Paths from DNA damage and signaling to genome rearrangements via homologous recombination

Abstract

DNA damage is a constant threat to genome integrity. DNA repair and damage signaling networks play a central role maintaining genome stability, suppressing tumorigenesis, and determining tumor response to common cancer chemotherapeutic agents and radiotherapy. DNA double-strand breaks (DSBs) are critical lesions induced by ionizing radiation and when replication forks encounter damage. DSBs can result in mutations and large-scale genome rearrangements reflecting mis-repair by non-homologous end joining or homologous recombination. Ionizing radiation induces genetic change immediately, and it also triggers delayed events weeks or even years after exposure, long after the initial damage has been repaired or diluted through cell division. This review covers DNA damage signaling and repair pathways and cell fate following genotoxic insult, including immediate and delayed genome instability and cell survival/cell death pathways.

Keywords: DNA damage; DNA repair; Genome instability; Radiation.

Fig. 1.

IR causes DNA damage directly and through ROS production, causing a wide variety of genetic changes. Some changes appear immediately after exposure and are most likely targeted effects. IR also induces delayed effects that can lead to the same types of genetic changes, but are likely non-targeted effects and may reflect changes in specific organelles or processes that have broad impact on genome stability.

Fig. 2.

Core factors in DNA repair and DDR networks. Ionizing radiation causes DNA damage that activates PIKs (bold) which transmit signals to both downstream and upstream targets that regulate DNA repair by aNHEJ, cNHEJ, and HR, and activate checkpoint response pathways that arrest the cell cycle and trigger programmed cell death pathways, all of which regulate cell fate.

Fig. 3.

DSB repair pathways. DSB repair pathway choice is controlled by resection, mediated by several nucleases. cNHEJ involves little or no resection, aNHEJ limited resection to expose microhomologies near the DSB (black rectangles), and HR involves extensive resection creating long, 3’ single-stranded tails that invade homologous sequences (typically sister chromatids) that serve as accurate repair templates.

Fig. 4.

IR-induced DHR. (A) Cells carry a single copy of a direct repeat HR substrate with two inactive copies of GFP (GFP⁻). HR creates GFP⁺ products that retain both copies or delete one of the copies via crossover or single-strand annealing. (B) GFP⁻ cells treated with IR produce colonies that are GFP⁻ (parental, non-recombinant), fully GFP⁺ (immediate HR (C) Moderate to high doses of X-rays stimulate DHR at high frequencies. (D) Low (cGy) doses of X-rays stimulate DHR at high frequencies and induce an adaptive response to a later challenge dose of 500 cGy. Data compiled from refs. [169, 185].