Complement activation in thrombotic thrombocytopenic purpura

BACKGROUND

Ultra-large von Willebrand factor and deficiency of its cleaving protease are important factors in the events leading to thrombotic microangiopathy; however, the mechanisms involved are only partly understood. Whereas pathological activation of the alternative complement pathway is linked to atypical hemolytic uremic syndrome, the role of complement activation in thrombotic thrombocytopenic purpura (TTP) is unknown. The aim of this study was to investigate whether signs of complement activation are characteristic of TTP.

PATIENTS AND METHODS

Twenty-three patients with TTP (18 women, median age 38 years) and 17 healthy controls (13 women, median age 38 years) were included. Complement parameters (C3, Factors H, I, B and total alternative pathway activity) together with complement activation fragments (C3a) or complexes (C1rs-INH, C3bBbP, sC5b9) were measured by ELISA or RID. ADAMTS13 activity and anti-ADAMTS13 inhibitory antibodies were measured by the VWF-FRET73 assay.

RESULTS

Increased levels of C3a, and SC5b9 were observed in TTP during acute episodes, as compared with healthy controls. Decreased complement C3 levels indicative of complement consumption occurred in 15% of acute TTP patients. Significant decrease of complement activation products C3a and SC5b9 was observed during plasma exchange (PEX). The sustained presence of anti-ADAMTS13 inhibitory antibodies in complete remission was associated with increased complement activation.

CONCLUSION

These data document in an observational study the presence of complement activation in TTP. Further investigation is needed to determine its potential pathogenetic significance.

Common large partial VWF gene deletion does not cause alloantibody formation in the Hungarian type 3 von Willebrand disease population

BACKGROUND

Type 3 von Willebrand disease (VWD) is an autosomal recessive bleeding disorder, characterized by virtually undetectable plasma von Willebrand factor (VWF) and consequently reduced plasma factor VIII levels. Genetic mutations responsible for type 3 VWD are very heterogeneous, scattered throughout the VWF gene and show high variability among different populations.

METHODS

Twenty-five severe VWD patients were studied by direct sequencing of the 51 coding exons of the VWF gene. The total number of VWD type 3 families in Hungary is 24, of which 23 were investigated.

RESULTS

Fifteen novel mutations were identified in 31 alleles, five being nonsense mutations (p.Q1238X, p.Q1898X, p.Q1931X, p.S2505X and p.S2568X), four small deletions and insertions resulting in frame shifts (c.1992insC, c.3622delT, c.5315insGA and c.7333delG), one a large partial deletion (delExon1-3) of the 5'-region, four candidate missense mutations (p.C35R, p.R81G, p.C295S, p.C623T) and one a candidate splice site mutation (c.1730-10C>A). Six previously described mutations were detected in 17 alleles, including the repeatedly found c.2435delC, p.R1659X and p.R1853X. Only one patient developed alloantibodies to VWF, carrying a homozygous c.3622delT.

CONCLUSION

We report the genetic background of the entire Hungarian type 3 VWD population. A large novel deletion, most probably due to a founder effect, seems to be unique to Hungarian type 3 VWD patients with high allele frequency. In contrast to previous reports, none of the five patients homozygous for the large partial deletion developed inhibitors to VWF. This discrepancy raises the possibility of selection bias in some of the reports.