Hepatitis C Study Could Lead to Development of New Drugs, Yale Researchers Find

Yale researchers have found that the hepatitis C virus (HCV) uses a unique method of shutting down a cell’s protein synthesis, while continuing to produce viral proteins, possibly leading to a target for new drugs that would block HCV infection without harming body tissues.

The study, published in a recent issue of Science, also provides new clues on how certain messenger RNAs (mRNAs) cause protein synthesis to begin.

“We were trying to see how hepatitis C and other viruses initiate synthesis of proteins,” said Jennifer Doudna, the Henry Ford II Professor of Molecular Biophysics and Biochemistry at Yale. “We know that the virus requires a large piece of RNA called an internal ribosome entry site (IRES) to recruit the cell’s protein-making machinery. We wanted to find out how this big piece of RNA in hepatitis C recruits the ribosome for making the viral proteins found in hepatitis.”

HCV, a human pathogen and worldwide health threat, kills about 10,000 people a year in the United States. This high death toll is often due to side effects such as chronic liver disease, cirrhosis or liver cancer. Doudna said current drugs to combat the virus have not been very effective, so many research studies are aimed at finding new targets.

Doudna and associates, Jeffrey Kieft and Kaihong Zhou, collaborated with Joachim Frank and other researchers at the Howard Hughes Medical Institute to look at direct images of the IRES RNA bound to a ribosomal subunit called 40S. Using a technique called cryo-electron microscopy (cryo-EM), single particles were visualized and the images were collated and averaged. These images were then fed into a computer program, which sorts and orients them to generate a three-dimensional reconstruction of the particle.

“What we found was that the IRES RNA binds to the ribosome and appears to place the viral template for protein synthesis in the correct position on the ribosome,” said Doudna, who is also a researcher at the Howard Hughes Medical Institute. “The 40S subunit consists of three domains, called the ‘body,’ ‘head’ and ‘platform.’”

“It looks as if the IRES is pushing down on the head of the ribosome and causing it to reach down and fuse with the platform and the shoulder region,” Doudna adds. “So the IRES is essentially clamping the ribosome around the messenger RNA.”

Doudna said the clamping could cause the viral messenger RNA to lock into the correct site on the ribosome, so that the ribosome is forced to initiate production of a viral protein.

“This is the first time any kind of initiation complex with the eukaryotic ribosome-the cell’s protein-synthesizing machinery-has been visualized,” said Doudna. “A long-term goal of this work is to understand the molecular basis for protein synthesis in hepatitis. It’s already clear that hepatitis C and a number of other viruses utilize a method for protein synthesis that’s different from the mechanism used in the host cell. That already makes it a target for drugs that could serve as an effective treatment for HCV.”

Doudna’s collaborators on the study included Kaihong Zhou and Jeffrey S. Kieft at Yale; Joachim Frank, Christian M.T. Spahn and Robert A. Grassucci of the Howard Hughes Medical Institute at the Wadsworth Center; and Pawel Penczek of the Howard Hughes Institute and the State University of New York at Albany.