Genomic Instability Mechanisms in Fragile Sites

Common fragile sites (CFSs) are specific chromosomal regions that become prone to breakage when cells experience impediments to DNA replication. These loci often coincide with large, late-replicating domains that contain few replication origins and are subject to high levels of transcription. Under conditions of replication stress—such as those induced by oncogene activation or DNA polymerase inhibition—replication forks may stall or collapse within CFSs, leaving segments of the genome under-replicated as cells enter mitosis. To prevent lethal chromosome missegregation, cells employ a specialised salvage pathway known as mitotic DNA synthesis (MiDAS) to complete replication at these under-replicated gaps. Failures in replication completion or in subsequent repair pathways lead to chromosome gaps, ultrafine bridges, deletions and complex structural rearrangements. The resulting genomic instability is a hallmark of many cancers and contributes to tumour evolution by driving copy number variants and chromosomal translocations. Recent work has begun to reveal how replication origin scarcity, head-on transcription–replication encounters, 3D chromatin architecture and the controlled action of structure-selective endonucleases converge to determine CFS stability, with potential implications for targeted therapeutic strategies in oncology.

Research from Nature Portfolio

Recent studies have defined the polymerase dynamics that underpin MiDAS at under-replicated loci. In human cells challenged by oncogene-induced replication stress, an ordered switch occurs between specialised gap‐filling polymerases (REV1 and polymerase ζ) and the replicative polymerase δ. This polymerase-switch ensures completion of DNA synthesis at common fragile sites during mitosis, and its disruption markedly compromises cell survival under stress, suggesting that targeted inhibition of MiDAS components may sensitise cancer cells.

Other work has uncovered a fragility signature that integrates 3D genome organisation with replication timing and transcriptional output. Core instability regions within CFSs map to topologically associating domain (TAD) boundaries harbouring large, highly transcribed genes that exhibit delayed replication under mild stress. The confluence of TAD boundaries and replication timing delays pinpoints the most vulnerable genome segments, offering a predictive framework for mapping novel fragile sites and understanding the spatial determinants of replication stress-induced breakage.

Genomic Instability Mechanisms in Fragile Sites publication trend

The graph below shows the total number of articles in genomic instability mechanisms in fragile sites across all publications each year (not limited to Nature Index journals).

Technical terms

Common fragile site (CFS): A chromosome region that is susceptible to breakage under replication stress and typically replicates late in S phase.

Replication stress: A cellular state in which DNA synthesis is impeded, often by oncogene activation, DNA damage or shortage of nucleotides.

Mitotic DNA synthesis (MiDAS): A salvage pathway activated in early mitosis to complete DNA replication at under-replicated genomic regions.

Topologically associating domain (TAD): A self-interacting chromatin domain within which genomic regions preferentially contact each other, influencing replication and transcription patterns.

Break-induced replication (BIR): A DNA repair mechanism that restarts collapsed replication forks by strand invasion and conservative DNA synthesis.

References

1. Mitotic DNA synthesis in response to replication stress requires the sequential action of DNA polymerases zeta and delta in human cells. Nature Communications (2023).
2. 3D genome organization contributes to genome instability at fragile sites. Nature Communications (2020).
3. Completing genome replication outside of S phase. Molecular Cell (2023).
4. GEN1 promotes common fragile site expression. Cell Reports (2023).
5. High-resolution mapping of mitotic DNA synthesis regions and common fragile sites in the human genome through direct sequencing. Cell Research (2020).