ADAMTS13 cleaves plasmatic von Willebrand factor (VWF) between Tyr1605 and Met1606 and regulates thereby the hemostatic activity of VWF. Mutations in the ADAMTS13 gene leading to severe ADAMTS13 deficiency have been found in patients with congenital thrombotic thrombocytopenic purpura (TTP). We have analyzed the ADAMTS13 gene defects in two brothers with hereditary TTP [Antoine et al, Brit. J. Hematol., 2003] where we detected a total of six nucleotide exchanges causing point mutations. On the maternal allele we found an accumulation of five amino acid substitutions (R7W, Q448E, P618A, A732V, R1336W) and on the paternal allele a stop mutation (Q44X) leading to premature protein termination in the propeptide region. Both brothers were double heterozygotes with < 3% of ADAMTS13 activity, whereas their asymptomatic parents have ~ 50% activity. Four (R7W, Q448E, P618A, A732V) of the five maternal mutations constitute single nucleotide polymorphisms (SNP) but R1336W was identified as novel rare mutation in the second cub domain. To evaluate the biologic phenotype of a given haplotype, e.g. the functional significance of the presence of the various SNPs, we analyzed the functional impact of the individual mutations on ADAMTS13 antigen levels and ADAMTS13 activity. A series of mutant ADAMTS13 molecules was expressed which contained either single amino acid substitutions or combinations of mutations with each other. We found that the common SNPs R7W, Q448E and A732V, as single mutations, had either no or only a minor impact on ADAMTS13 secretion and ADAMTS13 activity, whereas P618A and R1336W conferred a dominant adverse effect on ADAMTS13 secretion levels. Co-expression of SNPs R7W or Q448E with SNP P618A lead to improved ADAMTS13 secretion levels and could therefore partly attenuate the detrimental effect of P618A. Concomitant expression of all four SNPs reconstituted secretion levels similar to wild-type implicating a complex synergistically interaction of SNPs located in different ADAMTS13 domain regions, however, functional activity was impaired to 50%. Mutation R1336W was shown to be, as a single amino acid exchange, responsible for reduced ADAMTS13 antigen levels, but in contrast to P618A, the negative effect of R1336W was rather enhanced by the co-expression of R7W and Q448E, than rescued, leading to the total absence of ADAMTS13 secretion from the maternal allele. Our findings provide for the first time evidence that fairly common SNPs, dependent on the presence or absence of other mutations, may differently modulate functional ADAMTS13 protease levels.