Nijmegen breakage syndrome

Expanding the phenotypic spectrum of LIG4 pathogenic variations: neuro-histopathological description of 4 fetuses with stenosis of the aqueduct

Severe ventriculomegaly is a rare congenital brain defect, usually detected in utero, of poor neurodevelopmental prognosis. This ventricular enlargement can be the consequence of different mechanisms: either by a disruption of the cerebrospinal fluid circulation or abnormalities of its production/absorption. The aqueduct stenosis is one of the most frequent causes of obstructive ventriculomegaly, however, fewer than 10 genes have been linked to this condition and molecular bases remain often unknown. We report here 4 fetuses from 2 unrelated families presenting with ventriculomegaly at prenatal ultra-sonography as well as an aqueduct stenosis and skeletal abnormalities as revealed by fetal autopsy. Genome sequencing identified biallelic pathogenic variations in LIG4, a DNA-repair gene responsible for the LIG4 syndrome which associates a wide range of clinical manifestations including developmental delay, microcephaly, short stature, radiation hypersensitivity and immunodeficiency. Thus, not only this report expands the phenotype spectrum of LIG4-related disorders, adding ventriculomegaly due to aqueduct stenosis, but we also provide the first neuropathological description of fetuses carrying LIG4 pathogenic biallelic variations.

Infections in Inborn Errors of Immunity with Combined Immune Deficiency: A Review

Enhanced susceptibility to microbes, often resulting in severe, intractable and frequent infections due to usually innocuous organisms at uncommon sites, is the most striking feature in individuals with an inborn error of immunity. In this narrative review, based on the International Union of Immunological Societies' 2022 (IUIS 2022) Update on phenotypic classification of human inborn errors of immunity, the focus is on commonly encountered Combined Immunodeficiency Disorders (CIDs) with susceptibility to infections. Combined immune deficiency disorders are usually commensurate with survival beyond infancy unlike Severe Combined Immune Deficiency (SCID) and are often associated with clinical features of a syndromic nature. Defective humoral and cellular immune responses result in susceptibility to a broad range of microbial infections. Although disease onset is usually in early childhood, mild defects may present in late childhood or even in adulthood. A precise diagnosis is imperative not only for determining management strategies, but also for providing accurate genetic counseling, including prenatal diagnosis, and also in deciding empiric treatment of infections upfront before investigation reports are available.

A novel, ataxic mouse model of ataxia telangiectasia caused by a clinically relevant nonsense mutation

Ataxia Telangiectasia (A-T) and Ataxia with Ocular Apraxia Type 1 (AOA1) are devastating neurological disorders caused by null mutations in the genome stability genes, A-T mutated (ATM) and Aprataxin (APTX), respectively. Our mechanistic understanding and therapeutic repertoire for treating these disorders are severely lacking, in large part due to the failure of prior animal models with similar null mutations to recapitulate the characteristic loss of motor coordination (i.e., ataxia) and associated cerebellar defects. By increasing genotoxic stress through the insertion of null mutations in both the Atm (nonsense) and Aptx (knockout) genes in the same animal, we have generated a novel mouse model that for the first time develops a progressively severe ataxic phenotype associated with atrophy of the cerebellar molecular layer. We find biophysical properties of cerebellar Purkinje neurons (PNs) are significantly perturbed (e.g., reduced membrane capacitance, lower action potential [AP] thresholds, etc.), while properties of synaptic inputs remain largely unchanged. These perturbations significantly alter PN neural activity, including a progressive reduction in spontaneous AP firing frequency that correlates with both cerebellar atrophy and ataxia over the animal's first year of life. Double mutant mice also exhibit a high predisposition to developing cancer (thymomas) and immune abnormalities (impaired early thymocyte development and T-cell maturation), symptoms characteristic of A-T. Finally, by inserting a clinically relevant nonsense-type null mutation in Atm, we demonstrate that Small Molecule Read-Through (SMRT) compounds can restore ATM production, indicating their potential as a future A-T therapeutic.

New insights in phenotype and treatment of lung disease immuno-deficiency and chromosome breakage syndrome (LICS)

We report five patients with lung disease immuno-deficiency and chromosome breakage syndrome (LICS) but without recurrent infections and severe immunodeficiency. One patient had extended survival to 6.5 years. Hematopoietic stem-cell transplantation failed to cure another patient. Our findings suggest that the immunological abnormalities can be limited and do not fully explain the LICS phenotype.

DNA Repair Syndromes and Cancer: Insights Into Genetics and Phenotype Patterns

DNA damage response is essential to human physiology. A broad spectrum of pathologies are displayed by individuals carrying monoallelic or biallelic loss-of-function mutations in DNA damage repair genes. DNA repair syndromes with biallelic disturbance of essential DNA damage response pathways manifest early in life with multi-systemic involvement and a high propensity for hematologic and solid cancers, as well as bone marrow failure. In this review, we describe classic biallelic DNA repair cancer syndromes arising from faulty single- and double-strand DNA break repair, as well as dysfunctional DNA helicases. These clinical entities include xeroderma pigmentosum, constitutional mismatch repair deficiency, ataxia telangiectasia, Nijmegen breakage syndrome, deficiencies of DNA ligase IV, NHEJ/Cernunnos, and ERCC6L2, as well as Bloom, Werner, and Rothmund-Thompson syndromes. To give an in-depth understanding of these disorders, we provide historical overview and discuss the interplay between complex biology and heterogeneous clinical manifestations.

Nijmegen breakage syndrome: case report and review of literature

Nijmegen Breakage Syndrome (NBS) is a rare autosomalrecessive DNA repair disorder characterized by genomic instability andincreased risk of haematopoietic malignancies observed in morethan 40% of the patients by the time they are 20 years old. The underlying gene, NBS1, is located on human chromosome 8q21 and codes for a protein product termed nibrin, Nbs1 or p95. Over 90% of patients are homozygous for a founder mutation: a deletion of five base pairs which leads to a frame shift and protein truncation. Nibrin (NBN) plays an important role in the DNA damage response (DDR) and DNA repair. DDR is a crucial signalling pathway in apoptosis and senescence. Cardinal symptoms of Nijmegen breakage syndrome are characteristic: microcephaly, present at birth and progressive with age, dysmorphic facial features, mild growth retardation, mild-to-moderate intellectual disability, and, in females, hypergonadotropic hypogonadism. Combined cellular and humoral immunodeficiency with recurrent sino-pulmonary infections, a strong predisposition to develop malignancies (predominantly of lymphoid origin) and radiosensitivity are other integral manifestations of the syndrome. The diagnosis of NBS is initially based on clinical manifestations and is confirmed by genetic analysis. Prenatal molecular genetic diagnosis is possible if disease-causing mutations in both alleles of the NBN gene are known. No specific therapy is available for NBS; however, hematopoietic stem cell transplantation may be one option for some patients. Prognosis is generally poor due to the extremely high rate of malignancies. We present here a case of Nijmegen breakage syndrome associated with Hodgkin lymphomas and Combined variable immunodeficiency.

Growth Hormone's Links to Cancer

Several components of the GH axis are involved in tumor progression, and GH-induced intracellular signaling has been strongly associated with breast cancer susceptibility in genome-wide association studies. In the general population, high IGF-I levels and low IGF-binding protein-3 levels within the normal range are associated with the development of common malignancies, and components of the GH-IGF signaling system exhibit correlations with clinical, histopathological, and therapeutic parameters in cancer patients. Despite promising findings in preclinical studies, anticancer therapies targeting the GH-IGF signaling system have led to disappointing results in clinical trials. There is substantial evidence for some degree of protection against tumor development in several animal models and in patients with genetic defects associated with GH deficiency or resistance. In contrast, the link between GH excess and cancer risk in acromegaly patients is much less clear, and cancer screening in acromegaly has been a highly controversial issue. Recent studies have shown that increased life expectancy in acromegaly patients who attain normal GH and IGF-I levels is associated with more deaths due to age-related cancers. Replacement GH therapy in GH deficiency hypopituitary adults and short children has been shown to be safe when no other risk factors for malignancy are present. Nevertheless, the use of GH in cancer survivors and in short children with RASopathies, chromosomal breakage syndromes, or DNA-repair disorders should be carefully evaluated owing to an increased risk of recurrence, primary cancer, or second neoplasia in these individuals.

Epidermal Nbn deletion causes premature hair loss and a phenotype resembling psoriasiform dermatitis

Nijmegen Breakage Syndrome is a disease caused by NBN mutations. Here, we report a novel function of Nbn in skin homeostasis. We found that Nbn deficiency in hair follicle (HF) progenitors promoted increased DNA damage signaling, stimulating p16^Ink4a up-regulation, Trp53 stabilization and cytokines secretion leading to HF-growth arrest and hair loss. At later stages, the basal keratinocytes layer exhibited also enhanced DNA damage response but in contrast to the one in HF progenitor was not associated with pro-inflammatory cytokines expression, but rather increased proliferation, lack of differentiation and immune response resembling psoriasiform dermatitis. Simultaneous Nbn and Trp53 inactivation significantly exacerbated this phenotype, due to the lack of inhibition of pro-inflammatory cytokines secretion by Trp53. Altogether, we demonstrated novel functions of Nbn in HF maintenance and prevention of skin inflammation and we provide a mechanistic explanation that links cell intrinsic DNA maintenance with large scale morphological tissue alterations.

Circulating T Cells of Patients with Nijmegen Breakage Syndrome Show Signs of Senescence

Purpose

The Nijmegen breakage syndrome (NBS) is an inherited genetic disorder characterized by a typical facial appearance, microcephaly, growth retardation, immunodeficiency, and a strong predisposition to malignancies, especially of lymphoid origin. NBS patients have a mutation in the NBN gene which involves the repair of DNA double-strand breaks (DSBs). Here we studied the peripheral T cell compartment of NBS patients with a focus on immunological senescence.

Methods

The absolute numbers and frequencies of the different T cell subsets were determined in NBS patients from young age till adulthood and compared to age-matched healthy individuals (HI). In addition, we determined the expression of senescent T cell markers and the signal joint T cell receptor excision circles (sjTRECs) content.

Results

Our results demonstrate that NBS patients have reduced T cell numbers. NBS patients showed lower numbers of αβ⁺ T cells, but normal γδ⁺ T cell numbers compared to HI. Concerning the αβ⁺ T cells, both CD4⁺ as well as CD8⁺ T cells were excessively reduced in numbers compared to aged-matched HI. In addition, NBS patients showed higher frequencies of the more differentiated T cells expressing the senescent cell marker CD57 and did not express co-stimulatory molecule CD28. These effects were already present in the youngest age group. Furthermore, NBS patients showed lower sjTREC content in their T cells possibly indicative of a lower thymic output.

Conclusions

We conclude that circulating T cells from NBS patients show signs of a senescent phenotype which is already present from young age on and which might explain their T cell immune deficiency.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-016-0363-5) contains supplementary material, which is available to authorized users.

Cancers Related to Immunodeficiencies: Update and Perspectives

The life span of patients with primary and secondary immunodeficiency is increasing due to recent improvements in therapeutic strategies. While the incidence of primary immunodeficiencies (PIDs) is 1:10,000 births, that of secondary immunodeficiencies are more common and are associated with posttransplantation immune dysfunction, with immunosuppressive medication for human immunodeficiency virus or with human T-cell lymphotropic virus infection. After infection, malignancy is the most prevalent cause of death in both children and adults with (PIDs). PIDs more often associated with cancer include common variable immunodeficiency (CVID), Wiskott-Aldrich syndrome, ataxia-telangiectasia, and severe combined immunodeficiency. This suggests that a protective immune response against both infectious non-self-(pathogens) and malignant self-challenges (cancer) exists. The increased incidence of cancer has been attributed to defective elimination of altered or "transformed" cells and/or defective immunity towards cancer cells. The concept of aberrant immune surveillance occurring in PIDs is supported by evidence in mice and from patients undergoing immunosuppression after transplantation. Here, we discuss the importance of PID defects in the development of malignancies and the current limitations associated with molecular pathogenesis of these diseases and emphasize the need for further knowledge of how specific mutations can modulate the immune system to alter immunosurveillance and thereby play a key role in the etiology of malignancies in PID patients.

Chromosomal Instability and Molecular Defects in Induced Pluripotent Stem Cells from Nijmegen Breakage Syndrome Patients

Nijmegen breakage syndrome (NBS) results from the absence of the NBS1 protein, responsible for detection of DNA double-strand breaks (DSBs). NBS is characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. Here, we show successful reprogramming of NBS fibroblasts into induced pluripotent stem cells (NBS-iPSCs). Our data suggest a strong selection for karyotypically normal fibroblasts to go through the reprogramming process. NBS-iPSCs then acquire numerous chromosomal aberrations and show a delayed response to DSB induction. Furthermore, NBS-iPSCs display slower growth, mitotic inhibition, a reduced apoptotic response to stress, and abnormal cell-cycle-related gene expression. Importantly, NBS neural progenitor cells (NBS-NPCs) show downregulation of neural developmental genes, which seems to be mediated by P53. Our results demonstrate the importance of NBS1 in early human development, shed light on the molecular mechanisms underlying this severe syndrome, and further expand our knowledge of the genomic stress cells experience during the reprogramming process.

Copy number deletion of RAD50 as predictive marker of BRCAness and PARP inhibitor response in BRCA wild type ovarian cancer

OBJECTIVE

To identify novel prognostic and therapeutic markers for PARP inhibitors in BRCA wild type ovarian cancer (OvCa).

METHODS

BRCAness status was defined by analyzing whole-exome deep sequencing data from 220 BRCAwt OvCa cases in TCGA. Thirty-three DNA-repair genes were screened in an integrated manner for BRCA-independent mechanism of BRCAness using multiple-dimensional genomic data. Publicly available databases and siRNA knock-down were used for external validation and evaluation of drug response in OvCa cell lines.

RESULTS

In 220 BRCAwt OvCa patients, tumors exhibiting the BRCAness signature have enhanced OS (HR [95% CI]=0.33 [0.15-0.69], P=0.004) and PFS (HR [95% CI]=0.51 [0.24-1.08], P=0.077), strongly suggesting a BRCA-independent mechanism of drug sensitivity in those patients. Systematic screening of driving molecular events of BRCAness revealed that RAD50 deletion is a marker of BRCAness. The RAD50 deletion occurred in 18% of BRCAwt OvCa patients. RAD50 deletion led to its decreased mRNA expression in tumors (fold change=0.63, P=3.56×10(-13)). In BRCAwt patients, RAD50 deletion was associated with significantly better OS (HR [95% CI]=0.44 [0.25-0.78], P=0.005) and PFS (HR [95% CI]=0.60 [0.37-0.99], P=0.044), adjusted by age and stage. Knockdown of RAD50 expression augmented OvCa cell's responses to cisplatin and olaparib. Among 19 OvCa cell lines, the RAD50 copy number deletion is significantly associated with better responses to two structurally distinct PARPis (i.e. olaparib and rucaparib).

CONCLUSION

Our study identified the copy number deletion of RAD50 as a candidate marker for survival and response to PARPis in BRCAwt OvCa tumors.

VRK1 phosphorylates and protects NBS1 from ubiquitination and proteasomal degradation in response to DNA damage

NBS1 is an early component in DNA-Damage Response (DDR) that participates in the initiation of the responses aiming to repair double-strand breaks caused by different mechanisms. Early steps in DDR have to react to local alterations in chromatin that are induced by DNA damage. NBS1 participates in the early detection of DNA damage and functions as a platform for the recruitment and assembly of components that are sequentially required for the repair process. In this work we have studied whether the VRK1 chromatin kinase can affect the activation of NBS1 in response to DNA damage induced by ionizing radiation. VRK1 is forming a basal preassembled complex with NBS1 in non-damaged cells. Knockdown of VRK1 resulted in the loss of NBS1 foci induced by ionizing radiation, an effect that was also detected in cell-cycle arrested cells and in ATM (-/-) cells. The phosphorylation of NBS1 in Ser343 by VRK1 is induced by either doxorubicin or IR in ATM (-/-) cells. Phosphorylated NBS1 is also complexed with VRK1. NBS1 phosphorylation by VRK1 cooperates with ATM. This phosphorylation of NBS1 by VRK1 contributes to the stability of NBS1 in ATM (-/-) cells, and the consequence of its loss can be prevented by treatment with the MG132 proteasome inhibitor of RNF8. We conclude that VRK1 regulation of NBS1 contributes to the stability of the repair complex and permits the sequential steps in DDR.

Identification and characterization of a novel gene encoding the NBS1 protein in Pyricularia oryzae

The ascomycete Pyricularia oryzae (teleomorph: Magnaporthe oryzae) causes one of the most serious diseases known as rice blast. The Nijmegen breakage syndrome protein (NBS1) is essential for DNA repair; thus, we studied the P. oryzae NBS1 homolog (PoNBS1). A PoNBS1 null mutant exhibited high sensitivity to DNA damage-inducing agents. The mutant also exhibited the retarded hyphal growth, and induced abnormal conidial germination and shape, but showed normal appressorium formation. The phenotypes of the null mutant were complemented by introducing the cDNA of PoNBS1 driven by a TrpC promoter of Aspergillus nidulans. In addition, the null mutant similarly complemented with the PoNBS1 cDNA lacking the FHA domain that had a normal phenotype except for hyphal growth. These results suggest that PoNBS1 is involved in DNA repair and normal development in P. oryzae. Moreover, the FHA domain of PoNBS1 participates in normal hyphal growth.

Nbn gene inactivation in the CNS of mouse inhibits the myelinating ability of the mature cortical oligodendrocytes

Nijmegen Breakage Syndrome (NBS) is a recessive genetic disorder characterized by immunodeficiency, elevated sensitivity to ionizing radiation, chromosomal instability, microcephaly, and high predisposition to malignancies. Since the underlying molecular mechanisms of the NBS microcephaly are still obscure, thus our group previously inactivated the Nbn gene in the central nervous system (CNS) of mice by nestin-Cre targeting gene system, and generated Nbn(CNS-del) mice. Interestingly, the newborn Nbn(CNS-del) mice exhibit obvious microcephaly, which is accompanied by severe ataxia and balance deficiency. In this study presented here, we report that Nbn-deficiency induces the enhanced apoptosis of the mature oligodendrocytes at postnatal day 7, which further affects the myelination of the nerve fibers of cerebrum and corpus callosum.The distinct regulatory roles of Ataxia telangiectasia mutated (ATM) signaling and protein kinase B(Akt)/the mammalian target of Rapamycin (AKT/mTOR) signaling are responsible for the enhanced apoptosis of the Nbn-deficient oligodendrocytes. In addition, a series of transcriptional factors including histonedeacetylase (HDAC), zinc finger protein 191 (ZFP-191) and myelin sheath regulatory factor (MRF) play distinct roles in regulating the myelination of the Nbn-deficient oligodendrocytes. Based on these results, it concludes that ATM-Chk2-P53-P21 signaling pathway and the AKT/mTOR signaling pathway are both responsible for the enhanced apoptosis of the Nbn-deficient oligodendrocytes. HDAC, ZFP-191, and MRF are also involved in the pathogenesis of the hypomyelination of the Nbn-deficient oligodendrocytes.

Breakpoint sites disclose the role of the V(D)J recombination machinery in the formation of T-cell receptor (TCR) and non-TCR associated aberrations in T-cell acute lymphoblastic leukemia

Aberrant recombination between T-cell receptor genes and oncogenes gives rise to chromosomal translocations that are genetic hallmarks in several subsets of human T-cell acute lymphoblastic leukemias. The V(D)J recombination machinery has been shown to play a role in the formation of these T-cell receptor translocations. Other, non-T-cell receptor chromosomal aberrations, such as SIL-TAL1 deletions, have likewise been recognized as V(D)J recombination associated aberrations. Despite the postulated role of V(D)J recombination, the extent of the V(D)J recombination machinery involvement in the formation of T-cell receptor and non-T-cell receptor aberrations in T-cell acute lymphoblastic leukemia is still poorly understood. We performed a comprehensive in silico and ex vivo evaluation of 117 breakpoint sites from 22 different T-cell receptor translocation partners as well as 118 breakpoint sites from non-T-cell receptor chromosomal aberrations. Based on this extensive set of breakpoint data, we provide a comprehensive overview of T-cell receptor and oncogene involvement in T-ALL. Moreover, we assessed the role of the V(D)J recombination machinery in the formation of chromosomal aberrations, and propose an up-dated mechanistic classification on how the V(D)J recombination machinery contributes to the formation of T-cell receptor and non-T-cell receptor aberrations in human T-cell acute lymphoblastic leukemia.

Rare key functional domain missense substitutions in MRE11A, RAD50, and NBN contribute to breast cancer susceptibility: results from a Breast Cancer Family Registry case-control mutation-screening study

INTRODUCTION

The MRE11A-RAD50-Nibrin (MRN) complex plays several critical roles related to repair of DNA double-strand breaks. Inherited mutations in the three components predispose to genetic instability disorders and the MRN genes have been implicated in breast cancer susceptibility, but the underlying data are not entirely convincing. Here, we address two related questions: (1) are some rare MRN variants intermediate-risk breast cancer susceptibility alleles, and if so (2) do the MRN genes follow a BRCA1/BRCA2 pattern wherein most susceptibility alleles are protein-truncating variants, or do they follow an ATM/CHEK2 pattern wherein half or more of the susceptibility alleles are missense substitutions?

METHODS

Using high-resolution melt curve analysis followed by Sanger sequencing, we mutation screened the coding exons and proximal splice junction regions of the MRN genes in 1,313 early-onset breast cancer cases and 1,123 population controls. Rare variants in the three genes were pooled using bioinformatics methods similar to those previously applied to ATM, BRCA1, BRCA2, and CHEK2, and then assessed by logistic regression.

RESULTS

Re-analysis of our ATM, BRCA1, and BRCA2 mutation screening data revealed that these genes do not harbor pathogenic alleles (other than modest-risk SNPs) with minor allele frequencies>0.1% in Caucasian Americans, African Americans, or East Asians. Limiting our MRN analyses to variants with allele frequencies of <0.1% and combining protein-truncating variants, likely spliceogenic variants, and key functional domain rare missense substitutions, we found significant evidence that the MRN genes are indeed intermediate-risk breast cancer susceptibility genes (odds ratio (OR)=2.88, P=0.0090). Key domain missense substitutions were more frequent than the truncating variants (24 versus 12 observations) and conferred a slightly higher OR (3.07 versus 2.61) with a lower P value (0.029 versus 0.14).

CONCLUSIONS

These data establish that MRE11A, RAD50, and NBN are intermediate-risk breast cancer susceptibility genes. Like ATM and CHEK2, their spectrum of pathogenic variants includes a relatively high proportion of missense substitutions. However, the data neither establish whether variants in each of the three genes are best evaluated under the same analysis model nor achieve clinically actionable classification of individual variants observed in this study.

An adult patient with Nijmegen Breakage Syndrome and Hodgkin's Lymphoma

BACKGROUND

Nijmegen Breakage Syndrome (NBS) is a rare autosomal recessive DNA repair disorder characterized by immune deficiency, microcephaly, mental retardation and a disposition for the development of hematological malignancies. So far, mostly pediatric patients have been described, since the underlying condition is often fatal before adulthood. Many patients diagnosed with Hodgkin lymphoma (HL) due to this DNA repair defect receive reduced treatment followed by early progression and fatal outcome.

CASE PRESENTATION

We describe here a 26-year old male caucasian patient with NBS who presented with multi organ failure due to HL. Immediate intensive chemotherapy lead to complete remission and reversed organ failure.

CONCLUSION

We show that application of standard chemotherapy can lead to long-term disease free survival in patients with a DNA repair disorder. Furthermore, we describe here, to the best of our knowledge, the first adult patient with NBS and HL.

DNA damage and oxidative injury are associated with hypomyelination in the corpus callosum of newborn Nbn(CNS-del) mice

Nijmegen breakage syndrome (NBS), caused by mutation of the Nbn gene, is a recessive genetic disorder characterized by immunodeficiency, elevated sensitivity to ionizing radiation, chromosomal instability, microcephaly, and high predisposition to malignancies. To explore the underlying molecular mechanisms of NBS microcephaly, Frappart et al. previously inactivated Nbn gene in the central nervous system (CNS) of mice by the nestin-Cre targeting gene system and generated Nbn(CNS-del) mice. Here we first report that Nbn gene inactivation induces the defective proliferation and enhanced apoptosis of the oligodendrocyte precursor cells (OPCs), contributing to the severe hypomyelination of the nerve fibers of the corpus callosum. Under conditions of DNA damage and oxidative stress, the distinct regulatory roles of ATM-Chk2 signaling and AKT/mTOR signaling are responsible for the defective proliferation and enhanced apoptosis of the Nbn-deficient OPCs. In addition, specific HDAC isoforms may play distinctive roles in regulating the myelination of the Nbn-deficient OPCs. However, brain-derived neurotrophic factor and nerve growth factor stimulation attenuates the oxidative stress and thereby increases the proliferation of the Nbn-deficient OPCs, which is accompanied by upregulation of the AKT/mTOR/P70S6K signaling pathway. Taken together, these findings demonstrate that DNA damage and oxidative stress resulting from Nbn gene inactivation are associated with hypomyelination of the nerve fibers of corpus callosum.

The defect in humoral immunity in patients with Nijmegen breakage syndrome is explained by defects in peripheral B lymphocyte maturation

Patients with an immunodeficiency in the course of Nijmegen breakage syndrome (NBS) that is caused by mutations in the NBN/NBS1 gene are prone to recurrent infections and malignancies, due to a defective DNA double-strand breaks repair mechanism. Four-color flow cytometry was used to analyze changes in B lymphocyte subsets reflecting the most important stages of peripheral B cell maturation. It was demonstrated that the humoral immune defect observed in NBS patients was caused by reduced numbers of B lymphocytes, but also by their aberrant maturation. Reduced relative and absolute counts of naïve and memory B cells were accompanied by a significant accumulation of the natural effector B lymphocytes. The elevated proportion of IgM-only memory and reduced proportion of IgM-negative cells within the memory B cell pool suggests that there is class-switch recombination defect in this population of cells in NBS patients, resulting in inadequate production of immunoglobulins. Because of the reduced T-cell counts, the T-cell dependent antigen response is severely impaired resulting in a lower frequency of memory B-cells. The T-cell independent B-cell differentiation pathway seems less affected. The reduced IgG and IgA levels in patients with NBS are caused both by ineffective class switch, at least due to poor T cell help, and low number of memory B cells. This study illustrates that the NBN gene product nibrin plays an important role at different levels in the B-cell system.

Nijmegen breakage syndrome (NBS)

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive syndrome of chromosomal instability mainly characterized by microcephaly at birth, combined immunodeficiency and predisposition to malignancies. Due to a founder mutation in the underlying NBN gene (c.657_661del5) the disease is encountered most frequently among Slavic populations. The principal clinical manifestations of the syndrome are: microcephaly, present at birth and progressive with age, dysmorphic facial features, mild growth retardation, mild-to-moderate intellectual disability, and, in females, hypergonadotropic hypogonadism. Combined cellular and humoral immunodeficiency with recurrent sinopulmonary infections, a strong predisposition to develop malignancies (predominantly of lymphoid origin) and radiosensitivity are other integral manifestations of the syndrome. The NBN gene codes for nibrin which, as part of a DNA repair complex, plays a critical nuclear role wherever double-stranded DNA ends occur, either physiologically or as a result of mutagenic exposure. Laboratory findings include: (1) spontaneous chromosomal breakage in peripheral T lymphocytes with rearrangements preferentially involving chromosomes 7 and 14, (2) sensitivity to ionizing radiation or radiomimetics as demonstrated in vitro by cytogenetic methods or by colony survival assay, (3) radioresistant DNA synthesis, (4) biallelic hypomorphic mutations in the NBN gene, and (5) absence of full-length nibrin protein. Microcephaly and immunodeficiency are common to DNA ligase IV deficiency (LIG4 syndrome) and severe combined immunodeficiency with microcephaly, growth retardation, and sensitivity to ionizing radiation due to NHEJ1 deficiency (NHEJ1 syndrome). In fact, NBS was most commonly confused with Fanconi anaemia and LIG4 syndrome. Genetic counselling should inform parents of an affected child of the 25% risk for further children to be affected. Prenatal molecular genetic diagnosis is possible if disease-causing mutations in both alleles of the NBN gene are known. No specific therapy is available for NBS, however, hematopoietic stem cell transplantation may be one option for some patients. Prognosis is generally poor due to the extremely high rate of malignancies.

Zespół Nijmegen (Nijmegen breakage syndrome; NBS) jest rzadkim schorzeniem z wrodzoną niestabilnością chromosomową dziedziczącym się w sposób autosomalny recesywny, charakteryzującym się przede wszystkim wrodzonym małogłowiem, złożonymi niedoborami odporności i predyspozycją do rozwoju nowotworów.

Choroba występuje najczęściej w populacjach słowiańskich, w których uwarunkowana jest mutacją założycielską w genie NBN (c.657_661del5). Do najważniejszych objawów zespołu zalicza się: małogłowie obecne od urodzenia i postępujące z wiekiem, charakterystyczne cechy dysmorfii twarzy, opóźnienie wzrastania, niepełnosprawność intelektualną w stopniu lekkim do umiarkowanego oraz hipogonadyzm hipogonadotropowy u dziewcząt. Na obraz choroby składają się także: niedobór odporności komórkowej i humoralnej, który jest przyczyną nawracających infekcji, znaczna predyspozycja do rozwoju nowotworów złośliwych (zwłaszcza układu chłonnego), a także zwiększona wrażliwość na promieniowanie jonizujące. Wyniki badań laboratoryjnych wykazują: (1) spontaniczną łamliwość chromosomów w limfocytach T krwi obwodowej, z preferencją do rearanżacji chromosomów 7 i 14, (2) nadwrażliwość na promieniowanie jonizujące lub radiomimetyki, co można wykazać metodami in vitro, (3) radiooporność syntezy DNA, (4) hipomorficzne mutacje na obu allelach genu NBN, oraz (5) brak w komórkach pełnej cząsteczki białka, nibryny. Małogłowie i niedobór odporności występują także w zespole niedoboru ligazy IV (LIG4) oraz w zespole niedoboru NHEJ1. Rodzice powinni otrzymać poradę genetyczną ze względu na wysokie ryzyko (25%) powtórzenia się choroby u kolejnego potomstwa. Możliwe jest zaproponowanie molekularnej diagnostyki prenatalnej jeżeli znane są obie mutacje będące przyczyną choroby. Nie ma możliwości zaproponowania specyficznej terapii, ale przeszczep szpiku może być alternatywą dla niektórych pacjentów. Generalnie prognoza nie jest pomyślna z uwagi na wysokie ryzyko rozwoju nowotworu.

Correct end use during end joining of multiple chromosomal double strand breaks is influenced by repair protein RAD50, DNA-dependent protein kinase DNA-PKcs, and transcription context

During repair of multiple chromosomal double strand breaks (DSBs), matching the correct DSB ends is essential to limit rearrangements. To investigate the maintenance of correct end use, we examined repair of two tandem noncohesive DSBs generated by endonuclease I-SceI and the 3' nonprocessive exonuclease Trex2, which can be expressed as an I-SceI-Trex2 fusion. We examined end joining (EJ) repair that maintains correct ends (proximal-EJ) versus using incorrect ends (distal-EJ), which provides a relative measure of incorrect end use (distal end use). Previous studies showed that ATM is important to limit distal end use. Here we show that DNA-PKcs kinase activity and RAD50 are also important to limit distal end use, but that H2AX is dispensable. In contrast, we find that ATM, DNA-PKcs, and RAD50 have distinct effects on repair events requiring end processing. Furthermore, we developed reporters to examine the effects of the transcription context on DSB repair, using an inducible promoter. We find that a DSB downstream from an active promoter shows a higher frequency of distal end use, and a greater reliance on ATM for limiting incorrect end use. Conversely, DSB transcription context does not affect end processing during EJ, the frequency of homology-directed repair, or the role of RAD50 and DNA-PKcs in limiting distal end use. We suggest that RAD50, DNA-PKcs kinase activity, and transcription context are each important to limit incorrect end use during EJ repair of multiple DSBs, but that these factors and conditions have distinct roles during repair events requiring end processing.

Primary immunodeficiency diseases associated with neurologic manifestations

Primary immunodeficiency diseases (PID) are a heterogeneous group of inherited disorders of the immune system, predisposing individuals to recurrent infections, allergy, autoimmunity, and malignancies. A considerable number of these conditions have been found to be also associated with neurologic signs and symptoms. These manifestations are considered core features of some immunodeficiency syndromes, such as ataxia-telangiectasia and purine nucleoside phosphorylase deficiency, or occur less prominently in some others. Diverse pathological mechanisms including defective responses to DNA damage, metabolic errors, and autoimmune phenomena have been associated with neurologic abnormalities; however, several issues remain to be elucidated. Greater awareness of these associated features and gaining a better understanding of the contributing mechanisms will lead to prompt diagnosis and treatment and possibly development of novel preventive and therapeutic strategies. In this review, we aim to provide a brief description of the clinical and genetic characteristics of PID associated with neurologic complications.

The neuro-glial-vascular interrelations in genomic instability symptoms

A hallmark of neurodegenerative diseases is impairment of certain aspects of "brain functionality", which is defined as the total input and output of the brain's neural circuits and networks. A given neurodegenerative disorder is characterized by affected network organization and topology, cell numbers, cellular functionality, and the interactions between neural circuits. Neuroscientists generally view neurodegenerative disorders as diseases of neuronal cells; however, recent advances suggest a role for glial cells and an impaired vascular system in the etiology of certain neurodegenerative diseases. It is now clear that brain pathology is, to a very great extent, pathology of neurons, glia and the vascular system as these determine the degree of neuronal death as well as the outcome and scale of the neurological deficit. This review article is focused on the intricate interrelations among neurons, glia, the vascular system, neuronal cells, and the DNA damage response. Here I describe various aspects of neural and glial cell fate and the vascular system in genomic instability disorders including ataxia telangiectasia (A-T) and Nijmegen breakage syndrome.

Introduction to telomeres and telomerase

Telomeres are ends of chromosomes that play an important part in the biology of eukaryotic cells. Through the coordinated action of the telomerase and networks of other proteins and factors, the length and integrity of telomeres are maintained to prevent telomere dysfunction that has been linked to senescence, aging, diseases, and cancer. The tools and assays being used to study telomeres are being broadened, which has allowed us to derive a more detailed, high-resolution picture of the various players and pathways at work at the telomeres.

Malignancies in the setting of primary immunodeficiency: Implications for hematologists/oncologists

Many primary immunodeficiency disorders (PIDD) are associated with elevated risks for different types of cancer. Defective immunosurveillance mechanisms in PIDD and infection with oncogenic viruses (eg, Epstein Barr, herpesvirus 8) seem to have significant contributory roles in many cases. Non-Hodgkin lymphoma and Hodgkin disease are two of the most common PIDD-associated malignancies. The impact of PIDD-associated malignancy has increased in recent years in parallel with improved patient with PIDD survival and longevity, due largely to effective immunoglobulin replacement therapy. Epidemiologic data, clinical patterns, and management considerations of the common PIDD-associated cancers are reviewed.

ATM limits incorrect end utilization during non-homologous end joining of multiple chromosome breaks

Chromosome rearrangements can form when incorrect ends are matched during end joining (EJ) repair of multiple chromosomal double-strand breaks (DSBs). We tested whether the ATM kinase limits chromosome rearrangements via suppressing incorrect end utilization during EJ repair of multiple DSBs. For this, we developed a system for monitoring EJ of two tandem DSBs that can be repaired using correct ends (Proximal-EJ) or incorrect ends (Distal-EJ, which causes loss of the DNA between the DSBs). In this system, two DSBs are induced in a chromosomal reporter by the meganuclease I-SceI. These DSBs are processed into non-cohesive ends by the exonuclease Trex2, which leads to the formation of I-SceI–resistant EJ products during both Proximal-EJ and Distal-EJ. Using this method, we find that genetic or chemical disruption of ATM causes a substantial increase in Distal-EJ, but not Proximal-EJ. We also find that the increase in Distal-EJ caused by ATM disruption is dependent on classical non-homologous end joining (c-NHEJ) factors, specifically DNA-PKcs, Xrcc4, and XLF. We present evidence that Nbs1-deficiency also causes elevated Distal-EJ, but not Proximal-EJ, to a similar degree as ATM-deficiency. In addition, to evaluate the roles of these factors on end processing, we examined Distal-EJ repair junctions. We found that ATM and Xrcc4 limit the length of deletions, whereas Nbs1 and DNA-PKcs promote short deletions. Thus, the regulation of end processing appears distinct from that of end utilization. In summary, we suggest that ATM is important to limit incorrect end utilization during c-NHEJ.

When a chromosome is fragmented by multiple double-strand breaks (DSBs), each set of DSB ends needs to be matched correctly during repair to avoid chromosomal rearrangements. Considering the case of two tandem DSBs, if the ends of different breaks (incorrect ends) are used for repair, loss of the intervening DNA can occur. Alternatively, when the ends of a single DSB (correct ends) are used for repair, the original order of the chromosome is restored. Deficiencies in the factors ATM and Nbs1, as seen in patients with Ataxia Telangiectasia and Nijmegen Breakage Syndrome, respectively, have been associated with elevated chromosome rearrangements and cancer predisposition. Hence, we examined the possibility that these factors may be important for the usage of correct ends during repair of multiple DSBs. For this, we developed a reporter system to examine end usage during repair of two tandem DSBs in mammalian chromosomes and found that disruption of ATM or Nbs1 leads to elevated usage of incorrect ends. Furthermore, we found that the role of ATM during end usage depends on a repair pathway called classical non-homologous end joining (c-NHEJ). We suggest that ATM suppresses genome rearrangements via limiting incorrect end utilization during c-NHEJ.

High prevalence of primary ovarian insufficiency in girls and young women with Nijmegen breakage syndrome: evidence from a longitudinal study

CONTEXT

Nijmegen breakage syndrome (NBS) is a severe chromosomal instability disorder characterized by microcephaly, growth retardation, immune deficiency, and predisposition for malignancy. It is caused by hypomorphic mutations in the NBN gene, which product belongs to the protein complex critical for processing DNA double-strand breaks during mitotic and meiotic recombination. Data on gonadal function in patients with NBS are limited.

OBJECTIVE

Growth and sexual development, along with hormonal assays, were evaluated in girls and young women with NBS homozygous for c.657_661del5 mutation.

STUDY DESIGN AND PATIENTS

The group comprised 37 girls and young women with NBS (ages, 0.17-24.25 yr), followed between 1993 and 2008. Patients were divided into three age groups: 1) 1-3 yr; 2) 4-9 yr; and 3) 10 yr and older. Growth, puberty, concentrations of gonadotropins and 17-beta-estradiol, bone age, and pelvic ultrasound were assessed.

RESULTS

None of the patients presented a typical growth spurt; the adult height ranged between the 3rd and 25th centiles. Median bone age was delayed by 4.05 yr. Pubarche reached stadium P2 in eight patients and P3 in two patients. In all but one girl, thelarche did not exceed Th2, with low 17beta-estradiol levels. Gonadotropin levels showed a biphasic pattern, with median FSH values of 55.0, 10.9, and 81.9 IU/liter, and LH of 3.2, 0.8, and 21.0 IU/liter in consecutive age groups. Ultrasound visualized small ovaries or solid streaks and the hypoplastic uterus.

CONCLUSIONS

Primary ovarian insufficiency and the associated hypergonadotropic hypogonadism are hallmark manifestations in girls and young women with NBS. Our findings emphasize the need for long-term endocrinological and interdisciplinary supervision of these patients.

Associations between NBS1 polymorphisms, haplotypes and smoking-related cancers

Constituents of tobacco smoke can cause DNA double-strand breaks (DSBs), leading to tumorigenesis. The NBS1 gene product is a vital component in DSB detection and repair, thus genetic variations may influence cancer development. We examined the associations between NBS1 polymorphisms and haplotypes and newly incident smoking-related cancers in three case-control studies (Los Angeles: 611 lung and 601 upper aero-digestive tract (UADT) cancer cases and 1040 controls; Memorial Sloan-Kettering Cancer Center: 227 bladder cancer cases and 211 controls and Taixing, China: 218 esophagus, 206 stomach, 204 liver cancer cases and 415 controls). rs1061302 was associated with cancers of the lung [adjusted odds ratio (OR(adj)) = 1.6, 95% confidence interval (CI): 1.2, 2.4], larynx (OR(adj) = 0.56, 95% CI: 0.32, 0.97) and liver (OR(adj) = 1.7, 95% CI: 1.0, 2.9). Additionally, positive associations were found for rs709816 with bladder cancer (OR(adj) = 4.2, 95% CI: 1.4, 12) and rs1063054 with lung cancer (OR(adj) = 1.6, 95% CI: 1.0, 2.3). Some associations in lung and stomach cancers varied with smoking status. CAC haplotype was positively associated with smoking-related cancers: lung (OR(adj) = 1.7, 95% CI: 1.1, 2.9) and UADT (OR(adj) = 2.0, 95% CI: 1.1, 3.7), specifically, oropharynx (OR(adj) = 2.1, 95% CI: 1.0, 4.2) and larynx (OR(adj) = 4.8, 95% CI: 1.7, 14). Bayesian false-discovery probabilities were calculated to assess Type I error. It appears that NBS1 polymorphisms and haplotypes may be associated with smoking-related cancers and that these associations may differ by smoking status. Our findings also suggest that single-nucleotide polymorphisms located in the binding region of the MRE-RAD50-NBS1 complex or microRNA targeted pathways may influence tumor development. These hypotheses should be further examined in functional studies.

Chronic noninfectious necrotizing granulomas in a child with Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is a chromosomal breakage disorder with characteristic physical features, chromosomal instability, and combined immunodeficiency. It is closely related to other chromosomal breakage disorders like ataxia telangiectasia. Noninfectious granulomatous inflammation refractory to treatment is a relatively common feature in ataxia telangiectasia. Herein we report a patient with NBS who developed chronic refractory necrotizing granulomatous ulcerations and review the pathophysiology of NBS and noninfectious granulomas in primary immunodeficiency syndromes.

A divalent FHA/BRCT-binding mechanism couples the MRE11-RAD50-NBS1 complex to damaged chromatin

The MRE11-RAD50-NBS1 (MRN) complex accumulates at sites of DNA double-strand breaks in large chromatin domains flanking the lesion site. The mechanism of MRN accumulation involves direct binding of the Nijmegen breakage syndrome 1 (NBS1) subunit to phosphorylated mediator of the DNA damage checkpoint 1 (MDC1), a large nuclear adaptor protein that interacts directly with phosphorylated H2AX. NBS1 contains an FHA domain and two BRCT domains at its amino terminus. Here, we show that both of these domains participate in the interaction with phosphorylated MDC1. Point mutations in key amino acid residues of either the FHA or the BRCT domains compromise the interaction with MDC1 and lead to defects in MRN accumulation at sites of DNA damage. Surprisingly, only mutation in the FHA domain, but not in the BRCT domains, yields a G2/M checkpoint defect, indicating that MDC1-dependent chromatin accumulation of the MRN complex at sites of DNA breaks is not required for G2/M checkpoint activation.

[Nijmegen Breakage syndrome]

Nijmegen Breakage syndrome is a rare, autosomal recessive disorder characterized by severe, combined immunodeficiency, recurrent sinopulmonary infections, chromosomal instability, radiosensitivity, predisposition to malignancy, a "bird-like" facial appearance, progressive microcephaly, short stature, and mental retardation. The syndrome is caused by mutations in the NBS1 gene, which encodes a DNA-repair protein, named nibrin. The authors summarize current knowledge on molecular genetics, diagnostic characteristics and therapeutic options of this inborn error of innate immunity.

Nijmegen breakage syndrome: Long-term monitoring of viral and immunological biomarkers in peripheral blood before development of malignancy

Selected viruses and immune parameters were monitored in 57 patients with Nijmegen breakage syndrome as a proposed tool for early detection of changes preceding development of malignancy. The following parameters were analysed: (1) viral infections; (2) monoclonal proteins; and (3) B-cell and T-cell receptor gene rearrangements in peripheral blood lymphocytes. Viral infections were detected in 68.4% of patients with a predominance of EBV (63.2%), followed by HBV (19.2%) and HCV (8.8%). Monoclonal gammopathy detected in 38.6% of cases correlated with the presence of EBV DNA (p=0.002) and HCV RNA (p=0.04). Clonal Ig and/or TCR gene rearrangements occurred in 73.9% of patients. The presence of at least one of the studied parameters preceded the development of malignancy in 22 patients. Systematic PCR analysis for viral infections and Ig/TCR gene rearrangements, supplemented by detection of monoclonal proteins, is advantageous in monitoring NBS patients before severe complications of the disease, including cancer, appear.

Loss of ATM impairs proliferation of neural stem cells through oxidative stress-mediated p38 MAPK signaling

Ataxia-telangiectasia (A-T) is a genetic disorder caused by a mutation of the Atm gene, which controls DNA repair, cell cycling, and redox homeostasis. Even though oxidative stress has been implicated in the neurological anomalies in A-T, the effects of ATM loss on neural stem cell (NSC) survival has remained elusive. In this study, we investigated the effects of oxidative stress on NSC proliferation in an animal model for A-T neurodegeneration. We found that cultured subventricular zone neurosphere cells from Atm(-/-) mice show impaired proliferation, as well as intrinsic elevation of reactive oxygen species (ROS) levels, compared with those from Atm(+/+) mice. We also show that increasing the levels of ROS by H(2)O(2) treatment significantly reduces Atm(+/+) neurosphere formation and proliferation. In Atm(-/-) neurosphere cells, the Akt and Erk1/2 pathways are disrupted, together with enhanced activity of the p38 mitogen-activated protein kinase (MAPK). Treatment of these cells with the antioxidant N-acetyl-L-cysteine (NAC) or with a p38 MAPK inhibitor restores normal proliferation and reduced expression of p21(cip1) and p27(kip1) in the Atm(-/-) NSCs. These observations indicate that ATM plays a crucial role in NSC proliferation, by activating Akt and Erk1/2 pathways and by suppressing ROS-p38 MAPK signaling. Together, our results suggest that p38 MAPK signaling acts as a negative regulator of NSC proliferation in response to oxidative stress. These findings suggest a potential mechanism for neuronal cell loss as a result of oxidative stress in NSCs in progressive neurodegenerative diseases such as A-T.

Genetic variation in the NBS1, MRE11, RAD50 and BLM genes and susceptibility to non-Hodgkin lymphoma

BACKGROUND

Translocations are hallmarks of non-Hodgkin lymphoma (NHL) genomes. Because lymphoid cell development processes require the creation and repair of double stranded breaks, it is not surprising that disruption of this type of DNA repair can cause cancer. The members of the MRE11-RAD50-NBS1 (MRN) complex and BLM have central roles in maintenance of DNA integrity. Severe mutations in any of these genes cause genetic disorders, some of which are characterized by increased risk of lymphoma.

METHODS

We surveyed the genetic variation in these genes in constitutional DNA of NHL patients by means of gene re-sequencing, then conducted genetic association tests for susceptibility to NHL in a population-based collection of 797 NHL cases and 793 controls.

RESULTS

114 SNPs were discovered in our sequenced samples, 61% of which were novel and not previously reported in dbSNP. Although four variants, two in RAD50 and two in NBS1, showed association results suggestive of an effect on NHL, they were not significant after correction for multiple tests.

CONCLUSION

These results suggest an influence of RAD50 and NBS1 on susceptibility to diffuse large B-cell lymphoma and marginal zone lymphoma. Larger association and functional studies could confirm such a role.

NBN 657del5 heterozygous mutations and colorectal cancer risk in the Czech Republic

The most frequent Nijmegen breakage syndrome (NBS)-causing mutation is a 5-base pair deletion in gene coding for nibrin (NBN 657del5), which results in a non-fully functional protein product and is particularly frequent in Central and Eastern Europe. Recent studies have investigated whether NBN 657del5 carriage may predispose to an increased risk of different types of cancer. The Czech Republic has one of the highest incidences of colorectal cancer in the world as well as high incidence of NBS. To assess whether NBN 657del5 associates with an increased risk of sporadic colorectal cancer, we have screened 771 colorectal cancer patients, 614 controls with negative colonoscopy and 818 healthy blood donors from the Czech Republic. There were no significant differences between the frequencies of heterozygous carriers among the three groups. The present results do not provide any evidence that the exceeding risk of CRC in this population is attributable to the high frequency of heterozygous carriage of the NBN 657del5.

Human RAD50 deficiency in a Nijmegen breakage syndrome-like disorder

The MRE11/RAD50/NBN (MRN) complex plays a key role in recognizing and signaling DNA double-strand breaks (DSBs). Hypomorphic mutations in NBN (previously known as NBS1) and MRE11A give rise to the autosomal-recessive diseases Nijmegen breakage syndrome (NBS) and ataxia-telangiectasia-like disorder (ATLD), respectively. To date, no disease due to RAD50 deficiency has been described. Here, we report on a patient previously diagnosed as probably having NBS, with microcephaly, mental retardation, 'bird-like' face, and short stature. At variance with this diagnosis, she never had severe infections, had normal immunoglobulin levels, and did not develop lymphoid malignancy up to age 23 years. We found that she is compound heterozygous for mutations in the RAD50 gene that give rise to low levels of unstable RAD50 protein. Cells from the patient were characterized by chromosomal instability; radiosensitivity; failure to form DNA damage-induced MRN foci; and impaired radiation-induced activation of and downstream signaling through the ATM protein, which is defective in the human genetic disorder ataxia-telangiectasia. These cells were also impaired in G1/S cell-cycle-checkpoint activation and displayed radioresistant DNA synthesis and G2-phase accumulation. The defective cellular phenotype was rescued by wild-type RAD50. In conclusion, we have identified and characterized a patient with a RAD50 deficiency that results in a clinical phenotype that can be classified as an NBS-like disorder (NBSLD).