Potential genetic predisposition for anthracycline-associated cardiomyopathy in families with dilated cardiomyopathy.

OBJECTIVE: Anthracyclines are successfully used in cancer treatment, but their use is limited by their cardiotoxic side effects. Several risk factors for anthracycline-associated cardiomyopathy (AACM) are known, yet the occurrence of AACM in the absence of these known risk factors suggests that other factors must play a role. The purpose of this study was to evaluate whether a genetic predisposition for dilated cardiomyopathy (DCM) could be a potential risk factor for AACM. METHODS: A hospital-based registry of 162 DCM families and two hospital-based registries of patients with cancer treated with systemic cancer therapy (n>6000) were reviewed focusing on AACM. Selected patients with AACM/DCM families with possible AACM (n=21) were analysed for mutations in cardiomyopathy-associated genes and presymptomatic cardiological evaluation of first-degree relatives was performed. RESULTS: We identified five DCM families with AACM and one patient with AACM with a family member with a possible early sign of mild DCM. Pathogenic MYH7 mutations were identified in two of these six families. The MYH7 c.1633G>A (p.Asp545Asn) and c.2863G>A (p.Asp955Asn) mutations (one double mutant allele) were identified in a DCM family with AACM. The MYH7 c.4125T>A (p.Tyr1375X) mutation was identified in one patient with AACM. CONCLUSIONS: This study further extends the hypothesis that a genetic predisposition to DCM could be a potential risk factor for AACM.

MUTYH gene variants and breast cancer in a Dutch case­control study.

The MUTYH gene is involved in base excision repair. MUTYH mutations predispose to recessively inherited colorectal polyposis and cancer. Here, we evaluate an association with breast cancer (BC), following up our previous finding of an elevated BC frequency among Dutch bi-allelic MUTYH mutation carriers. A case­control study was performed comparing 1,469 incident BC patients (ORIGO cohort), 471 individuals displaying features suggesting a genetic predisposition for BC, but without a detectable BRCA1 or BRCA2 mutation (BRCAx cohort), and 1,666 controls. First, for 303 consecutive patients diagnosed before age 55 years and/or with multiple primary breast tumors, the MUTYH coding region and flanking introns were sequenced. The remaining subjects were genotyped for five coding variants, p.Tyr179Cys, p.Arg309Cys, p.Gly396Asp, p.Pro405Leu, and p.Ser515Phe, and four tagging SNPs, c.37-2487G>T, p.Val22Met, c.504+35G>A, and p.Gln338His. No bi-allelic pathogenic MUTYH mutations were identified. The pathogenic variant p.Gly396Asp and the variant of uncertain significance p.Arg309Cys occurred twice as frequently in BRCAx subjects as compared to incident BC patients and controls (p=0.13 and p=0.15, respectively). The likely benign variant p.Val22Met occurred less frequently in patients from the incident BC (p=0.03) and BRCAx groups (p=0.11), respectively, as compared to the controls. Minor allele genotypes of several MUTYH variants showed trends towards association with lobular BC histology. This extensive case­control study could not confirm previously reported associations of MUTYH variants with BC, although it was too small to exclude subtle effects on BC susceptibility.

Gene expression profiling assigns CHEK2 1100delC breast cancers to the luminal intrinsic subtypes.

CHEK2 1100delC is a moderate-risk cancer susceptibility allele that confers a high breast cancer risk in a polygenic setting. Gene expression profiling of CHEK2 1100delC breast cancers may reveal clues to the nature of the polygenic CHEK2 model and its genes involved. Here, we report global gene expression profiles of a cohort of 155 familial breast cancers, including 26 CHEK2 1100delC mutant tumors. In line with previous work, all CHEK2 1100delC mutant tumors clustered among the hormone receptor-positive breast cancers. In the hormone receptor-positive subset, a 40-gene CHEK2 signature was subsequently defined that significantly associated with CHEK2 1100delC breast cancers. The identification of a CHEK2 gene signature implies an unexpected biological homogeneity among the CHEK2 1100delC breast cancers. In addition, all 26 CHEK2 1100delC tumors classified as luminal intrinsic subtype breast cancers, with 8 luminal A and 18 luminal B tumors. This biological make-up of CHEK2 1100delC breast cancers suggests that a relatively limited number of additional susceptibility alleles are involved in the polygenic CHEK2 model. Identification of these as-yet-unknown susceptibility alleles should be aided by clues from the 40-gene CHEK2 signature.

Increased MUTYH mutation frequency among Dutch families with breast cancer and colorectal cancer.

Homozygous and compound heterozygous MUTYH mutations predispose for MUTYH-associated polyposis (MAP). The clinical phenotype of MAP is characterised by the multiple colorectal adenomas and colorectal carcinoma. We previously found that female MAP patients may also have an increased risk for breast cancer. Yet, the involvement of MUTYH mutations in families with both breast cancer and colorectal cancer is unclear. Here, we have genotyped the MUTYH p.Tyr179Cys, p.Gly396Asp and p.Pro405Leu founder mutations in 153 Dutch families with breast cancer patients and colorectal cancer patients. Families were classified as polyposis, revised Amsterdam criteria positive (FCRC-AMS positive), revised Amsterdam criteria negative (FCRC-AMS negative), hereditary breast and colorectal cancer (HBCC) and non-HBCC breast cancer families. As anticipated, biallelic MUTYH mutations were identified among 13% of 15 polyposis families, which was significantly increased compared to the absence of biallelic MUTYH mutations in the population (P = 0.0001). Importantly, six heterozygous MUTYH mutations were identified among non-polyposis families with breast and colorectal cancer. These mutations were identified specifically in FCRC-AMS negative and in HBCC breast cancer families (11% of 28 families and 4% of 74 families, respectively; P = 0.02 for both groups combined vs. controls). Importantly, the 11% MUTYH frequency among FCRC-AMS negative families was almost fivefold higher than the reported frequencies for FCRC-AMS negative families unselected for the presence of breast cancer patients (P = 0.03). Together, our results indicate that heterozygous MUTYH mutations are associated with families that include both breast cancer patients and colorectal cancer patients, independent of which tumour type is more prevalent in the family.

Discovering moderate-risk breast cancer susceptibility genes.

To date, five moderate-risk breast cancer susceptibility genes have been convincingly identified: CHEK2, ATM, BRIP1, PALB2, and NBS1. Moderate-risk breast cancer alleles confer increased breast cancer risks of two to fourfold compared to the 10% risk in the general population. In contrast to the high-risk BRCA1 and BRCA2 genes, moderate-risk genes typically have a limited number of variants that confer breast cancer risks. The prevalence of the variants usually varies widely among different geographical or ethnic populations, ranging from essentially absent up to 1.5% (i.e. 'rare' variants). Since moderate-risk breast cancer alleles are clinically not recognizable when inherited as single mutant, one usually encounters them in a polygenic setting and consequently in incomplete cosegregation with the breast cancer phenotype. As a result, discovery of moderate-risk breast cancer genes requires conclusive statistical evidence from association studies of hundreds of breast cancer cases and population-matched controls.

Distinct gene mutation profiles among luminal-type and basal-type breast cancer cell lines.

Breast cancer has for long been recognized as a highly diverse tumor group, but the underlying genetic basis has been elusive. Here, we report an extensive molecular characterization of a collection of 41 human breast cancer cell lines. Protein and gene expression analyses indicated that the collection of breast cancer cell lines has retained most, if not all, molecular characteristics that are typical for clinical breast cancers. Gene mutation analyses identified 146 oncogenic mutations among 27 well-known cancer genes, amounting to an average of 3.6 mutations per cell line. Mutations in genes from the p53, RB and PI3K tumor suppressor pathways were widespread among all breast cancer cell lines. Most important, we have identified two gene mutation profiles that are specifically associated with luminal-type and basal-type breast cancer cell lines. The luminal mutation profile involved E-cadherin and MAP2K4 gene mutations and amplifications of Cyclin D1, ERBB2 and HDM2, whereas the basal mutation profile involved BRCA1, RB1, RAS and BRAF gene mutations and deletions of p16 and p14ARF. These subtype-specific gene mutation profiles constitute a genetic basis for the heterogeneity observed among human breast cancers, providing clues for their underlying biology and providing guidance for targeted pharmacogenetic intervention in breast cancer patients.

Association of rare MSH6 variants with familial breast cancer.

Germline mutations in the mismatch repair genes MLH1, MSH2, MSH6, and PMS2 predispose to Lynch syndrome (also known as hereditary non-polyposis colorectal cancer). Recently, we have shown that the CHEK2 1100delC mutation also is associated with Lynch syndrome/Lynch syndrome-associated families albeit in a polygenic setting. Two of the ten CHEK2 1100delC positive Lynch syndrome families additionally carried a pathogenic MLH1 or MSH6 mutation, suggesting that mutations in mismatch repair genes may be involved in CHEK2 1100delC-associated cancer phenotypes. A phenotype of importance is hereditary breast and colorectal cancer (HBCC), with the CHEK2 1100delC mutation present in almost one-fifth of the families-again in a polygenic setting. In order to evaluate the involvement of MSH6 in polygenic CHEK2 cancer susceptibility, we, here, have analyzed the entire MSH6 coding sequence for genetic alterations in 68 HBCC breast cancer families. Rare MSH6 variants, with population frequencies below 1%, were identified in 11.8% of HBCC breast cancer families, whereas the same variants were identified in only 1.5% of population controls, suggesting that rare MSH6 variants are associated with HBCC breast cancer (P < or = 0.00001). However, screening of the entire MSH6 coding sequence in 68 non-HBCC breast cancer families showed a similar association (8.8 vs. approximately 1.4% in controls, P < or = 0.001), suggesting that rare MSH6 variants are not confined to HBCC breast cancer. Together, our data suggest that rare MSH6 variants may predispose to familial breast cancer. However, none of the rare MSH6 variants are obviously pathogenic, suggesting that a more subtle disease mechanism may operate in breast carcinogenesis.

Family history, genetic testing, and clinical risk prediction: pooled analysis of CHEK2 1100delC in 1,828 bilateral breast cancers and 7,030 controls.

If breast cancers arise independently in each breast the odds ratio (OR) for bilateral breast cancer for carriers of CHEK2 1100delC should be approximately 5.5, the square of the reported OR for a first primary (OR, 2.34). In the subset of bilateral cases with one or more affected relatives, the predicted carrier OR should be approximately 9. We have tested these predictions in a pooled set of 1,828 cases with 2 primaries and 7,030 controls from 8 studies. The second primary OR for CHEK2 1100delC carriers was 6.43 (95% confidence interval, 4.33-9.56; P < 0.0001), significantly greater than the published estimate for a first primary (P < 0.001) but consistent with its square. The predicted increase in carrier OR with increasing numbers of affected relatives was seen using bilateral cases from the UK (P(trend) = 0.0003) and Finland (P(trend) = 0.37), although not using those from the Netherlands and Russia (P = 0.001 for heterogeneity between countries). Based on a standard genetic model, we predict lifetime risks for CHEK2 1100delC carrier and noncarrier daughters of bilateral breast cancer cases of 37% and 18%, respectively. Our results imply that clinical management of the daughter of a woman with bilateral breast cancer should depend on her CHEK2 1100delC carrier status. This and other moderate penetrance breast cancer susceptibility alleles, together with family history data, will thus identify increasing numbers of women at potentially very high risk. Before such predictions are accepted by clinical geneticists, however, further population-based evidence is needed on the effect of CHEK2 1100delC and other moderate penetrance alleles in women with a family history of breast cancer.

CHEK2 1100delC and male breast cancer in the Netherlands.

Mutations in the breast cancer susceptibility genes BRCA1, BRCA2, and CHEK2 are known risk factors for female breast cancer. Mutations in BRCA1 and BRCA2 also are associated with male breast cancer (MBC). Similarly, it had been suggested in the original CHEK2 identification report that the CHEK2 1100delC mutation confers an increased risk for MBC. Here, we have evaluated the risk of CHEK2 1100delC for MBC by genotyping CHEK2 1100delC in 23 familial and 71 unselected Dutch MBC cases. None of the 23 familial MBC cases carried the CHEK2 1100delC mutation. In contrast, CHEK2 1100delC was present in 3 of the 71 (4.2%) unselected MBC cases, which was significantly more prevalent than the 1.1% Dutch population frequency assessed in 1,692 individuals (P = 0.05, OR = 4.1, 95% CI 1.2-14.3). Our data suggest that, in the Netherlands, CHEK2 1100delC is associated with an increased risk for MBC.

Deleterious CHEK2 1100delC and L303X mutants identified among 38 human breast cancer cell lines.

The CHEK2 protein plays a major role in the regulation of DNA damage response pathways. Mutations in the CHEK2 gene, in particular 1100delC, have been associated with increased cancer risks, but the precise function of CHEK2 mutations in carcinogenesis is not known. Human cancer cell lines with CHEK2 mutations are therefore of main interest. Here, we have sequenced 38 breast cancer cell lines for mutations in the CHEK2 gene and identified two cell lines with deleterious CHEK2 mutations. Cell line UACC812 has a nonsense truncating mutation in the CHEK2 kinase domain (L303X) and cell line SUM102PT has the well-known oncogenic CHEK2 1100delC founder mutation. Immunohistochemical analysis revealed that the two CHEK2 mutant cell lines expressed neither CHEK2 nor P-Thr(68) CHEK2 proteins, implying abrogation of normal CHEK2 DNA repair functions. Cell lines UACC812 and SUM102PT thus are the first human CHEK2 null cell lines reported and should therefore be a major help in further unraveling the function of CHEK2 mutations in carcinogenesis.

CHEK2 1100delC is a susceptibility allele for HNPCC-related colorectal cancer.

PURPOSE: The pathogenic CHEK2 1100delC variant is firmly established as a breast cancer susceptibility allele. Dutch CHEK2 1100delC breast cancer families frequently also include colorectal cancer cases, and the variant is particularly prevalent among breast cancer families with hereditary breast and colorectal cancer. Yet, it is still unclear whether CHEK2 1100delC also confers a colorectal cancer risk independent of its breast cancer risk. EXPERIMENTAL DESIGN: CHEK2 1100delC was genotyped in the index cases of 369 Dutch colorectal cancer families that had been excluded for familial breast cancer. The cohort included 132 cases with familial adenomatous polyposis (FAP) and FAP-related disease, and 237 cases with hereditary nonpolyposis colorectal cancer (HNPCC) and HNPCC-related disease. RESULTS: None of the FAP/FAP-related cases carried the CHEK2 1100delC variant. In contrast, CHEK2 1100delC was present in 10 of 237 (4.2%) HNPCC/HNPCC-related cases that was significantly more prevalent than the 1.0% Dutch population frequency (odds ratio, 4.3; 95% confidence interval, 1.7-10.7; P = 0.002). Nine of the 10 CHEK2 1100delC colorectal cancer cases met the revised Amsterdam and/or Bethesda criteria. The 10 CHEK2 1100delC colorectal cancer families had a high-risk cancer inheritance pattern, including 35 colorectal cancer cases, 9 cases with polyps, and 21 cases with other tumor types. CONCLUSION: Our analysis provides strong evidence that the 1100delC variant of CHEK2 confers a colorectal cancer risk in HNPCC/HNPCC-related families, supporting the hypothesis that CHEK2 is a multiorgan cancer susceptibility gene.

MDM2 SNP309 accelerates familial breast carcinogenesis independently of estrogen signaling.

A single nucleotide polymorphism (SNP309T>G) in the intronic promoter of MDM2 was recently found to accelerate carcinogenesis in early-onset cancer cases. This cancer acceleration presumably was due to increased SP1 binding, resulting in enhanced MDM2 transcriptional activation by estrogens. We evaluated MDM2 SNP309 in 343 familial breast cancer cases with known mutation status for CHEK2 1100delC, BRCA1 and BRCA2. Cancer acceleration was indeed observed in early-onset familial breast cancer cases (diagnosed

Exon expression arrays as a tool to identify new cancer genes.

BACKGROUND: Identification of genes that are causally implicated in oncogenesis is a major goal in cancer research. An estimated 10-20% of cancer-related gene mutations result in skipping of one or more exons in the encoded transcripts. Here we report on a strategy to screen in a global fashion for such exon-skipping events using PAttern based Correlation (PAC). The PAC algorithm has been used previously to identify differentially expressed splice variants between two predefined subgroups. As genetic changes in cancer are sample specific, we tested the ability of PAC to identify aberrantly expressed exons in single samples. PRINCIPAL FINDINGS: As a proof-of-principle, we tested the PAC strategy on human cancer samples of which the complete coding sequence of eight cancer genes had been screened for mutations. PAC detected all seven exon-skipping mutants among 12 cancer cell lines. PAC also identified exon-skipping mutants in clinical cancer specimens although detection was compromised due to heterogeneous (wild-type) transcript expression. PAC reduced the number of candidate genes/exons for subsequent mutational analysis by two to three orders of magnitude and had a substantial true positive rate. Importantly, of 112 randomly selected outlier exons, sequence analysis identified two novel exon skipping events, two novel base changes and 21 previously reported base changes (SNPs). CONCLUSIONS: The ability of PAC to enrich for mutated transcripts and to identify known and novel genetic changes confirms its suitability as a strategy to identify candidate cancer genes.

A multicenter study of cancer incidence in CHEK2 1100delC mutation carriers.

The CHEK2 1100delC protein-truncating mutation has a carrier frequency of approximately 0.7% in Northern and Western European populations and confers an approximately 2-fold increased risk of breast cancer. It has also been suggested to increase risks of colorectal and prostate cancer, but its involvement with these or other types of cancer has not been confirmed. The incidence of cancer other than breast cancer in 11,116 individuals from 734 non-BRCA1/2 breast cancer families from the United Kingdom, Germany, Netherlands, and the United States was compared with that predicted by population rates. Relative risks (RR) to carriers and noncarriers were estimated by maximum likelihood, via the expectation-maximization algorithm to allow for unknown genotypes. Sixty-seven families contained at least one tested CHEK2 1100delC mutation carrier. There was evidence of underreporting of cancers in male relatives (422 cancers observed, 860 expected) but not in females (322 observed, 335 expected); hence, we focused on cancer risks in female carriers. The risk of cancers other than breast cancer in female carriers was not significantly elevated, although a modest increase in risk could not be excluded (RR, 1.18; 95% confidence interval, 0.64-2.17). The carrier risk was not significantly raised for any individual cancer site, including colorectal cancer (RR, 1.60; 95% confidence interval, 0.54-4.71). However, between ages 20 to 50 years, the risks of colorectal and lung cancer were both higher in female carriers than noncarriers (P = 0.041 and 0.0001, respectively). There was no evidence of a higher prostate cancer risk in carriers than noncarriers (P = 0.26), although underreporting of male cancers limited our power to detect such a difference. Our results suggest that the risk of cancer associated with CHEK2 1100delC mutations is restricted to breast cancer, although we cannot rule out a small increase in overall cancer risk.

Thirteen new p53 gene mutants identified among 41 human breast cancer cell lines.

The p53 tumor suppressor gene is frequently mutated in breast cancer. Here, we used direct sequencing to screen the complete coding sequence of the p53 gene from 41 human breast cancer cell lines. We identified 32 cell lines (78%) with a p53 gene alteration that predicted a change in the encoded protein. Thirty-one of these mutations were accompanied by loss of the other p53 allele. All mutations but one were unique and 27 mutations had previously been identified in uncultured human cancers. Ten mutations were predicted to encode a truncated p53 protein and 22 missense mutations were identified. p53 transcript expression was analyzed by semi-quantitative RT-PCR and p53 protein expression was determined by Western blotting. Our analyses revealed three p53 expression patterns: wild-type p53 cell lines had normal transcript levels and low or no detectable protein expression; cell lines with a p53 truncating mutation had low transcript levels and low or no detectable protein expression; and cell lines with a p53 missense mutation had highly variable transcript and protein expression levels. As a whole, our data represent a p53 mutation profile in breast cancer cell lines, providing a model for structural, functional and pharmacological studies on p53 in human cancer.

BRCA1 mutation analysis of 41 human breast cancer cell lines reveals three new deleterious mutants.

Germ line mutations of the BRCA1 gene confer a high risk of breast cancer and ovarian cancer to female mutation carriers. The BRCA1 protein is involved in the regulation of DNA repair. How specific tumor-associated mutations affect the molecular function of BRCA1, however, awaits further elucidation. Cell lines that harbor BRCA1 gene mutations are invaluable tools for such functional studies. Up to now, the HCC1937 cell line was the only human breast cancer cell line with an identified BRCA1 mutation. In this study, we identified three other BRCA1 mutants from among 41 human breast cancer cell lines by sequencing of the complete coding sequence of BRCA1. Cell line MDA-MB-436 had the 5396 + 1G>A mutation in the splice donor site of exon 20. Cell line SUM149PT carried the 2288delT mutation and SUM1315MO2 carried the 185delAG mutation. All three mutations were accompanied by loss of the other BRCA1 allele. The 185delAG and 5396 + 1G>A mutations are both classified as pathogenic mutations. In contrast with wild-type cell lines, none of the BRCA1 mutants expressed nuclear BRCA1 proteins as detected with Ab-1 and Ab-2 anti-BRCA1 monoclonal antibodies. These three new human BRCA1 mutant cell lines thus seem to be representative breast cancer models that could aid in further unraveling of the function of BRCA1.

Are ATM mutations 7271T-->G and IVS10-6T-->G really high-risk breast cancer-susceptibility alleles?

Two mutations of the ATM gene were recently suggested to confer breast cancer risks similar to mutations of BRCA1 or BRCA2. Here, we set out to confirm these findings in 961 families with non-BRCA1/BRCA2 breast cancer from diverse geographical regions. We did not detect the ATM 7271T-->G mutation in any family. The ATM IVS10-6T-->G mutation was detected in eight families, which was similar to its frequency among population-matched control individuals (pooled Mantel-Haenszel odds ratio = 1.60; 95% confidence interval = 0.48 to 5.35; P = 0.44). Bayesian analysis of linkage in the ATM IVS10-6T-->G-positive families showed an overall posterior probability of causality for this mutation of 0.008. We conclude that the ATM IVS10-6T-->G mutation does not confer a significantly elevated breast cancer risk and that ATM 7271T-->G is a rare event in familial breast cancer.

The CHEK2 1100delC mutation identifies families with a hereditary breast and colorectal cancer phenotype.

Because of genetic heterogeneity, the identification of breast cancer-susceptibility genes has proven to be exceedingly difficult. Here, we define a new subset of families with breast cancer characterized by the presence of colorectal cancer cases. The 1100delC variant of the cell cycle checkpoint kinase CHEK2 gene was present in 18% of 55 families with hereditary breast and colorectal cancer (HBCC) as compared with 4% of 380 families with non-HBCC (P<.001), thus providing genetic evidence for the HBCC phenotype. The CHEK2 1100delC mutation was, however, not the major predisposing factor for the HBCC phenotype but appeared to act in synergy with another, as-yet-unknown susceptibility gene(s). The unequivocal definition of the HBCC phenotype opens new avenues to search for this putative HBCC-susceptibility gene.

Variants in CHEK2 other than 1100delC do not make a major contribution to breast cancer susceptibility.

We recently reported that a sequence variant in the cell-cycle-checkpoint kinase CHEK2 (CHEK2 1100delC) is a low-penetrance breast cancer-susceptibility allele in noncarriers of BRCA1 or BRCA2 mutations. To investigate whether other CHEK2 variants confer susceptibility to breast cancer, we screened the full CHEK2 coding sequence in BRCA1/2-negative breast cancer cases from 89 pedigrees with three or more cases of breast cancer. We identified one novel germline variant, R117G, in two separate families. To evaluate the possible association of R117G and two germline variants reported elsewhere, R145W and I157T with breast cancer, we screened 737 BRCA1/2-negative familial breast cancer cases from 605 families, 459 BRCA1/2-positive cases from 335 families, and 723 controls from the United Kingdom, the Netherlands, and North America. All three variants were rare in all groups, and none occurred at significantly elevated frequency in familial breast cancer cases compared with controls. These results indicate that 1100delC may be the only CHEK2 allele that makes an appreciable contribution to breast cancer susceptibility.

Low-penetrance susceptibility to breast cancer due to CHEK2(*)1100delC in noncarriers of BRCA1 or BRCA2 mutations.

Mutations in BRCA1 and BRCA2 confer a high risk of breast and ovarian cancer, but account for only a small fraction of breast cancer susceptibility. To find additional genes conferring susceptibility to breast cancer, we analyzed CHEK2 (also known as CHK2), which encodes a cell-cycle checkpoint kinase that is implicated in DNA repair processes involving BRCA1 and p53 (refs 3,4,5). We show that CHEK2(*)1100delC, a truncating variant that abrogates the kinase activity, has a frequency of 1.1% in healthy individuals. However, this variant is present in 5.1% of individuals with breast cancer from 718 families that do not carry mutations in BRCA1 or BRCA2 (P = 0.00000003), including 13.5% of individuals from families with male breast cancer (P = 0.00015). We estimate that the CHEK2(*)1100delC variant results in an approximately twofold increase of breast cancer risk in women and a tenfold increase of risk in men. By contrast, the variant confers no increased cancer risk in carriers of BRCA1 or BRCA2 mutations. This suggests that the biological mechanisms underlying the elevated risk of breast cancer in CHEK2 mutation carriers are already subverted in carriers of BRCA1 or BRCA2 mutations, which is consistent with participation of the encoded proteins in the same pathway.