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Hematology, 2005; 10 Supplement 1: 92 /99
CURRENT THERAPIES IN HEMOGLOBINOPATHIES
Sickle cell disease: A multigenic perspective of a single gene disorder
ABDULLAH KUTLAR
Professor of Medicine, Medical College of Georgia, Director, Sickle Cell Center, Augusta, GA
Half a century has elapsed since the elucidation of the
molecular basis of the sickle hemoglobin mutation in
the mid 50s. Although significant progress has been
made in our understanding of the disease and in the
development of new therapies, many questions are
still unanswered, and a cure remains elusive. This is
particularly evident in the clinical heterogeneity of
sickle cell disease (SAD), which ranges from a mild
clinical course with survival into the 6th and 7th
decades to a very severe presentation with significant
organ damage and death at relatively young ages [1].
Most of the advances in unraveling the phenotypic
heterogeneity of SCD have been made in the past 30
years, after the dawn of the molecular era. Studies in
this period have clearly shown the importance of high
hemoglobin F (Hb F) determinants and a-thalassemia
as modifiers of disease severity. The impact of these
modifiers has been well established and will not be
discussed in detail here [1]. We will rather try to
summarize recent advances on the effect of nonglobin genetic modifiers on the phenotype of SCD.
This process has been greatly facilitated by the
completion of the Human Genome Project, which
has identified a large number of single nucleotide
polymorphisms (SNPs) in and around many genes
that can potentially impact on different aspects of
SCD pathophysiology. The realization that a-thalassemia and variation in Hb F levels alone do not
account for the vast clinical diversity of the disease has
provided further impetus to such an endeavor.
The well-known consequences of the sickle mutation and its downstream effects are: (1) a chronic
hemolytic anemia, (2) episodic vasoocclusion with
resultant characteristic painful episodes, and (3)
chronic organ damage. The clinical phenotype of
SCD is highly variable; complications such as acute
chest syndrome, stroke, leg ulcers, and avascular
necrosis do not occur in all of the patients suggesting
that factors beyond the b6 Glu 0/Val mutation play a
ISSN 1024-5332 print/ISSN 1607-8454 online # 2005 Taylor & Francis
DOI: 10.1080/10245330512331390069
role in the pathogenesis of these syndromes. In the
past two decades, contributions by Hebbel et al. and
others, has established that SCD is an ‘‘inflammatory
state’’ and that the endothelium is activated [2]. In
more recent years, evidence has accumulated that
decreased nitric oxide (NO) bioavailability, contributed by scavenging of NO by cell free hemoglobin, a
product of the hemolytic process, plays a significant
role in the vascular pathobiology of SCD [3]. Thus,
the emerging view of the pathogenesis of many
complications of SCD involves complex interactions
between sickle reticulocytes, neutrophils, monocytes,
and the endothelium. It therefore follows that many
factors that are important in the pathways leading to
inflammation, cell adhesion, NO metabolism, vascular reactivity and coagulation will be important in the
pathophysiology of the disease and that variations in
the expression of a number of molecules in these
pathways will contribute to the heterogeneity of SCD.
The approach to the study of this complexity in the
post-genome era should involve several different
methodologies including: (1) analysis of polymorphisms in ‘‘candidate genes’’ by high throughput methods and the association of these polymorphisms with
distinct phenotypic features of the disease, (2) the
study of differential gene expression in various tissues
using cDNA microarrays, and (3) the application of
proteomics. The tissues and cells that are particularly
important in this regard include blood cells (neutrophils, monocytes, reticulocytes), bone marrow, endothelial cells, and liver tissues. A candidate gene
approach should include a large number of patients
with and without a certain complication (e.g., stroke)
to determine the role of these polymorphisms as risk
factors. Pooling studies to discover the role of genome
wide SNPs as risk factors for certain complications of
the disease is also emerging as a viable approach. In all
of these studies, careful characterization of the phenotype remains of critical importance. A study of
Sickle cell disease
Table I. Frequency of various complications among patients with
SCD.
Phenotype
Stroke/CVA
Retinopathy
Acute Chest Syndrome
Gallstones
Splenic sequestration
Nephropathy/microalbuminuria
Priapism
Avascular Necrosis
Leg ulcers
Frequency
11%
3%
30 /50%
30 /36%
5 /6%
30%
6 /42%
50%
4 /6%
sickle cell patients from different populations is likely
to yield important information because of the differences of the genetic backgrounds in these populations
and its potential implications on the disease phenotype. The important disease related ‘‘phenotypes’’
should include features such as stroke and CNS
disease, frequency of vasoocclusive episodes (VOEs),
frequency of the acute chest syndrome, avascular
necrosis of the hips and shoulders, gallbladder disease
and cholecystectomy, leg ulcers, renal involvement,
priapism, and retinopathy. Table I lists these important ‘‘sub-phenotypes’’ of SCD and their frequency in
the SCD patient population in the U.S.
For the past several years, our center and many
other centers have focused on the study of non-globin
genetic modifiers of sickle cell disease. Some of the
findings of these early studies will be summarized
below.
Hb F and Hb F response to hydroxyurea
It is well established that fetal hemoglobin has an
ameliorative effect on SCD because the gamma chains
of Hb F are excluded from the deoxyhemoglobin S
polymer. Because of this potent anti-sickling effect,
enhancing Hb F production has been a major
therapeutic goal in SCD. In fact, this goal has been
achieved to a large extent by the use of the S-phase
specific chemotherapeutic agent, hydroxyurea [4].
Several genetic determinants are known to contribute to the heterogeneity of baseline Hb F levels in
SCD. Haplotypes of the bS chromosomes are among
the most extensively studied. Senegalese and Asian /
Indian haplotypes are associated with higher Hb F
levels, and therefore a milder clinical and hematologic
phenotype compared to the other African haplotypes
(Benin, Bantu, and Cameroon). The common feature
of the Senegal and Asian haplotypes is the presence of
a C 0/T polymorphism in the promoter of the Gg gene
(/158 C 0/T, detectable by the restriction enzyme
Xmn I). The search for other cis-acting regulatory
elements in the b-globin cluster has failed to yield
consistent results. Thus, it has become clear that bS
haplotypes and variations in cis-acting elements
associated with different haplotypes are only partially
93
responsible for the variation seen in Hb F levels
among SCD patients. This has prompted the search
for other trans-acting regulatory elements controlling
Hb F levels. One of these putative elements is a
quantitative trait locus (QTL) on the X-chromosome
(Xp22), which presumably controls the production of
F-cells. This F-cell production locus (FCP) and the
/158 C 0/T polymorphism in the Gg promoter are
estimated to account for /50% of the variation in Hb
F in SCD [5]. Other QTLs that influence Hb F levels
have been located at 6q22.3 /23.2, and 8q [6 /8]. In a
study of 180 single nucleotide polymorphisms (SNPs)
in 38 candidate genes in 280 SCD patients, the
strongest association with Hb F levels was found
with SNPs in the 6q 22.3 /23.2 region. Detailed
analyses of this region, identified 12 SNPs in the
introns of four genes, associated with a 20/30%
variation in Hb F [9]. The genes in question were
phosphodiesterase 7 (PDE7), microtubule-associated
protein 7 (MAP7), peroxisomal biogenesis factor 7
(PEX7) and mitogen-activated protein kinase 5
(MAP3K5). Although the precise mechanisms of
this effect have not yet been identified, there is some
data to suggest that the product of these genes may be
involved in the regulation of g-globin gene expression
in various cellular systems (K562 cells).
In a collaborative study between Boston University
and the Medical College of Georgia Sickle Cell
Centers, genetic determinants of Hb F response to
hydroxyurea was studied in 214 African /American
SCD patients. Two hundred twenty six SNPs in 46
candidate genes associated with Hb F regulation and
drug metabolism were analyzed. SNPs in two genes,
in CYP2C9, a member of the cytochrome P 450
family and in Aquaporin 9 (AQP9), a membrane
channel that stimulates urea transport and transports
uncharged solutes, were associated with a good
response to hydroxyurea [10].
Genetic polymorphisms as risk factors for stroke
Stroke occurs in 11% of US sickle cell patients by 20
years of age [11]. Increased TCD (Transcranial
Doppler) velocities of blood flow (/200 cm/sec) in
major intracranial arteries has been shown to be a
predictor of stroke risk in children with SCD. The
value of TCD as a surrogate marker of cerebrovascular disease and a predictor of stroke risk has been
validated in the randomized STOP trial, and a
therapeutic intervention based upon this risk stratification (prophylactic transfusions in children with high
TCD velocities) has been proven to reduce the stroke
risk [12]. Despite the validation of the utility of TCD
as a clinical tool for predicting stroke risk, biologic
factors underlying the development of cerebrovascular
disease and stroke in the SCD population are poorly
understood. The presence of a-thalassemia has been
shown to be protective of cerebrovascular disease and
94
A. Kutlar
stroke [11]. Several recent studies have sought an
association between certain genetic polymorphisms
and stroke risk in SCD. Driscoll et al. [13] found a
higher rate of stroke in siblings with SCD (P /
0.0012), suggesting the role of genetic factors. A
recent study has found an association with HLA
genotypes and stroke (HLA DRB1*0301 and HLA
DRB1*0302 genotypes were associated with increased stroke risk); of particular interest is the
association of certain genotypes with distinct subtypes
(large vessel vs. small vessel stroke) in a retrospective
study of the CSSCD population [14,15]. This latter
study found an association between an Interleukin-4
receptor polymorphism (IL-4R S503P), TNF-a 308G, and b-adrenergic receptor-2 (ADRb2 Q27E)
polymorphisms and large vessel stroke. In the small
vessel stroke group, VCAM T -1594C polymorphism
and low density lipoprotein receptor LDLR NcoI
polymorphism emerged as risk factors. The combination of TNF-a -308 GG homozygosity and the IL4RS503P polymorphism conferred a particularly
strong risk for large vessel stroke (Odds ratio /5.5)
in this study. Another recent study [16] reported a
protective role for VCAM G1238C polymorphism
against symptomatic stroke (odds ratio /0.35, P/
0.04). Steinberg et al. [17] studied 113 SCD patients
with a history of stroke and 493 controls. They found
that polymorphisms in four genes were associated
with stroke risk: Klotho (KL), TGF-beta receptor
(TGFbR3), Annexin 2 (ANXA2), and bone morphogenic protein 6 (BMP6). The same group of investigators utilized a Bayesian network approach to
analyze gene-gene interactions to develop a risk model
for stroke in SCD. They analyzed 235 SNPs in 80
candidate genes in1398 unrelated subjects with SCD.
In addition to four clinical variables including athalassemia and Hb F, they found that SNPs in 11
genes interacted in a complex manner to modulate
stroke risk. These included SNPs in BMP6, Transforming Growth Factor beta-receptors (TGFbR2 and
TGFbR3), and P-Selectin (SELP). Using this model
to predict stroke in a different group of SCD patients,
they reported an overall predictive accuracy of 98.2%
[18]. A recently completed study at our center is
looking at the association of high TCD (as a risk
factor for ischemic stroke) with 28 genetic polymorphisms in 25 candidate genes in 630 patients
(230 high TCD, 400 normal TCD) in STOP and
STOP II trials. The candidate genes include those
associated with coagulation and thrombophilia (Factor V, Factor VII, Factor XIII, prothrombin, thrombomodulin, fibrinogen, PAI-1, MTHFR), endothelial
cell function and inflammation (VCAM-1, ICAM-1,
selectins, TNF-a, Apo A and Apo E), platelet function and activation (GpIIb/IIIa, GpIb IX-V, GpIa/
IIIa), and vascular reactivity (ACE). The analysis of
polymorphisms was accomplished by a high throughput SNP genotyping method using the MALDI-TOF
Table II. Association of SNPs with stroke in SCD.
SNP
VCAM1 G1238C
VCAM1 T-1594C
IL4R S503P
TNF-a -308
LDLR/Nco1
ADRB2 Q27E
HLA genes
BMP6
TGFBR2
TGFBR3
P-selectin
Effect
Protective
Permissive
Permissive
Protective
Protective
Protective
Protective and
permissive
Permissive
Permissive
Permissive
Permissive
Reference
Taylor et al. 2002
Hoppe et al. 2004
Hoppe et al. 2004
Hoppe et al. 2004
Hoppe et al. 2004
Hoppe et al. 2004
Hoppe et al. 2002
Sebastiani et al. 2005
Sebastiani et al.
Sebastiani et al.
Sebastiani et al.
based MassarrayTM system (Sequenom, San Diego,
CA). Data analyses are currently ongoing. A summary
of SNPs associated with stroke risk is summarized in
Table II.
Avascular necrosis
Studies of large patient populations such as the
CSSCD (Cooperative Study of Sickle Cell Disease)
have shown that a-thalassemia, age, high hematocrit,
and frequent VOEs are risk factors for the development of avascular necrosis (AVN) of the femoral head
in SCD [19]. We undertook a survey of the frequency
of the thermolabile MTHFR (methylenetetrahydrofolate reductase) C677T mutation in the sickle cell
population, because of its association with elevated
serum homocysteine levels and resultant vascular
complications and in particular, its relationship to
avascular necrosis. Overall, MTHFR was present in
16% of the sickle cell patients in our center (1.8%
homozygote). There was a strong association of the
presence of MTHFR with AVN with 35.6% of the
AVN patients having the MTHFR mutation as
opposed to only 12.9% of sickle cell patients without
AVN (P /0.006). Furthermore, the presence of concomitant a-thalassemia and MTHFR was found to be
additive in terms of the risk of developing AVN [20].
This association however was not confirmed in the
high Hb F population of Kuwaiti sickle cell patients,
suggesting that different genetic factors may be
operative in different populations [21]. Some other
smaller studies in different patient populations have
also failed to show an association between this
MTHFR polymorphism and the risk of avascular
necrosis in SCD [22 /24]. A recently published study
[25] analyzed 442 patients with AVN and 455 SCD
controls from the CSSCD cohort. They studied SNPs
in 66 candidate genes and found significant association of AVN with 7 SNPs in 7 genes. These are:
BMP6, TGFBR2, TGFBR3, EDN1 (Endothelin-1),
ERG (v-ets erythroblastosis virus E26 oncogene like),
KL (Klotho), and ECE1 (Endothelin converting
Sickle cell disease
Table III. R485K Polymorphism and catheter induced thrombosis.
R485K
Thrombosis n/10
Control n/10
6
3
1
2
2
6
95
Table IV. UGT1A1 Polymorphism and bilirubin levels in sickle cell
disease.
7/7 (n/22) 6/7 (n/35) 6/6 (n /10)
///
///
///
P/0.006; Odds Ration/4.9.
enzyme 1). The precise mechanism(s) whereby variation in these genes causes AVN is not yet understood.
Catheter induced thrombosis and factor V
R485K polymorphism
First described as a neutral polymorphism in the
Factor V gene in nucleotide 1628 in exon 10 by
Gandrille et al. the Arg 0/Val substitution at residue
485 was found at high frequency (32.4%) in subSaharan Africans [26]. This polymorphism was later
associated with a risk of thrombosis and coronary
artery disease in Asian populations and reported to
lead to a mild activated protein C (APC) resistance
[27]. A study of the frequency of this polymorphism
in our sickle cell population showed a similar gene
frequency of 0.29 (45.1% heterozygous, 6.6% homozygous). Although no association of this polymorphism was established with some complications of SCD
(frequent VOEs, acute chest syndrome, leg ulcers,
AVN, and priapism), interestingly the presence of this
mutation was associated with an increased risk of
central venous catheter associated thrombosis (P/
0.006, odds ratio 4.9, Table III) [28].
UGT1A1 polymorphism and bilirubin levels
The gene UGT1A1 encodes the enzyme UDP Glucuronosyl Transferase-1 that mediates the glucuronidation of bilirubin. A common polymorphism in the
TATA box of this gene was found to be associated
with a decreased enzyme activity and indirect hyperbilirubinemia (Gilbert’s syndrome). The sequence
A(TA)6TAA is presumed the wild type while
A(TA)7TAA is associated with Gilbert’s syndrome.
The co-inheritance of this (TA)7 polymorphism with
hereditary hemolytic anemias, such as hereditary
spherocytosis, b-thalassemia, and G-6-PD deficiency
results in marked hyperbilirubinemia and increased
incidence of gallstones [29]. We, and others, have
studied the frequency of this polymorphism in the
sickle cell population and its impact on bilirubin levels
and incidence of gallstones and cholecystectomy [30 /
34]. In all of these studies, an association was found
between the (TA)7 genotype and bilirubin levels;
those with 7/7 had the highest mean bilirubin levels
compared to the genotypes with 6/6 and 6/7. Our
results are summarized in Table III. Furthermore, an
impact of the 7/7 genotype on the results of hydro-
Retics
Hb F
Total bilirubin
Cholecystectomy
10.9
6.6
5.6
59%
11.4
11.
3.8
51%
11.3
8.4
3.8
60%
xyurea therapy was recently reported by Heeney et al.
[35]. These authors reported a normalization of the
bilirubin levels in sickle cell patients with the 6/6
genotype upon treatment with hydroxyurea in contrast to those with 7/7 and 6/7 genotypes whose
bilirubins failed to fall to normal levels. An interesting
aspect of the effect of this polymorphism was reported
by Haverfield et al. in the Jamaican study. These
investigators found an association between the (TA)7/
(TA)7 genotype and symptomatic gallstones except
for the younger cohort. Similarly in our study, in
multivariate analyses, in the younger age group ( B/10
years of age), Hb F levels were found to be a more
important determinant of bilirubin levels then the
UGT1A1 genotype. These observations indicate that
the influence of genetic modifiers on a particular
phenotype may vary with age in patients with SCD.
Other phenotypes
Few studies exist on genetic risk factors that may be
associated with complications such as acute chest
syndrome, pulmonary hypertension, leg ulcers, and
priapism. Sharan et al. and reported an association of
acute chest syndrome in females with a T-786C
polymorphism in the endothelial NO synthase gene
[36]. In one study in 45 patients with pulmonary
hypertension as judged by a tricuspid regurgitant jet
of /2.5 m/sec, an association was found with SNPs
in BMPR2 (bone morphogenic protein receptor 2)
and ADCY6 (adenylate cyclase 6) [37]. In a study of
148 patients with SCD and history of priapism and
529 controls without priapism, it was shown that a
polymorphism in the klotho (KL) gene was associated
with an odds ratio of 2.6. [38]. In a study by AshleyKotch et al. 2005, longevity (survival /50 years of
age) was associated with three SNPs in the klotho
gene and SNPs in the nitric oxide synthase 2
(NOS2A) and (TGFbR2) genes [39]. No studies
have yet been reported showing an association of
polymorphisms with a risk of sickle nephropathy.
Gene expression profiling
Gene expression profiling utilizing cDNA microarrays
is a relatively novel method, particularly in terms of its
applications to the study of SCD. The quality of the
data obtained from microarray studies depends upon
several factors: (1) a meticulous study design, parti-
96
A. Kutlar
Table V. Demographic and hematologic data in mild vs. severe
SCD patients (mean values).
Mild (n/8) Severe (n/4) P-value
Gender
Age
Crisis/year
Hb g dL 1
HCT%
MCV fl
Retics%
WBC/109 l 1
ANC (per ml)
Platelets/109 l 1
Total Bilirubin mg dL 1
Hb F%
Figure 1. Granulocyte gene expression profiling: Severe and mild
patients vs. controls.
cularly a clear and unambiguous definition of the
‘‘phenotype(s)’’ to be studied, (2) elimination of
technical/methodological pitfalls (quantity/quality of
the RNA, methods of specimen collection and processing, conditions and timing of hybridization, elimination of intra-patient variation by repeated
sampling, etc.), and (3) expertise in bioinformatics
in the interpretation of vast amounts of data generated. In patients with SCD, the tissues that will be
6 M, 2 F
35.5
1.1
8.9
27.4
91
10.7
11.6
6005
376
3.8
6.3
2 M, 2 F
25
5.8
8.6
24.9
96
10.8
11.2
5850
341
6.2
10.7
0.03
0.0002
0.6
0.4
0.4
0.8
0.8
0.9
0.6
0.2
0.2
amenable to gene expression profiling studies include
peripheral blood (PB) cells, endothelial cells (EC)
(circulating EC, as well as EC from various tissues),
bone marrow (BM), and liver tissue.
Few published reports exist in this area. These
include the study of gene expression profiling from the
kidney tissue in transgenic sickle cell mice [40],
profiling of cultured human pulmonary artery endothelial cells exposed to plasma from patients with
SCD and acute chest syndrome and patients with
SCD in steady state [41,42], a study of differential
gene expression in PB cells from SCD patients with
different crisis frequency [43], and a study of gene
expression profiles form mononuclear cells in SCD
patients on and off hydroxyurea (HU) therapy [44].
Jison et al. purified mononuclear cells from PB in 27
patients with SCD in steady state, 10 of which were
on HU and 13 normal controls. They demonstrated
differential gene expression of 112 genes involved in
heme metabolism, cell cycle regulation, antioxidant
and stress responses, inflammation, and angiogenesis.
HU did not significantly alter mononuclear cell gene
expression. Klings et al. exposed cultured human
pulmonary artery endothelial cells to plasma from
normal volunteers as well as to plasma from SCD
patients at steady state and SCD patients with acute
chest syndrome. They found that 50 genes were
differentially expressed in endothelial cells upon
exposure to plasma from SCD patients in steady state
compared to normal plasma. These genes involved
cholesterol biosynthesis, lipid transfer, cellular stress
response, and extracellular matrix proteins. Upon
exposure to plasma from SCD patients with acute
chest, another 58 genes were differentially expressed.
We have conducted preliminary studies of gene
expression with commercially available cDNA microarrays (Affymetrix U95, Affymetrix, Santa Clara, CA)
from PB neutrophils and reticulocytes in SCD patients with discordant rates of painful episodes in an
effort to elucidate the factors contributing to different
crisis frequency. We analyzed the gene expression
profiles of neutrophils from four patients with ‘‘se-
Sickle cell disease
vere’’ disease ( /3 vaso-occlusive episodes [VOE] per
year), eight patients with ‘‘mild’’ disease (B/3 VOE/
year) and compared these to each other and to the
gene expression profiles of neutrophils from five age
and sex matched, healthy, non-sickle cell, African /
American individuals. A summary of demographic
features, mean hematologic values, and Hb F in
‘‘mild’’ and ‘‘severe’’ SCD patients is shown in Table
V. Figure 1 depicts differences in gene expression
patterns between non-sickle cell controls and mild
and severe SCD patients. In general, a larger number
of genes were differentially expressed between ‘‘mild’’
patients vs. controls, compared to that between
‘‘severe’’ vs. ‘‘mild’’ patients. Out of the differentially
expressed genes (314 genes for severe vs. control, 718
genes for mild vs. control), those with greater than
two fold expression were analyzed with the gene
MAPP software for localization into biological pathways. Genes related to cellular proliferation, growth
and maintenance, DNA repair, DNA replication, and
cell cycle progression were expressed at significantly
higher levels in SCD patients compared to controls.
The most significant finding was the significantly
higher expression of genes leading to NFkB activation
and inhibition of apoptosis: IAP-1 (increased 6.7 fold
and 4.7 fold in mild and severe patients respectively),
IkB (decreased 0.14 fold and 0.3 fold), Apaf-1
(decreased 0.4-fold in mild), and c-jun (decreased
0.4-fold in severe); Traf-2 (TNF receptor associated
factor-2; increased 3.5-fold and 2-fold); genes in the
MAPK signaling pathway: ERK-2 (increased 3.5-fold
97
and 2-fold), MAP2K3 (increased 3.5-fold and 2fold). These data show that neutrophils in SCD
patients are activated with higher expression of genes
in the TNF, MAPK, and NFkB pathways consistent
with an inflammatory state. Since neutrophil apoptosis is considered critical for the resolution of inflammation, delayed or inhibited apoptosis of neutrophils
would further maintain this inflammatory state, even
during the so-called ‘‘steady state: of the disease.
Further analyses and identification of differentially
expressed genes and pathways between ‘‘mild’’ patients vs. controls and ‘‘severe’’ vs. ‘‘mild’’ patients is
in progress. A list of some of the differentially
expressed genes in neutrophils from ‘‘severe’’ and
‘‘mild’’ patients is shown in Table VI. We conclude
that the analyses of gene expression in neutrophils can
be a useful tool in identifying pathways and genes that
distinguish SCD patients from controls and in differentiating mild and severe phenotypes.
The application of the microarray technology to the
study of SCD is still in its infancy. Although the
preliminary data look interesting, these results need to
be confirmed and validated through a large number of
experiments in a large number of patients. The
available data appear promising and show the feasibility of the application of this technology to SCD.
Conclusion
The development of high throughput genotyping
methods, the microarray technology, and the comple-
Table VI. Differentially expressed genes in severe vs. mild SCD patients.
Fold Difference
11.6
9.5
6.4
5.4
4.7
4.6
4.4
Gene ID
NAD Kinase
GPR43
BTG family
HLA-G2.2
FLJ13052
Similar to tumor necrosis factor
HLA-G1
3.8
3.2
HLA-G2.1
IFI30
3.1
3.0
2.6
2.4
HLA-DRB1
KIAA0991
Helix-loop-helix zipper protein
Aryl Hydrocarbon receptorinteracting protein
2.4
2.3
RPL28
A20
2.1
DUT
Gene Function
P value
Signal Tx, Energy Metabolism, Detox Rx
0.005
7 Transmem domain rec for signal tx for a neuroendocrine pp, Galanin 0.004
Cell cycle regulation, antiproliferative
0.003
0.002
0.005
Inhibits TRAIL (1.2) mediated apoptosis
0.007
Role in inhibiting natural killer cell function, tumors can escape from 0.004
immsurv.
0.006
Catalyzes disulfide bond reduction for proteolysis of the internalized 0.005
proteins. Const. Expressed in Ag pre cells and induced by IFNg in
other Imp in Ag presentation
0.005
Rearranged with MLL in therapy induced AML
0.009
NFkB activation by inhibiting the I-kB proteins
0.004
Aryl hydrocarbon receptor (AHR) / HSP90 complex translocates to 0.002
nucleus, HSP90 dissociates and activated AHR dimerizes with ARNT
w then bind enhancer elements to regulate transcription of xenobiotic
metabolic enzymes
Structural mammalian ribosomal protein
0.002
Inhibits NFkB activity and TNF mediated apoptosis. Critical for
0.004
limiting inflammation by terminating TNF induced NFkB responses.
TNF dramatically increases A20 expression in all ts.
Hydrolyzes d UTP to d UMP. Limits d UMP incorporation into DNA 0.008
during replication and repair and protects from fragmentation and
death.
98
A. Kutlar
tion of the Human Genome Project have already
begun to revolutionize medicine and biology. The
application of these techniques to the study of SCD is
in its very early stages and is likely to yield novel
information.
A candidate gene approach and study of genomewide SNPs and their association with certain complications of SCD is expected to further clarify the basis
of clinical/phenotypic heterogeneity of this single gene
disorder. The pros and cons of a candidate gene
approach vs. genome wide association studies need to
be balanced carefully. A candidate gene approach is
limited by our current knowledge of the pathophysiology and by necessity may omit some of the genetic
influences that may contribute to the genotype in
question. It is also of utmost importance to carefully
and unambiguously define the phenotype. Genomewide studies are likely to provide some information on
novel genetic influences; however, there limitation is
the unrealistically large sample size that may be
required. Application of methods such as the Bayesian
networks may be important in providing information
on gene /gene interactions.
Although the association studies and the results of
microarray data reviewed above are far from conclusive, some interesting points and recurring themes
have emerged. Many complications of SCD can be
related to variation in genes in the TGF-beta super
family and genes that play a role in vascular reactivity,
inflammation, and apoptosis. These are emerging also
from the microarray data. A study of gene expression
profiling coupled with proteomics will shed further
light into the functional aspects and differences
between patients. This information will not only
lead to a better understanding of the pathogenesis
and pathophysiology of many complications of the
disease, but will also likely result in the identification
of novel therapeutic targets and discovery of new
genes with prognostic and therapeutic implications.
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