Merkel cell polyomavirus
A Microbial Biorealm page on the genus Merkel cell polyomavirus
Virus; dsDNA viruses, no RNA stage; Polyomaviridae; polyomavirus; unclassified polyomavirus (Merkel cell polyomavirus)
Description and significance
Merkel cell polyomavirus is a human tumor virus. It is suspected to be the cause of about 80% of a rare skin cancer called Merkel cell carcinoma. (5) This virus is a relatively recent discovery, thus there are no medications or vaccines against this virus or Merkel cell carcinoma. However, avoiding excessive sun exposure and using appropriate sun lotion can help prevent the virus from mutating and causing cancerous tumors. (3) Blood tests have shown that the majority of adults have already been exposed to the virus and continue to carry it as an asymptomatic infection. (7)
MCV has a 5.4 kbp circular double stranded DNA genome and is a non-enveloped virus. The virus typically integrates at different locations within the human genome in different tumors. Once the virus is integrated into the DNA within the tumor, it cannot be cut out from its host cell. Instead, it must replicate as the host cell replicates. Sequencing of the virus from Merkel cell cancers has revealed this characteristic. The virus typically has tumor-specific mutations that shorten the MCV T antigen. These mutations are the reason why the virus can no longer replicate independently. (3)
Cell structure, metabolism & life cycle
MCV virus-like particles resemble typical icosahedral polyomavirus capsids that are 55–58 nm diameter. (7) Once they are integrated into the host cell DNA, they lose their ability to replicate independently. They are typically integrated into the host cell DNA at different locations. (5)
Ecology (including pathogenesis)
MCV is typically found in about 80% of Merkel cell carcinoma, an agressive neuroendocrine skin cancer linked to lymphoid malignancies such as chronic lymphocytic leukemia. It can also be found in healthy tissues of people without Merkel cell carcinoma. (5) It can also be found in respiratory secretions, suggesting a possible respiratory trasmittance. (1) However, viruses found in Merkel cell carcinoma tumors are nontransmissible because the virsus is integrated into the host DNA and cannot replicate independently. (5) MCV mainly infects the elderly (or persons over age 50) or immunosuppressed individuals, such as AIDS patients. (2)
Patients with Merkel cell carcinoma tumors that are infected with MCV will notice a painless lump that expands rapidly. Prevention involves avoiding sunburns and using sun lotion, like other skin cancers. (3)
MCV is believed to be the cause of Merkel cell carcinoma tumors that are infected with the virus because inhibition of viral oncoproteins cause cells from the infected tumors to die. Cells from tumors that are not infected with the Merkel cell polyomavirus are not affected by this inhibition. (4)
One study found that patients with Merkel cell polyomavirus infected tumors actually had a higher survival rate than patients without Merkel cell polyomavirus infected tumors. This finding provides more evidence that the virus is the cause of the Merkel cell carcinoma tumors in the infected patients. This shows that the detection of the virus may actually lead to a better outcome. Researchers can now take these recent findings and search for potential cures. (6)
(1) Bialasiewicz S., Lambert S. B., Whiley D. M., Nissen M. D., Sloots T. P. "Merkel cell polyomavirus DNA in respiratory specimens from children and adults." Emerging Infectious Diseases. 2009. Volume 15(3).
(4) Houben, R., Shuda, M., Weinkam, R., Schrama, D., Feng, H., Chang, Y., Moore, P. S., Becker, J. C. "Merkel Cell Polyomavirus-Infected Merkel Cell Carcinoma Cells Require Expression of Viral T Antigens." Journal of Virology. 2010. Volume 84(14). p. 7064-7072.
(5) Shuda, M., Arora, R., Kwun, H. J., Feng, H., Sarid, R., Fernández-Figueras, M. T., Tolstov, Y., Gjoerup, O., Mansukhani, M. M., Swerdlow S. H., Chaudhary, P. M., Kirkwood, J. M., Nalesnik, M. A., Kant, J. A., Weiss, L. M., Moore, P. S., Chang, Y. "Human Merkel cell polyomavirus infection I. MCV T antigen expression in Merkel cell carcinoma, lymphoid tissues and lymphoid tumors." International Journal of Cancer. 2009. Volume 126(6). p. 1243-1249.
(6) Sihto, H., Kukko, H., Koljonen, V., Sankila, R., Böhling, T., Joensuu, H. "Clinical Factors Associated With Merkel Cell Polyomavirus Infection in Merkel Cell Carcinoma." Journal of the National Cancer Institute. 2009. Volume 109(13). p. 938-945.
(7) Tolstov, Y. L., Pastrana, D. V., Feng, H., Becker, J. C., Jenkins, F. J., Moschos, S., Chang, Y., Buck, C. B., Moore, P. S. "Human Merkel cell polyomavirus infection II. MCV is a common human infection that can be detected by conformational capsid epitope immunoassays." Journal of Virology. 2009. Volume 125(6). p. 1250-1256.