Novel genes and transcription factors in placental development--a workshop report

Homeobox genes in the human placenta: Twists and turns on the path to find novel targets

Fetal growth restriction (FGR) is a clinically important human pregnancy disorder that is thought to originate early in pregnancy and while its aetiology is not well understood, the disorder is associated with placental insufficiency. Currently treatment for FGR is limited by increased surveillance using ultrasound monitoring and premature delivery, or corticosteroid medication in the third trimester to prolong pregnancy. There is a pressing need for novel strategies to detect and treat FGR at its early stage. Homeobox genes are well established as master regulators of early embryonic development and increasing evidence suggests they are also important in regulating early placental development. Most important is that specific homeobox genes are abnormally expressed in human FGR. This review focusses on identifying the molecular pathways controlled by homeobox genes in the normal and FGR-affected placenta. This information will begin to address the knowledge gap in the molecular aetiology of FGR and lay the foundation for identifying potential diagnostic and therapeutic targets.

Expression of Homeobox Gene HLX and its Downstream Target Genes are Altered in Placentae From Discordant Twin Pregnancies

A discordant twin gestation, in which one fetus is significantly growth restricted, compared to the other normal twin, is a unique model that can be used to elucidate the mechanism(s) by which the intrauterine environment affects fetal growth. In many model systems, placental transcription factor genes regulate fetal growth. Transcription factors regulate growth through their activation or repression of downstream target genes that mediate important cell functions. The objective of this study was to determine the expression of the placental HLX homeobox gene transcription factor and its downstream target genes in dizygotic twins with growth discordance. In this cross-sectional study, HLX and its downstream target genes' retinoblastoma 1 (RB1) and cyclin kinase D (CDKN1C) expression levels were determined in placentae obtained from dichorionic diamniotic twin pregnancies (n = 23) where one of the twins was growth restricted. Fetal growth restriction (FGR) was defined as small for gestational age with abnormal umbilical artery Doppler indices when compared with the normal control co-twin. Homeobox gene HLX expression was significantly decreased at both the mRNA and protein levels in FGR twin placentae compared with the normal control co-twin placentae (p < .05). Downstream target genes CDKN1C and RB1 were also significantly decreased and increased, respectively, at both the mRNA and protein levels in FGR twin placentae compared with normal control co-twin placentae (p < .05). Together, these observations suggest an important association between HLX transcription factor expression and abnormal human placental development in discordant twin pregnancies.

The role of homeobox genes in the development of placental insufficiency

Intrauterine growth restriction (IUGR) is an adverse pregnancy outcome associated with significant perinatal and pediatric morbidity and mortality, and an increased risk of chronic disease later in adult life. While a number of maternal, fetal and environmental factors are known causes of IUGR, the majority of IUGR cases are of unknown cause. These IUGR cases are frequently associated with placental insufficiency, possibly as a result of placental maldevelopment. Understanding the molecular mechanisms of abnormal placental development in IUGR associated with placental insufficiency is therefore of increasing importance. Here, we review our understanding of transcriptional control of normal placental development as well as human IUGR associated with placental insufficiency. We also assess the potential for understanding transcriptional control as a means for revealing new molecular targets for the detection, diagnosis and clinical management of IUGR associated with placental insufficiency.

Downstream targets of homeobox gene HLX show altered expression in human idiopathic fetal growth restriction

Fetal growth restriction (FGR), a clinically significant pregnancy disorder, is poorly understood at the molecular level. This study investigates idiopathic FGR associated with placental insufficiency. Previously, we showed that the homeobox gene HLX is expressed in placental trophoblast cells and that HLX expression is significantly decreased in human idiopathic FGR. Here, we used the novel approach of identifying downstream targets of HLX in cell culture to detect potentially important genes involved in idiopathic FGR. Downstream targets were revealed by decreasing HLX expression in cultured trophoblast cells with HLX-specific small interfering RNAs to model human idiopathic FGR and comparing these levels with controls using a real-time PCR-based gene profiling system. Changes in candidate HLX target mRNA levels were verified in an independent trophoblast cell line, and candidate target gene expression was assessed in human idiopathic FGR-affected placentae (n = 25) compared with gestation-matched controls (n = 25). The downstream targets RB1 and MYC, cell cycle regulatory genes, showed significantly increased mRNA levels in FGR-affected tissues compared with gestation-matched controls, whereas CCNB1, ELK1, JUN, and CDKN1 showed significantly decreased mRNA levels (n = 25, P < 0.001, t-test). The changes for RB1 and CDKN1C were verified by Western blot analysis in FGR-affected placentae compared with gestation-matched controls (n = 6). We conclude that cell cycle regulatory genes RB1, MYC, CCNB1, ELK1, JUN, and CDKN1C, which control important trophoblast cell functions, are targets of HLX.

Maternal and fetal variants in the TGF-beta3 gene and risk of pregnancy-induced hypertension in a predominantly Latino population

OBJECTIVE

We sought to determine whether polymorphisms in the transforming growth factor (TGF)-beta3 gene are associated with risk of pregnancy-induced hypertension (PIH) in case-control mother-baby dyads.

STUDY DESIGN

Patients (n = 136) and control subjects (n = 169) were recruited from our hospital. We genotyped 4 TGF-beta3 polymorphisms and examined association with PIH using logistic regression, adjusting for parity, maternal age, gestational age at delivery, fetal (or maternal) genotypes for the polymorphism in question, and the 3 other polymorphisms within the TGF-beta3 gene.

RESULTS

Only 1 of the TGF-beta3 polymorphisms (rs11466414) was associated with PIH. Mothers who carried a baby with a minor allele were at decreased risk (odds ratio(multi-locus adj), 0.32; 95% confidence interval, 0.14-0.77). Maternal TGF-beta3 variants had no effect on risk of PIH.

CONCLUSION

A fetal TGF-beta3 polymorphism (rs11466414) is associated with PIH in a predominantly Hispanic population.

Homeobox gene HLX1 expression is decreased in idiopathic human fetal growth restriction

Fetal growth restriction (FGR) is a clinically significant pregnancy disorder in which the fetus fails to achieve its full growth potential in utero. Identifiable causes of FGR account for approximately 30% of cases, but the remainder are idiopathic and are frequently associated with placental malfunction. Previously, we isolated the homeobox gene HLX1 and provided evidence for a regulatory role in normal placental development. Here, we investigated whether placental HLX1 expression levels are changed in placentas from idiopathic FGR pregnancies. Real-time polymerase chain reaction quantitation showed reduced HLX1 mRNA levels with advancing gestation age (preterm control placentas, 27 to 35 weeks, 1.1 +/- 0.3, n = 13, versus term placentas 36 to 41 weeks, 0.74 +/- 0.02, n = 12, P < 0.005). FGR-affected placentas had significantly lower levels of HLX1 expression compared with gestation age-matched controls (0.36 +/- 0.07 versus 1.05 +/- 0.2, n = 25, P < 0.001). Immunoblotting with a rabbit polyclonal HLX1 antibody revealed reduced levels of HLX1 in FGR-affected placentas compared with controls (481.07 +/- 12.3 versus 2766.7 +/- 30.3, n = 10, P < 0.001). Immunohistochemistry showed a qualitative decrease in HLX1 immunoreactivity in FGR-affected term placentas compared with controls. This is the first demonstration that a homeobox transcriptional regulator shows altered expression in an important human placental disorder, suggesting that decreased HLX1 levels contribute to the abnormalities in placental developmental seen in idiopathic FGR.

Estrogenic endocrine disruptive components interfere with calcium handling and differentiation of human trophoblast cells

During development, calcium (Ca) is actively transported by placental trophoblasts to meet fetal nutritional and the skeletal mineralization needs. Maternal exposure to estrogenic pesticides, such as 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) and methoxychlor (MTC), has been shown to result in reproductive disorders and/or abnormal fetal development. In this study, we have examined the effects of exposure of trophoblastic cells to MTC and DTT, in comparison to 17beta-estradiol (E2) and diethylstilbestrol (DES), to test the hypothesis that cellular Ca handling is a target for these endocrine disruptive components. Treatment with DDT, MTC, DES, or E2 increased cellular Ca uptake, and the expression of trophoblast-specific human Ca binding protein (HCaBP) was down-regulated by both MTC and DDT. Treatment with MTC, DDT, and DES inhibited cell proliferation, induced apoptosis, and suppressed expression of several trophoblast differentiation marker genes. These effects were reversed by overexpression of metallothionein IIa, a gene highly responsive to cadmium and other metals. These results strongly suggest that trophoblast Ca handling functions are endocrinally modulated, and that their alteration by candidate endocrine disruptors, such as MTC and DDT, constitutes a possible pathway of the harmful effects of these components on fetal development.

Structure and regulation of the murine Mash2 gene

Transcription factors of the basic helix-loop-helix family such as Mash2 are essential for adequate differentiation of the trophoblast. Disruption of the Mash2 gene leads to early intrauterine death caused by placental insufficiency with an absent spongiotrophoblast and an underdeveloped chorion. The aim of the present study was to analyze the structure of the murine Mash2 gene, to screen a broad spectrum of organs for its expression, and to investigate placental Mash2 expression at different gestational ages. The RNase protection assay identified, in addition to the postulated Mash2 mRNA, two unexpected Mash2 transcripts that could be confirmed by a 5' rapid amplification of cDNA ends. However, all three transcripts were detectable exclusively in murine placenta and not in other organs, such as the ovary, uterus, skin, lung, femur, skeletal muscle, kidney, skull, adrenal gland, tongue, stomach, spleen, skin, testis, or pancreas. Sequence analysis disclosed an additional transcription start site upstream of exon 2. Placental Mash2 mRNA is measurable at all investigated stages of gestation. After its initial detection on Day 8.5 postcoitum (p.c.; set to 100%; 100.0% +/- 28.4%), the Mash2 mRNA concentration increases significantly and reaches a maximum of 812.0% +/- 69.7% on Day 12.5 p.c. The second half of gestation is marked by a more than 8-fold Mash2 decrease by Day 18.5 p.c. (77.0% +/- 28.4%). A 36.9% +/- 4.7% level of placental Mash2 mRNA is measurable at term.