Tumor ploidy determination in low-pass whole genome sequencing and allelic copy number visualization using the Constellation Plot.

Ploidy determination across the genome has been challenging for low-pass-WGS tumor-only samples. We present BACDAC, a method that calculates tumor ploidy down to 1.2X effective tumor coverage. Allele fraction patterns displayed in the Constellation Plot verify tumor ploidy and reveal subclonal populations. BACDAC outputs a metric, 2NLOH, that when combined with ploidy better distinguishes near-diploid from high-ploidy tumors. Validated using TCGA, BACDAC had good agreement with other methods and 88% agreement with experimental methods. Discrepancies occur mainly when BACDAC predicts diploidy with subclones rather than high-ploidy. Applied to 653 low-pass-WGS samples spanning 12 cancer subtypes, BACDAC calls 40% as high-ploidy.