Diagnose der thrombotischthrombozytopenischen Purpura

Zusammenfassung

Die TTP ist durch das generalisierte Auftreten von hyalinen Thromben in der Mikrozirkulation gekennzeichnet. Bei allen bekannten Fällen der angeborenen TTP ist die von-Willebrand-Faktor-spaltende Protease (VWF-CP) stark vermindert. Auch bei einem aktuell noch umstrittenen Prozentsatz von Patienten mit erworbener TTP ist diese Proteaseaktivität infolge Autoantikörperbildung nicht nachweisbar oder stark vermindert (T<5%). Daher gehört neben der konventionellen Diagnostik die Untersuchung der VWF-CP und der Nachweis gegen sie gerichteter Antikörper zur diagnostischen Aufarbeitung von Fällen mit thrombotischer Mikroangiopathie. Allen Methoden zum Nachweis der VWF-CP gemeinsam ist der initiale Verdau des Substrates (gereinigter VWF, rekombinanter VWF, Plasma ohne endogene VWF-CP, Fragmente mit spezifischer Spaltstelle oder Patientenplasma mit eigener endogener Protease). Der Nachweis des Verdaus erfolgt mittels Analyse der residualen Multimere, des Autretens spezifischer Spaltprodukte, der Bestimmung der residualen VWF:CB oder des residualen VWF:RCo, oder mit fragmentspezifischen monoklonalen Antikörpern. Wirklich schnell (<30 min) funktioniert die Messung im Cone-and-plate(let)-Aggregometer. Nach Zusatz einer geringen Plasmamenge eines TTP-Patienten kommt es zu starker Erhöhung von Thrombozytenadhäsion und -aggregation einer Normalperson, während beide Aktivitäten bei anderen thrombotischen Mikroangiopathien infolge Verdünnungseffekt geschwächt werden. Die Molekulargenetik spielt eine wichtige Rolle für die Unterscheidung zwischen angeborener und erworbener TTP.

Summary

As hallmark of TTP, generalized hyaline thrombi in the patient’s microcirculation is known. These thrombi are composed of platelets and VWF. A severe defect of the VWF cleaving protease (VWF-CP) was found in all known patients with the inherited form of TTP. In contrary, although a severe deficiency of VWF-CP is specific for the acquired form, too, only a fraction of these patients is characterized by a severe deficiency. In most patients with a severe deficiency autoantibodies directed against VWF-CP is detectable in plasma. However, many patients with acquired TTP do not show any severe deficiency. Because treatment differs in inherited and acquired forms and as persistance of autoantibodies during clinical remission is of prognostic value, the determination of the activity of VWF-CP and of antibodies against VWF-CP are important parts in the workup of patients with TTP. In all methods for the determination of the activity of VWF-CP the first step is proteolysis of a specific substrate for the protease. In the second step the activity of the protease is measured by analysis of the residual VWF multimers, by the generation of specific fragments, by using the residual VWF:CB or VWF:RCo as marker of the loss of multimers or with help of specific monoclonal antibodies. In less than 30 min the cone and plate(let) aggregometer helps to distinguish between different forms of thrombotic microangiopathies. While adhesion and aggregation of platelets from a healthy person are clearly enhanced after addition of a small amount of plasma from a TTP patient, both characteristics are weakened by plasma from patients with other forms of thrombotic microangiopathy (dilution effect). Molecular genetics are established methods in the differentiation between inherited and acquired forms of TTP in those cases without autoantibodies against VWF-CP.

Schlüsselwörter

Thrombotisch thrombozytopenische Purpura - thrombotische Mikroangiopathie - VWF-spaltende Protease - ADAMTS-13

Keywords

Thrombotic thrombocytopenic purpura - thrombotic microangiopathy - VWF-cleaving protease - ADAMTS-13

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