[LUM#11] Viruses that cause cancer

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

Two million. That is the number of cancer cases caused each year by... viruses. And which 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 tobacco, diet, sun exposure, alcohol, and genetic inheritance immediately come to mind, we often overlook another formidable cause: viruses. These silent carcinogens can often be prevented.

The link between viral infections and cancer was first identified in the 19th century. "In the 1840s, an Italian doctor noticed that cervical cancer, which was very prevalent among prostitutes, was almost non-existent among nuns, " explains Ignacio Bravo. This distribution is similar to that of sexually transmitted infections, suggesting that this cancer may be linked to an infectious agent...

Papillomavirus

It was not until 1973 that a German virologist, Harald zur Hausen, identified the virus responsible for cervical cancer: the papillomavirus. "Papillomaviruses, actually," Samuel Alizon points out, "because we have since identified around twenty strains of human papillomavirus, or HPV, responsible for the development of cervical cancer."

Viruses transmitted during sexual intercourse that are very widespread. "It is estimated that more than 80% of adults will be infected with oncogenic human papillomaviruses during their sexual lives, " says the specialist. 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 cancers

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 the development of lymphoma in 1 in 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 work by international collaborators aimed at better understanding cancers of infectious origin. "These are the ones we are best able to prevent because we can use traditional methods of infectious disease control such as vaccination and screening, " say the researchers.

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, " say the researchers. 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 cancer and condylomas in men, and on the other hand, men can be carriers of the virus without being sick and transmit it to their partners during sexual intercourse."

This is a real public health issue, but also a research issue. If vaccination coverage were to increase significantly, the prevalence of HPV16 and 18, which are targeted by the vaccine, would decrease sharply and could even be eradicated. "The vaccination context is exerting new selection pressure on HPV, and the entire papillomavirus ecosystem is being impacted without us knowing the potential consequences of these upheavals, " explain the specialists. "It is therefore essential to continue seeking to better understand these viruses, so that we can always stay one step ahead."

From viruses to cancer

How can a viral infection cause cancer? "Cancer always starts with an abnormal cell, either linked to an internal cause (a mutation passed on in the genetic makeup) or an external cause (environmental factors or infections). These abnormal cells accumulate other mutations, and cancer develops," explains Ignacio Bravo. Viruses can cause chronic infections that trigger the immune system. This immune response 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."