McArdle Blog L7 ~ 09/23/2013
Research Snapshot: Dr. Bill Sugden and the Epstein Barr Virus
Did you know that almost a fifth of all new cases of human cancers every year are caused by an infectious agent, such as bacteria or viruses? Epstein Barr Virus or Human Herpesvirus 4, commonly called EBV, is one such virus, and in fact was the first human virus to be associated with cancer. EBV causes the common, benign disease infectious mononucleosis, but can also cause lymphomas such as Burkitt’s lymphoma and other B-cell lymphomas, as well as carcinomas such as nasopharyngeal and gastric carcinomas. Burkitt’s lymphoma is the most common childhood and adolescence cancer in equatorial Africa.
Dr. Bill Sugden – who prefers to go by Bill – has been studying the Epstein Barr Virus for more than forty years. He recalls that “it was 1973 when I went to Stockholm to study EBV. It seemed likely even then that EBV was a human tumor virus, and I wanted to study a human tumor virus”. Over the years Bill and the people in his laboratory have worked to understand how EBV infects and persists within cells in our bodies and what role the virus plays in causing and sustaining the cancers it is associated with. Understanding how the virus contributes to the survival of the tumors it causes can lead to the development of therapies for these cancers. Recently, Bill and his colleagues have found that microRNAs (miRNAs) expressed by EBV sustain several EBV-induced lymphomas.
EBV is a peculiar virus in that it maintains its DNA as a plasmid – i.e. as a piece of DNA that is physically distinct from cellular DNA – in infected cells, such as tumor cells. One infected cell may contain multiple copies of EBV DNA. The viral plasmids are replicated along with cellular DNA during the S-phase of the cell cycle, and when the infected cells divide, the plasmids are partitioned equally and non-randomly to the daughter cells. This process allows EBV to maintain itself in infected cells.
But, surprisingly, research by Dr. Asuka Nanbo, a post-doctoral fellow in Bill’s lab, has shown that not all the EBV plasmids in a cell are replicated every cell cycle, which implies that over time EBV plasmids should be lost from a population of dividing cells (See Image 1).
However, this is not the case in cancer cells, which continue to harbor EBV DNA even while proliferating. Therefore, EBV must be providing a selective advantage to infected cells, which allows these cells to outgrow the cells that have lost the viral DNA. Dr. Dave Vereide, a former graduate student in Bill’s lab, engineered certain EBV-positive cancer cell lines to express a mutated version of the viral protein EBNA1. When cells containing EBV plasmids expressed this mutant EBNA1 protein, they lost the viral plasmids and died by apoptosis. This finding – that forcing the loss of EBV genomes from infected cells, such as lymphoma cells, leads to their death – confirmed the hypothesis that EBV provides survival factors to infected cancer cells. But what are these survival factors provided by EBV?
miRNAs are small RNAs expressed by many organisms, from humans to viruses. They play a role in regulating protein levels by binding to messenger RNAs (mRNAs) and either inhibiting their translation into proteins or causing their degradation. Dr. Ya-Fang Chiu, a post-doctoral fellow in Bill’s lab, says, “The only viral genes expressed in a number of EBV-derived lymphomas are the protein EBNA1, a couple of small RNAs called EBERs and a set of viral microRNAs called BARTs. We were very curious whether these EBV miRNAs contributed to tumor cell survival.” Along with Dr. Vereide and Mitch Hayes, a research specialist in the lab, Dr. Chiu found that if they expressed EBV miRNAs in EBV-positive cells and then forced the loss of EBV DNA from these cells, they did not die, but continued to proliferate (see Image 2). Thus, the death of infected cells caused by forcing the loss of EBV DNA could be rescued by expressing the EBV BART microRNAs in those cells.
These findings by Bill’s laboratory have opened the possibility of developing therapies for EBV-induced cancers. Understanding how tumor cells depend upon EBV for their survival makes it possible to plan potential treatment for cancers sustained by EBV. For example, since the viral protein EBNA1 is absolutely essential for the viral DNA to persist in infected cells, inhibiting the functions of this protein could be a way to disrupt the maintenance of EBV DNA in tumor cells and lead to the destruction of the tumor. The intrinsic advantage of targeting an EBV protein is that cellular functions need not be affected by the therapeutic strategies. “I began my career in cancer research in 1965 with the goal of developing a treatment for some form of cancer,” says Bill, “and it is encouraging to be at a point where we have learned enough about EBV to identify possible treatments so that we can now try to bring them into practice.”