[LUM#11] Viruses That Cause Cancer

[LUM Magazine, Podcast] - Published on January 20, 2020 in Health, Social Issues & Disabilities, Science and Society

Two million. That’s the number of cancer cases caused each year by… viruses. And these cases could therefore be prevented by limiting the spread of these infectious agents. Samuel Alizon and Ignacio Bravo, researchers at the Infectious Diseases and Vectors: Ecology, Genetics, Evolution, and Control (Mivegec) laboratory, explain.

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What contributes to the development of cancer? While we tend to think of smoking, diet, sun exposure, alcohol, or genetic predisposition, we often overlook a particularly dangerous cause: viruses. These are silent carcinogens, yet it is often possible to protect ourselves against them.

The link between viral infections and cancer was first identified as early as the 19th century. “In the 1840s, an Italian doctor noticed that cervical cancer, which was very common among prostitutes, was almost nonexistent among nuns, ” explains Ignacio Bravo. A distribution pattern similar to that of sexually transmitted infections suggests that this cancer might be linked to an infectious agent…

Human papillomavirus

It wasn’t until 1973 that a German virologist, Harald zur Hausen, identified the virus responsible for cervical cancer: the papillomavirus. “Papillomaviruses, actually,” clarifies Samuel Alizon, “because we have since identified about twenty strains of human papillomavirus, or HPV, responsible for the development of cervical cancer.”

Viruses transmitted through sexual contact that are very common. “It is estimated that more than 80% of adults will be infected with oncogenic human papillomaviruses during their sexually active lives,” the specialist explains. “More than 90% of these infections will be harmless, but unfortunately, some will lead to the development of cancer.” Thousands of new cases of cervical cancer are diagnosed each year, including 3,000 in France.

1 in 10 cases of cancer

Human papillomaviruses, which are responsible for 100% of cervical cancers, can also cause cancers of the anus, vulva, vagina, and throat. And they are not the only ones capable of causing cancer: the Epstein-Barr virus, which causes mononucleosis, leads to lymphoma in 1 out of every 10,000 cases. Hepatitis B and hepatitis C viruses can cause liver cancer. “In total, oncoviruses are responsible for one in ten cancers worldwide,” explains Samuel Alizon. “They represent a major public health challenge due to their enormous prevalence.”

To shed new light on these viruses, Samuel Alizon and Ignacio Bravo are coordinating multidisciplinary research involving international collaborators aimed at better understanding cancers of infectious origin. “These are the cancers we are best able to prevent because we can rely on traditional methods of controlling infectious diseases, such as vaccination and screening, ” the researchers say.

Public health issue

The HPV vaccine has been available in France since 2006. “There are three vaccines that protect against HPV16 and HPV18, which are the most oncogenic strains,” the researchers explain. Currently, authorities recommend vaccinating young girls before they become sexually active. “For now, vaccination coverage remains very low in France despite these recommendations: only 20% of teenage girls are vaccinated, ” explains Samuel Alizon, who also laments the fact that young boys are not vaccinated as well: “On the one hand, papillomaviruses can also cause cancers and genital warts in men; on the other hand, men can carry the virus without being sick and transmit it to their partners during sexual intercourse.”

This is a genuine public health issue, but also a research challenge. For if vaccination coverage were to increase significantly, the prevalence of HPV types 16 and 18—which the vaccine targets—would drop sharply and they might even be eradicated. “The vaccination landscape is exerting a new selective pressure on HPV, and the entire papillomavirus ecosystem is being impacted, though we do not yet know the potential consequences of these changes, ” explain the specialists. “It is therefore essential to continue striving to better understand these viruses, so that we can always stay one step ahead.”

From Virus to Cancer

How can a viral infection cause cancer? “At the root of cancer, there is always an abnormal cell, caused either by an internal factor (a mutation passed down through the genetic makeup) or an external factor (environmental stressors or infections). These abnormal cells accumulate further mutations, and cancer develops, ” explains Ignacio Bravo. Viruses can cause chronic infections that trigger the immune system. This immune reaction is sometimes accompanied by a significant inflammatory response. “This sustained inflammation is itself a risk factor for cancer because it can lead to genetic mutations that cause cancer.”