Abstract
DNA sequence variants with allele fractions below 1% are difficult to detect and quantify by sequencing owing to intrinsic errors in sequencing-by-synthesis methods. Although molecular-identifier barcodes can detect mutations with a variant-allele frequency (VAF) as low as 0.1% using next-generation sequencing (NGS), sequencing depths of over 25,000× are required, thus hampering the detection of mutations at high sensitivity in patient samples and in most samples used in research. Here we show that low-frequency DNA variants can be detected via low-depth multiplexed NGS after their amplification, by a median of 300-fold, using polymerase chain reaction and rationally designed 'blocker' oligonucleotides that bind to the variants. Using an 80-plex NGS panel and a sequencing depth of 250×, we detected single nucleotide polymorphisms with a VAF of 0.019% and contamination in human cell lines at a VAF as low as 0.07%. With a 16-plex NGS panel covering 145 mutations across 9 genes involved in melanoma, we detected low-VAF mutations (0.2-5%) in 7 out of the 19 samples of freshly frozen tumour biopsies, suggesting that tumour heterogeneity could be notably higher than previously recognized.