Role of the ataxia-telangiectasia gene (ATM) in breast cancer. A-T heterozygotes seem to have an increased risk but its size is unknown

Expanded carrier screening: A current perspective

Prenatal carrier screening has expanded to include a large number of genes offered to all couples considering pregnancy or with an ongoing pregnancy. Expanded carrier screening refers to identification of carriers of single-gene disorders outside of traditional screening guidelines. Expanded carrier screening panels include numerous autosomal recessive and X-linked genetic conditions, including those with a very low carrier frequency, as well as those with mild or incompletely penetrant phenotype. Therefore, the clinical utility of these panels is still subject of debate. Priority should be given to carrier screening panels that include a comprehensive set of severe childhood-onset disorders. Psychosocial support and genetic couseling should be available prior to screening and for the return of positive results. Systems are needed to reduce the risk of misinterpreting results. Finally, attention should be paid on the impact of expanded carrier screening on health care organizations and burden of cost.

Utility of DNA, RNA, Protein, and Functional Approaches to Solve Cryptic Immunodeficiencies

PURPOSE

We report a female infant identified by newborn screening for severe combined immunodeficiencies (NBS SCID) with T cell lymphopenia (TCL). The patient had persistently elevated alpha-fetoprotein (AFP) with IgA deficiency, and elevated IgM. Gene sequencing for a SCID panel was uninformative. We sought to determine the cause of the immunodeficiency in this infant.

METHODS

We performed whole-exome sequencing (WES) on the patient and parents to identify a genetic diagnosis. Based on the WES result, we developed a novel flow cytometric panel for rapid assessment of DNA repair defects using blood samples. We also performed whole transcriptome sequencing (WTS) on fibroblast RNA from the patient and father for abnormal transcript analysis.

RESULTS

WES revealed a pathogenic paternally inherited indel in ATM. We used the flow panel to assess several proteins in the DNA repair pathway in lymphocyte subsets. The patient had absent phosphorylation of ATM, resulting in absent or aberrant phosphorylation of downstream proteins, including γH2AX. However, ataxia-telangiectasia (AT) is an autosomal recessive condition, and the abnormal functional data did not correspond with a single ATM variant. WTS revealed in-frame reciprocal fusion transcripts involving ATM and SLC35F2 indicating a chromosome 11 inversion within 11q22.3, of maternal origin. Inversion breakpoints were identified within ATM intron 16 and SLC35F2 intron 7.

CONCLUSIONS

We identified a novel ATM-breaking chromosome 11 inversion in trans with a pathogenic indel (compound heterozygote) resulting in non-functional ATM protein, consistent with a diagnosis of AT. Utilization of several molecular and functional assays allowed successful resolution of this case.

Variation of ATM Gene Expression in Peripheral Blood Cells of Sporadic Breast Carcinomas in Iranian Patients

The ataxia telangiectasia mutated gene (ATM), candidate for breast cancer susceptibility gene, encode a 350-kDa protein belongs to the core components of DNA-damage response machinery. Female AT carriers have at least 5-fold increase risk for breast cancer. Reduction in ATM expression is shown in multiple studies in breast tissues. We aimed to perform a research to measure the ATM mRNA expression in peripheral blood cells in breast cancer patients. Peripheral blood sample from 40 newly diagnosed, histologically confirmed female breast cancer patients was collected before surgery. Total RNA was isolated from blood cells using the RNX-Plus solution and reverse transcribed into cDNA. Real-time PCR was performed using the 2(-ΔΔCT) method to calculate relative changes in gene expression by REST software. The Relative Quantitation (RQ) mean was 1.27 with the min. and max. equal to 0.20 and 3.34, respectively. Calculation of patient frequencies in different groups revealed that 17.5% had reduced expression lower than two fold decreases and 15% high expression more than two fold increases, but according to REST software there was no up-regulation or down-regulation compared to normal females. The findings of multiple studies consistent with this study indicate that the ATM gene may play an important role in breast cancer development and progression, and ATM expression is down-regulated in breast cancer tissues. Although, some of the results do not support a suppressor role for ATM in the development of sporadic breast cancer, 17.5% of our patients had under expression of ATM mRNA less than two fold relative to control.

Genome Instability in Ataxia Telangiectasia (A-T) Families: Camptothecin-Induced Damage to Replicating DNA Discriminates between Obligate A-T Heterozygotes, A-T Homozygotes and Controls

Previously we used the topoisomerase I inhibitor camptothecin (CPT), which kills mainly S-phase cells primarily by inducing double strand breaks (DSBs) in replication forks, to show that ataxia telangiectasia (A-T) fibroblasts are defective in the repair of this particular subclass of DSBs. CPT treated A-T cells reaching G2 have abnormally high levels of chromatid exchanges, viewed as prematurely condensed G2 chromosomes (G2 PCC), compared with normal cells where aberrations are mostly chromatid breaks. Here we show that A-T lymphoblastoid cells established from individuals with different mutations in the ATM gene also exhibit increased levels of chromosomal exchanges in response to CPT, indicating that the replication-associated DSBs are misrepaired in all these cells. From family studies we show that the presence of a single mutated allele in obligate A-T heterozygotes leads to intermediate levels of chromosomal exchanges in CPT-treated lymphoblastoid cells, thus providing a functional and sensitive assay to identify these individuals.

Screening women at high risk of breast cancer on the basis of evidence

Geneticists are able to identify the risk of breast cancer. Strategies on offer include prevention, early diagnosis by screening, and prophylactic surgery. This paper analyses the evidence for offering screening. The radiation dose of mammography has been measured, but the risk is not fully known. Mammography screening of women of 40-50 years in the normal population has known effect. Little evidence is available for women under 40 years or for women with genetic susceptibility to breast cancer. Dense parenchymal pattern is associated with high grade cancers, and is both a risk factor and a reason for impaired screening sensitivity. Whether this applies to younger women or women at high risk is speculative. The pathological features of the cancers in gene carriers show differences from those occurring in normal women. This work should be correlated with imaging features. There is no literature to support the use of newer imaging methods in these women. Ultrasound and MRI avoid radiation and may be useful in dense breasts. SestaMIBI and PET scanning are not yet mature enough for screening, and may never have such a role. Any newer modality must be subjected to a formal randomised trial before being offered to screen women at high risk.

Radiation and breast carcinogenesis

With the possible exception of radiation-induced leukemia, more is known about radiation-induced breast cancer than any other malignancy. Fourteen cohort studies have provided quantitative information on the level of risk following a wide range of doses in different populations around the world. Comprehensive studies have been conducted in Canada, Germany, Japan, Sweden and other Nordic countries, the United Kingdom, and the USA [Table I in text]. Key features are the linearity in the dose response (i.e., a straight line adequately fits the observed data), and the effect modification of age at exposure (i.e., risk is inversely related to exposure age and exposures past the menopausal ages appear to carry a very low risk); and the minimal effect of fractionating dose on subsequent risk. A recent combined analysis of almost 78,000 women and 1,500 breast cancer cases from eight cohorts confirmed the downturn in risk at the highest dose levels (related in part to the killing of cells rather than transformation) and that fractionation of dose has little influence on risk, at least on an absolute scale. It is not known whether persons predisposed to cancer are at enhanced risk of radiation-induced breast cancer from low-dose exposures, although this seems unlikely. New data on the effects of high doses following childhood exposures will be forthcoming from long-term studies of the survivors of childhood cancer.

Cancer in patients with ataxia-telangiectasia and in their relatives in the nordic countries

BACKGROUND

Epidemiologic studies of the families of patients with ataxia-telangiectasia (A-T), a recessive genetic neurologic disorder caused by mutation of the ATM gene, suggest that heterozygous carriers of an ATM mutation are at increased risk of cancer. A population-based study of cancer incidence in A-T families with unbiased selection and tracing of relatives would confirm this hypothesis.

METHODS

We conducted a study in the Nordic countries of 1218 blood relatives of 56 A-T patients from 50 families. The relatives were identified from population registries, and the occurrence of cancer was determined from cancer registry files in each country and compared with national incidence rates. All statistical tests were two-sided.

RESULTS

Among the 56 patients with A-T, we observed six cases of cancer (four leukemias and two non-Hodgkin's lymphomas) compared with 0.16 expected, yielding a standardized incidence ratio (SIR) of 37 (95% confidence interval [CI] = 13 to 80). Among the 1218 relatives, 150 cancers were recorded, with 126 expected (SIR = 1.19; 95% CI = 1.01 to 1.40). Invasive breast cancer occurred in 21 female relatives of A-T patients (SIR = 1.54; 95% CI = 0.95 to 2.36), including five of the 50 mothers (all of whom are obligate ATM mutation carriers) (SIR = 7.1; 95% CI = 2.3 to 17). Relatives who were less likely to be carriers of a mutant ATM allele had no increase or only a modest, statistically nonsignificant increase in the risk of breast cancer. There was no evidence of increased risk for cancer at any other site.

CONCLUSIONS

We confirmed the previously recognized high risk of lymphoma and leukemia in A-T patients. Our data are also consistent with an increased risk of breast cancer among blood relatives of A-T patients. The epidemiologic findings suggest, however, that, even if ATM mutations are responsible for some breast cancer cases, ATM is a relatively weak genetic risk factor for the disease.

Functional link of BRCA1 and ataxia telangiectasia gene product in DNA damage response

BRCA1 encodes a familial breast cancer suppressor that has a critical role in cellular responses to DNA damage. Mouse cells deficient for Brca1 show genetic instability, defective G2-M checkpoint control and reduced homologous recombination. BRCA1 also directly interacts with proteins of the DNA repair machinery and regulates expression of both the p21 and GADD45 genes. However, it remains unclear how DNA damage signals are transmitted to modulate the repair function of BRCA1. Here we show that the BRCA1-associated protein CtIP becomes hyperphosphorylated and dissociated from BRCA1 upon ionizing radiation. This phosphorylation event requires the protein kinase (ATM) that is mutated in the disease ataxia telangiectasia. ATM phosphorylates CtIP at serine residues 664 and 745, and mutation of these sites to alanine abrogates the dissociation of BRCA1 from CtIP, resulting in persistent repression of BRCA1-dependent induction of GADD45 upon ionizing radiation. We conclude that ATM, by phosphorylating CtIP upon ionizing radiation, may modulate BRCA1-mediated regulation of the DNA damage-response GADD45 gene, thus providing a potential link between ATM deficiency and breast cancer.

Substrate specificities and identification of putative substrates of ATM kinase family members

Ataxia telangiectasia mutated (ATM) phosphorylates p53 protein in response to ionizing radiation, but the complex phenotype of AT cells suggests that it must have other cellular substrates as well. To identify substrates for ATM and the related kinases ATR and DNA-PK, we optimized in vitro kinase assays and developed a rapid peptide screening method to determine general phosphorylation consensus sequences. ATM and ATR require Mn(2+), but not DNA ends or Ku proteins, for optimal in vitro activity while DNA-PKCs requires Mg(2+), DNA ends, and Ku proteins. From p53 peptide mutagenesis analysis, we found that the sequence S/TQ is a minimal essential requirement for all three kinases. In addition, hydrophobic amino acids and negatively charged amino acids immediately NH(2)-terminal to serine or threonine are positive determinants and positively charged amino acids in the region are negative determinants for substrate phosphorylation. We determined a general phosphorylation consensus sequence for ATM and identified putative in vitro targets by using glutathione S-transferase peptides as substrates. Putative ATM in vitro targets include p95/nibrin, Mre11, Brca1, Rad17, PTS, WRN, and ATM (S440) itself. Brca2, phosphatidylinositol 3-kinase, and DNA-5B peptides were phosphorylated specifically by ATR, and DNA Ligase IV is a specific in vitro substrate of DNA-PK.

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.