Enzyme Defective in Bleeding Abnormality

Applied Genetics News, August, 2002

Researchers the University of California, San Diego (UCSD) School of Medicine (9500 Gilman Dr., La Jolla, CA 92093; Tel: 858/534- 2230; Website: www.ucsd.edu) found that mutations in an enzyme, sialyltransferase ST3Gal-IV, greatly reduced levels of von Willebrand factor (VWF) in mice, by a mechanism also seen in some human patients with clinical signs of bleeding abnormalities and abnormally low levels of VWF. This study was published in the July 23, 2002 issue of the Proceedings of the National Academy of Sciences (PNAS).

The ST3Gal-IV enzyme normally acts by adding a sialic acid to certain blood factors that regulate clotting. The mutated version is unable to produce this sialic acid.

The level of VWF, a blood element involved in clotting, is a major determinant of the severity of von Willebrand disease (VWD), which is the most prevalent bleeding disorder in humans, affecting more than three million Americans. VWD patients have such symptoms as easy bruising, heavy or prolonged menstrual periods, frequent nosebleeds or prolonged bleeding after injury, surgery, childbirth, or dental work.

The study's senior author, Jamey D. Marth, a UCSD professor of cellular and molecular medicine and an investigator with the Howard Hughes Medical Institute, notes that the level of VWF in both mouse and human blood is highly variable, but must stay within a narrow range to avoid either excess bleeding or dangerous clotting that can lead to heart attack or stroke.

"Variability in VWF levels has been found to be a major determinant of VWD severity and abnormally high VWF level is reported as a risk factor for stroke and heart attack," Marth says. "What we've identified is a new regulatory control point that modifies the level of clotting factors within blood."

The research team found that 30 mice with the mutated gene bled for more than 4 min, on average, following a small cut to the tail, as compared with 30 normal mice that averaged 50 sec. The investigators also analyzed mouse blood samples for levels of VWF and Factor VIII, another blood component that regulates clotting. They determined that mice with mutated ST3Gal-IV had 30% of normal blood platelets, and from 10 to 50% of normal VWF and Factor VIII levels. The mice appeared to have a syndrome similar to human VWD.

"When VWF levels are too low in humans, blood doesn't clot fast enough," Marth says. "Many of these individuals will show up in hematology clinics to ask why they bleed excessively, bruise easily, or don't heal quickly."

The researchers studied 136 UCSD Medical Center patients with complaints of bleeding abnormalities. Of these, 117 patients had normal VWF levels. Of 19 patients with low VWF levels, 5 were found to have the same deficiency in sialic acid on VWF as did the mutant mice. The researchers are conducting further tests with these patients to determine if they lack ST3Gal-IV, and if reductions in ST3Gal-IV may prevent arteriosclerosis, stroke, and heart attack.