reliably whether a person has sickle trait. In addition, the hemoglobin
electrophoresis will detect hemoglobin C and ?-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 ?-globin gene. Consequently, the amino acid valine replaces
glutamic acid at the sixth position in the ?-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 ?s-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 ?s-6 valine of one tetramer and
the ?-85 phenylalanine and ?-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 ?-gene
cluster. Duplicate genes duplicate upstream of the ?-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 ?-globin
subunit increases. The ?-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, ?-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.
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