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The telomerase reverse transcriptase elongates telomeres to prevent replicative senescence. This process requires exposure of the 3′-end, which is thought to occur when two sister telomeres are generated at replication completion. Using two-dimensional agarose gel electrophoresis (2D-gels) and electron microscopy, we found that telomeric repeats are hotspots for replication fork reversal. Fork reversal generates 3′ telomeric ends before replication completion. To verify whether these ends are elongated by telomerase, we probed de novo telomeric synthesis in situ and at replication intermediates by reconstituting mutant telomerase that adds a variant telomere sequence. We found variant telomeric repeats overlapping with telomeric reversed forks in 2D-gels, but not with normal forks, nontelomeric reversed forks, or telomeric reversed forks with a C-rich 3′-end. Our results define reversed telomeric forks as a substrate of telomerase during replication.

Centromeres are key elements for chromosome segregation. Canonical centromeres are built over long-stretches of tandem repetitive arrays. Despite being quite abundant compared to other loci, centromere sequences overall still represent only 2 to 5% of the human genome, therefore studying their genetic and epigenetic features is a major challenge. Furthermore, sequencing of centromeric regions requires high coverage to fully analyze length and sequence variations, and this can be extremely costly. To bypass these issues, we have developed a technique, named CenRICH, to enrich for centromeric DNA from human cells based on selective restriction digestion and size fractionation. Combining restriction enzymes cutting at high frequency throughout the genome, except within most human centromeres, with size-selection of fragments >20 kb, resulted in over 25-fold enrichment in centromeric DNA. High-throughput sequencing revealed that up to 60% of the DNA in the enriched samples is made of centromeric repeats. We show that this method can be used in combination with long-read sequencing to investigate the DNA methylation status of certain centromeres and, with a specific enzyme combination, also of their surrounding regions (mainly HSATII). Finally, we show that CenRICH facilitates single-molecule analysis of replicating centromeric fibers by DNA combing. This approach has great potential for making sequencing of centromeric DNA more affordable and efficient and for single DNA molecule studies.

Here we provide a detailed protocol for the enrichment of telomeric repeats from mouse and human cells. The procedure consists of two successive rounds of digestion with frequently cutting restriction enzymes followed by size fractionation. Around 2 mg of genomic DNA is required, and the procedure lasts 5-6 d and yields preparations enriched800-fold in telomeres. The purified material is suitable for single-molecule analysis of telomere structure, visualizing telomere replication and recombination intermediates by electron microscopy or performing molecular combing at telomeric repeats. No special skills are required for the enrichment procedure, while some assistance is needed in harvesting a large number of plates in a timely fashion at the beginning of the procedure. A smaller-scale version of the protocol that involves one round of digestion and purification requires 200 µg of DNA and enriches telomeres ~50-fold in 4 d is also provided. The latter can be combined with specific labeling for single-molecule analysis of replicating DNA or for long-read sequencing analysis of telomeric repeats. The procedure described here can be adapted to the enrichment of other repetitive elements, based on the use of restriction enzymes that do not cut into the repeat of interest.

Centromeres are key elements for chromosome segregation. Canonical centromeres are built over long-stretches of tandem repetitive arrays. Despite being quite abundant compared to other loci, centromere sequences overall still represent only 2 to 5% of the human genome, therefore studying their genetic and epigenetic features is a major challenge. Furthermore, sequencing of centromeric regions requires high coverage to fully analyze length and sequence variations, which can be extremely costly. To bypass these issues, we have developed a technique based on selective restriction digestion and size fractionation to enrich for centromeric DNA from human cells. Combining enzymes capable of cutting at high frequency throughout the genome, except within most human centromeres, with size-selection of >20 kb fragments resulted in over 25-fold enrichment in centromeric DNA. Sequencing of the enriched fractions revealed that up to 60% of the enriched material is made of centromeric DNA. This approach has great potential for making sequencing of centromeric DNA more affordable and efficient and for single DNA molecule studies.

Extrachromosomal telomeric circles are commonly invoked as important players in telomere maintenance, but their origin has remained elusive. Using electron microscopy analysis on purified telomeres we show that, apart from known structures, telomeric repeats accumulate internal loops (i-loops) that occur in the proximity of nicks and single-stranded DNA gaps. I-loops are induced by single-stranded damage at normal telomeres and represent the majority of telomeric structures detected in ALT (Alternative Lengthening of Telomeres) tumor cells. Our data indicate that i-loops form as a consequence of the exposure of single-stranded DNA at telomeric repeats. Finally, we show that these damage-induced i-loops can be excised to generate extrachromosomal telomeric circles resulting in loss of telomeric repeats. Our results identify damage-induced i-loops as a new intermediate in telomere metabolism and reveal a simple mechanism that links telomere damage to the accumulation of extrachromosomal telomeric circles and to telomere erosion., Extrachromosomal circular DNA made of telomeric repeats have been found to have an effect on telomere maintenance. By combining electron microscopy with a telomere purification procedure the authors identify damage-induced i-loops as a key intermediate in telomere circle formation.

Moderate DNA damage resulting from metabolic activities or sub-lethal doses of exogenous insults may eventually lead to cancer onset. Human 46BR.1G1 cells bear a mutation in replicative DNA ligase I (LigI) which results in low levels of replication-dependent DNA damage. This replication stress elicits a constitutive phosphorylation of the ataxia telangiectasia mutated (ATM) checkpoint kinase that fails to arrest cell cycle progression or to activate apoptosis or cell senescence. Stable transfection of wild type LigI, as in 7A3 cells, prevents DNA damage and ATM activation. Here we show that parental 46BR.1G1 and 7A3 cells differ in important features such as cell morphology, adhesion and migration. Comparison of gene expression profiles in the two cell lines detects Bio-Functional categories consistent with the morphological and migration properties of LigI deficient cells. Interestingly, ATM inhibition makes 46BR.1G1 more similar to 7A3 cells for what concerns morphology, adhesion and expression of cell-cell adhesion receptors. These observations extend the influence of the DNA damage response checkpoint pathways and unveil a role for ATM kinase activity in modulating cell biology parameters relevant to cancer progression.