Congenital Hemolytic Anemias

Question 1

Describe the important features of the RBC membrane.

Answer 1

• The highly organized RBC membrane has evolved over time to allow the RBC to traverse the microcirculation while keeping its structural integrity (see figure)
• The membrane is composed of a fluid lipid bilayer that makes up almost 50% of the RBC membrane and contains both free cholesterol and phospholipids
• A two-dimensional network of skeletal and transmembrane proteins is also a major part of the membrane
• Maintaining membrane cohesion and mechanical stability within the membrane is essential for the RBC to undergo the necessary shape changes during its long circulatory lifespan

Question 2

What are the most important proteins of the RBC membrane and what happens when these are disrupted?

Answer 2

• The most important skeletal proteins, which interact at the membrane surface only, are spectrin, ankyrin and protein 4.2
• The principal transmembrane protein, which traverses the lipid bilayer, is band 3
• Mutations in various membrane and skeletal proteins that result in either decreased membrane cohesion or mechanical instability lead to:
  
  • membrane surface area loss
  • decreased red cell life span
  • anemia in a variety of inherited red cell membrane disorders
• The most common RBC membrane disorders are:
  
  • hereditary spherocytosis (HS)
  • hereditary elliptocytosis (HE)
  • hereditary pyropoikilocytosis (HPP)
  • Southeast Asian ovalocytosis (SAO)
• Diagnosis is based largely upon clinical features and morphological characteristics on examination of the peripheral blood smear

Question 3

Describe the pathophysiology and presentation of Hereditary spherocytosis? What is the treatment?

Answer 3

• Hereditary spherocytosis is the most common inherited RBC membrane disorder, affecting up to 1 in 3000 individuals of northern European descent
• In over 75% of cases, the disorder is inherited in an autosomal dominant fashion
• Mutations that result in ankyrin deficiency account for the majority of cases in hereditary spherocytosis, followed by spectrin and less commonly, band 3 deficiency
  
  • Deficiencies in these proteins lead to:
    
    • loss of mechanical stability, membrane loss and formation of spheroid, fragile RBCs that are selectively trapped in the spleen and undergo hemolysis
• Clinically, typical hereditary spherocytosis results in:
  
  • variable degrees of anemia
  • jaundice
  • gallstones
  • splenomegaly
• Treatment includes supportive therapy, transfusions as needed and splenectomy

Question 4

What is the pathophysiology and presentation of hereditary elliptocytosis? How is this different from hereditary pyropoikilocytosis?

Answer 4

• Found most commonly in individuals of African or Mediterranean descent
• Hereditary elliptocytosis and its variants are most commonly inherited in autosomal dominant fashion
  
  • Hereditary elliptocytosis is a both a genetically and clinically heterogeneous disorder
  • Most commonly, mutations that lead to a spectrin deficiency result in weakened lateral linkages to the skeletal membrane, a mechanistically unstable membrane and formation of elliptocytes
• Hereditary elliptocytosis ranges in clinical severity from asymptomatic carrier state to severe, life threatening anemia
• Hereditary pyropoikilocytosis is a clinically severe variant resulting from thermal sensitivity of the RBCs

Question 5

What is the pathophysiology and presentation of Southeast Asian ovalocytosis?

Answer 5

• Autosomal dominant disorder found most commonly in malaria endemic areas
• A single gene mutation resulting in band 3 deficiency is the only known cause of Southeast Asian ovalocytosis
  
  • This band 3 mutation leads to marked rigidity of the RBC membrane and the disorder’s morphological RBC features, yet it results in minimal to almost no hemolysis and anemia

Question 6

What are the major RBC metabolic pathways?

Answer 6

• Due to their lack of nuclei and mitochondria, RBCs must rely on 2 major metabolic pathways, the pentose phosphate shunt and glycolysis, to ensure they meet the metabolic requirements for sustaining their intravascular life cycle (see figure)
  
  • The pentose phosphate shunt relies on the breakdown of glucose to generate reducing potential, in the form of NADPH, for protecting the RBC against oxidative stress
    
    • In the oxidative phase of this reaction, the enzyme glucose-6-phosphate dehydrogenase (G6PD) plays a major role in the conversion of glucose-6-phosphate to generate NADPH equivalents
• Mature red blood cells are incapable of producing energy through the normal oxidative pathways (Kreb cycle) due to the absence of nuclei and mitochondria
• Instead, red blood cells depend on glycolysis, which is the anaerobic conversion of glucose by the Embden-Meyerhof pathway, for the generation and storage of high-energy phosphates
  
  • The veolocity of glycolysis depends in part on 3 major rate-limiting enzymes in the pathway:
    
    • hexokinase
    • phosphofructokinase
    • pyruvate kinase

Question 7

What is the underlying pathophysiology of most red blood cell enzyme deficiencies? How is this diagnosed?

Answer 7

• Abnormalities that result in RBC enzyme deficiencies, known as RBC “enzymopathies”, are associated with phenotypic variability
• The lack of characteristic morphological changes in the RBC differentiates this group of disorders from RBC membrane disorders and disorders of hemoglobin
• In general, red blood cell enzyme disorders cause chronic non-spherocytic hemolytic anemia (CNSHA)
  
  • The lack of sufficient energy and other metabolic derangements result in shortened lifespan of the mature RBC
  • The degree of hemolysis is variable and dependent on the relative importance of the affected enzyme and the functional properties of the variant enzyme
• Diagnosis is made upon quantifying specific enzyme activity in the RBC in setting of clinical suspicion

Question 8

Describe the mechanism and presentation of G6PD deficiency. What is the treatment?

Answer 8

• Represents one of the most common hematological disorders worldwide, caused by more than 300 variants in the G6PD gene that encodes the enzyme
  
  • A major role of the enzyme is to help generate NADPH for the reduction of glutathione
  • Reduced glutathione ensures that the RBC is protected from oxidative stress
  • This process is variably impaired depending on the specific genetic mutation, rendering the RBC susceptible to oxidation-induced hemoglobin damage
• The G6PD gene is encoded on the X chromosome, and thus, inheritance is X-linked
  
  • Although males are most severely affected, female carriers may exhibit symptoms in the setting of lyonization
• Most clinically important variants result in acute intermittent hemolysis
• Oxidative stress and triggers (e.g. infections, certain foods and medications) may result in:
  
  • fever
  • GI symptoms
  • jaundice
  • hematuria
  • variable degrees of anemia
• Treatment is supportive but may include transfusion therapy for severe, life-threatening anemia

Question 9

Describe the mechanism and presentation of Pyruvate kinase (PK) deficiency. What is the treatment?

Answer 9

• Most common cause of non-spherocytic hemolytic anemia due to defective glycolysis in the Caucasian population
• PK deficiency results in 2 major metabolic derangements, ATP depletion and an increase in 2,3-diphosphoglycerate (DPG) production
  
  • Importantly, an increase in RBC 2,3-DPG content leads to a rightward shift in the oxygen dissociation curve, thus ameliorating the impact of anemia by facilitation oxygen delivery
• PK deficiency is inherited in an autosomal recessive manner and results in:
  
  • variable anemia
  • jaundice
  • splenomegaly
• As with hereditary spherocytosis and other RBC enzyme deficiencies, a history of hyperbilirubinemia is common in the neonatal period
• Affected adults are at risk for developing gallstones and leg ulcers
• Treatment involves transfusion support as needed as well as splenectomy

Question 10

What are some less common enzyme deficiencies that can cause RBC dysfunction?

Answer 10

• A number of other clinically important RBC disorders result from deficiencies of enzymes in the Embden-Meyerhof pathway
• These result from deficiencies of major pathway enzymes, including:
  
  • hexokinase
  • aldolase
  • glucose-6-phosphate isomerase
  • triosephosphate isomerase
  • phosphoglycerate kinase
• In general, these disorders of glycolysis are inherited in an autosomal recessive manner and result in various degrees of non-spherocytic hemolytic anemia