Sickle Cell Anemia:
The Whole Story
Tim R. Randolph, PhD, MT(ASCP), CLS(NCA)
Department of Biomedical Laboratory Science
Doisy College of Health Sciences
Saint Louis University
Sickle Cell Anemia
Objectives
– By the end of this session the participants will
be able to:
Describe the molecular biology associated with the
genetic mutation and protein polymerization
Discuss abnormal lab findings associated with
sickle cell disease and trait
Discuss symptoms and treatment for patients with
sickle cell disease and trait
Thalassemias vs Hemoglobinopathies
Thalassemia
– Genetic mutation (435)
Globin gene(s)
– Lack of gene expression
– Decreased normal Hb
Anemia
– Low RBC, Hb, Hct, MCH, &
MCHC
Increased RBC divisions
Smaller RBC
– Low MCV
Increases Hb/RBC
– Normalization of MCH and
MCHC
Hemoglobinopathies
– Genetic mutation (1065)
Globin gene(s)
– Normal gene expression
– Release of abnormal Hb
Low oxygen saturation
Hb polymerization
– Vaso-occlusion
Hemolysis
– Anemia
– Normal MCV, MCH, &
MCHC
Sickle Cell Anemia
Hemoglobin S
– Definition
A genetic point mutation resulting in one nucleotide
change and a single amino acid substitution that
changes the Hgb molecule from Hgb A to Hgb S
– Etiology
Single nucleotide substitution (A >>T) resulting in
a change in one codon (GAG>>GTG) at the 6th
position on the beta gene and the substitution of a
valine (+0) for a glutamic acid (-1) at the 6th
position of the beta chain - (a2b2GLU>>VAL)
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NORMAL
0
1
2
3
4
5
6
MET VAL HIS LEU THR PRO GLU
ATG GTG CAC CTG ACT CCT GAG
SICKLE
ATG GTG CAC CTG ACT CCT GTG
MET VAL HIS LEU THR PRO VAL
Sickle Cell Anemia
Deoxyhemoglobin S Polymer Structure
Charge and size prevent
6b Glu from binding.
Paired strands of deoxy Hb S
(crystal structure)
Deoxyhemoglobin S
14-stranded polymer
(electron micrograph)
Hydrophobic pocket for 6b Val
Wishner, JMB 1975
Dykes, Nature 1978;
JMB 1979
Crepeau, PNAS 1981
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Hemoglobin S
– Basic Genetics
Simple Mendelian genetics
– Two beta genes
Two genotypes
– Homozygous
HbSS = Sickle Cell Anemia/Disease (SCD)
– Heterozygous
HbAS = Sickle Cell Trait (SCT)
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Hemoglobin S
– Homozygous
Affected alleles
–2
Affected individuals
– African Blacks
Pathogenesis
– Nucleotide substitution (A>T) >> amino acid substitution
(GLU>VAL) >> +1 charge >> DeoxyHgb >> structural
change >> polymerization >> Sickle shaped RBCs >>
vaso-occlusion >> Splenic removal >> Anemia, stroke,
MI, organ infarcts, increased infections
Sickle Cell Anemia
Clinical Features
– Begin at 6 months of age
– Pain in fingers and toes (vaso-occlusion)
– Other vaso-occlusive crises
Triggered by acidosis, hypoxia, dehydration, infection and fever,
and exposure to extreme cold
Bones, lungs, liver, spleen, penis, eyes, central nervous system
(CNS), and urinary tract for 4-5 days to weeks
Acute stroke occur approximately 600/100,000 patients per year
Silent infarcts occur at a rate of 20% of HbSS children
– Enlarged spleen
Rapid trapping and dramatic anemia
Increased bilirubin with jaundice & increased retics
Functional hyposplenism (Howell-Jolly, Pappenheimer)
– Frequent infections
Sickle Cell Anemia
Diagnosis
– Microscopic
Normo/normo anemia, Sickle cells, Target Cells,
NRBC/Poly/retic
– Solubility
Turbid (insoluble)
– Electrophoresis
Band at S (homozygote) (>80%)
No band at A
Hgb F = 1-20%, Hgb A2 = 2-5%
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 Hemoglobin S
 Homozygous
 Treatment
 Prophylactic
 Immunizations (pneumococcus)
 Antibiotics (penicillin G) (daily till age 5 or 6)
 Hydroxyurea, Butyrates, 5-Azacytadine (AzaC)(HbF)
 Crisis Management
 Pain management (analgesics/narcotics)
 IV fluids
 Oxygen
 Anemia Management
 Transfusions
Sickle Cell Anemia
Hemoglobin S
– Homozygous
Prognosis
– Good if followed closely by a physician
– Many are reaching retirement age
Sequelae
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Acute chest syndrome (25% of deaths)
Stroke
Silent stroke
Priapism
Gallstones
Renal failure
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Mechanism of
Hydroxyuria
– Generates Nitric Oxide
(NO)
– Activates NO/cGMP
signaling pathway
– Upregulate g-globin
gene expression
Sickle Cell Anemia
Mechanism of Butyrate
– Epigenetic modified DNA
Acetylation of histones
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–
–
–
–
Acetylation of lysine residues on histones
Removes positive charges
Reduces affinity between histones and DNA
Opens chromatin
RNA polymerase & transcription factors bind promoter
easier
– Acetylation enhances transcription & activates genes
– Histone deacetylation represses transcription &
silences genes
– Butyrate acetylates genes
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Mechanism of 5-Azacytidine & Decitabine
– Epigenetic modified DNA
Hypomethylation of promoters
– Methylation of cytosine nucleotides in promoters silences
genes
– Drugs are cytidine analogs > incorporate into DNA >
covalently bind DNMT > deplete DNMT >
hypomethylation
– Hypomethylation of promoters activates genes
Sickle Cell Anemia
Hemoglobin S
– Heterozygous
Affected alleles - (1)
Affected individuals - Same as homozygote
Pathogenesis - Same but rarely occurs
Description of blood smear - Target cells
Laboratory tests
– Solubility - Positive
– Hemoglobin electrophoresis
Band at A and S with a little at F and A2
Sickle Cell Anemia
Sickle Cell Trait
– Evidence that carriers are not totally
asymptomatic
Adverse events can occur under extreme stress
–
–
–
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Military boot camp
HS, college and pro athletes
Recreational athletes
Runners
Rhabdomyolysis
Some evidence of minor ongoing joint & muscle
pain
Sickle Cell Anemia
Summary of sickle cell
– Point mutation causes a single amino acid change
– Alters deoxyHgb structure causing polymerization &
sickle cell formation
– Sickle cells cause vaso-occlusion leading to stroke,
micro-infarctions and organ death
– Hemolytic anemia causes splenomegaly and
hyposplenism leading to increased infections
– Treated with immunizations, antibiotic, gamma chain
activators and transfusions
Thank You
Questions?????