Classification of the Disorders of Hemoglobin

 

 

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

Classification of the Disorders of Hemoglobin

Bernard G. Forget and H. Franklin Bunn

Cold Spring Harb Perspect Med

2013; doi: 10.1101/cshperspect.a011684

Subject Collection

Hemoglobin and Its Diseases

Hemoglobin Variants: Biochemical Properties and
Clinical Correlates

The Prevention of Thalassemia
Antonio Cao and Yuet Wai Kan

Christopher S. Thom, Claire F. Dickson, David A.
Gell, et al.

Classification of the Disorders of Hemoglobin

Bernard G. Forget and H. Franklin Bunn

The Switch from Fetal to Adult Hemoglobin

Vijay G. Sankaran and Stuart H. Orkin

Evolution of Hemoglobin and Its Genes

Development of Gene Therapy for Thalassemia

The Molecular Basis of

-Thalassemia

Douglas R. Higgs

Ross C. Hardison

The Search for Genetic Modifiers of Disease
Severity in the

-Hemoglobinopathies

Guillaume Lettre

World Distribution, Population Genetics, and
Health Burden of the Hemoglobinopathies

Thomas N. Williams and David J. Weatherall
Iron Metabolism: Interactions with Normal and
Disordered Erythropoiesis

Tomas Ganz and Elizabeta Nemeth

Pluripotent Stem Cells in Research and Treatment
of Hemoglobinopathies

Natasha Arora and George Q. Daley

Pathophysiology and Clinical Manifestations of
the

-Thalassemias

Arthur W. Nienhuis and David G. Nathan

Arthur W. Nienhuis and Derek A. Persons
-Thalassemia, Mental Retardation, and

Myelodysplastic Syndrome

Richard J. Gibbons
-Thalassemia Intermedia: A Clinical Perspective
Khaled M. Musallam, Ali T. Taher and Eliezer A.
Rachmilewitz

Hematopoietic Stem Cell Transplantation in
Thalassemia and Sickle Cell Anemia

Guido Lucarelli, Antonella Isgrò, Pietro Sodani, et
al.

For additional articles in this collection, see

http://perspectivesinmedicine.cshlp.org/cgi/collection/

Copyright © 2013 Cold Spring Harbor Laboratory Press; all rights reserved

a
b
b
a
b
Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

Classification of the Disorders
of Hemoglobin

Bernard G. Forget1 and H. Franklin Bunn2

1Section of Hematology, Department of Medicine, Yale School of Medicine, New Haven,
Connecticut 06520-8028
2Hematology Division, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical
School, Boston, Massachusetts 02115

Correspondence: hfbunn@rics.bwh.harvard.edu

Over the years, study of the disorders of hemoglobin has served as a paradigm for gaining
insights into the cellular and molecular biology, as well as the pathophysiology, of inherited
genetic disorders. To date, more than 1000 disorders of hemoglobin synthesis and/or struc-
ture have been identified and characterized. Study of these disorders has established
the principle of how a mutant genotype can alter the function of the encoded protein,
which in turn can lead to a distinct clinical phenotype. Genotype/phenotype correlations
have provided important understanding of pathophysiological mechanisms of disease.
Before presenting a brief overview of these disorders, we provide a summary of the struc-
ture and function of hemoglobin, along with the mechanism of assembly of its subunits,
as background for the rationale and basis of the different categories of disorders in the
classification.

An impressive degree of molecular “engineer-

ing” was necessary for the evolution of a
multisubunit protein that higher organisms re-
quire for optimal oxygen homeostasis and buf-
fering of acidic metabolic waste products. Each
globin subunit must form a stable linkage with
heme (ferroprotoporphyrin IX) situated on the
external surface of the protein so that oxygen
in the RBC cytosol can bind reversibly to the
hemes’ iron atoms. Moreover, the hydrophobic
cleft into which the heme is inserted must be
able to protect the Fe2þ heme iron from oxida-

tion to Fe3þ, which is incapable of binding ox-
ygen.3 Delicate noncovalent interactions be-
tween unlike globin subunits are required for
the hemoglobin tetramer a2b2 to bind and un-
load oxygen in a cooperative manner, thereby
assuring maximal transport to actively metab-
olizing cells. This phenomenon is reflected by
a sigmoid oxygen-binding curve that depends
on hemoglobin tetramer having two quaternary
structures: the T or deoxy conformer that has
low oxygen affinity and the R or oxy conformer
that has high oxygen affinity. Further fine-

3For more detailed information, see Dailey and Meissner (2013).
Editors: David Weatherall, Alan N. Schechter, and David G. Nathan

Additional Perspectives on Hemoglobin and Its Diseases available at www.perspectivesinmedicine.org
Copyright # 2013 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a011684
Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

1

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

B.G. Forget and H.F. Bunn

tuning of hemoglobin function comes from its
allosteric behavior triggered by the binding of
two small effector molecules 2,3-BPG, and pro-
tons to specific sites on the T structure distant
from the heme groups.4 To endow the blood
with high oxygen carrying capacity hemoglo-
bin must be stuffed into flexible circulating
RBCs. A remarkably high degree of solubility
is required for hemoglobin to achieve an intra-
cellular concentration of (cid:2)34 g/dl or 5 mM
(tetramer).

To reach such a high corpuscular hemoglo-
bin concentration it is essential that a-globin as
well as b-globin (or g-globin) mRNA be ex-
pressed at very high levels during erythroid dif-
ferentiation. Moreover, a- and non-a-globin
synthesis must be closely matched. As explained
in Nienhuis and Nathan (2012), subunit imbal-
ance is central to the pathophysiologyof the thal-
assemias. Free a-globin subunits are particularly
toxic to erythroid cells. This threat is alleviated
by the presence of a-hemoglobin stabilizing
protein (AHSP), a chaperone that is expressed
at high levels in erythroid cells and binds specif-
ically and tightly to heme-intact a-globin sub-
units (Kihm et al. 2002; Feng et al. 2004; Mollan
et al. 2010, 2012). The AHSP protects the cell
from potentially toxic oxidized (Fe3þ) heme un-
til it is reduced to functional Fe2þ heme by cy-
tochrome b5 reductase. On encountering a free
heme-intact b-globin subunit, the a-globin
dissociates from AHSP to form the extremely
stable ab dimer. As discussed later in this
work, this process is facilitated by electrostatic
attraction between positively charged a-globin
subunits and negatively charged b-globin sub-
units.

In view of the multiple molecular con-
straints that are required for high-level produc-
tion of fully functional and highly soluble he-
moglobin, it is no surprise that Murphy’s law
is in full force: whatever can go wrong will.
As discussed briefly here, and in much more
detail in Thein (2013), Higgs (2013), Nienhuis
and Nathan (2012), and Musallam et al. (2012),

4For more detailed information on hemoglobin function,
see Schechter (2013).

mutations of globin genes that impair synthesis
give rise to thalassemia and anemia of varying
degree. In addition, well-defined clinical and
hematologic phenotypes are associated with
mutations that alter the structure of globin sub-
units, discussed in more detail in Thom et al.
(2013). Impairment of hemoglobin solubility
can be caused either by the formation of intra-
cellular polymers (sickle cell disease) or by the
development of amorphous precipitates (con-
genital Heinz body hemolytic anemia). Abnor-
malities of oxygen binding can lead either to
erythrocytosis (high O2 affinity mutants) or to
cyanosis (low O2 affinity mutants). Some glo-
bin mutants have structural alterations within
the heme pocket that result in oxidation of
the heme iron and pseudocyanosis because of
methemoglobinemia.

GENERAL CLASSIFICATION OF
HEMOGLOBIN DISORDERS

Hemoglobin disorders can be broadly classified
into two general categories (as listed in Table 1):

1. Those in which there is a quantitative defect
in the production of one of the globin sub-
units, either total absence or marked reduc-
tion. These are called the thalassemia syn-
dromes.

2. Those in which there is a structural defect in

one of the globin subunits.

The majority of human hemoglobin mu-
tants were discovered as an incidental finding,
unassociated with any hematologic or clinical
phenotype. However, a number of a- and b-glo-
bin mutants are associated with distinct clinical
phenotypes. These fall into five broad catego-
ries: the sickle syndromes (SS, SC, Sb0-thalasse-
mia, and Sbþ-thalassemia); unstable mutants
causing congenital Heinz body hemolytic ane-
mia; mutants with high oxygen affinity result-
ing in erythrocytosis; low oxygen affinity mu-
tants and the M hemoglobins causing cyanosis;
and mutants associated with a thalassemia phe-
notype. Most of these disorders are inherited
genetic defects, but there are some defects that
are acquired or occur de novo.

2

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

Classification of Hemoglobin Disorders

II. QUALITATIVE DISORDERS OF GLOBIN

STRUCTURE: STRUCTURAL VARIANTS OF
HEMOGLOBIN
A. Sickle cell disorders
SA, sickle cell trait
SS, sickle cell anemia/disease
SC, HbSC disease
S/b thal, sickle b-thalassemia disease
S with other Hb variants: D, O-Arab, other
SF, Hb S/HPFH

B. Hemoglobins with decreased stability

(unstable hemoglobin variants)

Mutants causing congenital Heinz body hemolytic

anemia

Acquired instability—oxidant hemolysis: Drug-

induced, G6PD deficiency

C. Hemoglobins with altered oxygen affinity
High/increased oxygen affinity states:
Fetal red cells
Decreased RBC 2,3-BPG
Carboxyhemoglobinemia, HbCO
Structural variants

Low/decreased oxygen affinity states:
Increased RBC 2,3-BPG
Structural variants

D. Methemoglobinemia
Congenital methemoglobinemia:
Structural variants
Cytochrome b5 reductase deficiency

Acquired (toxic) methemoglobinemia

E. Posttranslational modifications
Nonenzymatic glycosylation
Amino-terminal acetylation
Amino-terminal carbamylation
Deamidation

Table 1. Classification of hemoglobin disorders

I. QUANTITATIVE DISORDERS OF GLOBIN
CHAIN SYNTHESIS/ACCUMULATION

The thalassemia syndromes
A. b-Thalassemia
Clinical classification:
b-Thalassemia minor or trait
b-Thalassemia major
b-Thalassemia intermedia

Biochemical/genetic classification:
b0-Thalassemia
bþ-Thalassemia
d-Thalassemia
g-Thalassemia
Lepore fusion gene
db-Thalassemia
1 gdb-Thalassemia
HPFH
“Dominant” b-thalassemia (structural variants with

b-thalassemia phenotype)

b-Thalassemia with other variants:
HbS/b-thalassemia
HbE/b-thalassemia
Other

B. a-Thalassemia
Deletions of a-globin genes:
One gene: aþ-thalassemia
Two genes in cis: a0-thalassemia
Two genes in trans: homozygous aþ-thalassemia

( phenotype of a0-thalassemia)

Three genes: HbH disease
Four genes: Hydrops fetalis with Hb Bart’s

Nondeletion mutants:
Hb Constant Spring
Other

C. De novo and acquired a-thalassemia
a-Thalassemia with mental retardation syndrome (ATR):
Due to large deletions on chromosome 16 involving

the a-globin genes

Due to mutations of the ATRX transcription factor

gene on chromosome X

a-Thalassemia associated with myelodysplastic

syndromes (ATMDS):

Due to mutations of the ATRX gene

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

THE THALASSEMIA SYNDROMES

The thalassemia syndromes are inherited dis-
orders characterized by absence or markedly
decreased accumulation of one of the globin

subunits of hemoglobin. In the alpha (a)-thal-
assemias, there is absent or decreased pro-
duction of a-globin subunits, whereas in the
beta (b)-thalassemias, there is absent or re-
duced production of b-globin subunits. Rare

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

3

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

B.G. Forget and H.F. Bunn

thalassemias affecting the production of delta
(d)- or gamma (g)-globin subunits have also
been described but are not clinically signif-
icant disorders. Combined deficiency of d þ
b-globin subunits, or of all of the b-like glo-
bin subunits also occurs. These disorders are
described in detail in Thein (2013) and Higgs
(2013).

THE b-THALASSEMIAS

The b-thalassemias can be subclassified into
those in which there is total absence of normal
b-globin subunit synthesis or accumulation,
the b0-thalassemias, and those in which some
structurally normal b-globin subunits are syn-
thesized, but in markedly decreased amounts,
the bþ-thalassemias. The molecular basis of the
b-thalassemias is very heterogeneous (see Thein
2013), with over 200 different mutations having
been described. In general, the mutations caus-
ing b-thalassemia are point mutations affect-
ing a single nucleotide, or a small number of
nucleotides, in the b-globin gene. Rare deletion
forms of b-thalassemia have also been de-
scribed. One of these deletions is caused by “un-
equal” crossing over between the linked and
partially homologous d- and b-globin genes,
resulting in the formation of a fusion db-globin
gene, the Lepore gene, that has a low level of
expression. Large deletions involving part or
all of the b-globin gene cluster are responsible
for the db-thalassemias, the 1 gdb-thalassemias,
and the hereditary persistence of fetal hemoglo-
bin (HPFH) syndromes.

Despite the marked heterogeneity in the
molecular basis of the b-thalassemias, the clin-
ical phenotype of these disorders is relatively
homogeneous because of their common patho-
physiology: deficiency of HbA tetramers and
excess accumulation of free a-subunits incapa-
ble of forming hemoglobin tetramers because of
deficiency of b-like globin subunits (see Nien-
huis and Nathan 2012). In heterozygotes (b-
thalassemia trait or b-thalassemia minor), there
is mild to moderate hypochromic microcytic
anemia, without evidence of hemolysis, whereas
in homozygotes or compound heterozygotes

(b-thalassemia major), there is usually a severe
transfusion-dependent hemolytic anemia asso-
ciated with marked ineffective erythropoiesis
resulting in destruction of erythroid precursor
cells in the bone marrow.

A less common clinical phenotype is re-
ferred to as b-thalassemia intermedia. In this
clinical disorder, there is a moderate to severe,
partially compensated, hemolytic anemia that
does not require chronic transfusion therapy
to maintain a satisfactory circulating hemo-
globin level in the affected patient, although
occasional transfusions may be required if the
anemia worsens because of associated com-
plications. Such patients have a milder disease
because there is less severe a- to non-a-globin
subunit imbalance than in typical b-thalasse-
mia major patients, resulting in less accumu-
lation of free a-subunits that cause the ineffec-
tive erythropoiesis. There are different possible
causes for such a lessened a- to non-a-globin
subunit imbalance, including: inheritance of
milder bþ-thalassemia mutations with less se-
vere than usual deficiency of b-globin subunit
production; coinheritance of a form of a-thal-
assemia; or coinheritance of another genetic
trait associated with increased production of
the g-subunit of fetal hemoglobin. Most pa-
tients with b-thalassemia intermedia carry
two mutant b-globin genes: they have a geno-
type typical of b-thalasemia major, but the phe-
notype is modified by one of the factors listed
above. However, rare cases of b-thalassemia in-
termedia are caused by heterozygosity for a
single mutant b-globin gene associated with
the production of a highly unstable b-globin
subunit that causes RBC damage in a fashion
similar to excess free a-subunits; this is the
so-called “dominant b-thalassemia” (Thom et
al. 2013).

The db-thalassemias are associated with to-
tal deficiency of b-globin subunit production,
but are clinically milder than typical cases of b0-
thalassemia, because there is an associated per-
sistent high level of expression of the g-subunit
of fetal hemoglobin that decreases the degree
of a-subunit excess. The 1 gdb-thalassemias are
associated with neonatal hemolytic anemia,
but this resolves during the first few months

4

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

of life and the associated phenotype in adults is
that of b-thalassemia trait or b-thalassemia mi-
nor. The syndrome of hereditary persistence of
fetal hemoglobin (HPFH) is not, strictly speak-
ing, a form of b-thalassemia because it is not
associated with significant a- to non-a-globin
subunit imbalance, but is characterized by high
levels of persistent g-globin production and is
frequently considered within the spectrum of
db-thalassemia.

THE a-THALASSEMIAS

In contrast to the b-thalassemias, which are
usually caused by point mutations of the b-glo-
bin gene, the a-thalassemia syndromes are usu-
ally caused by the deletion of one or more a-
globin genes and are subclassified according to
the number of a-globin genes that are deleted
(or mutated): one gene deleted (aþ-thalasse-
mia); two genes deleted on the same chromo-
some or in cis (a0-thalassemia); three genes
deleted (HbH disease); or four genes deleted
(hydrops fetalis with Hb Bart’s). Nondeletion
forms of a-thalassemia have also been charac-
terized but are relatively uncommon. These dis-
orders are discussed in detail in Higgs (2013).
Clinically, the deletion of only one of the
four a-globin genes is not associated with
significant hematologic abnormalities and is
sometimes called the “silent carrier” state for
a-thalassemia. The deletion of two a-globin
genes can occur in two forms: (1) on the same
chromosomes, or in cis; or (2) on opposite chro-
mosomes, or in trans, which is the homozygous
state for the single gene deletion, or homozygous
aþ-thalassemia. The cis genotype is particular-
ly common in Asian populations, whereas the
trans genotype is highly prevalent in persons of
black/African ancestry. The clinical phenotype
is similar with both genotypes and consists
of mild hypochromic, microcytic anemia, with-
out hemolysis, somewhat analogous to that of
b-thalasssemia trait, but somewhat less severe.
The deletion (or markedly decreased expression)
of three a-globin genes is associated with the
syndrome of HbH disease, a compensated he-
molytic anemia that usually does not require

Classification of Hemoglobin Disorders

treatment by RBC transfusion. The basis of
the hemolysis is the excess accumulation of b-
globin subunits that self-associate to form
b-chain tetramers or HbH. In contrast to the
situation in the b-thalssemias in which excess
a-globin subunits rapidly form insoluble ag-
gregates, excess b-globin chains can form sol-
uble tetramers (HbH). However, HbH is rela-
tively unstable and does precipitate as RBCs age,
forming inclusion bodies that damage RBCs
and shorten their lifespan (see Higgs 2013).
The deletion of all four a-globin genes is usually
fatal during late pregnancy or shortly after birth.
This condition is called hydrops fetalis with Hb
Bart’s. Hb Bart’s is a tetramer of four g-globin
subunits and is ineffective as an oxygen trans-
porter: it has a very high oxygen affinity, similar
to that of myoglobin, and does not release ox-
ygen to the tissues under physiologic condi-
tions. Therefore, the infant, whose RBCs lack
HbF or HbA and contain mostly Hb Bart’s, suf-
fers severe hypoxia resulting in hydrops fetalis.
Rare cases have been rescued by intrauterine
transfusions, but these children subsequently
require lifelong transfusion support similar to
that required by children with b-thalassemia
major.

One form of nondeletion a-thalassemia,
called Hb Constant Spring (HbCS), is particu-
larly prevalent in southeast Asia. It is caused by a
chain termination mutation, which results in the
synthesis of an elongated a-globin subunit that
accumulates at very low levels in RBCs of affect-
ed individuals. When coinherited with the a0-
cis deletion on the other chromosome, there re-
sults a form of HbH disease that is more severe
than the typical HbH disease associated with
full deletion of three a-globin genes (see Higgs
2013).

In addition to these forms of a-thalassemia
inherited in a pattern consistent with Mendelian
genetics, there are two a-thalassemia syndromes
that are caused by de novo or acquired muta-
tions affecting expression of the a-globin genes:
(1) the a-thalassemia with mental retardation
syndrome (ATR); and (2) acquired a-thalasse-
mia (HbH disease) associated with myelodys-
plastic syndromes (ATMDS). These syndromes
are described in detail in Gibbons (2012).

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

5

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

B.G. Forget and H.F. Bunn

THE a-THALASSEMIA WITH MENTAL
RETARDATION SYNDROME

There are two subtypes of this syndrome: (1)
one is associated with very large deletions in-
volving the a-globin genes and adjacent genes
on chromosome 16, that are not inherited from
a parent and appear to have arisen de novo dur-
ing embryogenesis (the ATR-16 syndrome); and
(2) the other is associated with structurally nor-
mal a-globin genes and is caused by germline
mutations in a transcription factor gene located
on the X chromosome (the ATR-X syndrome).
The encoded transcription factor has been
called ATRX and is an important regulator of
a-globin gene expression.

ACQUIRED a-THALASSEMIA (HbH DISEASE)
ASSOCIATED WITH MYELODYSPLASTIC
SYNDROMES (MDS)

A small number of patients with MDS, a clonal
bone marrow disorder characterized by dis-
turbed hematopoiesis with cytopenias and ab-
normal myeloid differentiation, develop RBC
abnormalities consistent with acquired HbH
disease. The a-globin genes in such patients
are structurally normal, but their expression is
severely impaired. The molecular basis for the
decreased a-globin gene expression in this con-
dition has been determined to be acquired
somatic mutations of the ATRX gene, the same
gene that is mutated in the syndrome of a-thal-
assemia with mental retardation. The impair-
ment of a-globin gene expression is more severe
in the ATMDS syndrome than in the ATRX syn-
drome.

QUALITATIVE DISORDERS OF
GLOBIN STRUCTURE

Sickle Cell Disorders

Sickle hemoglobin (HbS) results from an ami-
no acid substitution at the sixth residue of
the b-globin subunit: b6-Glu ! Val. Approxi-
mately 8% of African Americans are heterozy-
gous for this hemoglobin variant, a condition
called sickle cell trait or HbAS. In equatorial
Africa, where malaria is endemic, the prevalence

of HbAS is much higher and can reach over 30%
in some populations because of survival advan-
tage of HbAS heterozygotes from complica-
tions of falciparum malaria. RBCs of persons
with HbAS typically have 40% HbS and 56% –
58% HbA. Individuals with HbAS are typically
asymptomatic; severe hypoxia is required for
them to experience manifestations of sickle cell
disease, called sickling.

The basis of sickling in patients homozygous
for the disorder, called sickle cell anemia or
HbSS, is polymerization of deoxy-HbS resulting
in the formation of multistranded fibers that
create a gel and change the shape of RBCs
from biconcave discs to elongated crescents.
The polymerization/sickling reaction is revers-
ible following reoxygenetion of the hemoglobin.
Thus, an RBC can undergo repeated cycles of
sickling and unsickling. There are two major
pathophysiological consequences of sickling:

1. Repeated cycles of sickling damage the red
blood cell membrane leading to abnormali-
ties of permeability and cellular dehydration,
eventually causing premature destruction of
RBCs and a chronic hemolytic anemia.

2. Sickled RBCs are rigid, increase blood viscos-
ity and obstruct capillary flow, causing tissue
hypoxia and, if prolonged, cell death, tissue
necrosis/infarction, and progressive organ
damage. Acute episodes are often called
vaso-occlusive pain crises.

The pathophysiology and clinical manifesta-
tions of sickle cell disease are described in detail
in Schechter and Elion (2013) and Serjeant
(2013). There can be a great deal of variability
in the severity of the clinical manifestations of
the various sickle cell syndromes in patients
with the same b-globin gene genotype and
even between siblings in the same family. This
variability can be caused by the coinheritance of
a number of different genetic modifiers of the
disease, including coinheritance of a-thalasse-
mia, quantitative trait loci affecting the level of
production of
fetal hemoglobin and other
traits, as discussed in Lettre (2012). In general,
the clinical severity of the different sickle cell
syndromes correlates directly with the amount

6

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.

Downloaded from

http://perspectivesinmedicine.cshlp.org/

on February 15, 2013 – Published by Cold Spring Harbor Laboratory

Press

and concentration of HbS in the red cell. The
kinetics of sickling are markedly concentration
dependent, the speed of the reaction being in-
versely proportional to about the 30th power of
the starting HbS concentration. Thus, a small
increase in intracellular HbS concentration,
such as that resulting from loss of intracellular
water and cellular dehydration, can have a major
effect on the speed of the polymerization reac-
tion. The amount and type of other non-S he-
moglobins in the red cell can also influence the
extent or rate of sickling, and thus clinical se-
verity. HbSC disease is associated with signifi-
cant clinical manifestations. Why do SC pa-
tients often have vaso-occlusive manifestations
as dramatic as those with SS disease, whereas
AS individuals have virtually no morbidity?
HbC and HbA copolymerize to an identical de-
gree with HbS (Bunn et al. 1982). Two indepen-
dent factors conspire to make HbSC a disease.
The presence of HbC in the red cell greatly en-
hances potassium efflux and red cell dehydra-
tion, which increases corpuscular hemoglobin
concentration (Bunn et al. 1982) and promotes
polymerization. Sickling in SC patients is fur-
ther enhanced by the fact that they have (cid:2)50%
HbS, whereas HbAS individuals have (cid:2)40%
HbS. This difference is because a-globin sub-
units bind more readily to negatively charged
bA-subunits than to positively charged bC-sub-
units, as explained further at the end of this
work. Other clinically significant sickle cell syn-
dromes include: sickle/b0-thalassemia, sickle/
bþ-thalassemia, HbSD disease, and HbSO-
Arab disease. HbD and HbO-Arab participate
more readily than HbA in copolymer formation
with HbS.

A striking example of how another hemo-
globin can dramatically influence sickling is the
condition that results from coinheritance of
HbS with hereditary persistence of fetal hemo-
globin: HbSF or S/HFPH. This condition, in
which there is (cid:2)70% HbS and (cid:2)30% HbF,
is clinically asymptomatic and is a notable ex-
ception to the general rule that clinical severity
is directly related to the amount (or %) of HbS
in the RBC. In this condition, the HbF is dis-
tributed in a pancellular fashion so that every
RBC contains (cid:2)30% HbF and, because HbF

Classification of Hemoglobin Disorders

does not participate at all in polymer forma-
tion, this is sufficient to prevent sickling of all
RBCs under physiologic conditions. This situa-
tion is in contrast to what occurs when variable
amounts of HbF are present in HbSS disease or
the sickle/b-thalassemia syndromes. In the lat-
ter disorders, the HbF is distributed in a hetero-
cellular fashion, in which the HbF is present
in only a subset of cells called F cells, leaving
the other cells that lack HbF unprotected from
the antisickling effects of HbF. Nevertheless, a
marked increase in the proportion of F cells in
such cases can have a beneficial effect on the
clinical manifestation of these sickling disor-
ders. Therefore, there is a great deal of interest
and ongoing research to develop strategies for
enhancing HbF synthesis and F-cell production
as a therapeutic modality for sickling disorders
(see Sankaran and Orkin 2013).

UNSTABLE HEMOGLOBIN VARIANTS

A substantial minority of Hb mutants have sub-
stitutions that alter the solubility of the molecule
in the red cell. The intraerythrocytic precipitated
material derived from the unstable abnormal Hb
is detectable by a supravital stain as dark globu-
lar aggregates called Heinz bodies (HBs). These
intracellular inclusions reduce the life span of
the red cell and generate a hemolytic process of
varied severity called congenital Heinz body he-
molytic anemia. When a red cell hemolysate of
an affected individual is heated to 508 C or treat-
ed with 17% isopropanol, a precipitate usually
develops. The hemoglobin electrophoresis often
reveals an abnormal banding pattern. Defini-
tive diagnosis requires analysis of either globin
structure or DNA sequence. For more informa-
tion on congenital Heinz body hemolytic ane-
mia and other hemoglobin variants summa-
rized in this section, see Thom et al. (2013).

HEMOGLOBIN VARIANTS WITH
ALTERED OXYGEN AFFINITY

Over 25 hemoglobin variants have been en-
countered in individuals with erythrocytosis.
Amino acid substitutions cause an increase in
oxygen affinity usually because of impairment

Cite this article as Cold Spring Harb Perspect Med 2013;3:a011684

7

g
r
o

.
e
n

i
c
i

d
e
m
n

i
s
e
v
i
t
c
e
p
s
r
e
p
w
w
w

.