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Transcriptional stalling in B-lymphocytes: a mechanism for antibody diversification and maintenance of genomic integrity.

Sun J, Rothschild G, Pefanis E, Basu U - Transcription (2013)

Bottom Line: B cells utilize three DNA alteration strategies-V(D)J recombination, somatic hypermutation (SHM) and class switch recombination (CSR)-to somatically mutate their genome, thereby expressing a plethora of antibodies tailor-made against the innumerable antigens they encounter while in circulation.Of these three events, the single-strand DNA cytidine deaminase, Activation Induced cytidine Deaminase (AID), is responsible for SHM and CSR.Recent advances, discussed in this review article, point toward various components of RNA polymerase II "stalling" machinery as regulators of AID activity during antibody diversification and maintenance of B cell genome integrity.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology; College of Physicians and Surgeons; Columbia University; New York, NY USA.

ABSTRACT
B cells utilize three DNA alteration strategies-V(D)J recombination, somatic hypermutation (SHM) and class switch recombination (CSR)-to somatically mutate their genome, thereby expressing a plethora of antibodies tailor-made against the innumerable antigens they encounter while in circulation. Of these three events, the single-strand DNA cytidine deaminase, Activation Induced cytidine Deaminase (AID), is responsible for SHM and CSR. Recent advances, discussed in this review article, point toward various components of RNA polymerase II "stalling" machinery as regulators of AID activity during antibody diversification and maintenance of B cell genome integrity.

Show MeSH
Figure 1. class switch recombination at the Immunoglobulin Heavy Chain locus. (A) The configuration of the unrearranged immunoglobulin heavy chain locus in immature B cells according to NG_005838.1. Here, VH, DH and JH represent the various unrearranged gene segments that will generate the VDJ exon following V(D)J recombination and are followed by the various constant regions genes (Cμ-α, yellow boxes). Each constant region gene is preceded by a switch sequence (Sμ-α, black oval); switch sequences are non-coding regions transcribed by their own transcriptional regulatory promoter elements. Two important regions that have enhancer functions and influence various recombination events in the IgH locus are shown in a blue box and are labeled as Eμ and Eα (also known as 3′ regulatory region, 3′RR). (B) Following V(D)J recombination, various B cell signaling pathways induce transcription at switch sequence promoter regions. Transcription at the upstream switch sequence Sμ is constitutive whereas transcription at the downstream switch sequence (in this case, Sγ1) is induced due to activation of its promoter elements by various signaling pathways. (C) A schematic of a simplified IgH locus that is poised to undergo CSR to IgG1 following transcription activation at Sμ and Sγ1. A region of the switch sequences, known as the core switch region (G-rich on the non-template strand), is capable of forming stable RNA/DNA hybrids that lead to ssDNA structure “R-loop” formation. (D) Transcription at switch sequences induces formation of R-loops which become targets for AID activity. AID converts cytidine residues to uracils, that are then recognized by the base excision pathway uracil DNA deglycosylase (UNG). (E) UNG activity induces generation of abasic residues that are then cleaved by the apurinic endonuclease family of proteins (APE1/2) to generate DNA double strand breaks (DSBs) at both upstream (Sμ) and downstream (Sx, in this case, Sγ1) switch sequences. (F) Recognition of these two DSBs by two cellular DNA damage repair pathways known as non homologous end-joining (NHEJ) and alternative end joining (AEJ) leads to joining of the two distant switch sequences that have DSBs leading to the completion of CSR. (G) The final configuration of the antibody heavy chain molecule coding mRNA is shown.
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Figure 1: Figure 1. class switch recombination at the Immunoglobulin Heavy Chain locus. (A) The configuration of the unrearranged immunoglobulin heavy chain locus in immature B cells according to NG_005838.1. Here, VH, DH and JH represent the various unrearranged gene segments that will generate the VDJ exon following V(D)J recombination and are followed by the various constant regions genes (Cμ-α, yellow boxes). Each constant region gene is preceded by a switch sequence (Sμ-α, black oval); switch sequences are non-coding regions transcribed by their own transcriptional regulatory promoter elements. Two important regions that have enhancer functions and influence various recombination events in the IgH locus are shown in a blue box and are labeled as Eμ and Eα (also known as 3′ regulatory region, 3′RR). (B) Following V(D)J recombination, various B cell signaling pathways induce transcription at switch sequence promoter regions. Transcription at the upstream switch sequence Sμ is constitutive whereas transcription at the downstream switch sequence (in this case, Sγ1) is induced due to activation of its promoter elements by various signaling pathways. (C) A schematic of a simplified IgH locus that is poised to undergo CSR to IgG1 following transcription activation at Sμ and Sγ1. A region of the switch sequences, known as the core switch region (G-rich on the non-template strand), is capable of forming stable RNA/DNA hybrids that lead to ssDNA structure “R-loop” formation. (D) Transcription at switch sequences induces formation of R-loops which become targets for AID activity. AID converts cytidine residues to uracils, that are then recognized by the base excision pathway uracil DNA deglycosylase (UNG). (E) UNG activity induces generation of abasic residues that are then cleaved by the apurinic endonuclease family of proteins (APE1/2) to generate DNA double strand breaks (DSBs) at both upstream (Sμ) and downstream (Sx, in this case, Sγ1) switch sequences. (F) Recognition of these two DSBs by two cellular DNA damage repair pathways known as non homologous end-joining (NHEJ) and alternative end joining (AEJ) leads to joining of the two distant switch sequences that have DSBs leading to the completion of CSR. (G) The final configuration of the antibody heavy chain molecule coding mRNA is shown.

Mentions: Upon exposure to antigen, B cells undergo two types of controlled DNA mutation. Through somatic hypermutation (SHM) B cells incorporate point mutations in their variable gene exons to encode antigen-specific antibodies, while class switch recombination (CSR) rearranges the constant region genes, permitting them to express constant region exons downstream of IgM (e.g., the IgG series, IgE, IgA). Immunoglobulins are composed of two distinct polypeptides, the heavy chain and the light chain. The heavy chain is encoded by the immunoglobulin heavy chain locus (IgH), the light chain (IgL) by one of two separate loci, Igκ and Igλ. IgH undergoes CSR as well as SHM whereas the IgL loci only undergo SHM. In this review, we focus on genetic alterations in IgH, predominantly because more analysis has been performed on these sequences with respect to the mutagenesis process involving DNA single-strand and double-strand break formation and repair during SHM and CSR, respectively.2-5 A schematic representation of the antibody-expressing IgH locus is shown in Figure 1.


Transcriptional stalling in B-lymphocytes: a mechanism for antibody diversification and maintenance of genomic integrity.

Sun J, Rothschild G, Pefanis E, Basu U - Transcription (2013)

Figure 1. class switch recombination at the Immunoglobulin Heavy Chain locus. (A) The configuration of the unrearranged immunoglobulin heavy chain locus in immature B cells according to NG_005838.1. Here, VH, DH and JH represent the various unrearranged gene segments that will generate the VDJ exon following V(D)J recombination and are followed by the various constant regions genes (Cμ-α, yellow boxes). Each constant region gene is preceded by a switch sequence (Sμ-α, black oval); switch sequences are non-coding regions transcribed by their own transcriptional regulatory promoter elements. Two important regions that have enhancer functions and influence various recombination events in the IgH locus are shown in a blue box and are labeled as Eμ and Eα (also known as 3′ regulatory region, 3′RR). (B) Following V(D)J recombination, various B cell signaling pathways induce transcription at switch sequence promoter regions. Transcription at the upstream switch sequence Sμ is constitutive whereas transcription at the downstream switch sequence (in this case, Sγ1) is induced due to activation of its promoter elements by various signaling pathways. (C) A schematic of a simplified IgH locus that is poised to undergo CSR to IgG1 following transcription activation at Sμ and Sγ1. A region of the switch sequences, known as the core switch region (G-rich on the non-template strand), is capable of forming stable RNA/DNA hybrids that lead to ssDNA structure “R-loop” formation. (D) Transcription at switch sequences induces formation of R-loops which become targets for AID activity. AID converts cytidine residues to uracils, that are then recognized by the base excision pathway uracil DNA deglycosylase (UNG). (E) UNG activity induces generation of abasic residues that are then cleaved by the apurinic endonuclease family of proteins (APE1/2) to generate DNA double strand breaks (DSBs) at both upstream (Sμ) and downstream (Sx, in this case, Sγ1) switch sequences. (F) Recognition of these two DSBs by two cellular DNA damage repair pathways known as non homologous end-joining (NHEJ) and alternative end joining (AEJ) leads to joining of the two distant switch sequences that have DSBs leading to the completion of CSR. (G) The final configuration of the antibody heavy chain molecule coding mRNA is shown.
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Related In: Results  -  Collection

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Figure 1: Figure 1. class switch recombination at the Immunoglobulin Heavy Chain locus. (A) The configuration of the unrearranged immunoglobulin heavy chain locus in immature B cells according to NG_005838.1. Here, VH, DH and JH represent the various unrearranged gene segments that will generate the VDJ exon following V(D)J recombination and are followed by the various constant regions genes (Cμ-α, yellow boxes). Each constant region gene is preceded by a switch sequence (Sμ-α, black oval); switch sequences are non-coding regions transcribed by their own transcriptional regulatory promoter elements. Two important regions that have enhancer functions and influence various recombination events in the IgH locus are shown in a blue box and are labeled as Eμ and Eα (also known as 3′ regulatory region, 3′RR). (B) Following V(D)J recombination, various B cell signaling pathways induce transcription at switch sequence promoter regions. Transcription at the upstream switch sequence Sμ is constitutive whereas transcription at the downstream switch sequence (in this case, Sγ1) is induced due to activation of its promoter elements by various signaling pathways. (C) A schematic of a simplified IgH locus that is poised to undergo CSR to IgG1 following transcription activation at Sμ and Sγ1. A region of the switch sequences, known as the core switch region (G-rich on the non-template strand), is capable of forming stable RNA/DNA hybrids that lead to ssDNA structure “R-loop” formation. (D) Transcription at switch sequences induces formation of R-loops which become targets for AID activity. AID converts cytidine residues to uracils, that are then recognized by the base excision pathway uracil DNA deglycosylase (UNG). (E) UNG activity induces generation of abasic residues that are then cleaved by the apurinic endonuclease family of proteins (APE1/2) to generate DNA double strand breaks (DSBs) at both upstream (Sμ) and downstream (Sx, in this case, Sγ1) switch sequences. (F) Recognition of these two DSBs by two cellular DNA damage repair pathways known as non homologous end-joining (NHEJ) and alternative end joining (AEJ) leads to joining of the two distant switch sequences that have DSBs leading to the completion of CSR. (G) The final configuration of the antibody heavy chain molecule coding mRNA is shown.
Mentions: Upon exposure to antigen, B cells undergo two types of controlled DNA mutation. Through somatic hypermutation (SHM) B cells incorporate point mutations in their variable gene exons to encode antigen-specific antibodies, while class switch recombination (CSR) rearranges the constant region genes, permitting them to express constant region exons downstream of IgM (e.g., the IgG series, IgE, IgA). Immunoglobulins are composed of two distinct polypeptides, the heavy chain and the light chain. The heavy chain is encoded by the immunoglobulin heavy chain locus (IgH), the light chain (IgL) by one of two separate loci, Igκ and Igλ. IgH undergoes CSR as well as SHM whereas the IgL loci only undergo SHM. In this review, we focus on genetic alterations in IgH, predominantly because more analysis has been performed on these sequences with respect to the mutagenesis process involving DNA single-strand and double-strand break formation and repair during SHM and CSR, respectively.2-5 A schematic representation of the antibody-expressing IgH locus is shown in Figure 1.

Bottom Line: B cells utilize three DNA alteration strategies-V(D)J recombination, somatic hypermutation (SHM) and class switch recombination (CSR)-to somatically mutate their genome, thereby expressing a plethora of antibodies tailor-made against the innumerable antigens they encounter while in circulation.Of these three events, the single-strand DNA cytidine deaminase, Activation Induced cytidine Deaminase (AID), is responsible for SHM and CSR.Recent advances, discussed in this review article, point toward various components of RNA polymerase II "stalling" machinery as regulators of AID activity during antibody diversification and maintenance of B cell genome integrity.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology; College of Physicians and Surgeons; Columbia University; New York, NY USA.

ABSTRACT
B cells utilize three DNA alteration strategies-V(D)J recombination, somatic hypermutation (SHM) and class switch recombination (CSR)-to somatically mutate their genome, thereby expressing a plethora of antibodies tailor-made against the innumerable antigens they encounter while in circulation. Of these three events, the single-strand DNA cytidine deaminase, Activation Induced cytidine Deaminase (AID), is responsible for SHM and CSR. Recent advances, discussed in this review article, point toward various components of RNA polymerase II "stalling" machinery as regulators of AID activity during antibody diversification and maintenance of B cell genome integrity.

Show MeSH