Molecular Human Reproduction, Vol. 5, No. 10, 893-895,
October 1999
© 1999 European Society of Human Reproduction and Embryology
Debate |
What factors regulate HCG production in Down's syndrome pregnancies?
Screening for Down's syndrome using HCG concentrations – a common practice but still and enigma
Department of Biological Chemistry, The Silberman Institute of Life Sciences, The Hebrew University, Jerusalem, Israel and Department of Obstetrics and Gynaecology, Division of Gynaecologic Oncology, University of Toronto, Toronto, Ontario, Canada
Trisomy 21, phenotypically manifested as Down's syndrome (DS), is the most common autosomal trisomy among live-born babies, and a major known genetic cause of mental retardation. Trisomy 21 pregnancies are associated with changes in maternal serum concentrations of human chorionic gonadotrophin (HCG) and its ß subunit. These variables are used, in combination with 
-fetoprotein and oestriol, as markers for screening low-risk populations (aged <35 years) (Bogart et al., 1987
; Haddow et al., 1992, 1994, 1998). In the first and second trimesters of DS pregnancies, the maternal serum concentrations of free ßHCG and total HCG respectively, are increased while free HCG concentration is not altered (Spencer, 1993; Brizot et al., 1995). Additional markers are being investigated but these still lack any evidence for a better predictive value than the more common triple test (Wald et al., 1992; Price et al., 1998; Renier et al., 1998). Moreover, since the addition of unconjugated oestriol to the double test adds no further advantage (Extermann et al., 1998), the role of HCG in the detection of triploidy (Goshen et al., 1994), as well as other possible molecular markers, is further supported.
Despite the worldwide use of the triple test for prenatal DS screening, almost no data regarding the biomolecular mechanism, which underlines the high HCG concentrations in DS pregnancies, are available. A better understanding of this pathomechanism may enable us to identify unique factors, that mediate the increased HCG concentrations, thus hopefully enabling us to have a better prenatal screening test for DS pregnancies. While trying to sort out this enigma, one may wish to review what is known on HCG regulation, go ahead with the molecular data underlying the multisystemic nature of DS, and hopefully be able to combine the two.
The regulation of HCG synthesis and secretion has been intensively investigated. However, regulation of HCG expression has been almost exclusively studied in choriocarcinoma cell lines. Cytotrophoblast differentiation into syncytiotrophoblast was shown to be the main factor that leads to HCG production (Hay, 1988). Examination of villi from trisomy 21 in the third trimester placentae demonstrated trophoblastic hypoplasia with a resting cytotrophoblastic layer (Jauniaux et al., 1998). Accordingly, placental immaturity was one of the theories proposed to explain the endocrinological changes in the maternal serum of trisomy 21 pregnancies (Chard, 1991). No association between high HCG concentrations and fetal growth retardation has been found, as would have been expected in the case of placental insufficiency (Luckas et al., 1998).
Since DS is essentially a multisystemic disease resulting by overdose of a very small part of the human genome, a possible role for the function of putative transcription factors affecting other chromosomal areas is highly tentative. Data are available to support such a putative pathophysiological mechanism. For example, an homologous factor responsible for neurogenesis in the Drosophila was recently identified to be highly conserved among human, mouse and the fruit fly, and to be mapped to the human chromosome 21q22.2 (Chrast et al., 1997). The skeletal anomalies associated with DS were also implicated as originating from an overdose of another transcription factor on chromosome 21 (Sumarsono et al., 1996). Other transcription factors have been proposed to exert their phenotypic action either through the control of gene expression (Labudova et al., 1998), by regulating chromatin (Chen et al., 1996), or through the regulation of adhesion molecules (Paoloni-Giacobino et al., 1997).
It is only recently that data regarding the biomolecular characteristics of HCG production in DS pregnancies has become available. Recently, placental expression of HCG subunits in early pregnancies with DS was measured using Northern blot analysis of RNA extracted from 11–15 week placentas of DS pregnancies (Brizot et al., 1995). The authors failed to show any difference in mRNA of both HCG subunits extracted from DS placental tissue when compared with normal specimens. Therefore, they suggest that the increase in maternal serum HCG concentrations in DS is not due to an alteration in placental mRNA expression of HCG subunits, but rather a consequence of a post-transcriptional change. However, cytotrophoblasts isolated from second trimester DS placentae revealed elevated HCG excretion, and increased HCGß mRNA concentrations resembling the amounts found for first trimester cytotrophoblasts (Eldar-Geva et al., 1995). The discrepancies between these two studies are problematic, and may be due to methodological differences in the study design; the former utilizing a crude placental specimen for extracting HCG mRNA as opposed to the latter study that used cytotrophoblast cells in culture. An additional possible explanation for the discrepancy in the two studies may be related to the different placental age used in both studies. Brizot et al. used first trimester placentae while Eldar-Geva et al. used second trimester placentae. However, cytotrophoblasts purified from placentae of different gestational age, have been shown to resemble invasive (Kliman et al., 1990) and biochemical (Goshen et al., 1996) characteristics of first trimester placenta. Therefore, these purified cytotrophoblasts may still reflect the status of first trimester HCG production.
In order to resolve this enigma, one may aim to specifically look at the HCG gene activation which might be altered in DS. The synthesis of ß chains is the rate-limiting factor in HCG synthesis. Multiple copies of structurally-related LHß/HCGß genes are located on the human chromosome 19q.13.3 (Boorstein et al., 1982). Six HCGß genes are recognized (Graham et al., 1987), four of which are expressed in the placenta (Miller-Lindholm et al., 1997), while the expression of HCGß5 and HCGß3, being the main genes expressed in placenta, at a ratio of ~7:1 (Talmadge et al., 1984). Therefore, we have recently investigated the HCGß5 promoter as a candidate for the regulation of HCG synthesis (Goshen et al., 1999). In this study we were able to document, for three out of the six DS-derived fibroblasts cell cultures, which were transfected with the HCGß5 promoter construct, an augmented HCGß promoter activity. We suggested that elevated HCGß mRNA in DS-isolated cytotrophoblasts may be induced by an enhanced promoter activation. This mechanism is the first one proposed to explain the association between high HCG maternal serum concentrations and DS pregnancies.
One may postulate that a transcription factor, located on the extra segment of chromosome 21, may up-regulate HCGß transcription by interacting with its promoter. As a matter of fact, since the over-expression of any particular gene on chromosome 21 in trisomic cells is by 1.5-fold, while the average increase of HCG concentrations is higher than that, one may suspect that other processes regulating HCG maternal-serum concentrations are involved. However, a positive transcription factor encoded by a gene located on chromosome 21, may outbalance a negative inhibitory factor competing for the same DNA sequence. Such a competition can explain a higher than 1.5-fold increase.
The issue of such a transcription factor may have many more implications than that of understanding the mechanism underlying the high HCG concentrations. If such a factor will be identified, it may be used to have a rapid assessment of DS existence in any tissue or blood sample, prior to or instead of being subjected to karyotyping. Moreover, this may broaden our understanding of the complexity in DS phenotypic–genotypic correlation. Last but not least, it may help us understand why are we doing what we do, and possibly explain other examples of high HCG concentrations in other aneuploidies and altered pregnancy outcome.
Acknowledgments
I wish to thank Mr. Nathan Blieman on behalf of the Medallion Properties Inc. for their generous support in the Canada International Scientific Exchange Program (CISEPO) Project as well as the generous grants provided by the Halbert Foundation and the American Physician Fellowship Inc. for Medicine in Israel (APF).
Notes
To whom correspondence should be addressed at: Division of Gynaecologic Oncology, Toronto-Sunnybrook Regional Cancer Centre, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 3M5
This debate was previously published on Webtrack 75, July 1, 1999
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