De novo telomere addition at chromosome breaks: Dangerous Liaisons

To Join or Not to Join: Decision Points Along the Pathway to Double-Strand Break Repair vs. Chromosome End Protection

The regulation of DNA double-strand breaks (DSBs) and telomeres are diametrically opposed in the cell. DSBs are considered one of the most deleterious forms of DNA damage and must be quickly recognized and repaired. Telomeres, on the other hand, are specialized, stable DNA ends that must be protected from recognition as DSBs to inhibit unwanted chromosome fusions. Decisions to join DNA ends, or not, are therefore critical to genome stability. Yet, the processing of telomeres and DSBs share many commonalities. Accordingly, key decision points are used to shift DNA ends toward DSB repair vs. end protection. Additionally, DSBs can be repaired by two major pathways, namely homologous recombination (HR) and non-homologous end joining (NHEJ). The choice of which repair pathway is employed is also dictated by a series of decision points that shift the break toward HR or NHEJ. In this review, we will focus on these decision points and the mechanisms that dictate end protection vs. DSB repair and DSB repair choice.

Emerging non-canonical roles for the Rad51-Rad52 interaction in response to double-strand breaks in yeast

DNA double-strand break repair allows cells to survive both exogenous and endogenous insults to the genome. In yeast, the recombinases Rad51 and Rad52 are central to multiple forms of homology-dependent repair. Classically, Rad51 and Rad52 are thought to act cooperatively, with formation of the functional Rad51 nucleofilament facilitated by the mediator function of Rad52. Several studies have now identified functions for the interaction between Rad51 and Rad52 that are independent of the mediator function of Rad52 and affect a seemingly diverse array of functions in de novo telomere addition, global chromosome mobility following DNA damage, Rad51 nucleofilament stability, checkpoint adaptation, and microhomology-mediated chromosome rearrangements. Here, we review these functions with an emphasis on our recent discovery that the Rad51-Rad52 interaction influences the probability of de novo telomere addition at sites preferentially targeted by telomerase following a double-strand break (DSB). We present data addressing the prevalence of sites within the yeast genome that are capable of stimulating de novo telomere addition following a DSB and speculate about the potential role such sites may play in genome stability.

Telomeric small RNAs in the genus Caenorhabditis

Telomeric DNA is composed of simple tandem repeat sequences and has a G-rich strand that runs 5' to 3' toward the chromosome terminus. Small RNAs with homology to telomeres have been observed in several organisms and could originate from telomeres or from interstitial telomere sequences (ITSs), which are composites of degenerate and perfect telomere repeat sequences found on chromosome arms. We identified Caenorhabditis elegans small RNAs composed of the Caenorhabditis telomere sequence (TTAGGC)_n with up to three mismatches, which might interact with telomeres. We rigorously defined ITSs for genomes of C. elegans and for two closely related nematodes, Caenorhabditis briggsae and Caenorhabditis remanei Most telomeric small RNAs with mismatches originated from ITSs, which were depleted from mRNAs but were enriched in introns whose genes often displayed hallmarks of genomic silencing. C. elegans small RNAs composed of perfect telomere repeats were very rare but their levels increased by several orders of magnitude in C. briggsae and C. remanei Major small RNA species in C. elegans begin with a 5' guanine nucleotide, which was strongly depleted from perfect telomeric small RNAs of all three Caenorhabditis species. Perfect G-rich or C-rich telomeric small RNAs commonly began with 5' UAGGCU and 5' UUAGGC or 5' CUAAGC, respectively. In contrast, telomeric small RNAs with mismatches had a mixture of all four 5' nucleotides. We suggest that perfect telomeric small RNAs have a mechanism of biogenesis that is distinct from known classes of small RNAs and that a dramatic change in their regulation occurred during recent Caenorhabditis evolution.