Low maternal serum isthmin-2 levels in the first trimester may identify pregnancies at increased risk of subsequent preeclampsia or fetal growth restriction, with experimental findings linking the placenta-derived protein to extravillous trophoblast invasion.
Researchers analyzed samples from 3 prospective pregnancy cohorts, using the POPS study, which recruited 4,512 nulliparous patients with singleton pregnancies, 4,212 of whom were followed through delivery. Blood was collected at approximately 12, 20, 28, and 36 weeks of gestation. Using a case-cohort design, the researchers measured 2,904 maternal serum proteins at approximately 12 weeks and evaluated the findings in the independent POPS2 cohort and the IMPACT cohort.
The researchers evaluated 4 outcomes: preeclampsia (PE) without fetal growth restriction (FGR), FGR without PE, PE with FGR, and a composite of any of the 3 outcomes. They also compared the prognostic discrimination of theISM2 protein with established placental biomarkers and examined its association with uterine artery blood flow. The researchers then investigated ISM2 expression and function using human trophoblast stem cells, placental organoids, and a cell line lacking endogenous ISM2.
At about 12 weeks, low ISM2 was the strongest disease-associated signal and the only protein associated with all 4 outcomes, ranking first for PE alone, FGR alone, and the composite outcome and second for PE with FGR. Its summed rank across the 4 outcomes was 5 vs. 88 for the next-highest protein.
ISM2 levels remained lower among cases at approximately 20 weeks, did not differ between cases and controls at 28 weeks, and were higher among cases at 36 weeks. At about 12 weeks, ISM2 showed greater discrimination for the composite outcome compared with 5 established placental biomarkers measured in the same samples. Its discrimination was stronger for composite outcomes resulting in preterm rather than term birth. Lower ISM2 at about 12 weeks was also associated with greater uterine artery resistance at about 20 weeks among cases but not controls.
The association was reproduced in both validation cohorts. Among the first 2,595 patients with outcome data in POPS2, ISM2 had the lowest summed rank across the 4 outcomes and was the strongest signal for the composite outcome. ISM2 showed greater discrimination compared with placental growth factor. Among the individual outcomes, its discrimination was greatest for pregnancies complicated by both PE and FGR. In the IMPACT study, first-trimester ISM2 levels were lower among those who subsequently experienced PE complicated by preterm birth or FGR and showed greater discrimination compared with placental growth factor.
The researchers assessed whether the clinical association was consistent with a functional role for ISM2 in trophoblast invasion. Placental ISM2 messenger RNA expression was approximately 1,000-fold higher than the median expression across other tissues, and ISM2 expression increased approximately 15-fold when human trophoblast stem cells differentiated into extravillous trophoblast (EVT). Reducing ISM2 expression by more than 90% prevented normal EVT differentiation and abolished invasion from 3-dimensional placental organoids into Matrigel.
Adding recombinant ISM2 following knockdown restored EVT morphology in a dose-dependent manner, while ISM2 knockdown reduced invasive capacity. Conversely, expressing ISM2 in cells that did not normally produce the protein increased migration, and conditioned media containing ISM2 also increased invasion. The researchers suggested that secreted ISM2 was necessary for EVT invasion and sufficient to promote migration in cells lacking endogenous ISM2.
The clinical analyses were focused on northern European cohorts and the association requires assessment in other populations. The study did not determine why first-trimester ISM2 levels were lower among patients who subsequently developed PE or FGR. Although the findings identified effects of ISM2 on trophoblast invasion and cell migration, the mechanisms underlying those effects remains unknown.
“[W]e show that low ISM2 in the first trimester of pregnancy had the strongest association with the subsequent experience of PE and/or FGR among 2,904 proteins studied in maternal serum,” wrote lead study author Ruoyu Miao, Department of Obstetrics and Gynaecology at the University of Cambridge, and colleagues.
Full disclosures of the study authors can be found in the study.
Source: Nature Medicine
