Robbins pg. 634 to 642

Question 1

How do red cells protect themselves against oxidative injuries (and subsequent hemolysis)?

Answer 1

glutathione and the hexose monophosphate shunt

Question 2

Write out the glutathione pathway

Answer 2

1) G6PD oxidizes G6P to 6-phosphogluconate as NADP is reduced
2) Glutathione reductase reduces GSSG to GSH as NADPH is re-oxidized
3) Glutathione peroxidase reduces H2O2 to 2H20 as GSH is re-oxidized

Question 3

MOI of G6PD deficiency?

Answer 3

X-linked (males at higher risk)

Question 4

While there are hundred of variations of G6PD that are mostly harmless, the majority of clinically relevant cases are the result of two mutations:

Answer 4

1) G6PD-

2) G6PD Mediterranean

Question 5

What is G6PD- common in?

Answer 5

10% of American blacks

Question 6

The hemolysis caused by G6PD is cyclic/episodic. What brings on an episode of hemolysis?

Answer 6

1) The most common triggers are infections, in which
oxygen-derived free radicals are produced by activated
leukocytes. Many infections can trigger hemolysis; viral
hepatitis, pneumonia, and typhoid fever are among those
most likely to do so.

2) The other important initiators are
drugs and certain foods. The oxidant drugs implicated are
numerous, including antimalarials (e.g., primaquine and
chloroquine), sulfonamides, nitrofurantoins, and others.

Some drugs cause hemolysis only in individuals with the
more severe Mediterranean variant.

The most frequently
cited food is the fava bean, which generates oxidants when
metabolized. “Favism” is endemic in the Mediterranean,
Middle East, and parts of Africa where consumption is
prevalent.

Uncommonly, G6PD deficiency presents as
neonatal jaundice or a chronic low-grade hemolytic anemia
in the absence of infection or known environmental

Question 7

T or F. Oxidants cause both intravascular AND extravascular hemolysis

Answer 7

T.

Question 8

What are Heinz bodies?

Answer 8

Exposure of G6PD deficient red cells to high levels of oxidants causes the cross-linking of reactive sulfhydryl groups on globin chains, which become denatured and form membrane-bound
precipitates known as Heinz bodies.

Heinz bodies can damage the membrane sufficiently to cause intravascular hemolysis

Question 9

What are bite cells?

Answer 9

As inclusion-bearing red cells pass through the splenic cords, macrophages pluck out the Heinz bodies. As a result of membrane damage, some of these partially devoured
cells retain an abnormal shape, appearing to have a bite taken out of them.

Other less severely damaged cells become spherocytes due to loss of membrane surface
area. Both bite cells and spherocytes are trapped in splenic cords and removed rapidly by phagocytes.

Question 10

What are the symptoms of acute intravascular hemolysis?

Answer 10

• anemia
• hemoglobinemia
• hemoglobinuria

Question 11

What is the timeline of symptoms of acute intravascular hemolysis in G6PD deficient patients following oxidant exposure?

Answer 11

2-3 days (worse in meiterrranean variant)

Question 12

Why would a G6PD episode be self-limited?

Answer 12

because only older red cells are at risk, and lysis ceases when only younger red cells remain even if the oxidant remains

Question 13

Are symptoms of chronic hemolysis (splenomegaly, cholelithiasis) present in G6PD deficiency?

Answer 13

No, because this is not a chronic hemolytic disease- only an intermittent one

Question 14

What is sickle cell disease caused by?

Answer 14

a common hereditary hemoglobinopathy
caused by a point mutation in β-globin that promotes
the polymerization of deoxygenated hemoglobin,
leading to red cell distortion, hemolytic anemia, microvascular
obstruction, and ischemic tissue damage

Question 15

Describe the composition of hemoglobin.

Answer 15

Hemoglobin is a tetrameric
protein composed of two pairs of globin chains, each with
its own heme group.

Normal adult red cells contain mainly
HbA (α2β2), along with small amounts of HbA2 (α2δ2) and
fetal hemoglobin (HbF; α2γ2).

Question 16

What is the mutation responsible for sickle cell disease?

Answer 16

Sickle cell disease is caused by a point mutation in the sixth codon of β-globin that
leads to the replacement of a glutamate residue with a valine residue.

Question 17

What is sickle cell trait?

Answer 17

About 8% to 10% of African Americans, or roughly 2
million individuals, are heterozygous for HbS, a largely asymptomatic condition known as sickle cell trait.

The offspring
of two heterozygotes has a 1 in 4 chance of being homozygous for the sickle mutation, a state that produces
symptomatic sickle cell disease (MOI=AR).

HbS= a2BS2

Question 18

Why would HbS protect against malaria?

Answer 18

Metabolically active intracellular parasites consume O2
and decrease intracellular pH, both of which promote
hemoglobin sickling in AS red cells. These distorted and
stiffened cells may be cleared more rapidly by phagocytes
in the spleen and liver, helping to keep parasite
loads down.

• Another effect of sickling is that it impairs the formation
of membrane knobs containing a protein made by the
parasite called PfEMP-1. These membrane knobs are implicated in adhesion of infected red cells to endothelium,
which is believed to have an important pathogenic role in cerebral malaria.

Question 19

T or F. G6PD deficiency and thalassemias

also protect against malaria

Answer 19

T, by increasing the clearance and decreasing the adherence of infected red cells, possibly by raised levels of oxidant stress and causing
membrane damage in the parasite-bearing cells.

Question 20

What are the main pathogenic manifestations of sickle cell?

Answer 20

chronic hemolysis, microvascular occlusions, and tissue
damage

all caused by the tendency of HbS molecules to
stack into polymers and sickle when deoxygenated

Question 21

What variables affect the rate and degree of sickling?

Answer 21

-Interaction of HbS with the other types of hemoglobin in the
cell
-Mean cell hemoglobin concentration (MCHC).
-Intracellular pH.
-Transit time of red cells through microvascular beds.

Question 22

How does interaction of HbS with other types of hemoglobin in the cell affect the rate and degree of sickling?

Answer 22

The presence of normal HbA (or HbF) inhibits HbS polymerization (i.e. in carriers)- thus, no sickling in heterozygotes unless severely hypoxic

NOTE: HbF inhibits the
polymerization of HbS even more than HbA; hence, infants do not become symptomatic until they reach 5
or 6 months of age, when the level of HbF normally falls.

Question 23

What is HbC?

Answer 23

Another variant hemoglobin in which lysine is substituted for glutamate in the sixth amino acid residue of β-globin

HbC is also common in regions where HbS is found; overall, about
2% to 3% of American blacks are HbC heterozygotes and about 1 in 1250 are compound HbS/HbC heterozygotes.

Question 24

Would the presence of HbC (as in HbSC patients) result in disease? Why? What is this disease called?

Answer 24

In HbSC red cells the percentage of HbS is 50%, as compared with only 40% in HbAS cells. Moreover, HbSC cells tend to lose salt and water and become dehydrated,
which increases the intracellular concentration
of HbS. Both of these factors increase the tendency for HbS to polymerize.

As a result, individuals who are compound heterozygotes for HbS and HbC have a
symptomatic sickling disorder (termed HbSC disease), but it is milder than sickle cell disease

Question 25

How does mean cell hemoglobin concentration (MCHC) affect the rate and extent of hemolysis?

Answer 25

Higher HbS
concentrations increase the probability that aggregation
and polymerization will occur during any given period of deoxygenation. Thus, intracellular dehydration,
which increases the MCHC, facilitates sickling.

Question 26

Conversely,

conditions that decrease the MCHC reduce the disease severity. When could this happen?

Answer 26

This occurs when the individual is homozygous for HbS but also has coexistent
α-thalassemia, which reduces Hb synthesis and leads to
milder disease.

Question 27

How does intracellular pH affect the rate and degree of sickling?

Answer 27

pH reduces the oxygen affinity of hemoglobin, thereby increasing the fraction of deoxygenated HbS at any given oxygen tension and augmenting the tendency for sickling

Question 28

How does transmit time of red cells through microvascular beds affect the rate and degree of sickling?

Answer 28

much of the pathology of sickle cell disease
is related to vascular occlusion caused by sickling within
microvascular beds.

Transit times in most normal microvascular
beds are too short for significant aggregation
of deoxygenated HbS to occur, and as a result sickling is confined to microvascular beds with slow transit
times.

Question 29

What places in the body are commonly affected by sickle cell? Why?

Answer 29

Blood flow is sluggish in the normal spleen and
bone marrow, which are prominently affected in sickle cell disease, and also in vascular beds that are inflamed.

The movement of blood through inflamed tissues is
slowed because of the adhesion of leukocytes to activated
endothelial cells and the transudation of fluid
through leaky vessels. As a result, inflamed vascular beds are prone to sickling and occlusion.

Question 30

What do HbS polymers do to red cell structure?

Answer 30

As HbS polymers grow, they herniate
through the membrane skeleton and project from the cell
ensheathed by only the lipid bilayer.

This severe derangement
in membrane structure causes the influx of Ca2+ ions,
which induce the cross-linking of membrane proteins
and activate an ion channel that permits the efflux of K+
and H2O.

Question 31

What does repeated cyclic episodes of sickling cause?

Answer 31

With repeated episodes of sickling, red cells
become increasingly dehydrated, dense, and rigid. Eventually, the most severely damaged cells are converted
to end-stage, nondeformable, irreversibly sickled
cells, which retain a sickle shape even when fully oxygenated.

The severity of the hemolysis correlates with the percentage
of irreversibly sickled cells, which are rapidly
sequestered and removed by mononuclear phagocytes
(extravascular hemolysis). Sickled red cells are also
mechanically fragile, leading to some intravascular hemolysis

Question 32

T or F. Microvascular occlusions are not related to the

number of irreversibly sickled cells in the blood.

Answer 32

T, but instead
may be dependent upon more subtle red cell membrane
damage and local factors, such as inflammation or vasoconstriction,
that tend to slow or arrest the movement of
red cells through microvascular beds

Question 33

Why would sickle cells be more likely to bind to vasculature (and cause occlusion) during inflammatory states?

Answer 33

Sickle red cells express higher than normal
levels of adhesion molecules and are sticky. Mediators
released from granulocytes during inflammatory reactions
up-regulate the expression of adhesion molecules on endothelial
cells and further enhance the tendency
for sickle red cells to get arrested during transit through the microvasculature.

Question 34

What role does NO play in sickle cell?

Answer 34

Free hemoglobin released from lysed sickle red
cells can bind and inactivate NO, which is a potent vasodilator
and inhibitor of platelet aggregation. The reduction in active NO leads to increased vascular tone (narrowing
vessels) and enhances platelet aggregation, both of which
may contribute to red cell stasis, sickling, and (in some instances) thrombosis.

Question 35

What are ‘pain crises’?

Answer 35

Vaso-occlusive crises, also called pain
crises, are episodes of hypoxic injury and infarction that
cause severe pain in the affected region. Although infection,
dehydration, and acidosis (all of which favor sickling)
can act as triggers, in most instances no predisposing cause
is identified

Question 36

What are common sites of pain crises?

Answer 36

The most commonly involved sites are the
bones, lungs, liver, brain, spleen, and penis.

In children, painful bone crises are extremely common and often difficult to distinguish from acute osteomyelitis.

Question 37

What is priapism?

Answer 37

Priapism affects

up to 45% of males after puberty and may lead to hypoxic damage and erectile dysfunction.

Question 38

Other symptoms of sickle cell?

Answer 38

• stroke
• retinopathy leading to loss of visual acuity
• pulmonary dysfunction
• susceptibility to encapsulated bacteria

Question 39

What are sequestration crises?

Answer 39

Sequestration crises occur in
children with intact spleens. Massive entrapment of sickle
red cells leads to rapid splenic enlargement, hypovolemia, and sometimes shock.

NOTE: In early childhood, the spleen is enlarged up to 500 gm by red pulp congestion, which is caused by the trapping of sickled
red cells in the cords and sinuses. With time,
however, the chronic erythrostasis leads to splenic infarction fibrosis, and progressive shrinkage, so that by adolescence or early adulthood only a small nubbin of fibrous splenic tissue is
left; this process is called autosplenectomy

Question 40

How is sickle cell confirmed?

Answer 40

1) Mixing a blood sample with an oxygen consuming
reagent, such as metabisulfite, which induces sickling of red cells if HbS is present

2) Hemoglobin electrophoresis to exclude HbSC disease
3) Prenatal diagnoses via DNA by amniocentesis

Question 41

Outlook for sickle cell patients?

Answer 41

About 90% survive to age 20

50% survive to 50

Question 42

Treatment for sickle cell?

Answer 42

hydroxyurea

Question 43

How does hydroxyurea work to combat sickle cell?

Answer 43

1) increases HbF levels
2) anti-inflammatory effects

these help decrease crises in children and adults

Question 44

What are thalassemia syndromes?

Answer 44

a heterogeneous group
of disorders caused by inherited mutations that decrease the synthesis of either the α-globin or β-globin chains
that compose adult hemoglobin, HbA (α2β2), leading to
anemia, tissue hypoxia, and red cell hemolysis related to the imbalance in globin chain synthesis.

Question 45

Where are thalassemia common?

Answer 45

Mediterranean basin, Middle East, tropical Africa, India, and Asia

Question 46

Why are thalassemias so common?

Answer 46

like sickle cell, they seem to protect heterozygote carriers against malaria

Question 47

What are the common mutations relating to B-thalassemia?

Answer 47

(1) β0 mutations, associated with absent
β-globin synthesis, and

(2) β+ mutations, characterized by
reduced (but detectable) β-globin synthesis

Question 48

T or F. Most mutations of B thalassemia are point mutations

Answer 48

T.

Question 49

What is the most common cause of B+ thalassemia?

Answer 49

splicing mutations that lie within introns mostly (some are in exons)

These mutations create an “ectopic” splice site within an intron. Because the flanking normal splice sites remain, both normal and abnormal splicing occurs and some normal β-globin
mRNA is made, resulting in β+-thalassemia.

Question 50

How can splicing mutations cause B0 thalassemia?

Answer 50

Some of these mutations destroy the normal RNA splice junctions and completely prevent the production of normal
β-globin mRNA, resulting in β0-thalassemia.

Question 51

What do promoter region mutations do?

Answer 51

reduce transcription
by 75% to 80%. Some normal β-globin is synthesized; thus, these mutations are associated with
β+-thalassemia.

Question 52

What do chain terminator mutations cause? Subtypes?

Answer 52

These are the most common cause of β0-thalassemia. Two subtypes of mutations fall into this category.

The most common type creates a new
stop codon within an exon; the other introduces small
insertions or deletions that shift the mRNA reading frames (frameshift mutations). Both block translation and prevent the synthesis of any function B-globin

Question 53

Impaired β-globin synthesis results in anemia by two

mechanisms. What are they?

Answer 53

1) The deficit in HbA synthesis produces “underhemoglobinized” hypochromic, microcytic
red cells with subnormal oxygen transport capacity.

2) Even more important is the diminished survival of red cells and their precursors, which results from the imbalance
in α- and β-globin synthesis.

Question 54

What do unpaired a-chains do in B-thalassemia?

Answer 54

precipitate within red cell precursors, forming insoluble inclusions. These inclusions cause a variety of untoward
effects, but membrane damage is the proximal cause of most red cell pathology. Many red cell precursors succumb to membrane damage and undergo apoptosis. In severe β-thalassemia, it is estimated that 70% to 85% of red cell
precursors suffer this fate, which leads to ineffective erythropoiesis.

Those red cells that are released from the marrow also contain inclusions and have membrane damage,
leaving theme prone to splenic sequestration and extravascular
hemolysis.

Question 55

How does B-thalassemia affect bone?

Answer 55

Erythropoietic drive
in the setting of severe uncompensated anemia (inefficient erythryopoiesis) leads to massive erythroid hyperplasia in the marrow and extensive
extramedullary hematopoiesis. The expanding mass
of red cell precursors erodes the bony cortex, impairs bone
growth, and produces skeletal abnormalities.

Question 56

Where else are masses common in B-thalassemia?

Answer 56

spleen, and lymph nodes, and in extreme cases produces
extraosseous masses in the thorax, abdomen, and pelvis.

The metabolically active erythroid progenitors steal nutrients
from other tissues that are already oxygen-starved,
causing severe cachexia in untreated patients

Question 57

What is another complication of ineffective erythropoietin?

Answer 57

excessive absorption of dietary iron.

Question 58

How does ineffective erythropoietin cause excessive dietary iron absorption?

Answer 58

Ineffective

erythropoiesis suppresses hepcidin- secondary hemochromatosis can follow

Question 59

Would the presence of an a-globin deficiency help or hurt the prognosis of a B-thalasemic patient?

Answer 59

Helps, it improves the

effectiveness of erythropoiesis and red cell survival by lessening the imbalance in α- and β-chain synthesis

Question 60

How does B-thalassemia present?

Hemoglobin levels?

Answer 60

The anemia manifests 6 to 9 months after birth as hemoglobin synthesis switches
from HbF to HbA. Causes growth retardation and death if untreated.

In untransfused patients, hemoglobin
levels are 3 to 6 gm/dL. The red cells may completely lack
HbA (β0/β0 genotype) or contain small amounts (β+/β+ or
β0/β+ genotypes). The major red cell hemoglobin is HbF,
which is markedly elevated. HbA2 levels are sometimes
high but more often are normal or low.

Question 61

Other clinical manifestations?

Answer 61

• enlarged cheekbones

- hepatosplenomegaly due to extramedullary hematopoiesis

Question 62

Recognition of β-thalassemia trait is important for two

reasons:

Answer 62

(1) it superficially resembles the hypochromic
microcytic anemia of iron deficiency, and

(2) it has implications
for genetic counseling.

Question 63

How would you differentiate between B-thalassemia minor and iron deficiency?

Answer 63

The increase in HbA2 is diagnostically
useful, particularly in individuals (such as women of childbearing
age) who are at risk for both β-thalassemia trait
and iron deficiency.

check serum ferritin, iron-binding capacity

Question 64

T or F. Normally, there are four α-globin genes, and the severity
of α-thalassemia depends on how many α-globin genes are
affected

Answer 64

T. As in β-thalassemias, the anemia stems both from a lack of adequate hemoglobin and the presence of excess
unpaired globin chains (β, γ, and δ), which vary in type at different ages.

Question 65

What is hemoglobin Barts?

Answer 65

In newborns with α-thalassemia, excess
unpaired γ-globin chains form γ4 tetramers known as hemoglobin
Barts, whereas in older children and adults excess β-globin chains form β4 tetramers known as HbH.

Question 66

T or F. hemolysis and ineffective

erythropoiesis in a-thalassemia are less severe than in β-thalassemias. Why or why not?

Answer 66

T. Because
free β and γ chains are more soluble than free α chains and
form fairly stable homotetramers, .

Question 67

What is the most common cause of a-thalassemia?

Answer 67

gene deletion

Question 68

What is a-thalassemia trait?

Answer 68

α-Thalassemia trait is caused by the deletion of two α-globin genes from a single chromosome
(α/α −/−) or the deletion of one α-globin gene from each of the two chromosomes (α/− α/−)

both present the same way- only difference is genetic risks for offspring

Question 69

How does a-thalassemia trait present?

Answer 69

The clinical picture in α-thalassemia trait is identical to that
described for β-thalassemia minor, that is: 
-small red cells
(microcytosis), 
-minimal or no anemia, and 
-no abnormal
physical signs. 
-HbA2 levels are normal or low.

Question 70

What is HbH disease?

Answer 70

HbH disease is caused by deletion of three α-globin genes.

most common in Asian populations.

Question 71

What is HbH? Why does it form?

Answer 71

with only one working α-globin gene, the synthesis of α chains is markedly reduced, and tetramers of β-globin, called HbH, form.

Question 72

Why does HbH cause tissue hypoxia?

Answer 72

HbH has an extremely high affinity for oxygen and therefore

is not useful for oxygen delivery, leading to tissue hypoxia disproportionate to the level of hemoglobin.

Question 73

How does HbH affect red cells?

Answer 73

HbH is prone to oxidation, which causes
it to precipitate and form intracellular inclusions that promote red cell sequestration and phagocytosis in the
spleen. The result is a moderately severe anemia resembling
β-thalassemia intermedia.

Question 74

Deletion of all four a-globin genes causes what?

Answer 74

Hydrops fetalis

Question 75

What happens in hydrops fetalis?

Answer 75

In the fetus, excess γ-globin chains
form tetramers (hemoglobin Barts) that have such a high
affinity for oxygen that they deliver little to tissues. Survival
in early development is due to the expression of ζ chains,an embryonic globin that pairs with γ chains to form a functional ζ2γ2 Hb tetramer.