Stepwise requirements for polymerases δ and θ in theta-mediated end joining.

Timely repair of chromosomal double-strand breaks is required for genome integrity and cellular viability. The polymerase theta-mediated end joining pathway has an important role in resolving these breaks and is essential in cancers defective in other DNA repair pathways, thus making it an emerging therapeutic target. It requires annealing of 2-6 nucleotides of complementary sequence, microhomologies, that are adjacent to the broken ends, followed by initiation of end-bridging DNA synthesis by polymerase θ.

Templated insertions-DNA repair gets acrobatic.

Deletions associated with the repair of DNA double-strand breaks is a source of genetic alternation and a recognized source of disease-causing mutagenesis. Theta-mediated end joining is a DNA repair mechanism, which guarantees deletions by its employment of microhomology (MH) alignment to facilitate end joining. A lesser-characterized templated insertion ability of this pathway, on the other hand, is associated with both deletion and insertion.

Poly(ADP) ribose polymerase promotes DNA polymerase theta-mediated end joining by activation of end resection.

The DNA polymerase theta (Polθ)-mediated end joining (TMEJ) pathway for repair of chromosomal double strand breaks (DSBs) is essential in cells deficient in other DSB repair pathways, including hereditary breast cancers defective in homologous recombination. Strand-break activated poly(ADP) ribose polymerase 1 (PARP1) has been implicated in TMEJ, but the modest specificity of existing TMEJ assays means the extent of effect and the mechanism behind it remain unclear. We describe here a series of TMEJ assays with improved specificity and show ablation of PARP activity reduces TMEJ activity 2-4-fold.

Telomere replication-When the going gets tough.

Telomeres consist of repetitive tracts of DNA that shield a chromosome's contents from erosion and replicative attrition. However, telomeres are also late-replicating regions of the genome in which a myriad of replicative obstructions reside. The obstacles contained within telomeres, as well as their genomic location, drive replicative stalling and subsequent fork collapse in these regions.

EXO1 resection at G-quadruplex structures facilitates resolution and replication.

G-quadruplexes represent unique roadblocks to DNA replication, which tends to stall at these secondary structures. Although G-quadruplexes can be found throughout the genome, telomeres, due to their G-richness, are particularly predisposed to forming these structures and thus represent difficult-to-replicate regions. Here, we demonstrate that exonuclease 1 (EXO1) plays a key role in the resolution of, and replication through, telomeric G-quadruplexes.

Telomere fusions and translocations: a bridge too far?

Telomere fusions inevitably arise as a cell's last-ditch effort to protect exposed chromosomal ends when telomeres are lost due to aging-associated erosion, breakage, failed replication, or a plethora of other cellular mistakes. Fusion of an exposed chromosomal end to another telomere presumably presents a superficially attractive option to the cell as opposed to the alternative of the impending degradation of the unprotected chromosomal terminus. However, when allowed to progress to mitosis these fusion events subsequently foster non-disjunction or bridge:breakage events - both of which drive highly pathogenic genomic instability and additional chromosomal translocations.

CtIP is essential for telomere replication.

The maintenance of telomere length is critical to longevity and survival. Specifically, the failure to properly replicate, resect, and/or form appropriate telomeric structures drives telomere shortening and, in turn, genomic instability. The endonuclease CtIP is a DNA repair protein that is well-known to promote genome stability through the resection of endogenous DNA double-stranded breaks.