**2. Placenta transcriptome in normal pregnancy**

of mother and fetus growth [8]. These factors including placental hormones and growth factors lead to the regulation of gene expression critical for placenta plasticity and functions including angiogenesis, immune response, decidua invasion, endocrine regulation and fetal

The investigation of a complex organ such as placenta requires an approach without *a priori* that can be provided by high throughput methods such as microarray, ribonucleic acidsequencing (RNA-Seq) and/or quantitative reverse transcription-polymerase chain reaction (qRT-PCR) technologies, which markedly changed our analysis of tissue physiology and pathophysiology. This field is expanding since the investigation of keywords including placenta, transcriptomic and gene expression on PubMed database revealed a progression of the number of publications per years from 1 in 2001 to 55 in 2017 (**Figure 1**). The transcriptomic studies have enabled identification of specific genes involved in the progression of the gestation and the outcome of complications. These gene signatures may be used as new biomarkers

In this chapter, we will review the literature focusing on the gene expression profiling of placenta tissue and isolated cells during the progression of the gestation and pathological

**Figure 1.** Number of publications associated with "placenta, transcriptome and gene expression".

nutrition and growth [9].

28 Placenta

pregnancies.

for maternal and fetal complications of pregnancy.

#### **2.1. Gene expression and anatomic organization of placenta**

Human placenta is a feto-maternal organ composed of the fetal part (also known as chorion) and the maternal part (also known as decidua). The chorion is composed of trophoblasts, cytotrophoblasts and syncytiotrophoblasts, whereas the decidua contains enlarged endometrial stromal cells (epithelium) and leukocytes populations, thus suggesting that these two parts of placenta tissue are functionally different.

Few studies have investigated the transcriptomic profile of the different areas of the placenta tissue (**Table 1**). As shown in **Table 1**, two studies have investigated gene expression in amnion, chorion, decidua and villus parenchyma of at term placenta using microarray or RNA-sequencing. A core of gene expression patterns was observed in the different areas of the placenta; they are related to histology categories [10]. Microarray analysis revealed major differences among amnion, chorion and villus parenchyma. It showed that the gene encoding


**Table 1.** Transcriptomic analysis of tissue component of the placenta.

Mucin 1 (MUC1) gene was strongly expressed in the amnion, whereas a cluster of up-modulated genes associated with signal transduction, cell differentiation and immunity categories was found in chorion. Among these genes, tissue remodeling genes, genes induced by interferon and major histocompatibility complex (MHC) genes were strongly up-modulated, suggesting a role for chorion in the plasticity and the immune tolerance of the placenta during pregnancy. Hypoxia-inducible factor (HIF)-1α, a transcription factor known to induce transcription of genes involved in glycolysis, erythropoiesis and angiogenesis [12], was expressed at low level in villus sections of placenta. In contrast, HIF-1α was up-modulated in amnion chorion and decidua [11]. It is likely that these discrepancies are related to exposition to different gradients of oxygen, which modulate HIF-1α expression.

**Species Placenta and gestational age**

Human First trimester

Human First trimester (6–7 weeks) Third trimester (279 days)

> (9–12 weeks) Third trimester

(14–16 and 18–19, 21, 23–24 weeks) and third trimesters (37–40 weeks)

day 10.5, 12.5, 15.5 and 17.5

Human First

Human Second

Mice Embryonic

(45–59 days) Second trimester (109–115 days) **Technique Results References**

Gene Expression Profiling of Placenta from Normal to Pathological Pregnancies

[15]

31

http://dx.doi.org/10.5772/intechopen.80551

[16]

[17]

[18]

[19]

• 1st or 2nd trimesters versus term: cell division, mitosis, DNA metabolism, pregnancy, response to chemical, immune response and regulation of cell cycle • Genes included in gestational regulation of the Wnt

qRT-PCR • Cysteine dioxygenase (CDO) mRNA is up-modulated in at term placentas compared to 1st trimester placentas

Microarray • 7519 genes differentially expressed between 1st and 3rd

Microarray • Little changes on gene expression observed during 14 to

dissolution and lipid metabolism

metabolism for E 12.5 compared to E 15.5

E 15.5 compared to E 12.5

**Table 2.** Transcriptomic analysis of uncomplicated normal pregnancies at different ages of pregnancy.

chemosensory perception

• Biological processes up-regulated in 1st trimester: cell proliferation, cell differentiation and angiogenesis • Biological processes up-modulated in 3rd trimester: cell surface receptor-mediated signal transduction, G-protein mediated signaling, ion transport, neuronal activities and

• 3D separation observed for 17 imprinted genes of 1st and 3rd trimester placentas, suggesting epigenetic modifications

• 418 genes differentially modulated at the third trimester (37–40 weeks) compared to mid gestation (14–24 weeks) • These genes are involved in differentiation, motility, transcription, immunity, angiogenesis, extracellular matrix

• Up-regulation of genes associated with angiogenesis fatty acid metabolism and transport for E 10.5 compared to E 12.5 • Up-regulation of genes associated with hormonal control and ribosomal proteins for E 12.5 compared to E 10.5 • Up-regulation of genes associated with cell cycle and RNA

• Up-regulation of genes associated with cellular transport for

• Up-regulation of genes associated with cell cycle control and RNA metabolism for E 17.5 compared to E 15.5

Microarray • 500 genes common to the 1st and 2nd trimesters • 836 genes specific to the 1st trimester • 264 genes specific to the 2nd trimester

pathway

trimesters

24 weeks

Microarray • 599 genes differentially modulated

Although these studies based on at term placentas are limited, they suggest that distinct tissue transcriptional programs exist in placenta. In addition, some inter-individual variations of the placenta structures have been observed during the second and the third trimester of the pregnancy [13]. Further investigations must include placentas at the different age of the gestation to precisely map gene expression in different areas of placenta during pregnancy.

#### **2.2. Evolution of placenta gene expression during pregnancy**

During pregnancy, the placenta gene expression is continuously modulated to adapt immune response to tolerance necessity and to modify metabolism according to pregnancy requirement. Hence, the early stages of the pregnancy are characterized by trophoblast invasion, which leads to placentation. During the mid-gestation, the placenta adapts its gene expression to maintain the fetal growth and the organ development. Finally, during the third trimester, placenta must provide enough nutriments for the growing of the fetus. Some evidence suggests that preterm birth is related to changes in gene expression during pregnancy [14].

The evolution of placenta gene expression during the different gestational stages is summarized in **Table 2**. The five studies investigated gene expression at different gestational ages in humans and mice. They identify common gene pathways involved in cell differentiation, immune response and angiogenesis (**Table 2**). In first trimester, the few genes that are found modulated are strongly involved in cell proliferation vascular development and angiogenesis. At mid-gestation several genes associated with cycle control, including cyclin family, are found up-modulated [17, 19]. There is an enrichment in gene ontology (GO) terms related to growth of the placenta, the set-up and the maturation of villous with blood vessels during first and second trimester as compared with third trimester [15, 20]. In addition, GO terms for cell differentiation and communication are also observed and are related to the organization of placenta structure and plasticity [21]. All studies observed a major change in the gene expression pattern at the end of the pregnancy. The third trimester is associated with the over-expression of genes involved in apoptosis, oxidative stress response and inflammatory process [17, 18], whereas the second and third trimesters share the up-modulation of genes associated with immune response. It has been recently shown that the gene expression program of placentas at the third trimester exhibits activation of the immune response and an increase of oxygen-rich maternal blood in placenta, which reflect labor and delivery route [22, 23].


Mucin 1 (MUC1) gene was strongly expressed in the amnion, whereas a cluster of up-modulated genes associated with signal transduction, cell differentiation and immunity categories was found in chorion. Among these genes, tissue remodeling genes, genes induced by interferon and major histocompatibility complex (MHC) genes were strongly up-modulated, suggesting a role for chorion in the plasticity and the immune tolerance of the placenta during pregnancy. Hypoxia-inducible factor (HIF)-1α, a transcription factor known to induce transcription of genes involved in glycolysis, erythropoiesis and angiogenesis [12], was expressed at low level in villus sections of placenta. In contrast, HIF-1α was up-modulated in amnion chorion and decidua [11]. It is likely that these discrepancies are related to exposition to dif-

Although these studies based on at term placentas are limited, they suggest that distinct tissue transcriptional programs exist in placenta. In addition, some inter-individual variations of the placenta structures have been observed during the second and the third trimester of the pregnancy [13]. Further investigations must include placentas at the different age of the gestation to precisely map gene expression in different areas of placenta during pregnancy.

During pregnancy, the placenta gene expression is continuously modulated to adapt immune response to tolerance necessity and to modify metabolism according to pregnancy requirement. Hence, the early stages of the pregnancy are characterized by trophoblast invasion, which leads to placentation. During the mid-gestation, the placenta adapts its gene expression to maintain the fetal growth and the organ development. Finally, during the third trimester, placenta must provide enough nutriments for the growing of the fetus. Some evidence suggests that preterm birth is related to changes in gene expression during pregnancy [14]. The evolution of placenta gene expression during the different gestational stages is summarized in **Table 2**. The five studies investigated gene expression at different gestational ages in humans and mice. They identify common gene pathways involved in cell differentiation, immune response and angiogenesis (**Table 2**). In first trimester, the few genes that are found modulated are strongly involved in cell proliferation vascular development and angiogenesis. At mid-gestation several genes associated with cycle control, including cyclin family, are found up-modulated [17, 19]. There is an enrichment in gene ontology (GO) terms related to growth of the placenta, the set-up and the maturation of villous with blood vessels during first and second trimester as compared with third trimester [15, 20]. In addition, GO terms for cell differentiation and communication are also observed and are related to the organization of placenta structure and plasticity [21]. All studies observed a major change in the gene expression pattern at the end of the pregnancy. The third trimester is associated with the over-expression of genes involved in apoptosis, oxidative stress response and inflammatory process [17, 18], whereas the second and third trimesters share the up-modulation of genes associated with immune response. It has been recently shown that the gene expression program of placentas at the third trimester exhibits activation of the immune response and an increase of oxygen-rich maternal blood in placenta, which reflect labor and delivery

ferent gradients of oxygen, which modulate HIF-1α expression.

**2.2. Evolution of placenta gene expression during pregnancy**

route [22, 23].

30 Placenta

#### **2.3. Term labor and gene expression**

The molecular mechanisms regulating the initiation of labor are still poorly understood. The key role of genes involved in prostaglandin synthesis and inflammatory responses in cervix, myometrium and chorio-membranes has been reported in laboring women as compared with non-laboring women [24]. **Table 3** summarized the studies evaluating placenta gene expression from preterm versus term labor with vaginal or cesarean deliveries. The commonly modulated genes are involved in immune response and apoptosis. Oros et al. showed a balance of immune modulators with an increased expression of tumor necrosis factor (TNF) and interleukin (IL)-6 and a decrease in interferon (IFN)-γ expression [26]. These results are likely related with cytokine production of at term placentas [30] and M1 polarization shift of macrophages [31]. Another study showed that gene expression associated with oxidative stress is elevated [25]. It has been previously observed that oxidative stress in placenta explants and production of inflammatory cytokines are related [25]. The TNF levels are elevated during labor, suggesting that TNF may be a biomarker of preterm labor [32, 33]. Based on gene expression in placenta, epidemiological analysis suggested a genetic predisposition to spontaneous preterm labor and preterm birth [34]. Nevertheless, additional investigation of transcriptomic analyses must be necessary to better understand the role of gene expression

Gene Expression Profiling of Placenta from Normal to Pathological Pregnancies

http://dx.doi.org/10.5772/intechopen.80551

33

MicroRNAs are small non-coding 20–24 nucleotides that target and regulate numerous genes [35]. They are expressed by tissues and cells but can also circulate in blood. They can be used as biomarkers [36]. It has been reported that more than 500 microRNAs are produced by the human placenta and a part of them are placenta specific [37]. Their expression varies with the gestational stages; 191 microRNAs are differentially modulated in placentas from first versus third trimester [38]. During the first trimester, microRNAs are associated with angiogenesis, anti-apoptosis and oncogenesis categories, whereas microRNAs from third trimester are related to cell differentiation and tumor suppression categories. Luo et al. found that miR-378a-5p is up-modulated in first and second trimesters but not in third trimester in placenta tissue [39]. The variations of microRNAs expression during different gestational ages suggest that they may regulate specific functions during pregnancy. However, the studies investigating microRNAs in total tissue in normal pregnancy are scarce and further investigations are necessary to better characterize microRNA signature and their temporal expression pattern in

The alteration of gene expression pattern in placenta tissue may reflect metabolic disorders associated with pregnancy such as gestational diabetes mellitus or pathologies that compromise pregnancy success such as preeclampsia [40]. We will summarize main findings reported in preeclampsia and gestational diabetes mellitus that represent the two pathologies mainly

Preeclampsia is a severe placenta disease that occurs in 3–5% of pregnant women and is a source of complications for mother and fetus [41–43]. The preeclampsia is initiated at the time of trophoblast invasion and remodeling of the spiral arteries during the first and early second trimester of pregnancy [41, 44, 45]. Initial studies have shown that elevated levels of various placental proteins in maternal blood such as FLT1 (fms-related tyrosin kinase 1), sENG (soluble engolin) and PGF (placental growth factor) in early pregnancy may be predictive of preeclampsia development [46] but the presence of false negative rates limits their clinical use [47]. The use of high throughput methods such as microarray is likely useful to identify new biomarkers and potential therapeutic targets. **Table 4** summarizes transcriptional studies of

in term labor.

**2.4. Placenta microRNAs in pregnancy**

the placenta at different ages of gestation.

investigated by transcriptomic analyses.

**3.1. Preeclampsia**

**3. Placenta transcriptome in pathological pregnancy**


**Table 3.** Transcriptomic analysis of uncomplicated pregnancies from preterm or term labor.

and production of inflammatory cytokines are related [25]. The TNF levels are elevated during labor, suggesting that TNF may be a biomarker of preterm labor [32, 33]. Based on gene expression in placenta, epidemiological analysis suggested a genetic predisposition to spontaneous preterm labor and preterm birth [34]. Nevertheless, additional investigation of transcriptomic analyses must be necessary to better understand the role of gene expression in term labor.
