ReiNF4rcing repair pathway choice during cell cycle

Abstract Class Switch Recombination (CSR) involves the introduction of double stranded breaks (DSBs) at the switch regions of the immunoglulin heavy chain (Igh) locus by the enzyme Activation Cytidine Deaminse (AID). AID can also act as a general mutator targeting other loci in the genome which can then either be repaired faithfully or in an error-prone fashion introducing mutations and potentially initiating B cell lymphoma. The factors contributing to the choice of repair pathway are not fully understood. Here we tested the hypothesis that repair pathway choice is influenced by differential accessibility and expression levels of target loci across cell cycle. More specifically in the context of CSR we tested whether differential regulation of gene accessibility across cell cycle is an important determinant for AID binding and subsequent repair pathway choice as different repair pathways predominate at different stages of cell cycle. Using 3D-FISH in conjunction with Immunofluorescence we observed that AID target genes that are faithfully repaired are more accessible (found in euchromatic regions) in the G2 phase of the cell cycle then genes that are frequently mutated. In contrast, those genes which are repaired in an error prone fashion are more accessible in the G1 phase of cell cycle. Since Homologous Recombination mediated repair (HR), which is a faithful repair mechanism, occurs in G2 we speculate that accessibility of these genes at this stage of cell cycle facilitates action by this repair pathway. Conversely, genes that are more accessible during the G1 phase of cell cycle will be repaired by the non-homologous end joining (NHEJ) repair pathway and therefore are more likely to be mutated. Thus, HR could be the pathway by which faithful repair is accomplished and use of the NHEJ pathway on the other hand could contribute to the introduction of dangerous DNA mutations that might lead to B cell transformation and cancer. To connect differences in accessibility with repair pathway usage, we used a mouse model carrying a hypomorphic mutation in BRCA2, a protein involved in HR. This is the first mouse model impaired in HR that eludes embryonic lethality and allows inspection of the role of this pathway in maintaining genomic stability in splenocytes undergoing CSR. Our preliminary investigations indicate that in Brca2 mutant B cells not only is the integrity of fathfully repaired loci compromised, but the Igh locus is also damaged. Taken together these results support our hypothesis and further indicate that the HR pathway is involved in repairing Igh. Given that approximately 95% of lymphomas are of B cell origin and many of these are associated with AID mediated breaks, it is crucial for us to understand the factors that influence targeting and repair. Disclosures No relevant conflicts of interest to declare.

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DSB repair pathway choice is regulated by recruitment of 53BP1 through cell cycle-dependent regulation of Sp1

Cell Reports ◽

10.1016/j.celrep.2021.108840 ◽

2021 ◽

Vol 34 (11) ◽

pp. 108840

Author(s):

Michelle L. Swift ◽

Kate Beishline ◽

Samuel Flashner ◽

Jane Azizkhan-Clifford

Keyword(s):

Cell Cycle ◽

Repair Pathway ◽

Dsb Repair ◽

Pathway Choice ◽

Cycle Dependent

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Faculty Opinions recommendation of A cell cycle-dependent regulatory circuit composed of 53BP1-RIF1 and BRCA1-CtIP controls DNA repair pathway choice.

Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature ◽

10.3410/f.717972354.793473075 ◽

2013 ◽

Author(s):

Qiang Pan-Hammarström ◽

Andrea Björkman

Keyword(s):

Cell Cycle ◽

Dna Repair ◽

Repair Pathway ◽

Regulatory Circuit ◽

A Cell ◽

Pathway Choice ◽

Cycle Dependent

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Repair pathway choice for double-strand breaks

Essays in Biochemistry ◽

10.1042/ebc20200007 ◽

2020 ◽

Vol 64 (5) ◽

pp. 765-777 ◽

Cited By ~ 2

Author(s):

Yixi Xu ◽

Dongyi Xu

Keyword(s):

Cell Cycle ◽

Double Strand Breaks ◽

Homologous Region ◽

Gene Repair ◽

Repair Pathway ◽

Dna Double Strand Breaks ◽

Environmental Radiation ◽

Strand Breaks ◽

Dna End Resection ◽

End Resection

Abstract Deoxyribonucleic acid (DNA) is at a constant risk of damage from endogenous substances, environmental radiation, and chemical stressors. DNA double-strand breaks (DSBs) pose a significant threat to genomic integrity and cell survival. There are two major pathways for DSB repair: nonhomologous end-joining (NHEJ) and homologous recombination (HR). The extent of DNA end resection, which determines the length of the 3′ single-stranded DNA (ssDNA) overhang, is the primary factor that determines whether repair is carried out via NHEJ or HR. NHEJ, which does not require a 3′ ssDNA tail, occurs throughout the cell cycle. 53BP1 and the cofactors PTIP or RIF1-shieldin protect the broken DNA end, inhibit long-range end resection and thus promote NHEJ. In contrast, HR mainly occurs during the S/G2 phase and requires DNA end processing to create a 3′ tail that can invade a homologous region, ensuring faithful gene repair. BRCA1 and the cofactors CtIP, EXO1, BLM/DNA2, and the MRE11–RAD50–NBS1 (MRN) complex promote DNA end resection and thus HR. DNA resection is influenced by the cell cycle, the chromatin environment, and the complexity of the DNA end break. Herein, we summarize the key factors involved in repair pathway selection for DSBs and discuss recent related publications.

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ALTERED FIDELITY OF MITOTIC CHROMOSOME TRANSMISSION IN CELL CYCLE MUTANTS OF S. CEREVISIAE

Genetics ◽

10.1093/genetics/110.3.381 ◽

1985 ◽

Vol 110 (3) ◽

pp. 381-395

Author(s):

Leland H Hartwell ◽

David Smith

Keyword(s):

Cell Cycle ◽

Gene Product ◽

Mitotic Chromosome ◽

Mitotic Recombination ◽

Chromosome Loss ◽

Temperature Sensitive ◽

Repair Pathway ◽

Dna Metabolism ◽

Temperature Sensitive Mutants ◽

Chromosome Transmission

ABSTRACT Thirteen of 14 temperature-sensitive mutants deficient in successive steps of mitotic chromosome transmission (cdc2, 4, 5, 6, 7, 8, 9, 13, 14, 15, 16, 17 and 20) from spindle pole body separation to a late stage of nuclear division exhibited a dramatic increase in the frequency of chromosome loss and/or mitotic recombination when they were grown at their maximum permissive temperatures. The increase in chromosome loss and/or recombination is likely to be due to the deficiency of functional gene product rather than to an aberrant function of the mutant gene product since the mutant alleles are, with one exception, recessive to the wild-type allele for this phenotype. The generality of this result suggests that a delay in almost any stage of chromosome replication or segregation leads to a decrease in the fidelity of mitotic chromosome transmission. In contrast, temperature-sensitive mutants defective in the control step of the cell cycle (cdc28), in cytokinesis (cdc3) or in protein synthesis (ils1) did not exhibit increased recombination or chromosome loss.—Based upon previous results with mutants and DNA-damaging agents in a variety of organisms, we suggest that the induction of mitotic recombination in certain mutants is due to the action of a repair pathway upon nicks or gaps left in the DNA. This interpretation is supported by the fact that the induced recombination is dependent upon the RAD52 gene product, an essential component in the recombinogenic DNA repair pathway. Gene products whose deficiency leads to induced recombination are, therefore, strong candidates for proteins that function in DNA metabolism. Among the mutants that induce recombination are those known to be defective in some aspect of DNA replication (cdc2, 6, 8, 9) as well as some mutants defective in the G2 (cdc13 and 17) and M (cdc5 and 14) phases of the mitotic cycle. We suggest that special aspects of DNA metabolism may be occurring in G2 and M in order to prepare the chromosomes for proper segregation.

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