Immunoglobulin Heavy Chain Switch Research Articles

CtIP-mediated DNA resection is dispensable for IgH class switch recombination by alternative end-joining

To generate antibodies with different effector functions, B cells undergo Immunoglobulin Heavy Chain (IgH) class switch recombination (CSR). The ligation step of CSR is usually mediated by the classical nonhomologous end-joining (cNHEJ) pathway. In cNHEJ-deficient cells, a remarkable ∼25% of CSR can be achieved by the alternative end-joining (Alt-EJ) pathway that preferentially uses microhomology (MH) at the junctions. While A-EJ-mediated repair of endonuclease-generated breaks requires DNA end resection, we show that CtIP-mediated DNA end resection is dispensable for A-EJ-mediated CSR using cNHEJ-deficient B cells. High-throughput sequencing analyses revealed that loss of ATM/ATR phosphorylation of CtIP at T855 or ATM kinase inhibition suppresses resection without altering the MH pattern of the A-EJ-mediated switch junctions. Moreover, we found that ATM kinase promotes Alt-EJ-mediated CSR by suppressing interchromosomal translocations independent of end resection. Finally, temporal analyses reveal that MHs are enriched in early internal deletions even in cNHEJ-proficient B cells. Thus, we propose that repetitive IgH switch regions represent favored substrates for MH-mediated end-joining contributing to the robustness and resection independence of A-EJ-mediated CSR.

The Chromatin Reader ZMYND8 Regulates Igh Enhancers to Promote Immunoglobulin Class Switch Recombination

SummaryClass switch recombination (CSR) is a DNA recombination reaction that diversifies the effector component of antibody responses. CSR is initiated by activation-induced cytidine deaminase (AID), which targets transcriptionally active immunoglobulin heavy chain (Igh) switch donor and acceptor DNA. The 3′ Igh super-enhancer, 3′ regulatory region (3′RR), is essential for acceptor region transcription, but how this function is regulated is unknown. Here, we identify the chromatin reader ZMYND8 as an essential regulator of the 3′RR. In B cells, ZMYND8 binds promoters and super-enhancers, including the Igh enhancers. ZMYND8 controls the 3′RR activity by modulating the enhancer transcriptional status. In its absence, there is increased 3′RR polymerase loading and decreased acceptor region transcription and CSR. In addition to CSR, ZMYND8 deficiency impairs somatic hypermutation (SHM) of Igh, which is also dependent on the 3′RR. Thus, ZMYND8 controls Igh diversification in mature B lymphocytes by regulating the activity of the 3′ Igh super-enhancer.

DNA Replication Origins in Immunoglobulin Switch Regions Regulate Class Switch Recombination in an R-Loop-Dependent Manner

Class switch recombination (CSR) at the immunoglobulin heavy chain (IgH) locus generates antibody isotypes. CSR depends on double-strand breaks (DSBs) induced by activation-induced cytidine deaminase (AID). Although DSB formation and repair machineries are active in G1 phase, efficient CSR is dependent on cell proliferation and S phase entry; however, the underlying mechanisms are obscure. Here, we show that efficient CSR requires the replicative helicase, the Mcm complex. Mcm proteins are enriched at IgH switch regions during CSR, leading to assembly of facultative replication origins that require Mcm helicase function for productive CSR. Assembly of CSR-associated origins is facilitated by R loops and promotes the physical proximity (synapsis) of recombining switch regions, which is reduced by R loop inhibition or Mcm complex depletion. Thus, R loops contribute to replication origin specification that promotes DSB resolution in CSR. This suggests a mechanism for the dependence of CSR on S phase and cell division.

Hotspots for hormone response elements in Sα switch regions of immunoglobulin heavy chain loci; how estrogen may influence class switch recombination (CSR) and IgA/IgG isotype expression by activated B cells

Abstract B cell immune responses differ between genders. To investigate mechanisms responsible for these differences, we activated murine B cells with mitogens in vitro in the presence or absence of hormones. We found that additions of estradiol and thyroid hormones to mitogen-activated B cell cultures altered IgA/IgG expression patterns. Chromatin immunoprecipitation (ChIP) analyses then showed that estrogen receptor (ER) associated with the anti-apoptotic bcl-2 gene in LPS-stimulated murine B cells, but the interaction was inhibited by estrogen, possibly explaining variant isotype expression patterns. In search of additional mechanisms, we queried mouse and human immunoglobulin heavy chain switch (S) regions for potential hormone response elements (HRE). In both human and mouse loci, the DNA sequences of Sμ and Sα encompassed an extraordinary number of HRE. For example, there were more than 70 potential estrogen response elements (ERE, G/AG/AT/CT/CG/AnnnTGAC/GC) in Sα regions of both human and mouse loci, yet not a single ERE in Sγ2. Results suggest that hormones impact antibody isotype expression by both indirect and direct mechanisms; they might alter B cell growth, but might also up- and down-regulate class switch recombination (CSR) by direct receptor interactions with Sμ and Sα.

Virtual Screening and Molecular Docking Study of Bloom’s Syndrome Protein (BLM) for Finding Potential Lead Drug Candidate

Increased levels of locus-specific mutations within the BLM result in development of Bloom Syndrome and patients are found to be immune deficient. HRDC domain amino acid Lys1270 is presumably to play role in mediating interactions with DNA. Single point mutation of Lys1270 (K1270V) reduces the potency of Double Holliday junction (DHJ) DNA unwinding so BLM lead to its functional loss. Quadruplex formation have role in immunoglobulin heavy chain switching and inhibiting RecQ helicases activity in-vitro in BLM. Variety of G-Quadruplex ligands are employed by molecular docking for arriving at lead compound identification. The scoring function of docking results describes protein-ligand interaction and it conjointly instructed that docking of ligand at mutational binding site shows some repressing function to make potential lead drug molecule. So as to know the elaborated purposeful functional mechanism of protein and to relate impact of mutation with function and activity; dock screening, hit identification and lead optimization facilitate in design of lead drug compound.DOI: http://dx.doi.org/10.3126/ijasbt.v2i2.10086Int J Appl Sci Biotechnol, Vol. 2(2): 152-159

BCL6 breaks occur at different AID sequence motifs in Ig–BCL6 and non-Ig–BCL6 rearrangements

AbstractBCL6 translocations are common in B-cell lymphomas and frequently have chromosomal breaks in immunoglobulin heavy chain (IgH) switch regions, suggesting that they occur during class-switch recombination. We analyze 120 BCL6 translocation breakpoints clustered in a 2156-bp segment of BCL6 intron 1, including 62 breakpoints (52%) joined to IgH, 12 (10%) joined to Ig light chains, and 46 (38%) joined to non-Ig partners. The BCL6 breaks in Ig–BCL6 translocations prefer known activation-induced cytosine deaminase (AID) hotspots such as WGCW and WRC (W = A/T, R = A/G), whereas BCL6 breaks in non-Ig rearrangements occur at CpG/CGC sites in addition to WGCW. Unlike previously identified CpG breaks in pro-B/pre-B-cell translocations, the BCL6 breaks do not show evidence of recombination activating gene or terminal deoxynucleotidyl transferase activity. Both WGCW/WRC and CpG/CGC breaks at BCL6 are most likely initiated by AID in germinal center B-cells, and their differential use suggests subtle mechanistic differences between Ig–BCL6 and non-Ig–BCL6 rearrangements.

Erroneous class switching and false VDJ recombination: Molecular dissection of t(8;14)/MYC-IGH translocations in Burkitt-type lymphoblastic leukemia/B-cell lymphoma

The chromosomal translocation t(8;14)(q24;q32) with juxtaposition of MYC to enhancer elements in the immunoglobulin heavy chain (IGH) gene locus is the genetic hallmark of the majority of Burkitt lymphoma and a subset of Diffuse large B-cell lymphoma patients. Around 3% of adult B-lineage acute lymphoblastic leukemia (ALL) patients show this aberration. Flow cytometry mostly reveals a “mature B-ALL” or “Burkitt-type” ALL immunophenotype. Using long-distance PCR for t(8;14)/MYC-IGH fusion, we investigated bone marrow, peripheral blood and a few other samples with suspected Burkitt-ALL or mature B-ALL and identified 133 MYC-IGH-positive cases. The location of the chromosomal breaks in the IGH joining and the 8 different switch regions was determined using a set of long-distance PCRs. The chromosomal breakpoints with the adjacent MYC regions on 8q24 were characterized by direct sequencing in 49 cases. The distribution of chromosomal breaks among the IGH joining and switch regions was the following: JH 23.3%, M 21.8%, G1 15.0%, G2 7.5%, G3 3.8%, G4 4.5%, A1 12.8%, A2 3.8%, E 7.5%. Two breakpoint clusters near MYC were delineated. There was no clear correlation between the degree of somatic hypermutation and the chromosomal break locations. Epstein Barr virus was detected in 5 cases (4%). This detailed and extensive molecular analysis illustrates the molecular complexity of the MYC-IGH translocations and the detected distribution of breakpoints provides additional evidence that this translocation results from failed switch and VDJ recombinations. This study may serve as a model for the analysis of other IGH translocations in B-cell lymphoma.

ATMIN Is Required for Maintenance of Genomic Stability and Suppression of B Cell Lymphoma

SummaryDefective V(D)J rearrangement of immunoglobulin heavy or light chain (IgH or IgL) or class switch recombination (CSR) can initiate chromosomal translocations. The DNA-damage kinase ATM is required for the suppression of chromosomal translocations but ATM regulation is incompletely understood. Here, we show that mice lacking the ATM cofactor ATMIN in B cells (ATMINΔB/ΔB) have impaired ATM signaling and develop B cell lymphomas. Notably, ATMINΔB/ΔB cells exhibited defective peripheral V(D)J rearrangement and CSR, resulting in translocations involving the Igh and Igl loci, indicating that ATMIN is required for efficient repair of DNA breaks generated during somatic recombination. Thus, our results identify a role for ATMIN in regulating the maintenance of genomic stability and tumor suppression in B cells.

ATMIN: A New Tumor Suppressor in Developing B Cells

In the current issue of Cancer Cell , Loizou et al. report that the deletion of ATM Interactor (ATMIN) in developing B cells leads to profound genomic instability and clonal B cell leukemia, revealing a novel tumor suppressor function for ATMIN.

Abstract PL3-1: Biology of myeloma: Basis for novel drugs

Abstract The outcome of Multiple Myeloma (MM) patients has significantly improved over the last decade, and this is mainly due to the efficacy of novel drugs, such as Thalidomide, Lenalidomide, and Bortezomib. Nevertheless, the majority of patients relapses and become eventually refractory to all available treatments. Therefore, drugs with novel mechanism of action are urgently needed in order to improve the outcome of relapsed MM patients. The increase knowledge in MM biology is already contributing to a more specific drug design, and we have recently learned that in the pathogenesis of MM, as important as the malignant cells themselves, is their interaction with the microenvironment. MM requires a multistep transformation process that implies the sequential generation of primary Ig translocations, chromosomal instability (including mutations –i.e: RAS-, and deletions – i.e: RB-), and secondary translocations. Most primary immunoglobulin gene translocations occur early in the pathogenesis of MM. These translocations, which are mediated by errors in immunoglobulin heavy-chain switch recombination, result in the juxtaposing of an immunoglobulin enhancer and oncogene. On the basis of IgH translocations, MM patients can be divided into 5 subgroups: 1) D-type cyclins: Cyclin D1 on 11q23, Cyclin D3 on 6p21 and Cyclin D2 on 12p13 (25% of cases); 2) MMSET/FGFR3 proteins (4p16.3) (15%); 3) B-zip transcription factors: c-maf on 16q23 and mafB on 20q11 (15%); 4) other IgH translocations (20%); and 5) No IgH translocations (25%). Secondary oncogenic events may involve genes different from the Ig locus, as well as the 14q32 region, as occur in the c-myc translocations: t(8;14), t(2;8), t(8;22), t(14;20). Some of these molecular events represent potential therapeutic targets. Thus the t(4;14) translocation generates a constitutive activation of the oncogenic receptor tyrosine kinase FGFR3 with subsequent phosphorylation of the antiapoptotic STAT3 signalling pathway. Therefore, the use of inhibitors of the FGFR3 tyrosine kinase as well as inhibitors of cyclin dependent kinases could be attractive therapeutic targets. Similarly C-maf, that is over expressed in MM patients with t(14;16) as well as in some MM cases lacking this translocation, also represents a potential target. The second area of MM pathogenesis that has important implications for treatment intervention is the interaction between the malignant cell and the bone marrow microenvironment. MM cells adhere to the extra cellular matrix proteins and bone marrow stromal cells through a series of adhesion molecules, such as the β1-integrin family (VLA-4, VLA-5 and VLA-6) as well as ICAM-1 and VCAM-1. Adhesion of myeloma cells to BM microenvironment induces a cell-adhesion-mediated drug resistance. Interruption by downregulating the interactions between the tumor cell and its microenvironment can potentially halt MM cell growth and proliferation, and be of benefit to patients with MM. The binding of MM cell to BM microenvironment also induces the transcription and secretion of cytokines (TNFa, IL-6, IGF-1, SDF1a, VEGF), by both the plasma cells and BMSC, triggering signalling pathways (such as the RAF/MEK/MAPK, PI3K/AKT, and JAK/STAT pathways), that promote cell proliferation and prevent apoptosis. In this area several targeted oriented drugs are already at early phases of clinical investigation, and these includes: 1. Agents against receptors present in plasma cells (PC) which can be targeted by specific drugs or monoclonal antibodies: cell death receptors (such as DR4-6, FAS and BCL2); TK receptors, VEGFR; TACI, IL6-R, IGF-1R or the CD56 or CD40 antigens; 2. Inhibitors directed to signalling pathways including Farnesil Transferase (Tipifarnib), RAF (RAF 265), STAT3 (Atiprimod), mTor (RAD001) or AKT (Perifosine); and 3. Drugs that interfere with the interactions between PC and microenvironment such as the new Proteasome inhibitors, such as NPI-0052 and PR-171 (Carfilzomib), novel IMIDs (Pomalidomide), HDAC and Hsp90 inhibitors. Unfortunately, the expectations raised by some of these agents have not been so far confirmed in the clinic. It is probable that these targeted directed drugs will be more effective in science based combinations with other agents which have already shown clear efficacy in MM. Citation Information: Clin Cancer Res 2010;16(7 Suppl):PL3-1

Analysis of clonotypic switch junctions reveals multiple myeloma originates from a single class switch event with ongoing mutation in the isotype-switched progeny

AbstractMultiple myeloma (MM) is a cancer of plasma cells (PCs) expressing immunoglobulin heavy chain (IgH) postswitch isotypes. The discovery of earlier stage cells related to postswitch PCs, called preswitch clonotypic IgM (cIgM) cells led to the hypothesis that cIgM cells may be MM progenitors, replenishing the tumor throughout malignancy. cIgM cells may do this by undergoing class switch recombination (CSR), a process detectable in postswitch PCs as multiple IgH switch junctions associated with a single clonotypic IgH V/D/J. We addressed this with a specific clonotypic-switch polymerase chain reaction (PCR), informative for 32 of 41 cases. Here we made 2 significant discoveries: (1) in all cases, we detected only a single clonotypic switch fragment that persists over time (1-7.6 years), and (2) we detected ongoing mutation upstream of the switch junction in 5 of 6 patients, often targeting the intronic enhancer, a key control region in IgH expression. The presence of a single, unchanging clonotypic switch junction suggests that cIgM cells are not MM-PC progenitors; rather, postswitch PCs arise from a single cIgM cell, and MM-PC progenitors reside in the postswitch population. Furthermore, mutations revealed here provide a new marker to identify MM-PC progenitors and aggressive clones that evolve throughout malignancy.

Structure and Consequences of IGH Switch Breakpoints in Burkitt Lymphoma

The t(8;14) MYC/IGH breakpoint is the hallmark translocation of human Burkitt lymphoma (BL). The translocation breakpoint most often involves the immunoglobulin heavy-chain switch regions and is thought to be brought about by an aberrant class switch recombination (CSR) event. During CSR in normal germinal center B cells, DNA double-stranded breaks are introduced in Smu and one of the downstream switch regions (Sgamma, Salpha, or Sepsilon) that are juxtaposed and ligated to form the switch junction, with deletion of the intervening DNA. In contrast, aberrant switch recombination in BL exclusively involves only one switch region, resulting in a perfect reciprocal translocation. A functional consequence of this type of translocation is that IgM expression from the chromosome affected by the translocation is not necessarily disrupted.

The Human RecQ Helicases, BLM and RECQ1, Display Distinct DNA Substrate Specificities

RecQ helicases maintain chromosome stability by resolving a number of highly specific DNA structures that would otherwise impede the correct transmission of genetic information. Previous studies have shown that two human RecQ helicases, BLM and WRN, have very similar substrate specificities and preferentially unwind noncanonical DNA structures, such as synthetic Holliday junctions and G-quadruplex DNA. Here, we extend this analysis of BLM to include new substrates and have compared the substrate specificity of BLM with that of another human RecQ helicase, RECQ1. Our findings show that RECQ1 has a distinct substrate specificity compared with BLM. In particular, RECQ1 cannot unwind G-quadruplexes or RNA-DNA hybrid structures, even in the presence of the single-stranded binding protein, human replication protein A, that stimulates its DNA helicase activity. Moreover, RECQ1 cannot substitute for BLM in the regression of a model replication fork and is very inefficient in displacing plasmid D-loops lacking a 3'-tail. Conversely, RECQ1, but not BLM, is able to resolve immobile Holliday junction structures lacking an homologous core, even in the absence of human replication protein A. Mutagenesis studies show that the N-terminal region (residues 1-56) of RECQ1 is necessary both for protein oligomerization and for this Holliday junction disruption activity. These results suggest that the N-terminal domain or the higher order oligomer formation promoted by the N terminus is essential for the ability of RECQ1 to disrupt Holliday junctions. Collectively, our findings highlight several differences between the substrate specificities of RECQ1 and BLM (and by inference WRN) and suggest that these enzymes play nonoverlapping functions in cells.

TransCGH: Rapid Identification and High-Resolution Mapping of Balanced IgH Translocations in Archival DNA Using Custom Oligonucleotide Arrays.

Array based comparative genomic hybridization (CGH) has revolutionized the study of chromosomal imbalances but generally is incapable of detecting balanced genomic rearrangements like reciprocal translocations, which play central roles in the pathogenesis and diagnosis of lymphomas, leukemias and other tumors. The precise identification of immunoglobulin heavy chain (IgH) translocation partners, for example, is essential for the classification of B cell lymphomas and for predicting prognosis in plasma cell neoplasms like multiple myeloma. Using IgH translocations as a model for balanced genomic rearrangements, we have developed a simple modification of array CGH that we call translocation-CGH (transCGH) and that enables the rapid identification of IgH translocation partners and precise mapping of translocation-associated breakpoints to unprecedented resolution. To render IgH translocations detectable on CGH arrays, genomic DNA from test and reference samples is modified prior to array hybridization in an enzymatic linear amplification reaction that employs a single IgH joining (JH) or switch (Sμ/Sα/Sε) region primer, resulting in specific amplification of any fusion partner sequences that may be inserted (via translocation or other rearrangement) downstream of the IgH primer. Using a single tiling-density oligonucleotide array representing such common IgH partner loci as MYC, BCL2 and CCND1 (cyclin D1), transCGH successfully identified and mapped to ∼100bp resolution an assortment of known IgH fusion breakpoints in various cell lines and primary lymphomas, including JH-CCND1 breakpoints in MO2058 and Granta 519 cell lines (mantle cell lymphoma), a cytogenetically cryptic Sα-CCND1 fusion in U266 (myeloma), JH-MYC and Sμ-MYC breakpoints in MC116 and Raji (Burkitt lymphoma), and JH-BCL2 breakpoints in DHL16 (large cell lymphoma; minor cluster region) and in an archival case of follicular lymphoma (major breakpoint region). We then used transCGH to analyze 4 archival cases of mantle cell lymphoma and one t(11;14)-positive case of B cell prolymphocytic leukemia, all of which lacked PCR-detectable translocation breakpoints at the CCND1 major translocation cluster (MTC). Five novel CCND1 translocation breakpoints were identified and mapped to ∼100bp resolution, allowing the rapid design of patient-specific PCR primers for amplification, sequencing, and confirmation of the predicted breakpoints. One breakpoint mapped to within 500bp of the MTC, whereas the other 4 were scattered across a ∼150kb region flanking the MTC. To our knowledge, this represents the largest series of non-MTC mantle cell lymphoma breakpoint sequences reported to date. It also illustrates how transCGH can facilitate the rapid cloning of previously unidentified IgH translocation breakpoints dispersed over very large genomic regions. Because transCGH requires only genomic DNA and can simultaneously detect both balanced IgH translocations and genomic imbalances at ultra-high resolution on the same array, it may become a useful alternative to molecular cytogenetic methods (e.g. FISH) for clinical testing of B cell and plasma cell neoplasms. transCGH also will facilitate the development of highly sensitive breakpoint-specific PCR assays for detecting minimal residual disease. Finally, because the primer used in the linear amplification reaction is fully customizable, transCGH can readily be adapted to identify and map other balanced translocations (or more complex genomic fusions) that involve non-IgH loci, provided that one of the fusion partners is known.

Microfluidic Chips for Detecting the t(4;14) Translocation and Monitoring Disease during Treatment Using Reverse Transcriptase-Polymerase Chain Reaction Analysis of IgH-MMSET Hybrid Transcripts

Diagnosis platforms incorporating low-cost microfluidic chips enable sensitive, rapid, and accurate genetic analysis that could facilitate customized therapies tailored to match the vulnerabilities of any types of cancer. Using ex vivo cancer cells, we have detected the unique molecular signature and a chromosomal translocation in multiple myeloma. Multiple myeloma is characterized by IgH rearrangements and translocations that enable unequivocal identification of malignant cells, detected here with integrated microfluidic chips incorporating genetic amplification via reverse transcriptase-polymerase chain reaction and capillary electrophoresis. On microfluidic chips, we demonstrated accurate and versatile detection of molecular signatures in individual cancer cells, with value for monitoring response to therapy, detecting residual cancer cells that mediate relapse, and evaluating prognosis. Thus, testing for two clinically important molecular biomarkers, the IgH VDJ signature and hybrid transcripts signaling the t(4;14) chro-mosomal translocation, with predictive value in diagnosis, treatment decisions, and monitoring has been efficiently implemented on a miniaturized microfluidic system.

Treatment of Newly Diagnosed Multiple Myeloma Based on Mayo Stratification of Myeloma and Risk-Adapted Therapy (mSMART): Consensus Statement

Treatment of Newly Diagnosed Multiple Myeloma Based on Mayo Stratification of Myeloma and Risk-Adapted Therapy (mSMART): Consensus Statement

Bone Involvement in Multiple Myeloma Patients Carrying the T (4;14) Chromosomal Translocation.

Chromosomal translocations involving the immunoglobulin heavy chain switch region (IgH) are quite common in multiple myeloma (MM), and some of them can reliably predict disease outcome. In particular, t(4;14) chromosomal abnormality is one of the most adverse prognostic factors for response duration and survival after high dose therapy and autologous stem cell transplantation. Despite the dismal prognosis, however, in this subset of patients, bone involvement, as evaluated by spine MRI, is relatively infrequent, at variance to what has been observed in other MM subtypes according to TC classification. In the present study we aimed at further testing this hypothesis by analyzing the extent of whole bone involvement in patients showing t(4;14) chromosomal translocation as compared to negative patients. For this purpose, 50 newly diagnosed MM patients (32M, 18F, median age = 54 yrs) underwent evaluation of total skeletal X-ray, whole spine MRI and, at the same time, quantification of markers of bone resorption (urinary NTX, PYR and DPYR and serum crosslaps) and bone formation (bone alkaline phosphatase-BAP and osteocalcin) was performed. Using a real-time PCR assay to detect the presence of IgH/MMSET fusion gene as a surrogate marker for t(4;14), we found 15 patients carrying this chromosomal abnormality, 7 of whom (46%) were also positive for the deletion of chromosome 13, this abnormality was detected in 11/35 (31%) patients who proved negative for IgH/MMSET hybrid transcript. The two groups of patients did not differ significantly in terms of median age, distribution of M protein isotype and light chain, beta-2 microglobulin, bone marrow plasma cell infiltration and disease stage. Spinal MRI was negative in 3/15 (20%) t(4;14) positive patients as compared to 12% t(4;14) negative patients; skeletal involvement, however, was more pronounced in t(4;14) positive patients (median skeletal score = 6.24, as compared to 3.58 in t(4;14) negative cases, p= 0.00). These data were confirmed by the evaluation of bone resorption markers; specifically, serum crosslaps were significantly increased in patients with t(4;14) abnormality compared to negative individuals (7984 pmol/L±1682SE vs 5123pmol/L±783SE p=0.04). Conversely, no difference in bone formation markers was found in the two groups of patients. Our results indicate that, despite a spinal involvement at MRI that is comparable to what is observed in negative patients, individuals who are t(4;14) positive show a more pronounced bone resorption pattern, this in contrast to what has been reported so far, but in line with the aggressive features of the disease in these patients.

Decoupling of normal CD40/interleukin-4 immunoglobulin heavy chain switch signal leads to genomic instability in SGH-MM5 and RPMI 8226 multiple myeloma cell lines

The processes mediating genomic instability and clonal evolution are obscure in multiple myeloma (MM). Acquisition of new chromosomal translocations into the switch region of the immunoglobulin heavy chain (IgH) gene (chromosome 14q32) in MM, often heralds transformation to more aggressive disease. Since the combined effects of CD40 plus interleukin-4 (IL-4) mediate IgH isotype class switch recombination (CSR), and this process involves DNA double strand break repair (DSBR), we hypothesized that CD40 and/or IL-4 activation of MM cells could induce abnormal DNA DSBR and lead to genomic instability and clonal evolution. In this study, we show that MM cell lines that are optimally triggered via CD40 and/or IL-4 demonstrate abnormal decoupling of IL-4 signal transduction from CD40. Specifically, CD40 alone was sufficient to trigger maximal growth of tumor cells. We further demonstrate that CD40 triggering induced both DNA DSBs as well as newly acquired karyotypic abnormalities in MM cell lines. Importantly, these observations were accompanied by induction of activation induced cytidine deaminase expression, but not gross apoptosis. These data support the role of abnormal CD40 signal transduction in mediating genomic instability, suggesting a role for the CD40 pathway and intermediates in myelomagenesis and clonal evolution in vivo.

Molecular heterogeneity of sporadic adult Burkitt-type leukemia/lymphoma as revealed by PCR and cytogenetics: correlation with morphology, immunology and clinical features

Chromosomal translocations involving the MYC oncogene are a hallmark of Burkitt lymphoma but they are only found in a varying frequency in mature Burkitt-type acute lymphoblastic leukemia (B-ALL). We have investigated samples of 56 sporadic Burkitt leukemia/lymphoma patients for the translocations t(8;14)(q24;q32), t(2;8)(p11;q24) and t(8;22)(q24;q11). Long PCR was used for detecting the immunoglobulin heavy chain (IgH) translocation and cytogenetics and/or fluorescence in situ hybridization for detecting the 'variant' MYC translocations. A total of 29 samples (51.8%) were t(8;14)-positive by long PCR. Approximately one-third had a chromosomal breakpoint in the IgH joining region while the others had breakpoints in the IgH switch regions. Among them were two cases with a previously unreported MYC translocation into the IgE switch region. Long PCR was more reliable compared to conventional cytogenetics for detecting the t(8;14). Epstein-Barr virus was detected in high copy number in two (3.6%) t(8;14)-positive cases by real-time quantitative PCR. Human herpesvirus 8 was not detected in any case by nested PCR. A typical L3 or L3-compatible cytomorphology was highly predictive (>80%) but not specific of a MYC translocation. A total of 34 patients were treated according to the GMALL B-ALL therapy protocols and there was no significant difference in overall survival between patients with or without t(8;14).

Overexpression of transcripts originating from the MMSET locus characterizes all t(4;14)(p16;q32)-positive multiple myeloma patients

AbstractMultiple myeloma (MM) is a B-lineage malignancy characterized by diverse genetic subtypes and clinical outcomes. The recurrent immunoglobulin heavy chain (IgH) switch translocation, t(4;14)(p16;q32), is associated with poor outcome, though the mechanism is unclear. Quantitative reverse-transcription–polymerase chain reaction (RT-PCR) for proposed target genes on a panel of myeloma cell lines and purified plasma cells showed that only transcripts originating from the WHSC1/MMSET/NSD2 gene are uniformly dysregulated in all t(4;14)POS patients. The different transcripts detected, multiple myeloma SET domain containing protein (MMSET I), MMSET II, Exon 4a/MMSET III, and response element II binding protein (RE-IIBP), are produced by alternative splicing and alternative transcription initiation events. Translation of the various transcripts, including those from major breakpoint region 4-2 (MB4-2) and MB4-3 breakpoint variants, was confirmed by transient transfection and immunoblotting. Green fluorescent protein (GFP)–tagged MMSET I and II, corresponding to proteins expressed in MB4-1 patients, localized to the nucleus but not nucleoli, whereas the MB4-2 and MB4-3 proteins concentrate in nucleoli. Cloning and localization of the Exon 4a/MMSET III splice variant, which contains the protein segment lost in the MB4-2 variant, identified a novel protein domain that prevents nucleolar localization. Kinetic studies using photobleaching suggest that the breakpoint variants are functionally distinct from wild-type proteins. In contrast, RE-IIBP is universally dysregulated and also potentially functional in all t(4;14)POS patients irrespective of fibroblast growth factor receptor 3 (FGFR3) expression or breakpoint type.