Formation Of Dicentric Chromosomes Research Articles

Mechanism of Action and Use of Radiomimetic Compounds (Part 2)

Radiomimetic compounds, similarly to ionising radiation can directly or indirectly cause DNA damage. Two major groups of these compounds, alkylating agents and antimetabolites, have already been discussed in the first part of this article. The topic of this article is an overview of radiomimetic substances of biological origin; they are grouped and discussed upon their chemical structure. Some bacteria, belonging to the class of Actinobacteria can produce compounds with radiomimetic property as part of their defence mechanisms, such as bleomycins, enediynes, streptonigrins, etc. Radiomimetic compounds of bacterial origin can be divided into three main groups: radiomimetic glycopeptides, enediynes and quinone antibiotics. Each of them induces double-strand DNA breaks. Some of them work through their reactive radicals and the molecules are also transformed when they break DNA. Others, such as bleomycin and similar glycopeptides, have an enzyme-like catalytic effect as the molecule regenerates itself after interacting with DNA thus the same molecule can create new DNA breaks again. The damage caused by radiation and by bleomycin is very similar: double DNA strand breaks occur in close proximity to each other, below the lethal dose of the cell, so as the cell does not die, the DNA repair process is activated, which can lead to the formation of dicentric chromosomes and other detectable DNA alterations. This review briefly summarises the mechanism of action of bacterial radiomimetic compounds and their benefit.

Postirradiation temperature influences DSB repair and dicentric chromosome formation-potential impact for dicentric chromosome analysis in interlaboratory comparisons.

The objective was to investigate the influence of different pre-storage temperatures in the dicentric chromosome analysis (DCA) protocol (22°C vs. 37°C) by using γ-H2AX+53BP1 foci as a marker for deoxyribonucleic acid (DNA) double-strand break (DSB) damage induction and repair and the formation of dicentric chromosomes as a result of mis-repair. Repair of γ-H2AX+53BP1 DSB foci was absent in samples that were incubated for 2h at 22°C after exposure of 0.5 and 1.2Gy. When 0.5- and 1.2-Gy-exposed samples were incubated at 37°C for 2h, there was an average decline of 31 and 52% of DSB foci, respectively. This indicated that DNA repair occurred. There was a 27% decrease in dicentric chromosome yield at 1.2Gy and a 15% decrease at 3.5Gy after post-irradiation incubation for 2 h at 37°C relative to the observed dicentric frequencies at 22°C. Recommended to re-phase: our data suggested that there were more open DSBs after a 2-h incubation at 22°C, which contributed to more mis-repair and dicentric formation from the start of culture. Our findings are corroborated by publications showing that lesion interaction based on enzymatic activity is suppressed below 21°C. As such temperature variations can be a source of variation in DCA during interlaboratory comparison studies, we propose to establish a common guide for the standardisation of pre-culture conditions in cytogenetic dosimetry proficiency testing.

Abstract PR012: SETD2 safeguards the genome against isochromosome formation

Abstract Chromosome mis-segregation during cell division occurs in diverse tumor types, promoting aneuploidy and intratumoral genetic heterogeneity. While the mechanisms that govern chromosome segregation are well-established, how changes in the activities of tumor suppressors or oncogenes drive mitotic errors remain poorly understood. Loss of the tumor suppressor SETD2, the methyltransferase responsible for tri-methylation of histone 3 lysine 36 (H3K36me3), correlates with clear cell renal cell carcinoma (ccRCC) tumors exhibiting extensive intratumoral heterogeneity. Yet, how loss of H3K36me3 or SETD2 promotes heterogeneity is unknown. Here, we show that loss of SETD2 promotes chromosome mis-segregation during mitosis and interphase DNA bridges driven by the formation of dicentric chromosomes. Cytogenetic analyses revealed that these dicentrics loss were largely comprised of mirror-imaged isodicentric chromosomes that contain two active centromeres. In addition to isodicentrics, cells lacking SETD2 or H36K36me3 generated iso- and acentric chromosomes, however loss of SETD2 protein additionally caused chromosome instability. These data directly link loss of a tumor suppressor to these mutable chromatin structures that initiate intratumor heterogeneity by promoting gross chromosomal rearrangements. Citation Format: Frank M. Mason, Anteneh T. Tebeje, Emily S. Kounlavong, Rashmi Dahiya, Logan Vlach, Tiffany Guess, Ruhee Dere, Ryoma Ohi, Peter Ly, Cheryl L. Walker, W. Kimryn Rathmell. SETD2 safeguards the genome against isochromosome formation [abstract]. In: Proceedings of the AACR Special Conference: Advances in Kidney Cancer Research; 2023 Jun 24-27; Austin, Texas. Philadelphia (PA): AACR; Cancer Res 2023;83(16 Suppl):Abstract nr PR012.

Identification of small-sized intrachromosomal segments at the ends of INV-DUP-DEL patterns.

The mechanism of chromosomal rearrangement associated with inverted-duplication-deletion (INV-DUP-DEL) pattern formation has been investigated by many researchers, and several possible mechanisms have been proposed. Currently, fold-back and subsequent dicentric chromosome formation has been established as non-recurrent INV-DUP-DEL pattern formation mechanisms. In the present study, we analyzed the breakpoint junctions of INV-DUP-DEL patterns in five patients using long-read whole-genome sequencing and detected 2.2-6.1 kb copy-neutral regions in all five patients. At the end of the INV-DUP-DEL, two patients exhibited chromosomal translocations, which are recognized as telomere capture, and one patient showed direct telomere healing. The remaining two patients had additional small-sized intrachromosomal segments at the end of the derivative chromosomes. These findings have not been previously reported but they may only be explained by the presence of telomere capture breakage. Further investigations are required to better understand the mechanisms underlying this finding.

P029: Are Reed-Sternberg cells stuck in mitosis by short nucleoplasmic bridges as a consequence of centromeric instability?

Background: Multinucleated giant tumor cells are frequently observed in lymph nodes of untreated lymphoma patients. Malignant Hodgkin lymphoma (HL) cells i. e. small Hodgkin and large bi- or multinucleated Reed-Sternberg (HRS) cells are characterized by genomic instability and downregulation of numerous key regulators of e.g. cell cycle, spindle apparatus, cytokinesis and cell lineage differentiation. So far, the formation of HRS cells remains obscure. We have demonstrated previously that telomere instability is involved in the proliferation of small cells into large binucleated cells. Here, we have used a collection of HL cell lines to analyze the successive steps involved in the transformation of the mononucleated into the multinucleated RS-like cells. Materials and Methods: Seven HL cell lines and 30 HL lymph nodes were used. Cytokinesis-block micronucleus assay and telomere and centromere staining followed by the M-FISH technique were employed to analyze chromosomal instability. DNA repair pathways were investigated. Results: We demonstrate that telomere dysfunction in HL cell lines is associated with the formation of dicentric chromosomes with both centromeres in close proximity, but also with a high rate of formation of micronuclei. Two morphologically different types of nucleoplasmic bridges (NPBs) were observed: (1) Long NPBs related to telomere dysfunction and chromosome fusions, and (2) short NPBs related to centromere breakpoints with configurations of cells looking like “stuck” together as binucleated cells. Short NPBs and binucleated cells were seen in the cultures of all HL cell lines. However, each HL cell line was characterized by an individual signature of the proportion of long and short NPBs, which correlated with centrosome amplification and formation of stable binucleated cells. Similar mechanisms were identified in HL patient lymph nodes prior to treatment, thus underscoring the biological significance of our findings in cell cultures. Transcriptome analysis confirmed the variations in the DNA repair pathways regarding the rate of large cells among the different HL cell lines. Conclusion: The formation of dicentric chromosomes related to centromere instability, and short NPBs were associated with the emergence of binucleated cells through different mechanisms. Our findings open a novel route to understanding the transition from mononucleated cells to multinucleated cells in HL and in other B-cell lymphomas.

DETERMINATION OF THE OCCURENCE OF NUCLEAR ANOMALIES IN DANIO RERIO PERIPHERAL BLOOD ERYTH- ROCYTES DEPENDING ON THE DOSE OF IONIZING RADIATION

Ionizing radiation induces double-stranded breaks in the DNA structure, following by the formation of dicentric chromosomes. With the passage of subsequent postradiation mitoses, dicentric chromosomes are unevenly distributed to the poles of the cell, as a result of which nuclear anomalies of various types are formed, distinguishable at the light-optical level. The aim of this study was to assess the occurrence of nuclear anomalies in the peripheral blood erythrocytes of freshwater fish Danio rerio after exposure to ionizing radiation, as well as to assess the possibility of using these organisms as model animals in radiobiological studies. To determine the frequency of occurrence of nuclear anomalies in peripheral blood erythrocytes of freshwater fish Danio rerio, fish were exposed to X-ray radiation at doses of 0.0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0 Gy. 48 hours after irradiation in fish peripheral blood erythrocytes by light microscopy, four types of nuclear anomalies were identified: micronuclei, nuclear protrusions, nucleoplasmic bridges, dumbbell-shaped nuclei. The frequency of detection of micronuclei and nuclear protrusions was found to be dose-dependent (r = 0.9245, p < 0.05 and r = 0.9062, p < 0.05 respectively), while the appearance of nucleoplasmic bridges and dumbbell-shaped nuclei did not correlate with the dose. More than that, the frequencies of micronuclei and nuclear protrusions detected after irradiation of fish at doses of more than 4 Gy (4.0, 6.0 and 8.0 Gy) significantly differed from the control values. Thus, Danio rerio can be used as a laboratory test system for radiobiological research, for example, to determine the effect of the developed radioprotectors and radiosensitizers, despite the extreme radioresistance of these organisms. It is advisable to use micronuclei or nuclear protrusions as cell markers, given, however, that the identification of these markers will allow detecting radiation exposure only in doses above 4 Gy.

Relative Biological Effectiveness and Non-Poissonian Distribution of Dicentric Chromosome Aberrations after Californium-252 Neutron Exposures of Human Peripheral Blood Lymphocytes.

Cells exposed to fast neutrons often exhibit a non-Poisson distribution of chromosome aberrations due to the high ionization density of the secondary reaction products. However, it is unknown whether lymphocytes exposed to californium-252 (252Cf) spectrum neutrons, of mean energy 2.1 MeV, demonstrate this same dispersion effect at low doses. Furthermore, there is no consensus regarding the relative biological effectiveness (RBE) of 252Cf neutrons. Dicentric and ring chromosome formation was assessed in human peripheral blood lymphocytes irradiated at doses of 12-135 mGy. The number of aberrations observed were tested for adherence to a Poisson distribution and the maximum low-dose relative biological effectiveness (RBEM) was also assessed. When 252Cf-irradiated lymphocytes were examined along with previously published cesium-137 (137Cs) data, RBEM values of 15.0 ± 2.2 and 25.7 ± 3.8 were found for the neutron-plus-photon and neutron-only dose components, respectively. Four of the five dose points were found to exhibit the expected, or close to the expected non-Poisson over-dispersion of aberrations. Thus, even at low doses of 252Cf fast neutrons, when enough lymphocyte nuclei are scored, chromosome aberration clustering can be observed.

RNF8 promotes high linear energy transfer carbon-ion-induced DNA double-stranded break repair in serum-starved human cells

The cell-killing effect of radiotherapy largely depends on unrepaired DNA double-stranded breaks (DSBs) or lethal chromosome aberrations induced by DSBs. Thus, the capability of DSB repair is critically important for the cancer-cell-killing effect of ionizing radiation. Here, we investigated the involvement of the DNA damage signaling factors ataxia telangiectasia mutated (ATM), ring finger protein 8 (RNF8), and RNF168 in quiescent G0/G1 cells, which are expressed in the majority of cell populations in tumors, after high linear energy transfer (LET) carbon-ion irradiation. Interestingly, ATM inhibition caused a substantial DSB repair defect after high-LET carbon-ion irradiation. Similarly, RNF8 or RNF168 depletion caused a substantial DSB repair defect. ATM inhibition did not exert an additive effect in RNF8-depleted cells, suggesting that ATM and RNF8 function in the same pathway. Importantly, we found that the RNF8 RING mutant showed a similar DSB repair defect, suggesting the requirement of ubiquitin ligase activity in this repair pathway. The RNF8 FHA domain was also required for DSB repair in this axis. Furthermore, the p53-binding protein 1 (53BP1), which is an important downstream factor in RNF8-dependent DSB repair, was also required for this repair. Importantly, either ATM inhibition or RNF8 depletion increased the frequency of chromosomal breaks, but reduced dicentric chromosome formation, demonstrating that ATM/RNF8 is required for the rejoining of DSB ends for the formation of dicentric chromosomes. Finally, we showed that RNF8 depletion augmented radiosensitivity after high-LET carbon-ion irradiation. This study suggests that the inhibition of RNF8 activity or its downstream pathway may augment the efficacy of high-LET carbon-ion therapy.

The Potential Effect of Different Doses of Ionizing Radiation on Genes and Disease.

Aim:The chromosomal aberrations induced by radiation appear about nonrandomly distributed across the whole genome. Previous studies have shown that chromosomes with high DNA content are less frequently involved in the formation of symmetrical translocations and dicentric chromosomes than expected, whereas smaller chromosomes are more frequently involved. We hypothesized that these translocation regions are linked to radiation sensitivity.Materials and methods:We investigated the frequencies of chromosome translocations induced by radiation exposure and adjusted the results according to chromosome length. We specifically analyzed whole blood samples from 3 participants. The samples were irradiated using 60Co at doses of 0.5, 1, 2.5, and 5 Gy. Traditional Giemsa-trypsin-Wright band staining was performed to identify the translocations in the chromosomes, and results were compared with microarray data generated in our previous study.Results:Our analysis indicated that chromosomes 9q were the most sensitive to translocations after various doses of radiation, and such translocations occurred in the euchromatin region. Chromosomes 1, 9, 15, and 17 were more sensitive to radiation doses of 0.5 Gy. This observation could be useful when selecting sensitive reference chromosomes in the low-dose region. The results of expression profiling analysis for radiation-sensitive regions were similar to those of chromosome translocation analysis.Conclusion:This study shows that some chromosomes or genomic regions are more sensitive to alteration by radiation exposure.

Development of electronic training and telescoring tools to increase the surge capacity of dicentric chromosome scorers for radiological/nuclear mass casualty incidents

Dicentric chromosome assay (DCA) is most frequently used for estimating the absorbed radiation dose in the peripheral blood lymphocytes of humans after occupational or incidental radiation exposure. DCA is considered to be the “gold standard” for estimating the absorbed radiation dose because the dicentric chromosome formation is fairly specific to ionizing radiation exposure and its baseline frequency is extremely low in non-exposed humans. However, performance of DCA for biodosimetry is labor intensive and time-consuming making its application impractical for radiological/nuclear mass casualty incidents. Realizing the critical need for rapid dose estimation particularly after radiological/nuclear disaster events, several laboratories have initiated efforts to automate some of the procedural steps involved in DCA. Although metaphase image capture and dicentric chromosome analysis have been automated using commercially available platforms, lack or an insufficient number of these platforms may pose a serious bottleneck when hundreds and thousands of samples need to be analyzed for rapid dose estimation. To circumvent this problem, a web-based approach for telescoring was initiated by our laboratory, which enabled the cytogeneticists around the globe to analyze and score digital images. To further increase the surge capacity of dicentric scorers, we recently initiated a dicentric training and scoring exercise involving a total of 50 volunteers at all academic levels without any prerequisite for experience in radiation cytogenetics. Out of the 50 volunteers enrolled thus far, only one outlier was found who overestimated the absorbed radiation dose. Our approach of training the civilians in dicentric chromosome analysis holds great promise for increasing the surge capacity of dicentric chromosome scorers for a rapid biodosimetry in the case of mass casualty scenarios.

Establishment and Characterization of a Reliable Xenograft Model of Hodgkin Lymphoma Suitable for the Study of Tumor Origin and the Design of New Therapies.

To identify the cells responsible for the initiation and maintenance of Hodgkin lymphoma (HL) cells, we have characterized a subpopulation of HL cells grown in vitro and in vivo with the aim of establishing a reliable and robust animal model for HL. To validate our model, we challenged the tumor cells in vivo by injecting the alkylating histone-deacetylase inhibitor, EDO-S101, a salvage regimen for HL patients, into xenografted mice. Methodology: Blood lymphocytes from 50 HL patients and seven HL cell lines were used. Immunohistochemistry, flow cytometry, and cytogenetics analyses were performed. The in vitro and in vivo effects of EDO-S101 were assessed. Results: We have successfully determined conditions for in vitro amplification and characterization of the HL L428-c subline, containing a higher proportion of CD30−/CD15− cells than the parental L428 cell line. This subline displayed excellent clonogenic potential and reliable reproducibility upon xenografting into immunodeficient NOD-SCID-gamma (−/−)(NSG) mice. Using cell sorting, we demonstrate that CD30−/CD15− subpopulations can gain the phenotype of the L428-c cell line in vitro. Moreover, the human cells recovered from the seventh week after injection of L428-c cells into NSG mice were small cells characterized by a high frequency of CD30−/CD15− cells. Cytogenetic analysis demonstrated that they were diploid and showed high telomere instability and telomerase activity. Accordingly, chromosomal instability emerged, as shown by the formation of dicentric chromosomes, ring chromosomes, and breakage/fusion/bridge cycles. Similarly, high telomerase activity and telomere instability were detected in circulating lymphocytes from HL patients. The beneficial effect of the histone-deacetylase inhibitor EDO-S101 as an anti-tumor drug validated our animal model. Conclusion: Our HL animal model requires only 103 cells and is characterized by a high survival/toxicity ratio and high reproducibility. Moreover, the cells that engraft in mice are characterized by a high frequency of small CD30−/CD15− cells exhibiting high telomerase activity and telomere dysfunction.

Independent Mechanisms Lead to Genomic Instability in Hodgkin Lymphoma: Microsatellite or Chromosomal Instability †.

Background: Microsatellite and chromosomal instability have been investigated in Hodgkin lymphoma (HL). Materials and Methods: We studied seven HL cell lines (five Nodular Sclerosis (NS) and two Mixed Cellularity (MC)) and patient peripheral blood lymphocytes (100 NS-HL and 23 MC-HL). Microsatellite instability (MSI) was assessed by PCR. Chromosomal instability and telomere dysfunction were investigated by FISH. DNA repair mechanisms were studied by transcriptomic and molecular approaches. Results: In the cell lines, we observed high MSI in L428 (4/5), KMH2, and HDLM2 (3/5), low MSI in L540, L591, and SUP-HD1, and none in L1236. NS-HL cell lines showed telomere shortening, associated with alterations of nuclear shape. Small cells were characterized by telomere loss and deletion, leading to chromosomal fusion, large nucleoplasmic bridges, and breakage/fusion/bridge (B/F/B) cycles, leading to chromosomal instability. The MC-HL cell lines showed substantial heterogeneity of telomere length. Intrachromosmal double strand breaks induced dicentric chromosome formation, high levels of micronucleus formation, and small nucleoplasmic bridges. B/F/B cycles induced complex chromosomal rearrangements. We observed a similar pattern in circulating lymphocytes of NS-HL and MC-HL patients. Transcriptome analysis confirmed the differences in the DNA repair pathways between the NS and MC cell lines. In addition, the NS-HL cell lines were radiosensitive and the MC-cell lines resistant to apoptosis after radiation exposure. Conclusions: In mononuclear NS-HL cells, loss of telomere integrity may present the first step in the ongoing process of chromosomal instability. Here, we identified, MSI as an additional mechanism for genomic instability in HL.

Centromere Destiny in Dicentric Chromosomes: New Insights from the Evolution of Human Chromosome 2 Ancestral Centromeric Region

Dicentric chromosomes are products of genomic rearrangements that place two centromeres on the same chromosome. Due to the presence of two primary constrictions, they are inherently unstable and overcome their instability by epigenetically inactivating and/or deleting one of the two centromeres, thus resulting in functionally monocentric chromosomes that segregate normally during cell division. Our understanding to date of dicentric chromosome formation, behavior and fate has been largely inferred from observational studies in plants and humans as well as artificially produced de novo dicentrics in yeast and in human cells. We investigate the most recent product of a chromosome fusion event fixed in the human lineage, human chromosome 2, whose stability was acquired by the suppression of one centromere, resulting in a unique difference in chromosome number between humans (46 chromosomes) and our most closely related ape relatives (48 chromosomes). Using molecular cytogenetics, sequencing, and comparative sequence data, we deeply characterize the relicts of the chromosome 2q ancestral centromere and its flanking regions, gaining insight into the ancestral organization that can be easily broadened to all acrocentric chromosome centromeres. Moreover, our analyses offered the opportunity to trace the evolutionary history of rDNA and satellite III sequences among great apes, thus suggesting a new hypothesis for the preferential inactivation of some human centromeres, including IIq. Our results suggest two possible centromere inactivation models to explain the evolutionarily stabilization of human chromosome 2 over the last 5-6 million years. Our results strongly favor centromere excision through a one-step process.

"Perfect" designer chromosome V and behavior of a ring derivative.

Perfect matching of an assembled physical sequence to a specified designed sequence is crucial to verify design principles in genome synthesis. We designed and de novo synthesized 536,024-base pair chromosome synV in the "Build-A-Genome China" course. We corrected an initial isolate of synV to perfectly match the designed sequence using integrative cotransformation and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated editing in 22 steps; synV strains exhibit high fitness under a variety of culture conditions, compared with that of wild-type V strains. A ring synV derivative was constructed, which is fully functional in Saccharomyces cerevisiae under all conditions tested and exhibits lower spore viability during meiosis. Ring synV chromosome can extends Sc2.0 design principles and provides a model with which to study genomic rearrangement, ring chromosome evolution, and human ring chromosome disorders.

Establishment of Induced Pluripotent Stem Cells from Lymph Node B Cells and Induction of Aid Expression in Their Differentiation into Hematopoietic Stem Cells

Induced pluripotent stem cells (iPSCs) have been established from a variety of somatic cells not only for regenerative medicine but also for studies of the pathogenesis of inherited genetic diseases or neoplasms. In iPSCs established from T cells, the rearrangement of the T cell receptor (TCR) of the established T cell-derived iPSCs (TiPSCs) took over that of the original T cells. If B cell-derived iPSCs (BiPSCs) could be similarly established from mature B cells or plasma cells, the rearrangement of the B cell receptor (BCR) of the BiPSCs would take over that of the original B cell. BiPSCs were recently established from peripheral blood B cells by the simultaneous transfection of Yamanaka factors (Oct3/4, Sox2, Klf4, c-Myc) together with ectopic expression of the myeloid transcription factor CCAAT/enhancer-binding-protein-α (C/EBPα) using a Sendai virus vector (Bueno C, et al. Leukemia 2016).We were able to also establish BiPSCs with immunoglobulin heavy chain (IgH) gene rearrangement from normal B cells purified from lymph nodes using a method different from the above. The main points of our method are consecutive transfection of a high concentration of Yamanaka factors into B cells, which were pre-stimulated with IL-21 and CD40L, using a retrovirus vector, and centrifugations of the cells after their activation by IL-4, IL-2, and CD40-ligand (CD40L) on a retronectin coated plate. We further established doxycycline-controlled (Tet-off system) activation-induced cytidine deaminase (AID)-induced BiPSCs (BiPSCs-A). AID is an enzyme that initiates somatic hypermutation (SHM) and class-switch recombination (CSR) in B cells. We hypothesized that the origin of a myeloma cell is a reprogrammed mature B cell, in which reciprocal chromosome translocation occurs by double stranded breakage (DSB) of DNA induced by AID activation in the nonproductive (nonfunctional) allele of chromosome 14. First, we did not detect an increase in dicentric chromosome (DIC) formation, which is evidence of DSB of DNA, in the BiPSCs-A.We next analyzed the ability of these BiPSCs to differentiate into hematopoietic stem cells (HSCs). Both the parental BiPSCs and BiPSCs-A were capable of differentiating into HSCs as judged by confirmation of CD34 expression and colony-formation of macrophages, granulocytes, and erythrocytes from CD34-positive cells. However, these cells were negative for CD38, CD43, and CD45; we therefore think that these CD34+/CD38-/CD43-/CD45- cells might be hematoendothelial cells as Maxim proposed previously (Vodyanik MA, et al. Blood 2006). Furthermore, both BiPSCs with induced AID expression and BiPSCs without induced AID expression were capable of differentiating into HSCs. Based on the findings regarding the differentiation of BiPSCs-A into HSCs and their retention of the IgH gene rearrangement, there is a possibility that the induction of AID expression might induce chromosomal translocations in the process of differentiation of these BiPSCs into HSCs and further into B cells in experiments using mouse. Thus these BiPSCs might be useful in elucidating the tumor origin of abnormal B cells in B cell tumor formation. These cells might be especially useful in understanding multiple myeloma, which is thought to originate from germinal center (GC) or post-GC B cells and has a productive (functional) allele with IgH gene rearrangement that produces M-protein, and another nonproductive (nonfunctional) allele of chromosome 14. DisclosuresNo relevant conflicts of interest to declare.

Concurrent Amplification of MYC and 1q21 in Multiple Myeloma: Focal and Segmental Jumping Translocations of MYC

IntroductionMultiple myeloma (MM) is characterized by an accumulation of secondary chromosome aberrations which lead to cytogenetically heterogeneous subpopulations of cells in the subclonal progression of this disease. Copy number aberrations (CNAs) represent a large proportion of these aberrations and include both numerical and structural chromosome aberrations which involve unbalanced translocations, amplifications, insertions, and deletions.Secondary MYC translocations are thought to play a role in driving the transition from MGUS to MM, and during tumor progression additional secondary CNAs of both MYC (8q24) and 1q21 are found in relapsed patients. When CNAs are taken into account the percentage of myeloma cases where MYC is deregulated is 55%, making it the most common genetic event in MM (Walker B. et al., Nature Comm, 2015). In fact, high resolution SNP arrays have shown that somatic CNAs of MYC and 1q21 are two of the top ten CNAs in all cancers (Beroukhim R. et al., Nature, 2010). Two types of amplification predominate by SNPs array, either large arm-length amplifications (25% of the genome) or small focal CNAs (10% of the genome). Unfortunately, the origins of the complex structural chromosomal aberrations which underlie these types CNAs have not been well characterized in MM.To better understand the possible relationship between MYC and 1q21 CNAs we undertook a comprehensive metaphase cytogenetic analysis of 26 diagnostic specimens showing concurrent MYC and 1q21 CNAs by G-banding.MethodsLocus specific FISH probes and Spectral karyotyping were used to define and follow the progression of aberrations occurring in these two regions. Locus specific FISH probes for selected regions of chromosomes 1 and 8 pericentromeric regions and the regions surrounding 1q21 and 8q24 were utilized.ResultsThree primary aberration patterns were observed:The most frequent pattern involved copy number gains in both regions, apparently occurring independently of each other (sixteen patients).The second most frequent pattern showed unbalanced segmental jumping translocations (SJTs) of 8q and the MYC locus to different receptor chromosomes. These unbalanced translocations of MYC were found in seven patients involving varying sized segments of 8q, and included both intra and inter-chromosomal translocations. Surprisingly, three patients in this group showed clones with the co-amplification of MYC and 1q21 on the same derivative chromosome, where 8q and MYC were in essence piggybacking on jumping translocations of 1q12 (JT1q12) to receptor chromosomes.Finally, the third and most complex structural aberration pattern involved the breakage-fusion-bridge cycle mechanism, which results in focal amplifications of genes. This mechanism typically results from telomere shortening and loss and subsequent formation of dicentric chromosomes. Evidence of dicentric chromosomes 8 and the breakage fusion break amplification of MYC was identified in three patients and, strikingly, in one case was found to be initiated by a JT1q12 to 8qter. Therefore, this case identifies a novel alternative mechanism for the focal amplification of the MYC locus whereby the JT1q12 initiates sister chromatid fusion at 8q24.ConclusionWe demonstrate for the first time the mechanistic coupling of MYC and 1q21 amplifications involving sequential jumping translocations of 8q and JT1q12s. These findings provide evidence for the origin of both arm-length and focal amplifications of MYC, and that these aberrations can co-evolve in the same clones. The concurrent amplification of MYC and 1q21 suggests pericentromeric instability, and to a lesser extent telomeric instability, play a role in driving CNAs of both of these genomic lesions in MM. DisclosuresDavies:Celgene: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Morgan:Univ of AR for Medical Sciences: Employment; Celgene: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria; Bristol Meyers: Consultancy, Honoraria; Janssen: Research Funding.

Molecular mechanisms by which in vivo exposure to exogenous chemical genotoxic agents can lead to micronucleus formation in lymphocytes in vivo and ex vivo in humans

The purpose of this review is to summarise current knowledge on the molecular mechanisms by which in vivo exposure to exogenous chemical genotoxins in humans induces micronuclei (MNi) and other nuclear anomalies in lymphocytes in vivo and ex vivo after nuclear division in vitro. MNi originate from acentric chromosome fragments and/or whole chromosomes that are unable to engage with the mitotic spindle and/or fail to segregate properly to the daughter nuclei during anaphase. The lagging fragments or whole chromosomes are surrounded by membrane and become MNi. Acentric fragments are caused by failure of repair or mis-repair of DNA strand breaks which may be induced by chemicals that (i) damage the phosphodiester backbone of DNA, and/or (ii) inhibit the DNA damage response mechanisms or repair of DNA strand breaks and/or (iii) cause DNA replication stress due to DNA adduct or cross-link formation. MNi originating from lagging whole chromosomes may be induced by chemicals that cause defects in centromeres or the mitotic machinery. Mis-repair of chemically-induced DNA breaks may also cause formation of dicentric chromosomes and nucleoplasmic bridges (NPBs) between daughter nuclei in mitosis. NPBs may break and initiate recurring breakage-fusion-bridge cycles and chromosomal instability. The review also explores knowledge on (i) the routes by which lymphocytes in the human body may be exposed to genotoxic chemicals, (ii) kinetics of MNi expression in lymphocytes in vivo and ex vivo in the lymphocyte cytokinesis-block micronucleus (L-CBMN) assay and (iii) current evidence on the efficiency of the L-CBMN assay in detecting in vivo exposure to chemical genotoxins and its concordance with MNi expression in epithelial tissues. The review also identifies important knowledge gaps (e.g. effect of nanomaterials; interactions with nutritional deficiencies etc.) regarding mechanisms by which in vivo chemical genotoxin exposure may cause MNi formation in lymphocytes in vivo and ex vivo in lymphocytes.

Analysis of chromosome translocation frequency after a single CT scan in adults.

We recently reported an increase in dicentric chromosome (DIC) formation after a single computed tomography (CT) scan (5.78–60.27 mSv: mean 24.24 mSv) and we recommended analysis of 2000 metaphase cells stained with Giemsa and centromere-FISH for dicentric chromosome assay (DCA) in cases of low-dose radiation exposure. In the present study, we analyzed the frequency of chromosome translocations using stored Carnoy's-fixed lymphocyte specimens from the previous study; these specimens were from 12 patients who were subject to chromosome painting of Chromosomes 1, 2 and 4. Chromosomes 1, 2 and 4 were analyzed in ∼5000 cells, which is equivalent to the whole-genome analysis of almost 2000 cells. The frequency of chromosome translocation was higher than the number of DICs formed, both before and after CT scanning. The frequency of chromosome translocations tended to be higher, but not significantly higher, in patients with a treatment history compared with patients without such a history. However, in contrast to the results for DIC formation, the frequency of translocations detected before and after the CT scan did not differ significantly. Therefore, analysis of chromosome translocation may not be a suitable assay for detecting chromosome aberrations in cases of low-dose radiation exposure from a CT scan. A significant increase in the frequency of chromosome translocations was not likely to be detected due to the high baseline before the CT scan; the high and variable frequency of translocations was probably due to multiple confounding factors in adults.

Increase in dicentric chromosome formation after a single CT scan in adults.

Excess risk of leukemia and brain tumors after CT scans in children has been reported. We performed dicentric chromosome assay (DCAs) before and after CT scan to assess effects of low-dose ionizing radiation on chromosomes. Peripheral blood (PB) lymphocytes were collected from 10 patients before and after a CT scan. DCA was performed by analyzing either 1,000 or 2,000 metaphases using both Giemsa staining and centromere-fluorescence in situ hybridization (Centromere-FISH). The increment of DIC formation was compared with effective radiation dose calculated using the computational dosimetry system, WAZA-ARI and dose length product (DLP) in a CT scan. Dicentric chromosome (DIC) formation increased significantly after a single CT scan, and increased DIC formation was found in all patients. A good correlation between the increment of DIC formation determined by analysis of 2,000 metaphases using Giemsa staining and those by 2,000 metaphases using Centromere-FISH was observed. However, no correlation was observed between the increment of DIC formation and the effective radiation dose. Therefore, these results suggest that chromosome cleavage may be induced by one CT scan, and we recommend 2,000 or more metaphases be analyzed in Giemsa staining or Centromere-FISH for DCAs in cases of low-dose radiation exposure.

Cytogenetic evidence that DNA topoisomerase II is not involved in radiation induced chromsome-type aberrations

ICRF-187 (Cardioxane™, Chiron) is a catalytic inhibitor of DNA topoisomerase II (Topo II), proposed to act by blocking Topo II-mediated DNA cleavage without stabilizing DNA-Topo II-“cleavable complexes”. In this study ICRF-187 was used to evaluate the potential involvement of DNA topoisomerase II in the formation of the radiation-induced chromosome-type aberrations in the G0 phase of the cell cycle in human lymphocytes from three healthy male donors. This is based on many evidences that DNA topoisomerases are involved in DNA recombination, mainly of illegitimate type (non-homologous) both in vitro and in vivo. The results obtained clearly indicated that ICRF-187 did not induce per se any chromosomal damage. When challenged with the non-catalytic Topo II poison VP-16 (etoposide), which acts by stabilizing the “cleavable complex” generating “protein concealed” DSB’s and thus chromosomal aberrations, it completely abolished the significant induction of chromosome-type aberrations and formation of dicentric chromosomes. This indicates that ICRF-187 acts effectively as catalytic inhibitor of Topo II. On the other hand, when X-ray treatments were challenged with ICRF-187 using experimental conditions as for VP-16 treatments, no modification of the incidence of chromosome-type aberrations and dicentric chromosomes was observed. On this basis, we conclude that Topo II is not involved in the formation of X-ray-induced chromosome-type aberrations and dicentric chromosomes in human lymphocytes in the G0 phase of the cell cycle.