Abstract
Transcription of the switch (S) regions of immunoglobulin genes in B cells generates stable R-loops that are targeted by Activation Induced Cytidine Deaminase (AID), triggering class switch recombination (CSR), as well as translocations with c-MYC responsible for Burkitt's lymphomas. In Saccharomyces cerevisiae, stable R-loops are formed co-transcriptionally in mutants of THO, a conserved nuclear complex involved in mRNP biogenesis. Such R-loops trigger genome instability and facilitate deamination by human AID. To understand the mechanisms that generate genome instability mediated by mRNP biogenesis impairment and by AID, we devised a yeast chromosomal system based on different segments of mammalian S regions and c-MYC for the analysis of chromosomal rearrangements in both wild-type and THO mutants. We demonstrate that AID acts in yeast at heterologous S and c-MYC transcribed sequences leading to double-strand breaks (DSBs) which in turn cause chromosomal translocations via Non-Homologous End Joining (NHEJ). AID–induced translocations were strongly enhanced in yeast THO null mutants, consistent with the idea that AID–mediated DSBs depend on R-loop formation. Our study not only provides new clues to understand the role of mRNP biogenesis in preventing genome rearrangements and the mechanism of AID-mediated genome instability, but also shows that, once uracil residues are produced by AID–mediated deamination, these are processed into DSBs and chromosomal rearrangements by the general and conserved DNA repair functions present from yeast to human cells.
Highlights
Transcription is a source of genetic instability in eukaryotic cells
Cytosine residues deaminated by Activation Induced Deaminase (AID) at S regions are converted into uracils, which are processed by base excision repair (BER) and/or mismatch repair (MMR) pathways into double-strand breaks (DSBs) responsible for class switch recombination (CSR) [17,18,19]
Our data suggest that in yeast AID is able to mediate deamination of cytosines in transcribed DNA that are channeled into DSBs as it occurs in mammalian B cells, leading to reciprocal chromosomal translocations
Summary
Transcription is a source of genetic instability in eukaryotic cells (reviewed in [1]). Transcription of the switch (S) regions generates DNA intermediates in which the C-rich template strand forms stable R-loops and the non-template G-rich strand can form secondary structures (reviewed in [7]). Current models of CSR suggest that AID triggers the process by deaminating cytosines in the non-template DNA strand displaced by the R-loop [13,14,15]. Cytosine residues deaminated by AID at S regions are converted into uracils, which are processed by base excision repair (BER) and/or mismatch repair (MMR) pathways into DSBs responsible for CSR [17,18,19].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
