Tumor Viruses: Study with Video Lessons, Practice Problems & Examples

Tumor viruses, or oncogenic viruses, can lead to cancer by integrating their genome into host cells or by producing proteins that disrupt normal cell functions. For example, human papillomavirus (HPV) produces E6 and E7 proteins that inhibit tumor suppressors like p53 and Rb, promoting cancer development. Other viruses, such as hepatitis B and C, are linked to liver cancer, while HIV is associated with Kaposi sarcoma due to its immunosuppressive effects. Understanding these mechanisms is crucial for cancer prevention and treatment strategies.

Tumor Viruses

Hi. In this video, I'm going to be talking about tumor viruses. There are many types of tumor viruses, which we also call oncogenic viruses. These are viruses that can cause cancer. They don't always do this, but they have the potential. Somehow, these viruses can do this in a couple of different ways. One way is by actually having the genome integrate into the host cells. The viral genome comes in, infects, and integrates somewhere into the host cell genome, which can be harmful. For example, the virus might integrate its entire genome into something like RAS, which we've discussed, the GTPase. If it's in RAS, that's obviously going to disrupt function, and we know RAS is mutated in many different cancers. This could be extremely harmful and could easily lead to cancer. But other viruses don't necessarily have to integrate to cause or lead to cancer. They could just have proteins that interfere with normal cell functions, leading to cancer. For instance, if you have a protein that blocks a tumor suppressor's activity, you're going to lose the function of the tumor suppressor, resulting in the loss of activity, which can lead to cancer. You can have proteins that interfere with cell cycle control. For example, increased cell cycle control, beneficial for viral replication, or decreased apoptosis, beneficial for the virus. You can imagine how the virus has proteins that decrease apoptosis and increase the cell cycle, allowing the virus to replicate without dying. Other viruses have proteins that can take over the cell's replicative machinery, so cell replication or DNA replication, in order to replicate the genome. So, the virus infects the cell so that it can replicate itself. It has to take advantage of the cell's mechanisms to replicate, meaning these proteins are going to take over all of that machinery and say, "No, I'm going to use it for myself and I'm going to overuse it and overexpress it, so that I can generate a lot of myself," allowing the virus to produce a lot of itself. Here are some example viruses; this is not an exhaustive list, but these are some that you probably should be familiar with. Examples include hepatitis B and C, associated with liver cancer; papillomaviruses, with the most familiar being human papillomavirus, which there's a vaccine against. This can cause cervical cancer as well as head, neck, and throat cancer in both men and women, so it's not just a female cancer. You also have the herpesvirus, which can cause an interesting cancer called Kaposi sarcoma. This is most often associated with retroviruses, not because the retrovirus actually causes Kaposi sarcoma, but because the retrovirus leads to AIDS, a syndrome that significantly decreases your immune system's ability to fight off infections, and then you can get these herpesvirus infections that lead to this cancer. We often say HIV is associated with Kaposi sarcoma, but it's because it lowers your immune system, causing these other viral infections. I mentioned there were some proteins that could interfere with cell functions. I want to introduce a couple. In human papillomavirus, you have a protein called E6 and a protein called E7. When human papillomavirus infects a cell, these proteins are expressed and then interfere with the cell's functions. For instance, the E6 protein can bind the transcription factor P53, a tumor suppressor, leading to its destruction. Destroying a tumor suppressor obviously can promote cancer. Then you have the E7 protein, which can bind to the Retinoblastoma Tumor Suppressor. When it binds to it, it does not destroy it, but it does inhibit its function. Again, inhibiting this tumor suppressor, losing that activity, results in a high probability that a cancer cell or tumor cell will form. So that's an overview of tumor viruses or oncogenic viruses. With that, let's now move on.

Many viruses can cause cancer.

• A. True
• B. False

Which of the following viruses can cause head, neck, and throat cancer?

• A. Hepatitis B
• B. Human Papillomavirus
• C. Herpesvirus
• D. Retroviruses

Here’s what students ask on this topic:

What are tumor viruses and how do they cause cancer?

Tumor viruses, also known as oncogenic viruses, are viruses that can lead to cancer. They cause cancer through two main mechanisms: integrating their genome into the host cell's DNA or producing proteins that disrupt normal cell functions. When the viral genome integrates into the host DNA, it can disrupt genes that regulate cell growth, such as RAS. Alternatively, viral proteins can interfere with tumor suppressors like p53 and Rb, leading to uncontrolled cell division and cancer. Examples include human papillomavirus (HPV), which produces E6 and E7 proteins that inhibit tumor suppressors, and hepatitis B and C viruses, which are linked to liver cancer.

How does human papillomavirus (HPV) contribute to cancer development?

Human papillomavirus (HPV) contributes to cancer development primarily through its E6 and E7 proteins. The E6 protein binds to the tumor suppressor p53, leading to its degradation. This loss of p53 function prevents the cell from undergoing apoptosis in response to DNA damage. The E7 protein binds to the retinoblastoma protein (Rb), inhibiting its function. Rb normally controls cell cycle progression, so its inhibition leads to uncontrolled cell division. These disruptions in cell cycle regulation and apoptosis contribute to the development of cancers such as cervical cancer, as well as head, neck, and throat cancers.

What types of cancers are associated with hepatitis B and C viruses?

Hepatitis B and C viruses are primarily associated with liver cancer, also known as hepatocellular carcinoma. Chronic infection with these viruses leads to ongoing liver inflammation and damage, which increases the risk of developing liver cancer. The viral proteins can also interfere with normal cell functions, promoting cancerous changes. Hepatitis B virus (HBV) can integrate its DNA into the host genome, further increasing the risk of cancer. Preventing and treating these viral infections is crucial for reducing the incidence of liver cancer.

How does HIV infection lead to Kaposi sarcoma?

HIV infection leads to Kaposi sarcoma indirectly by severely weakening the immune system. HIV targets and destroys CD4+ T cells, which are crucial for immune defense. This immunosuppression makes the body more susceptible to infections, including those caused by human herpesvirus 8 (HHV-8), the direct cause of Kaposi sarcoma. HHV-8 infects endothelial cells and promotes their uncontrolled growth, leading to the characteristic lesions of Kaposi sarcoma. Therefore, while HIV itself does not directly cause Kaposi sarcoma, it creates an environment where HHV-8 can thrive and induce cancer.

What role do viral proteins play in the development of cancer?

Viral proteins play a crucial role in the development of cancer by interfering with normal cellular functions. For example, the E6 and E7 proteins of human papillomavirus (HPV) inhibit tumor suppressors p53 and Rb, respectively. This inhibition prevents the cell from undergoing apoptosis and allows uncontrolled cell division. Other viral proteins can interfere with cell cycle control, increase cell proliferation, and decrease apoptosis, all of which contribute to cancer development. By hijacking the host cell's machinery, these proteins create an environment conducive to viral replication and tumor formation.