"This is a major development that no doubt will profoundly influence the
field of hepatitis C research," says NIAID Director Anthony S. Fauci,
M.D. "Having an infectious HCV clone available will greatly facilitate
studies on virus evolution, pathogenesis and host immune response. It
will enable scientists to better understand the factors and mechanisms
that determine whether virus is cleared from the body or produces a
When the hepatitis C virus was first discovered in 1989, scientists
believed they had found the cause of the many cases of hepatitis, often
acquired through blood transfusions, that could not be attributed to
either the hepatitis A or B viruses. Since then, researchers have
detected HCV genetic sequences and antibodies in patients with classic
non-A, non-B hepatitis, and screening the blood supply for HCV has
lowered significantly the incidence of this disease.
Nevertheless, scientists have been unable to grow the virus efficiently
in tissue culture or purify it using other laboratory procedures,
necessary steps in the process of verifying that a suspected infectious
agent is able to cause disease. Therefore, questions have remained
about whether the HCV genetic sequences found in patients truly
represent the infectious agent, or are defective or non-infectious
versions of the virus.
Charles M. Rice, Ph.D., and colleagues at Washington University in St.
Louis, Mo., and at the Food and Drug Administration (FDA) in Bethesda,
Md., have now answered these questions. Their findings appear in the
July 25, 1997 issue of the journal Science.
The researchers extracted HCV RNA, the virus's genetic material, from
the serum of an individual infected with the virus. They then made
thousands of DNA "clones" of the HCV RNA which, in turn, were
transcribed back into RNA and injected into the livers of chimpanzees to
see if they could cause infection. Chimpanzees are the only known
non-human host for HCV.
After 34 of the most promising RNA transcripts failed to cause infection
in the chimpanzees, Dr. Rice and his colleagues analyzed the sequences
of numerous HCV clones to determine a "consensus sequence" of highly
conserved regions within the HCV genome. The researchers then
constructed a full-length "consensus clone," reflecting the consensus
sequence. RNA transcripts of the consensus clone, which had also been
tagged with a unique molecular identification marker, were then injected
directly into the livers of two healthy chimpanzees.
In the weeks following injection, the researchers detected signs of
infection in the chimpanzees, including enzymes produced in response to
liver damage, antibodies to HCV and the presence of HCV RNA. Liver
biopsies of each animal also showed tissue damage typical of that seen
in HCV-infected chimpanzees. Identification markers on the HCV RNA
revealed that the consensus clone had caused infection.
"The RNA levels in the chimps increased steadily in the weeks following
inoculation," says Dr. Rice. "If the RNA had come from the original
inoculum, instead of from new virus particles, its concentration would
have decreased. The RNA also was resistant to RNase, an enzyme that
breaks down naked RNA. The enzyme does not degrade RNA that is inside
The new finding should be a boon to HCV research. "Viral clones can now
be used to establish infections and study the genetics of HCV
replication in laboratory animals," says John La Montagne, Ph.D.,
director of NIAID's Division of Microbiology and Infectious Diseases.
"Importantly," he adds, "large quantities of infectious HCV RNAs can be
produced and used to find ways to grow the virus in cell culture."
Dr. Rice's research is supported in part through one of four Hepatitis C
Cooperative Research Center grants awarded by NIAID in 1996. These
Centers are multidisciplinary, collaborative research units designed to
generate the knowledge needed to devise preventive and therapeutic
strategies for hepatitis C infection.
HCV causes approximately 150,000 cases of acute viral hepatitis each
year in the United States. Recovery from infection is rare and between
70 and 90 percent of infected persons become chronic carriers of the
virus. According to the Centers for Disease Control and Prevention,
chronic liver disease due to HCV causes between 8,000 and 10,000 deaths
and leads to about 1,000 liver transplants each year in the United
NIAID, a component of the National Institutes of Health (NIH), supports
research on AIDS, tuberculosis and other infectious diseases, as well as
allergies and immunology. NIH and FDA are agencies of the U.S.
Department of Health and Human Services.