JUPITER, Fla. – Scientists at the Florida campus of TheScripps Research Institute have discovered a molecular interaction between astructural hepatitis C virus protein (HCV) and a protein critical to viralreplication.
This new finding strongly suggests a novel method ofinhibiting the production of the virus and a potential new therapeutic targetfor hepatitis C drug development.
The study was published in the January 2010 issue of theJournal of General Virology.
Hepatitis C virus infects between 130 and 170 million peopleworldwide. Hepatitis C is an infectious disease affecting the liver, caused bythe hepatitis C virus (HCV). The infection is often asymptomatic, but onceestablished, chronic infection can progress to scarring of the liver(fibrosis), and advanced scarring (cirrhosis) which is generally apparent aftermany years. In some cases, those with cirrhosis will go on to develop liverfailure or other complications of cirrhosis, including liver cancer or lifethreatening esophageal varices and gastric varices.
Because current HCV treatments are only partially effective,a number of alternative molecular mechanisms are actively being pursued aspossible drug targets.
These new data underline the essential role of the viralprotein known as "core" as a primary organizer of the infectious HCV particleassembly and support a new molecular understanding of the formation of theviral particle itself.
"While our finding that the HCV core interacts with thenon-structural helicase protein was not totally unexpected, this had not reallybeen confirmed until this study," Scripps Florida Professor Donny Strosberg,who led the study, says in a statement. "But the most exciting part is that smallmolecule inhibitors of dimerization (the joining of two identical subunits) ofcore actually inhibit interaction between core and helicase, thus possiblypreventing production of an infectious viral particle."
One of the critical problems of finding inhibitors for thehepatitis C virus is that it mutates at such prodigious rates. An RNA virussuch as hepatitis C can mutate at a rate estimated as high as one million timesthat of DNA viruses such as the herpes virus.
Looking closely at the core interaction with itself,Strosberg developed several novel quantitative assays or tests for monitoringthese protein-protein interactions with the specific goal of identifyinginhibitors of the core dimerization, which would block virus production.
"People have been dreaming about inhibiting protein-proteininteractions, as a new El Dorado for finding novel drug targets," saysStrosberg, "but few conclusive studies have emerged, except in the virus-hostarea."
Core is particularlyimportant in the assembly of the hepatitis C nucleocapsid or capsid, anessential step in the formation of infectious viral particles; the nucleocapsidis the virus genome protected by a protein coat.
"In one sense, the ongoing issue with hepatitis C is thatthere are still so very few drugs to treat the virus and very few tools tostudy it," Strosberg points out. "We set out to develop new tools and to identify anew target – core, the capsid protein. By targeting the interactions of corewith itself or other proteins, we could reduce the problem of rapid mutationnot only because the core protein mutates significantly less, but also becausemutations that would affect the interface between core and itself or otherproteins would often be more likely to deactivate the virus, in contrast tomutations in viral enzymes which often lead to increased resistance to drugs."
Last year, Strosberg developed a novel quantitative test formonitoring these protein-protein interactions with the specific goal ofidentifying inhibitors of the core dimerization, which would block virusproduction. Strosberg and his colleagues uncovered peptides derived from thecore protein of hepatitis C that inhibit not only dimerization of the coreprotein, but also production of the actual virus.
That earlier study led to the discovery of non-peptidicsmall organic molecules that strongly inhibited HCV production, one of which,SL201, was used in the new study.
In the new study, Strosberg and his colleagues focused onnon-structural proteins that provide functions relating to HCV production, inparticular NS3 helicase. The scientists' findings support a growing body ofevidence that this protein participates in the assembly and production ofinfectious viral particles. The interaction of the core protein with thisnon-structural protein also confirms core as a key organizer of virus assemblyand suggests it acts to facilitate the packaging and integration of the newlysynthesized viral RNA.
The new research, however, led to the discovery of twopeptides that inhibited HCV production by 68 percent and 63 percent,respectively; a third related peptide showed 50 percent inhibition. When addedto HCV-infected cells, each of the three peptides blocked release but notreplication of infectious virus; viral RNA levels were reduced by seven fold.Strosberg notes that the efficacy of small molecules like these can often beimproved over initial levels.
"After we'd finished our work, we learned that FrankChisari—one of the leading experts on HCV who works at Scripps Research in LaJolla—had been looking at similar peptides using a very different approach,"notes Strosberg. "One of his peptides was the same as ours—it also inhibited virusproduction. It's an incredible coincidence and a confirmation of our study."
Thefirst author of the study, "Dimerization-Driven Interaction of Hepatitis CVirus Core Protein with Ns3 Helicase," is Guillaume Mousseau of ScrippsResearch. Additional authors include Smitha Kota, S. and Virginia Takahashi ofScripps Research, and David Frick of the University of Wisconsin, Milwaukee.