Researchers identify clotting protein which causes hepatitis B

Toronto, ON July 7, 2003 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 was 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% 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.

“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 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.

The results included:

– Patients with severe chronic hepatitis B expressed fgl2 protein in their livers in association with fibrin clots resulting in liver cell death.
– Patients with mild chronic hepatitis B had no evidence of fgl2 expression or clotting within their livers.
– Mice with the gene producing clotting protein experienced blood clotting, and died within four to seven days of hepatitis after being infected with the corona virus. Blood tests 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% of the mice were still alive 14 days after being infected.

The researchers say the discovery of this new clotting factor may have implications for other viruses such as the human corona virus that causes SARS. The clotting protein is a logical target for molecular manipulation, and offers hope for developing newer treatment approaches for patients with chronic viral hepatitis. It is also the first time that blocking a clotting protein has been considered for use in the treatment of hepatitis B. Finally, if inhibitors of the clotting molecule that work in hepatitis B patients are developed, the researchers say that perhaps the same inhibitors might work in SARS patients.