A novel NGS library preparation method to characterize native termini of fragmented DNA.

Biological and chemical DNA fragmentation generates DNA molecules with a variety of termini, including blunt ends and single-stranded overhangs. We have developed a Next Generation Sequencing (NGS) assay, XACTLY, to interrogate the termini of fragmented DNA, information traditionally lost in standard NGS library preparation methods. Here we describe the XACTLY method, showcase its sensitivity and specificity, and demonstrate its utility in in vitro experiments. The XACTLY assay is able to report relative abundances of all lengths and types (5' and 3') of single-stranded overhangs, if present, on each DNA fragment with an overall accuracy between 80-90%. In addition, XACTLY retains the sequence of each native DNA molecule after fragmentation and can capture the genomic landscape of cleavage events at single nucleotide resolution. The XACTLY assay can be applied as a novel research and discovery tool for fragmentation analyses and in cell-free DNA.

A ligation-based single-stranded library preparation method to analyze cell-free DNA and synthetic oligos.

BACKGROUND: Cell-free DNA (cfDNA), present in circulating blood plasma, contains information about prenatal health, organ transplant reception, and cancer presence and progression. Originally developed for the genomic analysis of highly degraded ancient DNA, single-stranded DNA (ssDNA) library preparation methods are gaining popularity in the field of cfDNA analysis due to their efficiency and ability to convert short, fragmented DNA into sequencing libraries without altering DNA ends. However, current ssDNA methods are costly and time-consuming. RESULTS: Here we present an efficient ligation-based single-stranded library preparation method that is engineered to produce complex libraries in under 2.5 h from as little as 1 nanogram of input DNA without alteration to the native ends of template molecules. Our method, called Single Reaction Single-stranded LibrarY or SRSLY, ligates uniquely designed Next-Generation Sequencing (NGS) adapters in a one-step combined phosphorylation/ligation reaction that foregoes end-polishing. Using synthetic DNA oligos and cfDNA, we demonstrate the efficiency and utility of this approach and compare with existing double-stranded and single-stranded approaches for library generation. Finally, we demonstrate that cfDNA NGS data generated from SRSLY can be used to analyze DNA fragmentation patterns to deduce nucleosome positioning and transcription factor binding. CONCLUSIONS: SRSLY is a versatile tool for converting short and fragmented DNA molecules, like cfDNA fragments, into sequencing libraries while retaining native lengths and ends.

Chromosome-scale shotgun assembly using an in vitro method for long-range linkage.

Long-range and highly accurate de novo assembly from short-read data is one of the most pressing challenges in genomics. Recently, it has been shown that read pairs generated by proximity ligation of DNA in chromatin of living tissue can address this problem, dramatically increasing the scaffold contiguity of assemblies. Here, we describe a simpler approach ("Chicago") based on in vitro reconstituted chromatin. We generated two Chicago data sets with human DNA and developed a statistical model and a new software pipeline ("HiRise") that can identify poor quality joins and produce accurate, long-range sequence scaffolds. We used these to construct a highly accurate de novo assembly and scaffolding of a human genome with scaffold N50 of 20 Mbp. We also demonstrated the utility of Chicago for improving existing assemblies by reassembling and scaffolding the genome of the American alligator. With a single library and one lane of Illumina HiSeq sequencing, we increased the scaffold N50 of the American alligator from 508 kbp to 10 Mbp.

The mutagenic footprint of low-fidelity Pol I ColE1 plasmid replication in E. coli reveals an extensive interplay between Pol I and Pol III.

ColE1 plasmid replication is unidirectional and requires two DNA polymerases: DNA polymerase I (Pol I) and DNA polymerase III (Pol III). Pol I initiates leading-strand synthesis by extending an RNA primer, allowing the Pol III holoenzyme to assemble and finish replication of both strands. The goal of the present work is to study the interplay between Pol I and Pol III during ColE1 plasmid replication, to gain new insights into Pol I function in vivo. Our approach consists of using mutations generated by a low-fidelity mutant of Pol I (LF-Pol I) during replication of a ColE1 plasmid as a footprint for Pol I replication. This approach allowed mapping areas of Pol I replication on the plasmid with high resolution. In addition, we were able to approximate the strandedness of Pol I mutations throughout the plasmid, allowing us to estimate the spectrum of the LF-Pol I in vivo. Our study produced the following three mechanistic insights: (1) we identified the likely location of the polymerase switch at ~200 bp downstream of replication initiation; (2) we found evidence suggesting that Pol I can replicate both strands, supporting earlier studies indicating a functional redundancy between Pol I and Pol III (3) we found evidence pointing to a specific role of Pol I during termination of lagging-strand replication. In addition, we illustrate how our strand-specific footprinting approach can be used to dissect factors modulating Pol I fidelity in vivo.

Structural Variation Detection by Proximity Ligation from Formalin-Fixed, Paraffin-Embedded Tumor Tissue.

The clinical management and therapy of many solid tumor malignancies depends on detection of medically actionable or diagnostically relevant genetic variation. However, a principal challenge for genetic assays from tumors is the fragmented and chemically damaged state of DNA in formalin-fixed, paraffin-embedded (FFPE) samples. From highly fragmented DNA and RNA there is no current technology for generating long-range DNA sequence data as is required to detect genomic structural variation or long-range genotype phasing. We have developed a high-throughput chromosome conformation capture approach for FFPE samples that we call Fix-C, which is similar in concept to Hi-C. Fix-C enables structural variation detection from archival FFPE samples. This method was applied to 15 clinical adenocarcinoma- and sarcoma-positive control specimens spanning a broad range of tumor purities. In this panel, Fix-C analysis achieves a 90% concordance rate with fluorescence in situ hybridization assays, the current clinical gold standard. In addition, novel structural variation undetected by other methods could be identified, and long-range chromatin configuration information recovered from these FFPE samples harboring highly degraded DNA. This powerful approach will enable detailed resolution of global genome rearrangement events during cancer progression from FFPE material and will inform the development of targeted molecular diagnostic assays for patient care.

Random mutagenesis by error-prone pol plasmid replication in Escherichia coli.

Directed evolution is an approach that mimics natural evolution in the laboratory with the goal of modifying existing enzymatic activities or of generating new ones. The identification of mutants with desired properties involves the generation of genetic diversity coupled with a functional selection or screen. Genetic diversity can be generated using PCR or using in vivo methods such as chemical mutagenesis or error-prone replication of the desired sequence in a mutator strain. In vivo mutagenesis methods facilitate iterative selection because they do not require cloning, but generally produce a low mutation density with mutations not restricted to specific genes or areas within a gene. For this reason, this approach is typically used to generate new biochemical properties when large numbers of mutants can be screened or selected. Here we describe protocols for an advanced in vivo mutagenesis method that is based on error-prone replication of a ColE1 plasmid bearing the gene of interest. Compared to other in vivo mutagenesis methods, this plasmid-targeted approach allows increased mutation loads and facilitates iterative selection approaches. We also describe the mutation spectrum for this mutagenesis methodology in detail, and, using cycle 3 GFP as a target for mutagenesis, we illustrate the phenotypic diversity that can be generated using our method. In sum, error-prone Pol I replication is a mutagenesis method that is ideally suited for the evolution of new biochemical activities when a functional selection is available.

Interplay between base excision repair activity and toxicity of 3-methyladenine DNA glycosylases in an E. coli complementation system.

DNA glycosylases carry out the first step of base excision repair by removing damaged bases from DNA. The N3-methyladenine (3MeA) DNA glycosylases specialize in alkylation repair and are either constitutively expressed or induced by exposure to alkylating agents. To study the functional and evolutionary significance of constitutive versus inducible expression, we expressed two closely related yeast 3MeA DNA glycosylases - inducible Saccharomyces cerevisiae MAG and constitutive S. pombe Mag1 - in a glycosylase-deficient Escherichia coli strain. In both cases, constitutive expression conferred resistance to alkylating agent exposure. However, in the absence of exogenous alkylation, high levels of expression of both glycosylases were deleterious. We attribute this toxicity to excessive glycosylase activity, since suppressing spMag1 expression correlated with improved growth in liquid culture, and spMag1 mutants exhibiting decreased glycosylase activity showed improved growth and viability. Selection of a random spMag1 mutant library for increased survival in the presence of exogenous alkylation resulted in the selection of hypomorphic mutants, providing evidence for the presence of a genetic barrier to the evolution of enhanced glycosylase activity when constitutively expressed. We also show that low levels of 3MeA glycosylase expression improve fitness in our glycosylase-deficient host, implying that 3MeA glycosylase activity is likely necessary for repair of endogenous lesions. These findings suggest that 3MeA glycosylase activity is evolutionarily conserved for repair of endogenously produced alkyl lesions, and that inducible expression represents a common strategy to rectify deleterious effects of excessive 3MeA activity in the absence of exogenous alkylation challenge.