The von Willebrand Factor-Cleaving Protease (ADAMTS-13) and the Diagnosis of Thrombotic Thrombocytopenic Purpura (TTP)

Relationship of Thrombospondin 1 to von Willebrand Factor and ADAMTS-13 in Sickle Cell Disease Patients of Arab Ethnicity

BACKGROUND

Thrombospondin 1 (TSP-1) is a multifunctional glycoprotein secreted by platelets. In sickle cell disease (SCD), TSP-1 promotes red cell adhesion to the endothelium by binding to von Willebrand factor (vWF) and inhibiting its degradation by the protease ADAMTS-13. We investigated a possible correlation between TSP-1, vWF and ADAMTS-13 in adult and pediatric SCD patients.

METHODS

Using commercially available ELISA kits, TSP-1, vWF and ADAMTS-13 levels were measured in 59 SCD patients (20 children and 39 adults) and compared with 59 age- and sex-matched controls. Associations between TSP-1 and parameters of interest were analyzed using Pearson's correlation coefficient.

RESULTS

Although TSP-1 levels were higher in adult and pediatric SCD patients than in controls, the increase was not statistically significant (p > 0.05). We found a significant positive correlation between TSP-1 and platelet count in both adult (r = 0.402, p = 0.01) and pediatric (r = 0.589, p = 0.01) patients, which is expected due to increased platelet activation in SCD. There was a positive correlation between TSP-1 and vWF in normal adults (r = 0.305, p = 0.049) and children (r = 0.633, p = 0.005) but not in patients (p > 0.05). A significant negative correlation between TSP-1 and ADAMTS-13 activity (r = -0.41, p = 0.01) was found in adult patients. Also, a significant negative correlation between TSP-1 and ADAMTS-13/vWF antigen ratio in both normal controls (r = -0.595, p = 0.009) and patients (r = -0.493, p = 0.032) is reported for the pediatric group.

CONCLUSIONS

Our findings confirm the inhibitory effects of TSP-1 on ADAMTS-13 activity in adult SCD patients. The negative correlation reported between TSP-1 and ADAMTS-13/vWF antigen ratio in pediatric subjects suggests a possible protective mechanism in younger individuals, although this is not related to the presence of SCD. This work emphasizes the impact of age on interpreting results related to the regulation of vWF expression and interaction with TSP-1 and ADAMTS-13 in SCD.

Evaluation of von Willebrand factor and ADAMTS-13 antigen and activity levels in sickle cell disease patients in Kuwait

Sickle cell disease (SCD) is a severe form of hemolytic anemia characterized by chronic hemolysis and is associated with increased thrombotic risk. Elevated von Willebrand factor (vWF) levels in SCD have been attributed to increased secretion and impaired processing by its cleaving protease ADAMTS-13. In this study we measured vWF and ADAMTS-13 antigen and activity levels in our SCD patients. Hematological and biochemical parameters for 59 SCD patients (20 children and 39 adults) were analyzed and compared to 59 age- and sex-matched controls. Commercially available ELISA kits were used to measure vWF and ADAMTS-13 antigen and activity levels in patients and controls. Patients had significantly higher levels of vWF (p < 0.006) and ADAMTS-13 activity (p < 0.006) compared to controls. When patients were analyzed according to age and genotype, adult patients (23 SS and 16 Sβ⁰thal) maintained higher vWF antigen levels (p < 0.001), but with reduced ADAMTS-13 activity to vWF:Ag ratio (p < 0.003) compared to controls. Pediatric patients (8 SS and 12 Sβ⁰thal) had comparable vWF antigen levels to controls (p > 0.05), but had higher levels of ADAMTS-13 activity (p < 0.011) and ADAMTS-13 activity to vWF:Ag ratio (p < 0.038). Age is an important factor to consider when vWF and ADAMTS-13 proteins are analyzed among our patients. Increased vWF in adult patients may be attributed to increased production and resistance of vWF to proteolysis rather than ADAMTS-13 deficiency. This outcome was not seen in pediatric patients as higher ADAMTS-13 activity maintained vWF antigen at comparable levels to normal controls.

Acute myocardial infarction caused by tumor-associated thrombotic thrombocytopenic purpura: case report

OBJECTIVE

To describe a case with acute myocardial infarction caused by gastric carcinoma-associated thrombotic thrombocytopenic purpura.

CLINICAL PRESENTATION AND INTERVENTION

A 79-year-old man was admitted with abdominal pain and pyrexia. He later developed cardiac complications and microangiopathy that indicated worsening progression. Manifold evidence confirmed the diagnosis of myocardial infarction caused by thrombotic thrombocytopenic purpura. The patient was treated mainly with plasma transfusion incorporated with steroids.

CONCLUSION

This case should remind physicians to consider microangiopathy as a differential diagnosis in patients with unexplained cardiac symptoms or atypical presentation. Early diagnosis and treatment are helpful in decreasing the sequelae of this syndrome.

Thrombotic thrombocytopenic purpura in childhood: an uncommon but life-threatening cause of thrombocytopenia

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder characterised by microangiopathic haemolytic anaemia, thrombocytopenia and signs of ischaemic organ dysfunction such as neurological or renal impairment and fever. The diagnosis of TTP should be considered in any child presenting with thrombocytopenia, in particular those children with microangiopathic haemolysis, atypical immune thrombocytopenia purpura or Evan's syndrome. Distinguishing TTP from haemolytic uraemic syndrome is difficult, but where there is doubt about the diagnosis, a presumptive diagnosis of TTP should be made to allow potentially life-saving therapy with therapeutic plasma exchange. Recent advances in the molecular basis of the disease have resulted in assays for ADAMTS-13 activity, inhibitor levels and ADAMTS-13 mutation analysis. These assays help to distinguish TTP from haemolytic uraemic syndrome. However, the performance characteristics of these assays in the diagnosis and management of TTP are yet to be defined.

Molecular biology of ADAMTS13 and diagnostic utility of ADAMTS13 proteolytic activity and inhibitor assays

ADAMTS13, a reprolysin-like metalloprotease, limits platelet-rich thrombus formation in the small arteries by cleaving von Willebrand factor (vWF) at the Tyr1605-Met1606 peptide bond. Deficiency of plasma ADAMTS13 activity, due to either an inherited or an acquired etiology, may lead to a potentially lethal syndrome, thrombotic thrombocytopenic purpura (TTP). Molecular cloning and characterization of the ADAMTS13 gene have provided further insight into the structure-function relationships, biosynthesis, and regulation of the ADAMTS13 protease, in addition to understanding the pathogenesis of TTP and perhaps other thrombotic disorders. ADAMTS13 consists of a short propeptide, a typical reprolysin-like metalloprotease domain, followed by a disintegrin-like domain, first thrombospondin type 1 (TSP1) repeat, Cys-rich domain, and spacer domain. The carboxyl terminus of ADAMTS13 has seven more TSP1 repeats and two CUB domains. ADAMTS13 is synthesized mainly in hepatic stellate cells, but also in vascular endothelial cells. Recognition and cleavage of vWF require the proximal carboxyl terminal domains, but not the middle and distal carboxyl terminal domains. Cleavage of vWF appears to be modulated by shear force, binding to platelet or platelet glycoprotein-1balpha, heparin, inflammatory cytokine (interleukin-6), and chloride ion. At the site of thrombus formation, the ADAMTS13 may be inactivated by thrombin, plasmin, and factor Xa. Having a sensitive and specific assay for ADAMTS13 activity is not only critical to understand the basic biology of ADAMTS13 protease, but also to facilitate a more timely and accurate clinical diagnosis of TTP, and to initiate potentially life-saving plasma exchange therapy. Although many assays have been developed and tested for clinical applications, the fluorescent resonance energy transfer-vWF73 assay appears to be the simplest and most promising assay to date.