Abstract
Upshaw–Schulman syndrome (USS) is caused by severe ADAMTS13 (a disintegrin and metalloproteinase
with a thrombospondin type 1 motif, member 13) deficiency due to homozygous or compound
heterozygous mutations in the ADAMTS13 gene. Previous studies suggest three possible
disease mechanisms: (1) reduced secretion of ADAMTS13 variants, (2) impaired proteolytic
activity, (3) defective biosynthesis due to nonsense-mediated decay. Expression studies
have failed to establish a clear genotype/phenotype correlation that could explain
the significant variability in the age of onset and patients' clinical courses. In
this study, we investigated ADAMTS13 sequence variations in 30 USS patients and identified
31 disease-causing mutations; among them 10 novel variants. While none of the recombinant
proteins exhibited significant retention in the endoplasmic reticulum, secretion and
activity analysis revealed defective release for all but one missense mutant. The
latter exhibited normal secretion but impaired activity due to inactivation of the
catalytic domain. Truncated mutants showed secretion and residual activity even though
the patients suffered from a severe phenotype. The expression systems which we used
may not be appropriate here, as they do not assess nonsense-mediated decay causing
degradation of mRNA. In some patients, phenotypic severity could be explained by the
combined effects of two mutations. Genetic screening in combination with in vitro
characterization of ADAMTS13 variants from both alleles is a valuable tool to predict
the phenotypic severity of USS. When necessary, supplementary methods, such as kinetics
under flow conditions and mRNA processing assays, can be included. Such data are helpful
to identify patients who are at high risk for severe attacks and therefore might benefit
from prophylactic treatment.
Keywords
ADAMS/ADAMTS13 - von Willebrand factor - thrombotic thrombocytopenic purpura - thrombocytopenia
