Great obstetrical syndromes is a collective name for several complications of pregnancy that affect >15% of pregnancies. They may confer adverse pregnancy outcomes and maternal and fetal morbidity, and require close medical monitoring and treatment. The etiology, pathogenesis, and outcome of these syndromes are obscure in the majority of cases. All appear during mid-to-late pregnancy with no reliable biomarkers for early detection and possibly prevention at present. This article focuses on the quest for early reliable markers for preeclampsia and gestational diabetes mellitus (GDM) development, mainly on the involvement of microRNA in the pathogenesis and its possible role as an early biomarker for disease development.
Great obstetrical syndromes is a collective name for several complications of pregnancy, including preeclampsia, gestational diabetes mellitus (GDM), intrauterine growth restriction, preterm labor, preterm premature rupture of membranes, late spontaneous abortion, and abruptio placentae. These complications collectively affect at least 15% of pregnancies , and may confer adverse pregnancy outcome and maternal and fetal morbidity, and require close medical monitoring and treatment. Their etiology, pathogenesis, and outcome are obscure in the majority of cases. All appear during mid-to-late pregnancy with no reliable biomarkers for early detection and possibly prevention at present.
The early detection of maternal conditions that lead to harmful pregnancy complications, such as preeclampsia, would enable reliable accurate diagnosis and close monitoring, which can lower the risk for the mother as well as the fetus. In spite of considerable advances, this mission remains a major challenge.
This article focuses on the quest for early reliable markers for preeclampsia and GDM development, mainly on the involvement of microRNA (miRNA) in the pathogenesis of those syndromes and its possible role as an early biomarker for detection. Of interest, several of the mechanisms and pathways involved in preeclampsia and GDM described in this article overlap with other obstetrical syndromes and are mentioned here.
Preeclampsia is a pregnancy-associated multisystem disorder characterized by hypertension, proteinuria, and other systemic disturbances in the second half of pregnancy, during labor, or in the early period after delivery . Between 5% and 8% of pregnancies are affected, making it a leading direct cause of maternal and neonatal morbidity and mortality worldwide . The clinical diagnosis of preeclampsia is carried out by the presence of elevated blood pressure (≥140/90 mmHg or mean arterial pressure ≥105 mmHg) and proteinuria (≥300 mg of protein in 24-h urine collection or ≥1 + protein on a urine dipstick). The pathophysiology of preeclampsia is not fully understood. Abnormalities in the development of the placental vasculature early in pregnancy, weeks to months before the development of clinical manifestations of the disease, are well documented . These abnormalities can result in placental underperfusion, and possibly hypoxia and ischemia. Observational data support the hypothesis that placental underperfusion, hypoxia, and/or ischemia may lead to the release of circulating antiangiogenic factors that can cause widespread maternal systemic endothelial dysfunction (increased vascular permeability, vasoconstriction, activation of coagulation system, and hemolysis), resulting in hypertension, proteinuria, and other clinical manifestations of preeclampsia . In some studies, the hypoxia-inducible factor (HIF) protein levels are significantly elevated in the preeclamptic placenta. Furthermore, the restriction of placental perfusion in several animal species has resulted in a preeclampsia-like illness , and experimental hypoxia is also correlated with features of preeclampsia. These data tightened the theory that placental hypoxia may contribute to the pathogenesis of preeclampsia and lead to the syndrome.
GDM is defined as glucose intolerance with onset or first recognition during pregnancy . The prevalence of GDM varies substantially between populations with a range of 1.7–11.6% . Commonly used screening and diagnostic tests involve a 50-g oral glucose challenge test, followed by a 100-g oral glucose tolerance test (OGTT) for women who test positive on the first test . Though there is a strong correlation between maternal overweight and GDM development , the precise mechanisms underlying gestational diabetes remain unknown. During early pregnancy, increases in estrogens, progestins, and other pregnancy-related hormones lead to lower glucose levels. As gestation progresses, however, postprandial glucose levels steadily increase as insulin sensitivity steadily decreases. The inability of pancreatic cells to overcome the insulin resistance created leads to the appearance of GDM. Several genes, including TCF7L2, GCK, KCNJ11, CDKAL1, IGF2BP2, IRS1, and MTNR1B, are thought to modulate pancreatic islet β-cell function and are significantly associated with GDM risk .
Early detection of preeclampsia
In recent decades, there has been an enormous effort to find a serum marker for the early detection of preeclampsia. Soluble vascular endothelial growth factor (VEGF) receptor-1, soluble placental growth factor (phosphatidylinositol-glycan biosynthesis class F protein, PIGF), and soluble endoglin (an antiangiogenic protein) have been proposed as potential markers. However, the results have been disappointing due to a low detection rate and detection at later gestational ages . Following the observation that circulating, cell-free fetal (cff) DNA levels rise in pathologies involving ischemia/hypoxia , it has been suggested that plasma circulating cff DNA levels may represent a potential marker of preeclampsia, probably related to the aforementioned placental ischemia. Currently screening for preeclampsia by cff DNA is not a possibility due to the lack of a universal fetal marker and standardization of techniques .
Early detection of GDM
Several studies attempting to find a reliable biomarker for GDM detection were published recently. Amniotic fluid insulin and C-peptide in early amniocentesis were tested without any disease-predicting value . Adiponectin, high-sensitivity C-reactive protein (CRP), and placental lactogen demonstrated a modest classification performance with GDM in the first trimester . A combination of serum visfatin and maternal characteristics identified >65% of pregnant women who developed GDM, at a false-positive rate of 10% . Elevated tissue plasminogen (t-PA) and low high-density cholesterol (HDL) cholesterol levels were shown to be independent predictors of GDM . Glycosylated fibronectin predicted GDM occurrence with a positive predictive value of 63% and a negative predictive value of 95% . All studies were conducted on a small number of patients without demonstrating a clinically sufficient diagnostic yield.
Early detection of GDM
Several studies attempting to find a reliable biomarker for GDM detection were published recently. Amniotic fluid insulin and C-peptide in early amniocentesis were tested without any disease-predicting value . Adiponectin, high-sensitivity C-reactive protein (CRP), and placental lactogen demonstrated a modest classification performance with GDM in the first trimester . A combination of serum visfatin and maternal characteristics identified >65% of pregnant women who developed GDM, at a false-positive rate of 10% . Elevated tissue plasminogen (t-PA) and low high-density cholesterol (HDL) cholesterol levels were shown to be independent predictors of GDM . Glycosylated fibronectin predicted GDM occurrence with a positive predictive value of 63% and a negative predictive value of 95% . All studies were conducted on a small number of patients without demonstrating a clinically sufficient diagnostic yield.
MicroRNAs
miRNAs, a class of endogenous small noncoding RNAs (about 22 nt in size), play pivotal posttranscriptional regulatory roles in normal physiological functions by targeting messenger RNAs (mRNAs) for cleavage or translational repression. miRNA contributes to the regulatory network by the complementary binding of its “seed sequence” (nucleotides 2–8 of the miRNA itself starting from the 5′ end) and targets in the 3′ untranslated region (3′ UTR) of mRNAs. The altered expression of specific miRNAs has been reported to be associated with a number of diseases . Most miRNAs can regulate the translation of a large number of different mRNAs, and each mRNA can possess multiple binding sites for single or several different miRNAs. The current most comprehensive miRNA database (miRBase) lists 2578 human mature miRNAs (and 1872 precursors). A computational analysis predicts that >60% of human genes are potential targets of miRNAs suggesting that all cellular pathways may be regulated by miRNAs. However, direct experimental evidence defining mRNA targets of miRNA regulation has been reported for only part of these miRNA–target interactions ( Figs. 1 and 2 ).
In recent years, abundantly and differentially expressed miRNA species in placental samples have been reported, which opened up the possibility of finding new effective markers for the early diagnosis and/or monitoring of preeclampsia . Several studies have identified miRNAs that are present in the placenta and predominantly synthesized in trophoblasts. miRNAs have acquired a strong position in the list of factors influencing the different aspects of trophoblast biology, such as proliferation, syncytialization, and invasion, all physiological processes that, when dysregulated, can lead to placental malfunction and to placental-related diseases such as preeclampsia .
Placental miRNAs
The human placenta exhibits a specific miRNA expression pattern that dynamically changes during pregnancy and is reflected in the maternal plasma . So far, >130 miRNAs have been reported to be dysregulated in preeclampsia, out of which only 20 are named in at least two independent studies . This may be due to relatively small study sizes (up to 30 patients), intrinsic patient variations, different definitions of preeclampsia, inclusion criteria and variability in the methodologies, for example, in the RNA extraction protocols, statistical analyses, or others ( Table 1 ).
| Sample | Subtype | Description |
|---|---|---|
| DNA | Cell-free fetal DNA (cff DNA) | Increased cff DNA level was reported in several preeclampsia studies. Inconsistent results in others and knowledge gaps regarding to what affects its production, metabolism, and clearance are subjects to current larger multicentered studies. |
| RNA | MicroRNA | Differential expression of >130 microRNAs was correlated with preeclampsia. Whether such microRNAs are bystander markers of hypoxia, or are directly involved in the pathogenesis of preeclampsia, needs to be clarified. |
| Proteins | Soluble endothelial receptors, antiangiogenic factors | VEGF receptor, PIGF, soluble endoglin, and other hypoxia-related factors have been detected at increased levels in preeclampsia. At present, their accuracy falls far short of sensitivities and likelihood ratios required for clinical use |
miR-210: a specific case
miR-210 appears to be the most highly expressed miRNA in trophoblast cells and in placental tissue, and evidence indicates its involvement in the control of trophoblast proliferation and invasion . The upregulation of miR-210 in the plasma of preeclampsia patients was noticed, with a greater increase in more severe preeclampsia than in its milder form . In addition, the ectopic expression of miR-210 inhibited the migration and invasion capability of cultured trophoblast cells in a transwell assay. Similar to miR-210, miR-21 and miR-16 are highly dysregulated in preeclampsia. Both miRNAs are located in cancer-associated genomic sites and considered “oncomiRs” (miRNAs associated with cancer) and are able to regulate invasion, and their targets are mainly tumor suppressors ( Table 2 ).
