Pathologic blood clotting is a leading cause of morbidity and mortality in the developed world, underlying deep vein thrombosis,
myocardial infarction, and stroke. Genetic predisposition to thrombosis is still poorly understood, and we hypothesize that
there are many additional risk alleles and modifying factors remaining to be discovered. Mammalian models have contributed
to our understanding of thrombosis, but are low throughput and costly. We have turned to the zebrafish, a tool for high throughput
genetic analysis. Using zinc finger nucleases, we show that disruption of the zebrafish antithrombin III (at3) locus results in spontaneous venous thrombosis in larvae. Though homozygous mutants survive into early adulthood, they eventually
succumb to massive intracardiac thrombosis. Characterization of null fish revealed disseminated intravascular coagulation
in larvae secondary to unopposed thrombin activity and fibrinogen consumption, which could be rescued by both human and zebrafish
at3 cDNAs. Mutation of the human AT3 reactive center loop abolished the ability to rescue, but the heparin-binding site was dispensable.
These results demonstrate overall conservation of AT3 function in zebrafish, but reveal developmental variances in the ability
to tolerate excessive clot formation. The accessibility of early zebrafish development will provide unique methods for dissection
of the underlying mechanisms of thrombosis.