McArdle Blog L7 ~ 2/25/2014
Research Snapshot: PREVENTION IS BETTER WITH A CURE: The search for a treatment for Human Papilloma Virus infections
Fourteen million. That’s the estimated number of Americans who became infected with Human Papilloma Virus (HPV) in 2013, according to the Centers for Disease Control & Prevention (CDC). That’s greater than the combined populations of the states of Wisconsin, Minnesota and Iowa.
In most cases, our immune system is able to naturally get rid of HPV infections, but sometimes the virus can stay on in our body for years. When infections by HPV types are not cleared by our immune system, it can increase our risk for certain kinds of cancer. These types of HPVs are called high-risk HPVs. In fact long-term infection by HPVs leads to approximately 18,000 new cases of cancer every year in the United States.
While there does exist a vaccine that protects against infection by several high-risk HPVs, there is no treatment to eliminate existing HPV infections. Dr. Paul Lambert and graduate student Laurel Lorenz, here at the McArdle Laboratory for Cancer Research, are trying to change that.
“The idea is to try and develop the means to eliminate pre-existing high risk HPV infections,” says Lorenz. “We think this would be of great value in reducing the risk of cancer among patients already infected with these high risk HPVs.”
During long-term, or persistent, infection the HPV DNA exists as circular pieces of DNA that are physically distinct from the cellular DNA in infected cells. As the cells containing the HPV DNA – or genomes – grow and divide to form new cells, the viral genomes are also replicated and then distributed to the new cells. The HPV genomes are said to be “maintained” in these infected cells.
One way to eliminate the risk of cancer that develops from long-term high-risk HPV infections is to get rid of the viral DNAs from our cells. Lorenz explains that “If we can identify viral and cellular genes that play crucial roles in the maintenance of HPV genomes in cells, we might be able to devise new strategies to treat these persistent HPV-infections.”
Dr. Lambert and Lorenz initially focused on one high-risk HPV strain, HPV16, and one HPV16 gene: the E6 gene that contains the blueprint for – or encodes – the protein E6.
There has been previous research looking into what role HPV16 E6 protein plays in maintenance of HPV DNA in cells, but those studies used DNA from different HPV strains. This would be the first study to look at the HPV16 E6 protein and whether it played a role in the maintenance of the HPV16 virus DNA itself. This was important because the roles of other HPV proteins – such as E7 – vary between strains, and it is now known that the HPV16 E6 protein can act differently if paired with genomes from other HPV strains.
Lorenz designed modified, or mutated, versions of the HPV16 genome, such that the E6 protein either ended earlier than usual or had parts of it deleted. Could these HPV16 genomes containing mutant E6 proteins be used to identify roles of E6 that are necessary for the maintenance of HPV16?
When the non-mutated or wild type (WT) genomes were introduced into cells, 5/9 of populations tested maintained HPV16 genomes. When the HPV16 genomes containing the mutated versions of E6 were tested, the results were mixed. Some of the mutant genomes were not able to be maintained in cells, but others were maintained in a similar percentage of populations tested as the WT genome.
Turns out, Lorenz had predicted this would happen. It has been known for some time that the HPV E6 protein leads to the destruction of a cellular tumor-suppressor protein called p53. The loss of p53 due to E6 was thought to allow the HPV-infected cells to continue to grow and divide.
For the most part, the mutant E6 proteins that could maintain HPV16 genomes in cells also caused the degradation of cellular p53 protein. But not all of them did.
One version of the HPV16 genome – we will call it HPV16 mutant X – did not cause a decrease in levels of p53, but this mutant genome was maintained when introduced into cells. What was going on?
One possibility was that while the p53 protein was still present, it was inactive. The authors turned to a chemical compound called Actinomycin D. If p53 is active in cells, they tend to stop going through the cell cycle when Actinomycin D is added to their growth medium. If, on the other hand, p53 is inactive in these cells, Actinomycin D has a far less drastic effect.
When Lorenz grew normal cells with active p53 and added Actinomycin D, a portion of the cells were stuck at an initial stage of the cell cycle. When cells harboring WT HPV16 or mutant X were grown with Actinomycin D, comparatively fewer cells became stuck at the early stage of the cell cycle.
So, it appears that inactivating, and not necessarily destroying, the p53 protein is necessary for HPV16 maintenance. A test for this hypothesis is to introduce HPV16 genomes that cannot be maintained in cells having active p53 into cells where the p53 protein is already inactive. Are these previously “unmaintainable” HPV16 genomes now maintained?
Indeed, they are. Lorenz found that while introducing the mutant HPV16 genomes into cells with active p53 yielded no populations with extrachromosomal viral genomes, introducing them into cells where p53 was inactivated meant that viral plasmids could be detected in 38% to 50% of the populations.
So, what does it all mean? “Possible treatment strategies,” says Lorenz, “and you don’t have to reinvent the wheel.” Because p53 is such a well-studied and important protein in the context of both normal development and cancer, we know of several activators already.
“In fact, we are testing some compounds right now for their ability to inhibit HPV infections by activating p53,” concludes Lorenz. And we will keep hoping she succeeds.