Extraplacental human fetal tissues express mRNA transcripts encoding the human chrorionic gonadotropin‐β subunit protein

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Abstract
The glycoprotein hormone human chorionic gonadotropin (hCG) is synthesized in large quantities by the developing placenta, reaching peak concentrations in maternal blood during the late first trimester and early midtrimester of pregnancey. In general it is believed that the β‐subunit of this dimeric hormone is expressed in pituitary gonadotropes, thyrotropes, and trophoblasts, while the β‐subunit is expressed exclusively by trophoblasts. Studies from our laboratory and other laboratories have shown that some midtrimester human fetal tissues, in addition to the placenta, can synthesize proteins that appear to be very similar to the β‐subunit of hCG. To define precisely the nature of this putative hCG‐β‐subunit in extraplacental fetal tissues, we have examined the mRNA from a variety of human fetal and adult tissues using nucleic acid hybridization and reverse transcription‐polymerase chain reaction (PCR) methods. Our results demonstrate that midtrimester fetal kidney and adrenal tissues contain hCG‐β mRNA transcripts at concentrations comparable to that of placenta, while fetal lung, brain, muscle, and adult adrenal contain only trace to undetectable levels of hCG‐β mRNA. By restriction endonuclease mapping of PCR fragments from fetal tissue cDNAs, we show that the hCG‐β transcript expressed in midtrimester human fetal organs is a bone fide copy of hCG‐β gene No. 5 of the β‐subunit gene family located on chromosome 19.