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1 Department of Cancer Genetics, Roswell Park Cancer Institute, Buffalo, New York; 2 Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California; 3 Translational Oncology Laboratory, Barts and the London Queen Mary's School of Medicine and Dentistry, London, United Kingdom; 4 Department of Viruses, Hormones, and Cancer, Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark; and 5 Strangeways Research Laboratory, University of Cambridge, Cambridge, United Kingdom
Requests for reprints: Richard A. DiCioccio, Department of Cancer Genetics, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263. Phone: 716-845-8059; Fax: 716-845-1698. E-mail: richard.dicioccio{at}roswellpark.org
| Abstract |
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| Introduction |
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Increased copy number of chromosome 20q, the region bearing STK15, has been widely reported in sporadic and hereditary ovarian tumors by comparative genome hybridization (15-24). Moreover, amplification of the putative oncogene, STK15, has been reported in sporadic and hereditary ovarian tumors as well as in ovarian cancer cell lines (9, 12). These data suggest a role for STK15 in the pathogenesis of ovarian cancer.
Two STK15 polymorphisms, F31I (dBSNP reference rs2273535) and V57I (dBSNP reference rs1047972), are in evolutionarily conserved regions of the NH2-terminal domain (4). F31I and V57I are caused by single nucleotide substitutions, 449t/a and 527g/a, respectively (National Center for Biotechnology Information reference sequence NM003600). The functional significance of V57I is unknown. However, the I31 allele of F31I was preferentially amplified and associated with aneuploidy in human colon tumors (14). The same allele also was associated with faster growth of cultured cells and experimental tumors relative to the F31 allele. Thus, the I31 allele may be more oncogenic than the F31 allele and involved in human tumor susceptibility. To learn more about the involvement of STK15 in the etiology of ovarian cancer, we investigated the F31I and V57I polymorphisms for associations with risk of invasive ovarian cancer in three population-based ovarian cancer case-control studies from the United Kingdom, United States, and Denmark. We also studied two additional polymorphisms: a c/g change at nucleotide 287 in the 5' untranslated region (UTR; dBSNP reference rs732417) and a g/c change at nucleotide 1,891 in the 3' UTR (dBSNP reference rs8173). The functional significance of either UTR single nucleotide polymorphism is unknown, but either may have a regulatory function. Furthermore, by genotyping multiple single nucleotide polymorphisms across the gene, we were able to estimate haplotype frequencies in cases and controls and to examine associations of haplotypes with risk.
| Materials and Methods |
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The MALOVA study is a population-based, Danish case-control study of ovarian cancer. Eligible cases were women ages 35 to 79 years diagnosed with an ovarian tumor from December 1994 to May 1999. The study included 18 different hospitals from the municipalities of Copenhagen and Frederiksberg as well as the counties of Copenhagen, Frederiksborg, Roskilde, Vestsjælland, Storstrøm, Funen, southern Jutland, and northern Jutland. In total, 692 women with invasive epithelial ovarian cancer, 225 with ovarian borderline tumors, and 1,088 with benign ovarian tumors were enrolled. Controls were drawn from the general female population within the study area (ages 35-79 years) selected at random using the computerized Central Population Register. In total, 2,116 controls were recruited into the study. The study was approved by the local scientific ethical committee. DNA was extracted from blood using the QIAamp DNA Blood Mini Kit (Qiagen, Valencia, CA). Samples from 443 cases and 1,130 controls were available for this study.
The U.S. subjects were ascertained from the Family Registry for Ovarian Cancer of six counties in northern California. Included were ovarian cancer cases (n = 323) from Caucasians (ages 20-64 years) diagnosed between March 1, 1997 and July 31, 2001 and age/racially matched healthy female controls (n = 427). Buccal rinses were obtained from 83 cancer cases and 55 controls and bloods from all other subjects. DNA was purified from peripheral blood leukocytes using the Puregene Kit (Gentra Systems, Minneapolis, MN). DNA was isolated from exfoliated cells in buccal mouthwash rinses as described previously (25).
Genotyping
Genotyping was carried out using Taqman according to manufacturer's instructions. Each assay was carried out using 10 ng DNA in a 5 µL reaction with primers at 900 nmol/L and probes at 200 nmol/L concentrations. Primer and probe sequences and assay conditions used for each polymorphism analyzed are shown in Table 1. All reactions were carried out using 384-well arrays with 12 duplicate samples in each plate for quality control. There were no discordant genotypes in duplicates. Genotypes were called using the Allelic Discrimination sequence detection software (Applied Biosystems, Warrington, United Kingdom). DNA samples that did not give a clear genotype result at the first attempt were not repeated because this is a high-throughput process. Hence, there are variations in the number of samples successfully genotyped for each polymorphism.
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Statistical Analysis
In the controls, deviations of the genotype frequencies from those expected under Hardy-Weinberg equilibrium were assessed by
2 tests (1 df). Genotype frequencies in cases and controls were compared for each study separately using
2 tests (2 df). Data were pooled and genotype frequencies were compared in cases and controls using unconditional logistic regression with terms for genotype and study and an appropriate likelihood ratio test. Genotype-specific risks with the common homozygote as the baseline comparator were estimated as odds ratios (OR) by unconditional logistic regression. We also tested for rare allele dominance using
2 tests (1 df) for each study separately and unconditional logistic regression for the pooled data. Haplotype frequencies were estimated using the hapipf command implemented in STATA (StataCorp, College Station, TX). This function calculates allele/haplotype frequencies using log-linear modeling embedded within an expectation maximization algorithm (26). The expectation maximization algorithm handles the phase uncertainty and the log-linear modeling allows testing for disease association. STATA also allows haplotype frequencies to be compared between cases and controls while allowing for differences in haplotype frequencies by study using a likelihood ratio test.
| Results |
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| Discussion |
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We have found some evidence that the I31 allele of STK15 is associated with a modest risk of invasive ovarian cancer acting in a dominant manner. However, this result needs to be interpreted with some caution. Firstly, the overall comparison of genotype frequencies in cases and controls was not significant, even at the 5% level. Furthermore, the test for a dominant effect was only marginally significant (P = 0.03 in combined data), so the possibility of a type I statistical error must be considered. Some authors have suggested that stringent criteria should be applied to statistical tests for genetic association (e.g., P < 0.0001) because of the large number of candidate polymorphisms across the human genome. However, 9,000 cases and 9,000 controls would be needed to detect a dominant allele with risk of 1.17 with 80% power at this level of significance. Hidden population stratification is an alternative explanation for a spurious association. This occurs when allele frequencies differ between population subgroups and cases and controls are drawn differentially from those subgroups. However, it seems unlikely that population stratification is relevant in this investigation because the cases and controls in the three studies reported here were drawn from the same ethnic groups. Furthermore, the existence of significant population stratification that has resulted in a false genetic association has never been empirically shown (28).
On the other hand, STK15 is a good candidate for ovarian cancer susceptibility. Moreover, of the four polymorphisms tested (F31I, V57I, 5' UTR c/g, and 3' UTR g/c), F31I was the only one with a significant result and is the one most likely to alter protein function. F31I is a structurally nonconservative, aromatic/aliphatic amino acid substitution and occurs in an evolutionarily conserved region of the NH2-terminal domain of STK15 (4). The NH2-terminal domain has been proposed to function in translocation of STK15 from cytoplasm to centrosome during mitosis (7). Recently, it was reported that the I31 isoform of STK15 bound less efficiently than the F31 isoform to UBE2N, an ubiquitin-conjugating enzyme that colocalizes with STK15 at centrosomes during mitosis (14). Moreover, the F31 isoform and UBE2N colocalized to centrosomes more efficiently than the I31 isoform and UBE2N. Conceivably, reduced efficiency in subcellular localization of the I31 isoform of STK15 during mitosis may affect normal chromosome segregation, resulting in genomic changes in daughter cells that lead to cell transformation and tumor formation. V57I also occurs in an evolutionarily conserved region of the NH2-terminal domain of STK15. The effect of this polymorphism on function has not been investigated. However, V57I is a structurally conservative, aliphatic/aliphatic amino acid change and would not be predicted to have a major effect on protein function. Furthermore, we found no evidence from the haplotype analysis that a specific haplotype tagged by a combination of these four single nucleotide polymorphisms was associated with risk and therefore might be acting as a marker for another important single nucleotide polymorphism in linkage disequilibrium.
Clinical genetic testing for cancer is currently limited to genes with the rare, highly penetrant, cancer predisposition alleles such as BRCA1 and BRCA2. Testing for alleles such as F31I is not indicated because the increase in disease risk (if confirmed) is low and the predictive value of the genotype is poor. However, if enough weakly penetrant alleles are eventually identified to account for substantial increased risk, then genotyping a panel of such loci may be useful for risk determination (29). Individuals with a panel genotype profile of increased risk would be candidates for cancer prevention measures or for frequent surveillance for early detection of disease. Current prevention measures for ovarian cancer include periodic oral contraceptive use prior to menopause and prophylactic oophorectomy after child-bearing years (3033). Present early detection methods for ovarian cancer such as testing for CA125 antigen in blood and ultrasound lack sensitivity and specificity for routine use in the general population but may be effectively employed in women with increased disease risk (31).
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
6 Electronic data sources: Locus Link: http://www.ncbi.nlm.nih.gov/LocusLink/; dBSNP: http://www.ncbi.nlm.nih.gov/SNP/; Genbank: http://www.ncbi.nlm.nih.gov/Genbank/. ![]()
Received 2/27/04; revised 5/ 3/04; accepted 5/14/04.
| References |
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q13.3 and a pseudogene STK6P to 1q41
q42. Cytogenet Cell Genet 1997;79:2013.[Medline]
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