N Engl J Med 2014; 370:2534-2536June 26, 2014DOI: 10.1056/NEJMcibr1404324
For decades, researchers have unsuccessfully sought to understand the mechanisms underlying the onset of parturition. The apparent variation in mechanisms between other mammals and humans has further confounded these efforts. And yet, it does not make sense that an event so essential for successful reproduction and the propagation of evolutionarily related species would have radically diverged in signaling mechanisms. Multiple lines of evidence now suggest that an inflammatory process is involved in triggering parturition, a process that is modulated and influenced by maternal, fetal, and placental endocrine events. However, another important factor may be cell-free fetal DNA.
In 1998, Lo et al.1 observed cell-free fetal DNA in maternal plasma at concentrations of 3.4 to 6.2% of the total cell-free DNA in plasma, along with a dramatic increase in levels of cell-free fetal DNA between early and late gestation. Subsequent studies have shown that cell-free fetal DNA is derived from placental trophoblasts as they undergo cellular apoptosis and necrosis. Others studies have documented an increase by a factor of 12 in cell-free fetal DNA levels throughout gestation, with levels peaking at the end of pregnancy,2 and an inverse correlation between cell-free fetal DNA levels and the interval between blood collection and birth.3
Confirming the relationship between increased levels of cell-free fetal DNA and parturition is the observation that pregnant women who deliver prematurely were found to have twice the levels of cell-free fetal DNA as compared with women in a control group or those with threatened preterm labor.4Other studies have shown that levels of cell-free fetal DNA above the 95th percentile are associated with increased rates of preterm delivery. Notably, levels of cell-free fetal DNA have been reported to be 30% higher in twin gestations; such increases could be responsible for the shortened gestational length in multifetal pregnancies.
The presence of cell-free fetal DNA derived from fetal or placental sources is not unique to human pregnancies, since cell-free fetal DNA has also been detected in the plasma of pregnant mice, cows, horses, sheep, and primates. In mice and macaques, levels of cell-free fetal DNA have been reported to increase markedly during late gestation and then drop to undetectable levels after birth.5,6
The association between elevated levels of cell-free fetal DNA and parturition is interesting, but how might these observations be mechanistically linked? The pattern-recognition receptor TLR9, which is activated by free-circulating hypomethylated DNA, is an excellent candidate linker. TLR9 agonists have induced preterm deliveries when injected into pregnant mice. Fetal DNA injected into pregnant mice produces gestational losses (resorptions), which do not occur when adult DNA is used or when TLR9-deficient mice are injected with fetal DNA.7
Multiple previous studies have provided compelling support for the premise that spontaneous parturition is mediated by the activation of inflammation-related signaling pathways, leading to the increased secretion of proinflammatory cytokines and chemokines, the influx of neutrophils and macrophages into the pregnant uterus, and increased production of uterine-activation proteins, including the oxytocin receptor, prostaglandin F receptor, and connexin 43. These events also stimulate the activity of matrix metalloproteinases and the release of uterotonins, leading to cervical ripening, membrane rupture, and phasic myometrial contractions.
The missing link has been the fetal or placental signal that triggers these proinflammatory events in the absence of microbial invasion and intrauterine infection. In aggregate, several studies provide solid support for the hypothesis that increased levels of cell-free fetal DNA, which are released during placental maturation or senescence and peak at the end of gestation, activate the innate immune system through stimulation of TLR9 (or another DNA-sensing pattern-recognition receptor), leading to parturition (Figure 1FIGURE 1Proposed Signaling Pathway Leading from Placental Production of Cell-free Fetal DNA to Parturition.). If the hypothesis is true, this mechanism is potentially responsible for triggering the onset of parturition in all mammals.
Disclosure forms provided by the author are available with the full text of this article at NEJM.org.
This article was updated on June 26, 2014, at NEJM.org.
From the Vincent Center for Reproductive Biology, Department of Obstetrics and Gynecology, Massachusetts General Hospital, Boston.
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