The expression of the H-19 and IGF-2 genes during human embryogenesis and placental development

The H-19 gene in mice is maternally imprinted and its ectopic expression causes prenatal lethality. We have recently identified H-19 transcript in differentiating human placental cells and showed that its expression increases concomitantly with differentiation of cytotrophoblasts in vitro. Placental and embryonal specimens were collected from conception products derived from normal first and second trimester pregnancy terminations. We investigated the abundance of H-19 mRNA throughout placental development in vivo and compared it to the expression of other genes linked to placental differentiation. Furthermore, the expression of H-19 transcript in different organs of human fetuses, aborted during the second trimester, was examined by RNA isolation from separated fetal organs. Since IGF-2 is known to play an important role in embryogenesis, identical blots were hybridized with IGF-2 probe. H-19 expression in human placenta from the different trimesters of pregnancy remains practically constant. A high amount of H-19 gene product was found in the fetoplacental unit with the highest level measured in the adrenal gland. These findings argue that H-19 gene may play a role in human embryogenesis.

Interaction between choriocarcinoma cell line (JAr) and human cytotrophoblasts in vitro

Cytotrophoblasts (from term placentae) and cells from the choriocarcinoma cell line JAr were cultivated either separately or in co-culture for 72 h. RNA was isolated from the cell cultures and Northern blots were developed using equal amounts of RNA. The RNA was hybridized with cDNA probes for CG alpha, CG beta and hPL. Corresponding m-RNAs were detected in the three RNAs except for hPL m-RNA which was absent from JAr cells RNA. The abundance of CG alpha and CG beta m-RNA in the RNA of the co-culture was higher than their accumulative abundances in the RNAs from cytotrophoblasts and JAr cells cultured alone and the abundance of hPL m-RNA in the RNA of the co-cultures was as high as that in the RNA from cytotrophoblasts cultured alone. On the basis of previous findings (Hochberg et al, 1991), it can be assumed that the cytotrophoblasts in the co-cultures are responsible for the increase in hormonal m-RNA production. It could be calculated that the abundances of the CG alpha, CG beta and hPL m-RNAs in the RNA which originated in the cytotrophoblast nuclei were 20, 100 and 10-fold higher respectively in the co-culture compared to those in the culture of cytotrophoblasts. This effect is limited to certain genes only as the concentration of the 92kD collagenase m-RNA and uPA (urokinase type plasminogen activator) m-RNA, which are both produced in cytotrophoblasts to a much higher extent than in JAr cells, and are not increased by cultivating the cytotrophoblasts with JAr cells in co-culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Parental imprinting of the human H19 gene

It has only recently become clear that genetic imprinting plays an important role in human embryogenesis and in processes leading to the development of pediatric cancers and other human diseases. Using a unique human tissue, the androgenetic complete hydatidiform mole, we established that the maternally inherited allele of the imprinted H19 gene is expressed. Our results also show that the paternal allele of the human IGF-II gene, a gene suspected to be parentally imprinted in humans, is expressed.

Transcription of the H19 gene in differentiating cytotrophoblasts from human placenta

Placental differentiation is closely correlated with the appearance of specific proteins, yet factors regulating cytotrophoblast differentiation are unknown. One strategy employed to search for such factors makes use of differential screening of cDNA libraries. For this purpose, cytotrophoblasts were isolated from human term placentae and cultured for 24 and 120 hr. cDNA libraries were constructed from the cell's RNA, and differential screening resulted in the isolation of three identical clones highly expressed after 120 hr. A DNA sequencing of 139 bp at the 3' end of these clones and a search of the data bank revealed that the sequence was identical to the parallel domain in the human H19 gene. This highly conserved gene is unusual in that it may not encode a protein. In the mouse, its RNA was shown to accumulate to high levels in embryonic tissues of endodermal and mesodermal origin. Our present findings imply that, in humans, the H19 gene is efficiently expressed in placental tissue and differentiated cytotrophoblasts, which are of ectodermal origin. RNA blot hybridization revealed a unique bimodal pattern of expression for the H19 gene in cultured cytotrophoblasts. The modulation in expression of H19 during cytotrophoblast growth was not due to the increase in the number of multinuclear cells. Size fractionation of cytotrophoblasts by centrifugal elutriation revealed that H19 expression is correlated with the stage of cell differentiation. We therefore propose that H19 transcripts might play a regulatory role in the process of cytotrophoblast differentiation.

Differentiation of choriocarcinoma cell line (JAr)

Choriocarcinoma, a highly malignant tumor arising from the trophoblast, comprises a heterogenous population of cells including cytotrophoblasts, intermediate trophoblasts, and syncytiotrophoblasts. In order to investigate trophoblast differentiation, we used centrifugal elutriation to separate cells from the JAr choriocarcinoma cell line according to their size and to further show that the resultant cell populations differ in their stage of differentiation. Two % of the cell population consists of large, multinuclear cells, which display the highest level of chorionic gonadotropin (CG) mRNAs. The increase in the CG beta mRNA with cell size is a consequence of the transcriptional mechanism, since agents which induce differentiation in JAr cells, i.e., methotrexate, increase the level of CG alpha and CG beta transcripts, cause a shift in cell size, and result in the formation of multinuclear cells. The multinuclear cells in the JAr population arise, at least partly, from kariokinesis without cytokinesis.