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Blood, Vol. 91 No. 5 (March 1), 1998:
pp. 1777-1783
Intracellular Hemoglobin S Polymerization and the Clinical
Severity of Sickle Cell Anemia
By
William N. Poillon,
Bak C. Kim, and
Oswaldo Castro
From the Center for Sickle Cell Disease and Department of Pediatrics
and Child Health, Howard University College of Medicine, Washington,
DC.
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ABSTRACT |
Recent work has enabled us to quantitate the four
variables (2,3-DPG concentration, pHi, non-S
hemoglobin composition, and O2 saturation) that modulate
the equilibrium solubility (csat) of Hb S inside sickle
erythrocytes (SS RBCs). Using measured values of mean corpuscular
hemoglobin concentration (MCHC), 2,3-DPG concentration, and %Hb (F+A2), along with estimates of pHi
and the  csat due to partial oxygenation of SS RBCs in
the microcirculation, we calculated the mean polymer fraction
(fp) in erythrocytes from 46 SS homozygotes. Values of
fp derived from the conservation of mass equation ranged
from 0.30 to 0.59. MCHC and %Hb F were major determinants of the
magnitude of fp; 2,3-DPG concentration and pHi
also contributed, but to a lesser extent. A clinical severity score
(CSS) was assigned to each patient based on mean hospitalization rate.
There was a weak, but statistically significant, negative correlation
between fp and steady state hematocrit (P = .017), but none between fp and whole blood hemoglobin
concentration (P = .218). Although there was no correlation
between fp and mean number of hospitalization days per
year, patients with the greatest number of admissions and
hospitalization days were found only among those who had an
fp > 0.45. All five patients who died during the
follow-up period (median, 7 years; range, 3 to 10 years) had fp values  0.48. However, patients with few admissions,
low hospitalization days, and long survivals occurred at all
fp levels. These results suggest that the clinical course
of homozygous SS disease cannot be predicted by mean fp
calculations, which assume a homogeneous distribution of the five
variables that modulate intraerythrocytic polymerization. A
heterogeneous distribution is more likely; so the amount of polymerized
Hb S could vary considerably among cell populations. Factors such as
membrane abnormalities and endothelial cell interactions may also
contribute to clinical severity.
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INTRODUCTION |
THE PRIMARY CAUSE of the clinical
symptomatology of sickle cell anemia is the intracellular
polymerization of sickle hemoglobin (Hb S) that occurs when sickle
erythrocytes (SS RBCs) are partially deoxygenated under the hypoxic
conditions of the microcirculation. This, in turn, makes SS RBCs less
deformable and ultimately results in the debilitating microvascular
occlusions and hemolytic anemia characteristic of the disease. Recent
work1-3 has enabled us to quantitate the four variables
(2,3-DPG concentration, pHi, non-S hemoglobin
composition, and O2 saturation) that modulate the
equilibrium solubility (csat) of Hb S inside sickle
erythrocytes (SS RBCs).
The requirements for therapeutic inhibition of Hb S gelation have been
set forth by Sunshine et al,4 who showed that there is a
relationship between the kinetics of polymerization (td) and the solubility (csat) under various cellular
conditions. They also showed the extent by which td must be
increased to provide an amelioration of homozygous sickle cell disease
to the less severe clinical conditions of S/ +-thal,
S/HPFH, and A/S trait. Although there was a strong
correlation with non-Hb S hemoglobin composition, there was only a weak
one at constant hemoglobin concentration. A detailed analysis of the nucleation-controlled polymerization that underlies sickling has been
elucidated by these same investigators.5,6
Attempts by a variety of investigators7-16 to correlate
hematologic parameters with the clinical severity of various sickling disorders have, for the most part, been only partially successful. Some
earlier studies used only painful crises to score illness severity.17-19 Others20,21 used a composite
vasoocclusive severity score to assign weighted values to the presence
of various vasoocclusive events in a cohort of patients. This
assessment of clinical severity showed no correlation with any
laboratory parameters; however, the clinical severity score (CSS) and
erythrocyte adherence to endothelial cells were strongly
correlated.22
Eaton et al23 showed in a series of reports that the
polymer content of the sickle erythrocyte depends on hemoglobin
concentration and composition,24 as well as O2
saturation.25 The quantitative relationships among these
three cellular variables and the equilibrium aggregation of deoxy-Hb S
may be found in Eaton and Hofrichter.5,6 Subsequently,
Noguchi et al,26-31 using the conservation of mass equation
of Ross et al32 and the activity coefficients of Ross and
Minton,33 were able to quantitate intraerythrocytic polymer fraction (fp) by a theoretical analysis of Hb S solubility
that accounted for these three cellular variables. A series of reports that used this framework to quantitate intracellular polymer content in
various sickling syndromes,31,34 as well as in erythrocytes from individual patients,35 has shown an inverse
correlation between fp and whole blood hemoglobin
concentration, a hematologic index of hemolytic severity. Furthermore,
in the latter study,35 a visual analogue scale (VAS) was
used on a cohort of 30 patients to indicate perceived disease severity.
The VAS score showed significant positive correlations with the
calculated values of fp at both the venous pO2
(40 mm Hg) and P50.
In addition to the three major determinants of the polymerization
tendency of sickle erythrocytes cited above, we have shown that two
other cellular variables (2,3-diphosphoglycerate [2,3-DPG] concentration and intracellular pH [pHi]) exert separate,
but interdependent, effects on the equilibrium solubility
(csat) of unliganded Hb S.1 In a separate
study, we were able to quantitate the sparing effect of non-S
hemoglobins on the solubility of partially liganded Hb S in the region
of pathophysiologic interest (25% to 70% saturation2). It
was found that hemoglobins F and A2 are equipotent in their
effects on csat, as are hemoglobins A and C, but to a
lesser extent. Thus, all five cellular variables (2,3-DPG,
pHi, non-S hemoglobins, O2 saturation, and mean
corpuscular hemoglobin concentration [MCHC]) that determine the
polymerization tendency of sickle erythrocytes can now be quantitated
by use of the requisite experimental values.3
Our earlier study3 showed that depletion of intracellular
2,3-DPG produced a consistent reduction in the sickling tendency of
erythrocytes from four sickle cell anemia patients with widely differing hematologic features. Furthermore, estimates were made of the
decrease in fp evoked by loss of 2,3-DPG by using
appropriate values of the four variables that interact to affect
polymerization in a 2,3-DPG-dependent manner (2,3-DPG concentration,
pHi, O2 saturation, and MCHC).
In the present study, we used similar laboratory data, plus those for
non-S hemoglobin composition, for a cohort of 46 homozygous sickle cell
anemia patients, to quantitate all five cellular variables that
modulate the intracellular polymer content of SS RBCs. We then
attempted to correlate the values of fp so obtained with hemolytic and vasoocclusive severity for each patient.
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MATERIALS AND METHODS |
General methods.
Venous blood from 46 SS patients in the steady state was collected in
standard heparin Vacutainer tubes and sampled within 2 hours. Informed
consent was obtained before blood collection. All patients were adults,
age 18 years or greater (mean age, 30.4 ± 8.9 years; range, 18 to
51 years). The male to female ratio was 0.64. No patient had undergone
a blood transfusion for at least at least 3 months before blood
collection. Hemoglobin concentrations were measured with Drabkin's
reagent36 and used, along with the spun hematocrit, to
calculate MCHC. Red blood cell adenosine triphosphate
(ATP) and 2,3-DPG concentrations were measured using Sigma kits (Sigma,
St Louis, MO). Hemoglobins F and A2 were
quantitated in hemolysates by alkali denaturation37 and
elution from diethylaminoethyl (DEAE)-cellulose
columns,38 respectively. Intracellular pH (pHi) was estimated by use of the factor relating pHi to 2,3-DPG
concentration.3
Mean polymerization tendency of sickle erythrocytes from individual
patients.
A quantitative approach to sickle cell disease severity must take into
account the extent of polymer formation at equilibrium in the
circulating erythrocytes of individual SS patients. Of the five
cellular variables that determine intracellular polymer content,3 three (2,3-DPG concentration, % non-S
hemoglobins [F and A2], and MCHC) were measured directly.
Values of pHi were estimated from the 2,3-DPG concentration
and the loss of Bohr protons due to partial ligation and
depolymerization at the O2 tension of the microcirculation
(pO2 = 20 mm Hg1). The O2
saturation corresponding to this partial pressure ( 25%) was assumed
to be the same for each patient's erythrocytes. Knowledge of four of these five variables (2,3-DPG concentration, pHi, non-S Hb
composition, and O2 saturation) permits one to determine
csat, the equilibrium solubility of intraerythrocytic Hb S
at 25% O2 saturation, without measuring it
directly.3
One can derive increments of csat for each of these four
parameters by use of various empirical relationships we deduced (ie, the interdependence of 2,3-DPG concentration and
pHi1,3 and the sparing effect of non-S
hemoglobins2,24,39) and the effect of
ligation on solubility deduced by others.23 Thus: csat = csato + csat2,3-DPG + csatHb(F+A2) + csat25%O2 satn. The relevant
csat is then obtained as the sum of the increments due to
cellular modulators of solubility plus csato,
the intraerythrocytic solubility for unliganded, 2,3-DPG-saturated Hb S
at pH 7.41, which has a value of 18.0 g/dL (this baseline solubility
was deduced in Poillon and Kim1).
For our patient with the lowest fp (0.30), each of these
increments can be estimated: keeping in mind that the Bohr effect for
SS red blood cells is about twice that for AA red blood cells (ie,
 0.99 and 0.42, respectively40) and that this
translates into a loss of approximately four and two Bohr protons on
complete ligation, one can estimate the pH decrement for 25%
oxygenation of SS red blood cells as follows: the Bohr protons released
at this saturation = 25/89 × 4H+/tetramer = 1.12H+/tetramer, where 89% is the O2
saturation at which fp = 0. Then pH/H+ = 0.28 × 1.12/4 = 0.078 pH unit, where 0.28 is
the pH change for release of all four Bohr protons at 89%
O2 saturation1 and csat/pH = 10.8 × 0.078 = 0.85 g/dL (where 10.8 g/dL is the
increment in csat per pH unit1).
Effects of non-S hemoglobins (F and A2) and the degree of
ligation on csat are as follows:
csat/Hb(F+A2) = 0.334 × 18.5 = 6.18 g/dL (where 0.334 is the csat increment for 1%
Hb[F+A2]2 and 18.5 is the % Hb[F+A2] for this patient; csat/25%
O2 saturation = 2.46 g/dL [derived from the empirical
relationship between csat and O2 saturation
deduced in Sunshine et al25]). Thus, the overall
solubility at 25% O2 saturation for Hb S in SS
erythrocytes from this particular patient is: 18.0 0.85 + 6.18 + 2.46 = 25.8 g/dL.
The conservation of mass equation fp = cp(ct csat)/ct(cp csat) shows the relationship among three intracellular
concentrations in determining the polymer content of sickle
erythrocytes at any degree of oxygenation: cp, the polymer
concentration, 69.3 g/dL (taken from Sunshine et al24);
ct, the intracellular hemoglobin concentration or MCHC,
which has a value of 31.8 g/dL for this particular patient; and
csat, the equilibrium solubility, which has a value of 25.8 g/dL here. Substitution of these values into the conservation of mass
equation gives fp = 0.30.
This equation has general use for estimating the mean fp
for unfractionated SS erythrocytes. That is, the two variables that show the strongest correlation with fp (see Results) are
non-S hemoglobin composition and MCHC. Thus, one must have accurate values for these parameters to obtain a reliable estimate of
fp. For the other two parameters, O2 saturation
is held constant at 25% (corresponding to a csat of
2.46 g/dL) and the decrement in pH evoked by 2,3-DPG and H+
loss at 25% O2 saturation (0.078 pH unit) is
considered constant and corresponds to a decrement in csat
of 0.85 g/dL.
Our findings that polymer fraction correlates well with both Hb F (Fig
1) and MCHC (Fig 2) have been shown by us2 and by others.24,39 Although these are strong associations, the
simultaneous variation in other parameters that influence
fp may introduce a certain amount of noise to the overall
expression of polymer fraction.
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| Fig 1.
Polymer fraction fp versus Hb F concentration
for a cohort of homozygous SS patients (n = 46). Calculation of
fp is described in the text. Linear regression analysis of
these data gave values of r = 0.710 and P < 10 4, indicating a highly significant association between
these variables.
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| Fig 2.
fp versus intracellular hemoglobin
concentration (r = 0.677, P < 104,
indicating a highly significant association between these variables).
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Assessment of clinical severity.
Because painful crisis is the most common acute event experienced by SS
patients, the number of pain episodes requiring hospitalization and
administration of narcotic analgesics was used to assess clinical severity for the cohort of 46 patients who were followed for up to 10 years (1986 to 1995). The median follow-up time was 7 years, with a
range of 3 to 10 years. The mean number of admissions and mean
hospitalization days were determined retrospectively. For the purposes
of this study, we assumed each hospital admission to be due to painful
crisis because 87% of all hospital discharges of sickle cell patients
listed sickle cell crisis as a discharge diagnosis during the follow-up
period. A modification of the vasoocclusive severity score of Hebbel et
al20 was used to assign an index of clinical severity for
each patient. Our CSS was assigned solely on the basis of
hospitalizations and did not include contributions from organ
dysfunction due to specific vasoocclusive events. To compute the
average number of hospitalizations per year, we divided the number of
hospital admissions during the follow-up period (regardless of length
of hospital stay) by the number of years of follow-up. Then, CSS (Table
1, column 10) was assigned as follows: 15 patients with no
hospitalizations or less than one per year, 0 points; those with means
of 1 to 5 per year (25 patients), 1 point; 6 to 10 per year (5 patients), 2 points; more than 10 per year (1 patient), 3 points. A
second clinical severity measurement was also used: the mean number of
hospital days per year (Table 1, column 9), obtained by dividing total
number of days each subject was an inpatient during the follow-up
period by the number of years of follow-up.
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Table 1.
Mean Values of Red Blood Cell Variables That Affect
Polymerization and Clinical Severity for a Cohort of Adult Sickle
Cell Anemia Patients
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Statistical methods.
Analyses of variance were performed and used unpaired Student's
t-test (two tailed). To show linear relationships between fp and other variables, both ordinary least squares linear
regression and nonparametric tests (Spearman rank correlation) were
performed. These tests then generated correlation coefficients and
P values (two tailed) for the slope. Except where noted, all
correlation coefficients and P values are reported for ordinary
linear regression.
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RESULTS |
Mean values for laboratory and clinical parameters.
Because of
its length, the table showing composite laboratory and clinical data
for the entire cohort of 46 SS patients is not shown here in full
detail. Instead, we have compiled in Table 1 the mean values for three of the five cellular variables (MCHC, 2,3-DPG concentration, and % Hb[F+A2]) that interact to
determine polymer content, as well as those for whole blood hemoglobin
and ATP concentrations and for hospitalization days and clinical
severity score. The mean values of hematocrit, intracellular hemoglobin concentration (MCHC), %Hb F, and %Hb A2 shown in Table 1
(26.1 ± 4.3, 33.7 ± 1.5 g/dL, 5.5% ± 4.4%, and 3.0% ± 0.6%, respectively) are in the range found in other studies of
this nature.7-15 Mean values for ATP and 2,3-DPG
concentrations are 1.4 ± 0.2 and 6.1 ± 0.8 mmol/L,
respectively. These values are elevated by 25% relative to those
for normal adult blood, as was shown in our earlier study1
and by Steinberg et al.13 The mean value of hospitalization
days per year was 21.8 ± 27.8 and the mean value of CSS was
0.83 ± 0.71.
Dependence of polymer fraction on Hb F concentration and MCHC.
Linear regression plots of fp as a function of %Hb F and
MCHC are shown in Figs 1 and
2, respectively. The corresponding
correlation coefficients were 0.710 and 0.677, and the slopes were
significantly different from zero (P < 10-4) in
each case. Thus, a highly significant correlation exists between
intracellular polymer content and non-S hemoglobin composition, as well
as intracellular hemoglobin concentration. The strong dependence of
polymer fraction on Hb F concentration and MCHC is well known and has
been documented by us2 and by others.24,39
By contrast, for a plot of fp versus 2,3-DPG concentration
(data not shown), there was no association between these two variables (correlation coefficient = 0.199; P = .185).
Hemolytic and clinical severity: Is there a correlation with
polymerization tendency?
An attempt was made to correlate polymer fraction fp with
hematocrit and whole blood hemoglobin concentration, the laboratory parameters that best reflect hemolytic severity in sickle cell anemia.
The linear regression plot of fp versus hematocrit
(Fig 3), gave a correlation coefficient of
0.350 and a P value of .0173 for the slope, indicating a
negative relationship between fp and hematocrit; for the
Spearman rank correlation test, the values were r = 0.403 and P = .0054. The linear regression plot of
fp versus whole blood hemoglobin concentration (Fig
4) also showed a negative correlation coefficient of 0.198.
However, the P value was .187, indicating no statistically
significant association between fp and hemoglobin
concentration in this group of patients; nonparametric tests gave
essentially the same results (r = 0.225; P = .133) indicating no statistically significant association between
fp and hemoglobin concentration in this group of patients.
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| Fig 3.
fp versus hematocrit (r = 0.350,
P = .0173, indicating a significant association between these
variables).
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| Fig 4.
fp versus whole blood hemoglobin
concentration (r = 0.198, P = .187, indicating
no association between these variables).
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An attempt was made to correlate fp with hospitalization
data as an index of clinical severity. The linear regression plot of
hospitalization days/yr versus fp
(Fig 5) gave a correlation coefficient of 0.089 and a P value of .555 for the slope. Thus, there was no statistically significant association between these two
variables. Certain trends were discernible however: (1) No patient with
fp less than 0.45 had a high number of mean hospitalization days per year (Fig 5), and (2) these patients also had CSS values of 0 or 1 and none had values of 2 or 3 (Fig 6).
On the other hand, patients with high fp did not
necessarily have high hospitalization rates; instead, many had few mean
hospital days (Fig 5) and low clinical severity scores (Fig 6). This
suggested that a high fp could be a necessary, but not
sufficient condition, for high vasoocclusive severity. Other factors
such as membrane abnormalities or endothelial adhesion tendency
probably influence severity in patients with high fp, as
well.
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| Fig 5.
Hospitalization days per year versus fp
(r = 0.089, P = .555, indicating no association
between these variables).
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| Fig 6.
Clinical severity score (derived from hospitalization
frequency data [see footnote in Table 1 for details]) versus
fp (no statistical analysis of these data was possible).
The figure shows the distribution of fp for patients
examined over the range of clinical severity observed (0 to 3).
Although all 46 patients were included, some patients (n = 16) with identical CSS and fp appear as a
single point. For this reason, the number of points is less than 46.
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Figure 6 shows a plot of the clinical severity score (see
footnote in Table 1) versus polymer fraction. These data
are not amenable to statistical analysis and are shown only to
demonstrate that values of 0 and 1 for CSS encompass fp
values throughout the range observed (0.30 to 0.59). By contrast, there
were a few patients (six in all) with CSS values of 2 or 3, and these
tended to have high values of fp (0.47 to 0.58).
Six of the 46 patients were lost to follow-up soon after the study. For
the remaining 40 patients, the median follow-up period was 7 years
(range, 3 to 10 years). Five of these patients have died of sickle cell
disease complications. The polymer fractions were 0.48 in erythrocytes
from three of these patients and 0.54 and 0.58 for the other two. No
patient with a polymer fraction less than 0.48 died during the
follow-up period.
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DISCUSSION |
The underlying pathophysiologic events in sickle cell anemia are
chronic hemolysis and microvascular occlusion. The principal clinical
manifestations arising from these events are anemia, acute painful
crises, and organ dysfunction. The marked variability in severity of
symptoms among SS patients has made it difficult to establish a
meaningful clinical severity scoring system for this
disease.41 Although a single measurable parameter for
assessing clinical severity does not exist, a useful index that
assigned weighted scores to the presence of a group of vasoocclusive
events was devised by Hebbel et al20 for this purpose.
Because painful episodes requiring hospitalization are the most
frequent vasoocclusive event in this disease,42 we used
only such hospitalization data to assign a clinical severity score for
each patient evaluated and did not assess the contribution of organ
damage to the clinical picture.
We have been able to quantitate the interaction among the five cellular
variables (2,3-DPG concentration, pHi, non-S hemoglobin composition, O2 saturation, and MCHC) that modulate the
polymerization tendency of sickle erythrocytes (Table 1) in a cohort of
homozygous SS patients (n = 46). There was an excellent correlation
between the calculated values of fp and two parameters:
%Hb F and MCHC (Figs 1 and 2). Thus, these independent cellular
variables influence polymerization tendency in a strong and predictable
fashion: that is, fp varies inversely with Hb F
concentration and directly with MCHC. We next attempted to correlate
the calculated values of fp for these patients with indices
of hemolytic and clinical severity. Linear regression plots of
fp versus hematocrit (Fig 3), whole blood hemoglobin
concentration (Fig 4), and hospitalization
days/yr (Fig 5) were made. An inverse correlation was found between
fp and hematocrit, which would be expected if low values of
fp were associated with lower hemolytic rates and vice
versa. The correlation between fp and whole blood
hemoglobin concentration was not significant (Fig 4). These results are
discrepant with those of Keidan et al,35 who showed a
strong negative correlation between fp and hemoglobin
concentration. Although we can offer no explanation for this anomaly,
it may be due to variation in the number of patients evaluated (this
study, n = 46; Keidan et al, n = 30).
The correlation between fp and vasoocclusive severity
measured by hospitalization data, however, was not good. Whereas
fp > 0.45 appeared to be necessary for a severe clinical
course and perhaps also for short survival, many SS patients with high
fp had a mild clinical course. The only other study in
which a correlation between fp and clinical severity for a
cohort of homozygous SS patients was attempted is that of Keidan et
al35 in which laboratory and clinical parameters for 30 patients were evaluated. In this case, the solubility of mixtures of Hb
S with non-S hemoglobins, as a function of O2 saturation,
was estimated by use of theoretical assumptions regarding the
thermodynamics of gelation.28,30,34 Keidan et
al35 calculated values of fp at pO2 = 0 (0% saturation) and pO2 = 40 mm Hg (corresponding to
the oxygen saturation of the venous circulation); these values
correlated well (P < .05) with a visual analogue scale used
by each patient to indicate perceived disease severity. A strong
inverse correlation (P < .01) was also found between
fp at pO2 = 40 mm Hg and whole blood hemoglobin
concentration, suggesting that polymerization tendency at the venous
pO2 is a determinant of the hemolytic rate in individuals homozygous for Hb S.
We can offer several reasons for the disparity between our findings and
those of Keidan et al.35 First, our calculations of
fp used measured values of the cellular variables that
modulate the solubility of partially liganded Hb S (2,3-DPG
concentration, non-S hemoglobin composition, and MCHC). It is the
interplay of these variables, along with pHi and
O2 affinity, that determine csat, the
solubility of monomeric Hb S inside the partially oxygenated sickle
erythrocyte. Second, our patient population was skewed toward
individuals with the greatest disease severity, as our clinic tends to
attract such patients. Third, different criteria were used to assess
disease severity.
Because the microvascular occlusion that underlies the pathophysiology
of sickle cell anemia is polymerization-dependent, the polymer fraction
of erythrocytes from individual patients should correlate with disease
severity. Our results seem to indicate, however, that the clinical
course of homozygous SS disease in individual patients cannot be
predicted exclusively by fp, which assumes a homogeneous
distribution of the five cellular variables that modulate
intraerythrocytic polymerization. This suggests that the assumption of
a uniform distribution in the cell population of the variables that
modulate polymer formation (intracellular hemoglobin concentration,
2,3-DPG concentration, pHi, non-S hemoglobin composition,
and O2 saturation) may not be valid.26,27
Because polymerization is far from equilibrium in the majority of cells
in circulating erythrocytes,43,44 a
quantitative approach to disease severity would require a kinetic
analysis and knowledge of the distribution of delay times
(td) for intracellular polymerization, which has not yet
been done. However, the well-known supersaturation
relationship45 relates td to ci and
csat [(1/td =   (ci/csat)n, where is a
kinetic constant and n has values of 30 to 40] so that kinetic
parameters should correlate with equilibrium parameters. Distributions
of fp at equilibrium would also be helpful, but such data
are not available. One is left, then, with the measurement of whole
cell average parameters (ie, pHi, 2,3-DPG concentration, non-S hemoglobin composition, and MCHC).
It has been amply demonstrated46,47 that the red blood cell
population in sickle cell anemia is not homogeneous,43,44 but contains cells of widely varying Hb F content, 2,3-DPG, and total
hemoglobin concentration (MCHC). Thus, the amount of polymerized Hb S
varies considerably among the cell population, and our calculated values of fp represent only a weight-mean average.
Accordingly, the red blood cell heterogeneity in individual patients
implies that some cells are much higher in MCHC than others; some are devoid of Hb F, while others are rich in it; and cells show
considerable variability in 2,3-DPG concentration and O2
saturation. The net result is that fp varies considerably
and the mean fp for all cells does not provide a reliable
yardstick to measure disease severity.
| |
FOOTNOTES |
Submitted November 6, 1996;
accepted October 20, 1997.
Address reprint requests to Oswaldo Castro, MD, Center for
Sickle Cell Disease, Howard University, 2121 Georgia Ave, Washington, DC 20059.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. section 1734 solely to indicate this fact.
| |
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Abstract
Introduction
Abstract