Intracorpuscular Defects: The Hemoglobinopathies

Chapter 11

Heme Moiety

• Combination of iron and protoporphyrin ring

• Suspended in the center of polypeptide chains, so iron can exist in ferrous state

Globin Variants

• Chromosome 16: alpha (α) and zeta (ζ)

• Chromosome 11: beta, epsilon, gamma, delta

Sickle Cell Anemia

• Substitution of valine for glutamic acid in the sixth position from the NH2 - terminal end of the β chain

• HbS constitutes 80% of hemoglobin content

• Expressed as α2β2^6Val

• Prominent in african (Sickle Cell Disease is not immune Plasmodium malaria, only trait/carrier)

• When oxygen tension decreases, cells become sickling. Repeated sickling permanently damage the membrane.

• Hallmark features are hemolytic crisis, aplastic crisis particularly to parvoviruses, vaso-occlusion crisis(vessel blockage that are very painful), ischemic tissue injury (tissues & organs damage) including autosplenectomy (nonfunctional), dactylitis (swelling of hands & feet). Stroke occurs up to 22% of patients

• Screening use high-performance liquid chromatography (HPLC) or isoelectric focusing (IEF), electrophoresis on cellulose acetate & citrate agar

• Treat by stem cell transplantation, research into gene therapy with CRISPR

Susceptible to infections due to

1. MPS blockage by increased hemolysis

2. Blockage in sinusoids of the liver & spleen

3. Splenic dysfunction

4. Deficient opsonic activity

Three Principles in management of painful crisis

1. Adequate rehydration

2. Pain relief using analgesics

3. Antibiotic to treat underlying illness such as infection

Vasculopathy

• vaso-occlusion, and tissue ischemia can happen anywhere in the body.

• Acute chest syndrome is the second most common cause of hospital admission and complication of surgery & anesthesia

Hyposthenuria

• Inability of the kidney to concentrate urine (low specific gravity)

• Earliest nephropathy in sickle cell disease

• Hyperuricemia (high uric acid) and hematuria

Sickle Cell Trait

• HbA to HbS ratio is 60:40, expressed as α2β1β1^6Val

• Usually asymptomatic, but occasionally episodes of hematuria and hyposthenuria occur

Sickle Disease Morphology

• Sickled cells, target cells, fragmented red cells, polychromasia, nucleated red cells, siderotic granules

• Increased reticulocytes, except during an aplastic crisis

• Leukocytosis with left-shift, thrombocytosis

Electrophoresis

• Hemoglobin disease has no HbA, 80% HbS & ~20% HbF

• Carriers have 60% HbA, 40% HbS

• HbD and HbG both migrate to the same position as HbS at alkaline pH

• HbE and HbO_Arab migrate to the same position as HbC at alkaline pH

Hemoglobin C Disease

• Lysine replaces glutamic acid in the 6th position

• Electrophoresis shows 95% HbC & HbA₂, 7% HbF

• Morphology includes bar crystal, microspherocytes, target cells, schistocytes

Hemoglobin D Disease & Trait

• Glutamine replaces glutamic acid at position 121 of beta chain

• Most common variant is HbD_punjab also called HbD_{Los Angeles}

• HbD is nonsickling soluble so it’s asymptomatic and peripheral smear is normal except for few target

• HbD comigrates with HbS and HbG at alkaline pH, but comigrates with HbA at acid pH

Hemoglobin E Disease & Trait

• Glutamic acid replaces lysine at position 26 of beta globin

• Occurs commonly in Southeast Asia (Thailand, Cambodia, Laos)

• Alkaline electrophoresis reveals approximately 95% to 97% HbE plus A2 , and the remainder of the hemoglobin is Hb

• HbE comigrates with HbC and HbOArab at alkaline pH but migrates with HbA at acid pH

Hemoglobin O_Arab Disease

• Glutamic acid replaces lysine at position 121 of beta globin

• Common in North Africa, Balkan

• Migrates electrophoretically with HbC, HbE, and HbA2 at alkaline, while comigrates HbA in acidic pH

Unstable Hemoglobins

• Hemoglobin variants in which amino acid substitutions or deletions weaken the binding forces of the molecule structure. This causes denature and precipitate hemoglobin Heinz bodies

• Cannot diagnose with electrophoresis or high-performance liquid chromatography. The best method is gene sequencing, or heat stability test

Methemoglobinemia

• Clinical condition where methemoglobin is greater than 1%

• 3 common causes are hemoglobin M variant, NADH deficiency, toxic substances

• Treat with methylene blue, hyperbaric oxygen therapy, RBC transfusion