A genome-wide linkage study of mammographic density, a risk factor for breast cancer
- Celia MT Greenwood1,
- Andrew D Paterson2, 3,
- Linda Linton4,
- Irene L Andrulis5, 6,
- Carmel Apicella7,
- Apostolos Dimitromanolakis2,
- Valentina Kriukov4,
- Lisa J Martin4, 8,
- Ayesha Salleh4,
- Elena Samiltchuk2,
- Rashmi V Parekh2,
- Melissa C Southey9,
- Esther M John10,
- John L Hopper7,
- Norman F Boyd4, 8 and
- Johanna M Rommens2, 6Email author
© Greenwood et al.; licensee BioMed Central Ltd. 2011
Received: 26 July 2011
Accepted: 21 December 2011
Published: 21 December 2011
Mammographic breast density is a highly heritable (h2 > 0.6) and strong risk factor for breast cancer. We conducted a genome-wide linkage study to identify loci influencing mammographic breast density (MD).
Epidemiological data were assembled on 1,415 families from the Australia, Northern California and Ontario sites of the Breast Cancer Family Registry, and additional families recruited in Australia and Ontario. Families consisted of sister pairs with age-matched mammograms and data on factors known to influence MD. Single nucleotide polymorphism (SNP) genotyping was performed on 3,952 individuals using the Illumina Infinium 6K linkage panel.
Using a variance components method, genome-wide linkage analysis was performed using quantitative traits obtained by adjusting MD measurements for known covariates. Our primary trait was formed by fitting a linear model to the square root of the percentage of the breast area that was dense (PMD), adjusting for age at mammogram, number of live births, menopausal status, weight, height, weight squared, and menopausal hormone therapy. The maximum logarithm of odds (LOD) score from the genome-wide scan was on chromosome 7p14.1-p13 (LOD = 2.69; 63.5 cM) for covariate-adjusted PMD, with a 1-LOD interval spanning 8.6 cM. A similar signal was seen for the covariate adjusted area of the breast that was dense (DA) phenotype. Simulations showed that the complete sample had adequate power to detect LOD scores of 3 or 3.5 for a locus accounting for 20% of phenotypic variance. A modest peak initially seen on chromosome 7q32.3-q34 increased in strength when only the 513 families with at least two sisters below 50 years of age were included in the analysis (LOD 3.2; 140.7 cM, 1-LOD interval spanning 9.6 cM). In a subgroup analysis, we also found a LOD score of 3.3 for DA phenotype on chromosome 12.11.22-q13.11 (60.8 cM, 1-LOD interval spanning 9.3 cM), overlapping a region identified in a previous study.
The suggestive peaks and the larger linkage signal seen in the subset of pedigrees with younger participants highlight regions of interest for further study to identify genes that determine MD, with the goal of understanding mammographic density and its involvement in susceptibility to breast cancer.
Mammographic density (MD), adjusted for age and body mass index (BMI) is a strong risk factor for breast cancer. The radiographic appearance of the breast on mammography varies among women and reflects differences in breast tissue composition and X-ray attenuation characteristics . Fat is radiologically lucent  and appears dark on a mammogram, whereas connective and epithelial tissues are radiologically dense and appear light. The area of radiological dense tissue is often expressed as a percentage of the total breast area (percent mammographic density (PMD)). Compared to women of the same age and BMI with little or no density, PMD of ≥75% is associated with a four- to five-fold increased risk of breast cancer and an increased risk of all of the proliferative lesions that are thought to be precursors of breast cancer . The increased breast cancer risk associated with high PMD does not differ by age, menopausal status, or race/ethnicity and cannot be explained by the 'masking' of cancers by dense tissue . Extensive MD is relatively common and estimates of the associated attributable risk suggest that about a third of breast cancer may be explained by density in more than 50% of the breast [4, 5].
Twin studies have shown that, after adjustment for other factors such as age, parity, menopausal status, body weight and hormonal use, about 60% of the variance in PMD is explained by genetic factors [6, 7]. Both the dense area of the mammogram and the non-dense area have been found to be heritable to a degree similar to PMD. About two thirds of the negative correlation between dense and non-dense area was explained by the same genetic factors influencing both traits, but in opposing directions . To find genetic determinants of variation in MD, we have carried out a genome-wide linkage study with over 1,400 nuclear families with the aim of examining previously reported loci and of identifying new loci that influence MD.
Materials and methods
Recruitment of study subjects
Recruitment of families for linkage
Families, n (%)
Women with complete data for analysis, n (%)
Australian site of the BCFR
Australian Twin & Sister Study
Northern California site of the BCFR
Ontario site of the BCFR
Ontario families from other sources
Measurement of mammographic density
Mammograms were obtained from at least two sisters in each family. If a woman had breast cancer, the selected mammogram (from the contra breast) had been taken at or before the diagnosis of breast cancer. We sought that the ages at mammogram for two sisters should be within 5 years of each other to minimize the requirement for age-adjustment within a sibship. This was achieved in more than 95% of the families. The cranio-caudal view in each mammogram was digitized and sent to Toronto where all images were measured by one reader (NFB) using a computer-assisted thresholding method. Using Cumulus software, thresholds were set that defined the edge of the breast and outlined the areas of dense tissue. The pixels within these thresholds defined, respectively, the total area of the breast and the area of dense tissue (DA), from which percent density (PMD) was calculated. The non-dense area (NDA) of the mammogram was also calculated from these measurements . This measurement method has been used to define differences in risk of breast cancer associated with mammographic density [2–5] and generated the evidence of heritability that motivated the present study .
Images were measured in batches of approximately 100 images at a time. Within each batch, 10 images were duplicated, placed in random order within the set, and read twice in a blind fashion. Some images were also re-read in different batches, in order to assess reliability of the PMD measurements both within and between batches. The correlation between the two reads in the same batch for PMD was estimated to be 0.90 (Additional file 1: supplementary Figure 2) and correlation for DA was 0.92. In addition, there were 10 images that were scored three times in three different batches, and the intraclass correlation between batches was estimated to be 0.902 by variance components analysis.
DNA, genotyping and data cleaning
DNA was obtained from the BCFR biorepositories or was extracted from whole blood or lymphoblastoid cell lines using Gentra Puregene Blood Kit (Qiagen, Inc., Toronto, ON, Canada) or QIAamp DNA Blood Maxi Kit (Qiagen) according to the supplier's instructions. DNAs were quantified with Quant-iT PicoGreen dsDNA Reagent (Life Technologies, Inc., Burlington, ON, Canada) using fluorimetry measured with a SpectraMax Gemini EM instrument (Molecular Devices).
Genotyping for linkage analysis was performed by the Center for Inherited Disease Research (CIDR) http://www.cidr.jhmi.edu using the Illumina Infinium II Human Linkage-12 panel. Out of 6,090 SNPs, 413 were removed due to poor clustering, leaving 5,677 SNPs with an overall genotype missing rate of 0.073%. After examining Mendelian errors and estimating relationships [11–13], 49 pedigrees were modified to create half siblings when necessary, 6 larger pedigrees were created by merging two families (this information was subsequently confirmed from source site), 1 family was excluded due to sisters appearing unrelated, two families were removed due to a sample mix-up, and one sibling that appeared unrelated was removed from a larger pedigree (Additional file 1: supplementary Figure 3). After adjusting the pedigree structures, genotypes demonstrating Mendelian inheritance inconsistencies  were set to missing for all individuals in the families concerned; this involved removing a total of 475 genotypes on the autosomes. We removed all X chromosome data for six women where all genotypes were homozygous or had unusually high rates of missing genotypes.
Genotypes were missing in less than 20 individuals for 97.4% of the markers; and the poorest marker failed in 81 of 3,952 individuals. Ten or fewer marker genotypes were missing in 91% of the genotyped individuals. Allele frequencies were estimated from all genotyped individuals. Only 1.9% of the markers had minor allele frequency (MAF) less than 0.10, and 76% of the markers had MAF over 0.3; average heterozygosity across SNPs was 43.7%. Multipoint marker informativity remained very stable across the genome, ranging from 0.6 to 0.8.
Although self-reported race/ethnicity was available, we estimated population structure by using Eigenstrat  on one individual per family, combined with 1,207 HapMap phase 3 samples  from 10 populations, and our 5,677 SNPs. SNP alleles were flipped when necessary to match the strand used by CIDR. We found, as expected, that the majority of the samples overlapped with the HapMap CEU group, and furthermore that self-reported race/ethnicity matched well with clusters of Asian and African descent from the HapMap samples (Additional file 1: supplementary figure 4). Principal components (PCs) were estimated for all study participants from the eigenvectors, and then a Caucasian subgroup was defined as those with the first PC ≥0.003 and the second PC ≥0.
Physical marker locations were obtained from NCBI Build 36 http://www.ncbi.nlm.nih.gov/. The Rutgers linkage map was obtained from Rutgers Map Version 2 http://compgen.rutgers.edu/. For markers not included in the Rutgers data set, genetic distances were interpolated at the Rutgers site.
Genotyping of individual single-nucleotide polymorphisms (SNPs) to attempt validation of association was performed with allele-specific fluorescent probes in Taqman® SNP Genotyping Assays (C_334499_10 for rs723149 from Applied Biosystems, Foster City, CA, USA) as recommended using a 96-well format. End-point fluorescence was measured with the plate reader component of the 7900HT Real Time PCR System (Applied Biosystems) and aided by Taqman® Genotyper software for allele discrimination with call rates > 98%. A portion of the samples (4%) was run in duplicate and corresponded to individuals used in the linkage study to assure quality control and permit assessment across genotyping platforms. The concordances of replicate genotypes and cross platform genotypes were > 99%.
Linkage analysis was performed using the Merlin (version 1.1.2)  variance components method on MD after adjustment for known covariates. Since variance components linkage analysis is very sensitive to non-normal distributions of traits , the MD measurements were transformed to reduce skewness and shrink outliers towards the center of the distributions. A square root transformation was used for PMD, a log transformation for total non-dense area (NDA), and quantile normalization was used for total dense area (DA) since there were several extremely high values for DA. In addition, two extremely large values for NDA were winsorized to the 99th percentile. Trait distributions were unaffected by breast cancer status as mammograms were measured only from the opposite breast, prior to or at a diagnosis. Linear models and generalized additive models were used to model the relationships between the MD scores and known covariates, and the residuals were used in the linkage analysis. Family relationships were not taken into account when fitting the linear models. In general, the expected relationships between MD and covariates were seen. We also estimated heritability in Merlin  and in SOLAR version 4.2.7 , for the whole sample and for several subgroups. The model used assumed no dominance variance, and allowed for a polygenic effect. There were too few non-Caucasian families to perform variance components linkage analysis separately, however, linkage analysis was repeated in the subgroup of families estimated to be Caucasian by Eigenstrat . To estimate whether our linkage results were exceptional, simulations were performed in Merlin for all autosomes. For these simulations, the phenotypes and pedigree structures were retained, and genotype data were generated by using the estimated allele frequencies. Simulated genotypes were considered unknown if actual genotypes were unknown. Analysis of the simulated data was performed for the residuals from the square root transformation of PMD.
Although this was primarily a linkage study, we calculated evidence for association using the orthogonal test for quantitative traits implemented in QTDT  at 5,677 markers. This test focuses on within-family evidence for association, is robust to any differences in allele frequencies between families due to population stratification, and assumes an additive genetic model.
There were 1,616 pedigrees assembled for this study from existing collections, including 4,526 individuals with DNA. Data verification and cleaning included checking for monozygotic twin status (if monozygotic, one twin was removed), gender consistency, whether the mammogram was readable, whether relevant covariate data were available, and whether reported family relationships were consistent with the genotyping. Pedigrees were adjusted when data genotyping supported alternative relationships. A total of 1,415 families were retained for analysis containing 6,638 individuals, of whom 4,993 were women (Additional file 1: supplementary Figure 3).
There were 3,952 genotyped individuals, including siblings, parents and the sisters (3,253) with analysis information (namely genotypes, MD measures, and epidemiologic covariate information). Almost half the families (47%) were recruited from Australia (Additional file 1: supplementary table 2), 35% from Ontario and 18% from California. The families were mostly nuclear, with 77% with three or more women, and 19% with four or more women with complete analysis information. Parental DNA was rarely available; neither of the parents was genotyped in 84% of the families (Additional file 1: supplementary table 2).
Description of individuals with complete information
Age at mammogram, year
Self-reported ethnicity, n (%)
Age at menarche, year
Pregnant ever, n (%)
Number of live births, n (%)
Postmenopausal, n (%)
Past or current hormone therapy use, n (%)
Body mass index
Personal history of breast cancer, n (%)
Coefficients (standard errors) for covariates in multi-variable linear model predicting the square root of percent mammographic density, overall and by study site
All sites together (n= 3,253)
Australia (n= 1,534)
Age at mammogram
Weight squared (centered at 70 kg)
Number of live births
History of hormone therapy use
Estimates of heritability from SOLAR (standard error)
Number of families
Families from Australia
Families from Northern California
Families from Ontario
Genetic marker characteristics
After removing inconsistent and failed markers, 5,677 SNPs remained for analysis (See Materials and Methods). Tests for Hardy-Weinberg equilibrium (HWE)  were performed using one randomly chosen genotyped woman per family. When only Caucasian families are included in these tests (excluding 105 families containing 315 individuals), there is no evidence for deviation from HWE (Additional file 1: supplementary figure 5). For this subset analysis, Caucasian ethnicity was defined as individuals who clustered with the HapMap CEU individuals in Eigenstrat PC analysis .
We then performed several different analyses of ancillary phenotypes and subgroups to investigate whether the evidence for linkage was stronger under different assumptions.
Weight and height
Weight and height are partially determined by genetic factors. We considered the possibility that by including weight and height in the models for calculating residuals, we may have minimized genetic effects due to pleiotropy. Linkage was re-estimated using residuals to models containing all predictors except weight and height; no additional linkage peaks were identified. Conversely, we also considered the possibility that we were not adequately capturing the relationships between PMD and weight, height, and age at mammogram in our linear models. We therefore fitted generalized additive models  for these variables, including a bivariate smooth for weight and height, but again no additional linkage peaks were identified.
Menstrual, reproductive and hormonal variables
MD decreases with age and is influenced by many factors that alter endogenous sex hormone levels, such as the number of pregnancies, hormone therapy use, and menopausal status. Although in our primary analysis, we had analyzed residuals to linear models adjusted for these factors, we hypothesized that the magnitude of the potential effect of a locus on MD measurements could vary as a function of these reproductive and hormonal variables. To explore this hypothesis, we used our linear models with covariates to obtain a predicted PMD value for each individual, and then we calculated the smallest difference in predicted PMD among all pairs of sisters in each family. We then divided the families into two equally-sized groups based on this predicted PMD difference. Hence, in one group, age, menstrual and reproductive values led to predictions of more similar PMD for the sisters, whereas in the other group, larger PMD differences were predicted by the covariate model; this concept is illustrated in Additional file 1: supplementary figure 6. Evidence for linkage was calculated in these two subgroups, and results are also shown in Figure 2. In families containing sisters with dissimilar predictions, we saw a linkage peak for the DA phenotype on chromosome 12p11.22-q13.11 (max LOD = 3.30 between rs2061192 and rs10785424, 1-LOD drop interval from 56.0 cM - 64.0 cM, Figure 2B). Linkage in this subset implies that sharing of markers in this region is associated with more similarity in DA, among families where MD measures are expected to be quite variable due to covariates. Although this analysis used the total dense area phenotype rather than PMD, a reduced signal was also seen when we analyzed PMD (Figure 2A). DA, like PMD, is positively associated with breast cancer risk and may have genetic determinants that are not shared with the non-dense area of the breast. Our signal is within a broad linkage signal spanning an approximately 90 cM region (2-LOD drop interval) described by Vachon and colleagues . Their signal appeared to contain two peaks, one with a maximum LOD score of 2.47 with a 1-LOD drop interval from 19 cM to 46 cM and a second peak with a maximum LOD score of 2.45 with a 1-LOD drop interval from 55 cM to 101 cM (Vachon, personal communication). Vachon and colleagues employed a model with PMD measures adjusted for covariates but did not evaluate dense area as a separate phenotype.
Refining the PMD phenotype
To explore the impact of phenotype-covariate modelling on the linkage results, we refit our linear models for the square root of PMD phenotype using a different parameterization that was suggested by the Pike model . In light of this model, which relates cumulative exposure to 'breast tissue age' with breast cancer incidence rates, breast tissue age can be expected to be greatest at menarche, to decrease with each pregnancy, and to decline rapidly through the menopausal period. Therefore, we defined separate covariates to estimate the decrease in PMD with each year at each stage in life (that is, (1) between menarche and the first live birth, (2) during the child-bearing years, (3) post child-bearing years, and (4) post-menopause). Covariates for the number of pregnancies and use of hormonal therapy were included. In addition, we fit these models using generalized estimating equations to adjust for the effects of within-family correlation when estimating parameters. The signal on chromosome 7p is enhanced by using these residuals for the phenotype, with the peak at rs1029482 rising to 3.29 (1-LOD interval 59.2 - 67.3 cM) (Figure 2A, new covariate model). Eighteen percent of our simulations had a peak LOD score over 3.5, thus the evidence for linkage to this locus is still not genome-wide 'significant' .
Selected tests of within-family association with at least one phenotype with P-value ≤0
Minor allele/MAF in our data
Allele with higher MD
In order to see whether the linkage signal on chromosome 7p was explained by this association, we imputed haplotypes near rs723149 by using PLINK , and created a covariate based on the probable count of haplotype AA at rs1486155 and rs723149. This covariate was then included in the linkage analysis, but was found to have minimal impact. The LOD score fell by 0.21 for PMD_res, from 2.69 to 2.48.
Genotype information at rs723149 of additional sets of women with MD measurements and covariate information was available. These included a non-overlapping set of 235 women from Ontario and Australia  selected for extreme values for MD that had been genotyped with the Illumina Sentrix Human 1M BeadChip platform and an additional group of 789 unrelated women from Ontario (with characteristics as for the recruited family participants) genotyped with a Taqman® SNP Genotyping Assay specifically for rs723149. No evidence for association with PMD was observed in these 1,024 women (MAF = 0.42, linear regression with 0,1, 2 coding for number of copies of allele A, β = -0.077 (SE = 0.083), P = 0.35, for association with the square root of PMD adjusted for age, age squared, weight, weight squared, height, height squared, parity, and menopausal status).
Phenotype definition is critical in linkage analysis. The extent of MD can be measured by the percent of the breast that is dense or by the total area that is dense. DA is highly correlated with PMD, but has a skewed distribution. Nevertheless, after transformation and adjustment for covariates, repeated reads of the same mammogram showed very high correlations for DA and for PMD. Therefore, we examined the evidence for linkage to both these phenotypes, and found evidence for suggestive linkage at multiple locations. Furthermore, since MD is known to vary through life, genes that appear to influence MD in mid-life could reflect early influences on MD at the time the breast forms in adolescence, or through subsequent changes or rates of change with increasing age, parity and the menopause, or through influences on both formation and age change in MD . Our study was designed so that there was rarely more than five years difference between the ages at mammogram for sister pairs in a family; hence the study was partially matched within families for the large effect of age on MD. In addition, MD measures were adjusted for age prior to linkage analysis.
Given the number of study participants and available pedigree information, we had estimated that we would have 80% power to obtain a LOD score in the range of 3 with a locus that could explain 25% of the heritability of PMD. Despite the large collection of families and careful consideration of mammographic density parameters, our primary linkage analysis did not yield LOD scores that exceeded desired thresholds set by genome-wide gene-dropping simulations. With different modeling conditions, three loci on chromosomes 7p14.1-p13, 12p11.22-q13.11, and 7q32.3-q34, showed LOD scores that approach or exceed 3. However, given that these results were obtained after multiple analyses, these signals should be considered as 'suggestive'.
Chromosome 7p: With the development of a model for covariate effects that was inspired by Pike and colleagues , evidence for linkage increased on chromosome 7p, with a maximum LOD score of 3.29. This increase associated with a more careful phenotype definition is suggestive that there may be loci influencing aspects of MD or MD changes, but also that the ideal parameterization for MD is still unknown. The 1-LOD drop interval bounded by rs1949880 and rs2054789 corresponds to a 7.2 Mb region containing 72 genes, including small clusters of snoRNA and piRNA genes toward its proximal boundary. Of interest at the proximal boundary, are the insulin-like growth factor binding protein genes, IGFBP1 and IGFBP3, both of which have been hypothesized to be involved in mammographic density (and in breast cancer and other cancers) and have been considered in previous association studies examining MD phenotypes [31–35]. Of these, two studies included more than 1,000 unrelated women [32, 3] with investigation of the common genetic variation in these genes, but they did not reveal consistent evidence of association with MD phenotypes. Also, a recent meta-analysis of 4,877 women did not identify association with MD in this region . As linkage analysis may be less sensitive to allele frequency issues, these genes remain as interesting candidates. Another gene of potential interest within this region includes the ras-related gene, v-ral simian leukemia viral oncogene homolog A (RALA) with its implicated role in signalling and growth.
Chromosome 7q: When we examined families containing younger sisters (two sisters with mammograms under age 50 years), the peak on chromosome 7q became stronger. This peak with a 1-LOD drop interval bounded by rs4728251 and rs1476640, corresponds to 9.6 Mb containing 69 genes. Phenotypes at younger ages are expected to display a stronger genetic component  but there was no evidence that PMD h2 varied by age in our previous twin studies [30, 37], and thus age should continue to be considered in future studies. A gene of potential interest within this region is a member of the RAS oncogene family, RAB19.
Chromosome 12: On chromosome 12, a maximum LOD score of 3.3 was seen in families where the sisters would be expected to have dissimilar DA, after adjustment for factors affecting sex hormone levels. Such predicted differences could be due to: differences in the numbers of pregnancies; ages at menopause or menarche; weight; or height. Linkage in this context would identify families where phenotypes are more similar than expected, and this could imply a larger potential genetic effect. The 1-LOD interval of our linkage signal, bounded by rs1909160 and rs1978161 corresponds to a large physical distance of 16.7 Mb encompassing the centromere and contains 80 genes. Genes of potential interest within this region include the vitamin D receptor (VDR) and collagen type II (COL2A1) given their association to breast biology.
We did not detect any significant evidence for linkage on chromosome 5 as was reported in a previous study of 89 multi-generation pedigrees , which, like our study, included mostly Caucasian women; however, there may have been differences in the family ancestries or characteristics of the source populations that are not obvious.
That high mammographic density is associated with risk for breast cancer motivated our study, with anticipation that some gene determinants of MD may be candidates for involvement or development of malignancy . Many groups have pursued genome-wide linkage studies for breast cancer using non-BRCA1/2 high risk families, more recently with families of confined ancestries (see , and within) after family sets of diverse origins did not yield strong linkage signals [39, 40]. Suggestive evidence for a chromosome 7 locus was identified in one study , however, this region is more proximal occurring at 7q21.11-q21.3, and does not overlap with the suggestive MD locus detected in our younger women set. Large scale association studies have also been carried out, several with discovery or first-stage study sizes that exceed 1,000 cases of breast cancer [41–46] leading to loci for consideration, with considerable family risk remaining to be explained . We note that none of our suggestive linkage peaks observed coincide with the candidate genes and their local SNPs that reached genome-wide significance in these large studies.
Despite a reliable and plausible intermediate phenotype for breast cancer risk, we were unable to identify new loci with a strong influence on MD, nor could we confirm a previously reported region on chromosome 5p13-p14. However, we did identify signals on chromosomes 7p, 7q, and 12 that warrant further investigation. Notably, the evidence for linkage among younger women on 7q, as well as thepeak on chromosome 12 overlapping a previously-identified region may prove to be interesting.
CMTG is the Pharmaprix Weekend to End Cancer Career Scientist. ADP holds a Canadian Research Chair in the Genetics of Complex Diseases. LJM is a recipient of a New Investigator Award from the Canadian Institutes of Health Research. MCS is a National Health and Medical Research Council (NHMRC) Senior Research Fellow and a Victorian Breast Cancer Research Consortium Group Leader. JLH is an Australian Fellow of the NHMRC and a Victorian Breast Cancer Research Consortium Group Leader.
Center for Inherited Diseases Research
Genome-Wide Association Study
logarithm of odds
minor allele frequency
principal components analysis
percent mammographic density
This work was supported by the National Cancer Institute, National Institutes of Health under RFA-CA-06-503 and through cooperative agreements with members of the Breast Cancer Family Registry (BCFR) and Principal Investigators from Cancer Care Ontario (U01 CA69467), Cancer Prevention Institute of California (U01 CA69417), University of Melbourne (U01 CA69638), and the Research Triangle Institute (RTI) Informatics Support Center (RFP No. N02PC45022-46). The content of this manuscript does not necessarily reflect the views or policies of the National Cancer Institute or any of the collaborating centers in the BCFR, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government or the BCFR. The work was also supported by the National Institutes of Health (CA102659), Canadian Breast Cancer Research Alliance (016442) and the National Health and Medical Research Council of Australia.
For the initial family analyses, genotyping services were provided by the Center for Inherited Disease Research (CIDR). CIDR is funded through a federal contract from the National Institutes of Health to The Johns Hopkins University, contract number HHSN268200782096C.
Participants of the Breast Cancer Family Registry sites in Ontario, Northern California and Australia, Twins and Sisters Study of Ontario, Ontario Young Women's Study, Weekend to End Breast Cancer and Twins and Sisters Study of Australia are gratefully acknowledged. Study coordination by E. Satariano and informatics support by D. Harris of RTI are also gratefully acknowledged.
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