1.Von Willebrand Factor Proteolysis by ADAMTS13

8/20/2019 1.Von Willebrand Factor Proteolysis by ADAMTS13

1/10

von Willebrand Factor Proteolysis by ADAMTS13

Derrick J Bowen

Affiliation: Department of Haematology, School of Medicine, Cardiff University, Cardiff, UK

Submission date: 27th July 2009, Revision date: 1st September 2009, Acceptance date: 5th September 2009

A B S T R A C T

The plasma glycoprotein von Willebrand factor (VWF) is essential for effective hemostasis. It mediates platelet adhesion

and aggregation at a site of vascular damage. Additionally, it transports coagulation factor VIII (an important protein of the

clotting cascade) in the blood, shielding the coagulation factor from inactivation. VWF is found in plasma, platelets,

vascular subendothelium and endothelial cells; its presence throughout the vasculature reflects its fundamental role in

blood clot formation. Plasma VWF circulates as homopolymers (multimers) of different lengths, with the size differences

arising principally through cleavage by the metalloprotease ADAMTS13 (a disintegrin and metalloprotease withthrombospondin repeats). ADAMTS13-mediated VWF proteolysis contributes to the regulation of VWF bioactivity: newly

synthesized ultralarge multimers are highly thrombogenic in comparison to smaller proteolysed forms. The regulation of VWF

multimer size by ADAMTS13 is essential for normal hemostatic function, as evidenced by the pathological states that occur

when proteolysis is deficient or excessive: ADAMTS13 deficiency is the basis for the life-threatening disorder thrombotic

thrombocytopenic purpura (TTP), in which ultralarge VWF multimers in the circulation predispose to the spontaneous

formation of platelet aggregates, with potentially fatal consequences; conversely, too much proteolysis of VWF underlies

one form of thehemorrhagic disorder von Willebrand disease (VWD): hemostasis is severely compromised by the absence of

large (and sometimes intermediate) multimers in the circulation due to their rapid breakdown by ADAMTS13. This review

aims to provide the clinician or scientist new to this subject area with an overview of ADAMTS13-mediated VWF proteolysis

and the known key factors that can affect this important biochemical process in health and disease.

Keywords: von Willebrand factor, von Willebrand disease, ADAMTS13, proteolysis, thrombotic thrombocytopenic purpura

(TTP)

Correspondence: Derrick J Bowen, Department of Haematology, School of Medicine, Cardiff University, Heath Park,

Cardiff CF14 4XN, UK. E-mail: [email protected]

INTRODUCTION

The formation of a blood clot at a site of vascular

injury is a complex physiological process in which the

glycoprotein von Willebrand factor (VWF) plays a

fundamental role. Clot formation initiates when blood

contacts subendothelial structures, and circulating

platelets adhere, triggering further platelet aggrega-

tion to produce the primary hemostatic plug or soft

clot. During this process, platelet activation provides a

procoagulant surface that supports the activity of key

protein complexes in the coagulation cascade. The

latter produces fibrin polymers that become covalently

cross-linked into the soft clot, thereby reinforcing and

stabilizing it. Formation of the platelet-rich primary

hemostatic plug (primary hemostasis) and clot stabili-

zation (secondary hemostasis) are tightly regulated

processes, defects in which give rise to hemorrhagic or

thrombotic disorders.

Platelet arrest, adhesion, and aggregation are cen-tral to primary hemostasis. At low shear rates, multi-

ple ligand–receptor interactions may participate,

including those involving VWF. However, at high

shear rates, there appears to be an absolute depen-

dency on VWF. VWF interacts with platelets via two

platelet receptors: glycoprotein Iba (GPIba) [1] and

integrin aIIbb3 (previously referred to as platelet

glycoprotein IIb/IIIa or GPIIb/IIIa) [2]. GPIba is

unique among platelet receptors in that it does not

require prior platelet activation in order to bind itsligand (VWF); in contrast, aIIbb3 does. VWF interac-

tion with GPIba is rapid and reversible, whereas

interaction with aIIbb3 is slow and irreversible [3, 4].

At high shear rates, the VWF–GPIba interaction

tethers platelets long enough for activation and

subsequent irreversible binding via aIIbb3. Although

GPIba is competent to bind VWF without platelet

activation, there is apparently no significant interac-

tion between the two proteins in circulating blood.

This is explained, at least in part, by the requirement

for a critical level of shear stress [4].

Following platelet arrest and adhesion, aggregationof further platelets involves a number of ligand–

receptor interactions, among which VWF– aIIbb3 and

JOURNAL OF COAGULATION DISORDERS REVEW ARTICLE

JCD 2009; 000:(000). Month 2009 1 www.slm-hematology.com


2/10

fibrinogen– aIIbb3 are important. VWF and fibrinogen

act in concert, serving distinct but synergistic roles in

promoting platelet aggregation [5]. At high shear

rates, surface-bound VWF appears to be capable of

reversible association with circulating VWF, and the

resulting homotypic multimer assemblies may provide

a further contribution to the arrest of passing platelets[6]. At pathophysiological shear rates, transient VWF-

mediated platelet aggregation can occur that is

dependent upon GPIba but independent of aIIbb3 and

fibrinogen [7]. The latter may be highly relevant as a

pathological mechanism leading to vessel occlusion at

a site of stenosis, where vessel narrowing may vastly

increase shear stress. However, it may also be relevant

in a normal setting: as the soft clot is laid down, the

lesion aperture narrows while blood pressure remains

unchanged. Exiting blood may therefore be exposed to

ever-increasing shear stress, possibly permitting acti-

vation-independent, VWF–GPIba-mediated plateletaggregation, and this may facilitate lesion closure.

Collagen contributes to the overall process in at least

two ways: first, it may potentiate the ability of VWF

that is bound to it to arrest platelets; second, it can

interact directly with any of several receptors on the

platelet surface, thereby adding to platelet–suben-

dothelium interactions.

While the intricacies and timeline of events in

primary hemostasis may not yet be fully delineated,

the detailed studies to date have identified key

features of VWF involvement and emphasize the

central role that this protein plays at high shearstress. VWF has evidently evolved to ‘‘catch’’ platelets

from the passing blood under the adverse circumstance

of high shear.

In addition to its role in primary hemostasis, VWF

contributes towards secondary hemostasis: it transports

and protects coagulation factor VIII (FVIII) in the

circulation [8]. FVIII is an essential protein in the

intrinsic coagulation cascade and is necessary for

efficient fibrin production. FVIII deficiency is character-

ized by a breakdown of secondary hemostasis, resulting

in clinically significant bleeding (hemophilia A). Thus,

VWF facilitates secondary hemostasis by contributing tothe maintenance of circulating FVIII levels.

von WILLEBRAND FACTOR

VWF is synthesized by endothelial cells [9] and

megakaryocytes [10]. It is produced as a pre-pro-

protein comprising 2813 amino acids. As part of post-

translational processing, the pre- and pro-sequences

(22 and 741 amino acids respectively) are removed,

leaving the mature VWF subunit (2050 amino acids)

(Figure 1) [11, 12]. Prior to removal of the pro-

sequence, pro-VWF forms dimers via C-terminal dis-

ulfide bonds, and these dimeric units subsequently joinvia their N-termini to give polymers (multimers) [13].

The pro-peptide, which is needed for multimerization

of dimers [14], is removed leaving multimers composed

entirely of mature VWF subunits (Figure 1). Post-

translational processing additionally includes exten-

sive glycosylation involving the addition of 12 N-

linked and 10 O-linked carbohydrate moieties

(Figure 1) [15] and sulfation of specific N-linked

carbohydrates [16]. Internal homologies within the

mature subunit give rise to repeated structural

domains [17] (Figure 1). The A1 domain contains the

binding site for GPIba (and also for heparin and

sulfated glycolipids); the C1 domain contains the

ligand motif (Arg-Gly-Asp, RGD) for integrin aIIbb3;

the A3 domain (and possibly also the A1 domain) is

considered to be the binding site for collagen; and the

D9 –D3 domains bind FVIII (Figure 1).

VWF produced in endothelial cells can be stored in

Weibel–Palade bodies [18] or secreted. Secretion can

occur by a constitutive pathway that does not require

stimulation by secretagogues, or by a regulated path-way that does [19]. The former releases multimers

directly upon synthesis; the latter releases the Weibel–

Palade stores. Endothelial VWF can be secreted

apically into the bloodstream or basolaterally into

the subendothelial matrix. Only arteries, arterioles,

and large veins have subendothelial deposits of VWF

[20]; in capillaries, VWF may be deposited basolater-

ally in response to endothelial cell activation [21].

Plasma VWF appears to be derived principally from

endothelial cells [22]. Platelet VWF is stored in a-

granules from which it is released upon platelet

activation [23]. As much as 15% of the total VWF inblood is contained within the platelet compartment

[24]. VWF is therefore found in four physiological

compartments—plasma, platelets, endothelial cells,

and the subendothelium—and this no doubt reflects

its important role in primary hemostasis.

Stimulated release of VWF from its storage orga-

nelles places ultralarge multimers, which are highly

thrombogenic, directly at the site of need (Figure 3).

In the case of endothelial cells, these acutely released

ultralarge multimers rapidly form strings and net-

works that are anchored on to the cell surface via

integrin aVb3 [25]. Such immobilization would exposethem to shear stress in the passing blood flow. This

may potentiate platelet binding; however, it would

also promote VWF proteolysis by the metalloprotease

ADAMTS13 (a disintegrin and metalloprotease with

thrombospondin repeats) [26].

ADAMTS13

ADAMTS13 is synthesized predominantly in the liver

[27]; however, other tissues and cells (including

endothelial cells and platelets) also produce the

enzyme [28, 29]. The primary translation product is

1427 amino acids long and comprises a signal peptide(33 amino acids), a propeptide (41 amino acids), and a

multidomain mature protein (1353 amino acids) [30]

Journal of Coagulation Disorders



3/10

(Figure 2). The latter includes a metalloprotease

domain, disintegrin domain, cysteine-rich and spacer

region, and several thrombospondin type 1 motifs, all

of which characterize the enzyme as a member of the

ADAMTS family [30]. The metalloprotease domain has

the consensus sequence HEXXHXXGXXHD character-istic of certain zinc-dependent metalloendopeptidases

[31], a predicted calcium ion binding site (Glu83,

Asp173, Cys281), and a ‘‘met turn’’ in which Met249

supports the active site zinc ion. These features are

characteristic of the ‘‘metzincin’’ family of metalloen-

dopeptidases [31], which achieve optimal activity with

both zinc and calcium ions. Cooperative activation of ADAMTS13 has been demonstrated by zinc and

Figure 2. Structure of ADAMTS13. Pre- and pro-sequences are followed by a metalloprotease domain containing the active site, adisintegrin domain, a TSP-1 (thrombospondin-1) motif, a Cys-rich region, a spacer domain, seven additional TSP-1 motifs, and two CUBdomains

Figure 1. von Willebrand factor structure. (A) Relative extent of pre-, pro-, and mature VWF. (B) Pre-pro-VWF domain structure andposition of carbohydrate moieties in the mature subunit. Internal homologies within the protein give rise to repeated domains (A1–D4); D 9

is a partial homolog of the D domains; CK is a cysteine-rich region (cysteine-knot); H and D represent N-linked and O-linked glycosylationsites respectively. The A2 domain contains the ADAMTS13 cleavage site at Tyr1605–Met1606, the Tyr1584Cys polymorphic residue (*) thataffects proteolysis, two N-linked carbohydrates (Asn1515 and Asn1574, of which Asn1574 may influence proteolysis), and five O-linkedcarbohydrates. The numeric scale indicates amino acid residue number. (C) Location of VWF functional domains involved in ligandbinding and multimer formation (RGD represents the arg-gly-asp motif, which is the ligand for integrin aIIbb3). (D) Formation of VWFmultimers. Mature VWF dimerizes via C-termini and dimers form multimers via N-termini. ADAMTS13 proteolysis within any of the constituentA2 domains yields multimeric forms found in plasma

www.slm-hematology.com 3 JCD 2009; 000:(000). Month 2009


4/10

calcium together [32]. The C-terminus of ADAMTS13

contains two CUB domains that are common to many

proteins involved in developmental processes, but

whose function in ADAMTS13 is uncertain (CUB

abbreviates: complement component Clr/Cls, Uegf

and bone morphogenic protein 1 domain). Circulating

ADAMTS13 is highly glycosylated [33, 34] and is

primarily full length (pre-pro-sequences attached) [35].

ADAMTS13 cleaves VWF at the peptide bond between

Tyr1605 and Met1606, located in the VWF A2 domain(Figure 1) [36–38]. This proteolysis gives rise to multi-

mers of different lengths (Figure 1) and underlies the

characteristic triplet structure of plasma VWF [39]. The

proteolysis is important because it appears to be the

predominant physiological mechanism through which

multimer size is regulated. Although ADAMTS13 and

VWF both circulate in plasma, their interaction leading

to proteolysis is limited by a requirement for VWF to

undergo shear stress, the basis for which may be

stretching of the protein and exposure of the otherwise

buried A2 domain [26, 40].

Therefore, shear stress, on the one hand, promotesVWF interaction with platelets, while on the other, it

enhances cleavage by ADAMTS13. These two opposing

phenomena could contribute to the control of clot

growth (Figure 3), particularly when ADAMTS13 is

available from the various local sources (plasma,

endothelial cells, and activated platelets). Evidence that

excess cleavage has a negative impact on VWF hemo-

static function is provided by certain forms of the

bleeding disorder von Willebrand disease (VWD) that

result from a mutant VWF that is highly susceptible to

ADAMTS13 proteolysis [36]. Conversely, a deficiency or

dysfunction of ADAMTS13 leads to the survival of

uncleaved, or partially cleaved, ultralarge VWF in

plasma, which predisposes to the spontaneous formation

of intravascular platelet aggregates and is the biochem-

ical basis of the life-threatening disorder thrombotic

thrombocytopenic purpura (TTP) [37, 38, 41].

It is presently unclear which sites on ADAMTS13

and VWF contribute towards the interaction of the

two proteins under physiologic conditions. Data to

date support the involvement of several sites of

interaction on both proteins, at least under static/

denaturing conditions. The isolated metalloprotease

domain of ADAMTS13 is ineffective in cleaving VWF[42, 43]; efficient proteolysis requires the presence of

the non-catalytic domains. The disintegrin-like domain

Figure 3. A model of events in primary hemostasis, illustrated by a single site of vascular damage at which blood exits the vessel. (A) In anintact blood vessel, VWF is compartmentalized: ultralarge, highly thrombogenic multimers are localized to the a-granules of platelets andWeibel–Palade bodies of endothelial cells; plasma VWF (which comprises smaller, less thrombogenic forms) is prevented fromimmobilization by the endothelial layer, which also masks matrix-bound subendothelial VWF (if present). (B) Vessel damage allows exitingblood to contact the subendothelial matrix (containing collagen to which plasma VWF can bind) and matrix-bound VWF (if present). (C)In the earliest stages, platelets arrest and adhere to immobilized VWF. (D) Platelet adhesion is followed by activation and release of a-granule contents including ultralarge VWF multimers. Local agonists may stimulate endothelial cells to release ultralarge VWF multimersfrom Weibel–Palade bodies. (E) VWF-mediated platelet recruitment and ADAMTS13-mediated VWF proteolysis may take placeconcurrently during subsequent stages. The former would contribute to clot growth; the latter would limit it. Both are favored by highshear, which may itself be facilitated by the narrowing of the lesion aperture as the soft clot evolves. Factors that enhance proteolysis(such as the cysteine 1584 variant of VWF or, to a lesser extent, blood group O) or inhibit proteolysis (such as the serine 475 variant of

ADAMTS13) may imbalance VWF-mediated platelet recruitment and predispose to a hemorrhagic or thrombotic tendency respectively.For clarity, peripheral constituents of the blood have been omitted from (D) and (E)




5/10

appears to interact with VWF via Arg349 and Leu350

in ADAMTS13 and Asp1614 and Ala1612 in VWF [44].

The ADAMTS13 TSP-1 (first motif) and Cys-rich

domains, respectively, appear to bind VWF regions

Gln1624–Val1630 and Ile1642–Gln1652 [43, 45]. The

spacer domain binds a C-terminal portion of the VWF

A2 domain (residues 1653–1668) [46]. The carboxy-

terminal TSP-1 repeats and CUB domains may mod-

ulate ADAMTS13–VWF interaction [46, 47]. Data from

mutagenesis experiments suggest that ADAMTS13 is

relatively flexible and does not require a fixed spacing

between its catalytic site and the distal VWF binding

sites in the Cys-rich and spacer domains [45]. This

‘‘elasticity’’ may facilitate interaction between the two

proteins in flow conditions.

A model has been proposed in which ADAMTS13–

VWF interaction comprises two components acting in

concert: tight binding is provided by the Cys-rich and

spacer domain interactions, whereas the weaker

interactions (in particular of the disintegrin domain)

may assist in positioning the VWF scissile bond into

the active site cleft of the metalloprotease domain [44].

FACTORS INFLUENCING VWF PROTEOLYSIS

BY ADAMTS13

Various factors can affect the efficiency of VWF

cleavage by ADAMTS13. Those that contribute

towards the normal spectrum of proteolysis include

shear stress, VWF glycosylation, amino acid variants

in both proteins, and ligand–VWF interaction; theseare discussed in detail below. Additionally, there are

factors that underlie the extremes of proteolysis found

in the pathologies of TTP and VWD. These are

summarized briefly below, but are reviewed in detail

elsewhere [48–52].

Shear Stress

The velocity of blood flow is dependent upon several

parameters including blood pressure, vessel diameter,

and fluid viscosity. Within a given vessel, flow differs

according to the axial location within the lumen: it is

zero at the vessel wall and increases towards the lumencentre [53]. This gives rise to shear stress, produced by

the faster movement of one layer over an adjacent layer.

In the circulatory system, shear stress is highest in the

microvasculature [54, 55]. In vessels occluded by an

atherosclerotic plaque, shear stress at the site of stenosis

can greatly exceed even the highest normal levels [56].

ADAMTS13 does not proteolyse VWF significantly

under static conditions, but does cleave it slowly in vitro

in the presence of denaturing agents [37]. However, at

shear stress levels equal to those found in the micro-

vasculature, proteolysis is extremely rapid and does not

require denaturants [26, 38, 57]. In a physiologicalsetting, ADAMTS13 therefore functions efficiently

under the very conditions in which VWF functions to

recruit platelets and, as noted earlier, this may provide a

mechanism contributing to the control of clot growth.

ADAMTS13 can limit the activation-independent, VWF–

GPIba-mediated aggregation of platelets that takes place

at pathologically elevated shear stress [7] and may

thereby provide a control mechanism against thrombo-

genesis at sites experiencing such shear.

VWF Glycosylation

Studies prior to the discovery of ADAMTS13 pro-

vided evidence that the carbohydrate component of

VWF may protect the protein from degradation [58].

Subsequent studies have indicated that VWF glycosy-

lation can affect ADAMTS13-mediated proteolysis.

ABO blood group sugars—A, B, and H—are attached

to the N-linked carbohydrates of VWF [59, 60]

(Figure 1) and appear to alter the rate of

ADAMTS13 cleavage to a small extent: proteolysis

was faster for VWF of blood group O compared withnon-O (O > B . A > AB); however, the differences

between blood groups were not considerable [61]. VWF

from the plasma of individuals with the Bombay

phenotype (characterized by a failure to attach A, B,

and H sugars to carbohydrate structures) was proteo-

lysed more rapidly than that of both blood group O and

blood group AB (Bombay . O . AB) [62].

The physiological significance of these findings is

uncertain. Plasma VWF level correlates with ABO

blood group (O , A , B , AB) and is approximately

25% less in blood group O than in non-O [63, 64]. The

rank order and magnitude of proteolysis in the ABOblood groups do not correlate with those of VWF level;

therefore, proteolysis is unlikely to explain the

differences in the amount of VWF between these blood

groups. The increased VWF proteolysis associated

with blood group O is small; however, it could have a

subtle deleterious effect on primary hemostasis. In

conjunction with a low plasma VWF level and/or other

deleterious factors, this may compromise clot forma-

tion (Figure 3) [48].

Global removal of VWF N-linked carbohydrates has

been shown to increase affinity for, and proteolysis by,

ADAMTS13, and to enable proteolysis in the absenceof denaturant [65]. Mutation of the two asparagine

residues vicinal to the Tyr1605–Met1606 cleavage site

demonstrated that loss of the carbohydrate moiety at

Asn1574, but not Asn1515, increased the susceptibility

of VWF to proteolysis and allowed cleavage in the

absence of denaturant [65]. Thus, VWF N-linked

carbohydrates collectively may help to support a

globular conformation, and Asn1574 may additionally

influence accessibility of the scissile Tyr1605–Met1606

bond to the ADAMTS13 active site.

In contrast, N-linked carbohydrates on ADAMTS13

do not appear to modulate its activity: they can beenzymatically removed without affecting proteolysis.

However, they do appear to be necessary for efficient



6/10

secretion of the metalloprotease and may promote

correct conformational maturation of the enzyme

during its biosynthesis [34].

Amino Acid Variants

Both VWF and ADAMTS13 show amino acid varia-tion in the normal population. Several coding sequence

variants have been described in ADAMTS13, including

Arg7Trp, Gln448Glu, Pro475Ser, Pro618Ala, Arg625His,

Ala732Val, and Ala900Val [41]. The Pro475Ser variant,

which is prevalent in the Japanese population [66] but

absent or rare in other populations (Caucasians, Afro-

Americans, Chinese) [67, 68], has a significant effect on

proteolytic activity. The serine allele is associated with

very low ADAMTS13 activity despite normal secretion.

Based upon allele frequencies at the genetic level, as

many as 10% of the Japanese population may be

heterozygous for Ser475 and consequently may have

decreased ADAMTS13 levels [66].

The polymorphism Pro618Ala in ADAMTS13 also has

a significant effect on ADAMTS13 level, mediated by a

deleterious effect of the Ala618 allele on secretion and

proteolytic activity [69]. Ala732Val has a small but

demonstrable effect, the valine allele correlating with

impaired catalytic activity [69].

Studies on the interplay between ADAMTS13 amino

acid variants have indicated that the same polymorph-

isms can be either positive or negative modifiers

depending on the allelic combination present.

Additionally, polymorphisms may interact synergisti-cally and not just additively [69]. These observations

may be relevant to normal variations in hemostatic

function (although this has not been formally investi-

gated), and there is evidence supporting their impor-

tance in the phenotypic expression of TTP [69].

Amino acid variation in VWF can also affect

ADAMTS13-mediated proteolysis. The Tyr1584Cys

variation in the VWF A2 domain has been extensively

studied. The cysteine allele is associated with mildly

enhanced proteolysis [70, 71], the basis for which may

be the formation of new inter- or intramolecular

disulfide linkages that alter the conformation of theA2 domain and enhance access to the cleavage site [40,

72, 73]. That Cys1584 is necessary for enhanced

proteolysis and not simply linked to a causative

change elsewhere in VWF has been shown by compar-

ing the coding sequences of the protein in related

individuals whose plasma VWF showed either normal

or increased proteolysis. Two sequence variants were

unique to VWF that showed increased proteolysis:

Arg484 and Cys1584. Arg484, in the absence of

Cys1584, did not influence VWF proteolysis, indicating

that Cys1584 is necessary for the effect [74].

The mildly enhanced proteolysis associated withCys1584 is considerably greater than that of blood

group O. It is not additive with that of blood group O

[71], possibly due to Cys1584 inducing a conforma-

tional change in the A2 domain that abolishes the

blood group effect. As discussed above, increased

proteolysis of VWF may be detrimental to hemostatic

function (Figure 3). Cys1584 is enriched in type 1

VWD [72, 75]; however, whether this relates to its

effect on VWF proteolysis is uncertain. There is

adequate evidence showing that, in addition to

increased proteolysis, the variant is associated with

decreased VWF level, increased clearance of the

protein, and decreased VWF functionality [64, 76]. It

is likely that the prevalence of Cys1584 in type 1 VWD

reflects all of these deleterious effects [77].

The amino acid variation Val1565Leu located in the

VWF A2 domain has also been shown to affect

proteolysis, however to a much smaller extent than

Tyr1584Cys [71]. The rarer leucine allele correlates

with a minor increase in proteolytic susceptibility;

however, the magnitude of the effect is so small that it

may have no physiological relevance.

Interestingly, both Tyr1584 and Val1565 are highly

conserved in VWF from different species, they each

occur in regions that are homologous between species,

and they are near to the ADAMTS13 scissile bond [71].

These features provide an argument for a direct effect

of the variant residues Cys1584 and Leu1565 on VWF

proteolysis.

The possibility that the amino acid polymorphisms

Asp1472His, Gln1571His, Pro1601Thr, and Gly1643Ser

in the VWF A domains influence ADAMTS13-mediatedproteolysis has been investigated using recombinant

expressed proteins. The advantage of this approach is

that potential confounding variables are standardized,

thereby allowing comparison solely of the effect of each

amino acid allele. Using recombinant VWF and

recombinant ADAMTS13, His1472, His1571, and

Thr1601 in VWF all conferred mild resistance to

proteolysis, whereas Ser1643 potentiated proteolysis

[78]. The disadvantage of recombinant studies is that

VWF and ADAMTS13 expressed in vitro may differ

from their in vivo counterparts in a number of ways (for

example in terms of glycosylation) that could influencethe results. In this context, it is interesting to note that,

in contrast to the above result, His1472 appeared to

have no effect on proteolysis when plasma VWF, rather

than the recombinant protein, was used [71].

In combination, these detailed studies indicate that

ADAMTS13-mediated VWF proteolysis appears to be

highly dependent upon the amino acid variants present

at certain influential positions in both proteins. One

important inference from this is that the rate of

proteolysis could be quite different between two

individuals who have similar VWF protein levels and

also similar ADAMTS13 protein levels, potentiallyleading to differences in hemostatic efficiency despite

the similar phenotypic values. This may contribute to




7/10

normal variation in healthy individuals, and it may be

relevant to the phenotypic expression of TTP and VWD.

VWF–Ligand Binding

X-ray crystallography studies indicate that the VWF

A1 domain undergoes large structural changes onbinding with GPIba [79]. Additionally, proteolysis of a

recombinant peptide containing the VWF A1A2A3

region was increased when a mutant fragment of

GPIba bound [80]. Together, these observations sug-

gest that binding of GPIba to VWF increases the

susceptibility of the latter to proteolysis by

ADAMTS13 via a conformational change. Heparin, a

glycosaminoglycan stored in the secretory granules of

mast cells and released into the blood at a site of

vascular damage, also increases VWF proteolysis [80].

The obsolete antibiotic ristocetin, which is used in the

laboratory to trigger platelet–VWF aggregation,

causes increased proteolysis [61]. For both heparinand ristocetin, the mechanism of the effect may be

through the induction of conformational changes upon

binding to VWF.

CLINICAL SIGNIFICANCE

ADAMTS13-mediated proteolysis is a natural part of

VWF processing and, as is the case for all other

biochemical processes, displays a normal range in the

population [81]. At the extremes of the normal range,

the level of proteolysis may well predispose to

thrombotic (low level) or hemorrhagic (high level)

tendencies. Beyond theses extremes, the overt patho-logical disorders of TTP and a subset of VWD arise.

TTP is caused by a marked deficiency of ADAMTS13

activity, brought about either by mutation of the

relevant gene (ADAMTS13 , located at 9q34) [41] or by

acquired antibodies arising from an autoimmune

response to the protein [81, 82]. In the case of the

former (hereditary TTP), heterogeneous mutations

have been characterized in ADAMTS13 , including

missense and nonsense point mutations, splice site

mutations, and deletions [51]. Characterization of

antibodies in acquired TTP has shown that diverse

ADAMTS13 epitopes may be targeted [83]. Irrespectiveof the root cause of ADAMTS13 deficiency, the

outcome is the survival of ultralarge VWF multimers

in the circulation, leading to spontaneous platelet

aggregation with potentially disastrous consequences

[84]. These include damage or failure of the major

organs fed by the circulatory system (heart, kidneys,

brain, eyes). Five classical symptoms are indicative of

TTP—neurologic disturbance, kidney failure, fever,

thrombocytopenia, and microangiopathic hemolytic

anemia. The last is believed to arise from shear-

induced damage of red blood cells passing the platelet

aggregates (for a review, see [85]).In contrast to the above, excessive VWF proteolysis

by ADAMTS13 may predispose to bleeding. This is the

characteristic feature of a subset of type 2A VWD

(‘‘group 2’’ type 2A) [86]. In this subset, missense point

mutations in the region of the VWF gene (VWF ,

located at 12p12–13) encoding the A2 domain bring

about amino acid changes that cause the protein to be

proteolysed without the need for shear stress. These

amino acid substitutions may induce conformational

changes that alter the accessibility of the cleavage site

within the VWF A2 domain, or they may directly alter

ADAMTS13–VWF interaction without structural per-

turbation [40, 87, 88]. In the circulation, the mutant

VWF is spontaneously and rapidly cleaved by

ADAMTS13, resulting in the loss of large (and some-

times intermediate) multimer forms [86, 87, 89]. This,

together with the possibility that ultralarge multimers

secreted at a time of hemostatic challenge may be

cleaved far more rapidly than normal, severely

compromises hemostatic function. Characteristic clin-

ical features include mucocutaneous bleeding, epis-taxis, easy bruising, bleeding after dental extraction,

heavy menstrual periods, and excessive blood loss

during childbirth (for a review, see http://www.nhlbi.

nih.gov/guidelines/vwd/index.htm).

For both TTP and VWD, the parameters that

influence VWF proteolysis by ADAMTS13 may con-

tribute towards differences in penetrance and severity.

This is likely to be most important in the case of

mutations in either protein that cause a mild defi-

ciency: the additive or synergistic effect of negative

modifiers acting on such mutations could tip the

balance in favor of clinical presentation. This iscertainly believed to be the case for the Tyr1584Cys

polymorphism in VWF: the cysteine allele is found in

association with phenotypes ranging from asympto-

matic to overt type 1 VWD.

SUMMARY

In an undamaged, healthy vessel, blood is maintained

in a liquid state by several key phenomena: the

endothelial surface lining the lumen is anticoagulant,

endothelial cells themselves secrete anticoagulant

molecules, hemostatic proteins in the blood circulate

in inactive forms, and platelets, although primed readyfor clot formation, do not do so because the appro-

priate triggers are unavailable to them.

All this changes when vascular damage occurs and

the various signals that activate clot formation come

into play. Amid the complex array of biochemical

activities that occur, VWF capture of platelets at the

site of damage is fundamental. The efficiency of the

entire process of clot formation reflects a considerable

number of variables. VWF proteolysis by ADAMTS13

is just one biochemical process in the complex milieu,

but it is of pivotal importance, as evidenced by the

severe pathologies arising from its perturbation.Proteolysis is itself influenced by many variables,

including the amount of ADAMTS13 and VWF in the



8/10

blood, shear stress, amino acid polymorphisms within

each protein, VWF–GPIba binding, and ABO blood

group. These provide for a large number of possible

permutations, some of which may be important to the

phenotypic expression of TTP or VWD.

Disclosure: The author has no financial interests todisclose related to the contents of this article.

REFERENCES1. Martin SE, Marder VJ, Francis CW, Loftus LS, Barlow GH.

Enzymatic degradation of the factor-VIII-von-Willebrand pro-

tein: a unique tryptic fragment with ristocetin cofactor activity.

Blood. 1980;55(5):848–858.

2. Girma JP, Kalafatis M, Piétu G, et al . Mapping of distinct von

Willebrand factor domains interacting with platelet GPIb and

GPIIb/IIIa and with collagen using monoclonal antibodies.

Blood. 1986;67(5):1356–1366.

3. Savage B, Saldivar E, Ruggeri ZM. Initiation of platelet adhesion

by arrest onto fibrinogen or translocation on von Willebrand

factor. Cell . 1996;84(2):289–297.4. Doggett TA, Girdhar G, Lawshé A, et al . Selectin-like kinetics

and biomechanics promote rapid platelet adhesion in flow: the

GPIb(alpha)-vWF tether bond. Biophys J . 2002;83(1):194–205.

5. Ruggeri ZM, Dent JA, Saldivar E. Contribution of distinct

adhesive interactions to platelet aggregation in flowing blood.

Blood. 1999;94(1):172–178.

6. Savage B, Sixma JJ, Ruggeri ZM. Functional self-association of

plasma von Willebrand factor during platelet adhesion under

flow. Proc Natl Acad Sci USA. 2002;99(1):425–430.

7. Donadelli R, Orje JN, Capoferri C, Remuzzi G, Ruggeri ZM. Size

regulation of von Willebrand factor-mediated platelet thrombi by

ADAMTS13 in flowing blood. Blood. 2006;107(5):1943–1950.

8. Foster PA, Fulcher CA, Marti T, Titani K, Zimmerman TS. A

major factor VIII binding domain resides within the amino-

terminal 272 amino acid residues of von Willebrand factor. J Biol Chem. 1987;262(18):8443–8446.

9. Jaffe EA, Hoyer LW, Nachman RL. Synthesis of von Willebrand

factor by cultured human endothelial cells. Proc Natl Acad Sci

USA. 1974;71(5):1906–1909.

10. Nachman R, Levine R, Jaffe EA. Synthesis of FVIII antigen by

cultured guinea-pig megakaryocytes. J Clin Invest. 1977;60(4):

914–921.

11. Verweij CL, Diergaarde PJ, Hart M, Pannekoek H. Full-length

von Willebrand factor (vWF) cDNA encodes a highly repetitive

protein considerably larger than the mature vWF subunit.

EMBO J . 1986;5(8):1839–1847. Erratum in EMBO J .

1986;5(11):3074.

12. Bonthron DT, Handin RI, Kaufman RJ, et al . Structure of pre-

pro-von Willebrand factor and its expression in heterologous

cells. Nature. 1986;324(6094):270–273.13. Wagner DD, Marder VJ. Biosynthesis of von Willebrand protein

by human endothelial cells: processing steps and their intracel-

lular localization. J Cell Biol . 1984;99(6):2123–2130.

14. Verweij CL, Hart M, Pannekoek H. Expression of variant von

Willebrand factor (vWF) cDNA in heterologous cells: require-

ment of the pro-polypeptide in vWF multimer formation. EMBO

J . 1987;6(10):2885–2890.

15. Titani K, Kumar S, Takio K, et al . Amino acid sequence of human

von Willebrand factor. Biochemistry. 1986;25(11):3171–3184.

16. Carew JA, Browning PJ, Lynch DC. Sulfation of von Willebrand

factor. Blood. 1990;76(12):2530–2539.

17. Mancuso DJ, Tuley EA, Westfield LA, et al . Structure of the gene

for human von Willebrand factor. J Biol Chem. 1989;264(33):

19514–19527.

18. Wagner DD, Olmsted JB, Marder VJ. Immunolocalization of von

Willebrand protein in Weibel–Palade bodies of human endothe-

lial cells. J Cell Biol . 1982;95(1):355–360.

19. Sporn LA, Marder VJ, Wagner DD. Differing polarity of the

constitutive and regulated secretory pathways for von

Willebrand factor in endothelial cells. J Cell Biol . 1989;108(4):

1283–1289.

20. van der Kwast TH, Stel HV, Cristen E, Bertina RM, Veerman

EC. Localization of factor VIII-procoagulant antigen: an immu-

nohistological survey of the human body using monoclonal

antibodies. Blood. 1986;67(1):222–227.21. Takeuchi M, Nagura H, Kaneda T. DDAVP and epinephrine-

induced changes in the localization of von Willebrand factor

antigen in endothelial cells of human oral mucosa. Blood. 1988;

72(3):850–854.

22. Nichols TC, Samama CM, Bellinger DA, et al . Function of von

Willebrand factor after crossed bone marrow transplantation

between normal and von Willebrand disease pigs: effect on

arterial thrombosis in chimeras. Proc Natl Acad Sci USA. 1995;

92(7):2455–2459. Comment in Proc Natl Acad Sci USA.

1995;92(7):2428–2432.

23. Koutts J, Walsh PN, Plow EF, Fenton JW 2nd, Bouma BN,

Zimmerman TS. Active release of human platelet factor VIII-

related antigen by adenosine diphosphate, collagen, and throm-

bin. J Clin Invest. 1978;62(6):1255–1263.

24. Howard MA, Montgomery DC, Hardisty RM. Factor-VIII-relatedantigen in platelets. Thromb Res. 1974;4(5):617–624.

25. Huang J, Roth R, Heuser JE, Sadler JE. Integrin aVb3 on human

umbilical vein endothelial cells binds von Willebrand factor

strings under fluid shear stress. Blood. 2009;113(7):1589–1597.

26. Dong JF, Moake JL, Nolasco L, et al . ADAMTS-13 rapidly

cleaves newly secreted ultralarge von Willebrand factor multi-

mers on the endothelial surface under flowing conditions. Blood.

2002;100(12):4033–4039.

27. Uemura M, Tatsumi K, Matsumoto M, et al . Localization of

ADAMTS13 to the stellate cells of human liver. Blood. 2005;

106(3):922–924.

28. Liu L, Choi H, Bernardo A, et al . Platelet-derived VWF-cleaving

metalloprotease ADAMTS-13. J Thromb Haemost. 2005;3(11):

2536–2544.

29. Shang D, Zheng XW, Niiya M, Zheng XL. Apical sorting of ADAMTS13 in vascular endothelial cells and Madin–Darby

canine kidney cells depends on the CUB domains and their

association with lipid rafts. Blood. 2006;108(7):2207–2215.

30. Zheng X, Chung D, Takayama TK, Majerus EM, Sadler JE,

Fujikawa K. Structure of von Willebrand factor-cleaving

protease (ADAMTS13), a metalloprotease involved in thrombotic

thrombocytopenic purpura. J Biol Chem. 2001;276(44):41059–

41063.

31. Gomis-Rüth FX. Structural aspects of the metzincin clan of

metalloendopeptidases. Mol Biotechnol . 2003;24(2):157–202.

32. Anderson PJ, Kokame K, Sadler JE. Zinc and calcium ions

cooperatively modulate ADAMTS13 activity. J Biol Chem. 2006;

281(2):850–857.

33. Ricketts LM, Dlugosz M, Luther KB, Haltiwanger RS, Majerus

EM. O-fucosylation is required for ADAMTS13 secretion. J Biol

Chem. 2007;282:17014–17023.

34. Zhou W, Tsai HM. N-Glycans of ADAMTS13 modulate its

secretion and von Willebrand factor cleaving activity. Blood.

2009;113(4):929–935.

35. Soejima K, Nakamura H, Hirashima M, Morikawa W, Nozaki C,

Nakagaki T. Analysis on the molecular species and concentra-

tion of circulating ADAMTS13 in blood. J Biochem. 2006;139(1):

147–154.

36. Dent JA, Berkowitz SD, Ware J, Kasper CK, Ruggeri ZM.

Identification of a cleavage site directing the immunochemical

detection of molecular abnormalities in type IIA von Willebrand

factor. Proc Natl Acad Sci USA. 1990;87(16):6306–6310. Erratum

in Proc Natl Acad Sci USA. 1990;87(23):9508.

37. Furlan M, Robles R, Lämmle B. Partial purification and

characterization of aprotease from human plasma cleaving von

Willebrand factor to fragments produced by in vivo proteolysis.

Blood. 1996;87(10):4223–4234.




9/10

38. Tsai HM. Physiologic cleavage of von Willebrand factor by a

plasma protease is dependent on its conformation and requires

calcium ion. Blood. 1996;87(10):4235–4244.

39. Fischer BE, Thomas KB, Schlokat U, Dorner F. Triplet structure

of human von Willebrand factor. Biochem J . 1998;331(Pt 2):483–

488.

40. O’Brien LA, Sutherland JJ, Weaver DF, Lillicrap D. Theoretical

structural explanation for Group I and Group II, type 2A vonWillebrand disease mutations. J Thromb Haemost. 2005;3(4):796–

797.

41. Levy GG, Nichols WC, Lian EC, et al . Mutations in a member of

the ADAMTS gene family cause thrombotic thrombocytopenic

purpura. Nature. 2001;413(6855):488–494.

42. Ai J, Smith P, Wang S, Zhang P, Zheng XL. The proximal

carboxyl-terminal domains of ADAMTS13 determine substrate

specificity and are all required for cleavage of von Willebrand

factor. J Biol Chem. 2005;280(33):29428–29434.

43. Gao W, Anderson PJ, Majerus EM, Tuley EA, Sadler JE. Exosite

interactions contribute to tension-induced cleavage of von

Willebrand factor by the antithrombotic ADAMTS13 metallo-

protease. Proc Natl Acad Sci USA. 2006;103(50):19099–19104.

44. de Groot R, Bardhan A, Ramroop N, Lane DA, Crawley JT.

Essential role of the disintegrin-like domain in ADAMTS13

function. Blood. 2009;113(22):5609–5616.

45. Gao W, Anderson PJ, Sadler JE. Extensive contacts between

ADAMTS13 exosites and von Willebrand factor domain A2

contribute to substrate specificity. Blood. 2008;112(5):1713–1719.

46. Majerus EM, Anderson PJ, Sadler JE. Binding of ADAMTS13 to

von Willebrand factor. J Biol Chem. 2005;280(23):21773–21778.

47. Zhang P, Pan W, Rux AH, Sachais BS, Zheng XL. The

cooperative activity between the carboxyl-terminal TSP1 repeats

and the CUB domains of ADAMTS13 is crucial for recognition of

von Willebrand factor under flow. Blood. 2007;110(6):1887–1894.

48. Bowen DJ, Collins PW. Insights into von Willebrand factor

proteolysis: clinical implications. Br J Haematol . 2006;133(5):457–

467.

49. Bowen DJ. Molecular genetics of von Willebrand disease. In:Encyclopaedia of Life Sciences (ELS). Chichester: John Wiley &

Sons Ltd; 2008. DOI: 10.1002/9780470015902.a0021447.

50. Collins PW, Cumming AM, Goodeve AC, Lillicrap D. Type 1 von

Willebrand disease: application of emerging data to clinical

practice. Haemophilia. 2008;14(4):685–696.

51. Levy GG, Motto DG, Ginsburg D. ADAMTS13 turns 3. Blood.

2005;106(1):11–17.

52. Feys HB, Deckmyn H, Vanhoorelbeke K. ADAMTS13 in health

and disease. Acta Haematol . 2009;121(2–3):183–185.

53. Long DS, Smith ML, Pries AR, Ley K, Damiano ER.

Microviscometry reveals reduced blood viscosity and altered

shear rate and shear stress profiles in microvessels after

hemodilution. Proc Natl Acad Sci USA. 2004;101(27):10060–

10065. Erratum in Proc Natl Acad Sci USA. 2004;101(39):14304.

54. Lipowsky HH, Kovalcheck S, Zweifach BW. The distribution of blood rheological parameters in the microvasculature of cat

mesentery. Circ Res. 1978;43(5):738–749.

55. Lipowsky HH. Microvascular rheology and hemodynamics.

Microcirculation. 2005;12(1):5–15.

56. Strony J, Beaudoin A, Brands D, Adelman B. Analysis of shear

stress and hemodynamic factors in a model of coronary artery

stenosis and thrombosis. Am J Physiol . 1993;265(5 Pt 2):H1787–1796.

57. Tsai HM, Sussman II, Nagel RL. Shear stress enhances the

proteolysis of von Willebrand factor in normal plasma. Blood.

1994;83(8):2171–2179.

58. Federici AB, Elder JH, De Marco L, Ruggeri ZM, Zimmerman

TS. Carbohydrate moiety of von Willebrand factor is not

necessary for maintaining multimeric structure and ristocetin

cofactor activity but protects from proteolytic degradation. J

Clin Invest. 1984;74(6):2049–2055.

59. Sodetz JM, Paulson JC, McKee PA. Carbohydrate composition

and identification of blood group A, B, and H oligosaccharide

structures on human Factor VIII/von Willebrand factor. J Biol

Chem. 1979;254(21):10754–10760.

60. Matsui T, Titani K, Mizuochi T. Structures of the asparagine-

linked oligosaccharide chains of human von Willebrand factor.

Occurrence of blood group A, B, and H(O) structures. J Biol

Chem. 1992;267(13):8723–8731.

61. Bowen DJ. An influence of ABO blood group on the rate of

proteolysis of von Willebrand factor by ADAMTS13. J ThrombHaemost. 2003;1(1):33–40.

62. O’Donnell JS, McKinnon TA, Crawley JT, Lane DA, Laffan MA.

Bombay phenotype is associated with reduced plasma-VWF

levels and an increased susceptibility to ADAMTS13 proteolysis.

Blood. 2005;106(6):1988–1991.

63. Gill JC, Endres-Brooks J, Bauer PJ, Marks WJ Jr, Montgomery

RR. The effect of ABO blood group on the diagnosis of von

Willebrand disease. Blood. 1987;69(6):1691–1695.

64. Davies JA, Collins PW, Hathaway LS, Bowen DJ. Effect of von

Willebrand factor Y/C1584 on in vivo protein level and function

and interaction with ABO blood group. Blood. 2007;109(7):2840–

2846.

65. McKinnon TA, Chion AC, Millington AJ, Lane DA, Laffan MA.

N-linked glycosylation of VWF modulates its interaction with

ADAMTS13. Blood. 2008;111(6):3042–3049.66. Kokame K, Matsumoto M, Soejima K, et al . Mutations and

common polymorphisms in ADAMTS13 gene responsible for von

Willebrand factor-cleaving protease activity. Proc Natl Acad Sci

USA. 2002;99(18):11902–11907.

67. Ruan CH. The frequency of P475S polymorphism in von

Willebrand factor-cleaving protease in the Chinese population

and its relevance to arterial thrombotic disorders. Thromb

Haemost. 2004;91:1258–1259.

68. Bongers TN, De Maat MPM, Dippel DWJ, Uitterlinden AG,

Leebeek FWG. Absence of Pro475Ser polymorphism in ADAMTS-

13 in Caucasians. J Thromb Haemost. 2005;3:805.

69. Plaimauer B, Fuhrmann J, Mohr G, et al . Modulation of

ADAMTS13 secretion and specific activity by a combination of

common amino acid polymorphisms and a missense mutation.

Blood. 2006;107(1):118–125.70. Bowen DJ, Collins PW. An amino acid polymorphism in von

Willebrand factor correlates with increased susceptibility to

proteolysis by ADAMTS13. Blood. 2004;103(3):941–947.

71. Davies JA, Bowen DJ. The association between the L1565

variant of von Willebrand factor and susceptibility to proteolysis

by ADAMTS13. Haematologica. 2007;92(2):240–243.

72. O’Brien LA, James PD, Othman M, et al . Association of

Hemophilia Clinic Directors of Canada. Founder von

Willebrand factor haplotype associated with type 1 von

Willebrand disease. Blood. 2003;102(2):549–557.

73. Sutherland JJ, O’Brien LA, Lillicrap D, Weaver DF. Molecular

modeling of the von Willebrand factor A2 Domain and the effects

of associated type 2A von Willebrand disease mutations. J Mol

Model . 2004;10(4):259–270.

74. Keeney S, Grundy P, Collins PW, Bowen DJ. C1584 in von

Willebrand factor is necessary for enhanced proteolysis by

ADAMTS13 in vitro. Haemophilia. 2007;13(4):405–408.

75. Bowen DJ, Collins PW, Lester W, et al . UK Haemophilia Centre

Doctors’ Organization. The prevalence of the cysteine1584

variant of von Willebrand factor is increased in type 1 von

Willebrand disease: co-segregation with increased susceptibility

to ADAMTS13 proteolysis but not clinical phenotype. Br J

Haematol . 2005;128(6):830–836.

76. Davies JA, Collins PW, Hathaway LS, Bowen DJ. von

Willebrand factor: evidence for variable clearance in vivo

according to Y/C1584 phenotype and ABO blood group. J

Thromb Haemost. 2008;6(1):97–103.

77. Davies JA, Collins PW, Hathaway LS, Bowen DJ. C1584: effect

on von Willebrand factor proteolysis and von Willebrand factor

antigen levels. Acta Haematol . 2009;121(2–3):98–101.

78. Pruss CM, Notley CR, Hegadorn CA, O’Brien LA, Lillicrap D.

ADAMTS13 cleavage efficiency is altered by mutagenic and, to a



10/10

lesser extent, polymorphic sequence changes in the A1 and A2

domains of von Willebrand factor. Br J Haematol . 2008;143(4):

552–558.

79. Huizinga EG, Tsuji S, Romijn RA, et al . Structures of glycopro-

tein Ibalpha and its complex with von Willebrand factor A1

domain. Science. 2002;297(5584):1176–1179.

80. Nishio K, Anderson PJ, Zheng XL, Sadler JE. Binding of platelet

glycoprotein Ibalpha to von Willebrand factor domain A1stimulates the cleavage of the adjacent domain A2 by

ADAMTS13. Proc Natl Acad Sci USA. 2004;101(29):10578–10583.

81. Tsai HM, Lian ECY. Antibodies to von Willebrand factor-

cleaving protease in acute thrombotic thrombocytopenic pur-

pura. N Engl J Med. 1998;339:1585–1594.

82. Furlan M, Robles R, Galbusera M, et al . von Willebrand factor-

cleaving protease in thrombotic thrombocytopenic purpura and

the hemolytic-uremic syndrome. N Engl J Med. 1998;339(22):

1578–1584.

83. Klaus C, Plaimauer B, Studt JD, et al . Epitope mapping of

ADAMTS13 autoantibodies in acquired thrombotic thrombocy-

topenic purpura. Blood. 2004;103(12):4514–4519.

84. Moake JL, Rudy CK, Troll JH, et al . Unusually large plasma

factor VIII:von Willebrand factor multimers in chronic relapsing

thrombotic thrombocytopenic purpura. N Engl J Med. 1982;307(23):1432–1435.

85. Tsai HM. Current concepts in thrombotic thrombocytopenic

purpura. Annu Rev Med. 2006;57:419–436.

86. Lyons SE, Bruck ME, Bowie EJ, Ginsburg D. Impaired

intracellular transport produced by a subset of type IIA von

Willebrand disease mutations. J Biol Chem. 1992;267(7):4424–

4430.

87. Tsai HM, Sussman II, Ginsburg D, Lankhof H, Sixma JJ, Nagel

RL. Proteolytic cleavage of recombinant type 2A von Willebrand

factor mutants R834W and R834Q: inhibition by doxycycline and

by monoclonal antibody VP-1. Blood. 1997;89(6):1954–1962.

88. O’Brien LA, Sutherland JJ, Hegadorn C, et al . A novel type 2A

(Group II) von Willebrand disease mutation (L1503Q) associated

with loss of the highest molecular weight von Willebrand factor

multimers. J Thromb Haemost. 2004;2(7):1135–1142.89. Lyons SE, Cooney KA, Bockenstedt P, Ginsburg D.

Characterization of Leu777Pro and Ile865Thr type IIA von

Willebrand disease mutations. Blood. 1994;83(6):1551–1557.



1.Von Willebrand Factor Proteolysis by ADAMTS13

Documents

Transcript of 1.Von Willebrand Factor Proteolysis by ADAMTS13