From California, a wildlife model for carcinogenesis

Cancer is too complex to be studied only in laboratory animals, in genetically identical strains and under controlled conditions. Real life is not a laboratory; in real life cancer arises from complex interactions between multiple genes and environmental factors that cannot be easily replicated. One solution is to extrapolate precious findings on carcinogenesis that can be translated to humans from naturally-occurring cancers in non-laboratory, wild animals, which spend the whole life in the world and not inside a laboratory cage. Today’s special patient is a bulky, marine mammal from California: the sea lion. What can we learn from it?

A sea lion population in California, on the USA west coast, has a high prevalence of urogenital carcinoma. Sea lions are more “accessible” compared to other marine mammals, as they spend some periods on land. Of course, cancer can be studied in a variety of non-laboratory animals that are closer to us and easier to monitor over time, for example our pet friends, like domestic dogs and cats. Dog cancer in particular shares interesting genetic and molecular features with human cancer. Nonetheless, researchers are particularly interested in sea lions and not only because they like challenges: wild animals are probably the most informative models for carcinogenesis, because they are naturally exposed to a mix of contaminants and dietary factors that domestic animals are not.

What are the causes of urogenital carcinoma in sea lions?

Urogenital carcinoma affects adults and sub-adults of both sexes, with a mean age of approximately 8 years old. The cancer originates in the genital tract, but metastasizes in other sites and organs, including lymph nodes, kidney, urinary bladder, liver, lungs and spleen. As it often happens, there is not only one cause, but many. Variation in hormone receptor expression and other genetic factors are probably associated with carcinogenesis, but there are also external and environmental con-causes.

The exposure to environmental contaminants, like polycyclic aromatic hydrocarbon (PAH) and organochlorine (OC) groups of compounds, has been associated with the aetiology of many cancers, both in animals and humans. Such ubiquitous and persistent pollutants end up in the sea because of their large use in chemical industries or as components of pesticide. These compounds were found in high levels around California coasts and in sea lions as well: due to their lipophilic nature, they are also transferred to the offspring during lactation.

The disease has also been associated with viral infection. A herpes virus was found to be present in sea lions with cancer to a greater extent with respect to healthy ones and with the highest prevalence in the genital tract, suggesting that it is sexually transmitted and possibly correlates with the disease.

What do sea lions and women have in common?  

Apparently…nothing! However, when they get cancer, some similarities emerge. Urogenital carcinoma has a lot in common with cervical cancer in women, especially age of recurrence (in both cases this is not a disease typical of old age, but rather of adult life) and association with viral infection (HPV in women). Furthermore, there are histological and molecular features shared between the two cancers.

This is the point of the whole research: extrapolating findings that are useful not only for the safeguard of wild and often endangered population, but also for humans. Comparative oncology is a field of increasing interest: Vitares supports research projects to treat cancer in wildlife models, with the awareness that it is an exciting opportunity to create new collaborations between human and veterinary oncologists.

Reference:
Helen, M., et al. (2015). Common cancer in a wild animal: the California sea lion (Zalophus californianus) as an emerging model for carcinogenesis.