Blood, Vol. 93 No. 12 (June 15), 1999:
pp. 4242-4247
Familial Overexpression of 
Antithrombin Caused by an Asn135Thr
Substitution
T.A. Bayston,
A. Tripodi,
P.M. Mannucci,
E. Thompson,
H. Ireland,
A.C. Fitches,
L. Hananeia,
R.J. Olds, and
D.A. Lane
From the Imperial College School of Medicine, London, UK; "A.
Bianchi Bonomi" Hemophilia and Thrombosis Center, IRCCS Maggiore
Hospital and University, Milan, Italy; and the Dunedin School of
Medicine, University of Otago, Dunedin, New Zealand.
We have investigated the basis of antithrombin deficiency in an
asymptomatic individual (and family) with borderline levels (
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 
antithrombin (the very high affinity fraction)
greatly increased (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 antithrombin in regulating coagulation.
