The hepatitis C virus (HCV) infects more than 170 million people worldwide and leads to both acute and chronic liver diseases. Since its discovery several decades ago, the insidious human pathogen has stymied the quest for anti-viral therapies by refusing to reproduce in test tubes for more than a few hours or days, denying scientists an efficient virus production and infection system for experimental research.
Now, in a landmark study by Florida State University biologists that could bolster the development of anti-viral therapies for HCV—as well as for related RNA viruses such as West Nile and influenza—Assistant Professor Hengli Tang and doctoral student/co-author Heather B. Nelson have discovered the molecular mechanism that inhibits HCV replication in vitro after its host cells become crowded and stop dividing.
What’s more, their groundbreaking discovery came about as a result of the new test they developed that can quickly and easily monitor HCV replication in the laboratory.
Finally, after Tang and Nelson uncovered the reason for suppression of the virus in cell culture—in a nutshell: not enough nucleotide molecules, the building blocks of HCV—they then adapted an existing cell technology to remedy the problem right in the test tube.
The Tang-Nelson study and a description of the innovative technologies they devised to enable and track it will appear in the Feb. 8 edition of the Journal of Virology.
"Our findings could prove critical to research on HCV’s complex virus-host cell interactions and lead to better, targeted treatments," Tang said. "Currently, any nucleotide starvation therapies, used primarily to treat cancer, can inhibit replication by depriving viral agents of their molecular building blocks. However, those therapies may impact healthy cells, as well, causing undesired side effects."
In the human liver, the parasitic HCV makes copies of its genetic material by hijacking nucleotides—the little molecules produced by its dividing host cells. It is only in the liver that pools of nucleotides remain available to HCV in sufficient supply after the host cells reached confluence (stop dividing). Not so in test tubes, say the FSU researchers.
To address the shortage of HCV building blocks in vitro, their unique adaptation of an existing cell technology enabled the introduction of nucleoside molecules to a culture of liver cancer cells. The nucleosides then converted to the essential nucleotide molecules that Tang calls the missing link. In turn, the nucleotides generated in vitro replication of infectious HCV particles that continued even after host cell confluence—as it does in the liver.
That’s not all. "Our new cell line also allows us to rapidly identify and isolate drug-resistant HCV mutants in vitro and to screen for anti-viral drug candidates," Tang said. "This will help researchers better study the mechanism of drug resistance, a big problem with this virus and others such as HIV (human immunodeficiency virus) that mutate quickly."
Underpinning everything, Tang says, is their novel, easy-to-use assay. It can track mutant strains of HCV in a week or less while other assays take weeks or months.
"Our assay, for which FSU has filed a provisional patent application, employs a new reporter cell line, which means the cells give out a detectable signal when certain events happen inside them," said Tang. "In this case, they emit a green fluorescence whenever HVC is replicating. The fluorescence is tracked in the cell culture through a technique known as flow cytometry, which employs a machine equipped with a laser and lights that follows the green to find the virus."
Between earning his Ph.D. at the University of California-San Diego in 1998 and joining FSU’s biological science faculty in 2004, Tang served as a lead researcher in an industry setting, seeking targeted anti-viral therapies primarily for HIV.
"I find it particularly rewarding to play a part in research that may actually help somebody soon," he said.
The Tang-Nelson study at FSU—"Effect of Cell Growth on Hepatitis C Virus (HCV) Replication and a Mechanism of Cell Confluence-Based Inhibition of HCV RNA and Protein Expression"—was supported in part by a grant from the American Heart Association.