Somatic hypermutation in low-grade mucosa-associated lymphoid tissue- type B-cell lymphoma

The origin of low-grade mucosa-associated lymphoid tissue (MALT)-type B- cell lymphoma is still unclear. Using a novel two-step procedure, we have sequenced the Ig VH genes expressed by cells from four patients with gastric low-grade MALT-type lymphoma. The nucleotide sequences of the complementarity determining region 3 (CDR3) of the genomic DNA were first amplified using consensus oligonucleotide primers, then sequenced. Based on the CDR3 sequence amplified from each MALT lymphoma, individual tumor-specific primers were synthesized and used directly in the polymerase chain reaction (PCR) to analyze the sequences of their Ig heavy-chain variable region. When compared with the germ-line sequence, many nucleotide substitutions, mainly in the CDRs, were found in the variable gene sequences of the four MALT lymphomas. The mutations showed a high replacement-to-silent ratio and were distributed in a way which suggested that the tumor cells had been positively selected through their antigen receptor. Our findings indicate that the MALT-type lymphoma B cells are hypermutated postgerminal center lymphocytes that have undergone antigen selection.     

Regulation of human neutrophil aggregation: comparable latent times, activator sensitivities, and exponential decay in aggregability for FMLP, platelet-activating factor, and leukotriene B4

We have recently described a flow cytometry technique, whose sensitivity allows direct measurements of latent times before the onset of aggregation, and of rates, maximal extents, and reversibility of aggregation (J Leuk Biol 50:434, 1991). We report here that activators which stimulate sustained cellular signaling associated with increases in intracellular calcium (ionomycin) or protein kinase C activation (phorbol myristate acetate, PMA) cause complete (> or = 98%) and irreversible neutrophil aggregation, with latent times for the onset of aggregation inversely proportional to the activator concentration. In contrast, the receptor-specific activators leukotriene B4 (LTB4), formyl peptide FMLP, and platelet-activating factor (PAF) gave only partial and reversible aggregatory responses, limited by the following similar properties: latent times of 4.5 seconds +/- 1.5 seconds, independent of activator concentration; similar concentrations for onset of aggregation (approximately 1 nmol/L) that increased over a similar broad range of activator concentration, with one-half maximal rates of aggregation at 10 nmol/L to 30 nmol/L, corresponding to reported dissociation constant values; comparable limited recruitment and spontaneous reversibility of aggregation; absence of interactivator synergism; and similar exponential decays in activated cell stickiness (refractoriness), with t1/2 = 15 to 30 seconds. Variable cross- desensitization was seen between LTB4 and FMLP depending on donor and activator concentrations. In vivo, these properties are expected to provide localization of the aggregatory response, minimizing the otherwise detrimental effects of circulating activated neutrophils.     

Heterogeneity of breakpoints of 11q23 rearrangements in hematologic malignancies identified with fluorescence in situ hybridization

Twenty-four patients whose cells contained a variety of 11q23 rearrangements, including translocations, insertions, and an inversion, were studied using fluorescence in situ hybridization with cosmid, phage, and plasmid probes mapped to 11q22-24. In 17 patients, the breakpoints of the common 11q23 translocations involving chromosomes 4, 6, 9, and 19 as well as some uncommon translocations involving 3q23, 17q25, 10p11, and an insertion 10;11 were all located in the breakpoint cluster region of the MLL gene, regardless of age, phenotype of disease, or involvement of a third chromosome. The breakpoints in 11q23 in the other 7 patients with a t(7;11)(p15;q23), inv(11)(p11q23), t(4;11)(q23;q23), der(5)t(5;11)(q13;q23), ins(10;11)(p11;q23q24), t(11;14)(q23;q11), or t(11;18;11) (p15;q21;q23) were located either centromeric to CD3D or telomeric to THY1. Thus, although most 11q23 rearrangements, involve the same breakpoint cluster region of MLL, there is heterogeneity in the breakpoint in some of the rare rearrangements.     

Cytogenetic and molecular delineation of a region of chromosome 7 commonly deleted in malignant myeloid diseases

Loss of a whole chromosome 7 or a deletion of the long arm, del(7q), are recurring abnormalities in malignant myeloid diseases. To determine the location of genes on 7q that are likely to play a role in leukemogenesis, we examined the deleted chromosome 7 homologs in a series of 81 patients with therapy-related or de novo myelodysplastic syndrome or acute myeloid leukemia. Our analysis showed that the deletions were interstitial and that there were two distinct deleted segments of 7q. The majority of patients (65 of 81 [80%]) had proximal breakpoints in bands q11-22 and distal breakpoints in q31-36; the smallest overlapping deleted segment was within q22. The remaining 16 patients had deletions involving the distal q arm with a commonly deleted segment of q32-33. To define the proximal deleted segment at 7q22 at a molecular level, we used fluorescence in situ hybridization with a panel of mapped yeast artificial chromosome (YAC) clones from 7q to examine 15 patients with deletion breakpoints in 7q22. We determined that the smallest overlapping deleted segment is contained in a well- defined YAC contig that spans 2 to 3 Mb. These studies delineate the region of 7q that must be searched to isolate a putative myeloid leukemia suppressor gene, and provide the necessary cloned DNA for more detailed physical mapping and gene isolation.