Risk of breast cancer in ataxia-telangiectasia

Polymorphisms in DNA repair genes, medical exposure to ionizing radiation, and breast cancer risk

An epidemiologic study was conducted to determine whether polymorphisms in DNA repair genes modify the association between breast cancer risk and exposure to ionizing radiation. Self-reported exposure to ionizing radiation from medical sources was evaluated as part of a population-based, case-control study of breast cancer in African-American (894 cases and 788 controls) and White (1,417 cases and 1,234 controls) women. Genotyping was conducted for polymorphisms in four genes involved in repair of radiation-induced DNA damage, the double-strand break repair pathway: X-ray cross-complementing group 3 (XRCC3) codon 241 Thr/Met, Nijmegen breakage syndrome 1 (NBS1) codon 185 Glu/Gln, X-ray cross-complementing group 2 (XRCC2) codon 188 Arg/His, and breast cancer susceptibility gene 2 (BRCH2) codon 372 Asn/His. Allele and genotype frequencies were not significantly different in cases compared with controls for all four genetic polymorphisms, and odds ratios for breast cancer were close to the null. Combining women with two, three, and four variant genotypes, a positive association was observed between breast cancer and number of lifetime mammograms (P(trend) < 0.0001). No association was observed among women with zero or one variant genotype (P = 0.86). Odds ratios for radiation treatments to the chest and number of lifetime chest X-rays were slightly elevated but not statistically significant among women with two to four variant genotypes. The study has several limitations, including inability to distinguish between diagnostic and screening mammograms or reliably classify prediagnostic mammograms and chest X-rays in cases. Prospective studies are needed to address whether common polymorphisms in DNA repair genes modify the effects of low-dose radiation exposure from medical sources.

Excess risk for contralateral breast cancer in CHEK2*1100delC germline mutation carriers

We detected a significant excess risk for CHEK2*1100delC mutation carriers to develop a contralateral breast tumor, OR = 6.5 (95% CI 1.5-28.8, p = 0.005). The highest percentage of mutation carriers was detected among those bilateral breast cancer patients who had received radiation treatment for their first breast tumor. These results warrant prolonged medical surveillance and may indicate a clinically important interaction between CHEK2 heterozygosity and radiation in the development of contralateral breast cancer.

Dangers and unreliability of mammography: breast examination is a safe, effective, and practical alternative

Mammography screening is a profit-driven technology posing risks compounded by unreliability. In striking contrast, annual clinical breast examination (CBE) by a trained health professional, together with monthly breast self-examination (BSE), is safe, at least as effective, and low in cost. International programs for training nurses how to perform CBE and teach BSE are critical and overdue.

Radiation induces genomic instability and mammary ductal dysplasia in Atm heterozygous mice

Ataxia-telangiectasia (AT) is a genetic syndrome resulting from the inheritance of two defective copies of the ATM gene that includes among its stigmata radiosensitivity and cancer susceptibility. Epidemiological studies have demonstrated that although women with a single defective copy of ATM (AT heterozygotes) appear clinically normal, they may never the less have an increased relative risk of developing breast cancer. Whether they are at increased risk for radiation-induced breast cancer from medical exposures to ionizing radiation is unknown. We have used a murine model of AT to investigate the effect of a single defective Atm allele, the murine homologue of ATM, on the susceptibility of mammary epithelial cells to radiation-induced transformation. Here we report that mammary epithelial cells from irradiated mice with one copy of Atm truncated in the PI-3 kinase domain were susceptible to radiation-induced genomic instability and generated a 10% incidence of dysplastic mammary ducts when transplanted into syngenic recipients, whereas cells from Atm(+/+) mice were stable and formed only normal ducts. Since radiation-induced ductal dysplasia is a precursor to mammary cancer, the results indicate that AT heterozygosity increases susceptibility to radiogenic breast cancer in this murine model system.

Ataxia-telangiectasia, cancer and the pathobiology of the ATM gene

Ataxia-telangiectasia (A-T) is a pleiotropic inherited disease characterized by neurodegeneration, cancer, immunodeficiencies, radiation sensitivity, and genetic instability. Although A-T homozygotes are rare, the A-T gene may play a role in sporadic breast cancer and leukemia. ATM, the gene responsible for A-T, is homologous to several cell cycle checkpoint genes from other organisms. ATM is thought to play a crucial role in a signal transduction network that modulates cell cycle checkpoints, genetic recombination, apoptosis, and other cellular responses to DNA damage. New insights into the pathobiology of A-T have been provided by the creation of Atm-/- mice and by in vitro studies of ATM function. Analyses of ATM mutations in A-T patients and in sporadic tumors suggest the existence of two classes of ATM mutation: null mutations that lead to A-T and dominant negative missense mutations that may predispose to cancer in the heterozygous state.

Risk of breast cancer and other cancers in heterozygotes for ataxia-telangiectasia

Mortality from cancer among 178 parents and 236 grandparents of 95 British patients with ataxia-telangiectasia was examined. For neither parents nor grandparents was mortality from all causes or from cancer appreciably elevated over that of the national population. Among mothers, three deaths from breast cancer gave rise to a standardized mortality ratio of 3.37 (95% confidence interval (CI): 0.69-9.84). In contrast, there was no excess of breast cancer in grandmothers, the standardized mortality ratio being 0.89 (95% CI: 0.18-2.59), based on three deaths. This is the largest study of families of ataxia-telangiectasia patients conducted in Britain but, nonetheless, the study is small and CIs are wide. However, taken together with data from other countries, an increased risk of breast cancer among female heterozygotes is still apparent, though lower than previously thought.

Study design of randomized controlled clinical trials of breast cancer screening

Evaluation of population screening must be based on a randomized clinical trial (RCT) with the study population randomized into two arms: an intervention group invited to screening and a control group not invited to screening. Reduced mortality in the intervention group is evidence of a benefit from screening. Individual randomization is the ideal, but cluster randomization is often used for logistical and ethical reasons. The use of volunteer subjects is methodologically acceptable, but results cannot be generalized. Seven RCTs of breast cancer screening by mammography have been carried out in the United States, Canada, Sweden, and Scotland. All the studies, except the Canadian, were designed to assess the effect of screening across a wide range of ages at entry. The question of the efficacy of breast cancer screening at younger ages (< 50 years) arose early, after the first results were reported. To address this question, basic elements of the screening protocol must be considered when interpreting the results; these are screening modality (e.g., mammography with or without physical examinations), interscreening interval, and number of screening rounds. This article examines the possible influence of these factors and reviews the design choices and the characteristics of the seven RCTs.

Radiation risk from screening mammography of women aged 40-49 years

Although direct evidence of carcinogenic risk from mammography is lacking, there is a hypothetical risk from screening because excess breast cancers have been demonstrated in women receiving doses of 0.25-20 Gy. These high-level exposures to the breast occurred from the 1930s to the 1950s due to atomic bomb radiation, multiple chest fluoroscopies, and radiation therapy treatments for benign disease. Using a risk estimate provided by the Biological Effects of Ionizing Radiation (BEIR) V Report of the National Academy of Sciences and a mean breast glandular dose of 4 mGy from a two-view per breast bilateral mammogram, one can estimate that annual mammography of 100,000 women for 10 consecutive years beginning at age 40 will result in at most eight breast cancer deaths during their lifetime. On the other hand, researchers have shown a 24% mortality reduction from biennial screening of women in this age group; this will result in a benefit-to-risk ratio of 48.5 lives saved per life lost and 121.3 years of life saved per year of life lost. An assumed mortality reduction of 36% from annual screening would result in 36.5 lives saved per life lost and 91.3 years of life saved per year of life lost. Thus, the theoretical radiation risk from screening mammography is extremely small compared with the established benefit from this life-saving procedure and should not unduly distract women under age 50 who are considering screening.

Efficacy and feasibility of breast shielding during abdominal fluoroscopic examinations

RATIONALE AND OBJECTIVES

The purpose was to measure radiation exposure to the breasts during abdominal fluoroscopic examinations and evaluate the efficacy of breast shielding with a leaded vest.

MATERIALS AND METHODS

Sixty-six women underwent routine abdominal fluoroscopic examinations. During the examinations one breast was covered with a leaded shield. Radiation doses to both breasts were measured with a thermoluminescent dosimeter. The amount of radiation at the skin of the shielded breast was then compared with that at the skin of the nonshielded breast.

RESULTS

Radiation exposure to the breasts varied substantially with the type of examination being performed and with the individual patient. The average radiation level at the skin of the unshielded breast was 119 mR (range, 0-6,320 mR), compared with 59.6 mR (range, 0-1,640 mR) at the shielded breast. The average reduction in radiation exposure was 50% with shielding.

CONCLUSION

Although the average level of radiation exposure to the breast during abdominal fluoroscopic examinations is generally low, use of a leaded vest can further reduce radiation to the breast for different types of examinations.

The genetics of familial breast cancer and their practical implications

A small proportion of breast cancer (perhaps about 5%) and a higher proportion of early onset cases are due to the inheritance of mutations in dominant susceptibility genes which confer a high lifetime risk of the disease. This would equate to about 1250 cases per year in the U.K. and 9000 in the U.S.A. Even within these cases, there is genetic heterogeneity, i.e. there are several genes involved, each giving rise to different patterns of other cancers associated with the familial breast cancer. One such gene (p53) has been identified and a second (BRCA1) has been precisely mapped in the human genome, but further breast cancer predisposition genes remain to be identified. In addition, there are other genes which confer a lower risk of the disease, but may account for a larger proportion of cases, the most important example to date being ataxia telangiectasia. The identification of these genes will enable the entity of familial breast cancer to be more precisely defined and has implications for management of gene carriers with breast cancer and their relatives who are at risk. A major consideration in this new area of cancer genetics is that the identification of gene carriers may become possible on a large scale and this raises ethical and social issues.