Prothrombin deficiency is an autosomal recessive disorder associated with moderate or severe bleeding tendency. In this study, a three-month-old boy with non-consanguineous parents was referred for convulsions because of intracerebral hemorrhage. Standard coagulation tests revealed that the patient's plasma prothrombin activity was 12%, while his father's and mother's levels were 55% and 70%, respectively. Analysis of the prothrombin gene revealed that this patient is a compound heterozygote for two missense mutations: one maternally inherited point mutation in the propeptide (p.Arg4Gln) and one paternally inherited mutation in the kringle-2 (p.Arg220Pro) domain. Structural analysis was performed and confirmed that the resulting mutations were inferred to respectively affect the cleavage of the propeptide from the Gla domain, and the stability of the kringle-2 domain, both resulting in a severe hypoprothrombinemia. In unusually bleeding newborn of non-consanguineous parents, rare severe homozygous bleeding disorders need to be considered to facilitate early diagnosis and treatment.
Antibody inhibitors against human thrombin are rare and have remained poorly characterized. We report the case of a 40-yr-old patient who developed a potent thrombin inhibitor revealed by mild bleeding symptoms and marked prolongation of most laboratory clotting times. After two years of evolution, he died from cerebral hemorrhage. The inhibitor, a polyclonal IgG, was associated with hematological and immunological criteria of autoimmune disorder. Antithrombin IgG was isolated from the patient's plasma by protein A- and thrombin-affinity chromatography. Fab fragments inhibited amidolytic activity of alpha thrombin, and thrombin-thrombomodulin catalyzed protein C activation with a Ki of approximately 10(-8) M in a noncompetitive manner. Alpha to gamma conversion of thrombin resulted in a moderate loss of affinity for the inhibitor. Upon complex formation of thrombin with staphylocoagulase or alpha 2-macroglobulin (alpha 2M), inhibition was decreased by two orders of magnitude and acquired an apparent competitive character. In Western blot experiments, the antibody reacted with active alpha-thrombin, did not react with chloromethylketone-inhibited thrombin and reacted with a lower affinity with iPr2P-thrombin. The inhibitor did not block thrombin binding to benzamidine-, heparin-, or fibrin-Sepharose, but displaced proflavin from its complex with thrombin. Taken together, these results indicate that the patient's autoantibody recognized a conformational structure which includes, at least in part, the apolar binding site adjacent to the catalytic site of thrombin.
Le deficit constitutionnel en prothrombine (facteur II de la coagulation) est une maladie tres rare, transmise de facon autosomique et recessive. Le deficit complet, probablement letal, n’a pas ete rapporte chez l’homme. Une quarantaine de familles ont ete decrites dans le monde, caracterisees par un deficit severe ou modere, lie a une anomalie quantitative (hypoprothrombinemie) ou qualitative (dysprothrombinemie). L’expression clinique est tres variable. Les deficits quantitatifs s’accompagnent de manifestations hemorragiques qui peuvent etre graves, en particulier chez les homozygotes ou heterozygotes composites ou elles sont assez bien correlees a la concentration plasmatique de prothrombine. Le risque hemorragique est beaucoup plus variable en cas de dysprothrombinemie, et mal correle a l’activite biologique de la prothrombine mesuree dans le plasma. Les anomalies moleculaires sont des mutations reparties sur toute la longueur du gene. Les dysprothrombinemies sont caracterisees par des anomalies du site catalytique de la thrombine ou des anomalies des sites de clivage de la prothrombine par le FXa, le plus souvent associees a des manifestations hemorragiques graves, ou encore par des anomalies de l’exosite I ou du site de liaison au sodium de la thrombine, avec pas ou peu de manifestations cliniques. L’identification des defauts moleculaires a l’origine des deficits constitutionnels et leur caracterisation fonctionnelle ont permis de progresser de facon notable dans la connaissance de la relation structure-fonction de la molecule.
