Dormant origin signaling during unperturbed replication

Abstract

Mechanisms that limit origin firing are essential as the ˜50,000 origins that replicate the human genome in unperturbed cells are chosen from an excess of ˜500,000 licensed origins. Computational models of the spatiotemporal pattern of replication foci assume that origins fire stochastically with a domino-like progression that places later firing origins near recent fired origins. These stochastic models of origin firing require dormant origin signaling that inhibits origin firing and suppresses licensed origins for passive replication at a distance of ∼7-120 kbp around replication forks. ATR and CHK1 kinase inhibitors increase origin firing and increase origin density in unperturbed cells. Thus, basal ATR and CHK1 kinase-dependent dormant origin signaling inhibits origin firing and there appear to be two thresholds of ATR kinase signaling. A minority of ATR molecules are activated for ATR and CHK1 kinase-dependent dormant origin signaling and this is essential for DNA replication in unperturbed cells. A majority of ATR molecules are activated for ATR and CHK1 kinase-dependent checkpoint signaling in cells treated with DNA damaging agents that target replication forks. Since ATR and CHK1 kinase inhibitors increase origin firing and this is associated with fork stalling and extensive regions of single-stranded DNA, they are DNA damaging agents. Accordingly, the sequence of administration of ATR and CHK1 kinase inhibitors and DNA damaging agents may impact the DNA damage induced by the combination and the efficacy of cell killing by the combination.

Keywords: ATR kinase; CHK1 kinase; DNA replication; Dormant origins.

Figure 1.

We propose that there are two thresholds of ATR kinase signaling. ATR kinase-dependent dormant origin signaling in which a minority of ATR molecules are active may be essential for DNA replication in unperturbed cells. DNA damage-induced ATR kinase-dependent checkpoint signaling in which a majority of ATR molecules are active is critical for the phosphorylation of several thousand substrates that function in DNA replication and repair, chromatin remodeling, transcription, protein synthesis and degradation, cell cycle progression and cell death. ATR kinase inhibition blocks dormant origin signaling and increases origin firing and increases origin density in unperturbed cells. ATR kinase inhibitors AZD6738, VX970 and BAY1895344 are in clinical trials.

Figure 2.

We hypothesize that the two thresholds of ATR and CHK1 kinase signaling are distinguished, at least in part, by spatial limitations on signaling. At the lower threshold of ATR and CHK1 kinase signaling, a minority of ATR and CHK1 molecules are active and these limit dormant origin firing around active replication forks – this would be the dormant origin signaling mechanism.