DNA damage processing defects and disease

Detection of radiosensitive subpopulations ex-vivo with Raman microspectroscopy

Although significant advances in understanding the molecular drivers of acquired and inherited radiosensitivity have occurred in recent decades, a single analytical method which can detect and classify radiosensitivity remains elusive. Raman microspectroscopy has demonstrated capabilities in the objective classification of various diseases, and more recently in the detection and modelling of radiobiological effect. In this study, Raman spectroscopy is presented as a potential tool for the detection of radiosensitivity subpopulations represented by four lymphoblastoid cell lines derived from individuals with ataxia telangiectasia (2 lines), non-Hodgkins lymphoma, and Turner's syndrome. These are classified with respect to a population with mixed radiosensitivity, represented by lymphocytes drawn from both healthy controls, and prostate cancer patients. Raman spectroscopic measurements were made ex-vivo after exposure to X-ray doses of 0 Gy, 50 mGy and 500 mGy, in parallel to radiation-induced G2 chromosomal radiosensitivity scores, for all samples. Support vector machine models developed on the basis of the spectral data were capable of discrimination of radiosensitive populations before and after irradiation, with superior discrimination when spectra were subjected to a non-linear dimensionality reduction (UMAP) as opposed to a linear (PCA) approach. Models developed on spectral data acquired on samples irradiated in-vitro with a dose of 0Gy were found to provide the highest level of performance in discriminating between classes, with performances of F1 = 0.92 ± 0.06 achieved on a held-out test set. Overall, this study suggests that Raman spectroscopy may have potential as a tool for the detection of intrinsic radiosensitivity using liquid biopsies.

Rate of individuals with clearly increased radiosensitivity rise with age both in healthy individuals and in cancer patients

BACKGROUND

The question of an age dependence of individual radiosensitivity has only marginally been studied so far. Therefore, we analyzed blood samples of healthy individuals and cancer patients of different ages to determine individual radiosensitivity.

METHODS

Ex vivo irradiated blood samples of 595 individuals were tested. Chromosomes 1, 2 and 4 were stained by 3-color fluorescence in situ hybridization and aberrations were analyzed. Radiosensitivity was determined by the mean breaks per metaphase (B/M).

RESULTS

Healthy individuals (mean age 50.7 years) had an average B/M value of 0.42 ± 0.104 and an increase of 0.0014B/M per year. The patients (mean age 60.4 years) had an average B/M value of 0.44 ± 0.150 and radiosensitivity did not change with age. In previous studies we found that from a value of 0.6B/M on an individual is considered to be distinctly radiosensitive. The portion of radiosensitive individuals (B/M > 0.6) increased in both cohorts with age.

CONCLUSION

Individual radiosensitivity rises continuously with age, yet with strong interindividual variation. No age related increase of radiosensitivity can be demonstrated in patients due to the strong interindividual variation. However among old cancer patients there is a higher probability to have patients with clearly increased radiosensitivity than at younger age.

Clinical course and therapeutic implications for lymphoid malignancies in Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS, MIM #251260) is an autosomal recessive chromosomal instability disorder. Majority of patients affected are of Slavic origin and share the same founder mutation of 657del5 within the NBN gene encoding protein involved in DNA double-strand breaks repair. Clinically, this is characterized by a microcephaly, immunodeficiency and a high incidence of pediatric malignancies, mostly lymphomas and leukemias. Anticancer treatment among patients with NBS is challenging because of a high risk of life threatening therapy-related toxicity including severe infections, bone marrow failure, cardio- and nephrotoxicity and occurrence of secondary cancer. Based on systemic review of available literature and the Polish acute lymphoblastic leukemia database we concluded that among patients with NBS, these who suffered from clinically proven severe immunodeficiency are at risk of the complications associated with oncological treatment. Thus, in this group it reasonable to reduce chemotherapy up to 50% especially concerning anthracyclines methotrexate, alkylating agents and epipodophyllotoxines, bleomycin and radiotherapy should be omitted. Moreover, infection prophylaxis using intravenous immunoglobulin supplementation together with antifungal and antibacterial agent is recommended. To replace radiotherapy or some toxic anticancer agents targeted therapy using monoclonal antibodies and kinase inhibitors or bone marrow transplantation with reduced-intensity conditioning should be considered in some cases, however, this statement needs further studies.

Gly322Asp and Asn127Ser single nucleotide polymorphisms (SNPs) of hMSH2 mismatch repair gene and the risk of triple-negative breast cancer in Polish women

Triple-negative breast cancer (TNBC) is characterised by worse clinical outcome and poor prognosis. The alterations in the oncogenes and tumor suppressor genes as well as microsatellite instability (MSI) have been associated with breast cancer development. It is knowledge that the most common mechanism inducing MSI in many cancer is genomic rearrangements found in the hMSH2 (human MutS homolog 2) gene. In this report we genotyped two polymorphisms of hMSH2 DNA repair gene in 70 TNBC patients and 70 age-matched cancer-free women using RFLP–PCR. The following polymorphisms were studied: an A/G transition at 127 positions producing an Asn/Ser substitution at codon 127 (the Asn127Ser polymorphism, rs17217772) and a G/A transition at 1032 position resulting in a Gly/Asp change at codon 322 (the Gly322Asp polymorphism, rs4987188). We found an association between the hMSH2 Asp/Asp and Gly/Asp genotypes and TNBC occurence. Variant Asp allele of hMSH2 decreased cancer risk [odds ratio (OR) 0.11; 95 % confidence interval (CI) 0.05–0.21]. The risk of TNBC in the carriers of the Gly322Gly–Asn127Ser combined genotype was increased (OR 3.71; 95 % CI 1.36–10.10). However the risk of TNBC was not alter by polymorphism Asn127Ser of the hMSH2 gene. The Gly322Asp polymorphism of the hMSH2 gene may be linked with TNBC occurrence in Polish women.

X-ray repair cross-complementing protein 1 and 3 polymorphisms and susceptibility of breast cancer in a Jordanian population

OBJECTIVES

To elucidate the contribution of x-ray repair cross-complementing (XRCC) protein 1 399Gln, XRCC3 241M, and XRCC3-5'-UTR polymorphisms to the susceptibility of breast cancer (BC) in a Jordanian population.

METHODS

Forty-six formalin fixed paraffin embedded tissue samples from BC diagnosed female patients, and 31 samples from the control group were subjected to DNA sequencing. Samples were collected between September 2013 and December 2014.

RESULTS

The XRCC1 Arg399Gln genotype did not exhibit any significant correlation with the susceptibility of BC (odds ratio [OR]=1.45, 95% confidence interval [CI]: 0.60-3.51) (p=0.47). Likewise, XRCC3 M241T genotype did not show significant correlation with BC (OR=2.02, 95% CI: 0.50-8.21) (p=0.40). However, distribution of XRCC3-5'UTR (rs1799794 A/G) genotype showed a significant difference between the patient and control group (OR=0.73, 95% CI: 0.06-8.46) (p=0.02).

CONCLUSION

The XRCC3-5'UTR (rs1799794) G allele frequency was higher in cancer patients while XRCC1 (rs25487) and XRCC3 (rs861539) did not show any significant correlation with susceptibility of BC in the selected Jordanian population. Contribution of other environmental factors should be studied in future works, as well as the response of cancer therapy.

Fanconi Anemia: Overview of the Disease and the Role of Hematopoietic Transplantation

Fanconi anemia (FA) is an inherited bone marrow failure syndrome characterized by congenital abnormalities and chromosomal breakages with the occurrence of hematological and solid malignancies. FA is the most common type of inherited bone marrow failure and poses tremendous challenges. FA patients are uniquely hypersensitive to hematopoietic stem cell transplantation (HSCT) conditioning agents due to the underling chromosomal instability. HSCT has shown important progress in the last years, especially after the introduction of fludarabine and the reduction of cyclophosphamide in the preparative regimen. For patients with HLA-identical-related donors HSCT should be performed as first-line therapy, for patients with alternative donors HSCT remains a therapy with increased morbidity and mortality.

WRN translocation from nucleolus to nucleoplasm is regulated by SIRT1 and required for DNA repair and the development of chemoresistance

When defective or absent, Werner syndrome protein (WRN) causes a genetic premature aging disorder called Werner syndrome. Several studies have reported that defects in WRN function are responsible for not only progeria syndrome but also genomic instability via the deregulation of DNA repair, replication, recombination, and telomere stability. Given the importance of WRN in the repair process, we herein investigated the potential role of WRN in drug response by evaluating the DNA repair following exposure to cisplatin in human cancer cell lines. We found that the down-regulation of SIRT1 and inhibition of SIRT1 deacetylase activity blocked the translocation of WRN from the nucleolus to the nucleoplasm in response to genotoxic stresses. In addition, cells expressing low levels of WRN responded favorably to cisplatin, whereas cells expressing high levels responded poorly to cisplatin. The forced expression of WRN protein in chemosensitive cells resulted in an approximately two-fold increase in cell viability in response to cisplatin compared with vector controls and promoted DNA repair, while WRN-deficient cells accumulate unrepaired double-strand breaks following cisplatin exposure. These results suggest that WRN is regulated by SIRT1 and increased expression of WRN might be one of the determinants for the development of chemotherapeutic drug resistance.

Polymorphisms in DNA repair genes and breast cancer risk in Russian population: a case-control study

Genetic variation in DNA repair genes can alter an individual's capacity to repair damaged DNA and influence the risk of cancer. We tested seven polymorphisms in DNA repair genes XRCC1, ERCC2, XRCC3, XRCC2, EXOI and TP53 for a possible association with breast cancer risk in a sample of 672 case and 672 control Russian women. An association was observed for allele A of the polymorphism XRCC1 (R399Q) rs25487 (co-dominant model AA vs. GG: OR 1.76, P = 0.003; additive model OR 1.28, P = 0.005; dominant model: OR 1.29, P = 0.03; recessive model OR 1.63, P = 0.008). Allele T of the polymorphism ERCC2 (D312N) rs1799793 was also associated with breast cancer risk (co-dominant model TT vs. CC: OR 1.43, P = 0.04; additive model OR 1.21, P = 0.02; dominant model: OR 1.30, P = 0.02), but the association became insignificant after applying Bonferroni correction. No association with breast cancer was found for the remaining SNPs. In summary, our study provides evidence that polymorphisms in DNA repair genes may play a role in susceptibility to breast cancer in the population of ethnical Russians.

DNA repair: the link between primary immunodeficiency and cancer

The adaptive component of the immune system depends greatly on the generation of genetic diversity provided by lymphocyte-specific genomic rearrangements. V(D)J recombination, class switch recombination (CSR), and somatic hypermutation (SHM) constitute complex and vulnerable processes that are orchestrated by a multitude of DNA repair pathways. When inherited defects in certain DNA repair proteins are present, lymphocyte development can be compromised and, consequently, patients can develop primary immunodeficiencies (PIDs). PID patients often have a strong predisposition for cancer development as a result of genomic instability generated from defective DNA repair mechanisms. Tumors of lymphoid origin are one of the most common PID-associated cancers, likely due to DNA lesions resulting from defective V(D)J, CSR, and SHM. In this review, we describe PID syndromes that confer an increased risk for cancer development. Furthermore, we discuss the role of the affected proteins in tumorigenesis/lymphomagenesis.

Chronic myeloid leukemia 2011: successes, challenges, and strategies--proceedings of the 5th annual BCR-ABL1 positive and BCR-ABL1 negative myeloproliferative neoplasms workshop

This report is based on the presentations and discussions at the 5th annual BCR-ABL1 positive and BCR-ABL1 negative myeloproliferative neoplasms (MPN) workshop, which took place immediately following the 52nd American Society of Hematology (ASH) meeting in Orlando, Florida on December 7th-8th, 2011. Relevant data which was presented at the ASH meeting as well as all other recent publications were presented and discussed at the workshop. This report covers front-line therapies of BCR-ABL1-positive leukemias, in addition to addressing some topical biological, pre-clinical and clinical issues, such as new insights into genomic instability and resistance to tyrosine kinase inhibitors (TKIs), risk stratification and optimizing molecular monitoring. A report pertaining to the new therapies and other pertinent preclinical and clinical issues in the BCR-ABL1 negative MPNs is published separately.

Chronic myeloid leukemia cells refractory/resistant to tyrosine kinase inhibitors are genetically unstable and may cause relapse and malignant progression to the terminal disease state

BCR-ABL1 kinase-induced chronic myeloid leukemia in chronic phase (CML-CP) usually responds to treatment with ABL tyrosine kinase inhibitors (TKIs) such as imatinib, dasatinib, and nilotinib. In most patients TKIs reduce the leukemia cell load substantially, but some leukemia cells, for example leukemia stem cells (LSCs), are intrinsically refractory to TKIs. In addition, some patients who respond initially may later become resistant to TKIs due to accumulation of point mutations in BCR-ABL1 kinase. LSCs or their progeny, leukemia progenitor cells (LPCs), at some stage may acquire additional genetic changes that cause the leukemia to transform further to a more advanced blast phase (CML-BP), which responds poorly to treatment and is usually fatal. We postulate that LSCs and/or LPCs refractory or resistant to TKIs may be 'ticking time-bombs' accumulating additional genetic aberrations and eventually 'exploding' to generate additional TKI-resistant clones and CML-BP clones with complex karyotypes.

Association of common variants in mismatch repair genes and breast cancer susceptibility: a multigene study

Background

MMR is responsible for the repair of base-base mismatches and insertion/deletion loops. Besides this, MMR is also associated with an anti-recombination function, suppressing homologous recombination. Losses of heterozygosity and/or microsatellite instability have been detected in a large number of skin samples from breast cancer patients, suggesting a potential role of MMR in breast cancer susceptibility.

Methods

We carried out a hospital-based case-control study in a Caucasian Portuguese population (287 cases and 547 controls) to estimate the susceptibility to non-familial breast cancer associated with some polymorphisms in mismatch repair genes (MSH3, MSH4, MSH6, MLH1, MLH3, PMS1 and MUTYH).

Results

Using unconditional logistic regression we found that MLH3 (L844P, G>A) polymorphism GA (Leu/Pro) and AA (Pro/Pro) genotypes were associated with a decreased risk: OR = 0.65 (0.45-0.95) (p = 0.03) and OR = 0.62 (0.41-0.94) (p = 0.03), respectively.

Analysis of two-way SNP interaction effects on breast cancer revealed two potential associations to breast cancer susceptibility: MSH3 Ala1045Thr/MSH6 Gly39Glu - AA/TC [OR = 0.43 (0.21-0.83), p = 0.01] associated with a decreased risk; and MSH4 Ala97Thr/MLH3 Leu844Pro - AG/AA [OR = 2.35 (1.23-4.49), p = 0.01], GG/AA [OR = 2.11 (1.12-3,98), p = 0.02], and GG/AG [adjusted OR = 1.88 (1.12-3.15), p = 0.02] all associated with an increased risk for breast cancer.

Conclusion

It is possible that some of these common variants in MMR genes contribute significantly to breast cancer susceptibility. However, further studies with a large sample size will be needed to support our results.

Topo IIIalpha and BLM act within the Fanconi anemia pathway in response to DNA-crosslinking agents

The Bloom protein (BLM) and Topoisomerase IIIalpha are found in association with proteins of the Fanconi anemia (FA) pathway, a disorder manifesting increased cellular sensitivity to DNA crosslinking agents. In order to determine if the association reflects a functional interaction for the maintenance of genome stability, we have analyzed the effects of siRNA-mediated depletion of the proteins in human cells. Depletion of Topoisomerase IIIalpha or BLM leads to increased radial formation, as is seen in FA. BLM and Topoisomerase IIIalpha are epistatic to the FA pathway for suppression of radial formation in response to DNA interstrand crosslinks since depletion of either of them in FA cells does not increase radial formation. Depletion of Topoisomerase IIIalpha or BLM also causes an increase in sister chromatid exchanges, as is seen in Bloom syndrome cells. Human Fanconi anemia cells, however, do not demonstrate increased sister chromatid exchanges, separating this response from radial formation. Primary cell lines from mice defective in both Blm and Fancd2 have the same interstrand crosslink-induced genome instability as cells from mice deficient in the Fancd2 protein alone. These observations demonstrate that the association of BLM and Topoisomerase IIIalpha with Fanconi proteins is a functional one, delineating a BLM-Topoisomerase IIIalpha-Fanconi pathway that is critical for suppression of chromosome radial formation.

Multiple-pathway analysis of double-strand break repair mutations in Drosophila

The analysis of double-strand break (DSB) repair is complicated by the existence of several pathways utilizing a large number of genes. Moreover, many of these genes have been shown to have multiple roles in DSB repair. To address this complexity we used a repair reporter construct designed to measure multiple repair outcomes simultaneously. This approach provides estimates of the relative usage of several DSB repair pathways in the premeiotic male germline of Drosophila. We applied this system to mutations at each of 11 repair loci plus various double mutants and altered dosage genotypes. Most of the mutants were found to suppress one of the pathways with a compensating increase in one or more of the others. Perhaps surprisingly, none of the single mutants suppressed more than one pathway, but they varied widely in how the suppression was compensated. We found several cases in which two or more loci were similar in which pathway was suppressed while differing in how this suppression was compensated. Taken as a whole, the data suggest that the choice of which repair pathway is used for a given DSB occurs by a two-stage "decision circuit" in which the DSB is first placed into one of two pools from which a specific pathway is then selected.

Computational exploration of the activated pathways associated with DNA damage response in breast cancer

Molecular signaling events regulate cellular activity. Cancer stimulating signals trigger cellular responses that evade the regulatory control of cell development. To understand the mechanism of signaling regulation in cancer, it is necessary to identify the activated pathways in cancer. We have developed RepairPATH, a computational algorithm that explores the activated signaling pathways in cancer. The RepairPATH integrates RepairNET, an assembled protein interaction network associated with DNA damage response, with the gene expression profiles derived from the microarray data. Based on the observation that cofunctional proteins often exhibit correlated gene expression profiles, it identifies the activated signaling pathways in cancer by systematically searching the RepairNET for proteins with significantly correlated gene expression profiles. Analyzing the gene expression profiles of breast cancer, we found distinct similarities and differences in the activated signaling pathways between the samples from the patients who developed metastases and the samples from the patients who were disease free within 5 years. The cellular pathways associated with the various DNA repair mechanisms and the cell-cycle checkpoint controls are found to be activated in both sample groups. One of the most intriguing findings is that the pathways associated with different cellular processes are functionally coordinated through BRCA1 in the disease-free sample group, whereas such functional coordination is absent in the samples from patients who developed metastases. Our analysis revealed the potential cellular pathways that regulate the signaling events in breast cancer.

The ATM-mediated DNA-damage response: taking shape

Cellular responses to DNA damage are crucial for maintaining homeostasis and preventing the development of cancer. Our understanding of the DNA-damage response has evolved: whereas previously the focus was on DNA repair, we now appreciate that the response to DNA lesions involves a complex, highly branched signaling network. Defects in this response lead to severely debilitating, cancer-predisposing "genomic instability syndromes". Double strand breaks (DSBs) in DNA are potent triggers of the DNA-damage response, which is why they are used to study this pathway. The chief transducer of the DSB signal is the nuclear protein kinase ataxia-telangiectasia mutated (ATM). Genetic, biochemical and structural studies have recently provided insights into the ATM-mediated DSB response, reshaping our view of this signaling pathway while raising new questions.

RepairNET: A bioinformatics toolbox for functional exploration of DNA damage response

DNA damage response is one of the essential cellular mechanisms to maintaining the genomic integrity of the cell. Aberrations in the mechanism of DNA damage response often result in cancer. We describe here RepairNET, a protein-protein interaction network associated with the DNA damage response. RepairNET is assembled from the published literature by using a protocol that involved computational data mining of the MEDLINE and manual curation. This network represents the current knowledge on the intrinsic signaling pathways related to the DNA damage response process. RepairNET currently contains more than 1,200 proteins with over 2,300 functional interactions. A number of web-interface tools have been implemented to facilitate a user-friendly environment. The users can navigate through the cellular network associated with the DNA damage response via a Java-based interactive graphical interface. In order to help users explore the functional relationships between the interacting proteins, we have assigned functional domains to the proteins in RepairNET based on their sequences. A total of 365 unique functional domains are assigned. RepairNET is available online at http://guanyin.chem.temple.edu/RepairNET.html. It could become an essential resource center for cancer research, providing clues to understanding the functional relationship between proteins in the network, and to building scientific models for the mechanism of DNA damage response and cancer proliferation.

Gene expression and DNA repair in progeroid syndromes and human aging

Human progeroid syndromes are caused by mutations in single genes accelerating some but not all features of normal aging. Most progeroid disorders are linked to defects in genome maintenance, and while it remains unknown if similar processes underlie normal and premature aging, they provide useful models for the study of aging. Altered transcription is speculated to play a causative role in aging, and is involved in the pathology of most if not all progeroid syndromes. Previous studies demonstrate that there is a similar pattern of gene expression changes in primary cells from old and Werner syndrome compared to young suggesting a presence of common cellular aging mechanisms in old and progeria. Here we review the role of transcription in progeroid syndromes and discuss the implications of similar transcription aberrations in normal and premature aging.

The cell-type-specificity of inherited predispositions to tumours: review and hypothesis

Most hereditary predispositions to tumours affect only one particular cell type of the body but the genes bearing the relevant germ-line mutation are not cell-type-specific. Some predisposition syndromes include increased risks of lesions (developmental or tumourous) of unrelated cell types, in any individual predisposed to the main lesion (e.g. osteosarcoma in patients predisposed to retinoblastoma). Other predispositions to additional lesions occur only in members of some families with the predisposition to the basic lesion (e.g. Gardner's syndrome in some families suffering familial adenomatous polyposis). In yet other predisposition syndromes, different mutations of the same gene are associated with markedly differing family-specific clinical syndromes. In particular, identical germline mutations (e.g. in APC, RET and PTEN genes), have been found associated with differing clinical syndromes in different families. This paper reviews previously suggested mechanisms of the cell-type specificity of inherited predispositions to tumour. Models of tumour formation in predisposition syndromes are discussed, especially those involving a germline mutation (the first 'hit') of a tumour suppressor gene (TSG) and a second (somatic) hit on the second allele of the same TSG. A modified model is suggested, such that the second hit is a co-mutation of the second allele of the TSG and a regulator which is specific for growth and/or differentiation of the cell type which is susceptible to the tumour predisposition. In some cases of tumour, the second hit may be large enough to be associated with a cytogenetically-demonstrable abnormality of the part of the chromosome carrying the TSG, but in other cases, the co-mutation may be of 'sub-cytogenetic' size (i.e. 10(2)-10(5) bases). For the latter, mutational mechanisms of frameshift and impaired fidelity of replication of DNA by DNA polyerases may sometimes be involved. Candidate cell-type-specific regulators may include microRNAs and perhaps transcription factors. It is suggested that searching the introns within 10(5)-10(6) bases either side of known of exonic mutations of TSGs associated with inherited tumour predisposition might reveal microRNA cell-type-specific regulators. Additional investigations may involve fluorescent in situ hybridisations on interphase tumour nuclei.

Cotranscriptionally Formed DNA:RNA Hybrids Mediate Transcription Elongation Impairment and Transcription-Associated Recombination

Genetic instability, a phenomenon relevant for developmentally regulated processes, cancer, and inherited disorders, can be induced by transcription. However, the mechanisms of transcription-associated genetic instability are not yet understood. Analysis of S. cerevisiae mutants of THO/TREX, a conserved eukaryotic protein complex functioning at the interface of transcription and mRNA metabolism, has provided evidence that transcription elongation impairment can cause hyperrecombination. Here we show, using hpr1Delta mutants, that the nascent mRNA can diminish transcription elongation efficiency and promote recombination. If during transcription the nascent mRNA is self-cleaved by a hammerhead ribozyme, the transcription-defect and hyperrecombination phenotypes of hpr1Delta cells are suppressed. Abolishment of hyperrecombination by overexpression of RNase H1 and molecular detection of DNA:RNA hybrids indicate that these are formed cotranscriptionally in hpr1Delta cells. These data support a model to explain the connection between recombination, transcription, and mRNA metabolism and provide a new perspective to understanding transcription-associated recombination.

Malondialdehyde, a product of lipid peroxidation, is mutagenic in human cells

Malondialdehyde (MDA) is an endogenous genotoxic product of enzymatic and oxygen radical-induced lipid peroxidation whose adducts are known to exist in DNA isolated from healthy human beings. To evaluate the mutagenic potential of MDA in human cells, we reacted MDA with pSP189 shuttle vector DNA and then transfected them into human fibroblasts for replication. MDA induced up to a 15-fold increase in mutation frequency in the supF reporter gene compared with untreated DNA. Sequence analysis revealed that the majority of MDA-induced mutations occurred at GC base pairs. The most frequent mutations were large insertions and deletions, but base pair substitutions were also detected. MDA-induced mutations were completely abolished when the adducted shuttle vector was replicated in cells lacking nucleotide excision repair. MDA induction of large deletions and the apparent requirement for nucleotide excision repair suggested the possible involvement of a DNA interstrand cross-link as a premutagenic lesion. Indeed, MDA formed interstrand cross-links in duplex plasmids and oligonucleotides. Substrates containing the sequence 5'-d(CG) were preferentially cross-linked, consistent with the observation of base pair substitutions in 5'-d(CG) sites in the MDA-induced mutation spectrum. These experiments provide biological and biochemical evidence for the existence of MDA-induced DNA interstrand cross-links that could result from endogenous oxidative stress and likely have potent biological effects.

ATM and related protein kinases: safeguarding genome integrity

Maintenance of genome stability is essential for avoiding the passage to neoplasia. The DNA-damage response--a cornerstone of genome stability--occurs by a swift transduction of the DNA-damage signal to many cellular pathways. A prime example is the cellular response to DNA double-strand breaks, which activate the ATM protein kinase that, in turn, modulates numerous signalling pathways. ATM mutations lead to the cancer-predisposing genetic disorder ataxia-telangiectasia (A-T). Understanding ATM's mode of action provides new insights into the association between defective responses to DNA damage and cancer, and brings us closer to resolving the issue of cancer predisposition in some A-T carriers.

Fanconi anaemia

Fanconi anaemia (FA) is an autosomal recessive disease characterised by congenital abnormalities, defective haemopoiesis, and a high risk of developing acute myeloid leukaemia and certain solid tumours. Chromosomal instability, especially on exposure to alkylating agents, may be shown in affected subjects and is the basis for a diagnostic test. FA can be caused by mutations in at least seven different genes. Interaction pathways have been established, both between the FA proteins and other proteins involved in DNA damage repair, such as ATM, BRCA1 and BRCA2, thereby providing a link with other disorders in which defective DNA damage repair is a feature. This review summarises the clinical features of FA and the natural history of the disease, discusses diagnosis and management, and puts the recent molecular advances into the context of the cellular and clinical FA phenotype.