Vol 8. Issue 10 / March 24, 2008
Scientists Develop Process to Disrupt Hepatitis C Virion Production
By Jeff Worley
Scientists at The Scripps Research Institute's Scripps Florida facilities have discovered a method to disrupt the production of infectious virus particles that cause hepatitis C, a blood-borne liver disease. This discovery might be a first step in developing new and more effective therapies against the hepatitis C virus (HCV). Current anti-virals are ineffective for many patients infected with the viral strains most prevalent in the United States.
HCV is a significant human pathogen, infecting more than three percent of the world's population. The incidence of infection in the United States has been estimated to be as high as 4 million cases. In the March issue of the journal PLoS Pathogens, Timothy Tellinghuisen, an assistant professor in the Department of Infectology at Scripps Florida, and his colleagues describe how they used mutations of the viral NS5A phosphoprotein to disrupt virus particle production at an early stage of assembly. NS5A has long been proposed as a regulator of events in the HCV life cycle, but exactly how it orchestrates these events has been unclear.
"The interesting thing about this mutant is that while it triggers totally normal RNA replication, it causes severe defects in the output of infectious virus—in fact, it releases no infectious virus that we can detect," says Tellinghuisen. "And though this discovery isn't a cure for HCV, it is an important research tool that stops the assembly pathway." Total disruption of the replication process would be a cure for the disease, he adds, and that's the team's long-term goal.
HCV infection is roughly five to seven times more prevalent than HIV, underscoring the pandemic nature of HCV infection. HCV occurs when blood from an infected person enters the body of someone who is not infected. Most new HCV infections are due to illegal drug injections and sharing needles. However, those who had blood transfusions prior to blood donor screening in 1991, healthcare workers who had needle stick accidents, and hemodialysis patients are also at risk for developing HCV infection. The virus predominantly infects the liver, and following many decades of virus reproduction serious disease such as hepatitis (liver inflammation), cirrhosis (liver scarring), and carcinoma (liver cancer) develop. Ultimately, HCV infection destroys the liver, resulting in death. Attempts at curing HCV infections with drug therapy have been only marginally successful.
Before more effective therapies can be developed, scientists need to understand, at the molecular level, the detailed mechanisms HCV uses to infect cells, replicate itself, assemble progeny virus, and exit the cell. Each of these processes could potentially be a target for a new drug to eliminate HCV infection. HCV, like all viruses, requires the normal cellular machinery for its replication and has developed strategies to utilize normal cell physiology for its own benefit (often to the detriment of the host).
"Those proteins assemble in the cell to make a structure called a replicase that then copies the viral RNA," Tellinghuisen explains. "We measured that RNA accumulation and observed no defect in RNA replication, but found, surprisingly, that no infectious viral particles were released from the cells." The team also found that no viral RNA nor nucleocapsid protein are released from cells, indicating that an early event in virus assembly had been affected.
Using genetic mapping and biochemical analyses, the authors were able to show that their deletion altered a phosphorylation signal controlling the switch from RNA replication to virus particle assembly. This signal was attributed to the activity of a cellular kinase that when inhibited by genetic or chemical means led to a reduction in infectious virus production without altering HCV RNA replication.
This project was funded by a Career Development Award from the National Institutes of Allergy and Infectious Diseases of the National Institutes of Health, and by the State of Florida.
Send comments to: mikaono[at]scripps.edu