For decades, popular belief suggested that sharks possessed an innate immunity to cancer. This notion sparked interest in their potential to offer insights into cancer prevention or treatment. While the concept of a cancer-free shark is compelling, scientific inquiry has sought to understand the true relationship between sharks and cancer.
The Reality of Shark Cancer
Despite persistent myths, scientific observations confirm that sharks do, in fact, develop cancer. Tumors and cancerous growths have been documented in various shark species, appearing in different organs such as the kidneys, liver, and cartilage. The misconception of shark immunity likely stemmed from the historically low incidence of observed tumors compared to other animal species. However, the rarity of detected cases does not equate to complete immunity. Researchers acknowledge that while cancer is less common in sharks than in many other vertebrates, they are not impervious to the disease.
Distinct Biological Features
The seemingly lower incidence of cancer in sharks has prompted scientists to investigate their unique biological characteristics. Sharks possess a robust and highly evolved immune system, refined over 400 million years of evolution. This sophisticated defense mechanism may contribute to their ability to resist certain diseases, including cancerous malignancies. Genetic studies have revealed unique modifications in shark immunity genes, such as legumain and Bag1, which are linked to cancer in humans but appear to have protective functions in sharks.
Sharks also have a cartilaginous skeleton, unlike the bony skeletons of most vertebrates. Cartilage itself is naturally avascular, meaning it lacks blood vessels, and contains compounds that can inhibit the formation of new blood vessels. Since tumors require a blood supply to grow beyond a certain size, the presence of these anti-angiogenic factors in their cartilage might offer a degree of protection against tumor development. Sharks also exhibit a remarkably low rate of genetic mutation between generations, the lowest recorded for any vertebrate species. This reduced mutation rate could inherently lower the chances of random genetic errors that lead to cancer.
Exploring Anti-Cancer Mechanisms
Research has focused on specific compounds found in sharks that might contribute to their cancer resistance. One notable substance is squalamine, a naturally occurring steroid-polyamine conjugate initially isolated from the liver of the dogfish shark. Squalamine has shown anti-angiogenic properties by inhibiting the growth of new blood vessels that tumors need to thrive. It achieves this by affecting endothelial cells, blocking their proliferation and migration.
Shark cartilage has also been extensively studied as a source of anti-angiogenic compounds. While crude shark cartilage extracts have not proven effective, purified factors from cartilage have demonstrated the ability to inhibit angiogenesis in laboratory settings. These compounds, including certain glycosaminoglycans like chondroitin sulfate, are thought to disrupt tumor growth by preventing the formation of new blood vessels and potentially stimulating immune responses. Some studies have explored peptides from shark cartilage and proteins from the shark’s epigonal organ, which is involved in immune cell production, for their potential to induce programmed cell death (apoptosis) in cancer cells.
Insights for Human Medicine
Despite the scientific interest in shark biology, no shark-derived compounds have been proven to cure or effectively treat human cancer. Clinical trials involving shark cartilage extracts have largely yielded unsatisfactory results, failing to demonstrate significant benefits for patients. The promotion of crude shark cartilage as a cancer cure has been widely refuted by scientific evidence. This has led to concerns about patients abandoning proven medical treatments in favor of unverified remedies.
Consuming shark products, including cartilage supplements, is ineffective and can pose health risks due to contaminants like mercury. The real value of studying sharks lies not in direct therapeutic application of their body parts, but in understanding their unique biological and genetic mechanisms. Researchers continue to investigate these pathways, hoping to identify novel targets or strategies for developing new cancer therapies. This research emphasizes the importance of preserving shark populations, as they may hold fundamental biological secrets that could inform future medical advancements.