Sickle Cell Disease

Question 1

Hb changes in SCD

Answer 1

• Mutation in B chain leading to Glu-> Val substitution at 6 AA position (HbS) or Glu-> Lys (HbC)
• HbS polymerization is promoted by low O2 tension, low pH, dehydration, increased temperature (latter two based on their affects on the O2 curve- shift to the right)
• HbS polymerization is inhibited by HbF

Question 2

RBC sickling

Answer 2

• Due to polymerization of HbS: in deoxy form the hydrophobic Val are exposed and will aggregate w/ Phe residues of nearby Hb chains causing polymerization of the chains and membrane rigidity (polymers bind to band 3 on membrane)
• This causes membrane damage due to fragmentation and generation of ROS
• Membrane damage leads to exposure of cryptic protein sequences which adhere to vascular endothelium
• Phosphatidyl serine is misplaced to outer membrane and causes pro-thrombotic effect

Question 3

Loss of ion balance in sickled cells

Answer 3

• Losing membrane means losing Na/K pumps (cell cannot regenerate lost proteins)
• Leads to loss of K from cell (and thus loss of H20-> dehydration)
• Sickled cells are not deformable and unable to pass through narrowed vessels
• They are in part responsible for increasing blood viscosity and decreasing blood flow
• SCD patients have 2x the venous viscosity (@ high Hct), thus having anemia (low Hct) partially protects the vasculature

Question 4

Vascular occlusion

Answer 4

• Causes the main clinical symptoms
• Less dense RBCs adhere to venous endothelium, producing partial obstruction to flow
• Dense cells lodge behind the partial obstruction and occlude the vessel
• Hypoxia induced sickling and obstruction spreads retrograde
• This chronic process leads to ischemia, infarction and necrosis
• Manifests as pain, organ damage/failure

Question 5

Hemolysis

Answer 5

• Associated w/ pulmonary hypertension (due to expression of endothelin1 and lack of NO), stroke, leg ulcers, priapism (constitutively erect penis)
• Endothelin1 (ET1) is a potent vasoconstrictor of pulmonary capillary beds, and levels are increased in hypoxia (which occurs due to hemolysis)
• ET1 levels always elevated in SCD, rise sharply before acute inflammatory conditions (acute chest syndrome)
• Increased ET1 and decreased NO together result in vasoconstriction, increasing capillary transit time, endothelial expression of VCAM1 (adhesion protein)
• This increases RBC adherence and enhancing microvascular obstruction

Question 6

Nitric Oxide (NO) 1

Answer 6

• Potent vasodilator generated from Arginine, can be inactivated by free Hb in blood stream (reduced by Hb to NO3-) or arginase
• NO reduces adhesive actions of blood cells and endothelium and decreases expression of VCAM on endothelium
• NO is used for pulmonary vasodilation via inhalation (systemic use not effective due to inactivation by Hb->metHb)
• NO can produce ROS

Question 7

Nitric Oxide (NO) 2

Answer 7

• Arginine levels are low in SCD patients, and drop during acute chest syndrome (ACS)
• NO3- increased in ACS, suggesting depletion of NO
• Lysed RBCs liberate Arginase which destroys arginine and renders endothelium unable to produce more NO
• Reduced NO levels means it cannot prevent platelet activation and aggregation, and also cannot prevent expression of VCAM/ICAM, P/E-selectin on endothelial surfaces
• Taken together this means that low NO leads to vasoconstriction and easily clotable blood, exacerbating the already static flow

Question 8

Common types of SCD

Answer 8

• Sickle cell anemia (homozygous HbS, AKA HbSS)
• Sickle-C disease (double heterzoygote)
• S B+ thalassemia (double heterozygote)
• S Bo thalassemia (double heterozygote)

Question 9

Manifestations of sickling

Answer 9

• Chronic hemolytic anemia
• Intermittent acute events (crises)
• Increased susceptibility to bacterial infection (due to asplenia or abnormal complement activation)
• Specific organ syndromes due to infarction/necrosis

Question 10

Acute pain episodes

Answer 10

• Most often due to marrow ischemia (poorly localized)
• Only a minority of patients are severely affected and required hospitalization (skewed distribution w/ each genotype)
• Associated w/ decreased survival
• Management is relief w/ proper analgesics

Question 11

Acute chest syndrome

Answer 11

• Acute pulmonary infiltrate w/ cough and fever, difficult to distinguish infarction from infection
• A major cause of morbidity and mortality in SCD
• Can be due to infection (usually mycoplasma, chlamydia, viral), or infarction
• Worsening hypoxia causes vicious cycle of increasing Hb polymerization, sickling, and thus increasing lung infarction and pain
• Can lead to adult respiratory distress syndrome (ARDS) and fibrosis

Question 12

Fat embolism syndrome

Answer 12

• BM necrosis can release marrow elements into the circulation causing acute inflammation rxn against the marrow fat
• This leads to onset of fever, tachypnea, severe hypoxia and altered mental status

Question 13

Organ damage syndromes 1

Answer 13

• Proliferative sickle retinopathy: earlier onset in sickle-C disease. Progresses through 5 stages and results in retinal detachment and blindness
• Auto-splenectomy: due to ischemic necrosis, leads to susceptibility of infection from encapsulate organisms
• Cholelithiasis (bili stones): due to increased bili levels from hemolysis
• Avascular necrosis of bone (osteonecrosis): Vasoocclusion of BM sinuses leads to ischemia/necrosis. Necrosed BM is easily infected leading to ostomyelitis

Question 14

Organ damage syndromes 2

Answer 14

• Renal manifestations: glomerular hypertrophy, increased renal blood flow and GFR, papillary necrosis, hyposthenuria (can’t conserve H2O), renal insufficiency, loss of EPO
• HbSS mean survival is mid-40s, HbSC is late 60s. Stroke is leading cause of death in children, renal failure and chronic organ damage is leading cause of death in adults

Question 15

Rx of SCD

Answer 15

• Supportive care: hydration, pain management, infection Rx, organ complications
• Transfusions for acute (anemia and hypoxia) and chronic (suppress Hb S and stroke prevention) complications (don’t forget risks- Fe overload, transfusion reactions, allo-immunizations, viral transmission)
• Hydroxyurea increases HbF production, in turn decreasing HbS polymerization, obstruction, and membrane damage/hemolysis
• Hydroxyurea used only to alleviate crises
• Other agents to increase HbF expression: 5-azadeoxycytidine, butyrate
• BM transplantation and gene therapy