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Cancer Prevention Research Program [C. M. U., J. B., C. M. V., J. D. P.] and Biocomputing Shared Resource [E. A. G.], Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, and Department of Epidemiology [C. M. U., C. M. V., J. D. P.] and Department of Environmental Health, Center for Ecogenetics and Environmental Health [F. M. F.], University of Washington, Seattle, Washington 98195
| Abstract |
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| Introduction |
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TS (EC 2.1.1.45) is an enzyme that catalyzes the conversion of
deoxyuridylate to deoxythymidylate by simultaneous conversion of
5,10-methylenetetrahydrofolate to dihydrofolate (Fig. 1)
. Thus, TS is essential for the
provision of a nucleotide required for both DNA synthesis and repair.
TS is an essential enzyme in proliferating cells and is also an
important target for a variety of chemotherapeutic drugs, including
5-FU. Thus, TS plays a major role in cancer therapy and possibly in
cancer prevention.
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To date, only one functional polymorphism in the TS gene has been reported, and its prevalence in Caucasians is unknown (9) . We undertook a search for new genetic polymorphisms in TS by screening public databases of ESTs and identified a common 6-bp deletion in the 3'-UTR of the TS gene.
| Materials and Methods |
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Verification of a TS Candidate Polymorphism in a Caucasian
Population.
The presence of a TS candidate polymorphism was verified in
a Caucasian population (n = 95). These individuals were
initially recruited for a case-control study in Minnesota in 19911994
that was approved by the University of Minnesota Institutional Review
Board. DNA was obtained from buffy coat as part of the study protocol,
part of which was specifically focused on the role of genetic
variability in the etiology of colorectal neoplasia.
Genomic DNA was extracted from peripheral WBCs using the Puregene kit
(Gentra Systems, Minneapolis, MN). A RFLP spanning the 6-bp insertion
or deletion was used to verify the existence of the TS
3'-UTR polymorphism at bp 1494. The presence of the 6 bp creates a
DraI restriction site. The fragment containing the
polymorphism was amplified by PCR using primers
5'CAAATCTGAGGGAGCTGAGT3' and 5'CAGATAAGTGGCAGTACAGA3' in a reaction
containing 10 mM Tris (pH 8.3), 50
mM KCl, 2.5 mM
MgCl2, 150 µM
deoxynucleotide triphosphates, 300 nM each
primer, 100 ng of genomic DNA, and 1unit of AmpliTaq DNA polymerase (PE
Biosystems, Foster City, CA). The cycling conditions were: 1 cycle of
94°C for 5 min; 30 cycles of 94°C for 30 s, 58°C for 45 s, and 72°C for 45 s; and 1 cycle of 72°C for 5 min.
The amplified fragments were digested with DraI and the
products separated on a 3% NuSieve agarose gel. The expected fragment
sizes are 70 bp and 88 bp for the wild-type allele and 152 bp for the
mutant allele.
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analysis was performed to
test for agreement with Hardy-Weinberg equilibrium.
| Results |
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| Discussion |
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ESTs are submitted to the databases as part of the overall genomic analysis and are subject to fewer checks than accepted genomic or coding sequences. Therefore, sequencing errors can be quite common; this requires several quality control measures to increase the likelihood that an observed sequence variation constitutes a true polymorphism rather than a sequencing error. The measures used in this study include (a) the exclusion of sequences with a multitude of differences from the reference sequences (these may be error prone, but may also be expressed from a second related gene with significant sequence similarity); (b) exclusion of regions of an EST with multiple matching errors; (c) restriction to sequences with a very low likelihood of a random match, based on a probability calculated by BLAST; and (d) consideration only of candidate polymorphisms that occur in at least two ESTs from independent tissue sources.
The approach of screening EST databases for polymorphisms is naturally limited by the number of available ESTs and thus is more applicable to widely expressed genes and for common polymorphisms. Similarly, only the transcribed region can be investigated, but this is usually the genomic region of greater interest. A limitation of this approach is that ESTs are usually cloned from the 3' end [beginning at the poly(A) sequence] and reverse transcription usually does not yield full-length clones. Thus, ESTs containing 3' sequences are overrepresented. The very 3' end of an mRNA is noncoding but can contain sequences determining mRNA stability. Although the significance of mutations in the 3'-UTR is not as obvious as mutations resulting in amino acid changes, mRNA turnover can be affected. Differences in mRNA turnover alter steady-state levels of a given mRNA, which, in turn, determines protein expression levels.
TS is essential in the regulation of the balanced supply of the four nucleotides required for the normal replication of DNA and plays a central role in folate metabolism. The importance of an adequate folate supply for cancer prevention has been demonstrated in several recent studies, showing an elevated risk of colon cancer and colorectal adenoma (11, 12, 13, 14, 15, 16, 17) , pancreatic cancer (18) , and possibly breast cancer (19, 20, 21) associated with low folate status, particularly if combined with high alcohol consumption. The observed association with cancer risk may be attributable to a pathway associated with nucleotide synthesis; folate deficiency can result in deoxynucleotide triphosphate pool disturbances in rats (22 , 23) , in incorporation of uracil instead of thymidine into DNA, and subsequently in chromosome breaks attributable to transient nicks (24 , 25) . It is very likely that these effects are mediated by decreased TS activity attributable to a lack of substrate.
Inhibition of TS has antiproliferative effects, a mechanism used by several "antifolate" chemotherapeutic drugs, particularly 5-FU and raltitrexed. It has been shown that resistance to 5-FU, as well as patient survival, is often associated with increased TS expression (26, 27, 28, 29, 30) . Thus, functional genetic polymorphisms in TS could be relevant for cancer treatment.
In summary, we report here on a new, common polymorphism in the TS gene identified using a public EST database. The presence of this polymorphism has been confirmed in a Caucasian population. Potential effects of the TS 6-bp deletion at bp 1494 on function have not yet been investigated. Although the 3'-UTR of a gene is not translated into protein, it often plays an important role for maintaining mRNA stability. If changes in the secondary mRNA structure (e.g., folding) take place, translation can also be affected. Others have shown that a common point-mutation polymorphism in the 3'-UTR (polyadenylation signal) of the N-Acetyltransferase 1 (NAT1) gene is associated with altered enzyme activity in bladder and colon tissues (31) . It is possible that a deletion of 6 bp in the 3'-UTR affects mRNA stability or secondary mRNA structure and could thus ultimately affect protein levels of TS or response to up-regulation of this enzyme. If this polymorphism is associated with alterations in enzyme activity, it could be of major importance for cancer chemotherapy and possibly for cancer prevention.
| Acknowledgments |
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| Footnotes |
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1 This work was supported by the National
Institute of Environmental Health Sciences Center P30, ES-07033. ![]()
2 To whom requests for reprints should be
addressed, to Cancer Prevention Research Program, Fred Hutchinson
Cancer Research Center, 1100 Fairview Avenue N, MP-900, Seattle, WA
98109-1024. Phone: (206) 667-7617; Fax: (206) 667-7850; Email: nulrich{at}fhcrc.org ![]()
3 The abbreviations used are: EST, expressed
sequence tag; UTR, untranslated region; TS, thymidylate synthase; 5-FU,
5-fluorouracil; NCBI, National Center for Biotechnology Information;
BLAST, Basic Local Alignment Search Tool. ![]()
4 Internet address:
http://www.ncbi.nlm.nih.gov/dbEST/. ![]()
5 Internet address:
http://www.hgmp.mrc.ac.uk/omim/. ![]()
Received 5/ 3/00; revised 9/28/00; accepted 10/ 5/00.
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