Cancer Viruses

Hepatitis B Virus Particles (red): The hepatitis B virus has been linked to liver cancer in people with chronic infections. CDC

Researchers have long attempted to elucidate the role that viruses play in causing cancer. Worldwide, cancer viruses are estimated to cause 15 to 20 percent of all cancers in humans. Most viral infections however, do not lead to tumor formation as several factors influence the progression from viral infection to cancer development. Some of these factors include the host's genetic makeup, mutation occurrence, exposure to cancer causing agents, and immune impairment. Viruses typically initiate cancer development by suppressing the host's immune system, causing inflammation over a long period of time, or by altering host genes.

Cancer Cell Properties

Cancer cells have characteristics that differ from normal cells. They all acquire the ability to grow uncontrollably. This can result from having control of their own growth signals, losing sensitivity to anti-growth signals, and losing the ability to undergo apoptosis or programmed cell death. Cancer cells don't experience biological aging and maintain their ability to undergo cell division and growth.

Cancer Virus Classes

 Human papilloma virus. BSIP/UIG/Getty Images

There are two classes of cancer viruses: DNA and RNA viruses. Several viruses have been linked to certain types of cancer in humans. These viruses have varying ways of reprlication and represent several different virus families.

DNA Viruses

• The Epstein-Barr virus has been linked to Burkitt's lymphoma. This virus infects B-cell lymphocytes of the immune system and epithelial cells. Burkitt's lymphoma is a form of lymphatic system cancer that impairs immunity.
• The hepatitis B virus has been linked to liver cancer in people with chronic infections. Chronic infections cause liver damage and disease (cirrhosis), which increases the chances of caner development.
• Human papilloma viruses (HPVs) have been linked to cervical cancer. They also cause warts and benign papillomas. HPVs are commonly transmitted through sexual activity, however most infections don't lead to cancer.
• Human herpes virus-8 has been linked to the development of Kaposi sarcoma. Kaposi sarcoma causes patches of abnormal tissue to develop in various area of the body including under the skin, in the lining of the mouth, nose, and throat or in other organs.
• Merkel cell polyomavirus (MCV) is associated with Merkel-cell carcinoma (MCC). MCC is a rare form of skin cancer that is caused by a mutated form of the MCV found in cancerous Merkel cells. Merkel cells are deep epidermal cells involved in touch sensation.

RNA Viruses

• Human T lymphotrophic virus type 1 (HTLV-I), a retrovirus, has been linked to T-cell leukemia. The virus is found in body fluids and can be transmitted through sexual contact, blood transfusions, and from mother to fetus.
• The hepatitis C virus has been linked to liver cancer in people with chronic infections.

Cancer Viruses and Cell Transformation

Transformation occurs when a virus infects and genetically alters a cell. The infected cell is regulated by the viral genes and has the ability to undergo abnormal new growth. Scientists have been able to discern some commonality among viruses that cause tumors. The tumor viruses change cells by integrating their genetic material with the host cell's DNA. Unlike the integration seen in prophages, this is a permanent insertion in that the genetic material is never removed. The insertion mechanism can differ depending on whether the nucleic acid in the virus is DNA or RNA. In DNA viruses, the genetic material can be directly inserted into the host's DNA. RNA viruses must first transcribe RNA to DNA and then insert the genetic material into the host cell's DNA.

Cancer Virus Treatment

Peter Dazeley/Photographer's Choice/Getty Images

Insight into the development and spread of cancer viruses has lead scientists to focus on preventing potential cancer development by either preventing viral infection or by targeting and destroying the virus before it causes cancer. Cells that are infected by viruses produce proteins called viral antigens which cause the cells to grow abnormally. These antigens provide a means by which virus infected cells can be distinguished from healthy cells. As such, researchers are attempting to find therapies that would single out and destroy virus cells or cancerous cells while leaving non-infected cells alone.

Current cancer treatments, such as chemotherapy and radiation, kill both cancerous and normal cells. Vaccines have been developed against some cancer viruses including hepatitis B and human papilloma viruses (HPV) 16 and 18. Multiple treatments are required and in the case of HPV 16 and 18, the vaccine does not protect against other forms of the virus. The largest impediments to vaccination on a global scale appears to be treatment cost, multiple treatment requirements, and the lack of proper storage equipment for the vaccines.

Cancer Virus Research

Scientists and researchers are currently focusing on ways to use viruses to treat cancer. They are creating genetically modified viruses that specifically target cancer cells. Some of these viruses infect and replicate in cancer cells, causing the cells to stop growing or shrink. Other studies focus on using viruses to improve immune systemresponse. Some cancer cells produce certain molecules that prevent the host's immune system from recognizing them. The vesicular stomatitis virus (VSV) has been shown not only to destroy cancer cells, but to halt their production of immune system inhibiting molecules.

Researchers have also been able to show that brain cancers can be treated with modified retroviruses. As reported in Medical News Today, these therapeutic viruses can cross the blood-brain-barrier to infect and destroy cancerous brain cells. They also function to enhance the immune system's ability to identify brain cancer cells. Although human trials are underway concerning these types of virus therapies, further studies must be done before virus therapies can be used as a significant alternative cancer treatment.

_Sources:

• _{Paddock, Catharine. “Cancer: Virus fuels immune system to attack brain tumors.” Medical News Today, 4 Jan. 2018, www.medicalnewstoday.com/articles/320529.php.}
• _{“Viruses that can lead to cancer.” American Cancer Society, 11 July 2016, www.cancer.org/cancer/cancer-causes/infectious-agents/infections-that-can-lead-to-cancer/viruses.html.}
• _{Zur Hausen, H. “Viruses in human cancers.” Science, 27 Nov. 1991, pp. 1167–1173.}