Mammalian primitive erythrocytes: neither ﬁsh nor fowl

Mammalian primitive erythroblasts undergo enucleation in the circulation, thus refuting the long-standing perception that primitive erythroblasts remain nucleated and are more similar to nucleated avian, ﬁsh, and reptile red cells than to deﬁnitive red cells of mammals. This report provides new evidence that accelerated apoptosis of neutrophils and their precursors is an important mechanism for neutropenia in systemic lupus erythematosus.

D uring mammalian embryogenesis, erythropoiesis progresses through distinct phases, each phase producing cells with dramatically different characteristics. When cardiac contractions begin in mice embryos at embryonic day 8.25 (E8.25), "primitive" erythroblasts, developed in yolk sac blood islands, enter the circulation. 1,2 However, by E12.5 "definitive" erythrocytes, produced in the fetal liver, begin to circulate and quickly prevail as the dominant erythroid phenotype. 3 While definitive erythroblasts synthesize adult hemoglobins and enucleate, primitive red cells are larger, contain embryonic and adult hemoglobins, and have been thought not to undergo enucleation during their life span. Although several earlier observations hinted that in mouse embryos a population of large, enucleated cells might be circulating, 4 the longstanding perception has been that primitive mammalian erythrocytes retain their nuclei and are thus more similar to nucleated avian, fish, and reptile red cells 5 than to definitive red cells of mammals.
However, in this issue, Kingsley and colleagues (page 19) report quantitative data showing that between E12.5 and E16.5 primitive erythroblasts progressively enucleate in circulation. Further, they observed that enucleated, primitive cells can be detected up to 5 days after birth. Using antibodies to specific regions of murine embryonic ␤H1-globin and adult ␤major-globin, they were able to differentiate yolk sac-derived primitive red cells. These antibodies, in combination with nuclear staining, identified 3 distinct peripheral blood cell populations in E13.5 and E15.5 fetuses: a nucleated population expressing embryonic ␤H1-globin, an enucleated population lacking ␤H1-globin, and surprisingly, an enucleated population expressing ␤H1-globin. Small numbers of enucleated ␤H1-globinexpressing cells were initially detected at E12.5. By E16.5, all of the ␤H1-globinexpressing cells were enucleated. Morphometric analysis of cell area revealed that both nucleated and enucleated ␤H1-globinexpressing cells were 100 m 2 in size and about 3-fold larger than definitive erythrocytes. Importantly, the disappearance of circulating nucleated primitive cells was due to their progressive enucleation and not loss from the bloodstream.
These findings provide a persuasive argument refuting the currently held view that primitive mammalian erythropoiesis resembles avian and reptilian erythropoiesis more than definitive mammalian erythropoiesis. Indeed, the authors delineate a number of important similarities between murine primitive and definitive erythropoiesis. Both differentiation programs exhibit maturation with enucleation. Additionally, prior to extrusion, nuclei condense and move to the plasma membrane, coincident with loss of intermediate filaments. 6 Yet one striking difference in the differentiation programs is that primitive erythroblasts appear to undergo terminal differentiation in circulation, while definitive erythroblasts mature extravascularly within 3-dimensional erythroblastic islands, closely associated with macrophages and extracellular matrix proteins. A number of new questions can now be asked. Do circulating primitive erythroblasts require contact with macrophages of the reticuloendothelial system for enucleation? What is the trigger for enucleation? Are molecular mechanisms of chromatin condensation, cytoskeletal remodeling, and nuclear extrusion similar or different in primitive and definitive erythroblasts ? Do membrane mechanical properties of  enucleated primitive cells differ from those of  definitive cells, suggesting unique interactions  among transmembrane and cytoskeletal

W. Conrad Liles, Gordon Starkebaum, and David C. Dale UNIVERSITY OF WASHINGTON
This report provides new evidence that accelerated apoptosis of neutrophils and their precursors is an important mechanism for neutropenia in systemic lupus erythematosus.
I n this issue of Blood, Matsuyama and colleagues (page 184) report a novel mechanism for the pathogenesis of neutropenia in systemic lupus erythematosus (SLE). SLE is a relatively common disorder with a prevalence of 50 cases per 100 000 population and a female-male ratio of approximately 10:1. Neutropenia, defined as a blood neutrophil count less than 1.8 ϫ 10 9 /L, occurs in about 50% of individuals with SLE. Although usually mild (ie, 1.0-1.8 ϫ 10 9 /L), SLE-related neutropenia is considered to be an important factor predisposing these individuals to bacterial infections. Several mechanisms have been identified for the pathogenesis of neutropenia in SLE. Approximately two thirds of patients have antineutrophil autoantibodies (usually immunoglobulin G [IgG]). 1 Neutropenia is more common in individuals with either anti-Ro autoantibodies. 2 Several previous studies have implicated Fas (CD95)-mediated apoptosis of circulating neutrophils, monocytes, and lymphocytes of patients with SLE. [3][4] Marrow hypoplasia and Fas-mediated apoptosis of CD34 ϩ hematopoietic progenitor cells are additional factors contributing to SLE-associated cytopenias. 5 The report by Matsuyama et al provides further insight into the pathogenesis of neutropenia in SLE by describing abnormalities in another mediator of apoptosis, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study involving 28 patients with SLE and 8 healthy controls, serum TRAIL levels were inversely proportional to blood neutrophil counts. Expression of TRAIL receptor 3, a decoy receptor for TRAIL, also was lower in neutropenic patients than in patients or controls without neutropenia. In vitro, TRAIL induced apoptosis in neutrophils. Corticosteroid therapy reduced expression of TRAIL on T cells and enhanced expression of Fas-associating protein with death domain-like interleukin-1␤converting enzyme (FLICE)-inhibitory protein (FLIP), an antiapoptotic protein, by neutrophils. TRAIL may also play an important role in immunoregulation, and dysregulation of TRAIL or its cognate receptor may contribute to the development of systemic autoimmune diseases, such as SLE. 6 For example, mice deficient in TRAIL have been previously shown to be susceptible to the development of accelerated autoimmune disease.
This study by Matsuyama et al also adds to the growing evidence that neutropenia in SLE is chiefly attributable to accelerated apoptosis of mature neutrophils. It suggests to us that careful studies of the pathogenic mechanisms responsible for the cytopenias associated with SLE may provide the basis for development of novel and effective therapeutic approaches for clinical management of this severe autoimmune disorder.   Compared with "healthy" vessels, the extracellular matrix of atheromatous plaques includes increased content of type-EIIIA-domain-containing fibronectin. Apolipoprotein E-null mice engineered to produce fibronectin lacking this alternatively spliced exon exhibit reduced atherosclerosis. Diminished hypercholesterolemia and reduced macrophage foam cell formation may contribute to this phenotype.
V ariations in the molecular composition of the extracellular matrix (ECM) play important roles during vascular remodeling in pathophysiologic processes (ie, vasculogenesis and angiogenesis, wound healing, and vascular obstructive lesion formation). 1 Fibronectin is a ligand for fibrin, heparin, collagen, and several integrins implicated in the recruitment of blood leukocytes to the arterial endothelium. 2 While several splice-variant forms of fibronectin have been identified, the 2 major forms are plasma fibronectin (pFN), which is produced by hepatocytes, and cellular fibronectin (cFN), which is produced locally by different cell types and deposited and assembled into the ECM. 2 A single gene encodes for distinct fibronectin moieties depending on alternative RNA splicing of exons encoding for the V/CS-1 segment (also known as the V exon), extra type III repeat segments (known as ED-A and ED-B in humans, or EIIIA and EIIIB in rodents), and the (V ϩ C) region. pFN lacks extra type III repeat segments, whereas cFN contains variable amounts of these alternative domains.
Although a causal link between fibronectin and cardiovascular pathobiology has been difficult to establish because null mutations for fibronectin cause embryonic lethality, 3  Recently, analysis of fibronectin conditional knock-out mice revealed a role of pFN in thrombus initiation, growth, and stability. 6 Now, Tan and colleagues (page 11) directly test the function of EIIIA-FN in atherosclerosis by engineering EIIIA-null (EIIIA Ϫ/Ϫ ) mice lacking the EIIIA exon. They crossed EIIIA Ϫ/Ϫ mice with atherosclerosis-prone apolipoprotein E-null (ApoE Ϫ/Ϫ ) mice and found as much as a 67% reduction in aortic atherosclerosis in doubly deficient fat-fed EIIIA Ϫ/Ϫ ApoE Ϫ/Ϫ mice compared with ApoE Ϫ/Ϫ controls. An intriguing observation that deserves further examination is that EIIIA Ϫ/Ϫ ApoE Ϫ/Ϫ females display significant protection at all time points assayed (8, 12, and 16 weeks of fat feeding), while males are protected only after 16 weeks. Compared with ApoE Ϫ/Ϫ controls, both male and female EIIIA Ϫ/Ϫ ApoE Ϫ/Ϫ mice displayed diminished total plasma cholesterol levels, a reduction that is specific to the very low density lipoprotein fraction. Increased EIIIA-FN expression was found in both the plasma and in endothelial cells and macrophages within atherosclerotic lesions of ApoE Ϫ/Ϫ mice. Moreover, in vitro foam cell formation by ApoE Ϫ/Ϫ macrophages was associated with increased EIIIA-FN mRNA expression, and lipid accumulation in EIIIA Ϫ/Ϫ ApoE Ϫ/Ϫ macrophages was reduced by 31% compared with ApoE Ϫ/Ϫ controls.
Collectively, the study by Tan and colleagues convincingly demonstrates an atherogenic role of EIIIA-FN and suggests that this form of fibronectin is functional in both plasma lipoprotein metabolism and in macrophage foam cell formation. Future studies are warranted to elucidate additional systemic mechanisms and processes within the vessel wall by which EIIIA-FN may contribute to atherosclerosis. For example, the possibility that EIIIA-FN may interact with specific lipoprotein fractions and may affect leukocyte recruitment should be investigated. Moreover, because endothelial and smooth muscle cells cultured on different ECM components display significant differences in proliferation, migration, and apoptosis, comparing the phenotypic properties of vascular cells cultured on EIIIA-FN, EIIIB-FN, and fibronectin lacking
S ystemic inflammation shifts the hemostatic balance in the direction of thrombosis. This has long been recognized by physicians who treat patients with cancer, infections, autoimmune diseases, and pregnancy, as these systemic conditions (as well as many others) are associated with a hypercoag-ulable state. Thus the link between thrombosis and inflammation continues to be the focus of considerable attention, both in the laboratory and in the clinic.
Interactions between platelets, leukocytes, and the endothelium lie at the interface between thrombosis and inflammation, and these interactions are profoundly influenced by the actions of inflammatory cytokines. Examples of cytokines with procoagulant influence include tumor necrosis factor ␣ (TNF-␣), interleukin-6 (IL-6), and IL-8, which, among many effects, are known to stimulate expression of tissue factor by monocytes and macrophages (TNF-␣), 1 increase platelet reactivity and induce fibrinogen and plasminogen activator inhibitor-1 (PAI-1) expression by the liver (IL-6), 1,2 and activate endothelial cells (TNF-␣ and IL-8). 2,3 Building on this theme, Bernardo and colleagues in this issue of Blood (page 100) increase our understanding of the mechanisms by which TNF-␣, IL-6, and IL-8 exert their procoagulant influence, which turns out to involve the blood clotting protein von Willebrand factor (VWF) and ADAMTS-13, an enzyme involved in the proteolytic processing of VWF and deficient in the disease thrombotic thrombocytopenic purpura (TTP).
VWF synthesized by endothelial cells either is constitutively secreted into the circulation in the form of low-molecular-weight multimers or is stored in Weibel-Palade bodies, and in response to endothelial cell agonists is released in the form of ultralarge multimers (ULVWF). ULVWF is the most adhesive and reactive form of VWF and may lead to spontaneous platelet aggregation if not further processed by the ADAMTS-13 metalloprotease. Lack of ULVWF cleavage by ADAMTS-13 is thought to be the primary defect underlying TTP.
In this study, Bernardo and colleagues investigated whether ULVWF release from endothelial cells and its subsequent cleavage by ADAMTS-13 are affected by the inflammatory cytokines TNF-␣, IL-6, and IL-8, as these processes could represent potential links between inflammation and thrombosis. First, using an elegant system in which cultured endothelial cells are subjected to defined flow stress in vitro, the authors demonstrate that TNF-␣ and IL-8 (but not IL-6) stimulate the release of ULVWF strings from human umbilical vein endothelial cells (HUVECs) in a dose-dependent manner. As HUVECs lack an IL-6 receptor, the absence of response with IL-6 was not surprising; when the experiment was repeated with IL-6 precomplexed with soluble IL-6 receptor, ULVWF release was seen (although not to the level of the other cytokines).
The authors then turn their attention to whether these cytokines can alter the ability of ADAMTS-13 to cleave ULVWF. They show that IL-6 (but not TNF-␣ or IL-8) greatly abrogates the ability of ADAMTS-13 to cleave ULVWF strings under flowing, but not static, assay conditions. As this interesting effect is still observed when partially purified ADAMTS-13 is pretreated with IL-6, the authors speculate that IL-6 may physically impair binding of ADAMTS-13 to ULVWF under the shear stress of flowing conditions. As the authors point out, this observation is For personal use only. on July 24, 2018. by guest www.bloodjournal.org From especially interesting in light of previous studies demonstrating elevated IL-6 levels in patients with TTP, 4 and in coronary thrombi and atherosclerotic plaques. 5 This important study provides insight into potential new mechanisms by which the inflammatory process is able to shift the hemostatic balance in favor of thrombosis. Cytokines present in a variety of pathologic conditions may influence both the release of ULVWF from endothelial cells and its subsequent processing by ADAMTS-13, thus allowing ULVWF to persist long enough to induce platelet adhesion and aggregation, and ultimately lead to thrombosis. New information regarding regulation of the interface between inflammation and thrombosis is always welcome as it eventually may lead to new points of therapeutic intervention for patients with inflammation-associated coagulopathies. s

Richard Sullivan OREGON HEALTH AND SCIENCE UNIVERSITY
Evidence is growing that many types of ion channels and other molecules once thought to be restricted to the nervous system are expressed in hematopoietic cells, where they may function at various levels of differentiation.

I n 1978, Miller et al 1 inserted electrodes into
a Guinea pig megakaryocyte and showed that when electric current was injected, the cell fired a biphasic action potential exactly as a neuron would do under the same conditions. Voltage-dependent calcium and potassium channels identical to those found in some nerve and muscle cells were later shown to underlie its action potential. 2 Long distracted by the dazzling electric currents of neurons and myocytes, electrophysiologists had traditionally dismissed most other tissues-includ-ing blood-as "nonexcitable" and therefore unworthy of their attention. But the action potential of the Guinea pig megakaryocyte, which blurred this convenient distinction, was hard to ignore. Why did blood cells need ion channels? What business did they have generating electricity?
Inspired by the electronics of this curious bone marrow cell, and enabled by refinements in the patch-clamp technique that permitted characterization of single ion channels, electrophysiologists started around 1990 to tinker with other blood cells. They found to their great surprise that many of the same depolarizing (sodium and calcium) and hyperpolarizing (potassium and chloride) channels that had originally been considered the exclusive domain of neurons or muscle cells actually operated in hematopoietic cells as well. We now recognize that blood cells, like neurons, generate and store electric potential energy. Through the cooperative behavior of a panoply of ion channels, they harness this energy to carry out a diverse range of hefty biophysical work, including volume regulation, cell movement, and degranulation. It is likely that whenever a blood cell like a platelet or a neutrophil does any "heavy lifting," electric energy is spent through the operation of many of the same ion channels that exist in brain and muscle but that perform totally different tasks in those tissues.
In the current issue of Blood, Steidl and colleagues (page 81) take this concept a step further by reporting that human CD34 ϩ cells express not only genes encoding many of the ion channels found in the brain, but also a variety of other proteins whose roles have been primarily defined in the nervous system. These include numerous neuromediators, receptors, kinases, phosphatases, and other proteins involved in the regulation of ion channels, neurotransmitter release, or other aspects of neuronal behavior. Of particular interest is their finding that many components of the exocytic machinery involved in neurotransmitter release at axon termini are ex pressed by CD34 ϩ cells. This observation supports the view that agonist-mediated exocytosis in hematopoietic cells may share more similarities than differences at the molecular level with the highly specialized exocytosis that takes place in neurons. In their paper, Steidl et al go to great lengths to show that many neurobiologic genes are not only expressed at both the mRNA and protein level in For personal use only. on July 24, 2018. by guest www.bloodjournal.org From CD34 ϩ cells, some of them exhibit their predicted functions in these cells.
Since many of the ion channels that coexist in the brain and the blood carry out disparate tasks in these tissues, it is not surprising that many other "neurobiologic" proteins may have been adapted by nature for different roles in hematopoietic cells. Defining their functions in prolif-erative and mature blood cells is now the challenge. The blood-brain barrier is weakening. s

Michael Detmar HARVARD MEDICAL SCHOOL
Vascular endothelial growth factor (VEGF) unexpectedly induces the expression of the Down syndrome critical region protein-1 (DSCR1), an inhibitor of the calcineurin signaling pathway, in endothelial cells, leading to down-regulation of several inflammatory genes after initial stimulation.
V ascular endothelial growth factor (VEGF) is a potent angiogenic growth factor that has become the prime target of antiangiogenic cancer therapy. Hesser and colleagues (page 149) report the unexpected ability of VEGF to down-regulate the inflammatory response after an initial stimulation, by temporarily inducing the expression of the Down syndrome critical region protein-1 (DSCR1) in endothelial cells. This finding reveals potential new approaches to control inflammatory events at the vasculature.
Using a gene expression profiling approach, Hesser et al observed that DSCR1 was the most highly up-regulated gene in several types of human endothelial cells after VEGF treatment. DSCR1 is one of many gene products that have been associated with Down syndrome and belongs to a family of proteins that interact with the phosphatase calcineurin A (CnA). The authors showed that in human endothelial cells DSCR1 repressed several inflammatory genes by inhibiting nuclear translocation of nuclear factor of activated T cells (NFAT)-a transcription factor that is regulated by CnA. Previous studies had shown the ability of DSCR1 to inhibit CnA in other cell types, and Hesser et al observed that this was also the case in endothelial cells. So it appears that VEGF up-regulation of DSCR1 leads to the inhibition of CnA, and therefore prevents the translocation of NFAT to the nucleus and the transcription of proinflammatory genes. These genes include COX2, E-selectin, and tissue factor; so modulation of DSCR1 might represent a novel therapeutic approach to control inflammation at the level of the vasculature. CnA is also the target of the potent immunosuppressive drug cyclosporine A (CsA), which is widely used to inhibit transplant rejection. CsA blocks CnA signaling, however, by a different mechanism than DSCR1, and in vitro studies indicate that CsA inhibits lymphocyte functions at low concentrations that do not affect endothelial cell signaling. The long-term therapeutic use of CsA has been associated with a number of adverse effects that include nephrotoxicity, renal vascular damage, and hypertension. 1 The sites of major vascular damage by CsA overlap with the sites of physiologic expression of VEGF and VEGF receptors in healthy adults. It will therefore be of interest to determine whether physiologic levels of VEGF expression in adult renal tissues are sufficient to induce DSCR1 expression. If so, a synergistic inhibition of CnA signaling by DSCR1 combined with CsA might contribute to chronic CsA-induced renal toxicity.
VEGF inhibitors are being developed and have been tested in the clinic as antiangiogenic therapies for cancer and other diseases. The discovery of DSCR1 as a major target of VEGF signaling in endothelial cells therefore raises important questions regarding the biologic consequences of long-term inhibition of VEGF signaling. Does blockade of VEGF signaling result in impaired expression of DSCR1 in the tumor endothelium or in normal vasculature? If so, the decreased activity of DSCR1 might contribute to the frequently observed increase in E-selectin and tissue factor levels in patients treated with VEGF signaling inhibitors, 2 which cause vascular injury and other effects. This pathway could also underlie other side effects, such as the increased thrombosis or proteinuria that have been observed in clinical trials with inhibitors of VEGF bioactivity. 3 Administration of compounds that mimic the biologic activity of DSCR1, such as CsA, might compensate for the potential decrease in DSCR1 expression in patients who undergo chronic anti-VEGF therapy.
Finally, the findings of Hesser et al raise the question of whether the increase in DSCR1 expression observed in individuals with Down syndrome or Alzheimer disease 4 might affect CnA activity in their endothelial cells, and whether such a potential vascular perturbation might contribute to disease
I t is currently thought that factor XI is essential for propagation of coagulation after small amounts of thrombin are generated by the tissue factor pathway. Factor XI is activated by thrombin preferentially on platelet membranes, which leads through activation of factor X to further thrombin generation, even after the clot has formed. The additional amount of thrombin then activates thrombin-activatable fibrinolysis inhibitor, which results in stabilization of the clot.
Factor XI is a homodimer of 80-kDa subunits linked by a disulfide bond. Thrombin cleaves in each subunit an Arg369-Ile370 bond yielding a heavy chain consisting of 4 "apple domains" and a light chain containing the catalytic site. The production of the homodimer is essential for the secretion of factor XI from producing cells, and for its function on the surface of platelets where one of the subunits was suggested to bind glycoprotein Ib embedded in lipid rafts and the other subunit to factor IX, its substrate. 1,2 Factor XI deficiency, an injury-related bleeding tendency, was first described in 1953 by Rosenthal et al 3 in 2 sisters and their maternal aunt and was considered to be transmitted in an autosomal dominant fashion. A later study distinguished between individuals with "major" and "minor" deficiencies represented by factor XI levels of less than 20 U/dL and 30 to 65 U/dL, respectively. 4 This observation was consistent with an autosomal recessive pattern of inheritance. However, because some of the patients with a minor deficiency in this and other studies did bleed following injury, the designation "dominant" or "recessive" mode of inheritance has become less important. What remains important is that patients with a major deficiency are at a significantly greater risk of bleeding following injury than patients with a minor deficiency, specifically at sites where local fibrinolysis is present (oral mucosa, nose, urinary tract). 5 The deficiency has been reported in sporadic cases from many parts of the world but was found to be particularly common in Jews of Ashkenazi (European) origin. There are 2 mutations, Glu117Stop and Phe283Leu, that predominate in this population with allele frequencies of 0.0217 and 0.0254, respectively. Altogether, 49 mutations have been published to date, among which dysfunctional (cross-reacting material-positive) deficiencies are extremely rare.
In this issue, Kravtsov and colleagues (page 128) describe 2 novel alterations in the factor XI gene that present a new mechanism for factor XI deficiency. The mutations, Gly400Val and Trp569Ser, abolish secretion of the mutant proteins from transfected fibroblasts, and in cotransfection experiments reduce the secretion of wild-type factor XI by 50%. Formation of heterodimers consisting of wild-type and mutant proteins was also demonstrable. The heterozygotes harboring these mutations had factor XI activity and antigenicity ranging between 10 to 58 U/dL and some had significant bleeding. These new data characterize for the first time a dominant-negative effect of 2 mutations that produce nonsecretable but dimerizable factor XI that can trap normal factor XI intracellularly, resulting in plasma factor XI levels below the range observed in heterozygotes and in bleeding. M cKay and colleagues (page 159) describe a careful, systematic evaluation of bleeding problems in a very large kindred with the Quebec platelet disorder (QPD). The pathophysiology of QPD is well defined. It is an autosomal dominant trait with increased megakaryocyte expression and storage of urokinase-type plasminogen activator (u-PA). The u-PA generates plasmin, causing degradation of platelet fibrinogen and other ␣-granule proteins important for hemostasis. Measurements of platelet u-PA and ␣-granule fibrinogen degradation products allow accurate identification of affected family members and clear distinction from unaffected family members. To assess the nature and severity of bleeding problems among family members, McKay and colleagues developed a questionnaire specific for QPD. Their study has important lessons for hematologists who investigate and manage patients with bleeding disorders.
First, the methodology is a model for clinical research. Too often clinical research is based on sound laboratory methods, but the patient observations are not quantitative and may not be reproducible. In this study the patient observations are made with rigorous attention to quantitative analysis.
Second, McKay and colleagues documented that some bleeding symptoms, such as very large bruises and bruises that tracked downward, occurred exclusively among affected family members, while the frequency of other bleeding symptoms, such as nosebleeds that lasted longer than 15 minutes, were not different between affected and unaffected family members. These observations remind us that healthy people do bleed, a simple but often overlooked fact, and that characterization of bleeding as abnormal may be difficult. Instruments such as this questionnaire provide the ability to define and measure abnormal bleeding. These instruments will allow quantitative estimates of the risk of bleeding with inherited disorders, comparable with our current ability to estimate risks for thrombosis in patients with inherited thrombophilia traits.
Third, the quantitative assessment by McKay and colleagues documented heterogeneity of bleeding manifestations among affected and unaffected family members. In such a large kindred, this may be expected because bleeding symptoms ultimately result from the interactions among multiple risk factors. For example, some unaffected family members who reported bleeding symptoms may have had undiagnosed von Willebrand disease type 1, a common risk factor for excessive bleeding. Some family members affected by QPD who reported less bleeding may have also inherited the factor V Leiden trait or another prothrombotic trait, which could diminish the risk for bleeding. The use of quantitative measures of bleeding symptoms will allow greater understanding of the interactions of multiple common inherited traits on the risks for excessive bleeding.
Fourth, the types of bleeding symptoms manifested by patients with QPD are intriguing. We teach our students that clinical evaluation can distinguish patients who have abnormalities of primary hemostasis, such as platelet disorders, from patients who have abnormali-ties of coagulation, such as hemophilia. We say that abnormalities of primary hemostasis are manifested by the prompt occurrence of mucocutaneous bleeding, while abnormalities of coagulation are manifested by delayed bleeding with hemarthroses and large visceral hematomas. Patients with QPD had both types of bleeding. In addition to mucocutaneous bleeding, affected family members commonly reported joint bleeds and bleeding that began 12 hours or more after trauma. These clinical manifestations are consistent with the abnormalities of QPD, since it is not only a disorder of platelet function with abnormalities of aggregation but also a disorder of fibrin clot formation and fibrinolysis.
Therefore read this article not only to learn about the clinical manifestations of a rare inherited platelet abnormality, read it to learn how the application of quantitative and reproducible clinical assessments can reveal new insights into hematologic disorders. s