Anastazja Grabarz

Faithful DNA repair is essential to avoid chromosomal rearrangements and promote genome integrity. Nuclear organization has emerged as a key parameter in the formation of chromosomal translocations, yet little is known as to whether DNA repair can efficiently occur throughout the nucleus and whether it is affected by the location of the lesion. Here, we induce DNA double-strand breaks (DSBs) at different nuclear compartments and follow their fate. We demonstrate that DSBs induced at the nuclear membrane (but not at nuclear pores or nuclear interior) fail to rapidly activate the DNA damage response (DDR) and repair by homologous recombination (HR). Real-time and superresolution imaging reveal that DNA DSBs within lamina-associated domains do not migrate to more permissive environments for HR, like the nuclear pores or the nuclear interior, but instead are repaired in situ by alternative end-joining. Our results are consistent with a model in which nuclear position dictates the choice of DNA repair pathway, thus revealing a new level of regulation in DSB repair controlled by spatial organization of DNA within the nucleus.

A DNA double strand break (DSB) is a highly toxic lesion, which can generate genetic instability and profound genome rearrangements. However, DSBs are required to generate diversity during physiological processes such as meiosis or the establishment of the immune repertoire. Thus, the precise regulation of a complex network of processes is necessary for the maintenance of genomic stability, allowing genetic diversity but protecting against genetic instability and its consequences on oncogenesis. Two main strategies are employed for DSB repair: homologous recombination (HR) and non-homologous end-joining (NHEJ). HR is initiated by single-stranded DNA (ssDNA) resection and requires sequence homology with an intact partner, while NHEJ requires neither resection at initiation nor a homologous partner. Thus, resection is an pivotal step at DSB repair initiation, driving the choice of the DSB repair pathway employed. However, an alternative end-joining (A-EJ) pathway, which is highly mutagenic, has recently been described; A-EJ is initiated by ssDNA resection but does not require a homologous partner. The choice of the appropriate DSB repair system, for instance according the cell cycle stage, is essential for genome stability maintenance. In this context, controlling the initial events of DSB repair is thus an essential step that may be irreversible, and the wrong decision should lead to dramatic consequences. Here, we first present the main DSB repair mechanisms and then discuss the importance of the choice of the appropriate DSB repair pathway according to the cell cycle phase. In a third section, we present the early steps of DSB repair i.e., DSB signaling, chromatin remodeling, and the regulation of ssDNA resection. In the last part, we discuss the competition between the different DSB repair mechanisms. Finally, we conclude with the importance of the fine tuning of this network for genome stability maintenance and for tumor protection in fine.

The choice of the appropriate double-strand break (DSB) repair pathway is essential for the maintenance of genomic stability. Here, we show that the Bloom syndrome gene product, BLM, counteracts CtIP/MRE11-dependent long-range deletions (>200 bp) generated by alternative end-joining (A-EJ). BLM represses A-EJ in an epistatic manner with 53BP1 and RIF1 and is required for ionizing-radiation-induced 53BP1 focus assembly. Conversely, in the absence of 53BP1 or RIF1, BLM promotes formation of A-EJ long deletions, consistent with a role for BLM in DSB end resection. These data highlight a dual role for BLM that influences the DSB repair pathway choice: (1) protection against CtIP/MRE11 long-range deletions associated with A-EJ and (2) promotion of DNA resection. These antagonist roles can be regulated, according to cell-cycle stage, by interacting partners such as 53BP1 and TopIII, to avoid unscheduled resection that might jeopardize genome integrity.

International audience