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Mutations in the BRAF gene at or near the p. V600 locus are informative for therapy selection, but current methods for analyzing FFPE tissue DNA generally have a limit of detection of 5% variant allele frequency (VAF), or are limited to the single variant (V600E). These can result in false negatives for samples with low VAFs due to low tumor content or subclonal heterogeneity, or harbor non-V600 mutations. Here, we show that Sanger sequencing using the NuProbe VarTrace BRAF assay, based on the Blocker Displacement Amplification (BDA) technology, is capable of detecting BRAF V600 mutations down to 0.20% VAF from FFPE lymph node tissue samples. Comparison experiments on adjacent tissue sections using BDA Sanger, immunohistochemistry (IHC), digital droplet PCR (ddPCR), and NGS showed 100% concordance among all 4 methods for samples with BRAF mutations at ≥ 1% VAF, though ddPCR did not distinguish the V600K mutation from the V600E mutation. BDA Sanger, ddPCR, and NGS (with orthogonal confirmation) were also pairwise concordant for lower VAF mutations down to 0.26% VAF, but IHC produced a false negative. Thus, we have shown that Sanger sequencing can be effective for rapid detection and quantitation of multiple low VAF BRAF mutations from FFPE samples. BDA Sanger method also enabled detection and quantitation of less frequent, potentially actionable non-V600 mutations as demonstrated by synthetic samples.

Nanopore sequencing is more than 10-fold faster than sequencing-by-synthesis and provides reads that are roughly 100-fold longer. However, nanopore sequencing’s 7.5% intrinsic error rate renders it difficult to call somatic mutations with low variant allele frequencies (VAFs) without significant false positives. Here, we introduce the Oncogene Concatenated Enriched Amplicon Nanopore Sequencing (OCEANS) method, in which variants with low VAFs are selectively amplified and subsequently concatenated for nanopore sequencing. OCEANS allows accurate detection of somatic mutations with VAF limits of detection between 0.05% and ≤ 1%. We constructed 4 distinct multi-gene OCEANS panels targeting recurrent mutations in acute myeloid leukemia, melanoma, non-small-cell lung cancer, and hepatocellular carcinoma. Comparison experiments against Illumina NGS showed 99.79% to 99.99% area under the receiver-operator curve for these panels on clinical FFPE tumor samples. Furthermore, we identified a significant number of mutations below the standard NGS limit of detection in clinical tissue samples using each OCEANS panel. Comparison against digital PCR on 10 of putative mutations at ≤1% VAF showed 9 concordant positive calls with VAFs between 0.02% and 0.66%. By overcoming the primary challenge of nanopore sequencing on detecting low VAF single nucleotide variant mutations, OCEANS is poised to enable same-day clinical sequencing panels.

Whole exome sequencing (WES) is used to identify mutations in a patient’s tumor DNA that are predictive of tumor behavior, including the likelihood of response or resistance to cancer therapy. WES has a mutation limit of detection (LoD) at variant allele frequencies (VAF) of 5%. Putative mutations called at ≤5% VAF are frequently due to sequencing errors, therefore reporting these subclonal mutations incurs risk of significant false positives. Here we performed ~1000x WES on fresh-frozen and formalin-fixed paraffin-embedded (FFPE) tissue biopsy samples from a non-small cell lung cancer patient, and identified 226 putative mutations at between 0.5% and 5% VAF. Each variant was then tested using NuProbe NGSureTM, to confirm the original WES calls. NGSure utilizes Blocker Displacement Amplification to first enrich the allelic fraction of the mutation and then uses Sanger sequencing to determine mutation identity. Results showed that 52% of the 226 (117) putative variants were disconfirmed, among which 2% (5) putative variants were found to be misidentified in WES. In the 66 cancer-related variants, the disconfirmed rate was 82% (54/66). This data demonstrates Blocker Displacement Amplification allelic enrichment coupled with Sanger sequencing can be used to confirm putative mutations ≤5% VAF. By implementing this method, next-generation sequencing can reliably report low-level variants at a high sensitivity, without the cost of high sequencing depth.

DNA sequence variants with low allele frequencies below 1% are difficult to detect and quantitate by sequencing, due to the intrinsic error of sequencing-by-synthesis (NGS). Unique molecular identifier barcodes can in principle help NGS detect mutations down to 0.1% variant allele frequency (VAF), but require extremely high sequencing depths of over 25,000x, rendering high sensitivity mutation detection out of reach for most research and clinical samples. Here, we present the multiplex blocker displacement amplification (mBDA) method to selectively enrich DNA variants by an average of 300-fold in highly multiplexed NGS settings. On a 80-plex human single nucleotide polymorphism panel, mBDA achieves a 0.019% VAF limit of detection for single nucleotide variants, using only 250x sequencing depth, and detects human cell line contamination down to 0.07%. Using this technology, we constructed a 16-plex melanoma NGS panel covering 145 actionable mutations across 9 genes, and applied it to 19 fresh/frozen tumor biopsy tissue samples with high tumor fractions. We found low VAF mutations (0.2% to 5%) in 37% of the samples (7/19, 95% confidence interval 19%-58%). These results suggest that tumor heterogeneity could be significantly more pervasive than previously recognized, and can contribute significantly to acquired drug resistance to targeted therapies. We also validate mBDA panels on clinical cell-free DNA samples from lung cancer patients.