HCV is a significant human pathogen, infecting more than three percent of the worlds 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 virusin fact, it releases no infectious virus that we can detect, says Tellinghuisen. And though this discovery isnt 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 thats the teams 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).
The Tellinghuisen team, which includes Research Assistants Katie L. Foss and Jason Treadaway, has focused recent efforts on NS5A to understand the regulation of events used by the virus to assemble infectious copies of itself and exit the cell. NS5A is a three-domain protein, which means it is comprised of three compactly folded regions roughly 50 to 300 amino acids in length. The requirement of domains I and II for RNA replication is well documented. NS5A domain III, however, is not required for RNA replication, and the function of this region in the HCV life cycle is unknown.
Using standard molecular biology, the researchers removed from domain III of NS5A a coding sequence corresponding to roughly 15 amino acids. Then they generated a clone of the virus, transcribed the RNA from that clone, and purified the RNA. This RNA, which is directly infectious, was then transfected into a liver cell line where it produced all the HCV proteins that are encoded by that RNA genome.
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.
These data provide the first evidence for a function of domain III of NS5A and implicate NS5A as an important regulator of the RNA replication and virion assembly of HCV, Tellinghuisen says. The ability to uncouple virus production from RNA replication may be useful in understanding HCV assembly and may become therapeutically important.
Charles M. Rice, head of the Center for the Study of Hepatitis C at Rockefeller University, comments, This is a spectacular advance linking a specific phosphorylation event by a cellular kinase to hepatitis C virus assembly. Remarkably, the target is a viral nonstructural protein, NS5A, and the data point to a pivotal role for this protein in regulating RNA amplification and infectious virus production. These new data make this multifunctional protein an even more attractive target for developing new anti-virals for treating hepatitis C.
|Contact: Keith McKeown|
Scripps Research Institute