Background: Cytokines play important roles in antiviral action. We examined whether polymorphisms of IFN-γ,TNF-α and IL-10 affect the susceptibility to and outcome of severe acute respiratory syndrome (SARS). Methods: A case-control study was carried out in 476 Chinese SARS patients and 449 healthy controls. We tested the polymorphisms of IFN-γ,TNF-α and IL-10 for their associations with SARS. Results: IFN- γ+874A allele was associated with susceptibility to SARS in a dose-dependent manner (P < 0.001). Individuals with IFN-γ +874 AA and AT genotype had a 5.19-fold (95% Confidence Interval [CI], 2.78-9.68) and 2.57-fold (95% CI, 1.35-4.88) increased risk of developing SARS respectively. The polymorphisms of IL-10 and TNF-α were not associated with SARS susceptibility. Conclusion: IFN-γ+874A allele was shown to be a risk factor in SARS susceptibility. © 2006 Po Chong et al; licensee BioMed Central Ltd.published_or_final_versio

Wai Po Chong

WK Eddie Ip

Gloria Hoi Wan Tso

Man Wai Ng

Wilfred Hing Sang Wong

Helen Ka Wai Law

Raymond WH Yung

Eudora Y Chow

KL Au

Eric YT Chan

Wilina Lim

JS Malik Peiris

Yu Lung Lau

JS Peiris

YL Lau

CM Booth

JW Chan

GM Leung

WK Ip

H Zhang

VS Chan

CA Biron

NJ Makhatadze

MS Mulligan

DF Fiorentino

KW Moore

G Inoue

JR Panuska

TJ Standiford

Y Kalechman

HW Tso

Z Ben-Ari

JR Kerr

V Pravica

C Scagnolari

B Sainz Jr

DM Turner

E Crawley

AG Wilson

BMC Infectious Diseases

Springer - Publisher Connector

BioMedCentralBMC Infectious Diseases
ssOpen AcceResearch article
The interferon gamma gene polymorphism +874 A/T is associated 
with severe acute respiratory syndrome
Wai Po  Chong†1, WK Eddie Ip†1, Gloria Hoi Wan Tso1, Man Wai Ng1, 
Wilfred Hing Sang Wong1, Helen Ka Wai Law1, Raymond WH Yung2, 
Eudora Y Chow3, KL Au4, Eric YT Chan5, Wilina Lim6, JS Malik Peiris7 and 
Yu Lung Lau*1
Address: 1Department of Paediatrics and Adolescent Medicine, The Hong Kong Jockey Club Research Centre, The University of Hong Kong, 
Pokfulam, Hong Kong SAR, China, 2Department of Pathology, Pamela Nethersole Youde Hospital, Hong Kong SAR, China, 3Department of 
Pathology, United Christian Hospital, Hong Kong SAR, China, 4Princess Margaret Hospital, Hong Kong SAR, China, 5Queen Mary Hospital, The 
University of Hong Kong, Hong Kong SAR, China, 6Government Virus Unit, Department of Health, Hong Kong SAR, China and 7Department of 
Microbiology, The Hong Kong Jockey Club Research Centre, Pokfulam, The University of Hong Kong, Hong Kong SAR, China
Email: Wai Po  Chong - h9820905@graduate.hku.hk; WK Eddie Ip - WIP@partners.org; Gloria Hoi Wan Tso - h98083405@hkusua.hku.hk; 
Man Wai Ng - ivy_natsu@yahoo.com.hk; Wilfred Hing Sang Wong - whswong@hkucc.hku.hk; Helen Ka Wai Law - hkwlaw@hkucc.hku.hk; 
Raymond WH Yung - rwhyung@ha.org.hk; Eudora Y Chow - chowe@ha.org.hk; KL Au - klau@ha.org.hk; Eric YT Chan - eytchan@ha.org.hk; 
Wilina Lim - wllim@pacific.net.hk; JS Malik Peiris - malik@hkucc.hku.hk; Yu Lung Lau* - lauylung@hkucc.hku.hk
* Corresponding author    †Equal contributors
Abstract
Background: Cytokines play important roles in antiviral action. We examined whether
polymorphisms of IFN-γ,TNF-α and IL-10 affect the susceptibility to and outcome of severe acute
respiratory syndrome (SARS).
Methods: A case-control study was carried out in 476 Chinese SARS patients and 449 healthy
controls. We tested the polymorphisms of IFN-γ,TNF-α and IL-10 for their associations with SARS.
Results: IFN-γ +874A allele was associated with susceptibility to SARS in a dose-dependent manner
(P < 0.001). Individuals with IFN-γ +874 AA and AT genotype had a 5.19-fold (95% Confidence
Interval [CI], 2.78-9.68) and 2.57-fold (95% CI, 1.35-4.88) increased risk of developing SARS
respectively. The polymorphisms of IL-10 and TNF-α were not associated with SARS susceptibility.
Conclusion: IFN-γ +874A allele was shown to be a risk factor in SARS susceptibility.
Background
Severe acute respiratory syndrome (SARS) is an infectious
disease caused by SARS coronavirus [1] with >8000 cases
and 774 deaths reported in 2003 [2]. Much progress has
been made in understanding SARS coronavirus but the
pathogenesis is still unclear [3]. It was reported that old
age, diabetes mellitus and heart disease were risk factors
for adverse prognosis of SARS [4-6], however, little is
known about the contribution of genetic factors. We have
demonstrated that genetic haplotypes associated with low
serum mannose-binding lectin (MBL) were associated
with SARS [7] and our findings were recently replicated
[8]. Recently, homozygotes for CLEC4M tandem repeats
Published: 04 May 2006
BMC Infectious Diseases2006, 6:82 doi:10.1186/1471-2334-6-82
Received: 07 February 2006
Accepted: 04 May 2006
This article is available from: http://www.biomedcentral.com/1471-2334/6/82
© 2006Po Chong et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), 
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Page 1 of 4
(page number not for citation purposes)
BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82were reported to be less susceptible to SARS in Hong Kong
Chinese [9].
Cytokines are known to be important in antiviral action.
Interferon (IFN)-γ from T and natural killer (NK) cells is
important in driving the T helper cell type 1 (Th1)
responses. It also activates monocytes and macrophages,
which in turn take part in antiviral responses by produc-
ing free radicals and pro-inflammatory cytokines like
tumor necrosis factor (TNF)-α. [10]. TNF-α then regulates
expression of neutrophil-endothelial cell adhesion mole-
cules and chemokines, which recruit leukocytes to the site
of infection [11-13]. Thus, IFN-γand TNF-α play impor-
tant role in antiviral response and inflammation.
Interleukin 10 (IL-10) is an antiinflammatory cytokine
that inhibits the activation and effector function of Th1
cells, monocytes, and macrophages [14]. IL-10 appears to
limit and ultimately terminate inflammatory responses by
blocking the expression of a number of pro-inflammatory
cytokines and chemokines [15]. In animal model, IL10
counteracts the inflammatory response by inhibiting TNF-
α production and neutrophil activation, and leads to a
reduction of the lung tissue injury [16]. Thus, IL-10 plays
an important role in regulating many immune and
inflammatory processes. Various studies showed that a
high IL-10 level would result in suppression of innate host
defense and lead to increasing susceptibility of the host to
various microbes and death [17-19].
In this study, we hypothesized that the polymorphisms of
the cytokine genes, i.e. IFN-γ +874A/T, TNF-α -308G/A,
IL-10 -1082G/A and -592A/C, might be associated with
SARS. These genes were chosen based on their functions
in antiviral response and inflammation regulation that
may be involved in SARS pathogenesis and their polymor-
phisms based on their potential regulation on gene
expression (Table 1). We tested our hypotheses in 476
SARS patients and 449 healthy controls and found that
polymorphism of IFN-γ +874A allele was associated with
susceptibility to SARS in a dose-dependent manner.
Methods
Patient populations
The study was approved by the Clinical Research Ethics
Committee of the Institutional Review Board of the Uni-
versity of Hong Kong/Hospital Authority Hong Kong
West Cluster and included 476 Chinese patients with
SARS (201 male, mean age = 39.8 ± 15.2) and 449 ethni-
cally matched healthy controls from Red Cross (273 male,
mean age = 29.1 ± 10.4). At least 95% of the patients were
documented with SARS-CoV antibody seroconversion
and/or detectable SARS-CoV RNA in respiratory secretions
by RT-PCR.
Genotyping
IFN-γ +874A/T, IL-10 -1082G/A and -592A/C were geno-
typed by TaqMan system (Applied Biosystems, Foster
City, CA, USA) as described previously [20]. TNF-α -308
G/A was also genotyped by TaqMan system with same
condition. The sequences of the primers were 5'-CCT GGT
CCC CAA AAG AAA TG-3' and 5'-TCT TCT GGG CCA CTG
ACT GA-3' and the probes were 6-FAM-TTG AGG GGC
ATG GGG ACG G-TAMRA and VIC-TTG AGG GGC ATG
AGG ACG GG-TAMRA.
Statistical analysis
The frequencies of genotypes and alleles of the 4 single
nucleotide polymorphisms (SNPs) were compared
between the SARS patients and healthy controls by 3 × 2
and 2 × 2 chi square test respectively. In case of signifi-
cance, logistic regression was used for calculating OR with
95% CI and corresponding P-values between groups by
controlling age and sex as covariables. The genotypes of
all SNPs were tested for Hardy-Weinberg equilibrium
(HWE) by chi square test.
Results and discussion
Our case-control study genotyped the 4 SNPs IFN-γ
+874A/T, TNF-α -308G/A, IL-10 -1082G/A and -592A/C
in 476 Chinese patients with SARS and 449 healthy con-
trols. The genotype distributions and allele frequencies of
these 4 SNPs were shown in Table 2. The IFN-γ +874A
allele was overrepresented in SARS patients (83.1%) when
compared with the controls (66.3%) (P < 0.001). It was
also significantly associated with susceptibility to SARS in
a dose-dependent manner (P < 0.001), i.e. individuals
with IFN-γ +874 AA and AT genotype had an odds ratio
(OR) of 5.19 (95% CI, 2.78-9.68) and 2.57 (95% CI,
1.35-4.88) in developing SARS respectively. However, no
significant correlation was observed in SNPs of IL-10 and
TNF-α. All SNPs were in Hardy-Weinberg equilibrium
(HWE) (P > 0.05) in SARS patients and controls by chi
square test, except IL-10-592A/C.
IFN-γ +874A allele has been previously reported to be
associated with infectious diseases such as tuberculosis,
hepatitis B virus infection, and parvovirus infection [20-
22], revealing its potential role of function in host defense
against microbial infections. The mechanism by which
the IFN-γ +874A/T allele influences the susceptibility to
Table 1: Polymorphisms of the genes genotyped
Genes SNPs rs number References
IFN-γ IFN-γ +874 A/T rs2430561 [23]
IL-10 IL-10 -1082 A/G rs1800896 [23,26-27]
IL-10 -592 A/C rs1800872
TNF-α TNF-α -308G/A rs1800629 [28]Page 2 of 4
(page number not for citation purposes)
BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82SARS may depend on its role in the regulation of IFN-γ
production. The T allele of IFN-γ +874A/T provides a
binding site for the transcription factor nuclear factor-κB
(NF-κB), which is able to regulate IFN-γ expression [23].
It is possible that low IFN-γ production may impair their
anti-viral response against SARS-CoV, rendering these
individuals more susceptible to this virus infection. Our
observation that IFN-γ +874A allele was significantly asso-
ciated with SARS-CoV infection suggests a genetic risk fac-
tor for SARS. The role of IFN-γ in antiviral response against
SARS-CoV has also been supported by recent studies
showing that IFN-γ can inhibit the replication of SARS-
CoV in combination with IFN-β in vitro [24,25].
IL-10 and TNF-α SNPs were also included in this study.
They were chosen due to their potential regulation on pro-
tein expression level [26-28]. However, our present data
did not show any significant association of these SNPs
with SARS (Table 2). Nevertheless, we cannot exclude the
role of IL-10 and TNF-α as the susceptibility genes for
SARS, because other SNPs in these 2 genes may also be
involved in gene expression regulation. Further associa-
tion studies on other SNPs, which could alter the gene
expression level are required to ascertain the relationship
of IL-10 and TNF-α in SARS.
We have also compared the genotype and allele frequen-
cies of all the polymorphisms between the death group
and survival group of the SARS patients (Table 3). How-
ever, no significant association was established.
Conclusion
We demonstrated that IFN-γ +874A allele was signifi-
cantly associated with SARS susceptibility in a dose
dependent manner. Due to its role in regulating IFN-γ
expression [15], this allele may be involved in the patho-
genesis of SARS by altering the IFN-γ production.
Competing interests
The author(s) declare that they have no competing inter-
ests.
Authors' contributions
WPC and WKEI: Genotyping, data analyses, drafting the
manuscript
GHWT: Genotyping
MWN and WHSW: Data analyses, drafting the manuscript
Table 3: Genotype frequencies among survival and death SARS 
cases
SNP Death (n = 57) Survival (n = 415) P
Number (%)
Genotype
IFN-γ +874 NS
A/A 41 (71.9) 289 (69.6)
A/T 13 (22.8) 112 (27.0)
T/T 3 (5.3) 14 (3.4)
IL-10-1082 NS
A/A 52 (91.2) 383 (92.3)
A/G 4 (7.0) 31 (7.5)
G/G 1 (1.8) 1 (0.2)
IL-10-592 NS
A/A 28 (49.1) 214 (51.6)
A/C 21 (36.8) 165 (39.8)
C/C 8 (14.0) 36 (8.7)
TNF-a-308 NS
GG 46 (80.7) 353 (85.1)
GA 11 (19.3) 59 (14.2)
AA 0 (0) 3 (0.7)
NS = not significant.
Table 2: Allele frequencies and genotype frequencies in SARS 
patients and controls*
SNP SARS 
(n = 476)
Control 
(n = 449)
OR (95% CI) P
Number (%)
Genotype
IFN-γ +874 <0.001
A/A 332 (69.8) 203 (45.2) 5.19 (2.78 – 9.68)
A/T 127 (26.7) 189 (42.1) 2.57 (1.35 – 4.88)
T/T 17 (3.6) 57 (12.7) Reference
IL-10 -1082 NS
A/A 439 (92.2) 411 (91.5) ..
A/G 35 (7.4) 38 (8.5) ..
G/G 2 (0.4) 0 (0) ..
IL-10 -592 NS
A/A 244 (51.3) 209 (46.6) ..
A/C 188 (39.5) 214 (47.7) ..
C/C 44 (9.2) 26 (5.8) ..
TNF-a -308 NS
GG 403 (84.7) 377 (83.9) ..
GA 70 (14.7) 70 (15.6) ..
AA 3 (0.6) 2 (0.5) ..
Allele
IFN-γ +874 <0.001
A 791 (83.1) 595 (66.3) 2.23 (1.75 – 2.83)
T 161 (16.9) 303 (33.7)
IL-10 -1082 NS
A 913 (95.9) 860 (95.8) ..
G 39 (4.1) 38 (4.2) ..
IL-10 -592 NS
A 676 (71.0) 632 (70.4) ..
C 276 (29.0) 266 (29.6) ..
TNF-a -308 NS
G 876 (92.0) 824 (91.8) ..
A 76 (8.0) 74 (8.2) ..
NS = not significant.
*P-value and OR (95% CI) were calculated with the use of logistic 
regression models, adjusted with sex and age.Page 3 of 4
(page number not for citation purposes)
BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82HKWL, RWHY, EYC, KLA, EYTC: Sample collection, revis-
ing for medical content
WL and JSMP: Sample collection, providing virological
data
YLL: Study design, conception and co-ordination, drafting
the manuscript
All authors contributed to writing of the final manuscript
All authors read and approved the final manuscript
Acknowledgements
This work is supported by the Outstanding Researcher Awards (YLL & 
JSMP), Postgraduate Studentships (WPC, GHWT, MWN) from the Univer-
sity of Hong Kong, the Research Fund for the Control of Infectious Dis-
eases (03040302) from the Health, Welfare and Food Bureau of theHong 
Kong SAR Government and Edward Sai Kim Hotung Paediatric Education 
and Research Fund.
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The interferon gamma gene polymorphism +874 A/T is associated with severe acute respiratory syndrome

Chong, WP

Ip, WKE

Tso, GHW

Ng, MW

Wong, WHS

Law, HKW

Yung, RWH

Chow, EY

Au, KL

Chan, EYT

Lim, W

Peiris, JSM

Lau, YL

HKU Scholars Hub

Title The interferon gamma gene polymorphism +874 A/T isassociated with severe acute respiratory syndromeAuthor(s)Chong, WP; Ip, WKE; Tso, GHW; Ng, MW; Wong, WHS; Law,HKW; Yung, RWH; Chow, EY; Au, KL; Chan, EYT; Lim, W; Peiris,JSM; Lau, YLCitation Bmc Infectious Diseases, 2006, v. 6Issued Date 2006URL http://hdl.handle.net/10722/45169Rights Creative Commons: Attribution 3.0 Hong Kong LicenseBioMed CentralBMC Infectious DiseasesssOpen AcceResearch articleThe interferon gamma gene polymorphism +874 A/T is associated with severe acute respiratory syndromeWai Po  Chong†1, WK Eddie Ip†1, Gloria Hoi Wan Tso1, Man Wai Ng1, Wilfred Hing Sang Wong1, Helen Ka Wai Law1, Raymond WH Yung2, Eudora Y Chow3, KL Au4, Eric YT Chan5, Wilina Lim6, JS Malik Peiris7 and Yu Lung Lau*1Address: 1Department of Paediatrics and Adolescent Medicine, The Hong Kong Jockey Club Research Centre, The University of Hong Kong, Pokfulam, Hong Kong SAR, China, 2Department of Pathology, Pamela Nethersole Youde Hospital, Hong Kong SAR, China, 3Department of Pathology, United Christian Hospital, Hong Kong SAR, China, 4Princess Margaret Hospital, Hong Kong SAR, China, 5Queen Mary Hospital, The University of Hong Kong, Hong Kong SAR, China, 6Government Virus Unit, Department of Health, Hong Kong SAR, China and 7Department of Microbiology, The Hong Kong Jockey Club Research Centre, Pokfulam, The University of Hong Kong, Hong Kong SAR, ChinaEmail: Wai Po  Chong - h9820905@graduate.hku.hk; WK Eddie Ip - WIP@partners.org; Gloria Hoi Wan Tso - h98083405@hkusua.hku.hk; Man Wai Ng - ivy_natsu@yahoo.com.hk; Wilfred Hing Sang Wong - whswong@hkucc.hku.hk; Helen Ka Wai Law - hkwlaw@hkucc.hku.hk; Raymond WH Yung - rwhyung@ha.org.hk; Eudora Y Chow - chowe@ha.org.hk; KL Au - klau@ha.org.hk; Eric YT Chan - eytchan@ha.org.hk; Wilina Lim - wllim@pacific.net.hk; JS Malik Peiris - malik@hkucc.hku.hk; Yu Lung Lau* - lauylung@hkucc.hku.hk* Corresponding author    †Equal contributorsAbstractBackground: Cytokines play important roles in antiviral action. We examined whetherpolymorphisms of IFN-γ,TNF-α and IL-10 affect the susceptibility to and outcome of severe acuterespiratory syndrome (SARS).Methods: A case-control study was carried out in 476 Chinese SARS patients and 449 healthycontrols. We tested the polymorphisms of IFN-γ,TNF-α and IL-10 for their associations with SARS.Results: IFN-γ +874A allele was associated with susceptibility to SARS in a dose-dependent manner(P < 0.001). Individuals with IFN-γ +874 AA and AT genotype had a 5.19-fold (95% ConfidenceInterval [CI], 2.78-9.68) and 2.57-fold (95% CI, 1.35-4.88) increased risk of developing SARSrespectively. The polymorphisms of IL-10 and TNF-α were not associated with SARS susceptibility.Conclusion: IFN-γ +874A allele was shown to be a risk factor in SARS susceptibility.BackgroundSevere acute respiratory syndrome (SARS) is an infectiousdisease caused by SARS coronavirus [1] with >8000 casesand 774 deaths reported in 2003 [2]. Much progress hasbeen made in understanding SARS coronavirus but thepathogenesis is still unclear [3]. It was reported that oldage, diabetes mellitus and heart disease were risk factorsfor adverse prognosis of SARS [4-6], however, little isknown about the contribution of genetic factors. We havedemonstrated that genetic haplotypes associated with lowserum mannose-binding lectin (MBL) were associatedwith SARS [7] and our findings were recently replicated[8]. Recently, homozygotes for CLEC4M tandem repeatsPublished: 04 May 2006BMC Infectious Diseases2006, 6:82 doi:10.1186/1471-2334-6-82Received: 07 February 2006Accepted: 04 May 2006This article is available from: http://www.biomedcentral.com/1471-2334/6/82© 2006Po Chong et al; licensee BioMed Central Ltd.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Page 1 of 4(page number not for citation purposes)BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82were reported to be less susceptible to SARS in Hong KongChinese [9].Cytokines are known to be important in antiviral action.Interferon (IFN)-γ from T and natural killer (NK) cells isimportant in driving the T helper cell type 1 (Th1)responses. It also activates monocytes and macrophages,which in turn take part in antiviral responses by produc-ing free radicals and pro-inflammatory cytokines liketumor necrosis factor (TNF)-α. [10]. TNF-α then regulatesexpression of neutrophil-endothelial cell adhesion mole-cules and chemokines, which recruit leukocytes to the siteof infection [11-13]. Thus, IFN-γand TNF-α play impor-tant role in antiviral response and inflammation.Interleukin 10 (IL-10) is an antiinflammatory cytokinethat inhibits the activation and effector function of Th1cells, monocytes, and macrophages [14]. IL-10 appears tolimit and ultimately terminate inflammatory responses byblocking the expression of a number of pro-inflammatorycytokines and chemokines [15]. In animal model, IL10counteracts the inflammatory response by inhibiting TNF-α production and neutrophil activation, and leads to areduction of the lung tissue injury [16]. Thus, IL-10 playsan important role in regulating many immune andinflammatory processes. Various studies showed that ahigh IL-10 level would result in suppression of innate hostdefense and lead to increasing susceptibility of the host tovarious microbes and death [17-19].In this study, we hypothesized that the polymorphisms ofthe cytokine genes, i.e. IFN-γ +874A/T, TNF-α -308G/A,IL-10 -1082G/A and -592A/C, might be associated withSARS. These genes were chosen based on their functionsin antiviral response and inflammation regulation thatmay be involved in SARS pathogenesis and their polymor-phisms based on their potential regulation on geneexpression (Table 1). We tested our hypotheses in 476SARS patients and 449 healthy controls and found thatpolymorphism of IFN-γ +874A allele was associated withsusceptibility to SARS in a dose-dependent manner.MethodsPatient populationsThe study was approved by the Clinical Research EthicsCommittee of the Institutional Review Board of the Uni-versity of Hong Kong/Hospital Authority Hong KongWest Cluster and included 476 Chinese patients withSARS (201 male, mean age = 39.8 ± 15.2) and 449 ethni-cally matched healthy controls from Red Cross (273 male,mean age = 29.1 ± 10.4). At least 95% of the patients weredocumented with SARS-CoV antibody seroconversionand/or detectable SARS-CoV RNA in respiratory secretionsby RT-PCR.GenotypingIFN-γ +874A/T, IL-10 -1082G/A and -592A/C were geno-typed by TaqMan system (Applied Biosystems, FosterCity, CA, USA) as described previously [20]. TNF-α -308G/A was also genotyped by TaqMan system with samecondition. The sequences of the primers were 5'-CCT GGTCCC CAA AAG AAA TG-3' and 5'-TCT TCT GGG CCA CTGACT GA-3' and the probes were 6-FAM-TTG AGG GGCATG GGG ACG G-TAMRA and VIC-TTG AGG GGC ATGAGG ACG GG-TAMRA.Statistical analysisThe frequencies of genotypes and alleles of the 4 singlenucleotide polymorphisms (SNPs) were comparedbetween the SARS patients and healthy controls by 3 × 2and 2 × 2 chi square test respectively. In case of signifi-cance, logistic regression was used for calculating OR with95% CI and corresponding P-values between groups bycontrolling age and sex as covariables. The genotypes ofall SNPs were tested for Hardy-Weinberg equilibrium(HWE) by chi square test.Results and discussionOur case-control study genotyped the 4 SNPs IFN-γ+874A/T, TNF-α -308G/A, IL-10 -1082G/A and -592A/Cin 476 Chinese patients with SARS and 449 healthy con-trols. The genotype distributions and allele frequencies ofthese 4 SNPs were shown in Table 2. The IFN-γ +874Aallele was overrepresented in SARS patients (83.1%) whencompared with the controls (66.3%) (P < 0.001). It wasalso significantly associated with susceptibility to SARS ina dose-dependent manner (P < 0.001), i.e. individualswith IFN-γ +874 AA and AT genotype had an odds ratio(OR) of 5.19 (95% CI, 2.78-9.68) and 2.57 (95% CI,1.35-4.88) in developing SARS respectively. However, nosignificant correlation was observed in SNPs of IL-10 andTNF-α. All SNPs were in Hardy-Weinberg equilibrium(HWE) (P > 0.05) in SARS patients and controls by chisquare test, except IL-10-592A/C.IFN-γ +874A allele has been previously reported to beassociated with infectious diseases such as tuberculosis,hepatitis B virus infection, and parvovirus infection [20-22], revealing its potential role of function in host defenseagainst microbial infections. The mechanism by whichthe IFN-γ +874A/T allele influences the susceptibility toTable 1: Polymorphisms of the genes genotypedGenes SNPs rs number ReferencesIFN-γ IFN-γ +874 A/T rs2430561 [23]IL-10 IL-10 -1082 A/G rs1800896 [23,26-27]IL-10 -592 A/C rs1800872TNF-α TNF-α -308G/A rs1800629 [28]Page 2 of 4(page number not for citation purposes)BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82SARS may depend on its role in the regulation of IFN-γproduction. The T allele of IFN-γ +874A/T provides abinding site for the transcription factor nuclear factor-κB(NF-κB), which is able to regulate IFN-γ expression [23].It is possible that low IFN-γ production may impair theiranti-viral response against SARS-CoV, rendering theseindividuals more susceptible to this virus infection. Ourobservation that IFN-γ +874A allele was significantly asso-ciated with SARS-CoV infection suggests a genetic risk fac-tor for SARS. The role of IFN-γ in antiviral response againstSARS-CoV has also been supported by recent studiesshowing that IFN-γ can inhibit the replication of SARS-CoV in combination with IFN-β in vitro [24,25].IL-10 and TNF-α SNPs were also included in this study.They were chosen due to their potential regulation on pro-tein expression level [26-28]. However, our present datadid not show any significant association of these SNPswith SARS (Table 2). Nevertheless, we cannot exclude therole of IL-10 and TNF-α as the susceptibility genes forSARS, because other SNPs in these 2 genes may also beinvolved in gene expression regulation. Further associa-tion studies on other SNPs, which could alter the geneexpression level are required to ascertain the relationshipof IL-10 and TNF-α in SARS.We have also compared the genotype and allele frequen-cies of all the polymorphisms between the death groupand survival group of the SARS patients (Table 3). How-ever, no significant association was established.ConclusionWe demonstrated that IFN-γ +874A allele was signifi-cantly associated with SARS susceptibility in a dosedependent manner. Due to its role in regulating IFN-γexpression [15], this allele may be involved in the patho-genesis of SARS by altering the IFN-γ production.Competing interestsThe author(s) declare that they have no competing inter-ests.Authors' contributionsWPC and WKEI: Genotyping, data analyses, drafting themanuscriptGHWT: GenotypingMWN and WHSW: Data analyses, drafting the manuscriptTable 3: Genotype frequencies among survival and death SARS casesSNP Death (n = 57) Survival (n = 415) PNumber (%)GenotypeIFN-γ +874 NSA/A 41 (71.9) 289 (69.6)A/T 13 (22.8) 112 (27.0)T/T 3 (5.3) 14 (3.4)IL-10-1082 NSA/A 52 (91.2) 383 (92.3)A/G 4 (7.0) 31 (7.5)G/G 1 (1.8) 1 (0.2)IL-10-592 NSA/A 28 (49.1) 214 (51.6)A/C 21 (36.8) 165 (39.8)C/C 8 (14.0) 36 (8.7)TNF-a-308 NSGG 46 (80.7) 353 (85.1)GA 11 (19.3) 59 (14.2)AA 0 (0) 3 (0.7)NS = not significant.Table 2: Allele frequencies and genotype frequencies in SARS patients and controls*SNP SARS (n = 476)Control (n = 449)OR (95% CI) PNumber (%)GenotypeIFN-γ +874 <0.001A/A 332 (69.8) 203 (45.2) 5.19 (2.78 – 9.68)A/T 127 (26.7) 189 (42.1) 2.57 (1.35 – 4.88)T/T 17 (3.6) 57 (12.7) ReferenceIL-10 -1082 NSA/A 439 (92.2) 411 (91.5) ..A/G 35 (7.4) 38 (8.5) ..G/G 2 (0.4) 0 (0) ..IL-10 -592 NSA/A 244 (51.3) 209 (46.6) ..A/C 188 (39.5) 214 (47.7) ..C/C 44 (9.2) 26 (5.8) ..TNF-a -308 NSGG 403 (84.7) 377 (83.9) ..GA 70 (14.7) 70 (15.6) ..AA 3 (0.6) 2 (0.5) ..AlleleIFN-γ +874 <0.001A 791 (83.1) 595 (66.3) 2.23 (1.75 – 2.83)T 161 (16.9) 303 (33.7)IL-10 -1082 NSA 913 (95.9) 860 (95.8) ..G 39 (4.1) 38 (4.2) ..IL-10 -592 NSA 676 (71.0) 632 (70.4) ..C 276 (29.0) 266 (29.6) ..TNF-a -308 NSG 876 (92.0) 824 (91.8) ..A 76 (8.0) 74 (8.2) ..NS = not significant.*P-value and OR (95% CI) were calculated with the use of logistic regression models, adjusted with sex and age.Page 3 of 4(page number not for citation purposes)BMC Infectious Diseases 2006, 6:82 http://www.biomedcentral.com/1471-2334/6/82HKWL, RWHY, EYC, KLA, EYTC: Sample collection, revis-ing for medical contentWL and JSMP: Sample collection, providing virologicaldataYLL: Study design, conception and co-ordination, draftingthe manuscriptAll authors contributed to writing of the final manuscriptAll authors read and approved the final manuscriptAcknowledgementsThis work is supported by the Outstanding Researcher Awards (YLL & JSMP), Postgraduate Studentships (WPC, GHWT, MWN) from the Univer-sity of Hong Kong, the Research Fund for the Control of Infectious Dis-eases (03040302) from the Health, Welfare and Food Bureau of theHong Kong SAR Government and Edward Sai Kim Hotung Paediatric Education and Research Fund.References1. 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The interferon gamma gene polymorphism +874 A/T is associated with severe acute respiratory syndrome

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