Coagulation activation and inflammation in sickle cell disease-associated pulmonary hypertension

BACKGROUND

Pulmonary hypertension (PHT) is common in sickle cell disease (SCD). The purpose of this study was to determine whether markers of coagulation activation and inflammation are associated with PHT in SCD.

DESIGN AND METHODS

This cross-sectional study was performed using a cohort of patients followed at an adult Sickle Cell Clinic. Pulmonary artery systolic pressure was determined by Doppler echocardiography, and the diagnosis of PHT was defined using age, sex and body mass index-adjusted reference ranges. Clinical laboratory examinations, including hematologic studies and biochemical tests, as well as various measures of coagulation activation, endothelial activation and inflammation, were conducted on SCD subjects and on healthy, race-matched control subjects without SCD.

RESULTS

Patients with SCD (n=76) had higher plasma levels of markers of coagulation (thrombin-antithrombin complex, prothrombin fragment F1+2, D-dimer) and endothelial (soluble vascular endothelial cell adhesion molecule, sVCAM) activation compared with control subjects (n=6). SCD patients with PHT (n=26) had significantly higher levels of sVCAM compared with those patients without PHT (n=50). Although PHT patients showed increased plasma measures of coagulation activation, the differences were not statistically significant when compared to those of patients without PHT. HbSS patients with PHT also had a trend towards higher levels of other inflammatory cytokines (interleukins 6, 8 and 10) than HbSS patients without PHT. There was a modest negative correlation between hemoglobin and plasma measures of coagulation and endothelial activation, and modest positive correlations between markers of hemolysis and plasma measures of coagulation and endothelial activation.

CONCLUSIONS

SCD patients with PHT have higher levels of markers of endothelial activation and other inflammatory markers than patients without PHT. A trend towards an increased level of markers of coagulation activation was observed in SCD patients with PHT compared with that in patients without PHT. Markers of hemolysis are associated with coagulation activation and endothelial dysfunction in SCD patients. Clinical trials of anticoagulants and anti-inflammatory agents are warranted in SCD patients with PHT.

Phase I study of magnesium pidolate in combination with hydroxycarbamide for children with sickle cell anaemia

In sickle cell anaemia, red cell dehydration increases intracellular HbS concentration and promotes sickling. Higher erythrocyte magnesium reduces water loss through negative regulation of membrane transporters. Hydroxycarbamide (also known as hydroxyurea) reduces sickling partly by increasing intracellular HbF. Combining drugs with distinct mechanisms could offer additive effects. A phase I trial combining oral magnesium pidolate and hydroxycarbamide was performed to estimate the maximum tolerated dose (MTD) and toxicity of magnesium. Cohorts of three children with HbSS, who were on a stable dose of hydroxycarbamide (median 28.5 mg/kg/d), received magnesium pidolate for 6 months beginning at 83 mg/kg/d. The dose was escalated by 50% for subsequent cohorts. Laboratory evaluations were performed at 0, 3, 6 and 9 months. Sixteen children (aged 4-12 years) participated. All four dose-limiting toxicities (grade III diarrhoea and abdominal pain) occurred within the first month of starting magnesium. Additionally, diarrhoea grades I (n = 1) and II (n = 3), and abdominal pain grade II (n = 3) occurred. Hydroxycarbamide dose reduction or interruption was not required. The MTD for magnesium pidolate used in combination with hydroxycarbamide was 125 mg/kg/d. KCl co-transporter activity declined after 3 months of magnesium pidolate (P = 0.02). A phase II study is needed to investigate the efficacy of this drug combination.

Erythrocyte adhesion is modified by alterations in cellular tonicity and volume

We tested the hypothesis that dehydration-induced alterations in red blood cell (RBC) membrane organisation or composition contribute to sickle cell adhesion in sickle cell disease (SCD). To examine the role of RBC hydration in adhesion to the subendothelial matrix protein thrombospondin-1 (TSP), normal and sickle RBCs were incubated in buffers of varying tonicity and tested for adhesion to immobilised TSP under flow conditions. Sickle RBCs exhibited a decrease in TSP binding with increasing cell hydration (P<0.005), suggesting that cellular dehydration may contribute to TSP adhesion. Consistent with this hypothesis, normal RBCs showed an increase in TSP adhesion with increasing dehydration (P<0.01). Furthermore, increased TSP adhesion of normal RBCs could also be induced by isotonic dehydration using nystatin-sucrose buffers. Finally, TSP adhesion of both sickle RBCs and dehydrated normal RBCs was inhibited by the anionic polysaccharides, chondroitin sulphate A and high molecular weight dextran sulphate, but not by competitors of CD47-, band 3-, or RBC phosphatidylserine-mediated adhesion. More importantly, we found increased adhesion of nystatin-sucrose dehydrated normal mouse RBCs to kidney capillaries following re-infusion in vivo. In summary, these findings demonstrate that changes in hydration can significantly impact adhesion, causing normal erythrocytes to display adhesive properties similar to those of sickle cells and vice versa.

Pathology of Berkeley sickle cell mice: similarities and differences with human sickle cell disease

Because Berkeley sickle cell mice are used as an animal model for human sickle cell disease, we investigated the progression of the histopathology in these animals over 6 months and compared these findings to those published in humans with sickle cell disease. The murine study groups were composed of wild-type mixed C57Bl/6-SV129 (control) mice and sickle cell (SS) mice (alpha-/-, beta-/-, transgene +) of both sexes and between 1 and 6 months of age. SS mice were similar to humans with sickle cell disease in having erythrocytic sickling, vascular ectasia, intravascular hemolysis, exuberant hematopoiesis, cardiomegaly, glomerulosclerosis, visceral congestion, hemorrhages, multiorgan infarcts, pyknotic neurons, and progressive siderosis. Cerebral perfusion studies demonstrated increased blood-brain barrier permeability in SS mice. SS mice differed from humans with sickle cell disease in having splenomegaly, splenic hematopoiesis, more severe hepatic infarcts, less severe pulmonary manifestations, no significant vascular intimal hyperplasia, and only a trend toward vascular medial hypertrophy. Early retinal degeneration caused by a homozygous mutation (rd1) independent from that causing sickle hemoglobin was an incidental finding in some Berkeley mice. While our study reinforces the fundamental strength of this model, the notable differences warrant careful consideration when drawing parallels to human sickle cell disease.

Variation in hospitalizations and hospital length of stay in children with vaso-occlusive crises in sickle cell disease

OBJECTIVE

As a measure of morbidity of vaso-occlusive crises (VOC) in sickle cell disease, we used a national hospital discharge database, the Kids' Inpatient Database (KID), to determine the number of hospitalizations and hospital length of stay (LOS) by age.

PROCEDURE

Nationally weighted hospital discharges for VOC in children with sickle cell disease were analyzed using data from 2,500 hospitals in the Healthcare Cost and Utilization Project (HCUP) KID. Number of discharges and hospital LOS by age group were analyzed. Multiple linear regression was performed to analyze the effect of age on hospital LOS.

RESULTS

There were 20,271 hospital discharges for children with sickle cell disease and VOC. Mean age (SD) at time of admission was 10.6 (5.38) years with 49% being males. The overall average LOS was 4.4 days. There were differences in the number of hospital discharges by age group (P < 0.001). In addition, there was a difference in LOS by age group (P < 0.0001). After controlling for potential confounders, older age was associated with a longer LOS (P < 0.0001).

CONCLUSIONS

Older children with sickle cell disease and VOC have increased hospitalizations and longer LOS. This age effect should be considered when measuring the effect of an intervention on hospital utilization in these children.

The effect of magnesium on length of stay for pediatric sickle cell pain crisis

OBJECTIVES

To determine the effect of intravenous (IV) magnesium sulfate on the length of stay (LOS) for children admitted with sickle cell pain crisis.

METHODS

This was a single arm study, with historical controls. Hemoglobin SS or Hemoglobin Sbeta degrees children admitted for pain crisis were eligible. Two sequential treatment protocols of IV magnesium sulfate were added to standard inpatient therapy of IV fluids, opioids, and nonsteroidal anti-inflammatory drugs. The initial 12 children received 40 mg/kg (maximum of 1.5 g) of IV magnesium in the emergency department (ED), and eight and 16 hours later. Seven subsequent children received 40 mg/kg (maximum of 2.5 g) of IV magnesium in the ED, and every eight hours for up to four inpatient days. The main outcome measure was LOS, measured in days. Differences in LOS were analyzed between the admission with magnesium, and the two previous admissions meeting the same criteria, using a Wilcoxon signed-rank test for nonparametric paired data.

RESULTS

Nineteen children were enrolled. The results of the two protocols were combined for the analysis. A shorter LOS was found on study admissions (median LOS, 3.0 days compared with 5.0 and 4.0 days for the previous two admissions; p = 0.006).

CONCLUSIONS

IV magnesium appears to decrease the LOS for children with sickle cell pain crisis.

Hypoxia-induced acute lung injury in murine models of sickle cell disease

Vaso-occlusive events are the major source of morbidity and mortality in sickle cell disease (SCD); however, the pathogenic mechanisms driving these events remain unclear. Using hypoxia to induce pulmonary injury, we investigated mechanisms by which sickle hemoglobin increases susceptibility to lung injury in a murine model of SCD, where mice either exclusively express the human alpha/sickle beta-globin (halphabetaS) transgene (SCD mice) or are heterozygous for the normal murine beta-globin gene and express the halphabetaS transgene (mbeta+/-, halphabetaS+/-; heterozygote SCD mice). Under normoxia, lungs from the SCD mice contained higher levels of xanthine oxidase (XO), nitrotyrosine, and cGMP than controls (C57BL/6 mice). Hypoxia increased XO and nitrotyrosine and decreased cGMP content in the lungs of all mice. After hypoxia, vascular congestion was increased in lungs with a greater content of XO and nitrotyrosine. Under normoxia, the association of heat shock protein 90 (HSP90) with endothelial nitric oxide synthase (eNOS) in lungs of SCD and heterozygote SCD mice was decreased compared with the levels of association in lungs of controls. Hypoxia further decreased association of HSP90 with eNOS in lungs of SCD and heterozygote SCD mice, but not in the control lungs. Pretreatment of rat pulmonary microvascular endothelial cells in vitro with xanthine/XO decreased A-23187-stimulated nitrite + nitrate production and HSP90 interactions with eNOS. These data support the hypotheses that hypoxia increases XO release from ischemic tissues and that the local increase in XO-induced oxidative stress can then inhibit HSP90 interactions with eNOS, decreasing *NO generation and predisposing the lung to vaso-occlusion.

Increased erythrocyte adhesion in mice and humans with hereditary spherocytosis and hereditary elliptocytosis

Mice with disruptions of the red blood cell (RBC) cytoskeleton provide severe hemolytic anemia models in which to study multiorgan thrombosis and infarction. The incidence of cerebral infarction ranges from 70% to 100% in mice with alpha-spectrin deficiency. To determine whether mutant RBCs abnormally bind adhesive vascular components, we measured adhesion of mouse and human RBCs to immobilized human thrombospondin (TSP) and laminin (LM) under controlled flow conditions. Mutant RBCs had at least 10-fold higher adhesion to TSP compared with normal RBCs (P <.006). Mutant relative to unaffected RBC adhesion to LM was significantly (P <.01) increased as well. Treatment of RBCs with the anionic polysaccharide dextran sulfate inhibited mutant RBC adhesion to TSP (P <.001). Treatment of RBCs with antibodies to CD47 or the CD47-binding TSP peptide 4N1K did not inhibit TSP adhesion of RBCs. Previously, we have shown that infarcts in alpha-spectrin-deficient sph/sph mice become histologically evident beginning at 6 weeks of age. TSP adhesion of RBCs from 3- to 4- and 6- to 8-week-old sph/sph mice was significantly higher than RBCs from adult mice (> 12 weeks old; P <.005). While the mechanism of infarction in these mice is unknown, we speculate that changes in RBC adhesive characteristics contribute to this pathology.

Patient-controlled analgesia for sickle cell pain crisis in a pediatric emergency department

OBJECTIVE

To determine whether a protocol to start patient-controlled analgesia (PCA) in the emergency department (ED-PCA) would shorten the length of time between narcotic bolus doses and PCA initiation as compared with standard inpatient initiation of PCA (IP-PCA). Also, to compare patient satisfaction and inpatient length of stay for the 2 groups.

METHODS

To improve care, we developed a protocol to institute ED-PCA after an initial bolus dose of narcotics. This was a nonrandomized pilot study. Patient records were reviewed for location of PCA initiation, time from narcotic bolus to initiation of PCA, and length of stay. A brief patient/parent satisfaction survey was collected.

RESULTS

Sixty-nine records were reviewed. Patients treated using the protocol had initiation of PCA therapy within 35 +/- 7 minutes from the last bolus narcotic dose in the emergency department versus 211 +/- 17 minutes for nonprotocol patients. Forty-eight of 50 patient surveys indicated preference for starting ED-PCA; 2 did not have a preference. No complications were identified in either group.

CONCLUSIONS

A protocol to initiate PCA for sickle cell patients in a pediatric emergency department shortened the time of its initiation and was preferred by patients.

Effects of 5-aza-2'-deoxycytidine on fetal hemoglobin levels, red cell adhesion, and hematopoietic differentiation in patients with sickle cell disease

Fetal hemoglobin (HbF) decreases polymerization of sickle hemoglobin (HbS) and improves outcomes in sickle cell disease (SSD). Therefore, a therapeutic goal in SSD is pharmacologic reactivation of HbF. Silencing of the gamma-globin (HbF) gene is associated with DNA methylation. The cytosine analog 5-aza-2'-deoxycytidine (decitabine) hypomethylates DNA by inhibiting DNA methyltransferase. We examined if subcutaneous decitabine could increase HbF levels and improve SSD pathophysiology without cytotoxicity. Eight symptomatic SSD patients resistant or intolerant of standard treatment with hydroxyurea received decitabine 0.2 mg/kg subcutaneously 1 to 3 times per week in 2 cycles of 6-week duration. Treatment decreased neutrophils and increased mean HbF (6.5% to 20.4%, P <.0001) and mean total hemoglobin (76 to 96 g/L [7.6 to 9.6 g/dL], P <.001). Features of vaso-occlusive crisis pathophysiology such as red cell adhesion, endothelial damage, and coagulation pathway activity significantly improved. gamma-Globin gene promoter methylation decreased, and platelets and the proportion of megakaryocytes and erythroid cells in the marrow increased without a decrease in marrow cellularity, consistent with a DNA hypomethylating, noncytotoxic mechanism of action. Weekly subcutaneous decitabine produces cumulative increases in HbF and total hemoglobin through a noncytotoxic mechanism of action. Chronic dosing and sustained increases in hemoglobin F and total hemoglobin levels may be possible. Further studies in SSD and thalassemia are indicated.

PECAM-1 negatively regulates GPIb/V/IX signaling in murine platelets

Platelet adhesion at sites of vascular injury is mediated, in part, by interaction of the platelet plasma membrane glycoprotein (GP) Ib/V/IX complex with von Willebrand Factor (VWF) presented on collagen-exposed surfaces. Recent studies indicate that GPIb/V/IX may be functionally coupled with the Fc receptor gamma (FcR gamma)-chain, which, by virtue of its cytoplasmic immunoreceptor tyrosine-based activation motif, sends activation signals into the cell. Platelet endothelial cell adhesion molecule-1 (PECAM-1) is an inhibitory receptor that has previously been shown to negatively regulate platelet responses to collagen, which transduces activation signals via the GPVI/FcR gamma-chain complex. To determine whether PECAM-1 might similarly regulate signals emanating from GPIb/FcR gamma, we compared activation and aggregation responses to VWF of PECAM-1-positive and PECAM-1-deficient murine platelets. PECAM-1 and the FcR gamma-chain became rapidly tyrosine phosphorylated in platelets following botrocetin-induced VWF binding, but FcR gamma-chain tyrosine phosphorylation was delayed in PECAM-1-positive, versus PECAM-1-deficient, platelets. PECAM-1-deficient platelets were hyperaggregable to VWF, exhibited enhanced spreading and, under conditions of arterial flow, formed markedly larger thrombi on immobilized VWF than did wild-type platelets. Taken together, these data support the notion that engagement of the GPIb complex, in addition to sending activation signals, also initiates a negative feedback loop involving PECAM-1 that controls the rate and extent of platelet activation.

Single-chain antibody fragments derived from a human synthetic phage-display library bind thrombospondin and inhibit sickle cell adhesion

The enhanced adhesion of sickle red blood cells (RBCs) to the vascular endothelium and subendothelial matrix likely plays a significant role in the pathogenesis of vaso-occlusion in sickle cell disease. Sickle RBCs have enhanced adhesion to the plasma and extracellular matrix protein thrombospondin-1 (TSP) under conditions of flow in vitro. In this study, we sought to develop antibodies that bind TSP from a highly diverse library of human single-chain Fv fragments (scFvs) displayed on filamentous phage. Following 3 rounds of phage selection of increasing stringency 6 unique scFvs that bound purified TSP by enzyme-linked immunosorbent assay were isolated. Using an in vitro flow adhesion assay, 3 of the 6 isolated scFvs inhibited the adhesion of sickle RBCs to immobilized TSP by more than 40% compared with control scFvs (P <.001). Furthermore, scFv TSP-A10 partially inhibited sickle RBC adhesion to activated endothelial cells (P <.005). Using TSP proteolytic fragments to map the binding site, we showed that 2 of the inhibitory scFvs bound an epitope in the calcium-binding domain or proximal cell-binding domain of TSP, providing evidence for the role of these domains in the adhesion of sickle RBCs to TSP. In summary, we have isolated a panel of scFvs that specifically bind to TSP and differentially inhibit sickle RBC adhesion to surface-bound TSP under flow conditions. These scFvs will be useful reagents for investigating the role of the calcium and cell-binding domains of TSP in sickle RBC adhesion.

Delayed hemolytic transfusion reaction/hyperhemolysis syndrome in children with sickle cell disease

OBJECTIVE

Alloimmunization in patients with sickle cell disease (SCD) has a reported incidence of 5% to 36%. One complication of alloimmunization is delayed hemolytic transfusion reaction/hyperhemolysis (DHTR/H) syndrome, which has a reported incidence of 11%. In patients with SCD, clinical findings in DHTR/H syndrome occur approximately 1 week after the red blood cell (RBC) transfusion and include the onset of increased hemolysis associated with pain and profound anemia. The hemoglobin (Hb) often drops below pretransfusion levels. In many reported adult cases, the direct antiglobulin test (DAT) remains negative and no new alloantibody is detected as the cause for these transfusion reactions. To date, few pediatric cases have been reported with this phenomenon. The objective of this study was to describe the clinical and laboratory findings of a case series in children who had SCD and experienced a DHTR/H syndrome at our institution.

METHODS

An 11-year retrospective chart review of patients with discharge diagnosis of SCD and transfusion reaction was performed. DHTR/H syndrome was defined as the abrupt onset of signs and symptoms of accelerated hemolysis evidenced by an unexplained fall in Hb, elevated lactic dehydrogenase, elevated bilirubin above baseline, and hemoglobinuria, all occurring between 4 and 10 days after an RBC transfusion. Patient characteristics, time from transfusion, symptoms, reported DAT, new autoantibody or alloantibody formation, laboratory abnormalities, and complications were recorded. Patients with acute transfusion reactions were excluded.

RESULTS

We encountered 7 patients who developed 9 episodes of DHTR/H syndrome occurring 6 to 10 days after RBC transfusion. Each presented with fever and hemoglobinuria. All but 1 patient experienced pain initially ascribed to vaso-occlusive crisis. The DAT was positive in only 2 of the 9 episodes. The presenting Hb was lower than pretransfusion levels in 8 of the 9 events. Severe complications were observed after the onset of DHTR/H: acute chest syndrome, n = 3; pancreatitis, n = 1; congestive heart failure, n = 1; and acute renal failure, n = 1.

CONCLUSIONS

DHTR/H syndrome occurs in pediatric SCD patients, typically 1 week posttransfusion, and presents with back, leg, or abdominal pain; fever; and hemoglobinuria that may mimic pain crisis. Hb is often lower than it was at the time of original transfusion, suggesting the hemolysis of the patient's own RBCs in addition to hemolysis of the transfused RBCs; a negative DAT and reticulocytopenia are often present. Severe complications including acute chest syndrome, congestive heart failure, pancreatitis, and acute renal failure were associated with DHTR/H syndrome in our patients. DHTR/H in the pediatric sickle cell population is a serious and potentially life-threatening complication of RBC transfusion. It is important to avoid additional transfusions in these patients, if possible, because these may exacerbate the hemolysis and worsen the degree of anemia. DHTR/H syndrome must be included in the differential of a patient who has SCD and vaso-occlusive crisis who has recently had a transfusion.

Altered erythrocyte endothelial adherence and membrane phospholipid asymmetry in hereditary hydrocytosis

The risk for thrombosis is increased in patients with hereditary hydrocytosis, an uncommon variant of hereditary stomatocytosis. Erythrocytes from 2 patients with hydrocytosis were studied to gain insight into the mechanism of thrombosis in this disorder. Erythrocytes demonstrated abnormal osmotic scan ektacytometry and decreased erythrocyte filtration rates. There was also a mild increase in adherence of erythrocytes to endothelial monolayers in a micropipette assay. Adhesion of erythrocytes to the subendothelial matrix proteins thrombospondin and laminin, however, was not significantly increased. Percentages of hydrocytosis erythrocytes and reticulocytes with phosphatidylserine exposed on the outer surfaces were increased in both patients compared with healthy controls, indicating altered membrane phospholipid asymmetry. Increased phosphatidylserine exposure accelerating thrombin-forming processes has been proposed as a mechanism for thrombosis in sickle cell disease and beta-thalassemia and may play a similar role in hereditary hydrocytosis.

L-4F, an apolipoprotein A-1 mimetic, dramatically improves vasodilation in hypercholesterolemia and sickle cell disease

BACKGROUND

Hypercholesterolemia and sickle cell disease (SCD) impair endothelium-dependent vasodilation by dissimilar mechanisms. Hypercholesterolemia impairs vasodilation by a low-density lipoprotein (LDL)-dependent mechanism. SCD has been characterized as a chronic state of inflammation in which xanthine oxidase (XO) from ischemic tissues increases vascular superoxide anion (O2*-) generation. Recent reports indicate that apolipoprotein (apo) A-1 mimetics inhibit atherosclerosis in LDL receptor-null (Ldlr-/-) mice fed Western diets. Here we hypothesize that L-4F, an apoA-1 mimetic, preserves vasodilation in hypercholesterolemia and SCD by decreasing mechanisms that increase O2*- generation.

METHODS AND RESULTS

Arterioles were isolated from hypercholesterolemic Ldlr-/- mice and from SCD mice that were treated with either saline or L-4F (1 mg/kg per day). Vasodilation in response to acetylcholine was determined by videomicroscopy. Effects of L-4F on LDL-induced increases in endothelium-dependent O2*- generation were determined on arterial segments via the hydroethidine assay and on stimulated endothelial cell cultures via superoxide dismutase-inhibitable ferricytochrome c reduction. Effects of L-4F on XO bound to pulmonary arterioles and content in livers of SCD mice were determined by immunofluorescence. Hypercholesterolemia impaired vasodilation in Ldlr-/- mice, which L-4F dramatically improved. L-4F inhibited LDL-induced increases in O2*- in arterial segments and in stimulated cultures. SCD impaired vasodilation, increased XO bound to pulmonary endothelium, and decreased liver XO content. L-4F dramatically improved vasodilation, decreased XO bound to pulmonary endothelium, and increased liver XO content compared with levels in untreated SCD mice.

CONCLUSIONS

These data show that L-4F protects endothelium-dependent vasodilation in hypercholesterolemia and SCD. Our findings suggest that L-4F restores vascular endothelial function in diverse models of disease and may be applicable to treating a variety of vascular diseases.

Clinical and hematologic benefits of partial splenectomy for congenital hemolytic anemias in children

OBJECTIVE

To assess the role of partial splenectomy for symptomatic children with various congenital hemolytic anemias.

SUMMARY BACKGROUND DATA

The use of total splenectomy for symptomatic children with congenital hemolytic anemias is restricted by concern of postsplenectomy sepsis. A partial splenectomy is an alternative procedure, although its utility remains incompletely defined.

METHODS

This longitudinal cohort study followed 25 symptomatic children with various congenital anemias who underwent partial splenectomy. Sixteen children had hereditary spherocytosis (HS), and nine children had other erythrocyte disorders. Outcome measures were clinical and laboratory hemolysis, splenic phagocytic and immune function, and splenic regrowth as measured by ultrasonography. Discrete parameters were compared using the Student test.

RESULTS

Partial splenectomy was successful in all 25 children, with minimal morbidity. Follow-up ranged from 7 months to 6 years (mean 2.3 +/- 1.5 years). Following surgery, children with HS had increased hemoglobin values, decreased reticulocyte and bilirubin levels, and preserved splenic function. Most children without HS had decreased symptoms of hypersplenism and splenic sequestration. Over time, variable rates of splenic regrowth were noted, although regrowth did not necessarily correlate with recurrent hemolysis.

CONCLUSIONS

In children with hereditary spherocytosis, a partial splenectomy appears to control hemolysis while retaining splenic function. In children with other congenital hemolytic anemias, a partial splenectomy appears to control symptoms of hypersplenism and splenic sequestration.

Bleeding disorders: A common cause of menorrhagia in adolescents

OBJECTIVE

To determine the frequency of underlying bleeding disorders in adolescents with menorrhagia.

STUDY DESIGN

We retrospectively reviewed the charts of all girls, aged 10 to 19 years, who presented to our children's hospital for inpatient or outpatient evaluation of menorrhagia between January 1990 and November 1998.

RESULTS

At presentation, 9 of the 71 girls (13%) had thrombocytopenia (platelet count <150,000/microL; range, 5000-106,000/microL). The most common causes for thrombocytopenia were immune thrombocytopenic purpura (n = 5) and myelosuppression caused by chemotherapy (n = 2). Of 14 girls who underwent a more detailed hemostatic evaluation, 8 were given a diagnosis of a hereditary coagulation disorder: 6 had platelet function defects and 2 had type 1 von Willebrand disease. Excessive menstrual bleeding commonly results in anemia. One half of the total group had anemia (hemoglobin <12.0 g/dL). Seven girls (10%) had potentially life-threatening anemia (hemoglobin <5.0 g/dL).

CONCLUSIONS

Acquired and congenital bleeding disorders are common causes of menorrhagia in adolescent girls. Severe anemia is a frequent complication of menorrhagia. We recommend that adolescents without thrombocytopenia who present with menorrhagia receive a comprehensive hemostatic evaluation, including testing for von Willebrand disease and platelet function defects.

Hydroxyurea therapy decreases the in vitro adhesion of sickle erythrocytes to thrombospondin and laminin

The adhesion of sickle erythrocytes to the vascular endothelium and subendothelial matrix probably contributes to the pathogenesis of vaso-occlusive disease. The chemotherapeutic agent hydroxyurea (HU) decreases the frequency of vaso-occlusive crises in patients with sickle cell disease. However, the exact mechanism(s) of HU's effect on vaso-occlusive crises is not fully understood. The goal of this study was to determine the effect of HU therapy on the adhesion of sickle erythrocytes to the subendothelial matrix proteins thrombospondin (TSP) and laminin under conditions of flow in vitro. Erythrocytes from patients with severe sickle cell disease on HU therapy (n = 14) had significantly less adhesion to TSP (687 +/- 92 erythrocytes/mm2, mean +/- SE) than untreated patients with severe disease (n = 18, 1176 +/- 117 erythrocytes/mm2, P = 0.003). In addition, there was significantly less adhesion of erythrocytes to immobilized laminin in patients treated with HU (1695 +/- 293 erythrocytes/mm2) than in the untreated patients (2590 +/- 296 erythrocytes/mm2, P = 0.02). Erythrocytes from an additional nine patients with severe sickle cell disease were studied both before and after initiation of HU therapy. Erythrocytes from these patients became less adhesive to both TSP (P = 0.001) and laminin (P = 0.01), a change that was sustained in most patients throughout the duration of the study (2 months to > 12 months). This study suggests that HU modulates the adhesive phenotype of sickle erythrocytes, an effect that may be in addition to, or independent of, other known effects of HU, such as an increase in fetal haemoglobin level.

Low-molecular-weight heparin in thrombotic disease in children and adolescents

PURPOSE

The use of unfractionated heparin (UFH) in children is problematic. In adults, subcutaneous low-molecular-weight heparin (LMWH) is as effective as UFH in the treatment of thrombosis. Because pediatric data are limited, the authors studied the use of enoxaparin in children.

PATIENTS AND METHODS

Nineteen children (ages 18 days to 19 years; median age, 40 months) with indications for thrombosis treatment or prophylaxis were studied. Six patients (median age, 33 months), treated on a protocol that included pharmacokinetic studies, initially received enoxaparin 1 mg/kg subcutaneously every 12 hours; doses then were adjusted until target plasma levels of 0.5 to 1.2 anti-Xa U/mL were achieved. The records of 13 additional patients treated with enoxaparin off study were reviewed.

RESULTS

In the first six patients, enoxaparin pharmacokinetics was found to be similar to that in adults; once targeted levels were achieved, these remained stable. Among all 19 subjects, 14 had treatment of active thrombosis and 5 underwent thrombosis prophylaxis. For treatment of thrombosis, enoxaparin 1 mg/kg initially was administered subcutaneously every 12 hours. Target anti-Xa levels were achieved with 0.55 to 1.5 mg/kg every 12 hours (mean, 0.98 mg/kg; median, 1.0 mg/kg) in 1 to 7 days (median, 1 day). All patients in the treatment group had clinical improvement within 2 to 5 days, and 12 had follow-up radiological studies that confirmed this. For prophylaxis, enoxaparin was given at 1 mg/kg subcutaneously every 24 hours. No new thrombi were clinically evident in this group. There was no major bleeding with enoxaparin; one patient had transient mild mucosal oozing.

CONCLUSION

In this limited population, enoxaparin seems to be a safe, effective, and convenient alternative to UFH in children and adolescents. The adult therapeutic target range of 0.5 to 1.2 anti-Xa U/mL is readily achievable with a starting dose of 1 mg/kg every 12 hours in most children. Initial close monitoring with plasma anti-Xa activity should be done and doses adjusted to achieve target range, particularly in neonates. In the population of this study, enoxaparin seems as effective as UFH in the period immediately thrombotic episode. These results should be confirmed in the ongoing randomized trial comparing LMWH with UFH in children.

The carboxy-terminal cell-binding domain of thrombospondin is essential for sickle red blood cell adhesion

Sickle red blood cells (SS-RBCs) have enhanced adhesion to the plasma and subendothelial matrix protein thrombospondin-1 (TSP) under conditions of flow in vitro. TSP has at least four domains that mediate cell adhesion. The goal of this study was to map the site(s) on TSP that binds SS-RBCs. Purified TSP proteolytic fragments containing either the N-terminal heparin-binding domain, or the type 1, 2, or 3 repeats, failed to sustain SS-RBC adhesion (<10% adhesion). However, a 140-kD thermolysin TSP fragment, containing the carboxy-terminal cell-binding domain in addition to the type 1, 2, and 3 repeats fully supported the adhesion of SS-RBCs (126% +/- 25% adhesion). Two cell-binding domain adhesive peptides, 4N1K (KRFYVVMWKK) and 7N3 (FIRVVMYEGKK), failed to either inhibit or support SS-RBC adhesion to TSP. In addition, monoclonal antibody C6. 7, which blocks platelet and melanoma cell adhesion to the cell-binding domain, did not inhibit SS-RBC adhesion to TSP. These data suggest that a novel adhesive site within the cell binding domain of TSP promotes the adhesion of sickle RBCs to TSP. Furthermore, soluble TSP did not bind SS-RBCs as detected by flow cytometry, nor inhibit SS-RBC adhesion to immobilized TSP under conditions of flow, indicating that the adhesive site on TSP that recognizes SS-RBCs is exposed only after TSP binds to a matrix. We conclude that the intact carboxy-terminal cell-binding domain of TSP is essential for the adhesion of sickle RBCs under flow conditions. This study also provides evidence for a unique adhesive site within the cell-binding domain that is exposed after TSP binds to a matrix.

Potential therapeutic approaches for the treatment of vaso-occlusion in sickle cell disease

Vaso-occlusion is the major cause of morbidity and mortality in sickle cell disease. The pathogenesis of vaso-occlusion likely involves many complex steps related to both the primary event of deoxyhemoglobin S polymerization and the many resultant pathologic changes in both the sickle erythrocyte and the vascular endothelium. Several therapeutic approaches have been designed to reduce the extent of deoxyhemoglobin S polymerization. Although direct chemical inhibition of deoxyhemoglobin S polymerization has proven difficult, strategies to increase levels of fetal hemoglobin or reverse sickle erythrocyte dehydration have been more successful. Although gene therapy is actively being pursued, the ability to cure sickle cell disease is currently limited to bone marrow transplantation with its attendant toxicities and limitations. Because the pathophysiology of vaso-occlusion in sickle cell disease is complex, its treatment will likely be optimized using a multifactorial therapeutic approach.

Type 2M von Willebrand disease: F606I and I662F mutations in the glycoprotein Ib binding domain selectively impair ristocetin- but not botrocetin-mediated binding of von Willebrand factor to platelets

von Willebrand disease (vWD) is a common, autosomally inherited, bleeding disorder caused by quantitative and/or qualitative deficiency of von Willebrand factor (vWF). We describe two families with a variant form of vWD where affected members of both families have borderline or low vWF antigen levels, normal vWF multimer patterns, disproportionately low ristocetin cofactor activity, and significant bleeding symptoms. Whereas ristocetin-induced binding of plasma vWF from affected members of both families to fixed platelets was reduced, botrocetin-induced platelet binding was normal. The sequencing of genomic DNA identified unique missense mutations in each family in the vWF exon 28. In Family A, a missense mutation at nucleotide 4105T --> A resulted in a Phe606Ile amino acid substitution (F606I) and in Family B, a missense mutation at nucleotide 4273A --> T resulted in an Ile662Phe amino acid substitution (I662F). Both mutations are within the large disulfide loop between Cys509 and Cys695 in the A1 domain that mediates vWF interaction with platelet glycoprotein Ib. Expression of recombinant vWF containing either F606I or I662F mutations resulted in mutant recombinant vWF with decreased ristocetin-induced platelet binding, but normal multimer structure, botrocetin-induced platelet binding, collagen binding, and binding to the conformation-sensitive monoclonal antibody, AvW-3. Both mutations are phenotypically distinct from the previously reported variant type 2MMilwaukee-1 because of the presence of normal botrocetin-induced platelet binding, collagen binding, and AvW-3 binding, as well as the greater frequency and intensity of clinical bleeding. When the reported type 2M mutations are mapped on the predicted three-dimensional structure of the A1 loop of vWF, the mutations cluster in one region that is distinct from the region in which the type 2B mutations cluster.

Mechanisms of stroke in sickle cell disease: sickle erythrocytes decrease cerebral blood flow in rats after nitric oxide synthase inhibition

The etiology of stroke in sickle cell disease is unclear, but may involve abnormal red blood cell (RBC) adhesion to the vascular endothelium and altered vasomotor tone regulation. Therefore, we examined both the adhesion of sickle (SS)-RBCs to cerebral microvessels and the effect of SS-RBCs on cerebral blood flow when the nitric oxide (NO) pathway was inhibited. The effect of SS-RBCs was studied in the rat cerebral microcirculation using either a cranial window for direct visualization of infused RBCs or laser Doppler flowmetry (LDF) to measure RBC flow. When fluorescently labeled human RBCs were infused into rats, SS-RBCs had increased adhesion to rat cerebral microvessels compared with control AA-RBCs (P = .01). Next, washed SS-RBCs or AA-RBCs were infused into rats prepared with LDF probes after pretreatment (40 mg/kg intravenously) with the NO synthase inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), or the control isomer, D-NAME. In 9 rats treated with systemic L-NAME and SS-RBCs, 5 of 9 experienced a significant decrease in LDF and died within 30 minutes after the RBC infusion (P = .0012). In contrast, all control groups completed the experiment with stable LDF and hemodynamics. Four rats received a localized superfusion of L-NAME (1 mmol/L) through the cranial window followed by infusion of SS-RBCs. Total cessation of flow in all observed cerebral microvessels occurred in 3 of 4 rats within 15 minutes after infusion of SS-RBCs. We conclude that the NO pathway is critical in maintaining cerebral blood flow in the presence of SS-RBCs in this rat model. In addition, the enhanced adhesion of SS-RBCs to rat brain microvessels may contribute to cerebral vaso-occlusion either directly, by disrupting blood flow, or indirectly, by disturbing the vascular endothelium.

Hydroxyurea therapy in children severely affected with sickle cell disease

HYPOTHESIS

The major clinical manifestations of sickle cell disease (SCD) are hemolytic anemia, predisposition to infection, and recurrent vaso-occlusive episodes resulting in pain, organ dysfunction, or both. There has been no satisfactory treatment for children with recurrent severe painful episodes caused by SCD. Hydroxyurea is an antimetabolite drug shown in adults with SCD to increase fetal hemoglobin levels and reduce the symptoms of SCD. We hypothesized that hydroxyurea therapy in children with severe (defined as > or = 3 vaso-occlusive events per year) SCD could improve hematologic parameters and reduce vaso-occlusive events.

OBJECTIVE

To assess the safety and efficacy of hydroxyurea for the treatment of severe SCD in children.

STUDY DESIGN

After obtaining informed consent, we initiated hydroxyurea therapy at a dosage of 10 to 20 mg/kg per day in 15 patients with severe SCD (hemoglobin SS, hemoglobin SS-alpha thalassemia, or hemoglobin S-beta0-thalassemia). Doses were escalated as tolerated. Patients were monitored with bimonthly physical examinations and monthly laboratory measures. To assess the impact of hydroxyurea on clinical symptoms, we recorded the number of inpatient days for each patient during the period of treatment and compared it with the number of inpatient days for the 12- to 24-month period before institution of hydroxyurea therapy, using the subject as his or her own control subject.

RESULTS

Thirteen patients received hydroxyurea for a median of 24 months (range, 6 to 39 months). The mean dose of hydroxyurea was 21.4 +/- 5.2 mg/kg. Treatment with hydroxyurea induced statistically significant increases in the total hemoglobin concentration, mean corpuscular volume, and percentage of hemoglobin F, and a decrease in the serum concentration of bilirubin. Toxic effects included three episodes of a reversible myelotoxic reaction, two of which required transfusion of packed erythrocytes. For the entire group, hospitalization decreased from a prehydroxyurea median of 3.9 +/- 2.0 days per month to 1.1 +/- 2.1 days per month of therapy (p = 0.09). For those subjects (n = 10) completing at least 1 year of treatment, the decrease in hospitalization from 4.1 +/- 2.2 to 1.0 +/- 1.7 days per month was significant (p = 0.03).

CONCLUSIONS

In this pilot trial, hydroxyurea treatment of severe SCD in children was associated with improved hematologic parameters, acceptable toxic effects, and a trend to reduced hospitalization. Hydroxyurea appears to be a safe and potentially effective agent for the treatment of severe SCD in children. A prospective, controlled trial to investigate the efficacy of hydroxyurea in children is therefore warranted.