Nijmegen Breakage Syndrome: A Progress Report

T Lymphocytes in Patients With Nijmegen Breakage Syndrome Demonstrate Features of Exhaustion and Senescence in Flow Cytometric Evaluation of Maturation Pathway

Patients with Nijmegen Breakage Syndrome (NBS) suffer from recurrent infections due to humoral and cellular immune deficiency. Despite low number of T lymphocytes and their maturation defect, the clinical manifestations of cell-mediated deficiency are not as severe as in case of patients with other types of combined immune deficiencies and similar T cell lymphopenia. In this study, multicolor flow cytometry was used for evaluation of peripheral T lymphocyte maturation according to the currently known differentiation pathway, in 46 patients with genetically confirmed NBS and 46 sex and age-matched controls. Evaluation of differential expression of CD27, CD31, CD45RA, CD95, and CD197 revealed existence of cell subsets so far not described in NBS patients. Although recent thymic emigrants and naïve T lymphocyte cell populations were significantly lower, the generation of antigen-primed T cells was similar or even greater in NBS patients than in healthy controls. Moreover, the senescent and exhausted T cell populations defined by expression of CD57, KLRG1, and PD1 were more numerous than in healthy people. Although this hypothesis needs further investigations, such properties might be related to an increased susceptibility to malignancy and milder clinical course than expected in view of T cell lymphopenia in patients with NBS.

Investigating microcephaly

1. Microcephaly is a clinical finding, not a 'disease', and is a crude but trusted assessment of intracranial brain volume. 2. Developmental processes reducing in utero neuron generation present at birth with 'Primary microcephaly'. 3. 'Secondary microcephaly' develops after birth and predominantly reflects dendritic or white matter diseases. 4. Microcephalic conditions have a heterogeneous aetiology, but increasingly genomic tests are available that allow an exact diagnosis.

PC3 (BTG2/TIS21) possible role in chromosome instability syndromes

Chromosome instability syndromes (CIS) are autosomal recessive genetic disorders associated with defects in cell cycle regulation following DNA damage. Although most of the proteins involved in these syndromes have been identified as part of the MRN complex, little is known about their physiological functions and their interactions with other molecules that might explain the wide clinical presentation found in CIS patients. Here we discuss several observations suggesting that PC3 (BTG2/TIS21) - a protein involved in G1-S checkpoint progression control - might play a role in these pathologies.

DNA double-strand break repair, immunodeficiency and the RIDDLE syndrome

DNA double-strand break (DSB) repair is an essential cellular process required to maintain genomic integrity in the face of potentially lethal genetic damage. Failure to repair a DSB can trigger cell death, whereas misrepair of the break can lead to the generation of chromosomal translocations, which is a known causative event in the development or progression of cancer. DSBs can be induced following exposure to certain exogenous agents, such as ionising radiation or radiomimetic chemicals, as well as occurring naturally as intermediates of normal physiological processes, in particular during B and T cell antigen receptor assembly. Human syndromes with deficiencies in DSB repair commonly exhibit immunodeficiency, highlighting the critical nature of this pathway for development and maturation of the immune system. In this article we review the different pathways utilized by the cell to repair DSBs and how an inherited defect in some of the genes that are critical regulators of this process can be the underlying cause of human disorders associated with genome instability and immune system dysfunction. We focus on a newly described human immunodeficiency disorder called radiosensitivity, immunodeficiency dysmorphic features and learning difficulties (RIDDLE) syndrome, with particular reference to the function of the defective gene, RNF168. We also consider the implications of this finding on the mechanisms controlling development of the immune system.

Nijmegen breakage syndrome complicated with primary cutaneous tuberculosis

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive chromosomal instability syndrome characterized by severe immunodeficiency, growth retardation, microcephaly, a distinct facial appearance, and a high predisposition to lymphoid malignancy. We report a 7-year-old white girl with NBS associated with cutaneous tuberculosis. The patient presented with multiple red-brown, centrally scaring plaques on the leg and had neither pulmonary nor systemic manifestation of tuberculosis. Polymerase chain reaction testing using Mycobacterium genus- and Mycobacterium tuberculosis species-specific primers confirmed the clinical diagnosis of cutaneous tuberculosis. This is the first report describing the simultaneous presentation of NBS and cutaneous tuberculosis.

Dual functions of Nbs1 in the repair of DNA breaks and proliferation ensure proper V(D)J recombination and T-cell development

Immunodeficiency and lymphoid malignancy are hallmarks of the human disease Nijmegen breakage syndrome (NBS; OMIM 251260), which is caused by NBS1 mutations. Although NBS1 has been shown to bind to the T-cell receptor alpha (TCRα) locus, its role in TCRβ rearrangement is unclear. Hypomorphic mutations of Nbs1 in mice and patients result in relatively mild T-cell deficiencies, raising the question of whether the truncated Nbs1 protein might have clouded a certain function of NBS1 in T-cell development. Here we show that the deletion of the entire Nbs1 protein in T-cell precursors (Nbs1(T-del)) results in severe lymphopenia and a hindrance to the double-negative 3 (DN3)-to-DN4 transition in early T-cell development, due to abnormal TCRβ coding and signal joints as well as the functions of Nbs1 in T-cell expansion. Chromatin immunoprecipitation (ChIP) analysis of the TCR loci reveals that Nbs1 depletion compromises the loading of Mre11/Rad50 to V(D)J-generated DNA double-strand breaks (DSBs) and thereby affects resection of DNA termini and chromatin conformation of the postcleavage complex. Although a p53 deficiency relieves the DN3→DN4 transition block, neither a p53 deficiency nor ectopic expression of TCRαβ rescues the major T-cell loss in Nbs1(T-del) mice. All together, these results demonstrate that Nbs1's functions in both repair of V(D)J-generated DSBs and proliferation are essential for T-cell development.

[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.

Unique morphological spectrum of lymphomas in Nijmegen breakage syndrome (NBS) patients with high frequency of consecutive lymphoma formation

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder characterized by microcephaly, immunodeficiency, radiation hypersensitivity, chromosomal instability and increased incidence of malignancies. In Poland 105 NBS cases showing mutations in the NBS gene (nibrin, NBN), have been diagnosed, approximately 53% of which have developed cancer, mainly (>90%) lymphoid malignancies. This study is based upon the largest reported group of NBS-associated lymphomas. The predominant lymphoma types found in these 14 NBS children were diffuse large B cell lymphoma (DLBCL) and T cell lymphoblastic lymphoma (T-LBL/ALL), all showing monoclonal Ig/TCR rearrangements. The spectrum of NBS lymphomas is completely different from sporadic paediatric lymphomas and lymphomas in other immunodeficient patients. Morphological and molecular analysis of consecutive lymphoproliferations in six NBS patients revealed two cases of true secondary lymphoma. Furthermore, 9/13 NBS patients with lymphomas analysed by split-signal FISH showed breaks in the Ig or TCR loci, several of which likely represent chromosome aberrations. The combined data would fit a model in which an NBN gene defect results in a higher frequency of DNA misrejoining during double-strand break (DSB) repair, thereby contributing to an increased likelihood of lymphoma formation in NBS patients.

Successful treatment of diffuse large B-cell non-hodgkin lymphoma with modified CHOP (cyclophosphamide/doxorubicin/vincristine/prednisone) chemotherapy and rituximab in a patient with Nijmegen syndrome

A 17-year-old Croatian boy with Nijmegen breakage syndrome (NBS) who developed diffuse large B-cell non-Hodgkin lymphoma is presented. The majority of the patients with this rare autosomal recessive disease are of Slavic origin and, in most of them, the disease is caused by NBS1 mutation 657del5, as was found in our patient. Nijmegen breakage syndrome is characterized by microcephaly, growth retardation, abnormal facial appearance, spontaneous chromosomal rearrangements, immunodeficiency, and a high predisposition to cancer development, predominantly lymphoma. Because of increased sensitivity to radiation therapy and chemotherapy, the treatment of malignancies in patients with NBS can be difficult. To our knowledge, our patient is the first with NBS reported in the literature who was successfully treated for diffuse large B-cell lymphoma with the anti-CD20 monoclonal antibody rituximab in addition to a modified dose of CHOP (cyclophosphamide/doxorubicin/vincristine/prednisone) chemotherapy. He has been in complete remission for 3 years after finishing the treatment.

Nijmegen Breakage Syndrome

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disease, characterized by microcephaly, growth retardation, immunodeficiency, chromosome instability, radiation sensitivity, and a strong predisposition to lymphoid malignancy. The gene responsible for the development of this syndrome (NBS1) was mapped on chromosome 8q21. The product of this gene--nibrin--is a protein with 95 kDa molecular weight (p95). The same mutation in the NBS1 gene (deletion 657del5) was detected in most of the evaluated patients. In this chapter, we describe the analysis of the literature and our results on clinical and immunological features and genetic evaluation of 21 NBS patients.

The role of NBS1 in DNA double strand break repair, telomere stability, and cell cycle checkpoint control

The genomes of eukaryotic cells are under continuous assault by environmental agents and endogenous metabolic byproducts. Damage induced in DNA usually leads to a cascade of cellular events, the DNA damage response. Failure of the DNA damage response can lead to development of malignancy by reducing the efficiency and fidelity of DNA repair. The NBS1 protein is a component of the MRE11/RAD50/NBS1 complex (MRN) that plays a critical role in the cellular response to DNA damage and the maintenance of chromosomal integrity. Mutations in the NBS1 gene are responsible for Nijmegen breakage syndrome (NBS), a hereditary disorder that imparts an increased predisposition to development of malignancy. The phenotypic characteristics of cells isolated from NBS patients point to a deficiency in the repair of DNA double strand breaks. Here, we review the current knowledge of the role of NBS1 in the DNA damage response. Emphasis is placed on the role of NBS1 in the DNA double strand repair, modulation of the DNA damage sensing and signaling, cell cycle checkpoint control and maintenance of telomere stability.

NBS1 Knockdown by Small Interfering RNA Increases Ionizing Radiation Mutagenesis and Telomere Association in Human Cells

Hypomorphic mutations which lead to decreased function of the NBS1 gene are responsible for Nijmegen breakage syndrome, a rare autosomal recessive hereditary disorder that imparts an increased predisposition to development of malignancy. The NBS1 protein is a component of the MRE11/RAD50/NBS1 complex that plays a critical role in cellular responses to DNA damage and the maintenance of chromosomal integrity. Using small interfering RNA transfection, we have knocked down NBS1 protein levels and analyzed relevant phenotypes in two closely related human lymphoblastoid cell lines with different p53 status, namely wild-type TK6 and mutated WTK1. Both TK6 and WTK1 cells showed an increased level of ionizing radiation-induced mutation at the TK and HPRT loci, impaired phosphorylation of H2AX (gamma-H2AX), and impaired activation of the cell cycle checkpoint regulating kinase, Chk2. In TK6 cells, ionizing radiation-induced accumulation of p53/p21 and apoptosis were reduced. There was a differential response to ionizing radiation-induced cell killing between TK6 and WTK1 cells after NBS1 knockdown; TK6 cells were more resistant to killing, whereas WTK1 cells were more sensitive. NBS1 deficiency also resulted in a significant increase in telomere association that was independent of radiation exposure and p53 status. Our results provide the first experimental evidence that NBS1 deficiency in human cells leads to hypermutability and telomere associations, phenotypes that may contribute to the cancer predisposition seen among patients with this disease.

Nijmegen breakage syndrome: clinical manifestation of defective response to DNA double-strand breaks

Nijmegen breakage syndrome is a rare autosomal recessive genetic disease belonging to a group of disorders often called chromosome instability syndromes. In addition to a characteristic facial appearance and microcephaly, patients suffering from Nijmegen breakage syndrome have a range of symptoms including radiosensitivity, immunodeficiency, increased cancer risk and growth retardation. 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 framehift and protein truncation. The protein nibrin/Nbs1 is suspected to be involved in the cellular response to DNA damage caused by ionising irradiation, thus accounting for the radiosensitivity of Nijmegen breakage syndrome. We review here some of the more recent findings on the NBS1 gene and discuss how they impinge on the clinical manifestation of the disease.

Molecular disruption of the MRN(95) complex induces radiation sensitivity in head and neck cancer

OBJECTIVES/HYPOTHESIS

The goal of the project was to develop a novel treatment strategy for head and neck cancer that induces radiation sensitivity. We hypothesized that the normal cellular DNA repair response in head and neck squamous cell carcinoma after radiation therapy can be blocked by a dominant negative disruption of the functioning MRN(95) protein complex. To test this hypothesis, we have developed a novel molecular therapy that inhibits the MRN(95) complex in tumor cells. Disruption the MRN(95) complex and thus DNA repair should result in enhanced tumor killing after classic external-beam radiation therapy.

STUDY DESIGN

Experiments with human head and neck squamous cell carcinoma cell lines in vitro were performed.

METHODS

Recombinant adenovirus vectors carrying the genes for enhancing radiation were generated. Human head and neck squamous cell carcinoma cells were treated with recombinant adenovirus vectors carrying the mutated p95 gene (p95-300), which contains the C-terminus 300 amino acids of the Nbs1(p95) protein. Tumor cells were also treated with adenovirus vector carrying full-length p95 protein or DL312 control virus; then all cell lines were subjected to 2 Gy irradiation. Cell growth curves were determined through colorimetric tetrazolium salt assay.

RESULTS

Both the Ad-p95-300 and Ad-p94-his (full-length wild-type gene) demonstrated significant antitumor effect alone and in combination with radiation therapy compared with control samples. Cell cycle analysis demonstrated a shift toward the G2/M phase of the cell cycle. Analysis of telomerase activity demonstrated a significant decrease in telomerase activity after molecular therapy alone, and a greater decrease when combined with radiation therapy.

CONCLUSION

Adenovirus-mediated mutant or full-length p95 molecular therapy demonstrated efficacy for the treatment of head and neck squamous cell carcinoma in vitro. This novel molecular therapy strategy induced significant radiation sensitization, induced a relative G2/M arrest, and decreased telomerase activity, all of which enhance the benefit of radiation therapy.

Recruitment of NBS1 into PML oncogenic domains via interaction with SP100 protein

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder characterized by microcephaly, chromosomal instability, radiation sensitivity, and an increased incidence of malignancies. NBS1, the protein responsible for NBS, forms a complex with MRE11 and RAD50, and plays a vital role in DNA repair, cell cycle checkpoint, and telomere maintenance. Here, we show that a BRCA carboxyl terminus (BRCT) domain-containing region of NBS1 interacts with a nuclear dots-associated protein, SP100. The SP100 and NBS1 proteins co-localized in PODs and APBs in normal human fibroblast MRC5 and ALT line VA13 at G2 phase, respectively. Introduction of PML and SP100 into NT2 cells, which express no detectable amount of PML or SP100 proteins, resulted in localization of NBS1 in ectopically expressed PODs. These results indicate that NBS1 is recruited into PODs via interaction with SP100 protein. Thus, interaction between the NBS1 and SP100 proteins may be involved in genomic stability and telomere maintenance.

Heterogeneity of humoral immune abnormalities in children with Nijmegen breakage syndrome: an 8-year follow-up study in a single centre

During an 8-year period of observation, defects of immune responses were characterized and monitored in 40 of 50 Polish children with Nijmegen breakage syndrome referred to the Children's Memorial Health Institute in Warsaw. The following parameters were determined at diagnosis: (1) concentrations of serum IgM, IgG, IgA; (2) concentrations of IgG subclasses; and (3) lymphocyte subpopulations. In addition, naturally acquired specific antibodies against Streptococcus pneumoniae were determined in 20 patients with a history of recurrent respiratory infections. During follow-up, total serum immunoglobulins and IgG subclasses were monitored systematically in 17 patients who did not receive immunomodulatory therapy. Moreover, anti-HBs antibody response was measured after vaccination of 20 children against HBV. We found that the immune deficiency in NBS is profound, highly variable, with a tendency to progress over time. Systematic monitoring of the humoral response, despite good clinical condition, is essential for early medical intervention.

Chromosome breakage syndromes and cancer

There exist numerous genetic disorders, marked by chromosome instability, that are strikingly associated with various cancers. Both the chromosomal instabilities and neoplastic outcomes are related to abnormalities of DNA metabolism, DNA repair, cell-cycle governance, or control of apoptosis. Among these diseases are ataxia telangectasia and Nijmegen breakage syndrome, with increased incidences of lymphomas. Bloom syndrome, Werner syndrome, and Rothmund-Thompson syndrome, each characterized by a DNA helicase defect, are associated with early incidences of different cancers. Other diseases combining the phenotype of chromosomal instabilities and neoplastic development are Fanconi anemia and breast cancers associated with mutant BRCA1 and BRCA2 genes. The cloning of the encoding genes and the characterization of their products have resulted in partial understanding of the pathways of cellular DNA surveillance and maintenance of genomic rectitude. The exact pathways fully linking the genetic defect mechanisms to the eventual development of various neoplasias remain to be elucidated, but progress in defining the molecular genetics of these entities suggests that many of them are disorders of DNA recombination. Each defect involves a separate protein in these complex pathways.

Differing responses of Nijmegen breakage syndrome and ataxia telangiectasia cells to ionizing radiation

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder. Originally thought to be a variant of ataxia telangiectasia (AT), the cellular phenotype of NBS has been described as almost indistinguishable from that of AT. Since the gene involved in NBS has been cloned and its functions studied, we sought to further characterize its cellular phenotype by examining the response of density-inhibited, confluent cultures of human diploid fibroblasts to irradiation in the G(0)/G(1) phase of the cell cycle. Both NBS and AT cells were markedly sensitive to the cytotoxic effects of radiation. NBS cells, however, were proficient in recovery from potentially lethal damage and exhibited a pronounced radiation-induced G(1)-phase arrest. Irradiated AT cells showed no potentially lethal damage and no G(1)-phase arrest. Both cell types were hypersensitive to the induction of chromosomal aberrations, whereas the distribution of aberrations in irradiated NBS cells was similar to that of normal controls, AT cells showed a high frequency of chromatid-type aberrations. TP53 and CDKN1A (also known as p21(Waf1)) expression was attenuated in irradiated NBS cells, but maximal induction occurred 2 h postirradiation, as was observed in normal controls. The similarities and differences in cellular phenotype between irradiated NBS and AT cells are discussed in terms of the functional properties of the signaling pathways downstream of AT involving the NBS1 and TP53 proteins.

Insulin-like growth factor I receptor is expressed at normal levels in Nijmegen breakage syndrome cells

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder sharing a pleiotropic phenotype with ataxia-telangiectasia (A-T), including increased radiosensitivity and cancer disposition. Insulin-like growth factor I receptor (IGF-IR) expression is reportedly decreased in A-T cells, which is thought to contribute to its increased radiosensitivity. In this study, we investigated whether the same mechanism underlies the radiosensitivity of NBS cells. GM7166VA7 cells lacking NBS1 protein displayed a phenotype of increased radiosensitivity, while the introduction of NBS1 cDNA conferred radioresistance comparable to normal cells. IGF-IR expression levels were essentially the same among normal, NBS, and NBS1-complemented NBS cells. There was no significant difference between NBS and NBS1-complemented cells in activation of major downstream pathways of IGF-IR upon IGF-I stimulation, including phosphatidylinositol-3(') kinase (PI3-K) and mitogen-activated protein kinase (MAPK). Collectively, IGF-IR-related events are unlikely to be disrupted in NBS cells, and therefore, defects in IGF-IR signaling do not explain the increased radiosensitivity of NBS cells.

Decreased immunoglobulin class switching in Nijmegen Breakage syndrome due to the DNA repair defect

Nijmegen breakage syndrome (NBS) is a rare chromosomal-instability syndrome associated with defective DNA repair. Approximately 90% of NBS patients are homozygous for a truncating mutation of the NBS1 gene. As development of the immune system relies on recombination, which involves repair of DNA breaks, one might predict that mutations in the NBS1 gene could cause immunodeficiency. We immunologically investigated the world's largest series of NBS patients (n = 74), confirmed immunodeficiency, and found a discrepancy between relatively normal IgM concentrations, and decreased IgG and IgA concentrations. In addition, a significant relation between low IgA and low IgG levels was found. These data are compatible with a defective class switching in NBS and can be explained by a role of the NBS1 protein in DNA repair, signal transduction, cell cycle regulation or apoptosis.

Homologous recombinational repair of DNA ensures mammalian chromosome stability

The process of homologous recombinational repair (HRR) is a major DNA repair pathway that acts on double-strand breaks and interstrand crosslinks, and probably to a lesser extent on other kinds of DNA damage. HRR provides a mechanism for the error-free removal of damage present in DNA that has replicated (S and G2 phases). Thus, HRR acts in a critical way, in coordination with the S and G2 checkpoint machinery, to eliminate chromosomal breaks before the cell division occurs. Many of the human HRR genes, including five Rad51 paralogs, have been identified, and knockout mutants for most of these genes are available in chicken DT40 cells. In the mouse, most of the knockout mutations cause embryonic lethality. The Brca1 and Brca2 breast cancer susceptibility genes appear to be intimately involved in HRR, but the mechanistic basis is unknown. Biochemical studies with purified proteins and cell extracts, combined with cytological studies of nuclear foci, have begun to establish an outline of the steps in mammalian HRR. This pathway is subject to complex regulatory controls from the checkpoint machinery and other processes, and there is increasing evidence that loss of HRR gene function can contribute to tumor development. This review article is meant to be an update of our previous review [Biochimie 81 (1999) 87].

V(D)J rearrangement in Nijmegen breakage syndrome

Repair of DNA double-strand breaks is essential for maintenance of genomic stability, and is specifically required for rearrangement of immunoglobulin (Ig) and T cell receptor (TCR) loci during development of the immune system. Abnormalities in these repair processes also contribute to oncogenic chromosomal rearrangements that underlie many lymphoid malignancies. Nijmegen breakage syndrome (NBS) is a rare autosomal recessive condition characterized by immunodeficiency, radiation sensitivity, and increased predisposition to lymphoid cancers bearing oncogenic Ig and TCR locus translocations. NBS patients fail to produce nibrin, a protein required for the nuclear localization and function of a DNA repair complex that includes Mre11 and Rad50. Mre11 has biochemical properties that suggest a potential role in V(D)J recombination. We studied V(D)J recombination in NBS cells in vitro and in vivo, using cell lines and peripheral blood leukocyte DNA from NBS patients. We found that NBS cells were competent to rejoin signal substrates with normal efficiency and high fidelity. Coding substrates were similarly rejoined efficiently, and coding end structures appeared normal. In B cells from NBS patients, the spectrums of IgH CDR3 regions were diverse and normally distributed. Moreover, the lengths and composition of Igkappa VJ joins and IgH VDJ joins derived from NBS and normal subjects were indistinguishable. Our data indicate that nibrin plays no essential role in V(D)J recombination and is not required for the generation of an apparently diverse B cell repertoire.

Clinical presentation and mutation identification in the NBS1 gene in a boy with Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder which belongs to the group of inherited chromosomal instability syndromes. The clinical characteristics include severe microcephaly, a dysmorphic facies, and immunodeficiency with predisposition to malignancies. While the cellular characteristics of ataxia teleangiectasia (AT) and NBS are similar, the clinical findings are quite distinct. NBS patients show characteristic microcephaly, which is rare in association with AT and they do not develop ataxia and teleangiectasia. Recently, the gene mutated in NBS has been identified. Here we report a 5-year-old Bosnian boy with severe microcephaly. Because of multiple structural aberrations involving chromosomes 7 and 14 typical for AT (MIM 208900) and NBS (MIM 251260), AT was diagnosed. We suggested the diagnosis of NBS because of the boy's remarkable microcephaly, his facial appearance, and the absence of ataxia and teleangiectasia. DNA analysis was performed and revealed that the boy is homozygous for the major mutation (657de15) in the NBS1 gene. This finding confirms the diagnosis of NBS in our patient and offers the possibility to perform a most reliable prenatal diagnosis in a further pregnancy.

Nijmegen breakage syndrome-associated T-cell-rich B-cell lymphoma: case report

In 1981 Weemaes et al. first described the Nijmegen breakage syndrome (NBS), a rare autosomal recessive disorder characterized by stunted growth, microcephaly, immunodeficiency, spontaneous chromosome instability, and a peculiar predisposition to cancer development. Most NBS-related malignancies are lymphomas, but their pathologic features have rarely been specified. We report here the case of a northern Italian 8-year-old child who, 2 years after the diagnosis of NBS, developed a diffuse large B-cell lymphoma (T cell-rich B-cell lymphoma variant). The histological and immunobiological features of the lymphoma population are analyzed and discussed in detail.

Therapy for non-Hodgkin lymphoma in children with primary immunodeficiency: analysis of 19 patients from the BFM trials

BACKGROUND

Non-Hodgkin lymphomas (NHL) represent an important complication of primary immunodeficiency (ID), posing new therapeutic challenges in this patient population. This study analyzes clinical data and therapy results of pediatric patients with primary ID and NHL in three consecutive NHL-BFM trials.

PROCEDURE

Retrospective analysis of children with primary ID and NHL, treated according to protocol NHL-BFM, was performed regarding clinical presentation, diagnostic features, therapy, and outcome.

RESULTS

From October, 1986, to April, 1997, 19 of 1,413 newly diagnosed patients with NHL were registered as suffering from primary ID. Age at diagnosis of NHL was lower in patients with ID. Six patients suffered from humoral ID, 13 patients from combined ID (ataxia teleangiectasia n = 3; Nijmegen breakage syndrome n = 4; PNP deficiency n = 1; IL2 receptor defect n = 1, other combined ID n = 4). Thirteen lymphomas were of B-cell and six of T-cell-lineage. Four of thirteen patients with combined ID were diagnosed with T-NHL, nine with B-NHL. Two of six patients with humoral ID presented with T-NHL and four with B-NHL. NHL entities differed significantly between ID and non-ID patients (P < or = 0.01): centroblastic and immunoblastic lymphomas (31.6% vs. 8.1%), anaplastic large cell lymphoma (26.3% vs. 10.7%), Burkitt lymphoma and B-ALL (21% vs. 47. 8%). Seventeen patients received polychemotherapy. Therapy-related toxicity was increased in ID- compared to non-ID-patients. Three patients died of sepsis; three died of tumor progression; one patient relapsed; one died of BMT-related toxicity; one died of second malignancy. Ten patients are in first continuous remission after a median follow-up of 4 years.

CONCLUSIONS

Curative treatment of NHL in the presence of primary ID is possible and should be attempted.

Association of BRCA1 with the hRad50-hMre11-p95 complex and the DNA damage response

BRCA1 encodes a tumor suppressor that is mutated in familial breast and ovarian cancers. Here, it is shown that BRCA1 interacts in vitro and in vivo with hRad50, which forms a complex with hMre11 and p95/nibrin. Upon irradiation, BRCA1 was detected in discrete foci in the nucleus, which colocalize with hRad50. Formation of irradiation-induced foci positive for BRCA1, hRad50, hMre11, or p95 was dramatically reduced in HCC/1937 breast cancer cells carrying a homozygous mutation in BRCA1 but was restored by transfection of wild-type BRCA1. Ectopic expression of wild-type, but not mutated, BRCA1 in these cells rendered them less sensitive to the DNA damage agent, methyl methanesulfonate. These data suggest that BRCA1 is important for the cellular responses to DNA damage that are mediated by the hRad50-hMre11-p95 complex.

Mammalian X-ray-sensitive mutants which are defective in non-homologous (illegitimate) DNA double-strand break repair

In all organisms multiple pathways to repair DNA double-strand breaks (DSB) have been identified. In mammalian cells DSB are repaired by two distinct pathways, homologous and non-homologous (illegitimate) recombination. X-ray-sensitive mutants have provided a tool for the identification and understanding of the illegitimate recombination pathway in mammalian cells. Two (sub-)pathways can be distinguished, the first mediated by DNA-PK-dependent protein kinase (DNA-PK), and the second directed by the hMre11/hRad50 complex. A variety of mutants impaired in DSB repair by illegitimate recombination, with mutations in Ku, DNA-PKcs, XRCC4 or nibrin, have been described. Herein, the characterization of these mutants with respect to the impaired cellular function and the molecular defect is provided. Further studies on these mutants, as well as on new mutants impaired in as-of-yet unidentified pathways, should be helpful to a better understanding of DSB repair and of the processes leading to genome instability and cancer.

The hMre11/hRad50 protein complex and Nijmegen breakage syndrome: linkage of double-strand break repair to the cellular DNA damage response

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder characterized by increased cancer incidence, cell cycle checkpoint defects, and ionizing radiation sensitivity. We have isolated the gene encoding p95, a member of the hMre11/hRad50 double-strand break repair complex. The p95 gene mapped to 8q21.3, the region that contains the NBS locus, and p95 was absent from NBS cells established from NBS patients. p95 deficiency in these cells completely abrogates the formation of hMre11/hRad50 ionizing radiation-induced foci. Comparison of the p95 cDNA to the NBS1 cDNA indicated that the p95 gene and NBS1 are identical. The implication of hMre11/hRad50/p95 protein complex in NBS reveals a direct molecular link between DSB repair and cell cycle checkpoint functions.

G2 repair in Nijmegen breakage syndrome: G2 duration and effect of caffeine and cycloheximide in control and X-ray irradiated lymphocytes

Lymphocytes from a patient with the Nijmegen breakage syndrome (NBS/NBS) and his parents (NBS/+) have been analyzed to identify possible disturbances in chromosomal G2 repair. The study included the determination of G2 duration and the analysis of the chromosomal aberration frequencies in lymphocytes with/without caffeine and cyclohexemide (CHM) treatments during G2, under control and X-irradiated conditions. Under control conditions, NBS/NBS lymphocytes showed that the basal chromosomal damage as well as the damage detected in G2, with caffeine treatment, and the G2 duration were higher than cells from an age-matched control. In X-irradiated NBS/NBS lymphocytes, the basal and G2 chromosome aberration frequencies were higher than in the controls; however, no significant differences in G2 duration were detected between these two type of cells. Under X-irradiated conditions, NBS/+ lymphocytes showed that while the level of chromosomal damage in G2 and the duration of this cell cycle phase were similar to the control cells, the frequency of unrepaired chromosomal lesions was higher than in the control lymphocytes. No significant differences in chromosomal damage and G2 duration were detected in NBS/+ lymphocytes compared to the control cells, under control conditions. CHM treatment, which induces an increase in G2 duration, decreased the basal spontaneous and X-ray induced chromosome aberration frequency in NBS/NBS and NBS/+ lymphocytes. These results suggest that NBS lymphocytes might be affected by some disturbances in their ability to extend the G2 duration, which may be influencing their DNA repair efficiency in this phase of the cell cycle.

Genotype-phenotype relationships in ataxia-telangiectasia and variants

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity, and cancer predisposition. A-T cells are sensitive to ionizing radiation and radiomimetic chemicals and fail to activate cell-cycle checkpoints after treatment with these agents. The responsible gene, ATM, encodes a large protein kinase with a phosphatidylinositol 3-kinase-like domain. The typical A-T phenotype is caused, in most cases, by null ATM alleles that truncate or severely destabilize the ATM protein. Rare patients with milder manifestations of the clinical or cellular characteristics of the disease have been reported and have been designated "A-T variants." A special variant form of A-T is A-TFresno, which combines a typical A-T phenotype with microcephaly and mental retardation. The possible association of these syndromes with ATM is both important for understanding their molecular basis and essential for counseling and diagnostic purposes. We quantified ATM-protein levels in six A-T variants, and we searched their ATM genes for mutations. Cell lines from these patients exhibited considerable variability in radiosensitivity while showing the typical radioresistant DNA synthesis of A-T cells. Unlike classical A-T patients, these patients exhibited 1%-17% of the normal level of ATM. The underlying ATM genotypes were either homozygous for mutations expected to produce mild phenotypes or compound heterozygotes for a mild and a severe mutation. An A-TFresno cell line was found devoid of the ATM protein and homozygous for a severe ATM mutation. We conclude that certain "A-T variant" phenotypes represent ATM mutations, including some of those without telangiectasia. Our findings extend the range of phenotypes associated with ATM mutations.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Radiation induction of p53 in cells from Nijmegen breakage syndrome is defective but not similar to ataxia-telangiectasia

p53-mediated signal transduction after exposure to ionizing radiation was examined in cells from patients with Nijmegen breakage syndrome (NBS), an autosomal recessive disease characterized by microcephaly, immunodeficiency, predisposition to malignancy, and a high sensitivity to ionizing radiation. NBS cells accumulated p53 protein in a dose-dependent fashion, with a peak level 2 hrs after irradiation with 5 Gy. However, the maximal level of p53 protein in NBS cells was constantly lower than in normal cells. Moreover, this attenuation of p53 induction was confirmed by decreased levels of p21WAF1 protein, which is transcriptionally regulated by p53 protein. This defective induction of p53 protein in NBS is similar to that in ataxia-telangiectasia (AT), although the induced levels of p53 protein in NBS appeared to be the intermediate between normal cells and AT cells. This moderate p53 induction in NBS cells is consistent with the relatively mild radiation sensitivity and the abnormal cell cycle regulation post-irradiation, as present in NBS. Furthermore, all NBS cell lines used here exhibited time courses of p53 induction similar to normal cells, which is in contrast with p53 induction in AT cells, where the maximum induction shows a delay of approximately 2 hrs compared with normal cells. These evidences suggest a different function of each gene product in an upstream p53 response to radiation-induced DNA damage.

Molecular processes and radiosensitivity

BACKGROUND

DNA double-strand breaks (DSB) are the most genotoxic lesions induced by ionizing radiation. At least 2 different pathways for DSB repair have been identified, homologous and non-homologous recombination.

METHODS

Studies on X-ray-sensitive mutants have led to the identification of several genes involved in processing of DSB in bacteria, yeast and mammalian cells.

RESULTS AND CONCLUSION

In mammalian cells non-homologous recombination is the main pathway for DSB repair, while the role of homologous recombination in DSB repair awaits clarification. It is known that, in addition to DNA repair, other safeguards control the human cellular response to ionizing radiation, such as cell cycle regulation and mechanisms involved in scavenging of free radicals produced by ionizing radiation.

Examination of telomere lengths in muscle tissue casts doubt on replicative aging as cause of progression in Duchenne muscular dystrophy

The mean telomere length (TL) of somatic cells indicates their replicative age. In comparison with normal leukocytes (-0.03 kbp/y, 6.2 kbp at 80 y), we found advanced TL shortening in premature aging due to ataxia-telangiectasia or the Nijmegen chromosomal breakage syndrome. Duchenne muscular dystrophy (DMD) has been related to replicative senescence of satellite cells (SCs) caused by increased fiber turnover. Therefore, we determined TLs in DMD muscle. Because the regenerated fiber nuclei are produced by SCs. telomeres of both fiber and SC nuclei should be shortened. In DMD the SC number is increased. We determined that up to the age of 7 y the sum of fiber and SC nuclei should be large enough (73%) for the detection of TL shortening. Normal muscle fibers have negligible turnover rates, and, as expected, we did not find age-related TL shortening (10-83 y, n = 24, 8.3 +/- 0.5 kbp). Surprisingly, there was only slight TL shortening in patient muscles (DMD, 0.3-4.8 y, n = 4, 8.3 +/- 0.7 kbp; 5-7 y, n = 7, 7.9 +/- 0.4 kbp; limb-girdle muscular dystrophy 2C, 13 y, 7.6 kbp; Becker muscular dystrophy, 7 y, 8.5 kbp). Similarly, the peak positions of the telomere blots varied only slightly (DMD, 10.0 +/- 0.9 kbp; normal: 10.7 +/- 0.9 kbp). In accordance with our TL findings we derived less than 4 annual doublings per SC from published histologic data on DMD.

Chromosome instability with bleomycin and X-ray hypersensitivity in a boy with Nijmegen breakage syndrome

We report on a Mexican boy with microcephaly, short stature, and a high frequency of chromosome aberrations with rearrangements involving chromosomes 7 and 14, typical of ataxia telangiectasia (AT) patients. He had neither ataxia nor telangiectasia, and his immunological status and serum alpha feto protein (AFP) level were normal. Bleomycin hypersensitivity, which has been demon-strated in AT patients, was tested in the patient using AT and normal subjects for comparison. The frequency of spontaneously occurring chromosome aberrations in lymphocyte cultures was significantly higher in the patient and the AT patient than in the normal subject. Four cells from the patient showed structural rearrangements involving chromosomes 7 or 14, with breakpoints typical for AT. When exposed to 5.0 micrograms bleomycin, the lymphocytes from the AT patient showed the highest sensitivity to this agent; our patient had an intermediate sensitivity. In both patients several rearrangements involving chromosomes 7 and 14 were scored, while none were observed in the normal subject. A colony survival assay (CSA) [Huo et al., 1994: Cancer Res 54:2544-2547], using a lymphoblastoid cell line (LCL) derived from our patient, showed a survival fraction (SF) of 7%, which is in the same range as in AT patients. The clinical picture, together with the cytogenetic and radiosensitivity results, suggests that our patient fits the variable spectrum of Nijmegen breakage syndrome.

A variant of the Nijmegen breakage syndrome with unusual cytogenetic features and intermediate cellular radiosensitivity

We report the first Italian case of Nijmegen breakage syndrome (NBS). The proband is an immunodeficient, microcephalic, 11 year old boy with a "bird-like" face. He developed a T cell rich B cell lymphoma. Spontaneous chromosomal instability was detected in T and B lymphocytes and fibroblasts; chromosomes 7 and 14 were only sporadically involved in the rearrangements and no clonal abnormality was present. The patient appeared to be sensitive both to ionising radiation and to bleomycin, although his sensitivity did not reach the level of AT reference cells. After bleomycin treatment, inhibition of DNA synthesis was low when compared with normal cells, but higher than observed in an AT reference strain. Moreover, cell cycle analysis, after drug exposure, showed a progressive reduction in the percentage of S phase cells, but the G1 arrest, found in normal cells, was not observed. On clinical evaluation our patient shares features with NBS subjects, but cytogenetic and cell biological data do not completely overlap with those reported in Nijmegen breakage syndrome. The ethnic origin of our patient might account for these differences, as expression of different allelic forms at the NBS locus.

The defect in the AT-like hamster cell mutants is complemented by mouse chromosome 9 but not by any of the human chromosomes

X-ray sensitive Chinese hamster V79 cells mutants, V-C4, V-E5 and V-G8, show an abnormal response to X-ray-induced DNA damage. Like ataxia telangiectasia (AT) cells, they display increased cell killing, chromosomal instability and a diminished inhibition of DNA synthesis following ionizing radiation. To localize the defective hamster gene (XRCC8) on the human genome, human chromosomes were introduced into the AT-like hamster mutants, by microcell mediated chromosome transfer. Although, none of the human chromosomes corrected the defect in these mutants, the defect was corrected by a single mouse chromosome, derived from the A9 microcell donor cell line. In four independent X-ray-resistant microcell hybrid clones of V-E5, the presence of the mouse chromosome was determined by fluorescent in situ hybridization, using a mouse cot-1 probe. By PCR analysis with primers specific for different mouse chromosomes and Southern blot analysis with the mouse Ldlr probe, the mouse chromosome 9, was identified in all four X-ray-resistant hybrid clones. Segregation of the mouse chromosome 9 from these hamster-mouse microcell hybrids led to the loss of the regained X-ray-resistance, confirming that mouse chromosome 9 is responsible for complementation of the defect in V-E5 cells. The assignment of the mouse homolog of the ATM gene to mouse chromosome 9, and the presence of this mouse chromosome only in the radioresistant hamster cell hybrids suggest that the hamster AT-like mutant are homologous to AT, although they are not complemented by hamster chromosome 11.

Severe microcephaly with normal intellectual development: the Nijmegen breakage syndrome

A brother and sister are described with severe microcephaly of prenatal onset, normal intellectual and motor development, chromosomal breakage and cellular immunodeficiency, which is characteristic of the autosomal recessive condition, Nijmegen breakage syndrome. The proband was a girl who presented at 15 months, with normal developmental milestones and an extremely small head circumference of 36 cm. Twenty per cent of her lymphocytes showed spontaneous translocations involving chromosome 7p13, 7q35, 14q11, and 14q32. The lymphocytes also showed excessive x ray induced chromosome damage. She had T cell lymphopenia, but normal immunoglobulins, and a normal alpha fetoprotein. A brother was born shortly after her diagnosis was made. He also had extreme microcephaly of 28 cm, with similar spontaneous and x ray induced chromosomal breakage, and T cell lymphopenia. Neither child has clinical evidence of immunodeficiency. To test the hypothesis that Nijmegen breakage syndrome and ataxia telangiectasia are allelic disorders, haplotype analysis was carried out in the family using DNA markers spanning the AT locus on chromosome 11q22. The affected boy had a different haplotype from his affected sister. Thus in this family, the Nijmegen breakage syndrome is not allelic to the ataxia telangiectasia locus on chromosome 11q, and the two conditions are genetically distinct. The normal intellect in these children raises questions about normal brain development in the presence of severe microcephaly.

Genetic haplotyping of ataxia-telangiectasia families localizes the major gene to an approximately 850 kb region on chromosome 11q23.1

The genotyping data given localize the major A-T gene to an approximately 850 kb region. They also localize the group A A-T gene (ATA) to a region that contains the approximately 850 kb region. They are compatible with linking A-TFresno to 11q22-23. NBS-V2 does not link to this region. Four non-linking families contain only single affecteds, suggesting that these may be spontaneous mutations rather than evidence for an A-T gene outside the 11q22-23 region. Finally, two other non-linking families contain recombinant haplotypes that are compatible with a second A-T gene at 11q22-23, slightly distal to the approximately 850 kb region. However, convincing evidence for a second gene is still lacking.