1
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Rangel V, Sterrenberg JN, Garawi A, Mezcord V, Folkerts ML, Calderon SE, Garcia YE, Wang J, Soyfer EM, Eng OS, Valerin JB, Tanjasiri SP, Quintero-Rivera F, Seldin MM, Masri S, Frock RL, Fleischman AG, Pannunzio NR. Increased AID results in mutations at the CRLF2 locus implicated in Latin American ALL health disparities. Nat Commun 2024; 15:6331. [PMID: 39068148 PMCID: PMC11283463 DOI: 10.1038/s41467-024-50537-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2023] [Accepted: 07/10/2024] [Indexed: 07/30/2024] Open
Abstract
Activation-induced cytidine deaminase (AID) is a B cell-specific mutator required for antibody diversification. However, it is also implicated in the etiology of several B cell malignancies. Evaluating the AID-induced mutation load in patients at-risk for certain blood cancers is critical in assessing disease severity and treatment options. We have developed a digital PCR (dPCR) assay that allows us to quantify mutations resulting from AID modification or DNA double-strand break (DSB) formation and repair at sites known to be prone to DSBs. Implementation of this assay shows that increased AID levels in immature B cells increase genome instability at loci linked to chromosomal translocation formation. This includes the CRLF2 locus that is often involved in translocations associated with a subtype of acute lymphoblastic leukemia (ALL) that disproportionately affects Hispanics, particularly those with Latin American ancestry. Using dPCR, we characterize the CRLF2 locus in B cell-derived genomic DNA from both Hispanic ALL patients and healthy Hispanic donors and found increased mutations in both, suggesting that vulnerability to DNA damage at CRLF2 may be driving this health disparity. Our ability to detect and quantify these mutations will potentiate future risk identification, early detection of cancers, and reduction of associated cancer health disparities.
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Affiliation(s)
- Valeria Rangel
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
| | - Jason N Sterrenberg
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
| | - Aya Garawi
- School of Biological Sciences, University of California, Irvine, Irvine, CA, USA
| | - Vyanka Mezcord
- Center for Applied Biotechnology Studies, Department of Biological Science, California State University Fullerton, Fullerton, CA, USA
| | - Melissa L Folkerts
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
| | - Sabrina E Calderon
- School of Biological Sciences, University of California, Irvine, Irvine, CA, USA
| | - Yadhira E Garcia
- Department of Pharmaceutical Sciences, School of Pharmacy & Pharmaceutical Sciences, University of California, Irvine, CA, USA
| | - Jinglong Wang
- Division of Radiation and Cancer Biology, Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, USA
| | - Eli M Soyfer
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
| | - Oliver S Eng
- Division of Surgical Oncology, Department of Surgery, University of California, Irvine, Irvine, CA, USA
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
| | - Jennifer B Valerin
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
| | - Sora Park Tanjasiri
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
- Department of Health, Society and Behavior, University of California, Irvine, Irvine, CA, USA
| | - Fabiola Quintero-Rivera
- Department of Pathology and Laboratory Medicine, University of California, Irvine, Irvine, CA, USA
- Department of Pediatrics, University of California, Irvine, Irvine, CA, USA
| | - Marcus M Seldin
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
| | - Selma Masri
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
| | - Richard L Frock
- Division of Radiation and Cancer Biology, Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, USA
| | - Angela G Fleischman
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA
| | - Nicholas R Pannunzio
- Division of Hematology/Oncology, Department of Medicine, University of California, Irvine, Irvine, CA, USA.
- Department of Biological Chemistry, University of California, Irvine, Irvine, CA, USA.
- Chao Family Comprehensive Cancer Center, University of California, Irvine, Irvine, CA, USA.
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2
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Liu D, Hsieh CL, Lieber MR. The RNA tether model for human chromosomal translocation fragile zones. Trends Biochem Sci 2024; 49:391-400. [PMID: 38490833 PMCID: PMC11069435 DOI: 10.1016/j.tibs.2024.02.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2023] [Revised: 02/06/2024] [Accepted: 02/16/2024] [Indexed: 03/17/2024]
Abstract
One of the two chromosomal breakage events in recurring translocations in B cell neoplasms is often due to the recombination-activating gene complex (RAG complex) releasing DNA ends before end joining. The other break occurs in a fragile zone of 20-600 bp in a non-antigen receptor gene locus, with a more complex and intriguing set of mechanistic factors underlying such narrow fragile zones. These factors include activation-induced deaminase (AID), which acts only at regions of single-stranded DNA (ssDNA). Recent work leads to a model involving the tethering of AID to the nascent RNA as it emerges from the RNA polymerase. This mechanism may have relevance in class switch recombination (CSR) and somatic hypermutation (SHM), as well as broader relevance for other DNA enzymes.
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Affiliation(s)
- Di Liu
- Institute of Molecular and Translational Medicine (IMTM), and Department of Biochemistry and Molecular Biology, Xi'an Jiaotong University Health Science Center, and Key Laboratory of Environment and Genes Related to Diseases, Xi'an Jiaotong University, Ministry of Education, Xi'an, Shaanxi 710061, China
| | - Chih-Lin Hsieh
- USC Norris Comprehensive Cancer Center, Department of Urology, University of Southern California, Los Angeles, CA 90089-9176, USA
| | - Michael R Lieber
- USC Norris Comprehensive Cancer Center, Departments of Pathology and Laboratory Medicine, of Molecular Microbiology and Immunology, of Biochemistry and Molecular Medicine, and in the Section of Molecular and Computational Biology, University of Southern California, Los Angeles, CA 90089-9176, USA.
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3
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Pannunzio N, Rangel V, Sterrenberg J, Garawi A, Mezcord V, Folkerts M, Caulderon S, Wang J, Soyfer E, Eng O, Valerin J, Tanjasiri S, Quintero-Rivera F, Masri S, Seldin M, Frock R, Fleischman A. Increased AID Results in Mutations at the CRLF2 Locus Implicated in Latin American ALL Health Disparities. RESEARCH SQUARE 2023:rs.3.rs-3332673. [PMID: 37790327 PMCID: PMC10543404 DOI: 10.21203/rs.3.rs-3332673/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/05/2023]
Abstract
Activation-induced cytidine deaminase (AID) is a B cell-specific base editor required during class switch recombination and somatic hypermutation for B cell maturation and antibody diversification. However, it has also been implicated as a factor in the etiology of several B cell malignancies. Evaluating the AID-induced mutation load in patients at-risk for certain types of blood cancers is critical in assessing disease severity and treatment options. Here, we have developed a digital PCR (dPCR) assay that allows us to track the mutational landscape resulting from AID modification or DNA double-strand break (DSB) formation and repair at sites known to be prone to DSBs. Implementation of this new assay showed that increased AID levels in immature B cells increases genome instability at loci linked to translocation formation. This included the CRLF2 locus that is often involved in chromosomal translocations associated with a subtype of acute lymphoblastic leukemia (ALL) that disproportionately affects Latin Americans (LAs). To support this LA-specific identification of AID mutation signatures, we characterized DNA from immature B cells isolated from the bone marrow of ALL patients. Our ability to detect and quantify these mutation signatures will potentiate future risk identification, early detection of cancers, and reduction of associated cancer health disparities.
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4
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Sterrenberg JN, Folkerts ML, Rangel V, Lee SE, Pannunzio NR. Diversity upon diversity: linking DNA double-strand break repair to blood cancer health disparities. Trends Cancer 2022; 8:328-343. [PMID: 35094960 PMCID: PMC9248772 DOI: 10.1016/j.trecan.2022.01.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2021] [Revised: 12/20/2021] [Accepted: 01/03/2022] [Indexed: 10/19/2022]
Abstract
Chromosomal translocations arising from aberrant repair of multiple DNA double-strand breaks (DSBs) are a defining characteristic of many cancers. DSBs are an essential part of physiological processes in antibody-producing B cells. The B cell environment is poised to generate genome instability leading to translocations relevant to the pathology of blood cancers. These are a diverse set of cancers, but limited data from under-represented groups have pointed to health disparities associated with each. We focus on the DSBs that occur in developing B cells and propose the most likely mechanism behind the formation of translocations. We also highlight specific cancers in which these rearrangements occur and address the growing concern of health disparities associated with them.
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5
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Liu D, Lieber MR. The mechanisms of human lymphoid chromosomal translocations and their medical relevance. Crit Rev Biochem Mol Biol 2021; 57:227-243. [PMID: 34875186 DOI: 10.1080/10409238.2021.2004576] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
Abstract
The most common human lymphoid chromosomal translocations involve concurrent failures of the recombination activating gene (RAG) complex and Activation-Induced Deaminase (AID). These are two enzymes that are normally expressed for purposes of the two site-specific DNA recombination processes: V(D)J recombination and class switch recombination (CSR). First, though it is rare, a low level of expression of AID can introduce long-lived T:G mismatch lesions at 20-600 bp fragile zones. Second, the V(D)J recombination process can occasionally fail to rejoin coding ends, and this failure may permit an opportunity for Artemis:DNA-dependent kinase catalytic subunit (DNA-PKcs) to convert the T:G mismatch sites at the fragile zones into double-strand breaks. The 20-600 bp fragile zones must be, at least transiently, in a single-stranded DNA (ssDNA) state for the first step to occur, because AID only acts on ssDNA. Here we discuss the key DNA sequence features that lead to AID action at a fragile zone, which are (a) the proximity and density of strings of cytosine nucleotides (C-strings) that cause a B/A-intermediate DNA conformation; (b) overlapping AID hotspots that contain a methyl CpG (WRCG), which AID converts to a long-lived T:G mismatch; and (c) transcription, which, though not essential, favors increased ssDNA in the fragile zone. We also summarize chromosomal features of the focal fragile zones in lymphoid malignancies and discuss the clinical relevance of understanding the translocation mechanisms. Many of the key principles covered here are also relevant to chromosomal translocations in non-lymphoid somatic cells as well.
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Affiliation(s)
- Di Liu
- Department of Pathology & Laboratory Medicine, Department of Biochemistry & Molecular Biology, Department of Molecular Microbiology and Immunology, and Section of Computational Biology in the Department of Biological Sciences, USC Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA
| | - Michael R Lieber
- Department of Pathology & Laboratory Medicine, Department of Biochemistry & Molecular Biology, Department of Molecular Microbiology and Immunology, and Section of Computational Biology in the Department of Biological Sciences, USC Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA
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6
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Liu D, Loh YHE, Hsieh CL, Lieber MR. Mechanistic basis for chromosomal translocations at the E2A gene and its broader relevance to human B cell malignancies. Cell Rep 2021; 36:109387. [PMID: 34260910 PMCID: PMC8327686 DOI: 10.1016/j.celrep.2021.109387] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2021] [Revised: 03/31/2021] [Accepted: 06/21/2021] [Indexed: 11/22/2022] Open
Abstract
Analysis of translocation breakpoints in human B cell malignancies reveals that DNA double-strand breaks at oncogenes most frequently occur at CpG sites located within 20-600 bp fragile zones and depend on activation-induced deaminase (AID). AID requires single-stranded DNA (ssDNA) to act, but it has been unclear why or how this region transiently acquires a ssDNA state. Here, we demonstrate the ssDNA state in the 23 bp E2A fragile zone using several methods, including native bisulfite DNA structural analysis in live human pre-B cells. AID deamination within the E2A fragile zone does not require but is increased upon transcription. High C-string density, nascent RNA tails, and direct DNA sequence repeats prolong the ssDNA state of the E2A fragile zone and increase AID deamination at overlapping AID hotspots that contain the CpG sites at which breaks occur in patients. These features provide key insights into lymphoid fragile zones generally.
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Affiliation(s)
- Di Liu
- Departments of Pathology, Biochemistry & Molecular Biology, and Molecular Microbiology & Immunology, and Section of Molecular & Computational Biology (Department of Biological Sciences), USC Norris Comprehensive Cancer Center, University of Southern California and USC Keck School of Medicine, Los Angeles, CA, USA
| | - Yong-Hwee Eddie Loh
- USC Libraries Bioinformatics Services, University of Southern California and USC Keck School of Medicine, Los Angeles, CA, USA
| | - Chih-Lin Hsieh
- Department of Urology, USC Norris Comprehensive Cancer Center, University of Southern California and USC Keck School of Medicine, Los Angeles, CA, USA
| | - Michael R Lieber
- Departments of Pathology, Biochemistry & Molecular Biology, and Molecular Microbiology & Immunology, and Section of Molecular & Computational Biology (Department of Biological Sciences), USC Norris Comprehensive Cancer Center, University of Southern California and USC Keck School of Medicine, Los Angeles, CA, USA.
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7
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The molecular basis and disease relevance of non-homologous DNA end joining. Nat Rev Mol Cell Biol 2020; 21:765-781. [PMID: 33077885 DOI: 10.1038/s41580-020-00297-8] [Citation(s) in RCA: 201] [Impact Index Per Article: 50.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/08/2020] [Indexed: 12/26/2022]
Abstract
Non-homologous DNA end joining (NHEJ) is the predominant repair mechanism of any type of DNA double-strand break (DSB) during most of the cell cycle and is essential for the development of antigen receptors. Defects in NHEJ result in sensitivity to ionizing radiation and loss of lymphocytes. The most critical step of NHEJ is synapsis, or the juxtaposition of the two DNA ends of a DSB, because all subsequent steps rely on it. Recent findings show that, like the end processing step, synapsis can be achieved through several mechanisms. In this Review, we first discuss repair pathway choice between NHEJ and other DSB repair pathways. We then integrate recent insights into the mechanisms of NHEJ synapsis with updates on other steps of NHEJ, such as DNA end processing and ligation. Finally, we discuss NHEJ-related human diseases, including inherited disorders and neoplasia, which arise from rare failures at different NHEJ steps.
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8
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Rebolledo-Jaramillo B, Ziegler A. Teneurins: An Integrative Molecular, Functional, and Biomedical Overview of Their Role in Cancer. Front Neurosci 2018; 12:937. [PMID: 30618566 PMCID: PMC6297388 DOI: 10.3389/fnins.2018.00937] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2018] [Accepted: 11/28/2018] [Indexed: 12/16/2022] Open
Abstract
Teneurins are large transmembrane proteins originally identified in Drosophila. Their essential role in development of the central nervous system is conserved throughout species, and evidence supports their involvement in organogenesis of additional tissues. Homophilic and heterophilic interactions between Teneurin paralogues mediate cellular adhesion in crucial processes such as neuronal pathfinding and synaptic organization. At the molecular level, Teneurins are proteolytically processed into distinct subdomains that have been implicated in extracellular and intracellular signaling, and in transcriptional regulation. Phylogenetic studies have shown a high degree of intra- and interspecies conservation of Teneurin genes. Accordingly, the occurrence of genetic variants has been associated with functional and phenotypic alterations in experimental systems, and with some inherited or sporadic conditions. Recently, tumor-related variations in Teneurin gene expression have been associated with patient survival in different cancers. Although these findings were incidental and molecular mechanisms were not addressed, they suggested a potential utility of Teneurin transcript levels as biomarkers for disease prognosis. Mutations and chromosomal alterations affecting Teneurin genes have been found occasionally in tumors, but literature remains scarce. The analysis of open-access molecular and clinical datasets derived from large oncologic cohorts provides an invaluable resource for the identification of additional somatic mutations. However, Teneurin variants have not been classified in terms of pathogenic risk and their phenotypic impact remains unknown. On this basis, is it plausible to hypothesize that Teneurins play a role in carcinogenesis? Does current evidence support a tumor suppressive or rather oncogenic function for these proteins? Here, we comprehensively discuss available literature with integration of molecular evidence retrieved from open-access databases. We show that Teneurins undergo somatic changes comparable to those of well-established cancer genes, and discuss their involvement in cancer-related signaling pathways. Current data strongly suggest a functional contribution of Teneurins to human carcinogenesis.
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Affiliation(s)
| | - Annemarie Ziegler
- Center for Genetics and Genomics, Facultad de Medicina, Clínica Alemana Universidad del Desarrollo, Santiago, Chile
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9
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Pannunzio NR, Lieber MR. Concept of DNA Lesion Longevity and Chromosomal Translocations. Trends Biochem Sci 2018; 43:490-498. [PMID: 29735400 DOI: 10.1016/j.tibs.2018.04.004] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2018] [Revised: 04/07/2018] [Accepted: 04/08/2018] [Indexed: 01/11/2023]
Abstract
A subset of chromosomal translocations related to B cell malignancy in human patients arises due to DNA breaks occurring within defined 20-600 base pair (bp) zones. Several factors influence the breakage rate at these sites including transcription, DNA sequence, and topological tension. These factors favor non-B DNA structures that permit formation of transient single-stranded DNA (ssDNA), making the DNA more vulnerable to agents such as the enzyme activation-induced cytidine deaminase (AID) and reactive oxygen species (ROS). Certain DNA lesions created during the ssDNA state persist after the DNA resumes its normal duplex structure. We propose that factors favoring both formation of transient ssDNA and persistent DNA lesions are key in determining the DNA breakage mechanism.
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Affiliation(s)
- Nicholas R Pannunzio
- University of Southern California Keck School of Medicine, Norris Comprehensive Cancer Center, Los Angeles, CA 90033, USA
| | - Michael R Lieber
- University of Southern California Keck School of Medicine, Norris Comprehensive Cancer Center, Los Angeles, CA 90033, USA; Department of Pathology, Keck School of Medicine of USC, Los Angeles, CA 90033, USA; Department of Molecular Microbiology and Immunology, Keck School of Medicine of USC, Los Angeles, CA 90033, USA; Department of Biochemistry and Molecular Biology, Keck School of Medicine of USC, Los Angeles, CA 90033, USA; Section of Molecular and Computational Biology, University of Southern California, Los Angeles, CA 90089, USA.
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10
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Huet S, Sujobert P, Salles G. From genetics to the clinic: a translational perspective on follicular lymphoma. Nat Rev Cancer 2018; 18:224-239. [PMID: 29422597 DOI: 10.1038/nrc.2017.127] [Citation(s) in RCA: 93] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Follicular lymphoma (FL) is the most frequent indolent B cell lymphoma and is still considered to be incurable. In recent years, whole-exome sequencing studies of large cohorts of patients have greatly improved our knowledge of the FL mutational landscape. Moreover, the prolonged evolution of this disease has enabled some insights regarding the early pre-lymphoma lesions as well as the clonal evolution after treatment, allowing an evolutionary perspective on lymphomagenesis. Deciphering the earliest initiating lesions and identifying the molecular alterations leading to disease progression currently represent important goals; accomplishing these could help identify the most relevant targets for precision therapy.
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Affiliation(s)
- Sarah Huet
- Cancer Research Center of Lyon, INSERM 1052 CNRS5286, 'Clinical and experimental models of lymphomagenesis' Team, Equipe labellisée Ligue Contre le Cancer Oullins, France
- Hospices Civils de Lyon, Centre Hospitalier Lyon-Sud, 165 chemin du Grand Revoyet, Pierre Bénite 69495, France
- Université Lyon-1, ISPB-Faculté de Pharmacie de Lyon, Lyon, France
| | - Pierre Sujobert
- Cancer Research Center of Lyon, INSERM 1052 CNRS5286, 'Clinical and experimental models of lymphomagenesis' Team, Equipe labellisée Ligue Contre le Cancer Oullins, France
- Hospices Civils de Lyon, Centre Hospitalier Lyon-Sud, 165 chemin du Grand Revoyet, Pierre Bénite 69495, France
- Université Lyon-1, Faculté de Médecine et de Maïeutique Lyon-Sud Charles Mérieux, Oullins, France
| | - Gilles Salles
- Cancer Research Center of Lyon, INSERM 1052 CNRS5286, 'Clinical and experimental models of lymphomagenesis' Team, Equipe labellisée Ligue Contre le Cancer Oullins, France
- Hospices Civils de Lyon, Centre Hospitalier Lyon-Sud, 165 chemin du Grand Revoyet, Pierre Bénite 69495, France
- Université Lyon-1, Faculté de Médecine et de Maïeutique Lyon-Sud Charles Mérieux, Oullins, France
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11
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Pannunzio NR, Lieber MR. AID and Reactive Oxygen Species Can Induce DNA Breaks within Human Chromosomal Translocation Fragile Zones. Mol Cell 2017; 68:901-912.e3. [PMID: 29220655 DOI: 10.1016/j.molcel.2017.11.011] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2017] [Revised: 10/02/2017] [Accepted: 11/10/2017] [Indexed: 01/07/2023]
Abstract
DNA double-strand breaks (DSBs) occurring within fragile zones of less than 200 base pairs account for the formation of the most common human chromosomal translocations in lymphoid malignancies, yet the mechanism of how breaks occur remains unknown. Here, we have transferred human fragile zones into S. cerevisiae in the context of a genetic assay to understand the mechanism leading to DSBs at these sites. Our findings indicate that a combination of factors is required to sensitize these regions. Foremost, DNA strand separation by transcription or increased torsional stress can expose these DNA regions to damage from either the expression of human AID or increased oxidative stress. This damage causes DNA lesions that, if not repaired quickly, are prone to nuclease cleavage, resulting in DSBs. Our results provide mechanistic insight into why human neoplastic translocation fragile DNA sequences are more prone to enzymes or agents that cause longer-lived DNA lesions.
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Affiliation(s)
- Nicholas R Pannunzio
- USC Norris Comprehensive Cancer Center, University of Southern California Keck School of Medicine, 1441 Eastlake Avenue, Rm. 5428, Los Angeles, CA 90089, USA
| | - Michael R Lieber
- USC Norris Comprehensive Cancer Center, University of Southern California Keck School of Medicine, 1441 Eastlake Avenue, Rm. 5428, Los Angeles, CA 90089, USA.
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12
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Khan FA, Ali SO. Physiological Roles of DNA Double-Strand Breaks. J Nucleic Acids 2017; 2017:6439169. [PMID: 29181194 PMCID: PMC5664317 DOI: 10.1155/2017/6439169] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2017] [Accepted: 09/24/2017] [Indexed: 12/20/2022] Open
Abstract
Genomic integrity is constantly threatened by sources of DNA damage, internal and external alike. Among the most cytotoxic lesions is the DNA double-strand break (DSB) which arises from the cleavage of both strands of the double helix. Cells boast a considerable set of defences to both prevent and repair these breaks and drugs which derail these processes represent an important category of anticancer therapeutics. And yet, bizarrely, cells deploy this very machinery for the intentional and calculated disruption of genomic integrity, harnessing potentially destructive DSBs in delicate genetic transactions. Under tight spatiotemporal regulation, DSBs serve as a tool for genetic modification, widely used across cellular biology to generate diverse functionalities, ranging from the fundamental upkeep of DNA replication, transcription, and the chromatin landscape to the diversification of immunity and the germline. Growing evidence points to a role of aberrant DSB physiology in human disease and an understanding of these processes may both inform the design of new therapeutic strategies and reduce off-target effects of existing drugs. Here, we review the wide-ranging roles of physiological DSBs and the emerging network of their multilateral regulation to consider how the cell is able to harness DNA breaks as a critical biochemical tool.
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Affiliation(s)
- Farhaan A. Khan
- School of Clinical Medicine, Addenbrooke's Hospital, University of Cambridge, Hills Road, Cambridge CB2 0SP, UK
| | - Syed O. Ali
- School of Clinical Medicine, Addenbrooke's Hospital, University of Cambridge, Hills Road, Cambridge CB2 0SP, UK
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13
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Abstract
Analysis of chromosomal translocation sequence locations in human lymphomas has provided valuable clues about the mechanism of the translocations and when they occur. Biochemical analyses on the mechanisms of DNA breakage and rejoining permit formulation of detailed models of the human chromosomal translocation process in lymphoid neoplasms. Most human lymphomas are derived from B cells in which a DNA break at an oncogene is initiated by activation-induced deaminase (AID). The partner locus in many cases is located at one of the antigen receptor loci, and this break is generated by the recombination activating gene (RAG) complex or by AID. After breakage, the joining process typically occurs by non-homologous DNA end-joining (NHEJ). Some of the insights into this mechanism also apply to translocations that occur in non-lymphoid neoplasms.
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Affiliation(s)
- Michael R Lieber
- USC Norris Comprehensive Cancer Center, Room 5428, University of Southern California Keck School of Medicine, 1441 Eastlake Avenue, MC9176, Los Angeles, California 90089-9176, USA
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14
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Meta-Analysis of DNA Tumor-Viral Integration Site Selection Indicates a Role for Repeats, Gene Expression and Epigenetics. Cancers (Basel) 2015; 7:2217-35. [PMID: 26569308 PMCID: PMC4695887 DOI: 10.3390/cancers7040887] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2015] [Revised: 10/21/2015] [Accepted: 11/02/2015] [Indexed: 01/08/2023] Open
Abstract
Oncoviruses cause tremendous global cancer burden. For several DNA tumor viruses, human genome integration is consistently associated with cancer development. However, genomic features associated with tumor viral integration are poorly understood. We sought to define genomic determinants for 1897 loci prone to hosting human papillomavirus (HPV), hepatitis B virus (HBV) or Merkel cell polyomavirus (MCPyV). These were compared to HIV, whose enzyme-mediated integration is well understood. A comprehensive catalog of integration sites was constructed from the literature and experimentally-determined HPV integration sites. Features were scored in eight categories (genes, expression, open chromatin, histone modifications, methylation, protein binding, chromatin segmentation and repeats) and compared to random loci. Random forest models determined loci classification and feature selection. HPV and HBV integrants were not fragile site associated. MCPyV preferred integration near sensory perception genes. Unique signatures of integration-associated predictive genomic features were detected. Importantly, repeats, actively-transcribed regions and histone modifications were common tumor viral integration signatures.
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15
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Martin-Guerrero I, de Prado E, Ardanaz M, Martin-Arruti M, Garcia-Orad C, Guerra I, Ruiz I, Zabalza I, Garcia-Orad A. Methylation of CpG sites in BCL2 major breakpoint region and the increase of BCL2/JH translocation with aging. AGE (DORDRECHT, NETHERLANDS) 2015; 37:94. [PMID: 26335622 PMCID: PMC5005837 DOI: 10.1007/s11357-015-9834-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/30/2015] [Accepted: 08/20/2015] [Indexed: 06/05/2023]
Abstract
The BCL2 breakage mechanism has been shown to be highly dependent on DNA methylation at the major breakpoint region (MBR) CpG sites. We recently described an increased frequency of BCL2/ JH translocation with aging. It is known that methylation levels change with aging. The present study aimed to determine whether methylation alterations at CpG sites of BCL2 MBR were the cause of increased breakages with aging. We analyzed the methylation levels of three CpG sites on the region by pyrosequencing and studied if methylation levels and/or polymorphisms affecting CpG sites were associated with an increase of translocations. We observed that although the methylation levels of MBR CpG sites were higher in individuals with BCL2/JH translocation, in contrast to our expectations, these levels decreased with the age. Moreover, we show that polymorphisms at those CpG sites leading to absence of methylation seem to be a protective factor for the apparition of translocations.
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Affiliation(s)
- Idoia Martin-Guerrero
- Department of Genetics, Physical Anthropology and Animal Physiology, Faculty of Medicine and Odontology, University of the Basque Country UPV/EHU, Barrio Sarriena sn, 48940 Leioa, Bizkaia Spain
| | - Elena de Prado
- Department of Genetics, Physical Anthropology and Animal Physiology, Faculty of Medicine and Odontology, University of the Basque Country UPV/EHU, Barrio Sarriena sn, 48940 Leioa, Bizkaia Spain
| | | | | | - Cristina Garcia-Orad
- Assistance to Primary Health Care Center—Torrent 1, Hospital General Valencia, Valencia, Spain
| | | | - Irune Ruiz
- Donostia University Hospital, Donostia, Spain
| | | | - Africa Garcia-Orad
- Department of Genetics, Physical Anthropology and Animal Physiology, Faculty of Medicine and Odontology, University of the Basque Country UPV/EHU, Barrio Sarriena sn, 48940 Leioa, Bizkaia Spain
- BioCruces Health Research Institute, Barakaldo, Spain
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16
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Blombery PA, Wall M, Seymour JF. The molecular pathogenesis of B-cell non-Hodgkin lymphoma. Eur J Haematol 2015; 95:280-93. [DOI: 10.1111/ejh.12589] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/18/2015] [Indexed: 12/17/2022]
Affiliation(s)
| | - Meaghan Wall
- Victorian Cancer Cytogenetics Service; St Vincent's Hospital Melbourne; University of Melbourne; Fitzroy Vic. Australia
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17
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Lu Z, Lieber MR, Tsai AG, Pardo CE, Müschen M, Kladde MP, Hsieh CL. Human lymphoid translocation fragile zones are hypomethylated and have accessible chromatin. Mol Cell Biol 2015; 35:1209-22. [PMID: 25624348 PMCID: PMC4355534 DOI: 10.1128/mcb.01085-14] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2014] [Revised: 11/26/2014] [Accepted: 01/16/2015] [Indexed: 12/19/2022] Open
Abstract
Chromosomal translocations are a hallmark of hematopoietic malignancies. CG motifs within translocation fragile zones (typically 20 to 600 bp in size) are prone to chromosomal translocation in lymphomas. Here we demonstrate that the CG motifs in human translocation fragile zones are hypomethylated relative to the adjacent DNA. Using a methyltransferase footprinting assay on isolated nuclei (in vitro), we find that the chromatin at these fragile zones is accessible. We also examined in vivo accessibility using cellular expression of a prokaryotic methylase. Based on this assay, which measures accessibility over a much longer time interval than is possible with in vitro methods, these fragile zones were found to be more accessible than the adjacent DNA. Because DNA within the fragile zones can be methylated by both cellular and exogenous methyltransferases, the fragile zones are predominantly in a duplex DNA conformation. These observations permit more-refined models for why these zones are 100- to 1,000-fold more prone to undergo chromosomal translocation than the adjacent regions.
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Affiliation(s)
- Zhengfei Lu
- USC Norris Comprehensive Cancer Ctr., Los Angeles, California, USA
| | - Michael R Lieber
- USC Norris Comprehensive Cancer Ctr., Los Angeles, California, USA
| | - Albert G Tsai
- USC Norris Comprehensive Cancer Ctr., Los Angeles, California, USA
| | - Carolina E Pardo
- Department of Biochemistry & Molecular Biology, University of Florida College of Medicine, Gainesville, Florida, USA
| | - Markus Müschen
- Department of Laboratory Medicine, University of California, San Francisco, San Francisco, California, USA
| | - Michael P Kladde
- Department of Biochemistry & Molecular Biology, University of Florida College of Medicine, Gainesville, Florida, USA
| | - Chih-Lin Hsieh
- USC Norris Comprehensive Cancer Ctr., Los Angeles, California, USA
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18
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Laffleur B, Denis-Lagache N, Péron S, Sirac C, Moreau J, Cogné M. AID-induced remodeling of immunoglobulin genes and B cell fate. Oncotarget 2015; 5:1118-31. [PMID: 24851241 PMCID: PMC4012742 DOI: 10.18632/oncotarget.1546] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
Survival and phenotype of normal and malignant B lymphocytes are critically dependent on constitutive signals by the B cell receptor (BCR) for antigen. In addition, either antigen ligation of the BCR or various mitogenic stimuli result in B cell activation and induction of activation-induced deaminase (AID). AID activity can in turn mediate somatic hypermutation (SHM) of immunoglobulin (Ig) V regions and also deeply remodel the Ig heavy chain locus through class switch recombination (CSR) or locus suicide recombination (LSR). In addition to changes linked to affinity for antigen, modifying the class/isotype (i.e. the structure and function) of the BCR or suddenly deleting BCR expression also modulates the fate of antigen-experienced B cells.
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19
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Umiker BR, McDonald G, Larbi A, Medina CO, Hobeika E, Reth M, Imanishi-Kari T. Production of IgG autoantibody requires expression of activation-induced deaminase in early-developing B cells in a mouse model of SLE. Eur J Immunol 2014; 44:3093-108. [PMID: 25044405 DOI: 10.1002/eji.201344282] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2013] [Revised: 06/13/2014] [Accepted: 07/09/2014] [Indexed: 11/06/2022]
Abstract
Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the presence of pathogenic IgG antinuclear antibodies. Pathogenic IgG autoantibody production requires B-cell activation, leading to the production of activation-induced deaminase (AID) and class switching of IgM genes to IgG. To understand how and when B cells are activated to produce these IgG autoantibodies, we studied cells from 564Igi, a mouse model of SLE. 564Igi mice develop a disease profile closely resembling that found in human SLE patients, including the presence of IgG antinucleic acid Abs. We have generated 564Igi mice that conditionally express an activation-induced cytidine deaminase transgene (Aicda(tg) ), either in all B cells or only in mature B cells. Here, we show that class-switched pathogenic IgG autoantibodies were produced only in 564Igi mice in which AID was functional in early-developing B cells, resulting in loss of tolerance. Furthermore, we show that the absence of AID in early-developing B cells also results in increased production of self-reactive IgM, indicating that AID, through somatic hypermutation, contributes to tolerance. Our results suggest that the pathophysiology of clinical SLE might also be dependent on AID expression in early-developing B cells.
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Affiliation(s)
- Benjamin R Umiker
- Graduate Program in Immunology, Sackler School of Graduate Biomedical Sciences, Tufts University School of Medicine, Boston, MA, USA; Department of Integrative Physiology and Pathobiology, Sackler School of Graduate Biomedical Sciences, Tufts University School of Medicine, Boston, MA, USA
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20
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Yao Y, Des Marais TL, Costa M. Chromatin Memory in the Development of Human Cancers. GENE TECHNOLOGY 2014; 3:114. [PMID: 25606572 PMCID: PMC4297643] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Cancer is a complex disease with acquired genomic and epigenomic alterations that affect cell proliferation, viability and invasiveness. Almost all the epigenetic mechanisms including cytosine methylation and hydroxymethylation, chromatin remodeling and non-coding RNAs have been found associate with carcinogenesis and cancer specific expression profile. Altered histone modification as an epigenetic hallmark is frequently found in tumors. Understanding the epigenetic alterations induced by carcinogens or infectious agents may help us understand early epigenetic changes prior to the development of cancer. In this review, we focus on chromatin remodeling and the associated histone modifiers in the development of cancer; the application of these modifiers as a cancer therapy target in different clinical trial phases is also discussed.
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Affiliation(s)
- Yixin Yao
- Department of Environmental Medicine New York University, New York, USA,Corresponding author: Yixin Yao, Department of Environmental Medicine, New York University, New York, USA; Tel: 845-731-3517;
| | | | - Max Costa
- Department of Environmental Medicine New York University, New York, USA,Department of Biochemistry and Molecular Pharmacology, New York University Langone Medical Center, Tuxedo, New York, USA
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21
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Tsai CT, Yang PM, Chern TR, Chuang SH, Lin JH, Klemm L, Müschen M, Chen CC. AID downregulation is a novel function of the DNMT inhibitor 5-aza-deoxycytidine. Oncotarget 2014; 5:211-23. [PMID: 24457556 PMCID: PMC3960202 DOI: 10.18632/oncotarget.1319] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2013] [Accepted: 11/23/2013] [Indexed: 11/25/2022] Open
Abstract
Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutation (SHM) and class switch recombination (CSR) in immunoglobulin genes. However, AID can also cause mutations in host genes and contribute to cancer progression and drug resistance. In this study, molecular docking showed the interaction of free 5-aza-CdR and Zebularine (Zeb) with AID. However, only 5-aza-CdR-incorporated ssDNA bound to the active site of AID and inhibited AID expression through proteasomal degradation. 5-aza-CdR demonstrated cytotoxicity against AID-positive and -negative hematopoietic cancer cells. In contrast, Zeb exhibited a cytotoxic effect only in AID-negative cells due to its inability to inhibit AID expression. This differential effect might be due to the DNMT1 stabilization induced by AID, thus restricting the ability of Zeb to deplete DNMT1 and induce tumor suppressor genes (TSGs), such as p21, in AID-positive cells. Moreover, the in vivo anticancer effect of 5-aza-CdR but not Zeb in AID-positive hematopoietic cancer cells was demonstrated. The study not only displays the association of AID and DNMT1 and identifies a novel biological function of AID, but also provides novel information regarding the use of DNMT inhibitors to treat AID-positive hematopoietic cancers.
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Affiliation(s)
- Chiou-Tsun Tsai
- Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Pei-Ming Yang
- Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Ting-Rong Chern
- Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Shu-Hui Chuang
- Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan
| | - Jung-Hsin Lin
- School of Pharmacy, College of Medicine, National Taiwan University, Taipei, Taiwan
- Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
- Institute of Biomedical Science, Academia Sinica, Taipei, Taiwan
| | - Lars Klemm
- Department of Laboratory Medicine, University of California San Francisco, San Francisco, California
| | - Markus Müschen
- Department of Laboratory Medicine, University of California San Francisco, San Francisco, California
| | - Ching-Chow Chen
- Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan
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22
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Matthews AJ, Zheng S, DiMenna LJ, Chaudhuri J. Regulation of immunoglobulin class-switch recombination: choreography of noncoding transcription, targeted DNA deamination, and long-range DNA repair. Adv Immunol 2014; 122:1-57. [PMID: 24507154 PMCID: PMC4150736 DOI: 10.1016/b978-0-12-800267-4.00001-8] [Citation(s) in RCA: 102] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Upon encountering antigens, mature IgM-positive B lymphocytes undergo class-switch recombination (CSR) wherein exons encoding the default Cμ constant coding gene segment of the immunoglobulin (Ig) heavy-chain (Igh) locus are excised and replaced with a new constant gene segment (referred to as "Ch genes", e.g., Cγ, Cɛ, or Cα). The B cell thereby changes from expressing IgM to one producing IgG, IgE, or IgA, with each antibody isotype having a different effector function during an immune reaction. CSR is a DNA deletional-recombination reaction that proceeds through the generation of DNA double-strand breaks (DSBs) in repetitive switch (S) sequences preceding each Ch gene and is completed by end-joining between donor Sμ and acceptor S regions. CSR is a multistep reaction requiring transcription through S regions, the DNA cytidine deaminase AID, and the participation of several general DNA repair pathways including base excision repair, mismatch repair, and classical nonhomologous end-joining. In this review, we discuss our current understanding of how transcription through S regions generates substrates for AID-mediated deamination and how AID participates not only in the initiation of CSR but also in the conversion of deaminated residues into DSBs. Additionally, we review the multiple processes that regulate AID expression and facilitate its recruitment specifically to the Ig loci, and how deregulation of AID specificity leads to oncogenic translocations. Finally, we summarize recent data on the potential role of AID in the maintenance of the pluripotent stem cell state during epigenetic reprogramming.
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Affiliation(s)
- Allysia J Matthews
- Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York, USA; Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, New York, New York, USA
| | - Simin Zheng
- Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York, USA; Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, New York, New York, USA
| | - Lauren J DiMenna
- Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York, USA; Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, New York, New York, USA
| | - Jayanta Chaudhuri
- Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York, USA; Immunology and Microbial Pathogenesis Program, Weill Cornell Graduate School of Medical Sciences, New York, New York, USA.
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23
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Alt FW, Zhang Y, Meng FL, Guo C, Schwer B. Mechanisms of programmed DNA lesions and genomic instability in the immune system. Cell 2013; 152:417-29. [PMID: 23374339 PMCID: PMC4382911 DOI: 10.1016/j.cell.2013.01.007] [Citation(s) in RCA: 346] [Impact Index Per Article: 31.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2012] [Indexed: 12/15/2022]
Abstract
Chromosomal translocations involving antigen receptor loci are common in lymphoid malignancies. Translocations require DNA double-strand breaks (DSBs) at two chromosomal sites, their physical juxtaposition, and their fusion by end-joining. Ability of lymphocytes to generate diverse repertoires of antigen receptors and effector antibodies derives from programmed genomic alterations that produce DSBs. We discuss these lymphocyte-specific processes, with a focus on mechanisms that provide requisite DSB target specificity and mechanisms that suppress DSB translocation. We also discuss recent work that provides new insights into DSB repair pathways and the influences of three-dimensional genome organization on physiological processes and cancer genomes.
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Affiliation(s)
- Frederick W Alt
- Departments of Genetics and Pediatrics, Harvard Medical School, Boston, MA 02115, USA.
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24
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BCL6 breaks occur at different AID sequence motifs in Ig-BCL6 and non-Ig-BCL6 rearrangements. Blood 2013; 121:4551-4. [PMID: 23476051 DOI: 10.1182/blood-2012-10-464958] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
BCL6 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.
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