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Divisions of Clinical Research and Human Biology, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109-1024 [N. M. S., E. A. O.]; Public Health Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109-1024 [K. E. M., J. R. D., D. R. D.]; and School of Public, Health and Community Medicine, Department of Epidemiology, University of Washington, Seattle, Washington 98195 [K. E. M., J. R. D.]
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Abstract |
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22 were classified as long (L). For (GGC)n repeats, those < 17 were classified as short, and those 17 were classified as long. Women with a cumulative (CAG)n repeat size of 43 showed a modest increase in risk for breast cancer [odds ratio (OR), 1.3; 95% confidence interval (CI), 1.0–1.7]. Women with a (GGC)n long (L) allele and those with a 33 cumulative repeat size had a decreased risk of breast cancer (OR, 0.7; 95% CI, 0.5–0.9). Among women homozygous for the (CAG)n short (S) allele and those with any (GGC)n L allele, an increased risk of breast cancer in relation to ever use of oral contraceptives [OCs; OR = 1.9 (95% CI, 1.0–3.6) and OR = 1.7 (95% CI, 0.9–3.5), respectively] was observed. An increased risk for OC use, however, was not observed among women with the CAG L or GGC S allele. This study, one of the first to examine both (CAG)n and (GGC)n in a population-based study for its relation to breast cancer risk, suggests a reduced risk in young women with (GGC)n repeat lengths of 17. In addition, these data suggest that AR repeat length may be partly responsible for the increased risk for early-onset breast cancer in women who use OCs, although these findings need replication in other populations. |
Introduction |
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The androgen receptor protein is a member of the nuclear receptor subfamily of steroid receptors that functions as a transcription factor to regulate the transactivation of hormone-responsive genes and is thus of specific interest. The AR3 gene contains two trinucleotide repeats within exon 1; a polyglutamine tract encoded by a (CAG)n repeat, and a polyglycine tract encoded by a (GGT)3GGG(GGT)2(GGC)n sequence, known as the (GGN)n repeat (1 , 2) . Most of the domain, inclusive of the repeats, is necessary for full wild-type activity of the gene (3) , and both repeats have been considered as risk factors for cancer (4) .
Initial cancer-related studies of AR repeat variation focused exclusively on association with prostate cancer risk. Studies suggest a 2–3-fold increase in risk, particularly of aggressive disease, in men with short versus long (CAG)n repeat alleles (5, 6, 7) . Each additional (CAG)n repeat has been shown to correlate with a 3–14% decrease in prostate cancer risk (7 , 8) . The length of the polyglutamine chain has specifically been shown to correlate inversely with transcriptional activity (9, 10, 11) . In addition, men with extreme (CAG)n repeat expansion, associated with spinal and bulbar muscular atrophy or Kennedy’s disease, experience androgen insensitivity, presumably due to loss of transactivation (12 , 13) . Subsequently, several studies have linked (GGC)n repeat length with prostate cancer risk (7 , 14, 15, 16, 17) .
Recently, the role of AR repeat length and breast cancer risk has been evaluated in several sets of women. A particular focus has been the hypothesized association between AR repeat and mutations in the BRCA1/2 genes. In a study of 165 cases and 139 controls, all of whom carried confirmed germ-line mutations in the BRCA1 gene, Rebbeck et al. (18)
reported an increased risk of breast cancer among women with at least one long (repeat length 28) (CAG)n allele, compared with women with two short alleles (Hazard Ratio, 3.7; 95% CI, 1.6–8.4). Rebbeck et al. (18)
also noted a correlation between earlier age at diagnosis and longer repeat length. However, a study of BRCA1 and BRCA2 mutation carriers of Ashkenazi Jewish origin, a subset of whom had breast cancer, demonstrated no association with breast cancer risk (19)
. When women from breast/ovarian cancer families were examined in a case-only analysis, Menin et al. (20)
found no association with (CAG)n repeat length and age at diagnosis or tumor type.
When women from the general population are considered, the results are equally inconclusive. A study by Giguère et al. (21)
of 255 incident cases and 461 controls from Quebec City reported that women with short CAG alleles (
39 repeats totaled from both alleles) have a 50% reduction in risk compared with women for whom the sum of the repeats is 40 (OR, 0.5; 95% CI, 0.3–0.83). The association was largely confined to postmenopausal women and was not affected by family history (21)
. Haiman et al. (22)
assessed (CAG)n in a larger number of cases and controls, the vast majority of whom were postmenopausal, and found little evidence of an association in the aggregate; however, they did find an increased breast cancer risk among women with a first-degree family history who carried a (CAG)n repeat allele of 22, and they also presented results according to menopausal status. A study of 508 cases and 426 controls from Britain, however, showed no association (23)
. The study of Dunning et al. (23)
is difficult to compare with those of Giguère et al. (21)
and Haiman et al. (22)
. It included two cohorts of women; the first was a series of prospectively ascertained patients from a single hospital who were <71 years of age at diagnosis. A set of matched anonymous controls was also included with this set. The second series was a retrospectively ascertained group of patients identified through the East Anglian Cancer Registry as part of the Anglian Breast Cancer Study and comprised patients < 55 years of age together with a set of random controls. The study did not assess postmenopausal women separately, and family history information was not included, nor was BRCA1/2 status reported.
To date, only one population-based study has examined the role of AR polymorphisms in young women, irrespective of BRCA1 status. Spurdle et al. (24) reported no association between the risk of breast cancer and (CAG)n polymorphisms in a study of 368 Australian cases diagnosed before the age of 40 years and 284 controls. The study, however, did not consider the role of the (GGC)n repeat. The two studies to date that have addressed the role of (GGC)n repeat length and breast cancer risk, although largely negative, focused exclusively on women across a wide spectrum of age and/or women who are likely BRCA1/2 carriers (19 , 23) . In the present study, therefore, we consider the role of polymorphisms with the (CAG)n and (GGC)n repeats in a population-based, case-control study of 524 cases diagnosed before age 45 years and 461 age-matched controls.
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Materials and Methods |
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Interviews were completed on 642 cases (84.0%) and 608 controls (90.5% telephone screener rate x 81.5% interview response rate = 73.8% overall response rate). Blood collection occurred in two phases. As part of the initial case-control study, funding was provided to attempt early blood draw (preceding adjuvant therapy) on at least 50% of the cases (and a smaller proportion of controls) for the purpose of investigating endogenous hormone levels. Blood collection from the remaining women was completed as part of a subsequent genetic-epidemiology study. Altogether, blood was collected from 540 cases (84.1%) and 476 controls (78.3%). Of these, 337 (62.4%) case samples and 176 (37.0%) control samples were obtained at the time of the original study, and 203 (37.6%) case samples and 300 (63.0%) control samples were obtained as part of the subsequent study (which was begun approximately 2 years later). DNA was unavailable for this study for 16 cases and 14 controls due to low DNA yields.
Analysis for both (CAG)n and (GGC)n repeats was performed on samples from 524 cases (81.6%) and 462 controls (76.0%). Complete results were generated for 524 cases and 461 controls for the (CAG)n repeat and 515 cases and 443 controls for the (GGC)n repeat. There was one control sample for which (GGC)n repeat results were not ascertained due to PCR failure, one control sample in which only a single allele was determined for the (CAG)n repeat, and 9 case and 18 control samples for which only one allele of the (GGC)n repeat could be determined. For these subjects, variables designating CAG or GGC repeat genotype were coded as unknown when totally unclassifiable but coded into specific categories when possible [i.e., a subject with one undetermined (GGC)n allele and one long (GGC)n would be classified as unknown for the three-level variable that stratifies genotypes into short-short, short-long, and long-long but could be classified as having at least one long allele in the two-level variable that compares those with the short-short genotype versus those with the long-long/short-long genotype]. Statistical analyses were performed both including and excluding these women.
Molecular Genetics.
DNA samples were extracted from frozen buffy coats using standard protocols and sent blinded to the laboratory for genotyping, with each plate containing a mixture of cases and controls, unknown to the laboratory technician. The (CAG)n and (GGC)n repeats of the AR were amplified using nested primers as described previously (4)
. To assess (CAG)n repeat size, 25 ng of germ-line DNA was amplified in a 10-µl volume for 30 cycles using the following conditions: initial denaturation at 96°C for 60 s; followed by 30 s each at 94°C, 60°C, and 72°C. The product was diluted 1:100, 5 pmol of IR770dATP were added (Boehringer Mannheim), and a second round of PCR amplification was carried out using an annealing temperature of 62°C for 30 cycles. The (GGN)n repeat was amplified as described previously (4)
.
Products were resolved on denaturing polyacrylamide gels using a Li-Cor Model 4200 automated infrared DNA sequencer (Li-Cor, Lincoln, NE). To assess PCR product size, every fourth lane (17 lanes in total on each gel) was loaded with a size standard encompassing 15 PCR products ranging in size from 50 to 425 bp. Genotypes were determined with commercially available SAGA software (Li-Cor). Sizing is accurate to within 0.2 bp. In addition, for both CAG and GGC, individuals with previously determined, known-sized alleles were included once on every gel. For CAG, this was an individual with 26- and 15-bp alleles. For GGC, this was an individual with 16- and 12-bp alleles. Because of difficulties in reproducibly amplifying both alleles for the GGC samples, all putatively homozygote (GGC) samples were run twice to verify allele status. Samples were scored as homozygotes only if a single dark band was visible on both gels. We did occasionally observe a fainter band in a sample lane that, based on size, could have been a poorly amplifying secondary allele. In such cases, to be conservative, only the one allele was scored, and the second allele was scored as unknown. This scenario involved only a small fraction of samples (9 cases and 18 controls).
Five µl of a 1:50 dilution of the initial PCR product were used as the template for the second round of amplification, following the addition of 2.5 pmol of IR700dATP. Products were resolved on denaturing polyacrylamide gels using a Li-Cor Model 4200 automated infrared DNA sequencer (Li-Cor). Genotypes were determined with commercially available SAGA software (Li-Cor). As described previously, (CAG)n repeat alleles of <22 were classified as short, and those 22 and larger were classified as long (4 , 24) . (GGC)n repeats of <17 units within the (GGN)n repeat were classified as short, whereas those 17 and larger were classified as long (4) .
We assessed repeatability of genotyping for both cases and controls by blind duplication of a set of a set of six samples that the laboratory technician was unaware of. For (CAG)n, there was perfect agreement in all genotyping calls. For (GGC)n, there was perfect agreement for five of six samples. For the sixth sample, we were able to call only one of the two alleles, and there was agreement on that result in both assays. The second allele was very faint on one assay and missing on the other and hence was simply called as an X or unknown. Thus, for genotyped alleles called (representing 12 for CAG and 11 for GGC), we noted perfect agreement.
BRCA1 and BRCA2 Analysis.
A subset of the samples analyzed in this study had been analyzed previously for germ-line mutations in the BRCA1 and BRCA2 genes (27
, 28)
. Cases screened for mutations in BRCA1/2 (n = 146) were targeted based on having been diagnosed before 35 years and/or having a first-degree family history of breast cancer. Ten cases were found to have germ-line mutations that were considered likely to be disease associated (27
, 28)
. In addition, 240 controls with a first-degree family history or who were between the ages of 40 and 44 years at reference date were tested for BRCA1, and 37 controls with a first-degree family history were tested for BRCA2. No disease-associated mutations were found in controls.
Statistical Analysis.
The t test was used to compare the mean lengths of alleles in cases versus controls and to compare the mean ages at diagnosis in cases according to genotype. The (CAG)n and (GGC)n repeats were assessed as categorical variables, based on both the mean number of repeats and previously published results (4
, 24)
.
To assess the relationship between the categories of repeats and the risk of breast cancer (and also to assess the association between risk factors such as OC use and the risk of breast cancer according to genotype), we used logistic regression to obtain ORs as estimates of the relative risk and 95% CIs (29) . All analyses were completed using Stata statistical software.
Because reference age and year were matching variables for the frequency matching used in the original study, all risk estimates presented are age- and reference year-adjusted (both as continuous variables). Both established and suspected risk factors for breast cancer were evaluated for their potential confounding influence on the analyses of AR genotype in relation to breast cancer risk and on the analyses of risk factors within genotype. Potential confounders were evaluated by single variable addition to the models as well as simultaneous consideration of all potential confounders, and a change of 10% or more in an OR was set as the criterion for confounding. The following variables were evaluated as potential confounders: race; history of a live birth; age at first live birth; OC use (ever use, duration of use, recency of use); age at menarche; menopausal status; lifetime weekly average alcohol consumption; family history of breast cancer; quartile of body mass index; and history of ever having a mammogram.
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Results |
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Tested cases and controls did not differ significantly with regard to reference age or race (white versus non-white; Table 1
). However, cases were more likely to have reported a family history of breast cancer than controls, particularly a first-degree family history, which was reported by 16.8% of cases but only 6.9% of controls.
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. The distribution and its peaks are similar to those seen in other studies (18, 19, 20
, 22)
. The mean number of (GGC)n repeats within the (GGN)n repeat was 15.4 in cases and 15.6 in controls, respectively (P = 0.13), and ranged from 3 to 18. For (GGC)n, the most common allele was 16 repeats, which was observed in 70.5% of cases and 69.2% of controls. As per previous studies, (CAG)n repeat lengths of <22 were classified as short (S), and those of 22 and larger were classified as long (L). For (GGC)n, repeats of <17 were classified as short, and those of 17 or larger were classified as long (7
, 23
, 24)
.
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AR Repeats and Breast Cancer Risk.
In all analyses, (CAG)n and (GGC)n alleles were assessed in terms of S and L genotypes and considered as categorical variables. Compared with women with the CAG S/S genotype, there was a suggestive but modest increase in the risk of breast cancer for women with the S/L (OR, 1.3; 95% CI, 0.9–1.8) and L/L genotypes (OR, 1.2; 95% CI, 0.8–1.7; Table 2
). Similar results were observed for the risk of breast cancer in relation to having one or two L alleles [the S/L and L/L genotypes (OR, 1.3; 95% CI, 0.9–1.7)], compared with women with a S/S genotype. Women with a total (CAG)n repeat number (the sum of both alleles) of 43 or more versus 42 or less were at a similarly increased but not statistically significant risk (OR, 1.3; 95% CI, 1.0–1.7).
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The combined effects of the (CAG)n and (GGC)n repeats were also considered. We observed no obvious joint effects but note that our modest sample size would have limited our ability to detect any but the strongest of effects.
The small number of non-Caucasian women in the study (59 cases and 49 controls) precluded analysis of effect modification by race. However, assessment of AR genotype with risk of breast cancer among Caucasian women alone gave virtually identical results compared with those of the study overall (data not shown). The small number of women under the age of 35 years (69 cases and 79 controls) precluded analysis of age-specific effects. Whereas the associations with the (GGC)n repeat in women 35 years of age and older was similar to effects observed in the study overall, the magnitude of the associations with the (CAG)n repeat in the women age 35 years and older was slightly decreased compared with those seen in the study as a whole (data not shown). None of the potential confounders examined substantively altered the associations between AR genotypes and the risk of breast cancer.
Risk Factor Analysis.
A selection of breast cancer risk factors known to pertain to hormonal pathways was assessed for variation in effect by genotype. Factors for which we observed no discernible variation in association with the risk of breast cancer according to (CAG)n or (GGC)n genotype included age at menarche, history of a birth, age at first live birth, number of births, and body mass index (data not shown). An evaluation of these relationships by levels of the cumulative number of (CAG)n or (GGC)n repeats was similarly unremarkable.
There was some variation in the association between OC use and breast cancer risk by genotype for both (CAG)n and (GGC)n repeats (Tables 3
and 4
). Specifically, among women who were (CAG)n S/S homozygotes and among women who were either heterozygotes or homozygotes for a (GGC)n L allele, an increased risk for breast cancer in relation to ever having used OCs was observed [OR = 1.9 (95% CI, 1.0–3.6) and OR = 1.7 (95% CI 0.9–3.5), respectively]. Neither ever use nor specific features such as duration, recency, and age of first use appeared to be related to breast cancer risk in women with an L allele in (CAG)n or with the (GGC)n S/S genotype.
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], and among the women carrying at least one (GGC)n L allele, risks were increased in relation to OC durations of less than 5 years (OR, 1.8; 95% CI, 0.9–4.0) and 5 to <10 years (OR, 2.2; 95% CI, 1.0–5.1), but not in relation to OC use of 10 years or more (OR, 1.0; 95% CI, 0.4–2.5; Table 4
). Among both groups, there were no obvious trends of increasing risk in relation to increased duration of OC use, and the magnitudes of the effects for specific duration categories, although increased, were quite similar to the magnitude of the effects observed in relation to "ever use." Among women with the (CAG)n S/S genotype, a >2-fold increase in risk was observed among those who last used OCs within 10 years before reference date; risk decreased to a 1.6-fold excess among those whose most recent use was 10 years or more before the reference date. Among women with one or two (GGC)n L alleles, risk estimates varied by recency but not in any apparent dose-response pattern.
In women with the S/S genotype for (CAG)n, estimates of the risk of breast cancer in relation to varying ages of first use ranged from 50% to 100% increases in risk. These effects were broadly similar to the increased risk that was observed for ever use of OCs within this group. Risk increased slightly from 1.5 for OC use beginning after age 21 years (95% CI, 0.6–3.7) to a 2-fold increase in risk for first use at ages 18–21 years and for use that began before age 18 years, but there was no statistically significant trend. Among women with at least one L (GGC)n repeat, breast cancer risk was increased for those whose first OC use occurred between ages 18 and 21 years (OR, 2.4; 95% CI, 1.1–5.2), but not for those whose first use began before age 18 years or for those whose first use was after age 21 years.
We assessed whether family history modified the effects of the various genotypes on the risk of breast cancer. Although risk estimates fluctuated to some extent by family history strata, these differences were neither consistent, substantive, nor statistically significant. The results were also unaffected by BRCA1 and BRCA2 status. Associations did not change when women with likely disease-associated BRCA1 and BRCA2 germ-line mutations were (separately and together) excluded from the analysis (data not shown). We note, however, that only a subset of the study population was screened for BRCA1 (39%) and BRCA2 (19%) mutation status.
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Discussion |
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The magnitude of the increased risk we observed with the (CAG)n genotype was broadly similar to the findings of a population-based study of breast cancer in Australian women under 40 years of age (24)
. In a study of 368 cases and 284 controls, Spurdle et al. (24)
observed an OR of 1.4 for the relationship between breast cancer risk and carrying a long (CAG)n allele (defined as 22 repeats), but the CI included 1.0, indicating that the result was not statistically significant (OR, 1.41; 95% CI, 0.95–2.09). A recent Canadian study of 255 cases, 272 general population controls, and 189 hospital controls observed an approximate 50% reduction in the risk of breast cancer for women with a smaller cumulative number of (CAG)n repeats (21)
. This reduction in risk was primarily confined to postmenopausal women; there was little evidence of any association in premenopausal women. A larger study of British women (508 cases and 426 controls) found no significant or consistent association between (CAG)n repeat length and breast cancer risk (23)
. Neither of these last two studies is readily comparable with the data presented here, however, because these studies focused primarily on older women in contrast to the younger age of our study population. To date, the only study that has shown a significant increased risk of breast cancer in relation to longer AR (CAG)n repeat length is that of Rebbeck et al. (18)
, who reported an increased breast cancer risk in relation to the presence of (CAG)n alleles of repeat length 28 in a population of BRCA1 mutation-positive women. Our study included very few known BRCA1 or BRCA2 carriers, and exclusion of those women did not affect overall results.
Neither of the two prior studies to date that have examined (GGC)n repeat length and breast cancer risk reported any association (19
, 23)
. The results presented here, which indicate that among young women, the presence of an L (GGC)n allele (repeat length 17) is associated with a significantly reduced risk of breast cancer compared with the homozygous S genotype, are the first. Specifically, our results demonstrate a significant 30% reduction in breast cancer risk for heterozygotic carriers of a long (GGC)n allele and a 50% reduction in risk for those carrying two long (GGC)n alleles. These results, as with the (CAG)n repeat results, may not be directly comparable with previous studies because the latter have focused on women of all ages and/or specifically on BRCA1/2 carriers (19
, 23)
.
We did not observe any combined effects for (CAG)n and (GGC)n when they were considered jointly. However, we note that the sample size for such analyses was likely insufficient.
Interestingly, whereas no previous studies have observed an effect in terms of breast cancer, several studies do link (GGC)n repeat length with prostate cancer risk (7 , 14, 15, 16, 17) . Only one study (31) failed to find an association between GGC repeat length and prostate cancer risk. These data suggest a nonspecific role for the (GGC)n repeat in risk of hormonal cancers overall.
Whereas a number of risk factors were found not to vary in effect by AR genotype, there was some suggestion of modification by genotype of the effect of OCs on breast cancer risk. Specifically, among women who were homozygous for the (CAG)n S allele and among women with any L (GGC)n allele, those who ever used OCs had a respective 90% and 70% excess risk of breast cancer compared with those of the same genotype who never used OCs. No association with OC use was seen among women with any L (CAG)n allele and women homozygous for the S (GGC)n allele. Although there was some fluctuation in risk estimates for aspects of OC use such as duration and recency within the two genotypic groups that exhibited an increased risk in relation to ever use of OCs, there was no substantive evidence that the increased risk seen for ever use was further altered or explained by increased duration of use or time since last use. This persistent elevation in risk across increasing duration of use, for instance, could suggest that for women with certain genotypic profiles, any exposure to OCs, regardless of the duration of use, might be related to an increased risk of breast cancer.
Our results for OC use should be considered within the context of the multicenter study from which these women were drawn (25) . The multicenter study evaluated the relationship between OCs and breast cancer risk in 1648 cases and 1505 controls under the age of 45 years (25) . Main findings from that study included the fact that breast cancer risk was modestly increased in relation to ever use of OCs and that risk did not increase further in relation to duration of use. In both data from the Seattle study site used for analyses of AR and the larger multicenter study results reported by Brinton et al. (25) , similar increases in risk were seen for ever users of OCs [OR = 1.3 (95% CI, 0.9–1.8) and OR = 1.3 (95% CI, 1.1–1.5), respectively; data not shown]. The results presented here suggest that a proportion of the overall increase in risk related to OC use may be attributable to AR genotype. They do not, however, exclude the possibility of a role for other genes and/or additional environmental exposures.
Our data are consistent with a role for AR repeats and interactions with hormones relevant to breast growth and development. However, no precise model can easily be drawn that explains our results in aggregate, due in part to the still incomplete understanding of the precise functional role that the AR repeats play in hormone regulation (32) . The role of androgens themselves in breast cancer is also not well understood, with higher levels of adrenal androgens noted in women who develop the disease in the postmenopausal years compared with those who develop it earlier in life (33) . (CAG)n repeat expansion has consistently been associated with a reduction in the ability of the AR to activate transcription (9 , 10 , 34) . However, analysis of (CAG)n repeat reduction and function of the neighboring (GGN)n repeat have only been partly investigated. Within the (GGN)n repeat, the non-normal (GGC)n repeat distribution found in our study, largely due to the high frequency of 16- and 17-repeat alleles (observed in 69.9% and 15.3% of the population, respectively), may provide some insight. Gao et al. (34) hypothesized that preference for a particular repeat size may be due to the physical and structural organization of the protein. Their study found a decrease in transcription activation levels in the absence of either the (GGN)n or (CAG)n repeat or in the presence of (CAG)n repeat lengths other than 20 (35) . However, earlier studies contradict this result, showing increased transcription activation when the (CAG)n repeat was eliminated or reduced in size (9 , 10) .
A few studies have attempted to directly assess the possible association between AR (CAG)n repeat length and endogenous hormone levels. Ancillary to their main case-control study, Haiman et al. (22) examined the relationship of the (CAG)n repeat length with circulating (serum) steroid hormone levels in approximately 450 postmenopausal controls. Although no associations with repeat length were statistically significant, there were suggestive relationships with testosterone and dihydroepiandrosterone sulfate. A small study of 239 premenopausal women recently reported an association between short repeat alleles and higher testosterone levels in the follicular phase of the menstrual cycle (35) . At least one large study in men has demonstrated a significant relationship between percentage change in free testosterone and increased number of (CAG)n repeats (36) . Clearly, additional studies are needed before any conclusions can be drawn regarding the functional significance of normal length variation, particularly with regard to the (GGC)n repeat length within the (GGN)n repeat.
The results of this study should be assessed within the context of its limits. We note specifically that there are differences between tested and untested women. Tested cases were more likely to be alive, older, and to have a less advanced stage of cancer than untested cases. Thus, the generalizability of our results, although from a population-based study, must be viewed with caution because the underrepresentation in our study population of cases with advanced disease could result in potential bias if there are associations between genotype and prognosis. In addition, tested women, particularly controls, were more likely to use OCs than untested women (P = 0.01 for controls and P = 0.026 for cases). The fact that tested controls were more likely to be OC users could suggest that observed risk estimates may be underestimates of the true magnitude of risk. In addition, the total number of cases in many cells was limited. Repetition of these analyses with larger data sets allowing for larger numbers of women in various strata is warranted to best understand the relationship between AR polymorphisms, breast cancer, and OC use.
A further limitation of the study is that the (GGC)n repeat genotype categories were not in compliance with Hardy-Weinberg equilibrium. Rather, we observed an excess of both LL and SS homozygotes compared with SL heterozygotes. Several factors may account for this. It is formally possible that the observed polymorphisms in the AR gene may influence mating, fertility, or fecundity and/or mortality, any of which could in turn influence changes in the gene frequencies in a population. It is more likely that the explanation is methodologic. We speculate that the presence of a single nucleotide polymorphism in one of the (GGC)n repeat primer sites could lead to an overestimation of homozygotes at the expense of heterozygotes by preferential amplification of the allele without the change. DNA sequencing of several putative homozygote samples did not reveal any such single nucleotide polymorphism (data not shown). We also note that there was a small set of 27 samples for which the presence of a second allele could neither be confirmed nor denied due to the existence of a broad "stutter band" on the gel associated with a different molecular weight allele. The presence of such stutter bands is commonly observed when amplifying segments of DNA that contain repeats. It is possible that inclusion of these samples could have altered our results slightly. However, reanalysis of the data after exclusion of individuals for whom only one genotype was present did not alter our results (data not shown). Finally, we note that the fact that X-inactivation occurs randomly early in development means that there will be variation in AR polymorphism expression between women with the same genotype in any organ of interest. This added degree of variation might account, at least in part, for the variability in results between studies.
This study does have a number of important strengths. Because our assessment of AR repeat effects on breast cancer risk includes a large and unselected proportion of all women with breast cancer before age 45 years during a defined time period and in a defined geographic area, it produces results that are applicable to the general population of women under age 45 years. In addition, the wealth of data collected for these women allowed for the consideration of important potential confounding and effect-modifying factors.
The strength of the association we observe between the AR (GGC)n repeat and the risk of breast cancer and the fact that this is the first study to examine this in relation to younger onset breast cancer suggest that additional studies of this association are needed. This is the first study, to our knowledge, that has shown an association between OC use and breast cancer risk among women with certain AR genotypes. These findings also warrant replication, although their interpretation is limited somewhat by our sample size. Resolution may ultimately require the pooling of data sets for questions such as this. These results lend further support to the need for more detailed functional studies aimed at understanding the role of AR repeats in the etiology of breast cancer in young women.
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Acknowledgments |
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Footnotes |
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1 Supported in part by Grants and Contracts R01-CA-63697, R01-CA-63705, N01-CP9567, R01-CA-59736, and N01-CN-67009 from the National Cancer Institute. 
2 To whom requests for reprints should be addressed, at Fred Hutchinson Cancer Research Center, Division of Public Health Sciences, P. O. Box 19024, MP-381, Seattle, WA 98109-1024. Phone: (206) 667-4632; Fax: (206) 667-5948; E-mail: kmalone{at}fhcrc.org
3 The abbreviations used are: AR, androgen receptor; OR, odds ratio; CI, confidence interval; OC, oral contraceptive.
Received 6/28/02; revised 11/ 4/02; accepted 11/25/02.
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 S. N. Birrell, L. M. Butler, J. M. Harris, G. Buchanan, and W. D. Tilley Disruption of androgen receptor signaling by synthetic progestins may increase risk of developing breast cancer FASEB J, August 1, 2007; 21(10): 2285 - 2293. [Abstract] [Full Text] [PDF]
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 F. F Brockschmidt, M. M Nothen, and A. M Hillmer The two most common alleles of the coding GGN repeat in the androgen receptor gene cause differences in protein function J. Mol. Endocrinol., July 1, 2007; 39(1): 1 - 8. [Abstract] [Full Text] [PDF]
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 M. Hietala, T. Sandberg, A. Borg, H. Olsson, and H. Jernstrom Testosterone levels in relation to oral contraceptive use and the androgen receptor CAG and GGC length polymorphisms in healthy young women Hum. Reprod., January 1, 2007; 22(1): 83 - 91. [Abstract] [Full Text] [PDF]
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W. Wang, E. M. John, and S. A. Ingles Androgen Receptor and Prostate-Specific Antigen Gene Polymorphisms and Breast Cancer in African-American Women Cancer Epidemiol. Biomarkers Prev., December 1, 2005; 14(12): 2990 - 2994. [Abstract] [Full Text] [PDF]
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 K. L. Terry, I. De Vivo, L. Titus-Ernstoff, M.-C. Shih, and D. W. Cramer Androgen Receptor Cytosine, Adenine, Guanine Repeats, and Haplotypes in Relation to Ovarian Cancer Risk Cancer Res., July 1, 2005; 65(13): 5974 - 5981. [Abstract] [Full Text] [PDF]
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 W. Somboonporn and S. R. Davis Testosterone Effects on the Breast: Implications for Testosterone Therapy for Women Endocr. Rev., June 1, 2004; 25(3): 374 - 388. [Abstract] [Full Text] [PDF]
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E. O. Lillie, L. Bernstein, S. A. Ingles, W. J. Gauderman, G. E. Rivas, V. Gagalang, T. Krontiris, and G. Ursin Polymorphism in the Androgen Receptor and Mammographic Density in Women Taking and Not Taking Estrogen and Progestin Therapy Cancer Res., February 15, 2004; 64(4): 1237 - 1241. [Abstract] [Full Text] [PDF]
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A. Liede, W. Zhang, M. L. D. L. Matsuda, A. Tan, and S. A. Narod Androgen Receptor Gene Polymorphism and Breast Cancer Susceptibility in the Philippines Cancer Epidemiol. Biomarkers Prev., September 1, 2003; 12(9): 848 - 852. [Abstract] [Full Text] [PDF]
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