3 Sickle Cell Epidemiology bS : Single a.a. substitutionGlutamic acid ValineMost common single gene disorder in African Americans1/375 affected (homozygous)1/12 are heterozygous carriers (~8%)Also affects other ethnicities: India, Middle East, Hispanic
4 SICKLE CELL SYNDROMESMolecular pathology postulated by Pauling in the late 1940sMutation known for 50 yearsUnfinished tasksExplaining the clinical disorder based on the molecular defectRational and targeted treatments – still few despite detailed knowledge of the molecular defect
5 Molecular Pathophysiology Intracellular [Hb] ~ g/dLDeoxygenation allows interaction of bS subunits via abnormal hydrophobic regions (valines)Non-covalent bond with other bS in the RBCFormation of 14 stranded helical fiber
6 Delay time ≈ k / C15The time that elapses between the deoxygenation of hemoglobin S and the formation of polymer is inversely proportional to the intracellular concentration (C) of deoxyhemoglobin, raised to the 15th power
7 Polymerization phase is sensitive to: O2 concentration Hgb concentrationpHIonic strength(At salt concentrations spanning the physiologic range, solubility increases with ionic strength, but decreases markedly at high ionic strength.)Induction of Red-Cell Sickling by Polymerization of Deoxyhemoglobin S.As red cells traverse the microcirculation, oxygen is released from oxyhemoglobin (red circles), generating deoxyhemoglobin (purple circles). The diagram at the left of the figure shows molecules of hemoglobin S, with the globular 2 S2 tetramer shown as a flat circle. Deoxygenation of hemoglobin S induces a change in conformation in which the subunits move away from each other. The hydrophobic patch at the site of the 6 valine replacement, shown as a projection, can bind to a complementary hydrophobic site on a subunit of another hemoglobin tetramer, shown as an indentation. This interaction is necessary for the formation of polymer, depicted as the interaction of three deoxyhemoglobin S molecules on one strand (dark purple) with three deoxyhemoglobin S molecules on another strand (light purple). At the bottom, a high-resolution model, prepared by Drs. Robert Josephs, S.J. Watowich, and L.J. Gross, shows the interaction of three deoxyhemoglobin S molecules on one strand with three deoxyhemoglobin S molecules on another strand. The subunits are pale yellow-green, and the subunits are gray (lighter in the foreground and darker in the background). The heme groups are shown as red spheres. Also shown are contacts between foreground subunits involving 6 valine (blue) on one strand and the hydrophobic acceptor site (bright green) on the other strand. Only one of the two 6 valine sites in each hemoglobin S tetramer makes this contact.The diagram in the middle shows the assembly of deoxyhemoglobin S into a helical 14-strand fiber, shown as a twisted rope-like structure. The equation shows the time that elapses, or delay time (td), between the deoxygenation of hemoglobin S and the concerted formation of polymer. The delay time is inversely proportional to the intracellular hemoglobin concentration (C), raised to about the 15th power; k denotes an experimental constant. The photograph at the bottom is a high-resolution electron micrograph of a fiber, provided by Dr. Stuart Edelstein.As deoxyhemoglobin S polymerizes and fibers align, the red cell is distorted into an elongated banana or "sickle" shape, as shown in the diagram at the right. The photograph at the bottom is a scanning electron micrograph of a reversibly sickled cell, provided by Dr. James White.Bunn HF. NEJM (11)
9 Cellular Pathophysiology of Sickle Cell syndromes Polymerization leads to:Distortion of Cell shapeDamage to RBC MembraneAbnormal permeability Irreversible sicklingImpairment of RBC flow = InfarctionDecreased red cell number = Anemia
10 Anemia Most common feature of sickle cell disease Often ignored as pathologicModerate to severe in almost all patientsDegree of anemia reflects clinical severityEpisodic acute anemiaAnemia in Sickle Cell Disease per se is not viewed as a complication that requires treatment, but anemia has been associated with some of the major complications of the disease.
11 Sickle complications Vaso-occlusive crisis Cerebrovascular disease Splenic sequestrationSepsis due to functional aspleniaAcute chest syndrome
15 A 3 year old boy presents to the ER with a 12-hour history of fever to 38°C. He is slightly irritable but looks well. Despite IV antiobiotics, his fever continues to rise, his blood pressure falls, and his extremities become cold, with purple discoloration.
17 Functional asplenia Functional Asplenia Liver spleen scans from two children with HbSS. The scan at left shows functioning splenic tissue in the left upper quadrant along with normal hepatic uptake. The scan on the right shows normal hepatic uptake but absence of splenic function resulting from repeated splenic infarction.FunctionalAsplenia
18 Sepsis Prevention: the most effective drug for sickle cell is… Volume 314: June 19, Number 25Prophylaxis with oral penicillin in children with sickle cell anemia. A randomized trialMH Gaston, JI Verter, G Woods, C Pegelow, J Kelleher, G Presbury,H Zarkowsky, E Vichinsky, R Iyer, JS Lobel, and et al.PROPS I Prophylactic Penicillin StudyMulticenter randomized double-blind placebo-controlled trial“Prophylactic therapy with oral penicillin by four months of age decreases the morbidity and mortality associated with pneumococcal septicemia.”
19 Vaccination: Important for adult sickle cell and all splenectomized patients!! “Catch-up” vaccination if Prevnar series not complete
20 A fourteen year old girl with sickle cell disease comes to clinic because her left side is weak. She is immediately transferred to the ICU for exchange transfusion.
23 Sickle Cell Disease: Cross Section of Internal Carotid Artery NormalIntimalhyperplasiaThen there is large vessel disease that accounts for clinical stroke in Sickle Cell Disease, and this is an example of large vessel disease in the internal carotid artery of an 18-year-old. This slide shows the tremendous hyperplasia of the intima in this carotid, and this can grow to the point of actually occluding this very large vessel. Of course, this would then result in a stroke.
24 Age at 1st stoke in sickle syndromes Age at first CVA. Data from the 3,647 patients used to calculate incidence rates were used to determine CVA-free survival curves. The estimated age at first CVA was significantly different for SS and SC patients (P < .001; Fig 1). The chances of having a first CVA by 20 years of age, 30 years of age, and 45 years of age were estimated at 11%, 15%, and 24%, respectively, for SS patients and 2%, 4%, and 10%, respectively, for those with SC.
25 Stroke PreventionMost clinical strokes occur in children with increased cerebral blood vessel flow velocitiesFlow measured by transcranial DopplerultrasoundSTOP Trial - red cell transfusions reduce the risk of stroke in children with TCD > 200 cm/secThis diagram is from an article by Dr. Adams and his colleagues in 1990 and shows the placement of the transducer for TCD flow velocity measurements. The internal carotid, the middle cerebral, and the anterior cerebral arteries can be examined using TCD ultrasonography.Dr. Adams and his colleagues have reported the utility of TCD in predicting stroke risk for children with sickle cell anemia. Early studies showed that children with sickle cell anemia have higher baseline TCD flow velocities than normal age-matched children, thought to be related, at least in part, to their low hematocrit. In 315 children without stroke, 17.5% (55/315) had TCD flow velocity in the middle cerebral artery over 170 cm/sec and 7.9% (25/315) had TCD velocity of 200 cm/sec or greater. During 64 months of follow-up 17 patients had a stroke. Twelve of the strokes occurred in patients with TCD velocity of at least 170 cm/sec and 10 in patients with TCD velocity greater than or equal to 200 cm/sec, hence the designation conditional and abnormal. Based on these data, the STOP trial demonstrated that monthly blood transfusions significantly reduce the risk of primary stroke for children with SCA whose TCD velocity is greater than or equal to 200 cm/sec.
27 Long term treatment of sickle cell disease HydroxyureaHematopoietic stem cell transplantationGene therapy
28 HydroxyureaHydroxyurea decreases crises in patients with severe sickle cell disease.In 299 adults with severe sickle cell anemia:Hydroxyurea provided ~50% decrease in:frequency of hospitalizationincidence of pain, acute chest syndrome, and blood transfusionsIn good responders:hemolysis and leukocyte counts fellhemoglobin concentrations increasedHb F increased from 5 % to 9% overallHb F increased to 18 % in top quartile of respondersN Engl J Med 1995;332:
29 Stem Cell Transplantation Stroke riskAcute chest syndrome riskInexorable accrual ofchronic end organ damage (including CHF, pulmonary hypertension, iron overload)Transplant-related mortalityInfertilitySecondary malignancyGraft vs. host diseaseComplicationsof sickle cellComplicationsof transplantation