Austin P. So

High-Throughput Droplet Digital PCR System for Absolute Quantitation of DNA Copy Number

Digital PCR enables the absolute quantitation of nucleic acids in a sample. The lack of scalable and practical technologies for digital PCR implementation has hampered the widespread adoption of this inherently powerful technique. Here we describe a high-throughput droplet digital PCR (ddPCR) system that enables processing of ∼2 million PCR reactions using conventional TaqMan assays with a 96-well plate workflow. Three applications demonstrate that the massive partitioning afforded by our ddPCR system provides orders of magnitude more precision and sensitivity than real-time PCR. First, we show the accurate measurement of germline copy number variation. Second, for rare alleles, we show sensitive detection of mutant DNA in a 100 000-fold excess of wildtype background. Third, we demonstrate absolute quantitation of circulating fetal and maternal DNA from cell-free plasma. We anticipate this ddPCR system will allow researchers to explore complex genetic landscapes, discover and validate new disease associations, and define a new era of molecular diagnostics.

A robust targeted sequencing approach for low input and variable quality DNA from clinical samples

Next-generation deep sequencing of gene panels is being adopted as a diagnostic test to identify actionable mutations in cancer patient samples. However, clinical samples, such as formalin-fixed, paraffin-embedded specimens, frequently provide low quantities of degraded, poor quality DNA. To overcome these issues, many sequencing assays rely on extensive PCR amplification leading to an accumulation of bias and artifacts. Thus, there is a need for a targeted sequencing assay that performs well with DNA of low quality and quantity without relying on extensive PCR amplification. We evaluate the performance of a targeted sequencing assay based on Oligonucleotide Selective Sequencing, which permits the enrichment of genes and regions of interest and the identification of sequence variants from low amounts of damaged DNA. This assay utilizes a repair process adapted to clinical FFPE samples, followed by adaptor ligation to single stranded DNA and a primer-based capture technique. Our approach generates sequence libraries of high fidelity with reduced reliance on extensive PCR amplification—this facilitates the accurate assessment of copy number alterations in addition to delivering accurate single nucleotide variant and insertion/deletion detection. We apply this method to capture and sequence the exons of a panel of 130 cancer-related genes, from which we obtain high read coverage uniformity across the targeted regions at starting input DNA amounts as low as 10 ng per sample. We demonstrate the performance using a series of reference DNA samples, and by identifying sequence variants in DNA from matched clinical samples originating from different tissue types.Cancer diagnostics: Targeted DNA sequencing for low-quality tumor samplesA new DNA sequencing technology enables comprehensive genetic analyses of poor-quality tumor samples. Hanlee Ji from Stanford University in California, USA, together with colleagues from a company he cofounded called TOMA Biosciences, tested the performance of a targeted sequencing assay known as oligonucleotide-selective sequencing (OS-Seq). They used the “in-solution” version of OS-Seq, which involves a pre-processing step to remove any damaged DNA and then sequences target regions of the genome to look for duplications, insertions or deletions of DNA segments. Using archival specimens (which often contain low quantities of degraded DNA) from patients with lung and colorectal cancer, the researchers showed they could detect sequence variants in a panel of 130 cancer-related genes. The findings suggest the OS-Seq assay could help inform treatment decisions for cancer patients, even with clinical specimens of low quality.

Abstract 4019: A PCR-bias free capture-based library preparation platform permitting highly accurate and sensitive CNA detection in tumor molecular profiling and liquid biopsy

Next Generation Sequencing is increasingly implemented as a diagnostic test to identify actionable mutations in cancer patient samples. However, for routine diagnostics, tumor DNA is extracted from formalin-fixed, paraffin-embedded (FFPE) samples, which yields low quantity of damaged DNA. Inability to accurately repair the ends of these DNA fragments impairs adapter ligation by standard double stranded ligation methods. The resulting low yield of adapter-ligated DNA introduces the need for whole-genome PCR amplification prior to target capture. The drawback of such PCR amplification is the introduction of PCR biases, causing reduced sensitivity in the detection of copy number alterations (CNAs), an important biomarker for targeted therapy. To address the need for a library preparation platform that performs well with low quality and quantity DNA, and without relying on massive PCR amplification, we developed an improved, in-solution, version the OS-Seq targeted enrichment assay. OS-Seq circumvents the reliance on PCR amplification by using a single-stranded adapter ligation approach. Damaged bases induced by formalin fixation are removed by excision instead of attempting repair, and then DNA is denatured prior to adapter ligation. This method of adapter ligation result in yields of ~50% for low quality samples, eliminating the need for whole genome PCR. OS-Seq directly uses the adapter-ligated DNA in a linear targeted primer-extension, followed by low-cycle post-capture PCR expansion with Illumina bridge-PCR primers prior to library sequencing. We investigated the PCR duplication rate of the OS-Seq libraries by including an 11-mer random barcode to track unique molecules. We found that most input molecules were present in the sequencing reads at only one copy. Further, we demonstrate a linear correlation between the amount of DNA input (ranging from 1 to 600 ng) and the number of unique molecules sequenced (R2=0.94). Importantly, we show that this low PCR bias allows OS-Seq to detect CNAs in Coriell and Horizon Diagnostic cell lines highly concordant to digital PCR detection (R2=0.96). Further, we present CNA calling on cell line DNA sonicated to 200 bp fragments at 10 ng DNA input, mimicking cell-free DNA. In addition to CNA detection, OS-Seq detects SNVs with a sensitivity of 92-97% and a specificity of 100% down to 5% VAF. In conclusion, the OS-Seq library preparation method relies on single stranded adapter ligation and in-solution target capture, which generates uniform coverage with minimal PCR requirement, resulting in highly sensitive CNA calling. Note: This abstract was not presented at the meeting. Citation Format: Anna Vilborg, Yosr Bouhlal, Ryan Koheler, Daniel Mendoza, Federico Goodsaid, Yannick Pouliot, Austin So, Francisco De La Vega, Hanlee Ji. A PCR-bias free capture-based library preparation platform permitting highly accurate and sensitive CNA detection in tumor molecular profiling and liquid biopsy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4019. doi:10.1158/1538-7445.AM2017-4019

Abstract 54: TOMA OS-seq: A high efficiency targeted re-sequencing strategy for detecting gene amplifications, rearrangements, indels and point mutations in tumor DNA isolated from FFPE solid tumors and plasma

Critical genetic errors, namely specific mutations, are required for the development and maintenance of cancer. Molecular diagnostics that leverage the breadth of next-generation sequencing (NGS) to identify this mutational landscape are therefore becoming an important clinical tool in the precision treatment of cancer. However, widespread adoption of NGS technologies in the clinic is hampered by the complexity of the NGS workflow, and the resulting limits in analytical capabilities arising from noise introduced by poor step-wise yields and bias from extensive application of PCR. To address these deficiencies, we introduce TOMA OS-Seq, a complete targeted resequencing workflow designed specifically for the requirements of the clinical laboratory. Based on oligo-selective sequencing (OS-seq), a panel of 96 genes - each with diagnostic or prognostic value - can be sequenced end-to-end from as little as 1 ng of input material, enabling the analysis of DNA purified from fine-needle aspirates and plasma. Utilizing a minimal number of high efficiency processing steps to create a targeted library, this simple workflow minimizes the requirement for PCR amplification, preserving the quantitative relationship among the loci interrogated in the original sample. TOMA OS-Seq is therefore capable of detecting copy number alterations, in addition to rearrangements/fusions, insertions/deletions, and single nucleotide variants. Citation Format: Yosr Bouhlal, Alexander McKenzie, Austin P. So. TOMA OS-seq: A high efficiency targeted re-sequencing strategy for detecting gene amplifications, rearrangements, indels and point mutations in tumor DNA isolated from FFPE solid tumors and plasma. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Integrating Clinical Genomics and Cancer Therapy; Jun 13-16, 2015; Salt Lake City, UT. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(1_Suppl):Abstract nr 54.

Abstract 2712: Joint somatic mutation and germline variant identification and scoring from tumor molecular profiling and ct-DNA monitoring of cancer patients by high-throughput sequencing

Cancer tumor profiling by targeted resequencing of actionable cancer genes is rapidly becoming the standard approach for selecting targeted therapies and clinical trials in refractory cancer patients. In this clinical scenario, a tumor sample is obtained from an FFPE block and sequenced by targeted next-generation sequencing (NGS) to uncover actionable somatic mutations in relevant cancer genes. Some of the challenges that arise in analyzing tumor-derived NGS data include distinguishing between somatic and germline variants in the absence of normal tissue data, recognizing pathogenic germline variants, and identifying sequencing errors (which occur at about 0.5% rate). Additional challenges arise when considering other clinical applications of NGS such as sequencing cell-free tumor DNA (cf-DNA) from plasma samples to monitor disease response or disease recurrence. Here we present a principled approach to identify both single-nucleotide and small insertion/deletion somatic mutations and germline variants from NGS data of tumor tissue that leverages the allelic fraction patterns in tumors and prior information from external databases through the use of a Bayesian Network algorithm. Our approach allows us to score each putative mutation or variant with respect to its probability of belonging to each variant class, versus classification as a sequencing error. The method enables the joint calling of related samples form the same patient, such as cases where a cf-DNA sample and primary tumor sample are both profiled improving sensitivity and specificity. We validated our method by analyzing data obtained with the TOMA OS-Seq targeted sequencing RUO assay for 98 cancer genes from a mixture of well-known genomes, patient case triads (where normal, tumor and cf-DNA are available), and a retrospective analysis of tumor patient data that underwent clinical tumor profiling for therapy selection. Citation Format: Francisco M. De La Vega, Ryan T. Koehler, Yannick Pouliot, Yosr Bouhlal, Austin So, Federico Goodsaid, Sean Irvine, Len Trigg, Lincoln Nadauld. Joint somatic mutation and germline variant identification and scoring from tumor molecular profiling and ct-DNA monitoring of cancer patients by high-throughput sequencing. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2712.

Abstract 625: The frequency of gene amplifications in cancer revealed by a droplet digital PCR (ddPCR) based pan-cancer gene panel test

The development of molecular assays designed to detect gene amplifications has largely been hampered by technical challenges such as limited DNA quantity and tumor heterogeneity, which demand methods of very high precision and sensitivity. The recent introduction of affordable digital PCR platforms, such as droplet digital PCR (ddPCR), that are capable of providing single molecule resolution of target abundances thus provides a unique opportunity to address this gap in molecular diagnostics. A ddPCR-based test was therefore developed under CLIA-CAP guidelines to determine the amplification status of 12 commonly amplified genes targeted by FDA approved drugs. Termed the Amplinome Test, this test was applied to 49 clinical samples received over a period of 6 months and compared to the results obtained from a commercially available clinical sequencing test applied that also reports copy number alterations (CNAs). An overall concordance rate of 90% (532/588 calls) was observed between the two tests across the entire gene set, 5 of which were identified as amplified. There were 56 discordant copy number amplifications between the two testing modalities. The vast majority (55/56) of discordant calls arose from gene amplifications identified by the Amplinome test, but not detected by sequencing, indicating an 11-fold increase in sensitivity in detecting amplifications. One discordant call (HER3) was identified as amplified via sequencing, but confirmed to be unamplified via FISH. At the patient level, the Amplinome test identified 5-fold more patients as having actionable amplifications in at least one of the 12 assayed genes versus clinical sequencing (29 vs. 6). Clinical management was altered in 14 of the 29 patients with an actionable CNA identified on the Amplinome test; those patients received targeted therapy directed against the amplified gene. The ddPCR-based Amplinome test thus provides a highly sensitive method for measuring gene amplifications in cancer that alters patient management, and suggests that the prevalence of actionable amplifications may be significantly underestimated by standard clinical next-generation sequencing tests. Citation Format: Austin P. So, Amy Wong, Jennifer Pecson, Girish Putcha, Gregory Jensen, Michael Lucero, Gary Stone, Jason Gillman, Pravin Mishra, David Loughmiller, Derrick S. Haslem, Lincoln Nadauld. The frequency of gene amplifications in cancer revealed by a droplet digital PCR (ddPCR) based pan-cancer gene panel test. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 625. doi:10.1158/1538-7445.AM2015-625

Abstract A2-41: The Amplinome™ Test – A droplet digital PCR (ddPCR) based pan-cancer test to assess gene amplification status in solid tumors

Gene amplifications constitute >30% of all actionable mutation types present in cancer. However, despite their prevalence and the availability of targeted drug therapies, the development of molecular assays to detect gene amplifications has been limited by technical challenges such as limited DNA quantity and tumor heterogeneity. To address this, we describe the Amplinome™ test, a test that leverages the high precision and accuracy of droplet digital PCR (ddPCR) to determine the amplification status of 12 commonly amplified genes targeted by FDA approved drugs. Twelve taqman assays were designed to target AURKA, BRAF, CDK4, CDK6, EGFR, ERBB2/HER2, ERBB3/HER3, FGFR1, JAK2, MET, SRC, and VEGFA, and duplexed with the reference gene RNaseP (Epoch, Bothell, WA). Each duplexed assay was then assembled with 2 μL of purified DNA and analyzed via ddPCR (Bio-rad QX-200 ddPCR Platform, Hercules CA). Genes were identified as “amplified” if the observed target:reference ratio was >1.48 ( p The Amplinome test identified 29 of 49 patients having actionable amplifications in at least one of the 12 assayed genes. In contrast, only 6 of 49 patients were identified as having amplifications in these genes through a commercially available sequencing test. An overall concordance rate of 90% (532/588 calls) between the two tests was observed. Discordant calls (56/588) arose from amplifications identified by the Amplinome test but not sequencing (55/56). One discordant call (HER3) was identified as amplified via sequencing, but confirmed to be unamplified via FISH. The observed 10-fold increase in sensitivity versus a sequencing-based clinical test suggests that the prevalence of actionable amplifications may be significantly underestimated in cancer. This sensitivity, combined with its ability to provide results within an actionable timeframe ( Citation Format: Amy Wong, Jennifer Pecson, Girish Putcha, Lincoln Nadauld, Austin P. So. The Amplinome™ Test – A droplet digital PCR (ddPCR) based pan-cancer test to assess gene amplification status in solid tumors. [abstract]. In: Proceedings of the AACR Special Conference on Translation of the Cancer Genome; Feb 7-9, 2015; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(22 Suppl 1):Abstract nr A2-41.

Abstract A2-40: TOMA OS-seq™: A high efficiency targeted resequencing strategy for detecting gene amplifications, rearrangements, indels, and point mutations in tumor DNA isolated from FFPE solid tumors and plasma

Critical genetic errors, namely specific mutations, are required for the development and maintenance of cancer. Molecular diagnostics that leverage the breadth of next-generation sequencing (NGS) to identify this mutational landscape are therefore becoming an important clinical tool in the precision treatment of cancer. However, widespread adoption of NGS technologies in the clinic is hampered by the complexity of the NGS workflow, and the resulting limits in analytical capabilities due to noise introduced by poor step-wise yields and bias arising from extensive application of PCR. To address these deficiencies, we introduce TOMA OS-Seq™, a complete targeted resequencing workflow designed specifically for the requirements of the clinical laboratory. Based on oligo-selective sequencing, a panel of 96 genes - each with diagnostic or prognostic value - can be sequenced end-to-end from as little as 1 ng of input material, enabling the analysis of DNA purified from fine-needle aspirates and plasma. Utilizing a minimal number of high efficiency processing steps to create a targeted library, this simple workflow minimizes the requirement for PCR amplification, preserving the quantitative relationship among the loci interrogated in the original sample. TOMA OS-Seq™ is therefore capable of detecting of copy number alterations, in addition to rearrangements/fusions, insertions/deletions, and single nucleotide variants. Citation Format: Yosr Bouhlal, Alexander McKenzie, Austin P. So. TOMA OS-seq™: A high efficiency targeted resequencing strategy for detecting gene amplifications, rearrangements, indels, and point mutations in tumor DNA isolated from FFPE solid tumors and plasma. [abstract]. In: Proceedings of the AACR Special Conference on Translation of the Cancer Genome; Feb 7-9, 2015; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(22 Suppl 1):Abstract nr A2-40.

Abstract 4885: TOMA OS-Seq: A clinically oriented targeted resequencing method for detecting all mutation classes in DNA isolated from FFPE, FNAs and plasma

Critical genetic errors, namely specific mutations, are required for the development and maintenance of cancer. Molecular diagnostics that leverage the breadth of next-generation sequencing (NGS) to identify this mutational landscape are therefore becoming an important clinical tool in the precision treatment of cancer. However, the widespread clinical adoption of NGS technologies is hampered not only by the challenges of assaying from various clinical matrices, but also by the complexity of the workflows utilized to create, analyze, and annotate an NGS library. To address these challenges, an adaptation of oligo-selective sequencing (OS-Seq) was developed with a workflow designed specifically for the requirements of the clinical laboratory. Termed TOMA OS-Seq)a panel of 96 genes - each with diagnostic or prognostic value - can be sequenced end-to-end from as little as 1 ng of input material, enabling targeted library construction from diverse sources such as fine-needle aspirates (FNAs), plasma as well as formalin-fixed paraffin embedded (FFPE) tissue. The minimized number of high-efficiency steps reduces the requirement for PCR, allowing the identification of copy number alterations (CNAs), re-arrangements, insertions and deletions (indels), as well as single nucleotide variants (SNVs) from these matrices. The performance of TOMA OS-Seq) was assessed using a panel of cell lines harboring known variants in all classes of mutations. Specifically, comparison with a 12-gene droplet digital PCR (ddPCR) based test (the Amplinome Test) demonstrated a high correlation (R2 > 0.95) of CNA measurements via TOMA OS-Seq) with ddPCR, when further challenged with FNAs. The application of TOMA OS-Seq) in assaying all mutation classes in circulating tumor DNA from plasma demonstrates the robustness and flexibility of this platform as clinically oriented targeted resequencing method. Citation Format: Yosr Bouhlal, Alexander McKenzie, Austin P. So. TOMA OS-Seq: A clinically oriented targeted resequencing method for detecting all mutation classes in DNA isolated from FFPE, FNAs and plasma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4885. doi:10.1158/1538-7445.AM2015-4885

Abstract 4859: Ultra-sensitive detection of rare mutants by droplet digital PCR with conventional TaqMan assays

Molecular tests for genetic mutations play an important role in the diagnosis of cancer. Somatic mutations that drive the pathological features of most tumors have increasing promise as biomarkers for cancer prognosis and therapeutic efficacy. The detection of somatic mutations poses an analytical challenge due to the heterogeneous nature of most samples, where a gene carrying a mutation may differ from the highly abundant wild type sequence by only a single nucleotide. Although a variety of methods exist for mutation analysis, many have poor selectivity and fail to detect mutant sequence below 1 in 100 wildtype sequences. Methods that provide better discrimination and quantitation of somatic mutations are desirable. Here we present a simple strategy using droplet digital™ PCR (ddPCR™) for the detection of somatic mutations with high selectivity and sensitivity. Based on the simple principle of sample partitioning into water-in-oil microdroplets, this ddPCR method increases the abundance of a mutant DNA sequence up to 20,000 times compared to an equivalent bulk PCR reaction. Using conventional TaqMan chemistries and workflow, selectivities of up to 1/100,000 can readily be achieved in any laboratory. Here we present results on the use of ddPCR for the detection and quantitation of several clinically important mutations, including KRAS, c-KIT D816V and JAK2 from clinical samples such as bone marrow aspirates and FFPE. Results from ddPCR are compared to those of conventional approaches including allele specific real-time PCR and sequencing. This ddPCR method may play an important role in the earlier detection of cancer, monitoring the progress of disease and response to therapeutics. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4859. doi:1538-7445.AM2012-4859