Whether through condoms or abstinence, most methods to prevent sexually transmitted diseases have a common logic: keep the pathogen out of the body altogether. While this approach is certainly reasonable, it does not help the countless people who, for any number of reasons, cannot control their sexual circumstances.
Now, Judy Lieberman, HMS professor of pediatrics at Children’s Hospital Boston and the Immune Disease Institute, who is also a senior investigator at the IDI, has overseen the development of a topical treatment that, in mice, disables key genes necessary for herpesvirus transmission. Based on RNA interference (RNAi), the treatment cripples the virus in a molecular one–two punch, simultaneously disabling the bug’s ability to replicate and the host cell’s capacity to take up the virus.
The treatment is just as effective when applied from one week before to a few hours after exposure to the virus. So the basic biology of the prophylactic is responsive to real-world demands.
“People have been trying to make a topical agent that can prevent transmission, a microbicide, for many years,” said Lieberman. “But one of the main obstacles for this is compliance. One of the attractive features of the compound we developed is that it creates in the tissue a state that’s resistant to infection, even if applied up to a week before sexual exposure. This aspect has a real practicality to it. If we can reproduce these results in people, this could have a powerful impact on preventing transmission.”
The findings appear in the Jan. 22 issue of Cell Host and Microbe.
Developing RNAiThe World Health Organization estimates that approximately 536 million people worldwide are infected with herpes simplex virus type 2 (HSV-2), the most common strain of this sexually transmitted pathogen. Women are disproportionately affected, with potentially serious consequences. The virus can be passed easily from mother to child during birth, and untreated infants face the risk of brain damage and death. While HSV-2 alone is not life-threatening in adults, infection does increase a person’s vulnerability to other viruses such as HIV.
In order for the herpesvirus to infect the host, two conditions must be met. First, the virus must be able to enter and take over host cells. Second, the virus must then reproduce itself. Lieberman’s topical treatment uses RNAi to foil both of these events.
RNAi, a biological process that was identified barely a decade ago, has transformed the field of biological research. A breakthrough that earned its discoverers the Nobel Prize in 2006, RNAi is a natural cellular process that occurs in all cells of all multicellular organisms to regulate the translation of genetic information into proteins. By introducing tiny RNA molecules into cells, researchers can target a gene of interest and block that gene’s ability to build protein molecules. For all intents and purposes, that gene is disabled.
While RNAi has profoundly accelerated the ability of scientists to probe and interrogate cells in a Petri dish, therapeutic breakthroughs have proven far more problematic. Researchers have had a difficult time delivering these tiny RNA molecules to the desired cells and tissues in a living organism.
Modifying a delivery technique that Lieberman and colleagues developed in 2005, she and postdoctoral fellow Yichao Wu and junior researcher Deborah Palliser (who now heads her own lab at Albert Einstein College of Medicine) treated mice with strands of RNA that were fused to cholesterol molecules, which made it possible for the molecules to pass through the cell membranes. When applied in the form of a topical solution, these RNA molecules could then be fully absorbed into the vaginal tissue, protecting the mice against a lethal dose of administered virus.
One RNA molecule in the topical solution targeted the herpes gene UL29, which the virus needs to replicate. So knocking out the gene inactivates the virus. Another RNA molecule targeted Nectin-1, a surface protein on cells in the vaginal tissue. Nectin-1 acts as a kind of host gatekeeper to which the virus binds to pass into the cell. Without Nectin-1, the virus simply cannot infect cells.
Either RNA molecule delivered by itself would be sufficient to block the virus, but together in this RNAi cocktail, the host tissue becomes a force that pulls up the drawbridge to block the enemy’s entrance while brandishing weaponry to decimate the invaders if they make it through.
“As far as we could tell, the treatment caused no adverse effects, such as inflammation or any kind of autoimmune response,” said Lieberman. “And while knocking out a host gene can certainly be risky, we didn’t see any indication that temporarily disabling Nectin-1 interfered with normal cellular function.”
Lieberman was recently awarded a grant from the Massachusetts Life Sciences Center to collaborate with a corporate partner to build on these results, developing a topical microbicide that might be suitable for human use.
In addition, she is investigating how this same approach might be used to treat HIV. The work is part of a multi-institutional program funded by the National Institutes of Health that includes researchers at the Tulane National Primate Research Center, St. George’s Hospital in London, and Alnylam Pharmaceuticals in Cambridge.
Students may contact Judy Lieberman at lieberman@idi.harvard.edu for more information.
Conflict Disclosure: The authors report no conflicts of interest.
Funding Sources: The National Institutes of Health
