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1 Channing Laboratory, Department of Medicine, Brigham and Women's Hospital; 2 Department of Adult Oncology, Dana-Farber Cancer Institute; Departments of 3 Epidemiology and 4 Nutrition, Harvard School of Public Health, Boston; and 5 Department of Public Health, University of Massachusetts, Amherst, Massachusetts
Requests for reprints: Wendy Y. Chen, Channing Laboratory, Harvard Medical School, 181 Longwood Avenue, Boston, MA 02115. Phone: 617-525-2225; Fax: 617-525-2008. E-mail: wendy.chen{at}channing.hardvard.edu
| Abstract |
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| Introduction |
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The gene encoding VDR is known to have several polymorphisms. The FOK1 restriction enzyme identifies a polymorphic site (T
C transition) in exon 2 at the 5'-end of the VDR gene. The presence of this site ("f") allows protein translation to begin from the first initiation codon rather than from the second ("F"), resulting in a protein that is three amino acids longer. The longer protein (f allele) is a less active transcriptional activator (10, 11), and the ff genotype has been associated with decreased bone mineral density in multiple ethnic groups (12-17). The BSM1 restriction enzyme identifies a polymorphic site at an intron at the 3'-end which is in linkage disequilibrium with several other polymorphisms, including APA1, TAQ1, and the variable-length poly(A) (ref. 18). Although functional data have been inconclusive for BSM1, several small studies evaluating BSM1 have reported significant associations with breast cancer risk (19-21), suggesting the need to replicate these results in larger populations. For this analysis, we examined the association between the FOK1 and BSM1 VDR polymorphisms and breast cancer risk in a case-control study nested within the Nurses' Health Study. In addition, we assessed whether the relation varied by vitamin D intake or plasma vitamin D levels.
| Materials and Methods |
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Nested Case-Control Study
Eligible cases included women who provided blood samples and did not have a prior history of cancer (excluding nonmelanoma skin cancer) at the time of blood draw and were subsequently diagnosed with breast cancer between the date of the return of the blood sample and June 1, 2000. We asked women for permission to obtain medical records to confirm the diagnosis of cancer. Medical records were obtained for 98% of these participants. Because the confirmation rate was >99%, we included all reported incident breast cancer cases. Estrogen receptor (ER) and progesterone receptor (PR) status was abstracted from medical records. Both invasive and in situ cancers were included.
Controls were chosen from among women who provided a blood sample but did not report a diagnosis of any type of cancer up to and including the 2-year interval during which the case was diagnosed. One control was matched to each breast cancer case on the basis of age (±1 year), menopausal status (pre- versus post- versus unknown), recent use of postmenopausal hormones (yes versus no), month and time of day (±2 hours) of blood collection, and fasting status (<10 hours or unknown versus
10 hours since last meal). For postmenopausal women who did not use postmenopausal hormones at blood draw, a second control was also selected. In some instances, samples were available only for one member of a case-control pair, and these were still included in the analysis.
VDR Genotyping
All samples were genotyped using the ABI PRISM 7900HT Sequence Detection System (Applied Biosystems, Foster City, CA), in 384-well format. The 5' nuclease assay (TaqMan) was used to distinguish the alleles of the VDR gene at the restriction enzyme sites Fok1 and Bsm1, involving a T
C (Fok1) and a C
T (Bsm1) transition. The PCR amplifications were carried out on 5 to 20 ng DNA using 1x TaqMan universal PCR master mix (no Amp-erase UNG). Additional details of the TaqMan primers, probes, and conditions for genotyping assays are available on request. Genotyping was done by laboratory personnel blinded to case-control status, and blinded quality control samples were inserted to validate genotyping procedures. Concordance for blinded samples was 100% (64 of 64) for BSM1 and was 97% (64 of 66) for FOK1.
Plasma Vitamin D Levels
Plasma vitamin D levels were available on a subset of the cases diagnosed between 1990 and 1996 and their matched controls. 25 hydroxyvitamin D (25(OH)D) was analyzed in three batches: batches 1 and 2 by Dr. Michael Holick (Boston University School of Medicine, Boston, MA) and batch 3 by Dr. Bruce W. Hollis (Medical University of South Carolina, Charleston, SC). Assay methods have been discussed in detail previously (23). Briefly, plasma samples in batches 1 and 2 were extracted with absolute ethanol, and the extract was then treated with a protein-binding assay with a high affinity for 25(OH)D. Plasma samples in batch 3 were done by RIA. Mean coefficients of variation for 25(OH)D were 17.6% (batch 1), 16.4% (batch 2), and 8.7% (batch 3). 1,25(OH)2D was analyzed by Dr. Hollis in a single batch using RIA with radioiodinated tracers (24). The mean coefficient of variation for 1,25(OH)2D was 7.3%.
Statistical Analyses
Women were divided into three groups on the basis of genotype, ff, Ff, and FF or bb, Bb, BB. Simple conditional logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals. Multivariate conditional regression was used to control for other breast cancer risk factors, which were determined from the Nurses' Health Study questionnaires prior to the blood draw, including age at menopause (<45, 45-49, 50-55, >55), age at menarche (<12, 12, 13, >13), parity/age at first birth [nulliparous, age at first birth <25 (1-3 children), age at first birth 25-29 (1-3 children), age at first birth >29 (1-3 children), age at first birth <25 (4 or more children), age at first birth
25 (4 or more children)], average daily alcohol consumption (0, 0-9.9, 10-19.9,
20 g/d), body mass index (quartiles), family history of breast cancer in a first-degree relative (yes or no), and history of benign breast disease (yes or no). Menopausal status and postmenopausal hormone use were determined by questionnaires at the time of blood draw. Cumulative average vitamin D intake, including vitamin D from dietary sources and supplements, was calculated from the semiquantitative food-frequency questionnaire administered in 1980, 1984, 1986, and 1990 to all Nurses' Health Study participants (3). Variables included in the multivariate models were ones that were associated with breast cancer risk in the Nurses' Health Study and could confound the VDR and breast cancer association, as well as commonly accepted breast cancer risk factors. Several risk factors such as oral contraceptive use, physical activity, and smoking status were evaluated but were not included because they did not show strong associations within our cohort nor did they influence the associations under study. Unconditional logistic regression yielded results similar to those of conditional analyses; therefore, unconditional regression was used for the analyses by ER/PR status to increase power with the inclusion of unmatched cases and controls.
Analyses were done stratifying cases and controls by menopausal status, vitamin D intake, and 25(OH)D and 1,25(OH)2D plasma levels and stratifying cases by ER/PR status. We also cross-classified BSM1 by FOK1 genotypes. For analyses by 1,25(OH)2D, women were stratified by the median level. For 25(OH)D, we used batch-specific medians to account for batch-to-batch variation. Interactions were evaluated with a Wald test for the cross-product interaction term. A two-sided P value of <0.05 was used to determine statistical significance. All analyses were done in SAS version 8.0.
| Results |
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2 test). The genotype frequencies for FOK1 and BSM1 were comparable to those reported in other breast cancer studies with predominantly Caucasian populations (19, 21, 25).
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| Discussion |
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Bone mineral density has been positively associated with breast cancer risk in epidemiologic studies (30, 31), and it would seem paradoxical that the ff genotype could be associated with both decreased bone density and increased breast cancer risk. Bone density has been considered a surrogate for a woman's lifetime estrogen exposure, such that women with lower bone mineral density have less estrogen exposure and therefore lower breast cancer risk. However, the association between bone mineral density and the ff genotype would not be based upon a sex steroid hormonal mechanism. Vitamin D deficiency or other factors decreasing the effectiveness of the VDR system can upset the calcium/phosphate balance and lead to ostemalacia and osteoporosis (4). Our findings support the hypothesis that the antiproliferative effects of vitamin D could decrease breast cancer risk, because women with the ff genotype have the less active form of the VDR and would thus be expected to derive less benefit from vitamin D and have higher breast cancer risk and lower bone mineral density.
In several previous studies, polymorphisms in FOK1 have not been associated with breast cancer risk (19, 21, 25), but two (19, 25) of the three studies had <150 cases each. Because the ff genotype occurs in <20% of the population and the Ff heterozygote was not associated with increased risk, larger sample sizes would be necessary to detect a moderate association of the ff polymorphism with cancer risk. To improve power, the Curran et al. study (25) evaluated allele frequencies, rather than genotype frequencies, which may dilute the ability to detect an effect because we observed an association primarily among carriers of ff. We had more than twice the number of cases and controls than previous studies and thus greater power to detect an association. We did not find a significant interaction between BSM1 and FOK1 genotypes. If anything, the FOK1 association was apparent primarily among carriers of a B allele. In the study by Guy et al. (21), the F allele of FOK1 seemed to augment the risk they observed with the bb genotype, although this would be contrary to the functional data on the F allele.
Studies on the association between the BSM1 polymorphism and breast cancer risk have not been consistent. Two studies showed an increased risk of breast cancer with the B allele (19, 20), whereas other studies found no difference (32) or a decreased risk (21). The two largest studies observed no association (Newcomb et al., 32: 420 cases/402 controls) or decreased risk (Guy et al. 21: 398 cases/427 controls). BSM1 genotype frequencies vary considerably across ethnic groups, making results difficult to compare across studies. For example, 43% of 241 controls were bb in a British case-control series, whereas 91% of 169 controls were bb in a Taiwanese hospital-based case-control series (20). Functional studies have not conclusively shown an effect of BSM1 polymorphisms on VDR function or coding sequence, mRNA stability, osteocalcin levels or changes in bone mineral density in response to treatment (17, 29, 33, 34). Several other polymorphisms at the 3'-end of VDR that are closely linked with BSM1 have been implicated in breast cancer risk, including APA1 (20, 25) and TAQ1 (25). Given that none of these are known to be functional polymorphisms, it is hypothesized that these nonfunctional "marker" alleles are in linkage disequilibrium with the truly functional allele. The varying degree of linkage disequilibrium between these marker alleles and the functional allele might explain the variations in the strengths of associations seen across studies (29). Given the large area of linkage disequilibrium at the 3'-end and the absence of consistent functional data, it is difficult to interpret the body of data relating BSM1 to breast cancer risk, although our data do not support an important association in a primarily Caucasian population.
Although 1,25(OH)2D is considered to be the more biologically active metabolite and functions as the ligand for VDR, 25(OH)D may be hydroxylated to 1,25(OH)2D at various target tissues, including the breast, and therefore may be more representative of intracellular levels (35, 36). Nevertheless, we did not find that the disease/genotype relation varied significantly by 25 or 1,25(OH)2D or vitamin D intake. We did control for batch-to-batch variations in 25(OH)D levels, but assay variability and seasonal and geographic variations may have interfered with our ability to detect an association. Contrary to our a priori hypothesis, the breast cancer risk associated with the ff genotype was higher among women with higher cumulative vitamin D intake. This may be due to chance, but should be investigated in other studies. We did not have detailed data on routine sunlight exposure.
The strengths of our study include its prospective nature, large size, and our ability to control for most of the important breast cancer risk factors. Our population was mainly Caucasian; thus, we were unable to evaluate these relationships in other racial groups. Power was limited to evaluate interaction effects by subgroups of plasma vitamin D levels and genotype.
In conclusion, we found an increased risk of breast cancer among women with the ff genotype. This relationship did not seem to be modified by menopausal status, ER/PR status, BSM1 genotype, or plasma vitamin D levels, although power was limited for several of these subanalyses. Our findings on FOK1 and the increasing evidence demonstrating a protective effect of higher vitamin D levels on breast cancer risk suggest that the vitamin D pathway is a potentially important mediator of breast cancer risk. The VDR may represent an important target for breast cancer prevention outside of the known hormonal mechanisms.
| 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.
Received 4/21/05; revised 6/23/05; accepted 7/20/05.
| References |
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,25-dihydroxyvitamin D3 receptor as a mediator of transrepression of retinoid signaling. J Cell Biochem 1997;67:28796.[CrossRef][Medline]
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