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CHAPTER 8
Familial and acquired
polycythaemia
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8.1
Familial polycythaemia
Josef Prchal, Victor Gordeuk
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CHAPTER 8.1 • Familial polycythaemia
1. Introduction
Our understanding of the regulation of erythropoiesis has improved in recent years.
The elucidation of the pathophysiology of congenital polycythaemias (erythrocytoses)
provides new information regarding the molecular control of red cell production and
leads to precise diagnosis. In this chapter, we review the pathophysiology of
erythropoiesis and those clinical congenital disorders wherein the molecular basis
has been defined. These can be divided into three categories.
1. Familial polycythaemia may be due to an intrinsic red cell precursor defect that
leads to increased erythropoiesis despite low erythropoietin (Epo) levels. This
is called primary familial polycythaemia and is often due to a gain-of-function
mutation in the erythropoietin receptor gene.
2. Familial polycythaemia may be due to inherited defects that cause tissue hypoxia
and a secondary increase in erythropoietin concentrations. Examples include
haemoglobins with high oxygen affinity, defects that lead to
methaemoglobinaemia or deficiency of 2,3-BPG.
3. Familial polycythaemia may be due to inherited defects that lead to increased
erythropoietin concentrations in the setting of normal tissue oxygenation. These
conditions of abnormal hypoxia sensing may result from mutations in the von
Hippel-Lindau (VHL) gene, the gene for proline hydroxylase domain protein 2 or
the gene for hypoxia inducible factor-2α.
Once thought to be benign, it is now clear that many forms of familial polycythaemia
are complicated by thrombotic and other non-erythroid manifestations and increased
mortality. Optimal therapy has not been determined. While polycythaemia vera (PV)
is the most frequent primary polycythaemic disorder and is acquired, about 5% of
PV patients have relatives with PV or with other myeloproliferative disorders. This
suggests that these families inherit a yet-to-be identified gene(s) that increases
the acquisition of PV somatic mutation(s).
2. Regulation of erythropoiesis as it relates to familial polycythaemia
2.1 Stages of erythropoiesis
The production of erythrocytes, erythropoiesis, is a tightly regulated process, but
all aspects of its regulation are not fully elucidated. Much remains to be learned
from uncovering the molecular basis of congenital and acquired mutations that disrupt
the control of erythropoiesis. The process of erythropoiesis can be viewed as
having three stages. In the initial stage, pluripotent haematopoietic progenitors
transform to committed erythroid precursors. In the second stage, erythroid
progenitors proliferate by a process that is largely regulated by Epo and is made
effective by the appearance of its receptor (EpoR). Expression of EpoR peaks on the
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surface of early erythroblasts during this stage and then declines. In the third or
terminal stage, late erythroid precursors undergo enucleation and removal of
nucleotides and other remnants of organelles that may be toxic to mature circulating
erythrocytes. Defects in erythropoiesis that lead to polycythaemia invariably occur
at the second stage of erythropoiesis.
2.2 Alterations in regulation that lead to polycythaemia
In this review we will concentrate only on those defects that have been demonstrated
to lead to, or potentially could lead to, polycythaemia, i.e. increased red cell mass.
Polycythaemia is synonymous with the term “erythrocytosis”, and since no consensus
on usage has been reached, we will refer to individual entities by the conventional
usage associated with them.
Polycythaemias can be classified as being appropriate or inappropriate (Table 1A).
Table 1: Classification of polycythaemia by two approaches
A. Classification according to appropriateness (tissue oxygenation)
Congenital
Acquired
Appropriate polycythaemias (tissue hypoxia present)
Haemoglobins with high affinity for oxygen
Methaemoglobinaemia
Low erythrocyte 2,3-biphosphoglycerate levels
Cardiac and pulmonary disorders characterised
by hypoxia
Exposure to cobalt
Smoking and increased carboxyhaemoglobin
Inappropriate polycythaemias (tissue hypoxia absent)
Primary and familial congenital polycythaemia
Congenital polycythaemias due to abnormal
hypoxia sensing
Polycythaemia vera
Ectopic secretion of erythropoietin by tumours
B. Classification according to whether or not erythropoietin is increased
Congenital
Acquired
Primary (erythropoietin not increased; defect intrinsic to erythroid progenitor cells)
Primary and familial congenital polycythaemia
Polycythaemia vera
Secondary (erythropoietin increased)
Congenital heart disease with right to left
shunting
Methaemoglobinaemia
Haemoglobins with high affinity for oxygen
Low erythrocyte 2,3-biphosphoglycerate levels
Congenital polycythaemias due to abnormal
hypoxia sensing
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Acquired cardiac and pulmonary disorders
characterised by hypoxia
Exposure to cobalt or cyanate
Smoking and increased carboxyhaemoglobin
Ectopic secretion of erythropoietin by tumours
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Appropriate polycythaemias (1) are defined by physiologically intact upregulation of
Epo production to meet tissue oxygen demand, which then drives augmented
erythropoiesis. These disorders may be inherited or acquired conditions that lead to
tissue hypoxia. Inappropriate polycythaemias (1) are caused by inappropriate
increases of erythropoietin signalling such as seen in congenital or acquired primary
polycythaemias, in polycythaemia of disordered hypoxia sensing, in ectopic production
of erythropoietin by tumours. Dysregulation of angiotensin II signalling in renal
transplant patients might also be considered an inappropriate polycythaemia (2, 3).
Polycythaemias can also be classified as being primary or secondary (Table 1B).
Primary polycythaemias are characterised by a defect within the progenitors that
leads to Epo hypersensitivity or independence, which drives the polycythaemia
phenotype. These disorders are characterised by a low Epo level and can be divided
into two categories:
1. Congenital gain-of-function mutations of the EPOR gene lead to primary familial and
congenital polycythaemia (PFCP) wherein the Epo signal is augmented by the mutation.
2. In PV, a somatic (acquired) mutation located downstream of the physiological
stage of EpoR activation, i.e. the JAK2V617F gain-of-function mutation, contributes
to the pathogenesis.
Secondary polycythaemias are largely Epo-driven disorders. There are however other
stimulators of erythropoiesis including cobalt, angiotensin II, and insulin-like
growth factor 1 (IGF-1) (2). Some congenital defects lead to dysregulation of Epo
production and polycythaemia due to inappropriately high Epo levels. Some of these
congenital disorders have features of both primary and secondary polycythaemia
(i.e. Chuvash polycythaemia).
2.3 Regulation of erythropoietin production
Epo is the principal regulator of erythropoietin signalling as evidenced by its
modulation of erythroid proliferation, prevention of apoptosis, and promotion of
erythroid differentiation (reviewed by Krantz (4)). Regulation of Epo production can
be mediated by hypoxia or be independent of hypoxia.
2.3.1 Molecular basis of hypoxia sensing
Hypoxia inducible factor transcription factors
Hypoxia influences erythropoiesis, energy metabolism, vasculogenesis, iron metabolism,
tumour promotion and other processes (reviewed by Hirota (5) and (6)). The
transcription factors, hypoxia inducible factor (HIF)-1 and HIF-2, are induced in hypoxic
cells and bind to a cis-acting nucleotide sequence referred to as the hypoxiaresponsive element (HRE), first identified in the 3’-flanking region of the human EPO
gene (7). Many hypoxia-inducible genes are directly regulated by HIFs. Dr. Semenza’s
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group showed that ~ 3% of all genes expressed in endothelial tissue are HIF-1 regulated
(8). HIFs are heterodimeric transcription factors composed of an HIF-α subunit and
an HIF-β subunit (Figure 1). The HIF-β subunit belongs to the basic helix-loop-helix
(bHLH)-containing PER-ARNT-SIM (PAS)-domain family of transcription factors. Only
the HIF-1α and HIF-2α subunits are regulated by hypoxia and exist exclusively in
HIF. Thus HIF-α is the key subunit determining the hypoxia-modulated amount and
activity of HIF and the transcription of hypoxia-inducible genes.
HIF-1α
The half-life of HIF-1α in the cell is only minutes under normoxic conditions, as the
protein is rapidly degraded by the VHL protein ubiquitin-proteasome pathway (9).
Figure 1: Hypoxic and normoxic control of HIF-1a
HIF-1a regulation
HSP90
O2-dependent
pathway
HIF-1α
O2-independent
pathway
[O2]
[O2]
HIF-1a
p300
HIF-1b
[O2]
Elongin C
E2
VHL
Elongin C
E2
RACK1
HIF-1a
HIF-1a
Ubi
Ubi
Transcriptional
regulation
26S Proteasome
Overview of HIF-1a regulation. The left panel depicts the oxygen dependent pathway. In the middle of
the cartoon, in the presence of oxygen, HIF-1a is proline-hydroxylated, which allows interaction with
elongins C and E2, and VHL protein leading to ubiquitinisation and destruction of HIF-1a in the
proteasome. The left portion of the cartoon depicts the lack of HIF-1a degradation in hypoxia, with
formation of HIF-1a and HIF-1b dimer, resulting in transcriptional regulation of HIF controlled genes.
The right panel depicts the oxygen independent pathway wherein RACK1 interacting with elongins C and
E2 promote ubiquitinisation and destruction of HIF-1a in the proteasome.
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This is initiated by posttranslational hydroxylation of proline at position 564 of the
HIF-1α molecule by one of several iron-containing proline hydroxylases. Proline 564
hydroxylation facilitates the binding of HIF-1α to VHL protein and subsequent
degradation. Under hypoxic conditions, proline 564 of HIF-1α is not hydroxylated.
The unmodified protein escapes VHL-binding, ubiquitination, and degradation, but
rather is translocated to the cell nucleus where it dimerises with HIF-1β to form HIF1 and then activates transcription through binding to specific HREs. Another
regulatory step involves O2-dependent hydroxylation of asparagine at position 803
of HIF-1α, which requires the enzyme FIH-1 (factor inhibiting HIF-1). Hydroxylation
of asparagine 803 during normoxia blocks the binding of two other transcription
factors, p300 and CBP, to HIF-1 resulting in inhibition of HIF-1 mediated gene
transcription. Under hypoxic conditions, asparagine 803 of HIF-1α is not hydroxylated,
p300 and CBP can bind to the HIF-1 dimer, and transcriptional activation of HIF-1
target genes occurs. In summary, the targeting and subsequent polyubiquitination
of HIF1α requires pVHL, iron, O2 and proline hydroxylase activity, and, as depicted
in Figure 1, this complex constitutes the oxygen sensor (10, 11).
HIF-2α
HIF-1α and HIF-2α exhibit high sequence homology but have different mRNA
expression patterns. HIF-1α is expressed ubiquitously whereas HIF-2α expression is
restricted to certain tissues (5, 12). Both HIF-1α and HIF-2α are regulated through
identical mechanism by hypoxia and form a dimer with the same HIF-1β subunit.
The kidneys are the main site of Epo production, i.e. renal interstitial cells, and HIF1 is the principal regulator of EPO transcription in this organ (5). In other tissues,
such as the liver (14), which generates ~ 20% of circulating Epo, and the brain (13),
EPO gene transcription is HIF-2-regulated (12). The discovery of an iron-responsive
element in the 5′ untranslated region of HIF2α reveals a novel regulatory link
between iron availability and HIF-2α expression (15) that may impact control of
erythropoiesis. Thus, when iron supply is limited, HIF-2α would decrease, and when
iron is abundant, hepatic HIF-2α would increase, enhancing liver-synthesised Epo
production and promoting erythropoiesis (16).
Hypoxia independent regulation of HIF
O2-dependent regulation of the HIF-1α subunit is mediated by prolyl hydroxylases,
VHL protein, and a proteosomal complex as described above. A regulatory pathway
of HIF that is independent of hypoxia has also been described. This mechanism involves
the receptor of activated protein kinase C (RACK1) as an HIF-1α-interacting protein
that promotes prolyl hydroxylase/VHL-independent proteasomal degradation of HIF-
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1α. RACK1 competes with heat shock protein 90 (HSP90) for binding to the PAS-A
domain of HIF-1α. HIF-1α degradation is abolished by RACK1 loss-of-function.
RACK1 binds to the proteasomal subunit, elongin-C, and promotes ubiquitination of
HIF-1α (Figure 1). Thus, RACK1 is an essential component of an O2/PHD/VHLindependent mechanism for regulating HIF-1α (17).
3. Primary familial polycythaemia (low Epo levels and normal tissue
oxygenation)
Primary familial and congenital polycythaemia (PFCP) is the only recognised type
of primary familial polycythaemia. It is characterised by an autosomal dominant mode
of inheritance, and less frequently, by the occurrence of sporadic cases (18-21). This
can be contrasted with Chuvash polycythaemia where the inheritance is autosomal
recessive. The clinical features of PFCP include absence of predisposition to the
development of acute leukaemia or other myeloproliferative disorders, absence of
splenomegaly, normal white blood cell and platelet counts, low plasma Epo levels,
normal haemoglobin-oxygen dissociation curve (indicated by a normal P50) and
hypersensitivity of erythroid progenitors to exogenous erythropoietin in vitro (2025). PFCP is generally thought to be a benign condition, but it has been reported
to be associated with a predisposition to cardiovascular problems, such as
hypertension, coronary artery disease, and cerebrovascular events, and these events
are not clearly related to an elevated haematocrit (26-28). Association with
cardiovascular disease has not been described in all series.
The distal cytoplasmic region of the EPOR, in association with SHP-1, is required for
down-regulation of Epo-mediated activation of JAK2/STAT5 proteins (29, 30). Thus far,
more than 10 mutations of the Epo receptor gene (EPOR) have been convincingly linked
with PFCP (20-25), 3155. In general, these mutations result in truncation of the EPOR
cytoplasmic carboxyl terminal leading to loss of its negative regulatory domain,
resulting in a gain-of function of EPOR. Additional missense EPOR mutations have been
described that are not clearly linked to PFCP or other disease phenotypes (21, 55).
Absence of polycythaemic phenotype in some patients with EPOR mutation is suggestive
of a role played by gene modifiers or epigenetic factors in phenotypic penetrance (32).
4. Secondary familial polycythaemia (elevated Epo levels due to tissue
hypoxia)
4.1 High oxygen affinity haemoglobin mutants
Affinity of haemoglobin for oxygen is expressed as the P50, which is the partial
pressure of oxygen in blood at which 50% of the haemoglobin is saturated with
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oxygen. An abnormally low P50 reflects an increased affinity of haemoglobin for
oxygen. During oxygenation and deoxygenation, there is considerable movement along
the α1/β2 interface (33). Several haemoglobin mutants have substitutions affecting
this interface. Other haemoglobin variants have amino acid substitutions involving
the C-terminal residues of the β chain or of the 2, 3 BPG binding sites. All these
substitutions can affect the cooperative nature of oxygen binding with haem, the
change from T to R state and vice versa, and in turn can change the affinity of
haemoglobin for oxygen. The majority of mutations affecting oxygen affinity result
in high affinity haemoglobin variants that lead to relative tissue hypoxia and
compensatory secondary appropriate polycythaemia. There are 91 haemoglobin
variants, listed on the globin server, known to be associated with high affinity for
oxygen (http://globin.bx.psu.edu/hbvar/menu.html; last accessed December 24,
2008). Thus an autosomal dominant polycythaemia with normal to elevated Epo levels
is suggestive of a mutant haemoglobin with high affinity for oxygen (33). The therapy
of this compensatory polycythaemia by phlebotomies is ill advised.
Haemoglobin F binds oxygen normally but in the cell has high oxygen affinity due
to reduced binding of 2,3-BPG (see below).
4.2 2, 3-BPG deficiency
Congenitally low erythrocyte 2, 3 biphosphoglycerate (2, 3 BPG) level can occur because
of deficiency of the red cell enzyme 2, 3-BPG mutase (34, 35). This is an extremely
rare autosomal recessive condition. This disorder should be suspected in the case of
isolated polycythaemia (without any feature of myeloproliferative disorders such as
progressive increase of RBC mass, high platelet and granulocyte counts and
splenomegaly), absence of a family history and low P50 (signifying high oxygen
affinity). A haemoglobin mutation needs to be ruled out first. In these cases, the red
cells will have high oxygen affinity; however, unlike high affinity haemoglobin, the
oxygen affinity of the haemolysate is normal and the level of 2, 3-BPG is very low.
4.3 Congenital methaemoglobinaemias
There are three types of hereditary methaemoglobinaemias (reviewed by Gregg & Prchal
(36)). Two are inherited as autosomal recessive traits: cytochrome b5R deficiency and
cytochrome b5 deficiency. The third type is an autosomal dominant disorder, haemoglobin
M (Hb M) disease in which there is a mutation of one of the globin genes. All
congenital methaemoglobinaemias are associated with suboptimal delivery of oxygen
per red cell and this may result in compensatory secondary appropriate polycythaemia.
The resulting polycythaemia is typically mild and its treatment is not advisable as it
would only decrease tissue oxygen delivery and lead to tissue hypoxia.
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5. Disorders of hypoxia sensing (elevated Epo levels and normal tissue
oxygenation)
5.1 Chuvash polycythaemia
This disorder was the first hereditary condition of augmented hypoxia sensing to
be recognised. Chuvash polycythaemia, the only known endemic polycythaemia, is
an autosomal recessive hereditary polycythaemia. The Chuvash people reside in the
mid-Volga River region in Russia where Chuvash polycythaemia affects hundreds of
people, making it the most common congenital polycythaemia (37). Outside of
Chuvashia, this form of familial polycythaemia has also been found sporadically in
diverse ethnic and racial groups (38-40), and a high prevalence of this disorder was
reported in the Italian island of Ischia (41). In a study of five Chuvash families with
polycythaemia, a mutation of VHL (598CÆT) was found in the affected individuals
(42). This mutation impairs the interaction of pVHL with HIF-1α, thus reducing the
rate of ubiquitin-mediated destruction of HIF-1α. As a result, the level of the HIF1 increases and leads to increased expression of target genes including endothelin1, EPO, plasminogen activator inhibitor (PAI), transferrin, transferrin receptor and
VEGF (42-45). The VHL598CÆT mutation also impairs the interaction of pVHL with
HIF-2α (46), and an animal model of the Chuvash polycythaemia mutation indicates
that it may be increased levels of HIF-2α rather than HIF-1α that mediate the
increased Epo levels and polycythaemia in this condition (47). Chuvash polycythaemia
is characterised by an intact response to hypoxia despite increased basal expression
of a broad range of hypoxia-regulated genes in normoxia. Unexpectedly, the
erythroid progenitors are hypersensitive to Epo; thus Chuvash polycythaemia shares
features of primary as well as secondary polycythaemia.
Chuvash polycythaemia is a unique VHL syndrome characterised by homozygous
germline mutation of VHL leading to predisposition to development of varicose veins,
benign vertebral haemangiomas, low systemic blood pressures, pulmonary hypertension,
thrombosis, bleeding, cerebral vascular events, and increased mortality (Table 2).
Peripheral blood profiling indicates lower white blood cell and platelet counts than
controls, lower CD4 counts, elevated levels of both pro-inflammatory and antiinflammatory cytokines, and altered plasma thiol levels with elevated homocysteine
and glutathione and low cysteine concentrations (44, 45, 48-50). Despite increased
expression of HIF-1α and VEGF in normoxia, Chuvash polycythaemia patients do not
display a predisposition to tumour formation. Imaging studies of 33 Chuvash
polycythaemia patients revealed unsuspected cerebral ischaemic lesions in 45% but
no tumours characteristic of VHL syndrome (44). It is not known whether therapy with
aspirin or phlebotomy in patients with Chuvash polycythaemia prevents complications.
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Table 2: Characteristics of Chuvash polycythaemia
Molecular
Homozygous VHL598CÆT
Decreased capture of HIF-α by VHL protein
Decreased degradation of HIF-α by the ubiquitin-proteasome pathway
Increased HIF-α under normoxic conditions
Altered transcription of multiple HIF-regulated genes
Increased plasma concentrations of products of HIF-regulated genes
Endothelin-1
Erythropoietin
Plasminogen activator inhibitor-1
Vascular endothelial growth factor
Transferrin
Transferrin receptor
Physical findings
Ruddy complexion
Varicose veins, increased prevalence
Heart murmur, increased prevalence
Clubbing of digits, increased prevalence
Lower systemic blood pressure
Higher pulmonary artery pressure on echocardiogram
Benign vertebral haemangiomas on MRI
Cerebral ischaemic lesions on MRI
Complete blood count
Polycythaemia (erythrocytosis)
Lower white blood cell count
Lower platelet count
Immunologic profile
Increased plasma concentrations of Th-1 cytokines (GM-CSF, IFN-γ, IL-2 and IL-12, TNF-α)
Increased plasma concentrations of Th-2 cytokines (IL-4, IL-5, IL-10, IL-13)
Normal ratio of Th-1 to Th-2 cytokines
Lower CD4 counts
Thiol metabolism
Increased plasma concentrations of homocysteine, glutathione and cysteinylglycine
Reduced plasma concentration of cysteine
Clinical course
No increase in incidence of renal cell carcinoma, pheochromocytoma or haemangioblastoma
Increased peptic ulcers
Increased major thromboses
Increased haemorrhage
Increased cerebrovascular accidents
Increased mortality
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Homozygosity for VHL 598CÆT occurs sporadically in Caucasians in the United States
and Europe and in people of Southeast Asian (Indian subcontinent) ancestry (3841). Thrombotic complications have been reported in some of these individuals.
To address the question of whether the VHL 598CÆT substitution occurred in a single
founder or resulted from recurrent mutational events, haplotype analysis was
performed on subjects bearing the VHL 598CÆT mutation and normal unrelated
individuals from Chuvash, Asian, Caucasian, Hispanic and African-American ethnic
groups (51). These studies indicated that in most individuals, the VHL 598CÆT
mutation arose in a single ancestor between 12,000 and 51,000 years ago. However,
a polycythaemic family in Turkey had a different haplotype indicating that the VHL
598CÆT mutation in this family occurred independently (52).
In contrast, autosomal dominant mutations of the VHL gene cause VHL syndrome
(53). Heterozygotes for dominant VHL mutations are at increased risk of developing
haemangioblastomas, renal cell carcinoma, pheochromocytoma, pancreatic endocrine
tumours, and endolymphatic sac tumours when they acquire a somatic mutation
in the normal VHL allele. Some patients with VHL syndrome also develop acquired
polycythaemia (53). Increased expression of HIF-1 and possibly VEGF may underlie
the VHL tumour predisposition syndrome and the development of haemangioblastoma
and renal cell carcinoma. However, the absence of a predisposition to tumourigenesis
in Chuvash polycythaemia patients implies that deregulation of HIF-1 and VEGF
may not be sufficient to cause predisposition towards tumour formation in VHL
syndrome.
Most heterozygotes for VHL598CÆT do not have polycythaemia. However, an
English patient has been described who was a heterozygote for VHL 598CÆT (40),
although the inheritance of a deletion of a VHL allele or a null VHL allele in a trans
position was not excluded in this patient. Subsequently, two VHL heterozygous
patients with polycythaemia were described in whom a null VHL allele was more
rigorously excluded (52, 54); one of these patients also had ataxia-telangiectasia
(54). Some patients with congenital polycythaemia have proven to be compound
heterozygotes for the Chuvash mutation, VHL 598CÆT, and other VHL mutations
including 562CÆG, 574CÆT, 388CÆG, and 311GÆT (39, 52, 54).
5.2 Non-Chuvash germline VHL mutations
Non-Chuvash germline VHL mutations also cause polycythaemia. A Croatian boy was
homozygous for VHL 571CÆG, the first example of a homozygous VHL germline
mutation other than VHL 598CÆT causing polycythaemia (39). Additionally, a
Portuguese girl was a compound heterozygote for VHL 562CÆG and VHL 253CÆT
(54, 55). A few cases of congenital polycythaemia with mutations of only one nonChuvash polycythaemia VHL allele have been described. Two Ukrainian children with
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polycythaemia were heterozygotes for VHL 376GÆT, but the father with the same
mutation was not polycythaemic (38). Peripheral blood erythroid progenitors from
the children and father were hyper-responsive to recombinant Epo in in vitro
clonogenic assays in a way similar to that seen in Chuvash polycythaemia patients.
5.3 Proline hydroxylase deficiency
A family with a proline hydroxylase domain protein 2 (PHD2) mutation (950CÆG)
was recently described in which heterozygotes for this mutation have mild or
borderline polycythaemia (56). Since then, four additional patients with unexplained
polycythaemia who are heterozygote carriers of different mutations in PHD2 have
been reported (two frameshift mutations, 606delG and 840_841insA, both located
in exon 1, and two nonsense mutations, 1112GÆA and 1129CÆT, in exon 3) (57,
58). One of these patients had a major thrombotic event (thrombosis of the
sagittal sinus) (58). These observations underscore the importance of defective HIF
signalling in the genesis of polycythaemic disorders.
5.4 HIF-2a gain-of-function mutations
Recently, a family was described in which members with polycythaemia were heterozygous
carriers of a HIF-2α Gly537Trp mutation which served to stabilise the HIF-2α protein
(59). The same group subsequently reported four additional polycythaemic patients
with heterozygous Met535Val or Gly537Arg mutations in the HIF-2α gene (60). The
patients tend to present at a young age with elevated serum Epo. These findings support
the importance of the proline hydroxylase, HIF-2α, VHL axis in human Epo regulation
and in the pathogenesis of familial anaemias due to abnormal hypoxia sensing.
5.5 Congenital polycythaemias of yet unidentified defect with elevated or
inappropriately normal levels of Epo
Not all patients with congenital polycythaemias with normal or elevated Epo levels do
have VHL, PHD2 or HIF-2α mutations, and the molecular basis of polycythaemia in these
cases remains to be elucidated. It is not clear why in some families, the polycythaemia
is dominantly inherited (61), in others recessively, and in some it is sporadic and, why
in families with the same mutation the phenotype can be different (55, 62). Lesions
in genes linked to hypoxia-independent regulation of HIF as well as oxygen-dependent
gene regulation pathways are leading candidates for mutation screening in polycythaemic
patients with normal or elevated Epo without VHL, PHD2 or HIF-2α mutations.
6. Familial polycythaemia vera and other myeloproliferative disorders
There is growing evidence that the acquired primary polycythaemic disorder
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polycythaemia vera, which is caused by somatic mutation(s), often has a familial
predisposition (63); in some families other myeloproliferative disorders are also
present (64-65). It is estimated that about 5% of PV patients have relatives with
PV or with other myeloproliferative disorders (65). This suggests the existence of
germ-line mutations that either facilitate the somatic mutation(s) or contribute to
somatic acquired mutations to result in familiar clustering of myeloproliferative
disorders. The molecular basis of these interactions yet remains to be defined. This
suggests that these families inherit a yet-to-be identified gene(s) that increases
the acquisition of PV somatic mutation(s).
Acknowledgements
Supported in part by grant nos. 2 R25 HL003679-09 (VG) and 1 R01 HL079912-03 (VG) from
NHLBI, by Howard University GCRC grant no 2MOI RR10284-11 (VG) from NCRR, NIH, Bethesda,
MD and R01HL50077-14 (JP) and 1P01CA108671-03 (JP).
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Multiple Choice Questionnaire
To find the correct answer, go to http://www.esh.org/iron-handbook2009answers.htm
1. Mutation of which of the following genes has not been associated
with congenital polycythaemia?
a) VHL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b) HIF-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c) EPOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d) EPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ÆT
2. Chuvash polycythaemia is caused by homozygosity for the 598CÆ
mutation in the VHL gene. Which of the following is a recognised
manifestation of Chuvash polycythaemia?
a) Benign vertebral hemangiomas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b) Pheochromocytoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c) Hypertension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d) CNS cerebellar haemangioblastoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Which of these polycythaemias is caused by a somatic mutation(s)?
a) Primary familial congenital polycythaemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b) HIF-2 mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c) Polycythaemia vera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d) Chuvash polycythaemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Which of these congenital polycythaemic states is associated with
low erythropoietin level and augmented erythropoietin signaling?
a) Primary familial congenital polycythaemia
(due to EPO receptor mutations) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b) Chuvash polycythaemia (598CÆT VHL mutation) . . . . . . . . . . . . . . . . . . . . . . . . . . .
c) HIF-2 mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
d) Proline hydroxylase mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Which of these polycythaemia states is inherited as an autosomal
dominant disorder?
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a)
b)
c)
d)
Polycythaemia vera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chuvash polycythaemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mutations of globin gene causing high haemoglobin oxygen affinity . . . . . . .
Cytochrome b5 reductase mutations causing methaemoglobinaemia
and polycythaemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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