247. Congenital Hemolytic Anemias, Sickle Cell Disease

Question 1

What are the 4 key proteins in the RBC membrane and what is their role?

What genes are mutated in the following diseases, and what morphology is the result?

• Hereditary Spherocytosis (HS)
• Hereditary Elliptocytosis (HE)
• Hereditary Pyropoikilocytosis (HPP)
• Southeast Asian Ovalocytosis (SAO)

Which is most commonly inherited? What is their inheritance pattern? Tx?

Answer 1

1. Spectrin: self associate into heterotetramers, support bilayer, regulates integral protein mobility
2. Ankyrin: anchors spectrin to Band 3
3. Protein 4.2
4. Band 3: principle transmembrane protein transversing lipid bilayer (anion exchanger - Cl, bicarb)

HS: defect in ANKYRIN&raquo_space; spectrin > band 3, protein 4.2 = surface membrane loss = spherocytes & increased splenic trapping = hemolysis

• most commonly inherited (N Europe)
• AD inheritance
• tx: supportive, transfusions, splenectomy (get trapped in spleen and undergo hemolysis)

HE: defect in SPECTRIN, protein 4.1 = defective spectrin self-association or junctional complex causing membrane instability = elliptocytes

• African or Mediterraneans
• AD inheritance

HPP: same as HE but more membrane instability (various size/shape)
- severe variant due to increased thermal sensitivity of RBCs (AD)

SAO: defect in band 3 = membrane rigidity and decreased deformability

• AD disorder in malaria endemic areas
• minimal to no hemolysis/anemia

Question 2

What are the two pathways of RBC metabolism?

What is the most common RBC enzyme disorder? Function of that enzyme, inheritance, triggers, tx

What is the most common cause of non-spherocytic hemolytic anemia due to defective glycolysis in the Caucasian population? What is the effect of this enzyme deficiency? Inheritance pattern, tx

Answer 2

1. Pentose Phosphate Shunt: breakdown glucose to generate reducing potential (NADPH) to protect against oxidative stress
2. Glycolysis: convert glucose to pyruvate to make ATP (sole energy source)

Enzyme disorders: non-spherocytic hemolytic anemia

G6PD Deficiency

• fx: help generate NADPH to reduce glutathione to protect cell from ox stress
• XLR due to location of G6PD on X Ch
• trigger: ox stress (infections, fava beans, sulfa meds)
• normocytic anemia
• tx: supportive, transfusions if severe

PK Deficiency

• fx: last step of glycolysis to make ATP
• results in ATP depletion and increase in 2,3-DPG (right shift in O2 dissociation to improve O2 offloading to tissue)
• AR inheritance
• Tx: transfusion support, splenectomy

Question 3

When does hemoglobin switching occur?

Answer 3

1. Embryonic to fetal switch: at W10, switch from epsilon-globin (zeta-2, epsilon-2) to gamma-globin (alpha-2, gamma-2)
2. Fetal to adult switch (alpha-2, gamma-2) to (alpha-2, beta-2)

Question 4

Sickle Cell Disease

• cause
• regions with highest prevalence (what is effect)
• pathophysiology (what causes sickling)
• effects of sickle cell
• dx
• tx

Answer 4

Cause: single nucleotide substitution on 6th codon of beta globin gene = replaces glutamic acid with valine (beta-S globin)
Regions: areas endemic for malaria (Africa, India) - protective effect
Balanced Polymorphism: heterozygous state protects against malaria, homozygous state assoc with premature death

Sickling occurs in taut/deoxygenated state, depending on:

• conc of HbS
• Oxygenation status
• conc of non-sickle Hb (HbF, A, A2 have inhibitory effect)
• cation homeostasis and hydration status (activated K efflux channel - GARDOS, and KCl co-transporter promote polymerization activated by sickling or acidosis, causing water efflux from RBC and more sickling)
• acidosis (low pH)

Effects

• hemolysis = anemia (plasma-free Hb is a sink for NO = vasomotor dysregulation, relative VC)
• vaso-occlusion (in post-capillary venules of every end organ - acute chest syndrome, dactylitis, functional asplenia, aplastic crises)
• tx: supportive (oxygen, hydration, pain meds - NSAIDs/opioids), blood transfusions/ABx when needed (protect from encapsulated organisms), HYDROXYUREA (increase HbF), STEM CELL TRANSPLANT IS CURATIVE (need sibling matching)

Emerging tx

• crizalizumab: P-selectin inhibitor = improve RBCs
• Voxelotor: Hb allosteric modifiers
• L-glutamine: antioxidant (less ox stress)