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Institute of Human Genetics [R. V., A. R., K. Sp.] and Department of Pediatric Oncology/Hematology [G. H., H. G. v. E., K. Se.], Charité, Humboldt-University, 13353 Berlin, Germany, and Molecular Genetics and Gene Mapping Centre, Max-Delbrueck-Centre, 13092 Berlin, Germany [A. R.]
| ABSTRACT |
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| Introduction |
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NBS is a chromosomal instability disorder characterized by microcephaly, immunodeficiency, radiation sensitivity, and high susceptibility to lymphoid malignancy. The gene product, nibrin, is a member of the hMRE11/RAD50 protein complex involved in DNA double-strand break repair and recombination (5) . Recently it has been shown that nibrin and ATM participate in a common pathway (6, 7, 8) . Thus far, two domains have been found in the NH2-terminal region of the proteina FHA and a BRCT, both spanning the first 200 amino acids of nibrin (9) that are also present in a number of other proteins involved in the cell cycle control (10 , 11) . On the basis of epidemiological data, it has been suggested that NBS heterozygotes also have an elevated cancer risk (12) similar to AT or other syndromes associated with immune deficiencies (4) . The findings that the ATM gene is involved in the pathogenesis of B-CLL (13 , 14) and T-cell prolymphocytic leukemia (15) as well as in breast cancer (16) implicate its role as a tumor suppressor gene. The high predisposition of NBS patients to lymphoid malignancy and the fact that the NBS and AT are indistinguishable at the cellular level (17) stimulated us to initiate a study that purposes to answer whether the NBS1 gene is involved in the pathogenesis of ALL and whether it influences the course of the disease and so has its place among the tumor suppressor genes.
| Materials and Methods |
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| Results and Discussion |
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T in exon 3 at nucleotide position 278 leading to mutation S93L was detected in patient 2897 with BCP-ALL. Another transition G
A in exon 3 at nucleotide position 283 leading to D95N was identified in patient 976 with BCP-ALL. Surprisingly, we found that four additional leukemic cell samples from unrelated ALL patients were heterozygous for the A
G at position 511 leading to I171V substitution in exon 5. These three newly identified mutations were not detected among 110 (S93L and D95N)) and among 220 (I171) control chromosomes analyzed. In one additional leukemic cell sample, a transition C
T at position 643 in exon 6 leading to amino acid change R215W was identified. This amino acid substitution was also found in nine probands of Slavic origin from a population-based study performed to estimate the prevalence of the major NBS1 mutation 657del5 (18)
. However, despite the fact that this exchange results in a substitution of a basic to a nonpolar amino acid, it remains unclear whether this is a causative mutation or a rare sequence variant.
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In comparison with patients with wt NBS1, children with mutated NBS1 tend to have late relapses (71.4% versus 45%) and an adverse outcome. Only 3 of 7 children with mutated NBS1 (43%) are in second complete remission relapse in contrast to 23 of 40 with wt NBS1 (58%). No significant differences could be detected between patients with mutated and wt NBS1 regarding sex, age at initial disease (median, 6.3 versus 5.5 years) and relapse (median, 9.2 versus 9.1 years), duration of last remission (2.91 versus 2.5 years), WBC (median, 6.4 versus 5.35 109/liter) and peripheral blast cell count (median, 0.4 versus 0.6 109/liter), site stratification into therapy groups, and frequency of bone marrow transplantation.
Three of the amino acid substitutions found here are of special interest, because they occur in the two currently known domains of nibrin (S93L and D95N in the FHA and I171V in the BRCT), which are believed to be involved in protein-protein interactions and thus are certainly important for nibrin function. In addition, the D95N and I117V substitutions affect amino acids known to be conserved in a number of proteins containing the FHA and BRCT domains (10 , 11) . The mutations described in our study are point mutations, unlike all mutations found thus far in NBS patients, which are truncating mutations and occur downstream of the FHA and BRCT domains (9) . Nevertheless, these point mutations could still inactivate some nibrin functions. Patients with AT also exhibit mostly truncating mutations (19) . In contrast, most of the ATM mutations found in patients with B-CLL, T-cell prolymphocytic leukemia, and breast cancer are missense mutations (13 , 15 , 20) . Given the high incidence of heterozygous NBS1 mutations in relapsed childhood ALL found here, we are presently analyzing a larger cohort of 60 patients with initial ALL. Thus far, we have found two samples heterozygous for the major NBS1 mutation 657del5 and one sample heterozygous for another point mutation, V210F in exon 6. Two of these mutations turned out to be of germ-line origin and one was a somatic mutation, substantiating our observations further.4
To investigate whether the NBS1 gene acts as a tumor suppressor gene, we searched for a second mutation in the seven patients. Because we failed to detect additional sequence variants, we analyzed the seven ALL samples and the corresponding DNA samples from normal cells obtained at remission for loss of heterozygosity with five highly polymorphic markers adjacent to the NBS1 gene. The samples were mostly heterozygous, or there was no difference between the genotypes of the leukemic cell samples and the germ-line DNA for the polymorphisms tested, thus excluding any major deletion of the NBS1 gene (Table 2)
. No loss of heterozygosity of the AT gene has been found in B-CLL patients, although reduction of ATM protein was demonstrated in most of the analyzed cases (13)
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This study was supported by a grant from the Deutsche Forschungsgemeinschaft to Karl Sperling and André Reis and by a grant from the Deutsche Krebshilfe to Karlheinz Seeger and Günter Henze. ![]()
2 To whom requests for reprints should be addressed, at Institute of Human Genetics, Friedrich-Alexander-University Erlangen-Nürnberg, Schwabachanlage 10, 91054 Erlangen, Germany. Phone: 49-9131-8522318; Fax: 49-9131-209297; E-mail: reis{at}humgenet.uni-erlangen.de ![]()
3 The abbreviations used are: ALL, acute lymphoblastic leukemia; NBS, Nijmegen breakage syndrome; AT, ataxia teleangiectasia; ATM, AT-mutated; FHA, fork-head-associated domain; BRCT, breast cancer COOH-terminal domain; B-CLL, chromic lymphocytic leukemia; BCP, B-cell precursor; wt, wild-type. ![]()
Received 2/22/00. Accepted 3/13/01.
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