Preeclampsia is a hypertensive and inflammatory condition of pregnancy which carries an increased risk of thrombosis and subsequent organ failure. Preeclampsia is also dangerous for the fetus, carrying an increased risk of growth restriction and stillbirth. Preeclampsia occurs in around to 4.6% of pregnancies worldwide1, with over 70,000 of these pregnancies resulting in death for the pregnant woman/person and half a million of them resulting in infant deaths each year. If left untreated, preeclampsia can develop into eclampsia, which is an even more dangerous disorder than preeclampsia. Despite the obvious need for therapeutics in this area, there are currently no treatments beyond symptom management. The only cure for preeclampsia is the delivery of the baby or, more precisely, the delivery of the placenta. This often means iatrogenic preterm delivery, for which preeclampsia carries an increased risk (adjusted odds ratio 5.30).2
The pathogenesis of preeclampsia is not fully elucidated but is thought to involve several potentially concurrent and intertwined pathogenic routes. The first is the inadequate remodelling of the uterine spiral arteries. These tightly coiled arteries supply blood to the placenta and must be remodelled by extravillous trophoblasts and immune cells into wide, low-pressure vessels (Figure 1). Failure of this process is associated with placental damage and hypoxia. Furthermore, the placenta has been shown to release anti-angiogenic factors such as the soluble vascular endothelial growth factor receptor 1 (sFlt-1), and a proinflammatory environment prevails.
Figure 1. Schematic representation of spiral artery remodelling. Extravillous trophoblast cells (EVTs) migrate out from the tops of anchoring placental villi and invade into the decidual spiral arteries. Here they work with immune cells to remodel the arteries into wide, high conductance, low pressure vessels. EVTs line the lumen of remodelled vessels, replacing the endothelium. In preeclampsia this remodelling is incomplete and does not reach the myometrium, leaving high pressure, low conductance vessels such as the spiral artery depicted on the left side of the image.
A biomarker does exist for preeclampsia and is used widely in clinical practice. The ratio between sFlt-1 and placental growth factor (PlGF) has been shown to distinguish well between preeclamptic and normal pregnancies.3,4 However, in practice the sFlt-1:PlGF ratio is utilised only in the third trimester, and has been shown to be more effective at predicting short term outcomes.5 This late predictive window is despite preeclampsia having its roots much earlier in pregnancy, which would suggest that earlier prediction might be possible. This is where researchers have turned to extracellular vesicles (EVs), which are released from the placenta and from all cells, possibly allowing for peripheral sampling to give a window into early preeclampsia pathogenesis.
Indeed, a study by McElrath et al. (2020) identified a panel of altered proteins within later first trimester circulating EVs that differed between those pregnancies which went on to develop preeclampsia and those which did not.6 Interestingly, the analysis in this study identified two clusters of preeclamptic patients long before diagnosis. The first cluster was enriched for alterations in proteins involved in coagulation and platelet degranulation and a second cluster enriched for dysregulated immune responses (e.g., complement proteins). This second cluster was associated with a more severe preeclamptic phenotype, with significantly earlier gestational age at delivery and higher proteinuria, and a trend towards worsened hypertension.6 This gives hope that not only could an EV-based biomarker predict preeclampsia in early pregnancy, but that severity may also be predicted, guiding clinical management.
NX Prenatal is driving forward with this research, leading in the development of an EV-based biomarker test to predict preeclampsia in early pregnancy. Brian Brohman, CEO of NX Prenatal, reflects on NX Prenatal’s work to date and its potential implications:
"Our studies to date suggest that first trimester risk stratification and sub-typing are feasible with EV-derived multiplexed protein panels from maternal plasma. The potential impact of this approach for the future optimisation and development of prophylactic and therapeutic interventions for preeclampsia compels us to actively pursue commercialisation of these tests. We also now understand that EVs are providing us a snapshot of important molecular crosstalk occurring at the utero-placental interface, enabling further breakthroughs in conditions such as placenta accreta."
EVs have been shown to hold power in the prediction and stratification of preeclampsia, which begs the question: what roles do EVs play in pathogenesis? This is particularly of interest, given these EVs were found to contain inflammatory proteins.6
One route of investigation can be found by considering that placental removal is an effective treatment for preeclampsia. It follows, therefore, that factors promoting disease are released from preeclamptic placentas. Indeed, there is evidence that syncytiotrophoblast EVs from preeclamptic placentas cause endothelial damage7,8, partly via the modulation of nitric oxide synthase. Placental EVs in preeclampsia reduce the availability of nitric oxide (a key vasodilator and antioxidant) for endothelial cells by carrying less nitric oxide synthase compared to EVs from healthy placentas, and by carrying increased miR-155 which reduces expression of endothelial nitric oxide synthase.8,9
In addition to potentially decreasing nitric oxide availability for endothelial cells, syncytiotrophoblast EVs from preeclamptic placentas also have a prothrombotic effect, inducing platelet clotting.10 Given that there are more syncytiotrophoblast EVs released in preeclampsia11,12, especially in early-onset preeclampsia13, the impact of placental-derived EVs in preeclampsia could be a significant contributor to pathogenesis.
If EVs contribute to preeclampsia pathogenesis, could other EVs be harnessed for treatment? One way in which EVs could be used therapeutically in preeclampsia is to deliver beneficial molecules; for example, the delivery of the long non-coding RNA H19. Preeclamptic placentas have higher levels of the miRNA let-7c which downregulates FOXO1, potentially inhibiting extravillous trophoblast migration (See Figure 1 for the importance of this).14 H19 binds to let-7c, preventing its binding to FOXO1. Mesenchymal stem cell EVs containing H19 improved migration and invasion of an extravillous trophoblast model cell line, suggesting that H19 could be a promising therapeutic.14 Some EVs in their native state are also beneficial to trophoblast migration and proliferation, such as mesenchymal stem cells from the chorion and umbilical cord, and those from endometrial cells.15-17 Human umbilical cord mesenchymal stem cell EVs also increased endothelial cell tube formation in vitro by modulating vascular endothelial growth factor signalling, potentially combatting some of the negative impacts of preeclampsia on endothelia.18-20 These studies provide hope that EV based treatments for preeclampsia could prove fruitful.
As research into EV biomarkers and therapeutics for preterm birth and preeclampsia continues to grow, there are ongoing focused efforts to pursue relevant tests, for example by NX Prenatal who have made significant progress in their biomarker discovery and validation efforts. Such developments are not without challenges, given the cause of preterm birth is largely unknown and is likely to be multifactorial, as are the causes of major risk factors such as preeclampsia. This is likely because both are endpoints which may be arrived at by more than one route of pathogenesis. However, it is a pathway worth pursuing, with potential significant benefits for many. For successful biomarker discovery and therapeutic success then, stratification of patient populations is required to ensure that results are not clouded by varying routes of pathogenesis.21 Therefore, identifying tools to adequately stratify patients for this purpose is likely key to advancing this research to clinically meaningful endpoints.
Learn more about Extracellular Vesicles as Predictors of Premature Birth