Page 1980 - Williams Hematology ( PDFDrive )

1954 Part XII: Hemostasis and Thrombosis Chapter 114: Control of Coagulation Reactions 1955

from fibrinogen by thrombin, and APC levels in healthy nonsmok- phospholipid, it reduces the specific clotting activity and susceptibil-
ing adults, suggesting APC is a significant regulator of basal thrombin ity to APC. Normal plasma contains a mixture of factors V1 and V2.
activity. 15,158 Removal of the carbohydrate attached to factor V increases the rate of
Factors V and VIII are synthesized as large single-chain precursor inactivation of factor Va by APC, although the clinical significance of
coagulation cofactors of Mr 330,000, consisting of three homologous A this phenomenon is unknown. 185
domains (A1, A2, and A3) and two homologous C domains (C1 and C2) APC resistance with no identifiable genetic or acquired abnormal-
with a very large intervening, generally nonhomologous domain, desig- ities is well described in patients with venous and arterial thrombosis
nated the B domain, that connects the A2 and A3 domains (Chap. 113). and, at least for research purposes, should be therefore examined in
Activation of the inactive precursor form of the two cofactors V and patients with a suspected thrombophilia. Further studies are needed to
VIII involves limited proteolysis. 23,159–164 Factor V activation involves identify the causes of APC resistance in such patients. 186–188 One major
cleavages at Arg709, Arg1018, and Arg1545 by thrombin, factor Xa, or challenge involves defining the normal range for the clotting assays that
other proteases. 23,164–168 Cleavage at Arg1545 is the key step for gener- are actually used to characterize APC resistance and the multiple plasma
ating factor Va activity because this proteolysis releases the B domain analytes or nonplasma assay components that are present in the assays.
that blocks binding of factor Xa to factor Va. 164,169 The various forms For example, activated partial thromboplastin time-based assays are not
of factor Va (see Chap. 113, Fig. 113–11) are composed of two poly- equivalently sensitive as are dilute tissue-factor-based assays to plasma
peptide chains, one bearing the A1-A2 domains and the other bearing high-density lipoprotein (HDL) levels or oral contraceptive use. 189–191
the A3-C1-C2 domains. Although generally similar to factor V activa- Plasma variables, such as elevated prothrombin levels, 192,193 may affect
tion, factor VIII activation (see Fig. 113–13) involves formation of a the response to APC by inhibiting APC anticoagulant actions. Endoge-
heterotrimer of polypeptide chains containing the A1 domain, the A2 nous thrombin potential assays involving dilute tissue factor as the
domain, and the A3-C1-C2 domains, respectively. In contrast to het- procoagulant initiator provide additional tools for defining and charac-
erodimeric factor Va, heterotrimeric factor VIIIa is intrinsically unsta- terizing APC resistance and extend the tools for shedding light on the
ble as a consequence of spontaneous dissociation of the A2 domain. 170 gray area of APC resistance found in some thrombosis patients that is
not linked to currently known factors.
Factors Va and VIIIa as Substrates for Activated Protein C
Irreversible proteolytic inactivation of factors Va and VIIIa by APC ACTIVATED PROTEIN C ANTICOAGULANT
can be accomplished by proteolysis at Arg506 and Arg306 in factor Va
and Arg562 and Arg336 in factor VIIIa (see Chap. 113, Figs. 113–11 COFACTORS
and 113–13). 23,171–173 Currently, the most common identifiable venous APC anticoagulant activity is enhanced by a number of factors that may
2+
thrombosis risk factor involves a mutation of Arg506 to Gln in fac- be termed APC anticoagulant cofactors; these include Ca ions; certain,
tor V that results in APC resistance (Chap. 130). The complexities of but not all, phospholipids; protein S; factor V; certain glycosphingolip-
APC-dependent inactivation of factor Va and VIIIa are compounded ids; and HDL.
by the number of different molecular forms of Va and VIIIa that can be
generated by limited proteolysis by a variety of proteases and by their Phospholipids as Activated Protein C Cofactors
differing susceptibilities to APC and to the different APC cofactors. Certain phospholipids, such as phosphatidylserine, phosphatidyletha-
nolamine, and cardiolipin, enhance the anticoagulant activity of APC.
Activated Protein C Resistance In addition, phosphatidylethanolamine and cardiolipin stimulate the
APC resistance is defined as an abnormally reduced anticoagulant APC anticoagulant pathway activities much more than they stimulate
response of a plasma sample to APC (Chap. 130) and can be caused by the procoagulant pathway activities. 194–197
many potential abnormalities in the protein C anticoagulant pathway.
Such abnormalities could include defective APC cofactors, defective Protein S as Activated Protein C Cofactor
APC substrates, or other molecules that interfere with the normal func- Protein S structure–activity relationships are informed by much bio-
tioning of the protein C anticoagulant pathway (e.g., autoantibodies chemical work and the large number of mutations. 71,198 Protein S, as
against APC, APC cofactors, or APC substrates). an anticoagulant APC cofactor, forms a 1:1 complex with APC and
A report of familial venous thrombosis associated with APC resis- enhances by 10- to 20-fold the rate of APC’s cleavage at Arg306 in fac-
tance without any identifiable defect in four Swedish families led to an tor Va but not the Arg506 cleavage. 181,182 Part of the mechanism for this
174
intensive search for a genetic explanation that was soon found to involve activity of protein S may be related to its ability to bring the active site
replacement of G by A at nucleotide 1691 in exon 10 of the factor V gene of APC closer to the plane of the phospholipid membrane on which the
which causes the amino acid replacement of Arg506 by Gln. 175–177 This APC–protein S complex is located when the complex is formed. 199,200
factor V variant, like the prothrombin variant nt G20210A, arose in a Protein S also facilitates the action of APC against factor VIIIa. Protein
201
single white founder some 18,000 to 29,000 years ago 178,179 and is known S enhances APC’s action, in part at least, by ablating the ability of factor
as Gln506-factor V or factor V Leiden. This mutation is currently a com- Xa to protect factor Va from APC. The GLA domain, thrombin-
202
mon, but not the only, cause of APC resistance (Chap. 130). sensitive region, and EGF1 and EGF2 domains of protein S are impli-
The molecular mechanism for APC resistance of Gln506-factor V cated in binding APC for expression of anticoagulant activity by the
is based on the fact that the variant molecule is inactivated 10 times APC–protein S complex. 198,203–206 Cleavage of the thrombin-sensitive
slower than normal Arg506-factor Va. 23,177,180–182 The variant factor Va region by thrombin abolishes normal binding of protein S to phospho-
exhibits only a partial resistance to APC because cleavage at Arg306 in lipid and its normal APC-cofactor anticoagulant activity. 205,207
factor Va also occurs, causing complete loss of factor Va activity.
Plasma and recombinant factor V can exist in two biochemically Factor V as Activated Protein C Cofactor
distinct forms, designated factor V1 and factor V2 that differ in N-linked Factor V apparently can have anticoagulant as well as procoagulant
carbohydrate on Asn2181, near the phospholipid binding region of the properties because it enhances the anticoagulant action of APC against
C2 domain as factor V2 has none. 183,184 Because the N-linked carbohy- factors VIIIa and Va in a reaction in which protein S acts synergisti-
drate appears to decrease the apparent affinity of factor V1 or Va1 for cally with factor V. 23,208–211 Cleavage at Arg1545, which optimizes factor

Kaushansky_chapter 114_p1949-1966.indd 1955 9/18/15 10:05 AM