Human chorionic gonadotropin-β gene expression in first trimester placenta

The hCGβ gene family contains six genes linked in tandem on chromosome 19 and labeled β genes 7, 8, 5, 1, 2, and 3. Previous studies on a small number of placentas have indicated that β gene 5 was the most highly expressed gene during the first trimester of pregnancy, followed by genes 3 and 8. β genes 7, 1, and 2 were expressed at very low levels. The purpose of this study was to determine 1) whether this pattern of expression was typical during normal pregnancy by sampling a large number of first trimester placentas, and 2) whether there was a correlation between gestational age and the pattern of hCGβ gene expression. Total RNA from 27 first trimester placentas varying in age from 6-16 weeks was reverse transcribed into complementary DNA. The complementary DNA was amplified by PCR, and the amount of DNA representative of each hCGβ gene was quantified by Genescan analysis. In 14 of the 27 placentas, hCGβ gene 5 accounted for 50% or more of the total β messenger RNA expressed. β gene 3 was expressed at levels ranging from 1-42% of the total, and β gene 8 expression ranged from 12-32% of the total. Gene 7 expression was less than 3% of the total β expression in all 27 placentas. Although there appeared to be a trend toward lower expression of β gene 3 in placentas beyond 10 weeks gestational age, there was no correlation of the pattern of β expression with placental age. β gene expression was also examined in two blighted ova a spontaneous abortion sample, and a hydatidiform mole as well as in cultured JAR choriocarcinoma cells. With the exception of JAR cells, these abnormal tissues had low levels of gene 3 expression, but these levels were within the range of the patterns observed in normal placentas. These data suggest that it is the total amount of hCGβ gene expression rather than the expression of individual β genes that is important for the maintenance of normal pregnancy.