The characterization of LINE-1 elements in the human placenta and their role in preeclampsia

Abstract

Long interspersed nuclear elements (LINE-1), a type of retrotransposon, are genetic elements in the human genome that can self-amplify and integrate back into the genome, increasing their copy number. Their mode of action is called retrotransposition, and it is known to affect the human genome in many ways: generating insertional mutations, genomic instability, alterations in gene expression, and contribute to genetic innovation. These elements are usually silenced in somatic tissue because of their mutagenic potential. I have identified that the evolutionary youngest and human-specific LINE-1 elements are expressed and active in human placental trophoblast cells and that LINE-1 elements are over-expressed and possibly over-active in certain patients of the pregnancy disorder pre-eclampsia (PE). PE is a pregnancy-specific complex disorder in which a combination of poor placentation and placental stress leads to hypertension and proteinuria in the mother following the 20th week of gestation. PE affects 2-8% of all pregnancies. Despite PE being the leading cause of maternal and neonatal mortality and morbidity, the etiology remains unknown. One major problem in PE research is that PE is a multi origin disorder, exhibiting different phenotypes before the onset of the disease. I hypothesized that LINE-1 overexpression and activity in an immune-mediated subtype of PE patients contributes to PE pathogenesis. I conducted RNA sequencing in 10 PE and 8 control primary human trophoblast and have identified that human-specific LINE-1 elements, L1HS, are over expressed in 3/10 patients. Using RT-PCR, western blot, and immunohistochemistry methods, I have shown that LINE1s are expressed in primary human trophoblasts at the RNA and protein levels and in all three subtypes of trophoblast cells. Using a LINE-1 retrotransposition reporter assay, I showed that trophoblast cells contain all the necessary conditions to support LINE-1 retrotransposition and that there is greater retrotransposition activity in the PE-model trophoblast cell line. In addition, I identified that LINE-1 over expression in a subset of PE patients induces the expression innate immune response, suggesting a mechanism of early pathogenesis induction in a subtype of PE patients. Lastly, I also showed that in PE, a TE-derived miRNA, has-miR-576-5p, is upregulated, among other newly discovered dyregulated miRNAs. The expression of placenta fuction-relevant genes that are predicted targets of has-miR-576-5p was investigated, but no dysregulation was determined. Overall, this research revealed novel information about the somatic expression and activity of LINE-1 in the human placenta, and for the first time provided insight into the potential role of LINE-1 in PE pathogenesis.