Variants in DNA double-strand break repair genes and breast cancer susceptibility

We performed genetic association studies in a population-based breast cancer case-control study analysing polymorphisms in genes involved in homologous recombination (NBS1, RAD52, RAD51, XRCC2 and XRCC3) and non-homologous end-joining (KU70/80 and LIG4). These DNA double-strand break repair genes are candidates for breast cancer susceptibility. Genotype results were available for up to 2205 cases and 1826 controls. In the homologous recombination (HR) pathway, genotype frequencies differed between cases and controls for two polymorphisms in XRCC3; T241M (P=0.015) and IVS5 A>G at nt 17893 (P=0.008). Homozygous carriers of M241 were associated with an increased risk [odds ratio (OR) MM versus TT=1.3 (95% confidence interval (CI) 1.1-1.6)], while the rare allele of IVS5A>G was associated with a dominant protective effect [OR AG versus AA=0.8 (0.7-0.9)]. The association of a rare variant in XRCC2 (R188H) was marginally significant [P=0.07; OR HH versus RR=2.6 (1.0-6.7)]. In the non-homologous end-joining (NHEJ) pathway, a polymorphism in LIG4 (T>C at nt 1977) was associated with a decrease in breast cancer risk [P=0.09; OR CC versus TT=0.7 (0.4-1.0)]. No significant association was found for 12 other polymorphisms in the other genes studied. For XRCC3, we found evidence for four common haplotypes and four rarer ones that appear to have arisen by recombination. Two haplotypes, AGC and GGC, were associated with non-significant reductions in breast cancer risk, and the rare GAT haplotype was associated with a significantly increased risk. These data provide some evidence that variants in XRCC2 and LIG4 alter breast cancer risk, together with stronger evidence that variants of XRCC3 are associated with risk. If these results can be confirmed, understanding the functional basis should improve our understanding of the role of DNA repair in breast carcinogenesis.     

Common alleles at 6q25.1 and 1p11.2 are associated with breast cancer risk for BRCA1 and BRCA2 mutation carriers

Two single nucleotide polymorphisms (SNPs) at 6q25.1, near the ESR1 gene, have been implicated in the susceptibility to breast cancer for Asian (rs2046210) and European women (rs9397435). A genome-wide association study in Europeans identified two further breast cancer susceptibility variants: rs11249433 at 1p11.2 and rs999737 in RAD51L1 at 14q24.1. Although previously identified breast cancer susceptibility variants have been shown to be associated with breast cancer risk for BRCA1 and BRCA2 mutation carriers, the involvement of these SNPs to breast cancer susceptibility in mutation carriers is currently unknown. To address this, we genotyped these SNPs in BRCA1 and BRCA2 mutation carriers from 42 studies from the Consortium of Investigators of Modifiers of BRCA1/2. In the analysis of 14 123 BRCA1 and 8053 BRCA2 mutation carriers of European ancestry, the 6q25.1 SNPs (r(2) = 0.14) were independently associated with the risk of breast cancer for BRCA1 mutation carriers [hazard ratio (HR) = 1.17, 95% confidence interval (CI): 1.11-1.23, P-trend = 4.5 × 10(-9) for rs2046210; HR = 1.28, 95% CI: 1.18-1.40, P-trend = 1.3 × 10(-8) for rs9397435], but only rs9397435 was associated with the risk for BRCA2 carriers (HR = 1.14, 95% CI: 1.01-1.28, P-trend = 0.031). SNP rs11249433 (1p11.2) was associated with the risk of breast cancer for BRCA2 mutation carriers (HR = 1.09, 95% CI: 1.02-1.17, P-trend = 0.015), but was not associated with breast cancer risk for BRCA1 mutation carriers (HR = 0.97, 95% CI: 0.92-1.02, P-trend = 0.20). SNP rs999737 (RAD51L1) was not associated with breast cancer risk for either BRCA1 or BRCA2 mutation carriers (P-trend = 0.27 and 0.30, respectively). The identification of SNPs at 6q25.1 associated with breast cancer risk for BRCA1 mutation carriers will lead to a better understanding of the biology of tumour development in these women.     

Polymorphism of the homologous recombination repair genes RAD51 and XRCC3 in breast cancer

The RAD51 protein and its paralog, XRCC3, play an important role in the repair of DNA double-strand breaks (DSBs) by homologous recombination. Since DSBs may contribute to the pathogenesis of breast cancer and variability in DNA repair genes may be linked with some cancers, we performed a case-control study (135 cases and 175 controls) to check the association between the genotypes of the Thr241Met polymorphism of the XRCC3 gene and the 135G>C polymorphism of the RAD51 gene and breast cancer occurrence and progression. Genotypes were determined in peripheral blood lymphocytes by RFLP-PCR. We did not find any association between either polymorphism singly and breast cancer occurrence. Both polymorphisms were not related to tumor size, estrogen and progesterone receptors status, cancer type and grade. However, the Thr241Met genotype of the XRCC3 polymorphism slightly increased the risk of local metastasis in breast cancer patients (OR 2.56, 95% CI 1.27-5.17). The combined Thr241Met/135G>C genotype decreased the risk of breast cancer occurrence (OR 0.22, 95% CI 0.08-0.59). Our results suggest that the variability of the DNA homologous recombination repair genes RAD51 and XRCC3 may play a role in breast cancer occurrence and progression, but this role may be underlined by a mutual interaction between these genes.     

Cancer in the contralateral breast after radiotherapy for breast cancer.

BACKGROUND. Patients with breast cancer have a threefold increase in the risk that a second breast cancer will develop. Radiation treatment for the initial cancer can result in moderately high doses to the contralateral breast, possibly contributing to this heightened risk. METHODS. We conducted a case-control study in a cohort of 41,109 women diagnosed with breast cancer between 1935 and 1982 in Connecticut. We reviewed the medical records of 655 women in whom a second breast cancer developed five or more years after the initial tumor and compared their radiation exposure with that of 1189 matched controls from the cohort who did not have a second cancer. The dose of radiation to the contralateral breast was estimated from the original radiotherapy records. Among the exposed women, the average radiation dose to the contralateral breast was 2.82 Gy (maximum, 7.10). RESULTS. Overall, 23 percent of the women who had a second breast cancer and 20 percent of the controls had received radiotherapy (relative risk of a second breast cancer associated with radiotherapy, 1.19). Among women who survived for at least 10 years, radiation treatment was associated with a small but marginally significant elevation in the risk of a second breast cancer (relative risk, 1.33); the risk increased significantly with the dose of radiation. An increase in risk in association with radiotherapy was evident only among women who were under 45 years of age when they were treated (relative risk, 1.59) and not among older women (relative risk, 1.01). CONCLUSIONS. Radiotherapy for breast cancer contributes little to the already high risk of a second cancer in the opposite breast. Fewer than 3 percent of all second breast cancers in this study could be attributed to previous radiation treatment; the risk, however, was significantly increased among women who underwent irradiation at a relatively young age (less than 45 years). Radiation exposure after the age of 45 entails little, if any, risk of radiation-induced breast cancer.     

Common variation in BRCA2 and breast cancer risk: a haplotype-based analysis in the Multiethnic Cohort

It is well established that rare mutations in BRCA2 predispose to familial breast cancer, but whether common variants at this locus contribute more modest risk to sporadic breast cancer has not been thoroughly investigated. We performed a haplotype-based study of BRCA2 among women in the Multiethnic Cohort Study (MEC), genotyping 50 SNPs spanning 109.4 kb of the BRCA2 gene. Twenty-one haplotype-tagging SNPs (including seven missense SNPs) were selected to predict the common BRCA2 haplotypes and were genotyped in a breast cancer case-control study nested in the MEC (cases, n=1715; controls, n=2502). Compared to non-carriers, we observed nominally significant positive associations for homozygous carriers of specific haplotypes in blocks 2 (haplotype 2c: OR=1.50; 95% CI, 1.08-2.09) and 3 (haplotype 3d: OR=1.50; 95% CI, 1.01-2.24). These results could be explained on the basis of a single marker in intron 24 (SNP 42: rs206340) that was correlated with these haplotypes and the homozygous state was associated with a significantly increased risk of breast cancer (AA versus GG genotypes: OR=1.59, 95% CI, 1.18-2.16; nominal P=0.005). This association was modestly stronger among women with advanced disease (OR=2.00, 95% CI, 1.30-3.08; P=0.002). In this exploratory analysis, we found little indication that common variation in BRCA2 dramatically impacts sporadic breast cancer risk. However, a significant elevation in risk was observed among approximately 6% of women who carried a specific haplotype pattern and may harbor a susceptibility allele at the BRCA2 locus.     

Rare germline mutations in PALB2 and breast cancer risk: a population-based study

Germline mutations in the PALB2 gene are associated with an increased risk of developing breast cancer but little is known about the frequencies of rare variants in PALB2 and the nature of the variants that influence risk. We selected participants recruited to the Women's Environment, Cancer, and Radiation Epidemiology (WECARE) Study and screened lymphocyte DNA from cases with contralateral breast cancer (n = 559) and matched controls with unilateral breast cancer (n = 565) for PALB2 mutations. Five pathogenic PALB2 mutations were identified among the cases (0.9%) versus none among the controls (P = 0.04). The first-degree female relatives of these five carriers demonstrated significantly higher incidence of breast cancer than relatives of noncarrier cases, indicating that pathogenic PALB2 mutations confer an estimated 5.3-fold increase in risk (95% CI: 1.8-13.2). The frequency of rare (<1% MAF) missense mutations was similar in both groups (23 vs. 21). Our findings confirm in a population-based study setting of women with breast cancer the strong risk associated with truncating mutations in PALB2 that has been reported in family studies. Conversely, there is no evidence from this study that rare PALB2 missense mutations strongly influence breast cancer risk.     

Characterization of BRCA1 and BRCA2 deleterious mutations and variants of unknown clinical significance in unilateral and bilateral breast cancer: the WECARE study

BRCA1 and BRCA2 screening in women at high‐risk of breast cancer results in the identification of both unambiguously defined deleterious mutations and sequence variants of unknown clinical significance (VUS). We examined a population‐based sample of young women with contralateral breast cancer (CBC, n=705) or unilateral breast cancer (UBC, n=1398). We identified 470 unique sequence variants, of which 113 were deleterious mutations. The remaining 357 VUS comprised 185 unique missense changes, 60% were observed only once, while 3% occurred with a frequency of >10%. Deleterious mutations occurred three times more often in women with CBC (15.3%) than in women with UBC (5.2%), whereas combined, VUS were observed in similar frequencies in women with CBC and UBC. A protein alignment algorithm defined 16 rare VUS, occurring at highly conserved residues and/or conferring a considerable biochemical difference, the majority located in the BRCA2 DNA‐binding domain. We confirm a multiplicity of BRCA1 and BRCA2 VUS that occur at a wide range of allele frequencies. Although some VUS inflict chemical differences at conserved residues, suggesting a deleterious effect, the majority are not associated with an increased risk of CBC. © 2010 Wiley‐Liss, Inc.     

Prenatal diethylstilbestrol exposure and cancer risk in women

In the Diethylstilbestrol [DES] Combined Cohort Follow-up, the age- and calendar-year specific standardized incidence ratio [SIR] for clear cell adenocarcinoma [CCA] was 27.6 (95% confidence interval [CI] 7.51-70.6) for the exposed women. The SIR for breast cancer was 1.17 (95% CI 1.01-1.36) and the hazard ratio [HR] adjusted for birth year and cohort for comparison with the unexposed was 1.05 (95% CI 0.79-1.41). The SIR for pancreatic cancer was 2.43 (95% CI 1.21-4.34) and the adjusted HR for comparison with unexposed women was 7.16 (95% CI 0.84-61.5). There was little evidence of excess risk for other sites. There appeared to be a deficit in risk for endometrial cancer among the exposed (SIR 0.61; 95% CI 0.35-0.98), and an excess in the unexposed (SIR 1.55; 95% CI 0.95-2.40); the adjusted HR was 0.45 (95% CI 0.22-0.93) for the internal comparison. There was no overall excess cancer risk in exposed women compared with general population rates (1.06; 95% CI 0.95-1.17) or with unexposed participants (adjusted HR 1.03; 95% CI 0.84-1.25). These data do not support the suggestion that there is a diathesis of cancers in DES exposed female offspring The excess risk of breast and pancreatic cancers that we observed is concerning and warrants continued follow-up and mechanistic investigation. Environ. Mol. Mutagen. 60:395–403, 2019. Published 2017. This article is a US Government work and is in the public domain in the USA.     

MC1R, ASIP, TYR, and TYRP1 gene variants in a population-based series of multiple primary melanomas

Allelic variants of the low-penetrance melanoma gene MC1R increase the risk of both melanoma and non-melanoma skin cancer. Common variants of the genes ASIP, TYR, and TYRP1, which regulate the melanogenic pathway, have also been shown to associate with melanoma. In this population-based study, we investigated SNPs of MC1R, ASIP, TYR, and TYRP1 as risk factors for development of multiple primary melanomas (MPM) in 388 Norwegian cases. The MPM patients had a significantly higher likelihood of carrying any MC1R variant than the control group of 420 blood donors [86.8 vs. 78.3%, OR = 1.73, and confidence intervals (CI) 1.18-2.52]. When MC1R variants were analyzed individually, Asp84Glu and Arg151Cys were significantly more frequent among the MPM cases than among the controls (OR = 5.77, CI 1.97-16.90, and OR = 1.80, CI 1.36-2.37, respectively). In addition, there was an allele dose-dependent increase in MPM risk for carriers of red hair color (RHC) MC1R variants. The AH haplotype of ASIP was also a significant risk factor for MPM development (OR = 1.72 and CI 1.12-2.49), whereas no association was observed for previously reported risk variants of the TYR and TYRP1 genes. In summary, by using a population-based material of high-risk melanoma cases, we demonstrate a significant effect of both MC1R RHC variants and an ASIP haplotype, but could not replicate an association with postulated risk SNPs of TYR and TYRP1.     

The risk of contralateral breast cancer in daughters of women with and without breast cancer

We aimed to estimate the 15‐year and lifetime risks of contralateral breast cancer in breast cancer patients according to the age of diagnosis of the first cancer and the history of breast cancer in the mother. The risks of contralateral breast cancer were estimated for all 78,775 breast cancer patients in the Swedish Family‐Cancer Database (age at diagnosis of first breast cancer <70 years). The risk of experiencing a contralateral breast cancer within 15 years of diagnosis was 8.4% [95% confidence interval (CI): 8.1–8.7%] for women with an unaffected mother, was 12% (95%CI: 11–13%) for a woman with a mother with unilateral breast cancer and was 13% (95%CI: 9.5–17%) for women with a mother with bilateral breast cancer. In early‐onset diagnosed women (<50 years) with an unaffected mother, the risk of contralateral breast cancer until age 80 was 23% (95%CI: 20–26%) and for late‐onset (50–69 years) diagnosed women it was 17% (95%CI: 14–21%). In a woman with a mother with an early‐onset unilateral breast cancer, risk of contralateral breast cancer by age 80 was 35% (95%CI: 25–46%). Women with a mother with early‐onset bilateral breast cancer had 31% (95%CI: 12–67%) lifetime risk of contralateral breast cancer. The risk of contralateral breast cancer is higher for daughters of breast cancer patients than for daughters of women without breast cancer. Maternal cancer history and age at onset of first breast cancer in women should be taken into account when counseling breast cancer patients about their risk of contralateral breast cancer.     

Radiosensitization effects of berberine on human breast cancer cells

Berberine, an isoquinoline derivative alkaloid, has recently been shown to have antitumor activity. The present study aimed to investigate the effects of the concomitant administration of berberine and radiation on breast cancer. The effects of berberine on the radiosensitivity of MCF-7 and MDA-MB-468 cells were evaluated by using cell clonogenic assays. Cells pre-treated with berberine or dimethyl sulfoxide (DMSO) for 24?h were irradiated using a Faxitron Cabinet X-ray System to deliver the indicated doses (0, 1, 2, 3 and 4?Gy). Changes in cell cycle distribution were determined by flow cytometry. γ-H2AX foci were detected by immunofluorescence staining. The levels of Ku70, Ku86 and RAD51 proteins were evaluated by western blot analysis. We observed that berberine increased the MCF-7 and MDA-MB-468 cell radiosensitivity with cell clonogenic assays. the radiation-induced G2/M cell cycle delay was reduced in the MCF-7 cells pre-teated with berberine. Berberine pre-treatment prolonged the persistence of DNA double-strand breaks in the MCF-7 cell line. In comparison with the control cells, the protein levels of RAD51 were decreased in the MCF-7 and MDA-MB-468 cells treated with berberine, and in the cells pre-treated with 15?μM berberine for 24?h, the level of RAD51 protein decreased significantly at the indicated time-points (0, 2, 6 and 24?h) following X-ray exposure. In conclusion, berberine sensitizes human breast cancer cells to ionizing radiation by inducing cell cycle arrest and the downregulation of the homologous recombination repair protein, RAD51. Berberine may be a promising radiosensitizer for the treatment of breast cancer.     

A comprehensive haplotype analysis of CYP19 and breast cancer risk: the Multiethnic Cohort

The CYP19 gene encodes for aromatase (P450arom), a key steroidogenic enzyme that catalyzes the final step of estrogen biosynthesis. Apart from rare mutations in CYP19 which result in severe phenotypes associated with estrogen insufficiency, little is known about whether common variation in CYP19 is associated with risk of hormone-related diseases. In this study, we employed a haplotype-based approach to search for common disease-associated variants in this candidate breast cancer susceptibility gene among African-American, Hawaiian, Japanese, Latina and White women in the Multiethnic Cohort Study (MEC). We utilized 74 densely spaced single-nucleotide polymorphisms (SNPs) (one every approximately 2.6 kb) spanning 189.4 kb of the CYP19 locus to characterize linkage disequilibrium (LD) and haplotype patterns among 69-70 individuals from each ethnic population. We detected four regions of strong LD (blocks 1-4) that were quite closely conserved across populations. Within each block there was a limited diversity of common haplotypes (5 to 10 with a frequency >/=5%) and most haplotypes were observed to be shared across populations. Twenty-five haplotype-tagging SNPs (htSNPs) were selected to predict the common haplotypes with high probability (average Rh2=0.92) and genotyped in a breast cancer case-control study in the MEC (cases, n=1355; controls, n=2580). We first performed global tests for differences in risk according to the common haplotypes and observed significant haplotype-effects in block 2 [P=0.01; haplotypes 2b (OR=1.23; 95% CI, 1.07-1.40), 2d (OR=1.28; 95% CI, 1.01-1.62)]. We also found a common long-range haplotype comprised of block-specific haplotypes 2b and 3c to be associated with increased risk of breast cancer (haplotype 2b-3c: OR=1.31; 95% CI, 1.11-1.54). Our findings suggest the hypothesis that women with the long-range CYP19 haplotype 2b-3c may be carriers of a predisposing breast cancer susceptibility allele.     

Prevalence of germline truncating mutations in ATM in women with a second breast cancer after radiation therapy for a contralateral tumor

Patients treated with conservative surgery and radiation therapy for early-stage breast cancer develop a contralateral breast cancer at a rate of approximately 0.75% per year. Ataxia-telangiectasia (AT) is an autosomal recessive disease that is characterized by increased sensitivity to ionizing radiation (IR) and cancer susceptibility. Epidemiologic studies have suggested that AT carriers, who comprise 1% of the population, may be at an increased risk for developing breast cancer, particularly after exposure to IR. To test this hypothesis, we analyzed blood samples from 57 patients who developed a contralateral breast cancer at least 6 months after completion of radiation therapy for an initial breast tumor. A cDNA-based truncation assay in yeast was used to test for heterozygous mutations in the ATM gene, which is responsible for AT. No mutations were detected. Our findings fail to support the hypothesis that AT carriers account for a significant fraction of breast cancer cases arising in women after exposure to radiation. Genes Chromosomes Cancer 27:124-129, 2000.     

RAD51C is a susceptibility gene for ovarian cancer

A homozygous mutation in the RAD51C gene was recently found to cause Fanconi anemia-like disorder. Furthermore, six heterozygous deleterious RAD51C mutations were detected in German breast and ovarian cancer families. We screened 277 Finnish familial breast or ovarian cancer patients for RAD51C and identified two recurrent deleterious mutations (c.93delG and c.837+1G>A). These mutations were further genotyped in 491 familial breast cancer patients, 409 unselected ovarian cancer patients and two series of unselected breast cancer cases (884 from Helsinki and 686 from Tampere) and population controls (1279 and 807, respectively). The mutation frequency among all breast cancer cases was not different from the controls (4 out of 2239, 0.2% versus population controls 2 out of 2086, 0.1%, P= 0.7). In the Helsinki series, each mutation was found in four cases with personal or family history of ovarian cancer. No mutations were found among cases with familial breast cancer only, four out of the eight carriers did not have family history of breast cancer. The mutations associated with an increased risk of familial breast and ovarian cancer (OR: 13.59, 95% CI 1.89-97.6, P= 0.026 compared with controls), but especially with familial ovarian cancer in the absence of breast cancer (OR: 213, 95% CI 25.6-1769, P= 0.0002) and also with unselected ovarian cancer (OR: 6.31, 95% CI 1.15-34.6, P= 0.033), with a significantly higher mutation rate among the familial cases (two out of eight, 25%) than the unselected ovarian cancer cases (4 out of 409, 1%) (OR: 33.8, 95% CI 5.15-221, P= 0.005). These results suggest RAD51C as the first moderate-to-high risk susceptibility gene for ovarian cancer.     

Effect of EME1 exon variant Ile350Thr on risk and early onset of breast cancer in southern Chinese women

Essential meiotic endonuclease 1 homolog 1 (EME1) is a key DNA repair protein that participates in the recognition and repair of DNA double-strand breaks. Deficiency of the EME1 gene can lead to spontaneous genomic instability and thus contribute to tumorgenesis. We hypothesized that the exon variants of EME1 confer genetic susceptibility to breast cancer. In a case-control study of 748 breast cancer patients and 778 normal controls, we analyzed the association between two exon variants of EME1 (i.e.,Ile350Thr: rs12450550T > C and Glu69Asp: rs3760413T > G) and breast cancer risk. We found that compared to the common Ile/Ile genotype, the Thr variant genotypes (Thr/Ile + Thr/Thr) conferred a 1.47-fold increased risk of breast cancer (OR=1.47, 95% CI=1.13-1.92). The variant Ile350Thr was also associated with early onset of breast cancer (r = -0.116, P = 0.002). The mean age of onset was 44.4 years for Thr/Thr genotype carriers and 46.5 years for Thr/Ile genotype carriers, which was significantly lower than that (49.4 years) for Ile/Ile genotype carriers (P = 0.006). Moreover, no significant association was observed between the Glu69Asp variant and breast cancer risk. Our findings suggest that the EME1 variant Ile350Thr contributes to an increased risk and early onset of breast cancer.     

Human RAD50 deficiency: Confirmation of a distinctive phenotype

DNA double‐strand breaks (DSBs) are highly toxic DNA lesions that can lead to chromosomal instability, loss of genes and cancer. The MRE11/RAD50/NBN (MRN) complex is keystone involved in signaling processes inducing the repair of DSB by, for example, in activating pathways leading to homologous recombination repair and nonhomologous end joining. Additionally, the MRN complex also plays an important role in the maintenance of telomeres and can act as a stabilizer at replication forks. Mutations in NBN and MRE11 are associated with Nijmegen breakage syndrome (NBS) and ataxia telangiectasia (AT)‐like disorder, respectively. So far, only one single patient with biallelic loss of function variants in RAD50 has been reported presenting with features classified as NBS‐like disorder. Here, we report a long‐term follow‐up of an unrelated patient with facial dysmorphisms, microcephaly, skeletal features, and short stature who is homozygous for a novel variant in RAD50. We could show that this variant, c.2524G > A in exon 15 of the RAD50 gene, induces aberrant splicing of RAD50 mRNA mainly leading to premature protein truncation and thereby, most likely, to loss of RAD50 function. Using patient‐derived primary fibroblasts, we could show abnormal radioresistant DNA synthesis confirming pathogenicity of the identified variant. Immunoblotting experiments showed strongly reduced protein levels of RAD50 in the patient‐derived fibroblasts and provided evidence for a markedly reduced radiation‐induced AT‐mutated signaling. Comparison with the previously reported case and with patients presenting with NBS confirms that RAD50 mutations lead to a similar, but distinctive phenotype.     

Roles of Chk2/CHEK2 in guarding against environmentally induced DNA damage and replication-stress

Checkpoint kinase 2 (human CHEK2; murine Chk2) is a critical mediator of the DNA damage response and has established roles in DNA double strand break (DSB)-induced apoptosis and cell cycle arrest. DSBs may be invoked directly by ionizing radiation but may also arise indirectly from environmental exposures such as solar ultraviolet (UV) radiation. The primary forms of DNA damage induced by UV are DNA photolesions (such as cyclobutane pyrimidine dimers CPD and 6-4 photoproducts) which interfere with DNA synthesis and lead to DNA replication fork stalling. Persistently stalled and unresolved DNA replication forks can “collapse” to generate DSBs that induce signaling via Chk2 and its upstream activator the ataxia telangiectasia-mutated (ATM) protein kinase. This review focuses on recently defined roles of Chk2 in protecting against DNA replication-associated genotoxicity. Several DNA damage response factors such as Rad18, Nbs1 and Chk1 suppress stalling and collapse of DNA replication forks. Defects in the primary responders to DNA replication fork stalling lead to generation of DSB and reveal “back-up” roles for Chk2 in S-phase progression and genomic stability. In humans, there are numerous variants of the CHEK2 gene, including CHEK2*1100delC. Individuals with the CHEK2*1100delC germline alteration have an increased risk of developing breast cancer and malignant melanoma. DNA replication fork-stalling at estrogen-DNA adducts and UV-induced photolesions are implicated in the etiology of breast cancer and melanoma, respectively. It is likely therefore that the Chk2/CHEK2-deficiency is associated with elevated risk for tumorigenesis caused by replication-associated genotoxicities that are exacerbated by environmental genotoxins and intrinsic DNA-damaging agents.     

Breast cancer susceptibility: current knowledge and implications for genetic counselling

Breast cancer is the most common malignancy in women in the Western world. Except for the high breast cancer risk in BRCA1 and BRCA2 mutation carriers as well as the risk for breast cancer in certain rare syndromes caused by mutations in TP53, STK11, PTEN, CDH1, NF1 or NBN, familial clustering of breast cancer remains largely unexplained. Despite significant efforts, BRCA3 could not be identified, but several reports have recently been published on genes involved in DNA repair and single nucleotide polymorphisms (SNPs) associated with an increased breast cancer risk. Although candidate gene approaches demonstrated moderately increased breast cancer risks for rare mutations in genes involved in DNA repair (ATM, CHEK2, BRIP1, PALB2 and RAD50), genome-wide association studies identified several SNPs as low-penetrance breast cancer susceptibility polymorphisms within genes as well as in chromosomal loci with no known genes (FGFR2, TOX3, LSP1, MAP3K1, TGFB1, 2q35 and 8q). Some of these low-penetrance breast cancer susceptibility polymorphisms also act as modifier genes in BRCA1/BRCA2 mutation carriers. This review not only outlines the recent key developments and potential clinical benefit for preventive management and therapy but also discusses the current limitations of genetic testing of variants associated with intermediate and low breast cancer risk.     

Breast cancer risk and germline genomic profiling of women with neurofibromatosis type 1 who developed breast cancer

NF1 mutations predispose to neurofibromatosis type 1 (NF1) and women with NF1 have a moderately elevated risk for breast cancer, especially under age 50. Germline genomic analysis may better define the risk so screening and prevention can be applied to the individuals who benefit the most. Survey conducted in several neurofibromatosis clinics in the United States has demonstrated a 17.2% lifetime risk of breast cancer in women affected with NF1. Cumulated risk to age 50 is estimated to be 9.27%. For genomic profiling, fourteen women with NF1 and a history of breast cancer were recruited and underwent whole exome sequencing (WES), targeted genomic DNA based and RNA‐based analysis of the NF1 gene. Deleterious NF1 pathogenic variants were identified in each woman. Frameshift mutations because of deletion/duplication/complex rearrangement were found in 50% (7/14) of the cases, nonsense mutations in 21% (3/14), in‐frame splice mutations in 21% (3/14), and one case of missense mutation (7%, 1/14). No deleterious mutation was found in the following high/moderate‐penetrance breast cancer genes: ATM, BRCA1, BRCA2, BARD1, BRIP1, CDH1, CHEK2, FANCC, MRE11A, NBN, PALB2, PTEN, RAD50, RAD51C, TP53, and STK11. Twenty‐five rare or common variants in cancer related genes were discovered and may have contributed to the breast cancers in these individuals. Breast cancer predisposition modifiers in women with NF1 may involve a great variety of molecular and cellular functions.