Performance of Immunohistochemistry to Detect Rb/p53 Alterations in High-Grade Neuroendocrine Neoplasms: Staining...

Rb and p53 status helps distinguishing well from poorly differentiated high-grade neuroendocrine neoplasms (HG-NENs) and may predict response to platinum-based chemotherapy. Historically, gene and protein profile correlations for Rb and p53 were performed on frozen samples using high-throughput techniques efficient for copy number determination. This situation differs from routine practice in which NGS is not available everywhere and small deletions may be hard to detect on formalin-fixed paraffin-embedded (FFPE) samples with a gene panel approach. This study aimed at describing the immunohistochemical (IHC) staining patterns of Rb, p16, and p53 in a large cohort of well and poorly differentiated lung and pancreatic HG-NENs, and comparing the performance of Rb/p53 NGS profiling with IHC to detect molecular alterations. Rb, p16, and p53 protein status were determined by IHC in 132 HG-NENs (79 pulmonary and 53 pancreatic). NGS was performed on a subset of 63 tumors. Three IHC patterns were observed in HG-NENs: (i) loss of expression (Rb^loss = 45.4% with p16^strong expression in most cases, p53^loss = 32.6%), (ii) variable expression (Rb^var = 40.8% including 12 cases with p16^strong, p53^var = 37.1%), (iii) strong expression (Rb^strong = 13.6%, p53^strong = 30.3%). NGS testing and IHC agreed in 76.2% and 82.5% of cases for Rb and p53, respectively. Abnormal Rb & p53 IHC profiles, highly suggestive of a molecular alteration, were missed by NGS in 42% and 20.5% of cases, respectively. In conclusion, IHC is a reliable, rapid, and cost-effective screening method in HG-NENs for Rb/p53 status.