Sickle Cell Disease

Question 1

Describe the structure of hemoglobin.

Answer 1

• Hemoglobin consists of 2 alpha and 2 beta globin polypeptide chains
• A heme group, composed of an iron complex within a protoporphyrin ring (ferroprotoporphyrin IX) is linked covalently at a specific site to each chain (see figure)
• In the reduced or ferrous state, the heme group binds reversibly to gaseous ligands such as oxygen or carbon monoxide

Question 2

How is the hemoglobin gene expressed? What are its components?

Answer 2

• In humans, 2 gene clusters direct the production of hemoglobin during fetal and postnatal development (see figure)
• Alpha locus, which consists of the embryonic z gene and 2 adult a genes
• Beta locus, which consists of the embryonic e gene as well as the Gg, Ag, d and b genes
• Both loci are controlled by major regulatory elements located upstream of the structural genes

Question 3

What is hemoglobin switching and what are the resulting phenotypes?

Answer 3

• Two major globin gene switches occur in development, the embryonic to fetal switch and the fetal to adult switch
• This occurs at 2 important gene clusters that regulate the production of the globin chains that comprise hemoglobin:
  
  • b-globin gene locus on chromosome 11
  • a-globin gene locus on chromosome 16
• Genes are arranged in the order of expression during embryonic, fetal and postnatal development
  
  • After early embryonic development, a globin gene expression predominates on the a locus
  • However, genes at the b locus are expressed differentially throughout early development and the postnatal period and continue to be expressed into adulthood
• Importantly, b globin gene expression does not occur until the postnatal period
• Isoforms that are formed from the expression of genes in the b locus other than the b gene also represent functional hemoglobins. Distribution of hemoglobin in children over 6 months of age and adults follows the usual pattern:
  
  • Hemoglobin A (a2b2): 90 to 97%, adult hemoglobin (HbA)
  • Hemoglobin F (a2g2): ~1%, fetal hemoglobin (HbF)
  • Hemoglobin A2 (a2d2): ~2% (HbA2)

Question 4

What are the most common hemoglobin mutations and what are the associated conditions?

Answer 4

• More than 500 structural hemoglobin variants have been discovered to date through population surveys, most of which are not clinically relevant
  
  • point mutations
  • nucleotide insertions
  • deletions
  • crossovers
• These mutations may affect hemoglobin solubility, synthesis or oxygen affinity, and the resulting hemoglobin disorders are often classified into the following:
  
  • Sickle cell syndromes (clinically most relevant) ® Hb SS, SC, S/b thalassemia
  • Structural mutants resulting in thalassemia phenotype ® Hb E, Lepore, Constant Spring
  • Unstable hemoglobins ® over 100 described, may result in congenital Heinz body hemolytic anemia
  • Hemoglobins with abnormal affinity for oxygen ® may result in low and high affinity oxygen binding

Question 5

What kind of mutation leads to sickle cell disease?

Answer 5

• Sickle cell disease (SCD) is the most clinically important hemoglobinopathy in the US
• A single nucleotide substitution (GTG for GAG) in the 6th codon of the b globin gene results in the replacement of glutamic acid by valine
  
  • The mutant globin chain is referred to as bS globin
• The regions with the highest prevalence of sickle cell trait correspond to areas endemic for malaria, suggesting a protective effect against malaria as a result of the mutation
• The mutation is an example of balanced polymorphism since the heterozygous state has a protective effect against endemic malaria but the homozygous state is associated with premature death from complications related to the disease

Question 6

What is the epidemiology of sickle cell disease?

Answer 6

• The scope of African-Americans affected by SCD in the United States is large
• One in 12 is a carrier for the sickle cell trait
• One in 500 births is affected by SCD, accounting for approximately 75,000 to 80,000 people currently with the disease

Question 7

What is the phenotypic outcome results from the sickle cell hemoglobin mutation?

Answer 7

• Intracellular polymerization of sickle hemoglobin (Hb S) under deoxygenated state is the primary event in the pathogenesis of SCD
• Process is dependent on the following intracellular factors:
  
  • Hb S concentration: correlation exists between gelation and Hb S concentration
  • Oxygenation status: polymerization occurs only during deoxygenated state
  • Concentration of other non-sickle Hb: Hb F, A and A2 have direct inhibitory effect
  • Cation homeostasis and hydration status: activated K+ efflux channel (Gardos) and KCl- co-transporter promote polymerization
  • pH concentration: solubility of deoxygenated Hb S is lowest between 6.0 and 7.2

Question 8

What is the implication of heterozygosity for sickle cell? What are the common genotypes that lead to clinically significant sickling syndromes?

Answer 8

• Heterozygosity for the sickle mutation (sickle cell trait) is clinically insignificant for the most part
• SCD encompasses all genotypes in which at least one b globin gene carries the sickle mutation
• The most common genotypes that result in clinically significant sickling syndromes include homozygous SCD or compound heterozygous states in which the sickle cell mutation is co-inherited with either the HbC mutation or a b thalassemia mutation
  
  • Hb C is the result of another single nucleotide substitution at the 6th codon of the b gene that results in the replacement of lysine for glutamine acid
  • Hb C induces relative intracellular dehydration and precipitates sickling in the presence of Hb S
• Any mutation in the b gene that results in decreased or absent production of b chains and thus, a thalassemia phenotype, may also precipitate sickling in the presence of Hb S
• Although there are exceptions, the following genotypes are listed in their usual order of clinical severity:
  
  • Hb SS disease
  • Hb S/b0 thalassemia
  • Hb SC disease
  • Hb S/b+ thalassemia

Question 9

What is the pathophysiology of sickl cell disease?

Answer 9

• Decreased solubility of Hb S under hypoxic conditions and increased polymerization result in the classic “sickle form”
  
  • This is due to deoxygenated Hb S polymers that form highly ordered fiber aggregates, elongate the cell and distort it
• Sickle red blood cells are prone to hemolysis and have a shortened life span, resulting in anemia
• Because of decreased deformability, sickle cells also cause vaso-occlusion, mostly in the post-capillary venules (log jamming effect).
• Vaso-occlusion due to red blood cell adhesion is the classic pathophysiologic process in SCD but may be affected by the following additional factors:
  
  • Inflammation and abnormal leukocyte-endothelial interactions
  • Platelet activation and trapping
  • Nitric oxide dysregulation and abnormal vasomotor tone
  • Damaged endothelium and coagulation activation
  • Reperfusion injury and oxidative stress

Question 10

How is sickle cell disease diagnosed?

Answer 10

• SCD may be diagnosed at birth by abnormal testing on mandatory newborn state screening
• In older children, anemia and reticulocytosis develops by 4 to 6 months of age
• The clinical manifestations of SCD comprise a spectrum of complications that result from vaso-occlusion and affect virtually every organ
• Pain episodes, which result from bone infarction and may be severe, represent the hallmark of SCD and occur throughout all age groups
• Functional asplenia and infection with encapsulated organisms also remain lifelong risks
• Other early complications include:
  
  • acute chest syndrome
  • splenic sequestration
  • stroke and aplastic crises (due to parvovirus B19 infection)
• Late complications and end organ damage resulting in:
  
  • avascular necrosis
  • retinopathy
  • gallstones
  • renal insufficiency
  • cardiopulmonary disease
• Late complications mostly in adulthood, although they may appear in late childhood.

Question 11

What is the treatment of sickle cell disease?

Answer 11

• The treatment of SCD consists mostly of supportive measures, which includes:
  
  • judicious use of oxygen
  • increased hydration
  • pain medications
  • blood transfusions
  • antibiotics when needed
• Agents aimed at increasing Hb F production, such as hydroxyurea, are currently used in patients with severe disease to modify the pathophysiologic process in SCD
• Stem cell transplant remains the only curative option to date, although factors such as availability of appropriate donors remain an important obstacle