Transcriptomic analysis of placental tissue aids in understanding the pathogenic mechanisms of SARS-CoV-2 in pregnancy at the cellular level

By Tarun Sai LomteJan 24 2023Reviewed by Danielle Ellis, B.Sc.

In a recent study posted to bioRxiv*, researchers explored the pathogenic mechanisms of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in pregnancy using digital spatial profiling of placental tissue.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

Several studies have reported the development of a pre-eclampsia-like syndrome in pregnant females exposed to SARS-CoV-2. Vasculopathy and inflammation are elevated in the placental tissues of patients with coronavirus disease 2019 (COVID-19). Placental dysfunction is the root cause of pre-eclampsia. Pre-eclampsia is categorized into early-onset (< 34 weeks of gestation) and late-onset (> 34 weeks) according to the time of symptom detection.

Early-onset pre-eclampsia is linked to poor placentation leading to placental malperfusion and dysfunction, albeit its causes remain unclear. Maternal pathologic factors leading to placental dysfunction, such as endothelial dysfunction and systemic hypertension, are thought to contribute to late-onset pre-eclampsia.

About the study

In the present study, researchers used whole-transcriptomic digital spatial profiling of placentae to understand the mechanisms of SARS-CoV-2 pathogenesis in pregnancy. The study involved non-vaccinated pregnant individuals who were COVID-19 symptomatic within their third trimester. Archived placental tissues collected from 2016 to 2018 served as controls.

SARS-CoV-2 and control groups were matched on gestational age, maternal age, and comorbidities. Placental size, weight, macroscopic changes, fetal sex and weight, and delivery method were recorded. A trained pathologist analyzed placental specimens, and tissue microarrays (TMAs) were prepared from the samples.

Investigations were focused on seven samples from the SARS-CoV-2 group due to the demise of three fetuses. Likewise, one sample from the controls was excluded due to low quality. TMA slides were stained using fluorescent markers to characterize cell types and histologic features. TMAs were hybridized using a whole-transcriptome atlas (WTA)-barcoded probe set of approximately 18,000 genes.

Raw data were normalized to 134 negative probes in the WTA probe set. Differential gene expression analysis was separately performed for villous core stroma cells and trophoblasts containing regions of interest between SARS-CoV-2 and control groups. Pathway enrichment analyses were performed, and the gene set enrichment was clustered and visualized.

Findings

Placenta cores were obtained from seven participants who tested COVID-19-positive in the past 15 days pre-delivery and nine controls. No significant differences were noted in placental weight, fetal weight, maternal age, or gestational age between SARS-CoV-2-positive subjects and controls. Three neonates from the SARS-CoV-2 group and four from controls were delivered pre-term.

Four SARS-CoV-2-positive subjects and five controls had comorbidities, including gestational diabetes mellitus, bipolar disorder, hyperthyroidism, and hypothyroidism. Placentae from three SARS-CoV-2-positive individuals showed infarcts. Placentae from two controls exhibited hypoplasia leading to pre-term delivery.

There was no evidence of SARS-CoV-2 viral load in the placenta cores from COVID-19 subjects. The SARS-CoV-2 group had a significantly higher number of cluster of differentiation (CD68)-positive immune cells in the terminal villi than controls. Principal component analysis (PCA) revealed the separation of trophoblasts and villous core stroma cells between the two groups.

There were more differentially-expressed genes (DEGs) in response to SARS-CoV-2 at the anchoring villi than at the terminal villi for villous core stroma cells and trophoblasts. Further, DEGs at the anchoring and terminal villi were highly overlapping. However, there was minimal overlap in DEGs between villous core stroma cells and trophoblasts.

Pathway enrichment analysis revealed significant activation of several pre-eclampsia- and oxidative stress-associated pathways. Villous core stroma cells displayed positive enrichment of immune-related pathways. Trophoblasts showed positive enrichment of estrogen response, peroxisome pathway, and E2F targets.

Overall, 1641 and 1339 genes were significantly up- and down-regulated, respectively, in trophoblasts at the terminal and anchoring villi in response to SARS-CoV-2. Notably, pregnancy-associated plasma protein A (PAPPA), PAPPA2, and placental alkaline phosphatase (ALPP) were upregulated. Downregulated genes included complement component C1Q, osteopontin, caveolin 1, coagulation factor XII, and several collagens.

Pathways related to blood pressure and vascular tension were upregulated, and those involving collage deposition, coagulation, vasoconstriction, allograft rejection, and complement pathways were downregulated. In villous core stroma cells at anchoring and terminal villi, 1026 genes were upregulated, and 2697 were downregulated in response to SARS-CoV-2; Pathways related to oxidative stress, methylglyoxal levels, and nitrosative stress were enriched in them.

Conclusions

Taken together, the researchers illustrated the unique response of villous core stroma cells and trophoblasts from the terminal and anchoring villi of the placenta to SARS-CoV-2. Examining transcriptional changes in these cells revealed the enrichment of pre-eclampsia-related pathways. This suggests that the placenta in pregnant subjects with SARS-CoV-2 adopts a transcriptional profile aligning with that observed in pregnant females with pre-eclampsia.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.