When heat casts a spell on the DNA damage checkpoints

Abstract

Peregrine Laziosi (1265-1345), an Italian priest, became the patron saint of cancer patients when the tumour in his left leg miraculously disappeared after he developed a fever. Elevated body temperature can cause tumours to regress and sensitizes cancer cells to agents that break DNA. Why hyperthermia blocks the repair of broken chromosomes by changing the way that the DNA damage checkpoint kinases ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) are activated is an unanswered question. This review discusses the current knowledge of how heat affects the ATR-Chk1 and ATM-Chk2 kinase networks, and provides a possible explanation of why homeothermal organisms such as humans still possess this ancient heat response.

Keywords: ATM; ATR; Chk1; Chk2; DNA damage checkpoint; hyperthermia.

Figure 1.

Activation of ATM and ATR at 37°C. (a) Activation of ATM. The Mre11–Rad50–Nbs1 (MRN) complex recruits ATM kinase to a broken chromosome. Association of ATM with Nbs1 and the release of short DNA fragments by the Mre11 nuclease within the MRN complex activate ATM. Inactive ATM is a homodimer linked by a disulfide bond (inset). Active, monomeric ATM phosphorylates Chk2 kinase at threonine-68 (T68) and the histone-variant H2AX at serine-139 (S139). Dependent on the cell cycle stage, 53BP1 regulates the conversion of a blunt-ended break into a break with single-stranded DNA tracks. (b) Activation of ATR. The ATRIP subunit directs ATR kinase to single-stranded DNA covered by the trimeric replication protein A (RPA). Activation of ATR requires the independent recruitment of the Rad9–Rad1–Hus1 ring (9–1–1) and TopBP1. The tail domain of Rad9 and the ATR-activation domain of TopBP1 both stimulate ATR kinase. Active ATR phosphorylates Chk1 kinase at serine-345 (S345) and the RPA32 subunit or RPA.

Figure 2.

Activation of ATM and ATR at temperatures above 40°C. (a) Activation of ATR. Nucleolin (N) binds to replication protein A (RPA) after its heat-induced release from the nucleolus. This reprograms ATR activation. Despite the presence of the Rad9–Rad1–Hus1 ring (9–1–1) and TopBP1, heat-activated ATR does not modify RPA32. Chk1 is phosphorylated at several sites in addition to serine-345 (S345). (b) Cell cycle specificity of histone H2AX phosphorylation. In G1 and G2, ATM kinase phosphorylates the histone variant H2AX in response to DSBs or high levels of reactive oxygen species (ROS). During S phase, the related kinase DNA-PK modifies H2AX to stabilize stalled replication forks. (c) Activation of ATM. The Mre11–Rad50–Nbs1 complex moves from the nucleus to the cytoplasm (inset) in the response to heat stress. Despite the absence of the MRN complex, ATM kinase is activated in a yet unknown way by heat and phosphorylates Chk2 kinase at threonine-68 (T68). The recruitment of 53BP1 is delayed. Heat, inflammation and increased metabolism increase the levels of ROS which can open the disulfide bond of the inactive ATM dimer thereby activating ATM. DNA double-strand breaks may be a secondary consequence of high ROS levels.

Figure 3.

The anti-cancer function of heat activation of ATM and ATR. Elevated temperatures inside solid tumours and increased levels of reactive oxygen species (ROS) reprogramme ATR and ATM signalling. This impairs the repair of DNA double-stranded breaks (DSBs) which accumulate in quickly dividing cancer cells. ATR and ATM cause tumour regression by inducing cell death (apoptosis) and by imposing a cell cycle arrest. At the same time, normal tissue is protected.