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Morphologic transformation of follicular lymphoma is associated with somatic mutation of the translocated Bcl-2 gene.

Follicular lymphoma (FL) is a low-grade B-cell non-Hodgkin's lymphoma (NHL) that frequently transforms into diffuse aggressive NHL. The majority of FLs display a t(14; 18) translocation that places the bcl-2 gene into juxtaposition with the lg heavy-chain (H) gene locus. Morphologically transformed malignant FL cells retain their t(14;18) translocation and may acquire additional genetic abnormalities. We analyzed serial biopsy specimens from eight patients with FL for secondary alterations of the rearranged bcl-2 gene in the breakpoint and open reading frame (ORF) regions. Two cases of FL showed no histologic alteration in the second biopsy, and six cases of FL showed morphologic transformation to diffuse large-cell lymphoma (DLL) in the second biopsy. Polymerase chain reaction (PCR) amplification, cloning, and sequencing of the junctional region of the hybrid bcl-2/IgH genes showed identical nucleotide sequences in multiple biopsy specimens of FL that did not show morphologic transformation. In patients in whom FL cells underwent morphologic transformation, FL and autologous DLL cells displayed identical bcl-2/IgH gene nucleotide sequences in five cases and different sequences in one case. In the case for which FL and DLL cells showed different bcl-2/IgH junctional sequences, DLL cells incorporated larger bcl-2 and Ig-joining (JH) gene fragments than the corresponding FL cells, suggesting that DLL clones developed by a distinct t(14; 18) translocation rather than by alteration of the hybrid bcl-2/IgH gene detected in the FL cells. In all eight cases, neither FL nor DLL cells showed alterations of bcl-2 gene sequences in the breakpoint region, suggesting high conservation of the bcl-2 gene during both t(14; 18) translocation and morphologic transformation of the FL cells. PCR single-strand conformation polymorphism (SSCP) and sequence analyses were performed for identification of structural alterations of the bcl-2 gene in the ORF region corresponding to the 239-amino acid p26-bcl-2 alpha protein. A total of 11 point mutations of the ORF were detected in DLL cells of three transformed NHLs, but no alteration of the ORF was detected in FL cells. Four of 11 mutations, at positions 29, 46, 59, and 106, yielded amino acid replacements. These findings demonstrate that FL and DLL cells may be clonally related or unrelated. They also show that transformation of FL cells may be associated with somatic point mutations of the bcl-2 proto-oncogene ORF sequence resulting in alteration of the p26-bcl-2 alpha gene product.

Amino Acid Sequence↗

Three-dimensional structure determination of an anti-2-phenyloxazolone antibody: the role of somatic mutation and heavy/light chain pairing in the maturation of an immune response.

The three-dimensional structure of the Fab fragment of an anti-2-phenyloxazolone monoclonal antibody (NQ10/12.5) in its native and complexed forms has been determined at 2.8 and 3.0 A resolution, respectively. Identification of hapten-contacting residues has allowed us to evaluate the contribution of individual somatic point mutations to maturation of the immune response. In particular, amino acid residues 34 and 36 of the light chain, which are frequently mutated in antibodies with increased affinity for 2-phenyloxazolone, are shown to interact directly with the hapten. We propose that the strict maintenance of certain amino acid sequences at the potentially highly variable VL-JL and VH-D-JH junctions observed among anti-2-phenyloxazolone antibodies is due largely to structural constraints related to antigen recognition. Finally, the three-dimensional model of NQ10/12.5, which uses the typical light chain of primary response anti-2-phenyloxazolone antibodies but a different heavy chain, allows an understanding of how, by preserving key contact residues, a given heavy chain may be replaced by another, apparently unrelated one, without loss of hapten binding activity and why the V kappa Ox1 germline gene is so frequently selected amongst the other known members of this family.

Amino Acid Sequence↗

Toward a survey of somatic mutation of the NF1 gene in benign neurofibromas of patients with neurofibromatosis type 1.

Neurofibromatosis type 1 (NF1), a common autosomal dominant disorder caused by mutations of the NF1 gene, is characterized by multiple neurofibromas, pigmentation anomalies, and a variety of other possible complications, including an increased risk of malignant neoplasias. Tumorigenesis in NF1 is believed to follow the two-hit hypothesis postulated for tumor-suppressor genes. Loss of heterozygosity (LOH) has been shown to occur in NF1-associated malignancies and in benign neurofibromas, but only few of the latter yielded a positive result. Here we describe a systematic approach of searching for somatic inactivation of the NF1 gene in neurofibromas. In the course of these studies, two new intragenic polymorphisms of the NF1 gene, a tetranucleotide repeat and a 21-bp duplication, could be identified. Three tumor-specific point mutations and two LOH events were detected among seven neurofibromas from four different NF1 patients. Our results suggest that small subtle mutations occur with similar frequency to that of LOH in benign neurofibromas and that somatic inactivation of the NF1 gene is a general event in these tumors. The spectrum of somatic mutations occurring in various tumors from individual NF1 patients may contribute to the understanding of variable expressivity of the NF1 phenotype.

Adult↗

[The comparative analysis of gene and structural somatic mutations in inhabitants of Orel district areas contaminated with radionuclides as a result of Chernobyl accident].

The results of comparative analysis of gene and structural mutations found in peripheral blood lymphocytes of inhabitants of Orel district areas contaminated with radionuclides as a result of Chernobyl accident are presented. The average level of 137Cs contamination in those areas ranged about 22-113 kBq/m2. In the study group was found the enhanced frequency of somatic cells with gene and structural mutations compared with laboratory control level by synchronous applying a T-cell receptor (TCR) loci mutation assay and cytogenetic analysis of unstable aberrations. The case-control comparison was carried out using the measured mutation frequencies and cases of various thyroid gland sickness recognized by ultrasonic examination. The cytogenetic assay did not show the statistical difference between healthy group and subjects with thyroid gland sickness. The average frequency of TCR loci mutation cells in the subjects with thyroid gland sickness was found to be statistically higher comparing with healthy persons. This finding was true for each study region and for Orel district in total. The subgroup of subject exposed in utero in 1986, soon after accident was analyzed. Both cytogenetic and TCR loci mutation assays shown enhancement of average mutation frequency in somatic cells in the subjects of this subgroup with thyroid gland sickness comparing with healthy persons.

Cesium Radioisotopes↗

Somatic mutation in MECP2 as a non-fatal neurodevelopmental disorder in males.

Rett syndrome is a cause of severe learning disability in girls and is associated with a characteristic history and movement disorder. It is an X-linked dominant condition associated with mutations of the MECP2 gene on the distal part of the X-chromosome. If present in a male conceptus, the mutation is usually lethal. We present evidence to show that males can be affected by Rett syndrome. In the boy presented, this situation came about because cells containing the MECP2 mutation existed alongside a normal cell line. Somatic mosaicism could explain the occurrence of other X-linked dominant disorders in males, when they would normally be lethal.

Chromosomal Proteins, Non-Histone↗

Somatic mutations of the translocated bcl-2 gene are associated with morphologic transformation of follicular lymphoma to diffuse large-cell lymphoma.

BACKGROUND: Ninety percent of low-grade follicular lymphomas (FLs) carry the t(14;18) translocation. This event juxtaposes the bcl-2 oncogene to the immunoglobulin (Ig) heavy-chain gene and leads to bcl-2 gene overexpression. Morphologic transformation of FL to high-grade lymphoma is associated with multiple secondary chromosomal abnormalities of the neoplastic cells. DESIGN: To analyze whether additional structural alterations of the translocated bcl-2 gene are associated with morphologic transformation of FL, we PCR-amplified, cloned, and sequenced the major breakpoint region (MBR) and the open reading frames (ORF) of the translocated bcl-2 oncogene in six paired samples of FL and subsequent diffuse large-cell lymphoma (DLL). RESULTS: In five cases, FL and DLL cells were clonally related, as suggested by the identical MBR sequences, but in one case they were different. PCR single-strand conformation polymorphism (SSCP) and sequence analyses were performed for identification of structural alterations of the bcl-2 gene in the OFR region corresponding to the 239 amino acid p26-bcl-2a protein. In three of the six patients, a total of 11 point mutations of the ORF were detected in the DLL cells. Four of them, at positions 29, 46, 59, and 106, yielded amino acid replacements. CONCLUSIONS: These findings demonstrate that FL and DLL cells may be clonally related or unrelated. They also show that transformation of FL cells can be associated with somatic point mutations of the bcl-2 oncogene ORF sequence resulting in alteration of the p26-bcl-2a gene product.

Base Sequence↗

Antibody feedback and somatic mutation in B cells: regulation of mutation by immune complexes with IgG antibody.

In response to an appropriate antigenic stimulus, and with help from T lymphocytes, naive B cells differentiate into plasmacytes which produce the primary (germline-encoded) IgM and IgG antibody with low affinity for the antigen. The isotype switch from IgM to IgG coincides with the burst of germinal center reaction and the onset of somatic hypermutation. Here we propose that formation of immune complexes between the residual antigen and the primary IgG antibody, which activate complement and localize specifically in the network of follicular dendritic cells, provides an important signal for triggering the mutation mechanism in germinal center B cells. This hypothesis has been supported by studies on immunogenicity of immune complexes in vivo. The experiments have included an immunization with pre-formed antigen/IgG antibody complex and/or an administration of IgG antibody shortly after the antigen injection. Either of these strategies, which are known to augment the germinal center formation, resulted in earlier onset of somatic mutation and increased mutation frequency in VDJ rearrangements in antigen-reactive B cells, provided that help from T cells was also present. It is presumed that the antigen/antibody/complement complex is able to deliver this important signal by cross-linking of antigen receptor with the CD21/CD19/CD81 molecules on B cells. As a corollary, the signaling by immune complexes may lower the threshold of cell activation determined by receptor affinity for antigen and stimulate diverse V-gene repertoire of B-cell clones in germinal centers.

Animals↗

Somatic mutation and clonal expansion of B cells in an antigen-driven immune response.

The variable (V) regions of three closely related monoclonal antibodies produced by hybridomas which had been isolated from a single mouse were sequenced at the level of the mRNA. The sequences and the restriction analysis of the immunoglobulin loci carried by the hybridoma cells indicate that the antibodies are derived from cells belonging to a single B cell clone. The sequence data imply a high frequency and stepwise occurrence of somatic point mutations in the expressed V region genes and substantial clonal expansion of B cells in the mouse. The mutations appear to be randomly introduced into heavy and light chain V region genes. Mutations are also seen in the complementarity determining regions which may thus have been involved in the selection of the cells producing the three antibodies.

Amino Acid Sequence↗

Overexpression of the N-ras proto-oncogene, not somatic mutational activation, associated with malignant tumors in transgenic mice.

We have produced transgenic mice that carry a foreign gene construct consisting of the N-ras proto-oncogene driven by the mouse mammary tumor virus (MMTV) long terminal repeat. Overexpression of the normal N-ras gene is associated with development of hyperplasias and tumors in a variety of tissues. The tumors are clearly malignant, as evidenced by the presence of metastatic lesions. Extensive analysis of the foreign ras gene in these tumors by use of polymerase chain reaction and sequencing demonstrates in all cases the absence of somatically acquired mutations at those codons normally associated with activation of the ras genes. Thus, these tumors develop from overexpression of the proto-oncogene rather than the presence of the mutated oncogene. These data demonstrate that overexpression of a protooncogene of the ras family can predispose cells in vivo to fully malignant behavior.

Animals↗

Somatic mutation frequencies in Tradescantia stamen hairs treated with relatively low thermal neutron fluxes.

Young inflorescence-bearing cuttings of Tradescantia clone BNL 02, a blue/pink heterozygote, were treated with thermal neutrons for 12-120 s in the Irradiation Tube set in the Heavy Water Facility of the KUR Reactor of the Research Reactor Institute, Kyoto University. In this facility, practically all components of the neutron flux were thermal neutrons. The thermal neutron fluxes measured simultaneously with gold foils were 2.90-33.3 x 10(10) nth cm-2, and the gamma-ray contamination per 10(10) nth cm-2 was 2.58 x 10(-4) C/kg (1.0 R). The induced somatic pink mutation frequencies in the stamen hairs increased linearly with thermal neutron flux at a rate of 1.69 pink mutant events per 10(4) hair-cell divisions per 10(10) nth cm-2 in Experiments 1 and 2, but at a higher rate of 3.87 pink mutant events per 10(4) hair-cell divisions per 10(10) nth cm-2 in Experiment 3. The relatively small thermal neutron fluxes applied were calculated to have resulted in absorbed doses of 16.8-193 mGy, and the absorbed doses of contaminating gamma rays were 28.0-321 mGy. Therefore, these mutation frequencies could be converted into 109 and 251 pink mutant events per 10(4) hair-cell divisions per gray, including the contaminating gamma rays, and were 2.14-4.92 times higher than the 51.0 pink mutant events per 10(4) hair-cell divisions per gray for relatively small doses of acute X rays which was obtained in earlier studies. The relative biological effectiveness of heavy particles (p, alpha and 7Li) produced by thermal neutrons was calculated to be between 11.0 and 35.4 compared with acute X rays rather than with 18-20-h exposure to gamma rays, the value of 11.0 being considered to be more reliable.

Dose-Response Relationship, Radiation↗

Multiple endocrine neoplasia type 2A with the identical somatic mutation in medullary thyroid carcinoma and pheochromocytoma without germline mutation at the corresponding site in the RET proto-oncogene.

A germline mutation either in exon 10 or 11 of the RET proto-oncogene is found in the majority of patients with multiple endocrine neoplasia type 2A (MEN 2A). A 41-year-old female patient was referred for further evaluation of incidentally discovered right adrenal tumor. She had bilateral adrenal pheochromocytomas and medullary thyroid carcinomas detected by endocrinological and radiological examination, and diagnosed as MEN 2A. Molecular genetic testing of the RET exons 10 and 11 exhibited the identical somatic missense mutation at codon 634 in both tumors but did not confirm germline mutations in the corresponding sites. Possible mechanisms for tumorigenesis in this patient are discussed.

Adrenal Gland Neoplasms↗

A somatic mutation in the 5'UTR of BRCA1 gene in sporadic breast cancer causes down-modulation of translation efficiency.

Mutations in the 5' UTR which cause increment/decrement of translation efficiency have been recently described as a novel molecular mechanism of disease. Alterations in the consensus sequence for the translation initiation may promote context-dependent leaky scanning of ribosomes and/or initiation from a downstream AUG codon. Initiation of translation from a downstream in-frame AUG codon in BRCA1 gene was recently identified in normal cells and possibly in breast cancer. Here we present further insight into BRCA1 translational pathophysiology investigating the role of the canonical structure of the initiation consensus sequence of BRCA1. We have analysed the effect of a somatic point mutation (117 G>C) in position -3 with respect to the AUG of the BRCA1 gene, identified in a highly aggressive sporadic breast cancer. We constructed chimeric genes encoding the luciferase reporter sequence downstream of the wild type or the mutated BRCA1 5'UTR. These transcripts were tested for their activity in in vitro and in vivo systems. In in vitro transcription/translation assays the estimated translation efficiency of the construct with the mutated BRCA1 5'UTR was 30-50% lower than that with the wild type BRCA1 5'UTR. The same chimeric genes were analysed for their expression in vivo by transient transfection in human cells. While the two constructs were equally transcribed, the plasmid carrying the mutated sequence produced 70% less luciferase activity compared to the wild type sequence. Finally, to obtain a direct evaluation on translational efficiency in vivo, we analysed mRNA translation on translationally active and non-active ribosomes separated from transfected cells. Mutant mRNA was partially localized in subpolysomal particles analytically confirming a polysome recruitment defect. Thus, characterization of BRCA1 5'UTR and translation efficiency seems to provide new insight into BRCA1 role in breast and ovarian cancer pathogenesis.

5' Untranslated Regions↗

Survey of somatic mutations in tuberous sclerosis complex (TSC) hamartomas suggests different genetic mechanisms for pathogenesis of TSC lesions.

Tuberous sclerosis complex (TSC), an autosomal dominant disease caused by mutations in either TSC1 or TSC2, is characterized by the development of hamartomas in a variety of organs. Concordant with the tumor-suppressor model, loss of heterozygosity (LOH) is known to occur in these hamartomas at loci of both TSC1 and TSC2. LOH has been documented in renal angiomyolipomas (AMLs), but loss of the wild-type allele in cortical tubers appears to be very uncommon. Analysis of second, somatic events in tumors for which the status of both TSC1 and TSC2 is known is essential for exploration of the pathogenesis of TSC-lesion development. We analyzed 24 hamartomas from 10 patients for second-hit mutations, by several methods, including LOH, scanning of all exons of both TSC1 and TSC2, promoter methylation of TSC2, and clonality analysis. Our results document loss of the wild-type allele in six of seven AMLs, without evidence of the inactivation of the second allele in many of the other lesions, including tumors that appear to be clonally derived. Laser-capture microdissection further demonstrated loss of the second allele in all three cellular components of an AML. This study thus provides evidence that, in both TSC1 and TSC2, somatic mutations resulting in the loss of wild-type alleles may not be necessary in some tumor types-and that other mechanisms may contribute to tumorigenesis in this setting.

Clone Cells↗

Genetic dissection of lupus pathogenesis: Sle3/5 impacts IgH CDR3 sequences, somatic mutations, and receptor editing.

Sle3/5 is a lupus susceptibility locus identified on mouse chromosome 7 of the New Zealand Black/New Zealand White (NZB/NZW)-derived NZM2410 strain. Based on previous observations, this locus appears to contribute to lupus pathogenesis through its impact on diversification of immune responses. To understand how Sle3/5 affects somatic diversification of humoral responses, we analyzed IgH rearrangements preferentially encoding hapten-reactive IgG1 repertoires after immunization and assessed peripheral IgH VDJ recombination activities in C57BL/6 (B6) mice congenic for Sle3/5 (B6.Sle3/5). In addition to altered somatic V(H) mutation profiles, sequences from B6.Sle3/5 mice exhibited atypical IgH CDR3 structures characteristic of autoreactive B cells and consistent with peripheral B cells bearing putatively edited receptors. Significant expression of Rag genes and circular V(H)D gene excision products were detected in splenic mature B cells of B6.Sle3/5 but not B6 mice, showing that peripheral IgH rearrangements occurred beyond allelic exclusion. Taken together, on the nonautoimmune background, Sle3/5 affected V(H)DJ(H) junctional diversity and V(H) mutational diversity and led to recombinational activation of allelically excluded IgH genes in the periphery. Such impact on somatic IgH diversification may contribute to the development of autoreactive B cell repertoires. This is the first report to present evidence for significant association of a lupus susceptibility locus, which has been mapped to a chromosomal region in which no Ig genes have been identified, with somatic IgH sequence diversity and peripheral H chain receptor editing or revision without relying upon Ig transgene strategies.

Animals↗

The effects of a range of anti-cancer drugs in the white-ivory somatic mutation test in Drosophila.

The white-ivory test is based upon the reversion of the X-linked, recessive eye-colour mutation white-ivory to wild-type. Somatic reversion of white-ivory results in clones of red facets in the white-ivory eyes of eclosing adult flies. 4 anti-cancer drugs, bleomycin, adriamycin, mitoxantrone and cyclophosphamide were found to significantly increase the white-ivory reversion frequency whereas cytosine arabinoside, methotrexate and vincristine did not give positive results.

Animals↗

Somatic mutations of the MEN1 tumor suppressor gene in sporadic gastrinomas and insulinomas.

Gastrinomas and insulinomas are frequent in multiple endocrine neoplasia type 1 (MEN1). The MEN1 tumor suppressor gene was recently identified. To elucidate the etiological role of the MEN1 gene in sporadic enteropancreatic endocrine tumorigenesis, we analyzed tumors (28 gastrinomas and 12 insulinomas) from 40 patients for MEN1 gene mutations and allelic deletions. One copy of the MEN1 gene was found to be deleted in 25 of 27 (93%) sporadic gastrinomas and in 6 of 12 (50%) sporadic insulinomas. MEN1 gene mutations were identified in 9 of 27 (33%) sporadic gastrinomas and 2 of 12 (17%) insulinomas and were not seen in corresponding germ-line DNA sequence. A specific MEN1 mutation was detected in one gastrinoma and in the corresponding germ-line DNA of a patient who had no family history of MEN1. Somatic MEN1 gene mutations and deletions play a critical role in the tumorigenesis of sporadic gastrinomas and may also contribute to the development of a subgroup of insulinomas.

Adolescent↗

Constitutional and somatic mutations in the WT1 gene in Wilms' tumor patients.

Wilms' tumor development, like most tumors, involves multiple genetic alterations affecting diverse genes. Only one of these has thus far been identified, the Wilms' tumor 1 (WT1) gene on 11p13, which functions as a tumor suppressor gene. We assessed the involvement of the WT1 gene constitutionally and somatically in 26 Wilms' tumor patients. Of these, the clinical data suggest a constitutional pre-disposition in 12 bilateral cases and 7 cases with early onset. We employed Southern blot analysis and PCR-based markers for analyses of somatic allelic losses in chromosome bands 11p13, 11p15 and 16q and screened for point mutations in exons 2-10 of the WT1 gene with denaturing gradient gel electrophoresis (DGGE). Of the 12 cases with multiple tumors, 1 exhibited a constitutional 11p13 deletion and a somatic stop mutation in exon 4 of the WT1 gene and 2 harbored constitutional mutations in the WT1 gene: a pre-mature stop codon in exon 6 in a boy with bilateral cryptorchidism and bilateral Wilms' tumors and an intragenic deletion in a girl with bilateral WT. Three additional bilateral tumors displayed WT1 rearrangements or allelic losses with 11p13 markers. Four of 7 patients with an early onset of unilateral tumor had losses of 11p13, though no WT1 mutations were detected. Two of the remaining cases that did not show any somatic or constitutional 11p13 alterations had Beckwith-Wiedemann syndrome, known to involve the 11p15 region.

Alleles↗

Evolution of somatic mutations in mammary tumors in transgenic mice is influenced by the inherited genotype.

BACKGROUND: MMTV-Wnt1 transgenic mice develop mammary hyperplasia early in development, followed by the appearance of solitary mammary tumors with a high proportion of cells expressing early lineage markers and many myoepithelial cells. The occurrence of tumors is accelerated in experiments that activate FGF proto-oncogenes or remove the tumor suppressor genes Pten or P53, implying that secondary oncogenic events are required for progression from mammary hyperplasia to carcinoma. It is not known, however, which oncogenic pathways contribute to Wnt1-induced tumorigenesis - further experimental manipulation of these mice is needed. Secondary events also appear to be required for mammary tumorigenesis in MMTV-Neu transgenic mice because the transgene in the tumors usually contains an acquired mutation that activates the Neu protein-tyrosine kinase. METHODS: cDNA or DNA from the mammary glands and mammary tumors from MMTV-Wnt1, MMTV-Wnt1/p53-/-, MMTV-Neu transgenic mice, and newly generated MMTV-Wnt1/MMTV-Neu bitransgenic mice, was sequenced to seek activating mutations in H-Ras, K-Ras, and N-Ras genes, or in the MMTV-Neu transgene. In addition, tumors from bitransgenic animals were examined to determine the cellular phenotype. RESULTS: We found activating mutations at codons 12, 13, and 61 of H-Ras in just over half of the mammary tumors in MMTV-Wnt1 transgenic mice, and we confirmed the high frequency of activating mutations of Neu in tumors in MMTV-Neu transgenic mice. Tumors appeared earlier in bitransgenic MMTV-Wnt1/MMTV-Neu mice, but no Ras or MMTV-Neu mutations were found in these tumors, which were phenotypically similar to those arising in MMTV-Wnt1 mice. In addition, no Ras mutations were found in the mammary tumors that arise in MMTV-Wnt1 transgenic mice lacking an intact P53 gene. CONCLUSIONS: Tumorigenic properties of cells undergoing functionally significant secondary mutations in H-Ras or the MMTV-Neu transgene allow selection of those cells in MMTV-Wnt1 and MMTV-Neu transgenic mice, respectively. Alternative sources of oncogenic potential, such as a second transgenic oncogene or deficiency of a tumor suppressor gene, can obviate the selective power of those secondary mutations. These observations are consistent with the notion that somatic evolution of mouse mammary tumors is influenced by the specific nature of the inherited cancer-promoting genotype.

Animals↗