Familial overexpression of beta antithrombin caused by an Asn135Thr substitution.
We have investigated the basis of antithrombin deficiency in an asymptomatic individual (and family) with borderline levels (approximately 70% antigen and activity) of antithrombin. Direct sequencing of amplified DNA showed a mutation in codon 135, AAC to ACC, predicting a heterozygous Asn135Thr substitution. This substitution alters the predicted consensus sequence for glycosylation, Asn-X-Ser, adjacent to the heparin interaction site of antithrombin. The antithrombin isolated from plasma of the proband by heparin-Sepharose chromatography contained amounts of beta antithrombin (the very high affinity fraction) greatly increased (approximately 20% to 30% of total) above the trace levels found in normals. Expression of the residue 135 variant in both a cell-free system and COS-7 cells confirmed altered glycosylation arising as a consequence of the mutation. Wild-type and variant protein were translated and exported from COS-7 cells with apparently equal efficiency, in contrast to the reduced level of variant observed in plasma of the affected individual. This case represents a novel cause of antithrombin deficiency, removal of glycosylation concensus sequence, and highlights the potentially important role of beta antithrombin in regulating coagulation.
Bayston
T A
TA
Imperial College School of Medicine, London, UK.
Tripodi
A
A
Mannucci
P M
PM
Thompson
E
E
Ireland
H
H
Fitches
A C
AC
Hananeia
L
L
Olds
R J
RJ
Lane
D A
DA
D000328:Adult; D000595:Amino Acid Sequence; D000990:Antithrombin III / Q000737:chemistry / Q000235:genetics*; D020152:Antithrombin III Deficiency / Q000235:genetics*; D000991:Antithrombins / Q000032:analysis; D001216:Asparagine / Q000235:genetics*; D016384:Consensus Sequence; D005260:Female; D006031:Glycosylation; D006801:Humans; D007525:Isoelectric Focusing; D008297:Male; D008875:Middle Aged; D009154:Mutation; D017422:Sequence Analysis, DNA; D013912:Threonine / Q000235:genetics*; D013927:Thrombosis / Q000235:genetics
D000991:Antithrombins; D013912:Threonine; D001216:Asparagine; D000990:Antithrombin III