HER2 over-expressing high grade endometrial cancer expresses high levels of p95HER2 variant

Abstract Background Subsets of high grade endometrial cancer (EnCa) over-express HER2 (ERBB2), yet clinical trials have failed to demonstrate any anti-tumor activity utilizing trastuzumab, an approved platform for HER2 positive breast cancer (BrCa). A truncated p95HER2 variant lacking the trastuzumab binding site may confer resistance. The objective of this investigation was to characterize the expression of the p95HER2 truncated variant in EnCa. Materials and methods With institutional approval, 86 high grade EnCa tumors were identified with tumor specimens from surgeries performed between 2000 and 2011. Clinical data were collected and all specimens underwent tumor genotyping, HER2 immunohistochemistry (IHC, HercepTest®), HER2 fluorescent in situ hybridization (FISH), along with total HER2 (H2T) and p95HER2 assessment with VeraTag® testing. Regression models were used to compare a cohort of 86 breast tumors selected for equivalent HER2 protein expression. Results We identified 44 high grade endometrioid and 42 uterine serous carcinomas (USC). IHC identified high HER2 expression (2 + or 3 +) in 59% of the tumors. HER2 gene amplification was observed in 16 tumors (12 USC, 4 endometrioid). Both HER2 gene amplification and protein expression correlated with H2T values. High p95HER2 expression above 2.8 RF/mm2 was observed in 53% ( n = 54) with significant correlation with H2T levels. When matched to a cohort of 107 breast tumors based on HercepTest HER2 expression, high grade EnCa presented with higher p95 levels ( p < 0.001). Conclusions These data demonstrate that compared to BrCa, high grade EnCa expresses higher levels of p95HER2 possibly providing rationale for the trastuzumab resistance observed in EnCa.