Hot Topics I: A Potpourri of Current Issues and Controversies in Thrombosis and Hemostasis

Welcome to a special issue of Seminars in Thrombosis and Hemostasis. Characteristically, each issue of Seminars in Thrombosis and Hemostasis is theme-driven, with each new issue devoted to a particular theme of relevance to thrombosis and hemostasis. The current issue of Seminars in Thrombosis and Hemostasis is a little different, in so far as the theme on this occasion is uncharacteristically less specific and rather represents a broader potpourri of current issues and controversies in thrombosis and hemostasis. Nevertheless, it is not intended that this issue of Seminars in Thrombosis and Hemostasis be comprehensive in its dealing with potential issues and controversies in our field of interest, as this would be a weighty tome indeed. Rather, the content of the current issue of Seminars in Thrombosis and Hemostasis reflects a broad range of topics considered to be topical or potentially controversial by the contributing authors and the guest editor alike. We hope that you enjoy this slight diversion from the norm.

The first article on offer was prepared in response to several recent and past events. First and foremost was the author's recognition that the first published description of this assay that we now call the von Willebrand factor collagen binding (VWF:CB) assay was 21 years ago, in 1986, by Brown and Bosak.[1] The last time that I prepared a review specifically on the VWF:CB assay was in 2002,[2] also for Seminars in Thrombosis and Hemostasis, and quite a lot has happened since then. The second event (or rather a series of ongoing events) that prompted this article was ongoing communication with many respondents over these years who continue to express confusion regarding the VWF:CB assay itself, as well as its place within the diagnostics and therapy of von Willebrand disease (VWD). This confusion is somewhat bemusing to me. On the one hand, one might expect that the situation should be reasonably clear, particularly given the amount of literature on the VWF:CB assay over the past 15 years. On the other hand, several developments continue to cloud the situation. The third major event to prompt this article was again rather a series of events, and that was the plethora of recent studies and publications regarding genetic mutations in VWD and the resulting ability to better identify “true” cases of VWD, enabling the reassessment of these cases in terms of comparative phenotypic test profiles. To my mind, these studies confirm the difficulties in diagnosing and characterizing VWD using phenotypic assessment and support more than ever a place for the VWF:CB assay within the diagnostics and therapy of VWD. Thus, the question can now be asked: Has the VWF:CB, at “age 21,” finally matured and come of age? Read the first article and all will be revealed.

The next article, by Favaloro and colleagues, to some extent carries on from the preceding one and more broadly considers the use of certain tests and processes for undertaking pharmacokinetic studies of VWF concentrates in people with VWD. The authors here make several proposals for a new approach to future studies to permit the development of better tools that will enhance the assessment of, and permit more appropriate comparisons for, the potential utility and clinical efficacy of VWF factor concentrates for use as therapy for VWD. This chapter also provides a review and summary of previous pharmacokinetic studies in VWD. The new proposals for consideration are better qualification of multimer patterns and better use of phenotypic assays (VWF:Ag, VWF:RCo, and VWF:CB), along with the use of assay ratios (i.e., RCo/Ag and CB/Ag) to help identify VWF functional discordance, both in therapeutic product composition (i.e., in vitro composition labeling data for VWF concentrates) and in pharmacokinetic studies (i.e., ex vivo data to help assess potential utility and clinical efficacy of VWF concentrates). This can only be reasonably achieved if assay methodologies are appropriately standardized, and the authors also propose additional work to be performed in this important area.

The next article, by Lippi and colleagues, provides an extensive review of genomics and proteomics in venous thromboembolism. In particular, they propose that a reliable approach based on genomics and proteomics might be effective to construct a rational personalized medicine framework that can be applied to individual patients in the preclinical, clinical, and therapeutic settings of venous thrombosis. They report that the number of inherited thrombophilic disorders for which genetic testing is available has increased enormously and also that the test methods have changed considerably over recent times. Moreover, the field of pharmacogenomics, particularly for evaluating individual patient sensitivity to oral anticoagulant therapy, such as warfarin, has also evolved in parallel. They conclude that, provided extensive counseling is used to maximize the benefit and minimize the risks, results of genetic testing combined with ethnic, clinical, environmental, and psychological factors would assist in clinical decision making and build a solid bridge toward personalized medicine. They also propose that, in the very near future, proteomics would allow us to construct highly informative protein profiles in health and disease and thus assist in predicting and diagnosing disease and identifying targets for new therapeutic agents. The topic of genomics, proteomics, and pharmacogenomics is not only topical but also one that tends to divide workers in the field. For example, the FDA[3] has recently posted a position paper on its Web site, which states that the “FDA recognizes the importance of pharmacogenomics and encourages its use in drug development.” At the same time, there is some fear among clinicians that the requirement for pharmacogenomic testing may become universally mandated with regard to warfarin sensitivity, and that this is premature and unlikely to be cost-effective and yield its projected utility. In particular, it has to be recognized that pharmacogenomic-related warfarin sensitivity is only part of the story, with race-related genetic variability, drugs, diet, and comorbidity also contributing to warfarin sensitivity. Moreover, extraneous issues such as clinically applied dosing patterns, patient compliance, and even the basic laboratory testing processes for the most commonly applied test, the international normalized ratio (INR), all contribute to the individual efficacy of warfarin therapy. Finally, the current cost-effectiveness of warfarin pharmacogenomics is by no means clear.[4]

The article by Enjeti and colleagues describes the detection and measurement of microparticles, small membrane-bound vesicles that are generated from cells of different origin, notably in terms of thrombosis and hemostasis from platelets and endothelial cells. These bear at least some characteristics of the parent cell, and elevated levels have been described in cardiovascular states, thrombotic conditions, and cancer. The article contrasts and compares various methods for their detection of microparticles, including flow cytometry, enzyme-linked immunoassays, functional assays, and also notes some of the newer approaches being investigated. The authors conclude that, although it has evolved as an important vascular biology research tool and is gathering much interest among workers in thrombosis and hemostasis, the detection and measurement of microparticles needs further evaluation and refinement before it becomes truly useful as a clinical tool.

The article by Othman discusses the differential identification of platelet-type (PT)-VWD and type 2B VWD. This is an interesting story of “nonidentical twins” caused by two different genetic abnormalities but which evolve into similar phenotypes. PT-VWD has also been referred to in the past as pseudo-VWD. PT-VWD and type 2B VWD share common bleeding phenotypes and also common laboratory test phenotypes. Both PT-VWD and type 2B VWD result in gain of function for (i.e., hyperresponsive) binding of plasma VWF to its platelet ligand, glycoprotein 1b α (GPIBA). However, type 2B VWD results from a functionally abnormal VWF molecule, whereas PT-VWD is caused by hyperresponsive platelets due to defects in the platelet GPIBA gene. The laboratory discrimination between the two disorders can be a challenge because simple phenotypic testing will not differentially identify the disorders, and the more complex testing approaches are often poorly applied. For example, platelet function-based studies might be helpful but require particular technical experience and fresh blood samples. Definitive diagnosis is critical for treatment decisions, and Othman argues that this can be most definitively achieved by identifying the gene defect at either the VWF or GPIBA loci. She suggests that a systematic international molecular genetic study would also be helpful to address the question of whether PT-VWD is being systematically misdiagnosed as type 2B VWD. Such a study can be facilitated by an international online database/disease registry to enhance international awareness about this otherwise long-recognized diagnostic dilemma.

The debate about which phenotypic laboratory methods can be used to discriminate between the two conditions in a screening process was otherwise recently illustrated in a series of reports and correspondence.[5] [6] [7] [8] [9] [10] In the diagnosis of PT-VWD or type 2B VWD, the key feature that raises the question about the possibility of either disease is an enhanced ristocetin-induced platelet-agglutination (RIPA) assay, which reflects the increased platelet-VWF binding characteristics of both disorders. There are two subsequent nongenetic laboratory procedures that can then be used to potentially discriminate PT-VWD from type 2B VWD. The simpler of these is the cryoprecipitate challenge test, which is based on the fact that (washed) PT-VWD platelets (and not type 2B platelets) will aggregate upon the addition of cryoprecipitate. The slightly more complex option is performance of a RIPA-based plasma/platelet mixing study, which is based on the fact that (washed) PT-VWD platelets (but not type 2B platelets) aggregate in normal plasma with a low concentration of ristocetin and that (washed) normal platelets aggregate in the presence of type 2B (but not PT-VWD) plasma at similarly low concentrations of ristocetin.[10] Thus, either of these tests can be useful in the discrimination process but are only likely to provide useful results when performed carefully, or interpreted appropriately, and perhaps only in experienced laboratories. An additional limitation is the risk of false-positive PT-VWD identification as platelets from some type 2B patients may aggregate in response to cryoprecipitate challenge.[6]

The article by Franchini and colleagues discusses the role of ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin-1-like domains), the enzyme that regulates the size of VWF multimers, in multisystem acquired thrombotic microangiopathy (TMA) and other pathologic conditions distinct from the more often recognized and discussed cases of congenital and acquired thrombotic thrombocytopenic purpura (TTP). Their comprehensive review looks at secondary thrombotic microangiopathies that may occur with bone marrow and organ transplantation, as a result of malignant tumors, with infections, coincident with autoimmune disorders, related to medication use, with pregnancy, and a variety of other pathologic conditions. They conclude that the overall evidence from existing literature suggests that a decrease in plasma ADAMTS-13 activity is not specifically restricted to patients with hereditary or congenital TTP. Furthermore, if a severe deficiency in plasma ADAMTS-13 activity is a diagnostic feature of classic TTP, there is growing evidence suggesting that a substantial proportion of patients with acquired TMA may have mildly or moderately decreased plasma ADAMTS-13 levels. Nevertheless, they suggest that further epidemiologic studies with larger patient populations are needed to confirm whether mild/moderate decreases in plasma ADAMTS-13 levels may also occur in other pathologic conditions different from inherited or acquired TMA and to evaluate whether the decrease of this enzyme may be involved in the pathogenesis or is merely an epiphenomenon of the primitive underlying disorder.

The article by Mina and colleagues discusses the association between disorders of endocrine function and hemostasis. The authors reviewed the content of a large number of review articles to identify such associations. Abnormalities of hemostasis, platelets, and endothelium, and the presence of microparticles, the abnormal expression of adhesion molecules, and elevated VWF are all associated with cardiovascular disease and are also features of various endocrine disorders, including diabetes and its complications, insulin resistance, polycystic ovary syndrome, and various thyroid disorders. Related causes and associated factors, including obesity, alcohol, hyperlipidemia, omega fatty acids, vitamin D, serotonin, insulin-like growth factors, angiotensin-converting enzyme, and C-reactive protein, are also discussed in this review.

The article by Levi and de Jonge reviews the clinical relevance of plasma expanders on coagulation. Patients with severe bleeding are often treated with colloids as plasma replacement fluids. These may include dextrans, gelatin-based solutions, or starches. Many of these agents will affect the hemostatic system beyond their effect on hemodilution, and the ensuing impairment of coagulation is obviously undesirable in patients with major blood loss. However, there is considerable controversy regarding whether these anticoagulant effects truly translate into clinically relevant effects, such as increased blood loss, increased transfusion requirements, the need for surgical (re)exploration, organ dysfunction, or mortality. This chapter describes the effects of various plasma replacement solutions on the coagulation system and reviews the controlled clinical studies using different plasma expanders and evaluating clinically significant end points. The authors conclude that most plasma expanders do have marked effects at various points in the hemostatic system and that there are significant differences between various plasma replacement fluids, but that clinically relevant effects on bleeding are mostly present only if large volumes are infused or if the patient has a concomitant or preexistent hemostatic impairment.

The next article, by Lippi and colleagues, reviews the topic of thrombosis and hemostasis as it applies to the very young and nicely complements a previous review by Franchini[11] on aging and hemostasis. In the current review, the authors recognize that the physiology of hemostasis in pediatric patients differs widely from that in adults. Because the hemostatic system does not achieve full maturation by 3 to 6 months of age, the authors assert that the correct interpretation of hemostasis test results in young patients, along with a deep understanding of the normal postnatal development in the human coagulation system, are essential prerequisites to the proper investigation of thrombotic and hemorrhagic problems in pediatric patients and to prevent misclassification of children as having defects of factors and inhibitors of the coagulation system.

The last article in this issue of Seminars in Thrombosis and Hemostasis is by Douma and Kamphuisen who review the benefits and limitations of thrombolytic therapy for venous thromboembolism (VTE) and who ask the pertinent question, “Is it worthwhile?” VTE has a potentially fatal outcome, with an estimated annual incidence of 1 per 1000 persons, and includes the conditions deep venous thrombosis (DVT) and pulmonary embolism (PE) and so is a serious and frequently occurring disease. Although VTE can be effectively treated with heparin and vitamin K antagonists, there are short-term as well as long-term sequelae that characterize the clinical course of VTE, and there is an ongoing debate if more aggressive therapy, such as thrombolytic drugs, should be used to achieve faster clot lysis in pursuit of reducing mortality and long-term sequelae. Thrombolytic therapy activates plasminogen to form plasmin and causes accelerated lysis of thrombi. Although some VTE patients may benefit from thrombolytic treatment, the indications for more aggressive therapy in these patients are still controversial. Any clinical advantage of thrombolytic therapy must be weighed against an increased risk of major bleeding. From the current evidence-based research, the authors make several differential conclusions depending on the clinical presentation and taking into account the relative risks (e.g., major bleeding) versus the potential benefit.

I sincerely thank all the contributors to this special issue of Seminars in Thrombosis and Hemostasis for their excellent contributions, and each of us sincerely hope that you, the reader, enjoy the collected articles.

REFERENCES

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• 2 Favaloro E J. von Willebrand factor (VWF) collagen binding (activity) assay (VWF:CBA) in the diagnosis of von Willebrand's disorder (VWD): A 15-year journey.  Semin Thromb Hemost. 2002;  28 191-202
• 3 U.S. Food and Drug Administration . Center for Drug Evaluation and Research. Genomics at FDA.  Available at: http://www.fda.gov/cder/genomics/ Accessed October 17, 2007; 
• 4 Veenstra D L. The cost-effectiveness of warfarin pharmacogenomics.  J Thromb Haemost. 2007;  5 1974-1975
• 5 Enayat M S, Guilliatt A M, Lester W, Wilde J T, Williams M D, Hill F G. Distinguishing between type 2B and pseudo-von Willebrand disease and its clinical importance.  Br J Haematol. 2006;  133 664-666
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• 9 Othman M, Lillicrap D. Distinguishing between non-identical twins: platelet type and type 2B von Willebrand disease.  Br J Haematol. 2007;  138 665-666
• 10 Favaloro E J, Patterson D, Denholm A et al.. Differential identification of a rare form of platelet-type (pseudo-) von Willebrand disease (VWD) from type 2B VWD using a simplified ristocetin-induced-platelet-agglutination mixing assay and confirmed by genetic analysis.  Br J Haematol. 2007;  139 623-626
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