Sickle cell Anemia- Biochemical defect, inheritance, frequency, clinical manifestations, diagnosis and treatment.

1. Biochemistry for Medics
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2. Normal Structure of hemoglobin
Normal hemoglobin has four subunits called
globins.
Adult hemoglobin has two α (α 1and α 2) and two
β (β 1 and β 2) globin chains.
Each globin chain has an associated heme prosthetic
group, which is the site of oxygen binding and release.
The quaternary structure of HbA can be described
as a dimer of α 1 β 1 and α 2 β 2 dimers.
The αβ dimers move relative to one another during
the binding and release of oxygen.
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3. Structure of Hemoglobin
All globin chains have similar
primary sequences.
The secondary structure of
globin chains consists of
approximately 75 percent α-
helix.
The similar primary sequence
promotes a similar tertiary
structure in all globins that is
called the globin fold, which is
compact and globular in overall
conformation.
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4. Normal Structure of hemoglobin
Hemoglobin must remain soluble at high
concentrations within the red blood cell to support
normal oxygen binding and release properties.
This is made possible by a distribution of amino acid
side chains in which hydrophobic residues are
sequestered in the interior core of the folded globin
subunits, while hydrophilic residues dominate the
water-exposed surface of the globin fold.
The disk-shaped heme prosthetic group is inserted
into a hydrophobic pocket formed by the globin.
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5. Sickle cell disease- Biochemical
Defect
In Sickle cell disease, also called as sickle cell anemia, a
single nucleotide alteration (point mutation) in the β globin
gene of hemoglobin causes a change of thymine for adenine
(GAG to GTG.), at the sixth codon of the ß gene.
This change encodes Valine instead of Glutamic acid in the
sixth position on the ß-globin molecule.
The charge at this site is altered and allows for
polymerization of hemoglobin under conditions of hypoxia.
 These properties are responsible for the profound clinical
expressions of the sickling syndromes.
 The mutant β globin chain is designated as βs , and the
resulting hemoglobin is referred as HbS.
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6. Sickle cell disease- Biochemical Defect
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7. Sickle cell disease- Biochemical Defect
The continuous formation and destruction of sickled cells contributes
to severe hemolytic anemia. These rigid cells may initiate small vessel
occlusions.
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8. Sickle cell disease- Inheritance
It is a homozygous recessive
disorder.
It occurs in individuals who have
inherited two abnormal genes (one
from each parent) that code for the
synthesis of β chain of the Hb
molecule.
 Heterozygotes have on normal and
one sickle cell gene.
The blood cells of these individuals
contain both HbS and HbA.
These individuals have sickle cell
trait and they usually do not show
the clinical symptoms and Biochemistry For Medics
2/17/2013
can have a 8
normal life span.

9. Sickle cell disease-Frequency
Sickle cell disease has been recognized
in the malarial areas of the world.
In malarial areas there is a selective
advantage to the heterozygote (Hb AS),
Individuals with sickle cell trait have
lower levels of Plasmodium falciparum
parasitemia, higher hemoglobin
counts, and less severe reinfections than
individuals with homozygous Hb A.
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10. Protection against Malaria ?
Sickling might protect a person from malaria
by either of the following:
 Making it more difficult for the parasite to
metabolize or to enter the sickled cell
By accelerating Sickling parasitized cells are
quickly removed
While children with sickle cell trait Hb SA
seem to have a milder form of falciparum
malaria, those with homozygous Hb S have a
severe form that is associated with very high
mortality rate.
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11. Protection against Malaria ?
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12. Sickle cell disease-Frequency
Sickle cell disease is
most common in
individuals of African
descent but is seen in
Hispanics, Arabians, Indian
s, and whites.
In the United States the
incidence is 1 in 625 live
births to African-
Americans.
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13. Biochemical Basis for Sickling
The substitution of the nonpolar Valine for a
charged glutamate forms a protrusion on the β
–globin that fits in to a complementary site on
the α –chain of another hemoglobin molecule
in the cell.
At low oxygen tension, HbS polymerizes
inside the red blood cells, first forming a gel,
then subsequently assembling in to a net work
of fibrous polymers that stiffen and distort the
cell, producing rigid misshapen erythrocytes-
sickle shaped erythrocytes.
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15. Biochemical Basis for Sickling
After recurrent episodes of sickling, membrane
damage occurs and the cells are no longer capable of
resuming the biconcave shape upon reoxygenation.
Thus, they become irreversibly sickled cells (ISCs).
From 5-50% of RBCs permanently remain in the
sickled shape.
The life span of an erythrocyte homozygous for HbS
is approximately 20 days compared to 120 days for a
normal RBC.
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16. Factors affecting Sickling of Red
blood cells
The extent of Sickling depends on the
factors that increase the proportion of HbS
in the deoxy state. These factors include –
Decreased oxygen tension as a result of
high altitude or flying in nonpressurized
plane,
 Increased pCO2
 Decreased pH and
Increased concentration of 2, 3
bisphosphoglycerate.
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17. Consequences Of Sickling
 Reduced life span of red blood cells
Sickled cells frequently block the flow of blood in
the narrow capillaries.
The interruption in the supply of oxygen leads to
tissue anoxia, causing pain and eventually death of
cells (infarction) in the vicinity of blockage.
Because of the disruption of the red cell
membrane, the increased adhesiveness of sickle
reticulocyte, and the increased leukocyte count
there is a thrombotic coagulopathy associated
with sickle cell anemia that contributes to the
severity of the disease.
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18. Sickle cell disease(SCD)- Clinical
Manifestations
The presenting symptoms of SCD involve pain and
anemia.
SCD usually manifests early in childhood. For the
first 6 months of life, infants are protected largely by
elevated levels of Hb F
The disease is associated with growth retardation,
delayed sexual maturation, and being underweight
Anemia is universally present. It is chronic and
hemolytic in nature and usually very well tolerated.
Anemia may be complicated with Megaloblastic
changes secondary to folate
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19. Sickle cell disease(SCD)- Clinical
Manifestations- Cont.
The spleen enlarges in the latter part of the first
year of life.
Pneumococcal infections are common in childhood.
The most common clinical picture during adult life is
vasoocclusive crisis.
Many individuals with Hb S disease experience
chronic low-level pain, mainly in bones and joints.
The acute chest syndrome consisting of chest
pain, fever, tachypnea, leukocytosis, and pulmonary
infiltrates may be there
Central nervous system involvement is one of the
most devastating aspects of SCD.Medics
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20. Sickle cell disease(SCD)- Clinical
Manifestations- Cont.
The heart is involved due to chronic anemia and micro
infarcts.
Chronic hemolysis with hyperbilirubinemia is
associated with the formation of bile stones.
Cholelithiasis may be asymptomatic or result in acute
cholecystitis, requiring surgical intervention Repeated
infarction of joints, bones, and growth plates leads to
aseptic necrosis
Blood in the pulmonary circulation is deoxygenated,
resulting in a high degree of polymer formation.
Pulmonary hypertension may develop
The kidneys lose concentrating capacity
Leg ulcers are a chronic painful problem
 Chronic organ damage- Most common are lungs,
kidneys, liver, skeleton and skin.
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21. Laboratory Findings
1) A complete blood cell count usually
reveals an increased reticulocytes count (5–
15%),
2) Total Leukocyte count (12,000–
20,000/mm3), upper limit of normal or
greater.
3) Differential Leukocyte count- normal (or
predominance of neutrophils
4) Mean corpuscular volume (MCV) normal
(unless thalassemic hemoglobin is present),
5) Hb- mild to moderate anemia (5–9g/dL),
6) Platelet count- normal to increased,
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22. Laboratory Findings-Contd.
7) Blood smear- A diagnosis of sickle cell disease can
be suspected by examination of the peripheral blood
film and that shows the presence of target
cells, poikilocytes, hypochromasia, sickle red
cells, nucleated RBCs, and Howell-Jolly bodies
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23. Laboratory Findings-Contd.
8) Sickle solubility test-A mixture of hemoglobin S
(Hb S) in a reducing solution (such as sodium
dithionite) gives a turbid appearance, whereas
normal Hb gives a clear solution.
9)Hemoglobin electrophoresis-The substitution
of the nonpolar Valine for the charged Glutamic
acid results in decreased mobility of the HbS in
the electric field as compared to HbA. This altered
mobility is due to the presence of less negative
charge on the two β –globin chains
10) HPLC-The diagnosis is confirmed by high-
pressure liquid chromatography (HPLC).
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24. Laboratory Findings-Contd.
10) Bone marrow is markedly hyper plastic with
erythroid predominance.
11) Radiological studies may reveal characteristic
bony findings of sickle cell disease in the
vertebral bodies, mild expansion of the marrow
cavities, osteoporosis, and possibly sclerosis of
the long bones and femoral heads.
12) Liver function tests, as well as
BUN, creatinine, and serum electrolytes are also
required to be performed.
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25. Treatment
Treatment strategies include the following 7 goals:
(1) management of vasoocclusive crisis,
(2) management of chronic pain syndromes,
(3) management of the chronic hemolytic anemia,
(4) prevention and treatment of infections,
(5) management of the complications and the
various organ damage syndromes associated with
the disease,
(6) prevention of stroke, and
(7) detection and treatment of pulmonary
hypertension.
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26. Treatment (Contd.)
Vasoocclusive crisis is treated with vigorous hydration
and analgesics.
Pain control is best achieved by the administration of
opioids.
Prevention of infection improves chances of survival in
SCD.
Allogeneic bone marrow transplantation (BMT) can cure
the disease, but it is difficult to decide which patients
should be offered BMT.
Therapy with hydroxyurea- Hydroxyurea increases the
production of Hb F, which retards gelation and sickling.
Blood transfusion is indicated only in specific situations
Surgical care is limitedBiochemistry For Medics
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27. Prevention of sickle cell disease
Screen for Hb S at birth. This method of
case finding allows institution of early
treatment and control.
Prenatal diagnosis is also available. The
laboratory procedures employed in
prenatal testing are sensitive and rapid.
Prenatal testing must be accompanied
with genetic and psychological counseling.
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28. Prognosis
Because this is a lifelong disease,
prognosis is guarded.
The goal is to achieve a normal life span
with minimal morbidity.
As therapy improves, the prognosis also
improves
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29. Sickle cell trait
About 8% of African Americans and about 30% of
Nigerians are heterozygous for sickle cell trait.
 HbS heterozygotes have minimal clinical
problems
 Overall life expectancy is similar to normal
individuals
 Individuals with sickle cell trait may develop
sickle cell crisis if they become severely hypoxic
 May have recurrent episodes of painless
hematuria due to impaired ability to form
concentrated urine.
These individuals are not placed at high risk.
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30. Sickle beta Thalassemia
Commonly encountered from Mediterranean
countries as well as from central Africa
Patients have congenital hemolytic anemia of
varying severity, accompanied by splenomegaly in
70 % of cases.
Sickle β0 Thalassemia have vaso- occlusive
manifestations comparable to homozygous SS
disease.
Sickle β+ Thalassemia have less severe
anemia, fewer pain crisis and less organ damage
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31. Sickle C disease
Hb electrophoresis reveals 50% HbS and 50%
Hb C
 One of the beta chain gene is defective for
Hb S and the other is defective for Hb C( The 6th
position in beta chain is replaced by Lysine in
place of Glutamic acid
There is increased tendency for SC cells to
sickle cell as compared to sickle cell trait
Occasional painful crisis or organ infarcts
 High rate of complications during pregnancy
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