Altered vascular reactivity in sickle hemoglobinopathy. A possible protective factor from hypertension

No Increase in Masked Hypertension Prevalence in Children With Sickle Cell Disease in France

BACKGROUND

An important prevalence (32%-45%) of masked hypertension has been reported in children with sickle cell disease (SCD). Stroke screening is well established using transcranial Doppler (TCD) ultrasound. The objectives of our proof-of-concept study in childhood SCD were to evaluate the prevalence of hypertension and its relationships with cerebral vasculopathy (TCD velocity) and to further evaluate in a subgroup of children the correlations of cardiovascular autonomic nervous system indices with TCD velocity.

METHODS

Ambulatory blood pressure measurement (ABPM) and TCD velocity were obtained in children with SCD and in a restricted sample, cardiac sympathovagal balance using heart rate variability analyses, baroreflex sensitivity, and pulse wave velocity were measured.

RESULTS

In 41 children with SCD (median age 14.0 years, 19 girls, SS/Sβ + thalassemia/SC: 33/2/6), ABPM results showed masked hypertension in 2/41 (5%, 95% confidence interval, 0-11) children, consistent with the prevalence in the general pediatric population, elevated blood pressure (BP) in 4/41 (10%) children, and a lack of a normal nocturnal dip in 19/41 children (46%). Children with increased TCD velocity had lower nocturnal dipping of systolic BP. In the 10 participants with extensive cardiovascular assessment, increased TCD velocity was associated with parasympathetic withdrawal and baroreflex failure. Exaggerated orthostatic pressor response or orthostatic hypertension was observed in 7/10 children that was linked to parasympathetic withdrawal.

CONCLUSIONS

Autonomic nervous system dysfunction, namely loss of parasympathetic modulation, of SCD contributes to increase TCD velocity but is not associated with an increased prevalence of masked hypertension.

CLINICAL TRIALS REGISTRATION

NCT04911049.

Cystatin C-derived estimated glomerular filtration rate in children with sickle cell anaemia

BACKGROUND

Sickle cell disease is the most common inherited blood disorder in humans and constitutes a major public health burden. It is a multisystemic condition with long-term renal complications. Early detection of sickle cell nephropathy and initiation of appropriate interventions are associated with improved survival and quality of life. This study aimed to compare the cystatin C-derived estimated glomerular filtration rate (GFR) of the study groups and also, to correlate the clinical features of chronic kidney disease (CKD) with decreased GFR in children with sickle cell anaemia (SCA).

METHODS

This hospital-based cross-sectional analytic study recruited 86 SCA subjects in steady-state and 86 age and sex-matched healthy HbAA controls aged 1-14 years who attended the Paediatric Haematology and Outpatient clinics of Federal Medical Centre Bida over six months. Data were collected using a semi-structured questionnaire, and participants' length/height, weight, and blood pressure were measured using standard procedures. Blood samples were drawn for serum cystatin C assay via the sandwich enzyme-linked immunosorbent assay (ELISA) technique. Filler's equation was used to calculate the glomerular filtration rate.

RESULTS

There was a significant difference in the mean cystatin C-derived GFR between the two groups, i.e. 116 ± 30mL/min/1.73m2 vs. 106 ± 24mL/min/1.73m2 for the SCA and control groups, respectively (p = 0.017). The prevalence of supernormal GFR (i.e. GFR > 140mL/min/1.73m2) and decreased GFR (i.e. GFR < 90mL/min/1.73m2) was 19.8% and 22.1%, respectively, in children with SCA. There was no significant association between the age at diagnosis of SCA, blood transfusions, blood pressure, packed cell volume and presence of peripheral oedema with decreased GFR in the study subjects.

CONCLUSIONS

Supernormal GFR is common in children with SCA and there is no significant association between clinical features of CKD with decreased GFR. Regular evaluation of renal function is, however, recommended in children with SCA for early detection and treatment of renal complications in order to halt the progression to end-stage kidney disease (ESKD).

Clinical characteristics and risk factors of relative systemic hypertension and hypertension among sickle cell patients in Cameroon

Increased blood pressure (BP) has been associated with higher risk of stroke and mortality in Sickle Cell Disease (SCD). We investigated risk factors associated with Relative Systemic Hypertension (RSH) or systemic hypertension in SCD patients in Cameroon. Using R, Multivariate multinomial logistic regression modeling was used to examine the effects of the demographic, anthropometric, clinical, and laboratory factors to determine risk factors. A total of 815 individuals with SCD, including 380 (46.6%) males were analyzed. At baseline, the median age [interquartile range] was 18.0 [12.0-25.0] years, ranging from 3 to 66 years. Approximately three-quarters of the patients (n = 645; 79.1%) had normal BP, 151 (18.5%) had RSH and 19 (2.3%) had hypertension. Age (P < 0.001) and gender (P = 0.022) were significantly different across the BP categories. Weight (P < 0.001), height (P < 0.001), BMI (P < 0.001), pulse pressure (P = 0.020), history of stroke (P = 0.012), hemoglobin level (P = 0.002), red blood cell count (P = 0.031), creatinine (P < 0.001), and (estimated glomerular filtration rate) eGFR (P = 0.002) was also significantly different across the three BP categories. After adjustment, the significantly associated factors of RSH in the SCD patients were age [OR = 1.03, (95% CI = 1.01-1.06), P < 0.010], male gender [OR = 1.54, (95% CI = 1.04-2.27), P = 0.029], BMI [OR = 1.10, (95% CI = 1.04-1.17), P = 0.001]. After adjustment, the independent variables significantly associated factors of Hypertension in the SCD patients were age [OR = 1.05, (95% CI = 1.01-1.10), P = 0.034], male gender [OR = 3.31, (95% CI = 1.04-10.52), P = 0.042], BMI [OR = 1.14, (95% CI = 1.01-1.29), P = 0.027]. Creatinine was significantly associated with RSH [OR =1.31 (1.05-1.63), P = 0.016]. SCD patients with RSH or hypertension maybe at increased risk of renal dysfunction. We found relatively high prevalence of RSH and hypertension (20.8%) in SCD patients in Cameroon. Tailored Interventions that consider major risk factors (age, gender, and BMI) may lower BP pressure and prevent severe complications.

Reduced blood pressure in sickle cell disease is associated with decreased angiotensin converting enzyme (ACE) activity and is not modulated by ACE inhibition

Background

Sickle cell disease (SCD) incurs vaso-occlusive episodes and organ damage, including nephropathy. Despite displaying characteristics of vascular dysfunction, SCD patients tend to present relatively lower systemic blood pressure (BP), via an unknown mechanism. We investigated associations between BP and renin-angiotensin-system (RAS) components in SCD and determined whether an inhibitor of angiotensin converting enzyme (ACE; often used to slow SCD glomerulopathy) further modulates BP and RAS components in a murine model of SCD.

Methods

BP was compared in human subjects and mice with/without SCD. Plasma angiotensin II, ACE and renin were measured by immunoassay. BP was reevaluated after treating mice with enalapril (25 mg/kg, 5x/week) for 5 weeks; plasma and organs were stored for angiotensin II and ACE activity measurement, and quantitative real-time PCR.

Results

Diastolic BP and systolic BP were significantly lower in patients and mice with SCD, respectively, compared to controls. Reduced BP was associated with increased plasma renin and markers of kidney damage (mice) in SCD, as well as significantly decreased plasma ACE concentrations and ACE enzyme activity. As expected, enalapril administration lowered BP, plasma angiotensin II and organ ACE activity in control mice. In contrast, enalapril did not further reduce BP or organ ACE activity in SCD mice; however, plasma angiotensin II and renin levels were found to be significantly higher in enalapril-treated SCD mice than those of treated control mice.

Conclusion

Relative hypotension was confirmed in a murine model of SCD, in association with decreased ACE concentrations in both human and murine disease. Given that ACE inhibition has an accepted role in decreasing BP, further studies should investigate mechanisms by which ACE depletion, via both Ang II-dependent and alternative pathways, could contribute to reduce BP in SCD and understand how ACE inhibition confers Ang II-independent benefits on kidney function in SCD.

Cardiovascular changes in children with sickle cell crisis

BACKGROUND

Sickle cell anaemia is characterised by frequent, sometimes serious events referred to as "crisis". Cardiopulmonary consequences such as pulmonary hypertension and myocardial ischaemia may accompany a serious crisis.

OBJECTIVE

To determine the cardiovascular changes that occur during a severe sickle cell crisis.

METHODS

A cross-sectional comparative study of sickle cell anaemia in children (5-17 years) admitted during a severe crisis (cases) and those in steady state (controls) was conducted over a 2-year period. Effects of the crisis on the cardiopulmonary system were assessed. The diagnosis of myocardial ischaemia was made using electrocardiography and serological cardiac biomarkers, while cardiac dysfunction and the presence of pulmonary hypertension were determined using echocardiography. The presence of systemic hypertension and tachycardia was also evaluated.

RESULTS

A total of 176 patients were recruited, 92 in steady state (male:female ratio, 1.2:1) and 84 in severe crisis (male:female ratio, 1.3:1). The mean age was 10.4 ± 3.2 years for steady state and 10.5 ± 3.4 years for those in crisis. The mean heart rate in crisis was higher than in steady state (p < 0.0001). The blood pressures (systolic, p < 0.0001, diastolic, p < 0.0001, mean, p < 0.0001) as well as myocardial ischaemia scores (p < 0.0001) were higher in patients with crisis than in those in steady state. Similarly, conduction abnormalities, pulmonary hypertension, and ventricular dysfunction were more prevalent in the crisis than in the steady state.

CONCLUSION

The present data suggest that sickle cell crisis results in a derangement of clinical, electrocardiographical, and echocardiographical parameters in children with sickle cell anaemia. Further research on these cardiovascular events may improve the overall care of these patients.

Posterior reversible encephalopathy syndrome secondary to asymptomatic poststreptococcal glomerulonephritis in a child with sickle cell anemia: a case report

BACKGROUND

Posterior reversible encephalopathy syndrome is a neurotoxic condition that occurs as a result of the failure of posterior circulatory autoregulation in response to acute changes in blood pressure. Overperfusion with resultant disruption of the blood-brain barrier results in vasogenic edema, but not infarction. Posterior reversible encephalopathy syndrome can be the presenting feature of postinfectious glomerulonephritis, which has been reported in approximately 5% of hospitalized children, and it has been reported in very few cases of adult patients with sickle cell anemia. We report a very rare case of posterior reversible encephalopathy syndrome that occurred in a child with sickle cell anemia. This presentation should be differentiated from other neurologic manifestations that occur in patients with sickle cell anemia, because management is totally different.

CASE PRESENTATION

We report what is to our knowledge the first reported case of a 9-year-old Saudi girl with sickle cell anemia who developed posterior reversible encephalopathy syndrome secondary to asymptomatic poststreptococcal glomerulonephritis. This occurred after full recovery from acute chest syndrome and severe vaso-occlusive crisis.

CONCLUSIONS

The purpose of this report is to emphasize that all efforts should be made to explore the causes of different neurologic manifestations that occur in patients with sickle cell anemia, because this will require different pathways of management.

Prevalence of relative systemic hypertension in adults with sickle cell disease in Ghana

Individuals with sickle cell disease particularly with the homozygous (SS) genotype historically have relatively low blood pressure. Nonetheless, they develop vasculopathy-associated organ dysfunction and the risk of organ dysfunction increases at blood pressures that are normal in the general population. This phenomenon is termed relative systemic hypertension (RSH) with a systolic blood pressure range of 120–139 mmHg, and diastolic blood pressure range of 70–89 mmHg. The significance of RSH lies in its association with renal insufficiency, pulmonary hypertension, stroke and propensity to progress to systemic hypertension. We conducted a retrospective chart review of 1,000 adults with sickle cell disease at the Ghana Institute of Clinical Genetics, to determine the prevalence of RSH in sickle cell disease in Ghana and associated complications. We found a high prevalence of RSH and hypertension with a relatively low frequency of renal insufficiency. Pulse pressure, a predictor of mortality, was higher in males of all ages. We anticipate that providing an estimate of the burden of RSH will heighten its recognition and clinical management among health care providers.

[Kidney and hemoglobinopathy]

Sickle-cell disease (SCD), one of the most common severe monogenic disorders into the world, is associated with an increased frequency of chronic kidney disease. SCD is caused by a point mutation in the gene encoding β globin gene which leads to the formation of hemoglobin S that polymerises after deoxygenation. HbS polymerisation is associated with erythrocyte rigidity and vaso-occlusive episodes that play a central role into SCD pathogenesis. The spectrum of renal diseases during SCD is broad and includes various renal manifestations which become more apparent with increasing age. Underlying pathophysiological processes involved in sickle cell nephropathy are multifactorial but endothelial dysfunction related to chronic hemolysis is a key factor contributing to renal involvement. Our review focuses on the pathogenesis and on the spectrum of renal manifestations occurring in SCD patients.

Abnormalities in aortic properties: a potential link between left ventricular diastolic function and ventricular-aortic coupling in sickle cell disease

Left ventricular (LV) diastolic dysfunction in patients with sickle cell disease (SCD) is associated with increased mortality. However, its mechanisms are not well known, preventing the development of effective therapies. We hypothesized that patients with SCD have altered aortic properties despite normal blood pressure, which may contribute towards the development of diastolic dysfunction. We studied 31 stable adult patients with SCD (32 ± 7 years) and 12 healthy controls of similar age (29 ± 10 years) who underwent echocardiography and cardiovascular magnetic resonance (CMR) imaging on the same day. Echocardiographic measurements of mitral inflow and mitral annulus velocities were used to evaluate LV diastolic function. CMR imaging included standard LV function evaluation and myocardial tissue characterization as well as velocity-encoded images of the ascending aorta to measure aortic diastolic cross-sectional area, distensibility, as well as peaks and volumes of the global, forward and backward blood flow rate. Compared to controls, SCD patients had increased aortic diastolic area, global stroke volume, and both forward and backward flow, while aortic distensibility and peripheral blood pressure were similar. Furthermore, peak backward flow rate and volume were able to discriminate between patients with and without diastolic dysfunction. Our findings show that some aortic properties are altered in SCD patients and may be associated with diastolic dysfunction despite normal systolic blood pressure. If confirmed in larger studies, these aortic changes could be a novel therapeutic target to prevent or delay the development of LV diastolic dysfunction in SCD and thus potentially improve outcomes in these patients.

Arterial Stiffness and Peripheral and Central Blood Pressure in Patients With Sickle Cell Disease

Blood pressure (BP) in patients with sickle cell disease (SCD) has been reported to be lower than in persons in the general population. Data on arterial stiffness, which is an important risk factor for the progression of BP, are inconclusive for this patient population. Forty-five adult patients with SCD and 40 controls matched for sex, age, and body mass index were studied. Brachial systolic BP (SBP) and diastolic BP (DBP) were significantly lower in the patient group (SBP 115.1±13.8 mm Hg vs 121.9±11.3 mm Hg and DBP 68.5±8.0 mm Hg vs 80.6±9.1 mm Hg, P<.05, respectively). Augmentation index (AIx), however, was significantly higher in SCD patients compared with healthy controls (24.9±9.6 for patients vs 12.4±10.8 for controls, P<.001), while carotid femoral pulse wave velocity was comparable between the two groups. The study shows that mechanisms other than arterial elasticity are involved in the low BP phenotype of patients with SCD.

Genome-wide meta-analysis of systolic blood pressure in children with sickle cell disease

In pediatric sickle cell disease (SCD) patients, it has been reported that higher systolic blood pressure (SBP) is associated with increased risk of a silent cerebral infarction (SCI). SCI is a major cause of neurologic morbidity in children with SCD, and blood pressure is a potential modulator of clinical manifestations of SCD; however, the risk factors underlying these complications are not well characterized. The aim of this study was to identify genetic variants that influence SBP in an African American population in the setting of SCD, and explore the use of SBP as an endo-phenotype for SCI. We conducted a genome-wide meta-analysis for SBP using two SCD cohorts, as well as a candidate screen based on published SBP loci. A total of 1,617 patients were analyzed, and while no SNP reached genome-wide significance (P-value<5.0 x 10(-8)), a number of suggestive candidate loci were identified. The most significant SNP, rs7952106 (P-value=8.57 x 10(-7)), was in the DRD2 locus on chromosome 11. In a gene-based association analysis, MIR4301 (micro-RNA4301), which resides in an intron of DRD2, was the most significant gene (P-value=5.2 x 10(-5)). Examining 27 of the previously reported SBP associated SNPs, 4 SNPs were nominally significant. A genetic risk score was constructed to assess the aggregated genetic effect of the published SBP variants, demonstrating a significant association (P=0.05). In addition, we also assessed whether these variants are associated with SCI, validating the use of SBP as an endo-phenotype for SCI. Three SNPs were nominally associated, and only rs2357790 (5' CACNB2) was significant for both SBP and SCI. None of these SNPs retained significance after Bonferroni correction. Taken together, our results suggest the importance of DRD2 genetic variation in the modulation of SBP, and extend the aggregated importance of previously reported SNPs in the modulation of SBP in an African American cohort, more specifically in children with SCD.

Pressão arterial em crianças portadoras de doença falciforme

OBJETIVO: Avaliar os valores da pressão arterial (PA) em crianças portadoras de doença falciforme (DF). MÉTODOS: Estudo observacional unicêntrico descritivo de 70 crianças portadoras de DF. Os valores da PA obtidos foram classificados conforme as V Diretrizes Brasileiras de Hipertensão Arterial. Os pacientes foram distribuídos segundo o genótipo em grupo HbSS e HbSC e segundo a faixa etária: grupo I (três anos a quatro anos e 11 meses), grupo II (cinco anos a oito anos e 11 meses) e grupo III (nove anos a 13 anos e 11 meses). Na análise estatística, aplicou-se o teste t de Student e a ANOVA, sendo significante p<0,05. RESULTADOS: A média e o desvio padrão (DP) das medidas da PA sistólica (PAS) (mmHg) foram 95,9±11,45 e da PA diastólica (PAD) 62,6±7,78. As médias da PA por faixa etária foram: grupo I, PAS 91,2±5,78 e PAD 61,5±7,15; grupo II, PAS 97,3±10,86 e PAD 64,4±7,89; e grupo III, PAS 100,0±9,88 e PAD 61,5±4,94. Observou-se que 5,7% dos pacientes apresentavam hipertensão arterial (HA) e 8,6% eram pré-hipertensos. A média dos valores da PAS e PAD entre os pacientes HbSS e HbSC não diferiu. CONCLUSÕES: Novos estudos devem ser realizados para avaliar PA em pacientes com DF e detectar as possíveis causas de HA nesses pacientes.

Dynamic cerebral autoregulation in homozygous Sickle cell disease

BACKGROUND AND PURPOSE

Sickle cell disease (SCD) is associated with cerebral hyperperfusion and an increased risk of stroke. Also, both recurrent microvascular obstruction and chronic hemolysis affect endothelial function, potentially interfering with systemic and cerebral blood flow control. We addressed the question whether cerebrovascular control in patients with SCD is affected and related to hemolysis.

METHODS

Systemic and cerebrovascular control were studied in 18 patients with SCD and 10 healthy subjects. Dynamic cerebral autoregulation was evaluated by transfer function analysis assessing the relationship between mean cerebral blood flow velocity and mean arterial pressure.

RESULTS

Normal baroreflex sensitivity and postural cardiovascular reflex responses indicated integrity of systemic cardiovascular control. In the low- (0.07 to 0.15 Hz) frequency region, mean arterial pressure variability was comparable for both groups, but a larger mean cerebral blood flow velocity variability in SCD (6.1 [4.6 to 7.0] versus 4.2 [2.6 to 5.2] [cm x s(-1)](2) x Hz(-1); P<0.05) indicated a reduced capacity to buffer the transfer of blood pressure surges to the cerebral tissue. Impairment of dynamic cerebrovascular control was confirmed by a reduced mean arterial pressure-to-mean cerebral blood flow velocity transfer function phase lead in SCD versus healthy subjects (32+/-17 degrees versus 50+/-19 degrees , P<0.05) that was unrelated to the severity of hemolysis.

CONCLUSIONS

In patients with SCD, dynamic cerebral autoregulation is impaired but appears unrelated to hemolysis.

Arterial Blood Pressure and Hyperviscosity in Sickle Cell Disease

Abnormal rheologic behavior of sickle cells is the result of increased viscosity of the blood caused by the polymerization of hemoglobin S and the resultant production of dense, dehydrated sickle erythrocytes. As the viscosity of sickle cells increases, there is a negative impact on blood flow, which contributes to the vascular occlusion process, the hallmark of the sickling disorders. Blood flow is directly proportional to the blood pressure and inversely proportional to the blood viscosity. Blood flow has important implications for the diagnosis and management of hypertension in sickle cell patients and for transfusion therapy for the acute and chronic complications of this disease.

Effect of fetal hemoglobin on microvascular regulation in sickle transgenic-knockout mice

In sickle cell disease, intravascular sickling and attendant flow abnormalities underlie the chronic inflammation and vascular endothelial abnormalities. However, the relationship between sickling and vascular tone is not well understood. We hypothesized that sickling-induced vaso-occlusive events and attendant oxidative stress will affect microvascular regulatory mechanisms. In the present studies, we have examined whether microvascular abnormalities expressed in sickle transgenic-knockout Berkeley (BERK) mice (which express exclusively human alpha- and beta(S)-globins with <1% gamma-globin levels) are amenable to correction with increased levels of antisickling fetal hemoglobin (HbF). In BERK mice, sickling, increased oxidative stress, and hemolytic anemia are accompanied by vasodilation, compensatory increases in eNOS and COX-2, and attenuated vascular responses to NO-mediated vasoactive stimuli and norepinephrine. The hypotension and vasodilation (required for adequate oxygen delivery in the face of chronic anemia) are mediated by non-NO vasodilators (i.e., prostacyclin) as evidenced by induction of COX-2. In BERK mice, the resistance to NO-mediated vasodilators is associated with increased oxidative stress and hemolytic rate, and in BERK + gamma mice (expressing 20% HbF), an improved response to these stimuli is associated with reduced oxidative stress and hemolytic rate. Furthermore, BERK + gamma mice show normalization of vessel diameters, and eNOS and COX-2 expression. These results demonstrate a strong relationship between sickling and microvascular function in sickle cell disease.

Arterial stiffness and wave reflections in patients with sickle cell disease

We tested the hypothesis that lower blood pressure and increased vasodilatation reported in sickle cell disease (SCD) patients with hemoglobin SS genotype (SS) are translated by lower arterial stiffness determined by pulse wave velocity (PWV) and wave reflections assessed by augmentation index (AI). We enrolled 20 SS (8 females; 12 male) patients closely matched for age, gender, height, and body mass index to 20 subjects with hemoglobin AA genotype (AA). Carotid-femoral PWV (PWV(CF)) and carotid-radial PWV (PWV(CR)) were recorded with the Complior device. Aortic AI was derived from pressure wave analysis (SphygmocoR). PWV(CF) and PWV(CR) were lower in SS than in AA (4.5+/-0.7 m/s versus 6.9+/-0.9 m/s, P<0.0001 and 6.6+/-1.2 m/s versus 9.5+/-1.4 m/s, P<0.0001, respectively). AI was lower in SS than in AA (2+/-14% versus 11+/-8%, P=0.02). Multivariate analysis revealed that both PWV(CF) and PWV(CR) were negatively associated with hemoglobin SS type and positively related to mean arterial pressure (MAP), whereas AI was positively associated with MAP and total cholesterol (all P<0.0001). Multivariate analysis restricted to SS indicated a positive association between PWV(CF) and PWV(CR) with age but a negative association with MAP (R2=0.57 and 0.51, respectively, both P<0.001), whereas MAP and heart rate were independently associated with AI (R2=0.65, P<0.001). This study provides the first evidence that SCD is associated with both lower arterial stiffness and wave reflections. SS patients have a paradoxical negative association between PWV and MAP, suggesting that low MAP does not protect them against arterial stiffness impairment.

Brain injury in children with sickle cell disease: Prevalence and etiology

Our objective was to evaluate the relationship between brain injury by magnetic resonance imaging (MRI) and vasculopathy by magnetic resonance angiography (MRA) in children with hemoglobin SS, the most serious form of sickle cell disease. We reviewed imaging for all 146 SS patients imaged at St. Jude Children's Research Hospital since 1993. Standard MRI criteria were used to identify cystic infarction, leukoencephalopathy, encephalomalacia, or atrophy. Standard MRA criteria were used to identify arterial tortuousity (limited vasculopathy), and stenosis or occlusion (extensive vasculopathy). At an average age of 10 years, the estimated prevalence of infarction, ischemic damage, or atrophy in SS patients was 46%, and of vasculopathy was 64%. Only 28% of patients were normal by both modalities, and patients abnormal by MRA often were abnormal by MRI (p < 0.00001). Patients with cystic infarction had limited vasculopathy, whereas patients with encephalomalacia had stenosis or occlusion (p < 0.0001). Large arteries were affected in 31% of brain injury patients, whereas small arteries are inferred to be abnormal in up to 69% of patients with brain injury. The degree of vasculopathy is closely related to the degree of brain injury, implying that vasculopathy is prodromal to most forms of brain injury in hemoglobin SS.

Oxygen radical inhibition of nitric oxide-dependent vascular function in sickle cell disease

Plasma xanthine oxidase (XO) activity was defined as a source of enhanced vascular superoxide (O(2)( *-)) and hydrogen peroxide (H(2)O(2)) production in both sickle cell disease (SCD) patients and knockout-transgenic SCD mice. There was a significant increase in the plasma XO activity of SCD patients that was similarly reflected in the SCD mouse model. Western blot and enzymatic analysis of liver tissue from SCD mice revealed decreased XO content. Hematoxylin and eosin staining of liver tissue of knockout-transgenic SCD mice indicated extensive hepatocellular injury that was accompanied by increased plasma content of the liver enzyme alanine aminotransferase. Immunocytochemical and enzymatic analysis of XO in thoracic aorta and liver tissue of SCD mice showed increased vessel wall and decreased liver XO, with XO concentrated on and in vascular luminal cells. Steady-state rates of vascular O(2)( *-) production, as indicated by coelenterazine chemiluminescence, were significantly increased, and nitric oxide (( *)NO)-dependent vasorelaxation of aortic ring segments was severely impaired in SCD mice, implying oxidative inactivation of ( *)NO. Pretreatment of aortic vessels with the superoxide dismutase mimetic manganese 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin markedly decreased O(2)( small middle dot-) levels and significantly restored acetylcholine-dependent relaxation, whereas catalase had no effect. These data reveal that episodes of intrahepatic hypoxia-reoxygenation associated with SCD can induce the release of XO into the circulation from the liver. This circulating XO can then bind avidly to vessel luminal cells and impair vascular function by creating an oxidative milieu and catalytically consuming (*)NO via O(2)( small middle dot-)-dependent mechanisms.

Reactive species in sickle cell disease

The red cell is a relatively abundant locus of both free radical generation and reaction. Erythrocytes have a high content of unsaturated membrane lipids, a rich oxygen supply and are densely packed with redox-active hemoglobin residues. In response, red cells have a highly evolved and well-integrated network of oxidant defense mechanisms that lend an ability to withstand oxidative stress. In the case of congenital hemoglobin mutations that underlie sickle cell disease, they become very susceptible to free radical-mediated injury by virtue of enhanced endogenous rates of production of reactive species and impairment of tissue free radical defense mechanisms. In sickle cell disease, a combination of these susceptibility factors are hypothesized to lead to an overall impairment of vascular function, in large part due to loss of "bioactive" nitric oxide via the free radical-mediated consumption of this vasoactive molecule.

Impaired nitric oxide-mediated vasodilation in transgenic sickle mouse

Transgenic sickle mice expressing human beta(S)- and beta(S-Antilles)-globins show intravascular sickling, red blood cell adhesion, and attenuated arteriolar constriction in response to oxygen. We hypothesize that these abnormalities and the likely endothelial damage, also reported in sickle cell anemia, alter nitric oxide (NO)-mediated microvascular responses and hemodynamics in this mouse model. Transgenic mice showed a lower mean arterial pressure (MAP) compared with control groups (90 +/- 7 vs. 113 +/- 8 mmHg, P < 0.00001), accompanied by increased endothelial nitric oxide synthase (eNOS) expression. N(G)-nitro-L-arginine methyl ester (L-NAME), a nonselective inhibitor of NOS, caused an approximately 30% increase in MAP and approximately 40% decrease in the diameters of cremaster muscle arterioles (branching orders: A2 and A3) in both control and transgenic mice, confirming NOS activity; these changes were reversible after L-arginine administration. Aminoguanidine, an inhibitor of inducible NOS, had no effect. Transgenic mice showed a decreased (P < 0.02-0.01) arteriolar dilation in response to NO-mediated vasodilators, i.e., ACh and sodium nitroprusside (SNP). Indomethacin did not alter the responses to ACh and SNP. Forskolin, a cAMP-activating agent, caused a comparable dilation of A2 and A3 vessels ( approximately 44 and 70%) in both groups of mice. Thus in transgenic mice, an increased eNOS/NO activity results in lower blood pressure and diminished arteriolar responses to NO-mediated vasodilators. Although the increased NOS/NO activity may compensate for flow abnormalities, it may also cause pathophysiological alterations in vascular tone.

Sickle Cell Anemia as a Possible State of Enhanced Anti-Apoptotic Tone: Survival Effect of Vascular Endothelial Growth Factor on Circulating and Unanchored Endothelial Cells

The biologic processes of apoptosis and angiogenesis are linked in endothelial biology because some endothelial cell growth factors also exert anti-apoptotic effects. We studied whether apoptosis is occurring in circulating endothelial cells (CEC) that have lost the survival signals derived from anchorage to extracellular matrix. Consistent with this expectation, 64% ± 16% of CEC from normal donors showed evidence of apoptosis (by morphology and TdT-mediated dUTP nick end labeling [TUNEL] assay). However, only 30% ± 15% (P &lt; .001 v normal) of CEC from donors with sickle cell anemia were apoptotic. Vascular endothelial growth factor (VEGF) levels were significantly (P = .001) higher in plasma of sickle donors (120.1 ± 81.4 pg/mL) than that of normal donors (37.6 ± 34.6 pg/mL), and there was an inverse correlation between VEGF and CEC apoptosis (r = .612,P = .001). Consistent with stimulation by VEGF, CEC from sickle donors exhibited increased expression of vβ3. In vitro experiments showed that VEGF inhibits apoptosis for cultured endothelial cells that are kept unanchored and not allowed to re-establish attachment to extracellular matrix, thus demonstrating that VEGF provides survival signals independent of its ability to promote matrix reattachment. These data suggest the hypothesis that sickle cell anemia is a state of enhanced anti-apoptotic tone for endothelial cells. If true, this has implications for disease pathobiology, particularly the development of neovascularizing retinopathy.

Sickle Cell Anemia as a Possible State of Enhanced Anti-Apoptotic Tone: Survival Effect of Vascular Endothelial Growth Factor on Circulating and Unanchored Endothelial Cells

The biologic processes of apoptosis and angiogenesis are linked in endothelial biology because some endothelial cell growth factors also exert anti-apoptotic effects. We studied whether apoptosis is occurring in circulating endothelial cells (CEC) that have lost the survival signals derived from anchorage to extracellular matrix. Consistent with this expectation, 64% ± 16% of CEC from normal donors showed evidence of apoptosis (by morphology and TdT-mediated dUTP nick end labeling [TUNEL] assay). However, only 30% ± 15% (P < .001 v normal) of CEC from donors with sickle cell anemia were apoptotic. Vascular endothelial growth factor (VEGF) levels were significantly (P = .001) higher in plasma of sickle donors (120.1 ± 81.4 pg/mL) than that of normal donors (37.6 ± 34.6 pg/mL), and there was an inverse correlation between VEGF and CEC apoptosis (r = .612,P = .001). Consistent with stimulation by VEGF, CEC from sickle donors exhibited increased expression of vβ3. In vitro experiments showed that VEGF inhibits apoptosis for cultured endothelial cells that are kept unanchored and not allowed to re-establish attachment to extracellular matrix, thus demonstrating that VEGF provides survival signals independent of its ability to promote matrix reattachment. These data suggest the hypothesis that sickle cell anemia is a state of enhanced anti-apoptotic tone for endothelial cells. If true, this has implications for disease pathobiology, particularly the development of neovascularizing retinopathy.

Platelet activation in patients with sickle cell disease

Vascular occlusion and vasculopathy underlie much of the morbidity in patients with sickle cell anaemia. Platelets may play a role in this vasculopathy. Samples from 43 patients with sickle cell disease (SCD) were examined for evidence of platelet activation using fluorescent-labelled monoclonal antibodies and flow cytometry. There was increased expression of activation-dependent antigens on the platelets from patients with SCD compared to those from both Caucasian and African-American controls. In addition, SCD patients had increased levels of platelet microparticles. Platelets are activated in patients with sickle cell disease. The contribution of platelet activation to sickle cell pathophysiology is under active investigation in our laboratories.

Natural history of blood pressure in sickle cell disease: risks for stroke and death associated with relative hypertension in sickle cell anemia

PURPOSE

Blood pressure in individuals who have sickle cell disease has been reported to be lower than published normal values. We determine whether and to what degree this is true, using data obtained as part of a large natural history study.

PATIENTS AND METHODS

Blood pressure was measured annually for 3,317 subjects with sickle cell disease who were 2 years old or older. Values obtained were compared with those reported by the National Health and Nutrition Examination Survey I and II (NHANES I and II). They were further analyzed with respect to age, sex, height, weight, hematologic diagnosis, blood urea nitrogen and creatinine, stroke, and death.

RESULTS

Blood pressure was significantly lower in subjects with sickle cell anemia than published norms for age, race, and sex, a difference that increased with age. It correlated with body mass index, hemoglobin, measures of renal function and age, but the strength of the correlation varied among age and sex subgroups. The risk for occlusive stroke increased with systolic but not diastolic pressure. Mortality was related to elevated blood pressure in males (P < 0.05) and to a lesser extent in females (P = 0.10). In subjects with hemoglobin SC disease, blood pressure also deviated from normal but to a lesser degree.

CONCLUSION

Blood pressure is generally lower than normal in individuals with sickle cell anemia. Those with high values relative to this population had an increased risk of stroke and death. Blood pressure should be monitored but values obtained must be assessed relative to the lower values expected for patients with this disease. Those with blood pressure values above 140/90 mm Hg should be evaluated and considered for treatment.

Sickle erythrocytes, after sickling, regulate the expression of the endothelin-1 gene and protein in human endothelial cells in culture

The molecular defect in sickle cell disease resides in the beta globin gene, with consequent defects in erythrocytes only, suggesting that the vascular occlusion and vasomotor instability which characterize this disease are the result of interactions between abnormal sickle erythrocytes and cells of the blood vessel wall. We explored whether sickle erythrocytes may have effects on vascular tone, exclusive of adhesion events. Exposure of human endothelial cells in culture to previously sickled sickle erythrocytes resulted in a four to eight-fold transcriptional induction of the gene encoding the potent vasoconstrictor endothelin-1 (ET-1). Unsickled sickle erythrocytes or normal erythrocytes exposed to "sickling" conditions had no effect on ET-1 gene induction. Contact of the sickled erythrocytes with the endothelium was not required. Elevations in the ET-1 transcript peaked at 3 h after exposure and persisted for up to 24 h. Four to sixfold increases in the amount of ET-1 peptide was released into the medium surrounding the endothelial cells after exposure to sickled sickle erythrocytes. This is the first demonstration of the regulation of gene expression in endothelial cells as a result of interaction with sickle cells, with induction of genes encoding vasoconstrictors. Furthermore, these findings suggest that sickle erythrocytes may have the capacity to affect local vasomotor tone directly.

Daily variability in resting levels of cardiovascular variables in normal subjects and those with homozygous sickle cell disease

Measurements were made of cardiovascular variables and oral temperature in 16 male subjects with homozygous sickle cell disease (SS) and in 17 matched controls (AA) at 10.00 a.m., 1.00 p.m. and 4.00 p.m. All subjects were in a rested state throughout. At 10.00 a.m., mean arterial pressure was lower, while heart rate, total forearm blood flow and cutaneous red cell flux in the forearm were higher in SS than AA. Vascular resistance in total forearm and forearm skin, calculated by dividing arterial pressure by blood flow or red cell flux, were lower in SS but hand cutaneous red cell flux and vascular resistance were not significantly different in SS and AA. In both SS and AA, there were parallel increases over the three sessions, in mean arterial pressure (by approximately 12 and 10%, respectively) forearm vascular resistance (by approximately 17 and 27%) and hand cutaneous vascular resistance and hand cutaneous resistance (by approximately 2240 and 350%) whereas forearm blood flow and hand cutaneous red cell flux fell. By contrast, forearm cutaneous resistance showed no change during the day in SS, but increased progressively in AA (by approximately 75%). These results indicate that, during the day, there is progressive vasoconstriction in forearm muscle and hand skin in SS and AA and also in forearm skin of AA that contributes to a progressive rise in the resting level of mean arterial pressure. We suggest this daily variability should be considered in studies of cardiovascular function: within a given study they should be performed at the same time of day.

Is "relative" hypertension a risk factor for vaso-occlusive complications in sickle cell disease?

Supine arterial blood pressure measurements of 89 patients with homozygous sickle cell disease and normal renal function were compared with those of an age-, race-, and sex-matched normal population and with those of individuals who had similar levels of anemia due to beta thalassemia. Consistent with previous reports, sickle cell patients had significantly lower blood pressure than the normal population. However, within most age groups, sickle cell patients tended to have higher than expected blood pressure than individuals with similar or less severe degrees of anemia. Furthermore, the authors have found an association between cerebrovascular accident and elevated blood pressure in men, even in a range of systolic and diastolic pressures that would be considered normal by conventional standards. These results reiterate the intricate relationship that exists between factors governing red cell rheology and microvascular tone. They also raise the possibility that "relative" hypertension may be associated with other vaso-occlusive manifestations of the sickle cell syndromes.