Fig. 1

Schematic illustration of the examined human endogenous retroviruses, ERVWE1, the adjacent endogenous retroelement HERV-H at chromosome 7q21.2, and two HERV-W loci on chromosomes 4 and 21. Human endogenous retrovirus ERVWE1 includes 5′LTR containing the enhancer/promoter sequence, 3′LTR, an intron of cellular origin located downstream of 5′LTR, the Δgag-Δpol gene and the env gene with an intact open reading frame. Two RNA variants of ERVWE1 can be produced (depicted by bold lines): full-length RNA, which includes all three genes mentioned above and monocistronic spliced mRNA containing the env gene. The Syncytin-1 protein can be produced only from the spliced form. ERVWE1 itself integrated into MaLR LTR whose parts are located on both sides of the ERVWE1, Trophoblast-specific enhancer (TSE) located upstream of the ERVWE1 5′ LTR contains a GCM1 binding site and is required for placenta-specific expression of ERVWE1. Another retroelement in the close proximity of ERVWE1, the human endogenous retrovirus HERV-H, consists of 5′ and 3′ LTR and the gag-pol gene, and is integrated within the LTR9b retroelement. In addition, we examined expression of two HERV-W elements on chromosomes 4 and 21. Both contain 5′ and 3′ LTRs, an intron of cellular origin located downstream of 5′LTR, the ∆gag-∆pol gene and the ∆env gene. Pairs of primes used for the qRT-PCR analysis are depicted by arrows. Length-scale bar indicates 1 kbp. Chromosomal positions assigned according to the genome assembly GRCh38/gh38, December 2013