- Viruses are microscopic intracellular pathogens that rely on living host cells to replicate and employ the cellular machinery to generate new viral particles.
- The process of viral replication can lead to alterations in the host cell's genes, potentially contributing to the development of cancer.
- Oncoviruses are a category of viruses capable of inducing cancer.
- Approximately 20% of all human cancer cases are attributed to viral infections.
- While numerous viruses can induce tumors in animals, there are currently only seven viruses known to be associated with human cancers and classified as oncogenic viruses.
Table of Contents
Introduction
- These viruses encompass the hepatitis B virus (HBV), hepatitis C virus (HCV), human papillomavirus (HPV), Epstein-Barr virus (EBV), human herpesvirus 8 (HHV8), Merkel cell polyomavirus (MCPyV), and HTLV-1.
- HBV and HCV are responsible for roughly 80% of hepatocellular carcinoma (HCC), the most prevalent form of liver cancer.
- High-risk HPV strains are the primary culprits behind cervical cancer, as well as other anogenital neoplasms and a significant portion of head and neck tumors.
- EBV is linked to nasopharyngeal carcinoma, Hodgkin's lymphoma, and Burkitt's lymphoma.
- HHV8, also known as Kaposi's sarcoma-associated herpesvirus (KSHV), is the cause of Kaposi's sarcoma, which is frequently observed in individuals with acquired immunodeficiency syndrome (AIDS).
- MCPyV is responsible for Merkel cell carcinoma, while HTLV-1 is the agent responsible for adult T-cell lymphoma.
Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV)
- HBV and HCV have the potential to initiate liver infections, which in certain cases may progress to liver cancer.
- These viruses are typically contracted through the sharing of needles for drug injection, engaging in unprotected sexual activity, or receiving a transfusion involving contaminated blood.
- Medical professionals employ medication to treat HBV and HCV infections.
- HCV infections can often be successfully cleared with several months of treatment.
- While medication doesn't provide a cure for HBV, it can significantly reduce the risk of liver damage and the development of liver cancer.
- A vaccine is available for the prevention of HBV, but there is currently no vaccine for HCV. Those at a heightened risk of HBV transmission, such as individuals with HIV, individuals who use illicit drugs by injection, or healthcare workers, should consider getting vaccinated.
Kaposi Sarcoma-Associated Herpesvirus (KSHV)
- KSHV, a herpes virus, has the potential to induce Kaposi sarcoma, a cancer affecting blood vessels, as well as two forms of lymphoma.
- The risk of developing cancer due to KSHV is heightened in individuals with compromised immune systems resulting from factors such as organ transplantation, undergoing chemotherapy, or having AIDS.
- Transmission of the virus can occur through sexual contact, as well as via blood and saliva.
Merkel Cell Polyomavirus (MCV)
- MCV is a prevalent virus that can infect the skin. Typically, it remains asymptomatic and does not result in cancer.
- However, in certain individuals, MCV can trigger a rare form of skin cancer known as Merkel cell carcinoma.
Human Papillomavirus (HPV)
- HPV comprises a collection of over 200 viruses, with approximately a dozen known to have the potential to cause cancer.
- The transmission of HPV primarily occurs through vaginal or anal sexual contact.
- In many cases, HPV infections spontaneously clear up and do not result in any health issues. However, some individuals may remain infected.
- Persistent infection with cancer-causing HPV strains can lead to various cancers, including those of the cervix, vulva, vagina, penis, anus, tonsils, or tongue.
- HPV vaccines are available to prevent HPV infection. Health authorities recommend these vaccines for young women up to the age of 26 and young men up to the age of 21.
Human T-Cell Lymphotropic Virus Type 1 (HTLV-1)
- HTLV-1 targets T cells, a subset of white blood cells, and has the potential to induce leukemia and lymphoma.
- HTLV-1 can be transmitted through various routes, such as from mother to child during childbirth or breastfeeding, sharing needles with infected individuals, organ transplantation, and engaging in unprotected sexual activity.
- Approximately 2% to 5% of individuals carrying the virus may develop adult T-cell leukemia or experience other related health issues.
- The reasons why some individuals develop leukemia while others do not remain unclear, and the symptoms and disease progression can vary widely from person to person.
- Currently, there is no known cure or specific treatment for HTLV-1, making it a lifelong condition. However, regular medical checkups can help reduce the risk of developing cancer.
Epstein-Barr Virus (EBV)
- EBV is a prevalent virus, and the majority of individuals experience infection at some stage in their lives. Typically, those infected with EBV remain in good health without experiencing symptoms.
- However, in some cases, EBV can lead to mononucleosis and more severe illnesses, including viral meningitis and pneumonia.
EBV is also associated with the following cancers:
- EBV has been linked to various types of cancer, including Burkitt’s lymphoma, nasopharyngeal carcinoma (a cancer affecting the upper throat), Hodgkin’s and non-Hodgkin’s lymphoma, T-cell lymphomas, post-transplant lymphoproliferative disorder (characterized by an excessive number of white blood cells), and leiomyosarcoma (a cancer occurring in soft tissue).
- While there is currently no vaccine available to prevent EBV infection, you can take measures to protect yourself by refraining from activities like kissing or sharing drinks, food, or personal items with individuals who carry the virus.
- If you contract EBV, there is no specific treatment. However, you can alleviate symptoms by staying well-hydrated, getting sufficient rest, and taking over-the-counter medications to manage pain and fever.
Mechanisms of Viral Oncogenesis
The molecular processes involved in viral oncogenesis are intricate and may encompass:
- The initiation of chronic inflammation.
- The disruption of the host's genetic and epigenetic stability and balance.
- Interference with the cellular DNA repair mechanisms, resulting in instability of the genome.
- Dysregulation of the cell cycle.
- The ability of oncogenic DNA viruses to integrate their genetic material into the host cell's chromosomes, causing genetic abnormalities.
- The activation of cellular signaling pathways by viral "oncoproteins," which can modify the expression of cellular genes and microRNAs at the transcriptional or post-transcriptional levels. These oncoproteins can also destabilize or deactivate proteins responsible for controlling cell polarity, signal transduction, immune responses, and apoptosis.
- Genetic and epigenetic changes induced by infection and replication of oncogenic viruses may lead to the emergence and proliferation of cancer stem cells. These cells play a pivotal role in the initiation, advancement, metastasis, recurrence, and resistance to chemotherapy in cancers.
Understanding the significance and the underlying molecular mechanisms of specific cellular genes and signaling pathways in viral oncogenesis is currently a focal point of extensive research endeavors.
