We feel that this report looks a lot better single-spaced. A Brief History of
Sickle Cell Disease Sickle Cell Disease in African Tradition Sickle cell disease
has been known to the peoples of Africa for hundreds, and perhaps thousands, of
years. In West Africa various ethnic groups gave the condition different names:
Ga tribe: Chwechweechwe Faute tribe: Nwiiwii Ewe tribe: Nuidudui Twi tribe:
Ahotutuo Sickle Cell Disease in the Western Literature Description of Sickle
Cell Disease In the western literature, the first description of sickle cell
disease was by a Chicago physician, James B. Herrick, who noted in 1910 that a
patient of his from the West Indies had an anemia characterized by unusual red
cells that were "sickle shaped". Relationship of Red Cell Sickling to
Oxygen In 1927, Hahn and Gillespie showed that sickling of the red cells was
related to low oxygen. Deoxygenation and Hemoglobin In 1940, Sherman (a medical
student at Johns Hopkins) noted a birefringence of deoxygenated red cells,
suggesting that low oxygen altered the structure of the hemoglobin in the
molecule. Protective Role of Fetal Hemoglobin in Sickle Cell Disease Janet
Watson, a pediatric hematolist in New York, suggested in 1948 that the paucity
of sickle cells in the peripheral blood of newborns was due to the presence of
fetal hemoglobin in the red cells, which consequently did not have the abnormal
sickle hemoglobin seen in adults. Abnormal Hemoglobin in Sickle Cell Disease
Using the new technique of protein electrophoresis, Linus Pauling and colleagues
showed in 1949 that the hemoglobin from patients with sickle cell disease is
different than that of normals. This made sickle cell disease the first disorder
in which an abnormality in a protein was known to be at fault. Amino Acid
Substitution in Sickle Hemoglobin In 1956, Vernon Ingram, then at the MRC in
England, and J.A. Hunt sequenced sickle hemoglobin and showed that a glutamic
acid at position 6 was replaced by a valine in sickle cell disease. Using the
known information about amino acids and the codons that coded for them, he was
able to predict the mutation in sickle cell disease. This made sickle cell
disease the first known genetic disorder. Preventive Treatment for Sickle Cell
Disease Hydroxyurea became the first (and only) drug proven to prevent
complications of sickle cell disease in the Multicenter Study of Hydroxyurea in
Sickle Cell Anemia, which was completed in 1995. How Does Sickle Cell Cause
Disease? The Mutation in Hemoglobin Sickle cell disease is a blood condition
primarily affecting people of African ancestry. The disorder is caused by a
single change in the amino acid building blocks of the oxygen-transport protein,
hemoglobin. This protein, which is the component that makes red cells
"red", has two subunits. The alpha subunit is normal in people with
sickle cell disease. The ß-subunit has the amino acid valine at position 6
instead of the glutamic acid that is there normally. The alteration is the basis
of all the problems that occur in people with sickle cell disease. The schematic
diagram shows the first eight-of the 146 amino acids in the ß-globin subunit of
the hemoglobin molecule. The amino acids of the hemoglobin protein are
represented as a series of linked, colored boxes. The lavender box represents
the normal glutamic acid at position 6. The dark green box represents the valine
in sickle cell hemoglobin. The other amino acids in sickle and normal hemoglobin
are identical. The molecule, DNA (deoxyribonucleic acid), is the fundamental
genetic material that determines the arrangement of the amino acid building
blocks in all proteins. Segments of DNA that code for particular proteins are
called genes. The gene that controls the production of the ß-subunit of
hemoglobin is located on one of the 46 human chromosomes (chromosome #11).
People have twenty-two identical chromosome pairs (the twenty-third pair is the
unlike X and Y-chromosomes that determine a person's sex). One of each pair is
inherited from the father, and one from the mother. Occasionally, a gene is
altered in the exchange between parent and offspring. This event, called
mutation, occurs extremely infrequently. Therefore, the inheritance of sickle
cell disease depends totally on the genes of the parents. If only one of the ß-globin
genes is the "sickle" gene and the other is normal, the person is a
carrier. The condition is called sickle cell trait. With a few rare exceptions,
people with sickle cell trait are completely normal. If both ß-globin genes
code for the sickle protein, the person has sickle cell disease. Sickle cell
disease is determined at conception, when a person acquires his/her genes from
the parents. Sickle cell disease cannot be caught, acquired, or otherwise
transmitted. The hemoglobin molecule (made of alpha and ß-globin subunits)
picks up oxygen in the lungs and releases it when the red cells reach peripheral
tissues, such as the muscles. Ordinarily, the hemoglobin molecules exist as
single, isolated units in the red cell, whether they have oxygen bound or not.
Normal red cells maintain a basic disc shape, whether they are transporting
oxygen or not. The picture is different with sickle hemoglobin. Sickle
hemoglobin exists as isolated units in the red cells when they have oxygen
bound. When sickle hemoglobin releases oxygen in the peripheral tissues,
however, the molecules tend to stick together and form long chains or polymers.
These polymers distort the cell and cause it to bend out of shape. When the red
cells return to the lungs and pick up oxygen again, the hemoglobin molecules
resume their solitary existence (the left of the diagram). A single red cell may
traverse the circulation four times in one minute. Sickle hemoglobin undergoes
repeated episodes of polymerization and depolymerization. This
"Ping-Pong" alteration in the state of the molecules damages the
hemoglobin and ultimately the red cell itself. Polymerized sickle hemoglobin
does not form single strands. Instead, the molecules group in long bundles of 14
strands each that twist in a regular fashion, much like a braid. These bundles
self-associate into even larger structures that stretch and distort the cell. An
analogy would be a water ballon that formed ice sickles that extended and
stretched the ballon. The stretching of the rubber of the ballon is similar to
what happens to the membrane of the red cell. Despite their imposing appearance,
the forces that hold these sickle hemoglobin polymers together are very weak.
The abnormal valine amino acid at position 6 in the ß-globin chain interacts
weakly with the ß globin chain in an adjacent sickle hemoglobin molecule. The
complex twisting, 14-strand structure of the bundles produces multiple
interactions and cross-interactions between molecules. On the other hand, the
weak nature of the interaction opens one strategy to treat sickle cell disease.
Some types of hemoglobin molecules, such as that found before birth (fetal
hemoglobin), block the interactions between the hemoglobin S molecules. All
people have fetal hemoglobin in their circulation before birth. Fetal hemoglobin
protects the unborn and newborns from the effects of sickle cell hemoglobin.
Unfortunately, this hemoglobin disappears within the first year after birth. One
approach to treating sickle cell disease is to rekindle production of fetal
hemoglobin. The drug, Hydroxyurea induces fetal hemoglobin production in some
patients with sickle cell disease and improves the clinical condition of some
patients. The Sickle Red Cell The schematic diagram shows the changes that occur
as sickle or normal red cells release oxygen in the microcirculation. The upper
panel shows that normal red cells retain their biconcave shape and move through
the microcirculation (capillaries) without problem. In contrast, the hemoglobin
polymerizes in sickle red cells when they release oxygen, as shown in the lower
panel. The polymerization of hemoglobin deforms the red cells. The problem,
however, is not simply one of abnormal shape. The membranes of the cells are
rigid due in part to repeated episodes of hemoglobin polymerization/depolymerization
as the cells pick up and release oxygen in the circulation. These rigid cells
fail to move through the microcirculation, blocking local blood flow to a
microscopic region of tissue. Amplified many times, these episodes produce
tissue hypoxia (low oxygen supply). The result is pain, and often damage to
organs. The damage to red cell membranes plays an important role in the
development of complications in sickle cell disease. Robert Hebbel at the
University of Minnesota and colleagues were among the first workers to show that
the heme component of hemoglobin tends to be released from the protein with
repeated episodes of sickle hemoglobin polymerization. Some of this free heme
lodges in the red cell membrane. The iron in the center of the heme molecule
promotes formation of very dangerous compounds, called oxygen radicals. These
molecules damage both the lipid and protein components of the red cell membrane.
As a consequence, the membranes become stiff. Also, the damaged proteins tend to
clump together to form abnormal clusters in the red cell membrane. Antibodies
develop to these protein clusters, leading to even more red cell destruction (hemolysis).
Red cell destruction or hemolysis causes the anemia in sickle cell disease. The
production of red cells by the bone marrow increases dramatically, but is unable
to keep pace with the destruction. Red cell production increases by five to
ten-fold in most patients with sickle cell disease. The average half-life of
normal red cells is about 40 days. In-patients with sickle cell disease, this
value can fall to as low as four days. The volume of "active" bone
marrow is much expanded in-patients with sickle cell disease relative to nomal
in response to demands for higher red cell production. The degree of anemia
varies widely between patients. In general, patients with sickle cell disease
have hematocrits that are roughly half the normal value (e.g., about 25%
compared to about 40-45% normally). Patients with hemoglobin SC disease (where
one of the ß-globin genes codes for hemoglobin S and the other for the variant,
hemoglobin C) have higher hematocrits than do those with homozygous Hb SS
disease. The hematocrits of patients with Hb SC disease run in low- to
mid-thirties. The hematocrit is normal for people with sickle cell trait. How Do
People Get Sickle Cell Disease? Sickle cell disease is an inherited condition.
The genes received from one's parents before birth determine whether a person
will have sickle cell disease. Sickle cell disease cannot be caught or passed on
to another person. The severity of sickle cell disease varies tremendously. Some
people with sickle cell disease lead lives that are nearly normal. Others are
less fortunate, and can suffer from a variety of complications. How Are Genes
Inherited? At the time of conception, a person receives one set of genes from
the mother (egg) and a corresponding set of genes from the father (sperm). The
combined effects of many genes determine some traits (hair color and height, for
instance). One gene pair determines other characteristics. Sickle cell disease
is a condition that is determined by a single pair of genes (one from each
parent). Inheritance of Sickle Cell Disease The genes are those which control
the production of a protein in red cells called hemoglobin. Hemoglobin binds
oxygen in the lungs and delivers it to the peripheral tissues, such as the
liver. Most people have two normal genes for hemoglobin. Some people carry one
normal gene and one gene for sickle hemoglobin. This is called "sickle cell
trait". These people are normal in almost all respects. Problems from the
single sickle cell gene develop only under very unusual conditions. People who
inherit two genes for sickle hemoglobin (one from each parent) have sickle cell
disease. With a few exceptions, a child can inherit sickle cell disease only if
both parents have one gene for sickle cell hemoglobin. The most common situation
in which this occurs is when each parent has one sickle cell gene. In other
words, each parent has sickle cell trait. Figure 1 shows the possible
combination of genes that can occur for parents each of whom has sickle cell
trait. Figure 1. (ABOVE) Inheritance of sickle genes from parents with sickle
cell trait. As shown in the graphic, the couple has one chance in four that the
child will be normal, one chance in four that the child will have sickle cell
disease, and one chance in two that the child will have sickle cell trait. A
one-in-four chance exists that a child will inherit two normal genes from the
parents. A one-in-four chance also exists that a child will inherit two sickle
cell genes, and have sickle cell disease. A one-in-two chance exists that the
child will inherit a normal gene from one parent and a sickle gene from the
other. This would produce sickle trait. These probabilities exist for each child
independently of what happened with prior children the couple may have had. In
other words, each new child has a one-in-four chance of having sickle cell
disease. A couple with sickle cell trait can have eight children, none of whom
have two sickle genes. Another couple with sickle trait can have two children
each with sickle cell disease. The inheritance of sickle cell genes is purely a
matter of chance and cannot be altered. Do Factors Other Than Genes Influence
Sickle Cell Disease? Sickle cell disease is quite variable in itself. Other
blood conditions can influence sickle cell disease, however. One of the most
important is thalassemia. One form of thalassemia, called ß-thalassemia,
reduces the production of normal hemoglobin. A person with one normal hemoglobin
gene and one thalassemia gene has thalassemia trait (also called thalassemia
minor). Parents who have sickle cell trait and thalassemia trait have one chance
in four of having a child with one gene for sickle cell disease and one gene for
ß-thalassemia (Figure 2). This condition is sickle ß-thalassemia. The severity
varies. Some patients with sickle ß-thalassemia have a condition as severe as
sickle cell disease itself. People of Mediterranean origin who have a sickle
condition most often have sickle ß-thalassemia. Figure 2. (BELOW ON LAST PAGE)
Inheritance of hemoglobin genes from parents with sickle cell trait and
thalassemia trait. As illustrated, the couple has one chance in four that the
child will have the genes both for sickle hemoglobin and for thalassemia. The
child would have sickle ß-thalassemia. The severity of this condition is quite
variable. The nature of the thalassemia gene (ßo or ß+) greatly influences the
clinical course of the disorder. Another disorder that interacts with sickle
cell disease is "hemoglobin SC disease". The abnormal hemoglobin C
gene is relatively harmless. Even people with two hemoglobin C genes have a
relatively mild clinical condition. When hemoglobin C combines with hemoglobin
S, the result is "hemoglobin SC disease". On average, hemoglobin SC
disease is milder than sickle cell disease. However, some patients with
hemoglobin SC disease have a clinical condition as severe as any with sickle
cell disease. The reason for the marked variability in the clinical course of
hemoglobin SC disease is unknown. We do know that the tendency of hemoglobin C
to produce red cell dehydration is a major reason that the combination of
hemoglobins S and C is so problematic. Figure 3. (ABOVE) Inheritance of
hemoglobin genes from parents with sickle cell trait and hemoglobin C trait. As
illustrated, the couple has one chance in four that the child will have the
genes both for sickle hemoglobin and for hemoglobin C. The child would have
hemoglobin SC disease. Most patients with hemoglobin SC disease have a milder
condition than occurs with sickle cell disease (two sickle genes).
Unfortunately, some patients run a clinical course that is undistinguishable
from sickle cell disease. Are There Tests That Can Tell Me Whether I Have Sickle
Cell Trait? The answer is yes. Routine "blood counts" commonly
performed in doctors' offices do not give hints about the presence of sickle
cell trait. The blood counts of most people with sickle cell trait are normal.
Only a special test, called a "hemoglobin electrophoresis" indicates
reliably whether a person has sickle trait. In addition, the hemoglobin
electrophoresis will detect hemoglobin C and ß-thalassemia. How Can I Be Tested
for Sickle Cell Trait? Most large hospitals and clinics can perform routine
hemoglobin electrophoresis. Smaller laboratories send the test to commercial
firms for testing. If you are concerned about the possibility of having sickle
cell trait, you should speak with your doctor. Overview Everyone with sickle
cell disease shares the same gene mutation. A thymine replaces an adenine in the
DNA encoding the ß-globin gene. Consequently, the amino acid valine replaces
glutamic acid at the sixth position in the ß-globin protein product. The change
produces a phenotypically recessive characteristic. Most commonly sickle cell
disease reflects the inheritance of two mutant alleles, one from each parent.
The final product of this mutation, hemoglobin S is a protein whose quaternary
structure is a tetramer made up of two normal alpha-polypeptide chains and two
aberrant ßs-polypeptide chains. The primary pathological process leading
ultimately to sickle shaped red blood cells involves this molecule. After
deoxygenation of hemoglobin S molecules, long helical polymers of HbS form
through hydrophobic interactions between the ßs-6 valine of one tetramer and
the ß-85 phenylalanine and ß-88 leucine of an adjacent tetramer. Deformed,
sickled red cells can occlude the microvascular circulation, producing vascular
damage, organ infarcts, painful crises and other such symptoms associated with
sickle cell disease. Although everyone with sickle cell disease shares a
specific, invariant genotypic mutation, the clinical variability in the pattern
and severity of disease manifestations is astounding. In other genetic disorders
such as cystic fibrosis, phenotypic variability between patients can be traced
genotypic variability. Such is not the case, however, with sickle cell disease.
Physicians and researchers have sought explanations of the variability
associated with the clinical expression of this disease. The most likely causes
of this inconstancy are disease-modifying factors. I have reviewed the role of
some of these factors, and tried to ascertain the clinical importance of each.
Fetal Hemoglobin Augmented post-natal expression of fetal hemoglobin is perhaps
the most widely recognized modulator of sickle cell disease severity. Fetal
hemoglobin, as its name implies is the primary hemoglobin present in the fetus
from mid to late gestation. The protein is composed of two alpha-subunits and
two gamma-subunits. The gamma-subunit is a protein product of the ß-gene
cluster. Duplicate genes duplicate upstream of the ß-globin gene encodes fetal
globin. Fetal hemoglobin binds oxygen more tightly than does adult hemoglobin A.
The characteristic allows the developing fetus to extract oxygen from the
mother's blood supply. After birth, this trait is no longer necessary and the
production of the gamma-subunit decreases as the production of the ß-globin
subunit increases. The ß-globin subunit replaces the gamma-globin subunit in
the hemoglobin tetramer so that eventually adult hemoglobin replaces fetal
hemoglobin as the primary component red cells. HbF levels stabilize during the
first year of life, at less than 1% of the total hemoglobin. In cases of
hereditary persistence of fetal hemoglobin, that percentage is much higher. This
persistence substantially ameliorates sickle cell disease severity. Mechanism of
Protection Two properties of fetal hemoglobin help moderate the severity of
sickle cell disease. First, HbF molecules do not participate in the
polymerization that occurs between molecules of deoxyHbS. The gamma-chain lacks
the valine at the sixth residue to interact hydrophobically with HbS molecules.
HbF has other sequence differences from HbS that impede polymerization of
deoxyHbS. Second, higher concentrations of HbF in a cell infer lower
concentrations of HbS. Polymer formation depends exponentially on the
concentration of deoxyHbS. Each of these effects reduces the number of
irreversibly sickle cells (ISC). Hemoglobin F Levels and Amelioration of Sickle
Cell Disease The level of HbF needed to benefit people with sickle cell disease
is a key question to which different studies supply varying answers. Bailey
examined the correlation between early manifestation of sickle cell disease and
fetal hemoglobin level in Jamaicans. They concluded that moderate to high levels
of fetal hemoglobin (5.4-9.7% to 39.8%) reduced the risk for early onset of
dactylics, painful crises, acute chest syndrome, and acute splenic
sequestration. Platt examined predictive factors for life expectancy and risk
factors for early death (among Black Americans). In their study, a high level of
fetal hemoglobin (*8.6%) augured improved survival. Koshy et al. reported that
fetal hemoglobin levels above 10% were associated with fewer chronic leg ulcers
in American children with sickle cell disease. Other studies, however, suggest
that protection from the ravages of sickle cell disease occur only with higher
levels of HbF. In a comparison of data from Saudi Arabs and information from
Jamaicans and Black Americans, Perrine et al. found that serious complications
occurred only 6% to 25% as frequently in Saudi Arabs as North American Blacks.
In addition mortality under the age of 15 was 10% as great among Saudi Arabs.
Further, these patients experienced no leg ulcers, reticulocyte counts were
lower and hemoglobin levels were higher on average. The average a fetal
hemoglobin level in the Saudi patients ranged between 22-26.8%. This is more
than twice that reported in studies mentioned above. Kar et al. compared
patients from Orissa State, India to Jamaican patients with sickle cell. These
patients also had a more benign course when compared with Jamaican patients. The
reported protective level of fetal hemoglobin in this study was on average
16.64%, with a range of 4.6% to 31.5%. Interestingly, ß-globin locus haplotype
analysis shows that the Saudi HbS gene and that in India have a common origin
(see below). These studies suggest that the level of fetal hemoglobin that
protects against the complications of sickle cell disease depend strongly on the
population group in question. Among North American blacks, fetal hemoglobin
levels in the 10% range ameliorate disease severity. The higher average level of
fetal hemoglobin could contribute to the generally less severe disease in
Indians and Arabs. Another study that suggests only a small role at best for
fetal hemoglobin as a modifier of sickle cell disease severity was reported by
El-Hazmi. The subjects were Saudi Arabs in whom a variety of symptoms associated
with sickle cell disease were assessed to form a "severity" index. The
author concluded that among his patients, no correlation existed between HbF and
the severity index. However, his analysis has a fundamental flaw. El-Hazmi
failed to examine the effect of HbF on each of these symptoms individually.
Their important information and an association between fetal hemoglobin levels
specific disease manifestations could be concealed in his data. However, the
study reinforces the conclusion that fetal hemoglobin levels most likely work in
conjunction with other moderating factors to determine clinical severity
in-patients with sickle cell disease. Alpha-Thalassemia Concurrent alpha-thalassemia
has also been examined as a modifier of sickle cell disease severity. Alpha-thalassemia,
like sickle cell disease, is a genetically inherited condition. The loss of one
or more of the four genes encoding the alpha globin chain (two each on
chromosome 16) produces alpha-thalassemia. A gene deletion most commonly is at
fault. The deletion results from unequal crossover between adjacent alpha-globin
genes during the prophase I of meiosis I. Such a crossover leaves one gamete
with one alpha-gene and the other gamete with three alpha genes. Upon
fertilization the zygote can have 2, 3, 4, or 5 alpha genes depending on the
make up of the other parental gamete. In people of African descent, the most
common haploid gamete of this type is alpha-thal-2 in which there is one
deletion on each of the number 16 chromosomes in the patient. Heterozygotes for
this allele, therefore, have three alpha genes (one alpha gene on one of the
number 16 chromosomes, two alpha genes on the other). Embury et al. (1984)
examined the effect of concurrent alpha-thalassemia and sickle cell disease.
Based on prior studies, they proposed that alpha-thalassemia reduces
intraerythrocyte HbS concentration, with a consequent reduction in
polymerization of deoxyHbS and hemolysis. They investigated the effect of alpha
gene number on properties of sickle erythrocytes important to the hemolytic and
rheological consequences of sickle cell disease. Specifically they looked for
correlations between the alpha gene number and irreversibly sickled cells, the
fraction of red cells with a high hemoglobin concentration (dense cells), and
red cells with reduced deformabilty. The investigators found a direct
correlation between the number of alpha-globin genes and each of these indices.
A primary effect of alpha-thalassemia was reduction in the fraction of red blood
cells that attained a high hemoglobin concentration. These dense cells result
from potassium loss due to acquired membrane leaks. The overall deformability of
dense RBCs is substantially lower than normal. This property of alpha-thalassemia
was confirmed by comparison of red cells in people with or without 2-gene
deletion alpha-thalassemia (and no sickle cell genes). The cells in the
nonthalassemic individuals were denser than those from people with 2-gene
deletion alpha-thalassemia. The difference in median red cell density produced
by alpha-thalassemia was much greater in-patients sickle cell disease. Reduction
in overall hemoglobin concentration due to absent alpha genes is not the only
mechanism by which alpha-thalassemia reduces the formation of dense and
irreversibly sickled cells. In reviewing the available literature, Embry and
Steinburg suggested that alpha-thalassemia moderate's red cell damage by
increasing cell membrane redundancy. This protects against sickling-induced
stretching of the cell membrane. Potassium leakage and cell dehydration would be
minimized. These two papers by Embury et al. give some insight into the
moderation of sickle cell disease severity by alpha thalassemia. Some
deficiencies exist, nonetheless. The first paper makes no mention of the patient
pool. Unspecified are the number of patients used, their ethnicity, or their
state of health when blood samples were taken. This information would help
establish the statistical reliability of the data, and its applicability across
patient groups. Despite these limitation, the work provides important insight
into the mechanisms by which alpha-thalassemia ameliorates sickle cell disease
severity. Ballas et al reached different conclusions regarding alpha thalassemia
and sickle cell disease than did Embury et al . They reported that decreased red
blood cell deformability was associated with reduced clinical severity of sickle
cell disease. Patients with more highly deformabile red cells had more frequent
crises. They also found that fewer dense cells and irreversible sickle cells
correlated inversely with the severity of painful crises. Like Embury et al.,
Ballas and colleagues found alpha thalassemia was associated with fewer dense
red cells. In addition, Ballas' group found that alpha thalassemia was
associated with less severe hemolysis. However they reached no clear conclusion
concerning alpha gene number and deformability of RBC except to note that the
alpha thalassemia was associated with less red cell dehydration. The two studies
are not completely at odds. Both state that concurrent alpha-thalassemia reduces
hemolytic anemia. They agree that this occurs through reduction in the number of
dense cells, a number directly related to the fraction of irreversibly sickled
cells. Embury et al. concludes that through this mechanism red blood cell
deformability is increased. The investigators diverge, however, on the
relationship to clinical severity of dense cells and rigid cells. Ballas et al.
asserts that both the reduction of dense cells and rigid cells contribute to
disease severity.