Differential expression of endogenous retroviruses in equine placenta: preliminary results

Endogenous retroviruses (ERVs) are proviral phases of exogenous retroviruses that have co-evolved with vertebrate genomes for millions of years [1]. Conservation of ERV genes through evolution suggests beneficial effects to their hosts’ survival. An example of such positive selection is demonstrated by the syncytin genes that have a key role in placentation [2]. Until now, all the characterized syncytins have been associated with the highly invasive placentation type: endotheliochorial, synepitheliochorial and hemochorial [3]. Only recently, a study has found a retroviral env gene (EqERV) with syncytin-like properties in horses, having epitheliochorial placenta, and appraise its expression in different equine tissues (spleen, liver, lung, kidney and placenta), demonstrating that it is expressed at higher level in the placenta than in the other tissues [4]. Considering the type of placentation in equine species and the possible role of this gene, the exact localization of the EqERVs env gene expression could be relevant in the understanding of equine reproductive physiology and pathology. The aim of the current study was to show if there is a further different EqERVs env expression pattern in various areas of the same placentas (amnion, cervical star, corion). In order to better investigate the expression of EqERVs in target tissues, an improvement in the detection of the EqERVs env gene was performed, through an in silico analysis. Primers were designed on the env region of the candidate full-length ERV. Placentas were collected immediately after an eutocic delivery and a sampling of specific areas (amnion, cervical star, corion) of each placenta was carried out. Total RNA was extracted, treated with DNase I and reverse transcribed into cDNA, before qPCR reaction. The expression ratio of the gene of interest was normalized relatively to two reference genes. Among the selected full-length EqERVs, we found only one candidate showing an ORF longer than 300 aa suitable for downstream investigation. qPCR assay showed that this gene is regularly highly expressed in the placenta as expected for a candidate syncytin-like gene. Additionally, a different EqERV env pattern of expression also was found between amnion, cervical star and corion. These findings suggest a possible role of EqERV in placental tissue, which deserves further investigation. Moreover, because in horses there is no syncytiotrophoblast layer at their maternal-fetal interface, a putative immunological role of this EqERV in relation with maternal-fetal tolerance could be hypothesized. A possible interference with diagnostic activity for exogenous Equine Infectious Anemia virus should also be evaluated.