Elucidation of the molecular mechanism of the breakage-fusion-bridge (BFB) cycle using a CRISPR-dCas9 cellular model

Chromosome instability (CIN) is frequently observed in many tumors. The breakage-fusion-bridge (BFB) cycle has been proposed to be one of the main drivers of CIN during tumorigenesis and tumor evolution. However, the detailed mechanisms for the individual steps of the BFB cycle warrants further investigation. Here, we demonstrated that a nuclease-dead Cas9 (dCas9) coupled with a telomere-specific single-guide RNA (sgTelo) can be used to model the BFB cycle. First, we showed that targeting dCas9 to telomeres using sgTelo impeded DNA replication at telomeres and induced a pronounced increase of replication stress and DNA damage. Using Single-Molecule Telomere Assay via Optical Mapping (SMTA-OM), we investigated the genome-wide features of telomeres in the dCas9/sgTelo cells and observed a dramatic increase of chromosome end fusions, including fusion/ITS+ and fusion/ITS-.Consistently, we also observed an increase in the formation of dicentric chromosomes, anaphase bridges, and intercellular telomeric chromosome bridges (ITCBs). Utilizing the dCas9/sgTelo system, we uncovered many novel molecular and structural features of the ITCB and demonstrated that multiple DNA repair pathways are implicated in the formation of ITCBs. Our studies shed new light on the molecular mechanisms of the BFB cycle, which will advance our understanding of tumorigenesis, tumor evolution, and drug resistance.

Multiplex structural variant detection by whole-genome mapping and nanopore sequencing

Identification of structural variants (SVs) breakpoints is important in studying mutations, mutagenic causes, and functional impacts. Next-generation sequencing and whole-genome optical mapping are extensively used in SV discovery and characterization. However, multiple platforms and computational approaches are needed for comprehensive analysis, making it resource-intensive and expensive. Here, we propose a strategy combining optical mapping and cas9-assisted targeted nanopore sequencing to analyze SVs. Optical mapping can economically and quickly detect SVs across a whole genome but does not provide sequence-level information or precisely resolve breakpoints. Furthermore, since only a subset of all SVs is known to affect biology, we attempted to type a subset of all SVs using targeted nanopore sequencing. Using our approach, we resolved the breakpoints of five deletions, five insertions, and an inversion, in a single experiment.

Customized optical mapping by CRISPR-Cas9 mediated DNA labeling with multiple sgRNAs

Whole-genome mapping technologies have been developed as a complementary tool to provide scaffolds for genome assembly and structural variation analysis (1,2). We recently introduced a novel DNA labeling strategy based on a CRISPR-Cas9 genome editing system, which can target any 20bp sequences. The labeling strategy is specifically useful in targeting repetitive sequences, and sequences not accessible to other labeling methods. In this report, we present customized mapping strategies that extend the applications of CRISPR-Cas9 DNA labeling. We first design a CRISPR-Cas9 labeling strategy to interrogate and differentiate the single allele differences in NGG protospacer adjacent motifs (PAM sequence). Combined with sequence motif labeling, we can pinpoint the single-base differences in highly conserved sequences. In the second strategy, we design mapping patterns across a genome by selecting sets of specific single-guide RNAs (sgRNAs) for labeling multiple loci of a genomic region or a whole genome. By developing and optimizing a single tube synthesis of multiple sgRNAs, we demonstrate the utility of CRISPR-Cas9 mapping with 162 sgRNAs targeting the 2Mb Haemophilus influenzae chromosome. These CRISPR-Cas9 mapping approaches could be particularly useful for applications in defining long-distance haplotypes and pinpointing the breakpoints in large structural variants in complex genomes and microbial mixtures.

Single-molecule analysis of subtelomeres and telomeres in Alternative Lengthening of Telomeres (ALT) cells

BACKGROUND

Telomeric DNA is typically comprised of G-rich tandem repeat motifs and maintained by telomerase (Greider CW, Blackburn EH; Cell 51:887-898; 1987). In eukaryotes lacking telomerase, a variety of DNA repair and DNA recombination based pathways for telomere maintenance have evolved in organisms normally dependent upon telomerase for telomere elongation (Webb CJ, Wu Y, Zakian VA; Cold Spring Harb Perspect Biol 5:a012666; 2013); collectively called Alternative Lengthening of Telomeres (ALT) pathways. By measuring (TTAGGG) n tract lengths from the same large DNA molecules that were optically mapped, we simultaneously analyzed telomere length dynamics and subtelomere-linked structural changes at a large number of specific subtelomeric loci in the ALT-positive cell lines U2OS, SK-MEL-2 and Saos-2.

RESULTS

Our results revealed loci-specific ALT telomere features. For example, while each subtelomere included examples of single molecules with terminal (TTAGGG) n tracts as well as examples of recombinant telomeric single molecules, the ratio of these molecules was subtelomere-specific, ranging from 33:1 (19p) to 1:25 (19q) in U2OS. The Saos-2 cell line shows a similar percentage of recombinant telomeres. The frequency of recombinant subtelomeres of SK-MEL-2 (11%) is about half that of U2OS and Saos-2 (24 and 19% respectively). Terminal (TTAGGG) n tract lengths and heterogeneity levels, the frequencies of telomere signal-free ends, and the frequency and size of retained internal telomere-like sequences (ITSs) at recombinant telomere fusion junctions all varied according to the specific subtelomere involved in a particular cell line. Very large linear extrachromosomal telomere repeat (ECTR) DNA molecules were found in all three cell lines; these are in principle capable of templating synthesis of new long telomere tracts via break-induced repair (BIR) long-tract DNA synthesis mechanisms and contributing to the very long telomere tract length and heterogeneity characteristic of ALT cells. Many of longest telomere tracts (both end-telomeres and linear ECTRs) displayed punctate CRISPR/Cas9-dependent (TTAGGG) n labeling patterns indicative of interspersion of stretches of non-canonical telomere repeats.

CONCLUSION

Identifying individual subtelomeres and characterizing linked telomere (TTAGGG) n tract lengths and structural changes using our new single-molecule methodologies reveals the structural consequences of telomere damage, repair and recombination mechanisms in human ALT cells in unprecedented molecular detail and significant differences in different ALT-positive cell lines.

Comprehensive Analysis of Human Subtelomeres by Whole Genome Mapping

Detailed comprehensive knowledge of the structures of individual long-range telomere-terminal haplotypes are needed to understand their impact on telomere function, and to delineate the population structure and evolution of subtelomere regions. However, the abundance of large evolutionarily recent segmental duplications and high levels of large structural variations have complicated both the mapping and sequence characterization of human subtelomere regions. Here, we use high throughput optical mapping of large single DNA molecules in nanochannel arrays for 154 human genomes from 26 populations to present a comprehensive look at human subtelomere structure and variation. The results catalog many novel long-range subtelomere haplotypes and determine the frequencies and contexts of specific subtelomeric duplicons on each chromosome arm, helping to clarify the currently ambiguous nature of many specific subtelomere structures as represented in the current reference sequence (HG38). The organization and content of some duplicons in subtelomeres appear to show both chromosome arm and population-specific trends. Based upon these trends we estimate a timeline for the spread of these duplication blocks.