Researchers identify clotting protein which causes hepatitis B

Discovery offers ‘new hope to patients by paving the way for future therapies that will change the course of hepatitis’

A protein molecule that contributes to the severity of chronic viral hepatitis in humans, and which may also be implicated in SARS, has been identified by a team of scientists from Toronto General and St. Michael’s Hospitals. This data is published in the July 1 issue of the Journal of Clinical Investigation.

The protein, called Fgl2/fibroleukin prothrombinase, is a newly discovered protein which causes blood to clot in the livers of humans with viral hepatitis. In animal trials, this same protein causes blood to clot in the livers of mice that are exposed to the corona virus.

Blood or fibrin clots are the clumps that result from coagulation of the blood, and may cause partial or complete blockage of a blood vessel. This deprives the tissue of normal blood flow and oxygen, resulting in damage or death of the tissue.

Corona viruses are named for their corona-like (halo) appearance in electron micrographs. They account for 50 per cent of all upper respiratory infections in humans, and a mutant form of the virus is a cause of Severe Acute Respiratory Syndrome (SARS). The virus can be fatal and has been shown to cause liver damage in mice.

The research in this study provides compelling evidence of the role of the clotting protein molecule fgl2 in viral chronic hepatitis, and also raises the question of whether the same protein molecule is also important in SARS infection of human lung tissue.

“This offers new hope to patients by paving the way for future therapies that will change the course of hepatitis,” says Dr. Philip Marsden, a Nephrology specialist at St. Michael’s Hospital, Professor of Medicine at University of Toronto, and the Keenan Chair of Medical Research at St Michael’s Hospital. “Our work represents an innovative new approach to combating viral disease. Therapies to date have focused on getting rid of the virus, but this work points the way to blocking the damage the virus does.”

“These studies confirm previous studies in mice of the importance of fgl2 or the clotting protein in viral disease,” says Dr. Gary Levy, Medical Director of the Multi-Organ Transplant Program at Toronto General Hospital, University Health Network and Professor of Medicine at the University of Toronto. “Antibodies are now being generated to neutralize fgl2 activity which we hope will be useful in treating patients with viral hepatitis, and may be of value to patients with SARS. We are now examining patients with SARS for fgl2 activity and the possible use of neutralizing antibodies for their treatment.”

The work originally began in the laboratory of Dr. Gary Levy when his team isolated the protein molecule from the livers of corona virus-infected mice, and found that the molecule has unique and novel clotting aspects. Specifically, the protein is expressed by immune cells only when triggered by the corona virus. Once produced, fgl2 directly cleaves prothrombin to thrombin resulting in a fibrin clot at the site of acute viral infection. The same gene has been identified in humans and in the present article is shown to be triggered by the human virus Hepatitis B, resulting in liver damage. Interestingly, only patients with severe Hepatitis B Virus Infection in contrast to patients with inactive viral infection and normal controls expressed fgl2.

In order to test the effect of the protein on the severity of hepatitis, knock-out mice were created which did not have the gene that produces the clotting protein. These mice were infected with the corona virus and compared to corona virus infected mice that had the gene for the protein. All the mice became infected by the corona virus but only mice which carried the gene became ill and ultimately succumbed to the infection.

In addition to the tests on mice, liver biopsies from 23 patients with severe chronic hepatitis B and 13 patients with minimal chronic hepatitis (36 patients in total) were tested for a link between amounts of the clotting protein and the severity of chronic hepatitis B.

Results

• Patients with severe chronic hepatitis B: These patients expressed fgl2 protein in their livers in association with fibrin clots resulting in liver cell death.
• Patients with mild chronic hepatitis B: These patients had no evidence of fgl2 expression or clotting within their livers.
• Mice with the gene producing clotting protein: These mice experienced blood clotting, and died within four – seven days of hepatitis after being infected with the corona virus. Blood test showed signs of liver inflammation and liver cell death.
• Mice without the gene failed to generate blood clotting in response to corona virus infection and 40 per cent of the mice were still alive 14 days after being infected.

Implications

• A new clotting factor has been found which is important in viral infection;
• This clotting protein is important in human hepatitis B as well as in mice where it is triggered by the corona virus. It may therefore have implications for the human corona virus which causes SARS;
• The clotting protein is a logical target for molecular manipulation and offers the hope for developing newer treatment approaches for patients with chronic viral hepatitis;
• This is the first time that blocking a clotting protein has been thought of in the treatment of hepatitis B; typically hepatitis treatments focus on killing the virus or blocking the growth of the virus;
• If we can develop inhibitors of the clotting molecule that work in hepatitis B patients, perhaps the same inhibitors might work in SARS patients.

Funding

This work was funded by grants from the Canadian Institute of Health Research, Heart and Stroke Foundation of Canada, and Dr. Levy is the recipient of the CIHR Novartis Research Chair in Transplantation.

St. Michael’s Hospital is a Catholic teaching and research hospital, fully affiliated with the University of Toronto, specializing in inner city health, heart and vascular disease, trauma/neurosurgery, diabetes comprehensive care, minimal access therapeutics, and neurological and musculoskeletal disorders.

Toronto General Hospital is a partner in University Health Network, along with Toronto Western and Princess Margaret Hospitals. The scope of research and complexity of cases at Toronto General Hospital has made it a national and international source for discovery, education and patient care. It has one of the largest hospital-based research programs in Canada, with major research projects in cardiology, transplantation, surgical innovation, infectious diseases, and genomic medicine. Toronto General Hospital is a teaching hospital affiliated with the University of Toronto.

To arrange interviews with Dr. Levy, please contact:
Alex Radkewycz or Kim Garwood, Public Affairs
Toronto General Hospital, University Health Network
Phone: Alex – 416-340-3895
Kim – 340-4800 ext. 6753

To arrange interviews with Dr. Philip Marsden, please contact:
Tracy MacIsaac, Media Relations
St. Michael’s Hospital
Phone: 416-864-5047
Page (via locating): 416-864-5431