Salk Institute for Biological Studies


Clodagh  O'Shea

Clodagh O'Shea

Associate Professor
Molecular and Cell Biology Laboratory
William Scandling Developmental Chair



Clodagh O'Shea, an associate professor in the Molecular and Cell Biology Laboratory, is employing the help of a small DNA virus, called adenovirus, to both understand and treat cancer. Normally, cellular replication is tightly controlled. However, both DNA viruses and tumor cells sabotage such controls to drive their respective pathological propagation, albeit with one small difference: In tumor cells the key cellular players are targeted via mutations, while in infected cells viral proteins achieve the same end. Not surprisingly, many of the cellular targets are the same. Dr. O'Shea lab is exploiting this overlap to help address three key questions:

What are the critical cellular targets and pathways that drive deregulated growth?

Human tumors acquire a myriad of mutations, which makes it difficult to pinpoint the critical therapeutic targets. In contrast, Adenovirus encodes a relatively small number of proteins that overcome all the cellular checkpoints which normally prevent aberrant replication. Hence, uncovering the cellular targets of these viral proteins is a powerful strategy with which to identify key cellular pathways that may also be deregulated in tumorigenesis. In addition, viral proteins can provide novel insights into how such targets could be modulated for cancer therapy. Dr. O'Shea's lab is currently using this paradigm to gain new insights into the p53 tumor suppressor pathway, PI-3 Kinase/mTOR signaling and RNA export/processing in tumorigenesis.

What is the human growth deregulation program and how do we uncouple it for tumor therapy?

Tumor mutations act together within integrated and complex cellular networks to elicit aberrant replication. Unfortunately, the overlapping and interconnected nature of cellular networks implies that therapies which target any single tumor mutation are likely to be ineffective. But how do we determine the correct combinations of therapeutic targets that will uncouple a pleiotropic human growth deregulation program? Just as viral proteins can be used to identify discrete tumor targets, Dr. O'Shea's lab is exploring whether viral infection can be exploited to reveal the overall program for human growth deregulation. Using a systems biology approach, they are determining the key molecular signatures that are common to both infected primary cells and tumor cells. With the help of viral mutants, RNAi and chemical genetics, this is also a powerful experimental platform in which to test and identify combination therapies that selectively abort aberrant replication, but leave normal cells unharmed.

Can we manipulate viruses as novel therapeutic agents that trigger the rapid lytic death of tumor cells but leave normal cells unharmed?

The overlap between the tumor and viral growth deregulation programs can also be exploited to develop viruses that replicate selectively in tumor cells, killing them from the inside. This approach is called oncolytic viral therapy, something that is of particular interest to Dr. O'Shea. Defective viruses that are unable to inactivate critical normal cell checkpoints selectively replicate in tumor cells in which these checkpoints are inactivated by mutations. Defining how tumor cells complement selectively the replication of defective viruses can also reveal unexpected tumor targets, such as altered tumor RNA export as the therapeutic target of ONYX-015. Oncolytic viruses offer a novel and potentially self-perpetuating cancer therapy: Each time a virus homes in on a cancer cell and successfully replicates, the virus ultimately kills the cancer cell by bursting it open to release thousands of viral progenies, which have the potential to seek out remaining tumor cells and distant micro-metastases.

"One of the burning questions in contemporary cancer research is, 'What are the critical therapeutic targets that uncouple aberrant growth and survival?' I am employing viruses to pin down key cellular processes that are dysfunctional in cancer cells and to develop novel, virus-based cancer therapies."

If a cell suffers non-repairable injury to its genetic material, or cell growth starts to go astray, the tumor suppressor protein 53 pulls the emergency brake, a built-in "auto-destruct" mechanism that eliminates abnormal cells from the body before they can cause disease, including cancer. To sidestep the cell's suicide program—a process called apoptosis—tumor cells need to inactivate p53, which turns on genes that mediate cell cycle arrest and apoptosis.

Similarly, adenovirus, which causes upperrespiratory infections, needs to get p53 out of the way to multiply successfully; therefore it brings along a viral protein, E1B-55K, which binds and degrades p53 in infected cells. Without E1B-55K to inactivate p53, adenovirus should only be able to replicate in p53-deficient tumor cells, making it the perfect candidate for oncolytic cancer therapy. Oncolytic viruses offer a novel and potentially self-perpetuating cancer therapy: Each time a virus infects a cancer cell and successfully multiplies, the virus ultimately kills the cancer cell by bursting it open to release thousands of viral progeny. The next generation seeks out remaining tumor cells and distant micro-metastases but leaves normal cells unharmed.

Clinical trials found such viruses to be safe and promising. Contrary to all expectations, however, patient responses did not correlate with the p53 status of their tumors. When O'Shea followed up on this unexpected finding, she discovered that the inability of the E1B-55K–mutant virus to replicate in normal cells was not because the virus failed to degrade p53. Instead, adenovirus brings along another protein, E4-ORF3, which neutralizes the p53 checkpoint through a completely different mechanism. It prevents p53 from binding to its target genes in the genome by modifying histone proteins, the "spools" around which DNA winds. With its access denied, p53 is powerless to pull the trigger on apoptosis. O'Shea and her team are now exploiting this new viral protein as a powerful tool to both pinpoint and connect critical new targets in the cellular p53 tumor suppressor network and to develop the next generation of oncolytic viruses.

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