Variation in the Selenoenzyme Genes and Risk of
Advanced Distal Colorectal Adenoma
Ulrike Peters,1,2Nilanjan Chatterjee,3Richard B. Hayes,3Robert E. Schoen,4
Yinghui Wang,1Stephen J. Chanock,5and Charles B. Foster6
1Public Health Science, Fred Hutchinson Cancer Research Center;
University of Washington, Seattle, Washington;
NIH, Department of Health and Human Services, Rockville, Maryland;
5Core Genotype Facility, National Cancer Institute, NIH, Department of Health and Human Services, Gaithersburg,
2Department of Epidemiology, School of Public Health,
3Division of Cancer Epidemiology and Genetics, National Cancer Institute,
4University of Pittsburgh, Pittsburgh, Pennsylvania;
6Division of Pediatrics, The Cleveland Clinic, Cleveland, Ohio
Background: Epidemiologic and animal studies provide
evidence for a chemopreventive effect of selenium on
colorectal cancer, which may be mediated by the
antioxidative and anti-inflammatory properties of
selenoenzymes. We therefore investigated whether
genetic variants in selenoenzymes abundantly
expressed in the colon are associated with advanced
colorectal adenoma, a cancer precursor.
Methods: Cases with a left-sided advanced adenoma
(n = 772) and matched controls (n = 777) screen negative
for polyps based on sigmoidoscopy examination were
randomly selected from participants in the Prostate,
Lung, Colorectal, and Ovarian Cancer Screening Trial.
The underlying genetic variation was determined by
resequencing. We genotyped 44 tagging single nucle-
otide polymorphisms (SNP) in six genes [glutathione
peroxidase 1-4 (GPX1, GPX2, GPX3, and GPX4),
selenoprotein P (SEPP1), and thioredoxin reductase 1
(TXNRD1)] to efficiently predict common variation
across these genes.
Results: Four variants in SEPP1 were significantly
associated with advanced adenoma risk. A rare variant
in the 5¶ region of SEPP1 (-4166C>G) was present in
nine cases but in none of the controls (exact P = 0.002).
Three SNPs located in the 3¶ region of SEPP1, which
is overlapping with the promoter region of an
antisense transcript, were significantly associated with
adenoma risk: homozygotes at two SEPP1 loci (31,174
bp 3¶ of STP A>G and 43,881 bp 3¶ of STP G>A) were
associated with increased adenoma risk [odds ratio
(OR), 1.48; 95% confidence interval (95% CI), 1.00-2.19
and OR, 1.53; 95% CI, 1.05-2.22, respectively] and the
variant SEPP1 44,321 bp 3¶ of STP C>T was associated
with a reduced adenoma risk (CT versus CC OR, 0.85;
95% CI, 0.63-1.15). Furthermore, we observed a
significant 80% reduction for advanced colorectal
adenoma risk for carriers of the variant allele at
TXNRD1 IVS1-181C>G (OR, 0.20; 95% CI, 0.07-0.55;
Ptrend = 0.004). Consistent with the individual SNP
results, we observed a significant overall association
with adenoma risk for SEPP1 and TXNRD1 (global
P = 0.02 and 0.008, respectively) but not for the four
Conclusion: Our study suggests that genetic variants
at or near the SEPP1 and TXNRD1 loci may be
associated with advanced colorectal adenoma. As this
is the first study to comprehensively investigate this
hypothesis, confirmation in independent study popu-
lations is needed.
(Cancer Epidemiol Biomarkers Prev
An increasing number of studies suggests that the
essential trace element selenium is a preventive agent
for colorectal carcinogenesis. Evidence arises from the
Nutritional Prevention of Cancer Trial, a randomized
study to evaluate selenium supplementation and skin
cancer prevention, which observed, as secondary end-
point, a 61% reduction in colorectal cancer (1). This
association was further confirmed, although slightly
attenuated, when based on 2 additional years of follow-
up (2). Furthermore, several blood-based observational
studies, including ours (3), support a beneficial effect of
selenium on colorectal carcinogenesis (3-10).
Important biological activities of the essential trace
element selenium are mediated through the function of
selenoenzymes. Selenium is incorporated into the active
center of selenoenzymes as selenocysteine, which was
only recently discovered as the 21st naturally occurring
amino acid (11). Compared with other amino acids,
selenocysteine occurs infrequently in a small number of
proteins (12); however, it is located in the active center
of the selenoenzymes. The unique redox characteristics
of selenocysteine confer important antioxidant properties
to these selenoenzymes, which can reduce reactive
oxygen species and thereby prevent damage of impor-
tant biomolecules, including DNA, RNA, lipids, proteins,
and membranes; reactive oxygen species–induced DNA
damage is known to promote tumor progression
(13-18). Because of the direct contact of the colonic
epithelial cells with microbial- and food-derived reactive
oxygen species, the gastrointestinal tract may be partic-
ularly susceptible to oxidative damage (19-23). In
addition, selenium and selenoenzyme activity may
Cancer Epidemiol Biomarkers Prev 2008;17(5). May 2008
Received 12/29/07; revised 2/20/08; accepted 2/25/08.
Requests for reprints: Ulrike Peters, Public Health Science, Fred Hutchinson Cancer
Research Center, P.O. Box 19024, 1100 Fairview Avenue North, M4-B402, Seattle, WA
98109-1024. Phone: 206-667-2450; Fax: 206-667-7850. E-mail: email@example.com
Copyright D 2008 American Association for Cancer Research.
reduce inflammatory processes, known triggers for
colorectal carcinogenesis (24, 25).
In our study, we focus on the six antioxidative
selenoenzymes, glutathione peroxidase 1-4 (GPX1,
GPX2, GPX3, and GPX4; refs. 1-4), selenoprotein P
(SEPP1), and thioredoxin reductase 1 (TXNRD1), which
are expressed in the gastrointestinal tract (26-30).
Interestingly, the gastrointestinal tract expresses all four
common GPX (GPX1-GPX4), which may suggest a
significant role in colonic function (e.g., as a barrier
for reactive oxygen species; refs. 26, 31, 32). Support for
the importance of these selenoenzymes comes from an
animal study showing that targeted disruption of
both cytosolic and gastrointestinal GPX (GPX1 and
GPX2, respectively) genes results in accumulation of
lipid hydroperoxides (33) and high susceptibility to
inflammation and colon cancer in mice (25, 34).
Preliminary data suggest that genetic variants in some
of these selenoenzymes may affect their function and
cancer risk; however, only a limited number of the
genetic variants in selenoenzymes has been identified or
studied so far.
To investigate the association between polymorphisms
in six selenoenzymes genes abundantly expressed in the
colon (GPX1-GPX4, SEPP1, and TXNRD1) and advanced
colorectal adenoma risk, we conducted a case-control
study nested within the Prostate, Lung, Colorectal and
Ovarian Cancer Screening Trial. In this large and well-
characterized population, case and control status were
identified following a standard protocol. To capture the
underlying genetic variation across each of the six genes,
we resequenced all functionally important regions and
genotyped selected tagging single nucleotide polymor-
phisms (tagSNP). Gene-environmental interactions were
explored for important factors, such as serum selenium
concentration and smoking.
Materials and Methods
Study Population. This case-control study was nested
within the Prostate, Lung, Colorectal and Ovarian Trial,
which was designed to evaluate selected methods for the
early detection of these cancers and to investigate
etiologic factors and early markers of cancer (35, 36). In
brief, the Prostate, Lung, Colorectal and Ovarian Trial
recruited 154,952 men and women ages 55 to 74 years at
10 centers in the United States (Birmingham, AL; Denver,
CO; Detroit, MI; Honolulu, HI; Marshfield, WI;
Minneapolis, MN; Pittsburgh, PA; Salt Lake City, UT;
St. Louis, MO; and Washington, DC) between 1993 and
2001. Participants were randomized to routine care or
screening for prostate cancer (prostate-specific antigen
testing and digital rectal examination), lung cancer (chest
X-ray), colorectal cancer (sigmoidoscopy), and ovarian
cancer (CA125 testing and transvaginal ultrasound). If
the sigmoidoscopy identified polyps or other suspect
lesions, participants were advised to get further follow-
up examination through their own medical care pro-
viders, which usually resulted in a full colonoscopy
with polypectomy or surgical procedures, if indicated.
All medical and pathologic reports of the follow-up
examinations were obtained and coded by trained
medical record abstractors. Written informed consent
was obtained from participants and the trial received
approval from the institutional review boards of the U.S.
National Cancer Institute and the 10 study centers.
Identification of Cases and Controls. Cases and
controls for this study were selected from participants
randomized to the screening arm between September
1993 and September 1999, who had undergone a
successful sigmoidoscopic examination at baseline
(insertion to at least 50 cm with >90% of mucosa visible
or a suspect lesion identified), completed a baseline
risk factor questionnaire, donated a blood sample, and
consented to participate in etiologic studies (n = 42,037).
Of these participants, we excluded 4,834 with a self-
reported history of cancer (except basal-cell skin cancer),
ulcerative colitis, Crohn’s disease, familial polyposis,
colorectal polyps, or Gardner’s syndrome. We randomly
selected 772 cases for study from among 1,234 cases with
advanced distal adenoma (z10 mm in size, containing
high-grade dysplasia, or villous characteristics) in the
distal colon (descending colon and sigmoid or rectum)
and 777 controls with a negative screening sigmoidosco-
py (that is, no polyp or other suspect lesion; n = 26,651)
frequency matched to cases by gender and self-reported
ethnicity. Five cases and four controls either had no DNA
or had discrepancies on repeated fingerprint analyses
and were excluded from all analysis, leaving 767 cases
and 773 controls for the study. Approximately 63% of the
cases had at least one distal adenoma considered to be
histologically aggressive (high-grade dysplasia or villous
Genotype Analysis. To determine the underlying
genetic variation in each gene (GPX1-GPX4, SEPP1,
and TXNRD1), we resequenced the promoter regions, 5¶
and 3¶ untranslated regions, including the selenocysteine
insertion sequence, as well as all exons and intron-exon
boundaries in a multiethnic panel of 102 subjects,
including 31 Caucasians and 24 African Americans
(details are described elsewhere; ref. 37). To efficiently
predict the common variation across each gene, we
selected tagSNPs based on the resequencing data using
the method developed by Clayton.7The selection criteria
were minimum haplotype r2= 0.9 and minor allele
frequency > 5% in Caucasians or African Americans,
the predominant ethnic groups in this study population.
We preselected any nonsynonymous SNPs and those
located in the selenocysteine insertion sequence as
tagSNPs. TagSNPs were determined separately within
the subgroup of Caucasians and African Americans, with
the goal to maximize the overlap between tagSNPs to
minimize the total number of tagSNPs.
In total, 44 tagSNPs (GPX1, n = 6; GPX2, n = 6; GPX3,
n = 11; GPX4, n = 7; SEPP1, n = 6; and TXNRD1; n = 8)
were successfully genotyped. These 44 tagSNPs resulted
in substantial coverage of the common SNPs (minor
allele frequency > 5%). The average r2and minimal r2
are as follows: GPX1, 0.73 and 0.33; GPX2, 0.81 and
0.22; GPX3, 0.83 and 0.62; GPX4, 0.92 and 0.53; SEPP1,
0.73 and 0.38; and TXNRD1, 0.72 and 0.20, respectively.
The fraction of common SNPs captured by the tagSNPs
with r2z 0.8 and r2z 0.5 is 0.75 and 0.75 for GPX1,
0.59 and 0.73 for GPX2, 0.67 and 1.0 for GPX3, 0.88 and
Cancer Epidemiology, Biomarkers & Prevention
Cancer Epidemiol Biomarkers Prev 2008;17(5). May 2008
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Genetic Variants in Selenoenzyme and Colorectal Adenoma
Cancer Epidemiol Biomarkers Prev 2008;17(5). May 2008