A new function for oncoproteins of DNA tumor viruses

oncoproteinsOncogenes of DNA tumor viruses encode proteins that cause cells to divide incessantly, eventually leading to formation of a tumor. These oncoproteins have now been found to antagonize the innate immune response of the cell (link to paper).

Most cells encountered by viruses are not dividing, and hence do not efficiently support viral DNA synthesis. The genomes of adenoviruses, polyomaviruses, and papillomaviruses encode proteins that cause cells to divide. This effect allows for efficient viral replication, because a dividing cell is producing the machinery for DNA synthesis. Under certain conditions, infections by these viruses do not kill cells, yet they continue to divide due to the presence of viral oncoproteins. Such incessant division gives the cells new properties – they are called transformed cells – and they may eventually become a tumor.

These so-called viral oncoproteins include large T antigen (of SV40, a polyomavirus); E6 and E7 (papillomavirus), and E1A (adenovirus). These viral proteins kick cells into mitosis by inactivating cell proteins (such as Rb, pictured) that are normally involved in regulating cell growth. The cells divide, and in the process produce proteins involved in DNA replication, which are then used for viral replication. These oncoproteins accidentally cause tumors: the replication of none of these viruses is dependent on transformation or tumor formation.

Cells transformed with T, E6/E7, or E1A proteins are commonly used in laboratories because they are immortal. An example is the famous HeLa cell line, transformed by human papillomavirus type 18 (which originally infected Henrietta Lacks and caused the cervical tumor that killed her). Another commonly used transformed cell line is 293 (human embryonic kidney cells transformed by adenovirus E1A). It’s been known for some time that when DNA is introduced into normal (that is, not transformed) cells, they respond with an innate response: interferons are produced. In contrast, when DNA is introduced into the cytoplasm of a transformed cell, there is no interferon response.

To understand why HeLa and HEK 293 cell lines did not respond to cytoplasmic DNA, the authors silenced the viral oncogenes by disrupting them with CRISPR/Cas9. The altered cells produced interferon in response to cytoplasmic DNA. Furthermore, they produced new transformed lines by introducing genes encoding E6, E7, E1A, or T into normal mouse embryonic fibroblasts. These new transformed cells failed to respond to cytoplasmic DNA.

Cytoplasmic DNA is detected in cells by an enzyme called cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase) together with an adaptor protein known as STING (stimulator of interferon genes). When cytoplasmic DNA is detected by this system, the antiviral interferons are produced. The viral oncoproteins were found to directly bind STING, but not cGAS. A five amino acid sequence within E1A and E7 proteins was identified that is responsible for overcoming the interferon response to cytoplasmic DNA. When this sequence was altered, interaction of the oncoprotein with cGAS was reduced, and antagonism of interferon production in response to cytoplasmic DNA was blocked.

These findings provide a new function for the oncoproteins from three DNA tumor viruses: antagonism of the interferon response to cytoplasmic DNA. Normally DNA is present in the cell nucleus, and when it is detected in the cytoplasm, this is a signal that a virus infection is underway. The cytoplasmic DNA is sensed by the cGAS-STING system, leading to interferon production and elimination of infection. A herpesvirus protein has been identified that binds to STING and blocks interferon responses to cytoplasmic DNA. Clearly antagonism of the cGAS-STING DNA sensing system is of benefit to DNA viruses.

An interesting question is what selection pressure drove the evolution of viral oncogenes. One hypothesis, described above, is that they are needed to induce a cellular environment that supports viral DNA synthesis. The other idea, favored by the authors of this new work, is that oncogenes arose as antagonists of innate immune signaling. But I can’t imagine these DNA viruses without oncogenes, because they would not be able to replicate very efficiently. Could both functions have been simultaneously selected for? Why not – the same five amino acid sequence that binds cGAS also binds cellular proteins (such as Rb), disrupting their function and leading to uncontrolled cell growth!

Virus Watch: Cancer Killing Viruses

Guest host Lynda Coughlan reviews how oncolytic viruses, which specifically kill tumor cells, are designed and how they work.

TWiV 400: Harold ‘400’ Varmus, a scientist for all seasons

The TWiV team is together in New York City for a conversation with Nobel Laureate Harold Varmus about his remarkable career in science.

You can find TWiV #400 at microbe.tv/twiv, or listen/watch below.

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TWiV 395: The cancer thief

Vincent, Rich and Kathy speak with Stephen Russell about his career and his work on oncolytic virotherapy – using viruses to treat cancers. Recorded before an audience at ASV 2016 at Virginia Tech in Blacksburg, Virginia.

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TWiV 374: Discordance in B

TWiVOn episode #374 of the science show This Week in Virology, the TWiVniks consider the role of a cell enzyme that removes a protein linked to the 5′-end of the picornavirus genome, and the connection between malaria, Epstein-Barr virus, and endemic Burkitt’s lymphoma.

You can find TWiV #374 at microbe.tv/twiv.

TWiV 359: A Blossom by any other name

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You can find TWiV #359 at www.microbe.tv/twiv.

TWiV 339: Herpes and the sashimi plot

On episode #339 of the science show This Week in Virology, tre TWiV amici present three snippets and a side of sashimi: how herpesvirus inhibits host cell gene expression by disrupting transcription termination.

You can find TWiV #339 at www.microbe.tv/twiv.

A transmissible cancer of soft-shell clams

Mya_arenariaA leukemia-like cancer is killing soft-shell clams along the east coast of North America. The cancer is transmitted between animals in the ocean, and appears to have originated in a single clam as recently as 40 years ago.

Hemic neoplasm is a disease of marine bivalves that is characterized by proliferation of morphologically and functionally aberrant hemocytes, the cells that circulate in the circulatory fluid of mollusks. A newly identified LTR retrotransposon called Steamer correlates with neoplastic disease in clams. LTR retrotransposons are DNA sequences in the genome that are thought to be precursors to retroviruses. The normal clam genome contains 10-20 copies of Steamer, compared with 150-300 copies in neoplastic hemocytes.

Integration of retroviruses into the genome is a known mechanism for disrupting cellular growth control, leading to uncontrolled proliferation and ultimately development of cancer. Whether Steamer causes neoplasia, by integrating near an oncogene, can be determined based on where this retroelement has inserted in the clam genome.

Analysis of 12 Steamer integration sites revealed that 7 were present in neoplastic samples from clams collected at sites in New York, Maine, and Prince Edward Island, Canada. Examination of nuclear and mitochondrial DNA revealed that cancerous hemocytes collected from clams at all three sites contain similar mutations that are not present in normal tissues. These observations indicate that these neoplasms  are nearly genetically identical and could not have arisen from the hosts. The cancer probably originated in a single clam and was then transmitted to other animals.

Cells from different vertebrates are usually rejected by the host immune system, which recognizes foreign cells by the major histocompatibility (MHC) system. However, mollusks do not have MHC, which may explain why tumor cells can be transmitted among clams. Two other transmissible tumors have been described: the canine venereal tumor, which is sexually passed among dogs, and Tasmanian devil facial tumor disease, transmitted by biting. In both cases MHC molecules are low in tumor cells, explaining why they are not rejected by the recipient animal.

How cancer could spread among clams hundreds of miles apart is not known. Clams are filter feeders, which may lead to uptake of neoplastic cells released into the water by diseased animals. Movement of clams by humans might also have played a role in dissemination of disease along the northeastern US seaboard.

There are no known contagious human cancers, and it is unlikely that the steamer clam neoplasia could be transmitted to humans. It is not known how extensively hemic neoplasia will spread among soft-shell clams, or whether the disease could spread to other bivalves.

TWiV 320: Retroviruses and cranberries

On episode #320 of the science show This Week in Virology, Vincent speaks with John Coffin about his career studying retroviruses, including working with Howard Temin, endogenous retroviruses, XMRV, chronic fatigue syndrome and prostate cancer, HIV/AIDS, and his interest in growing cranberries.

You can find TWiV #320 at www.microbe.tv/twiv.

TWiV 290: Baylor goes viral

On episode #290 of the science show This Week  in VirologyVincent meets up with Janet Butel and Rick Lloyd at Baylor College of Medicine to talk about their work on polyomaviruses and virus induced stress.

You can find TWiV #290 at www.microbe.tv/twiv.