Molecular characterisation of a putative endogenous retrovirus cDNA isolated from human placental tissue

The human genome comprises of abundant DNA sequences related to endogenous retroviruses (ERV) and a variety of solitary long terminal repeats (LTRs). Substantial numbers of intact retroviral particles have been detected by electron microscopy in normal human placental villous tissue particularly in syncytiotrophoblast. Understanding the molecular structure, organisation and distribution of these ERV sequences may lead to elucidation of their possible dual function at the foetal-maternal interface; proliferation and differentiation of cytotrophoblast and induction of local pregnancy-associated immune suppression thus allowing survival of the foetal allograft. In this study, antibody probes were used to screen a human placental expression library and cDNA clones isolated were characterized by polymerase chain reaction, Southern blot hybridisation, DNA cloning and partial nucleotide sequencing. A specific 1.7kb-cDNA clone was isolated from a human placental expression library. Further characterisation showed this clone represents a single copy gene, approximately 9-10kb and did not hybridise to the env region of ERV3 human endogenous retrovirus. The 1.7kb-cDNA clone may represent a provirus co-expressed with cellular sequences.