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Inducing and Detecting Mitotic DNA Synthesis at Difficult-to-Replicate Loci
Lorenza Garribba1, Wei Wu1, Özgün Özer1
1Center for Chromosome Stability, University of Copenhagen, Copenhagen, Denmark.
Abstract:
Our conventional understanding of the process of DNA replication is that it occurs in the S-phase of the cell division cycle. However, during investigations into the mechanism by which common fragile sites (CFSs) drive genome instability, we observed that some DNA synthesis was still occurring in early mitosis at these loci. This curious phenomenon of mitotic DNA synthesis (which we now term "MiDAS") appears to be a form of break-induced DNA replication (BIR), a DNA repair process based on homologous recombination that has been characterized in detail only in lower eukaryotes. During MiDAS, it is proposed that parts of the human genome that are not fully replicated when cells enter mitotic prophase complete their replicative cycle at that point. To date, the loci that most depend upon this process are those whose replication can be affected by oncogene-induced DNA replication stress (RS), most notably, CFSs. From our studies, it is clear that the successful completion of MiDAS at CFSs can minimize chromosome missegregation and nondisjunction. Nevertheless, it is still not clear which loci that can undergo MiDAS, whether MiDAS is associated with mutations or genome rearrangements, or whether MiDAS really is a form of BIR. In this review, we describe methods for detecting MiDAS both in prometaphase cells and directly on isolated metaphase chromosomes. In addition, we have included methods for combining MiDAS detection either with immunofluorescence (IF) detection of proteins that are recruited to the MiDAS loci, or with fluorescence in situ hybridization using probes that target specific genomic loci.
Insights
Mitotic DNA synthesis (MiDAS) occurs in early mitosis at common fragile sites (CFSs), functioning as a break-induced replication repair mechanism. This process helps minimize chromosome missegregation during cell division.
Area of Science:
- Genetics
- Cell Biology
- Genomics
Background:
- Conventional understanding places DNA replication solely in S-phase.
- Common fragile sites (CFSs) are linked to genome instability.
- Investigations into CFSs revealed unexpected DNA synthesis during mitosis.
Purpose of the Study:
- To investigate the phenomenon of DNA synthesis occurring in early mitosis.
- To characterize this novel process, termed Mitotic DNA synthesis (MiDAS).
- To explore the role of MiDAS in genome stability, particularly at CFSs.
Main Methods:
- Observation of DNA synthesis in early mitosis at CFS loci.
- Characterization of MiDAS as a potential form of break-induced DNA replication (BIR).
- Development and description of methods for detecting MiDAS in prometaphase cells and on metaphase chromosomes.
- Integration of MiDAS detection with immunofluorescence and fluorescence in situ hybridization.
Main Results:
- Identified and termed "Mitotic DNA synthesis" (MiDAS) occurring at CFSs during early mitosis.
- Proposed MiDAS as a break-induced DNA replication (BIR) process.
- Demonstrated that successful MiDAS completion at CFSs can reduce chromosome missegregation and nondisjunction.
- Highlighted that loci dependent on MiDAS are often affected by replication stress induced by oncogenes.
Conclusions:
- MiDAS is a novel DNA replication process occurring during mitosis, particularly at CFSs.
- MiDAS appears to be a form of break-induced DNA replication (BIR) crucial for completing replication of stressed genomic regions.
- Successful MiDAS is vital for maintaining genome stability by preventing chromosome missegregation.
- Further research is needed to identify all MiDAS loci and its association with mutations or rearrangements.
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