Preventing replication fork collapse to maintain genome integrity

Abstract

Billions of base pairs of DNA must be replicated trillions of times in a human lifetime. Complete and accurate replication once and only once per cell division cycle is essential to maintain genome integrity and prevent disease. Impediments to replication fork progression including difficult to replicate DNA sequences, conflicts with transcription, and DNA damage further add to the genome maintenance challenge. These obstacles frequently cause fork stalling, but only rarely cause a failure to complete replication. Robust mechanisms ensure that stalled forks remain stable and capable of either resuming DNA synthesis or being rescued by converging forks. However, when failures do happen the fork collapses leading to genome rearrangements, cell death and disease. Despite intense interest, the mechanisms to repair damaged replication forks, stabilize them, and ensure successful replication remain only partly understood. Different models of fork collapse have been proposed with varying descriptions of what happens to the DNA and replisome. Here, I will define fork collapse and describe what is known about how the replication checkpoint prevents it to maintain genome stability.

Mutant p53 establishes targetable tumor dependency by promoting unscheduled replication

“…Supporting the central role of GOF p53-mediated transactivation of CHK1 in preventing collapse of replication forks, it was observed that a CHK1 inhibitor (Figure 2, G and H) or inhibition of CHK1 expression by transcriptional inactivation of GOF p53 leads to collapse of replication forks ( Figure 5G). It is known that the collapse of replication forks leads to either cell death or gross genomic rearrangement due to incomplete genome duplication (25,26). GOF p53-mediated fork stabilization, therefore, should allow the cells to evade these events detrimental to cell growth, although the cells enter S phase as early as p53-null cells.…”

“…36,56), increase in cyclin A expression leads to firing of more origins of replication (Figure 1 and Supplemental Figure 9F), whereas increased CHK1 expression prevents their collapse (Figure 2), thus quickening genome duplication ( Figure 3) and proliferation of cells with genetic abnormalities, as evidenced by micronuclei formation (Figure 4 and Figure 5E). In contrast, due to their inability to upregulate cyclin A and CHK1 expression, p53-null cells or cells expressing GOF-deficient p53 mutants activate fewer origins than cells with GOF p53 (Figure 1) and face a vulnerability to collapse of replication forks (Figure 2), which is known to cause incomplete genome duplication, gross genome rearrangement, and cell death (25,26).…”

“…Collapse of replication forks is known to induce either cell death or gross chromosomal rearrangement due to incomplete DNA replication (25,26). Consistently, experiments utilizing a mouse xenograft model revealed that inhibition of CHK1 reduced the size of tumors generated from a human lung cancer cell line with GOF p53 mutation more efficiently than GOF p53 knockdown cells and selectively reduced proliferation of cultured lung , p53R172H-KI (R172H), and WT (p53WT) mice (B) and mock-depleted (shGFP) or p53-depleted (shp53) lung cancer cell lines H1048 (D), which harbors R273C, and VMRC, which harbors R175H (F), are shown by bar graphs.…”

Functions of SMARCAL1, ZRANB3, and HLTF in maintaining genome stability

“…Stalled forks need to be stabilized to prevent nuclease cleavage or degradation to maintain genome integrity (Cortez, 2015). In situations when there is no converging replication fork, the stalled fork must also restart to complete DNA synthesis.…”

FANCM, BRCA1, and BLM cooperatively resolve the replication stress at the ALT telomeres

“…Nonetheless, the wellrecognized sources of replication stress include unrepaired DNA lesions, mis-incorporated ribonucleotides, unique DNA sequences that are prone to form secondary structures (e.g., G-quadruplex or G4), collision of the replication fork with the transcriptional machinery, an RNA-DNA hybrid (or R-loop) that is formed between a nascent RNA and the adjacent displaced single-stranded DNA (ssDNA), common fragile sites (CFS), and tightly packed genomic regions, such as heterochromatin (2). Because of these constant challenges faced by the replisome, mammalian cells have developed elaborate and complex strategies to resolve the replication stress and ensure the successful completion of DNA replication (1)(2)(3)(4).…”