Expression and Production of Human Chorionic Gonadotropin (hCG) in the Normal Secretory Endometrium: Evidence of CGB7 and/or CGB6 Beta hCG Subunit Gene Expression

We have previously confirmed glandular cell CGB and CGA subunit mRNA gene expression as well as the expression of their dimeric and single-subunit human chorionic gonadotropin (hCG) proteins in normal secretory transformed endometrium. The objective of this study was to investigate the endometrial epithelial gene locus of the human hCG/LH gene cluster from CGB genes responsible for gene expression. For this study, endometrial specimens were selected from women characterized using our endometrium score and hCG staining index that had normal secretory transformed endometrium and optimal hCG staining. Using full-length CGB mRNA sequence analysis, we found that epithelial CGB is (co)expressed as the product of gene locus CGB7 and CGB6 (48%), as single CGB7 (42%), or to a lower percentage as single CGB6 (10%). In addition to known differences between these genes and CGB5, the nucleotide sequence of the mRNA differs between CGB7 and CGB6 in the untranslated promoter region and in translated exon 2. Immunohistochemical results show that endometrial joint CGB7 and CGB6, single CGB7, and single CGB6 mRNA expression lead to the release of endometrial hCG. Gene-specific antibodies for CGB7 reveal secretory endometrial hCG production, which is not observed for gene-specific CGB5 antibodies, whereas the placenta is positive for CGB5 and negative for CGB7 antibody as revealed by immunohistochemistry and Western blot hCG isoform analysis. Only endometrial CGB7 expression seems to be supported specifically by secretory endometrial transcription factors. In conclusion, epithelial hCG is expressed and produced as CGB7 and/or CGB6 but not CGB5, and it is produced together with CGA as a secretory transformation marker in the normal secretory phase endometrium.