Expression of the human chorionic gonadotropin-beta gene cluster in human pituitaries and alternate use of exon 1.

Abstract

Previous studies have indicated that in addition to other glycoprotein hormones, the pituitary gland produces small amounts of hCG beta, the classical pregnancy and tumor marker. At the gene transcription level, definitive proof for hCG beta messenger ribonucleic acid transcription was still lacking, largely due to the 90% homology to hLH beta at the DNA sequence level, which renders specific hCG detection in the presence of a vast excess of LH difficult. We investigated both the presence of hCG beta messenger ribonucleic acid and the protein itself in normal human female postmenopausal (n = 4) and male pituitaries (n = 2). Reverse transcription-PCR and subsequent restriction enzyme analysis revealed that the hCG beta 3, 5, 7, and 8 genes coding for genuine hCG beta were transcribed in all pituitaries. Additionally, three alternatively spliced gene products derived from hCG beta genes 1 and 2 were detected and verified by single strand sequencing of the complementary DNAs. The most abundant fragment (244 bp) showed a point mutation (T-->A) in the splice donor site for the first intron, resulting in an alternate use of exon 1 and a frame shift in the open reading frame that might give rise to a hypothetical protein, 132 amino acids in length. With regard to protein synthesis, we confirmed the pituitary as the site of production for hCG beta by reverse phase high performance liquid chromatography and subsequent immunoradiometric assays, including a monoclonal antibody directed against the unique C-terminal extension of hCG beta.