ATM mutations in sporadic lymphoid tumours

Chromosome abnormalities with prognostic impact in B-cell chronic lymphocytic leukemia

The detailed analysis of the biologic features led to a rapid increase in clinically relevant information in CLL. The recognition of the prognostic role of IgVH hypermutation status and related phenotypic changes (CD38, ZAP-70 expression) as well as of chromosome abnormalities defined by cytogenetic analysis enabled a refined classification of the disease. Improvements in karyotyping and the introduction of fluorescence in situ hybridization (FISH) in routine hematological diagnostics raised the detection rate of chromosomal aberrations to approx. 60-80% in CLL. Among them, deletions of 17p and 11q have been associated with unfavorable prognosis. The deletion of the p53 locus (17p13) was described as the strongest independent predictor for aggressive behavior, resistance to chemotherapy and early death. On the contrary, an isolated deletion at 13q14 or a normal karyotype was related with a long survival. Classical and molecular cytogenetic analysis became an important tool for individual risk estimation. Unlike any other approaches, cytogenetic monitoring reflects the genetic heterogeneity and clonal growth dynamics during the course of the disease.

Molecular pathology of ataxia telangiectasia

Ataxia telangiectasia (A-T) is one of a group of autosomal recessive cerebellar ataxias. Presentation is usually by the age of 2 years and ataxia of both upper and lower limbs develops, such that by early teenage most patients require a wheelchair for mobility. Speech and eye movement are also affected. Other important features are t(7;14) translocations, immunodeficiency, a high serum alpha fetoprotein concentration, growth retardation, telangiectasia-most noticeably on the bulbar conjunctiva-and a very high risk of developing a lymphoid tumour. Patients also show an increased sensitivity to ionising radiation. The classic form of A-T results from the presence of two truncating ATM mutations, leading to total loss of the ATM protein, a protein kinase. Importantly, A-T shows clinical heterogeneity, including milder forms where neurological progression may be slower or of later onset. In these cases there is a correlation between the preservation of neurological function, decreased radiosensitivity, and the degree of retained ATM protein kinase activity. Considerable scope remains for understanding the progress of the disorder in relation to the types of ATM mutation present.

Insights into the multistep transformation process of lymphomas: IgH-associated translocations and tumor suppressor gene mutations in clonally related composite Hodgkin's and non-Hodgkin's lymphomas

Clonally related composite lymphomas of Hodgkin's lymphoma (HL) and Non-Hodgkin's lymphoma (NHL) represent models to study the multistep transformation process in tumorigenesis and the development of two distinct tumors from a shared precursor. We analyzed six such lymphomas for transforming events. The HLs were combined in two cases with follicular lymphoma (FL), and in one case each with B-cell chronic lymphocytic leukemia, splenic marginal zone lymphoma, mantle cell lymphoma (MCL) and diffuse large B-cell lymphoma (DLBCL). In the HL/FL and HL/MCL combinations, BCL2/IGH and CCND1/IGH translocations, respectively, were detected in both the HL and NHL. No mutations were found in the tumor suppressor genes FAS, NFKBIA and ATM. The HL/DLBCL case harbored clonal replacement mutations of the TP53 gene on both alleles exclusively in the DLBCL. In conclusion, we present the first examples of molecularly verified IgH-associated translocations in HL, which also show that BCL2/IGH or CCND1/IGH translocations can represent early steps in the pathogenesis of composite HL/FL or HL/MCL. The restriction of the TP53 mutations to the DLBCL in the HL/DLBCL case exemplifies a late transforming event that presumably happened in the germinal center and affected the fate of a common lymphoma precursor cell towards development of a DLBCL.

Carrier frequency of mutation 657del5 in the NBS1 gene in a population of polish pediatric patients with sporadic lymphoid malignancies

Nijmegen breakage syndrome (NBS) is a human autosomal recessive disease characterized by genomic instability and enhanced cancer predisposition, in particular to lymphoma and leukemia. Recently, significantly higher frequencies of heterozygous carriers of the Slavic founder NBS1 mutation, 657del5, were found in Russian children with sporadic lymphoid malignancies, and in Polish adults with non-Hodgkin lymphoma (NHL). In addition, the substitution 643C>T (R215W) has also been found in excess among children with acute lymphoblastic leukemia (ALL). In an attempt to asses the contribution of both mutations to the development of sporadic lymphoid malignancies, we analyzed DNA samples from a large group of Polish pediatric patients. The NBS1 mutation 657del5 on one allele was found in 3 of 270 patients with ALL and 2 of 212 children and adolescents with NHL; no carrier was found among 63 patients with Hodgkin lymphoma (HL). No carriers of the variant R215W were detected in any studied group. The relative frequency of the 657del5 mutation was calculated from a total of 6,984 controls matched by place of patient residence, of whom 42 were found to be carriers (frequency = 0.006). In the analyzed population with malignancies, an increased odds ratio for the occurrence of mutation 657del5 was found in comparison with the control Polish population (OR range 1.48-1.85, 95% confidence interval 1.18-2.65). This finding indicates that the frequency of the mutation carriers was indeed increased in patients with ALL and NHL (p < 0.05). Nonetheless, NBS1 gene heterozygosity is not a major risk factor for lymphoid malignancies in childhood and adolescence.

The Ataxia-telangiectasia mutated gene and breast cancer: gene expression profiles and sequence variants

The role of the Ataxia-telangiectaisa mutated (ATM) gene, as a risk factor for breast cancer has been a consistent theme in the literature since the first reports by Swift and colleagues who reported that ATM heterozygotes in AT families had increased risks of developing breast cancer. Loss of heterozygosity at the ATM locus has been reported in 30-40% of breast tumours and 50-70% show altered ATM protein levels. Germline ATM sequence variants have been reported in breast cancer cases, however, it is difficult to fully evaluate the increased risk associated with their presence. The potential role of such variants needs to be further assessed, together with functional studies to model their impact on ATM function.

Pathogenesis of Hodgkin's lymphoma

In Hodgkin's lymphoma (HL), the B cell origin of the tumour cells, the Hodgkin and Reed-Sternberg (HRS) cells, has been disclosed by molecular single cell analysis about 10 yr ago. This finding formed the basis for various studies aimed to better understand the pathogenesis of this peculiar malignancy and the pathophysiology of the HRS cells. Work of our groups in this regard was focussed recently on two main topics, namely the study of differential gene expression in HRS cells and the pathogenesis of composite lymphomas. Composite lymphomas are combinations of HL and B cell non-Hodgkin lymphomas, that turned out to be often clonally related. By molecular analysis of several composite lymphomas for potential transforming events, we identified examples of both shared as well as distinct transforming events. Comparing gene expression profiles of HL-derived cell lines with the corresponding profiles from other B cell lymphomas and normal B cell subsets revealed a global down-regulation of the B cell-specific gene expression signature in HRS cells. Moreover, we identifed aberrant expression and activity of multiple receptor tyrosine kinases in HRS cells of classical and to a lesser extend lymphocyte predominant HL, which appears to be a unique feature of HL, and may offer novel strategies for treatment.

ATM and ataxia telangiectasia

Ataxia telangiectasia (AT) has long intrigued the biomedical research community owing to the spectrum of defects that are characteristic of the disease, including neurodegeneration, immune dysfunction, radiosensitivity and cancer predisposition. Following the identification of mutations in ATM (ataxia telangiectasia, mutated) as the underlying cause of the disease, biochemical analysis of this protein kinase has shown that it is a crucial nexus for the cellular response to DNA double-stranded breaks. Many ATM kinase substrates are important players in the cellular responses that prevent cancer. Accordingly, AT is a disease that results from defects in the response to specific types of DNA damage. Thus, although it is a rare neurodegenerative disease, understanding the biology of AT will lead to a greater understanding of the fundamental processes that underpin cancer and neurodegeneration.

H2AX: the histone guardian of the genome

At close hand to one's genomic material are the histones that make up the nucleosome. Standing guard, one variant stays hidden doubling as one of the core histones. But, thanks to its prime positioning, a variation in the tail of H2AX enables rapid modification of the histone code in response to DNA damage. A role for H2AX phosphorylation has been demonstrated in DNA repair, cell cycle checkpoints, regulated gene recombination events, and tumor suppression. In this review, we summarize what we have learned about this marker of DNA breaks, and highlight some of the questions that remain to be elucidated about the physiological role of H2AX. We also suggest a model in which chromatin restructuring mediated by H2AX phosphorylation serves to concentrate DNA repair/signaling factors and/or tether DNA ends together, which could explain the pleotropic phenotypes observed in its absence.

Single-cell analysis of loss of heterozygosity at theATM gene locus in Hodgkin and Reed-Sternberg cells of Hodgkin's lymphoma:ATM loss of heterozygosity is a rare event

Hodgkin's lymphoma (HL) is a lymphoid malignancy characterized by the presence of rare neoplastic cells, Hodgkin and Reed-Sternberg (HRS) cells, scattered among a predominant population of inflammatory cells. On the basis of previously reported cytogenetic analyses, the ATM (ataxia-telangiectasia mutated) gene at 11q22-23 has been implicated in the etiology of HL. We therefore developed a single-cell PCR approach to detect ATM loss of heterozygosity (LOH) in HRS cells. Three microsatellites were investigated; 1 localized inside the ATM gene and the remaining 2 in close proximity. In 2 of the 15 lymph node samples, an allelic loss of the ATM gene locus was detected. ATM protein expression was examined in 8 cases (including 1 of the 2 cases with LOH) by immunohistochemistry. In the case associated with an allelic loss, the ATM protein was absent in the HRS cells, whereas in the 7 remaining cases, without detectable LOH at the ATM locus, nuclear ATM expression was observed. In the 2 HL cases with LOH, the ATM gene was sequenced following whole genome amplification of DNA isolated from microdissected HRS cells. In 1 of these 2 cases, a splice site mutation in the second ATM allele was found. This mutation could generate a premature termination codon leading to a marked instability and a rapid degradation of the resulting ATM mRNA transcripts. This latter event could explain the loss of the expression of the ATM protein in HRS cells as detected by immunohistochemistry in this particular case. As previously reported in some B-cell lymphomas, our results suggest that ATM genetic anomalies could play a role in the pathogenesis of a subset of HL cases.

Control of Atm-/- thymic lymphoma cell proliferation in vitro and in vivo by dexamethasone

AIM

Ataxia telangiectasia (A-T) is an autosomal recessive disease in humans caused by mutations in the Atm (A-T mutated) gene. The disease involves multiple organ systems, and is associated with a high incidence of leukemias and lymphomas that develop in childhood. We have reported previously that thymic lymphoma development in Atm knockout (Atm-/-) mice is associated with elevated spontaneous DNA synthesis in thymocytes, and that dexamethasone (Dex) attenuates the elevated DNA synthesis and prevents thymic lymphoma development. The primary objectives of the present study were (1) to investigate possible mechanisms underlying the tumor-suppressing effect of Dex on Atm-/- thymic lymphoma cells, and (2) to determine whether Dex is an effective tumor-suppressing treatment in mice bearing transplanted Atm-/- thymic tumors.

METHODS

Establishment of a number of Atm-/- thymic lymphoma (ATL) cell lines from Atm-/- mice, cell proliferation assays, cell cycle analyses, Western blotting and Hoechst nuclear staining were used to analyze the effects of Dex on Atm-/- thymic lymphoma cells. Atm-/- tumor cells were transplanted into the right flanks of Atm+/+ mice prior to the initiation of Dex treatment.

RESULTS

Atm-/- tumor cells were highly sensitive to Dex, both in culture and in vivo as ectopic tumors in mice. In cultured ATL-1 cells, Dex induced apoptosis, arrested the cell cycle at the G1 phase and downregulated NF-kappaB and multiple cell cycle regulators, while upregulating the NF-kappaB inhibitor IkappaBalpha. In Atm+/+ mice transplanted subcutaneously with ATL-1 cells, tumor growth was either prevented completely or significantly suppressed by Dex treatment.

CONCLUSIONS

Our findings identify potential mechanisms by which Dex affects the proliferation and survival of ATL-1 cells in culture, and provide evidence that Dex can suppress the proliferation of Atm-/- thymic lymphoma cells growing in the body. Together these results add to our earlier published data suggesting that the cellular pathways regulated by Dex may be promising therapeutic targets for prevention and treatment of thymic lymphomas in A-T individuals.

Cancer risk according to type and location ofATMmutation in ataxia-telangiectasia families

Epidemiological studies have indicated that ataxia-telangiectasia (AT) heterozygotes in AT families have an increased risk of cancer, particularly of breast cancer (BC). However, in BC case-control studies, no significant differences were found in the frequency of ATM mutations between patients and controls. In such studies missense mutations were found more frequently than truncating mutations, suggesting that the cancer risk depends on mutation type. To investigate this possibility, we assessed the risk of BC according to the type and position of the ATM truncating mutation in extended AT families. DNA or RNA that had been isolated from blood or buccal cells of AT children and their relatives was screened for ATM germ-line mutations using restriction endonuclease fingerprinting, the protein truncation test, fluorescence-assisted mismatch analysis, and direct sequencing. The standardized incidence ratio of cancer associated with ATM heterozygosity status and type of mutation was estimated. We tested for genotype-phenotype correlations by simulations, permuting mutations among parental branches. No significant difference was found in the relative risk of breast cancer or any other type of cancer based on mutation type. However, the occurrence of BC may be associated with truncating mutations in certain binding domains of the ATM protein (e.g., P53/BRCA1, beta-adaptin, and FAT domains; P = 0.006). In this limited sample set, the presence of missense or truncating ATM mutations was not associated with different cancer risks. The risk of BC appeared to be associated with the alteration of binding domains rather than with the length of the predicted ATM protein.

Expression of ATM protein and its relationship with p53 in pancreatic carcinoma with tissue array

ATM protein anticipates in the initiation of the DNA repair signal pathway and also mediates cell cycle arrest and repair. ATM deficiency predictably results in radiosensitivity, germ cell degeneration, chromosomal instability, immunodeficiency, and an extreme predisposition to tumors. Moreover, studies found that ATM is the upstream gene of the p53 pathway and would phosphorylate p53 directly after DNA damage, which would suppress tumorigenesis. Expression of ATM and p53 in 167 pancreatic cancer and 101 control specimens, benign lesions, and normal pancreata were detected by high-throughput tissue microarray and immunohistochemistry while seeking the role of ATM in the initiation and development of pancreatic carcinoma as well as its relationship with p53. We found that the positive rates of ATM and p53 expression in pancreatic carcinoma and its relative control specimen were 67.7% (113/167) and 82.2% (83/101) (P < 0.05) and 57.5% (96/167) and 5.0% (5/101) (P < 0.01), respectively. ATM positive staining is significantly relative to age and infiltration (P < 0.05), while the expression of p53 was significantly associated with tumor differentiation, lymph node metastasis, and nerve infiltration (P < 0.05). Expression of ATM and p53 was positively correlated. These findings suggest that expression of ATM deficiency may increase the transformative ability of pancreatic cancer cells. ATM may also cooperate with p53 in the repair of cell damage.

H2AX haploinsufficiency modifies genomic stability and tumor susceptibility

Histone H2AX becomes phosphorylated in chromatin domains flanking sites of DNA double-strand breakage associated with gamma-irradiation, meiotic recombination, DNA replication, and antigen receptor rearrangements. Here, we show that loss of a single H2AX allele compromises genomic integrity and enhances the susceptibility to cancer in the absence of p53. In comparison with heterozygotes, tumors arise earlier in the H2AX homozygous null background, and H2AX(-/-) p53(-/-) lymphomas harbor an increased frequency of clonal nonreciprocal translocations and amplifications. These include complex rearrangements that juxtapose the c-myc oncogene to antigen receptor loci. Restoration of the H2AX null allele with wild-type H2AX restores genomic stability and radiation resistance, but this effect is abolished by substitution of the conserved serine phosphorylation sites in H2AX with alanine or glutamic acid residues. Our results establish H2AX as genomic caretaker that requires the function of both gene alleles for optimal protection against tumorigenesis.

ATM and related protein kinases: safeguarding genome integrity

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

Germ-lineATM gene alterations are associated with susceptibility to sporadic T-cell acute lymphoblastic leukemia in children

A major feature of ataxia-telangiectasia (A-T) is an increased risk of cancer, particularly of lymphoid malignancies. We studied ATM gene involvement in leukemic cells derived from 39 pediatric T-cell acute lymphoblastic leukemias (ALLs). Two types of sequence changes--truncating and missense--were identified in 8 T-cell ALL samples: 3 truncating changes, all previously identified in A-T (R35X, -30del215, 2284delCT), and 3 missense variants (V410A, F582L, F1463C) were found, none associated with loss of heterozygosity (LOH). In all patients studied, the mutation was present in the germ-line. A-T carriers, defined by the finding of truncating mutations, were found to be 12.9 times more frequent than in the normal population (P = 0.004). A normally ethnically matched population was screened for the 3 missense variants, and their frequency was significantly more prevalent (4.9-fold excess) than in the normal population (P = 0.03). Our data suggest there is some evidence of an association between missense alterations in the ATM gene and T-cell ALL. A significant difference in the mean age at diagnosis of T-cell ALL was noted between patients harboring an ATM sequence change and those with no change, 5.4 years and 9.7 years, respectively (P = 0.001). No ATM alterations were identified in relapse samples, indicating that ATM does not play a role in disease progression. The high prevalence of germ-line truncating and missense ATM gene alterations among children with sporadic T-cell ALL suggests an association with susceptibility to T-cell acute leukemia and supports the model of predisposition to cancer in A-T heterozygotes.