Claudin-18 expression in gastric type adenocarcinoma and HPV-associated adenocarcinoma of the uterine cervix

Aims: Claudin-18 (CLDN18) is both a marker for the gastric phenotype and a therapeutic target. However, little is known about its immunoexpression in endocervical adenocarcinomas (ECAs), particularly as detected using the clone 43-14A antibody, or about the gene expression of its isoforms in ECAs.
Methods and results: We examined CLDN18, HIK1083, p16 and Rb expression by immunohistochemistry and high-risk human papillomavirus (HR-HPV) mRNA by in situ hybridization (ISH) in 121 ECAs, including 35 HPV-independent adenocarcinomas (gastric type [GAS], n = 24; non-GAS, n = 11) and 86 HPV-associated ECAs. We also analysed mRNA expression of the CLDN18.1 (lung type) and CLDN18.2 (gastric type) isoforms by quantitative polymerase chain reaction (qPCR) in selected cases. CLDN18 positivity was detected in 8/24 (33%) GASs, 0/11 (0%) non-GASs and 2/86 (2%) HPV-associated ECAs, with positivity defined as staining in ≥75% of tumour cells, as in gastric cancer. When a 5% cut-off was used, CLDN18 positivity was detected in 22/24 (92%) GASs, 0/11 (0%) non-GASs and 6/86 (7%) HPV-associated ECAs; CLDN18 expression was thus significantly associated with GAS histology (P < 0.0001). Among the 6 cases of HPV-associated ECAs with CLDN18 expression (ranging from 5% to 80%), the histological patterns included a mix of usual and mucinous features in 4 cases, pure usual type in 1 and villoglandular variant in 1. Otherwise features such as p16 overexpression and the Rb partial loss pattern were consistent with those of HPV-associated ECAs. Six of 22 (27%) CLDN18-positive GASs were also positive for p16, but their other features—such as CLDN18 expression and the Rb preserved pattern—were the same as in p16 negative GASs. Expression of CLDN18.2 mRNA but not CLDN18.1 mRNA was confirmed in both GASs and HPV-associated ECAs.
Conclusions: CLDN18 (43-14A) emerged as a potential diagnostic and therapeutic marker for GAS. A minor subset of HPV-associated ECAs also can be immunoreactive for CLDN18 and express CLDN18.2 mRNA, suggesting divergent gastric phenotypic differentiation. The caution is that GAS and HPV-associated ECAs can share overlapping histological features and similar expression of CLDN18 and p16.