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Review
. 2019 Dec 19;12(1):26.
doi: 10.3390/cancers12010026.

When RAD52 Allows Mitosis to Accept Unscheduled DNA Synthesis

Camille Franchet  1 , Jean-Sébastien Hoffmann  1
Affiliations
Review

When RAD52 Allows Mitosis to Accept Unscheduled DNA Synthesis

Camille Franchet et al. Cancers (Basel). .

Abstract

Faithful duplication of the human genome during the S phase of cell cycle and accurate segregation of sister chromatids in mitosis are essential for the maintenance of chromosome stability from one generation of cells to the next. Cells that are copying their DNA in preparation for division can suffer from 'replication stress' (RS) due to various external or endogenous impediments that slow or stall replication forks. RS is a major cause of pathologies including cancer, premature ageing and other disorders associated with genomic instability. It particularly affects genomic loci where progression of replication forks is intrinsically slow or problematic, such as common fragile site (CFS), telomeres, and repetitive sequences. Although the eukaryotic cell cycle is conventionally thought of as several separate steps, each of which must be completed before the next one is initiated, it is now accepted that incompletely replicated chromosomal domains generated in S phase upon RS at these genomic loci can result in late DNA synthesis in G2/M. In 2013, during investigations into the mechanism by which the specialized DNA polymerase eta (Pol η) contributes to the replication and stability of CFS, we unveiled that indeed some DNA synthesis was still occurring in early mitosis at these loci. This surprising observation of mitotic DNA synthesis that differs fundamentally from canonical semi-conservative DNA replication in S-phase has been then confirmed, called "MiDAS"and believed to counteract potentially lethal chromosome mis-segregation and non-disjunction. While other contributions in this Special Issue of Cancers focus on the role of RAS52RAD52 during MiDAS, this review emphases on the discovery of MiDAS and its molecular effectors.

Keywords: DNA replication; RAD52; chromosome instability; genome instability; mitotic DNA synthesis; replication stress.

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Conflict of interest statement

The authors have no affiliations or financial involvement with any entity with a financial interest or conflict with the material in this review.

Figures

Figure 1
Figure 1
Illustrations of MiDAS at CFSs and telomeres in human cells. (A) From [19]: original and first published experiment showing EdU incorporation in mitotic cells. Asynchronously growing cells treated with 0.3 µM Aphidicolin for 24 h were labeled with EdU for 45 min, fixed and stained with a phospho-H3 (Ser10) specific antibody. DNA was counterstained with 4′,6-Diamidine-2′-phenylindole dihydrochloride (DAPI). The presence of EdU signals on p-H3-positive mitotic chromosomes can be observed. (B) From [23]: H1299 cells expressing a BRCA2 shRNA were pulsed with EdU for 40 min and processed for detection of EdU (green) and immunofluorescence staining with anti-FANCD2 antibody (red) on metaphase spreads. DNA was counterstained with DAPI. (C) From [24]: Pol η depletion leads to MiDAS at telomeres in U2OS cells treated for 24 hr with aphidicolin and then for 40 min with EdU. EdU (green) and Fluorescence in situ hybridization with the telomeric probe TTAGGG (red) were detected on metaphase spreads. DNA was counterstained with DAPI. Scale bar, 10 μm.
Figure 2
Figure 2
Model for MiDAS function and mechanism. Successful replication completion of CFS loci by MiDAS could limit the formation of ultrafine anaphase bridges (UFBs) which compromise the partition and integrity of chromosomes and reduce the transmission of DNA damage (53BP1 NBs) to the next generation of cells. One model proposed that MiDASoccurrs through a break-induced replication (BIR)-like process after cohesin degradation. Persisting stalled replication forks at difficult-to-replicate region within CFS upon mild replicative stress could result in under-replicated DNA (illustrated as a white locus on chromosomal arm) and trigger its collapse by the action of SLX4 in complex with the MUS81-EME1 endonuclease in early mitosis. The vicinity of the sister chromatid at the collapsed fork renders unnecessary an extensive homology search and a HR process to start while a limited end resection favors the single-stranded DNA annealing activity of RAD52 into regions of micro-homology and the subsequent POLD3-dependent conservative DNA repair synthesis (EdU incorporation in a single chromatid).
Figure 3
Figure 3
Three major responses to excessive RS that represent potential targets for cancer therapy. If genetic instability is important for tumor development, conversely excessive chromosome instability appears to suppress tumorigenesis and extreme genetic instability correlates with improved cancer outcome, suggesting that karyotypic diversity required to adapt to selection pressures might be balanced in tumors against the risk of excessive instability. We propose that three major mechanism known to respond to replicative stress could counteract karyotypic diversity, contribute to tumor progression and constitute important novel targets in cancer therapy: (i) stabilization of stalled forks by high expression of Chk1, Claspin and Timeless; (ii) overexpression of Pol θ that functions in a back-up alternative repair pathway at resected double-strand breaks when homologous recombination is deficient to repair broken forks; (iii) MiDAS mediated by RAD52.
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