Establishment and evaluation of digital PCR methods for HER2 copy number variation in breast cancer

Accurate measurement of human epidermal growth factor receptor 2 (HER2) copy number variation (CNV) is very important for guiding the tumor target therapy in breast cancer. Digital PCR (dPCR) is a sensitive and an absolute quantitative method, which can be used to detect HER2 CNV. Three HER2 exon-specific digital PCR assays along with three new reference genes assays (homo sapiens ribonuclease P RNA component H1 (RPPH1), glucose-6-phosphate isomerase (GPI), and chromosome 1 open reading frame 43 (C1ORF43), on different chromosomes) were established and validated by using standard reference material, 8 different cell lines and 110 clinical Formalin-fixed and paraffin-embedded (FFPE) samples. DPCR can achieve precise quantification of HER2 CNV by calculating the ratio of HER2/reference gene. The positive and negative coincidence rates were 98% (53/54) and 95% (53/56), respectively, compared with fluorescence in situ hybridization (FISH) diagnostic result 110 of FFPE samples. The common reference gene CEP17 used for FISH diagnostic was not suitable as single reference gene for HER2 CNV measurements by dPCR. The best practice of HER2 CNV determination by dPCR is to conduct the three duplex assays of H1 (HER2 exon 4) with the proposed three new reference genes, with a positive cut-off value of H1/RPPH1 ≥ 2.0 or H1/averaged reference gene ≥ 2.0. The proposed dPCR method in our study can accurately provide absolute copy number of HER2 and reference gene on an alternative chromosome, thus avoiding false negative caused by polysomy of chromosome 17. The improved molecular typing and diagnosis of breast cancer will better guide clinical medication.