111P - Development of digital PCR for accurate measurement of HER2 amplification in 184 gastric cancer patients

Background

Immunohistochemistry (IHC)/in situ hybridization (ISH) are widely used methods in many pathology laboratories to assess human epidermal growth factor receptor 2 (HER2) status in gastric cancer (GC) patients. Nowadays, next generation sequencing (NGS) technologies are adopted to precisely predict absolute gene amplification levels. However, these tissue-based tests are suffering from intratumoral heterogeneity and inter-laboratory technical biases.

Methods

We set up a novel probe-based Absolute Q digital PCR assays for quantitative measurement of HER2 amplification using the formalin-fixed paraffin embedded (FFPE) tissue samples and the results were compared with IHC/ISH and NGS results in 184 gastric adenocarcinoma samples. HER2 amplifications were determined with HER2 TaqMan® probe hydrolysis assays within exon 22 and was multiplexed by the ratios of HER2 copy number (CN) to reference gene CN (HER2/RNaseP ratio).

Results

With digital PCR, the median HER2 CN were 1.79 copies/μL (ranged from 0.02 to 48.87 copies/μL). With NGS, HER2 amplifications were detected in 28 cases (15.2%). By IHC, HER2 positive (3+) was found in 25 cases (13.6%) and equivocal (2+) cases were observed in 15 cases (8.2%), in which ISH conformed amplifications in 13 cases. The concordance rate between IHC (3+ or 2+ and HER2 amplified) and NGS was 71.3%. In HER2-amplified cases by NGS, the median CN measured with digital PCR were 9.7 copies/μL (ranged from 1.95 to 48.87 copies/μL). In statistical analyses, Pearson correlation coefficients between NGS and digital PCR was 0.916, suggesting strong correlation. Given HER2 amplification by NGS as the standard reference, digital PCR predicted HER2 amplification with a 98.7% specificity, 92.9% sensitivity, 92.9% positive predictive value, and 98.7% negative predictive value. The AUC values for predicting HER2 amplification by digital PCR were 0.989.

Conclusions

With newly designed novel probes, digital PCR accurately measured HER2 CN with high sensitivity and specificity.

Editorial acknowledgement

Clinical trial identification

Legal entity responsible for the study

The authors.

Funding

The Ministry of Health & Welfare, Republic of Korea.

Disclosure

All authors have declared no conflicts of interest.