The DNA replication checkpoint prevents PCNA/RFC depletion to protect forks from HLTF-induced collapse in human cells

The DNA replication checkpoint is crucial for maintaining genome stability after genotoxic stress; without it, stalled DNA replication forks cannot restart normally, excess DNA replication origins are activated, DNA damage and single-stranded DNA (ssDNA) accumulate, S phase does not finish, and cells die. Preventing excess origin firing suppresses all these effects. Here, we show in human cells that when replication is not restrained by a functional checkpoint, excess DNA synthesis sequesters the processivity factor PCNA and its loader, replication factor C (RFC), preventing normal fork restart. Nascent DNA ends unprotected by RFC/PCNA are attacked by the helicase-like transcription factor (HLTF), causing irreversible replication fork collapse and hyperaccumulation of ssDNA. This explains how the checkpoint stabilizes stalled replication forks and has implications for how origin firing is normally coordinated with fork progression. Loss of HLTF suppresses fork collapse and cell lethality in checkpoint-deficient cells, which has implications for how resistance to anti-checkpoint therapies may arise.