Hemoglobinopathies

Hemoglobinopathies are types of intracorpuscular defects

leading to the production of an abnormal hemoglobin or to an
aberration of hemoglobin synthesis

Abnormal hemoglobins
- Most are clinically insignificant with no physiologic
consequence

- Most abnormalities occur in the β chain with abnormalities

in
this chain more likely to cause disease because we have only
two genes that encode the β chains, but we have four genes
that encode the α chains.

- Most variants arise from the substitution of a single amino

acid in the  globin chain.
- Changes may also arise from multiple substitutions, insertions or
deletions, frame shift mutations, cross-over, and fusions of
subunits.
- If an individual is homozygous for a structural gene in the β
chain, the individual is said to have the disease or anemia
- If the individual is heterozygous, they are said to have the
trait, and 50% or less of the hemoglobin will be abnormal.
Sickle cell disease
- Hemoglobin S: position 6 on the  chain has a valine (nonpolar)
substituted for the normal glutamic acid (polar).
- Carriers of the gene, when parasitized by Plasmodium
(causes malaria), cells containing HbS will sickle quickly, either
killing the parasite or causing RBCs to be sequestered in the
spleen and destroyed.
- Therefore, having the gene provides a certain protection against
malaria.
Pathophysiology of the disease
- When oxygenated, Hb S is soluble, but when oxygen tension
decreases, Hb S in the deoxyhemoglobin state polymerizes
into insoluble aggregates leading to sickled cells.

- This leads to increased blood viscosity

which leads to decreased circulation
and increased exposure to low oxygen.

- This, in turn, leads to more sickling.

- The small microvasculature may

become clogged with the rigid sickle
cells leading to hypoxia and infarction
of organs and a “sickle cell crisis”.
Molecular changes of HbS

Heme

Valine
Molecular and cellular changes of HbS

Decreased PO2

Permanent damage to RBC

Cell ⟺ endothelium interactions

- Upon reoxygenation, the RBC may return to its original
shape.
- With repeated sickling damages the permeability of the
RBC membrane leading to premature death of the cell.

- In addition, after repeated sickling events, the cells become

irreversibly sickled and are removed by the spleen.

- Early in childhood, the spleen loses its function due to

splenic atrophy and necrosis from repeated ischemic (blood
supply decreased due to blockage of the small vasculature)
crises.

-Thus, these young patients are more subjected to infections.

-The liver and bone marrow then take over destruction of

abnormal cells.
-Hb S has a decreased affinity for oxygen, leading to a shift to
the right in the oxygen dissociation curve.
- This, however, creates more deoxyhemoglobin, and hence,
more sickling.
Clinical findings
-The disease is diagnosed early at about 6 months of age
when hemoglobin F is replaced with Hb S rather than Hb A.
- Homozygous individuals frequently do not live beyond
middle age.
- Chronic hemolytic anemia.
- RBC survival may decrease to 14 days.
- Increased bilirubin turnover leads to gallstones.
Lab findings
- Normochromic, normocytic anemia (6-10 g/dl Hb).
- 10-20% reticulocytes
- RBCs are sickled cells
- Bone marrow: normoblastic hyperplasia
Diagnosis: peripheral blood smear, Hb electrophoresis,
solubility tests, sodium metabisulfite will cause the cells to
sickle by deoxygenating the blood.

Therapy
- No known effective long term
therapy, hoping to develop drugs
that can inhibit Hb S polymerization
- Bone marrow transplant
- Gene therapy

Sickle cell trait (heterozygous for Hb S)

-Usually the patient has no problems because >50% of their
hemoglobin is Hb A with some occasional problems upon
exposure to severe hypoxia

Diagnosis: Hb electrophoresis or ttt with sodium metabisulfite

Hemoglobin C disease
- Lysine is substituted for glutamic acid at position 6 on the 
chain.
- Hb C has decreased solubility and in the deoxyhemoglobin
state, the RBCs form intracellular crystals leading to a rigid
RBC with a decreased survival time (33-35 days).
- The disease is usually asymptomatic.
Lab findings
- Slight ↑in reticulocytes - Hb C crystals
Diagnosis: Hb electrophoresis
S/C disease
- Both  chains are abnormal, therefore, Hb A is absent and
the disease is almost as severe as in Hb S disease
- Clinically, it is similar to those of mild sickle cell anemia
- Can be differentiated from Hb S by Hb electrophoresis.
Hb D disease and trait
- A glutamine replaces glutamic acid at position 121 on the 
chain
- Both homozygous and heterozygous states are asymptomatic
- When combined with S to form D/S, D potentiates the
polymerization of deoxyhemoglobin leading to sickling and
mild anemia.
Hb E disease and trait
- A glutamic acid replaces lysine at position 26 on the  chain
leading to a slightly unstable hemoglobin with oxidant stress.
- Hb E has a decreased affinity for oxygen leading to a shift to
the right in the oxygen dissociation curve
- Homozygous individuals have a mild microcytic anemia with
decreased RBC survival, target cells and increased osmotic
fragility
- Heterozygous individuals are symptomless
Unstable hemoglobin disorders
- Contain amino acid changes in internal portions of Hb chains
leading to decreased stability
-They are characterized by precipitation of the abnormal Hb
as Heinz bodies which leads to increased cell rigidity,
membrane damage, and RBC hemolysis.
- They are only found in the heterozygous state since the
homozygous state is incompatible with life
Hemoglobin variants with altered oxygen affinity
- Amino acid substitutions in the globin chains close to the
heme pocket may affect the ability of the hemoglobin to
carry oxygen
- This also occurs with substitutions near the 2, 3 DPG binding
site
Hb M variants
- Are characterized by permanent methemoglobin formation
because iron is stabilized in the Fe +3 state.
Thalassemias
-They are a heterogeneous group of genetic disorders with
variable levels of severity.
- Individuals with homozygous forms are severely affected
and die early in childhood without treatment
- The disorders are due to mutations that decrease the rate of
synthesis of one of the two globin chains ( or ).
- The genetic defect may be the result of:
- A mutation in the noncoding introns of the gene resulting in
inefficient RNA splicing to produce mRNA, and therefore,
decreased mRNA production
-The partial or total deletion of a globin gene
- A mutation in the promoter leading to decreased expression
- A mutation at the termination site leading to production of
longer, unstable mRNA
- A nonsense mutation.
- Any of these defects lead to:
- An excess of the other normal globin chain
- A decrease in the normal amount of hemoglobin made
- Development of a hypochromic, microcytic anemia
 thalassemia
- The disease manifests itself when the switch from  to 
chain synthesis occurs several months after birth.
- There may be a compensatory increase in  and  chain
synthesis resulting in increased levels of Hb F and A2.
- The genetic background of  thalassemia is heterogeneous
and may be roughly divided into two types:
1- 0 in which there is complete absence of  chain
production which is common in the Mediterranean.
2- + in which there is a partial block in  chain synthesis.
- At least three different mutant genes are involved:
+1 →10% of normal  chain synthesis occurs
+2 → about 50% of normal  chain synthesis occurs
+3 →50% of normal  chain synthesis occurs
-The clinical expression of the different gene combinations (1
from mother and 1 from father) are as follows:
- 0/0, +1/ +1, or 0/ +1,+2,or +3 = thalassemia major (Cooley's
anemia), the most severe form of the disease.

- Imbalanced synthesis leads to decreased total RBC

hemoglobin production and a hypochromic, microcytic
anemia.

- Excess  chains precipitate causing hemolysis of RBC

precursors in the bone marrow leading to ineffective
erythropoiesis causing severe anemia.

- In circulating RBCs,  chains may also precipitate leading to

pitting in the spleen.
- Untreated individuals die early, usually of cardiac failure
(due to overwork and hemochromatosis).
- Lab. findings include:
- hypochromic, microcytic anemia
- basophilic stippling from  chain precipitation
- increased reticulocytes and nucleated RBCs

- Serum iron and ferritin are normal to increased and there is

increased saturation

- Chronic hemolysis leads to increased bilirubin and gallstones

- Hemoglobin electrophoresis shows increased Hb F, variable
amounts of Hb A2, and no to very little Hb A

-Therapy: transfusions plus iron chelators to prevent

hemochromatosis and tissue damage from iron overload,
beside the trials of gene therapy.
-  +2, or 3 homozygous = thalassemia intermedia
- Heterozygosity of 0, or + = thalassemia minor
- Mild hypochromic, microcytic anemia
- Patients are usually asymptomatic with symptoms occurring
under stressful conditions such as pregnancy.
-  thalassemia may also be found in combination with any of
the hemoglobinopathies (S, C, or E) leading to a mild to
severe anemia depending upon the particular combination.
 thalassemia
-The disease is manifested immediately at birth
-There are normally four alpha chains, so there is a great
variety in the severity of the disease.
- At birth there are excess  chains and later there are excess 
chains.
-These form stable, nonfunctional tetramers that precipitate
leading to decreased RBC survival.
-The disease is usually due to deletions of the  gene and occasionally to
a functionally abnormal  gene.
- Since one gets two genes from each parent, there are four types of 
thalassemia:
- Loss of ONE gene → silent carrier (-a/aa).
- Loss of TWO genes → thalassemia minor (trait) (-a/-a) or (--/aa) with
mild anemia.
- Loss of THREE genes → Hemoglobin H (--/-a) → accumulation
of β chains → association of β chains in groups of 4 → Hb H (has
a higher affinity for O2 and precipitates in older cells) → anemia
may be chronic to moderate to severe.
- Loss of FOUR genes → Hemoglobin Barts (--/--) (NO α chains
produced, only γ chains present → association of 4 γ chains →
hydrops fetalis which is fatal with stillbirth or death within hours
of birth.
- Hemoglobin Barts (4) forms and has such a high affinity for O2
that no O2 is delivered to the tissues.
- Hb S/  thalassemia: symptomless to moderate anemia
Delta/beta (/) thalassemia (Hereditary persistence of Hb F)
- Both  and  chains are absent with no or little compensatory increase
in  chain synthesis.
- This leads to 100% Hb F and mild hypochromic, microcytic anemia
- Since Hb F has an increased affinity for O2, this results in polycythemia.

Hemoglobin Constant Spring

- Formed by a combination of two structurally abnormal  chains (each
elongated by 31 amino acids at the COOH end) and two normal 
chains.
- Homozygous individuals have mild hypochromic, microcytic anemia
similar to a mild a  thalassemia.
Hemoglobin Lepore
- A normal  chain plus a - hybrid (N-terminal , and C-terminal ).
- There is ineffective synthesis of the hybrid chain leading to  chain
excess and the same problems seen in  thalassemia.
- Homozygous individuals have a mild to severe hypochromic,
microcytic anemia
- Heterozygous individuals are asymptomatic.