the paper here: Larger Mammalian Body Size Leads to Lower Retroviral Activity
Oxford University press release: Viral relics show cancer's 'footprint' on our evolution