Genomic origin, fragmentomics, and transcriptional properties of long cell-free DNA molecules in human plasma.

Recent studies have revealed an unexplored population of long cell-free DNA (cfDNA) molecules in human plasma using long-read sequencing technologies. However, the biological properties of long cfDNA molecules (>500 bp) remain largely unknown. To this end, we have investigated the origins of long cfDNA molecules from different genomic elements. Analysis of plasma cfDNA using long-read sequencing reveals an uneven distribution of long molecules from across the genome. Long cfDNA molecules show overrepresentation in euchromatic regions of the genome, in sharp contrast to short DNA molecules. We observe a stronger relationship between the abundance of long molecules and mRNA gene expression levels, compared with short molecules (Pearson's r = 0.71 vs. -0.14). Moreover, long and short molecules show distinct fragmentation patterns surrounding CpG sites. Leveraging the cleavage preferences surrounding CpG sites, the combined cleavage ratios of long and short molecules can differentiate patients with hepatocellular carcinoma (HCC) from non-HCC subjects (AUC = 0.87). We also investigated knockout mice in which selected nuclease genes had been inactivated in comparison with wild-type mice. The proportion of long molecules originating from transcription start sites are lower in Dffb-deficient mice but higher in Dnase1l3-deficient mice compared with that of wild-type mice. This work thus provides new insights into the biological properties and potential clinical applications of long cfDNA molecules.

Epigenetic analysis of cell-free DNA by fragmentomic profiling.

Cell-free DNA (cfDNA) fragmentation patterns contain important molecular information linked to tissues of origin. We explored the possibility of using fragmentation patterns to predict cytosine-phosphate-guanine (CpG) methylation of cfDNA, obviating the use of bisulfite treatment and associated risks of DNA degradation. This study investigated the cfDNA cleavage profile surrounding a CpG (i.e., within an 11-nucleotide [nt] window) to analyze cfDNA methylation. The cfDNA cleavage proportion across positions within the window appeared nonrandom and exhibited correlation with methylation status. The mean cleavage proportion was ∼twofold higher at the cytosine of methylated CpGs than unmethylated ones in healthy controls. In contrast, the mean cleavage proportion rapidly decreased at the 1-nt position immediately preceding methylated CpGs. Such differential cleavages resulted in a characteristic change in relative presentations of CGN and NCG motifs at 5' ends, where N represented any nucleotide. CGN/NCG motif ratios were correlated with methylation levels at tissue-specific methylated CpGs (e.g., placenta or liver) (Pearson's absolute r > 0.86). cfDNA cleavage profiles were thus informative for cfDNA methylation and tissue-of-origin analyses. Using CG-containing end motifs, we achieved an area under a receiver operating characteristic curve (AUC) of 0.98 in differentiating patients with and without hepatocellular carcinoma and enhanced the positive predictive value of nasopharyngeal carcinoma screening (from 19.6 to 26.8%). Furthermore, we elucidated the feasibility of using cfDNA cleavage patterns to deduce CpG methylation at single CpG resolution using a deep learning algorithm and achieved an AUC of 0.93. FRAGmentomics-based Methylation Analysis (FRAGMA) presents many possibilities for noninvasive prenatal, cancer, and organ transplantation assessment.

Single-molecule sequencing reveals a large population of long cell-free DNA molecules in maternal plasma.

In the field of circulating cell-free DNA, most of the studies have focused on short DNA molecules (e.g., <500 bp). The existence of long cell-free DNA molecules has been poorly explored. In this study, we demonstrated that single-molecule real-time sequencing allowed us to detect and analyze a substantial proportion of long DNA molecules from both fetal and maternal sources in maternal plasma. Such molecules were beyond the size detection limits of short-read sequencing technologies. The proportions of long cell-free DNA molecules in maternal plasma over 500 bp were 15.5%, 19.8%, and 32.3% for the first, second, and third trimesters, respectively. The longest fetal-derived plasma DNA molecule observed was 23,635 bp. Long plasma DNA molecules demonstrated predominance of A or G 5' fragment ends. Pregnancies with preeclampsia demonstrated a reduction in long maternal plasma DNA molecules, reduced frequencies for selected 5' 4-mer end motifs ending with G or A, and increased frequencies for selected motifs ending with T or C. Finally, we have developed an approach that employs the analysis of methylation patterns of the series of CpG sites on a long DNA molecule for determining its tissue origin. This approach achieved an area under the curve of 0.88 in differentiating between fetal and maternal plasma DNA molecules, enabling the determination of maternal inheritance and recombination events in the fetal genome. This work opens up potential clinical utilities of long cell-free DNA analysis in maternal plasma including noninvasive prenatal testing of monogenic diseases and detection/monitoring of pregnancy-associated disorders such as preeclampsia.

Genome-wide detection of cytosine methylation by single molecule real-time sequencing.

5-Methylcytosine (5mC) is an important type of epigenetic modification. Bisulfite sequencing (BS-seq) has limitations, such as severe DNA degradation. Using single molecule real-time sequencing, we developed a methodology to directly examine 5mC. This approach holistically examined kinetic signals of a DNA polymerase (including interpulse duration and pulse width) and sequence context for every nucleotide within a measurement window, termed the holistic kinetic (HK) model. The measurement window of each analyzed double-stranded DNA molecule comprised 21 nucleotides with a cytosine in a CpG site in the center. We used amplified DNA (unmethylated) and M.SssI-treated DNA (methylated) (M.SssI being a CpG methyltransferase) to train a convolutional neural network. The area under the curve for differentiating methylation states using such samples was up to 0.97. The sensitivity and specificity for genome-wide 5mC detection at single-base resolution reached 90% and 94%, respectively. The HK model was then tested on human-mouse hybrid fragments in which each member of the hybrid had a different methylation status. The model was also tested on human genomic DNA molecules extracted from various biological samples, such as buffy coat, placental, and tumoral tissues. The overall methylation levels deduced by the HK model were well correlated with those by BS-seq (r = 0.99; P < 0.0001) and allowed the measurement of allele-specific methylation patterns in imprinted genes. Taken together, this methodology has provided a system for simultaneous genome-wide genetic and epigenetic analyses.

Detection and characterization of jagged ends of double-stranded DNA in plasma.

Cell-free DNA in plasma has been used for noninvasive prenatal testing and cancer liquid biopsy. The physical properties of cell-free DNA fragments in plasma, such as fragment sizes and ends, have attracted much recent interest, leading to the emerging field of cell-free DNA fragmentomics. However, one aspect of plasma DNA fragmentomics as to whether double-stranded plasma molecules might carry single-stranded ends, termed a jagged end in this study, remains underexplored. We have developed two approaches for investigating the presence of jagged ends in a plasma DNA pool. These approaches utilized DNA end repair to introduce differential methylation signals between the original sequence and the jagged ends, depending on whether unmethylated or methylated cytosines were used in the DNA end-repair procedure. The majority of plasma DNA molecules (87.8%) were found to bear jagged ends. The jaggedness varied according to plasma DNA fragment sizes and appeared to be in association with nucleosomal patterns. In the plasma of pregnant women, the jaggedness of fetal DNA molecules was higher than that of the maternal counterparts. The jaggedness of plasma DNA correlated with the fetal DNA fraction. Similarly, in the plasma of cancer patients, tumor-derived DNA molecules in patients with hepatocellular carcinoma showed an elevated jaggedness compared with nontumoral DNA. In mouse models, knocking out of the Dnase1 gene reduced jaggedness, whereas knocking out of the Dnase1l3 gene enhanced jaggedness. Hence, plasma DNA jagged ends represent an intrinsic property of plasma DNA and provide a link between nuclease activities and the fragmentation of plasma DNA.

Fragment Ends of Circulating Microbial DNA as Signatures for Pathogen Detection in Sepsis.

BACKGROUND: Nuclear-derived cell-free DNA (cfDNA) molecules in blood plasma are nonrandomly fragmented, bearing a wealth of information related to tissues of origin. DNASE1L3 (deoxyribonuclease 1 like 3) is an important player in shaping the fragmentation of nuclear-derived cfDNA molecules, preferentially generating molecules with 5 CC dinucleotide termini (i.e., 5 CC-end motif). However, the fragment end properties of microbial cfDNA and its clinical implication remain to be explored. METHODS: We performed end motif analysis on microbial cfDNA fragments in plasma samples from patients with sepsis. A sequence context-based normalization method was used to minimize the potential biases for end motif analysis. RESULTS: The end motif profiles of microbial cfDNA appeared to resemble that of nuclear cfDNA (Spearman correlation coefficient: 0.82, P value 0.001). The CC-end motif was the most preferred end motif in microbial cfDNA, suggesting that DNASE1L3 might also play a role in the fragmentation of microbe-derived cfDNA in plasma. Of note, differential end motifs were present between microbial cfDNA originating from infection-causing pathogens (enriched at the CC-end) and contaminating microbial DNA potentially derived from reagents or the environment (nearly random). The use of fragment end signatures allowed differentiation between confirmed pathogens and contaminating microbes, with an area under the receiver operating characteristic curve of 0.99. The performance appeared to be superior to conventional analysis based on microbial cfDNA abundance alone. CONCLUSIONS: The use of fragmentomic features could facilitate the differentiation of underlying contaminating microbes from true pathogens in sepsis. This work demonstrates the potential usefulness of microbial cfDNA fragmentomics in metagenomics analysis.

Comparison of Single Molecule, Real-Time Sequencing and Nanopore Sequencing for Analysis of the Size, End-Motif, and Tissue-of-Origin of Long Cell-Free DNA in Plasma.

BACKGROUND: Recent studies using single molecule, real-time (SMRT) sequencing revealed a substantial population of analyzable long cell-free DNA (cfDNA) in plasma. Potential clinical utilities of such long cfDNA in pregnancy and cancer have been demonstrated. However, the performance of different long-read sequencing platforms for the analysis of long cfDNA remains unknown. METHODS: Size biases of SMRT sequencing by Pacific Biosciences (PacBio) and nanopore sequencing by Oxford Nanopore Technologies (ONT) were evaluated using artificial mixtures of sonicated human and mouse DNA of different sizes. cfDNA from plasma samples of pregnant women at different trimesters, hepatitis B carriers, and patients with hepatocellular carcinoma were sequenced with the 2 platforms. RESULTS: Both platforms showed biases to sequence longer (1500 bp vs 200 bp) DNA fragments, with PacBio showing a stronger bias (5-fold overrepresentation of long fragments vs 2-fold in ONT). Percentages of cfDNA fragments 500 bp were around 6-fold higher in PacBio compared with ONT. End motif profiles of cfDNA from PacBio and ONT were similar, yet exhibited platform-dependent patterns. Tissue-of-origin analysis based on single-molecule methylation patterns showed comparable performance on both platforms. CONCLUSIONS: SMRT sequencing generated data with higher percentages of long cfDNA compared with nanopore sequencing. Yet, a higher number of long cfDNA fragments eligible for the tissue-of-origin analysis could be obtained from nanopore sequencing due to its much higher throughput. When analyzing the size and end motif of cfDNA, one should be aware of the analytical characteristics and possible biases of the sequencing platforms being used.

Identification and characterization of extrachromosomal circular DNA in maternal plasma.

We explored the presence of extrachromosomal circular DNA (eccDNA) in the plasma of pregnant women. Through sequencing following either restriction enzyme or Tn5 transposase treatment, we identified eccDNA molecules in the plasma of pregnant women. These eccDNA molecules showed bimodal size distributions peaking at ∼202 and ∼338 bp with distinct 10-bp periodicity observed throughout the size ranges within both peaks, suggestive of their nucleosomal origin. Also, the predominance of the 338-bp peak of eccDNA indicated that eccDNA had a larger size distribution than linear DNA in human plasma. Moreover, eccDNA of fetal origin were shorter than the maternal eccDNA. Genomic annotation of the overall population of eccDNA molecules revealed a preference of these molecules to be generated from 5'-untranslated regions (5'-UTRs), exonic regions, and CpG island regions. Two sets of trinucleotide repeat motifs flanking the junctional sites of eccDNA supported multiple possible models for eccDNA generation. This work highlights the topologic analysis of plasma DNA, which is an emerging direction for circulating nucleic acid research and applications.

Orientation-aware plasma cell-free DNA fragmentation analysis in open chromatin regions informs tissue of origin.

Cell-free DNA (cfDNA) in human plasma is a class of biomarkers with many current and potential future diagnostic applications. Recent studies have shown that cfDNA molecules are not randomly fragmented and possess information related to their tissues of origin. Pathologies causing death of cells from particular tissues result in perturbations in the relative distribution of DNA from the affected tissues. Such tissue-of-origin analysis is particularly useful in the development of liquid biopsies for cancer. It is therefore of value to accurately determine the relative contributions of the tissues to the plasma DNA pool in a simultaneous manner. In this work, we report that in open chromatin regions, cfDNA molecules show characteristic fragmentation patterns reflected by sequencing coverage imbalance and differentially phased fragment end signals. The latter refers to differences in the read densities of sequences corresponding to the orientation of the upstream and downstream ends of cfDNA molecules in relation to the reference genome. Such cfDNA fragmentation patterns preferentially occur in tissue-specific open chromatin regions where the corresponding tissues contributed DNA into the plasma. Quantitative analyses of such signals allow measurement of the relative contributions of various tissues toward the plasma DNA pool. These findings were validated by plasma DNA sequencing data obtained from pregnant women, organ transplantation recipients, and cancer patients. Orientation-aware plasma DNA fragmentation analysis therefore has potential diagnostic applications in noninvasive prenatal testing, organ transplantation monitoring, and cancer liquid biopsy.

Single-Molecule Sequencing Enables Long Cell-Free DNA Detection and Direct Methylation Analysis for Cancer Patients.

BACKGROUND: Analysis of circulating tumor DNA has become increasingly important as a tool for cancer care. However, the focus of previous studies has been on short fragments of DNA. Also, bisulfite sequencing, a conventional approach for methylation analysis, causes DNA degradation, which is not ideal for the assessment of long DNA properties and methylation patterns. This study attempted to overcome such obstacles by single-molecule sequencing. METHODS: Single-molecule real-time (SMRT) sequencing was used to sequence plasma DNA. We performed fragment size and direct methylation analysis for each molecule. A methylation score concerning single-molecule methylation patterns was used for cancer detection. RESULTS: A substantial proportion of plasma DNA was longer than 1 kb with a median of 16% in hepatocellular carcinoma (HCC) patients, hepatitis B virus carriers, and healthy individuals. The longest plasma DNA molecule in the HCC patients was 39.8 kb. Tumoral cell-free DNA (cfDNA) was generally shorter than nontumoral cfDNA. The longest tumoral cfDNA was 13.6 kb. Tumoral cfDNA had lower methylation levels compared with nontumoral cfDNA (median: 59.3% vs 76.9%). We developed and analyzed a metric reflecting single-molecule methylation patterns associated with cancer, named the HCC methylation score. HCC patients displayed significantly higher HCC methylation scores than those without HCC. Interestingly, compared to using short cfDNA (area under the receiver operating characteristic [ROC] curve, AUC: 0.75), the use of long cfDNA molecules greatly enhanced the discriminatory power (AUC: 0.91). CONCLUSIONS: A previously unidentified long cfDNA population was revealed in cancer patients. The presence and direct methylation analysis of these molecules open new possibilities for cancer liquid biopsy.

Dnase1l3 deletion causes aberrations in length and end-motif frequencies in plasma DNA.

Circulating DNA in plasma consists of short DNA fragments. The biological processes generating such fragments are not well understood. DNASE1L3 is a secreted DNASE1-like nuclease capable of digesting DNA in chromatin, and its absence causes anti-DNA responses and autoimmunity in humans and mice. We found that the deletion of Dnase1l3 in mice resulted in aberrations in the fragmentation of plasma DNA. Such aberrations included an increase in short DNA molecules below 120 bp, which was positively correlated with anti-DNA antibody levels. We also observed an increase in long, multinucleosomal DNA molecules and decreased frequencies of the most common end motifs found in plasma DNA. These aberrations were independent of anti-DNA response, suggesting that they represented a primary effect of DNASE1L3 loss. Pregnant Dnase1l3-/- mice carrying Dnase1l3+/- fetuses showed a partial restoration of normal frequencies of plasma DNA end motifs, suggesting that DNASE1L3 from Dnase1l3-proficient fetuses could enter maternal systemic circulation and affect both fetal and maternal DNA fragmentation in a systemic as well as local manner. However, the observed shortening of circulating fetal DNA relative to maternal DNA was not affected by the deletion of Dnase1l3 Collectively, our findings demonstrate that DNASE1L3 plays a role in circulating plasma DNA homeostasis by enhancing fragmentation and influencing end-motif frequencies. These results support a distinct role of DNASE1L3 as a regulator of the physical form and availability of cell-free DNA and may have important implications for the mechanism whereby this enzyme prevents autoimmunity.

Characteristics of Fetal Extrachromosomal Circular DNA in Maternal Plasma: Methylation Status and Clearance.

BACKGROUND: Although the characterization of cell-free extrachromosomal circular DNA (eccDNA) has gained much research interest, the methylation status of these molecules is yet to be elucidated. We set out to compare the methylation densities of plasma eccDNA of maternal and fetal origins, and between small and large molecules. The clearance of fetal eccDNA from maternal circulation was also investigated. METHODS: We developed a sequencing protocol for eccDNA methylation analysis using tagmentation and enzymatic conversion approaches. A restriction enzyme-based approach was applied to verify the tagmentation results. The efficiency of cell-free fetal eccDNA clearance was investigated by fetal eccDNA fraction evaluations at various postpartum time points. RESULTS: The methylation densities of fetal eccDNA (median: 56.3%; range: 40.5-67.6%) were lower than the maternal eccDNA (median: 66.7%; range: 56.5-75.7%) (P = 0.02, paired t-test). In addition, eccDNA molecules from the smaller peak cluster (180-230 bp) were of lower methylation levels than those from the larger peak cluster (300-450 bp). Both of these findings were confirmed using the restriction enzyme approach. We also observed comparable methylation densities between linear and eccDNA of both maternal and fetal origins. The average half-lives of fetal linear and eccDNA in the maternal blood were 30.2 and 29.7 min, respectively. CONCLUSIONS: We found that fetal eccDNA in plasma was relatively hypomethylated compared to the maternal eccDNA. The methylation densities of eccDNA were positively correlated with their sizes. In addition, fetal eccDNA was found to be rapidly cleared from the maternal blood after delivery, similar to fetal linear DNA.

Single Cell and Plasma RNA Sequencing for RNA Liquid Biopsy for Hepatocellular Carcinoma.

BACKGROUND: Human plasma contains RNA transcripts released by multiple cell types within the body. Single-cell transcriptomic analysis allows the cellular origin of circulating RNA molecules to be elucidated at high resolution and has been successfully utilized in the pregnancy context. We explored the application of a similar approach to develop plasma RNA markers for cancer detection. METHODS: Single-cell RNA sequencing was performed to decipher transcriptomic profiles of single cells from hepatocellular carcinoma (HCC) samples. Cell-type-specific transcripts were identified and used for deducing the cell-type-specific gene signature (CELSIG) scores of plasma RNA from patients with and without HCC. RESULTS: Six major cell clusters were identified, including hepatocyte-like, cholangiocyte-like, myofibroblast, endothelial, lymphoid, and myeloid cell clusters based on 4 HCC tumor tissues as well as their paired adjacent nontumoral tissues. The CELSIG score of hepatocyte-like cells was significantly increased in preoperative plasma RNA samples of patients with HCC (n = 14) compared with non-HCC participants (n = 49). The CELSIG score of hepatocyte-like cells declined in plasma RNA samples of patients with HCC within 3 days after tumor resection. Compared with the discriminating power between patients with and without HCC using the abundance of ALB transcript in plasma [area under curve (AUC) 0.72)], an improved performance (AUC: 0.84) was observed using the CELSIG score. The hepatocyte-specific transcript markers in plasma RNA were further validated by ddPCR assays. The CELSIG scores of hepatocyte-like cell and cholangiocyte trended with patients' survival. CONCLUSIONS: The combination of single-cell transcriptomic analysis and plasma RNA sequencing represents an approach for the development of new noninvasive cancer markers.

Jagged Ends of Urinary Cell-Free DNA: Characterization and Feasibility Assessment in Bladder Cancer Detection.

BACKGROUND: Double-stranded DNA in plasma is known to carry single-stranded ends, called jagged ends. Plasma DNA jagged ends are biomarkers for pathophysiologic states such as pregnancy and cancer. It remains unknown whether urinary cell-free DNA (cfDNA) molecules have jagged ends. METHODS: Jagged ends of cfDNA were detected by incorporating unmethylated cytosines during a DNA end-repair process, followed by bisulfite sequencing. Incorporation of unmethylated cytosines during the repair of the jagged ends lowered the apparent methylation levels measured by bisulfite sequencing and were used to calculate a jagged end index. This approach is called jagged end analysis by sequencing. RESULTS: The jagged end index of urinary cfDNA was higher than that of plasma DNA. The jagged end index profile of plasma DNA displayed several strongly oscillating major peaks at intervals of approximately 165 bp (i.e., nucleosome size) and weakly oscillating minor peaks with periodicities of approximately 10 bp. In contrast, the urinary DNA jagged end index profile showed weakly oscillating major peaks but strongly oscillating minor peaks. The jagged end index was generally higher in nucleosomal linker DNA regions. Patients with bladder cancer (n = 46) had lower jagged end indexed of urinary DNA than participants without bladder cancer (n = 39). The area under the curve for differentiating between patients with and without bladder cancer was 0.83. CONCLUSIONS: Jagged ends represent a property of urinary cfDNA. The generation of jagged ends might be related to nucleosomal structures, with enrichment in linker DNA regions. Jagged ends of urinary DNA could potentially serve as a new biomarker for bladder cancer detection.

Fetal mitochondrial DNA in maternal plasma in surrogate pregnancies: Detection and topology.

OBJECTIVES: Due to the maternally-inherited nature of mitochondrial DNA (mtDNA), there is a lack of information regarding fetal mtDNA in the plasma of pregnant women. We aim to explore the presence and topologic forms of circulating fetal and maternal mtDNA molecules in surrogate pregnancies. METHODS: Genotypic differences between fetal and surrogate maternal mtDNA were used to identify the fetal and maternal mtDNA molecules in plasma. Plasma samples were obtained from the surrogate pregnant mothers. Using cleavage-end signatures of BfaI restriction enzyme, linear and circular mtDNA molecules in maternal plasma could be differentiated. RESULTS: Fetal-derived mtDNA molecules were mainly linear (median: 88%; range: 80%-96%), whereas approximately half of the maternal-derived mtDNA molecules were circular (median: 51%; range: 42%-60%). The fetal DNA fraction of linear mtDNA was lower (median absolute difference: 9.8%; range: 1.1%-27%) than that of nuclear DNA (median: 20%; range: 9.7%-35%). The fetal-derived linear mtDNA molecules were shorter than the maternal-derived ones. CONCLUSION: Fetal mtDNA is present in maternal plasma, and consists mainly of linear molecules. Surrogate pregnancies represent a valuable clinical scenario for exploring the biology and potential clinical applications of circulating mtDNA, for example, for pregnancies conceived following mitochondrial replacement therapy.

Size-tagged preferred ends in maternal plasma DNA shed light on the production mechanism and show utility in noninvasive prenatal testing.

Cell-free DNA in human plasma is nonrandomly fragmented and reflects genomewide nucleosomal organization. Previous studies had demonstrated tissue-specific preferred end sites in plasma DNA of pregnant women. In this study, we performed integrative analysis of preferred end sites with the size characteristics of plasma DNA fragments. We mined the preferred end sites in short and long plasma DNA molecules separately and found that these "size-tagged" ends showed improved accuracy in fetal DNA fraction estimation and enhanced noninvasive fetal trisomy 21 testing. Further analysis revealed that the fetal and maternal preferred ends were generated from different locations within the nucleosomal structure. Hence, fetal DNA was frequently cut within the nucleosome core while maternal DNA was mostly cut within the linker region. We further demonstrated that the nucleosome accessibility in placental cells was higher than that for white blood cells, which might explain the difference in the cutting positions and the shortness of fetal DNA in maternal plasma. Interestingly, short and long size-tagged ends were also observable in the plasma of nonpregnant healthy subjects and demonstrated size differences similar to those in the pregnant samples. Because the nonpregnant samples did not contain fetal DNA, the data suggested that the interrelationship of preferred DNA ends, chromatin accessibility, and plasma DNA size profile is likely a general one, extending beyond the context of pregnancy. Plasma DNA fragment end patterns have thus shed light on production mechanisms and show utility in future developments in plasma DNA-based noninvasive molecular diagnostics.

Preferred end coordinates and somatic variants as signatures of circulating tumor DNA associated with hepatocellular carcinoma.

Circulating tumor-derived cell-free DNA (ctDNA) analysis offers an attractive noninvasive means for detection and monitoring of cancers. Evidence for the presence of cancer is dependent on the ability to detect features in the peripheral circulation that are deemed as cancer-associated. We explored approaches to improve the chance of detecting the presence of cancer based on sequence information present on ctDNA molecules. We developed an approach to detect the total pool of somatic mutations. We then investigated if there existed a class of ctDNA signature in the form of preferred plasma DNA end coordinates. Cell-free DNA fragmentation is a nonrandom process. Using plasma samples obtained from liver transplant recipients, we showed that liver contributed cell-free DNA molecules ended more frequently at certain genomic coordinates than the nonliver-derived molecules. The abundance of plasma DNA molecules with these liver-associated ends correlated with the liver DNA fractions in the plasma samples. Studying the DNA end characteristics in plasma of patients with hepatocellular carcinoma and chronic hepatitis B, we showed that there were millions of tumor-associated plasma DNA end coordinates in the genome. Abundance of plasma DNA molecules with tumor-associated DNA ends correlated with the tumor DNA fractions even in plasma samples of hepatocellular carcinoma patients that were subjected to shallow-depth sequencing analysis. Plasma DNA end coordinates may therefore serve as hallmarks of ctDNA that could be sampled readily and, hence, may improve the cost-effectiveness of liquid biopsy assessment.

Sequencing Analysis of Plasma Epstein-Barr Virus DNA Reveals Nasopharyngeal Carcinoma-Associated Single Nucleotide Variant Profiles.

BACKGROUND: Nasopharyngeal carcinoma (NPC) is strongly associated with Epstein-Barr virus (EBV) infection. Plasma EBV DNA is a validated screening tool for NPC. In screening, there are some individuals who do not have NPC but carry EBV DNA in plasma. Currently it is not known from screening if there may be any genotypic differences in EBV isolates from NPC and non-NPC subjects. Also, low concentrations of EBV DNA in plasma could pose challenge to such EBV genotypic analysis through plasma DNA sequencing. METHODS: In a training dataset comprised of plasma DNA sequencing data of NPC and non-NPC subjects, we studied the difference in the EBV single nucleotide variant (SNV) profiles between the two groups. The most differentiating SNVs across the EBV genome were identified. We proposed an NPC risk score to be derived from the genotypic patterns over these SNV sites. We subsequently analyzed the NPC risk scores in a testing set. RESULTS: A total of 661 significant SNVs across the EBV genome were identified from the training set. In the testing set, NPC plasma samples were shown to have high NPC risk scores, which suggested the presence of NPC-associated EBV SNV profiles. Among the non-NPC samples, there was a wide range of NPC risk scores. These results support the presence of diverse SNV profiles of EBV isolates from non-NPC subjects. CONCLUSION: EBV genotypic analysis is feasible through plasma DNA sequencing. The NPC risk score may be used to inform the cancer risk based on the EBV genome-wide SNV profile.

Topologic Analysis of Plasma Mitochondrial DNA Reveals the Coexistence of Both Linear and Circular Molecules.

BACKGROUND: Cellular mitochondrial DNA (mtDNA) is organized as circular, covalently closed and double-stranded DNA. Studies have demonstrated the presence of short mtDNA fragments in plasma. It is not known whether circular mtDNA might concurrently exist with linear mtDNA in plasma. METHODS: We elucidated the topology of plasma mtDNA using restriction enzyme BfaI cleavage signatures on mtDNA fragment ends to differentiate linear and circular mtDNA. mtDNA fragments with both ends carrying BfaI cleavage signatures were defined as circular-derived mtDNA, whereas those with no cleavage signature or with 1 cleavage signature were defined as linear-derived mtDNA. An independent assay using exonuclease V to remove linear DNA followed by restriction enzyme MspI digestion was used for confirming the conclusions based on BfaI cleavage analysis. We analyzed the presence of BfaI cleavage signatures on plasma DNA ends in nonhematopoietically and hematopoietically derived DNA molecules by sequencing plasma DNA of patients with liver transplantation and bone marrow transplantation. RESULTS: Both linear and circular mtDNA coexisted in plasma. In patients with liver transplantation, donor-derived (i.e., liver) mtDNA molecules were mainly linear (median fraction, 91%; range, 75%-97%), whereas recipient-derived (i.e., hematopoietic) mtDNA molecules were mainly circular (median fraction, 88%; range, 77%-93%). The proportion of linear mtDNA was well correlated with liver DNA contribution in the plasma DNA pool (r = 0.83; P value = 0.0008). Consistent data were obtained from a bone marrow transplantation recipient in whom the donor-derived (i.e., hematopoietic) mtDNA molecules were predominantly circular. CONCLUSIONS: Linear and circular mtDNA molecules coexist in plasma and may have different tissue origins.

Enrichment of fetal and maternal long cell-free DNA fragments from maternal plasma following DNA repair.

OBJECTIVE: Cell-free DNA (cfDNA) fragments in maternal plasma contain DNA damage and may negatively impact the sensitivity of noninvasive prenatal testing (NIPT). However, some of these DNA damages are potentially reparable. We aimed to recover these damaged cfDNA molecules using PreCR DNA repair mix. METHODS: cfDNA was extracted from 20 maternal plasma samples and was repaired and sequenced by the Illumina platform. Size profiles and fetal DNA fraction changes of repaired samples were characterized. Targeted sequencing of chromosome Y sequences was used to enrich fetal cfDNA molecules following repair. Single-molecule real-time (SMRT) sequencing platform was employed to characterize long (>250 bp) cfDNA molecules. NIPT of five trisomy 21 samples was performed. RESULTS: Size profiles of repaired libraries were altered, with significantly increased long (>250 bp) cfDNA molecules. Single nucleotide polymorphism (SNP)-based analyses showed that both fetal- and maternal-derived cfDNA molecules were enriched by the repair. Fetal DNA fractions in maternal plasma showed a small but consistent (4.8%) increase, which were contributed by a higher increment of long fetal cfDNA molecules. z-score values were improved in NIPT of all trisomy 21 samples. CONCLUSION: Plasma DNA repair recovers and enriches long cfDNA molecules of both fetal and maternal origins in maternal plasma.