Tumor Viruses: Videos & Practice Problems

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

Tumor viruses, also known as oncogenic viruses, are a group of viruses that have the potential to cause cancer in host organisms. These viruses can induce cancer through various mechanisms, primarily by integrating their viral genome into the host cell's DNA or by producing proteins that disrupt normal cellular functions.

One significant way tumor viruses can lead to cancer is through the integration of their genome into critical areas of the host genome. For example, if a viral genome integrates into the RAS gene, which is known to be mutated in many cancers, it can disrupt the normal function of this important GTPase, potentially leading to uncontrolled cell growth and cancer development.

Alternatively, some tumor viruses do not integrate their genome but instead produce proteins that interfere with the host's cellular mechanisms. For instance, viral proteins may inhibit the activity of tumor suppressors, which are crucial for regulating cell growth and preventing tumor formation. If a virus produces a protein that blocks the function of a tumor suppressor, the loss of this regulatory activity can increase the risk of cancer. Additionally, these viral proteins can manipulate the cell cycle, promoting cell division or reducing apoptosis (programmed cell death), which benefits viral replication and survival.

Several well-known viruses are associated with specific types of cancer. Hepatitis B and C viruses are linked to liver cancer, while human papillomavirus (HPV) is known to cause cervical cancer, as well as head, neck, and throat cancers in both men and women. The herpes virus is associated with Kaposi sarcoma, particularly in individuals with compromised immune systems, such as those with HIV/AIDS, which increases susceptibility to opportunistic infections.

Focusing on HPV, two critical proteins, E6 and E7, play a significant role in cancer development. The E6 protein binds to the tumor suppressor protein p53, leading to its degradation, which removes a vital checkpoint in the cell cycle and promotes uncontrolled cell division. Meanwhile, the E7 protein binds to the retinoblastoma (Rb) tumor suppressor, inhibiting its function without destroying it. This inhibition further disrupts cell cycle regulation, increasing the likelihood of tumor formation.

In summary, tumor viruses can lead to cancer through genomic integration or by producing proteins that disrupt normal cellular functions, highlighting the complex interplay between viral infections and cancer development.

Many viruses can cause cancer.

• A. True
• B. False

Answer: (A)

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

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

Answer: (B)

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.