Fruit Flies May Provide Answers to How HPV-induced Cancers Work

A team of researchers from the Zhang lab at the University of Missouri have created the first fruit fly model of human papillomavirus (HPV) induced human cancer. Through the analysis of these fruit flies, scientists hope to better understand the mechanism that allows HPV to cause cancer as well as identify therapies that can potentially treat HPV-induced cancers, according to a press release.

Dr. Bing Zhang, PhD, professor of Biological Sciences and expert on fruit fly (Drosophila melanogaster) genetics at the University of Missouri, and corresponding study author, said, “This is the first in vivo model of an HPV-induced cancer in fruit flies. This new model will allow scientists to exploit a powerful genetic system to understand the molecular and biochemical pathways involved in tumor growth and malignancy caused by HPV as well as screen for potential drug targets.”

The study, led by Dr. Mojgan Padash, PhD, postdoctoral fellow at the University of Missouri and the University of Oklahoma was a collaborative effort; Dr. Vanessa Auld, PhD, professor in the Department of Zoology at the University of British Columbia and Dr. Lawrence Banks, PhD, of the International Centre for Genetic Engineering and Biotechnology in Italy also assisted in the research.

Around 80 million people in the United States alone are infected with HPV, according to the Centers for Disease Control and Prevention (CDC). According to a study analyzing the role of HPV in the development of cervical cancer, “Cervical cancer is one of the leading world causes of cancer morbidity and mortality in women, with more than 98% related to a human papillomavirus (HPV) infection origin.”

In previous studies of mice and human cells, researchers found that the virus uses epithelial skin as a way to enter the body, and once inside, it produces several oncoproteins, which are able to transform normal cells into tumor cells, according to the press release. Viral oncoprotein E6 is found in almost all HPV-postive cancers because it is a contributing factor when it comes to the later stages of tumor formation and metastasis. E6 interacts with normal human proteins, causing them to “lose control.”

The aforementioned human proteins in this interaction are called E6AP, or E6-associated proteins, due to the fact that E6 partners up with E6AP in this process. When combined, they target other human proteins that contain a PDZ domain which then causes them to undergo degradation. Without the normal human proteins, epithelial cells “become disorganized and start uncontrolled proliferation,” according to the press release. A particular protein that tends to be heavily targeted by E6 for degradation is called Magi.

In the new study, viral E6 and human E6AP proteins were introduced to just the wing and the eye epithelial cells in fruit flies. The combined proteins resulted in severe abnormalities within the epithelial cells. In addition, the same suite of PDZ proteins that are targeted by E6 in humans showed reduced levels. The researchers found that the version of the Magi protein found in the fruit flies were still the major target of degradation, just like in human epithelia. “Further experiments done in human cell lines with the fruit fly version of the Magi protein yielded similar results, indicating that the mechanism is highly conserved,” according to the press release.

When speaking of the implications of the findings, Dr. Padash said, “This finding prompted us to see if expression of Magi could suppress the E6-medicated cellular abnormalities in fruit fly, which it did, again reinforcing that Magi is an important player.”

The scientists found that the co-expression of viral E6 and human E6AP were insufficient in causing tumors within flies, despite the noted cellular abnormalities. These findings were consistent with the findings of HPV E6-induced cancer found in humans, according to the study authors.

Presented with these findings, Dr. Padash hypothesized, “In humans, there is a period of 15-20 years from the time of HPV infection to the development of cancer, suggesting that cooperation between E6 and E6AP is not sufficient to induce cancer. It is thought that mutations in another oncoprotein, called Ras, may contribute to the E6-mediated tumorigenesis in humans.”

The scientists then proceeded to express the viral E6 and human E6AP in the presence of a mutated version of the oncoprotein, Ras, in the fruit flies in order to test their hypothesis. The combination of all three proteins resulted in the development of malignant tumors in the fruit flies that “metastasized from the eyes to the rest of the body,” according to the press release.

Dr. Padash said, “The take home message is that the same key molecular players that underlie HPV E6-mediated cancer in humans do the same things in flies, which suggests that the mechanism is highly conserved. Practically speaking, this means we can now use this fly model to identify other essential components or elements that contribute to E6-mediated tumorigenesis.”

The researchers demonstrated the potential of these findings when they identified an insulin receptor that interacts through E6, found when they conducted an initial genetic screen. After identifying the insulin receptor, they then introduced it to fruit flies that had both viral E6 and human E6AP proteins, which resulted in cell proliferation. The suggests “that insulin may play a role in cancer progression induced by HPV,” according to the press release.

When speaking of how the meaning of these results, Dr. Banks, commented, “From a basic science point of view, it shows that the mechanisms by which the HPV E6 oncoprotein targets essential cellular regulatory pathways are conserved across evolution, suggesting that these targets, as exemplified by Magi, are of fundamental importance in controlling cell growth and proliferation.” He then added, “From a more practical point of view, the power of this model is that it can be used now to screen for inhibitors of other pathways, which have the potential to translate into therapies for HPV-induced cancers.”