Thrombomodulin mutations in atypical hemolytic-uremic syndrome.
BACKGROUND
The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.


METHODS
We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.


RESULTS
Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.


CONCLUSIONS
Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.
Delvaeye
Mieke
M
VIB-K.U.Leuven Vesalius Research Center, Leuven, Belgium.
Noris
Marina
M
De Vriese
Astrid
A
Esmon
Charles T
CT
Esmon
Naomi L
NL
Ferrell
Gary
G
Del-Favero
Jurgen
J
Plaisance
Stephane
S
Claes
Bart
B
Lambrechts
Diether
D
Zoja
Carla
C
Remuzzi
Giuseppe
G
Conway
Edward M
EM
D000293:Adolescent; D000328:Adult; D002648:Child; D003167:Complement Activation / Q000235:genetics*; D003179:Complement C3b; D017244:Complement Factor I; D003170:Complement Pathway, Alternative / Q000502:physiology; D004252:DNA Mutational Analysis; D006463:Hemolytic-Uremic Syndrome / Q000235:genetics* / Q000276:immunology; D006579:Heterozygote; D006801:Humans; D008875:Middle Aged; D020125:Mutation, Missense; D010375:Pedigree; D020641:Polymorphism, Single Nucleotide; D018180:Thrombomodulin / Q000235:genetics* / Q000378:metabolism; D055815:Young Adult
D018180:Thrombomodulin; D003179:Complement C3b; D017244:Complement Factor I